JPS61138615A - Removal of free formaldehyde - Google Patents

Abstract

PURPOSE:To remove the titled substance continuously without causing the enlargement and unevenness of the particles of a crosslinked ureaformaldehyde polymer (UF), by transferring a dispersion of UF particles and an aqueous solution of urea successively to a continuous mixer, a bucket conveyor, and a neutralization step. CONSTITUTION:A UF particle dispersion and an aqueous solution of urea are mixed with each other in a continuous mixer at an average residence time of <=3min, and the mixture is supplied continuously to a bucket conveyor consisting of buckets fixed to an endless belt, etc. The liquid mixture is discharged from the bucket conveyor, and neutralized with sodium hydroxide to remove the free formaldehyde from the UH particle dispersion. The time necessary from the mixing to the neutralization is preferably 5-30min.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field] The present invention relates to a method for removing free formaldehyde from a stock solution of a cross-linked urea-formaldehyde polymer particle dispersion.

[Conventional technology]

Fine cross-linked urea formaldehyde polymer particles (hereinafter abbreviated as UF particles), in which particles with a particle size of 0.1 to 1.0 μm are aggregated to 1.0 to 30 μm, are used as a paper filler for the purpose of improving the opacity of paper. It is used. UF like this
As a method for producing particles, for example, there is a method in which a urea formaldehyde initial condensate and an acidic aqueous solution are mixed in a specific ratio, and the mixture is fed onto an endless belt to produce the particles continuously (Japanese Patent Publication No. 57-26686). These particles are usually added to the pulp slurry during the papermaking process as an aqueous dispersion or dry powder. However, since UF particles contain unreacted or free formaldehyde generated by hydrolysis, they cause problems such as environmental pollution during the papermaking process, and the resulting paper also contains formaldehyde, which limits its uses. Ta.

As a method for removing free formaldehyde in UF particles, Japanese Patent Publication No. 57-27905 discloses that urea is added in an amount equivalent to or more than the free formaldehyde to a UF particle dispersion stock solution, and neutralized within 3 minutes to 1 hour after addition. A method for removing free formaldehyde is disclosed. In this method, the urea treatment time from the addition of urea to neutralization is important; if it is too short, free formaldehyde cannot be removed sufficiently, and if it is too long, the IJF particles become fat and become a paper filler. Appropriate particle size as 0, 1~
It ends up being larger than 1.0μ. Therefore, the above-mentioned time is 3 minutes or more and 1 hour or less, preferably 5 minutes or more and 30 minutes or less.

[Problem that the invention seeks to solve]

In the above-mentioned free formaldehyde removal method, a urea aqueous solution is generally added to the UF particle dispersion stock solution and then mixed with stirring, and a batch-type complete mixing tank is generally used as a mixing device for this purpose.

In other words, in a continuous mixing device, there is a distribution in the residence time of the liquid,
As a result, the obtained UF coarse particles have a particle size distribution, and a large number of particles exceeding 1.0 μm, which is the upper limit of the preferable particle size as a paper filler, are produced, resulting in a decrease in performance as a paper filler. In this respect, in a batch-type complete mixing tank, there is no residence time distribution in the tank, so the variation in particle size is extremely small.

However, as long as a batch-type mixing tank is used, U
The F coarse particle production process also has to be batch-based, which is disadvantageous in terms of productivity. That is, although the production of UF coarse particles itself can be carried out continuously by the above-mentioned method using a belt, etc., the process of removing free formalin using urea must be carried out batchwise.

It is conceivable to install two or more batch-type mixing tanks and use them sequentially to achieve continuity, but not only would the switching operation be complicated, but it would be difficult to receive the UF particle dispersion liquid from multiple tanks. It is necessary to balance the charging time, the urea treatment time, and the discharge time of the dispersion after treatment, which is also complicated and problematic in practice.

[Means for solving problems]

The present inventor has been researching a method for continuously removing free formaldehyde using urea, and found that under certain conditions, even if a continuous mixing device was used, there was no substantial variation in the particle size of the UF coarse particles. We have discovered that urea treatment can be carried out on urea, and have arrived at the present invention.

That is, the present invention comprises a step of continuously mixing a cross-linked urea-formaldehyde polymer particle dispersion stock solution in which free formaldehyde is dissolved and an aqueous urea solution under conditions such that the average residence time in a mixing device is less than 3 minutes; b. Continuously supplying the mixed liquid to the bucket conveyor; c. Discharging the dispersion liquid from the bucket conveyor; d.
This is a method for removing free formaldehyde from a cross-linked urea-formaldehyde polymer dispersion stock solution, which comprises the step of neutralizing the discharged dispersion liquid.

Hereinafter, typical embodiments of the method of the present invention will be explained in order of steps.

