CURING SYSTEM

The cross-linking reaction is carried out after the resin has been applied to the glass fibre. In practice the curing is carried out either at elevated temperatures of about 100°C where press mouldings are being produced, or at room temperature in the case of large hand lay-up structures. 

Benzoyl jreroxide is most eommonly used for elevated temperature curing. The peroxide is generally supplied as a paste ( 50%) in a liquid such as dimethyl phthalate to reduce explosion hazards and to facilitate mixing. The curing cycle in pressure moulding processes is normally less than five minutes. 

In the presence of certain aromatic tertiary amines such as dimethylaniline, benzoyl peroxide will bring about the room temperature cure of general purpose polyester resins. 

More frequently either methyl ethyl ketone peroxide or cyclohexanone peroxide is used for room temperature curing in conjunction with a cobalt compound such as a naphthenate, octoate or other organic solvent-soluble soap. The peroxides (strictly speaking polymerisation initiators) are referred to as catalysts and the cobalt compound as an accelerator . Other curing systems have been devised but are seldom used. 

Commercial methyl ethyl ketone peroxide (MEKP) is a mixture of compounds and is a liquid usually supplied blended into dimethyl phthalate, the mixture 

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Peroxide curing systems Peroxide decomposers Peroxide initiators Peroxides... 

Resin additives Resin cements Resin component Resin cure systems Resin curing agents Resin formation Resin-in-pulp Resinoid Resinols Resins... 

Sulfur concrete Sulfur copolymers Sulfur cures Sulfur cure system Sulfur dichloride... 

Foam. PhenoHc resin foam is a cured system composed of open and closed ceUs with an overall density of 16—800 g/cm. Principal appHcations are in the areas of insulation and sponge-like floral foam. The resins are aqueous resoles cataly2ed by NaOH at a formaldehyde phenol ratio of ca 2 1. Free phenol and formaldehyde content should be low, although urea may be used as a formaldehyde scavenger. 

Catalyst Selection. The low resin viscosity and ambient temperature cure systems developed from peroxides have faciUtated the expansion of polyester resins on a commercial scale, using relatively simple fabrication techniques in open molds at ambient temperatures. The dominant catalyst systems used for ambient fabrication processes are based on metal (redox) promoters used in combination with hydroperoxides and peroxides commonly found in commercial MEKP and related perketones (13). Promoters such as styrene-soluble cobalt octoate undergo controlled reduction—oxidation (redox) reactions with MEKP that generate peroxy free radicals to initiate a controlled cross-linking reaction. 

Formulation Design for Free-Radical Cured Systems... 

Diamine curatives were the first cross-linking agents for fluorocarbon mbbers. They are corrosive to mild steel molds and have been replaced in many appHcations by the bisphenol or other more recent cure systems. Nevertheless, some diamines are stiU used for food-contact appHcations of fluorocarbon mbbers and in zinc-free cures of halobutyl mbbers for pharmaceutical stoppers. Methylene dianiline and triethylene tetramine are cross-linking agents for ethylene—acryflc elastomers. 

Nitrile mbber compounds have good abrasion and water resistance. They can have compression set properties as low as 25% with the selection of a proper cure system. The temperature range for the elastomers is from —30 to 125°C. The compounds are also plasticized using polar ester plasticizers. The main dilemma is the selection of a heat-stable, nonfugitive plasticizer that also gives good low temperature properties. 

Vulcani2ation is a chemical process for improving an elastomer compound s performance. However, in most cases not all of the desired properties reach their optimum levels simultaneously. One of the mbber compounder s key responsibiHties is to achieve a balance of the most important property requirements by the proper selection of cure system (chemical) and time—temperature cure cycle (physical). 

Erequendy, the curing equipment available, ie, presses, autoclaves, LCM lines, etc, do not allow the curing conditions to be varied as desired, so the compounder must design a cure system compatible with the existing equipment while also meeting the compound performance requirements. 

Heat resistance is iafluenced by both the type and extent of cure. The greater the strength of the chemical bonds ia the cross-link, the better is the compound s heat resistance. Peroxide cure systems, which result ia carbon—carbon bonds, result ia a range of sulfur cross-links varyiag from 1 to > 30 sulfur atoms per cross-link, and heat resistance improves as the number of more thermally stable short cross-links predominates. This is an important factor ia designing the desired cure system. 

Another cure system consideration is the compound scorch behavior. Prior to vulcanisation, mbber is plastic-like and can be processed iato desired shapes such as tires, hoses, belts, or other articles. The time available to accomplish this processiag depends largely on the cure system and is referred to as the scorch time. If a compound cures prematurely duriag the processiag step, it usually becomes useless scrap. Therefore, a key requirement of the vulcanisation step is to minimise premature vulcanisation or scorch (Fig. 4). 

Amine Cross-Linking. Two commercially important, high performance elastomers which are not normally sulfur-cured are the fluoroelastomers (FKM) and the polyacrylates (ACM). Polyacrylates typically contain a small percent of a reactive monomer designed to react with amine curatives such as hexamethylene-diamine carbamate (Diak 1). Because the type and level of reactive monomer varies with ACM type, it is important to match the curative type to the particular ACM ia questioa. Sulfur and sulfur-beating materials can be used as cure retarders they also serve as age resistors (22). Fluoroelastomer cure systems typically utilize amines as the primary cross-linking agent and metal oxides as acid acceptors. 

