This invention relates to degraded, particulate starches that are hydrophobically modified and especially effective in reducing the porosity of paper, improving the resistance to penetration by liquids and improving the surface strength when applied as a paper surface size. Blends of a degraded, particulate starch and a film former are also effective as a paper surface size to provide these properties.
The use of surface sizing in paper is known to provide several beneficial attributes to the paper and processing thereof including paper strength, retarding liquid penetration into the sheet and the quality and ease of printing on the paper. Starches are one of the most used raw materials in the paper industry and are often added in the wet end of the papermaking machine or as a surface size or coating.
Various paper grades require a low level of porosity to air in order to feed properly in copiers and sorting machines for example. Other paper grades require a highly continuous film on their surface that will resist penetration by non-aqueous fluids.
Surprisingly, it has been found that use as a paper surface size of a degraded, particulate starch which is hydrophobically modified, is especially effective in reducing paper porosity, resisting the penetration of liquids and improving surface strength as well as being cost effective.
Now, in accordance with this invention, it has been found that selected degraded, particulate starches that are hydrophobically modified are particularly useful as paper surface sizes to provide improved properties, particularly reduced porosity of paper, resistance to liquid penetration and increased surface strength.
More particularly, this invention relates to a method of providing paper with improved surface sizing properties comprising applying to the surface of a paper substrate an effective amount of a surface sizing composition which comprises a degraded, particulate, hydrophobic starch wherein the starch is modified with a hydrophobic hydrocarbon group of 5 to 23 carbon atoms at about 1 to 20% substitution level by weight of bound hydrophobe based on the weight of dry starch, the volume average size of the hydrated starch particles is at least about 20 microns and the volume fraction of the hydrated starch particles at 1% weight concentration in water is at least about 5%. Typically, the starch base is degraded to a water fluidity (WF) of from about 10 to 80 or the hydrophobically modified and/or particulate starch is degraded using an equivalent amount of degradation agent and substantially the same reaction conditions.
It has also been found that blends of a degraded, particulate starch and a film forming material are effective at improving surface sizing properties such as porosity reduction, resistance to liquid penetration and surface strength when used as a surface size on paper. More particularly, the level of degradation, the volume average size and the volume fraction of hydrated starch particles at 1% solids that are useful for the degraded, particulate starch are the same as that described above for the hydrophobically modified, degraded, particulate starch. A film former (film forming material) is intended to mean a polymer which helps to provide improved surface sizing properties such as porosity reduction, resistance to liquid penetration and surface strength, when added to the degraded particulate starch. The film former may be a hydrophobically modified starch, wherein the starch is modified with a hydrophobic hydrocarbon group of 5 to 23 carbon atoms at about 1 to 20% substitution level by weight of bound hydrophobe based on the weight of dry starch. Other illustrative film formers that may be used are modified starch, alginate, pectin, carboxymethylcellulose, polyvinyl alcohol, xanthan gum, rhamsan gum and welan gum. Typical modified starches include hydroxyalkylated starch with the alkyl group possessing 1 to 4 carbon atoms, oxidized, enzyme converted, thermally converted, acetylated and cationized starches. The degraded, particulate starch:film former blends may be used in amounts of from about 1:99 to 99:1 parts by weight of starch per part by weight of film former.
The selected starches provided by this invention and useful in surface sizing paper are degraded, hydrophobically modified particulate starches. The terms xe2x80x9csurface sizingxe2x80x9d, xe2x80x9csurface applicationxe2x80x9d and xe2x80x9cpaper coatingxe2x80x9d as used herein refer to the use or application of the starch composition of this invention on paper to provide properties including reduced porosity, resistance to penetration by liquids (e.g. water and aqueous solutions, inks, oils, solvents, greases, and silicone fluids) and improved surface strength.
All starches and flours (hereinafter xe2x80x9cstarchxe2x80x9d) may be suitable for use as a base material herein and may be derived from any native source. A native starch as used herein, is one as it is found in nature. Also suitable are starches derived from a plant obtained by breeding techniques including crossbreeding, translocation, inversion, transformation or any other method of gene or chromosome engineering to include variations thereof. In addition, starch derived from a plant grown from artificial mutations and variations of the above generic composition which may be produced by known standard methods of mutation breeding are also suitable herein.
