Lichang Wang

Surface characterization of electrochemically oxidized carbon fibers

High strength PAN-based carbon fibers were continuously electrochemically oxidized by applying current to the fibers serving as an anode in 1% wt aqueous KNO3. Progressive fiber weight loss occurred with increasing extents of electrochemical oxidation. XPS studies (C 1s and O 1s) indicated that the oxygen/carbon atomic ratio rose rapidly to 0.24 as the extent of electrochemical oxidation was increased from 0 to 133 C/g and then remained almost constant as the extent of electrochemical oxidation rose to 10 600 C/g. Fitting the C 1s spectra demonstrated that the rise in surface oxygenated functional groups was mainly due to an increase in carboxyl (COOH) or ester (COOR) groups. An increase in the intensity of the O 1s peak (534.6–535.4 eV) after electrochemical oxidation corresponded to chemisorbed oxygen and/or adsorbed water. Electrochemical oxidation increased surface activity by generating more surface area via the formation of ultramicropores, and by introducing polar oxygen-containing groups over this extended porous surface. FT-IR spectra showed a broad peak at about 1727 cm−1 from C=O stretching vibrations of carboxyl and/or ketone groups, the relative intensity of which increased significantly with the extent of electrochemical oxidation. Post-oxidation heat-treatments in flowing nitrogen at 550°C for 30 min. caused further weight losses due to decarboxylation of carboxyl groups and other reactions in which oxygenated functions decomposed. These weight losses increased with the extent of electrochemical oxidation. This demonstrated that more oxygenated groups formed on the internal pore surfaces as pores increasingly penetrated deeper into the fibers with increased electrochemical treatment. Weight loss depended on the heat treatment temperature since different types of carbon–oxygen surface groups were formed during the electrochemical oxidations. Different functions have different abilities to decarboxylate or decarbonylate. The amount of Ag+ and NaOH uptake by electrochemically oxidized fibers rapidly decreased as the temperature of the post heat treatment increased to 550°C. Beyond 550°C the progressive decrease in Ag+ adsorption and NaOH uptake continued at a slower rate and approached 0 μmol/g after heating to 850°C. Conversely, after heat treatment I2 adsorption showed a marked increase as the treatment temperature was raised. Thermal decomposition of carbon–oxygen complexes within the pore structure leads to a lower hydrophilicity of the pore surface. The extensive micropore surface area generated by electrochemical oxidation becomes more accessible to I2 as CO2 and CO evolve. Very narrow pores (<10 A diameter) blocked by hydrogen bonding and oxygenated functions become more open. XPS analyses illustrated that the surface oxygen content decreased significantly after heat-treating to 550 or 850°C and was lowest after the 850°C treatment.

Adsorption of precious metal ions onto electrochemically oxidized carbon fibers

Electrochemically oxidized carbon fibers (ECF) adsorbed a prodigious amount of Ag+ in contrast to oxygen plasma and nitric acid treated carbon fibers. The amount of adsorbed Ag+ reached 3700 μmol/g after 5652 C/g of electrochemical oxidation. This value approaches the 4050 μmol/g of Ag+ which adsorbed onto steam-activated Kenaf-based carbon (with a surface area of 1284 m2/g determined by N2/BET) under the same adsorption conditions. ECF oxidized to 9540 C/g adsorbed more than its own weight of Ag+ (12 608 μmol/g). These fibers exhibited a surface area of 115 m2/g (CO2–DR). Two different reactions occurred during Ag+ adsorption. These reactions were ion exchange adsorption between Ag+ and acidic functions (carboxyl) and redox adsorption between Ag+ and reducing functions such as catechol groups on these electrochemically oxidized fibers (ECF). The redox capability was expressed by the reaction electric potential (E) using the Nernst equation. High resolution XPS C 1s spectra of ECFs (level of oxidation 5300 C/g), before and after Ag+ adsorption, showed that the carbon atoms present in phenolic, alcohol or ether groups and those present in carbonyl or quinone groups increased after Ag+ adsorption. X-ray diffraction and X-ray photoelectron spectroscopy (XPS) Ag 3d spectra of the ECF showed that adsorbed Ag+ was reduced to Ag0 after both Ag+ adsorption and subsequent post-heat treatment of the fibers under N2 at 550°C for 30 min. Only about one-third as much Au3+ adsorption occurred versus the extent of electrochemical oxidation as was observed for Ag+. This ratio matches the requirement that three electrons are required to convert Au3+ to Au0 versus one to convert Ag+ to Ag. High resolution angle resolved XPS (ARXPS) Pd 3d and Pt 4f spectra show that there are two different Pd oxidation states and three different Pt oxidation states present after adsorption of Pd2+ and Pt2+ onto ECF. The peak areas as a function of take off angle showed that substantial amounts of Pd0 and Pt0 are present in addition to Pd2+ and Pt2+ and Pt4+ on the outermost surface regions of oxidized fibers.

Wood Enhancement Treatments I. Impregnation of Southern Yellow Pine with Melamine-Formaldehyde and Melamine-Ammeline-Formaldehyde Resins

Abstract Southern yellow pine (SYP) was impregnated with melamine-formaldehyde (MF) or melamine-ammeline-formaldehyde (MAF) resins. The dimensional stability, strength properties, weathering, fire resistance and chemical resistance of the impregnated wood were examined. The wood treated with two commercial MF resins or with a synthesized MAF resin exhibited greatly enhanced dimensional stability, fire resistance and resistance to weathering. The chemical resistance increased moderately.

