High performance RIM fascia.

High performance RIM fascia 

Two new RIM polymers for fascia have been developed which provide significant improvements in processing and performance over commercial polyurethane/polyurea fascia systems. These polymers combine improved filler technology with new chemistries to yield a painted surface quality within five "DOI" units of steel, and they exhibit improved dimensional stability, without sacrificing low temperature impact strength.

Compositional modifications of polyurethane precursors provide these benefits in polyurethane/polyurea (PU/urea) RIM polymers. This chemistry yields high performance fascia desired by the automotive industry.

Polyurea RIM systems, adapted from body panel technology, provide these same desirable characteristics and additionally offer the potential for self-release unassisted by external mold release agents. The increased cost of polyurea RIM chemistry is partially compensated by higher productivity. Performance comparisons show that fascia made from either high performance PU/urea or polyurea RIM systems are equal or superior in properties and appearance to engineering thermoplastics presently being used for flexible fascia.

RIM has become the dominant process for producing plastic fascia in North America, consuming an estimated 66% of the approximately 170 million pounds of polymer used for this application in 1988. Blends of polycarbonate and polybutylene terephthalate resins (PC/PBT) share about 21% of this market to produce rigid bumper systems. Thermoplastic olefin (TPO) and thermoplastic co-polyester resins (TPPE) share the remainder of the market.

Industry surveys indicate that the dominance of RIM fascia will continue through 1992, based on the proven ability of the RIM process to rapidly produce large and complex fascia. Styling trends suggest that the size of an average fascia will increase twenty percent during this period, further emphasizing the value of this RIM processing characteristic.

The high productivity for large part molding has resulted from the industry's conversion to RIM chemistry based on chain extension with diethyl toluene diamine (DETDA) and from the incorporation of internal mold release (IMR) into formulations[ref.1]. Such RIM systems contain both dydroxyl functional polyols and amine, and produce both urethane and urea bonds upon reaction with prepolymers of methylene diphenyl diisocyanate (MDI). These PU/urea polymers provide rapid cure and strength at demold, allowing fast cycle time and low molding scrap.

Milled glass filler has also become widely used to economically provide stiffness and dimensional stability in thin parts (3.5mm). Filled RIM fascia typically contain 15% of 1/16 inch milled glass fiber to lower heat sag, coefficient of linear thermal expansion (CLTE), and moisture distortion.

While these composites have met OEM performance standards, remaining undamaged after five mile per hour barrier impacts at -29 [degrees] C, the automotive industry desires further improvements in polymer performance and quality, specifically, matching the surface quality of painted steel, and reducing the distortion of RIM polymers on exposure to environmental moisture.

Unfilled engineering thermoplastics, although costing about 60% more than RIM polymers, have been shown to provide these desired characteristics. TPPE resin manufacturers are also promoting superior impact at low temperatures, even though RIM has proven performance at -29 [degrees] C.

In response to these needs, Dow Chemical began a program to develop superior-performing, cost effective RIM polymers. This work has resulted in the identification of improved filler technology and two RIM fascia polymers that provide high surface quality, low moisture distortion and CLTE, and lower heat sag than commercial PU/urea RIM systems. Additionally, impact strength of painted RIM polymers at -29 [degrees] C has been shown to be essentially equal to painted commercial TPPE fascia.

Surface quality

Although glass filler provides desired increases in stiffness and improved dimensional stability, the DOI of painted reinforced RIM (RRIM) is typically 15 to 20 DOI units less than painted steel and unfilled polymer (table 1). Details of the coatings and method of application used in generating this data are shown in table 2.

This reduction in surface quality is particularly noticeable in RRIM body panels, containing about 25% flake glass, used on otherwise steel-clad vehicles. A development emphasis has thus been the identification of alternate fillers which do not detract from appearance.

This work has resulted in the identification of filler packages which provide a distinctness of image within five units of painted steel, and which maintain desired performance[ref.2]. These fillers have also been found to provide the same advantageous characteristics in RRIM fascia, with the added benefit of lowering cost.

