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41KGK · 1-2 2014www.kgk-rubberpoint.de

Recycling · rubber waste particles · EPDM · liquid polymers · activation · sulphur vulcanization

The recycling of ethylene-propylene-diene-rubber (EPDM) waste was rea-lized by activation of EPDM rubber was-te particles surface with liquid EPDM polymers (LP) of low molecular weight. The optimal ratio of the waste EPDM rubber to LP was investigated whit re-gards to the original EPDM properties. The degree of unsaturated ethylidene norbornene diene ter-monomer in LPs plays an important role in the activati-on process and the cross-linking density of the reactivated materials and there-fore the mechanical properties.

Wiederverwertung von EPDM-Gummimehl durch Aktivie-rung mit flüssigen Polymeren Recycling · Altgummimehl · EPDM · flüssige Polymere · Aktivierung · Schwefelvulkanisation

Eine Recyclingstudie Ethylen-Propylen-Dien (EPDM) Gummimehl wurde durch Aktivierung von Altgummimehlparti-keln (EPDM-P) mit flüssigen niedriger molekularen EPDM-Polymeren (LP) durchgeführt. Es wurde das optimale Verhältnis von EPDM-Gummimehl/LP und Aktrivierungsagenzien ermittelt, um originären mechanischen EPDM Ei-genschaften zu erhalten. . Der Grad an ungesättigtem Ethyliden-norbornen-Termonomer entscheidet maßgeblich über die Vernetzungsdichte des entste-henden Vulkanisates und so dessen me-chanischen Eigenschaften.

Figures and Tables:By a kind approval of the authors.

Nowadays recycling of waste rubbers is an important research topic for many industries with major implications in en-vironmental protection and economy. According to an estimate study made in Germany in 2006 by the “Fraunhofer Ins-titute” for Environmental, Safety and Energy Technology (UMSICHT), the waste rubber amount is on the order of about 125,000 t/year [1] approximately 14 % of total used raw material. In some cases, this percentage can reach values of about up to 50 % from the raw material, depending on the production process.

Recycling or reclaiming processes are very difficult and not completely under-stood until now, due to the irreversible three-dimensional cross linking of rubbers [2]. Most research studies that involve the reprocessing and recycling of vulcanized rubber waste are related to diene rubber types, i.e. copolymers of butadiene, isopre-ne, acrylonitrile and styrene - (NBR, SBR). The studies for recycling of Ethylene-pro-pylene-diene rubber (EPDM) products are significantly fewer although EPDM is one of the most used synthetic rubbers be-cause of its very good resistance to weat-hering, heat, oxygen and ozone [3]. The usual recycling methods for EPDM waste are based on devulcanization techniques, which assume that crosslinks are partially broken by chemical, thermo-chemical, physical, biological, or radiation-chemical process or intensive grinding in the pre-sence of oxygen, or reclaiming agents. Such treatments lead to rubber powders with partially chemically activated surface that can be used to substitute virgin rub-ber in blends with other polymers up to 45 %, without affecting the processability and physical properties of the blends [4]. Earlier studies in this area used different methods for reutilization of EPDM rubbers such as: amines as devulcanization agent [5-8], microwave energy [9,10], ultrasonic devulcanization [11-14]. Disadvantages of the mentioned processes result from the fact that not only crosslinks are cleaved, but also partially the polymer chains. This leads to branched macromolecules with broader molecular weight distributions than the original elastomer and thus has a

considerable influence on the mechanical properties of final materials. Moreover the accelerators remaining in the waste rub-ber play also an important role in the recy-cling process of waste rubber. De and co-workers [15,16] investigated the effects of the ground waste EPDM on the Mooney viscosity, cure characteristics and mecha-nical properties of the EPDM virgin rubber. They observed that at higher percent (ca. 50 %) of the ground waste rubber into virgin rubber, the maximum rheometric torque decreases, phenomena which can be attributed to the sulphur migration from the virgin matrix rubber to waste particles rubber with a decrease in appa-rent crosslink density of material. Hamed et. al. [17] performed similar studies on SBR rubber containing ground vulcanisate and also observed migration of accelerator fragments from the waste rubber phase to fresh rubber matrix causing a decrease of scorch time. They showed that samples containing virgin rubber, sulfur and ground waste rubber (accelerated-sulfur vulcanizated) without an accelerator, no-netheless exhibits acceleration of cure [17]. First studies using a liquid polymer to activate the ground rubber waste particles were described by Stark [18]. He found that by applying a small amount of a li-quid unsaturated polymer with a curing agent on the surface of a particulate vul-canised rubber, these could be used at high concentration as an additive to virgin rubber with only moderate loss of physical properties [18].

