Process for preparing polyalkoxymethylmelamines

Polyalkoxymethylmelamines are prepared by reacting melamine with formaldehyde and then alkylating the resultant polymethylolmelamine with a lower monohydric alcohol, having 1 to 4 carbon atoms, especially methanol, in the presence of a solid "superacid" catalyst.

BACKGROUND OF THE INVENTION 
Polyalkoxymethylmelamines, especially polymethoxymethylmelamines, have been 
well known for their usefulness in serving as cross-linking agents in 
surface coatings and for treating paper and textile materials. See, for 
example, U.S. Pat. Nos. 2,715,619, 4,101,520; Canadian Pat. No. 773,170. 
Alkylation, especially methylation, of polymethylolmelamines improves the 
melamine-formaldehyde condensation products in their solobility 
characteristics, stability before application or curing, and efficiency in 
reacting with compounds containing active H atoms. 
The alkylated polymethylolmelamines may be represented by the general 
formula 
EQU MF.sub.m R.sub.n 
where 
M=melamine 
F=combined formaldehyde, --CH.sub.2 O-- 
R=alkyl group with 1 to 4 C atoms, CH.sub.3 being the most important 
When M is lower than 4 and n/m ratio is lower than 2/3 or 0.67, their 
preparation is relatively easy. For most of the applications for which 
they are used, however, the degree of polymerization of the 
melamine-formaldehyde condensates should be kept low, preferably 
monomeric. For this reason, the reaction conditions for their preparation 
must be so chosen as not to promote undue polymerization. For condensates 
with m higher than 4 and n/m ratio higher than 2/3, methylation becomes 
laborious. Since the alkylation or ether formation reaction is reversible, 
the water formed during alkylation tends to drive the reaction backward 
and reach an equilibrium. In order to obtain fully-methylated products, 
water must be substantially removed from the reaction mass. As a rule, a 
second methylation step is carried out to complete the reaction and obtain 
the desired product. Strongly acidic conditions are required to effect the 
alkylation; but to remove the water by distillation, the pH should be 
above neutral in order to prevent polymerization. Repeated acidification 
and neutralization, with the accompanying salt formation, render the 
operation inconvenient. 
Usually, nitric acid or hydrochloric acid is used as the catalyst for 
alkylation. (See, e.g., British Plastics, Feb. 1943, p. 518; U.S. Pat. No. 
3,322,762, 4,101,520, 4,293,692; Canadian Pat. No. 773,170.) These acids 
require very careful handling. Hydrochloric acid is very corrosive to the 
reaction vessels. Nitric acid, being oxidizing, is dangerous to use with 
the oxidizable compounds formaldehyde and methanol. Furthermore, these 
acids are not anhydrous. The water introduced with the acid is unfavorable 
to the alkylation reaction equilibrium. 
A process for preparing methylated polymethylol melamines using an organic 
acid cation exchange resin as the catalyst has been disclosed (U.S. Pat. 
No. 3,488,350) without the usual acidification. It is, however, not very 
efficient. A large amount of the catalyst, more than 10% by weight of the 
total reactions mass, is required. 
SUMMARY OF THE INVENTION 
It has now been found that by using a new class of solid "superacid" 
catalysts, alkylation of polymethylolmelamines can be efficiently and 
conveniently carried out without the need for neutralizing all the acid 
present in the reaction mass. 
The superacid catalysts have been discovered in recent years (see G.A. 
Olah, et al., Science, 206, 1979, pp. 13-20). Their acidity has been 
estimated to be orders of magnitude higher than the mineral acids. Some of 
them are solids, such as perfluorinated resin sulfonic acid (e.g., Du 
Pont's Nafion-H), SbF.sub.5 -TiO.sub.2 -SiO.sub.2, SbF.sub.5 -SiO.sub.2 
-Al.sub.2 O.sub.3 (see M. Hino and K. Arata, Chem. Soc. Japan Chem. 
Letters, 1979, pp. 1259-60). More recently developed solid superacid 
catalysts can be prepared more conveniently and more economically, notably 
"sulfate-treated" inorganic oxides, such as zirconia, titania, and iron 
oxide (see R.A.Rajadhyaksha et al., Ind. Eng. Chem. Res, 26 (9) 1987, pp. 
1743-6; J. Am. Oil Chem. Soc., 65 (5) 1988, pp. 793-7; M. Hino and K. 
Arata, Chem. Soc. Japan. Chem. Letters, 1979, pp. 477-80; 1981, pp. 
1671-2; J. Chem. Soc. Comm., 1979, pp. 1148-9; 1980, pp. 851- 2). As 
catalysts for the preparation of the alkylated melamine-formaldehyde 
condensates, they are far superior to the mineral acids. They are stronger 
in acidity, they are not corrosive to the equipment. They are easily 
removable from the reaction mass, and they can be recycled. 
The catalyst loading can be varied between 2% to 20%, preferably between 5 
to 15%, based on the total weight of the reaction mass. The reaction 
temperature can be varied between 20.degree.-60.degree. C. Higher 
temperatures increase the rate of alkylation, but also tend to promote the 
formation of melamine resin polymers, while lower temperatures unduly 
extend the necessary reaction time. 
The used catalyst can be regenerated, after filtration from the reaction 
mixture, by washing with methanol and preferably recalcining as is known 
in the art.

