Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 93/07337 2 1 2 0 ~i 2 4 PCl /US92/0823b
POLYUREA-REINFORCE:D FIBER-BASED MATERIALS
FIELD OF THE INVENTION
The present invention relates to reinforced fiber-based materials such
as reinforced fiberboards and reinforced paperboards, and containers made
therefrom.
BACKGROUND OF THE INVENTION
Piberboards, including corrugated and non-corrugated palperboa~ds are
useful for an extremely wide variety of applications, but particularly for making
containers such as packaging and shipping containers. Modern techniques for
mal~ng such containers involve not only manufacturing the requisite fiberboard
material but also cutting and shaping of one or more sheets of the fiberboard in~o
"box blanks" that are folded into the corresponding container shape. Box blanks are
typically designed with multiple scored lines and the like so that the bl2nk can be
readily forrned into a container by merely folding the box blank in an ordered
manner along the scored lines. Regardless of the container design, the forming of
a substantially planar box blank into a corresponding three-dimensional container
involves subjecting the fiberboard to a plurality of folds.
One drawback to many fiberboards, including paperboard, is their poor
rigidity when wet. To overcome this shortcoming, manufacturers have tried various
ways of reinforcing fiberboard and rendering ~he fiberboard nonabsolptive for
liquids. Examples of such reinfo~cement include impregnati3lg or coating the
fiberboard wi~h paraffin or other polymeric material.
_. While paraffin coating substandally decreases the tendency of the
fiberb~ard to absorb water, malcing paraf~ln-reinforced corrugated paperboard
popular for use in packaging vegetables as~d meats. Unfortunately, paraffin has the
disadvantage of being readily softened by moderately eleva~ed temperatures. Also,
while paraffin coating can sometimes enhance the compressive strength of the
fiber~oard and resistance to puncturing, the enhancement may not be sufficient for
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many uses. ln view of the shortcomings of reinforcing fiberboard using paraffin,other polymenc resins, particularly various thermoset materials, have been
considered for this purpose. Many cured therrnosets have the advantage of being
very rigid. As a result, fiberboards reinforced with cured thermosets tend to have
S high resistance to compression. Unfortunately, many currently favored thermosets
are extremely brittle after being fully cured and fracture when subsequently creased
or folded. Such fracturing of the thermoset reinforcing agent can readily extend to
the fiberboard itself, thereby seriously reducing the integrity of the container made
therefrom along edges and at corners.
Phenolics have received the greatest attention, particularly as a
reinforcing agent for corrugated paperboard. Representative U.S. Patents disclosing
use of phenolics include Patent Nos. 3,886,019, 4,096,935, 4,051,277 and
4,096,305 to Wilkenson et al. These patents disclose the application of a thin film
of phenolic resin to surfaces of linerboards and corrugated medium that will be
adhered together to form the corrugated paperboard. Since ve~y little resin
penetrates into the thickness dimension of the underlying paperboard, the outer
surfaces of the c~rrugated paperboard are free of resin. After adhering together the
linerboards and colTugated medium, the corrugated paperboard can be cut, scored,and slotted to make box blanks. Full curing of the resin is delayed until after the
box blanks have been folded to make cartons.
Various thermoset Uends of phenolics with other resins have also been
tried in an attempt to reduce the brittleness of phenolic alone. Representative U.s.
patents include Nos. 3,687,767 to Reisman et al. (pbenol-aldehyde), 3,607,598 toLeBlanc et al. (phenol-aldehyde plus polyvinylalcohol), 3,616,163 to Reisman
(phenoi-aldehyde resole), 3,619,341 to Elmer (phenol-aldehyde resole), 3,619,342to Burke (phenol-aldehyde resole), 3,697,365 to Reisman et al. (resole phenolic plus
an organosilyl compound), 3,682,762 to LeBlanc (resole phenolic plus
polyaminoalkyl substituted organosiloxane), 3,617,427 to LeBlanc (aminoplast-
modified phenol-aldehyde resole), 3,617,428 to Carlson (aminoplast with
phenolaldehyde resole), and 3,617,429 to LeBlanc (aminoplast plus phenol-aldehyde
and polyvinylalcohol).
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Despite these developménts, even phenolic blends tend to be
unacceptably brittle, which imposes certain limitadons on manufacturing processes.
