Oxidative aging of thermoplastic polyimide films

Two linear polyimides were subjected to accelerated isothermal aging at temperatures between 300 and 400°C. Losses in toughness correlated well with changes in glass transition temperature (T_g) of the films, but not with weight loss. Both materials have extrapolated lifetimes of 100,000 h near their T_g's. © 1995 John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

A simple dilatometer for thermoset cure shrinkage and thermal expansion measurements

A simple capillary and bulb mercury dilatometer designed for specific volume measurements on thermoset resin systems during the curing reaction and as a function of temperature is described. The design, calibration, operation, data treatment, and error analysis are presented in detail, with data on the bisphenol A dicyanate resin system used as an example. Particular attention is directed at experimental difficulties such as monomer/prepolymer degassing, filling the dilatometer under vacuum, adhesive distortion of the curing resin on the dilatometer bulb, and the dilatometer bulb to capillary connection problem. © 1994 John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

Dynamic mechanical analysis of rubber toughened epoxy resins

Dynamic mechanical analysis (DMA) was used to characterize cured epoxy resin formulations from ?150°C to temperatures above their α transitions. The resins were aromatic amine and aliphatic amine cured and were modified with carboxylterminated acrylonitrile-butadiene (CTBN) rubbers to improve their toughness, A DuPont 981 dynamic mechanical analyzer was used to measure the modulus and mechanical loss factor (tan δ) of the samples. Changes in the α and β transitions in the scan of tan δ as a function of temperature were related to changes in the formulation. Relations were also sought between changes in the DMA data and fracture and impact toughness of the cured formulations obtained using an instrumented impact test. Impact tests were performed at ?196°C and at room temperature. Results indicate that fracture toughness and the dynamic mechanical properties are affected by the amount of rubber, the compatibility of the rubber and epoxy, and changes in the curing agent stoichiometry.     

Fracture behavior of coated/uncoated graphite-epoxy composites

The static delamination behavior of graphite/epoxy composite specimens subjected to mode I tensile opening (using UDCB

1 Uniform double cantilever beam.

specimens), and pure mode II shear loading (using ENF

2 End-notched flexural.

specimens) were studied. The graphite epoxy composites for the study were made from commercially treated fibers, with and without an electropolymerized interlayer. The mode I fracture energy (G_IC) was found to be significantly higher (more than 50 percent) for the coated fibers. However, this improvement was accompanied by a high reduction (more than 3 times) in the mode II fracture energy (G_IIC). This effect is apparently related to poor adhesion between the interlayer and the epoxy resin, which may be corrected by use of a “top layer” of appropriate composition to form chemical bonds between the phases. The fracture toughness (K_IC) of composites made with commercially treated fibers was also evaluated, using double side-notched specimens.     

Glass fiber-reinforced epoxy composites

Glass fiber-reinforced epoxy composites were prepared from the matrix resins tetraglycidyl diaminodiphenylmethane

1 Systematic name: N,N,N′,N′-Tetrakis(2,3-epoxypropyl)-4,4′-diaminodiphenylmethane.

(TGDDM) and tetraglycidyl bis(o-toluidino)-methane

2 Systematic name: N,N,N′,N′-Tetrakis(2,3-epoxypropyl)-4,4′-bis(o-toluidino)methane.

(TGMBT) using various amines like 4,4′-diaminodiphenylmethane (DDM), 4,4′-diaminodiphenylsulfone (DDS) and diethylene triamine (DETA) as curing agents. The fabricated laminates were evaluated for their mechanical and dielectrical properties and chemical resistance. The composites prepared using an epoxy fortifier (20 phr) showed significant improvement in the mechanical properties.     

