In automotive RIM today, the magic words are 'low density.' (polyurethane materials)

Propelled by weight and cost savings, low-density RIM, RRIM, and SRIM polyurethanes are on a fast track in a growing spectrum of automotive applications. The trend toward low density has been most evident so far in interior applications. Reinforced PUR and polyurea LD-SRIM has been used commercially in door panels since the late 1980s (see PT, Feb. '94, p. 104). Today, applications on 1994 and '95 model vehicles from all three Detroit OEMs also include package shelves, seat pans, seat backs, and quarter panels. LD-RRIM has appeared in only one North American commercial application - and door panel on the '94 Corvette. Still other potential uses for LD-RRIM or LD-RRIM are sun visors, instrument-panel retainers, and consoles.

Elastomeric LD-RIM technology is just now emerging, though there is strong indication from the two major players - Dow Plastics and Miles Inc. - that LD-RIM bumper fascias will debut on 1995 models.

CHALLENGING THERMOPLASTICS

LD reinforced RIM substrates are rapidly growing in fabric-covered interior trim. Advantages over thermoplastics such as ABS and PP include reduced weight, consolidation of manufacturing steps, and the use of aluminum tooling versus steel.

Attractive features of the RIM process include the ability to mold directly onto vinyl, cloth, or carpet surface materials and to combine the foam substrate, fabric covering, and even fasteners in one in-mold operation. Upon removal from the mold, parts are essentially ready for vehicle assembly.

ICI Polyurethanes development associate Paul Mackey says that in replacing PP or ABS in these applications, LD-SRIM offers part-weight reduction of about 40%. Specific gravity of both current LD-SRIM and LD-RRIM systems is typically 0.6 g/cc vs. thermoplastics at 1.0 g/cc or more), and suppliers aim to reduce density to 0.3-0.5 g/cc. "ABS is over-engineered for some of these applications," adds Mackey, and LD-SRIM reportedly offers more consistent properties over a broader range of temperatures than does ABS. While the cost per pound for LD-SRIM systems is higher, the lightweight composite foam uses less material. "In the end, depending on the density of the part, the material cost is generally lower," Mackey says.

Unlike conventional RIM elastomers, LD-SRIM and LD-RRIM are foam composites. The key difference between RRIM and SRIM is the reinforcement-continuous-strand glass mat in SRIM and 1/16-in. milled fiberglass or high-aspect-ratio minerals like wollastonite in RRIM.

Standard RIM equipment can be used with both types of reinforcements. In the case of LD-SRIM, precut glass mat is placed in the tool cavity prior to open or closed-pour injection. For LD-RRIM, lance-cylinder pumping equipment is recommended to handle the reinforcing fibers blended in with the polyol.

Use of environmentally acceptable blowing agents is no longer an issue, points out Miles automotive v.p. Hans Kogelnik, as these composite foams are now water-blown.

Though they're thermosets, PUR RIM materials do not suffer a liability where recycling is concerned. There are at least three methods of recycling RIM materials: 1) incorporating glycolyzed liquid RIM polyols; 2) adding powdered regrind to the liquid resin stream; or 3) using granulate as filler between layers of glass mat in LD-SRIM.

According to Dow automotive market manager Jane Warren, a key factor that will influence use of LD-RIM technologies is the expected imminent adoption of new federal standards for head impact and side impact. Meeting the new standards, she explains, will require components with additional functionality (i.e., incorporating an air bag in the instrument panel), which adds weight. Says Warren, "Manufacturers will look to reduce mass. We expect to see these systems [LD-SRIM, LD-RRIM! increase in use because of their lower mass. Total part cost will be critical as to which system is used."

Weight aside, industry reports have it that LD-SRIM has outperformed ABS in some OEMS' side-impact tests because of the way it deforms under impact.

LD-SRIM VS. LD-RRIM

A significant advantage of LD-SRIM, according to ICI's Mackey and to Rich Rossio, BASF group leader for technical services, is that its physical properties - particularly impact strength and dimensional stability under high heat and humidity - are better than those of LD-RRIM, owing to the superior reinforcing of continuous-strand glass mat. "RRIM is designed for applications where it is poured onto a decorative coverstock (vinyl, cloth, or carpet). Because of its low impact strength it needs a continuous surface to meet OEM requirements," says Mackey. Also, LD-RRIM parts must often be thicker: Interior door panels are at least 10-15% thicker with LD-RRIM than with LD-SRIM, he reports.

Part-weight reduction relative to ABS can approach 42% for LD-SRIM, vs. an average of about 13% with LD-RRIM, according to Mackey. Dow's Warren says LD-RRIM can do better than that, depending on the door panel design: "We have seen up to 30% weight reduction with some of the lower density LD-RRIM systems."

