Title:
Surface sizing with sizing agents and glycol ethers
Kind Code:
A1


Abstract:
In this invention a composition is applied to the surface of paper comprising a glycol ether based polymer that provides enhanced ink-jet print quality. The advantage of using the composition is that it improves ink-jet printing on the paper without significant loss of water hold-out. More specifically, the paper is uncoated and has been treated on the surface with the following materials and dried to the point of usefulness. These materials are:
  • 1) one or more compounds that increase water hold-out. These are known as sizing agents,
  • 2) starch,
  • 3) A glycol ether based polymer.



Inventors:
Varnell, Daniel F. (Wilmington, DE, US)
Application Number:
11/652232
Publication Date:
07/10/2008
Filing Date:
01/10/2007
Primary Class:
Other Classes:
162/164.3
International Classes:
D21H21/16
View Patent Images:



Primary Examiner:
KILIMAN, LESZEK B
Attorney, Agent or Firm:
Joanne Mary Fobare Rossi (Hercules Incorporated Hercules Plaza 1313 North Market Street, Wilmington, DE, 19894-0001, US)
Claims:
1. An uncoated cellulosic paper having a front and a back surface, having been treated on at least one surface with: a) at least one sizing agent present in an amount of between 0.01 to 0.3% based on dry weight of paper; b) starch present in an amount of between 1 to 8% based on dry weight of paper c) a glycol ether based polymer present in an amount less than 0.025% based on dry weight of paper and optionally, a divalent metal salt.

2. The cellulosic paper of claim 1 wherein the glycol ether based polymers primary backbone is selected from polyethylene glycol, or polypropylene glycol.

3. The cellulosic paper of claim 1 wherein the glycol ether based polymers have glycidyether functional groups.

4. The cellulosic paper of claim 1 wherein the glycol ether based polymers primary backbone is selected from polyethyleneglycol diglycidyl ether or polypropylene glycol diglycidyl ether.

5. The cellulosic paper of claim 1 wherein the sizing agent is selected from alkyl succinic anhydride, ketene dimers, or alkenyl ketene dimers.

6. An aqueous composition for surface treating paper through a size press, which composition contains: a) Sizing agent in an amount of 0.03-1.0%, b) starch present in an amount of 1.5-12% c) a glycol ether based polymer present in an amount of less than 0.25% of the composition.

7. The composition of claim 6 wherein the glycol ether polymers primary backbone is either polyethylene glycol or polypropylene glycol.

8. The composition of claim 6 wherein the glycol ether polymers primary backbone Is selected from polyethyleneglycol diglycidyl ether or polypropylene glycol diglycidyl ether.

9. The composition of claim 6 wherein the glycol ether based polymers have glycidyl ether based functional groups.

10. The composition of claim 6 wherein the sizing agent is selected from alkyl succinic anhydride, ketene dimers, or alkenyl ketene dimers.

11. Process for improving ink-jet printing properties of paper which comprises treating paper with a composition of claim 6.

12. Process of claim 11 wherein the glycol ether based polymer is selected from polyethylene glycol, polypropylene glycol, polyethyleneglycol diglycidyl ether, or polypropylene glycol diglycidyl ether.

13. Process of claim 11 wherein the glycol ether based polymers have glycidyl ether functionality.

14. The process of claim 12 wherein the sizing agent is a reactive sizing agent.

15. Process of claim 11 wherein the sizing agent is selected from alkyl succinic anhydride, ketene dimers, or alkenyl ketene dimers.

16. The cellulosic paper of claim 1 wherein the sizing agent is a combination of a reactive dimer sizing agent and a polymeric latex sizing agent.

17. The process of claim 12 wherein the sizing agent is a combination of a reactive dimer sizing agent and a polymeric latex sizing agent.

18. Aqueous composition containing a glycol ether based polymer and an emulsified sizing agent where the glycol ether based polymer is present in an amount of 3-50% based on amount of sizing agent.

19. The cellulosic paper of claim 1 wherein the starch is present in an amount of between 2 to 7%, and the glycol ether based polymer is present in an amount less than 0.02%.

Description:

FIELD OF THE INVENTION

This invention describes the improvement of uncoated fine paper by treatment of the surface of the paper with polymers based on glycol ethers used in combination with starch and paper sizing agents. The invention results in paper with enhanced ink-jet print quality. The enhanced qualities include less mottle on the printed images, less strike-through to the back side of the paper when printed, and enhanced optical density of printed images.

BACKGROUND OF THE INVENTION

Glycol ether based polymers including those end capped with epoxy groups have been added to paper for many years. As far back as at least 1972, U.S. Pat. No. 3,873,354A (1972) describes the addition of humectants along with salts, such as sodium chloride and calcium chloride, to paper to be used for electroreprographic (or electrostatic) printing, that is copy paper. The humectants named included “polyethylene glycols and polymers . . . ”. The humectant must be water soluble or water miscible. The size press is mentioned as a means of application and “another additive which will typically be added to the size press solution is a conventional binder such as a starch, gum, casein, polyvinyl alcohol or acetate, animal glue, etc.”. The goal was to reduce the static build-up on the paper. The level of addition of salts was recited but not the level of humectant. However, in an example, about 0.05% of the paper weight of polyethylene glycol was added. This is greater than the level use by applicant herein. In addition the use of adducts of polyethylene glycol are not mentioned nor is the use of polypropylene glycol or adducts of polypropylene glycol mentioned. The latter two are not water soluble. There is no mention of the use of sizing agents in the paper.

