Cellulose graft copolymers. II. Graft copolymerization of ethyl acrylate with γ-irradiated cellulose from acetone–water systems

Ethyl acrylate was graft-copolymerized from acetone–water systems with γ-irradiated, purified cotton cellulose. The scavenging of the free radicals in the irradiated cellulose by water, acetone, and water–acetone systems was determined by electron spin resonance spectroscopy. The ESR spectra of free radicals, scavenged by water and acetone, were recorded by the use of a time-averaging computer attached to the ESR spectrometer, in which the ESR spectrum of the irradiated cellulose, which had been immersed in water and/or acetone, was electronically subtracted from the ESR spectrum of the irradiated cellulose control. For both water and acetone, the ESR spectra of the scavenged free radicals were singlets. This indicated that free radical sites formed on carbon C₁ or C₄ on radiation-initiated depolymerization, which would generate singlet ESR spectra, were readily accessible to these solvents. The maximum scavenging of the radicals was observed when irradiated cellulose was immersed in acetone–water solution which had a composition of 25/75 vol-%. The scavenging of the free radicals in irradiated cellulose when immersed in acetone–water solutions was less than when immersed in methanol–water solutions. Also, the extent of graft copolymerization of ethyl acrylate from acetone solutions with irradiated cellulose was less than that of ethyl acrylate from methanol solutions. These differences were probably due to differences in the diffusion rates of acetone and methanol into the cellulosie structure. The Trommsdorff-type effect in the acetone solutions would be less than in the methanol solutions, since acetone is a better solvent for poly(ethyl acrylate) than methanol.     

Silicone-acrylic hybrid aqueous dispersions of core–shell particle structure and corresponding silicone-acrylic nanopowders designed for modification of powder coatings and plastics. Part I – Effect of silicone resin composition on properties of dispersions and corresponding nanopowders

Aqueous silicone-acrylic dispersions with core–shell particle structure can be obtained in the process of emulsion polymerization of acrylic or methacrylic monomers in previously synthesized silicone resin dispersions. If the glass transition temperature (T_g) of the shell is around +120 °C or higher, drying of such dispersions leads to “nanopowders” which can be applied as impact modifiers for powder coatings and plastics due to the presence of low T_g silicone resin contained in the hybrid nanoparticles. The aim of our study was to investigate the effect of silicone resin composition on the properties of dispersions and the corresponding nanopowders what, in turn, was expected to influence the properties of powder coatings modified with such nanopowders. Silicone resin dispersions (DSI) were synthesized by emulsion polymerization of three silicone monomers: octamethylcyclotetrasiloxane (D4), methyltrimethoxysilane (METMS) and methacryloyltrimethoxysilane (MATMS) in the presence of dodecylbenzenesulphonic acid playing the role of both surfactant and polymerization catalyst. Silicone-acrylic hybrid dispersions (DASI) having core–shell particle structure confirmed by TEM were further obtained by emulsion polymerization of methyl methacrylate in DSI, and eventually nanopowders (NP-DASI) were produced by spray-drying of DASI. A designed experiment was conducted where the different proportions of D4, METMS and MATMS were used in DSI synthesis and a range of properties of DSI, DASI and NP-DASI were tested. A significant effect of starting silicone monomers composition (reflected in silicone resin structure) on dispersion particle size was observed what could be explained by differences in their hydrophobicity. SEM investigations revealed that NP-DASI were produced in the form of 1–10 μm agglomerates of round-shaped nanoparticles of ca. 120 nm in size. Two clear glass transition temperatures (T_g) of NP-DASI were identified by DSC: one attributed to silicone part – around −120 °C – and the other attributed to poly(methyl methacrylate) (PMM) part – around +120 °C. T_g attributed to silicone part decreased with increased share of D4 and MATMS in the silicone monomers composition while T_g of PMM part showed a minimum for specific composition of silicone monomers.     