Typical sources for the starch are cereals, tubers, roots, legumes and fruits. The native source can be corn, pea, potato, sweet potato, banana, barley, wheat, rice, sago, amaranth, tapioca, arrowroot, canna, sorghum, and waxy or high amylose varieties thereof. As used herein, the term xe2x80x9cwaxyxe2x80x9d is intended to include a starch containing at least about 95% by weight amylopectin and the term xe2x80x9chigh amylosexe2x80x9d is intended to include a starch containing at least about 40% by weight amylose. Also included are derivatized or modified starches such as cationic and anionic starches.
The starches of this invention generally are converted or degraded starches, particularly those in which the base or native, unmodified and non-particulate starch has a water fluidity (WF) of from about 10 to 80, particularly about 30 to 70, or the modified and/or particulate starch is degraded using an equivalent amount of degradation agent and substantially the same reaction conditions. The measurement for water fluidity as described herein is made using a Thomas Rotational Shear type Viscometer in accordance with standard procedures such as disclosed in Zwiercan, et al., U.S. Pat. No. 4,499,116 issued Feb. 12, 1985, which patent is incorporated by reference herein. The starch may be converted or degraded to the selected water fluidity using known procedures such as oxidative hydrolysis, acid hydrolysis or enzyme hydrolysis. A particularly useful method of converting starch involves the use of hydrogen peroxide with a manganese catalyst such as potassium permanganate as disclosed in U.S. Pat. No. 4,838,944 issued Jun. 13, 1989 to L. Kruger. Another useful starch conversion process involves the use of a metal-based coordination complex catalyst, such as a manganese complex, as disclosed in U.S. Pat. No. 5,833,755 issued Nov. 10, 1998 to P. Schlom, et al. The starch conversion processes as disclosed in both the ""944 and ""755 patents are incorporated herein by reference.
Degradation of the starch can be performed before or after treatment that will create a particulate starch upon hydration (i.e. by cooking) and/or hydrophobic substitution. The degree of degradation or hydrolytic treatment of the modified and/or particulate starch is the same as used to achieve a 10 to 80 WF for a native, unmodified and non-particulate starch base.
The starches of this invention are formed into particulate starches. By particulate starch it is meant that the starch, after hydration, retains some granular structure such that there remains discrete particles. The starches of this invention have a volume average size of hydrated particles of at least about 20 microns, particularly from about 20 to 300 microns, more particularly from about 30 to 200 microns, most particularly from about 40 to 150 microns. Additionally, the number of particles in the particulate starch, represented by the volume fraction of hydrated starch particles at 1% solids, is at least about 5%, particularly at least about 10%, more particularly at least about 20%, and most particularly at least about 30%. Particulate starches can be prepared by a number of techniques known in the art including chemical crosslinking, physical modification, physical association (i.e., crystallization) and/or hydration under controlled conditions. Chemical crosslinking is a particularly suitable way to form particulate starches and this may include treatment with any of a number of multi-functional crosslinking agents known in the art and disclosed for example in xe2x80x9cStarch Derivatives: Production and Usesxe2x80x9d by M. Rutenberg and D. Solarek, Starch: Chemistry and Technology, Chapter X, pp. 324-332, 1984. Such crosslinking agents include bifunctional etherifying and/or esterifying agents such as epichlorohydrin, bis-xcex2-chloroethyl ether, dibasic organic acids, phosphorus oxychloride, trimetaphosphate (i.e., the alkali and alkaline earth metal salts), and linear mixed anhydrides of acetic and di- or tribasic carboxylic acids. Another useful crosslinking agent is sodium hypochlorite, which when used in the proper amount and under pH conditions of at least 11 provides crosslinked starch as disclosed in Solarek et al., U.S. Pat. No. 5,523,339 issued Jun. 4, 1996, which patent is incorporated by reference herein. Particularly suitable crosslinking agents are epichlorohydrin, phosphorus oxychloride, adipic-acetic anhydrides and sodium trimetaphosphate, most particularly epichlorohydrin.
One technique for physically modifying the starch to form the particulate starch is the thermal inhibition of granular starch by heat treatment as disclosed in U.S. Pat. No. 5,718,770 issued to M. Shah, et al. on Feb. 17, 1998.
