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The history of Redux® and the Redux bonding process
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Int. J. Adhesion and Adhesives 17 (1997) 287--301
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The history of Redux® and the Redux bonding process John A. Bishopp Hexcel Composites, Duxford, Cambridge CB2 4019, UK (Accepted 3 May 1997)
As I drove back the heavens opened wide to reveal a chorus and trumpets making Handel-like noises (de Bruyne, N.A. 'My Life', Midsummer Books, 1996). These were Norman de Bruyne's feelings when he returned from the de Havilland works in Hatfield, on 9th April 1936, with a cheque for £1000 and a consultancy to research into reinforced phenol formaldehyde resins for use in propeller manufacture. This decision by de Havilland turned out to be one of considerable importance as it led directly to the acceptance and use of structural adhesive bonding in many, if not all, aircraft from the mid-1940s to the present day. © 1998 Elsevier Science Ltd. All rights reserved (Keywords: A. phenolic; A. toughened adhesives; B. aluminium and alloys; adhesion/adhesives history)
INTRODUCTION The history of Redux and the Redux bonding process is inextricably linked with the history of N o r m a n de Bruyne (Figure 1). Therefore it is appropriate, initially, to concentrate on the man himself, his early years and on the research work which led to the formation of Aero Research Limited and to the discovery of a structural adhesive and bonding process which are still, very much, in use today.
In 1932 he founded The Cambridge Aeroplane Construction Company, at the original Marshall's Aerodrome, Cambridge, in a shed rented from the late Sir Arthur Marshall. Here he designed and built a four-seater monoplane--the Snark
(Figure 2). However, things did not proceed smoothly. The original application for an airworthiness approval was thrown out by the Royal Aircraft Establishment (RAE)
N O R M A N A D R I A N DE B R U Y N E (1904-1997) Norman Adrian de Bruyne MA, PhD, FInstP, FRAeS, FRS, was born in Chile; his father was Dutch and his mother English. The family moved to England in 1906 and in 1923 de Bruyne went up to Trinity College, Cambridge, to read Physics. As a post-graduate he studied in the Cavendish Laboratory under Lord Rutherford and was awarded his PhD in 1930. In 1931, he became Junior Bursar at Trinity, a post that he held for 13 years. The knowledge of administration gained in this position, coupled with his love of flying and aircraft (he gained his pilot's licence and artificer qualifications whilst still a student), stood him in good stead when he set up, probably, his most important c o m p a n y - - A e r o Research Limited. AERO R E S E A R C H L I M I T E D : T H E E A R L Y DAYS Right from the start, de Bruyne was a frustrated aircraft designer. He was totally dissatisfied with the complacency shown in aircraft design during the late 1920s and early 1930s, and was convinced he could do a lot better. Unlike so many other people with similar feelings, he did something about it.
Figure 1 Norman de Bruyne (on the ladder) in the late 1930s; inspecting the U F plant at Duxford
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997 287
The history of Redux: J.A. Bishopp
Figure 2 The Snark as the structure was, apparently, too light! Further, most of their experience on stress analysis was on biplanes, not on a monoplane construction. In June 1934, the RAE were finally persuaded to test the Snark's fuselage to destruction. The results led to the design being approved by RAE's Airworthiness Department and on 16th December the Snark successfully completed its maiden flight; an Airworthiness Certificate was granted in March 1935. In April 1936, the Snark was sold to the Air Ministry who stationed it at R A F Duxford for research purposes.
At the same time as dealing with all the problems caused by the RAE, de Bruyne was consolidating his company. April 1934 saw its name being changed to Aero Research Limited (or A R L as it was usually known) and the employment of George S. Newell, the other key figure to the development of Redux. Then, in May 1935, de Bruyne purchased a 50acre field in Duxford and moved the company there (Figure 3), where it has remained ever since, in spite of about seven or eight further name changes! Following the sale of the Snark, work commenced on the Ladybird--a mid-wing, single-seated plane which
Figure 3 ARL: the workforce (de Bruyne is taking the picture and George Newell is top left) and the buildings in 1936
288
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997
The history of Redux: J.A. Bishopp was intended to be a low-cost machine. However, owing to the need to carry out the basic research required by de Havilland, work stopped and the design and the partly built aircraft were sold to J. N. Maas; Ladybird eventually got her maiden flight at the new Marshall's Aerodrome on 6th January 1938. GORDON AEROLITE® AND AEROLITE® Over the first two years, de Bruyne's research consultancy with de Havilland led to the development of the Aerolite range of urea-formaldehyde 'wood glues' and to Gordon Aerolite, one of the first manmade, organ!c composite materials. Although they were totally separate areas of research and development, in their own way, both were key to the development of the first, synthetic, structural adhesive--Redux--and the Redux process.
Gordon Aerolite The initial research into reinforced phenolic resins for propeller manufacture was carried out in conjunction with the Bakelite Company (of England), who manufactured the phenol-formaldehyde resins. The starting point was an extension of the work associated with the production of cord-reinforced motor tyres: the use of cotton fibres to reinforce cured phenolic mouldings. When, however, it proved impossible to obtain a uniform composite material, the project was abandoned until a Mr Gordon, an undergraduate at Trinity College, suggested the use of unidirectional flax fibres in place of cotton. This approach produced an extremely strong material whose specific gravity was half that of aluminium; it was called Gordon Aerolite. The first R&D contract based on the use of this novel material was the fabrication of the spar for the Bristol Blenheim. However, it was not only its lightweight properties but also the uncertainty over the supply of aluminium during the Second World War, that gave Gordon Aerolite its role as the first structural composite material to be considered seriously for mass aircraft construction. This involved the design and building of an experimental Spitfire fuselage2'3, the RAE having given the material its enthusiastic approval 4 for such a venture. In the event, the composite Spitfire (Figure 4) was not needed. However, 30 Miles Magister tailplanes were successfully constructed out of this material. The knowledge that the finished Gordon Aerolite components used to stick to the mould, during the demoulding process, was to be remembered and used to good effect during the R&D programme which eventually led to Redux.
Aerolite From 1937, in order to put the company on a sounder financial footing, de Bruyne needed a product range which would inject capital into the organisation. Not surprisingly, in view of de Bruyne's interest in the science of adhesion, the team at ARL started work on the development of synthetic glues for wood.
Figure 4 Working on the Gordon Aerolite Spitfire cockpit
Casein-based glues had, indeed, been used in wooden aircraft since about 1917. It was claimed that such bonded structures had an in-built useable-life monitoring system: once the pilot could detect the smell of sour milk, the bond was in danger of catastrophic failure! de Bruyne considered the commercially available glues (phenolic-based systems were then only seen as matrices for moulding compounds) and decided that there was a market for the existing urea-formaldehyde (UF) glues, in aircraft applications. Therefore, on 3rd March 1937 he decided to proceed with the production of a suitable UF adhesive. Despite a serious question of ownership, with ICI 5, the Air Ministry approved this adhesive for aircraft usage on 22nd April 1937. In May 1937, under the tradename of Aerolite, it was launched onto the market. When, m 1939, a method for in-line quality assessment of the degree of condensation was introduced and it was discovered that formic acid was a significant improvement as the catalyst for the system, Aerolite became the only adhesive accepted by the RAF. It is, today, still a standard wood-working adhesive. It was during the early evaluation of Aerolite, in November 1937, that the importance of surface pretreatment, prior to adhesive bonding, came to light. The British Aircraft Manufacturing Company at Feltham reported that they were experiencing poor adhesion when trying to produce bonded plywood joints. Tests on plywood from the Duxford laboratories did not show this phenomenon. The problem was finally attributed to a 'case hardening' effect which could only be alleviated by the removal of the surface of the plywood prior to bonding. This problem was seen again, in 1938, when de Bruyne and his colleagues were replacing the caseinbonded structures in a Desoutter monoplane. In this instance, when fresh plywood was being bonded to the original spruce longerons and stiffeners, poor adhesion was again experienced. The solution was, again, to remove the top surface prior to bonding. Thus another interfacial 6Problem was solved by surface pretreatment . This important lesson would have to be addressed once more following the development of Redux, in order to gain general acceptance of structural bonding by the aero industry.
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997 289
The history of Redux: J.A. Bishopp Thus, from the two disparate R&D areas came the key factors for structural adhesive development and usage--'toughened' phenolics and surface pretreatment.
