Oxidized polyethylene wettability

The wettability, and hence ability to bond, of oxidized polyethylene decreases quickly upon heating it to 85°C (66). Apparently, oxygen-containing groups in the surface spontaneously turn inward toward the bulk of the sample, so that the surface energy of the material is reduced and the hydrocarbon character of the surface is increased. At room temperature, the loss of bondability is slower since the chains have less mobility for this redistribution. 

The wettability, and hence ability to bond, of oxidized polyethylene decreases quickly upon heating it to 85°C (66). Apparently, oxygen-containing groups in the surface spontaneously turn inward toward the bulk of the sample. 

UV irradiation on a polymer surface produces chemical modification as well as wettability and bondability improvement. It causes chain scission and oxidation on polymer surfaces. -iven in the presence of an inert gas [45]. Carbonyls are found to be introduced onto polyethylenes on UV irradiation. Sivram et al. [46] have used photochemical treatments for surface modification of polymers. They have generated surfaces of vaying surface energies by simple organic reactions. 

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]. 

The modification of the chemical composition of polymer surfaces, and thus their wettability with chemical substances, can be realized in different ways electric discharges more commonly called Corona effect, oxidation by a flame, plasma treatment, UV irradiation and also UV irradiation under ozone atmosphere. Numerous studies have been devoted to the effects of these different treatments. More recently, Strobel et al. [204] compared the effects of these treatments on polypropylene and polyethylene terephthalate using analytical methods such as E.S.C.A., F.T.I.R., and contact angle measurements. They demonstrated that a flame oxidizes polymers only superficially (2-3 nm) whereas treatment realized by plasma effect or Corona effect permits one to work deeply in the polymer (10 nm). The combination of UV irradiation with ozone flux modifies the chemical composition of the polymers to a depth much greater than 10 nm, introducing oxygenated functions into the core of the polymer. 

Strong joints with epoxy adhesives can be made to polyethylene surfaces which have been oxidized by a variety of techniques (4, 5). The general belief has been that the presence of polar groups on the polymer surface creates an affinity for the polar epoxy adhesive which improves wettability and results in a strong adhesive joint. 

The present generally accepted procedure for structurally bonding polyethylene with an adhesive is to oxidize the polyethylene surface this raises its surface free energy and improves the wettability of the polyethylene by the adhesive [2]. An improvement in bond strength is thus obtained. We are not, however, aware of any previous work in which the strength levels reported were comparable to those which we have obtained. 

Natural, unmodified montmorillonite-Na (MMT-Na) has cation exchange capacity, typically 80-90 mequiv/100 g. Although some polymers, such as polyethylene oxide or polyvinylpyrrolidone, are of sufficient polarity to be able to directly exfoliate unmodified MMT-Na, organic modification of the layered clay is usually required to render the hydrophilic surface of the clay more hydrophobic and thus more compatible with most polymers, thereby improving the wettability and dispersibility of the clay in the polymer matrix. 

The effects of air or oxygen plasma on polymer films have been reported. In a comparative study with polypropylene (PP) and polyethylene (PE), higher levels of oxygen incorporation were achieved in PP than with PE. In this method, the initial step involves formation of radicals on the top of the layer of the polymer surface. These can react with each other to initiate cross linking or branching or result in surface oxidation. The use of nonthermal plasma treatment to polymer surfaces to enhance wettability and adhesion has been reported. The use of Corona discharge and dielectric discharge has also been reported for polymer modification [78]. 

Treatment with UV/ozone has been used as a means of removing organic contaminants from different polymer surfaces. However, UV/ozone treatment has also been used to increase the wettability of poly(ethylene terephthalate) (PET), polyethylene (PE), polypropylene, different rubbers (vulcanized styrene-butadiene-SBR, unvulcanized styrene-butadiene-SBS). This UV/ozone treatment results in an increase in the surface energy of the polymer through oxidation of the polymer. 

Flame Treatment - In adhesive bonding, a surfaee preparation technique in which the plastic is briefly exposed to a flame. Flame treatment oxidizes the surface through a free radical mechanism, introducing hydroxyl, carbonyl, carboxyl, and amide functional groups to a depth of 4-6 nm, and produces chain scissions and some cross-linking. Commonly used for polyolefins, polyacetals, and polyethylene terephthalate, flame treatment increases wettability and interfacial diflfusivity. 

Oxidation which produces wettable, polar groups on the surface has been extensively applied to polyethylene.Also surface grafting is carried out for the same purpose as well as treatment with alkali and acid, halogenation, sulfonation etc. Also crosslinking of polyethylene can be effected with fluorine. 

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