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.     

A study of acetoacetoxyethyl methacrylate hydrolysis in acrylic latex polymers as a function of pH

Acetoacetoxyethyl methacrylate (AAEM) is an ambient crosslinking monomer that, when incorporated into architectural coatings binders, provides coatings with improved hardness, scrub, stain, and dirt pick-up resistance. This study details the use of NMR spectroscopy to explore the hydrolysis profile of AAEM as a function of pH, type of neutralizer, and glass transition temperature of the binder. We have determined that AAEM has a hydrolysis profile that is independent of latex polymer pH relevant to coatings (pH 7–10). Lower T _g latex polymers enable the diffusion of ammonia into the binder nanoparticles converting a larger amount of the acetoacetoxy moiety to the enamine form; this approach also allows for measuring the distribution of enamine and AAEM form in the latex polymer particle. The consequence of these findings is that AAEM may be utilized at a lower pH than previously envisioned without consequence to the hydrolysis profile of the acetoacetoxy functionality.     

Functional latex and thermoset latex films

We review advances in the design and development of functional latex particles that can be used to form crosslinked coatings. Our emphasis is on understanding fundamental principles, of the formation and aging of latex films, of crosslinking of polymer films, of the reaction mechanisms that lead to bond formation, and of the competition between bond formation and polymer diffusion in latex films. These principles form the basis for the design of modern coatings that combine high performance with environmental compliance.     

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.     

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.     

Crosslinking of isocyanate functional acrylic latex with telechelic polybutadiene. III. Application of a diffusion model

The diffusion and reaction of amino-telechelic polybutadiene (PBD-NH₂) in poly(styrene/n-butyl acrylate/TMI®) (PSBT) was studied. A monodisperse poly(St/BA/TMI) seed latex was prepared by semicontinuous emulsion polymerization. Two core/shell latices were also prepared semicontinuously, using the seed latex as the core and poly(St/BA) as the shell. These monodisperse latices were mixed with equivalent amounts of the telechelic PBD-NH₂ artificial latex before casting into films. The consumption of the TMI in these films was monitored by FTIR as a function of time and the NH₂/TMI ratio. The results showed that without the PSB shell, the PSBT particles (80 nm radius) could be penetrated by the PBD-NH₂ completely. A 24 nm PSB shell was found to act effectively as a barrier to preventing the penetration of the PBD-NH₂ inside the particles. This was consistent with previous TEM results, indicating that the crosslinking between the isocyanate in the PSBT particles and the amine in the PBD-NH₂ particles provided the driving force for the chain diffusion. A diffusion model was established for the PSBT/PBD-NH₂ system. Assuming steady-state diffusion, the effective diffusion coefficients were calculated based on the experimental data. This lead to the estimation of the surface coverage of the PBD-NH₂ on the PSBT particles in the latex films. A film formation and crosslinking mechanism was proposed for the PSBT/PBD-NH₂ latex blend system. In the absence of crosslinking reactions, the two incompatible polymers tended to completely phase separate during the film formation. However, with the crosslinking reactions, the PBD-NH₂ will be bound to the PSBT particle surfaces, forming a PSBT-PBD copolymer interphase. This interphase facilitates the diffusion of the PBD-NH₂ into the PSBT particles. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 985–993, 1998     

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.     

The influence of latex blend composition on crosslinking and mechanical properties

Model reactive latices were synthesized by semicontinuous emulsion copolymerization of n‐butyl methacrylate and acetoacetoxyethyl methacrylate or dimethylaminoethyl methacrylate. The two functional latices were then blended in various ratios to study the influence of blend composition on crosslinking and mechanical properties of the resulting films. Crosslinking was quantified through swelling measurements. It was found that the crosslink density increased with increasing amounts of acetoacetoxy‐functional polymer. In addition, the crosslink density exhibited two maxima, at 30/70 and 70/30 (acetoacetoxy‐functional latex/amino‐functional latex) blend compositions. The mechanical properties of the films were quantified by dynamic mechanical analysis (DMA). It was shown that optimal mechanical properties occurred when the particles packed most efficiently at the 30/70 and 70/30 blend compositions. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3774–3779, 2007     

Evaluation of the shrinking‐core model for examining the kinetics of film formation in a reactive latex blend

We prepared reactive latex blends from two copolymer latices comprised of n‐butyl methacrylate (n‐BMA) with acetoacetoxyethyl methacrylate and n‐BMA/dimethylaminoethyl methacrylate to study the kinetics of film formation. We generated thin films by blending equal weights of the two latices. The films were then cured at temperatures ranging from 50 to 90°C. The extent of the crosslinking reaction was calculated from the crosslink density, which was determined from swelling measurements of the films in toluene. The shrinking‐core model, a diffusion/reaction model, which was originally derived for combustion reactions of coal particles, was adopted to calculate the diffusion coefficient (D_e) and reaction rate constants from the extent of the reaction with time data. This model system exhibited a diffusion‐controlled regime above 70°C and a reaction‐controlled regime at temperatures below 70°C. In the reaction‐controlled regime, the shrinking‐core model predicted D_e for the system, which was in agreement with literature values for n‐BMA. In the diffusion‐controlled regime, the model predicted a lower apparent value for D_e but with an activation energy that was close to that obtained for n‐BMA. The model was also used to examine the kinetics of the crosslinking reaction. The kinetic rate constants for the crosslinking reaction were also determined. The activation energy for the crosslinking reaction was 18.8 kcal/mol, which compared reasonably with the activation energy of 22.8 kcal/mol determined for the reaction between the functional monomers as small molecules. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3659–3665, 2006     

