8.2.1 PVC polymerisation, structure and morphology

The basic repeat unit for PVC is:

All types of PVC are formed via free-radical polymerisation of vinyl chloride monomers using peroxide catalysts. Use of this type of catalyst normally produces an atactic polymer (e.g. polystyrene). However, due to the relatively large size and electronegativity of the chlorine atom, commercial PVC contains about 55 % syndiotacticity. This is located in sequences around 5–12 repeat units long; these sequences are long enough to crystallise, producing ~ 10 % crystallinity. Crystallites are small (~ 10 nm), and do not form spherulites, so PVC can be transparent. They vary considerably in size and perfection, melting over a temperature range from 110 to 240 °C.

Most PVC is produced by suspension polymerisation. Because PVC is insoluble in its own monomer, the morphology of PVC grains is complex. The grains themselves are 100–150 µm in diameter, and irregular in shape, as shown in the scanning electron micrographs in Fig. 8.1. When a grain is cut open (Fig. 8.1(b)) primary particles (∼ 1 μm in diameter) are revealed together with a considerable amount of porosity. Further examination reveals a pericellular membrane, produced from the suspending agent, which coats each grain, while the primary particles consist of smaller particles known as domains, around 10 nm in size. The crystallinity exists within these domains.

In emulsion polymerisation a latex containing spherical primary particles (usually with a size range of around 0.3−1.0 μm) is produced. This is then dried to produce aggregates of primary particles, described as secondary particles or grains, which are 30–60 μm in diameter.

Clearly, when nanofillers are added to PVC, their location with respect to the structures described is of considerable importance, as discussed later.

8.2.2 PVC fusion and processing

All rigid PVC and some flexible PVC is melt processed, mainly by extrusion, but compression moulding, injection moulding and calendering are also used. During processing, it is necessary to convert individual grains into a continuous solid structure. This process is known as fusion or gelation. The fusion, or gelation, of PVC can be defined simply as the conversion of PVC grains, which will be mixed with the required additives, to produce a product with the desired properties. In practice, the process is significantly more complex. Important work in the area was carried out by Allsopp,³ who studied morphology changes during the processing of PVC by a variety of techniques, and proposed two possible mechanisms for PVC fusion. Fusion requires both heat and shear, and the level of shear determines which mechanism occurs. In the comminution mechanism, the PVC grains are broken down into primary particles, additives are distributed, and the primary particles are then fused together to form a melt. This occurs in more aggressive high-shear equipment such as Brabender rheometers and Banbury mixers. In compression moulding, which is a very low-shear process, primary particles can be fused together within grains, but it is difficult to fuse grains together adequately, except at very high temperatures at which degradation can become a problem. For extrusion and two-roll milling, shear is higher than in compression moulding, and fusion occurs through the mechanism described by Allsopp as the compaction, elongation, densification and fusion (CDFE) mechanism.

The CDFE mechanism, as illustrated in Fig. 8.2, has the following stages:

An unusual feature of PVC is the fact that it is processed below its highest melting temperature. Thermal analysis shows that melting and reformation of crystallinity have an important role in fusion. This led Covas et al.⁴ to develop the CDFE mechanism further to incorporate crystallisation. The results are summarised in Fig. 8.3. During processing, typically at 170 °C to 210 °C, some of the crystals will melt, whilst the remaining more perfect crystals, referred to as primary crystals, will be annealed. When a sufficient number of crystals have melted, the material will recrystallise during cooling to produce secondary crystallinity, which links the original structures to produce a strong material. Summers et al.⁵ also used crystallisation to explain the results obtained by a rheological method used to measure fusion.

Although initial studies in this area related to rigid PVC, it has subsequently been shown that similar considerations apply to the fusion of flexible PVC.⁶ It should be noted that some flexible PVC is processed via a plastisol process using emulsion polymer. Although the process is different, the melting and crystallisation processes discussed previously still occur. When nanofillers are added, it is important that they are well dispersed in the homogeneous melt.

