The shrinkage rate of nail-free glue during curing is one of the core factors affecting its adhesive performance. This process involves volume changes caused by molecular restructuring as the glue transforms from a liquid or paste to a solid state. Improper control of the shrinkage rate can lead to various adhesive failures.
Interfacial damage caused by shrinkage stress is the primary risk. When nail-free glue cures, the molecular chains form a three-dimensional network structure through cross-linking reactions, and the shortening of interatomic distances causes volume shrinkage. If the adhesive layer is firmly adhered to the substrate, this shrinkage is converted into shear stress, which is concentrated at the adhesive interface. When the stress exceeds the adhesion between the adhesive layer and the substrate, delamination occurs. For example, in the bonding of metal and glass, if the glue shrinkage rate is too high, the difference in thermal expansion coefficients of the metal substrate may further exacerbate stress concentration, leading to edge lifting or overall peeling.
Substrate deformation and dimensional accuracy loss are also closely related to the shrinkage rate. For thin or flexible substrates, such as plastic sheets, films, or lightweight alloys, the shrinkage stress of the glue may force irreversible deformation of the substrate. In electronic component packaging, excessive shrinkage of nail-free glue can cause precision circuit boards to bend, leading to broken component leads or poor contact. Furthermore, in optical device bonding, shrinkage-induced deformation can alter the optical path, causing blurred images or reduced light source efficiency.
The formation of internal defects is also a concern. During shrinkage, if impurities, air bubbles, or unevenly cured areas exist within the glue layer, stress will preferentially release at these vulnerable points, forming microcracks. Over time, these cracks may expand into macroscopic fissures, leading to seal failure or a sharp drop in mechanical strength. For example, in waterproof sealing applications, cracks can become channels for moisture penetration, causing short circuits or corrosion problems.
The shrinkage rates of different types of nail-free glue vary significantly, directly affecting their applicable scenarios. Epoxy resin-based nail-free glues, due to their ring-opening polymerization characteristics, have lower shrinkage rates and are suitable for high-precision bonding; while acrylic resin-based nail-free glues, due to their double-bond addition polymerization, have higher shrinkage rates, but these can be partially reduced through modification techniques. Polyurethane nail-free glues achieve a balance between shrinkage and elasticity by adjusting the flexibility of the molecular chains, making them suitable for dynamic load scenarios.
The curing process is crucial for controlling shrinkage. While rapid curing improves efficiency, it doesn't allow enough time for molecular chains to rearrange, leading to stress concentration during shrinkage. For example, excessively high UV-cured nail-free glue may crack due to rapid curing; however, staged curing (such as low-temperature pre-curing followed by high-temperature complete curing) allows the molecular chains to gradually arrange themselves in an orderly manner, significantly reducing shrinkage. Furthermore, adding inorganic fillers (such as nano-silica) or toughening agents can release stress through physical filling or flexible chain segment movement, further optimizing shrinkage performance.
Environmental factors and substrate characteristics must also be considered. High temperatures accelerate glue curing, potentially amplifying the shrinkage effect; while low temperatures may result in residual stress due to incomplete curing. For porous substrates (such as wood and gypsum board), due to differences in adhesive absorption, nail-free glue with matching shrinkage rates must be selected to avoid localized stress concentration. For dense substrates (such as metal and glass), it is crucial to consider the compatibility of the adhesive layer's and substrate's coefficients of thermal expansion to prevent additional stress caused by temperature changes.
The shrinkage rate of nail-free glue is a key parameter in adhesive reliability design. Through material selection, process optimization, and environmental adaptation, a balance between shrinkage rate and adhesive performance can be achieved, thereby meeting the stringent requirements for strength, precision, and durability in various scenarios.