Crosslinking of isocyanate functional acrylic latex with telechelic polybutadiene. II. Film formation/crosslinking

The mechanism of film formation and crosslinking in a poly(methyl methacrylate/n-butyl acrylate/TMI®) (PMBT) and amino-telechelic polybutadiene (PBD-NH₂) latex blend was studied. Films cast from the latex blends were ultramicrotomed and examined by transmission electron microscopy. The PBD-NH₂ was found to be located primarily at the surface of the PMBT particles due to a crosslinking reaction. Without the possibility of crosslinking (when TMI was absent in the acrylic copolymer), the PBD-NH₂ only formed segregated phases in the acrylic polymer matrix due to the incompatibility of the two polymers. While this PBD-NH₂ could be extracted with toluene, the PBD-NH₂ crosslinked at the PMBT particle surfaces was not extractable. FTIR results showed that the isocyanate groups in the PMBT were completely consumed when the PBD-NH₂ concentration was sufficiently high, indicating that the PBD-NH₂ had diffused and reacted inside the PMBT particles. Thus, the crosslinking reaction was considered to be the driving force for the diffusion of the PBD-NH₂ into the PMBT particles. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 977–984, 1998     

The diacetone acrylamide crosslinking reaction and its influence on the film formation of an acrylic latex

Waterborne colloidal polymers (i.e. latex) represent a promising alternative to organic solvent-based systems in coatings applications. The development of mechanical strength and hardness is often enhanced by chemical crosslinking that creates a three-dimensional network. If extensive crosslinking occurs within the particles prior to their coalescence, however, interdiffusion will be prevented. A weaker product will result. We have explored the inter-relationship between coalescence, crosslinking, and surfactant exudation in an acrylic latex containing diacetone acrylamide exploiting the “keto-hydrazide” crosslinking reaction. The complementary use of spectroscopic techniques on a model system determined that the crosslinking reaction yields an imine, not an enamine as has been proposed in some literature. Gel fraction measurements were used to probe the rate and amount of crosslinking and identified a slower rate in larger particles, suggesting that the transport of crosslinking agent is rate-limiting. The keto-hydrazide reaction was found to be acid catalyzed and favored at lower water concentration. Measurement of the latex pH relative to the polymer mass fraction during film formation clarified the expected point of onset for crosslinking in relation to particle packing. Atomic force microscopy was used to follow surface leveling relative to the competing influence of crosslinking. The rate and total amount of surfactant exudation were found to be influenced by crosslinking, particle deformability (as determined by the temperature relative to the polymer glass transition temperature), and the evaporation rate (as controlled by the relative humidity). There is evidence that surfactant exudation can be triggered by the particle deformation that occurs at film formation temperatures well above the glass transition temperature. 2007 Roon Awards winner, presented at 2007 FutureCoat! Conference, sponsored by Federation of Societies for Coatings Technology, October 3–5, 2007, in Toronto, Ontario, Canada.     

Isocyanate-functionalized latexes: Film formation and tensile properties

Latexes functionalized with isocyanate groups were prepared by carrying out the emulsion terpolymerization of dimethyl meta-isopropenyl benzyl isocyanate (TMI®) with methyl methacrylate and n-butyl acrylate. The film formation of these latexes and the tensile properties of the resulting latex films were studied. The effect of TMI concentration on the film properties was investigated. The locus of the isocyanate groups in the latex particles was controlled by using different polymerization processes. The locus of the functional groups was found to greatly influence the tensile properties of the latex films. Triethyl amine was used as an external catalyst to cure the TMI polymer films. One-component self-curable systems capable of undergoing crosslinking at ambient temperatures were developed by incorporating small amounts of methacrylic acid into the recipe. These systems exhibited significant improvement in tensile properties upon curing. In addition, the shelf-stability of these latexes was found to be excellent. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1869–1884, 1997     

Crosslinking vs. interdiffusion rates in melamine-formaldehyde cured latex coatings: A model for waterborne automotive basecoat

Designing optimal formulations for automotive waterborne basecoats can be fairly complex, often requiring knowledge of events that occur at the molecular level. The ultimate performance of the coating can depend upon the success with which this knowledge is applied. We examine a system in which an aqueous dispersion of an acrylic latex with -OH functionality reacts with a melamine derivative when heated. We use fluorescence-labeling and energy transfer measurements to obtain information on the relative rates of crosslinking and interparticle polymer diffusion in these films. We show that temperature and particle morphology play an important role in the development of film properties. Finally, these energy transfer experiments provide information on the location of the melamine-formaldehyde resin in the dry film before the onset of crosslinking. This system can serve as a model for waterborne basecoat development in many automotive applications. Presented at the 1998 Annual Meeting of the Federation of Societies for Coatings Technology, on October 15, 1998, in New Orleans, LA. Department of Chemistry, 80 St. George St., Toronto, Canada M5S 3H6. Finishes Division, 377 Fairall St., Ajax, ON Canada L1S 1R7.     