For example, in all the phenolic-blend patents recited above, curing (thermosetting)
of the resin is performed only ~ç~ corrugating the medium fiberboard or even later
5 such as after the corrugated paperboard is scored along fold lines. This means, for
example, that resin-coated paperboard destined to become the corrugated medium
cannot be cured before it is passed through a corrugating machine. As a result,
conventional thermoset-impregnated medium paperboard cannot be made up and
cured in one location and supplied to another location for corrugating and
10 incorporation into corrugated paperboard using conventional machinery. Also,
interposition of resin-applying and resin-curing machinery into existing production
lines for manufacturing corrugated paperboard is expensive. These and other
problems with existing methods can unacceptably increase the costs of products
formed thereby, such as reinforced corrugated paperboard and cartons.
15Another disadvantage inherent in using phenolic blends according to
the above~ited references is that considerable amounts of resin must be used to
obtain satisfactory reinforcement. Typical resin loading levels are 2 to 10 w/w
perca~t resin per dq mass of linerboards and S to 15 w/w percent resin per dry mass
-~ ~ of corrugated medium. Since these resins are expensive, it would be advantageous
- 20 to have loading levels as low as possible, such as no greater than about 5 w/w
percent.
~ ~ SUMMARY OF THE INVENTION
- According to one aspect of the present invention, a polyurea-reinforced
25 fiber-based material is provided which comprises, at least, a single ply of a fibrous
material impregnated with polyurea on at least one of the faces (i.e., major surfaces)
of the ply. Each such impregnadon, termed herein a "polyurea-impregnated stratum. of fibers", extends depthwise from the corresponding face into the thickness
dimension of the ply no greater than about one-half the thickness dimension. That
. :.
30 is, the ply of fibrous material can have a polyurea-impregnated stratum on dther or
both faces. However~ whether a stratum is located on either or on both faces, a
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portion of the thickness dimension is left unimpregnated with polyurea. Therefore,
if such a stratum is located on both faces, each stratum has a thickness dimension
preferably no greater than about one-third the thickness dimension of the ply.
Although the fibers comprising the ply can be any of a wide variety
of fibers, including hydrophilic and hydrophobic fibers, they are preferably wood
pulp fibers. In any event, the fibers should contain residual moisture in a
concentration of about 3 to about 15% wlw relative to the dry mass of the fibers.
The fibers are preferably organized into a sheetlike web having a porosity sufficient
to absorb liquid polyisocyanate resin applied to the web for the purpose of forming
a polyurea-impregnated stratum. Most preferably, the wood pulp fibers are in theform of a paperboard.
Polyurea-reinforced fiber-based materials according to the present
invention exhibit surprisingly high ring-crush strengths at low loading levels of
polyurea. For example, a polyurea-reinforced paperboard according to the presentinvention contains a loading level of polyurea of about 5% w/w or less, yet exhibits
ring-crush strength equal to ring-crush strengths of analogous polymer-reinforc~d
materials known in the prior art having loading levels of polymel at least twice as
high, about 10% or more.
Leaving a portion of the thickness dimension unimpregnated with
polyurea contributes to the ability of the polyurea-reinforced material according to
the present invention, despite the fact that the polyurea is fully "cured", to be folded
and creased without necessarily fracturing. This is in contrast to analogous prior-art
materials that are generally so brittle that folding, and especially creasing, will cause
fracture of the material along the fold line. In fact, materials according to the
present invention are sufficiently foldable that they can be passed through a
corrugating-machine without fracturing. At a given ling-crush strength, the
foldability of polyurea-reinforced fiber-based materials according to the present
invention is higher than the foldability of prior-art materials having equal ring~rush
strengths.
Hence, to make a polymer-reinforced fiber-based material according
to the present invention, less fiber and polymer are required to achieve a desired
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ring-crush strength and foldability than are required to make analogous prior-art
materials.
According to another aspect of the present invention, the polyurea-
reinforced fiber-based material can comprise multiple web plies superposedly adhered
5 together, wherein at least one of the faces of at least one of the plies has a polyurea-
impregnated stratum. Hence, the present invention encompasses polyurea-reinforced
"corrugated paperboard" comprising at least one "linerboard" and at least one
"corrugated medium paperboard", wherein at least one of said plies has at least one
polyurea-impregnated stratum. prefcrably, but not necessarily, the corrugated
10 medium contains one or more of the polyurea-impregnated strata. Such corrugated
paperboard can also be comprised of more than one corrugated medium, each
sandwiched between and adhered to coextensive linerboards.
