Moisture sorption-desorption-resorption characteristics and its effect on the mechanical behavior of the epoxy system

Thermosetting epoxy resins are attractive materials for many engineering applications, as they are low in density, with excellent mechanical properties and easily fabricated by processes such as injection molding, extrusion and vacuum forming. However, the hostile hygrothermal environment can degrade the epoxy system. In this study, moisture sorption-desorption-resorption characteristics of the DGEBA/DDA epoxy system have been investigated by the hygrothermal aging and molecular dynamic (MD) simulation. Also, the effects of moisture on the mechanical behavior of the epoxy system have been studied by the uniaxial tensile test and a scanning electron microscopy (SEM), for the unaged, moisture saturated, completely desorbed and moisture re-saturated specimens, respectively. Results show that the moisture diffusion in epoxy system is not only dependent on the hygrothermal conditions, but also on the specimen thickness and hygrothermal history. Due to the effect of the hygrothermal aging, both the tensile elastic module and tensile strength of the studied epoxy system have been reduced, that is, the absorbed moisture has deleterious effects on the physical properties of epoxies and can, therefore, greatly compromise the performance of an epoxy-based component.     

玻纤增强环氧树脂复合材料的酸雨循环老化性能与机理

针对潮湿和酸雨多发地域的航空器复合材料老化问题,模拟酸雨和湿热(普通热水)环境,研究了单向玻纤增强环氧树脂复合材料(UGFREC)在酸雨和湿热环境下的吸湿-干燥循环老化行为;分析了不同循环次数下的UGFREC的吸水动力学;采用力学测试装置、热力学分析装置和扫描电镜,分别表征酸雨和湿热循环老化前后UGFREC的弯曲性能、层间剪切性能、冲击强度、动态粘弹性、玻璃化转变温度和冲击破坏形貌;初步给出反映酸雨和湿热循环过程中基体树脂与纤维间的界面应力变化模型。     

单向玻纤增强BMI复合材料的酸雨和温热老化行为

针对潮湿和酸雨多发地域的航空器用复合材料老化问题,模拟酸雨和温热(普通热水)环境,研究了单向玻璃纤维增强双马来酰亚胺树脂(BM I)复合材料(UGFRBC)吸水行为,通过红外光谱仪、力学测试装置、热力学分析装置分别表征了老化前后复合材料基体的结构、弯曲性能、层间剪切性能、动态粘弹性和玻璃化转变温度,分析了酸雨与温热环境下吸湿率对复合材料力学性能影响,建立了一种预测该复合材料酸雨和温热老化力学性能的经验公式。     

Evaluation of structural changes in epoxy systems by moisture sorption-desorption and dynamic mechanical studies

This work forms a basis for relating moisture solubilities and dynamic mechanical properties to the hygrothermal history of epoxy systems. Two different classes of epoxy systems were investigated; a low-performance epoxy (DGEBA-TETA) and a high-performance system (TGDDM-DDS) commonly used in carbon fiber composites. An increase in the equilibrium moisture content from TGDDM-DDS epoxy samples having various DDS compositions was observed as a result of thermal cycling in a liquid environment. Interpretation of the experimental results suggest that hygrothermal interactions produce changes in the epoxy network structure and result in the observed moisture sorption behavior.     

CE/EP/CF复合材料的湿热性能研究

采用溶液预浸渍法分别制备了两种碳纤维(CF)增强环氧树脂(EP)改性氰酸酯树脂(CE)(CE/EP/CF)复合材料,研究了该复合材料的吸湿行为及湿热环境对其力学性能和微观结构的影响。结果表明,CE/EP基体具有比EP更小的吸湿能力;湿热环境对CE/EP/CF复合材料的纵向拉伸强度影响不大,但对其层间剪切强度的影响较为显著。     

Dynamic mechanical study of epoxy,epoxy/glass,and glass/epoxy/wood hybrid composites aged in various media

