Technical Information • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Rev. 2, January 2008

A Review of Fast-Cure Systems for Vamac® Elastomers

Abstract Vamac® ethylene acrylic elastomers (AEM) are used in numerous automotive sealing applications requiring resistance to service fluids over a broad temperature range. Current AEM grades are based on copolymers of ethylene and methylacrylate and terpolymers of ethylene, methylacrylate and an acidic cure site monomer. The most common AEM vulcanization systems employ a combination of peroxide and coagent for the dipolymers and a combination of aliphatic diamine salts with organic accelerators for the terpolymers. Diamine-cured terpolymers must be postcured.

Efforts are underway to develop alternative vulcanization systems that will significantly reduce the press cure time of both dipolymers and terpolymers increasing manufacturing productivity. The status and results of the developments on alternative fast cure systems for AEM terpolymers with improved scorch safety are reviewed in this presentation.

Introduction AEM elastomers have been commercially available since 1975 and since that time their usage has increased in critical automotive applications such as engine seals, transmission seals, torsional dampers and hoses. The attributes most responsible for this growth include a unique balance of high- and low-temperature properties (175°C to -40°C), and resistance to many automotive service fluids. Existing grades of AEM terpolymers are based on gum polymers containing a medium level of methylacrylate, Vamac® G (AEM-G), and a high level of methylacrylate, Vamac® GLS (AEM-GLS). The fluid resistance of these elastomers depends on their methylacrylate to ethylene ratio. Higher methylacryate levels increase the polarity of the polymer and the resistance to hydrocarbon oils. In general, the volume swell of AEM-GLS polymers in hydrocarbon fluids is approximately one-half the volume swell obtained with AEM-G polymers. Both AEM-G and AEM-GLS terpolymers contain an organic acid cure site monomer and are vulcanized with diamines. The terpolymer structure and the curing mechanism are shown in Figure 1. The diamine vulcanization system requires an organic accelerator, normally a guanidine, to control pH. During press cure, the diamine reacts with the acid cure site producing an amide crosslink. In the chemical environment of the AEM terpolymer backbone, the amide linkage is converted with heat and time to an imide. This reduction of the amide to the imide must take place prior to placing a part under stress; therefore, a postcure is required. To increase productivity and expand the capabilities of parts based on AEM terpolymers, alternative vulcanization systems have been developed which show significantly faster rates of cure than current vulcanization systems. Compounds have been prepared in the Durometer A range of 55 to 75, which are not only faster curing but also safer, based on Mooney Scorch values.

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Figure 1 – Vamac® Terpolymer Curing Mechanism ~~(CH 2-CH2)x~~ ~~(CXH 2-CH)Y~~ ~~ (CH-CH) Z~~ | | | C=O O=C C=O | | | OCH 3 OH OR Ethylene Methyl Acrylate Cure Site + H2N-(CH 2)6-NH 2 Hexamethylene Diamine ~~~~CH-CH~~~~ ~~~~CH-CH~~~~ | | | | O=C C=O O=C C=O

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| | NH OR N | | (CH 2) +H2O (CH 2)6+ROH | | NH OR N | | O=C C=O O=C C=O

Postcure

| | ~~~~CH-CH~~~~ ~~~~CH-CH~~~~

Amide Crosslink Imide Crosslink Fast, Safe Vulcanization Systems for AEM Terpolymers The basic components of the fast, safe vulcanization systems developed for AEM terpolymers are show in Table 1.

Table 1 – Basic Components of Fast, Safe Vulcanization Systems Vulcanization System Diamines: Hexamethylenediamine carbamate (Diak #1, Vulcafac HDC)

Ethylenediamine/methyisobutlyketimine (Epicure 3502) Ethylenediamine

Peroxides: Dicumyl Peroxide (Di-Cup®) a-a-bis(t-butyl peroxy)diisopropylbenzene (Vulcup) Accelerators: Diphenyl Guanidine (DPG) Di-orthotoyl Guanidine (DOTG) Coagents: Ricon® 152 (Low Molecular Weight Polybutadiene Polymer) Retarders: Salicylic acid (Retarder W) Armeen® 18D (Secondary Amine)

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ODR and MDR measurements at 177°C and 190°C were used to compare reaction rates and Mooney Scorch at 121°C and 135°C were used to judge compound storage stability. The practical utility of the compounds was determined by examining selected physical properties which included: stress-strain, tear strength, compression set, compression stress relaxation and thermal stability. The control AEM composition selected was a nominal 55 to 60 Durometer A hardness based on AEM-G. The composition of the control compound and the properties averaged over several trials are shown in Table 2.

