Looking for the right mineral filler for plastics

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Published: Monday, 28 January 2013

Fillers were first introduced into the plastics industry as a cost-saving device. But, as Ajay Kulshreshtha discovers, their myriad properties perhaps should be more correctly focused on improving performance and driving innovation

By Ajay Kulshreshtha

The plastic industry cannot survive without the use of fillers, additives or reinforcements. Commodity resins, such as PVC, polystyrene, polyethylene and polypropylene are a major part of the modern plastic industry and these materials have been sold and used as essentially pure resins.

Mineral fillers were first introduced in a bid to reduce costs as prices escalated amid sporadic shortages of resins and petroleum feedstocks.

But the use of mineral fillers in the polymer matrix is not simply limited to cost considerations as they can play a vital role in performance enhancement to become “functional fillers” not merely “fillers”.

Apart from improving the intrinsic properties of resins, inherent deficiencies, which are usually present, can also be compensated for by using appropriate mineral fillers. Industrial mineral fillers have also proved helpful in introducing a completely new property to the polymer.





It can be argued that, in the modern context, mineral fillers are as inseparable from plastics as plastics are from our daily life. Such multifunctional use has certainly paved the way for an efficient and widespread utilisation of fillers to provide properties in plastic products to the specific requirements of the user.

Mineral filler selection is based on the needs of processors or manufacturers, and is dependent on cost, price and quality together with the ease of processing of resins. There may be a class of manufacturer who would like to move up the value chain by providing innovative solutions to customer requirements and to do this they may require a special mineral filler to give their products the desired properties.

For example, a plastic tape manufacturer whose product is priced on a per-length basis, may need a mineral filler that can help increase the length of his final product, while still using the same amount of resin. Similarly, a plastic film manufacturer, whose selling price is on a per-kilogramme basis, may require a filler that helps increase the weight of the final product. The benefits in cost here are obvious.

Both producers will see the economies of their products enhanced by adding a mineral filler, while its standard properties remain unchanged.

Another example could be plastics producers looking to produce cheaper varieties of goods from recycled plastics. They would only require mineral fillers to reduce costs, sometimes at the expense of quality.





The reverse of this would be manufacturers willing to pay higher prices for fillers as they do not want to see any deviation in the quality standards of their products. A manufacturer involved in making engineered plastic products, for example for the automotive industry, would not be prepared to compromise the properties of his product, but would more likely prefer a more efficient mineral filler to produce even more superior products that could give him an advantage over his competitors.

Plastic producers tailor their products to the demands of the ultimate customer, so the question of how to select a mineral filler to satisfy such requirements becomes crucial.

The plastics industry is flooded by a huge number of products with a variety of uses, ranging from run-of-the-mill to exclusive bespoke products. The large number of industrial minerals available and the performance requirements of plastic products makes the task of selecting a suitable filler difficult.





Newer varieties of fillers are also emerging as polymer technologists and mineral researchers develop even more suitable grades of mineral fillers to meet the ever-rising requirements of the plastic industry.

Mineral filler selection should, therefore, not only be based on cost reduction, but also on improving the performance of the composite. Another consideration will be to improve the rate of production after incorporating the mineral filler due to the increase in thermal conductivity resulting in faster moulding cycles and fewer rejects due to warpage for certain items. This will result in greater productivity and economies of scale.

However, it is wrong to base decisions on cost alone. For example, the incorporation of fillers into thermoplastics (compounding) is a
costly process, sometimes totally cancelling out any financial savings.





Principals of mineral filler selection

In general, mechanical properties such as stiffness, tensile and impact strength, appearance, smoothness and optical properties of particulate-filled polymer composites are strongly dependent on shape, size and distribution, the surface area of filler particles in the matrix polymer and good adhesion at the interface surface.





