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The manufacture and properties of titanium dioxide pigments for plastics applications

Abstract

TiO2 pigment manufacturers offer a wide range of sulfate and chloride high-performance titanium dioxide (TiO2) pigments for plastics applications. These pigments provide opacity and durable whiteness to a wide variety of materials ranging from weatherable PVC siding, through aesthetically pleasing automotive components, to the thinnest of polyolefin films. This article discusses the features of TiO2 pigments that make them suitable for optimal performance in plastics applications.

Titanium Dioxide – physical features that make it an outstanding white pigment for plastics

Light scattering (1)

Visible light travels through different substances at different speeds. The speed of light through any substance relative to the speed of light through a vacuum is defined as its refractive index:

Where c is the speed of light in a vacuum and v is the velocity of light in the substance.

When a ray of visible light passes from one substance to another and the two substances have different refractive indices, the light changes direction. The greater the difference in refractive index between the two media, the greater is the change in direction, and consequently, if one of the substances is present as a polymer (low refractive index) and the other as a pigment (high refractive index), the more the incident light will be scattered, and the more opaque and white will be that composite substance. The bigger the difference between these refractive indices, the greater will be the scattering of light.

Two allotropes of titanium dioxide, rutile and anatase, are commercially suitable for use as pigments. This is because they have very high refractive indices and can scatter electromagnetic radiation efficiently - especially in the visible region of the spectrum.

The refractive indices shown in Figure 1 allow comparison of rutile and anatase TiO2 with other materials which might be considered for use as opacifying pigments. Although some materials have refractive indices approaching those of TiO2, the best combination of high refractive index and ease/cost of production is provided by TiO2.

Figure 1: Refractive Indices of Actual and Potential White Pigments

When the refractive indices of TiO2 are compared to those of polymer resins (Figure 2), the difference in refractive index between pigment and polymer - the requirement for effective reflectivity - is evident.

Figure 2: Refractive Indices of TiO2 and Some Common Polymers

In addition to the requirement for high reflectivity, to deliver opacity efficiently, a pigment must also be present in a well-dispersed particulate form with the correct average particle size and a narrow particle size distribution. For TiO2 pigments, the optimal particle size for visible light scattering is about 0.25 microns. Therefore ideally, all TiO2 particles would be monodispersed with particle size of 0.25 microns. In practice however, TiO2 is made with a range of particle sizes centered on the optimal average size.

The amount of oversize pigment particles must be minimized if a high gloss surface finish is required. “Oversize” TiO2 particles are TiO2 particles greater than about 0.4 microns. The presence of a high proportion of these particles, which can protrude from the surface of a blown film, for example, would make it difficult to obtain high gloss.

Weathering Protection (4)

Titanium dioxide is an intrinsic semiconductor. The energy required to promote an electron from the valence to conduction band is 3.0 eV for rutile and 3.2 eV for anatase. Both the excited-state electron, and the hole left in the conduction band, are highly mobile within the TiO2 lattice. There are several fates for the excited-state electrons and holes:

  • The hole/electron pair may simply recombine, emitting a small amount of heat, and without damaging the polymer matrix surrounding the pigment particle.
  • The hole or electron may encounter an impurity with a variable oxidation state within the TiO2 lattice which can passivate the hole or electron by changing its oxidation state. No degradation of the polymer is caused by this type of event.
  • The hole or electron may encounter an oxygen or water molecule adsorbed at the surface of the TiO2. This can cause the formation and liberation of highly reactive radical species such as hydroxy and peroxy radicals as shown schematically in Figure 3 with the potential to cause degradation of the surrounding polymer.

Figure 3: Formation of hydroxy and peroxy radicals at the surface of rutile TiO2

Hydroxy and peroxy radicals liberated into the polymer matrix near the surface of the TiO2 are extremely destructive. They can initiate the radical degradation processes that lead to the discoloration and the loss of mechanical integrity of plastics materials. These degradative processes are often autocatalytic, so unless preventative measures are taken, it is easy for degradation to continue and worsen. The progression of weathering in TiO2 pigmented plastics materials is shown in schematic summary in Figure 4. There are two mechanisms which can operate depending on whether the TiO2 pigment is acting in a protective or pre-degradant role. When the presence of TiO2 pigment acts to increase the rate of degradation, the process is called “photocatalytic”, but when the TiO2 pigment acts to retard the rate of degradation, the process is called “photochemical”.

