Cure Cycles

During the vulcanization, the volatile species formed are by-products of the peroxide. Typical cure cycles are 3—8 min at 115—170°C, depending on the choice of peroxide. With most fluorosihcones (as well as other fluoroelastomers), a postcure of 4—24 h at 150—200°C is recommended to maximize long-term aging properties. This post-cure completes reactions of the side groups and results in an increased tensile strength, a higher cross-link density, and much lower compression set. 

The type of varnish used in the process depends on the kraft paper manufacturer and basis weight of the papers the machine, temperature, and control (scraper bars, squeeze roUs) used the method of cutting the paper to size the laminate being produced (post-forming or regular) and the press-cure cycle (see Laminated materials, plastic). 

Vulcani2ation is a chemical process for improving an elastomer compound s performance. However, in most cases not all of the desired properties reach their optimum levels simultaneously. One of the mbber compounder s key responsibiHties is to achieve a balance of the most important property requirements by the proper selection of cure system (chemical) and time—temperature cure cycle (physical). 

Resin Cure. Resin cure systems yield carbon—carbon cross-links and, consequendy, thermally stable materials. Butyl mbber vulcanised with resins are used as tire-curing bladders, and have a life of 300—700 curing cycles at steam temperature of 175°C at about 20 m/cycle. 

These three cure systems have in common the need for a two-step cure cycle to generate the best cured properties. The first step is the appHcation of heat and pressure in a mold to shape the article (press cure). The second step is a high temperature oven cycle at atmospheric pressure to obtain the final cured properties. 

Anhydride name CAS Registry Number Melting point or viscosity at 25°C M ol wt Recommended concentration, phr" Curing cycles ASTM deflection temperature, °C... 

Benzoyl peroxide is most commonly used for elevated temperature curing. The peroxide is generally supplied as a paste (-50%) in a liquid such as dimethyl phthalate to reduce explosion hazards and to facilitate mixing. The curing cycle in pressure moulding processes is normally less than five minutes. 

The methylated maleic acid adduct of phthalic anhydride, known as methyl nadic anhydride VI, is somewhat more useful. Heat distortion temperatures as high as 202°C have been quoted whilst cured systems, with bis-phenol epoxides, have very good heat stability as measured by weight loss over a period of time at elevated temperatures. The other advantage of this hardener is that it is a liquid easily incorporated into the resin. About 80 phr are used but curing cycles are rather long. A typical schedule is 16 hours at 120°C and 1 hour at 180°C. 

Fig. I. Silicone contamination, as measured by XPS, as a function of distance from the edge of a silicone tape after a heal treatment to simulate a curing cycle. The dashed line represents the silicone level that would produce delamination. Adapted from Ref. [11).

Bonded patch Flat or curved surface field repair Not suitable for high temperature or critical parts Fast but depends on cure cycle of adhesive Low to medium... 

All of the types of repairs described can be accomplished using electron/X-ray curing and suitable electron-curable adhesive systems. The advantages ol using an electron accelerator are faster curing cycles, short turn-around time, and higher-temperature-resistant bonds, cured at ambient temperatures. 

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. 

Inspection. After the final adhesive cure cycle, the bonded assembly is removed from the tool and inspected. The first and most cursory inspection is a simple visual check. Major problems such as crushed core and significantly mis-located details can be seen with the unaided eye. A lack of continuous adhesive squeeze-out ( flash or spew ) around the periphery of details can be indicative of insufficient adhesive and subsequent voids. 

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. 

For a typical assembly, first part qualification begins with a rigorous dimensional check and painstaking prefit of all details on the bond tool. The assembly details are placed on the tool without adhesive, close contact between bond surfaces is verified and any detail or tool interference is corrected prior to proceeding. This is followed by fabrication of a verification film , or a simulated bond cure cycle of the assembly to allow measurement of the adhesive bondline thickness. 

Simply bonding a stiffener to a panel with adhesive is certainly a very feasible and natural procedure with typical composite structure construction. We have not discussed any procedure for joining parts except co-curing. Alternatively, to use film adhesive for bonding parts together, we simply cut a sheet or film of adhesive to the proper size, place it between the two parts that we wish to bond together, and then go through a cure cycle that causes the adhesive to adhere to both the stiffener and to the panel itself. We can also mechanically fasten any stiffener we like to a panel. 

---