Moisture absorption behavior of epoxies and their S2 glass composites

The influence of moisture exposure on the behavior of three toughened epoxy–amine systems (scrimp resins SC11, SC15, and SC79, Applied Poleramic, Inc., Benicia, CA) was investigated. Neat resin samples were conditioned by immersion in distilled water at 71°C and in an environmental chamber at 85% relative humidity and 87.8°C until saturation. The equilibrium weight gain ranged from 1.8 to 3.8% for the resins. The long-chain, low-crosslink-density epoxy system (SC11) absorbed the highest amount of water and was saturated first, and it was followed by the medium-crosslink-density (SC15) and high-crosslink-density materials (SC79). The moisture diffusivity decreased with the increasing crosslink density of the resins. The percentage reduction of the glass-transition temperature (T_g) at equilibrium moisture absorption was highest for the low-crosslink molecule. The percentage reductions for the medium-crosslink and higher crosslink systems were comparable. A net weight loss after drying was observed for the SC11 and SC79 resin systems. Fourier transform infrared analysis confirmed the segment breakage and leaching of molecules from the epoxy–amine network. The effects of moisture cycling on T_g were dependent on the epoxy–amine morphology. During the drying stage, T_g increased to a value higher than that of the unaged dry systems. The S₂ glass composite samples were conditioned under identical conditions for the resin system. Composite systems absorbed less moisture than the neat resins as expected. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Moisture effects on the behavior of graphite/polyimide composites

The effect of moisture on the material behavior of Magnamite® IM7 Graphite/Avimid® K3B thermoplastic polyimide composite laminates has been investigated. Laminates consisting of a 62 vol% fiber were laid up with several different stacking sequences: [90]₁₀ or unidirectional, [0₂/90]_S or thin cross-ply, [0₂/90₂/0₂]_S or thick cross-ply, [45/0/-45/90]_S or quasi-isotropic, and [0/90] laminates. In this study, the glass-transition temperature, T_g, and the intralaminar fracture toughness, G_IC, were measured for dry and moisture-saturated unidirectional samples. When laminates were saturated with moisture, the value of T_g was found to decrease from its baseline (dry) value, but recovered upon redrying the samples. This observation is consistent with the effects of moisture on the T_g of other polymer composites. Permanent toughness losses have not been observed in samples conditioned in room-temperature 75% and 100% relative humidity environments. However, during long-term conditioning of cross-ply and quasi-isotropic samples in liquid water, transverse cracks initiated in the absence of an applied mechanical load. Moisture uptake curves for conditioning in room-temperature liquid water, 80°C (176°F) liquid water, and at room temperature and 75% relative humidity were used to calculate Henry's Law constants and diffusion coefficients. Non-Fickian behavior, consisting of a postsaturation increase in moisture uptake, was observed in crossply and quasi-isotropic laminates and might be due to the observed transverse cracking.     

Effects of alkaline conditioning and temperature on the properties of glass fiber polymer composite rebar

This article investigated long term alkaline conditioning and temperature on the physical and mechanical properties of glass fiber‐reinforced polymer (GFRP) composite rebar for structural applications. The GFRP rebar was immersed in alkaline solution (pH ≈ 13) for 23 months at 23°C, and for 24 months at 60°C. The moisture absorption was found to be 0.34% at 23°C after 23 months, and 0.76% at 60°C after 24 months. At both temperatures, moisture absorption did not reach equilibrium which was attributed to two stages non‐Fickian behavior. Glass transition temperature (T_g) of the polymer matrix of rebar that conditioned at 23°C was found to be decreased because of plasticization, whereas T_g of the rebar that conditioned at 60°C was remained greater than the T_g of control rebar due to nonplasticization effect. Shear strength was retained by 83.5% at 23°C and 80.5% at 60°C, flexural strength was retained by 81% at 23°C and 69% at 60°C, and tensile strength was retained by 91.2% at 23°C and 74.3% at 60°C. It was revealed that durability of GFRP rebar in alkaline environment was controlled by the absorbed moisture; this was because the load transfer efficiency of fiber/matrix interface is vulnerable to moisture. POLYM. COMPOS., 37:3181–3190, 2016. © 2015 Society of Plastics Engineers     

