Rigid or Unplasticized PVC – Pipe & Fittings Using SMI Precipitated and Ground Calcium Carbonates
Polyvinyl Chloride (PVC) is a versatile polymer well suited for a
variety of pipe applications ranging from large sewer piping to
residential drain waste vent (DWV) piping,
as well as smaller potable water piping and electrical conduit.
Applications such as DWV are not pressure rated, which reduces the
impact resistance required to
function in the application. Others, such as residential potable water,
carry ratings appropriate to their intended application. More recently,
chlorinated PVC (CPVC) has
been approved in some municipalities for use in hot water piping because
of its superior high-temperature properties versus PVC.
Both Specialty Minerals Inc.’s (SMI’s) ground calcium carbonates (GCCs) and precipitated calcium carbonates (PCCs)
contribute to the ultimate performance properties of this wide variety of piping applications. Product choice depends on the
specific application and the property requirements of that application. For maximum property development and formulation
flexibility, our surface-treated carbonates are designed to give good dispersion along with excellent incorporation in the
Calcium carbonate improves the base properties of polyvinyl chloride by adding stiffness to the polymer matrix and
improving impact resistance as particle sizes become smaller. Calcium carbonate also improves compounding performance by
helping disperse various ingredients into the PVC powder blend and improves processing by making polymer flow
The relationship between a calcium carbonate’s particle size and the impact strength of the finished rigid PVC part
also applies to extruded pipe and injection-molded fittings. The object is to produce an acceptable part at the lowest
possible cost. This is achieved by minimizing the level of ingredients that add cost to the PVC formulation, such as impact
The ideal is to eliminate the use of impact modifier. This can be achieved in larger diameter pipes using a formulation
containing a 2-3 micron calcium carbonate. The impact specification is usually more difficult to meet in a small diameter
pipe, thus a finer calcium carbonate must be used—usually in the range of 1-2 microns. In applications that require an
impact modifier, the amount of modifier can be minimized by using a very fine, submicron PCC such as
Calofort® S, or Ultra-Pflex® PCCs. This approach is especially useful in demanding applications such
as pipe fitting formulations and large diameter corrugated drainpipe.
In modern life, we take the long-term performance benefits of PVC pipe for granted.
PVC pipe functions without corrosion or leaking and repairs are simple to perform compared
to cast iron or copper piping. To assure the purity of potable water supplies, ingredients for
water-system piping used in the United States must be certified by the NSF (National Sanitation Foundation).
SMI’s calcium carbonates are NSF-certified for plastics piping. These products include:
Hi-Pflex® 100, Super-Fil®, Vicron® 15-15, Super-Pflex® 100,
Super-Pflex® 200, and Ultra-Pflex®
Choosing an SMI product generally depends on pipe diameter as well as the specific requirements of the application.
The selector chart below gives some general guidelines.
PVC Pipe Fittings
Visit a home improvement store and look down the plumbing aisle for PVC fittings.
You will see a myriad of fittings for pipe connection applications. To assure leak-proof,
durable connections, fittings need to be molded to close tolerances with a predictable margin
of shrinkage in the PVC compound.
SMI’s PCCs, including Ultra-Pflex®, Calofort® S, and
Super-Pflex® 200 possess the quality
and performance needed for these demanding applications. All these products are certified by NSF
for potable water applications.
Impact Strength Fundamentals: Particle Size Effect and Impact / Stiffness Balance
Particle Size and Impact Strength
size of the filler used can have a dramatic effect on the impact
strength of a PVC compound. The graph below shows the relationship
between impact strength and filler particle size for calcium carbonate
in rigid PVC.
The relationship is not linear. When the carbonate
size is larger than 1 micron, the impact strength is relatively low.
However, as the carbonate size is reduced into the submicron range, the
impact strength of the compound increases dramatically.
The low impact strength imparted by particles larger
than 1 micron is a function of points of weakness in the polymer
matrix. Likewise, the submicron particles impart enhanced impact
strength because they are points where stresses can be relieved in the
Particle Size Distribution and Impact Strength: Precipitated vs. Ground Calcium Carbonates
the median or average particle size alone is not enough to ensure high
impact strength. The impact strength of PVC or other polymer is limited
by the size of the largest particles present. Two fillers with equal
median particle sizes, but different particle size distributions, will
confer different impact strengths.
An advantage of precipitated calcium carbonates
(PCCs) over ground calcium carbonates (GCCs) is the narrower particle
size distribution of the PCCs. The particle size distributions of a PCC
and a GCC of the same median particle size are compared in this plot:
The precipitated calcium carbonate has fewer
large particles than the ground, and the size of the largest particles
is smaller for the PCC than for the GCC. This can be seen in scanning
electron micrographs. Here are a PCC and a GCC, each with a median
particle size of 0.7 microns:
Specialty Minerals Inc.’s (SMI’s) submicron PCC fillers such as Calofort® S, and Ultra-Pflex®
PCCs are particularly effective in building impact strength in rigid
PVC because of their steep particle distribution curves. The PCCs do not
have the large particles found in the GCCs, reducing stress
concentrator points in the polymer matrix.
theory, one can get the best of both worlds with a high-aspect ratio
filler of very small particle size because impact strength is a function
of particle size and stiffness is a function of aspect ratio.
