Calcium carbonate’s polymer promise

Published: Tuesday, 22 November 2011

Mineral fillers offer cost reduction – and sometimes enhanced properties – to polymer formulations. Siddhartha Roy outlines ground calcium carbonate’s changing role in the growing polymer market

It is estimated that about 5% of the Earth’s crust is some form of calcium carbonate, either as calcite, limestone, chalk, marble or aragonite or through impure forms like dolomite. While some minor portion of naturally occurring calcium carbonates have formed due to geological processes, the majority has its origins from animal life.

Calcium carbonate is the main constituent of shells of marine animals and exoskeletons of myriad creatures, which gradually accumulated on the sea floor over many eons and later transformed by inexorable geological processes to form the mighty Himalayas, the iconic white cliffs of Dover and many other calcium carbonate deposits worldwide1. Calcium carbonate is one of the most useful minerals and has been used by mankind for 40,000 years for a wide range of applications.

Mining & production

The vast majority of calcium carbonate used in industry is extracted by mining or quarrying. Pure calcium carbonate (eg. for food or pharmaceutical use), can be produced from a pure quarried source (usually marble). The mined minerals are ground into fine powders and classified according to particle size (ground calcium carbonate - GCC). The purity is very much dependant of the quality of the mine deposits. Ultrafine particle size grades are termed as fine ground calcium carbonate (FGCC).

Alternatively, calcium oxide is prepared by calcining crude calcium carbonate. Water is added to give calcium hydroxide (milk of lime). Insoluble particles can be separated and the milk of lime carbonated with the CO2 obtained during calcination. The carbon dioxide precipitates the desired calcium carbonate from the milk of lime, is filtered, dried and pulverized. This is referred to in the industry as precipitated calcium carbonate (PCC). PCC is normally costlier than GCC of an equivalent particle size.

Both GCC and PCC are available in coated (activated) versions which are of interest in the plastics industry.

Calcium carbonate as filler

Fillers are insoluble minerals which increase the bulk of polymers and rubbers. They play many roles in polymer systems and can be broadly categorised as reinforcing and non-reinforcing (Table 1).

Cost reduction

Costly and exotic filler materials like glass fibre, carbons fibres and nanotubes are not in the scope of this article. Mineral fillers like talc are primarily added for stiffening and improvement of polymer properties. Though it is cheaper than the polymers it is used to fill, the addition of talc does not necessarily bring about cost reduction (see ‘volume cost’).

Calcium carbonate’s primary role is cost reduction, with improvements in properties (electricals, smooth extrusion, matt finish, etc.) being added bonuses. At high levels of filler addition (normally where reduction in cost is the primary target) key physical properties like tensile strength, impact strength and hoop stress are adversely affected.

An educated compromise has to be made to balance the cost reduction targeted and the property levels which should not be breached for the application. This is especially true for PVC applications where maximum calcium carbonate is used and addition of filler carries marginal incremental incorporation costs. In India, PVC pipes are around 70% of the PVC market and the largest calcium carbonate market in the plastics sector.

Calcium carbonate in PVC pipes

Historically, PCC has been the filler of choice for PVC pipes, profiles, windows and other construction applications. Unplasticised PVC processing requires comparatively sophisticated and expensive twin screw extruders (compared to single screw extruders used with most other plastics).

The purity of the filler has a direct impact on the life of the screws and barrel of the twin screw extruder. The presence of abrasive silicates and other impurities drastically reduces the operational life, forcing costly replacements of screws and barrels. The operational life of a screw and barrel set can vary from 15-25,000 hours depending on the build quality. This life can be halved if excessive amounts of abrasive fillers are used. Regular purity GCC inherently contains more abrasive impurities than PCC where these can be filtered from the milk of lime. Thus PCC has generally been more preferred, as screw barrel replacements every one and a half to two years used to cost thousands of dollars.

For better dispersability and surface finish, coated or activated PCC is popular. The coatings are normally stearic acid-based, but titanate compounds and other functional chemicals are also used. The coatings modify the surface tension properties of the filler to come closer to the surface tension of the PVC melt, aiding dispersion and encapsulation. Activation also reduces the oil absorption, but this is of more relevance with plasticised applications. Coated PCC is costlier and is normally used for better quality and performance-guaranteed pipes.

