Polybenzoxazole Fibers Research Articles

Effects of Drawing and Heat-Treatment Conditions on the Structure and Mechanical Properties of Polyhydroxyamide and Polybenzoxazole Fibers

We report the preparation of polybenzoxazole (PBO) fiber from polyhydroxyamide (PHA) precursor fiber which is free from strong acid such as polyphosphoric acid. We prepared the PHA fibers with different spin-draw ratios (SDRs) using a wet-spinning method and the PBO fibers with an SDR of 3.5 (SDR-3.5 PBO fibers) were prepared by various heat-treatment temperatures, and investigated their morphology, crystalline structure, and mechanical properties. The simultaneous thermogravimetric analysis-mass spectrometry (STA-MS) and field-emission scanning electron microscopy (FE-SEM) results confirmed that the diameter of the SDR-3.5 PBO fiber was much smaller than that of the SDR-3.5 PHA fiber, due to the release of water during the thermal cyclization reaction which forms the PBO structure. The wide-angle Xray diffraction (WAXD) pattern of the SDR-3.5 PBO fiber heat-treated at 350 °C (SDR-3.5 PBO 350 fiber) showed two peaks, at 2θ=14.83 ° and 24.38 °, and the diffraction angles dropped with increasing heat-treatment temperature. In addition, the initial modulus and tensile strength of the SDR-3.5 PBO fiber heat-treated at 550 °C (SDR-3.5 PBO 550 fiber) were found to be 19.1 GPa and 449.2 MPa, which were much higher than those of the SDR-3.5 PHA fiber, 9.3 GPa and 227.0 MPa, respectively.

Erosion characteristics and mechanical behavior of new structural hybrid fabric reinforced polyetherimide composites

In this work, a new structural hybrid fabric reinforced polyetherimide composite was prepared using a solution impregnation method. The effects of fabric orientation, i.e., the angle between the projected line of the impact path of the solid particle on the specimen surface and the axis of the polybenzoxazole (PBO) fibers, and weft density on the erosion behavior of the prepared hybrid composites were investigated. The results show that the erosion rates of the hybrid composites decreased with increasing fabric orientation angle and weft density. No obvious crimp of carbon fibers was observed in the cross-section of a hybrid fabric prepreg sheet, therefore composites with excellent mechanical properties can be obtained. The carbon fibers were covered with PBO fibers and resin, and these protect the carbon fibers from damage well. The prepared hybrid composites showed better erosion resistance than carbon fiber reinforced thermoplastics and better mechanical properties than PBO fiber reinforced thermoplastics. Dynamic mechanical analysis showed that the prepared hybrid composite has high heat resistance.

Methodology for Detecting Residual Phosphoric Acid in Polybenzoxazole Fibers

Because of the premature failure of in-service soft-body armor containing the ballistic fiber poly[(benzo-[1,2-d:5,4-d']-benzoxazole-2,6-diyl)-1,4-phenylene] (PBO), the Office of Law Enforcement Standards (OLES) at the National Institute of Standards and Technology (NIST) initiated a research program to investigate the reasons for this failure and to develop testing methodologies and protocols to ensure that these types of failures do not reoccur. In a report that focused on the stability of the benzoxazole ring that is characteristic of PBO fibers, Holmes, G. A.; Rice, K.; Snyder, C. R. J. Mater. Sci. 2006, 41, 4105-4116, showed that the benzoxazole ring was susceptible to hydrolytic degradation under acid conditions. Because of the processing conditions for the fibers, it is suspected by many researchers that residual phosphoric acid may cause degradation of the benzoxazole ring resulting in a reduction of ballistic performance. Prior to this work, no definitive data have indicated the presence of phosphoric acid since the residual phosphorus is not easily extracted and the processed fibers are known to incorporate phosphorus containing processing aids. Methods to efficiently extract phosphorus from PBO are described in this article. Further, characterization determined that the majority of the extractable phosphorus in PBO was attributed to the octyldecyl phosphate processing aid with some phosphoric acid being detected. Analysis by matrix assisted laser desorption ionization of model PBO oligomers indicates that the nonextractable phosphorus is attached to the PBO polymer chain as a monoaryl phosphate ester. The response of model aryl phosphates to NaOH exposure indicates that monoaryl phosphate ester is stable to NaOH washes used in the manufacturing process to neutralize the phosphoric acid reaction medium and to extract residual phosphorus impurities.

