Polyethylene surface pretreatments

The described reactive adhesives are suitable for bonding of nearly all metal and nonmetal materials used in industry, trade and even in the private sector. They are characterized by good to very good adhesive properties on correspondingly prepared surfaces (Section 7.1.2) as well as by stress-related strengths. Some plastics, especially polyethylene, however require special measures regarding their surface pretreatment Here, we refer to Section 9.2. 

Wool [57,78] suggests that these principles could be used to develop pretreatments which give a highly ramified, fractal surface to which high adhesion by mechanical interlocking would be expected. Consider a blend of polyethylene with a second phase, perhaps starch, amenable to removal by selective attack or dissolution. Above a critical concentration some of the second phase particles will be connected, forming a fractal structure. Treatment of the polyethylene surface, then, will leave fractal voids, receptive to an adhesive, such as a liquid epoxy resin. 

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. 

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. 

An atmospheric pressure plasma ion bombardment treater together with an atmospheric CO2 composite snow spray cleaning module (see Figure 7.9) was positioned above an un-treated low density polyethylene (LDPE) material used in microelectronic devices (for use within military and aerospace electronic systems) for surface pretreatment prior to application of a Loctite 401 cyanoacrylate adhesive, and separately a Loctite 3553 light-cure acrylic adhesive. 

Thermal aging is another simple pretreatment process that can effectively improve adhesion properties of polymers. Polyethylene becomes wettable and bondable by exposing to a blast of hot ( 500°C) air [47]. Melt-extruded polyethylene gets oxidized and as a result, carbonyl, carboxyl, and hydroperoxide groups are introduced onto the surface [48]. 

Surface-water samples are usually collected manually in precleaned polyethylene bottles (from a rubber or plastic boat) from the sea, lakes, and rivers. Sample collection is performed in the front of the bow of boats, against the wind. In the sea, or in larger inland lakes, sufficient distance (about 500 m) in an appropriate wind direction has to be kept between the boat and the research vessel to avoid contamination. The collection of surface water samples from the vessel itself is impossible, considering the heavy metal contamination plume surrounding each ship. Surface water samples are usually taken at 0.3-1 m depth, in order to be representive and to avoid interference by the air/water interfacial layer in which organics and consequently bound heavy metals accumulate. Usually, sample volumes between 0.5 and 21 are collected. Substantially larger volumes could not be handled in a sufficiently contamination-free manner in subsequent sample pretreatment steps. 

Polyethylene parts are decorated by silk screening, hot stamping, or dry offset printing. For satisfactory printing, the surface must be oxidized by hot air, flame, chlorination, sulfunc acid-dichromate solution, or electronic bombardment. Hot air or flame methods are used with molded parts flame or electronic methods with films. Inks specially made for polyethylene give best results. Roll-leaf hot stamping does not require pretreatment of the suifaoe. 

Polymers of MMA, AAc, and MAA were grafted onto an ultrahigh molecular weight polyethylene (UHMWPE) fiber surface after pretreatment with electron beam irradiation [31]. Sundell et al. [32] pretreated a PE film with electron beams to facilitate the graft polymerization of vinyl benzylchloride onto the substrate. The inner surface of porous PE hollow fiber had also been modified by grafting of glycidyl methacrylate (GMA) polymer after electron beam irradiation [33]. 

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. 

Polyethylene terephthalate cannot be solvent-cemented or heat-welded. Adhesives are the prime way of joining PET to itself and to other substrates. Only solvent cleaning of PET surfaces is recommended as a surface treatment. The linear film of polyethylene terephthalate (Mylar) provides a surface that can be pretreated by alkaline etching or plasma for maximum adhesion, but often a special treatment such as this is not necessary. An adhesive for linear polyester has been developed from a partially amidized acid from a secondary amine, reacted at less than stoichiometric with a DGEB A epoxy resin, and cured with a dihydrazide.72... 

Polyethylene oxide). This ether-rich polymer (PEO) apparently forms hydrogen bonds with silanol groups, with concomitant adsorption of a PEO layer at silica surfaces. In the case of CZE, the fused silica capillaries are pretreated with 0.1 M NaOH and 0.1 M SDS at the beginning of each day. The typical coating protocol is to flush the capillary with 1.0 M HC1, followed by a solution of 0.2% PEO, then washing with an electrophoretic buffer. The coating process has to be repeated before each run. The EOF is reduced by 60-70%, and the columns thus treated work well for basic proteins. 

The direct way to increase the number of radicals on a substrate surface is to treat it with stable radical solutions. Nitroxyl radicals of the 2,2,6,6-tetramethylpiperidine series have been found to considerably improve durability of adhesive bonds (62,63). For instance, when sticking together high pressure polyethylene, fluoro-plastic F-4 and polyvinylchloride, pretreatment of their surfaces with nitroxyl radical solutions improved the durability of adhesive bonds by 160-170%. This result is considerably better than that obtained by treatment of the aforesaid polymer surfaces with the most active adhesion modifiers known (64). 

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). 

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. 

The coating consists of polyvinylpyrrolidone or cellulosic derivatives deposited by dipping the polymeric substrate into the diluted polymer solution (Scheme 12.2). Since most of the usual polymeric substrates (acrylic resin, polyethylene, etc.) are hydrophobic, they must be pretreated before the coating process. The polymeric substrate surface have to be more polar than the aqueous solutions. Increasing their surface energy impHes the formation of a uniform coating. As... 

Fibre surface modification. The surface energy is closely related to the hydrophilicity of the UgnoceUulosic fibres. Use of dispersing agents, such as stearic acid or a mineral oil. The dispersion of lignocellulosic fibres can be improved by pretreatment with lubricants or thermoplastic polymers. An addition of 1-3 per cent stearic acid is sufficient to achieve a maximum reduction in size and number of aggregates in PP and polyethylene [7]. The use of stearic acid in HDPE/wood fibres was reported to improve the fibre dispersion and the wetting between the fibre and the matrix [9]. 

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. 

---