Properties of EVM Compounds in Relation to the Vinyl Acetate Content of the Polymer 1

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1 ELASTOMERE UND KUNSTSTOFFE ELASTOMERS AND PLASTICS EVA EAM ACM Oil resistance Flexibility Damping Radiation crosslinking Typical properties of ethylene-vinyl acetate (EVM) copolymers were determined. These properties were compared with the chemically similar polyacrylate polymers and ethylene-methylacrylate copolymers. Common tendencies relating to the composition were established. A reduction in the ethylene content caused an increase in oil resistance and a fall in low-temperature flexibility. A new process which enables higher viscosities to be achieved helps to remove certain difficulties in processing. EVM is a cost-effective alternative to polyacrylate rubber grades. Eigenschaften von EVA- Mischungen in AbhaÈngigkeit vom Vinylacetatgehalt der Polymere EVA EAM ACM OÈ lresistenz FlexibilitaÈ t DaÈ mpfung strahlungsreduzierte radikalische Quervernetzung Typische Eigenschaften von Ethylen-Vinylacetat-Copolymeren (EVA) wurden ermittelt. Diese Eigenschaften wurden verglichen mit den chemisch aè hnlichen Polyacrylat- und -methylacrylat-copolymeren. Gemeinsame Tendenzen in Bezug auf die Zusammensetzung wurden gefunden. Die Reduzierung im Ethylengehalt fuè hrt zu einem Anstieg in der OÈ lresistenz und einem Abfall in der TieftemperaturflexibilitaÈ t. Ein neuer Prozeû, der das Entstehen hoè herer ViskositaÈ ten ermoè glicht, traè gt dazu bei, gewisse Verarbeitungsschwierigkeiten zu uè berwinden. EVA ist eine kostenguè nstige Alternative zu Polyacrylat- Kautschuktypen. Properties of EVM Compounds in Relation to the Vinyl Acetate Content of the Polymer 1 H. Meisenheimer, Leverkusen (Germany) Ethylene (E) and vinyl acetate (VA) are two monomers which can be easily copolymerised in any ratio possible. The properties of the corresponding polymers, provided there is a random distribution of the two monomers in the polymer chain, are determined by the ratio of ethylene to vinyl acetate. Polymers which are in a rubber-like state are designated EVM and such polymers are marketed by Bayer AG under the tradename Levapren. Increasing the vinyl acetate content leads to a change in crystallisation point, glass transition temperature and polarity. The voluminous acetate group, as a lateral group, disturbs the crystallisation of the methylene groups which would ideally like to form a helix. The greater the disturbance of crystallisation, the lower the melting point. At the same time, however, the glass transition temperature increases (Figure 1). The corresponding EVM grades are in a rubber state at between around 40 and 80 % VA content, i. e. in this range there is little or no crystallisation and the glass transition temperature is below 0 8C. Knowledge of the glass transition and crystallising points enables many of the vulcanisates' properties to be explained. If the lines representing glass transition temperature and crystallising point are extrapolated, it can be deduced that crystallisation will cease completely at a 55 % VA content and the glass temperature alone will determine the low-temperature behaviour. For this reason, three laboratory products were included in the test. 1 Paper given at a DKG-meeting, Hamburg, December 10, 1998 Figure 1. Effect of the VA content on crystallinity, melting point and glass transition temperature 724 KGK Kautschuk Gummi Kunststoffe 52. Jahrgang, Nr. 11/99

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3 Table 1. Test compound used Polymer 100 Rhenovin DDA Rhenogran P 50 3 Stearic acid 1 Carbon black N Rhenogran TAC/S 3.5 Perkadox 14/40 4 Table 2. VA content, density and viscosity of Levapren grades Vinyl acetate % Density g=cm 3 Viscosity ML 1 4/ 100 8C Levapren Levapren 500 HV Levapren 600 HV Levapren 700 HV Levapren 800 HV Lev. KA The change in properties of the homologous EVM grades was investigated on this simple formulation (Table 1). A major problem in any comparison of rubber grades is a possible change in parameters. In this case the polymer was exchanged 1:1 under the assumption