U.S. Pat. No. 4,210,412 describes the use of “polyhydric alcohol” in cellulose based textiles. The additive is used to swell the cellulose so it will accept dyes during a transfer printing operation. What is called polyhydric alcohol includes “polyalkylene glycols such as polyethylene glycol of average molecular weight of 200 to 4000, polypropylene glycol of average molecular weight of 400 to 5000, polyethylene glycol/polypropylene glycol block copolymers of average molecular weight of 400 to 5000 . . . mono- and diethers . . . and diesters . . . ” The addition levels were not given; but, the addition levels used in the examples were higher than the current invention. For example, the level used in Example 2 had polyethylene glycol added at 7.5% of the substrate. This is far greater than used in the current invention

US Patent application 20050104947 A1 describes treatments for paper to reduce the amount of cockle and curl and also to provide an ink-jet image recording method. The treated paper is defined by its level of water absorption and a ratio of wet to dry tensile strength. The cockle and curl tendencies are lowered by decreasing the hydrogen bonds between fibers in the paper. The desired paper is obtained by two methods. One method deals with the treatment of the pulp fibers before the sheet is formed such that the treatment compounds are located between fibers. Among the compounds that can be used are polyethylene glycol polypropylene glycol block copolymers, polyoxyethylene fatty acid esters, and other nonionic surfactant type materials. Also materials such as monoglycidyl ethers can be used. A second method is to add to the paper a heat-curable material or a thermoplastic material. Again the material must be located between fibers and in this case adhere fibers together. Epoxy resins are given as one class of suitable additives. Thus, there is no surface treatment of a paper sheet. The application expands on what can be done by saying that the sheet can be treated by surface sizing to apply a nonionic surfactant. Furthermore, the treatment is also said to preferably include polyvalent metal salts such as calcium chloride or a cationic polymer such as cationic starch. Either material can be added at levels that provide 0.1 to 2 g/m2 of material to the sheet. The sheet weights of interest are 60 to 128 g/m2. Therefore, the range of percentage addition is from 0.08 to 3.3%.

The addition of polyethylene glycol and polypropylene glycol to the wet end of a paper machine is described in U.S. Pat. No. 5,240,562 (issued in 1993 to Procter and Gamble). The patent points out that polyhyroxy compounds like polyethylene glycols and polypropylene glycols having weight average molecular weights from about 200 to 4000 can be used with quatemary ammonium compounds to give tissue paper a good combination of softness and absorbency. This patent only covers the wet-end addition of the polyhydroxy compounds before the paper is dried. There is no mention of the impact on ink-jet printing. The quatemary ammonium compound and the polyhydroxy compound were premixed.

There are numerous patents concerning coated paper where a reactive polymer hardener is added to the ink absorptive layer. A wide variety of materials to crosslink the absorptive layer are known and they include diglycidyl ethers of glycol ether based polymers, such as polyethylene glycol digylcidyl ether (PEGDGE). In US Patent 20050202188A1 PEGDGE is used in a ink-jet absorptive coating layer to crosslink polyvinyl alcohol.

However, coated papers are very different from the paper of the current invention. A coated paper has one or more relatively thick layers of treatment on one or both sides of a preformed paper. Such papers find use in high-end printing application including high-end ink-jet printing applications such as photographic paper. For ink-jet applications, coated grades have an absorbent layer composed of either a polymer that absorbs a high volume of liquid ink or a high level of filler that can adsorb a high volume of liquid ink. There are two uses of glycol ether based polymers used in such coatings. First they may be added as an absorbent material and secondly, diglycidyl ethers of glycol based polymers may be added to improve surface strength. European Patent 0634284 discusses both such uses. The instant invention does not cover coated papers.

There are also other uses for polyglycol ethers in paper coatings. U.S. Pat. No. 5,746,814 describes the addition of the following compounds as desizing agents in a more complex composition for coating of paper: poly(alkylene glycol); poly(propylene oxide)-poly(ethylene oxide) copolymers; fatty ester modified compounds of glycerol and poly(ethylene glycol); and poly(oxyalkyene)modified compounds of sorbitan esters, fatty amines, alkanol amides, castor oil, fatty acid, fatty alcohol. The overall composition contains solvent, binder, desizing agent, anticurl agent, defoamer, biocide, antistatic agent, lighffastness promoting agent, and a filler. All of the coating applications fall into a different class of paper from the current invention and the coating compositions are significantly different as noted above.

US application, 20050013949 A1 concerns a coated paper. However, its description of the base paper on which an ink absorbing layer would be applied is described as. “Base paper employed for the paper support of the present invention is made employing wood pulp as a main raw material . . . It is possible to incorporate into base paper sizing agents such as . . . strengthening agents such as starch . . . moisture retention agents such as polyethylene glycol, dispersing agents and softening agents . . . ”

U.S. Pat. No. 6,203,899B1 describes a printing medium comprising a liquid absorbent base material, an ink-receiving layer provided on the base material, which comprises a pigment, a binder and a cationic substance, and a surface layer provided on the ink receiving layer. Here again the concern is a coated paper. It is described as fibrous pulp and a filler impregnated with a crosslinking substance. The crosslinking substance is crosslinked to form nonswelling paper. The crosslinking substance can be an epoxy. PPGDGE is specifically described. Along with the crosslinking substance can be a polymer having a reactive group. Starch is one example of the polymer. The amount of starch is in the range that can be applied by a size press. The ratio of crosslinking substance, starch, to crosslinking agent, PPGDGE, is from 100:1 to 1:1 by weight. The range is different from the current invention where our ratio under preferred conditions is about 400 to 1 or less. A polyvalent metal salt is mentioned as an option for crosslinking certain materials. The divalent metal ions that are optional additives of the invention of this disclosure do not crosslink starch.

