Fabrication technology optimization of oriented cotton stalk-polypropylene film boards

Abstract: Natural fiber plastic composites (NFPCs) have been given considerable interest by researchers in recent years because of the increasing price of petroleum, ecological concerns, and the impending depletion of fossil fuels. NFPC is a product which can be made from natural fibers (such as wood, cotton stalk, kenaf, jute and banana fiber) and plastics (such as high density polyethylene, polypropylene, polyvinyl chloride and thermoset plastics). In order to use cotton stalk resource effectively and make high performance straw-based composites, this paper presented a potential use of long cotton stalk bundles as matrices and isotactic polypropylene (iPP) films as adhesives for the fabrication of oriented cotton stalk-polypropylene film boards (OCPBs). Firstly, the long cotton stalk bundles were oven-dried, modified by polymeric methylene diphenyl diisocyanate (PMDI) resin, and then divided into 2 equal portions. Each portion formed into a single unidirectional layer and was pre-pressed at room temperature. Then, the pre-pressed layers and iPP films were stacked to form a mat on a stainless steel caul with a wooden mold frame. The plastic films were placed between the pre-pressed layers and on the outside surfaces of the mat. The number of iPP films introduced at each layer was adjusted according to the targeted film content in that panel. Another caul was placed on top of the mat after forming, and 2 mold-releasing papers were used to prevent adhesion between the mat and the caul. Finally, the formed mat was hot-pressed between 2 steel plates preheated to a certain temperature, and then consolidated to the desired thickness. The whole process was program-controlled. After reaching the final thickness and maintaining for a certain duration, the mat was gradually cooled to room temperature. The pressed panel was then trimmed to a panel size of 300 mm×300 mm×10 mm with a density of 0.7 g/cm3. The tested specimens were made according to GB/T 17657 (2013) standards. The effects of film content (5%, 10%, 15%, 20% and 25%), hot pressing temperature (165, 175, 185, 195 and 205℃), hot pressing time (11, 13, 15, 17 and 19 min) and density (0.5, 0.6, 0.7, 0.8 and 0.9 g/cm3) on the properties of OCPBs including modulus of rupture (MOR), modulus of elasticity (MOE), internal bond (IB) and thickness swelling (THS) were evaluated. The optimum results of the single-factor experiment were further optimized by an orthogonal experiment (L9 (3,4)). The interface between cotton stalk fibers and iPP films was also investigated by a scanning electron microscope. Results showed that the optimal hot pressing parameters were the following: film content was 15%, hot pressing temperature was 185℃, hot pressing time was 15 min, and density was 0.7 g/cm3. The MOE, MOR, IB and THS of the boards reached 60.60, 5 074.4, 1.48 MPa and 2.53%, respectively. The properties of OCPBs satisfied the requirements of OSB/2 standard. Scanning electron micrographs showed that cotton stalk fibers were compressed and deformed during hot pressing, and wrapped with molten plastics. A mechanical interlock structure between cotton stalk fibers and iPP films was also observed, and that would be the main reason to effectively improve the mechanical properties and thickness swelling of the composite panels. The study can be a reference for fabricating NFPCs with agricultural biomass and plastic film as raw materials.