Swelling and glass transition relations for epoxy matrix material in humid environments

Abstract

SynopsisAn experimental investigation has established coherent data sets describing effects of moisture absorption upon swelling and glass transition of an epoxy resin. Equilibrium moisture absorption levels and resultant swelling can be represented by power law functions of relative humidity and quantity of absorbed moisture, respectively. Depression of glass transition temperature is shown to be described by a relationship derived from free-volume theory. INTRODUCTIONIt is an accepted fact that epoxy matrix composites absorb moisture and that this absorption can alter behavior of certain pr0perties.l The absorption process has been defined in various ways but is reasonably well described by classical diffusion law.2 Altered behavior due to this absorption has been predicted for both loaded and unloaded structures due to swelling of the matrix.3 Methods have been developed which incorporate this hygrothermal conditioning effect into thermal stress analy~is.~ However, coherent data sets describing the swelling behavior have not been reported.Altered mechanical behavior has been observed in matrix dominated properties such as compression strength and shear m o d u l u~.~ Behavior alteration is particularly apparent at elevated temperatures. This has been related to depression of glass transition temperature caused by the moisture absorption.6 It has also been shown that moderate temperature transients can alter the subsequent diffusion behavior of laminates containing certain amounts of moi~ture.~ The importance of these temperature effects requires a coherent data set and theory describing glass transition shifts due to moisture absorption. This paper presents experimental findings of swelling behavior, thermal expansion behavior, and changes in glass transition of an epoxy matrix material resulting from moisture absorption. Theories describing these properties are discussed, and modifications to currently accepted views are proposed. EXPERIMENTALNarmco 5208 epoxy resin was used for all of the investigations discussed in this report. Specimens for all tests were rectangles having a thickness of about 0.035 in. (0.89 mm) and were cut from a neat resin casting. The resin was cured using the cycle shown in Table I.Special care was taken to assure that all specimens were completely dry at the start of testing. The resin casting was kept in a desiccator following cure until individual specimens could be cut with a Micromatic Wafering Saw. The individual specimens were then postcured for 4 hr at 400°F (204OC) and placed immediately into a room-temperature desiccator.One group of specimens was maintained in the reference dry condition. The remainder were placed in groups of three into each of several hygrothermal conditioning environments. The various stagnant-air, constant humidities were provided in laboratory jars containing appropriate saturated salt solutions. The constant temperatures resulted from placing the jars in ovens. Table I1 identifies the environmental conditioning matrix.Specimens from each environmen...

Influence of absorbed water on the dielectric properties and glass-transition temperature of silica-filled epoxy nanocomposites

“…Some researchers have claimed that absorbed water molecules forming double hydrogen bonds would cause an increase of T g [12]. However, other papers reported that the absorbed water in epoxy materials would lead to a decrement of the glass transition temperature due to the plasticizing effect of water [10,12,[13][14][15][16]. Our loss tangent results support the viewpoint that T g of epoxy samples is decreasing with increasing relative humidity.…”

Probing the molecular interactions in the diffusion of water through epoxy and epoxy–bismaleimide networks

“…[1][2][3][4] One major deficiency of products based on these cured resins is the absorption of relatively large amounts of water in high humidity environments, which brings about a general deterioration of properties. At equilibrium, a typical TGDDM/ DDS-cured resin may absorb between 4.0 and 7.0 wt % water, [5][6][7] depending on the molar ratio of the two components, the curing conditions and, obviously, the water vapor activity of the environment. The absorbed moisture acts as a very effective plasticizer, lowering the T g value of the cured resin, which is estimated to be approximately 20°C for every 1% absorbed water.…”

“…Moreover, water sorption may cause irreversible damage to the material by the formation of microcracks through repeated sorption/desorption cycles. 4,6,7 NMR studies have indicated that the water dispersed within an epoxy network may be present as bound water (hydrogen-bonded) and as mobile water (free). 8,9 Recent work has shown that the levels of both bound and free water can be estimated by Fourier transform infrared (FTIR) spectroscopy in the mid-infrared (MIR; 4000 -400 cm Ϫ1 ) and near-infrared (NIR; 8000 -4000 cm Ϫ1 ) ranges.…”

Thermomechanical and hygroelastic properties of an epoxy system under humid and cold-warm cycling conditions

“…In the case of water, c is negative. In other words, water penetration in the polymer is accompanied by a contraction of the mixture but this contraction is not strong enough to inhibit the swelling totally [22].…”