An important characteristic of the starches of this invention is the volume average size and volume fraction of the hydrated starch particles, both of which are influenced by the conditions used in preparing the particulate starch, e.g., cook conditions or the degree of crosslinking. This is important in providing the starch with suitable properties, particularly when it is being used in surface sizing paper to reduce porosity, provide resistance to liquid penetration and improve surface strength. The level of chemical crosslinking that is useful in providing the starches of this invention with desired volume average particle size and volume fraction at 1% solids after hydrating will range from about 0.05 to 5.0 crosslinks/1000 anhydroglucose units and more particularly from about 0.1 to 1.5 crosslinks/1000 anhydroglucose units.
In addition to being converted or degraded, the particulate starches of this invention are hydrophobically modified with hydrocarbon groups of at least 5 carbon atoms, more particularly from 5 to 23 and most particularly from 8 to 20 carbon atoms. In a particularly suitable embodiment, the hydrophobic hydrocarbon group will be an ester or ether substituent and may comprise saturated or unsaturated hydrocarbon groups and may contain some branching with unbranched hydrocarbon groups being particularly suitable. It should also be understood that the ester or ether substituents may contain other groups in addition to the hydrocarbon chains as long as such groups do not interfere with the net hydrophobic properties of the substituent.
The preparation of starch ester and ether derivatives is well known and has been carried out for many years. U.S. Pat. No, 2,661,349 issued on Dec. 1, 1953 to C. Caldwell, et al. describes hydrophobic starch derivatives such as starch alkyl or alkenyl succinates. This patent describes an aqueous method in which such derivatives are prepared using a standard esterification reaction wherein the reagent and starch suspended in water are mixed under alkaline conditions. Other methods for preparing the starch derivatives are known in the art and disclosed for example in the ""349 patent as well as in xe2x80x9cModified Starches: Properties and Usesxe2x80x9d, edited by O. Wurzburg, 1986, Chapter 9, pp. 131-147 and U.S. Pat. No. 5,672,699 issued on Sep. 30, 1997 to R. Billmers, et al.
Reagents used in preparing the hydrophobic starch esters generally are organic acid anhydrides having one of the following formulas: 
wherein R is a dimethylene or trimethylene group or the corresponding unsaturated group, e.g., ethenyl; R1 is a linear, branched or cyclic alkyl, alkenyl, aralkyl or aralkenyl group having 3 to 21 carbon atoms; and R2 and R3 are independently a linear, branched or cyclic alkyl, alkenyl, aralkyl or aralkenyl group having 5 to 23 carbon atoms.
Another suitable class of reagents for preparing starch ester derivatives includes imidazolides or N,Nxe2x80x2-disubstituted imidazolium salts of carboxylic or sulfonic acids such as those described in U.S. Pat. No. 4,721,655, issued Jan. 26, 1988 to P. Trzasko having the general formula: 
wherein Z is xe2x80x94COxe2x80x94 or xe2x80x94SO2xe2x80x94, A1 comprises a hydrocarbon of at least 5, more particularly 5 to 23 carbon atoms, R4 is H or C1-C4 alkyl, R5 is C1-C4 alkyl and Xxe2x88x92 is an anion.
A class of reagents useful as etherifying reagents are described in U.S. Pat. No. 2,876,117 issued on Mar. 3, 1959 to E. Paschall and comprise the reaction product of epihalohydrin with a tertiary amine having the structure: 
wherein R6 and R7 are independently H or a C1-C4 alkyl and A2 comprises a hydrocarbon group of at least 5, more particularly 5 to 23 carbon atoms.
Another type of hydrophobic reagent which can be used to produce starch ethers has the following formula: 
Another type of hydrophobic reagent which produces a starch ether is described below, wherein R9 and R10 are either a hydrogen or linear, branched or cyclic alkyl, alkenyl, aralkyl or aralkenyl groups having 3 to 21 carbon atoms. 
The hydrophobic hydrocarbon and particularly the ester or ether derivatives of this invention as described herein will comprise from about 1 to 20% and particularly from about 3 to 12% by weight of bound derivative or substituent based on the weight of dry starch.
In some cases, a single agent may be used to crosslink and to hydrophobically modify the starch.
The hydrophobically modified starch esters or ethers of this invention are useful in surface sizing or coating paper to provide improved surface sizing properties including reduced paper porosity, resistance to liquid penetration and surface strength, particularly the esters. The starch surface sizing composition will be used for this purpose in amounts of from about 0.5 to 15% by weight, particularly from about 2 to 6% by weight, based on weight of the paper substrate.
