Influence of External Additives and Environmental Factors on the Strength of Adhesives
Hits: 692 img
PH Value
For some adhesives, the PH value is closely related to the pot life of the adhesive and affects the bonding strength and bonding lifespan. Strong acids and alkalis, especially when they have a significant impact on the bonding materials, are often harmful to bonding. Porous fibrous materials such as wood and paper are more vulnerable. The curing process of thermosetting phenolic resins and urea - formaldehyde resins is greatly affected by the PH value. During curing, p - toluenesulfonic acid or phosphoric acid is added to phenolic resins, and ammonium chloride or hydrochloric acid is added to urea - formaldehyde resins. In cases where a low acidity is not desired but bonding is required, it is advisable to choose neutral resorcinol - formaldehyde resins. Pretreating the wood surface with an alkali generally results in a strong joint. However, attention must be paid to the PH value of the adhesive layer, which has a greater impact on the adhesive layer than on the bonded surface.
Cross - linking
The cohesive strength of polymers increases with the increase in cross - linking density. However, if the cross - linking density is too high, the polymer will become hard and brittle, reducing its impact resistance. The strength of cross - linked polymers is closely related to the number of cross - linking points and the length of cross - linked molecules. As the number of cross - linking points increases, the cross - linking distance shortens, and the length of cross - linked molecules decreases, the cross - linked polymer becomes harder and more brittle.
Solvents and Plasticizers
The bonding strength of solvent - based adhesives is affected by the amount of residual solvent in the adhesive layer. When there is a large amount of solvent, although the wettability is good, the cohesive force of the adhesive becomes smaller, reducing the cohesive strength. When there is a strong affinity between the adhesive polymers, the bonding strength increases as the solvent evaporates. When there is no affinity between them, the adhesion of the adhesive is greater when some solvent remains, and the strength decreases as the solvent evaporates. For example, polyvinyl acetate can bond polyethylene after adding a small amount of solvent. Plasticizers act similarly to solvents. In some cases, adding an appropriate plasticizer can enable bonding even when it was not possible before. However, over time, the plasticizer will either volatilize or exude to the surface. As the amount of plasticizer decreases, the bonding strength continuously decreases. Conversely, the plasticizer in the adherend may also migrate into the adhesive layer, softening the adhesive and causing it to lose its cohesive bonding strength, or accumulate at the interface, leading to the separation of the bonding interface.
Fillers
Adding fillers to adhesives has several functions, including increasing the cohesive strength of the adhesive, adjusting the viscosity or processability (such as thixotropy), improving heat resistance, adjusting the coefficient of thermal expansion or shrinkage, increasing the fillability of gaps, imparting electrical conductivity, reducing costs, and improving other properties.
Decomposition
During use, the decomposition of adhesives is an important factor leading to a decrease in bonding strength. Water, heat, radiation, acids, alkalis, and other chemical substances can cause the decomposition of adhesives. The reaction of polymers with water to decompose is called hydrolysis. Heating can often lead to the cross - linking of polymers. The hydrolytic resistance of polymers varies depending on the chemical bonds in their molecules. Most water - soluble polymers are prone to hydrolysis. Polymers that are insoluble in water hydrolyze very slowly. The water - adsorption capacity of polymers plays an important role in hydrolysis, and the crystallinity and chain conformation of polymers also significantly affect hydrolysis. Trace amounts of acids or alkalis can accelerate the hydrolysis of some polymers. For example, polyester - type condensation resins are easily hydrolyzed when in contact with acids or alkalis. The moisture resistance of epoxy resins varies significantly depending on the type of curing agent and the use environment. Epoxy resins cured with polyamides are damaged due to the hydrolysis of amide bonds; epoxy resins cured with polyanhydrides disintegrate due to the breakage of ester bonds; polyurethanes are often damaged due to the hydrolysis of ester bonds. Polymers with ether bonds and carbon - carbon bond structures, such as phenolic resins, styrene - butadiene rubber, and nitrile - butadiene rubber, are not easily hydrolyzed and have good water resistance. Excessive heating of polymers will cause the following changes: (1) decomposition of polymer molecules; (2) continued cross - linking; (3) escape of volatile and migratory components. The results of these processes will lead to a decrease in the cohesive strength of the adhesive or a reduction in the interfacial force. At high temperatures, polymers undergo degradation and cross - linking. Degradation breaks the polymer molecular chains, reduces the molecular weight, and decreases the strength of the polymer. Cross - linking forms new chemical bonds between molecules, increases the molecular weight, and increases the strength of the polymer. However, continuous cross - linking of polymers at the bonding joint will make the polymer brittle and deteriorate the joint strength.
