low viscous hydrophilic processing additives for extrusion of polyethylene at reduced temperatures

Low Viscous Hydrophilic Processing Additives forExtrusion of Polyethylene at Reduced Temperatures

Oleg Kulikov,1 Klaus Hornung,1 Manfred Wagner21 University of the Federal Armed Forces Munich, LRT-7 and Polymerphysik, 85577 Neubiberg, Germany

2 Technical University Berlin, LRT-7 and Polymerphysik, Fasanenstr. 90, D-10623 Berlin, Germany

Industry is using fluorinated polymer processing addi-tives (PPA) to delay the onset of sharkskin to higherrates of extrusion of PE resins. Yet it is necessary tokeep elevated temperatures during extrusion to reduceapparent melt viscosity. We propose to use low vis-cous PPA made from reacting mixtures of polyethyleneglycol with organic polyacids, phosphoric acid, andpolyesters of oxiacids of Phosphorus. Surprisingly,extrusion pressures and apparent viscosity with thenovel PPA at reduced temperatures are less, than atelevated temperatures. In total, extrusion pressurescan be reduced 2–5 times for concentrations of PPAfrom 0.1 to 0.5 wt%, while sharkskin melt fracturecan be eliminated for concentrations of PPA above0.02 wt%. Extrusion with the novel PPA at reducedtemperatures potentially increases productivity, re-duces production cost, and allows processing of PEresins of higher MW and highly filled polymer composi-tes. POLYM. ENG. SCI., 50:1236–1252, 2010. ª 2010 Society ofPlastics Engineers

INTRODUCTION

Advantages of Extrusion of Polyolefin Resins atReduced Temperatures

Extrusion of thermoplastic polymers is used for pellet-

izing, fiber spinning, sheet and film manufacturing, pipe,

tube and profile forming, as well as for wire and cable

insulation [1]. Opposite to extrusion of metals [2], the

extrusion of polymers goes ordinarily at temperatures

above the melting point of the material. High molecular

weight (MW) polymers can be used to produce articles

with enhanced mechanical properties and chemical stabil-

ity, but flow of such polymers inside the processing

equipment and through the forming die is impeded by

very high friction losses because of high viscosity of

melts. Viscosity of thermoplastic polymers decreases with

heating of the melt, and therefore processing of polymers

with high MW is commonly performed at temperatures

that are far above the melting point. At such temperatures,

molten organic polymers rapidly react with oxygen and

chemically decay with the consequence of declining me-

chanical and optical properties. Additionally, at high tem-

peratures polymer flow is unstable to bubble and helical

instabilities in film blowing, and to draw resonance in

fiber spinning and film casting [3–9]. Production rates in

processing by extrusion depend very much on the temper-

ature and the achievable rate of cooling of the extrudate.

Reduced temperatures of extrusion resulting in shorter

cooling times allow an increase in productivity, especially

for film blowing, blow molding, and forming of pipes of

large diameter to transport gas and water. Since polymers

are subject to thermal degradation, reduced extrusion tem-

peratures would also allow a smaller degree of macromo-

lecular degradation. Crystallization of molten polymer

and therefore optical and mechanical properties, as well

as gas permeability of PO films depends on the rate of

cooling and deformation of the polymer matrix. In many

cases, extrusion at reduced temperatures would help to

decrease the size of crystallites and to improve transpar-

ency of extruded films from PO resins.

PO resins are the biggest and the fastest growing poly-

mer family of commodity polymers. Polyethylene (PE)

resins represent about 40% of all polymers produced in

the world, whereas the share of Polypropylene (PP) resins

is about 22%. The PE resins made from modern catalysts

(Ziegler-Natta and metallocen) are cheaper (price differ-

ence is about 10%), cleaner, and mechanically stronger in

comparison with PE made from older catalyst (chromium

oxide) [10], but extrusion rate and processability are lim-

ited by the onset of the sharkskin instability [11, 12].

Melt Index (MI) is an indication of viscosity of the poly-

mer. The MI is measured by the grams of polymer that

are extruded through a narrow orifice held at 1908C with

a specified weight. If the MI is low, the apparent viscosity

is high and vice versa. LDPE is characterized by strong

Correspondence to: Oleg Kulikov; e-mail: [email protected]

Contract grant sponsor: German Science Foundation (Deutsche

Forschungsgemeinschaft).

DOI 10.1002/pen.21645

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2010 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2010

shear thinning, and therefore it can be extruded through a

forming die at relatively low pressures in comparison to

LLDPE of the same MI. Therefore film-blowing machines

designed for extrusion of LDPE and LDPE/LLDPE blends

cannot be used for extrusion of LLDPE and LLDPE/

LDPE blends. The extrusion pressure would be reduced if

the extrusion die has a wider die gap. Film blowing with

such dies requires high drawdown and blowup ratios.

Drawdown is defined as the ratio of the die gap to the

film gauge. The blowup ratio in film blowing is defined

as the ratio of the largest bubble diameter to the diameter

of the die. The ultimate drawdown rate of known PE res-

ins is limited by the onset of a melt flow instability

known as draw resonance, and for LLDPE the critical

drawdown ratio can be as low as 2 [13]. Additionally, at

high drawdown ratios long molecules are stretched and

predominantly oriented along the machine direction so

that the produced PE film manifests anisotropic mechani-

cal properties. The anisotropy of the mechanical proper-

ties of PE film is considered as undesirable for many

applications.

Increasing temperatures of LLDPE melt to reduce its

apparent viscosity and to delay the onset of sharkskin

melt fracture generally necessitates lower rates of film

formation because of bubble instability as well as limita-

tions of air-cooling and heat transfer, e.g. see [14, 15]. To

overcome this discrepancy, Hutchinson and Blanchard

[16] proposed to heat up the tip of the extrusion die so

that only the surface layer of the extrudate is heated up

and therefore bubble instability is reduced. Quite opposite,

Cogswell [17] proposed to cool down a surface layer of

the extrudate at the die exit to temperatures not more than

158C above its solidification point to delay an onset of

the sharkskin melt fracture. In extrusion of polymers with

narrow MWD, e.g. LLDPE, HDPE, and fluorinated PO

resins, the extrudate starts to slip along the die surface at

some critical rate of extrusion. Fields and Wolf [18] pro-

posed a method of producing articles with smooth surface

at extrusion rates above the critical extrusion rate for the

onset of slip. Extrusion temperatures are supposed to be

at least 1008C above the melting point of the polymer. To

our knowledge, this method to suppress sharkskin melt

fracture and to reduce pressures in extrusion is practically

used for processing of polymers with narrow MWD (e.g.

HDPE), by blow molding, but has its limitations because

of the ‘‘die drool’’, i.e. accumulation of polymer at the

die exit.

Conventional Polymer Processing Additives

Processing of neat polymers by extrusion is rarely

possible. Instead, it is a common practice to formulate

compositions containing polymer and a variety of addi-

tives in relatively small, but critical amounts, see e.g

[19–21]. Foremost among these additives are lubricants

to reduce extrusion pressure and eliminate sharkskin melt

fracture as well as the stick-slip instability [22]. In the

early 1960s, DuPont Canada accidentally discovered that

fluorinated polymer added in a small amount to a

LLDPE resin works as slip agents and postpones the

onset of the sharkskin melt fracture. Mechanism of trans-

fer of fluorinated polymer processing additive (PPA) to

the die wall is investigated in [23]. It was demonstrated

that during the extrusion of PPA/LLDPE blends, droplets

of fluoropolymer that are in close proximity to the sur-

face of the die first coat the die entrance, and then they

flow as streaks toward the die exit under the influence of

shear field. Fluorinated polymers are still in use as PPA,

e.g. Viton from DuPont, Dynamar from 3M, Kynar from

Arkema, and Tecnoflon from Solvay Solexis [24]. They

are typically employed at concentrations of about 250 to

3000 parts per million (ppm) based on the mass of the

thermoplastic material. During extrusion of the blend of

LLDPE with PPA, the fluorinated polymer deposits at

the die walls, displaces LLDPE and induces slip of the

melt along the coated surface. Because of the slip the

pressure that is necessary to extrude molten LLDPE

through the die is reduced by about 20% from the refer-

ence values for extrusion of LLDPE without adding the

fluorinated PPA.

According to Slattery and Giacomin [25], the plastics

industry spends at least 2% of the cost of the resins used

in extrusion on PPA to delay the sharkskin melt fracture.

The total cost of the fluorinated additives used by the

industry in 1993 was about $200 Million. The main

problem, but not the only one, caused by the commercial

use of fluorinated polymers as PPA, is a tendency for

plate-out of decomposed fluorinated polymer on the ex-

truder screw and/or the die exit, i.e. die build-up. The

problem is often severe, requiring shutdown of the equip-

ment and extensive clean-ups. Fluorinated polymers are

extremely hydrophobic, and their use as PPA enhances

the hydrophobic properties of the PE film, as well as

the accumulation of static electrical charge at the film

surface.

Deposits of fluorinated PPA inside the processing

equipment can be essentially reduced and the accumula-

tion of static electrical charge at the PE surface can be

avoided as well as the amounts of fluorinated polymers

needed to suppress sharkskin in extrusion of LLDPE can

be diminished (by about a factor of three) if polyethyl-

ene glycol (PEG) is added to the polymeric material to-

gether with the fluorinated PPA. The use of such syner-

gistic combinations is well known, see e.g [26–35]. PPAs

consisting of blends of PEG with fluorinated polymers

for extrusion of PO resins are available commercially

under trade names of Kynar, Dynamar, and Viton.

Nevertheless, fumes from burning of the fluorinated poly-

mers, e.g. during incineration of LLDPE film produced

with fluorinated PPA, are toxic and potentially cancerous.

As the fumes are Fluorine containing gasses, they deplete

the ozone layer of our planet and their use, even at

reduced amounts, in the composition of PPA cannot be

desirable.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2010 1237

Mechanisms of Lubrication and Sharkskin Elimination

A cyclical instability in peeling of pressure-sensitive

adhesive (PSA) tapes that involves alternate storage and

dissipation of elastic energy is well known, e.g. in peeling

of PSA tapes from glass surface [36]. This is a so called

‘‘stick-slip’’ mode of peeling when smooth ‘‘adhesive’’

failure at the adhesive-glass interface is alternated with

‘‘cohesive’’ separation within an adhesive layer. It is easy

to see some similarity in periodic structures at the PSA

tapes in the ‘‘stick-slip’’ mode of peeling and sharkskin

defects at the surface of an extrudate.

Conventional theories predict that adhesion of PSA to

substrate should be proportional to its surface free energy

but Newby and Chaudhury [37] demonstrated that adhe-

sion on fluorocarbon surface is significantly larger than on

some hydrocarbon surfaces, although the fluorocarbon sur-

face has the lowest surface energy. Later they clearly evi-

denced slip motion of adhesive on elastic substrate (poly-

dimethylsiloxanes) just before it peels out using tracking

fluorescent particles placed at the interface, see [38, 39].

Amouroux et al. [40] demonstrated that amplitude of

interfacial slip motion in peeling of an acrylic tape from a

silicone elastomer is correlated to the ratio of an elastic

component of the complex Young modulus E0 and a vis-

cous component E00 of the silicone elastomer. The slip

motion is decreased for the reduced ratio E0/E00 that is forreduced elasticity of the elastomer.

