An emulsion polymerisation comprises water, an initiator (usually water-soluble), a water-insoluble monomer and a colloidal stabiliser, which may be added or may be formed in situ. The main locus of polymerisation is within the monomer-swollen latex particles which are either formed at the start of polymerisation, or may be added initially (in which case one has seeded emulsion polymerisation). The term ‘emulsion polymerisation’ is a misnomer, arising for historical reasons: the process was originally developed with the aim of polymerising emulsion droplets (although in fact this does not occur). The starting emulsion is not thermodynamically stable, although the final product is colloidally and thermodynamically stable.
An ab initio emulsion polymerisation involves the emulsification of one or more monomers in a continuous aqueous phase and stabilisation of the droplets by a surfactant. In a seeded emulsion polymerisation, one starts instead with a preformed seed latex. Usually, a water soluble initiator is used to start the free-radical polymerisation. The locus of polymerisation is within submicron polymer particles (either formed during the process or added at the start), which are swollen with monomer during the polymerisation process, and dispersed in the aqueous phase. The final product is a latex comprising a colloidal dispersion of polymer particles in water. Ab initio emulsion polymerisation differs from suspension, mini- and micro-emulsion polymerisations in that the particles form as a separate phase during the polymerisation process.
The fact that particles in an emulsion polymerisation are small, much smaller than those in a (conventional) emulsion, indicates that polymerisation does not occur in the monomer droplets. If a surfactant is used in the system, above the critical micelle concentration, then micelles form. A micelle is an aggregate of ∼102 surfactant molecules, usually spherically shaped with the dimension of a few nanometers. If present, micelles are the locus of the commencement of polymerisation, because they are much more numerous than the monomer droplets, and thus much more likely to capture aqueous-phase radicals generated from the initiator: micellar nucleation. Consistent with this, an increase in surfactant concentration results in an increase in the number of formed particles. If there is no added surfactant, or the system is below the critical micelle concentration, a latex can still form, stabilized by entities formed from the initiator. Particle formation is by the collapse (coil-to-globule transition) of aqueous-phase oligomers to form particles by homogeneous nucleation.
The polymer particle size is much smaller than those formed in a suspension polymerisation, and also much smaller than the original monomer droplets. It is essential to be aware that the polymer colloids which are the result of an emulsion polymerisation contain many polymer chains in each particle (despite the not uncommon misconception that there is only one chain per particle). Two observations make this apparent. First, the size of a typical polymer colloid, ∼102 nm, is very much greater than the volume that could be occupied by a single polymer chain of the molecular weight (∼106) typical of that found in emulsion polymerisations. Second, when one considers that most particles have at least one growing radical in them and that the growth time of a single chain is orders of magnitude less than the time during which the latex particle is polymerizing.
Polymerisations may be categorised by both the polymerisation mechanism (e.g., radical polymerisation, anionic polymerisation, etc.), and by the polymerisation technique (e.g., solution polymerisation, emulsion polymerisation, etc.). A third factor is how the reactor is operated: in batch mode, or by adding monomers during the process (semi-continuous), or by continuous operation. Mechanism, technique and process strategies (mode of operation) all have an influence on the rates of polymerisation and the characteristics of the formed polymer. It is also possible to distinguish an emulsion polymerisation related to rate, development of molar mass and chemical composition, and the effects of the process strategy adopted.
Although polymer latices are the primary focus of this section it should be recognised that water dispersible polymers may also be used in water-based coating systems. Polyurethanes, polyesters, alkyds and epoxies, of low-to-moderate molecular weights prepared by step growth polymerisation and then dispersed in water, can have advantages of toughness in relation to film formation temperature as compared with emulsion polymers. Pseudo-latex dispersions of polyurethanes, for example, can be prepared without surfactant addition, at sizes as low as 20-100 nm and offer low film formation temperatures as a consequence of their water-swollen and plasticised nature.
