high-temperature sintering of rubber powder...effects of ground rubber on mixing behavior, curing...

Article

Grinding of ethylene–propylene–dienemonomer vulcanizates:High-temperaturesintering of rubberpowder

Eduard Prut, Dmitry Solomatin, Olga Kuznetsova,Larisa Tkachenko and Dmitry Khalilov

AbstractThe grinding under the action of shear deformations of vulcanizates of oil-filled ethylene–propylene–diene elastomer cured by a sulfur-based system up to different degrees ofcross-linking was studied. Monodisperse powders with a predominant fraction of particlesizes 0.315–0.63 mm were obtained. The effects of the cross-link density of the originalvulcanizate on the content of sol fraction and the cross-link density of the rubber powderwere estimated. The possibility of producing molding materials from rubber powders withenhanced rigidity using the method of high-temperature sintering was shown. The depen-dence of the structure and properties of molding materials on the parameters of rubberpowders was studied. The mechanisms of the grinding and high-temperature sintering areproposed.

KeywordsElastomers, vulcanizates, grinding, rubber powders, molding materials, mechanicalproperties

Introduction

Recently, the importance of recycling of waste materials has been increased worldwide.

The automotive and transportation industries are the biggest consumers of raw rubber.

Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia

Corresponding author:

Dmitry Solomatin, Semenov Institute of Chemical Physics, Russian Academy of Sciences, Kosygin str. 4,

Moscow 119991, Russia.

Email: [email protected]

Journal of Elastomers & Plastics

2015, Vol. 47(1) 52–68

ª The Author(s) 2013

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DOI: 10.1177/0095244313489905

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Rubber waste is usually generated during the manufacture of the industrial products and

disposal of post-consumer (retired) products (for the most part, scrap tires).

The development of effective recycling technologies continues to be an interesting

motive because earlier efforts to recycle rubber wastes, such as incineration, pyrolysis,

and landfill, ended up with ecological and quality problems. Reutilization of powdered

waste rubber seems to be a viable economic alternative, which is also environmentally

friendly.

Recycling of tires may follow different ways such as1:

� retreading (truck and passenger tires);

� using of tires as a whole (artificial rafts and cover foil weights) or in parts

(building blocks);

� grinding;

� pyrolysis (to oils, monomers, and carbon black (CB));

� reclaiming (decross-linking for mixing into fresh rubbers).

Note that the landfill disposal is not even mentioned above, as it is being envir-

onmentally problematic (leakage of pollutants, breeding grounds for rats and mosqui-

toes)2,3 and thus in many countries banned.

Our survey deals mostly with grinding and reclaiming as they represent the major

material recycling options. Moreover, they are often combined to guarantee the neces-

sary compatibility between ground tire rubber (GTR) and matrix polymer as discussed

later. The rubber recycling usually concentrates on discarded tires.

This is due to the fact that almost 70% of all natural and synthetic rubbers ‘‘is con-

sumed’’ in the tire manufacture.1,4 The related recycling methods were already the topics

of review articles,4�6 which, however, are dated back by one decade.

Ethylene–propylene–diene monomer (EPDM) rubber in the form of discarded auto-

motive parts and roofing contributes much to the environmental pollution. Given that

EPDM products contain high oil and filler loadings, it is possible that EPDM powder

may act as low-modulus reinforcing fillers.

Powdered waste rubber has been used as fillers in blends with virgin rubbers and

thermoplastics7–20 and also for modifying concrete and asphalt.21,22 The processibility of

this rubber compound depends not only on the polymer type, average molecular weight,

molecular weight distribution, and temperature but also on the type and concentration of

filler, polymer–filler interaction, processing aids, and scorchiness of compound.23,24

Effects of ground rubber on mixing behavior, curing characteristics, and mechanical

properties of virgin rubbers like natural rubber (NR) and styrene–butadiene rubber were

previously reported.25–28 Earlier researchers studied the rheological properties of virgin

EPDM rubber29 and EPDM compounds filled with CB and oil.30–32 Both the virgin rubber

and filled compounds show pseudoplasticity. Bhaumik et al.33 reported the rheological

behavior of EPDM blends with brominated butyl rubber. Studies were also conducted on

blending of powdered specialty elastomers with general-purpose rubbers.34,35 Processibil-

ity and migration of curatives in waste rubber-filled compounds were studied by a number

of researchers.28,36,37 Jacob et al. reported that a ground vulcanizate of known composition

