R. Ramaswamy

Reactive compatibilization of a nitrile rubber/phenolic resin blend: Effect on adhesive and composite properties

Resole phenolic resins containing various p-cresol (PC) to phenol (P) mol ratios were prepared and characterized. These phenolic resins were blended with nitrile rubber (NBR) and the measurements of adhesive joint strength, stress–strain properties, DSC, TGA, DMA, TEM, and SEM were performed using a 50 : 50 NBR/phenolic resin blend. It was observed that the adhesive joint strength and the mechanical properties of the blend enhanced significantly on incorporation of p-cresol into the phenolic resin, and the optimum p-cresol/phenol mol ratio was in the vicinity of 2 : 1. Observation of a more continuous phase and the increase in Tg of the rubber region in the blend indicated increased reactivity and compatibilization of NBR with phenolic resin as p-cresol was incorporated. The effect of silica filler on the properties of the nitrile rubber/phenolic resin blend was also studied without and with p-cresol modification and the results suggest that silica filler take not only the role of a reinforcing filler in the nitrile–phenolic–silica composite, but also a role as surface compatibilizer of the blend components.

Epoxidized hydroxy-terminated polybutadiene — synthesis, characterization and toughening studies

Abstract Hydroxy-terminated polybutadiene was epoxidized using performic acid generated in situ. Different grades of epoxidized hydroxy-terminated polybutadiene ( ehtpb ) were prepared by adjusting reaction conditions such as temperature, time and molar ratios of formic acid and hydrogen peroxide. The ehtpb was characterized by chemical and spectroscopic methods. EHTPB was found to be a good toughening agent for epoxy resins, which are brittle at room temperature. The mechanical properties of a toughened epoxy with ehtpb as the toughening agent were evaluated. Lap shear strength and T-peel strength were observed to increase with increasing ehtpb content, pass through a maximum at an ehtpb content of about 10 parts per 100 parts epoxy resin (pphr) and then decrease. The enhancement of mechanical properties was attributed to the higher toughness produced by the dispersed rubber particles. At higher ehtpb content, the rubber phase became continuous, and the system exhibited a fall in mechanical properties due to the flexibilization effect. The glass transition temperature ( T g ) of the cured epoxy resin was not affected by the incorporation of up to 10 pphr ehtpb but higher levels of ehtpb decreased T g due to flexibilization.

Thermal decomposition characteristics of Alder-ene adduct of diallyl bisphenol A novolac with bismaleimide: effect of stoichiometry, novolac molar mass and bismaleimide structure

The addition-cured blends of diallyl bisphenol A formaldehyde resin (ABPF) with various bismaleimides (BMIs) were evaluated for thermal stability and degradation behavior by thermogravimetric analysis (TGA). TGA of the blend of ABPF and 2,2-bis 4-[(4-maleimido phenoxy) phenyl] propane (BMIP) with varying maleimide to allylphenol stoichiometry indicated that the thermal stability of the system was only marginally improved by the increase in BMI stoichiometry in the blend. The effect of BMI structure on thermal stability was studied using four different BMIs, viz. bis (4-maleimido phenyl) methane (BMIM), bis (4-maleimido phenyl) ether (BMIE), bis (4-maleimido phenyl) sulfone (BMIS) and BMIP. TGA showed a two stage decomposition pattern for BMIS system and a single stage for all the other three. The thermograms of BMIM and BMIE were identical and superior to that of BMIS; the latter showing a relatively poor performance at lower temperatures. Compared to the BMI-adduct of monomeric diallyl bisphenol A (DABA), the polymeric analog viz. ABPF system exhibited better thermal stability. Non-isothermal kinetic analyses of the different systems showed the decomposition occurring in at least two kinetic steps. The computed activation energy exhibited a direct correlation to the relative thermal stability of the systems.

Adhesive and thermal characteristics of maleimide-functional novolac resins

A novel, addition-curable maleimide-functional novolac phenolic resin was evaluated for adhesive properties such as lap shear strength and T-peel strength using aluminium adherends, when thermally self-cured and cocured with epoxy resins. The adhesive properties of the self-cured resin, although inferior at ambient temperature, improved at high temperature and were found to depend on the cure conditions. When cocured with epoxy resin, the adhesive properties improved significantly and showed a strong dependence on the nature of the epoxy resin used, on the stoichiometry of the reactants, on the concentration of imide groups in the phenolic resin, and on the extent of polymerization of the maleimide groups. Optimum adhesive properties were obtained for novolac resins with a moderate concentration of maleimide groups, taken on a 1 : 1 hydroxyl–epoxy stoichiometry with a novolac epoxy resin. In comparison to the conventional novolac, the imide–novolac contributed to improved adhesion and better adhesive property retention at higher temperature when cured with the novolac–epoxy resin.

