Thermosetting Plastic Substrates

In general, unfilled thermosetting plastics tend to be harder, more brittle, and not as tough as thermoplastics. Thus, it is common practice to add filler to thermosetting resins. These fillers can affect the nature of the adhesive bond (either positively or negatively) and are a possible source of lot-to-lot and supplier-to-supplier variability. 

The surface of thermoset materials may be of slightly different chemical character than the material beneath the surface because of surface inhibition during cure or reaction of the surface with oxygen and/or humidity in the surroundings. By abrading the surface, a more consistent material is available for the adhesive to bond. 

Similar surface abrasion processes can be applied on all thermosetting plastics. Mechanical abrasion methods consist of abrasion by fine sandpaper, carborundum or alumina abrasives, metal wools, or steel shot. The following surface treatment procedure is usually recommended for most thermosetting plastics  

Grit- or vapor-blast, or abrade with 100- to 300-grit emery cloth. 

The roughness of the abrasion media can vary with the hardness of the plastic. Usually, this is not a critical parameter except where decorative surfaces are important. 

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Thermosetting Foam Substrates Most thermosetting plastics are not particularly difficult to bond. Obviously, solvent cementing is not suitable for bonding thermosets to themselves, since they are not soluble. In some cases solvent solutions can be used to join thermoplastics to thermosets. In general, adhesive bonding is the only practical method of joining a thermoset to itself or to a non-plastic material. Epoxies or modified epoxies are the most widely used adhesives for thermosets (1). 

With the flexibility of the blown film extrusion process, one-step production of wide multilayer films is possible. Incorporating a built-in adhesive layer, PVDF films can be laminated directly onto various thermo and thermoset plastics or primed metal substrates. PVDF films can protect the esthetics of UV sensitive substrates while also providing a thermal, chemical, and abrasion-resistant barrier. Recently, white PVDF films have been commercialized that are of full opacity in the UV range of 290 00 Most recently,... 

Chem. Descrip. Oil-free sat. polyester resin In xylene Uses Polyester for high-sollds, low-VOC thermoset coatings, high-sollds baking enamels, coatings (machinery, metal furniture, appliances, automotive OEM, plastic substrates)... 

As a one coat finish for plastics, acrylic urethanes have one major limitation, the aggressive solvent content of the acrylic and the polyisocyanate resins themselves. This limits their use to coatings for thermosetting substrates and a small number of TPA substrates such as ABS, PA (polyamide), RIM polyurethane and PPO (jmlyphenylene oxide). Even then, the system should be evaluated to ensure that no degradation of the plastic s impact resistance has taken place. Consult the TPA chapter for discussion on plastic substrates. 

Adhesives can also stress crack or craze certain plastics and this can also indnce premature failure of the substrate. This generally happens whilst the adhesive is uncured and the plastic part is pre-stressed due to abrupt changes of section. The liquid adhesive softens and weakens the plastic leading to the formation of cracks and the liqnid adhesive then penetrates these cracks causing fnrther damage. With fast-curing adhesives, this is less likely to occur as once the adhesive has cured it is essentially a thermoset plastic (or thermoplastic) itself and is therefore inert. Amorphons thermoplastics are more prone to stress cracking than others and so it is important therefore to ensure that the adhesive is compatible with the substrate. 

The substance and significance of the thermal expansion of plastics is discussed in more detail below. The reason is that MID are multimaterial systems, consisting of polymer substrate (thermoplastic, and in the future thermoset plastics as well), metal (e.g., Cu-Ni-Au), and electronic devices. Fig. 2.7 illustrates this, taking an MID conductor structured by laser direct structuring by way of example. 

Polymer thick films also perform conductor, resistor, and dielectric functions, but here the polymeric resias remain an iategral part after cuting. Owiag to the relatively low (120—165°C) processiag temperatures, both plastic and ceramic substrates can be used, lea ding to overall low costs ia materials and fabrication. A common conductive composition for flexible membrane switches ia touch keyboards uses fine silver particles ia a thermoplastic or thermoset polymeric biader. 

Development of a glossy, aesthetic and unpainted roof module on the Smart Roadster by DaimlerChrysler AG s Smart. The two-piece, removable roof, built by ArvinMeritor, is surfaced with a thermoformable three-layer film (Lexan by GE Plastics) that can be co-moulded with either thermoplastic or thermoset substrates. The film exhibits 95% gloss retention after the equivalent of 10 years of Florida sun exposure. Weight saving is 50% versus a painted steel roof system. 

Embossing is used for producing a tactile texture or pattern on plastics sheet or film. As the process involves the use of heat and pressure to texture a semifinished substrate, embossing is largely limited to thermoplastic materials. However, it can be adapted to thermoset composites, such as melamine-impregnated sheet stock. 

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