High speed corrugating adhesive incorporating solubilized cellulosic fiber

Corrugating adhesives of the carrier, no-carrier and carrier-no-carrier type having excellent dry pin adhesion, green strength and improved water resistance are prepared with solubilized fiber as a complete or partial substitute for starch. The adhesive composition contains solubilized fiber and, optionally, from about 0.1% to about 99% added starch by weight based on total solubilized fiber and added starch. When the adhesive is the carrier type or the carrier-no-carrier type, the fiber, preferably corn fiber, can be solubilized in situ during the process of preparing the carrier phase.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to corrugating adhesives which contain
 solubilized plant fiber, such as corn fiber. More particularly, the
 invention relates to high speed corrugating adhesives of the carrier,
 no-carrier or carrier-no-carrier type which are prepared by incorporating
 solubilized corn fiber in the adhesive formulation to obtain excellent pin
 adhesion and improved water resistance.
 2. Description of Related Art
 Adhesives used in manufacturing corrugated board are usually comprised of
 starch, caustic, a boron containing compound and, where water resistance
 is needed, a waterproofing resin. The main binder of corrugating paper is
 the starch which is gelatinized in the corrugating process as it
 penetrates the paper fiber. The other components, namely, the caustic,
 boron containing compound, and waterproofing resin are auxiliary agents
 which modify the basic properties of the starch.
 Caustic, usually in the form of sodium hydroxide, directly affects the
 gelatinization point temperature of the starch. Gelatinization point
 temperature is often referred to as gel point or gel temperature and is
 reduced from its natural value of 165.degree. F. (74.degree. C.) for
 unmodified corn starch to a temperature in the range from about
 138.degree. F. (59.degree. C.) to about 160.degree. F. (71.degree. C.) by
 the addition of caustic. Caustic also enhances penetration of the
 gelatinized starch into the surface paper fibers allowing for better
 bonding.
 The boron containing compounds perform multiple tasks. The most important
 function is developing the adhesive tackiness that is crucial in the
 formation of the "green bond". The green bond is the bond which holds the
 components of the corrugated board together until final heat curing of the
 adhesive system. It relies solely on the viscosity of the adhesive to
 maintain the integrity of the product during curing. Corrugating machine
 speeds therefore are limited by the rate of viscosity increase (i.e., rate
 of green bond formation) in the bond line between the liner and the
 corrugated medium. The oxygen atoms attached to the boron form stronger
 bonds, sometimes called boron bridges, between the starch and cellulose
 hydroxyl groups. Boron containing compounds also act as buffering agents
 in the presence of caustic and help maintain the viscosity stability of
 the adhesive paste.
 It is known that polyhydroxy compounds other than starch, such as polyvinyl
 alcohol, will respond with the boron containing compounds in a manner
 similar to that with starch. Boron containing compounds and polyvinyl
 alcohol are believed to interact synergistically to form strong bonds.
 Like other polyhydroxy compounds, polyvinyl alcohol in the presence of
 starch will develop adhesive tackiness faster in the presence of boron,
 which means that the corrugator can operate at higher machine speeds.
 Water proof or water resistant resins are considered to be an optional
 component of adhesives, and although some corrugating plants may not need
 them, most plants are prepared to use them when needed. Many types of
 thermo-setting resins are suitable. The most common of them are derived
 from urea-formaldehyde, ketone-formaldehyde or melamine-formaldehyde. When
 heat and pressure is applied to them in a corrugating machine, they will
 condense to form an excellent water resistant network. When polyvinyl
 alcohol is added to adhesives containing these resins, it participates in
 cross-linking which will improve the water resistant bonds.
 In the corrugating process, adhesive is commonly applied to the tips of the
 flutes of a corrugated medium. Then a noncorrugated flat paper liner is
 applied against the adhesive coated flutes as they pass between a
 corrugating roll and a pressure roll. The resulting product has the
 corrugating medium on one side and a flat liner on the other side and is
 called a single-faced portion. The single-faced portion may be used "as
 is" (called a "single facer" board) or adhesive may be applied to the
 flute tips of the single-faced portion and a second flat sheet can be
 applied in the same manner as the first in what is called a "double-facer"
 or a "double-backer" operation. The second liner sheet is treated with
 heat and reduced pressure (relative to the pressure used to make a
 single-faced portion) immediately following contact with the adhesive.