Sorption and transport of water vapor in nylon 6,6 film

The sorption and transport of water in nylon 6,6 films as functions of the relative humidity (RH) and temperature were studied. Moisture‐sorption isotherms determined gravimetrically at 25, 35, and 45°C were described accurately by the GAB equation. Water‐vapor transmission rates were enhanced above ≈ 60–70% RH, primarily due to the transition of the polymer from glassy to rubbery states. The glass transition temperatures (T_g's) of nylon 6,6 were measured at various moisture contents using differential scanning calorimetry. The results showed that the sorbed water acted as an effective plasticizer in depressing the T_g of the polyamide. Fourier transform infrared spectroscopy (FTIR) was utilized to characterize the interaction of water and the nylon. Evidence from FTIR suggested that the interaction of water with nylon 6,6 took place at the amide groups. Based on the frequency shift of the peak maxima, moisture sorption appeared to reduce the average hydrogen‐bond strength of the N H groups. However, an increase was seen for the CO groups. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 197–206, 1999     

WATER SORPTION ISOTHERMS AND GLASS TRANSITION PROPERTIES OF GELATIN

ABSTRACT

The water sorption isotherms of gelatin of different molecular weights (317,700, 228,900, and 197,400) were determined at 50°C using an isopiestic method. The sorption isotherms were modeled using the Brunauer–Emmett–Teller (BET) and Guggenheim–Anderson–deBoer (GAB) equations. The BET and GAB equations were able to predict the equilibrium moisture content (EMC) with a mean relative error of 5.2 and 5.0%, respectively. The BET monolayer moisture content varied from 4.81 to 5.70% (d.b.) while modeling with the GAB equation predicted monolayer moisture content of 6.14–7.58% (d.b.) depending upon molecular weight. The monolayer moisture content increased with increasing molecular weight. Studies on the effect of moisture content on the “rheological glass transition temperature” (T _g ) showed a smooth increase in the value of T _g as a function of increasing concentration of gelatin solids. This varied from 7 to 35°C at 75% and 97% solids, respectively for the protein sample with MW = 317,700. Pinpointing of the T _g was implemented with the technique of small deformation dynamic oscillation. It was proposed that the “rheological” T _g is the point between the glass transition region and the glassy state. It acquires physical significance by identifying the transition from free volume phenomena of the polymeric backbone in the glass transition region to an energetic barrier to motions in the glassy state involving stretching and bending of chemical bonds.     

Hydrolysis and blistering of cyanate ester networks

The hydrolysis of a cyanate ester network made from the monomer 2,2′-bis(4-cyanatophenyl) isopropylidene (bisphenol A dicyanate homopolymer) was studied. Hydrolysis reactions were performed isothermally at temperatures from 150 to 180°C under conditions of excess water. The kinetics of the reaction were characterized by the decrease in T_g as measured by differential scanning calorimetry. The rate of change of T_g was found to be adequately described as first order in T_g, which is an indirect measure of the concentration of crosslink junctions. The activation energy of the reaction was found to be 115 kJ/mol. In addition, moisture-conditioned, glass-reinforced laminate samples were heated and the time to delamination or blistering was recorded as a function of temperature. The blister time at solder temperatures (T = 220–260°C) was modeled using the above kinetic results. Heat transfer to the laminate was considered and the criteria used for blister time was the time at T = T_g of the sample. At lower temperatures (T < 220°C), loss of water from the laminate is sufficiently fast to prevent blistering. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 107–113, 1997     

Revisiting the thermal relaxations of poly(vinyl alcohol)

