Plastics flame treatment

Plastics Flame treatment Weak boundary layer removal and surface oxidation... 

Uses. The principal use of magnesium hydroxide is in the pulp (qv) and paper (qv) industries (52). The main captive use is in the production of magnesium oxide, chloride, and sulfate. Other uses include ceramics, chemicals, pharmaceuticals, plastics, flame retardants/smoke suppressants, and the expanding environmental markets for wastewater treatment and SO removal from waste gases (87). 

If corona, plasma, or flame treatment is chosen as the surface treatment, it is important to bond quickly after the treatment. Waiting several hours will reduce the effectiveness of the treatment. In some cases, attempts to bond 24 h after the treatment can give the same poor bonding results as if the plastic had never been surface treated. If surface oxidation is not possible, priming the surface with a chlorinated polyethylene primer is a second choice [95]. 

In flame treatment, an oxidizing flame is deployed so as to impinge on the surface for a very short time (thus avoiding melting the plastic). As time is... 

Flame treatment Surface pretreatment method, especially for plastics, by means of an acetylene, propane or butane flame burning in excess oxygen. Results in improved surface wettability by the adhesive due to the chemical entrapment of oxygen atoms in the polymer surface. 

Some plastics, notably polyolefins and acetals, are, however, highly resistant to bonding and need separate treatment to activate the surface. Commonly used treatment processes are flame treatment, electronic treatments such as corona discharge and plasma discharge, and chemical treatment. 

In flame treatment, plastic objects such as bottles and film are passed through an oxidizing gas flame. Momentary contact with the film causes oxidation of the surface, which makes it receptive to material used in decorating the product. 

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. 

These techniques vary considerably in terms of the plastic and the application. For example, adhesives are widely used. There are solvent systems for most TPs, but not thermosets. Monomeric or polymerizable cements can be used for most TPs and TSs. There are certain plastics with outstanding chemical resistance, such as the polyolefins, that preclude the use of many cements they generally require some form of surface treatment prior to adhesion, such as flame treatment. 

Covalent chemical bonds can form across the interface and are Ukely to occur in cross-Unked adhesives and thermoset coatings. This type of bond is usually the strongest and most durable. However, they require that mutually reactive chemical groups should exist. Some surfaces, such as previously coated surfaces, wood, composites, and some plastics, contain various functional groups that under appropriate conditions can produce chemical bonds with the adhesive material. There are ways to intentionally generate these conditions, such as by surface treatment of plastics with techniques like corona or flame treatment. 

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. 

Polyolefins, such as polyethylene, polypropylene and polymethyl pentene, as well as polyformaldehyde and polyether, may be more effectively treated with a sodium dichromate-sulfuric acid solution. This treatment oxidizes the surface, allowing better wetting by the adhesive. Flame treatment and corona discharge have also been used. Table 7.20 shows the relative joint strength of bonded polyethylene and other plastic substrates pretreated by these various methods. 

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. 

Flame treatment is mainly used for pre-treating plastic articles of fairly thick section, such as blow-moulded bottles and thermo-formed tubs. Its early application to polyolefin film treatment was not sustained because of process control difficulties and safety problems however, these have been overcome in recent times, and flame treatment competes with Corona discharge treatment in this area once again. 

The Corona discharge treatment is especially suitable for the continuous treatment of plastic films, whereas the flame treatment is generally preferred for treating thicker sections snch as bottles. Chromic acid is sometimes used prior to the metal plating of plastics and for treating complex shapes. These three methods are discussed in more detail in Pre-treatments of polyolefins. 

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. 

To obtain good results on these plastics, it is necessary to modify the surface of the plastic, to cross-link the weak layer into the plastic below and to oxidize it, thus creating polar groups and a higher energy surface. The preferred treatments for doing this are corona discharge (for plastic films) and flame treatment (for moulded articles). 

In flame treatment the oxidizing portion of a gas flame (6-10 mm from the tip of the blue inner cone) is brought into contact with the surface of the plastic for a brief period (0.02-0.1 s). The flame temperature is high (1100-2800 °C) and the flame is a plasma. Thus the plastic is modified by oxidation by the plasma and by recombination of free radicals created in the polymer with free radicals in the plasma. Changes are observed to a depth of 4-9 nm and wetting and adhesion are improved. This is a popular technique for improving the adhesion of printing inks to the surfaces of moulded polyolefin containers. 

A technique similar to flame polishing can be used to treat certain plastic surfaces before coating or adhesives bonding. See flame treatment surface treatment. 

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