Polypropylene surface pretreatments

Analysis of the Non Metallized. Pretreated Polypropylene. In a previous paper (1), we have shown that for very short treatment times (23 ms) in N2 or NH3 plasma, the first observed effect of the plasma was an increase of the dispersive component (y ) of the polypropylene surface tension. Since almost no nitrogen nor oxygen were detected by XPS for treatment times shorter than 0.7 s, it was concluded that the plasma had first a physical effect rather than a chemical one, although the efficiency of the treatment on the Al-PP adhesion was high (as proven by the use of a scotch-tape test). 

A perfluoroacrylate monomer was flash evaporated at 100 millitorr and exposed to polypropylene fibers pretreated in a plasma field within one second while the fabric was traveling at 50 m/min. The condensed monomer layer was then cured in-line by electron beam radiation within 100 milliseconds resulting in a 0.1 pm perfluoroacrylate coating on the material surface. The product had an adequate repellency for both water and oil and a surface energy of 27 dyne/cm. 

The post-grafting reaction provides a strong and irreversible adhesion between the grafted layer and the substrate, since a covalent bond is estabhshed. We grafted N-vinylpyrrolidone (NVP) monomer onto a polypropylene (PP) surface pretreated with cold N2 plasma (Scheme 12.1). After the plasma treatment, the modified surface was dipped in aqueous NVP solution for several periods of time (1-50 h) and at various temperatures (60-80°C). 

Surface pretreatment of cellulosic fibers, processing timeand temperature of cellulose-containing polypropylene [153]... 

Like the fluorocarbon polymers discussed above, polyolefins such as low- and high-density polyethylene, polypropylene and poly (4-methyl 1-pentene) cannot usually be joined by adhesives to give reproducible high strengths unless some form of surface pretreatment is first employed. 

Figure 4.4 Effect of a trichloroethylene vapour surface pretreatment on the strength of polypropylene joints [33].

PP (Polypropylene) - the use of this polymer has recently become far more widespread because it can now be satisfactorily coated following surface pretreatment. It is resistant to most organic solvents and is stable to relatively high temperatures. This makes the use of low-bake convertible coatings possible. 

Cast film extrusion is used in manufacturing polypropylene films and requires greater surface pretreatment power density (possibly 2-3 times) compared to other polyolefin films. With blown film extrusion processes, polyethylene films are typically used and require pretreatment on both sides. Considerable amounts of slip additives, used to lubricate the surface of these films for processing ease, can be prevalent within the resin and migrate to the surface of the film within a few days after extrusion. Although there is potential for the additive to mask-over treatment, it is far more important to surface treat immediately after extrusion, since it will be practically impossible to do so after additive migration to improve surface properties sufficiently for ink, coating, or lamination adhesion. 

Waterbased primers have been introduced to reduce solvent output from TPO component coatings in Europe. As these do not contain efficient adhesion-promoting ingredients for TPO substrates, the result is strictly dependent on the chemical pretreatment. Commercial experience with these waterbased prodncts has been limited to polypropylene surfaces that have been perfectly flamed and thus the scope of this technology at present is limited to relatively simply shaped parts. New experimental primers with inherent adhesion to TPO are under evaluation and will allow for substitution of their solventbased counterparts in the very near future. 

Whereas PVA fleeces are used only in primary cells polyamide fleeces compete with polyolefin, preferably polypropylene fleeces. The latter are more stable at higher temperatures and do not contribute to electrolyte carbonation, but they wet only after a pretreatment either by fluorination [131] or by coating and crosslinking with hydrophilic substances (e.g., polyacrylic acid [132]) on the surface of the fiber. 

For this reason much attention is being applied to surface science and to the interaction of coating materials with intended substrates. Some plastics— particularly polyethylene, polypropylene—and the silicone rubbers, cannot be covered with high-performance coatings without their surfaces being pretreated. 

Metallized polypropylene (PP) is used today in many different fields such as automotive, decoration, electrical. In order to obtain a good adhesion between the aluminium and the polymer a pretreatment of the film prior to the metallization is necessary. Indeed, the very extreme surface of the polymer has to be modified in order to prepare it to a good adhesion with the metal. Thus the polymer is placed in a low pressure plasma of nitrogen with a corona discharge configuration of electrodes, and the metallization is carried out in situ after the pretreatment in nitrogen. This process, which simulates an industrial polymer film treatment has proven a great efficiency for very short treatment times (23 ms) (1).However, the mechanisms responsible for the improvement of adhesion are not totally explained yet. 

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). 

Plastics can have a layer of metallic aluminium deposited on the surface by vacuum. Although aluminium is basically a bright silver, it may be lacquer tinted to give a wide range of metallic colours. Polyester, polystyrene, urea and phenol formaldehyde, and polypropylene are readily metallised. Other plastics need a pretreatment. 

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. 

Figure 8 Positive ion ToF-SiMS spectrum from the surface of pretreated (electrochemically oxidized) polypropylene, illustrating the typicai signai-to-noise ratio of poiymer fragments at low mass and additive molecular ions at high mass (see text). The peak at nVz133 is due to primary Cs+ ions. Reproduced with permission from Vanden Eynde X (2001) Quantitative analysis of polymer surfaces. In Vickerman JC and Briggs D (eds.), ToF-SIMS Surface Analysis by Mass Spectrometry, Ch.16. Manchester SurfaceSpectra/IM Pubiications SurfaceSpectra/IM Publications.)...

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. 

Good dispersion of mica into hydrophobic resins like polypropylene is difficult because of this hydrophyllic nature of the mica surface. Poor interaction at the surface of mica and polypropylene results in lowered mechanical and thermal properties. These property reductions are caused by the absence of good interfacial bonding and also by the presence of air and water in the interfacial microvoids. Treatment of mica with organofunctional silanes will place a hydrophobic surface on mica that can form weak bonds with polypropylene. This phenomena is described in many papers and bulletins (23-27). In one bulletin (27), coupling agents PC-IA and PC-IB (produced by OSi Specialties) were used in a 4 1 ratio. Table 14.28 lists the changes in mechanical properties obtained with mica pretreated with 1 and 2 wt% of a PC-IA/PC-IB blend. 

Much of the recent development work on ATH is in the development of smaller particle size, less dust, and more translucency problems also present with antimony oxide. Surface coating with titanates and other materials has been tried successfully. In one case, a 40% ATH-filled low-density polyethylene for foamed pipe lines uses 0.5% isostearoyl titanate and in another, a Japanese company has developed a "self-extinguishing" polypropylene compound for household heater junction boxes that uses 30% ATH pretreated with 1% titanate in combination with halogen, to achieve a UL 94 V-1 rating [127]. 

Polyolefins. Polyethylene, polypropylene, and polybutene can be bonded only after treatment to increase the surface energy, generally by oxidation, and make the surfaces receptive to bonding. Pretreatment can be carried out with an oxidizing flame, with oxidizing chemicals, or by electrical discharge. 

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