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Home All Updates (164) Moisture-cure polyur
Moisture-cure polyurethane binders for particulates are produced using a wide range of chemistries for both the polyol and isocyanate components, depending on the requirements of the final composite product. The “active” agent in the binder is the prepolymer, which may be composed of any variety of polyol and isocyanate reactants at a stoichiometry to achieve a “% free NCO, ” typically between 3-25%. The polyol component may be polyether, polyester, polybutadiene, or natural oil based. The isocyanate may be selected from the entire range of conventional chemistries, TDI, various isomers of pure MDI, polymeric MDI, and aliphatic types. Many critical physical properties of the final composite are dictated by the polyol and isocyanate choice, and by the molecular weight of the prepolymer. However, the natural viscosity of the prepolymer increases rapidly with increasing molecular weight and functionality. For instance, a simple prepolymer of polypropylene glycol polyether diol and MDI will have a viscosity of 1, 000-2, 000 cps at a free NCO in the 8-10% range. Increasing functionality through either polyol or isocyanate choice can result in prepolymer viscosities several multiples of these figures at the same free NCO range. Since many particulate bonding processes rely on mixing the particles with the moisture-cure binder at ambient temperature and humidity using relatively low-shear mixing equipment, binder viscosities over 1, 000 cps are problematic for uniform incorporation into the mix. The conventional resolution to this problem is to incorporate a non-reactive hydrocarbon diluent into the formulation at levels ranging from 10% to (in some cases) over 30%. Hydrocarbons of choice are typically naphthenic and aromatic process oils, which are relatively inexpensive and exhibit fairly high compatibility with the prepolymer, along with good viscosity depression characteristics. However, particularly at higher levels, the addition of hydrocarbon diluents creates other problems: • Incompatibility in the liquid phase • Incompatibility in the cured (solid) phase • Reduction of physical properties by plasticization or extension • Increase in VOC content • Problematic regulatory status (with aromatic oils in particular) In addition, the price of these types of hydrocarbons has been quite volatile in recent years, and the availability and consistency has been negatively affected by changes in refining processes and capacity. Given the limitations of current prepolymer and diluent technology, the “perfect” diluent would have the following characteristics: • Efficient viscosity depression • Compatibility with a wide range of polyol and isocyanate chemistries • No effect on final cured properties • No addition of VOCs • No regulatory problems • Ready availability • Low cost One diluent that could potentially meet all of these criteria is water. In fact, this transition has already been made in the coatings and adhesives industries that are going from solvent-soluble polymers to emulsions of the same types of polymer backbones. For more details & prices for PU Binder please email us at :
  • 2015-11-14T05:09:06

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