Final Latex Research Articles

Effect of interfacial phenomena on the development of particle morphology in a polymer latex system

Seeded batch emulsion polymerization was carried out using monodisperse polystyrene latex particles as seed and methyl methacrylate as monomer at different seed polymer/monomer weight ratios. The final composite latex particles were found to have a mixture of more than one nonuniform morphology. A thermodynamic analysis and a mathematical model were derived to describe the free energy changes corresponding to various possible thermodynamically unstable morphologies during the course of the polymerization. The mechanism of changes in particle morphology during the polymerization is proposed

The effect of sodium methacrylate on the soapless emulsion copolymerization of methyl methacrylate and n‐butyl acrylate

AbstractThe soapless emulsion polymerization of methyl methacrylate (MMA) and n‐butyl acrylate (BA) containing various concentrations of sodium methacrylate (NaMA) or methacrylic acid (MMA) is studied. The hydrosoluble yields in final latexes are not larger than 1.3–5%, depending on the concentration of NaMA used. Below 25% conversion, the change of conversion with reaction time follows the square rule and the particle size is proportional to the 2/3 power of time. Above 25% conversion, serious gel effect occurs, and the conversion follows the seventh power on time and the growth of particle diameter obeys the 2.5 power on time. The multiple glass transition (Tg) occur below 20% conversion, where monomer droplets exist. NaMA added induces more Tgs. The effect of molecular weight of the copolymers obtained on Tg (even the molecular weight distributions were shown to be shouldertype bimodal) is estimated to be insignificant. Thus, the heterogeneity of copolymer compositions for multiple Tgs is ascribed to be caused from neither the molecular weight heterogeneity nor the shifts in compositions due to the difference of the monomer reactivity ratios. Referring to the results mentioned, we assume the sublayer surrounding the particle, rich with SO4− and COO− groups, and the concentration gradients of monomers in particles to illustrate particle morphology. In addition, the relatively hydrophilic sublayer is proposed to be closely relative with the occurrence of the composition heterogeneity in particles.

Role of itaconic acid in latex particle nucleation

AbstractItaconic acid, a carboxylated water‐soluble monomer, was examined in terms of its role in latex particle nucleation when copolymerized with acrylate monomers. From emulsion polymerizations in which the particle number concentrations were followed as a function of monomer conversion, it was found that the greater the amount of itaconic acid present, the larger the final latex particle diameter. This was interpreted as the result of the lower number of primary particles being initially nucleated. The above observations can be explained by the incorporation of the hydrophilic itaconic acid into acrylate oligomeric radicals that initially form. This incorporation increases the water‐solubility of the radicals such that a lower proportion of these radicals is likely to precipitate to form primary particles.

Microemulsion polymerization of styrene

Microemulsion polymerization of styrene initiated by water-soluble potassium persulphate produced microlatexes which are stable at room temperature even after a year of preparation. The effects of initiator concentration, polymerization temperature, as well as monomer concentration on the polymerization kinetics and molecular weight distribution were investigated. The microemulsion polymerization rate shows only two reaction intervals as a function of monomer conversion. It increased to a maximum at about 20% conversion and then decreased on further polymerization. The final latexes obtained are characterized by high molecular weight of polystyrene ( $$\bar M$$ w: 5−10x106) and small particle diameter (D: 20–40 nm). It is calculated that each latex consists of two to four polystyrene chains.

Emulsifier‐free emulsion copolymerization of styrene and butyl acrylate. I. Kinetic studies in the absence of surfactant

AbstractSoapless emulsion copolymerization of styrene (S) and n‐butyl acrylate (BuA) has been investigated using two types of initiator and different comonomer feed mixtures. When using K2S2O8 as initiator, the particle size and size distribution of the final latexes (500 nm and 1.003, respectively) is not significantly affected by the comonomer feed composition, whereas the molecular weight and surface characteristics were found to sharply change at high butyl acrylate content. Based on the most probable particle nucleation mechanism and type of chain termination in the monomer swollen particles, a tentative explanation of these results has been proposed. Replacing persulfate by a carboxylic initiator (4‐4′‐azobiscyanopentanoic acid) results in the formation of stable particles as α observed with the persulfate, provided the aqueous phase pH is fixed in between 6 and 7. Results on the initiator residue location as a function of the conversion point out that the particle flocculation mechanism is strongly significant in the preparation of such latexes.

