Лако-красочные материалы — производство

A special case in adhesion is to some extent represented by the strong adherence that can often be observed between two surfaces of polymer materials of similar or dissimilar types that are in intimate contact with each other. Owing to their structure, physical low-energy interactions are thought to be effective here, while chemical interactions are hardly assumed to be involved. During the 1960s, Voyutskii ex­plained this phenomenon [11], his diffusion theory being based on a fact that has often been forgotten, or not been taken into account. In terms of the molecular structure of their components, polymer materials are ‘living’ entities, especially in a state of low-grade crosslinking or no crosslinking at all, rather than being static systems. Between the points of entanglement, an intense motion of the molecular chains takes place in the form of atomic rotation, at least above the glass-transition temperature, because the free volume allows them to do so, although the polymer as a material itself is in a state of stiffness and rigidity. When this movement takes place, it cannot be excluded that molecular parts leave the original surface and diffuse into an adjacent polymer.

This situation is very easy to demonstrate. Suppose you remove the protective cover of a modern piece of eraser that is made from a highly flexible plastic material to which a filler was added in order to provide for friction. You then place the rubber onto the body of a cheap ballpoint pen or (if you have one) an older fountain-pen made from celluloid. If you then leave this arrangement for about a year you will (at least in the case of the celluloid object) discover, surprisingly, that the parts have become completely welded together ‘by themselves’.

Joining well flashed-off adherents coated with contact adhesive or dispersion adhesive on both sides, respectively is most effective if, even for only a short period of time, a high contact pressure is applied. Here, the thermoplastic polymer layers with very smooth surfaces created from the liquid come into intimate contact with each other, and a seamless diffusion process between the molecules from both sides will take place (see Section 4.2). This results in a ‘green strength’ — that is, the strength of the bond is determined immediately after assembly but will increase during time (e. g. hours or days) as diffusion is a relatively slow-speed process. The same applies to pressure-sensitive adhesives characterized by high molecular mobility, as both by means of yield effects taking place on rough surfaces and by diffusion processes taking place between polymers, they build up adhesion over the course of hours, days or weeks. Ultimately, they will have created a green strength that is quite considerable from the outset, leading to an increase in peel strength of 50-70%.

It appears, therefore, that diffusion theory may be used to explain some interesting adhesion phenomena. Later in this chapter, we note that the molecular mobility which occurs in solid-state polymers, as postulated by Voyutskii, might also be very important for other types of adhesive systems (see Section 3.4).

3.2.3