Surface energy engineering plastics

TPX has a number of unique properties and features, but it has remained a specialty engineering polymer. It has the lowest density of any plastic material 0.835 g/cw . Although crystalline, it is totally transparent due to the amorphous and crystalline phases having the same density. It has very low surface energy and outstanding optical and acoustic properties. A current growth area is in films. Mitsui has a separate tradename Opulent for films of poly 4-methyl pent-l-ene. 

With the exception of the polyolefins and other low surface energy plastics, they cope with almost all common engineering alloys and many plastics. The stress cracking of some plastics is a hazard for some versions of these adhesives. None of them bonds rubbers satisfactorily and polyurethane plastics may prove difficult. 

Silicone rubber, polytetrafluoroethylene (PTFE), Acetal and the polyolefin plastics (polypropylene, polyethylene) are always a challenge to the adhesive engineer due to the low surface energy of these materials. Whilst the detailed consideration of surface tension is more in the province of the physicist than the engineer, wetting (the establishment of contact) plays a significant role in adhesion. 

When a designer is selecting an adhesive for a specific application, the engineering properties of the individual plastic will be considered carefully. All too often, however, the data supplied by the plastic manufacturer will include melting point, mould shrinkage, tensile modulus, hardness, dielectric properties, water absorption, density and thermal conductivity but almost never the surface energy of the plastic, which is one of the key properties required for the adhesive application engineer. 

Low-surface-energy plastics such as the polyolefin family will always be a challenge to the adhesive application engineer, especially if the adhesive bond line is to be subjected to peel loading. Bonding to difficult plastics and the wetting of adhesives is discussed in more detail in Section 6.1. 

Polymers used for engineering purposes tend to be relatively resistant to chemical attack, but the strength of certain polymers can fall dramatically in the presence of particular environments through the process of environmental stress cracking. The problem is very specific to certain polymer environment combinations such as PMMA with alcohols and polyalkenes in detergents and can lead to premature failure at very low stresses through either plasticization or a reduction in surface energy. 

---