Water Resistant Materials for Food-Safe Uses

The present disclosure is directed to water or moisture-resistant paper or paper-containing substrates safe for use as food and beverage containers and utensils, as well as preparation and use of the same.

Various methods are known for treating paper and paper containing materials to impart liquid-repellant or liquid-proof properties. A number of the known methods for treating paper to render it more water-repellent use paper sizing or surface sizing. Various compositions have been described as suitable for surface sizing but are limited with respect to their ability to produce paper that is sufficiently water-repellant or waterproof so as to remain intact and legible when wet.

There remains a need in the art for a cellulosic composition (e.g., paper) that is weatherproof and is safe for use with food and beverages. In addition, a need exists for such a composition that can be easily written on with a pen or pencil, can be re-pulped and recycled, and is compatible with a wide variety paper stocks (particularly recycled stock). Finally, there remains a need in the art for compositions and methods that can provide paper having the above-described properties with water-based materials (i.e., environmentally friendly chemicals) capable of imparting the requisite water resistance and wet strength. The present disclosure fulfills these needs and provides further related advantages.

Generally, the present disclosure relates to food safe water resistant material in combination with paper or a paper containing material(s). Accordingly, one embodiment provides a food safe article comprising a substrate comprising a plurality of cellulose fibers, and a food safe water resistant material, wherein the food safe water resistant material impregnantly covers at least a portion of a surface of the substrate.

Another embodiment provides use of a food safe article for consuming, serving, transporting, storing, or disposing of food wherein the food safe article comprises a substrate comprising a plurality of cellulose fibers, and a food safe water resistant material, wherein the food safe water resistant material impregnantly covering at least a portion of a surface of the substrate. Yet another embodiment provides a method for preparing a food safe article, the method comprising:

These and other aspects of this disclosure will be evident upon reference to the following detailed description of the disclosure.

Food safe water/moisture resistant or water/moisture proof paper or paper-containing substrates, as well as preparation and use thereof, are disclosed herein below. The particulars described herein are by way of example and are only for purposes of illustrative discussion of embodiments of the present disclosure. The use of any and all examples, or exemplary language (e.g., “such as” or “for example”) provided herein is merely intended to better illuminate the disclosure and does not pose a limitation on the scope of the disclosure as claimed. No language in the specification should be construed as indicating any non-claimed element is essential to the practice of the disclosure. Further, all methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.

The use of the alternative (e.g., “or”) should be understood to mean one, both, or any combination thereof of the alternatives. The various embodiments described above can be combined to provide further embodiments. Groupings of alternative elements or embodiments of the disclosure described herein should not be construed as limitations. Each member of a group may be referred to and claimed individually, or in any combination with other members of the group or other elements found herein.

Each embodiment disclosed herein can comprise, consist essentially of, or consist of a particular stated element, step, ingredient, or component. As used herein, the term “comprise” or “comprises” means “includes, but is not limited to,” and allows for the inclusion of unspecified elements, steps, ingredients, or components, even in major amounts. As used herein, the phrase “consisting of” excludes any element, step, ingredient, or component that is not specified. As used herein, the phrase “consisting essentially of” limits the scope of the embodiment to the specified elements, steps, ingredients, or components, and to those that do not materially affect the basic and novel characteristics of the claimed disclosure.

The terms “a,” “an,” “the,” and similar articles or terms used in the context of describing the disclosure (especially in the context of the following claims) are to be construed to cover both the singular and the plural (i.e., “one or more”), unless otherwise indicated herein or clearly contradicted by context. Ranges of values recited herein are intended to serve as a shorthand method of referring individually to each separate value falling within the range. In the present description, any concentration range, percentage range, ratio range, or integer range is to be understood to include the value of any integer within the recited range and, when appropriate, fractions thereof (such as one tenth and one hundredth of an integer), unless otherwise indicated. Also, any number range recited herein relating to any physical feature, such as size or thickness, are to be understood to include any integer within the recited range, unless otherwise indicated. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein.

The term “about” has the meaning reasonably ascribed to it by a person of ordinary skill in the art when used in conjunction with a stated numerical value or range, i.e., denoting somewhat more or somewhat less than the stated value or range, to within a range of ±20% of the stated value; ±19% of the stated value; ±18% of the stated value; ±17% of the stated value; ±16% of the stated value; ±15% of the stated value; ±14% of the stated value; ±13% of the stated value; ±12% of the stated value; ±11% of the stated value; ±10% of the stated value; ±9% of the stated value; ±8% of the stated value; ±7% of the stated value; ±6% of the stated value; ±5% of the stated value; ±4% of the stated value; ±3% of the stated value; ±2% of the stated value; or ±1% of the stated value.