Effect of crosslink structures on dynamic mechanical properties of natural rubber vulcanizates under different aging conditions

The effects of crosslink structures on the dynamic mechanical properties (DMPs) of unfilled and carbon black N330‐filled natural rubber (NR) vulcanizates cured with conventional (CV), semiefficient (SEV), and efficient (EV) cure systems and having about the same total crosslink densities were investigated before and after aerobic and anaerobic aging at 100°C. The three unfilled NR vulcanizates cured with the CV, SEV, and EV systems had about the same mechanical loss factor (tan δ) values at about 0°C but showed some apparent differences in the tan δ values in the order EV > SEV > CV at relatively high temperatures of 40–80°C before aging. However, N330‐filled NR vulcanizates gave higher tan δ values than the unfilled vulcanizates and showed little effect of the crosslink types on the tan δ at different temperatures over the glass‐transition temperature (T_g) before aging. Aerobic heat aging increased the T_g and tan δ values of the vulcanizates over a wide range of temperatures from ?80 to 90°C that was mainly due to the changes in the total density and types of crosslinks. The unfilled vulcanizates cured with the CV system showed the greatest change in DMP because of their poor resistance to heat aging. Aerobic heat aging of NR vulcanizates caused a more significant change in the DMP than anaerobic heat aging because of the dominant effect of the oxidative degradation during aerobic heat aging on the main‐chain structure, crosslink structures, and DMPs of the vulcanizates. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 710–718, 2001     

H01双基推进剂老化的动态力学性能表征

用动态热机械分析仪测定了65℃下双基固体推进剂H01的老化性能。在低温段（-10～40℃）,随老化时间的增加tanδ值有明显下降,β松弛峰也越来越明显。H01的α松弛的损耗角正切tanδ的峰温、动态柔量J＇和J＂的值以及动态模量E＇和E＂、动态柔量J＇和J＂主曲线的叠合垂直位移因子与老化时间存在一定规律。TG-DTG试验表明,随着老化时间的增加,试样在183℃时的质量损失逐渐减小,说明增塑剂含量随老化时间增加而减小。因此,除了因结构松弛造成的“物理老化”外,这是造成上述各力学损耗量随老化时间下降的又一主要原因。     

Influence of cure via network structure on mechanical properties of a free-radical polymerizing thermoset

An improved understanding has been achieved regarding the relationships among cure chemistry, network structure, and final physical properties of vinyl ester (VE) resins, a thermoset polymer often used as the matrix of fiber reinforced polymers. Mechanical properties of the polymer are found to depend on both cure schedule and cure formulation. The possibilities of phase separation and micro-gel formation being the cause for these differences in mechanical properties are examined. The VE/styrene (S) system does not phase separate under the conditions studied. Though bulk properties of the resin are unaffected by the details of the cure, the microscopic morphology, in particular the type of cross-link formed (intermolecular bond or intramolecular bond), is sensitive to both cure temperature and initiation mechanism as determined by cure formulation. An analysis of cure kinetics shows that both temperature and initiation mechanism affect the apparent ‘reaction order’ of the VE/S system as determined by the autocatalytic equation. This apparent reaction order is interpreted as being an indication of the degree of heterogeneity in the resin. By controlling cure temperature and cure formulation, it is possible to minimize the apparent reaction order and thereby optimize physical properties. Finally, a theory is adapted from other non-network polymer systems to qualitatively describe how cure temperature and initiation mechanism may alter the heterogeneity in network structure via micro-gel formation and how these changes in structure affect changes in the mechanical properties.     

Effects of water content on the properties of starch/poly(ethylene–vinyl alcohol) blends

The physical properties of unmodified starch, poly(ethylene vinyl alcohol), glycerol, and water mixtures are reported. Thermal and melt-flow properties of the preprocessed, physically mixed materials were determined along with the tensile properties and morphology of injection-molded microtensile samples. Melt-flow properties were measured by a capillary rheometer, and the water content was varied from 4 to 18%. The morphology, rheology, and tensile properties are all highly related to the percentage of water present. A transition in the tensile properties and morphology of the blends was observed at approximately 11% moisture content. © 1994 John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

Behavior of amine-modified urea–formaldehyde-bonded wood joints at low formaldehyde/urea molar ratios