The cNsh resistance and foldability of materials according to the
present invention permit the mateAals to be prepared at one location, including full
15 cuAng, and used at a different location. For example, it is possible to manufacture
polyu~a-impregnated medium pa~rbcard at one plant and shîp the ~rd to
a second plant at which the pap rboard is corrugated for making into corrugated
py~rboard. It is also possible for fully cured polyurea-reinforced corrugated
paperboard according to the present invendon to be made at one location, then cut,
20 sco~ed, and folded to make cartons at another location. In otha words, the end-user
of the material does not have to be concerned with curing the material, in contrast
- ~- to end-users of analogous prior-art materials.
As another aspect of the present invention, methods are provided for
manufacturing such polyurea-reinforced fiber-based materials. In a representative
25 embodiment, polyisoeyanate resin is applied to one or both !faces of a fibrous web
at a loading~level that ensures that the resin does not penetrate into the thickness
dimension of the we~ more than about half the thickness dimension (if applied to- only one fæ) or about one-third the thickness dimension (if applied to both faces).
- Hence, the m~cimal loading level (the m~nilude of which will, of course, depend
30 upon the particular nature of the web) is dictated by the necessity to leave a portion
of the thickness dimension of the web unimpregnated with the resin. Although heat
:
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and pressure are not required to cure polyisocyanate, curing of the polyisocyanate
to form polyurea preferably occurs by application of heat and pressure.
BRIEP DESCRIPIlON OF THE DRAWINGS
FIG. 1 is a plot of the dry ring crush strength of polyurea-reinforced
paperboards according to the present invention compared to prior-art reinforced
paperboards.
FIG. 2 is a bar graph of the data of FIG. 1 together with plots of the
foldability of polyurea-reinforced paperboards according to the present invendoncompared to prior-art reinforced paperboards.
DETAILED DESCRIPIlON
In a method according to the p~esent invention, a liquid polyisoeyanate
resin is controllably applied to either the obverse or the reverse faces, or both, of a
sheetlike fibrous web. The polyisocyanate is subsequently cured to transform e~ch
polyisocyanate-impregnated face into a polyurea-impregnated stratum. Each
polyurea-impregnated stratum does not extend through the thickness dimension of the
web. In other words, even if the web possesses a polyurea-impregnated stratum onboth faces, the web retains a non-impregnated stratum within the thickness dimension
of the web.
As referred to herein, a Hsheetlike fibrous web" can comprise woven
, or nonwoven fibers. Consistent with a sheetlike confonnation, such a web has a
length dimension, a width dimension, an obverse face, a re~rerse face parallel to the
~- obverse face, and a thickness dimension extending between the obverse and reverse
25 faces. As is typical with fibrous webs, the thichless dimension is porous.
~ Representative fibers, not intended to be limi~ng, comprising the web
are hydrophilic fibers such as cellulosic fibers ~e.g., cotton, wood pulp, rayon),
carbohydrate fibers, polyvinyl alcohol fibers~ substituted cellulosic fibers, glass
fibers, mineral fibers, proteinaceous fibers (e.g., silk); and hydrophobic fibers such
as sized wood pulp, ~otton, or rayon fibers, polyethylene fibers, polypropylene
hbs, polyesw fibers, nylon fiber-, polyvinylace~te fibers, ~rea~ed glass fibers, and
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ararnid fibers; and mixtures of these fibers. If the fibers are synthetic polymeric
fibers, the fibers can be spun-bonded or heat-bonded.
The fibrous web should contain about three to about fifteen percent
moisture relative to the dry mass of the web. Such an amount of moisture is not
S necessarily sensed as Hwetnessn. In fact, virtually all fibrous materials, especially
hydrophilic materials, have a certain amount of moisture associated with the
constituent fibers and fiber molecules. The stated range of about 3 to about lS w/w
percent moisture is a typicat residual moisture range for most hydrophilic fibers such
as cellulosic fibers. If the fibers are hydrophobic, they may not contain sufflcient
lO residual moisture. In that case, additional moisture may have to be added, such as
by applying steam to the fibers or to the web made therefrom.
A 'polyurea-reinforced fiberboardN is a product according to the
present invention made from a sheetlike web of fibers. When the sheetlike web used
to make the fiberboard is comprised substantial1y of wood pulp fibers, the product
lS is referred to as a "polyurea-reinforced paperboardH.
