Mechanical properties of parts manufactured from prestaged thermosetting towpreg

A prestaging process consisting of passing thermoset towpreg through a tunnel oven and roller apparatus was developed to address the difficulties inherent in making thick-walled thermosetting composite parts: aging, moisture uptake, and low resin viscosity during lay-up; and residual stresses, warpage, thermal degradation, and excessive void nucleation during the final cure. Unidirectional laminates were fabricated from prestaged and as-received AS4/3501-6 towpregs to understand the relationship between the parts' mechanical properties and the amount of prestaging (degree of cure) of the towpregs. In this manner, the feasibility of manufacturing with prestaged thermosetting towpreg was confirmed. Ultrasonic nondestructive evaluation showed that the parts manufactured from prestaged towpreg were uniform and relatively low in voids as compared to parts manufactured with as-received towpreg. This was confirmed with scanning electron microscopy. Using three-point bending tests, a drop in mechanical properties was found in parts manufactured from towpreg prestaged beyond 19% degree of cure. However, the parts were well consolidated and had predictable mechanical properties up to approximately 40% degree of cure. The equivalent limits on degree of cure from a companion processing study are between 15% and 22%.     

Material systems for rapid manufacture of composite structures

A manufacturing process is described that builds complex composite parts using a layered building process in which each layer of pre‐preg composite is laid and cured as the build progresses. In order to employ on‐line curing without molds, resin technologies that provide fast curing at room temperature—ultraviolet curable and epoxy/polyamide—were investigated. UV‐curable resins were tested for their ability to “shadow” cure by exposing carbon fiber composites to ultraviolet light to determine if the cure propagated from areas directly exposed to areas under fibers. Though ultraviolet curing showed advantages in cure time and low volatile production, very minimal “shadow” curing was achieved. A low temperature curing epoxy/polyamide mixture was tested for the effects of cure temperature, cure time, and mix ratio on the final degree of cure (%DOC) and glass transition temperature (T_g). Layers were made using different resin mixtures, partially cured, and used to build layered parts to determine curing characteristics during the lay‐up process. In the epoxy/polyamide mixtures, mix ratio had little effect on the reaction rate but did affect the T_g. A kinetic model was established for the resin epoxy/polyamide system for optimizing processing conditions during fabrication. However, the model failed to correctly predict the fabrication. The reaction of the material was different during the fabrication process than during the isothermal cure due to the presence of oxygen. During the build process, the degree of cure in each layer increased significantly over the prestaged degree of cure in less time than theoretically predicted. However, the final resin properties, such as T_g, were still below the specifications for high performance parts.     

On-line consolidation of thermoplastic towpreg composites in filament winding

A study of the use of thermoplastic powder coated towpregs in filament winding was conducted. A suitable technique for manufacturing parts with adequate on-line consolidation was identified through an experimental investigation that compared several techniques. This process utilized a single compaction roller on-line consolidation head with hot air guns to obtain consolidation at the lay-down point on the mandrel. The addition of a heated pultrusion die between the creel system and the filament winder was found to produce the best process. Nylon 11/E-glass towpreg was used in this study. The effect of winding speed and compaction force on the quality of the resulting parts was determined in order to optimize the process. Only the speed was found to have a significant effect, with better quality parts being made at higher speed. Observations were also made on parts manufactured with polypropylene and E-glass towpreg, as well as on parts wound with helical patterns.     

Processing parameters for filament winding thick-section PEEK/carbon fiber composites

The consolidation pressure and winding speed for thermoplastic filament winding were studied. Thermoplastic composite parts were manufactured from tape prepreg (APC-2); powder-coated, semi-consolidated towpreg; and commingled fiber towpreg. The material used was carbon fiber (AS-4) (60 vol%) in a PEEK matrix. The parts made were open-ended cylinders of the three materials, 177.8-mm ID, 228.6 mm long, 17 plies thick with a 0° lay-up angle; and rings, 50 plies of APC-2 thick, 6.35 mm wide (one strip wide), 177.8-mm ID, and a lay-up of 0°. Their quality was determined by surface finish and void percentage. The tubes made from APC-2 appeared to have the best quality of the three prepregs. For the rings, the speed of lay-down had a significant effect (at a 99% confidence level) on both the final width of the parts and on the percentage of voids. The pressure of the roller had a significant effect on the final widths at a 99% confidence level, but a significant effect on the percentage of voids at only a 95% confidence level.     