The degraded, hydrophobically modified, particulate starches as described herein are useful as paper surface sizes to provide improved properties especially reduced porosity of paper, resistance to liquid penetration and surface strength. Particularly useful starches of this type are those having a volume average size, as described herein, of hydrated starch particles of at least about 20 microns, particularly from about 20 to 300 microns, more particularly from about 30 to 200 microns, most particularly from about 40 to 150 microns.
The starch surface sizing material can be applied to the paper using known methods of application which commonly involve application to the surface of a paper web by a size applicator such as a conventional twin roll size press, tub size press, calender water box, pre-metering size press or gate roll. With applicators other than the pre-metering size press and gate roll, dry paper is passed through a flooded nip and a solution or dispersion of the surface sizing material and other functional chemicals contact both sides of the paper. Excess liquid is squeezed out in the press and the paper is redried and cured. With the pre-metering size press and gate roll, the solution or dispersion of the sizing material and other functional chemicals are metered onto an applicator roll which then applies the solution or dispersion to the surface of the paper, which is then redried and cured.
The surface size composition of the present invention may be successfully utilized for the surface sizing of paper and paperboard prepared from all types of both cellulosic and non-cellulosic fibers, and combinations thereof. Also included are sheet-like masses and molded products prepared from combinations of cellulosic and non-cellulosic materials derived from synthetics such as polyamide, polyester and polyacrylic resin fibers as well as from mineral fibers such as asbestos and glass. The hardwood or softwood cellulosic fibers which may be used include bleached and unbleached sulfate (Kraft), bleached and unbleached sulfite, bleached and unbleached soda, neutral sulfite, semi-chemical, groundwood, chemi-groundwood, and any combination of these fibers. In addition, synthetic cellulosic fibers of the viscose rayon or regenerated cellulose type can also be used, as well as recycled waste papers from various sources.
All types of pigments and fillers may be added in the usual manner to the paper product which is to be surface sized. Such materials include without limitation clay, talc, titanium dioxide, calcium carbonate, calcium sulfate and diatomaceous earths. Stock additives, such as defoamers, pitch dispersants, slimicides, etc., as well as other sizing compounds, can also be used with the surface sizing mixtures described herein. Further, less expensive traditionally used starches may be blended in to obtain a more cost effective surface sizing starch.
In addition to the use of degraded, hydrophobically modified, particulate starch as described above as a surface sizing composition, blends of a degraded, particulate starch with a film forming material are also found to be effective in providing improved surface sizing properties such as porosity reduction, resistance to liquid penetration and surface strength when used as a surface size on paper. The level of degradation, the volume average size of hydrated starch particles and the volume fraction of hydrated starch particles at 1% solids that are useful on the starch are the same as that described previously for the hydrophobically modified, degraded, particulate starch. That is, the degradation level of the native, unmodified and non-particulate starch base is typically from about 10 to 80 WF, particularly from about 30 to 70 WF, or the modified and/or particulate starch is degraded using an equivalent amount of degradation agent and substantially the same reaction conditions. The volume average size of hydrated starch particles is generally at least about 20 microns, particularly from about 20 to 300 microns, more particularly from about 30 to 200 and most particularly from about 40 to 150 microns. Additionally, the number of particles in the particulate starch is represented by a volume fraction of hydrated starch particles at 1% solids of at least about 5%, particularly at least about 10%, more particularly at least about 20%, most particularly at least about 30%. Particulate starch can be formed as noted above, by chemical crosslinking, physical modification, physical association or hydrating under controlled conditions with chemical crosslinking being a particularly suitable method. The film former may be a hydrophobically modified starch, wherein the hydrophobe can be an ester or ether substituent comprising a saturated or unsaturated hydrocarbon chain of at least 5 and more particularly 5 to 23 carbon atoms and there is about 1 to 20%, particularly from about 3 to 12%, by weight of bound hydrophobe based on the weight of dry starch. Other illustrative film formers that may be used are modified starch, alginate, pectin, carboxymethylcellulose, polyvinyl alcohol, xanthan gum, rhamsan gum and welan gum. Typical modified starches include hydroxyalkylated starch with the alkyl group possessing 1 to 4 carbon atoms, oxidized, enzyme converted, thermally converted, acetylated and cationized starches. Particularly suitable film formers are hydrophobically modified starch, alginate, carboxymethylcellulose, polyvinyl alcohol and pectin. The particulate starch:film former blends may be used in amounts of from about 1:99 to 99:1 parts by weight of starch per part by weight of film former and more particularly from about 30:70 to 70:30 starch to film former.
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