Toughening of epoxy resin using acrylate‐based liquid rubbers

Carboxyl‐terminated poly(2‐ethylhexyl acrylate) (CTPEHA) liquid rubbers of different molecular weights and functionalities (LR‐1 to LR‐6) were synthesized by bulk and solution polymerization techniques. The liquid rubbers were characterized by nonaqueous titration, vapor pressure osmometry, and gel permeation chromatography. The CTPEHA oligomers were prereacted with the epoxy resin, and the modified epoxy networks were made by curing with an ambient‐temperature curing agent. The impact properties of the modified epoxy networks were evaluated, and the effects of molecular weight, functionality of the liquid rubber, and ductility of the matrix on the impact strength of the modified networks were investigated. The morphology of the toughening behavior was analyzed using a scanning electron microscope. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 716–723, 2000     

Study of impact properties and morphology of 4,4′‐diaminodiphenyl methane cured epoxy resin toughened with acrylate‐based liquid rubbers

Randomized carboxyl poly(2‐ethylhexyl acrylate) (A‐1) and randomized epoxy poly(2‐ethylhexyl acrylate) (B‐1) rubbers were synthesized in the form of liquid rubber by a solution polymerization technique. The liquid rubbers A‐1 and B‐1 were characterized by ¹H NMR and IR spectroscopic analysis, non‐aqueous titration, viscosity measurements and gel permeation chromatography. The liquid rubbers A‐1 (M?_n = 3900 g mol^?1), B‐1 (M?_n = 4100 g mol^?1) and a (1:1) mixture of A‐1 and B‐1 were pre‐reacted with epoxy resin separately and the modified epoxy networks were made by curing with high temperature curing agent. The modified epoxy networks were evaluated by unnotched Izod impact testing. The morphology and toughening behaviour were analysed by scanning electron microscopy. Optimum properties were obtained with the mixture of A‐1 and B‐1. Copyright © 2003 Society of Chemical Industry     

Influence of aliphatic amine epoxy hardener on the adhesive properties of blends of mono‐carboxyl‐terminated poly(2‐ethylhexyl acrylate‐co‐methyl methacrylate) with epoxy resin

The adhesive properties have been investigated in blends of mono‐carboxyl‐terminated poly(2‐ethylhexyl acrylate‐co‐methyl methacrylate) with diglycidyl ether of bisphenol A and three different aliphatic amine epoxy hardener. The adhesives properties are evaluated in steel alloy substrate using single‐lap shear test. The copolymers are initially miscible in the stoichiometric blends of epoxy resin and hardener at room temperature. Phase separation is noted in the course of the polymerization reaction. Different morphologies are obtained according to the amine epoxy hardener. The most effective adhesive for steel–steel joints in single‐lap shear test is the blends using 1‐(2‐aminoethyl)piperazine (AEP) as hardener. This system shows the biggest lap shear strength. However, the modified adhesives show a reduction in the mechanical resistance. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Acrylate‐based liquid rubber as impact modifier for epoxy resin

Carboxyl‐terminated poly(2‐ethyl hexyl acrylate) (CTPEHA) having various molecular weights were synthesized by bulk polymerization in the form of liquid rubber. The liquid rubbers (LR‐1 to LR‐4) were characterized by ¹³C‐NMR spectroscopic analysis, nonaqueous titration, and vapor‐pressure osmometry (VPO). The liquid rubber having the lowest molecular weight (M?_n = 3600) was prereacted with the epoxy resin and the modified epoxy networks were made by curing with an ambient temperature curing agent. The modified epoxy networks containing different concentrations of CTPEHA were evaluated with respect to their thermal and impact properties. The optimum properties were obtained at about 10–15 phr of CTPEHA concentration (phr stands for parts per hundred parts of epoxy resin). Fracture surface analysis by scanning electron microscopy (SEM) indicated the presence of a two‐phase microstructure. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1792–1801, 2001     

Application of a series of novel chain-extended ureas as latent-curing agents and toughening modifiers for epoxy resin

