Chorappan Pavithran

Cardanol as a dispersant plasticizer for an alumina/toluene tape casting slip

Abstract Cardanol, a naturally occurring C15 alkyl chain substituted phenol obtained from cashew nut shell liquid (CNSL), is used as a dispersant and plasticizer for PMMA binder based alumina tape casting slips in toluene medium. The best dispersion of an alumina powder with BET surface area of 10.4 m2/g occurs at a cardanol concentration 2 wt.% of the powder at which the cardanol molecules form a monolayer on the particles with an end-on-adsorbed configuration through phenolic hydroxyl anchored on the surface. Cardanol added in excess of the dispersing agent acts as a plasticizer for PMMA, as revealed by significant decrease in the Tg of the polymer, reduction in the slurry viscosity and increase in the tape flexibility. Green tapes with good flexibility (failure strain, 10–20%) and strength (5.7–7 MPa) were obtained using a binder content 12 wt.% of alumina and employing a plasticizer to binder ratio in the range 0.66–0.82 by weight. The green tapes were thermally debinded with or without extraction of the plasticizer with methanol. The solvent extraction of the plasticizer prior to thermal debinding, however, did not show any effect on the density (∼97% TD) of the tapes sintered at 1500°C.

Gelcasting of Alumina from Acidic Aqueous Medium using Acrylic Acid

Abstract An aqueous gelcasting process for alumina using an acrylic acid monomer has been investigated. Concentrated aqueous alumina slurries with pH 3 were set in moulds at 80°C during 10–15 min by polymerisation of the monomer contained in it using ammonium persulphate as free-radical initiator. The green density maximum (57% TD, corrected for organic content) was obtained at ∼6 wt% binder content, at which the green strength (by diametrical compression test) measured was 11.5 MPa. The green bodies with binder content ⩾4 wt% were amenable to conventional machining operations such as lathing, milling and drilling. The total linear shrinkage of 55 vol% A16SG alumina slurry was 1.4% during drying and 15–17% during binder removal and sintering to 97% TD.

Bifunctionalized hybrid silica spheres by hydrolytic cocondensation of 3-aminopropyltriethoxysilane and vinyltriethoxysilane.

Facile, surfactant free synthetic strategy for bifunctionalized hybrid silica spheres (HS) with structural ordering is presented. HS was prepared by casting and drying of stable siloxane solution from hydrolytic co-condensation of 3-aminopropyltriethoxysilane (AS) and vinyltriethoxysilane (VS) with AS:VS mole ratio of 1:3 in ethanol/water mixture. Spheres of size in the range of 250 nm to 2.5 microm were produced by adjusting the concentration of reacted siloxane solution using ethanol. Characterization by FTIR, XRD, TGA, and DSC revealed that the HS was formed from coprecipitation of fully condensed polyhedral oligomeric silsesquioxane (POSS) bilayer and incompletely condensed siloxanes (SIL) produced during drying. Formation of POSS bilayer was confirmed by intercalating and stabilizing POSS within a smectite clay and characterizing the modified clay. XRD, FTIR, SEM, and HRTEM of HS heated to 170 degrees C revealed transformation from disordered into ordered lamellar structure of POSS bilayer assembly and siloxane network due to rearrangement and densification of low melting SIL.

Microvesicles through Self-Assembly of Polystyrene−Clay Nanocomposite

Polystyrene-clay nanocomposite (PCN) particles, which exhibited concentration dependent self-assembling property in THF were synthesized by in situ intercalative polymerization of styrene with vinyl POSS amine-modified organoclay. While the particles from dilute solution of 0.001 mg mL(-1) showed a lateral dimension of 190-800 nm and thickness of 120 nm, microvesicles of diameter of 2.5-3.5 microm and average membrane thickness of 85 nm were produced from solution concentration of 2.5 mg mL(-1) by consuming the particles. The particle possessed a sandwich structure consisting of polystyrene(PS)-POSS-intercalated clay tactoid of thickness of 12.6 nm at the core and PS chains growing from the tactoid surfaces on either side, exposing the hydroxylated edges of the silicate layers. Vesicle was formed by edge-edge association of the silicate layers so that the layers lie flat along the vesicle membrane. Guest-encapsulated vesicles were obtained when prepared from solutions containing guest molecules.

