PU Composites: the cure for lighter cars

Composites and joining technologies were some of the highlights in the automotive technical session at UTECH Europe 2018. Simon Robinson pulls some of the strands together.

Polyurethane matrix systems can make automotive parts lighter and as strong as those made from conventional materials, while being faster and easier to produce. Many raw materials suppliers and machinery companies have been working hard to develop new ways of replacing more traditional materials in automotive components.

Car makers are being forced away from polluting, high power, liquid fuel engines; in the future, zero-emission vehicles with heavy batteries and electrical motors will be important. The heavy weight of batteries will need to be offset by using lighter, stronger components in other parts of the car’s construction. As a result, composite materials will become increasingly important in the automotive industry.

Tough competition

Polyurethane matrix materials offer a range of useful properties, but they have to compete against more well-established technologies and materials. The key competing technologies are glass-fibre or carbon-fibre reinforced polyester, vinyl esters and epoxy resins.

Polyurethanes offer an important benefit over existing products such as epoxies: they have low viscosity. Additionally, polyurethane matrix materials can produce composites with in-use properties that are often as good as, if not better than, similar composites made with vinyl ester or polyester resins.

Bart Vangrimde, technical manager for PU composites at Huntsman, told delegates about his company’s work on snap-cure polyurethane resins for use in resin transfer moulding.

The liquid moulding technique allows highly structural, complex, lightweight parts to be made. This is because the cure starts at a closely defined time, often a few tens of seconds after the polyurethane system is injected into the mould. The delay gives the low viscosity system time to wet all of the reinforcement, and also flow into the corners of the mould, before curing occurs.

Don’t move the fibres

He explained the fibres may make up 50-60% of the volume of the part. ‘The resin viscosity must be low so that it can wet the fibres without pushing them out of the way,’ he said. ‘And once the mould is full, the resin must cure quickly.’

The process makes parts with tight dimensional tolerances, he explained. ‘There are minimal finishing operations.’

Vangrimde added that there has to be a balance between the speed of this part of the production cycle, and the need to keep fibres in place. ‘Typically, the part processor manipulates mould temperature or the catalyst level. This manipulation changes the injectable time and also directly influences how quickly the part will reach full cure.’

Conventional composites can be difficult to use in long runs of parts that need to have short cycle times. ‘Because automotive parts are made in large series, the time available to inject each part is minimal,’ he said. ‘But if the resin is injected too quickly, flow can move the fibres, which alters the part’s properties. Very short injection times make it difficult to make large parts and, because the injection pressure is high, moulds, tooling and presses can be expensive.’

Snap to it

He said that in response to these difficulties, Huntsman has developed a new class of polyurethane resins. ‘These combine a low stable viscosity for injection with a snap cure,’ he explained. ‘The low and stable viscosity enables manufacturing of large complex parts with high fibre volume fractions at moderate pressures. The snap cure catalysis ensures cure times and high productivity.’

Huntsman first developed Vitrox resins with a high glass transition temperature (Tg).

This is the temperature at which the physical properties of viscoelastic materials such as polyurethane start to change. As these polymers are heated through the glass transition temperature, they become more flexible and tougher. Some thermoplastics such as polyethylene will start to melt and turn into a liquid at a higher temperature, but thermoset polymers such as conventional polyurethanes will begin to chemically degrade as the temperature rises because they cannot melt.

The new materials have a standard Tg of 130°C, a low initial viscosity and snap cure characteristics, and good toughness. ‘The materials have good durability at 130°C Tg and – what is truly unique – is that we built into the system the tuneable Vitrox catalysis, offering extended injection time and snap cure,’ Vangrimde added. ‘The Tg can vary between 180°C and 130°C. The lower Tg materials have higher tensile modulus and greater tensile elongation at break than the higher Tg materials.’

They also have higher fracture toughness, a measure of how much energy is needed to force a crack through a component. ‘It is not quite at the level of thermoplastics, but for a thermoset resin it is best in class,’ he said. The stiffness of the materials is sufficient for structural applications, he added.

You’ve got to be tough

Vangrimde used a rheology test to outline the material’s snap-cure properties. Vitrox with a viscosity of 30 mPa was loaded into a twin-plate rheometer, preheated to 100°C. The viscosity stayed low for about 50s, and then there was a very rapid rise in viscosity as the material cured.

The material was tested at a larger scale in an HP-RTM process with plate moulds. A variety of reinforcements, with and without binders, were tested. ‘The parts could be demoulded at about 130s from the end of injection when the mould was heated at 100°C,’ he said.

The matrix was also tested for the quality of the fibre interface bonding in a number of interlaminar shear test experiments. In these, short beams of composite were bent, and the new Vitrox materials were able to produce composites with interlaminar shear strength of 77MPa for glass fibre, and 68 MPa for carbon fibre.

Vangrimde explained that the test formulation included internal mould release. The glass composite was reinforced by a unidirectional E-glass non-crimp fabric (NCF) with a weight/unit area of 1,134 g/m2. The carbon fibre plates were made of biaxially oriented NCF, using six layers of 300 g/m2 to result in a 2 mm composite with 51% fibre volume.

Enter the matrix

The low viscosity of the uncured Vitrox resins means that relatively little pressure is needed to fill moulds, so the systems can be used to manufacture sandwich panels.

