The Curing Process

Thermosetting resins are processed by means of a change in their molecular structure, in which, under certain circumstances, the individual molecular chains can be made to link up in an irregular fashion, forming a solid infusible network. This is called curing , and will happen, in time, by normal processes (in fact, it was this phenomenon which first attracted the interest of researchers in the last century). For industrial purposes, curing (or cross-linking ) can be activated by means of special chemicals, heat, or irradiation. 

UVA is part of natural sunlight and most ordinary light bulbs will produce a small amount of UVA. Human skin will tan under UVA but at low intensities UVA is 

UVB is also found in sunlight and this has a shorter wavelength and a higher energy and thus is more aggressive than UVA. Many suntan lotions will filter out UVB. 

UVC has the highest energy and is therefore the most harmful of the UV wavelengths and can produce skin cancer. Lamps producing this wavelength should be properly shielded to protect operators. 

In addition to the three traditional UV categories, there are two further categories within the electromagnetic spectrum  

This is at the visible end of the UV spectrum (the human eye can see light in the range 400-700 nm) and is not as harmful as UVA except at high intensities. 

---

During the press operation, which is actually a form of compression mol ding, the resin-treated laminate pHes are heated under pressure and the resins cured. The initial heating phases cause the resin to melt and flow into voids in the reinforcing ply and bond the individual pHes together. The appHed heat simultaneously causes the resin to polymerize and eventually to cross-link or gel. Therefore, resin viscosity reaches a minimum during the press cycle. This is the point at which the curing process becomes dominant over the melt flow process. Dynamic mechanical and dielectric analyses (11) are excellent tools for study of this behavior. 

The relative effectiveness of nucleating agents in a polymer can be determined by measuring recrystallization exotherms of samples molded at different temperatures (105). The effect of catalyst concentration and filler content has been determined on unsaturated polyesters by using dynamic thermal techniques (124). Effects of formulation change on the heat of mbber vulcanization can be determined by dsc pressurized cells may be needed to reduce volatilization during the cure process (125). 

To make a decorated plate, the mold is opened shortly after the main charge of molding compound has been pressed iato shape, the decorative foil is laid ia the mold on top of the partially cured plate, ptinted side down, and the mold closed again to complete the curing process. The melamiae-treated foil is thus fused to the molded plate and, as with the decorative lamiaate, the overlay becomes transparent so that the ptinted design shows through yet is protected by the film of cured resia. 

At this point in the process, thermoplastic and chlorosulfonated polyethylene (CSPE) membranes are complete and are ready for packaging. In the case of ethylene—propylene—diene monomer (EPDM), the curing step occurs before the membrane is ready for packaging. The curing process is accomphshed by placing the membrane in a large vulcanizer where the material is heated under pressure to complete the cure. 

Accelerator activators are chemicals required to initiate the acceleration of the curing process. They also improve the polymer compound quaUty. 

Phenohc resins (qv), once a popular matrix material for composite materials, have in recent years been superseded by polyesters and epoxies. Nevertheless, phenohc resins stiU find considerable use in appHcations where high temperature stabiHty and fire resistance are of paramount importance. Typical examples of the use of phenoHc resins in the marine industry include internal bulkheads, decks, and certain finishings. The curing process involves significant production of water, often resulting in the formation of voids within the volume of the material. Further, the fact that phenoHcs are prone to absorb water in humid or aqueous conditions somewhat limits their widespread appHcation. PhenoHc resins are also used as the adhesive in plywood, and phenohc molding compounds have wide use in household appliances and in the automotive, aerospace, and electrical industries (12). 

Catalytic curing agents initiate resin homopolymerization, either cationic or anionic, as a consequence of using a Lewis acid or base in the curing process. The Lewis acid catalysts frequently employed are complexes of boron trifluoride with amines or ethers. 

Specific additives Catalysts organo-Sn, -Ti, -Pt, -Zn, -Rh Control of the rate of the curing process. 

Resorcinol additions just prior to use provide acceleration, though they are still not the most cost effective nor technically effective use of resorcinol. In-line addition of a PRF resin Just before application of the resin to the adherend achieves better results at less than half the cost of resorcinol chemical, in most cases and it is easier to handle. A variant of this concept is application of the resorcinol and PF resins to the substrate separately but in a way that permits them to come into contact during the curing process. Such methods are sometimes called honeymoon systems. 

The process interaction in cast plastic products is mainly involved with the curing processes and with mold filling problems. Voids and porous sections are a frequent problem with castings because the mold filling is done at atmospheric pressure, or low pressure, and if the product has thin sections to fill, the flow may be a problem. 

During the curing process, some of the nitrites are converted to nitric oxide. This combines with the myoglobin proteins in the muscle of the meat to form the deep red nitric oxide myoglobin, which causes cured meats such as ham to turn pink during the smoking process. 

---