DRYING OF LATEX FILMS OF POLY(VINYL ACETATE)

ABSTRACT

Drying of Poly(vinyl acetate) latex films has wide application in the industries of synthetic fibers, adhesives, coatings, paints, etc. In this investigation, drying of Poly(vinyl acetate) latex film was studied experimentally in a drying tunnel where the air velocity and temperature were controlled. The water evaporation rate was obtained by weighing the latex film during the drying process. The weight loss of latex was measured for different polymer concentrations, film thicknesses, drying areas, temperatures, and air velocities. Results emphasize the important role of these parameters on the relative water transport from the latex film. Increasing air velocity and temperature leads to a significant increase of the weight loss of latex and drying rate. Changing other parameters affects only the rate of weight loss but not the constant drying rate.     

天然胶乳厚胶膜热风干燥与红外干燥对比研究

采用失重法研究了天然胶乳厚胶膜在热风与红外线两种干燥方式下的干燥过程。研究结果表明,在相同条件下,红外干燥的干燥速率常数远大于热风干燥,采用红外干燥可大大缩短干燥时间,干燥温度和厚度均显著影响干燥过程,红外干燥对温度更敏感。通过动力学拟合结果,得到了干燥动力学方程,可以预测干燥时间。     

VAc/BA/NaAA共聚物乳液的合成及性能研究

采用半连续种子乳液聚合法制备了固含量较高（40%）、稳定性较好（常温贮存期6个月以上）的VAc/BA/NaAA共聚物乳液。通过DLS、GPC、接触角测试仪等对乳液粒径、聚合物相对分子质量、接触角进行测试和表征,并测试了乳液保水率、乳胶膜吸水率、拉伸强度等性能。研究结果表明：适量功能性单体的引入,对乳液稳定性、保水性有明显改善;同时乳胶膜的亲水性和力学强度也有所提高。     

Drying of semicrystalline polymers: mathematical modeling and experimental characterization of poly(vinyl alcohol) films

A mathematical model was developed to predict the drying mechanism of semicrystalline polymers involving multiple solvents. Since drying of semicrystalline polymers can be accompanied by changes in polymer degree of crystallinity, the model integrates crystallization kinetics and the Vrentas-Duda diffusion model to provide a better understanding of the mechanism. The model considers the effect of external conditions such as temperature, film shrinkage and diffusion and evaporation of multiple solvents during drying. Poly(vinyl alcohol) (PVA)/water/methanol was chosen as a test system. The drying kinetics of PVA films swollen in water and methanol were investigated using gravimetric techniques. The model predicts that higher temperatures, lower film thicknesses and lower methanol to water ratios increase the drying rate. The model predictions were compared with experimental data and showed good agreement.     

Drying of latex films and coatings: Reconsidering the fundamental mechanisms

The two existing theories describing drying of latex films or coatings are reconsidered. Subsequently, a novel mathematical drying model is presented, the simulations of which can match and explain experimental drying rate data of two previous investigations with latex films. In contrast to previous model studies, but in agreement with observations, simulations suggest that during the falling rate period of the drying process of a latex film, a porous skin of partly coalesced latex particles is indeed formed, which limits transport of water vapour from the receding air–liquid interphase to the surface of the film. The value of the effective diffusion coefficient of water vapour in the dry and partly coalesced layer (7 × 10⁻⁷ m²/s at 19–24 °C), the adjustable parameter of the model for the falling rate period, was found to be independent of initial wet film thickness (89–1322 μm), latex particle size (500–600 nm), initial polymer volume concentration (19–47 vol.%), and molecular weight of latex polymer (not quantified). Simulations also demonstrate that the transition from a constant to a falling drying rate in all cases takes place when the polymer volume concentration of the latex film is equal to that of hexagonal closest packed monodisperse spheres (74 vol.%). Consequently, the model has predictive properties and model inputs are only needed on the specific experimental (or field) conditions of interest. The effects on drying time of variations in relative humidity, wet film thickness, initial polymer volume concentration, and air flow velocity are simulated and analysed using the new model.     