To balance the requirements of high versatility and low manufacturing cost for the production of emulsion polymers (large volume applications, like paints, adhesives, paper coating and carpet backing) the discontinuous semi-batch process is widely used. Semi-batch means that initially only a portion of the water, monomers and emulsifiers is charged into the reactor, polymerisation is started and the remainder of the ingredients is added over a period of time until the desired filling volume is reached. The most common temperature range for emulsion polymerisation is 60–100 °C. The reactors used are normally agitated stainless-steel vessels, ranging in size from 20 to 100 m3. After the end of monomer addition non-reacted monomers are further polymerised, often using a redox initiator system. Other volatile organic compounds like monomer impurities or by-products from polymerisation are removed, most commonly by steam distillation. Afterwards any coagulum is removed by filtration and post additions of other ingredients may be made along with final adjustment of the latex properties, such as pH and solids content.
The polymer provides many of the performance features needed for specific coating applications for example, adhesion to the substrate, toughness and elasticity to resist mechanical impact, like scratching, abrasion or yield stress; stability against chemicals, water resistance and so on. In the market different types of emulsion polymers are established:
- copolymers of styrene and acrylic esters (styrene acrylics)
- copolymers of methacrylic esters and acrylic ester (pure acrylics)
- homopolymers and copolymers of vinyl acetate
- polyurethane dispersions (PUD)
- Polyols/OH-functional polymer
The particular properties of the different classes mean that each dominates specific areas of the coating sector.
For exterior applications on mineral substrates, like architectural paints or textured finishes, usually styrene acrylics are preferred. They have the highest resistance to saponification and thus do not undergo hydrolysis if coated on not fully cured highly alkaline substrates, like concrete or lime cement. In addition, they bring low water absorption, good adhesion to the substrate and high pigment binding capacity.
Pure acrylics and PUDs are used especially in low pigment volume concentration (low PVC) applications for example, clear-coats, varnishes or high gloss paints. Since these coatings contain only little or even no pigment they have to demonstrate their low susceptibility to UV-degradation.
Homo- and copolymers of vinyl acetate are in general the most cost efficient type and dominate the price-sensitive segment of interior paints.
The choice of emulsifier plays an important role in this respect, too. Quite often mixtures of different types are used to optimise the overall stability of emulsion polymers during production and processing.
Dispersion polymerization is a form of precipitation polymerization in which the precipitating polymer is kept dispersed in the polymerization medium using a polymeric steric stabilizer. It starts with a homogeneous solution of monomer, initiator, and stabilizer. The polymer formed being insoluble in the polymerization medium separates out from the latter forming a dispersion. In order to obtain coagulum-free dispersions, the polymeric stabilizer should adsorb strongly at the interface between dispersed polymer particle and polymerization medium. One way to achieve this is to use as stabilizer an amphipathic diblock copolymer, a graft or a comb copolymer in each of which one of the component polymer is soluble in the dispersion medium, whereas the other is insoluble. The latter adheres strongly to the dispersed polymer particle anchoring the stabilizer onto the particle. Anchoring also occurs when the insoluble part is miscible with the dispersed polymer. The part of the stabilizer soluble in the dispersion medium, referred to as the stabilizing moiety, extends out of the particle into the medium and prevents particle coagulation by steric stabilization mechanism.
The most commonly used laboratory water-soluble initiators are potassium, sodium and ammonium persulfates. Next in line are the water-soluble azo-compounds, especially those with an ionic group, such as 2,2’-azobis(2-methylpropionamidine) dihydrochloride. Another important group are the peroxides (benzoyl peroxide, cumene hydroperoxide). In cases where the polymerisation should be performed at lower temperatures (less than 50 °C), a redox system can be used. Lower polymerisation temperature gives the advantage of lowering chain branching and crosslinking in the synthesis of rubbers. Usually the redox couple reacts quickly to produce radicals, and thus one or both components must be fed during the course of the emulsion polymerisation process. For this reason, redox initiators are very useful for safety in commercial emulsion polymerisations because, in the case of a threatened thermal runaway (uncontrolled exotherm), the reaction can be quickly slowed by switching off the initiator feed.
