Accelerated aging versus realistic aging in aerospace composite materials. V. The effects of hot/wet aging in a structural epoxy composite

Samples of an aerospace‐grade, carbon‐fiber epoxy composite (Hexcell 8552/IM7) were subject to long‐term (≈ 1 year) hot/wet aging and thermal spiking under different humidity levels and temperature conditions related to the “in‐service” conditions seen in military aircraft. Changes to the chemical and physico‐chemical structure of the composite were analyzed by a range of experimental techniques including gravimetric analysis, FTIR, and DMA, to compare the effects of the various aging conditions. The results indicated that, while the chemical changes (as seen by FTIR) in this well cured composite appeared to be only significant at the surface, they did appear to have a deeper influence on some of the major physical property changes observed, such as microcracking, glass‐transition‐temperature (T_g) variations, and Tan δ curves. These physical changes could not be fully explained by standard water‐absorption effects alone but could also be influenced by chemical changes similar to those seen at the surface. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Processing robustness for a phenylethynyl‐terminated polyimide composite

The processability of a phenylethynyl‐terminated imide resin matrix (PETI‐5) composite was investigated. Unidirectional prepregs were made through the coating of an N‐methylpyrrolidone solution of an amide acid oligomer (PETAA‐5/NMP) onto unsized IM7 fibers. Two batches of prepregs were used: one was made by the National Aeronautics and Space Administration in house, and the other was from an industrial source. The composite processing robustness was investigated with respect to the prepreg shelf life, the effect of B‐staging conditions, and the optimal processing window. The prepreg rheology and open hole compression (OHC) strengths were not to affected by prolonged ambient storage (i.e., up to 60 days). Rheological measurements indicated that the PETAA‐5/NMP processability was only slightly affected over a wide range of B‐stage temperatures (from 250 to 300°C). The OHC strength values were statistically indistinguishable among laminates consolidated under various B‐staging conditions. An optimal processing window was established with response surface methodology. The IM7/PETAA‐5/NMP prepreg was more sensitive to the consolidation temperature than to the pressure. A good consolidation was achievable at 371°C (700°F)/100 psi, which yielded a room‐temperature OHC strength of 62 ksi. However, the processability declined dramatically at temperatures below 350°C (662°F), as evidenced by the OHC strength values. The processability of the IM7/PETI‐5 prepreg was robust. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3212–3221, 2006     

Cure kinetics of a polymer‐based composite friction material

In the present article, cure kinetics of a commercially available composite friction material used in railroad vehicles is investigated using the rheometer measurements. Effect of ingredients of friction material compound, including rubber matrix, phenolic resin, and fillers, on overall cure kinetics of friction compound is also investigated by comparing the cure kinetics of friction material and rubber matrix compound. A phenomenological model and an Arrhenius‐type equation is developed for cure kinetics and induction time of both friction material and rubber matrix. The parameters of the models are extracted from experimental data, using the rheometer at different temperatures and utilizing appropriate optimization method. The good agreement between experimental measurement and models prediction indicates the good performance of the models developed in this study. The results demonstrate that phenolic resin and fillers have dominant effects on the overall cure behavior of the friction material compound. A comparison between the present results and other published data based on the differential scanning calorimetry (DSC) shows a reasonable agreement as well. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 100: 9–17, 2006     

Thickness‐dependent sorption and effects of physical aging in a polyimide sample

The solubility of a relatively noninteracting gas (CH₄) was measured and compared for samples of vastly different thicknesses (25.4 vs 0.1 μm) to investigate the possibility of thickness‐dependent sorption and physical aging in glassy polymers. Changes in the sorption due to physical aging in these dense samples were observed and compared. To further compare the tendency of free‐volume relaxations in samples of different thicknesses, a variety of conditioning sorption tests were performed in which increases in the CH₄ sorption due to high‐pressure CO₂ exposure were observed and then compared after sample degassing. The reduction in CH₄ solubility was related to excess free‐volume relaxations, and response comparisons of samples of widely different film thicknesses supported the notion of thickness‐dependent physical aging in glassy polymers. To the authors' knowledge, this was the first direct observation of a significant differences in gas solubility in glassy polymers due to the film thickness and aging time. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1115–1121, 2005     