A process for prestaging thermosetting towpreg

A method for prestaging thermosetting towpreg consisting of passing the material through a tunnel oven was developed to address the difficulties inherent in making thick-walled thermosetting composite parts: aging, moisture uptake, and low resin viscosity during lay-up; and residual stresses, warping, thermal degradation, resin gradients, and excessive void nucleation during the final cure. AS-4 carbon fiber in a 3501-6 epoxy matrix was used to test the continuous prestaging process. With prestaging, significant reductions in towpreg bulk, voids, and total enthalpy were achieved. Cure shrinkage also was achieved. The final towpreg product had good drape and greatly reduced tack. These changes addressed many of the aforementioned difficulties. A relation between residence time at 200°C (the process temperature) versus percent degree of cure (%DOC) was developed. Manufacturing characteristics such as drape, tack, and bulk were related to %DOC. A processing window of 15 to 22 %DOC yielded towpreg with optimal characteristics. Prestaged towpreg would be well suited to manufacturing with advanced fiber placement (AFP) type equipment. Initial trials in industry have been promising.     

Investigation of curing characteristics of carbon fiber/epoxy composites cured with low‐energy electron beam

To solve the penetration depth of carbon fiber/epoxy prepreg and irradiation dose uniformity by low‐energy E‐Beam under 125 keV, the both‐side irradiation curing of prepreg was investigated. The results show that there is little thermal effect during the low‐energy electron beam irradiation curing process, even though the irradiation dosage reached 300 kGy, only 46.2°C can be tested on the prepreg surface. Due to the low curing temperature, the degree of cure of prepreg was only 61.8% at 300 kGy level of irradiation, and the glass‐transition temperature (T_g) was only 48.6°C. The degree of cure and T_g can be increased sharply by thermal postcure. After being postcured at 160°C for 30 min, the degree of cure and the T_g of prepreg reached 98.5% and 170.4°C, respectively. Interlaminar shear strength testing result indicate that the fabrication process of the composite layer by layer curing by the low‐ energy E‐Beam is a promising cure approach. POLYM. COMPOS., 36:1731–1737, 2015. © 2014 Society of Plastics Engineers     

Monitoring the reaction progress of a high‐performance phenylethynyl‐terminated poly(etherlmide). Part II: Advancement of glass transition temperature

The cure schedule for carbon fiber‐reinforced, phenylethynyl‐terminated Ultem™ (GE Plastics) composites was studied in an attempt to optimize the resultant glass transition temperature, T_g. Reaction progress and possible matrix degradation were monitored via the T_g. On the basis of previous research, matrix degradation induced T_g reduction was expected for increases in cure time or temperature beyond approximately 70 minutes at 350°C. Using the central composite design (CCD) of experiment technique, composite panels, neat resin, and polymer powder‐coated tow (towpreg) were cured following various cure schedules to allow for the measurement of the glass transition temperatures resulting fronm cure time and temperature variations. The towpreg and neat resin specimens were cured in a differential scanning calorimeter. The glass transition temperatures of all specimens were measured via differential scanning calorimetry; the composite glass transition temperatures were also measured with dynamic mechanical thermal analysis. The composite panels and towpreg specimens showed similar trends in T_g response to cure schedule variations. Composite and towpreg glass transition temperatures increased to a plateau with increasing cure time and temperature, whereas, the neat resin showed an optimal T_g followed by T_g reduction with increasing cure time and temperature. The optimal neat resin T_g occurred within a cure time and temperature significantly below that required to maximize the composite and towpreg glass transition temperatures.     