 Starch-based adhesives which can be of the carrier, no-carrier and
 carrier-no-carrier type are commonly used in processes for manufacturing
 corrugated paper board. In carrier type adhesives, a portion of the starch
 (or dextrin) forms a carrier, often known as the gelatinized phase, which
 suspends the balance of the starch which is in an ungelatinized state.
 Under conditions of heat and pressure, the ungelatinized starch is rapidly
 hydrated and gelatinized to increase quickly the viscosity and adhesivity
 of the adhesive composition. In no-carrier type adhesives, all of the
 starch is slightly cooked or swollen with heat and caustic soda for
 viscosity. Finally, carrier-no-carrier type adhesives have a portion of
 the starch which forms a carrier and is responsible for about one half of
 the viscosity and the remaining viscosity is obtained by slightly swelling
 the uncooked starch.
 One of the most important properties of a corrugating adhesive is green
 strength. Green strength is the characteristic that holds the paper
 together until the full strength of an adhesive develops. The green
 strength contributed by conventional starch adhesives is fairly low, but
 the addition of substances like polyvinyl alcohol improve the property as
 noted above. Increased levels of green strength, sometimes called early
 tack, allow corrugating machine operators to increase the speed of their
 machines without sacrificing the quality of the product.
 Synthetic polymers that affect green strength tend to be expensive and
 their prices follow the volatility of the petroleum market. These polymers
 also are perceived by the public as less desirable than natural products.
 Moreover, the use of such polymers can require additional processing steps
 which adds expense for the corrugator.
 Another important property of corrugating adhesives is known as dry pin
 adhesion. This is a measure of bond strength and the percentage of paper
 failure versus adhesive failure.
 According to U.S. Pat. No. 4,941,922, fibers can be added to starch-based
 corrugating adhesives to enhance adhesion and dispersion and yield
 improved adhesive characteristics including increased waterproofness, dry
 strength, viscosity and adhesiveness. The fibers disclosed generally are
 insoluble fibers which have certain mechanical properties in that they
 provide interlocking filaments. Suitable sources of fiber are described as
 cellulosic fiber including wood, paper, cotton and rayon; fibers of
 synthetic origin including Nylon, polyester, polypropylene, Lycra Spandex,
 Vyrene, Vinyon, Dynel, Saran, Creslan, acrylic, polyethylene, Teflon,
 tetrafluoroethylene, glass, Corlon and metallic fibers. Fibers generated
 during the wheat or corn milling processes are also said to be suitable.
 According to the patent, the fibers typically are suspended initially in
 the uncooked starch portion, suspended with the carrier portion, suspended
 prior to the carrier-no-carrier (or no-carrier) swell or added to the
 final adhesive mixture prior to storage or use. There is no experimental
 data in the patent, however, describing the use of fibers generated during
 the corn milling process and the patent does not disclose the extraction
 of hemicellulose from fibers of any kind. Furthermore, the fibers must be
 present in an amount from about 2.2 to about 22 pounds of fiber per 100
 pounds of corrugating adhesive and they must be large, having a diameter
 of between about 0.0005 inches and about 0.02 inches and a length of
 between about 0.025 inches and 0.25 inches.
 It has now been found that solubilized plant fiber, particularly corn
 fiber, a readily available component of corn kernels and hulls, and other
 plant materials, can be used to enhance dry pin adhesion in starch-based
 corrugating adhesives. The use of solubilized plant fibers also provides
 excellent green strength, particularly at low viscosities, which allows
 for a wider range of viscosities compared with adhesives based primarily
 on starch. The solubilized fiber can replace some or all of the added
 starch in typical adhesives and can reduce the cost of the adhesive.
 It has also been found that flatter board can be produced with adhesives
 containing plant fibers. This is advantageous when the board is cut and
 printed. The board also has less fractured liners and cracked scores
 compared with board produced using adhesives without plant fiber.