There are seven principal classes of accelerators and several miscellaneous products that do not fit into these classes. In addition, many proprietary blends of several accelerators are sold which are designed as cure packages for a specific appHcations. Choosing the best cure system is a responsibiUty of the mbber chemist and requites extensive knowledge of each accelerator type and its appHcabiUty in each elastomer. Table 5 shows a rule of thumb comparison of the scorch/cure rate attributes for the five most widely used classes of accelerators used in the high volume diene-based elastomers. 

It is common practice in the mbber industry for a compounder to use combinations of several accelerators in developing a cure system. Typically these cure systems are comprised of a primary accelerator and one or more secondary types. Primary accelerators are generally the thiazole and sulfenamide classes the secondary types (kickers) are the thiurams, dithiocarbamates, guanidines, and to a much lesser extent, certain amines and the dialkylphosphorodithioates (20). 

Natural mbber usually contains sufficient levels of naturally occurring fatty acids to solubilize the zinc salt. However, if these fatty acids are first extracted by acetone, the resultant "clean" natural mbber exhibits a much lower state of cure. Therefore, to ensure consistent cure rate, fatty acids are usually added. Synthetic mbbers, especially the solution polymers, do not contain fatty acids and requite thein addition to the cure system. 

Another commercially available retarder for sulfur vulcanization is based on an aromatic sulfenamide. Like CTP, this product is most effective ki sulfenamide cure systems, but it also works well ki thiazole systems. Performance properties are generally not affected except for a slight modulus kicrease. In some cases this feature allows for the use of lower levels of accelerator to achieve the desked modulus with the added potential benefits of further scorch delay and lower cost cure system (23). 

Curatives and accelerators are combkied to achieve desired performance properties through cure system design. 

Eig. 8. Sulfiir-based cure system designs where conventional systems are polysulfidic, EV systems are mono-/disulfidic, and semi-EV systems are clean polysulfidic. A shows pendent sulfide groups terminated by accelerator B, monosulftde cross-links C, disulfide cross-links D, polysulftde cross-links... 

Conventional cure systems use relatively high levels (2.5 + phr) of sulfur combkied with lower levels of accelerator(s). These typically provide high initial physical properties, tensile and tear strengths, and good initial fatigue life, but with a greater tendency to lose these properties after heat aging. 

In contrast, the EV cure systems employ much lower levels of free sulfur (0.1—1.0 phr) or they use sulfur donors such as TMTD or DTDM combkied with higher accelerator levels. The short mono- and disulfide cross-links that form often do not exhibit the excellent physical properties of the conventional systems but they do retain thek properties much better after aging. 

Examples of Cure Systems in NR, SBR, and Nitrile Rubber. Table 6 offers examples of recipes for conventional, semi-EV, and EV cure systems ia a simple, carbon black-filled natural mbber compound cured to optimum (t90) cure. The distribution of cross-links obtained is found ia Figure 9 (24). 

Eig. 12. SBR cure systems (A = conventional, B = semi-EV, C = EV) relative to conventional cure at 100 where I I shows aged elongation retained ,... 

Fig. 13. Relationship of nittile rubber cure systems where DCP is dicumyl peroxide MBTS, benzothiazyl disulfide ZrJDMD, zinc dimethyldithiocarbamate ...

Cure Systems of Butyl Rubber and EPDM. Nonhalogenated butyl rubber is a copolymer of isobutjiene with a small percentage of isoprene which provides cross-linking sites. Because the level of unsaturation is low relative to natural mbber or SBR, cure system design generally requites higher levels of fast accelerators such as the dithiocarbamates. Examples of typical butyl mbber cure systems, thein attributes, and principal appHcations have been reviewed (26). Use of conventional and semi-EV techniques can be used in butyl mbber as shown in Table 7 (21). 

Table 7. Conventional and Semi-EV Cure Systems for Butyl Rubber ...

Other ingredients besides the elastomer and the cure system itself influence cure and scorch behavior. Usually the effect of a material on cure is pH-dependent. Ingredients which are basic in nature tend to accelerate the rate of both scorch and cure, whereas acidic materials exhibit the opposite effect. 

There are many ways to measure these properties and some of them are proprietary. However, most laboratory tests are standardized by American Standard Testing Methods (ASTM). Many of them are interactive to various degrees. The rate and state of vulcanization is especially important to consider for components of heavier and thicker tines. The heat used to vulcanize the tine in a mold under pressure requites time to penetrate from both sides of the giant tine to the innermost portions. Securing a balanced state of cure, ie, the maximizing of physical properties in all the components, results in the innermost components having a faster rate of cure. The peripheral compounds should have a cure system which holds its physical properties well when overcured. 

Zinc oxide and stearic acid are used to activate the curing system as well as to preserve cured properties when overcuring, which is curing beyond the point of time and temperature at which maximum properties are obtained. 

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