In recent years, there have been two trends which have allowed GCC to make headway into this large market:

- Availability of better quality GCC from many sources, not only from industry leaders like Omya and Imerys (some of whose high performance GCCs are considerably more expensive than PCC). Purity and tighter particle size distribution have considerably improved. The limestone source is crucial but advances in mining, crushing and classification technologies have brought about good improvements;

- There has been a proliferation of machining companies offering replacement twin screw barrels and screws at much lower costs than previously. When the twin screw extruder technology for PVC debuted in the 1960s in Europe, there were only a handful of firms which could afford the CNC lathes required for the parallel and conical twin screw machines. The leader was Cincinnati Milacron, which was a CNC machine manufacturer itself. In the 1990s and the current millennium, such CNC machines are available at a fraction of the original costs, and screw machining is becoming a commonplace operation.

Thus better quality GCC and cheaper screw barrels have opened the doors for GCC into this huge market.

PVC pipe manufacture

Mixing is typically achieved through high speed heater and cooler mixing sets. Friction generated by the mixing blades rotating at high speed (800-1,000 rpm) heats up the PVC resin, fillers, stabilisers, pigments and other additives charged through the heater mixer lid. The mixing and heating action of the blades ensures a homogeneous dryblend in which all of the additives melt and mix at around 110-120ºC. The hot mix is cooled in a higher capacity water jacketed cooler mixer to bring temperatures down to ~ 50ºC for conveying to storage silos/bins and then to the extrusion shop.

In PVC pipe manufacture there is no pelletizing step as is common for other filled plastics, as twin screw extruders are capable of receiving dryblend feed. Complete homogenisation of the melt is achieved by the twin screw action much like the compounding action of intensive melt mixing equipment used for rubber and other plastics.

In the extrusion process, the ready to extrude drybend is conveyed to the extruder hopper from where it is dosed into the heated screw/barrel system. Most high output lines have a vacuum degassing zone to remove residual moisture, volatiles and entrapped air from the crumbly melt which is then finally mixed and metered at a steady rate to the die head and forming dies.

Twin screw technology keeps processing temperatures down to 140-170ºC with the final polish being given at die exit at 200ºC for a very short residence time.

The pipe (or profile) shape emerging out of the die is sized by either vacuum or air pressure and enters a spray cooling tank. The cooled pipe is gripped by the haul off (caterpillar) pads and pulled at a constant rate which, along with the extrusion speed, determines the pipe wall thickness. A travelling saw or circular cutter cuts the pipe into the desired length.

There are various levels of sophistication in each downstream process, but rock-steady extruder output and smooth constant pulling speed are the cornerstones of producing a good pipe.

PVC is a heat sensitive polymer, explosively releasing HCl if overheated and inadequately stabilised. Twin screw extruders are ideal for high output processing of this sensitive melt. Pipe outputs of 800-1,000 kgs/hr are now commonplace, although 250-450 kgs/hr is the norm (150 tpm).

Thus a medium-size factory with four extrusion lines is capable of producing 600 tpm, which is sizeable for plastic processing. Filler requirements would be 50-150 tpm dependent on how aggressive they are on filler usage (conservative is 5%, aggressive is 15-20%). There are many such medium-size factories, and the major players in PVC pipes have upwards of 20 lines churning out 4-5,000 tpm (50-60,000 tpa). It is easy to understand why this sector is such a large market for CaCO3.

CaCO3 in plasticised PVC

PVC is a versatile polymer as it can be plasticised to various degrees of softness, from semi-rigid to latex rubber-like softness. The plasticised PVC applications are manifold, and calcium carbonate finds applications in most of them. It is excluded from all transparent polymer applications as any mineral filler renders a PVC formulation translucent/opaque.

Unlike rigid PVC, nearly all plasticised PVC is pelletized in a melt compounding process where the recipe is extruded into strands, cooled and chopped into 2-3mm pellets. A plasticised dryblend is too sticky to flow compared to unplasticised PVC. The major sectors using substantial tonnages of calcium carbonate are:

- Wire and cable insulation

- Films, sheets and floorings

- Coated textiles (leathercloth, rexine)

Wire & cable

The inorganic nature of mineral fillers improves the electrical resistance of most cable coatings. Kaolin and calcined clays have been used widely for boosting the volume resistivity of PVC insulation, but have been gradually replaced by the much cheaper PCC. PCC is preferred over GCC for purity considerations for wire and cable, but high quality GCC is also used.

