Preparation of crystalline n-methylol-(meth) acrylamide

In a process for the preparation of crystalline N-methylol(meth)acrylamide from (meth)acryamide and paraformaldehyde, the reaction of the solid reactants to give a product melt and the subsequent crystallization thereof in a reactor while subjecting the reaction mixture to mechanical shear load is preferably carried out in a self-purging screw reactor or a disk reactor. The reaction of the reactants and the crystallization of the product melt are preferably carried out without added solvent. A catalyst, in particular an alkali metal carbonate or a trialkylamine, can be used for the reaction of the reactants.

This application is a 35 U.S.C. 371 of PCT/EP95/02594, filed Jul. 5, 1995. 
The present invention relates to a process for the preparation of 
crystalline N-methylol(meth)acrylamide from paraformaldehyde and 
(meth)acrylamide in the presence of a suitable catalyst. 
N-Methylol(meth)acrylamide is used as a monomer component with crosslinking 
properties in a number of polymers, predominantly in polymer emulsions, 
which are used as adhesives and coating materials, as binders for paints 
and fibers and as material for textile or paper coating. 
Processes for the preparation of crystalline N-methylolacrylamide are 
known. Thus, U.S. Pat. No. 3,064,050 describes such a process in which 
acrylamide and paraformaldehyde in highly concentrated aqueous solution 
are reacted using a basic catalyst. The disadvantages of this process are 
a) the relatively low product yields of about 50% which are obtained in 
one operation, b) the recycling of the mother liquors which is therefore 
required and c) the drying of the product obtained by filtration. 
U.S. Pat. No. 2,864,861 discloses a process for obtaining 
N-methylolacrylamide which dispenses with the use of any solvents. In this 
known process, a product melt is produced in a stirred kettle from the 
solid reactants paraformaldehyde and acrylamide using a catalytic amount 
of triethylamine and is allowed to solidify to solid N-methylolacrylamide 
after the end of the reaction by cooling. If this process were scaled up 
to a large industrial plant, it would not be possible to achieve the 
following: 
a) to mix the reactants uniformly; 
b) to remove the heat of reaction rapidly in order to avoid an 
b) to remove the heat of reaction rapidly in order to avoid an accumulation 
of heat and hence the danger of polymerization; 
c) to produce an N-methylolacrylamide of constant quality and 
d) to discharge the N-methylolacrylamide in free-flowing form from the 
stirred kettle. 
It is an object of the present invention to provide a solvent-free process 
for the preparation of N-methylol(meth)acrylamide in which the 
abovementioned features are also achievable on the large industrial scale. 
We have found that this object is achieved, according to the invention, by 
the processes defined in the claims. In particular, (meth)acrylamide and 
paraformaldehyde are reacted using a suitable catalyst in one process step 
to give crystalline N-methylol(meth)acrylamide by carrying out the 
reaction in a reactor, preferably in a self-purging screw apparatus. The 
reactor is designed in such a way that the reaction mixture is exposed to 
significant shear loads, as in screw extruders or in disk mixers. These 
apparatuses are known per se, and the degree or the intensity of 
mechanical mixing for the reaction and the crystallization is therefore 
also defined. Preferably used reactors are those which transmit to the 
reaction mixture a shear load which, based on the power consumption per 
unit volume of content, is of the same order to magnitude as that of the 
commercial apparatuses stated in the Examples, the material contained 
being the same. 
Below, (meth)acrylamide is to be understood as meaning acrylamide and/or 
methacrylamide. Suitable catalysts are tertiary amines or alkali metal 
carbonates, in particular potassium carbonate. This process, which can be 
operated by either a continuous or batchwise procedure, gives a 
free-flowing product with small quality fluctuations. The mode of 
operation of the Discotherm B used for carrying out the present process 
will be described in detail below: 
Highly self-purging reactors are preferably used for the preparation of 
N-methylol(meth)acrylamide. Such a reactor is, for example, a Discotherm B 
from List AG, 4422 Arisdorf, Switzerland, in which the required residence 
time can be realized. 
The Discotherm B (DTB) is a batchwise or continuous kneading apparatus. It 
consists of a horizontal cylindrical barrel with a concentric stirring 
shaft on which disk elements are mounted in planes at right angles to the 
axis and kneader-mixer bars on the outer circumference. Static 
counter-hooks which clean the shaft and the disk elements are fastened in 
the barrel. The degree of self-purging of the apparatus is 90%. The 
remaining 10% are purged by means of product movements. The axial 
transport is ensured by the arrangement of the disk elements with 
kneader-mixer bars and by the shape of the counter-hooks. Axial mixing is 
small, so that a narrow residence time range can be realized. The barrel, 
shaft and disk elements are heatable and coolable. The degree of filling 
(preferably 60-80%) can be set by means of the height of an adjustable 
overflow weir at the apparatus discharge. 
BRIEF DESCRIPTION OF THE DRAWING 
FIG. 1 depicts an apparatus which may be used with a preferred embodiment 
of the invention (see Example 2). "M" denotes various drive units. "1" 
denotes a first reactor and "2" denotes a second reactor. "3" and "4" 
denote differential metering balances which may aid in metering reactant 
and catalyst to the first reactor 1. "5" denotes a pipeline which allows 
flow of product from the first reactor 1 to the second reactor 2. "6" 
denotes a vertical conveying screw used to continuously discharge product 
from the second reactor 2. "7" depicts a pipe used to bring heating water 
to the first reactor 1 and "8" depicts a pipe for removing the heating 
water from the first reactor 1. Similarly, "9" depicts a pipe used to 
bring cooling water to the second reactor 2 and "10" depicts a pipe for 
removing the cooling water from the second reactor 2. "11" depicts a pipe 
used to channel the flow of product from the conveying screw 6.

