Multi-Zone Drying Schemes for Lowering the Residual Solvent Content during Multi-Component Drying of Semicrystalline Polymers

The development of a glassy skin in multicomponent semicrystalline polymer systems limits the diffusion of solvents out of the system and increases residual solvent levels. Based on the results of a mathematical model that we had previously developed, we have proposed a multi-zone drying scheme aimed at lowering the residual solvent levels by taking into account the effect of interactions between the various solvents as predicted by the model. This article focuses on the application of this model to develop optimal drying schemes and to verify the effectiveness of these predictions using experimental techniques. The mathematical model developed previously to study the diffusion of multiple solvents and changes in the crystallinity of semicrystalline polymer systems during drying incorporates many features including Vrentas-Duda diffusion theory, solvent-induced crystallization kinetics, as well as glass transition effects and skinning of the film. The multi-zone drying system was developed by varying the drying temperature in each zone as well as changing the partial pressure of individual solvents during the drying process. The effectiveness of the multi-zone drying schemes predicted by the model was validated experimentally using thermogravimetric methods. The polymer-solvent system chosen was a poly(vinyl alcohol)-water-methanol system. Our experimental data suggested that the multi-zone drying schemes were superior to a single-zone drying system through direct comparison. Further examination of the mathematical model yielded individual solvent profiles and these data reaffirmed our conclusions that a multi-zone drying scheme has the ability to reduce the effect of solvent trapping and thus lower the overall residual solvent content.     

Multi-Zone Drying Schemes for Lowering the Residual Solvent Content during Multi-Component Drying of Semicrystalline Polymers

The development of a glassy skin in multicomponent semicrystalline polymer systems limits the diffusion of solvents out of the system and increases residual solvent levels. Based on the results of a mathematical model that we had previously developed, we have proposed a multi-zone drying scheme aimed at lowering the residual solvent levels by taking into account the effect of interactions between the various solvents as predicted by the model. This article focuses on the application of this model to develop optimal drying schemes and to verify the effectiveness of these predictions using experimental techniques. The mathematical model developed previously to study the diffusion of multiple solvents and changes in the crystallinity of semicrystalline polymer systems during drying incorporates many features including Vrentas-Duda diffusion theory, solvent-induced crystallization kinetics, as well as glass transition effects and skinning of the film. The multi-zone drying system was developed by varying the drying temperature in each zone as well as changing the partial pressure of individual solvents during the drying process. The effectiveness of the multi-zone drying schemes predicted by the model was validated experimentally using thermogravimetric methods. The polymer-solvent system chosen was a poly(vinyl alcohol)-water-methanol system. Our experimental data suggested that the multi-zone drying schemes were superior to a single-zone drying system through direct comparison. Further examination of the mathematical model yielded individual solvent profiles and these data reaffirmed our conclusions that a multi-zone drying scheme has the ability to reduce the effect of solvent trapping and thus lower the overall residual solvent content.     

Mechanistic investigation of drying regimes during solvent removal from poly(vinyl alcohol) films

The mechanism of removal of solvents such as water and methanol from semicrystalline poly(vinyl alcohol) films was investigated using various thermal analysis techniques to obtain a mechanistic understanding of the drying process. Various drying regimes were identified, which correspond to different rate‐controlling steps, and characterized by differences in drying rates and mechanisms. The kinetics of solvent removal was measured gravimetrically using thermogravimetric analysis (TGA). The drying kinetics studies yielded four distinct drying regimes hypothesized to be due to (1) removal of free solvent, (2) elimination of bound solvent, (3) solvent removal during the rubbery‐glassy transition, and (4) drying when the polymer is in the glassy state. The amounts of free and bound water present in the polymer were measured using differential scanning calorimetry and compared with the TGA results, and they were found to verify the proposed hypothesis. The rubbery‐glassy transition observed in TGA results was confirmed using dynamic mechanical analysis. The thickness of the films was also measured as a function of drying time using thermomechanical analysis. The drying rate plots were found to be qualitatively similar to the plots of changes in film thicknesses as functions of drying time. The results of these various techniques were analyzed to propose a comprehensive mechanism of solvent removal from poly(vinyl alcohol) films. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1913–1920, 1999     

The effects of water soluble polymers on paper coating consolidation

The consolidation process of wet paper coating layers has been generally divided into several stages by the definitions of first critical concentration (FCC) and second critical concentration (SCC), however few works have been done on the kinetics of water expelled during drying and its influence on the critical concentrations. In this paper, the effects of three kinds of water soluble polymers, i.e., carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA) and modified coating starch, on the consolidation of model kaolin clay-latex coatings were investigated. On-line water evaporation loss rate, together with the level of FCC and time needed to reach FCC, was characterized. The results showed that water evaporation loss weight increased almost linearly with drying time during the initial wet coating layer consolidation stage. On-line water evaporation rate and the value of FCC decreased with water soluble polymers addition, while the time needed to reach FCC became longer. Among the three polymers studied, CMC was the most effective, then PVA and starch.     

