Лако-красочные материалы — производство

It is now well known that alterations and modifications of the adhesive and/or adherend can be found in the vicinity of the interface leading to the formation of an interfacial zone exhibiting properties (or properties gradient) that differ from those of the bulk materials. The first approach to this problem is due to Bikerman [24], who stated that the cohesive strength of a weak boundary layer (WBL) can always be considered as the main factor in determining the level of adhesion, even when the failure appears to be interfacial. According to this assumption, the adhesion energy G is always equal to the cohesive energy Gc(WBL) of the weaker interfacial layer. This theory is based primarily on prob­ability considerations showing that the fracture should never propagate only along the adhesive-substrate interface for pure statistical reasons and that cohesive failure within the weaker material near the interface is a more favorable event. Therefore, Bikerman has proposed several types of WBLs, such as those resulting from the presence at the interface of impurities or short polymer chains.

Two main criticisms against the WBL argument can be invoked. First there is much experimental evidence which shows clearly that purely interfacial failure does occur for many different systems. Second, although the failure is cohesive in the vicinity of the interface in at least one of the materials in contact, this cannot neces­sarily be attributed to the existence of a WBL. According to several authors [25,26], the stress distribution in the materials and the stress concentration near the crack tip certainly imply that the failure must propagate very close to the interface, but not at the interface.

However, the creation of interfacial layers has received much attention in recent years and has led to the concept of ‘‘thick interface’’ or ‘‘interphase,’’ widely used in adhesion science [27]. Such interphases are formed whatever the nature of both adhesive and substrate, their thickness being between the molecular level (a few angstroms or nanometers) and the microscopic scale (a few micrometers or more). Many physical, physicochemical, and chemical phenomena are responsible for the formation of such interphases, as shown from examples taken from our own recent work [28]:

1. The orientation of chemical groups or the overconcentration of chain ends to minimize the free energy of the interface [29]

2. Migration toward the interface of additives or low-molecular-weight fraction [30]

3. The growth of a transcrystalline structure, for example, when the substrate acts as a nucleating agent [31]

4. Formation of a pseudoglassy zone resulting from a reduction in chain mobility through strong interactions with the substrate [32]

5. Modification of the thermodynamics and/or kinetics of the polymerization or cross-linking reaction at the interface through preferential adsorption of reac­tion species or catalytic effects [33,34].

It is clear that the presence of such interphases can strongly alter the strength of multicomponent materials and that the properties of these layers must not be ignored in the analysis of adhesion measurement data. A complete understanding of adhesion, allow­ing performance prediction, must take into account potential formation of these boundary layers.