-By way of exarnple and not intended to be limiting, representative basis
weights of webs comprising wood pulp fibers (i.e., "paperboards") range from about
10 to about 90 pounds per thousand square feet. It will be appreciated that, since
- ~ ~different fiber materials have different specific gravity values and since webs made
20 from different fiber materials may have different densities, suitable basis weight
~anges for other types of fibers may be diff~rent from the stated range for wood pulp
fibers.
NPolyisocyanatesN as used herein are liquid resins characterized as
having at least t vo isocyanate (-NCO) groups per molecule, rendering the molecules
25 polyfunctional and capable of covalently inter~onnecting with each other. Candidate
polyisocyanates can be selected from a group consisting of aliphatic, aromadc, and
alicyclic diisocyanates and other polyisocyanates generally known in the art as being
capable of forming polyurea. The polyisocyanate resin can comprise a mixture of
- ~polyisocyanates. Preferred polyisocyanates are formulations known in the art as
30 "PMDI" (a mixture of polyisocyanate oligomers) and an emulsifiable PMDI
formulation known as NEMDI". EMDI generally reacts with water to form polyurea
: ~ .
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at a faster rate than PMDI. This is because water is less soluble in PMDI than in
EMDI. A representative PMDI formulation is "PAPI 2027" manufactured by the
Dow Chemical Corp., Midland, Michigan. A representative EMDI formulation is
"XI-242" supplied by ICI United states, Inc., Wilmington, Delaware.
S The polyisocyanate used for treating webs according to the presentinvention can be in either a "neat" (undiluted) form or contain a diluent. suitable
diluents comprise organic solvents miscible with the polyisocyanate. If used, the
amount of solvent is generally within a range of about 5 to 20% w/w, relative to the
mass of the polyisocyanate. A preferred solvent is propylene carbonate, principally
because it is substantially odorless, and colorless, has low viscosity, low toxicity,
low vapor pressure at room temperature, and low flarnmability (boiling point: 242C;
flashpoint: 132C). The high boiling of propylene carbonate is particularly
advantageous because this solvent is thereby prevented from vaporizing under curing
conditions of elevated temperature and pressure. Other organic solvents can also be
used, so long as possible drawbacks of those other solvents, such as toxicity, low
boiling point, or flammability, can be accommodated. Candidate alternative solvents
include, but are not limited to, aromatics such as benzene, halogenated benzenes,
nitrobenzenes, alkylbenzenes such as toluene and xylenes, halogenated lower
aliphatics, ethers, ketones, alkyl acetates, and other alkylene carbonates
A benefit of diluting the polyisocyanate is reduced cost, since
polyisocyanates such as PMDI are relatively expensive compared to the cost of the
solvent. It has been found that diluting the polyisocyanate as described above
generally does not cause any substantial corresponding decrease in degree of
reinforcement compared to neat polyisocyanate.
Adding solvent to the polyisocyanate tends to reduce the viscosity of
the relatively-viscous neat polyisocyanate, which can improve the rate or increase the
depth of penetration of the resin into the web of a polyisocyanate at a particular
loading level. These results can be advantageous especially in high-speed processes
for producing reinforced fiber-based materials according to the present invention.
Each polyurea impregnated stratum typically extends the length and
width dimensions of the web parallel to the obverse and reverse faces of the web.
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When a polyurea-impregnated stratum is located on only one face of the web, the
impregnated stratum preferably has a thickness dimension no greater than about half
the web thickness dimension and preferably between one-third and one-half the web
thickness dimension. When a polyurea-impregnated stratum is located on both faces
S of the web, the strata each have a thickness dimension no greater than about one-
third the web thickness dimension. In either case, a portion of the thiclcness
dimension of the web is leR unimpregnated with polyurea.
Although fully impregnating the thickness dimension of the web may
yield a fiber-based material having even greater crush resistance, leaving at least a
portion of the thickness dimension of the web without any polyurea, according to the
present invention, provides a unique combination of crush strength and flexibility.
Accordingly, if too much of the thickness dimension is impregnated with polyurea,
the fiber-based material can become too brittle for certain uses. If too little of the
thickness dimension is impregnated, the material may e~hibit insufficient crush
resistanoe forcertain uses.
