Лако-красочные материалы — производство

PVA and EVA products are often sold with rather limited information. Often solids content, viscosity range, and pH are the only real specifications given. Minimum filming temperature (or in some cases, glass transition temperature) may be quoted, together with comonomer type, if any, and some brief application recommendations. Manufacturers may in production test for more properties than they publish, especially grit content, particle size, and unreacted monomer. Grit is the material retained by a standard sieve and comprises oversized particles up to beads, skins, and pieces of reactor wall fouling which have found their way through the system. Most adhesives are filtered prior to packing, but this is less easy and less important than in lower-viscosity paint grades. The high viscosity of many adhesive products makes the use of fine screens an economic impossibility because of the slow speed of filtration.

Viscosity is a property sometimes difficult to assess, as figures can be measured on any one of several types of viscometer. One common type is the rotating disk viscometer, which must be used in a container big enough to eliminate wall effects. The main alternative is the cup-and-bob viscometer, where the viscous drag of the liquid between stationary and rotating concentric cylinders is indicated by a spring-loaded pointer moving over a dial.

For use with high-speed applicators, high shear cone and plate viscometer results may be quoted as secondary information. Many poly(vinyl alcohol)-stabilized products are comparatively insensitive to shear and give broadly similar results with different types of viscometer. This behavior under shear is known as Newtonian and is a feature, inter alia, of large particles with a narrow particle size distribution. High shear viscosity testing also indicates if there is sufficient mechanical stability to allow application by knife or roller, although this is not usually a problem with colloid-stabilized emulsion polymers and adhesives.

Particle size range is often from 200 to 4500 nm or more, with one or more peaks. Multipeak distribution may indicate agglomeration at some stage of the preparation, and microscopy can be used to show if the peaks are of single particles or of an agglomerated mass of smaller ones. Freeze-thaw resistance is called for in many countries. The key here is to avoid the higher degrees of hydrolysis in the poly(vinyl alcohol) (not greater than 98% hydrolyzed), and ensure that sufficient stabilizer is present to cover the surfaces. Nonionic surfactants will act as antifreezes and suppress the freezing point and it is often of value to quote the behavior of the adhesive at —5°C and —20°C. Residual monomer should also be checked for quality control purposes and kept below the specified level. Gas-liquid chromatography is the favored method of analysis, but bromination is also widely used.

Application tests by their nature are often specific to the materials to be bonded and the application machinery in use. For packaging applications it is important that sufficient strength is generated within seconds of the bond being formed to hold the surfaces in position until the adhesive dries. Testers are available that apply a measured, standardized film to a series of kraft paper pieces. A second sheet of kraft paper (or whatever may be appropriate) is applied and the papers are peeled apart at specified time intervals. The time at which the surface of the paper is first torn off (as contrasted to the earlier tests, in which the surfaces are partially covered with adhesive) is noted. Short times are necessary for high-speed machinery. High viscosity, or suitable rheology to create resistance to parting of the adhesive layer, is obviously of value and is called wet tack or ‘‘grab.’’ As most of the initial drying is through wicking of the water in the adhesive into the paper or board to be bonded, standardization of the paper used in this test is vital. Wood bond strength is more concerned with ultimate strength and the test pieces are usually allowed to dry thoroughly for 24 to 48 h or more. Small beechwood slips giving a controlled area of overlap are generally used, although end bonding of beech dowels is an alternative. In each case, bond strength is measured using a tensiometer and it should be noted that the overlap method gives the strength to resist shearing in a direction parallel to the wood surface, while the dowel method tests the resistance at right angles to the wood. In both cases the wood should fail before the adhesive, although if the adhesive fails, the strength measured may be a function of the amount of plasticizer or softening comonomer used in the preparation.

Ethylene-vinyl acetate adhesives are used for many of the same applications as externally plasticized PVAs but have especially good performance in the field of poly(vinyl chloride) (PVC) lamination to hardboard and chipboard. Again, an adhesive is sought where the mode of failure is the cohesion of the wood. Drying is slow in this case because the PVC foils are largely impervious to water. Wicking into the substrate and vapor loss are the only modes of water removal. Hence it is essential that bonds be matured for a suitable period prior to test. Various test methods exist; the simplest uses 5-cm-wide strips of PVC foil laminated to plywood. This is dried and matured and suspended inverted with the plywood at 45° to the horizontal. Weights may be suspended from the width of the PVC, using a clamp or a firm clip to which the weights are attached. This is simply extended to high-temperature peel strengths by placing the test piece within an oven, say at 70° C. The distance that the specimen has peeled after 30 min can be measured. Weights employed are usually 350 g or 500 g, but greater weights are possible. Ninety-degree peel tests can be conducted using tensile-strength testing equipment, but they are more difficult to conduct at temperatures above ambient.