Radiation-induced emulsion polymerization of vinyl acetate and styrene

Studies have been made of the γ-induced emulsion polymerization of styrene and comparisons made with chemically initiated emulsion polymerization. The polymerization proceeded smoothly to high conversions at 0 and 60°C, the reaction showing a high G (monomer) value. Complete conversions were obtained with total doses of less than 0.05 Mrad. In accordance with the behavior expected of systems having a constant rate of initiation, the molecular weight was found to decrease with decreasing temperature. The molecular weight and particle size distributions were narrower than those obtained in chemically initiated emulsion polymerizations at the same temperature. The radiation-induced emulsion polymerization of vinyl acetate proceeded smoothly at temperatures in the range 0–50°C to give polymers of much higher molecular weight than these obtained from chemically initiated polymerizations at the same temperature. Complete conversion was attained after a dose of 0.02 Mrad for latices approaching 50% solids. The elimination of ionic endgroups in the poly(vinyl acetate) radicals tends to drive the polymerization from the aqueous phase, resulting in faster rates and higher molecular weights than are obtained from chemically initiated systems. Rates of polymerization were found to be independent of temperature and the molecular weight of the polymer to be independent of dose rate. Latices of poly(vinyl acetate) of high solids content were evaluated for latex and film properties and found to have improvements over commercially available samples in both areas, especially in clarity of film and scrub resistance. A number of acrylate and maleate esters were copolymerized with vinyl acetate in a radiation-initiated emulsion system. High molecular weight copolymers were produced after low dose.     

PVAc Microspheres via Semicontinuous Emulsion Polymerization: Synthesis,Characterization, Kinetic,and Surface Morphology Studies

Poly (vinyl acetate) (PVAc) latexes are economically important products with many desirable features. They are used as adhesives for porous materials in various processing stages of industries. Synthesis parameters have an important role on the physico-chemical properties of PVAc latexes such as: viscosity, average molecular weight, degree of polymerization, and surface morphology. In this work, PVAc was prepared via semicontinous emulsion polymerization (delayed monomer and initiator addition process) in the presence of ammonium persulfate (APS) as conventional anionic initiator, poly (vinyl alcohol) (PVA) as stabilizer, and sodium lauryl sulfate (SLS) as anionic emulsifier. The surface morphology of PVAc microspheres was, examined using a scanning electron microscope (SEM) and atomic force microscope (AFM). It is evident from the SEM photographs that all the particles became microspheres and are uniform in shape. The use of AFM for imaging of polyvinyl acetate confirms a typical sphere polymer. The effect of changes in the different parameters such as concentration of emulsifier, initiator concentration, and presence or absence of buffer on the vinyl acetate (VAc) conversion, the steady state polymerization rate, the viscosity-average molecular weight, and the final latex viscosity of synthesized PVAc were investigated. The effects of anionic emulsifier on the synthesized PVAc are also compared with those obtained by the nonionic emulsifier. The comparison indicated that the VAc monomer conversion and the final latex viscosity of the anionic system were higher than for the nonionic system but the viscosity-average polymer molecular weight of the anionic system was lower than that of the nonionic system. The adhesive strength of the synthesized PVAc latex was examined and the load and deflection data were reported.     

Semicontinuous emulsion polymerization of vinyl acetate: Effect of ethoxylation degree of nonionic emulsifiers

Poly(vinyl acetate) latices were prepared in the presence of an ammonium persulfate initiator, 10–50 mol of an ethoxylated nonylphenol nonionic emulsifier, and a poly(vinyl alcohol) colloid stabilizer by applying semicontinuous emulsion polymerization (delayed monomer and initiator addition process) in a laboratory scale similar to industrial practice. Two approaches were applied: the molar concentration of the nonionic emulsifier was kept constant and the weight ratios in the polymerization recipe varied or the weight ratios were kept constant. The effects of the change in the ethoxylation degree of the emulsifier to the final latex viscosity, average polymer molecular weight, polymer grafting degree, surface tension of the latex, and the surface free energy of the dried films were investigated. It was determined that the resultant latex viscosity decreases and the viscosity‐average polymer molecular weight increases with increase of the nonionic emulsifier ethoxylation degree. The increase of the ethoxylation degree does not seriously affect the surface tension of the resultant latex or the surface free energy of the dried poly(vinyl acetate) films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 844–851, 2002     

Film formation of monodispersed polystyrene latex at high temperature

This report discusses the drying behavior of monodispersed polystyrene latex at elevating temperature with particular attention to the relationship between water evaporation rate and morphological evolution during the film formation process. At the first stage, water evaporation rate was less influenced by the skin film formed at the latex/air interface, which was consistent with Croll's model. During this stage, a drying front advanced from the top film toward the bulk dispersion. At the final stage of film formation, the water evaporation rate was less than that of the initial stage, and another drying front developed from the interior region outside the system. Two distinct boundaries corresponding to the opposite directions of the second drying front between completely dried region and wet region were found if the film was peeled off the container surface. Besides, some particular morphologies were found in the completely dried region, which was likely related to preferable coalescence among the particles induced by capillary force because of water evaporation. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1835–1840, 2001     

