Redux is the generic name of a family of phenyl–formaldehyde/polyvinyl–formal adhesives developed by Aero Research Limited (ARL) at Duxford, UK, in the 1940s, subsequently produced by Ciba (ARL). The brand name is now used for a range of epoxy adhesives manufactured by Hexcel. The name is a contraction of REsearch at DUXford.
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Devised at ARL by Dr. Norman de Bruyne and George Newell in 1941 for use in the aircraft industry, the adhesive is used for the bonding of metal-to-metal and metal-to-wood structures. The adhesive system comprises a liquid adhesive and a powder hardener.
The first formulation available was Redux Liquid E/Formvar, comprising an adhesive (Redux Liquid E) and a hardener (Formvar), and after its initial non-aviation related application of bonding clutch plates on Churchill and Cromwell tanks, it was used by de Havilland from 1943 to the early 1960s, on, among other aircraft, the Hornet, the Comet and the derived Nimrod, and the Dove, Heron and Trident. It was also used by Vickers on the Viking and by Chance Vought on the F7U Cutlass.
Typically, Redux would be used to affix stiffening stringers and doublers to wing and fuselage panels, the resulting panel being both stronger and lighter than a riveted structure. In the case of the Hornet it was used to join the aluminium lower-wing skin to the wooden upper wing structure, and in the fabrication of the aluminium/wood main wing spar, both forms of composite construction made possible by the advent of Redux.
After initially supplying de Havilland only, ARL subsequently produced a refined form of Redux Liquid E/Formvar using a new liquid component known as Redux K6, and a finer-grade (smaller particle-size) powder, and this was later made generally available to the wider aircraft industry as Redux 775/Powder 775, so-named because it was sold for aircraft use to specification DTD 775*. Available for general non-aerospace use it was called Redux Liquid K6/Powder C.
Redux 775/Powder 775 was joined in 1954 by the subsequent Redux Film 775/Powder 775 system, used from 1962 by de Havilland (later Hawker Siddeley and subsequently British Aerospace) on the DH.125 and DH.146. Other users included Bristol (on the Britannia), SAAB (on the Lansen & Draken), Fokker (on the F.27), Sud Aviation (on the Alouette II/III), Breguet and Fairchild, the film-form having the advantage of greater gap-filling ability with no loss of strength over Redux 775/Powder 775, allowing for wider tolerances in component-fit, as well as easier handling and use.
Other Redux adhesives available included Redux 318, a fire- and heat-resistant Nomex polyamide paper covered on one side with a tacky film of dry, high-strength adhesive, and Redux 322, a high-strength structural adhesive flexible film, both being used in the British Aircraft Corporation/Sud Aviation Concorde supersonic airliner.[1]
Currently, Redux 420 and Redux 501 are now marketed as Araldite 420 and Araldite 501.
* DTD = Directorate of Technical Development
To use Redux, a thin film of the liquid adhesive is applied to both mating surfaces and then dusted-with or dipped-in the powder hardener in an approximate ratio by weight of 1 part liquid to 2 parts hardener. The coated joints are then allowed to stand for not less than 30 minutes and not more than 72 hours before the components are brought together under elevated pressure and temperature. The curing process is by condensation and a typical figure for Redux 775/Powder 775 is 30 minutes at 145 °C (293 °F) under a pressure of 100 lbf/in2 (690 kPa). This is not critical and variations in curing-time and/or temperature may be used to increase shear and creep strength at temperatures above 60 °C (140 °F). Extending the curing-cycle gives benefits in fatigue strength at some cost in the room-temperature peel strength, the practical limit for aluminium alloys being approx 170 °C (338 °F) for one hour, due to the possibility of affecting the alloy's mechanical properties.
Strength of bonds to materials other than aluminium:
Tensile shear of 0.5 inch (12.7 mm) lap joints at room temperature:
1 = HK31A-H24
2 = ICI Titanium 130