Particle size distribution

Smaller particles tend to increase the values of properties such as tensile strength, modulus and hardness. Coarser particles can give compounds less strength than an unfilled material. Top cut (d98) and mean particle size (d50) of powdered filler are the two important parameters in establishing the particle size distribution (PSD) of mineral fillers. The PSD graph is also an essential tool to understanding their distribution pattern.

The coarsest particles pack to produce the gross volume of the system. As successively finer material is added, it occupies the spaces between the largest particles so that the total volume is not expanded. Each additional finer material also has a void volume which, in turn, may be occupied by a still finer material. The resulting geometric progression then depends on the smallest amount of space left in the system and on very wide ranges of particles size to achieve a high degree of packing. All particles have essentially the same density, and therefore, percent finer by weight, and volume are equal.

Maximum packing is obtained with a balanced or particular distribution of particle sizes, whereas minimum packing will occur if all the particle are the same size.





Chemical composition

Chemical reactivity remains the chief concern for mineral filler selection.

Salt - calcium carbonate, dolomite

Products of acids and bases are salts. The most widely used filler, calcite (CaCO3), is a salt which differs considerably with barite (BaSO4). It is generally derived from limestone. This is a sedimentary rock mass, which when metamorphosed, is called marble. Typically of a weak acid and weak base, calcium carbonate has very little resistance to even the weakest organic, and has a fairly high water solubility and is reactive with a great variety of chemical reagents. Dolomite is sometimes used as a filler, but is more reactive than calcite due to the presence of magnesium.

Silicates - wollastonite, mica, talc, asbestos, kaolin, silica, feldspar, glass

Silicates vary widely in their characteristics, but as a rule, all - with the exception of wollastonite - have good resistance to acids and poor-to-excellent resistance to alkalis depending on the disposition of the silica and tetrahedral in the crystal lattice. Glass is a synthetic silicate, as it is a precipitated product made with sodium silicate solutions and insolubilising cations such as calcium. Its composition differs considerably from natural products, and due to small particle sizes and high surface area, it is less acid and alkali resistant. Talc is a good acid- and alkali-resistant material because it is chemically inert.

There may be occasions when plastic producers are using the wrong mineral filler due to a lack of sufficient information and knowledge. This article aims to show the important characteristics of some of the more-common mineral fillers, which will assist in understanding the basics in the selection of mineral fillers. However, as there is no rule-of-thumb for suitability or criteria because of the involvement of so many other factors, this remains a complex subject.





Calcium carbonate

Limestone is the main raw material for producing calcium carbonate. It is also found in nature in crystalline form, which is known as calcite. Naturally ground calcium carbonate (GCC) is available as dry ground (size 200- 325 mesh) or wet ground (Size d50 - 0.7 to 12 µm and d98 - 10 to 44 µm). Its particles are generally rhombohedral or prismatic in shape. While there are more than 300 crystal shapes for calcite alone, it is generally an irregular particle with a low surface area and is non-toxic, non-irritating and odourless. It lacks water of crystallisation and its PSD is controllable for optimal packing in each polymeric system. It can be easily coated in dry form in a high-intensity mixture to improve the plastic’s melt rheology. These fillers can be smoothly mixed into formulations, sometimes aiding the mixing of other ingredients. It also reduces shrinkage during moulding and curing, for example in no-shrink reinforced polyester sheet moulding compounds. It has a relatively low stiffness, even with high loading, and is usually stable over a wide temperature range.

The disadvantage in using GCC is that when it is attacked by acids, CO2 is evolved and soluble salts are formed. Many plastics, such as epoxy and polyester, successfully and thoroughly wet and bind CaCO3 so that the compound, even though highly loaded, resists acid attack. On heating at 800 - 900oC, CO2 is evolved and CaO is formed. In polyethylene and polystyrene CaCo3 loading tends to make the product more brittle. An additional problem is that the triangular crystal shape of CaCO3 provides little reinforcing action compared with strongly reinforcing materials. In PP, talc and asbestos fillers have higher stiffness, flexural modulus and deflection temperature than CaCO3. However, CaCo3 in PP has better impact resistance, possibly due to better bonding between the polymer and calcium carbonate.