Prut et al. 53

shows the characteristics of reinforcing fillers, possibly by virtue of the high CB content,

when incorporated into gum (unfilled) EPDM matrix.38 It has been reported that the addi-

tion of cross-linked particles to a virgin rubber compound results in decreased die swell, a

smoother surface finish, and better shape retention.39 To improve the adhesion between

virgin rubber and powdered rubber vulcanizate, the surface modification of the powder

was carried out and the surface-modified powders were studied.40–42

There are reports on the utilization of rubber compounds-containing GTR, obtained

by the cryogenic or ambient grinding of worn-out tires.27,40,41,43–45

Phadke and De46 observed that cryogenically ground waste rubber can be vulcanized to

an acceptable level by compounding with additional sulfur and an accelerator, whereas the

incorporation of additional activators caused only marginal improvement in their physical

properties. It is believed that waste rubber vulcanizates contain unreacted zinc oxide and

stearic acid, which take part in further curing.

The aim of this work was to study the potential of ground waste EPDM produced from

the completely characterized oil-filled EPDM vulcanizates with different cross-link den-

sities that allowed one to estimate the influence of their properties on the parameters of

rubber powder (RP) and related materials.

Experimental

Vulcanizates used in this work were obtained from EPDM Dutral TER 4535 (Polimeri

Europa, Italy) with 50% oil added in the course of synthesis and 32% propylene units; the

Mooney viscosity of polymer was as high as 32 (at 125�C). 5-Ethylidene-2-norbornene

(4�5%) was used as a diene component of EPDM.

Sulfur-based system of the following composition (in parts per hundred) was used as a

curative: Sulfur (1.0�4.0 phr), tetramethylthiuram disulfide (0.73 phr), di-(2-benzthiazo-

lyl) disulfide (0.25 phr), zinc oxide (2.53 phr), and stearic acid (1.0 phr) (BINAGroup,

Moscow, Russia).

The initial vulcanizates were prepared by mixing EPDM with the sulfur-based curatives

containing different amounts of sulfur for 10 min at room temperature on a Plastograph EC

(Brabender GmbH & Co. KG, Dusseldorf, Germany) followed by molding for 15 min at

190�C and 10 MPa.

The resulting vulcanizates were ground at 150�155�C by fourfold pass of the material

over a rotor disperser constructed on the base of a single-screw extruder, in the Institute of

Chemical Physics, Russian Academy of Sciences, Russia.47�49 The rotor disperser has the

following technical characteristics: the screw diameter of 32 mm, the length/diameter ratio

of 11, and the rotation velocity of 45 rpm. To create an intensive shear deformation, the

rotor disperser was equipped with a grinding head in the form of a cam rotor rotating inside

a riffled cylinder.50

The particle size distribution of RP was determined by the method of vibratory

sieving.51

The molding materials based on RP were obtained by the method of high-temperature

sintering both in the presence and in the absence of additional curatives.52 Curatives were

introduced into RP at room temperature for 10 min on a Plastograph EC mixer. The

54 Journal of Elastomers & Plastics 47(1)

concentration of sulfur in curatives was as high as 1.0 phr. The sintering was carried out in

a press (15 min at 10 MPa and 190�C).

Sol–gel analysis of the initial vulcanizates, RP, and molding materials was performed

by extraction with acetone in a Soxhlet apparatus (Gerhardt Analytical Systems, Ger-

many) for 8 h at 56�C, after which the samples were dried to the constant weight. Then

these samples were purified in a Soxhlet apparatus by extraction with toluene for 12 h at

111�C and again were dried to the constant weight. The results were averaged by three

samples. The gel content in the samples was calculated by the following equation

Gc %ð Þ ¼ mi

ms

� �� 100 ð1Þ

where mi is weight of the insoluble fraction and ms is weight of the sample.

The equilibrium swelling degree of gel fraction Q1 was determined by swelling in

cyclohexane for 48 h followed by drying for 24 h.