A series of novel chain-extended ureas (U_i) containing poly(ethylene glycol) (PEG) spacers with different molecular weights were synthesized and the reactive mechanism, reactive activity, impact strength, dynamic mechanical behavior, morphology, and storage properties of the epoxy resin/chain-extended urea system were studied. Experimental results revealed that the impact strength of the epoxy resin modified with chain-extended urea containing a PEG flexible spacer with molecular weight of 1000 is 9.5 times higher than that of the epoxy resin/dicyandiamide system. Results also show that the molecular weight of PEG in chain-extended ureas hardly has any influence on the reactive activity of the chain-extended ureas. The storage life of the epoxy resin/U_i system can be delayed to 38 h at 50°C. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 339–347, 1998     

Toughening of epoxy resin by modification with 2‐ethylhexyl acrylate–acrylic acid copolymers

2‐Ethylhexyl acrylate–acrylic acid copolymers, ie carboxyl randomized poly(2‐ethylhexyl acrylate) (CRPEHA) (LR‐1 to LR‐6), with different molecular weights and functionality were synthesized. The liquid rubbers were characterized by FTIR spectroscopic analysis, non‐aqueous titration, vapour pressure osmometry (VPO) and viscosity measurements. All the liquid rubbers were reacted with the epoxy resin in 10:100 weight ratio using triphenyl phosphine as a catalyst. The modified epoxy networks were made by reacting the homogeneous prereacted resin with an ambient temperature hardener, triethylene tetramine (HY 951). The effect of the molecular weight and functionality of the liquid rubbers on the thermal and impact properties of the modified networks was investigated. © 2000 Society of Chemical Industry     

Elastomer modification of epoxy based film adhesives: adhesive and mechanical properties

Three approaches were employed to improve the flow and sandwich bonding properties of a nylon-carrier supported film adhesive based on carboxyl terminated butadiene acrylonitrile (CTBN)-modified novolac epoxy resin. These included the addition of a commercial acrylate flow modifier, replacement of novolac epoxy partly with solid diglycidyl ether of bisphenol A (DGEBA) resins, and replacement of CTBN partly with an epoxy functional acrylate terpolymer (EPOBAN). Adhesive properties such as lap shear strength (LSS), T-peel strength (TPS) and flatwise tensile strength (FTS) on honeycomb core bonded sandwich specimens were evaluated using aluminium adherends. The addition of the flow modifier in low concentrations enhanced the flexibility of the system and resulted in a marginal increase in LSS, TPS and FTS. Replacing novolac epoxy partly with solid DGEBA resulted in a less brittle system with enhanced LSS and TPS, but with reduced FTS due to the decreased flow characteristics. A substantial increase in FTS was observed when CTBN was partly replaced with EPOBAN. The introduction of EPOBAN resulted in good flow and fillet properties and the optimum FTS was obtained for the composition based on 25/75 CTBN/EPOBAN ratio. Mechanical properties of selected systems were also studied in addition to adhesive properties.     

Curing and toughening of a styrene‐modified epoxy resin

A commercially available epoxy resin (E907) formulated with a viscosity‐reducing styrene monomer and several additives was subjected to thermal cure studies and mechanical property measurements. Thermoplastic poly(arylene ether sulfone) (PES) and poly(arylene ether phosphine oxide) (PEPO) with reactive amine or hydroxyl end groups were utilized to toughen and co‐cure with the system. The cure cycle was optimized and the networks were analyzed via differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical analyzer, scanning electron microscopy, sol–gel extractions, and fracture toughness. A model epoxy resin was prepared from a tetrafunctional epoxy, e.g., MY722, difunctional EPON828, styrene monomer, and benzoyl peroxide initiator (BPO), and was evaluated as a control to assess the possible role of the styrene monomer. The optimized cure cycle for E907 was 6 h at 93°C, followed by a postcure of 2 h at 204°C. The fracture toughness of E907 was increased only marginally with PES and PEPO. In contrast, the model epoxy resin demonstrated a positive effect due to the styrene monomer and BPO and exhibited significantly increased fracture toughness with PES modification. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1504–1513, 2001     