Permeability of semicrystalline polymers to toluene/methanol mixture

In this article, we analyze the effect of the alcohol content in gasoline on the permeability of four semicrystalline polymers used in automobile fuel systems. More specifically, we are interested in the methanol/toluene mixture as a “binary” model of a complex gasoline to understand the selectivity brought about, in front of these solvents, by the nature of the polymer. We developed a permeation cell coupled to a chromatograph to analyze the composition of escaping toluene and methanol. These experiments allow us to demonstrate the strong “positive synergy” that exists between the flows of the methanol and the toluene when they are mixed, compared to the flows of these solvents on their own. This phenomenon is notably highlighted on polymers of very different kinds (PA12, PVDF, HDPE, EVOH) and, in light of recent theoretical developments, we can consider that this property is general, because this “positive synergy” is a consequence of the evolution of the solubility of the mixture of solvents in the polymer film, and more particularly, of the highly positive value of the Flory‐Huggins interaction coefficient between methanol and toluene. These experimental data allow one to better understand and predict the permeability behavior of the polymers in front of complex gasolines. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2727–2733, 2003     

Permeability of semicrystalline polymers to toluene/methanol mixture

In this article, we analyze the effect of the alcohol content in gasoline on the permeability of four semicrystalline polymers used in automobile fuel systems. More specifically, we are interested in the methanol/toluene mixture as a “binary” model of a complex gasoline to understand the selectivity brought about, in front of these solvents, by the nature of the polymer. We developed a permeation cell coupled to a chromatograph to analyze the composition of escaping toluene and methanol. These experiments allow us to demonstrate the strong “positive synergy” that exists between the flows of the methanol and the toluene when they are mixed, compared to the flows of these solvents on their own. This phenomenon is notably highlighted on polymers of very different kinds (PA12, PVDF, HDPE, EVOH) and, in light of recent theoretical developments, we can consider that this property is general, because this “positive synergy” is a consequence of the evolution of the solubility of the mixture of solvents in the polymer film, and more particularly, of the highly positive value of the Flory‐Huggins interaction coefficient between methanol and toluene. These experimental data allow one to better understand and predict the permeability behavior of the polymers in front of complex gasolines. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 676–682, 2004     

Experimental Studies and Modelling of Sorption and Diffusion of Water and Alcohols in Cellulose Acetate

The sorption characteristics of water, methanol and ethanol vapours in cellulose acetate (CA) films were measured by microgravimetry. The sorption isotherms for water vapours in the CA film at different temperatures in the range 20–40°C do not obey the Flory equation over the whole water activity range. At high water activities, the sorption extent increases much faster with the water activity than it should according to the Flory approach. The isotherms over the whole water activity range can be fitted well by the ENSIC model, a new mechanistic model developed to account for the possibility of solvent cluster formation in the polymer material. Similar behaviour was observed for ethanol, which shows a lower tendency to form clusters, but higher affinity to CA. The sigmoidal shape found for the methanol sorption isotherm suggests a strong sorption on CA sites at low methanol activities. The Guggenheim–Anderson–De Boer equa-tion fitted well this isotherm. The diffusion coefficient, which was calculated from Fickian sorption kinetics at different solvent activities by curve fitting, was constant for water but increased with ethanol content in the membrane according to an exponential relationship characterized by a limit diffusion coefficient and a plasticization coefficient. The limit diffusion coefficient for water was two orders of magnitude larger than that for ethanol, but the activation energy for ethanol was twice as large. Methanol diffusion was only Fickian at a low solvent activity; the diffusion coefficient was one order of magnitude lower than that for water. © of SCI.     

