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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to an all natural, starch-based corrugating adhesive
providing water resistance without containing water resistance additives
or chemical crosslinkers. More particularly, this invention is directed to
a corrugating adhesive composition comprising an aqueous dispersion of a
selected gelatinized carrier starch and a raw ungelatinized starch wherein
the carrier starch has an amylose content of greater than 60% by weight
and the cooked or gelatinized amylose content is at least about 15% by
weight of the total starch content in the adhesive.
The procedure employed in the production of corrugated paperboard usually
involves a continuous process whereby a strip of paperboard is first
corrugated by means of heated, fluted rolls. The protruding tips on one
side of this fluted paperboard strip are then coated with an adhesive, and
a flat sheet of paperboard, commonly known in the trade as a facing, is
thereafter applied to these tips. By applying heat and pressure to the two
paperboard strips thus brought together, an adhesive bond is formed
between the strips. The above-described procedure produces a single-faced
board in that the facing is applied to only one surface thereof. If a
double-faced paperboard in which an inner fluted layer sandwiched between
two facings is desired, a second operation is performed wherein the
adhesive is applied to the exposed tips of the single-faced board and the
adhesive-coated tips are then pressed against a second facing in the
combining section of the corrugator under the influence of pressure and
heat. The typical corrugating process and the operation and use of
corrugators in general are described in U.S. Pat. Nos. 2,051,025 and
2,102,937 issued on Aug. 18, 1936 and Dec. 31, 1937 respectively to Bauer.
A major concern in such corrugating processes is the selection of the
appropriate adhesive, as its properties will affect not only the strength
and stability of the final corrugated product, but also the parameters,
such as corrugator speed, within which the process can be operated. Thus,
such adhesives are chosen in light of the specific requirements of the
process and the properties desired in the ultimate corrugated paperboard.
The adhesives most commonly used in corrugating are starch-based adhesives
which are popular because of their desirable adhesive properties, low cost
and ease-of preparation.
The most fundamental of starch corrugating adhesives is an alkaline
adhesive which is comprised of raw ungelatinized starch suspended in an
aqueous dispersion of cooked starch (carrier). The adhesive is produced by
gelatinizing starch in water with sodium hydroxide (caustic soda) to yield
a primary mix of gelatinized or cooked carrier, which is then slowly added
to a secondary mix of raw (ungelatinized) starch, borax and water to
produce the fully formulated adhesive. In the corrugating process, the
adhesive is applied (usually at between 25.degree. and 55.degree. C.) to
the tips of the fluted paper medium or single-faced board, whereupon the
application of heat causes the raw starch to gelatinize, resulting in an
instantaneous increase in viscosity and tack and formation of the adhesive
bond.
It is often desired or necessary in the manufacture of corrugated
paperboard that the adhesive yield water resistant bonds which can
withstand extended exposure to high humidity, water, melting ice and the
like. A number of approaches have been devised to produce water resistant
corrugating adhesives. One method involves preparation of an acidic,
starch-based adhesive wherein urea-formaldehyde together with an acidic
catalyst is added to the composition to produce a crosslinked starch and
provide water resistant bonds in corrugated board. Another more commonly
followed method involves the use of alkaline curing starch-based adhesives
wherein formaldehyde-based chemicals are added as crosslinking additives
to produce water resistant bonds.
It has been known for many years that a corrugating adhesive whose carrier
portion is prepared from a high amylose starch (i.e., at least 35 to 40%
amylose) is superior to one prepared from pearl starch, which contains
about 27% amylose, because a carrier can be produced having improved
rheological and film-forming properties, and increased moisture
resistance. However, despite this knowledge, the ability to provide useful
water resistant starch-based corrugating adhesives has been primarily
directed to compositions containing synthetic additives,
formaldehyde-based crosslinkers or other starch crosslinking agents. This
is true with alkaline corrugating adhesives containing high amylose
carrier starches which provide little or no water resistance when used
without crosslinking additives. In other words, to get significant or
reasonable amounts of water resistance even when using high amylose
starches, crosslinking additives have been required.
