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The Use of Thermal Analysis in Polymer Characterization
Published in Nicholas P. Cheremisinoff, Elastomer Technology Handbook, 2020
Gelation determinations are important since beyond the gel point the thermosetting system will not flow and processing becomes difficult. Weak weld lines, trapped gases, and cosmetic defects from prematurely hardened resin in the shot can all result. DSC can measure degree of conversion but supplemental techniques are required to evaluate the physical changes that occur at gelation. Schneider et al.78 report on experiments that combine DSC and torsional braid analysis (TBA) to establish the degree of conversion at the gel point. More work on gelation and cure determinations by DMA will be reviewed in Section VI. The time to gelation, tgel, has been shown by this author to be a key factor in accounting for cosmetic defects. Two supposedly equivalent phenolic materials were utilized in long production runs; one resin produced a markedly higher percentage of rejected parts due to hard spots, localized areas on the part surface that reached gelation before the cavity had filled completely. Figure 25 shows a conversion vs. time plot for the two materials based on an evaluation of cure kinetics. This shows a faster cure and therefore a shorter tgel for the more troublesome material.
Sol−Gel Processing
Published in Mohamed N. Rahaman, Ceramic Processing, 2017
The clusters grow until they begin to impinge on one another and gelation occurs by the linking of these clusters by a percolation process. Near the gel point, bonds form at random between the nearby clusters, linking them into a network. Structural changes that occur as the gel point is approached are illustrated schematically in Figure 15.14. The gel point corresponds to the percolation threshold, when a single cluster (called the spanning cluster) appears that extends throughout the sol. The spanning cluster coexists with the sol, containing many smaller clusters that gradually become attached to the network. It reaches across the vessel that contains it, so the sol does not pour when the vessel is tipped. By creating a continuous solid network, the spanning cluster is responsible for an abrupt rise in the viscosity and the appearance of an elastic response to stress. Figure 15.15 shows changes in the viscosity for acid-catalyzed and base-catalyzed TEOS systems described in Table 15.5. The sudden increase in viscosity is generally used to identify the gel point in a crude way [7]. The gel point is often defined as the time at which the viscosity is observed to increase rapidly or the time to reach a given viscosity (e.g., 1000 Pa.s).
Linear and Non-Linear Rheological Properties of Foods
Published in Dennis R. Heldman, Daryl B. Lund, Cristina M. Sabliov, Handbook of Food Engineering, 2018
Ozlem C. Duvarci, Gamze Yazar, Hulya Dogan, Jozef L. Kokini
Crosslinking polymers undergo phase transitions from liquid to solid at a critical extent of reaction, which is called gelation. Gel point is defined as the moment at which a polymer/biopolymer system changes from a viscous liquid (sol) to an elastic solid (gel) (Ross-Murphy, 1995a, and 1995b). It can be determined from rheological properties such as steady shear viscosity for the liquid state and equilibrium shear modulus for the solid state (Gunasekaran and Ak, 2000). The polymer is considered to be at the gel point where its steady shear viscosity is infinite and its equilibrium modulus is zero (Winter and Chambon, 1986).
Experimental studies and mathematical modeling of the curing reaction of bioinspired copolymers
Published in Green Chemistry Letters and Reviews, 2018
N. Chen, N. Lee, S. A. Bortolato, D. M. Martino
In light of all these possible applications of the copolymer, the rational design of polymeric materials can be enriched by the development of recursive mathematical models that simulate the photo-induced crosslinking (curing reaction) allowing the prediction of the molecular properties of the polymer as a function of the synthetic conditions. The curing reaction is crucial for the application of virtually any polymeric material, hence it is important to absolutely identify the nature of the reaction (22). Polymer curing is a complex process that includes several steps but generally is evidenced by a rapid increase in molecular weight as the reaction progresses since chains are fused in an infinite molecular weight reticulum. The rapid and irreversible transformation in which the polymeric material passes from a viscous liquid to an elastic gel state, indicating the commencement of the reticulum appearance, is often called a gel point (23).
Rheological characterization of chitosan/starch blends by varying polyols and amylopectin content
Published in Journal of Dispersion Science and Technology, 2019
Marilia M. Horn, Virginia C. Amaro Martins, Ana Maria De Guzzi Plepis
The gelation point is a gradual transition from a viscoelastic liquid to a viscoelastic solid that can be determined by rheology. The gel point is one of the most important physical parameters of the gelation process and can be easily determined from rheological data, since it coincides with the point at which G′ intersects G″.[30]Figure 5 shows a typical curve of the storage modulus and the loss modulus as a function of temperature for CS73GL blend.