The Huntsman Automotive team gave Urethanes Technology International an overview of some of the issues driving change in the way that polyurethane can be used to reduce the noise, vibration and ride harshness (NVH) which passengers experience in a modern car.
Noise and vibrations come from a number of different sources said Andy Walton, business manager, automotive & flexible foam, EAIME (Europe, Africa, India and Middle East) in a telephone interview with Huntsman.
Peters, market sector manager automotive acoustics, Huntsman added, “There are a number of potential noise sources; road surface and vehicle construction leads to structure-borne noise, the engine and transmission generate airborne noise That noise needs to be attenuated or tuned to meet the OEM’s demands and branding rules. OEMs use inner dash and under carpet insulation to combat structure-borne noise and engine bay absorbers to reduce airborne noise. They also use engine bay absorbers for both in-car noise as well as environmental pass-by noise.”
Walton said: “Polyurethanes play important roles in minimising the effects of NVH in the cab, passenger compartment or cockpit of modern cars”
Inside the cab
Polyurethane foam can be found in a number of places inside the cab. Some areas, such as carpet backing are similar to non-automotive uses but the formulations are very different, as are the tasks the backing is asked to perform.
Residential carpet underlay has to provide comfort and durability for the life of the carpet. Automotive carpet underlay must also provide noise insulation for the life of the car.
Peters said: “Carpet underlay and choice is driven by [the] OEM and there is some difference. Some German manufactures are using viscoelastic foam, which is better at damping low frequencies. However, high resilient (HR) foam is the most common insulation foam for many OEMs.”
Peters added: “OEM, Tier 1 suppliers or the specialist cockpit designers, typically don’t give a complete specification for the component. The specification is likely to specify the PU material properties of the underlay and an acoustic target.”
“The design process might typically involve Tier 1 suppliers modelling the cockpit, and we can suggest polyurethane systems that will meet the desired thickness, loss factors, sound absorption target and so on of the carpet,” Peters said.
“Typically, automotive carpet underlay is made from high resilience polyurethane foam and will have a density of between 45-65 kg/m³. Viscoelastic carpet underlay is usually a bit higher in density and ranges from 55-85 kg/m³. The foam’s insulation behaviour and loss factors are important criteria here,” said Peters.
“One of the drivers for insulation is the mass layer that is applied to acoustic problem areas. The foam acts as the decoupler between that mass and the vehicle floorpan. The actual weight of the foam is not the determining factor here,” he added.
Huntsman said that historically, when car production and supply was localised within, there were strong regional variations between components. Today, although automotive production may be based on the same continent that the car is eventually sold in, the supply chain is increasingly global. This movement to globalised supply chains is leading towards increasingly standardised components and specifications for parts within OEMs. Increasingly, US vehicles are being built with parts that meet European specifications, for example, and the differences are becoming increasingly cosmetic.
“OEMs design the NVH specification to fit with the brand of car. The driving and passenger experiences are tied closely to the brand of the car,” said Walton
Standards are driving innovation
“In Europe and North America, weight has to be kept as low as possible while retaining the acoustic properties that the brand demands,” Walton said. He explained that in Europe, the thrust of automotive development is shaped by EU-wide carbon dioxide emission targets. There is a maximum allowable emission of 130g/km CO2 and an average of 95g/km across manufacturers’ fleets by 2021.
“In Europe, car models that fail to reach the standard will face a surcharge that will either be absorbed by the OEM or passed on to the customer,” said Walton. “This could hit large vehicles,” Walton suggested.
Robert Sawitski, marketing manager for the Americas, Huntsman explained that in the US, new 2012 Corporate Average Fuel Economy (CAFE) regulations are driving innovation. Many OEMs will have to nearly double the fuel economy of their fleets over the next decade if they wish to meet the new 2025 targets. “Like Europe, OEMs in the US will face penalties for failing to comply with the new CAFE regulations,” Sawitski stated.
The new CAFE standards in the US are pushing OEMs to increase their use of smaller engines. “Without the weight of the heavier engines to absorb the structure-borne noise, OEMs will have to improve the insulation of the cockpit,” Sawitski said.
“The need to save weight to improve fuel efficiency or reduce emissions is forcing car makers to think more innovatively about how they offer improving ride quality while reducing the weight of the vehicle,” said Peters.
Huntsman stated that polyurethane foam can be applied in a large number of places: as part of the dash assembly, under the bonnet or as carpet panels. “Because polyurethane reacts quickly, and is fairly easily foamed in the geometry of the floorpan, it can be used in ways that other materials cannot,” said Peters.
He added: “For example, it is possible to spot apply high-density PU mass layer over noise or vibration hot spots. This selective application can have useful weight savings compared to earlier technology such as vacuum formed EPDM (ethylene propylene diene monomer rubber) or EVA foam sheets which have been used as sound deadening materials. The PU elastomer used as mass layer is densified by using mineral fillers like barium sulphate, in order to achieve high surface mass with a relatively thin layer, leaving room for the foam decoupler.”
“Because car designs are modelled for vibration, it is possible to spot apply an anti-vibration coating of polyurethane over hotspots, avoiding quiet points. This can potentially save several kilogrammes of material per car,” said Peters.
Designing a car’s cockpit is a complicated process, areas such as vibration prediction are left to experts.
Increasingly, under the bonnet, low density foam (below 20 kg/m³) is used to absorb engine noise. The foam is produced in blocks, of which the slices are thermoformed, between two layers of non-woven cover. This type of PU offers significant weight reduction over historically used textile fibres. “It is easier to reduce the effect of high frequency vibrations and much harder at lower frequencies below 500Hz,” Peters said.
Sawitski explained, that fit-and-finish is increasingly important to automotive companies. “Polyurethane carpet underlay gives superior fit-and-finish, and coupled with its acoustic insulation properties, PU has a big advantage over other materials,” he added.
Sawitski explained that, in the cockpit, there is a lot of focus on seating.
“If car makers can lower the height of the seat by an inch (2.5cm), they could remove one inch of steel and glass that make up the roof module without making the cockpit feel small and cramped, Sawitski said. He added: “Huntsman has developed thin seat foam technology that will deliver the same comfort as a thick pad.”
“It is important to not only retain comfort in thin seats, but to also improve it,” Sawitski said.