8.2.3 PVC degradation

If PVC were made up solely of (-CH₂ CHCl-) repeat units, its stability would be reasonably good. However, in practice, chemical defect structures are present – labile chlorine, unsaturation, head-to-head units, and peroxide and hydroperoxide groups. The latter in particular act as sites for the initiation of degradation. The first stage of degradation is the formation of macroradicals, but these undergo a rapid ‘unzipping‘ reaction leading to the production of hydrogen chloride, which is both corrosive and acts as a catalyst for degradation, and results in the formation of conjugated unsaturation along the polymer chain, i.e.

Conjugated chains absorb UV radiation, but as the number of conjugated units increases, the conjugation starts to absorb in the visible region, and the PVC changes in appearance, producing the well-known sequence of colours white/yellow/orange/brown/black. Without a stabiliser, this degradation can start at temperatures as low as 100 °C. It will be seen later that certain nanofillers can accelerate this process, while others can act as stabilisers.

8.2.4 PVC formulation and additives

As a plastic material, PVC is unique in the extent to which it is formulated. Additives are normally introduced by high-speed mixing in which the powder is stirred at speeds of the order of 3000 rpm. Solid additives are usually added first, followed by liquid ones. Solid additives coat the PVC grains, becoming located in hollows on the grain surfaces, while liquid additives are absorbed and located in porous regions. The high speed results in shear heating to above the glass transition temperature of the PVC (~ 80 °C), and mixing is normally continued until the temperature reaches 120 °C, when the mix is discharged into a cooling chamber. The resulting powder blend may be processed directly to make a product or extruded to make a pelletised compound for subsequent processing.

When plastisol technology is used the PVC is suspended in a liquid plasticiser, and additives are introduced by mixing with the liquid state.

The main additives used in PVC formulations are summarised below.

8.4.1 Montmorillonite (MMT)

The most widely used nanofiller for PVC is the layered clay, montmorillonite (MMT), both in its natural form as sodium montmorillonite (Na-MMT), and as organomodified (OMMT) grades. PVC/MMT nanocomposites were reviewed by Pagacz and Pielichowski in 2009.⁸ Organomodification of MMT was discussed in detail in this paper, which also gives a useful summary of salts used for MMT modification. Most researchers have used melt compounding to disperse nanoclays, with varying degrees of success. However, solution blending,^{1, 9, 10}polymerisation^11–17 and plastisols have also been used.¹⁸ One problem associated with these nanofillers arises because the most common organomodifiers for MMT are quaternary ammonium compounds, and the ammonium cations can actually contribute to PVC degradation.⁸ A variety of other modifiers have therefore been investigated.

Another of the problems associated with the formation of PVC nanocomposites is related to the morphology of PVC. This is illustrated in Fig. 8.4, which shows transmission electronic microscopy (TEM) micrographs of (a) melt compounded, (b) solution blended and (c) plastisol based nanocomposites containing Cloisite 30B OMMT.

Figure 8.4(a) shows intercalation and some exfoliation of nanoclay. Light areas without filler are PVC primary particles, which have not been destroyed. These areas could have been reduced by increasing the processing temperature, but full intercalation was not achieved because the PVC did not fully melt. When a melt-processed sample is dissolved in tetrahydrofuran, all morphology and crystallinity are destroyed, and a high level of exfoliation can be achieved (Fig. 8.4(b)). This is confirmed by the disappearance of the low-angle peaks observed by X-ray diffraction. When a nanocomposite is produced from a plastisol containing emulsion PVC, again there is some exfoliation (Fig. 8.4(c)), but less than in the solution-blended sample. In other words, the formation of PVC nanocomposites with high levels of exfoliation is difficult via melt compounding. Matuana²⁰ addressed the problem of achieving the appropriate PVC morphology for good nanoclay dispersion, using a torque rheometer, and showed that when nanoclay is introduced at the onset of fusion, when the PVC particles are reduced in size, better dispersion and enhanced mechanical properties are obtained. Industrial applications of this approach in, for example, extrusion need to be addressed.

When nanofillers are added during polymerisation, exfoliation can be achieved much more readily.^{12, 13, 16, 17}

8.4.2 Other clays

Other clays which have been used as nanofillers in PVC include Laponite,²¹ bentonite,^22–25 hectorite^{23, 25} and kaolinite.²⁶ Vandevyver and Eicholz²⁴ recognised the problems associated with dispersing nanofillers in suspended PVC, so they focussed on mixing sodium bentonite with an emulsion PVC latex, taking advantage of the ability of water to cause exfoliation. The mixture produced was spray dried, and its behaviour after milling and pressing, and in plastisols, was investigated.