The major use of CaCO3 is in the inner and outer sheathing of power cables. The insulated cable conductors are made up into a circular cross-section by filling up in between spaces with circular or shaped strands of highly filled (or scrap PVC) and then coated with an inner sheathing to maintain the circular shape. The cable is finished with a thicker outer sheathing which protects the cable system from transport, installation and service hazards.

PVC is formulated by parts per hundred resin basis (PHR) like rubber. Some typical cable formulations are outlined in Table 2.

Films & sheets

PVC films and sheets range in thickness from a few microns (cling film, shrink wrap) to several mm thick (industrial curtains, chemical tank linings, floor tiles). Thicknesses less than 0.25mm (1,000 gauge) are termed films to differentiate from the thicker sheets.

Calendaring and extrusion are the major conversion processes. Sophisticated four-roll and five-roll calendars can produce high quality films and sheeting in the 0.1-0.3mm range at very high speeds (80-120 metres/minute). The product can be embossed, printed and lacquered into a wide range of products like shower curtains, decorative and protective foils, stationery items and many more. Care has to be taken in CaCO3 grade selections as abrasiveness can badly affect the polished and crowned calendar roll surfaces. PCC and activated PCCs are preferred over GCC.

Extruded sheets and blown film are more tolerant of GCC. Floor tiles are an example where the GCC is the major constituent, being used at levels of 300-400 PHR. Extruded sheet can have thicknesses as high as 5mm which can only be achieved in calendaring by compression moulding multiple layers of calendared foil. Blown film is a low-cost route.

Coated textiles

This is one application where, historically, GCC is preferred over PCC. Coated fabrics are processed by two main techniques: lamination, and spread coating.

PVC resin is commercially available in two forms. Suspension grades are the bulk, but emulsion grades (of which paste grade resins are an important part), are also significant. Emulsion grades are much finer in particle size and the paste grade variant has non-porous particles which form a paste with plasticisers. Such grades are much more expensive than suspension resin.

Lamination involves joining the fabric layer to a PVC film by an adhesive. The adhesive is a thin layer of PVC paste and the assembly is heat-fused and embossed into many attractive products. They range from low-cost rexine and book binding cloth to high class, water resistant soft luggage fabric. Thus the low-cost rexine is made in a similar fashion to a Louis Vuitton high handbag and luggage fabric. The PVC films and foils are mainly calendared from suspension resin, though the cheaper end products use blown film.

In spread coating the PVC plastisol or paste is spread over the fabric substrate by a doctor knife and cured continuously in an oven at ~180-200ºC. The paste is fused into a film and can be printed and embossed.

How can the spread coating process using expensive paste grade resins be competitive with laminated leathercloth from suspension resin? It is a low-pressure process carried out at atmospheric pressures, and the conversion equipment is much lower in capital costs than a calendaring train and high-speed laminator. The main balancing factor is that while calendaring and extrusion have to use PCC, costing $250-350/tonne, spread coaters are quite comfortable using low quality GCC available at pit heads at $60-80/tonne. The transportation costs sometimes rival the run of mine costs.

The GCC need not be very pure nor of fine particle size as the spread coaters grind the paste on three roll mills prior to coating. The more abrasive GCC does much less damage to the doctor knife used for spreading the paste on the fabric than it would to expensive extrusion equipment. Furthermore, the high levels of GCC thicken the paste so that it does not penetrate and wet out the fabric being coated. Spread coaters normally apply a thick base coat loaded with GCC (100-150 PHR) and top it off with a thin higher quality topcoat with lower or no GCC content. The resultant leathercloth could be cheaper to produce than the suspension resin system thanks to the low cost of GCC.

Spread coating has a more sophisticated avatar of transfer coating where the top coat is first applied on a release paper and then the base coat, adhesive tie coat and then the fabric. The entire assembly is fused in a series of ovens. Foaming agents are introduced in the middle layer for foaming and a wide variety of high quality foamed leathercloth and cushion vinyl products can be offered.

The general rule is that GCC is used in the base/foam layers and PCC or no filler on the tin top coat. Thus we can say that this is one industry kept afloat by the availability of low cost GCC.

CaCO3 filler in other polymers

In all other polymers (except liquid resins like polyesters) calcium carbonate has to be melt compounded into the polymer matrix. Unlike PVC resin, which is a powder, other major thermoplastics are available as pellets. The compounder has to mix the filler in fixed proportions (75:25 for 25% filler loading as opposed to the PHR system in PVC), and melt compounded in intensive mixing systems. It started with adaptations of rubber technology with Banbury mixers, roll mills, sheet slabbing and cooling and dicing. Continuous mixers like Buss Ko kneaders and Farrell continuous mixers replaced the batch Banbury process. The modern method is to use co-rotating twin screw extruders with interchangeable screw elements.