Starch-based corrugating adhesives containing high amylose carrier starch
and providing improved and superior green bond strength and tack are
disclosed in U.S. Pat. No. 4,787,937 issued Nov. 29, 1988 to C. Leake.
High amylose starches are also disclosed in U.S. Pat. No. 4,912,209 issued
Mar. 27, 1990 to C. Leake et al., as being useful carrier starches in
corrugating adhesives providing faster bond development and higher
corrugator running speeds.
The use of high amylose starch in water resistant corrugating adhesives has
been disclosed in U.S. Pat. Nos. 3,284,381 issued to L. Hickey et al. on
Nov. 3, 1966 and 3,532,648 issued to D. Wilhelm on Oct. 6, 1970. However
both references require the use of chemical crosslinkers commonly based on
formaldehyde to provide desired water resistance. Other patents which
disclose the use of crosslinking agents or other water resistance
additives include U.S. Pat. No. 3,728,141 issued Apr. 17, 1973 to D.
Ray-Chaudhuri et al. involving the use of a diacetone
acrylamide-formaldehyde condensate, U.S. Pat. No. 3,944,428 issued Mar.
16, 1976 to J. Schoenberg et al. which is directed to an
acetoacetamide-formaldehyde condensate and U.S. Pat. No. 4,009,311 issued
Feb. 22, 1977 to J. Schoenberg which shows the reaction product of a
ketone, formaldehyde and a secondary amine as a crosslinking agent.
More recently, due to the uncertainty of the safety of formaldehyde,
efforts have been made to reduce the levels of exposure to formaldehyde.
U.S. Pat. No. 4,366,275 issued on Dec. 28, 1982 to M. Silano et al.
employs an acetone-formaldehyde condensate crosslinking additive low in
free formaldehyde in an alkaline corrugating adhesive. U.S. Pat. No.
5,079,067 issued on Jan. 7, 1992 to S. Willzing discloses that the levels
of free formaldehyde in alkaline starch containing corrugating adhesives
can be reduced by reacting the free formaldehyde with a nitrogen base and
urea.
Other patents disclose water resistant corrugating adhesives which use
additives that are formaldehyde free. These patents include U.S. Pat. No.
4,775,706 issued Oct. 4, 1988 to C. Iovine et al. which uses a latex
polymer of a halohydrin quaternary ammonium monomer, U.S. Pat. No.
5,055,503 issued Oct. 8, 1991 to C. Leake et al. disclosing a
polysaccharide graft copolymer containing a reactive aminoethyl halide
group and U.S. Pat. No. 5,190,996 issued Mar. 2, 1993 to M. Foran et al.
showing an epihalohydrin-amine condensate crosslinking agent.
Despite the advances made to reduce or eliminate the level of formaldehyde
exposure, the corrugating industry is still searching for effective means
of providing water resistance to corrugated paperboard products using
formaldehyde-free alkaline curing starch-based adhesives.
Accordingly, it is an object of the present invention to provide a water
resistant, all natural (i.e. without crosslinkers), formaldehyde free,
alkaline starch-based adhesive composition which is useful in the
manufacture of corrugated paperboard and in which the water resistance is
developed solely through the starch and does not rely on the addition of
synthetic crosslinkers.
SUMMARY OF THE INVENTION
It has now been found that an all natural, starch-based, alkaline
corrugating adhesive free of formaldehyde and other crosslinking and water
resistance additives and having good water resistant properties is
provided by a composition comprising an aqueous dispersion of a selected
gelatinized carrier starch and a raw ungelatinized starch wherein the
carrier starch is a high amylose starch having greater than 60% by weight
of amylose content and further where the gelatinized starch amylose
content is at least about 15% by weight of the total starch content in the
adhesive composition.
In one preferred embodiment, the high amylose carrier starch used in the
corrugating adhesive is a substantially pure starch extracted from a plant
source having an amylose extender genotype, the starch comprising less
than 10% amylopectin determined by butanol fractionation/exclusion
chromatography measurement.