8.4.3 Calcium carbonate

Various types of calcium carbonate (ground calcite, ground whiting and ground limestone) with mean particle sizes ranging from 1 to 30 μm are the most common fillers for PVC. More specialist (and expensive) precipitated calcium carbonates (mean particle size < 1 μm) are also used when good mechanical properties are required, particularly for rigid formulations. It is therefore unsurprising that nano-sized calcium carbonates have received a considerable amount of attention. Nanocomposites have been produced by polymerisation,^{27, 28} solution blending²⁹ and melt processing.^30–39 A novel polymerisation method involved synthesising nano-porous PVC particles then reacting calcium hydroxide and carbon dioxide inside these to produce PVC/CaCO₃ nanocomposites, which were finally melt blended.⁴⁰

8.4.4 Silica

Silica is another type of nanofiller which has been used in PVC. Incorporation techniques include polymerisation. ^{17, 28} There has been a specific focus on the surface coating of silica particles to improve compatibility with the PVC matrix^41–46 in melt-compounded nanocomposites.

8.4.5 Layered double hydroxides

Layered double hydroxides (LDHs), specifically hydrotalcite, although not necessarily in the form of nanoscale particles have, since 2000, found a significant application as secondary stabilisers in PVC.^{47, 48} They are usually found in stabiliser/lubricant packages, which are added to PVC formulations. Hydrotalcites are layered structures, but, unlike MMTs, have anions between the layers, which can be replaced by Cl^– ions produced during PVC degradation, thus retarding PVC degradation. Because of the benefits of hydrotalcites, hydrotalcite nanofillers are obviously of interest. In one of the earliest papers in this area, Wang et al.⁴⁹ compared nanocomposites prepared by in situ polymerisation and melt blending, and demonstrated that the former performed better in all respects. The same group also investigated nanocomposites containing various LDHs, and carried out further work on PVC/hydrotalcite nanocomposites prepared by in situ polymerisation.^{50, 51} LDH nanocomposites have also been prepared by melt processing,⁵² solution intercalation⁵³ and using a novel method in which the filler was delaminated by lauryl ether phosphate in tetrahydrofuran and then the suspension produced was added to PVC.⁵⁴ LDH nanofillers have also been modified by grafting with toluene diisocyanate to improve dispersion.⁵⁵

8.4.6 Carbon nanotubes

There are some reports of the use of carbon nanotubes (CNTs) in PVC, although the addition of a costly filler to an inexpensive commodity thermoplastic polymer can only be justified for very specialist applications, as discussed later. Solution blending has been used for the preparation of nanocomposites.⁵⁶

8.4.7 Other nanofillers

The use of a variety of other nanofillers in PVC has been reported. These include antimony trioxide,⁵⁷ polyhedral oligomeric silsesquioxane (POSS),^{58, 59} calcium sulphate,^{60, 61} calcium phosphate⁶² and zinc oxide.⁶³ Finally, combinations of two different fillers have been used on a number of occasions. These include CNTs with wood-flour, to improve the performance of commercially available wood-filled PVC,⁶⁴ and nano-calcium carbonate and wood-flour.⁶⁵ Nanoclays have been used with metallic oxides, which affect combustion and smoke suppressant properties,⁶⁶ and with zeolite to improve thermal stability.⁶⁷

8.5.1 Thermal stability

The thermal stability of PVC compounds may be improved or reduced by nanofillers. Frequently thermal stability has been assessed by thermogravimetric analysis,⁸ but in practice tests routinely used to assess the thermal stability of PVC compounds (e.g. the Congo red test, oven ageing and torque rheometry) are more important. Unsurprisingly, different nanofillers have different effects. Generally, MMTs reduce thermal stability, calcium carbonate and other clays can improve performance, but the effect of hydrotalcite is the most positive.

