Surface pretreatments abrasion treatments

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). 

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

Peeling, one of the most delicate pretreatments, is achieved industrially by abrasion, high-pressure steam, treatment with sodium hydroxide solution, or mechanically. Abrasion is effected by rough, moving surfaces which remove the outer surface of the product, but it has the drawback of considerable loss of raw material. Steam peeling consists of heating the product to... 

The most common technique involves abrading the substrate surface with hard powders (preferably diamond), or ultrasonic treatment in a slurry of an abrasive powder (preferably diamond grit) in an organic liquid for a prolonged period. This type of pretreatment leads to embedding of the abrasive particulates into the surface as well as mechanical damage to the substrate, both of which are believed to enhance the nucleation density. The maximum nucleation enhancement is observed when diamond grit is used, and this has been attributed to implantation of diamond particles... 

Composite materials can be prepared for bonding in several ways. Common procedures include mechanical abrasion of the cured composite surface and the use of a peel ply. This is a removable layer that is applied prior to curing the composite and then stripped to reveal a textured surface prior to adhesive bonding abrasion is sometimes used in addition to remove residues that may have been left by the peel ply (see Fibre composites - joining). There are also other surface treatments available, but aU aerospace bonding and pretreatment processes will require high standards of control. 

Polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) parts are generally joined by adhesives. Surface treatments recommended specifically for PBT include abrasion and solvent cleaning with toluene. Gas plasma surface treatments and chemical etch have been used where maximum strength is necessary. Solvent cleaning of PET surfaces is recommended. The linear film of polyethylene terephthalate (Mylar ) surface can be pretreated by alkeiline etching or plasma for msiximum adhesion, but often a special treatment is unnecessary. Commonly used adhesives for both PBT and PET substrates are isocyanate-cured polyesters, epoxies, and urethanes. Polyethylene terephthalate cannot be solvent cemented or heat welded. 

Various surface treatment processes have been developed to ensure adhesion of adhesives, coatings, or print to polymeric surfaces. These treatments are both mechanical and chemical. They include abrasive cleaning, chemical etching, corona discharge, flame treating, plasma treatment, and laser pretreatment. (See also surface treatment.)... 

Recently there has been increased use of surface coating materials, i.e. printing inks, metallization, or painting, to enhance the appearance and surface-related properties of plastics (e.g. barrier properties to gases or UV radiation, or increased abrasion resistance). Usually such treatment is accompanied by pretreatment to modify the surface of the polymer and to ensure adhesion of the surface coating to the substrate. 

Blackman et al. [71] compared the use of conventional abrasion pretreatments with corona discharge and oxygen plasma treatments. Again, the need to optimise the surface polarity was confirmed. As in the previous work, both adhesive and matrix chemistry influenced the optimum plasma conditions. For carbon/PEEK... 

Whilst mechanical abrasion does not appear to produce a surface topography suitable for mechanical interlocking to occur to any significant extent, chemical pretreatments can result in such surfaces. However, care is needed to ensure that the resulting surface layer is not too weak or friable and, even with the advent of modern surface-specific analytical equipment, the question of whether the treatment has induced chemical, as well as topographical, changes remains a difficult point to resolve. 

Simple abrasion is usually a very poor surface treatment for fluorocarbon polymers. This may be seen from the results shown in Table 4.2 where joint strengths for PTFE and PCTFE, bonded using an epoxy-polyamide adhesive, are given and the fluorocarbons have been pretreated by either abrasion or a... 

The technological implications of the above points are basically that, to attain good initial joint strengths, in many instances no surface treatment is necessary and even when the state of the incoming substrate material is such that relatively thick layers of protective greases, mill scale, etc. need to be removed then a simple degreasing and, possibly, mechanical abrasion, pretreatment are all that is required. 

When mechanical pretreatment is used, abrade the surface lightly. Fine grades of sand or abrasive paper will minimize penetration. The correct chemical treatment depends on the type of metal, which is deposited during the plating process. 

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