Micro‐void toughening of thermosets and its mechanism

Void toughening is studied using epoxy resin. Voids were produced mechanically without any chemical agents so that it was possible to isolate the role of micro‐voids from other factors. Toughness of the epoxy resin owing to voids was improved over that of the control by an order of magnitude. The toughening mechanism was found to be strongly related to intervoid distance and, hence, to void‐to‐crack distance. Large deformation bands between the crack and voids were deduced to be the source of toughening. The intervoid distance was directly measured on scanning electron microscope photos. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1290–1295, 2005     

Synthesis of rosin‐based imidoamine‐type curing agents and curing behavior with epoxy resin

Rosin is an abundantly available natural product. The characteristic fused ring structure of rosin acids is analogous to that of some aromatic compounds in rigidity, and makes rosin and its derivatives potential substitutes for those aromatic compounds. In the study reported, the synthesis of biobased curing agents containing imide structure using rosin and the cure reaction were investigated. Rosin‐based imidoamine‐type curing agents were synthesized, and the chemical structure was confirmed using ¹H NMR, Fourier transform infrared and electrospray ionization spectroscopy. The curing behavior with diglycidyl ether of bisphenol A epoxy was studied using differential scanning calorimetry. The thermal mechanical properties and thermal stability of the cured epoxy resins were evaluated using dynamic mechanical analysis and thermogravimetry, respectively. The results indicate that the curing behavior of the rosin‐based curing agents is similar to that of curing agents with analogous structures. Cured products have good thermal stability due to the presence of the imide group and the bulky hydrogenated phenanthrene ring structure. Rosin acids have a great potential in the synthesis of epoxy curing agents as replacements for some of the current commercial aromatic or cycloaliphatic analogues. Copyright © 2010 Society of Chemical Industry     

Curing behavior and thermal property of epoxy resin with boron‐containing phenol‐formaldehyde resin

The curing reaction of bisphenol‐A epoxy resin (BPAER) with boron‐containing phenol–formaldehyde resin (BPFR) was studied by isothermal and dynamic differential scanning calorimetry (DSC). The kinetic reaction mechanism in the isothermal reaction of BPAER‐BPFR was shown to follow autocatalytic kinetics. The activation energy in the dynamic cure reaction was derived. The influence of the composition of BPAER and BPFR on the reaction was evaluated. In addition, the glass transition temperatures (T_gs) were measured for the BPAER‐BPFR samples cured partially at isothermal temperatures. With the curing conditions varying, different glass transition behaviors were observed. By monitoring the variation in these T_gs, the curing process and the thermal property of BPAER–BPFR are clearly illustrated. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1054–1061, 2000     

环氧固化剂改性甲基环戊二胺的合成与性能研究

采用胺甲基化法对甲基环戊二胺(TAC)进行加成改性,合成了一种新型脂环胺类环氧固化剂,研究了反应温度与时间对固化剂性能的影响,并对TAC改性前后其固化物的物理力学性能进行了测试。结果表明:TAC改性后,其固化物耐冲击性及粘接性能均有明显提高,冲击强度提高159%,ABS剪切强度提高35.6%,Al片剪切强度提高47.5%。     

Behavior of polyamidoamine dendrimers as curing agents in bis‐phenol A epoxy resin systems

Polyamidoamine (PAMAM) dendrimers with different generations (0–5) were investigated as curing agents in epoxy resin systems. Flory's gelation theory and the Avrami equation were used to predict the cure behavior of epoxy resin/PAMAM/imidazole at various temperatures and PAMAM concentrations. The theoretical prediction is in good agreement with the experimental results obtained from the dynamic torsional vibration method. Copyright © 2004 Society of Chemical Industry     

Preparation of poly(1,4‐cyclohexylenedimethylene phthalate)s and their use as modifiers for aromatic diamine‐cured epoxy resin

