Cured at atmospheric pressure

Zinc oxide is a common activator in mbber formulations. It reacts during vulcanization with most accelerators to form the highly active zinc salt. A preceding reaction with stearic acid forms the hydrocarbon-soluble zinc stearate and Hberates water before the onset of cross-linking (6). In cures at atmospheric pressure, such as continuous extmsions, the prereacted zinc stearate can be used to avoid the evolution of water that would otherwise lead to undesirable porosity. In these appHcations, calcium oxide is also added as a desiccant to remove water from all sources. 

The authors of Ref. [9] used the cross-linked epoxy polymers based on diglycidyl ether of bisphenol A (ED-22) of anhydride curing (EP-2) [11]. Two series of EP-2 are used - one of them was cured at atmospheric pressure (EP-2-200) and another at hydrostatic pressure that allowed to obtain 10 samples of EP-2, differing by cross-linked networks topology [11]. 

AEM polymers, both terpolymers and copolymers, must be cured under pressure to avoid sponging. Pressures of 413.7 kPa and higher, preferably higher, are recommended for curing Vamac compounds without sponging. High viscosity compounds with fast cure systems can be molded at the lower end of the pressure range. If compression set is not a consideration, the viscosity of a terpolymer-based compound can be increased sufficiently with ionic crosslinks to be cured at atmospheric pressure [3]. 

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. 

The process interaction in cast plastic products is mainly involved with the curing processes and with mold filling problems. Voids and porous sections are a frequent problem with castings because the mold filling is done at atmospheric pressure, or low pressure, and if the product has thin sections to fill, the flow may be a problem. 

No one curing process posesses all the virtues and those which may appear to be most desirable may be rejected on grounds of initial cost and maintenance of the equipment. For example even an autoclave curable rubber lined vessel can be cured in open steam at atmospheric pressure and this method can be adopted while the autoclave is down for repairs and maintenance. 

Open steam cure - A method of vulcanization in which the steam is in direct contact with the product being vulcanized at atmospheric pressure. 

The polyester resin used in this study, MR 13006 (Aristech Corporation), was supplied as a 60-wt% solution in styrene monomer. The epoxy resin, a digly-cidyl ether of bisphenol A (Epon 828), was obtained from Shell Chemical Company. The reactive liquid rubber, an amino-terminated butadiene-acrylonitrile copolymer (ATBN 1300 x 16), was provided by the BFGoodrich Company. The resin was mixed with additional styrene monomer to maintain the ratio of reactive unsaturation in the polyester-to-styrene monomer at 1 to 3. We added 1.5 wt% of tert-butylperbenzoate initiator to the solution, which we then degassed under vacuum. The mixture was poured between vertical, Teflon-coated, aluminum plates and cured under atmospheric pressure at 100 °C. In the modified compositions, the rubber was first dissolved in the styrene monomer, and then all the other components were added and the solution cured as described. In all the compositions, the ratio of the amine functions with respect to the epoxy functions was kept at 1 to ensure complete cure of the epoxy. 

Cure of the laminate takes place at atmospheric pressure, which helps to make the process simple and inexpensive. However, a high level of fibre content is not possible and hence high mechanical performance properties are not achievable. This does not mean they are unacceptable it simply results in thicker laminates than those from processes which can achieve high fibre content. The extra material cost is balanced against low capital cost, etc. 

An alternative plasma treatment technique is the glow discharge method that can be done at atmospheric pressure. Fluoroplastic films were treated by glow discharge in helium atmosphere. Strips of the treated and untreated films were bonded to 0.2-mm-thick aluminum foils using a urefliane adhesive cured at 100 °C for 15 min. The treatment conditions and bond strengths of the samples are given in Table 5.19. 

Network structure and reaction mechanisms in high pressure vulcanisation (HPV) and peroxide vulcanisation of BR was studied by 13C solid-state NMR [43]. Different samples of polybutadiene (51% trans, 38% cis, and 11 % vinyl) were peroxide cured with dicumyl peroxide on a silica carrier and by the HPV conditions of 250 °C and 293 MPa. The 13C NMR spectra from peroxide and HPV cures were compared to a control samples heated to 250 °C for 6 minutes under atmospheric pressure. Although no new isolated strong peaks were detected in either the peroxide or HPV vulcanisations, small increases in both spectra were observed at 29.5, 36.0, 46.5, and 48.0 ppm. These peaks compare favourably with calculated shifts from structures that arise from main chain radical addition to the pendent vinyl groups. These assignments are further reinforced by the observation that the vinyl carbon concentration is substantially reduced during vulcanisation in both peroxide and HPV curing. Two peaks at 39.5 and 42.5 ppm appear only in the peroxide spectrum. Cis-trans isomerisation was absent in both cures. 

Thin-fllm samples of specific Isothermal cure histories were prepared for dynamic tensile testing on the Rheovibron by compression molding 0.6 to 0.7 mm thick samples at a pressure of 14 MPa. The samples were air quenched after various times at constant cure temperature. The thin-fllms were then mounted In the Rheovibron and the temperature Increased from room temperature to approximately 31 0 C at 2 C/m1n In a dry nitrogen atmosphere. 

Fig. 34.15 shows the thermal expansion of the RT-cured adhesives under several pressures measured using the bellows vessel. The adhesives expanded greatly, even though they had been cured. The maximum volume expansion under atmospheric pressure was about four-fold at 90 °C. Above 100 °C, it seems that volume increased again. However, the volume expansion was caused by leaked gas it was confirmed that the gas leaked from the specimen by thermodilatometry with the glass syringe (Fig. 34.4). Since the microcapsules in the adhesive were expanded about eight-fold, only 1-2 MPa of hydrostatic pressure was apphed from the microcapsules to the matrix resin, as shown by the intersection in Fig. 34.14.

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