THE DEVELOPMENT OF REDUX AND THE REDUX PROCESS As has already been stated, it was the observation that Gordon Aerolite components had a very strong propensity to adhere to the mould which led directly to the evaluation of filled phenol-formaldehyde (PF) resins as adhesives for bonding metal structures; unmodified/unfilled phenolic resin matrices never showed this tendency. Uninterrupted research into this phenomenon was the result of a fortuitous dispute, in 1941, with the Ministry for Aircraft Production. This ended ARL's involvement with Gordon Aerolite and released de Bruyne and Newell to focus on the development of structural adhesives. It was argued that the use of flax fibre in Gordon Aerolite not only reinforced the inherently brittle phenolic matrix but also aided in the escape of volatiles from the matrix during cure; phenolic resoles cure by a condensation reaction which releases water vapour. However, any adhesive development would require a reinforcing filler which was simpler to use than flax fibres. During the latter half of 1941, Newell and de Bruyne prepared solvent-based, one-component phenolic adhesives which contained a finely divided inorganic filler (micanite) and a modified castor oil.
Simple lap-shear joints, with Gordon Aerolite substrates, were prepared and tested to destruction7. Strengths of no higher than 700 lbs in -z (4.8 MPa) were recorded. This approach was abandoned. Poly(vinyl formal) resin (PVF)--Formvar® ex the Shawinigan CorporationS--is a sintering polymer of high softening point and was known to be compatible with phenolic resins. It was also believed to be able to 'soak up water '9 associated with the cure of the phenolic. PVF powder was, therefore, almost arbitrarily and certainly fortuitously chosen as the replacement for both flax and micanite powder. Initially, solution adhesives, comprising blends of PF and PVF, were prepared but this approach was abandoned until the launch of the Redux range of onecomponent, friction-bonding adhesives during the late 1940s/early 1950s. Adhesive films were then prepared, where a de Bruyne or Newell finger was used to coat a suitable resole onto either side of a thin PVF film which had been cast from solution. Aluminium lap-shear joints were prepared and tested; steel and wooden adherends were also, occasionally, used. December 5th, 1941 saw the optimisation of this process with strengths of 1250 lbs in 2 (8.6MPa) being achieved (Figure 5). This product was designated Redux, standing for Research at Duxford. On the day before, a liquid/powder joint had been produced by coating the PF resin onto the substrate and then sprinkling a powdered rubber onto the liquid surface to give a liquid-to-powder ratio of about 7:3. The failing loads of 60 lbs in -2 were very disappointing. It wasn't until 24th February 1942 that de Bruyne and Newell prepared another set of liquid/powder lap...................
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INT. J. ADHESION A N D ADHESIVES V o l u m e 17 N u m b e r 4 1997
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shear joints, this time using their standard PF resin (later designated Redux Liquid E) and Formvar powder. This was the final breakthrough• Figure 6 shows the relevant page from the laboratory notebook (Experiment 3), strengths of 2000 to 2200 lbsin -2 (13.8 to 15.3MPa) being attained• These strengths were confirmed a couple of days later. At that time, values of 30001bsin -2 (20.7 MPa) were also attained on mild steel substrates 7. A few months later this product (which took over the mantle of the Redux designation) gained Ministry approval for use in aircraft construction. To complete the outline history, in May 1954 a film version was produced (see below) and, in 1955, Redux Liquid and Powder (later designated Redux 775 Liquid and Redux 775 Powder) and the Redux bonding
process, where the PVF powder is sprinkled onto a substrate already coated with the PF liquid (Figure 7), were qualified to DTD 775 as the first adhesive approved for bonding primary structures in aircraft. It is still in use today--55 years after its invention.
THE CHEMISTRY OF REDUX As has been indicated, the chemistry of the new adhesive was very simple. Redux is a two-component system--a neat phenolic resole based on phenol and formaldehyde [Figure 8(a)] and a poly(vinyl formal) resin having a weight-average molecular weight range of about 20 000 to 50 000 (Figure 9).
INT. J• ADHESION AND ADHESIVES Volume 17 Number 4 1997
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Figure 10 Reaction of PF resole with PVF resin
Figure 8(b) shows the crosslinked structure formed when further formaldehyde reacts with phenol and/or any of the initial structures to crosslink the structure through further methylene bridge formation, with the accompanying loss of water and formaldehyde. Figure 10 shows the reaction between the poly(vinyl formal) resin and the phenolic resin l°. Although the adhesive is a blend of liquid and powder, it is known that a certain degree of solution of the powder in the liquid will take place at the powder/liquid boundary. Thus, on completion of cure, some, at least, of the dissolved PVF should be chemically bound to the three-dimensional phenolic network. This would explain the inherent toughness seen with this adhesive, the phenolic matrix accounting for the system's superlative environmental resistance.
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997
The history of Redux: J.A. Bishopp
Figure 11
The Sea Hornet showing the folded wings
PLYWOOD B
LIGHT ALLOY EXTRUSION
THIN PLYWOOD BONDED TO METAL SPARBOOM Figure 12 Schematic of the bonded reinforcements in the Hornet/ Sea Hornet wings
REDUX BONDING: THE WAR YEARS The properties and results associated with Redux were presented to the Society of British Aircraft Constructors (SBAC) in 1942 but, before the RAF could benefit from this technique, the Army stepped in to use Redux to bond thousands of Cromwell and Churchill tank clutch plates. The bonded version proved to have a tenfold increase in life over riveted plates. It wasn't until 28th July 1944 that the first bonded, structural components in an aircraft successfully completed their maiden flight 11. The aircraft was the de Havilland Hornet which, together with its naval cousin, the Sea Hornet (Figure 11), possessed a wing structure
relying heavily on 'Reduxed' wood/metal components (Figure 12). The ribs and spars contained many such joints to enable the wing to carry the very high predicted tensile stresses. Thus, by structural bonding, speed was not sacrificed to increased weight. Further, with the requirement to fold the wings of the Sea Hornet, for on-board stowage, extra bonded strengthening devices were included in the proximity of the wing hinges (Figure 13). Similar bonding techniques were used to reinforce areas of the wooden fuselage where internal elements were to be attached. Finally, and most importantly, where exceptionally high levels of stress were expected, for example at the attachment points for the tail wheel and arrestor hook to the rear fuselage bulkhead, metalto-metal structural bonding was employed for the first time to build up a reinforcing layer of integrally bonded aluminium sheet. The importance of the surface pretreatment lessons learned when bonding plywood had not been forgotten, and a method for cleaning aluminium prior to painting was adapted for use when Redux bonding. The recommendation was, and still is, to degrease in trichloroethyle_ne, etch in chromic/sulfuric acid to DTD 915A j2 and, if necessary, anodise in chromic acid in accordance with Specification 910C. Although the date when this procedure was adopted is lost in the mists of time 13, it was certainly well before 1950; a 1953 paper 14 shows that these original recommendations had been confirmed by other workers 15 prior to May 1950.
STRUCTURAL BONDING: EDUCATION After the war, the use of structural bonding became more general but not universal; a problem still seen today. Not unnaturally, one of the main stumbling blocks to widespread acceptance was the lack of
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997 293
The history o f Redux: J.A. Bishopp
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In order to redress the situation, de Bruyne, amongst others, gave 'Summer Schools' (Figure 14) during the post-war years to pass on to the next generation of engineers the benefits of structural, and hence, Redux bonding. These sessions 14'16-17, as well as earlier publications 18, covered such important technical areas as the improvement in fatigue (Figures 15 and 16) and, probably, the first examination of the relationship between joint strength and joint dimensions. This was, essentially, a plot of the so-called t/l ratio (which is the ratio of the thickness of the substrate, t, to the length of the overlap of the bonded area, /) against mean failure stress for that particular joint configuration 19. Also, the opportunity was not missed to give a good, hard sell for structural bonding by stressing the improved economics. One such example, savings in labour and costs in stringer manufacture, is given in Figure 17.
STRUCTURAL BONDING: THE IMMEDIATE POST-WAR YEARS
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knowledge, both of the subject and the techniques needed to obtain load-bearing, bonded, structural components, by industry in general. The achievements of the pioneers had been obtained the hard way, by trial and error.
294
In spite of all the problems and thanks to the educative process mentioned above, the immediate post-war years did see something of an expansion in structural bonding, based on the Redux process. As has been mentioned, the old ideas of producing a solution of the PVF polymer in the PF resole were resurrected. These adhesives began to replace the liquid/powder systems for bonding friction materials generally to mild steel, in the production of braking systems, drive shafts, clutch plates, etc. 2°. Redux Liquid and Powder was also used in the sports goods industry, particularly in the manufacture of skis. Here, the versatility of the system was key in producing composite skis containing such varied substrates as wood, steel, aluminium, polyethylene, glass and rubber. Apart from the production of bonded brakes and clutch plates, the automotive industry, in general, was slow on the uptake, with only decorative bonding in, for example, the facia panel taking place. It was in the young aerospace industry, however, that the biggest strides were made. The de Havilland Dove was, in 1946, the first, all-metal, passenger aircraft to be built; it and the later Heron relied heavily on the use of the Redux bonding. So did the Comet 1 in the early 1950s (Figure 18) and the Fokker F.27 Friendship a few years later21'22; stiffened stringers and window assemblies (schematics are given in Figures 19 and 20) are just two examples. A better idea as to the extent of structural bonding can be seen in the completed wing panel (Figure 21). Although several other European aircraft manufacturers--such as SAAB, Sud Aviation, Breuget Aviation and Short Brothers--made extensive use of Redux-bonded structures in their aeroplanes, very little of this technology passed over the Atlantic to the United States of America. The notable exceptions were the Fairchild Engine and Airplane Corporation, who built and bonded the F.27 under licence and Chance Voght with their F4 Corsair series, F7 Cutlass and Regulus guided missile.
