Mineral modifiers take on new roles Minerals used as fillers in plastic compounds have traditionally been used to reduce material costs by replacing a portion of the polymer with a less expensive material. However, nowadays many functional fillers or mineral modifiers are required to modify processing characteristics or finished part properties. Many are now also being used to reduce the level of more expensive additives such as pigments, flame retardants and impact modifiers. Jennifer Markarian reports on the changing face of fillers.

May / June 2004

Minerals used in plastics include naturally occurring minerals such as calcium carbonate, talc, clays, barium sulphate, diatomite, mica and wollastonite, as well as synthetic grades and nano-sized minerals. In 2002, the North American market consumed 1.8 billion kg (4 billion lbs) of these minerals, while the European market was slightly smaller, notes Steve Van Kouteren, principal at Principia Partners Consulting, a marketing and business consulting firm for the plastics and related industries. In some applications, minerals continue to be used as a conventional fillers to reduce material costs by replacing a portion of the polymer with a less expensive material. However, minerals today are primarily considered functional fillers or mineral modifiers that serve to modify processing or finished part properties. Mineral modifiers may also be used to reduce the level of more expensive additives such as pigments, flame-retardants and impact modifiers. In the last 10-15 years, mineral producers have focused on increasing value and functionality by tailoring mineral properties through controlling particle size distribution and treating particle surfaces. "Companies are identifying new areas where minerals can be used as 'additives' rather than 'fillers'; the traditional line between additive and filler is blurring," comments Bob Nash, marketing manager for Performance Minerals at Imerys.
When added to plastics, minerals generally increase density, stiffness and surface hardness; improve temperature resistance and reduce shrinkage by lowering the coefficient of linear thermal expansion (CLTE). "As polymers continue to compete with other polymers and other materials, formulators are seeing that minerals can enhance particular properties to meet end-use requirements," explains Mr. Van Kouteren. Mineral modifiers are growing faster than the overall plastics market, with 5-10 % AAGR (average annual growth rate) depending on the mineral type, notes Mr. Van Kouteren. A key driver for growth is that minerals are increasingly being used to help meet the growing need for improved temperature resistant polymers. Mineral fillers increase the temperature resistance of polymers such as polypropylene and polyamides, allowing them to compete with more expensive materials such as ABS or PBT, respectively, in applications such as automotive interiors. In the electronics area, which is growing at a rate of over 10% per year, minerals are being used to increase temperature resistance of plastics both during use and during fabrication of parts such as connectors, notes Carl Ecket, principal at Principia. Another high growth application for minerals is in wood-plastics composites (WPC), which is growing at a rate of more than 20% per year, he adds. Minerals are added to WPC formulations to improve properties such as impact, heat distortion temperature, and creep under load. Minerals such as talc can also increase lubricity and improve processing. Use of flame retardant minerals, including alumina trihydrate, antimony trioxide, magnesium hydroxide, borates, and nanoclays, is increasing with the trend towards non-halogenated flame-retardants and increased use of flame retarded plastics in areas such as construction, adds Mr. Ecket.

Omya AG supplies calcium carbonate for the PVC industry.

Key physical characteristics of a filler include mean particle diameter, particle size distribution, aspect ratio, surface area, dispersibility, and inherent colour. While various minerals can be added to a polymer to obtain a given performance characteristic, each may differ in the amount of property enhancement as well as in variables such as loading level and cost. In determining a formulation, close relationships between suppliers and customers are key to creating functional fillers that will solve problems. "Suppliers of functional fillers must be aware of what the customer wants and, in turn, what their customers are really looking for," says Alan Minty, director of marketing and business development at Dynatec. One area for mineral producers to consider is the desire of compounders and masterbatch producers to increase throughput of the compounding step even with high mineral loadings. Dispersion and wettability can be improved with surface modification and by controlling particle size distribution, enabling increased throughput, says Mr. Nash. Franklin Industrial Minerals developed a calcium carbonate to aid compound bulk flow into the extruder by reducing bridging and agglomeration. Fillers have historically impeded flow through feeding systems, but the improved product aids pneumatic flow of both the filler and the pre-blended compound, says Rob Whitaker, technical director at Franklin Industrial Minerals. With the continuing trend towards downgauging and thin-walling, mineral modifiers must be more advanced to help deliver the same properties in a thinner part. Mineral suppliers must also respond to changes in polymer use. For example, lower-density, 'stickier' resins require development of improved antiblocks, not just increased antiblock levels, proposes Mr. Nash. As minerals add more functionality, mineral producers are acting more as additive suppliers and need to provide more technical support service to formulators.

