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Specialty Minerals Precipitated Calcium Carbonates in Sealants and Adhesives

Precipitated calcium carbonates (PCCs) are a special class of calcium carbonates used in high-performance sealants and adhesives. They provide various functionalities related to particle size and coating type.

Most calcium carbonates in adhesives and sealants are ground calcium carbonates (GCCs), typically 3 microns in size or larger. Their main function is as a filler, extending more expensive ingredients and lowering costs. GCCs increase the viscosity of the product, but the viscosity is non-thixotropic, so there is no control of slump or sag. 

Precipitated calcium carbonates also act as fillers in sealants. However, because of their much smaller sizes, down to nano particals of 60 nonometers or 0.06 microns, they are chiefly used as the primary rheological additive in high-performance products. PCCs add reinforcement and make the product stronger. They are used at loadings of 10 to 50 percent.


Precipitated Calcium Carbonates for Sealant and Adhesive Rheology
Ultrafine PCCs build a true thixotropic internal structure in the sealant or adhesive, with the structure resulting from both particle-to-particle and particle-to-matrix interactions. The structure becomes stronger as the number of particles increase. The ultrafine PCCs recommended for sealants have small particle sizes–0.06 to 0.15 micron–and high surface areas. This gives rise to a large number of particles capable of interacting and building rheological structure when used in moderate to high loadings. This rheological structure adds viscosity, yield value, control of slump and sag, and ease of application.

This rheological structure is shear-thinning with the particle interactions breaking and viscosity decreasing as force is applied. As for example, during extrusion, spraying, or other method of application. Once the force is removed, the structure quickly reforms and prevents the applied sealant from sagging or slumping.

Only high surface area, small particle sized PCCs give this sag control. Larger PCCs, in the 0.7 to 3 micron or larger size range, add viscosity but will not provide the sag or slump control needed for application on vertical surfaces.  

Precipitated Calcium Carbonates are a primary choice for thixotrope products that contain 10 percent or more mineral solids. At this level, there are a sufficient number of particles to produce the rheological structure. Sealants, which are almost always highly filled, are a natural application for PCCs. In addition to serving as the sole or primary rheological additive, PCCs can be used to replace part of more expensive thixotropes such as fumed silica, organoclays, castor oil derivatives, and polyamides, reducing costs.


Coated Precipitated Calcium Carbonates
Most high-performance sealants and adhesives are non-aqueous, either solvent or plasticizer based, or are 100% solids. The ultrafine or nano PCCs used in these types are almost always coated with a fatty acid (such as stearic acid) or with a proprietary organic coating. The coating makes the PCC more oleophilic and less hydrophilic, allowing the particles to wet easier, while increasing the dispersibility of the PCC in the polymer, and making the PCC more compatible with the polymer. Together, this increases the rheological efficiency of the PCC and the rheological stability of the product. 

In addition, SMI manufactures uncoated ultrafine nano PCCs for high performance water-based sealants. These uncoated nano PCCs are also used in butyl rubber-based products.


Relating PCC Dispersion to Efficiency and Stability
It is important to make sure that the Precipitated Calcium Carbonate used is fully dispersed, so the full rheological potential is realized. The higher the amount of work applied during dispersion, the greater the viscosity and sag control developed.

Another important reason for fully dispersing the PCC is viscosity stability. If the PCC is not fully dispersed in the sealant, it will be in the form of small agglomerates of PCC particles. As the sealant sits on the shelf, the agglomerates will be slowly wetted out by the polymer, plasticizer, and/or solvent. This increases the number of individual PCC particles, which increases the internal structure, viscosity, and extrusion time.


PCC and GCC Combinations: Adjusting Rheology and Extrusion Time
Most sealants contain a combination of precipitated and ground calcium carbonates. The GCCs add viscosity, but the viscosity is not thixotropic. The graph below depicts rheological profiles of three PVC plastisols with different calcium carbonates.


Filler Type and Rheological Profile

Sealant A is unfilled, with lowest viscosity and no thixotropy.  Sealant B contains GCC only. Its viscosity is higher than A’s, but it has very little thixotropy—not enough for sag and slump control. Sealant C contains the same level of calcium carbonate as does B, but about two-thirds of the filler is PCC. The viscosity is higher again and the large space between the up and down curves shows the high degree of thixotropy built into the PVC plastisol, which gives excellent sag control.

