Identification of nucleation loci in emulsion polymerization processes. II. A systematic study of liquid–liquid emulsions

Identification of the main nucleation sites and characterizing them in terms of their size and composition is prime objective of this research effort. Different nucleation sites have been proposed as the most likely nucleation sites by various researchers. Recent advances in the development of the spectroscopic techniques enable comprehensive characterization of the emulsion at the beginning of the reaction. In this paper, we present the experimental evidence of the existence of a previously unidentified nano‐droplet population of size range 30 to 100 nm in diameter using spectroscopy. Presence of about seventy to eighty percent of the dispersed phase in the nano‐droplet population and large interfacial area make them the most probable particle nucleation loci in emulsion polymerization processes. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2858–2866, 2006     

Inducing the stable polymorph using heterogeneous primary nucleation

Polymorphism plays an important role in crystallization of active pharmaceutical ingredients (APIs). At the earlier stage of development of a new API, a polymorph screening is performed to find out its different polymorphic forms. In particular, it is interesting to obtain the stable form. However, the form which nucleates spontaneously from solution is often a metastable form. This phenomenon corresponds to the so-called Ostwald rule of stages which is explained by the difference existing between the nucleation kinetics of the metastable and stable forms. The paper shows how a control of the heterogeneous primary nucleation can favour the nucleation of a more stable form. Several heterogeneous supports were tested with an API. The efficient supports did not act as templates but reduced the metastable zone widths and facilitated the primary nucleation of the different polymorphs. Optimising the operating conditions, i.e. mainly support concentration and supersaturation with respect to each polymorph modification, has allowed the preferential nucleation of the stable form. In the absence of heterogeneous support, a metastable form was systematically obtained. Since the interactions with the heterogeneous support do not need to be polymorph specific, the approach can a priori be used to perform the polymorph screening of any API, in addition to the usual in house screening methods.     

Aerosol formation by heterogeneous nucleation in wet scrubbing processes

The phenomenon of aerosol formation is often watched in industrial wet scrubbing processes especially if strong acid gases are absorbed in aqueous solutions. Although the basic principles of aerosol formation are well known in general, there exist nearly no specific rules for the design of industrial processes in which aerosol formation can be expected. There are two reasons for the fragmentary knowledge concerning aerosol formation in wet scrubbing processes. Firstly, it is a very complex task to describe the formation and the growth of aerosols theoretically under typical conditions of industrial processes. Secondly, no reliable experimental data, especially for the validation of theoretical work, has been available to date. To bridge this gap, a semi-technical wet scrubbing plant has been developed and erected in which the formation and the growth of aerosols under realistic conditions can be studied and the characteristic aerosol parameters can be measured in situ. It is shown that HCl-aerosols are formed in a quench cooler for hot flue gases of about 200°C even at low HCl feed concentrations of about l000 mg m⁻³, but only in the presence of foreign nuclei. The aerosol droplets in the quench cooler grow up to 1–1.5 μm and can be enlarged in a direct-contact-cooling column up to about 2.5 μm. Furthermore, a strategy for aerosol evaporation is discussed.     

高频脉冲电场作用下乳状液液滴动力学模型

基于乳状液中液滴在电场中的受力情况的分析,建立了W/O乳状液中液滴振荡固有频率公式,从理论角度分析和解释了高频脉冲电场对W/O乳状液的破乳机理,从力学的角度分析了最佳频率存在的理论基础,并推导出了最佳频率的计算公式。该固有频率与液滴尺寸、界面张力、黏度、温度、密度等有关。为了验证最佳频率公式的可靠性,用正辛烷乳状液进行了室内实验。实验结果表明,理论最佳频率公式的预测结果与实验测得的最佳频率结果吻合,经修正后与白油实验结果也非常接近。     

乳液聚合中成核机理的研究进展

主要介绍了在乳液聚合反应中,各种成核过程的机理及其对乳液聚合反应和性能的影响;分析了胶束成核、均相成核、液滴成核的产生条件与影响因素;介绍了成核机理的最新研究进展.     

