Model Adherend Surface Effects on Epoxy Cure Reactions

Adherend surface effects on the amine cure of epoxy resins were investivated using finely divided aluminum oxide as high surface area models for aluminum. Calorimetric analysis of simplified crosslinking systems revealed significantly faster reactions which led to lower glass transition temperature materials for activated aluminum oxide filled samples. A monofunctional amine and epoxy were then utilized to obtain soluble reaction products amenable to molecular characterization. These studies similarly showed an increase in the rate of epoxy consumption in the presence of activated aluminum oxide which was attributed to both an increase in the rate of amine addition to epoxy as well as to epoxy homopolymerization. The latter was not observed in the unfilled mixtures. Such changes in reaction mechanism at the adherend surface have implications for the strength and durability of actual adhesive bonds.     

Interphase Composition in Aluminum/Epoxy Adhesive Joints

Model epoxy/aluminum adhesive joints were constructed with a geometry that allowed cracks to be propagated extremely close to the adhesive/adherend interface. The joints were fractured in air and the fracture surfaces analyzed using angle resolved X-ray photoelectron spectroscopy. Fracture occurred in a manner that left a significant amount of aluminum oxide on the epoxy side of the fracture surface and very little epoxy on the aluminum side. Aliphatic amine curing agent found associated with the aluminum oxide on both the adhesive and the adherend sides of the fracture surface was protonated by the acidic hydroxyls present in the aluminum hydroxide. Moreover, catalysis of the curing reaction by these hydroxyls resulted in an increased degree of crosslinking in the regions of the adhesive very close to the oxide surface. Thus, the aluminum oxide surface modified the structure of the adhesive in the near surface regions and resulted in the formation of a distinct interphase region with a composition different from that of the bulk adhesive.     

Interphase Composition in Aluminum/Epoxy Adhesive Joints

Model epoxy/aluminum adhesive joints were constructed with a geometry that allowed cracks to be propagated extremely close to the adhesive/adherend interface. The joints were fractured in air and the fracture surfaces analyzed using angle resolved X-ray photoelectron spectroscopy. Fracture occurred in a manner that left a significant amount of aluminum oxide on the epoxy side of the fracture surface and very little epoxy on the aluminum side. Aliphatic amine curing agent found associated with the aluminum oxide on both the adhesive and the adherend sides of the fracture surface was protonated by the acidic hydroxyls present in the aluminum hydroxide. Moreover, catalysis of the curing reaction by these hydroxyls resulted in an increased degree of crosslinking in the regions of the adhesive very close to the oxide surface. Thus, the aluminum oxide surface modified the structure of the adhesive in the near surface regions and resulted in the formation of a distinct interphase region with a composition different from that of the bulk adhesive.     

环氧树脂/活化纳米氧化铝复合材料的固化动力学

采用氢氧化钠溶液将纳米氧化铝表面活化,利用Y-缩水甘油醚氧丙基三甲氧基硅烷(KH560)和环氧树脂对活化纳米氧化铝进行表面处理而引入环氧基团.采用差示扫描量热法(DSC)研究复合材料的非等温固化行为,分析了活化纳米氧化铝及其环氧功能化产物对环氧树脂固化动力学参数与机理的影响,运用等转化率方法包括Friedman方法与K...     

Analysis of the epoxy polymerization process with piperidine as the initiator

Measurements of the rate of heating from a polymerization reaction of a popular Epoxy compound, EPON 828, with piperidine as the initiator were made until no further heat output was detectable. The results were obtained using a microcalorimeter at a temperature of 27.5°C. Three experiments were run where the amine equivalents relative to the epoxy equivalents were 0.034, 0.100, and 0.170. At these ratios, the amine was insufficient to bond with all the epoxide rings. Each amine bond produces an oxide ion, which in turn reacts with other epoxide rings creating another oxide ion. This propagation reaction continues until all the epoxy groups are opened and bonded. The heat rate from the epoxy reaction started at a value proportional to the amine content but then it accelerated rapidly by over an order of magnitude and then decelerated after about 50% completion. The heat rate profiles were found to fit a combination of three mechanisms operating during the polymerization. The first stage is generation of an oxide ion by amine reaction and by a hydrogen ion exchange between the unreacted amine and a hydroxyl group present in the EPON 828 molecule. The second stage is the rapid acceleration in heat rate due to a build up of an ether bond from the reaction of the oxide ion with an epoxy group. The peak heating rate occurred when the epoxy rings and ether bonds were equal. The last stage is a classic diffusion process, which is the only mechanism left to allow reaction after the other mechanisms have dissipated. The paper generates rate equations and discusses specific issues arising from the heat rate database. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 5066–5086, 2006     