It was proposed and subsequently established that wrapping of red oak wood crossties with epoxy impregnated glass fiber composites will impart longer service life and better stiffness and strength characteristics to these hybrid ties than conventional ones and will help them better withstand environmental extremes. The objective was to understand the degrading effects of aqueous (distilled water), saline (NaCl), acidic (HCl), and alkaline (NaOH) solutions, as well as accelerated aging and freeze/thaw cycling environments on the dynamic and static mechanical properties of these hybrid materials (i.e., wood, wrapped with fiber reinforced resin) and their components. Also micrographs of composite samples, obtained through scanning electron microscopy (SEM), were studied to determine the failure mechanism of composite specimens aged in different environments. Results showed that immersion in aging media lowered the glass transition temperature (T_g) and enhanced apparent phase separation in the samples because of polymer plasticization. In water immersion, the T_g and the stiffness increased with time owing to continued resin curing. At ambient temperature, sustained load had little effect on the mechanical behavior of the aged samples. The extent of degradation was the least for samples aged in salt solution. Soaking in room‐temperature acid solution was most damaging to pure red oak wood samples. Six‐cycle aging did not damage the neat resin or the hybrid samples, whereas it damaged pure wood specimens. Therefore, the composite wrapping around the wood core of the hybrid sample protected it sufficiently, thereby preventing damage to the hybrid specimen during the aging process. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Effects of hygrothermal aging on the thermal–mechanical properties of vinylester resin and its pultruded carbon fiber composites

Hygrothermal aging was carried out on vinyl ester (VE) resin cast and its pultruded carbon fiber reinforced composite (CF/VE) by immersing them in distilled water at 65 and 95°C. Hygrothermal aging effects on the samples were studied in terms of thermal–mechanical properties, as well as moisture absorption behavior, interfacial adhesion, and transverse mechanical properties. Moisture absorption behaviors of the VE casts and the CF/VE composites were characterized as Fickian behavior. Dynamic mechanical thermal analysis (DMTA) tests showed that the tan δ peak temperatures of the VE casts and CF/VE composites decreased with immersion time at 65 and 95°C. Moreover, there existed a splitting in the tan δ peaks at 95°C, which was reversible and could be recovered by dehydration. Three‐point flexural test indicated that flexural strengths of both the VE casts and the composites decreased by hygrothermal aging with a trend related to their moisture absorption behaviors, while flexural modulus of the composites was less affected. The ILSS of the CF/VE composites was also depressed by deterioration in interfacial adhesion, which was proved by the interfacial adhesion parameters, A and α. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers     

Hygrothermal cycling effects on the durability of phenolic based composites

Work has been performed to investigate the thermal and mechanical properties of carbon fiber/phenolic resin composites as engineering materials for the aerospace industry. These materials are cost effective while displaying excellent temperature and fire resistance as well as good mechanical properties. All phenolic and epoxy composite specimens used here were prepared by resin transfer molding (RTM) to model a cost‐effective process. Hygrothermal cycling effects on the property changes of phenolic composites were evaluated through thermal, mechanical, and morphological tests. The fracture performance of a phenolic composite modified with a silicone‐based additive decreased after fewer hygrothermal cycles than unmodified phenolic and epoxy composites. Results from dynamic mechanical analysis (DMA) experiments showed that the modified phenolic composite was more significantly affected by the hygrothermal cycling than the unmodified phenolic composites. Fatigue tests showed that the phenolic composites that were not exposed to hygrothermal cycling had more resistance to fatigue cycles than the epoxy composites.     

Study on the damage behavior of carbon fiber composite after low-velocity impact under hygrothermal aging

In this article, T800 carbon fiber/epoxy resin composite was subjected to hygrothermal aging. By analyzing the mass change, surface morphology before and after aging, infrared spectra, and dynamic mechanical properties, the effect of hygrothermal aging on the composite properties was studied. The hygrothermal aging of the composite after low-velocity impact, the effects of environmental factors on the damaged area, and the post-impact compression properties of composites were studied. The results showed that the saturation moisture absorption rate of the composite after aging (71°C constant temperature) was 0.88%. Upon increasing the impact energy, an indentation appeared before the inflection point at 35 J. When the impact energy was less than 15 J, aging did not affect invisible damage. Above this, the damaged area and number of internal cracks and defects in the composite were increased. After aging, the compressive strength of composite laminates with impact damage decreased obviously. During the aging stage, the residual compressive strength of the sample was the lowest in the moisture saturated state, and hygrothermal aging had little effect on the compression failure mode after impact.     

