Flame treatment, polymer adhesion

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... 

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. 

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. 

There are many different methods for modifying polymer surfaces to improve their adhesion and wetting properties. They include chemical etching and oxidation, ion bombardment, plasma treatments, flame treatment, mechanical abrasion and corona-discharge treatments (1.2). Especially flame and corona treatments are widely used for the modification of polyolefin surfaces to enhance, for instance, their printabilify. Despite the widespread use of such processes in industry, the understanding of the fundamental processes which occur at the polymer surface is very limited. This is undoubtedly due to the shallow depth to which the polymer is modified, typically 5 nm or less. 

Polymer surface modifications are omnipresent in applications where the surface properties of materials with favorable bulk properties are insufficient. By altering the surface characteristics using physical or chemical modification the desired surface properties may be achieved. Such treatments are required e.g. to enhance printability of films, the adhesion of paints, metal or other coatings, biocompatibility, protein resistances/reduced biofouling, etc. The diverse approaches met in practice include, among others, wet chemical and gas phase chemistry, plasma or corona, UV/ozone and flame treatments. In most cases surface chemical modification reactions take place that alter the surface energy in a desired way. For example,... 

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. 

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. 

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. 

Liquid etchants can be used for chenucal modification or dissolving surface contamination. Etchants effectively treat irregularly shaped objects that are difficult to treat by other adhesion-promoting processes such as corona or flame treatment. A number of etching solutions and procedures have been developed for specific polymeric surfaces. The choice of the liquid etchant depends on the polymers. Polyolefins are usually treated by oxidizing acids such as chromic, sulfuric, nitric, or mixtures of these. Fluorocarbons are usually treated by sodium-napthalene etching solution. 

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. 

The dominating chemical pretreatment used in Europe for molded automotive TPO components is flame treatment. The effectiveness of the flame treatment is checked by surface tension measurement and should be a minimum of 45 dynes/cm to ensure acceptable adhesion with solvent-based paints. Chemically, the adhesion effect of flaming is based on a controlled oxidization of the CH2-and CH3 units (7). Also, a degradation of the polymer and polymer melting can occur therefore the flaming needs to be carried out within an upper and lower temperature limit. 

Most polymeric surfaces are hydrophobic in nature. In order to improve adhesion (adhesion with other surfaces, adhesion with paints or heparin for biomedical appUcations), this trait must be modified [31]. The most common method of doing this is by oxidation of the surface, which can be carried out by either corona discharge, flame treatment, plasma polymerization at the surface, grafting reactions, or blending the polymer with reactive surfactants that enrich at polymer interfaces. It has been shown that benzophenone xmder ultraviolet irradiation can abstract hydrogen from a polymer surface ... 

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