Figure 4: Photocatalytic (a-c) and Photochemical (d-f) Degradation during exposure of a pigmented film to ultraviolet radiation

Figure 4(a) represents the pigmented film very early in its exposure to ultraviolet radiation. The surface of the film is intact and the TiO2 pigment particles (grey spheres) are well distributed in the polymer matrix. As exposure continues (Figure 4(b)), radicals are produced by the effect of UV on the TiO2 particles and degradation of the polymer matrix near the surface of the TiO2 particles is initiated. Pigment particles near the surface of the film are more photoactive because the intensity of the UV light decreases as a function of depth into the film. A “cavity of degradation” grows around each pigment particle until these cavities meet and the top-most layer of the film is lost (Figure 4(c)). The surface loses gloss, appears “chalky”, and mechanical strength of the film decreases. Early in the photochemical process, we observe that the surface of the polymer film begins to degrade by direct irradiation with UV (Figure 4(d)). The TiO2 pigment now plays a protective role, absorbing and passivating the UV energy. As exposure continues, polymer is lost from the film between TiO2 particles, while the polymer beneath each TiO2 particle remains. Overall, the rate of degradation of the film is reduced compared to the photocatalytic case. Some gloss and mechanical strength is lost, but the useful life of the film is extended by the TiO2 pigment. Prodegradant types of TiO2 pigments are those based on anatase, or rutile pigments which have no surface coating at all, or an inappropriate, or poorly applied, non-coherent surface coating. Of course some applications have no requirement for resistance to weathering and the design of pigments for these applications does not include features for photochemical stability. Durable types of TiO2 pigment are based on the rutile form (Chloride Process rutile is slightly more stable to weathering than Sulfate Process rutile), and tend to have higher amounts of coating. These coatings often include a silica component.

Pigment Production

Pigmentary TiO2 is made by either of two methods: the Sulfate Process or the Chloride Process. In the Sulfate Process, which is shown schematically in Figure 5, ilmenite ore, mined from natural deposits found in China, Australia, Brazil, Norway (among others), is purified by a sequence of crystallization, filtration and washing techniques before being calcined to produce crystalline titanium dioxide. Either anatase or rutile TiO2 can be produced by the Sulfate Process. The required crystal form can be selected by choice of calciner additives to promote or inhibit rutilisation, and careful control of calcination conditions: temperature as a function of length along the calciner, residence time and so on. Crystal size, and thus particle size, is controlled by the concentration of crystal nuclei added before calcination as well as domain modifying additives and calcination conditions.

Figure 5: The Sulfate Process

In the Chloride Process, which is shown schematically in Figure 6, pigmentary titanium dioxide pigment with extremely low levels of transition metal impurities is produced, and this has benefits for the color of the pigment - Chloride Process pigments generally have a more blue tone than Sulfate Process pigments. The Chloride Process uses a starting material which is higher in titanium content than the Sulfate Process. It is convenient to use beneficiated ilmenite slag where a large proportion of the iron in the ilmenite ore, FeTiO3, has already been removed to be used in steel-making. This slag is reacted with chlorine gas in the presence of carbon in a fluidized bed reactor. This exothermic reaction produces titanium tetrachloride, TiCl4. Titanium tetrachloride is a liquid at ambient temperature and pressure, allowing it to be separated from impurities by distillation. The high-purity TiCl4 is then oxidized in the presence of certain additives – commonly metal halides - that control crystal growth kinetics and ensure a high degree of rutilisation. The chlorine produced at oxidation is recycled to be used again in the chlorination step.

Figure 6: The Chloride Process

The properties of most TiO2 pigments, made by the Sulfate or Chloride processes, are further tailored by application of coatings to the surface of the TiO2 crystals (Figure 7). These coatings are normally hydrous oxides of elements such as aluminum and silicon that have refractive indices that are much lower than TiO2. The roles of the coatings are to prevent compaction of the pigment during storage and transportation, to encourage rapid redispersion of the pigment during compounding, and to minimize the photochemical activity of the TiO2 in its final application. The hydrous oxides are precipitated from precursor salts e.g. aluminum sulfate, sodium aluminate, sodium silicate etc. In order to precipitate the hydrous oxides in the most effective form, the TiO2 manufacturer adjusts the pH and temperature and this can encourage agglomeration of the pigment particles. Consequently, various milling and grinding stages are necessary to ensure that the TiO2 pigment particles are well separated and stable with respect to compaction before the pigment can be released for sale.

The principal difference between rutile TiO2 pigments made by the Sulfate and Chloride Processes is color. As mentioned above, the color of Chloride Process pigments is sometimes preferred. But there are other differences. Chloride TiO2 rutile pigments are more abrasive than Sulfate process rutile pigments. This can have an effect on the wear of compounding equipment, for example by reducing the lifetime of extruder screws by wearing the flight tips, or increasing the amount of metal pick-up when using a high speed mixer to prepare a dry-blend or pre-mix with consequences for color.

