Effect of Processing Conditions on Mechanical Properties of Pretreated Gelatin Samples

Abstract

The mechanical behavior of seven gelatin samples, chemically crosslinked by bifunctional agents have been studied. The latter were two aldehydes (glyoxal and glutaric aldehyde), two diepoxides (1,2,3,4-butadiene diepoxide and 1,2,7,8-diepoxyoctane) as well as three diisocyanates (1,4-diisocyanato butane, 1,6-diisocyanato hexane and 1,12-diisocyanato dodecane). The first four agents lead to both physical and chemical network in gelatin, whereas the last three-to only chemical one. Several sets of unoriented samples, room conditioned or dry, as well as annealed at various annealing temperatures and duration, were prepared. In addition, samples were drawn in a swollen state and subsequently dried at fixed ends showing draw ratios between λ = 1 and A = 9. The dependences of the universal microhardness, indentation modulus as well as the Vicker's microhardness on the annealing temperature and duration were obtained for room conditioned and dry unoriented crosslinked gelatin samples. An attempt was make to establish a relationship between the indentation modulus as obtained from universal microhardness measurements and the Young's modulus as obtained from static mechanical tests and it was found that this relationship strongly deviates from the theoretically derived one which is explained by the extremely high microhardness of gelatin. The dependence of the indentation modulus on the molecular weight between two crosslinking points for annealed room conditioned samples showed a clear minimum due to the competition between the two processes, taking place with the decrease of the network density - loosening of the network and increasing of the degree of crystallinity. For the oriented samples it was found that the drawability strongly depends on the type of networks present in the sample and the highest values were obtained for the samples, containing only chemical network and the sample, crosslinked with 1,2,3,4-butadiene diepoxide (characterized by the loosest network among the samples with both types of networks). Finally, the dependences of the Young's modulus, tensile strength and elongation at break on the draw ratio were obtained from static mechanical tests.