A detailed investigation of the mechanical properties of polybenzoxazole fibers within soft body armor

Using the modified-single fiber test developed by Holmes and colleagues (J Appl Polym 2008), a detailed analysis of fibers extracted from soft body armor comprised of polybenzoxazole (PBO) fibers was performed. The data indicate that hydrolytic degradation of these ballistic fibers is accompanied by degradation associated with folding (or fatigue-induced degradation) and an undefined degradation mechanism associated with vest use that appears to target the horizontal yarns of the alternating 0°/90° woven layers. These additional failure mechanisms have the potential to create localized regions in the PBO soft body armor which are significantly lower than the homogeneous degradation expected from uniform hydrolysis. Results also indicate that the absence of ballistic penetrations in the initial study conducted at the National Institute of Standards and Technology may be associated with using the properties of the fibers from the back panel of a compromised vest as representative of the properties in the front panel that was penetrated. Analysis of a field return vest showed the front panel to be significantly more degraded than the back panel.

Preparation of polybenzoxazole fibers via electrospinning and postspun thermal cyclization of polyhydroxyamide

AbstractPolybenzoxazole (PBO) fibers with a submicron diameter were successfully prepared by electrospinning its precursor, polyhydroxyamide (PHA), solutions to obtain the PHA fibers first, followed by appropriate thermal treatments for cyclization reaction. BisAPAF‐IC PHA with two different molecular weights (MWs) were synthesized from a low temperature polymerization of 2,2′‐bis(3‐amino‐4‐hydroxyphenyl) hexafluoropropane (BisAPAF) and isophthaloyl chloride (IC). Using dimethylacetamide (DMAc) and tetrahydrofuran (THF), solvent effects on the electrospinnability of PHA solutions were investigated. For balancing the solution properties, it was found that DMAc/THF mixture with a weight ratio of 1/9 was the best cosolvent to prepare smooth PHA fibers; uniform PHA fibers with a diameter of 325–720 nm were obtained by using 20 wt % PHA/(DMAc/THF) solutions. For a fixed PHA concentration, solutions with a lower MW of PHA yielded thinner electrospun fibers under the same electrospinning condition. After obtaining the electrospun BisAPAF‐IC PHA fibers, subsequent thermal cyclization up to 350 °C produced the corresponding thermally stable BisAPAF‐IC PBO fibers with a diameter of 305–645 nm. The structure of the precursor fibers and the fully cyclized fibers were characterized by FTIR. For the cyclized BisAPAF‐IC PBO fibers, thermogravimetric analysis showed a 5% weight loss temperature at 523 °C in nitrogen atmosphere. The interconnected fiber structure in the BisAPAF‐IC PBO fiber mats was irrelevant to the curing process, but resulted from the jet merging during the whipping process as revealed by the high speed camera images. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 8159–8169, 2008

MALDI−TOF MS Study of Aromatic Polybenzoxazole Fibers

The evaporation-grinding MALDI−TOF MS sample preparation method and evolved gas analysis/GC/MS (EGA/GC/MS) were used to study the cyclodehydration of polybenzoxazole and poly(benzoxazoleamide) precursors. Precursor fibers were heated from 100 to 300 °C while evolved gases were monitored by GC/MS; after each heating, samples were removed for MALDI−TOF MS analysis. Changes observed in MALDI−TOF mass spectra result from cyclodehydration, along with end group, and structure modifications (due to decarboxylation and branching). Evolved gas analysis/GC/MS and thermogravimetric analysis data indicate mass loss (from chain rupture) and evolution of low mass material which lead to branching. Cured polybenzoxazole fibers display an overall predominance of branched species with minor reduction in molecular weight whereas cured poly(benzoxazoleamide) fibers yield linear species as the predominant product with only minor branching observed and significant reductions in molecular weight. Results from this study indicate that a combination of MALDI−TOF MS and EGA/GC/MS has great potential for examination of thermal, chemical, and photodegradation pathways of high molecular weight, condensation polymers.

飛行船用高比強度の膜材料の開発

Progress in materials technology has produced a much more durable synthetic fabric envelope for the non-rigid airship. Flexible materials are required to form airship envelopes, ballonets, load curtains, gas bags and covering rigid structures. Polybenzoxazole fiber (Zylon) and polyalirate fiber (Vectran) show high specific tensile strength, so that we developed membrane using these high specific tensile strength fibers as a load carrier. The main material developed is a Zylon or Vectran load carrier sealed internally with a polyurethane bonded inner gas retention film (EVOH). The external surface provides weather protecting with, for instance, a titanium oxide integrated polyurethane or Tedlar film. The mechanical test results show that tensile strength 1,000 N/cm is attained with weight less than 230g/m2. In addition to the mechanical properties, temperature dependence of the joint strength and solar absorptivity and emissivity of the surface are measured.