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.     

Film formation of latices with crosslinked particles

Summary The mechanical strength of latex films is caused by the diffusion of chain segments across particle boundaries and the formation of entanglements. This process should be hindered when the film is formed from crosslinked particles with an average network chain length M_c smaller than the entanglement length M_e. This assumption is verified by tensile tests of of crosslinking. In films of too highly crosslinked particles no mechanical strength is developed by annealing above the glass transition temperature.     

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.     

Effects of surface functional groups on film formation and tensile strength development in reactive latex systems

The effect of surface functional groups on chain interdiffusion at the interfacial zone of reactive latexes was investigated. A series of latexes with different surface functionalities was prepared by batch or shot-growth emulsion polymerizations. The particle surface properties were varied by changing the number density of functional groups. The rate of tensile strength development decreases with the increasing number density of surface functional groups, which indicated that inter-particle crosslinking restricted the interdiffusion of polymer chains during film formation and annealing. The average diffusion coefficient D of polymer chains across particle interfaces was obtained from dynamic mechanical analysis and compared well with the results from the rate of tensile strength development. The magnitude of D of the reactive latex film was lower than that of the homopolymer film. The lower chain mobility of reactive latexes prevented mechanical strength development.     

Acetoacetoxy functionalized natural rubber latex capable of forming cross-linkable film under ambient conditions

This work demonstrates that natural rubber (NR) latex particles containing acetoacetoxy (AcAc) groups are able to undergo cross-linking upon film formation at ambient temperature by reaction with glutaraldehyde (GTA). Natural rubber latex grafted with poly(acetoacetoxyethyl methacrylate) (NR-g-PAAEM) was synthesized by seeded emulsion polymerization, using benzoyl peroxide (BPO) as an initiator in free radical polymerization. The degree of grafting of PAAEM in the graft copolymers was evaluated by ¹H NMR technique. Transmission electron microscope (TEM) was used for investigation the particle morphology of the grafted NR latex. Since the AcAc groups are intentionally attached to the NR particles providing sites of cross-linking at ambient temperature, the cross-linking ability of these sites by reaction with GTA was then investigated. The results revealed that the latex film of NR-g-PAAEM with the addition of GTA had a much higher tensile strength in comparison with the film without GTA. The surface morphology of the NR-g-PAAEM latex film formed in the absence and presence of the GTA cross-linker was also investigated using atomic force microscopy (AFM). By GTA addition into the NR-g-PAAEM latex before film formation, an increase in the root-mean-square (RMS) roughness of the surface of the latex film was observed. Moreover, it was also observed that the NR-g-PAAEM films with the addition of GTA had higher activation energy for thermal degradation than that without the cross-linker. This confirms that the cross-linking reaction took place in the NR-g-PAAEM latex film as a result of its reaction with the GTA.     

Crosslinking of latex polymers

The physicomechanical properties and processes of thermal crosslinking of latex acrylic polymer films containing functional groups of different chemical nature were studied. Improvement of properties characteristic of latex copolymers containing methylolamide groups is explained by orientated location of hydrophilic groups on the surface of latex particles during emulsion copolymerisation, that, in its turn, leads to a more ordered arrangement of macrochains. An optimum content of methylolamide groups as regards crosslinking processes was established and explained by steric location of these groups on the surfaces of latex particles. A similar optimum is not observed for crosslinking by watersoluble resin of latex copolymers containing groups of hydrophobic glycidylmethacrylate.     

Polymer blend latex films: Miscibility and polymer diffusion studied by energy transfer

Fluorescence non-radiative energy transfer experiments were used to study latex blend films composed of high molar mass poly(butyl acrylate-co-methyl methacrylate) (PBA-co-MMA) and much lower molar mass PBA-co-MMA latex of the same chemical composition (50:50 BA:MMA by weight). These blends take advantage of the strong chain length dependence of T_g so that the particles consisting of oligomeric polymer (“low-M”) have a much lower T_g than the corresponding high-M latex. This type of blend represents a useful strategy for obtaining latex coatings with a reduced VOC content. Here we report on experiments which follow the rate at which the low-M polymer mixes via diffusion with the high-M polymer in the latex films. The high-M latex are doubly labeled, containing both donor and acceptor dyes covalently bound to the PBA-co-MMA backbone. Diffusion of the unlabeled low-M polymer into this phase dilutes the dyes, increasing their separation and lowering the quantum efficiency for energy transfer.