Effect of crosslinker reaction rate on film properties for thermoset coatings

With thermoset coatings that employ room temperature crosslinkers, there is often a competition between film formation and crosslinking. In order to meet the early use/handling requirements for the coated substrate, formulators sometimes need to use crosslinkers with very fast room temperature reaction rates. However, when the coatings are based on aqueous latex resins, sufficient time must be allowed for latex particle coalescence to occur prior to crosslinking, or the resulting films will be partially or completely precross-linked, which is detrimental to many coatings properties. In this paper the effect of the crosslinker reaction rate on various film properties of coatings based on acrylic and acrylic/styrene latex resins will be investigated using several model carbodiimide-functional crosslinkers which have different reaction rates. Presented at the 76th Annual Meeting of the Fedeation of Societies for Coatings Technology on October 14–16, 1998, in New Orleans, LA. 727 Norristown Rd., Spring House, PA 19477, E-mail: RSCWTB@rohmhaas.com.     

The effects of latex coalescence and interfacial crosslinking on the mechanical properties of latex films

The effects of latex coalescence and interfacial crosslinking on the mechanical properties of latex films were extensively investigated by means of several series of model latexes with varying backbone polymer crosslinking density and interfacial crosslinking functional groups. It was found that the tensile strength of crosslinked model latex films increased with increasing gel content (i.e. crosslinking density) of latex backbone polymers up to about 75% and then decreased with further increase in gel, while their elongation at break steadily decreased with increasing gel content. These findings showed that latex particle coalescence was retarded above a gel content of about 75% so that the limited coalescence of latex particles containing gel contents higher than 75% prevented the tensile strength of crosslinked latex films from increasing by further crosslinking the latex backbone polymers. This was contrary to the theory of rubber elasticity that the tensile strength increases with increasing molecular weight and crosslinking density. This limitation was found to be overcome by the interfacial crosslinking among latex particles during film formation and curing. This paper will discuss the effects of both latex backbone polymer and interfacial crosslinking on latex film properties. It will also discuss the development of self-curable latex blends and structured latexes containing co-reactive groups: oxazoline and carboxylic groups.     

Surface and interfacial FTIR spectroscopic studies of latexes. IX. The effect of homopolymer and copolymer structures on surfactant mobility in Sty/BA latices

The effects of homopolymer and copolymer compositions and structures in styrene/n-butyl acrylate (Sty/BA) latices on sodium dioctyl sulfosuccinate (SDOSS) surfactant mobility and its preferential concentration at the film–air (F–A) and film–substrate (F–S) interfaces were examined using attenuated total reflectance Fourier transform infrared (ATR FTIR) spectroscopy. It appears that the SDOSS concentration at the F–S interface is highest when the Sty/BA feed ratio is 50/50, and the excess of Sty results in migration of SDOSS surfactant to the F–A interface. This behavior is attributed to the increased glass transition temperature and diminished compatibility between surfactant molecules and copolymer latex. This study also shows that the primary factors that influence exudation to either F–A or F–S interfaces are surface tension of the substrate, glass transition temperature, water flux during coalescence, and compatibility between latex components. © 1995 John Wiley & Sons, Inc.     

Skin Development during the Film Formation of Waterborne Acrylic Pressure-Sensitive Adhesives Containing Tackifying Resin

Tackifying resins (TR) are often used to improve the adhesive properties of waterborne pressure-sensitive adhesives (PSAs) derived from latex dispersions. There is a large gap in the understanding of how, and to what extent, the film formation mechanism of PSAs is altered by the addition of TR. Herein, magnetic resonance profiling experiments show that the addition of TR to an acrylic latex creates a coalesced surface layer or “skin” that traps water beneath it. Atomic force microscopy of the PSA surfaces supports this conclusion. In the absence of the TR, particles at the surface do not coalesce but are separated by a second phase composed of surfactant and other species with low molecular weight. The function of the TR is complex. According to dynamic mechanical analysis, the TR increases the glass transition temperature of the polymer and decreases its molecular mobility at high frequencies. On the other hand, the TR increases the molecular mobility at lower frequencies and thereby promotes the interdiffusion of latex particles to create a skin layer. In turn, the skin layer is a barrier that prevents the exudation of surfactant to the surface. The TR probably enhances the coalescence of the latex particles by increasing the compatibility between the acrylic copolymer and the solids in the serum phase.     

