Thermosetting precursors materials

The thermoset precursor material must not be heated for long in the barrel, or it will cure prematurely and set solid. Barrels are therefore quite short L/D in the range 12 to 14). 

Carbon fibers can be produced from a wide variety of precursors in the range from natural materials to various thermoplastic and thermosetting precursors Materials, such as Polyacrylonitrile (PAN), mesophase pitch, petroleum, coal pitches, phenolic resins, polyvinylidene chloride (PVDC), rayon (viscose), etc. [42-43], About 90% of world s total carbon fiber productions are polyacrylonitrile (PAN)-based. To make carbon fibers from PAN precursor, PAN-based fibers are generally subjected to four pyrolysis processes, namely oxidation stabilization, carbonization and graphitiza-tion or activation they will be explained in following sections later [43]. 

Core-shell polymers were commercially introduced as impact modifiers for poly(vinyl chloride) PVC, in the 1960s. They are produced by a two-stage latex emulsion polymerization technique (Cruz-Ramos, 2000). The core is a graftable elastomeric material, usually crosslinked, that is insoluble in the thermoset precursors. Typical elastomers used for these purposes are crosslinked poly(butadiene), random copolymers of styrene and butadiene,... 

The cure cycle is the temperature vs time schedule used to polymerize the thermoset precursors. The selection of an adequate cure cycle has several purposes. What is desired is to obtain the final part without strains exceeding design tolerances, with a uniform conversion (usually close to the maximum possible conversion), without degradation produced by the high temperatures attained during the cure, with convenient morphologies (in the case of heterogeneous materials), and all this, must be achieved in the minimum possible time for economic reasons. 

The matrix allows the necessary positioning of the fibers, transfers the load to the fibers and distributes the stress among them, and is also responsible for protecting the reinforcement from the environment. However, the matrix is often the weakest component of a composite. One important parameter for the material properties is the fiber-matrix interface (or interphase), which guarantees the stress transfer from fiber to fiber via the matrix. The interface/interphase is a finite thin layer with its own (very often unknown) physical and chemical properties that depend on the fiber-matrix combination. Because of the low viscosity of the thermoset precursors, they wet the reinforcement better than a thermoplastic polymer. 

Savings in cost most often accrue from the injection moulding of thermosets (such as polyesters), instead of thermoplastics, because thermoset polymer is usually cheaper than thermoplastic and the cycle times are sometimes shorter (the material cross-links in the mould so that cooling is not necessary before the part is removed). The purpose of the screw is to plasticize and homogenize the precursor material (which may contain short fibres) in preparation for injection into a heated mould in which cure takes place. The basic machine is similar to that described above for thermoplastics, but there are important differeiKes in detail (see Figure 7.40) ... 

Thermoset polymers are those whose precursors are heated to an appropriate temperature for a short time, so that tli wilt flow as a viscous liquid a diemkal aoss-linldiig reaction then causes the liquid to solidify U> form an infu le mass. The precursor materials nu be of low molecular weight some precursors, after mixing, will flow and cross-link at room temperature. 

The literature is abundant in data on the photochemical behavior of thermosetting matrices based composites. Most of the reported data focused on two main issues (a) the UV-cured composites and nanocomposites and (b) the photochemical degradation of these materials upon UV exposure. In the first case, the UV radiation is employed in the synthesis of various materials starting from thermoset precursors included in complex formulations that may also contain— besides fillers, whether fibers or particles— initiators, plasticizers, compatibilizing agents, UV absorbers. 

Glassy, or vitreous, carbon is a black, shiny, dense, brittle material with a vitreous or glasslike appearance (10,11). It is produced by the controUed pyrolysis of thermosetting resins phenol—formaldehyde and polyurethanes are among the most common precursors. Unlike conventional artificial graphites, glassy carbon has no filler material. The Hquid resin itself becomes the binder. 

Analysis of Thermoset Materials, Precursors and Products, Martin J. Forrest, Rapra Technology Ltd. Polymer/Layered Silicate Nanocomposites, Masami Okamoto, Toyota Technological Institute. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

If a large number of branches exist that connect all of the backbone molecules into a three-dimensional network, the material will not flow when heated, and it is considered a thermoset resin. Vulcanized rubber is an example where the sulfur linkages create a three-dimensional network, converting the precursor rubber into a solid thermoset material. Crosslinked backbone chains are shown in Fig. 2.8(e). When extruding many thermoplastics, the polymer can undergo chemical reactions to form small amounts of crosslinked material. Partial crosslinking is a problem with some PE resins that contain residual double bonds that are made using... 

Structural modifications were envisioned early to overcome these limitations. A first improvement was outlined by preparing copolymers, which were soluble in the state of full imidation, mainly poly(ester-imide)s and poly(amide-imide)s [2,4, 5]. As an alternative to these conventional copolymers, addition polyimides were developed in the 1970s as a new class of thermosetting materials. Thus, bismaleimides, bisnadimides, and end-capped thermocurable polyimides were successfully developed and marketed [6,7]. These resins were the precursors of the modern PMR (polymeric monomer reactants) formulations [8]. 

In contrast to gas-phase carbonization, most thermosetting resins, such as phenol-formaldehyde and furfuryl alcohol, and also cellulose can be converted to carbon materials by solid-phase carbonization. When the carbonization of most of these precursors proceeds rapidly, the resultant carbon materials become porous. If the carbonization is performed so slowly that the resultant carbonaceous solids can shrink completely, the so-called glass-like carbons are produced, which contain a large number of closed pores. 

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