Experimental Characterization of Polyurethane Adhesive: Independence of Environmental and Mechanical Aging

Abstract

Abstract Assessment of the durability of elastomers for structural applications has been of much interest among the literary circles for quite some time. All polymeric materials undergo degradation when in contact with aggressive environmental agents like humidity, water, and temperature. This rate of degradation should increase with the growing intensity of the ambient environmental agents involved. On the other hand, during service life, these materials are also subjected to mechanical loading in one way or the other. The rate and extent of degradation can be accessed by scrutinizing the changes in constitutive behavior of material through mechanical and chemical properties. Accelerated thermal, hydrolytic, and hygrothermal aging are among the most common modes of process-related degradation, leading generally to chain scission, cross-linking phenomena, and reduced resistance to fracture stress and strain. In this experimental study, our goal is to separate the environmental degradation from mechanical damage. A polyurethane based (PUB) adhesive was aged in thermo-oxidative (0%RH), hydrolytic aging, and hygrothermal (80%RH) aging environments. The damage and decay mechanisms have been used to draw a distinction between environmental degradation and mechanical damage. Material characterization included cyclic tensile tests, gravimetric measurements to observe water-uptake, FTIR and scanning electron microscopy (SEM) tests on as-received and aged samples. Aging was conducted at three different temperatures (60°C, 80°C and 95°C) and four different exposure durations (1, 10, 30, and 90 days). This work confirms that environmental degradation is separable from mechanical damage. Data collected during gravimetric measurements indicate that during aging in desalinated water, water uptake increases linearly with the square root of the aging duration, which agrees with Fick’s second law. Increasing water uptake is proportional to decreasing tensile strength and toughness of the material. The total mechanical and environmental damage is a synergized effect of all exposure conditions and parameters involved i.e. aging time, temperature, oxygen, water, and humidity. The chemistry and mechanics of the polymer degradation were found to be in good agreement with each other.