Poly(1,4‐cyclohexylenedimethylene phthalate) s, prepared by the reaction of phthalic anhydride and 1,4‐cyclohexane dimethanol (35/65 or 73/27 mol % cis/trans or trans alone), have been used to improve the toughness of bisphenol‐A diglycidyl ether epoxy resin cured with 4,4′‐diaminodiphenyl sulfone. The aromatic polyesters include poly(cis/trans‐1,4‐cyclohexylenedimethylene phthalate) (PCP) based on a commercial cyclohexanedimethanol, poly(trans‐1,4‐cyclohexylenedimethylene phthalate) (trans‐PCP) and poly(cis/trans‐1,4‐cyclohexylenedimethylene phthalate) (cis‐rich PCP) prepared from a cis‐rich diol. The polyesters used were soluble in the epoxy resin without solvents and were effective as modifiers for toughening the cured epoxy resin. For example, the inclusion of 20 wt% of PCP (MW 6400 g mol⁻¹) led to an 80% increase in the fracture toughness (K_IC) of the cured resin with no loss of mechanical and thermal properties. The toughening mechanism is discussed in terms of morphological and dynamic viscoelastic behaviours of the modified epoxy resin system. © 2000 Society of Chemical Industry     

Delamination mechanism of high‐voltage coil insulators made from mica flakes and thermosetting epoxy resin

To clarify the delamination mechanism of high‐voltage coil insulators made from mica flakes and epoxy resin due to static mechanical stress, the relationships between the shear strength of the insulator and the physical properties of the component materials were studied. The mechanism of their delamination was thought to be either a lack of epoxy resin between the mica flakes, interface failure between the mica flakes and the epoxy resin, or cleavage of the mica flakes. The first two mechanisms were discounted because the shear strength of the insulator was found to be independent of both the contact angle of the corresponding liquid epoxy resin on the mica flakes and the critical surface tension of the epoxy resin. Furthermore, the shear strength of the model insulator was improved by using an epoxy resin with a higher bending elastic modulus, implying that the delamination mechanism in this system is the cleavage of mica flakes. Therefore, the epoxy resin should have a high elastic modulus to ensure high delamination resistance, that is, the temperature to which the insulators are exposed should be lower than the glass transition temperature of the corresponding epoxy resin. Optical microscope studies also supported these results. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2164–2169, 2001     

Use of an epoxidized oil‐based resin as matrix in vegetable fibers‐reinforced composites

Hemp fibers were used as natural reinforcement in composites of thermosetting vegetal oil based resin. Boards with fibers content from 0 to 65 vol % were produced by hot pressing. The mechanical properties were investigated with flexural testing. The effect of effect has been observed on both modulus and strength, indicating a good fiber–matrix interfacial adhesion, which was confirmed by means of scanning electron microscopy observations. Dynamic mechanical analysis also showed an important reinforcement effect in the polymer rubbery region, where at 180°C the storage modulus increased from 17 MPa for the neat resin to 850 MPa for 65 vol % fiber content composites. It also revealed an glass transition temperature decrease when fiber amount in the composite increased. Additional experiments based on differential scanning calorimetry show a weakly accelerated cure when fibers content increases, which usually lead to a lower T_g. But, this phenomenon alone cannot explain the observed T_g change. Contact angle on hemp evolution with time for the resin components show that anhydride is totally absorbed after a few seconds, whereas contact angle of epoxydized oil decreases slowly. This indicates probably a preferential anhydride absorption that leads to a lower amount of anhydride in the matrix and as a consequence to a reduced T_g. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4037–4043, 2006     

One‐component epoxy resin with imine as water‐initiated latent hardener: Improvement of the mechanical and adhesive properties by the addition of methacrylate copolymer