Waterborne polyurethane dispersions obtained by the acetone process: A study of colloidal features

Waterborne polyurethane (PU) dispersions were prepared from isophorone diisocyanate (IPDI), 2‐bis(hydroxymethyl) propionic acid (DMPA), 1,4‐butane diol (BD), poly(propylene glycol) (PPG), and triethylamine (TEA) by means of phase inversion through the acetone process. Changes in DMPA content, initial PU content in acetone, phase‐inversion temperature, evaporation conditions, and solvent nature were found to have a great impact on dispersion properties. Using a DMPA concentration of 0.30 mmol/g_pol, stable PU dispersions could only be obtained when the initial PU content in acetone was at least 60 wt %, and phase‐inversion temperature was lower than 30°C. However, when increasing the PU content to 75 wt %, stable dispersions were obtained using DMPA concentrations three times lower. Finally, viscosity curves during the water addition step as well as a phase diagram were determined to understand the particle formation mechanism. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Gelpermeationschromatographie von anionischen polyurethan-dispersionen

Aqueous PMMA latices with a narrow particle-size distribution (PSD) as well as dispersions of anionic polyurethanes (PUR) with different PSD were studied by sizeexclusion chromatography (SEC) using a copolymer of oligoethyleneglycol, glycidylmethacrylate and pentaerythrol dimethacrylate (Fractogel TSK) as packing material. Elution volume of the peak maximum and peak width depend on the ionic strength of the eluent. The PSD of the dispersions was also determined by ultracentrifugation. The peak maximum obtained by SEC corresponds best with the number average particle diameter D?_N or with D₅₀ (diameter at which 50% of the mass are recorded) as was shown with the PUR dispersions. Solvents and other additives which are included in the PUR dispersions can also be identified by SEC.     

Optimized dispersion of nanoparticles for biological in vitro and in vivo studies

Background

The aim of this study was to establish and validate a practical method to disperse nanoparticles in physiological solutions for biological in vitro and in vivo studies.

Results

TiO₂ (rutile) dispersions were prepared in distilled water, PBS, or RPMI 1640 cell culture medium. Different ultrasound energies, various dispersion stabilizers (human, bovine, and mouse serum albumin, Tween 80, and mouse serum), various concentrations of stabilizers, and different sequences of preparation steps were applied. The size distribution of dispersed nanoparticles was analyzed by dynamic light scattering and zeta potential was measured using phase analysis light scattering. Nanoparticle size was also verified by transmission electron microscopy. A specific ultrasound energy of 4.2 × 10⁵ kJ/m³ was sufficient to disaggregate TiO₂ (rutile) nanoparticles, whereas higher energy input did not further improve size reduction. The optimal sequence was first to sonicate the nanoparticles in water, then to add dispersion stabilizers, and finally to add buffered salt solution to the dispersion. The formation of coarse TiO₂ (rutile) agglomerates in PBS or RPMI was prevented by addition of 1.5 mg/ml of human, bovine or mouse serum albumin, or mouse serum. The required concentration of albumin to stabilize the nanoparticle dispersion depended on the concentration of the nanoparticles in the dispersion. TiO₂ (rutile) particle dispersions at a concentration lower than 0.2 mg/ml could be stabilized by the addition of 1.5 mg/ml albumin. TiO₂ (rutile) particle dispersions prepared by this method were stable for up to at least 1 week. This method was suitable for preparing dispersions without coarse agglomerates (average diameter < 290 nm) from nanosized TiO₂ (rutile), ZnO, Ag, SiO_x, SWNT, MWNT, and diesel SRM2975 particulate matter.

Conclusion

The optimized dispersion method presented here appears to be effective and practicable for preparing dispersions of nanoparticles in physiological solutions without creating coarse agglomerates.     

Application of statistical experimental strategies to the process optimization of waterborne polyurethane

A modified acetone process for the preparation of aqueous polyurethane (PU) dispersion is investigated. PU dispersions were prepared by polyaddition of isophorone diisocyanate (IPDI) to poly(tetramethylene ether glycol) (PTMG) and dimethylolpropionic acid (DMPA), followed by neutralization of pendant COOH groups with triethylamine (TEA). The resulting prepolymer chain was then extended through reaction with an ethylenediamine (EDA) derivative bearing sulfonate groups. The effect of such preparation conditions as catalyst (dibutyltin dilaurate, DBTDL) concentration, the acetone/PU ratio, phase-inversion temperature, agitation rate, and water-addition rate on the average particle size of aqueous PU anionomer dispersions is systematically studied using fractional factorial design and response surface methodology. Fractional factorial analysis indicates that the effects of the acetone/PU ratio, phase-inversion temperature, water-addition rate as well as the two factor interactions of DBTDL concentration and phase-inversion temperature, of the acetone/PU ratio and phase-inversion temperature, and of acetone/PU ratio and agitation rate are the key variables influencing average particle size of PU dispersions. Empirical models for average particle size are fitted and plotted using central composite experimental design as contour diagrams in order to facilitate examination of the average particle size results. The results show that for a 100 g PU anionomer containing 0.311 wt % COOH and 1.51 wt % SO₃H groups, a minimum number-average particle size of the dispersion ～ 25 nm can be obtained under an acetone/PU ratio, water-addition rate, phase-inversion temperature, catalyst concentration, and agitation rate of 3.65, 2 mL/min, 50°C, 150 ppm and 350 rpm, respectively.     

