Microcapsules of low-formaldehyde melamine/formaldehyde resins

Details Microcapsules of low-formaldehyde melamine/formaldehyde resins Microcapsules of low-formaldehyde melamine/formaldehyde resins

1999-08-04

2001-07-17

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

06261483

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the preparation of microcapsules by polycondensation of a melamine/formaldehyde resin mixture in water, in which the material forming the capsule core is dispersed, in the presence of an anionic protective colloid, and the melamine/formaldehyde resin mixtures used therein. The present invention furthermore relates to the microcapsules obtained by the novel process and to their use for pressure-sensitive recording systems.
2. Description of the Background
Microcapsules which are used for pressure-sensitive recording materials must be impermeable and must also have a narrow size distribution, since otherwise, when they are further processed, deficiencies occur in the copying papers. The encapsulation process and the starting materials therefore have to meet high requirements.
As a rule, partially methylated resins of 1 mol of melamine and from 5 to 6 mol of formaldehyde are used as starting materials for the wall material since they lead to particularly impermeable and uniform capsules.
However, such resins have a relatively high residual formaldehyde content, which it is intended to avoid since the microcapsules are often prepared directly by the paper maker.
Melamine/formaldehyde resins converted to the methyl ether form and prepared with a relatively low formaldehyde content are disclosed in EP-A-0 008 683 for the preparation of coatings. When used as a starting material for microcapsules, however, they are unsatisfactory since they lead to nonuniform capsule walls.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for the preparation of microcapsules which has a lower total pollution by formaldehyde. In particular, it is intended to provide a melamine/formaldehyde resin mixture which is distinguished by a low formaldehyde content and nevertheless can be processed to give good microcapsules. In addition, the microcapsule dispersions should likewise have a low formaldehyde content.
We have found that this object is achieved by a process for the preparation of microcapsules by polycondensation of a melamine/formaldehyde resin mixture in water, in which the material forming the capsule core is dispersed, by choosing a melamine/formaldehyde resin mixture which is composed of
a) an aqueous solution of a condensate which is composed of
a1) 1 mol of melamine
a2) from 2 to 5 mol of formaldehyde and
a3) from 0 to 0.5 mol of a melamine derivative which can undergo an addition or condensation reaction with formaldehyde and
a4) from 1 to 5 mol of a C
1
-C
3
-alkanol and
b) from 0.01 to 1 mol of a water-soluble primary, secondary or tertiary amine or ammonia.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Melamine/formaldehyde resin converted to the methyl ethyl form and having a molar ratio of from 6.5 to 8 mol of formaldehyde per mol of melamine are generally known, and a process for their preparation is described, for example, in EP-A-0 008 683.
The formaldehyde may be used both as paraformaldehyde and, preferably, in aqueous solution.
As a rule, the melamine is methylolated with an aqueous formaline solution which contains from 30 to 50% by weight of formaldehyde and was brought to pH of from 7 to 10, in a molar ratio of from 1:3.5 to 1:7 over a period of from 5 minutes to one hour at from 60 to 110° C., and the resulting suspension of the methylolmelamine is etherified with from 10 to 20 mol of a C
1
-C
3
-alkanol per mol of melamine at from 30 to 60° C. in the presence of a strong acid. Since atmospheric pressure is generally employed, the formaldehyde content of the resulting resin mixture is lower than that of the mixture of the starting materials. The alkanol is also used in excess, since the etherified content is lower.
For the condensation reaction of melamine with formaldehyde, it is possible to add from 0 to 0.5 mol, per mol of melamine, of a melamine derivative which can undergo an addition or condensation reaction with formaldehyde.