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997
The history of Redux: J.A. Bishopp
Figure 14
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The Chance Voght designers took a 1941 idea of de Bruyne, modified it and produced an incredibly light and strong fuselage which was, essentially, a sandwich panel where end-grain balsa wood was Redux-bonded to aluminium skins (Metalite®)--a
technique, later, to be used in the construction of aircraft flooring. This is the first example of a metal-skinned sandwich structure being used in aeroplane construction (the Mosquito used the same concept but the skins were of wood). With the advent of Redux the possibility of using metal-skinned sandwich panels containing an aluminium
INT. J. ADHESION A N D ADHESIVES Volume 17 Number 4 1997
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honeycomb became a reality, opening up the possibility of producing significantly lighter aircraft without sacrificing the engineering properties of the bonded components 14. To improve the adhesion of the Redux adhesive to the honeycomb core, a primer was introduced (Redux 120--an emulsion of the PVF
Apart from the introduction of Redux 120, several other changes were made over the 10 or so years following the launch of Redux and it is at this point that the history begins to get somewhat confused. The main cause is that industry used the term Redux to apply to all the post-war variants of the initial liquid and powder system and, hence, distinguishing which product was used in which application is now difficult. One point is irrefutable. The only change to the formulation of the PF resin took place in the mid1940s, i.e. well before the adhesive was qualified to DTD 775. All references to the qualified product, therefore, must be to an adhesive containing the reformulated resole, Redux K6, and not to Liquid E--the post-war designation given to the original PF. However, as far as can be discovered, only de Havilland and Chance Voght carried on using Liquid E after its replacement (Redux K6 or Redux 775) became available. In de Havilland's case their usage of Liquid E carried on until 1963--i.e. until they started to use Redux Film--well into the build of the C o m e t 4. The reasoning behind the change from Liquid E to Redux K6 was to produce a less alkaline, more stable liquid resin of significantly improved shelf-life. The end result, most fortuitously, was to produce a resin which also imparted significantly higher lap-shear performance to the bonded structure: an increase to about 53001bsin -2 (36.5MPa). The two formulations are compared in Table 1.
SpANWISE STRINGERS IN WINGS
ALL LONGITUDINAL STRINGERS OF FUSELAGE SHELL
ALL LONGITUDINAL STRINGERS OF FUSELAGE SHELL
LOCAL DOUBLINGS AND REINFORCINGS AROUND WINDOW FRAMES
FIN AND ELEVATORS
CANOPY STRUCTURE
ALL STIFFENERSIN PRESSURE FLOOR "REDUX" BONDED TO FLOOR SHELL
'Redux' Bonding in the Comet The very wide use of the 'Rcdux" bonding process in the construction of the Comet is indicated in this cut-away drawing, reproduced by kind permission of Ihe de Havilland Aircraft Co. Lid, The use of the "Redux" process in the Comet is so extensive that it has not been found possible to indicate all the structures for which it is be[ng u ~ d .
IN AILERON STRUCTURE
VERTICAL STIFFENER FLANc~ES AND DOUBLERSIN WINGS
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18
Exploded diagram o f the Comet 1 showing areas of Redux bonding
INT. J. A D H E S I O N A N D A D H E S I V E S V o l u m e 17 N u m b e r 4 1997
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The history of Redux: J.A. Bishopp Table 1 Comparison of the formulation of the two resoles used in Redux Property
Redux Liquid E
Redux K6 (Redux 775 Liquid)
P:F molar ratio Phenol:NaOH molar ratio on manufacture pH (approximate) Solids content (wt%) Colour on manufacture Solvent
hl.57 1:0.097
hl.43 1:0.11
10 62±2 Dark red/purple
8 70±5 Straw/light brown
IMS
IMS
The situation is further complicated when the powder is considered. The first material used was an unsieved version of Formvar 15/95E from the Shawinigan Company in Canada. Shawinigan were eventually bought out by Monsanto who retained the tradename and the individual powder designations. It was quickly found that the mechanical properties of the bonded structure depended heavily on the liquidto-powder ratio in the joint; the particle-size distribution of the powder was critical to controlling this parameter. If the distribution were significantly skewed to the fine end of the particle range (e.g. < 150/~m), the PF surface was instantly 'blinded', preventing further powder pick-up and leading to tow mechanical performance. If, on the other hand, the distribution were skewed to the large end of the particle range (e.g. >1200 to 2000#m), the joint became over-rich in PVF polymer and the bonded structure lost the benefits imparted by the cured phenolic matrix. By the mid-1940s, the particle-size distribution had been optimised, the powder component reflecting this in its new designation of Formvar 15/95E-30/52 Grade. This indicated that the bulk ( > 65%) of the powder fell in the range of 300 to 5001tm (i.e. BS 30 to BS 52mesh). In recent days, the 30/52 requirement has risen to > 80%. In these early days, the raw powder was worked inhouse, at Duxford--i.e. the unsieved powder was ground and sieved or even dissolved and reprecipitated to obtain the maximum yields possible within the desired particle-size range. Unfortunately, no records were kept indicating which company received which powder. From the mid-1950s, however, all material supplied by Shawinigan/Monsanto was sieved, prior to shipment, to produce the 30/52 grade; the grinding process was abandoned and only reinstated in the late 1980s. It should be noted, though, that SIVA (an Italian-based supplier of PVF resins), see below, always used a grinding process in the production of their 30/52 grade. Once again, it was de Havilland who went their own way. They insisted, against all the advice from George Newell, on using a significantly larger particle size--in the belief, it is thought, that this enhanced the peel performance of the adhesive. This material, known as Powder D, had a particle-size distribution of 1670 to 2000/tm (BS 8 to BS 10mesh). The visual differences in the two can be clearly seen by comparing Figure 7 (30/ 52 grade material) and Figure 22 (8/10 grade).
Figure 19 Schematic of a typical bonded stringer
J
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Figure 20
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Schematic of a typical bonded window
Figure 21 A fully-bonded F27 wing
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997
297
The history of Redux: J.A. Bishopp
J
Figure 22
A beam assembly using Redux Liquid E and Powder D
Some people believed that the particle topographies of the two materials were also different but this is now thought to be unlikely: all PVF powders precipitated after solution polymerisation have a similar, rough structure. It is only when a high degree of grinding energy is applied to the particle that a surface smoothing takes place (Figure 23). This would not have happened in the production of these two different grades of polymer from the same source. This smooth finish was, however, prevalent in the SIVA product and was, indeed, thought to explain some of the latter-day variance in properties. A graphical representation of the various particlesize distributions is given in Figure 24. In all other aspects, the two versions were chemically identical
(Table 2). Table 2 also gives the comparison of other powders used in the Redux process. All were qualified, to DTD 775, as being chemically equivalent to the original Formvar material. Their history of usage can be summarised as follows23:
Table 2 Comparison of the properties of various poly(vinyl formal) resins
• Shawinigan/Monsanto Formvar 15/95E used exclusively from 1942 to 1969; • SIVA H.30 powder used as 'second source', alongside Formvar, from 1969 to 1970; • SIVA H.30 powder used exclusively from early 1971 to 1986 for aerospace work (many other powder combinations were used by the ski industry--but that's another story!); • Formvar powder available again in 1986 and used alongside SIVA material; • Monsanto withdraw product in 1988, stocks run out in 1991; • use of SIVA, again exclusively, until product is withdrawn in 1995; and • Chisso's Vinylec EC powder (ex Japan) qualified in 1995 and is now used exclusively for Redux 775 products. REDUX FILM: THE FINAL DEVELOPMENT? During 1953, experiments were carried out at Duxford to convert the Liquid/Powder system into a film adhesive. In essence a so-called half-web was produced where a film of the resole (Redux K6/ Redux 775 but never Redux Liquid E) was coated onto polyethylene film, powder was applied to the phenolic coating and the excess was allowed to drop off leaving a film which, by natural pick-up of powder, had the required liquid-to-powder ratio. Two half-webs were then consolidated together--powder face to powder face--to produce the Redux Film (Figure 25). The first prototype batch was produced on 31st May 1954 and over the next 10 years the full product range was launched. Needless to say, this range has been severely rationalised over the last 15 years so that only Redux 775 Film and Redux Film C remain. To ensure that the correct liquid-to-powder ratio was attained, the various films in the range needed special grades of the Formvar powder. The lightest film required the socalled 'Fines' or <150/~m (
Property
F o r m v a r 15/ 95E
H.30
Vinylec® EC
Weightaverage molecular weight Poly(vinyl alcohol) content (%) Poly(vinyl acetate) content (%) Poly(vinyl formal) content (%) Free acetic acid (%) Water content
24 000 to 40 000
20 000 to 50 000
not quoted
5.0 to 6.5
5.0 to 7.0
5.0 to 6.5
Film
9.5 to 13.0
9.5 to 13.0
9.5 to 13.0
> 82
> 80
> 81
< 0.25
< 0.3
< 0.3
< 1.0
< 0.5
<3
Solution viscositya (mPa s)
37 to 53
25 to 50
30 to 51
Redux Redux Redux Redux Redux MR Redux Redux Redux MRN Redux LRN Redux
(%)
Table 3 The Redux 775 Film range Areal weight (g m -2)
Carrier
775 775 M 775 L 775 R 775
370 245 156 370 245
30/52 40G Fines 30/52 40G
None None None Woven glass Woven glass
775 L R 775 R N 775
156 370 245
Fines 30/52 40G
Woven glass Woven Nylon Woven Nylon
775
156
Fines
Woven Nylon
C range
a Solution: 5 g of polymer made up to 100 ml with ethylene dichloride
298
Powder
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997
The same range as above but not released to aerospace specifications
The history of Redux: J.A. Bishopp
Figure 23
Electron micrographs of PVF powder showing the normal rough finish and the smooth surface following grinding
o
---- 30•62 Grade I Powder "D"
c o
.m u.
