Performance fillers
T
he volume cost of a raw material
input is the purchase cost of a
unit volume of the material.1 It is
extremely important to
understand the volume cost of
polymers and their additives as this plays a key
role in their selection for a particular
application.
Price is one of the first characteristics of a
polymer that a designer looks at before
specifying it as the material of construction.
Prices vary from time to time, sometimes
wildly, but tend to maintain their proportion
in respect to other polymers. The recent prices
of the commodity thermoplastics, using
average prices, are shown in Table 1.
While it would appear that UPVC is by far
the cheapest polymer, the natural question is:
why does it have such limited applications in
eg. moulded products? Assuming that UPVC
is as easy to mould as the other commodity
thermoplastics, why is it not used in
widespread applications such as plastic
buckets?
At this point we have to focus on an
important polymer property that is often
overlooked – fortunately this is not a property
which changes with time or location!
The density of a polymer (see Table 2) is
measured from a fully gelled and fused sample
and should not be confused with bulk density,
1
42
Mueritz
Volume cost
in plastics
applications
Mineral fillers are widely used in the plastics industry to
enhance polymer properties and reduce ingredient costs.
But, as Siddartha Roy demonstrates, over-use of fillers can
be a false economy
which is the apparent density of the granules
or powder that the polymer is sold and
measured as prior to processing.
Bulk density has more relevance to rate of
flow through the hopper throat of the
processing machine, tendency to bridge/stick
and other handling and storage
considerations. Bulk density can change
depending on particle size/shape, but density
of a polymer is constant.
When the volume cost is plotted, a
completely different picture appears.
Polypropylene now becomes the cheapest (see
Figure 1).
In the case of the household plastic bucket,
it is interesting to note that in India these
were first moulded in LDPE in the early
1960s. As HDPE became available in the late
‘60s, its lower volume cost was one of the
reasons why there was a wholesale shift by
bucket manufacturers to HDPE.
Of course the better stiffness and warm
water resistance of HDPE were major factors
for the shift, but the lower volume cost helped
Volume cost (Rs./litre) = Purchase cost (Rs./kg) x density (kg/litre or gm/cc)
industrial minerals
July 2011
Performance fillers
Table 1: Prices of commodity
thermoplastics*
Price
(Rs./kg)
Polymer
Abbreviation
Unplasticised PVC
UPVC
48
Plasticised PVC
FPVC
60
Low density
polyethylene
LDPE
70
High density
polyethylene
HDPE
67
Polypropylene
homopolymer
PP
68
Polypropylene
copolymer
PPCO
70
Polystyrene
PS
80
High impact
polystyrene
HIPS
82
Acrylonitrile
butadiene styrene
ABS
85
* Note for international readers: the taxation
structure in India tends to push up polymer
prices compared to international norms. Price is
so variable that a study can quickly become out
of date especially with volatile petroleum
markets. However, the comparative price ratios
between polymers are more stable even with
wild price fluctuations overall. The Indian Rupee
is approx 45 to a US$: thus PP Table 1 (Rs. 68/
kg) is about US$ 1,500/tonne.
to keep bucket moulding firmly in the
polyolefin family.
In the 1990s, polypropylene had also made
inroads into the bucket market; aided no
doubt by its lower volume cost though its
superior clarity, although stiffness and
temperature resistance were also factors.
It is smart marketing which has positioned
the clearer and stiffer PP bucket as a premium
product sold at higher prices than its HDPE
counterpart. As they say, “pricing depends on
marketing policy while costing depends on
facts”, and the fact is that the volume cost of
the higher-priced PP bucket is lower than the
HDPE one. That is to say that if PP and
HDPE are injected into the same bucket
mould volume, lesser amount of PP in
kilograms would be required.
It is a separate matter that the PP bucket
mould would be different with perhaps a
thinner wall to cash in on PP’s higher rigidity,
but the reality of better volume cost remains.
PVC has never been in the picture because of
its higher volume cost – if its volume cost had
been lower than the polyolefins, ways and
means would have been devised to mould
PVC into buckets!
This example is simplified and there are of
course many factors which have to be
considered to select the correct plastic for a
specific application, but the point to take
Figure 1: Polymer price vs. volume cost
90
Rs/Kg
80
2
70
away is that volume cost is a less understood
but extremely important factor.
On the basis of volume cost, when chief of
R&D of VIP Industries, a leading Indian
moulded luggage producer, the author
formulated a directive to the company’s
luggage designers that any new plastic
component should be designed with PP
copolymer unless the design intent could not
be met by the properties of PPCP. Only then
was there a need to look at other higher
volume cost polymers.
