Fast fastening with adhesives
Tom Shelley reports on bonding technologies that take only seconds to take effect.
While spot welding has for long been seen as the technology of choice for joining car components, particularly parts making up bodies in white, rapid bonding using adhesives is increasing its penetration of automotive as well as other markets, not least because it allows the very rapid bonding of plastic and composite parts, which cannot be spot welded.
We first heard just how fast bonding has become when we happened to encounter Julian Bond, R&D engineering manager for Parafix at the end of last month's Manufacturing Summit and asked if it was possible to bond parts together as fast or faster than using spot welding. He responded by saying they could bond parts, "In seconds" using adhesive tapes.
These provide vastly stronger bonds that the kinds of sticky tapes used round the average home or office, so they can have failure modes in which substrates fail before bonded joints. As regards time, Steve Barwick from Parafix described a recent job developing the technology to bond charcoal cloth onto a microporous PTFE vent where cloth die-cuts are spread out and bonded to the vents robotically, requiring less than a second for the whole operation, including bonding.
The company works with a range of suppliers of adhesive systems and machinery, including 3M, Tesa, Nitto Denko, Scapa, Advance Adhesive Tapes, Rogers, Laird Technologies, Accuplace and Adhesives Research. A lot of the leading edge development in structural bonding films, he said, is being undertaken by 3M, which he said, is "Developing all manner of technologies", including tapes for bonding composites.
These generally give a tack strength but have to be heat cured to achieve full strength. Pick and place robotic handling copes with the problems of handling thin, flexible adhesive tape components. In automotive applications, final cure can often be achieved in the paint curing oven. Barwick says that while designers of cars and mobile phones tend to be well aware of the advantages to be had from adhesive tape bonding: reduced assembly time, stronger bonds and the absence of stress concentrations or bosses to accept screws, smaller companies tend to be less aware.
There are even more adhesive bonding technologies coming down the line, especially for bonding composites for future vehicles and aircraft, according to Paul Burling, business development manager, and Dr Farshad Salamat-Zadeh, senior project leader, at TWI, who explained that success with bonding composites depends on the resin used in the composite, its surface treatment and compatibility with the adhesive.
There are many options that allow bonding to be undertaken more rapidly. For example thixotropic cyanoacrylates – 'Super glues', can provide an instantaneous bond but there should in addition be a structural adhesive to provide service strength. However, using something quick acting to hold a construction together allows it to be handled, without having to have it supported for a long period using expensive jigs and fixtures.
This is an accepted method of bonding panels to door frames for vehicles. The subsequent cure of the structural adhesive can be undertaken either at room temperature, or under the influence of elevated temperature, radio frequency radiation, infra red light or generally applied heat or ultra violet light, if a way can be found to shine it on the adhesive in the bond. One area of research at TWI is to apply microwaves to heat adhesives that possess hydroxyl or other polar groups.
Since high power microwaves couple with human tissue, particularly in the brain, it is necessary to apply the microwaves in a closed enclosure, which could become quite expensive to build if that which is to be bonded together is car sized. Curing in the case of cyanoacrylates is set off by moisture, which is why they are so good at bonding human skin. Threadlockers, on the other hand, cure by reacting with metal in the absence of oxygen. The most common structural adhesives are epoxies and acrylics.
Since it is not possible to get ultra violet light into a metal to metal or opaque composite to opaque composite joint, heat is the most favoured option. Curing rate, along with most other chemical reactions, doubles in speed with every 10°C rise in temperature, so a 24 hour cure at room temperature can be reduced to about 10 min at 95°C. Alternatively, hot melts, are applied molten and bond when they solidify. TWI has in the past looked at electron beam curing, which Dr Salamat Zadeh noted can cure in 3s to 4s, but, as he also noted, "not everyone has an electron beam facility".
As regards compatibility with the substrate, it is easy if the substrate resin is epoxy based and the adhesive is too, but there can be problems with a vinyl ester resin and an epoxy adhesive. One option is to co-cure, bonding using the resin basis of the composite. An 'Achilles heel' with adhesive bonding in critical applications is that it can be extremely difficult to detect poor bonding. Ultrasonic examination, for example, will not reveal a 'kissing' bond where two surfaces are in intimate contact, but not stuck together.
This is an argument for 'belt and braces' approaches, such as rivets to back up adhesive bonding, as in the chassis of the Lotus Elise, but this adds both cost and weight. The only way out is to develop a process that always works, and always follow the recipe exactly, which is another argument for robotic bonding, to ensure reproducible conditions and consistency. Dr Salamat Zadeh noted that end users often demand a 15 or 20 year service guarantee of service, but said that it is "Difficult" to ensure that degree of confidence.
Accelerated ageing tests are often relied on, as they are in electronics, but there is always a degree of uncertainty in relating performance at short times at elevated temperature and with other adverse stresses, with performance at longer times. Dr Salamat Zadeh observed that the most common cause of adhesive failure was, "Contamination", which can also be said to be true of metallurgical failures too. Cleanliness inside steel depends on the supplier but cleanliness in bonding depends on management in the factory where it is undertaken, which can be controlled.
Another problem is disassembly. If the construction is to be permanent, it is not too much a problem until the product comes to be recycled, when it is probably going to be crushed and/or fragmentised. Spot welding is not amenable to disassembly either but is still very widely used. The usual approach to disbanding joints is to apply more heat. There has been quite a lot of research on suitable strategies to make disassembly easier, without compromising assembly, but Dr Salamat-Zadeh observed that a proposed EU project on fast assembly and fast disassembly was never funded.
Jennifer Harrison's Namtec report, "Active disassembly" is still available for free download from the Materials KTN website but political interest seems to have waned and commercial interest has waned also. Nonetheless, there is no doubt that the use of adhesive bonding in the manufacture of motor vehicles and almost everything else is still advancing because of the inherent advantages in being able to join what can be otherwise incompatible materials in a way that reduces cost, transmission of vibration, weight, ingress of moisture and transmission of corrosion currents in the case of different metals.
Our TWI informants observed that there is typically 18kg of adhesives in the average car, and with the pressure to further reduce weight, especially for electric vehicles, rapid adhesive bonding of composites is clearly the way forward. Composite panels, all bonded, feature extensively in the design of both the Nissan Leaf and Tesla Motors seven seat Model S cars.
• It is now possible to bond parts in less than 1s
• If the assembly is to be able to withstand heavy structural loads, a fast acting bond can be used to tack the construction together in order to allow a structural adhesive to be cured
• Cure times of structural adhesives are now down to 10 mins in some cases, aided by heat • Research is underway to investigate microwave curing to cut times still further
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