CA1279159C - Lightweight paper and process for producing same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A lightweight paper having a superior opacity and printing opacity is provided, which lightweight paper comprises dry pulp, 0.015 to 1.2% by weight based on the pulp of an alumina polymer and 0.5 to 30X by weight based on the pulp of a coaggregate formed from agglomeratd particles of urea-formal-dehyde polymer (A) and agglomerated particles of hydrated silicic acid (B) in a ratio of (A):(B) of 5:95 to 95:5.

Description

1~79~59 TITLE OF THE INVENTION
Lightweight Paper and Process for Producing Same BACKCROUND OF THE INVENTION
1. Eield of the Invention This in~ention relates to a lightweight paper com-prising a coaggregate formed from agglomerated particles of urea-formaldehyde polymer and agglomerated particles of hydrated silicic acid, and a process for producing the same.

2. Description of Related Art Making printing paper and newspaper roll, light-weight has been conducted by making paper thinner, but there mag occur an obstacle of the so-called print through. Such print through includes show through due to reduction in the opacity of paper (hereinafter referred to as "opacity") and the so-called strike through that printed ink runs through paper and it is seen from its back surface (hereinafter the extent to which it is pre~ented will be referred to "printing opacity"). Either of the abo~e physical properties refer to a phenomenon that Print on the back surface is seen from the front surface to make reading of print on the front surface difficult; hence such physical properties are most important at the time of making lightweight paper.
Agglomerated particles of hydrated silicic acid are called white carbon and ha~e been used for pre~enting the print through (e.g. U.S. Patents 4132806, 4161455, 4157920 and 4202813) but they ha~e almost no effect of impro~ing the opacity, and moreo~er, the percentage fi~ation thereof ' ~79~9 relati~e to pulp at the time of paper-making is so weak that the retention thereof is low.
Further, agglomerated particles of urea-formaldehyde polymer ha~e been used for impro~ing the brightness of paper and the opacitY (e.g. see U.S. Patent No. 3909348), but the effect of impro~ing the printing opacit~ is still insufficient although the agglomerates of urea-formaldehyde polymer has a stronger fi~ability onto pulp than that of ~hite carbon and hence the percentage retention is higher.
SUMMARY OF THE INYENTION
The obiect of the present in~ention is to proYide a lightweight paper ha~ing superior physical properties mainly brought about by the function of highly impro~eing its printing opacity, while retaining the function of impro~ing the opacity and the high fi~ability, each of agglomerated particles of urea-formaldehyde polymer.
The present inventio~ in a-first aspect resides in a lightweight paper comprising dry pulp, 0.015 to 1.2~ by weight based on the pulp of an alumina polymer and 0.5 to 30% by weight based on the pulp of a coaggregate formed from agglomerated particles of urea-formaldehyde pol~mer (A) and agglomerated particles of hydrated silicic acid ~B) in a ratio of (A):(B) of 5:95 to 95:5.
The present inYentiOn in a second aspect resides in a process for producing a lightweight paper, which process comprises adding into a pulp slurry, an aluminium salt jD
a quantity required for forming 0.015 to 1.2% by weight based on drY pulp, of an alumina polymer and further adding 12~9~L~i9 agglomerated particles of urea-formaldeh~de polymer (A) and agglomerated particles of hydrated silicic acid (B) in qqanti-ties required for giYing a weight ratio uf (A):(B) of 5:95 to 95:5 and 0.5 to 30% by ~eight based on drY pulp, of (A) and (B), followed by paper-making.
DETAILED DESCRIPTION ~F PREFERRED EMBODI~ENTS
The agglomerated particles of urea-formaldehyde polymer used in the present in~ention are preferred to ha~e an a~erage particle diameter of 0.1 to 0.5 ~ and an aYerage diameter of the agglomerated particles of 1 to 15 ~.
If the average particle diameter is less than 0.1 ~ , the resulting agglomerated particles haYe a lo~ strength, so that when they are used in the process of paper-making, they collapse due to the pressure applied to the Paper during the process of paper-making to make insufficient the objecti~e function of impro~ing the printing opacit~. ~n the other hand, if the a~erage particle diameter exceeds 0.5 ~ , the resulting coaggregate of associated particles of urea-formal-deh~de polymer, white carbon and the alumina polymer ~herein-after abbreYiated to "teraggregaten) has a lo~ percentage retention in the resulting paper, and also converted paper after printing has a low opacity. The more preferable a~erage particle diameter is in the range of 0.12 to 0.3 ~
and such a range is usually employed. Further if the aYerage diameter of the agglomerated particles is less than l ~, the resulting teraggre~ate has a low retention in the result-ing paper, so that con~erted paper after printing has a low opacity. If the a~erage diameter of the agglomerated particles 3L~7 9~L5 9 exceeds 15 ~ , the resulting teraggregate has a high Fercentage fi~ation to paper, but it has a low dispensi-bility in paper so that the printing opacity, the opacity, etc. Iower. Further, the more preferable range of the a~erage diameter of agglomerated particles is in the range of 2 to 10 ~ and such a range is usuall~ employed.
The aboYe-mentioned agglomerated particles of urea-formaldehyde pol~mer are easily prepared according to a known, optional method. For e~ample, the agglomerated particles are obtained by uay of either a one-step process or a two-step process, and in either of the processes, the polymer particles are prepared so as to ha~e an optional ratio by mol of urea to formaldehyde. In more detail, in the case of the two-step process, a ~ater-soluble urea-formaldehyde condensate is firstly formed, follloued by curing the ~ater-soluble, initial condensate in the presence of a curing catal~st at an elevated temperature to thereb~
form agglomerated particles of the pol~mer.
~ urther, in the case of a one-step process, all components and additi~es used for the reaction are first added, and the reaction proceeds directly till the agglome-rated particles of the polymer are formed. In the respective cases, the resulting agglomerated particles of urea-formal-deh~de polymer is neutralized and uashed with uater to remove free formaldehyde, or before the neutralization, urea, ammonia, an ammonium salt, sulfurous acid or a sulfite salt is added and reacted to remoYe free formaldehyde, folloued by neutralization, and thereafter filtration or centrifugal . .

