The automotive sector is driving polyurethane growth in the US, a major market as people replace worn out vehicles with new state-of-the-art cars, vans and light trucks.
By Simon Robinson
Polyurethanes are playing an increasingly important role in the automotive sector. Traditionally, PU has been widely used in seating and interior comfort but now it is finding its way into structural parts and noise vibration and harshness control applications.
Polyurethane systems are used not only to produce customer-facing applications but also to speed the production process, cut costs and add value to the OEM or tier suppliers.
Information from the 2012 CPI End Use Market Survey of the Polyurethanes Industry in the US, Canada and Mexico which, Hanne Hirsimaki, of IAL Consultants, presented at the recent CPI meeting in Phoenix, showed just how important the automotive sector is to the polyurethane industry.
In 2012 there were 14.5m vehicles produced in the US, up 13% from 2011, according to IAL. That figure was driven by high levels of scrap, and a trend towards smaller, lighter vehicles, and greater fuel efficiency from the new Corporate Average Fuel Efficiency (CAFE) standards. IAL, and industry estimates from conversations at the CPI conference, put the volume of polyurethanes in cars at 16% of the total by weight. In 2012, according to IAL figures, that amounted to 1.343bn lbs (609 kt/year) up from 532 kt/year in 2010, but still short of the 757 kt/year in 2006, before the economic crash.
In addition to light-weighting, automotive producers see the ability to offer greener cars produced with higher levels of renewables as being an important characteristic that can help swing a sale. So they are encouraging all of their suppliers to adopt increasingly green materials and processes.
Polyurethane is playing a significant role and is moving from structurally trivial applications, such as sunshades, increasingly into automotive structural composites, as Bayer MaterialScience’s Mike Super, explained at a recent Society of Plastics Engineers (SPE) Automotive Composites Conference in Novi, Michigan.
His paper explains how polyurethane spray sandwich technology, which was originally developed for automotive sunshades, can be optimized for use in more demanding automotive applications such as load floors.
Polyurethane sandwich construction combines the low density of a honeycomb core with the high strength of a fibre-reinforced polyurethane skin to produce load-bearing parts. Super said these have very high flexural stiffness and good thermal properties. This combination makes them a lighter weight alternative to acrylonitrile butadiene styrene (ABS), polypropylene, sheet moulding compound (SMC) and wood products.
Sandwich parts made this way can meet OEM specifications for deflection and no permanent set at both room and elevated temperatures. They can also be produced faster than conventional alternatives, said Super.
Super explained that Baypreg two-component PU spray sandwich technology was originally invented to produce light-weight sunshades that could withstand high temperatures found near automotive windshields. In the standard production process, honeycomb cores which can be made of paper, thermoplastics, rigid foams or expanded polystyrene are sandwiched between natural and glass fibre mats to make parts from 6-30 mm thick.
The sandwich is sprayed from both sides with the two-component polyurethane system. These systems have low viscosity and this ensures the glass reinforcing mats are thoroughly wetted with the resin. Next, the composite is placed in a compression mould which is heated under pressure. The process binds the composite together.
Because the composites are low density and offer high stiffness for their weight, they can be used to replace conventional materials, which are heavier and may themselves require relatively heavy supports, said Super. Additionally, he said that the moulding process allows connection points to be included in the composites in the moulding step and that it is possible to mould carpeting onto the parts, further reducing manufacturing steps and reducing costs at the OEM.
Baypreg materials are being used in door panels, spare tyre covers, load floors, sun roof cassettes. It could be possible to make interior rear parcel shelves, and exterior applications such as tonnau covers for light trucks, said Super.
Super explained that work had been done by Bayer MaterialScience to produce a range of options for automotive parts producers. These include options for components with high renewables content, thin parts and light parts. Newer polyurethane formulations do not need to be nucleated, give longer open times and faster demould with better mould release than earlier versions. They are stiffer than the original formulation, Super said. A comparison is shown in table 1. Here stiffness at between 105.1 and 107.7N/mm for PUR 2 and 4 were significantly higher than the standard PUR1 formulation at 84.7N/mm and failure load at between 1779 and 1954N PUR 2 and 4 was much greater than then 1623N of standard PUR1 formulation and construction.
Increasing levels of glass mat/unit area will also increase stiffness but there will also be an increase in part weight, said Super.
If OEMs need to reduce weight and then, if space permits, increasing honeycomb thickness can lead to large increases in part stiffness and also an increase in renewables content, if a renewable core material is chosen.
Spray polyurethane composite parts such as these offer OEMs a wide range of options to meet their varied design needs, Super said.