In accordance with ideas of Hill et al. [41] based on

the apparent relation between adhesive failure and shark-

skin melt fracture we believe that sharkskin instability is

caused by swelling of the extrudate at the die exit, flow-

ing of the molten polymer around the die rim, stretching

of its surface layer outside the die, and periodical failures

in adhesion of the molten polymer at the die rim. Due to

stretching, the surface layer accumulates elastic energy

and releases it during the act of adhesion failure. The ad-

hesion failure propagates along the surface of the die as a

crack. At the die rim, the crack deviates from the die wall

into the melt and produces a seed crack, i.e. a notch. The

seed crack grows, ruptures the surface layer and creates a

valley at the surface of the extrudate. Upstream of the

crack, the molten PE decelerates, flows along the die rim,

and forms a ridge that will detach from the die in the

next act of adhesion failure. Periodical repetitions of the

melt fracture process create rough surface relief, i.e.

‘‘sharkskin’’, see [11] for details.

Actually, our tentative mechanism of sharkskin melt

fracture differs from a widely accepted explanation of

sharkskin proposed by Cogswell [42] only by a statement

that sharkskin melt fracture starts from some seed cracks

and these seed cracks originate from periodical adhesion

failures at the die surface. We claim that an onset of

sharkskin instability would be suppressed or substantially

delayed if the seed cracks are healed or prevented. In

extrusion of clay paste a similar fracture process may

happen and we proposed healing of seed cracks to sup-

press surface fracturing of an extrudate [43]. In particular,

we proposed a die with a core extended beyond the die

exit for defect-free extrusion of clay paste. In extrusion of

clay paste sliding friction of the extrudate on that core

outside the die generates back-pressure at the die exit.

The back-pressure heals seed cracks so that a smooth

extrudate can be produced. Similarly, healing of seed

cracks is used in industry to reduce shattering of glass

bottles and glass windows.

Our tentative mechanism of sharkskin explains experi-

mental observations of a ‘‘mysterious’’ delay of a shark-

skin onset in extrusion of LLDPE with additives of plate-

like nano-particles of organically modified clay [44] and

Boron Nitride [45] without slip inside the die. Indeed,

plate-like nanoparticles in a surface layer of the extudate

are oriented predominantly parallel to the die surface and

therefore they stop propagation of seed cracks in trans-

verse direction, i.e. into the extrudate, and an onset of

sharkskin melt fracture is delayed.

We believe that tiny peculiarities of the fracture pro-

cess, like: development of macroscopic fracture from mi-

croscopic seed cracks; slip of polymer along the die sur-

face just before it peels out; instability of shear cracks at

the boundary of two sliding bodies; and correlation of ad-

hesion of polymer melt with elasticity of the substrate,

are important and have to be taken into account. When

molten polymer peels out from the exit of a metal die and

slips along the surface just before the adhesion failure,

the crack that separates molten polymer from metal is

unstable to deviate from the boundary into the melt. Quite

opposite, the shear crack that originates at the die en-

trance and develops in the direction of melt flow at some

high extrusion rates does not deviate to inside the melt.

More over, we believe that successive propagation of

many such cracks shows up as slip of polymer inside the

die, i.e. ‘‘super flow’’ of melt.

If the die is coated by an elastic material, e.g. silicone

rubber [46], the crack of adhesion failure at the die exit

would not deviate from the boundary into the polymer but

rather in the opposite direction. No seed crack is produced

in this case and without seed cracks sharkskin melt frac-

ture would not develop. In [47] we coated an extrusion die

by Boron-cured silanol and demonstrated that sharkskin

melt fracture is suppressed if elasticity (E0/E00) of the coat-

ing is higher than elasticity of melt. Elasticity of the coat-

ing (accumulation of elastic energy) helps to reduce fric-

tion losses, and therefore for the higher elasticity the melt

better slips and we measure lower pressure at the extrusion

die. Onset of sharkskin melt fracture can be delayed also

in the case when the coating is essentially viscous in com-

parison with the melt [48]. In this case the coating works

like glue and dumps propagating of a crack of adhesion

failure at the die exit so that detachment of the melt from

the die surface goes steadily without oscillations.

Polymer melts show rubber-like behavior when

deformed rapidly, and we can expect to find similarities

between the slip of polymer melts and the slip of rubbers

1238 POLYMER ENGINEERING AND SCIENCE—-2010 DOI 10.1002/pen

along a solid boundary. Schallamach [49] has noticed that

the lateral force required to drag a glass lens across a rub-

ber slab is reduced, when macroscopic detachment waves

are formed. The Schallamach waves detach the rubber

from the hard surface and slowly propagate in the direc-

tion of relative displacement of rubber, i.e. from a leading

edge of a glass lens to its trailing edge. Similar structures

we observed during slip of clay [50] and molten LLDPE

[46] in transparent dies. These waves of macroscopic

detachment or ‘‘surface cavitation’’ propagate in the direc-

tion of melt flow.

Mechanism of sliding of solid elastic bodies is a sub-

ject of intensive research, e.g [51]. Gerde and Marder

[52] proposed that separation waves or self-healing

cracks, resembling bumps on rug, run along the interface

causing one solid elastic body to slip over the other. Ev-

ery crack in its propagation separates an elastic body from

the other one and results in a small relative displacement.

Actually, separation of the bodies can be of an atomic

scale. The propagation of many cracks manifests itself in

continuous slip. A flux of elastic energy is going to the

tip of every crack to break the contact bonds between

the bodies. At its tail, the banks of the crack collapse and

the bonds recover, at least partially. Low adhesion (weak

contact bonding) between the contacting bodies would

promote slip. In the referred model of slip, the shear

cracks propagate in the direction of bulk motion of an

elastic body along the rigid surface. These self-healing

shear cracks are radically different from the Schallamach

waves of macroscopic detachment, which propagate

slowly compared with the material wave speeds and show

macroscopic detachment from the boundary.

Our tentative mechanism of slip of molten polymer is

described in some more details in [11]. We believe that

slip of molten polymer can be explained in frames of the

model of Gerde and Marder. For continuous and stable

slip of melt along a solid surface shear cracks should

propagate in the direction of melt flow. So, development

of cracks from the die entrance to the die exit along the

metal can be responsible for a continuous slip at some

high rates of extrusion, i.e. for ‘‘super flow’’ of polymer

melt, while propagation of shear cracks in opposite direc-

tion, i.e. from the die exit to its entrance, is unstable.

Therefore, adhesion failures at the die exit are producing

sharkskin defects. For the die with an elastic coating at

the inside surface of the die the situation is reversed:

cracks are stable to develop from the die exit to the die

entrance and we observe stable slip of the melt but shear

cracks are unstable when they propagate from the die en-

trance to its exit and at some elevated rates of extrusion

we observe severe melt fracture of the extrudate instead

of the ‘‘super flow’’ of melt.

Core-Annual Flow

A technique to reduce friction losses in transportation

of very viscous fluids is long known in industry. It is a

‘‘core-annual flow’’ technique. Core-annual flow is the

pumping through a pipeline of a viscous liquid, such as

crude oil or bitumen, in a core surrounded by a lower vis-

cosity liquid such as water [53]. Normally, core-annual

flow is established by injecting water around viscous oil

being pumped in a pipeline. However, core-annual flow

can also be established above a certain shear rate in a

pipe flow that is high enough to break a water/oil emul-

sion and create a water-rich zone near the pipe wall that

acts as a lubricating film. Critical speeds for self-lubrica-

tion of water/oil emulsions are smaller when the viscosity

of the crude oil is higher. The amount of water used for

core-flow can be as low as 1% if water solvable salts are

used such as sodium-silicate, phosphates, borates, sul-

phates, carbonates, and mixtures thereof [54]. Ideal core-

annular flow is unstable to waves and disturbances at the

interface between the core and the lubricating film [55].

These waves tend to break up into droplets of the film liq-

uid, which disperses in the viscous phase. The lubricating

fluid having a low viscosity but a high elasticity sup-

presses inequalities in film thickness, reduces dispersion

of the film into the core of the flow and reduces friction

losses in the pipeline, see [56].

Gels and Greases in Lubrication

Bones are sliding along each other in joints of our

skeleton with a friction coefficient as low as 0.002. For

comparison, the friction coefficient of smooth steel

against ice and snow lubricated by a layer of water is

about 0.05. Taking a close look at a human hip-joint we

see that bones are coated by elastic cartilages and the gap

between the cartilages is filled by a synovial fluid. The

synovial fluid lubricates better than water, showing that

the low friction coefficient observed cannot result from a

hydrodynamic mechanism of lubrication [57]. The syno-

vial fluid is an elastic gel and a high MW (225 kDa) gly-

coprotein, namely lubricin, plays an important role there.

A lack of lubricin and a change of the synovial fluid into

a Newtonian fluid were observed in patients with rheuma-

toid arthritis, see Sokoloff [58] and Pieterse [59].

By definition a gel consists of an elastic network of a

solid substance with a liquid filling the inner space of the

network. The network holds the liquid in place through

its interaction forces and so gives the gel solidity, elastic-

ity, and coherence, but the gel is also wet and soft and ca-

pable of undergoing large deformation. Gels can be clas-

sified into four subgroups:

• Coagulated dispersions of colloidal particles which are

�1–1000 nm in size at low fractions of solids in a liq-

uid (down to a few percent).

• Rubber-like cross-linked polymers swollen in a liquid

solvent.

• Jellies, such as a solution of polymer molecules in a

liquid in which molecules are cross-linked due to

hydrogen or ionic bonds.


• Solutions of polymeric molecules of very high MW in

a liquid. Molecular entanglements work as physical

bonding between the molecules.

Grease is a semi-fluid to solid mixture of a fluid lubri-

cant, a thickener (a gelling agent), and additives. Industry

widely uses greases for lubrication of ball bearings and

for parts sliding along each other. Organo-clay, slaked

lime, silica fume, soaps of Lithium, Aluminum, Calcium,

Barium, and Sodium as well as diuretane polymers are

examples of thickeners. Additives enhance performance

and protect the grease and lubricated surfaces from corro-

sion and oxidation. Fluid lubricants used to formulate

grease are normally petroleum oils or synthetic fluids.

Examples of synthetic fluid lubricants are also diesters,

polyolesters, polyglycols, synthetic polycarbon fluids, sili-

cones, chlorofluorocarbon fluids, and tri-phosphate esters,

see e.g. [60].

Tertiary, i.e. tri-substituted, phosphate esters were first

introduced as antiwear and extreme-pressure additives for

lubricants in the 1930s. They are characterized by high

affinity to metals and adsorption activity on the frictional

surface. Molecules of the tertiary phosphate esters deposit

from a fluid lubricant to the frictional surface and form a

thin layer that prevents a contact of one metal part to

another and frictional welding of the parts. Nowadays, the

tertiary phosphate esters are used also as a fluid lubricant,

especially for aircrafts [61].

