Process of making finely divided opaque particles

Details Process of making finely divided opaque particles Process of making finely divided opaque particles

2000-07-14

2002-10-08

Michl, Paul R. (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

Reexamination Certificate

active

06462128

ABSTRACT:

BACKGROUND
This invention relates to a process of making colored finely divided opaque particles based on aminoformaldehyde resin having high surface area and being suitable as pigments in the fields of plastic, rubber, paint, paper coating and printing ink.
Processes for producing finely divided polymeric particles by adding a curing catalyst such as sulfuric acid to an aqueous solution of amino resin obtained by reacting urea or melamine with formaldehyde and then curing the resin with stirring, have been known in the past [GB 672763, GB 748484, U.S. Pat. No. 2,851,424]. However, these known processes possessed a number of shortcomings, since the amino-formaldehyde resin would rapidly harden in the presence of the curing catalyst to become insoluble and infusible. For instance, in these processes it is necessary to conduct the hardening reaction under strictly controlled conditions for obtaining a finely divided resin having a uniform particle size. Further, when it is intended to produce colored finely divided polymeric particles, there are imposed several restrictions like the class of dye that can be used and the time of its use in the case of the conventional processes, it being possible to obtain colored finely divided hardened resins only by adding water-soluble dyes when the amino resin is in an aqueous solution state and then hardening the resin.
Relatively recently, a number of approaches to improve the process of making finely divided polymeric particles has been disclosed. One approach entails the use of protective colloids, like water-soluble polymers which greatly increase the viscosity of aqueous solutions and make the hardening of the amino-formaldehyde resin more controllable [U.S. Pat. No. 3,981,845]. This method was used for making a highly dispersed insoluble and infusible urea-formaldehyde polymer having a specific surface area of more than 10 square meters per gram and an average particle size smaller than 5 microns.
The second approach includes the stage of formation of a water-in-oil emulsion when a water solution of urea-formaldehyde prepolymer is admixed with high shear to a non-solvent, like xylene [U.S. Pat. No. 3,928,272]. This process has been used for preparation of particles with an average diameter from 0.2 to 4 microns and a specific surface area up to 90 square meters per gram. A dispersing agent, like potassium tripolyphosphate, sodium hexamethaphosphate or sodium laurylsulfate is added to prevent the particles from clustering.
A third approach involves the preparation of a soluble and fusible aminoformaldehyde resin by reacting benzoguanamine or its mixture with melamine and formaldehyde in an aqueous medium, introducing this aqueous liquid to an aqueous solution of protective colloid with stirring to prepare a microemulsion, and adding a curing catalyst to said emulsion [U.S. Pat. Nos. 3,945,980, 4,069,176].
All three approaches lead to microspheres of mainly spheroidal shape consisting of crosslinked amino-formaldehyde polymer with an average particle size smaller than 5 microns. Because of the approximately spherical shape of the polymer particles having a substantially uniform size, the level of opacity of said products is limited.
In the present invention, a high degree of opacity is achieved with particles of an irregular shape of crosslinked amino-formaldehyde resin having high surface area and an average particle size below 1 micron. In addition, the level of opacity can be monitored and the shape (spherical or irregular) of particles changed, depending on the presence of surfactants (dispersing agents).
SUMMARY OF THE INVENTION
The instant invention provides novel highly dispersed particles of solid insoluble and infusible amino-formaldehyde condensation polymer in the form of a fluffy bright colored powder, said product having a specific surface area of more than 100 square meters per gram, said product essentially consisting of non-spherical microparticles having substantially uniform size and irregular shape (if no surfactants are used) or mainly spherical microparticles having mainly submicron size: up to 70% below 0.7 microns and up to 40% below 0.5 microns (if a surfactant or a mixture of surfactants are used).
A process for producing such finely divided particles in colored form comprises the steps of reacting in aqueous medium at a pH of 6-9 a member selected from the group consisting of either urea or benzoguanamine and a mixture of 100-0% by weight of urea and 0-100% by weight of benzoguanamine with formaldehyde in a ratio of 1 mol of the former to 1.3-3 moles of the latter to prepare an aqueous liquid of a soluble and fusible prepolymer, introducing this water-containing prepolymer into an organic solvent with boiling point below 80° C. and completely mixable with the water in a ratio of 70-30% by weight of prepolymer and 30-70% by weight of organic solvent, dyeing said mixture with a single fluorescent dye or a combination of several dyes, introducing with high shear the dyed resin-containing mixture in to an aqueous solution of protective colloid consisting o f 92-98.5 % of water, 1-3% of water-soluble polymer, 0-3% of a nonionic surfactant or a mixture of non-ionic and ionic surfactants and 0.5-2.0% of acidic curing catalyst at a reaction temperature in the range from 70-100° C., and thereafter separating the hardened resin from the suspension followed by drying and deaggregating the dried hardened resin.
DETAILED DESCRIPTION OF THE INVENTION
The dyed finely divided particles can be obtained in accordance with the present invention in the following manner. Either urea or benzoguanamine or a mixture of 100-0% by weight of urea and 0-100% by weight of benzoguanamine and 1.3-3 moles, preferably 1.5-2.5 moles, per mole thereof of formaldehyde are reacted at a pH range of 6-9, and preferably 7-8.5, and suitably a temperature in the range of 50-95° C., preferably 70-90° C. using water as the reaction medium to prepare an aqueous solution of a soluble and fusible prepolymer. The reaction time should appropriately be so long that the greater part of formaldehyde (about 90%) has the opportunity of reacting with either urea or benzoguanamine, but not so long that the molecular weight of the prepolymer becomes so high that it can not be homogeneously mixed with water-mixable organic solvent. Typical representatives of organic solvents are aliphatic ketones and alcohols, like acetone, methylethylketone, methanol or ethanol. The concentration of either urea or benzoguanamine or a mixture thereof and formaldehyde are selected so that the concentration of the prepolymer is in the optimum range of 20-35%, preferably 25-30% by weight.
Appropriately, the organic solvent is added to the prepolymer at a temperature in the range of 40-60° C. in a ratio of 70-30% by weight of prepolymer and 30-70% by weight of organic solvent. A ratio between the prepolymer and organic solvent is selected preferably so that complete solubility of fluorescent dyes in said mixture can be achieved. In the case where a water-soluble dye is used, a ratio of 70-50% by weight of prepolymer and 30-50% by weight of organic solvent is recommended. In the case where an oil-soluble (water-insoluble) dye is used, a ratio of 50-30% by weight of prepolymer and 50-70% by weight of organic solvent is recommended.
Appropriately, the typical representatives of water-soluble dyes are rhodamines, fluoresceines and some coumarins, containing ionic groups in their structure, like Basic Yellow 40. The typical representatives of oil-soluble (water-insoluble) dyes are coumarins, like Blankophor SOL, Solvent Yellow 135, naphthalimides and non-fluorescent polymer-soluble dyes, like heterocyclic compounds with structures I-IX or a combination of the above mentioned dyes in a ratio of 10-90% by weight of fluorescent dyes and 90-10% by weight of dye with structure (I)-(IX).
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