Sandwich construction makes it possible to reduce the amount of expensive carbon fibre needed to bear bending loads, he said. The resin can therefore help automotive designers to design lower cost lightweight composite parts.

Vangrimde showed this in some demonstrator spoiler parts made at the Frimo and Frunhofer IWKS centre in Hannau, Germany. The parts were used in the Callaway Corvette C7 GT3 race car that competed in the ADAC Race Masters Series.

After a moulded polyurethane core with inserts was created, it was wrapped with hybrid glass and recycled carbon fibre reinforcements. Vitrox resin was then injected into a closed mould containing the preform. The tool was preheated at 95°C to give a slightly longer injection time than at 100°C.

‘We shot 1600 g of resin at 25g/s, and when the part was demoulded, we had spoilers which were ready to race,’ he said. The spoilers are 1.8m wide. In addition, the spoiler contained a Frimo Street Shark surface on its underside. This improved part aerodynamics.

Wet sandwich

Sandwich panels featured prominently in the paper given by Alessandro Colella, product manager at Cannon. He told delegates about his company’s JetPreg process, which can be used to make components such as parcel shelves.

Colella focused on composites that were sandwiches made of a honeycomb cardboard core between two fiberglass mats impregnated with sprayed polyurethane. This process can be used to make load floors, parcel shelves and roofs. The sprayed layer of polyurethane acts as bonding agent, and improves rigidity of the composite structure, he added.

‘The real innovation lies in how the sandwich is impregnated on both sides,’ he said. ‘For big parts or very high productivities, the spray head is mounted on a six-axis robot, and the sandwich is firmly held by a gripper. The head sprays polyurethane on both sides of the sandwich. Then the part is placed in a press and cured.’

The machinery comprises a dosing unit to accurately produce the polyurethane system, and a spray head. ‘The heart of the system is the spray head,’ Colella said. ‘It is a Cannon LS10, and it is L-shaped and hydraulically operated. It can produce a polyurethane spray fan without air assistance.’

Removing compressed air from the equation makes the system much simpler, and uses less energy. Cannon avoided the use of compressed air by using special spray tips mounted on the mix head. ‘These enable a number of spray angles and different spray widths,’ Colella explained.

He said that the L50 spray head is light, and can be installed on relatively small robots. Despite this, the output of the mix head can be varied between high and low volumes.

Colella said that Cannon had been asked to try the system on a customer’s part.

Using a customer mould, Cannon was able to reduce the total spray time needed to impregnate both surfaces and their reinforcement to a total of 16 seconds. ‘This was a significant improvement,’ Colella said. ‘Then we studied how much material had been consumed and how much was deposited on the part.’

More consistency

They used two samples. ‘The reference panel came from the client and was made in series production, and the other Cannon sprayed with its JetPreg system,’ he said. ‘We cut the panels into 12 parts, and analysed the level of polyurethane impregnation on both sides of the samples.’

Samples from the reference panel were, on average, impregnated to a depth of 3.5 mm, and impregnation ranged from 1 mm to 8 mm. ‘The panel produced by JetPreg was impregnated to 3.4mm, and the range of impregnation was between 4.1 and 2.1 mm,’ he said. ‘Additionally, the material was applied in a much more homogeneous manner, and the surface quality of the JetPreg load floor was high as well.’

Cannon has a number of plants available for this composites technology. These range from a semi-manual plant for runs of 600 parts/day, and the next size up is an automated plant for 1,300 parts/day, which needs a single operator to remove the cut part from the frame. The largest machine in the range can handle 2,000 parts/day. This has a big press, and the mix head is mounted on a robot. A shuttling gripper holds the composite sandwich stable while sprayed. The composites can be as large as 2.5m x 1.2m, and the machine requires two operators to handle the sandwich and remove the part from the press.

Keep on producing

Away from reinforced polyurethanes, Joseph Berger, who works in project engineering and sales at Fil, described a machinery system that is used to make engine covers in polyurethane.

The system was based on a double rotary turntable, with 10 stations on each. Fil provided the dry-side equipment, and KraussMaffei supplied the wet side. Berger described a layout where the pouring robot is in the centre of the machinery and supplies both turntables.

‘There are number of six-axis robots spraying release agent and in-mould coating,’ he said. ‘The machinery set up also includes an exhaust system.’

This combination of machinery and materials made it possible to make composite parts in under 1 min cycles. He added that the plant is designed so that mould carriers can be changed without stopping production.

Philippe Michaud, managing director of Allrim, outlined his company’s Allrane BF45 polyurethane toughener for epoxy resin systems. It is designed for use in composites applications in the automotive sector.

He told delegates: ‘We have patented a bio-based toughener, BF45, which is a reactive toughener. It is bio-based and, importantly for automotive applications, it is free of endocrine disruptors and CMRs. It is Reach registered and there are no toxic labels on the product . It is a reactive polyurethane with hydroxyl end groups.’

Allrim recommends using the material at around 60°C, when its viscosity falls to about 100 Pa/s, to mix with epoxies. When the mixture solidifies, it becomes opaque because of phase separation. With vigorous mixing, Michaud said, the polyurethane domains are between 0.4 µm and 0.6 µm in diameter.