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The properties of filled thermoplastics strongly depend on how the filler is presented in the polymer, especially its degree of interaction with the host matrix and the nature and extent of mixing achieved. The method used to combine filler and polymer defines the microstructure developed, principally through exposure to the shear and elongational flow fields encountered during melt compounding.
Scope of Compounding Requirements
The properties of particulate-filled thermoplastic composites critically depend on the mixing procedures used to combine filler and polymer, the end processing techniques used to convert this compound into finished products, and the resulting structure developed in the composite. This entry will consider the former, which is usually undertaken by the judicious use of melt processing technologies, where the aim is to achieve uniform containment of filler in the polymer with the particle size being reduced to the minimum achievable level. There are many factors which can influence this requirement, in particular, the agglomeration tendency of the filler particles and the effectiveness of the melt mixing process in breaking down these interactions and creating a uniform distribution within the polymer matrix. Interparticle attraction is also affected by the surface chemistry of the filler, the presence of modifying surface treatments applied to the filler, and the polarity of the host polymer, including the presence of reactive or polar functional phases blended with the polymer to enhance interaction with the filler.
The importance of optimum mixing between filler and polymer is demonstrated by the effect of filler dispersion on physical properties of the composite. With inorganic fillers, such as calcium carbonate, poorly dispersed particles, evident as agglomerates, can act as stress concentrators causing large reductions in tensile and impact strength. Furthermore, the optical properties of pigments in polymers, including color strength, opacity, and gloss, and the efficiency of UV light stabilizers, strongly depend on the extent of filler dispersion.
Although many additives, such as pigments and heat and light stabilizers, are used in very small amounts (<3% by weight), a large number of fillers are added to thermoplastics at much higher addition levels, even as much as 60-80% by weight. This might be to create a wood effect with the polymer, maximize its resistance to combustion, or impart special magnetic or optical properties. Frequently, polymer masterbatches are prepared containing high concentrations of functional additives, such as pigments, which are subsequently diluted when added to virgin polymer during secondary processing. In some polymer-based formulations used for making injection-molded ceramics, on a volume basis, the inorganic filler level may greatly exceed the amount of polymer present, which is then removed during subsequent burnout. The presence of high loadings of filler poses several additional challenges during compounding in addition to effective dispersion, including the requirement for premixing of filler and polymer before melt compounding or, alternatively, controlled feeding of the filler directly into the melt compounding machine, together with a capability for the machinery to effectively melt and transport the compound through the process, especially when continuous compounding. A further directly related consideration is the dramatic increase in melt viscosity of the polymer, which occurs when significant loadings of filler are present. This results in high processing pressures and power requirements, resulting in the need for specially engineered, robust, and generally costly compounding plant.
In addition to the various material factors highlighted above, the design and operation of the melt compounding machinery is of prime importance in determining particle dispersion, or breakdown of agglomerates, with their subsequent randomization within the matrix. Often this requires development of high levels of shear stress during mixing, where interparticle attractions are strong, with controlled shear strain in the melt to optimize particle distribution and ultimate structure within the composite. However, there are some instances with shear-sensitive additives, such as hollow glass microspheres or carbon or glass fibers, where the application of excessive shear stress can result in significant filler damage with resulting loss of functionality, and a compromise must be achieved to obtain acceptable homogeneity with minimum additive breakdown.
Some fillers are thermally sensitive and can degrade or decompose during the compounding stage, due to both the time and temperature of exposure when melting the polymer. For example, such concerns apply with organic fillers, such as wood flour and alumina trihydrate fire retardant. The viscous dissipation of mechanical energy into shear heat can exacerbate this problem, especially where there is a need to develop high shear stress to effect filler dispersion or where filler loadings are high (causing an increase in polymer melt viscosity) or in high-throughput continuous compounding lines.
Moisture sensitivity is an issue relevant to all polymer melt processing operations. However, the presence of high surface area or hygroscopic fillers may significantly increase the need for pre-drying measures, either before compounding or through design of one or more devolatilization stages during the compounding process, to remove moisture (and/or other volatiles) present in the material, which would otherwise detrimentally affect the quality of the compound. For this purpose, vacuum- assisted venting zones are frequently used during continuous extrusion compounding.
There are many specialist-compounding situations, for example, with multiphase additive combinations involving the use of fillers. In this regard, particulate fillers may be combined with reinforcing fibers to control fiber orientation in the composite. Here there may be a conflict between achieving high levels of shear stress to disperse the filler and minimizing breakage to the shear-sensitive fiber. In order to obtain a balance between modulus increase, due to the presence of rigid fillers in thermoplastics and increased toughness from inclusion of rubbery additives, a combination of these two phases may be used. Whether or not the rubbery phase encapsulates the filler or exists as discrete particles has a direct effect on mechanical properties. This spatial distribution is influenced both by the relative physiochemical affinity of the filler and rubber and by the compounding methodology employed. Imparting electrical conductivity into thermoplastics is equally complex and can be achieved by inclusion of fillers such as graphite, carbon black, or metal powders into the polymer. However, in order to obtain electrical percolation (a conductive pathway) through the material, it requires judicious control of the amount of filler, the particle characteristics (size, shape, and inherent conductivity), and, very importantly, the microstructure developed during primary melt mixing and then after secondary processing to the finished product. Although outside the scope of this review, melt compounding considerations are central to the emerging field of polymer nanocomposites. This includes nanoscale fillers such as silicate layer (or nanoclay) structures, carbon nanotubes, graphene, and cellulose nanofibers. Amounts added to the polymer tend to be very low, sometimes less than 1 wt%, and may require specially modified compounding methods. More relevant to the present discussion, however, is the emerging interest in combining conventional macroscale fillers with nanoparticulates within the same compound, thereby achieving property benefits from each component.
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