that all the changes depend on the polymer used (Table 2). An EVM grade with 40 % VA and a density of 0:98 g=cm 3 differs considerably from a EVM grade containing 80 % VA and with a density of 1:11 g=cm 3. When interpreting the test results it must therefore always be borne in mind that an increase in VA content reduces the volume of polymer in the compound. than that measured at room temperature. At 70 8C, the chains are no longer hindered in their movement by these effects. The thermoplastic part softens and the Shore A hardness is reduced. Between 50 and 60 8% VA content there is no more crystallisation and the glass transition temperature is the lowest possible without crystals forming. Here, the change in hardness is at its smallest and the polymers are more rubber-like. Elasticity changes quite markedly, however. Rebound resiliance falls as the VA content rises to 70 % and increases again at 80 %. The compression set shown in Figure 3 behaves similarly to hardness. At high temperatures ± well above the products' Experimental All compounds were mixed in a GK 1.5E/1 laboratory internal mixer manufactured by Werner and Pfleiderer. Vulcanisation took place in a laboratory press. The vulcanisation time was t 95 5 min for 2 mm-thick test discs. The vulcanisation time was correspondingly adjusted for thicker discs. Tensile tests were carried out with S2 test pieces. Compression set was determined on test specimen 2 according to DIN. Temperature-dependent torsional vibrations were generated with an ARES device produced by Rheometric Scientific. TR tests were conducted using a Wallace device. Results The strength and elongation of all EVM vulcanisates are very similar (Figure 2). Only EVM with a 40 % VA content has lower strength. This is the result of the lower molecular weight that arises through the production process. Hardness is a different matter, however. Hardness measured at 70 8C is less likely to be affected by the proximity of the melting point and glass transition temperature Figure 2. Effect of the VA content on tensile strength, elongation at break, hardness and elasticity 726 KGK Kautschuk Gummi Kunststoffe 52. Jahrgang, Nr. 11/99

4 Figure 3. Effect of the VA content on compression set at different temperatures glass transition points ± all the EVM grades have outstanding rebound resilience and compression set is very low. At room temperature and below, crystallisation and glass transition affect rebound resilience. Best are the amorphous grades with a 60 ± 70 % VA content. EVM with 70 % VA is even better than the grade with 60 %. At 20 8C, however, all grades remain severely deformed with set values > 90 %. Another method for determining lowtemperature flexibility is the TR test. Figure 4 shows tensile retention as a function of VA content and temperature. As with change in hardness, the amorphous EVM grades with the lowest glass transition temperature produce a particularly good result. Effect of the VA content on immersion in various swelling media Figure 4. Effect of the VA content on the TR test Polymers with a higher VA content have higher polarity, which is why EVM becomes increasingly resistant to mineral oils as its VA content rises. ASTM Oil IRM 903, Engine Oil 20 W 20 and Gear Oil SAE 90 have similar swelling effects. The specimens were immersed for 70 h at 150 8C in ASTM Oil IRM 903, and for 7 days at 150 8C in Gear Oil SAE 90 and Engine Oil 20 W 20. The biggest changes in tensile strength, Figure 5, ultimate elongation, hardness and swelling, Figure 6, were for EVM with a 40 % VA content. As expected, the smallest changes were for EVM with an 80 % VA content. With technical oils, the additives they contain may be pose a problem. These usually vary according to the manufacturer and may alter the properties by reacting with the polymers, sometimes severely. A test with the oil to be used is therefore recommended. EVM with higher viscosity Figure 5. Effect of the VA content on tensile strength and elongation at break after immersion in oil In this summary one important property has been overlooked which may not interest the end user but which is fundamental to the manufacturer of rubber articles and that is the processability. EVM is used not only in the rubber industry but in adhesives as well. The very ªadhesionº that the adhesives user desires poses a great problem for the rubber processor. KGK Kautschuk Gummi Kunststoffe 52. Jahrgang, Nr. 11/99 727