The molecular dynamics of poly(vinyl alcohol) (PVA) were studied by dielectric spectroscopy and dynamic mechanical analysis in the 20–300°C range. The well-established plasticizing effect of water on the glass-transition temperature (T_g) of PVA was revisited. Improper water elimination analysis has led to a misinterpretation of thermal relaxations in PVA such that a depressed T_g for wet PVA films (ca. 40°C) has been assigned as a secondary β relaxation in a number of previous studies in the literature. In wet PVA samples, two different Vogel–Fulcher–Tammann behaviors separated by the moisture evaporation region (from 80 to 120°C) are observed in the low- (from 20 to 80°C) and high- (>120°C) temperature ranges. Previously, these two regions were erroneously assigned to two Arrhenius-type relaxations. However, once the moisture was properly eliminated, a single non-Arrhenius α relaxation was clearly observed. X-ray diffraction analysis revealed that the crystalline volume fraction was almost constant up to 80°C. However, the crystallinity increased approximately 11% when temperature increased to 180°C. A secondary β_c relaxation was observed at 140°C and was related to a change in the crystalline volume fraction, as previously reported. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Studies on the thermal properties of poly(phenylene sulfide amide)

Thermal properties of poly(phenylene sulfide amide) (PPSA) prepared using sodium sulfide, sulfur, and thiourea as sulfur sources which reacted with dichlorobenzamide (DCBA) and alkali in polar organic solvent at the atmospheric pressure, were studied. The glass transition temperature (T_g), melting point temperature (T_m), and melting enthalpy (ΔH_m) of the related polymers were obtained by use of differential scanning calorimetry analysis. The results are: T_g = 103.4–104.5°C, T_m = 291.5–304.7°C, and ΔH_m = 104.4–115.4 J/g. Thermal properties such as thermal decomposition temperature and decomposition kinetics were investigated by thermogravimetric analysis under nitrogen. The initial and maximum rate temperatures of degradation were found to be 401.5–411.7°C and 437–477°C, respectively. The parameters of thermal decomposition kinetics of PPSAs were worked out to be: activation energy of degradation was 135 to 148 kJ/mol and the 60-s half-life temperature was 360 to 371°C. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1227–1230, 1997     

Comparison of thermal techniques for glass transition assignment. II. Commercial polymers

The differential scanning calorimetry glass transition (DSC T_g), measured by ASTM test method E-1356, and the dynamic mechanical analysis glass transition (DMA T_g), measured using a new definition of the DMA T_g, generally agree within ±4°C for a wide variety of commercially available polymers. The DMA T_g is defined as the average of E′ and tan δ peak temperatures measured at a 1 rad/s oscillation frequency. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 191–195, 1997     

Moisture diffusion in epoxy systems

The moisture diffusion process of an epoxy system is studied as a function of epoxy‐amine stoichiometry and the resulting microstructure. Differences in diffusion behavior are related to the relative importance of diffusion through the low‐density and high‐density microstructural phases for different stoichiometries. Also, changes in saturation level with stoichiometry are explained by competing effects of free volume versus the content of the low‐density phase. Increasing the humidity level causes a corresponding increase in saturation level, while increasing the temperature causes more pronounced non‐Fickian behavior. The effects of absorbed moisture on the thermomechanical properties of the epoxies are also investigated. Reductions in the glass transition temperature, T_g, and moisture‐induced swelling strains are measured after exposure of samples to the three conditioning environments. Moisture‐induced swelling strains increase with increasing moisture content. The reductions in T_g range from 5 to 20°C and are generally larger for amine‐rich samples than for epoxy‐rich and stoichiometric samples. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 787–798, 1999     

