Redux (adhesive)

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".

History

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.

* DTD = "Directorate of Technical Development"

Usage

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.

Performance (typical) Redux 775

*Lap shear strength at ambient temperature = 34.0 MPa (4,930 lbf/in²)
*Young's Modulus (E) = 3.35 GPa (486,000 lbf/in²)
*Shear modulus = 1.20 GPa (174,000 lbf/in²)

Strength of bonds to materials other than aluminium:

Tensile shear of 0.5 in (12.7 mm) lap joints at room temperature:

*Bright mild steel of thickness 0.0625 in (1.6 mm) - mean failing stress = 4,980 lbf/in² (33.7 MPa, 3.50 kgf/mm²)

*Stainless steel of thickness 0.048 in (1.2 mm) - mean failing stress = 5,600 lbf/in² (38.6 MPa, 3.94 kgf/mm²)

*Magnesium alloy¹ of thickness 0.063 in (1.6 mm) - mean failing stress = 3,210 lbf/in² (22.1 MPa, 2.26 kgf/mm²)

*Commercially-pure titanium² of thickness 0.050 in (1.3 mm) - mean failing stress = 4,070 lbf/in² (28.1 MPa, 2.86 kgf/mm²)

¹ = HK31A-H24

² = ICI Titanium 130

References

*"Project 3 - Environmental Durability of Adhesive Bonds - Report No. 9 - Forensic Studies of Adhesive Joints - Part 2 Bonded Aircraft Structure" by A. Beevers - September 1995 - [http://www.adhesivestoolkit.com/Docs/test/Procedures%20and%20Recommended%20Practice_files/P3r9pt2.pdf]
* [http://www.admc.esrtechnology.com/Background/Manufacturing/Adhesive%20Bond%20Information%20John/COMET_P.pdf Usage on the de Havilland Comet]
* [http://uksaa-www.me.ic.ac.uk/awards.html The de Bruyne Medal]
* [http://www.aeroplanemonthly.com/aeroarchive/Aeronautical_Engineering_news_70081.html "Bonding with Redux" - (reprinted from "The Aeroplane" - Sept 1946)]
* [http://www.hexcel.com/News/Press+Releases/Archives/Redux+Anniversary.htm Hexcel "Redux film adhesive – 50th anniversary" Press Release]
* [http://www.hexcel.com/NR/rdonlyres/70EAD6FC-E47D-4600-BB6D-DFA0CB6C9B89/0/Redux775_eu.pdf Hexcel "Redux 775 Product Data"]