Calcium carbonate filler in a precipitated form is also used for applications requiring any combination of higher brightness, smaller particle size, greater surface area and lower abrasion, with higher purity generally available from ground natural products. Both GCC and PCC are available with stearate surface treatments for better compatibility with polymer matrices. Surface-coated grades with stearic acid and calcium stearate have improved rheological properties. Stearic acid is an adding operation, which does not require any chemical modifications, and it is different from silane treatments used in other minerals, such as wollastonite and mica.





Kaolin

Calcined clays ( Size d50 0.8 to 1.4 µm) are used in plastics for imparting specific functional properties, such as anti-blocking and infrared blocking, together with partial replacement of the more expensive titanium dioxide. Kaolin clays are available in various other grades as dry-ground, water-washed and delaminated coarse clay fraction from water washing, pulverising into thin, wide individual plates. With calcined clays, the surface hydroxyl group is partially or totally removed after heating and its controlled PSD and irregular shape makes it most suitable for anti-blocking properties in polyethylene and infrared blocking in polyesters.

Water-washed clay, usually soft clay, has been slurried in water and centrifuged or hydro-cycloned to remove impurities and produce specific particle-size fractions. Water-washed clays are often treated to improve brightness and this includes chemical bleaching and/or high-intensity magnetic separation to remove iron and titanium impurities. Kaolin clay is also available in surface-coated grades, for example with stearates or silanes.

Talc

Talc particle (100 to 500 mesh and finer grades ranging from 20 µm to 1 µm, d50 3.5 to 0.8 µm) is highly platy and soft in nature. Due to its lamellar structure, it is considered to be a reinforcing filler, distinguishing it from the other particulate mineral fillers. Its low cost identifies talc as an extender, while its high-aspect ratio helps to improve the compound’s performance properties.

Polymers filled with platy talc always exhibit a higher stiffness and creep resistance, both at ambient and increased temperatures. To create a homogenous composite, special melt compounding actions such as smearing, folding, stretching, wiping, compressing and shearing are required for proper compounding of fine talc particles to fully wet the particles with the molten polymer and to achieve a high degree of dispersion. Adding talc to polypropylene increases stiffness and high-temperature creep resistance, which is particularly applicable in industries such as automotive appliances.

The outstanding effectiveness of the platy talc in increasing the modulus of polypropylene, both at ambient and elevated temperature, together with minimum deterioration in other properties, makes it most suitable for rigid items. Impact properties of polyethylene are optimised by using talc, while studies have shown that platy talc in PVC increases the modulus of the polymer at both temperatures, with negligible loss in tensile strength. Surface-coated talc grades are also available.

Wollastonite

Wollastonite particles (size 100 mesh to 8 µm) are needle-shaped and are known for their high-aspect ratio (15:1 to 20:1). Producers have to be careful not to destroy this structure while grinding during dry processing. Powder grades are milled to a low-aspect ratio (3:1, 5:1), either from naturally low-aspect ratio ores or from high-aspect ratio ores that have been ground in a way that breaks the needles width-wise. Despite their low average-aspect ratios, powder grades can retain a significant portion of acicular particles. Both powder and acicular forms of wollastonite are available with surface coatings, usually silane. The silane treatment is a chemical function of the minerals. Performances of properties, particularly nylon 6/66 and PVC, are significantly enhanced by using suitable grades of wollastonite.

Mica

Mica (size 100 mesh to 325 mesh and micronised grades up to 10 µm) is a flaky material that can be ground dry or wet. Both air-milled mica powder and micronised grades are available. Most filler-grade mica is first collected as flakes by flotation from ore that contains several minerals. Dry-ground products are air-milled from flotation concentrate that has been partially or completely dried. Wet-ground products are ground in water using mills designed to delaminate the mica into flakes with a higher aspect ratio, sheen and slip compared with dry-ground mica.