PEG in Lubricants and PPA

Polyethylene glycols (PEG) and esters of PEG are rou-

tinely used in industry as a component of lubricants, cool-

ing and cutting fluids in machining and metal working,

e.g. [62, 63]. The use of PEG, polyethylene oxide (PEO),

i.e. very high MW PEG and esters of PEG as a process-

ing aid and a release agent in injection molding is also

known. DeJuneas et al. [64] disclose that additives of

PEG with MW from 600 to 20,000 Da in amounts from

0.02 to 0.05 wt% to PE resins for film blowing reduce the

incidence of breakdown at typical operating conditions.

The authors also mentioned in a description of the patent

that PEG at levels of 1–3 wt% is an antistatic agent for

PE films. Wolinski [65] discloses blends of PE and low

MW PEG (MW is from 1000 to 6500 Da) to provide heat

sealable PE film suitable for printing. Tikuisis et al. [66]

propose the use of PEG with MW from 10,000 to 50,000

Da in amounts from 100 to 2000 ppm as PPA. They

report that for extrusion of LLDPE resin that comprises

hindered phenol primary antioxidant and a Phosphorus-

containing secondary antioxidant, the use of PEG 35,000

results in less pressure at the extrusion die in comparison

with the conventional PPA, i.e. a fluoroelastomer/PEG

8000 blend with a weight ratio of 1–2. The conditioning

time to ‘‘clear melt fracture’’ was essentially the same for

both PPAs. Blong and Lavallee proposed to use PEG and

PEO in amounts up to 20 wt% as PPA for extrusion of

fluorinated polymers [67]. They report that extrusion of

fluorinated polymers with additives of PEG is possible at

reduced temperatures and without surface defects.

According to Duchesne and Bryce [68], the use of PEG

with MW of 400 and 3350 Da as an additive in the

amount 0.2 wt% for extrusion of LLDPE at 2108C does

not suppress sharkskin melt fracture. Chapman et al. [69]

report that the use of PEG 8000 at a concentration of 480

ppm as an additive to LLDPE does not suppress sharkskin

melt fracture within the observation time of about 1 h.

The use of esters of polyalcohols with number of car-

bon atoms from 2 to 6 (e.g. glycerol, pentaerythritol, dec-

aglycerol, glycol, PEG 400) and saturated fatty acids, e.g.

stearic acid, as PPA for processing of LLDPE was pro-

posed by Williams and Geick [70]. The inventors reported

an 18% pressure reduction in extrusion of LLDPE by use

of a 0.3 wt% blend of fatty acid esters as PPA at a tem-

perature of 2238C and the suppression of sharkskin. Bauer

et al. [71] proposed to use thermoplastic polyesters with

melting points below 1508C as PPA for fiber spinning,

film extrusion, and molding of PE resins. Dover Chemical

recently announced Doverlube FL-599, an ester of glycols

and fatty acids, for the use as PPA in processing of vari-

ous polymers: High Impact Polystyrene (HIPS), Polysty-

rene (PS), PE, Polypropylene (PP), Acril-Butane-Styrene

(ABS), and Polyvinylchloride (PVC). It was also reported

that this PPA improves transparency of PP, reduces

decomposition of polymers inside processing equipment,

and can be used as a component of purging blends for

cleaning of processing equipment. Esters of PEG with

MW below 1500 Da and boric acid are proposed by Sato

for the use as a release agent [72] and as a component of

a purging blend for cleaning of processing equipment

[73]. The use of a reacting mixture of PEG with boric

and phosphoric acids as a PPA to suppress sharkskin in

extrusion of LLDPE is proposed recently in [74].

The use of a blend of PEG and fine powder of mineral

particles with sizes from 3.5 to 12 lm as a processing aid

is also known. Corwin et al. [75] proposed the use of an

antigel composition in extrusion of PE films wherein the

antigel composition can be a blend of PEG with MW

from 200 to 4,000,000 Da with an inorganic antiblock

agent and a hindered phenolic antioxidant. Li and co-

workers [76] described the use of PEG as a processing

aid for extrusion of UHMW PE/PP blend. Li and co-

workers [77, 78] also report a suppression of sharkskin

melt fracture for additives of a binary blend of PEG and

diatomaceous earth in amounts from 0.5 to 3 wt%.

According to the authors, the blend works synergistically

and PEG alone does not suppress sharkskin melt fracture.

The use of blends and reactive mixtures of PEG and

chemical substances comprising Phosphorus and oxygen,

e.g. phosphoric acid, organo-phosphites, and organo-phos-

phates, as PPA to suppress sharkskin in extrusion of

LLDPE is proposed recently in [79]. Optionally, the reac-

tive mixture can comprise fumed silica and PEO, i.e.

PEG with very high MW, as a passive thickening agent.


Esters of PEG in Lubricants and PPA

Commercially available LLDPE grades comprise low

MW organo-phosphites as antioxidant additives. Such

antioxidants are described and classified in [80]. The

organophosphites deplete oxygen from the PE melt and

get converted to organo-phosphates, i.e. low MW esters

of alcohols and phosphoric acid. As stated earlier, organo-

phosphates have a high affinity to steel alloys and the sur-

face layer of steel parts gets in time saturated with Phos-

phorus oxide (PO). Saturation of steel alloys with PO, i.e.

phosphating of steel, is used in industry. Actually, the first

patent on phosphating by the use of phosphoric acid was

granted in 1907 to Coslett [81].

Ester condensation reactions in a mixture comprising

monomers, such as polyhydric alcohol and polyfunctional

acids are well known [82]. During the condensation reac-

tion, if the temperature of the reactive mixture is suffi-

ciently high and applied for a sufficient time, the compo-

sition would convert to a water stable alkyd resin. Noda

et al. [83] describe in a recent patent application the use

of reacting mixtures of polyhydric alcohol and a poly-

functional organic acids for manufacturing of films, fibers,

foams, molded, and extruded articles.

PEG reacts under heating with carboxylic acids and

anhydrides of the carboxylic acids, e.g. stearic acid, oxalic

acid, succinic acid, adipic acid, citric acid, maleic anhy-

dride, phthalic anhydride. Reactivity is higher with PEG

of lower MW as well as in presence of strong mineral

acids, e.g. sulfuric acid, and catalysts. If weights of the

reacting components are selected close to stoichiometry

and the mixture has in average more than two reacting

groups per molecule, the ester-condensate is a thermoset

at room temperature. In the presence of moisture and at

elevated temperatures, the product of the polycondensa-

tion reaction, i.e. ester-condensate, is in equilibrium with

reactants and both direct and reverse reactions occur so

that covalent bonding between molecules breaks and reap-

pears in time. Therefore at such conditions the blend of

the ester-condensate and reactants behaves under applied

stress not as a thermoset but as a visco-elastic fluid.

Reacting Mixtures of PEG with Polyacids as Novel PPA

We propose here novel PPA at concentrations from

0.02 to 0.5 wt% for processing of polymers at reduced

temperatures. Actually, our proposal is an application and

development of the ‘‘core-annual flow’’ transportation idea

of viscous oil to the extrusion of plastics. Due to our pro-

posal molten polymer is blended inside the extruder with

a low-viscous lubricant. Both components do not dissolve

in each other, and when such an emulsion flows through

a long and narrow channel, the low-viscous component

actively migrates to the wall of the channel. In contrast to

the original ‘‘core-annual flow’’ idea we propose to use as

a low-viscous component a reacting mixture that com-

prises PEG with MW of about 300 to 4000 Da and at

least one of following reactants: organic acid, anhydride

of organic acid, phosphoric acid or Phosphorus oxide, and

low MW ester of oxiacids of Phosphorus.

Because of the higher affinity to metal, the low-viscous

component displaces molten polymer at the wall and

forms a lubricating film. At the wall of the channel the

reacting mixture turns into grease-like material that resists

to flow, separates molten polymer from the wall and

lubricates the wall so that the melt slips along the lubri-

cating film. The thickness of the lubricating film and the

efficiency of lubrication may be enlarged if some amount

of a passive thickening agent, e.g. fumed silica and PEO,

is blended with PEG. Indeed, because of friction with the

wall, nanoparticles of silica and long molecules of PEO

accumulate at the die wall and help to turn the low-vis-

cous reacting mixture into a viscoelastic gel. The PPA

may be applied in concentrations from 0.02 to 0.5 wt%

and with the advantage that there is no need to use a mas-

ter batch. Instead, the additive is supplied as pellets, spray

or by flow of liquid PPA directly into the extruder.

As indicated above, nowadays synergistic blends of

PEG with fluorinated polymers are used in industry to

improve extrusion of PO resins with narrow MWD. We

show here that fluorinated polymers are not a necessary

component of PPA. The use of reacting mixtures can be a

cheap alternative to the existing fluorinated polymer/PEG

blends, and can be especially useful for extrusion of PO

resins with high amounts of fillers. The PPA can be used

as a component of the master batch comprising the fillers

to improve their dispersion in polymer matrix. Opposite

to fluorinated polymers, the novel PPA does not accumu-

late inside the processing equipment and at the die exit,

but rather at long exposure to heat and oxygen it disinte-

grates to substances of low MW, and evaporates from the

equipment. The fumes they potentially generate by incin-

eration or heating are nontoxic. Viscous fluorinated PPA

stay trapped inside the polymer matrix, and only a small

fraction of the PPA is used to form a lubricating layer at

the die surface. In contrast, low viscous reacting mixtures

made of PEG actively migrate to the outside of the melt

and form a lubricating film inside the die around the vis-

cous core of molten polymer.

EXPERIMENTS

Experimental Methodology and Materials Used

In a series of experiments on extrusion we used

LLDPE resins from ExxonMobil Chemicals with narrow

MWD, LL1201 XV (density 0.925 g per cc, melting point

1238C, and MI ¼ 0.7 g/10 min) and LL 1001 XV (0.918

g per cc, melting point 1208C and MI ¼ 1 g/10 min).

One grade of HDPE resin was used, HPA 020 (0.952 g

per cc, melting point 1278C and MI ¼ 0.07 g/10 min).

These grades have antioxidant additives. To produce a

master batch with our PPA, we used LLDPE LL6301RQ


(melting point 1258C MI ¼ 5.0 g/10 min) from Exxon-

Mobil Chemical supplied as a coarse powder. We also

used for comparison LDPE LL166 BA (0.923 g per cc,

melting point 1108C and MI ¼ 0.2 g/10 min) with a wide

MWD from ExxonMobil Chemicals. We used the follow-

ing chemicals in our experiments:

• PEG 200, PEG 300, PEG 400, PEG 600, PEG 1000,

PEG 1500, PEG 2000, PEG 4000, PEG 6000, PEG

8000, PEG 10,000, and PEO with MW above

5,000,000 Da,

• Sorbitol,

• Oxalic acid, adipic acid, succinic acid, citric acid, ste-

aric acid, phthalic anhydride, and phosphoric acid from

Aestar.

• Fumed silica ‘‘Aerosil 300’’ from Degussa.

• Fluorinated PPAs: Viton Free Flow SC-PW and Viton

Free Flow Z100 from company DuPont, Kynar PPA

from Arkema, Dynamar E-15653 from 3M.

• Hostanox PAR 62, aromatic organophosphite (antioxi-

dant) from Clariant.

For extrusion of PE resins, we used a single screw ex-

truder from the Extrudex (Germany) with flood feeding.

