Hygrothermal effects of epoxy resin. Part II: variations of glass transition temperature

Abstract

Three epoxy systems (DGEBA+mPDA, TGDDM+DDS, and Fiberite 934 TM) were used to investigate glass transition temperature ( T g) variation of epoxy under hygrothermal environment exposure. Materials were immersed in distilled water at constant temperatures of 45 oC, 60 oC, 75 oC, and 90 oC for water absorption and then desorbed at different temperatures. Thermomechanical analysis (TMA) and differential scanning calorimetry (DSC) were employed to determine T g changes at different hygrothermal stages. The investigations revealed the following results: i) the change of T g does not depend solely on the water content absorbed in epoxy resins, ii) T g depends on the hygrothermal history of the materials, iii) for a given epoxy system, higher values of T g resulted for longer immersion time and higher exposure temperature, and iv) the water/resin interaction characteristics (Type I and Type II bound water) have quite different influence on T g variation. A sorption model and collateral evidence introduced in Part I of the series were used to interpret and explain T g variation in epoxy resin systems. Both Type I and Type II bound water influence T g variations, albeit in different ways. Type I bound water disrupts the initial interchain Van der Waals force and hydrogen bonds resulting in increased chain segment mobility. So Type I bound water acts as a plasticizer and decreases T g. In contrast, Type II bound water contributes, comparatively, to an increase in T g in water saturated epoxy resin by forming a secondary crosslink network. The experimental T g values encompass the combined effect of the two water-resin interaction mechanisms described briefly in the preceding text and in detail in Part I of this paper series. The often-cited polymer-diluent model used to predict T g variation of polymers exposed diluent media is lacking when a dual sorption mechanism is involved during hygrothermal exposure process.