5 that the compounds produced in this way when extruded varied from glossy (without pre-crosslinking) to completely matt (with pre-crosslinking). Below, there is a comparison of precrosslinked X-EVM, conventionally manufactured EVM and a blend of the two polymers with a ratio of 50:50. All three varieties give almost the same results, e. g. Figure 8. Immersion in various swelling media such as IRM 903, SAE 90 and Hydraulic Fluid NF 46 changes the vulcanisate properties of the tested compounds equally. Slight deviations can be interpreted as resulting from the natural scatter of the measurements. Figure 6. Effect of the VA content on hardness and change in volume after immersion in oil It makes certain compounds extremely difficult to separate from the roll and they sometimes stick to the internal mixer. One reason for this behaviour is the low viscosity of the polymer. In the existing production plant it was not possible to increase the viscosity or molecular weight. Pre-crosslinked polymers are known to improve processing behaviour. Improve here means good mill release and better stability during profile extrusion. With the aid of ionising irradiation, radicals were induced into the polymer matrix, which then reacted and formed a network whose structure could be controlled. The reaction could be regulated in such a way that any degree of crosslinking and viscosity could be achieved. Increasing the radiation led to a rise in crosslinking and thus in the viscosity of the polymer. That naturally caused an increase in the viscosity of the rubber compound. At the same time, the green strength and elongation at break fell. Irrespective of this, the vulcanisate properties measured were almost identical (Figure 7). It was, however, observed Figure 7. Polymer, compound and vulcanisate tensile strength; dependence on the radiation dose Comparison of EVM with EAM and ACM If the bond configuration of vinyl acetate is slightly altered, the isomeric methyl acrylate is obtained. Copolymers of ethylene and methyl acrylate are designated EAM, whereas homoplymeric acrylates are ACM. EVM is chemically isomeric to EAM. It is interesting to note both the expected similarities and the differences between the polymer groups. An acrylate rubber was included in the investigation for comparison. The polymers tested are listed in Table 3. Typical rubber grades have an acrylate content of > 60 %.The composition of polymers 3 and 4 was determined with NMR. The compounds all have a similar recipe. The only difference is that the crosslinking system was adapted to the recommendations of the different rubber manufacturers. The crosslinking system plays an important role in determining the processing and vulcanisate properties of the end product. Without a reactive lateral group, saturated copolymers are mainly crosslinked with organic peroxides. It can be seen in Figure 9 that EVM vulcanisates have outstanding properties even without post-cure. Thermal post-treatment is necessary in the case of EAM, however, and is thus recommended. This applies particularly to amine crosslinking which depends on post-cure for good results. The same applies to ACM to a lesser extent. It can clearly be seen from Figure 9 that in terms of compression set after 70 h at 150 8C, EVM with a 60 and 70 % VA content is superior to the other 728 KGK Kautschuk Gummi Kunststoffe 52. Jahrgang, Nr. 11/99