Miniemulsion copolymerization of styrene–methyl methacrylate

AbstractMiniemulsion copolymerization of 50 : 50 weight fraction of styrene–methyl methacrylate monomer, using hexadecane as the cosurfactant, was carried out in both unseeded and seeded polymerizations. Effects of the hexadecane concentration and the ultrasonification time on the conversion–time curves and particle size of the final latex were investigated for unseeded polymerization. The kinetic and particle size distribution results showed that an increase in hexadecane concentration and ultrasonification time cause faster polymerization rate and smaller particle size. The mechanism of mass transport from miniemulsion droplets to polymer particles was also investigated for seeded polymerization. For this purpose a monomer miniemulsion was mixed with a fraction of a previously prepared miniemulsion latex particles prior to initiation of polymerization, using residual oil‐soluble initiator in the seed latex. The concentration of hexadecane and a water‐insoluble inhibitor (2,5 di‐tert‐butyl hydroquinone) in the miniemulsions were the main variables. Seeded polymerizations were also carried out in the presence of miniemulsion droplets containing a water‐insoluble inhibitor and water‐soluble initiator. The inhibitor concentration and the agitation speed during the course of polymerization were the experimental variables. The kinetic and particle size results from these seeded experiments suggested that collision between miniemulsion droplets and polymer particles may play a major role in the transport of highly water‐insoluble compounds.

Emulsifier‐free emulsion copolymerization of styrene and butyl acrylate. II. Kinetic studies in the presence of ionogenic comonomers

AbstractBatch emulsifier‐free copolymerizations of styrene (S) and butyl acrylate (BuA) have been performed for a S/BuA weight ratio = 50/50 in the presence of two types of functional comonomers [methacrylic acid (MAA) at different pHs] or potassium sulfopropylmethacrylate (SPM) and two initiators [potassium persulfate or 4–4′azobiscyanopentanoic acid (AZO)]. The use of AZO/MAA system results in the formation of polymer particles with only surface carboxylic end groups. The particle size of the final latexes can be adjusted with the MAA concentration, provided the polymerization is carried out at pH > 6.5. However, the higher the MAA concentration, the sooner the polymerization levels off in conversion. With the K2S2O8/SPM system, particles bearing only sulfate and sulfonate groups are produced and the polymerization is complete. In that case, the particle size of the final latexes is smaller than with the previous system and 30% of the SPM is fixed on the particle surface, instead of 10% with MAA. Using SPM, a too high functional monomer concentration results in the latex destabilization caused by the formation of a large amount of polyelectrolytes. Kinetic studies indicate that most of the functional monomer is incorporated onto the particle surface during the last 30% conversion of the polymerization. A tentative explanation of such a behavior is discussed, based on the existence of two polymerization loci in the latex system.

Kinetics of emulsion copolymerization of vinyl acetate with butyl acrylate

With ammonium persulphate as initiator and Aerosol MA and Aerosol OT as emulsifiers, batch copolymerization of butyl acrylate and vinyl acetate (VAc) was investigated at 60°, varying only the initial monomer feed composition. Accurate kinetic (polymerization rates, sequence distribution) and colloidal data (particle size and distribution) were collected on the final latexes and as a function of conversion. Polymerization in the water phase was not negligible for VAc and experimental data were well fitted by simulations taking into account the monomer partition between water and organic phases. Moreover, the simulation of the particle monomer swelling makes possible determination of the overall propagation rate constant, its changes as a function of conversion and, as a consequence, to derive from kinetic ( R p) and colloidal ( N p) data, the changes in the average number of radicals per particle, n . The lower polymerization rates observed for copolymerizations, by comparison with the homopolymerizations, can be explained in terms of a change in the overall propagation rate constant as well as in n . Experimental data and theoretical simulations, which fit quite well, are in agreement with homogeneous particle nucleation mechanisms and particle structuration.

