Particulate-filled thermoplastic

In particulate-filled thermoplastics, the matrix is the load-bearing component and all deformation processes take place in the matrix. Particulate fillers are, in most cases, not capable of carrying any substantial portion of the load due to the absence of interfacial friction as the means of stress transfer. This is evidenced by the lack of broken particles on the surfaces of fractured filled thermoplastics. Hence, it seems appropriate to start this volume with a brief overview of the basic structural levels and manifestation of these levels in governing the mechanical properties of semicrystaUine thermoplastics used in compounding. 

General Concepts of Yielding in Particulate Filled Thermoplastics... 

Consider a particulate-filled thermoplastic consisting of a random distribution of uniform size spherical inclusions of diameter d. Although they are ran-... 

Pukanszky, B., Vanes, M., Maurer, H.J., Voros, G.Y., Micromechanical deformations in particulate filled thermoplastics volume strain measurements, /. Mater. Sci. 29 (1994) 2350-2355. 

Twin-screw extruders are used most often for the production of particulate filled thermoplastics [166]. A wide variety of machines are available in the market. 

Particulate-filled thermoplastic polyolefins are used in vehicle applications. Talc, calcium carbonate, and kaolin are typical fillers. Fillers do not change the transition characteristics of the plastic. Fillers increase the temperature-dependent elastic moduli of the plastics, increasing the forces required to form the plastics in the plateau-temperature regions. Matched-mold thermoforming is necessary if the product cannot be formed using a conventional pressme box over a single-surface mold. 

Pukanszky, B. et al.. Micromechanical deformations in particulate filled thermoplastics— Volume strain measurements. Journal of Materials Science, 1994. 29(9) 2350-2358. 

Since very few rigorous theories are available in the literature to enable one to predict the bulk rheological properties of particulate-filled viscoelastic polymeric fluids, in this section we present primarily the experimental observations reported in the literature on the rheological behavior of particulate-filled thermoplastic polymers and elastomers. 

The use of glass fibers for producing fiber-reinforced thermoplastic composites has long been practiced in industry. The use of glass fibers in thermoplastic polymers requires special attention in that the fibers, when mixed with a molten polymer, orient in certain directions, thus producing some unique mechanical properties not achievable with particulate-filled thermoplastic polymers. 

Nanocomposites are composed of a polymer matrix and layered silicate platelets having approximately 1 nm thickness and large aspect ratio. In the last two decades, nanocomposites have attracted much attention from both industry and academia, because they may offer enhanced mechanical and/or physical properties (e.g., high modulus and high heat-distortion temperature) that are not readily available from conventional particulate-filled thermoplastic polymers. One of the advantages of such nanocomposites lies in that the concentration of layered silicates required is much lower (say, less than 7 wt%) than that (e.g., 40-60wt%) required for the conventional particulate-flUed thermoplastic composites to achieve a similar property enhancement. The lower specific gravity of nanocomposites as compared to conventional thermoplastic composites can offer potential cost benefits as well. 

Hancock M Filled Thermoplastics. In Rothon R (ed) Particulate filled polymer composites. Longman Scientific Technical, Harlow (UK), p 279... 

Lutz JT Jr (1989) Thermoplastic polymer additives. Marcel Dekker, New York Miyata S, Imahashi T, Anabuk H (1980) J Appl Polym Sci 25 415 Hancock M (1995) Filled thermoplastics. In Rothon RN (ed) Particulate-fiUed polymer composites. Longman, Harlow, p281... 

Shear yield behaviour of polymer melts containing plate-like filler particles is also prevalent and is clearly shown in Fig. 8 for talc-filled polystyrene. In this system an estimate was made of shear yield values, which were found to increase with increasing particle loading and decreasing particle size. These results are compared with reported yield values for other particulate-filled polymers in Table 2. It is evident that shear yield values also depend on the particle type and thermoplastic matrix used. 

PE-PEP diblock were similar to each other at high PE content (50-90%). This was because the mechanical properties were determined predominantly by the behaviour of the more continuous PE phase. For lower PE contents (7-29%) there were major differences in the mechanical properties of polymers with different architectures, all of which formed a cubic-packed sphere phase. PE-PEP-PE triblocks were found to be thermoplastic elastomers, whereas PEP-PE-PEP triblocks behaved like particulate filled rubber.The difference was proposed to result from bridging of PE domains across spheres in PE-PEP-PE triblocks, which acted as physical cross-links due to anchorage of the PE blocks in the semicrystalline domains. No such arrangement is possible for the PEP-PE-PEP or PE-PEP copolymers (Mohajer et al. 1982). 

Whilst many of these areas fall outside the scope of this chapter, particulate polymer composites are becoming increasingly complex and commonly require more than just inclusion of a filler or particle additive in order to achieve optimum properties. For example, rubber modification of mineral-filled thermoplastics to yield a balance of enhanced toughness and stiffness, is an area of commercial importance. In these ternary-phase systems, there is not only a requirement to attain good dispersion of the filler component, but also a need for breakdown of the rubbery inclusion to yield the most effective size and spatial location within the composition. Whilst this may depend to a large extent on characteristics of the material s formulation, it can also be influenced by the material s compounding route. 

Fig. 5.70 Sections of a mica filled thermoplastic are shown in the optical (A and B) and TEM (C) micrographs. The platy mineral filler particles are aligned with the polymer. The particulate texture of the mica (black) in the TEM cross section reflects the effects of diamond knife fracture of individual mica flakes.

---