ASC: Weighting game

Innovative adhesives will help automotive manufacturers take full advantage of the emerging generation of lightweight materials

Adhesives are likely to play a crucial role in helping cars to comply with future environmental legislation.

Tough carbon emission targets, particularly in Europe and North America, are giving car makers a headache: in North America, Corporate Average Fuel Economy (Cafe) regulations insist that each car fleet has an average fuel consumption of 54.5 miles/gallon by 2025; and, in Europe, the relevant legislation stipulates that cars emit no more than 130g of carbon dioxide (CO2)/km by next year, and no more than 95g/km by 2021.

Reducing the rolling resistance of tyres will help and so will boosting engine efficiency. However, the number one target is weight 
reduction. An industry rule of thumb says that every 100kg reduction in car weight cuts carbon emissions by around 7g and improves fuel economy by about 0.25 litres/km.

So how do adhesives fit in? Manufacturers and suppliers believe that structural adhesives can help across the spectrum. At the “nuts and bolts” end, they will help to replace rivets and other fasteners, which will have a cumulative effect on weight reduction. However, they may also be the critical “enabling technology” that allows the industry to embrace the emerging generation of new weight-saving materials: aluminium, composites and carbon fibre.

Innovative adhesives are needed to bond different substrates Copyright: Dow Automative Systems

Different materials
Danielle Hunter, market development manager for adhesives at Bayer MaterialScience, says the supply industry has been studying the “unmet needs” of the automotive sector.

“The use of structural adhesives will increase in future as these new substrates are introduced. This may require new products to be developed – in both raw materials and finished adhesives,” she says.

The most important priority, she says, is to improve the adhesive bonding between dissimilar materials – whether it is steel and carbon fibre, or steel and aluminium.

“If you have two coefficients of thermal expansion in two materials, the different expansion and contraction rates need a bond to be flexible, but also have high strength,” she says. And this will require a technological balancing act, as adhesives are not usually both strong and flexible.

In these multi-materials applications, adhesives are likely to be used in conjunction with traditional fasteners such as rivets – though these will be used in much lesser quantities than they are now.

Another factor is the electro-coating (E-coating) process, which applies paint to the car body then bakes it in an oven. An adhesive such as an epoxy or hybrid epoxy will happily withstand the 170°C, 20-minute journey through the oven. However, it does this as it sits between two steel surfaces – and will not necessarily be able to cope with dissimilar substrates.

Of course, adhesives can be applied once the car body has come out of the oven, but the industry would like to carry out as many processes as possible before the E-coat process for more efficient production.

Hunter says that Bayer MaterialScience is working on a number of projects, but cannot reveal details as they are all in the early stages. She points to one aspect where adhesives will give a distinct advantage for joining dissimilar materials. “When you bond aluminium and steel together, that bond surface area could be a potential site for corrosion, but adhesives can help to alleviate this,” she says.

Bayer MaterialScience is concentrating its attention on aluminium, arguing that this is the “next phase” of substrate change – ahead of likely later introductions of composites and carbon fibre. She cites the Ford F-150 pick-up truck as an example of a mass market vehicle that has made the move to aluminium. “If you look at a traditional car company like Ford, and see a typical car moving to an aluminium structural body, that’s a tell-tale sign of where things are going,” she says.

Many other carmakers are embracing the use of aluminium. Japanese manufacturer Honda has developed an innovative new way of joining steel to aluminium. The technique, which combines adhesives with a new physical structure, is already being used in the Acura RLX model in the US to make a door – which has an aluminium outer panel and a steel inner panel.

First, the panels are designed using Honda’s “3D Lock Scheme” (3DLS) structure, which layers and hems the two metals together. Then, an adhesive with low elastic modulus (which can easily “stretch”) is used to bond them. The shape of the combined panel ensures that the gap between the metals is completely filled with adhesive. The technique eliminates a spot welding process.

Honda has not revealed the nature of the adhesive used.

In a pickle
Aluminium may be far lighter than steel, but it has an Achilles heel: it has low corrosion resistance, so its surface must be treated (or “pickled”) in order to prepare it for any kind of coating, including adhesives.

Researchers at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) in Germany have developed a better, safer way to do this, by incorporating the harmful pickling chemicals into a tape.

Aluminium parts are traditionally pickled in a bath. This has health and safety implications, due to the large volume of corrosive chemicals used. Portions of the surface can be treated with pastes and sprays, but this does not completely overcome safety and handling issues.

“There’s a demand for local treatment of aluminium, so why not do it in the form of a tape?” says Malte Burchardt, of the adhesives and interface research department at IFAM.

The pickling tape has taken three years to develop and draws on Fraunhofer’s expertise in adhesive formulation and bonding. “The motivation was that when we bond for customers we must pre-treat our surfaces properly,” he says.

The tape has been tested by comparing the performance of treated and untreated aluminium surfaces. In one case, the initial strength of a polyurethane (PU) adhesive bond was greater with the treated surface and it retained its bond strength after exposure to acidic salt spray. The untreated sample failed the test.

The tape itself incorporates the pickling agent and a special adhesive to make it stick to the aluminium surface. The formulation is water-based (just like traditional pickling), rather than organic solvent-based.

When the tape is removed from the surface, no trace of its contents must remain there, according to Malte Kleemeier, of Fraunhofer IFAM’s adhesives and polymer chemistry department, who was heavily involved in the tape’s development.

“The surface must be completely clean,” he says. “If you leave any residue behind, this is where the coating or adhesive would fail.” This includes removing all the pickling agent, as otherwise the surface would continue pickling.