PVC films (Photo courtesy of Omya AG).

Surface treatments or coatings are primarily used to increase mineral dispersion by making it more compatible with the host resin system. Fatty acids such as metallic stearates and stearic acid can improve mixing and dispersion, reduce resin viscosity, and improve resin stability by acting as an acid scavenger, explains Don Mills, sales director of the industrial minerals business unit at Huber Engineered Materials, a business unit of the J.M. Huber Corporation. For silicates, organo-functional silanes act as coupling agents between the mineral and the resin, resulting in better dispersion and improved strength properties. While in the past, in-situ coating during compounding has been used as a low-cost method, technical advances are making pre-treated minerals more cost-effective. "Compounders and formulators are becoming increasingly aware of the performance benefits provided by pre-treatment of minerals. Advancements in the controlled reaction of functional surface treatments has increased the effectiveness of the coupling reactions, reduced oligomeric grit formation, controlled VOC emissions and optimized economics," says Mr. Mills. The efficiency of stearic acid surface treatment of calcium carbonate has also improved dramatically in the last few years, comments Scott Brann, president of Heritage Plastics, a compounder specializing in calcium carbonate concentrates. The treatment process is now better controlled, producing just the right amount of stearic acid on the surface without leaving any unbonded stearic acid that might cause problems in downstream processes such as extruder die lip buildup. Further research and development in surface treatments continues, particularly in understanding surface interactions at the molecular level, adds Mr. Van Kouteren.

 

Calcium carbonate production (Photo courtesy of Huber Engineered Materials). Calcium carbonate laboratory (Photo courtesy of Huber Engineered Materials).

Calcium carbonate
Calcium carbonate, mined in the form of limestone, marble or chalk, has long been used as a filler to reduce costs in PVC applications, such as wire and cable and flooring. However, in the last 10 to 15 years, calcium carbonate producers have developed products with surface coatings to improve dispersion and finer particle sizes to improve polymer properties. In PVC applications such as window profiles, calcium carbonate can improve gloss and provide some impact resistance, potentially allowing formulators to reduce elastomeric impact modifier levels, explains Gil Morieras marketing manager for plastics at Omya AG, which provides Hydrocarb 95 T for PVC applications. In addition to improving surface finish and stiffness-impact balance, calcium carbonates can improve throughput and edge definition of PVC profiles. In polyolefins, calcium carbonate is used in microporous breathable films used for applications such as infant nappies (diapers) and adult hygienic applications. The polyolefin films contain about 50 percent calcium carbonate by weight so that, when stretched, micropores or voids are formed in the structure to increase water vapour transmission. In other polyolefin film applications, 10 to 30 percent calcium carbonate can reduce costs by increasing productivity and by improving properties such as stiffness and impact resistance, which allows downgauging. Minerals increase thermal conductivity, enabling the film or moulded part to be cooled more quickly. Since cooling is often the limiting step in polyolefin fabrication, line speeds can be increased and productivity increased 10 to 30 percent, comments Mr. Brann. "An additional benefit is that reducing the polymer content can improve a product's 'green' image, which is important in some countries," says Mr. Morieras. In polyolefin sheet thermoforming, adding about 30 percent calcium carbonate to homopolymer PP can improve stiffness and impact resistance so that the blend gives the same performance as PS or PVC. The PP/calcium carbonate formulation has the same shrinkage characteristics as PS and can use the same thermoforming equipment and tooling, adds Mr. Morieras. Commercial products targeted for polyolefins include Imerys' Filmlink and Omya's Omyafilm.

Talc
Talc, or hydrous magnesium silicate, is found in four different particle shapes, but only the platelet form is used in the plastics industry. In automotive applications, talc enhances the properties of polypropylene to allow its use in replacing engineering thermoplastics, notes Richard Clark, senior industry manager for polymers at Luzenac. Talc's platy structure tends to orient when injection moulded, leading to improved stiffness. Talc increases dimensional stability of plastic parts by lowering the coefficient of linear thermal expansion (CLTE) so that parts will shrink less in cold temperatures and have a lower tendency to warp in hot temperatures. Aesthetics of interior automotive components are becoming increasingly critical, notes Mr. Clark.

