A range of structural adhesives that can be used under the most extreme conditions could lead to not only safer, but lighter and more environmentally friendly cars. Paula Wallace reports.
FOR decades car makers have relied on adhesives to reinforce welds in car bodies. While conventional adhesives can withstand the mechanical stresses of everyday driving they have one major drawback: they can not withstand a serious crash because of their brittleness.
When examined under extreme conditions through crash testing, two pieces depending on conventional adhesives will inevitably split like a zipper – leaving the parts solely reliant on the nuts and bolts of the welds to literally hold cars together. And while this means heavier, more costly, higher emitting vehicles, car makers don’t use these adhesives in critical crash-safety components.
The problem car makers have faced is that until now, no adhesive has been strong or dependable enough to withstand extreme crash scenarios without rupturing. But innovation from Henkel’s international Adhesives Technologies platform has led to the development of a range of structural adhesives so strong they can be used under the most extreme conditions, do not rupture in a crash and perform equally well in all other aspects.
Pulling together a team of specialists from around the world, Henkel systematically investigated the relationship between the chemical composition of the adhesives, their internal structure (morphology) and the resultant mechanical properties.
The result is not only a safer car, but a lighter one, as when specialised adhesives are used as reinforcement, thinner metal panels can be used which make the car lighter. The lighter the car, the less fuel needed to power it and ultimately, the less damaging carbon dioxide emissions each car will emit.
“Several automotive OEMs are already using structural adhesives optimised for crash resistance,” said Henkel’s general manager automotive division, Greg Tricker-Reissig.
“In Australia, Henkel supplies many and varied new generation, structural adhesives which provide significant benefits over traditional adhesives.”
Optimised structural adhesives are used in such areas as roof-to-bodyside, wheel arch construction, and floor pan to name a few.
The benefits of such applications include:
The removal of more spot welds – resulting in significant cost savings – because there is greater engineering confidence in adhesive performance;
The ability to automate;
Better crash performance, contributing to higher crash ratings, from the New Car Assessment Program (NCAP), without the need to add steel mass to the car body; and
Aluminium use, due to the insulating properties and higher flexibility required to prevent galvanic corrosion and accommodate differential expansion of mixed metals respectively.
Evolution of adhesive structures
Structural adhesives usually consist of a brittle adhesive matrix with spherical islands of soft rubber-like polymers. These islands only begin to form when the adhesive is curing, a process in which the originally uniform mix of the two constituents separates almost completely.
The experts use the term ‘conventional rubber-toughened’ for this kind of morphology. Crash resistant adhesives, however, call for a much more complex structure.
The key to crash resistance is the morphology of the cured adhesive. Henkel has developed a method of distributing soft particles, only a few hundred nanometres in size, uniformly throughout the matrix before curing.
Dr. Rainer Schonfeld, in charge of Research/Polymer Chemistry at Henkel explains that this novel technology means that experts can precisely control the adhesive’s morphology and internal interfaces.
“Experts can tell exactly how the interfaces between the hard and soft constituents will affect the mechanical properties of the adhesive,” said Dr. Schonfeld.
“With this technology, Henkel specialists have created a new generation of crash resistant structural adhesives, with especially high thermal stability and a crash resistance that is more than ten times better than that of reinforcing adhesives.
“This has been demonstrated in various ways, including drop tower experiments in which the experts test the mechanical strength of metal-to-metal bonds,” he said.
In the drop tower test, a roughly 500 kilogram weight is dropped from a height of about two metres onto the test specimen. Henkel experts use this method to investigate the crash behaviour of test specimens produced by bonding or spot welding two sheet-steel panels with a top-hat profile.
In the test, the bonded test specimens crumple accordion style over a certain distance, starting at the top. The impact energy is absorbed well, and the bond line holds.
By comparison, the exclusively spot welded test bodies show greater deformation and energy absorption is less effective.
Uptake in automotive industry
Car manufacturers can now obtain adhesives geared precisely to their requirements and their production processes. The first crash resistant Terokal was brought to market in 2004 in the USA and was used by Ford in its F-150 pickup. The F-150 was given the highest crash resistance rating in its class.
Despite these advances, vehicle manufacturers are not yet exploiting the full potential of crash-resistant adhesives.
While welding has been widely accepted for decades and is a highly developed technique, engineers have been slow to adopt adhesive bonding technologies.
Yet bonded joints are in fact capable of almost entirely replacing the classical spot welds – just a few spot welds would be sufficient to fix the bonded body components until the adhesive has cured. In addition to enhanced safety, this also generates considerable potential for savings.
“Generally speaking, it is true that the Australian automotive and manufacturing markets have been shrinking. They are also under enormous cost-competitive pressure, in part as a consequence of reduced volumes,” said Greg Tricker-Reissig.
“Most automotive OEMs actively pursue total cost reduction initiatives and, because of this, rely heavily on their supply base.
“The full benefits, though, of these technologies can only be realised if they are considered early enough in the design cycle, so it can take many years after concept introduction for them to be implemented into production.
“As a facility’s investment for employing structural adhesives is not particularly high, the benefits can be enjoyed as much for plants producing low volumes as those in Asia, Europe and the US producing greater volumes,” said Tricker-Reissig.
Today, it is already standard practice for engineers to design and develop new car bodies on computers and simulate their mechanical properties. However, the thinness of adhesive bond lines still represents a major challenge for conventional numerical modelling techniques.
By 2010 however, experts aim to further develop the finite element method and other techniques for these applications in order to be able to provide the desired computational support for the reliability of bonding.
According to Tricker-Reissig, traditional structural adhesives for metal bonding already differ significantly in price due to many factors, including the varying specifications against which they are supplied.
“The uptake of any new technology, however, is generally dependent upon the net perceived value associated with the change,” he said.
“Customers adopting such technologies in their operations are, rather than replacing existing adhesives, usually replacing or augmenting mechanical fasteners. In this case, the value of the solution must consider the impact on total manufactured cost.”
Aside from in automotive applications, Henkel’s crash resistant structural adhesive technology lends itself to the joining of a variety dissimilar materials, such as plastics, lightweight metals and conventional materials. It provides additional benefits over mechanical fastening such as joint fatigue resistance, and in-flange corrosion resistance (including galvanic corrosion).
“Any application where lightweight, durability and strength are properties required in combination will benefit from this technology,” said Tricker-Reissig.
“In addition to automotive, industries that could potentially benefit might include aviation/aerospace, marine, trucks/buses, cycles, and general construction.”
Henkel, www.henkel.com.au.
.jpg)