Accelerated aging versus realistic aging in aerospace composite materials. I. The chemistry of thermal aging in a low‐temperature‐cure epoxy composite

Samples of an aerospace‐grade epoxy composite (M20/IM7) are subject to long‐term (～ 1 year) thermal aging at temperatures of 70°C, 120°C, 170°C, and 200°C (in air) and the changes to the chemical and physicochemical structure of the composite are analyzed by a range of different techniques, including gravimetric analysis, FTIR, DSC, and DMA to compare the effects of different severities of degradation treatment. The results show that at the lower temperatures, the oxidative degradation changes are very selective for chemical defect groups, particularly near the sample surfaces. However, at the higher temperatures, combinations of further cure reactions and generalized oxidative degradation changes (again from the surface inwards) make for a highly complex ageing pattern for this particular composite material. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4291–4303, 2006     

Modification of high‐performance polymer composite through high‐energy radiation and low‐pressure plasma for aerospace and space applications

In this investigation, attempts are made to modify a high‐performance polymer such as polybenzimidazole (PBI) (service temperature ranges from ?260°C to +400°C) through high‐energy radiation and low‐pressure plasma to prepare composite with the same polymer. The PBI composites are prepared using an ultrahigh temperature resistant epoxy adhesive to join the two polymer sheets. The service temperature of this adhesive ranges from ?260°C to +370°C, and in addition, this adhesive has excellent resistance to most acids, alkalis, solvents, corrosive agents, radiation, and fire, making it extremely useful for aerospace and space applications. Prior to preparing the composite, the surface of the PBI is ultrasonically cleaned by acetone followed by its modification through high‐energy radiation for 6 h in the pool of a SLOWPOKE‐2 (safe low power critical experiment) nuclear reactor, which produces a mixed field of thermal and epithermal neutrons, energetic electrons, and protons, and γ‐rays, with a dose rate of 37 kGy/h and low‐pressure plasma through 13.56 MHz RF glow discharge for 120 s at 100 W of power using nitrogen as process gas, to essentially increase the surface energy of the polymer, leading to substantial improvement of its adhesion characteristics. Prior to joining, the polymer surfaces are characterized by estimating surface energy and electron spectroscopy for chemical analysis (ESCA). To determine the joint strength, tensile lap shear tests are performed according to ASTM D 5868–95 standard. Another set of experiments is carried out by exposing the low‐pressure plasma‐modified polymer joint under the SLOWPOKE‐2 nuclear for 6 h. Considerable increase in the joint strength is observed, when the polymer surface is modified by either high‐energy radiation or low‐pressure plasma. There is further significant increase in joint strength, when the polymer surface is first modified by low‐pressure plasma followed by exposing the joint under high‐energy radiation. To simulate with spatial conditions, the joints are exposed to cryogenic (?196°C) and high temperatures (+300°C) for 100 h. Then, tensile lap shear tests are carried out to determine the effects of these environments on the joint strength. It is observed that when these polymeric joints are exposed to these climatic conditions, the joints could retain their strength of about 95% of that of joints tested under ambient conditions. Finally, to understand the behavior of ultrahigh temperature resistant epoxy adhesive bonding of PBI, the fractured surfaces of the joints are examined by scanning electron microscope. It is observed that there is considerable interfacial failure in the case of unmodified polymer‐to‐polymer joint whereas surface‐modified polymer essentially fails cohesively within the adhesive. Therefore, this high‐performance polymer composite could be highly useful for structural applications in space and aerospace. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1959–1967, 2006     

Thermomechanical characteristics of benzoxazine–urethane copolymers and their carbon fiber‐reinforced composites