Unfortunately, it is very difficult to produce a filler with both
properties because the critical size is the largest dimension of the
filler particle and a very small particle, in order to have a high
aspect ratio, would also have to be extremely thin, thus becoming quite
fragile. Still, some interesting property balances can be achieved by
using the finest platy fillers available—such as SMI’s ultrafine talcs,
notably UltraTalc® 609 —with increased levels of impact modifier. Click here to download a SMI publication on the use of these ultrafine talcs in formulating a strong and stiff rigid PVC.
With this approach, it is possible to produce a
rigid PVC with high performance, enabling it to replace
acrylonitrile-butadiene-styrene (ABS) in many applications.
More on Impact Strength Fundamentals
section of the SMI web site contains a number of pages that describe
the basics of impact strength in polymers.
Impact Strength in Thermoplastics: Functional Fillers and Their Properties
The Role of Fillers in Thermoplastics
are used for a wide variety of reasons. They can extend resin, increase
stiffness and strength, improve impact performance, and shorten cycle
times. They prevent hang-up in dies and neutralize the products of
degradation. Fillers can also be used to add color, opacity, and
conductivity to a compound. Unique property combinations can be achieved
through the use of fillers.
Traditionally a filler was a low-cost material of
relatively large particle size that lowered a formulation’s cost simply
because it was less expensive than the other ingredients in the
formulation. Today a “filler” can be a true performance additive.
Advances in compounding technology allow the use of much finer fillers
that could not be used in the past. Today’s filler products are tailored
for specific applications and designed to deliver value in new and
Types of Fillers Used in PVC and Other Thermoplastic Polymers
80 percent of the filler used in PVC in the U.S. is calcium carbonate.
Titanium dioxide is second at around 12 percent, followed by calcined
clay at about 5 percent. The remaining few percent is taken up by other
materials including glass and talc.
Calcium carbonate products are available in a wide
range of sizes. They are produced by grinding limestone and by
precipitation. The precipitation process can produce true nanoparticles
of calcium carbonate (defined as less than 100 nanometers or 0.1 micron)
whereas the grinding process is typically limited to an average
particle size of around 1 micron. Titanium dioxide is used as a white
pigment and UV stabilizer. Calcined clay goes into wire and cable
formulations where it improves electrical properties. The remaining
fillers find their role in a variety of specialty applications.
Primary Filler Properties: Particle Size and Shape
fillers tend to be described by their properties which influence the
filler’s performance in a resin system. Normally the first two
considered are pigment particle size and shape.
- Particle Size
- Most fillers are grouped and ranked by their particle size. There are
a number of different ways to measure and report particle size. When
comparing two fillers, one must make sure that the comparisons are made
using equivalent measurement techniques. Even small differences can be
significant, especially where fine fillers are involved.
term “particle size” is itself misleading. Even a small sample of a
filler will contain many particles of different sizes. What we are
actually dealing with is a particle size distribution. Most data sheets
give the average size or the midpoint (median) value in their product’s
“Top size” is another term used to describe a
filler’s particle size. It is a carry-over from the manufacture of
coarser, screened stone, where the “top size” of the product was the
finest screen that all the material would pass through (that is, the one
on the top of the stack of screens). Its definition and applicability
become less clear when dealing with fine fillers. Classifiers do not
produce perfect top cuts and fine fillers tend to stick together causing
agglomerates that act like coarser particles. It is technically more
appropriate to speak of a “95 percent finer than” or “99 percent finer
- Particle Shape - Filler
particles come in a variety of shapes as well as sizes: spheres, rods,
platelets, and irregular shapes of varying proportions. Here are
scanning electron micrographs of three of Specialty Minerals Inc.’s
(SMI’s) functional filler products—ground calcium carbonate (GCC), two
precipitated calcium carbonates (PCCs), and talc.
One sub-feature of shape that has a significant influence on a composite’s physical properties is aspect ratio. The aspect ratio of a filler particle is the ratio between the particle’s largest and smallest dimensions.
In the case of a rod, it would be length divided by
diameter. In the case of a talc platelet, it would be length divided by
thickness. A sphere would have an aspect ratio of 1:1, while a platelet
or fiber can be 20:1. Shape plays an important role in defining a
filler’s reinforcing characteristics.
Some Additional Filler Properties
Other filler or extender pigment properties that influence performance include:
– Abrasion is a function of hardness and particle size. This
relationship to particle size is not linear. Coarser particles are much
more abrasive to plastics processing equipment than finer ones. The
contribution of impurities must also be considered.
– A filler can contribute to the color of a compound. If color is
important, the pigment package must be adjusted to compensate for the
effect of the filler. The dry color of the filler can be misleading. A
bright white filler may produce a gray color in the polymer. The only
way to know how a filler will affect color is to test it in the
- Specific Gravity – The
specific gravity of the filler must be taken into account when
calculating the cost of a compound because the parts produced are sold
on the basis of volume, not weight. Most mineral fillers have a
relatively high specific gravity and will therefore raise the specific
gravity of the compounds they are used in. As a result, it will take
more pounds of highly filled compound to make the finished part.
- Surface Treatment
– Most of the fillers sold to PVC applications are surface treated.
This treatment is usually a fatty acid, such as stearic acid. A coating
can improve the dispersion of the filler particles during melt
compounding, reduce the adsorption of other formulation ingredients onto
the filler’s surface, improve the filler’s dry flow properties, and
change its processing characteristics.