Definitions used in the present disclosure are meant and intended to be controlling in any future construction unless clearly and unambiguously modified in the examples or when application of the meaning renders any construction meaningless or essentially meaningless. In cases where the construction of the term would render it meaningless or essentially meaningless, the definition should be taken from Webster's Dictionary, 3Edition or a dictionary known to those of ordinary skill in the art.

“Food safe” refers to an article or coating that complies with government regulations related to food safety. For example, “food safe” includes, but is not limited to, an article or coating that comply with regulations of the U.S. Food and Drug Administration (“USFDA”), the U.S. Drug administration (“USDA”), European Food Safety Authority (“EFSA”), the China Food and Drug Administration (“CFDA”), the Canadian Food Inspection Agency, and the like. “Food safe” may include compliance with Title 21 of the Code of Federal Regulations (e.g., 21 CFR §§ 174.5-178.3950)

The term “water resistant” refers to a material's ability to resist the penetration of water to some degree, despite prolonged exposure to moisture or a wet environment. More specifically, this means that the food safe article resists falling apart when wet and also maintains a substantially intact and undisturbed form. The water resistant character of the food safe article is largely a function of water repellency and wet strength. Water repellency refers to the ability of the food safe article to resist wetting (i.e., invasion of water or moisture into the matrix of cellulose fibers of the article through capillary action).

“Impregnantly” refers to a physical blend of elements, materials, polymers, and/or fibers that is substantially homogenous. That is, in certain embodiments, a food safe water resistant material “impregnantly” covering a surface means the food safe water resistant material is substantially dispersed throughout that surface and is not limited to a layer (e.g., a coating layer) separate from and on top of that surface (i.e., the food safe water resistant material penetrates the surface to at least some degree). In some embodiments, “impregnantly” means the food safe water resistant material substantially saturates and/or permeates the surface of the substrate and is intermingled throughout the cellulose fibers of a surface of the substrate.

As used herein, “article” or “food safe article” is an object used for consumption, service, transport, storage, and/or disposal of food or food related goods (e.g., condiments, seasoning, sauces, colorants, additives, etc.). In some embodiments, a “food safe article” may include a dish, plate, bowl, cutlery, placemat, coaster, straw (see, e.g.,), cup (see, e.g.,), cup lid () platter, mug, canteen, cruet, napkin ring, napkin, bib, crockery, fork, spoon, knife, basket, wrapper, jar, can, liner, bag, container (see, e.g.,), container lid, canister, box, bottle, cover, saucer, basin, bin, bucket, tray, jug, ladle, packaging, insert, carrier, sack, cooler, warmer, table cloth, carafe, flask, decanter, pitcher, funnel, cutting board, pot, vessel, urn, ewer, spork, packet, pouch, carton, and the like.

“Wet strength” refers to the tensile strength of an article when permeated or soaked with water, the strength being provided by bond between the components of the system (e.g., inter-fiber bonds, fiber-fiber cross-links, fiber-polymer cross-links, polymer-polymer cross-links, etc.) having resistance to attack by water. Without wishing to be bound by theory, strength is believed to be related to entanglement of fibers as well as addition of natural polymers and synthetic resin to pulp slurry during the manufacturing process, which creates a resistance to swelling, protects existing fiber bonds and forms new water resistant bonds. Wet strength can be determined by Tappi Test Method T456 and is routinely expressed as the ratio of wet to dry tensile force at break. Wet strength can be measured as the peak tensile force (in Newtons) at breakage for an article soaked in distilled water for a controlled period of time (e.g., 5 minutes; referred to as “wet strength method”).

In one embodiment, a food safe article comprising a substrate comprising a plurality of cellulose fibers, and a food safe water resistant material, wherein the food safe water resistant material impregnantly covers at least a portion of a surface of the substrate is presented. In certain embodiments, the food safe article comprises at least one cross-link between two or more components (e.g., cellulose fibers, polymers, and combinations thereof).

