Highly Filled Thermoplastics

Technically adequate thermal conductivity figures are in the region of 1 to 20 Wm K and require the admixture of weighted quantities of fillers and additives. Filler systems can be ceramic materials (thermally conductive but electrically insulating) and metallic materials (thermally and electrically conductive) and diverse modified forms of carbon [4, 31]. The high proportion of fillers alters behavior radically compared to that of nonmodified thermoplastics see Fig. 2.10. 

FIGURE 2.10 Dependency of material properties on filler content 

In principle, compounds for MID can be manufactured with a very wide variety of fillers from the groups of metallic (only for MID processes that do not require wet-chemical conductor metallization) or ceramic materials. At this time some fillers are coming into widespread use on account of their verifiably excellent thermal properties, offering considerable benefits for MID production (e.g., reduction in hotspots for lead-free soldering processes) and MID use (e.g., integrated thermal management in highly loaded MID such as LED modules). The most important of these fillers are 

MID with heat conductivity ratings up to about 20 Wm K are possible these days, depending on filler content and also on particle shape and size. In comparison, unmodified thermoplastics (see Section 2.3.1) have thermal conductivity values in the region of 0.15 Wm K so selective modification with fillers up to about 60% by volume clearly benefits the processing and use phases. Thermal conductivity shows an overproportional increase as filler content rises (Fig. 2.11). Over and above filler content, the fillers themselves can be classified in a number of different ways. 

Filler geometry affords one system of classification based purely on the mechanics of reinforcement  

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Below the development and employment of high-filled thermoplastics is examined, which represent an economical alternative for the production of bipolar plates with lower processing costs and more affordable raw materials for the following reasons ... 

Bigg, D.M. (1984) Complex rheology of highly filled thermoplastic melts, Proc. DC Inti. Congress on Rheology in Mexico, Adv. in Rheology, 3,429-37. 

The direct manufacture of prototype molds using the rapid prototyping process offers significant advantages (see also Section 4.7). By LaserCUSING and adapted processes, molds without restriction for prototype and small-series production can be used. Also, highly filled thermoplastics of up to 70% GF can still be manufactured to several thousand injection molded parts on standard injection molding units. 

Hordijk A C, Schoolderman C, Van der Heijden A E D M (2001) Highly filled thermoplastic elastomers (TPEs) - processing and rheology, Polym Rheol Conf, Shrewsbury, IXT13/1-P13/9 (CA 140 376339). 

For filled thermoplastics (30-40% by mass of chalk, ash or asbestos), complex shear may, as reported in [235], provide an increase of apparatus productivity by 40-80%, or if the flow rate is to be constant, the pressure in the molding instrument may be reduced by at least 20-30%. It is to be noted that while some extra power is required to create the complex shear conditions, the total power consumption of the apparatus as a whole may be reduced, on the power per unit of product basis, due to the high extrusion rates [233]. 

In [332] it was noted that the strength of samples cut out at different locations of an article made from filled thermoplastics by pressure molding may differ widely — which is due to the non uniform orientation of the polymer at different locations of the mold. The very high strength parameters of composites with PMF in molded specimens are obviously also due to orientation effects, while for standard mixed samples of similar composition (that is, a matrix which, apart from the filler, contains some superhigh molecular polyethylene imitating the PMF coats) the... 

Binary combinations, i.e. filled thermoplastics or blends of two polymers, exhibit either increased stiffness or enhanced fracture resistance [17,18]. High fill-... 

It has been suggested that the three-dimensional network structures discussed above, which are believed to occur from particle interactions at high filler loadings, may, in the case of plate-like particles, lead to anisotropic shear yield values [35]. Although this effect has not been substantiated experimentally, further theoretical interpretation of shear yield phenomena in talc- and mica-filled thermoplastics has been attempted [31,35]. 

To conclude this brief digression into history, we may point out one more important aspect the high efficiency of the combined shear in molding of filled thermoplastics. One of the first works in this field was 31) which described experiments carried out with polypropylene filled with a disperse aggregate calcium carbonate (chalk) and a short-fiber material-asbestos. 

Results given in Table VII show that the viscosity versus shear rate variation of these three Santoprene grades fits the power law over the entire range from 10 to 5200 s . Both the viscosity and the extrudate swell at constant shear rate increase with decreasing rubber particle content (or increasing hardness). Thus, these olefinic thermoplastic vulcanizates essentially behave like highly filled fluids in flow. When compared with unvulcanised rubber (see previous sections) or polypropylene - EPDM blends (32), the extrudate swell appears low and there is no Newtonian viscosity plateau. 

Chem. Descrip. Thermoplastic acrylic based on BA/MMA Uses Acrylic for paper coatings, wallpaper ground coats, highly filled formulations... 

In 1994, Monsanto patented a process to recycle aU the components of postconsumer nylon 6,6 carpet, without separation, into a filled thermoplastic product suitable for injection molding [62, 63]. It used a twin-screw extruder to accomplish high-intensity mixing of the thermoplastic from carpet samples. The recycled material contained 35-67 wt% nylon, 8-21 wt% polypropylene, 5-29 wt % SBR, and 10-40 wt % inorganic filler. In one study, no compatibilizer was used [62]. The carpet samples were fed directly into a twin-screw extruder operating at about 250-260°C and at a shear rate of 200-400 s The tensile... 

The rheology of filled polymers has been reviewed extensively [44,45], In general, viscosity curves of highly filled polymers show a yielding behavior at low shear rates followed by a power-law behavior at high shear rates [44], For most of the filled thermoplastics with small particles such as glass beads, calcium carbonate, talc, and carbon black, etc., the viscosity increases with the filler concentration. For some filled systems, however, the viscosity increases with the filler content up to the critical concentration, then decreases [46] or becomes little dependent on the filler concentration [47], This is particularly true for glass fiber-filled polymers. 

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