Cure-dependent loading rate effects on strength and stiffness of particle-reinforced thermoset polymers

Abstract

Thermoset polymers are widely used in the civil engineering field, especially as adhesives, coatings, and matrices for composite materials. During the in-situ application of these materials, thermal activation is typically not possible, resulting in an undefined curing state of the system, which depends on the reaction kinetics and ambient conditions. These resins can also react at low temperatures; however, a certain amount of time is needed to reach a fully cured state. This characteristic complicates the assessment of the mechanical properties and the prediction of long-term behavior. In this work, primarily the loading rate effect on strength is analyzed for different curing states, which are derived from relevant in-situ conditions while also the effect of stiffness is discussed. The degrees of cure have been assessed by means of DSC measurements after submitting the samples to specific curing and post-curing protocols. Two products, one epoxy-based and one vinyl ester-based, have been studied. The tensile strength of these materials was characterized at various loading rates, monitoring the strain using digital image correlation (DIC). The results reveal that the loading rate effect on strength and stiffness strongly depends on the polymer’s curing degree. Finally, simplified models able to describe the dependencies between the degree of cure of the thermosets and the rate effects on strength and stiffness are proposed as an essential element for realistic numerical simulations of incompletely cured thermosets in real world applications. • The post-curing of thermoset polymers leads to improved mechanical properties. • Epoxy with a high degree of cure presents a more ductile behavior. • The rate effect on strength and stiffness depends on the curing of the thermoset. • The loading rate effect increases with the degree of cure of the thermoset for the epoxy. • The loading rate effect decreases with post-curing treatments for the vinyl ester.