Particle formation in supercritical fluids—Scale-up problem

The process of antisolvent precipitation of particles, termed solution enhanced dispersion by supercritical fluids (SEDS™), is applied to precipitate the model drug, paracetamol, from ethanol solutions. In the SEDS process the substrate solution is quickly mixed in a mixing chamber of the coaxial two-component nozzle with an antisolvent, represented in this case by the supercritical CO₂. Resulting partially mixed, highly supersaturated solution is introduced into the precipitation vessel as a jet, which generates intensive circulation of residual fluids that dilute the fresh supersaturated solution. Nucleation starts in the nozzle chamber, whereas particle growth completes the process in the precipitation vessel. The process is carried out above the mixture critical pressure; the antisolvent is thus completely miscible with the solvent. Under such conditions the macro-, meso-, and micro-mixing processes can affect the particle size distribution (PSD) and should be considered when the process is scaled up. Scaling up considerations of the precipitation process are based on scale-up rules, CFD simulations and experimental data for paracetamol precipitation. In simulations the model presented earlier (Ba?dyga et al., 2004) that is based on the population balance equation and CFD modelling of compressible flow processes is applied. Results of experimental investigations and numerical simulations are applied to verify scale-up strategies for the SEDS processes.     

Poly(methyl methacrylate) and polyacrylonitrile dispersions stabilized by gelatin

Methyl methacrylate was polymerized in an aqueous medium in the presence of gelatin using potassium persulfate as initiator. The dispersion mode of polymerization, when the monomer is completely miscible with water, was investigated and compared with an emulsion process, which proceeds at higher monomer concentration. Spherical and relatively uniform polymer particles were formed. Macroscopic precipitation of polymer is prevented by combination of the steric stabilization by grafted gelatin and of repulsive electrostatic interactions from the initiator residues attached to the particle surface. Static and dynamic light scattering have been used to determine the molar mass (molar mass of the whole dispersion particle, M_wD ～ 10⁸-10⁹ g mol^?1) and hydrodynamic radius (R_hD ～ 50-120 nm) of the particles. The number of particles per unit volume does not depend on overall monomer concentration, and it is higher, and therefore the particle size is smaller, than that observed for the soapless emulsion polymerization. The addition of gelatin may be thus used to modify the particle size. Acrylonitrile dispersions were prepared under similar conditions. Unlike methyl methacrylate, this monomer does not swell the polymer particles. While poly(methyl methacrylate) particles are spherical and relatively uniform, the polyacrylonitrile dispersions consist of polydisperse aggregates of tiny polymer particles.     

聚氨酯-含氟丙烯酸酯复合乳液的制备及其表面性能

为了得到低表面自由能的聚氨酯-丙烯酸酯乳胶膜,以2,2,3,4,4,4-六氟丁醇甲基丙烯酸酯(FA)、苯乙烯(St)、丙烯酸丁酯(BA)为单体,在交联聚氨酯溶液(PU)中通过溶液聚合相转化法制得阳离子含氟聚氨酯-丙烯酸酯复合乳液(FPUA).通过FT-IR、TEM、粒径分析及接触角测试对聚合物结构、乳胶粒径及形态、乳胶膜表面性能进行了研究.研究表明,含氟丙烯酸酯的引入使聚氨酯-丙烯酸酯乳胶膜的表面自由能降低50%以上,常温固化的FPUA乳胶膜的表面自由能小于0.0172 J&#8226;m^-2;该复合乳液的粒子形态呈球形,粒径约为220 nm.     

