Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1999-08-26
2002-02-12
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C521S056000, C528S355000, C528S408000
Reexamination Certificate
active
06346592
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a procedure for producing polymer particles with a narrow size distribution in the range 0.5-15 &mgr;m by means of free radical polymerization of vinyl monomer(s), which is(are) easily soluble in the polymerization medium, while the polymer precipitates out with subsequent nucleation of the particles.
There is an increasing need for small polymer particles with a narrow size distribution within chromatography, biological separation, catalyst production, carriers for catalysts, immunoassays, peptide synthesis, coating powders, toners, spacing indicators, viscosity modifiers, gloss modifiers, ion exchangers and in many other areas. The small polymer particles can also be used as seed in the production of larger particles of vinyl monomers or resins. In this connection, the seed may only constitute a small part of the overall particle so that the properties may be completely different from the initial particle. The areas of application of the particles increase considerably with activation with functional groups which, in themselves, provide the special properties or which can be used as coupling points for other groups/compounds.
Small polymer particles mean particles in the size range 0.5 to 15 micrometres. The term monodisperse particles means that all particles are of identical size. Since such systems have homogeneous properties, this may be advantageous for some applications but in most cases it is sufficient to have a narrow distribution, for example with a variation coefficient, CV, in the range 5-25%.
2. Description of Related Art
Conventional particles are produced by suspension polymerization, emulsion polymerization, seed polymerization, activated seed polymerization, dispersion polymerization and related techniques.
Suspension polymerization results in particles with a wide size distribution, often 10 to 200 micrometres, which makes it necessary to classify the particles and remove the desired fraction, which is thus usually only obtained in small quantities. In standard emulsion polymerization, particles are mostly produced with a diameter in the range 0.1 to 1 micron and it is not common to make particles larger than 1 micron with this technique. Norwegian patent application no. 139410 describes a so-called two-stage swelling technique, activated swelling, and the subsequent polymerization which makes possible the production of monodisperse particles in a wide size range. The disadvantage of this technique is that each swelling stage is very time-consuming. This is on account of the long time it takes to disperse the swelling reagent in water and transport it through the water phase and into the particles which are to grow. If not all the swelling agent is swelled in and drops remain in the water phase, the result is large polymer particles, which must be removed from the main product before use.
Dispersion polymerization (precipitation polymerization) can be regarded as a special type of deposition polymerisation and was intended, from the start, as an alternative to traditional emulsion polymerisation. The typical components of a dispersion polymerisation process are monomer(s), an initiator, a steric stabiliser, possibly also a co-stabiliser and a medium in which these components easily dissolve. Typical dispersion polymerisation processes are carried out in non-polar organic media such as hydrocarbons. Recently, polar media such as alcohols, mixtures of alcohols, water and ethers have also been taken into use. The polymer which is formed, most commonly with free radical or anionic polymerisation, is, however, insoluble in the medium and is stabilised against coagulation with the steric stabiliser. The reason why particles with a narrow distribution are formed with this polymerisation technique is not fully known. However, some explanations have been given (for example, K. E. Barrett, Br. Polym. J., 5, 259, 1973 and E. Shen et al., J. of Pol. Sci., 32, 1087, 1994).
One of the disadvantages of dispersion polymerisation is that the reactions traditionally take a very long time. In Shen et al. (Journal of Polymer Science; Part A: Polymer Chemistry, Vol. 31, 1393-1402, 1993), polymerisation times of 48 hours for the production of PMMA particles are reported. The reason for this long time is that the temperature has a decisive effect on the size of the particles which are formed. A higher temperature for increased polymerisation speed also produces an increased quantity of free radicals and increased solubility of the oligomer/polymer. This also results in the particle size increasing and a tendency for the particle distribution to be wider.
None of the references stated solves the problem of producing polymer particles which are easily activated, i.e. which contain a chemical functionality or substances which allow the particles to be used directly for special applications and further modifications. Nor do the references provide a complete solution to how the organic media are to be recirculated for reuse. With the large quantities of solvent and stabiliser used in the dispersion polymerisation process, this is hardly advantageous from a financial or an environmental standpoint. Shen et al. decanted the reaction medium from the particles and used it again after having corrected it for unconverted monomer and undecomposed initiator. This was reported as having been done three times with only a slight increase in particle diameter. The fact that the problem of reuse has not been fully solved, together with the disadvantageously long cycle time for production, makes the described methods unfavourable for use in practical applications, for example industrial production. Commercial exploitation of dispersion polymerisation (precipitation polymerisation) in polar organic media requires a shorter polymerisation time than that which has traditionally been described.
BRIEF SUMMARY OF THE INVENTION
Surprisingly, it turned out to be possible to use a required temperature, most advantageously lower than the average polymerisation temperature in the process, at the start of polymerisation (during the deposition/formation of the particles) to control the size of the particles and then to increase the temperature to achieve a satisfactory conversion speed without this resulting in an unfavourable size and distribution of the particles produced. The reaction time could thus be reduced to times of about 10 hours for tests with methyl methacrylate, which, for approximately 90% conversion, previously took about 48 hours. This makes the new process more flexible than the process previously reported. In addition, the present invention makes it possible to use a varying concentration of initiator for the production of a certain particle size without varying any process parameters other than the temperature.
DETAILED DESCRIPTION OF THE INVENTION
The parameters for controlling the particle size in a dispersion polymerisation system are the polymerisation medium, the polymerisation temperature, the initiator type and concentration, the stabiliser type and concentration and the monomer type and concentration.
When the solubility of the oligomer/polymer molecule increases in the medium, the chain length before precipitation will increase. The size of the particles which are formed will thus increase. The solubility of the oligomer/polymer molecules also increases with increasing temperature. Thus the chain length at which the molecules precipitate out will increase. The concentration of radicals will also increase with increasing temperature with a consequent increase in the decomposition speed of the initiator. Increased radical concentration results in the speed at which the chains precipitate out increasing. At the same time, the adsorption of the stabiliser on the precipitated/agglomerated particles will decrease on account of the increased solubility of the stabilising agent in the medium. As the initiator concentration increases, the radical concentration increases and thus also the speed at which the
Pedersen Steinar
Sæthre Bård
Norsk Hydro ASA
Wenderoth , Lind & Ponack, L.L.P.
Wu David W.
Zalukaeva Tanya
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