US20070215301A1

US20070215301A1 - Method for making a low density multi-ply paperboard with high internal bond strength

Info

Publication number: US20070215301A1
Application number: US11/686,742
Authority: US
Inventors: Daniel T. Bunker; Donald D. Halabisky; Shahrokh A. Naieni
Original assignee: Weyerhaeuser Co
Current assignee: Weyerhaeuser NR Co
Priority date: 2006-03-17
Filing date: 2007-03-15
Publication date: 2007-09-20
Also published as: CA2581898A1; CN101054781A; EP1835075A1; JP2007254948A; BRPI0702166A

Abstract

Methods for improving the internal bond strength of paperboard with greater than 25 percent crosslinked fiber in at least one ply are described. In the methods, additives are added to the slurry in various combinations and order while maintaining the ionic demand of the slurry at less than zero. Paperboard with high ZDT, Scott Bond and Taber Stiffness is obtained.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Application No. 60/783,624 filed Mar. 17, 2006.

FIELD

The present application relates to a method for increasing the bond strength in a multi-ply paperboard that has high crosslinked cellulose fiber present in at least one of the plies.

SUMMARY

This application is directed to a method improving the internal bond strength of paperboard with greater than 25 percent crosslinked fiber in at least one ply. In the method, additives are added to the slurry in various combinations and order while maintaining the ionic demand of the slurry at less than zero. Paperboard with high ZDT, Scott Bond and Taber Stiffness is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the pilot line.

DESCRIPTION

In single or multi-ply paperboard where the inner plies contain greater than approximately 25 percent crosslinked cellulose fiber the density of the stratum will drop below 0.4 g/cc. As a result, the internal bond strength can drop so low as to not only be well below levels required for converting the paperboard into packaging products but also below the level where conventional methods of increasing the internal strength cannot provide enough increase to meet minimum levels needed for converting. This effect can occur either in the entire structure or some fraction within the structure. The present application provides a method for increasing the internal bond of low density paperboard back into the range which is useable for converting.
In this application, the use of high concentrations of wet end additives have been demonstrated while producing low density paperboard.
A distinguishing characteristic of the present application is that at least one ply of the paperboard, whether a single-ply or a multiple-ply structure, contains crosslinked cellulose fibers and strength enhancing additives such as mechanically refined fiber, anionic and cationic starches and other additives to offset the board strength lost by adding the crosslinked cellulosic fibers. The crosslinked cellulosic fibers increase the bulk density of the insulating paperboard characteristics of the board. The paperboard also contains chemical pulp fibers. As defined herein, chemical pulp fibers useable in the present application are derived primarily from wood pulp. Suitable wood pulp fibers for use with the application can be obtained from well-known chemical processes such as the kraft and sulfite processes, with or without subsequent bleaching. Softwoods and hardwoods can be used. Details of the selection of wood pulp fibers are well known to those skilled in the art. For example, suitable cellulosic fibers produced from southern pine that are useable in the present application are available from a number of companies including Weyerhaeuser Company under the designations C-Pine, Chinook, CF416, FR416, and NB416. A bleached Kraft Douglas Fir pulp (D. Fir), and Grande Prairie Softwood, all manufactured by Weyerhaeuser are examples of northern softwoods that can be used. Mercerized fibers such as HPZ and mercerized flash dried fibers such as HPZ III, both manufactured by Buckeye Technologies, Memphis Tenn., and Porosinier-J-HP available from Rayonier Performance Fibers Division, Jessup, Ga. are also suitable for use in the present application when used with crosslinked cellulose fibers. Other non crosslinked cellulose fibers include chemithermomechanical pulp fibers (CTMP), bleached chemithermomechanical pulp fibers (BCTMP), thermomechanical pulp fibers (TMP), refiner groundwood pulp fibers, groundwood pulp fibers, TMP (thermomechanical pulp) made by Weyerhaeuser, Federal Way, Wash., and CTMP (chemi-thermomechanical pulp) obtained from NORPAC, Longview, Wash., sold as a CTMP NORPAC Newsprint Grade, jet dried cellulosic fibers and treated jet dried cellulosic fibers manufactured by the Weyerhaeuser Company by the method described in U.S. application Ser. No. 10/923,447 filed Aug. 20, 2004. These fibers are twisted kinked and curled. Additional fibers include flash dried and treated flash dried fibers as described in U.S. Pat. No. 6,837,970,
Suitable crosslinking agents for making crosslinked fibers include carboxylic acid crosslinking agents such as polycarboxylic acids. Polycarboxylic acid crosslinking agents (e.g., citric acid, propane tricarboxylic acid, and butane tetracarboxylic acid) and catalysts are described in U.S. Pat. Nos. 3,526,048; 4,820,307; 4,936,865; 4,975,209; and 5,221,285 The use of C₂-C₉polycarboxylic acids that contain at least three carboxyl groups (e.g., citric acid and oxydisuccinic acid) as crosslinking agents is described in U.S. Pat. Nos. 5,137,537; 5,183,707; 5,190,563; 5,562,740; and 5,873,979.
Polymeric polycarboxylic acids are also suitable crosslinking agents for making crosslinked fibers. These include polymeric polycarboxylic acid crosslinking agents are described in U.S. Pat. Nos. 4,391,878; 4,420,368; 4,431,481; 5,049,235; 5,160,789; 5,442,899; 5,698,074; 5,496,476; 5,496,477; 5,728,771; 5,705,475; and 5,981,739. Polyacrylic acid and related copolymers as crosslinking agents are described U.S. Pat. Nos. 5,549,791 and 5,998,511. Polymaleic acid crosslinking agents are described in U.S. Pat. No. 5,998,511 and U.S. Pat. No. 6,582,553. CHB405, a citric acid crosslinked cellulose fiber and CHB505, a polyacrylic acid crosslinked cellulose, both commercially available from Weyerhaeuser Company, Federal Way, Wash. were used in this work.
In single or multi-ply paperboard construction a mixture of wood pulp fibers and crosslinked cellulose fibers are used. In one embodiment the crosslinked cellulosic fibers are present in at least one layer at a level of 25 to 80 percent by total fiber weight of the ply. In another embodiment the crosslinked fibers are present at a level of 40 to 75 percent by total fiber weight of the layer and in yet another embodiment they are present at a level of 50 to 70 percent by total fiber weight of the layer.