The cross-link density (n) of gel fraction of EPDM was obtained using the following

equation53:

n ¼ � ln 1� Vkð Þ þ Vk þ wV 2k

Vs V1=3

k � Vk=2� � ð2Þ

where Vk is volume fraction of elastomer in the swollen sample, Vs is molar volume of

solvent, and w ¼ 0.315 is the Huggins elastomer–solvent interaction parameter.54,55 The

final values of cross-link density were averaged for three measurements.

Mechanical tests were carried out with plates 1 mm thick obtained by molding for 10 min

at 190�C and 10 MPa followed by cooling under pressure with the rate of ~15 K/min. The

dumbbell-shaped samples with the working part 35� 5 mm2 in size were cut from the plates.

The tests were performed on an Instron-1122 (Instron, USA) tensile test machine in the ten-

sion mode at room temperature and with the upper traverse velocity of 50 mm/min. Tensile

modulus E (from the initial slope of curves in the stress–strain (s�ε) diagrams) along with

sb and εb values were determined. The results were averaged by six to eight samples.

Results and discussion

Grinding of vulcanizates

As noted in Introduction section, the grinding of EPDM-based vulcanizates was carried

out using high temperature simulated distillation (HTSD) method47,48 realized in a spe-

cial unit, a rotor disperser constructed on the base of a single-screw extruder. The oper-

ation of this disperser is based on repeated deformations of grinding material in gaps

between the grinding elements (cams) and the housing walls. The structure of the grind-

ing rubber on its pass throughout the disperser zones was studied previously.56 It was

found that the compacted material is fed into a grinding head and is subjected to further

grinding. Note that the fractional composition of RP is determined in many respects by

the material density.

Prut et al. 55

The following scheme of the grinding process will be considered below. Let us

assume that, upon grinding, two competitive processes take place: the fracture and

formation of particles smaller in size compared to pristine particles (grinding) and

aggregation of particles (Figure 1). Let all particles have spherical shape with radius r;

thus the number of particles per unit volume would be n0 � 14pr3=3

, and, in volume V,

n � V4pr3=3

. Let us assume that, in the dispersion medium, the particles move indepen-

dently from each other.

The change in the number of particles n with the grinding time t can be determined

using the following equation

dn

dt¼ k1n� k2n ð3Þ

where the first term, k1n, in equation (3) describes the grinding rate and is determined by

the ratio of the energy of material deformation W to the particle fracture energy A, i.e.

k1 ¼W/A. For simple shear, W ¼ Zg2, where Z is effective viscosity and g is shear rate.57

The energy consumed in the material dispersion A presents the sum of energy

expended in the sample fracture Af and the formation of fresh surface s 4pr2nV¼ 3 s

r(s is

surface tension)

A ¼ Ap þ 3sr

ð4Þ

The second term, k2n, in equation (3) describes the aggregation rate, where

k2 ¼ 4p p _g V

V0and p is the probability of favorable collisions (0 < p < 1).58 If V � V0, then

k2 ¼ 4p p _g.

Figure 1. Model of grinding process.


The rate of particle formation approaches the aggregation rate as the grinding time

increases, and the radius of the resulting particles r* is expressed as follows

r� ¼ 12ps

pZ_g 1� 4pAp

pZ _g

� � ð5Þ

Thus, the radius of formed particles depends on shear deformation, fracture energy

and surface energy.

The grinding of vulcanized rubbers occurs under the joint mechanical and thermal

action. The thermal and mechanical degradation of polymer materials is inevitable under

such conditions and results in the degradation products. Therefore, for development of

new ecologically safe technologies of grinding, the problems of reducing mechanical

load and temperature as well as changing the conditions of dispersion or other processes

of structurization of vulcanized rubbers must be solved.

Thus, the composition and properties of vulcanizates used for producing rubber

powders are of great importance. The characteristics of the initial vulcanizates used for

production of RP are listed in Table 1.

As can be seen from Table 1, the content of sol fractions extracted using acetone and

toluene remains unchanged as the sulfur concentration grows, and the cross-link density

increases. In this case, the content of sol fractions extracted by acetone is approximately

three times higher compared to the content of sol fractions extracted by toluene that is

related to the presence of 50% paraffin oil in the elastomer. As a result of extraction with

acetone, the major portion of organic compounds not incorporated into a three-

dimensional network (antioxidants, plasticizers, products of their reactions, softeners,

activators, and modifiers) is removed from the vulcanizate. On extraction with toluene,

the network fragments and rubber molecules not incorporated into the three-dimensional

network are eliminated.49

The topological structure of vulcanizate characteristic of the connectivity of structural

elements is irregular and contains free chain ends, loops, and entanglements.56 The

presence of cyclic structures is the most important topological characteristic of network.