Kinetics of water absorption and hygrothermal aging rubber toughened poly(butylene terephthalate) with and without short glass fiber reinforcement

The objectives of this paper were to investigate the water absorption and hygrothermal aging behavior of rubber‐toughened poly(butylenes terephthalate) (RT‐PBT) with and without short glass fiber (SGF) reinforcement. The rubbers used in the study were AX8900 and EXL2314, both of which are acrylate‐based terpolymer. The effect of the hygrothermal aging on its fracture properties was also studied. The kinetics of the water absorption study were carried out on the injection‐molded samples of the RT‐PBTs and the SGF‐reinforced rubber‐toughened PBT (SGF‐RT‐PBT) at three immersion temperatures, 30, 60 and 90°C, for a total of 450 h. The study of the deterioration caused by the hygrothermal aging was conducted by investigating the fracture parameters and flexural properties of all the materials as both hygrothermally aged (HA) and redried state (RD). The modes of the failure of HA and RD samples were studied using the scanning electron microscopy (SEM) technique. It was found that all the samples conformed to Fickian behavior and the kinetics of absorption exhibited a strong dependency on the rubber types, presence of SGF, as well as the immersion temperature. Generally, SGF‐RT‐PBT showed a better resistance to hygrothermal aging than that of RT‐PBT and PBT, though a declining trend was observed in the fracture parameters, K_c and G_c. However, an opposite observation was exhibited in the flexural properties in some, but not all cases. Finally, the results obtained from SEM micrographs showed that permanent damage occurred in the materials and the hygrothermal aging had suppressed the plastic deformation ability of the PBT matrix and both types of impact modifiers where brittle failure was observed. Fiber pull‐out was apparently the failure mode of the SGF‐reinforced materials. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 506–516, 2004     

Effect of moisture absorption on the dynamic mechanical properties of short carbon fiber reinforced nylon 6, 6

A study of hygrothermal aging in terms of the kinetics of moisture absorption by nylon 6,6 and its carbon fiber reinforced composites has been carried out. The single free phase model of absorption has been applied to the kinetic data and thereafter the values of diffusivity have been evaluted. The diffusivity was found to be dependent on the conditioning temperatures and the volume fraction of fibers. Dynamic mechanical properties of unaged and aged samples were studied using a free resonance torsion pendulum which covers a temperature range of 350°C. Incorporation of carbon fibers has led to an increase in structural rigidity of the nylon 6,6 matrix especially at higher temperatures. This was reflected by the sharp increase in the relative shear modulus as the glass transition temperature of nylon 6,6 is appoached. Absorbed moisture was observed to plasticize the polymer matrix and decreased the temperatures of all the transitions. For instance, the α-transition was shifted by almost 95°C. The intensities of the transition peaks of both unaged and aged samples were found to decrease with fiber volume fraction. Increasing the conditioning temperatures has resulted in a reduction of the shear storage modulus and this effect was found to be more pronounced in the reinforced nylon 6,6. This has been attributed to the increase in the extent of degradation at the fiber-matrix interface.     

A Comparative Study on the Mechanical Properties of Jute and Sisal Fiber Reinforced Polymer Composites

The aim of the present study is to investigate and compare the mechanical properties of raw jute and sisal fiber reinforced epoxy composites with sodium hydroxide treated jute and sisal fiber reinforced epoxy composites. This is followed by comparisons of the sodium hydroxide treated jute and sisal fiber reinforced composites. The jute and sisal fibers were treated with 20% sodium hydroxide for 2 h and then incorporated into the epoxy matrix by a molding technique to form the composites. Similar techniques have been adopted for the fabrication of raw jute and sisal fiber reinforced epoxy composites. The raw jute and sisal fiber reinforced epoxy composites and the sodium hydroxide treated jute and sisal fiber reinforced epoxy composites were characterized by FTIR. The mechanical properties (tensile and flexural strength), water absorption and morphological changes were investigated for the composite samples. It was found that the sodium hydroxide treated jute and sisal fiber reinforced epoxy composites exhibited better mechanical properties than the raw jute and raw sisal fiber reinforced composites. When comparing the sodium hydroxide treated jute and sisal fiber reinforced epoxy composites, the sodium hydroxide treated jute fiber reinforced composites exhibited better mechanical properties than the latter.     

