Volume 4 - Loctite.ph

Shortening bond overlaps reduces both the bending and the differentialshearing effects which are present in lap joints (see table on page 80).If larger bond areas are needed to carry the load, it is better to increasebond width rather than bond overlap. Increased width adds very little tostress distribution in the bond.Thicker bondlines make the joint more compliant to shear stress. Theextra thickness spreads the shear strain over a larger dimension,resulting in less unit strain on the adhesive and, therefore, less stressconcentration (see table on page 80). This is similar to using a lowermodulus adhesive; a more compliant joint results in both cases.By using a combination of the methods described previously, stressconcentrations in plastic lap joints can be reduced to levels comparableto those in steel joints. For example, a joint made with an adhesivehaving a modulus of 20,000 psi, and overlap of 0.25 inch and a bondlinethickness of 0.010 inch would have a maximum stress factor of 1.64.This compares favorably with the value for a typical bonded steel joint.Good wet out gives good bondGood wetting of the substrate surface is essential for developingreliable bonds. Adhesives that do not wet the surface will not spreadout and fill substrate surface irregularities.Wetting occurs when the surface tension of the liquid adhesive islower than the critical surface tension of the substrates being bonded.If this condition is not met, the liquid does not spread, but forms around droplet on the surface, much like water beads up on a newlywaxedcar.Wetting of plastic surfaces is much more complex than wetting cleanmetal surfaces. Plastics and adhesives are both polymeric materialsand thus have similar physical properties, including wetting tensions.Plastic bonded joints do not have the large difference between thecritical surface tension of the substrate and that of the adhesive, whichinsures wetting for metals. In addition, many plastics have notoriouslylow critical wetting tensions. Polyethylene (PE) and polypropylene (PP),with critical surface tensions of 31 and 29 dynes/cm respectively,present serious wetting challenges for most adhesives. Other plasticssuch as polystyrene (PS) and polyvinyl chloride (PVC) have highercritical surface tensions and present less of a problem.The Loctite ® Design Guide for Bonding Plastics, Volume 4 81