Adhesive Characteristics of Alder-Ene Adduct of Diallyl Bisphenol a Novolac and Bisphenol a Bismaleimide

Diallyl bisphenol A–formaldehyde copolymer (ABPF) was addition cured with bisphenol A bismaleimide (BMIP) making use of the Alder-ene reaction at high temperatures. The lap shear strength (LSS) of the system was found to depend on the conditions of cure and the stoichiometry of the reactants. Moderate cross linking achieved at a 1:1 maleimide:allylphenol stoichiometry and a stepwise cure, up to a maximum of 250 °C for 2 h, was found to be the most effective in achieving the optimum LSS properties. The system exhibited greater than 100% retention of the LSS at temperatures up to 250 °C. Matrix modification using polysulfone (PS) and polycarbonate (PC) resulted in a remarkable improvement in the adhesive characteristics, although the high-temperature retention was marginally adversely affected. The performance advantage both at room temperature (RT) and at high temperature was greater in the case of PS modification, showing an optimum improvement at 20% loading as against PC modification, exhibiting maximum properties at 10% loading. Scanning electron microscopy (SEM) analysis confirmed that the fine dispersion of PS, rather than large size nodules found in PC, was conducive for the better performance of the former. Dynamic mechanical analysis (DMA) corroborated the observations made in SEM. The existence of co-continuous phases of thermoplastic, matrix and thermoplasticdissolved matrix was evidenced in the PS modification and a clear phase separation was evident in the case of the PC modified system, manifesting independent glass transitions by the individual phases.

Epoxy–imide resins from N-(4- and 3-carboxyphenyl)trimellitimides. I. Adhesive and thermal properties

Epoxy–imide resins were obtained by curing Araldite GY 250 (diglycidyl ether of bisphenol-A and epichlorohydrin; difunctional) and Araldite EPN 1138 (Novolac–epoxy resin; polyfunctional) with N-(4- and 3-carboxyphenyl)trimellitimides derived from 4- and 3-aminobenzoic acids and trimellitic anhydride. The adhesive lap shear strength of epoxy–imide systems at room temperature and at 100, 125, and 150°C was determined on stainless-steel substrates. Araldite GY 250-based systems give a room-temperature adhesive lap shear strength of about 23 MPa and 49–56% of the room-temperature adhesive strength is retained at 150°C. Araldite EPN 1138-based systems give a room-temperature adhesive lap shear strength of 16–19 MPa and 100% retention of room-temperature adhesive strength is observed at 150°C. Glass transition temperatures of the Araldite GY 250-based systems are in the range of 132–139°C and those of the Araldite EPN 1138-based systems are in the range of 158–170°C. All these systems are thermally stable up to 360°C. The char residues of Araldite GY 250- and Araldite EPN 1138-based systems are in the range of 22–26% and 41–42% at 900°C, respectively. Araldite EPN 1138-based systems show a higher retention of adhesive strength at 150°C and have higher thermal stability and Tg when compared to Araldite GY 250-based systems. This has been attributed to the high crosslinking possible with Araldite EPN 1138-based systems arising due to the polyfunctional nature of Araldite EPN 1138.

Effect of elastomer modification on the adhesive characteristics of maleimide-functional phenolic resins

The effect of addition of elastomeric modifiers on the adhesive properties like lap shear strength and T-peel strength of an addition curable, maleimide functional novolac phenolic resin (PMF), self-cured and cocured with a novolac epoxy resin, was studied using aluminium adherends. The modifiers used were (1) two grades of carboxyl terminated butadiene acrylonitrile copolymer (CTBN) of different molecular weights, (2) a low molecular weight, epoxidized hydroxyl-terminated polybutadiene, and (3) a high molecular weight acrylate terpolymer containing pendant epoxy functionality. The adhesive properties, when examined as a function of the varying concentrations of the additives, ranging from 10 to 30 parts per hundred parts (phr) of the resin, were found to depend on the nature of the matrix being modified as well as on the nature and concentration of the elastomer. The adhesive properties at ambient temperature of the self-cured, highly brittle PMF resin were dramatically improved by the inclusion of all the elastomers, the increase being substantial in the case of high molecular weight CTBN. For the more rigid, less ductile, epoxy-cured PMF system, the adhesive properties were marginally improved by the high molecular weight CTBN, whereas the other elastomers were practically ineffective. For both self-cured and epoxy-cured PMF systems, the inclusion of these elastomers generally decreased the high-temperature adhesive properties, implying impairment of thermal characteristics, evidenced also from their dynamic mechanical spectra. The presence of phase-separated elastomer particles in the modified systems has been evidenced from scanning electron micrographs.

