Silicone-polyacrylate composite latex particles. Particles formation and film properties

Composite latex particles with a polydimethylsiloxane PDMS core and a poly(methyl methacrylate-co-n-butyl acrylate) P(MMA-BA) copolymer shell were synthesized by seeded emulsion polymerization using the PDMS latex as the seed. The compatibility between the two polymer phases was changed by introducing vinyl groups in the latex core. Monomer conversions and particle size evolution were monitored to see the influence of the nature of the core functionality on the polymerization kinetics and on the extent of secondary nucleation. Particle morphology was characterized by cryo-transmission electron microscopy. The P(MMA-BA) copolymer formed a regular shell around the PDMS seed, whereas nonuniform coatings were formed when vinyl functionalities were introduced into the seed. Films were produced from the latexes, and their surface property was analyzed by X-ray photoelectron spectroscopy and contact angle measurements. It was shown that the PDMS component segregated to the polymer/air interface and that the extent of segregation depended on the original particles structure. Because PDMS has a very low glass transition temperature, it can easily diffuse throughout the film material. However, protected by an acrylic shell, polymer diffusion is significantly hindered and the film then displays all the characteristic properties of the acrylic copolymer. The surface composition of the films formed by the structured particles which PDMS core was not totally covered by the polyacrylate, was found to be intermediate between the composition of the films issued from the core-shell latexes and that of the films produced from blends of pure polyacrylate and PDMS latexes.     

Effect of a coalescing aid on the earliest stages of polymer diffusion in poly(butyl acrylate-co-methyl methacrylate) latex films

In this article we use fluorescence resonance energy transfer (FRET) to investigate how a classic coalescing aid, such as 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (Texanol?) (TX), acts on the earliest stages of polymer diffusion as the latex film is still drying. In our approach, we temporarily arrest the drying process of a partially wet latex film by sealing it in an airtight chamber previously cooled to near the latex T_g. At these conditions, we are able to effectively stop the drying process and the polymer diffusion. FRET measurements at various locations on such a sample provide us information about the mechanism operating at the initial stages of polymer diffusion as the latex film is still drying. We complete our study with FRET measurements carried out at longer aging times on predried latex films. We analyze our diffusion data in terms of free volume theory and propose a mechanism that can account for the results obtained.     

The onset of polymer diffusion in a drying acrylate latex: how water initially retards coalescence but ultimately enhances diffusion

The diffusion of polymer chains across the interface between distinct latex particles is the final step in latex film maturation. This step drives the transformation of a honeycomb of compacted latex particles bound by weak surface forces into a mechanically robust film. Knowledge of the onset of this diffusion process is limited. We have examined film formation in butyl acrylate-methyl methacrylate copolymer latex containing 1 wt% methacrylic acid. These films dry via a propagating drying front. We were able, via fluorescence resonance energy transfer measurements, to determine the extent of polymer interdiffusion at 23°C as a function of distance from the edge of the drying front for a series of partly wet latex films. Our apparatus allows us to arrest the latex drying process and to extract interdiffusion information from sub-millimeter regions of the drying film. We have tracked the latex drying process and subsequent polymer diffusion as a function of humidity. We find that adjacent to the drying front, increasing humidity initially delays the onset of interdiffusion, but once this initial barrier is overcome increasing humidity increases the rate of diffusion. This transition occurs within 1–2 mm of the drying front.