In some embodiments, the food safe article has a wet strength greater than 0 Newtons. In some more specific embodiments, the food safe article has a wet strength greater than 100 Newtons. In some embodiments, the food safe article has a wet strength greater than 200 Newtons, greater than 300 Newtons, greater than 400 Newtons, greater than 500 Newtons, greater than 600 Newtons, greater than 700 Newtons, greater than 800 Newtons, greater than 900 Newtons, greater than 1,000 Newtons, greater than 1,100 Newtons, greater than 1,200 Newtons, greater than 1,300 Newtons, greater than 1,400 Newtons, greater than 1,500 Newtons, greater than 1,600 Newtons, greater than 1,700 Newtons, greater than 1,800 Newtons, greater than 1,900 Newtons, greater than 2,000 Newtons, greater than 2,100 Newtons, greater than 2,200 Newtons, greater than 2,300 Newtons, greater than 2,400 Newtons, greater than 2,500 Newtons, greater than 2,600 Newtons, greater than 2,700 Newtons, greater than 2,800 Newtons, greater than 2,900 Newtons, or greater than 3,000 Newtons. In any of the forgoing examples, wet strength can be measured using the wet strength method or the Tappi Test Method T456.

“Alkyl” refers to a straight or branched hydrocarbon chain radical consisting solely of carbon and hydrogen atoms, which is saturated or unsaturated (i.e., contains one or more double (alkenyl) and/or triple bonds (alkynyl)), having, for example, from one to twenty-four carbon atoms (C-Calkyl), four to twenty carbon atoms (C-Calkyl), six to sixteen carbon atoms (C-Calkyl), six to nine carbon atoms (C-Calkyl), one to fifteen carbon atoms (C-Calkyl), one to twelve carbon atoms (C-Calkyl), one to eight carbon atoms (C-Calkyl) or one to six carbon atoms (C-Calkyl) and which is attached to the rest of the molecule by a single bond, e.g., methyl, ethyl, n-propyl, 1-methylethyl (iso propyl), n butyl, n pentyl, 1,1-dimethylethyl (t butyl), 3-methylhexyl, 2-methylhexyl, ethenyl, prop-1-enyl, but-1-enyl, pent-1-enyl, penta-1,4-dienyl, ethynyl, propynyl, butynyl, pentynyl, hexynyl, and the like. Unless stated otherwise specifically in the specification, an alkyl group is optionally substituted.

“Alkylamino” refers to the group-NRR′, where R and R′are each independently either hydrogen or alkyl, and at least one of R and R′is alkyl. Alkylamino includes groups such as piperidino wherein R and R′ form a ring. The term “alkylaminoalkyl” refers to-alkylene-NRR′.

“Alkylene” refers to a straight or branched divalent or multivalent hydrocarbon chain linking the rest of the molecule to a radical group or linking two or more radical groups, consisting solely of carbon and hydrogen, which is saturated or unsaturated (i.e., contains one or more double and/or triple bonds), and having from one to twelve carbon atoms, e.g., methylene, ethylene, propylene, n-butylene, ethenylene, propenylene, n-butenylene, propynylene, n-butynylene, and the like. The alkylene chain is attached to the rest of the molecule and/or radical group(s) through a single or double bond. The points of attachment of the alkylene chain to the rest of the molecule and/or to the radical group(s) can be through one carbon or any two carbons within the chain. Unless stated otherwise specifically in the specification, an alkylene chain is optionally substituted.

“Haloalkylene” refers to an alkylene, as defined above, wherein at least one H is replaced by a halogen radical, for example, fluoro, chloro, bromo, iodo, or combinations thereof. Unless otherwise stated specifically in the specification, a haloalkylene group is optionally substituted.

“Cycloalkyl” refers to a stable non-aromatic monocyclic or polycyclic carbocyclic radical consisting solely of carbon and hydrogen atoms, which may include fused or bridged ring systems, having from three to fifteen carbon atoms, preferably having from three to ten carbon atoms, and which is saturated or unsaturated and attached to the rest of the molecule by a single bond. Monocyclic radicals include, for example, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl. Polycyclic radicals include, for example, adamantyl, norbornyl, decalinyl, 7,7-dimethyl-bicyclo[2.2.1]heptanyl, and the like. A “cycloalkylene” is a divalent or multivalent cycloalkyl, which typically connects one portion a molecule to a radical group or connects two or more radical groups. Unless otherwise stated specifically in the specification, a cycloalkyl (or cycloalkylene) group is optionally substituted.