非均匀成核法石墨表面改性的研究

以ZrOCl2·8H2O为前驱物,采用非均匀成核法对鳞片石墨进行了表面改性处理,并研究了pH值(2.4～5.1)、前驱物溶液的浓度(0.005～0.1mol·L-1)、石墨与前驱物溶液的混合方式等工艺因素对石墨表面改性效果的影响。结果表明:1)石墨悬浮液先与浓度为0.025mol·L-1的前驱物溶液充分混合,然后滴加氨水调节pH值为3.6是非均匀成核法改性石墨的最佳工艺条件,能在石墨表面形成均匀的ZrO2包覆层;2)石墨经过表面改性后,在其表面均匀包覆的ZrO2包覆层提高了其抗氧化性和分散性,减少了团聚。     

Synthesis of aluminium titanate by means of isostructural heterogeneous nucleation

The synthesis of tialite was carried out by means of the solid state reaction technique using heterogeneous nucleation. The influence of the temperature of synthesis and simultaneous sintering (1250–1350 °C) on the yield of reaction and densification of the systems was investigated. Reference samples, consisting of stoichiometric mixture of titanium and aluminium oxides as well as the samples containing such additives as MgO, SiO₂ and MgTi₂O₅ were prepared. Applying the DSC analysis the synthesis temperatures of systems differing in the substrate composition were determined. The phase composition of synthesis products was determined by means of XRD analysis combined with Rietveld's method. The density of the obtained materials was determined by the Archimedes method. SEM observations enabled microstructures and homogeneity of sinters to be compared.On the basis of the conducted research it was found that using oxide additives containing magnesium cations allows the synthesis efficiency exceeding 95% as early as at the temperature of characterized by a homogeneous microstructure with a lower number and a smaller size of microcracks.     

Secondary nucleation of styrenated hydroxyl-functionalized latexes

In this study, secondary nucleation in emulsion polymerization of butyl acrylate (BA), styrene (St), and 2-hydroxyethyl methacrylate (HEMA), was investigated. Different molar ratios of BA and St were used. Average particle size, particle size distribution (PSD), polymerization kinetics, glass transition temperature (T_g), and surface tension were monitored and compared. All synthetic latexes were prepared through seeded semi-batch emulsion polymerization process. Three different BA to St ratios (1:1, 1:2, 2:1) were combined with eight different HEMA contents (0, 1, 3, 5, 10, 20, 30, 40 mol%) in monomer composition, respectively. In the case of high HEMA content (10–40 mol%) it is found that secondary nucleation is mainly affected by HEMA. Secondary nucleation at low HEMA content (1–5 mol%) is dependent on the BA to St monomer ratio, and consequently a reflection of z-mer hydrophobicity.     

Batch foaming of poly(ε-caprolactone) using carbon dioxide: Impact of crystallization on cell nucleation as probed by ultrasonic measurements

The effect of crystallization on the foaming of poly(ε-caprolactone), a semi-crystalline biodegradable polymer, was studied using batch processing and CO₂ as the physical blowing agent (PBA). Previous work on the same system suggested that the formation of high cell population densities of small open cells was crystalline fraction dependent. In this work, ultrasonic sensors were used to investigate the effect of crystallization on the cell nucleation mechanism through the monitoring of ultrasonic velocity, attenuation and specific volume characteristics. Analysis of the ultrasonic data allowed to determine the crystallization temperature and thus determined the effect of crystallization on the pressure at which phase separation occurred (degassing pressure). Clearly, the presence of crystals leads to a 5-10-fold increase in the degassing pressure with respect to the amorphous system. In addition, it was shown that the talc content did not significantly influence the degassing pressure for a full amorphous system. However, talc particles can drastically affect the degassing conditions through their role of precursors for crystal formation. Different mechanisms are proposed as possible explanations for the enhancement of cell nucleation in such semi-crystalline polymer systems.     

Fibrous ceramic membrane constructed by mullite whiskers for the treatment of oil-in-water emulsions

Ceramic membranes with high porosity and excellent separation efficiency are necessary for the efficient treatment of large-scale wastewaters. However, the conventional ceramic membranes are usually prepared by particles-packing, which inhibits the advances of separation efficiency because of the low porosity and connectivity. Here, a fibrous ceramic membrane with mullite whiskers-interlocked structure was prepared by gas-solid reaction. The effects of aluminum fluoride (AlF₃) on the formation and growth of mullite whiskers, and then the permeability and selectivity of the ceramic membranes were investigated. With the increase of AlF₃ contents, the mullite phase evolved from needle-like, rod-like to flake-like structure, thus the catalyst accelerated the growth of mullite whiskers in the diameter direction. For the ceramic membrane sintered at 1400°C, the porosity increased from 58% to 76% while the average pore sizes increased from 0.65 to 3.93 μm because of the whisker-constructed structures. For the ceramic membrane sintered at 1450°C, the emulsion flux increased stably from 295 L/(m²·h) to 992 L/(m²·h) with the increase of trans-membrane pressure, and the oil rejection exceeded 98%. Thus, this study provides a feasible strategy for the preparation of ceramic membranes with high porosity and excellent separation performances.     