Moisture sorption/desorption behavior of various manmade cellulosic fibers

The kinetics of dynamic water vapor sorption and desorption on viscose, modal, cotton, wool, down, and polyester fibers and lyocell knit fabrics were investigated according to the parallel exponential kinetics (PEK) model. The total equilibrium moisture regain (M_inf(total)) in all the materials decreased with increasing temperature. However, the partial equilibrium fast sorption, determined by PEK simulation at 60% relative humidity (RH) and 36°C, was larger than that at 20°C, whereas the partial equilibrium slow sorption was smaller. The characteristic times in fast sorption (τ₁) and in slow sorption (τ₂) for lyocell were reduced when the conditions were changed from 60% RH and 20°C to 36°C, whereas those for the other fibers increased. Lyocell exhibited the highest M_inf(total) value and the lowest τ₁ and τ₂ values, and this suggested high equilibrium moisture content and fast moisture uptake/release, that is, high moisture accessibility for lyocell. The relationships between the moisture regain, hysteresis, water retention capacity, and Brunauer–Emmett–Teller surface volume in the materials were also examined. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1621–1625, 2005     

The Use of Dynamic Mechanical Methods to Study the Effect of Absorbed Water on Temperature-dependent Properties of an Epoxy Resin-Carbon Fibre Composite

Dynamic mechanical methods were used to study the effect of absorbed moisture on the properties of an epoxy resin matrix CFRP. The glass transition temperature (T_g)of the matrix resin, determined as the onset of the characteristic fall in dynamic modulus with increasing temperature, was found to decrease with increasing moisture content. Maximum shifts in T_g of 80 to 90°C, relative to the dry material, were observed for a resin moisture content of 5.2% by weight. The effects of sample geometry, fibre orientation, and frequency of oscillation, on the dynamic mechanical properties are discussed. Results are given of an analysis of the observed dependence of T_g on water content using two theoretical models.     

Synthesis and properties of novel poly(coumarin‐amide)s

A novel monomer diacid, 6,6′‐methylenebis(2‐oxo‐2H‐chromene‐3‐carboxylic acid), was synthesized and used in a direct polycondensation reaction with various aromatic diamines in N‐methyl‐2‐pyrrolidone solution containing dissolved LiCl and CaCl₂, using triphenyl phosphite and pyridine as condensing agents to give a series of novel heteroaromatic polyamides containing photosensitive coumarin groups in the main chain. Polyamide properties were investigated by DSC, TGA, GPC, wide‐angle X‐ray scattering, viscosity, and solubility measurements. The copolymers were soluble in aprotic polar solvents, and their inherent viscosities varied between 0.49 and 0.78 dL g^?1. The weight‐average and number‐average molecular weights, measured by gel permeation chromatography, were 27,500–43,900 g mol^?1 and 46,500–66,300 g mol^?1, respectively, and polydispersities in the range of 1.48–1.69. The aromatic polyamides showed glass‐transition temperatures (T_g) ranging from 283 to 329°C and good thermal properties evidenced by no significant weight loss up to 380°C and 10% weight loss recorded above 425°C in air. All the polyamides exhibited an amorphous nature as evidenced by wide‐angle X‐ray diffraction and demonstrated a film forming capability. Water uptake values up to 3.35% were observed at 65% relative humidity. These polymers exhibited strong UV‐vis absorption maxima at 357–369 nm in DMSO solution, and no discernible photoluminescence maxima were detected by exciting with 365 nm. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Conversion—temperature—property relationships in thermosetting systems: Property hysteresis due to microcracking of an epoxy/amine thermoset—glass fiber composite

A single specimen of an epoxy/amine thermoset—glass fiber composite was examined, using a freely oscillating torsion pendulum operating at ∼ 1 Hz, for different conversions (as measured by T_g) from T_g0 = 0°C to T_g∞ = 184°C during cooling and heating temperature scans. T_g was increased for successive pairs of scans by heating to higher and higher temperatures. The data were used in two ways: (i) vs. temperature for a fixed conversion to obtain transitions, modulus, and mechanical loss data, and (ii) by crossplotting to obtain isothermal values of the mechanical parameters vs. conversion (T_g). Hysteresis between cooling and subsequent heating data was observed in temperature scans of essentially ungelled material (T_g < 70°C) and was attributed to spontaneous microcracking. Hysteresis was analyzed in terms of the following three parameters: T_crack, the temperature corresponding to the onset of microcracking on cooling; T_heal, the temperature at which the specimen heals on subsequent heating; and the difference between isothermal cooling and heating data vs. conversion. Results were incorporated into a more general conversion—temperature—property diagram which serves as a framework for relating transitions (relaxations) to macroscopic behavior. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 39–53, 1997     

