Chemical curing

The outstanding performance characteristics of the resins are conveyed by the bisphenol A moiety (toughness, rigidity, and elevated temperature performance), the ether linkages (chemical resistance), and the hydroxyl and epoxy groups (adhesive properties and formulation latitude, or reactivity with a wide variety of chemical curing agents) (see also Phenolic resins). 

In order to increase the capacity of a production line especially by reducing the necessary press times, adhesive resins with a reactivity as high as possible should be used. This includes two parameters (1) a short gelation time and (2) a rapid bond strength increase, and this even at a low degree of chemical curing. 

Thermosetting epoxy and polyurethane chemically-cured liquid resins can provide, among other characteristics, superior abrasion resistance coatings. Solvent-free formulation applied by hot spray techniques can achieve film thicknesses of up to 5 mm. 

In the past chemical cure linings have been employed on a wide scale. These linings, usually based on natural rubber or acrylonitrile-butadiene rubber consist of a standard lining compound with a chemical activator such as dibenzylamine incorporated in the formulation. Prior to the application of the lining to the substrate, the individual sheets of rubber are dipped or brush coated with carbon disulphide or a solution of a xanthogen disulphide in a solvent. The carbon disulphide or xanthogen disulphide permeates the rubber and combines with the dibenzylamine to form an ultra-fast dithiocar-bamate accelerator in situ, and thus the rubber rapidly vulcanises at ambient temperature. 

Such chemical cure methods have been used for many years but, due to the inherent hazards of flammability and toxicity of the system, are not accepted in several countries. Also, if all the exposed working surfaces have not been properly treated, unvulcanised areas may occur which would rapidly deteriorate when the lining is put into service. 

Even though potential memory exists in all TPs, polyolefins, neoprenes, silicones, and other cross-linkable TPs are example of plastics that can be given memory either by radiation or by chemically curing. Fluorocarbons, however, need no such curing. When this phenomenon of memory is applied to fluorocarbons such as TFE, FEP, ETFE, ECTFE, CITE, and PVF2, interesting high-temperature or wear-resistant applications become possible. 

The physical properties of radiation and peroxide cured polysiloxanes have been compared by several investigators [402]. Vinyl-substituted (0.14 mol%) radiation-cured polysiloxane is found to have better strength properties than the chemically cured analogue. The phenyl substitution (7.5 mol%) has only marginal effect. The physical properties of radiation and thermally cured sUica-filled polyvinylethyldimethylsiloxane compounds are found to be similar. 

In chemically-cured materials, one example of an initiator/activator system is hydrogen peroxide as initiator, ascorbic acid as activator and cupric sulphate as co-activator. In light-cured materials, camphorquinone is used as a visible-light photochemical initiator, sodium p-toluene-sulphinate as activator and ethyl 4-dimethylaminobenzoate as photoaccelerator. 

The Myth of the Chemical Cure, Basingstoke Palgrave Macmillan,... 

Note 1 Curing is achieved by the induction of chemical reactions which might or might not require mixing with a chemical curing agent. 

In 1939, Rene Dubos, Waksman s former postdoctoral student, extracted two chemicals, tyrocidine and gramicidin, from the soil germ Bacillus brevis. These chemicals cured bacterial infections in cattle but were too toxic for humans. This discovery prompted a number of scientists to expand the search for microbes in the soil, microbes capable of making chemicals that could kill disease-causing bacteria in humans. 

Compared with chemical cross-linking of PE, radiation curing produces a different product in many respects. The chemical cross-linking is done at temperatures near 125°C (257°F), where the polymer is in the molten state. Consequently, the cross-link density in the chemically cross-linked polyethylene is almost uniformly distributed, while there are relatively few cross-links in the crystalline fraction of the radiation cross-linked PE. The crystalline fraction of the radiation-processed polyethylene is greater than that in the chemically cured product. ... 

The retention of the maximum tensile strength at elevated temperatures is greater for radiation cured than for chemically cured natural rubber. The physical properties after high-temperature aging are not improved, however. Lower flex life and higher abrasion resistance of radiation cross-linked NR were reported. Other properties, such as permanent set, hardness, and resilience, were found to be nearly equal. 

Irradiation of carbon-black-reinforced polychloroprene compounds produced a maximum tensile strength of 20 MPa (2,900 psi) at a dose of 20 Mrad (200 kGy), which is a value obtained typically from chemically cured compounds. The addition of 20 phr of N,N -hexamethylene-bis methacrylamide as a prorad in the above compound produced a tensile strength of 18 MPa (2,610 psi) at a dose of 7 Mrad (70 kGy). Further addition of 6 phr of hexachlo-roethane caused the deterioration of the tensile strength by 50% at the 7 Mrad (70 kGy) dose. ... 

Compared to the chemically cured adhesive systems in common use today, this enzymatic binder has certain advantages ... 

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