1 2~ ;9 separation to reco~er agglomerated particles of urea-formal-dehyde polymer in the form of a cake, or drYing the particles in a conYentional manner such as spray drYing, further air drYing and other contact and con~ectional drYing, etc. In the case where the agglomerated particles of urea-formal-dehyde polymer are used in the form of a cake or used in the form of a slurry by redispersing the cake, in ~ater milling is carried out before the agglomerated particles are made into a cake-form, to adjust the a~erage diameter of the agglo-merated particles preferably to 2 to 10 ~. ~urther in the case where the agglomerated particles of urea-formaldehyde polymer are obtained in a dry state, milling is carried out after drying to adjust the a~erage diameter of the agglome-rated particles preferably to 2 to 10 ~. The filtrate obtained by the abo~e-mentioned filtration or centrifugal separation is used as water for raw materials or water for adjustment at the prior step.
The curing catalyst usable in the preparation of the agglomerated particles of urea-formaldehyde polymer includes acidic catalysts, for esample, a mineral acid such as sulfuric acid, phosphoric acid, hydrochloric acid, nitric acid, organic acids ha~ing medium PK ~alues less than 4 such as fnrmic acid, o~alic acid, maleic acid, succinic acid, chloroacetic acid or the like acid. Further, sulfamic acid or a water soluble ammonium hydrogensulfate of the formula RNH3S04H twherein R represents hydrogen, alkyl group, cycloalkyl group, hydrosyalkYl group, aralkyl group, aryl group, etc.) may be also used. The water-soluble ammonium ~L~79~L~9 hydrogen sulfate refers to methylammunium hydrogensulfate, ethylammonium hydrogensulfate, hydrogyethylammonium hydrogen-sulfate, phen~la~monium hydrogensulfate, benzylammonium hydrogensulfate, etc.
In order to form particles ha~ing a preferable particle diameter in the production of the agglomerated particles o~ urea-formaldehyde pol~mer, it is adqantageous to add a water-soluble, organic high molecular weight polymer ha7ing a function of a protectiqe colloid, to a water-soluble, initial urea-formaldehYde condensate in ad~ance of forming Particles of a mixed aqueous solution of urea and formaldehyde. The water-soluble, organic high molecular weight polymer having a function of a protecti~e colloid referted to herein means natural substances such as starch, gelatin, hide glue, tragacanth gum, agar, acacia gum, etc., modified natural substances such as alkali metal salts e.g.
sodium salts, potassium salts, etc. of carbo~ymethyl cellulose, carbo~yethyl cellulose, etc., alkali metal salts of methyl cellulose, ethyl cellulose, ~ -hydro~yethyl cellulose, alginic acid, etc., polyqinyl alcohol, polyqinyl pyrohidone, pol~acr~lic acid, polymethacrylic acid and alkali metal salts thereof, copolymers of maleic acid with styrene or butqlene and salts thereof, salts of homopol~mer or copolymers of qinylpyridine, etc. The quantity of the protectiqe colloid agent used is generally in the range of about 0.1 to 10% by weight (hereinafter ~ means ~ by weight), preferably 0.5 to 5X based on the weight of urea and formaldehyde, although it dePends on its kind.

