Post-cure Temperature Research Articles

UV radiation curing of epoxidized palm oil–cycloaliphatic diepoxide system induced by cationic photoinitiators for surface coatings

Epoxidized palm oil (EPO) can be cured with ultraviolet (UV) radiation using either radical, cationic or hybrid initiation. Cationic curing system has been chosen in this study due to the fact that epoxy groups present in EPO can be utilized directly to form cross links. Curing was carried out in a 20 cm wide UV IST machine. Triaryl sulphonium salts and ferrocenium salt were used as cationic photo initiators. Formulations were developed based on selected grades of EPO. Other materials employed included different types and concentrations of photo initiators, monomers and photo-sensitizer. The effect of post-cure conditions was also studied. The properties of the cured film, namely, pendulum hardness, percentage of gel content and tensile strength were determined. It was found that triarylsulphonium hexafluorophosphate exhibited low solubility in EPO. Addition of a vinyl ether monomer to the formulation did not enhance pendulum hardness and gel content of the cured films. Post-cure temperature had no significant effect on the properties of the cured film. The FTIR studies showed evidence of the ring opening of the epoxide groups present in the formulation.

Characterisation of a glass-fibre reinforced vinylester to steel joint for use between a naval GRP superstructure and a steel hull

The use of polymer composites in naval applications is becoming more common, particularly due to the potential weight savings. The mechanical response of a prototype joint between a glass-fibre reinforced polymer superstructure and a steel hull formed via a resin infusion process was examined and subsequently modified to improve performance through a combined programme of modelling and mechanical testing. The GRP–steel interface was shown to be critical to the mechanical performance of the joint. The effects of post-cure temperature and surface preparation on the GRP–steel interface were evaluated through the use of the four-point bend delamination test. The results show interfacial toughness to increase greatly with surface roughness for low surface roughness up to a particular roughness above which no further increase is obtained. A similar increase is observed for low temperature post-cure but for higher temperatures a significant decrease is observed.

複合材料 ＣＦ／Ｅｐｏｘｙ複合材料の機械的特性に及ぼすポストキュアの効果

This paper presents the effect of post-cure on mechanical properties of CF/Epoxy composites. At first, the shrinkage strain and the mechanical properties such as Vickers hardness of resin, transverse modulus and strength are measured for carbon/epoxy unidirectional laminates after four-hour thermal treatment (post-cure process) at temperatures ranging from 70°C to 200°C. Then, the effect of post-cure temperature on transverse cracking behavior in a cross-ply laminate is experimentally investigated. The change in the residual stress due to post-cure is also estimated for the cross-ply laminate. Finally, the fracture surfaces of transverse cracks are observed using the scanning electron microcopy (SEM) to reveal the effect of post-cure on fracture behavior. It is found that the unidirectional laminate shrinks in only the transverse direction after the post-cure process. The hardness of the resin increases with increasing post-cure temperature below 150°C while transverse modulus and strength are independent of post-cure temperature below 150°C. Transverse cracking in post-cured cross-ply specimens initiates at lower stresses than the critical stresses in virgin specimens. The transverse crack density is predicted by a Weibull's probabilistic failure model. It is deduced from SEM observation of fracture surfaces that the fiber/resin interface is weakened by thermal treatment that may lead to reduction in the transverse strength at 200°C.

Effect of exposure intensity and post-cure temperature storage on hardness of contemporary photo-activated composites

Objectives: The effect of variation in post-exposure storage temperature (18 vs. 37°C) and light intensity (200 vs. 500 mW/cm 2) on micro-hardness of seven light-activated resin composite materials, cured with a Prismetics Mk II (Dentsply) light activation unit, were studied. Methods: Hardness values at the upper and lower surfaces of 2 mm thick disc shaped specimens of seven light-cured resin composite materials (Herculite XRV and Prodigy/Kerr, Z100 and Silux Plus/3M, TPH/Dentsply, Pertac-Hybrid/Espe, and Charisma/Kulzer), which had been stored dry, were determined 24 h after irradiation with a Prismetics Mk II (Dentsply) light activation unit. Results: Hardness values varied with product, surface, storage temperature, and curing light intensity. In no case did the hardness at the lower surface equal that of the upper surface, and the combination of 500 mW/cm 2 intensity and 37°C storage produced the best hardness results at the lower surface. Conclusions: Material composition had a significant influence on surface hardness. Only one of the seven products (TPH) produced a mean hardness values at the lower surface >80% of the maximum mean upper surface hardness obtained for the corresponding product at 500 mW/cm 2 intensity/37°C storage temperature when subjected to all four test regimes. Despite optimum post-cure storage conditions, 200 mW/cm 2 intensity curing for 40 s will not produce acceptable hardness at the lower surface of 2 mm increments of the majority of products tested.