EXAMPLE 1 
Batchwise preparation of N-methylolacrylamide 
The starting materials acrylamide, paraformaldehyde and potassium carbonate 
are in the form of free-flowing powders. 13.78 kg of acrylamide, 6.12 kg 
of paraformaldehyde and 0.1 kg of potassium carbonate are introduced into 
a Discotherm B, type designation DTB-40-Batch (free volume about 60 l), 
and are mixed at a speed of 50 min.sup.-1. The reactor is heated to 
50.degree. C. After about 30 minutes, the reaction is complete and the 
N-methylolacrylamide is in the form of a clear melt. The reactor is then 
cooled to about 20.degree. C. so that the N-methylol-acrylamide melt 
begins to crystallize. After a cooling time of 45 minutes, the 
N-methylolacrylamide has completely crystallized out and is discharged 
from the reactor in the form of a fine-grained powder. The product 
obtained has a melting point of 53.degree.-57.degree. C. 
EXAMPLE 2 
Continuous preparation of N-methylolacrylamide 
The structure of the apparatus is shown in the Figure. The various drive 
units are denoted there in general by M. 20.67 kg/h of acrylamide, 9.18 
kg/h of paraformaldehyde and 0.15 kg/h of potassium carbonate are metered 
continuously into the reactor 1 of the type Discotherm B (DTB 40-Conti) 
with the aid of two differential metering balances 3 and 4 of the type 
K-Tron Soder (total throughput 30 kg/h). Paraformaldehyde and potassium 
carbonate are metered in as a mixture. The reactor 1 is heated by hot 
water (50.degree. C.) which flows in through the pipe 7 and flows away 
again through pipe 8. The shaft speed is 50 min.sup.-1. At a degree of 
filling of about 60% and a residence time of &gt;35 min, the solids react to 
give a clear melt, which flows, under the influence of gravity, 
continuously through a pipeline 5 heated to 50.degree. C. with hot water 
into the second reactor 2. The second reactor 2 is likewise a DTB 40-Conti 
which is thermostated at 20.degree. C. by cooling water flowing in through 
the pipe 9 and flowing away through pipe 10, so that the melt 
crystallizes. At a shaft speed of 25 min.sup.-1, a degree of filling of 
60% and a residence time of &gt;35 min, a fine-grained solid is obtained at 
the end of the reactor and is discharged continuously via the pipe 11 by 
means of a vertical conveying screw 6.