Recycling of EPDM Rubber Waste Powder by Activation with liquid Polymers

AuthorsAna Maria Lepadatu, Simona Asaftei, Norbert Vennemann, Osnabrück Corresponding author: Dr. C. Simona AsafteiUniversity of OsnabrückBarbarastraße 7D-49069 Osnabrück GermanyE-Mail: [email protected], [email protected]


42 KGK · 1-2 2014 www.kgk-rubberpoint.de

Considering the above mentioned is-sues, in the present work, we report an easy method to activate the ground was-te rubber particles surface by means of a low molecular weight polymer (LP) which is highly compatible with the EPDM waste rubber particles (EPDM-P) and also suita-ble for sulphur crosslinking. The prepara-tion of EPDM-P/LP composites with LP which contains different degree of unsa-turation, as well as the vulcanisation of them with different amount of curative is reported in this study. The content of 5-ethylidene-2-norbornene diene termo-nomer (ENB) of the LP was 4.5 % and 9.5 % and the amount of ground EPDM waste rubber was varied from volume fraction from 0.5 to 0.9 of EPDM-P. The results showed that ENB and curative system content have a high influence upon me-

chanical properties and crosslink density of the compounds. The properties of the resulting composites were investigated by physical, chemical and mechanical methods.

Experimental part

MaterialsThe ethylene-propylene diene rubber (EP-DM) waste was ground at room tempera-ture using a typical sulfur-cured EPDM vulcanizate, supplied by M.D.S. Meyer GmbH, Germany. Mean particles size of the powder was about 700-850 μm with a density of 1.19 g/cm3 determinate by Ela-test®, Brabender® GmbH& Co.KG, Gema-ny. Liquid polymer Trilene® 67 (LP-A) Type ethylene-propylene-ethylidene norborne-ne (EBN) with specific gravity 0.86, mole-

cular weight 7,700 Da, 9.5 % diene, ethyle-ne/propylene ratio 45/55 and liquid poly-mer Trilene® 66 (LP-B) Type ethylene-pro-pylene-ethylidene norbornene (EBN): with specific gravity 0.84, molecular weight 8,000 Da, 4.5 % Diene, ethylene/propylene ratio 45/55 were supplied by Lion Copolymer,USA. The zinc oxide (ZnO), stearic acid, N-Cyclohexyl-2-benzothiazo-le sulfenamide (CBS) were commercial grade as used in rubber industry. All che-micals were used as received.

Preparation of vulcanisatesThe preparation of LP/EPDM-P com-pounds was carried out in a laboratory internal mixer using conventional mixing procedures that involve two stages: in the first stage the mixing of LP and EPDM-P was carried out with a fill factor of 0.7, at a chamber temperature of 100 °C and a rotor speed of 40 rpm for 15 min. In the second stage the blends were mixed fol-lowed by addition of the activators (ZnO and stearic acid), and crosslinking agents (CBS and sulfur), in the same mixer at a chamber temperature of 90 °C, with 40 rpm for 7 min (Chart 1).

The Table 1 and 2 summarize the com-positions of samples used in this study. The samples listed in Table 1 were obtai-ned using two different types of LP, type A and B (4.5 % and 9.5 % ENB respec-tively) with a constant ratio of curative system. The content of waste rubber particles (EPDM-P) was varied from 0.5 to 0.9 volume fractions. In Table 2 the content of curative was varied system and the amount of LP and EPDM-P con-tent was kept constant (volume fraction 0.8 EPDM-P).

Measurement of mechanical and mor-phological properties of LP/EPDM-P compositesThe cure characteristics of the com-pounds were determined at 180 °C using a Dynamic Moving Die Rheometer of

Chart 1: Schematic representation of chemical activation of EPDM rubber waste particles (EPDM-P) with liquid polymers (LP). Step 1. Mixing of LP/EPDM-P (pre-batch) at 100 °C, 15 min; Step 2. Mixing of pre-batch with curative system, at 90 °C, 7 min; Step 3. Vulcanisation at 180 °C.