Surface and interfacial FTIR spectroscopic studies of latexes. V. The effects of copolymer composition on surfactant exudation

The influence of copolymer structure on the magnitude of surfactant enrichment to the film—air and film—substrate interfaces of latex films prepared on a polytetrafluoroethylene (PTFE) substrate is investigated. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR FTIR) is used to elucidate the surfactant enrichment by the comparison of butyl acrylate/methyl methacrylate/methacrylic acid (BA/MMA/MAA) and ethyl acrylate/methacrylic acid (EA/MAA) latex systems prepared with various anionic surfactants. It is found that, in all cases, the magnitude of exudation of the sulfonate-containing surfactants manifested by the presence of S—O bending and scissors modes is reduced in the case of the BA/MMA/MAA latex. Similar behavior is observed when the spectra of films prepared on a liquid mercury substrate are compared. It is believed that this behavior results from the longer aliphatic n-butyl groups present in this copolymer that enhances compatibility by providing a greater opportunity for hydrophobic surfactant—copolymer interactions. The butyl groups may also reduce excess interfacial free energy by orienting themselves toward the film interfaces, which, in turn, will reduce the surface tension-induced driving force for surfactant exudation. © 1993 John Wiley & Sons, Inc.     

Heterogeneous latices as binders in porous particle structures

Heterogeneous carboxylated styrene-butadiene (S/Bu) latices were prepared by a two-stage polymerization process, using three seeds of polystyrene with different molecular weights. The second-stage polymer was a copolymer with a fixed S/Bu-ratio of 1 and a methacrylic acid (MAA) content of either 1 or 10 wt %. It has been found that the morphology of the films made from these latices influenced the modulus in the rubbery region of these films. The heterogeneous latices were used as binders in porous structures based on micron-sized kaolin particles. Such structures are typically employed as paper coatings. Polyester substrates were coated with aqueous suspensions containing the kaolin particles and the heterogeneous latex. The coatings were dried at room temperature, which corresponds to the rubbery region of the latex films. It was found that a higher modulus (which is determined here by the morphology of the latex film) in the rubbery region of the films was associated with coating layers with higher porosity, greater light scattering ability, and higher coating gloss. This is interpreted as being the result of a retarded shrinkage of the coating layers during the drying of these structures due to the increase in modulus of the latex films. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 661–670, 1997     

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.     

Stability and compatibility in blends of silicone and vinylacrylic polymer emulsions

Special, unexpected properties may arise from blending of polymer emulsions, depending on the miscibility and stability of the resulting dispersions on the one hand, and the morphology of the films obtained upon drying on the other hand. In the field of water-repellent paints, blends of silicone and vinylacrylic copolymer emulsions have been previously shown to be a tricky means to associate binding ability and water permeability properties in the same material. It is currently assumed that the resulting morphology and physical properties of films obtained from these blends upon drying is partially controlled by the compatibility of the two kinds of polymers. In the present paper, the crucial problem of stability and compatibility in blends of silicone and vinylacrylic copolymer emulsions is investigated; more particularly, the effect of pH of the vinylacrylic copolymer latex, as well as the influence of the stabilizing system of both emulsions, on the stability of emulsion blends is qualitatively described. As regards the morphology of the films, the strong incompatibility of the two polymers is shown, which results in large macroscopic heterogeneities in the dried composite films. Furthermore, semi-quantitative angle-resolved ESCA measurements have evidenced a pronounced diffusion of the silicone species towards the surface of the samples. Investigations into different ways to increase compatibility of the two phases is presented. It is shown that phase separation at the macroscopic scale can be controlled either by lowering the interfacial tension between the two phases by grafting silane compatibilizers on to the latex particles, or by decreasing the molecular mobility of the silicone phase by cross-linking. Diffusion of the silicone chains towards the surface of the composite films can only be significantly restricted by cross-linking of this species.     

