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Polyurethane Sealants with Specialty Minerals Precipitated Calcium Carbonates

Urethane-based sealants are one of the most commonly used sealants in both construction and transportation markets because of their long service life, excellent flexibility (which allows joint movement), and good adhesion to many substrates. In construction, polyurethane sealants are used in the manufacture of insulating glass windows for high-performance joint sealing in concrete and metal buildings, and for sealing roofing membranes. They are very popular for homeowner DIY (do-it-yourself) and OTC (over-the-counter) sales to builders, for blocking drafts and air leaks around foundations, windows, and electrical outlets. In original equipment manufacturer (OEM) auto and truck production, urethane sealants are used for windshield glazing and in the aftermarket for installing replacement windshields. Urethanes are also used for seam sealing in body repair shops.

Chemically, a polyol, usually a polyether polyol, reacts with a diisocyanate (such as toluene diisocyanate) to form a polyurethane. For sealant use, a prepolymer, a partially reacted polyurethane with an excess of isocyanate groups, is formed; the reaction completes upon application. Most urethane sealants are formulated as single-component types where the prepolymer is blocked by one of a variety of moieties to avoid pre-cure. After the sealant is applied, available moisture in the air causes the unblocking of the prepolymer and the reaction proceeds to completion. In two-component urethane sealants, the prepolymer and polyol are reacted, often with the use of a catalyst, just before application.

A newer type of polyurethane is the silylated polyurethane—known as SPUR—in which a reactive silane is used for cross-linking. SPUR sealants cure at room temperature, with improved adhesion and physical properties.


Polyurethane and SPUR Sealants with SMI Precipitated Calcium Carbonates
Specialty Minerals Inc. (SMI) manufactures a variety of coated precipitated calcium carbonates (PCCs) for use as both the key rheological additive and as a functional filler in polyurethane and silylated polyurethane sealants. With the ability to form shear thinning, thixotropic structures in these sealants, precipitated calcium carbonates build viscosity and yield value, controlling extrusion, sag and slump. PCCs are also reinforcing fillers, adding to the strength of the sealants. The PCCs used in sealants are ultrafine or true nano particles, coated to improve dispersion and compatibility in the polymer.

The grades of SMI PCCs recommended for both one- and two-part polyurethane sealants include:

  • Thixo-Carb® 500 PCC – the most hydrophobic PCC with lowest moisture, making it ideal for single-component moisture-curing systems. Thixo-Carb® 500 PCC can be used without drying in many systems, with excellent package stability. If predrying is wanted for ultimate stability, this PCC releases the small amount of moisture it contains very quickly, so it can be ready to use much faster than would be the case with conventional PCCs. Click here to see surface moisture and drying rate results for this and some other SMI PCCs. Thixo-Carb® 500 PCC is also able to provide rheological control in phenoxy and other blocked polymers, which are normally difficult to thicken. Sealants using Thixo-Carb® 500 will have lower modulus and higher flexibility. It is a 0.15 micron ultrafine PCC coated with a proprietary treatment and is manufactured in SMI’s Adams, Massachusetts plant.
  • Calofort® SV PCC – develops highest efficiency under normal shear conditions. This nano, 70 nanometer (0.07 micron) PCC is stearic acid coated and is manufactured in SMI’s plant in Birmingham, U.K. It can be used in one- and two-part products.
  • Ultra-Pflex® PCC – a high-efficiency thixotrope with excellent uniformity. It is a 70 nanometer (0.07 micron) PCC manufactured in Adams, Massachusetts, for single- and two-component sealants.
  • Thixo-Carb® 300 PCC – is for two-part formulations where a high modulus is desired. It is a coated, ultrafine PCC with a 0.15 micron median particle size manufactured in Adams.
  • Thixo-Carb® HP PCC – is a high-surface area nano PCC for two-part systems. When fully dispersed, this PCC produces highest efficiency, flexibility and toughness, so the use of other expensive thixotropes can be reduced or eliminated. This PCC requires high-shear dispersion equipment. It is a 0.06 micron coated PCC manufactured in Adams.

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

To read more about PCC’s properties in sealants and about all SMI PCCs for sealants click here and then click here for information on how PCC properties such as particle size, hydrophobicity, and surface moisture play a role in your application. The selector chart for SMI PCCs in sealant and adhesive applications can be viewed by clicking here.


SMI Ground Calcium Carbonates for Polyurethane Sealants
Ground calcium carbonates (GCCs) and ground dolomites (ground calcium magnesium carbonates) are added to polyurethane sealant formulations to serve at least two key functions: they increase solids and replace polymer for cost reduction and added non-thixotropic viscosity. Uncoated fillers are used in most sealants, but in polyurethanes, It may be advantageous to use coated GCCs, as they have lower moisture for better shelf stability, will disperse more readily, and have lower are absorptions, which allows higher loadings than which the uncoated resins.

Coated GCCs for polyurethanes are the Hi-Pflex® 100 – East and Hi-Pflex® 100 – West GCCs, stearic acid-coated, 3.5 microns in size, manufactured in Adams, Massachusetts, and Lucerne Valley, California, and Pfinyl® 402 GCC, a 5.5 micron coated GCC, also manufactured in Adams, Massachusetts. Uncoated GCCs include Vicron® and Marblewhite®, manufactured in Massachusetts and California. Dolocron® ground dolomites are manufactured in SMI’s plant in Canaan, Connecticut.

Click here to read more about SMI calcium carbonates for sealants.

 

 

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