Because of environmental concerns, aqueous polymer dispersions are important binding raw materials for plasters and coatings. Particles with phase-separated morphology, like core-shell copolymers, are responsible for unique and tailor-made mechanical and physico-chemical properties. These copolymers are gaining interest in coatings and in the thick coatings industries. Morphology, preparation, properties and comparison of heterogeneous polymers with random species are discussed in this article.
IntroductionDuring the last four decades, polymer dispersions prepared in an emulsion polymerization process were widely introduced as binders in waterborne (W/B) coatings, plasters and renderings. Final product parameters are very dependent on type and properties of polymers applied. Often, tailor-made polymers with proper monomer choice and type of morphology are used as raw materials to fulfill the most demanding applications. Acrylic and EVA copolymers are developed to overcome the drawbacks of traditional vinyl acetate copolymers such as water sensitivity, rapid hydrolysis in the presence of alkalis, poor elasticity and limited adhesion in humid environment. Acrylates are very versatile because of monomer availability. There are commercialized all-acrylic copolymers, styrene-acrylics, VeoVa-acrylics and polymers using different types of chemistry like polyurethane-acrylics, epoxide-acrylics and silicone-acrylics with very different mechanical, physico-chemical and optical properties.
For certain coating applications it is important to develop polymers characterized by a large difference between glass transition temperature (Tg) and minimum film-forming temperature, MFFT (Tg » MFFT). In the case of random acrylic copolymers this difference is very moderate; the EVA copolymers are an exception and generally have a much lower MFFT than Tg. Because of this feature EVA copolymers were introduced in the 1980s as binders in low-emission waterborne paints produced for the Scandinavian market.
Simultaneously, in the last decades a big effort was made to create various polymers with different types of morphology.(1-6) The idea of particle designing attracted the attention of scientists and was successfully developed. Currently, such polymers in the form of water dispersions or redispersible powder are produced by leading manufacturers and are available commercially. Heterogeneous polymers gained important applications in many industries and in some cases are responsible for unique, exceptional properties of the final products.
Heterogeneous Polymer DispersionsIn the classic one-stage emulsion polymerization process, a pre-prepared mixture of monomers is added step by step to the aqueous solution of surfactants, initiator and other additives. Because of the same content of monomer feed, a random copolymerization process is favorable. Different polymer properties may be achieved using different monomers and changing their content in the monomer mixture added to the reaction vessel. Another possibility is to influence the molecular weight range and polymer particle size.
If the polymerization process is conducted in two or more stages with a differentiated monomer feed, it is possible to obtain heterogeneous (phase-separated) polymer particles with a different monomer composition for the outer and inner parts. This sequential polymerization technique is a powerful tool for polymer structure creation and allows the preparation of dispersions, polymer films and coatings with the desired balance of physical, chemical and mechanical properties.(7, 8) Phase-separated polymerization process, both two-stage and multi-stage, is widely described in the chemical literature.(5, 8, 9)
Typically, on the laboratory scale, the emulsion polymerization process is carried out in the reactor immersed in a thermostatic bath to maintain temperature at 60 – 80 ºC and equipped with stirrer, reflux condenser, thermometer, nitrogen inlet and dropping funnel. Heterogeneous lattices are prepared in the polymerization process of two or more pre-emulsions of different monomer composition and degree of functionality. De-ionized water is used as a polymerization medium in this multi-step semi-continuous process with a pre-emulsion feed. After a short period of time for nucleation of the polymer particles, the core pre-emulsion mixture is fed into the first step. Between the first and second feeds a pause is made. The concentration of the anionic emulsifier in the second pre-emulsion mixture is reduced in order to avoid the production of a second generation of particles.(10) The acid (functional) monomer and the chain-transfer agent are preferably added to the second (last) pre-emulsion feed. After the end of the last feed the reaction system is maintained at 80 ºC for one hour to reduce the residual monomer content.
The final particle morphology is greatly influenced by the method of monomer addition, their relative hydrophilicity and compatibility. Core-shell species are the best known heterogeneous particles; other morphologies include inverted core-shell, currant-buns, raspberry, half moon, confetti-like or multi-lobed structures.(11) Very often the “core-shell” term is used as a common name for all different heterogeneous, phase-separated, particles.
The film-forming properties of latex and the performance of the film are strongly related to the morphology of the polymer particles. More ordered core-shell copolymers, in contrast to random species, are generally characterized by bigger elongation and lower tensile strength. Minimum film-forming temperature (MFFT) is dependent on the glass transition temperature (Tg) of the polymer shell.(7)
Low-Tg (mean value) heterogeneous copolymers with hard shell and soft core, in contrast to the similar random copolymers, could easily be transformed in the spray drying process into free-flowing powders used as cement additives for flexible concrete preparations.(5, 7, 8) Another concept of core-shell morphology is applied in coating practice – copolymers characterized by soft shell and hard core are very efficient binders in low-VOC or VOC-free paints, varnishes and plasters formulations.(6, 12) Contrary to random acrylic species, the heterogeneous polymers have relatively high Tg (good antiblocking, hardness, low dirt pick-up) and relatively low MFFT values (thus a reduced need for coalescing agents).
Heterogeneous Copolymer ApplicationThe first industrial applications started in the last two decades, mostly in solventless interior and exterior emulsion paints and in waterborne enamels with good antiblocking properties. In the beginning, the higher polymer cost was a limitation. Now, because of technology development and production volume increase, the price difference between core-shell and random copolymers is almost negligible, and phase-separated polymers are used much more frequently. Taking into account environmental concerns and VOC regulations, the solvent-free formulations are becoming important and that means additional possibilities for core-shell copolymers, even in standard facade paints and plasters.