Effect of molecular weight distribution on polymer diffusion during film formation of two-component high-/low-molecular weight latex particles

We describe fluorescence resonance energy transfer (FRET) studies of film formation by a new type of two-component latex particles. These particles consist of a miscible blend of two components that have a similar composition but very different molecular weights. In our approach, we used sequential seeded emulsion polymerization to generate (in situ) a fraction of oligomer in poly(butyl acrylate-co-methyl methacrylate) P(BA-MMA) seed particles that contained a relatively high molecular weight (high-M) dye-labeled polymer. In this way we could systematically change the molecular weight distribution of polymer inside the particles. We varied the amount and the molecular weight of the oligomers. For latex films cast from these two-component particles, we studied the diffusion rate of the high molecular weight polymer by FRET. These measurements revealed that oligomers promoted diffusion rate during latex film formation (oligoplasticization). We analyzed our diffusion data in terms of the Fujita–Doolittle free-volume model and showed that higher molecular weight oligomers are less efficient as plasticizers. In separate experiments, oligomers with similar molecular weights as those in the two-component particles were introduced via latex blending. We compared oligoplasticization in latex blends films with that in the two-component particles films. Finally, we investigated the rheological behavior of the two-component polymers with compositions adjusted to have a common T_g (2 °C). The higher the molecular weight of the oligomer, the more that had to be added to achieve T_g = 2 °C. All of the oligomers were much shorter than the entanglement length and act as diluents of the entanglements in the high-M polymer. We found that incorporating larger amounts of oligomers with a higher molecular weight resulted in a more pronounced drop in polymer viscosity, associated with the decrease in the entanglement density.     

Surface and interfacial FTIR spectroscopic studies of latexes. IV. The effect of surfactant structure on the copolymer–surfactant interactions

The interactions between sodium dodecylbenzene sulfonate (SDBS) and the components of an ethyl acrylate/methacrylic acid (EA/MAA) copolymer latex were examined and the influences of using D₂O as the synthetic suspension medium were investigated. Whereas it is found that D₂O has no detectable influence on the fully coalesced latex films, the film coalescence conditions are shown to significantly affect the nature of surfactant interactions within the film matrix. When films are prepared and stored under controllable low atmospheric water-vapor concentrations, hydrogen-bonding interactions between the surfactant SO₃^?Na⁺ groups and the copolymer acid functionality dominate. However, coalescence and storage of the films under higher relative humidity conditions results in a displacement of these interactions in favor of the hydrated form of the surfactant. It is also shown that the presence of an aromatic ring adjacent to the surfactant sulfonate group exerts an influence on the nature of the SO₃^? ··· HOOC interactions within the copolymer matrix. Relative to sodium dioctyl sulfosuccinate (SDOSS), the aromatic group near hydrophilic end of SDBS increases the strength of the S? O bond in the presence of acid interactions. © 1993 John Wiley & Sons, Inc.     

Interfacial aspects of strength development in poly(methyl methacrylate)-based latex systems

Film formation from poly(methyl methacrylate) (PMMA) latex and PMMA copolymer latex incorporating N-(iso-butoxymethyl)acrylamide (IBMA) or methacrylic acid (MAA) has been investigated in terms of the development of tensile strength as a function of annealing time and temperature. Tensile strength is developed through a combination of macromolecular interdiffusion and interfacial crosslinking. The relative rates of interdiffusion vs. crosslinking reactions were studied as a function of temperature and the chemical nature and concentration of the IBMA and MAA functional groups. For low concentrations of these two functional monomers it appears that polymer chain interdiffusion between adjacent latex particles during the film formation process dominates the kinetics of strength development. However, at higher IMBA and MAA concentrations, the higher glass transition temperature at the latex particle surface and intraparticle crosslinking hinders interdiffusion, as reflected by differences in the power law exponent values obtained from the log-log dependence of tensile strength on annealing time. The power law exponents were higher in the case of PMMA than for both IBMA- and MAA-containing copolymers. There was a greater influence temperature on the tensile behavior for the MAA copolymer system as compared to the IBMA copolymer. In the interfacially crosslinked latex polymer system, there is competition between the interdiffusion and crosslinking mechanisms in determining the final mechanical strength of films during the annealing process. © 1995 John Wiley & Sons, Inc.     

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.     

