Polyethylene flame treatment

Papirer et al. used ATR, XPS, and SIMS to determine the effect of flame treatment on adhesion of polyethylene and polypropylene to styrene/butadiene (SBR) rubber [8]. Each flame treatment consisted of a 75-ms pass over a circular burner. The distance between the upper flame front and the polymer was kept fixed al 8 mm. A band was observed near 1720 cm" in the ATR spectra and assigned to carbonyl groups this band increased in intensity as the number of flame... 

Briggs et al. also used XPS to investigate flame treatment of low-density polyethylene [35]. They found that a normal flame treatment of PE resulted in... 

Fig. 30. C(ls) XPS spectra of high-density polyethylene before and after flame treatment. Reproduced by permission of VNU Science Press B.V. from Ref. [8].

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]. 

Oxidation by flame treatment for polyolefins exposure to a flame of methane, propane or butane and oxygen in excess for a very short time (less than 0.2 seconds) to create oxidation and reactive sites such as hydroxyl, carbonyl, carboxyl... Particularly used for polyethylene and polypropylene. 

Oxidation by ultra-hot-air treatment for polyolefins exposition to a blast of hot air (roughly 500°C) for a short time to oxidize the surface and create reactive sites such as hydroxyl, carbonyl, carboxyl, amides... Rather similar to flame treatment, it is particularly used for polyethylene and polypropylene. 

The treatment with a fuel gas-oxygen flame (propane/butane or acetylene with excess oxygen, recognizable by the blue coloration of the flame) results in a chemical and physical surface modification, also with oxidative effects. This method is particularly suitable for handycraft applications, because of its low effort and expenditures. The flame treatment time is in the range of seconds, the distance of the flame to surface should be approximately 5-10 cm. In the case of thermoplastics like polyethylene and polypropylene, care should be taken that surface melting is avoided. 

Certain surfaces require some form of pretreatment prior to printing. Foil usually requires primer wash, and polyethylenes and polypropylenes need to have the surface oxidised. The corona process is almost invariably used for films and flame treatment for bottles. Whether a surface has received treatment or not can be detected by immersing it in water and observing whether or not the water runs off. An oxidised surface has a lower wetting angle. If the surface is not printed soon after treatment another treatment may be necessary. Inks will not key onto non-oxidised PE and PP and will be removed when a self-adhesive tape test is employed. 

Flame treatment is a commercial process to render polyolefins and polyethylene terephthalate adherable. The polymer article (e.g., film) is passed over an oxidizing flame formed by of an oxygen-rich (relative to stoichiometry) mixture of hydrocarbon gas. Variables affecting the extent of oxidation include the flame characteristics (e.g., excess oxygen) and the speed of the article movement. Gas flame... 

Polyolefins such as polyethylene and polypropylene must be surface treated before being printed. The most effective way is in an integrated machine where surface treatment takes place right before printing. A time lapse will mean that the treatment will lose some of its effect. Three methods are used flame treatment, corona discharge, and chemical treatment. Flame treatment is considered the most practical and most widely used. 

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. 

The growth in the use of low-density polyethylene after the Second World War was restricted by the fact that it was very difficult to print upon or bond. This led to the development of a number of pre-treatments including the use of chromic acid. Flame treatment and corona discharges. Under the correct conditions, these pre-treatments are aU highly effective. ... 

After the Second World War, it soon became apparent that low-density polyethylene (LDPE) would be a useful packaging material. However, one serious problem was the difficulty in obtaining good adhesion to the polymer. This led to much research and development to obtain effective pre-treatments for LDPE. By about 1950, a number of pre-treatments had been developed, including the use of chlorine - - UV radiation, chromic acid etching. Flame treatment and Corona discharge treatment. 

Flame treatment is used to improve the adhesion to relatively thick polyolefin objects. For many years, the method has been used to improve the adhesion of printing ink to polyethylene bottles. More recently, flame treatment has been used to improve the paint adhesion (see Paint constitution and adhesion) to polypropylene car bumpers. 

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. 

Flame treatment. Polyester and polyethylene films are commonly exposed to flame treatment to increase bondability. Here, an oxidizing flame briefly ( 0.01-0.1 s) impinges on the surface (69,70). XPS analysis (71) has shown that amide surface groups are generated, as well as typical oxidation fiinctionahty. Flame-treated films maintain bondabihty better than those that have been given corona treatment. Moreover, for all types of treatment, it is best to bond surfaces as soon as possible after treatment. 

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]. 

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. 

As indicated above, the surface treatment process may be responsible for the removal of such material and exposing the bulk of the polymer underneath the segregated layer. Such a situation can be illustrated by the XPS spectra of Fig. 10.4 fi om PVC in its extruded form and after flame treatment. The extrudate is protected by a thin film of low density polyethylene (LDPE), and upon removal of this film the spectrum (O Fig. 10.4a) is dominated by carbon and oxygen with significant contributions from PVC-spedfic elements such as chlorine, lead, and zinc. On flame treatment (O Fig. 10.4b), the carbon level is reduced to a certain extent... 

The low energy surface of some polymers can create problems with adhesion. Critical surface tension values for some common materials are given in the following table. Substrates such as the polyolefins, polypropylene and polyethylene are extremely difficult surfaces on which to achieve good adhesion. Techniques such as flame treatment have been developed, which oxidise the surface of the componenL but these still tend to be used in conjunction with the standard chlorinated polyolefin primer in a belt and braces approach to adhesion. The most reliable form of polyolefin pretreatment is plasma surface treatment. 

Polyethylene and PP have a surface tension of 30 dynes/cm, which has to be raised to a minimum of 38 dynes/cm, but preferably raised to 42 dynes/cm, in order to render their surfaces adherable. Flame treatment is the most widely used method of pretreatment. It is flexible and reliable if carefully controlled. It enables uneven and curved surfaces to be treated. It uses a mixture of air at 138—345 kPa and a fuel gas at a (low) pressure of 1.7 kPa. The gas can be butane, propane, natural gas (methane), and coal gas. 

In addition to flame treatment and low-pressure plasma treatment, fluorine treatment has gained some importance as an alternative method (12). This method is very effective and even gives excellent results on polyethylene or polypropylene homopolymer even when flaming does not provide sufficient adhesion. Many automotive parts such as interior door handles and interior switches have been successfully treated by this method. For the fluorination process to work effectively, the parts need to be positioned in a reactor to which a vacuum is applied. After approximately 20 seconds or so, the parts are exposed to 10 percent fluorine gas in air at 500 mbar (7.0 psi) pressure. The excess of fluorine and the... 

A paper of interest as to the surface properties of polyethylene after treatment has been published by D. Briggs et al.25 The surface of polyethylene in some cases containing an anti-oxidant was subjected to flame treatment for 1.2 s. The oxidation appeared quite mild and was similar to several treatments by chromic acid. 

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