Redux adhesives

Phenolic based adhesives are outstanding in their wetting characteristics and strength. In the unmodified form, however, they tend to be brittle. To overcome this problem, phenolic based adhesives usually contain a rubber modifier to increase the crack resistance of the adhesive. The first modified phenolic contained polyvinyl butyral and was known as the "Redux" adhesive. Many modern phenolic adhesives contain nitrile rubbers (copolymers of acrylonitrile and butadiene) as the modifiers. An example of a phenolic based film adhesive is AF-30 which is listed in Table 1 along with its physical properties. This film... 

The main application of poly (vinyl formal) is as a wire enamel in conjunction with a phenolic resin. For this purpose, polymers with low hydroxyl (5-6%) and acetate (9.5-13%) content are used. Similar grades are used in structural adhesive (e.g. Redux) which are also used in conjunction with phenolic resin. Poly(vinyl formal) finds some use as a can coating and with wash primers. Injection mouldings have no commercial significance since they have no features justifying their use at current commercial prices. 

PVF-P adhesives have been used at de Havilland in Hatfield, England since 1936 for the manufacture of air screws [199]. Aero Research Ltd. of Duxford, England, which became CIBA (A.R.L.), originally developed them ([200], pp. 76-83). Currently, the Redux business and trademark belongs to the Hexcel Composites Ltd. and is applied to the original polyvinyl-phenolic alloys as well as epoxies and bismaleimide adhesives that give structural performance at even higher temperatures [201]. 

One of the first applications of the Redux process was in construction of structural aluminum parts for the de Havilland bomber Sea Hornet produced during World War II ([198], pp. 80-81 [202,203]). In the original fonn, it consisted of a two-part adhesive including a low viscosity liquid phenolic known... 

Bishopp, J.A., A brief history of the redux bonding process, structural adhesives in engineering V, 5th Int. Structural Adhesives in Engineering Conference (SAE-V). Bristol, UK, 1998. 

De Bruyne himself was undoubtedly no small factor in the spread of adhesive bonding through the European aerospace community. In addition to co-inventing the Redux family of adhesives, he was involved in the conceptual development of the Mosquito and numerous other aircraft and sponsored (and taught) a series of educational sessions on the benefits of structural bonding during the post-war years. 

Allen and Alsalim 22 compared the effect of various pretreatments of stainless steel (martensitic structure) on the torsional shear strength of napkin ring joints formed with an epoxy adhesive (Redux 319 (Bonded Structures Ltd.)). They concluded that... 

The bonded assemblies were produced using two adhesives. The first, which will be discussed in more detail here, is the matrix epoxy resin employed either to bond the composite adherends (in the case of lap shear and L specimens) or to overlaminate onto a balsa sandwich or monolithic composite base in the case of top hat specimens. The second is a more ductile epoxy resin (Redux 420), used to bond the composite adherends. Figure 1... 

When the aldehyde in this reaction is formaldehyde, the product is poly(vinyl formal). This polymer is, however, made directly from poly(vinyl acetate) and formaldehyde without separating the alcohol. The product with low hydroxyl (5-6%) and acetate (9.5—13%) content (the balance being formal) is used in wire enamel and in structural adhesives (e.g., Redux). In both applications the polymer is used in conjunction with phenolic resins and is heat cured. 

Phenolic resoles provided the backbone chemistry for the first structural adhesives for bonding metal substrates these were the Redux (/Jesearch at Dnxford) range of adhesives. They are invariably supplied as solutions, usually in industrial methylated spirits or ethanol, which effectively precludes their use in hot melt adhesive formulations. 

Most phenoUc resole adhesives are supplied in liquid form where the adhesive is dissolved in snitable solvent(s). However, there are some exceptions where the adhesive can be supplied as a readily handleable film. Notable examples are Redux 775 Film (resole/PVF) aud the 3 Ms range, which includes Scotch-Weld AF 6, AF 10, AF 31, AF 32, and so on (resole/NBR). 

Application and utilisation of these film adhesives Structural thermosetting film adhesives The film is cut to the desired size and laid on one of the substrates (refer to Fig. 92). Parts are assembled and hot pressed in an autoclave or a hot press, for instance for a one component epoxy adhesive for 30 min at 120°C, under a pressure of 100-300 kN/m for REDUX 312, or 60 min at 170°C with REDUX 308 or REDUX 322. 

Although Redux is still in use today, it has largely been displaced by adhesives supplied in film form. The advantages of film adhesives over the Redux system are twofold (1) they are supplied at exactly the desired weight, and (2) the formulation is precisely controlled (i.e., the ratio of phenolic resin to vinyl resin). As a consequence the aircraft manufacturer is less dependent on the skill of the individual applying the adhesive. 

The first of this new generation of structural adhesives, developed in the UK by Norman de Bruyne and George Newell of Aero Research Limited, Duxford, Cambridge, was Redux (/fesearch at Durford), which was based on a formulated P/F resin and was the first synthetie adhesive for bonding structural components construeted from composite and metaUie materials. 

It was the seareh for a suitable adhesive to bond Gordon Aerolite , a structural composite developed in 1936 and comprising a phenolic resin matrix reinforced with flax fibres, which led to the development of Redux and the Redux Process. 

Figure 15 Application of Redux liquid and powder adhesive to an airframe component.

It is no small example of the trust that the aircraft manufacturers had in adhesive bonding that bonded structural components using Redux were in operational aircraft within 2 years of the product s development. 

It was the former, however, which, initially, proved to be of great interest to the aircraft industry. The essence of an idea of de Bruyne was worked on by Chance Vought, in the United States of America. They used Redux 775 Liquid and Powder adhesive to bond thin aluminium skins to end-grain balsa wood sheets in order to produce sandwich panels. These panels were called Metalite (Fig. 29). 

Although many non-structural adhesives ean be solvent or water based, in aerospace applications, this format is usually the preserve of the surface protection and corrosion inhibiting primers. However, as indicated, there are exceptions and these are often those adhesives which are based on phenolic resoles (for example, Redux 775 Liquid) or on polyimides (for example, PMR -2). 

Film adhesives Film adhesives are supplied as foils and can be cut to the shape and size of the area to be bonded. The matrices, therefore, are solid at room temperature but, critically, will liquefy but do not cross-link on moderate heating. Thus, on heating from ambient to cure temperature they will first melt, then flow and displace the air in the bond and, in so doing, wet the substrates to be bonded and finally cure to an infusible solid. This can be readily seen by examining the dynamic viscosity traces for three typical film adhesives Redux 312, Redux 319 and Redux 322 (Fig. 44). 

Table 4 Chemistries employed in the Redux range of stmctural adhesives.

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