 In the present specification and claims, all parts and percentages are by
 weight unless otherwise specified, and the term "parts of adhesive" is
 used to refer to the total weight of adhesive, including water, starch and
 all chemicals. The term "parts of carrier phase" is used to refer to the
 total weight of the carrier phase, including water, starch and all
 chemicals. The terms "total starch" and "total carbohydrate" refer to the
 total cumulative weight of unmodified starch, modified starch and dextrin
 present, i.e. all of the carbohydrates.
 SUMMARY OF THE INVENTION
 The corrugating adhesive composition of the invention is a carrier,
 no-carrier or carrier-no-carrier corrugating adhesive which contains
 solubilized fiber and, optionally, from about 0.1% to about 99%,
 preferably from about 10% to about 95%, added carbohydrate by weight based
 on total solubilized fiber and carbohydrate. The fiber can be derived from
 various plant materials including wood and agricultural products, and a
 preferred source of fiber is corn. It is recognized by applicants that
 starch, protein and other impurities will be present with fiber which has
 not been completely purified, and in such cases the fiber will contain
 from about 4% to about 60% starch.
 Solubilized fiber can be added to the primary or secondary mixer in the
 preparation of a carrier type corrugating adhesive or it can be added at
 any stage in the preparation of a no-carrier or a carrier-no-carrier
 corrugating adhesive. In the preferred embodiment, a fiber such as corn
 fiber is added to the primary mixer and solubilized in situ in the
 preparation of a carrier or carrier-no-carrier corrugating adhesive.
 According to the preferred embodiment, the carrier type corrugating
 adhesive of the invention is made by the following process:
 1. Water, corn fiber and, optionally, added carbohydrate, are mixed in a
 primary mixer and heated for at least about 1 minute and preferably from
 about 1 minute to about 25 minutes at a temperature of from about
 115.degree. F. (46.degree. C.) to about 180.degree. F. (82.degree. C.),
 preferably from about 125.degree. F. (52.degree. C.) to about 145.degree.
 F. (63.degree. C.):
 2. An aqueous solution of caustic is added to attain a pH from about 8 to
 about 14, preferably from about 12 to about 14, and mixing is continued
 for at least about 10 minutes and preferably from about 10 to about 40
 minutes;
 3. Additional water is added and mixing is continued for at least about 1
 minute and preferably from about 1 to about 15 minutes, most preferably
 from about 5 to about 8 minutes, to make a carrier phase;
 4. A secondary mixer is charged with water, heated to from about 70.degree.
 F. (21.degree. C.) to about 120.degree. F. (49.degree. C.), preferably
 from about 90.degree. F. (32.degree. C.) to about 105.degree. F.
 (41.degree. C.) and borax is added;
 5. Solubilized fiber and, optionally, unmodified starch and/or modified
 starch and/or dextrin is added, and waterproof resin can be added as an
 option, to make a suspended phase and the contents are mixed for from
 about 3 to 25 minutes;
 6. The contents of the primary mixer are gradually added to the secondary
 mixer with continuous mixing. This step typically is carried out over a
 period of about 5 to 20 minutes.
 A single tank system works just as well as the two-part system described
 above.
 The corn fiber which is added to the primary mixer can be any kind of corn
 fiber. Suitable fibers include crude fiber, typically described as feed,
 and more finished products such as dietary corn fiber which is made for
 human consumption. The insoluble residue of the solubilized fiber in the
 adhesive and/or in a carrier phase preferably has a particle size of less
 than about 0.005 inch, a geometric mean size of less than about 0.00005
 inch and a median size of less than about 0.00005 inch.
 DETAILED DESCRIPTION OF THE INVENTION
 The corrugating adhesive of the invention is an aqueous emulsion which
 employs solubilized fiber essentially to replace some or all of the starch
 component. The fiber can be pre-solubilized with caustic at a pH from
 about 8 to about 14 and a temperature from about 80.degree. F. (27.degree.
 C.) to about 220.degree. F. (104.degree. C., under pressure using a jet
 cooker) before it is added to the adhesive mix, or it can be solubilized
 in situ when the conditions (pH and temperature) used to make the adhesive
 will cause solubilization. Higher pH levels generally will increase the
 rate of solubilization, as will higher temperatures.