Ionic Demand Balance

The technology relies on the ability to balance the ionic demand in the wet end of the paper machine such that 1) anionic polymeric materials can be retained on the fibers and fines without excess remaining in the water system, 2) the fibers and system do not pass through the zero charge point which destabilizes retention and drainage 3) since pulp fibers are anionic, some cationic material can be added, however, adding too much cationic material without balancing the excess anionic demand will either cause the fibers to flocculate reducing formation and/or cause the drainage to drop, impacting the runnability.
Each of the components used in the paperboard containing crosslinked fiber in this disclosure has a specific charge density typically measured by ionic demand titration. A Mutek PCD—Titrator was used for the particle charge titration coupled with the PCD 02 Particle Charge Detector for measuring the ionic demand of the component or fiber furnish. The method was performed according to a procedure from A.E. Staley Manufacturing, a subsidiary of Tate and Lyle, Decatur, Ill. The method is as follows.

1. Turn the Mutek on using the power switch on the back of the instrument.

2. Place 10 mL of a well mixed sample in the sample vessel. Insert the plunger and washer into the vessel. The sample consistency should be no more than 0.83. Thick stock samples should be diluted.
3. With the instrument turned on, the plunger should move up and down and a mV potential should be displayed. The sign of the potential (+ or −) indicates whether the sample is cationic (+) or anionic (−).
4. Titrate the sample with the appropriate titrant until the mV potential reads 0 mV (PolyDADMAC is the cationic polymer and is used to titrate anionic samples; PVSK or PESNa are the anionic polymers used to titrate the cationic samples). A buret or syringe can be used to deliver the titrant to the sample. Titration should not be conducted with more than 4 mL of titrant since higher volumes will give inaccurate measurements. If the sample requires more than 4 mL of titrant, the sample should be diluted or more concentrated titrant should be used.
5. Record the amount of titrant used to titrate the sample. To calculate the demand of the system, use the following equation:
“Ionic Demand” (ueq/L)=(mL titrant)×(% titrant dilution)×(sample dilution)
Ionic Demand refers to the amount of anionic or cationic charge required to neutralize the counter ion charge and is expressed in meq/g or ueq/kg. For example, an additive with an ionic demand of +2.2 meq/g has an anionic demand of 2.2 meq/g; an additive with an ionic demand of −1.8 meq/g has a cationic demand of 1.8 meq/g. Specific components whose ionic demand was measured by the Mutek method are noted in Table 1, other component values are from suppliers.

	TABLE I

	Component	Ionic Demand

Fully bleached Softwood kraft pulp	~−0.015	meq/g total
Fully bleached Softwood kraft pulp	~−0.0015	meq/g available
CHB405	~−0.43	meq/g total
CHB405	~−0.015	meq/g available
Kymene ® 557H	~+2.2	meq/g
Hercobond ® 2000	~−1.8	meq/g

STA-LOK ® 300

~+0.3

RediBOND ® 3050	~−0.19	meq/g
RediBOND ® 2038	~+0.24	meq/g
STA-LOK ® 330	~+0.41	meq/g
PPD M-5133	~+16	meq/g
GALACTASOL ® SP813D	~+2.3	meq/g

With reference to the table, the difference between the total and available ionic demand represents the amount of charge that is internal to the fiber that is not accessible to polymers of molecular weight above 300,000 g/mole. For papermaking, the available ionic demand is more representative of the results obtained in practice than the total ionic demand.
The situation is further complicated in a paper machine wet-end where dilution water from outside sources and/or wash water from pulp mill bleaching stages contain ionic materials, (both dissolved and dispersed), is used to control consistency of the pulp slurry. In integrated mills where excess ionic materials are present, materials added to the pulp slurry to increase internal bond strength can be consumed by the excess ionic materials. Also, the available ionic sites on pulp wilt also depend on how much refining has been done and on the basic fiber morphology, i.e. the smaller the fiber or partial fiber the higher available surface area, and therefore the higher available ionic demand.
In general it may be stated that the fiber slurry is anionic to start with and should remain anionic through the paper making process i.e. the ionic demand of the slurry should less than zero.
Mechanically refined fiber can be added to the slurry to increase the strength of the paperboard. In one embodiment the mechanically refined fiber has a Canadian Standard Freeness of less than 125 mL CSF, a curl index of ⅓ or less of the unrefined fiber and a kink angle of ½or less of the unrefined fiber.
In one embodiment mechanically refined fiber is added to the slurry followed by the addition of an anionic starch and then followed by addition of a cationic fixative. After each addition step the slurry ionic demand is less than zero. The slurry is deposited on a foraminous support, dewatered forming a web and dried to form a paperboard.
In one embodiment the total starch level on dry fiber is from 50 to 120 lb/t. In another embodiment the total starch level on dry fiber is from 60 to 100 lb/t. In yet another embodiment the total starch level is 80 to 90 lb/t.
Cationic fixatives such as cationic starch (e.g. STA-LOK® 300, STA-LOK® 330 and RediBOND®2038) have a low anionic demand i.e. less than 1 meq/g. Other cationic additives such as Kymene®557H have a high anionic demand (+2.2 meq/g). In one embodiment the cationic fixative has an anionic demand of greater than zero but less than one meq/g. In another embodiment the cationic fixative has an anionic demand of from 1 meq/g to 10 meq/g.
The paperboard of the present application may be one of several structures. In one embodiment the paperboard is a single ply structure, in another the paperboard is a two-ply structure and in yet another embodiment the paperboard is a multi-ply structure.
In the method, the addition order of the additive can vary. As stated earlier, in one embodiment, mechanically refined fiber is added to the slurry followed by the addition of an anionic starch and then followed by addition of a cationic fixative. After each addition step the slurry ionic demand is less than zero. The slurry is deposited on a foraminous support, dewatered forming a web and dried to form a paperboard. In another embodiment mechanically refined fiber is added to the slurry followed by the addition of a cationic fixative and then followed by addition of an anionic starch. After each addition step the slurry ionic demand is less than zero. The slurry is deposited on a foraminous support, dewatered forming a web and dried to form a paperboard. In yet another embodiment, mechanically refined fiber is added to the slurry followed by the addition of an anionic starch and then followed by addition of first cationic fixative, followed by adding a second cationic fixative. After each addition step the slurry ionic demand is less than zero. The slurry is deposited on a foraminous support, dewatered forming a web and dried to form a paperboard. In each case, the first cationic fixative may have an anionic demand of from 1 meq/g to 10 meq/g and the second fixant may have an anionic demand of greater than zero but less than 1.