When formulating a sealant, enough PCC is added to give the desired rheology. GCC is then added to take the solids up to the desired level. The ratio between PCC and GCC can be adjusted to change the rheological profile, especially the extrusion rate or spray viscosity. More PCC will increase the high shear viscosity, reducing the extrusion rate and increasing extrusion time; less PCC and more GCC reduces the high shear viscosity and extrusion time.

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Specialty Minerals Precipitated Calcium Carbonates for Sealants and Adhesives
Specialty Minerals Inc. (SMI) has been manufacturing nano and ultrafine Precipitated Calcium Carbonates for more than 25 years. Nano particles are defined as less than 100 nanometers or 0.1 micron in size. SMI’s nano PCCs are true nanos—60 and 70 nanometers, or 0.06 and 0.07 microns. Other ultrafine grades are near-nano, 150 nanometers or 0.15 microns. SMI manufactures a range of different size grades with different coatings in the U.S. and in the U.K. 

SMI’s fine and medium PCCs, 0.7 microns to about 3 microns, can be used in adhesives and sealants to add viscosity. The fine, 0.7 micron product is good for sealants or adhesives used on horizontal surfaces where good viscosity build is desired but where sag control is not a priority. Coated versions of these products, which are especially useful in moisture curing systems because of their greater hydrophobicity, are manufactured in the U.S.

In order of increasing particle size, the SMI PCC products for sealants and adhesives are:

  • Thixo-Carb® HP  nano PCC– is a 0.06 micron, stearic acid coated PCC. When used on high shear dispersion equipment, this PCC has the best rheological efficiency. It is manufactured in SMI’s plant in Adams, Massachusetts.
  • Calofort® SV and SM nano PCCs – are 0.07 micron, stearic acid coated PCCs manufactured in Birmingham, U.K. SM is the general purpose grade while SV is processed to give better efficiency.
  • Ultra-Pflex® nano PCC – is a 0.07 micron, stearic acid coated PCC. This PCC is a general-purpose grade of broad utility with high uniformity. It is manufactured in Adams.
  • Calofort® U and Multifex-MM® PCCs – are uncoated nano PCCs, 0.07 micron in size. Both are manufactured in Birmingham, U.K. and sold as Multifex-MM® in North America and Calofort® U in the rest of the world.
  • Thixo-Carb® 300 ultrafine PCC – is a 0.15 micron PCC with a proprietary coating PCC. It has the highest efficiency in polysulfides and butyl rubber sealants Mmanufactured in Adams, Massachusetts.
  • Thixo-Carb® 500 ultrafine PCC– is a unique PCC with the highest hydrophobicity and lowest moisture content for urethanes, silicones and other moisture curing systems. It is a 0.15 micron PCC with a proprietary coating and is manufactured in Adams, Massachusetts.
  • Super-Pflex® 100 fine PCC– is a 0.7 micron, stearic acid-coated PCC used in sealants for horizontal applications. It is made in Adams.
  • Albafil® fine PCC– is an uncoated 0.7 micron PCC for water-based products. Produced in Adams.
  • Albacar® 8101 medium PCC – is an uncoated 1.4 micron PCC. It is a scalenohedral, starburst-type crystal which gives high surface area and high oil absorption. It significantly increases non-thixotropic viscosity build and is manufactured in Adams.
  • Super-Pflex® 1430 medium PCC – is a stearic, acid-coated Albacar 8101. It is a 1.4 micron scalenohedral PCC with high surface area and is manufactured in Adams.
  • Albacar® 5970 medium PCC – is a 1.9 micron, uncoated scalenohedral PCC manufactured in Adams.
  • Calopake® 5970 – is a 1.9 micron, uncoated scalenohedral PCC manufactured in Birmingham, U.K.

Click on a product’s name to download the technical data sheet for the product.

“Understanding How to Select the Right PCC for Sealants” web page will help you decide which of the SMI PCC products is best for your particular sealant or adhesive. Click here to read and click here to view the PCC product selector chart for various sealant types.

 

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