As the polyisocyanate resin is applied to the web, the resin usually
absorbs rapidly into the pores of the thickness dimension of the web. The depth of
absorption is controlled by precisely controlling the "loading" of polyisocyanate on
the surface of the web. As used herein, "loading" and "loading level" refer to the
- 20 mass of polyisocyanate resin (or the mass of polyurea, after the polyisocyanate is
cured) applied to a face of the web, relative to the mass of the web. Of course, a
particular loading level of polyisocyanate resin will penetrate to different depths in
- ~ the thichess dimensions of different webs, including webs made of different fibers.
Hence, different webs can accommodate different polyisocyanate loaiding levels
before the requisite penetration Iimits are exceeded. By way of examplè, not
intended to-be limiting, a PMDI rcsin such as "PAPI 2027" when applied to
paperboard at about a four to five percent w/w loading level will penetrate a
maximum of about 1/3 the thickness dimension of the paperboard. When this resin
is appliied to paperboard at an eight percent w/w loaiding, the resin penetrates nearly
the entire thickness dimension. When applied at twenty percent w/w, the paperboard
becomes fully saturated with the resin. For any type of web, simple cross-sectional
.''~ .
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examination of the thickness dimension of an impregnated web using a microscope
will enable one to readily determine the particular loading level that will produce a
particular depth of penetration of the resin.
It will be appreciated that controlling the loading level involves
applying the polyisocyanate resin in a manner whereby the mass of polyisocyanateresin applied per unit area of the web is precisely controlled. The liquid can be
applied to the web by any of various liquid-application methods including, but not
limited to, gravure printing, roller coating, and spraying. The preferred application
method is gravure printing because it has been found that this method provides more
precise control of resin loading on the web surface than other methods.
"Curing" of a polyisocyanate resin in the presence of water (present
as residual moisture in the web) conve~s the polyisocyanate resin to polyurea which
is a type of thermoset material. Curing of polyisocyanate resin occurs via
polyaddition and crosslinking reactions of the polyisocyanate molecules by reactions
involving water as well as other molecules in the web that have -OH substituent
groups available for reaction.
Curing can occur at room temperature, but the time required (several
days) may be inconvenient. One way to increase the rate of curing is to increasetemperature and/or pressure. However, the curing temperature must not be so highthat damage to the resin, polyurea, or web results. A general range for curing
temperature is room temperature up to about 232C (450F). A general range for
curing pressure is from about zero up to about 1000 psig. With paperboards to
which PMDI resin has been applied, curing is preferably conducted at about 204C(400F) and about 800 psig for a time from about four seconds to about five minutes.
The preferred curing time at 200C and 800 psig is about 40 seconds. Of course,
since elevated temperature and pressure increase the rates of the curing reacdons, the
higher the temperature and/or pressure, the less time required to achieve the same
degree of cure.
Curing at elevated temperatures and pressures can be effected in any
of various devices adapted to controllably apply heat and pressure. Candidate curing
devices include, but are not limited to, platen presse~, continuous belt presses, and
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autoclaves (steam). If necessary, curing can be performed by a regimen that includes
two or more short applications of pressure rather than a continuous application for
the entire time required to achieve full cure.
As is known in the art, curing of polyisocyanate can be accelerated by
the addition of a catalyst to the resin. In general, any of various catalysts effective
for use with polyisocyanates will work. Examples of catalysts, not in any way
intended to be limiting, are dibutyltin dilaurate and "DABCO", which would be
added to the resin in an amount ranging from about 0.1 lo about 1.0% w/w relative
to the mass of the polyisocyanate. However, we have determined that a catalyst is
usually not required, especially if curing is effected by application of both heat and
pressure.
A polyu~ea-reinforced fiber-based material according to the present
invention comprises at least one fibrous sheetlike web. When the polyurea-
reinforced fiber-based material is comprised of only web or "ply", the ply comprises
at least one substantially continuous polyurea-impregnated stratum of fibers located
within the thickness dimension of the web. The impregnated stratum can be located
on either the obverse or reverse face of the web or on both faces.
A polyurea-reinforced fiber-based material according to the present
invention can be comprised of only one ply or more than one ply. In such multiple-
ply materials, it is not necessary that all the plies have a polyurea-impregnated
stratum. The present invention encompasses multiple-ply materials wherein only one
ply thereof has at least one polyurea-impregnated stratum. The present invention also
encompasses multiple-ply materials wherein multiple plies each have at least onepolyurea-impregnated stratum. Each stratum need not have the same loading level.ln multiple-p!y materials according to the present invention, each ply
can be made from the same or a different fibrous web. The webs need not all havethe same basis weight, thicla~ess, porosity, or texture.
