Stock material or miscellaneous articles – Composite – Of wax or waxy material
Reexamination Certificate
2001-10-11
2003-07-01
Acquah, Samuel A. (Department: 1711)
Stock material or miscellaneous articles
Composite
Of wax or waxy material
C264S004100, C264S004300, C264S004330, C264S004700, C428S402210, C528S068000, C528S073000, C523S201000, C524S801000
Reexamination Certificate
active
06586107
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to microcapsules having walls comprising polyurea obtained by reaction of polyisocyanates, guanidine compounds, and amines, to processes for producing these microcapsules, and to the carbonless copy papers produced from the microcapsules.
Carbonless copy papers are coated on their back with a layer of microcapsules. The microcapsules contain a solution in oil of a color former, so called because it is capable, in the presence of acidic developers, of effectuating a change in color from colorless to colored within a few seconds.
When a capsule-coated paper, then, is placed on a developer-primed paper in such a way that the capsules are in direct contact with the developer, a colored image will appear on the developer side upon writing on the back of the capsuled paper, causing the capsules to burst under the pressure of the writing implement and to release the color former.
In one version of this process, papers coated with capsules are directly aftercoated with a developer layer (to provide two-layer self-contained paper) or capsules are directly mixed with developer and coated onto paper (to provide single-layer self-contained paper). In the case of these SC papers, an image can be created directly on the coated side by mechanical pressure, for example, by placing a sheet of uncoated paper on top and writing on it.
Carbonless copy papers are used worldwide, differing essentially only in the nature of the microcapsule wall used. Customary are gelatin, melamine-formaldehyde, and polyurethane or polyurea capsules. Polyurea capsules are customarily formed by an interfacial polyaddition process, the basic traits of which will now be outlined.
Generally, a suitable color former is dissolved in an oil by heating and a polyisocyanate is added. In addition, an aqueous solution of a protective colloid, for example, polyvinyl alcohol, is prepared. An emulsifying apparatus is then used to convert the oil and water phases into a fine oil-in-water emulsion for which the particle size is substantially equal to the average diameter of the desired capsules.
In a concurrent or subsequent operation, this emulsion is admixed with an aqueous solution of an amine or an amine-like substance (hereinafter referred to as “crosslinker”), which causes a rapid reaction of oil-dissolved isocyanate to take place at the oil/water phase boundary, to form a thin polyurea film. This polyaddition can be made virtually quantitative by supplying heat and allowing appropriate time for reaction. This process is known as an interfacial process. The end result obtained is isocyanate-free oil droplets dispersed in water which are surrounded by a resilient polyurea wall and are known as microcapsules. Typical processes of this kind and capsules resulting therefrom are described, for example, in U.S. Pat. No. 5,225,118, U.S. Pat. No. 5,164,126, EP-A 780,154, U.S. Pat. No. 5,635,211, U.S. Pat. No. 5,643,506, and EP-A 535,384.
The prior art production of microcapsules having polyurea walls by an interfacial process requires a hydrophobic oil phase containing a polyisocyanate as primary wall former. The thickness of the later capsule wall is directly proportional to the fraction of the primary wall former, i.e. the polyisocyanate.
EP-A-727 251 (counterpart of U.S. Pat. No. 5,635,211) describes microcapsules that are prepared using crosslinkers containing guanidine compounds and have a good performance profile. The stability of these microcapsules is now to be improved further. Especially when used for preparing self-contained papers, the microcapsules come into direct contact with the developer and have to have high stability, especially tightness. Otherwise, the capsules will become inadvertently damaged during the manufacture of such papers, releasing the color former and staining the paper.
Overcoming such deficiencies is an object of the present invention.
SUMMARY OF THE INVENTION
This object is achieved by microcapsules for which the walls comprise reaction products of polyisocyanates with guanidine compounds and amines.
DETAILED DESCRIPTION OF THE INVENTION
The guanidine compounds and amines are used as crosslinkers.
Useful guanidine compounds for producing microcapsules according to this invention include, for example, guanidine compounds of the formula (I)
where
or their salts with acids.
The salts may be, for example, salts of carbonic acid, nitric acid, sulfuric acid, hydrochloric acid, silicic acid, phosphoric acid, formic acid, and/or acetic acid. Salts of guanidine compounds of the formula (I) may be used in combination with inorganic bases in order that the free guanidine compounds of the formula (I) may be obtained from the salts in situ. Useful inorganic bases for this purpose include, for example, alkali metal and/or alkaline earth metal hydroxides and/or alkaline earth metal alkoxides. Preference is given to aqueous solutions or slurries of these bases, especially aqueous sodium hydroxide solution, aqueous potassium hydroxide solution, and aqueous solutions or slurries of calcium hydroxide. Combinations of a plurality of bases may also be used.
It is frequently advantageous to use the guanidine compounds of the formula (I) as salts, since they are commercially available in that form and free guanidine compounds are in some instances sparingly soluble in water or lack stability in storage. When inorganic bases are used, they may be used in stoichiometric, substoichiometric, or superstoichiometric amounts, based on salts of guanidine compounds. Preference is given to using 10 to 100 equivalent % of inorganic base (based on salts of the guanidine compounds). The addition of inorganic bases has the consequence that, for microencapsulation, guanidine compounds having free NH
2
groups are available in the aqueous phase for reaction with the polyisocyanates in the oil phase. For microencapsulation, salts of guanidine compounds and bases are advantageously added separately to the aqueous phase.
Preference is given to using guanidine or salts of guanidine with carbonic acid, nitric acid, sulfuric acid, hydrochloric acid, silicic acid, phosphoric acid, formic acid, and/or acetic acid.
It is particularly advantageous to use salts of guanidine compounds with weak acids. These are in equilibrium with the corresponding free guanidine compound in aqueous solution as a consequence of hydrolysis. The free guanidine compound is consumed during the encapsulation process and is constantly regenerated according to the law of mass action. Guanidine carbonate exhibits this advantage to a particular degree. When salts of guanidine compounds with weak acids are used, there is no need to add inorganic bases to release the free guanidine compounds.
Useful guanidine compounds of the formula (I) for the present invention may also be prepared by ion exchange from their water-soluble salts according to the prior art using commercially available basic ion exchangers. The eluate from the ion exchanger can be neutralized directly for capsule wall formation by mixing it with the oil-in-water emulsion. Guanidine carbonate is most preferred.
For example, sufficient guanidine compound can be used so that 0.2 to 4.0 mol of free NH
2
groups are introduced into or released in the water phase in the form of guanidine compounds per mole of NCO groups present as polyisocyanate in the oil phase. This amount is preferably 0.5 to 1.5 mol. When guanidine compounds are used in a substoichiometric amount, free NCO groups remain after the reaction with the polyisocyanate. These then generally react with water, which is usually not critical since this reaction gives rise to new, free amino groups capable of crosslinking.
The guanidine compounds are preferably used in the form of aqueous solutions. The concentration of such solutions is not critical and is generally limited only by the solubility of the guanidine compounds in water. Useful aqueous solutions of guanidine compounds are 1 to 20% by weight in strength, for example.
Useful amines as an additional crosslinker component include polyami
Klug Günter
Weisser Jürgen
Acquah Samuel A.
Bayer Aktiengesellschaft
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