“Heteroalkylene” refers to an alkylene group, as defined above, comprising at least one heteroatom (e.g., N, O, P or S) within the alkylene chain or at a terminus of the alkylene chain. In some embodiments, the heteroatom is within the alkylene chain (i.e., the heteroalkylene comprises at least one carbon-heteroatom-carbon bond). In other embodiments, the heteroatom is at a terminus of the alkylene and thus serves to join the alkylene to the remainder of the molecule (e.g., M1-H-A-M2, where M1 and M2 are portions of the molecule, H is a heteroatom and A is an alkylene). Unless stated otherwise specifically in the specification, a heteroalkylene group is optionally substituted.

“Haloheteroalkylene” refers to a heteroalkylene group, as defined above, wherein at least one H is replaced by a halogen radical, for example, fluoro, chloro, bromo, iodo, or combinations thereof. Unless otherwise stated specifically in the specification, a haloheteroalkylene group is optionally substituted. “Cycloheteroalkylene” refers to a heteroalkylene group, as defined above, further comprising a cycloalkylene as define above (e.g., M1-H-A-Cy-M2, where M1 and M2 are portions of the molecule, H is a heteroatom, A is an alkylene, and Cy is a cycloalkylene. Unless otherwise stated specifically in the specification, a cycloheteroalkylene group is optionally substituted.

“Aryl” refers to a ring system comprising at least one carbocyclic aromatic ring. In some embodiments, an aryl comprises from 6 to 18 carbon atoms. The aryl ring may be a monocyclic, bicyclic, tricyclic or tetracyclic ring system, which may include fused or bridged ring systems. Aryls include, but are not limited to, aryls derived from aceanthrylene, acenaphthylene, acephenanthrylene, anthracene, azulene, benzene, chrysene, fluoranthene, fluorene, as-indacene, s-indacene, indane, indene, naphthalene, phenalene, phenanthrene, pleiadene, pyrene, and triphenylene. Unless stated otherwise specifically in the specification, an aryl group is optionally substituted.

As used herein, “arylene” refers to a divalent or multivalent aryl group which links a portion of a molecule to a radical group, two or more radical groups, or a portion of a first molecule to a portion of a second molecule. Unless stated specifically otherwise, an arylene is optionally substituted.

“Heteroaryl” refers to a 5- to 14-membered ring system radical comprising one to thirteen carbon atoms, one to six heteroatoms selected from the group consisting of nitrogen, oxygen and sulfur, and at least one aromatic ring. For purposes of this disclosure, the heteroaryl radical may be a monocyclic, bicyclic, tricyclic or tetracyclic ring system, which may include fused or bridged ring systems; and the nitrogen, carbon or sulfur atoms in the heteroaryl radical may be optionally oxidized; the nitrogen atom may be optionally quaternized. Examples include, but are not limited to, azepinyl, acridinyl, benzimidazolyl, benzothiazolyl, benzindolyl, benzodioxolyl, benzofuranyl, benzooxazolyl, benzothiazolyl, benzothiadiazolyl, benzo[b][1,4] dioxepinyl, 1,4-benzodioxanyl, benzonaphthofuranyl, benzoxazolyl, benzodioxolyl, benzodioxinyl, benzopyranyl, benzopyranonyl, benzofuranyl, benzofuranonyl, benzothienyl (benzothiophenyl), benzotriazolyl, benzo[4,6] imidazo[1,2-a] pyridinyl, carbazolyl, cinnolinyl, dibenzofuranyl, dibenzothiophenyl, furanyl, furanonyl, isothiazolyl, imidazolyl, indazolyl, indolyl, indazolyl, isoindolyl, indolinyl, isoindolinyl, isoquinolyl, indolizinyl, isoxazolyl, naphthyridinyl, oxadiazolyl, 2-oxoazepinyl, oxazolyl, oxiranyl, 1-oxidopyridinyl, 1-oxidopyrimidinyl, 1-oxidopyrazinyl, 1-oxidopyridazinyl, 1-phenyl-1H-pyrrolyl, phenazinyl, phenothiazinyl, phenoxazinyl, phthalazinyl, pteridinyl, purinyl, pyrrolyl, pyrazolyl, pyridinyl, pyrazinyl, pyrimidinyl, pyridazinyl, quinazolinyl, quinoxalinyl, quinolinyl, quinuclidinyl, isoquinolinyl, tetrahydroquinolinyl, thiazolyl, thiadiazolyl, triazolyl, tetrazolyl, triazinyl, and thiophenyl (i.e. thienyl). “Heteroarylene” is a divalent or multivalent heteroaryl radical. Unless stated otherwise specifically in the specification, heteroaryl and heteroarylene groups are optionally substituted.

