CA2547276A1 - Coated mechanical pulp paper - Google Patents

Abstract

A paper having a brightness of at least about 81% is provided. The paper comprises pulp having at least about 40% lignin, and a coating comprising a pigment to increase the brightness.

Description

Coated Mechanical Pulp Paper FIELD OF INVENTION

[0001 ] The present invention generally relates to a process for producing paper using mechanical pulp and more particularly relates to a process for producing paper with a brightness and printing performance approaching that of papers made from chemical pulps.

BACKGROUND OF THE INVENTION

[0002] Pulps used in the manufacturing of paper can be either so-called chemical pulp or mechanical pulp. Chemical pulp refers to pulp that has been made with a chemical process wherein a majority of the non-cellulose constituents of the wood, including lignin and hemi-cellulose, have been removed. A typical unbleached chemical pulp may contain up to between 4 and 5% non-cellulose constituents while a fully bleached chemical pulp may contain less than 1% of these constituents. As lignin tends to yellow paper chemical pulp is used for the production of high brightness papers such as so-called offset papers. The offset paper brightness is typically from 85% to 92%. Chemical pulp is however expensive to produce for reasons including that there is only about 45% yield with regard to the initial wood feedstock.

[0003] In a mechanical pulp process, the wood is ground to produce pulp and there is no removal of lignin and hemi-cellulose, therefore most of the wood is converted to paper, presenting a more efficient use of the wood resource.
Mechanical based processes include numerous variations to the basic mechanical process. For example, a Thermo-Mechanical Process (TMP) provides heat to the pulp making process and Thermo-Chemi-Mechanical Process (TCMP) provides a small chemical component. Other variations include the use of pressurized groundwood and Refiner Mechanical Pulp (RMP). It will apparent to one skilled in the art that the above variations to a basic mechanical process do not represent all possible variations.

[0004] AII mechanical based processes however leave the majority of the lignin and hemi-cellulose in the pulp. With the retention of lignin the basic brightness of paper produced from mechanical pulp of approximately 58% is lower than that produced from chemical pulp. This brightness can be raised to approximately 80% through bleaching steps. However, there is still a visible brightness difference with offset papers.

[0005] In addition, a mechanical pulp paper, without surface treatment, contains on its surface a large amount of loosely bound fibers that could be easily removed by the high viscosity inks used in the offset printing process usually applied with chemical pulp offset papers, contaminating thereby the printing equipment leading to lower printing quality and performance.

[0006] Due to the above reasons mechanical pulp papers were traditionally limited to lower value end uses such as newsprint, advertisement inserts or flyers, low price magazines etc.

[0007] There is therefore a need to provide an economic alternative to chemical pulp containing offset papers, with improved brightness and printing performance while maximizing the use of wood resource.

SUMMARY OF THE INVENTION

[0008] The present invention relates to a coated paper and a method of manufacturing the same. The coated paper is made from mechanical pulp and the resultant paper has improved optical and printing performance more comparable to that of papers made from chemical pulps.

[0009] It is an object of the invention to provide an improved brightness for a paper based on mechanical pulp. It is a further object to produce a mechanical pulp based paper providing offset printing performance comparable to that of papers based on chemical pulp.

[0010] According to one aspect of the invention a paper having a brightness of at least about 81 % is provided. The paper comprises pulp having about 28% lignin, and a coating comprising a pigment to increase the brightness.

[0011] According to another aspect of the invention a paper having a brightness of at least about 81 % is provided. The paper comprises mechanical pulp, and a coating comprising a pigment to increase the brightness.

[0012] According to another aspect of the invention a coating for use in manufacturing a coated paper having a brightness of at least about 81 % that is made using mechanical pulp is provided. The coating comprises a pigment to increase the brightness, and a binder.

[0013] According to another aspect of the invention a method of making a paper based on mechanical pulp is provided. The method comprises the steps of screening the pulp to remove particles of a size defined by the screen from the pulp, bleaching the pulp to increase the brightness, washing the pulp, forming a paper from the pulp, pressing the paper with a shoe press, and coating the paper with a coating having at least a pigment and a binder, wherein the paper has a brightness of at least about 81 %.

[0014] This summary of the invention does not necessarily describe all features of the invention.