Mica is widely used in plastics as it has several advantages, such as excellent dimensional stability of moulded parts, low warp, planar reinforcement, reduced creep, excellent electrical properties, reduced wear of processing equipment, increased heat-distortion temperature, reduced flammability, low permeability, and excellent resistance to weathering and corrosive attack by acids or alkalis. Its disadvantages, however, are that it lowers the strength and impact toughness of the composite.

High-aspect ratio mica products are an outstanding reinforcing agent for all thermoplastic materials. Mica is also used in ABS to improve the adhesion of an electroplated metal coating, while mica-reinforced nylon 6/66 blends have found applications in injection-moulded automotive parts. It also used in extruded pipes as the flakes provide both longitudinal and circumferential reinforcement due to their preferential alignment parallel to the surface.

Market growth

The demand of fillers is driven by the consumption of plastics. In the US, President Obama’s re-election has removed some economic uncertainty and market observers believe the US industry will return to a surplus situation for plastics products, moulds and machinery, having previously shown a deficit. Despite the global economic slowdown, plastics and polymer consumption has shown an average annual growth figure of 5% worldwide and is expected to reach 227m tonnes by 2015.

Annual global GDP growth is expected to be around 3.4% through to 2020, although polyethylene (grades HDPE, LLDPE and LDPE) is expected to grow by 4.4 % annually through to 2020. Mineral filler demand, which is directly related to the consumption of plastics and polymers, it is expected to at least match that up to 2020 as there will be additional growth on account of a rise in global population.

Figure 8 shows the annual average growth rate of different resins, and gives a fairly good indication of the growth in PVC, PP and PE, the main three sectors of major mineral filler consumption. On the basis of this graph, growth in PVC is expected to be slightly less than 1%, PP slightly higher at 1.5 %, with PE growing by 4.4%.

Mineral fillers will also benefit because of the increase in production capacities of oil and feedstock for resins following the formation of newer petrochemical complexes in Middle Eastern countries such as Oman, Saudi Arabia and the UAE. China, India and the Middle East will be the major global players, where expansion and augmentation of existing petrochemical capacity is expected to take place during the coming five years.

The global distribution of plastics consumption shows Asia, excluding Japan, as the major user with a 30% share, North America on 26%, western Europe with a 23% share and Japan with 6.5% of the global market.

Innovations through nano technology

Plastic producers are always looking for new uses of their products or for new formulations to improve their performance, with fillers in a ‘nano-scale’ bringing enhanced functionality.

An example of this is a nano calcium carbonate filler, which maintains the transparency of plastics and is of great use for food packaging. Nano calcium carbonate fillers also increase the stiffness of plastics and exhibit better anti-wear and friction-reducing properties. The cost of such nano-fillers is higher than that of bulk form, however, so there is an impact on production budgets.

Plastic producers are increasingly turning to nano fillers as they provide remarkable properties at low filler loads. But these processes still present some technical difficulties and the challenge now is to find solutions via other control measures such as compatibilisers, surface treatment and compounding with ultrasonic techniques.

Nano clay can be used in the plastics industry by employing compatibilisers and surface treatment. This can triple the modulus, maintain low-temperature impact, increase melt viscosity at low shear and reduce melt viscosity at high shear. Although homogenous dispersion in nano clay is challenging, it is not impossible.

Dispersion of fillers at a low level has been achieved without difficulties, but higher-level loading presents a greater test as the particle-to-particle interaction is too great.

All particulate fillers have a tendency to attract because of what is called the London or van der Waal forces, according to Amit Dharia, president at Transmit Technology Group, Dallas, US. If the filler is platelet, it has two different charges on opposite sides and hence different layers will attract. If this attraction is higher, say through water bridges of hydrogen bonding, it is very difficult to delaminate stake of layers, as happens in HAR talc or MMT. The ionic charges have to be treated with compounds, so the layers are easy to separate. The key here is to wet the filler surface. The higher the surface area and the smaller the size, the more difficult this is.