Лако-красочные материалы — производство

The measurement of the strength of adhesion between adherend and adhesive requires a measurement of the intermolecular forces of attraction; this is not currently possible. This aspect of quality control is therefore essentially reduced to assessing the adherend surface characteristics prior to bonding, although some post-bonding simple mechanical tests are appropriate.

Weak or loose surface layers are to be avoided, as are excessive amounts of water vapour, hydrocarbons or other surface contam­inants.

Quality control activities must therefore address:

(1) removing weak or loosely bound surface layers

(2) reducing surface contamination to a minimum

(3) verifying the desired surface texture or oxide layer.

The methods used in addressing these activities may include:

Visual inspection. Simple optical methods and electron microscopy can be used for examining surfaces. The latter method can be used for assessing optimum oxide formation, where this is critical, or aluminium and metal alloys, as well as general morphological characteristics.

Wettability tests. Surface wettability may be readily assessed simply but subjectively by measurement of the contact angle. If the surface is clean, it is readily wetted and a drop of water will spread out rather than remain as a discrete droplet. This method cannot really be used to detect small variations in quality, but rather the gross effects. It does not lend itself to use on very rough or porous surfaces such as concrete.

Surface potential difference. Absorbed layers of contamination on metallic surfaces can be detected by measurement of variations in the amount of energy required for an electron to leave the surface. The Fokker Contamination Tester developed from this principle is claimed to be very useful in the optimization of various surface treatment processes and routine quality control activities (6).

Mechanical test procedures. As discussed in Chapter 4, mechanical tests of adhesion must introduce a tensile force at the adherend/adhes — ive interface. Peel tests can therefore discriminate rapidly between different surface pretreatments, or between surfaces carrying differ­ent amounts of surface contamination, particularly after environmen­tal exposure. The same is true of the fracture mechanics test specimen configuration such as the wedge cleavage test (Fig. 4.14(c)). The Boeing wedge test was developed as a cheap and sensitive method of discriminating between variations in aluminium adherend surface preparation, and the concept has been extended to other metals and even other materials. Tensile pull-off tests have traditionally been used in connnection with concrete substrates, in which a steel or aluminium dolly is bonded to a prepared concrete surface. The torque required to pull the dolly off the surface can indicate something about the surface treatment only if failure is experienced at the interface — an outcome which rarely happens.