THERMODEXTRINIXED AND OXIDIZED RYEGRASS NANOSTARCH APPLIED IN ACRYLIC PAINT

Starches are hydrocolloids widely used in the food and non-food industry. This polysaccharide can be applied in native or modified form to meet the required techno-functional properties. The paints are matrices that use starch as a thickening agent, and the starch's morphological, rheological and structural characteristics determine the best performance of this biopolymer, highlighting the native ryegrass nanostarch. This work aimed to use thermodextrinized and oxidized ryegrass starch in acrylic paint, evaluating the formation of carbonyl and carboxyl groups, mechanical power (MP) and instrumental texture (firmness and adhesiveness) as dependent variables. The starch was extracted from ryegrass grains by the aqueous method. A Central Composite Design (P <0.10; R2 >0.70) was used as an experimental tool, with the independent variables being TDT (thermodextrinization temperature - 116 to 184 °C), ACC (active chlorine content - 0 to 10 % w/w) and MSC (modified starch concentration - 0.1 to 6 % w/w). Commercial acrylic paint was diluted with distilled water (1:1 w/w) to evaluate the performance of starch as a thickener. The carbonyl and carboxyl contents varied between 0.494 and 2.785 % and 0.025 and 2.675 %, respectively. The carbonyl content was favored by ACC (β2 = 0.33; P <0.001). The highest carboxyl contents were quantified in nanostarches subjected to the lowest TDT (β11 = –0.45; P <0.001) and by the interaction between TDT and ACC (β12 = 0.20; P <0.001). The presence of carbonyl and carboxyl groups indicates the affinity of the biopolymer with water, allowing better mixing and interaction between the ryegrass nanostarch and the paint's acrylic (synthetic) resin. The formation of carboxyls and carbonyls is desirable for the modified nanostarch. Still, high degrees of oxidation are not desirable, given the low viscosity obtained in the test with a higher degree of oxidized starch. MP presented values between 27.68 and 79.08W, being favored by MSC (β3 = 8.45; P <0.001) and interaction between TDT and MSC (β13 = 0.61; P <0.001). Firmness and adhesiveness presented values between 0.16 and 0.35 N and 0.25 to 0.45 N, respectively. It was observed that MSC in paint was the most important variable, with the linear term (β3 = 0.025; P = 0.004) for firmness and the quadratic term (β33 = –0.022; P <0.001) for adhesiveness. Firmness determines the force required for fluid displacement and directly correlates to matrix viscosity and MP. The adhesiveness is related to the interaction between the fluid particles to avoid dripping and spatter, favoring adherence to the surface on which it will be applied. The optimum point was obtained with a TDT of 144 °C, an ACC for starch oxidation of 5 % and an MSC of 6 %, resulting in high values of MP and firmness and minimum values of carboxyl groups and adhesiveness. This solution was found with a desirability degree of 74.90 %. Using biopolymers subjected to thermodextrinization and oxidation processes is advantageous for the paint industry, as they are more environmentally sustainable and add value to the starch industry.