Лако-красочные материалы — производство

As with any other technical component or material, the most frequently asked question about adhesive bonds concerns its strength, defined as the maximum stress or strain under which the bond will fail or be destroyed. The ultimate tensile strength of adhesive joints is determined almost exclusively in tensile shear tests using single­lap shear specimens that are easily manufactured (see Section 7.2.2). Butt joints are sensibly not used because the adhesives have a relatively low strength under tensile load, particularly with high-strength materials. The mean breaking strength, defined as the ratio ofthe failure load to the bonded area, can be determined by means ofthe tensile shear test.

In the examples shown in Figure 7.27, mild steel (St 37) was alternatively treated with the following surface preparation methods: degreasing with acetone in an ultrasonic bath (US); blasting with shot blast glass (SBG) or grit blast glass (GBG); shot blast ceramic (SBC); grit blast corundum (GBK); shot blast steel (SBS); or grit blast steel (GBS). It is easily recognized that different variants of a surface preparation method alone can have an influence on the strength behavior, at least with certain

adhesives, although the grain size of all blast compounds was within a range of 100 to 250 pm.

The adhesives used in this investigation were Ciba-Geigy AW 106/HV953U, Degussa Agomet U 4, and Kommerling Korapur 666. Although these adhesives each have a different chemical base, they have comparable viscosity (so that wetting failure will not occur, despite the surfaces being very rough). After curing, however, they have different strength and failure patterns, respectively. In the unaged condi­tion, the epoxy system AW 106 had tensile shear strength values >20 N mm2 and displayed a cohesive failure pattern near the adhesive-substrate interface (surface near cohesive failure; SCF). Purely cohesive failure was not expected to occur. Agomet U 4 (acrylate adhesive) and Korapur 666 (polyurethane adhesive), which were used for comparison, each had a considerably lower inherent strength. In the unaged condition, due to a high content of fillers, these adhesives generally had cohesive failure patterns, as illustrated in Table 7.1. The effect of blasting with different compounds compared to reference values obtained with acetone degreasing (with ultrasound) is also indicated (as relative response).

It was also shown that the type of blast compound used influenced the tensile shear strength of the bond only in the case of the epoxy resin adhesive system. Due to a cohesive failure pattern near the adhesive-substrate interface, failure occurred within the zone of influence of the pretreated surface. The tensile shear strength values measured in this test run changed depending on the blast compound used — that is, the overall behavior of the bond was dominated by the behavior of the substrate-adhesive interface. The tensile shear strength was considerably impaired by the SBG surface preparation, but when using GBG, SBC, SBS or GBS it was considerably increased. Compressed air blasting with GBK slightly reduced the tensile shear strength. In the current literature [9], the mean arithmetic roughness Ra

7.7 Applications of Test Methods in Structural Adhesive Bonding Table 7.1 Characteristics of fracture surfaces of steel bondings.

(the surface preparation methods are classified according to the increase in rough­ness induced) was reported to have an influence on the tensile shear strength of steel samples bonded with epoxy resin adhesive. However, this effect was not recognized in the test run described in Table 7.1.

Within the standard deviation, those samples bonded with polyurethane and acrylate adhesives did not respond to the very intense surface preparation methods. These commercial adhesives have a high filler content, and therefore the polymer-substrate interface has a higher strength compared to the bulk; hence, cohesive failure will occur outside the zone of influence of the blasted surface. Consequently, whilst it is not possible to evaluate the effect ofthese different surface preparation methods, it should be noted that this behavior may drastically change as soon as these bondings are exposed to humidity (see Section 7.7.7).

It has become clear that whilst these simple tensile shear tests are adequate for determining certain parameters, they barely apply to different bond dimensions or surface conditions of other adherent materials. In general, values of ultimate tensile strength are not suitable for the dimensioning of components because, in simple terms, a component should not be evaluated by the maximum load under which it breaks. However, the results of tensile shear tests do provide useful indications for the evaluation of adhesive bonds, provided that only one influencing parameter is changed while all other parameters (adherent, surface condition, sample dimen­sions) are maintained.

This may be illustrated with a completely different adhesive system. The data in Figure 7.28 show the relatively low tensile shear strength of a pressure-sensitive adhesive (PSA) setting by UV irradiation, where the degree of crosslinking is a function of the exposure dose. As shown, an increase in crosslinking results in a considerable increase in shear strength, but this is limited by the chemical reactivity of the adhesive [10].

First, as can be seen in Figure 7.28, the absolute load-bearing capacity of this PSA (and others) is considerably less than that of structural adhesives. Furthermore, it becomes clear that the load capacity can be increased with a rise in UV irradiation intensity. In this adhesive system, as long as it is exposed to UV radiation, crosslinking reactions are induced in the formerly thermoplastic adhesive via chemically incorporated photoinitiators (see Section 5.7.2.6). However, the use of this effect to provide an overall assessment of the adhesive is limited because a higher degree of crosslinking may induce modifications of its other properties (see Section 7.7.3).

The tensile shear test is also suitable for the comparison of the temperature resistance of adhesives in defined assemblies (Figure 7.29) [11].

Figure 7.29 provides an overview of the general temperature behavior of structural adhesives. It is well known that the ultimate tensile strength values are also contingent upon the stress conditions prevailing in the bond-line, which in turn depend upon the plastic deformability of the adhesive. Possibilities for the plastification of high-temperature-resistant adhesives are limited, and therefore when performing tensile shear tests, low ultimate tensile strength values will be obtained.

As illustrated in Figure 7.29, a strength maximum occurred within a temperature range between 0 °C and 100 °C for almost all adhesives. This was followed by a decrease, with both events being recorded within the Tg ranges of the adhesives.

Tensile shear tests are also well suited to the evaluation ofdeformation behavior of adhesives. Displacement ofthe adherents during the test can be measured using fine — strain extensometers; the typical shear stress-shear strain curves shown in Figure 7.30 may be very useful in the evaluation of adhesives [12]. In this example,

5 Stainless steel
-50	250
200	30C

Test temperature ( C)

1-6: hot bonded together curing adhesives

8: cold bonded together curing adhesives

Material: Al Cu Mg 2 clad

Overlap length: 12 mm

Figure 7.29 Tensile shear strength of metal bondings as a function of temperature.

high-strength adhesives were used to join 6 mm-thick adherents with a single overlap length of 5 mm (to provide for a uniform stress distribution). The data in Figure 7.30 show that, up to a shearing of 0.1, there was a near-linear shear stress-shear strain behavior. At a higher shear deformation, the stress built up at a lower rate, which indicates that plastic deformation had occurred. Plastic deformation, however, may only be detected by means of the residual shear deformation following relief of the load (not shown here). This allows local peak stresses to be prevented, for example in single lap joints at the ends of the overlaps.

Figure 7.30 Shear stress-shear strain curves (EN ISO 11003-2) of hot-setting structural adhesives.

It should be noted, however, that the deformation rules of organic polymers only apply to a maximum deformation of 1-2% of the total deformation capacity. Higher deformations may induce irreversible damage, the degree of which has not yet been determined. Therefore, the deformations should not be greater than the shearing tangent values displayed in Figure 7.30 (up to 0.1).

To summarize, simple tensile shear tests with short-term loading are an efficient, but rather limited — test method for the general evaluation ofadhesives and adhesive bonds.

7.7.2