Hence, the topological junctions (entanglements) should have a significant influence on

the material properties. In the first approximation, the total number of junctions per unit

Table 1. Contents of sol fractions extracted by acetone and toluene and cross-link densities ofEPDM-based vulcanizates.

Content of curative S (phr)

Content of sol fraction (%)

n � 105 (mol/cm3)Acetone Toluene

1 39.9 10.8 7.62 39.9 11.8 8.53 37.8 12.4 9.04 40.5 9.3 10.6

EPDM: ethylene–propylene–diene monomer.

Prut et al. 57

volume is the sum of chemical and topological junctions. One of the characteristics of the

topological structure of polymer network is the presence of sol fraction that is related to

the specific features of the network formation. As noted above, the cross-link density

increases with sulfur concentration, whereas the content of sol fraction extracted by

toluene remains practically unchanged. This difference is most probably due to the fact

that the rubber vulcanization is carried out in an oil-filled medium. This changes both the

concentration of curatives owing to their distribution in rubber and oil and the number of

topological junctions.

Figure 2 shows the s�ε diagrams of vulcanizates obtained at different concentrations

of sulfur. The observed S-shaped patterns are shown by nonvulcanized rubbers (curve 1)

and vulcanizates (curves 2�5). The effect of cross-link density on the stress and elonga-

tion values manifests itself at distinct portions of s�ε curves. The s values for vulcani-

zates differ from those for nonvulcanized samples even at low deformations. The

increase in the cross-link density results in changes of s and ε values at the final stage

of deformation.

The observed variations can be interpreted as follows.59 The tension of a nonpolar

elastomer with a low cross-link density leads to the extension and straightening of inter-

junction chains in the direction of applied force. The change in the free energy of the

system under rather low deformations has the entropy nature and is related to changes

in chain conformations. Hence, the difference between the s�ε diagrams of nonvulca-

nized and vulcanized samples follow from the entropy factor.

The random distribution of interjunction chains by length and their space orientation

under deformation give rise to the ultimate straightening of the shortest chains. Further

0

1

2

0 500 1000

σ (M

Pa)

ε (%)

1

23

45

1 - [S] 0 phr.2 - [S] 1phr.3 - [S] 2phr.4 - [S] 3phr.5 - [S] 4phr.

Figure 2. s�ε diagrams of vulcanizates of oil-filled EPDM at different concentrations of sulfur: (1)0 phr, (2) 1 phr, (3) 2 phr, (4) 3 phr, and (5) 4 phr. s�ε: stress–strain; EPDM: ethylene–propylene–diene monomer.


deformation of the system is possible, if these maximally straightened chains redistribute

the stress on less loaded chains. Thus, it can be suggested that the difference in the s�εdiagrams at the final stage of deformation depending on cross-link density is related both

to the increasing number of chemical junctions and to the internal energy.

The effect of the cross-link density on the process of fracture of vulcanizates was con-

sidered in a great many works; however, this problem is not solved up to now.59,60

The elongation at break (εb) for vulcanizates prepared from the initial EPDM

decreases with increasing concentration of curative (Figure 3(a)). This fact confirms the

theoretical models.59,60

0 1 2 3 4

Content of sulfur(phr)

0

1

2

3

E (

MPa

)

(c)

0

250

500

750

1000

0 1 2 3 4

ε b (%

)

Content of sulfur(phr)0 1 2 3 4

Content of sulfur(phr)

Nonvulcanized molding materialsVulcanized moldingmaterialsInitial vulcanizates



(a)

0

1

2

3

(b)

σ b (

MPa

)

Figure 3. (a) εb, (b) sb, and (c) E of initial vulcanizates, nonvulcanized and vulcanized moldingmaterials on the base of RP versus sulfur concentration in the initial vulcanizates. εb: elongation atbreak; sb: tensile strength; E: tensile modulus.