Molecular Structure of Polymer/Metal Interphases

The molecular structure of interphases in aluminum/epoxy and steel/epoxy adhesive joints was characterized using infrared spectroscopy. In one series of experiments, adhesive joints were prepared by curing beams of epoxy against aluminum or steel substrates. When the joints were cooled to room temperature, the residual stresses were sufficient for crack propagation along the interface. The adhesive and substrate failure surfaces were then analyzed with reflection-absorption infrared spectroscopy (RAIR), attenuated total reflection infrared spectroscopy (ATR) and X-ray photo-electron spectroscopy (XPS). When an epoxy/anhydride adhesive was cured against aluminum substrates primed with an aminosilane coupling agent, amide and imide groups were formed in the interphase. Chemical reaction between the primary amine of the primer and the anhydride of the curing agent precluded chemical bridge formation between the primer and adhesive. Metal cations from the 2024 aluminum substrate reacted with the anhydride to form carboxylate salts on the surface. When an epoxy/tertiary amine adhesive was cured against steel substrates, evidence of oxidation of the primary amine to imine was observed in the interphase.     

Molecular Structure of Polymer/Metal Interphases

The molecular structure of interphases in aluminum/epoxy and steel/epoxy adhesive joints was characterized using infrared spectroscopy. In one series of experiments, adhesive joints were prepared by curing beams of epoxy against aluminum or steel substrates. When the joints were cooled to room temperature, the residual stresses were sufficient for crack propagation along the interface. The adhesive and substrate failure surfaces were then analyzed with reflection-absorption infrared spectroscopy (RAIR), attenuated total reflection infrared spectroscopy (ATR) and X-ray photo-electron spectroscopy (XPS). When an epoxy/anhydride adhesive was cured against aluminum substrates primed with an aminosilane coupling agent, amide and imide groups were formed in the interphase. Chemical reaction between the primary amine of the primer and the anhydride of the curing agent precluded chemical bridge formation between the primer and adhesive. Metal cations from the 2024 aluminum substrate reacted with the anhydride to form carboxylate salts on the surface. When an epoxy/tertiary amine adhesive was cured against steel substrates, evidence of oxidation of the primary amine to imine was observed in the interphase.     

固化剂对环氧玻璃鳞片涂层性能的影响

以双酚A环氧树脂(E20)为主要成膜物,层片状玻璃鳞片和云母氧化铁为主要填料,辅助以铝粉、氧化铁红颜料,以及各种助剂和溶剂,再分别加入固化剂脂肪族胺加成物、聚酰胺TY-650,制得高固体分厚浆型环氧玻璃鳞片涂料。通过常规的力学性能(涂层的硬度、耐冲击性、柔韧性、附着力等)和耐腐蚀性测试,对比了不同固化剂对环氧玻璃鳞片涂层性能的影响。研究结果表明,脂肪族胺加成物与环氧树脂交联固化的玻璃鳞片涂层的硬度、附着力以及交流阻抗值都较高,具有良好的综合性能,加入铝粉对基体可起到阴极保护作用。     

Fast and facile synthesis of boehmite nanofibers

A quick, simple route was established for producing boehmite nanofibers by direct reaction between activated aluminum and hot water. Naturally, an oxide layer around aluminum particles prevents aluminum core to react with water. In the present work, the oxide layer was destroyed, using mechanical energy, to facilitate the reaction. Hot water increased the kinetic of reaction which accelerates hydrogen generation rate at the interface of aluminum and just-produced layer of boehmite, whose outward flow prohibits passive boehmite layer from being formed on aluminum particles. Boehmite particles suspended in water were self-assembled using no surfactant and/or template, constructing a fiber structure with a diameter less than 10 nm and an average length of 150 nm in less than 40 s. Average crystallite size of resultant fibers was 4.3 nm and its specific surface area was about 283.6 m²/g.     