Mechanically Refined Fiber and High Levels of Starch

Fiber and polymer binders were applied to low density board so that internal bond strength increases by 100% or more with 10% or less increase in density. The effect of refining on freeness and ionic demand is shown in Table 11.

TABLE II

Effect Of Refining On Ionic Demand

				Kink	Ionic
		CSF,	Curl	angle,	Demand,*
#Test	Fiber Description	mL	Index	°/mm	meq/g

1	LV Lodgepole Pine -	720	0.25	92	−0.0008
	Unrefined
2	LV Lodgepole Pine - EW	550	0.10	46	−0.0069
3	LV Lodgepole Pine - EW	275	0.07	31	−0.0118
4	LV Doug. Fir - Unrefined	675	0.23	64
5	LV Doug. Fir - EW	85	0.07	28	−0.0114
6	LV Lodgepole Pine - EW	65	0.05	18	−0.0167
7	LV Lodgepole Pine	33	0.05	21	−0.0114

*Fiber only
LV, Longview
EW, Escher Wyss
VB, Valley Beater

Each of the following Examples were generated as follows:

1. Handsheets formed using typical handsheet making equipment with an extension to reduce the forming consistency.
2. 250 gsm OD fiber.
3. 60% CHB405 (crosslinked fiber), dispersed independently; several methods were used interchangeably (Valley beater with no load, lab disk refiner with 1-2 amps over no load and a pilot scale deflaker. Mechanical dispersion was done to improve formation.
4. 40% Douglas Fir refined to 400 ml CSF; pH was adjusted to 7.
6. 4 #/t Aquapel sizing agent.
7. 5 #/t Kymene®557H.
8. 25 #/t cationic starch, (STA-LOK® 300)

The above formulation serves as a control; adjustments to the chemistry are noted in each Example.

As defined herein, mechanically refined fiber (MRF) is mechanically refined wood pulp for example, Lodgepole Pine having a Canadian Standard Freeness <125 mL, a index ½ or less of unrefined starting fiber and a kink angle of ½ or less of the unrefined starting fiber. Curl Index and kink angle were determined using a Fiber Quality Analyzer (FQA) as published in the Journal of Pulp and Paper Science 21(11):J367 (1995). Mechanically refined fiber can be generated to meet these criteria by different refining methods which have different impact on conventional fiber properties. Table III shows the effect on Z-direction tensile and density of various formulations with mechanically refined fiber. ZDT was determined by TAPPI 541.

TABLE III

Effect Of Mechanically Refined Fiber Addition On Strength Properties

	Den-
	sity	Change in	ZDT,	Δ ZDT,
Description	g/cc	Density	kPa	%

100% D. Fir	0.628	138	498	1138
Control (as above)	0.264		40.22
10% valley beater mechanically	0.292	10.6%	95.38	137
refined fiber replacing
10% D. Fir
5% Escher Wyss mechanically	0.274	3.7%	78.14	94
refined fiber replacing 5% D. Fir
5% Escher Wyss mechanically	0.274	3.7%	110	174
refined fiber replacing 5% D. Fir
5% Escher Wyss mechanically	0.259	−1.9%	52.86	31.4
refined fiber replacing 5% D. Fir
Control	0.232		24.8
5% Escher Wyss mechanically	0.238	2.5%	68.95	178
refined fiber replacing D. Fir
5% mechanically refined fiber	0.242	4.3%	85.5	244
replacing D. Fir (double disk
refined)
Control	0.255		40.22
5% Valley Beater mechanically	0.265	3.9	69.81	73.5
refined fiber replacing D. Fir
5% Valley Beater mechanically	0.256	0.4	80.15	99.3
refined fiber replacing D. Fir

Δ indicates “change in”

Internal bond strength can be increased by replacing some of the cationic starch with a higher ionic strength molecule such as Kymene® as shown in the following example.

50% CHB405.

50% Lodgepole Pine refined to 400 mL CSF.

10 #/t Kymene® 557H from Hercules.
10 #/t Stalok 400 cationic starch from Staley.

Mechanically refined Lodgepole pine fiber refined at 50 ml CSF using an Escher Wyss laboratory refiner.

In this formulation the level of the Kymene®557H with an ionic demand of +2.2 meq/g was doubled and the STA-LOK® 300 cationic starch with an ionic demand of +0.3 meq/g was reduced by 60%. As noted from the table, significant increases in ZDT bond strength and Scott Bond can be obtained by this method

TABLE IV

Effect On Strength Of Partial Replacement Of Cationic Starch
With A Higher Ionic Demand Polymer

Mechanically					Scott
refined fiber	Density	%	ZDT	%	Bond	%
% by wt.	g/cc	Increase	kPa	Increase	J/m²	Increase

0%	0.222		23		98
10%	0.234	+5.4%	62	+170%	135	+38%
20%	0.260	+14.6%	125	+443%	173	+76.5%

