Лако-красочные материалы — производство

There exist a number of sources listing the general factors involved in adhesive selection and the performance properties to consider (e. g. 2-5). It has already been emphasised that the adhesive represents only one part in a joining process which involves, among other things, a consideration of the needs of the various materials in the bonded assembly. For example, thin sheet material and polymer composite materials require either a toughened product or a flexibilised adhesive with high strain to failure in order to accommodate gross adherend strain under load. Similarly concrete adherends may benefit from being bonded with flexible and relatively low modulus products in order to reduce interfacial stress concentrations which may initiate surface fracture of the concrete. On the other hand, sustained loading may lead to excessive deformation and creep unless the adhesive is relatively unmodifed and therefore highly cross-linked, whilst also possessing a high Tg. A summary check-list of adhesive considerations is offered in Table 5.2; clearly, there exist property trade-offs.

Obviously some adhesive mechanical property data must be specified together with a detailed elaboration of the test procedures used for obtaining them, preferably over a range of conditions.

Procedures for the determination of important properties are essential for the selection of an appropriate product and for joint design and analysis. We have seen that adhesive polymers are sensitive to temperature and moisture, and to the rate at which stress is applied. Their bondline behaviour in joints depends greatly upon the system stiffness, and upon more subtle variations such as bondline thickness. It is therefore important that test methods are relevant to the real application, the fabrication conditions, the actual materials to be joined, and so on. Many standard test methods exist, for both strength and durability assessment, and these were discussed in Chapter 4.

For cold-curing epoxides wide variations in adhesive material properties are possible, with different combinations of resin, hardener, filler, and the multitude of modifiers. Products which cure at ambient temperature cannot achieve the same performance as is obtained by curing at elevated temperature. For products cured at room temperature their Tss, at 40-50 °С initially, are relatively low and may be lowered even further by absorbed water, in liquid or vapour form. This may also be accompanied by a reduction in strength and modulus. Thus the use of materials with a slow and small water uptake is to be preferred, which implies a fairly highly cross-linked formulation. Such considerations do of course depend upon the performance and durability expectations in service. Whilst the environmental durability of joints can often be improved enormously by the surface pretreatment methods employed (see Chapters 3 and 4), the adhesive must be selected carefully to ensure long term durability in consideration of the modes and duration of loading, and the environmental conditions. Ideally the adhesive should be fairly tolerant of poor surface pretreatment procedures.

The working characteristics of the adhesive relevant to the application conditions must be determined. For instance viscosity is often temperature, shear-rate and time-dependent, and this will influence the choice of dispensing equipment, the method of application, the usable life and the open time. The viscosity should therefore be regulated bearing in mind the adherend rugosity and surface pretreatment, the method and location of application, and the cure temperature and duration of application. A thixotropic material may be required for application to vertical or soffit surfaces. Generally, relatively thick bondlines are encountered so that the adhesive should be able to cure in thick and/or uneven layers. It should also be remembered that for about every 8 °С change in temperature, the reaction rate changes by a factor of two, so that the full cure time for an adhesive may vary widely depending upon ambient temperature; adequate strength may however be conferred within a fairly short time. Finally, it is usually important that the adhesive should undergo negligible shrinkage on cure (polyester — based adhesives can shrink substantially).

It is important that the adhesive manufacturer should be assessed for the quality and consistency of his products. Quality control test data should be available for each batch of adhesive, which requires the prior definition of the necessary characteristics and the means of assessing them. Adhesive materials should be packed in suitable containers ready for mixing, and each container should be durably and legibly marked with appropriate information. An instruction sheet should also accompany any delivery of adhesive, detailing information which includes product chemical type, storage con­ditions, mixing and application instructions, advice on compatible primers, curing profiles, safety instructions, and so on (e. g. see Appendix).