What factors exactly affect the strength change of adhesives?（1）
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Polarity
Generally speaking, the polarity of the molecules of the adhesive and the adherend affects the bonding strength. However, it doesn't imply that an increase in the polarity of these molecules will necessarily enhance the bonding strength.
From the perspective of polarity, in order to improve the bonding strength, it is more advisable to change the polarity of the surface in the interface area rather than altering the polarity of all the molecules of the adhesive and the adherend. For instance, after polyethylene, polypropylene, and polytetrafluoroethylene undergo plasma surface treatment, numerous polar groups such as hydroxyl groups, carbonyl groups, or carboxyl groups are generated on their surfaces, which significantly improves their bondability.
Molecular Weight
The molecular weight (or degree of polymerization) of a polymer directly influences the intermolecular forces among polymer molecules. The magnitude of these intermolecular forces determines the melting point and boiling point of the substance. For polymers, it determines their glass transition temperature (Tg) and melting point (Tm). Therefore, whether a polymer serves as an adhesive or an adherend, its molecular weight has an impact on the bonding strength.
Generally speaking, the relationship between molecular weight and bonding strength is applicable only to linear polymers without branches, and it can be categorized into two types. In the first type, cohesive failure of the adhesive occurs across the entire range of molecular weights. In this case, the bonding strength increases as the molecular weight increases, but it remains constant once the molecular weight reaches a certain value. In the second type, the failure mode varies depending on the molecular weight. Cohesive failure occurs within the range of low molecular weights. As the molecular weight increases, the bonding strength increases. When the molecular weight reaches a certain value, the cohesive force of the adhesive is equal to the adhesive force, resulting in mixed failure. When the molecular weight increases further, the cohesive force exceeds the adhesive force, and the wettability is poor, leading to interfacial failure. As a result, the bonding strength of the adhesive reaches its maximum value at a specific molecular weight.
Side Chains
The side groups on long-chain molecules are crucial factors determining the properties of polymers. Considering the intermolecular forces, the effect of the branches of polymers is as follows: when the branches are short, increasing the branch length reduces the intermolecular forces. Once the branches reach a certain length, crystallization starts, and increasing the branch length enhances the intermolecular forces. This is likely the reason for either a decrease or an increase in the bonding strength.
pH Value
For certain adhesives, there is a relatively close relationship between their pH value and the pot life of the adhesive, which affects the bonding strength and the service life of the bond. Generally, strong acids and strong bases, especially when the acid or base has a significant impact on the bonding material, are often detrimental to bonding. In particular, fibrous materials such as porous wood and paper are more susceptible to such influences.
Since the curing process of thermosetting phenolic resins and urea-formaldehyde resins is greatly affected by the pH value, a relatively high acidity is often required. For example, during the curing process, p-toluenesulfonic acid or phosphoric acid is added to the phenolic resin, and ammonium chloride or hydrochloric acid is added to the urea-formaldehyde resin. Therefore, in situations where a large acidity is not desired but bonding is necessary, it is appropriate to select the neutral m-phenol formaldehyde resin.
Pre-treating the surface of wood with an alkali can generally result in a firm joint. However, it is also necessary to pay attention to the pH value of the adhesive layer, which has a greater impact on the adhesive layer than on the bonded surface.