On-line Control Studies of Continuous Emulsion Polymerization Systems

A redesigned latex reactor train configuration for the continuous production of emulsion poly (vinyl acetate) is being employed in order to test and implement on-line computer control stategies. Multivariable computer control of the final latex polymer particle size and monomer conversion is achieved using several algorithms which manipulate the feedrate of initiator to the first reactor of the train, and the flow split of the monomer and water feedrates between the first two reactors. The control algorithms include simple single loop feedback, multivarable, and optimal controllers. Both set-point changes and disturbance rejection (mainly water soluble impurities or oxygen contamination) are being covered. Control is implemented using a VAX 11/750 with a PDP-11 front end system interfaced to our pilot plant stainless steel 1-gallon reactors.

On-line Particle Size and Conversion Determination in Continuous Latex Reactor Trains

Particle number and particle size distribution are key parameters in emulsion polymerization because they not only affect the polymerization rate during the reaction but they also determine application properties of the final latex such as stability, viscosity, film forming properties, etc. Because of their important role a control scheme for latex reactors cannot be complete without the control of these two parameters. Therefore there is a need for accurate and fast on-line measurements of the number of particles and the particle size distribution in the latex. On line turbidity and density measurements will be demonstrated to clearly follow polymerization property trajectories and their potential for on-line latex reactor computer control will be further illustrated by experimental evidence from our pilot plant 1 gallon stainless steel reactor train.

The determinants of latex monodispersity in emulsion polymerizations

The kinetic factors that determine the monodispersity of polystyrene latices produced by emulsion polymerization were explored, both experimentally and theoretically, in order to understand why certain surfactants (e.g., Aerosol MA) generate latices with a higher degree of monodispersity than others (e.g., sodium dodecyl sulfate). Three surfactants, Aerosol MA, Aerosol OT, and sodium dodecyl sulfate, were found experimentally to generate latices whose particle size distributions at the end of the nucleation period, although different, were all positively skewed. This skewness is in conformity with the coagulative nucleation theory. Experiments also showed that the free radical entry rate coefficient in the subsequent growth of the latex particles was inversely related to the number of latex particles generated, the latter being smaller for aerosol MA than for sodium dodecyl sulfate. Theoretical calculations based on coagulative nucleation theory showed that neither the duration of the nucleation period nor the form of the particle production rate curves was an important determinant of monodispersity at long times (although it may exert large effects at earlier times). In contrast, the monodispersity of the final latex produced was found theoretically to be critically dependent upon the entry rate parameter, the coefficient of variation of the latex being reduced as the entry rate coefficient increased. It can be inferred that Aerosol MA favors the production of a monodisperse latex primarily because it generates relatively few latex particles during nucleation: as a consequence, the free radical entry rate coefficient is relatively high (varying approximately inversely with particle number), which is conducive to the production of a monodisperse latex. Thus those properties of a surfactant that are able directly or indirectly to influence growth kinetics in emulsion polymerizations are important determinants of the size and size distribution of the latex particles ultimately generated. © 1987 Academic Press, Inc. All rights reserved.

TEM analysis of core/shell latex morphology

Transmission electron microscopy is an excellent method to study particle size and particle morphology of latexes. Special sample techniques are utilized in the study of latexes and often more than one technique is needed to answer questions concerning the structure of the particle. If the latex is of a core/shell type the study becomes more complex. Sample preparation for studying latexes include staining with osmium tetroxide to crosslink and stain unsaturated polymers, cold stage if the latex is too soft to examine at room temperature, negative staining to increase contrast shadowing to determine the sag of the particle at room temperature, ultramicrotoming, cryoultramicrotoming or a combination of techniques such as staining and cold stage.The latex investigated was the first stage seed latex of poly (butylacrylate - butadiene) and the final latex a core-shell of poly(butylacrylate - butadiene)/poly(methyl methacrylate). Because of its unsaturation, poly(butadiene) can be preferentially stained with osmium tetroxide.

