Reactive Cure Sites

The acrylic rubbers, being saturated polymers, cannot be vulcanized by conventional accelerated sulphur systems. On the other hand the homopolymers, such as poly(ethyl acrylate) and such copolymers as those of n-butyl acrylate with acrylonitrile could be cross-linked by such agents as  

strong bases such as sodium metasilicate pentahydrate, also believed to react at the a-methylenic position  

diamines, believed to cause cross-linking through aminolysis of the ester group. 

For various reasons these materials were not very satisfactory and this led to the concept of introducing another monomer in order to provide a more suitable reactive site. Many such cure-site monomers have been described in the patent literature but most of them fall into one of four groups  

Of the dienes, butadiene appears to have been the earliest to have been evaluated. Although it allowed accelerated sulphur vulcanization this was to the detriment of heat and oil resistance. More recently, non-conjugated dienes such as those used with EPDM rubbers have been quoted in the patent literature. These include dicyclopentadiene and methyl cyclopentadiene. A French patent (Tellier and Grimaud, 1968) quotes the use of tetrahydrobenzyl acrylate (VIII). 

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With exposure to moisture, hydrolysis of the cross-linking agent leads to the silanol reactive cure site, so such materials must be compounded and stored free of moisture.66... 

FIGURE 5.2 Polyaciylate typical reactive cure sites. 

HyTemp Nipol Reactive Cure Site Volume Swell % Gehman Tioo °C Mooney Sp. Cr. Viscosity ... 

Reactive Cure Site Temperature Chemicals Corporation... 

Two kinds of monomers are present in acryUc elastomers backbone monomers and cure-site monomers. Backbone monomers are acryUc esters that constitute the majority of the polymer chain (up to 99%), and determine the physical and chemical properties of the polymer and the performance of the vulcanizates. Cure-site monomers simultaneously present a double bond available for polymerization with acrylates and a moiety reactive with specific compounds in order to faciUtate the vulcanization process. 

In order to enhance the reactivity of the chlorine atom, a second reactive monomer can be adopted giving dual cure sites. According to the Hterature, the second monomer can contain carboxyl (22—24), cyanoalkyl (25), hydroxypropyl (26), or epoxy groups (27,28). 

Double-Bond Cure Sites. The effectiveness of this kind of reactive site is obvious. It allows vulcanization with conventional organic accelerators and sulfur-based curing systems, besides vulcanization by peroxides. Fast and controllable vulcanizations are expected so double-bond cure sites represent a chance to avoid post-curing. Furthermore, blending with other diene elastomers, such as nitrile mbber [9003-18-3] is gready faciUtated. 

Radiation curable polymer systems are based on the same chemical structural design as the conventional polymer systems, but certain modifications are made in order to accommodate reactive unsaturation sites necessary for a radiation-induced free radical curing mechanism. Examples of these modifications of conventional polymer structures to form radiation curable polymers are as follows ... 

In the phenol/formaldehyde reaction, the rates for the addition and condensation reactions are in the ratio of 1 42. The overall activation energy is 84-100 kJ/mol. The p site in this acid-catalyzed reaction is about 2.4 times more reactive than the o site of the phenol. In general, then, p-methylol phenols are produced, but these are not as attractive commercially as the o,o -methylol-rich novolaks. The curing of novolak should, of course, occur as quickly as possible, which assumes that the reactive p sites are available for reaction (see below). 

Reactive halogen-containing, cure-site monomers have dominated the commercial scene. From 1948 to 1963 2-chloroethyl vinyl ether (IX) was used almost exclusively ... 

The limited amount of cure sites and reactivity in EPDM requires higher levels of curatives than do the diene polymers ENB grades require 0.5-2 phr sulfur or equivalent amounts of sulfur donors (e.g., DTDM). They can be cured by sulfur and thiazoles/sulfena-mides, but with these systems the cure rate may be unsatisfactory (except for fast curing grades). 

Polyacrylate elastomers may consist of one or more of the following monomers ethyl acrylate, EA butyl acrylate, BA and methoxy ethyl acrylate MEA, the structures of which are given in Figure 5.1. These are coupled with cure sites, which may be reactive halogens, epoxy or carboxyl groups, or combinations thereof, as seen in Figure 5.2. [1,6,11]. 

During press cure the primary diamine reacts with the acid cure site producing an amide crosslink. In the chemical environment of the AEM terpolymer backbone the amide crosslinks are reactive and are converted with time and temperature to imide crosslinks. Since the reduction of the amide to the imide needs to occur before exposing a functional part to high temperatures and stress, a post-cure is required. A separate post-cure step can minimize press-cure times, increases vulcanizate modulus, tensile strength and hardness, and generally results in compression set values <20% (70 h at 150°C). Alternative post-cure cycles are... 

Di- and tri-functional thiol (or mercapto) molecules are used as cross-linkers for halogen-containing polymers. Like the bisphenols, they may require an accelerator, depending on the inherent reactivity of the halogen cure site and the particular... 

Obviously, a series of products are formed depending on whether epoxide or amine is present in excess. As the stoichiometric relationship approaches one equivalent of epoxide per equivalent of reactive amine sites, the molecular weight approaches infinity and a three-dimensional network polymer is obtained. However, at higher ratios of the curing agent, a linear thermoplastic polymer is obtained. 

Halobutyl Cures. Halogenated butyls cure faster in sulfur-accelerator systems than butyl bromobutyl is generally faster than chlorobutyl. Zinc oxide-based cure systems result in C—C bonds formed by alkylation through dehydrohalogenation of the halobutyl to form a zinc chloride catalyst (94,95). Cure rate is increased by stearic acid, but there is a competitive reaction of substitution at the halogen site. Because of this, stearic acid can reduce the overall state of cure (number of cross-links). Water is a strong retarder because it forms complexes with the reactive intermediates. Amine cure may be represented as follows ... 

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