3.Machining and processing HPL
Contact adhesives can be solvent adhesives as well as dispersion adhesives which are processed using contact bonding. Polymers are used as bonding agents for this type of adhesive; after evaporation of the solvent they change from an amorphous to a crystalline state after a certain time, which greatly increases their strength. First a uniform layer of adhesive is applied to both bonding surfaces. Pieces are left for the solvent to evaporate until the adhesive feels dry, that means it is no longer tacky when touched and only has a low level of immediate adhesion. As with solvent adhesives, at least one of the surfaces which are to be bonded has to be permeable to solvents, otherwise it can take a very long time until the adhesive reaches its final strength.
The adhesive can be applied by hand using a notched trowel and automatically with spray units (hot or cold). In addition, these adhesives can be applied to HPL and substrate using pouring units. When applying adhesives with a notched trowel, the directions of application on substrate and HPL have to be at right angles. Sufficient curing is important (finger test!). Open time can be reduced by accelerated drying of the adhesive films, ensuring to avoid overdrying. Contact adhesives require short but firm contact pressure to ensure safe bonding. This can be done e.g. with a hand-held pressing roller and or a roller press for narrow areas. Care has to be taken that no stresses occur in the material when bonding substrate and HPL.