The emulsion polymerisation process is often used for the (co-)polymerisation of monomers, such as vinyl acetate, ethylene, styrene, acrylonitrile, acrylates and methacrylates. Conjugated dienes, such as butadiene and isoprene, are also polymerised on a large industrial scale with this method. One of the advantages of emulsion polymerisation is the excellent heat exchange due to the low viscosity of the continuous phase during the whole reaction. Examples of applications are paints, coatings (including paper coatings), adhesives, finishes and floor polishes.
During the manufacturing of paints shear stress is applied through dispersing and pumping process steps. In addition, pigments and extenders can release multivalent cations that destabilize the colloidal paint system. To improve the stability of emulsion polymers usually minor amounts of monomers, like unsaturated acids (e.g. acrylic acid, methacrylic acid, itaconic acid), are copolymerised in addition. In the alkaline paint they are deprotonated. The negative charge sitting on the particle surface increases the resistance against agglomeration.
One of the common functional monomers to induce crosslinking, is 2-hydroxyethyl methacrylate (HEMA). This is a highly water-soluble monomer compared to typical co-monomers applied in waterborne coatings (butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate). This large difference in water solubility results in strong variations of the co-monomer ratio between the different phases of the polymerisation mixture.
Emulsion polymerisation is frequently used to create core–shell particles, which have a layer structure. Core–shell products are in use in the coatings industry, in photographic and printing materials and in the production of high impact materials (a core of rubbery polymer and a shell of a glassy engineering plastic). In recent years, considerable interest has arisen in the preparation of block copolymers in emulsion polymerisation through the use of controlled radical polymerisation mechanisms. The formation of block copolymers within a latex particle can lead to interesting new morphologies and can lead to new latex applications.
Using waterborne systems extra formulation components are often needed. The high latent heat of evaporation of water leads to long drying times such that, commercially volatile co-solvents have been used to reduce this problem and to aid plasticization of high Tg polymers. Current pressures are to choose environmentally more acceptable co-solvents or to eliminate them altogether. When lower Tg polymers are chosen at the outset, their film surfaces tend to be tacky. Composite low Tg and high Tg formulations can overcome this problem especially when core-shell morphologies are employed.
In waterborne polymers, although homogeneous particles meet the requirements of many of the applications, heterogeneous particles provide advantages in the more demanding cases. Thus, 2-phase soft–hard particles have been used for coatings, which combine a low minimum film-forming temperature and a high blocking resistance. Waterborne polymer–polymer hybrids (e.g., alkyd–acrylic, polyurethane–acrylic, epoxy–acrylic) have been developed in an attempt to combine the positive properties of both polymers, avoiding their drawbacks. Structured latex particles are also used to overcome the limitations of some copolymerisation systems. One interesting example is the styrene (S)/vinyl acetate (VAc) system which consists of monomers having complementary properties. Although the system does not copolymerize (rS = 55 and rVAc = 0.01 (Odian, 2004), structured latex particles of the corresponding polymers can enhance the mechanical and resistance properties of the latex films due to the PS, while keeping the film-forming properties at room temperature of the PVAc.
The formation of a film arises from the ‘coalescence’ (compaction, deformation, cohesion, polymer chain interdiffusion and cross-linking) of the individual polymer particles, which are initially held apart by stabilising forces electrostatic and/or steric, resulting from the charged polymer chain end groups or adsorbed surfactant polymer. These forces and others resisting particle deformation, are overcome upon evaporation of the continuous phase water.
