The present invention relates to a method for separa¬ tion of synthetic water soluble polymers by a filtration 5. process into two solu ions, one of which is recirculated for re-polymerisation. The invention also relates to a plant comprising a polymerizat on reactor and a unit for separation of produced synthetic water soluble polymers.

Natural polymers such as proteins, latex of natural 10. rubber, cellulose etc often have very weLl def ned molecular weights. In contrast to this synthetic polymers usually have a broad molecular we ght distribution and n many cases only the fractions containing polymer of high molecul¬ ar weight have the for the intended use desired favourable 15. propert es. It is not always possible to control polymer synthesis with available technology in order to obtain only the desired high molecular weight fractions. In addi¬ tion to undesired low molecular weight polymer and bypro¬ ducts, unreacted monomers are also often present n the 20. synthesized resins.

The present invention offers a method for separat on of synthetic water soluble polymers to obtain the desired h gh molecular weight fractions w th advantageous properties and the method further includes recirculation of the low

25. molecular weight fraction to a polymerization step.

The molecular weight of synthet c polymers depends on several variables such as monomers, type of polymer reaction, reaction time and temperature etc. For polymers in solution the molecular weight distr bution nearly always 30. has the form of a curve. The curve will of course vary from polymer to polymer and may have one or more peaks. It has, for example, been found that water soluble urea-for¬ maldehyde resins have a molecular weight d stribution th two clearly def ned and specific peaks. According to the 35. present method the desired high molecular weight fraction of such a resin can be separated from the undes red ones which can be re-used.

The method of the present invention is generally appl cable to separation of des red high molecular weight

fractions of any water soluble synthetic resin. However, the method is particularly advantageous for water soluble resins used in paper production and the following discussion will thus be directed, although not limited, to such resins. 5. Water soluble resins used in paper production are for example urea-formaldehyde resins, meLamine-forma Ldehyde resins and polyamidoamine-epi chLorohydrin resins. These resins are used as wet strength agents for paper and it has been found that the effect on the wet strength is ob-

10. tained only by the high molecular weight fraction. The low molecular weight fraction is not retained in the paper, but circulated in the closed white water system. For form¬ aldehyde based resins the high temperature of the water results in hydrolysis of this low molecular weight fraction

15. and release of free formaldehyde which may cause environmen¬ tal problems. Polyamidoamine-epichlorohydrin resins have a higher molecular weight than the formaldehyde based resins and the- Low molecular weight fraction of this resin also- has a Lower effect on the wet strength. Further, these

20. resins contain monomeric chlorinated byproducts which also may cause en ironmental problems.

When the above described resins are treated according to the method of the invention solutions of high molecular weight polymers are obtained which give a very good ^" wet

25. strength effect and which also g ve a considerable reduction of formaldehyde and other non-desired compounds in the water and a r at paper m lls.

Since urea-formaldehyde resins are the predominant wet strength resins and also those which tend to cause

30. the severest environmental problems their upgrading accord¬ ing to the nvention is of particular importance and these resins will thus be discussed more in detail. As stated above, free formaldehyde is released from urea-formaldehyde resins at paper production due to hydrolysis. Further,

35. the utilized original resins always contain free formal¬ dehyde as well ^' and th s s accumulated and circulated n the white water system. Since alL concentrated solutions obtained by the separation according to the nvention are more effective, compared th the original res n, the added

amount required for a certain wet strength effect is thus considerably reduced. The content of unreacted monomers, for example formaldehyde, and low molecular weight bypro¬ ducts is also lower n a concentrate solution than in the 5. original resin which leads to a double reduction of unwanted products in the wh te water system. As an example it can be mentioned that an original urea-formaldehyde resin may contain 7% free formaldehyde, based on the dry weight of the resin, and a concentrate sol.ution of the same urea-for- 10. maldehyde resin obtained accord ng to the present method wi ll contain only about 4%. Normally added amounts of urea-- formaldehyde resins during paper production are 10 to 20 kg per ton paper. Using a concentrate of the invention the same wet strength effect is obta ned with only 6 to 15. 12 kg dry weight resin. Th s means that "added" free formal¬ dehyde s reduced from 0.7 to 1.4 kg to 0.24 to 0.48 kg, e a reduction with 60 to 70%. This reduction is further ncreased by the fact that the concentrate contains much less of easily hydrolyzed low molecular weight resin. 20. Likewise, for anionic bisulphite modified urea-formaldehyde resins which, as has been found, contain nearly half of the sulphur amount in monomeric products with formaldehyde can be treated according to the present method to remove these products wh ch, besides not having any wet strength 25. effect, are detrimental to the z-potential of the stock. Use of the present method for separation of the desired high molecular weight fraction of urea-formaldehyde, and melamine-formaldehyde resins, thus not only leads to an effective wet strength resin product but also to such a 30. product which is advantageous from an environmental and occupational point of view.