Effect of surface nucleation on isothermal crystallization kinetics: Theory, simulation and experiment

Surface nucleation of poly(l/d-lactide) at the interface with aluminum was studied by performing isothermal DSC analysis of amorphous samples of varying thickness between 100 °C and 130 °C. To ensure complete wetting of the aluminum surface, a hot melt laminating process was used to prepare the samples. Theoretical aspects of surface crystallization kinetics were explored and the resulting model was compared with the results of Monte-Carlo simulations. Three stages of surface crystallization were identified depending on the growth geometry: (1) impingement-free growth, (2) increasingly laterally-constrained transverse growth, and (3) interstitial growth. By fitting the Monte-Carlo simulation to the experimental half-times of crystallization the surface nucleation concentration and the bulk nucleation rate was estimated at 4 different temperatures. It was found that both surface nucleation concentration and the bulk nucleation concentration decrease with increasing crystallization temperature.     

Modeling evidence in support of coagulative nucleation theory

An extensive lumped model was developed for emulsion polymerization. It incorporated all of the complex processes: aqueous‐phase radical balances for all radical species arising from initiator decomposition and from exit; determination of radical number inside the particles by balance among rates of radical entry into, exit from, and termination inside the particles; determination of the monomer concentration inside the particles and in the aqueous phase by a thermodynamic equation; and particle formation by micellar, homogeneous, and coagulative nucleation. Model validation was done for the system with styrene (monomer), potassium persulfate (initiator), and sodium dodeceyl sulfate (emulsifier) and for the variables, which included the duration of nucleation, conversion at the end of nucleation, and total number of particles formed. The validation process revealed that coagulation during nucleation needed to be included in the model, even for emulsifier concentrations above the critical micelle concentration. The model predictions were in good quantitative agreement with the experimental data. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Inhibition of oxidation in 10% oil-in-water emulsions by β-carotene with α- and γ-tocopherols

The effects of low concentrations of β-carotene, α-, and γ-tocopherol were evaluated on autoxidation of 10% oil-in-water emulsions of rapeseed oil triacylglycerols. At concentrations of 0.45, 2, and 20 μg/g, β-carotene was a prooxidant, based on the formation of lipid hydroperoxides, hexanal, or 2-heptenal. In this emulsion, 1.5, 3, and 30 μg/g of γ-tocopherol, as well as 1.5 μg/g of α-tocopherol, acted as antioxidants and inhibited both the formation and decomposition of lipid hydroperoxides. Moreover, at a level of 1.5 μg/g, γ-tocopherol was more effective as an antioxidant than α-tocopherol. At levels of 0.5 μg/g, both α- and γ-tocopherol significantly inhibited the formation of hexanal but not the formation of lipid hydroperoxides. Oxidation was effectively retarded by combinations of 2 μg/g β-carotene and 1.5 μg/g γ- or α-tocopherol. The combination of β-carotene and α-tocopherol was significantly better in retarding oxidation than α-tocopherol alone. While γ-tocopherol was an effective antioxidant, a synergistic effect between β-carotene and γ-tocopherol could not be shown. The results indicate that there is a need to protect β-carotene from oxidative destruction by employing antioxidants, such as α- and γ-tocopherol, should β-carotene be used in fat emulsions.     

乙烯基硅氧烷改性丙烯酸酯乳液聚合的动力学及成核机理分析

用种子乳液聚合法研究了硅氧烷改性丙烯酸酯的乳液聚合,对影响种子乳液聚合动力学的因素进行讨论,并分析了乳液聚合的成核机理.实验结果表明：反应温度、引发剂浓度、乳化剂浓度和有机硅氧烷用量对有机硅改性丙烯酸酯乳液聚合动力学有较大的影响.反应温度越高,引发剂浓度越高、乳化剂浓度越高、有机硅氧烷用量相对较小,乳液聚合反应的转化率越高;此外,体系的pH值在6～8之间时也有利于反应的进行.种子乳液聚合中R_P∝［E］^0.72,R_P∝［I］^0.56,表观活化能E_a为143.92 kJ•mol^-1.种子乳液聚合初期,反应主要是单体液滴成核;进入壳层反应时,反应成核主要是以胶束成核为主.     

Production of quasi-monodisperse emulsions with large droplets using a micromachined device

The development and operation of a modified method for the preparation of quasi-monodisperse (CV<16%) emulsions are reported. The device uses a microfabricated-nozzle (MN) array, produced by micromachining technology, to achieve a higher rate of emulsification [76.8 m³/(m²·h)] than previously reported for microchannel (MC) emulsification methods. The dispersed phase is extruded into a continuous phase through the MN, and the emulsions droplets are formed by viscous drag force of the continuous phase. The droplet diameter decreased with an increase in the flow velocity and viscosity in the continuous phase, which was explained by the theoretical model. The range of droplet diameters produced (130 to 370 μm) was larger than in previously reported MC emulsification, membrane emulsification, and shear-rupturing methods.     