U.S. Pat. No. 6,706,320B2 describes modifying a surface containing a polymeric material. In one claim, it is said the substrate can be cellulosic and in another claim it is said the modifying agent can be a crosslinker. The crosslinker can be an epoxy compound such as diglycidyl ethers of polyols. However, the process of the patent requires the presence of a polyamine as a surface modifying agent and that the polyamine vaporize in the process. The epoxy compound reacts with the amine.

U.S. Pat. No. 5,746,814A includes polyethyleneglycol diglycidyl ether as one of the compounds that can be added to prevent paper from curling. An elaborate composition for treating paper is described with a wide range of levels of each component. There are nine components in the composition and they can include water, starch as a binder, and PEGDGE as an anticurl or a desizing agent. Taking the maximum of starch in the composition and the minimum of PEGDGE one gets a ratio of 150:1. This is different from that of the current invention. Furthermore the current invention does not require many of the other components listed such as antistatic agent or desizing agent or lightfastness promoting agent. The composition is for treating paper that has already been imaged.

Hercules Incorporated has a patent (U.S. Pat. No. 6,207,258 B1) on the use of divalent metal salts with sizing agents. It also had a patent (U.S. Pat. No. 6,051,107A) on mixtures of reactive and nonreactive sizing agents as pre-mixes and in paper treatment compositions. The later patent was not maintained.

SUMMARY OF THE INVENTION

In general one embodiment of the invention involves a paper composition having applied to its surface a composition comprising a glycol ether based polymer that provides enhanced ink-jet print quality. The advantage of the paper composition is improved ink-jet printing without significant loss of water hold-out.

More specifically, this first embodiment of the invention is an uncoated paper (based on wood pulp or other cellulose product based) that has been treated on the surface with at least the following materials and dried to the point of usefulness. These materials are:

1) one or more compounds that increase water hold-out. These are known as sizing agents,

2) starch,

3) a glycol ether based polymer.

The sizing agent will be present in an amount of 0.01% to 0.3% based on dry weight of paper.

The starch will be present in an amount of 1 to 8% based on dry weight of paper.

The glycol ether based polymer will be present in an amount of less than 0.025% based on dry weight of paper.

One preferred composition is paper surface-treated with starch, sizing material, a glycol ether based polymer and a divalent metal salt. The sizing materials and glycol ether based polymers are defined below along with the addition levels.

The paper of the invention has improved ink-jet print qualities of less mottle of printed areas, less strike-through of ink, and possibly increased optical density over paper not treated with all the components of the invention.

The sizing agents, as defined below, provide water hold-out. They may also improve the sharpness and optical density of ink-jet printed images. The addition of sizing agents to paper is common. Likewise starch is a standard additive to paper and thus, the invention in one embodiment is the addition of sizing agent and starch with glycol ether based polymers, such as polypropylene glycol diglycidyl ether. Another option is that the paper as described above also contains a divalent metal salt as described in Hercules patent U.S. Pat. No. 6,207,258 B1, incorporated by reference.

Any uncoated paper can be used, but a preferred paper is uncoated “fine” paper for printing and writing applications. Such paper typically contains bleached wood pulp, precipitated calcium carbonate, and starch along with other materials deemed beneficial for preparing the paper or enhancing paper properties or reducing the cost of the paper. A typical uncoated fine paper has a basis weight of 70 to 80 grams per square meter.

For the present invention, the three or four components of the invention, listed above, may be added either separately or together to the surface of the paper. Fine paper for printing and writing is usually treated before it leaves the paper machine on which it was made.

A clear distinction should be made between surface treatments and coatings. Both are applied to paper that is already formed. Both can be used to enhance printing properties. A treatment is meant to modify the surface of the paper but the general structure remains mostly unchanged. A coating creates a new surface. More material is applied with a coating than a treatment. Coatings are usually made up of a large percentage of inorganic filler or pigment such as silica or clay or calcium carbonate. A coating covers the entire surface of the paper substrate blocking the porous nature of the paper and leveling the surface. With a “treatment”, the composition of the base sheet still has a large influence on final properties. For example, more wet-end sizing leads to a higher surface tension in the final paper versus a low level of sizing. With a “coating”, the internal chemistry may affect the application of the coating but does not have a direct effect on the final properties of the paper. For example, internal sizing is covered up by a coating and will not directly change the surface tension of the final coating unless it migrates through the coating.

The method of applying the materials used in this invention is not crucial as long as the application method for the materials is controllable and leads to the desired results. The most preferred method of addition is for the starch to be dissolved in water by cooking and for the other components to be added to the starch solution and then for the resulting composition (the treatment composition) to be applied to the paper with a paper machine size press.

A second embodiment of the invention is the composition used to treat the paper. It is defined as a starch based solution containing at least one paper sizing agent and at least one glycol ether based polymer and optionally a soluble divalent metal salt such as calcium chloride and optionally other additives common to the treatment of paper. The treatment composition allows for all of the additives to be applied simultaneously to the paper. This is an aqueous composition that contains:

sizing agent in an amount of 0.03-1%,

starch present in an amount of 1.5-12%,

glycol present in an amount of less than 0.25%.

