There is no one-size fits all material or approach. ‘We need to understand case by case each application and so we have wide array of TPU grades specifically designed and chosen for some of the extremes,’ he said.
By tuning the hard and soft segments, it is possible to change the properties of the polyurethanes. Some are more sensitive to polar fluids such as modern synthetic fuels and additive packages, which are very chemically aggressive. Chemical resistance quite often informs the choice of the material for the seal.
Trelleborg makes its own proprietary materials, Saur said. 'We synthesise all our materials ourselves… we have our own synthesis plant and produce a wide range of material properties from more elastomeric polyurethanes to the more thermoplastic grades.'
This flexibility is important in world where specification changes can be invisible. 'We need low swell and ideally no swell grades,’ he said. ‘That's why we need to understand the applications, the media and the specify fluid that the customer wants to use. We qualify our material with the fluids they select.'
He added that the major challenges occur when fluid suppliers change the specification and composition of their fluids. ‘They may be sold under the same brand name, but if a supplier changes an additive package, every now and then we see unexpected behaviour from our seals,' he said. 'From a commercial perspective seals are a C part, and probably don’t show up in the cost breakdown for an entire component. But from a performance perspective they are an A+ part. Usually, when you see a machine leaking oil, people think it is the seal, but this may very often be the symptom and not the root cause of the problem.'
For growth in hydraulic seals, Trelleborg is looking at the off-highway, agricultural and construction industries, where the broad performance range and tolerance of polyurethane seals is attractive. Elastomeric and PTFE seals are only used where they are really needed.
Trelleborg is interested in next-generation compounds to accommodate a broad range of use conditions, not least because hydrolysis and exposure to polar media such as water will continue to be a challenge for polyurethane seals. ‘Sealing against polar materials at high temperatures is one of the biggest challenges we see,' Saur said.
Hydrolysis resistance may seem like a strange requirement for seals that are designed to be used with engineered, oil-based fluids but, the reality is that in the mine or on the construction site people will use whatever liquid is available, and there is always water in the hydraulic fluid. ‘Hydraulics are often not maintained to the OEM's specification,' he said.
Fundamentally, there are a number of materials limitations that limit the number of applications for TPUs, Saur said. A broader operating temperature window would permit their wider use.
‘Currently, TPU is used at temperatures between –20°C and 90°C,’ Saur said. ‘We are trying to stretch this to 130°C or 135°C or even 140°C, and down to –35°C or even lower. At low temperatures, we run into challenges from glass transition temperature, as the seals stop behaving in a rubbery way and start behaving like a thermoplastic. This is why there are few TPU seals in the cold parts of long-distance aircraft – we can't accommodate –60°C for extended periods, and aircraft makers require that.'
Ground-based applications are not quite that tough, but there are still limitations. 'There are excavators that have to run in Siberia, and also in Dubai,’ he said. ‘We are trying to stretch the performance windows at these extremes.'
In the world of domestic automotive applications, Lubrizol's Santamaria said his company had seen demand for protective films take off over the past couple of years. Paint protection films are transparent films that are applied by dealerships to the most vulnerable parts of a car. 'They help protect the paintwork from scratches from small stones in the road, and are popular in the US because of the coarse grit and salt used to melt snow and ice on the road,' he said.
Stick to it
TPU films like this could also find their way inside the car, where they might complement existing applications in gear levers, and some synthetic leather seats that have a TPU top layer. ‘This is a positive trend, even if fewer cars are sold in the future and concepts like shared cars grow,’ he said. ‘In this case, the performance and durability of materials is going to be more and more important.’
If car sharing continues to grow in importance, the durability and interior finish will also increase because shared cars are likely to be less well cared for than a car with a single owner. They will need to use robust, good looking surfaces which can withstand frequent cleaning and some misuse.
'Frequent cleaning can lead to scratched interiors, and after a year, without the right material choices, the interior could become unattractive,’ he explained. ‘If OEMs choose a material that can withstand scratching or cleaning products, this will give value in the longer term. Maintenance is money, so we see a push for more durable materials for shared mobility vehicles. We see a trend for very durable materials like TPU becoming be more popular than PVC.'
Self-healing films could also prove very useful in these environments. ‘Any valuable surface could be protected with paint protection films,’ Santamaria believes. ‘Coatings are irreversible, but TPU PPF can be removed without residue. Self-healing is a very strong point for superficial scratches. These can be cured with hot water or a hairdryer, and the film adhesive can be formulated to allow it to be removed without residue.
Hot-melt polyurethane adhesives are another area where Lubrizol is investing a good deal of effort. These are 100% solids, so there is no water or solvent to evaporate, and it is possible to modify the adhesives’ properties to a significant degree.
‘[Products with] extremely low activation temperatures and very low hardness are a very good combination or properties for textile lamination,’ Santamaria said. ‘This market has been in the hands of liquid adhesives, but hot melts are gaining ground because of their sustainability.'
Successful materials are likely to melt and be active at about 60°C, and have a low viscosity at that temperature. 'It's a technical challenge,’ he said. ‘TPU is quite amorphous, and there is not usually a steep drop in viscosity with temperature. You have to play with the formulation to narrow the window. The material could be polyester or polyether based, depending on the working conditions of the final product.