S. Packirisamy

Morphology, mechanical properties, and failure topography of semi-interpenetrating polymer networks based on natural rubber and polystyrene

Semi-interpenetrating networks (semi-IPNs) were prepared from natural rubber (NR) and polystyrene (PS) by the sequential method. In these semi-IPNs the NR phase was crosslinked while the PS phase was uncrosslinked. Different initiating systems such as dicumyl peroxide (DCP), benzoyl peroxide (BPO), and the azobisisobutyronitrile (AIBN) system were used for polymerizing the PS phase. The blend ratio was varied by controlling the swelling of NR in the styrene monomer. The mechanical properties of the semi-IPNs, namely, density, tensile strength, tear strength, elongation at break, tension set, tensile set, impact strength, and hardness, were determined. The morphology of different IPNs was studied using scanning electron microscopy. A compact morphology with a homogeneous phase distribution was observed in the semi-IPNs. The properties of the semi-IPN do not change much with the initiating system. However, in most cases, the DCP initiating system showed slightly superior performance. The tensile and tear-strength values of the IPNs were found to increase with increasing plastomer content. The crosslink density of the semi-IPNs also increased with increase in the polystyrene content. The experimental values were compared with theoretical models such as series, parallel, Halpin Tsai, Coran, Takayanaki, Kerner, and Kunori. The tensile and tear-fracture surfaces were examined using a scanning electron microscope. The fracture patterns were correlated with the strength and nature of the failure.

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.

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.

Thermal degradation kinetics of poly(methylvinylsilylene-co-styrene)

Abstract Thermal degradation kinetics of poly(methylvinylsilylene-co-styrene) copolymers, viz., PMVSS-I to PMVSS-V obtained by reacting methylvinyldichlorosilane (MVDCS) and styrene in 1:0.25, 1:0.5, 1:1, 1:3 and 1:7 mole ratios under dechlorination conditions, using sodium, was studied by thermogravimetry. The homopolymer, poly(methylvinylsilane) (PMVS), synthesized from MVDCS using sodium was also subjected to the above study for comparative evaluation. The kinetic parameters for thermal degradation, viz., activation energy ( E ) and pre-exponential factor ( A ) for the above polymers were estimated by non-isothermal kinetic methods such as Mac Callum–Tanner (M–T), Horowitz–Metzger (H–M), Madhusudhanan–Krishnan–Ninan (MKN) and Coats–Redfern (C–R). The order for thermal degradation of PMVS was found to be almost 0. In the case of the copolymers, the order was 1 for PMVSS-I and 2 for PMVSS-II to PMVSS-V. The observed difference in the order for thermal degradation of PMVSS-I when compared to the other copolymers is attributed to the presence of polysilyl linkages in PMVSS-I. It was found that the activation energy and pre-exponential factor showed an increase in trend with increase in concentration of styrene in the copolymer system.

Characterization of free carbon in the as-thermolyzed Si-B-C-N ceramic from a polyorganoborosilazane precursor

Polyorganoborosilazane ((B[C2H4-Si(CH3)NH]3)n) was synthesized via monomer route from a single-source precursor and thermolyzed at 1300 °C in argon atmosphere. The as-thermolyzed Si-B-C-N ceramic was characterized using X-ray diffraction (XRD) and Raman spectroscopy. The crystallization behavior of silicon carbide in the as-thermolyzed amorphous Si-B-C-N matrix was understood by XRD studies, and the crystallite size calculated using Scherrer equation was found to increase from 2 nm to 8 nm with increase in dwelling time. Concomitantly, Raman spectroscopy was used to characterize the free carbon present in the as-thermolyzed ceramic. The peak positions, intensities and full width at half maximum (FWHM) of D and G bands in the Raman spectra were used to study and understand the structural disorder of the free carbon. The G peak shift towards 1600 cm−1 indicated the decrease in cluster size of the free carbon. The cluster diameter of the free carbon calculated using TK (Tuinstra and Koenl) equation was found to decrease from 6.2 nm to 5.4 nm with increase in dwelling time, indicating increase in structural disorder.

Adhesive Joining of Metal to Metal and Metal to Ceramic by Ceramic Precursor Route

Inorganic ceramic adhesives (geopolymers) based on aluminosilicate matrix are versatile candidates for bonding metals to metals or metals to ceramics. On curing, they result in an amorphous, crosslinked, impervious, acid resistant 3D-structures. Alkali activated aluminosilicate based ceramic adhesive was developed for bonding metals to ceramics and metal to metal, for high temperature applications. The bonding is achieved at 175°C for 3 hrs, by solid state reaction of alkaline solution of allkalisilicate precursor with the refractory filler, contributing to the bulk aluminosilicate matrix. Lap shear strength of 2-4 MPa was obtained for bonding stainless steel. The XRD patterns show the amorphous nature of the aluminosilicate matrix, with mullite formation at higher temperatures. Thermogravimetric analysis shows that the weight loss is only due to the removal of water from the system by means of evaporation and polycondensation of Si-OH groups and Al-OH groups. This is followed by structural reorganisation in which aluminium ions are incorporated into the silicate chains forming the Si-O-Al network resulting in the bonding with the metallic surface. The system can withstand the maximum operational temperatures of the substrates and can be used for bonding different metallic or ceramic, joints/interfaces for RLV-TD/TSTO.

Thermal Barrier Coating on Metallic Substrates by Preceramic Route

A low cost, easily processable, multi layered functionally graded ceramic coating of thickness ~1000 µm was developed to protect metallic substrates from the risk of thermal oxidation. It consists of a four layered functionally graded coating consisting of aluminide layer followed by an intermediate zirconia as thermal insulative layer, an alumina layer serving as buffer layer with the outer most high emissivity layer to provide the required emissivity. All the layers were brush coated and the specimens were cured at 150°C. The solar absorptivity and emissivity of the coating was found to be 0.82 and 0.88 respectively. 15CDV6 plate of 150 x 150 x 5 mm was coated with multilayer thermal barrier coating of thickness ~ 1 mm. The coated sample was subjected to a heat flux of 8.5 W/cm2 for 1035 secs to evaluate the thermo-responsive behaviour of the coating. Maximum back wall temperature measured was 299°C. The coating methodology is simple compared to complicated plasma techniques which can be applied on complex shaped substrates and all operations are carried out at low temperatures, below 150°C ensuring no deterioration of structural properties of the substrate.