Commercially available organically modified MMTs tend to have adverse effects on PVC thermal stability,⁸ although effects can be reduced to some extent by changing the stabilisers present in the PVC formulation.⁶⁸ Wan et al.⁶⁹ suggested that discolouration rather than degradation occurred when OMMT was used in PVC, but this is not a widely held view. Attempts have been made to avoid the problem of poor stability by developing alternative modifiers. It has been shown that the incorporation of silica into a modifier improves thermal stability^{67, 70} Sterky et al.⁷¹ showed that, unlike normal cationic modifiers, non-ionic intercalants did not affect thermal stability. Another approach is the use of chelating agents to modify surface-treated MMT.⁷² Added metallic oxides also produced favourable results.⁶⁶ Other modifications have been discussed in the literature.⁸

Nano-calcium carbonate,^{35, 73} kaolinite²⁶ and sodium bentonite²⁴ also appear to improve the thermal stability of PVC compounds to some extent.

Consistently with the use of layered double hydroxides, such as hydrotalcite, as secondary stabilisers, these compounds appear to be the most effective nanofillers for enhancing PVC stability.^{49, 51, 53, 55, 72–76}

8.5.2 Mechanical properties

One of the most common benefits sought by the incorporation of nanofillers in a polymer is an improvement in mechanical properties. It should be remembered that rigid PVC itself has good strength and stiffness compared with other commodity thermoplastics (tensile strength 60 MPa and Young‘s modulus 2.8 GPa), providing that it is processed correctly. Toughness is typically improved by the addition of impact modifiers, as discussed in Section 8.2.4. The mechanical properties of PVC nanocomposites have been reported extensively. As with any filler, the modulus normally increases. Improvements in tensile properties can be achieved, but, unless dispersion is good, toughness and elongation can often decrease.

The mechanical properties of MMT/PVC nanocomposites have been discussed previously.⁸ It is generally concluded that improvements can be achieved at low levels of MMT, but they are relatively modest compared with improvements seen for other polymers. In recent work the use of OMMT with a silane coupling agent has produced impact strengths higher than that of PVC alone when the clay content was < 7 phr.⁷⁷

Nano-calcium carbonate was found to be effective in increasing toughness in nanocomposites.³⁷ In order to enhance toughness, specific filler treatments have been used on a number of occasions. Examples include grafted nanotubes⁷⁸ and nano-calcium carbonate coated with silicone rubber.⁷⁹

8.5.3 Flammability and smoke suppression

Due to the presence of chlorine, rigid PVC does not burn readily but does produce smoke on burning. Flexible PVC burns more readily due to the presence of relatively large quantities of plasticiser (Section 8.2.4). Nanofillers that reduce the amount of smoke produced or make flexible PVC, particularly in applications like cables, more difficult to burn are of significant interest. A number of workers have investigated the use of MMTs to improve the fire performance of PVC.⁸ A considerable amount of research has been carried out by Beyer and co-workers^{25, 80–82} but results remain inconclusive, and, as observed previously, significant discolouration (dehydrochlorination) of the PVC occurs. However, modified flexible PVC nanocomposites also containing halloysite, an aluminosilicate (nanotube-based) filler, were found to exhibit enhanced flame retardancy.⁸³ It has also been shown using cone calorimetry that flame retardancy and smoke suppression were significantly improved by the addition of OMMT to PVC/wood-flour composites.⁸⁴

Modified OMMTs in which sodium ions were replaced by Cu^{2 +} have been shown to have improved flame retardancy.^{85, 86} The Cu^2 + ions were also found to be effective in smoke suppression. Fe-modified OMMTs have also shown improved flame retardant properties.⁸⁷ A detailed study of OMMT with added zinc, copper and molybdenum oxides has been carried out, and has shown that the metal oxides have a more significant effect than OMMT in improving fire and smoke suppression.⁸⁸

The beneficial effects of hydrotalcite on the thermal stability of PVC have already been discussed. It also appears to improve, very often in a modified form, fire performance. Nanofillers that appear promising include LDH/ZnO combinations, which reduced smoke density and increased the limiting oxygen index.^{89, 90} Phosphate-modified hydrotalcite has been shown to reduce smoke emissions.⁹¹

Zinc hydroxystannate (ZnSn(OH)₆) is well known as a flame-retardant/smoke suppressant for PVC, and nano-(ZnSn(OH)₆) has been shown to provide even better performance.⁹²

