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1Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 1, 2004

Use of Ground Tyre Rubber (GTR) in Thermoplastic Polyolefin Elastomer Compositions

Use of Ground Tyre Rubber (GTR) in Thermoplastic

Polyolefin Elastomer Compositions

E. Lievana and J. Karger-Kocsis

Institut für Verbundwerkstoffe GmbH (Institute for Composite Materials), Kaiserslautern

University of Technology, POBox 3049, D-67653 Kaiserslautern, Germany

ABSTRACT

A novel method was proposed to produce thermoplastic elastomers containing

ground tyre rubber (GTR). The new proprietary technology is based on the fact that bitumen and related materials may contribute to the devulcanisation of

GTR and act at the same time as plasticiser and compatibiliser in the blends.

Blends containing polyolefins, olefinic type rubbers and GTR treated with

bitumen showed outstanding mechanical properties and met the requirements

with thermoplastic rubbers. It was established that the conditions of the GTR

treatment by bitumen strongly affect the performance of the compounds.

Scanning electron microscopy (SEM) results revealed a good adhesion between

the GTR particles and the surrounding matrix. The apparent particle size of

GTR was reduced owing to the treatise with bitumen, which is an indirect hint

for GTR devulcanisation.

1. INTRODUCTION

The disposal of worn tyres and their economic recycling mean a great

challenge nowadays. Table 1 lists the disposal statistics of post-consumer

tyres in the European Union for 2000. This table also indicates the forecast (see

arrows) for the near future.

Material recycling is preferred not only due to legislative actions but pushed

forward also by strong economic arguments. The energy equivalent of 1 kg

tyre is ca. 128 MJ. Its recycling by energy generation results, however only in

30 MJ. On the other hand less than 3.4 MJ energy is required to produce 0.5-

0.75 kg of ground tyre rubber (GTR)(1). Owing to highly advanced grinding

methods GTR fractions in reproducible good quality are available on the

market.

GTR is mainly used in low-tech, less demanding applications as already

indicated in Table 1. A promising way of ‘upcycling’ GTR is to incorporate

or transform it into thermoplastic elastomers (TPEs). This strategy is based on

the fact that GTR powder can well be dispersed in suitable fresh rubbers, which

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2 Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 1, 2004


constitute one major component of blend-type thermoplastic rubbers. Further,

in case of TPEs produced by dynamic vulcanisation the rubber particles are

finely dispersed (particle size less than 1 µm) in thermoplastic (mostlycomposed of polyolefins) matrices(2). However, such fine fraction of GTR can

hardly be produced, not even by cryogenic techniques. The adhesion between

the GTR particles and matrix polymers is usually very weak. This is because

of the crosslinked structure of GTR that hinders the molecular entanglement.

The other problem when using GTR in polymer blends is related with its

particle size, as mentioned above. Irrespective to these problems several

attempts were made to produce thermoplastic rubbers by adding GTR into the

corresponding recipes(3-10). It was early recognized that the crumb rubber,

GTR should be - at least partially - devulcanised. For devulcanisation,reclaiming various routes were followed, and especially thermo mechanical

and thermo chemical treatments were applied (e.g. (9-10)). The other option was

to functionalise the GTR and thus improve the compatibility toward the matrix

(e.g. (11)).

The outcome of our former works(9-10) can be summarized as follows. In order

to produce TPEs which contain GTR and exhibit competitive properties, the

GTR must be partially descrosslinked (in order to facilitate molecular

entanglement) and in addition, fresh rubber showing compatibility towardboth GTR and polyolefins should be added. Incorporation of fresh rubber

which encapsulates the filler (here GTR) particles has been found as a very

promising way also for filled polyolefins, and especially for polypropylenes(12).

Table 1 Disposal of post-consumer tyres in the European Union(1)

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The working hypothesis of this study was that bitumen is a suitable reclaiming

agent for GTR. Note that bitumen may absorb and react with sulfur. Further

on, bitumen is a good plasticizer for both GTR and polyolefins.

2. EXPERIMENTAL

Materials

As polyolefin low density polyethylene waste (LDPErec.

) was selected. LDPErec.

was produced from used greenhouse films and had the following composition:

LDPE: 65-70%, linear low density polyethylene (LLDPE): 12-17%, ethylene/ vinyl acetate copolymer (EVA): 12-15%, E-modulus: 180 MPa, tensile

strength: 16 MPa, ultimate tensile elongation: 500%, melt flow index (MFI ):

0.29 and 0.95 g/10 min at 190 and 230oC, respectively, under a load of 2.16 kg.

The particle size of the GTR was between 0.4 to 0.7 mm. In some formulations

a finer GTR fraction (particle size < 0.4 mm) was also tested. As fresh rubber

ethylene/propylene/diene monomers (EPDM) containing rubber (Buna® EP

G 6470 of Bayer) served. Characteristics of the ethylene rich EPDM and GTR

are disclosed in our previous works(9-10). The bitumen used was an EN 70/100 grade

according to the standard EN 12591.