Water effect on the morphology of EVOH copolymers

The effect of water on the morphology of four ethylene vinyl alcohol copolymers (EVOH) with different ethylene contents was studied by differential scanning calorimetry (DSC). EVOH film samples equilibrated in controlled atmospheres at different relative humidities (RH) and 23°C were analyzed. Under dry conditions, the glass transition temperature (T_g) was unaffected by copolymer ethylene content. As RH increases, T_g decreases. It seems that the presence of water within the polymer matrix results in plasticization of the polymer. T_g varies from around 50°C (dry) to below room temperature. EVOH copolymers are glassy polymers when dry and rubbery polymers at high RHs. Fox and Gordon–Taylor's equations well describe T_g depletion at low water uptake, although severe water gain results in a considerable T_g decrease, which is not predicted by these theories. Melting temperature, T_m, and enthalpy, ΔH_m, were also analyzed. When dry, T_m decreases as ethylene content increases. No significant water effect was found on either T_m or ΔH_m. Hence, crystallinity seems to be unaffected by water presence. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1201–1206, 1999     

Novel conductive adhesives for surface mount applications

Electrically conductive adhesives (ECAs) have been explored as a tin/lead (Sn/Pb) solder alternative for attaching encapsulated surface mount components on rigid and flexible printed circuits. However, limited practical use of conductive adhesives in surface mount applications is found because of the limitations and concerns of current commercial ECAs. One critical limitation is the significant increase of joint resistance with Sn/Pb finished components under 85°C/85% relative humidity (RH) aging. Conductive adhesives with stable joint resistance are especially desirable. In this study, a novel conductive adhesive system that is based on epoxy resins has been developed. Conductive adhesives from this system show very stable joint resistance with Sn/Pb‐finished components during 85°C/85% RH aging. One ECA selected from this system has been tested here and compared with two popular commercial surface mount conductive adhesives. ECA properties studied included cure profile, glass transition temperature (T_g), bulk resistivity, moisture absorption, die shear adhesion strength, and shift of joint resistance with Sn/Pb metallization under 85°C/85% RH aging. It was found that, compared to the commercial conductive adhesives, our in‐house conductive adhesive had higher T_g, comparable bulk resistivity, lower moisture absorption, comparable adhesion strength, and most importantly, much more stable joint resistance. Therefore, this conductive adhesive system should have better performance for surface mount applications than current commercial surface mount conductive adhesives. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 399–406, 1999     

The effect of composition on the properties of nylon 612 copolymers

The series of nylon 612 copolymers was synthesized from caprolactam (C) and laurolactam (L) at 145°C. The 50/50 C/L molar ratio copolymer was found to have the minimum melting temperature (T_m ) for the series. The glass transition temperatures (T_g 's) of the copolymers were affected by the crystallinity of the copolymers. The T_g was at a minimum for the 50/50 copolymer for crystalline samples. However, for amorphous samples there was a decrease in T_g with increasing L content. Percent crystallinity was determined by differential scanning calorimetry and X-ray techniques. It was found that the degree of crystallinity was at a minimum for copolymers of 70/30 to 40/60 C/L ratios. Coefficients of linear thermal expansion (CLTE) were obtained for the copolymers at 10°C intervals from 20 to 70°C for dry and from 20 to 50°C for samples conditioned at 50% relative humidity and 50°C. The dry samples gave lower initial values, but had a greater temperature dependence than the conditioned samples. As expected, the CLTE was found to be lowest for samples exhibiting the highest crystallinity. The tensile strengths and moduli decreased rapidly with increasing L up to the 70/30 C/L ratio after which they remained relatively constant. Elongations reached maximums between 70/30 and 40/60 C/L ratios. An inverse relationship was found between crystallinity and impact strength.     