[0015] Other aspects and advantages of the invention, as well as the structure and operation of various embodiments of the invention, will become apparent to those ordinarily skilled in the art upon review of the following description of the invention in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings wherein:

[0017] FIGURE 1 shows a flow chart of a paper making process in accordance with an embodiment of the present invention;

[0018] FIGURE 2 shows a flow chart of a refining process in accordance with another embodiment of the present invention; and [0019] FIGURE 3 shows a flow chart of a screening process in accordance with another embodiment of the present invention.

[0020] DETAILED DESCRIPTION

[0021 ] The following description is of a preferred embodiment.

[0022] The instant invention presents a coated paper formulation that uses mechanical pulp. The invention further encompasses a method of producing this paper.

[0023] It is an object of the invention to provide a paper that is an alternative to offset paper grades. It is a further object of the invention to provide a mechanical pulp based paper with improved optical properties, improved printing performance, a lower basis weight and increased opacity.

[0024] The paper of the instant invention is based on mechanical pulp. A
paper produced according to embodiments of the instant invention has a brightness of at least about 81 %, a basis weight of between 48.8 and 81 grams per square meter, an opacity of between 85 and 96, a thickness between 81 and 130 microns and a porosity superior to 100 mI/s.

Process [0025] A flow chart of a paper making process 100 according to an embodiment of the invention is shown in Figure 1. The process 100 considers the basic steps in a paper making process and does not necessarily contain all steps of a paper making process as would be apparent to one skilled in the art. Further particular steps of the process 100, as will be discussed, have been tailored to the requirements of the embodiments of the invention.

[0026] At step 102 a mechanical pulp is prepared using a Thermo-Mechanical Pulp (TMP) process. In the current embodiment a mixture of spruce and fir is used for the pulp. Pulp made from the above woods using a TMP process will typically have a lignin content of about 28%. It will be apparent to one skilled in the art that the exact lignin content is a function of both the particular pulping process and the wood feedstock. It is noted that other mechanical based processes are within the scope of the invention.

[0027] Figure 2 presents a flow chart of a typical TMP process 200. As it is important that all poorly refined fibers be removed from the final pulp to minimize printing press contamination particular care in the pulping process is taken in this regard. It is noted that the TMP process 200 includes two streams of pulp after the impregnating step 202. Each pulp stream passes through a primary refining 204 and a secondary refining 206.

[0028] After refining of the pulp at step 102 it is screened at step 104. The screening step 104 is designed to remove any rejects from the pulp prior to it being bleached and washed. Details of the screening step 104 are presented in Figure 3.

[0029] In Figure 3 the flow of pulp is depicted with arrows. Solid arrows indicate the flow of pulp to screens, "dash-dot" arrows indicate pulp that has passed a secondary and rejects screening and a continuous "dot" arrow indicates rejects. The rejects line 305 is used only occasionally when the rejects refiners or screens are overloaded or down for maintenance.

[0030] Refined pulp 307 is provided to a first screen line 301 via a first latency chest 304 and to a second screen line 303 via a second latency chest 316.
The pulp provided to the latency chest 304 and 316 includes both pulp from the refining step 102 and occasionally rejects from the screening operations.
With regard to the first screen line 301 pulp is communicated from the first latency chest 304 to the primary screen 306. From the primary screen 306 it is communicated to secondary screen (1) 308 and (2) 310. Pulp that has passed the secondary screen (1) 308 is communicated to the unbleached pulp storage 314. The pulp is then communicated to the unbleached pulp storage 320 of the screen line 303. Pulp that passes the secondary screen 310 is passed directly to the pulp storage 320. The final screen provided in the first line 301 is a rejects screen 312. The proper balance of the operational parameters of the secondary screen line 303 provides a way to control the strength and porosity required for the paper being produced by the current embodiment.

[0031 ] Rejects from the first screen line 301 and the second screen line 303 are fed back into the screening process via the rejects storage tank 330. The two rejects refiner lines 332 and the three rejects screens 312, 318 and 336 provide an additional means for controlling the pulp quality to optimize stiffness, strength and porosity of the paper of the current embodiment.
[0032] The screening process step is designed in conjunction with the washing step that will be described shortly such that paper strength and opacity are maximized while maintaining high paper porosity to ensure appropriate offset printing quality.

