Plastic and nonmetallic article shaping or treating: processes – Encapsulating normally liquid material – Liquid encapsulation utilizing an emulsion or dispersion to...
Reexamination Certificate
2002-07-10
2004-04-13
Sergent, Rabon (Department: 1711)
Plastic and nonmetallic article shaping or treating: processes
Encapsulating normally liquid material
Liquid encapsulation utilizing an emulsion or dispersion to...
C264S004330, C264S004700, C264S004100, C427S213310, C427S213320
Reexamination Certificate
active
06719931
ABSTRACT:
The invention relates to a process for preparing a dispersion of microcapsules by condensing partially methylated melamine-formaldehyde resin in water, in which the essentially water-insoluble material forming the capsule core is present in dispersion, in the presence of an anionic protective colloid. The invention further relates to the dispersions of microcapsules obtained by this process and to their use to prepare printing inks and paper-coating compositions.
Microdisperse particles which may have diameters in the range from about 0.1 to 100 &mgr;m, have found broad application in various fields. They are used, for example, as solid beads in polishing and/or cleaning compositions, as spacers in printing inks, as scale indicators for medical microscopy investigations, etc. In addition to the solid beads, microcapsules whose core material may comprise liquid, solid or gaseous substances which are essentially insoluble or insoluble in water are known. Examples of customary materials for the capsule walls are melamine-formaldehyde polymers, polyurethane, gelatin, polyamides, or polyureas. A widespread use is that of oil-filled microcapsules to prepare carbonless copying papers.
For this application, the oil-filled microcapsules are incorporated into paper-coating compositions with which paper substrates are coated. The nowadays customary high coating speeds require the paper-coating compositions to be of low viscosity, which in turn necessitates a low viscosity in the microcapsule dispersions. Nevertheless, the capsule concentration of the dispersions should be as high as possible in order to avoid an unnecessarily wet operation. In order to achieve a good color strength yield, moreover, a very narrow capsule size distribution is sought.
Owing to their preparation, dispersions of microcapsules comprising amino resins, such as melamine-formaldehyde resins, contain a greater or lesser amount of free formaldehyde. For reasons of ecology and occupational hygiene, the aim should be to keep the formaldehyde content as low as possible without adversely affecting other properties of the microcapsule dispersions. A distinction should be made here between the formaldehyde content of the dispersion itself and the formaldehyde content of the material coated with the dispersion. A low concentration of free formaldehyde in the aqueous capsule dispersion does not necessarily mean that a determination of the formaldehyde content in the coated material by means, for example, of cold-water extraction in accordance with DIN EN 645 and DIN EN 1541 will find low formaldehyde levels.
To reduce the formaldehyde content it is common to add formaldehyde scavengers to microcapsule dispersions based on melamine-formaldehyde resins. Ammonia, urea, ethyleneurea, and melamine, which are more or less effective in reducing the residual formaldehyde content in the capsule dispersion, are among the formaldehyde scavengers most commonly used.
EP-A-0 383 358 and DE-A-38 14 250 disclose light-sensitive materials comprising microcapsules whose walls are formed from melamine-formaldehyde resins. In order to remove excess formaldehyde, urea is added at the curing stage.
In the techniques described in EP-A-319 337 and U.S. Pat. No. 4,918,317, urea is added toward the end of curing.
EP-A-0 415 273 describes the preparation and use of mono- and polydisperse solid-bead particles of melamine-formaldehyde condensate. The use of ammonia, urea or ethyleneurea is proposed for binding the formaldehyde released during condensation.
Melamine-formaldehyde resin microcapsules notable for their uniform capsule size and imperviousness are known from EP-A-0 218 887 and EP-A-0 026 914. These capsule dispersions, however, still contain residual free formaldehyde, whose presence is undesirable in the context of further processing. EP-A-0 026 914 therefore recommends binding the formaldehyde, after curing, using ethyleneurea and/or melamine as formaldehyde scavengers.
