Thermoplastic Substrates

Unlike the thermosetting resins, the thermoplastic resins will soften on heating or on contact with solvents. They will then harden on cooling or on evaporation of the solvent from the material. This is a result of the noncrosslinked chemical structure of thermoplastic molecules. The following are important characteristics of thermoplastic resins that can affect their joining capability. 

Thermoplastic materials often have a lower surface energy than do thermosetting materials. Thus, physical or chemical modification of the surface is necessary to achieve acceptable bonding. This is especially true of the crystalline thermoplastics such as polyolefins, linear polyesters, and fluoropolymers. Methods used to increase the surface energy and improve wettability and adhesion include 

The reasons that these surface treatments improve the adhesion of epoxy adhesive to plastic substrates are summarized in Table 16.8. 

As with metal substrates, the effects of plastic surface treatments decrease with time, so it is important to carry out the priming or bonding as soon as possible after surface preparation. The surface preparation methods suggested in App. F are recommended for conventional adhesive bonding. Greater care must be taken in cleaning thermoplastics than 

TABLE 16.8 Characterization of Common Surface Treatments for Polymers56 

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Resolution Performance Products, Starting Formulation 4008, Adhesive for Thermoplastic Substrate, Houston, TX, August 2003. 

Starting formulations for an epoxy adhesive for polyolefin, polyester, and thermoplastic substrates... 

The process by which vapor-deposited material forms two-dimensional arrays of spherical particles just beneath the substrate surface has been studied in detail. A model has been proposed in which the numbers and sizes of particles are determined by the coupled processes of particle growth (by capture of diffusing molecules) together with particle coalescence. Expressions have been derived for particle size and number density as functions of deposition parameters. Experimental evidence is presented in support of the model for the case of selenium physically vapor-deposited onto a heated thermoplastic substrate. Finally, the technological application of the deposit morphology as dry microfilm is reviewed. 

Figure 2. Plan view transmission electron micrograph of spherical selenium particles grown in a thermoplastic substrate. (Reproduced, with permission, from Ref. 2. Copyright 1981, American Institute of Physics.)...

Allan, J. 1993. The molecular binding of inherently conducting polymers to thermoplastic substrates. M.S. thesis, Clemson University, Clemson, SC. 

This is a processing method, by which decor moldings can be produced by the injection molding process, requiring just a few working steps. The moldings are composed of a thermoplastic substrate and decor material, which mainly consists of frhn or textiles. 

Door and Pillar Trim Door and pillar trim did not come widely into use in automobiles until the 1960s [1]. A variety of materials, particularly thermoplastic substrates such as PP and ABS PP or ABS covered with vinyl skin, cloth, or carpet covered thermoset RIM substrates and covered natural fiber substrates are used in their construction. The trend is toward lighter materials that flow easily and are easier to mold during processing, to reduce the mass and therefore the material use [3]. The thermoplastic substrates are easily recycled. If a noncompatible covering material is added, the recycling becomes more difficult. There has been an effort to develop covered systems that are more recyclable for instance, PP substrate with a TPO skin. 

Welding processes are of two main types thermal and solvent. By careful application of heat or solvent to a thermoplastic substrate, one may liquefy the surface resin and use it to form the bond. The bond strength is determined by diffusion of polymer from one snr-face into another instead of by the wetting and adsorption of an adhesive layer. It is possible to weld plastics of different types. However, for both thermal and solvent welding, the success of the process will be heavily determined by the compatibility of the polymers being joined. 

Diffusion Adhesion arises through the interdiffusion of molecules in the adhesive and adherend. Solvent or heat welding of thermoplastic substrates is considered to be due to diffusion of molecules. 

A primerless adhesive offering excellent adhesion to a variety of thermoset and thermoplastic substrates and tolerant of wide cure cycle variation. 

In the case of two-component composites such deflashing steps may become critical, if the components have different coefficients of thermal expansion and becanse of a strong increase of brittleness in case of thermoplastic substrates. 