Copolymers of polybenzoxazine (BA‐a) and urethane elastomer (PU) with three different structures of isocyanates [i.e., toluene diisocyanate (TDI), diphenylmethane diisocyanate, and isophorone diisocyanate], were examined. The experimental results reveal that the enhancement in glass transition temperature (T_g) of BA‐a/PU copolymers was clearly observed [i.e., T_g of the BA‐a/PU copolymers in 60 : 40 BA‐a : PU system for all isocyanate types (T_g beyond 230°C) was higher than those of the parent resins (165°C for BA‐a and ?70°C for PU)]. It was reported that the degradation temperature increased from 321°C to about 330°C with increasing urethane content. Furthermore, the flexural strength synergism was found at the BA‐a : PU ratio of 90 : 10 for all types of isocyanates. The effect of urethane prepolymer based on TDI rendered the highest T_g, flexural modulus, and flexural strength of the copolymers among the three isocyanates used. The preferable isocyanate of the binary systems for making high processable carbon fiber composites was based on TDI. The flexural strength of the carbon fiber‐reinforced BA‐a : PU based on TDI at 80 wt % of the fiber in cross‐ply orientation provided relatively high values of about 490 MPa. The flexural modulus slightly decreased from 51 GPa for polybenzoxazine to 48 GPa in the 60 : 40 BA‐a : PU system. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Accelerated aging versus realistic aging in aerospace composite materials. II. Chemistry of thermal aging in a structural composite

Samples of an aerospace structural epoxy composite (8552/IM7) were subject to long‐term (≈ 1 year) thermal aging at temperatures of 70°, 120°, 170°, and 200°C (in air). The changes to the chemical and physicochemical structure of the composite were analyzed by a range of different techniques, including gravimetric analysis, Fourier transform infrared (FTIR), and dynamic mechanical analysis (DMA) to compare the effects of different severities of degradation treatment. The results highlighted the large differences in chemical effects between the surface and the interior of the composite with very minor changes in the latter even at quite high aging temperatures and long aging times. The oxidative changes at the surface, however, varied from highly selective molecular changes for particular chemical groups at the lower aging temperatures (70° and 120°C), to quite general and extensive oxidative degradation at the higher aging temperatures (170° and 200°C). The results indicated that the mechanical changes in an aged composite of this type will vary greatly with the material thickness and surface protection as well as the aging temperature the composite is exposed to. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3221–3232, 2006     

Thermally stimulated creep (TSCr) study of viscoelastic behavior and physical aging of a polymeric matrix composite for spacecraft structures

Thermally Stimulated Creep (TSCr) mechanical spectroscopy has been used to analyze molecular movements in KMU‐4lcarbon/epoxy composite material around the glass transition temperature. This technique is powerful to characterize the microstructure and micromechanical properties of the epoxy matrix and their evolution upon thermal aging. Three cooperative submodes have been distinguished by resolving the fine structure of the material complex α‐retardation mode. The elementary processes constituting this mode possess activation enthalpies and preexponential factors that strongly depend on the thermal history of the sample. The activation parameters of the composite are subject to perceptible evolution due to postcuring degradation. The α‐mode associated complex spectrum shifts towards higher temperatures by 27°C as a consequence of a series of quenching in the temperature range 260 to 0°C; the material shows a rise in the fragility and a deterioration in the crack‐growth resistance qualities. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 342–350, 2002     

Study of thermal stability and ablation behavior of carbon/epoxy‐novolac composites

The use of carbon/epoxy‐novolac composites as advanced ablative materials for insulation of exit cone of solid‐propellant rocket nozzles are studied. In this article, three types of carbon fabrics are used and their composites are prepared by use of impregnation and hand lay‐up methods. To study the thermal stability and ablation behavior, these composites are tested by thermal tests such as thermogravimetric analysis (TGA) and oxyacetylene standard flame tests; the latter test is one of the most important standard tests of ablative materials. The test apparatus is made according to American standard, ASTM‐E‐285‐80, and over 33 polymeric composites and 3 steel specimens were carried out according to its standards. It is found that the composites that are made up of C‐9750 fabric (high‐strength carbon fabric) in comparison with steel and the other types of carbon fabric specimens have the highest thermal stability and the best ablation behavior. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2455–2461, 2003     

Magnetic‐field‐assisted electrospinning highly aligned composite nanofibers containing well‐aligned multiwalled carbon nanotubes

