Cure systems, cross-link

In the maleimide cure, the cross-linking agent is -y -phenjlenedimaleimide [3006-93-7] HVA-2. This system has two significant advantages Htharge is not required for high cross-link density and low compression set may be obtained. The accelerators are weak bases, ie,... 

Use Curing or cross-linking agent for polymeric systems, especially epoxy resins intermediate for stabilizers and antioxidants. 

The more traditional method to make laminates uses separate plies combined with adhesives, which can be divided into groups—molten, water-based and solvent-based. Wax and polythene extrusion are the main molten laminants. Water-based glues are often used to combine paper and foil. Solvent-based adhesives include the polymethanes, but recent developments use water dispersions and molten curing systems to replace the solvent systems. Cross-linking reactions develop high heat and product resistance in all these adhesives. 

Radiation Curing. The cross-linking of acrylate-modified polyester resins with U V radiation or an electron beam is an energy-saving alternative to the heat-curing systems. Reactive diluents (e.g., polyfunctional acrylates) are required to adjust the viscosity for application although they increase reactivity, they reduce the flexibility and substrate adhesion of the paint film. 

Although not widely studied, the simplest case is that of peroxide cured or cross-linked systems, where unsaturation can take part in the curing readion. Mori has reported very good results with some unsaturated carboxylic acids (e.g., CH2=C(CH3)C00C2H4C00H) in a calcium carbonate-filled unsaturated polyester resin [15]. Some of the results are presented in Table 6.1. 

Accelerator (1611) n. (1) Any substance used in small proportion which increases the speed of a chemical reaction. In the paint industry, the term usually indicates materials that hasten the curing or cross-linking of a resin system. In the polyester resin field, it covers more specifically an additive... 

Basically, compression molding consists of forcing a combination of a resin and a curing, or cross-linking, agent into a mold by means of pressure (Fig. 9-1). Heating and the applied pressure cause the system to cross-link, which results in thermosetting. 

An important difference between solid hydroxy- and thiol-terminated polymers is that since the latter are of relatively low molecular weight it is possible to prepare base polymers which are slightly branched (by including a trihalide in the polymerization system) but still easily handled. When the branched material is cured a cross-linked product is obtained and this has impoved compression set resistance. In contrast, if a trihalide is included in the polymerization system and the resulting polymer is not cleaved by a hydrosulphide/sulphite treatment, the product is too tough to be worked on a rubber mill. 

Elastomeric Modified Adhesives. The major characteristic of the resins discussed above is that after cure, or after polymerization, they are extremely brittie. Thus, the utility of unmodified common resins as stmctural adhesives would be very limited. Eor highly cross-linked resin systems to be usehil stmctural adhesives, they have to be modified to ensure fracture resistance. Modification can be effected by the addition of an elastomer which is soluble within the cross-linked resin. Modification of a cross-linked resin in this fashion generally decreases the glass-transition temperature but increases the resin dexibiUty, and thus increases the fracture resistance of the cured adhesive. Recendy, stmctural adhesives have been modified by elastomers which are soluble within the uncured stmctural adhesive, but then phase separate during the cure to form a two-phase system. The matrix properties are mosdy retained the glass-transition temperature is only moderately affected by the presence of the elastomer, yet the fracture resistance is substantially improved. 

In this case, the components are mixed, the pH adjusted to about 6.0 with sodium hydroxide, and the solution appHed to the textile via a pad-dry-cure treatment. The combination of urea and formaldehyde given off from the THPC further strengthens the polymer and causes a limited amount of cross-linking to the fabric. The Na2HP04 not only acts as a catalyst, but also as an additional buffer for the system. Other weak bases also have been found to be effective. The presence of urea in any flame-retardant finish tends to reduce the amount of formaldehyde released during finishing. 

The resulting bisepoxy compounds are cross-linked cold with polyamines, if necessary with added accelerators. A hot cure can either be accomphshed with amines or anhydrides (eg, phthaUc acid anhydride). If suitable initiators are present, EP systems can also be cross-linked by radiation. 

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