High temperature adhesives for structural applications - a review

The requirement for high temperature adhesives in the aero engine industry is assessed and the reasons for the choice of polyimide adhesives as prime candidates are discussed. Cure chemistry of these resins is describe and the recent development of polyimides that cure by addition mechanisms is reviewed. Currently available adhesives are listed and two typical formulations (FM34 and A7F) are discussed in more detail giving some “as cured” and “thermally aged” properties. The necessity for the provision of special accessory materials and curing facilities in order to process these adhesives at the required temperatures is explained.     

端氨基树枝状大分子PAMAM/环氧树脂体系固化物的性能

以端氨基树枝状大分子PAMAM作为环氧树脂固化剂, 通过拉伸试验、 冲击试验、 DSC、 TGA研究了配比和固化温度对PAMAM与环氧树脂E-44的固化物性能的影响。 结果表明, 最佳固化温度为140℃, 但随着固化温度升高, 配比的影响表现出不同的规律: 80℃固化时, 最佳配比为0.47, 此时拉伸强度和冲击强度最佳, 玻璃化转变温度最高, 交联密度最大; 而在80℃以上固化时, 最佳配比逐渐向低配比方向移动, 140℃固化时, 最佳配比为0.28, 此时拉伸强度和冲击强度最佳, 玻璃化转变温度最高, 交联密度最大。固化物的密度和体积收缩率都是配比为0.47时最大, 而热稳定性都是配比为0.28时最佳。利用滴定法测定了固化物的固化度, 结果表明, 随着固化温度的升高, 低配比体系的固化度迅速提高并接近化学计量点配比体系的固化度。      

紫外光固化导电胶基体固化动力学分析

紫外光固化导电胶作为新型电子连接材料,具有环保、节能等优点。导电胶中基体胶的固化性能是影响导电胶使用可靠性的重要因素。利用实时红外分析研究环氧树脂基体胶的固化过程和固化过程的动力学特征。研究表明基体胶的固化是通过体系中-C=C-双键的互联来实现,随固化反应的进行,双键的相对浓度降低。光引发剂的添加量影响体系的固化速度和固化程度,光引发剂的添加量存在阈值Cmax。     

Preparation of Microcellular Epoxy Foams through a Limited‐Foaming Process: A Contradiction with the Time–Temperature–Transformation Cure Diagram

3D cross‐linking networks are generated through chemical reactions between thermosetting epoxy resin and hardener during curing. The curing degree of epoxy material can be increased by increasing curing temperature and/or time. The epoxy material must then be fully cured through a postcuring process to optimize its material characteristics. Here, a limited‐foaming method is introduced for the preparation of microcellular epoxy foams (Lim‐foams) with improved cell morphology, high thermal expansion coefficient, and good compressive properties. Lim‐foams exhibit a lower glass transition temperature (T_g) and curing degree than epoxy foams fabricated through free‐foaming process (Fre‐foams). Surprisingly, however, the T_g of Lim‐foams is unaffected by postcuring temperature and time. This phenomenon, which is related to high gas pressure in the bubbles, contradicts that indicated by the time–temperature–transformation cure diagram. High bubble pressure promotes the movement of molecular chains under heating at low temperature and simultaneously suppresses the etherification cross‐linking reaction during post‐curing.     

GFRP厚板制件固化过程固化度分布

采用真空导入模塑工艺(VIMP) 制备了85 mm 厚玻璃纤维增强环氧树脂层合板, 单面刚性模具加热固化, 沿铺层厚度方向设置热电偶, 进行了实时固化温度监测, 发现固化时厚度方向存在明显的温度差异。通过DSC方法得到等温环氧树脂固化度-时间实验数据, 建立了基于自催化反应模型的等温固化反应动力学方程, 模型计算值和实验值符合良好; 提出了时间离散分步计算法, 对非等温固化条件下, 厚度方向的固化度分布进行了计算。结果表明: 固化过程中厚度方向固化度存在差异, 短时间的后固化可以消除此差异。该方法可以模拟出由温度差异导致的固化度的不均匀分布, 用于指导优化固化工艺。      