The method of the invention further comprises recir- culation of the at the separat on obtained solution of low molecular weight compounds for re-polymerization. This 35. recirculation for re-polymerization is preferably carried out after an upgrad ng step. The invention thus offers a technically and commercially advantageous method of pro¬ ducing desirable high molecular weight fractions of water soluble synthetic polymers wh le at the same time providing

for re-use of the low molecular we ght fract on.

According to the present invention an aqueous feed solution of a water soluble synthetic resin is charged to an ultra-filtrat on membrane unit n which the feed 5. solution is separated into two solutions, a concentrate, which mainly comprises the polymer molecules with high molecular weight, and a permeate, which comprises the po¬ lymer molecules of low molecular weight, monomers and bypro¬ ducts, said permeate subsequently being rec rculated to 0» a polymerization step.

In the ultrafiltration unit membranes ith a suitable cut-off for retention of the desired high molecular weight fractions are used and these will of course vary with the specific polymer and the desired fractions. Suitable types

15. of membranes are for example polysulphones, cellulose ace¬ tates, polyamides, vinyl chloride-acryloni tri le copolymers and poLy( nylidene fluoride) membranes. The membrane units may for example have the form of plate-and-frame modules, but other types of membrane units can of course also be

20. used. The membranes are suitably subjected to a pre-treat- ment with a diluted solution of the actual resin to be separated prior to the separation which helps in forming a secondary membrane layer. For urea-formaldehyde resins the desired high molecular we ght fraction s in the range

25. of 2000 to 4000 and the separation is thus carried out to give essentially this fraction as the membrane-retained component, ie as the concentrate or retentate. As a guide it can be mentioned ^" that for this separation membranes of the above mentioned type with cut-offs of 20000 to 200000

30. are suitably used. For other resins membranes ith cut-offs in the range of 200000 to 400000 can generally be used.

The dry content of the feed soLution to the ultrafilt¬ ration unit should usually be in the range of from 8 to 25 per cent by weight. The process is generally operated

35. at pressures of about 1.0 to 15 bar and the flux through the membranes is ncreased by increased temperature. Care must however be taken that the chosen temperature is not harmful to the res n or the membrane. For membranes of the mentioned type and separation of for example urea-foi—

maldeh de, melamine-for aldehyde and polyaminopolyamide- epichlorohydrin resins temperatures within th ^' e range of 30 to 45°C are suitably used.

The filtration can advantageously be carried out 5- in such a way that the concentrate from the membrane unit is recirculated to the same via a feed tank and made to pass the membrane unit a number of times until the desired degree of concentration is obtained. Alternatively, the concentrate can of course be subjected to treatment in 10. several membrane units n series.

The aqueous solution containing polymer of lower molecular weight, the filtrate or permeate, obtained from the ultrafiLtration unit, which n the following wilL be termed the UF-permeate, is brought back to the polymeriza- • * tion reactor for the original resin for re-polymerization to give an economic process without loss of material.

The separation in the ultra-f ltration unit is suit¬ ably such that at Least 5 per cent by weight of the original dry resin content, ie higher and lower molecular weight 20. polymer fractions, is separated off in the permeate solu¬ tion. Depending on the amount of material separated off n the permeate solution and the amount of water associated ith this an upgrading step for the UF-permeate can be included before the recirculation to the polymerization 25. reactor. If the amount is small, between about from 5 to 10 per cent by weight, the UF-permeate can be directly transferred for re-polymerization. Otherwise, an upgrading of the UF-permeate is suitably carried out in order not . to disrupt the water balance in the poIymeri zati on-sepa ra- 0. tion system too much and also to avoid building up unaccep¬ table amounts of unwanted products in this system.

