Starch has been used for a variety of coating and adhesive applications for textiles and wood products. Its utility is due in part to its ready availability, low cost, and hydrophilic nature. Since starch-based coatings, sizings, and adhesives require water for application, they are not generally considered suitable for protective purposes because of their sensitivity to water (i.e., dissolution or swelling).
For example, allyl starch was examined extensively by Wilham et al. (J. Poly. Sci., Vol. 7, pp. 1403-1410 (1963)). His conclusions regarding use of allyl starch in coating applications were negative because the material had limited solvent solubility and was difficult to apply. The films were brittle and had poor water resistance. Hydroxyalkyl ethers are commercially available but are not useful in protective coatings due to poor water resistance. Such ethers are of low DS (0.05-0.1) since higher substitute ethers are difficult purify and filter (Kirk Othmer Vol. 21, pp. 502-505).
Starch esters such as starch acetates at low acetyl content (degree of substitution &lt;1) are useful in stabilizing aqueous starch dispersions/emulsions for food use. At higher acetyl contents (degree of substitution &gt;1) these materials yield clear films but continue to exhibit brittleness and are damaged by organic solvents. Thus, like starch ethers, starch esters would not protect substrates against organic solvents and water. Neither starch esters nor starch ethers are therefore expected to be efficacious in protective coating applications.
Because starch is relatively economical, readily available, and a renewable resource, it would be desirable to develop a starch-based coating that would protect against abrasion and have resistance to common organic solvents and water.
Protective coatings can be either thermoplastic or thermosetting. Each type has its own set of advantages. Thermoplastic coatings are easier to handle. For example, a starch or cellulose ester would be dissolved in an organic solvent and applied to the substrate in a variety of ways. The main disadvantage of thermoplastic coatings is that they are easily dissolved or swollen by solvents. Thermosetting coatings are usually much harder to handle and not all substrates can tolerate high temperatures or strict humidity conditions often needed. Despite their difficulty in handling, thermosetting coatings offer much greater resistance to abrasion and solvents.
It would thus be desirable to combine the advantages of thermoplastic and thermosetting coatings. Ease of handling and enhanced cured-film properties would accrue.
Photo-activated curing of a thermoplastic coating containing grafted ethylenic unsaturation can be used to accomplish this. In the presence of a photo-initiator, exposure to ultraviolet radiation can induce interchain reactions between pendent ethylenic unsaturated groups to yield a three dimensional polymeric network with greatly improved resistance to abrasion and solvents (see, for example, U.S. Pat. Nos. 4,839,230 and 4,565,857).
It is thus the objective of this invention to describe the preparation and use of novel photo-curable, starch ester-urethanes that can be applied from organic solvents and used to form protective coatings with enhanced solvent and scratchresistant properties.