Effect of surface treatment of carbon fibers on the mechanical properties of polypropylene-based composites

Abstract

Carbon fiber reinforced plastics are widely used in areas of technology requiring high specific properties of the products (specific strength and rigidity), chemical resistance, and elevated heat resistance. An increase in the mechanical properties of such plastics can be obtained by increasing the adhesion of the polymer binder to the carbon fibers, and this adhesion in turn depends substantially on the state of the surface of the fibers. Studies have been published dealing with the surface activation of carbon fibers by liquid-phase [1, 2] and gas-phase [3] oxidants; this makes it possible to increase the bonding strength in the carbon-fiber-binder system and, correspondingly, the strength of carbon fiber reinforced plastics. However, the indicated methods of surface treatment of carbon fibers either are expensive (ozonization) or involve the use of chemically corrosive liquids which cause corrosion of the equipment and give rise to unfavorable working conditions. In the present work, we studied the effect of surface treatment of carbon fibers from Ural-LO-24 (TU 6-06-31442- 85) hydrated cellulose with atmospheric oxygen on the properties of thermoplastic carbon fiber reinforced plastics. The polymer matrix used was 21030 polypropylene GOST 26996-86. The carbon fiber reinforced plastics were prepared by combining the polymer matrix and carbon fibers chopped into segments up to 10 mm in a multiple-unit extruder at a temperature of 473-493 K. During the surface treatment, the carbon fibers were held in a furnace in an atmosphere of air at temperatures of 673-873 K. In addition to the surface treatment of the carbon fibers, the latter were also dressed with an aqueous solution of polyvinylpyrrolidone with a molecular mass of 12,600. The results of the study of the strength of carbon fibers subjected to surface treatment with atmospheric oxygen showed that above 873 K, a substantial loss of fiber mass (> 5%) and a decrease of their breaking strength are observed. At temperatures of 673-823 K, an increase in strength is observed that may be due to the burning out, during the treatment of the carbon fibers, of foreign surface deposits and defect regions, and to a decrease in the number of dangerous cracks on their surface. A similar dependence was found in a study of the properties of composites obtained from polypropylene and surfacetreated carbon fibers (Fig. 1). Oxidation of carbon fibers by atmospheric oxygen causes an increase in the strength and rigidity of the composites. The optimal treatment time of the reinforcing filler was 15-20 min at 823 K (Table 1). The increase in the strength of the composite is explained by a strengthening of the carbon fibers as a result of the improvement of their structure. Also possible is the formation on their surface, during the treatment, of active oxygen-containing groups; this results in an increased adhesive interaction of the binder and filler and an increase in the tensile strength of the composites. Surface oxidation of carbon fibers also results in the formation of a brittle layer on their surface; this causes a decrease in the impact elasticity of polypropylene-based carbon-reinforced plastics (see Table 1) and in some cases limits the areas of their application. The same effect has been observed earlier in the reinforcement of thermosetting binders with surface-treated carbon fibers [4]. To reduce the drop in impact elasticity of the composites by producing on the surface of the carbon fibers an intermediate layer having an affinity for the polymer binder and filler, the fibers were dressed with an aqueous solution ofpolyvinylpyrrolidone. The optimal amount of dressing deposited on the fiber was determined to be 2 mass %. Table 2 lists the values of the properties of carbon-fiber plastics reinforced with dressed carbon fibers and also with carbon fibers oxidized with atmospheric oxygen, then dressed. Analyzing the tabular data, one can conclude that the maxi