Importance to the plastics
formulator
The consideration of volume cost is even more
important when polymers are compounded
with additives. The density of the final
product can change considerably especially
when mineral fillers are added primarily to
reduce costs.
Volume cost and its implications are not
properly understood by many entrepreneurs,
formulators and people undertaking cost
reduction/value engineering. It is vital to
understand its implications before embarking
on cost reduction exercises.
Most plastic products are sold by volume.
They are priced either per piece (mouldings)
or per unit length (pipes, cables, tape) – thus
the costing and pricing are for fixed volumes.
As the plastic raw materials are always
purchased per unit weight, the tendency is to
calculate cost on a per kilogram basis, and the
finished product is priced accordingly to the
weight per piece.
However, in the marketplace, competitive
pressures often force the entrepreneur to offer
discounts to protect market share. The
discount is normally a percentage of the
existing selling price, which, in the majority of
cases is the realisation on volume basis.
Table 2: Polymer density (kg/lt)
60
Polymer
Density (kg/lt)
UPVC
1.38
50
FPVC
1.25
LDPE
0.92
40
HDPE
PP
0.96
0.90
PPCO
0.905
PS
1.05
HIPS
1.05
ABS
1.05
UPVC
FPVC
LDPE
HDPE
PP
PPCO
PS
HIPS
ABS
Rs./Kg
48
60
7O
67
68
70
80
82
85
Rs/Ltr.
67.2
75
64.4
64.32
61.2
63.35
84
86.1
89.25
Explanatory note for international readers: While ratios between polymer prices would be roughly the same globally, the price ratios of PVC resin and important compounding
ingredients like calcium carbonate fillers, stabilisers and pigments could be very different from country to country. This is because the taxation tariffs are quite different for
polymers and minerals. Also the mineral filler prices have a major transportation cost element especially for the cheaper GCC grades.
I have intentionally calculated the costing with individual compounding ingredients in a classical lead stabilised twin screw pipe formulation. The calcium carbonate price is for a
precipitated uncoated grade. One pack stabiliser lubricant systems are the norm in India as it is globally, but there is a paucity of density data for such one packs. It is more
accurate to calculate volume cost with well documented densities of the separate ingredients. An interested reader could redo the calculations with their local price data and the
one pack density if it is known. My guess is though the percent cost reductions may vary, the pattern will be in line with my workings.
July 2011
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43
Performance fillers
3
Table 3: Summary of volume costs for PVC pipes
0 PHR
10 PHR
20 PHR
30 PHR
40 PHR
50 PHR
Formulation cost Rs/kg
50.08
46.53
43.86
41.32
39.30
37.39
Volume cost Rs/lt
69.99
67.94
66.55
64.83
63.51
62.05
% Reduction in cost/kg
7.08
12.42
17.48
21.53
25.33
% Reduction in cost/lt
2.94
4.91
7.37
9.26
11.35
Figure 2: Volume cost vs. per kg cost
Rs. 71.00
Formulation Cost Rs./kg
Rs. 71.00
Rs. 66.00
11.35%
Rs. 66.00
Rs. 61.00
Rs. 61.00
Rs. 56.00
Rs. 56.00
Rs. 51.00
Rs. 51.00
Rs. 46.00
25.33%
Rs. 41.00
Rs. 36.00
Formulation Cost Rs/Kg
Volume Cost Rs/Ltr
Rs. 46.00
Rs. 41.00
0 PHR
Rs.50.08
Rs.69.99
10 PHR
Rs.46.53
Rs.67.94
20 PHR
Rs.43.86
Rs.66.55
30 PHR
Rs.41.32
Rs.64.83
40 PHR
Rs.39.30
Rs.63.51
50 PHR
Rs.37.39
Rs.62.05
Volume Cost Rs/Litre
If costs are calculated on a per kilo basis, often
the reduction in cost by adding fillers/extenders
is calculated as a percentage of original
formulation cost. The savings may be translated
into a price reduction based on this percentage.
After some time the entrepreneur realises that
he is sustaining losses as the reduction in
volume cost was nowhere near the per kilo cost
reduction on which the discounts were based,
especially when mineral fillers are the main cost
reducing input. All mineral fillers have a higher
density than most plastics.
Rigid PVC pipes are a prime example. The
ease with which calcium carbonate can be
loaded and processed by modern twin screw
extruders has led to mindless loading of fillers in
a desperate bid to reduce costs. The pitfalls are
many as is illustrated by the calculations in
Tables 3 and 4.2
It is interesting to note that even though these
are theoretical calculations, the predicted
density is quite near the actually measured
density, with the difference being a few points
in the third decimal place. Rarely do we find
errors in the second decimal place. Assuming
that the pipe is gelled fully and has no voids, the
density figures predicted are quite close to actual
densities.