Treated wollastonite, a fibrous calcium silicate mineral, is priced about $0.30 per pound, which is one-third the cost of treated milled glass fiber. The RIM industry is thus in the process of replacing milled glass with wollastonite in selected fascia and body panel systems. While wollastonite provides attractive economics and a noticeable improvement in surface quality (table 1), the DOI of wollastonite-filled RIM polymer is still inferior to painted metal.

Further investigations have identified a platelet-type filler package that yields a painted RRIM substrate having a DOI within five units of steel and unfilled RIM polymer, and within three DOI units of unfilled copolyester thermoplastic resin. This "DOI" filler costs $0.32 per pound, and thus retains the economic advantage of wollastonite.

Fascia mechanical properties

RIM fascia materials discussed in this article were molded into plaques via the conditions described in table 3. Compositional properties of these polymers are shown in table 4 with mechanical properties of these polymers and a TPPE resin shown in table 5. The controls are a commercial unfilled "50,000 psi" flexural modulus PU/urea RIM system and a 25,000 psi base flexural modulus PU/urea RIM polymer with 15% milled glass.

Low-cost wollastonite provides essentially identical mechanical performance to milled glass in the "25K" RIM system at an 11% by weight higher loading.

The effects of substituting the "DOI" filler (which has very little difference in x and y dimensions) for milled glass in this "25K" polymer are as expected; differences between properties parallel and perpendicular to flow are reduced as the composite becomes more isotropic.

Combining improved polyurethane precursors with the "DOI" filler yields a composite having mechanical properties similar to conventional PU/urea polymers. In addition, the flexural modulus ratio of this high performance material (HP-PU/urea) is reduced, indicating improved thermal stability over a broad temperature range.

The polyurea fascia polymer, reinforced with the "DOI" filler, reflects lower elongation and higher modulus at a lower hard segment content, as is common with polyurea polymers which tend toward increased phase mixing over polyurethane RIM materials[ref.3].

The properties of a thermoplastic co-polyester resin, obtained from commercial fascia, were also examined. Tensile strength and flexural modulus at -29 [degrees] C and room temperature of this TPPE are quite similar to unfilled PU/urea RIM polymer. However, the TPPE resin suffers a loss of modulus at elevated temperature, having a high flexural modulus ratio of 5.1:1. The relatively high density of this TPPE should also be noted.

Heat sag

Thermal stability of the fascia materials suggested by the -29 [degrees] C/+ 70 [degrees] C flexural modulus ratio shown in table 5 is more clearly illustrated with the measurement of heat sag (figure 1). Specimens were tested according to ASTM D-3769-85, using an overhang of 150 mm (6 inches) and were exposed for one hour.

While all of the polymers provide an industry standard 15.2 mm maximum sag after an exposure at 121 [degrees] C (250 [degrees] F), only filled HP-PU/urea and polyurea RIM polymers meet this requirement at 135 [degrees] C (275 [degrees] F). The superior thermal stability of polyurea RIM polymers becomes more apparent after heat sag testing at 149 [degrees] C (300 [degrees] F). The lower heat sag of HP-PU/urea and polyurea RIM fascia reflect value in part production efficiency, where increasing the paint baking oven temperature would allow higher line speeds.

Coefficient of linear thermal expansion

The use of filler provides a desirable reduction of coefficient of linear thermal expansion (CLTE) as seen in table 6. The uniform CLTE of unfilled PU/urea RIM polymer reflects the amorphous characteristic of this material. Fibrous filler provides a significant reduction in the CLTE parallel to the direction of flow, but produces minimal effect perpendicular to flow. Flake-like "DOI" filler yields a significant reduction in both parallel and perpendicular flow orientation.

The temperature dependent CLTE of the TPPE resin is also seen in table 6. The temperature induced dimensional change of this thermoplastic is quite similar to unfilled RIM at higher temperatures.

