Step cure

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. 

In the vulcanisation of thick rubber articles the temperature may be stepped up in stages to ensure adequate heat penetration to the interior before the external surface begins to cure. Such a cure schedule is termed a step cure. Step post cures are used with thick section rubber articles to ensure that volatiles are released gradually without causing damage, such as porosity, due to an over rapid release. 

The standard process cycle for polymer matrix composites is a two-step cure cycle, as seen in Figure 8.1. In such cycles the temperature of the material is increased from room temperature to the first dwell temperature and this temperature is held constant for the first dwell period ( 1 hour). Afterward, the temperature is increased again to the second dwell temperature and held constant for the second dwell period (2-8 hours). After the second dwell, the part is cooled down to room temperature at a constant rate. Because there are two dwell periods, this type of cure cycle is referred to as a two-step cure cycle. The purpose of the first dwell is to allow gases (e.g., entrapped air, water vapor, or volatiles) to escape and to allow the matrix to flow, which leads to compaction of the part. Thus, the viscosity of the matrix must be low during the first dwell. Typical viscosity versus temperature profiles of polymer matrices show that as the temperature is increased, the viscosity of the polymer decreases until a minimum viscosity is reached. As the temperature is increased further, the polymer begins to cure rapidly and the viscosity increases dramatically. The first dwell temperature must be chosen judiciously so that the viscosity of the resin is low while the cure is kept to a minimum. 

Figure 8.4 shows the model predictions and experimental data for degree of cure during the manufacturer s recommended cure (MRC) cycle for IM6/3100. The MRC cycle is a two-step cure with the second dwell at 182°C. Note that most of the reaction occurs during the second dwell period. Full cure is reached during the latter half of the second dwell. 

Mechanical testing of the three-step cure specimens indicated that no sacrifice in properties resulted from the modification of the process cycle. The retainment of mechanical properties (transverse strength and modulus) coupled with the reduction in dimensionless curvature for the three-step cure cycles investigated provides another suitable cure cycle modification for reduction of residual stresses in composite materials. Overall processing time has not been increased beyond that specified in the MRC cycle. Thus, with no increase in process time and comparable mechanical properties, the residual stresses have been reduced by more than 20 percent in comparison to the MRC cycle baseline data. 

Figure 8.20 Three-step cure cycles and MRC cycle for IM6/3100...

Figure 8.21 Dimensionless curvature (tf) for MRC and three-step cure cycles...

NR055X and 703 polyamic resin material were separately coated onto cleaned silicon substrates so as to produce a resin layer v2y thick. Each sample t e was subsequently heated on a hot plate for 10 minutes at 85 C to remove solvent from the films to produce the condensed phase amic acid. Specimens were sequentially step cured as follows 10 minutes at 200°C 30 minutes at 300 0 on a hot plate having a lid in a N flow and 30 minutes at 400 C in a closed tube furnace in a flowing stream of N. (in separate experiments, a flowing stream of forming gas, i.e., 80% N2-20% H2> was used for the 300°C and 400°C cures). ESCA measurements were made after each curing step in order to evaluate differences caused by increasing the temperature of cure. 

Figure I. The C-h spectra for two polyimides (a) PI 5878 after 120°C solvent removal drying cycle (b) PI 5878 after step cures 325° in 2V (c) PI 2525 after 95°C solvent removal drying cycle (d) PI 2525 after step cures to 325°C in N,.

Hexamethylene tetramine is an important chemical. Its most important application is as a source of formaldehyde for crosslinking phenolic molding powders, shell molding resins, and two-step curing resins for chip board. Quatemization of HMTA with an alkyl chloride (R-Cl) gives a family of bactericides (developed by Dow) for use in latex paints and as a dermatitis preventative in water-soluble cutting oil. Hexamine is used in pharmaceutical formulations to combat urinary tract infections and also as an intermediate in the production of chloramphenicol. 

El-ghayoury A, Hofmeier H, de Ruiter B, Schubert US (2003) Combining covalent and noncovalent cross-linking a novel terpolymer for two-step curing applications. Macromolecules 36(11) 3955—3959... 

Steps Cure Cycle Steps Cure Cycle... 

Storm, R. S. 1980. One-step curing method for manufacture of neutron absorbing plates. U.S. Patent 4198322. Stoto, T., L. Zuppiroli, and J. Pelissier. 1985. Absence of defect clusters in electron irradiated boron carbide. Radiat. Effects 90 161-170. 

Moderating the cure conditions, for example, employing a step cure instead of a rapid high-temperature shock cure. 

Polyimides Higher temperature stability compared to epoxies. High ionic purity. Reduced bleedout. Trapped solvent can produce voids under large ICs. Multi-step curing required to volatilize solvent. High-stress materials. May absorb moisture in cured condition. Cannot be B-staged. 

Cure cycles for adhesives should not be interrupted. Step curing should be performed in the same oven without exposing the adhesive to ambient air between steps. Moisture absorption in partially cured adhesives affects the curing process and alters properties. The effects of interrupting the cure cycle and exposing an epoxy encap-sulant to air moisture were shown by Naito and Todd. ... 

The 3BS cured paste has higher solid phase density than that of the pure 3BS phase. Such a high value can be obtained if the paste contains considerable amounts of PbO and Pb. Except for the paste cured at 90 °C for 4 h the pastes of Group III (Table 8.1), produced from the same initial 3BS paste by two-step curing with steam treatment, have densities about 6.50 g cm after curing. The decrease in solid phase density indicates that conversion of 3BS crystals into 4BS ones occurs through incorporation of water in the crystals. There is no other substance in the paste that would reduce the crystal density. 

E. Asmussen, A. Peutzfeldt, Two-step curing influence on conversion and softening of a dental polymer, Dent. Mater. 19 (2003) 466 70. 

J.W.V. van Dijken, A 6-year clinical evaluation Class I poly-acid modified resin com-posite/resin composite laminate and resin composite restorations cured with a two-step curing technique. Dent Mater. 19 (2003) 423-428. 

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