Laminates, surface pretreatment

This general principle is as true for wooden substrates as it is for the high tech carbon fibre laminates. Indeed, it was with the bonding of wood in aircraft structures that the requirement for surface pretreatment first came to light. 

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. 

As a general procedure, the first step is preparation of the substrate, which may include cleaning, sizing and applying thin overlay by lamination to cover surface imperfections. Metal surfaces often need a primer coat to assure a good bond of the first coat to the metal. The first coat is then applied to the pretreated substrate. 

Considerable evidence exists indicating that the acidity of an oxide surface can vary according to the pretreatment. For example, Finlayson and Shah [12] used flow microcalorimetry to characterize the oxidized surfaces of three aluminum specimens that had received different pretreatments. They found that the surface chemistry of the three samples was considerably different but was dominated by Lewis base sites in all cases. The peel strength of ethylene/acrylic acid copolymers laminated against the substrates increased as the basicity of the substrate and the acrylic acid content of the co-polymer increased. 

HDPE is approximately a threefold better moisture barrier than LDPE of equivalent thickness. Polythene surfaces need pretreatment prior to printing or adhesive lamination. 

Base units can be obtained in steel, wood, or plastic laminates. The steel in the steel units should be heavy gauge, e.g., 18 gauge, with a pretreatment to reduce the corrosive effects of chemicals. Painting all surfaces with a durable, baked on, chemically resistant paint finish will also help minimize chemical effects. In better units, this is an epoxy coating. Some individuals continue to prefer wooden laboratory furniture. Because of cost, solid wooden furniture is not an economic choice, but durable wooden (or plastic) veneer furniture is available which can meet most safety requirements. Although wood may be more absorbent to liquids than steel, it is less reactive and more resistant to a very wide range of... 

Epoxy does not adhere well to untreated copper surfaces.This means that some type of treatment must be applied to the innerlayer before lamination. One option is to use double-treated copper, discussed earlier. Double-treated copper has a rough surface with a treatment supplied by the material vendor. Many ML-PWB fabricators report excellent results with double-treated copper. Others report problems with contamination and difficulty with rework. The alternative to using pretreated copper foil is to use a chemical treatment after etching. 

The perforated skins are usually pretreated by grit abrasion and then, occasionally, primed with a surface protection primer such as Redux 122 or with an adhesion-promotion primer. The adhesive is epoxy based and can be in film or in paste form. The acoustic panel so formed is either bonded directly to the engine casing or is first closed with a glass-reinforced epoxy laminate to promote stiffness and this GRP skin is then bonded directly to the casing. 

As may be seen from Tables 2.3 and 2.5, the surfaces of plastics, rubbers and fibre composites have low surface free energies (typically less than about 50 mJ/m ), in distinction from those of metals, with energies generally greater than about 500 mJ/m. With many plastics, plastic laminates and rubbers it is possible to obtain reasonable or even good adhesive joint strengths with little or no pretreatment. This is shown by the results in Table 4.1. 

SA(500)-NEt2 was found to be remarkably active to afford the double alkylated product in 93% yield (entry 1). Decreasing the pretreatment temperature of silica-alumina reduced the catalytic activity of SA(T)-NEt2 (entries 1-4). It is to be noted that this reaction scarcely proceeded with either SA(500) or homogeneous amines (entries 8 and 9). A mixture of SA(500) and triethy-lamine showed a catalytic activity, but the yield of the product was very low (entry 10). It is indicated that both the surface acid site and the amine group in SA(500)-NEt2 without acid-base neutralization act as catalytically active species. The reaction pathway might be similar to the case of 1,4-addition of nitriles (Scheme 1.10a). 

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