DISPERSIONI

Una polimerizzazione in emulsione ab initio comporta l’emulsificazione di uno o più monomeri in una fase liquida continua (acqua) e la stabilizzazione delle goccioline da parte di un tensioattivo. In una polimerizzazione in “seeded emulsion polymerisation”, si inizia invece con un lattice preformato di nuclei. Di solito, un iniziatore solubile in acqua viene utilizzato per iniziare la polimerizzazione dei radicali liberi. Il locus di polimerizzazione è all’interno di particelle polimeriche submicroniche (formate durante il processo o aggiunte all’inizio), che sono gonfiate con monomero durante il processo di polimerizzazione e disperse nella fase acquosa. Il prodotto finale è un lattice costituito da una dispersione colloidale di particelle polimeriche in acqua. La polimerizzazione in emulsione ab initio differisce dalla polimerizzazione in sospensione, mini e microemulsione in quanto le particelle si formano come una fase separata durante il processo di polimerizzazione.

Il fatto che le particelle in una polimerizzazione in emulsione siano piccole, molto più piccole di quelle di una emulsione (convenzionale), indica che la polimerizzazione non si verifica nelle goccioline di monomero. Se nel sistema viene utilizzato un tensioattivo, al di sopra della concentrazione critica di micelle, si formano micelle. Una micella è un aggregato di ∼102 molecole di tensioattivo, di solito a forma sferica con la dimensione di pochi nanometri. Dopo la formazione, le micelle sono il luogo dell’inizio della polimerizzazione, poiché sono molto più numerose delle goccioline di monomero e quindi molto più probabilità di catturare i radicali di fase liquida generati dall’iniziatore: nucleazione micellare. Coerentemente con ciò, un aumento della concentrazione di tensioattivo provoca un aumento del numero di particelle formate. Se non viene aggiunto tensioattivo o il sistema è al di sotto della concentrazione critica di micelle, può ancora formarsi un lattice, stabilizzato da entità formate dall’iniziatore. La formazione di particelle avviene mediante il “collasso” (coil-to-globule transition) di oligomeri in fase acquosa per formare particelle mediante nucleazione omogenea.

La dimensione delle particelle è molto più piccola di quelle formate in una polimerizzazione in sospensione, e anche molto più piccola delle goccioline monomeriche originali. È essenziale essere consapevoli del fatto che i colloidi polimerici che sono il risultato di una polimerizzazione in emulsione contengono molte catene polimeriche in ciascuna particella (nonostante il non comune malinteso che esiste solo una catena per particella). Due osservazioni lo rendono evidente. Innanzitutto, la dimensione di un colloide polimerico tipico, ∼102 nm, è molto maggiore del volume che potrebbe essere occupato da una singola catena polimerica del peso molecolare (∼106) tipica di quella riscontrata nelle polimerizzazioni in emulsione. In secondo luogo, se si considera che la maggior parte delle particelle ha almeno un radicale in crescita in esse e che il tempo di crescita di una singola catena è di ordini di grandezza inferiori al tempo durante il quale la particella di lattice polimerizza.

Le polimerizzazioni possono essere classificate sia dal meccanismo di polimerizzazione (ad es. polimerizzazione via radicale, polimerizzazione anionica, ecc.), sia dalla tecnica di polimerizzazione (ad es. polimerizzazione in soluzione, polimerizzazione in emulsione, ecc.). Un terzo fattore è il funzionamento del reattore: in modalità batch o aggiungendo monomeri durante il processo (semi-continuo) o mediante funzionamento continuo. Meccanismo, tecnica e strategie di processo (modalità di funzionamento) influenzano tutti i tassi di polimerizzazione e le caratteristiche del polimero formato. È anche possibile distinguere una polimerizzazione in emulsione correlata alla velocità, allo sviluppo della massa molare e della composizione chimica e agli effetti della strategia di processo adottata.

Sebbene i lattici polimerici siano l’obiettivo principale di questa sezione, si dovrebbe riconoscere che i polimeri disperdibili in acqua possono essere utilizzati anche nei sistemi di rivestimento a base acqua. Poliuretani, poliesteri, alchidici ed epossidici, con pesi molecolari bassi (o moderati) preparati mediante polimerizzazione a crescita graduale e quindi dispersi in acqua, possono avere vantaggi in termini di tenacità e temperatura di formazione del film rispetto ai polimeri di emulsione. Le dispersioni pseudo-lattice di poliuretani, ad esempio, possono essere preparate senza aggiunta di tensioattivo, a dimensioni comprese tra 20 e 100 nm e offrono basse temperature di formazione del film a causa della loro natura plastificata e il rigonfiamento d’acqua.

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.

Anche la scelta del tensioattivo (emulsionante) svolge un ruolo importante nel processo di sintesi. Abbastanza spesso vengono utilizzate miscele di diversi tipi per ottimizzare la stabilità complessiva dei polimeri in emulsione durante la produzione e la fase di stoccaggio.
Surfactant-free
La polimerizzazione in dispersione è una forma di polimerizzazione in precipitazione in cui il polimero precipitante viene mantenuto disperso nel mezzo di polimerizzazione usando uno stabilizzatore polimerico sterico. Si inizia con una soluzione omogenea di monomero, iniziatore e stabilizzatore. Il polimero formato essendo insolubile nel mezzo di polimerizzazione si separa da quest’ultimo formando una dispersione. Al fine di ottenere dispersioni prive di coagulo, lo stabilizzatore polimerico dovrebbe adsorbire fortemente all’interfaccia tra particelle polimeriche disperse e mezzo di polimerizzazione. Un modo per ottenere ciò è utilizzare come stabilizzatore un copolimero a blocchi anfifilico, graffato o un copolimero a pettine in ciascuno dei quali uno dei polimeri componenti è solubile nel mezzo di dispersione, mentre l’altro è insolubile. Quest’ultimo aderisce fortemente alla particella polimerica dispersa ancorando lo stabilizzatore sulla particella. L’ancoraggio si verifica anche quando la parte insolubile è miscibile con il polimero disperso. La parte dello stabilizzatore solubile nel mezzo di dispersione, indicata come parte stabilizzante, si estende dalla particella nel mezzo e impedisce la coagulazione delle particelle mediante un meccanismo di stabilizzazione sterica.

Gli iniziatori idrosolubili di laboratorio più comunemente usati sono i persolfati di potassio, sodio e ammonio. I successivi sono gli azocomposti idrosolubili, in particolare quelli con un gruppo ionico, come il 2,2′-azobis (2-metilpropionamidina) diidrocloruro. Un altro gruppo importante sono i perossidi (perossido di benzoile, idroperossido di cumene). Nei casi in cui la polimerizzazione deve essere eseguita a temperature più basse (inferiori a 50 °C), è possibile utilizzare un sistema redox. Una temperatura di polimerizzazione più bassa offre il vantaggio di ridurre la ramificazione della catena e il grado di reticolazione nella sintesi di lattici elastomerici. Di solito la coppia redox reagisce rapidamente per produrre radicali, e quindi uno o entrambi i componenti devono essere alimentati durante il processo di polimerizzazione dell’emulsione. Per questo motivo, gli iniziatori redox sono molto utili per la sicurezza in impianto nella polimerizzazioni in emulsione perché, nel caso di un innalzamento pericoloso della temperatura (esotermia incontrollata), la reazione può essere rapidamente rallentata disattivando l’alimentazione dell’iniziatore.

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.

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.

Cross-Linking
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.