Untersuchung über die agglomeration von polymerisatlatices

Different polymer latices are often incompatible, even when the emulsifying agent is the same in each. If one polymer has a proportion of hydrophilic groups (type ?A”? latex) and the other does not (type ?S”? latex), a mixture of the two latices may coagulate within seconds of mixing. Quite small proportions of type ?A”? will cause agglomeration of the particles of a type ?S”? latex. If 1% of a latex prepared from ethyl acrylate and acrylic acid (95:5) is added to a poly(butylacrylate) latex, D?_w, = 800 Å, and the pH is adjusted to 8–9, the resulting latex is characterized by D?_n, = 6300 Å, D?_w, = 7300 Å; addition of any given type ?S”? latex increases the average particle size still more. The mechanisms involved were studied by observing changes in particle size distribution, by the use of polymerizable dyes as markers, and by electron microscopy. The results of these investigations are : The latices are protected only in some respect by the emulsifier. The protective ionic doublelayer formed by the emulsifier around a type ?S”? particle repels other particles, but not type ?A”? particles. If a collision occours between particles of type ?A”? und type ?S”? they stick together; this process is repeated as often as a type ?S”? particle hits the surface of ?A”?. All the type ?S”? particles of the associate combine to one big particle in such a way that the ?A”? particle stays at the surface and remains active for further agglomeration.     

Structure and functional properties of glycerol-plasticized starch/TiO<Subscript>2</Subscript> nanocomposite materials obtained through resonant wave mixing

Resonant wave mixing is a promising technology based on employing non-linear waves to intensify heat and mass transfer and enhancing dispersion processes in mixed materials. In the present work, for the first time, resonant wave mixing was used to prepare film-forming dispersions based on gelatinized maize starch and spherical TiO₂ nanoparticles (0.5–1.5 wt%) synthesized by sol–gel technique. Then, nanocomposite films were obtained by solution casting method. The dynamic viscoelastic properties, including relaxation spectra of the film-forming dispersions were investigated by oscillatory squeeze film rheometry, while the structure of the nanocomposite films was studied by X-ray diffraction, FTIR spectroscopy and scanning electron microscopy. The mechanical, water-related and UV-protective properties of the film materials were evaluated. It was shown that nanofiller incorporation enhanced the density of the 3D network structure of a gelatinized starch dispersion. The resonant wave treatment favored homogenous dispersion of the TiO₂ nanoparticles in the nanocomposites. All nanocomposite film samples displayed higher tensile strength and lower water vapor permeability in comparison with starch films without the nanofiller. The obtained nanocomposites possessed UV-protective properties, which could be potentially applied to produce biodegradable packaging materials with improved functional characteristics.     

Amorphization of Thiamine Mononitrate: A Study of Crystallization Inhibition and Chemical Stability of Thiamine in Thiamine Mononitrate Amorphous Solid Dispersions

This study investigated thiamine degradation in thiamine mononitrate (TMN):polymer solid dispersions, accounting for the physical state of the vitamin and the recrystallization tendency of TMN in these dispersions. Results were compared with those from solid dispersions containing a different salt form of thiamine (thiamine chloride hydrochloride (TClHCl)). TMN:polymer dispersions were prepared by lyophilizing solutions containing TMN and amorphous polymers (pectin and PVP (polyvinylpyrrolidone)). Samples were stored in controlled temperature and relative humidity (RH) environments for eight weeks and monitored periodically by X-ray diffraction and high performance liquid chromatography (HPLC). Moisture sorption, glass transition temperature (T_g), intermolecular interactions, and pH were also determined. Similar to the TClHCl:polymer dispersions, thiamine was more chemically labile in the amorphous state than the crystalline state, when present in lower proportions in amorphous TMN:polymer dispersions despite increasing T_g values, when environmental storage conditions exceeded the T_g of the dispersion, and when co-formulated with PVP compared to pectin. When thiamine remained as an amorphous solid, chemical stability of thiamine did not differ as a function of counterion present (TMN vs. TClHCl). However, storage at 75% RH led to hydration of thiamine:PVP dispersions, and the resulting pH of the solutions as a function of thiamine salt form led to a higher chemical stability in the acidic TClHCl samples than in the neutral TMN samples.     

