Low-modulus adhesives

It is necessary that the adhesive retain some resiliency if the thermal expansion coefficients of the adhesive and adherend cannot be closely matched. At room temperature, a standard low-modulus adhesive may readily relieve stress concentration by deformation. At cryogenic temperatures, however, the modulus of elasticity may increase to a point where the adhesive can no longer effectively release the concentrated stresses. At low service temperatures, the difference in thermal expansion is very important, especially since the elastic modulus of the adhesive generally decreases with falling temperature. 

Most conventional low-modulus adhesives and sealants, such as polysulfides, flexible epoxies, silicones, polyurethanes, and toughened acrylics, are flexible enough for use at intermediate low temperatures such as -40°C. Low-temperature properties of common structural adhesives used for applications down to -129°C are illustrated in Fig. 15.9, and the characteristics of these adhesives are summarized in Table 15.12. 

The width of the lap shear specimen is generally 1 in. The recommended length of overlap, for metal substrates of 0.064-in thickness, is 0.5 + 0.05 in however, it is recommended that the overlap length be chosen so that the yield point of the substrate is not exceeded. In lap shear specimens, an optimum adhesive thickness exists. For maximum bond strengths, the optimum thickness varies with adhesives of different moduli (from about 2 mils for high-modulus adhesives to about 6 mils for low-modulus adhesives).5... 

Thermal interface materials are slurries of thermally conductive particles, usually diamond or metallic oxides, suspended in liquid or low modulus adhesive. They are designed to be flexible or slightly fluid to maintain good thermal contact during temperature cycling. 

Stresses in adhesive-attached die become critical as the size of the die increases and as dissimilar adherends are used. Silicon die, smaller than 100-mil square, attached with high-modulus adhesives to leadframes or to other substrates have been quite reliable even when large differences in the CTEs of the adherends exist. However, for large devices (greater than 300-mil square) low-modulus adhesives are necessary to absorb or dissipate stresses. ... 

Low-modulus adhesives, on the other hand, have low deflections, low bending stresses, and high radii of curvature. For example, silicones and other elastomeric adhesives have low deflections of approximately 0.01 pm and radii of curvature of over 125,000 mm. 

Take a very rigid material, like glass, and bond two pieces together using a very low modulus adhesive. The low modulus will encourage any stress to be distributed uniformly over the bond area. The objective in structural bonding is to transfer and distribute stress and strain in such a way that the integrity of the structure is maintained. 

The selection of a particular adhesive type is for reasons other than modulus and associated stress distrihution. This might be for example, specific adhesion, chemical resistance, setting speed, gap fill capability, durability, heat resistance, fire performance, electrical properties, thermal conductivity, colour, toxicity or price. Low modulus adhesives are used very successfully in low stress applications to accommodate differential thermal expansion in applications like bonding glass to aluminium in double glazing assembly. 

The plastic components must be installed without introducing localised stresses and adequate provision must be made to accommodate movement resulting from thermal expansion (thick-layer bonding using a low-modulus adhesive). 

Low strength, low-modulus adhesive sealants are used in building construction waterproofing applications where maximum extensibility is required in expansion joints to accommodate movement. They maintain the seal by stretching easily without losing adhesion to the substrates. 

In the case of bonded high modulus composites, failure at low load levels due to interply delamination within the composites is a common problem. The use of low modulus adhesives minimizes this problem. 

Mixed modulus joints have been proposed in the past to improve the stress distribution and increase the joint strength of high modulus adhesives. The stiff, brittle adhesive should be in the middle of the overlap, while a low modulus adhesive is applied at the edges prone to stress concentrations. Sancaktar and Kumar (2000) used rubber particles to toughen the part of the adhesive located at the ends of the overlap and increase the joint strength. The concept was studied with the FE method and proved experimentally. Fires et al. (2003) also proved with an FE analysis and experimentally with two different adhesives that the mixed adhesive method gives an improvement in joint performance. Temiz (2006) used an FE analysis to study the influence of two adhesives in double-lap joints under bending and found that the technique... 

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