While BASF offers both LD-SRIM and LD-RRIM technologies, Rossio acknowledges that LD-RRIM suffers a cost disadvantage. Mile you sometimes get even lower specific gravity with the latest LD-RRIM technology, you cannot get lower part weight because of the need for higher wall thickness," he explains. Dow's Warren says wall thickness depends on the part and its design requirements. "Typical wall thickness of an LD-SRIM part is in the range of 2.5-4.0 mil. With LD-RRIM, depending on the design, typical wall thickness is about 3.3 mil."

Pound for pound, LD-SRIM costs more than LD-RRIM. ICI's Mackey pegs overall material costs at around $2.80/lb and $2.60/lb, respectively. That's because glass mat costs around $1.50/lb, compared with milled-glass fibers at around 80cents/lb. Some suppliers, like Dow, are seeking to partially replace the milled glass in LD-RRIM systems with lower-cost mineral fillers, such as wollastonite at about 35cents/lb. Still, Mackey cautions that "since you need to have higher density and greater wall thickness with LD-RRIM, you may end up with higher part cost."

All suppliers concede that LD-RRIM offers certain processing advantages, such as no handling or trimming of glass mat. Miles' Kogelnik does not regard handling glass mat as much of a problem for U.S. processors. "People here have been doing it for years and have developed ways to make it easier, such as setting up mat-cutting tables that limit the glass waste." (Miles offers both LD-SRIM and LD-RRIM technologies.)

Dow's Warren make the additional point that LD-RRIM is a one-step molding process, whereas current LD-SRIM technology requires first encapsulating the glass mat and then adding the vinyl coverstock as a secondary step. Still, she says, developmental work could eventually lead to a one-step LD-SRIM technique.

BASF's Rossio credits LD-RRIM with being better suited to applications such as door panels which have deep draws, deep ribs, or deep attachments. "Glass mat won't get in there, whereas the short glass fibers will." On the other hand, he adds, "We have been able to mold an LD-SRIM door panel with 5-6 in. draw on the arm rest without a problem, which is pretty remarkable."

NEW DEVELOPMENTS

Internal mold release, lower density, and higher strength and heat resistance are all goals of materials suppliers' R&D programs.

* BASF recently expanded its Elastolit SR LD-SRIM product line with the introduction of a system that incorporates an internal mold release (IMR) to permit faster demolding. Use of the IMR also eliminates the time required to spray external mold release, reportedly contributing to productivity improvement of up to 20%. The system works with aluminum or steel molds.

* Like BASF, Dow also has developed an IMR technology that's said to increase productivity by at least 18%. Dow reports no sacrifice in polymer reactivity, adhesive bond strength to PVC, processability, or thermal and mechanical properties. Warren says the IMR LD-SRIM and LD-RRIM systems are designed for aluminum molds that have aluminum inserts. The company also aims to tailor the technology for molds with steel inserts.

* Miles, too, has developed IMR systems, which can run in all types of tool materials and provide up to 45 releases with no touch-up of the tool, according to Kogelnik. "These IMR systems go down in density to 0.4 g/cc with no effect on reactivity, bonding to substrate, etc.," he says.

* ICI also offers IMR technology in low-density systems. In addition, ICI has developed LD-SRIM systems capable of molding composites down to 0.27 g/cc. These ultralightweight composites are said to achieve weight savings over 55% in door panels while maintaining acceptable physical properties, according to Mackey. Cost savings are also enhanced. Material cost for a door panel with the ultra-light LD-SRIM material is $1.57, compared with $2.34 for ABS and $2.94 for LD-RRIM, he reports.

* Dow has a new lower-density LD-RRIM based on a new, proprietary resin formulation (see Table 1).

TABLE 1 - LD-RRIM MECHANICAL
PROPERTIES(a)

                           0.6           0.4
Property                   g/cc          g/cc

Weight % Glass
(1/16-in. Milled)          15.8          13.2
Flex. Modulus, MPa,
Parallel to Flow           655           635
Perpendicular              565           600
Heat-Distortion Temp.
(1.82 MPa), C              43            47
(0.45 MPa), C              73            82
Notched Izod Impact
@23 C, J                   1.81          3.00
@-29 C, J                  1.79          1.58

(a) At 3.2 mm thickness. (Source: Dow Plastics)

* BASF, according to Rossio, is looking to improve properties for some new application areas, such as package trays. "We are aiming to develop systems that have higher strength properties, as OEMS are looking for this in order to mount speakers. This may mean a higher specific gravity." While OEMS are willing to accept 0.8 g/cc in order to get a high flexural modulus, Rossio does not believe that's a realistic expectation. "We are now working on a material that will allow you to achieve a flexural modulus above 1 million psi, which I don't think is achievable with a 0.8-g/cc material."