Preparation and properties of nonionic polyol dispersion from terpinene‐maleic ester‐type epoxy resin

A nonionic epoxy‐based polyol (NTP) which can be used in place of the commonly used polyol dispersions to prepare two‐component waterborne polyurethanes was synthesized with terpinene‐maleic ester‐type epoxy resin (TME), polyethylene glycol (PEG), and trimethylopropane (TMP) in the presence of sulfuric acid as catalyst. The synthesis process was tracked with gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) by investigating the changes of molecular weight and glass transition temperature (T_g) of the product. FTIR was used to characterize the chemical structure of NTP. Major technical parameters of NTP were as follows: hydroxyl value 100 mg/g, hydroxyl group content 3.04%, T_g 4.03°C, and viscosity 150 mPa s (40% solid content). Effect of molecular weights and dosages of PEG on stability of NTP dispersion was examined by particle size analyses. It was found that stable dispersion was obtained when using PEG6000 as hydrophilic chain and its dosage ≥8% by the weight of TME. The average particle size of the prepared dispersion was about 200 nm from particles size and TEM analyses. The NTP dispersion showed characteristic of shear thinning, which indicated it was a pseudoplastic fluid. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Role of Surface Area, Primary Particle Size, and Crystal Phase on Titanium Dioxide Nanoparticle Dispersion Properties

Characterizing nanoparticle dispersions and understanding the effect of parameters that alter dispersion properties are important for both environmental applications and toxicity investigations. The role of particle surface area, primary particle size, and crystal phase on TiO₂ nanoparticle dispersion properties is reported. Hydrodynamic size, zeta potential, and isoelectric point (IEP) of ten laboratory synthesized TiO₂ samples, and one commercial Degussa TiO₂ sample (P25) dispersed in different solutions were characterized. Solution ionic strength and pH affect titania dispersion properties. The effect of monovalent (NaCl) and divalent (MgCl₂) inert electrolytes on dispersion properties was quantified through their contribution to ionic strength. Increasing titania particle surface area resulted in a decrease in solution pH. At fixed pH, increasing the particle surface area enhanced the collision frequency between particles and led to a higher degree of agglomeration. In addition to the synthesis method, TiO₂ isoelectric point was found to be dependent on particle size. As anatase TiO₂ primary particle size increased from 6 nm to 104 nm, its IEP decreased from 6.0 to 3.8 that also results in changes in dispersion zeta potential and hydrodynamic size. In contrast to particle size, TiO₂ nanoparticle IEP was found to be insensitive to particle crystal structure.     

Zum kautschukelastischen verhalten von polyurethan-ionomeren

Dispersions of cationic and anionic ionomers of polyurethanes were prepared by the acetone method. Characteristic changes of the viscosity were observed during the addition of water. This change was studied with a cationic ionomer. The ionomers are mainly associated dimeric species in solution of acetone. During the addition of water the ions are solvated while the hydrophobic segments are increasingly associated. At the maximum of the observed viscosity the apparent molecular weight M_max ≈ 3 M_min. In the absence of solvents the ionomers behave like crosslinked materials even if no covalent crosslinks are present. The modulus at small elongations is a linear function of the square of the concentration of the cations. It is concluded that two ionic centers are required per crosslink. The anionic ionomers were also chemically crosslinked since an excess of isocyanate was used. A linear increase of the modulus was observed with increasing amount of chemical crosslinks, while the concentration of ions and hydrogen bonds was constant. The crosslinks formed by ions can be suspended by swelling with water. The portion of the modulus caused by ionic crosslinking can be computed from the difference of the moduli in the dry and swollen state.     

The influence of the ionic concentration, concentration of the polymer, degree of neutralization and chain extension on aqueous polyurethane dispersions prepared by the acetone process

There are many variables in the preparation of aqueous polyurethane (PU) dispersions. Chemical and compositional variables such as carboxylic acid concentration, concentration of the polymer, degree of pre/post-neutralization of the carboxylic acids and chain extension that all impact solution properties such as particle size and viscosity. Another variable is the method by which the dispersion is prepared; two primary methods are currently employed in industrial manufacture, the prepolymer mixing process and the acetone process. This study evaluates the impact of the chemical variables on a given PU dispersion formulation prepared by the acetone process. Changes in carboxylic acid concentration, degree of pre/post-neutralization and chain extension were found to have the expected impacts on dispersion solution properties. Increased ionic concentration, and degree of pre-neutralization led to lower particle size and higher viscosity, increased degree of chain extension led to larger particle size and lower viscosity, increased post-neutralization increased both particle size and viscosity, and increased concentration of the polymer led to a viscosity increase without any change in particle size.     

