Pretreatment methods plastics

Sabreen S. Basics of surface wetting and pretreatment methods. Plastics Technical Reference Library. Jul. 16, 2001. The Sabreen Group. 

Methods for recycling used plastic materials are reviewed. Emphasis is placed on the research projects into chemical recycling methods for used plastics at the Leuna location. These include development of a process for the thermaL thermooxidative pretreatment of used plastic materials, utilisation of pretreated used plastic materials in the visbreaker by gasification and by hydrogenation and the production of wax oxidates from pretreated used plastics. The results are discussed. 

This method is used for curing coatings and inks on plastic cups, tubs, tubes or metal cans.11 The parts are placed on mandrels, which are attached to a rotating device. This device moves them through the individual stations feed, pretreat (for plastics, most frequently corona or flame), printing, curing and take-off. The printing is done by dry offset (see Section 7.5.3). 

Grinding, brushing or sanding (with the exception of the above-mentioned Saco method) do not cause chemical modifications of the material s surface. A clean surface results with a characteristic structure corresponding to the composition of the material, as shown in Figure 7.6. Therefore, physical and chemical pretreatment methods are aimed at the chemical modification of the surfaces. Thus, on the one hand it is possible to further enhance the adhesive forces for extremely high demands on bonded joints, and on the other hand, to make poorly bondable material (e.g., plastics) bondable at all. Since physical methods are mainly used in bonding of plastics, they are described in Section 9.2.4. 

Owing to the deformability of plastics - in particular of thermoplastics - mechanical pretreatment methods are applicable only to a very limited extent. So for example, if jet pressure is too high the blasting abrasive can be shot into the surface. For polyethylene and polypropylene for instance, the SACO-mefhod described in Section 7.1.2.1 has proven its worth. It develops a surface suitable for the formation of adhesive forces by means of chemically modified blasting abrasives (silication). 

Corona process Method for surface pretreatment of plastics, based on the entrapment of reactive atoms from the gas phase in the surface of plastics by high-voltage discharge. 

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. 

Many pretreatment techniques are used in practice (Table 8.2). The normal physical method used to improve the adhesive strength of the coating to the substrate is to slightly roughen the surface by solvent treatment, abrasion, or blasting. Some plastics (e.g., polyolefins) require special pretreatment methods processes that modify the surface molecular layers of the plastic to increase their polarity have proved suitable (e.g., flaming, immersion in an oxidizing acid, immersion in a benzophenone solution with UV irradiation, corona treatment, plasma treatment). 

Over the last 50 years many methods have been developed to pre-treat plastics and rubbers. Partly because of the much simpler formulations, pre-treatments for plastics have been the subject of much greater scientific interest. Our understanding of pretreatments for plastics is therefore much greater than that for rubbers. Some of the key studies on pre-treatments of plastics will therefore be outlined in Section 1.2.2. 

The chemical pretreatment methods mentioned are mostly used for TPOs, but in some cases can also be effective when used on polyamide (PA), polybutylene terephthalate (PBT), or other crystalline polymers, as well as some blends like poly(phenylene oxide)/polyamide (PPO/PA), are flamed. As a possible future trend, research is currently underway (plasma polymerization) attempting to combine a pretreatment for adhesion and to provide the surface conductivity on necessary on plastic parts for acceptable electrostatic application (15). 

The mechanical properties of composites are mainly influenced by the adhesion between matrix and fibers of the composite. As it is known from glass fibers, the adhesion properties could be changed by pretreatments of fibers. So special process, chemical and physical modification methods were developed. Moisture repel-lency, resistance to environmental effects, and, not at least, the mechanical properties are improved by these treatments. Various applications for natural fibers as reinforcement in plastics are encouraged. 

One might expect that the more developed porous layer produced by the PAA process would tend to provide a greater number of successful mechanical interlocking sites 54-86-135-136> and initiate a larger degree of plastic deformation in the resin upon failure than the FPL oxide. Test data i29) comparing PAA and FPL pretreated systems have supported this conclusion. Also, the oxides formed by the PAA pretreatment have exhibited better stability in wet environments 54). Hence, the PAA process has replaced the FPL etch as the method of choice for the pretreatment of aluminum for adhesion systems i32-133>. 

Some plasticizer mixes require pretreatment, such as saponification, but in most instances chromatographic separations can be accomplished with the mix. In addition to the usual identification of substances by organochemical analysis, other methods now being used include color tests, physical tests (determinations of boiling point and refractive index), and infrared and ultraviolet spectroscopy. 

The thermal degradation of waste HDPE can be improved by using suitable catalysts in order to obtain valuable products. However, this method suffers from several drawbacks. The catalysts are deactivated by the deposition of carbonaceous residues and Cl, N compounds present in the raw waste stream. Furthermore, the inorganic material contained in the waste plastics tends to remain with the catalysts, which hinders their reuse. These reasons necessitate a relatively high purity of waste plastics, containing very low concentrations of a contaminant. Thus, various pretreatments are required to remove all the components that may negatively affect the catalyst. 

Another method of improving the dispersibility of cellulose fibers in a thermoplastic matrix is described in U.S. Pat. No. 4,414,267 [36], according to which cellulose fibers (hardwood kraft, from 1 to 40% by weight of the final composite) are pretreated by slurrying them in water, contacting them with an aqueous suspension of a mixture of a vinyl chloride polymer and a plasticizer, and drying the thus-treated fibers. 

ASTM D638 Standard Test Method for Tensile Properties of Plastics is used to determine the tensile properties of unreinforced and reinforced plastics under defined conditions of pretreatment, temperature, humidity, and testing machine speed. Specimens are dumbbell-shaped and can be of any thickness up to 14 mm. However, test Method D8 82 is the preferred test method for testing films... 

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