EVOLUTION OF AN EXCELLENT LIGHTWEIGHTING TOOL PUR SANDWICH COMPOSITES Mike Super, Rollan Bradley, Craig Snyder Bayer MaterialScience LLC

Transcription

1 EVOLUTION OF AN EXCELLENT LIGHTWEIGHTING TOOL PUR SANDWICH COMPOSITES Mike Super, Rollan Bradley, Craig Snyder Bayer MaterialScience LLC Abstract This paper details how polyurethane spray sandwich technology, originally developed for automotive sunshades, has been optimized for use in more demanding automotive applications such as load floors. Polyurethane sandwich construction combines the light weight of a honeycomb core with the high strength of a fiber-reinforced polyurethane skin to produce loadbearing parts with very high flexural stiffness and excellent thermal properties, making it an attractive, lighter weight alternative to acrylonitrile butadiene styrene (ABS), polypropylene, sheet molding compound (SMC) and wood products. Sandwich construction parts can meet OEM-specific specifications for deflection and no permanent set at both room and elevated temperatures. The paper will present information on the deflection performance of different constructions with different systems to guide manufacturers on the best ways to hit specific targets such as cost, thickness or weight. The authors also explain how new formulations enable productivity improvements, including longer open times and shorter demolding times, which facilitate production of larger parts and reduce scrap. By matching core and facing materials, polyurethane spray sandwich construction technology can produce customized parts with the combination of lighter weight and stiffness needed to confidently replace heavier materials. The evolution of polyurethane spray sandwich technology The inherent versatility of polyurethane chemistry has helped make it the material of choice for innovative products in diverse markets for decades. Bayer s Baypreg two-component polyurethane spray sandwich technology system was originally developed in the mid-1990s as a material for use in automotive sunshades, due to its lightweight properties and stiffness retention at high temperatures generated in auto interiors by sunlight. Existing materials on the market at that time were heavier than desired and could have difficulty meeting the required level of thermal stability. Manufacturing process The manufacturing process is shown in Figure 1. The composite is manufactured by creating a sandwich structure in which honeycomb cores (paper, thermoplastics, rigid foams and expanded polystyrene, etc.) are sandwiched between natural and glass fiber mats for parts ranging from 6-30 millimeter (mm) thick. The sandwich is sprayed from both sides with the twocomponent polyurethane system. The low viscosity of the polyurethane mixture ensures that the mats are thoroughly wetted with the resin. Next, the composite is placed in a mold, where it is compression-molded. The polyurethane system then reacts and binds the components together. Page 1

2 Figure 1: Manufacturing process Customized lightweight composites for thin, lightweight auto parts Automakers and tier suppliers continue to seek reduced part weight while retaining optimal material performance. Indeed, increasingly stringent fuel standards are motivating the industry to focus on reducing vehicle weight and, ultimately, improving energy efficiency without compromising style and comfort. No part is too big or too small to be included in this weight reduction analysis. When considering polyurethane sandwich construction technology, finding the right mix of core and facing materials can produce customized parts with the combination of lighter weight and stiffness needed to confidently replace heavier materials. The polyurethane spray sandwich composite is thinner and lighter weight than competitive materials, such as ABS, polypropylene, SMC and wood products, and meets the same deflection requirements with no permanent set even at the elevated temperatures. The stiffness and thinness of a part made with the polyurethane system can lead to removing the additional weight of underlying supports, which are necessary for other materials. Additionally, the polyurethane technology offers attachment methods and techniques that are lighter. For instance, the technology makes it possible to create a screw area. In addition, handle features can be molded into the part. New, patented developments allow carpeting to be molded into a part eliminating a traditional manufacturing operation. The lightweight and thinner part provides designers the ability to optimize storage capacity. Automakers that put this thin, lightweight yet strong polyurethane technology to work in vehicle productions will also benefit from a simplified assembly process thanks to a reduction in the steps needed to produce the parts. Initial parts were sunshades. Today applications have expanded to door panels, spare tire covers, load floors, sun roof cassettes and other interior applications. Other application possibilities include interior rear parcel shelves and exterior light truck tonneau covers. Page 2