A
/ \ 1_ \
o
0 • :
Industry, in general, used considerable amounts of film for such varied applications as transporter belting (Sandvik), flooring and computer doors. Although the product line has long been rationalised, Redux 775 Film has been used on a continuous basis since the 1960s, particularly by the European aerospace industry. The following examples suffice: BAC 1-ll
I-o-Fines !
I-°-Orig in'l L~--P-0-wder4°°
zf
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G.
0
Figure 24
600
1000 1600 ParlJcle Size - prn
2000 POWDER CONVEYORSYSTEM /-
(a) VIBRATINGTRAY F
Particle-size distributionsfor criticalPVF powders
E
E
~
REIN EXTRUDER
The industry's change, from the liquid-and-powder system to the more user-friendly film version, did not take place overnight. Re-qualification in the aerospace industry is not a speedy affair and is never undertaken lightly. Indeed, the change at Fokker and at Short Brothers, Belfast, did not take place until 1995! However, the Handley Page Herald, Hawker Siddley 125, Armstrong Whitworth Argosy, Bristol Britannia, Sud Aviation Alouette and SAAB's Lansen and Draken aircraft all used the film version in their construction and, in 1963, de Havilland replaced Redux Liquid E and Powder D by Redux 775 Film in the Comet 4 and Nimrod aircraft. Further, without Redux 775 Film MRN, the strong, lightweight, large-area box construction of the Westland/Saunders Roe SRN4 hovercraft would not have been possible. Mintex and Melbourne Buses (Australia) were unique in switching from the specialised brake-bonding adhesives to Redux 775 Film in the production of drum brakes. With one notable exception, the automotive industry were again reluctant to commit to adhesive bonding. The main beams of Donald Campbell's world-speed record car, The Bluebird C.N.7 (Figure 26), were bonded sandwich structures 24 and undoubtedly saved his life when he crashed during the land-speed attempt on the salt flats in Utah, USA.
.....
TURRET ASSEMBLY
CARRIER PAYOFF (b) UNCONSOLIDATED PRODUCT ROLL
UNCONSOLIDATED PRODUCT ROLL
""
INFRA-RED DIRECT HEATERS SLITTER
CONSOLIDATORROLLS
Figure 25
Schematic of the current Redux Film machine
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997
299
The history of Redux: J.A. Bishopp FINAL THOUGHTS It is appropriate that Norman de Bruyne lived to see not only industry's general acceptance of structural adhesive bonding, which he pioneered with the invention of Redux, for joining an enormously varied range of loadbearing components, but' also saw the acceptance of Redux as the world benchmark against which all others are measured. It is interesting food for thought as to where Redux and Redux bonding would be today, on a world-wide basis, if the de Havilland Comets G-ALYP and GALYY had not crashed. Although Redux was completely exonerated by the court of inquiry--the cause being attributed to metal fatigue failure of the fuselage which resulted in explosive decompression--it took until 1955 to do so; G-ALYP crashed on 10th January 1951. By this time nitrile-phenolic and, particularly, epoxy-based adhesives had begun to gain a strong foothold for structural bonding and the expected expansion of the Redux process to the USA never took place. In Europe, however, major aeroplane constructors still specify Redux bonding for their new aircraft, to take advantage of its exceptional durability and its 55 year, proven track record.
ACKNOWLEDGEM ENTS The author is indebted to Norman de Bruyne's book 'My Life' for confirmation of the critical dates during the early years. Thanks are also due to Dr Peter Stark, former Research Manager at Ciba Polymers, for supplying copies of the crucial pages from Norman de Bruyne's laboratory notebooks. This enabled, once and for all, the confusion as to the date on which Redux was invented to be cleared up. Thanks are also extended to Andy Bush, now of The Welding Institute, Abington, Cambridge, for clarifying the situation with respect to the use of Redux K6/ Redux 775 Liquid and Redux Liquid E.
REFERENCES Figure 26 Preparation of one of the bonded main beams for The Bluebird
and Trident, BAe 146 and the Fokker 50 and 100 aircraft. Finally, to return to the very first batch of Redux Film which was produced. A small sample has been preserved and tested over the years to determine the effect of ageing on lap-shear performance. Figure 27 shows the results of these tests up to 19th February 1991. A further, and ultimate, bonded panel will be prepared on 31st May 2004 when the film sample will be exactly 50 years old: the indications are that the shear strength will still be above 30MPa.
300
1 2 3 4 5 6 7 8 9 10 11 12 13 14
de Bruyne, N.A. 'My Life', Midsummer Books, 1996 Aero Research Technical Note, No. 34, October 1945 Aircraft Production July 1945, p. 323 RAE Report, MT 56442, 1940 de Bruyne, N.A. 'My Life', Midsummer Books, 1996, p. 94 Aero Research Technical Note, No. 21, September 1942 de Bruyne, N.A. Laboratory Notebooks for 1941-1942 Fitzhugh, A.F., Lavin, E. and Morrison, G . O . J . Elect. Chem. Soc. 1953 100(8), 351-355 de Bruyne, N.A. 'My Life', Midsummer Books, 1996, p. 167 Monsanto Data Sheet, Formvar and Butvar®--Properties and Uses, MP-3-205-6, p. 31 Aero Research Technical Note, No. 39, March 1946 Process for Cleaning Aluminium and Aluminium Alloy Plating Prior to Painting, DTD 915A, December 1942 (reprinted September 1951) Evans, G.B. 40 years of structural adhesive bonding, CME, March 1985, pp. 23-27 de Bruyne, N.A. Structural adhesives for metal aircraft. Paper given to The Fourth Anglo-American Aeronautical Conference
INT. J, ADHESION AND ADHESIVES Volume 17 Number 4 1997
The history of Redux: J.A. Bishopp 35 3O 25
~3 ¢ql
2O
~
lO
N
5 0 May-54
I Nov-59
I May-65
I Nov-70
b Apr-76
I Oct-81
I Apr-87
Sep-92
Date of Test Figure 27
15
16 17
Performance of Redux 775 Film Batch 1 since manufacture
1953, published by The Royal Aeronautical Society, London, UK Eickner, H.W. and Schowalter, W.E. A Study of Methods for Preparing Clad 24S-T3 Aluminium Alloy Sheet Surfaces for Adhesive Bonding, Report 1813, Forest Products Laboratory, Wisconsin, USA, May 1950 de Bruyne, N.A. Redux in Aircraft, Aero Research Limited, January 1953 Schliekelmann, R.J. Production tools for Redux. In 'Proceedings of the Conference Bonded Aircraft Structures', 1957, published by Bonded Structures Limited, Cambridge, UK
18 19 20 21 22 23 24
de Bruyne, N.A. The Strength of Glued Joints, Aircraft Engineering, Workshop and Production Section, April 1944, pp. 115 118 Hexcel Composites, Redux Bonding Technology, February 1997, pp 18 20 Aero Research Technical Note, No. 203, November 1959 Aero Research Technical Note, No. 110, February 1952 Ciba-Geigy Technical Note, No. 3/1977 Bishopp, J.A. Hexcel Composites Internal Report on the History of Redux, 1995 Aero Research Technical Note, No. 212, August 1960
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997
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•
Int. J. Adhesion and Adhesives 17 (1997) 287--301
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~;, 1998 Elsevier Science Limited Printed in Great Britain. All rights reserved 0143-7496/98/$19.00
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ELSEVIER
PII: S0143-7496(97)00023-7
The history of Redux® and the Redux bonding process John A. Bishopp Hexcel Composites, Duxford, Cambridge CB2 4019, UK (Accepted 3 May 1997)
As I drove back the heavens opened wide to reveal a chorus and trumpets making Handel-like noises (de Bruyne, N.A. 'My Life', Midsummer Books, 1996). These were Norman de Bruyne's feelings when he returned from the de Havilland works in Hatfield, on 9th April 1936, with a cheque for £1000 and a consultancy to research into reinforced phenol formaldehyde resins for use in propeller manufacture. This decision by de Havilland turned out to be one of considerable importance as it led directly to the acceptance and use of structural adhesive bonding in many, if not all, aircraft from the mid-1940s to the present day. © 1998 Elsevier Science Ltd. All rights reserved (Keywords: A. phenolic; A. toughened adhesives; B. aluminium and alloys; adhesion/adhesives history)
INTRODUCTION The history of Redux and the Redux bonding process is inextricably linked with the history of N o r m a n de Bruyne (Figure 1). Therefore it is appropriate, initially, to concentrate on the man himself, his early years and on the research work which led to the formation of Aero Research Limited and to the discovery of a structural adhesive and bonding process which are still, very much, in use today.
