Assessment and Non-Destructive Evaluation of the Influence of Residual Solvent on a Two-Part Epoxy-Based Adhesive Using Ultrasonics

Abstract

Polymers are increasingly being used in higher demanding applications due to their ability to tailor the properties of structures while allowing for a weight and cost reduction. Solvents play an important role in the manufacture of polymeric structures since they allow for a reduction in the polymer’s viscosity or assist with the dispersion of fillers into the polymer matrix. However, the incorrect removal of the solvent affects both the physical and chemical properties of polymeric materials. The presence of residual solvent can also negatively affect the curing kinetics and the final quality of polymers. Destructive testing is mainly performed to characterize the properties of these materials. However, this type of testing involves using lab-type equipment that cannot be taken in-field to perform in situ testing and requires a specific sample preparation. Here, a method is presented to non-destructively evaluate the curing process and final viscoelastic properties of polymeric materials using ultrasonics. In this study, changes in longitudinal sound speed were detected during the curing of an aerospace epoxy adhesive as a result of variations in polymer chemistry. To simulate the presence of residual solvent, samples containing different weight percentages of isopropyl alcohol were manufactured and tested using ultrasonics. Thermogravimetric analysis was used to show changes in the decomposition of the adhesive due to the presence of IPA within the polymer structure. Adding 2, 4, and 6 wt.% of IPA decreased the adhesive’s lap shear strength by 40, 58, and 71%, respectively. Ultrasonics were used to show how the solvent influenced the curing process and the final sound speed of the adhesive. Young’s modulus and Poisson’s ratio were determined using both the longitudinal and shear sound speeds of the adhesive. Using ultrasonics has the potential to non-invasively characterize the quality of polymers in both an in-field and manufacturing settings, ensuring their reliability during use in demanding applications.

Electromechanical Properties of Smart Vitrimers Reinforced with Carbon Nanotubes for SHM Applications

“…Although the repair phenomena of these systems have been demonstrated in previous investigations [11], they cannot be categorized as smart materials since, by definition [12], they should react and respond to structural changes (e.g., damages) autonomously [13] and be assessed correctly and in time without reducing the safety and reliability of the structure to be addressed. Therefore, the development of Non-Destructive Testing (NDT) techniques is necessary to enhance the quality of the structures prior to and after their manufacturing [14]. Nevertheless, conventional NDT techniques do not offer continuous information on the health of a structure because they are performed in a particular period and not continuously [15].…”

Recyclable Multifunctional Nanocomposites Based on Carbon Nanotube Reinforced Vitrimers with Shape Memory and Joule Heating Capabilities

“…Epoxy resins, one of the most used thermosets, have widespread industrial applications such as coatings, adhesives, electronic encapsulants, and matrices for advanced composites [1] due to their excellent mechanical performance, dimension stability, adhesive capacity, and chemical resistance, among others [2]. The global epoxy resin demand will grow continuously by 4-6% for the next five years [3], to an estimated market of $37.3 billion by 2025 [4].…”

“…These two options imply further environmental degradation and improper utilization of resources, thus new material science research is required to produce novel epoxy-based formulations capable of retaining their properties while being repairable, recyclable, and/or reprocessable more sustainably. To achieve these goals, different strategies have been used in recent years based on covalent adaptable networks (CANs) [1,6,[8][9][10]. The use of CANs allows combining the traditional thermoset properties and the recyclable, re-shapable, and reprocessable attributes of thermoplastics [10].…”

“…The use of CANs derived from epoxy monomers has demonstrated remarkable potential as matrices for micrometric and nanometric reinforcements [1,6,9]. The use of these reinforcements into epoxy matrices leads to significant improvements in mechanical, electrical, or thermal properties [11][12][13][14][15], as well as to improve or achieve other functionalities such as Joule heating, self-healing, or shape memory [11,[16][17][18][19].…”