Guanamines, such as benzoguanamine and acetoguanamine, bisguanamines, such as adipo-, glutaro- or methylolglutarobisguanamine, and compounds which contain a plurality of fused aminotriazine nuclei are suitable for this.
Suitable alcohols for the etherification are propanol, ethanol and in particular methanol.
After the end of the etherification, working up is effected in a known manner, i.e. the mixture is neutralized or rendered weakly alkaline and the excess alcohol is distilled off, generally under reduced pressure, together with the water contained in the batch.
All water-soluble primary, secondary or tertiary amines and ammonia are suitable as component b) of the melamine/formaldehyde resin mixture.
Examples of suitable amines are:
monoalkylamines, such as methylamine, ethylamine, 3-propylamine, isopropylamine, butylamine, sec-butylamine, isobutylamine, 1,2-dimethylpropylamine or 2-ethylhexylamine,
dialkylamines, such as dimethylamine, diethylamine, dipropylamine, diisopropylamine, dibutylamine, diisobutylamine, di-sec-butylamine, di-2-ethylhexylamine, N-methyl-n-butylamine or N-ethyl-n-butylamine,
alkoxyalkylamines, such as 2-methoxyethylamine, bis-2-methoxyethylamine, 3-methoxypropylamine, 3-ethoxypropylamine or 3-(2-ethylhexyloxy)propylamine,
trialkylamines, such as trimethylamine, triethylamine, tri-n-propylamine, tributylamine, N,N-dimethylethylamine, N,N-dimethylisopropylamine, N,N-dimethylbenzylamine, and
hydroxylamines, such as mono- or diethanolamine, mono- or dipropanolamine, mono- or diisopropanolamine, N-mono- or N,N-di(C
1
-C
4
-alkyl)ethanolamines, -propanolamines or -isopropanolamines, such as N-mono- or N,N-dimethylethanolamine, -propanolamine or -isopropanolamine, N-mono- or N,N-diethylethanolamine, -propanolamine or -isopropanolamine, N-mono- or N,N-dipropylethanolamine, -propanolamine or -isopropanolamine, N-mono- or N,N-diisopropylethanolamine, -propanolamine or -isopropanolamine or N-mono- or N,N-dibutylethanolamine, -propanolamine or -isopropanolamine, N-(C
1
-C
4
-alkyl)diethanolamines, -dipropanolamines or -diisopropanolamines, such as N-methyldiethanolamine, -dipropanolamine or -diisopropanolamine, N-ethyldiethanolamine, -dipropanolamine or -diisopropanolamine, N-propyldiethanolamine, -dipropanolamine or -diisopropanolamine, N-isopropyldiethanolamine, -dipropanolamine or -diisopropanolamine or N-butyldiethanolamine, -dipropanolamine or -diisopropanolamine, triethanolamine, tripropanolamine, triisopropanolamine, N-(2-hydroxyethyl)pyrrolidine, N-(2- or 3-hydroxypropyl)pyrrolidine, N-(2-hydroxylethyl)piperidine, N-(2- or 3-hydroxypropyl)piperidine, N-(2-hydroxyethyl)morpholine, N-(2- or 3-hydroxypropyl)morpholine, N-(2-hydroxyethyl)piperazine, N-(2- or 3-hydroxypropyl)piperazine or N,N-dimethyl- or N,N-diethyl-N-(5-hydroxy-3-oxapentyl)amine.
The novel melamine/formaldehyde resin mixtures consisting of the etherified condensate a) and one or more water-soluble primary, secondary or tertiary amines or ammonia in the abovementioned amounts can advantageously be used for the preparation of microcapsules.
A melamine/formaldehyde resin mixture comprising an etherified condensate a) composed of
a1) 1 mol of melamine
a2) from 2.5 to 4.5 mol of formaldehyde and
a3) from 0 to 0.2 mol of a melamine derivative which can undergo an addition or condensation reaction with formaldehyde,
and the amine b)
is preferably chosen.
Melamine/formaldehyde resin mixtures comprising C
1
-C
6
-alkylamines and/or C
1
-C
6
-alkanolamines are likewise preferred.
In particular, those melamine/formaldehyde resin mixtures which contain from 0.01 to 0.5 mol, especially from 0.01 to 0.1, mol, based on 1 mol of melamine, of a water-soluble primary, secondary or tertiary amine or ammonia are preferred.
The novel melamine/formaldehyde resin mixtures are suitable as starting materials for the microencapsulation and lead to microcapsules having good properties. Since these mixtures have from the outset a much lower content of free formaldehyde than the conventional starting materials, they are easier to handle. Moreover, the nov

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