L4．00R20越野子午线轮胎的硫化测温

介绍14．00R20越野子午线轮胎的硫化测温过程,绘制测温曲线,利用胶料的活化能和测温数据计算各部位胶料的硫化程度。结果表明：轮胎硫化条件和各部位硫化程度较理想,胎侧中部的硫化程度最大,达到400％以上;胎冠肩部的硫化程度最小,不到100％。     

Curing kinetics and viscosity change of a two‐part epoxy resin during mold filling in resin‐transfer molding process

The curing kinetics and the resulting viscosity change of a two‐part epoxy/amine resin during the mold‐filling process of resin‐transfer molding (RTM) of composites was investigated. The curing kinetics of the epoxy/amine resin was analyzed in both the dynamic and the isothermal modes with differential scanning calorimetry (DSC). The dynamic viscosity of the resin at the same temperature as in the mold‐filling process was measured. The curing kinetics of the resin was described by a modified Kamal kinetic model, accounting for the autocatalytic and the diffusion‐control effect. An empirical model correlated the resin viscosity with temperature and the degree of cure was obtained. Predictions of the rate of reaction and the resulting viscosity change by the modified Kamal model and by the empirical model agreed well with the experimental data, respectively, over the temperature range 50–80°C and up to the degree of cure α = 0.4, which are suitable for the mold‐filling stage in the RTM process. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2139–2148, 2000     

Investigation of the curing behavior of a novel epoxy photo‐dielectric dry film (ViaLux™ 81) for high density interconnect applications

The objective of this work was to determine the cure kinetics of ViaLux™ 81 photo‐dielectric dry film and to optimize its curing schedule for the fabrication of sequentially built up high density interconnect‐printed wiring boards. Photosensitive epoxy materials such as the photo‐dielectric dry film studied herein have complicated curing regimes. This is attributed to the long lifetime of the curing catalyst that is generated by ultraviolet exposure. Dynamic differential scanning calorimetry (DSC) experiments revealed a two‐peak curing mechanism, which could not be separated at lower heating rates. The activation energies for the two cure events, calculated using the Kissinger method, were found to be 129 and 124 kJ/mol, respectively. A cure‐dependent activation energy was also determined using the isoconversional method, and a “model‐free” approach was adopted to simulate the evolution of degree‐of‐cure under dynamic and isothermal conditions. The results suggest that cure cycles of approximately 15 min at temperatures above 165°C can result in a degree‐of‐cure of 90% and above. This implies that faster fabrication is possible with either rapid thermal curing equipment or continuous cure surface mount technology furnaces. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 430–437, 2000     

Equivalent processing time analysis of glass transition development in epoxy/carbon fiber composite systems

The cure kinetics and glass transition development of a commercially available epoxy/carbon fiber prepreg system, DMS 2224 (Hexel F584), was investigated by isothermal and dynamic‐heating experiments. The curing kinetics of the model prepreg system exhibited a limited degree of cure as a function of isothermal curing temperatures seemingly due to the rate‐determining diffusion of growing polymer chains. Incorporating the obtained maximum degree of cure to the kinetic model development, the developed kinetic equation accurately described both isothermal and dynamic‐heating behavior of the model prepreg system. The glass transition temperature was also described by a modified DiBeneditto equation as a function of degree of cure. Finally, the equivalent processing time (EPT) was used to investigate the development of glass transition temperature for various curing conditions envisioning the internal stress buildup during curing and cooling stages of epoxy‐based composite processing. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 144–154, 2002; DOI 10.1002/app.10282     

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.     