 There are various options for employing the soluble fiber in the
 manufacture of corrugating adhesives. In a carrier type adhesive, the
 starch component that would normally be employed in the carrier phase can
 be partially or completely replaced with fiber and the fiber then can be
 solubilized in situ in the presence of a conventional amount of caustic.
 The suspended phase can then have a conventional amount of starch or some
 of the starch can be replaced with fiber which has been pre-solubilized.
 In a no-carrier type adhesive, some or all of the starch component can be
 replaced with pre-solubilized fiber. Carrier-no-carrier type adhesives can
 be prepared by solubilizing fiber in situ or by adding pre-solubilized
 starch, or both. Variations on the foregoing should be apparent to those
 skilled in the art.
 The corrugating adhesive of the invention is an aqueous emulsion which
 comprises water; from about 5 to about 35, preferably from about 15 to
 about 25, parts per 100 parts of adhesive of solubilized fiber and,
 optionally, an added carbohydrate component; and sufficient caustic to
 attain a pH from about 8 to about 14, preferably from about 12 to about
 14. When an added carbohydrate component is used, it is present in an
 amount from about 0.1 to about 99%, preferably from about 10 to about 95%
 by weight of the total solubilized fiber and added carbohydrate component.
 In the carrier type embodiment, the carrier phase comprises from about 2 to
 about 6 parts per 100 parts of adhesive of the solubilized fiber and/or
 added carbohydrate component and the remaining solubilized fiber and/or
 added carbohydrate component is in a suspended phase, provided that at
 least a portion of the suspended phase must include a carbohydrate
 component in order to have a carrier type system. When the adhesive is a
 no-carrier type adhesive, the solubilized fiber/added carbohydrate
 component is partially swollen to attain a viscosity from about 20 to
 about 40 seconds, preferably from about 25 to about 35 seconds, as
 determined by an orifice type viscometer, commonly known as a Stein Hall
 cup. Finally, in the carrier-no-carrier type embodiment from about 0.05 to
 about 0.10 parts of the solubilized fiber/added carbohydrate component
 (based on total solubilized fiber and added carbohydrate) is in a carrier
 phase and the remainder is partially swollen to attain a viscosity from
 about 30 to about 50 seconds, preferably from about 35 to about 45
 seconds, determined in the same manner as for the no-carrier embodiment.
 The adhesive of the invention can contain from about 0.2 to about 1 part
 per 100 parts of adhesive of a boron containing compound. A waterproofing
 or water resistant resin also can be incorporated in the adhesive in an
 amount from about 0.5 to about 5 parts per 100 parts of adhesive.
 In a preferred embodiment, the adhesive is a carrier type or
 carrier-no-carrier type adhesive and the fiber is corn fiber which is
 solubilized in situ during preparation of the carrier phase.
 The carrier type corrugating adhesive composition of the invention is made
 by separately preparing two intermediates and then combining them. One
 intermediate is called the carrier phase and the other is called the
 suspended phase.
 The carrier phase is made by admixing the fiber and the optional added
 carbohydrate component with water. These components can be added to the
 water together or in any order. The combined amount of fiber and optional
 added carbohydrate component which is added is from about 10 to about 30
 parts per 100 parts of carrier phase or from about 2 to about 6 parts per
 100 parts of adhesive.
 In the most preferred embodiment of the invention, the fiber is corn fiber
 which is employed in a carrier type adhesive. A suitable corn fiber is
 dietary corn fiber and it can be prepared according to U.S. Pat. Nos.
 4,994,115 or 5,073,201. The corn fiber is admixed with the other
 ingredients in the preparation of the carrier phase. The components are
 continuously mixed and heated to a temperature from about 115.degree. F.
 (46.degree. C.) to about 180.degree. F. (82.degree. C.). Sufficient
 caustic is then added to provide an alkaline pH while continuing mixing
 and maintaining heating. Mixing and heating are continued for a sufficient
 time to solubilize the corn fiber in situ. The pH should exceed about 10
 and preferably will exceed about 12. The sufficient time and temperature
 will generally be from about 10 to about 40 minutes at from about
 115.degree. F. (46.degree. C.) to about 180.degree. F. (82.degree. C.).