Prut et al. 59

The tensile strength (sb) is determined by elastic and relaxation properties of the

polymer system and is related to the cross-link density by the extremal dependence. At

the tension rate used in this work, only the second branch of curve was obtained: there

was a decrease in sb with increasing concentration of curative (Figure 3(b)).

In this case, E changes insignificantly owing to the presence of oil in the sample studied

(Figure 3(c)). The cross-link density for gel fractions of samples was determined from the

equilibrium swelling. As shown, the cross-link densities calculated from the data on the

equilibrium swelling and modulus do not coincide.61 The observed discrepancy is proba-

bly related to the fact that, in the formula for calculation of cross-link density from E, no

account was taken for the front factor, which may change because of the presence of oil in

rubber.62

The obtained vulcanizates were grinded on a rotor disperser. As found previously, the

particle size decreases with increasing deformation.57,63 By analyzing the powder structure

in the course of grinding, it was shown that the vulcanizate is compacted before the entry

into the grinding head of disperser: the smaller the pristine particle size, the higher is the

degree of compaction. The density of material fed into the grinding unit determines in

many respects the fractional composition of the resulting powder. The coarse fractions of

vulcanizates formed a looser material before the entry into the grinding head.

The fractional composition of RP is given in Table 2. As can be seen, the fraction with

the particle size d from 0.315 to 0.63 mm is predominant in the studied materials. The

particle size depends on shear stress, since the cross-link density affects the fracture

energy as shown in the figures (Figure 2). The surface energy is also sensitive to changes

in the cross-link density. Thus, from equation (5), it follows that the radius of formed

particles must change significantly, which was proved experimentally.

The content of particles with d < 0.315 mm in RP varies with the sulfur content in the

pristine vulcanizates and passes through the maximum: the content of particles with

d < 0.315 mm increases to 17.6% as the sulfur content changes from 1 to 3 phr, and at

4 phr, the content of this fraction reduces to the minimum.

Based on the data of the dispersion analysis, in further experiments, the fraction of RP

particles 0.315�0.63 mm in size was used predominantly. Figure 4 shows the images of

the particle surface for this fraction. It is evident that the RP is also similar at the

microscale. In spite of the different cross-link densities of the pristine vulcanizates, the

Table 2. Fractional composition of RP.a

Vulcanizate

Content of RP particles different in size (%)

d < 0.315 (mm) 0.315 < d < 0.63 (mm) d > 0.63 (mm)

RP-1 8.9 90.6 0.5RP-2 15.9 83.5 0.6RP-3 17.6 81.5 0.9RP-4 1.9 97.3 0.8

RP: rubber powder.aRPs differ in the sulfur content in the initial vulcanizates (1, 2, 3, and 4 phr, respectively).


related powders are practically identical as to the particle shape and structure: the parti-

cles have a smooth surface with steps in some regions.

As noted above, the process of RP grinding is not reduced to purely mechanical

fracture. The mechanical action initiates various mechanochemical processes. The

intensity of chemical processes occurring upon RP production can be characterized using

sol�gel analysis. On grinding, the three-dimensional network is broken up with forma-

tion of linear products and a sol fraction. The contents of sol fractions extracted by acet-

one and toluene for RP produced from vulcanizates with different cross-link densities are

given in Table 3. For the initial vulcanizates differing in the cross-link density, the con-

tent of sol fractions extracted by acetone remains practically unchanged. The content of

sol fractions extracted by acetone from RP increases by 3�8%. It is different with a sol

fraction extracted by toluene. The content of sol fractions extracted by toluene for RP is

approximately two times higher compared to the initial vulcanizates and tends to

decrease with increasing cross-link density. At the same time, the cross-link density in

RP is higher compared to that for vulcanizates.

The obtained results allow one to formulate some concepts on the mechanism of

dispersion of vulcanizates in the rubber-like state by HTSD method. The experimental

study of the mechanism of multiple fractures of elastomers under plunger extrusion

showed that the dispersion in the complex-stressed state begins only at some critical

Figure 4. Images of the RP surface with particle size of 0.315�0.63 mm. RPs differ in the sulfurcontent in the initial vulcanizates ((a) 1 phr, (b) 2 phr, (c) 3 phr, and (d) 4 phr, respectively). RP:rubber powder.

