Plastic and nonmetallic article shaping or treating: processes – Encapsulating normally liquid material – Liquid encapsulation utilizing an emulsion or dispersion to...
Reexamination Certificate
1999-07-26
2001-05-01
Nutter, Nathan M. (Department: 1711)
Plastic and nonmetallic article shaping or treating: processes
Encapsulating normally liquid material
Liquid encapsulation utilizing an emulsion or dispersion to...
C264S004300, C264S004330, C264S004700, C427S213300, C427S213310, C427S213330, C427S213340, C428S402200, C428S402210
Reexamination Certificate
active
06224795
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a process for the preparation of microcapsules by the condensation of melamine-formaldehyde resins and/or the methyl ethers thereof in water, in which the substantially water-insoluble material forming the core of the capsules is dispersed, in the presence of an anionic protective colloid at pH's ranging from 3 to 6.5 by the formation of the microcapsules at temperatures ranging from 20° to 50° C. followed by curing of the shell of the capsules at from >50 to 100° C. It also relates to the microcapsules produced by the process and to the use thereof in pressure-sensitive recording systems.
DISCUSSION OF THE BACKGROUND
EP-A 0-383 358 and DE-A 3,814,250 teach light-sensitive materials comprising microcapsules whose walls are composed of a melamine-formaldehyde resin. In order to remove the excess formaldehyde, urea is added during curing, which takes place from pH 5.5 and 60° C. upwards.
Urea is likewise added in the processes that are described in EP-A 319,337 and U.S. Pat. No. 4,918,317, toward the end of curing.
Microcapsules of melamine-formaldehyde resins which are distinguished by uniform size and impermeabity of the capsules, are disclosed in EP-A 0,026,914 and EP-A 0,218,887. The capsule dispersions produced by this process still contain residual free formaldehyde, the presence of which is undesirable during subsequent processing. EP-A 0,026,914 therefore recommends binding the formaldehyde subsequently to curing with imidazolidin-2-one and/or melamine acting as formaldehyde scavengers. This makes it possible to lower the formaldehyde content of the dispersion but has no influence on the formaldehyde that is liberated during subsequent processing or on the formaldehyde content of the coated papers. Attention should also be drawn to the fact that the addition of imidazolidin-2-one acting as formaldehyde scavenger has no influence on the shell of the microcapsules that are already formed.
SUMMARY OF THE INVENTION
It is thus an object of the present invention to provide a process for the preparation of low-formaldehyde dispersions of microcapsules which avoids the aforementioned drawbacks. In particular, the novel dispersions of microcapsules should emit distinctly less formaldehyde during manufacture and subsequent processing thereof, as also applies to the recording systems, and should nevertheless exhibit good performance properties. Such demands include, for example, impermeability of the shell of the capsule and narrow size distribution of the capsules.
Accordingly, we have found a process for the preparation of microcapsules by the condensation of melamine-formaldehyde resins and/or the methyl ethers thereof in water, in which the substantially water-insoluble material forming the core of the capsule is dispersed, in the presence of an anionic protective colloid at pH's ranging from 3 to 6.5 by the formation of the microcapsules at temperatures ranging from 20° to 50° C. followed by curing of the shell of the capsule at from >50 to 100° C., in which from 10 to 200 wt % of urea, whose amino groups are optionally linked to an ethylene or propylene bridge, based on the weight of melamine-formaldehyde resin, are added prior to curing. The percentage is based on the content of resin and not on its aqueous solution, in which it is usually employed.
DETAILED DESCRIPTION OF THE INVENTION
The process of the invention is generally carried out by finely emulsifying the core material to be encapsulated in an aqueous solution of the anionic protective colloid having a pH from 3 to 6.5, and the drop size can be adjusted depending on the final usage of the capsules. The aqueous solution of the melamine-formaldehyde resin and/or the methyl ethers thereof is added continuously or portionwise to the emulsion used as initial batch at temperatures ranging from 20° to 45° C. with stirring.
