Surface preparation polyolefins

Near-field scanning techniques are relative newcomers, and the basis for their interpretation is less well established. However, AFM has opened up new perspectives for morphological studies, particularly given that excessive surface damage in soft specimens can be avoided by use of non-contact or intermittent contact modes. Its sensitivity to surface topography nevertheless makes AFM prone to artefacts when used to observe surfaces prepared by microtoming, and its effective depth of field is limited compared with SEM. On the other hand, if lamellar surfaces can be prepared such that the surface relief (or hardness, friction variations) is representative of the bulk texture, very striking detail can be recorded at the nanometre scale in deformed polyolefins [11]. 

To obtain a usable adhesive bond with polyolefins, the surface must be treated. A number of surface preparation methods, including flame, chemical, plasma, and primer treatments, are in use. Figure 16.4 illustrates the epoxy adhesive strength improvements that can be made by using various prebond surface treatments to change the critical surface tension of polyethylene. 

Epoxy and nitrile-phenolic adhesives have been used to bond polyolefin plastics after plasma surface preparation. Shear strengths in excess of 3000 psi have been reported on... 

Ultraviolet Radiation Exposure - In adhesive bonding, a surface preparation technique in which the substrate is irradiated with high intensity UV light. Exposure to UV radiation results in chain scissions, crosslinking, and oxidation of the polymer surface. The effectiveness of this technique is dependent on the wavelength of radiation used. It is commonly used for polyolefins. Also called UV exposure. 

R.L. Ayres, D.L. Shofner, Preparing polyolefin surfaces fm inks and adhesives. SPE J. 28,... 

Polyolefin materials can be effectively bonded only if the surface is first oxidized. Polyethylene and polypropylene can be prepared for bonding by holding the flame of an oxyacetylene torch over the plastic until it becomes glossy, or else by heating the surface momentarily with a blast of hot air. It is important not to overheat the plastic, thereby causing deformation. The treated plastic must be bonded as quickly as possible after surface preparation. 

Adhesives manufacturers are continually trying to develop adhesives to meet the needs of industry. One group of plastics that have been difficult to bond are polyolefins and related low-energy substrates (see Surface energy). They could not be bonded without elaborate surface preparation such as Flame treatment or Plasma pre-treatment, Corona discharge treatment or oxidative chemical methods. 

Epoxy and nitrile-phenolic adhesives have been used to bond these plastics after surface preparation. The surface can be etched with a sodirim sulfiiric-dichromate add solution at an elevated temperature. Flame treatment and corona discharge have also been used. However, plasma treatment has proven to be the optimum siuface process for these materi2ds. Shear strengths in excess of 3000 Ib/in have been reported on polyethylene treated for 10 min in an oxygen plasma and bonded with an epoxy adhesive. Polyolefin materials can also be thermally welded, but they cannot be solvent cemented. 

Thermoplastic fibres often are more difficult to wet (see adsorption theory. Table 13.3). This is especially tme for the thermoplastics such as polyolefins and linear polyesters. Methods used to increase the wettability and improve adhesion include the surface treatments shown in Table 13.7. The effects of surface treatments generally decrease with time, so it is important to carry out adhesive bonding as soon as possible after surface preparation. 

This is excellent, except with thermoplastics and rubber where performance is substantially reduced. Low surface-energy materials (polyolefines, fluo-ropolymers and silicone rubbers, for example) may not be bonded without special surface preparation. 

Lord Corporation introduced adhesives containing methacrylated phosphate monomers that gave much-improved thermal and atmospheric durability, and Dymax Corporation introduced their aerobic acrylics that were less sensitive to inhibition by atmospheric oxygen. Dow Automotive, 3M, and Loctite recently introduced two-part acrylic-based adhesives that can bond many low-surface-energy plastics, including many grades of polypropylene, polyethylene, and thermoplastic polyolefins without special surface preparation (see Section 4.2.2 for a description of this technology). 

