Hollow latex
As hollow latex (English hollow latex ; also opaque polymer or polymeric micro-hollow particles ) is defined as a special form of multiphase polymer particles .
These are mostly polymer dispersions made from core-shell particles, produced by a multistage emulsion polymerization , the core of which is swollen in a further process step. When it dries, the water diffuses out of the swollen core, which collapses and leaves a cavity in the particle.
The main application of these polymer dispersions is the partial replacement of classic white pigments such as titanium dioxide in emulsion paints and paper coatings . The main producer is Rohm and Haas , which sells the dispersions under the trade name Ropaque.
production method
There are several different processes for the production of hollow latices, but currently only the swelling of multiphase polymer particles is of greater importance. The other processes are currently of subordinate interest or represent special forms of encapsulation processes.
Swelling of multiphase polymer particles
This multi-stage process, patented by Rohm & Haas, can be used to produce hollow latex particles with a diameter of a few hundred nanometers to a few micrometers . The principle is the swelling of the core of a core-shell particle, which collapses when the dispersion dries and leaves a cavity. The individual process steps are:
- Production of an aqueous dispersion of the core polymer by emulsion polymerization of acrylic and methacrylic acid esters with approx. 30 to 40% methacrylic acid . The polymer particles produced in this way can be swollen by the action of monovalent bases ( ammonia , caustic soda , etc.). Instead of methacrylic acid, acrylamide , methacrylamide or their derivatives can also be copolymerized, the particles then being swellable by monobasic acids ( acetic acid , nitric acid , etc.). When these water-soluble monomers are copolymerized , a small amount of a polyfunctional monomer (e.g. ethylene glycol dimethacrylate , butanediol diacrylate and the like) must also be copolymerized as a crosslinking agent in order to enable the formation of a shell in the second step. It is also conceivable to dispense with the hydrophilic comonomers. The polymer formed is partially saponified by the action of strong bases, and the core swells.
- Polymerization of a shell made of a non-swellable (or only very little) swellable polymer onto the core latex. Esters of acrylic or methacrylic acid, styrene or also acrylonitrile can again be used as monomers . Small amounts of acrylic or methacrylic acid (a few percent) allow the formation of a more even shell. The polymerization of the core and shell must be in the acidic to max. neutral pH range. The glass transition temperature of the polymers should be selected so that the polymers are hard at the application temperature, but are deformable during the swelling.
- Swelling of the core-shell dispersion with a monovalent base (ammonia, sodium or potassium hydroxide , amines) at temperatures above the glass transition temperature of the shell polymer. Since the base has to diffuse through the shell into the core, this process step takes a certain amount of time (about half an hour to a few hours).
- Further shells can be polymerized onto the swollen particles.
The core makes up only a few percent of the mass fraction of the polymer particle. When the dispersion dries out, the core shrinks and leaves a cavity. The collapsed core polymer usually then covers the inner surface of the particle.
Swelling of homogeneous latices
This process can produce polymer particles with one or more cavities. The particle diameter is a few hundred nanometers. Compared to the process described above, this process is a little less complex, but the finished dispersion contains quite a high proportion of surfactants and salts, which can interfere with further use (e.g. in emulsion paints).
The starting point of the process is a latex containing carboxy groups produced by emulsion polymerization . This consists of a copolymer of styrene with methacrylic acid, with part of the styrene being replaced by other monomers such as methyl methacrylate , n - butyl acrylate , 1,3-butadiene or the like. can be replaced. Acrylic acid can also be used instead of methacrylic acid, but this may lead to problems with the homogeneity of the polymer particles. The methacrylic acid content is approximately between 5 and 14%.
The polymer dispersion thus obtained is stabilized by adding further surfactant and swollen at temperatures above the glass transition temperature of the copolymer by adding a base. Potash lye is mostly used as the base, but other monovalent bases (ammonia, ethanolamine) lead to similar results. When using polyvalent bases (e.g. calcium hydroxide ), the particles do not swell because of the ionic crosslinking between the carboxy groups and the polyvalent cations. By adding solvents such as toluene or monomeric styrene before adding the alkali, the polymer particles can be swollen, so that the base can act at lower temperatures.
After the alkali treatment, the pH is adjusted to the acidic range by adding an acid (e.g. hydrochloric acid, sulfuric acid or methacrylic acid). The acid treatment is again carried out at temperatures above the glass transition temperature.
The polymer particles treated in this way contain one or more cavities after the dispersion has dried, depending on the pH value during the alkali treatment. At very high pH values, only one or very few cavities are formed; as the pH value falls, the number of cavities increases and their diameter decreases.
Polymerization in the presence of an extender
This process is based on emulsion or suspension polymerization in the presence of an extender , i.e. an inert, non-polymerizable hydrocarbon.
