Cured Vamac

AEM polymers, both terpolymers and copolymers, must be cured under pressure to avoid sponging. Pressures of 413.7 kPa and higher, preferably higher, are recommended for curing Vamac compounds without sponging. High viscosity compounds with fast cure systems can be molded at the lower end of the pressure range. If compression set is not a consideration, the viscosity of a terpolymer-based compound can be increased sufficiently with ionic crosslinks to be cured at atmospheric pressure [3]. 

Commercial Forms. Eour different base polymers of VAMAC ethylene—acryhc elastomer are commercially available (Table 1). Until 1990, existing grades of ethylene—acryhc elastomers were based on a single-gum polymer. VAMAC G, defined as a terpolymer of 55% methyl acrylate, ethylene, and a cure-site monomer (5). In 1991, a higher methyl acrylate terpolymer, VAMAC LS, was introduced. The composition of this polymer was specifically chosen because it significantly increases the oil resistance of the polymer while minimizing losses in low temperature fiexibihty (6). 

A new family of peroxide-cured dipolymers was introduced in 1991. The peroxide cure provides copolymers that cure faster and exhibit good compression set properties without a postcure. The removal of the cure-site has also made the polymer less susceptible to attack from amine-based additives. By varying the methyl acrylate level in the dipolymer, two offerings in this family have been synthesized, VAMAC D and its more oil-resistant... 

A terpolymer rubber was introduced by Du Pont in 1975 (Vamac). This is based on ethylene, methyl acrylate and a third, undisclosed, monomer containing carboxylic acid groups to act as the cure site (see Section 11.9). 

More recently, in 1975, Du Pont introduced a terpolymer (Vamac) based on ethylene, methyl acrylate and a third monomer of undisclosed composition which contained a carboxylic acid group to provide a cure site for use with peroxides or amines. Both types of rubber exhibit good heat, oxygen and ozone resistance. 

Interestingly, later grades of Vamac to become available did not employ the cure site monomer, using instead a peroxide-curing system. Some of these copolymers also contained higher levels of methyl acrylate (up to 69%) to enhance the oil resistance. 

A Review of Fast-Cure Systems for VAMAC Elastomers, Technical Bulletin E.I. DuPont de Nemours and Company, Inc., Document reference VAM020507, 2002. 

Vamac is generally cured with diamines, with DPG as an accelerator, but it is also possible to crosslink this material with peroxides. 

Vamac G material, carbon black filled with a Diak/DOTG cure system... 

J. F. Hagman, Curing Mechanisms of Vamac, Bulletin EA-030.0684, DuPont Polymers, Stow, Ohio, 1980. 

H. J. Barager, K. Kammerer, E. McBride, L. C. Muschiatti, and Y. T. Wu, Increased Cure Rates of Vamac (AEM) Dipolymers and Terpolsuners using Peroxides, American Chemical Society Rubber Division, Cincinnati, Ohio, Oct. 2000. 

Ethylene acrylic mbber is manufactured by M/s Dupont USA under the trade name of Vamac, and is about half ethylene and half methylacrylate. A small amount of cure site monomer in the molecule provides the ability to cross-link chemically. This rubber is the combination of two major chemicals which give its unique balance of properties. For instance, the backbone structure of the polymer molecule is saturated, and so it is inherently resistant to ozone attack. The acrylic segment provides oil resistance, and the ethylene segment yields low temperature performance. The added feature of this mbber is that there is no halogen present to become corroded. There is slightly more tendency to swell than a homopolymer, such as polyacrylate or acrylonitrile mbber, but it is approximately equal to silicone, chloroprene and Hypolan (chlorosulfonated polyethylene) mbbers. 

The ethylene-methyl acrylate rubbers were introduced in 1975 (Vamac—Du Pont) and are prepared from ethylene, methyl acrylate and a small amount of a third monomer to provide a cure site. The polymer is said to have the following structure ... 

Is a post-cure impractical Choices Vamac D versus Vamac G (Vamac DLS versus Vamac GLS). 

Terpolymers must be post-cured to attain optimum properties. If a post-cure is either not practical or not desired, and excellent compression set is a requirement, the copolymer Vamac D grades are the preferred choice. 

Vamac terpolymers are usually cured with a combination of primary diamines and guanidine accelerators... 

The peroxide cure of Vamac copolymers requires a coagent. The two preferred peroxide/coagent packages are... 

Note Vamac dipolymers are cured with a combination of peroxide and coagent. 

Vamac will sponge if cured without pressure Air autoclave... 

Typical shrinkage values for Vamac compounds are around 2.5% after press-cure, increasing to 3.5% % after post-cure. Part shrinkage can be reduced by increasing the amount of nonvolatile filler, decreasing the level of plasticizer, reducing mold temperatures, and reducing the temperature and duration of post-cure. 

The primary use for Vamac in wire and cable is in ATH filled, flame resistant, nonhalogen jackets, usually peroxide cured (Table 6.32). The following typical jacket compounds are based on Vamac D and Vamac G. A color concentrate can be used in place of the carbon black. Armeen 18D and Vanfre VAM can be added to improve release properties but at the high filler levels associated with flame-retardant jacketing compounds, release is usually not a problem. 

Test slabs for GG were press-cured 15 min at 180°C and post-cured 4 h at 180°C. Test slabs far the remaining compounds were press-cured 5 min at ISff C and post-cured 4 h at 175°C. Data based on Vamac G. 

Struktol WB 42 at a 1 % level, combined with 1 % stearic acid, is a good combination for general purpose use in peroxide-cured AEM ethylene acrylic elastomer compounds. Vamac AEM stocks using hexamethylene diamine carbamate (Diak No. 1) as the curative, requires 1% stearic acid plus 0.3% of a stearamide such as Armeen 18D from Akzo and 0.8% of Struktol WS 180. These chemicals act as coagents for mill release. Struktol WB 42, Struktol WS 180, and Armeen 18D also serve as mold release agents. 

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