Gangping Wu

Effect of moisture on stabilization of polyacrylonitrile fibers

The method of modification through pre-stabilization stretching in the presence of plasticizers, such as succinic acid [1], boracic acid [2], dimethylformamide [3] and zinc chlorid [4] etc., has been adopted to prepare small-diameter carbon fibers, which contain fewer defects per unit volume. But the method is relatively complicated and at risk of introducing new flaws. To overcome the above drawbacks, the authors have introduced a modification into the stabilization process, and have chosen moisture in air as the plasticizer. The process is easy to control by damping the atmospheric air and facilitated to applied to continuous process. Foremost in importance is that the new species has not been used, which guaranteed the required cleanliness in the process. Experiments were performed on a PAN-based 3000filament batch, and the shrinkage of PAN fibers was monitored by the displacement of weight. Stabilization was carried out between the temperatures of 180– 220 ◦C, air flow of 0.3 m3/h. The stabilized fibers were then introduced into the carbonization furnace. The carbonization furnace was heated in high-purity nitrogen from room temperature to 1000 ◦C at a rate of 5 ◦C/min, and heating ceased when the temperature was reached. Finally the fibers were taken out when cooled down to room temperature. Experimental details have been published elsewhere [5] and the experimental setup was shown in Fig. 1. A special barothermohygrograph was utilized to examine the amount of moisture in damp air. The result from Fig. 2 show that the fibers can be more easily elongated with increasing R.H. in air. The total amount of length elongation was about 0.2% at 40% of R.H. as compared with 0.8% at 80%. This means that

Effect of hydrophobic group in polymer matrix on porosity of organic and carbon aerogels from sol–gel polymerization of phenolic resole and methylolated melamine

Abstract Organic and carbon aerogels were prepared by solution–sol–gel polymerization of phenolic resole and methylolated melamine followed by supercritical drying and pyrolysis. The hydrophobic group was incorporated into polymer matrix by adding m -cresol in the solution–sol–gel step and the effect of addition on porosity of organic and carbon aerogels was elucidated by nitrogen adsorption and density measurement. The ratios of m -cresol to phenolic resole (R) were changed from 0/7.5 to 2.5/7.5 while ratios of other components to phenolic resole remained unchanged. It is found that the total pore volumes of organic and carbon aerogels exhibit maxima at 1/7.5 and are determined by the total concentrations of reactants and cumulative volume shrinkages of gels from hydrogels to organic aerogels and from hydrogels to carbon aerogels respectively. The macropores in organic aerogels, developed in supercritical drying process, are determined by gel discrete particles–gel discrete particles interactions, which can be tuned by the incorporated hydrophobic groups. The micropores in carbon aerogels are generated by evolving volatile compounds in the pyrolysis process. The meso- and macropores in carbon aerogels are determined by (1) meso- and macropores in relevant organic aerogels; (2) volume shrinkages that convert macropores to mesopores and mesopores to micropores; (3) mass loss in pyrolyzing organic aerogels that increases sizes and volume of carbon aerogels; and (4) coarsening that converts mesopores to macropores to reduce interfacial energy. The mesopore size distributions of organic and carbon aerogels exhibit maxima at 1/7.5, which is also related to the actions of the hydrophobic groups. The BET surface areas of organic aerogels and external surface areas of carbon aerogels are determined by sizes and volumes of mesopores and the BET surface areas of carbon aerogels are determined by micro- and mesopores.

Structural heterogeneity and its influence on the tensile fracture of PAN-based carbon fibers

This communication reported an intuitive and convenient approach to detect the structural heterogeneity of carbon fibers by using Raman spectroscopy. As indicated by both the linear and mapping modes, the skin–core difference was enhanced with the rising temperature during the graphitization. This enhanced structural heterogeneity was observed to strongly influence the tensile fracture mode and the tensile properties of the graphitized fibers.

Structural evolution during the graphitization of polyacrylonitrile-based carbon fiber as revealed by small-angle X-ray scattering

On the basis of a Debye–Bueche correlation length analysis, the small-angle X-ray scattering (SAXS) intensity components due to different scatterers within polyacrylonitrile-based carbon fiber were determined and analyzed separately. According to Guiniers law and other related methods, an intensity component indicating a relatively large scatterer was ascribed to the amorphous structure within the boundaries of fibrils. Results indicated that the amorphous regions decreased in dimension and finally transformed completely into voids as the heat treatment temperature rose to 2773 K. The general trend for microvoids was a systematic change from many small voids to a few large voids, while the local density fluctuation within the samples weakened and finally faded away. In conclusion, the graphitization process of carbon fibers as revealed by SAXS is a systematic evolution from a quasi-two-phase system (fibril, amorphous region and microvoid within the fibril) of high-strength carbon fiber to the true two-phase structure (crystallite and microvoid) of high-modulus graphite fiber.

Preparation and characterization of a polyimide coating on the surface of carbon fibers

Abstract Organic, solvent-free polyamic acid sizing was coated onto T300 grade carbon fibers (3k) to prepare a polyimide (PI) coating having a high thermal stability and oxidative resistance. The surface of PI-coated carbon fibers was characterized by FTIR and SEM. The mechanical strength of the carbon fibers, thermal stability and oxidative resistance of the coating were also investigated. Results indicate that a continuous and uniform PI coating is formed on the surface of the carbon fibers. Compared to a carbon fiber coating with epoxy, the PI coating produces excellent thermal stability with onset decomposition and a 5% weight-loss temperatures of 567 and 619 °C, respectively. The tensile strength of PI-coated carbon fibers after thermal oxidation in air at 400 °C for 1 h has only a slight decrease of 6%, which is significantly lower than the decrease of 22% for epoxy-coated fibers.

Rheological and thermal properties of acrylonitrile-acrylamide copolymers: Influence of polymerization temperature

AbstractAn attempt was made to correlate the polymerization temperature and rheological and thermal properties of acrylonitrile (AN)-acrylamide (AM) copolymers. The copolymers were synthesized at different polymerization temperature. The copolymer structure was characterized by gel permeation chromatography (GPC) and Infrared spectrum (IR). The rheological and thermal properties were investigated by a viscometer and differential scanning calorimeter-thermogrametric (DSCTG) analysis, respectively. When the polymerization temperature increased from 41 °C to 65 °C, the molecular weight

Fracture mechanisms of polyacrylonitrile-based high-strength type carbon fibers

(\overline M _w )

Influence of Tension on the Oxidative Stabilization Process of Polyacrylonitrile Fibers

of copolymers decreased from 1,090,000 to 250,000, while its conversion increased from 18% to 63%, and the polymer composition changed slightly. To meet the requirements of carbon fibers, the rheological and thermal properties of products were also investigated. It was found that the relationship between viscosity and