Increased fracture toughness in nanoporous silica–polyimide matrix composites

Silica–polyimide nanocomposites were prepared by hot-pressing mixtures of polyimide and highly porous silica powder. The silica powder was produced using a sol-gel process that generates pores as small as 15 Å. The effects of loading, cure, and post-cure temperature on fracture toughness were investigated. The addition of silica particles improved the fracture toughness from 0.5 to a maximum of 1.9 MPa m0.5. However, fracture toughness dropped at silica weight percentages ≥30%. The cure and post-curing temperatures have a strong influence on toughness; post-curing exposure ≥400 °C reduced toughness. Transmission electron microscopy examination of the fracture surfaces indicated that the toughness improvements may occur at the nanometer scale due to crack pinning and branching induced by the nanoporous silica particles.

The effect of post-cure duration on the mode I interlaminar fracture toughness of glass-fibre reinforced vinylester

Abstract The effect of different post-cure conditions on the mode I fracture toughness of a vinylester resin and its glass-fibre reinforced composite counterpart has been studied. Two sets of parameters were investigated. The first was the post-cure duration at constant temperature; 90°C for 1, 4 and 24 h. The second, for resin toughness only, was a combination of post-cure temperature and duration; 90°C/4 h, 80°C/8 h and 70°C/16 h. The results show that the post-cure increases toughness. This trend is consistent between the pure resin and the fibre composite for all treatments, except at 90°C/24 h. It is believed that the prolonged post-cure duration of 24 h has weakened the bond strength at the fibre–matrix interface, thus reducing the effectiveness of toughness transfer from the matrix to the composite. The study concludes that the post-cure enhances the toughness of the glass-fibre/vinylester composite, mainly due to the increase of resin toughness.

NMR characterization of postcure temperature effects on the microstructures of Blackglas™ resin and ceramic

Blackglas™ is a novel siliconoxycarbide ceramic material with high levels of carbon incorporated in the atomic structure. This material has potential in the fabrication of low cost ceramic matrix composites with high temperature capability. Here we study the relationship between the microstructure of Blackglas™ resins and ceramics and the postcure temperature using solid-state NMR to determine explicitly the microstructure evolution from resin to ceramic. In this study, 29 Si solid-state NMR reveals a dependence between the postcure temperature and the microstructure of the resin, which then effects the resulting ceramic structure. The postcure temperature that appears to give optimal mechanical and oxidative properties of Blackglas™ ceramic is around 150°C. For a resin postcured at 250°C, the incorporation of oxygen into the resin network, and thermal redistribution reactions between Si–H and Si–O bonds, results in a dramatic and detrimental change of microstructure compared to that of resins postcured at 100°C and 150°C.

Thermal properties of phthalonitrile functional polybenzoxazines

Phthalonitrile functional polybenzoxazines show high thermal stability with temperature at 5% weight loss ( T 5%) up to 450°C for monofunctional benzoxazine-based polymers, and up to 550°C for polymers from the bifunctional precursors. High char yield up to 80% by weight is obtained by thermogravimetric analysis in an inert atmosphere at 800°C. Char yield up to 70% by weight at 600°C is achieved in air with T 5% in the 380–420°C range. Differential scanning calorimetry (DSC) is used to determine curing conditions. These resins do not require ultrahigh curing temperature as it is typical for the other phthalonitrile functional prepolymers and 250°C is used as a maximum post-cure temperature to achieve desired properties. Dynamic mechanical analysis (DMA) reveals unusually broad dependence of loss modulus vs. temperature with the maximum at 300°C for polybenzoxazines from bifunctional precursors and 278°C for polymer from monofunctional precursors. Additionally, these materials exhibit low flammability.

