Postcured

During the vulcanization, the volatile species formed are by-products of the peroxide. Typical cure cycles are 3—8 min at 115—170°C, depending on the choice of peroxide. With most fluorosihcones (as well as other fluoroelastomers), a postcure of 4—24 h at 150—200°C is recommended to maximize long-term aging properties. This post-cure completes reactions of the side groups and results in an increased tensile strength, a higher cross-link density, and much lower compression set. 

The steps followed in the precure are repeated in the postcure process, except that after the drying step the goods are shipped to a garment manufacturer who makes garments, presses them into the desired shape with creases or pleats, and then cures the amino resin on the completed garment. 

It is important that the amino resins used in the postcure process should ( /) not react with the fabric before it has been fashioned into a garment, and (2) release a minimum amount of formaldehyde into the atmosphere, especially while the goods are in storage or during the cutting and sewing operations. These requirements are met, at present, with the diethylene glycol modified DMDHEU resin. 

Pseudothermoplastic resin systems, which are formed as conventional thermoplastic materials and then cured or postcured in a manner similar to that used for thermosetting resins to enhance high temperature properties. 

E. Chang and E. Ma22one, "A New Non-Postcure Curative Package for Polyacrylate Elastomers", paper presented atMCA BjibberDivisions Detroit, Mich., Oct. 17—20, 1989. 

E. Chang and G. E. Duim, "Injection Molding of Polyacryhc Gasketing Requiring No Postcure", SAE Technical Paper Series 900202 Detroit, Mich., Feb. 26-Mar. 2,1990. 

A new family of peroxide-cured dipolymers was introduced in 1991. The peroxide cure provides copolymers that cure faster and exhibit good compression set properties without a postcure. The removal of the cure-site has also made the polymer less susceptible to attack from amine-based additives. By varying the methyl acrylate level in the dipolymer, two offerings in this family have been synthesized, VAMAC D and its more oil-resistant... 

Mold temperatures vary between 150—200°C, depending on the mol ding methods and part size. Parts can be molded in 1.5—10 min depending on the configuration and thickness of the part, the mold temperature, and the desired state of cure at demolding. Since most ethylene—acryflc parts are postcured, it is sometimes possible to demold partly cured articles and complete vulcanization in the postcuring oven. 

Figure 6-24 Postcuring Shapes of Various Unsymmetric Laminates (After Hyer [6-37])...

Epoxy (Amine-Cured) Bisphenol A-based epoxy resins used for composite fabrication are commonly cured with multifunctional primary amines. For optimum chemical resistance these generally require a heat cure or postcure. The cured resin has good chemical resistance, particularly to basic environments, and can have good temperature resistance. 

Euran Furan resins are thermosetting polymers derived from furfuryl alcohol and Furfural. The cure must be carefully controlled to avoid the formation of blisters and delaminations. To obtain optimum strength and corrosion resistance, furan composites must undergo a postcure schedule at carefully selected temperatures depending upon the laminate thickness. Equipment made with furan resins exhibits excellent resistance to solvents and combinations of acids and solvents. These resins are not for use in strong oxidizing environments. 

After synthesis, the modified carborane-siloxane gums were fabricated into shaped components using standard siloxane vulcanization and fabrication technology. Di-chlorobenzyl peroxide (1% by wt) was used as the cross-linking agent and mixed into the polymer formed in scheme 7. Shaped rubber components were readily prepared by compression molding operations at 70°C. Postcure operations were typically at 120°C for 24 hours. 

It has been established by a variety of techniques that aromatic cyanate esters cyclotrimerize to form cross-linked cyanurate networks.1 Analogously, the fluoromethylene cyanate monomers cure to cyanurate networks. In addition to the 19F-NMR spectra shown in Figure 2.3, evidence includes an up-field shift of the methylene triplet (1H-NMR, 0.21 ppm 13C-NMR, 9.4 ppm), the disappearance of the cyanate functional group (IR, 2165 cm4 13C-NMR, 111.9ppm) and the appearance of the cyanurate functional group (IR, 1580 and 1370 cm4 13C-NMR, 173.6 ppm).9 Typically, monomers are advanced to prepolymers by thermal treatment at 120°C or just above the melting point. The prepolymers are then cured at 175°C and are postcured at 225°C. 

In addition to esterification, these oils can also be reacted with isocyanates to make polyurethanes. Some of the most interesting products to be described below consist of mixed ester-urethane compositions, where the ester portion is made before gelation where the water can be evaporated easily, and the urethane component is added as a type of postcure. 

Figure 5. Transmission electron micrograph of poly[(CO,SA,TDI)-SIN-(S,DVB)], 10/90, after being fully polymerized and postcured. Oil phase is stained dark.

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