Straining phenomena of POY revealed by thermal retraction and other techniques

Partially oriented polyesters yarns (POY) were strained at different strain rates (0.03–12.00 min^?1) and temperatures above and below T_g (3–92°C). Thermal retraction, density, DSC, and WAXS techniques show that strain-induced crystallization takes place by straining at temperatures above as well as below T_g. Above T_g, depending upon the strain rate, two regimes are observed: Below the strain rate of 1.5 min^?1, the flow regime; the degree of crystallinity is reduced as the strain rate increases. Above the strain rate of 1.5 min^?1, the strain-induced crystallization regime; the degree of crystallinity increases as the strain rate increases. Thermal retraction, stress–relaxation, and sonic modulus techniques indicate that, upon cold straining, instead of the original T_g at 65–69°C, two glass transitions occur: an upper T_g (u) and a lower T_g (l). For POY strained at 3°C and at a strain rate of 10 min^?1, the values are 78°C and 37°C, respectively. The higher the strain rate and the lower the straining temperature, the large the difference between T_g (u) and T_g (l).     

Core–shell particles to toughen epoxy resins. I. Preparation and characterization of core–shell particles

A two-stage, multistep soapless emulsion polymerization was employed to prepare various sizes of reactive core–shell particles (CSPs) with butyl acrylate (BA) as the core and methyl methacrylate (MMA) copolymerizing with various concentrations of glycidyl methacrylate (GMA) as the shell. Ethylene glycol dimethacrylate (EGDMA) was used to crosslink either the core or shell. The number of epoxy groups in a particle of the prepared CSP measured by chemical titration was close to the calculated value based on the assumption that the added GMA participated in the entire polymerization unless it was higher than 29 mol %. Similar results were also found for their solid-state ^13C-NMR spectroscopy. The MMA/GMA copolymerized and EGDMA-crosslinked shell of the CSP had a maximum glass transition temperature (T_g) of 140°C, which was decreased with the content of GMA at a rate of −1°C/mol %. However, the shell without crosslinking had a maximum T_g of 127°C, which decreased at a rate of −0.83°C/mol %. The T_g of the interphasial region between the core and shell was 65°C, which was invariant with the design variables. The T_g of the BA core was −43°C, but it could be increased to −35°C by crosslinking with EGDMA. The T_g values of the core and shell were also invariant with the size of the CSP. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2069–2078, 1998     

Acid- and base-catalyzed epoxy-phenol thermosets from catechol,resorcinol and hydroquinone: A structure–property study

In order to better understand the design rules of epoxy–phenol thermosets we will report on the chemistry and (thermo)mechanical properties of cured epoxy–phenol thermoset films. Ortho-, meta- and para-isomers of dihydroxybenzene (DHB) were reacted with the diglycidyl ether of bisphenol A (DGEBA) in the presence of an acid catalyst or triphenylphosphine (PPh₃). The glass transition temperatures (T_g) of the cross-linked films decreases in the order of meta- (T_g = 115°C) > ortho- (T_g = 102°C) > para-DHB (T_g = 96°C) as measured by differential scanning calorimetry. Uniaxial tensile testing of cross-linked films showed excellent stress–strain behavior. The average ultimate strength values ranged from 65 to 82 MPa and the average values of the strain-at-break ranged from 4.8% to 6.9% at 25°C for all cross-linked films. When a PPh₃ was used, the network properties were profoundly different. The base catalyzed thermoset of DGEBA and meta-DHB shows a T_g of 85°C, which is 30°C lower than the T_g of the acid-catalyzed analog. Tensile films appear to be more ductile, as they exhibit a strain-at-break of 20%. The results of this study confirm that simple dihydroxybenzene hardeners can be used to prepare cross-linked films with excellent thermomechanical properties.