[0033] A mechanical pulp process generally produces a paper that has a large number of chemical residuals. Therefore the process of the current embodiment is designed to control these residuals so as not to lead to problems in the final paper making process. It is noted that the papers of the invention are to be used in offset, or higher quality, printing processes.
Therefore the pulp is thoroughly washed at step 105 and 108. Therefore the pulp is washed before and after bleaching the step 106. The washing steps 105 and 108 use approximately 6 m3 of fresh water per ton of pulp. This is in comparison to 1-2 m3 of water per ton of pulp that is used in a standard, post-bleach washing process. Use of this quantity of water has been found to adequately manage residuals in the pulp and subsequently in the finished paper.

[0034] The bleaching step 106 uses a mixture of H202 + caustic soda +
sodium silicates + DTPA and raises the brightness from around 50%, which is typicai of mechanical pulps, to about 78%.

[0035] Forming of the paper occurs at step 109. A very diluted aqueous mixture of bleached pulp, filler pigments and other chemicals is evenly spread over a set of forming fabrics. It is found that, at this stage, the pulp fiber orientation is most critical for the final paper dimensional stability required for some digital printing processes.

[0036] At step 110 the paper is pressed with a shoe press. A shoe press applies stationary pressure through a "shoe" located behind a rubber belt.

The paper is then pressed between this rubber belt and a felt belt located on the other side of the paper. A shoe press can apply pressures that are a factor of 10 larger than is possible with other press systems. The larger pressing pressure produced by a shoe press results in a higher density surface, which is desirable for the application of the coating. Namely, the final coating is more uniform and it is not absorbed into the paper. This eliminates the need for a pre-calender traditionally required for coating applications.
It should be mentioned that the higher density surface resulting from the shoe press results in a lower density through the thickness of the paper for the same basis weight of paper, yielding higher opacities. It is also found that the proper design and control of the operation of the shoe press as well as of the presses prior to and following it are important to ensure the symmetry of the paper surface necessary for good printing quality. This is particular to the process of the subject application.

[0037] The coating step 112 is performed using a film coater. In a film coater the coating slurry is provided to applicator rolls located on either side of the paper with the coating being deposited on the paper as a film as the paper passes between the applicator rolls. This method of coating provides for a uniform thin layer of coating at low coat weight as required for the papers made according to the process of the current embodiment. Other coating techniques such as blade coating are not desirable for use in the current invention as coating is generally unevenly applied at low coat weight. With blade coating, coating would collect in any valleys present between fibers providing both an uneven distribution and use of possibly excessive amounts of coating.

[0038] In the current embodiment between 3.5 and 4.5 glm2 of coating is applied at step 112 depending on the pulp quality, the characteristics of the paper making machines and the end-use requirements. It has been shown that the amount of coating can be reduced to 2 g/m2. The coating slurry composition according to embodiments of the invention will be described shortly. The slurry compositions will depend upon each coat weight.

[0039] After coating the paper is subjected to a calendering process at step 113. In the current embodiment, the calendering process is performed using from two to six steel and/or synthetic rolls to control the paper thickness to the level and uniformity required by the particular end-use. Paper thickness uniformity is central to the paper of the current embodiment, therefore it has to be well controlled within very tight limits.

[0040] The steps of the paper making process that are subsequent to calendering step 113 are provided by finishing step 116. In the finishing step 116, paper is cut into narrower rolls, where the width of each roll depends upon the final end-use, and wrapped in air and moisture impervious wrapping paper before delivering to customers. At finishing step 116, it is important to ensure an appropriate winding tension and slitting operation to avoid performance problems at pressrooms or converters.

Coatin [0041] A central aspect of the instant invention is the coating that is applied to the paper. This coating is important to the provision of the desired paper properties and influences the process 100.

[0042] Table I presents the formulation of four coatings that have been used according to embodiments of the invention.