EPDM-P Masterbatch LP/EPDM-P Vulcanized new compound

1. LP/EPDM-P2. Pre-batch/ curative system

3. Vulcanisation

surface of the particle surface of the particle in contact with LP cross-linking points

1 Tab. 1: Composition of LP/EPDM-P samples with different EPDM-P content and constant curative system.Ingredients Sample name

A1 A2 A3 A4 A5 B1 B2 B3 B4 B5Amount (phr)

LP 100 100

EPDM-P 140 210 330 560 1300 145 220 340 570 1300ZnO 2 2Stearic acid 1 1CBS 3 3Sulfur 1.2 1.2

2 Tab. 2: Composition of LP/EPDM-P samples with constant EPDM-P content and variable curative system Ingredients Sample name

A4 B4a b c d e f a b c d e f

Amount (phr)LP 100 100EPDM-P 560 570ZnO 0 2 4 6 8 10 0 2 4 6 8 10Stearic acid 0 1 2 3 4 5 0 1 2 3 4 5CBS 0 3 6 9 12 15 0 3 6 9 12 15Sulfur 0 1.

22.4

3.6

4.8

6.0

0 1.2

2.4

3.6

4.8

6.0



type D- MDR 3000 from MonTech GmbH, Buchen, Germany. The compounds were compression moulded at 180 °C with a pressure of 10 MPa using an electrical heated hydraulic press according to their respective cure time, t90.

Mechanical PropertiesMechanical properties like hardness we-re measured according to ASTM D 2240 ISO 7619 a Durometer Shore A (DIN 53505 EN ISO 868). Elongation at break and tensile strength tests were perfor-med using a universal testing machine (Zwick) in accordance with DIN 53504. The swelling measurements were perfor-med to obtain information about the apparent crosslink densities of the new compounds. Samples of known weight (m0) were immersed in toluene at 24 °C for 48h until the samples reached the maximum swelling. The solvent was re-placed after 24h with fresh toluene to remove the extracted components.

After 48h the samples were removed from toluene and the surfaces were quickly wiped with tissue paper and weighted to find the swollen weight of the samples (m). The samples were further dried at 50 °C for 48h, cooled in a desiccator and then weighted again (md). The polymer volume fraction, ΦP, was calculated by the Equation (1) according to the reference [19].

(1)

where m is the mass of the swollen sam-ple and md is the mass of the sample after drying, ρP and ρS are the sample and solvent densities, respectively.

The apparent crosslink density, υe, which represents the effective number of chains per unit volume, is calculated using Equation (2) according to the Flory-Rehner equation [20, 21].

(2)

where VS is the solvent molar volume (106.5 cm3/mol for toluene), χ is the EP-DM-toluene interaction parameter and is taken from literature as 0.49 [22].

Temperature Scanning Stress Relaxa-tion (TSSR) measurements were perfor-med to determine the crosslink density of the samples using a TSSR instrument from Brabender GmbH (Duisburg, Ger-many), by using the temperature scan-ning stress relaxation method. The TSSR

instrument consists of an electrically heated where the sample is placed bet-ween two clams, which are connected to a linear drive unit to apply a uniaxial ex-tension to the sample. A high quality sig-nal amplifier in combination with a high resolution AD-converter is used to detect and digitize the analogue signals of the high-resolution force transducer and the temperature sensor. All signals are trans-ferred to a personal computer. A soft-ware is used for treatment and evaluati-

on of the data as well as for the control of the test procedure.