The treatment composition consists predominantly of water in which a binder is dissolved. The binder is the predominant component aside from the water. The binder for this invention is predominantly starch. Other binders such a polyvinyl alcohol can be used in combination with the starch. With the binder, other additives are typically added such as sizing agents that hold out water and improve printing properties; optical brightening agents; dyes; and antistatic agents. Sometimes materials such as inorganic fillers are added. When fillers are added they do not constitute more than about 40% of the composition on a dry weight percent basis of the non-water components.

These compositions can also contain other common additives such as sodium chloride and defoamer. The advantage offered by the second embodiment of the invention (the treatment composition) is the simultaneous treatment of the paper with a composition that leads to the improvements listed above in the first embodiment of the invention. The composition must be such that it is uniform and it can be applied to the paper in a uniform manner.

Preferably, the glycol ether based polymers will be added to the treatment composition so that they are applied to the paper with the paper machine at the same time as the starch and sizing agent.

A third aspect of the current invention is a pre-mixture of the sizing agents and glycol ether based polymers and optionally a divalent metal salt. The pre-mixture is then added to the starch based solution just described. The pre-mixtures may be water based solutions or emulsions or dispersions. The glycol ether polymer will be 3-50% based on the amount of sizing agent.

A third embodiment of the invention comprises pre-mixtures of some of the additives that go into the surface treatment composition thus offering the convenience of adding several materials simultaneously to the mixture used to treat the paper. The pre-mixtures that are of interest for this disclosure will contain the following materials:

    • A water-based composition (i.e. a solution, latex, and or dispersion) of one or more paper sizing agents, and one or more glycol ether based polymers.

DETAILED DESCRIPTION OF THE INVENTION

The glycol ether based polymers used in this invention include polymers with repeat units of ethylene glycol or propylene glycol or combinations of them. There may be other functional groups along the backbone provided they do not account for more than approximately ten percent by weight of the final polymer structure. There may be branching along the backbone. The polymers can be end-capped with hydroxyl groups, ethers, gylcidyl ethers, esters, carboxylic acid groups, and other functionality so long as the end groups do not account for more than 25% of the weight of the average polymer molecule.

The polymer compositions may have one, two, or more types of functional end groups other than hydroxyl groups. There may also be one of more types of functional groups within the polymer backbone or grafted on to it.

Use of glycol ether based surfactants that fit the above criteria are part of this invention provided they meet the remaining conditions given below. Glycidyl ether end-capped polymers such as polyethyleneglycol digycidyl ether and polypropyleneglycol digycidyl ether are useful also.

The average molecular weight of the glycol ether based polymers of this invention shall be from approximately 350 g/mole to approximately 200,000. The polymer must be soluble or readily dispersible in water. The preferable average molecular weight is between 380 and 20,000 and most preferably between 500 and 2000 g/mole.

The addition level of the glycol ether based polymers of the invention is defined by the impact on sizing as defined in this invention and by concentration. The glycol ether based polymers of the invention should not cause greater than a 25% reduction in the level of sizing when utilized under at least some portion of the conditions of this invention. For example, polypropylene glycol (PPG) may be used at a low level but at a high level it interferes with sizing. The level of addition of the glycol ether polymers to the aqueous composition for surface treating the paper shall be less than 0.25%. The level of addition in a premix with sizing agent should be less than 50% of the active sizing agent by weight.

The paper substrate which is treated in the current invention can contain wood based pulp from ground wood to chemically bleached wood or a non-wood based pulp or a combination of pulps. The paper can also contain usual paper making inorganic fillers such as calcium carbonate or clay and may also contain organic fillers. The paper can also contain strength additives, retention additives, internal sizing agents and other common paper additives such as alum. The preferred grade of paper can be any type suitable for ink-jet printing which can include fine paper to white-top liner board. The basis weight of the paper can be anywhere from 40 g/m2 to 350 g/m2. The preferred paper is any type of printing and writing paper including roll fed to sheet fed papers. The most preferred is uncoated fine paper with a basis weight between 60 and 100 g/m2.

The starches that are suitable as part of the paper treatment can be of any kind provided they can be dissolved in water and applied to the paper. The starches can be from a variety of sources including corn, potato, rice, cassava root, and others used in paper making. Unmodified and modified starches can be used. Modified starches include oxidized, ethylated, cationic, anionic, amphoteric, hydrophobically modified, and others used in paper making. The preferred starches have reduced viscosities such that solutions of greater than 6% solids can be used on a paper machine size press. The most preferred are those with reduced viscosity that are also are oxidized, ethylated, cationic, or amphoteric. The range of starch treatment based on the dry weight of the final paper is 1% to 8.0%. More preferably from 2% to 7% and most preferably from 3% to 6%. The range of starch concentration in the treatment composition can be from 1.5% to 12% provided the viscosity allows for application to the paper substrate. More preferably the concentration will be between 3% and 11% and most preferably between 5% and 10%.

The surface treatment sizing agents suitable for the current invention include those that are termed reactive such as alkyl ketene dimers, alkenyl ketene dimers, and alkyl succinic anhydrides. Sizing agents termed unreactive are also suitable and may be mixed or used with the reactive sizing agents as described in a previous Hercules' patent (U.S. Pat. No. 6,051,107A). Unreactive sizing agents may be used on their own. Unreactive sizing agents include soluble polymers such as styrene-maleic anhydride based polymers, styrene-esterified maleic anhydride based polymers, styrene acrylic acid and styrene methacrylic acid based polymers and insoluble polymers such as polymer latexes commonly used in paper making such as poly(styrene/acrylic) resins, acrylonitrile/acylic resins and urethane polymers and insoluble polymer dispersions such as of ethylene/acrylic acid polymers.

