Advances in Low Density SMC for Automotive Class A Applications

Transcription

1 Advances in Low Density SMC for Automotive Class A Applications Jeffrey L. Klipstein Technical Service Specialist AOC ACCE SPE September, 2014 ABSTRACT Today the automotive market is pursuing lighter weight materials to reduce the overall weight of the vehicle. This is mainly due to the ever increasing demand to improve gas mileage. Past advances have shown that a lower density Sheet Molding Compound (SMC) based on fiberglass and polyester technology, can be manufactured with sufficient mechanical properties and weight reduction; but this has been limited to structural or non-class A applications. However, recent advancements have shown that a Class A surface with acceptable mechanical properties can be achieved in a 1.2 specific gravity, low density SMC system, based on fiberglass and unsaturated polyester technology. This paper will compare past advancements in lower density systems, along with an insight into future work that is currently in progress. BACKGROUND AND REQUIREMENTS Since the 1970s, when fuel crises forced automakers to find ways to reduce weight, sheet molding compound (SMC), a compression-molded blend of polyester or vinyl ester resins, specialty additives, various mineral fillers and chopped fiber glass, has been part of progress towards weight reduction. Historically SMC has offered up to a 30 percent lower weight than the same part made of steel. These applications include Class A exterior body panels, underbody heat shields, and structural parts. Automakers today expect the painted surface of vehicles to exhibit the same level of smoothness and gloss, regardless of the material of construction. (1) Today s technology for lighter weight materials is constantly advancing and traditional SMC materials need to keep pace. Today aluminum is the preferential material for light weighting high volume vehicles and carbon fiber composites are receiving serious consideration, but both come at a cost premium. Our focus has been to create a new glass reinforced low density SMC product with adequate mechanical properties and a Class A surface profile. Jim Devries, Staff Technical Specialist and Manager of the Manufacturing Research Dept. at the Ford Research Laboratory (Detroit, Mich.) pointed out two important issues regarding the carbon fiber SMC. One, Ford favors aluminum over carbon fiber and glass fiber SMC. If the cost of industrial-grade carbon fiber drops to $5 per lb, he noted, then it could become competitive, at least at higher production volumes. Two, Devries posed a tough question: If the cost of carbon fiber wasn t an issue, would carbon fiber suppliers be able to meet the resulting automotive demand that, for $5/lb carbon fiber, could be as high as 75 million lb (~34,000 metric tonnes) per year? (2) While aluminum is currently more expensive than the steel it replaces, it is substantially lower cost than carbon fiber composites. While more expensive than traditional Class A SMC on an equal volume basis, low density SMC is competitive with aluminum in terms of both cost and weight savings. Coupled with favorable manufacturing costs for volumes up to 70,000 units per year, it provides automakers with an attractive opportunity for saving cost and weight on lower volume and specialty vehicles. In order for low density SMC compound to compete, it must fit existing compounding and molding infrastructure, have qualities such as ease of molding within current parameters, good flow properties; lower energy consumption; good mechanical properties, a greater ability to dissipate heat and a smooth glossy surface. (3) The average consumer tends to prefer a smooth shiny surface on their car. This is highly subjective to an individual s eye. However for development purposes technically a Class A surface finish refers to the waviness in the surface of body panels in addition to the distinctness of image (DOI), orange peel (OP) and gloss level on the part. Class A specifications have tightened in the last 5 to 10 years but

