Insulation material gives car makers the edge

A new range of sound absorbing materials for the automotive sector is being launched this month, which will enable design engineers to totally rethink their approach to acoustic absorption in many parts of the vehicle. Dean Palmer reports A new range of sound absorbing materials for the automotive sector is due to be launched this month. The material was developed by technology company 3M, using its intimate understanding of the principles of sound transmission and the latest in advanced simulation and modelling technologies. The firm claimed its innovation will "allow car makers to totally rethink their approach to acoustic absorption in many parts of the vehicle". He continued: "The products that we are launching this month are our first acoustic materials that aren't based on a 3D non-woven fibre structure. They've been developed as a result of our investigations into the way different materials interact to absorb sound. We've found ways to greatly improve the absorption of certain materials at key frequencies, which will really help car makers with some difficult noise reduction challenges." The roots of the new technology can be traced back more than 25 years and actually started life in a sector far removed from automotive design. In the 1970s, engineers at 3M experimented with synthetic materials that would have good thermal insulation qualities. They felt that emerging techniques for the production of extremely fine polymer fibres could provide the key to the manufacture of a synthetic competitor to the best natural insulator: goose down. The result of this research was a rather clever composite material that used a non-woven network of extremely fine polypropylene fibres - as small as 2 to 5microns in diameter. Being so fine - a human hair, by comparison, is typically 75 microns across - these new fibres had an extremely high surface area in relation to their weight, and as a result, could trap a large quantity of air and provide the desired thermal insulation qualities. While their thermal properties were excellent, the fine fibres produced a material that was too weak to be used in the application that 3M had in mind: bad weather clothing. Its solution, inspired by the structure of natural materials like down, was to add a proportion of thicker, stronger fibres to the mix. By including around 30% (by weight) of polyester fibres with a thickness of around 25 microns, 3M created 'Thinsulate' insulation, a material that was light and durable but with excellent thermal insulation properties. A key advantage of the synthetic product over its natural competitors was the fact that the fibres used were hydrophobic. While down and other natural insulators tend to absorb water, losing their insulation properties and becoming extremely heavy, the new material would repel moisture, maintaining its effectiveness in the most difficult outdoor conditions. 3M brought it to market with great success and the material is still widely used today in coats, jackets, hats, gloves and footwear. 3M sought alternative uses for Thinsulate insulation and so the company's R&D engineers started to investigate the material's other properties. In the 1990s the company discovered another key benefit of the material - its ability to absorb sound. The characteristics of the product that make it a good thermal insulator - a complex structure that traps and holds a lot of air - also make it a superb absorber of sound. Sound waves entering the material have to take a tortuous route through it, and in the process, much of the sound's energy is lost. Convinced that the automotive sector would be an ideal market for a material that could deliver superb sound absorption with light weight, 3M approached car manufacturers to demonstrate the product. Andrew Christie, European manager for Thinsulate Acoustic Insulation at 3M recalled the company's initial efforts: "Results were decidedly mixed. The car company engineers were extremely impressed with the sound absorbing properties of the material, which were far in excess of the materials currently in use, but the cost was unacceptably high." So, said Christie, 3M's development team returned to their drawing boards and embarked on a two-pronged response. He explained: "First, they re-engineered the product specifically for the automotive sector, producing a material that cost less than the clothing variant, but still delivered high performance in vehicle applications. Second, they took a long hard look at the other properties of Thinsulate insulation that might help them to compete." They were in luck; the traditional material for sound absorption in cars is cotton shoddy - a felt material made from clothing industry scraps. Shoddy is extremely cheap, but it is also heavy, stiff and needs to be kept dry. Assembly of cotton shoddy can be difficult too, to insulate the complex shapes found behind a vehicle dashboard, for example, often requires many shoddy parts to be cut and individually bonded into place. However, Thinsulate insulation is strong, flexible and can be ultrasonically welded rather than bonded. As a result, 3M demonstrated to car makers that, by using the more expensive material, they would be able to reap a host of productivity benefits. The flexibility of Thinsulate insulation means it can be stretched or squashed into difficult areas where shoddy would simply tear. A single piece of Thinsulate could replace multiple pieces of shoddy, which, combined with the ability to weld it into position, save a lot of time in production. "By getting a good understanding of the new processes that the material would allow, we could go back to the car makers and show them that Thinsulate insulation could solve real problems for them," explained Christie. The first vehicular applications of Thinsulate insulation rolled off the production line in the mid-1990s. Since that time, 3M and its customers have continued to explore the potential for the material in automotive applications. They are still exploiting the material's unique characteristics to deliver unconventional solutions. Some of the latest examples make use of its hydrophobic characteristics to allow sound insulation to be installed in unusual areas of the vehicle. At the end of last year, the first European vehicle to have its door insulation installed in the 'wet' side of the door, rather than the 'dry' side began production. Putting the insulation here allowed less insulation to be used and cut assembly costs, while improving the overall acoustic performance of the assembly. While 3M's application engineers have been working hard, the firm's material scientists have not been idle. The version of Thinsulate insulation being installed in vehicles today is the fourth generation of the material to be produced since its introduction to the automotive industry. During that time, the material has evolved with its acoustic properties foremost in the minds of its developers. "Our engineers now have a much better understanding of the acoustic properties of the materials," said Christie. "That means we can match the acoustic performance more precisely to the needs of our auto customers. We now use statistical energy analysis software, a computational technique that allows us to predict the acoustic properties before we make a new material." 3M has also improved the heat resistance of the material to make it more appropriate for vehicle under-bonnet applications. The first such applications, in the diesel sector, where thermal conditions are less severe than with gasoline engines, were launched earlier this year. Boat building is another sector that is taking advantage of the material. As Christie said: "The marine sector has very different demands from automotive. They were traditionally using glass fibre materials to isolate engine rooms from living accommodation. These were heavy materials with much higher performance demand than you see in cars. We've developed a range of marine-specific Thinsulate insulation products that deliver improved thermal and acoustic properties at much lower weight." One UK luxury yacht designer has just adopted Thinsulate insulation in its latest craft, and managed to reduce weight by over half a tonne in the process - reducing fuel consumption and improving performance as a result. Even ambulances, caravans and motor homes are now using the insulation material. ointers * 3M's new range of sound insulation materials, based on Thinsulate, will be launched this month targeted at the automotive sector * The materials are 3M's first acoustic materials that are not based on a 3D non-woven fibre structure. 3M has discovered ways of greatly improving the absorption of the material at certain frequencies by looking closely at how different materials interact to absorb sound * Thinsulate insulation started life in outdoor clothing, but is now being used in caravans, motor homes, boat building, luxury yachts, ambulances and under-bonnet diesel engine applications Eureka Says: Innovation often arises when new applications are found for an existing technology or material. Since developing Thinsulate insulation, 3M has constantly 'tweaked' the material to offer customers in different sectors new properties and benefits