Selected aspects of epoxy adhesive compositions curing process

The paper focuses on selected parameters of curing process – temperature and time. The tests aimed at evaluating the impact of short-term thermal recuring on 1050A and 2017A aluminium alloy sheet adhesive joints strength. Joints were formed with two different adhesives, the main component of which was in both cases epoxy resin Epidian 53 and two different cure agents – poliamineamide C (PAC) and triethylenetetraamine (PF) curing agents. Curing conditions – first curing time, recuring time and recuring temperature – were modified for each of the four tests conducted. For the sake of comparative analysis, adhesive joints were subjected to a single-stage cure cycle at ambient temperature. A two-stage cure cycle of both Epidian 53 compositions at 80?°C for 1 and 2?h produces a material of different mechanical properties than the same material which submits a single-stage cure cycle at ambient temperature, as well as at 60?°C for 30?min. Simultaneously, Epidian 53/PF/100:50 composition proves to produce higher joint strength after recuring than Epidian 53/PAC/100:80; the strength of a joint formed with the former composition increases up to 50% when compared with joints subjected to a single-stage cure cycle. Moreover, tests show that recuring of the adhesive joint formed with both compositions at 60?°C for 30?min does not have a considerable influence on either 1050A or 2017A aluminium adhesive joint strength.     

Comparison of the curing kinetics of the RTM6 epoxy resin system using differential scanning calorimetry and a microwave‐heated calorimeter

The cure of a commercial epoxy resin system, RTM6, was investigated using a conventional differential scanning calorimeter and a microwave‐heated calorimeter. Two curing methods, dynamic and isothermal, were carried out and the degree of cure and the reaction rates were compared. Several kinetics models ranging from a simple nth order model to more complicated models comprising nth order and autocatalytic kinetics models were used to describe the curing processes. The results showed that the resin cured isothermally showed similar cure times and final degree of cure using both conventional and microwave heating methods, suggesting similar curing mechanisms using both heating methods. The dynamic curing data were, however, different using two heating methods, possibly suggesting different curing mechanisms. Near‐infrared spectroscopy showed that in the dynamic curing of RTM6 using microwave heating, the epoxy‐amine reaction proceeded more rapidly than did the epoxy‐hydroxyl reaction. This was not the case during conventional curing of this resin. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3658–3668, 2006     

Cationic electron‐beam curing of a high‐functionality epoxy: effect of post‐curing on glass transition and conversion

This paper reports on the cationic electron‐beam curing of a high‐functionality SU8 epoxy resin, which is extensively used as a UV‐curing negative photoresist for micro‐electronics machine systems (MEMS) applications. Results show that elevated post‐curing treatment significantly increased both the conversion and the glass transition. The degree of conversion and the glass transition temperature were measured by using Fourier‐transform infrared (FTIR) spectroscopy and modulated differential scanning calorimetry (MDSC^®), respectively. The glass transition temperature (T_g), which has been observed to be dependent on the degree of conversion, reaches a maximum of 162 °C at 50 Mrad and post‐curing at 90 °C. The degradation pattern of the cured resin does not show much variation for exposure at 5 Mrad, but does show significant variation for 50 Mrad exposure at various post‐curing temperatures. A degree of conversion of more than 0.8 was achieved at a dosage of 30 Mrad with post curing at 80 °C, for the epoxy resin with an average functionality of 8 a feature simply not achievable when using UV‐curing. Copyright © 2004 Society of Chemical Industry     

Curing kinetics of medium reactive unsaturated polyester resin used for liquid composite molding process

The cure kinetics of medium reactivity unsaturated polyester resin formulated for Liquid Composite Molding process simulation was studied by Differential Scanning Calorimetry (DSC) under isothermal conditions over a specific range of temperature. For isothermal curing reactions performed at 100, 110, and 120°C, several influencing factors were evaluated using the heat evolution behavior of curing process. We propose two‐ and three‐parameter kinetic models to describe the cure kinetics of thermoset resins. Comparisons of the model solutions with our experimental data showed that the three‐parameter model was the lowest parameter model capable of capturing both the degree of cure and the curing rate qualitatively and quantitatively. The model parameters were evaluated by a non‐linear multiple regression method and the temperature dependence of the kinetic rate constants thus obtained has been determined by fitting to the Arrhenius equation. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Comparison of the curing kinetic behavior for two epoxy resin systems containing EPIKOTE 828–EPIKURE 3090 and DURATEK KLM 606A–DURATEK KLM 606B