~L279~59 Nest, an advantageous process for producing the agglomerated particles of urea-formaldehgde ~ill be described in detail.
Usually, a water-soluble, initial urea-formaldehyde condensate in a molar ratio of 1:1 to 1:2 is used as an intermediate raw material, which is obtained by reacting an aqueous solution of urea, formaldehYde and additives having a total concentration of about 20 to 75%, at a temperature of about 30 to 100C, at a pH of about 5 to 9 and for lO
minutes to 4 hours. As the protective colloid agent, poly-vinyl alcohol or sodium salt of carbosymethyl cellulose is used and this may be added at an optional time during the preparation of the water-soluble, iDitial urea-formal-dehyde condensate. As the subsequent step, to th0 initial condensate containing the protective colloid agent is added a solution of sulfuric acid or sulfamic acid at a temperature of room temperature to about lOO~C, uith stirring, till gelation occurs, follo~ed by roughl~ grinding the resulting agglomerated particles into those ha~ing a diameter of 1 to 2 mm by means of a pelletizer or a hammer mill, thereafter adding water ~ith stirring to obtain a slurry having a con-centration of the agglomerated particles of urea-formaldebyde polymer of 5 to 10%, successively neutralizing the resulting material with aqueous ammonia or an aqueous solution of an alkali such as sodium h~droside, milling the resulting material into agglomerated particles having a diameter of 2 to 10 ~ by means of a mill and dehydrating by means of a filtration-dehydrator to obtain cake-form, agglomerated ~27g~5~

particles of urea-Pormaldehqde po]~mer.
The agglomerated particles of hydrated silicic acid used in combination with the above agglomerated particles and others are preferred to ha~e a BET specific surface area (measured according to Brunauer Emette and Teller's method) of IO0 to 300 m2~g. If the BET specific surface area is less than 100 m2/g, con~erted paper obtained using the teraggregate formed from the agglomerated particles of hydrated silicic acid, the agglo~erated particles of urea-formaldeh~de polvmer and the alumina Pol~mer is insufficient in the function of impro~ing the printing opacity. On the other hand, if the BET specific surface area e~ceeds 300 m2/g, the association strength of the agglomerated particles of hydrated silicic acid is so weak that when the agglomerated part;cles are used at the step of paper-making, the agglomerated particles collapse due to the pressure applied to the paper during the step Df paper-making, and as a result, the aimed function of i~proving the printing opacity is insufficient. The more preferred BET specific surface area is in the range of 150 to 25n m2/g.
The agglomerated particles of silicic acid haqing such surface areas are easil~ prepared according to a known method. In general, the agglomerated particles can be obtained by reacting an alkali silicate and a mineral acid as well as a salt in an aqueous solution, and usually an aqueous solu-tion of a mineral acid prepared so as to have a concentration of 2 to 40 g/100cc is added to an aqueous solution of an alkali silicate prepared in advance so as to have a . .. , ~

~z~9~s9 concentration o~ 2 to 9.5 g/100cc as calculated in terms of silica. The addition reaetion is carried out at a tempera-ture of 65~C or higher. As the method of adding the aqueous solution of a mineral acid, there are a method of adding it continuously and a method of adding it in di~ided portions, but in the former case of continuous addition, a product haYing a more stable qualitY is liable to be obtained, and yet the operation is easier. Usuall~, in the case of adding it continuously, it is preferred to complete the addition at a time of 50 minutes or shorter. The BET specific surface area of the agglomerated partieles of hydrated silieic aeid depends mainl~ upon the addition rate of the mineral aeid i.e. the formation rate of partieles of hydrated silicic acid.
There is a tendency that the lo~er the addition rate, the smaller the BET specific surface area, ~hile the higher the addition rate, the greater the BET specific surface area.
The alkali silicate as the rau material of the abo~e agglomerated partieles of hydrated silieie aeid mag be those Nhieh ean be e~pressed bg SiO~/alkali ~molar ratio), and eurrently eommereially aYailable water glass mag be used as it is. Further, as the mineral aeid used as its ra~ material, mineral aeids sueh as sulfurie aeid, hydro-ehloric acid, nitrie aeid, etc. are usable, but sulfuric acid is suitable in the aspect of its effect upon the paper-making step.
In the preparation of the agglomerated particles of hydrated silicic acid, ~arious techniques alreadg employed for the preparation of the hydrated silicic acid referred to - , ~; , ,, ;
- ....