Sound of silence
Noise vibration and harshness is another important area for automotive producers. The perceived quality of a car increases with increasing quietness of the passenger compartment. Noise comes from the powertrain and external factors such as road condition, suspension and tyre noise. Sound can be transmitted through the body of the vehicle along pillars and rocker covers, for example.
Dow used the CPI meeting to outline how it has improved its Betafoam polyurethane technology with a series of new grades called Renue. This has 25% renewable content, said Allan James, marketing manager NAFTA composites.
James noted, that polyurethane foam can be used to effectively seal complex cavity shapes: it can be injected and reacts quickly to seal the intricate spaces within pillars, for example. The Dow material is a pre-polymer.
The pre-polymers are oligomeric liquid intermediates. They are produced by reacting an excess of either isiocyanate or alcohol so that the final pre-polymer is still a liquid and contains functional groups. This has a number of production benefits at OEM/ Tier suppliers, said James.
Firstly, there is much less free MDI in the unreacted state. This means that there is no requirement for full ventilation or a downdraft booth during installation.
Less expensive, but suitably effective local ventilation can be used instead.
Secondly, the pre- polymer approach enables an easier means for the reactivity of the foam to be closely controlled since all of the polyol is pre-reacted with the isocyanate. This leaves only the blowing reaction with water to complete the formation of the foam.
Thirdly, the conventional isocyanate and polyol polyurethane reaction is highly exothermic. Using a part-reacted polyol reduces the heat generation within the automotive part. This leads to better quality foam, said James, and also means that lower pressure and less expensive dispensing equipment is needed.
Additionally, the pre-polymer has also been modified to increase the available isocyanate functional groups of the unreacted pre-polymer. This leads to greater carbon dioxide generation when the isocyanate reacts with the water and so density is lower than with the conventional material.
Finally, Dow’s Betafoam Renue foam has over 25% renewable soya-based content in a new plasticiser that replaces the traditional oil-derived material.
Automotive producers like fast reactions, said James. If a reaction is fast then there will not be any need to slow down the production line to accommodate a chemical process. The material gels after about three seconds and loses its tack after 7 seconds. These times are well within the parameters demanded by automotive producers, said James.
Dow has developed a way of measuring sound deadening effects within automotive voids. This involves inserting a noise generator and a microphone and measuring how effective the foam is at blocking the sound over a wide range of frequencies.
James showed work in real-life pillars and rocker boxes. The studies showed that despite a change in density from 2lb/cuft (about 32kg/m³) to 1.4 lb/cu ft (22 kg/m³ noise reduction with Betafoam Renue was between 35 and 42dB, with the same level of noise reduction at the rocker.
A key drive in the automotive sector is to reduce the weight and volume of seating materials without compromising on ride quality inside the car. Also at the recent CPI conference, Sanyo outlined how some of its Primepol polyols can be used to help automotive producers meet some of these imperatives. Sanyo said it is possible to use its polyols to reduce foam density, and lower the volume of seat cushions without significant changes to ride comfort. This is due to the morphology of the foam. Sanyo said that foams produced using its polyols have hard segments homogenously dispersed with smaller domain size compared to conventional polyols. The company added that high reactivity and fast viscosity build-up in the foaming process – because of the polyols’s high hydroxyl number – helps to produce a fine phase-separated structure and enhances elastic behaviour.
The company added that the rate at which foam transmits vibrations at different vibration frequencies has a big impact on ride quality. It is important to avoid frequencies at around 6Hz because this induces feelings of nausea in passengers.
There are two approaches to this problem: one is to design seats and foams so that the resonant frequency is below 6Hz, and the other is to make the seat/foam combination transmit very low levels of vibration at this frequency.
Lowering frequency can be done by enhancing the elastic behaviour of the polyurethane foam. Generally, Sanyo said, foams with a more elastic nature are more suitable for seat cushions with frequencies lower than 6Hz. This is the most important route to modifying seat behaviour, said Sanyo.
Increased elastic behaviour can be coupled with cell structure modification to reduce air permeability of the foam. Cell structure modification increases the foam’s viscous response. This can be done by lowering air permeability. Therefore, a combination of these two approaches is effective for the improvement of vibration characteristics.
Sanyo said that its Primepol gives foams with MDI that have high levels of elastic properties derived from its high primary OH ratio and low monol levels and so vibration characteristics can be improved.
Sanyo showed some lab work, which it said proved this advantage.
The firm prepared foam samples with 100mm thickness by a hand mixing method according to one of our basic formulations that gives foam with core density of 60kg/m³. The isocyanate index was adjusted to give similar foam hardness. Foam physical properties including vibration characteristics are summarized in Table 2. As expected, by using Primepol, resonance frequency was down-shifted by 0.26Hz with a slight decrease in maximum transmissibility. As a result, it could cause approx. 30% reduction of transmissibility at 6Hz.