Glass-Transition Temperature Based on Dynamic Mechanical Thermal Analysis Techniques as an Indicator of the Adhesive Performance of Vinyl Ester Resin Benjamin Herzog, 1 Douglas J. Gardner, 2 Roberto Lopez-Anido, 3 Barry Goodell 2 1 APA—The Engineered Wood Association, Tacoma, Washington 98466 2 Wood Science and Technology and Advanced Engineered Wood Composites Center, University of Maine, Orono, Maine 04469 3 Department of Civil and Environmental Engineering and Advanced Engineered Wood Composites Center, University of Maine, Orono, Maine 04469 Received 22 September 2004; accepted 3 January 2005 DOI 10.1002/app.21868 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Vinyl ester resins are being used extensively as matrices in fiber-reinforced polymer composite materials, but their use as a structural adhesive has been limited. Initial studies investigating the durability of a vinyl ester as a wood adhesive showed unsatisfactory performance in comparison with other adhesives. In this work, the glass-transition tem- peratures (T g ’s) of a vinyl ester and a E-glass/vinyl ester composite material, fabricated by the Composites Pressure Resin Infusion System, were determined with dynamic me- chanical thermal analysis. The results indicated that the resin cured under ambient conditions had a much lower T g (60°C) than the postcured material (107°C). This sug- gested undercuring, that is, incomplete crosslinking, of the resin when it was cured at room temperature. E-glass/vinyl ester samples, however, showed virtually no difference in T g between room-temperature-cured and postcured samples. The exact reasons for this are not currently known but are thought to be both mechanical and chemical in nature. On the basis of the findings presented in this article, it can be concluded that if this vinyl ester resin is to be used as a structural adhesive, postcuring or formulation to ensure a high degree of crosslinking under ambient conditions is necessary. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2221–2229, 2005 Key words: adhesives; composites; glass transition; thermo- sets INTRODUCTION Vinyl ester resins are being used extensively as matri- ces in fiber-reinforced polymer (FRP) composite struc- tures. 1 Vinyl esters exhibit many desirable qualities, including mechanical properties comparable to those of epoxy resins, 2 excellent chemical resistance and tensile strength, 3 low viscosity (enabling room-tem- perature infusion), and cost competitiveness. 1 There are three general fabrication options used in the reinforcement of materials with an FRP composite: adhesive bonding of prefabricated sheets, wet lay-up of fabric, and resin infusion. In both wet lay-up and variations of the resin infusion processes, the resin serves a double function as an adhesive for the FRP– material interface and as a matrix for the fabric rein- forcement. 4 The characteristics of vinyl ester as a matrix for FRP composites have been well docu- mented. 5–12 The role of vinyl ester as an adhesive, however, has received less attention. 3,13,14 Hygrothermal cycling tests (ASTM D 2559) de- signed to investigate and characterize the durability of both FRP–wood and wood–wood bonds with vinyl ester resin as an adhesive were recently performed. 15 As part of these tests, bonded samples were repeat- edly saturated with water, subjected to steam (100°C), and dried (65.5°C); this resulted in significant swelling and shrinking of the wood material. In both cases, the performances of these bond lines were unsatisfactory in comparison with those of an epoxy adhesive and a phenol resorcinol formaldehyde resin, in that greater interface delamination rates were observed with the vinyl ester resin. In an attempt to explain the relatively poor performance of the vinyl ester resin, a determi- nation of the glass-transition temperature (T g ) of the resin was made with dynamic mechanical thermal analysis (DMTA). Correspondence to: D. J. Gardner (douglas.gardner@ umit.maine.edu). This is paper 2809 of the Maine Agricultural and Forest Experiment Station. Contract grant sponsor: New England Wood Utilization Research Program. Contract grant sponsor: McIntire Stennis Funds. Contract grant sponsor: National Science Foundation; contract grant number: CMS-0093678. Journal of Applied Polymer Science, Vol. 97, 2221–2229 (2005) © 2005 Wiley Periodicals, Inc.