Mineral Blends for Polypropylene: Combining
Value and Performance
Holly Hansen
Technical Director
Heritage Plastics, Inc.
1002 Hunt Street
Picayune, Mississippi
INTRODUCTION
Manufacturers of mineral concentrates (masterbatches)
specialize in combining minerals with carefully selected
polymers to produce formulations specific to targeted
applications. Mineral concentrates will typically contain up
to 80% for calcium carbonate and 60% for talc. The
percentages of mineral in the concentrate are kept as high
as possible to keep costs down and minimize dilution of the
prime resin being used. However, selecting the right resin
carrier for the concentrate is essential to ensure optimum
performance. Mineral concentrates offer the molder the
advantage of being able to use minerals, that are well
dispersed, easy to handle and readily let down into
polypropylenes to optimize performance and reduce costs.
This paper will revisit the benefits
provided by using mineral concentrates in polypropylene. It
will discuss the advantages that can be realized in
processing and in end product performance for calcium
carbonate, talc, and blends of the two minerals. This paper
will also highlight why mineral concentrates offer the most
flexible and cost effective method to incorporate minerals
into the injection molding process.
DISCUSSION AND RESULTS
Depending on the intended end use of the
product, desired application requirements can vary
dramatically. Much of this work was focused on a
polypropylene siding application so impact strength,
stiffness, and Coefficient of Linear Thermal Expansion (CLTE)
are discussed. Other applications will have other critical
requirements, but typically impact performance and part
stiffness are areas of primary concern.
Experimental Details: For this work, mineral
concentrates were hand blended with virgin polypropylene
resin to achieve several target mineral loadings for
evaluation. The calcium carbonate concentrate was 80%
mineral, and the talc concentrates were 60% mineral. Let
downs of individual minerals or blends were into the resin
being tested via hand mixing The carriers resins for the
concentrates were polypropylene and selected to ensure
optimum dispersion and end product performance. The
polypropylenes tested were all injection molding grades.
Samples were molded into the appropriate ASTM test parts
using a Niigata 35 ton all electric injection molder. Molded
samples were allowed to condition for 24 hours in an ASTM
temperature and humidity controlled lab. Izod impact was
done using notched samples and followed ASTM D256. Stiffness
was determined by measuring flexural modulus on a tensile
tester via ASTM D790 Method B, the 3 point test at a
deflection speed of 0.133"/minute. CLTE testing was done
according to ASTM E831-00. Samples were prepared by cutting
with a blade shear and sanding edges to be flat and
parallel. The samples were cooled to below -30o C and heated
to above 30oC at a heating rate of 5oC/min. Samples were
orientated in the TMA to measure thermal changes in the
direction of flow.
Impact performance will be influenced
by resin selection, processing conditions, mineral type and
loading level used. In some of our early work, we evaluated
the effect of calcium carbonate in homopolymer and two
different izod copolymer polypropylenes. Figure 1 shows that
impact strength is not adversely affected by the addition of
calcium carbonate and the data suggests that impact is
somewhat improved at the higher loading levels. Homopolymer
polypropylenes are not typically used for high impact
applications, but modest gains in strengths can be realized
with calcium carbonate addition. The two copolymers varied
in impact strength: Co-PP1 was marketed as a 3.5 Izod
product and Co-PP2 is marketed as a 0.95 Izod product. We
didn’t see this same trend in our lab tests, but believe
that optimized process conditions could help maximize the
Izod of both polypropylenes. The significant finding here is
that CaCO3 can be used at levels up to 35% by weight in
polypropylenes without adversely affecting impact strength.
This finding will be key in maintaining performance in parts
with reduced wall thickness when switching from resin only
systems to formulations that contain calcium carbonate.
Figure 1: Effect of CaCO3 on Izod Impact Strength
Talc is often
used in polypropylene to provide better CLTE and
stiffness. Talc however, has a negative effect on impact
strength as shown in Figure 2. We evaluated three talc’s
with varying particle sizes (1.7, 3.5, 10.5 micron) since
it had been suggested that finer particle sized talc’s are
better at maintaining impact strength. For this study,
that effect was not observed.
Figure 2:
Effect of Talc on Izod Impact Strength
Talc and
calcium carbonate were also blended together to see if
their combined effect could better balance performance
properties. In later sections, the blend effects on CLTE
and stiffness will be discussed. In Figure 3 calcium
carbonate again improves the impact strength of Co-PP1
while talc alone lowers it. The blends of talc (3.5 micron
particle size) and calcium carbonate were able to recover
some of the impact strength lost by the addition of the
talc. This data is useful if the goal is a balance of
performance properties such as impact, stiffness, and CLTE.
3
Figure 3:
Effect of CaCO3/Talc Blends on Izod Impact Strength
Part Stiffness
is significantly improved with the addition of minerals
into polypropylene. Figure 4 shows both talc and calcium
carbonate improve stiffness or flexural modulus and the
effect goes up with increasing loading level of mineral.
As suggested in literature, the smaller particle sized
talc shows the greatest improvement in part stiffness.
Figure 4:
Effect of Minerals of Part Stiffness
Blends of talc
and calcium also improved polypropylene part stiffness as
shown in Figure 5. There did not appear to be any
significant gains in stiffness as a function of blend
ratio. Improvements appear to be related to loading level
of mineral used. 4
Figure 5:
Effect of Talc/CaCO3
Blends on Part Stiffness
Coefficient of
Linear Thermal Expansion or CLTE monitors the expansion
and contraction of the part as in undergoes thermal
changes. Figure 6 shows the individual impact of talc (3.5
micron) versus calcium carbonate on CLTE. Talc with its
platy structure is much more efficient in minimizing
expansion and contraction of the final part due to
temperature changes. However, calcium carbonate is also
able to suppress temperature related changes in
polypropylene. Higher mineral loads were more efficient
than lower loadings. And blends of the two minerals, also
shown in Figure 6, were extremely effective in reducing
thermal changes.
Figure 6:
Effect of Minerals on Thermal Expansion and Contraction
SUMMARY
Three of the primary drivers for using minerals in
polypropylene are to reduce material costs, improve
productivity, and to optimize performance. Both talc and
calcium carbonate have unique thermal properties that
cause the parts to cool faster giving faster cycle times
and less warpage and shrinkage compared to resin-only
systems.
Optimized
performance in the final part can be achieved through
careful selection of mineral type, mineral loading level,
and resin type. Minerals can provide additional strength
in the final part to the extent that part geometry can be
reduced or thinwalled without sacrificing performance.
Highest impact strengths can be obtained when using
calcium carbonate and lowest CLTE is achieved with talc.
Both minerals provide added stiffness to the final part.
Blends of the two minerals can provide a balance of low
CLTE, high impact strength and stiffness. Molders can work
with mineral concentrate suppliers to balance desired end
use performance with the formulation of the masterbatch to
be used.