The most likely beneficiary will be the automotive industry, for cars with relatively small production runs: the tape would be applied for a few minutes, before adhesive is applied to those small areas of the part prior to bonding. It may also find use in car workshops, as a way of repairing or repainting portions of an aluminium component.

Adhesive raise cab comfort levels Copyright: John Deere

Carbon future
Many suppliers are already helping manufacturers to take advantage of aluminium’s weight-saving attributes. Dow Automotive Systems is working with BMW on its 7 Series model, which features an aluminium roof bonded to a steel body. Dow’s Betaforce structural adhesive technology is used to bond these two parts of the car together. No other joining processes are used in conjunction with it, says Dow.

“OEMs are looking to use different substrates where they can have the most impact on weight reduction,” says Tonja Sutton, global strategic market manager at Dow Automotive.

Reducing the weight of a car roof is an effective way of lowering a car’s centre of gravity, which makes it more stable. Unsurprisingly, it’s not just BMW that is looking at aluminium roofs. Dow has worked with other suppliers on similar concepts.

The design of the roof assembly is important, says Sutton. Different customers have different ways of designing the roof, so that it can accommodate the adhesive. This means that Dow must work closely with each customer to get that right. Many of these projects are for 2015-2016 models, she says.

“We’ve worked with a number of OEMs, who are looking to bond the roof in the body shop and then put it through the E-coat process. This is a cost-effective way to do it,” she says.

It means the adhesive must withstand both the high temperature of the E-coat oven and the flexing at the steel-aluminium interface. “The adhesive must manage the stresses that are generated,” says Sutton. “This will take some innovation.”

The two companies have gone beyond aluminium in BMW’s futuristic i3 city car, which makes extensive use of carbon fibre. Dow has developed a special grade of its Betaforce two-part PU structural adhesive to stick the carbon fibre elements of the passenger compartment together. In this case, it is bonding only carbon fibre parts and is not used to stick dissimilar materials together.

Dow is not revealing specific details of the adhesive, but Sutton says it was geared to fit BMW’s production needs.

“It’s a unique BMW process, and it needed a tailored product. The product we developed has improved cycle time and a more flexible open time,” says Sutton. Open time is the period between applying the adhesive and pressing the parts together.

“Bringing carbon fibre structural parts into the body shop is also something that we see coming,” she says. “As this happens, you’ll need composite materials that can go through thermal exposure – and an adhesive that can bond it.”

At one time, the idea of glueing parts of a car together – rather than riveting or welding them – would have seemed like madness. However, the pressing need for weight reduction, and the likelihood that this will be achieved by combining the benefits of several different innovative materials, is opening up real opportunity for the latest generation of adhesives. And, of course, for the future formulations that are still being investigated.

Agriculture makes greater use of structural adhesives
John Deere builds steel monsters for the agricultural and construction industries. It might seem surprising that a manufacturer of tractors, combine harvesters and cranes would have much need for adhesives. However, Nate Tortorella of the materials engineering department in John Deere’s product engineering centre in Waterloo, Indiana, says that adhesives have been on the rise in the industry for some time.

A good example is in the cabs themselves – which over time have become more sophisticated, with a higher degree of comfort.

“Consumers would complain about windshields squeaking or letting in dust,” he says. “Their feedback was driving a lot of our decisions.”

Now, cabs are sealed to maintain positive pressure. Adhesives and sealants are increasingly replacing traditional methods such as welding and mechanical fasteners such as nuts and bolts.

The company uses just about every kind of adhesive across its product range, including urethanes, methacrylates and epoxies. Tortorella says that acrylics are usually preferred to epoxies: other than their lower price, they have faster fixing times and can bond oily surfaces and different types of material.

He also cites the drive train as an area where adhesives and sealants are making headway, on components such as axle housings and differential casings.

Existing adhesives are meeting the company’s needs, he says, although there is still a wish list of enhanced products such as more oil-tolerant grades.

“We clean our mating surfaces by spraying them with iso-propyl alcohol. Then we apply the adhesive. Our manufacturing engineers would like to eliminate the cleaning step, so that there’s no need for surface preparation,” he says.

Tortorella says that John Deere’s network of dealers is well equipped to carry out repairs to its products. However, as adhesives start to be used in a more structural way, this may become more challenging for them.

“The dealers may not have the same experience with adhesives as they do with welding, or nuts and bolts – so we’ll need to educate them,” he says.

Automotive boost from developing regions
Emerging regions will drive the growth of adhesives in automotive, says Grand View Research, which recently published an overview of the global adhesives industry.

Anshuman Bahaguna, who wrote the report, estimates that the automotive sector accounts for 5.9% of total revenue and around 6.6% of the volume. This equates to around 384,000 tonnes and $1.4bn, respectively.

Volume will grow by 4.5% per year, to give a total of 522,000 tonnes in 2020. In terms of revenue, this is expected to expand by 6% annually, to reach a total of $2.1bn (a growth of 50% on today’s figure).

Bahaguna confirms that weight saving is a critical factor behind the increasing use of adhesives in automotive, driven mainly by US regulations on fuel efficiency.

At the same time, he says that emerging markets – especially the “BRICs” nations of Brazil, Russia, India and China – will take the lion’s share of the growth in automotive adhesives, due to their growing importance in automotive manufacturing. Other emerging nations to watch are Malaysia and Indonesia.

“Western Europe and North America are mature markets, so growth here will be more sluggish,” he says.