CA2046251A1

CA2046251A1 - Aqueous dispersions of urea/formaldehyde polymers and the use thereof

Info

Publication number: CA2046251A1
Application number: CA002046251A
Authority: CA
Inventors: Alfred Renner; Klaus Kramer
Original assignee: Individual
Current assignee: Novartis AG
Priority date: 1990-07-06
Filing date: 1991-07-04
Publication date: 1992-01-07
Also published as: JPH04226539A; EP0468917A3; EP0468917A2

Abstract

Aqueous dispersions of urea/formaldehyde polymers and the use thereof Abstract of the Disclosure Aqueous dispersions comprising 15-35 % by weight of a urea/formaldehyde polymer (UF
polymer), said dispersions having - a formaldehyde:urea (F:U) ratio of (1.25-1.40):1, - a content of free formaldehyde of less than 0.1 % by weight, - a specific surface area of 5-30 m2/g, - a pore volume of 2-4 cm3/g, - a mean particle size of 0.05-1 µm, and - a mean agglomerate size of 3-10 µm, are especially suitable for use as white pigments in paper manufacture.

Description

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Aqueous dispersions of urea/formaldehYde polymers and the use thereof The present invention relates to aqueous dispersions containing 15-35 % by weight of specific urea/formaldehyde polymers, to a process for their preparation and to the use thereof, in particular as white pigments in paper manufacture.

Urea/formaldehyde polymers (UF polymers) have long been known and are used in different fields. A known use of specific UF polymers is as white pigments in paper manufacture.

In US patent 3 909 348 there is disclosed a paper product containing 0.5-80 % by weight, based on dry pulp slurrv, of a nonporous particulate UF polymer in which the molar ratio of formaldehyde:urea (F:U) is from 1.3 to 1.8:1 and having a specific surface area, absorption capacity and mean agglomerate size. In this publication, the preferred sole exemplified F:U ratio of the UF polymer is 1.5:1. According to the teaching of this patent specification, the synthesised UF polymer is isolated from the reaction mixture, dried, and milled in the dry form to the desired mean agglomerate size. The particulate UF polymer so obtained is added to the pulp either in the dry form or as an aqueous slurry.
This prior art procedure has drawbacks. On the one hand, drying the polymer before milling requires a very high energy consumption. On the other hand, the dry product, which is suitable for use as white pigment, is a very fine powder with a bulk density of only about 60 g/l. Although special packaging techniques make it possible to achieve a maximum bulk density of about 150 gtl, the cost of transporting the white pigment from the polymer producer to the paper manufacturer is also very high, as the cost calculation is normally based on volume and not on weight. A further drawback ;s the dusting tendency of the finely particulate polymer during production and use.

In addition, it has been found that these products, in which the F:IJ ratio of the UF polymer is 1.5:1 or even higher, have a relatively high content of free formaldehyde, so that they do not in all respects meet the increasingly strict requirements for products having as low a content of free formaldehyde as possible.

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Surprisingly, it has now been found that all these shortcomings can be overcome by preparing UF polymers having a lower F:U ratio of (1.25-1.40):1 and afterwards, without drying the product, subjecting the product to wet milling. In this manner it is possible to prepare stable aqueous dispersions having a high solids content (up to ca. 35 % by weight) direct and without high energy consumption. Owing to the high solids content, the products can be transported cheaply and used direct in this form by the paper manufacturer. In the relatively low F:U ratio of the UF polymer defined above, the content of free formaldehyde is so low that the products satisfy all requirements. The known excellent properties of the IJF polymer as white pigment for paper all remain unchanged.

Accordingly, the invention relates to aqueous dispersions containing 15-35 % by weight of a urea/formaldehyde polymer (UF polymer), said dispersions having - a formaldehyde:urea (F:U) ratio of (1.25-1.40):1, - a content of free formaldehyde of less than 0.1 % by weight, - a specific surface area of 5-30 m2/g, - a pore volume of 2-4 cm3/g, - a mean particle size of 0.05-1 Jlm, and - a mean agglomerate size of 3-10 ~Im.

