Лако-красочные материалы — производство

Viscosity — the shear stress, t, required to displace an element of fluid as shown in Fig. 2.5 is directly proportional to the shear rate D = dvldy where v = velocity. The constant of proportionality is known as the coefficient of viscosity, t|, and fully defines a Newtonian fluid (see Fig. 2.6). However most structural adhesives exhibit pseudo-plastic characteristics together with a yield stress as shown in Fig. 2.7. This implies that they have gap filling properties as well as good workability. Typical flow curves determined at a shear rate of 10“1 s and 20 °С for a range of epoxy adhesives are illustrated in Fig. 2.8(14). Apparent viscosities range from 30 to 260 Pa s and yield stresses from 0 to 70 Pa. For ease of spreading, a viscosity within the range 20 to 150 Pa s at a shear rate of 10“1 s is desirable. For spreading on vertical surfaces a yield stress of at least 20 Pa is essential. Changes in ambient temperature and the inclusion of fillers can significantly alter flow properties as illustrated in Fig. 2.9(14).

Fig. 2.5. An element in a Newtonian fluid.

Newtonian

T = r)D

Adhesives are sometimes described as thixotropic. Thixotropy is a time dependent phenomenon brought about by the breakdown of the fluid structure during shearing. This results in a reduction in resistance to flow until an equilibrium level is reached (Fig. 2.10). After shearing has ceased the structure gradually reforms and thus thixotropy is reversible. Thixotropy aids wetting of the substrate during spreading, in adhesives which otherwise require a high viscosity.

Usable life — as soon as the resin and hardener components of a cold cure product are brought together they begin to react and the cross-linking process commences. The rate of cross-linking, and therefore hardening, depends on the reactivity of the formulation and the mobility of the molecules. After a time the mix becomes

stiff and unworkable and has come to the end of its usable or ‘pot’ life. Whereas pot life defines the limit of workability, gel time is taken to be the point at which solidification commences. In some circumstances the two values are very similar but pot life is the more meaningful. It may be assessed in a number of ways, e. g.

(a) the time taken for a specified quantity of material in a specified container to reach a given temperature(15)

(b) by measurement of the time at which joints made with the mixed material no longer give satisfactory performance(16)

(c) by assessing the time at which less than a specified area of filter paper is penetrated on wetting(I7)

(d) by measuring the time to reach a viscosity above which the mixed material becomes unworkable, e. g. at 150 Pa s.(14)

(e) by monitoring the change in viscosity with time and identifying a change point at which the gradient of the curve rapidly increases(18).

Table 2.4 shows how the pot life varies between several epoxy adhesives and according to the method of assessment.

Wetting ability. — The ability of an adhesive to wet a substrate surface (and the contact angle formed between liquid and solid) is fundamental to obtaining adhesion (Chapter 3). It is feasible to measure such contact angles for a stationary drop of liquid on a solid surface but the high filler contents of many structural adhesives mean that spreading is controlled by viscous forces. Adhesives being applied towards the end of their usable life tend to lack wetting ability.

Joint open time. — This starts when the adhesive has been applied to the parts to be joined and represents the time limit during which the joint should be closed, otherwise the strength of the bond may be reduced significantly. This is a function of the degree of hardening and reaction with the atmosphere itself. Open time may be reduced by high humidity and high temperature. The exposed surface of some hardeners e. g. aliphatic polyamines, can react with moisture and carbon dioxide in the atmosphere in a way which impairs adhesion between the two components to be joined. The effect can be minimised by lightly disturbing the surface of the wet adhesive immediately prior to closing the joint.