There is some volatile loss, but in a pipe
formulation this is a low percentage. The
graphical representation shows the big
difference in the reduction in cost when
measured per kilogram and the volume cost
(Figure 2).
By adding 50 PHR calcium carbonate, which
is not unusual in commercial grade PVC water
supply pipe (in India some processors sell such
pipes for water supply, albeit for irrigation), and
in the non-pressure applications like SWR, the
expected cost reduction appears to be a healthy
25%.
However, in actuality, the volume cost has
reduced only 11%. Such a high loading of filler
not only ruins the pipe impact strength and
pressure resistance, but the wear and tear on
costly twin screw equipment is severe. Thus it is
not worth sacrificing so much quality
deterioration and machine life reduction for a
mere 11% reduction in cost.
This should be understood by all PVC pipe
manufacturers and other sectors which rely on
dense mineral fillers primarily for cost
reduction. Of course nobody makes pipes with
0 PHR filler, and around 8-10 PHR filler is the
optimum level for good quality pipe
conforming to BIS 4985 (Bureau of Indian
Standards), which is in line with the DIN and
British Standards for pressure PVC pipes.
Screw barrel life is of acceptable levels, and it
is heartening to note that most of the quality
Rs. 36.00
Figure 3: The filler-based cost reduction trap
Reduce pipe costs to be
more competitive by
increasing filler loading
Reduce pipe prices based on
per kg. reduction in costs
Losses mount as
reduced price is not
matched by actual
cost reduction
conscious PVC pipe manufacturers have
persisted with such formulations and have been
successful in the long run.
It is when higher loadings are resorted to for
cost reduction that a vicious cycle starts. Let us
say a manufacturer increases his filler loading
from 10 PHR to 40 PHR. Relying on
formulation costing he expects a reduction of
15.5%, and so reduces the price of his pipes by
Pipe
business
collapses
Quality plummets, customer
confidence erodes, markets
shrink further
Despec pipe by reducing
thickness/increasing filler to
compensate for losses
15% from his BIS 4985 price.
However his cost per length of pipe has gone
down only by only 6.5% (the volume cost
reduction). Soon the producer finds out that he
is losing money, so what is the next step? More
filler loading coupled with decreasing the wall
thickness of the pipe, deteriorating quality even
further: and the downward spiral in quality and
shrinking returns continues (see Figure 3)3.
The pressure to cut costs is surely not restricted to Indian markets. I am sure worldwide that cost reduction of PVC pipes with fillers is endemic, but it should be within limits.
Addition of fillers is the easiest way out of the many avenues available to cut costs, but the dangers of doing so blindly without heed to volume costs would lead to disastrous
results similar to those illustrated in Figure 3.
July 2011
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45
Performance fillers
The author hopes those PVC processors
tempted to take the high filler route pause
and rethink their strategy. One of the reasons
that so many PVC pipe and profile extrusion
firms have collapsed and closed shop is that
they got caught in this vicious cycle: higher
filler loading, decreased wall thickness,
product failures, and compensation claims
– trapping the company with heavy losses.
What has been highlighted above is a most
dangerous trend. Many polymer applications
in India have faced declining demand due to
loss in confidence of the consumers because
of repeated failures of poor quality, cheap
products. Examples are too numerous, and it
is most saddening to persons and companies
who have worked so hard in establishing
such applications.
In the pipe field itself one can recall the
hammering HDPE pipes took in the early
1980s due to large scale failure of pipes made
from offgrade/scrap HDPE and sold to
prestigious government projects as prime
grade pipes. While HDPE pipe markets
languished because of the bad name, PVC
pipes surged ahead.
Even major companies like Polyolefin India
Ltd, a Hoechst licensee, were so badly
affected that they had to stop the
manufacture of their well established Hasti
brand HDPE pipes. It has taken two decades
for HDPE pipes to claw back to good
volumes, which involved consistent quality
and development of new application areas
like drip and sprinkler irrigation, gas piping,
large diameter sewerage pipes etc., as well as
consolidation in the core water supply sector
with good quality pipe with second
generation HDPE grades.
A dangerous fallout of mindless filler
loadings is when markets change from
pricing per piece, or – in the case of pipes –
per unit length of specified thickness to
pricing on a per kilo basis. Such a change
encourages higher filler loadings and should
be resisted by all discerning manufacturers.