This extruder has 4 axial grooves in its feeding zone to

ensure processing of raw reactor powders of PO resins

[84]. The grooves are 8 mm in width and their depth is

gradually decreasing from 2 mm to 0 in the machine

direction. The feeding zone of the extruder is cooled by

water. A big advantage of an extruder with a grooved

feeding section is that the mass rate of extrusion (MRE)

is less sensitive to pressure variations at the extrusion die

in comparison with smooth bore extruders. All parts of

the extruder that are in contact with molten polymer are

made from a special steel alloy (34CrAlNi7) and nitrided,

i.e. saturated by Nitrogen to harden a thin (about 0.1 mm)

surface layer of the parts. The extrusion die has a diame-

ter of 2 mm and a length of 60 mm and is also made

from nitrided steel. The 2 mm bore of the die was conju-

gated with a 508 cone having a diameter of 8 mm at the

entrance of the die. Extrudate comes out from the die in

the downward direction.

For extrusion of PE resins with PPA, we mixed pellets

of PE resin and PPA in a plastic drum by shaking and

rotating the drum. To ensure that PPA with low melting

temperatures, e.g. made with PEG, is distributed evenly

on the pellets we heated pellets of PE resins to about

908C. Before every extrusion trial we cooled the pellets

with the additives to room temperature.

A pressure transducer was arranged near the die en-

trance and its reading were used for data acquisition at

rate about 200 Hz. Pressure reduction (PR) [bar] data are

calculated by comparison of pressures in extrusions with

PPA (pressure measured or PM) and without any PPA

(reference pressure or RP), i.e. PR ¼ PM2RP. The refer-

ence pressure by use of the die 2 3 60 mm and neat

LLDPE is 398 bar. The data on pressure reduction [%] in

the figures are presented as relative values, i.e. as a ratio

of PR/RP. MRE was monitored in all experiments. In a

separate extrusion trial we measured extrusion pressure of

neat LLDPE vs. MRE. In the case of extrusion with PPA

when the measured MRE differs from the reference value

of MRE for extrusion without additives, the reference

pressure (RP) for pressure reduction (PR) calculations

was selected that corresponds to the measured MRE.

The ‘‘Flow Curve,’’ which is a plot of apparent shear

stress versus apparent shear rate, is commonly used for

rheological characterization of molten polymers. How-

ever, we discuss here sharkskin melt fracture and slip

phenomena, which may include friction and fracture. The

use of reduced quantities like apparent shear stress and

apparent shear rate in the characterization of friction and

fracture would be confusing. Therefore we present charac-

teristic curves of the directly measured quantities, i.e. the

pressure (P) at the die entrance versus the averaged extru-

date velocity (V) to characterize the flow in a circular die.

The average extrudate velocity (V) is derived from the

volumetric flow rate Q by

V ¼ 4Q

pd2(1)

where d is the diameter of the die. Below we will also

use a term ‘‘extrusion rate’’ instead of V. If needed, the

apparent shear stress ta can be calculated from the meas-

ured pressure P by

ta ¼ 4L

dP (2)

where L is the length of the die, and the apparent shear

rate _ca is given by

ga ¼32Q

pd3¼ 8V

d(3)

Fluorinated PPA and Fluorinated PPA/PEG Blends

As a reference we prepared and extruded blends of

Viton/LL1201, Dynamar/LL1201 at concentrations of 0.1

wt% of the additives, and Kynar/LL1201 at 0.2 wt% of

the additive. For industry it is important to know not only

the conditioning time ‘‘to clear the sharkskin melt frac-

ture,’’ but also the time to clean the processing equipment

from the additives used by purging with neat polymer.

Therefore, we first extruded 1 kg of the PPA/LLDPE

blend, and then we purged the extruder with neat LLDPE,

at a temperature of 1658C. The lubrication charts, i.e. per-

centage of pressure reduction as a function of time since

start-up of extrusion in the case of Viton and Dynamar

(concentration 0.1 wt%) and Kynar (concentration

0.2 wt%) are shown in Fig. 1. The time, when we

changed from feeding the PPA/LLDPE blend to feeding

neat LLDPE, is shown in Figs. 1, 2, and 3 by a vertical

line (approximately 2 h 20 min after start of the extru-

sion). These and further extrusion experiments were made


at an average extrudate velocity of about 40 mm/s and at

a temperature of 1658C if not otherwise indicated. Resi-

dence time, i.e. the time for polymer material in the ex-

truder, is about 15 min. With the use of fluorinated PPA

as well as other PPA in following extrusion trials the

sharkskin melt fracture was suppressed when Pressure

Reduction was above 18–20%.

As it is possible to see from Fig. 1, the use of Dyna-

mar results in slightly better lubrication than the use of

Viton, but conditioning time is longer in this case. We

observed accumulation of static electrical charge at the

extrudate, when Viton and Dynamar are used as PPA.

Using Kynar, which is a blend of some fluorinated PPA

with PEG, no static electrical charge of the extrudate was

observed in this experiment. We may also see that the use

of Kynar results in a higher lubrication in comparison

with the use of Viton or Dynamar, and the conditioning

time is the shortest. A similar improvement in lubrication

was observed when Viton FF Z100 blend was used as

PPA in a concentration of 0.2%. This blend also com-

prises some amount of PEG. We see from the experimen-

tal curves that conventional PPA based on fluorinated

polymers accumulates inside the extruder, and cannot be

removed by purging of processing equipment with neat

LLDPE. To clean up the extruder from Viton, Dynamar,

and Kynar, we had to purge it by a blend of LLDPE and

mica. The die was heated to 4508C in an electrical fur-

nace to burn off the fluorinated PPA from its surface after

extrusion trials with fluorinated PPA. A nitrided die from

a steel alloy 34CrAlNi7 can stay heating up to 5008Cwithout loosing Nitrogen.

To understand the mechanism of synergistic improve-

ment in lubrication by the use of blends of PEG and fluo-

rinated polymers, we prepared blends of Viton with PEG

8000 where fractions of Viton are from 10 to 80 wt%.

Neat Viton and PEG 8000 were also used as additives.

We made a sequence of extrusion trials with a gradual

increase of the fraction of Viton in the blend used as

PPA: First we extruded LLDPE with PEG 8000; next we

used LLDPE with a Viton/PEG 8000 with 10 wt% of

Viton, etc., till the final extrusion trial when only Viton

FIG. 1. Pressure Reduction (%) vs. extrusion time (hour) in extrusion

of LLDPE (LL1201 XV, comprises antioxidant AO) through the die 2 360 mm at temperature 1658C and averaged extrudate velocity 40 mm/s

with following additives: Viton (0.1 wt%, a solid line), Dynamar (0.1

wt%, s dotted line), Kynar (0.2 wt%, a dashed line), PEG 2000 with cit-

ric acid (PEG2KþCAþAO, 0.1 wt% of PPA, 2 wt% of CA, a dash-dotted

line), PEG 2000 (PEG2KþAO, 0.1 wt%, a dash-dot-dotted line).

FIG. 2. PPA (0.1 wt%) is a reacting mixture of PEG 10 000 with antiox-

idant (PEG10KþAO). Pressure reduction (%) vs. extrusion time (hour) in

of LLDPE with anti-oxidant (LL1201 XV) at temperature 1658C and

averaged extrudate velocity 40 mm/s through dies 2 3 20 mm: a fresh

nitrided die (N-layer, a solid line), the nitrided die exposed to 8 h of extru-

sion of LLDPE (LL1201 XV) with organo-phosphites (NþP-layer, a dot-

ted line), the die with removed nitrided layer (no N-layer, a dashed line).

FIG. 3. Pressure reduction (%) vs. extrusion time (hour) for extrusion

of LLDPE with anti-oxidant (LL1201 XV) through the die 2 3 60 mm

at temperature 1658C and at averaged extrudate velocity 40 mm/s. PPAs

(0.1 wt%): PPA is a reacting mixture of PEG 1500 with antioxidant (a

solid line), PEG 1500 with antioxidant and phosphoric acid (a dashed

line), PEG 1500 with antioxidant and organo-phosphite Hostanox PAR

62 (a dotted line) PEG 1500 with antioxidant, sorbitol and phosphoric

acid (a dash-dotted line).


was used as PPA. As seen from Fig. 1, it takes a while to

obtain a stationary pressure at the extrusion die. Extrusion

pressures and MRE at stationary conditions are presented

in Table 1. Using neat PEG 8000 as PPA, it took almost

8 h to obtain a stationary level of lubrication. As seen in

Table 1, MRE is stable vs. fraction of Viton in the PPA,

while pressure reduction reaches a maximum at a fraction

of Viton of about 30 wt%. This maximum value of pres-

sure reduction is indeed much larger than for neat Viton,

but it differs very little from the effect of neat PEG 8000

as PPA. From these measurements we may conclude that

the fluorinated polymer is a not necessary component of a

PPA, and the only justification for the use of fluorinated

polymers as a component of a PPA is the relatively short

conditioning time of the Viton/PEG blend.

Die Material

To estimate an impact of the die material on the effi-

ciency of PPA and the conditioning time ‘‘to clear shark-

skin melt fracture,’’ we made extrusion trials of LLDPE

with PEG 10,000 in concentration of 0.1 wt%. We used a

die of diameter 2 mm and length 20 mm fabricated from

nitrided steel 34CrAlNi7. The die was heated before the

trials to a temperature of 4508C and then washed in

water. One kilogram of the PPA/LLDPE blend was

extruded at 1658C with an extrusion rate of about 40 mm/

s followed by neat LLDPE to clean the extruder. The

pressure reduction curve derived from data acquisition of

a pressure transducer is presented in Fig. 2 by a full line

as a reference. In a similar way as for fluorinated PPA,

sharkskin is suppressed when the pressure reduction

exceeds about 20%. For the next experiment the die was

cleaned as before, but the extruder was purged for about

10 h with neat LLDPE before the blend of PEG/LLDPE

was fed to the extruder. The pressure reduction curve is

presented in Fig. 2 by a dotted line. The LLDPE grade

that we used for our experiments contains antioxidant,

and the die surface will be saturated by organo-phos-

phates after long extrusion of the LLDPE.

An increase of pressure reduction in comparison with a

fresh and clean die proves that the accumulation of the

organo-phosphates in the extruder helps to improve lubri-

cation. Then we treated the die by nitric acid to remove

the nitrided layer from the die surface, and in this case

the measured lubrication values were much less that

for the nitrided die (dashed line in Fig. 2). In a similar

experiment with the die treated by phosphoric acid, i.e.

with a phosphated die, we observed slightly improved

lubrication. The etched surface of the die is not as smooth

as for the original nitrided die, so the experiments are not

perfect but quantitatively we can testify that saturation of

the steel alloy by Nitrogen and Phosphorus improves

lubrication. In an extrusion experiments with a die of

dimensions 2 3 20 mm made from aluminum alloy we

obtained pressure reduction values that are compatible

with the values for the nitrided die of the same sizes,

whereas the use of a die made from stainless steel demon-

strated considerably less lubrication than the reference

experiment.