An emerging technology to cure fiber-reinforced composites: Electron curing

Electron accelerators are being widely used in industry to process polymeric materials. Their use to cure fiber-reinforced composites is an emerging technology, based on the work done here in Canada, and in France. The advantages of electron curing include ambient temperature curing with reduced internal stress, reduced curing times, and overall cost of savings. In this paper we present a brief review of our work with emphasis on the effects of dose and dose rate, temperature rise during curing, internal stress, voids, properties of matrix resins and a comparison of the properties of electron cured and thermally cured matrices and composites.Abbreviations: kW kilo-watt - eV electron volt - RTM resin transfer molding - SEC size exclusion chromatography - Gy Gray (1 Gy = 100 rad) - THF tetrahydrofuran - AMU atomic mass unit - T _g glass transition temperature     

Cure kinetics and rheology characterisation and modelling of ambient temperature curing epoxy resins for resin infusion/VARTM and wet layup applications

Ambient temperature curing epoxy resins are widely used as the matrix material in fibre reinforced plastics in the marine and wind energy sectors, where they are popular due to their relatively high mechanical performance yet low processing and tooling costs. To date, the characterisation of ambient curing epoxy resins has been limited to relatively simple measures, not suitable for use in heat transfer and flow process models. A complete cure kinetics and rheology model allows the prediction of the progression of degree of cure and viscosity for any time–temperature history. The progression of degree of cure of two epoxy resin systems was measured by differential scanning calorimeter and fitted to an nth order model incorporating vitrification effects. Viscosity was measured using an oscillatory rheometer and fitted to a model from the literature. The resulting cure kinetics and rheology model enables the improvement of resin infusion and wet layup processes by providing a thorough understanding of the interlinked relationship between time, temperature, degree of cure and viscosity.     

The development of a low temperature cure modified epoxy resin system for aerospace composites

It has been recognised within the composites industry that there are economic advantages in moving away from a conventional autoclave cure at high temperature. A low temperature cure (LTC) resin system has been developed that can be cured at 85 °C initially then subsequently further cured at 175 °C, for a short period. For such resins there is a requirement to achieve the same toughness properties as conventional thermoplastic toughened high temperature cured (HTC) material. The development of an LTC resin system has required a special catalyst systems and appropriate choice of a poly(ethersulphone) co-polymer, in terms of molecular structure, molecular weight and chain ends. Toughness development in thermoset–thermoplastic blends requires an appropriate development of phase morphology during cure. Morphology control has been achieved in LTC materials. Application of linear elastic fracture mechanics measurements of toughness in association with transmission electron microscopy and scanning electron microscopy of the fracture surface has demonstrated links between the toughness and the phase morphology and the chemistry. Small angle neutron scattering has enabled the development of the phase morphology during the low temperature curing process to be monitored.The presence of phase morphology in the resin means that the influence of phase or interface boundaries on the environmental stress cracking (ESC) properties might be a critical issue. This issue has been addressed and investigated for three environments, namely air, demineralised water and dichloromethane at 23 °C. The LTC material is shown to perform in a similar manner to a comparable conventional HTC resin system.     

Material characterization of a polyester resin system for the pultrusion process

In the present work, the chemo-rheology of an industrial “orthophthalic” polyester system specifically prepared for a pultrusion process is characterized. The curing behaviour is first characterized using the differential scanning calorimetry (DSC). Isothermal and dynamic scans are performed to develop a cure kinetics model which accurately predicts the cure rate evolutions and describes the curing behaviour of the resin over a wide range of different processing conditions. The viscosity of the resin is subsequently obtained from rheological experiments using a rheometer. Based on this, a resin viscosity model as a function of temperature and degree of cure is developed and predicts the measured viscosity correctly. The evolution of the storage and loss moduli are also measured as a function of time using the rheometer which provides an information about the curing as well as the gelation. The temperature- and cure-dependent elastic modulus of the resin system is determined using a dynamic mechanical analyzer (DMA) in tension mode. A cure hardening and thermal softening model is developed and a least squares non-linear regression analysis is performed. The variation in elastic modulus with temperature and phase transition is captured for a fully cured resin sample.     