Neuere wäßrige PUR-Systeme

A brief survey is given of results of reactions between isocyanate prepolymers and water. Then a variety of improvements concerning the preparation of aqueous dispersions of ionic and nonionic polyurethanes are discussed, namely: Advances in the solvent dispersion and melt dispersion processes, electrolyte and freeze/thawstable dispersions, direct dispersions of solid polyurethanes, and the “ketimine” and ?ketanzine”? processes. The solvent dispersion process is discussed in detail. New products have recently been developed for high quality coatings of textile, leather and flexible PVC as well as special adhesives. New applications for aqueous PUR dispersions are thought to be glass fiber sizing and cathodic dip coating (car bodies). New applications have also been found for emulsfield polyisocyanates and prepolymers as well as for aqueous PUR solutions. Other aqueous systems include micro capsule suspensions, support casts, as well as hydrophilic foams. Interest rigid foams can be made from W/O emulsions which contain inorganic substart dispersed or dissolved in the aqueous phase.     

Mesoglobules of thermoresponsive polymers in dilute aqueous solutions above the LCST

The colloidal stability and characteristics of particles formed by homopolymers of poly(N-vinyl caprolactam), poly(N-isopropyl acrylamide) and poly(vinyl methyl ether) in dilute aqueous solutions above the lower critical solution temperature, LCST, was followed by means of dynamic and static light scattering. Depending on the solution concentration, the homopolymers precipitate or form stable dispersions of monodisperse spherical particles. To obtain colloidally stable aggregates, also called mesoglobules, no stabilising agent was added. The stability of the mesoglobules upon time and dilution at temperatures above the LCST suggests that the particle surfaces possess a hydrophilic character. The size of the formed particles depends on the concentration and the heating rate of the solutions. However, internal structure and shape of mesoglobules are affected neither by the way, how the mesoglobules were prepared, nor by molar mass of individual macromolecules. Mesoglobules of PNIPAM obey the M∝R^2.7 scaling law. Origin of stability of the dispersions vs. expected precipitation is discussed.     

The effect of pH and ionic strength on the dispersion of carbon nanotubes in poly(acrylic acid) solutions

The dispersion of three kinds of acid‐treated carbon nanotubes (CNTs) in poly(acrylic acid) (PAA) aqueous solution of different pH and ionic strengths (varied by NaCl, KCl and ZnCl₂) was investigated by visual observation, zeta potential, particle size analysis, transmission electron microscopy and scanning electron microscopy. Visual observation revealed that the dispersion of CNTs acid treated at 60 °C for 3 h and at 80 °C for 2 h was poor in aqueous solutions with pH < 2 or pH > 12. The poor dispersion of acid‐treated CNTs may be improved by adding PAA. In particular, PAA improved the dispersion of CNTs with greater COOH content. In a low pH solution (pH 1.5), a higher PAA content resulted in poorer CNT dispersion while in a high pH solution (pH 12.5), a higher PAA content led to better CNT dispersion. For superior dispersion in a basic aqueous solution (pH 12.5), experimental data showed that a greater atomic radius or a higher cationic charge of the added salt may result in faster aggregation and thus precipitation of CNTs. Copyright © 2011 Society of Chemical Industry     

Über die flockung von hochverdünnten monodispersen polystyrol-dispersionen mit polyvinylpyridiniumsalzen

Mechanically preemulsified monomer styrene in water, without the use of an emulsifier, gave under definite reaction conditions almost monodisperse polystyrene dispersions, as shown in the electronmicrographs. Some of these dispersions were used in a concentration of 50 ppm to test the flocculation activity of polyvinylpyridinium-salts (PVP-salts). These in turn were prepared by the solution polymerisation of vinylpyridine and a subsequent quaternisation of the polymer with alkyl halides. The molecular weight of the PVP-salts showed a marked influence, especially on very small particle size of the dispersions. Among the inorganic salts, a strong effect of the valency of the cation was noticeable. Cs⁺, Na⁺ and K⁺ have only a little influence, whereas La⁺⁺⁺ and Al⁺⁺⁺ increase the flocculation activity enormously.