Inverse emulsion polymerization of acrylamide. II. Synthesis and characterization of copolymers with methacrylic acid

AbstractInverse emulsion copolymerization studies of acrylamide (Am) with methacrylic acid (MAA) are reported. Aqueous monomer solutions were emulsified in toluene with a blend of two surfactants (sorbitan sesquioleate and C18‐terminated acrylamide oligomers). Polymerization kinetics in presence of an oil‐soluble initiator (AIBN) were determined at 40°C as a function of methacrylic acid content and aqueous‐phase pH. Polymerization rates were found to be faster at basic pH than at acidic pH, which appeared to be related to the actual concentration of methacrylic acid in the aqueous phase. Monomer reactivity ratios have been derived as rA‐M = 0.58 ± 0.02 and rM‐A = 4.0 ± 0.10 at pH 4, rA‐M = 0.56 ± 0.005 and rM‐A = 0.15 ± 0.03 at pH 10. These differences were found to have an effect on the molecular characteristics of the copolymers. Initial emulsions and final inverse latexes displayed the same broad size distribution; under basic pH the particle size is relatively insensitive to the ionic comonomer concentration. Poor latex stability is characteristic of copolymer latexes prepared under acidic conditions. Based on these experimental results, some aspects of the polymerization mechanism are discussed.

Emulsion copolymerization of methyl methacrylate with ethyl acrylate, 1. Effect of the initiator concentration on the polymerization behaviour

AbstractThe emulsion copolymerization of methyl methacrylate with ethyl acrylate, initiated by ammonium persulfate in the presence of both anionic and nonionic emulsifiers, was kinetically investigated at 60°C by the conventional gravimetric method. In the range of low conversions the rate of polymerization was found to be proportional to the 0,5 power of the initiator concentration. Over the range of medium conversions the rate of polymerization is approximately proportional to the 0,35 power of the initiator concentration and the number‐average molecular weight is inversely proportional to the 0,4 power of the initiator concentration. The molecular weight increases up to 30% conversion and then it remains constant up to the total monomer consumption. It was found that the mean particle diameter decreases while the number of particles per volume unit, the conductivity of final latices, the coagulate yield and the number of radicals per particle increase with increasing concentration of the initiator.

Synthesis of poly(vinyl alcohol-g-4-vinylpyridine hydrochloride) for application as a surfactant in emulsion polymerization of methyl methacrylate

A redox initiation system composed of ceric ion and poly(vinyl alcohol) was used to successfully synthesize poly(vinyl alcohol-g-4-vinylpyridine hydrochloride). The aqueous solutions of this polycationic surfactant exhibited no sharp CMC value and tended to become turbid upon standing. This turbidity was related to the formation of crystalline-like aggregates in solution, and was thermally reversible. PMMA latices were unstable when formulated with turbid surfactant solutions, but when surfactant solutions were thermally equilibrated, the final PMMA latices were stable.

Inverse emulsion polymerization of acrylamide. I. Contribution to the study of some mechanistic aspects

AbstractThe inverse emulsion polymerization of aqueous solution of acrylamide in toluene has been studied at 40°C using a blend of surfactants as emulsifying system and oil soluble azo initiators. The azo compound partition between the phases has been measured and the effects of their nature and concentration on the polymerization kinetics have been investigated. The influence of other parameters on the kinetics and particle size of the inverse latex have also been investigated: the nature and amount of the emulsifier system, the stirring rate, and the presence of oil‐soluble inhibitor. The particle‐size analysis using electron microscopy or dynamic light‐scattering methods showed the presence of two populations of particles in the initial monomer emulsion and in the final inverse latex: one with very tiny particles (20 nm diam) and the other with larger particles (80–400 nm diam) which is highly polydispersed. The average size of these large particles undergoes a sharp decrease at a certain percent conversion depending upon the stirring rate. The evolution of the particle size distribution may result from a balance between coalescence and dispersion of the emulsion droplets under the effect of prevailing shear rate due to agitation.Concerning the initiation process, the very low solubility of the azo compound in the aqueous solution, together with the effect of the stirring rate and the presence of an oil‐soluble inhibitor on the polymerization kinetics lead to the conclusion that most of the initiaton originates from the capture of radicals or oligomeric radicals produced in the oil phase or in the interfacial layer.

