A Look At Glue For Aerospace Applications.
A tale of technology!
Predating the first powered flight, nearly all aircraft were primarily constructed from plywood panels which were glued & mechanically fixed using screws. The glue was a rather simple product, based mainly on raw materials derived from the food sector. They were not easy to use, they needed detailed surface preparation and in the winter months, were difficult to apply. The first major development in adhesives for the aircraft sector, came in the late 1930’s.
In the UK, Ciba Geigy started to produce Redux adhesive. These were a early form of the now common two part adhesives, based on phenol formaldehyde. The Redux adhesives were very easy to use and found their first application in bonding wing and fuselage stiffening stringers and later, both wing & fuselage panels. The resulting panel being both stronger and lighter than a riveted structure. Redux adhesive found particular use to join the aluminium wing skins, to the wooden substructures of the aircraft fuselage.
Another of the adhesive firsts was Tego film. Developed in Germany around 1930 it was initially used as a film glue for the manufacture waterproof plywood sheets. In its day, Tego film was a revolutionary product. It was composed of a very thin paper sheet impregnated with a modified phenolic resin. When the Tego film was placed between wooden veneers, heated and compressed, a strong and waterproof laminated plywood was formed.
The manufacture of plywood at this time used other adhesives, the earliest ones being casein resin & the earliest forms of PVA glues. These early adhesives were generally applied as water based emulsions and were not without issue, as it was soon discovered that they caused warping of thin veneers and this made it difficult to achieve a solid laminate without risk of voids. As the Tego film had the advantage of being used dry, it gave exceptionally high bond strengths, without risk of hidden weaknesses. This became an important factor in time, when it was used in aircraft construction, gluing the wood main wing spar.
In the late 50’s & early 60’s, and due primarily to developments in the US Space Programme, the use of adhesives in aerospace applications became better understood and widely accepted. These early epoxy, and silicone adhesives were used in several applications, mainly to bond glass observation panels and to glue insulation to the walls of the space capsules. It was not until the early 1970’s, that speciality adhesives became more common within the aerospace sector.
Moving to the 21st century, due primarily to many aircraft being constructed from carbon fibre, the use of adhesives is now more common than the use of mechanical fixings. No better place to see this is the design of military aircraft.
The Eurofighter Typhoon is a case in point. In order to deliver maximum stealth operation and protection from radar-based systems, this aircraft was designed and built around composite materials, all of which are bonded. Approximately 70% of the aircraft is manufactured from bonded carbon fibre composite, 12% from bonded glass reinforced plastic. Only 15% of the aircraft’s surface is metal! With the vast reduction in the amount of metal used in the airframe, adhesives are used extensively.
What is the future for adhesives within the aerospace sector? Well its certainly very bright and some would say that the future is already here. As the development of composite materials moves apace, the development of adhesives to bond these composites, most keep apace.
Of course, some challenges do remain. Specifically, methodologies to improve surface treatments on carbon fibre composites and most certainly, shorter gluing times. However, for aerospace adhesives, the only way is up.