A third technology is use of a starch excess. The general approach was to overcome the normal limits of effective wet-end starch, balancing the charge in the wet-end by adding excess anionic starch and fixing it to the fibers by adding cationic starch or other high charge density cationic polymers thus balancing the system to near neutral charge density. The neutralization was important to prevent excessive flocculation and large impacts on drainage.
Specifically, total starch content added to the wet can be increased to 2% to 5% based on dry fiber. Anionic starch such as RediBOND® 3050 supplied by National Starch & Chemical or Aniofax® AP25 supplied by Carolina Starches can be used. Cationic fixatives include common cationic starches like STA-LOK® 300 supplied by Staley Corp., Poly Aluminum Chloride (PAC) like Nalco ULTRION® 8187 or high charge density cationic polymers like M5133 and M5134, GALACTAOL® SP813D (anionic guar) and Kymene® 557H supplied by Hercules Corp. and Nalco NALKAT® 62060 (branched EPEDMA) Nalco NALKAT® 2020 (poly DADMAC). As used herein, a high ionic demand is represented by a polymer that has an ionic demand of 1 meq/g to 17 meq/g, either as an anionic demand or as a cationic demand. For example, Kymene®557H has an anionic demand of 2.2 meq/g and Hercobond®2000 has a cationic demand of 1.8 meq/g.
The level of anionic starch needed to obtain high strength development depends on the charge density and more importantly on the retention. Typically, 2% to 5% addition level based on dry fiber is adequate. The amount of cationic fixative depends entirely on the size of the polymer and the cationic charge density. As defined herein, a fixative is a charged polymer that ionically bonds to a molecule of the opposite charge. In general the higher the charge density the smaller the amount required and for equal charge density the larger the polymer the smaller the amount required.
The following data was based on laboratory handsheets of the following formulation:

1. Handsheets formed using typical handsheet making equipment with an extension to reduce the forming consistency.
2. 250 gsm OD fiber.
3. 60% CHB405, dispersed independently; several methods were used interchangeably, (Valley Beater with no load, lab disk refiner with 1-2 amps over no load condition, and a pilot scale deflaker). Mechanical dispersion is done to improve formation.
4. 40% Douglas Fir refined to 400 ml CSF.
5. pH adjusted to 7.
6. 5#/t Kymene® 557H.
7. 4#/t Aquapel 625 sizing agent.
8. 25#/t cationic starch (STA-LOK® 300)

Adjustments to the chemistry are noted in each Example.

EXAMPLE 1

The above described handsheet is the control. The following adjustments were made to the non-fiber portion of the furnish, 80 lbs/t (4%) Aniofac® AP25 was mixed with the fibers, followed by 20 lbs/t cationic starch STA-LOK® 300. Then Kymene®557H was added and the amount increased to 10 lbs/t. Last, before sheet making a blend of cationic starch STA-LOK® 300 and Aquapel 625 were added, the cationic starch was reduced to 20 lbs/t and the Aquapel was kept constant at 4 lbs/t. Handsheets were evaluated for density, ZDT and Scott Bond the results are in Table V.

TABLE V

	Density	ZDT	Scott Bond
Description	g/cc	kPa	J/m²

Control		0.243	45	80
4% Anionic Starch	Example 1	0.273	204	119
(Aniofac ® AP25)

EXAMPLE 2

The control handsheet as described above was adjusted as follows to the non-fiber portion. 40 lbs/t Aniofax AP25 was mixed with the fibers, followed by 20 lbs/t STA-LOK® 300 cationic starch. Kymene® 557H at 5#/t was added and the same combination of Stalok 300 and Aquapel 625 as in Example 1, i.e. 20 lb/t and 4 lb/t, respectively. Handsheets were evaluated for density, ZDT and Scott Bond; the results are in Table V1 combined with the results from Example 1.

TABLE VI

	Density	ZDT	Scott Bond
Description	g/cc	kPa	J/m²

Control		0.243	45	80
4% Anionic Starch*	Example 1	0.273	204	119
2% Anionic Starch*	Example 2	0.262	85	113

*Aniofac ® AP25

EXAMPLE 3

The handsheet formulation described in Example 2 was altered to contain mechanically refined fiber fibers so that the fiber portion of the furnish is:

60% CHB405.

35% Fully Bleached D. Fir refined to 400 mL CSF.

5% Valley beater mechanically refined fiber—fully bleached kraft Lodgepole Pine at ˜50 mL CSF.

The remainder of the additives are the same as in Example #2. The results are shown in Table V11.

TABLE VII

	Density	ZDT	Scott Bond
Description	g/cc	kPa	J/m²

Control		0.243	45	80
4% Anionic Starch*	Example 1	0.273	204	119
2% Anionic Starch*	Example 2	0.262	85	113
Control + 5%		0.274	78	106
Mechanically refined fiber
2% anionic starch + 5%	Example 3	0.283	122	148
mechanically refined fiber

*Aniofac ® AP25

EXAMPLE 4

Adjustments were made to the non-fiber portion of the of the handsheet formulation described in Example 3 (containing mechanically refined fiber) as follows: 100 lb/t Aniofax® AP25 (was blended with the fibers followed by 90 lb/t Nalco 8187 PAC, then 5 lb/t Kymene® 557H, 5 lb/t STA-LOK® 300 and 4 lb/t Aquapel 625. The results are shown in Table VIII.

TABLE VIII

	Density	ZDT	Scott Bond
Description	g/cc	kPa	J/m²

Control		0.243	45	80
Control + 5% mechanically		0.274	78	106
refined fiber
5% Anionic starch* + 4.5%	Example 4	0.304	142	214
PAC

*Aniofac ® AP25

EXAMPLE 5

Adjustments were made to the non-fiber portion of the of the handsheet formulation described in Example 3 (containing mechanically refined fiber) as follows: 50 lb/t Aniofax® AP25 was blended with the fibers followed by 8 lb/t Nalco 62060 poly, then 5 lb/t Kymene® 557H, 5 lb/t STA-LOK® 300 and 4 lb/t Aquapel 625. The results are shown in the Table IX.

TABLE IX

	Density	ZDT	Scott Bond
Description	g/cc	kPa	J/m²

Control		0.243	45	80
Control + 5% Mechanically		0.274	78	106
refined fiber
5% Anionic starch* + 4.5%	Example 4	0.304	142	214
PAC
2.5% Anionic starch* + 0.4%	Example 5	0.284	109	138
Poly DADMAC

*Aniofac ® AP25

EXAMPLE 6

Adjustments were made to the non-fiber portion of the of the handsheet formulation described in Example 3 (containing mechanically refined fiber) as follows: 100 lb/t Aniofax® AP25 was blended with the fibers followed by 6 lb/t Nalco 2020 poly, then 5 lb/t Kymene® 557H, 5 lb/t STA-LOK® 300 and 4 lb/t Aquapel 625. The results are shown in Table X.