Figure 7: The TiO2 Pigment Coating Process

In some applications where minimal abrasivity is required, anatase pigments can be used because anatase is much softer than either Chloride or Sulfate Process rutile TiO2. For example in the production of polypropylene fibers, which involves drawing the fibers at high speed through the narrow orifices of spinnerets, anatase is used as a low abrasivity delustering pigment to prolong the life-span of the spinnerets. Another advantage of anatase TiO2 pigment is that although it is more photochemically active than rutile TiO2, it absorbs less strongly than rutile in the near UV range of the spectrum. This is advantageous in materials where the presence of UV radiation is required – for example in materials that contain fluorescent whitening agents to provide a whiteness boost by absorbing UV light and re-emitting it as blue/white light.

Product Design

Pigment architecture

TiO2 pigment manufacturers work hard to make pigments that combine many properties: the right crystal size and crystal size distribution; high rutile content; the right particle size, and particle size distribution; stability with respect to compaction; low moisture content; passivation of photochemical activity, and so on, to provide pigments which are easily dispersed in the customer's application to impart opacity, tinting strength, gloss, good weather resistance.

The crystal sizes of Sulfate Process TiO2 pigments are controlled by careful addition of nucleating seeds and salts prior to calcination, and also by controlling the calcination temperature and residence time of the developing TiO2 as it progresses along the calciner. The crystal size of Chloride Process TiO2 pigments is mainly controlled by addition of salts at oxidation. Although the optimum crystal size for TiO2 pigments for opacity is about 0.25 microns, TiO2 pigments made for plastics applications are often intentionally made with a crystal size of 0.20 microns or less because this promotes scattering of blue light and imparts a more aesthetically pleasing 'blue' tone to white plastic articles which can otherwise have a slightly more yellow tone.

The properties of the TiO2 pigment are further modified by the application of coating materials to the surface of the TiO2 particles dispersed as an aqueous colloid. For pigments intended for use in plastics, alumina only, or silica with alumina are the most common types of hydrous oxide coatings. Alumina is always applied as the outermost layer of the coating because alumina provides only weak particle-to-particle interactions that resist compaction during storage and transportation, and make the particles easier to redisperse during subsequent compounding operations. The amount of coating material applied to plastics grades of TiO2 is intentionally minimized because the porous nature of the coating materials promotes absorption of atmospheric moisture. Subsequent release of this moisture from the pigment during compounding can severely compromise the ease of processing of the pigment/polymer system. High moisture content of a TiO2/polymer melt system in, say, an extruder barrel, impedes the smooth transfer of the melt along the barrel, and the melt can be forced to back-up causing the extruder to stall or the melt to exit via vents. Compounding becomes impossible. Even if a concentrate can be made with a high moisture content, when the concentrate is used, for example to pigment a blown film, moisture will be released, causing bubbles to form in the film. These bubbles may burst to make holes in the film (a phenomenon known as “lacing”) making the film unstable and prone to collapse during production. In other cases, the explosive release of moisture at the die can deposit small amounts of molten polymer onto the die-lip which char and fall back into the film to leave small black/brown specks. High moisture content of TiO2 pigments can also have a deleterious effect on color of some moisture sensitive polymers such as polycarbonate. Moisture causes hydrolysis of the polycarbonate macromolecules, and the products are yellow brown colored unsaturated species. In severe cases the viscosity of the polymer (such as polyethylene terephthalate for example) is reduced by hydrolysis such that shear can no longer be transferred to the melt system to produce a pigmented compound.

When durability is required, the most common pigment design strategy is to precipitate silica as a coherent, complete layer at the surface of the TiO2 particles. An alumina top-layer is always applied after the silica. This acts a physical barrier between the photochemically active surface of the TiO2 particle and the surrounding polymer matrix, reducing the number of reactive radicals (described above) from reacting with the polymer before they are passivated harmlessly. While conventional silica coatings provide very good durability, the best available technology is the dense silica type of coating. Dense silica is precipitated from sodium silicate at temperatures of about 80°C with agitation. The resulting silica coating layer completely encapsulates the pigment particle, isolating the pigment surface from the polymer very effectively.

In the final stages of pigment manufacturing, the nature of the pigment surface is modified by application of a surfactant (commonly referred to as the “organic”). The purpose of the organic treatment is to compatibilize the pigment surface - which is quite polar in nature due to the presence of oxides and hydroxides of alumina titania and silica - with the polymer chosen for pigmentation which is comparatively non-polar. Siloxanes have been found to work very well in this respect, and continue to be widely used in the production of general purpose plastics pigments. More recently, reactive C-8 surfactants have been used to make TiO2 pigments with superior dispersibility. These types of pigments allow users to operate production lines at the highest throughput rates, while providing excellent pigmentation of plastics materials. These organics are able to chemically bond to the surface of the TiO2 pigment particle via reactive “head” groups. This allows the non-polar C-8 “tail” of the organic to become anchored to the pigment surface – in exactly the right place to exert its effect as an internal and external lubricant.