Composite nanofiber meshes of well‐aligned polyacrylonitrile (PAN)/polyvinylpyrrolidone (PVP) nanofibers containing multiwalled carbon nanotubes (MWCNTs) were successfully fabricated by a magnetic‐field‐assisted electrospinning (MFAES) technology, which was confirmed to be a favorable method for preparation of aligned composite nanofibers in this article. The MFAES experiments showed that the diameters of composite nanofibers decreased first and then increased with the increase of voltage and MWCNTs content. With the increase of voltage, the degree of alignment of the composite nanofibers decreased, whereas it increased with increasing MWCNTs concentration. Transmission electron microscopy observation showed that MWCNTs were parallel and oriented along the axes of the nanofibers under the low concentration. A maximum enhancement of 178% in tensile strength was manifested by adding 2 wt % MWCNTs in well‐aligned composite nanofibers. In addition, the storage modulus of PAN/PVP/MWCNTs composite nanofibers was significantly higher than that of the PAN/PVP nanofibers. Besides, due to the highly ordered alignment structure, the composite nanofiber meshes showed large anisotropic surface resistance, that is, the surface resistance of the composite nanofiber films along the fiber axis was about 10 times smaller than that perpendicular to the axis direction. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41995.     

Influence of potassium humate on the swelling properties of a poly(acrylic acid‐co‐acrylamide)/ potassium humate superabsorbent composite

A novel superabsorbent composite, poly(acrylic acid‐co‐acrylamide)/potassium humate (PAA‐AM/KHA), was prepared by aqueous solution polymerization from acrylic acid, acrylamide, and potassium humate (KHA) with N,N′‐methylenebisacrylamide as a crosslinker and potassium peroxydisulfate as an initiator. The effects of incorporated KHA on the water absorbency, swelling rate, and reswelling capability were investigated. The swelling property of PAA‐AM/KHA in various saline solutions was studied systematically. The results show that the comprehensive properties and especially salt‐resistant ability of PAA‐AM/KHA were enhanced. There was a linear relationship between the saturated water absorbency and the minus square root of the ionic strength of the external medium, and the water absorbency of PAA‐AM/KHA in various salt solutions had the following order: NH₄Cl_(aq) = KCl_(aq) = NaCl_(aq) > MgCl_2(aq) > CaCl_2(aq) > AlCl_3(aq) > FeCl_3(aq). Moreover, the polymeric net structure of PAA‐AM/KHA was examined with respect to that of poly(acrylic acid‐co‐acrylamide). The results indicate that the polymeric net of PAA‐AM/KHA was improved by the introduction of a moderate amount of KHA into the superabsorbent composite and made more suitable for agriculture and horticulture applications. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

In vivo aging of gutta‐percha dental cone

Gutta‐percha cone is the most widely used material for root canal filling. The in vivo aging of this cone focus on the degradation of its main organic component, trans‐1,4‐polyisoprene, was studied. Aged cones (25 samples) from 2 to 30 years of root canal filling were extracted from different patients in the occasion of retreatment by mechanical way. The information about the aging time was given by the patients. Gel‐permeation chromatography (GPC) and infrared spectroscopy (FTIR) were the analytical techniques used. Polyisoprene degrades with time of aging, but in a slow process. Decrease in polymer molar mass from 5.7 × 10⁵ to 1.7 × 10⁵ g/mol was observed in polyisoprene from cone after 30 years of root canal filling and inside a noninfected tooth. In tooth with caries and periodontal infection, the decrease in molar mass is higher (4.6 × 10⁴ g/mol in cone with 10 years of aging). The production of carbonyl and hydroxyl groups in the aged material indicates that the process is oxidative, even in closed teeth. In these cases, the oxygen could be provided from tissue fluid. The degradation mechanism is complex and depends on many factors, besides time of root canal filling. The dental problem caused by the aging could be the production and migration of cytotoxic substances to periodontal ligament and the reduction on the canal sealing property due to the polymer weight loss. Both of them could contribute to the root canal treatment failure. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100:4082–4088, 2006     

Long‐term development of thermophysical and mechanical properties of cold‐curing structural adhesives due to post‐curing

The long‐term changes in the thermophysical and mechanical properties of a cold‐curing structural epoxy adhesive were investigated by accelerating the curing reaction by post‐curing at elevated temperatures. Experimental data concerning the glass transition temperature for periods of up to 7 years and tensile strength and stiffness measurements could be extrapolated for a period of up to 17 years. An existing model for the long‐term development of concrete properties was modified for the prediction of the long‐term mechanical properties of adhesives. The applicability of the acceleration procedure and the new model was confirmed by several verification procedures. Structural adhesives exhibit significant increases in glass transition temperature, strength and stiffness over the long term provided that joints are adequately sealed and protected from humidity and UV radiation. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

A study of localized curing of glass‐filled composites using microhardness measurements