TABLE X

	Density	ZDT	Scott Bond
Description	g/cc	kPa	J/m²

Control		0.243	45	80
Control + 5% Mechanically		0.274	78	106
refined fiber
5% Anionic starch* + 4.5%	Example 4	0.304	142	214
PAC
2.5% Anionic starch* + 0.4%	Example 5	0.284	109	138
Poly DADMAC
5% Anionic Starch* + 0.3%	Example 6	0.279	218	215
Poly

*Aniofac ® AP25

Single-ply handsheets designed to simulate the mid-ply of low density multi-ply paperboard were made. A 0.015 percent to 0.035 percent consistency slurry was used in these studies. Handsheet making equipment was standard 8″×8″ sheet mold modified with an extended headbox so that twice the normal volume of stock was used. This modification was necessary to improve handsheet formation when using materials designed to generate high bulk (e.g. crosslink fiber such as CHB405 and CHB505). Fiber weights are expressed as a weight percent of the total fiber dry weight; additives are based on weight of dry fiber.
A series of handsheets were made using different levels of wet-end additives, different addition order and some changes in fiber furnish to demonstrate the level of internal bond strength that could be generated by starch loading the web. The additives were added to the slurry in the order across each sample row and the slurry stirred after each addition.

Series 1.

The Table XI below shows the conditions and formulations used when making the series of handsheets

TABLE XI-A

Handsheet Formulation And Addition Order.

						Anionic	Cationic	Anionic		Cationic
				PVOH		Starch	Starch	Starch		Starch
	Target			Celanese	Mechanically	Avebe	STA-	Avebe		STA-
	Basis		D.	Celvol	Refined	Aniofax	LOK ®	Aniofax	Kymene ®	LOK ®	Aquapel
	wt.	CHB405	Fir	165SF	Fiber*	AP25	300	AP25	557H	300	650
Code	g/m²	%	%	%	%	#/t	#/t	#/t	#/t	#/t	#/t

1	250	60%	30%	5%	5%	0	0	0	5	25	4
2	250	60%	35%	0%	5%	0	0	0	5	25	4
3	250	60%	40%	0%	0%	40	20	0	5	20	4
4	250	60%	35%	0%	5%	40	20	0	5	20	4
5	250	60%	35%	0%	5%	0	20	40	5	20	4
6	250	60%	35%	0%	0%	80	20	0	10	10	4
7	250	60%	35%	0%	5%	80	20	0	10	10	4
8	250	60%	35%	0%	5%	0	10	80	15	10	4
9	250	60%	40%	0%	0%	0	0	0	5	25	4

*Lodgepole Pine refined with Valley Beater to 33 CSF

TABLE XI-B

Calculated Ionic Demand As Chemical Additions Are Made In Table XI-A

							Ionic				Ionic
					Ionic	Ionic	strength	Ionic	Ionic	Ionic	strength
					demand,	strength	with	strength	strength	strength	with
	Ionic	Ionic	Ionic	Ionic	total	with	STA-	with	with	with STA-	Aquapel
	demand	demand	demand	demand	pulp and	Aniofax ®	LOK ®	Aniofax ®	Kymene ®	LOK ®	650 - end
	CHB405	D. Fir	PVOH	MRF	particles	AP25	300	AP25	557H	300	point
Code	ueq/g	ueq/g	ueq/g	ueq/g	ueq/g	ueq/g	ueq/g	ueq/g	ueq/g	ueq/g	ueq/g

1	−9	−0.45	0	−0.57	−10.02	−10.02	−10.02	−10.02	−4.52	−0.89	−0.89
2	−9	−0.53	0	−0.57	−10.1	−10.1	−10.1	−10.1	−4.60	−0.97	−0.97
3	−9	−0.6	0	0	−0.96	−14.2	−11.3	−11.3	−5.8	−2.9	−2.9
4	−9	−0.53	0	−0.57	−10.1	−14.7	−11.8	−11.8	−6.3	−3.40	−3.40
5	−9	−0.53	0	−0.57	−10.1	−10.1	−7.20	−11.8	−6.3	−3.40	−3.40
6	−9	−0.53	0	0	−9.53	−18.7	−15.8	−15.8	−4.8	−3.38	−3.38
7	−9	−0.53	0	−0.57	−10.1	−19.3	−16.4	−16.4	−5.4	−3.94	−3.94
8	−9	−0.53	0	−0.57	−10.1	−10.1	−0.86	−0.86	−1.34	0.11	0.11
9	−9	−0.6	0	0	−0.96	−0.96	−0.96	−0.96	−4.1	−0.48	−0.48

Each handsheet was then coated with Polyvinyl Alcohol (PVA) coating, Celvol V24203 supplied by Celanese Ltd. The total coat weight was about 50 g/m²and was divided equally between each side of the sheet. The coating was added to the surface to facilitate testing Z-direction tensile (ZDT) and internal Scott Bond because low density structures without the coating tend to separate at the tape instead of the within the sheet.
Each sheet was evaluated for several physical properties including basis weight, caliper, ZDT, internal Scott Bond and Taber Stiffness (15°). Scott Bond and Taber Stiffness were determined by TAPPI T 569 om-00 and T 489 om-04, respectively. Table XII below shows the results for these key characteristics

TABLE XII

Physical Characteristics of Laboratory Handsheets as Described in Table
XI

	Basis		Z-direction	Scott	Taber
	Weight	Density	Tensile	Bond	Stiffness
Sample	g/m²	g/cm³	kPa	J/m²	g cm

1	297	0.267	124	158	375
2	296	0.274	78	106	316
3	306	0.262	85	113	433
4	303	0.283	122	147	387
5	303	0.281	121	142	388
6	305	0.273	204	119	403
7	301	0.284	179	128	374
8	303	0.284	140	116	394
9	301	0.264	40	67	364

