Bisphenol cure systems

Long-term compression set resistance is described in Figure 4. Lower set values are achievable by use of higher viscosity gumstock at comparable cross-link densities. Compression set resistance is also very dependent on the cure system chosen. The bisphenol cure system offers the best compression set resistance available today, as shown in Table 5. 

Extruded Articles. In extmded article compounding, the most important parameters are scorch safety and flow characteristics (53). The bisphenol cure system again offers the best scorch resistance of the available fluorocarbon elastomer cure systems. Good flow characteristics can be achieved through proper selection of gum viscosities. Also, the addition of process aids to the formulation can enhance the flow characteristics. Typical formulations for extmsion grade fluorocarbon elastomers are given iu Table 7. 

Although development of a formulation for a specific product and process requires a great deal of knowledge and experience, there are some basic rules typical of FKM compounding. The levels of acid acceptor (MgO) and activator (Ca(OII)2) in the bisphenol cure system strongly affect not only the cross-linked network as reflected by the physical properties of the material, but also the behavior of the compound during vulcanization. Therefore, the curing system must be optimized to achieve the best balance of properties. 

Peroxidic cure systems are applicable only to fluorocarbon elastomers with cure sites that can generate new stable bonds. Although peroxide-cured fluorocarbon elastomers have inferior heat resistance and compression set, compared with bisphenol cured types they develop excellent physical properties with little or no postcuring. Peroxide cured fluoroelastomers also provide superior resistance to steam, acids, and other aqueous solvents because they do not require metal oxide activators used in bisphenol cure systems. Their difficult processing was an obstacle to their wider use for years, but recent improvements in chemistry and polymerization are offering more opportunities for this class of elastomers [42]. 

FIGURE 11.1 Comparison of a standard formulation, formulation with bisphenol curing system, and formulation with an FKM not requiring post cure. (Courtesy of Daikin.)... 

Peroxide-cured fluoroelastomers of this type are claimed to exhibit a superior resistance to steam, hot water and mineral acids than amine and bisphenol cured systems as well as providing certain advantages in the curing of thick sections. 

Bisphenol curing systems are usually used for 0-ring applications [26]. Indeed, they exhibit a high resistance to high temperature compression set. Figure 20 [3,33,35] depicts the evolution of a 177 °C cxxre response by ODR, of a bisphenol AF (Bp-AF) ciuring poly(VDF-co-HFP) copolymer. 

First, fluoroelastomers were cured with amines. However, the scorchy nature of those cure systems as well as a rather poor compression set resistance led to the development of bisphenol cure systems [13,14]. Peroxide curing is performed thanks to the addition of a cure site monomer [4,15-18] for compositions that cannot be cured with bisphenol, such as high fluorine compositions, low-temperature polymers where HFP has been substituted by PMVE, and non-VF2 containing polymers such as FEPMs (TFE/P and FTP polymers), unless a bisphenol cure site has been added to the polymer [19,20]. 

Amine curatives such as DIAK 1 and DIAK 3 were first introduced in the late 1950s but were rapidly abandoned when the bisphenol cure system was introduced as it could give much better scorch safety and superior compression set resistance. 

The entire cure system is used in conjunction with metal oxides, which act as acid acceptors in all systems, capturing HF formed during vulcanization. In the case of bisphenol cure systems, they act as bases to dehydrofluorinate the backbone, and their concentration is essential to the cure response [14],... 

Since about half of the applications for fluoroelastomers are O-rings and gaskets, compression set resistance is a key property compounder that typically aim for a value of 20% after 70 h at 200°C for a 75 Shore A formulation. For higher fluorine types, compression set resistance is usually a bit worse, in the 30%-40% range. The major factor for optimizing compression set is the selection of the type and level of curative. Amine curatives are the least effective and led to the development of bisphenol curatives. Until recently, peroxide curatives could not provide the excellent compression set resistance offered by bisphenol cure systems. However, recent advances in development of new cure site monomers have addressed these issues, even offering the capability to reduce the post-cure cycles to only 1 h [15-18]. 

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. 