Flame treatment is widely used to prepare polyolefin surfaces for adhesive bonding, particularly in labelling operations. This method is purported to burn-off contaminants and weak boundary layers, and also produces surface oxidation. Although flame treatment can be readily automated on a container labelling line, it is very impractical for most product assembly operations. 

Polyethylene has a low surface energy (31 mN/m) and so will generally require surface preparation prior to bonding. The two-part acrylic polyolefin bonder showed good strengths in these trials [2] (Table 2.9). 

Du Pont has three types of Tedlar film, Type A, with one side treated for bonding Type B, with both sides bondable and Type S, which is untreated and is used as a release film (www2.dupont.com). Type B is used in laminating to metals, plastics, wood, and other materials it requires no further surface preparation for adhesive bonding. The methods for preparing the untreated film for adhesive bonding are similar to those of polyolefins and lluoropolymers (see Chapter 6). 

Many polymeric materials inherently have a low surface energy that results in poor surface adhesion or even complete adhesion failure. This makes it difficult for inks, paints, adhesives and other coatings to properly wet-out and adhere to the surface of these substrates. Proper surface preparation of these materials will increase surface energy, improve surface adhesion properties, and add value to the product and the process. However, one must keep in mind that it is the bulk mechanical properties of the polymer that control the interfacial forces, which in turn influence adhesion. We will be subsequently reviewing various substrate orientations, from oriented and metallized films to spunbonded polyolefins and molded polymers, in order to examine their bulk structures for their ability to endure mechanically-induced deformations to allow for surface roughening and chemical covalent bonds to achieve requisite adhesions. 

Many synthesis methods for nanopartides of these materials and thdr surface functionalization have been devdoped. Nanocomposites based on polyolefins were prepared by mdt compounding with nanoscaled ZnO and 1102. Amphiphilic copolymers and surfadants were used to stabilize pattides for film casting in polystyrene, " polycarbonate, and PMMA. By in situ partide preparation in MeOH and subsequent mixing with acetone and PMMA for solvent evaporation transparent ZnO/PMMA films could be realized even without further surface modification. Poly(acrylic add-co-sodium aaylate)/ZnO composite latex partides were obtained via inverse miniemulsion polymerization. ... 

Siloxane containing interpenetrating networks (IPN) have also been synthesized and some properties were reported 59,354 356>. However, they have not received much attention. Preparation and characterization of IPNs based on PDMS-polystyrene 354), PDMS-poly(methyl methacrylate) 354), polysiloxane-epoxy systems 355) and PDMS-polyurethane 356) were described. These materials all displayed two-phase morphologies, but only minor improvements were obtained over the physical and mechanical properties of the parent materials. This may be due to the difficulties encountered in controlling the structure and morphology of these IPN systems. Siloxane modified polyamide, polyester, polyolefin and various polyurethane based IPN materials are commercially available 59). Incorporation of siloxanes into these systems was reported to increase the hydrolytic stability, surface release, electrical properties of the base polymers and also to reduce the surface wear and friction due to the lubricating action of PDMS chains 59). 

This is a major achievement, mainly due to Basset and his group, in surface organometallic chemistry because it has been thus possible to prepare single site catalysts for various known or new catalytic reactions [53] such as metathesis of olefins [54], polymerization of olefins [55], alkane metathesis [56], coupHng of methane to ethane and hydrogen [57], cleavage of alkanes by methane [58], hydrogenolysis of polyolefins [59] and alkanes [60], direct transformation of ethylene into propylene [61], etc. These topics are considered in detail in subsequent chapters. 

Conventional bright-field TEM observations of polyolefins often require contrast enhancement, usually by staining with Ru04 or other suitable markers [17]. These accumulate in the amorphous phase, at lamellar surfaces and in cavities, and differential staining can reveal the phase distribution in blends. Staining also hardens the specimens, facilitating preparation of thin sections at room temperature (cryo-sectioning is required for unfixed polyolefins). 

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