Polymer diffusion and mechanical properties of films prepared from crosslinked latex particles

Wedescribe energy transfer (ET)measurements to follow polymer diffusion, as well as oscillatory dynamic mechanical measurements and tensile measurements, on films prepared from structured and unstructured latex particles consisting of a copolymer of butyl methacrylate and butyl acrylate with a T_g of 20°C. Structure was introduced in the form of a low level (1 mol%) of crosslinking, using seeded semi-continuous emulsion polymerization to control the locus of the crosslinking agent in the particles. Linear dynamic mechanical measurements showed the G′ and G″ were sensitive to the particle morphology, with particular sensitivity exhibited by the elastic modulus G′. The tensile properties were less sensitive to particle morphology; sufficient polymer diffusion occurs during film formation for the films to acquire strength and toughness. As expected, crosslinking increases strength but decreases elongation to break. Some interesting compromises could be found through control of the location of the crosslinked regions of the film. Dept. of Chemistry, 80 St. George St., Toronto, Ont., Canada M5S 3H6. F-27470 Serquigny, France.     

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.     

Formation and crosslinking of latex films through the reaction of acetoacetoxy groups with diamines under ambient conditions

We investigated the processes of film formation, polymer diffusion, and crosslinking of latex films at ambient temperature, using low T_g methacrylate latex bearing acetoacetoxy groups, and curing the systems with 1,6-hexanediamine as the crosslinker. The addition of diamine induces floc formation, which modifies the rheological properties of the dispersion and increases its drying rate when coated onto a substrate. The crosslinking reaction between diamine and acetoacetoxy groups occurs at a rapid rate, even in the dispersed state. Although the crosslinking reaction precedes polymer diffusion in the two systems we examined, latex films with relatively good solvent resistance are obtained. Department of Chemistry, Toronto, Ontario, Canada M5S 3H6. Department of Polymer Chemistry, P. O. Box 513, 5600 MB Eindhoven, The Netherlands.     

Preparation of polymer latex films by a flash casting technique

Conventional film casting methods have proved unsuitable for the preparation of thin (10–100 μ) films from surfactant-free polymer latices. A flash casting technique has been devised and successfully employed to cast films from a wide range of surfactant-free homopolymer, copolymer and core-shell polymer latices. Essentially, the method involves spraying the latex at a specially designed hotplate, the surface of which is coated with a thin layer of PTFE. The hotplate is maintained at a temperature between 393 and 473 K. When sufficient thickness of film has been built up, the hotplate is switched off and allowed to cool to the temperature at which the film is to be removed. For homogeneous latex films, the optimum temperature range for removal lies in the region of the glass transition temperature of the polymer concerned. Where this is below ambient, the appropriate cooling mixture is placed in a slush bath, situated to the rear of the hotplate, thereby cooling the hotplate to the correct temperature. The film is removed by gently peeling it from the surface. The surfaces of the films appeared smooth to the naked eye, but the sprayed surfaces of freshly prepared poly(n-butyl methacrylate) films were rough when examined by scanning electron microscopy. The surfaces smoothed out on ageing, resembling those of solvent-cast films after one months storage.     

A feasible method of preparation of block copolymer latex films with stable microphase separation structures

RAFT (reversible addition–fragmentation chain transfer) miniemulsion polymerization was engaged to engineer latex particle morphology. With this approach, a macromolecular amphiphilic RAFT agent with epoxy groups was synthesized that assembled onto the surface of monomer mini-droplets. It caused the polymer chains to grow inwards gradually in particles as polymerization proceeded. The batch polymerization of n-butyl acrylate (BA) followed by addition of styrene (St) led to the formation of PBA-b-PSt diblock copolymer shell–core latex, where epoxy groups were enriched on the particle surface. The shell–core ratio was varied feasibly by changing the mass of St. When the structured latexes were dried, epoxy groups underwent efficient curing reactions triggered by a thermal-latent curing agent (dicyandiamide) in a controlled manner, leading to the formation of bonded PBA blocks connecting the PSt blocks in adjacent particles. Mechanical tests show that the films behaved like ductile materials, whose modulus and elongation at break were functions of copolymer compositions. Furthermore, curing reaction was a very robust method of preserving film morphology which correlated well with that observed for the latex particles. The results demonstrated a feasible method of preparation of latex films with stable microphase separation structures and thus improved mechanical properties.     