The measurement with the TSSR-Me-ter starts with placing the sample into the electrically heated test chamber an initial temperature of 23 °C. At first, the initial strain of 50 % is applied to dumb-bell test piece. While the temperature remains constant the isothermal test period starts subsequently. During this time most of the short time relaxation process occurs and the sample reaches a

Fig. 1: MAS 1H-NMR spectra of: a) EPDM-P, b) cured LP with 9,5 % ENB and c) EPDM-P/LP vulcanized

1

Fig. 2: Rheometer curves of LP-A (full symbols) and LP-B/EPDM-P (open symbols) composites with constant curative system and variable EPDM-P content

2



nearly stable state. After the isothermal relaxation test period is finished, the sample is heated linearly at a constant rate of 2K/min, until the stress relaxation has been fully completed or rupture of the sample has occurred. The apparent crosslink density, υe, was determined from the maximum slope of the stress temperature curve in the initial part of the curve according to the Equations (3)

and (4) from literature [19]:

(3)

with

(4)

where υe is the apparent crosslink densi-ty, R is the universal gas constant, λ is

strain ratio, κ is the derivative of mecha-nical stress with respect to temperature, ρ is the mass density and Mc is defined as the average molar mass of the elastically active network chains.

Scanning Electron Microscopy (SEM) StudiesThis was carried out on tensile fracture surfaces of EPDM-P and EPDM-P/LP sam-ples using a JEOL JSM 6510 scanning electron microscope (JEOL GmbH Germa-ny). Cryogenically fractured surfaces we-re coated with a thin layer of gold prior to examination, using a Fine Coater JEOL JFC 1200.

1H-NMR Spectroscopy Magic angle spinning (MAS) 1H-NMR spectra were recorded on a Brucker AMX-500 spectrometer at a resonance fre-quency of 500 MHz. A 4 mm probe head was used with a MAS rate of 8 kHz. The spectra were acquired by means of a single-pulse excitation with 90° pulse of 5 μs and a recycling delay time of 5 s. The experiments were performed at 30 °C.

Results and discussion

Characterization of the covulcanizationThe presence of any double bonds in the recycled rubber powder, mentioned in the past by Giese and co-workers [2], can be used as reactive center for chemically modifying of waste particles surface for better grafting of it onto the virgin rub-ber matrix. The concentration of double bonds after vulcanization is an interes-ting factor in estimating chemical reacti-vity of vulcanizate and were performed by chemical methods [2], solid state 1H-NMR [23] and 13C labeled [24] study on chemical cross-links. The MAS 1H-NMR spectra of EPDM waste rubber particles are dominated by signals of aliphatic protons of the ethylene propylene mono-mers (EPM) backbone around 1 ppm. At large magnification of spectra the vinyl protons of ENB are still seen in the spec-tral range of 4-6 ppm, the signals of not reacted accelerators are also present in the spectral range of 7-9 ppm (Figure 1a).

Comparing the spectra of EPDM liquid polymers with 9,5 % ENB after cross-lin-king pure and mixed with waste rubber particles (volume fraction 0.8 EPDM-P) suggest that the sulphur has been largely converted. The remaining aromatic sig-nals in Figure 1a and 1c respectively are related to the not consumed accelerator fragments comparing with LP only Figure

Fig. 3 Hardness versus volume fraction of EPDM-P of LP/EPDM-P composite

Fig. 4 Tensile strength versus volume fraction of EPDM-P of LP/ EPDM-P composite

Fig. 5: Elongation at break versus volume fraction of EPDM-P of LP/EPDM-P composite

3

4

5



1b. The chemical shifts of the protons of ENB residues into EPDM-P are present as doublet signal by 5.5 and 5.3 ppm, respec-tively while with liquid polymers 9.5 % ENB the shifts were 5.6 and 5.4 ppm, res-pectively. The subsequent difference in chemical environment is sufficient to in-duce a shift of EBN which gives informati-on of the chemical structure of the cross-linking. This result demonstrates that af-ter vulcanisation between EPDM-P and LP a chemical reaction took place to form, via substitution at the α-position next to the residual double bond of ENB, sulfur-cross-linking network. The intensity of the un-saturated EBN resonances decreases as a result of the substitution of protons in al-lylic position and a proton from α-position of EBN by sulfur bonds.

Vulcanization curves plotted in Figure 2, showing the dependence of vulcaniza-tion time on the torque moment.

