Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2003-06-17
2004-04-20
Pezzuto, Helen L. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S272000, C526S318200, C526S318400, C526S318440, C526S328500, C526S329200, C526S329600
Reexamination Certificate
active
06723813
ABSTRACT:
RELATED APPLICATION
This application claims the priority of Japanese Patent application No. 2002-179256 filed on Jun. 20, 2002, which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of preparing an acrylic copolymer having an acid anhydride group.
2. The Prior Art
Recently, the acid/epoxy curing type is often used for a clear coating for the top coat of automobiles because of its durability under acidic rain. For a polymer having an acid group, a polymer having a so-called half ester unit prepared by using an alcohol for ring opening of a polymer having an acid anhydride group, described in Japanese Patent Laid-Open No. H03-287650, is often chosen for reactivity control reasons.
In order to obtain a good film appearance even with the minimized amount of the organic solvent contained, for the sake of reducing the burden on the environment, the viscosity of the polymer, an ingredient of the aforementioned clear coating, needs to be lowered. For this purpose, the number average molecular weight of the aforementioned polymer having an acid group is designed to be several thousands. A polymer with such a molecular weight can be obtained by using more than the usual amount of the initiator, which is a costly material. However, this is not a cost effective method.
It is known that the molecular weight of the obtained polymer can be lowered without the use of a large quantity of an expensive initiator by conducting polymerization at higher temperatures. However, the acrylic copolymer having an acid anhydride group obtained by simply conducting the polymerization at higher temperatures has a problem in that the obtained coating film occurs yellow discoloration when it is used in the clear coating and baked.
The object of the present invention is to provide a method of preparing, by means of high temperature polymerization without the use of a large amount of an expensive initiator, an acrylic copolymer having an acid anhydride group with a relatively low molecular weight that does not cause the problem of yellowing after baking when used in an acid/epoxy curing type clear paint.
BRIEF SUMMARY OF THE INVENTION
The method of preparing an acrylic copolymer having an acid anhydride group of the present invention is a method of preparing the copolymer with a number average molecular weight of 1,500-5,000 by polymerizing a monomer mixture of a radical polymerizable monomer having an acid anhydride group and another radical polymerizable monomer at a temperature of 150° C. or higher by using a radical initiator wherein the concentration of the monomer mixture is 50-75 wt %. Further, the abovementioned number average molecular weight may be 2,000-3,000, and the aforementioned monomer mixture concentration may be 55-70 wt %. Furthermore, the aforementioned radical initiator may be an active radical species that generates alkyl radicals.
DETAILED DESCRIPTION OF THE INVENTION
The method of preparing an acrylic copolymer having an acid anhydride group of the present invention uses as the raw material a monomer mixture of a radical polymerizable monomer having an acid anhydride group and another radical polymerizable monomer.
Examples of the aforementioned radical polymerizable monomer having an acid anhydride group include itaconic acid, anhydrous itaconic acid, anhydrous maleic acid, and anhydrous citraconic acid. Anhydrous maleic acid is preferable because of its reactivity.
Examples of the other radical polymerizable monomer include styrene, &agr;-methyl styrene, acrylates (such as methyl acrylate, ethyl acrylate, propyl acrylate, n, i, and t-butyl acrylate, 2-ethylhexyl acrylate, and lauryl acrylate), methacrylates (such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, n, i, and t-butyl methacrylate, 2-ethylhexyl methacrylate, and lauryl methacrylate), acrylamide and methacrylamide.
The amount of the radical polymerizable monomer having an acid anhydride group in the aforementioned monomer mixture is preferably 10-40 wt %, more preferably 15-30 wt %, of the total amount of the monomer.
In the method of preparing an acrylic copolymer of the present invention, the aforementioned monomer mixture concentration is characteristically 50-75%. This monomer mixture concentration refers to the ratio of the monomer mixture to the total amount of the solvent used for polymerization plus the monomer mixture. If the monomer mixture concentration is less than 50 wt %, then the problem of yellowing after baking occurs when the obtained acrylic copolymer is used for coating. Also, polymerization becomes difficult if it is more than 75 wt %. The lower limit of the preferable range of the aforementioned monomer mixture concentration is 55 wt %, and the upper limit is 70 wt %.
Selection of the solvent used in the aforementioned polymerization is not limited in particular, as long as it does not react with the acid anhydrate group. It is preferable to consider the polymerization temperature and the fact that the solvent is used for a thermosetting coating. If the radical polymerizable monomer having the acid anhydride group is solid, it is also preferable to choose those that can dissolve it. Specific examples of the aforementioned solvent used for polymerization include ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, isoamyl acetate, oxohexyl acetate, methylmethoxybutyl acetate, ethylethoxy propionate, and ethylene glycol monobutyl ether acetate.
The method of preparing an acrylic copolymer of the present invention uses a radical initiator. Selection of the aforementioned radical initiator is not limited in particular, but since the polymerization reaction is carried out at a high temperature as described later, i.e. 150° C. or higher, a radical initiator that is suitable for such a polymerization temperature, such as those with a half-life of 10 seconds to 15 minutes at 170° C., is preferable. The amount of the radical initiator used in the preparation method of the acrylic copolymer of the present invention can be reduced to 0-80% of the amount used for conventional acrylic copolymerization carried out at a temperature lower than 150° C.
Also, for the aforementioned radical initiator, a preferable choice is an active radical species that generates alkyl radicals, which has a weaker hydrogen drawing ability compared with the oxygen radical and phenyl radical. Examples of the preferable radical initiators that generate alkyl radicals include di-t-amyl peroxide and di-t-hexyl peroxide.
The polymerization temperature for the method of preparing an acrylic copolymer of the present invention is 150° C. or higher, preferably 160° C. or higher. The upper limit is not limited in particular, but 200° C. or lower is preferable when the selection of the solvent used and the design of the polymerization reactor vessel are taken into consideration. In order to carry out the aforementioned polymerization at 150° C. or higher, it is preferable to use a reactor vessel that can keep the reaction system in a sealed condition. When a reactor vessel that can seal the reaction system is used and a solvent having a boiling point lower than the polymerization reaction temperature is chosen, the pressure in the reactor vessel at the time of polymerization inevitably becomes higher than the atmospheric pressure. However, this pressure does not affect the properties of the acrylic polymer thus obtained.
The method of preparing an acrylic copolymer of the present invention can use conventionally well known procedures of acrylic polymerization. For example, a method is used in which the monomer mixture and the radical initiator are dropped into the solvent which has been heated up to the prescribed temperature. As necessary, aging for a prescribed time can be done, and a post-shot to reduce residual monomers can be done as well.
The number average molecular weight of the acrylic copolymer having an acid anhydride obtained by the preparation method of the present
Asai Tomohito
Ninomiya Yusuke
Yamada Shin-ya
Nippon Paint Company Ltd.
Pezzuto Helen L.
Townsend & Banta
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