Uncured thermosets

Thermoplastic polymers can be heated and cooled reversibly with no change to their chemical structure. Thermosets are processed or cured by a chemical reaction which is irreversible they can be softened by heating but do not return to their uncured state. The polymer type will dictate whether the compound is completely amorphous or partly crystalline at the operating temperature, and its intrinsic resistance to chemicals, mechanical stress and electrical stress. Degradation of the basic polymer, and, in particular, rupture of the main polymer chain or backbone, is the principal cause of reduction of tensile strength. 

The majority of composite fabrication processes are carried out with thermosetting resins. One must therefore keep track of both the cured and uncured resin. In what follows, first the overall mass balance equation for the resin phase will be developed, then a balance equation for the cured portion of the resin will be presented. 

As in all thermosetting polyimides, the diamine and the tetracarboxylic dianhydride employed to build the backbone can be varied. Alteration of the diamine, tetraacid or both, allow the modification of the polyimide s melting point and solubility. Of interest to the end user is the influence of chemical structure on the melting transition of the prepolymer and the Tg of the fully cured product. Lowering the uncured Tg means increasing flow and, in most... 

In Table 1, the moisture uptake of cured and uncured Navy P3-2300-PE resin after 24 hours of immersion is compared to a number of other high-temperature polymer resins. The moisture uptake of the Navy P3 oligomer is nearly identical to that of the commercial P3 thermoplastic. Other commercial thermoplastics, such as poly(ether ether ketone), with very similar chemical compositions, exhibit similarly low levels of moisture uptake. On the other hand, the Navy P3 resins absorbs about 85% less water than the commercial polyimide Kapton HN. Since thermosetting phenyl ethynyl end-capped polyimides have moisture uptake characteristics that are similar to Kapton HN, with around 3% weight gain on exposure to 95% relative humidity (77), void-free composites based on Navy P3 resins should exhibit greatly reduced moisture uptake compared to those based on thermosetting polyimides. 

Wet layup consists of placing a layer of dry reinforcement inside a mold and then applying an uncured, low-viscosity thermoset resin as shown in Fig. 2. Woven glass fibers are the prevalent reinforcing preform utilized in layup processes, although carbon and aramid fibers are also used to a lesser extent. Typical fiber volume fraction of composites manufactured via wet layup range between 30% and 50%. The resin can be poured, sprayed, or brushed on top of the preform layer either by hand or by machine. The fiber preform layer is rolled on or pressed after the application of resin to evenly distribute the resin and remove air pockets. Resin is applied on top after each layer of fiber mats is properly placed. This process is repeated until the desired thickness is reached. To provide a smooth surface finish on the mold side, a thin layer of mold release is often applied prior to starting the layup. Thereafter, pressure and heat are applied to allow the composite to cure. Pressure can be applied... 

Ben Abdelouahab J., A. El Bouardi, R. Granger, and J.M. Vergnaud. 2001. Repairing broken thermoset pieces using diffusional pre-treatment and cure with uncured resin. Polym. Polym. Composites. 9(8) 515-22. 

In thermosetting resins the reactive groups of the molecules from cross-Knks between the molecules during the fabrication process. The cross-linked or cured material cannot be softened by heating. Thermoset materials are usually supplied as a partially polymerized molding compound or as a liquid monomer-polymer misture. In this uncured condition they can be shaped with or without pressure and polymerized to the cured state with chemicals or heat. 

There is also the frequently-used term resin. Resins are synthetic or natural polymers that are liquid and sticky at room temperature (meaning, uncured in the case of the thermoset or of low molecular weight if thermoplastic). 

Used to describe an intermediate stage of reaction for a thermoset where the material softens when heated, and swells somewhat in the presence of certain liquids, but may not completely fuse or dissolve. Thermoset resins are usually supplied in this uncured state. 

This is another version of cold forming by utilizing a thermoformed plastic skin too impart an excellent surface and other characteristics (for weather resistance, etc.) to a cold-molded thermoset RP. For example, a TP sheet is placed in a matched mold cavity with an RP uncured material placed against the sheet. The mold is closed and the fast, room temperature curing resin system hardens. The finished product has a smooth TP-formed sheet backed-up with RP. 

CLASS Thermoset polymers, after curing (the uncured base resins are thermoplastic)... 

Fig, 7. Dynamic relative modulus (DMA) for the uncured (thermoplastic), curing, and fully cured (thermoset) gel coat. 

Synonyms Condensation prods., epoxy Epoxides, polymers, epoxy resins Epoxy compds. Ethers, cyclic, epoxides, polymers Plastics, epoxy Polyethers, epoxy resins Classification Polymer Definition A thermosetting resin based on the reactivity of the epoxide group Toxicology LD50 (oral, rat) 2200 mg/kg strong skin irritant in uncured state poison by inhalation moderately toxic by ingestion little or no toxicity in cured state Hazardous Decomp. Prods. Heated to decomp., emits toxic fumes of Cr... 

Figure 2.29. Schematic TXT cure diagram for a thermosetting system. The gelation and vitrification lines divide the liquid, rubbery and glassy state. Tgo, and Tgoo are the glass transition temperatures of the uncured and fully-cured resin, respectively gei Tg is the temperature at which gelation coincides with vitrification.

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