Gel Casting of Alumina using Boehmite as a Binder

Abstract The process for alumina gel casting was developed using an inorganic binder The monohydroxy aluminium oxide (boehmite, AlOOH) was incorporated with ultrafine alumina of particles (0·5 μ m) and the slurry rheology was studied and presented. The effect of boehmite in slurry viscosity was observed with respect to different amounts of boehmite and time. The alumina 54 vol% slurry with 10 wt% boehmite showed the viscosity of 880 mPa s at 93 s −1 . An external coagulating agent, HMTA, was incorporated with alumina–boehmite slurry and the effective change in slurry viscosity with respect to concentration and time was studied. The addition of HMTA results in faster gelation and the optimum concentration was determined as 0·21 mol L −1 . The alumina gelcast body was dried under humidity conditions at 40°C, RH 70%. The defect free dried green body was obtained and the total linear drying shrinkage was calculated as 3·2% and the green density observed was 59·3% of theoretical value. The sintered density of 98% (TD) was achieved at 1450°C in 2 h. The mechanical hardness of sintered alumina measured as 2286 kg mm −2 . The sintered ceramic showed an extremely fine grained microstructure with an average grain size μ m. The boehmite acts as an excellent binder and sintering aid for alumina ceramics.

A comparative evaluation of a novel flame retardant, 3-(tetrabromopentadecyl)-2,4,6-tribromophenol (TBPTP) with decabromodiphenyloxide (DBDPO) for applications in LDPE- and EVA-based cable materials

Flame retardation of polymeric materials for cables is becoming a statutory requirement due to governmental regulations to protect life and property from damages caused by fire. This and other factors such as the ever-increasing cost of existing flame retardants (FRs) have given rise to the search for better FRs. In this article, the suitability of an FR, 3-(pentadecyltetrabromo)-2,4,6-tribromophenol (TBPTP) developed from cardanol was evaluated for use in cable insulating and jacketing materials based on low-density polyethylene (LDPE) and ethylene vinyl acetate (EVA). The processability, mechanical properties, compatibility and miscibility, thermal behavior, flammability behavior, smoke generation, acid emission, aging characteristics etc., of the blends of the FR with LDPE and EVA were studied in comparison to those of decabromodiphenyl oxide (DBDPO), which is a standard FR used by the cable industry. Although TBPTP is found to be less thermally stable than is DBDPO, it exhibited better flame retardancy and has comparable thermal stability when blended with LDPE and EVA. Both LDPE-TBPTP and EVA-TBPTP blends produced less smoke than did the corresponding blends of DBDPO. In the case of the EVA-TBPTP blend, the percentage emission of smoke was almost negligible, placing EVA-TBPTP under the low smoke grade. Formulations containing a synergistic agent, promoter, and filler with the corresponding FR and polymer polymer along with an antioxidant were extruded out into wire and tested for cable properties. At 20% loading, the LOI values of the blends were 34.6 and 32.5, respectively, for the TBPTP-EVA and DBDPO-EVA blends. Vertical burning tests carried out with EVA-TBPTP cable showed that it is self-extinguishable. The processability of the compositions containing TBPTP were better than those of DBDPO. The improved processability was found to be due to the plasticising effect of TBPTP. SEM pictures of the blend showed excellent distribution of TBPTP in the polymer, indicating good compatibility and miscibility. Comparatively, DBDPO did not exhibit uniform distribution. The mechanical properties of the blends were within specifications of standard cable materials except that the % elongation of the DBDPO-LDPE blend was far too low. Aging studies also gave better properties for the TBPTP system than for those of the DBDPO system. The overall results show that the properties of EVA-TBPTP cable fall within specifications for the FARLS grade, whereas the EVA-DBDPO cable did not. In the case of LDPE, both TBPTP and DBDPO did not satisfy specifications for the FRLS grade, but the data indicate that they can be used as FRs. The superiority in properties of the TBPTP system over DBDPO is explained in terms of the structure of TBPTP characterized by the distribution of the flame-retardant element, bromine, almost evenly between the aliphatic and aromatic moieties of the molecule, which can, in contrast to the fully aromatic DBDPO, provide halogen over a wide range of temperatures to the combustion zone of the decomposing polymer. Moreover, the presence of the aliphatic segment assures improved processability and compatibility.