In plastics, ‘heavier’ does not mean
‘stronger’: physical properties are seriously
compromised in PVC products made heavy
by excessive filler additions.
Formulating polyolefins
With polyolefins, the situation is different.
Here fillers like talc and calcium carbonate
are added to improve stiffness to PP, or
desired properties like anti-fibrillation in
HDPE or PP raffia tape.
Incorporation of fillers in polyolefins is an
expensive process, requiring costly corotating twin screw extruders or
sophisticated equipment like Buss KoKneaders. Compounding costs for filling
polyolefins can be as high as Rs. 10-15/kg
(US$250-350/tonne), while in PVC the
increase in dry-blending cost with filler
addition is negligible.
Filled polyolefins (10-30%) are costlier
than the base polymer because compounding
costs outweigh the lower filler cost. The
volume costs go up sharply, but requirements
of better stiffness in auto components,
moulded furniture and other technical parts
Table 4: Volume costs of PVC formulations for PVC pipes
0 PHR Filler
Ingredient
10 PHR Filler
Price Rs/kg
Density kg/lt
PHR
(kg)
Cost
(Rs.)
Volume
(lt)
PHR
(kg)
Cost
(Rs.)
Volume
(lt)
PHR
(kg)
Cost (Rs.)
Volume
(lt)
72.46
PVC Resin K67
48
1.38
100.0
4,800
72.46
100
4,800
72.46
100
4,800
TBLS
120
7.2
0.8
96
0.11
0.8
96
0.11
0.9
108
0.13
DBLS
140
4.5
0.5
70
0.11
0.5
70
0.11
0.6
84
0.13
Lead stearate
100
2.1
0.4
40
0.19
0.4
40
0.19
0.5
50
0.24
Calcium stearate
80
1.1
0.4
32
0.36
0.4
32
0.36
0.5
40
0.45
Filler
10
2.7
0
0
0.00
10
100
3.70
20
200
7.41
Lubricant
140
0.95
0.3
42
0.32
0.3
42
0.32
0.4
56
0.42
TiO2
130
5.6
0.6
78
0.11
0.6
78
0.11
0.6
78
0.11
50
0.98
0.1
5
0.10
0.1
5
0.10
0.1
5
0.10
113.1
123.6
Carbon black
Total
5,163
73.77
5,263
77.47
5,421
81.45
Formulation cost
103.1
50.08
Density
46.53
Density
43.86
Density
Volume costs
69.99
1.398
67.94
1.460
66.55
1.517
30 PHR Filler
Ingredient
40 PHR Filler
50 PHR Filler
Price Rs/kg
Density kg/lt.
PHR
(kg)
Cost
(Rs.)
Volume
(lt)
PHR
(kg)
Cost
(Rs.)
Volume
(lt)
PHR
(kg)
Cost (Rs.)
Volume
(lt)
72.46
PVC Resin K67
48
1.38
100.0
4,800
72.46
100
4,800
72.46
100
4,800
TBLS
120
7.2
0.9
108
0.13
1
120
0.14
1
120
0.14
DBLS
140
4.5
0.6
84
0.13
0.65
91
0.14
0.65
91
0.14
Lead stearate
100
2.1
0.5
50
0.24
0.45
45
0.21
0.45
45
0.21
Calcium stearate
80
1.1
0.5
40
0.45
0.55
44
0.50
0.55
44
0.50
Filler
10
2.7
30.0
300
11.11
40
400
14.81
50
500
18.52
Lubricant
140
0.95
0.4
56
0.42
0.5
70
0.53
0.5
70
0.53
TiO2
130
5.6
0.6
78
0.11
0.6
78
0.11
0.6
78
0.11
50
0.98
0.1
5
0.10
0.1
5
0.10
0.1
5
0.10
143.9
153.9
Carbon black
Total
46
20 PHR Filler
5,521
85.16
5,653
89.01
5,753
92.72
Formulation cost
41.32
Density
39.30
Density
37.39
Density
Volume costs
64.83
1.569
63.51
1.616
62.05
1.659
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133.6
July 2011
Performance fillers
is the driving force for filler addition.
It is only at filler levels of over 40%, as in
filler masterbatches, that the cost per kilo
dips below polymer cost levels, but the
volume cost will be adverse. Thus normally
filler addition does not automatically lead to
cost savings with polyolefins as it does with
PVC. This is why polyolefin pipes cannot be
cheapened by adding filler, as in PVC, and it
is volume cost considerations which
determine this.
Glass-filled and fibre-filled polymers are a
special case with the fillers price sometimes
exceeding the polymer prices. It should be
obvious that glass filling is done purely to
improve mechanicals.
