What factors exactly affect the strength change of adhesives?（2）
Hits: 719 img
Cross-linking
The cohesive strength of a polymer increases with the increase in the cross-linking density. However, when the cross-linking density is too high, the polymer becomes hard and brittle, thus reducing the impact resistance of the polymer. The strength of a cross-linked polymer is closely related to the number of cross-linking points and the length of the cross-linked molecules. As the number of cross-linking points increases, the cross-linking distance shortens, and the length of the cross-linked molecules shortens, the cross-linked polymer will become harder and more brittle.
Solvents and Plasticizers
The bonding strength of solvent-based adhesives is certainly 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 strength of the adhesive decreases due to the reduced cohesive force of the adhesive. When there is a strong affinity between the adhesive polymer molecules, the bonding strength increases as the solvent evaporates. When there is no affinity between them, the adhesion of the adhesive is relatively high when some solvent remains, and the strength decreases instead as the solvent evaporates. For example, polyvinyl acetate cannot bond polyethylene, but it can bond after adding a small amount of solvent. Obviously, the solvent plays a role in increasing the affinity between the two.
Plasticizers have a similar effect to solvents. Sometimes, even in situations where bonding is not possible initially, adding an appropriate amount of plasticizer can enable bonding. However, over time, the plasticizer may either evaporate or exude to the surface. As the amount of plasticizer decreases, the bonding strength continuously decreases. Conversely, sometimes 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 the plasticizer may accumulate at the interface, causing the bonding interface to separate.
Fillers
Adding fillers to adhesives has the following functions:
(1) Increasing the cohesive strength of the adhesive;
(2) Adjusting the viscosity or processability (such as thixotropy);
(3) Improving the heat resistance;
(4) Adjusting the coefficient of thermal expansion or shrinkage;
(5) Increasing the fillability of gaps;
(6) Providing electrical conductivity;
(7) Reducing the cost;
(8) Improving other properties.
Crystallinity
The polycondensation state of polymer molecules with a high degree of crystallinity is regular. If the melting point is not high, heating the crystalline polymer will make the orderly molecular arrangement within the crystalline range become disordered, and the molecules will start to transition to a molten state. Therefore, polymers with a high degree of crystallinity are suitable for use as hot melts.
Decomposition
During the use process, the decomposition of the adhesive is an important factor leading to a decrease in the bonding strength. The causes of adhesive decomposition include water, heat, irradiation, acids, alkalis, and other chemical substances. The reaction of a polymer with water to cause decomposition is called hydrolysis. Heating often may lead to the cross-linking of the polymer. The hydrolytic resistance of a polymer varies depending on the chemical bonds in its molecules. Most water-soluble polymers are prone to hydrolysis. The hydrolysis of polymers that are insoluble in water is very slow, and the water absorption ability of the polymer plays an important role in hydrolysis. The hydrolysis of polymers is also significantly affected by crystallinity and the conformation of the chains. Since trace amounts of acids or alkalis can accelerate the hydrolysis of certain polymers, polyester condensation resins are easily hydrolyzed when they come into contact with acids or alkalis. The moisture resistance of epoxy resins varies significantly depending on the type of curing agent and the usage environment. Epoxy resins cured with polyamides are damaged due to the hydrolysis of amide bonds; epoxy resins cured with polybasic acid anhydrides are disintegrated due to the breakage of ester bonds; polyurethanes are also often damaged due to the hydrolysis of ester bonds. However, polymers with ether bonds and carbon-carbon bond structures, such as phenolic resins, styrene-butadiene rubber, and nitrile rubber, are not easily hydrolyzed and have good water resistance. Products-IOTA