 Lower temperatures generally correspond with longer times.
 The preferred solubilized fiber of the invention has residual insoluble
 material having a particle size in the adhesive of less than about 0.005
 inch, a geometric mean size of less than about 0.00005 inch and a median
 size of less than about 0.00005 inch.
 Following the removal of heat, mixing can be continued to allow uniform
 cooling. Water can be added at this stage to speed cooling.
 The suspended phase is made by admixing a carbohydrate component and,
 optionally, pre-solubilized fiber with heated water. The water is heated
 at a temperature from about 70.degree. F. (21.degree. C.) to about
 105.degree. F. (41.degree. C.) and is mixed continuously. The carbohydrate
 component and optional pre-solubilized fiber is added in an amount from
 about 10 to about 30 parts per 100 parts of the corrugating adhesive, and
 preferably in an amount from about 15 to about 25 parts per 100 parts of
 the adhesive.
 A boron containing compound such as boric acid or a boric acid salt must be
 added at some stage in the process to improve tack or stickiness. It may
 be added in the preparation of the carrier phase, the suspended phase, or
 in both phases. When a portion of the boron containing compound is added
 during the preparation of the carrier phase, it also improves the
 viscosity stability of the final adhesive product. Preferably, however,
 most of the boron containing compound is added to the suspended phase.
 When the boron containing compound is added during the preparation of the
 carrier phase, it is added in an amount from about 0.03 to about 1 part
 per 100 parts of carrier phase or in an amount from about 0.01 to about
 0.3 parts per 100 parts of adhesive. The boron containing compound added
 to the suspended starch phase is added in an amount to bring the total
 boron containing compound up to from about 0.2 to about 1 part per 100
 parts of the adhesive.
 The carrier phase is gradually added to a tank containing the suspended
 phase with continuous mixing of the contents of the tank. The temperature
 of the tank contents is maintained at from about 70.degree. F. (21.degree.
 C.) to about 120.degree. F. (49.degree. C.), preferably from about
 90.degree. F. (32.degree. C.) to about 105.degree. F. (41.degree. C.),
 during the gradual addition. The amount of the carrier phase added is from
 about 15 to about 50 parts per 100 parts of adhesive.
 A waterproofing or water resistant resin may be added to the suspended
 phase tank at any time in an amount from about 0.5 to about 5 parts per
 100 parts of adhesive. It is preferable to add the resin following
 completion of all of the other adhesive preparation steps.
 The individual components of the carrier type adhesive composition of the
 invention are described in detail below along with descriptions of their
 preferred embodiments.
 Fiber
 Corn fiber produced by the corn wet milling industry is suitable,
 especially dietary corn fiber which is sold under the designation
 PEERLESS.RTM. corn fiber by the Corn Products Unit of CPC International
 Inc., P.O. Box 8000, Englewood Cliffs, N. J. 07632 USA. Another preferred
 source of corn fiber is produced by the dry milling process, such as corn
 bran sold by Illinois Cereal Mills, 616 South Jefferson Avenue, Paris,
 Ill. 61944 as Code 97800 high fiber corn bran. (Dry millers may refer to
 corn fiber as corn bran. The terms corn fiber and corn bran are synonymous
 for purposes of this specification.) The corn fiber and/or bran must be
 ground to increase its surface area sufficiently to minimize the amount of
 residual insoluble material in the solubilized fiber and to allow the
 fiber to dissolve under relatively mild conditions. When the fiber is not
 ground to sufficient fineness, it dissolves poorly, particles float around
 in the preparation and a grainy adhesive is produced which plugs the hole
 in a Stein-Hall cup. Successful adhesives have been made when the fiber is
 sufficiently ground so that about 70% will pass through a -325 mesh screen
 (44 microns or 0.0017 inch).
 Carrier Phase Carbohydrate
 When a carbohydrate component is employed in the carrier phase, unmodified
 starch or dextrin can be used, but modified starch is preferred because it
 enables the dissolution of substantially more solids (up to twice the
 amount attainable with unmodified starch) in the adhesive composition and
 therefore contributes to the development of better pin adhesion and other
 adhesive characteristics. The reason is that modified starches, when
 pasted in water, are less viscous than their unmodified counterparts, and
 as a consequence they can "carry" more ungelatinized starch at practical
 viscosities.