 

Kaolin production at one of Imerys’ UK sites

Kaolin
Kaolin, also called clay or natural aluminosilicate, is platy with a high aspect ratio. Surface-treated, fine particle size kaolins compete with talc and mica for impact modification of injection moulded nylons in automotive applications. Surface-treated, calcined or heat-treated kaolins, such as Polarite 102A from Imerys, are also used in nylon automotive applications. Application requirements include high temperature and chemical resistance, dimensional stability, and a balance of rigidity and toughness. Some calcined kaolins add functionality to wire and cable applications. The chemical sites produced on the surface by the heat-treating process adsorb ions that are formed during cable production, effectively removing the ions from the insulation layer and improving electrical properties, explains Mr. Nash. Calcined kaolin is finding new use competing with silica as an antiblock additive for polyolefin films. Imerys' new InFilm 200 has good antiblocking and optical properties, reports Mr. Nash.

Barium sulphate
Barium sulphate, or barite, is the highest density and most chemically resistant mineral modifier. It is used extensively in applications where sound deadening and vibration control is a performance objective. In polyurethane foams, barium sulphate improves processing and increases density and resilience. Barium sulphates are also used in unsaturated polyesters and some thermoplastics. Dynatec introduced a new barium sulphate product line, SPARWITE® W-44C, which offers lower oil demand than traditionally seen in a 325 mesh barium sulphate, allowing increased loading levels and more uniform appearance to the finished product, notes Mr. Minty.

Alumina trihydrate
Fine particle size alumina trihydrate (ATH) is used extensively as a flame retardant in wire and cable and insulator products. ATH also adds arc-track resistance for high voltage insulator applications. Almatis introduced a new series of Lubral® silane-coated fine hydrates. The surface coating bonds the mineral to the resin, increasing properties such as tensile strength, and makes the mineral hydrophobic, slowing water migration in applications such as wire and cable jacketing, explains Terry Clever, technology manager for speciality hydrates at Almatis, formerly Alcoa World Chemicals. Almatis has patented a new on-line coating process that is more cost-efficient than typical off-line batch processes and more efficient in coating the particles than in-situ coating during compounding

PVC pipes (Photo courtesy of Omya AG).

Coarser particle size ATH is also the primary filler in solid-surface applications such as kitchen and bathroom solid-surfacing, which are typically 65% ATH in polyester, acrylic, or polyester-acrylic blends, notes Mr. Clever. Advantages of ATH in solid-surface products include class I flame retardancy, stain resistance and a lack of colour. Because the refractive index of ATH is similar to that of the polymer, it is translucent to light, making it easier for formulators to colour the polymer consistently.

Nanomaterials
Nano-sized minerals such as carbon black and fumed silica have been used in polymers for fifty or more years, but in the last decade much research has gone into new nanomaterials for polymers such as nanoclays and nanotalcs. These nanomaterials are in the embryonic growth stage, with commercial volumes well under 4.5 million kg (10 million pounds). "It will probably be a few years before we see significant growth. The industry is trying to find breakthrough applications to sustain use," comments Mr. Ecket. Nanomaterials have very high surface areas compared to naturally occurring minerals. This allows them to improve properties with much lower loading levels than other mineral modifiers, resulting in final parts with lower weight and density. For example, Dellite nanoclays have a surface area of 800 m2/g and can be used at levels of 3-5% compared to 20-30% of a standard filler, notes Valerio Cittadini, sales and technical assistant at Laviosa Chimica Mineraria. In addition to improving physical properties, nanomaterials have been shown to improve thermal stability, barrier properties and flame retardancy.

Ground calcium carbonate powder (Photo courtesy of Huber Engineered Materials).