Skin Development during the Film Formation of Waterborne Acrylic Pressure-Sensitive Adhesives Containing Tackifying Resin

Tackifying resins (TR) are often used to improve the adhesive properties of waterborne pressure-sensitive adhesives (PSAs) derived from latex dispersions. There is a large gap in the understanding of how, and to what extent, the film formation mechanism of PSAs is altered by the addition of TR. Herein, magnetic resonance profiling experiments show that the addition of TR to an acrylic latex creates a coalesced surface layer or “skin” that traps water beneath it. Atomic force microscopy of the PSA surfaces supports this conclusion. In the absence of the TR, particles at the surface do not coalesce but are separated by a second phase composed of surfactant and other species with low molecular weight. The function of the TR is complex. According to dynamic mechanical analysis, the TR increases the glass transition temperature of the polymer and decreases its molecular mobility at high frequencies. On the other hand, the TR increases the molecular mobility at lower frequencies and thereby promotes the interdiffusion of latex particles to create a skin layer. In turn, the skin layer is a barrier that prevents the exudation of surfactant to the surface. The TR probably enhances the coalescence of the latex particles by increasing the compatibility between the acrylic copolymer and the solids in the serum phase.     

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.     

Surface and interfacial FTIR spectroscopic studies of latexes. II. Surfactant–copolymer compatibility and mobility of surfactants

Whereas molecular level interactions between sulfonate groups of SDOSS surfactant and COOH groups of EA/MMA copolymer have been discussed in part I of this series, the major focus of this work is to establish the effect of compatibility on the distribution of surfactants at the film–air and the film–substrate interfaces. It is found that the exudation of anionic surfactants is inhibited in neutralized ethyl acrylate/methacrylic acid latex films. On the other hand, nonionic surfactants do not exhibit enrichment at the film interfaces. The inhibited exudation of anionic surfactants is attributed to the increased compatibility resulting from surfactant penetration into the swollen latex particles. This is followed by the formation of solubilized polymer–surfactant complexes through the adsorption of surfactant onto the hydrophobic polymer segments. The effect of neutralization of the carboxylic acid groups on the exudation of anionic surfactants suggests the formation of hydrophobic interactions that overwhelm surface tension effects and prevent surfactant enrichment at either interface.     

Investigation of the effect of precoalescence or postcoalescence crosslinking on film formation, properties, and latex morphology

Latexes have many product applications including functioning as a binder in coatings. For many years, coatings researchers in industry as well as in academe have been exploring various modes of crosslinking latexes. Quite often, the goal of preparing crosslinked latexes is to upgrade film properties relative to the film properties of uncrosslinked latexes. In the present report, the synthesis and properties of crosslinkable acrylic latexes prepared with either an internal crosslinker (1,3-butylene glycol dimethacrylate)—“precoalescence crosslinking”—or an external crosslinker (adipic dihydrazide)—“postcoalescence crosslinking”—at various levels of crosslinking were studied. For postcoalescence crosslinking, diacetone acrylamide was copolymerized into the latex to provide sites for subsequent reaction with adipic dihydrazide. Fundamental properties of films cast from the two types of latexes were systematically compared. These properties included gel content, dynamic mechanical properties, nano-indenter hardness and modulus, stress–strain properties as well as the characterization of latex morphology by atomic force microscopy (AFM). In addition, some specific end-use properties were determined. This study assesses the effect of type (precoalescence or postcoalescence) and level of crosslinking on the film formation process and the resulting fundamental and end-use properties as well as resulting latex film morphology. Presented at 2007 FutureCoat! Conference, sponsored by Federation of Societies for Coatings Technology, October 3–5, 2007, in Toronto, Ont., Canada.     

Effects of carboxyl group on the ambient self‐crosslinkable polyacrylate latices

Ambient curable carbonyl functional acrylic latices were synthesized by incorporating diacetone acrylamide as functional monomer into acrylic copolymer, adipic acid dihydrazide (ADH) was used as curing agent. In this work acrylic acid (AA) and acrylic acid homopolymer (PAA) were used to facilitate the crosslinking reaction. We found that the properties of latex film were different when use AA and PAA as the source of the carboxyl groups separately. The results from the characterization of carboxyl groups of the latex particles demonstrated that the distribution of the carboxyl group on the latex particle surface is optimal for the crosslinking reaction. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3948–3953, 2007     

