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Unformatted text preview: GLASS TRANSITION TEMPERATURE AS AN INDICATOR OF ADHESIVE PERFORMANCE OF VINYL ESTER RESIN Benjamin Herzog WSC 550 April 27, 2004 Background and Scope Hygrothermal cycling tests (ASTM D2559) on both FRP-wood and Hygrothermal FRPwood-wood bonds using vinyl ester resin as an adhesive were woodrecently performed. VPS, steam, elevated-temperature drying VPS, elevatedswelling/shrinking swelling/shrinking The performance of the vinyl ester bondlines was unsatisfactory The compared to other resin systems. In an attempt to explain the relatively poor performance of the vinyl In ester resin, a determination of the glass transition temperature (Tg) of the resin was done using dynamic mechanical thermal analysis (DMTA). Objective: to compare the Tg of room-temperature cured and postObjective: roompostcured vinyl ester resin and FRP and to assess factors, including Tg and degree of cure, which may influence the adhesive durability of the resin. 1 Glass Transition Temperature (Tg) Various definitions of Tg corresponding with Various various methods of Tg calculation. 1 1 – Glassy 2 Storage Modulus (E’) 2 – Glass Transition 3 – Rubbery 3 4 4 – Rubbery Flow 5 – Liquid Flow Glass Transition Region 5 Temperature Glass Transition Temperature (Tg) Various definitions of Tg corresponding with Various various methods of Tg calculation. Temperature at which a polymer undergoes the Temperature transition from a hard glass to a rubbery state. Tg used to estimate/establish Tg degree of cure degree maximal end-use temperature maximal When the Tg of a polymer is exceeded, certain When mechanical properties may be severely compromised. 2 Dynamic Thermal Mechanical Analysis The DMTA supplies an oscillating The force sinusoidal stress sinusoidal sinusoidal strain. sinusoidal Modulus, viscosity, and damping can Modulus, be calculated. Performed during a temperature ramp Performed changes in the flexural modulus as changes a function of temperature can be monitored. Allows for the identification of Tg and Allows Tm, and the determination of the effects of these changes on loadloadbearing characteristics. Methods and Materials vinyl ester resin and E-glass/vinyl ester FRP vinyl different curing strategies: different Sample a b c pc Resin Room temperature cure Immediate retest of ‘a’ samples Post-cure of ‘b’ samples: 99°C/2 hours Post-cure: 99°C/2 hours FRP Room temperature cure Immediate retest of ‘a’ samples Post-cure of ‘b’ samples: 80°C/6 hours Post-cure: 80°C/6 hours Oscillated at a frequency of 1 Hz. Oscillated 3-point bending mode 25°C – 175°C at a rate of 5°C/min. 25 Strain amplitude = 0.01%. Strain 3 Typical Results - Resin 9.5 9 Log E' (Pa) 8.5 8 7.5 7 0 20 40 60 80 100 120 140 160 180 200 Temperature (C) 3a 3b 3c 6pc Typical Results - FRP 10.6 10.4 10.2 Log E' (Pa) 10 9.8 9.6 9.4 9.2 0 20 40 60 80 100 120 140 160 180 200 Temperature (degrees C) 3a 3b 3c 4pc 4 No differences in Tg based on FRP curing strategy? Fibers are stiffer and carry more load than the matrix; the Fibers properties observed in samples loaded in three-point threebending may not be appreciably sensitive to variation in the mechanical properties of the matrix. The heat transfer through the FRP may be different than in The resin, during the DMTA temperature ramp, due to the fiber reinforcement acting as a thermal energy conduit. It is also possible that the surface chemistry of the fibers It react in such a way with the resin to produce a higher exotherm, and therefore, a higher degree of cross-linking, crossthan found in resin alone. Implications 1. Because of the competing reactions involving styrene and vinyl ester, the relationship between Tg and degree of cure is only direct for a specific cure temperature (Fink 2002). Our samples underwent a typical nonisothermal cure. Our Because of the difference between the Tg of ambient- and postBecause ambientpostcured samples, it is still concluded that room-temperature roomcured vinyl ester resin is associated with incomplete crosslinking. Undercured resin reduced adhesive qualities, mechanical Undercured reduced properties and durability performance. 5 Implications (Cont.) 2. The Tg of ambient-cured resin is lower than the test lower conditions of ASTM D2559. Changes in molecular free volume and the relative mobility Changes of individual molecules within the crosslinked lattice. The storage modulus drops sharply, and therefore the The compressive and shear strength of the material does as well. Other properties, such as water resistance, also decrease Other significantly above a resin’s Tg. resin’ Because the original hygrothermal specimens were Because subjected to temperatures above the Tg and water and saturation, adhesive properties may have been adversely saturation, affected. Recommendations Post-curing of vinyl ester resin is recommended higher degree Posthigher of cross-linking improved mechanical properties, adhesive crossimproved qualities, and durability performance of the resin. In addition, the Tg of the polymer will increase resulting in a higher In maximal end use temperature. Important to remember -- if used as an adhesive, the thickness of Important the vinyl ester bondlines may be less than the samples tested in this work lower exotherm during curing reaction lower lower lower degree of curing, lower Tg. Post-curing strategies need to be determined for large structural Postmembers, e.g., glued-laminated beams, if these materials include gluedvinyl ester resin. 6 Acknowledgments Dr. Jungil Son Dr. Douglas Gardner Dr. Roberto Lopez-Anido Dr. Barry Goodell Thank you for your attention. Questions? 7 ...
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