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Impregnation and coating of high-modulus polymer fibers: Effects on the compressive strength and other mechanical properties
Abstract
The compressive failure of high-modulus polymer fibers, such as poly(p-phenylene terephthalamide) (PPTA) and poly(p-phenylene benzobisthiazole) (PBZT), occurs at stresses an order of magnitude smaller than their tensile failure. A literature review is presented which covers the numerous theories and models for the compressive strength and failure of these fibers as well as of uniaxial composites, which are structurally similar and fail in an analogous fashion. It is pointed out that the smallest shear modulus of any material is a fundamental upper bound to its compressive strength. Physical combination of high-modulus polymers with rigid inorganic materials by impregnation or by deposition of coatings are presently the most promising routes to improved compressive strength and are the research topics presented in this dissertation. PPTA fibers, obtained by spinning from nematic solution, have been impregnated with up to 20 wt-% of a waterborne, highly crosslinkable melamine-formaldehyde resin and with approximately 5 wt-% of sodium silicate from a colloidal aqueous solution. PBZT fibers have also been impregnated with approximately 5 wt-% of sodium silicate. PPTA fibers, obtained by spinning from isotropic solution, have been impregnated with up to 40 wt-% of sodium silicate. None of the impregnations has led to improved compressive strength or other mechanical properties. Kevlar$\sp\circler$ 49 PPTA fibers and heat-treated PBZT fibers have been surface modified with siloxane and silicate functionalities in order to make fibers wettable by aqueous solutions of sodium silicate and to provide good adhesion to silicate coatings. The surfaces of as-received and modified fibers have been characterized by X-ray photoelectron spectroscopy. PBZT and PPTA fibers showed little or no decrease of their tensile properties as a consequence of the surface modification. Glass coatings have been applied to single filaments by dip-coating in aqueous sodium silicate solution followed by drying. Coated PPTA and PBZT fibers with shear moduli as high as 5 GPa have been prepared. The compressive strain at failure of PPTA filaments coated with a 0.5 $\mu$m thick silicate coating was raised to 0.6%, compared to 0.4% for uncoated filaments. These compressive failure strains correspond to compressive strengths of approximately 500 and 400 MPa, respectively.
Subject Area
Polymer chemistry|Materials science
Recommended Citation
Lietzau, Christian, "Impregnation and coating of high-modulus polymer fibers: Effects on the compressive strength and other mechanical properties" (1993). Doctoral Dissertations Available from Proquest. AAI9408302.
https://scholarworks.umass.edu/dissertations/AAI9408302