In 1932 he founded The Cambridge Aeroplane Construction Company, at the original Marshall's Aerodrome, Cambridge, in a shed rented from the late Sir Arthur Marshall. Here he designed and built a four-seater monoplane--the Snark
(Figure 2). However, things did not proceed smoothly. The original application for an airworthiness approval was thrown out by the Royal Aircraft Establishment (RAE)
N O R M A N A D R I A N DE B R U Y N E (1904-1997) Norman Adrian de Bruyne MA, PhD, FInstP, FRAeS, FRS, was born in Chile; his father was Dutch and his mother English. The family moved to England in 1906 and in 1923 de Bruyne went up to Trinity College, Cambridge, to read Physics. As a post-graduate he studied in the Cavendish Laboratory under Lord Rutherford and was awarded his PhD in 1930. In 1931, he became Junior Bursar at Trinity, a post that he held for 13 years. The knowledge of administration gained in this position, coupled with his love of flying and aircraft (he gained his pilot's licence and artificer qualifications whilst still a student), stood him in good stead when he set up, probably, his most important c o m p a n y - - A e r o Research Limited. AERO R E S E A R C H L I M I T E D : T H E E A R L Y DAYS Right from the start, de Bruyne was a frustrated aircraft designer. He was totally dissatisfied with the complacency shown in aircraft design during the late 1920s and early 1930s, and was convinced he could do a lot better. Unlike so many other people with similar feelings, he did something about it.
Figure 1 Norman de Bruyne (on the ladder) in the late 1930s; inspecting the U F plant at Duxford
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997 287
The history of Redux: J.A. Bishopp
Figure 2 The Snark as the structure was, apparently, too light! Further, most of their experience on stress analysis was on biplanes, not on a monoplane construction. In June 1934, the RAE were finally persuaded to test the Snark's fuselage to destruction. The results led to the design being approved by RAE's Airworthiness Department and on 16th December the Snark successfully completed its maiden flight; an Airworthiness Certificate was granted in March 1935. In April 1936, the Snark was sold to the Air Ministry who stationed it at R A F Duxford for research purposes.
At the same time as dealing with all the problems caused by the RAE, de Bruyne was consolidating his company. April 1934 saw its name being changed to Aero Research Limited (or A R L as it was usually known) and the employment of George S. Newell, the other key figure to the development of Redux. Then, in May 1935, de Bruyne purchased a 50acre field in Duxford and moved the company there (Figure 3), where it has remained ever since, in spite of about seven or eight further name changes! Following the sale of the Snark, work commenced on the Ladybird--a mid-wing, single-seated plane which
Figure 3 ARL: the workforce (de Bruyne is taking the picture and George Newell is top left) and the buildings in 1936
288
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997
The history of Redux: J.A. Bishopp was intended to be a low-cost machine. However, owing to the need to carry out the basic research required by de Havilland, work stopped and the design and the partly built aircraft were sold to J. N. Maas; Ladybird eventually got her maiden flight at the new Marshall's Aerodrome on 6th January 1938. GORDON AEROLITE® AND AEROLITE® Over the first two years, de Bruyne's research consultancy with de Havilland led to the development of the Aerolite range of urea-formaldehyde 'wood glues' and to Gordon Aerolite, one of the first manmade, organ!c composite materials. Although they were totally separate areas of research and development, in their own way, both were key to the development of the first, synthetic, structural adhesive--Redux--and the Redux process.
Gordon Aerolite The initial research into reinforced phenolic resins for propeller manufacture was carried out in conjunction with the Bakelite Company (of England), who manufactured the phenol-formaldehyde resins. The starting point was an extension of the work associated with the production of cord-reinforced motor tyres: the use of cotton fibres to reinforce cured phenolic mouldings. When, however, it proved impossible to obtain a uniform composite material, the project was abandoned until a Mr Gordon, an undergraduate at Trinity College, suggested the use of unidirectional flax fibres in place of cotton. This approach produced an extremely strong material whose specific gravity was half that of aluminium; it was called Gordon Aerolite. The first R&D contract based on the use of this novel material was the fabrication of the spar for the Bristol Blenheim. However, it was not only its lightweight properties but also the uncertainty over the supply of aluminium during the Second World War, that gave Gordon Aerolite its role as the first structural composite material to be considered seriously for mass aircraft construction. This involved the design and building of an experimental Spitfire fuselage2'3, the RAE having given the material its enthusiastic approval 4 for such a venture. In the event, the composite Spitfire (Figure 4) was not needed. However, 30 Miles Magister tailplanes were successfully constructed out of this material. The knowledge that the finished Gordon Aerolite components used to stick to the mould, during the demoulding process, was to be remembered and used to good effect during the R&D programme which eventually led to Redux.
Aerolite From 1937, in order to put the company on a sounder financial footing, de Bruyne needed a product range which would inject capital into the organisation. Not surprisingly, in view of de Bruyne's interest in the science of adhesion, the team at ARL started work on the development of synthetic glues for wood.
Figure 4 Working on the Gordon Aerolite Spitfire cockpit
Casein-based glues had, indeed, been used in wooden aircraft since about 1917. It was claimed that such bonded structures had an in-built useable-life monitoring system: once the pilot could detect the smell of sour milk, the bond was in danger of catastrophic failure! de Bruyne considered the commercially available glues (phenolic-based systems were then only seen as matrices for moulding compounds) and decided that there was a market for the existing urea-formaldehyde (UF) glues, in aircraft applications. Therefore, on 3rd March 1937 he decided to proceed with the production of a suitable UF adhesive. Despite a serious question of ownership, with ICI 5, the Air Ministry approved this adhesive for aircraft usage on 22nd April 1937. In May 1937, under the tradename of Aerolite, it was launched onto the market. When, m 1939, a method for in-line quality assessment of the degree of condensation was introduced and it was discovered that formic acid was a significant improvement as the catalyst for the system, Aerolite became the only adhesive accepted by the RAF. It is, today, still a standard wood-working adhesive. It was during the early evaluation of Aerolite, in November 1937, that the importance of surface pretreatment, prior to adhesive bonding, came to light. The British Aircraft Manufacturing Company at Feltham reported that they were experiencing poor adhesion when trying to produce bonded plywood joints. Tests on plywood from the Duxford laboratories did not show this phenomenon. The problem was finally attributed to a 'case hardening' effect which could only be alleviated by the removal of the surface of the plywood prior to bonding. This problem was seen again, in 1938, when de Bruyne and his colleagues were replacing the caseinbonded structures in a Desoutter monoplane. In this instance, when fresh plywood was being bonded to the original spruce longerons and stiffeners, poor adhesion was again experienced. The solution was, again, to remove the top surface prior to bonding. Thus another interfacial 6Problem was solved by surface pretreatment . This important lesson would have to be addressed once more following the development of Redux, in order to gain general acceptance of structural bonding by the aero industry.
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997 289
The history of Redux: J.A. Bishopp Thus, from the two disparate R&D areas came the key factors for structural adhesive development and usage--'toughened' phenolics and surface pretreatment.
