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Understanding How to Select the Correct Specialty Minerals Precipitated Calcium Carbonate for Sealants and Adhesives

Precipitated calcium carbonates (PCCs) are used in many high performance sealants and adhesives for rheological control, reinforcement, and as a cost-reducing filler. This page will help guide your selection of the best PCC for your product for some background, click here to read about Specialty Minerals Inc.’s (SMI’s) PCCs, how they work, and the role that the choice of dispersion equipment has on the efficiency of a PCC in sealants and adhesives.


Choosing a Specialty Minerals Precipitated Calcium Carbonate
SMI has developed a wide range of PCCs for use in sealants and some adhesives. Click here to see the list. Why are there so many different products? Because they have different particle sizes, degrees of agglomeration, surface areas, and coatings, which are used in various combinations to optimize the PCC’s performance in the range of polymers used as the bases for sealants.


Precipitated Calcium Carbonate Particle Size Effects
A PCC’s particle size and surface area have major effects on the properties of a sealant:

  • Rheological: as the PCC particle size is decreased and as the degree of PCC agglomeration is decreased, the PCC’s surface area increases and there is a greater number of individual PCC particles per unit of weight. This increased number of particles produces a stronger internal rheological structure, so viscosity, thixotropy, and sag or slump resistance increases.


And varying the PCC particle size also has a major effect on the degree of sag control developed:

  • Mechanical: as the particle size decreases, tensile strength and modulus increases, and ultimate elongation decreases.


Surface Property Effects
A key surface property is hydrophobicity, which affects the PCCs compatibility with the base polymer. As can be seen in the functionality chart above, SMI’s PCCs for sealants fall into three groups: two uncoated nano PCCs (hydrophilic), Thixo-Carb® 500 PCC (extremely hydrophobic), and the rest falling into a middle group of about equal hydrophobicities. The hydrophilic Calofort® U and Multifex-MM® PCCs are mainly used in water-based systems. The hydrophobic Thixo-Carb® 500 PCC is used in moisture sensitive systems.


Equilibrium Moisture
The middle group of SMI PCC’s will generally have about 0.15 to 0.2 percent of equilibrium surface moisture. For many sealant systems, this surface moisture has no effect. But in moisture-curing systems, the presence of this amount of moisture can cause pre-curing of the polymer in the package. For urethanes and silicones, it is common for the manufacturer to dry the PCC (and other fillers) before use, bringing the surface moisture below 500 ppm (0.05 percent). 

Thixo-Carb® 500 PCC is an exception. Because of its extreme hydrophobicity, it has very little surface moisture, generally coming out of the bag at a level below that of other PCCs when dried—less than 500 ppm. If further drying to essentially no moisture is wanted, then it can be done very quickly, in a fraction of the time needed for standard PCCs.  


Shear Thinning—Extrusion or Spray Viscosity
Another factor in PCC selection is how to the sealant is to be applied. Application method dictates the viscosity wanted at the high shear rates during use, as well as the degree of shear thinning. Yield value and high shear viscosity must be balanced for the sealant to work properly in the factory or the field.


In the same system, the SMI PCCs are shown to shear thin by different degrees, and result in different high shear viscosities. The lower viscosity at high shear may be needed when spraying and the higher viscosity at high shear rates when extruding. The high shear viscosity can also be changed by altering the relative amounts of PCC rheological additive and ground calcium carbonate (GCC) filler used in the formula. Click here for more details on this type of adjustment.


Which Specialty Minerals Precipitated Calcium Carbonate to use?
Click here to view the selector chart of which SMI PCC to use in a variety of sealant types, including acrylic and PVC plastisols, butyls and other elastomerics, polysulfides, polyurethanes, silicones, silyl-terminated polyether modified silicones, and water-based sealants.

 

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