A new concept for a 100% plastic prototype door panel has the potential to provide a weight reduction of up to 40% compared to conventional steel door panels but with equivalent performance (static strength), according to two JSP engineers who will present details of the prototype during the Automotive Composites technical session on April 26 at the SAE 2012 World Congress in Detroit.
The concept includes a composite sandwich panel combination of GFRP (glass fiber-reinforced polymer) and LACTIF, which is an expanded bead foam made from PLA (polylactic acid) and developed by JSP Corp. The door-panel prototype also offers an alternative to conventional steel door panel systems by using unsaturated polyester material of plant origin as part of the GFRP composite. Using this combination with PLA would yield a sustainable product containing up to 80% bio-based material, the engineers claim.
PLA resin is available in the market as plant-derived material in many cases and is recognized by the plastics industry as having the potential to be widely accepted as a bio-based material. Plastics are generally required to exhibit good environmental (heat, humidity, and UV) characteristics; PLA, however, has poor resistance, especially under high-temperature and high-humidity environments where it tends to hydrolyze.
PLA foam has a strong ability to adhere or bond to FRP, which generally consists of unsaturated polyester resin and glass fiber. Hand lay-up and spray-up methods, which include impregnation of liquid resin into glass fiber mat, shape molding, and hardening processes, are well-known manufacturing methods for FRP. It is also generally known that if expanded polystyrene (EPS) foam beads are used as a core part for a composite sandwich panel, styrene monomer, which is used as a constituent of unsaturated polyester resin, will melt the core material, resulting in an unacceptable product.
It has been confirmed that LACTIF has good resistance to styrene monomer and can be utilized as a core material in any variety of 3-D shapes, as well as in board or plank form.
The flexural strength of the proposed FRP/LACTIF composite panel was measured using a three-point bending load (span of 100 mm [3.9 in]). The 1-mm (0.04-in) flex/bending load of the prototype composite panel was significantly improved by a factor of five to seven at the same weight compared to that of a typical SMC board. The weight of the FRP/LACTIF panel also can be reduced by at least 50% while retaining the same flex/bending strength.
The prototype door panel was produced using a hand lay-up method, consisting of the sandwich panel of FRP/LACTIF/FRP for the outer panel and a single panel of FRP for the inner panel. For mass production, the light resin transfer molding (L-RTM) method could also be utilized. The prototype FRP door panel was produced using BIOMUP, a bio-based polyester resin produced by Japan U-Pica Co. Ltd. The combination of LACTIF and BIOMUP will allow for up to 80% bio-based materials to be used for this automotive composite door panel.
For the prototype design, a glass mat was used as a reinforcement fiber. Carbon fiber could also be used for the outer panel for improved strength and puncture resistance. The total percentage of bio-based material and types of reinforcement fibers can be optimized to meet the specific customer’s performance requirement or goal for renewable-material composition.
The analysis and evaluation of the completed prototype door panel demonstrated that the visual appearance of the door panel achieved the same level of quality as the original steel door panel. During the prototype construction trials, it was discovered that when alternative foam products (with poor or reduced ability to adhere/bond) were used as the core material, surface sink-marks and dimples were observed due to the fracturing or separation that occurs between the FRP and the alternative core materials. Uneven shrinkage of the FRP layer was also discovered when these alternative core materials were used. No unevenness was observed on the surface of the FRP/LACTIF composite prototype door panel.
While this concept FRP/LACTIF composite panel is still in the development stage, it has the potential to be applied to exterior parts such as the exterior door panel. Efforts are under way to benchmark current automotive production part designs and establish a partnership for the purpose of process optimization and higher-volume production studies. Marketing studies are also under way to understand the application potential for this process.
This article is based on SAE International technical paper 2012-01-0942 by Steven Sopher, Technical Director at JSP International in the U.S., and Hidehiro Sasaki, General Manager at Development Division for JSP Corp. in Japan.