颗粒状氯化天然橡胶穿透式干燥动力学模型

本试验通过氯化天然橡胶的薄层干燥试验,探讨风温、风速以及粒径对CNR含水量的影响规律,建立薄层干燥方程。试验结果表明,预测值与实测值一致性较好,所建数学模型可用于描述氯化天然橡胶的薄层干燥。     

纳米氧化锌对天然胶乳膜干燥及硫化性能影响研究

采用失重法和溶胀法研究了普通氧化锌和纳米氧化锌活化的天然胶乳膜的干燥动力学,以及干燥过程中胶乳膜交联密度变化.结果表明:天然胶乳厚胶膜在成膜后仍含有大量水分并且去除困难.纳米氧化锌活化胶膜的干燥速率明显高于普通氧化锌胶膜.干燥温度对干燥过程影响显著,干燥时间随干燥温度升高明显降低.随着干燥的进行,2种氧化锌活化的胶乳膜的交联密度均迅速上升,在相同条件下,纳米氧化锌活化胶乳膜交联密度均比普通氧化锌的大.纳米氧化锌对天然胶乳膜的干燥及硫化过程均有促进作用.     

RADICAL DESORPTION IN EMULSION POLYMERIZATION OF VINYL ACETATE

In the present paper a model equation for calculating the radical desorption rate constant,k_0,in the emulsion polymerization of vinyl acetate was suggested and the various parameters forevaluating k_0 were determined.Effects of reaction temperature,emulsifier concentration,initiator con-centration,monomer conversion and phase ratio on k_0 were studied.It indicates that the desorptionof radicals from latex particles to aqueous phase must be taken into account in the modelling ofemulsion polymerization for the monomers with higher hydrophilicity such as vinyl acetate.     

Improvement of film properties of vinyl acetate based emulsion polymers by using different types of maleic acid diesters

Two types of maleic acid diesters, dibutyl maleate (DBM) and dioctyl maleate (DOM) were used as comonomers in semicontinuous emulsion copolymerization of vinyl acetate (VAc) in order to improve the film properties of poly(vinyl acetate), PVAc emulsion polymer. The effects of the comonomer type and comonomer ratio on minimum film forming temperature (MFFT), glass transition temperature (T_g), polymer structure, molecular weights, water contact angle and water resistance of PVAc latex films were examined. It was found that MFFT and T_g of the PVAc emulsion polymer decreased by the presence of the maleic acid disters in copolymer composition. This decrease was more affected by the increasing content and alkyl chain length of the comonomers. The molecular weights of the emulsion polymers were also affected by the comonomers and their ratios. Moreover, hydrophobicity and water resistance of the PVAc latex films were increased by using DBM and DOM as comonomer.     

Effect of emulsion polymerization conditions of vinyl acetate on the viscosity fluctuation and gelation behavior of aqueous poly(vinyl alcohol) solution

Poly(vinyl alcohol) (PVA) was prepared by emulsion polymerization of vinyl acetate (VAc) at 40, 50, and 60°C. PVAs of various molecular parameters were dissolved in water at concentrations of 3, 5, and 7% (g/dL) and aged at 30 and 60°C. The effects of the molecular weight and polymerization temperature on the viscosity fluctuation and gelation behavior of aqueous PVA solutions were investigated. Viscosity was increased with increasing molecular weight when other parameters were held the same. PVA of superior molecular regularity due to the lower polymerization temperature of VAc had higher relative viscosity. Viscosity and its fluctuation of PVAs with aging time varied with the polymerization temperature of the precursors, concentration of the aqueous solutions, and aging time. For PVA with a number‐average degree of polymerization of 2800 prepared from poly(vinyl acetate) polymerized at 40°C, a sharp increase in viscosity was observed after 500 min of aging at a concentration of 7%. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1897–1902, 2001     

DRYING BEHAVIOR OF MUSHROOM SLICES

Drying of mushroom slices is an unsteady-state process under the control of diffusion race of water through the mushroom hyphae. Therefore, temperature, thickness of the mushroom slices and the ratio of the air film to mushroom resistance toward the diffusion of water control the drying rate. In this work, the initial thickness of mushroom slices was kept constant at 2 mm and the effects of structural parameters, temperature and humidity were investigated. Through the measurements of weight, center and surface temperatures of the mushrooms, and variation of the surface area, actual drying behavior of mushrooms under different dryer conditions were determined. Color of the dried mushroom slices were used as criteria for the evaluation and determination of the optimum conditions.     