In another preferred embodiment, the high amylose carrier starch used is a
degraded starch, particularly one obtained in a controlled process
employing hydrogen peroxide and a manganese salt catalyst in an alkaline
slurry reaction.
DETAILED DESCRIPTION OF THE INVENTION
The alkaline corrugating adhesive composition of this invention comprises a
raw ungelatinized starch, a gelatinized or cooked carrier starch, an
alkali base material and water.
The carrier starch component used in this corrugating adhesive is a
selected high amylose starch having greater than 60% and preferably at
least about 70% by weight total amylose content, i.e. including normal
amylose and low molecular weight amylose. When used throughout the
application, the term "amylose" by itself refers to total amylose which
includes both normal amylose and low molecular weight amylose. It is well
known that starch is composed of two fractions, the molecular arrangement
of one being linear and the other being branched. The linear fraction is
known as amylose and the branched fraction amylopectin. Starches from
different sources, e.g. potato, corn, tapioca, and rice, etc., are
characterized by different relative properties of amylose and amylopectin
components. Some plant species have been genetically developed which are
characterized by a large preponderance of one fraction over the other. For
instance, certain varieties of corn which normally contain about 22-28%
amylose have been developed which yield starch composed of over 40%
amylose. These hybrid varieties have been referred to as high amylose or
amylomaize.
While high amylose corn hybrids were developed in order to naturally
provide starches of high amylose content for commercial applications,
other starches which are useful include those derived from any plant
species which produces or can be made to produce a high amylose content
starch, e.g., corn, peas, barley and rice.
Besides having the high amylose content noted above, the starch used as the
carrier component in the corrugating adhesive of this invention will
preferably have a weight average molecular weight of from about 300,000 to
1,200,000 more preferably from about 400,000 to 1,000,000 and even more
preferably from about 500,000 to 850,000 determined using gel permeation
chromatography (GPC). Additionally, the carrier starch is characterized by
having an amylopectin content, as defined by a weight average molecular
weight greater than about 1,500,000 by gel permeation chromatography
(GPC), of less than about 40%, preferably less than about 30% and more
preferably less than about 20%, and a low molecular weight amylose
content, as defined by a GPC peak molecular weight of about 15,000 of from
about 5 to 30%, preferably from about 8 to 25%, all determined on the
fractionated components.
While any starch having the characteristics of amylose content and
preferably the molecular weight, as described herein, may be used as the
carrier starch component, particularly useful is an ae starch or
substantially pure starch extracted from a plant source having an amylose
extender genotype, the starch comprising less than 10% amylopectin.
Another especially useful starch is a degraded starch and particularly one
obtained in a controlled process employing hydrogen peroxide and a
manganese salt catalyst in an alkaline slurry reaction.
The ae starch which is useful as the carrier starch is derived from a plant
breeding population, particularly corn, which is a genetic composite of
germplasm selections and comprises at least 75% amylose, optionally at
least 85% amylose (i.e. normal amylose) as measured by butanol
fractionation/exclusion chromatography techniques. The starch further
comprises less than 10%, optionally less than 5%, amylopectin and
additionally from about 8 to 25% low molecular weight amylose. The starch
is preferably extracted in substantially pure form from the grain of a
starch bearing plant having a recessive amylose extender genotype coupled
with numerous amylose extender modifier genes. This ae starch and the
method of preparation are described in copending application Ser. No.
937,794 filed Aug. 28, 1992, which is incorporated herein by reference.
The starch used in producing the carrier ae starch is preferably obtained
from a modified ae maize population. However, starches from other plant
sources may be suitable for use in this invention, and starch compositions
blended or formulated from more than one starch source are also suitable
for use herein, provided that the composition is adjusted to the
appropriate ratios of amylose, amylopectin and low molecular weight
amylose. The ae genotype plant from which the plant is extracted may be
obtained by standard breeding techniques or by translocation, inversion or
any other method of chromosome engineering to include variations thereof
whereby the desired properties of the starch are obtained. Any plant
source which produces starch and which can breed to produce a plant having
ae homozygous genotype may be used.