The effect of post-cure heating on residual, unreacted monomer in a commercial resin composite

Objectives. This paper examined the influence of post-cure temperature on the amount of unreacted monomer remaining in a commercial light-cured resin composite restoration following initial light-curing and subsequent post-cure heating. Methods. Discs of composite were light-cured and then subjected to immediate post-cure heating (50, 75, 100, or 125°C for 7 min) or were left unheated (control). They were then placed in a solvent for two weeks at 37°C to extract the unreacted monomer. HPLC analysis was used to determine the amounts of TEGDMA, BIS-GMA, and ethoxylated BIS-GMA remaining after the different treatments. The amounts of each monomer leaching were compared using ANOVA with respect to the different curing treatments. Results. Even the lowest post-cure heat treatment (50°C) resulted in 80% reduction in remaining, unreacted TEGDMA, 75% reduction in BIS-GMA, and 77% lower ethoxylated BIS-GMA than the light-cured only control. Post-cure heating at 75°C and above resulted in the lowest amount of each type monomer remaining uncured in the polymer and did not significantly decrease with an increase in post-cure temperature for the most part. Significance. One of the main benefits of post-cure heating of resin composite restorations could be the enhancement of biocompatibility of these restorations as a result of the significant decrease in potentially leachable, unreacted monomer.

The influence of post-cure on properties of carbon/phenolic resin cured composites and their final carbon/carbon composites

Two-dimensional (2D) carbon/carbon (C/C) composites were prepared with phenol-formaldehyde resin and graphite fabric. After curing, polymer composites were post-cured in air at 160°C and 230°C for several hours and then all polymer composites were carbonized up to 1500°C. The effect of post-cure on the microstructure and fracture behavior of the resultant carbon/carbon composites was studied. The post-cure process was characterized by weight loss. This process promoted the crosslinking and condensation reactions and led to the formation of long-chain, cross-linked polymeric structures in the matrix. The post-cured composites had a greater density than the unpost-cured composite. This study indicates that a longer post-curing time and higher post-curing temperature would limit the shrinkage for the post-cured composites during carbonization. The improvement in linear shrinkage was 22% to 44%. This process also limited the formation of open pores and decreased the weight loss of the resultant C/C composites. The resultant C/C composites developed from post-cured composites had a greater flexural strength by 7 to 26% over that developed from unpost-cured composite.

Dynamic mechanical and thermal characterization of high-performance polybenzoxazines

Two polybenzoxazines are cured in an autoclave from the polyfunctional benzoxazine monomers, 8,8′-bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine) and 6,6′-bis(2,3-dihydro-3-phenyl-4H-1,3-benzoxazinyl) ketone. The density and tensile properties of these polybenzoxazines are measured at room temperature. Dynamic mechanical tests are performed to determine the Tg, crosslink density, and the activation enthalpy of the glass-transition process for these two polybenzoxazines. The effect of postcure temperature on the Tg of the polymers is investigated and discussed in terms of crosslink density. Fourier transform infrared (FTIR) spectroscopy is also applied for the molecular characterization of the curing systems. Thermal properties of these polybenzoxazines are studied in terms of isothermal aging and decomposition temperature via thermogravimetric analysis. These two polybenzoxazines show mechanical and thermal properties similar to or better than bismaleimides and some polyimides. They also show very high char yield after being carbonized in a nitrogen atmosphere. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3257–3268, 1999

High temperature thermal analysis characterization of honeycomb core

One of the most important processing operations in the fabrication of honeycomb composite structures is the forming of the honeycomb core. High performance honeycomb structures must be precisely formed, or contoured, in order to meet the strict dimensional specifications required by the end user of the core. However, in the composites industry today, there is no basic understanding of the high temperature behavior of honeycomb materials, which is fundamental to understanding the commercial processing of honeycomb composite structures. In this study, the thermal and mechanical responses of Nomex®-based honeycomb core were investigated and characterized through thermal analysis. In the ribbon direction of the core, specific post-cure temperatures were identified where the honeycomb core displayed a maximum degree of creep. In the expanded direction of the honeycomb, the core displayed no maximum-creep temperature. This behavior was attributed to the degradation of the node adhesive. A honeycomb creep time constant (τc) was also defined and evaluated as a function of the honeycomb post-cure temperature. From this analysis, the softening of the dip resin was determined to play a larger role in creeping and deforming the core than the mechanical properties of the Nomex substrate.