Table 2 – Control Compound Composition and Average Properties CONTROL COMPOUND Compound Composition Ingredient Parts Vamac® G (AEM-G) 100 Naugard® 445 2 Armeen® 18D 0.5 Stearic Acid 1.5 Vanfre® VAM 1 N774, SRF Black 60 Plasthall® P670 10 Diak™ #1 1.5 Diphenylguanidine 4 STOCK PROPERTIES Mooney Scorch @ 121°C T(3), m.m 9.4 T(10), m.m 14.4 Mooney Scorch @ 135°C T(10), m.m 6.6 ODR @ 177°C MH, in-lb 42 t(50), m.m 2.8 tc(90), m.m 11.9 Slope(1), in-lb/m.m 11.7 MH/2[t(50)-ts2], in-lb/m.m 13 MH/[tc(90)-ts2], in-lb-m.m 4 ODR @ 190°C MH, in-lb 64 t(50), m.m 2.1 tc(90), m.m 6.6 Slope(1), in-lb/m.m 20 Slope, MH/2[t(50)-ts2], in-lb/m.m 26 Slope, MH/[tc(90)-ts2], in-lb/m.m 11 PHYSICAL PROPERTIES – POSTCURED Cure: Press Cure 3 min. @ 190°C Post Cure 4 hrs @ 175°C

Stress/Strain and Durometer @ 23°C Durometer A, Pts 56 100% Modulus, psi (Mpa) 500 (3.4) Tensile Strength, psi (Mpa) 2150 (14.8) Elongation, % 346 Die C Tear Strength @ 23°C pli (kN/m) 188 (33) Compression Set, 70 hrs @ 150°C Press Cured, % 84 Post Cured, % 20 1Measured initial slope taken from the ODR curves

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Unless otherwise specified, vulcanization systems are compared in compounds with the same composition as the control, differing only in the vulcanization package. Table 3 compares the vulcanization rates and critical properties of AEM-G compounds vulcanized with three diamine-type curatives: hexamethylenediamine carbamate (Diak™ No. 1), ethylenediamine and ethylenediamine/ketimine (Epicure 3502). Typical vulcanization systems containing only the diamine and DPG or DOTG accelerator are compared to vulcanization systems containing 5 parts of 40% dicumylperoxide (Di-Cup® 40C) and 2 parts of low molecular weight polybutadiene (Ricon® 152) in addition to the standard package. None of these compounds contained a retarder. Selected cure rate data from Table 3 is plotted in bar graph form in Figures 2 through 5.

Table 3 – Comparison of Diamine Vulcanization Systems DIAMINE VULCANIZATION SYSTEMS Cure System Diak™ #1 1.0 1.5 1.0 Epicure 3502 3.0 5.0 3.0 Di-Cup® 40C 5.0 5.0 Ricon® 152 2.0 2.0 STOCK PROPERTIES ODR @177°C MH, in-lb 29 42 44 56 60 60 t(50), m.m 2.3 2.8 3.1 4.5 2.1 2.7 tc(90), m.m 9.8 11.9 12.5 13.8 5.6 6.2 Slope, in-lb/m.m 10 12 13 10 27 23 ODR @190°C MH, in-lb 30 44 47 59 56 61 t(50), m.m 1.6 1.9 2.0 2.8 1.4 1.8 tc(90), m.m 6.6 8.7 9.2 9.4 2.9 5.3 Slope, in-lb/m.m 17 20 25 20 50 41 Mooney Scorch @121°C t(3), m.m 8.7 9.2 12.3 15.9 8.3 11.8 t(10), m.m 14.0 15.7 20.2 >21 13.5 19.8 Mooney Scorch @135°C t(3), m.m 4.5 4.7 5.9 6.9 4.3 5.2 t(10), m.m 6.2 6.6 8.3 10.4 5.9 7.7 VULCANIZATE PROPERTIES – POST CURED Cure: Press Cure 3-minutes @190°C Post Cure 4-hours @175°C

Stress/Strain and Durometer @23°C Durometer A, pts 51 54 54 60 59 58 M(100), psi 300 405 365 535 530 530 Tb, psi 1710 2040 1800 1800 2030 1750 Eb, % 463 388 423 293 268 272 Die C Tear Strength @23°C pli 199 188 158 165 Compression Set – 70 hrs @150°C Press Cured 81 84 59 57 Post Cured 29 20 20 20 Increasing the hexamethylenediamine carbamate, HMDC, level from 1.0 to 1.5 parts produces a nominal 18% to 20% increase in cure rate as indicated by ODR slope. Cure rates for the basic ketimine systems are essentially the same as those for the basic HMDC systems but the ketimine systems are significantly less scorchy. The addition of dicumylperoxide and polybutadiene increases the cure rate (ODR slope) by 167% (177°C) to 199% (190°C) for HMDC and 80% (177°) to (63% (190°C) for the ketimine. The time to 90% cure, tc(90), at 190°C is decreased by a factor of three (3) for HMDC and 1.7 for the ketimine. Figures 2 and 3 compare the MH and ODR slope at 177°C and 190°C for HMDC. Figure 4, 5 and 6 compare cure rates for HMDA and ketimine fast cure systems. Both fast-cure systems show a slight decrease in scorch safety under storage conditions and at mold temperatures. The effect of the fast cure package on shelf stability and scorch rates at a molding temperature of 190°C is shown in Figures 7 and 8 respectively. A number of potential retarders were evaluated in an attempt to increase scorch safety. Salicylic acid and Armeen® 18D were found to be effective with salicylic acid having the least effect on cure rate and overall properties. In general the reduction in property values after press cure was