The manufacture of the majority of fluorocarbon elastomer gums includes the addition of an incorporated cure system comprising an organic onium cure accelerator, such as triphenylbenzylphosphonium chloride [1100-88-5] and a bisphenol cross-linking agent, such as... 

Internal mixing is widely used with fluorocarbon elastomers. Gumstocks and compounds that are particularly successful fall in the viscosity ranges discussed earlier, and use both incorporated bisphenol-type and peroxide cure systems. A typical internal mix cycle mns 6—8 min with a drop temperature of 90—120°C. The typical formulations in Tables 4 and 7 are readily mixed in an internal mixer. 

Curing Systems. The most commonly used vulcanizing agent for the polyethers not containing AGE, that is, ECH and ECH—EO, is 2-mercaptoimidazoline, also called ethylenethiourea [96-45-7]. Other commercially appHed curing agents include derivatives of 2,5-dimercapto-l,3,4-thiadiazole, trithiocyanuric acid and derivatives, bisphenols, diamines, and other substituted thioureas. 

The multiepoxy functionality of the epoxy novolaks (2.2 to >5 epoxy groups per molecule) (3) produce more tightly cross-linked cured systems having improved elevated temperature performance and chemical resistance than the difunctional bisphenol A-based resins. 

Some terpolymers contain an additional cure site monomer, for example, bromotetrafluorobutene, to permit crosslinking with peroxides. Peroxide curing gives vulcanisates more resistance to amine stabilisers in motor oils, more resistance to methanol containing motor fluids. Resistance to acids, aqueous media and steam is also improved. Compression set and heat resistance are slightly inferior to bisphenol A cure systems. 

The nucleophilic curing system is most common and is used in about 80% of all applications. It is based on the cross-linker (bisphenol AF) and accelerator (phase... 

Ten years ago, good model systems were found for epoxy networks. These were polymers based on monomeric diglycidyl ethers of some bisphenols cured by simple aromatic amines (primarily w-phenylenediamine — wPhDA). Polymers based on these reactants satisfied the requirements for such a model system in several points ... 

The nucleophilic curing system is most common and is used in about 80% of all applications. It is based on the cross-linker (bisphenol AF) and accelerator (phase transfer catalyst, such as phosphonium or amino-phosphonium salt). Both diaminic and bisphenol type cure systems are permitted by U.S. Food and Drug Administration (FDA) regulations governing rubber articles in contact with food. The diaminic curing system is also used in some coating and extrusion applications [42]. 

Solid fluorocarbon elastomers are commercially available as pure gum polymers or precompounded grades with bisphenol type curing system included. Some precompounded stocks include processing aids, adhesion promoters, or other application-specific additives. The relative strengths and weaknesses of commonly used curing systems are listed in Table 5.7. 

The setting of roll temperatures depends on the cure systems used. Typically, stocks with diamine (e.g., Diak No. 3) are calendered at temperatures of top and middle rolls set 15°C to 20°C (27°F to 36°F) lower than stocks with bisphenol and peroxide [51]. 

Epoxy resins produced by the reaction of bisphenol A and epichloro-hydrin are versatile polymers with several useful properties (subsection 2.2.2.1). However, one significant weakness is their brittle nature. Incorporation of plasticisers is not very useful. Dibutyl phthalate is an exception, showing good compatibility but offering only limited ability to flexibilise the resin. Moreover, plasticisers affect the mechanical properties and chemical resistance of the cured system. With polyurethanes it is possible to complement the flexibility of the epoxy system. Numerous attempts have been made to combine the two types to achieve beneficial modifications (Lee and Nivelle, 1967). These modifications proved successful under high-temperature cure but inferior results were obtained for ambient cures. 

Resin I contains roughly equal amounts of diglycidyl ether of bisphenol A (DGEBA) and an epoxy cresol novolac. Sufficient dicyandiamide (DICY) as a curing agent is present such that the amine/epoxy ratio is 0.85. Monuron is present as an accelerator. The supplier s recommend standard cure is two hours at 127°C. Previous work (1 ) has shown that this cycle produces a fully cured system, as indicated by the disappearance of the epoxide absorbance band in the infrared spectrum. 

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