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Manufacturing Techniques
Published in Sumit Sharma, Composite Materials, 2021
In general, the higher the temperature during curing, the shorter the cure time (short of burning the material, of course). Heat is required because (1) some catalysts and/or hardeners do not react below a critical temperature; (2) molecular mobility is necessary for contact of reactive chemical groups; (3) heat drives off volatiles from solvents and water (otherwise, voids occur; note that volatiles will not outgas if pressure is also being applied); and (4) resin flows more easily to obtain a uniform distribution. Pressure is required to consolidate (debulk) the fiber and matrix system and to squeeze out excess resin. A typical curing cycle of temperature vs. time of epoxy resin is shown in Figure 3.40. The timescale is several hours, and the temperature scale is hundreds of °F (also hundreds of °C). The curing cycle starts with a gradual temperature increase under vacuum conditions so that volatiles and water (vapor) can be driven off. Then, the temperature is gradually increased to the maximum curing temperature which is held for a couple of hours to develop a high degree of cross-linking along with pressure application to consolidate the laminae.
Polymeric Materials
Published in Heinz K. Müller, Bernard S. Nau, Fluid Sealing Technology, 2019
Heinz K. Müller, Bernard S. Nau
The cross-links are formed during the curing process and limit the deformation that occurs when the material is under load. The thermal motion of the chain segments between cross-links provides the restoring force when the elastomer is strained. Curing is the chemical reaction that forms the cross-links, which are usually strong covalent bonds, and normally takes place in a metal mold at elevated temperature and pressure. FPM and FFPM, and sometimes other elastomers, also require a post-cure, during which they are heated to yet higher temperature at atmospheric pressure for some hours. This allows reaction gases to diffuse out. Examples of curing agents are organic peroxides (peroxide cure), amines, sulfur (the original vulcanising agent used for natural rubber), and bisphenols (in fluoroelastomers). The density of cross-linking affects the elastomer properties and is largely determined by the amount of curing agent used; properties are also affected by the choice of the curing agent. The nature of the relationship between cross-link density and elastic modulus was demonstrated in the discussion of constitutive models of rubber elasticity.
Polyhexahydrotriazines: Synthesis and Thermal Studies
Published in Didier Rouxel, Sabu Thomas, Nandakumar Kalarikkal, Sajith T. Abdulrahman, Advanced Polymeric Materials, 2022
Nitish Paul Tharakan, J. Dhanalakshmi, C. T. Vijayakumar
A thermosetting polymer also known as a thermoset is a prepolymer material that cures irreversibly. The cure may be induced by heat, generally above 200°C, through a chemical reaction or suitable irradiation. Thermoset materials are usually liquid or malleable prior to curing and designed to be molded into their final form or used as adhesives. Others are solids like that of the molding compound used in semiconductors and integrated circuits. Once hardened, a thermoset resin cannot be reheated and melted to be shaped differently. Thermosetting resin may be contrasted with thermoplastic polymers which are commonly produced in pellets and shaped into their final product form by melting and pressing or injection molding.
Real-time inverse solution of the composites’ cure heat transfer problem under uncertainty
Published in Inverse Problems in Science and Engineering, 2020
K. I. Tifkitsis, A. A. Skordos
The manufacturing of fiber-reinforced thermosetting matrix composites involves several stages such as lay-up, draping, resin impregnation or consolidation and curing. The cure process is a non-linear heat transfer effect in which the thermosetting polymer resin reacts exothermically and is transformed from an oligomeric liquid to a glassy solid. The quality of the final part depends strongly on phenomena taking place during the cure governed by manufacturing process parameters and boundary conditions. The selection of cure process parameters is crucial for eliminating potential induced defects such as undercure or thermal overshoot in thick components.