Adhesives in the Automotive Industry

Thanks to innovations in adhesives, “adhesive-bonded cars” are safer and more economical than their forerunners. Adhesives have taken over many of the functions of traditional welding in the automotive industry.
For instance, in 2001, around 10 meters of adhesive bond-line were used for the body of the BMW 7 model; today about 15 times that amount is used. Modern adhesives have enabled automotive engineers to reduce spot welding by about 50%. Today about 9% of total annual adhesive production worldwide is used in vehicle construction.

Today a car contains up to 18kg of adhesives – and for good reason; crash tests have demonstrated that “adhesive-bonded cars “do better than “welded designs. One important factor is that adhesives do not affect the substrates used in assembly, whereas other mechanical fastening techniques including welding, riveting or bolting can affect the rigidity of assembly materials.

However, enhanced safety is not the only benefit of adhesives in vehicle production. When doors are welded, the outside is laboriously reworked to ensure a good finish. Glue-bonded doors, however, do not need reworking, resulting in lower production costs.

Adhesive use is not limited to automobile production but is widely used in other vehicles too. A typical railway carriage, for example, built between 1981 and 1993 used 10kg of adhesive; today some modern carriages weigh in with up to 500kg of adhesive – an increase of 5000%! While modern airliners such as the Airbus and Boeings use adhesives to bond up to 50% of the assembly.

The use of Adhesives in the Transportation and Marine Industries is quite extensive and includes thousands of applications in such areas as:

  • Aircraft & Aerospace ; Automotive Module; Sealing using Liquid Gasket & Sealants, Thread and Bearings Lockers; Electronic Circuit Board Protection using Liquid Potting and Encapsulants; Exterior Vehicle Trim; Interior Vehicle Trim; Powertrain and Under-the-Hood Components; vehicle Assembly; Rail, Marine, etc.

We’ll be discussing the following areas in more detail in our next issues:

  1. Bonding Metal Sheets in Vehicle Bodywork Construction
  2. Safer Cars with Adhesives
  3. Bonding Panes of Glass into car bodywork – Direct Glazing
  4. Aircraft Manufacture
  5. Rail Vehicle Manufacture
  6. Construction of Containers.

Starting today with:

BONDING METAL SHEETS IN VEHICLE BODYWORK CONSTRUCTION

assembly-carsCar bodies largely comprise steel sheets having a thickness of 0.6 to 0.8 mm. The trend in modern car bodies is towards a flatter design with as little wind resistance as possible, low weight and hence ultimately reduced fuel consumption. Flat com­ponents such as the hood and trunk, and also door and roof panels, are affixed to ridge -like struts and are hence stiffened.

The most common method for joining bodywork components, namely spot welding, cannot be used on the flat visible sides because this would result in unsightly points and these would require time-consuming work (filling) to repair. The use of adhesives to solve this problem was introduced about 40 years ago – adhesives are used to join the components together and the number of welding points was reduced to a small number at the edges.

The bonding process had to be introduced into the relevant production line sequence. The construction of car bodies puts extremely high demands on structural bonding technology:

Special surface pretreatment of the metal sheets, which is essential for creating high-strength bonds having high stability, is not possible on a mass production line. In addition, the substrates are also still coated with corrosion protection oils during the production process. These can only be removed before the final process step.

Key requirements of adhesives for use in vehicle bodywork construction are as follows:

  • Ability to form structural bonds with defined properties over the lifetime of the component under operating loads;
  • Stability at 170 to 230°C for ca. 30 minutes during the painting/lacquering process;
  • Resistance to running and washing away in the non-cured state;
  • Ability to be processed automatically;
  • Ability to penetrate for spot-welds as crack stoppers.

Only hot curing adhesives meet the above mentioned requirements. The adhesives specially developed for these applications (curing at 180 to 230°C) are formulated in such a way that in the non-cured state up to 20 weight percent of oil can be taken up via dissolution. However, in practice when using the adhesive, so much oil is forced away by the adhesive that immediate initial bonding of the adhesive is usually guaranteed. The remain­ing oil is taken up by the adhesive and forms part of the adhesive film.

The high temperatures accelerate the dissolution of the oil by the adhesive. The oil is essentially uniformly distributed in the cured adhesive film and does not diffuse back to the surface.

Crash tests make clear that the alternating fatigue strength and the energy absorption, even after aging, can be considerably improved by using adhesives for some of the joints, compared to wholly spot-welded constructions.

The current state-of-the-art involves a combination of bonding and spot welding, so-called spot-weld bonding, and this is used on a large scale for vehicle body­work construction.

The following adhesives are used for bonding sheet metal in vehicle bodywork construction: Plastisol adhesives, essentially filler-containing pastes comprising powder-form PVC, plasticizers and adhesion promoters.  On the down side, PVC plastisols give rise to environmental problems (PVC issue) when recycling the bonded components, and consequently have become increasingly replaced by alternative adhesives, such as epoxy resins.

For special applications, single component hot curing epoxy resin adhesives are used. 

Example applications are: stiffening and flange seams on hoods and trunks, doors and roof structures.

Bibliography: The Wolf Group Newsletter &

http://adhesivesmag.com/articles/assembly