Estudo do fenômeno de stress cracking nos polímeros biodegradáveis PHB e PHBV.

Abstract

This work investigated the stress cracking resistance (ESCR) of biodegradable polymers, poly (hydroxybutyrate) (PHB) and poly (hydroxybutyrate-co-valerate) (PHBV), in contact with an aqueous solution of sodium hydroxide (NaOH) as an active agent, using a concentration of 3 molar (3M). The specimens were prepared by the injection molding. Tensile tests were performed in static and dynamic conditions. During testing, the specimens were kept in contact with the NaOH and mechanical properties were monitored. In the static tensile experiments four deformation rates (0.1, 1.0, 2.0, and 5.0 mm/min) were used. Relaxation tests were conducted with tree levels of load (75%, 50%, and 25% of the maximum load determined in the static tension versus deformation tests); at the specified maximum load the active agent was applied on the surface of the test specimens and the effect of the relaxation of tension was monitored. After the tests the surfaces of the specimens were analyzed and images were recorded by digital microphotography. The results of stress versus strain tests showed an increase in the tensile strength of PHB and PHBV with the strain rates. Regarding the effect of NaOH on the surface appearance of PHB and PHBV, was verified the formation of microcracks. At lower deformation rates a reduction in strength resistance and deformation was observed, probably due to the longer contact time of the specimens with the NaOH solution. In fatigue tests a decrease in the number of cycles of specimens was verified in the presence of active agent.