The obtained data show that the in-crease of the EPDM-P content in the samples composition resulted in an in-crease of minimum torque (ML), repre-senting the viscosity of the compound. The higher values obtained for the maxi-mum torque MH in case of samples with LP-A in the composition can be assigned to the higher percent of ENB contained by the liquid polymer, which plays an important role upon crosslink density. The difference between MH and ML is re-lated to the samples crosslink density. The rise of the maximum torque could be thus attributed to the interaction bet-ween LP and EPDM-P. The optimum cure time (t90) increased with increasing EP-DM-P content in the compounds, sugges-ting that there are still active curatives in the EPDM-P (see Figure 1a) that may participate in the vulcanization process.

On the other hand the scorch time of LP/EPDM-P compounds, a measure of premature vulcanization, decreases with increasing the EPDM-P content. Similar behaviour was observed on the EPDM and SBR virgin rubber mixed with ground rubber. The work group of De et al. [15, 16], Gibala and Hamed [17] explained this effect by migration of accelerators from ground vulcanizates to the virgin rubber matrix. The group of Padke et al. [25] reported also a decrease in scorch time for cryogenically ground rubber (CGR) -natural rubber (NR) blends, which was attributed to the presence of unre-acted curatives and/or crosslinked pre-cursors (CGR) in the ground rubber.

The mechanical properties of LP/EP-DM-P composites strongly depend on ground waste rubber content. The varia-

tions of hardness, tensile strength and elongation at break with increasing volu-me fraction of EPDM-P in the LP/EPDM-P composites are shown in Figures 3, 4, 5.

The higher the content of EPDM-P in LP/EPDM-P compounds an increase of the hardness is observed (Figure 3). The tensile strength follows the same trend, but it also shows higher values for the samples with higher ENB content in the composition (Figure 4). The dependence of elongation at break on the volume fraction of EPDM-P (Figure 5) reveals a maximum value for the samples with LP-A containing 70 % of EPDM-P, followed by a sharp decrease. The samples with LP-B show only a slightly rise of the elongati-on. The increase of the mechanical pro-perties may be also attributed to the fil-ler contained in the EPDM-P.

Cure behaviour To study the influence of the curative system content on the cure rate of the LP/EPDM-P composites, rheometric stu-

dies were performed on samples with constant volume fraction of EPDM-P 0.8 (v/v %) and different curative system con-tent (Table 2). The corresponding rheo-meter curves are shown in Figure 6.

The samples A4a and B4a, without cu-rative system, are vulcanised to a small extent, which was probably caused by the residual curatives migrated from the EP-DM-P into the liquid phase. The maxi-mum torque of the compounds increased with increasing the amount of curative system as well as with the diene content of LP. At higher content of the curative system the curves remains constant, indi-cating that no crosslinking reactions take place anymore. By systematically increa-sing of the curative system, a linear incre-ase of the difference MH-ML can be obser-ved, whereas the scorch time remained almost constant. It can be concluded that the curative system together with the suitable amount of ENB contained in the LP enhance the activation of the surface particles giving higher crosslinking values.

6

Fig. 6: Rheometer curves of LP-A (full symbols) and LP-B/EPDM-P (open symbols) composites with different curative system ratio and constant content of EPDM-P

3 Tab. 3: Mechanical properties of the vulcanizates Mechanical properties

Sample nameA4 B4

a b c d e f a b c d e fHardness,Shore A

31 38 44 48 50 53 16 36 39 43 46 48

Tensilestrength,MPa

0.7 2.2 5.1 6.0 6.4 6.0 0.2 1.1 2.2 3.2 3.9 4.6

Elongationat break, %

80 256 309 323 298 275 14 130 181 227 246 253



Mechanical properties of composites such as: hardness, tensile strength and elongation at break are summarized in Table 3.

For all samples, hardness, tensile strength and elongation at break increa-se by increasing the curative system content; better values were obtained for samples containing LP-A. The samples containing LP-A and LP-B, respectively, revealed different mechanical properties, as consequence of the double bonds con-centration of the liquid polymers. It can be thus concluded that a suitable amount

of curatives together with higher con-tent of diene leads to a significant impro-vement of the mechanical properties.

Apparent crosslink density determinate by swelling and TSSR measurementsThe apparent crosslink density was de-termined by swelling measurements for the samples with LP-A and LP-B, contai-ning different volume fraction of EPDM-P and constant curative system. The appa-rent crosslink density, increases signifi-cantly with increasing the volume frac-tion of EPDM-P (Figure 7).