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7 Table 3. Composition and glass transition temperature of various EAM grades Polymer Manufacturer Acrylate content % Ter monomer % 1 Vamac D DuPont Vamac DLS DuPont Vamac G DuPont 55 4 (*) 26 4 Vamac GLS DuPont 61 5 (*) 21 5 Hytemp 4051 EP Nippon Zeon a) own measurements Glass transition temperature 8C a) Table 4: Test compound used Vamac G/GLS 100 ± Hytemp 4051 EP ± 100 Rhenovin DDA ± Paraff. amine (6) 0.5 ± Stearic acid 1 1 Black N HMDAC (7) 1.5 ± Vulkacit DOTG 4 ± Rhenocure Diuron ± 1.8 Rhenovin Na-stearate 80 ± 5 Figure 8. Pre-crosslinked EVM with 70 % VA compared to normal EVM with 70 % VA and a blend with a ratio of 50:50; change in properties after immersion in IRM 903 for 70 h at 150 8C Figure 9. Compression set after 70 hours at 150 8C; a ªtº indicates that the sample was postcured polymers. A very good value is achieved with 15 ± 16 % deformation without postcure. Residual deformation is acceptable for EAM after 4 h post-cure at 150 8C and for ACM after 4 h at 175 8C. Post-cure improved these values in some case by a factor of up to 3. For the sake of clarity the following figures show the vulcanisate properties after post-cure of EAM and ACM only. This additional aftertreatment is not necessary in the case of EVM. The properties were determined for EVM vulcanisates which were not post-cured. When the crosslinking of the copolymers is carried out with peroxides, the two EVM grades have better tensile strength than the isomeric EAM grades, slightly lower elongation at break and slightly higher hardness. Acrylates also achieve a high tensile strength when crosslinked with amines. All the rubber grades tested had good oil resistance at high temperatures. This oil resistance can be explained by the increasing polarity at higher acrylate contents. The quantity of maleic acid semiester, an equally polar compound, is simply added to the acrylate compound. The properties remaining after immersion in ASTM Oil IRM 903 for 70 hours at 150 8C can be easily explained by the polarity of the polymers (Figure 10). Without doubt polyacrylate rubber is the most resistant. Polycrylate vulcanisates have swollen the least after immersion in oil and the changes in strength, elongation and hardness are also smallest in their case. Volume swelling and change in hardness correlate very closely with one another. Where strength values are at the same level, the relative change can still vary significantly, however. The change in elongation at break is particularly large in polymers containing a low amount of acrylate. 730 KGK Kautschuk Gummi Kunststoffe 52. Jahrgang, Nr. 11/99

8 Although Oil SAE 90 does not cause as much swelling as IRM 903, the results are similar. Only with elongation at break are increases frequently recorded. The relative changes are smaller. Hydraulic Fluid NF 46 does not attack the polymers to any great extent (Figure 11). In the case of ACM, swelling is somewhat higher. Nonetheless all the vulcanisates can be regarded as having good resistance. Low-temperature properties Figure 10. Pre-crosslinked EVM with 70 % VA compared to normal EVM with 60 % VA, EAM and ACM; change in tensile strengthand elongation at break after immersion in IRM 903 for 70 h at 150 8C Figure 11. Pre-crosslinked EVM with 70 % VA compared to normal EVM with 60 % VA, EAM and ACM; compression set at various temperatures after 7 days In addition to having good properties at high temperatures the vulcanisates are also expected to retain their rubber-like properties at low temperatures. Various methods are applied to determine the changes in properties. Static properties are determined in terms of the compression set. Copolymers crosslinked with peroxides do not behave in a unified manner (Figure 11). EVM with a 70 % VA content is superior to its isomer EAM. Where there is a 60 % VA content, the results are the opposite. The non-peroxide systems are somewhat better. In the TR test (Figure 12) the copolymers crosslinked by peroxides are also somewhat inferior (Table 4). The temperatures correlate with the chemical composition of the polymers. Despite the lack of ethylene components ACM reaches the same height as X-EVM 70. A dynamic test was the last test method to be applied. In this torsional test, the numbers as absolute temperatures correspond to the TR test (Figure 13). Applications The similar properties of EVM and EAM make it inevitable that their applications overlap to a considerable extent. Substitution is always possible when a standard is met and additional benefits are expected. General studies on compounds were presented in 1994 [1]. The examples given were either already being successfully applied or about to reach development completion. Hoses Figure 12. Pre-crosslinked EVM with 70 % VA compared to normal EVM with 60 % VA, EAM and ACM in the TR test This formulation met the requirements for air intake hoses in the necessary tests. The mixing and processing behaviour of the raw compound was good and overall KGK Kautschuk Gummi Kunststoffe 52. Jahrgang, Nr. 11/99 731