THE DEVELOPMENT OF REDUX AND THE REDUX PROCESS As has already been stated, it was the observation that Gordon Aerolite components had a very strong propensity to adhere to the mould which led directly to the evaluation of filled phenol-formaldehyde (PF) resins as adhesives for bonding metal structures; unmodified/unfilled phenolic resin matrices never showed this tendency. Uninterrupted research into this phenomenon was the result of a fortuitous dispute, in 1941, with the Ministry for Aircraft Production. This ended ARL's involvement with Gordon Aerolite and released de Bruyne and Newell to focus on the development of structural adhesives. It was argued that the use of flax fibre in Gordon Aerolite not only reinforced the inherently brittle phenolic matrix but also aided in the escape of volatiles from the matrix during cure; phenolic resoles cure by a condensation reaction which releases water vapour. However, any adhesive development would require a reinforcing filler which was simpler to use than flax fibres. During the latter half of 1941, Newell and de Bruyne prepared solvent-based, one-component phenolic adhesives which contained a finely divided inorganic filler (micanite) and a modified castor oil.
Simple lap-shear joints, with Gordon Aerolite substrates, were prepared and tested to destruction7. Strengths of no higher than 700 lbs in -z (4.8 MPa) were recorded. This approach was abandoned. Poly(vinyl formal) resin (PVF)--Formvar® ex the Shawinigan CorporationS--is a sintering polymer of high softening point and was known to be compatible with phenolic resins. It was also believed to be able to 'soak up water '9 associated with the cure of the phenolic. PVF powder was, therefore, almost arbitrarily and certainly fortuitously chosen as the replacement for both flax and micanite powder. Initially, solution adhesives, comprising blends of PF and PVF, were prepared but this approach was abandoned until the launch of the Redux range of onecomponent, friction-bonding adhesives during the late 1940s/early 1950s. Adhesive films were then prepared, where a de Bruyne or Newell finger was used to coat a suitable resole onto either side of a thin PVF film which had been cast from solution. Aluminium lap-shear joints were prepared and tested; steel and wooden adherends were also, occasionally, used. December 5th, 1941 saw the optimisation of this process with strengths of 1250 lbs in 2 (8.6MPa) being achieved (Figure 5). This product was designated Redux, standing for Research at Duxford. On the day before, a liquid/powder joint had been produced by coating the PF resin onto the substrate and then sprinkling a powdered rubber onto the liquid surface to give a liquid-to-powder ratio of about 7:3. The failing loads of 60 lbs in -2 were very disappointing. It wasn't until 24th February 1942 that de Bruyne and Newell prepared another set of liquid/powder lap...................
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INT. J. ADHESION A N D ADHESIVES V o l u m e 17 N u m b e r 4 1997
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shear joints, this time using their standard PF resin (later designated Redux Liquid E) and Formvar powder. This was the final breakthrough• Figure 6 shows the relevant page from the laboratory notebook (Experiment 3), strengths of 2000 to 2200 lbsin -2 (13.8 to 15.3MPa) being attained• These strengths were confirmed a couple of days later. At that time, values of 30001bsin -2 (20.7 MPa) were also attained on mild steel substrates 7. A few months later this product (which took over the mantle of the Redux designation) gained Ministry approval for use in aircraft construction. To complete the outline history, in May 1954 a film version was produced (see below) and, in 1955, Redux Liquid and Powder (later designated Redux 775 Liquid and Redux 775 Powder) and the Redux bonding
process, where the PVF powder is sprinkled onto a substrate already coated with the PF liquid (Figure 7), were qualified to DTD 775 as the first adhesive approved for bonding primary structures in aircraft. It is still in use today--55 years after its invention.
THE CHEMISTRY OF REDUX As has been indicated, the chemistry of the new adhesive was very simple. Redux is a two-component system--a neat phenolic resole based on phenol and formaldehyde [Figure 8(a)] and a poly(vinyl formal) resin having a weight-average molecular weight range of about 20 000 to 50 000 (Figure 9).
INT. J• ADHESION AND ADHESIVES Volume 17 Number 4 1997
291
The history of Redux: J.A. Bishopp _ CH2
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Structure of a typical poly(vinyl formal) resin
H
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o\/o Figure 7 Application of Redux Liquid and Powder to a Fokker wing assembly (a)
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292
Figure 10 Reaction of PF resole with PVF resin
Figure 8(b) shows the crosslinked structure formed when further formaldehyde reacts with phenol and/or any of the initial structures to crosslink the structure through further methylene bridge formation, with the accompanying loss of water and formaldehyde. Figure 10 shows the reaction between the poly(vinyl formal) resin and the phenolic resin l°. Although the adhesive is a blend of liquid and powder, it is known that a certain degree of solution of the powder in the liquid will take place at the powder/liquid boundary. Thus, on completion of cure, some, at least, of the dissolved PVF should be chemically bound to the three-dimensional phenolic network. This would explain the inherent toughness seen with this adhesive, the phenolic matrix accounting for the system's superlative environmental resistance.
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997
The history of Redux: J.A. Bishopp
Figure 11
The Sea Hornet showing the folded wings
PLYWOOD B
LIGHT ALLOY EXTRUSION
THIN PLYWOOD BONDED TO METAL SPARBOOM Figure 12 Schematic of the bonded reinforcements in the Hornet/ Sea Hornet wings
REDUX BONDING: THE WAR YEARS The properties and results associated with Redux were presented to the Society of British Aircraft Constructors (SBAC) in 1942 but, before the RAF could benefit from this technique, the Army stepped in to use Redux to bond thousands of Cromwell and Churchill tank clutch plates. The bonded version proved to have a tenfold increase in life over riveted plates. It wasn't until 28th July 1944 that the first bonded, structural components in an aircraft successfully completed their maiden flight 11. The aircraft was the de Havilland Hornet which, together with its naval cousin, the Sea Hornet (Figure 11), possessed a wing structure
relying heavily on 'Reduxed' wood/metal components (Figure 12). The ribs and spars contained many such joints to enable the wing to carry the very high predicted tensile stresses. Thus, by structural bonding, speed was not sacrificed to increased weight. Further, with the requirement to fold the wings of the Sea Hornet, for on-board stowage, extra bonded strengthening devices were included in the proximity of the wing hinges (Figure 13). Similar bonding techniques were used to reinforce areas of the wooden fuselage where internal elements were to be attached. Finally, and most importantly, where exceptionally high levels of stress were expected, for example at the attachment points for the tail wheel and arrestor hook to the rear fuselage bulkhead, metalto-metal structural bonding was employed for the first time to build up a reinforcing layer of integrally bonded aluminium sheet. The importance of the surface pretreatment lessons learned when bonding plywood had not been forgotten, and a method for cleaning aluminium prior to painting was adapted for use when Redux bonding. The recommendation was, and still is, to degrease in trichloroethyle_ne, etch in chromic/sulfuric acid to DTD 915A j2 and, if necessary, anodise in chromic acid in accordance with Specification 910C. Although the date when this procedure was adopted is lost in the mists of time 13, it was certainly well before 1950; a 1953 paper 14 shows that these original recommendations had been confirmed by other workers 15 prior to May 1950.
STRUCTURAL BONDING: EDUCATION After the war, the use of structural bonding became more general but not universal; a problem still seen today. Not unnaturally, one of the main stumbling blocks to widespread acceptance was the lack of
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997 293
The history o f Redux: J.A. Bishopp
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In order to redress the situation, de Bruyne, amongst others, gave 'Summer Schools' (Figure 14) during the post-war years to pass on to the next generation of engineers the benefits of structural, and hence, Redux bonding. These sessions 14'16-17, as well as earlier publications 18, covered such important technical areas as the improvement in fatigue (Figures 15 and 16) and, probably, the first examination of the relationship between joint strength and joint dimensions. This was, essentially, a plot of the so-called t/l ratio (which is the ratio of the thickness of the substrate, t, to the length of the overlap of the bonded area, /) against mean failure stress for that particular joint configuration 19. Also, the opportunity was not missed to give a good, hard sell for structural bonding by stressing the improved economics. One such example, savings in labour and costs in stringer manufacture, is given in Figure 17.
STRUCTURAL BONDING: THE IMMEDIATE POST-WAR YEARS
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knowledge, both of the subject and the techniques needed to obtain load-bearing, bonded, structural components, by industry in general. The achievements of the pioneers had been obtained the hard way, by trial and error.
294
In spite of all the problems and thanks to the educative process mentioned above, the immediate post-war years did see something of an expansion in structural bonding, based on the Redux process. As has been mentioned, the old ideas of producing a solution of the PVF polymer in the PF resole were resurrected. These adhesives began to replace the liquid/powder systems for bonding friction materials generally to mild steel, in the production of braking systems, drive shafts, clutch plates, etc. 2°. Redux Liquid and Powder was also used in the sports goods industry, particularly in the manufacture of skis. Here, the versatility of the system was key in producing composite skis containing such varied substrates as wood, steel, aluminium, polyethylene, glass and rubber. Apart from the production of bonded brakes and clutch plates, the automotive industry, in general, was slow on the uptake, with only decorative bonding in, for example, the facia panel taking place. It was in the young aerospace industry, however, that the biggest strides were made. The de Havilland Dove was, in 1946, the first, all-metal, passenger aircraft to be built; it and the later Heron relied heavily on the use of the Redux bonding. So did the Comet 1 in the early 1950s (Figure 18) and the Fokker F.27 Friendship a few years later21'22; stiffened stringers and window assemblies (schematics are given in Figures 19 and 20) are just two examples. A better idea as to the extent of structural bonding can be seen in the completed wing panel (Figure 21). Although several other European aircraft manufacturers--such as SAAB, Sud Aviation, Breuget Aviation and Short Brothers--made extensive use of Redux-bonded structures in their aeroplanes, very little of this technology passed over the Atlantic to the United States of America. The notable exceptions were the Fairchild Engine and Airplane Corporation, who built and bonded the F.27 under licence and Chance Voght with their F4 Corsair series, F7 Cutlass and Regulus guided missile.