The level of surface treatment sizing agent in the final dry paper will range from 0.01% to 0.3% on a dry weight basis. Preferably the level is 0.02 to 0.2% and most preferably it is 0.03 to 0.1%.

All of the above levels are based on both sides of a base paper being treated. However, the invention is applicable to treatment of one or both sides. When only one side is being treated all of the above levels relating to the paper will be one half of the values listed.

The final paper may contain other additives that were included in the formation of the paper or were applied along with the surface treatment or separately from the surface treatment. The additives applicable are those which are usually utilized in paper. They include but are not limited to the following: inorganic and organic fillers such as calcium carbonate or hollow sphere pigments; optical brightening agents which are also known as fluorescent whitening aids, pigments; dyes, strength additives such as polyamidoamines, promoter resins such as polydimethyldiallylammonium chloride; adhesion promoting polymers such as styrene acrylic latexes and styrene maleic anhydride based polymers; and inorganic salts such as sodium chloride and calcium chloride. When the final paper contains a divalent metal salt it should be less than 0.25% of the paper weight.

The methods of applying the paper treatment composition of the current invention are not limited provided uniform controlled application is obtained. The treatment may be made to paper formed on a paper machine and then only partially dried or it can be made on a paper machine to dried paper or the treatment can be done separate from the paper machine to paper that was formed, dried, and moved. The preferred process is for paper to be formed with a paper machine, partially dried, and treated using a paper machine size press, and then for the paper to be dried again. The paper may be further modified by calendering.

The treatment composition, the second embodiment of the invention described above, has the same components just described. The starch solution in water must be of dissolved starch and the viscosity must be such that the solution, containing other components, can be applied to the paper. The viscosity and not the solids of the starch solution is the most critical factor; however, some starch materials can only be dissolved at very low concentrations before they become too thick to use. A paper maker will select a starch based on the properties it imparts to the paper as well as its ease of use and the ability to apply the desired level with the equipment being used. A preferred level of starch in the treatment composition is 3 to 12% and a most preferred level is 5 to 10%. Other materials such as polyvinyl alcohol may be used with the starch, if desired. The additional binder level will be less than the starch level.

Suitable sizing agents are described above. They are added to the starch solution. The level of sizing agent in the composition will depend on two factors: the level of the starch composition applied to the paper and the desired level of sizing treatment. Typically the level of starch addition is selected along with the starch solution concentration and then the level of sizing agent in the starch solution is adjusted to get the desired level of treatment. The level of sizing agent in the starch composition will be between 0.03 and 1% of the paper treatment composition based on the active component of the sizing agent.

The level of glycol ether based polymer in the treatment composition will be determined in the same manner as described for the sizing agent level. The level of glycol ether based polymer will be determined by the amount of starch solution applied and the desired level of the polymer to be applied. The level of glycol ether polymer in the paper treatment composition will be less than 0.25%.

The treatment composition may contain other materials as noted above for the paper. When the treatment composition contains a soluble divalent metal salt such as calcium chloride and magnesium chloride the level of the salt will be less than 20% of the starch level.

The sizing agents of interest were described above. One or more sizing agents can be combined. The form of the sizing agent will be as a solution emulsion or dispersion in water with suitable additives used to obtain desired stability. One or more glycol ether based polymers can be combined with the sizing agents. The sizing agent formulation may also contain a water soluble divalent metal salt such as calcium chloride or magnesium chloride.

The amount of glycol ether based polymer versus sizing agents can be from 3% to 50% polymer to active sizing agent. More preferably the level of glycol ether based polymer to active sizing agent will be from 4% to 35%. When added the level of divalent metal salt will be less than two times the total level of sizing agents.

DEFINITIONS

Sizing

Paper sizing refers to the ability of a paper to hold out a liquid or from preventing it from penetrating into or through the paper. Generally the liquid held out is water. Sizing values are specific to the test used. Compounds that are designed to increased the hold-out of liquids are known as sizing agents. Sometimes a specific type of sizing is referred to such as an oil sizing agent. For the current work, the sizing and sizing agents are defined in terms of the ability to hold out the water based ink solution used in the Hercules Sizing Test. The test is defined below. For a discussion on sizing, see Principles of Wet End Chemistry by William E. Scott, Tappi Press 1996, Atlanta, ISBN 0-89852-286-2

Optical Density

Optical density is a measure of the inverse of the amount of reflected light off a surface. Generally and for the purpose of this invention, it is a measure of light reflecting off of a black printed area on a sheet of paper. Optical Density (OD) equals −log10(reflectance). The test method utilized was to use commercial hand-held densitometer. It is further described below. The concept of optical density is explained in detail in an article by Allen Rushing found on the internet at www.loglight.com/concepts&tools.htm.

Mottle

Mottle, or more accurately print mottle, refers to the unevenness of a printed area. It can be blotchiness or variations in optical density. They are visible to the eye. The method used to measure the mottle will determine what value it has. The test method used is provided below. An example of print mottle is shown in FIG. 19.17 of Printing Fundamentals ed. Alex Glassman, Tappi 1985, Atlanta, ISBN 0-89852-045-2.

Strikethrough

Strike-through refers to the uneven penetration of ink through to the back side of a printed paper. It is not show through which is the ability to see the printed image from the back side of the sheet but rather is penetration of the ink through or almost through the sheet. The ink penetration is often uneven and the strike-through will have a speckled appearance on the back side of a large printed area. The method used to measure the mottle will determine what value it has. The test method used in the current invention is provided below.

TEST METHODS

Preparation of Samples

Paper samples for the examples below were prepared by either a laboratory method or with a pilot paper machine. The general procedures are described here. Specific details are listed with each example.