TPV is often used for dynamic seals for doors, seals in glass run channels, O-ring seals, and CVJ boots. Self-bonding TPV grades eliminate the need for adhesive primers for overmolding to thermoplastic substrates including nylon. High-flow Santoprene TPV seals are overmolded to encapsulate rear quarter windows. 

However, where sieving is not the preferred route to achieve film performance, for example when the substrate caimot be heated up to high temperatures as with most thermoplastic substrates, or where very high metal film thicknesses are involved then powders become unsuitable. Here, two component acrylic/isocyanate technology is more appropriate. 

Of the many thermal characteristic values measurable for polymers, two that are highly relevant for molded interconnect devices are glass transition temperature (for amorphous and partially crystalhne thermoplastics) and melting temperature Tu (for partially crystalline thermoplastics). They can be determined by dynamic mechanical analysis (DMA) or differential scanning calorimetry (DSC). Other characteristic properties of primary importance are (short-term and long-term) service temperature and heat deflection temperature, as are temperature response and heat conductivity and the thermal expansion of the thermoplastic substrate materials. 

In the hot-embossing process a heated die carrying the negative of the conductor layout presses a specially coated copper film on to a thermoplastic substrate, applying thermal loading and mechanical pressure. The die cuts out the film, forming a positive bond to the locally melted plastic close to the surface of the blank. Figure 3.16 shows the process chain in simplified, schematic form. 

Insert molding, or in-mold processing as it is sometimes known, is characterized by the combination of a printed or structured film with a thermoplastic substrate material. MID technology makes use of this property in a variety of process combinations. 

Other factors (e.g., power transferred, plating makeup, thermal conductivity of the materials, environment, load duration, conductor pitch) will have to be taken into account in MID technology for more in-depth consideration and computation of current-carrying capability. In the approaches used to date, moreover, no attention is given to the thermoplastic substrate materials and the different metallization processes. The latter in particular, however, lead to differences in conductivity due to microstructure, fluctuations in plating thickness, and differences in roughness. Future studies will provide more results in this area, and this in turn will lead to a more accurate assessment of current-carrying capability [54]. 

There is little to be gained by using relatively large, two-terminal devices on thermoplastic materials. Two-terminal devices such as chip resistors are based on ceramic bodies, and consequently they have extremely low coefficients of thermal expansion (Table 5.1). The difference in CTE vis- -vis the thermoplastic substrate material is highest for this type of component. Given the absence of conditionally elastic connector legs, this type of device is extremely stiff and consequently unable to prevent deformation at the point of connection. 

Special attention has to be paid to the properties specific to thermoplastic substrates in environmental testing. 

A test specimen is subjected to elevated temperature and relative humidity over a certain period of time. The parameters frequently used in practice for MID have temperature set to +85 °C and 85% relative humidity for this test. The thermoplastic substrates absorb water under these conditions, so the test enables insulating properties to be checked along with corrosion of the plating. Humidity heat testing is frequently followed up by an extra test of the metallization s adhesion. 

Localized heating coupled with embossing has been demonstrated by many researchers to fabricate microfeatures on thermoplastic substrates. However, this... 

The development of an alternative fabrication technique that would solve these issues would have commercial benefits. Researchers have demonstrated that embossing techniques offer solutions to many of the previously detailed problems [3]. Localized heating coupled with embossing has been demonstrated by many researchers to fabricate micro-features on thermoplastic substrates. However, this process produces undesired flash which must be removed with costly secondary operations [4]. Because the substrate is only locally heated, the material that is displaced during the embossing caimot be captured by the embossing tool to form a flat surface. It is possible to heat the entire surface of the substrate and reform the flash to a flat surface, but this is not desirable because it increases the heat input. 

Zuyev, Konstantin, Processing Studies In Reactive In-Mold Coating for Thermoplastic Substrates, PhD. Thesis (2004)... 

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