According to a preferred embodiment of the present invention the UF-permeate solution is thus upgraded to remove at least part of the water from the solution of * polymer material before its reci rcu I ati on to polymerization. This upgrading can for example be carried out by evaporation or socalled reverse osmosis membrane filtration. The upgrad¬ ing is preferably carried out by reverse osmosis as this process is comparatively cheap and advantageous with regard

to the Low thermal Load on the polymer material n the

UF-permeate. The reverse osmosis process is also advantag¬ eous with regard to the separation of unreacted monomers and byproducts, such as for example formaldehyde, which 5. is obtained hereby. The concentrated UF-permeate obtained by reverse osmosis is utilized in synthesis of the resin, ie recirculated for re-polymerization, while the -solution which has passed the membranes in the reverse osmosis unit, the reverse osmosis permeate which hereinafter wilL be

10. termed the RO-permeate, can be transferred to a feed tank and at least part of it can then be used as dilution Liquid during actual res n synthesis.

The treatment of the UF-permeate in a reverse osmosis membrane unit is carried out to remove part of the water

15. contained therein and suitably to give a solution having a dry content of at least 25 per cent by weight. Suitable temperatures at the reverse osmosis are from about 30 to 50°C and the pressure is suitably from about 20 to 60 bar.- The membranes in the reverse osmosis unit may suitably

20. consist of composite film material or poLybenzi mi dazolone or cellulose acetate.

When the present method is used for formaldehyde based resins it is possibLe, and advantageous, to add a formaldehyde binding agent to the feed to the ultrafiltra-

25. tion step. This addition is made in order to further reduce the formaldehyde content in the final product. It is also advantageous to add a formaldehyde binding agent to the UF-permeate in order to bind more formaldehyde to the RO— concentrate product. In this manner the total load of free

30. formaldehyde, in the products and the process, is further reduced. Preferably the formaldehyde binding agent is urea which forms dimethyloLureas with the formaldehyde.

The present invention also reLates to a plant com¬ prising a polymer zat on reactor, a first membrane unit

35. for fra ctionati on and purification of water soluble syn¬ thetic resins and which preferably also compr ses a second membrane unit for upgrading of the permeate from the separa¬ tion whereby the permeate outlet of the first membrane unit is connected to the second membrane unit v a a feed

t a n k .

A plant comprising two separation units and incor¬ porated within a plant for polymer synthesis according to the preferred embodiment of the invention wi ll be de- 5. scribed in detai l below with reference, to the attached drawing showing a schematic flow chart of such a plant.

The plant comprises a reactor 1 for synthesis of the water soluble polymer. The reactor is connected to a feed tank 2. From the feed tank 2 pipes 3 and 4 lead 10. to an ultrafiltration unit 5. From the concentrate side, indicated by C, of the unit the high molecular weight frac¬ tion can be removed- or the concentrate can via pipe 6 be returned to the feed tank 2 for subsequent further ultra¬ fi ltration. The permeate side, indicated by P, of the ultra- 15. fi ltration unit is connected to a second feed tank 7 which in turn is connected by way of pipes 8 and 9 to the second membrane unit, the reverse osmosis unit 10. The permeate side of the reverse osmosis unit can be connected to feed tank 2 via p pe 12a and to a waste water treatment plant 20. via pipe 12b whi le the concentrate side of the unit is connected to feed tank 7 by pipe 11 and connected to reactor 1 by valve 15 and pipe 16. From feed tank 2 high molecular weight product is obtained by pipe 3, valve 13 and pipe 14. Raw material for the resin synthesis is charged to 25. reactor 1 via pipe 17. The actual synthesis of water soluble syntheti c resins is a batch process but except for the polymerization the plant can be run continuously. Storage tanks which might be necessary, for example connected to ^' pipes 6 and 12 and between unit 5 and tank 7, have not 30. been shown in the drawing.