Toughening of Diglycidyl Ether of Bisphenol-A Epoxy Resin Using Poly (Ether Ether Ketone) with Pendent Ditert-Butyl Groups

Poly(ether ether ketone) (PEEKDT), hydroxyl terminated poly(ether ether ketone) (PEEKDTOH) and fluorine terminated poly (ether ether ketone) (PEEKDTF) with pendent ditert-butyl groups were synthesized by the nucleophilic substitution reaction of 4,4′-difluorobenzophenone with 2,5-ditert-butylhydroquinone in N-methyl-2-pyrrolidone medium using anhydrous potassium carbonate as catalyst. Diglycidyl ether of bisphenol-A epoxy resin was blended with PEEKDT, PEEKDTOH, and PEEKDTF, and cured with 4,4′-diaminodiphenylsulfone (DDS). The polymers formed heterogeneous blends before curing, and upon curing the polymers got dispersed in the epoxy matrix. The mechanical properties of the cured blends were slightly lower than that of the unmodified resin. The fracture toughness increased with the addition of ditert-butyl PEEK into epoxy resin and the extent of improvement was dependent on the type of modifier used. Hydroxyl terminated polymers gave up to 40% increase in fracture toughness. The dynamic mechanical spectrum of the blends showed only a single Tg due to the proximity of the glass transition temperature of modified PEEK and DDS cured epoxy resin.

Adhesive and Thermal Properties of Epoxy-Imide Resins Obtained from Different Diimide-Diacids: Structure-Property Correlations

ABSTRACT Different epoxy-imide resins were prepared through the reaction of epoxy resins namely, Araldite® GY 250 (difunctional; DGEBA) and Araldite® EPN 1138 (polyfunctional; novolac epoxy) with diimide-diacids such as 4,4′-bis(4-carboxyphthalimido)diphenylmethane, 4,4′-bis(4-carboxyphthalimido)diphenylsulphone, 3,3′-bis(4-carboxyphthalimido)diphenylsulphone, 4,4′-bis(4-carboxyphthalimido)diphenylether, 2,2-bis[4-(4-trimellitimidophenoxy)phenyl]propane, 1,13-bis(4-carboxyphthalimido)-4,7,10-trioxatridecane, and 1,6-bis(4-carboxyphthalimido)hexane in 1:1 carboxyl equivalent to epoxy equivalent ratio. The adhesive and thermal properties of these systems were evaluated to arrive at structure–property correlations. It is observed that epoxy-imides with more aliphatic moieties and (or) ether linkages give higher room temperature adhesive strength when compared to those with more aromatic moieties. But the latter systems give higher retention of room temperature adhesive strength at elevated temperatures when compared to that of the former systems. Thermogravimetric analysis shows that epoxy-imide systems are stable up to 360°C and char residues of GY 250- and EPN 1138-based systems at 800°C fall in the range of 14–38% and 23–48%, respectively, in nitrogen atmosphere. This has been attributed to the higher crosslinking possible for the latter systems due to the polyfunctional nature of EPN 1138.

Epoxy-Imide Resins from N-(4- and 3-Carboxyphenyl) Trimellitimides: Modified with Reactive Rubbers

ABSTRACT Epoxy-imide resins obtained through the reaction of epoxy resins such as Araldite® GY 250/Araldite® EPN 1138 with N-(4- and 3-carboxyphenyl)trimellitimides (IDA-I and IDA-II, respectively) have been modified with 10 wt% of epoxidized hydroxyl-terminated polybutadiene (EHTPB), 10 phr of carboxyl-terminated butadiene-acrylonitrile liquid copolymer (CTBN-L), and 10 phr of carboxyl-terminated butadiene-acrylonitrile solid copolymer (CTBN-S) without sacrificing much of their performance at elevated temperatures. Adhesive lap shear strength on stainless steel substrate at room temperature and at 100, 125, and 150°C has been evaluated for the modified and unmodified systems. CTBN-S offers a remarkable increase of 13 MPa and 8 MPa in the room temperature adhesive strength of GY 250-based system and EPN 1138-based system, respectively. EHTPB gives only a marginal improvement and CTBN-L offers an improvement by 4 MPa for GY 250-based system whereas CTBN-L reduces the adhesive strength of EPN 1138-based system. SEM studies suggest that in general, the modification with EHTPB and CTBN-L results only in improving the ductility of epoxy-imide systems, whereas the modification with CTBN-S results in phase separation of rubber particles in the epoxy-imide matrix.