9 Figure 13. Pre-crosslinked EVM with 70 % VA compared to normal EVM with 60 % VA, EAM and ACM in a torsional pendulum test; measuring frequency 1 Hz Figure 14. Damping of EVM with 50 and 70 % VA compared to EAM Table 5: Levapren in a hose formulation Levapren 700 HV X-EVM 70 ± 50 Rhenovin DDA-70 2 Paraff. amine (6) 1 Ca stearate 4 Stearic acid 1 Black N Polyadipate (11) ± 5 PAT (12) 0.75 Renofit TRIM/S 3 Polydispersion T(VC)D 40 P 6 Total ML 1 4/100 8C Vulcanisate properties Tensile strength MPa Elongation at break % Hardness Shore A Price index (EAM 128) the compound is less expensive than an EAM compound with a similar composition (Table 5). The reasons for changing the compound were: ± better processing ± more favourable price. Seals The example in Table 6 is a development intended to compete against silicone rubber. Particularly high thermal resistance was not required for this application. What was important was good colour fastness for light-coloured articles. The price was also attractive. Damping elements In an existing EAM compound the polymer should be substituted by EVM. Fig- Table 6: Levapren 700 HV for seals Levapren 700 HV 100 Aluminium silicate (14) 55 Stearic acid 1 Iron oxide 2 Rhenogran P 50 3 Rhenovin DDA Rhenofit TAC/S 3.5 Polydispersion T(VC)D 40 P 4 Total: 170 Vulcanisate properties : Vulcanisation in the press 10 min/180 8C Tensile strength Mpa 14.7 Elongation at break % 200 Hardness Shore A 81 Compression set 150 8C/70 h % 55 Table 7: Torsional vibration damper Levapren 500 HV 100 ± ± Levapren 700 HV ± 100 ± EAM 55 4 ± ± 100 Black N DOS (15) 15 MgO (16) 2 2 ± Stearic acid Rhenogran P ± Rhenovin DDA±70 2 PE wax 1 1 ± Aktiplast PP Rhenofit TAC/S 3 3 ± Polydispersion T(VC)D 40 P 6 6 ± HMDAC (7) ± ± 1.25 Vulkacit DOTG ± ± KGK Kautschuk Gummi Kunststoffe 52. Jahrgang, Nr. 11/99

10 ure 2 showed the elasticity of the various EVM grades. Two EVM grades were chosen. EVM with 50 % VA content and a rebound resiliance of 36 % and EVM with a 70 % VA content recording a figure of 14 % rebound resiliance. EVM with 70 % VA had poor elastic behaviour, or expressed the other way round it gave the strongest damping effect. The tan d of a compound (Table 7) for vibration dampers is given in Figure 14. The author Dr. Hermann Meisenheimer is working in the Elastomers R & D Department of Bayer AG, Leverkusen, Germany, and is responsible for Special Elastomers. Corresponding author Dr. H. Meisenheimer, KA-Forschung u. Entwicklung/ Spezialelastomere, Bayer AG, D Leverkusen Literature Appendix: Substances used Product Chemical composition Manufacturer (1) Rhenovin DDA-70 Styrenated diphenyl amine Rheinchemie (2) Rhenogran P 50 Polycarbodiimide Rheinchemie (3) Rhenofit TAC/S TAC Rheinchemie (4) Perkadox 14/ bis(tert.-butylperoxyisopropyl)- AKZO benzene (40 %) (5) Polydispersion T(VC)D40P 1.3-bis(tert.-butylperoxyisopropyl)- Rheinchemie benzene (40 %) (6) Armeen 18 D Paraffinic amine AKZO (7) Diak No 1 Hexamethylene diamine carbamate DuPont (8) Vulkacit DOTG N,N'-di-o-tolylguanidine Bayer AG (9) Rhenocure Diuron Rheinchemie (10) Rhenovin Na-Stearat 80 Na stearate Rheinchemie (11) Ultramoll III Polyadipic ester Bayer AG (12) PAT Processing aid WuÈ rtz-chemie (13) Rhenofit TRIM/S Trimethylol propane trimethacrylate Rheinchemie (14) Vulkasil A1 Sodium aluminium silicate Bayer AG (15) Edenol 888 DOS Henkel (16) Maglite DE MgO Merck (17) Aktiplast PP Fatty-acid Zn salt Rheinchemie [1] H. Meisenheimer, Paper presented at DKG-meeting, November , Bad Neuenahr. KGK Kautschuk Gummi Kunststoffe 52. Jahrgang, Nr. 11/99 733