DRYING BEHAVIOR OF MUSHROOM SLICES

Drying of mushroom slices is an unsteady-state process under the control of diffusion race of water through the mushroom hyphae. Therefore, temperature, thickness of the mushroom slices and the ratio of the air film to mushroom resistance toward the diffusion of water control the drying rate. In this work, the initial thickness of mushroom slices was kept constant at 2 mm and the effects of structural parameters, temperature and humidity were investigated. Through the measurements of weight, center and surface temperatures of the mushrooms, and variation of the surface area, actual drying behavior of mushrooms under different dryer conditions were determined. Color of the dried mushroom slices were used as criteria for the evaluation and determination of the optimum conditions.     

一种高强度双组分拼板胶PVAc乳液的制备

以聚乙烯醇为保护胶,配合乳化剂,将混合单体采用半连续乳液聚合法滴加制备一种高强度双组分拼板胶用聚醋酸乙烯酯(PVAc)乳液。研究了pH缓冲剂、聚乙烯醇、主单体和功能单体的种类及用量对乳液稳定性和耐水性等的影响。结果表明,NaHCO_3可保乳液聚合反应平稳,提高单体转化率和贮存稳定性;疏水型软单体叔碳酸乙烯酯的引入,可以有效地改善胶膜的柔韧性并提高乳液的固化速度、耐水性;适量甲基丙烯酸甲酯可明显降黏并改善乳液的施工便捷性;甲基丙烯酸和羟乙基丙稀酸酯的引入,可以改善乳液冻融稳定性和耐水强度的同时,还能调节拼板胶的使用期。     

小颗粒“胶乳法”氯化天然橡胶穿透干燥试验

通过氯化天然橡胶(CNR)的穿透干燥试验,探讨风温、风速以及粒径对“胶乳法”CNR干燥速率的影响规律,结果表明,温度和风速对含水量的下降速率影响较大,而粒度对其影响较小。建立了穿透干燥方程,对实验数据进行了拟合,结果显示其吻合性较好。红外光谱分析以及物理性能测试结果表明,“胶乳法”CNR物料的干燥温度应控制在100℃以下。     

Grafting and adsorption of poly(vinyl) alcohol in vinyl acetate emulsion polymerization

Poly(vinyl alcohol) is often used in vinyl acetate emulsion polymerization as a protective colloid, but its role is complex and controversial since it partakes in grafting reactions with the monomer, influencing process mechanisms, and affecting the colloidal properties of the latex. Furthermore, in industrial operations, the wide scatter of macromolecular properties of the commercial types of poly(vinyl alcohol) causes process irreproducibilities. In this work different types of polyvinyl alcohol were used to perform a series of polymerizations, and their kinetics were compared. A selective solubilization procedure was applied to separate the three fractions of poly(vinyl alcohol) in the final latex: free in the water phase, physically adsorbed onto the polymer particles and chemically grafted. These results were compared with those obtained from pure adsorption measurements of polyvinyl alcohol onto ‘emulsifier-free’ polyvinyl acetate dispersions. The rheological behavior of the different latexes was also compared, and the results were used to formulate an hypothesis on the interaction mechanisms acting in these systems.     

CHARACTERISTICS OF GELATIN BIOFILMS IN RELATION TO DRYING PROCESS CONDITIONS NEAR MELTING

ABSTRACT

Biofilms are edible films formed from biopolymers. In the casting process technique, the biofilm is obtained by the drying of a colloidal solution in the final process stage The aim of this work was to analyse the drying process of gelatin based biofilms, in room frontier conditions. The biofilms were prepared initially by dissolving gelatin in water and adding sorbitol as plasticizer. The filmogenic solution was dried in an automatic drier under various drying conditions, for 1mm film: air temperature (T= 35, 40 and 45 °C), relative humidity (?= 60 and 75 %) and air velocity (v= 3m/s). The drying kinetics was measured for 15, 30 and 45 % sorbitol concentrations. Sorption isotherms at 25, 35 and 45 °C were determined for the 15% sorbitol concentration films by the static gravimetric method. The glass transition and the melting temperature for various moisture contents were determined with a Differential Scanning Calorimeter TA 2010. The results showed an unusual behaviour for temperature dependence: decreasing drying kinetics with increasing air temperature. Also separation of gelatin and sorbitol were seen when the temperature increased. This phenomenon occurs when the film temperature during the drying process gets close to the melting temperature of the film.