First, UF coarse particles are produced by a known method, for example, the method disclosed in Japanese Patent Publication No. 57-26686. That is, after mixing formaldehyde aqueous solution, urea, carboxymethyl cellulose and water, the pH is adjusted to 7 with caustic soda water and reacted to obtain an initial condensate. This initial condensate liquid is continuously mixed with a several percent aqueous sulfuric acid solution using an in-line mixer and is fed onto an endless belt of acid-resistant rubber before the condensate begins to solidify. The liquid mixture solidifies on the belt.

The solid thus obtained is stirred to form a slurry, adding water if necessary, and then pulverized in a colloid mill to obtain a UF coarse particle dispersion stock solution. The dispersion solution obtained in this way usually contains free formaldehyde of 1,000 to 1
Contains 10,000pp.

In addition, the concentration K of the urea aqueous solution is not particularly limited, but is usually 1 to
It is 50% by weight.

The above-mentioned UF particle dispersion stock solution and urea aqueous solution are continuously mixed in a mixing device, but in this case, the average residence time in the mixing device needs to be less than 3 minutes, preferably less than 2 minutes. The average residence time is the value obtained by dividing the actual volume of the mixing device by the liquid supply rate (ie, in this case, the sum of the supply rates of the UF particle dispersion stock solution and the urea aqueous solution). If the average residence time is 3 minutes or more, the particle diameter after the UF coarse urea treatment will vary, and the performance as a paper filler will deteriorate.

Further, as a mixing device, an in-line mixer such as a static mixer is one of the preferable examples. Another preferred example is a small-capacity complete mixing tank that satisfies the above average residence time condition.

The mixing ratio of the UF particle dispersion stock solution and the urea aqueous solution is such that the amount of urea in the latter is at least equimolar to the free formaldehyde present in the former.

If the amount of urea is less than equimolar, free formaldehyde cannot be removed sufficiently. There are no particular restrictions on the mixing temperature, but it is usually room temperature to 70°C.

The mixed solution of the UF particle dispersion stock solution and the urea aqueous solution is then continuously supplied to a bucket conveyor. The bucket conveyor according to the present invention has a large number of buckets connected adjacently or at approximately equal intervals in an endless manner. A bucket is a container with an opening, and its shape is not limited. The material of the bucket is not particularly limited as long as it is corrosion resistant such as stainless steel or plastic, and glass-reinforced polyester resin is an example of a preferable material.

Typical methods for connecting buckets include fixing the buckets to an endless belt or chain. By driving these endless belts or endless chains, the bucket conveyor rotates continuously or intermittently at approximately constant speed.

The mixed liquid supplied to the bucket is discharged from the bucket after an appropriate period of time has elapsed. Since the bucket conveyor rotates endlessly, it is advantageous to discharge by gravity at the point where the buckets turn over. For this purpose, it is preferable to determine the rotational movement speed of the bucket in consideration of the time required from supplying the mixed liquid to the bucket until discharging it. Since the liquid mixture is a UF powder particle dispersion liquid, some of the UF powder particles may settle and cannot be discharged by gravity alone, but in that case, they can be discharged by normal means such as spraying water.

The discharged mixed liquid is neutralized with an alkali such as caustic soda. The time from mixing the UF' particle dispersion and the urea aqueous solution in the mixing device until neutralization is 3 minutes or more and 1 hour or less, preferably 5 minutes or more and 30 minutes or less. The amount of free formaldehyde in the dispersion thus obtained was 500 ppm.
It is as follows. If the time until neutralization is less than 3 minutes, free formaldehyde will not be removed sufficiently, and if it exceeds 1 hour, the UF powder particles will become enlarged and the average particle size will exceed 1.0 μm, which is not preferable. In this case, the residence time in the mixing device shall be represented by the average residence time.0 [Operation] In the present invention, since a continuous mixing device and a bucket conveyor are used in combination, free formaldehyde can be continuously removed. UF powder particles are less likely to enlarge or vary in particle size.

〔Example〕

This will be explained below using examples.

The amount of free formaldehyde was determined by the acetylacetone method, and the particle size of the UF powder particles was determined by electron micrographs.

Example 1 37% formaldehyde (hereinafter all percentages are expressed as weight % except for those related to paper whiteness) aqueous solution 38.4 parts (all percentages hereinafter expressed as weight percent), urea 18.9 parts,
Mix 0.7 parts of carboxymethyl cellulose and 42.0 parts of water, and adjust the pH to 70 with 20% caustic soda water.
The reaction was carried out at ℃ for 2 hours to obtain a urea formaldehyde initial condensate. 1 part of 2.7% sulfuric acid aqueous solution to 100 parts of this initial fastener.
After adding 80 parts and mixing, the mixture was left for 1 hour. The resulting gel was coarsely ground and stirred to form a slurry (at this time, the pH of the dispersion was 1.5) to obtain a dispersion stock solution (UF particle dispersion stock solution) consisting of crosslinked urea formaldehyde polymer particles. . The free formaldehyde in the aqueous phase of this UF particle dispersion stock solution was 5,500 ppm, and the irritating formaldehyde odor was extremely strong.