* Miles has a new PUR/polyurea hybrid LD-RRIM system that has better heat resistance than straight PUR. To demonstrate this, panels of both this hybrid system and a similar non-urea PUR system were molded at several thicknesses and densities. The average heat-sag value for the hybrid system was only 34% of that of the PUR system.

LD-RIM TOE TO TOE WITH TPO

In the past, standard elastomeric RIM has suffered from higher density and consequently higher cost than TPO in fascia applications. The evolution of LD-RIM over the past couple of years is changing all that, claims Miles' Kogelnik. "Low density puts a new perspective on RIM. We say you can get a 15-20% weight savings over typical RIM. We are working on a density of 0.90 g/cc."

The key is use of a higher level of nucleation, he explains. "Our patented technology involves using hollow microspheres, which allow you to put more air into the system with no bubble clustering in the mixhead." In its development efforts, he says, Miles' goal is to ensure that no changes in machinery or tooling would be required to use the LD-RIM system.

Miles is supplementing its current Bayflex 110 LD-RIM system with a new Bayflex 95 family of PUR/polyurea systems that combine new microsphere filler technology with a new isocyanate and new polyol blend. According to Kogelnik, it was developed to meet a O[degrees]F 5-mph impact test. "Our formulation gives excellent low-temperature impact performance," he says (Table 2).

TABLE 2 - PHYSICAL PROPERTIES OF LD-RIM WITH LOW-DENSITY
FILLERS (MEASURED PARALLEL TO MOLD FLOW DIRECTION)

                                           Bayflex 95        Bayflex 95
                        Bayflex 110-35   (18% Wollastonite   (8% Wollastonite
                        (18 %            + Low-Density       + Low-Density
                        Wollastonite)    Microsphere #1)     Microsphere #2)

Density, g/cc            1.13             0.98               0.95
Flexural Modulus, MPa    483              324                296
Tensile Strength, MPa    1.52             11.7               13.6
Elongation, %            142              110                155
Tear Strength,
Die C, kN/m              81.6             54.3               60.0
Heat Sag, mm,
1 hr @ 120 C,
152-mm overhang          36.5             9.0                10.7
5-mph Rheometric
Dart Impact, J, 23 C     18.4             15.7               19.0
-18C                     6.2              9.2                15.7
-30C                     2.4              4.3                8.9

Source: Miles Inc.

Dow is also using low-density microspheres fillers in place of the usual fillers such as wollastonite. Automotive group market manager Jim Davidson stresses that Dow's approach to low density allows molders to incorporate microspheres into existing RIM systems and tools. In a 2000-sq-in. fascia, he says, using 4-7% glass microspheres reduces weight of a typical filled RIM system from the usual 10 lb to 8 lb. The microspheres reduce specific gravity to 0.89 or 0.90 g/cc, compared with a typical filled RIM system at 1.12 g/cc. Typical TPOs have densities of 0.94 g/cc. And suppliers hope to push LD-RIM densities even lower.

The cost/lb of LD-RIM may be higher, but less urethane material is used, so that the final LD-RIM system can provide 18-20% material-cost savings, says Davidson. Yet another way cost savings can be achieved with glass microspheres is by downgauging. Relative to standard RIM, 18-20% reduction in wall thickness can be achieved without compromising properties such as stiffness, impact strength, and dimensional stability, he asserts. Wall thicknesses of 3.2 mils reportedly can be achieved with LD-RIM vs. a typical TPO wall thickness of 3.5 mils.

Degree of attrition or breakage of the microspheres during processing will depend on the RIM urethane chemistry and type of microspheres used. Selection of microspheres permits tailoring physical properties, too. "With glass microspheres, you can improve compressive properties, which results in better crashworthiness," explains Davidson. Miles' Kogelnik says his company has examined all types of spheres, including polymeric, glass, and ceramic types. "Which you use ultimately comes down to how much you are willing to pay. We recommend different microspheres depending on the part design and surface quality required."

LD-RIM retains standard RIM's paintability advantage over TPO, suppliers say. "RIM urethanes have an affinity for paint, eliminating the need for primers," says Davidson. He adds that special additives make RIM conductive so that paint is drawn to them in electrostatic coating operations.

Overall savings that can be achieved with LD-RIM vs. TPO today can approach about 30%, Davidson estimates. On a typical bumper fascia, downgauging wall thickness saves about $10. "By improving conductivity, you save another $3-10. By adding the glass microspheres to reduce density, you save another $2-3. Plus, you have the added factor of recyclability."

Davidson claims that there's no problem using regrind scrap. "You can use 10-12% of painted RIM regrind to make a new fascia without any loss of properties."

Miles' Kogelnik is optimistic: "In the future, we expect that a significant amount of LD-RIM will be used in bumper fascias." Both he and Davidson from Dow also see potential for LD-RIM in other applications such as body side panels, or fenders.