Mitsubishi Motors Corp has developed resin composite materials that are reinforced by bamboo fibers and can be used in automotive interiors.
This announcement was made at the 17th Polymer Material Forum, which was organized by the Society of Polymer Science, Japan and ran from Nov 27 to 28, 2008, at the International Congress Center Hiroshima in Hiroshima City, Hiroshima Prefecture, Japan (Lecture No: 2PA02).
This is the first time that bamboo fibers have been used as automotive materials. Mitsubishi Motors aims to reduce CO2 emissions by using the plant-based materials. It has been considering mass-producing the materials and "intends to reduce the cost to a level 1.5 times higher than existing materials," according to the company.
In order to process bamboos into fibers, they are longitudinally split into four strips, their joints are removed, and then they are crushed and fibrillated using a special (undisclosed) machine.
The existing fibrillating method involves blasting or steaming (hot steam is used to loosen the fibers). But the fibers processed by these methods were not applicable to automobiles because they produced acetic acid, formic acid, aldehyde and VOC (volatile organic compound). Meanwhile, the fibers processed by the new method are free from those problems, according to the company.
Mitsubishi Motors produced composite compounds using the bamboo fiber made by the new method and two kinds of resins, (1) PBS (polybutylene succinate) resin and (2) polyurethane resin. Both of the resins contain a substantial amount of plant-based materials.
The PBS resin (1) was produced by direct dehydration polycondensation of succinic acid generated by fermenting sugar or starch, and petroleum-based 1,4-butanediol. The bamboo fibers and the PBS fibers were entwined to create a mat, which was then hot-pressed. The weight ratio of plant-based materials is about 83%, according to the company.
The properties of the composite material have been confirmed to satisfy the requirements for automotive materials. It was actually used as tailgate trim on the "iMiEV" electric vehicles that were provided to Japanese power companies as evaluation vehicles in February 2008, according to Mitsubishi Motors.
However, there are some problems with the material (1). First, hydrolysis inhibitors, which are expensive, have to be added because the material is hydrolytically unstable. Second, it cannot be applied to all of the automotive parts because it has a low heat resistance (deflection temperature under load: 109°C). The company said it has been studying the possibility of using the material for volume production of the "iMiEV."
As for the resin (2), polyurethane is normally produced by reacting polyol component and isocyanate component. But Mitsubishi Motors replaced the majority of the polyol component with castor oil polyol and coconut oil glycerin to produce the polyurethane.
Binder resin was added to the bamboo fibers to form a mat, which was then placed in a metallic mold and hardened by injecting the polyol and isocyanate components using a reaction injection molder (RIM). The ratio of plant-based materials is about 60%, according to the company.
This composite material's deflection temperature under load is as high as 193°C. However, further improvements are required because its production cost is higher than that of PP (polypropylene), which is commonly used in vehicles, due to the slow production speed, according to the company. The material is intended for use as door trim base material, seatback board and ceiling board.
In respect to the reduction of CO2 emissions, the composite material made from the bamboo fibers and the PBS resin has achieved a reduction of 51% in lifecycle CO2 emissions (from the collection of raw materials to disposal) compared with existing petroleum-based PP resins. As for the material made from the bamboo fibers and the plant-based urethane resin, it achieved a reduction of about 28%, according to calculations made by Mitsubishi Motors.
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