Samples 1, 2 and 9 can be considered the controls for this experiment. Sample number 9 is a fiber formulation designed to deliver low density paper and uses typical wet end chemistry (i.e. cationic starch and Kymene®557H). The result is a very low Scott Bond, but typical Taber Stiffness. Sample 2, incorporates mechanically refined fiber in an effort to increase the internal bond and, by itself, results in an increase in ZDT and Scott Bond, but not enough to reach the targets needed for converting multi-ply paperboard. It is estimated the minimum necessary ZDT needed for converting is about 175-190 kPa.
Sample 1 incorporates mechanically refined fiber with a particle PVOH—known as a good binder but is hindered by issues with retention, cost and process reliability impacts. The increase in ZDT and Scott Bond for sample 1 begins to approach the amount needed for converting paperboard. Samples 3 and 4 show that by adding 4% total starch to the furnish the ZDT and Scott Bond essentially double. Adding mechanically refined fiber, Sample 4, gives an increase of about the same magnitude as it did to the original structure, Sample 2 v. Sample 4 and Sample 3 v. Sample 9.
Sample 5 shows reversing the order (i.e. adding cationic starch first then anionic starch) in which the cationic and anionic starch are added makes no difference to the strength development
Samples 6 and 7 are a case where the amount of anionic starch is doubled while the cationic starch remains constant. Kymeme® 557H, a higher charge density cationic polymer, is used to balance the additional anionic charge. The result is further increase in internal bond, increasing ZDT by 500%, (Sample 6) over the control and Scott Bond is unaffected by the additional starch and Kymene® 557H. Sample 7 shows that by adding mechanically refined fiber the effect on ZDT is negative in this case, yet the Scott Bond increases.
Sample 8 adjusts the source of cationic charge further, increasing the amount of Kymene®557H and decreasing the amount of cationic starch. In Table XI-B the ionic demand of the system crosses from negative to positive at the last point of cationic starch addition and from Table XII the corresponding ZDT and Scott bond are further reduced indicating that when the ionic demand exceeds zero the effectiveness of the ionic binding system is reduced.
The ZDT is reduced further, indicating that higher charge density polymer is less effective than cationic starch in adding internal bond strength. In general the impact of the starch loading on Taber Stiffness at 15° is small. For single ply handsheets this is reasonable because caliper is the dominating variable effecting bending stiffness. The impact of the starch loading on density is small enough that the increase in elastic modulus of the sheets due to the starch loading compensates for the small changes in caliper. In a multi-ply web the same response would be expected.

Series 2

A second set of handsheets was produced to determine the impact of using high charge density cationic polymers to retain additional anionic starch. It is thought that using higher charge density polymers less total starch would be necessary to achieve the same strength due to better retention. The result would reduce the risk of affecting drainage and formation by adding excess starch. Table XIII shows the formulations used in the experiments.

TABLE XIII

Handsheet Formulation and Addition Order

				Anionic	Cationic					Cationic
		D. Fir @		Avebe	STA-					STA-
	CHB	500 ml		Aniofax ®	LOK ®	Nalco	Nalco	Nalco	Kymene ®	LOK ®
	405	CSF	MRF	AP25	300	8187	62060	2020	557H	300
Code	Wt. %	Wt. %	Wt. %	#/t	#/t	#/t	#/t	#/t	#/t	#/t

1	60	35	5	80	20				5	5
2	60	35	5	50		25			5	5
3	60	35	5	50		40			5	5
4	60	35	5	100		60			5	5
5	60	35	5	100		90			5	5
6	60	35	5	50			4		5	5
7	60	35	5	50			8		5	5
8	60	35	5	100			6		5	5
9	60	35	5	100			12		5	5
10	60	35	5	50				4	5	5
11	60	35	5	50				8	5	5
12	60	35	5	100				6	5	5
13	60	35	5	100				12	5	5
14	0	100	0						5	25

All Codes at 250 g/m²target; all additives are on a dry fiber wt. basis
Aquapel 650 at 4 #/ton was used in all the studies (dry fiber weight basis)
MRF: Mechanically refined Fiber

Each handsheet was then coated with Polyvinyl Alcohol (PVA) coating, Celvol V24203 supplied by Celanese Ltd. The total coat weight was about 50 g/m²and was divided equally between each side of the sheet. The coating was added to the surface to facilitate testing Z-direction tensile (ZDT) and internal Scott Bond, because low density structures without the coating tend to separate at the tape instead of the within the sheet.
Each sheet was evaluated for several physical properties including basis weight, caliper, ZDT, Internal Scott Bond, Taber Stiffness (15°) and other. The table below shows the results for these key characteristics

TABLE XIV

Physical Characteristics Of Laboratory Handsheets As Described In
Table XIII

			Z-direction		Taber
	Basis Weight	Density	Tensile	Scott Bond	Stiffness
Code	g/m²	g/cm³	kPa	J/m²	g cm

1	306	0.287	186	132	392
2	317	0.285	107	137	408
3	314	0.278	83	130	394
4	311	0.287	118	164	388
5	320	0.304	142	214	400
6	303	0.286	87	122	373
7	309	0.284	109	138	392
8	304	0.272	107	104	380
9	306	0.276	118	141	389
10	305	0.281	100	112	384
11	308	0.282	103	141	394
12	308	0.279	218	215	390
13	307	0.276	122	109	394
14	270	0.628	498	329	116

From Table XII - control with normal strength additives, as a reference.

9	301	0.264	40	67	364

Sample code 9 from Tables X1 and XII above is the base case.
The impact of using PAC such as Nalco 8187 (codes 2-5) to retain the anionic starch in the presence of mechanically refined fiber is less than that of using cationic starch on ZDT, however the impact on Scott Bond is greater, suggesting that the PAC improves the retention of the mechanically refined fiber giving greater shear strength to the board.
For codes 6-9 using NALKAT®62060 a branched EPEDMA cationic polymer as a fixative, the impact at 2.5% anionic starch addition is roughly the same as the PAC Nalco 8187) but significantly less ZDT development relative to the cationic starch, Code 1. At the 5% anionic starch addition level there was no further significant gain
Use of the polyDADMAC (codes 10-13) as a fixative shows more promise than the other two cationic polymers at the 5% added starch dose where it exceeded (Code 12) the cationic starch in ZDT and Scott Bond development vs Code 1
The last code in Tables XIII and XIV, Code 14, is that of a normal density board, included for comparison. The higher ZDT and Scott Bond come at the expense of bending stiffness.
In general, at equal total starch levels it appears that more ZDT is developed when using combination of anionic and cationic starch than when using higher charge density cationic polymers in combination with anionic starch. However, both methods develop significant ZDT. Scott Bond has the opposite result. The shear strength appears to increase at a greater rate than the ZDT when using higher charge density cationic polymers in combination with anionic starch.
Finally, the impact of the starch loading, independent of the cationic fixative has little effect on the product density and therefore little impact on the bending stiffness.

