2 lubrİcants and mold release agents

8/2/2019 2 LUBRCANTS and MOLD RELEASE AGENTS

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Lubricants are added to most polymers at low levelsto increase the overall rate of processing or improvesurface parameters. They have been used in the past

to facilitate extrusion, injection, compression etc. ofmany polymers

Most used lubricants;

Ca and Zn stearates 45% Waxes 23,5%

Fatty acid amines 19%

Fatty acid esters12%

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Lubricants often improve the properties of the products;

- Brightness

- Heat stability in the course of a processing operation- Light stability

- Resistance to degradation by corrosive agents and towater absorption

- Better dispersion of additivies

- Improved electrical optical and mechanical propertiesetc.

Some examples;

Stearic acid and its Ca, Li, Ba, Al, Pb etc., salts,

Natural waxes Microcrystalline paraffins with high melting point

Mineral and vegetable oils

Whale fat

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A main function of lubricants is to prevent polymersthat have a tendency toward tackiness from stickingon metallic parts of processing machinery; this isexternal lubrication.

Another important function is to provide internal

lubrication to the polymer under processing toimprove the free flow of the melt bulk polymer byreducing friction between chain segments.

These two combined functions give a better outformthe processing equipment and also they facilitatemold release.

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The lubricant acts exactly as an oil in the case ofbearing lubrication and the spherical agglomeratesslide directly on a lubricant layer.

With a melt polymer a melt lubricant may formeither a homogeneous phase or a mixture of the twoseparate liquids according to their mutual miscibility.This is the reason that melt PVC and a lubricant mayform a two phase system at least , made of spherical

PVC agglomerates dispersed in a liquid solution ofPVC single molecules and the melt lubricant

or in the melt lubricant alone if it is not miscible withPVC

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The effect of lubrication on the processibility for three thpes of differently

polymerized PVC; c1: the emulsion polymerized polymer contains

emulsifiers that act as lubricant. Only this type of PVC with low molecular

weight can be processed without lubricant.

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Other important consideration in selection

1- mutual interaction between lubricants and other

additivies2- the way and conditions of processing polymer

compounds

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To be effective lubricant should be slightly

soluble in the polymer and should have some

cohesion with polymer molecules in order toexude too easily

It should be a long chain hydrocarbon ended by a

polar group

Its melting point should be such that the film

produced at the surface of the polymer particles

is strong enough

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According to their action1-external

2-internal

External Lubricant; very low solubility in the polymer; they exudepartially from the bulk,creating a separating layer between thepolymer and the metal surface of the processing machine.

Polar end of the lubricant molecules is strongly adsorbed to the metalsurface. The strength of the bond between a lubricant and the metalsurface depends on the nature of the metal surface so as to form astationary layer of lubricant molecules that then initiates the formation

of multimolecular layer as shown in fig. 2.6 ;The strength of the bond between a lubricant molecule and the metalsurface depends on the nature of the metal surface ; it determineswhether the adsorption is physical or chemical in nature. If thelubricant is an acid such as lauric acid it reacts with metals such as Cdand Zn but not with Ag,Al,Ni,Cr etc..

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Internal lubricants; which have a good affinity to thepolymer and reduce its viscosity in the melt state. Theyreduce the heat produced by the action of the friction

forces. They also affect elastic behaviour in the course ofsuccesive processing steps (swelling-melt fracture etc.)

It has also been observed that external lubr. extend theplastication time of PVC comp.s and that internalsreduce it.

The limit of the affinity between a polymer and a

lubricant is criterion for the diffrentiation

A lubricant begin to act as an external above a given

concentration in the polymer. If the affinity to the polymeris limited mechanical properties vary with its

concentration before the limited affinity is attained. In suchcases that substance functions as a plasticizer.

