Stiff light-weight paper
United States Patent [1 1 Opderbeck et al.  
 [451 Apr. 15, 1975 1 1 STIFF LIGHT-WEIGHT PAPER [73] Assignee: Feldmuhle Anlagen-nind produktionsgesellschaft mit beschrankter Haftung, Dusseldorf, Germany 1 [22] Filed: Mar. 16, 1972 [21] Appl. No.: 235,412  
 [30] Foreign Application Priority Data Mar. 18, 1971 Germany 2113216 [52] U.S. Cl. ..162/167; 117/155 UA; 117/155 L; 117/156; 162/168; 162/174; 162/184 [51] Int. Cl D2lh 3/52 [58] Field of Search 162/184, 135, 167, 166, 162/158, 174, 168; 117/155 L, 404,156;  
 [56] References Cited UNITED STATES PATENTS 2,691,604 10/1954 Priest 117/155 L X 3,096.231 7/1963 Griggs et a1 162/164 X 3,505,264 4/1936 Thocse et al 117/155 UA X 3.573136 3/1971 Barlow ll7/155 UA 3,597,313 8/1971 Williams et al. 162/167 X 3,682,632 8/1972 Arai et al 96/].5 X  
 Primary ExaminerS. Leon Bashore Assistant ExaminerWilliam F. Smith Attorney, Agent, or FirmHans Berman; Kurt Kelman [57] ABSTRACT Light-weight paper can be made as stiff as paper of 50% greater weight or more without loss of folding endurance by coating its surfaces with a freshly prepared mixture of polyvinyl alcohol or an analogous polyol or polyamine and a source of formyl groups such as a water-soluble precondensate of formaldehyde with melamine or urea in an aqueous solution viscous enough to prevent full penetration of the paper. The product has a core layer of low bulk density and surface layers of much greater density and toughness and can be processed successfully in xerographic and like copying equipment not normally capable of handling very light paper.  
 7 Claims, No Drawings STIFF LIGHT-WEIGHT PAPER This invention relates to paper. and particularly to paper suitable for use in xerographic copying machines and like equipment. and to a method of preparing the same.  
  Modern high-speed copying machines require relatively stiff paper which can be guided precisely through the machines. It is also necessary that such paper have I adequate opacity and some resiliency. These requirements could be met heretofore only by paper weighing approximately 80 90 g/m&#39;-&#39;. Such paper is much bulkier and heavier than onion skin. Documents and correspondence printed on the heavy paper require much storage space and can be mailed only at relatively high expense for postage. There is a well-recognized need for a paper capable of being handled by presently available copying equipment. yet much lighter than the papers employed until now.  
  The usual after-treatments available to the papermaker do not provide a suitable product when applied to a light base stock. lmpregnated paper is made stiff throughout so that its folding endurance and elasticity are impaired. and surface coatings tend to be heavier than the amount of base stock which they replace.  
  it has now been found that a paper meeting the partly contradictory requirements of stiffness. light weight. resiliency. and folding endurance can be obtained by preparing a laminar structure in which a core layer of low bulk density consisting essentially of cellulose fibers carries at least one surface layer which is tough and resilient so as to provide the necessary stiffness and folding endurance. Paper having a weight of 35 to 70 g/m&#34;. yet having adequate opacity. stiffness. and folding endurance has been prepared according to this invention.  
  At least one surface layer of preferred paper of the invention predominantly consists of a condensation product ofa polymer having a molecular weight greater than 2,000. and preferably of 30.000 to 90,000, and having available hydroxyl or amino groups in each of its repeating units. with a source of available formyl groups which cause cross linking of the polymer. The condensation reaction is performed on or in the outermost portion of the core layer so that the predominantly polymeric surface layer is bound to the cellulosic core layer by cellulose fibers extending into the surface layer.  
  The laminar structure of the paper is obtained by coating at least one face of a paper web consisting essentially of cellulose fibers with an aqueous mixture of the polymer and of the source of formyl groups in an amount corresponding to 1.5 to 3.0 grams of solids in the mixture per square meter of the coated face or faces. The condensation mixture must have adequate viscosity to prevent penetration of the core layer. Not more than about 12% by weight of the core layer should consist of the condensation product, and it is not normally practical to prevent penetration of the core layer by less than 3% of the condensation product.  
  If necessary, the condensation mixture may be provided with an inert thickener, such as bentonite, which does not enter into the reaction, or the base paper forming the core layer may be sized to impede penetration by the aqueous condensation mixture.  
  Under otherwise comparable conditions, better results are obtained with paper webs prepared prior to coating by straining an aqueous suspension of cellulose fibers of a major portion of the water content and collecting the fibers on a foraminous carrier such as a Fourdrinier wire or a perforated drum, as is conventional in itself, when the initial fiber suspension is alkaline. more specifically has a pH value of 7.2 to 9.7.  