 The modified starch which is used in accordance with the present invention
 can be mechanically, chemically or heat modified. Compared to unmodified
 starches, modified starches improve paste stability compared with pearl
 starch and they frequently possess superior physical properties such as
 increased solubility, better film forming, increased whiteness, improved
 gel strength, viscosity stability, increased adhesivity, improved
 resistance to shear and increased resistance to freeze-thaw degradation.
 Starches derived from other genetic forms of corn, such as high amylose
 and waxy corn as well as sorghum varieties would also be suitable for such
 applications. Suitable chemically modified starches include modified
 oxidized starch such as hypochlorite-oxidized starch, acid-thinned starch,
 ethylated starch, cross-bonded starch and others which have reduced
 molecular weight, higher fluidity and/or functional sub groups.
 Examples of chemically modified starches which can be used in the invention
 and are commercially available are SUREBOND.RTM. or STABLEBOND.RTM.
 modified starches which have residual carboxyl functionality and extreme
 uniformity and are sold by the Corn Products Unit of CPC International
 Inc., P.O. Box 8000, Englewood Cliffs, N.J. 07632 USA.
 Unmodified starches which can be used in the carrier phase are the same as
 those described in more detail below in the discussion of the suspension
 phase starch.
 The dextrins which can be used in the carrier phase are prepared by heating
 starch under various conditions as more fully described in Whistler, R. L.
 et al., Starch: Chemistry and Technology, 2nd edition, Chapter XX, pages
 596-607, Academic Press, Inc. (New York, 1984).
 Suspension Phase Carbohydrate
 The preferred carbohydrate for the suspension phase is unmodified starch
 which is a commodity chemical produced from the root, stem or fruit from a
 number of plants. It is a high molecular weight carbohydrate polymer which
 is comprised of linear and branched polysaccharide polymers. Modified
 starch and/or dextrin can be used as the suspension phase starch, but
 unmodified starch is more economical.
 Boron Containing Compound
 Any boron containing compound having free hydroxyl groups attached to the
 boron atoms can be used. The most commonly used compounds are commercial
 boric acid (ortho boric acid, H.sub.3 BO.sub.3 and its hydrated forms
 H.sub.3 BO3.xH.sub.2 O) and borax (sodium tetraborate decahydrate,
 Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O and other hydrate and anhydrous
 forms)
 Caustic
 Any strong base can be used, but the preferred bases are alkali metal
 hydroxides. The most preferred are sodium and potassium hydroxide.
 Waterproofing/Water Resistance
 Starch-based corrugating adhesives prepared with solubilized fiber have
 excellent waterproof characteristics compared with the same adhesive made
 without solubilized fiber. If more enhanced waterproofing or water
 resistance is desired, however, conventional resins which are employed in
 the corrugating industry can be used.
 Preferred waterproofing or water resistant resins include those which upon
 heating in basic media generate cross-linking species which react and
 cross-link with any available hydroxyl group in the starch, polyvinyl
 alcohol, hemicellulose or cellulose molecules. The cross-linking action
 reduces the hydrophilic nature and water-solubility of the solubilized
 fiber, carbohydrate and other polyhydroxy molecules by effectively
 removing the availability of hydroxyl groups to water and by developing
 hydrophobic, aliphatic cross-linking moieties. Condensation products from
 the reaction of a ketone and an aldehyde compound are suitable. These
 resins are characterized as polyether polymers, but can contain a variety
 of other monomers such as urea, melamine, and the like. The most preferred
 resins are acetone-formaldehyde resins, acetone-urea-formaldehyde resins
 and acetone-melamine-formaldehyde resins comprising about 1.5-30% by
 weight acetone, about 5-50% by weight formaldehyde and about 0-15% of a
 third monomer. A commercially available cross-linking resin which is
 suitable for use in the present invention is the ASTROMEL.RTM. series of
 resins manufactured by Astro Industries, Inc., 114 Industrial Boulevard,
 P.O. Box 2559, Morganton, N.C. 28655 USA.