SL~ 9~L~3 as white carbon such as techniques of adding sodium sulfate, sodium chloride, etc. or successi~elg raising the reaction temperature are applicable if necessary.
The agglomerated particles of hydrated silicic acid are obtained in the form of slurr~ as described abo~e, but if necessary, the resulting slurry mag be subjected to centrifugal dehgdration or filtration-dehgdration to obtain a cake-form material, or this cake-form material may be fur ther dried to obtain a po~derg material. Wben such a cake-form or powdery material is used, it is returned to a slurr y by adding ~ater.
In the practice of the present in~ention, it is necessary to add an aluminium salt for forming 0.015 to 1.2% based on drY pulp, of an alumina polymer, into a pulp slurry. If the proportion of the alumina polymer is less than 0.015% or e~ceeds 1.2X, the teraggregate formed fro~
the alumina polgmer, the agglomerated particles of urea-~ormaldehgde Pol~mer (A) and the agglomerated particles of hydrated silicic acid (B) has a low percentage retention thereof. in paper, so that it is impossible to acquire as sufficient opacity of con~erted paper after printing and opacity and an impro~ed brightness. The quantit~ of the alumina polymer formed is preferred to be in the range of 0.04 to 0.75~ . As the aluminium salt used for forming the alumina polYmer, alumiDium sulfate, aluminu~ chloride, sodium aluminate, etc. are usable, but use of aluminum sulfate is preferred in Yie~ of stabilized formation of the alumina polymer. Aluminum sulfate is e~pressed ~2~791~i9 by the formula A Q ~(SO4)3 and this is h~drolyzed in pulp slurry to form an aluminum hydro~ide polgmer haYing cations.
The quantity by weight of tbis aluminum hydro~ide polymer prepared refers to the quantitY calculated in terms of AQ 23 from the total quantit~ of the aluminum salt added and the aluminum salt contained in ~hite water circulated.
The agglomerated particles of urea-formaldehyde polymer (A) and the agglomerated particles of hydrated silicic acid (B) bear a negati~e potential in the suspension state in ~ater. It is presumed that the alumina polymer ha~ing cationic properties is adsorbed to these agglomerated particles having a negati~e potential to afford a cohesion by uhich the coaggregate of both the agglo~erated particles is formed, and at the same time, further reinforce the fi~-ability of the agglomerated particles of urea-formaldehyde polymer (A) onto pulp.
The present in~ention is characterized by paper-making using the agglomerated particles of urea-formaldehyde pol~mer (A) and the agglomerated particles of hydrated silicic acid (B) so as to gi~e a ratio bg weight of (A):(B) of 5:95 to 95:5 and a total ~eight of (A) and (B) of 0.5 to 30%
based on drY pulp. If the proportion of (A) is smaller in a ratio of 5:95, the teraggregate has a low percentage fi~ation thereof onto paper and the resulting conYerted paper is insufficient in the printing opacity, the brightness and the opacity. On the other hand, if the proportion of (B) is smaller in a ratio of 95:5, the resulting teraggregate has a high percentage fi~ation thereof onto paper, but the 1 1 ' , 127gl~9 resulting con~erted paper is insufficient in the printing opacit~. The ratio of (A):(B) is preferred to be in the, range of 20:80 to B~:20. ~urther the total weight o~ (A) and (B) is in the range of 0.5 to 30% based on drY pulp.
The reason is that if the total weight of (A) + ~B) is less than 0.5%, the resulting conYerted paper is lo~ in the printing opacit~, the opacity and the percentage of impro~e-ment in the brightness; hence the object cannot be achie~ed.
On the other hand, if the total weight of (A) and (B) e~ceeds 30~, the resulting converted paper has a low strength and the so-called powder drop from paper is obser~ed to occur;
hence it is impossible to sufficiently effect the function of paper. Thus the total weight of (A) and (B) is preferred to be in the range of 1 to 15%.
When the agglomerated particles (A) and (B) are added to pulp slurry, the agglomerated particles of h~drated silicic acid and the agglomerated particles of urea-formal-dehyde polymer have different percentages retention, respec-tively, as described later; hence the quantities of both the agglomerated particles are determined so that the ratio by weigbt of (A):(B) and the total weight of (A) and (B) ma~
fall in definite ranges, respecti~ely, taking into account the percentages retention thereof.
When an aluminum salt, the agglomerated particles of urea-formaldehyde polgmer (A) and the agglomerated particles of hydrated silicic acid (B3 are added to pulp slurry, the addition site thereof ma~ be an optional site between the refiner and the fan pump at the paper-making step, and both ~2791S9 the agglomerated particles added are desired to be uniformly dispersed in pulp slurry; thus con~entional methods may be employed for agitating and disPersing them. Further, the addition order thereof has no particular limitation, but it is preferred in the aspect of forming the coaggregate to mi~
the agglomerated particles of urea-formaldehYde polymer (A) with the agglomerated particles of hydrated silicic acid (B) in advance, followed by adding the mi~ture or to add both the agglomerated particles as successively as possible even when the7 are individuallg added. When the alu~inum salt and both the agglomerated particles are added to pulp slurry, they are each prepared in advance into an aqueous solution or slurr~ having a concentration in which the addition quantit7 is easily adiusted, but tbe concentration is preferred to be as low as possible, from the ~ie~point of a uniform dispersibility thereof in pulp slurry. Usually~
pulp slurry ha~ing added an aluminu~ salt and both the agglomerated particles each prepared so as to have a con-centration of lOX or lower is formed into a thin paper on the wire of an elongated wire paper machine, a cylindrical wire paper machine or a twin wire paper machine. Usually, by further dehYdrating the paper by means of a press roll, drying by means of a dr~er and finallY subjecting it to calender treatment, it is possible to easilg produce the lightweight paper of the present invention.
Depending on the use applications of the light-weight paper of the present invention, it is preferred to add an additive or an adjusting agent generally and conven-~L2 7~3~ ~i9 tionally used, into pulp slurry having added an aluminumsalt and both the agglomerated particles.
It is possible to usually add into pulp slurry, for e~ample, sizing agents such as rosin sizes, synthetic sizes, reactive sizes, paper strength-reinforciDg agents such as those of starch and gums, acr~lamides, ureas, melamines, chlorohydrins, etc., water filtration-impro~ing agents such as those of ethylene-imines, polyamides, acrylamides, etc., retention aids such as those of acrylamides,formation-improving adhesives, dyes, detergents, ~etting agents, pitch-control agents, etc. It is apparent that the present in~ention also comprises light~eight paper containing such other additi~es.
The abo~e-mentioned light~eight paper of the present in~ention has a function of highly impro~ing the printing opacity while Tetaining the function of impro~ing the opacity and a high figabilit~, and the abo~e-mentioned process for producing the lightweight paper of the prese~t in~ention is a process which makes eas~ the production of the lightweight paper ha~ing the abo~e-~0ntioned properties and is effecti~e for making paper lightweight.
The present inYention will be described by way of egamples in ~ore detail, but it should not be construed to be limited thereto.
Example A-1 -Into a flask ~ere fed water (20.00 parts by weight) (hereinafter all parts being by weight) and sodium ~d~ k r~ salt of carbogymethyl cellulose (Cellogen F-3H, ~ h~m~e of ~,2~91~