PVA/ZnO复合材料“骨架支撑”型孔道构建及铅离子吸附

通过原位共交联方法制备了多孔型PVA/tetra-ZnO(聚乙烯醇/四针状氧化锌)复合材料作为废水中Pb(II)的脱除吸附剂,研究发现,由于tetra-ZnO四条晶须臂具有互锁特性使其孔壁处形成了“骨架支撑”型孔道结构,从而有利于Pb(II)的传输扩散和吸附络合。复合材料在tetra-ZnO含量为7%时具有最大Pb(II)吸附量,随pH增大,吸附量逐渐增大,复合材料的吸附效应在pH为4.0左右时最为明显。与复合材料的孔径分布相反,吸附的Pb(II)在复合材料中由外至内呈逐渐递减的梯度分布。等温吸附线表明Langmuir模型具有更好的拟合性,其吸附机理为单分子层均质吸附。动力学过程拟合表明其吸附分多步进行,化学吸附和内扩散均具有重要作用。与纯PVA相比,复合材料的应力吸附稳定性和热稳定性得到大幅提高。     

Modification of the dynamic swelling behavior of poly(2-hydroxyethyl methacrylate) in water

The equilibrium and dynamic swelling behavior of glassy polymers immersed in solvents can be modified by controlling the history of the polymer sample, which includes prior swelling and the drying method, or by copolymerization with other monomers. In this paper, the swelling kinetics in water of ionic hydrogels of 2-hydroxyethyl methacrylate copolymerized with potassium 3-sulfopropylmethacrylate and/or ethylene glycol dimethacrylate have been studied at 23°C. The dimensional changes of a swelling polymer sheet can be controlled through incorporation of anisotropic stresses in the initially dry, glassy polymer. These anisotropic stresses do not affect the swelling kinetics as long as the sample is partially glassy. However, differences in the initial stresses cause sharply different swelling kinetics once the polymer becomes entirely rubbery, due to differences in dimensional changes. Increasing the percentage of ionic comonomer in the polymer increases the equilibrium degree of swelling and the water sorption rate without changing the time for equilibration or the swelling transport mechanism. In contrast, increasing the percentage of cross-linker in the polymer not only reduces the degree of swelling and water sorption rate, but also increases the equilibration time and shifts the water transport mechanism from Fickian diffusion to anomalous transport.     

The heterogeneous formalization of poly(vinyl alcohol) accompanying crosslinking reaction – I

The effect of the solvents of poly(vinyl alcohol) (PVA) species on the crosslinking reaction accompanying in the heterogeneous formalization of PVA was studied. The crosslinking reaction was prevented when PVA film was allowed to react with formaldehyde in the solution containing methanol, ethanol or propanol, whereas it occurred in the solution containing acetone. The crosslinking reaction occurred more easily in syndiotacticity-rich PVA than in atactic PVA (commercial PVA).     

In situ analysis of solvent/nonsolvent exchange and phase separation processes during the membrane formation of polylactides

Membrane formation of polylactides has been studied using in situ analysis techniques. An experimental method based on the use of dark ground optics and reflected light illumination is used to monitor the mass transfer and phase separation dynamics during for mation. Additionally, the phase separation and structure formation has been studied using optical microscopy. The results of the dark ground optics technique for the polymer/solvent/nonsolvent systems poly-L-lactide/chloroform/methanol and poly-DL-lactide/chloroform/methanol showed that the diffusion kinetics were similar for the semicrystalline poly-L-lactide (PLLA) and the amorphous poly-DL-lactide. The influence of the molecular weight of the polymers on the diffusion kinetics was found to be negligible. Increasing the polymer concentration of the casting solution decreased the rate of diffusion. The phase separation of poly-DL-lactide was studied with optical microscopy and found to proceed via liquid-liquid demixing. For poly-L-lactide solutions of relatively low concentration (5–6% w/w), phase separation proceeded via liquid-liquid demixing followed by crystallization. For more concentrated PLLA solutions, phase separation proceeded directly via solid-liquid demixing processes. Additionally, for 6% w/w solutions of poly-L-lactide in dioxane immersed in methanol, precipitation also occurred solely via solid-liquid demixing. © 1996 John Wiley & Sons, Inc.     