A mixture of styrene and / or acrylic or methacrylic acid esters with acrylic acid, for example, can serve as monomers, possibly with the addition of a chain transfer reagent to reduce the molar mass. The extender must be miscible with the monomer and more hydrophobic than it. The polymer formed must not be soluble in the extender. The boiling point should be above the maximum reaction temperature. An example of a possible extender is i- octane . In the course of the polymerization, particles swollen by the extender initially form; as the polymerization progresses, the polymer precipitates and is deposited in the areas of the particles near the surface. The result is a core / shell particle with the extender as the core and the polymer formed as the shell. By metering in and polymerizing a crosslinking monomer (e.g. a mixture of styrene and divinylbenzene ), the structure formed can be strengthened. The process can be applied as an emulsion polymerization or as a suspension polymerization.
When the dispersion thus obtained dries, the extender diffuses out of the particle and leaves a cavity. In principle, the same scheme is used to encapsulate dyes (or precursors thereof) for carbonless paper or fragrances for smell samples (e.g. in advertisements for cosmetics).
Applications
Hollow latices are mainly used as a (partial) substitute for classic white pigments (e.g. titanium dioxide ) in emulsion paints . Compared to other white pigments, the aim is to achieve a higher rub resistance of the colors with a lower need for binding agents. Another area of application is paper coating. Compared to mineral pigments, advantages in terms of “whiteness”, surface quality (smoother surface, since the hollow particles on the surface are flattened during calendering) and the lower weight per unit area of the coated papers can be achieved.
As a concrete additive , hollow latices should represent an alternative to air-entraining agents to increase frost resistance. The more uniform distribution of the particles and an improved compressive strength are stated as advantages.
Dispersed in epoxy resins , dried hollow latices can increase the impact strength analogous to the rubber particles otherwise used.
literature
- JW Vanderhoff, JM Park, MS El-Aasser: Preparation of Particles for Microvoid Coatings by Seeded Emulsion Polymerization . In: Polymer Latexes: Preparation, Characterization, and Applications . 1992, p. 272-281 (ACS Symposium Series Vol. 492).
- Elodie Bourgeat-Lami: Hollow Particles: Synthetic Pathways and Potential Applications . In: Abdelhamid Elaissari (Ed.): Colloidal Polymers . New York 2003, ISBN 0-8247-4304-0 , pp. 189-223 .
Individual evidence
- ↑ Patent US4427836 : Inventors: A. Kowalski, M. Vogel, RM Blankenship.
- ↑ Patent US4469825 : Inventor: A. Kowalski, M. Vogel.
- ↑ Patent US5157084 : Inventors: DI Lee, MR Mulders, DJ Nicholson, AN Leadbetter.
- ↑ M. Okubo, A. Ito, T. Kanenobu: Production of submicron-sized multi-hollow polymer particles by alkali / cooling method. In: Colloid and Polymer Science. 274, No. 8, 1996, pp. 801-804, doi: 10.1007 / BF00654677 .
- ↑ M. Okubo, A. Ito, A. Hashiba: Production of submicron-sized multi-hollow polymer particles having high transition temperatures by the stepwise alkali / acid method. In: Colloid and Polymer Science. 274, No. 5, 1996, pp. 428-432, doi: 10.1007 / BF00652464 .
- ↑ M. Okubo, M. Nakamura, A. Ito: Influence of the kind of alkali on the preparation of multi-hollow polymer particles by the alkali / cooling method. In: Journal of Applied Polymer Science. 64, No. 10, 1997, pp. 1947-1951, doi : 10.1002 / (SICI) 1097-4628 (19970606) 64:10 <1947 :: AID-APP9> 3.0.CO; 2-I .
- ↑ Patent US5360827 : Inventors: H. Touda, Y. Takagishi, M. Kaino.
- ↑ Patent US4910229 : Inventor: M. Okubo.
- ↑ J. Snuparek: In: Emulsion Copolymerization. Hüthig & Wepf, ISBN 3-85739-010-7 , p. 129 ff.
- ↑ H. Wiese, R. Rupaner: Influence of metal ions on the alkali-swelling behavior of carboxylated acrylic polymer latex. In: Colloid and Polymer Science. 277, No. 4, 1999, pp. 372-375, doi: 10.1007 / s003960050394 .
- ↑ Patent US4973670 : Inventors: CJ Mc Donald, Y. Chonde, WE Cohrs, DC MacWilliams.
- ↑ Patent US4049604 : Inventor: DS Morehouse College Bolton.
- ↑ Patent US4677003 : Inventor: GH Redlich, RW Novak.
- ↑ Rohm & Haas, Technical Information on Ropaque OP-62
- ^ Rohm & Haas, Technical Information on Ropaque HP-91.
- ↑ Patent DE102006008967A1 : Inventors: JH Schattka, H. Kautz, G. Löhden.
- ^ Reza Bagheri, Raymond A. Pearson: The use of microvoids to toughen polymers. In: polymer. 36, No. 25, 1995, pp. 4883-4885, doi: 10.1016 / 0032-3861 (95) 99306-F .
- ^ Reza Bagheri, Raymond A. Pearson: Role of particle cavitation in rubber-toughened epoxies: 1. Microvoid toughening. In: polymer. 37, No. 20, 1996, pp. 4529-4538, doi: 10.1016 / 0032-3861 (96) 00295-9 .