Influence of concentration and temperature on the flow behavior of oil-in-water emulsions stabilized by sucrose palmitate

To study the influence that concentration and temperature exert on the viscous behavior of emulsions stabilized by a sucrose ester (SE) of high hydrophilic-lipophilic balance (HLB), flow curves and droplet size distributions were determined. Flow curves of presheared emulsions always exhibited a shear-thinning behavior at intermediate shear rates, a tendency to a limiting viscosity at high shear rates, and a metastable region at low rates. This behavior can be fitted to a Carreau model. Both SE and oil concentrations increase emulsion viscosity as a result of a more structured system with a lower droplet size and polydispersity. An increase in temperature usually leads to a decrease in emulsion viscosity. However, at high oil concentration, coalescence and phase separation take place at low temperature. On the other hand, at high temperature, droplet bursting due to shear forces, leading to an increase in viscosity, may result. Despite the strong structural breakdown caused by steady shear, master flow curves may be obtained by using superposition methods.     

Structure-property relations in crack-resistant alkaline-earth aluminoborosilicate glasses studied by solid state NMR

The effect of the average ionic potential ξ = Ze/r of the network modifier cations on crack initiation resistance (CR) and Young's modulus E has been measured for a series of alkaline-earth aluminoborosilicate glasses with the compositions 60SiO₂–10Al₂O₃–10B₂O₃–(20−x)M₍₂₎O–xM’O (0 ≤ x ≤ 20; M, M’ = Mg, Ca, Sr, Ba, Na). Systematic trends indicating an increase of CR with increasing ionic potential, ξ, have been correlated with structural properties deduced from the NMR interaction parameters in ²⁹Si, ²⁷Al, ²³Na, and ¹¹B solid state NMR. ²⁷Al NMR spectra indicate that the aluminum atoms in these glasses are essentially all four-coordinated, however, the average quadrupolar coupling constant <C_Q> extracted from lineshape analysis increases linearly with increasing average ion potential computed from the cation composition. A similar linear correlation is observed for the average ²⁹Si chemical shift, whereas the fraction of four-coordinate boron decreases linearly with increasing ξ. Altogether the results indicate that in pure alkaline-earth boroaluminosilicate glasses the crack resistance/E-modulus trade-off can be tailored by the alkaline-earth oxide inventory. In contrast, the situation looks more complicated in glasses containing both Na₂O and the alkaline-earth oxides MgO, CaO, SrO, and BaO. For 60SiO₂–10Al₂O₃–10B₂O₃–10MgO–10Na₂O glass, the NMR parameters, interpreted in the context of their correlations with ionic potentials, are consistent with a partial network former role of the MgO component, enhancing crack resistance. Altogether the presence of MgO in aluminoborosilicate glasses helps overcome the trade-off issue between high crack resistance and high elasticity modulus present in borosilicate glasses, thereby offering additional opportunities for the design of glasses that are both very rigid and very crack resistant.     

Nucleation truncation by time-dependent supersaturation: a unified model for homogeneous and heterogeneous nucleation

In this paper, a unified model for heterogeneous, as well as for homogeneous two-dimensional nucleation and growth processes for electrochemical phase transitions is derived. It accounts for three different mechanisms of the termination of nucleation and growth: the geometric mechanism by surface exhausting, the stoichiometric mechanism by the consumption of active surface sites and the systemic mechanism by the time dependence of the supersaturation of the initial phase as the thermodynamic driving force of the phase transition. The special role of time-dependent supersaturation in truncating the nucleation process is discussed by means of numerical simulations of the model equations. It could be shown that some—until now—poorly understood experimental findings concerning phase transition kinetics at electrodes, as for example, the occurrence of instantaneous homogeneous nucleation at the mercury electrode or of non-integer Avrami slopes for heterogeneous nucleation at single crystal electrodes, can be explained easily in the frame of the presented model.     

Phase segregation,shear thinning and rheological behavior of crude oil-in-water emulsions

This work reports an experimental investigation of the oil segregation effect during the rheological evaluation of concentrated oil-in-water (O/W) emulsions. The results demonstrated that alkyl alcohols can significantly affect the rheological behavior of O/W emulsions. The addition of n-octanol and n-decanol to the emulsifying agent formulation produces a critical shear rate, promotes a sharp break in the flow curves and leads to a sharp viscosity drop, which was attributed to wall depletion. This phenomenon is related to the displacement of the dispersed phase away from the solid-boundary, which generates a low-viscosity oil-depleted layer that acts as a lubricating layer. The incidence of slippage depends on the alkyl-chain length and the concentration of the alcohol. The lubrication layer will promote energy savings during emulsified oil pumping and result in significant economic benefits to the petroleum industry.