[0043] According to the current embodiment the amount of coating is preferably between 3.5 and 4.5 g/m2 per paper side. Generally it is preferable to use the least amount of coating that can produce the desired paper properties as the coating costs more than wood fiber. It has been found that the amount of coating required varies with the quality of both the pulp and the paper making equipment. Further the amount of coating is a function of the application for which the paper is being supplied where a lower coating content results in a lower brightness, lower surface strength, higher thickness and higher porosity. According to an alternative embodiment the amount of coating is reduced to around 2 g/m2.

a [0044] The primary constituent of the coating is pigment, which in large part, provides for the brightness enhancement. As illustrated in Table 1 the primary pigment in examples 1, 2 and 3 is Ground Calcium Carbonate (GCC).
In the examples where GCC formed the primary pigment a delaminated coating clay was used as a second pigment. It was found that around twenty percent of delaminated clay is the optimal amount for printing quality and brightness gain, therefore example 1 is the preferred case. The coating of example 4 used a Bresilian clay as the primary pigment and GCC as the secondary pigment.

[0045] There are advantages and disadvantages associated with each possible coating pigment with the final selection being made with consideration to cost and paper quality. GCC provides a high brightness for the cost. Bresilian clay is brighter than Georgian clay and has been found to provide improved printing quality and a smoother surface. It however has a lower porosity and brightness, and is more expensive. The currently preferred coating composition does not include Bresilian clay.

[0046] The use of GCC in the coating process creates a high pH in the water and pulp systems around the paper machines. A water pH around 9 has been found. Since the lignin that is present in mechanical pulp loses brightness with increasing pH, steps have to be taken to maintain a pH at around neutral i.e. 7.0-7.2. The pH is lowered to around neutral through the addition of phosphoric acid, SOz, CO2 or other acids to prevent brightness loss. Further, while it is normal practice to re-use water from the paper forming steps 109 to 116 at the pulp preparing step 102 this is not the case in the current embodiment as the high pH of the water will kill the paper brightness.

[0047] After pigments the next major ingredient is binder where the binder holds the components of the coating together as well as provides the necessary surface strength and print quality. The binder is formed from a combination of latex and starch, generally comprising between 5 and 15%
latex where the selected latex should have a gel temperature (Tg) between 0 and 10 C. This gel temperature is chosen such that the binder provides as much strength as possible. Among the various latexes that suffice the above requirement SBR is the least expensive while acrylic latex provides higher light stability but at higher cost. With regard to the starch it may be either an oxidized or an ethylated starch or an equivalent type of starch with similar viscosity and binding strength. Examples 1 to 4 all use a combination of SBR
latex and ethylated corn starch.

[0048] In the current embodiment, the total of binder parts should be calculated from the following formulae to ensure adequate surface strength.
In addition, starch should not be higher than 12 parts to optimize brightness and viscosity.

Total Binder = A + B * Coat Weight Where A = 69.5 - 72.5 and B = 9.6 - 11.2 [0049] The balance of the components in the coating composition can be considered as minor components. A red dye, a blue dye, and a Fluorescence Whitening Agent are all added as minor components. These components are used to tailor the white shade of the final paper.

[0050] The cross-linker is operative with the starch where it accelerates the work thereof.

[0051] The water retention agent improves the spreading characteristics of the coating while preventing its premature drying on the applicator roll's surface.

[0052] Biocides are also added to the coating composition to prevent unwanted bio-activities.

[0053] The final minor components include a dispersant, a lubricant and caustic soda. The dispersant maintains the pigment in solution, maintains solubility and prevents the coating from settling. The lubricant facilitates coating application uniformity, prevents coating pick-up and creates a smoother paper calendering operation. Caustic Soda controls the final pH of the Coating Slurry.

The present invention will be further illustrated in the following examples.
Coating Class Ingredient 1( !o) 2( !o) 3(%) 4( !o) Ground Calcium Pigment 1 Carbonate (GCC) 54.87 76.59 71.09 7.55 Delaminated Pigment 2 Coating Clay 23.52 4.03 7.89 Bresilian coating Pigment 2 - alt. 67.89 Clay Binder 1 SBR Latex 9.41 11.29 12.64 12.07 Binder 2 Starch 9.41 4.84 5.53 9.05 Crosslinker 0.54 0.28 0.32 0.52 Water Retention 0.03 0.2 0.2 0.03 Agent Fluorescence Whitening 1.18 1.61 1.18 1.89 Agent Dye 1 0.002 0.002 0.002 0.002 Dye 2 0.001 0.001 0.001 0.001 Dispersant 0.16 0.16 0.16 0.15 Defoamer 0.24 0.24 0.24 0.23 Lubricant Calcium Stearate 0.55 0.56 0.55 0.53 Biocides minor component 0.01 0.03 0.03 0.01 Caustic Soda Sodium Hydroxide 0.08 0.17 0.17 0.08 Table 1 Coating Ingredients [0055] A paper produced according to embodiments of the invention provides for a printing quality that is comparable to offset printing paper. In one embodiment the paper of the invention may be used for digital printing ~i purposes. It has been found that the paper can be used in various Electrophotographic Printing Processes, using dry or liquid toners as well as in Ink Jet Printing Processes, using water or solvent based inks.