The co-rotating twin screw extruders are totally different from the counter-rotating parallel and conical twin screw machines discussed before. These operate at about 30-35 rpm (there is too much wear and tear at higher speeds). Co-rotating twin screws can run at much higher screw speeds (500-600 rpm) as the screws tend to float in the polymer matrix in the barrel and abrasion is not a major issue. Polymers like polyolefins, nylons, and styrenics are not as heat sensitive as PVC and are unaffected by such high screw speeds.

The outputs of sophisticated twin screw compounding extruders have reached phenomenal levels, with a 65mm machine delivering upwards of 500 kgs/hr compared to 120-150 kgs/hr of a similarly sized counter-rotating PVC machine. Such sophistication comes at a price. The high capital cost of this equipment adds $250-350/tonne to the processing cost of filled compounds. With PVC fillers incorporation costs are marginal, but in all other thermoplastics the high compounding costs brings a different set of economic dynamics.

Twin screw extruder, first developed by Cincinatti Milacron in the US
Cincinatti Milacron

Calcium carbonate in polyolefins

The polyolefin family includes low density and linear low density polyethylene (LDPE and LLDPE), high density polyethylene (HDPE), polypropylene homopolymer and copolymer (PPHo, PPCo), and polybutelene and thermoplastic elastomers like EPM and EPDM. They are the predominant polymer family accounting for the majority of consumption worldwide.

Unlike PVC, where nearly all applications use calcium carbonate, filler applications in polyolefins are quite restricted. The main reason is the high compounding costs and the increase in density by adding fillers. Thus except for some specific sectors like raffia tape, mineral fillers do not bring about substantial cost reductions in polyolefins. Fillers are primarily added to improve physical properties like stiffness, paintability, electrical properties and some others.

Calcium carbonate is used in conjunction with other mineral fillers, primarily talc - which is costlier than calcium carbonate, but is more effective as a reinforcement. Carefully chosen blends of talc and calcium carbonate are tailored to meet the application requirements.

Filled polypropylene is the bulk of the business with the automotive and moulded furniture markets using very large tonnages. Stiffness and paintability dictate that most of the PP used in a modern automobile will be filled compounds of talc/CaCO3/PP and even rubbers like EPM and EPDM. A very wide range of compounds have been developed with special additives to withstand the rigours of automotive exterior and interior requirements.

In moulded furniture CaCO3/talc PP compounds are widely used, especially in monoblock stackable chairs that are popular as garden furniture and mass seating. Competitive pressures have dictated a trend to thinwall the chairs, and filled PP is used to stiffen the thinner legs to support seating loads (this is quite important as obesity seems to be a worldwide disease).

Stadium seating, another major moulded furniture sector, relies more on properly compounded UV and light stabilisers to withstand the continuous sunlight exposure. Fillers are used to improve stiffness if necessary.

Siddhartha Roy, consultant, +919890366632, Roy is well known in Indian plastic circles. A chemical engineer from IIT Kharagpur (1968), he has worked with plastics all throughout his career. He was actively involved in development of PVC markets and applications, especially pipes and fittings. Roy worked with Shriram Vinyls, PRC (now DCW) and Chemplast, manufacturers of PVC resin and compounds. He was manager of a PVC pipes and fittings factory in Kuwait and helped Jain Pipes set up its production facilities. Roy was recently awarded the Fellowship by the governing council of IPI for his contribution to the plastics industry.

1 Calcium carbonate is possibly one of the three major minerals which has been formed by life, whether plant or animal. Coal originates from the massive forests in the Carboniferous age while peat is of more recent origin. These forests also helped in converting the high levels of CO2 in the young Earth’s atmosphere to oxygen, and the carbon is the major component of the fossilised remains. The other, if it can be termed a mineral, is petroleum, shale oil, natural gas (collectively known as liquid gold). How many billions of animals had to die so that their organic remains could be transformed into a thick hydrocarbon soup which seeped through vast tracts of porous sedimentary rocks to be trapped under impervious rocky domes which now bring untold riches to the countries lucky enough to possess such structures? Petroleum is probably the second largest naturally occurring liquid of substantial quantities on Earth. The first of course is water. More information on calcium carbonate geology, history, production and modern uses can be found here.