The microcapsules form in this first step. The shell thereof is then cured by raising the temperature. Curing of the shell of the capsule is found to take place from 50° C. upwards, so that >50° C., preferably 55° C. and more preferably 65° C. is used as the lower limit of the temperature range. Because it is an aqueous dispersion, curing should be carried out up to 100° C. and preferably up to 80° C. as the upper temperature limit. Depending on the pH of the dispersion, curing takes place at different rates, with dispersions having lower pH's between 3 and 5 optimally curing at temperatures ranging from 65° to 85° C. However above 50° C. curing is also clearly apparent in the weakly acid to neutral pH range.
The optimum temperatures with respect to the pH for the two steps capsule formation and curing can be readily determined by simple routine tests. An essential feature is that the urea and/or its cyclic derivatives are added prior to the curing step. This can be at the stage of preparing the emulsion of the core material or before or after the formation of the dispersion of microcapsules. The urea solutions are advantageously metered in together with the aqueous solution of the anionic protective colloid or preferably with the solution of the melamine-formaldehyde resin.
In addition to urea, which is preferably used, imidazolidin-2-one or tetrahydro-2-pyrimidone can be used.
Preferably from 50 to 150 wt % of urea, imidazolidin-2-one and/or tetrahydro-2-pyrimidone, based on the weight of melamine resin, are added prior to curing.
Suitable starting materials for the shell material are melamine-formaldehyde resins and/or the methyl ethers thereof, the ratio of melamine to formaldehyde being from 1:1.5 to 1:6, preferably from 1:3 to 1:6. These resins are N-methylolmelamine compounds or the methyl ethers thereof. The resins used in the process of the invention have to be miscible with water in all proportions without causing turbidity. For these reasons the ethers of methylol-melamines are particularly preferred. Methods of preparing the starting materials are known. Condensation of the resins takes place at pH's of from 3.0 to 6.5 and preferably from 3.5 to 5.5. The pH in the aqueous phase can be adjusted by adding acids, preferably formic acid. The method of precipitating the melamine-formaldehyde resin is somewhat dependent on the resin, so that there are somewhat different optimal pH's and/or temperatures for the formation of the microcapsules from the various resins.
Suitable core materials for the microcapsules are liquid, solid or gaseous, water-insoluble to substantially undissolved substances, and the following may be mentioned by way of example: liquids such as alkylnaphthalenes, partially hydrogenated terphenyls, aromatics such as xylene, toluene, dodecylbenzene, aliphatic hydrocarbons, such as gasoline and mineral oil, chlorinated paraffins, fluorocarbons, naturally occurring oils such as peanut oil, soybean oil, in addition adhesives, flavors, perfume oils, monomers such as (meth)acrylates, styrene, active substances such as agricultural pesticides, red phosphorus, inorganic and organic pigments, such as iron oxide pigments; in addition solutions or suspensions of colorants and primarily color formers and pigments in hydrocarbons such as alkylnaphthalenes, partially hydrogenated terphenyl, dodecylbenzene and other high-boiling liquids. The anionic protective colloids used are preferably water-soluble homopolymers or copolymers containing sulfo groups.
Suitable anionic protective colloids are preferably water soluble homopolymers or copolymers containing sulfo groups. Preference is given to homopolymers or copolymers containing sulfo groups which have a Fikentscher K value of from 100 to 170 or a viscosity of from 200 to 5000 mpa•s (measured on a 20 wt % strength aqueous solution at 23° C. using a Brookfield viscosimeter RVT, spindle No. 3, at 50 rpm).
Polymers having a K value of from 115 to 150 or a viscosity of from 400 to 4000 mpa•s are preferred. Such protective colloids are described in EP-A 0,026,914 and EP-A 0,562,344.
Polymers of sulfoethyl (meth)acrylate,
Biastoch Ralf
Frank Gabriele
BASF - Aktiengesellschaft
Nutter Nathan M.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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