Effect of layering pattern on the physical,mechanical, and thermal properties of jute/bagasse hybrid fiber‐reinforced epoxy novolac composites

In this study, randomly oriented short jute/bagasse hybrid fiber‐reinforced epoxy novolac composites were prepared by keeping the relative volume ratio of jute and bagasse of 1:3 and the total fiber loading 0.40 volume fractions. The effect of jute fiber hybridization and different layering pattern on the physical, mechanical, and thermal properties of jute/bagasse hybrid fiber‐reinforced epoxy novolac composites was investigated. The hybrid fiber‐reinforced composites exhibited fair water absorption and thickness swelling properties. To investigate the effect of layering pattern on thermomechanical behavior of hybrid composites, the storage modulus and loss factor were determined using dynamic mechanical analyzer from 30 to 200°C at a frequency of 1 Hz. The fracture surface morphology of the tensile samples of the hybrid composites was performed by using scanning electron microscopy. The morphological features of the composites were well corroborated with the mechanical properties. Thermogravimetric analysis indicated an increase in thermal stability of pure bagasse composites with the incorporation of jute fibers. The incorporation of hybrid fibers results better improvement in both thermal and dimensional stable compared with the pure bagasse fiber composites. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Thermal aging performance of glass fiber/polyphenylene sulfide composites in high temperature

In order to use the glass fiber reinforced polyphenylene sulfide composites (GF/PPS) in high temperature environments, thermal aging performance of two kinds of commercial grade PPS composites, reinforced by 40% glass fiber, PPS-G40 HM and 1140L4, in thermal aging temperature of 250°C was compared by tensile strength, oxidized layer, color, crystallization and melting behavior. The results showed that tensile strength of GF/PPS composites is significantly decreased with increasing of aging time below 200 h and the tensile strength of aged PPS-G40 HM is higher than that of aged 1140L4. The thickness of dark color area is increased with increasing of aging time. The thickness of oxidized layer of 1140L4 is thinner than that of PPS-G40 HM. However, the color of oxidized layer of PPS-G40 HM is lighter than that of 1140L4. The recrystallization in thermal aging results in the formation of crystal with higher melting point and increased melting temperature of GF/PPS composites. It is found that addition of epoxy resin can increase the initial mechanical property and improve the thermal aging performance of GF/PPS composites. A novel modified GF/PPS composite with higher thermal aging properties was obtained.     

Solid freeform fabrication of soybean oil-based composites reinforced with clay and fibers

Soybean oil/epoxy-based composites were prepared by an extrusion freeform fabrication method. These composites were reinforced with a combination of organically modified clay and fibers. The intercalated behavior of the epoxy resin in the presence of organo-modified clay was investigated by X-ray diffraction and transmission electron microscopy. The mixture of epoxidized soybean oil and EPON^® 828 resin was modified with a gelling agent to solidify the materials until curing occurred. The flexural modulus reached 4.86 GPa with glass fiber reinforcement at 50.6 wt% loading. It was shown that the fiber orientation followed the direction of motion of the writing head that deposited the resins and had an influence on the properties of the composite. The composites cured by curing agent jeffamine EDR-148 were found to have lower mechanical properties than those cured with triethylenetetramine, diethylenetriamine, and polyethylenimine. In addition, the effects of clay loading and fiber loading on mechanical properties of the composites were studied and reported.     

Effects of hygrothermal aging on anisotropic conductive adhesive joints: experiments and theoretical analysis

Epoxy-based anisotropic conductive adhesive film (ACF) joints have been used in a number of interconnect applications, including direct chip attachment, i.e., chip on glass, chip on ceramics, etc. The ACF joints can be subjected to high relative humidity environment and are susceptible to moisture sorption, especially at elevated temperatures. The long-term hygrothermal aging will induce irreversible changes to epoxy resin systems due to susceptibility of the polymer resin to hydrolysis, oxidation, etc. In this study, the hygrothermal environment was used as an accelerator for the degradation of ACF joints in chip-on-glass (COG) assemblies, which were fabricated in the form of single-lap joints. The effects of aging on the epoxy-based ACF joints were characterized by shear tests and scanning electron microscopy (SEM) at accelerated aging times of 125, 250, 375 and 550 h. The results show that the strength of ACF joints decreases and the fracture mechanism gradually changes with hygrothermal aging. In order to further interpret the hygrothermally-induced degradation to the ACF joints, an ACF joint aging model with hygrothermal environment has been developed, introducing a dimensionless parameter A, which was obtained from the interfacial fracture energy.     

