Surface energy polyolefin plastics

Sometimes primers can take the place of surface treatments. Two examples are with porous substrates and with certain plastic substrates. With weak porous substrates, such as wood, cement, or porous stone, the primer can be formulated to penetrate and bind weakly adhering material to provide a new, tightly anchored surface for the adhesive. Chlorinated polyolefin primers will increase the adhesion of coatings and adhesives to polypropylene and to thermoplastic olefins. The chlorine atoms in the outer surface of the primer increase surface energy and enhance adhesion of adhesives, sealants, and paints. 

This chapter identifies and discusses various epoxy adhesives and the processes that have been used to successfully bond or seal specific substrates. There are only a few materials that epoxy adhesives will not bond well. These uncooperative substrates are most notably low-surface-energy plastics, such as the polyolefins, fluorocarbons, and silicones. However, even these materials can be bonded effectively with epoxy adhesives if a prebond surface treating process is used to change the nature of the substrate surface. Of the other substrate materials, there are some that epoxy adhesives will bond more effectively than others. Table 16.1 lists substrates that generally provide excellent epoxy adhesive joints. 

Adhesives manufacturers are continually trying to develop adhesives to meet the needs of industry. One group of plastics that have been difficult to bond are polyolefins and related low-energy substrates (see Surface energy). They could not be bonded without elaborate surface preparation such as Flame treatment or Plasma pre-treatment, Corona discharge treatment or oxidative chemical methods. 

Different plastics present different painting problems but, on the whole, once clean they do not often present problems of paint adhesion. Exceptions are the polyolefin plastics, e.g. polyethene and polypropylene. Because these polymers lack polar groups, they have low energy surfaces which are difficult to wet and are not readily penetrated by solvents. In addition to this, the surface layer of the plastic, a few nanometres thick, is often different to the bulk, being low in molecular weight and weak. Paint adhering to this layer will pull the layer away and peel off, thus appearing to have poor adhesion. 

With plasma treatment, surface wettability can be readily induced on a variety of normally non-wettable materials as shown in Table P. 5. Certain polymeric surfaces, such as the polyolefins, become crosslinked during plasma treatment. The surface skin of polyethylene, for example, will become crosslinked so that if the polymer were placed on a hot plate of sufficient heat, the interior would turn to a molten liquid while the crosslinked outer skin held a solid shape. Other polymers have their critical surface energy affected in different ways. Plasma-treated polymers usually form adhesive bonds that are two to four times the strength of nontreated polymers. Table P.5 presents bond strength of various plastic adherends pretreated with activated gas and bonded with an epoxy or urethane adhesives. 

The adhesives will bond almost all materials (though a primer may be needed with some), except polyolefin plastics (eg Polythene) and other low surface-energy types such as fluoropolymers (eg Teflon) and silicone-based rubbers. Alkaline glass may cause premature bond failure and all glasses should be silane primed if at all possible, as this considerably improves the joint s humidity resistance. May stress crack stressed mouldings or susceptible plastics - polycarbonate, for example. 

Emulsions bond most materials which absorb water. Non-absorbent adherends may be bonded if the other surface provides an escape route for water. Thus, some adhesion can be obtained on most metal and plastics surfaces, though care should be taken to ensure that metals are adequately painted to prevent corrosion. Without special preparation, polyolefin plastics, other low surface-energy types such as PTFE and the silicone rubbers cannot be bonded. 

The family as a whole is very versatile. While performance on some plastics is only modest, the group may be used with advantage, on plastics materials sensitive to stress cracking. The polyolefins and other low surface-energy plastics (such as PTFE and some rubbers) present major difficulties unless specially prepared. 

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. 

Cyanoacrylate adhesives will bond most substrates to themselves and to each other. The few adherends which do not bond well with standard adhesives are polyethylene, polypropylene, EPDM rubber, plasticized PVC, teflon, and acidic surfaces. A few manufacturers sell modified adhesives which will bond some of these materials, such as EPDM and flexible PVC. Adhesion to low surface energy plastics like polyolefins and Teflon can be improved by an etching or oxidizing treatment. 

The application of aqueous formulations onto hydrophobic substrates such as polyolefins, other plastics, or waxy leaves is often problematic the low surface energy of these substrates leads to poor wetting behavior or even dewetting phenomena such as crater formation and crawling [82]. 

Lord Corporation introduced adhesives containing methacrylated phosphate monomers that gave much-improved thermal and atmospheric durability, and Dymax Corporation introduced their aerobic acrylics that were less sensitive to inhibition by atmospheric oxygen. Dow Automotive, 3M, and Loctite recently introduced two-part acrylic-based adhesives that can bond many low-surface-energy plastics, including many grades of polypropylene, polyethylene, and thermoplastic polyolefins without special surface preparation (see Section 4.2.2 for a description of this technology). 

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. 

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. 

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