  Suitable polymers soluble or dispersible in water and having available hydroxyl or amino groups in their repeating monomer units include polyvinyl alcohol. particularly polyvinyl alcohol practically free from acid moieties such as residual acetate. starch. ethoxylated starch. and related starch derivatives including products of oxidation and partial enzymatic hydrolysis. also polymerized hydroxyalkyl esters of acrylic and methacrylic acid. acrylamide and methacrylamide. also copolymerizates of the acrylic monomers mentioned with olefinically unsaturated compounds. such as ethylene. propylene. vinyl acetate. and vinyl chloride.  
  It has been found that the surface layer in the paper of the invention may additionally contain proteins such as casein. soybean protein, gelatin. in amounts not exceeding of the weight of the polymer described above. A significant increase in stiffness isachieved with 10% protein, but folding endurance is impaired by proteins in amounts exceeding 60% of the polymer because the surface layer is embrittled.  
  The source of a formyl group capable of reacting with the hydroxyl or amino groups of the polymer may be a lower alkanal or a substance which. in the presence of the polymers, yields alkanals. The sources of available formyl groups thus include formaldehyde, acetaldehyde. glyoxal, but also hexamethylenetetramine. and the water-soluble. liquid precondensates of formaldehyde with melamine and urea which are staple articles of commerce and contain at least the equivalent of one mole formaldehyde per amino group in the urea or melamine.  
  Cyanamide. sodium cyanamide. or calcium cyanamide. when present in the condensation mixture applied to the base paper. enhance the cross-linking effect of the aldehyde and significantly increase the bond strength between the polymer predominating in the surface layer and the cellulose which is the essential component of the core layer.  
  The condensation reaction can be accelerated. if so desired. by the presence of catalysts such as sodium chloride, ammonium chloride, hydrogen chloride, or Lewis acids.  
  The viscosity of the condensation mixture must be adequate to preventpenetration of the core layer, and is controlled most effectively by suitably selecting the molecular weight or degree of polymerization of the polymer having available hydroxyl or amino groups. The condensation mixture preferably should be capable of being applied to a paper web in continuous operation by the conventional equipment of the paper industry. particularly coating equipment. and must be capable of being applied in a thin, uniform layer since the condensation product should not usually amount to more than ten percent of the weight of the base stock per surface layer.  
  The necessary reproducible properties of the condensation mixture can be achieved unter industrial conditions only if the polymer and the cross-linking agent are mixed immediately prior to being applied to the base stock. or by applying the polymer and the I cross-linking agent sequentially, the more viscous polymer solution preferably being applied first. The lastmentioned procedure is advantageous in preventing the formation of hardened condensates in the equipment if the machinery has to be stopped.  
  The papers of the invention having surface layers predominantly consisting of the afore-described condensation products may be written or printed upon like good bond papers and have satisfactory opacity even when weighing only g/m The surface layers have sufficient electric conductivity to prevent a build-up of electrostatic charges and the resulting difficulties in separating sheets of a stack of papers of the invention from each other. Because the surface layers contain at least some cellulose fibers, they do not form a continuous film impermeable to air, and sheets of the paper can be fed by conventional suction equipment. The condensation products are thermally stable. do not turn yellow at elevated temperature, and the surface layers lack abrasiveness which might cause accumulation of dust and dirt in the processing equipment. The stiffness of the paper of the invention and its resistance to buckling are not impaired by the relatively high operating temperatures of many types of copying equipment which do not permit the use of thermoplastic coatings.  
  The bonding of the condensation product to the cellulose and other important. but less critical mechanical properties of the paper of the invention are particularly good when the base stock employed is prepared from aqueous stuff having an alkaline reaction. and preferably a pH of 7.2 to 9.7. ln such paper, the hydroxyl groups in the cellulose are most readily available for bonding to the condensation products of the invention.  
  The following Examples further illustrate the invention, but it will be understood that the invention is not limited thereto.  
 EXAMPLE 1 Pulp prepared from coniferous wood and from hardwood was ground in a weight ratio of &#39;60 parts to 40 parts to a drainage rate of 38S.R. and 6 parts calcium carbonate were added as a filler. An alkylketene dimer (Aquapel. Hercules Incorporated) and a cationic polyamide resin (Kymene 557, Hercules Incorporated) were added to the stuff as sizing agent and retention aide respectively, the pH was adjusted to 7.8-8.0, and a paper web was formed on a Fourdrinier type paper machine in the usual manner, the consistency of the initial cellulose fiber suspension and the speed of the machine being such as to normally produce paper having a weight of 50 g/m Near the dry end of the machine, the sized paper web containing approximately 93% airdry matter was coated on both sides on a doctor blade coater to a dry weight of 1.5-1.7 g/m with a homogeneous aqueous composition having a viscosity of 34 cp at 40C and prepared by mixing equal volumes ofa 120 g/l solution of commercial polyvinyl alcohol of high viscosity, 99.9% saponified, and of a 120 g/l solution of a commercial melamine formaldehyde precondensate containing an excess of formaldehyde over the three amino groups of the melamine. The two solutions were continuously pumped to a small mixing vessel from which they entered the trough of the coater, the dwell time in the mixing vessel and the trough being held to a minimum by permitting much of the mixture to overflow to waste so that very little condensation took place before the mixture was applied to the paper web, and its viscosity was uniform at the time of application.  
