Review on the effect of moisture and contamination on the interfacial properties of adhesive joints

Abstract

Adhesives play an important role in many key industrial sectors, such as the automotive industry, enabling the construction of lightweight, multi-material structures, combining polymers, composites and metals. However, adhesives are usually polymeric materials, which can be affected by environmental and working conditions, such as moisture and contamination. Although moisture and contamination degrade the adhesive, the failure of a bonded joint is often ultimately interfacial. Therefore, a literature review on the influence of those factors on the interfacial properties of adhesive joints is performed to understand the phenomena that take part in the degradation on adhesive joints when subjected to humid and contaminated environments, which can oftentimes be the case in factory conditions, especially for parts from third-party suppliers. The mechanisms and effects of moisture aging and contamination are presented, as well as experimental testing methods and practical case studies. It is concluded that both moisture and other contaminants may lead to a reduction in joint strength and catastrophic adhesive failure. Moisture absorption can occur through the adhesive, but in an adhesive joint, it can additionally occur through the substrate, the interface between the adhesive and the substrate or in the cracks and pores of the adhesive. After water ingresses into the adhesive, it decreases its mechanical properties and plasticizes it. However, in an adhesive joint, the water diffusion occurs much faster than in bulk adhesive due to the complementary diffusion paths, which typically leads to adhesive failure at the adhesive/substrate interface. Additionally, in an adhesive joint, water may induce the hydrolysis of the substrates or have other chemical interactions with them. Contaminants can diffuse through the joint or remain at the adhesive/ substrate interface. When they diffuse through the joint, they have consequences similar to those of water sorption. However, when they remain at the interface, they can produce locally debonded areas, which may lead to joint failure.

A review of the relationship between design factors and environmental agents regarding adhesive bonded joints

“…As already seen, the interaction with water in various ways alters the Tg. Generally, water absorption tends to reduce the Tg, as observed by several authors [136,[138][139][140]. This behavior of Tg with the percentage of absorbed moisture can be seen in Fig.…”

“…When the absorbed moisture in the adhesive is increased, the tensile strength of the adhesive declined. This behavior is probably related to the reduction of Tg and softening of the adhesive, which increases the ductility of the material, in addition to swelling, which can cause stresses and microcracks [4,101,136]. The rate at which the reduction occurs depends on other factors related to exposure to humidity, such as time, temperature and other environmental conditions.…”

“…Thus, understanding of the influence of water absorption on adhesive joints allows predicting the failure modes and thus adopting measures to improve their performance. Many studies have focused on the degradation caused by moisture in adhesive bonded joints [136,[144][145][146][147][148].…”

Effect of Water Ingress on the Mechanical and Chemical Properties of Polybutylene Terephthalate Reinforced with Glass Fibers

“…However, since these materials are still under development, further research is needed. Moisture generally promotes degradation of the polymer, which can happen even when subjected to usual atmospheric humidity [ 6 , 7 ]; therefore, it is crucial to understand how the properties of composite materials change when exposed to water.…”

“…Free water occupies the free spaces on the structure of the material, the polymeric chain, which means it does not change the volume of the material. Bound water can increase the volume of the material, as it forms chemical bonds with the polymer’s molecular chain [ 3 , 6 , 8 , 9 ]. The bound water is also responsible for the decrease in glass transition temperature, .…”