Characterizing semi-interpenetrating polymer networks composed of poly(vinyl chloride) and 5–15% of oligomeric MDI isocyanate cross-linked networks ☆

Semi-interpenetrating polymer networks (SIPNs) were prepared from PVC and 5‐15 wt.% of di-(4,4-diisocyanatophenyl)methane (MDI) oligomers by directly mixing the liquid MDI with small (150 mm dia.) porous (30% voids) unplasticized PVC particles at low temperatures followed by hot press curing. The tensile, flexural, and impact strengths increased significantly when these small amounts of isocyanate networks were created in PVC. These SIPN blends exhibited tan d peak temperatures and single distinct loss modulus, E 00 , peaks at temperatures lower than those of PVC which had been exposed to the same processing temperatures. These observations rule out the presence of large PVC domains distinct from PVC/isocyanate SIPN domains and pure thermoset domains. A substantial fraction of the isocyanate appears to exist in SIPN type phases in these blends. Considerable amounts of unextracted residue (about 30‐36%) remained after 48 h of continuous THF extraction of these SIPN blends. A crude correlation was noticed between the amounts of SIPN residue present and the mechanical strength improvements. Mathematical modeling of DMTA-derived Tg data by a Tg third power blend composition equation was employed to understand SIPN structures and the nature of PVC/isocyanate interactions. DMTA measurements of segmental mobility indicated that the isocyanate had a lower cross-link density when diluted in PVC than in the pure cured isocyanate. Fitting experimental Tg values gave parameters indicating that both the binary hetero-interactions (enthalpic effects) and the conformational redistributions (entropi c effects) during the binary hetero-interactions contributed to SIPN formation. q 2000 Elsevier Science Ltd. All rights reserved.

Effect of crosslinking on mechanical and viscoelastic properties of Semiinterpenetrating polymer networks composed of poly(vinyl chloride) and isocyanate crosslinked networks

Semiinterpenetrating polymer networks (SIPNs) of PVC/isocyanate/poly-triol were prepared by premixing small (150 μm dia.) porous (30% voids) unplasticized PVC particles, 10% by weight of isocyanate, and a triol at different OH/NCO mol ratios. Three types of isocyanates (methylene bis-phenyl diisocyanate (MDI), oligomeric MDI isocyanates (PAPI), and toluene diisocyanate (TDI) prepolymer/polytriol) were used. Two-roll milling was followed by hot-press curing. The tensile, flexural, and impact strengths increased when small amounts of crosslinked isocyanate networks were created in PVC. The isocyanate/polyol hydroxyl stoichiometry was varied, and the effects of crosslinking on the tensile, impact, and flexural strengths of PVC/isocyanate/triol SIPNs were examined. The strength increments were greater when the OH/NCO mole ratio went from 0 to 0.25, than when it went from 0.25 to 1.0. In many cases, increasing OH/NCO mol ratio from 0.5 to 1.0 decreased tensile, impact, and flexural strengths. Both PAPI and MDI (30% NCO content) gave bigger improvements in the these mechanical strengths than the TDI (only 9.7% NCO). These SIPN blends exhibited lower tan δ peak temperatures and a single distinct loss modulus, E″, peak values at lower temperatures than those of PVC that had been exposed to the same processing temperatures. Substantial amounts of isocyanate networks exist in SIPN phases according to DMTA studies. The OH/NCO ratio did not generally correlate with the decreases in the glass transition temperatures in these three sets of blends.

POLYURETHANE RESINS-TREATED WOOD PALLETS WHICH ARE DECONTAMINABLE OF CHEMICAL WARFARE AGENTS*

Wood pallets manufactured by impregnating and coating wood with selected polyurethane resins performed comparably to steel control pallets in decontamination of major chemical warfare agents before and after being subjected to various rough-handling and strength test procedures. Cost calculations showed that the wood pallets would be competitive with steel pallets. * See Ref. [1].

WOOD MADE DECONTAMINABLE OF CHEMICAL WARFARE AGENTS AFTER SUNLIGHT WEATHERING OR ABRASION

Methods of making wood decontaminable of major chemical warfare agents were investigated using phenol-formaldehyde and polyurethane resins. Selected phenol-formaldehyde resins impregnated in southern yellow pine gave enhanced dimensional stability and fire resistance properties without decreasing strength properties, but the decontaminability for one major chemical warfare agent was inadequate. Selected polyurethane resins impregnated in southern yellow pine gave enhanced dimensional stability and strength properties with only a minor deterioration in fire properties, and southern yellow pine, red oak, and aspen impregnated with these polyurethane resins were adequately decontaminated of all three major chemical warfare agents. A polyurethane resin that performs adequately as a coating material for this purpose was also identified. *See Ref. [1].

Reactions of Defined Oxidized Carbon Fiber Surfaces with Model Compounds and Polyurethane Elastomers

Ex-PAN carbon fiber surfaces, oxidized to varying degrees by HNO 3 , have been characterized by NaOH, dye and HCl uptake, ion scattering spectroscopy (ISS), and angleresolved X-ray photoelectron spectroscopy (ARXPS). Subsequent treatments with tetraethylenepentamine to introduce amino groups or epichlorohydrin to introduce epoxy groups have been thoroughly characterized. The efficiency of using these surface functions to bond polymers onto fiber surfaces has been investigated using model anhydrides and isocyanates (for amines) and diols and diamines (for epoxides). The fraction of surface-bound groups which react drops with an increase in molecular size of the species being grafted. Crosslinked elastomeric polyurethane layers with designed modulus values and thicknesses have been bonded to these fibers. Composites have been prepared (epoxy matrices). The impact strengths and interlaminar shear strengths (ILSS) were studied as a function of the interphase modulus and thickness. Impact strengths increased (even at 500–1500 A thicknesses). ILSS values depend on interphase modulus.