Acrylonitrile-capped poly(propyleneimine) dendrimer curing agent for epoxy resins: Model-free isoconversional curing kinetics,thermal decomposition and mechanical properties

Acrylonitrile-modified aliphatic amine adducts are often used as curing agents for room-temperature epoxy formulations (coatings, adhesives, sealants, castings, etc.), yet the curing reaction and properties of resultant epoxy systems still remain less fundamentally understood. Herein we systematically investigate our newly-developed acrylonitrile-modified multifunctional polyamine curing agent for bisphenol A epoxy resin (DGEBA): an acrylonitrile-capped poly(propyleneimine) dendrimer (PAN4). The impact of the molecular structure of PAN4 and a controlled poly(propyleneimine) dendrimer (1.0GPPI) on the curing reactivity, reaction mechanisms, thermal stability, viscoelastic response and mechanical properties of the epoxy systems are highlighted. Differential scanning calorimetry (DSC) confirms DGEBA/PAN4 shows markedly lower reactivity and reaction exotherm than DGEBA/1.0GPPI, and the model-free isoconversional kinetic analysis reveals that DGEBA/PAN4 has the generally lower reaction activation energy. To be quantitative, the progress of the isothermal cure is predicted from the dynamic cure by using the Vyazovkin equation. The isothermal kinetic prediction shows that DGEBA/PAN4 requires about 10 times longer time to achieve the same conversion than DGEBA/1.0GPPI, which agrees with the experimentally observed much longer gel time of DGEBA/PAN4. Subsequently, dynamic mechanical analysis shows that PAN4 results in the cured epoxy network with the lower β- and glass-relaxation temperatures, crosslink density, relaxation activation energy, enthalpy, entropy, but the higher damping near room temperature than 1.0GPPI. Finally, thermogravimetric analysis (TGA) demonstrates cured DGEBA/PAN4 is thermally stable up to 200 °C, and mechanical property tests substantiate that PAN4 endows the cured epoxy with much higher impact and adhesion strengths than 1.0GPPI. Our data can provide a deeper insight into acrylonitrile-modified aliphatic amine curing agents from the two good model compounds (PAN4 and 1.0GPPI).     

Characterization of melded carbon fibre/epoxy laminates

‘Melding’ is a novel in situ method for joining thermosetting composite structures, without the need of adhesives. Laminate joining is achieved using uncrosslinked resin matrix of the pre-preg. This study used Hexply914C pre-preg material to characterize melded CFRP structures produced using the melding method. A designated area of a laminate was maintained at temperatures below 40 °C retaining uncured (B-staged) material, while the remainder of the laminate was cured at 175 °C. After a 2.5 h cure cycle, the cured region showed a high degree of cure (0.88) and glass transition temperature (176 °C). The uncured area of the same laminate was cured in a second stage, simulating an in situ melded joint. By controlling the temperature and duration of the intermediate dwell and affecting minimum viscosity values prior to final cure, low values of porosity (<0.5%) were achieved. The mechanical properties of the resulting joint were consistent throughout the melded laminate. Flexural strength (1600 MPa), flexural modulus (100–105 MPa) and short beam strength (105–115 MPa) values observed where equivalent or greater than those found in the recommended autoclave cured control specimens. After the entire laminate was post cured, glass transition temperatures of 230 °C (peak tan δ) were observed in all areas of the laminate.     

Optimization of composite patch repair processes with?the?use of genetic algorithms

The aim of this contribution is the optimization of some parameters of the composite patch repair technique (CPR). This technique is mainly used by the aircraft industry, as it offers high reliability, short repair times and reduced cost in compare to other methods, such as the riveted joints. CPR consists of adhesively bonding thin composite patches over cracked or corroded areas with heat supply. As the polymer-matrix composite patch is heated, it cures and toughens. Proper curing insures structural reliability of the repair. Short duration curing cycles are of great importance for the aircraft availability. With the use of Genetic Algorithms, we design minimum time curing cycles. The optimization is subjected to the following constraints: (1) Maximum allowed temperature in order to avoid residual stresses, (2) Minimum temperature in order to initiate the cure reaction, (3) Sufficient degree of cure at the end of the process and (4) Maximum heat generation rate that can be achieved by the device. Our design vector contains the duration of the plateau stage of the cure cycle and the characteristic thermal profile. The degree of cure is estimated with the use of the Kamal cure rate model for thermosetting polymers. For the numerical time integration of the cure rate equation, a second order, implicit Runge-Kutta scheme is employed.     