Add the above TRP particle dispersion stock solution to 20 ml/min.
.

% urea aqueous solution at a rate of 1.11/min to a complete mixing tank with an actual volume of 301 and an outlet at the top.Therefore, the ratio of urea is 2 moles to 1 mole of free formaldehyde, and the average residence time is (2.7 minutes), the mixed liquid continuously discharged from the discharge port was supplied to a continuously rotating bucket conveyor. The bucket conveyor was made up of glass-reinforced polyester buckets with an actual internal capacity of about 201 ml, each of which was fixed to an endless chain so that the individual buckets were adjacent to each other, and the rotation speed was set to make one rotation every 40 minutes. Therefore, after supplying the mixed liquid, the bucket is turned over about half a turn, and the mixed liquid is discharged.

The mixed liquid discharged one after another from the bucket is placed in a container equipped with a stirrer and is continuously neutralized with 20% caustic soda.
An F particle dispersion was obtained. The time from mixing to neutralization was approximately 22 divisions. The free formaldehyde in the obtained UF particle dispersion was 216 ppm, and the average particle diameter of the UF powder particles was 0.75 μ.

Example 2 A UF particle dispersion was obtained in the same manner as in Example 1 except that the actual capacity of the complete mixing tank was 151 and the average residence time was 1.4 minutes. The free formaldehyde in this dispersion was 224 ppm, and the average particle size of UF powder particles is 01
It was 39μ.

Comparative Example 1 UF was carried out in exactly the same manner as in Example 1, except that the actual capacity of the complete mixing tank was 401, and therefore the average residence time was 3.6 minutes.
A particle dispersion was obtained. The free formaldehyde in this dispersion was 208 ppm, and the average particle size of the UF powder particles was 1.
．． It was 26μ.

Example 3 Instead of a complete mixing tank as a mixing device, the actual internal capacity is 1.5
A UF particle dispersion was obtained in exactly the same manner as in Example 1, except that a No. 6 static mixer was used (therefore, the average residence time was 0.14 minutes). The free formaldehyde in this dispersion was 226 ppm, and the average particle size of the UF powder particles was 0.41μ.

Application Example The results of paper making using the UF powder particles of Examples 1 to 3 and Comparative Example 1 are shown in Table 1.

The paper was made as follows. That is, N.

A12(SO4)a, 18H was added to 2000 parts of 1% loop slurry of 330 ml of freeness (C8F) containing 30 parts of BKP, 30 parts of TMP and 40 parts of RGP.
Add 2 parts of a 20.0% aluminum sulfate aqueous solution and stir for 2 minutes. Subsequently, 1 part of each UF powder particle based on the dry weight was added and stirred for 5 minutes to prepare the adjustment slurry IJ-
get. Next, paper was made using a TAPPI square sheet machine, and the wet paper obtained by press dehydration was dried using a drum dryer with a surface temperature of 110°C.
Pass through the calendar twice at rL, humidity 65%, temperature 2
Seasoning was performed for 24 hours in a constant humidity and constant room at 0'C.
I got processed paper.

The physical properties of these processed papers were measured and shown in Table 1.

Note that the physical properties were measured by the following method.

The basis weight was measured and calculated according to JIS (P-8111).

The stiffness was calculated by measuring the thickness of the paper according to JIS (P-8118) and using the formula (basis weight/thickness) x 1000.

The white paper opacity is based on the literature (Kamiba Technology Times, September 1970)
The procedure was carried out according to the method described in (Japan issue, pages 1 to 13).

From Table 1, Examples 1 to 3, which are within the scope of the present invention, all have excellent white paper opacity at °C. Large particle size and low paper opacity.

〔Effect of the invention〕

As is clear from the above, according to the present invention, free formaldehyde in the UF particle dispersion stock solution can be continuously removed without deteriorating the performance as a paper filler, and as a result, the UF powder particle manufacturing process can be completely continuous. It could be a process.

Claims (1)

[Claims] a. A cross-linked urea-formaldehyde polymer particle dispersion stock solution in which free formaldehyde is dissolved and a urea aqueous solution are continuously mixed under conditions such that the average residence time in a mixing device is less than 3 minutes. Step b. Continuously supplying the mixed liquid to the bucket conveyor; c. Discharging the dispersion liquid from the bucket conveyor; d.
A method for removing free formaldehyde from a stock solution of cross-linked urea-formaldehyde polymer particle dispersion, comprising the step of neutralizing the discharged dispersion.