On-line Computer Control of a Continuous Latex Reactor Train

During continious emulsion polymerization of many monomers. Mistained oscillat ions limit cycles occur in all the latex and polymer properties This creates severe problems with the commercial application of continuous reactors.A mathematical model, based on a particle age distribution analysis and incorporating detailed chemistry and physics of the Polymerization phenomena involved. is used to simulate the dynamic behaviour of these reactors Advanced control theory is shown to he inadequate for controlling these svstems. Rather. it shown that the limit cycle behaviour must be eliminated through redesign of the reactor train configuration using the dynamic model. The redesigned reactor Configuration is shown to eliminate the Oscillations . and to offer greatly increased flexibility in controlling the particle size and conversation.Experimental studies which demonstrate the improved dvnamic performance of the redesigned reactor system are carried out in a pilot plant stainless steel reactor train. On line measurements of conversion and particle size are obtained using densitometers and U.V spectrophotometers. Off line measurements of particle size are obtained by turbidity sped ra. size exclusion chromatography (SEC) and electron microskopy. and measurements of the molecular weight averages are obtained by gel permeation chromatomaphy (GPC) and low angle laser light scattring (LALLS).Multivariable computer control of the final latex particle size and thie monomer conversion is achieved using several algorithms which manipulate the feed rate of initiator to the first reactor and the split of the monomer and water feed rates between the first two reactors Control is achieved using VAX plus PDP front end system interfaced to the pilot plant reactors.

The formation of small scale granularities in latex particles

A series of latices were synthesized using emulsifier-free emulsion copolymerization of styrene and sodium styrene sulfonic acid. The final latex particles display an internal granular structure which can be ascribed to the primary particles present in the early stages of particle growth. In these systems, the primary particles appear to have maintained their integrity during the swelling and growth stage.

Some aspects of inverse emulsion copolymerization of acrylamide with methacrylic acid

Inverse emulsion copolymerization studies of acrylamide (Am) with methacrylic acid (MAA) are reported. Aqueous monomer emulsions in toluene were prepared with a blend of two surfactants (sorbitan sesquioleate - Cl8 - terminated acrylamide oligomers). Polymerization kinetics in presence of an oil soluble initiator (AIBN) were determined at 40°C as function of MAA content and aqueous phase pH : monomer reactivity ratios have been derived as rAm = 0.58 ± 0.02, rMAA = 4.40 ± 0.10 at pH 4 and rAm = 0.56 ± 0.005, rMAA = 0.15 ± 0.03 at pH 10. Particle size and stability of inverse monomer emulsions and final latexes were found to depend upon the aqueous phase pH, as well as the intrinsic viscosities of the copolymers; this suggests differences in polymerization mechanism versus the pH.

Batch emulsion copolymerization of n-butyl acrylate and methyl methacrylate in the presence of a nonionic surfactant

Batch emulsion copolymerization studies of n-butyl acrylate and methyl methacrylate have been investigated using potassium persulfate as initiator and a nonionic surfactant (REWOPAL HV 25) as emulsifier. Higher polymerization rates, shorter nucleation stage and complete conversion are obtained by comparison to the SDS-emulsified copolymerization. The kinetic and particle number data support a micellar-type mechanism for the formation of copolymer particles. Strong affinity between the two monomers and the nonionic surfactant results in significantly high solubilization of the surfactant in the initial monomer phase and its subsequent burial inside the final latex particles.