Water vapour permeability,diffusion and solubility in latex films

Water vapour diffusion D, solubility S and permeability P coefficients have been determined for films obtained from carboxylated styrene-butadiene (SB) copolymer latexes. The experimental method is water vapour sorption performed in the range 30–60d?C. Using the small angle neutron scattering (SANS) method and a selective labelling with D₂O vapour, it has been shown that water molecules mainly diffuse in the films through the particle-particle interfaces, which consist of a polar carboxyl-rich copolymer. It has been shown that the degree of cross-linking of the particles does not significantly affect the values of D and S. Moreover, the effect of neutralization conditions, regarding both the pH value of the initial latex and the nature of the neutralizing agent, has also been investigated. It has been found that D does not depend on these parameters, whereas S appears to be very sensitive to them. The results have been interpreted on the basis of the structural modifications of the films induced by neutralization. Finally, the hydrophilicity (or hydrophobicity) of the neutralizing agent has been identified as one of the key features for controlling the affinity of the latex film for water vapour and hence its permeability properties.     

Adhesion of latex films. Part III. Surfactant effects at various peel rates

In order to study the effect of surfactants on the adhesion of latex films, peel energy versus surfactant concentration curves were established at various peel rates. The main latex polymer was a methyl methacrylate (MMA)/ethyl acrylate (EA) copolymer synthesized in the presence of a hydrophilic polyester. Another polymer, less extensively studied, a styrene/butyl acrylate/methacrylic acid terpolymer, was also used for comparison purposes. The surfactants were either sodium dodecyl sulfate (SDS) or ethoxylated nonyl phenol containing 30 segments of ethylene oxide (NP30). The substrates were glass plates or poly(ethylene terephthalate) films. It was found that with SDS-containing films, whatever the substrate or the polymer, the curves went through a maximum, whereas with NP30 they went through a minimum, at medium or high peel rates. When the peel rate was decreased, the curves flattened out and at zero peel rate (extrapolated values), they became horizontal. The peel energies at zero peel rate were three to four times higher than the reversible works of adhesion. Qualitative interpretations are proposed for these results.     

Interdiffusion and crosslinking in thermoset latex films

Thermoset latex systems represent an attractive approach to obtaining the high performance needed in many different kinds of industrial coatings, while satisfying the growing requirement for environmental friendliness. In these coatings in the dispersed state, the reactive groups are packaged inside of polymer particles. These latex particles deform as the coating dries to form a transparent binder phase. The useful properties of mechanical strength, as well as scrub and solvent resistance, develop over time. This paper focuses on the idea that to achieve the desired properties in a thermoset latex coating, one has to pay proper attention to the relative rates of polymer diffusion and crosslinking in the coating. Strength in these films develops as a consequence of chains that connect crosslink points on opposite sides of interface formed between adjacent particles in the film. Thus polymer diffusion must precede extensive bond formation created by the crosslinking chemistry. This paper reviews fundamental concepts and then describes experiments in three separate systems. These experiments show that the formulator has three main strategies to vary the relative rates of these processes: 1. Catalyst strength and concentration will affect the reaction rate. 2. Polymer chain length will affect the polymer diffusion rate. 3. Temperature changes will normally have a larger affect on the polymer diffusion rate than on the crosslinking reaction rate. Presented at the 79th Annual Meeting of the Federation of Societies for Coatings Technology, on November 3, 2001, in Atlanta, GA. Department of Chemistry, Toronto, Ont., M5S 3H6, Canada.     

DRYING OF LATEX FILMS OF POLY(VINYL ACETATE)

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.