Urea–formaldehyde-bonded wood products are limited to interior nonstructural applications because of their poor durability under cyclic moisture or humid environments. The stability of solid-wood joints and particleboards can be enhanced by bonding with urea–formaldehyde adhesives modified with di- and trifunctional amines at an effective resin formaldehyde-to-urea mol ratio (F/U) of 1.6; however, particleboard formaldehyde emissions were not improved over those from boards made with unmodified adhesives and were unacceptably high. The relative effectiveness of selected modifications was investigated at resin form aldehyde-to-urea (F/U) molar ratios of 1.4 and 1.2 Solid-wood joints and particleboards made with modified adhesives, an unmodified adhesive, and a phenol formaldehyde (PF) resin were subjected to cyclic soak-dry (cyclic stress) treatments and moist-heat aging. Formaldehyde emissions from particleboards were also determined. At F/U 1.4, the resistance of solid-wood joints made with modified adhesives to cyclic stress and moist-heat aging was equal to that of PF-bonded joints and superior to that of joints bonded with unmodified adhesive. The resistance of particleboards made with modified adhesives was greater than that of boards made with unmodified adhesive but less than that of PF-bonded board. Solid-wood joints and particleboards made with F/U 1.4 resins performed better than did those made with F/U 1.2 resins. Particleboards made with F/U 1.2 resins had formaldehyde emissions well below the standard, and room temperature aging or bonding at high temperature reduced emissions substantially. © 1994 John Wiley & Sons, Inc. 1

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

The effects of morphology and hygrothermal aging on water sorption and transport in Kapton® polyimide

A comparison of water sorption and diffusion behavior in 2 mil, as-received, Kapton

1 = Trademark of E.I. Dupont de Nemours Inc.

film, and in otherwise identical but hygrothermally aged samples, suggests chemical differences between the samples. These differences can be related to known polyimide chemistry and the specific history encountered by the samples. Although the sorption isotherms are similar for an as-received 0.3 mil sample studied earlier and for the as-received 2 mil samples studied here, the diffusion coefficient of water in the as-received 2 mil sample is approximately 300% larger than in the as-received 0.3 mil sample. This large effect is believed to be related to the presence of small, paracrystalline aggregates with large aspect ratios. Differences in orientation measured by birefringence for the two samples suggest that the barrier properties of Kapton^® are strongly affected by the detailed morphological organization of the ordered aggregates. Comparison of the sorption isotherms for the as-received and hygrothermally aged 2-mil films suggests a significant increase in the hydrophillic nature of the aged film. This change is consistent with an apparent chemical reaction between water and uncyclized amic acid residues in the film. The tendency for water to cluster in as-received Kapton^® is essentially eliminated by the chemical modification associated with hygrothermal aging. The hygrothermal aging produces a dramatic reduction in the water vapor diffusion coefficient at low vapor activities but a rather modest change at high activities.     

Effects of Accelerated Thermo-Oxidative Aging on Properties of a Damped-Encapsulating Epoxy Adhesive

The effects of accelerated thermo-oxidative aging treatment on structure, hardness, morphology of fractured surface, and dynamic mechanical properties of a damped-encapsulating epoxy adhesive were investigated. FT-IR indicated the formation of a small quantity of amide groups. The hardness increases rapidly at earlier stages and then tends to stabilize when the thermo-oxidative aging time is over 300 hours. SEM graphs indicated a change from some toughness to rigidity. DMA analyses revealed that the Tg of the aged samples gradually shifted to higher temperatures, the maximum value of tanδ fluctuated in a narrow range, the tanδ spectra were greatly temperature dependent, and the tanδ spectrum showed a tendency of horizontal displacement with increased thermo-oxidative aging time.     

In situ cure monitoring of epoxy resins using fiber-optic Raman spectroscopy

Fiber-optic Raman spectroscopy was used to monitor the curing of epoxy resins in situ for eventual application to polymer composite processing. A 200-μm diameter quartz fiberoptic sensor immersed in liquid resin was used to obtain Raman spectra for a concentration series of diglicidyl ether of bisphenol-A in its own reaction product with diethylamine using an 820 nm continuous-wave diode laser excitation. A Raman peak at 1240 cm^?1 was assigned to a vibrational mode of the oxirane (epoxide) ring and its normalized intensity was found to be linearly related to the concentration of epoxide groups in the resin mixtures. Raman peaks at 1112 and 1186 cm^?1 associated with phenyl and gem-dimethyl resin backbone vibrations, respectively, did not change in intensity due to the curing reaction and were used as internal references to correct the Raman spectra for intensity changes due to density fluctuations and instrumental variations during the experiments. Fiber-optic Raman spectroscopy was used to monitor the extent of reaction in situ for the room-temperature cure of phenyl glicidyl ether with diethylamine. The extent of reaction of the epoxide groups calculated from the Raman spectra were in excellent agreement with kinetic data from Fourier transform near-infrared absorbance measurements made under the same conditions. © 1994 John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

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.     

