Heat-treated bamboo powder improves anti-mold performance of bamboo powder/polypropylene composites

Abstract: Bamboo fiber possesses excellent innate properties such as fast growing, high tensile modulus, high wear resistance, biodegradability, and is a cheaper substitute for glass fiber and carbon fiber. To make full use of bamboo resources in China, research and development of bamboo powder (BP) reinforced plastic is a good way. As we know, bamboo plastic composite (BPC) has broad applications including building, decoration, packing, and automobile manufacturing. BP contains high content of starch, protein, carbohydrate, fat, and other nutrients and thus is vulnerable to microorganisms. Researches showed that the anti-mold performance of BPC was inferior to other wood plastic composite (WPC) and the anti-mildew performance of BPC became worse with the increase of the BP content. The proliferation of mold not only affects the appearance, mechanical properties, and service life of the BPC, but also raises human health issues. Therefore, study on the anti-mold performance of BPC is of great significance. Heat treatment is a good modification method to improve the water resistance, dimension stability, and durability of plant fiber-based materials. In this paper, BP/polypropylene composites with 60% BP were prepared by hot pressed molding and the BP was heat-treated at the temperature of 150, 170 and 190 ℃ respectively for 120 min. Three common types of mould i.e. Aspergillus niger, Penicillium citrinum, Trichoderma viride were used to conduct mildew test. The effects of heat treatment on the BP chemical content, mass loss, and hygroscopicity were studied. The influences of heat treatment on the composites surface color, mechanical properties, surface wettability, and anti-mold property were also investigated. The results showed that the BP chemical content did not vary much at the heat temperature of 150 ℃, and the BP cellulose and hemicellulose content gradually decreased and the lignin content accordingly increased with the increase of heat temperature. The heat-treated time of 60, 120, and 240 min had less influence on the BP chemical content than the temperature ranging from 150 to 190 ℃. The increase of mass loss and the reduction of equilibrium moist content (EMC) for the heat-treated BP were directly related to the change of chemical composition. After heat treatment at 190 ℃ for 120 min, the BP holocellulose dropped by 7.00 percentage points, the lignin enhanced by 2.92 percentage points, the mass loss was 4.41 percentage points, and EMC was 6.39%. Compared with the untreated composite, the heat treated composites possessed lower surface wettability, lower bending properties, but higher color stability during mold test and stronger anti-mold performance. The 190 ℃ heat-treated composite became darker and slightly turned green and blue; the lightness change was -5.47, and the color change was 7.54; the bending strength and bending modulus were reduced by 9.79% and 5.37%, respectively. Compared to composite without mildew test, the lightness change, redness change, and yellowness change of tested composite were 0.87, -0.30 and 0.20, respectively, and the color change was only 0.94; the mold resistance value was decreased from 3.75 to 2.25 and the anti-mold effectiveness was increased to 40%. This paper provides experimental data and theoretical reference for the development and application of mold-resistant BPC and WPC.