Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Nitrogen-containing reactant
Reexamination Certificate
2002-05-02
2004-03-16
Truong, Duc (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Nitrogen-containing reactant
C528S423000, C528S422000, C528S254000
Reexamination Certificate
active
06706856
ABSTRACT:
The invention relates to crystalline melamine, more in particular to multicrystalline melamine powder and its use in amino-formaldehyde resins.
Melamine is prepared in various ways on an industrial scale. There are methods involving the crystallisation of melamine from an aqueous solution, there is a process in which melamine is obtained directly from a gaseous phase, and there is a method in which melamine is synthesised at a high pressure (7-25 MPa) and in which the melamine melt thus obtained is sprayed in an ammonia atmosphere and cooled. This last method yields a crystalline powder that may be used as such without further purification steps.
Crystalline melamine obtained according to the first method consists of a very pure melamine, but the crystals are relatively large, so that the dissolution rate in a solvent such as, for instance, water or a water/formaldehyde mixture is low. The melamine thus obtained is often ground to obtain more suitable smaller particles. While smaller particles do have a higher dissolution rate, they also have a lower bulk density and often poorer flow properties. As a result, the product obtained is not optimal in terms of the combination of dissolution rate, bulk density and flow properties. Melamine recovered directly from the gas phase is very fine and consequently has a poor bulk density and often poor flow properties. Crystalline melamine obtained according to the method involving spraying and cooling of a melamine melt in an ammonia atmosphere is a multi-crystalline melamine powder having good dissolution and reactivity properties in combination with reasonable flow properties.
Multicrystalline melamine powder consists of multicrystalline particles. This means that the larger particles (>20 &mgr;m) are composed of a multiplicity of small crystals, bonded together to form large porous particles. As a result, multicrystalline particles have both a high specific surface area normally associated with small particles while at the same time having advantages of larger crystals such as good flow properties. Scanning Electron Microscope pictures show a clear distinction between these particles and melamine crystallised from water. The particles obtained by spraying a melamine melt in an ammonia atmosphere have a cauliflower-like structure. The melamine crystallised from water contains a substantial amount of crystals having a crystal size greater than 50 &mgr;m.
A method for the preparation of multicrystalline melamine at high pressure in which a melamine melt is obtained that is cooled in an ammonia atmosphere is described inter alia in U.S. Pat. No. 4,565,867. In particular, this patent specification describes how urea is pyrolysed in a reactor at a pressure of from 10.3 to 17.8 MPa and a temperature of from 354 to 427° C. to produce a reactor product. This reactor product contains liquid melamine, CO
2
and NH
3
and is transferred as a mixed stream under pressure to a separator. In this separator, which is kept at virtually the same pressure and temperature as the said reactor, the said reactor product is separated into a gaseous stream and a liquid stream. The gaseous stream contains CO
2
and NH
3
off-gases and also melamine vapour. The liquid stream mainly comprises liquid melamine. The gaseous stream is transferred to a scrubber unit, while the liquid melamine is transferred to a product-cooling unit. In the scrubber unit, the said CO
2
and NH
3
off-gases, which contain melamine vapour, are scrubbed, at virtually the same pressure as the pressure of the reactor, with the molten urea needed for the process in order to preheat the urea and to remove the melamine present from the waste gases. Then the preheated molten urea, which contains the said melamine, is fed to the reactor. In the product cooler the liquid melamine is reduced in pressure and cooled with a liquid cooling medium to produce a solid melamine product without scrubbing or further purification. U.S. Pat. No. 4,565,867 preferentially uses liquid ammonia as the liquid cooling medium.
A drawback of the method according to U.S. Pat. No. 4,565,867 is that the melamine obtained has a yellowish color, as a result of which it cannot be used in all melamine applications.
Multicrystalline melamine obtained according to U.S. Pat. No. 4,565,867 can be used in amino-formaldehyde resins in which the color of the melamine is of minor importance. Amino-formaldehyde resins, such as for instance melamine-formaldehyde resins (MF), urea-formaldehyde resins (UF) and melamine-urea-formaldehyde (MUF) resins are generally known. U.S. Pat. No.-A-5120821 describes a method for the preparation of melamine-formaldehyde resins starting from melamine that still contains 2-8% of the impurities of the melamine preparation process. These impurities comprise small amounts of for instance ammeline, ammelide, ureidomelamine, melem and melam. An increase in this combination of impurities is unfavourable in particular for use in amino-formaldehyde resins for transparent applications. A too high content of oxygen-containing compounds for instance reduces the pH of the resin solution and this may result in unstable resins. The pH reduction is caused by, inter alia, the oxygen-containing compounds ammeline, ammelide and cyanuric acid, ARCs for short (
A
mmeline-
R
elated
C
ompounds).
The object of the present invention is to obtain improved crystalline melamine powder by means of a high-pressure melamine process in which melamine is obtained as a dry powder directly from a melamine melt. More in particular the object of the present invention is to obtain crystalline melamine powder by means of a high-pressure melamine process with a high dissolution rate in water, acceptable flow properties, a low content of oxygen-containing compounds and a good color.
Surprisingly, it has been found that amino-formaldehyde resins with strongly improved properties can be obtained by using melamine, obtained by means of a high-pressure process, showing a combination of properties comprising a high melam content.
The invention relates to multicrystalline melamine powder, in particular multicrystalline melamine powder obtained by means of a liquid-phase process, with the following properties:
APHA color less than 17
more than 1.5 wt. % melam
content of oxygen-containing components lower than 0.7 wt. %
a specific surface area of between 0.7 and 5 m
2
/g.
The melam concentration in the melamine powder is preferably greater than 2 wt. %, more in particular greater than 2.5 wt. %.
Preferably, the content of oxygen-containing compounds is lower than 0.4 wt. %. The ARC content among the oxygen-containing compounds is usually below 0.15 wt. %, preferably below 0.1 wt. % and in particular below 0.05 wt. %.
The specific surface area preferably lies between 0.9 and 3 m
2
/g.
A customary method for determining the color of melamine is the so-called APHA colorimetry. This involves the preparation of a melamine-formaldehyde resin with an F/M ratio of 3, a formaldehyde solution being used which contains 35 wt. % formaldehyde, between 7.5 and 11.2 wt. % methanol and 0.028 wt. % acid (as formic acid). The theoretical solids content of the solution is 56 wt. %. 25 g Melamine is dissolved in 51 g of the above solution by rapidly heating the mixture to 85° C. After about 3 minutes all melamine has dissolved. 2 ml of a 2.0 mol/l sodium carbonate solution is added to this solution, which is followed by stirring for 1-2 minutes. After this, the mixture is rapidly cooled to 40° C. The color is determined by means of a Hitachi U100 spectrophotometer with a 4 cm glass cuvette by subjecting the above-mentioned solution to absorbance measurements at a wavelength of 380 nm and 640 nm with demineralised water as blank in the reference cuvette.
The APHA color is calculated using the following formula:
APHA=f
*(
A
380
−A
640)
where A380=absorbance at 380 nm;
A640=absorbance at 640 nm;
f=calibration factor.
The calibration factor f is determined on the basis of absorbance measurements at 380 nm on calibration solutions prep
Aarts Veronika M. L. J.
Liekelema Koert
Tjioe Tjay T.
DSM N.V.
Pillsbury & Winthrop LLP
Truong Duc
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