In another embodiment of this invention, the starch carrier may be a
degraded or converted starch produced by techniques such as acid
conversion, oxidation, pyroconversions and enzyme conversion, all of which
are conventional and well known in the art.
One particularly preferred method of obtaining the converted or degraded
starch carrier involves a process employing hydrogen peroxide and a
catalytic amount of manganese ions in an alkaline slurry reaction. In
carrying out this process a granular starch slurry is provided, ordinarily
at 20-50% solids, and the pH is raised to about 11.0 to 12.5, preferably
11.5 to 12.0, with sodium hydroxide. In order to achieve improved
retention rates when using slurry temperatures below about room
temperature, (e.g., 0.degree.-20.degree. C.), it is preferable that the pH
level be maintained at the upper portion of the designated useful pH range
(pH 11.8-12.5). Sufficient potassium permanganate or other source of
manganese ions is added with stirring to the slurry to provide a
concentration of about 1 to 80 ppm (parts per million) of manganese ions
based on the amount of starch solids. If desired, the manganese ions can
be added to the slurry prior to the pH adjustment. The manganese ions
catalyze the hydrogen peroxide/starch reaction so that a desired amount of
degradation (thinning) of the granular starch can be effected in a
substantially shorter reaction period as opposed to a reaction without
manganese ions. Temperatures ranging from about 0.degree.-55.degree. C.
can be used in reacting the starch but a range of about
22.degree.-45.degree. C. is most practical and preferred in the process
with the higher temperatures requiring less time for completion of the
reaction. With most starches, the temperature should not be allowed to go
much above 50.degree. C. because of the possible initiation of
gelatinization. Hydrogen peroxide is introduced into the slurry with the
full amount added at once or preferably divided into several portions (5
to 6 or more with larger amounts of peroxide) and added at about one-hour
intervals. The amount of hydrogen peroxide employed is from 0.01 to 3.0%,
anhydrous, preferably 0.05 to 1.5%, based on starch solids. The reagent is
ordinarily introduced by the addition of sufficient 20-35% aqueous
hydrogen peroxide as is commonly supplied in commerce. The practitioner
can easily determine the details of the preferred manner of addition of
any of the reaction components.
The agitated slurry is usually held for a total of 4-18 hours at the
designated temperature after the first addition of the hydrogen peroxide.
In some cases reaction periods of 2 hours or less may be adequate. In
cases of very high degradations and/or low temperatures, the reaction
period may be extended to as much as 36 hours. Samples of the slurry can
be taken at periodic intervals and the viscosity of the starch determined
to monitor the progress of the starch degradation, with Ford Cup
viscosities of about 10 to 180 seconds, preferably about 10 to 100 seconds
and more preferably about 10 to 60 seconds being particularly desirable.
When the desired level of starch degradation is reached, a reducing agent
for example, sodium bisulfite or sodium metabisulfite, can be added to
remove any excess hydrogen peroxide.
In a variation of the described procedure, a portion, about one-third or
one-quarter of the hydrogen peroxide estimated or calculated to be needed
to reach the desired degradation, can be added to the slurry, allowing
time for its reaction with the starch. After the addition of 2 or 3 such
portions the degree of starch degradation can be determined and additional
hydrogen peroxide is added as needed or the reaction can be terminated. In
this variation the peroxide increments are reacted as they are added and
better control of the degradation end-point can be achieved. A potassium
iodide spot test can be used to confirm the presence or absence of
hydrogen peroxide in the test slurry.
When the desired degradation is reached, the slurry is adjusted to a pH
level of 5.0-6.0 using dilute hydrochloric acid or other common acid and
filtered. The filter cake is optionally washed with water, refiltered and
dried in any conventional manner.
In addition to the use of potassium permanganate, which is preferred, other
manganese salts may be used as well, provided they are sufficiently
water-soluble to supply the necessary catalytic concentration of manganese
ions. Thus also useful for example, are: manganous nitrate and manganous
sulfate. The amount of manganese ions needed to promote the degradation is
from about 1-80 ppm, preferably 3-50 ppm, based on starch solids. Amounts
less than 1 ppm do not appear to hasten the reaction significantly while
amounts of 55 to 60 ppm or more tend to yield lower reaction efficiencies
as compared to the efficiencies obtained using 5-50 ppm.
