Fabrication And Manufacturing Mechanism Of Oriented Cotton Stalk- Polypropylene Film Boards

Cotton stalk is a very important kind of crop straws, which has an annual production of 20 million tons in China,but its utilization rate is low. Cotton stalk has a high value of cellulose, its main chemical components are similar to wood, and can be used for the fabrication of non-wood products. Polypropylene(PP) film is a common kind of thermoplastics, which has a certain adhesiveness with the effects of temperature and pressure, and can be used as a raw material for the fabrication of artificial boards. In order to effectively use the rich cotton stalk resources and promote the added economic value of crop straws, this study used cotton stalks and polypropylene films as raw materials to fabricate oriented boards using hot pressing method. The effects of hot pressing parameters and surface treatments on the properties of oriented boards were investigated, the hot pressing parameters were optimized, and manufacturing mechanism of oriented boards was also analyzed. The results are obtained as following:(1) The effects of film content(5 %, 10 %, 15 %, 20 %, and 25 %), hot pressing temperature(165, 175, 185, 195, and 205 oC), hot pressing time(11, 13, 15, 17, and 19 min) and mat density(0.5, 0.6, 0.7, 0.8, and 0.9 g/cm3) on the properties of oriented boards including modulus of rupture(MOR), modulus of elasticity(MOE), internal bond(IB) and thickness swelling(THS) were evaluated. The optimum results of single-factor experiment were further optimized by an orthogonal experiment(L9(3, 4)). Results showed that the optimal hot pressing parameters were the following: film content was 15 %, hot pressing temperature was 185 oC, hot pressing time was 15 min, and mat density was 0.7 g/cm3. The MOR, MOE, IB, and THS of the boards reached 60.60 MPa, 5074.4 MPa, 1.48 MPa, and 2.53 %, respectively. The properties of oriented boards satisfied the requirements of oriented structural board standard.(2) The mechanical and water absorption properties of oriented boards made of PP film were better than that of polyethylene(PE) film. The best mechanical and water absorption properties of oriented boards were obtained with the use of 15 % PP film. The SPSS test showed that film contents had a significant influence on the properties of oriented boards.(3) The mechanical and water absorption properties of oriented boards were better than that of medium density fiberboard(MDF) and particleboard at film content of 15 %. Compared to MDF, the MOR, MOE and IB of 15 % PP based oriented boards increased 95.5 %(P<0.01), 63.7 %(P<0.01), and 41.0 %(P<0.01), respectively, while THS decreased 74.7 %(P<0.01). Compared to particleboards, the MOR, MOE and IB of 15 % PP based oriented boards increased 157.9 %(P<0.01), 84.5 %(P<0.01), and 48.0 %(P<0.01), respectively. Compared to MDF, the MOR, MOE and IB of 15% PE based oriented boards increased 46.2 %(P<0.01), 44.8 %(P<0.01), and 24.8 %(P<0.05), respectively, while THS decreased 72.0 %(P<0.01). Compared to particleboards, the MOR, MOE and IB of 15 % PE based oriented boards increased 92.9 %(P<0.01), 63.3 %(P<0.01), and 31.0 %(P<0.05), respectively.(4) Oriented cotton stalk-polypropylene film boards were surface-treated with three different kinds of coupling agents and were fabricated by hot pressing. Testing of mechanical and water absorption properties revealed that the MOR, MOE, IB, and THS of oriented boards were significantly improved by surface modification of polymeric methylene diphenyl diisocyanate(PMDI) compared with maleic anhydride polypropylene(MAPP) treated or vinyltriethoxy silane coupling agent(A171) treated boards. Compared with untreated boards, the MOR, MOE, and IB of 3 % PMDI treated boards increased 22.7 %(P<0.01), 15.0 %(P<0.01), 289.5 %(P<0.05), respectively, while THS decreased 89.4 %(P<0.01). The MOR, MOE, and IB of 3 % A171 treated boards increased by 12.8 %(P<0.01), 16.2 %(P<0.01), and 123.7 %(P<0.01), respectively, while THS decreased 59.7 %(P<0.01). The MOR, MOE, and IB of 2 % MAPP treated boards increased by 8.7 %(P<0.01), 17.0 %(P<0.01), and 40.9 %(P<0.01), respectively, while THS decreased 41.0 %(P<0.01).(5) The properties of compound surface treated boards were better than that of single surface treated boards, the performance of A171 and PMDI showed the best. The properties of surface-treated oriented boards increased significantly, especially the water resistant property.(6) The manufacturing mechanism of oriented cotton stalk-polypropylene film boards were analyzed by using scanning electron microscopy, fourier transform infrared spectrum, and X-ray vertical density profile meter. The results showed that cotton stalk fibers were deformed and wrapped by hydrophobic PP films, the molten plastics filled into the gap between cotton stalks instead of filling into the inner part of cotton stalk fibers, and a mechanical interlock structure was formed between the two joint interfaces, thus effectively improved the mechanical and water absorption properties of oriented boards.This study will not only contribute to the utilization of cotton stalks and plastic films, but also provide a theoretical basis for the fabrication of oriented biomass boards.