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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 polymer matrix.

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 more homogenous.

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 modifier.

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.

 

PVC Pipe
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® calcium carbonates.

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
The 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 polymer matrix.

 

Particle Size Distribution and Impact Strength: Precipitated vs. Ground Calcium Carbonates
Considering 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.


Impact/Stiffness Balance
In 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
This 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
Fillers 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 interesting ways.

Types of Fillers Used in PVC and Other Thermoplastic Polymers
Approximately 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
Mineral 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.
     

The 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 size distribution.

 

“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 than” size.

  • 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:

  • Hardness – 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.
  • Color – 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 compounded formulation.
  • 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.