Further description of this process and the components and conditions used
in carrying it out are disclosed in U.S. Pat. No. 4,838,944 issued Jun.
13, 1989 and which is incorporated herein by reference.
The high amylose starch used in the carrier starch component, as disclosed
herein, may comprise the entire starch portion of that component or it may
comprise a blend of at least about 25% and preferably at least about 50%
by weight of high amylose starch with other starches suitable for use in
corrugating adhesives. These blends are useful as long as the defined
characteristics of the carrier component, i.e. having a total amylose
content of greater than 60% by weight and other characteristics as
described herein, are satisfied. These other starches as well as the
starch in the raw component may be selected from the several starches,
native or converted, heretofore employed in starch corrugating adhesive
compositions including corn, potato, waxy maize, sorghum, wheat and
tapioca. Suitable starches, include, for example, those starches as well
as high amylose starches and the various derivatives of these starches.
Hence, the applicable starches which are useful in the compositions of
this invention as either carrier or raw starch include derivatives such as
ethers, esters, thin boiling types prepared by known processes such as
mild acid treatments, enzyme conversion, oxidation, etc and other starches
typically employed in corrugating adhesives. The carrier starch may be a
granular starch, a pre-gelatinized or cold water swellable starch or a
partially pre-gelatinized starch.
While the starch content can vary depending on several factors such as the
intended end use application and the type of starch used, it is important
that the amount of amylose in the gelatinized or cooked carrier starch
comprise at least 15% by weight, preferably from about 15 to 50% and more
preferably from about 18 to 40% by weight of the total starch content in
the adhesive. The total amount of starch employed including the
gelatinized or cooked carrier and the ungelatinized raw starch will
typically be in the range of about 10 to 50% by weight, based on the
weight of the composition. The ratio of the raw starch to carrier will
vary depending on properties desired and generally will range from about
1:1 to 10:1 depending on the nature of the starch and viscosity desired.
While the corrugating adhesive composition as described herein is primarily
directed to the particularly preferred embodiment of a composition
comprising a carrier starch and a raw starch, it may also include a no
carrier composition having just a single starch component comprising an
ungelatinized starch which upon subsequent treatment with alkali becomes
partially swollen. This single starch composition will comprise high
amylose starch i.e. starch having a total amylose content of grater than
60% by weight and blends with other starches as described herein provided
there is sufficient high amylose starch to provide at least 15% by weight,
and preferably at least 18% by weight of amylose based on the total weight
of starch in the adhesive. While varying amounts of up to 100% of amylose
content can be used, typical ranges will preferably use up to 75% of
amylose. The total amount of starch employed in the single starch
component composition will range from about 10 to 50% by weight, based on
the weight of the composition. Other components including the alkali,
boron-containing salt and water will be used in amounts as otherwise
described herein.
The adhesive composition also includes an alkali which is used in an
effective amount sufficient to provide the adhesive with a pH greater than
7, more particularly from about 7.5 to 13 and preferably from 10 to 13.
Typically this represents an amount of from about 0.3 to 5% and preferably
from about 1 to 4% by weight based on the weight of the starch.
The alkali (base) employed herein is preferably sodium hydroxide; however,
other bases may be used in partial or full replacement of the sodium
hydroxide and include, for example, alkali metal hydroxides such as
potassium hydroxide, alkaline earth hydroxides such as calcium hydroxide,
alkaline earth oxides such as barium oxides, alkali metal carbonates such
as sodium carbonate, and alkali metal silicates such as sodium silicate.
The alkali may be employed in aqueous or solid form.
Another common ingredient of corrugating adhesives is a boron-containing
salt, e.g., borax which is useful as a tackifier and which is optionally
used in effective amounts of up to about 5% by weight, based on the total
weight of starch. Additionally, any conventional non-che | | |
