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.