According to the drawing the plant is arranged to work in the following way, but other ways are also possible as disclosed earlier. The synthet c water soluble res n with a broad molecular weight distribution is produced 35. n reactor 1 and brought to feed tank 2. In feed tank 2 water is added. ^' The solution from feed tank 2 is passed to the ultrafi ltration unit 5, where it is separated into two solutions. The permeate solution comprising water, Low molecular we ght polymer, unreacted monomers and bypro-

ducts, passes the membrane and is in the drawing Led to the second feed tank 7. At low separation degrees this permeate solution could be Led directly to the reactor. The main part of the solution remains on the inlet side, 5. the concentrate side, of the membrane. To increase the concentration of this solution it can be led back to the feed tanks and by means of a pump, not shown in the drawing, once again forced to flow along the membrane surfaces in the uLtrafiLtration unit. When the solution has been treated

10. in this way during a certain time, the product, the concen¬ trated solution of desired high molecular weight polymer, is taken out from the feed tank by way of pipes 3 and 14.

The permeate obtained from the ultrafiltration mem¬ brane unit which is collected in the second feed tank 7

15. is brought to a second membrane unit 10 and concentrated by reverse osmosis. The concentrate of the UF-permeate is rec rculated for a certain time and made to flow along the membranes in the unit to make the desired amount of water pass these. The final concentrate from the reverse

20. osmosis unit is then returned to the reactor via pipes 8 and 15 and used as raw material for n.ew production of resin. The solution which passes the membranes in the second membrane unit consists mainly of water and part of it can be reused n the polymerization step.

25. The invention is further illustrated in the following examples which, however, are not intended to Limit the same. Parts and per cent relate to parts by weight and per cent by weight respect vely, unless otherwise stated. Example 1

30. Water soluble cationic urea-formaldehyde resin was diluted with water until the concentration of the solu¬ tion was 23% dry weight. 55.2 kg of this solution was con¬ centrated on an ultrafiltration plant with membranes of the type UF-PS-20, i.e. membranes of polysuIphone . The

35. solution was made to pass the ultrafilters until a solution with a concentration of 37.9% was obtained. The amount of concentrated solution was 26.2 kg. 27.6 kg permeate, solution which had passed the membranes, with a dry weight content of 10% was collected. 2.64 kg dry weight resin

had thus been separated from the original amount of 12.6 kg. The inlet pressure was 10 bar, the temperature 40°C and- the filtration was performed during 3 hours.

The amount of free formaldehyde in the original pro- 5. duct was 6.85% based on dry weight resin i.e. 0.87 kg in the 55.2 kg solution. The free amount of formaldehyde in the concentrate was 4.1% on dry weight res n i.e. 0,41 kg and in the permeate 19.1% based on dry weight i.e. 0.53 kg. The difference between totally found amount of formaldehyde 10. 0.94 kg and the added amount of 0.87 kg can be explained by a slight hydrolysis of the resin dur ng the filtration. Example 2

Water soluble., cationic urea-formaldehyde resin was diluted with water until the concentration of 8.1% dry

15. weight. 68.8 kg of this solution was concentrated on an ultrafi ltration plant with membranes of polysulphone. The solution was made to pass the ultrafilters until a solution with a concentrat on of 20.4% was achieved. The amount of concentrated solution obtained was 20.6 kg i .e. 4.2 kg

20. dry resin. 48.2 kg permeate solution which had passed the membranes with a dry content of 2.8%, 1.35 kg dry weight, had thus been separated from the or ginal amount of 5.6 kg.

The inlet pressure was 10 bar the temperature 40°C and the f ltration was performed during 1.5 hours.

25. The total added amount of free formaldehyde was

0.390 kg of which 0.230 kg was found in the concentrate and 0.171 kg was found in the permeate. Based on dry weig t resin the concentrate contained 5.47% free formaldehyde and the permeate 12.7% compared with the content of 6.9%

30. i the original product.

Example 3

Example 2 was repeated with 83.0 kg resin solution with 8.9% dry weight. After 2 hours at 40°C, 10 bar pressure, 24.2 kg concentrate with the dry content of 23.4% 35. and 58.8 kg permeate with dry content 2.95% were achieved. 0.510 kg free formaldehyde was added and 0.280 kg was found in the concentrate and 0.241 kg was found in the permeate. Based on dry weight resin the concentrate contained 4.9% free formaldehyde and the permeate 13.9% compared with

the content of 6.9% in the original product. Example 4

Concentrated resins from example 1 were tested as wet strength agents on a pilot paper machine. The pulp 5. consisted of 50% bleached softwood and 50% hardwood with 24°SR. pH was 4.5. Results :

