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Cure on Demand: Classic Formulation Challenges & New Developments

Posted on 7/5/2016 9:26:04 AM By Deb Bhattacharjee
  

Wouldn't you like to have an adhesive or sealant system with a long shelf and work life at ambient temperature to complete uniform distribution within substrates and subsequent assembly operations, while having rapid cure kinetics following a cure trigger event without compromising rheological properties, ultimate bond strength, mechanical properties and chemical resistance?

This has been a classic formulation challenge for ages in a variety of applications including automotive, construction, industrial, electronics and packaging applications and it remains as an area of active interest among researchers and significant new developments with several innovative solutions have been accomplished over the years1-5.

In reactive two component polyurethane or epoxy systems, several technologies using blocked catalysts or encapsulated reactant/catalysts were documented.  For example, a chemical adduct with a sharp decomposition temperature can be formed using a basic catalyst and an organic acid with specific pKa value, whereas a catalyst could be encapsulated inside a wax with a sharp melting point.  Below the melting point, the system would have excellent working/pot life but it cures extremely rapidly above the melting point when the catalyst is released.  The other approaches would include encapsulating catalyst with an organic shell which can be crushed with heat and shear.  Several examples, especially in electronics application, include encapsulation of one of the two components.

A variety of energy sources were investigated including UV, e-beam, light, heat and electromagnetic radiation.

Ultraviolet light has been described as a trigger, but it can only be used when at least one of the substrates is transparent to light.The cure of opaque, filed or thick cross section is problematic using UV triggered system.  Dual-cure systems using a combination of UV at the first stage followed by actinic radiation has been claimed to offer improvements6.

The development of new photo-latent base catalysts that release amidine-type catalysts creates new opportunities for radiation curing of conventional adhesive systems.

One part, snap curing, vinyl containing oligomers (urethane acrylate or epoxy acrylate with methacrylate monomer as diluent) with long worklife (e.g. 3 days at room temperature), and fast cure in 60 secs at temperature <120oC, have been used in electronic applications7.

Recently, Pojmanet. al. has been developing two approaches for cure on demand systems that can be one pot formulations with long shelf life 8-10.  One is coupling polymerization with Click Reactions and the other on “Frontal Polymerization”.  In frontal polymerization (FP), the monomer is polymerized by a zone of reaction propagating through the monomer.  The reaction zone is fueled by the heat given off by the polymerization of the monomer. The monomers selected for the adhesive is the main factor in determining the adhesive’s properties.  This allows rapid repair of holes in wood, floor and dry walls.  The hole can be filled with the putty, which has a shelf life of months to years.  Prof. Pojman demonstrated the usefulness of frontal polymerization to synthesize adhesives for wood and plastic-wood composites with rapid cure and excellent shear strength properties.


References:

  1. “Cure on demand adhesives and window module with cure on demand adhesives thereon”, US6613816B2, 09/02/2003
  2. “Light triggered cure on demand sealants”, WO2013090988A1, 06/27/2013
  3. “Fast cure polyurethane sealant composition containing titanium ester accelerators”, US Patent 4889903
  4. “Cure-on-Demand liquid sealant composition, process for the preparation thereof and uses thereof”, US20150232725A1, 08/20/2015  
  5. “Cure on demand adhesives for assembling shoe soles”, WO1999055753A1, 11/04/1999
  6. “Command-cure adhesives”, US Patent 7235593, 06/26/2007
  7. “Low temperature snap cure material with suitable work life”, EP1661928A1, 05/25/2006
  8. “Time-lapse and cure-on-demand polymerization for adhesives and fillers”, presented by Prof. John Pojman at the Spring Adhesive & Sealant convention at New Orleans, April 18-20, 2016
  9. “Free-radical frontal polymerization with microencapsulated monomers & initiators”, US20060142512A1, 06/29/2006
  10. “Cure-on demand wood adhesives using frontal polymerization of acrylates”, a          Thesis Submitted to the Graduate Faculty of the Louisiana State University and  Agricultural and Mechanical College in partial fulfillment of the  requirements for the degree of Master of Science in  The Department of Chemistry by TreyVon Holt B.S., Louisiana State University, 2009 December 2011.