DE 198 35 114 discloses dispersions of microcapsules based on melamine-formaldehyde resin, said resin being partially etherified and comprising a water-soluble primary, secondary or tertiary amine or ammonia. Prior to curing, urea is added as a formaldehyde scavenger.
DE 198 33 347 describes a process for preparing microcapsules by condensing melamine-formaldehyde resins and/or their methyl ethers, in which urea or urea whose amino groups are connected by an ethylene or propylene bridge is added as a formaldehyde scavenger prior to curing. Although the resultant dispersions are low in formaldehyde, the addition of urea prior to curing has an adverse effect on the stability of the microcapsules and on the viscosity of the microcapsule dispersion.
True, the addition of the abovementioned formaldehyde scavengers to the finished microcapsule dispersion or during the preparation of the microcapsule dispersion does generally lower the formaldehyde content of the dispersion. However, the formaldehyde content of papers coated with coating compositions comprising the microcapsule dispersions, as determinable by cold-water extraction, cannot be reduced below a certain limit even on adding large amounts of formaldehyde scavengers.
It is an object of the invention to provide a process for preparing a formaldehyde-reduced dispersion of microcapsules where the formaldehyde content of papers coated with the dispersion, as determinable by cold-water extraction, is very low. A further object is to provide low-viscosity microcapsule dispersions, especially low-viscosity microcapsule dispersions with a high solids content.
We have found that these objects are achieved by the process of the invention for preparing a dispersion of microcapsules by condensing a partially methylated melamine-formaldehyde resin having a molar ratio of melamine:formaldehyde:methanol of from 1:3.0:2.0 to 1:6.0:4.0 in water in which the essentially water-insoluble material forming the capsule core is present in dispersion in the presence of an alkali metal salt of a homopolymer or copolymer of 2-acrylamido-2-methylpropanesulfonic acid as protective colloid at a pH of from 3 to 6.5 by preforming the microcapsules at a temperature of from 20 to 50° C. and then curing the capsule wall at >50 to 100°C., which comprises adding from 5 to 100% by weight of melamine, based on the melamine-formaldehyde resin, in the course of curing.
It is assumed that, with the known methods of lowering the formaldehyde content by adding formaldehyde scavengers, such as ammonia, amines, etc., the formaldehyde released during the condensation is bound by the formation of an adduct from formaldehyde and scavenger. However, during the coating of paper substrates or the drying of the coated papers, this adduct is apparently cleaved back under the action of acidic groups, e.g., sulfonic acid groups of excess protective colloid which has not been incorporated into the capsule wall, so that formaldehyde continues to be determinable in the cold-water extract of the coated papers. In the case of the process of the invention, the formaldehyde released, the excess protective colloid, and the added melamine are bound irreversibly, the product that is formed presumably being deposited on the capsule walls. In addition to lowering the formaldehyde content as determinable by cold-water extraction, therefore, the process also achieves a reduction in the viscosity of the microcapsule dispersion, since the excess protective colloid, which is now present no longer in dissolved form in the aqueous phase but in chemically bonded form on the capsule wall, makes no contribution to the viscosity of the dispersion. Accordingly, the dispersions prepared in accordance with the invention may also, advantageously, be spray-dried, since the microcapsules exhibit essentially no “stickiness” with one another.
The process of the invention is generally conducted by combining the core material to be encapsulated, the partially methylated melamine-formaldehyde resin having a defined molar ratio of melamine:formaldehyde:methanol of from 1:3.0:2.0 to 1:6.0:4.0, preferably f
Eisermann Herbert
Hoffman Dietrich
BASF - Aktiengesellschaft
Sergent Rabon
Tran Thao
LandOfFree
Low-viscosity, melamine-formaldehyde resin microcapsule... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Low-viscosity, melamine-formaldehyde resin microcapsule..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Low-viscosity, melamine-formaldehyde resin microcapsule... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3236705