Sheet-Molded Polyolefin Natural Fiber Composites for Automotive Applications Muhammad Pervaiz, Mohini M. Sain* Advanced Wood Composite Group, Earth Science center/Forestry, University of Toronto, 33 Willcocks Street, Toronto, Ontario, M5S 3B3, Canada Fax: þ416 978 3834; E-mail: m.sain@utoronto.ca Received: November 25, 2002; Revised: March 11, 2003; Accepted: March 17, 2003; DOI: 10.1002/mame.200350002 Keywords: auto applications; density profile; mechanical properties; hemp fiber; polyolefin composites Introduction Over the last few years, ecological concerns have initiated a considerable interest in natural materials to produce ‘‘green’’ products. Traditionally, glass fibers have been used extensively to reinforce thermoplastics. They have also played a pivotal role in interior and exterior applications in cars due to their good impact strength. However, glass fiber mat thermoplastics (GMTs) have several environmental dis- advantages, because glass fibers are obtained from non- renewable resources, and a lot of energy is consumed in their production. These fibers are abrasive to processing equip- ment and bear potential health risks to production workers. These composites may transform into sharp splints as a result of collision or accident causing injury. Moreover, GMTs are not recyclable and their incineration generates a clinker-like mass that is hard to dispose off. On the other hand researchers and automobile companies are exploring natural fibers more extensively as an environ- mental friendly alternative to glass fibers. Most of the bast fibers being studied are obtained from flax, hemp, and kenaf. These fibers are renewable, non-abrasive to process equip- ment and can be incinerated at the end of their life cycle for energy recovery, because they are of good calorific value. They are also very much safer during handling and less suspected of affecting the human lung. Automotive applications represent the best opportu- nity for natural fiber-filled thermoplastics due to some Full Paper: The use of natural fibers as reinforcing filler in thermoplastics is a relatively new application and has great potential in replacing glass fiber products in automotive industry. However, most of the research in this area has been focused primarily on flax fiber. In the first part of the work presented here, hemp fiber non-woven mats are used exclu- sively in combination with a poly(propylene) matrix to study the mechanical properties of natural fiber mat thermoplastics (NMT) in the absence of binder. Film stacking was used as the method of preparation. The results show that hemp-based NMT have comparable or even higher strength properties as compared with conventional flax-based thermoplastics. A value of 63 MPa for the flexural strength is achieved at a fiber content of 64 wt.-%. The influence of the compression ratio on the mechanical properties and density of NMT is also re- ported. A definite increase in strength is observed with increa- sing compression together with a much more uniform density profile. In the second part of this study, a unique combination of random hemp fibers, non-woven mats and poly(propylene) films was employed in film stacking to evaluate strength pro- perties and economic implications. The same fiber content (64 wt.-%) was maintained in the final NMT by replacing 78 wt.-% of the mats by random fibers. Preliminary tests reveal better mechanical properties especially in terms of impact energy, which is 50 to 100% higher, as compared with different mats-only/poly(propylene) combinations. Further, a net saving of 40% in fiber cost is anticipated by replacing 78% non-woven mats with an equivalent amount of random fibers. Overall results of this study indicate that hemp-based NMT are promising candidates in automotive applications where high specific stiffness is required. Tensile Strength of different NMTs and GMT. Macromol. Mater. Eng. 2003, 288, 553–557 553 Macromol. Mater. Eng. 2003, 288, No. 7 ß WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2003 1438-7492/2003/0707–553$17.50þ.50/0