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997
The history of Redux: J.A. Bishopp
Figure 14
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ENDURANCE IN CYCLES Figure 15 riveting
Joint fatigue: the advantages of using bonding over
The Chance Voght designers took a 1941 idea of de Bruyne, modified it and produced an incredibly light and strong fuselage which was, essentially, a sandwich panel where end-grain balsa wood was Redux-bonded to aluminium skins (Metalite®)--a
technique, later, to be used in the construction of aircraft flooring. This is the first example of a metal-skinned sandwich structure being used in aeroplane construction (the Mosquito used the same concept but the skins were of wood). With the advent of Redux the possibility of using metal-skinned sandwich panels containing an aluminium
INT. J. ADHESION A N D ADHESIVES Volume 17 Number 4 1997
295
The history of Redux: J.A. Bishopp ~X
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polymer in the PF resin). It was applied to the honeycomb surface immediately prior to bonding.
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Economics of bonding; labour costs compared with
honeycomb became a reality, opening up the possibility of producing significantly lighter aircraft without sacrificing the engineering properties of the bonded components 14. To improve the adhesion of the Redux adhesive to the honeycomb core, a primer was introduced (Redux 120--an emulsion of the PVF
Apart from the introduction of Redux 120, several other changes were made over the 10 or so years following the launch of Redux and it is at this point that the history begins to get somewhat confused. The main cause is that industry used the term Redux to apply to all the post-war variants of the initial liquid and powder system and, hence, distinguishing which product was used in which application is now difficult. One point is irrefutable. The only change to the formulation of the PF resin took place in the mid1940s, i.e. well before the adhesive was qualified to DTD 775. All references to the qualified product, therefore, must be to an adhesive containing the reformulated resole, Redux K6, and not to Liquid E--the post-war designation given to the original PF. However, as far as can be discovered, only de Havilland and Chance Voght carried on using Liquid E after its replacement (Redux K6 or Redux 775) became available. In de Havilland's case their usage of Liquid E carried on until 1963--i.e. until they started to use Redux Film--well into the build of the C o m e t 4. The reasoning behind the change from Liquid E to Redux K6 was to produce a less alkaline, more stable liquid resin of significantly improved shelf-life. The end result, most fortuitously, was to produce a resin which also imparted significantly higher lap-shear performance to the bonded structure: an increase to about 53001bsin -2 (36.5MPa). The two formulations are compared in Table 1.
SpANWISE STRINGERS IN WINGS
ALL LONGITUDINAL STRINGERS OF FUSELAGE SHELL
ALL LONGITUDINAL STRINGERS OF FUSELAGE SHELL
LOCAL DOUBLINGS AND REINFORCINGS AROUND WINDOW FRAMES
FIN AND ELEVATORS
CANOPY STRUCTURE
ALL STIFFENERSIN PRESSURE FLOOR "REDUX" BONDED TO FLOOR SHELL
'Redux' Bonding in the Comet The very wide use of the 'Rcdux" bonding process in the construction of the Comet is indicated in this cut-away drawing, reproduced by kind permission of Ihe de Havilland Aircraft Co. Lid, The use of the "Redux" process in the Comet is so extensive that it has not been found possible to indicate all the structures for which it is be[ng u ~ d .
IN AILERON STRUCTURE
VERTICAL STIFFENER FLANc~ES AND DOUBLERSIN WINGS
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296
18
Exploded diagram o f the Comet 1 showing areas of Redux bonding
INT. J. A D H E S I O N A N D A D H E S I V E S V o l u m e 17 N u m b e r 4 1997
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The history of Redux: J.A. Bishopp Table 1 Comparison of the formulation of the two resoles used in Redux Property
Redux Liquid E
Redux K6 (Redux 775 Liquid)
P:F molar ratio Phenol:NaOH molar ratio on manufacture pH (approximate) Solids content (wt%) Colour on manufacture Solvent
hl.57 1:0.097
hl.43 1:0.11
10 62±2 Dark red/purple
8 70±5 Straw/light brown
IMS
IMS
The situation is further complicated when the powder is considered. The first material used was an unsieved version of Formvar 15/95E from the Shawinigan Company in Canada. Shawinigan were eventually bought out by Monsanto who retained the tradename and the individual powder designations. It was quickly found that the mechanical properties of the bonded structure depended heavily on the liquidto-powder ratio in the joint; the particle-size distribution of the powder was critical to controlling this parameter. If the distribution were significantly skewed to the fine end of the particle range (e.g. < 150/~m), the PF surface was instantly 'blinded', preventing further powder pick-up and leading to tow mechanical performance. If, on the other hand, the distribution were skewed to the large end of the particle range (e.g. >1200 to 2000#m), the joint became over-rich in PVF polymer and the bonded structure lost the benefits imparted by the cured phenolic matrix. By the mid-1940s, the particle-size distribution had been optimised, the powder component reflecting this in its new designation of Formvar 15/95E-30/52 Grade. This indicated that the bulk ( > 65%) of the powder fell in the range of 300 to 5001tm (i.e. BS 30 to BS 52mesh). In recent days, the 30/52 requirement has risen to > 80%. In these early days, the raw powder was worked inhouse, at Duxford--i.e. the unsieved powder was ground and sieved or even dissolved and reprecipitated to obtain the maximum yields possible within the desired particle-size range. Unfortunately, no records were kept indicating which company received which powder. From the mid-1950s, however, all material supplied by Shawinigan/Monsanto was sieved, prior to shipment, to produce the 30/52 grade; the grinding process was abandoned and only reinstated in the late 1980s. It should be noted, though, that SIVA (an Italian-based supplier of PVF resins), see below, always used a grinding process in the production of their 30/52 grade. Once again, it was de Havilland who went their own way. They insisted, against all the advice from George Newell, on using a significantly larger particle size--in the belief, it is thought, that this enhanced the peel performance of the adhesive. This material, known as Powder D, had a particle-size distribution of 1670 to 2000/tm (BS 8 to BS 10mesh). The visual differences in the two can be clearly seen by comparing Figure 7 (30/ 52 grade material) and Figure 22 (8/10 grade).