The dispersions of this invention may conveniently be prepared by (i) condensing forrnaldehyde and urea in a (F:U) ratio of (1.25-1.40):1 in an aqueous solution at pH 6-9 and a temperature of 40-100C, (ii) gelling the condensate by addition of a dilute aqueous acid at a temperature of 20- 100C, (iii) comminuting the gel and suspending the particles in water and neutralising the suspension by addition of a dilute base to ca. pH 7-9, (iv) subjecting the neutralised suspension to wet milling until the mean agglomerate size is attained, and (v) filtering and washing the milled product and, if appropriate, compressing the product to a filter cake, and thereafter (vi) dispersing the product by the action of mechanical shear forces.

Another suitable process comprises filtering and washing the neutralised product (iv) obtained from step (iii), if appropriate compressing said product to a filter cake and subjecting said filter cake to -3- ~ t~

(v) wet milling until the indicated mean agglomerate size is attained.

The preparation of particulate UF condensation polymers by gelation is commonly known and is described, for e7~ample, in the aforementioned US patent 3 909 348 or by A. Renner in Makromolekulare Chemie, 149, 1 (1971). Particularly suitable process conditions for steps (i) to (iii) of the process of this invention are described in US patents 4 018 741 and 4 101 ~21. As mentioned at the outset, in all prior art processes the product of step (iii) is filtered, dried and only then milled tO a powder.

The condensation step (i) is preferably ca~ied out at pH 6-8 and at a temperature of 50-90C, preferably at ca. 70C. The concentrations of the reactants is preferably chosen such that, before gelation, a ca. 20-30 % solution of the UF polymer forms.

Gelling agents which may be suitably used for obtaining the insoluble, infusible, completely crosslinked llF polymers in step (ii) are the customary acid catalysts, such as sulfuric acid, sulfamic acid, phosphoric acid, hydrochloric acid, nitric acid, organic acids of medium strength having a pKa value of less than 4, typically formic acid, oxalic acid, maleic acid, succinic acid, chloroacetic acid and the like. It is preferred to use sulfuric acid and, most preferably sulfamic acid, as acid catalyst.

The acid used as gelling agent is suitably used in amounts of 10-100, preferably 20-40, millimol of acid per mol of urea.

A preferred embodiment of the invention comprises carrying out the preparatis)n of the UF polymers in the presence of a protective colloid. Typical examples of such protective colloids are natural starches such as starch, gelatin, mucilage, tragacanth gum, gum arabic, modified natural substances such as carboxymethyl cellulose, the aL~ali metal salts of carboxymethyl cellulose, especially the sodium salt of carboxymethyl cellulose, methyl cellulose, ethyl cellulose"B-hydroxypropyl cellulose, aLkali metal alginates and the like, synthetic polymers such as polyvinyl alcohol, polyvinyl pyrrolidone, water-soluble polymers and copolymers of acrylic acid and methacrylic acid and their alkali metal salts, salts of copolymers containing maleic acid, of copolymers of styrene and maleic anhydride and the like. The amount of protective colloid will depend on the type, chemical structure and molecular weight thereof. Normally it will be added in amounts ranging from ca. 0.1 to 10 % by weight, based on the total weight of the urea and formaldehyde reactants. It is preferred to add the protective colloid in amounts ranging from ca. 0.5 to % ~

ca. 5 % by weight. In a particularly preferred embodiment of the invention, sulfamic acid is used as curing catalyst in conjunction with a protective colloid.

The particularly preferred protective colloid is sodium carboxymethyl cellulose. The protective colloid may be suitably added in step (i) or (ii) of the process of the invention.
Afterwards an aqueous solution of the acid is added, with stirring, to the reaction mixture in the temperature range from room temperature to ca. 100C until a crosslinked gel has formed. The gel is then comminuted, typically in an extruder or cutter granulator, a crusher or a roll mill.

The wet milling of the UF polymer is preferably effected in a bead mill by comminuting the milling stock with glass beads at high speed to the desired mean agglomerate size. A
mean agglomerate size of 5-8 llm is preferred. The fineness of the product can be controlled conveniently by suitable adjustment of the rate of flow and the size of the glass beads.