DESCRIPTION OF THE INVENTION INCLUDING PREFERRED EMBODIMENTS 
In preparing the alkylated melamine-formaldehyde condensates in general, 
and methylated melamine-formaldehyde condensates in particular, the 
polymethylolmelamines may be prepared as usual, by reacting melamine with 
formaldehyde under neutral or slightly alkaline conditions. However, in 
the present invention, it has been found to be more convenient to carry 
out the methylolation reaction in the presence of the alkylating alcohol, 
with a small amount of water (5 to 20% by weight of the total reaction 
mixture) in the reaction mixture. It is followed by alkylation with an 
excess of alcohol in the presence of the superacid catalyst. Excess of 
alcohol is required to drive the reaction forward according to the mass 
action law. The higher the alcohol/ formaldehyde ratio or alcohol/melamine 
ratio, the higher will be the n/m ratio in the finished product MFmRn. For 
products with m&gt;4 and n/m&gt;2/3, two-stage alkylation proves to be more 
economical than one-stage reaction with ultra high alcohol/melamine 
ratios. 
The following examples are set forth for the purpose of illustration only 
and are not to be construed as limitations on the present invention except 
as set forth in the appended claims. All parts and percentages are by 
weight. 
EXAMPLE A 
Zr(OH).sub.4 is prepared by hydrolyzing ZrOCl.sub.2.8H.sub.2 O with aqueous 
ammonia, followed by washing it free of alkali and drying at 100.degree. 
C., and powdering the precipitate to below 100 mesh grains. The zirconium 
hydroxide is treated with 1-N sulfuric acid, followed by drying and 
calcination at 575.degree. C. -650.degree. C. in a Pyrex tube in air for 3 
hours, and finally sealing until use. 
EXAMPLE B 
H.sub.4 TiO.sub.4 is prepared by hydrolyzing TiCl.sub.4 with aqueous 
ammonia followed by washing and drying. The precipitate is treated with 
sulfate ion by pouring 1-N H.sub.2 SO.sub.4 (30 ml) on to the dried 
titanium hydroxide (2 g) on a filter paper. The material is dried and 
powdered below 100 mesh, calcined in a Pyrex tube in air at 
600.degree.-650.degree. C. for 3 hours, and finally sealed in a glass tube 
until use. 
EXAMPLE C 
Fe(OH).sub.3 is precipitated by hydrolyzing ferric nitrate or chloride with 
aqueous ammonium hydroxide. It is washed and dired at 100.degree. C. for 
24 hours. The treatment of catalyst with sulfate ion is performed by 
pouring 30 ml of 0.5-N H.sub.2 SO.sub.4 to 2 g of the dried Fe.sub.2 
O.sub.3 on a filter paper. After drying (without washing the treated 
material with water) the material is powdered below 100 mesh, calcined in 
air at 500.degree. C. for 3 hours, and finally sealed until use. 
EXAMPLE 1 
Into a suitable reaction vessel equipped with thermometer, stirrer, and 
reflux condenser, methanol (320 parts, 10 moles) is introduced. The pH of 
the liquid is adjusted to 10.5-11.5 with a 50% NaOH solution. A charge of 
330 parts of paraformaldehyde (91% CH.sub.2 O, 10 moles) is then added, 
and the mixture is heated to 45.degree.-50.degree. C. to effect complete 
solution. Melamine (126 parts, 1 mole) is introduced and the reaction mass 
is heated to 57.degree. C. It exotherms to about 80.degree. C. The 
reaction mixture is held at this temperature for about 15 minutes. A 
second portion of methanol (198 parts) is added, and the reaction mass is 
then cooled to about 35.degree. C. A charge of 50 parts of sulfated 
zirconia superacid catalyst (as described in Example A) is introduced. The 
reaction mass is held at 35.degree.-40.degree. C. with stirring for 2 
hours, and filtered to separate out the solid catalyst. The pH of the 
liquid is adjusted to 9-10 with 50% NaOH, concentrated under vacuum (30mm 
Hg pressure or lower) to a terminal temperature of 77.degree. C. It is 
cooled to 35.degree. C. and subjected to a second methylation step with 
118 parts of methanol and 5 parts of fresh sulfated zirconia catalyst. The 
batch is held at 35.degree.-40.degree. C. with stirring for 1 hour. It is 
again filtered and the liquid adjusted to a pH of 9.5-10 and concentrated 
to a terminal temperature of about 100.degree. C. under reduced pressure 
(30 mm Hg). The liquid product is centrifuged to remove any solid 
materials present. Its molar composition of melamine : formaldehyde : 
methanol is found to be 1 : 6.1 : 5.0. 
EXAMPLE 2 
Example 1 is repeated in substantially every essential detail except that 
sulfated TiO.sub.2 catalyst (Example B) is used in place of sulfated 
ZrO.sub.2. Similar results are obtained. 
EXAMPLE 3 
Example 1 is again repeated except that sulfated Fe.sub.2 O.sub.3 is used 
as the methylation catalyst. Similar results are obtained. 
EXAMPLE 4 
The procedure of Example 1 is again followed except that the methanol is 
replaced by n-butanol. Butylated polymethylol melamine is recovered. 
EXAMPLE 5 
Example 1 is repeated except that in the second methylation step 315 parts 
of methenol and 7 parts of sulfated zirconia catalyst are used. The 
product has a molar composition of MF.sub.6.0 Me.sub.5.6. 
EXAMPLE 6 
Example 1is repeated except that in the second methylation step 79 parts of 
methanol and 4 parts of sulfated zirconia catalyst are used. The product 
has a molar composition of MF.sub.6.0 Me4.6.