Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1997-12-18
2001-10-30
Wilson, Donald R. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S287000, C526S070000
Reexamination Certificate
active
06310155
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the manufacture of beads of acrylic (co)polymers, in particular of beads suitable for the applications as coating.
It is well known that for certain coatings, for instance in certain dental applications, beads having a granulometry from 0.03 to 0.100 mm are required.
In particular the present invention relates to the manufacture of beads of acrylic (co)polymers, obtained by processes in aqueous suspension in the presence of suspending agents, utilizable as coating.
It is known that with the processes of polymerization in suspension, beads of polymers having an average diameter between 0.1-1 mm are obtained.
The granulometric size distribution is very large and generally one proceeds to separation by screening if a certain kind of average size is desired. However the drawback from the industrial point of view is that for particles for coating with the indicated sizes, the screening is not very effective and is complex and besides there is the further drawback that the yields are very low.
It is not therefore assumable from the industrial point of view a production by using these processes known in the art.
The polymerization in suspension is a reaction type which is performed in a system in which the monomer is suspended in the form of small drops in a continuous phase and polymerized using an initiator of radical type soluble in the monomer. The continuous phase is generally water.
The ratio between continuous phase (water) and discontinuous phase (monomer) is generally comprised between 1:1 and 3:1.
In the practical embodiment of this type of process it is necessary the employment of suspending stabilizers hindering the coalescence of the small drops of monomer in the most advanced steps of the polymerization.
As suspending stabilizers are used, in the most usual technique, hydrosoluble macromolecular compounds with affinity towards the monomer which, placing themselves at the interface between organic phase and aqueous phase, form a protective film hindering agglomeration of the particles.
At the end of the polymerization the suspending agent is removed from the surface of the polymer particles by washing with water.
The suspending agent is a key factor as its characteristics conditions the performances of the whole process both from the point of view of the quality of the final product and from the point of view of costs.
Polymerization processes in aqueous suspension of acrylic monomers are also known, in particular the one described in European patent application 457,356, which corresponds to U.S. Pat. No. 5,344,901 which is herein fully incorporated by reference, wherein particular polymers selected from the homopolymers of compounds of formula
are used as suspending and stabilizing agent of the aqueous suspension,
wherein: R
1
=H, CH
3
; R
2
and R
3
, equal or different, are H, alkyls C
1
-C
8
, and M=an alkaline or alkaline-earth metal, or copolymers of said compounds with acrylic monomers.
By using said suspending agents various advantages are obtained, such as:
high stability of the aqueous suspension, also operating with ratios water/acrylic monomers near unity
waste waters having a very low content in residual polymer
obtainment of acrylic polymers having high optical purity.
However, according to the examples of EP Patent 457,356 the obtained beads have average sizes from 0.2 to 0.3 mm. It is known that in the industrial plants, which are large-sized, there exist some bonds to the stirring, wherefore it is not industrially suggestable to act on this parameter to obtain fine particles for coating.
It was felt the need to have at one's disposal an industrial process with high yields making available beads of average sizes from 0.03 to 0.100 mm to be used as coating and at the same time capable of utilizing the advantages indicated for the suspending agent cited above.
It has now been unexpectedly and surprisingly found that this is possible using the process described hereunder.
SUMMARY OF THE INVENTION
The object of the present invention is therefore a process for the polymerization in aqueous suspension of acrylic monomers in the presence of a radical initiator soluble in the monomer and of a polymeric suspending agent selected from:
a) the homopolymers of a compound of formula:
wherein: R
1
is H or CH
3
; R
2
and R
3
, equal or different, are H or alkyls C
1
-C
8
, optionally branched when possible; M is an alkaline or alkaline-earth metal or ammonium and A is NH, O or NCH
3
,
b) the copolymers of said compound of formula I with 40% by weight at most of acrylic monomers,
characterized in that the polymerization aqueous phase is wholly or partly formed by the mother waters obtained after the separation of the acrylic polymer, said mother waters containing an organic phase which comprises said suspending agent and other products obtained during the polymerization, optionally charged with a further amount of said suspending agent, so as to have a polymerizable suspension containing at least 0.01% by weight and up to about 1% by weight of said suspending agent and at least higher than 1.5% by weight and up to about 5% by weight of the other products mentioned above, obtained during the polymerization. Preferably 0.03-0.3% by weight of suspending agent and 2-3% by weight of the other products obtained during the polymerization.
The suspending agents of formula (I) are prepared by means of the methods indicated in EP 457,356, also when A=O the same methods can be utilized. The acrylic polymer separates from the mother waters, for instance by centrifugation or filtering.
The process according to the invention can be carried out with the known modalities for polymerizations in aqueous suspension, i.e. operating with ratios between the aqueous phase and the acrylic monomers generally comprised between 1:1 and 3:1, in the presence of suspending agents and of a radical polimerization initiator at temperatures at which the decomposition of the initiators occurs, generally comprised from 50° C. to 120° C., as more specifically described in EP patent application 457,356.
The aqueous phase is formed wholly (100%) or partly, even of the order of 30-50% by weight, by the mother waters obtained by a previous polymerization, provided that the above indicated limits are respected.
Such mother waters obtained from the separation of the acrylic polymer at the end of the polymerization, generally contain, with respect to the initial ratio aqueous phase/monomers and under the reaction conditions used, from 1.5% to 5% by weight of an organic phase, determined as dry residue at 160° C., formed by the suspending agent and by “other products” obtained (e.g., the polymer contained in the mother liquors in the form of particles in emulsion) during the polymerization.
The suspending agent comes almost entirely from that initially introduced.
By wholly or partly recycling said mother waters, aqueous suspension of acrylic monomers are then obtained, which contain besides the suspending agent previously used also the above mentioned “other products” obtained during the polymerization and which result more stable.
The amount to be recycled must be at least such that, in relation- to the dry content at 160° C. of said mother waters, aqueous emulsions of acrylic monomers are obtained, containing at least the amounts by weight indicated above of said suspending agents, and at least the amounts by weight indicated above of said “other products” obtained during the polymerization.
It is possible to recycle from 30 to 100% by weight of said mother waters, in case with further amounts of water optionally containing fresh suspending agent.
The acrylic monomers which can be polymerized according to the process of the present invention are formed by C
1
-C
8
alkylacrylates or methacrylates such as, for instance, methyl(meth)acrylate, ethyl(meth)acrylate, propyl(meth)acrylate, isopropyl(meth)acrylate, butylacrylate, sec-butylmethacrylate, terbutylmethacrylate.
They can be used alone or in admixture with each o
Giberti Fabio
Relvini Pasquale
ELF Atochem Italia S.r.l.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Wilson Donald R.
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