As stated above, the wet milling can be cariied out after neutralisation upon conclusion of step (iii) or alternatively in step (v) after filtration. In the former case, the milled product is filtered and, if appropriate, compressed to a filter cake which preferably has a solids content of 15-40 % by weight and likewise falls within the scope of the invention. Such filter cakes of high solids content are also admirably suitable for transportation to the end user and can be dispersed on the spot before use, conveniently by the action of mechanical shear forces. Although the filter cakes have a relatively high water content, they are in appearance solids and can be handled as such.

The dispersions of this invention have almost unlimited stability in the claimedconcentration range and can be stored for some considerable time (up to six months) without difficulty. If the claimed concentration of the UF polymer is exceeded or not maintained, then sedimentation of the polymer may occur.

For the preparation of the dispersions of the invention it may also be convenient to use dispersants. Suitable dispersants are typically known phosphate or polyacrylate dispersants. Such agents are commercially available, for example under the tradename Calgon (ex Benkiser, Knappsack), Polysalz CAL (ex B~SF, Ludwigshafen), A 4775 (ex Stockhausen, Krefeld) or the experimental product Dispersant 7030 (ex BASF, Ludwigshafen).

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The use of dispersants makes it possible to lower the viscosity of the dispersion, if desired, or to prepare dispersions of higher solids content if the viscosity remains the same. The dispersant is preferably added in the last step of the preparatory process.

Preferred dispersions of this invention are those containing ca. 25 % by weight of the UF polymer.

The most preferred forrnaldehyde:urea (F:U) ratio of the UF polymer present in the dispersion is 1.26:1.

If the F:U ratio is lower than the claimed range (i.e. <1.25), then microporous particles result which have a smaller pore volume (ca. 0.5 cm3/g) and which are not suitable for use as white pigments for paper. If on the other hand the F:U ratio of the UF polymer is higher than 1.40, then the polymers obtained have too high a content of free formaldehyde and for this reason are undesirable. Only in the narrowly defined molar ratio of 1.26: 1.40 are UF polymers obtained which satisfy all criteria. These polymers have spherical submicron particles which are agglomerated to some de~ree, thereby producing the claimed pore volume which is substantially attributable ~o the pores between the agglomerated spherical submicron particles and which may be conveniently determined by mercury porosimetry.
Such UF polymers are disclosed in US patent 4 367 171 ~Example 1).

The dispersions of this invention preferably contain UF polymers containing less than 0.05 % by weight of free formaldehyde. Also preferred are UF polymers having a specific surface area of 7-25 m2/g and those having a pore volume of 3-3.5 cm3/g as well as having a mean particle size of 0.1-l llm, as such products ale particularly suitable for use as white pigments for paper.

Further possible utilities for the dispersions of this invention are the preparation of aqueous emulsion paints or aqueous coatings for paper. Such util;ties for UF polymers are disclosed in US patents ~ 307 005 and 3 367 171.

The preferred utility of the dispersions of the invention, however, is as white pigments for paper manufacture.

Any conventional type of paper pulp may be used for manufacturing the paper. Thus the - 6- ;~'d~

pulp may be a chemically treated pulp, typically a sulfate pulp, sulfide pulp, soda pulp or soda kraft pulp, or a semi-chemical pulp, a mechanically ground pulp, or a mixture thereof. Other pulps obtained from plants and rags are also suitable. In those cases where it is not necessary to use virgin pulp, waste paper by itself or in combination with virgin pulp can be processed. The waste paper cuttings can be added to the virgin pulp either in dry form or as an aqueous slurry.

It is preferred to use the dispersions of this invention in the paper without inorganic white pigments and without polymeric binders.

The paper containing UF white pigments can be manufactured in known manner. To this end the dispersions of the invention are added in a suitable amount to the pulp prior to the formation of sheets. The paper preferably contains from 0.5-10 % by weight, preferably from 1-5 % by weight, of l~F polymer, based on dry paper stock. A high filler retention and a very good white effect and high opacity are obtained with the polymers of this invenhon.

The invention is illustrated by the following Examples, in which percentages are by weight.

Example 1:
a) Preparation of the condensation polymer:
(CH20: urea = 1.26; sulfarnic acid as gelling agent) 1260 g of a 30 % aqueous solution of formaldehyde are diluted with 950 g of deionised water and heated to 7QC. Then 600 g of urea arè added and the pH is adjusted to 7.0 with a 10 % solution of sodium carbonate, and condensation is carried out for 2 hours at pH 7.0 and 70C. The reaction mixture is then cooled to 50C.