Product Added Wet strength Wet Strength

Amount breaking length 10. % !J3 'ά

Original 0 0..55 0 0..9933 13.7

1 . 0 1 . 22 16.9

1 . 5 ^" 1 . 41 18.6

15. 3 3..00 1 1..6633 19.7

Concentrate 0 0..55 11..0066 15.4

1 . 0 1 . 41 18.8

1 . 5 1 . 65 21.7 20. 33..00 11..8822 22.0

Example 5

55.8 kg of a permeate solution, being a mixture of permeates obtained after several repetitions of Example 1, with a dry content of 8.8%, i.e. 4.92 kg was charged to 25. a filtration unit for reverse osmosis. Thin film composite membranes were used". The process was carried out during 2 hours at 40 bar and 40°C. 20.8 kg RO-concentrate with a dry content of 23.0% i.e. 4.78 kg dry resin and 35 kg RO-per eate with a dry content of 0.2%, i.e. 0,07 kg were 30. collected.

Total added amount of free formaldehyde was 0.930 kg of which 0.413 kg was found in the RO-concentrate and 0.532 kg was found in the RO-permeate.

Synthesis of a urea-formaldehyde resin was performed

35. where the RO-concentrate was one of the raw materials.

Extra urea and formaldehyde and other ingredients were added according to standard recip e. The amount of RO-con- centrate was 20% dry weight of the total dry weight added.

The product achieved had standard properties and

gave good wet strength results. Examp le 6

61.6 kg of a permeate solution, with a dry content of 8.6% i.e. 5.3 kg dry weight was added to a R0-filtrat on 5. unit. Thin fi lm composite membranes were used. The process was carried out during 2.5 hrs at 40 bar and 40°C.

19.0 kg R0-concentrate with a dry content of 25% i.e. 4.8 kg dry resin and 42.6 kg RO-permeate with a dry content of 1.17% i.e. 0.5 kg were achieved.

10. This RO-concent rate was used as raw material for new synthesis. Some 24% of the total added amount of chemi¬ cals for the new systhesis was contributed from the RO-con- centrate. The new resin had standard propeπ ^' tes and gave good wet strength results.

15. Examp le 7

37.1 kg of a solution of an anionic urea-formal¬ dehyde resin containing sulphur molecules at a dry weight of 21% i.e. 7.8 kg was run through an ultrafiltration plant. After 4 hrs at 10 bar and a temperature of 40°C, 22.8 kg

20. of a concentrated solution with 29.4% dry weight, i .e. 6.70 kg and 14.3 kg permeate w th a dry content of 7.6% were collected i.e. 1.1 kg dry weight.

Some 30% of the added sulphur groups were parts of unwanted monomeric byproducts with a negative impact on 25. the f nal wet strength effect of the resin.

The total added amount of monomers containing sulphur was 0.122 kg. The concentrate contained 0.06 kg and the permeate 0.062 kg of monomers conta n ng sulphur. Based on dry weight resin the original product contained 1.56%,

30. the concentrate 0.89% and the permeate 5.6%.

These products were also tested for formaldehyde emission from cellulose fibres according to a standard procedure .

35. Sample Emission of formaldehyde in ug/mg dry product

orig nal product 92 concentrate 61

Ex a mp le 8

The relation between added amount of free formal¬ dehyde to the stock together with urea formaldehyde resin and amount of formaldehyde emitted in the drying air at 5. a paper mach ne was tested on a pilot machine.

The white water system was very closed and the test was performed at status quo.

Added amount Added amount of free Found amount of

10. w.s. agent CH_0 to the stock CH_0 in the air mq/min ppm ^'

1.6 21 0.49

2.0 55 0.61

2.4 68 0.96

15. 2.5 93 1.24

3.0 135 1.34

The same urea-formaldehyde resin but with different contents 20. of free formaldehyde was tested. A contained 7.14% free formaldehyde based on dry weight resin and product B con¬ tained 2.25% free formaldehyde based on dry weight.

Product Added amount Added amount Found amount of

25. % free formaIdehyde - dry weight formaIdehyde in the in the white water air g/mi n ppm ppm

30. A 2.44 55.8 43 1.08 B 2.50 17.5 15 0.41