Water sorption kinetics in poly(aryl ether ether ketone)

The kinetics of diffusion of water from different activity vapors and liquid phase have been investigated in glassy amorphous poly(aryl ether ether ketcne) (PEEK) films at the temperature of 60°C and in glassy semicrystalline PEEK sheets at different temperatures, respectively. In the case of the amorphous PEEK films (250 μm thick) the data at low activity levels were interpreted by means of a purely Fickian mechanism. At higher activity levels the material has shown the presence of a relaxation process; in this case the data have been interpreted using a model proposed by Berens and Hopfenberg. Equilibrium sorption isotherm is also reported. Liquid water sorption in semicrystalline (30%) PEEK sheets (2 mm thick) has been determined to follow the classical Fickian mechanism. The water uptake values obtained for both amorphous and semicrystalline PEEK, confirm the good moisture and liquid water resistance of this kind of high performance thermoplastic polymer.     

Effects of repeated water exchange and the molecular-weight distribution of PVA cast gels on the elution of polymers

The elution behaviors of polymers from hydrogels are studied using repeated water exchange on poly(vinyl alcohol) (PVA) gels. Physically cross-linked PVA gel was prepared using a cast-drying method from several powders with a high degree of hydrolysis (98.5 mol%) and varying degrees of polymerization (500, 1000, 1700, and 2400). The gels were immersed in water, which was then repeatedly exchanged with and without an immediate drying process. The elution of polymers from a PVA cast gel was estimated by quantitatively measuring the total carbon in the solvent water using a total organic carbon analyzer; a measurement was taken each time the solvent water was exchanged, and the amount of polymers eluted approached zero after sufficient water exchange. The total amount of polymers eluted is dependent on the degree of polymerization of the PVA powders. Additionally, the molecular-weight distributions of the PVA powders used for gelation and the polymers eluted in the solvent water were evaluated using a high performance liquid chromatography. The average molecular-weight of the eluted polymers was less than that of the PVA powders. The elution behaviors during repeated water exchange are discussed on the basis of the amount eluted and the molecular-weight distribution of the eluted polymers; these factors are dependent on the nature of the PVA powders.     

超声辐射对PVA/CB导电复合材料气敏性能的影响

以聚乙烯醇(PVA)为基体,填充各种类型炭黑导电粒子,制备了导电复合薄膜。研究了超声作用下导电复合薄膜在不同溶剂蒸气中的气敏性能。考察了PVA与炭黑的比例、炭黑种类、超声时间、超声功率等条件对导电薄膜响应性能与响应稳定性的影响,并对这种导电复合薄膜产生气敏响应性的机理作了分析。结果表明,以PVA/乙炔炭黑组成的导电复合薄膜具有较强气敏响应性。当将其置于丙酮、正丁醇、乙醇、水极性溶剂中,电阻急剧提高102~103倍,表现为正蒸气系数效应(PVC);随炭黑含量提高,复合材料薄膜室温电阻下降,对溶剂蒸气的响应强度提高;超声时间、功率对导电粒子分散行为及复合材料的气敏性有影响,随超声时间延长及功率增大,接枝改性效果明显,导电薄膜的响应性及重复稳定性提高。     

Non-equilibrium electrolyte permeability in poly(vinyl alcohol) membranes

Permeability of poly(vinyl alcohol) membranes was studied as a function of exposure time in 45% aqueous potassium hydroxide and quantity of residual water in film. The presence of non-equilibrium and equilibrium periods in electrolyte permeability of PVA films containing aqueous KOH solution or residual water were found and were explained from the viewpoint of change in the diffusion mechanism of permeability. The influence of cross-linking on permeability of PVA films was examined.     

Heterogeneous formalization of poly(vinyl alcohol)

The distributions of formaldehyde content from the surface to the inner part of poly(vinyl alcohol) (PVA) films formalized in a aqueous solution of sodium chloride and in a methanol/water mixture both containing formaldehyde and hydrochloric acid were investigated using rolled films. The formaldehyde content at the surface was higher than that in the inner part. When the PVA film was formalized in such a reaction condition that the rate of formalization was fast, a PVA film of high formaldehyde content at the surface and of large difference in the formaldehyde content between the surface and the inner part was prepared. Assuming that the formalization occurs in proportion to the amount of diffusing solution for formalization and that the diffusion obeys the FICK'S Law, the diffusion coefficients of the solution for formalization were estimated from the distribution of the formaldehyde content to be about 1.8 × 10^?6 cm²/min at 50°C for the methanol solution containing 5% formaldehyde, 18.1% water and 5% hydrochloric acid and 1.7% × 10^?6cm²/min at 50°C for an aqueous solution containing 1% formaldehyde, 10% sodium chloride and 5% hydrochloric acid.     