Surface modification of SiC powders by hydrolysed aluminium coating

SiC powders are surface modified to behave like alumina in aqueous suspensions by coating the powders with in situ generated hydrolyzed aluminium from dilute aqueous aluminum nitrate solutions in a pH range of 3–4.5 using hexa-methylene-tetramine as the base generator at ambient temperature. By examining the zeta potential and rheological properties of aqueous suspensions of the powder with different Al-coverage, it was observed that the coated powder begins to show alumina-like surface properties at an Al-coverage of 0.1 mg/m2, in contrast to 0.5 mg/m2 that was the minimum value reported earlier for observing the effect. This is explained by proposing a coating mechanism which proceeds through adsorption of a layer of cationic hydrolysed aluminum molecules, such as [Al(OH)(H2O)5]2+ and [Al2(OH)2(H2O)8]4+, during which the particles attain alumina-like surface charge properties. The modified powder retained alumina-like surface characteristics when stored under moist conditions and in acidic suspensions. The low value of Al-coverage and the resulting low increase (<10%) of powder surface area are advantageous for preparation of concentrated SiC and SiC–Al2O3 composite suspensions in acidic aqueous medium with rheological properties similar to that of their counterparts containing alumina only.

Gelcasting of alumina using urea-formaldehyde I. Preparation of concentrated aqueous slurries by particle treatment with hydrolysed aluminium

A treatment of α-alumina with hydrolysed aluminium (HA), generated in situ by homogenous hydrolysis of aqueous aluminium nitrate solution using hexamethylene tetramine (HMTA), has been evolved to enhance the surface activity of the powder towards complexation with H+ and thus to promote dispersion of its aqueous concentrated (>50 vol%) slurries using a mineral acid (nitric acid) with a view to prepare slurries having high solids loading and low viscosity for gelcasting. Viscosity minimum of the slurries was obtained at HA minimum (Al3+ added in the system)∼6×10−6 mol/m2 (BET surface area) of the powder. Slurries of solids loading >60 vol% having viscosity <1 Pa s at 93 s−1 could be prepared by HA treatment of α-aluminas of median particle size 0.3–0.8 μm to obtain green bodies with relative density of 65–71% by evaporational drying. ©

Micropatterned Surfaces through Moisture-Induced Phase-Separation of Polystyrene—Clay Nanocomposite Particles

We report micropatterned polystyrene-clay nanocomposite (PCN) surfaces with concavities by moisture-induced phase separation of PCN particles. Micropatterned film with concavity size of 800 nm to 1.3 microm and a high number density of 2 x 10(8) features/cm(2) was obtained by drop-casting PCN solution (20 mg/mL PCN/THF) under ambient relative humidity of 70-80%. It is proposed that water droplets were channeled through the hydrophilic interfaces between the PCN particles, and the two-dimensional array of concavities was formed by spontaneous phase separation due to the presence of rigid clay platelets. The concavity size and number density can be tuned by varying the solvent for PCN. Micropatterned film with concavity size in the range of 650 nm to 1.1 microm with a number density of 5 x 10(7) features/cm(2) was obtained using chloroform as solvent, whereas a concavity size of 150-740 nm and number density of 10(8) features/cm(2) were obtained using carbon disulfide.

Preparation of extrudable alumina paste by coagulation of electrosterically stabilized aqueous slurries

Abstract The present work demonstrates the conversion of a low viscous concentrated (55 vol.%) alumina slurry into a thick extrudable paste by in situ coagulation. Highly concentrated (55 vol.%) electrosterically stabilized aqueous alumina slurry prepared using polyacrylate dispersant was coagulated by in situ generation of acetic acid. Acetic anhydride was used for acid generation and the miscibility of acetic anhydride in aqueous medium was increased by polyethylene glycol. Addition of acetic anhydride >0.16 mol/l converts the slurry in to a stiff paste, within 5 min. The paste exhibits extrudable consistency. Incorporation of 1% PVA in the slurry provides additional strength to the extruded tubes against deformation and development of drying cracks. The tubes sintered to 97% TD and the sintered samples showed a fine grained microstructure.