Polymers yielding process

Further on, the rightfulness of application of the structural defect concept to polymers yielding process description will be considered. As a rule, previously assumed concepts of defects in polymers were primarily used for the description of this process or even exclusively for this purpose [4—11]. Theoretical shear strength of crystals was first calculated by Frenkel, basing on a simple model of two atoms series, displaced in relation to one another by the shear stress (Fig. 4.1a) [3]. According to this model, critical shear stress Tg is expressed as follows [3] ... 

Let us consider now the results of the concept [44, 45] application to polymers yielding process description. The yielding can be considered as polymer structure loss of its stability in the mechanical stresses field and the yield strain is measure of this process resistance. In Ref. [44], it is indicated that specific lifetime of suprasegmental structures t is coimected with AG as follows ... 

Hence, the stated above results shown that polymer yielding process can be described within the frameworks of the macrothermodynamical model. This is confirmed by the made in Ref. [44] conclusion about thermodynamical factor significance in those cases, when quasiequilibrium achievement is reached by mechanical stresses action. The existence possibility of struc-... 

Beloshenko, V. A., Kozlov, G. V. (1994). The Cluster Model Application for Epoxy Polymers Yielding Process. Mechanika Kompozitnykh Materialov, 30(4), 451 54. Balankin, A. S., Bugrimov, A. L. (1992). The Fractal Theory of Polymers Plasticity. S sokomolek. Soed. A, 34(10), 135-139. 

Kozlov, G. V, Yanovskii, Yu. G., Kamet, Yu. N. (2008). The Generalized Fractal Model of Amorphous Glassy Polymers Yielding Process. Mekhanika Kompozitsiormykh Materialov i Konstruktsii, 14(2), 174-187. 

The criterion (9.6) shows that amorphous glassy polymers yielding process is controlled by cluster structure stability loss (for more details see chapter four) and the condition of transition from shear to crazing can be written as follows [20] ... 

At solid body deformation the heat flow is formed, which is due to deformation. The thermodynamics first law establishes that the internal eneigy change in sample dU is equal to the sum of woik dW, carried out on a sample, and the heat flow dQ into sample (see the Eq. (4.31)). This relation is valid for any deformation, reversible or irreversible. There are two thermo-d5mamically irreversible cases, for which dQ = -dW, uniaxial deformation of Newtonian liquid and ideal elastoplastic deformation. For solid-phase polymers deformation has an essentially different character the ratio QIW is not equal to one and varies within the limits of 0.35 0.75, depending on testing conditions [37]. In other words, for these materials thermodynamically ideal plasticity is not realized. The cause of such effect is thermodynamically nonequilibrium nature or fractality of solid-phase polymers structure. Within the frameworks of fractal analysis it has been shown that this results to polymers yielding process realization not in the entire sample volume, but in its part only. 

Hence, the crosslinked polymers yielding process can be described within the frameworks of synergetics of a deformable body, namely by dissipative structure evolution. The quantitative identification of DS is obtained again within the frameworks of the cluster model [47]. 

The above batch process has undergone numerous refinements to improve yields, processing characteristics, purity, and storage stabiUty, but it remains the standard method of manufacture for these products. Recentiy a continuous process has been reported by Bayer AG (6) wherein the condensation is carried out in an extmder. The by-products are removed in a degassing zone, and the molten polymer, mixed with stabilizers, is subsequendy cracked to yield raw monomer. 

In these cases, the polymer remains processible in the gelled state, because it is in the form of discrete PSA particles dispersed in the reaction medium. However, once the particles are dried, redispersion may be difficult if strong interactions develop between the particle surfaces. Polymerization of the acrylic PSA directly on the substrate, as in the case of UV polymerization, can also yield a covalently crosslinked polymer that does not require any further coating steps [71]. 

Ebdon and coworkers22 "232 have reported telechelic synthesis by a process that involves copolymerizing butadiene or acetylene derivatives to form polymers with internal unsaturation. Ozonolysis of these polymers yields di-end functional polymers. The a,o>dicarboxy1ic acid telechelic was prepared from poly(S-s tot-B) (Scheme 7.19). Precautions were necessary to stop degradation of the PS chains during ozonolysis. 28 The presence of pendant carboxylic acid groups, formed by ozonolysis of 1,2-diene units, was not reported. 

The BP Chemicals polymer cracking process is based at Grangemouth in Scotland and uses mixed plastics as the raw material. The reactor uses a fluidised bed which operates at 500 °C in the absence of air, and under these conditions the plastics crack thermally to yield hydrocarbons. These vaporize and are carried away from the bed with the fluidising gas. Solid impurities such as metals from PVC stabilisers accumulate in the bed or are carried away in the hot gas to be captured by a cyclone further along in the plant. PVC decomposes to HCl and this is neutralized on a solid lime absorbent to yield CaCl2 which is disposed of in landfill. The purified gas is cooled to condense most of the hydrocarbon which can be employed as commercially useful distillate feedstock. The light hydrocarbons which are less easy to condense are compressed, reheated and recycled as fluidising gas. 

A wire crosshead die is used to manufacture wire coatings, which is illustrated in Fig. 11.10. This specialized die turns the melt flow 90° before it leaves the die. At this turn, the wire to be coated enters the melt stream and exits the die co-axially with the polymer. This process yields a seamless polymer coating around the wire. 

The polymerization proceeds under photo- [49,50],X-ray [51], and y-ray [52] irradiation in the dark in vacuo, in air, or even in water or organic solvent as the dispersant (nonsolvent) for the crystals, similar to the solid-state polymerization of diacetylene compounds [ 12]. The process of topochemical polymerization of 1,3-diene monomers is also independent of the environment surrounding the crystals. Recently, the thermally induced topochemical polymerization of several monomers with a high decomposition and melting point was confirmed [53]. The polymer yield increases as the reaction temperature increases during the thermal polymerization. IR and NMR spectroscopies certified that the polymers obtained from the thermally induced polymerization in the dark have a stereoregular repeating structure identical to those of the photopolymers produced by UV or y-ray irradiation. 

The advantages offered by this process are (1) a one-step synthesis to poly(methylchlorosilanes), (2) low cost dimers and (3) fiber processing that leads to phase pure SiC fibers. The primary drawbacks to the Dow-Corning process are (1) a low polymer yield (15-20%), (2) a multistep process and (3) the high cost of LiAUTj reduction of the chlorinated polymer. 

This polymer precursor (1) requires relatively inexpensive starting materials, (2) is quite stable in air, (3) offers good processability for polymer infiltration processing of composites, (4) provides excellent SiC ceramic yields and (5) high purity with controllable microstructures. However, one important drawback is the use of costly LiAlHzt. This polymer is now available commercially (Starfire Inc., NY). 

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