Nanoclays are produced commercially from montmorillonite (MMT) clay minerals by Italian company Laviosa Chimica Mineraria and U.S. companies, Nanocor and Southern Clay Products. In its untreated form, MMT platelets exist in clusters with very little surface area exposed. "The challenge is to create conditions favourable for the exposure of all this potential surface area to the polymer," says Southern Clay. This is done through exfoliation, or delaminating the clusters into primary platelets, and dispersion, or distributing the platelets homogeneously throughout the polymer. The compatibility of the clay surface treatment with the resin matrix and the melt blending conditions determine the degree of delamination and dispersion, adds the company. Southern Clay recommends compounding using twin screw extruders and feeding the clay downstream into the molten polymer rather than pre-blending. Nanoclays are being used commercially in resins such as nylon 6 and polypropylene for packaging, semi-crystalline nylon for ultra-high barrier containers and fuel systems, and polyolefins for automotive parts, fire retardant cable, electrical enclosures and housings, reports Nanocor. Using nanoclays in flame retardant polymer systems is a growing area. Since flame retardant additives typically reduce mechanical properties, adding nanoclays can help maintain mechanical properties. At the same time, nanoclays add anti-dripping properties to the system, explains Dr. Cittadini. Nanoclays also improve flame retardancy by forming surface char.
Nanova® LLC's NanoTalc® is a surface-modified nano-size talc. Currently it is produced in pilot plant quantities, with a demonstration plant planned for construction in 2004, says Roger Padden, vice-president of sales and marketing at Nanova LLC, a wholly owned subsidiary of Nanomat, Inc. Nanova uses a proprietary, patent-pending mechano-chemical synthesis process to reduce the size and increase the surface area of conventional talc. The process maintains talc's high aspect ratio, which gives improved physical properties such as impact resistance and rigidity to the modified polymer. Surface modification of the NanoTalc particles improves dispersion and prevents agglomeration. NanoTalc has a surface area of about 250 m2/g compared to conventional talc platelets that range from 8 to18 m2/g. This high surface area makes the NanoTalc very active, which will permit lower levels of other additives. As the additives are adsorbed to the wide surface of the nanotalc, they are available to more of the polymer matrix. The platelet form of the NanoTalc adds a barrier to gas and moisture transmission. Compared to conventional talcs, NanoTalcs improve dimensional stability and provide better scratch and mar resistance. NanoTalcs also provide smoother surfaces with excellent distinctness of image (DOI) of 90% or more, compared to 100% DOI achievable with a metallic or mirrored surface, comments Mr. Padden. DOI is key in applications such as automotive exteriors for moulded-in colour or primerless paints. Because NanoTalcs are expensive compared to conventional talcs, they will find use in applications where the materials can add value, such as automotive, appliances and computer housings, predicts Mr. Padden. "NanoTalcs fill a void where there had previously been no solution, such as in obtaining good DOI and will expand the market beyond where conventional talcs can go," he adds. Nanova technology is being extended to other mineral products. Pilot scale work and application testing is being conducted on alumina trihydrate (ATH), calcium carbonate (NanoCalc®), magnesium hydroxide, mica and borates. Preliminary results are providing high surface area materials with unique properties. It is expected that these nanosized materials will become high value additives in flame retardants, engineered resins, polystyrene and future nanocomposite materials, predicts Mr. Padden.

Contacts:
Almatis
Tel: +1 412 630 2800
Website: www.almatis.com

Dynatec Corporation Mineral Products Division
Tel:+1 403 261 3999
Website: www.dynatecminerals.com

Franklin Industrial Minerals
Tel: +1 615 259 4222
Fax: +1 615 726 2693
Website: www.frankmin.com

Heritage Plastics, Inc.
Tel: +1 800 245 4623
Website: www.heritage-plastics.com

Huber Engineered Materials
Tel: +1 877 949 5400
Website: www.hubermaterials.com

Imerys
Tel: +44 1726 74482
Website: www.imerys-perfmins.com

Laviosa Chimica Mineraria
Tel: +39 0586 434000
E-mail: additives@laviosa.it

Website: www.laviosa.it
Luzenac
Tel: +1 800 325 0299
Website: www.luzenac.com

Nanocor
Tel: +1 847 394 8844
Website: www.nanocor.com

Nanova, LLC
Tel: +1 724 978 2190
Website: www.nanomat.com

Omya
Tel: +41 62 789 2929
Website: www.omya.com

Principia Partners
Tel: +1 610 868 6140
Website: www.principiaconsulting.com

Southern Clay Products
Tel: +1 830 672 2891
Fax: +1 830 672 1903
Website: www.nanoclay.com