As used herein, “haloheteroarylene” refers to a heteroarylene group, as defined above, wherein at least one H is replaced by a halogen radical, for example, fluoro, chloro, bromo, iodo, or combinations thereof. Unless otherwise stated specifically in the specification, a haloheteroarylene group is optionally substituted.

The term “substituted” used herein means any of the above groups (e.g., alkyl, alkylene, alkylamino, alkylaminoalkyl, alkoxy, aryl, arylene, carbocyclyl, cycloalkyl, cycloalkylene, cycloheteroalkylene, haloalkyl, haloalkylene, haloheteroalkylene, heteroalkylene, heterocyclyl, heteroaryl and/or heteroarylene) wherein at least one hydrogen atom is replaced by a bond to a non-hydrogen atom such as, but not limited to: a halogen atom such as F, Cl, Br, and I; an oxygen atom in groups such as hydroxyl groups, alkoxy groups, and ester groups; a sulfur atom in groups such as thiol groups, thioalkyl groups, sulfone groups, sulfonyl groups, and sulfoxide groups; a nitrogen atom in groups such as amines, amides, alkylamines, dialkylamines, arylamines, alkylarylamines, diarylamines, N-oxides, imides, and enamines; a silicon atom in groups such as trialkylsilyl groups, dialkylarylsilyl groups, alkyldiarylsilyl groups, and triarylsilyl groups; and other heteroatoms in various other groups. “Substituted” also means any of the above groups in which one or more hydrogen atoms are replaced by a higher-order bond (e.g., a double-or triple-bond) to a heteroatom such as oxygen in oxo, carbonyl, carboxyl, and ester groups; and nitrogen in groups such as imines, oximes, hydrazones, and nitriles.

For example, “substituted” includes any of the above groups in which one or more hydrogen atoms are replaced with —NRR, —NRC(═O)R—NRC(═O)R, —NRC(═O)NRR, —NRC(═O)OR, —NRSOR, —OC(═O)NRR, —OR, —SR, —S OR, —SOR, —OSOR, —SOOR,═NSOR, and —SONRR. “Substituted” also means any of the above groups in which one or more hydrogen atoms are replaced with —C(═O)R, —C(═O)OR, —C(═O)NRR, —CHSOR, —CHSONRR. In the foregoing, Rand Rare the same or different and independently hydrogen, alkyl, alkoxy, alkylaminyl, thioalkyl, aryl, aralkyl, cycloalkyl, cycloalkylalkyl, haloalkyl, heterocyclyl, N-heterocyclyl, heterocyclylalkyl, heteroaryl, N-heteroaryl and/or heteroarylalkyl. “Substituted” further means any of the above groups in which one or more hydrogen atoms are replaced by a bond to an aminyl, cyano, hydroxyl, imino, nitro, oxo, thioxo, halo, alkyl, alkoxy, alkylaminyl, thioalkyl, aryl, aralkyl, cycloalkyl, cycloalkylalkyl, haloalkyl, heterocyclyl, N-heterocyclyl, heterocyclylalkyl, heteroaryl, N-heteroaryl and/or heteroarylalkyl group. In addition, each of the foregoing substituents may also be optionally substituted with one or more of the above substituents.

A “cellulose fiber” or “cellulose fibers” refer to fibrous molecules generally having the structure shown below:

wherein n is an integer greater than 1, for example ranging from 1 to 15,000. A typical example of a substrate comprising a plurality of cellulose fibers is cellulosic paper. Cellulosic paper may comprise fibers such as wood fibers, cotton fibers, as well as other cellulosic fibers, including recycled cellulosic fibers. Particular embodiments are directed to a substrate that is paper comprising cellulose fibers, for example, cellulosic fibers from recycled paper. The substrate is said to be impregnantly covered with a food safe water resistant material, when the material penetrates the surface of the substrate to at least some degree.

“Organo-silicon polymer” or “organosilicon” refers to an organometallic polymer having carbon-silicon bonds. Exemplary organo-silicon polymers can be found in products such as, e.g., FoamStar® ST 2446.

