Reinforcing polymers with strong/stiff fibers is nothing new. Such materials have been used pretty much since aircraft were first created. In those pioneering days, wings were reinforced with woven cotton, or silk fabric skins, impregnated with nitrocellulose ‘dope’ to seal against the wind, and laminated wood was reinforced with fabric bonded with adhesive. Although there have been a myriad of developments along the way from those early days to where we are now in the 21st century, the principles of reinforcement are much the same.
Fibers add strength and stiffness to an otherwise viscoelastic polymer that, without reinforcement, lacks the mechanical resilience needed to construct modern day aircraft. In general, the longer and more perfectly aligned the fibers, the more efficient they are at reinforcing the material. Layers of such reinforced material are laid down at prescribed angles on top of one another in the desired aero-component shape to build thickness and carry structural load. Recent developments include:
the invention and continuous improvement of carbon fibers formed from the controlled pyrolysis of polyacrylonitrile polymer fibers (a type of modified polyethylene fiber) or from coal tar pitch and
impregnating polymers, which the fibers reinforce.
Some refer to these polymers as merely the adhesive which holds and bonds the fibers in place, but in reality the polymer provides much more than that. More accurately, the polymer is referred to as the ‘matrix’. Fibers and matrix work together in synergy providing a ‘composite’ material with characteristic properties benefiting from the contribution of both elements. Matrix polymers have also received much attention in laboratories around the world over the intervening years and primarily two camps have developed that exploit chemistry in fundamentally different ways.
Thermosetting Polymers – Sensitive to Temperature Until Fully Cured
The first and, so far, most commercially used falls within the realm of thermosetting resins, such as epoxy. These are polymers that are almost fully cured, but not quite. Thermosetting polymers are soft and mobile until ‘set’ (cured) with a cross-linking reaction initiated by heat.
The low viscosity of these polymers allows the polymers to flow and impregnate between the reinforcing fibers. In some cases the polymer is very fluid and can be applied by a brush, or roller, or forced under moderate pressure into the weave of reinforcing fibers, displacing the entrained air and in so doing, filling the spaces between fibers with the watery, or honey-like, substance.
In other cases the thermosetting polymer is provided by the materials supplier already applied onto the reinforcing fabric, or on a tape of aligned fibers prepared as a ‘prepreg’. In this case the polymer has been partially cured or β-staged to increase its viscosity and aid prepreg stability. Thermosetting materials are sensitive to temperature and must be stored in a refrigerator to preserve their shelf life, otherwise the curing reaction may be prematurely initiated, rendering the material useless for further processing into parts.
Thermoplastics – Indefinite Shelf Life Without Refrigeration
The second camp is that of thermoplastic polymers. In the case of these materials, the chemistry has already been completed by the materials supplier and the long chain molecules are ready to provide maximum performance from the get-go. No further chemical reactions are necessary to achieve the full mechanical properties of the polymer. Consolidation of composite parts and bonding together of layers is achieved by simply heating, melting and cooling the material under a degree of contact pressure to achieve molecular entanglement and crystallinity. The shelf life of these materials under normal conditions is indefinite without the need for refrigeration.
Suppliers provide composite materials in prepreg forms based on woven fabrics or unidirectional tapes just as with thermosets, except that the materials are stiffer and more difficult to handle, lacking the tack and drape qualities of thermosets. It is largely for this reason (and considering cost) that thermoset based composites won the battle between these competing technologies in the 1980’s and 1990’s when key materials selection decisions were made for the new generation of commercial composite aircraft.
Thermosets were considered easier to process and more versatile in terms of processing options, than were thermoplastics. The initial interest in thermoplastic composites faded over this period and most of the suppliers who developed these materials divested interest in such materials, certainly for aerospace use.
The use of composites, both thermosetting and thermoplastic, in aircraft applications has grown steadily over the past 30-plus years. Just as materials have continued to advance, processing technologies have evolved as well.
Using aerostructures as an example, in the early days of composite technology much of the manufacturing was done by hand. As technology and input materials continue to develop, there is a gradual reduction of labor to the point of automation. In recent years, automated processing methods surpassed more labor intensive processing technologies for the first time with interiors and aero structures representing the largest growth areas.
Potential to Increase Production Build Rate
Advancing automated processing technologies (typically better suited for thermoplastics) is largely driven by the need to increase the production build rate of middle market aircraft, such as Boeing’s 737 and Airbus’ A320 type aircraft, to around 60 per month.
At these rates the prospects of utilizing the fast cycle time associated with a simple heat/cool process without the concerns around completing chemical reactions make thermoplastics highly attractive. There have also been developments in the way these materials are handled and processed, which aid the manufacture of complex parts.
Mechanically, both classes of polymer are substantially the same as much of the reinforcing effort comes from the fibers, which are consistent between these types of materials. The main differences relate to how the materials respond to impact loads. Thermoplastics are generally tougher than thermosets, although there is some ‘blurring of the edges’ as thermosets can be toughened and not all thermoplastics suited to aero-structures are themselves tough. Developments include the fine-tuning of the fiber/matrix interface to achieve the maximum benefit of the reinforcement offered by the fibers.