Лако-красочные материалы — производство

It is easily understandable that chemical bonds formed across the adhesive-substrate interface can greatly participate to the level of adhesion between both materials. These bonds are generally considered as primary bonds in comparison with physical interactions, such as van der Waals, which are called secondary force interactions. The terms primary and secondary stem from the relative strength or bond energy of each type of interaction. The typical strength of a covalent bond, for example, is on the order of 100 to 1000 kJ/mol, whereas those of van der Waals interactions and hydrogen bonds do not exceed 50 kJ/mol. It is clear that the formation of chemical bonds depends on the reactivity of both adhesive
and substrate. Different types of primary bonds, such as ionic and covalent bonds, at various interfaces have been evidenced and reported in the literature. The most famous example concerns the bonding to brass of rubber cured with sulfur, adhesion resulting from the creation of polysulfide bonds [80]. One of the most important adhesion fields involving interfacial chemical bonds is the use of adhesion promoter molecules, generally called coupling agents, to improve the joint strength between adhesive and substrate. These species are able to react chemically on both ends, with the substrate on the one side and the polymer on the other, thus creating a chemical bridge at the interface. The coupling agents based on silane molecules are the most common type of adhesion promoters [81]. They are widely employed in systems involving glass or silica substrates, and more particularly in the case of polymer-based composites reinforced by glass fibers. In addition to the improvement in joint strength, an important enhancement of the environmental resistance of the interface, in particular to moisture, can be achieved in the presence of such coupling agents.

The influence of chemical bonds on the joint strength G, and more precisely on the intrinsic adhesion fracture energy G0, defined earlier, has been analyzed in several studies. The most relevant and elegant work in this area was performed by Gent and Ahagon [82], who have examined the effect on the adhesion of polybutadiene to glass of chemical bonds established at the interface by using silane coupling agents. In these experiments the sur­face density of interfacial covalent bonds between the glass substrate and the cross-linked elastomer was varied by treating the glass plates with different mixtures of vinyl — and ethyl — terminated silanes. Obviously, both species form siloxane bonds on the glass surface. Moreover, it was assumed that the vinylsilane can react chemically with the polybutadiene during the cross-linking treatment of this rubber, where a radical reaction is involved. On the contrary, a chemical reaction between the ethyl group of the latter silane and the elastomer is unlikely. Therefore, Gent and Ahagon [82] have shown that the intrinsic peel energy G0 increases linearly with the surface concentration of vinylsilane, in good agreement with their assumptions, and thus proved the important effect of primary bonds on adhesive strength.

Another experimental evidence of the chemical bond effect on the interfacial strength is relative to the adhesion between two sheets of cross-linked polyethylene [83]. To control the number of chemical bonds at the interface, the assemblies were prepared as follows. First, polyethylene containing 2% by weight of dicumylperoxide (DCP) was molded into sheets at rather low temperature (120°C) to prevent the decomposition of DCP. Second, partial pre-cross-linking of the two separate polymer sheets was performed at 140°C for a given time. Since the decomposition kinetics of DCP is known at this temperature, the degree of cross-linking can be varied as a function of time. Finally, assemblies of the two resulting sheets are obtained under pressure by heating at 180° C to ensure the total decomposition of DCP. Hence this technique leads to complete cross-linking in the bulk of the assembly, the mechanical properties of which therefore remain constant, whereas the surface density of interfacial bonding can be varied. In agreement with previous results obtained by Gent and Ahagon [82], a linear relationship has been established between the peel energy G and the number of bonds v per unit interfacial area, insofar as v does not exceed 1 x 1013 bonds/cm2.

More recently, in a series of papers [84-86], Brown has analyzed the improvement in adhesion between two immiscible polymers [i. e., poly(methyl methacrylate) (PMMA) and polyphenylene oxide (PPO)] by the presence of polystyrene-PMMA diblock co­polymers. Since one of the blocks is PMMA and the other is polystyrene (PS), which is totally miscible with PPO, it was reasonably expected that the copolymer organizes at the interface, due to the fact that each block dissolves in the respective homopolymer. The molecular weight of these blocks is always superior to the critical molecular weight Me, for which entanglements of chains occur in the homopolymers. Experimentally, Brown employed partially or fully deuterated copolymers in order to be able to determine the deuterium on the fracture surface after separation by secondary-ion mass spectrometry (SIMS) and forward-recoil spectroscopy (FRES) [85]. A scission of the copolymer chains near the junction point of both blocks is observed, indicating that the diblock copolymers are well organized at the interface, whatever their molecular weights, with their junction accurately located at the PMMA-PPO interface. Moreover, Brown has proposed [86] a molecular interpretation of the toughness of glassy polymers, which can also be applied to the failure of interfaces between immiscible polymers. This approach stems from the idea that the cross-tie fibrils, which exist between primary fibrils in all crazes, can transfer mechanical stress between the broken and unbroken fibrils and thus strongly affect the failure mechanics of a craze. It is based on a simple model of crack tip stress concentration. Finally, assuming that all the effectively entangled chains in the material are drawn into the fibril, the fracture energy G of a polymer is found to be directly related to the square of both the areal density v of entangled chains and the force f required to break a polymer chain:

G — v2f 2 — (25)

where D is the fibril diameter and S is the stress at the craze-bulk interface, which is assumed to be constant. Brown has considered [86] that diblock copolymer-coupled inter­faces between PMMA and PPO are ideal experimental systems for testing the validity of his model. Indeed, a linear dependence of the interfacial fracture energy G on the diblock copolymer surface density v, in logarithmic scales, is observed for copolymers of different molecular weights. A slope of 1.9 ± 0.2 was found for the master straight line in good agreement with Eq. (25). Nevertheless, it is worth noting that Brown’s results involving chain scission at the interface and leading to a dependence of G on v2 are in contradiction with both previous examples, where linear relationships between G and v are established.