In other experiments, we charged a portion of about 1 g

of urea into the extruder to ‘‘deactivate’’ acids inside the

extruder and the die (2 3 60 mm) from nitrided steel dur-

ing purging of the extruder by neat LLDPE. Urea is

decomposing upon heating to ammonia and carbon diox-

ide, and ammonia reacts with phosphoric acid. Five hours

later we started extrusion of 2 kg of LLDPE with additives

(500 ppm) of PEG 1000 with silica fume (1 wt% from the

PEG). Only little pressure drop was detected (about 5%)

in the end of the extrusion time but after further purging

of the extruder by neat LLDPE for about 15 h the same

additives gave 16% of pressure reduction and partial sup-

pression of sharkskin. Next, after 15 more hours of purg-

ing the pressure reduction in a similar experiment was

about 18% with complete sharkskin suppression.

Organo-Phosphites and Organo-Phosphates

To prove the importance of having organo-phosphites

and organo-phosphates as a component of PEG based

PPA, we prepared mixtures of PEG 1500 with phosphoric

acid (PA), sorbitol (SO) and fumed silica (SF). The pres-

sure reduction curves are presented in Fig. 3: neat PEG

1500 (a solid line); 100 g of PEG 1500 mixed with 4 g of

PA (a dashed line); 100 g of PEG 1500 mixed with 25 g

of Hostanox PAR 62 (a dotted line); 100 g of PEG 1500

mixed with 8.6 g of phosphoric acid, 4 g of sorbitol and

1 g of boric acid (a dot-dashed line). The blends compris-

ing phosphoric acid and organo-phosphite were heated to

about 1608C in vacuum, and the reaction products were

used as PPA in concentrations 0.1 wt%. The experimental

curves presented in Figs. 1, 2, and 3 are derived from

data acquisition of a pressure transducer in extruding of 1

kg of the PPA/LLDPE blend and 2 kg of neat LLDPE.

To find the optimum amounts of organo-phosphite and

organo-phosphate, we prepared mixtures of PEG 1500

with phosphoric acid and PEG 1000 with organo-phos-

phite Hostanox PAR 62. The mixtures were heated to

1608C in vacuum, and the reaction products were used

as PPA in concentrations 0.1 wt%, and two sequences

of extrusion trials were made by extruding 1 kg of the

TABLE 1. PPA in a concentration 0.1 wt% is made by blending Viton

and PEG 8000.

Fraction of Viton

in PPA, wt% 0 10 20 40 60 80 100

Max. pressure

reduction (%)

47 48 50 50 43 38 26

Change in mass

rate (%)

0.3 20.2 20.3 0.3 20.2 21 20.2

Temperature is 165 8C. For all Tables extrusion of LLDPE with anti-

oxidant (LL1201 XV) and PPA is done at averaged extrudate velocity

40 mm/s through the die 2 3 60 mm from nitrided steel.


PPA/LLDPE blend and 2 kg of neat LLDPE with gradu-

ally increasing amounts of the organo-phosphites and

organo-phosphates in the PPA. The maximum pressure

reduction obtained during the extrusion trials is presented

in Fig. 4 vs. the ratio of the number of Phosphorus atoms

to the number of PEG molecules. Maximum efficiency of

lubrication is achieved when the molar ratio of reactants

is getting close to stoichiometry, i.e. for [P]/[PEG] ratios

from 1/3 to 2/3. The molar ratio of reactants in the case

of Hostanox PAR 62 has to be higher, probably because

not all organo-phosphite is converted to organo-phospate

during extrusion. We also observed a higher efficiency of

lubrication if the blend of Hostanox PAR 62 with PEG

1000 was heated in air to higher temperatures, i.e. 2608Cinstead of 1608C. It gives us a hint that for better lubrica-

tion, the organo-phosphite in the composition of the PPA

has to react with oxygen.

Impact of MW of PEG and Additives of Silica Fume

To determine an impact of MW and concentration on

the lubrication efficiency in extrusion of LLDPE LL1201

XV, we made several sequences of extrusion trials with

PEG of various MW at increasing concentrations of PEG

in the range from 250 to 4000 ppm, at 1658C and 40

mm/s extrusion rate. We prepared blends of PEG/LLDPE,

and measured extrusion pressures at stationary state, i.e.

in the case of 250 ppm PEG addition, the pressure read-

ings were taken after about 10 h of extrusion, for 500

ppm after 5 h and for 1000, 2000, and 4000 ppm after

2 h 30 min of extrusion. Pressure reduction data vs. MW

of PEG are presented in Fig. 5. At high concentrations of

PEG, maximum lubrication is observed for PEG 4000,

whereas at low concentrations of PEG, maximum lubrica-

tion is obtained for PEG with MW of 8000 or higher. We

believe that the pressure reduction at high concentrations

of low viscous PEG can be explained by separation from

the blend and migration of the low viscous component to

the die wall, i.e. the establishment of ‘‘core-annual flow,’’

see references above.

We demonstrated already that additives of PEO and

fumed silica improve lubrication with PPA from low vis-

cous PEG, see [85]. To determine the impact of fumed

silica (SF), we made several sequences of extrusion trials

with PPA made from of blends of PEG of various MW,

fumed silica (1%) and phosphoric acid, in the range of

PPA concentrations from 250 to 4000 ppm at 1658C and

40 mm/s extrusion rate. The extrusion trials were made in

a similar way as for neat PEG. Pressure reduction data vs.

MW of PEG are presented in Fig. 6. The maximum lubri-

cation at high concentrations of PPA is attainable for PPA

made from PEG 2000. By comparison Figs. 5 and 6 we

can see that the use of silica fume improves lubrication

for high concentrations of PEG with MW below 6000 but

reduces it for low concentrations of PEG with MW above

6000. As a reference we made a sequence of extrusion tri-

als of Viton/LLDPE blends at gradually increasing con-

centrations of Viton. In contrast to extrusion of PEG-

based PPA, pressure reduction for fluorinated polymers is

nearly independent of the amount of PPA under stationary

conditions, but the conditioning time is shorter at higher

amounts of Viton.

To investigate an impact of MW of PEG on the effi-

ciency of PPA for extrusion at reduced temperatures, we

prepared blends of PEG/LLDPE for the following MW of

PEG: 200, 600, 1000, 1500, 2000, 4000, 6000, 8000,

10,000. A sequence of extrusion trials with PEG of vari-

FIG. 4. Pressure reduction (%) for extrusion of LLDPE with antioxi-

dant (LL1201 XV) vs. a molecular ratio of Phosphorus atoms to PEG

molecules in PPA for the extrusion at temperature 1658C and at aver-

aged extrudate velocity 40 mm/s through the die 2 3 60 mm: PPA (0.1

wt%) comprises PEG 1500 with antioxidant and phosphoric acid

(PEG1.5KþAOþPA, a solid line and circles), PPA (0.1 wt%) comprises

PEG 1000 with antioxidant and organo-phosphite Hostanox PAR 62

(PEG1KþAOþOP, a dotted line and triangles).

FIG. 5. PPA is a reacting mixture of PEG with antioxidant. Pressure

reduction (%) vs. a molecular weight of PEG for extrusion of LLDPE

with antioxidant (LL1201 XV) through the die 2 3 60 mm at tempera-

ture 1658C and at averaged extrudate velocity 40 mm/s: PPA (0.025

wt%, a solid line), (0.05 wt%, a dashed line), (0.1 wt%, a dotted line),

(0.2 wt%, a dash-dotted line), (0.4 wt%, a dash-dot-dotted line).


ous MW in concentrations 0.5 wt% were made by extrud-

ing 1 kg of the PPA/LLDPE blend through the extruder

and the die, and then purging it by 2 kg of neat LLDPE

at an extrusion rate about 40 mm/s. Pressure reduction

from reference pressure without additives at a temperature

of 1388C of the PPA/LLDPE blends vs. MW of PEG are

presented in Table 2. We can see that the best lubrication

was achieved by the use of PEG with MW from 1000 to

4000 Da. For PEG 200 we did not observe suppression of

sharkskin, and we detected an 11% reduction in the MRE.

PEG is slightly soluble in molten LLPDE and it

reduces surface tension and viscosity of the melt. In

extrusion experiments with the use of PEG with molecu-

lar weights above 6000 Da local inclusions of PEG

appear as shallow depressions at the extrudate surface. In

our experiments with PPA from low viscous PEG (MW is

below 4000 Da) the extrudate has surface without detecta-

ble depressions. Therefore, we can conclude that all this

PPA is ejected from the polymer.

Reacting Mixtures of PEG and Organic Acids as PPA

PEG reacts with phosphoric acid and organic polya-

cids, and therefore it is possible to shorten conditioning

time by the use of a reacting mixture of PEG with the

acids [86]. To compare the efficiency of various organic

acids in the composition of PPA, we prepared reacting

mixtures of PEG 6000 with 2 wt% of following acids: ox-

alic acid, adipic acid, succinic acid, citric acid, and stearic

acid. The components of the reacting mixtures were fed

to the extruder as powders and extruded through a die

with 2 mm in diameter and 60 mm in length at a temper-

ature about 608C by the same extruder that we used for

extrusion experiments. Similar to extrusion of chocolate

[87], the extrudate is soft and plastic immediately after

melting and solidification of PEG inside the extruder but

in a short while (in about a minute) it gets solid and frag-

ile. So, the filaments produced were crashed after solidifi-

cation to small pieces (about 3 mm long) and mixed with

pellets of LLDPE for extrusion in amounts of 2 g of ev-

ery PPA to 1 kg of LLDPE. For comparison we also

extruded neat PEG 6000 and mixed it with LLDPE.

Extrusion trials were made at 1658C in a similar way as

it is described above. The measured maximum pressure

reduction values and conditioning times to clear sharkskin

are presented in Table 3 for neat PEG 6000 and the

blends of PEG 6000 with the acids. Oxalic and citric

acids show the shortest conditioning time. The use of cit-

ric acid improves lubrication, whereas the use of succinic,

adipic, and stearic acids worsens lubrication in compari-

son with the use of neat PEG 6000 for extrusion of

LLDPE with antioxidants, but the differences are small.

In another set of experiments we compared efficiency

of various organic acids in the composition of PPA when

the molar ratio of reactants is close to stoichiometry. We

prepared reacting mixtures of PEG 2000 with following

acids: oxalic acid, adipic acid, succinic acid, citric acid,

stearic acid, phosphoric acid, and phthalic anhydride.

Silica fume was used as a passive thickening agent in

concentration of 1 wt% and phosphoric acid (1.2 wt%)

was used as a catalyst and adhesion enhancer. We added

organic acids to PEG with fumed silica and phosphoric

acid under heating to about 1808C and intensive stirring.

Liquid PPA (1 g) was blended with 2 kg of hot pellets of

LLDPE to ensure homogeneous distribution of the addi-

tives on the pellets. Next, the pellets were cooled down to

room temperature and extruded through a die (2 3 60

mm) from nitrided steel. We used 2 kg of neat LLDPE

to purge the extruder after every extrusion trial.