Pilot Trial

The disclosure was further explored using a pilot paper machine where dynamic drainage and white water re-circulation could be used to improve the simulation of commercial application on a paper machine.
The fiber furnish used in all of the following examples was the same, only the chemical additives and the order of addition were changed. The fiber components were:

60% Weyerhaeuser CHB405

35% Weyerhaeuser fully bleached kraft D. Fir wet lap refined to ˜500 mL CSF

5% Douglas Fir refined to 85 ml CSF (Escher Wyss refined)

The chemical components were combinations of some or all of the following, levels and addition order are shown in Table XV.

Kymene® 557H supplied by Hercules Incorporated (cationic wet strength resin)

Aquapel® 650 supplied by Hercules Incorporated (AKD sizing agent)

Hercobond® 2000 supplied by Hercules Incorporated (anionic polyacrylamide, retention aid)

RediBOND® 3050 supplied by Hercules Incorporated (anionic starch)

RediBOND® 2038 supplied by Hercules Incorporated (cationic starch)

PPD M-5133 supplied by Hercules Incorporated (cationic high charge density polymer)

GALACTASOL® SP813D supplied by Hercules Incorporated (cationic guar gum)

The pilot paper machine was a standard Fourdrinier type single ply former. The design is such that there are several chemical addition points so that wet end additive effects can be studied. FIG. 1 shows the basic unit operations with the chemical addition points. indicated as lower case letters as in Table XV. The addition points have been labeled and should be used as a reference for the formulations shown in Table XV.
Other unit operations were changed to maximize the bulk, for example, lowering the amount of vacuum on the forming table suction boxes, lifting the Dandy rolls away from the web using only one wet press, a normal drying profile, no size press (typical solids entering dryer 32-36%) no calendaring and finally, samples were taken for evaluation at the reel (eliminating effect of reel tension)
For each different formulation, the machine was run 10 to 15 minutes after making adjustments and insuring the basis weight was on target. In this way, the white water was completely turned over and reach equilibrium with the new chemistry. Target basis weight was 200 g/m².
Each sample was then coated with Polyvinyl Alcohol (PVA) coating, Celvol V24203 supplied by Celanese Ltd. The total coat weight was about 22 g/m²and was divided equally between each side of the sheet. The coating was added to the surface to facilitate testing Z-direction tensile (ZDT) and Internal Scott Bond, because low density structures without the coating tend to separate at the tape instead of the within the sheet.

TABLE XV

Wet End Additives for Pilot Paper Machine Starch Loading Trials.

Code	b	c	d	e	g	h

1	5 #/t Kymene ®	2 #/t Hercobond			10 #/t RediBOND 2038	4.5 #/t Aquapel 650
	557H	2000
2	20 #/t	20 #/t RediBOND	5 #/t	2 #/t Hercobond	10 #/t RediBOND 2038	4.5 #/t Aquapel 650
	RediBond2038	3050	Kymene ®	2000
			557H
3	20 #/t RediBond	20 #/t RediBOND	5 #/t		10 #/t RediBOND 2038	4.5 #/t Aquapel 650
	2038	3050	Kymene ®
			557H
4	30 #/t RediBond	40 #/t RediBOND	5 #/t	2 #/t Hercobond	10 #/t RediBOND 2038	4.5 #/t Aquapel 650
	2038	3050	Kymene ®	2000
			557H
5	20 #/t RediBond	60 #/t RediBOND	10 #/t	2 #/t Hercobond	10 #/t RediBOND 2038	4.5 #/t Aquapel 650
	2038	3050	Kymene ®	2000
			557H
6	20 #/t RediBond	10 #/t Kymene ®	20 #/t	2 #/t Hercobond	10 #/t RediBOND 2038	4.5 #/t Aquapel 650
	2038	557H	RediBOND	2000
			3050
7	30 #/t RediBond	10 #/t Kymene ®	40 #/t	2 #/t Hercobond	10 #/t RediBOND 2038	4.5 #/t Aquapel 650
	2038	557H	RediBOND	2000
			3050
8	40 #/t RediBond	20 #/t RediBOND	5 #/t		10 #/t RediBOND 2038	4.5 #/t Aquapel 650
	2038	2038	Kymene ®
			557H
9	40 #/t	2 #/t	8.2 #/t		10 #/t RediBOND 2038	4.5 #/t Aquapel 650
	RediBOND 3050	M-5133	Hercobond
			2000
10	5 #/t Kymene ®	2 #/t Hercobond			10 #/t RediBOND 2038	4.5 #/t Aquapel 650
	557H	2000
11	30 #/t	10 #/t Kymene ®	40 #/t	2 #/t Hercobond	10 #/t RediBOND 2038	4.5 #/t Aquapel 650
	RediBOND 2038	557H	RediBOND	2000
			3050
12	30 #/t	5 #/t	40 #/t	2 #/t Hercobond	10 #/t RediBOND 2038	4.5 #/t Aquapel 650
	RediBOND 2038	Kymene ® 557H	RediBOND	2000
			3050
13	30 #/t	2.5 #/t Kymene ®	40 #/t	2 #/t Hercobond	10 #/t RediBOND 2038	4.5 #/t Aquapel 650
	RediBOND 2038	557H	RediBOND	2000
			3050
14	30 #/t	40 #/t RediBOND	5 #/t	2 #/t Hercobond	10 #/t RediBOND 2038	4.5 #/t Aquapel 650
	RediBOND 2038	3050	Kymene ®	2000
			557H
15	8 #/t	40 #/t RediBOND	5 #/t	2 #/t Hercobond	8 #/t GALACTASOL	4.5 #/t Aquapel 650
	GALACTASOL	3050	Kymene ®	2000	SP813D
	SP813D		557H
16	40 #/t	20 #/t RediBOND	5 #/t		4 #/t GALACTASOL	4.5 #/t Aquapel 650
	RediBOND 3050	2038	Kymene ®		SP813D
			557H

Lower case letters refer to the additive addition points in FIG. 1

Codes number 1 and 10 are controls for two different running days, code 14 is a repeat of code 4 on a different day and code 11 is a repeat of code 7 on a different day. The physical characteristics of the resultant paper are shown in Table XV.