Neural-fuzzy optimization of thick composites curing process

This article addresses the optimization of curing process for thick composite laminates. The proposed methodology aims at the evaluation of the thermal cycle promoting a desired evolution of the degree of cure inside the material. At the same time, temperature overshooting as well as excessive temperature and cure degree gradient through the thickness of the material are prevented. The developed approach is based on the integrated application of artificial neural networks and a fuzzy logic controller. The neural networks promptly predict the behavior of composite material during curing process, while the fuzzy logic controller continuously and opportunely adjusts the proper variations on the imposed thermal cycle. The results highlighted the efficiency of the method in comparison with the cure profiles dictated by the material suppliers. For thick laminates, a reduction of 35% of cure time and improvements of approximately 10% of temperature overshooting was obtained compared to conventional curing cycles. The method was validated by experimental tests.     

Thermal furnace and Ultraviolet assisted curing impact on SiOCH spin-on ultra low dielectric constant materials

In recent years, the continuous progression of ultra-large scale integration has driven the emergence of technological solutions. In particular, major challenges have been faced for the fabrication of interconnect structures, where ultra low dielectric constants are required to decrease the parasitic capacitances between metal lines. Porous material, obtained using the porogen approach, is the main candidate investigated. The curing process is critical for achieving a good control of final film structure. The integration of such material requires a good chemical and mechanical stability, particularly to maintain the structure integrity during the stressing steps: chemical mechanical polishing and packaging. In this work, Ultraviolet assisted thermal cure (or UV curing) is investigated as an alternative solution to the conventional thermal curing. Chemical and physical analyses reveal that the best porogen removal efficiency and the enhancement of matrix crosslinking are achieved when the material is UV cured. This crosslinking improvement (as indicated by higher Si-O-Si bond density in the fourier transformed infra-red spectra) can be correlated to better mechanical properties. Significantly better electrical properties (dielectric constant, leakage current and breakdown voltage) are obtained with better integrity (no moisture uptake after 1 week storage in humid atmosphere 85 °C/85% relative humidity) when the dielectric is optimally cured. Porosity evaluation reveals similar results between both curing processes with slightly larger pore size in the case of the UV cured film. Finally, a basic model is described to illustrate how the UV assisted thermal cure may improve the crosslinking in comparison to the thermal curing. Selective UV action is proposed to explain the curing process kinetics.     

Development of a resin curing model for UV nanoimprint

UV nanoimprint lithography uses UV light as an energy source. It is performed at room temperature and low pressure, and has its own merits as compared to thermal nanoimprint. In this paper, a measurement system was developed to measure the degree of resin curing in UV nanoimprint to improve our understanding of the resin solidification phenomenon. A curing model was then established based on the measurement results. The measurement system measured the degree of cure in real time and was composed of a Fourier transform infrared spectroscopy system, a UV light source, and an optical guide. Also, new UV-curable resins that had low viscosity values were developed for the UV nanoimprint process, and imprint tests using these resins were performed successfully. The curing model considered the UV irradiation time, power, and curing temperature, which are important parameters in the UV nanoimprint process. The degree of cure had an exponential relation to UV irradiation time, power, and temperature; thus, the curing model was expressed as an exponential function of the UV irradiation time, power, and temperature. The developed model was verified for various UV-curable resins.     

