Plastics surface preparation flame treatment

Historically, surface treatments to improve adhesion of coatings to plastics consisted of mechanical abrasion, solvent wiping, solvent swell that was followed by acid or caustic etching, flame treatment, or corona surface treatment. Each of these treatments has limitations, thus providing a strong driving force for the development of alternative surface preparation methods. Many of the common methods mentioned are accompanied by safety and environmental risks, increased risk of part damage, and expensive pollution and disposal problems. 

Flame and corona, although useful in oxidizing the surfaces of plastics, have limited utility in many applications. In addition, the transitory nature of these modifications prevents their widespread use in many applications. Corona treatment is limited to both the materials that are responsive to this method of surface preparation and the part configuration itself. Complex shapes cannot easily be treated, as the treatment quality is a function of the distance of the part from the electrode. Thus small-diameter holes and... 

As with metallic substrates, the effects of plastic surface treatments decrease with time. It is necessary to prime or bond soon after the surfaces are treated. Some surface treatments, such as plasma, have a long effective shelf life (days to weeks) between treatment and bonding. However, some treating processes, such as electrical discharge and flame treating, wiU become less effective the longer the time between surface preparation and bonding. 

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. 

Epoxy and nitrile-phenolic adhesives have been used to bond these plastics after surface preparation. The surface can be etched with a sodirim sulfiiric-dichromate add solution at an elevated temperature. Flame treatment and corona discharge have also been used. However, plasma treatment has proven to be the optimum siuface process for these materi2ds. Shear strengths in excess of 3000 Ib/in have been reported on polyethylene treated for 10 min in an oxygen plasma and bonded with an epoxy adhesive. Polyolefin materials can also be thermally welded, but they cannot be solvent cemented. 

There is increasing use of rigid plastics as substrates in place of metals. Plastic surfaces may be prepared by chemical cleaning and/or surface roughening with a mechanical blast. Flame treatment and corona treatment are also viable options. 

Rame treatment is also a popular method for surface preparation of plastics. The nonequilibrium gas phase produced by the flame also leads to surface oxidation thereby raising the surface energy [6,9]. Rame treatment can be automated or it can be done by manually with a hand held torch. However, if the flame is too far from the surface the film is improperly treated and if the flame is too close to the surface then it may char and degrade. Therefore, manual handheld torch results in less uniform treatment than automated systems. The gas mixture, flow rate, and treatment duration affect the resulting surface energy. 

Flame Treatment - In adhesive bonding, a surfaee preparation technique in which the plastic is briefly exposed to a flame. Flame treatment oxidizes the surface through a free radical mechanism, introducing hydroxyl, carbonyl, carboxyl, and amide functional groups to a depth of 4-6 nm, and produces chain scissions and some cross-linking. Commonly used for polyolefins, polyacetals, and polyethylene terephthalate, flame treatment increases wettability and interfacial diflfusivity. 

PE has a non-polar, nonporous, and inert surface. For this reason, adhesives cannot link chemically or mechanically to untreated PE surfaces. Although PE is relatively inert to most solvents, solvent cementing cannot be used. For bonding to itself or to other materials, an adhesive with a suitable surface preparation method must be used. There are a number of surface treatment techniques in use, including chemical, electronic, flame, and primer methods. Oxidation treatments are the most successful. These include immersion in a chromic-acid solution, exposure to corona discharge, flame oxidation, immersion in an aqueous solution of chlorine, or exposure to chlorine gas in the presence of ultraviolet light. The chromic acid oxidation method is probably the most convenient for use with molded plastic parts of diverse contour. 

Almost any substrate that will be powder coated, whether it is metal, medium-density fiberboard (MDF), or some other material, requires some kind of surface freafmenf. Surface preparafion (prefreafmenf) of wood and more specifically, MDF preparation consist of sanding, removal of confami-nanfs, and board condifioning. MDF boards fhaf will be coafed by UV curable powders should have a moisture content of 49%. If is also a common practice fo preheaf MDF boards prior to application of fhe UV powder. Mefal surfaces are always cleaned by washing and usually freated by a phosphafe primer. Conductive plastics are washed and dried and then coated without any further treatment. Nonconductive plastics are made conductive by applying a primer coat or treated by plasma or flame. ... 

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