The effect of addition of methacrylate polymer into a one‐component epoxy resin, containing Epikote 828 and diimine as a water‐initiated hardener, was examined. Although the cured epoxy resin in the presence of methyl methacrylate–butyl acrylate (MMA–BA) copolymer was very brittle, the resin containing MMA–BA–[γ‐(methacryloxy)propyl]trimethoxysilane (TMSMA) copolymer showed good mechanical and adhesive properties. The adhesive strength of the cured epoxy resin containing MMA–BA–TMSMA copolymer was much higher than that without its polymer. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1943–1949, 2005     

Curing kinetics and thermal property characterization of the bisphenol‐F epoxy resin and phthalic anhydride system

The curing kinetics of bisphenol‐F epoxy resin (BPFER) and curing agent phthalic anhydride, with N,N‐dimethylbenzylamine as an accelerator, were studied by differential scanning calorimetry (DSC). Analysis of DSC data indicated autocatalytic behaviour in the first stages of the cure for the system, and that this, could be well described by the model proposed by Kamal, which includes two rate constants, k₁ and k₂, and two reaction orders, m and n. The curing reaction in the later stages was practically diffusion‐controlled. To consider the diffusion effect more precisely, a diffusion factor, ??(α), was introduced into Kamal's equation. The glass transition temperatures (T_gs) of the BPFER/phthalic anhydride samples were determined by means of torsional braid analysis. The thermal degradation kinetics of cured BPFER were investigated by thermogravimetric analysis. © 2002 Society of Chemical Industry     

Toughness and strength improvement of diglycidyl ether of bisphenol‐A by low viscosity liquid hyperbranched epoxy resin

This article reports on the use of low viscosity liquid thermosetting hyperbranched poly(trimellitic anhydride‐diethylene glycol) ester epoxy resin (HTDE) as an additive to an epoxy amine resin system. Four kinds of variety molecular weight and epoxy equivalent weight HTDE as modifiers in the diglycidyl ether of bisphenol‐A (DGEBA) amine systems are discussed in detail. It has been shown that the content and molecular weight of HTDE have important effect on the performance of the cured system, and the performance of the HTDE/DGEBA blends has been maximum with the increase of content and molecular weight or generation of HTDE. The impact strength and fracture toughness of the cured systems with 9 wt % second generation of HTDE are 58.2 kJ/m² and 3.20 MPa m^1/2, which are almost three and two times, respectively, of DGEBA performance. Furthermore, the tensile and flexural strength can be enhanced about 20%. The glass transition temperature and Vicat temperature, however, are found to decrease to some extent. The fracture surfaces are evaluated by using scanning electron microscopy, which showed that the homogeneous phase structure of the HTDE blends facilitates an enhanced interaction with the polymer matrix to achieve excellent toughness and strength enhancement of the cured systems, and the “protonema” phenomenon in SEM has been explained by in situ reinforcing and toughening mechanism and molecular simulation. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2504–2511, 2006     

Synthesis of rosin‐based flexible anhydride‐type curing agents and properties of the cured epoxy

BACKGROUND: Although rosin acid derivatives have received attention in polymer synthesis in recent years, to the best of our knowledge, they have rarely been employed as epoxy curing agents. The objective of the study reported here was to synthesize rosin‐based flexible anhydride‐type curing agents and demonstrate that the flexibility of a cured epoxy resin can be manipulated by selection of rosin‐based anhydride‐type curing agents with appropriate molecular rigidity/flexibility. RESULTS: Maleopimarate‐terminated low molecular weight polycaprolactones (PCLs) were synthesized and studied as anhydride‐type curing agents for epoxy curing. The chemical structures of the products were confirmed using ¹H NMR spectroscopy and Fourier transform infrared spectroscopy. Mechanical and thermal properties of the cured epoxy resins were studied. The results indicate that both the epoxy/anhydride equivalent ratio and the molecular weight of PCL diol play important roles in the properties of cured resins. CONCLUSION: Rosin‐based anhydride‐terminated polyesters could be used as bio‐based epoxy curing agents. A broad spectrum of mechanical and thermal properties of the cured epoxy resins can be obtained by varying the molecular length of the polyester segment and the epoxy/curing agent ratio. Copyright © 2009 Society of Chemical Industry