Obtenção e caracterização de filmes poliméricos a partir de amido de milho e de gelatina recuperada de resíduos de couro curtido ao cromo (III)

Abstract

Polymers produced from renewable sources have drawn the scientific community attention due to their environmental advantages when compared to those from petroleum. Many of these polymers, such as those produced from starch and gelatin, have also the advantage of being easily degraded, which considerably reduces the impact caused by these materials when disposed. The total or partial reuse of hazardous wastes, such as chromed leather wastes (CLW), represents an environmental progress if compared to the mere disposal of these materials in hazardous wastes landfills. CLW contain protein from animal skin that can be partially extracted for further application as, for example, in fertilizer, or in the manufacture of polymeric materials. Taken that into account, in this work gelatin was extracted from CLW and used, in combination with corn starch and plasticizers, for the production of polymeric films. Gelatin was recovered through alkaline hydrolysis. The process was optimized in order to obtain a product with the highest protein/chromium ratio possible. The hydrolysis optimum condition was obtained from two 2k experimental designs. Within the limits of this study, the optimal condition was one that employed 2.0 g of MgO per 50 g of CLW and 250 mL of water in a process carried out for 6 h at 70°C with orbital agitation of 180 rpm. The gelatin, obtained in aqueous media, presented TKN content of 2186 mg/L and chromium content lower than 0.04 mg/L. The molar mass of gelatin from CLW was 140 kg.mol-1. This gelatin was then employed, with the addition of corn starch and glycerol or sorbitol, in the production of polymeric films through casting technique. When employed with corn starch for film production, gelatin from CLW exhibited a plasticizer behavior. It may have occurred due to the presence of free amino acids derived from the partial degradation of the protein polypeptide chain during hydrolysis, as well as due to the presence of salts from leather tanning process and from the hydrolysis. Glycerol proved to be a more effective plasticizer than sorbitol. Both plasticizers increased elongation at break, lowered tensile strength and made the films more hygroscopic. Thickness variation did not significantly change films mechanical properties. The increase in starch mass led to higher tensile strength values and reduced films hygroscopicity, hence reducing their solubility and monolayer moisture content. The increase in the films drying temperature made the drying process faster than the starch retrogradation process, decreasing films crystallinity and, consequently, reducing their tensile strength and increasing their elongation at break. Thermogravimetry of the films indicated that the addition of CLG gelatin to starch films increase the material’s residual mass due to high salts content of the CLW gelatin. When CLW gelatin is added to starch films it is also noticed at least two thermal events related to degradation, which may indicate a phase separation in the material. FTIR spectra indicated that the absorption bands of corn starch and CLW gelatin films are the same ones present in the films of these materials when not combined. It also indicates the presence of a system with phase separation. The chromium content in starch and CLW gelatin films did not characterize them as toxic materials. The films were applied as soil mulching, resisting to degradation for approximately 30 days. After use, if buried in the soil, they would degrade and act as nitrogen source, as fertilizers.