2 automakers and their Tier 1 suppliers normally are faced with relying on the subjectivity of the human eye for quality control of a Class A surface profile. (4) Various types of equipment are used to determine the smoothness and surface qualities of SMC substrates. A method particularly well suited for material development is a Laser Optical Reflected Image Analyzer (LORIA) developed by Ashland Chemical Company Composites Polymers Division, which is what we use for our surface profile evaluation. Most low density SMC at a 1.3 specific gravity is lacking in what most may consider to be a true Class A surface profile. A LORIA value more associated with a Class A surface profile would range in the 65 to 100 range for index and having a distinctness of image (DOI) from with an orange peel (OP) of 80 to 100 depending on the customer s preference. Ideally a LORIA value of 85, DOI of 80 and an OP of 8.0 would be considered a good base line. Note: lower LORIA index values and higher OP and DOI numbers are preferred. AOC has developed new Class A resin systems and B-sides (Thickening Systems) that when properly formulated offer the unique combination of improved mechanical properties and enhanced surface profiles. Normally when the mechanical properties are enhanced the surface profile suffers for a low density SMC compound of a 1.3 specific gravity or lower. The opposite normally happens; when the surface profile is enhanced the mechanical properties tend to suffer. Using the new technologies, materials can be manufactured incorporating both enhanced surface profiles and improvements to mechanical properties. Our goal is for the new low density SMCs to match or exceed the properties of a standard density compounds nearly as possible. EXPERIMENTAL The experimental section will review generic formulation breakdowns that incorporate the new low density Class A resins and their corresponding B- sides. Together these components of the experimental formulations give the desired Class A surface profile along with required mechanical properties. Formulation Components for Low Density SMC Abbreviations for Formulations: A1 - Using AOC Proprietary Resin System 1 with proprietary B-side 1 A2 - Using AOC Proprietary Resin System 2 with proprietary B-side 1 A3 - Using AOC Proprietary Resin System 3 with proprietary B-side 2 850LD CSP Composites Technology using AOC proprietary resin and B-side B1 - Using AOC Proprietary Resin System 4 with proprietary B-side 3 B2 - Using AOC Proprietary Resin System 3 with proprietary B-side 3 The SMC was manufactured on a 24" Finn and Fram SMC laboratory machine except for the CSP 850LD which was processed on their production equipment. The A and B-side pastes for each 24 formula were blended on a Cowles high shear mixing system. Laser Optical Reflected Image Analyzer (LORIA) was used for our surface profile measurements. Mechanical and Surface Property Testing Test panels for the sheet molding compounds were molded on a 100 ton hydraulic press in a 12" X 12" plaque tool for samples A1 through A3. The 850 LD mechanical data was supplied by CSP composites. The Mechanical property testing for compression molded panels was performed and compared to the Ford Low Density Structural Specification for Mechanical Properties WSS-M3D188-A. The LORIA profile data for a typical low density SMC is compared against the new low density SMC s which is represented in Table #1. The corresponding physical property data for both typical and new low density SMC's is represented in Table #2.

3 Table I LORIA Data Descrip on Index O P DOI Typical Structural LD A A LD As can be seen from the data comparison, the typical LD system has poor LORIA values, but good mechanical properties, where the new systems have both good values for LORIA and good mechanical properties. A Table 2 Mechanical Properties (5) UNSATURATED POLYESTER (UP), LOW DENSITY, NON CLASS 'A', WSS M3D188 A 40% GLASS FIBER REINFORCED, SHEET MOLDING COM POUND Typical Structual LD A1 A2 85OLD A3 Tensile Strength at Break, min 42 MPa (ISO R 527, 150 min x 10 x 2.5 +/ 0.2 mm specimen, 5 mm/ min test speed) Tensile Modulus at Break, min 6.8 GPa (ISO R 527, 150 min x 10 x 2.5 +/ 0.2 mm specimen, 5 mm/min test speed) Tensile Elonga on, min 0.8% (ISO R 527, 150 min x 10 x 2.5 +/ 0.2 mm specimen, 5 mm/min test speed) Flexural Modulus, min 5.2 GPa (ISO 178, 60 x 10 x 2.5 +/ 0.2 mm specimen, 40 mm support span, 2 mm/ minute test speed) Flexural Strength, min 115 MPa (ISO 178, 60 x 10 x 2.5 +/ 0.2 mm specimen, 40 mm support span, 2 mm/ minute test speed) Density g/cm3 (ISO 1183, Method A) Water Absorp on, max 1.3% no blisters Glass Fiber Content, 33 45% by Weight

