Automotive polyurethanes: From weight reduction to lightweight construction

Dr. Rolf Albach, Product Research, Bayer MaterialScience AG, Leverkusen, Germany

According to the legend John Montagu, 4. Earl of Sandwich invented the beef between toasts named sandwich after him in the 1760s. In the 21 century sandwiches have passed the limits of the kitchen for long and are now also part of the future of weight management for climate-conscious mobility. Our focus is here on what is in between the facings. The facings themselves are topics of their own.
The contribution of chemistry to the weight management in vehicles can be on three levels. These differ in the level of the value chain integration of innovation.
1. Density reduction of sandwich structures within the ­current design
    • Collaboration is only required on the testing and ­approval level
2. Innovation in the existing sandwich structure with some redesign or added value
    • Collaboration on TIER level is required from the design phase and approval on OEM level
3. Redesign of complete automotive parts by using new sandwich structures
    • Collaboration is required along all the value chain from design to launch

These approaches are illustrated by three examples.

 

Instrument panels: an example of parallel incremental weight saving innovation

Instrument panels are sandwich panels based on one layer of fiber-reinforced polypropylene, a flexible polyurethane foam core and a skin with a variety of materials from PVC over TPO to PUR (thermoplastic or elastomer, figure 1a). Recent developments include foamed PVC skins (figure 1b) and two-layer skin based on a solid elastomer layer and a foam layer produced e.g. by Recticel. The resulting sandwich is composed of two foam layers, a foamed thermoplastic frame and only a thin solid facing.

New airbag modules with more power and sensitive electronic assistance devices like head-up displays are hinged to sandwiches based on materials with continuously decreasing intrinsic mechanical strength.  Figure 2 illustrates this for the Bayfill range of semi-rigid and flexible foams. There is a natural decrease of compressive strength with density. Weight saving in both facings and in the polyurethane core is a challenge. Chemistry is partially able to overcome the power law of the density-related decrease in strength. Customization is possible. The redesign of the chemistry according to customer needs is behind the recent introduction of Bayfill systems for reduced weight (figure 3). The new Bayfill systems with improved flow permit, depending on the reference, 10-25 % weight savings. They are designed in a way that there is no need to adapt the other materials of the sandwich structure to the new material.

The parallel evolution of materials combined in one sandwich structure leads to a strong increase in diversity. This is in line with the desire of OEM marketing for differentiation. On the other hand it leads to complexity in production and in the design phase due to moving targets. The interaction between polymeric materials is both known and underestimated for long and remains a key challenge for the expansion of sandwich structures in automotive applications.

 

 Abb-1a-REM_Querschnitt_I-Ta Abb-1b-53IF33_REM_PVC_foam_ Abb-1c-Bayfill_mit_Spruehhaut
Figure 1a: REM photograph of a typical standard instrument panel cross section with glassfibre-reinforced PP (>1350 kg/m³, bottom), Bayfill open-cell polyurethane flexible foam (~120 kg/m³, center) and solid PVC (~1170 kg/m³, top)

Figure 1b: REM photographs of the interface between PUR foam (bottom) and a foamed PVC skin (top)

Figure 1c: REM photographs of the interface between PUR foam (bottom) and a two-layer PUR skin (top)

 

 Abb-2-Bayfill_Dichte_Stauchhaerte_deFigure 2: Density dependence of compressive strength for foams based on different Bayfill systems.
As predicted by theory the compressive strength of rigid open cell foams follow a power law “y ~ densityx“. According to theory the exponent for completely open-celled rigid foam with cubic cells is x=2. Cell wall contribution should decrease the exponent. REM pictures of Bayfill-based foams show that cell walls are 80–90% intact in 125 kg/m³ foam but exponents are in the range of X=2,1–2,3.
The figure shows a selection of Bayfill-based systems. It shows also a design trend: very hard “semi-rigid” foams got out of fashion for instrument panels.

 

The parallel evolution of materials combined in one sandwich structure leads to a strong increase in diversity. This is in line with the desire of OEM marketing for differentiation. On the other hand it leads to complexity in production and in the design phase due to moving targets. The interaction between polymeric materials is both known and underestimated for long and remains a key challenge for the expansion of sandwich structures in automotive applications.

 

Headliners: added value at constant weight
Headliners are sandwich panels based on a semi-rigid thermoformable polyurethane foam core and two layers of fiber-reinforced MDI-based polyurea. The sandwich is covered by a variety of decorative materials that may include another thin layer of polyurethane flexible foam (figure 4). The traditional approach to save weight is to reduce the density of the foam core and adapt fibers and isocyanate glue. In the beginning of this century most OEM in Europe turned from 30 kg/m³ and 26 kg/m³ (2 pcf) foam to 21 kg/m³ foam in the core of the sandwich. Currently this development has slowed down. Those who still use foam with 25-30 kg/m³ do it on purpose.

