CA1126438A - Aminoplast resin particles - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:
Aminoplast resin pigments for paper making comprising large surface area (>5 m2/g) particles, preferably of size below 200 µm, made from an aminoplast resin obtained by condensing formaldehyde with urea and/or melamine in the presence of 0.2 to 15 moles, per 100 moles of formaldehyde, of sulphite, phosphate, phosphite or borate radicals. The resin may be converted to the particulate form by gelling it in extended form by dilution with an aqueous acid, followed by drying, curing, and comminution of the gelled resin. To effect gelation in extended form the resin is preferably diluted to a resin solids content of less than 25% by weight by the addition of sufficient acid to cause gelation within 20 minutes.

Description

~6~8 Aminoplast rcs~n particles Urea-formaldehyde resin pigments ~re known and are sold as add.itives :Eor paper (see, for example, Paper Technology June :l.974 p.l64, Makromolekulare Chemie, 1971, 149 p.l-27, and UK Patent Speci:Eications 1239143, 1296246 , 131824~ and 1451973).
These pigments are prepared by treating a UF resin withr for preference, sulphamic acid and a surfactant. The pigments confer a good colour and improved opacity to papers, and improved bulk. Such pigments do, however, decrease the strength of the paper and axe not totally retained in the paper web durin~ papermaking.
We have now produced an improved pigment for paper-making, which is substantive to cellulose pulp, is therefore easily retained with little, if any, loss of paper strength and which can be easily and efficiently prepared.
In our German OLS Specifica-tion 2754525 aminoplast resin, eg.UF resin fibres having certain inorganic oxyacid radicals incorporated therein are described. These fibres are made by ~orming a concentrated aminoplast resin solution containing the inorganic oxyacid xadicals and then convertiny the resin solution into fibres, eg by centrifugal spinning.
We have now found that improved pigments for paper--making can be formed from aminoplast resins containing such inorganic oxyacid radicals. In order that the resin is suitable as a paper pigment it has to be converted to a particulate form having a high_ _ ___ . . . _ . _ . ~ ~
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aurface area, for example in exce~s of 5 m2/g. This can be aohieved by cau~ing the re~in to gel and polymerise in an "extended"
form oonsiatln~ of hi~h-area particles lightly bonded together which can, after ourin~, be comminuted to a powder form.
Aocordin~ly we provide particles of an aminoplas-t reain comprising a condensate of at least one amino compound selected from urea ~nd melamine with ~ormaldehyde containin~ 0.2 to 15 inorganic oxyacid radicals Yalectecl from aulphite, phosphate, pho~phata, phoaphite And borate radicals, per 100 methylene radicals in the re~in, said particles havin~ a surface area of at least 5 m2/~.
The aminoplast reain is a oondensate of urea and/or melamine with formaldehyde. ~he molar ratio of formaldehyde to amino groups is preferably in the range 0.6 to 1.2, particularly above 0.75 Where the aminoplast resin is a urea formaldehyde resln, which is to be pre~erred9 this corre~ponds to a ~ormalde-hyde: urea molar ratio range of 1.2 to 2.4, particularly above 1.5-~he amino compound ia generally initially condensed with formaldehyde in an aqueous medium, eg by the use of an a~ueoussolution of formaldehyde, ie formali~, under neutral or alkaline conditions. The inorganic oxyaoid radicals are incorporated at this stage, either before or durin~ this initial condensation.
The inorganic oxYacid radical3 may be incorporated by the addition of the appropriate acid or by addition of one or more salts that giYe rise to suoh radioals. Because of the need to avoid premature cro~slink~ng, a pH above about 6 i~ normally maintained during the initial conden~atlo~. Where the inor~anic oxyacid radieals are added as the acid, some alkali may also be required to maintain 3D the nece~ary p~. ~he lnor6anio oxyaoid radicals are thus prefer~
ably added a~ ~alts~ Example~ of suitable salts inolude soaium sulphite, sodium metabisulphite, sodium dihydro~en pho~phate, sodium hydro~en phosphite and sodium tetraborate (borax). Mixture3 of such ~alt~ may be used~ for ~xample ~odium sulphite in admixture with ~odium ~etabi~uIphite. I~ the case o~ ~ulphites, the radicals ~L~2~

may be introduced by the incorporation of the reaction product of formaldehyde and sulphites, eg by addition of sodium formaldehyde bisulphite. The reduction product thereof, viz sodium formaldehyde sulphoxylate, is readily oxidised to give sulphite radicals and so may also be used as a source of sulphite radicals.
The amount of inorganic oxyacid radicals used is from 0.2 to 15, preferably 0.5 to 10, particularly 0.7 to 5, moles per 100 moles of formaldehyde. During the condensation and curing of the aminoplast resin water is produced and the amino compound becomes linked by methylene bridges derived from the formaldehyde. Consequently the presence of x mole %
(based on formaldehyde) of inorganic oxyacid radicals during the condensation results in x inorganic oxyacid radicals per 100 methylene radicals in the aminoplast resin.
As mentioned hereinbefore, in order to obtain the resin in a particulate form having a high surface area, the resin may be gelled and polymerised in an extended form.
Formaldehyde resins are normally gelled, eg in fibre pro-duction as described in aforesaid OLS Specification 2754525, by adding to a concentrated resin solution, eg having a viscosity of 5 - 100 poise, a small amount of a curing catalyst, eg 5 ml of a 5~ aqueous phosphoric acid solution per 100 ml of the concentrated resin solution. Such a con-centrated resin solution may typically have a solids content of 55 - 70~ by weight.
In contrast thereto, in order to obtain gelation in the extended form, sufficient catalyst solution, option-ally together with additional water, is added to dilute the resin considerably, eg to a solids content below 25~ by weight.

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~~ - 3A -Also the amount of catalyst, which is a strong acid such as sulphuric, hydrochloric, or phosphoric acid, must be suffi-cient -to ensure rapid gelation. In particular sufficient acid should be added to ensure that the resin gels within 20 minutes of the time of acid addition. The maximum time permissible for gelation will depend on the extent of the condensation at the time of acid addition. Thus, during storage, condensation tends to continue slowly, with an observable ., . . , ; . . .... . .. . .

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increase in vi~cosity. Also in some resina the initial condens-ation may ha~e been continued to a greater extent than in other reein~. As a general guide, the amount of acid that i8 addedshould be 3ufficient to en3ure gelation within 10 minutes of acid addition if the resin has been condensed, at the time of acid addition, to an extsnt equivalent to that of a resin of the same composition which, at a aolid~ oontent of 660/o by weight, ha~ a viscosity of more thQn 50 poi~e~ Whore the re3:Ln has been condensed to a lee3er extent, ie equivalent to that of a re~in havin~ a vi~ooaity of less than 50 poise at 660/o by weiOEm 301id3, the amount of acid added chould be su~ficient to an3ure KeLation within 20 minute~.
It will be appreciated that 3imple experimentation will indicate how much acid is ne&ee3ary to obtain gelation in the extended form. ~he amount of acid required will generally be with-in the range 5 to 20% by wei~ht (expres3ed as 100% acid), based onthe welght of the re~in solids.
The aminopla~t resin produced by condensing the amino oompound and formaldehyde wiLl normally have a solids content in the range 30 to 50/0 by weight. ~orm~lly the re3in is concentrated 20 to a solids content within the range 55 to 7~/0 by weight, generally to above 60yO by weight. Omission of 3uch a~concentration ~tep presents 3torage problem3 owing to separation, on standing, of crystalline methylolated derivatives of the amino ¢ompound. Such cr~stalline derivative~ tend to be difficult to re~olvate. -However 25 where the re~in 19 used directly to produoe the pigme~t powder9 with no intermediate stora~e, such a conoentration 3tep i3 not e3sential.
~hu~ in order to enaure galation in the extended form the re~in is diluted, either with or witho~t an intermediate conoentration ~tap, to a ~olid~ content below about 25%, particularly to below 22/ol by weight. Insufficient dilution tends tc increa3e the aensity of the gelled resin ~a~ making the latter harder to com-minute into high area partioles. Also in~ufficient dilution tends to redu¢e the ~ur~ace area of the resultant particle~. If any given re~in i~ diluted ~y more than ~ certain exten-t, generally corresponding to a ~olid~ content below about lG% by weight~

~64313 precipitation of portions of the resin rapidly occurs. It is therefore preferred to dilute the resin to a solids content between 13 and 22% by weight. The dilution is preferably performed in stages by first diluting with water and then adding the appropriate amount of an aqueous solution of the acid to give the requisite concentration of acid catalyst and the requisite degree oE dilution.
Under these conditions, the resin system gels and polymerises in an extended form consisting of high-area particles lightly bonded together. During the gelation stage little, if any, aqueous material is expelled from the system;
this is unlike the normal gelation of aminoplast resins, where considerable syneresis can take place. The low density gelled mass (which may be in the form of a cast block) has then to be cured and disintegrated to form the pigment particles. Conveniently the cast block is first broken up into small pieces and dried, for example in an air oven at 50C. The dried gelled resin is then cured, typically by heating in an oven for 15 minutes to 4 hours at temperatures within the range 100 to 200C. Preferred curing conditions are 30 minutes to 2 hours at 110 to 140C. The cured resin mass typically has a density within the range 0.1 to 0.3g/cm3.
The cured resin is then comminuted to produce the pigment particles. The comminution, preferably by grinding or milling, is preferably conducted to produce particles having a size below 200 ~m, particularly below 100 ,um. The particles however preferably have a partic]e size above 5 ~m. The final grinding or milling may be done, advantageously, in water to produce a slurry suitable for addition to paper furnishes.

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3~3 The pigment particles may be used in paper made from mechanical and/or chemical ce,llulosic pulps but are of particular utility in papers based on chemical pulps, eg kraft or birch sulphate. The cellulose pulp may be used alone or in conjunction with synthetic fibrous materials such as polyolein fibres and urea-ormaldehyde fibres (for example as described in our German OLS Specification 2810299S.

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Paper compo~s;t;on~ nr~l~r~hl~r contain 1 to 25, particularly 2 to 15% by weight of the pigment particles.
The piqment powde~rs accordinc~ to the inven-tion are, particularly when used in conjunckion with multivalent ions such as A13~, Ti~, Fe3~, and Zr4 , readily retained on paper webs and improve opacity, colour, and in some instances the bulk.
The particles are substantive to cellulose and to other particulate matter in the paper furnish and so assist in the retention of other, for example inorganic, pi~ments and fillers. The strength of the paper web i5 less dimin-ished than with UF pigment particles unmodi~ied by the in-organic oxyacid radicals, and in some cases may be increased.
In the following examples all parts and percentages are expressed by weight.

Preparation of the modified resin 3560 parts of Eormalin (containing 36.6% formal-dehyde and 5.7% methanol) was mixed with 1303 parts of urea, to give a formaldehyde: urea molar ratio of 2:1 and warmed to 50C to effect dissolution. To this 5 parts of sodium sulphite Na~S03 and 186 parts of sodium metabisulphite Na2S205 were added, the pH adjusted to 9 and the mixture refluxed for 30 minutes. The pH was then reduced to 4.85 and the mixture refluxed for 45 minutes. The mixture was then cooled, neutralised and concentrated by vacu~m distillation to give a resin of solids content 65% having a viscosity of about 30 poise The resin contained about 4.6 sulphite radicals per 100 methylene radicals.

~;b ~Z6438 6~ -Preparation of gelled resin particles To 100 parts of the concentrated resin solution were added 255 parts of 3.33% phosphoric acid solution to give a solution having a resin solids content of about 18 and containing 13% H3PO~ based on the resin solids. The solution was poured into a casting mould wherein it gelled within about 9 minutes of the acid addition. The resin gelled in an extended form and was then .

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bro~en into lump~ which were dried overnight in an oven at 50C.
The dried resin wa~ then oured by heatin~ for 1 hour in an oven at 120 C and then ground in water to a partiole size below lO0 ~m.
Mioromeritio examination of the pigment ~howed that it had a surfaoe are of 12 m /g and a pore volume of 1.5 om3/g. ~he partiole den~ity was 0.47 g/om3 and the true density was 1.56 g/cm3.
A similar powder wa~ prepared in the ~ame way from a ~ re~in having a fo~maldehyde: urea molar ratio of 2:1 but unmodified by inorganio oxyaoid radioale. The two powder~ were u~ed a~ pigmen-t~ in paper hand~heet~ u~ing biroh ~ulphate pulp as the papermakin~ f D i~h.
Te~t of bur~t index, ~I, whioh i~ the bur~t pres~ure in K~m 2 divided by the ~ubstanoe in ~m ~ and bulk of the papers ga~e the following result~.
... _. _ __, _. _ _ _ - % partiole~ Partioles with Particle~ with no in paper ~ulphite radioals sulphite radicalc ~I ~ulk ~I
KNg-l cm g K~g-l cm3 g _ __ 0 3~45 1.46 3.45 1.46 ~54 1.60 2.99 1.73 _ _ _ 3.17 l.66 2-44 1.88 EX*MP E 2 Pigment partiolee were prepared a~ in example 1 but dilutin~ the oonoentrated sulpnite oontaining resin solution to a ~olido oont~nt of 15% by addition of 160 parts of water followed by 90 part~ of a 5% pho~phorio aoid solution. ~he amount of ~3P04 was thu~ about 7% of the re~ln ~olid~. ~e time to gelation was 18 minute~. --3xQxple 2 was repsated but the oonoentrated resin ~olution W2~ diluted direotly to the solid~ content of l~jo by addition of 250 part~ of the 5% pho~phoric aoid solution. ~he B ~ 30272/AA
amount of ~3P04 was thu~ about l~Yo based on the resin ~olids.
The time to ~elation was 5 minutes~
The cured, but unground, gelled reain masses producedin ~xamplea 1 to 3 eaoh had ~ density of about 0.2~ g/cm3.
50 part~ of a conoentrated aulphite modified reain solution produoed a~ in Example 1 but havin~ a solids content of 67% was mixed with 50 parts of a oonoentrated unmodified urea-fo~maldehyde re~in having a formaldehyde:urea molar ratio of 2:1 and a aolid~ content of 66%. ~he reaul-tant mixture of re~ina was thu0 equivalent to a sulphite modified resin containin3 about 2.3 sulphite radicala per 100 methylene radicals. The mixture was diluted by addition of 200 parta of water followed by 200 parta of a 5% phoflphoric acid solution to give a solution having a resin ~olids oontent of about 13yo and containing about 15% H ~04 based on the resin aolids. ~hi3 solution gelled in 10 minutea to give an extended mas~ which was dried and oured as in Exàmple 1 to give a mass of oured den~ity 0.14 g/om3.

Example 4 waa repeatad but diluting the mixed resin solution fir~t with 800 parts of water followed by 100 parts of the 5% pho~phorio acid ~olution. The final resin aolids content waa about 6,6% and, whlle the re~in gelledt it took 51 minutes to do ~o and the g~llad ma~B had a hard outer skin enclo~in~ a aoft damp coreO Satisfaotory pigment particle~ could ~ot be obtained therefrom.
EXrMP$~ 6 - Example 1 wa~ repeat6d ~ave that the cuxed sulphite modified resin ma~ wa~ ~ro~nd d~y to below 100 ~m a~d then further ~round in w~ter on a ball mill for 40 minute~. A ~lur~y of the pigment partiole~ waa made and added in varying amount~ to meohan-ioal pulp together with ~/0 aluminium sulphateO Papsr was made from the reaultant mixtures and the bur~t index and densi-ty of the papers flO formed were mea~ured. ~hie proceedure was repeated uaing as the oonoentrated resin .

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(a) the mix-ture of the unmodified re~in and the 3ulphite modified re3in a~ uaed in F~ample 4, (b) the unmodified re~in u~ed in Example 4.
The reault~ are shown in the table.

Compo~Itio~ ~ o~ p~per mol~ % S03" i-~ resin particles (ba~ed on -CH2-`

Partioles Pulp 0 2.3 4.6 % % SI denaity SI denslty BI density ENg-l ~ cm-3 K~g-l ~ cm~ 3 KNg-l g cm~3 _. _ _ . ~ _ _ 0 100 0.78 0.402 0.78 0.402 0.78 0.402 o~75 _ 0.78 0.409 0.87 o.414 _ 0. 70 _ 0.7l o. ~a9 o.77 o.400 Similar re~ults were obtained when the reains were cured for 30 minutsc at 120C instead of 1 hour.
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A urea-formaldehyde re~in modified by about 0.93 pho~phate radioal3 per 100 methylene radloal~ wa~ made a~ follow~:
650 part~ of fornalin (~6.4% fo~maldohyde, 5.~/o methanol, acidity 0.016%) were mixed with 237 part~ of urea (formaldehyde:
urea molar ratio 2:1) and brought to pX 7 with cau~tic soda solution.
25 11.6 part~ of ~odium dihydrogen phosphate (~aH2P04.2H20) were added and the mlxture hea~ed to 55 C, when further caustic 30aa was added to bring the pH to 6.05. The mixture wa~ refluxed for 30 minute3, acidified to pH 4.9 with formie aold 301ution and rsfluxed further for 50 mi~utee. The pH wa~ then ad~u3ted to 5.4 with c~u~tic soaa ~0 solution, cooled to 4QC and neutralised to pH 7 with more cau~tic ~oda ~olution~ The re~in wa~ con¢entratea by heating under vacuum to remove 285 pax~s of di~tillate.
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A ursa-formQldehyde re~in modified by about 0.93 pho3phite radi¢al3 per 100 meth~lene r~dical3 wa~ made as follow~:

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650 part~ of formalin and 237 part~ of urea, a~ used in oxample 7, were mixed with 16 part~ of disodlum hydrogan pho~phite (~a2H P03 . 5H20) and refluxed for 30 mlnutec. ~ormio aoid waa added to reduoe the pH to 4.85 and the mixture wao further refluxed for 44 minutes. Caustic ~oda wao then added to ohange the pH to 5.45, the reain wao oooled to 50C and ad~uoted to pH 6.95. rrhe re~in wa~ oonoentrated under vaouum, removin~
355 parto of distillate.
EXAMPL5_2 A urea formaldehyde resln modified by about 2.3 borate radioal~ per 100 methylene radioal~ wao made a~ follows:
650 parts of formalin, and 237 part~ of urea, a~ uæed in Example 7, were mixed and warmed to 40C. 2.3 part~ of borio aoid (H ~03) and 14.2 part8 of bor~x (Na2B407 lOH20) were di~-eolved in tha mlxture whioh wa~ then refluxed for 30 minutes atpH 8.05. Eormio acid wac added to bring the pH to 4.85 and -the mixture refluxed for a further 43 minutes. Caustio soda was added to modify the pH to 5.6, the ~olution cooled to 50C and finally adJu~ted with oaustio ~oda to p~ 7. r~he recin was oon-oentrated under vaouum to remove 345 parto of diotillate.
Pigment powders of hi~h ~urface area oould be mad~ bygelling tha resin~ of Example~ 7 - 9 in exte~ded form by tha method de~cribad in Example 1.

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Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the manufacture of particles suitable for use as pigments in paper making which comprises:
(A) reacting in an aqueous medium urea, formaldehyde and a source of inorganic oxyacid radicals selected from the group consisting of sulphite, phosphate, phosphite and borate radicals, said source being present in such a quantity that there are 0.2 to 15 moles, per 100 moles of formaldehyde, of said oxyacid radicals, to form an aqueous urea-formaldehyde resin solution;
(B) adding an aqueous acid to said resin solution, the amount of said acid being sufficient to cause said resin to gel in an extended form of a mass of high area particles lightly bonded together;
(C) drying the resultant gelled resin mass;
(D) curing said mass by heating at 100°C to 200°C;
and (E) comminuting said cured gelled resin mass to powder form.

2. A process according to Claim 1 in which the in-organic oxyacid radicals are sulphite radicals.

3. A process according to Claim 1 in which the source of inorganic oxyacid radicals is present in such a quantity that there are 0.5 to 10 moles of said inorganic oxyacid radicals per 100 moles of formaldehyde.

4. A process according to Claim 1 wherein the amount of aqueous acid added is sufficient to give an acidified resin solution having a resin solids content of less than 25% by weight.

5. A process according to Claim 4 wherein the acidified resin solution has a resin solids content between 13 and 22% by weight.

6. A process according to Claim 1 wherein the amount of acid added is sufficient to cause the resin to gel within 20 minutes.

7. A process according to Claim 6 wherein the amount of acid, expressed as 100% acid, added is between 5 and 20%
by weight of the resin solids.

8. A process according to Claim 1 wherein the cured gelled resin mass is comminuted to a particle size below 200 µm.

9. A process according to Claim 1 wherein the cured resin is comminuted by grinding in water.