FIG. 6. PPA is a reacting mixture of PEG, antioxidant and phosphoric

acid with fumed silica. Pressure Reduction (%) vs. a molecular weight

of PEG for extrusion of LLDPE with anti-oxidant (LL1201 XV) through

the die 2 3 60 mm at temperature 1658C and at averaged extrudate ve-

locity 40 mm/s: PPA (0.025 wt%, a solid line), (0.05 wt%, a dashed

line), (0.1 wt%, a dotted line), (0.2 wt%, a dash-dotted line), (0.4 wt%, a

dash-dot-dotted line).

TABLE 2. PPA in concentration 5000 ppm is PEG. Temperature is

1388C.

MW of PEG,

1000 Da 0.2 0.6 1 1.5 2 4 6 8 10 20

Max. pressure

reduction (%)

11 41 67 71 69 61 53 47 45 41

Change in mass

rate (%)

211 25 þ2 þ1 þ2 þ2 21 211 220 233

TABLE 3. PPA in a concentration 0.2 wt% is a reacting mixture of

PEG 6000 and organic acid (2 wt% of PEG). Temperature is 1658C.

Name of the blend CA RF OA StA AA SA

Max. pressure reduction, % 55 49 48 47 46 42

Conditioning time, min 36 77 33 36 36 37

CA is a blend of PEG 6000 with citric acid; RF is neat PEG 6000

(with anti-oxidant); OA is a blend with oxalic acid; StA is a blend with

stearic acid; AA is a blend with adipic acid; SA is a blend with succinic

acid. A conditioning time includes a residence time to transport pellets

in an extruder, ca. 15 min.


The measured values of maximum pressure reduction and

conditioning times to clear sharkskin are presented in

Table 4 for PEG 2000 with silica fume and phosphoric

acid as a reference, as well as for further blends with

organic acids. The use of citric acid and phthalic anhy-

dride, improves lubrication in comparison with reference

composition of PPA: PEG 2000 with silica fume (1 wt%

from PEG) and phosphoric acid (1.2 wt% from PEG).

Citric acid looks as the most attractive reagent to pre-

pare reacting mixtures with PEG. To find an optimum

amount of citric acid in the composition of PPA, we used

a blend of PEG 2000 with citric acid, phosphoric acid

(1.2 wt%), and silica fume (1 wt%). Conditioning times

to clear sharkskin and maximum pressure reduction values

are presented in Table 5 for various concentrations of cit-

ric acid. We see that with increasing concentration of

citric acid from 6.4 wt% to 9.6 wt% the conditioning time

gets short but at high concentrations lubrication is

reduced. We also prepared pellets of PEG 2000 with 2

wt% of citric acid and used the PPA in a concentration of

0.2 wt% for extrusion of LLDPE (LL1201 XV). The

resulting pressure reduction is presented in Fig. 1 by a

dash-doted line. For comparison we made extrusion by

the use of neat PEG 2000 as PPA, and the curve of pres-

sure reduction is also presented in Fig. 1 by a dot-dash-

doted line. From Fig. 1 we see that the blend of PEG

2000 with citric acid shows a very short conditioning time

similar to the use of Kynar at the same concentration, but

much better lubrication than any of the fluorinated PPA.

The PPA can be used in small concentrations. For exam-

ple, in extrusion of LLDPE with PPA made from PEG

2000 with 6.4 wt% of citric acid, 1 wt% of silica fume

and 1.2% of phosphoric acid in a concentration 250 ppm

pressure reduction was about 17% with total suppression

of sharkskin. This proves that fluoropolymer is not a nec-

essary component of a PPA, and much better results in

lubrication and suppression of the sharkskin during extru-

sion of LLDPE can be achieved by the use of novel PPA.

PEG-based PPA in Extrusion of LDPE and LLDPE

To compare an impact of PPA on the extrusion of PEs

with wide and narrow MWD, we used a blend of PEG

2000 with 1% of silica fume) in a concentration 0.5 wt%

for extrusion of LDPE (LD 166BA) and LLDPE (LL

1001 XV) at a temperature of 1658C and for extrusion

rates from about 4 to 100 mm/s. For comparison we

extruded these PE resins also without PPA. Characteristic

flow curves, i.e. curves of extrusion pressure at the die

versus extrusion rate are presented in Fig. 7. The best-fit

curves connect experimental points for extrusion of PE

resins without PPA: a full line with symbols of open

circles for LLDPE, and a dashed line with open squares

for LDPE. For extrusion of PE resins with PPA, the best-

fit curves connect experimental points: a full line with

symbols of full circles for LLDPE, and a dashed line with

full squares for LDPE. At the characteristic curve for

extrusion of LLDPE without PPA, the onset of sharkskin

(at 4 mm/s) and the onset of stick-slip (at 52 mm/s) insta-

TABLE 4. PPA in concentration 500 ppm is a reacting mixture of

PEG 2000 and organic acid, with additives of phosphoric acid (1.2 wt%

from PEG) as a catalyst and silica fume (1 wt% from PEG) as a

thickening agent. Temperature is 1658C.

Name of the blend RF2 PA CA PhAn OA NT StA AA SA

Max. pressure red (%) 38 40 39 36 33 33 30 27 25

Max. torque red (%) 35 23 36 26 29 34 33 28 31

Condit. Time, min 82 41 38 59 87 122 122 93 213

RF2 is a blend of PEG with silica fume (1 wt% from PEG) and phos-

phoric acid (1.2 wt% from PEG); PA is a mixture of RF2 with phos-

phoric acid (3.8 wt% from PEG); CA is a mixture of RF2 with citric

acid (9.6 wt% from PEG); PhAn a mixture of RF2 with phtalic anhy-

dride (14.8 wt% from PEG); OA is a mixture of RF2 with oxalic acid

(12.6 wt% from PEG); RF is PEG with silica fume (1 wt% from PEG)

without phosphoric acid; StA is a mixture of RF2 with stearic acid (14.2

wt% from PEG); AA is a mixture of RF2 with adipic acid; SA is a mix-

ture of RF2 with succinic acid. A conditioning time includes a residence

time to transport pellets through an extruder, ca. 15 min.

TABLE 5. PPA in concentration 500 ppm is a reacting mixture of

PEG 2000 and citric acid (wt% from PEG) with additives of phosphoric

acid (1.2 wt% from PEG) as a catalyst and silica fume (1 wt% from

PEG) as a thickening agent. Temperature is 1658C.

Amount of citric acid, wt% 0 (RF2) 3.2 6.4 9.6 12.8

Max. pressure reduction (%) 38 38 41 39 32

Max. torque reduction (%) 35 36 38 36 26

Conditioning time, min 82 65 74 38 66

RF2 is a blend of PEG with silica fume (1 wt% from PEG) and phos-

phoric acid (1.2 wt% from PEG). A conditioning time includes a resi-

dence time to transport pellets in an extruder, ca. 15 min.

FIG. 7. Characteristic flow curves that are pressure (bar) vs. extrusion

rate (mm/s) curves for extrusion of LLDPE with anti-oxidant (LL1001

XV, circles) and LDPE with no antioxidant, (LD166 BA, squares): with-

out additives (hollow circles and squares) and with additives of 0.5 wt%

of PPA comprised from PEG 2000 and 1 wt% of fumed silica (filled

circles and squares). Onset of sharkskin is marked by number 1, and

onset of stick-slip is marked by number 2 at the curve of extrusion of

LLDPE without additives.


bilities are marked by arrows. Opposite to extrusion of

neat LLDPE, extrusion of LLDPE with PPA is stable at

extrusion rates up to 80 mm/s, where the so-called elastic

instability (entrance melt fracture) occurs. While extrusion

of LLDPE is greatly improved by addition of the PPA

and pressures at the extrusion die are reduced 4–5 fold in

comparison with extrusion of LLDPE without PPA, there

is only a marginal pressure reduction in the case of extru-

sion of LDPE with PPA. We also made an extrusion trial

with a HDPE resin, HPA 020, and with 0.2 wt% of PPA

composed from PEG 2000 and 2 wt% of citric acid at the

temperature 1658C and for an extrusion rate 44 mm/s.

The HDPE grade comprises a big portion of HDPE resin

of low MW. Therefore after an extrusion time of 2 h

30 min, pressure reduction was only 39%.

Extrusion of LLDPE vs. Temperature

We compared the extrusion of LLDPE with and with-

out PPA at various temperatures: 130, 135, 145, 165, 185,

205, 225, 2358C. Characteristic flow curves, i.e. curves of

pressure at the extrusion die versus extrusion rate

(expressed as average velocity of the extrudate) for extru-

sion trials of LL1201 XV without PPA are presented in

Fig. 8 for two temperatures: 1458C (full line) and 2258C(dashed line). The best-fit curves connect experimental

data: symbols of open circles (1458C) and open squares

(2258C). At the characteristic curve for extrusion at

1458C, the onset of sharkskin (at 2 mm/s) and stick-slip

(at 27 m/s) instabilities are marked by arrows. For extru-

sion at higher temperatures onsets of surface instabilities

are delayed. However, for LLDPE with narrow MWD the

sharkskin instability appears at low rates of extrusion

(below 6 mm/s for LL1201 XV) even at temperatures as

high as 2258C. For experiments with PPA we mixed

under heating PEG 2000 with silica fume (1%) and added

5 g of this PPA per 1 kg of the LLDPE pellets. Extrusion

trials were made at the same temperatures as without the

PPA. In the range of extrusion rates from 2 to 45 mm/s

and for temperatures below 1858C, the measured pres-

sures showed little variations with temperature. Surpris-

ingly, we observed an increase of the extrusion pressures

at temperatures from 185 to 2358C. Characteristic curves

of extrusion of LLDPE with PPA for two temperatures:

145 (full line) and 2258C (dashed line) are presented in

Fig. 8. The best-fit curves connect experimental data:

symbols of full circles (1458C) and full squares (2258C).So, the extrusion pressures at the die can be considerably

less for reduced temperatures of extrusion than for ele-

vated temperatures, if LLDPE is blended with PPA. This

result is quite opposite to the extrusion of neat LLDPE,

where apparent viscosity drops at elevated temperatures.

Pelletizing of LLDPE Resins with Air Cooling

In our experiments extrusion is going downward from

the die, and the die is about 1 m above the floor. At

reduced temperatures the extrudate quickly cools down by

air convection and solidifies. To demonstrate an opportu-

nity to produce a pelletized master batch with some com-

positions of PPA from viscous PEG, we blended coarse

powder of LLDPE (LL6301RQ) with 3% of PEG 6000

and extruded it at temperature of about 1358C from a die

2 mm diameter. A solid strand was cut by a rotating knife

of a meat grinder to produce a pelletized master batch.

We also used a rotating knife of a meat grinder attached

to an electrical motor to cut strands directly at the hot die

face. Pelletizing of LLDPE material without PEG addi-

tives is hardly possible at low temperature of extrudate,

e.g. 1388C, not only because of high pressures at the die

but also because of sharkskin and pronounced die drool,

i.e. accumulation of molten PE resin at the die exit. PE

melt does not stick to the metal surface wetted by PPA.

Pellets cut by the rotating knife at higher temperatures,

e.g. 1658C, stick together and therefore pelletizing with

air cooling would be not possible at such temperatures.