product manufactured by naiichi Kogyo Seiyaku Kabushiki Kaisha) (0.325 part), follo~ed by dissol~ing these materials, adding a 37% aqueous solution of formaldehyde tl8.24 parts), heating the mi~ture to 70 ~C Nith stirring, adjusting, at the same time, the pH to 7.5 with an aqueous solution of NaOH, thereafter adding urea (9 parts), and subjecting the mi~ture to condensation reaction at 70~C for 2.0 hours to obtain an initial urea-formaldeh~de condensation reaction product, cooling this product to about 45 C, and rapidly and uniforml~ mi~ing it Nith a solution obtained by diluting 95% sulfuric acid (0.46 part) with ~ater ~15.73 parts).
After 10 seconds, the reaction mi~ture cured and at that time its temperature rose up to 61~C. The solidified material was then kept at about 60~C for one hour, follo~ed b~ roughlg grinding it into particles having a size of 1 ~ 2 mm by means of a cutter granulator~ adding water (100 parts) to obtain a slurry-for~ ~aterial, neutralizing it with a 20X aqueous solution of NaOH to make its p~ ~.5, milling the resulting slurry by means of a mill, and filtration-deh~drating it to obtain a white cake-form material (60.2 parts). A portion thereof was dried by hot air at 105~C for 2 hours and the concentration of the agglome-rated particles of urea-formaldeh~de pol~mer in the cake-form material was measured to give 20.1%. Thus, 12.10 parts of the agglomerated particles (A) were obtained. The agglomerated particles had an a~erage particle diameter of 0.2 ~ as measured by means of the photograph of an elec-tron microscope, and also had an a~erage diameter of \

.. ... . ..