TABLE XVI

Physical Characteristics of Pilot Paper Machine Samples in Table XV

				Geometric	Geometric
	Basis		Z-direction	Mean Scott	Mean Taber
	Weight	Density	Tensile	Bond	Stiffness
Code	g/m²	g/cm³	kPa	J/m²	g cm

1	228	0.271	161	198	135
2	256	0.273	194	242	163
3	251	0.283	204	258	161
4	245	0.274	219	252	166
5	237	0.284	265	236	147
6	236	0.276	241	227	146
7	239	0.279	232	236	146
8	262	0.288	199	246	190
9	236	0.260	134	189	156
10	231	0.271	192	227	140
11	230	0.295	350	354	127
12	229	0.290	360	340	131
13	234	0.288	319	323	131
14	244	0.292	339	340	144
15	228	0.288	305	297	134
16	243	0.273	269	345	146

Pilot Machine Versus Control

The effect of starch loading is basically the same, it is estimated that the target internal bond strength that would be enough for performance during converting is about 2× of the control samples. For the pilot trial ZDT doubled and Scott Bond increased 75%.
By loading the wet-end with between 2% and 4% total starch (anionic and cationic), ZDT can be increased by approximately 25% to 85% relative to the same furnish with conventional levels of cationic starch (Codes 2-8, 11-14).
Loading up to 2% anionic starch into the wet-end and using high charge density cationic polymer (code 9) to retain the starch little or no gain in ZDT or Scott Bond was achieved.
Loading the wet-end with up to 2% anionic starch and using cationic guar gum (codes 15 and 16) to improve retention as a substitute for cationic starch about 40%-50% increase in ZDT was obtained.
When changing the order of addition, indications were that adding the anionic starch after the cationic material resulted in better strength efficiency.
Starch loading resulted in an increase in density of <10% in all cases and had no significant impact on stiffness.

Claims

1. A method for forming at least one ply of a paperboard comprising the steps of:

forming a slurry of cellulose fibers comprising crosslinked fibers;

adding mechanically refined fiber;

adding an anionic starch subsequent to adding said mechanically refined fiber;

adding a cationic fixative subsequent to adding said anionic starch;

wherein, after each addition step, the slurry ionic demand is less than zero;

depositing said slurry on a foraminous support;

forming a fibrous web layer by withdrawing liquid from said slurry;

drying said web to form a paperboard.

2. The method of claim 1 wherein said crosslinked fibers are present at a level from 25 to 80 percent of the total fiber weight in at least one ply of said paperboard.

3. The method of claim 1 wherein the total starch level is 50 to 120 lb/t.

4. The method of claim 1 wherein the mechanically refined fiber has a CSF of less than 125 CSF, a curl index of ⅓ or less of the unrefined fiber and a kink angle of ½ or less of the unrefined fiber.

5. The method of claim 1 wherein the cationic fixative has an anionic demand of greater than zero but less than 1 meq/g.

6. The method of claim 1 wherein the cationic fixative has an anionic demand of from 1 meq/g to 10 meq/g.

7. The method of claim 1, wherein said paperboard is at least a two-ply board, said at least one ply containing said crosslinked fibers.

8. The method of claim 1, wherein said paperboard is at least a three-ply board, said at least one ply containing said crosslinked fibers.

9. A method for forming a paperboard comprising the steps of:

forming a slurry of cellulose fibers comprising crosslinked fibers;

adding mechanically refined fiber,

adding a cationic fixative and mixing with said slurry;

adding an anionic starch subsequent to adding said cationic fixative;

wherein, after each addition step, the slurry ionic demand is less than zero;

depositing said slurry on a foraminous support;

forming a fibrous web layer by withdrawing liquid from said slurry;

drying said web to form a paperboard.

10. The method of claim 9 wherein said crosslinked fibers are present at a level from 25 to 80 percent of the total fiber weight in at least one ply of said paperboard.

11. The method of claim 9 wherein the total starch level is 50 to 120 lb/t.

12. The method of claim 9 wherein the mechanically refined fiber has a CSF of less than 125 CSF, a index of ⅓ or less of the unrefined fiber and a kink angle of ½ or less of the unrefined fiber.

13. The method of claim 9 wherein the cationic fixative has an anionic demand of greater than zero but less than 1 meq/g.

14. The method of claim 9 wherein the cationic fixative has an anionic demand of from 1 meq/g to about 10 meq/g.

15. The method of claim 9, wherein said paperboard is at least a two-ply board, said at least one ply containing said crosslinked fibers.

16. The method of claim 9, wherein said paperboard is at least a three-ply board, said at least one ply containing said crosslinked fibers.

17. A method for forming at least one ply of a paperboard comprising the steps of:

forming a slurry of cellulose fibers comprising crosslinked fibers;

adding mechanically refined fiber;

adding an anionic starch subsequent to adding said mechanically refined fiber;

adding a first cationic fixative subsequent to adding said anionic starch;

adding a second cationic fixative subsequent to adding said first cationic fixative;

wherein, after each addition step, the slurry ionic demand is less than zero;

depositing said slurry on a foraminous support;

forming a fibrous web layer by withdrawing liquid from said slurry;

drying said web to form a paperboard.

18. The method of claim 17 wherein said crosslinked fibers are present at a level from 25 to 80 percent of the total fiber weight in at least one ply of said paperboard.

19. The method of claim 17 wherein the total starch level is 50 to 120 lb/t.

20. The method of claim 17 wherein the mechanically refined fiber has a CSF of less than 125 CSF, a curl index of ⅓ or less of the unrefined fiber and a kink angle of ½ or less of the unrefined fiber.

21. The method of claim 17 wherein the first cationic fixative has an anionic demand of from 1 meq/g to 10 meq/g.

22. The method of claim 17 wherein the second cationic fixative has an anionic demand of greater than zero but less than 1 meq/g.

23. The method of claim 17, wherein said paperboard is at least a two-ply board, said at least one ply containing said crosslinked fibers.

24. The method of claim 17, wherein said paperboard is at least a three-ply board, said at least one ply containing said crosslinked fibers.