How to Improve the Moisture Resistance of Adhesives and Sealants? Moisture resistance is a critical factor in the adhesives and sealants industry. It enhances the performance and durability across many industrial applications. When exposed to moisture, these materials can experience significant changes in their physical properties and adhesive strength. This potentially leads to joint failure. Hence, strategies for improving moisture resistance in adhesive systems are crucial. This article focuses on the bulk adhesive properties and the crucial interface between adhesive and substrate. Explore how polymer selection, chemical modifications, additives, and surface treatments can be optimized to enhance moisture resistance. Addressing the Bulk Adhesive or Sealant All polymers absorb moisture to some extent. In doing so they become plasticized by the water molecules. As a result, the bulk properties get changed. The glass transition temperature, tensile strength, and modulus lowers while elongation increases. In sealants, swelling and deformation are also noted. These properties generally recover on drying unless hydrolysis takes place. Even if the properties completely recover, the migration of moisture into the adhesive and possible preferential accumulation at the joint interface can result in loss of adhesion. To improve moisture resistance the formulator generally must operate on the bulk adhesive. This will occur mainly through modification or change in the bulk polymer and somewhat by modification or change of the fillers and additives in the formulation. The first step towards formulation or selection of a moisture-resistant adhesive is to choose a base polymer having a low diffusion coefficient and permeability to water. This helps in two ways: It reduces the rate of diffusion of moisture to the critical interphase between the substrate and the adhesive. It also reduces the effect on the bulk properties of the adhesive. There have been many investigations to determine the best chemical structures to provide resistance to moisture and hydrolysis. Attempts have been made to synthesize epoxy adhesives with improved water resistance by replacing some hydrogen atoms with fluorine.2 However, the cost and processing of such materials have restricted commercial development. For electronic sealants, it is highly desirable to keep moisture from penetrating critical areas. Hydrophobic polymers have been developed to accomplish this task. They are siloxyimides, fluorosilicones, fluoroacrylics, phenylated silicone, and silastyrene. Other things being equal, water permeates fastest through flexible polymers. Hence the moisture pick-up is generally much faster for flexible compounds than for more rigid types. Unfortunately, those polymers that provide the best resistance to ingress of moisture tend to be rigid, highly crosslinked systems. They form brittle adhesives with relatively low peel and impact strengths. Microvoids can also be formed within the polymer by clusters of water molecules, and a mechanism of damage is evident in thermoset resins in which the rigid crosslinked structure does not allow the matrix to relax after microvoid formation. There is great variation within types of adhesives because of differences in chemical linkages, formulation parameters, crosslinking density, etc. For example, the room temperature curing two-part epoxy adhesives are usually considered to have a lower level of performance than the heat-cured counterparts. However, investigators have shown that certain two-part, room-temperature curing epoxy adhesives can demonstrate excellent durability even after 12 years of tropical exposure.3,4 The performance of many two-part systems, however, can be improved by a heat treatment following room temperature cure to optimize crosslinking. Certain chemical linkages are susceptible to hydrolytic attack and, if present in an adhesive or sealant, are potential sites for irreversible reaction with water that has diffused into the joint. Such hydrolytic (chemical) degradation causes a permanent reduction in the cured physical properties. The functional groups present in the chains are hydrolyzed, resulting in both chain breaking and loss of crosslinking. Other things being equal, the rate of hydrolytic attack on any adhesive increases rapidly as the crosslinking density decreases. Epoxy resins cured with flexibilizing anhydrides, derived from long-chain aliphatic acids, will hydrolyze rapidly. Epoxy resins cured with short-chain, highly functional acid anhydrides such as methyl nadic anhydride yield a rigid network having a much lower permeability for water and a greatly reduced rate of hydrolytic attack under alkaline conditions. Additives More generally, formulation additives do not have a positive effect on moisture resistance, and they often may have a deleterious effect. Therefore, the selection of additives must go through a similar assessment process as the base polymer with respect to hydrolytic stability. Any polymeric additive used to modify the properties of the base resin should be considered for its possible effect on moisture resistance and environmental durability. Antihydrolysis agents Certain additives have been reported to improve the hydrolytic stability of polyurethanes. These additives are carbodiimides (e.g. Stabaxol® from LANXESS), especially non-leachable polycarbodiimides. The service life of polymers containing an "antihydrolysis" additive such as carbodiimide has been reported to be up to three times longer than unprotected products, even under the harshest conditions. This additive is claimed to protect a range of technically important polymers such as PUR, TPE, TPU, PET, PA and EVA from premature degradation due to hydrolysis.10 Fillers Sometimes filled adhesives will show better resistance to moisture resistance than unfilled adhesives. This is simply because incorporating inert fillers into the adhesive lowers the organic volume that can be affected by moisture. Aluminum powder seems to be particularly effective, especially on aluminum substrates. The filler can provide a reduction of shrinkage on cure, thermal expansion coefficient, and permeability to water and other penetrants. However, fillers do not always produce more durable bonds. Primers Primers tend to hinder adhesive-strength degradation in moist environments by providing corrosion protection to the adherend surface. A fluid primer that easily wets the interface presumably tends to fill in minor discontinuities on the surface. Organosilane, organotitanate, and phenolic primers have been found to improve the bond strength of many adhesive systems. Silanes and other coupling agents can be applied to various substrates or incorporated into an adhesive primer to serve as hybrid chemical bridges to increase the bonding between organic adhesive and inorganic adherend surfaces. Such bonding increases the initial bond strength and also stabilizes the interface to increase the durability of the resulting joint. However, moisture can diffuse through any polymeric primer, and eventually, it will reach the interface region of the joint. Therefore, the onset of corrosion and other degradation reactions can only be delayed by the application of a primer unless the primer contains corrosion inhibitors or it chemically reacts with the substrate to provide a completely new surface layer that provides additional protection. Chemical surface modification In considering the interface, one must contemplate not only the possibility of moisture disrupting the bond but also the possibility of corrosion of the substrate. Corrosion can quickly deteriorate the bond by providing a weak boundary layer before the adhesive or sealant is applied. Corrosion can also occur after the joint is made and, thereby, affect its durability. Mechanical abrasion or solvent cleaning can provide adhesive joints that are strong in the dry condition. However, this is not always the case when joints are exposed to water or water vapor. Resistance to water is much improved if metal surfaces can be treated with a protective coating before being bonded. Approaches for the development of water-resistant surface treatments include the application of inhibitors to retard the hydration of oxides or the development of highly crystalline oxides as opposed to more amorphous oxides. Standard chemical etching procedures, which remove surface flaws, also result in improved resistance to high humidity. A number of techniques have been developed to convert corrosion-prone clean surfaces to less reactive ones. Three common conversion processes are: Conclusion Resistance of adhesive joints to moisture can be improved either by preventing water from reaching the interface or by improving the durability of the interface itself. Several methods of minimizing degradation are possible. These include: Proper selection of a base polymer having a low water permeability and diffusion coefficient. Chemically modifying the adhesive or sealant to reduce water permeation. Incorporating inert fillers into the adhesive to lower the volume that can be influenced by moisture. Coating the exposed edges of the joint with very low permeability resins. Using primers or chemically treating the substrate surface to improve adhesion. This will thus protect the interface from the intrusion of water. Chemically alter the substrate surface prior to bonding. This will provide for better adhesion and corrosion protection. https://adhesives.specialchem.com/tech-library/article/improving-the-moisture-resistance-of-adhesives-and-sealants-part-ii