GTR Reclaiming

GTR was used without (reference, R) and with bitumen reclaiming performed

under various conditions. GTR/bitumen (1/1 ratio) was kept at T=120°C for

4 hours (A), at T=120°C for 4 hours followed by rolling in an open mill at

T=40°C for 40 min (B), and at T=170°C for 4 hours followed by rolling as in

(B) and finally mastication in the kneading chamber of a Brabender plasticorder

(model PL 2000) at T=160°C for 5 min (C). Due to the very low viscosity of the GTR/bitumen blend the required amount of EPDM was added also in the

kneading chamber in process C.

Apart of the above bacthwise operations, GTR was devulcanised also

continuously in a corotating laboratory twin-screw extruder (Brabender DSE

25). First the GRT powder was passed trough the extruder. The temperature

profile here was 90/110/120/140°C and the screw speed 80 rpm. Afterwards,

the extruded GTR powder was extruded again together with the bitumen (T=

80/90/100/110°C) and the final elastomer composition was produced in the 3rd

extrusion run (T=125/135/145/155°C, 20 rpm) - version D. Finally, some

compositions produced by the method D have been masticated in the Brabender

kneading chamber (T=160°C, t=8 min, 80 rpm) – version E.

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Tests were run mostly with a coarse GTR fraction (particle size: 0.4-0.7 mm).

In order to get information on the effect of particle size TPEs were produced

also by a fine GTR fraction (particle size: 0-0.4 mm). GTR reclaiming wasfollowed by the acetone extract (in some cases also chloroform was used).

Extraction was carried out for 8 hours (h) followed by drying the samples in

an air oven (50°C for 22 h) prior to weighing.

Recipe of the Thermoplastic Elastomers

Recipes of the blends were varied in the followed ranges:

LDPErec.= 40-50 partsEPDM =15-35 parts

GTR/bitumen (1/1)= 45-15 parts

Major goal with the recipe development was to reduce the amount of fresh

EPDM in the formulation.

Specimen Preparation and their Testing

Sheets were produced by compression moulding of the blends. The melttemperature during moulding agreed with that of the compounding. Specimens

were cut from the sheets and tested as described in Refs.(9-10).

3. RESULTS AND DISCUSSION

Reclaiming Efficiency

The acetone extract increased from the initial ca. 12% (untreated GTR) up to

ca. 30% due to one or other of the treatments used. It was found, however, that

the acetone extract does not reflect the devulcanisation accordingly. Its value

was strongly affected by the time elapsed between the reclamation and

extraction. In some cases the acetone extract increased when the reclaimed

GTR was stored for longer time. This ‘revulcanisation’ phenomenon forced us

to ‘dilute’ the reclaimed GTR either by bitumen or bitumen + EPDM. By this

way ‘covulcanisation’ instead of ‘revulcanisation’ is favoured.

Effect of Bitumen Reclaiming of GTR

Major criteria for a thermoplastic elastomer are: elongation at break > 100%

and compression set < 50%. It was demonstrated in our earlier works(9-10) that

the compression set requirement can be fulfilled with the above mentioned

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compositions, however, the ductility criteria not. That is the major reason why

we focus in this paper on the tensile mechanical response of the TPEs. The

other aspect is that the ultimate elongation is the best hint for the compatibilityof a multicomponent (and thus multiphase) polymer blend.

Data in Table 2 clearly demonstrate the beneficial effect of bituminous

treatment of GTR. One can also recognize that decomposing the GTR at higher

temperatures in molten bitumen is even more advantageous (cf. data achieved

by methods A and B).

Homogenisation of GTR + bitumen + EPDM by mastication (method C)

improved further the ultimate tensile strength and elongation. The fact thatwith increasing elongation the stress strongly increases already suggests the

rubbery appearance (strain-hardening effect) of the TPE compositions.

Effects of the Composition

As mentioned above our intention was to increase the GTR content at cost of

the fresh EPDM without penalty in the performance. Table 3 shows that this

is not an easy task as the mechanical data are the better the lower the GTR

amount in the corresponding composition is.

Table 2 Effects of the bituminous reclaiming of GTR on the tensile mechanical

properties of TPEs

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1R 2.5 - 4.5 071

2R 2.3 7.3 9.4 855

A 2.3 7.3 1.5 065

B 1.3 6.3 3.7 877

C 7.3 0.4 0.01 949E PDL:noit isopmoC

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Phase Structure

Scanning electronic microscopy (SEM) pictures taken from the cut surface of

the sheets produced by the recipes and reclaiming routes listed in Table 2, are

given in Figure 1. One can clearly see that the GTR particles are very poorly

bonded to the matrix when directly compounded in the absence (R1 - see

picture a in Figure 1) and presence of bitumen (R2 - see picture b in Figure 1).

The GTR particle debond from the matrix indicating for lacking interaction

between them.

A better bonding via formation of an interphase layer can be recognized for theformulations produced by methods A and especially by C (see pictures c and

d in Figure 1). Further, the cut surfaces of the recipes containing bitumen show

an increased ductility. This is due to the plasticising effect of bitumen, which

has good compatibility toward both thermoplastic and rubber components.

Even more striking is in picture d in Figure 1 that the apparent particle size of

the GTR is reduced (compare with those surfaces depicted in frames a and b).