An ESEM investigation of latex film formation in cement pore solution

Environmental scanning electron microscopy (ESEM) and complementary methods were employed to study the time dependent film formation of a latex dispersion in water and cement pore solution. First, a model carboxylated styrene/n-butyl acrylate latex dispersion possessing a minimum film forming temperature (MFFT) of 18 °C was synthesized in aqueous media via emulsion polymerization. Its film forming property was at a temperature of 40 °C, studied under an ESEM. The analysis revealed that upon removal of water, film formation occurs as a result of particle packing, particle deformation and finally particle coalescence. Film formation is significantly retarded when the latex dispersion is present in cement pore solution. This effect can be ascribed to adsorption of Ca²⁺ ions onto the surface of the anionic latex particles and to interfacial secondary phases. This layer of adsorbed Ca²⁺ ions hinders interdiffusion of the macromolecules and subsequent film formation of the latex polymer.     

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.     

Surface and interfacial FT–IR spectroscopic studies of latexes. VIII. The effect of particle and copolymer composition on surfactant exudation in styrene-n-butyl acrylate copolymer latex films

Polarized attenuated total reflection Fourier transform infrared (ATR FT–IR) spectroscopy was used to identify the mobility and surfactant exudation of sodium dioctyl sulfocuccinate (SDOSS) surfactant molecules to the film–air (F–A) and film–substrate (F–S) interfaces in styrene/n-butyl acrylate (Sty/n-BA) latex films. It was found that, depending upon the latex particle composition, the surfactant molecules could be driven to the F–A or F–S interfaces. The primary factors that governed the direction of exudation were the compatibility of the latex components, interfacial film-substrate surface tension, and the chemical composition of the latex particles. Concentration, as well as orientation, of the hydrophilic SONa⁺ surfactant ends changed as a function of depth and the latex particle composition. © 1993 John Wiley & Sons, Inc.     

On the role of colloidal crystal-like domains in the film forming process of a carboxylated styrene-butadiene latex copolymer

Environmental scanning electron microscopy (ESEM) was employed to study the mechanism of film formation of a carboxylated styrene-butadiene latex copolymer with a glass transition temperature (T_g) of 6 °C. ESEM allows the investigation of wet samples in their native state which is required to study the drying process of latex dispersions. The film forming process was tracked by time-dependent ESEM monitoring of the latex particle morphology and by observing the different stages occurring during the drying process. The focus of our study was an analysis of the three-dimensional (3D) arrangement of the latex particles and a comparison of their appearance on the surface and in the center of the coalesced film. It was found that in the course of film formation, the latex particles arrange in domains which are similar to colloidal crystals. Such domains occur at the stage of dense particle packing. Particle coalescence appears to begin first in these domains before a continuous and homogeneous film is formed which then spreads across the entire substrate. The results suggest that for our carboxylated styrene-butadiene copolymer the current model known for the film forming mechanism which includes four main steps should be complemented by two additional ones, namely the arrangement of particles in crystal-like domains and the beginning of coalescence within these domains. This specific behavior only occurs for monodisperse latices.     

Influence of a hydrogen-bonding co-monomer on polymer diffusion in poly(butyl acrylate-co-methyl methacrylate) latex films

Small amounts of hydrogen-bonding comonomers such as N-(2-methacryloxyethyl)ethylene urea (MEEU) are often included in latex particle synthesis to promote adhesion of latex films to metals and old surfaces. Little is known about how these monomers affect the latex film formation process. Here we examine the influence of 1-7 wt.% MEEU on butyl acrylate-methyl methacrylate copolymer latex films using fluorescence resonance energy transfer (FRET) measurements, in conjunction with donor- and acceptor-labeled latex particles, to study the rates of polymer diffusion in these films. The presence of MEEU in the copolymer leads to small increases in the polymer glass transition temperature (Tg). It also tends to retard the rate of polymer diffusion. This effect, however, is very sensitive to the humidity of the surrounding atmosphere. It appears that moisture taken up in the film minimizes the influence of MEEU groups on the rate of polymer diffusion.     