To achieve good film forming properties of paint or plaster, binder MFFT has to be low enough. Binder MFFT could be decreased by the addition of coalescing organic solvent. Because of environmental restrictions, this is not a recommended solution even though some coalescing agents are not treated as VOCs. Application of low MFFT and low Tg random polymer is possible but it means coating tackiness, poor anti-blocking resistance and extensive dirt pick-up. Core-shell copolymers are developed to overcome these drawbacks and presently, in some application fields, they are treated more as utility polymers than fine, tailored-made species. The core-shell approach was applied even to EVA copolymers, thus allowing the possibility of introducing a hard phase EVA into a soft EVA phase.(13) These heterogeneous EVA binders characterized by low MFFT and at the same time very high Tg values are suitable for low-emission dispersion paints, enamels and plasters.
The hardness and robustness of a coating are improved a great deal with the binder change from homogeneous to the heterogeneous. In some cases it is not enough, and extra curing properties are desired. For such reason a functionality, responsible for curing during the drying process, was added to the core-shell copolymer.(14) Lundsten and Lindberg tested three dispersions (random, core-shell and core-shell + functionality) in soft sheen interior paint; the scrub resistance, adhesion, mud cracking and König hardness of the core-shell + functionality version was excellent. A new dispersion was recommended for use in dispersion paints (interior and exterior), fillers and plasters.(14)
Waterborne enamels, because of European VOC Phase 2 (2010), have developed very fast in the last five years. Relatively low 2010 limits of volatile organic compounds,15 both in waterborne (130 g/L) and solventborne (300 g/L) systems together with a possibly more restrictive approach in the future are triggering the development of new technologies and application of phase-separated copolymers as a binder in new formulations. In the structured copolymers, the low Tg part contributes to elasticity and film formation while the high Tg part improves hardness.16 The core-shell structure is responsible for excellent anti-blocking properties of trim products. It is a very important factor for products used mainly for window frame and door painting.7
Other Applications of Heterogeneous CopolymersPhase-separated copolymers, mainly in the form of free-flowing redispersible powders, are used as specialties in some dry cement compositions yielding to the flexible mortars. Such mortars are characterized by outstanding elasticity (low Tg polymer core), improved adhesion (polar functional groups located mainly on the polymer shell) and lower water uptake (hydrophobic effect).(8, 17, 18) Similar, low-Tg, heterogeneous polymer powders, as added-value raw materials, find application as impact modifiers, flexibilizing agents in powder coatings or organic binders in powder adhesives.(19-21)
Other very fast developing application fields for heterogeneous polymers are biology, biochemistry and medicine. Highly sophisticated polymers are used as nanoscale carriers for drug molecules, catalysts and viruses. The potential for such applications of microemulsion dispersions is very high.(3, 4, 22, 23) Core-shell micro-emulsions may be prepared by a two-stage polymerization semi-continuous process of styrene, acrylates and itaconic acid yielding to high solid content dispersion of polymer characterized by small particles (below 30 nm) and high molar mass (over 2 x 106 g/mol).(4) Core-shell drug carriers were applied as a delivery system for lipophilic drugs. The transportation of anticancer drug (all-trans retinoic acid) encapsulated in core-shell nanoparticles was studied.(24)
Nano-sized polymer shells containing anti-corrosion inorganic core are very promising raw materials for anti-corrosion paint formulations. Due to their small size, shells are distributed evenly in the coating, giving improved protection level.(25) A mini-emulsion polymerization technique, in the presence of high shear devices, was used for the preparation of nano-sized polymer particles containing pigment or filler core.(26) Very fine inorganic particles of hydrophilic or hydrophobic nature may be enclosed by an organic shell thus forming extremely efficient pigments.
Nanocapsules of different structures containing active cores have been synthesized for use in nanostructured coatings with high wear resistance and superior weatherability. This technology has great industrial potential due to relatively low cost and properties improvement.(25)
A concept of core-shell structure was applied successfully in the case of pure inorganic compounds and particles containing an inorganic core and an inorganic shell were prepared and tested as catalysts and in the information storage technology.(27, 28)
Areas for Further DevelopmentFor very special applications, coatings have to be very hard and be flexible at the same time. Such properties may be obtained using core-shell polymers with an additional crosslinking process. After physical drying, an additional crosslinking process may be started to obtain the final parameters of the coating. For such purposes a typical chemical reaction(14) or a radiation curing step(6) may be used. A first solution for polymer extra crosslinking (typical chemical reaction) is applied in the industrial practice. For the second extra curing mechanism (radiation curing), a novel hybrid system may be proposed consisting of core-shell copolymers with some reactive unsaturation grafted on the polymer shell.
Another potentially important field of development is core-shell morphology containing inorganic nano particles. Inclusion of nano inorganic material into the polymeric structure may yield very special properties of new binders or pigments and create new applications. A recent work of K. Matyjaszewski’s group on gold nanoparticles core covered by crosslinked polymeric shell is a valuable example of current core-shell technology achievement.(29) The area of core-shell full of inorganic particles, although rather out of the scope of polymer chemistry and the topic of this paper, is also developing very fast and may result in new catalysts, pigments and information technology devices.
ConclusionsThe technology of waterborne polymers is still very promising and is developing fast mainly because of environmental reasons. More and more often a specialty polymer product from the 1990s is treated now as a utility. Polymers with heterogeneous morphology are used not only in medicine or special coatings, but also in the building industry, for example in high-quality exterior paints and plasters.
The addition of an extra curing mechanism to heterogeneous polymers allows the possibility to obtain tailor-made coatings that fulfill the most challenging demands. Micro-emulsion polymerization and micro-emulsion organic–inorganic polymerization are generating products with totally new properties.
This paper was presented at ACT’ 08 (Advances in Coatings Technology), November 2008, Warsaw, Poland. For more information, contact the author at email@example.com.