Plasticity, adherend

Dendrimers had a lower improvement effect than hyperbranches but the concentration needed for the same effect was an order of magnitude lower. Hyperbranches showed shear strength promotion on various adherends - plastic and metal. 

Solvents promote adhesion to solvent-sensitive adherends (plastic films and reconstituted cellulose films) by swelling and partially dissolving them, thus allowing the adhesive to wet and/or penetrate the surface. Coated adherends that resist adhesion, such as printed, lacquered, or waxed papers, may also be partially dissolved by solvents to promote adhesion. (Chlorinated solvents are particularly effective on waxed surfaces). 

Thus it is possible, by using closed-form analyses of varying complexity, to predict the stresses in simple lap joints. (This approach is termed continuum mechanics.) In many instances, such solutions may be deemed acceptable. However, two problems still remain to be solved if it is required to predict the strength of real joints. These may be summarized as end effects and material non-linearity (adhesive and adherend plasticity). We will look first at end effects for linear elastic systems. 

The effects of adherend yielding were investigated by modelling the adherends with elasto-plastic properties. The adhesive maximum principal stress distributions are shown in Fig. 46 for a case where the adherend properties correspond to a relatively low strength alloy (a 0 -2% proof stress of 110 MPa) and the adhesive is linearly elastic. At a very low load of 0 01 kN the distribution is identical to that in Fig. 45, since the adherends are still elastic. Under the action of tension and bending, the adherends begin to yield at an applied load of approximately 1 5 kN. At 3 kN, the adherend plastic deformation has had two effects on the adhesive stresses. Firstly, it has led to a reduction in the peak stress concentration at the end of the overlap, at point A, over and above that for the elastic case, as a result of the enhancement of... 

So far, we have only considered parallel-sided adherends in single- and double-lap joints. It has been shown that the mathematical treatment, whether it is by closed-form analytical methods or by finite-element techniques, is difficult if realistic results are to be obtained. For instance, it is essential to allow for adhesive and adherend plasticity if joint strength predictions are to be made. But, as shown in Fig. 5, there are several forms of the lap joint in which the adherends are not parallel-sided, constant-thickness sheets but can have a variety of forms. These deviants are an attempt to reduce the high stress and strain concentrations, which occur at the ends of the simple lap joint, by modifying the stiffness of the adherends. In these profiled joints, the load line direction must change and, in addition to the tensile stiffness, so the shear and bending stiffness of the adherends change. 

Fig. 2. Schematic of energy dissipation in a commonly used peel test. The energy dissipation can occur in the adhesive and/or the adherends. The extent of energy dissipation depends on the elasto-plastic properties of the adhesive and the adherends under the test conditions as well as the local stresses and strains near the crack tip.

Solvent-Based Adhesives—In these the adhesive flows because it is dissolved in an appropriate solvent, and solidification occurs on evaporation of the solvent. Good bonds are usually formed if the solvent attacks or actually dissolves some of the plastic adherend to produce a solvent-welded bond. 

Important factors regarding incoming adherends are the chemical and physical properties of the material. This can be especially important with adhesive bonding because different metal alloys have different surface oxidation, and different elastomers and plastics can have different additives and modifiers. With elastomeric and plastic substrates, lot-to-lot differences should be tested. Often a supplier will change formulations but still be within the requirements of the specification. The difference in formulation may have a profound effect on the quality of the ultimate adhesive bond. 

Thin or relatively weak materials such as plastics, rubber, or fabrics are sandwiched between the adherends and tested. 

Due to the considerably higher heat conductivity of metals in contrast to wood or plastics, it is advisable to preheat the adherends to be bonded to the temperature of the melt in order to achieve good adhesive strengths. (Hot-air gun, where appropriate, hair-dryer at highest heat level because of its electric conductivity never use a microwave oven )... 

Owing to their limited thermal resistance, bonding of plastics, in particular of thermoplastics, requires certain precautionary measures to avoid deformation of the adherends (use hot-melt adhesives with low processing temperature, e.g., on a polyamide basis). 

Solvent-based adhesives are adhesives with polymers dissolved or pasted in organic solvents. The solvents or solvent mixtures are only processing aids and have to be removed, either partly or completely, from the applied liquid adhesive layer through evaporation or penetration prior to the fixing of the adherends. The first case is necessary for solvent-impermeable materials (metals, glass, thermosetting plastics), the second case concerns porous and solvent-permeable materials (paper, cardboard, wood, leather). This process can be accelerated by heat supply. Solvents are mainly esters, ketones, if applicable, portions of different alcohols. The total solvent portion ranges between 75-85%. 

This positive or mechanical interlocking (it is also talked of mechanical adhesion ) is indeed a possibility of joining adhesive layer and adherend. It occurs preferably in the case of very rough and/or porous surfaces, for example, of papers, cardboards, wood, ceramics or plastic foams. This concept fails in the case of smooth surfaces, however, surfaces we call smooth can actually show a mountainous structure under the microscope. But such fine roughnesses can hardly contribute to sufficient mechanical interlocking. Thus another possibility must exist which enables the adhesive layer and the adherend to be j oined firmly and permanently. 

Apart from liquids, also solids, such as metals, glasses and plastics have surface tension. Due to the stiffness of these materials, it is invisible to the eye, but metrologically determinable. Thus, with the application of the adhesive, two partners with different surface tensions are joined - depending on the material of the adherend and the adhesive. 

In practice, the temperature-time curve schematically shown in Figure 7.16 does not only depend on the adhesive-related parameters, but also on the properties of the adherends, especially on their thermal conductivity. High thermal conductivity (e.g., metals) leads to shorter heating times than low thermal conductivity, as is to be found, for example, with plastics, glasses, wood. Even the dimensions of the adherends play a role. 

Adherend combinations metal/glass, metal/plastics... 

The strength values mentioned above depend on the respective construction conditions as well as on the stress duration. An example A plastic hook fixed to a tile by means of a pressure-sensitive adhesive and exposed to stress can come off in the course of time due to a failure of the adhesive layer (creeping). In the case of dynamic stress, the hook can break inside, the bonded joint remains unaffected. In this case, the adherend is the weaker link in the strength chain . 

Materials with different thermal stressability, for example, metal-plastic, glass-plastic Application of cold-curing adhesives or those with a curing temperature corresponding to that of the more temperature-sensitive adherend. 

In addition, the adherend strength is an important influence, which in many plastics accounts for only approximately 10% of the strength of metal materials. Since, due to the chemical relatedness of plastics and adhesive layers, the same or similar strength values can be assumed, butt joints are feasible, in contrast to metals (Chapter 11, 2nd rule). 

---