Polyurethanes in Automotive Seating

Topics Covered

Background

Function of Automotive Seating

Conventional Seat Design

Seat Evolution

Polyurethane Development

Design Development

The Full Foam Seat

The Design

The Target

Dynamic Creep

Ball Rebound Testing

Design Conclusions

Summary

Background

The design of automotive seating is attracting increased attention from vehicle manufacturers. They recognise that seat styling, comfort and safety contribute to the initial appeal of a vehicle and to ensuring customer satisfaction, while the durability of seating can be reflected in the vehicle's residual value.

Recent advances made by ICI Polyurethanes in flexible polyurethane foam technology have led to the formulation of new, low density, high performance polyurethane (PU) systems for automotive seating. Alongside this development programme, the company has extended its understanding of how key chemical components of the base materials used in the production of PU foam affect the final performance of the seat. This allows an accurate prediction of, seating foam performance, as well as long and short term comfort and durability, over time and under a range of climatic conditions, for both conventional and unsprung, thin dead pan seats.

Function of Automotive Seating

The function of an automotive seat is to support the body comfortably under both static and dynamic conditions. To achieve this, conventional seat designs have combined a number of separate mechanical suspension systems in the seat base. These systems are used to a degree in seat backs and other trim parts, but they are less sophisticated as the loads carried are significantly lower.

Conventional Seat Design

The conventional seat base uses a steel (or other metal) frame that acts as a chassis to support a series of springs and a PU foam layer or ‘topper pad’, which sits on top of the springs. To produce such a seat, expensive and time consuming multi-stage fabrication techniques have to be used.

Seat Evolution

Over recent years, vehicle manufacturers and first tier suppliers have focused on reducing production costs, and have initiated programmes to reduce the weight of the seat, while improving its ride comfort and durability. To complement these programmes, ICI Polyurethanes initiated its own research aimed at reducing the foam density and cushion thickness used in production, and at increasing its understanding of factors affecting both long-term and short-term seating comfort.

Polyurethane Development

Implementation of the Montreal Protocol in 1989 banned the use of CFC11 as an auxiliary blowing agent, which was used to improve processability and reduce foam densities. It was then necessary to increase foam densities to 55-6O kg.m-3 to meet vehicle manufacturers' seating specifications. ICI Polyurethanes has now developed special PU systems with enhanced performance characteristics, which meet or exceed the demands of vehicle manufacturers at significantly reduced foam thicknesses, at densities of around 40 kg m-3.

Design Development

To increase design speed and further enhance comfort, the company has also developed computer modelling techniques that can simulate both the static and dynamic response of the seat. From the results it is possible to select the precise blend of base materials to produce a foam that is suited to the unique characteristics of a vehicle. This offers significant advantages to seat manufacturers, as they can speed up development from design and low cost laboratory testing to full in car testing.

The Full Foam Seat

Despite these advances, the conventional seat design remains relatively expensive to produce and places restrictions on the optimisation of comfort and durability. ‘Full foam’ seating techniques, on the other hand, offer vehicle manufacturers the opportunity to fully exploit the advances made in PU technology. It is this technique that ICI Polyurethanes is using in the development of the ‘concept seat’.

The Design

In a full foam seat, the metal frame or chassis is replaced by a cradle or pan, which is often manufactured from impact modified glass-filled nylon, ABS or lightweight metal. The springs and topper pad are all replaced by a single PU foam core.

As the foam core now has to provide both vibration dampening and comfort, it has to be considerably thicker than a foam used in a conventional seat. To give an equivalent performance, the first generation of full foam seats generally needed a foam core thickness of approximately 13 cm. However, to achieve maximum design freedom and reduce the cost impact of the increased material usage, volumes need to be significantly reduced.

The Target

The target for the concept seat is to reduce the foam core thickness by 46%, to 7 cm. To achieve this, ICI has been studying the effect of this reduction on two key factors, foam durability and long term occupant comfort. This required special testing methods to be developed from the techniques used to improve conventional seating PU foams.

Dynamic Creep

Dynamic creep testing allows the physical change that occurs to the foam under driving conditions to be determined. Energy generated from prolonged vibrations of the occupant results in the foam losing height and hardness. Under certain conditions, this deformation can result in permanent damage to the foam structure.

Tests have shown that minimising the elastic modulus change and creep rate over time and a range of climatic conditions improves comfort, reduces fatigue and can allow a reduction in foam thickness. These findings have been confirmed by a number of global seat manufacturers, who have conducted subjective laboratory tests using real people.

Ball Rebound Testing

Ball rebound testing has become an automotive industry standard in Japan for evaluating and specifying seating comfort. The test is designed for analysis over a short time period and only provides an indication of static comfort or ‘showroom feel’. To give a true representation of sustained ride comfort, analysis of dynamic performance is required.

Conventional ball rebound theory states that, to retain comfort levels, thinner seats require higher ball rebound (foam ‘springiness’) values. ICI's latest research, however, considers the time dependent behaviour of seating foams and shows that seats exhibiting relatively low rebound performance can still give a high comfort ride, even under varying climatic conditions.

Design Conclusions

The company’s latest range of seating technologies has been designed to achieve optimal comfort at minimal densities, both initially and over time, under the most stringent conditions of varying temperature and humidity. They demonstrate that the net reduction in weight brought about b reducing the seat thickness outweighs an increase in density necessary to control foam performance, therefore offering seat manufacturer the potential for significant cost savings.

As these mathematical models and testing procedures are further developed, refined and combined with the advances in PU formulation technology, so the concept seat strategy will allow further improvements in comfort at ever low foam volumes and seat thickness.

Summary

The concept seat strategy combines many aspects of automotive design that are critical to vehicle manufacturers. It embraces the continuing need for reducing production costs, the need to increase the durability of the vehicle and the need to meet the demands of an increasingly sophisticated customer.

The concept seat, however, is not purely a notion around which production costs and speed to market times can be reduced. It embraces wide concerns on recycling and reducing the environmental impact of manufacturing processes.

The concept seat increases opportunities of component integration, which reduces component cost, reduces assembly time and improve the efficiency of recycling operations. Reduced foam volumes will reduce the consumption of base materials and increase the energy efficiency of production operations. The underlying research, although in its infancy, is already yielding extremely valuable information from which the benefits are only just beginning to be felt.

 

Primary author: Dr. Aun Tan

Source: Materials World, Vol. 6, no. 1, pp 15-16, January 1998.

 

For more information on Materials World please visit The Institute of Materials

 

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