低黏度环氧树脂固化度唯象模型及其在实际工程中的应用

采用等温和非等温差示扫描量热(DSC)对Huntsman1564/3486低黏度环氧树脂体系的固化反应进行了研究,建立了修正的Olivier固化度唯象模型,用于描述恒温条件下固化度温度时间关系。模型计算值与实验值符合良好。监测了基于实际工程应用背景85mm厚复合材料单面模具加热固化过程中制件厚度方向的温度情况,针对该过程提出了基于修正Olivier模型的适用于变温条件的时间离散分步计算法,并计算了固化过程厚度方向的固化度分布。     

Advanced cure monitoring by optoelectronic multifunction sensing system

In this work, a dual functionality fiber optic sensing system for thermoset cure monitoring has been developed and successfully tested. Principle of operation relies on the use of a standard optical fiber in single ended configuration containing a single in-fiber Bragg grating. The change in refractive index directly related to material density at fiber end and the variation in the wavelength reflected by the fiber grating have been monitored simultaneously, throughout an In-Mould curing process of an epoxy resin. A compact and low cost in-fiber dual wavelength demodulation system has been developed and experimental results demonstrate the capability to on line identify the degree of cure, gel point, residual stresses and the glass transition temperature of the cured resin.     

基于纸胶体系及烘干模式的水基胶固化时间测定及影响因素研究

目的 了解水基胶固化过程中水分散失的规律。方法 建立基于纸胶体系及烘干模式的固化时间测定方法，提出水基胶“有效固化”的概念，利用皮尔模型对水基胶干燥曲线进行拟合，并研究5种水基胶在不同干燥温度下的干燥曲线。结果 结果表明，皮尔模型可以较好地拟合水基胶的干燥曲线。固化率在80%～90%存在一个拐点，具体位置与干燥温度有关。当固化率达到70%时纸胶体系已基本完成黏合，对应的有效固化时间相对于完全固化时间可缩短74.60%～83.70%。在干燥温度一定时，不同水基胶的固化时间有一定的差异，其差异性主要表现在低温干燥区域。结论 综上所述，干燥温度一定时，低玻璃化转变温度、低粒径、高固含量、高黏度的水基胶的固化时间更短。     

A Dynamic Transient Model to Simulate the Time Dependent Pultrusion Process of Glass/Polyester Composites

The objective of this paper is to introduce a novel dynamic transient model to simulate the time dependent pultrusion process of glass/polyester composites. The model is able to simulate the resin curing process systematically. The resin curing process is divided in two liquid and gel-solid phases. Physical properties of the resin including resin specific heat, viscosity and thermal conductivity change by altering the resin temperature and the degree of cure. It is shown that in liquid and gel-solid phases, some of the resin physical properties have significant role in heat transfer phenomenon and affect simulation results. The physical and mechanical properties of fibers do not change during the curing process of composites; therefore, an equivalent material is introduced instead of the resin-fiber compound. The model simulates the heat generation during the resin curing process. The degree of cure of the resin, used for the resin viscosity calculation, is an important parameter indicating the final stage of simulation of resin curing process. The components of the model are integrated in a finite element method. As case studies, the process of pultrusion of circular, rectangular and I cross-sections are simulated by the model. The results show that the model is able to simulate the pultrusion process very well.     

Evolution of cure,mechanical properties,and residual stress during electron beam curing of a polymer composite

Evolution of cure, mechanical properties, and residual stress during E-beam (Electron Beam) processing was studied to evaluate the influence of process parameters – dose and dose per pass – using an epoxy reinforced with IM7 carbon fibers. The composite prepreg was also cured thermally to various cure levels and compared with the E-beam cured composite. Cure evolution changed substantially with irradiation condition; lower dose/pass and wider scanning width of the beam (for the same dose/pass) resulted in rapid curing. For a given degree of cure, the longitudinal (E₁₁) and transverse (E₂₂) moduli of the E-beam cured composite varied with dose/pass and were less than that of the thermally cured composite. Transverse strength and failure strain of the composite cured at 20 kGy/pass were higher than that of composites cured thermally and at other dose/pass conditions. E-beam curing resulted in lower residual stresses than thermal curing and lower dose per pass resulted in lower cure-induced residual stress than higher dose/pass.