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Saturated hydrocarbons, lubrication efficiencyincreases with the chain length. Solidparaffins(polyethylene waxes Mw: 2000-10000) are

more effective than parraffin oils or low-meltingparaffins (Mw:200-400)

Fatty acids;mainly as external lubricants. The highermolecular weight acids have a greater affinity toPVC. Stearic acid is the one most used (low price-

availability). Commercial materials used aslubricants usually contain from 40% to 97% stearicacid and the rest is made of palmitic, myristic andoleic acids.

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High molecular weight alcohols;differ from fattyacids in their actions. They have more affinity to

PVC and do not exude at concentrations used inpractice(~2%). They are considered internallubricants. ie: cetylstearyl alcohol.

Metal soaps: external lubricants and are among best

mold release. In some cases they improve heat andlight stability of PVC( ie: Li,Sr, Ca, AL,Ba, Pb)

Waxes: esters of fatty acid and high molecular-weight monofunctional alcohols. The two long

chains attached to the polar group produce very goodlubrications ie: wax from partially saponifiedmontanic acid,ester with C28 and C32 chains,stearylstearate.

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Fatty acid monoesters; good molding properties and actas secondary plasticizers, affecting the mech. properties.ie: butyl and octyl stearates

Partially esterified polyfunctional alcohols by fatty acids;3 different types

1-Hydrogenated tallow glycerides (HTG)

CH3(CH2)16COOCH2CH

3(CH

2)16

COOCH

CH3(CH2)16COOCH2

2- Glyceryl monostearates (GMS) or glycerylmonoricinoleates

3- Esters of sorbitol, ethylene glycol,propylene glycol,

pentaerythritol and high molecular weight alcohols. Actas internal lubri.

Fatty acid bisamides; have outstanding externallubrcation properties

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Some commercial products improves rheologicalproperties of the melt bulk polymer and

brilliance of the surface of finished products

without affecting machenial properties. Some of

them also act as antistatic agent.

Polyethylene (PE) Mw:2000 is recomended

Fatty acid amide,calcium stearate, sodium

stearate are some examples

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Normal rules for selection;

Metal soaps, mainly stearates, function as externallubricants, since they have low affinity to allpolymers

Long-chain fatty acids, alcohol and amides act as

internal lubr. for polar polymers such as PVC, pAetc. But have low aff. to non polar polymers(polyolefins)

Long chain dialkyl esters have med. aff. to mostpolymers and can act both int. and ext lub., they

often used to obtain a balanced lubrication High-Mw paraffin waxes function as external lubr.

For polar polymers,(low aff.) but they have high aff.to polyolefins then they used as int.lubr.

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Table 2.1; most used lubricants

Different lubr. are used together to obtain special properties forplastic materials. Total quantity used vary 1%-2%

They can be added during preparation of compounds, to theextruders or in the course of polymerization react.

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Mold release is facilitated by a better design ofmolds by keeping walls of the molds clean and

polished, and by reducing the friction coefficient

between plastic materials and molds

The use of mold release agents increases the

productivity of molding process such as

compression molding, injection molding,production of plastic bottles by extrusion

blowing, or injection blowing.

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Waxes, soaps, and silicones are used. Paraffin waxes are not very convenient

Soaps offer a better adhesion to metals because

of the polar part of their molecules. Howeverthey also can be removed by plastic materials,

since they have a long and bulky nonpolar chain

that is weakly attached to the metals surface by

only one point.

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Silicones, mainly polydimethylsiloxane, areexcellent mold-release agents. Their chain containsnumerous polar groups that adhere strongly to the

metal surface and their methyl groups pointing awayfrom the surface facilitate the flow of the melt plasticmaterial. Silicones function thus as a permanentseparation layer. Their heat stability is relatively high

(~300C) and no C film is formed when theydecompose. However their presence creates someproblems when impression or metallization is to bedone afterward on the surface of the products.

Mold-release agents are used in the form ofpowders, oils or jellies, solutions, sheets or films. Assheets, cellophane, PV AL, CA and so on, are used.

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2 lubrİcants and mold release agents

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