For the laboratory test, base papers were prepared ahead of time at Western Michigan University on their pilot paper machine. The papers were made without any size press treatment, that is no starch, sizing agent, or other additive was applied to the surface of the formed paper. The pulp used to make the papers was a 75%/25% by weight mixture of hardwood and softwood bleached craft pulp as is typical of what would be used to produce commercial copy paper. The papers also contained precipitated calcium carbonate, cationic starch, aluminum sulfate and a retention aid. Once made and dried the papers were cut into sheets and stored. The paper was later treated at the Hercules Research Center with a laboratory bench top puddle size press. The size press consisted of a horizontal set of ten inch pinched rollers, one rubber coated and one metal, through which the paper was fed. A puddle of the size press treatment was held by the rollers and dams on the top side of the rollers. The rollers were held together with 14 pounds of air pressure. The paper passed through the puddle as it was pulled by the rollers, and through the rollers, to give a controlled and uniform level of treatment. The level of treatment was controlled by the concentration of the treatment chemicals in the treatment solution which was generally a dissolved starch solution. The paper was captured below the two rollers and immediately dried on a drum drier set at about 100° C. The paper was dried to about a 3-5% moisture level.

The size press formulations were prepared by dissolving starch for 45 minutes at 95° C., cooling, holding the starch at 65° C. Generally the starch pH was adjusted to 7.5. To the starch was added other additives for this treatment such as salt, sizing agents, and the glycol ether based polymers used in this invention. Once the additives were in the starch solution the solution pH was readjusted to a pH of 7.5. Then the starch solution, still at 65° C. was used to treat the paper. For each base paper used the amount of solution picked up through the rollers was determined and the additive levels set accordingly. After drying, each sample was conditioned by aging at room temperature for seven days. The samples were also conditioned for at least 12 hours prior to being tested under the conditions they would be tested.

Other samples used in the examples below were prepared on Hercules' pilot paper machine. The paper was made with conditions similar to those described above for Western Michigan University. Again the goal was to make standard copy paper. On the Hercules paper machine the first drier section was followed by a size press and then another drier section and then a set of calendering rolls. The treatments of the invention were applied to the paper at the size press. A puddle size press mode was used. In the puddle mode, the liquid treatment solution was held along the rolls as a puddle through which the paper passed. The pilot machine process imitated the process of a large paper machine. As with laboratory studies a solution of cooked (dissolved) starch was used as the carrier for treatment chemicals.

Sizing Test

Descriptions of various sizing tests can be found in The Handbook of Pulping and Papermaking by Christopher J. Biermann Acedemic Press 1996, San Diego, ISBN 0-12-097362-6. Properties of Paper: An Introduction ed. William E. Scott and James C. Abbott Tappi Press 1995, Atlanta, ISBN 0-89852-062-2. The Hercules Sizing Test (HST) was used for the current work. It is described by Tappi Method T530. For the test results presented in this disclosure a solution containing 1% napthalene green dye and 1% formic acid was used as the penetrant. The end point of the test was set at 80% reflectance.

Ink-Jet Printing Conditions

Paper samples were printed with a Hewlett Packard HP6122 printer using HP45 black ink cartridges. The settings for the printer were: plain paper, normal print quality, color, no color enhancement. On each sample a solid black five inch by five inch square was printed and allowed to dry before being stacked.

Image Analysis for Mottle and Strike-Through

Mottle and strike-through are both variations of uniformity, mottle of black print and strike-through of the white back side of paper that has been printed. An image analysis method was used to quantify the variations. Images were captured by a dual light scanner at a resolution of 600 dpi in bit-map format. The images were then analyzed with a mottle analysis software package from Verity IA 2004 Multifunction Version 1.4.1 designed by Roy R. Rosenberger. The software does a stochastic analysis. Mottle values can be obtained for various target sizes. A paper by Roy R. Rosenberger of Appleton, Wis. explains the analysis. The paper “Stochastic Frequency Distribution Analysis as Applied to Mottle Measurement” can be obtained on the VerityIA web site www.verityia.com. For the results presented in this disclosure black printed samples were used and the luminance values of the printed areas were evaluated on a grey scale. In the analysis, three values are obtained: 1) the standard deviation of the standard deviation of luminance values of each target on a grey scale; 2) the mean of the standard deviation of the luminance values; and 3) the standard deviation of the means of the standard deviation of the luminance values. These values are multiplied together to obtain what is called a Mottle Number.

For the examples presented in this disclosure, one target size is reported. First the 600 dpi images were taken. Then each image was analyzed as a collection of squares (a target size) of 16 dots by 16 dots (a 0.67 mm by 0.67 mm square) across approximately a 55 mm by 55 mm area. Across the image, groups of four squares were combined into a larger square. Each large square was a target of 1.4 mm by 1.4 mm. The numbers for the analysis of the mottle was determined for the collection of 1.4 by 1.4 mm squares across the selected image area. The software provided a mottle number for the 1.4×1.4 mm targets. In the analysis, a higher value indicates more mottle. A lower value represents a more uniform image and is desirable.

The same procedure was used for quantifying strike-through.

Measuring Optical Density

As noted above, optical density is a reflectance measurement. For the values of the current disclosure, an optical densitometer from Graphics Microsystems Inc. was used. The model was a Cosar 200. Black optical densities were measured and reported. Six readings were taken for each sample and averaged. A higher optical density value represents less reflectance and thus a darker looking print.