Toughening of AT-Resins with a Polyphenylquinoxaline

Acetylene terminated (AT) resins are addition-curable thermoset materials which do not generate volatiles during cure and therefore can be fabricated into void-free structures. They retain good thermal and mechanical properties even after exposure to high humidity environments. Their use as composite matrix resins and adhesives has shown promise. These resins, however, are brittle. Molecular structure modifications and blending with thermoplastic modifiers have been used to improve their toughness. In this work, improvement in toughness has been sought through the use of a polyphenylquinoxaline (PPQ) modifier. The blended systems showed improvements in toughness, thermooxidative stability, and lap shear strength over the original AT-resins.     

Solvent replacement in the synthesis of persubstituted phosphonitrilic hydroquinone prepolymer materials

A desirable solvent replacement has been effected in the synthesis of persubstituted phosphonitrilic hydroquinone prepolymer materials. The replacement of carbon tetrachloride with cyclohexane has no adverse effect on the composition and quality of the material prepared. The overall yield of the reaction increases from 65 to 70% and the toxicity of the reaction mixture and of the waste generated from the reaction are greatly reduced. The particularly close match of solvent properties between carbon tetrachloride and cyclohexane makes this an excellent solvent replacement. Such changes to less-expensive, less-toxic, and less-environmentally damaging solvents are on the increase. A brief list of some known replacement solvents for use in polymer and coatings applications is presented. © 1994 John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

Physical and mechanical properties of highly plasticized pectin/starch films

Blends of citrus pectin and high amylose starch plasticized with glycerine were investigated to determine the effect of compositional variables on film properties. Several films with representative compositions were made from sugar beet and almond pectin, and tested for comparison. The films were cast from water onto glass plates, dried, and removed. Mechanical analysis was done using a Rheometrics RSA II solids analyzer. Increasing the glycerine concentration led to decreases in static modulus, dynamic modulus, and tensile strength, but to increases in elongation. Increasing levels of starch in the blend lowered the effect of glycerine on mechanical properties. Oxygen permeability of the films was extremely low. Sugar-beet pectin and almond pectin gave films with mechanical properties comparable to those made with citrus pectin. © 1994 John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

Polyamine-modified urea–formaldehyde-bonded wood joints. III. Fracture toughness and cyclic stress and hydrolysis resistance

The objective of this study was to improve the durability and stability of urea–formaldehyde-bonded wood products by decreasing the internal stress developed during the resin cure and by improving the ability of the cured system to withstand cyclic stresses. Urea–formaldehyde resins were modified either by incorporating urea-capped di-and trifunctional amines into the resin structure or by using the hydrochloride derivatives of some of these amines as the curing agent, or by both methods. This study supplements our previous work by examining the effects of additional amines and subjecting bonded products to additional testing. Solid wood joints bonded with a variety (7 of 15) of modified adhesives had resistance to cyclic stress superior compared to that of joints bonded with unmodified urea–formaldehyde adhesive; at least three of the modified adhesives approached the behavior of phenol–formaldehyde-bonded joints. Resistance to moist heat aging, although still inferior to that of phenol–formaldehyde-bonded joints, was significantly improved for joints bonded with modified adhesives over joints made with unmodified resins. The fracture behavior of joints made with modified adhesives was different from that of joints made with unmodified resins. The fracture energy was greater for joints made with three of four modified adhesives than for joints made with unmodified resins. Modified adhesives produced particleboards made with enhanced cyclic stress resistance. Formaldehyde emission from particleboards made with resins modified with urea-terminated amines was less than emission from boards made with unmodified resins. However, emissions from particleboards made with amine hydrochlorides were not improved compared to those from boards made with an ammonium chloride curing agent. © 1993 John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.