Extent of cure of hybrid composite systems is examined by conducting hardness measurements at different stages of the photopolymerization reaction and obtaining kinetic parameters that matched the experimental data. The materials are commercial dental composites based on bis[4‐(2‐hydroxy‐3‐methacryloyloxypropoxy)phenyl]propane resins with different photoinitiator concentrations as well as filler particle sizes and combinations. Samples (five per group) were made using nylon molds (2.5 × 5 mm) of the tested composites. The samples were light cured with a constant‐power light source for durations up to 20 s. After curing, all samples underwent Vicker's hardness testing of top and bottom surfaces. While there are significant differences in the polymerization behavior between the top and bottom locations for the tested composites, the corresponding growth exponent n, a kinetic parameter in the kinetic theory, is very close in all cases. For the tested materials the coefficient factor k is much lower for the bottom surfaces compared with the top surfaces. This reduction in the value of k is more severe for the material with a higher concentration of the photoinitiator as well as a higher percentage of glass filler particles in the wavelength range affecting the photopolymerization. It is argued that a relationship between k and the irradiation intensity can be used to quantify the decay of irradiated light with its penetration into the composites. The comparisons can be used to draw preliminary conclusions on the parameters controlling the effective depth of cure in a hybrid composite. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 426–431, 2005     

Fracture toughness of silica particulate‐filled epoxy composite

The relationship between the postcuring conditions and fracture toughness on three silica particulate‐filled epoxy composites was investigated. The glass transition temperature, T_g, and the fragility parameter, m, derived from the thermo‐viscoelasticity, were used to characterize the composites, which were postcured under various conditions. The glass transition temperature and fragility both depended on both of the curing conditions and the volume fraction of silica particles. The glass transition temperature increased with the postcuring time and temperature, while the fragility generally decreased as the volume fraction increased. There was no direct correlation between the glass transition temperature and fragility. The fracture toughness depended on both the glass transition temperature and fragility. The composites with a high glass transition temperature and low fragility had high fracture toughness. These results indicate that the glass transition temperature and fragility are useful parameters for estimating the fracture toughness of the silica particulate‐filled epoxy composites. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2261–2265, 2002     

Comparison between microwave and thermal curing of glass fiber–epoxy composites: Effect of microwave‐heating cycle on mechanical properties

The objective of this study was to compare the mechanical properties between epoxy composites cured by thermal heating and microwave heating. Epoxy‐anhydride resins reinforced with glass fiber were cured in a domestic microwave oven and in a thermal oven. Hardening agents included methyl tetrahydrophthalic anhydride and methyl hexahydrophthalic anhydride. Microwave curing was carried out at various conditions, including 1‐, 2‐, and 3‐step heating cycle, whereby each cycle employed different power level and time. Mechanical properties were tested according to ASTM standards. It is found that the microwave‐cured composites produced mechanical properties as good as the thermally cured composites. The 2‐ and 3‐step heating cycle used in the microwave curing process produced better mechanical properties higher than those obtained from the microwaved 1‐step and thermally curing process. This is attributed to the slow increase in temperature during the beginning of the microwave curing process whereby the very low power level was applied in the first cycle of the multistep heating process. This affected the slower rate of viscosity increment, resulting in better wettability of the glass fiber with enhanced interfacial adhesion between the fibers and the resins. The viscosity of resins affected the homogeneity of the crosslinked structure. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1059–1070, 2006     

Single‐pack,self‐curable,aqueous‐based polyurethane dispersion containing a disperse dye

An organic‐solvent‐soluble disperse dye was introduced into an aqueous phase by a reaction with a self‐emulsified, aqueous‐based polyurethane (PU), and this resulted in the formation of a homogeneous, aqueous polymeric dye dispersion. This aqueous polymeric dye was stable in a water phase with excellent color extension upon application. It was formulated with a latent curing agent, polyaziridine (e.g., TMPTA‐AZ), as a single‐component, self‐curable, aqueous polymeric dye. The curing reaction took place between PU carboxylic acid and the latent curing agent upon drying. A self‐cured polymeric dye was obtained with excellent color extension and fastness and resistance to organic solvents and water after drying. This single‐component, self‐curable, aqueous‐based PU system containing a dye has potential for printing, writing, and dyeing applications. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 2006 © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3741–3747, 2006