初期高温履歴を受けたコンクリートの長期強度発現性

The long term strength development of concrete cured under high temperature conditions in an early age was studied. High temperature conditions at curing were planned with systematic varying of the temperature-time factors in the experiment. The strength development of concrete was found to be affected by maximum temperature, pre-curing time, post-curing temperature and heating rate. The time dependence of the effect of curing temperature on the strength development of concrete was indicated. The strength of concrete at a long age strongly depends on the curing temperature at the time of 0 - 0.5 day since concrete is mixed up.

The development of glass fiber reinforced polystyrene for pultrusion. II: Effect of processing parameters for optimizing the process

AbstractThis paper presents a novel process developed to manufacture polystyrene (PS) pultruded composites. The effects of the processing parameters on the mechanical properties (tensile strength, flexural strength, flexural modulus, etc.) of the glass fiber reinforced PS composites by pultrusion have been studied. The processing parameters investigated include die temperature, pulling rate (in‐line speed), postcure temperature and time, filler type and content, and glass fiber content. The optimum die temperature was determined from DSC diagrams, molecular weight, and mechanical properties tests. It was found that the mechanical properties increased with increasing die temperature and glass fiber content, and with decreasing pulling rate. The optimum die temperature, pulling rate, and glass fiber content were determined to be 160–200°C, 20–60 cm/min and 50–60 vol%, respectively. The mechanical properties reached a maximum value at 10 phr kaolin, and then decreased. However, the properties decreased with increasing tale content. The mechanical properties increase at a suitable postcure temperature and time. Furthermore, the properties that decreased as a result of composite degradation after long postcure time are discussed.

Reaction-induced phase separation mechanisms in modified thermosets

The reaction-induced phase separation in amorphous thermoplastic—modified epoxy systems was observed in situ using methods of different observation windows: small angle X-ray scattering (SAXS), light transmission (LT) and light scattering (LS). The transmission electron microscopy (TEM) technique was concurrently used to get direct representations of morphologies at different levels of the phase separation process. The selected systems were bisphenol-A diglycidylether cured with either 4,4′-diaminodiphenylsulfone or 4,4′-methylenebis[3-chloro,2,6-diethylaniline] in the presence of polyetherimide. The phase separation mechanisms involved were found to be greatly dependent on the initial modifier concentration and on the ratio of the phase separation rate with respect to the polymerization rate. Experimental results showed that, for modifier concentrations close to the critical fraction, the system was directly thrown into the unstable region, even at a low polymerization rate, and phase separation proceeded by spinodal demixing. On the other hand, for off-critical compositions the homogeneous solution demixed slowly via the nucleation and growth mechanism. In spite of the evolution of the phase diagram with reaction extent, the system remains in the metastable state whatever the cure temperature. The cure temperature has a strong effect on the extent of phase separation, since sooner or later vitrification of the thermoplastic-rich phase occurs and stops the evolution of morphologies. A post-cure allows the phase separation process to go further and sub-particles can be generated depending on the precure and post-cure temperatures.

Mass loss in urethane/TEGDMA- and Bis-GMA/TEGDMA-based resin composites during post-cure heating

Objectives This research examined weight loss of commercial UDMA/TEGDMA- and Bis-GMA/TEGDMA-based resin composites at a variety of post-cure temperatures. Weight loss profiles of individual monomer components were also tested at elevated temperature: Bis-GMA, ethoxylated Bis-GMA (EBis-GMA), urethane dimethacrylate (UDMA), and triethyleneglycol dimethacrylate (TEGDMA). Methods Disc-shaped composite specimens (1 × 5 mm, approximately 50 mg) were light-cured and then isothermally post-cure heated in a thermogravimetric analysis (TGA) unit at either 50°, 75°, 100°, 125° or 150°C. A single specimen was made for each post-cure temperature for each product (a total of 10 discs). Individual monomer components were heated to 800°C. Filler and organic phase weight percentages were determined by ashing cured composite in the TGA. Weight loss differences between resin systems at various post-cure temperatures were analyzed using linear regression. Results For each type of composite, loss of volatile component increased with both elevated post-cure temperature as well as duration of heat exposure. Using recommended post-cure temperature and time (125°C for 7.5 minutes), there was no difference in weight loss profile between the two products: both exhibited 1.3% loss of resin component. After 10 min of heating, the Bis-GMA-based product always demonstrated a greater weight loss than the UDMA material. Weight loss could not be attributed to any specific monomer. Significance Specimen weight loss during post-cure heating may result in a depletion of leachable, unreacted material at the restoration surface, possibly enhancing material properties at that location. This decrease would also potentially reduce the biological impact of leachable materials. Loss of volatile components from post-cure heating would affect the accuracy of infrared spectroscopic techniques in determining monomer conversion values.