Figure 19 Schematic of a typical bonded stringer
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Figure 21 A fully-bonded F27 wing
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The history of Redux: J.A. Bishopp
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Figure 22
A beam assembly using Redux Liquid E and Powder D
Some people believed that the particle topographies of the two materials were also different but this is now thought to be unlikely: all PVF powders precipitated after solution polymerisation have a similar, rough structure. It is only when a high degree of grinding energy is applied to the particle that a surface smoothing takes place (Figure 23). This would not have happened in the production of these two different grades of polymer from the same source. This smooth finish was, however, prevalent in the SIVA product and was, indeed, thought to explain some of the latter-day variance in properties. A graphical representation of the various particlesize distributions is given in Figure 24. In all other aspects, the two versions were chemically identical
(Table 2). Table 2 also gives the comparison of other powders used in the Redux process. All were qualified, to DTD 775, as being chemically equivalent to the original Formvar material. Their history of usage can be summarised as follows23:
Table 2 Comparison of the properties of various poly(vinyl formal) resins
• Shawinigan/Monsanto Formvar 15/95E used exclusively from 1942 to 1969; • SIVA H.30 powder used as 'second source', alongside Formvar, from 1969 to 1970; • SIVA H.30 powder used exclusively from early 1971 to 1986 for aerospace work (many other powder combinations were used by the ski industry--but that's another story!); • Formvar powder available again in 1986 and used alongside SIVA material; • Monsanto withdraw product in 1988, stocks run out in 1991; • use of SIVA, again exclusively, until product is withdrawn in 1995; and • Chisso's Vinylec EC powder (ex Japan) qualified in 1995 and is now used exclusively for Redux 775 products. REDUX FILM: THE FINAL DEVELOPMENT? During 1953, experiments were carried out at Duxford to convert the Liquid/Powder system into a film adhesive. In essence a so-called half-web was produced where a film of the resole (Redux K6/ Redux 775 but never Redux Liquid E) was coated onto polyethylene film, powder was applied to the phenolic coating and the excess was allowed to drop off leaving a film which, by natural pick-up of powder, had the required liquid-to-powder ratio. Two half-webs were then consolidated together--powder face to powder face--to produce the Redux Film (Figure 25). The first prototype batch was produced on 31st May 1954 and over the next 10 years the full product range was launched. Needless to say, this range has been severely rationalised over the last 15 years so that only Redux 775 Film and Redux Film C remain. To ensure that the correct liquid-to-powder ratio was attained, the various films in the range needed special grades of the Formvar powder. The lightest film required the socalled 'Fines' or <150/~m (
Property
F o r m v a r 15/ 95E
H.30
Vinylec® EC
Weightaverage molecular weight Poly(vinyl alcohol) content (%) Poly(vinyl acetate) content (%) Poly(vinyl formal) content (%) Free acetic acid (%) Water content
24 000 to 40 000
20 000 to 50 000
not quoted
5.0 to 6.5
5.0 to 7.0
5.0 to 6.5
Film
9.5 to 13.0
9.5 to 13.0
9.5 to 13.0
> 82
> 80
> 81
< 0.25
< 0.3
< 0.3
< 1.0
< 0.5
<3
Solution viscositya (mPa s)
37 to 53
25 to 50
30 to 51
Redux Redux Redux Redux Redux MR Redux Redux Redux MRN Redux LRN Redux
(%)
Table 3 The Redux 775 Film range Areal weight (g m -2)
Carrier
775 775 M 775 L 775 R 775
370 245 156 370 245
30/52 40G Fines 30/52 40G
None None None Woven glass Woven glass
775 L R 775 R N 775
156 370 245
Fines 30/52 40G
Woven glass Woven Nylon Woven Nylon
775
156
Fines
Woven Nylon
C range
a Solution: 5 g of polymer made up to 100 ml with ethylene dichloride
298
Powder
INT. J. ADHESION AND ADHESIVES Volume 17 Number 4 1997
The same range as above but not released to aerospace specifications
The history of Redux: J.A. Bishopp
Figure 23
Electron micrographs of PVF powder showing the normal rough finish and the smooth surface following grinding
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Industry, in general, used considerable amounts of film for such varied applications as transporter belting (Sandvik), flooring and computer doors. Although the product line has long been rationalised, Redux 775 Film has been used on a continuous basis since the 1960s, particularly by the European aerospace industry. The following examples suffice: BAC 1-ll
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Particle-size distributionsfor criticalPVF powders
E
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The industry's change, from the liquid-and-powder system to the more user-friendly film version, did not take place overnight. Re-qualification in the aerospace industry is not a speedy affair and is never undertaken lightly. Indeed, the change at Fokker and at Short Brothers, Belfast, did not take place until 1995! However, the Handley Page Herald, Hawker Siddley 125, Armstrong Whitworth Argosy, Bristol Britannia, Sud Aviation Alouette and SAAB's Lansen and Draken aircraft all used the film version in their construction and, in 1963, de Havilland replaced Redux Liquid E and Powder D by Redux 775 Film in the Comet 4 and Nimrod aircraft. Further, without Redux 775 Film MRN, the strong, lightweight, large-area box construction of the Westland/Saunders Roe SRN4 hovercraft would not have been possible. Mintex and Melbourne Buses (Australia) were unique in switching from the specialised brake-bonding adhesives to Redux 775 Film in the production of drum brakes. With one notable exception, the automotive industry were again reluctant to commit to adhesive bonding. The main beams of Donald Campbell's world-speed record car, The Bluebird C.N.7 (Figure 26), were bonded sandwich structures 24 and undoubtedly saved his life when he crashed during the land-speed attempt on the salt flats in Utah, USA.
.....
TURRET ASSEMBLY
CARRIER PAYOFF (b) UNCONSOLIDATED PRODUCT ROLL
UNCONSOLIDATED PRODUCT ROLL
""
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CONSOLIDATORROLLS
Figure 25
Schematic of the current Redux Film machine
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The history of Redux: J.A. Bishopp FINAL THOUGHTS It is appropriate that Norman de Bruyne lived to see not only industry's general acceptance of structural adhesive bonding, which he pioneered with the invention of Redux, for joining an enormously varied range of loadbearing components, but' also saw the acceptance of Redux as the world benchmark against which all others are measured. It is interesting food for thought as to where Redux and Redux bonding would be today, on a world-wide basis, if the de Havilland Comets G-ALYP and GALYY had not crashed. Although Redux was completely exonerated by the court of inquiry--the cause being attributed to metal fatigue failure of the fuselage which resulted in explosive decompression--it took until 1955 to do so; G-ALYP crashed on 10th January 1951. By this time nitrile-phenolic and, particularly, epoxy-based adhesives had begun to gain a strong foothold for structural bonding and the expected expansion of the Redux process to the USA never took place. In Europe, however, major aeroplane constructors still specify Redux bonding for their new aircraft, to take advantage of its exceptional durability and its 55 year, proven track record.
ACKNOWLEDGEM ENTS The author is indebted to Norman de Bruyne's book 'My Life' for confirmation of the critical dates during the early years. Thanks are also due to Dr Peter Stark, former Research Manager at Ciba Polymers, for supplying copies of the crucial pages from Norman de Bruyne's laboratory notebooks. This enabled, once and for all, the confusion as to the date on which Redux was invented to be cleared up. Thanks are also extended to Andy Bush, now of The Welding Institute, Abington, Cambridge, for clarifying the situation with respect to the use of Redux K6/ Redux 775 Liquid and Redux Liquid E.
REFERENCES Figure 26 Preparation of one of the bonded main beams for The Bluebird
and Trident, BAe 146 and the Fokker 50 and 100 aircraft. Finally, to return to the very first batch of Redux Film which was produced. A small sample has been preserved and tested over the years to determine the effect of ageing on lap-shear performance. Figure 27 shows the results of these tests up to 19th February 1991. A further, and ultimate, bonded panel will be prepared on 31st May 2004 when the film sample will be exactly 50 years old: the indications are that the shear strength will still be above 30MPa.
300
1 2 3 4 5 6 7 8 9 10 11 12 13 14
de Bruyne, N.A. 'My Life', Midsummer Books, 1996 Aero Research Technical Note, No. 34, October 1945 Aircraft Production July 1945, p. 323 RAE Report, MT 56442, 1940 de Bruyne, N.A. 'My Life', Midsummer Books, 1996, p. 94 Aero Research Technical Note, No. 21, September 1942 de Bruyne, N.A. Laboratory Notebooks for 1941-1942 Fitzhugh, A.F., Lavin, E. and Morrison, G . O . J . Elect. Chem. Soc. 1953 100(8), 351-355 de Bruyne, N.A. 'My Life', Midsummer Books, 1996, p. 167 Monsanto Data Sheet, Formvar and Butvar®--Properties and Uses, MP-3-205-6, p. 31 Aero Research Technical Note, No. 39, March 1946 Process for Cleaning Aluminium and Aluminium Alloy Plating Prior to Painting, DTD 915A, December 1942 (reprinted September 1951) Evans, G.B. 40 years of structural adhesive bonding, CME, March 1985, pp. 23-27 de Bruyne, N.A. Structural adhesives for metal aircraft. Paper given to The Fourth Anglo-American Aeronautical Conference
INT. J, ADHESION AND ADHESIVES Volume 17 Number 4 1997
The history of Redux: J.A. Bishopp 35 3O 25
~3 ¢ql
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5 0 May-54
I Nov-59
I May-65
I Nov-70
b Apr-76
I Oct-81
I Apr-87
Sep-92
Date of Test Figure 27
15
16 17
Performance of Redux 775 Film Batch 1 since manufacture
1953, published by The Royal Aeronautical Society, London, UK Eickner, H.W. and Schowalter, W.E. A Study of Methods for Preparing Clad 24S-T3 Aluminium Alloy Sheet Surfaces for Adhesive Bonding, Report 1813, Forest Products Laboratory, Wisconsin, USA, May 1950 de Bruyne, N.A. Redux in Aircraft, Aero Research Limited, January 1953 Schliekelmann, R.J. Production tools for Redux. In 'Proceedings of the Conference Bonded Aircraft Structures', 1957, published by Bonded Structures Limited, Cambridge, UK
18 19 20 21 22 23 24
de Bruyne, N.A. The Strength of Glued Joints, Aircraft Engineering, Workshop and Production Section, April 1944, pp. 115 118 Hexcel Composites, Redux Bonding Technology, February 1997, pp 18 20 Aero Research Technical Note, No. 203, November 1959 Aero Research Technical Note, No. 110, February 1952 Ciba-Geigy Technical Note, No. 3/1977 Bishopp, J.A. Hexcel Composites Internal Report on the History of Redux, 1995 Aero Research Technical Note, No. 212, August 1960
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