 

Abb-3-Grafik-EN-4Figure 3.
The design strategy for achieving lower densities in thin wall foam is to control the kinetics of polymerization. High flowability reduces the force to overcome the friction a foam needs to fill a cavity. This is particularly important if the cavity has a high surface/volume ratio. The Reynolds number describes an important part of it, the ratio of momentum and internal friction, i. e. viscosity. The lower the viscosity the lower the internal pressure required to move a liquid polyurethane foam during polymerization. The figure shows the evolution of viscosity over time for a current standard Bayfill® foam system and a modern one for reduced weight.

 

Abb-4-Grafik-EN-5Figure 4:
Typical sandwich design of a simple “one-step” automotive headliner. A core of PUR foam is covered by glass mat-reinforced polyurea and decorative layers. In the “two-step” process a textile-covered flexible foam sheet will be attached.

 

We believe that another path is worthwhile exploring. We have shown that 40 kg/m³ foam will yield a similar bending performance as a 21 kg/m³ foam with twice the thickness (figure 5). This is an option to save height. Options for saving weight through a new balance of fibers, glue and foam are just being evaluated.

There may be further options for weight reduction in the interior acoustic package if the headliner with its large surface would bear more responsibility in noise management. Driven mainly by one OEM significantly airborne noise absorption has been added to the performance profile of semi-rigid thermoformable 21-26 kg/m³ foam. The open cell content of these foams is well above 90 % while the values for standard headliner foam are in the range of 80 %. The compression strength of noise-absorbing foam is accordingly lower. The sound absorption requires a redesign of the headliner for twice the standard thickness adding approx. 125 grams/m² to the weight of the headliner. Products are commercial globally since 2011 and there is still a strong momentum in continuous improvement. 
On easy option to improve acoustic performance is the combination of two or more different types of foam. Combining acoustic semi-rigid foam with flexible or other semi-rigid sound absorbing foam will improve the acoustic performance even without added thickness (figure 6). Refraction between the different sheets increases the path length through the foam. The effect is already commercially used in stacks of flexible slabstock foam for office noise absorbers. The use of rigid noise-absorbing foam adds opportunities for the structural design of such absorbers.

 

Abb-5-en-Bending-BaynatFigure 5: The power law of bending strength over density and the power law of bending strength over thickness match here to yield similar bending strengths for foams that differ by factors of two in thickness and density. The thicknesses of these laboratory samples are not relevant for headliners. For a typical headliner a thickness reduction from 8 mm to 4 mm could be considered.
The graph can also be used to estimate the potential savings in glue and glass when using high-density foam.

 

Abb-6-en-Baynat-acousticFigure 6: Acoustic absorption of noise-absorbing PUR foam using the Kundt’s tube. While the 22 kg/m³ has advantages in the low-frequency region over flexible foam (not shown) the disadvantage in high frequencies can be overcome by a combination with very low density PUR foam. This is relevant because electrical engines emit a different noise spectrum than combustion engines.
 

Abb-7-en-baynat-PIRFigure 7: It is not surprising that PIR chemistry is suitable for expanding the range of thermal stability of isocyanate-based polymers. Expectedly thermoformable PIR has a glass transition approx. 70 °C above thermoformable PUR. Typically PIR foams are considered stiff and friable, particularly when water is used as the sole blowing agent. Surprisingly it was possible to maintain the typical elongation at rupture at approx. 20 %.
 

In order to expand the application of thermoformable semi-rigid acoustic Baynat beyond the headliner application we added thermal stability to the Baynat product range (figure 7). Today the engine bay the acoustic package is dominated by felt, flexible foams
> 60 ­kg/m³ and PUR lightweight absorbers of 15 kg/m³ – all with hardly any structural performance. Space around the engine becomes scarce. Acoustic absorbers need to get closer to hot parts of the engine. This increases the need for temperature stability. Foam sheets with 40 kg/m³ density based on the new Baynat PIR materials sustained 180 °C–200 °C in our laboratories. The way from laboratory to series application is still long but promising.

Tailgate: From metal to polymers in crash-relevant parts?
The replacement of steel sheet by fiber-reinforced duromers or thermoplastic organic sheets in structural parts is in a sense the “holy grail” of lightweight construction. The concept has been applied in demonstrators and low volume cars like the Mercedes SL or the VW XL1.  Still the sandwich composed of organic sheets and energy absorbing foam belongs to the “impossible entry segments that include high-liability arenas such as automotive safety-equipment” (McKinsey Quarterly spring 2012).

Material cards are available and can be used in crash simulations; challenges like high-gloss-surfaces and the design of efficient hinges have been addressed (figure 8). Challenges remain, starting from the initial need for more space to the desire to integrate plastics into standard processes like high temperature coating. Bayer MaterialScience is committed to continue contributing to the solution of these challenges through polyurethanes in the future.

 

Abb-8-Grafik-EN-9Figure 8: Tailgate as a demonstrator for a sandwich structure designed for crash-relevant parts. The facing (red) is a Makroblend PC/PBT-based organic sheet. The black polyurethane foam is energy-absorbing foam well known in the automotive industry. The proposal for a support for the hinge is in the upper left edge of the picture (white).

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