Polytetrafluoroethylene surface energies

Plastic and elastomeric adherends are even more dependent than metals on surface preparation. Many of these surfaces are contaminated with mold-release agents or processing additives. Such contaminants must he removed before bonding. Becanse of their low surface energy, polytetrafluoroethylene, polyethylene, and certain other polymeric materials are completely unsuitable for adhesive bonding in their natnral state. The surfaces of these materials must be chemically or physically altered prior to bonding to improve wetting. 

Of the organic polymeric plastics, polytetrafluoroethylene (PTFE) was one of the earliest to be investigated as a solid lubricant because of its chemical inertness and its low surface energy. Kay and Tingle... 

Most plasma-treated hydrophobic surfaces of biomaterials are formed with tetra-fluoromefhane (CF4) plasma interactions [3, 4]. The modified surface represents a nonadherent polytetrafluoroethylene (PTFE)-like structure (-(CF2) -) with low surface energy that could vary from 20 mj mT down to only a few mj uT when the super-hydrophobic character is pronounced. The chosen operating conditions lead to a low fluorine atom density in the plasma, thus avoiding surface degradation. Such surfaces are applied in order to prevent the formation of the biofilm. 

A complete wetting of a solid is only possible for spontaneous spreading of a drop of the liquid at the surface, i.e. for = 0 or cos = 1. For a specific solid surface of low surface energy, a linear correlation is observed between cosO and the surface tension. This is demonstrated for polytetrafluoroethylene in Figure 3.6. The limiting value, i.e. cos = 1, is a constant for a solid and is called the critical surface tension of a solid, Xc- Therefore, only liquids with y < yc are able to spontaneously spread on surfaces and to wet them completely. 

The three solid surfaces employed allowed coverage of a range of contact angles from near 0° to greater than 90 . These low energy surfaces included polytetrafluoroethylene (TFE), acrylic, and nylon with majority of work being performed with the first two. 

The adsorption theory also shows that the adhesion forces of two materials are not reciprocal. For example, if a hquid epo resin adhesive is allowed to set on the surfece of polytetrafluoroethylene or polyethylene, a very weak adhesive joint is formed. If, by contrast, liquid polytetrafluoroethylene or polyethylene is applied to the surface of hardened epoxy resin, strong adhesive joints are obtained [17]. In practice, this theory is not free from contradictions either above all, it does not answer the question whether the difference in the surface energies between two materials is indicative of the intensity of the adhesion force [18]. 

Some polymers have very low surface energies (like polyethylene or polytetrafluoroethylene), and they require surface preparation to raise the surface energy. This can be done through chemical treatment, corona discharge, plasma treatment, flame treatment, and ultraviolet radiation [9]. 

Bond strength is relatively low due to the surface energy of polytetrafluoroethylene. The range of bond strength is 0.1-1.8 kg/cm depending on the type of adhesive. The adhesive can be applied in the form of a fluid by working it on the surface to achieve an even thickness. A more convenient alternative is two sided tapes to which the pressure sensitive adhesive has already been applied. All bubbles should be removed to lessen the chance of delamination and peeling. 

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. 

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