乙二胺四乙酸合成双子型阳离子表面活性剂

以乙二胺四乙酸、环氧氯丙烷和十二烷基-N,N-二甲基叔胺为原料合成双子（Gemini）型阳离子表面活性剂乙二胺四乙酸四酯季胺盐。研究了两种合成方法：一是乙二胺四乙酸先与环氧氯丙烷反应,生成的酯化产物再和十二烷基-N,N-二甲基叔胺反应生成产物乙二胺四乙酸四酯季胺盐;二是先将乙二胺四乙酸加入十二烷基-N,N-二甲基叔胺中,进行Lewis酸碱反应,乙二胺四乙酸负离子与产生的季铵盐结合为离子对,再加入环氧氯丙烷从而生成产物乙二胺四乙酸四酯季胺盐。研究了乙二胺四乙酸与环氧氯丙烷的摩尔比对合成过程及产物性能的影响,并测试了相关性能。产品显示了良好的发泡性和稳定性。     

FT-IR Microscopy Studies of Water Ingress into Aluminum-Adhesive Joints

FT-IR microscopy of the effect of humidity on epoxy thin films on anodized aluminum surfaces was performed in the absence of external stress. The oxide transformation was observed in the anodized samples. Moisture damage was greater in the untreated aluminum samples indicating that interfacial debonding occurs faster in these samples. Water damage was localized spatially. The only organic reaction seen was at the epoxy/air interface giving carboxylates at the surface.     

Effect of urethanes on base-catalyzed epoxy reaction

The effect of carbamate (urethane) groups on the curing reaction of an epoxy catalyzed by tertiary amine was studied by infrared (IR) and differential scanning calorimetry (DSC) analysis. It was found that the aromatic-connected carbamate (Ar-carbamate) accelerated the consumption rate of epoxy in the epoxy/tertiary amine/Ar-carbamate reaction system, where the tertiary amine and Ar-carbamate was found to form an active complex with stoichiometric ratio in the curing reaction. However, under the introduction of aliphatic-connected carbamate (Al-carbamate) into the reaction system, no complex formed with the tertiary amine during the reaction and therefore no acceleration in the reaction rate was observed.     

Processing and properties of resol and epoxy blends for resin transfer molding

Blends of epoxy and resol matrices for resin transfer molding (RTM) have been prepared and characterized with the double aim of improving the processability of resols and the fire resistance of epoxies. The reaction rate of the overall system is enhanced by the exothermic reaction between the epoxy and an aliphatic amine, which also promotes the resol reaction. In addition, the mechanical behavior of the resol is improved as the shear stress increases with the addition of epoxy resins. However, blending also produces a reduction of the intrinsic fire resistance of phenolic resins. The study demonstrated the feasibility of producing blends for matrices of composites processed by resin transfer molding. The recommended range for the epoxy content was determined through a balance of processability, fire resistance, and mechanical properties of the composites obtained. Moreover, the effects of fiber surface modifications were studied as a way to promote higher shear stresses in the epoxy-resol matrix composite without jeopardizing the fire resistance properties.     

聚醚胺/酚醛胺-环氧树脂体系的固化动力学研究

采用示差扫描量热法(DSC)研究了以聚醚胺/酚醛胺为固化剂的环氧树脂体系固化反应,在25~230℃范围内以不同的升温速率(5℃/min、10℃/min、15℃/min、20℃/min)对该体系的固化动力学参数分析。由Kisserger方程求得该体系固化反应的表观活化能为61.76 kJ/mol,频率因子A为7.1×107s-1;由Crane方程得出固化反应级数为1.116;并建立了固化动力学方程:-da/dty=k(1-a)1.116,其中。k=7.1×107exp(-7 429/t)。     

Cure kinetics of several epoxy–amine systems at ambient and high temperatures

Epoxy-crosslinker curing reactions and the extent of the reactions are critical parameters that influence the performance of each epoxy system. The curing of an epoxy prepolymer with an amine functional group may be accompanied by side reactions such as etherification. Commercial epoxy prepolymers were cured with different commercial amines at ambient as well as at elevated temperatures. Singularly, only epoxy–amine reactions were observed with diglycidyl ether of bisphenol-A (DGEBA)-based epoxides in our research even upon post-curing at 200°C. Etherification side reaction was found to occur at a cure temperature of 200°C in epoxides possessing a tertiary amine moiety. A combined goal of our research was to understand the effect of tougheners on the cure of epoxy–amine blend. To discern the effect of tougheners on the cure, core–shell rubber (CSR) particles were incorporated into the epoxy–amine blend. It was observed that CSR particles did not restrict the system from proceeding to complete reaction of epoxy moieties. Besides, CSR particles were found to accelerate the epoxy-amine reaction at a lower level of epoxy conversion. The lower activation energy of epoxy–amine reaction of CSR incorporated system compared to control supported the catalytic effect of CSR particles on the epoxy-amine reaction of epoxy prepolymer and amine blends.     