“Star polymer” refers to polymer having a multifunctional core or center from which at leastpolymer chains, arms, or backbones radially extend. The polymer chains, arms, or backbones can be chemically identical (homostars) or different (heterostars) and have variable length and may provide additional branching. The core or center may be an atom, molecule, or macromolecule. In some embodiments, the star polymer is from, for example, Hydropalat® WE 3322, FoamStar® ST 2446, and the like.

In some embodiments, the first polymer comprises a branched star polymer a fluorinated polymer, or combinations thereof. In some embodiments, the first polymer comprises a fluorinated polymer. Fluorinated polymer-based compositions include, for example, Hydropalat® WE 3370 and the like.

In some embodiments, the food safe water resistant material comprises a first polymer and a second component. The second component may be, for example, a second polymer, mineral oil, a silicone emulsion, 2,4,7,9-tetramethyl-decyn-5-diol (“TMDD”) or dioctyl sodium sulfosuccinate (“DOSS”), an additive for increasing slip, surface smoothness, or gloss, a diluent or wetting agent, a surfactant, a filler to provide block resistance, a pigment, a wax, an additive to enhance wet strength, or a combination thereof.

In some embodiments, the food safe water resistant material comprises mineral oil. In some embodiments, the food safe water resistant material comprises a silicone emulsion. In some embodiments, the food safe water resistant material comprises mineral oil, a silicone emulsion, or combinations thereof. Exemplary mineral oil compositions include, e.g., Foamaster® MO 2140, Foamaster® MO 2172, Foamaster® MO 2111 NC, Foamaster® MO 2185, and the like. Exemplary silicone emulsions include, for example, FoamStar® ED 2522(formerly Dehydran® SE 2) and the like (e.g., ultra-low SVOC silicone emulsions).

In some embodiments, the food safe water resistant material comprises 2,4,7,9-tetramethyl-decyn-5-diol (“TMDD”) or dioctyl sodium sulfosuccinate (“DOSS”). In some embodiments, the food safe water resistant material comprises 2,4,7,9-tetramethyl-decyn-5-diol. In some embodiments, the food safe water resistant material comprises dioctyl sodium sulfosuccinate. In some embodiments, the TMDD is from, for example Hydropalat® WE 3650 or the like. In some embodiments, the DOSS is from, for example, Hydropalat® E 3475.

In some embodiments, the food safe water resistant material comprises an additive for increasing slip, surface smoothness, or gloss (i.e., a “slip agent”). Exemplary additives for increasing slip, surface smoothness, or gloss include Efka® SL 3299, Efka® SL 3257 and the like.

In some embodiments, the food safe water resistant material comprises a surfactant. In some embodiments, the surfactant is a polyethylene glycol (PEG) and polypropylene glycol (PPG) block co-polymer. Exemplary surfactants include, e.g., Hydropalat® WE 3966 and the like.

In some embodiments, the food safe water resistant material comprises a diluent or wetting agent. In more specific embodiments, the diluent or wetting agent is a water soluble polyalkylene glycol (PEG, PPG etc.). Diluents and wetting agents include, for example,

Hydropalat® WE 3155 and the like.

A “filler to provide block resistance” refers to an additive included in the food safe water resistant material to prevent surfaces in contact in a stack of food safe articles (e.g., a roll of food safe sheet material) from sticking together. A filler to provide tooth for printability and writability refers to an additive included in the food safe water resistant material to impart to its surface a degree of texture or roughness required for printability or writability.

“Outthrows” refers to papers that are so manufactured or treated or are in such a form as to be unsuitable for recyclability or consumption as the specified by grade according to the Institute of Scrap Recycling Industries, Inc. (“ISRI”).

A “cross-link” refers to a covalently bonded molecular bridge or linkage between two or more components (e.g., between cellulose fiber(s) and cellulose fiber(s), between polymer(s) and polymer(s), between cellulose fiber(s) and polymer(s)). Both intra and inter-molecular covalent attachments of the aforementioned components and combinations are meant to be included.

“Cross-linking density” refers to a ratio of cross-linking moieties (i.e., isocyanate, isothiocyanate, aziridine, carbodiimide, etc.) to molecular weight of the cross-linking agent. A cross-linking agent having a higher cross-linking density has more cross-linking moieties than a cross-linking agent having a low cross-linking density when molecular weight is held constant. As used herein, the cross-linking density is expressed according to the following equation: