CA1161593A - Resinous material - Google Patents

Abstract

ABSTRACT

The present invention provides fibres of urea formaldehyde, melamine formaldehyde, phenol formaldehyde, and mixtures thereof, modified by inorganic oxyacid radicals derived from inorganic oxyacids of sulphur, salts of such inorganic oxyacids, phosphate radicals, phosphite radicals, borate radicals and mixtures thereof.

Description

7~61593 B 29219 ~EIS INVE~I0~ RELATES to a resinous material and in particular to a formaldehyde resin such as ~F, M~ or PF in fibre form and containing inorganic oxyacid radical3. It relates aIso to the manufacture of fi~rous sheet material, in particular paper, containing such ma-terial in fibre form.
Such a resin in fibre form and containing inorganio oxyacid radicals is believed to be a new artiole of manufacture. It preferably oompri~es or oonsists of a urea-formaldehyde resin. ~he inorganio oxyaoid radicals are preferably derived from sulphur, which i~ oapable of forming a great many oxyaoids, and the radioals are oonveniently the sulphite ~nd/or sulphonio aoid radioal~. The proportion of suoh radicals is ~uitably in the range 0.2 to 15% by mols or CH2 derived from formaldehyde.
The radioal-oontaining resin fibre oan be made by applying a suitabIe inorganio oxy oompound to the fibres after th~y have been formed, espeoially before any ouring or harding treatment. More ¢on ve~iently the inorganic oxy oompound is incorporated in the oourse of re~in-formatio~, preferably at an early stage of incomplete - .
oondensation. The inorganio oxy compound may be added in the form of one or more inorganio oxyacids, as one or more salts thereof, particularly one or more acid 3alts thereof, or as a mixture of one or more inorganio oxyacids with one or more salt3 thereof. A ~uitable proportion of the radioals can be introduced by having present 0.5 to 20~, espeoially 1 to l~/o~ by weight of sodium sulphite and/or metabisulphite on the resin solid~ to be fo~med in the mixture, or equi~alent proportions of other inorganio oxyacids and/or their com-pound~ (for example, pho~phate, phosphite, borate compounds). If desired, a formaldeh~de bi~ulphite or sulphoxylate oompound can be used. The re~in forming composieion preferably contain3 su~fioiont aoid, addition~l if neoes~ary to the inorganio aoid, to promote ouring. For example, 0.2 to 5.0% W/w of formio aoid or its equivalent ' `- ~ 2921~
1 1 ~1593 .

i9 suitable; ammonium salts of phosphoric, sulphuric and formic acia, and aluminium sulph2te, are also suitable.
The resin-formin3 composition is formed, while still flowable, into fibre~. This can be suitably done by conventional spinning of a viscous reain syrup into hot air ("dry-sp;nning") or into an acid bath ("wet-spinning"). Alternatively it can be formed by passing a fine stream or series of drops into a flowing resin-gellin~ liquid or by gas-fibrillation (in particular air-fibrillation) by me2ns of a co-current or transverse gas stream: the latter procedure conatitutes a further feature of the invention. As a ~urther alternative it can be spun by tack-spinning, by pullinæ a fiberis2ble material between two surfaces to which it adheres and subsequently severing the fibres from one or both of the surfacesO ~or e:cample, as described in ~K patent 1141207, tha resin may be moved into oontact with a pair of belt surfaces so as to deposit it therebetween, whereafter the surfaces of the belts are moved apart to form fibres and stretch them, and the fibres are detached and collected. In another suitable tack-spinning process fiberisable material is interposed between a porous surface and a second surface, the surfaces are caused to diverge so a~ to string fibres between them, the fibres are stabilised or sol-idified at least in part by fluid directed into or through the fibre-forming area from the opposite side of the porous surface to that on which the fibres are formed, and the fibres are separated at least from the 3eoond surface.
After formation, the fibres are normally fully cured by heat;ng at between 100 C and 200 C. I-t is, however, within the scope of the invention to effect a partial cure at this stage, especially if the fibrea are to be used in a process in which it is desirable that ~ l 61593 th~y should show chemical reaotivity.
Fibres acoording to the invention are suitable for paper making, especially when their a~erage diameter does not exceed 25 microns (1 mi¢ron i8 usually the smallest practicable diameter) and their average len~th is at least 1 mm, preferably over 2.5 mm. They can be the sole or major fibrous component of the paper or can be in blends (1-99yo by weight) with other fibres suoh as formaldehyde resin fibres containing no inorganio oxyacid radicals and/or with oellulose fibres ~uch as oonventional paper pulp and/or rag stock and/or ;10 other synthetlc polymer fibres and/or inorganic~fibres. The fibres ao¢ording to the invention adhere well to one another and also to other paper-makin~ fibres. This makes possible a gradation of paper prop-erties, from the low stiffness, high bulk papers when little or no fibre according to the invention is present, to stiff papers. A
particularly useful paper contains 1G-3C% W/w of long fibre (over 2 mm) according to the in~ention with short-fibre (under 2 mm) mechanical oellulose pulp and optionally 5 to 20Yo W/w of oxyacid-free urea-for~-aldehyde re~in fibre. When the paper contains cellulose fibre and 5 to 50~ W/w of fibre acoording to the invention, its water~resistanoe (in whioh the sense of wet-~trength or, depending on its composition, resistanoe to wetting~ is usefully greater than that of a corresponding all-oellulose paper.
Paper containing fibre according to the invention can contain con-vantio~al fi}lers, sizing agents9 oolourin~ matters and oan oar~y the usual coatings and be laminated with other sheet materials. Preferably such paper oontains one or mora mNltivalent cations for e~ample of iron, zirconium, caloium or (especially) aluminium, at a concentration in the ranB~ 0.5 to 5~/o W/w calculated as equivalent aluminium sulphate on the fibre~. Such cations are con~eniently introduced as soluble ; 30 e~lt~ in the paper-making pulp. Generally it appears that less sizing ~ 29219 ~ 3 ~ 3 agent i~ needed using the fibre~ aooor~ing to the invention than in all-¢ellulose papers.
~he method of making suoh papers typically comprise3 mixing the fibres with water and any other ingredients as set out hereinbefore, bringin~ the mixture to a sufficiently homogenesus state Por the grade of paper to be produoed, forming the mixture into the shape required and removing water from the shape. ~he mixing, homogenisation and water-removal can be carried out in machinery as used for dealing with cellulo~e pulp. ~he paper shapes are normally heated at over 60 C
to remove water, and their cohesion can be improved, especially when an inorganic multivalent cation such as aluminium is present, by heating at over 90 C, for example 110-140 C. If the paper is in eheet form, suoh heating oan be oarried out using the u~ual steam-heated oalender rolls. The adhesion obtained between the fibres will oPten be improved by en~uring a good degree of oompaotion of the paper sheet~ e.g. by rolling at the wet stage.
In the gas fibrillation prooess aooording to the invention a oon-tinuous stream of liquid resin (suoh a~ a ~F syxup oontaining 50-80yo W/w of solids in water) is direoted into the path of tran~versely-flowing or oo-currently-flowing ~as. The gas may issue from a nozzle in the throat of whioh its velooity may be as high as sonio velooity. ~he gas flow is preferably about perpendicular to the resin flow. Suitably the gas is air.
~- ~y way of example, if the resin stream in the zone of ga~ impineement is 0.1 to 2.0 mm in diameter and the resin visoosity is 5-100 (preferably 10-50) poise, the thread ~plit~ into fine fibres of diameter in the ran~e 2-30 miorons. ~he nozzle should be as olo~e to the resin stream as po~ible in order to mRximise oonver~ion to fibre. ~he fibre thioknesa is le~s the greater the air supply pressure and flow rate.
Above a oertain pressure, for given resin oharaoteristio~ and ~tream S

~ 29219 ~ ~1 61~93 diameter, the fibres become not thinner but shorter. A suita~le resin flow rate is 5-30 g/min with an air flow rate of 100-600 SC~H at air pres~ures of 10 80 psi~O ~he air nozzle can be ciroular3 rectangular, or of any ~hape that impinges upon the resin stream in a stable manner. It ha~ been found that good quality fibre with the minimu~
amount of ~hort results if the resin stream flows steadily and without breaking. A plurality Or resin streams oan be direoted into a ~ingle ga~ flow.
~he present invention accordingly provides a formaldehyde resin ln fibre form and oontaining inorganio oxyacid radicals.
The invention also provides a fo~maldehyde resin in fibre form and oontainin~ inorganic oxyacid radicals, wh~ch comprises the ~teps of formin~ a ¢oncentrated flowable resin having a viscosity in the range 5 to 100 poise, comprising urea formaldehyde, melamine formaldehyde, and/or phenol formaldehyde, oonverting the concentrated flowable resin into fibres havin6 therein or thereon inorganic oxyacid radicals, and curing the fibres.
The invention further prcvides fibrou~ material, in particular paper, whereof the fibres comprise a formaldehyde resin in fibres form and oontaining inor~anic oxyaoid radioals.
:;~ ~E 1 3267 ml of formalin (36.5% HC~O, 6.5% CH30E) was adjusted to pH
7.2 with NaOE solution, and 1303~ of urea, 20g of Na2S03 . 7H20 and 186~ Na2S205 were added (pH was then 9.1). The mixture was refluxed for 30 minutes, cooled to 70 C, acidified with formic acid to pH
4,8, and returned to reflux for one hour. It wa3 brou~ht to pH 5.8 by adding ~aOH, cooled to 50 C, and finally ad~usted to 7025 by the a~dition of further NaOH. ~he mixture was conoe~trated by vacuum ~0 evaporatio~ to ~ive a re~in ~olution of visoo~ity 21 poise at 29 C, ~ ~ 61~g3 and solids content about 75% W/w. ~ iB suitable for oonver~ion to fibres by ~ny of the methods de~cribed hereinbefore.
EXAl~IiE ?

(a) A ~ resin, of FslJ ratio 1.95 and solida content about 67% was adju~ted to a viscosity of 22 poi~e at 23 C by adding water and

2.5B~ of a 4C96 W/w solution of diammonium hydrogen phosphate, as a curing agent, for every 200g of resin. The final solid~ oontent was about 61% W/w. This was forced by air pxessure downwards throu~h an orifice at a rate of 21g/min and blown by a horizontal air jet from a rectan~ular nozzle (dimensions lmm x 8mm), u~ing an air pre~ure of 50 psig and a flow rate of 500 SC~EIo ~his produ¢ed a fine fibre whioh was blown into a chamber heated to 50 C. I!he fibre was ¢ollected, further dried at 60 C and cured by heatin~ at 120 C.
When di~per~ed and di~integrated in water as for paper-making, the ~ibres were 1-2 mm long and about 10 miorons in mean diameter.
(b,o) ~ibres were ~imilarly produced from the sulphite-modifiea re~in of l~bcample 1 and from a blend of equ~l amounts of the sulphite-modified resin and the ~ resin of F:~ ratio 1.95 of para~raph (a).
(d) In a further experiment the re~in of (a) was adju~ted to a visoosity of 32 poise and disoharged from an orifice at a rate of 12g/min~ A
4C~6 solution of ammonium sulphate was continuously mixed into the ~yetem at a rate of 2.5 g of solution per 200g or resin. ~he ~me sir nozzle wa~ used but at an air pres~ure of 30 p3ig (air rate 300 SCF~). A fibre was prepared whioh, whe2l di~integrated and disper~ed in water, wa~ on average about ~nm lon~ and of diameter about 15 mioron~.
.~:rE,,~
~e~
~hree ~ fibres were used, each fibrillated acoording to the ~ 29219 invention:
A a~ in paragraph a of Example 2 (sulphite-free);
a~ in paragraph b of E~ample 2 (all sulphite-containing);
C as in paragraph o of Example 2 (blend of sulphite-free and sulphite-containin~ resins) In addition the following cellulo~e fibres were u~ed:
D blea¢hed Kraft pulp, beaten to a Canadian Standard ~reenes~
of about 400.
Eaoh fibre was disintegrated in a standara manner using 2000 counter revolutions of a 3tandard laboratory pulp disinte~rator, the~ converted to paper of substance lO0 g/m2 on a standard pulp evaluation apparatus.
A paper made from fibre A oould just be removed from the wire screen of the apparatus but fell apart on dryin~.
Papers made from ~ and C could be handled more easily and when dried on a heated drum at 120 C oohered well to~ether.
A paper was prepared from C using, in the paper-making mixture, z~ W/w of A12(S04)3 based on the weight of fibre. ~his formed well on the wire, and was dxiad initially by pres~ing between absorbent ; ~ 20 sheets9 and finally in an oven at ~0 C. When tested it showed a bur~t index of 0.40 KN~ 1 (burst pressure in KNm ? divided by the ~ubstanoe in g m 2).
A mixture of equal amount~ of fibres ~ and D were formed into a series of papers; in ~ome of the samples 2% W/w of A12(S04)3 (ba~ed on the weight of fibre) wa~ added during disinte~ration of of the pulp. Stron~, stiff paper~ were obtained after drying. ~ho~e with A12(S04)3 added were partioularly stiff, eapeoially when dried at 120 C. ~y comparison, blend~ of A and D were limp, and lo~t fibre easily when the surfaoe was rubbed.
Papers prepared from blend~ of B and D were not readily wetted ~ 29219 by water, whereaa paper~ prepared from D alone absorbed a drop of water placed on the surface almost immediatel~.

A resin was prepared using the components desoribed in Example 1. ~he a¢idic conden~ation stage was performed at a pH of 4.8 for 4B minutes and the final p~ was 7.2. After vacuum concentration a resin of approximate solids content 8~/o and ~isoosity 45 poise at 23 C
was obtained.
A blend o~ equal parts of thi~ resin with a oon~entional ~F
re~in of ~:U ratio 1.95~ olids oontent 67% (as u~ed in Fxample 2(a)) was fibrillated using the prooedure of Example 2ta). Inoorporated into the resin mixture before spinning was 1.25% of an aqueous solution of a oatalyst. The catalyst solution, and the ouring sohedule after spinning, was varied as follows:
E catalyst : 4~/o diammonium pho~phate ouring time 4 hours at 120 C
F catalyst : 40% aluminum sulphate curing time 1 hour ar 120 C
G catalyst : 40yo formic acid curing time 7 hours at 120 C
~he fibres 80 produoed were of mean diameter 10 ~. ~hey were broken down by u~ing a laboratory disintegrator to a length of about 2mm, and inoorporated into paper~ (a) alone and (b) ln admixture with a mechanically ground wood pulp, using the standard procedure for making and testing paper hand sheets. ~luminium- sulphate (2% W/w on the fibre) was added. ~urst indices were measured on the papers, whioh had a ~ubstance of about 60g/m2 Fibre ~0 80 E 0.28 0.56 0.92 ) ~ur~t index ~ 0.17 0~75 0.88 ) KNm~2/ gm-2 G 0.12 0.38 0.67 ~s a comparison, sheets of paper were made using fibres produced ~ 29219 7~61~93 from a sulphite-free resin (see Example 2(a)) and meohRnical wood pulp. ~urst indices were as follows:
% W/w Mecharnical Pulp }ur~t I~d~x o.63 100 0.82 The examples ~how the superiority of the fibres of the present invention over unmodified ~F fibres, and that the fibres oan be used to improve the burst index of machanioal pulp.
E~ ~
~he sulphite-modified resin aa prepared in Example 4, and the unmodified reain as described in Example 2(a), were blended together in various proportions to produce a range of re~ins of different levels of sulphite-modifioation. ~hese were adjusted to a viscosity of 30 poise and spun into fibre usin~ the procedure of Example2 (d), except that the oatalyst solution used was a 40% solution in water of d diammonium phosphate. Eibres of mean diameter 10 ~ were produoed.
Papers were made as before from the fibres and from mixtures of the fibres with meohanical p~lp. Aluminium sulphate, 2% W/w on the fibres, wa~
added. ~he fibres and the properties of the papers are described below.
- FIBRES
5 parts ~ulphite resin : 95 parts ~F resin (About 0~5yo S03, ba~ed on ori~inal weight of reactants) I 15 part~ sulphite re~in : 85 parts ~F resin (~bout 1.5% S03-) J 25 parts sulphite resin : 75 part~ ~F re~in (About 2.5% S023-) K 35 parts sulphite re~in : 65 part~ ~F resin (~bout

3 3.5% S023-) , ` ~ 29219 ~159~

Paper Prope ties ' 100/o H : soft adhe3io~
80~, Me¢hanical pulp, 20% ~ : ~urst Index o.69 100~ soft adhe~ion 8~/o Me¢hani¢al pulp, 20Y I: ~urst Inde2 o.67 100/o J : soft adhesion 80~ Mechanioal pulp, 20/o J : Burst Index 0.70 5C% n ~ 50/o J: Bur~t Index 0.41 lOGyo K : firm adhe~ion 80Y Mechanical pulp, 2C% K: Burst IndexØ79 5~% ~ 5ao/ K : BNrst Index 0.47 A mixture of e~ual parts of a sulph1te-containing resin and a conventional UF resin, as used in Example 4, was adjusted in visco3ity to 35 poise. Eibres were prepared from this m~terial by centrifugal spinning as described in our Canadian Patent No. 111~974, issued March 2, 1982-l(Detailed ¢onditione : 3" rotor, speed

4,500 rpm, flow rate of resin 60 mls/min, temperature of surrounding air 75 C). Catalyst (1.25% of 4C% diammonium sulphate) was fed into the resin, and mixed, via a stati¢ mixer, continuously as the res~
was supplied to the.spinning disc. Long fib~es of diameter 12~
(avera~e) were produ¢ed. ~hese were ¢ured by heating at 120 C for four hourst and broken to len~ths less than 5mm before use.
Paper hand sheets were prepared from the fibre~ incorporating wit~
the diaintegrated fibres in water variou~ metalli¢ salts (all at a le~el of 2~o based on the amount of fibre). ~he papers were well ¢ompaoted while wet, pressed a¢cording to the Briti~h standard pro¢edure, d dried at 70 C. ~urst Indi¢es were measured as follows:
Added Salt Burst Index A12(S04)3 0.5~

~ 11 ~ 2g219 Caloiu~ formate 0.28 Ferrl¢ sulphate o.46 Zino nitraté 0.27 ~ EBA~MPLE 7 A re~in was prepared u~in~ the oomponents of Example l;
the aoidio condensation stage was performed at pH 4~9 for ~8 minutes. Urea wao added (208 parts) to brinOE the final F~
ratio to 1.72~ his was blended with a oonventional ~F resin of ~olids content 67% and F:~ ratio 1.7~ he mlxture waB ad~usted to a vi~cosity of 35 poi~e and oentri~ug~lly ~pun a~ in ~xample 6.
~he oataly~t used wa~ 6% of a 30% ~olution of diammonium pho~phate similar ~ibres were produoed, and reduoed to le~s than 6mm in length after ourin~ at 120 for 4 hours.
~ome of the fibres were made into paper as de~oribed previously (Example 6) to yield a paper with a bur~ ind~x of 0.51. ~he paper had oon~iderable strength when wetted by water; a bur~t index of 0.15 wa~ mea~ured on the wet paper.
A blend of the fibres with an equal amount of meohanioal pulp gave a paper with a burst index of 0.85.
~ ~lends of the fibre with Kraft pulp were oompared, as paper handsheets, with blends using u~modified ~F fibre~ similarly : ~ produced, Mea~ured burst indioes were as follows: .
Paper ` - ~ur~t Ind~ (ENm /gm : 25 8~ Kraft, 20~ unmodified ~F fibre~ 3.36 90% Eraft, loY unmodified ~F fibres 4.49 80yo ~ra~t, 20~ modified ~F fibres, acoording to invention 4.04 90% Kraft, 10% modified ~F fibre~, aocording to invention 5.03 ~1615g3 ~ 29129 In a further experiment to illustrata the use of the fibres of the present invention, a paper was preparea usin~ 30yo of modified nF
; fibre~, as p~epared above, 2~/o of unmodified UF fibres, aB used in the above experiment, and 5~/0 groundwood pulp. An excellent paper sample of burst index 0.68 KNm 2/&m 2 was obtained.
~X~MPLE 8 650 part~ by weight of formalin (36.4% formaldehyde, 5.7% methanol, acidity 0.015~) were mixed with 237 part~ of urea (formalde~yde :
urea ratio 2:1) and brou~ht to p~ 7 with caustic soda ~olution. 11.6 parts of Na~2P04.2H20 were added a~d the mixture heated to 55 C, when further NaOH solution was added to brin~ the p~ to 6.o5. ~he m~xture was refluxed for 30 minutes, acidified to pH 4.9 with formic ~oid solution and refluxed further for 50 minute~. ~he pH was then ad~usted to 5.4 with ¢austio soda solution, cooled to 40 C a~d ne~tralised to pE 7 with more NaOH solution. The re~in was conGentrated by heatin~ under vacuum to remove 285 parts of distillate.
~he resin was converted to fibres by using the centrifugal ~pinnin~
teohnique des¢ribed in Example 6, except that in thi~ case the cataly~t u~ed was a 30% solution of phosphorio acid. The fibres were oured at 120 C for 4 hours.
Paper handsheets were prepared as before from the di~inte~rated fibre (fibre length about 5mm, diameter 12~), and from mixtures of the fibre with ~roundwood pulp. ~he bur~t indices o~ the paper samples were a~ follows:
Groundwood ~urst Index 0 0.20 9 0.90 ~he fibres ~howed good self-adhe~ive properties.
3 ~a~E_~

1161593 ~ 29219 650 part~ of formalin and 237 part~ of urea, a~ used in the pravious example, were mixed with 16.0 part~ of ~a2EIl?03 . 5E20 and refluxed for 30 minutes. Fo~Dio aoid uas added to redu¢e the p~[ to 4.85 and the mixture was ~urther refluxed for 44 minute~
Caustio soda was then added to ohange the pH to 5.45, tha resin was oooled to 50 C and adjusted to pH 6.95. ~he ra~i~ waa oonoentrated under vaouum, removin~3~ 355 parts of distillate.
A blend of equal parts of this re~in with a oomre~tional ~F
resin (Formaldehyde: urea ratio l.g5:1) was centrifugally 3pUll as desoribad in Example 6. Fibres as produced were of me~
diameter 12 ~ a~ter ouring for 4 hours at 120 C. llfter reduoing in lell~th to about 4 mm, papers were prepared, as before, and demonstrated that the fibres showed the proparty of self adhesion.
Mixtures of the fibre~ with groundwood gave papers w1th the following propertias.
~9~ Bur3t Ihdex 0.57 o.7 0.87-EoU~P~E lo 650 parts of formalin, and 237 parts of urea, as u~ad in E5xample g, were mixed and warmed to 40 C. 2.3 part~ of H3Bo4 and 14.2 parts of l~a2B207 . 10 H20 were dissolved in the mixture which wa~
then rafluxed for 30 minutes (pH 8.05). FormiG aoid was added to bring the pH to 4.85 a~d the mixture ref~xed for a further 43 minute~.
Cau~tio soda solution wa~ added to aohieve a pH Or 5.6, the ~olution wa~ oooled to 50 C and flnally ad~u~tad (with oauetio soda solution) to pH 7Ø I~e resin was oonoentrated lmder vaouu~ to remove 345 partg of di~tillate.
~ Fibre~ wer~ oentrifugally ~pun a~ in Exas~ple 6. ~i!ho ¢atalyet ~ 29219 solu-tion used was 30yo pho~phorio acid. The fibre~ were cured at 120 C for an hour and shortened to a length of about 3mm. Papers were made as in Example 9. A paper containing lOO~o borate modified ~F fibre was easily formed and ~howed ~ood ~elf-adhesion. Papers containing groundwood pulp had the following burst indioea.
~o Groundwood~ Borate modified fibre ~ur~t_Index

5 5 0.75 0.85 0.89 PA
8 ~ovember 1977

Claims (8)

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

1. An amino-formaldehyde resin selected from the group consisting of urea-formaldehyde, melamine-formalde-hyde and mixtures thereof, the said resin being in fiber form and containing 0.2 to 15 moles of sulphite radicals per 100 moles of -CH2- derived from formaldehyde.

2. A wet-laid paper in which at least 1% by weight of the fibrous constituents comprise urea-formalde-hyde resin fibres made from a modified urea-formaldehyde resin, alone or in admixture with an unmodified urea-formaldehyde resin, said modified urea-formaldehyde resin being made by reacting urea and formaldehyde in the presence of a source of inorganic oxyacid radicals selected from the group consisting of sulphite, phosphate, phosphite, and borate radicals, the amount of said source being such that there are 0.2 to 15 of said inorganic oxyacid radicals per 100 methylene radicals in said urea-formaldehyde resin fibres.

3. A wet-laid paper according to Claim 2 wherein the urea-formaldehyde resin fibres have an average diameter of between 1 and 25 µm.

4. A wet-laid paper according to Claim 2 wherein the urea-formaldehyde resin fibres have a length of less than 6 mm and an average length of at least 1 mm.

5. A wet-laid paper according to Claim 2 wherein the modified urea-formaldehyde resin is made by reacting the urea and formaldehyde in the presence of 0.5 to 20%
by weight, based on the weight of the resin solids formed from said urea and formaldehyde, of a compound selected from at least one of sodium sulphite and sodium metabi-sulphite.

6. A wet-laid paper according to Claim 2 wherein the fibrous constituents comprise cellulose fibre in admixture with 5 to 50% by weight, based on the weight of said fibrous constituents, of the inorganic oxyacid radical containing urea-formaldehyde resin fibres.

7. A method of making paper comprising forming a mixture of discontinuous fibres with water, forming the mixture into sheet form and removing the water, wherein at least 1% by weight of the fibres are urea-formaldehyde resin fibres made from a modified urea-formaldehyde resin, alone or in admixture with an unmodified urea-formaldehyde resin, said modified urea-formaldehyde resin being made by reacting urea and formaldehyde in the presence of a source of inorganic oxyacid radicals selected from the group con-sisting of sulphite, phosphate, phosphite, and borate radicals, the amount of said source being such that there are 0.2 to 15 inorganic oxyacid radicals per 100 methylene radicals in said urea-formaldehyde resin fibres.

8. A method of making paper comprising a) forming a modified urea-formaldehyde resin by reacting urea and formaldehyde in the presence of a source of inorganic oxyacid radicals selected from the group consisting of sulphite, phosphate, phosphite, and borate radicals, b) forming said modified urea-formaldehyde resin, alone or in admixture with an un-modified urea-formaldehyde resin, into discontinuous inorganic oxyacid radical containing fibres having an average length of at least 1 mm and an average diameter between 1 and 25 µm, the amount of said source of inorganic oxyacid radicals being such that said urea-formaldehyde resin fibres contain 0.2 to 15 of said inorganic oxyacid radicals per 100 methylene units, c) mixing said inorganic oxyacid radical con-taining fibres with water and optionally other discontinuous paper making fibres to form a slurry so that the inorganic oxyacid radical containing fibres constitute at least 1% by weight of the fibrous material in said slurry, d) forming said slurry into sheet form, and e) removing the water therefrom.