~279~9 5.1 ~ as measured by means of a Coulter Counter (Model TA II
manufactured by Coulter Counter Inc.) and calculated.
The agglomerated particles of urea-formaldehyde polymer obtained in this esample are referred to as UF-l.
Examples A-2 to A-9 Agglomerated particles Df urea-formaldehYde polymer (hereinafter referred to as UF) were obtained in the same manner as in Example A-l except ~arying the tradename and amount of carboxymethyl cellulose sodium salt used and the clearance of colloid mill (TK Micolloidal L type, manufac-tured by Tokushu Kika Kogyo, Inc.) as set forth in Table X-1.
The results obtained are set forth in Table 1.

1 ~

.- .. ,~ ~ , ..

s9 Table X-1 Carbo~meth~l cellulose Clearance E~amples UF of colloid Tradenames Makers Amounts, mill, ,u,m A-l UF-1 Cellogen Daiichi Kogyo 0.325 35 . F-3H Seiyaku K.K.

A-2 UF-2 Hi-Sunlose Sanyo Kokusaku 0.500 35 H750 Pulp. Inc.

A-3 UF-3 Sunlose - do - 0.325 35 A-4 UF-4 F-3H As the above 0.325 20 A-5 UF-5 - do - - do - 0.325 50 A-6 UF-6 Sunlose Sanyo Kokusaku 0.500 35 PN-OlA Pulp. Inc.
A-7 UF-7 Cellogen As the above 0.170 35 f~ , F-3H

A-8 UF-8 H~-Sunlose As the abo~e 0.325 15 A-9 UF-9 Cellogen As the above 0.325 70 J~rn e~.r k .

~2~79159 Table 1 partl c1e Average Cake E~ample UF diameter ag~slomerated concentration . ( ~)parti)CIeS (%) A-1 UF-l 0.20 5.1 20.1 A-2 UE-2 0.12 5.2 19.3 A-3 UF-3 0.30 5.1 22.4 A-4 UE-4 0.21 2.2 20.3 A-5 U~-5 0.20 9.8 20.2 A-6 U~-6 0.09 4.9 17.9 A-7 UF-7 0.65 6.4 23.4 A-8 U~-8 0.15 0.87 19.7 A-9 UF-9 0.23 20.6 20.8 E~ample B-l Sodium silicate (246cc) ha~ing a molar ratio of SiO2/Na2O of 3.02 and a content of SiO2 of 19.5 g/lOOcc was diluted into 1,200 cc with water to prepare an aqueous solution of sodium silicate ha~ing a concentration of SiO2 of 4 g/lOOcc, as a raw material. Into a 2 Q flask equipped with heating and cooling means, an agitator and a thermometer was placed the abo~e aqueous solution of sodium silicate as a raw material, followed bg raising the temperature up to 90C with stirring at 1,000 rpm, and adding 2N sulfuric acid for 40 minutes while heating the mi~ture so as to keep 90C, to make the final pH 8Ø

~L279~L59 The resulting agglomerated particles of hydrated silicic acid (B) had a slurry concentration of about 3.8~ . This slurry was subjected to filtration-dehydration to obtain a cake-form material. A portion thereof was dried in hot air at 105O for 2 hours and the concentration of the cake was measured to giie 21.3%. Eurther the BET specific surface area was 150 m /g. The agglomerated particles of hydrated silicic acid obtained in this E~ample are referred to as WC-1.
E~amples B-2 to B-5 Agglomerated particles of hydrated silicic acid (hereinafter referred to as WC) were obtained in the same manner as in Example B-1 e~cept ~ar~ing the SiO2 concentra-tion of the aqueous solution of sodium silicate and the dropping speed of 2N sulfuric acid as indicated in Table X-2. The obtained results are set forth in Table 2.
Table X-2 E~arples ~C SjO2 co centration, Droppigg speed of B-l WC-1 4 40 1 g 127~;9 Table 2 BET specific surface Example WC n area (m'/g) concentration (%) B-1 WC-l 150 21.3 B-2 WC-2 250 21.5 B-3 WC-3 100 20.6 B-4 WC-4 80 22.8 B-5 WC-S 350 21.8 Examples 1 ~ 5 and Comparative examples 1~ 5 Among A-l ~ 5 and B-1~ 3 in the preparation Examples of the agglomeTated Particles of urea-formaldeh~de polymer (A) and the agglomerated particles of hydrated silicic acid (B), UF-1 and WC-l the values of physical propeTties of which are close to the central values within the limited ones were oombined to eYidence the use~ulness of the present in~ention.
An aqueous solution of aluminum sulfate ha~ing a concentration of 3.06~ as calculated in terms of AQ 23 (the proportion of alum;na being about 0.31% b~ weight based on dr~ pulp)(2.0 parts) was added into a 1% pulp slurr~ (20.00 parts) ha~ing blended therein NB- KP (25 parts), TMP (30 parts), RGP (20 parts) and deinked newspaper (25 parts) and having a beating degree (CSF) of 330 mQ , followed by agitating the mixture for 2 minutes, succesi~ely adding a pre~iously prepared, mixed slurry of UF-1 and ~C-1 2 ~