Emulsion polymers film formation refers to a dynamic process that transforms a deposited layer of stably suspended colloidal polymer particles into a continuous, mechanically coherent coating or film as it dries, usually in air. Current understanding of film formation consists of three stages: (i) consolidation, i.e. particle immobilization by multiple contacts with one another as solvent evaporates; (ii) compaction, i.e. elimination of pore space by progressive flattening of consolidated particles and by local rearrangement of particles – usually quite minor; (iii) cohesion, i.e. development of tensile strength and continuous polymer phase by inter-particle diffusion of polymer. As a drying coating transforms from stage (i) to stage (ii), the air-solvent menisci may recede into the pore space in the packing of polymer particles created by consolidation, followed by air that creates moist zones where liquid persists only in pendular rings around inter-particle contacts and perhaps in the smallest interstices or pore bodies. Either capillary force or van der Waals force, or both, flatten polymer particles against one another and thereby shrink their interstices; ultimately all of the solvent may evaporate except that trapped in isolated pore bodies of an almost fully compacted coating. In stage (iii), the interfaces between flattened particles disappear as polymer molecules interdiffuse across them in the process of coalescence by which the coating, or film, acquires permanent mechanical integrity. While conceptually film formation can be divided into stages, the whole sequence of events of microstructure evolution is continuous. Moreover, the entire process may not be traversed. The extent to which it is realized depends on the properties of the polymer, the types of additives in the initial dispersion, the conditions of drying, and the circumstances of any aging.
CONSOLIDATION – Evaporation, particle concentration and ordering.
Water evaporates from the latex surface, concentrating the latex solids content: the rate of evaporation has been determined as being the same as the rate of evaporation from water alone, or of water from a dilute solution of surfactant plus electrolyte, i.e. such as that which constitutes the aqueous phase of a latex prepared via an emulsion polymerisation. This first stage is the longest of the three and lasts until the polymer has reached approximately 60/70% volume fraction (dependent on the stability of the latex, 74% for close packed spheres) or until the surface area of the latex’s liquid/air interface starts to decrease as a result of, for example, solid film formation. Initially the particles move with Brownian motion but this ceases as the electrical double layers undergo significant interaction once a critical volume of the water has evaporated.
COMPACTION – Particle deformation
This starts when the particles first come into irreversible contact, and iridescence in the case of uniform sized, surfactant-free latices capable of colloidal crystal formation with its accompanying Bragg diffraction may be observed on the latex surface. The rate of evaporation per unit area of open wet latex remains constant, but the overall rate of evaporation decreases greatly during this stage. Reducing the rate of evaporation can lead to better quality films by allowing the particles more time to pack into an ordered structure before flocculation occurs. Casting at high temperatures gives the particles sufficient energy to overcome their mutual repulsion and the films are formed before the particles are fully ordered. Particle deformation occurs in soft latices, in some instances, even before particle contact as shown by the absence of a discontinuity in the rate of decrease of interparticle spacing at the volume fraction associated with the close packing of spheres in different modelling. The completion of particle deformation, marks the end of the second stage of film formation.
COHESION – Polymer chain diffusion across particle boundaries.
This stage starts with the initial formation of a continuous film. The remaining water leaves the film initially via any remaining interparticle channels and then by diffusion through the fused polymer skin, but the rate of evaporation eventually slows to (asymptotically) approach that of diffusion alone. The rate of water removal may be decreased by film additives that are impermeable (as a result of the increased diffusion path length) or hydrophilic (due to polar interactions). It is during this final stage that a soft latex becomes more homogeneous and gains its mechanical properties as polymer chain interdiffusion occurs (a process variously termed maturation, autohesion or further gradual coalescence) and particle interfaces tend to become less distinct. A drastic change in film properties is noted between stages II and III, as the initially brittle cohered particles become more ductile due to polymer chain entanglements.
The diffusion of polymer chains in a polymer matrix is strongly dependent on the molar mass of the chains. In terms of development of the cohesive strength, two opposing effects can be recognized, as follows. Polymer with a relatively low molar mass ensures facile diffusion of chains from one particle into the other, after coalescence of the particles in the film formation process. However, the effect of this interdiffusion on the strength development is not very large. Polymer with a higher molar mass is hindered in its diffusion to a larger extent. The contribution of this diffusion process to the development of the cohesive strength is much larger than in the case of low molar mass polymer.
A clear film is not necessarily completely dry but could contain water-filled domains significantly smaller than the wavelength of light.
A film of low solids content could dry faster than one of high solids content despite the lower quantity of water to be removed from the latter which, however, reaches the diffusion-controlled stage (i.e. surface closure) sooner, and then loses water more slowly.