In all of the above tests, it is not so much the absolute values that are of importance but rather the relative values versus control samples that were included in all tests. The control samples are described in each example.

EXAMPLES

Example 1

Demonstration of the Problem of Lowering Internal Sizing Levels When No Glycol Based Polymer is Present

Using the pilot paper machine process described earlier and the conditions listed here three different paper samples were prepared with different levels of internal sizing and only starch and sodium chloride applied at the size press. The sizing (liquid hold-out property) of the final paper was determined by the Hercules Sizing Test described above. The mottle and strike-through were determined on ink-jet printed black squares as described earlier.

The paper was made with 70:30 ratio of bleached craft hardwood and softwood as suitable for fine paper. 15% of a medium size precipitated calcium carbonate (Albacar 5970) was added based on the pulp solids. Likewise 0.75% cationic starch, 0.25% papermakers alum (Al2O3*14H20), 0.015% of an anionic polyacrylamide retention aid, and three different levels of alkyl succinic anhydride (ASA) sizing agent were added on the same basis. The ASA was added as a stable emulsion typical for papermaking. At the size press, each sample was treated with a water solution of 8% oxidized starch and 0.5% sodium chloride to give an addition of 4% starch and 0.25% sodium chloride to the final dry paper.

The following table lists the results. Less ASA sizing agent resulted in less sizing and more mottle and strike-through when printed.

SampleASA levelHST (sec)MottleStrike-Through
1.0.036%35.8412.0
2.0.046%695.1511.2
3.0.060%1312.682.61
HST = Hercules Sizing test

Example 2

Demonstration of the Problem of Increasing Surface Sizing Levels with No Glycol Based Polymer.

The same conditions of Example 1 were used. At the size press, Hercules imPress® ST900 sizing agent (ST900) was added along with the starch. ST900 is an emulsion containing alkenyl ketene dimer as a sizing agent. The levels of addition on a dry basis of the dimer to the final paper were 0 and 0.05%.

The results for sizing mottle and strike-through are listed in the following table. Adding some surface sizing increased the sizing but the mottle and strike-through were worse.

Sampledimer levelHST (sec)MottleStrike-Through
1.035.8412.0
2.0.05018927.018.3

Example 3

Addition of Premixed Polyethyleneglycol and Magnesium Chloride

Samples were prepared in the laboratory using a bench top size press and pre-made base sheet by the process described above. The base sheet contained 1.25#/ton of a solid AKD sizing agent applied as an emulsion (Hercules' Hercon® 70 sizing emulsion). It also contained 10% Albacar HO PCC filler. The compositions of the paper treatments applied at the size press were based on a 9% solution, in water, of a low viscosity oxidized starch (D-15F from Grain Processing Corporation). The pick-up of the base sheet of the starch solution was 54.0%. Therefore, the final paper contained approximately (54×0.09)/(100+(54×0.09)×100% or 4.64% starch. Addition levels of additives were based on this pick-up. To each formulation enough NaCl was added to give a final paper content of 0.25%. Enough reactive sizing agent, non-reactive sizing agent, glycol ether polymer and MgCl2 was added to give the desired levels of addition as listed in the following table. The reactive sizing agent used was a stable emulsion of liquid dimer based on a saturated alkyl fatty acid (AKD). The unreactive sizing agent was a styrene acidic emulsion type (SAE) consisting of a poly(styrene/butyl acrylate) emulsion (Hercules' Chromaset 800 product). The glycol ether polymer was a polyethylene glycol (PEG) of 1000 g/mole average molecular weight. Both the MgCl2 and PEG were pre-mixed with the dimer emulsion before being added to the starch solution. The results for the final paper sizing, mottle, strike-through and optical density (OD) are also listed in the table.

Values listed for materials are percent of final paper weight. The HST values are in seconds.

DimerSAEPEGMgCl2HSTMottleSt. Thr.OD
A.0.0350.0180049616.28.291.23
B.0.0450.0230051518.02.781.32
C.0.0350.0180.007044910.22.241.37
D0.0450.0230.00904772.10.601.42
E0.0350.01800.05347722.94.581.29
F.0.0450.02300.06851311.02.041.45
G0.0350.0180.0070.05337823.04.311.32
H0.0450.0230.0090.0684310.70.421.47

The addition of PEG improved mottle, strike-through, and optical density. The sizing was not compromized. Addition of MgCl2 lowered sizing a little but also improved mottle, strike-through, and optical density. Addition of both materials had a significant effect on sizing and at the lower level did not improve mottle. However, at the higher level mottle, strike-through, and optical density improved. The benefits of MgCl2 were known from previous work but the benefit obtained from PEG was surprising. Surprisingly, the benefits of divalent metals salts and PEG were additive.

Example 4

Addition of Polypropylene Glycol

Using the pilot paper machine described earlier with wet-end conditions consisting of 75/25 hard wood/soft wood, 16% PCC, 0.07% dimer from an AKD emulsion, alum, cationic starch, and anionic polyacrylamide retention aid the effect of the additions of starch with NaCl, liquid dimer emulsion and polypropyleneglycol (PPG) were tested. The level of starch, NaCl, and dimer in the final paper samples were constant and were 4%, 0.25%, and 0.035%, respectively.