3 Parts made using the polyurethane system can be found in vehicles from BMW, General Motors, Toyota, Honda, Ford and Nissan. Improved polyurethane systems Automakers and suppliers are also keenly interested in the most efficient and cost effective process for manufacturing a part. Advancements in polyurethane spray sandwich systems provide longer open times and shorter demolding times. This improved capability allows for larger parts to be processed and cycle times to be decreased which improves efficiency. In the traditional process, carbon dioxide gas is dissolved into the polyol to improve flow of the polyurethane. This was especially important when spraying a complex part with multiple dimensions. The new polyurethane systems do not require carbon dioxide nucleation. Both simple, flat part geometries and complicated geometries can now be made without nucleation. As a result, the manufacturing process is simplified, because you don t have to manage the material stream of purchasing/storing gas cylinders as well as maintenance of the nucleation system. The new polyurethane systems also release better from the mold. The result is lower production costs because of better demolding, more time between applications of mold release, additional time between touch-ups of mold release and less scrap due to cosmetic issues. Additionally, less mold build-up and longer usage times between mold cleanings provide more machine uptime. The new polyurethane systems provide the following improvements that make the sandwich composites more productive and lower costs: 1. Longer open times and faster demold times, 2. No nucleation, 3. Better release from the mold. Research for optimal parts To guide manufacturers on the best way to hit their specific targets such as cost, thickness or weight, specific research was needed regarding the stiffness and ultimately, the deflection performance of different compositions of the polyurethane spray sandwich technology. For this research, the new polyurethane systems, facing materials and honeycomb cores were combined to develop a matrix of material combinations. The materials used in the study are shown in Table 2. These combinations were tested to determine stiffness, part weight and biorenewable content. Table 2: List of resins, facing materials and honeycomb cores used in tests Resin system Fiberglass chopped strand mat-facing materials Paper honeycomb PUR 1 1 oz Honeycomb A kg/m3, Cell size: 0.5 PUR oz Honeycomb B kg/m3, Cell size: PUR 3 2 oz PUR oz Page 3

4 Resin comparison As a starting point, PUR 1 is the current standard polyurethane system. It requires the current carbon dioxide nucleation process. Three new polyurethane systems PUR 2, PUR 3 and PUR 4 that do not require nucleation were also added to the study. The objective was to compare stiffness, part weight and biorenewable content ratios. Additional conditions included a part thickness of 15 mm, Honeycomb B, 16 mm height and 2 ounces (oz) chopped strand material (glass mat). The results are shown below in Table 3. Table 3: Resin comparison New resins give higher stiffness than current standard. Part thickness, mm Total biorenewable, % Stiffness, N/mm Part weight kg/m2 Failure load, N PUR 1 with carbon dioxide nucleation Honeycomb B (16 mm) PUR 2 Honeycomb B (16 mm) PUR Honeycomb B (16 mm) PUR Honeycomb B (16 mm) The new systems PUR 2, PUR 3 and PUR 4 without nucleation showed higher stiffness than the original system, PUR 1, using nucleation. Otherwise, the property characteristics, such as biorenewable content and part weight, had similar profiles. Facing material comparison In this test, a PUR 2 resin, which does not require carbon dioxide nucleation, was used in order to determine the impact of different levels of glass mats (1 oz, 1.85 oz and 2 oz) on part stiffness, weight and biorenewable content. Other conditions were Honeycomb A, 16 mm core height; and a part thickness of 15 mm. Part stiffness increased as the ratio of glass mat increased. However, this added stiffness came at the expense of increased weight and reduced biorenewable material. Similar testing was done using PUR 2 with glass mat levels (1.85 oz, 2 oz and 2.85 oz); Honeycomb A, 13 mm core height and a part thickness of 12 mm. Table 4 shows the effect of facing material using these variables. As anticipated, the level of stiffness increased with the amount of glass mat used. This increased use of glass mat led to a heavier part with a lower biorenewable content. Page 4