[0056] While the invention has been described according to what is presently considered to be the most practical and preferred embodiments, it must be understood that the invention is not limited to the disclosed embodiments.
Those ordinarily skilled in the art will understand that various modifications and equivalent structures and functions may be made without departing from the spirit and scope of the invention as defined in the claims. Therefore, the invention as defined in the claims must be accorded the broadest possible interpretation so as to encompass all such modifications and equivalent structures and functions.

Claims (27)

1. A paper having a brightness of at least about 81 % comprising:
pulp having about 28% lignin; and a coating comprising, a pigment to increase the brightness.

2. A paper having a brightness of at least about 81 % comprising:
mechanical pulp; and a coating comprising a pigment to increase the brightness.

3. A paper having a brightness of at least about 81 % comprising:
thermo-mechanical pulp; and a coating comprising a pigment to increase the brightness.

4. A paper having a brightness of at least about 81% comprising:
chemi-mechanical pulp; and a coating comprising a pigment to increase the brightness.

5. The paper according to any of claims 1 to 4 wherein the coating further comprises:

a second pigment;
a binder; and minor constituents including at least one dye and a Fluorescence whitening agent to adjust a white shade of the paper.

6. The paper according to any of claims 1 to 4 wherein the paper has a basis weight between 48.8 and 81 grams per square meter.

7. The paper according to any of claims 1 to 4 wherein the paper has an opacity between 85 and 96.

8. The paper according to any of claims 1 to 4 wherein between 2 and 4.5 grams/m2 of coating are applied to the paper.

9. The paper according to any of claims 1 to 4 wherein the pigment is ground calcium carbonate.

10. The paper according to claim 5 wherein the binder includes a latex.

11. The paper according to claim 10 wherein the binder further includes a starch.

12. A coating for use in manufacturing a coated paper having a brightness of at least about 81 % that is made using mechanical pulp, the coating comprising:

a pigment to increase the brightness; and a binder.

13.The coating according to claim 12 further comprising:
a second pigment; and minor constituents including at least one dye and a Fluorescence whitening agent to adjust a white shade of the paper.

14. The coating according to claim 13 wherein the pigment is ground calcium carbonate.

15.The coating according to claim 13 wherein the binder includes a latex.

16. The coating according to claim 15 wherein the binder further includes a starch.

17.A method of making a paper based on mechanical pulp comprising the steps of:

screening the pulp to remove particles of a size defined by the screen from the pulp;

washing the pulp;

bleaching the pulp to increase the brightness;
washing the pulp;

forming a paper from the pulp;

pressing the paper with a shoe press; and coating the paper with a coating having at least a pigment and a binder, wherein the paper has a brightness of at least about 81 %.

18. A method according to claims 17 wherein the mechanical pulp is made using a thermo-mechanical process.

19. A method according to claims 17 wherein the mechanical pulp is made using a chemi-mechanical process.

20.The method according to any one of claims 17 to 19 wherein the washing steps use around 6 m3 of water per ton of pulp.

21.The method according to any one of claims 17 to 19 wherein the pulp is made from a combination of spruce and fir.

22.The method according to any one of claims 17 to 19 further comprising the step of calendering the paper.

23.The method according to any one of claims 17 to 19 wherein the step of coating applies between 2 and 4.5 grams of coating per m2 of paper.

24. Use of a paper according to claim 1 in an offset printing process.

25. Use of a paper according to claim 1 in a digital printing process.

26. The use according to claim 24 wherein the digital printing process is an electrophotographic printing process.

27.The use according to claim 24 wherein the digital printing process is an ink jet printing process.