Recent advances in stratification and film formation of latex films; attenuated total reflection and step-scan photoacoustic FTIR spectroscopic studies

Surfactant–latex molecular level interactions as well as transient effects during latex film formation play an important role in latex technology. This review article focuses on the siloxane effects on anionic sodium dioctylsulfosuccinate (SDOSS) surfactant exudation during latex coalescence and quantitative analysis of SDOSS distribution at the both film–air (F–A) and film–substrate (F–S) interfaces. Attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy was utilized for characterization of the interactions between SDOSS and styrene–butyl acrylate latex copolymers. In addition, studies of SDOSS stratification along with depth-profiling analysis during latex film formation utilizing step-scan photoacoustic (S²-PAS) FTIR spectroscopy illustrate that SDOSS content is enriched at the F–A interface and decreases as the penetration depth increases across the latex film thickness. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1321–1348, 1998     

Composite natural rubber based latex particles: a novel approach

The oil resistance of natural rubber (NR) film could be effectively improved by using the heterocoagulation of large NR particle with small polychloroprene (CR) particles. In the preparation of NR/CR composite particle with a core-shell structure, a nonionic surfactant whose molecule bears poly(ethylene oxide) (PEO) was adsorbed on CR particles and allowed to form complexes between PEO and indigenous surfactant (protein-lipid) on the NR particle surface. Composite latex particle obtained was characterised by particle size, zeta potential and glass transition temperature measurements and the data indicated the presence of CR on the outer layer of composite particle. Better oil resistance of film casted from heterocoagulated latex when compared to that of NR film confirmed the NR/CR core-shell structure. The epoxidised natural rubber (ENR), crosslinked ENR and/or skim latex particles were investigated in order to replace the use of CR in the heterocoagulation process.     

Synthesis and characterization of epoxy‐acrylate composite latex

A waterborne epoxy‐acrylate composite latex was synthesized. The effects of the concentration of the initiator, surfactant, and epoxy resin on the particle size, molecular weight, and grafting ratios of the composite latex were investigated. The increase of the concentration of the initiator and epoxy resin led to the decrease of the weight‐average molecular weight. The graft ratios increased with an increase in the initiator level and a decrease in the epoxy resin concentration whereas the variation of the concentration of the surfactant did not have much influence on the graft ratios. The increase in the initiator level caused the aggrandizement of the particle size, and the increase of the concentration of the surfactant and epoxy resin caused a decrease in the latex particle size. Fourier transform IR spectroscopy with attenuated total reflectance indicated that the epoxy resin molecules were enriched in the mold‐facing surface in the film from the composite latex. The differential scanning calorimetry analysis, dynamic mechanical analysis, and Instron test showed that the polymer films cast by the composite latex had lower tensile strength and glass transition than those by the blend latex. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1736–1743, 2002     

New waterborne acrylic binders for zero VOC paints

The development of a waterborne acrylic binder that can be formulated into zero VOC paints requires the tuning of film formation under difficult conditions, hardness, and flexibility properties. To meet these requirements, a model of the ‘ideal’ film was developed and polymers were synthezised to comply with this model. The choice of particle size and glass transition temperature (T _g) of the polymer phases were the key parameters in producing the desired film morphology. However, to ensure good mechanical properties, it was also crucial to optimize the interaction between the polymer phases by varying both the polymer composition and the stabilization of the latex. Presented at XXVIIIth FATIPEC Congress, organized jointly by the Hungarian Chemical Society (MKE) and the Polish Association of Chemical Engineers (SITPCHEM), in Budapest, Hungary, June 12–14, 2006.     

Influence of chain length in nonyl-phenol ethoxylate surfactants on the film formation behaviour of methylmethacrylate-2-ethylhexyl acrylate copolymer latexes: part 1. Differential scanning calorimetry and atomic force microscopy

A comparison of the film forming characteristics of methylmethacrylate-2-ethylhexyl acrylate latex copolymers stabilised with nonyl-phenol ethoxylate molecules of varying chain lengths is presented. The ability of the stabiliser to segregate and diffuse from the interfacial layer into the surrounding media influences both the rate of coalescence process and structure of the film formed. Dynamic mechanical analysis, minimum film formation temperature measurements, particle size analysis, differential scanning calorimetry (DSC) and atomic force microscopy reveal the complexity of the mechanism involved in the coalescence process. A model that describes the various stages of coalescence and compaction of the latex particles indicates the effects of chain length on the film forming properties. For the stabiliser with a chain length of 20, coalescence is observed at room temperature; whereas for the stabiliser with chain lengths of 30 and 40, coalescence only occurs if the films are raised above 315 K. For the longer chain stabilisers, the effect of stabiliser-stabiliser interaction inhibits the coalescence process and DSC data indicate the occurrence of crystalline phase structure in the thin film.     

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.