To demonstrate an opportunity to produce micropellets

by extrusion at reduced temperatures and pelletizing in

open air we used as a die set a pack of 7 tubes with thin

walls, i.e. injection needles soldered by silver alloy inside

a metal housing of 20 mm length. Two types of needles

were used: with outer diameter 0.45 and 0.8 mm. The

rotating knife of a meat grinder was used to cut strands at

the hot face of the die plate to thin disks or cylinders by

varying the rotation speed of the knife. Otherwise, we

used a rotating blade of a wood planner to cut strands of

LLDPE to pellets. In this case, extrusion was going at

1328C with a die of 2 or 4 mm diameter at averaged melt

velocity from 10 to about 40 mm/s. Two rotating rolls

were used to draw a strand of LLDPE and to supply it to

FIG. 8. Characteristic Flow Curves that are pressure vs. extrusion rate

curves for extrusion of LLDPE with anti-oxidant (LL1201 XV) at tem-

peratures 1458C (circles) and 2258C (squares) without additives (hollow

circles and squares) and with additives of 0.5 wt% PPA comprised from

PEG 2000 and 1 wt% of fumed silica (filled circles and squares). Onset

of sharkskin is marked by number 1, onset of stick-slip is marked by

number 2 and onset of elastic instability (gross melt fracture) is marked

by number 3 at the curve of extrusion of LLDPE without additives at

1458C.


the rotating blade. No cooling but air convection was used

to solidify molten PE before it comes to the rollers that

are located about half meter downward from the die.

Micropellets were produced as disks of 0.8 to 1 mm di-

ameter and 0.5 mm thickness. Such micropellets demon-

strate good flow properties and obviously can be used for

rotomolding or flood feeding of an extruder.

DISCUSSION

PEG-based PPA Instead of Fluorinated PPA

Industry is already using blends of fluorinated poly-

mers with PEG as PPA. However, we demonstrated that

fluorinated polymers are not a necessary component of

PPA. Viscous PPA made by blending fluorinated poly-

mers and PEG with MW about 8000 Da stay trapped

inside the polymer matrix, and only a small fraction of

the fluorinated PPA is used to form a lubricating layer at

the die surface. In contrast, PPA made from PEG actively

migrates to the outside of the melt and forms a lubricating

film. It can be blended with PO resins without master

batching as pellets, by spraying or flow of molten PPA

into the extruder. Such PPA can be used in concentrations

from 0.025 to 0.5 wt% to suppress sharkskin in extrusion

of LLDPE. In some cases a composition of novel PPA

that is liquid at room temperature can be recommended to

be used at low concentrations, i.e. from 0.025 to 0.1 wt%,

to suppress sharkskin at very moderate pressure reduc-

tions. Such compositions of PPA can be prepared from

PEG with MW from 300 to 600 Da.

Mechanisms of Lubrication

A tentative mechanism of slip is discussed shortly in

introduction. Commercially available LLDPE grades com-

prise low MW organo-phosphites as antioxidant additives.

The antioxidants deplete Oxygen from the PE melt and

get converted to organo-phosphates, i.e. low MW esters

of alcohols and phosphoric acid. Organo-phosphates are

well known for their high affinity to metal surfaces and

for their ability to saturate surface layers of steel alloys

with Phosphorus oxide (phosphating of steel). If PEG is

used as an additive to LLDPE resins, it reacts with these

esters at the surface of the die, and high MW esters

appear in the reaction of transesterification. Reaction rates

depend on the presence of catalysts. We used in our

experiments a nitrided die that is saturated by Phosphorus

and Oxygen in its surface layer. Therefore we observed

relatively short conditioning times, whereas with dies hav-

ing no active centers for hydrogen and covalent bonding

of the esters, e.g. a not-oxidized Chromium- or a Plati-

num-coated die, the conditioning times would be too long

for practical use. The esters of PEG and phosphoric acid

accumulate at the die inside and form a layer of a plastic

lubricant, i.e. grease. The grease is a blend of PEG and

high MW organo-phosphates that are thickening agents.

Due to high affinity to steel the organo-phosphates decel-

erate flow of the grease along the die inside and, there-

fore, a thicker lubricating film can be formed with less

concentrations of the lubricant. Passive, i.e. nonreacting,

thickening agents are used in industry as a component of

greases, and the use of them in the composition of PPA

with low viscous PEG would further improve lubrication

and shorten conditioning times ‘‘to clear sharkskin melt

fracture.’’

The use of simplified compositions of PPA that include

PEG and organo-phosphites has the obvious disadvantage

of long conditioning times. At least two chemical reac-

tions are involved in the formation of a lubricating layer

with the use of PEG and organo-phosphites: conversion

of the organo-phosphites to organo-phosphates, and a

reaction of transesterification. The conditioning time can

be considerably reduced if organo-phosphates are added

to the PE resin together with PEG. In our experiments,

the PPA that comprises organo-phosphates made from

sorbitol shows a short conditioning time.

Some organic acids readily react with PEG, and the

conditioning time would be short if the PPA is a reacting

mixture of PEG and such organic acid or anhydride of or-

ganic acid. By use of citric acid as an ingredient of PPA,

we observed not only short conditioning times but also

better lubrication. We may speculate that citric acid reacts

at the die surface with PEG and forms a gel-like sub-

stance that resists to flow, and therefore accumulates at

the die surface as a lubricating film of larger thickness

and of better lubricating performances.

Esters of PEG and citric or phosphoric acid are of

amber color and if the additives are used in high concen-

tration, i.e. 0.5 wt%, it is possible to see them at the die

exit as beads of a dark viscous fluid. It wets metal surface

and flows under forces of surface tension so that it can

coat a considerable part of the die. In time, PEG decom-

poses and evaporates from the die surface but deposits of

Phosphorus oxide remain. Such deposits at the die exit

are reduced or avoided at smaller concentrations of the

additives.

The proposed PPA is a development of a ‘‘core-annual

flow’’ idea that was originally used to reduce friction

losses in transportation of viscous bitumen and crude oils

in long pipelines. We believe that some reacting mixtures

of PEG with acids and comprising thickening agents can

be used in low amounts, i.e. 0.1–1%, instead of water also

to improve transportation of crude oils and bitumen dur-

ing a cold season in areas of northern climates.

Processing Equipment for the Use with PEG-Based PPA

The use of novel PPA is a challenging opportunity to

simplify processing of PO resins. The use of low viscous

PEG in the composition of PPA with existing machinery

has to be combined with changes in the die design for

film blowing, blow molding, and pipe extrusion to extract


and distribute evenly low viscous PPA: elongated dies,

circumferential grooves in the mandrel and housing near

the die exit, as well as tapered ridges of the die and an

elongated core. Kurtz in several patents [88–90] described

various designs of the extrusion die to delay the onset of

sharkskin instability that would also be beneficial for the

use with our PPA. There is no explanation for the geo-

metrical solution of the sharkskin problem in the patents

of Kurtz. We may speculate that accumulation of lubri-

cants like Zinc stearate in the grooves and tapered areas

of the die can contribute to the delay of sharkskin melt

fracture. In a film blowing experiment by use of a die

with elongated core we observed accumulation of low vis-

cous PPA between the PE film and the core. Obviously,

the pool of low viscous liquid between the core and the

PE film ensures homogeneous lubrication and take-up of

the PPA by the PE film. Otherwise, because of variations

in the PPA content and locale variation of friction losses

within a ring and slot dies, we may observe sporadic

zones of locale spurt of the extrudate and variations of

the film thickness.

In processing of pelletized PO resins that comprise

synergistic blends of fluorinated polymers with PEG as

PPA as well as our PEG-based PPA, conveying of pellets

in smooth bore extruders in conditions of flood-feeding

can get unstable or even terminate when a lubricant is fed

to the extruder. This phenomenon is known in industry as

‘‘screw-slip,’’ while it is actually a barrel slip. To ensure

stable conveying of pellets, a single-screw extruder with

grooves in its feeding zone is recommended as well as

starve-feeding [91] of the extruder, i.e. the extruder has to

be fed at a rate less than the capacity of the screw.

Novel Opportunities for Polymer Processing withPEG-based PPA

Extrusion at reduced temperatures potentially simplifies

a design of processing machines for pelletizing, film

blowing, and film casting as well as for tube and pipe

production. It helps to suppress bubble and helical insta-

bilities in film blowing, as well as draw resonance in film

casting and fiber spinning, see [92–94]. For extrusion at

reduced temperatures with the use of novel PPA less anti-

oxidants in the polymer can be used. Indeed, most effi-

cient antioxidants are aromatic and mixed alkyl-aril phos-

phites. When these substances react at high temperatures

with oxygen they form organo-phosphates that are toxic.

PEG itself readily reacts with oxygen and therefore it is

an antioxidant additive itself. So, the use of novel PPA

and reduced amounts of the organo-phosphites would not

only reduce material cost for polymer processing but also

make our environment cleaner. It has to be clearly under-

stood that composition of PPA has to be optimized for

the polymer grade and technology of processing. For

example, for film blowing of LLDPE the optimal compo-

sition of PPA is far from the composition that is optimal

for high lubrication.

We may speculate about an advantage to produce pel-

lets and micropellets by extrusion of PO resins at reduced

temperatures in conditions of air cooling. Some amount

of PEG will stay on the pellets and would be useful for

further processing of the polymers. Micropellets of oval

shape with sizes from 0.3 to 0.8 mm provide superior

handling and processing performances in rotomolding

[95]. Surprisingly, we observed shorter times to sinter

LLDPE particulates with additives of a composition that

is identical to the proposed PPA for extrusion of LLDPE

resins. Details of these experiments are published in [96].

CONCLUSIONS

We propose to use low viscous additives as lubricants

in processing of polyolefin (PO) resins with narrow mo-

lecular weight distribution (MWD) and low melt index

(MI) by extrusion at reduced temperatures, and we present

first experimental results illustrating the improved per-

formance of the novel polymer processing additives

(PPA). The PPA proposed is a reacting mixture of poly-

ethylene glycol (PEG) of MW from 300 to 10 000 Da

with reactants from a following list: citric and phosphoric

acids, phtalic anhydride, polyesters of oxiacids of Phos-

phorus. Silica fume and polyethylene oxide of high MW

are used in composition of PPA as thickening agents. The

PPA can be blended with pellets or powder of polymer

without master-batching. Such PEG-based PPA would be

a cheap alternative to existing blends of fluorinated poly-

mers and PEG that are used as PPA nowadays. Surpris-

ingly, we discovered that at reduced temperatures friction

losses in extrusion with our PPA can be less than at ele-

vated temperatures.

Further experimental and theoretical research is neces-

sary to optimize compositions of PPA for particular proc-

essing technologies and grades of PO resins, as well as to

get better insight into the physical and chemical phenom-

ena underlying the observed improvements in extrusion.

Experiments were made with LLDPE and HDPE, but we

believe that the PPA proposed can improve processing of

many other polymers with narrow MWD and low MI.

Some compositions of the proposed PPA, e.g. comprising

PEG and citric acid, would easily comply with pharma-

ceutical and foodstuff contact regulations.

ACKNOWLEDGMENTS

The authors thank Dr. Kalman Migler for presenting a

copy of his recent book on Instabilities in Polymer Proc-

essing and Prof. Hatzikiriakos for sending his publication

[97] related to their findings. They also thank Dr. Chris

Rauwendaal for discussion and his valuable comments,

ExxonMobil Chemicals for supplying polyethylene resins,

Clariant for samples of organo-phosphites, as well as 3M

and Arkema for samples of fluorinated PPAs.


