Autoclave bonding

RTV silicone Very flexible, withstand autoclaving, bond to low surface energy plastics like polyethylene and polypropylene (PP) (one) component versions are slow curing and (two) components require mixing... 

Large, complex shapes, where high bonding pressures (> 100 kPa) are required, have to be either clamped up (Fig. 60) or jigged up on an appropriate tool, utilising vacuum-bag techniques (Fig. 61) and this assembly is then cured in an autoclave. A typical autoclave bonding shop is shown in Fig. 62. 

Figure 62 Autoclave bonding shop railway system to take the tools to the autoclave (courtesy of BAE SYSTEMS, Broughton).

S.2.2.2. Composite adherends. Composite adherends are bonded in both the cured and uncured states. Wherever possible the adhesive and all adherends are cured simultaneously to avoid the added cost of additional autoclave cure cycles. In many cases this is not practical due to part size and complexity. Cured parts can be bonded to uncured parts, which is known as cobonding, and fully cured parts can be bonded together, which is known as secondary bonding. Adhesives for composites are formulated to be compatible with matrix resins in either cured or uncured states. 

The vast majority of metallic bonded assemblies are cured in autoclaves. A small number are cured using mechanical pressure application devices (clamps, presses) or by drawing a vacuum under a bag over the part in an oven. Mechanical pressure is rarely used for a few reasons. For all but completely flat assemblies it... 

Fig. 18 shows a trio of autoclaves. In the foreground is a bond assembly on a wheeled tool ready to be placed in the autoclave. A technician stands at a control console next to a two-level rack used for holding multiple bond tools in the autoclave. 

Fig. 18. Autoclaves and bond tools. Reproduced by permission of the Boeing Company.

First part qualification. First part qualification is a process performed the first time a new bonded assembly is manufactured or the first time a new tool is used to manufacture a bonded assembly. First part qualification provides assurance that all of the aspects that control bond assembly quality, such as the design dimensions, detail part manufacturing techniques, tool dimensions, layup procedures and autoclave cure cycle parameters are correct and will produce a bond assembly that meets the engineering requirements. 

The first type of bonded design for this application was the beaded doubler panel (Fig. 28). This design was fairly successful at addressing the problems with simple riveted structure but had two primary drawbacks. The area under the beads remained a single thickness sheet and was still prone to fatigue. Reducing the unbonded areas under the beads was not a solution because it reduced the overall stiffness of the panel. Secondly, tooling for these panels was complex and not very robust. Autoclave pressure applied to the beaded areas of the doubler would cause them to collapse, so thick frames were fabricated with cutouts for the beads to protect them. A rubber layer bonded to the surface of the frames... 

Platinum and rhodium-platinum and iridium-platinum alloys are frequently employed to line and sheath autoclaves, reactor vessels and tubes, and a wide range of equipment. Linings are generally 0-13 mm to 0- 38 mm thick, and for certain applications co-extruded platinum-lined Inconel or other metal reactor or cooling tubes are fabricated. In such cases the platinum is bonded to the base metal, but in all other instances platinum linings are of the loose type. 

Modern bonding systems usually consist of a primer coat, often with a secondary tie coat, plus a tacky solution to assist in the application of the rubber. The bonding systems currently in use are usually suitable both for autoclave vulcanisation and vulcanisation at 100°C with atmospheric pressure steam or hot water. Ambient vulcanisation bonding systems have to be chemically active at the lower temperatures and are therefore specialist in nature. 

Dimethyiacetamide (DMA) behaves the same way the interaction exothermicity is higher than with DMF. A differential thermal analysis of the 1 1 mixture of DMA/CCI4 shows two exothermic peaks at 91-97 and 97-172°C (maximum at 147°C), respectively. An experiment made in an autoclave shows that the final temperature reaches 450°C and a pressure of 12.8 bar that is bonded to a substantial gas release. If 1 % of iron is present, the first exotherm occurs at 71 C. 

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