~x~ 9 in a ratio by solids weight of the respective associated particles of 50:50 and haYing a concentration of 5% in total (40 parts) (the proportion of both the agglomerated particles being 10% based on drY pulp), and agitating the miature for 5 minutes to obtain a prepared slurr~, then subjecting it to paper-making b~ means of a TAPPI standard rectangular sheet machine, carrying out press-dehydration, dr~ing the result-ing wet paper on a drum drYer having a surface temperature of llO~C, thereafter twice passing it through under a linear pressure of 40 kg/cm, and seasoning it in a thermo-hygrostal at a humidity of 65% and a temperature of 20C for 24 hours to obtain a converted paper of E~ample 1.
With the converted paper obtained in E~ample 1, measurements of its physical properties such as basic ~eight, percentage retention, smoothness, bulk densitY, brightness, opacity, printing opacit~, etc. and calculations were carried out.
The results are shoun in Table 1.
Converted papers of Examples 2~v 5 and Comparative examples 1 ~ 4 were obtained in the same manner as that in the production of the converted paper uf Example 1 e~cept that the blending ratio of UF-1 and WC-1 was Yaried.
Further, a converted paper of Comparati~e e2amples 5 was obtained in the same manner as that in the process for producing the conYerted paper of E~ample 1 e~cept that the agglomerated particles were not added. ~ith the conYerted papers of E~amples 2 ~ 5 and ComparatiYe egamples 1 ~ 5, too, measurements of their physical properties and calculations were carried out in the same manner as that in the case of the converted paper of E~ample 1. The results are shown in Table 3.
In addition, the methods of measurements of physical properties of paper and calculations are as follows:
The basic weight was measured and calculated b~
the treatment according to JIS (P-8111).
The bulk densitY was determined b~ measurement of the thickness of paper according to JIS (P-B118) and cal-culation from an equation of basic ueight/thickness X ltOOO.
The smoothness was measured by menas of Bekk smooth-ness tester according to the method stated in JIS (P-8119) and TAPPI (Standard method T479).
The brightness was measured using a blue filter by means of Hunter meter. Opacit~ was measured according to JIS
(P-8138).
Printing opacit~ was measured according to a method described in a literature (Paper and Pulp Art Times, September, 1979, page 1-13).
The percentage retention of UF in the con~erted paper is calcula-ted by measuring the fi~ed amount of solids of UF added to dr~ pulp. In accordance with the method of TAPPI-T418 SU-72, the nitrogen content in paper (hereinafter referred to as N1 %) is measured and the nitrogen content in UF (hereinafter referred to as No%) is calculated. For e~ample, the No in the solids of UF-1 is 28%. The amount of UF in the conYerted paper (hereinafter referred to as UF-Y%) is calculated as follows:

~LZ 7 9~L~;9 UF-Y = N X l00 (%) Then, the percentage retention of UF (hereinafter referred to as UF-X~) is ca]culated as follows:

UF-Y
UF-X = X 100 (%) Amount of UF added to dr~ pulp The percentage retention of WC in the converted paper is calculated b~ measuring the content of hydrated silicic acid particles in paper ~hereinafter referred to as ~ 1~) in accordance with the method of TAPPI-T~13-ts-66 and the amount of hydrated silicic acid perticles added to drY
pulp ~hereinafter referred to as ~ 0%) is ealculated.
Percentage retention of WC = ~ 1 X l00 (%) In the measurement of percentage retentiOD of UF, the above N1 is determined by deducting the nitrogen content contained jD the pulp itself.
In the measurement of percentage retention of WC, the above ~ 1% is determined by deducting the ash content in converted paper obtained in the same procedure and condi-tions e~cept not adding WC.