REFERENCES

1. Z. Tadmor and C.G. Gogos, Principles of Polymer Process-ing, 2nd ed., Wiley, New York (2006).

2. B. Avitzur, Metal Forming. Encyclopedia of Physical Sci-ence & Technology, Academic Press, San Diego (1987).

3. R.E. Christensen, S. P. E. J., 18, 751 (1962).

4. J.C. Miller, S. P. E. Trans., 3, 134 (1963).

5. S. Kase and T. Matsuo, J. Polym. Sci. Part A., 3, 2541 (1965).

6. M.M. Denn, Ann. Rev. Fluid. Mech., 12, 365 (1980).

7. T. Kanai and J.L. White, Polym. Eng. Sci., 24, 1185 (1984).

8. W. Minoshima and J.L. White, J. Non-Newt. Fluid Mech.,19, 275 (1986).

9. J.L. White and H. Yamane, Pure Appl. Chem., 59, 193 (1987).

10. H.F. Mark and J.I. Kroschwitz, Ethylene Polymers. Encyclope-dia of Polymer Science and Technology, 2, Wiley, New York

(2003).

11. O.L. Kulikov, K. Hornung, and M. Wagner, in Advancesin Polymer Processing, Ch. 15, S. Thomas and W. Yang,

Eds., Wood Head Publ. (2009).

12. J.-F. Agassant, D. Arda, C. Combeaud, A. Merten, H.

Munstedt, M. Mackley, R. Laurent, and V. Bruno, Intern.Polym. Proc., 21, 239 (2006).

13. H.W. Jung and J.C. Hyun, in Polymer Processing Instabil-ities: Control and Understanding, Ch. 7, S.G. Hatzikiriakosand K.B. Migler, Eds., Marcel Dekker, New York (2005).

14. C.D. Han and J.Y. Park, J. Appl. Poly. Sci., 19, 3291

(1975).

15. C.D. Han and R. Shetty, IEC Fundam., 16, 49 (1977).

16. L.B. Hutchinson and R.R. Blanchard, U.S. Patent 4,080,138

(1978).

17. F.N. Cogswell, U.S. Patent 3,920,782 (1975).

18. R.T. Fields and C.F.W. Wolf, U.S. Patent 2,991,508 (1961).

19. J.T. Lutz Jr. and R.F. Grossman, Polymer Modifiers andAdditives, Marcel Dekker, New York (2001).

20. H. Zweifel, Plastic Additives Handbook, 5th ed., Hanser,

Munich (2001).

21. E.W. Flick, Plastics Additives: an Industrial Guide, 3rd ed.,Plastics Design Library, New York (2001).

22. E.C. Achilleos, G. Georgiou, and S.G. Hatzikiriakos,

J. Vinyl Addit. Technol., 8, 7 (2002).

23. S.B. Kharchenko, P.M. McGuiggan, and K.B. Migler,

J. Rheol., 47, 1523 (2003).

24. S.B. Kharchenko, K.B. Migler, and S.G. Hatzikiriakos,

Polymer Processing Instabilities: Control and Understanding,Ch. 8, S.G. Hatzikiriakos and K. Migler, Eds., Marcel Dekker,

New York (2005).

25. J.C. Slattery and A.J. Giacomin, U.S. Patent 5,637,268

(1997).

26. D.E. Priester, U.S. Patent 5,587,429 (1996).

27. R. Chiu, J.W. Taylor, D.L. Cooke, S.K. Goyal, and R.E.

Oswin, U.S. Patent 5,550,193 (1996).

28. T.J. Blong, M.P. Greuel, and C. Lavallee, U.S. Patent

5,830,947 (1998).

29. K. Focquet, G. Dewitte, and S.E. Amos, U.S. Patent

6,294,604 (2001).

30. S.S. Woods, U.S. Patent 6,734,252 (2004).

31. J. Briers, J.J. Cernohous, and R.R. Nuyttens, U.S. Patent

20,050,101,722 A1 (2005).

32. G.R. Chapman Jr. and S.R. Oriani, U.S. Patent 6,894,118

(2005).

33. S.R. Oriani, S. D’Uva, and V.P. Trilokekar, U.S. Patent

6,906,137 (2005).

34. B. Barriere, A. Bonnet, J. Laffargue, and G. Marot, U.S.

Patent 20,060,025,523 A1 (2006).

35. G.R. Chapman Jr. and S.R. Oriani, U.S. Patent 20,060,

116,477 A1 (2006).

36. D.W. Aubrey, G.N. Welding, and T. Wong, J. Appl. Polym.Sci., 13, 2193 (1969).

37. B.Z. Newby, M.K. Chaudhury, and H.R. Brown, Science,269, 1407 (1995).

38. B.Z. Newby and M.K. Chaudhury, Langmuir, 13, 1805

(1997).

39. B.Z. Newby and M.K. Chaudhury, Langmuir, 14, 4865

(1998).

40. N. Amouroux, J. Petit, and L. Leger, Langmuir, 17, 6510(2001).

41. D.A. Hill, T. Hasegawa, and M.M. Denn, J. Rheol., 34, 891(1990).

42. F.N. Cogswell, J. Non-Newtonian Fluid Mech., 2, 37,

(1977).

43. O. Kulikov and K. Hornung, J. Non-Newtonian Fluid Mech.,98, 107 (2001).

44. S.G. Hatzikiriakos, Polymer Processing Instabilities: Controland Understanding, Ch. 9, S.G. Hatzikiriakos and K. Migler,

Eds., Marcel Dekker, New York (2005).

45. S.G. Hatzikiriakos, N. Rathod, and E.B. Muliawan, Polym.Eng. Sci., 45, 1098 (2005).


47. O. Kulikov, K. Hornung, and M. Wagner, Rheol. Acta, 46,741 (2007).

48. O. Kulikov, J. Vinyl Addit. Techn., 11, 127 (2005).

49. A. Schallamach, Wear, 17, 301 (1971).


51. G. Lykotrafiti and A.J. Rosakis, Int. J. Fracture, 140, 213(2006).

52. E. Gerde and M. Marder, Nature, 413, 285 (2001).

53. D.D. Joseph and Y.Y. Renardy, Fundamentals of Two-FluidDynamics, Springer, New York (1993).

54. D.E. Broussard, P.R. Scott, and V.R. Kruka, U.S. Patent

3,977,469 (1976).

55. D.D. Joseph, J. Non-Newtonian Fluid Mech., 70, 187 (1997).

56. J.L. Lummus, U.S. Patent 3,434,485 (1969).

57. I.P. Herman, Ed., Physics of the Human Body, Springer,

Berlin (2007).

58. L. Sokoloff, Ed., The Joints and Synovial Fluid, I, Academic

Press, New York (1978).

59. N. Pieterse, Development of a Dynamic Hip Joint Simula-

tion Model, M.Sc. Thesis, University of Pretoria, Pretoria

(2006).


60. R.E. Booser, Ed., Tribology Data Handbook, CRC Press,

Boca Raton, Florida (1997).

61. L.R. Rudnick, Synthetics, Mineral Oils, and Bio-basedLubricants, CRC Press, Boca Raton, Florida (2006).

62. N. Gaylord, Polyalkylene Oxides and Other Polyethers,Interscience, New York (1963).

63. L.E. Mirci, S. Boran, V. Pode, and D. Resiga, J. Synth.Lubr., 2491, 51 (2007).

64. J.V. DeJuneas, G.L. McIntyre, and J.F. O’Horo Jr., U.S.

Patent 4,013,622 (1977).

65. L.E. Wolinski, U.S. Patent 3,222,314 (1965).

66. T. Tikuisis, G. Arnould, J. Bayley, P.S. Chisholm, and

S. Marshall, U.S. Patent 20,050,070,644 A1 (2005).

67. T.J. Blong and C. Lavallee, U.S. Patent 5,527,858 (1996).

68. D.J. Duchesne and V. Bryce, U.S. Patent 4,855,360 (1989).

69. G.R. Chapman Jr. and S.R. Oriani, U.S. Patent 7,001,951

(2006).

70. J.B. Williams and K.S. Geick, U.S. Patent 20,020,063,359

A1 (2002).

71. P. Bauer, U. Seeliger, and U. Faller, U.S. Patent 6,048,937

(2000).

72. S. Sato, U.S. Patent 20,040,083,925 A1 (2004).

73. S. Sato, U.S. Patent Appl. 20,040,132,878 A1 (2004).

74. O. Kulikov, R.U. Pat. 2,288,095 C1 (2006).

75. M.A. Corwin and G.N. Foster, U.S. Patent 4,540,538

(1985).

76. M. Xie, X. Liu, and H. Li, J. Appl. Polym. Sci., 100, 1282(2006).

77. J. Chen, X. Liu, and H. Li, J. Appl. Polym. Sci., 103, 1927(2007).

78. X. Liu and H. Li, Polym. Eng. Sci., 45, 898 (2005).

79. O. Kulikov, R.U. Patent Appl. 20,08,109,571/04(010359) (2008).

80. D.R. Stevenson, T.C. Jennings, M.E. Harr, and M.R.

Jakupca, U.S. Patent 7,320,764 (2008).

81. T.W. Coslett, U.S. Patent 870,937 (1907).

82. G.W. Becker and D. Braun, Kunststoff-Handbuch, 3(1), 15,Hanser Verlag, Munich (1992).

83. I. Noda, W.M. Allen, J.T. Knapmeyer, and M.M. Satkowski,

U.S. Patent 20,080,200,591 (2008).

84. E. Gruenschloss, Intern. Polym. Processing, 3, 226 (2003).

85. O. Kulikov, K. Hornung, and M.H. Wagner, PPS-24Abstracts, Salerno, Italy (2008).

86. O. Kulikov, K. Hornung, and M.H. Wagner, 2nd Int. Conf.on Advanced Tribology: Abstracts, Singapore (2008).

87. Y.W. Chen and M.R. Mackley, Soft Matter, 2, 304 (2006).

88. S.J. Kurtz, U.S. Patent 4,267,146 (1981).



91. C. Rauwendaal, Polymer Extrusion, 4th ed., Hanser Verlag,Munich (2001).

92. M. Wishman and G.E. Hagler, in Handbook of Fiber Chem-

istry, 2nd ed., Ch. 3, M. Lewin and E.M. Pearce, Eds., Mar-

cel Dekker, New York (1998).

93. R.J. Fisher and M.M. Denn, AIChE J., 22, 236 (1976).

94. B.M. Devereux and M.M. Denn, Ind. Eng. Chem. Res., 33,2384 (1994).

95. R.J. Crawford and M.P. Kearns, Practical Guide to Rota-

tional Moulding, Rapra Technology Shawbury, UK (2003).

96. O. Kulikov, K. Hornung, and M.H. Wagner, Int. Pol. Proc.,5, 452 (2009).

97. S.G. Hatzikiriakos and J.M. Dealy, Int. Pol. Proc., 8, 36

(1993).


low viscous hydrophilic processing additives for extrusion of polyethylene at reduced temperatures

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