Moisture distortion

In the family of plastic fascia materials, commercial PU/urea polymers exhibit a moderate tendency to absorb and desorb moisture. Figure 2 shows that unfilled RIM polymer can grow almost 1% after immersion for 10 days in water. This phenomena causes dimensional change with varying environmental conditions. With fascia attached to a car body, this moisture sensitivity may lead to visible distortion.

Milled glass and wollastonite provide minor reduction in RIM polymer growth after water immersion. However, the "DOI" filler contributes to a significant improvement in this property. Figure 2 also shows that the HP-PU/urea RIM system yields a further reduction in water-induced polymer growth. The combination of "DOI" filler and HP-PU/urea polymer lowers moisture distortion to a level approaching that of the thermoplastic co-polyester resin.

The highest stability to moisture distortion is achieved with the blend of polyurea RIM with the "DOI" filler which has less than 0.2% growth after immersion for 10 days in water.

Thus both HP-PU/urea and polyurea RIM polymers in combination with low cost "DOI" filler have a high potential of meeting the auto industry's desire for improved dimensional stability in flexible fascia. They provide lower moisture distortion, CLTE and heat sag without sacrificing surface quality.

Impact strength

RIM and milled glass filled RRIM fascia have a proven on-car history of meeting industry low temperature impact requirements. Laboratory impact comparisons of these materials and the other fascia compositions discussed in this article were made to quantify the relative effects of the low cost fillers and new RIM polymers. These results are illustrated in figure 3 for unpainted substrate, tested at -29 [degrees] C using both the Gardner dart-drop test (ASTM D-3029) and the MTS instrumented impact test (ASTM D-3763).

The impact characteristics of both unfilled RIM and TPPE polymers are essentially identical. Filler reduces the impact force at failure to about half that of the unfilled polymers.

However, in the real world, these fascia materials are painted. These polymers were thus coated with commercial primer, pigmented top coat, and clear coat according to industrial conditions shown in table 2.

Somewhat surprisingly, the low temperature painted impact force at failure of reinforced RIM PU/urea polymers actually increased slightly over unpainted RRIM substrate (figure 4). This could be attributed to improved phase segregation as the polymers were subjected to paint oven baking. The unfilled PU/urea polymer and RRIM polyurea system had only a minor decrease in impact strength after painting.

In-contrast, coating TPPE resin significantly degrades low temperature impact to a level where there is little difference in -29 [degrees] C impact force at failure of copolyester thermoplastic and PU/urea or RIM reinforced with "DOI" filler (figure 4).

Processing

Initial processing studies have shown that both HP-PU/urea and polyurea RIM polymers provide superior strength at demold over conventional PU/urea RIM. Release of the high performance PU/UREA polymer from clean steel fascia tools appears to be similar to today's commercial RIM systems. However, polyurea chemistry results in exceptional multiple release from bare metal, over 100 consecutive releases have been achieved. Further work is in progress to completely define the processing characteristics of these new RIM fascia materials.

Conclusions Continued improvements which illustrate the versatility of the polyurethane/polyurea polymers in meeting the increasing performance demands of the automotive industry have been made. During the past year, two RIM fascia polymers were identified that, when combined with improved filler technology, provide lower moisture distortion and CLTE, with thermal stability and high surface quality. These developments, when merged with the continuing efforts toward cost reduction and process improvements, maintain the polyurethane/urea polymers and the RIM process as the leading candidate for the production of tomorrow's fascia. [Tabular Data 1 to 6 Omitted] [Figure 1 to 4 Omitted]

References [1]Meyer, L.W. "A self-releasing RIM system for increased productivity, operational simplicity, and improved economics," proceedings of the SPI 6th International Technical/Marketing Conference, San Diego, California (November 2-4, 1983). [2]Martinez, E.C. "RIM - a process for the 1990s," International Congress and Exposition, Society of Automotive Engineers, Detroit, Michigan (February 27-March 2, 1989). [3]Willkomm, W.R. "Properties and processing of polyurea RIM," thesis, University of Minnesota, 1988.