This behaviour can be attributed to the interaction between the particle surface and the liquid polymer, proving that the powder has been incorporated into the three dimensional network. The crosslink density of the pure LP is very low when comparing with the samples which con-tain EPDM-P (Figure 7). Due to the low viscosity of the liquid polymers it is able to wet the surface of the ground rubber par-ticles and serve as a binder between par-ticles. The relation between the crosslink densities obtained from the swelling ex-periments is in good agreement with the data obtained from TSSR measurements.

Figure 8 shows the data obtained from swelling and TSSR measurements, for a set of samples which constant volu-me fraction 0.8 EPDM-P and different amount of curative system.

The increase of the curative system content leads to an increase of the cross-link density. This can be explained by the fact that the curative system has a signifi-cant effect in the consolidation of net-work structure. The results regarding the solvent swelling studies support the changes in torque, evidenced by the rheo-meter curves as presented in Figure 6.

Surface characterization by SEMThe morphologies observed at the frac-ture surface of tensile specimens for the EPDM vulcanized start material, ground rubber particle (EPDM-P), liquid polymer (LP) and composite LP-A/EPDM-P (volu-me fraction 0.8 EPDM-P) with and wit-hout curative system respectively were studied by scanning electron microscopy (SEM) and are shown in Figure 9. The pictures give an insight into the blend morphology regarding homogeneity and compactness of the LP/EPDM-P composi-tes. The EPDM start material from which are obtained the waste particles (EPDM-P) shows a good uniformity and homo-geneity (Figure 9a). The EPDM-P reveals the presence of spongier agglomerates with high surface roughness compared with the EPDM-P sample after pressing at 180 °C; the morphology of the sample changes after heating, the surface beco-mes smoother but with fissures brittle (Figure 9 b,c).The vulcanized LP-A (Figure 9f) presents a uniform and smooth sur-face. The interaction between LP-A and EPDM-P without activators/curative sys-tem leads to a brittle, failure surface containing voids which indicate the ab-sence of inter-rubber crosslinking (Figure 9e). The surface of the LP/EPDM-P com-posite (volume fraction 0.8 EPDM-P) with

Fig. 7: Volume fraction of EPDM-P versus crosslink density from swelling measurements

7

Fig. 8: Sulfur content versus crosslink density from swelling and TSSR measurements

8



Fig. 9: SEM images of: a) EPDM-vulcanized start material; b) EPDM-P c) EPDM-P pressed at 180 °C; d) LP-A/EPDM-P (volume fraction 0.8 EPDM-P) with curative system e) LP-A/EPDM-P (volume fraction 0.8 EPDM-P) without curative system; f) LP-A vulcanizated with curative system.

9

curative system after vulcanization show higher degree of compactness similar to the vulcanized start material (Figure 9d). The architectural homogeneity indicates much better state of dispersion with ef-ficient interfacial cross-linking. The com-pact structure of the LP/EPDM-P compo-site is manifested in the significant en-hancement of mechanical properties.

ConclusionsThe cure characteristics, crosslink density, morphology and mechanical properties of LP/EPDM-P composites have been studied in terms of EPDM-P loading, variation of ENB content in the LP and different ratios of the curative systems. ENB content and EPDM-P ratio plays an important role upon cure characteristics behaviour and mechanical properties. The maximum torque increases with increasing EPDM-P content as result of the interaction bet-ween EPDM-P and LP, while the scorch time decreases, as a migration of curati-ves from EPDM-P to LP phase. By using a volume fraction of 0.8 EPDM-P activated with the LP together with a suitable cura-tive system a new composite was obtai-ned (Table 3, A4d) with 80 % hardness, 66 % tensile strength and 58 % elongation at break from the values of the start ma-terial. This study provides an important framework to design new rubber compo-site with desirable properties.

Nevertheless, further studies regar-ding the characterization of activated EPDM-P with liquid polymers are neces-

sary to make plain and comprehensible the chemistry processes hidden behind.

AcknowledgementsWe express our thanks to the Deutsche Bundesstiftung Umwelt special to Dr. Jörg Lefevre and M.D.S. Meyer GmbH for the financial support of this work. The authors are also grateful to the Lion Copolymer Geismar (USA), LLC for the liquids polymers samples.

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