Larger GTR particles still debond, at least partially (cf. Figure 1 picture d and

Figure 2 picture a), whereas smaller ones are well incorporated (cf. Figure 2

picture b).

This phase structure corroborates that bitumen is a useful devulcanisation

agent and compatibiliser at the same time. Further by-side information is that

Table 3 Effect of the composition on the tensile mechanical properties of TPEs

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a b a b a b a b

2R 9.2 3.3 - 7.3 9.2 4.7 151 077

A 9.2 2.3 - 8.3 0.3 5.6 321 196

B 0.3 2.3 4.3 8.3 9.3 6.01 864 0201

E PDL:noit isopmoC .cer

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Figure1 SEM pictures from the cut surface of TPE specimens produced by the

methods R1 (a), R2 (b), A (c) and C (d). Note: the methods are listed in Table 2

Figure 2 SEM pictures taken from the cut surface of a TPE sheet produced by

method C Note: recipe and method are indicated in Table 2

(a)(b)

(d)(c)

(a) (b)

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the initial particle size of GTR is a key parameter in TPE formulations which

should be considered accordingly.

Our interest was focused next whether or not the positive results achieved by

the batch techniques can be transferred to continuous operations. Goal of the

extrusion experiments was to check the feasibility to reclaim the GTR and

produce TPEs with it on-line.

Extrusion Compounding

First attention was paid to the effect of GTR particle size and residence time

in the extruder. The latter was controlled by the revolution of the screws. Datain Table 4 show that decreasing initial particle size of the thermo mechanically

decomposed GTR (passed once through the extruder) improves the tensile

strength and elongation data of the TPEs. The same tendency is received with

increasing residence time (decreasing screw rotation speed) on the example of

the composition with fine GTR fraction (cf. Table 4).

Data related to the methods D and E in Table 4 demonstrate two basic effects.

First, adding bitumen is advantageous also under continuous extrusion. However,

the mechanical performance of the TPEs produced batchwise was superior tothose compounded continuously in the extruder. Second, additional high

temperature mastication of the blend (method E) strongly enhances the mechanical

properties, which reach the level of the TPEs produced discontinuously. These

findings suggest that TPEs can be, in fact, produced by on-line GTR reclaiming

continuously. To achieve this, however, a suitable screw configuration (e.g.

length/diameter ratio, kneading/shearing segments) along with setting the needed

residence time (e.g. via screw configuration and screw speed) are required.

The above results showed that bitumen is a suitable low-cost reclaiming agentfor GTR by means of which polyolefin-based TPEs of guaranteed quality can

be produced. It is assumed that the bitumen is ‘extracting’ the sulfur from the

GTR in which the sulfur crosslinks are broken under mechanical stresses

(induced by swelling and/or by shear forces). ‘Extraction’ means that

components of the bitumen cocrosslink with the GTR or crosslink themselves.

This ‘crosslink partitioning’ concept is supported by the fact that adding sulfur

in paving mixtures containing asphalt and GTR improves the mechanical

properties(13). As a consequence the GTR becomes less crosslinked and thus

capable of molecular entanglement. The outcome is a good adhesion betweenthe GTR particles and the surrounding matrix via a thick interphase layer. At

the same time the bitumen works as a plasticiser for the matrix composed of

polyethylene(14) and EPDM rubber.

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4. CONCLUSIONS

Based on this study devoted to produce thermoplastic polyolefin elastomers

(TPE) using ground tyre rubber (GTR), the following conclusions can be

drawn:

• Bitumen is a suitable reclaiming agent for GTR that additionally works as

compatibiliser between components of the recipe (LDPE+EPDM+GTR).

• Reclaiming activity of bitumen on GTR depends on the temperature and

storage time, as well as on the compounding conditions.

• The smaller is the particle size of GTR the better the mechanical properties

of the TPEs are.

• Continuous TPE production with on-line GTR decomposition is principally

viable.

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R x 08 2.4 - 5.4 032

D x 08 0.3 7.3 3.5 875

E x 08 5.4 9.4 1.7 726

R x 08 1.4 1.5 3.5 673

R x 02 3.4 1.5 7.5 324

R x 01 5.4 5.5 9.5 644

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Table 4 Effects of the initial particle size of GTR and the residence time during

compounding on the tensile mechanical behaviour of TPEs composed of LDPErec

:

EPDM: GTR/bitumen = 50:25:25 parts

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ACKNOWLEDGEMENT

The authors are thankful to the European Union (Inco-Copernicus “Recrupot”

- Contract No.: ICA2-CT-2001-10003) for the financial support of this project.

REFERENCES

1. V.L. Shulman (ETRA): presentation at the DIK workshop, Hannover, June 6-7,

2002.

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3. S. Al-Malaika and E.J. Amir: Polym. Degr. Stab. 26 (1989), 31-41.

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9. A. Radeshkumar and J. Karger-Kocsis: Plast. Rubb. Compos. 31 (2002), 99-105.

10. A. Radhesh Kumar, I. Fuhrmann and J. Karger-Kocsis: Polym. Degrad. Stab., 76

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14. A.H. Fawcett and T. McNally: Polymer 41 (2000), 5315-5326.

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