Effect of post-cure temperature and heat duration on monomer conversion of photo-activated dental resin composite

Objectives This research evaluated the relationship between post-cure heating temperature and duration on the monomer conversion of a commercial light-activated resin composite. Methods Disc-shaped composite specimens (10 × 1 mm) were initially light-cured and then submitted to one of the following post-cure conditions: none (control), 50°,75°, 100° or 125°C for 0.5,1, 3, 5, or 7 min. After curing, specimens were stored in the dark at room temperature. Monomer conversion of all specimens was then determined using infrared spectroscopy. Results Post-cure temperature was 12 times more influential than was heat duration on the extent of resin cure. The extent of cure increased linearly with an increase in post-cure temperature. Post-cure specimens that were heated between 3 and 7 min demonstrated equivalent cure values. All post-cure heating conditions resulted in significantly higher cure values than the light-cured only control. Significance Physical properties of post-cure heated composites may change with respect to aging and water, but monomer conversion values of post-cure heated materials remain unaltered. Results of this study validate the use of relatively high temperatures for increasing the extent of monomer conversion. Such increases may lead to more biocompatible restorations.

Effects on Dielectric Properties of Modified Bismaleimide Resins

Six modified bismaleimide (BMI) resin systems are developed. The modifier is diallyl bisphenol A, diallyl bisphenol A ether. TDE-85 epoxy/MNA anhydride, and styrene. respectively. In view of structure of those cured resins, relationships between structure and dielectric properties have been studied emphatically, while effects of postcure temperature and catalysts on dielectric properties were also shown. Results indicate that dielectric properties lie on structure of cured resin and postcure temperature; catalysts can strikingly improve the heat-resistance of materials, but has little effect on the dielectric properties. In addition, mechanical and thermal properties of neat resins were also shown.

A high-performance bismaleimide resin with good processing characteristics

A new bismaleimide (BMI) resin system, designated 4504, with excellent heat resistance and good mechanical properties for advanced composites was developed. The 4504 resin was made up of 4,4′-bismaleimidodiphenyl methane, diallyl bisphenol A, and desirable catalysts. The reactivity of 4504 was investigated by gel characteristics and differential scanning calorimetry (DSC). Data showed that 4504 had a long work life under 100°C, but would gel within 7 min or 40 s at 140 or 160°C, respectively. The glass transition temperature (Tg) and heat-deflection temperature (HDT) of the cured 4504 resin were 315 and 290°C, respectively, which were much higher than the postcure temperature (200°C). In addition, the cured resin is also tough. Thermogravimetric analysis (TGA) in a nitrogen atmosphere revealed that the neat resin was stable up to 450°C; its char yield at 700°C under anaerobic conditions was 29.4%. Carbon fiber T300 laminates based on 4504 were prepared and characterized. In the case of short-beam (SBS) strength, when tested at 230°C, 51% of the original room temperature strength was retained. © 1996 John Wiley & Sons, Inc.

Synthesis and characterization of polyfunctional naphthoxazines and related polymers

In pursuing the new polymers that could provide high mechanical properties and good thermal stabilities, a series of naphthoxazines are synthesized from different hydroxynaphthalenes with aniline and formaldehyde. The molecular structures are confirmed by NMR spectroscopy. After being polymerized in an autoclave, the naphthoxazine derived from 1,5-dihydroxynaphthalene is successfully cured to form the void-free resin. The density and tensile properties of these polynaphthoxazines are measured. Dynamic mechanical tests are performed to determine the Tg, crosslink density, and the activation enthalpy of the glass transition process for the polynaphthoxazines postcured in air at different temperatures. The effect of postcure temperature on the Tgs of the polynaphthzoxazines is investigated and discussed in terms of crosslink density. The polynaphthoxazine shows a Tg higher than the cure temperature. Fourier transform IR spectroscopy is applied for the molecular characterization of the curing systems. Thermal properties of these polynaphthzoxazines are studied in terms of the weight loss after isothermal aging in static air, the decomposition temperature from thermogravimetric analysis, and the change of dynamic storage moduli at high temperatures. © 1996 John Wiley & Sons, Inc.