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~ ,a ~; ~. 3 ~ o m m u) m o _ _ ~L27~g As apparent fsom Table 3, the converted papers of Examples 1 ~ 5 have higher percentages retention than those of the converted papers of ~omparative e~amples 1 ~ 4 although the basic weight, bulk density, smoothness, etc.
are the same, so that the former converted papers e~hibit higher printing opacities than those of particularly Com-parative e~amples 3 and 4 wherein the agglomerated particles are singly added, while retaining higher brightnesss and opacity.
Egamples 6 ~ 20 Paper-making was carried out in the same manner as in E~ample 1, fi~ing the simultaneous use ratio of the agglomerated particles of urea-formaldehyde polymer (A) to the agglomerated particles of h~drated silicic acid (B) to 60:40 and also fi~ing the total quantitY of these agglomerated particles added, to 10%, e~cept that the other factors were varied, followed by measurement of the ph~sical properties of the resulting converted papers and calculation. The results are shown in Table 4.
As apparent from Table 4, the converted papers obtained under the conditions of the present inventions are improved in any of brightness, opacity and printing opacity.

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_ _ 27 _ ~L~7 9 1~9 Examples 21 ~ 29 and Comparati~e e~ample 6 The instant E~amples and Comparative egample illustrate making paper lightweight.
In E~amples 21 ~ 29, conYerted papers were obtained under the same conditions and process as those in E~ample 1 e~cept that under the paper-making conditioned of E~ample 1, aluminum sulfate was added to pulp sturry, fol-lowed by further adding a maleic rosin size in a quantity of 0.15% by weight based on drY pulp; the quantities of UF-l and WC-l added were ~aried and the simultaDeous use ratio thereof was changed to 60:40; and the basic weight of paper was varied. Further, in comparati~e e~ample 6, con~erted paper ~eas obtained under the same conditions and process as those ;D E~amples 21~ 29 e~cept that U~-l, WC-1 and maleic rosin were not added and the basic weight of made paper was 50 g/m2 . The resulting respecti~e conYerted papers were subjected to the same e~aluation of physical properties as that carried out in E~ample 1, and further, a test of water absorption properties was carried out.
The paper-making conditions, ~easurements of physical properties and calculation results of these conYerted papers are shown in Table 5.
In addition, the test of water absorption properties was carried out by dropwise adding distilled water (0.04 mQ ) onto paper surface by means of a syringe and measuring the time by which water drops were absorbed and e~tinct on the paper surface.
As apparent from Table 5, by the simultaneous use ~Z79~LS9 of the agglomerated particles of urea-formaldehyde polymer and the agglomerated particles of h~drated silicic acid, it is possible to make paper lightweight, while retaining the .
opacit~ and the printing opacity, that is, without reducing the percentage print through.

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~. .

Claims (7)

1. A lightweight paper comprising a dry pulp, 0.015 to 1.2% by weight based on the pulp of an alumina polymer and 0.5 to 30% by weight based on the pulp of a coaggragate formed from agglomerated particles of urea-formaldehyde poly-mer (A) and agglomerated particles of hydrated silicic acid (B) in a ratio of (A):(B) of 5:95 to 95:5.

2. A lightweight paper according to claim 1 wherein said agglomerated particles of urea-formaldehyde polymer have an average particle diameter of 0.1 to 0.5 µ and an average diameter of said agglomerated particles of 1 to 15 µ.

3. A lightweight paper according to claim 1 wherein said agglomerated particles of hydrated silicic acid have a BET specific surface area of 100 to 300 m2/g.

4. A lightweight paper according to claim l wherein the ratio by weight based on said dry pulp of (A):(B) is in the range of 20:80 to 80:20 and the total weight based on said pulp of (A) and (B) is in the range of 1 to 15% by weight.

5. A lightweight paper according to claim 1 wherein said alumina polymer is formed from aluminum sulfate, alumi-num chloride or sodium aluminate.

6. A lightweight paper according to claim 1 wherein the content of said alumina polymer is in the range of 0.04 to 0.75% by weight based on said pulp.

7. A process for producing a lightweight paper, which process comprises adding into a pulp slurry, an aluminum salt in a quantity required for forming 0.015 to 1.2% by weight based on dry pulp, of an alumina polymer and further adding agglomerated particles of urea-formaldehyde polymer (A) and agglomerated particles of hydrated silicic acid (B) in quanti-ties required for giving a ratio of (A):(B) of 5:95 to 95:5 and 0.5 to 30% by weight based on dry pulp, of (A) and (B), followed by paper-making.