  The condensation reaction took place in the dryer section of the paper machine at about 125C.  
  The treated paper had a bulk density of 0.58-0.60 g/cm. When its section was inspected by the unaided eye or at low magnification, three distinct layers were clearly visible, but microscopic examination showed a minor, though significant amount of cellulose fibers in the surface layers and a small amount of condensation product in the core layer. The bulk density of the surface layers was approximately I, whereas that of the core layer was approximately 0.45.  
  The stiffness of the laminar product was 4-6 gem in the direction of web travel and 2.2-3.2 gcm at right angles to that direction as determined according to German standard method DIN-Norm 6723. A control sample of paper produced under otherwise the same conditions, but not provided with surface layers of condensed polymer, had stiffness values of 2-2.5 gem and 1.2-1.5 gcm respectively. The stiffness of the paper of the invention thus corresponded to that normally expected from a paper weighing 80-90 g/cm It was processed without difficulty on a xerographic copying machine adjusted for handling paper sheets of the heavier weight.  
 EXAMPLE 2 20 Parts (weight) fine groundwood pulp having a drainage rate of -82S.R. and bleached was ground with 40 parts long-fibered sulfate pulp. and combined with 25 parts hardwood pulp and 15 parts de-inked waste paper to a stuff having a drainage rate of 35-40S.R. Rosin size and alum were added for fully sizing the paper which was formed at pH 4.5-4.8.  
  As in Example 1, an aqueous composition was applied to the almost dry paper web on a doctor blade coater on both faces. The composition was mixed immediately prior to application as described in Example 1, and consisted, on a dry basis, of 50 parts (weight) bone gelatine, 50 parts of the polyvinyl alcohol of high viscosity employed in Example 1, 25 parts ureaformaldehyde precondensate, and 0.5 part sodium bentonite, also enough water manually controlled to permit application of a uniform coating varying in dry weight between 1.5 and 3.0 g/m per side in successive runs, while the base stock amounted to about 45 g/m The papers so produced were generally similar in their structure and their properties to the paper described in Example 1. The stiffness values increased with increasing weight of the surface layers to three times the corresponding values for the base stock. The stiffness-reducing effect of the groundwood in the stuff and of the acidic medium was balanced by the stiffnessenhancing effect of the gelatine in the coating. The amount of condensation product in the core layer varied with the viscosity of the coating solution between 3 and 12%.  
  All the papers produced in Example 2 were run successfully on the xerographic copying machine referred to in Example 1.  
 EXAMPLE 3 A paper web was formed on the above-mentioned paper machine in a conventional manner from stuff prepared from 40 parts long-fibered sulfate pulp, 50  
 parts birchwood sulfate pulp, and 10 parts waste paper free from cellulose. together with parts China clay. at pH 7.8. The sizing added to the stuff consisted of the afore-mentioned alkylketene dimer (Aquapel) and a pared immediately prior to application as described above.  
  The composition was cured at the highest temperature available on the dryer section used. and the coated paper was reeled while still hot to complete the cure. whereby each turn of coated paper in the reel was directly and releasably superimposed on the surface layer of another turn. In several successive runs. papers weighing 35-7O g/m were produced and had the necessary properties enumerated above.  
  The surface layers of the several papers prepared in Examples 2 and 3 had bulk densities close to 1.0 and slightly higher. while the densities of the core layers varied between 0.4 and 0.75. The stiffness values varied between twice and three times the corresponding values for the untreated base stock.  
  Further increase in the stiffness values was possible by further increasing the amount of cyanamide to as much as 50% of the weight of the melamineformaldehyde or urea-formaldehyde precondensate. Little could be achieved by less than 10% cyanamide. The use of cyanamide is most beneficial with base stock of 30 g/m or near this very low value. Cyanamide was replaced successfully in the procedure of Example 3 by equimolecular amounts of its sodium and calcium salts, and a somewhat stiffer paper was obtained when the polyvinyl alcohol was replaced by 50% gelatine.  
  Sulfate pulp generally gives better results than groundwood or sulfite pulp, but some pulp other than sulfate pulp is acceptable as exemplified in Example 2.  