recovered by post cure. Some of our observations with salicylic acid in the fast HMDC cure system are shown in Figures 9 and 10. The addition of salicylic acid resulted in a mild reduction in cure rate (slope), minor reduction in tc(90) and a significant increase in scorch safety. The rate of cure of the retarded, fast HMDC system, as determined by ODR slope, was still about 2.4 times greater than that of the standard HMDC cure system. There was essentially no change in maximum ODR torque with the addition of salicylic acid. In general, the addition of peroxide and coagent to diamine cure systems results in a slight increase in Durometer, a large increase in 100% modulus, a lower elongation at break and no change in tensile strength. The addition of salicylic acid had no effect on the physical properties of the post cured compounds (Figures 11 through 15). Compression sets at 150°C were equivalent to those of post-cured compounds with high HMDC levels and were all around 20%, Figure 14. Ethylenediamine is an effective low-cost curative for AEM polymers. Ethylenediamine is a flammable liquid and difficult to handle as such. However, it remains equally effective if it is added adsorbed on a carrier such as fumed silica or other common fillers. Ethylenediamine cures can also be accelerated by the addition of peroxide and polybutadiene. As can be seen in Figure 16, ethylenediamine alone yields cure rates essentially equivalent to those of a 1.5 part HMDC system. The addition of peroxide/coagent almost doubles the slope and reduces tc(90), even at a higher maximum torque. As with HMDC and ketimine, fast cure ethylenediamine systems produce vulcanizates with slightly higher Durometer, much higher 100% modulus and lower elongation at break than their non-accelerated counterparts (Figure 17). All the compounds and results discussed thus far were based on compounds formulated with 60 parts of SRF (N774) carbon black and 10 parts of plasticizer. Increasing the amount of carbon black or changing to a finer particle size FEF (N550) carbon black produces a faster curing compound with higher Durometer and strength properties, and a lower elongation at break. This trend was found to carry over to the fast cure systems. The trend for SRF carbon black is illustrated in Figures 18 and 19. Summary Diamine cure systems for AEM terpolymers can be effectively accelerated to increase productivity by the addition of selected peroxides and coagents. Increased scorch safety, both on the shelf and in the mold, can be attained through the incorporation of salicylic acid or excess Armeen® 18D. The strength properties of the compounds prepared from the accelerated diamine cure systems are equal to or better than those of the nonaccelerated vulcanizates. For most fast-cure compositions, compression sets are around 20% after post-cure. Compression sets for press-cured compounds prior to post-cure tend to be approximately 30% lower than equivalent nonaccelerated vulcanizates, i.e. 57% to 60% versus 80% to 85%. The only potential drawback for the fast-cure systems is the reduction in elongation. If the elongation at break for an accelerated AEM-based compound is acceptable, these cure systems offer an opportunity to reduce compound costs and increase productivity. The only potential drawback for the fast-cure systems is the reduction in elongation. If the elongation at break for an accelerated AEM-based compound is acceptable, these cure systems offer an opportunity to reduce compound costs and increase productivity.

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Figure 2 – Rate of Cure @177°C – Diak™ No. 1

Figure 3 – Rate of Cure @190°C – Diak™ No. 1

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Figure 6 – Fast Cure Systems: Time to 90% Cure

Figure 7 – Fast Cure Systems: Scorch Safety (Mooney Scorch)

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For further information please contact one of the offices below, or visit our website at www.dupontelastomers.com/vamac

Global Headquarters – Wilmington, DE USA Tel. +1-800-853-5515 +1-302-792-4000 Fax +1-302-792-4450

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The information set forth herein is furnished free of charge and is based on technical data that DuPont Performance Elastomers believes to bereliable. It is intended for use by persons having technical skill, at their own discretion and risk. Handling precaution information is given with theunderstanding that those using it will satisfy themselves that their particular conditions of use present no health or safety hazards. Sinceconditions of product use and disposal are outside our control, we make no warranties, express or implied, and assume no liability in connection with any use of this information. As with any material, evaluation of any compound under end-use conditions prior to specification is essential. Nothing herein is to be taken as a license to operate or a recommendation to infringe on patents. While the information presented here is accurate at the time of publication, specifications can change. Check www.dupontelastomers.com for the most up-to-date information. Caution: Do not use in medical applications involving permanent implantation in the human body. For other medical applications, discuss withyour DuPont Performance Elastomers customer service representative and read Medical Caution Statement H-69237. DuPont™ is a trademark of DuPont and its affiliates. Vamac® is a registered trademark of DuPont and brought to market by DuPont Performance Elastomers. Diak™ is a trademark of DuPont Performance Elastomers. Ricon® is a registered trademark of Sartomer Technology Company; Armeen® is a registered trademark of Akzo Nobel Chemicals; Naugard® is a registered trademark of Uniroyal Chemicals; Vanfre® is a registered trademark of R. T. Vanderbilt; Di-Cup® is a registered trademark of Hercules Powder Company; Plasthall® is a registered trademark of Hallstar Innovations Corp. Copyright © 2007 DuPont Performance Elastomers. All Rights Reserved.

(05/02) Printed in U.S.A. Reorder no: VME-A10616-00-B0108

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