4 Tables # 3 and 4 show how the new low density systems compare to a typical standard density SMC. Table 3 Mechanical Properties (6) UNSATURATED POLYESTER (UP), IMPACT MODIFIED, 28% WSS M3D184 A GLASS FIBER REINFORCED, SHEET MODLING COMPOUND Standard Density A1 A2 85OLD A3 Tensile Strength at Break, min 50 MPa (ISO R 527, 150 min x 10 x 2.5 +/ 0.2 mm specimen, 5 mm/min test speed) Flexural Modulus, min 6 GPa (ISO 178, 60 x 10 x 2.5 +/ 0.2 mm specimen, 40 mm support span, 2 mm/ minute test speed) Flexural Strength, min 116 MPa (ISO 178, 60 x 10 x 2.5 +/ 0.2 mm specimen, 40 mm support span, 2 mm/ minute test speed) * indicates ASTM D 792 Density g/cm3 (ISO 1183, Method A) * Water Absorp on, max 0.7% Glass Fiber Content, 26 31% by Weight Table 4 LORIA Data Description Index O P DOI Standard Density A A LD A Extensive paint and adhesive bonding performance testing is nearing completion. To date all results are looking positive.

5 Table 5 Cross Hatch Data and Photo Figure 6. In 1994, composite underground storage tanks are installed at Mobil service stations in Singapore. There is additional work going on to lower the specific gravity yet further, while maintaining mechanical properties and LORIA values. Tables 6 & 7 give some insight into our progress. Further work is required to boost Tensile and Flexural modulus.

6 Table 6 Mechanical Properties UNSATURATED POLYESTER (UP), LOW DENSITY, NON CLASS 'A', WSS M3D188 A 40% GLASS FIBER REINFORCED, SHEET MOLDING COMPOUND B1 B2 Tensile Strength at Break, min 42 MPa (ISO R 527, 150 min x 10 x 2.5 +/ 0.2 mm specimen, 5 mm/min test speed) Tensile Modulus at Break, min 6.8 GPa (ISO R 527, 150 min x 10 x 2.5 +/ 0.2 mm specimen, 5 mm/min test speed) Tensile Elonga on, min 0.8% (ISO R 527, 150 min x 10 x 2.5 +/ 0.2 mm specimen, 5 mm/min test speed) Flexural Modulus, min 5.2 GPa (ISO 178, 60 x 10 x 2.5 +/ 0.2 mm specimen, 40 mm support span, 2 mm/ minute test speed) Flexural Strength, min 115 MPa (ISO 178, 60 x 10 x 2.5 +/ 0.2 mm specimen, 40 mm support span, 2 mm/ minute test speed) Density g/cm3 (ISO 1183, Method A) Water Absorp on, max 1.3% no blisters Glass Fiber Content, 33 45% by Weight Table 7 LORIA Data Description Index O P DOI B B

7 In addition, the above mentioned systems have cold tool to cold part shrinkage values as follows in Table 8. The minus sign signifies expansion. Table 7 Shrinkage Data Description mils/in. A A A LD B B SUMMARY AND NEXT STEPS None of the sheet molding compounds discussed exhibited any issues during the manufacture and molding of the material. The new resins combined with their corresponding B- sides displayed 1.2 specific gravity results that were favorable to the Ford Low Density Structural Specification for Mechanical Properties WSS- M3D188-A and comparable to the surface parameters mentioned above for a Class A system. We are currently focusing on the A3 version. This version appears to offer the best path in maintaining good mechanical properties, while improving the LORIA surface profile. Production evaluations are underway. Additionally, the work that is ongoing to further reduce the relative density while maintaining mechanical properties and LORIA surface profiles looks encouraging. We believe that the work done to improve the mechanical properties and surface profile will increase the opportunity for the automotive industry to consider usage of low density polyester SMC in their future designs. It will additionally give other markets the opportunity to consider SMC as an alternative material for light weighting and surface profile. ACKNOWLEDGEMENTS I would like to acknowledge The AOC Closed Mold Technical Service Organization and The CSP Composites Group for making this paper possible. In addition I would like to acknowledge all the lab personnel for completing the required testing for this paper. BIBLIOGRAPHY 1. innovation-driving-automotive-smc SMC+Tackles+Structural+Auto+Partsa automotive-composites-taking-subjectivity-out -of-class-a-surface-evaluation 5. Ford Unsaturated Polyester (UP), Low Density, Non-Class A', WSS-M3D188-A 40% Glass Fiber Reinforced Sheet Molding Compound 6. Ford Unsaturated Polyester (UP), Impact Modified, 28% Glass Fiber Reinforced Sheet- Molding Compound, WSS-M3D184-A