Quality Control

Consistency of quality is of prime importance for TiO2 manufacturers. Customers demand the same performance, every time. Therefore a battery of tests are used in the Control Laboratory to ensure that the TiO2 pigment produced has the correct quality.

  • Composition of the pigments is measured using X-Ray Fluorescence spectroscopy to determine the amounts of coating materials that have been applied. Impurities such as transition metals are measured using atomic emission spectroscopy. Organics are measured by C (CO2) analysis using IR spectroscopy.
  • Moisture is measured using a simple weight loss test. The pigment is heated to 105°C and the weight lost as a function of heating time is attributed to moisture. When the weight of the sample ceases to decrease, the test is complete. The extent of lacing in a cast film is also used to assess the amount of volatile material released from the pigment during compounding.
  • Dispersion is measured using an aqueous filtration test, and by measuring the filter pressure value (FPV) of a PE-LD compound of the pigment. The FPV test measures the increase in internal pressure in an extruder due to the presence of hard-to-disperse residues introduced with the TiO2. Filtration of an aqueous suspension of the pigment through a 45 micron sieve is used to check for hard aggregates.
  • Tinting Strength and Undertone are measured in blue and grey tinted plastisols respectively. Tinting strength is used as a proxy measure for opacity.
  • Colour of the pigment is measured on a compressed tablet of the pigment powder, and as a compressed tablet of white PE-LD concentrate.

Achieving optimal pigmentary performance in plastics

While TiO2 is delivered as a finely divided powder comprised largely of discrete particles with a defined average particle size and a narrow particle size distribution, to work well as an opacifying pigment, these pigmentary particles must be uniformly dispersed in the final application. This allows a beam of light to pass repeatedly between the pigment particles and the surrounding matrix, optimizing the amount of refraction and reflection of the light, and thus optimizing opacity and whiteness. There are four stages in achieving optimal dispersion in the final application. These are: 1. Subdivision, 2. Initial wetting, 3. Dispersion, 4. Mixing. In practice there is a certain amount of overlap between these processes depending on the processing equipment which has been selected. The overall process is summarized in Figure 8.

Figure 8: Stages in obtaining dispersion of TiO2 pigment in polymer resin

Sub division of the basic raw materials involves separating pigment particles, which may have become slightly agglomerated during transportation and storage and introducing them to the polymer resin. This is sometimes done in a separate blending process using for example, a tumble blender or a high intensity mixer. The result is a simple admixture of pigment and resin. In the high intensity mixer, the pigment/polymer blend is heated by frictional forces. The surface of the polymer particles melt slightly and the pigment particles adhere to these surfaces – this can be helpful preparation for the next stage which is initial wetting of the pigment particles. In initial wetting, the polymer melts completely, and the molten polymer replaces the air voids between the pigment particles. At the same time shear forces are applied to aggregated pigment particles. The third stage of dispersion involves further application of shear forces to effect the reduction of agglomerated pigment particles to their smallest size, with complete wetting of the surface of the pigment particles down to the microporous scale. The speed with which complete dispersion can be obtained has implications for final product quality and production throughput rates, and TiO2 pigments which can disperse rapidly and completely are preferred. At the end of this stage the polymer is cooled and formed into pellets containing a high concentration of pigment to make a masterbatch or concentrate, or the pigmented polymer melt can be formed directly into the shape required for an end use – vinyl siding for example. The final stage in obtaining uniform pigmentation of an article using a masterbatch involves the distributive mixing of the masterbatch in a diluent resin. Since complete dispersion of the pigment is provided by the masterbatch producer, this final step involves melting the masterbatch particles, together with the diluent resin, and mixing these such that the pigment is uniformly distributed in the melt. The pigmented melt is then formed and cooled to produce the final article.

References

  1. There are many excellent discussions of this topic - see, for example, Balfour, J.G., J. Oil Col. Chem. Assoc., 73, 225-230, 1990
  2. Allen, N.S., Photostabilisation and Photosensitised Degradation of Polyolefins by Pigments, Ch8, 337-371, in Degradation & Stabilisation of Polyolefins, Ed Allen, N.S, 1983, Applied Science Publishers Ltd, 1983, ISBN 0-85334-194-X
  3. Peacock, A.J., Ch 5, 210, in Handbook of Polyethylene, Marcel Dekker, Inc., 2000, ISBN 0-8247-9546-6
  4. See for example, Wypych, G., Handbook of Material Weathering, 2nd Ed., ChemTec Publishing, 1995, ISBN 1-895198-12-7
  5. After Braun, J.H., Prog Org. Coat., 15, 249, 1987
  6. After Gagne, B.A., Titanium Dioxide Pigment in Plastics, TiInfo System, Section 3.4.5, 1991, hTioxide UK Ltd

Author Details

Neil Macdonald

Quality and Technical Manager, Lomon Billions

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