Table 3. Contents of sol fractions extracted by acetone and toluene and cross-link densities forRP fraction with d of 0.315�0.63 mm.a

RP type

Content of sol fraction (%)

n � 105 (mol/cm3)Acetone Toluene

RP-1 44.1 20.4 7.6RP-2 43.5 18.8 8.4RP-3 46.2 14.3 8.6RP-4 42.9 17.6 14.5

RP: rubber powder.aRPs differ in the sulfur content in the initial vulcanizates (1, 2, 3, and 4 phr, respectively).

Prut et al. 61

level of the rubber-like deformation accumulated by material.64 The accumulated elastic

energy results in the formation of new surface.

The possibility of multiple fractures depends on two factors: the elastic properties of

material and the heterogeneity of its structure providing the needed concentration of

mechanical stresses.

As noted above, the structure of vulcanizates is characteristic of the space heterogeneity

related to the random character of cross-linking. The applied mechanical loads favor the

development of overstresses on chains connecting highly cross-linked regions. The chain

scission occurs mainly near the surface of highly cross-linked regions, and the material

bulk remains almost intact, that is, on dispersion of vulcanizates, the fracture inside the

three-dimensional network is absent. The microcrack propagation from the surface into

the interior of particles is most probably hindered by network. This assumption explains

the fact that the cross-link densities of RP are higher compared with the original vulcani-

zates, and the content of sol fraction increases through scission of weak bonds.

The s–ε curves under tension (Figure 2) show that the deformation of samples with

different cross-link densities is of the same type. This allows one to suggest that the pow-

der particles are formed via the unique mechanism independent of the cross-link density

of the pristine vulcanizates; thus, the grinding of vulcanizates with different cross-link

densities results in powders with similarly shaped particles.

The decrease in the number of particles less than 0.315 mm in size in most cross-

linked vulcanizate (S ¼ 4 phr) is related to a more uniform network with a smaller num-

ber of defects.

Thus, the necessary condition for dispersion is sufficiently high cross-link density,

which allows the accumulation of the elastic energy during compression of the cross-

linked material. Once this condition is fulfilled, the vulcanizate is grinded rather easily.

Properties of molding materials based on RP

The structure and properties of materials obtained by powder molding technology

depend to a great extent on the characteristics of the initial powders. The compaction or

sintering, a definition more common for solid state physics, which is used in this work, is

one of most important processes in fabrication of block products from various materi-

als.65,66 The sintering presents a complex multistage kinetic process of powder molding.

Along with the determination of the mechanism of powder particles compaction, the

effect of the contacts between particles, their porosity, the role of friction in the molding

processes, the density distribution in particles, and so on must be estimated.

The production of molding materials from RP via sintering is described in the study

by Burford18 and Pittolo and Kruglitskii.67 The effect of powder molding parameters on

the tensile strength of molding materials was studied in a number of works.56,68�70 It was

found that, on molding of RP without additional curatives, the tensile strength depends

mainly on pressure and temperature. The molding time has a minor effect on the tensile

strength in the time range of 3�30 min. At temperatures above 200�C, the degradation

begins. The changes in tensile strength and εb depending on the curative content are

influenced by the molding temperature and pressure. At the same time, in the pressure


range from 1 to 50 MPa, the addition of curatives is accompanied by weak changes in the

properties of molding materials. In these experiments, RP were prepared by grinding of

worn tires based on a blend of NR and SBR using a cryogenic method. The particle size

of RP ranged from 0.3 to 0.9 mm. The analogous results were also obtained for RP

samples prepared by the grinding of vulcanizates on the base of chloroprene and butyl

rubbers.

The obtained results allow one to propose the following mechanism of compaction.

The scheme of the process is presented in Figure 5. At stage 1, the particles approach

each other and the crumb is compacted; at stage 2, the particles move relative to each

other with repacking and the fracture of aggregates; at stage 3, the particles are subjected

to movement (shear) relative to each other accompanied by their partial fracture, changes

in shape and internal porosity, and the boundaries between particles disappear; finally, at

stage 4, the interparticle bonds are formed followed by the formation of monolithic

plates.