Meanwhile a solution of 33.2 g of sulfamic acid (H3N+.SO-3) in 13Q0 g of H20 is prepared and is then also heated to 50C. This solution is added all at once to the precondensate, with vigorous stirring~ ~tirring is discontinued after 8 seconds, whereupon the reaction mixture becomes opaque and gels after 11 seconds, the temperature rising to 69C. This gel is left for 2 hours at 70C, comminuted, and suspended in 4 litres of water with a high-speed stirrer. The suspension is adjusted to pH 7.0 with a 10 % solution of sodium carbonate and then micronised to a mean agglomerate size of 5.5 llm in a "Dynomill"
(supplied by Bachofen, Basel). The treated pigment is isolated by filtration, giving 3035 g - 7 ~ 2 ~ ~

of a snow-white filter cake having a solids content of 20.8 %. The specific surface area of the pigment is 7.4 + 0.2 m2/g. The product has a primar~! particle size of 0.4 ~lm and a pore volume of 2.33 mVg. It contains 0.03 % of free formaldehyde.

b) Preparation of an aqueous dispersion (slurrY) and use thereof for paper manufacture bl) Viscosity experiments In this experiment, the product prepared according to a) is dried and the slurries required for the dispersion are prepared from the dried product to give a solids content of exactly 30 % by weight. It will be readily understood that as usual the aqueous dispersions are used direct for paper manufacture.

The following viscosities are measured at a solids content of 30 % by weight. The viscosity is determined without and with a dispersant.

?3 Viscosity* without dispersant at 30 % by weight solids content: 650 mPas.

Viscosity~
Amount of dispersant (%) Dispersant Calgon~ ) Polysalz CAL2) (8) A477s3) 0.5 35 4) 480 5) 3o 4) 1.0 40 520 40 1.5 45 680 40 2.0 55 810 50 2.5 60 790 55 3.0 65 740 60 1) polyphosphate ex Bankiser, Knappsack 2) polyacrylate ex BASF, Ludwigshafen 3) polyacrylate ex Stockhausen, Krefeld 4) spindle 3 5) spindle 4 * Brookfield RVT, 100 rpm b2) Paper sheet formin~
The UF polymer is used as 20.8 % slurry direct for the manufacture of wood-free paper.

(i) Stock preparation The seock used for sheet forming has the following composition:

80 % birch kraft pulp 20 % pine kraft pulp The stock is pulped in the pulper at 4 % SC (stock consistency). The pulping time is 10 min. Afterwards the pulp is beaten for 1 min. 20 sec. in a laboratory refiner at 1450 rpm (5.9 KW) to a freeness of 23SF:.

The pulp is dewatered by centrifugation for 2 minutes (mesh si~e of the sieve 2~0 llm, SC ca. 16.5 %) and stored at +5C.

(ii) Sheet forming (accordin to DIN 54358) Sheet forming is carried out on a "Rapid-Kothen" sheet forming system with distributor.

Specifically, 45 g atro (absolutely dry) of cooled stock are put into the distributor, bulked to 61 and stirred for 10 min. (SC - 0.7~ %).

The white pigment filler slurry (20.8 % by weight) is dispersed with a Chemacol stirrer for 1 min. at 12000 rpm. The white pigment suspension is added to the stock in the distributor and water is added to make up a volume of 91 (SC =
0.~ %)-The calculated amount of pulp is drawn off and fed into the Rapid-Kothen sheet former. The stock suspension is diluted in the sheet former to 0.015-0.043 %. The sheets are dried on the Rapid-Kothen sheet former for 10 minutes at 95C and under a vacuum of 800 mbar. The sheets obtained have a basis weight of ca.
50 g/m2. Paper containing no white pigment and containing 1.2, 2.4 and 3.6 % by weight of white pigment (based on dry pulp) is prepared.

(iii) Testing The white pigment retention is determined by nitrogen analysis according to the method of Kjeldahl. The sheets are tested for different properties:

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Standard without U/F Polymer white pigment 1 2 3 whitepigmentcontent 1.2 % 2.4 % 3.6 %
white pigment retention 71 62 73 basis weight (glm2) 49 50 49 50 w4h5i7enes)s (%), DIN 53145 ) 84.4 86.6 86.9 86.4 opacity (%), DIN 53146 70.5 72.0 72.7 74.5 thickness (~lm), DIN 53105 87 90 90 93 volume (cm3/g), DIN 53105 1.78 1.8 1.84 1.86 It is clearly evident from the Table that the addition of the l~F white pigment effects a marked increase in whiteness and opacity.

Example 2:
a) Preparadon of the condensation polymer (CH20: urea = 1.26; sulfamic acid as gelling agent; use of a protective colloid)10;78 g of a high molecular weight sodium carboxymethyl cellulose (Blauose 7 HFD, ex Aqualon) are dissolved in 930 g of deionised water and 1260 g of a 30 % aqueous solution of formaldehyde. The further procedure described in Example 1 is carried out to give, after neutralisation to pH 7.5,3507 g of a filter cake having a solids content of 18.0 %. The pigment has a specific surface area of 23.5 ~ 0.5 m2/g, a prirnary particle size of 0.15 llm, a pore volume of 3.35 ml/g and a content of free formaldehyde of 0.04 %.

Example 3:
a) Preparation of the condensation polymer (CH2O: urea = 1.26; H2SO4 as gelling agent; use of a protective colloid) preconden-sate: H2O deionised 930 g CH2O 30 % 1260 g Blauose 7 HFD 13 g urea 600 g gelation catalyst: H2SO4 (98 %) 25.5 g H2O 1300 g Procedure as in Examples 1 and 2.
gel tirne at 50C 12.3 s neutralisation to pH = 7.5 Yield: 4850 g of a filter cake having a solids content of 17.0 %
spec. surface area of the pigment: 18.9 + 0.4 m2/g free formaldehyde: 0.02 %
primary particle size: 0.25 ~Lm.

Example 4:
a) Preparation of the cnndensation polymer (CH20: urea = 1.33; sulfamic acid as gelling agent) preconden-sate: H2O deionised 933.1 g CH2O 30 % 1333 g urea 600 g gelation catalyst: H2O deionised 1310 g sulfamic acid 32.3 g Procedure as in Examples 1-3.
gel time at 50C: 11 s filter cake: 3893 g solids content 20.8 %
spec. surface area: 6.4 + 0.1 m2/g free formaldehyde: 0.055 %
primary particle size: 0.5 ~,lm.

b) Preparation of slurries:
The ~llter cake is milled direct in a ball mill (e.g. available from Retsch). The fluid suspension obtained has a non-Newtonian behaviour during the viscosity measurement.
The viscosity of the suspension can be substantially reduced with the aid of theexperimental Dispersant 7030 of BASF.

Viscosity of 20 % slurries as a function of the concentration of the dispersant and the rotation speed of the Brookfield viscosimeter.

Speed of rotation % by wt. 7030 10 20 50 100 [~pm]

0~0 1.550 800370 205 viscosity (mPa-s~
0.5 1.600 675220 100 1.0 910 365122 55 1.5 300 17596 63 2.0 210 15078 40 3.0 150 8040 30 The slulTy concentration can also be increased with the aid of the experimental Dispersant 7030 of BASF at an a.cceptable viscosity.

Slurries containing 1.5 % by weight of Dispersant 7030 of BASF: viscosity (mPa.s) as a function of the solids content:

Solids content 20 Ipm 50 rpm % by weight 20.8 485 210 viscosity 21.8 825 440 (mPa-s) 22.8 975 600 23.8 1.950~ 1.020 24.8 3.250~ 1.600 ~ heavily time-dependent 2 ~ ~
c) Paper manufacture Paper sheets are prepared on a laboratory paper machine which contain the amount of white pigment listed in the Table as 20 % slurry.
_ _ White pigment (% by wt.%) 0 1.00 2.15 basis weight (g/m2) 79.,7 79.7 80.2 thickness (~,lm), DIN 53105 103 109 106 whiteness (%), DIN 53145 (457 nm) 80.87 84.05 84.75 scattering coefficient (cm2/g), TAPPI 218 272 314.72 opacity (%), D~N 53146 71.3 75 5 78.2 Example 5:
a) Preparation of the condensation polymer (CH20: urea = 1.40; sulfamic acid as gelling agent) preconden-sate: H2O deionised 400 g CH2O (30 %) 700 g urea 300 g gelation catalyst: sulfamic acid 15.25 g H2O deionised 460 g The educts are reacted as described in the preceding Examples and the product has the following properties:
filter cake (30 %): 1250 kg spec. surface area: 19 m2/g freeformaldehyde: 0.1 %
primary particle size 0.2 ~,1m Example 6: Wet milling of the UF pol~lmer A 15 % by weight suspension of the llF polymer prepared according to Example la is milled, after neutralisation, in a bead mill as follows:

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The milling stock is comminuted in the bead mill with glass beads which are stirred at high speed. By adjusting the rate of flow and the size of the glass beads it is possible to control the finess of the product. The experiments are carried out in a small 1.5 Iitre laboratory bead mill. The following results are obtained using 2.0 mm glass beads:
Rate of flow Mean diameter of the (Measured with Cilas 81 litre/h agglomerates (llm) laser scattering) a. 8 5.7 b. 20 8.2 c. 40 9.5

Claims

1. An aqueous dispersion comprising 15-35 % by weight of a urea/formaldehyde polymer (UF polymer), said dispersion having - a formaldehyde:urea (F:U) ratio of (1.25-1.40):1, - a content of free formaldehyde of less than 0.1 % by weight, - a specific surface area of 5-30 m2/g, - a pore volume of 2-4 cm3/g, - a mean particle size of 0.05-1 µm, and - a mean agglomerate size of 3-10 µm.

2. A dispersion according to claim 1 comprising 20-30 % by weight, preferably about 25 % by weight, of the UF polymer.

3. A dispersion according to claim 1, wherein the UF polymer has a F:U ratio of 1.26:1.

4. A dispersion according to claim 1, wherein the UF polymer contain 0.05 % by weight of free formaldehyde.

5. A dispersion according to claim 1, wherein the UF polymer has a specific surface area of 7-25 m2/g.

6. A dispersion according to claim 1, wherein the UF polymer has a pore volume of 3-3.5 cm3/g.

7. A dispersion according to claim 1, wherein the UF polymer has a mean particle size of 0.1-1 µm.

8. A dispersion according to claim 1, wherein the UF polymer has a mean agglomerate size of 5-8 µm.

9. A process for the preparation of a dispersion according to claim 1, which comprises (i) condensing formaldehyde and urea in a (F:U) ratio of (1.25-1.40):1 in an aqueous solution at pH 6-9 and a temperature of 40-100°C, (ii) gelling the condensate by addition of a dilute aqueous acid at a temperature of 20-100°C, (iii) comminuting the gel and suspending the particles in water and neutralising the suspension by addition of a dilute base to ca. pH 7-9, (iv) subjecting the neutralised suspension to wet milling until the mean agglomerate size is attained, and (v) filtering and washing the milled product and, if appropriate, compressing the product to a filter cake, and thereafter (vi) dispersing the product by the action of mechanical shear forces.

10. A process for the preparation of a dispersion according to claim 1, which comprises (i) condensing formaldehyde and urea in a (F:U) ratio of (1.25-1.40):1 in an aqueous solution at pH 6-g and a temperature of 40-100°C, (ii) effecting gelation of the condensate by addition of a dilute aqueous acid at a temperature of 20-100°C, (iii) comminuting the gel and suspending the particles in water and neutralising the suspension by addition of a dilute base to ca. pH 7-9, (iv) filtering and washing the milled product and, if appropriate, compressing the product to a filter cake, and thereafter (v) subjecting the neutralised suspension to wet milling until the mean agglomerate size is attained.

11. A process according to either claim 9 or claim 10, which is carried out in the presence of a protective colloid.

12. A process according to claim 9 or claim 10, wherein the acid used in step (ii) is sulfuric acid or sulfamic acid.

13. A process according to claim 9 or claim 10, wherein a dispersant is subsequently added to the dispersion.

14. A filter cake obtainable by the process of claim 9 or claim 10 and preferably having a solids content of 15-40 % by weight.

15. Use of a dispersion as claimed in claim 1 as white pigment in paper manufacture.

FD 4.3/SZ/ac*/RU