8.5.4 Permeability

Nanofillers have been used to reduce gas and liquid permeation through a polymer by increasing the tortuosity of the diffusion path. Hectorite and bentonite clays have been shown to decrease oxygen permeation in flexible PVC by up to 77 %. A fivefold improvement in oxygen barrier properties by the addition of OMMT has been reported.⁹³ PMMA-grafted silica has also been used to reduce oxygen and water permeation.⁴³ Recent work has investigated the use of OMMTs to reduce plasticiser migration in flexible PVC.⁹⁴ It was found that the nanoclay reduced plasticiser migration slightly when compounds were melt compounded, but the effect was considerably greater after dissolving the nanocomposite in tetrahydrofuran, and producing a cast film, because clay dispersion was much better, as discussed in Section 8.4. A novel application of nanofillers is the coextrusion of a carbon nanotube coating to reduce the ingress of moisture into wood-filled composites.⁹⁵

8.5.5 Transition temperatures

PVC has three transition temperatures. The broad melting temperature discussed in Section 8.2.1 is generally unaffected by additives, being controlled by the syndiotacticity of the PVC and the processing conditions. The glass transition temperature of rigid PVC is approximately 80 °C, depending on the precise formulation; this reduces to temperatures from below ambient down to temperatures as low as − 50 °C as plasticiser concentration increases. PVC also has a β transition, detected by dynamic mechanical analysis. This is split into two, with maxima at − 50 °C and 0 °C (using a frequency of 110 Hz), attributed to the movement of small chain segments in the amorphous and crystalline regions, respectively.

The effect of nanofillers on PVC transition temperatures has been reported extensively in the literature, with research mainly focussing on the glass transition temperature in rigid PVC. One paper⁹⁶ commented on the effects of MMT on the β transition in a lightly plasticised PVC, which was found to become broader with a similar peak value as Na-MMT but moved to a lower temperature; this transition also broadened in the presence of OMMT. Reasons for this are unclear because the glass transition temperature was virtually unchanged in these compounds. Mkhabela et al.⁹⁷ suggested that the PVC melting temperature was increased in carbon nanotube/PVC composites, but the differential scanning calorimetry traces reported do not illustrate PVC melting.

Nanofillers have been reported as increasing, ^{13, 14, 22, 27, 30, 33, 36, 50, 77, 97–99} decreasing^{16, 54, 100} and having no significant effect^99–101 on the glass transition temperature of rigid PVC, but where changes are reported they generally seem to be no more than 2–3 °C, so probably the latter is nearer the truth in most cases.

Benderly et al.²³ observed no change in the glass transition temperature of pPVC with the addition of hectorite and bentonite clays.

The Vicat softening point is a penetrometer technique, and measures the temperature at which a loaded probe penetrates a sample by a specific amount. It is a technological measure of transition temperature and, in an unfilled largely amorphous polymer such as PVC, it would be expected to approximate to the glass transition temperature. In filled polymers, the fillers themselves restrict penetration of the probe, such that Vicat softening points of filled polymers are often much higher than conventional glass transition temperatures. The T_g of rigid PVC is relatively low at 80 °C, so additives that increase this are beneficial. There are significant increases in the Vicat softening point with the addition of Na-MMT^{13, 102} and nanoscale calcium carbonate.³⁰

8.5.6 Electrical properties

Multi- and single-walled carbon nanotubes have been used to reduce electrical resistance.⁵⁶ Increased electrical conductivity with a very low percolation threshold has been achieved by the use of multi-walled carbon nanotubes. ^{103, 104} PVC/graphite nanosheet/nickel nanocomposites have been developed for electrostatic charge dissipation.¹⁰⁵

8.5.7 Other properties

PVC nanocomposites have been developed to enhance various other properties. Magnetite^106–108 has been used to impart magnetic properties. UV resistance was achieved by using phthalocyanine-modified Laponite clay.²¹

Various commercially available nanofillers have been used to improve specific compressive strength, specific flexural modulus and the density of PVC foams.¹⁰⁹ It has been shown that nanoparticles can increase the orientation of crystalline and amorphous phases in oriented PVC films¹¹⁰ and modify plastisol rheology.²⁴ Nanofillers can also be effective during recycling. The properties of recycled PVC were shown to be improved by the addition of nanoclay,¹¹¹ and surface-modified nano-calcium carbonate can improve compatibility with polypropylene in mixed plastic waste so that material containing 10–20 % polypropylene can be produced.

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