Characterization of Poly(Aminosiloxane) Surface Chemistry on Aluminum Oxide

N-2-aminoethyl-3-aminopropyltrimethoxysilane (AAPS) was shown to react with aluminum oxide powder to form an amine/carbonate salt, as observed by diffuse reflectance infrared spectroscopy (DRIFT) and thermogravimetric analysis with mass spectroscopy (TGA-MS). TGA-MS, together with electron spectroscopy for chemical analysis (ESCA), reveal that the stoichiometric ratio of amine salt to free amine is higher on the surface of aluminum oxide powder than in a comparable neat film. In addition, TGA-MS shows that a nonstoichiometric ratio of CO₂H₂O is evolved upon heating the surface-treated powder (4.5/1), whereas the neat film evolves CO₂/H₂O at a ratio near unity. The high fraction of protonated amines, together with the higher ratio of CO₂/H₂O in the presence of alumina, is consistent with a proposed bonding mechanism which involves carbonate bridging between protonated amines and hydroxyl sites on the aluminum oxide surface.     

Impact resistance enhancement by adding epoxy ended hyperbranched polyester to DGEBA photocured thermosets

Epoxy-functionalized HBP was synthesized and used as toughening additive in epoxy based UV-curable formulations. The addition of the HBP-epox induced an increase of epoxy group conversion, probably through both an activated monomer mechanism, involving the residual hydroxyl groups, and a copolymerization reaction, involving the epoxy functionalities present on the surface of the HBP. Phase separation was reached during UV-Curing, which assured to maintain the good thermomechanical properties of the cured epoxy matrix improving the impact resistance.     

Promotion effect of bifunctional five‐membered cyclic dithiocarbonate on curing of one‐component epoxy resin by imines as latent initiator

This article deals with curing of a one‐component epoxy resin containing a bifunctional five‐membered cyclic dithiocarbonate 1 with an imine 2 as a latent initiator. When 1 was added to a mixture of epoxy resin and 2 , the curing rate and initial adhesive strength increased higher than that without 1 . It was supposed that the reaction of 1 and an amine released from 2 was much faster than the amine–epoxide reaction, and a thiol group formed by the reaction of 1 with the amine accelerated the amine–epoxide reaction. The adhesive strength of the one‐component epoxy resin containing 10 mol % of 1 exhibited the highest value. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 961–964, 2004     

Characterization of Poly(Aminosiloxane) Surface Chemistry on Aluminum Oxide

N-2-aminoethyl-3-aminopropyltrimethoxysilane (AAPS) was shown to react with aluminum oxide powder to form an amine/carbonate salt, as observed by diffuse reflectance infrared spectroscopy (DRIFT) and thermogravimetric analysis with mass spectroscopy (TGA-MS). TGA-MS, together with electron spectroscopy for chemical analysis (ESCA), reveal that the stoichiometric ratio of amine salt to free amine is higher on the surface of aluminum oxide powder than in a comparable neat film. In addition, TGA-MS shows that a nonstoichiometric ratio of CO₂H₂O is evolved upon heating the surface-treated powder (4.5/1), whereas the neat film evolves CO₂/H₂O at a ratio near unity. The high fraction of protonated amines, together with the higher ratio of CO₂/H₂O in the presence of alumina, is consistent with a proposed bonding mechanism which involves carbonate bridging between protonated amines and hydroxyl sites on the aluminum oxide surface.     

Blocked diisocyanate-modified epoxy resin: Mechanism and mechanical properties

The modification of bisphenol-A-based epoxy resin with isocyanates was investigated. To increase the crosslinking density by a urethane reaction and to conduct the normal cross-linking reaction between the epoxide and the amine during the cure cycle, blocking the isocyanates with active hydrogens is necessary, which may be deblocked and reacted at elevated temperature. The modified reactions involving diisocyanates and hydroxyl groups generated from epoxy cured with an amine system were studied. This mechanism can be identified by the variations of infrared spectra in the carbonyl group stretching region. Furthermore, the effect of the blocked isocyanate incorporation with amine on the reactivity of epoxy was also studied. The thermal and the mechanical properties were characterized. It was found that the glass transition temperature increased with an aromatic blocked-diisocyanate content. The mechanical properties were improved by blocked diisocyanate. The morphology was also investigated, which showed that a homogeneous structure existed in the modified system. © 1996 John Wiley & Sons, Inc.