Polyolefins surface pretreatment

A WBL can also be formed within the silicone phase but near the surface and caused by insufficiently crosslinked adhesive. This may result from an interference of the cure chemistry by species on the surface of substrate. An example where incompatibility between the substrate and the cure system can exist is the moisture cure condensation system. Acetic acid is released during the cure, and for substrates like concrete, the acid may form water-soluble salts at the interface. These salts create a weak boundary layer that will induce failure on exposure to rain. The CDT of polyolefins illustrates the direct effect of surface pretreatment and subsequent formation of a WBL by degradation of the polymer surface [72,73]. 

More recent viscoelastic 2C acrylate adhesives often adhere to low-energy surfaces as well, making them suitable for adhesion to polyolefins without pretreatment (3 M Scotch-Weld DP 8005). The 2C acrylate adhesives are often supplied in double cartridges (side-by-side cartridges), whereby the flow-mix principle is used. Users process the adhesive like a 1C system. These acrylate adhesives are transparent and UV-stable, making them suitable for braiding glass. They can also be painted over. 

The materials were developed in the early 1990 s to provide an easily paintable polyolefin requiring no surface pretreatment except cleaning. 

Thermoplastic polyolefin blend paintable without necessity of surface pretreatment STRUCTURE Co-continuous... 

Typical Applications Epoxies, phenolics, meleunines, nylons, PVC, acrylics, polyolefins, polyurethanes, nitrile rubbers and fiberglass-reinforced thermoplastics used to couple inorganic fillers or reinforcing materials with resins surface pretreatment of fillers and reinforcers. 

Mix R, Eriedrich JE, Inagaki N. Permanence of functional groups at polyolefin surfaces introduced by dielectric barrier discharge pretreatment in presence of aerosols. In Thomas M, Mittal KL, editors. Atmospheric pressure plasma treatment of polymers-relevance to adhesion. Beverly, MA Scrivener Puhlishing EEC 2013. 

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. 

Many chemical pretreatments have been reported for polyolefins but the only one of commercial importance is etching in chromic acid. It is typically used for the treatment of irregularly shaped or large objects when it would be difficult to use either flame or corona-discharge (Section 4.2.7) treatments. The chromic acid etches the polyolefin surfaces and results in loss of material into solution. 

As mentioned earlier, other plastics such as poly(vinyl chloride), poly (ethylene terephthalate), polyacetals, nylons and polyimides do not present such a severe problem to the adhesives technologist as do fluorocarbon polymers or polyolefins. Nevertheless, in order to obtain very high joint strengths approaching the cohesive strength of the substrate, some form of surface pretreatment is often necessary. 

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. 

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. 

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). 

Polyolefins, such as PE and PP, are commonly used in many applications in the biomedical sector. PE and PP can achieve biocompatible and antimicrobial properties using the suitable surface treatment [131, 132]. Many modification methods of the polymer surfaces have been employed, for example, techniques based on the plasma treatment [133]. A deposition of chitosan on the plasma-pretreated PP surface provides antifungal and antibacterial properties because chitosan exhibits an efficient antimicrobial activity [134]. If PE films were modified by a multistep process using plasma discharge, carboxylic groups and antibacterial agent can be developed over the surface. Immersion of these films into the solution of chitosan leads most likely to the adherence of a chitosan monolayer on the treated film. Small concentration of chitosan was enough for the induction of antimicrobial properties to the modified material [135]. 

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. 

SABRA n. Abbreviation or surface activation beneath reactive adhesives, a method of bonding plastics, such as polyolefins and polytetrafluoroethylene, that are normally unreceptive to adhesives without pretreatment. The method consists of mechanical abrasion of the surfaces to be joined to roughen their outer layers scission of bonds with creation of free radicals, and further reaction with primers in the liquid, vapor, or gaseous phase. An adhesive such as an epoxy is then applied. 

With plasma treatment, surface wettability can be readily induced on a variety of normally non-wettable materials as shown in Table P. 5. Certain polymeric surfaces, such as the polyolefins, become crosslinked during plasma treatment. The surface skin of polyethylene, for example, will become crosslinked so that if the polymer were placed on a hot plate of sufficient heat, the interior would turn to a molten liquid while the crosslinked outer skin held a solid shape. Other polymers have their critical surface energy affected in different ways. Plasma-treated polymers usually form adhesive bonds that are two to four times the strength of nontreated polymers. Table P.5 presents bond strength of various plastic adherends pretreated with activated gas and bonded with an epoxy or urethane adhesives. 

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