The role of Calcium Carbonate as a filler in Polymers. 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

1. Calcium carbonate fillers
Calcium carbonate’s
polymer promise
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
I
t 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
1
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.
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 at: http://calcium-carbonate.org.uk/calcium-carbonate.asp
40
industrial minerals
November 2011

2. Calcium carbonate fillers
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 110-120°C.
The hot mix is cooled in a higher capacity water
jacketed cooler mixer to bring temperatures
down to ~ 50°C 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.
Table 1: Fillers commonly used in plastics
Polymer
Filler
Type
Functionality
Polyolefins (LDPE, HDPE, PP)
Glass fibres
Reinforcing
Superior stiffness
Stiffening, paintability
Talc
Reinforcing
Calcium carbonate
Reinforcing in blends with talc, non- Stiffening, paintability, cost
reinforcing in large addition %
reduction in special cases
Thermosets (bulk moulding and
sheet moulding compounds)
Calcium carbonate
Non-reinforcing in large addition %
Cost reduction
Engineering plastics (nylon,
polycarbonate etc)
Glass fibres, carbon fibres, talc
Reinforcing
Enhanced stiffness and mechanicals
PVC unplasticised
(pipes, profiles)
Calcium carbonate
Non-reinforcing
Smooth extrusion, cost reduction
Plasticised PVC (wires and cables,
leathercloth
Calcium carbonate,
calcined clay
Non-reinforcing
Better electricals, cost reduction
Plasticised PVC (films and sheets,
coated fabrics, leathercloth)
Calcium carbonate
Non-reinforcing
Surface modification, cost reduction
November 2011
industrial minerals
41

3. Calcium carbonate fillers
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 140-170°C with the
final polish being given at die exit at 200°C 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:
42
industrial minerals
Table 2: Typical PVC formulations for cables
Ingredient
Insulation PHR
Outer sheath PHR
Inner sheath PHR
PVC resin K 67-72
100
100
100
Primary plasticisers
30-60
25-40
20-40
Secondary plasticisers
0-20
10-35
40-50
Stabilisers and
lubricants
3-4
4-5
5-6
PCC
5-20
25-50
>100
Pigments
1-1.5
0.5 (carbon black)
0.5 (carbon black)
• 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
~180-200°C. 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,
November 2011

4. Calcium carbonate fillers
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 counterrotating 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.
Calcium carbonate in polyolefins
The polyolefin family includes low density and
linear low density polyethylene (LDPE and
LLDPE), high density polyethylene (HDPE),
Cincinatti Milacron
Twin screw extruder, first developed by Cincinatti Milacron in the US
November 2011
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.
Contributor: Siddhartha Roy, consultant,
+919890366632, royplastech@rediffmail.com.
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.
industrial minerals
43