  The melamine and urea precondensates with formaldehyde are preferred sources of available formyl groups although formaldehyde and acetaldehyde may be used in corresponding amounts. However, the free aldehydes react with the polyvinyl alcohol at so high a rate that it is difficult to maintain desired and uniform operating conditions whereas the precondensates yield formaldehyde at a slow and readily reproducible rate. Hexamethylenetetramine is intermediate in its decomposition and reaction rate between the free aldehydes and the preferred precondensates.  
  The polyvinyl alcohol referred to in Examples 1 to 3 had a degree of polymerization of 1700-1800 and a viscosity of 44 centipoise at C in 4% aqueous solution as determined with a Brookfield viscosimeter at 100 r.p.m. of a No. 2 spindle. A polymer of lower viscosity may be employed either at higher concentrations or in combination with thickeners such as the bentonite mentioned.  
  Polyvinyl alcohol may be replaced at least partly in the above Examples by other polymers having a molecular weight of at least 2,000 and at least one available hydroxyl or amino group in each repeating monomer unit such as those mentioned above.  
  The properties of the surface layer or layers may also be modified by varying the ratio of hydroxyl or amino groups in the polymer to the formyl groups in the crosslinking agent. With polyvinyl alcohol and the preferred precondensates of melamine and urea. the weight ratio may be chosen between one and nine parts polyvinyl alcohol per part of the precondensate.  
  Obviously. many other modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.  
 What is claimed is:  
  1. A stack of sheets of laminar structure, each sheet consisting essentially of cellulose fibers and a condensation product of water-soluble polyvinyl alcohol and a water-soluble precondensate of formaldehyde with melamine or urea. the weight ratio of said polyvinyl alcohol to said precondensate in said condensation product being between 1:1 and 9:1. said sheet having a weight of 35 to grams per square meter, a core layer of said sheet consisting essentially of said cellulose fibers and not more than 12 percent by weight of said condensation product. and the sheet having at least one surface layer consisting essentially of said condensation product in an amount of 1.5 to 3.0 grams per square meter of said surface layer and being bound to said core layer by cellulose fibers extending from said core layer into said surface layer, the mechanical strength. stiffness. and bulk density of said surface layer being substantially greater than the corresponding properties of said core layer, the molecular weight of said polyvinyl alcohol being at least 2000. each sheet being directly and releasably superimposed on said surface layer of another sheet.  
  2. A stack as set forth in claim 1, wherein said surface layer contains a protein in an amount of 10 percent to 60 percent of the weight of said polymer in the condensation product present in said surface layer.  
  3. A stack as set forth in claim 1, wherein said condensation product further includes cyanamide, sodium cyanamide, or calcium cyanamide in an amount of 10 to 50% of the weight of said precondensate.  
  4. A stack as set forth in claim 1, wherein said sheet has two of said surface layers, said core layer being received between said surface layers.  
  5. A method of preparing a laminar paper which comprises:  
 a. separately feeding respective aqueous dispersions of polyvinyl alcohol having a molecular weight of 30,000 to 90,000 and of a pre-condensate of urea or melamine with formaldehyde as a source of formyl groups to a mixing zone;  
 b. mixing said dispersions in said zone;  
 c. discharging a major portion of the resulting mixture from said zone to waste;  
 d. continuously applying to at least one face of a web consisting essentially of cellulose fibers a coating of the remainder of said mixture in an amount corresponding to 1.5 to 3.0 grams of solids in said mixture per square meter of said at least one face,  
 1. the dry weight of said web after said applying being about 35 to 70 grams per square meter, 2. the viscosity of said remainder of said mixture being kept constant by said discharging of said major portion,  
 3. the viscosity of said applied remainder being sufficient to retain most of said mixture in a surface layer of said web while holding the amount of portion of said web after said evaporation of water from said other portion.  
  6. A method as set forth in claim 5, wherein said web contains water and is prepared prior to said applying by straining an aqueous suspension of said cellulose fibers of a major portion of the water content thereof, and collecting the fibers on a foraminous carrier, said suspension having a pH value of 7.2 to 9.7.  
  7. A method as set forth in claim 5, wherein said web is sized prior to said coating.  
  UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,878,038 Dat d April 15, 1975 Inventor s) FRITZ RBECK ET AL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 In the heading, line fig], change &#34;Feldmxihle Anlagen-nind produktionsgeselischaft mit beschrankter Haftung&#34; to Feldmflhle Anlagenund Produktionsgesellschaft mit beschrflnkter Haftung Signed and sealed this 1st day of July 1975.  
 At t &#39;5 St 1 C ZIARSHAIE, DANN RUTH C. III-150E Commissioner of Patents Attesting; S. i icer and Trademarks