5 Table 4: Effect of glass/weight facing material Increasing glass increases stiffness but increases weight and reduces bio content. Condition: PUR 2; Honeycomb A, 16 mm; Part thickness: 15 mm Variable: Glass weight: 1 oz; 1.85 oz; 2 oz Glass weight increase, % Stiffness increase, % Part weight increase, % Bio-renewable content decrease, % Condition: PUR 2; Honeycomb A, 13 mm; Part thickness: 12 mm Variable: Glass weight: 1 oz; 2 oz; 2.85 oz Glass weight increase, % Stiffness increase, % Part weight increase, % Bio-renewable content decrease, % Glass weight part weight Glass weight stiffness Glass weight bio-renewable % Core thickness and part thickness comparison It is important to understand the interaction of core thickness and part thickness and the effect on stiffness, weight and biorenewable composition. This test compared PUR 2, Honeycomb A, and 1 oz of chopped strand glass mat as the facing material. The variable was core thickness over a range from mm and part thickness of mm. An increase of the core thickness resulted in a large increase in stiffness. The biorenewable content increased and the part weight increased slightly. This indicates that the best way to achieve increased stiffness and biorenewable content is to increase the thickness of the honeycomb core. This trend also holds true when the amount of glass mat material is increased to 2 oz and is estimated to be similar even with glass weights as high as 2.85 oz. The net effect of changing core thickness and part thickness led to similar part characteristics. Page 5

6 Table 5: Effect of core thickness/part thickness Increasing core thickness results in significant increases in stiffness with slight increase in weight. Condition: PUR 2; Honeycomb A, 16 mm; 1 oz CSM Variable: Core thickness: 13 mm; 16 mm; 18 mm, 21 mm Part thickness: 12 mm; 15 mm; 17 mm; 20 mm Core thickness / part thickness, mm Part weight increase, % Stiffness increase, % Bio-renewable content increase, % 13 / / / / Condition: PUR 2; Honeycomb A, Variable: Core thickness: 16 mm; 13 mm Part thickness: 15 mm; 12 mm Core thickness / part thickness, mm Part weight increase, % Stiffness increase, % Bio-renewable content increase, % 13 / / Core thickness part weight Core thickness stiffness Core thickness bio-renewable % Page 6

7 Different designs to achieve specific objectives Based on the results from the research, the best construction to achieve a specific objective can be estimated. Table 6 shows the scenarios. Table 6: Different designs to achieve specific objectives Research can help estimate the best construction for thinnest part, lightest weight or highest bio-renewable content. Structure Part thickness, mm Total biorenewable, % Stiffness, N/mm Part weight kg/m2 Failure load, N For a thin part PUR 2 / PUR oz CSM Honeycomb B (6 mm / 8 mm) 2.85 oz CSM 5 / 7 16 / / For a light part PUR 2 / PUR 1 1 oz CSM Honeycomb B (17 / 19 mm) 1 oz CSM 16 / / / For highest bio percent PUR 2 / PUR 1 1 oz CSM Honeycomb A (17 / 19 mm) 1 oz CSM 16 / / / For example, to have a thinner part, more fiberglass reinforcement is used in the construction. In this case, the best construction is the PUR 2 resin system, 2.85 oz. chopped strand mat, Honeycomb B at a height of 6 mm, which results in a composite part with a thickness of 5 mm. If the objective is to have a lighter part, the best construction is the PUR 2 resin system, 1 oz chopped strand mat, Honeycomb B at a height of 17 mm. The result is a composite part that weighs 1.7 kg/m 2. If the objective is to have the highest biorenewable content, the best construction is the PUR 1 resin system, 1 oz chopped strand mat, Honeycomb A at a height of 17 mm. The result is a composite part with total biorenewable content of 41 percent. Summary The inherent versatility of polyurethane chemistry has helped make it the material of choice for innovative products in diverse markets for decades. Automakers and tier suppliers continue to seek reduced part weight while retaining optimal material performance with an eye toward biorenewable content and excellent cosmetics. Polyurethane spray sandwich technology offers a practical and multi-faceted option that can be formulated to meet each OEM s necessary targets. Lower production costs are also possible thanks to new polyurethane resin system advancements that provide longer open times and faster demold times, do not require nucleation and provide better release from the mold. Page 7

8 This research provides the following key takeaways: 1. Newer resin systems can improve part stiffness. In addition, they have been designed to have longer open times and shorter demold times to lower scrap and increase productivity. 2. Stiffness is not improved with nucleation. 3. Stiffness can be effectively improved by increasing the glass weight and core thickness. 4. If there is enough design flexibility to increase part thickness, the best way to optimize part stiffness and ultimately, part deflection, is to increase core thickness. This approach will not add significant weight or cost while increasing biorenewable content. Acknowledgements The authors also wish to acknowledge the contributions of Erika Zhu, Merle Lesko, Jack Taylor and Dave Rocco. Page 8