Microencapsulation of red phosphorus

Details Microencapsulation of red phosphorus Microencapsulation of red phosphorus

2002-05-24

2004-07-20

Niland, Patrick D. (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...

C523S200000, C523S205000, C523S208000, C524S541000, C524S542000, C524S593000, C524S594000, C524S595000

Reexamination Certificate

active

06765047

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to red phosphorus and more particularly to a method of microencapsulating red phosphorus.
SUMMARY OF THE INVENTION
A method for microencapsulating red phosphorus in polymerized urea resorcinol formaldehyde resin is disclosed. The method entails (i) preparing an aqueous mixture containing urea, resorcinol and formaldehyde, (ii) adding hexamethylene tetramine and optionally water to the mixture to yield a stabilized mixture (iii) dispersing red phosphorus in the stabilized mixture (iv) adding acid to the reaction mixture, and (v) collecting the resulting encapsulated red phosphorus. The product is suitable as flame retarding agent in molding compositions.
BACKGROUND OF THE INVENTION
Red phosphorus has long been known to be an extremely effective flame retardant, particularly for fibreglass-reinforced polyamides and a number of other plastics.
Thus the use of red phosphorus in reinforced polyamide molding compositions is described for example in DE-A 1 931 387.
However considerable risks are involved in handling red phosphorus, especially when working it into plastic molding compounds at elevated temperatures. Thus red phosphorus forms highly toxic phosphines by the action of atmospheric oxygen and humidity, particularly at temperatures up to over 300° C. which are typical for processing industrial plastics. Apart from their toxicity phosphines are extremely critical owing to their self-ignition potential, and there is an additional hazard of self-ignition of the red phosphorus itself.
Many attempts have therefore been made in the past to improve the handling properties of red phosphorus when it is worked into plastic molding compounds and to make the process safer.
For example attempts have been made to suppress phosphine formation by mixing metal salts into the red phosphorus. The salts may be metal oxides and hydroxides combined with other metal compounds or additives. This is described inter alia in DE-A 2 308 104, EP-A 278 555 and EP-A 557 222.
Articles have also been published describing the stabilisation of red phosphorus by encapsulation. Thus red phosphorus encapsulated by precipitation of silicon dioxide or application of melamine formaldehyde resin is known from DE-A 2 646 835, DE-A 19 619 701 and EP-A 052 217. Coating with phenol formaldehyde resin is disclosed in DE-A 2 625 673.
WO 87/00187 describes the combination of urea resorcinol and formaldehyde for microencapsulation of red phosphorus. The method comprises adding the encapsulation components to an aqueous suspension of red phosphorus, condensation of the reagents being achieved by lowering the pH. A drawback of the version described is that the phosphine liberation rate of the microencapsulated red phosphorus is still high, and also that the space-time yield of the reaction relative to red phosphorus is low. (The space-time yield is understood in this context as being the solids content of red phosphorus to be encapsulated in the total aqueous reaction mixture used which is converted per unit time; this may be described inter alia by the weight ratio of red phosphorus put into the reaction to the total water content). Another drawback of the reaction in dilute solution/suspension is that a relatively large amount of acid has to be added to start the condensation reaction, and this then has to be neutralized with correspondingly large quantities of alkali solution when reconditioning the production effluent.
The object of the invention is to find an alternative method of microencapsulating red phosphorus with urea resorcinol formaldehyde resin, which will firstly improve the properties of microencapsulated red phosphorus in terms of its phosphine liberation rate and secondly give a greatly improved space-time yield.
DETAILED DESCRIPTION OF THE INVENTION
It has been found that the set object of microencapsulating red phosphorus with urea resorcinol formaldehyde resin may be achieved by a method which is the subject of the invention, wherein
a mixture of
0.4 to 4 wt. % urea,
2 to 20 wt. % resorcinol and
5 to 150 wt. % formaldehyde,
each relative to the weight of red phosphorus used, is polymerised in aqueous phase in the presence of finely divided red phosphorus suspended by known mixing methods (Ullmann's Encyclopedia of Industrial Chemistry, VCH 1988, Vol. B2, page 25-1 to 25-33 and literature quoted therein; Ullmann's Encyclopedia of Industrial Chemistry, VCH Publishers Inc. 1992, Vol. B4, page 87-120 and page 167-180 and literature quoted therein) and 0.1 to 8 wt. % hexamethylene tetramine, relative to the weight of red phosphorus used, by adding sufficient acid to adjust the pH of the mixture to 0 to 5, the temperature of the reaction mixture preferably being kept at 25 to 150° C. for 0.5 to 24 hours and, according to the invention, the weight ratio of red phosphorus to water being adjusted to 1:0.3 to 1:3. “Water” refers here to the entire aqueous phase including the water content of any formaldehyde solution used. Here and in the following the percentages given for the substances used are percentages by weight. The reaction may be carried out at pressures from 0.8 to 12 bar. “Formaldehyde” is understood in this context as being either pure formaldehyde or formaldehyde solutions in water, which may further be mixed with 10 to 15 wt. % of a stabilizer such as methanol. The use of aqueous formaldehyde solutions has proved successful in the invention, a 37 wt. % solution stabilised with 10 to 15% methanol being particularly preferred. “Red phosphorus” refers in this context to any colored allotropic forms of phosphorus in finely divided form. The particles normally have a mean size of 0.1 to 100, preferably 0.2 to 50 and particularly preferably 10 to 50 &mgr;m. The particle size of the phosphorus may be adjusted by grinding, particularly by wet grinding with a ball or bead mill. A suitable grinding medium is either water or organic solvents such as toluene, petroleum, dimethylformamide, dimethylacetamide or formic acid.
It has been found that under the processing conditions of the invention mixtures preferably of 0.5 to 3.0% urea, 5 to 15% resorcinol, 10 to 100% formaldehyde and 1 to 6% hexamethylene tetramine relative to the red phosphorus used react smoothly to form very dense microcapsules which are superior to the described state of the art. The mixtures which are particularly preferred in this connection comprise 1 to 2.5% urea, 7 to 12% resorcinol, 25 to 90% formaldehyde and 2 to 6% hexamethylene tetramine, each relative to the red phosphorus employed.
Red phosphorus encapsulated by the method of the invention is made up of particles substantially of a core-shell structure.
It has also been found in the invention that particularly good results may be obtained by using reaction temperatures from 50 to 150° C. and particularly preferably from 70 to 150° C.
In addition to the conditions described, reaction times from 0.5 to 12 hours, especially from 1.5 to 5 hours, have proved particularly successful.
With regard to the space-time yield, which may be expressed by the weight ratio of red phosphorus put into the reaction to the water contained in the mixture, including the weight content of a formaldehyde solution used, it has been found that phosphorus: water weight ratios well above 1 in favor of the red phosphorus content may be used in the method of the invention.
It has been found, surprisingly, that raising the solids content of the reaction mixture does not cause the red phosphorus to form lumps consisting of agglomerated particles which are not completely encapsulated. The reduction of the weight content of water in relation to red phosphorus according to the invention in fact considerably lessens subsequent liberation of phosphine from the microencapsulated red phosphorus obtained by the method of the invention. According to the invention the best results may be obtained in this connection with a red phosphorus to water ratio of 1:0.3 to 1:2.5 or particularly preferably from 1:0.35 to 1:1.5.
It has also been found that the quality of encapsulate

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