In flexible PVC, considerations of volume
costs come into play. Large amounts of
plasticisers and extenders (secondary
plasticisers) are used. The volume cost
calculations are similar, though the
contraction in volume in flexible PVC
compounds is slightly more because of
volatile constituents in the liquid added.
A simple example of a soft PVC compound
stabilised with a mixed metal stabiliser/ESO
mix is shown in Tables 5-7. It is interesting
to note how the relative costs of the other
ingredients change in relation to PVC resin
when viewed from the volume cost angle.
Plasticisers like DOP, which per kilo is
much more expensive than resin, have always
been thought to be the reason why
plasticised PVC is costlier than RPVC. But
DOP, for example, is not that costly from the
volume cost viewpoint. In fact when PVC
prices had flared up, DOP was actually
cheaper than resin on a per litre basis.
Chlorinated paraffin (CP), a secondary
plasticiser that is very popular in India, is
cheaper than DOP.
As the tables demonstrate, an expected cost
reduction by tripling the filler loading is
considerably eroded on a volume cost basis.
Secondary plasticisers like the popular
chlorinated paraffin family have a higher
density than the primary plasticiser. The
higher the chlorination, the higher the
density and thus the lesser the cheapening
effect.
Apart from fillers, CP is the favoured cost
reduction tool. It takes considerable skill to
balance the compatibility with the
chlorination level of the CP selected with the
addition PHR to achieve an effective cost
reduction without compromising quality.
Compounding of moderately filled
plasticised PVC compounds can be handled
by single screw extruders. Unlike UPVC
compounds, normally plasticised PVC is
processed after a pelletising pass.
With single screw extruders, compounding
costs are low compared to filled polyolefins.
However, as filler loadings increase over
July 2011
Table 5: Volume cost of major ingredients
% cost of resin Density (kg/lt)
Volume cost
(Rs/lt)
% resin
volume cost
Ingredient
Cost (Rs/kg)
PVC
50
1.4
70
DOP
80
160
0.98
78.4
112
CP
50
62.5
1.25
62.5
89.29
Stabiliser
150
187.5
1.05
157.5
225
Filler
12
15
2.7
32.4
46.29
Table 6: Volume cost – formula 1
Product
PVC
Recipe (kg)
60
Cost (Rs/kg)
3,000
Litres
42.86
DOP
30
2,400
30.61
CP
15
750
12
Stabiliser
2
300
1.9
Filler
10
120
3.7
Total
117
6,570
91.08
Cost per unit
–
56.15/kg
72.14/lt
Table 7: Volume cost – formula 2
Product
PVC
DOP
Recipe (kg)
60
30
Cost (Rs/kg)
3,000
2,400
Litres
42.86
30.61
CP
15
750
12
Stabiliser
2
300
1.9
Filler
30
360
11.11
Total
137
6,810
98.49
Cost per unit
–
49.71/kg
69.15/lt
Reduction (from
formula 1)
–
11.48%
4.14%
40-50 PHR, even flexible PVC requires
intensive compounding equipment with
much higher compounding costs (examples:
cable sheathing compounds).
This cost increase has to be factored in for
determining the cost savings while boosting
filler levels in flexible PVC. Needless to say,
the reduction in volume costs compared to
unfilled/lightly filled formulations is less in
comparison to UPVC, as the base unfilled
compound has a lower density than that of
UPVC.
Instances of soft PVC products which are
sold by weight are too many for comfort for
discerning persons working for healthy
growth of the PVC industry. Agricultural
hoses, low quality cables, and some small
mouldings are sold by weight. Customers do
not realise until after using the product that
they have got less actual product, whether in
terms of per metre or numbers when he buys
such highly filled products with attractively
low per kilo prices.
The industry as a rule should discourage
per kilo prices for finished products,
although raw materials are always sold by
weight.
There are other ways of reducing costs
which do not impact quality and offer value
for money. The author hopes PVC processors
will explore and exhaust all of these other
routes before increasing filler levels. If so this
article on volume costs would have served its
purpose.
Contributor: Siddhartha Roy is a chemical
engineer from IIT Kharagpur (1968). He has
worked with plastics throughout his career
and 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 has managed a PVC pipes and fittings
factory in Kuwait and helped Jain Pipes (now
Jain Irrigation) set up their pipe production
facilities.
Roy headed R&D at VIP Industries, Nasik,
and is well versed in the processing of
polyolefins, styrenics, polyamides and PC. He
has been active in the Indian Plastics
Institute’s activities and was recently awarded
the Fellowship by the governing council of IPI
for his contribution to the plastic industry.
industrial minerals
47