The aim of the study is to determine the optimum cure temperatures and kinetics for two different epoxy resin systems without using solvent. Two resin systems consist of EPIKOTE 828® epoxy resin–EPIKURE® 3090 polyamidoamine curing agent and DURATEK® KLM 606A epoxy resin–DURATEK® KLM 606B polyamide curing agent. The ratio of resin to curing agent was kept as 1:1 for both the systems. Curing temperatures of both the systems were determined and kinetic parameters were calculated with respect to the experimental results following nth‐order kinetics. Then, a series of isothermal temperatures was applied to the resin systems in order to assess the cure process in terms of conversion, time, and temperature by using differential scanning calorimeter (DSC). The test results of both systems show that the rate of degree of cure for EPIKOTE 828® epoxy resin–EPIKURE® 3090 polyamidoamine curing agent system is approximately 10 times higher than that of DURATEK® KLM 606A epoxy resin–DURATEK® KLM 606B polyamide curing agent system at 230°C. POLYM. COMPOS., 28:762–770, 2007. © 2007 Society of Plastics Engineers     

Cure kinetics,glass transition temperature development,and dielectric spectroscopy of a low temperature cure epoxy/amine system

This article reports a study of the chemical cure kinetics and the development of glass transition temperature of a low temperature (40°C) curing epoxy system (MY 750/HY 5922). Differential scanning calorimetry, temperature modulated differential scanning calorimetry, and dielectric spectroscopy were utilized to characterize the curing reaction and the development of the cross‐linking network. A phenomenological model based on a double autocatalytic chemical kinetics expression was developed to simulate the cure kinetics behavior of the system, while the dependence of the glass transition temperature on the degree of cure was found to be described adequately by the Di Benedetto equation. The resulting cure kinetics showed good agreement with the experimental data under both dynamic and isothermal heating conditions with an average error in reaction rate of less than 2 × 10^?3 min^?1. A comparison of the dielectric response of the resin with cure kinetics showed a close correspondence between the imaginary impedance maximum and the calorimetric progress of reaction. Thus, it is demonstrated that cure kinetics modeling and monitoring procedures developed for aerospace grade epoxies are fully applicable to the study of low temperature curing epoxy resins. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Resin transfer molding (RTM) process of a high performance epoxy resin. I: Kinetic studies of cure reaction

The cure kinetics of a high performance PR500 epoxy resin in the temperature range of 160–197°C for the resin transfer molding (RTM) process have been investigated. The thermal analysis of the curing kinetics of PR500 resin was carried out by differential scanning calorimetry (DSC), with the ultimate heat of reaction measured in the dynamic mode and the rate of cure reaction and the degree of cure being determined under isothermal conditions. A modified Kamal's kinetic model was adapted to describe the autocatalytic and diffusion‐controlled curing behavior of the resin. A reasonable agreement between the experimental data and the kinetic model has been obtained over the whole processing temperature range, including the mold filling and the final curing stages of the RTM process.     

Optimal curing for thermoset matrix composites: Thermochemical and consolidation considerations

A design sensitivity method is used to find optimal autoclave temperature and pressure histories for curing of thermoset-matrix composite laminates. The method uses a finite element simulation of the heat transfer, curing reaction, and consolidation in the laminate. Analytical sensitivities, based on the direct differentiation method, are used within the finite element simulation to find the design sensitivities, i.e., the derivatives of the objectives function and the constraints with respect to the design variables. Standard gradient-based optimization techniques are then used to systematically improve the design, until an optimal process design is reached. In this study the objective is to minimize the total time of the cure cycle, while the constraints include a maximum temperature in the laminate (to avoid thermal degradation) and a maximum deviation of the final fiber volume fraction from its target value (to achieve proper consolidation). The simulations of curing process are performed for EPON 862/W epoxy under a conventional cure cycle, for both thin and thick parts. Time-optimal cure cycles are found using the optimization program. Simulations of fast-curing cycles are also examined. The optimal cycles are similar in form to conventional cure cycles, but give substantially shorter cure times. The entire scheme works automatically and efficiently, simultaneously adjusting multiple design variables at each iteration.