Barrier properties for short‐fiber‐reinforced epoxy foams

The barrier properties of short‐fiber‐reinforced epoxy foam are characterized and compared with unreinforced epoxy foam in terms of moisture absorption, flammability properties, and impact properties. Compression and shear properties are also included to place in perspective the mechanical behavior of these materials. Compared with conventional epoxy foam, foam reinforced with aramid fibers exhibits higher moisture absorption and lower diffusion, while glass‐fiber‐reinforced foam is significantly stiffer and stronger. In addition, the polymeric foam composites studied present superior fire‐resistance compared with conventional epoxy foam systems. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3266–3272, 2006     

Hygrothermal degradation of the composite laminates from woven carbon/SC‐15 epoxy resin and woven glass/SC‐15 epoxy resin

The degradation mechanism for hygrothermal aging of woven carbon‐epoxy and woven glass‐epoxy composite laminates was investigated in the micro‐scale. Interlaminar shear and cross laminar flexural tests were performed on notched and unnotched specimens to know the mechanical performance of the composite laminates. The Interlaminar Shear Stress (ISS) for both the composites was also evaluated and correlated with the number of hygrothermal cycles. Four‐point bending and tensile or compression shear loading configurations were also used. The stress at the onset of delamination (Delamination Damage Tolerance, DDT) was identified from the load‐deflection curve of the flexural specimens and correlated with the number of hygrothermal cycles. It was found that both the ISS and DDT decrease with the exposure time. Dimensional stability was almost unchanged throughout the aging process, although there was a very little moisture absorption (∼1.3%) in glass‐epoxy and carbon‐epoxy composite laminates. SEM photomicrographs of the delaminated surface show that failure occurs suddenly in a macroscopically brittle mode by crack initiation and propagation method. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers.     

Application of the thermal energy storage concept to novel epoxy–short carbon fiber composites

For the first time, multifunctional epoxy–short carbon fiber reinforced composites suitable for thermal energy storage technology were developed. Paraffin microcapsules (MC) and short carbon fibers (CFs) were added at different relative amounts to an epoxy matrix, and the microstructural and thermomechanical properties of the resulting materials were investigated. Scanning electron microscopy images of the composites showed a uniform distribution of the capsules within the matrix, with a rather good interfacial adhesion, while the increase in the polymer viscosity at elevated CF and MC amounts caused an increase in the void content. Differential scanning calorimetry tests revealed that melting enthalpy values (up to 60 J/g) can be obtained at high MC concentrations. The mixing and thermal curing of the composites did not lead to breakage of the capsules and to the consequent leakage of the paraffin out of the epoxy matrix. The thermal stability of the prepared composites is not negatively affected by the MC addition, and the temperatures at which the thermal degradation process begins were far above the curing or service temperature of the composites. Flexural and impact tests highlighted that the presence of MC reduces the mechanical properties of the samples, while CF positively contributes to retaining the original stiffness and mechanical resistance. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47434.     

Influence of Matrix Properties on Fragmentation Test

Physical aging was used to vary the mechanical properties of model single fiber composites without changing the chemistry at the interface in order to study how property changes affect the measurement of interfacial adhesion by the fragmentation test. The properties of epoxy matrix/AS4 single fiber composites driven to full cure (T_g = 166°C) are altered by annealing below T_g . Neat resin samples with identical thermal histories are tested. All aged panels show roughly the same embrittlement with aging characterized by an average 30% decrease in tensile failure strain and 7.3% increase in compressive yield relative to quenched samples. Fragmentation results indicated no change between aged and quenched samples. Results are discussed in terms of micromechanics models for the fragmentation test. Strain at fragmentation increased with aging. This was related to the residual stress state in the model composite and the possibility of the zero stress state of the single fiber composites increasing with thermal annealing.