Vt Bui

Epoxy/castor oil graft interpenetrating polymer networks

Full-interpenetrating polymer networks (IPNs) were prepared from epoxy and castor oil-based polyurethane (PU), by the sequential mode of synthesis and were characterized by different techniques: swelling test, scanning electron microscopy (SEM), thermomechanical analysis (TMA), thermogravimetric analysis (TGA), tensile test, and instrumented impact test. 2,4-Toluene diisocyanate (TDI) was used as a curing agent for castor oil, at a NO/OH ratio = 1.50. Diglycidyl ether of bisphenol A (DGEBA) was cured and crosslinked using 2,4,6-tris(dimethylaminomethyl)phenol (TDMP) at 1.5%, by weight, of epoxy resin. The homogeneous morphology of IPN samples of PU compositions up to 40%, by weight, revealed by SEM may be attributed to some extent to grafting of the PU phase onto the epoxy matrix, which results from the reaction between NCO groups in the PU phase with OH groups in the epoxy matrix. This has some synergistic effect on the thermal resistance and tensile properties of IPNs compared to those of the pure components, such as illustrated by the data from TGA and tensile tests. However, the grafting structure appears not to enhance their impact resistance, which probably requires the formation of rubbery particles of suitable size.

Energetic polyurethanes from branched glycidyl azide polymer and copolymer

Branched glycidyl azide polymer (GAP) and glycidyl azide–ethylene oxide copolymer (GEC) have been prepared by a degradation process using different polyols in the synthesis reaction. The azido homopolymers and copolymers were characterized by gel permeation chromatography and viscometry techniques. Energetic polyurethanes were then obtained from the curing of branched glycidyl azide polymers and copolymers using isophorone diisocyanate as a curing agent. The polyurethanes were characterized using thermomechanical analysis and tensile testing. The polyurethane copolymers have generally a higher elongation at break and a lower glass transition temperature than their GAP homopolymer counterparts. The polyol reactant used in the synthesis of GAP and GEC had an effect on the mechanical properties of the polyurethanes obtained from these polymers.

Radiation effects on aluminum-epoxy adhesive joints

Two epoxy adhesive types, Cole-Parmer and Devcon, were used for preparing aluminum-epoxy bondings. The adherend surfaces, of 30 mm in diameter, were prepared using grits of 120, 240, and 320 followed by a final grit of 400, according to the ASTM D897 standard. The curing was set at 72 h at room temperature. The samples were submitted to irradiation for different times in the pool of a SLOWPOKE-2 reactor which produced thermal neutrons, fast neutrons, and γ rays. The tensile properties of nonirradiated and irradiated samples were obtained with an Instron Tester, model 4206. The failure stress, about 11 MPa for nonirradiated samples, had a large decrease after a short period of irradiation and then constantly increased for longer irradiation periods. This may be explained by a predominant effect of crosslinking over chain scissions for higher irradiation doses. The density data and tensile properties of the bulk cured epoxy (Devcon) also supported the above findings. The presence of water on the bonding joints had an effect of exaggerating the irradiation effects on the strength of joints. The use of the adhesive failure modes to group the results into subgroups has permitted the reduction of the spread of the results from the tensile tests.

Physicochemical and Adhesion Characteristics of High-Density Polyethylene when Treated in a Low-Pressure Plasma under Different Electrodes

ABSTRACT The present investigation studys the effects of different electrodes such as copper, nickel, and stainless steel under low-pressure plasma on physicochemical and adhesion characteristics of high-density polyethylene (HDPE). To estimate the extent of surface modification, the surface energies of the polymer surfaces exposed to low-pressure plasmas have been determined by measuring contact angles using two standard test liquids of known surface energies. It is observed that the surface energy and its polar component increase with increasing exposure time, attain a maximum, and then decrease. The increase in surface energy and its polar component is relatively more important when the polymer is exposed under a stainless-steel electrode followed by a nickel and then a copper electrode. The dispersion component of surface energy remains almost unaffected. The surfaces have also been studied by optical microscopy and electron spectroscopy for chemical analysis (ESCA). It is observed that when the HDPE is exposed under these electrodes, single crystals of shish kebab structure form, and the extent of formation of crystals is higher under a stainless-steel electrode followed by nickel and then copper electrodes. Exposure of the polymer under low-pressure plasma has essentially incorporated oxygen functionalities on the polymer surface as detected by ESCA. Furthermore the ESCA studies strongly emphasize that higher incorporation of oxygen functionalities are obtained when the polymer is exposed to low-pressure plasma under a stainless-steel electrode followed by nickel and then copper electrodes. These oxygen functionalities have been transformed into various polar functional groups, which have been attributed to increases in the polar component of surface energy as well as the total surface energy of the polymer. Therefore, the maximum increase in surface energy results in stronger adhesion of the polymer when the polymer is exposed under a stainless-steel electrode rather than nickel and copper electrodes.

Electron beam modification of space durable polymeric nano-adhesive bonding of ultra-high temperature resistant polymer

Abstract This investigation highlights fabrication of ultra-high temperature resistant polymers such as polybenzimidazole (PBI) by high-performance nano-adhesive. High-performance nano-adhesive is prepared by dispersing carbon nano-fibers into ultra-high temperature resistant epoxy adhesive. Prior to fabrication of PBI, the surface of PBI is ultrasonically cleaned by acetone and then modified by atmospheric pressure plasma with 30, 60 and 90 s of exposure and low-pressure plasma with 30, 60, 120, 240 and 480 s of exposure. Surface characterization of the unmodified and modified PBI sheets is carried out by contact angle measurements and surface energy of the polymer is estimated. It is observed that the polar component of surface energy leading to total surface energy of the polymer increases significantly when exposed to atmospheric pressure plasma. Tensile lap shear strength of adhesive bonded PBI reveals that atmospheric pressure plasma is more useful than low-pressure plasma in terms of adhesive bond strength of PBI and increases further when fabricated by nano-carbon fibers dispersed epoxy adhesive. The nano-adhesive bonded PBI sheets are post-cured by electron beam radiation under the SLOWPOKE-2 nuclear reactor. Post curing under electron beam radiation further increases the adhesive bond strength considerably.

Transverse Compressibility of a Commingled Roving

The transverse compressibility of a commingled roving composed of continuous glass and polymer fibers was measured and compared with the respective transverse compressibility of a homogeneous polymer roving and of a homogeneous glass roving. The transverse compressibility of each homogeneous roving was fitted with an existing model developed for aligned fibers. The model parameters for each of the two homogenous roving were then applied in parallel and in series to reproduce the transverse compressibility of the commingled roving. The configuration applying the homogeneous polymer and glass roving in series reproduced best the transverse compressibility of the studied commingled roving.