Cohesive strength development
The process of cohesive strength development in a water-borne polymeric coating consists of two main mechanisms:
- interdiffusion of molecular polymer chains from one particle into another
- crosslinking, interfacial and residual
This process of cohesive strength development is the final stage in the complex process of film formation. The two preceding stages are the evaporation of water and the compaction/deformation of the latex particles.
In the development of water-borne coatings, a main area of current research activities is the crosslinking of the polymer film. The method of crosslinking determines to some extent the requirements with respect to polymer–polymer interdiffusion after film forming. One consists of a polymer that is to be crosslinked by a low molar mass crosslink agent. The other consists of two different polymers containing complementary reactive groups. Terms like interfacial crosslinking followed by residual crosslinking apply to the former of these examples, but hardly to the latter.
Crosslinking of polymers by low molar mass crosslink agents.
The most elementary form of a crosslinking water-borne coating is where the emulsion polymer contains functional groups that are crosslinked in a reaction with a low molar mass crosslink agent. The crosslinking agent will generally be added to the latex immediately prior to application on the substrate. This type of system is referred to as a two-component coating, for obvious reasons. In general the crosslink agent will reside in the aqueous phase.
Diffusion of the crosslinking agent into the polymer particles is crucial in order to obtain a homogeneously crosslinked film. One of the concerns here is that, upon coalescence of the particles, a relatively high concentration of crosslinking agent is present on the interface between the particles. This may result in a densely crosslinked film at the interface, which greatly reduces mobility of polymer chains across the interface, and may result in inhomogeneous crosslinking. The residual crosslinking is hindered to some extent. One solution to this problem is the homogeneous distribution of crosslink agent throughout the polymer phase. In the regular systems this will result in crosslinking of the latex particles before film formation. These crosslinked particles will not be able to undergo film formation, hence an inferior quality of the coating will be achieved. However, when the crosslinking reaction is intrinsically slow, but when its rate can be enhanced by some catalyst, this problem may be solved.
A very important characteristic of a dispersion polymer is the temperature at which it forms a clear and homogeneous film. This temperature is called the minimum film-forming temperature (MFFT). It can be determined experimentally with a special apparatus. This consists of a support that provides a temperature gradient along its length. The emulsion polymer is drawn down on it. After equilibration the transition from a cracked to a clear film determines the MFFT. The MFFT of an emulsion polymer is usually a few °C lower than its glass transition temperature (Tg). One reason is that a small amount of water dissolved in the latex particle acts as a plasticizer. Most coatings are applied under ambient conditions (either on a job site or in a factory) at typical temperatures between 5 and 40 °C. For obvious reasons the MFFT of paints and coatings should be lower than the temperature at the application side. The low end of the temperature range is defined by architectural coatings used outdoors that should still form a neat paint film under unfavourable conditions. The MFFT of paints can be temporarily lowered by use of a coalescing agent. This works by partitioning into the emulsion polymer particles, disrupting the packing of the polymer chains and thus lowering the MFFT. After film formation the coalescent evaporates. This principle is employed for certain applications where hard polymers are required. For example, wood coatings for door or window frames should not stick to each other when brought in contact by closing. In technical terms the coating should show sufficient block resistance.
Coalescents contribute largely to VOCs that are emitted after application. As already stated the clear trend in the coatings industry is directed towards environmentally friendly low-VOC coatings. Thus, in the last years new kinds of emulsion polymers have been developed that allow realization of the contradictory requirements of low MFFT on the one hand and good block resistance on the other without use of any coalescent: the so-called multiphase particles. They consist of at least two different polymers one with low MFFT and one with high MFFT. The particle structure can be varied largely by the choice of monomers and process conditions. The low MMFT can be adjusted to 0 °C which easily forms a film, even at low temperature without coalescent. The hard domains must be perfectly distributed in the soft phase matrix, they stick out of the film to impart block resistance. Multiphase particles technology is developed to achieve high performance low-VOC coatings.