The wet-end conditions were as follows:

    • The cationic starch used was Stalok 400 from Staley
    • The level of cationic starch was 0.75% of the final paper weight
    • AKD stands for Alkyl Ketene Dimer Sizing Agent. The commercial product used was Hercules Hercon 70 sizing agent.
    • The alum level was 0.25% based on final paper weight
    • The level of anionic polyacrylamide retention aid was 0.015% based on final paper weight
    • The temperature was 50° C.
    • The pH was 7.0
    • The basis weight of the paper formed was 75 grams per square meter of paper

The size press conditions were as follows:

    • The starch used was an oxidized corn starch called D15F from Grain Processing Corporation
    • The starch was cooked for 40 minutes at 95° C.
    • The starch solution concentration was 7.5%
    • The temperature of the size press solution was 65° C. The liquid dimer sizing agent used was Hercules imPress ST900 sizing agent.

The level of PPG added is listed below in the table of results. The PPG had an average molecular weight of 700 g/mole.

SamplePPG (%)HST (sec)MottleStr-Thr
 A.026816.451.6
B0.01%22914.637.4
C0.02%7012.017.2

PPG was only slightly effective at lowering mottle. It was effective at reducing strike-through. However, at 0.01% and 0.02% in the paper it reduced the level of sizing. Example 5 Addition of polyethyleneglycol diglycidyl ether and polypropylene glycol diglycidyl ether.

The pilot paper machine described above was used to test the effectiveness of PEG and PPG with glycidyl ether (epoxide) end groups. The conditions were the same as in example 4, except the wet-end sizing was alkyl succinic anhydride (ASA). The pH was 7.8 instead of 7.0

Paper was made with two levels of ASA, 0.036 and 0.041%. A 75/25 hard wood/soft wood mixture was used with 16% PCC filler, 0.75% cationic starch, 0.5% paper makers alum, and 0.015% anionic retention aid. The paper was then treated on the paper machine at the size press.

Polyethyleneglycol diglycidyl ether (PEGDGE) with an average molecular weight of about 350 g/mole and polypropylene glycol diglycidyl ether (PPGDGE) with an average molecular weight of about 400 g/mole were mixed with an oxidized starch and a liquid ketene dimer sizing agent made from an unsaturated alkyl fatty acid. The sizing agent was added as an emulsion to the starch solution. The glycol ether polymers were also added to the starch solution. The level of starch added to the paper was 4%. As is typical in fine paper manufacturing 0.25% NaCl was added to the paper by also adding NaCl to the size press starch solution. The levels of dimer and glycol ether polymers in the final papers are noted below.

(the levels of materials are listed in dry percent of the final paper weight.)

I. 0.036% ASA in the Base Sheet

SampleDimerPolymerHSTMottleStr. Th.OD
A0.05none1818.383.181.27
B0.065none19513.63.351.32
C0.050.01 PEGDGE1699.793.221.34
D0.0650.01 PEGDGE1812.800.7351.41
E0.050.02 PEGDGE1542.161.051.38
F0.0650.02 PEGDGE1922.240.8141.39
G0.050.01 PPGDGE1621.160.9211.43
H0.050.02 PPGDGE1261.510.7221.43

II. 0.041% ASA in the Base Sheet

SampleDimerPolymerHSTMottleStr. Th.OD
A0.04none1742.391.361.41
B0.055none1851.130.571.43
C0.040.0075 PEGDGE1581.170.621.42
D0.0550.0075 PEGDGE1800.9050.531.43
E0.04 0.015 PEGDGE1561.020.831.41
F0.055 0.015 PEGDGE1480.880.541.42
G0.040.0075 PPGDGE1640.820.711.43
H0.0550.0075 PPGDGE2010.940.651.43

Both PEGDGE and PPGDGE reduced mottle and strike-through. The benefit depends on the amount of mottle and strike-through of the paper without them. The samples with more internal ASA had less mottle and strike-through and therefore the amount of improvement was less. PPGDGE was more effective than PEGDGE. However, when 0.02% PPGDGE was used the sizing was reduced. At lower levels, it may also have a negative effect on sizing when a low level of sizing agent is used. As an added benefit, both epoxides enhanced optical density. PPGDGE was again more effective than PEGDGE.

Example 6

Addition of Alternative Functionalized Glycol Ether Polymers

Using the same conditions as Example 3, except for use of an 8% rather than a 9% starch solution, poly(ethylene glycol) bis (carboxymethyl) ether (PEGBCME) was evaluated. The polymer was obtained from Alrich and had an average molecular weight of 600. The results are displayed in the following table.

Values listed for materials are percent of final paper weight. The HST values are in seconds.

DimerPEGBCMEMottleSt. Thr.
A.0.04015.010.5
B.0.040.00754.7514.3
C.0.040.0151.135.25
PEGBCME reduced mottle and strike-through.

Example 7

Use of a pre-mixture

Using the same conditions as Example 3, except for use of a 10% starch solution instead of a 9% starch solution, a pre-mixture of sizing agent and glycol ether polymer was evaluated.

ImPress® ST900 sizing emulsion of Hercules was used as the sizing agent on its own and in pre-mixtures with PPG. The PPG was predispersed in water, 10% PPG and 90% water, with a sonicator. The PPG dispersion was mixed with the dimer emulsion. The ratio of dimer to PPG in the pre-mixtures was 10 to 1 and 5 to 1. The results of the evaluation are summarized in the following table.

DimerPPGHSTMottleSt.Thr.
A.0.035038428.98.56
C.0.0350.003539229.45.56
E.0.0350.00734916.64.83

PPG was premixed with a dimer emulsion and the mottle and strike-through performance of the treated paper was improved. With the lowest level of PPG, 0.0035%, there was little or no improvement of mottle but strikethrough improved. At a higher level the mottle and strikethrough performance of the paper improved. As noted in a previous example, the addition of too much PPG will reduce the level of sizing.