Drying of a system of multiple solvents: Modeling by the reaction engineering approach

Drying is a very important industrial operation in society. In drying, solute may dissolve in an aqueous solvent, a nonaqueous solvent or a mixture of solvents. Many mathematical models have been published previously to model drying of solute in water. The reaction engineering approach (REA) is known to be an easy‐to‐use approach. It can describe well many drying cases of water removal. Currently, no simple lumped model has been attempted to describe drying of porous materials containing a mixture of solvents. In this study, for the first time, REA is constructively implemented to model drying in a mixture of one aqueous and one nonaqueous solvent. The REA is applied here to model the drying of polyvinyl alcohol/methanol/water under constant and time‐varying environmental conditions. Similar to the relative activation energy of water, that of methanol is generated through one accurate drying run. For modeling the time‐varying drying, the relative activation energies are the same as those for modeling convective drying under constant ambient conditions but combined with the equilibrium activation energies at the corresponding humidity, methanol concentration, and temperature for each drying period. The REA is accurate to model drying of a solute in nonaqueous solvent as well as in a mixture of noninteracting solvents. In the future, spatially distributed REA for nonaqueous or mixtures of both aqueous and nonaqueous solvent will be explored for fundamental understanding and for practical application. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2144–2153, 2016     

Swelling effect of semicrystalline poly(vinyl alcohol) in hydrogen peroxide

The effect of hydrogen peroxide on the physicochemical properties of semicrystalline poly(vinyl alcohol) (PVA) has been investigated. Significant and irreversible increases in equilibrium water swelling, dissolved oxygen permeability, and surface wettability have been observed in PVA samples treated with concentrated hydrogen peroxide. Based on the small amount of carbonyl content detected and the crystallinity reduction in hydrogen-peroxide-treated poly(vinyl alcohol) samples, a mechanism involving a combination of hydrogen-peroxide-induced oxidative chain scission and dissolution of crystalline regions in poly(vinyl alcohol) is proposed for the observed swelling kinetics and associated changes in polymer properties.     

Controlled drug release through carboxymethyl‐chitosan/poly(vinyl alcohol) blend films

Carboxymethyl‐chitosan (CMCS)/poly(vinyl alcohol) (PVA) blend film was studied for an application as a coating material in the site‐specific drug delivery system. Films were prepared by blending varying amounts of 4 wt% CMCS with 4 wt% PVA, casting and drying at 50°C for 24 h. The cross‐sectional SEM micrograph of the film revealed that an increase in the amount of CMCS in the blend resulted in the film surface less smooth in the dry state and the network less uniform and more porous in the hydrated state, which became appreciable at 50%. The inclusion of CMCS in the blend also made the swelling of the films pH‐dependent, and lead to an increase in the degree of swelling with pH increase. When the permeation of three model drugs, salicylic acid, theophyline, and ornidazole, was studied using a static diffusion vessel, it followed a zero‐order kinetics and increased with an increase in the CMCS content in the blend, a decrease in the molecular weight of drug, an increase in the pH of medium, and a decrease in the film thickness. POLYM. ENG. SCI., 47:1373–1379, 2007. © 2007 Society of Plastics Engineers     

Separation of liquid mixtures by using polymer membranes. II. Water-alcohol separation by pervaporation through porous PVA membranes

The permeation and separation characteristics of alcohol/water systems through porous PVA membranes were investigated. Porous PVA membranes with different pore size and number were prepared by solution blending of PVA with several synthesized polymers or copolymers, such as polyacrylic acid, polyacrylamide, polyacrylonitrile, and methylmethacrylate-co-maleic anhydride, etc. Then casting, and finally extracting the blended polymers or copolymers by solvent from the membranes. The dependency of both permeation and separation on the molecular size and shape of the permeating species was dicussed qualitatively. Moreover, the permselectivity was investigated with attention to the feed composition of alcohol/water mixture and the effect of pore size and number. The selectivity was found to depend on the weight ratio and the molecular weight of polymer introduced to the membrane. When the weight ratio of polymer introduced into the membrane was larger than 0.1, methanol was permeated through membrane preferentially in methanol/water system, and the separation factor increased with increasing the methanol feed concentration. On the other hand, membrane had a selective permeability for water in the other alcohol/water systems. The influence of operating conditions was also studied.