Curing, of paints

Radiation-induced modification or processing of a polymer is a relatively sophisticated method than conventional thermal and chemical processes. The radiation-induced changes in polymer materials such as plastics or elastomers provide some desirable combinations of physical and chemical properties in the end product. Radiation can be applied to various industrial processes involving polymerization, cross-linking, graft copolymerization, curing of paints and coatings, etc. 

In industry, radiation is applied both as an initiator and as a control mechanism on one hand, and as a sustainer of reactions on the other. Among the many industrial uses of radiation, one may mention food preservation, curing of paints, manufacture of wood-plastic combinations, syntheses of ethyl bromide, of ion exchange materials, of various graft copolymers, and of materials for textile finishing. In addition, there are important uses of tracers in various process industries and in mining and metallurgy. 

Current practice in the ultraviolet curing of paints, adhesives, and inks relies heavily on the use of photoinitiators of this type. An alternative procedure involves the use of photosensitizers which undergo excitation at an irradiation frequency which is not effective on the monomer or initiator of direct interest. Transfer of energy from the excited photosensitizer takes place at a frequency at which the recipient molecule can absorb and subsequently decompose. Benzophenone is a common photosensitizer in radical reactions. 

In the applied technologies, the curing of paints or inks by electron beam or ultraviolet light has... 

In the manufacture of amino resins every effort is made to recover and recycle the raw materials. However, there may be some loss of formaldehyde, methanol, or other solvent as tanks and reactors are vented. Some formaldehyde, solvents, and alcohols are also evolved in the curing of paint films and the curing of adhesives and resins appUed to textiles and paper. The amounts of material evolved in curing the resins may be small so that it may be difficult to justify the installation of complex recovery equipment. However, in the development of new resins for coatings and for treating textiles and paper, emphasis is being placed on those compositions that evolve a niinimum of by-products on curing. 

Considerable interest has been aroused recently by the possibility of instant curing of paint films by means of electron irradiation. This process has its origin in the early work of Ballantine (10) and Charlesby (11), who found that polyesters irradiated in the presence of a monomer cross-linked rapidly with the production of firm gels. The process was further developed by the Ford Motor Company by the irradiation of paint films containing approximately 65 % of the polyester and 35 % of an added monomer (12a, b,). 300-keV electrons were used at dose rates of up to 20 Mrad/min. Unlike conventional polymerization, the process was not subject to the ordinary kinetic considerations, and it was found, for example, that the rate of curing was independent of intensity up to dose rates of the order of 100 Mrad/min. Resins tailor-made to particular applications will crosslink with added monomers. Even with the present limited materials and rather costly electron accelerators, there is evidence of the commercial attraction of the process, and it is likely that developments in new resins and cheaper electron generators will lead to wide acceptability of this technique. 

The technique of determining viscosity from the velocity of a rolling ball on an inclined plate or panel has been used largely for following the flow, drying, and curing of paints and other coatings (212-215), but can also be used for other materials. The system can be calibrated with standard oils or other fluids of known... 

---