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Polymer Uses of Talc
The best talcs are significantly more expensive than fillers such as calcium carbonates and have to bring extra benefits to justify their use. The main benefits of talc are a greater increase in stiffness and heat distortion temperature than observed with the same amount of a blocky (low aspect ratio) filler like calcium carbonate. These effects are particularly noticeable in semicrystalline thermoplastics, such as polyolefins, and, as a result, these polymers are the main markets for talcs. In particular, they are used in polypropylenes for various automotive applications and in household appliances. In some applications, they are used in conjunction with lower-cost fillers, such as calcium carbonates. The extreme softness is a benefit as it means that machine wear is less than with other fillers (although this can be compromised by the presence of hard impurities, notably quartz).
As described earlier, there are many grades of talc, with variations in particle size and aspect ratio being two key variables. Unfortunately, much of the published literature isn’t very specific about these properties, often just referring to low, medium, or high lamellarity and particle size. The difference in effect on stiffness between a calcium carbonate and two different aspect ratio talcs in a polypropylene matrix is demonstrated in Fig. 1.
Heat distortion (deflection) temperature is another key property of semicrystalline thermoplastics which benefits from the use of lamellar fillers like talc. This is illustrated in Table 1.
While it is generally held that their enhanced effect on stiffness and HDT is due to their platy nature, other factors also contribute, notably the relatively high stiffness of the plates themselves and their effect on polymer crystallinity.
Creep is an important limitation of thermoplastics but receives little attention in the general literature. One of the advantages of platy fillers like talc is a significant reduction in creep compared to blocky fillers. This is illustrated by values presented in Table 2.
The lamellar nature means that talc can significantly lower the permeability of polymers, especially if the particles are aligned to provide maximum effect, as in polymer film. This is illustrated by the data in Table 3, which compares the oxygen and water vapor transmission rate of polypropylene film containing talc with one containing a calcium carbonate.
Fig. 1 A comparison of calcium carbonate and two talc fillers of different aspect ratio on the tensile modulus of a homopolymer polypropylene compound
Table 1 The effect of talc lamellarity on the heat deflection temperature (HDT) of polypropylene
Table 2 A comparison of the effect of talc on the creep of a polypropylene composite
Table 3 A comparison of the effect of a lamellar talc on the permeability of homopolymer polypropylene film
Impact resistance is always an important property for thermoplastics but is a complicated property to describe. The failure mechanism and the effect of fillers and filler surface treatments vary considerably with the type of thermoplastic matrix involved. Glassy, amorphous polymers, such as polystyrene, behave much differently to semicrystalline polymers where the amorphous phase is ductile. Here, we only discuss the semicrystalline types, as they are the major market.
Impact resistance in these semicrystalline polymers can be affected by many talc properties, especially particle size, dispersion, and lamellarity, and these are not often sufficiently described in the literature on talc compounds to allow the different factors to be unambiguously quantified. It does seem that talcs in general reduce the impact strength of both homo- and copolymer polypropylene when used at levels above 10% w/w. What is clear is that the smaller size talcs give the best impact resistance, as shown in Fig. 2. (Note: this is a simplification for illustrative purposes only. It uses the average particle size; the largest size is as important and is assumed constant within the series used here.)
There is some evidence that very fine talcs actually increase impact resistance at loadings below 10% w/w, an effect that is believed to arise from polymer nucleation. This is illustrated in Fig. 3.
Deleterious effects on aging performance can be an issue for some talcs, especially in polypropylene which is particularly susceptible to such problems. These effects vary markedly from talc to talc and are mainly due to structural impurities such as aluminum and iron which can promote polymer degradation. It should be noted that the reactivity of these impurities can vary markedly, depending on their chemical environment, and so metal level alone is not a sufficient guide to performance. In some cases, surface treatments are used to deactivate these troublesome sites. This is illustrated in Fig. 4.
Fig. 2 The effect of talc particle size (d50) on Charpy impact resistance of a polypropylene copolymer (20% w/w talc loading)
Fig. 3 The effect of fine talc addition level on the impact resistance of a high impact grade of polypropylene
The main polymer use for talc fillers is in thermoplastic compounds for the automotive industry, and it is believed that these compounds have a high recycling rate. Many of the major automotive markets have stringent environment targets which encourage recycling. Thus, in the EU, the End of Life Vehicle Directive requires that a minimum of 95% of a vehicle’s weight must be reused or recycled. Talc-filled thermoplastics are recovered and reused for a variety of automotive applications, mainly in under bonnet parts, body arch liners, and cable harness parts. There is also downcycling into nonautomotive parts, such as water and sewage pipes, furniture feet, etc. According to the Industrial Minerals Association (ima-europe.eu), about 95% of the talc used in automotive applications in Europe is recycled in some way.
Fig. 4 Effect of talc surface modification on thermal stability of homopolymer polypropylene (days to failure at 150 °C)
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