Initially, on compaction of crumb, the particles draw together and during the process,

the structural deformations dominate, that is, the movement of particles relative to each

other, their repacking, and fracture of weak aggregates (associates). In this case, elastic

deformations prevail. The structural deformations are retarded with increasing load and

the relative movement (shear) of particles accompanied by their partial fracture, changes

in shape, and internal porosity becomes dominant. The boundaries between particles

disappear, the number of contacts and entanglements between structural elements

increases and, as a result, the monolithic plate will be formed.

Evidently, the compaction of crumb is rather a complex process. For example, if the

pristine product contains pores or they are formed in the course of compaction, there is

the maximum pore concentration, above which the monolithic plate cannot be formed.

It should be noted that in RP, some residual amount of curatives is still retained. Thus,

the molding materials (the fraction with d of 0.315�0.63 mm) were prepared both in the

presence and in the absence of additional curatives.

The contents of sol fractions extracted by acetone and toluene from molding materials

obtained from RP with different cross-link densities are presented in Table 4. The results

for molding materials in the presence and in the absence of the additional curatives are

Figure 5. Schematic representation of molding material compaction (see text).

Prut et al. 63

given. The contents of sol fractions were found to be practically identical for all molding

materials. The cross-link density in the molding materials remains almost unchanged

compared with the cross-link density in the pristine vulcanizates. The addition of cura-

tives to RP results in an increase in the cross-link density of molding materials owing to

formation of chains between highly cross-linked regions and particles.

Figure 6 shows the s�ε diagrams of nonvulcanized and vulcanized molding mate-

rials. It was shown that the curve patterns are practically independent of the cross-link

density of RP (Table 3) both in the presence and in the absence of additional curatives.

However, these patterns differ from the corresponding curves for the original vulcani-

zates at the final stage of deformation (Figure 2).

As shown, E (Figure 3(c)) and εb (Figure 3(a)) do not depend on the presence of

additional curatives on production of molding materials. The E and εb values seem to be

determined by the structure of powder particles characteristic of the presence of tie

chains between highly cross-linked regions. Hence, the dependences of E and εb of mold-

ing materials and the original vulcanizates on the sulfur concentration have the identical

patterns (Figure 3). However, the E values of molding materials are higher than the cor-

responding values for vulcanizates, and εb of molding materials are lower compared with

the εb of vulcanizates. This fact calls for further investigation.

Another situation is observed for changes in sb. The introduction of additional

curatives results in an increase in sb for molding materials and, at the minimum con-

centration of sulfur (1.0 phr), this value becomes equal to sb of the pristine vulcanizate

(Figure 3(b)).

Conclusions

This article presents various aspects of the grinding of oil-filled EPDM vulcanizates and

the RP application. It was shown that the fraction with the particle size from 0.315 to

0.63 mm is predominant in the studied vulcanizates. The intensity of chemical processes

occurring upon RP production was characterized by sol�gel analysis. The content of sol

Table 4. Contents of sol fractions extracted by acetone and toluene and cross-link densities ofmolding materials.a

Vulcanizates

Without additional curatives With additional curatives

Content of solfraction (%)

n � 105 (mol/cm3)

Content of solfraction (%)

n � 105 (mol/cm3)Acetone Toluene Acetone Toluene

RP-1 41.1 9.5 8.85 39.8 9.8 11.8RP-2 40.3 10.7 10.2 38.6 10.8 12.5RP-3 41.0 11.7 9.65 39.7 9.7 12.4RP-4 40.1 10.6 10.9 38.5 10.2 12.0

aRPs differ in the sulfur content in the initial vulcanizates (1, 2, 3, and 4 phr, respectively).


fractions in RP extracted by toluene was higher compared with the original vulcanizates

and tends to decrease with increasing cross-link density. The cross-link density in RP

was higher compared to that for original vulcanizates.

The production of molding materials from RP via sintering was studied. The molding

materials prepared in the presence and in the absence of the additional curatives were

investigated. The cross-link density in the molding materials was found to remain

unchanged unlike the cross-link density in the pristine vulcanizates. It was shown that

the addition of curatives to RP results in an increase in the cross-link density of molding

materials. The mechanisms of grinding and compaction were proposed.

Funding

The authors are thankful to the Ministry of Education and Science of the Russian Federation for the

financial support (contract 14.740.11.0372).

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high-temperature sintering of rubber powder...effects of ground rubber on mixing behavior, curing...

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