Flame treatments

Flame treatment is often used to change the surface characteristics of plastics. It involves passing the surface of the plastic through the oxidising portion of a natural gas flame. The surface is rapidly melted and quenched by the process some oxidation of the surface may occur at the same time. Exposure to the flame is only a few seconds. Flame treatment is widely used for PE and PP, but has also been applied to other 

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 

The data in Table 7.6 reveal a large increase in the bond strength of PVF and ECTFE after flame treatment. The fluorine-to-carbon ratio (F/C) of PVF remained unchanged but the oxygen-to-carbon (0/C) ratio increased significantly. In the case of PTFE, the F/C ratio actually increased which could explain the drop in the bond strength as a result of flame treatment. The flame probably removed contamination that had previously masked (covered) some of the F atoms on the surface. 

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. 

Lamination Temperature, °C Postheat treatment Bond strength, g/cm Faiiure interface 

FEP corona treatment atmosphere FEP film speed, m/min Bond strength, g/cm at heat seal temperature of 315 °C Bond strength, g/cm at heat seal temperature of 350 °C  

It is imperative to find the correct flame treatment suitable to the component in question. Key factors include the gas type, gas-to-air ratio, burner type, gas flow rate, flame distance, and flame passage speed (treatment time). Excessive treatment of the surface results in degradation of the PP, and therefore poor adhesion. Insufficient treatment, however, does not modify the surface adequately, thus leading to poor adhesion. It is often a rather delicate procedure to find the proper conditions, and great care has to be taken in the experimentation. 

During flame treatment, all hydrocarbon gases combine with oxygen to produce heat, as shown here for methane  

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Bonding. Surface treatment, such as chemical etch, corona, or flame treatments, is required for adhesive bonding of Tefzel. Polyester and epoxy compounds are suitable adhesives. 

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].

Brewis, D.M. and Mathieson, Flame treatment of polymers to improve adhesion. In Mittal, K.L. and Pizzi, A. (Eds.), Adhesion Promotion Techniques — Technological Applications. Dekker, New York, 1999, pp. 175-190. 

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]. 

Flame treatment is predominantly used with articles of relatively thick section, such as blow moulded bottles, although it has been applied to polyolefin films as well. The most important variables in the process are the air-gas ratio and their rate of flow, the nature of the gas, the separation between burner and surface, and the exposure time. 

Some physical techniques can be classified into flame treatments, corona treatments, cold plasma treatments, ultraviolet (UV) treatment, laser treatments, x-ray treatments, electron-beam treatments, ion-beam treatments, and metallization and sputtering, in which corona, plasma, and laser treatments are the most commonly used methods to modify silicone polymers. In the presence of oxygen, high-energy-photon treatment induces the formation of radical sites at surfaces these sites then react with atmospheric oxygen forming oxygenated functions. 

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. 

Flame is probably the oldest plasma known to humanity and flame treatment is one of the oldest methods used by industries for the modification of polymeric surfaces. Flame treatment is very often used to treat bulky objects. It is mainly employed to enhance the ink permeability on the polymer surface. Though a very simple set-up (comprising of a burner and a fuel tank) is required for this technique, a very high degree of craftsmanship is needed to produce consistent results. Oxidation at the polymer surface brought about by the flame treatment can be attributed to the high flame temperature range (1000-1500 °C) and its interaction with many exited species in the flame. For an efficient flame treatment, the variables like air-to-... 

Welding of ETFE parts can be done easily by spin welding, ultrasonic welding, and conventional butt-welding using flame and ETFE rod. The resins bond readily to untreated metals, but chemical etch corona and flame treatment can be used to increase adhesion further.21... 

PVF films often require a surface treatment to improve bonding to other materials. Among these are flame treatment,69 electric discharge,70 chemical etching, and plasma treatment.71... 

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. In this the surfaces are flamed, usually by application of a gas burner for a brief period. Factors such as the temperature of the flame (the ratio of gas to air in the fuel), distance of flame from the surface, and speeds of travel of flame and objects, all are critical—and small variations in any of these factors can lead to unsatisfactory results (that is, under-treatment, or overtreatment). Because of this, standardization and consistent results are best achieved through programmed control with robots. Even so it is difficult to treat more complex shapes satisfactorily, and normally such items would be primed after flame treatment—that is, two methods of promoting adhesion vould be used in combination. 

Evenness of treatment depends on factors such as the designs of the chamber and of the electrode, the levels of energy and pressure, and the period of time of exposure once established at satisfactory values, these parameters can be controlled fairly easily. The surface chemistry is similar to that with simple flame treatment but the use of a gaseous reactant means that even treatment of quite complex shapes is practicable. Besides even and repeatable results the method offers the additional advantage that adhesion promoters are not necessary—with associated savings in costs and enhanced environmental implications. 

Polymer manufacturers offer paintable grades of polypropylene or copolymer blends with talcum or other mineral fillers with such grades the tolerance of over-treatment seems to be increased—which can be advantageous, especially with flame treatment. 

Several techniques including corona discharge [1], plasma treatment [1,3,4], flame treatment [1], and irradiation with UV light in the presence of a UV sensitive gas [5-8] have been developed to modify the polymer surface. The principle of those surface treatment technologies is to introduce polar groups onto the polymer surface. This provides significant improvement of wettability, paintability, biocompatibility and also adhesion to other polymers or metals. 

Parts molded from polyetherimide can be assembled with all common thermoplastic assembly methods. Adhesives that are recommended include epoxy, urethane, and cyanoacrylate. However, service temperature must be taken into consideration in choosing an adhesive because PEI parts are generally used for high-temperature applications. Good adhesion can be effected by simple solvent wipe, but surface treatment by corona discharge, flame treatment, or chromic acid etch will provide the highest bond strengths. 

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