When the polyurea-reinforced fiber-based material is comprised of
- more than one ply, the plies are typically superposedly adhered together. Adhering
the plies together can be achieved by adhering non-impregnated faces to non-
imp~gnated faces, non-impregnated faces to impregnated faces, and impregnated
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faces to impregnated faces. The outermost faces of such multiple-ply materials need
not be the impregnated faces.
One example of a multiple-ply material according to the present
invention is a corrugated paperboard wherein at least one of the plies thereof has at
5 least one polyurea-impregnated stratum. As used herein, a "corrugated paperboard"
is a widely recognized product comprising at least two plies of paperboard adhered
together, where at least one of said plies is corrugated in a manner known in the art.
The corrugated ply is generally re~erred to as the "medium" paperboard. At leastone of said plies is not corrugated and is used as a facing sheet for the corrugated
10 paperboard. Hence, the non-corrugated ply is termed a "linerboard". Typical
corrugated paperboards are comprised of a corrugated medium sandwiched between
two linerboards adhered to the corrugated medium. The linerboard(s) of a
corrugated paperboard o~ten have a larger basis weight than the corrugated medium.
Any suitable adhesive can be used to adhere the linerboards to the corrugated
15 medium. A corrugated paperboard can also compAse multiple plies of co~ugated
medium separately inte~posed between plies of linerboards. Colrugated paperboards
- are widely used for making cartons and the like.
Since curing can occur at moderate temperatures, it has unexpectedly
been found that curing of the polyisocyanate applied to a paperboard can be
performed according to the present invention simultaneously with corrugation of the
paperboard. This is because conventional eolTugators impart a certain amount of
heat and pressure to the paperboard as the paperboard passes through the corrugator.
Simultaneous curing and corrugation is particularly advantageous when making
polyurea-impregnated eorrugated medium according to the compression strength of
polyurea-reinforced material eq~al to prior~art reinfor~ed materAal could be at~ained
by using less fiber in the polyurea-impregnated m~teAal according to the presentinvention. Use of less fiber can substantially reduce production and shipping costs.
The data plotted in FIG. 1 is also presented in FIG. 2 in bargraph
form.
Exarnples 1 through 16 were also subjected to fold~ng endurance tests
in order to assess the brittleness of the materials. Folding endurance tests as
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described hereinbelow are a commonly used assessment of the brittleness of a
material.
Folding endurance tests were performed on st~ips 1~2 inch wide and 6
inches long according to the TAPPI T511-OM-83 test procedure. Briefly, the
folding endurance test comprises holding one end of a test strip in a stationaryposition and applying a one-kilogram weight to the other end. While applying theweight, the length of the strip between the ends is repeatedly flexed over a 270 arc
until the strip breaks. Data is recorded as the number of flexes until break.
Fold-resistance data are plotted in PIG. 2. While PE/S exhibited the
greatest fold resistance, the fold resistance of material treated with PMDI (polyurea-
impregnated stratum) exhibited a fold resistance that was about the same as UF and
substantially better than PF. These results also reveal that, as loading level
increases, fold resistance decreases. Hence, a lower loading level of polyurea
(reladve to prior-art reinforcing agents) not only yields the sarne compression
strength as prior-art reinforcing agents at subs~andally higher loading levels, such
lower polyurea loading levels also provide better fold resistance at equal strength.
Therefore, at a ~iven compression strength, polyurea reinforced fiber-based materials
actually have less present invention because conventional process machinery can be
readily and inexpensively adapted to include a gravure coater, sprayer, or the like
without the need to add a separate curing device. In such an instance, the gravure
coater, sprayer, or the like is added to the process machinery upstream of the
corrugator. As the paperboard to which the resin has been applied passes thr~ughthe corrugator, the polyisocyanate undergoes curing simultaneously with impression
of corrugations into the paperboard.
As can be appre~,iated from the foregoing, the polyurea imparts a
subs~antial reinforcement to a fibrous web, enabling the polyurea-reinforced web to
e.xhibit a crush-resistance strength that is greater than the crush-resistance strength
of the corresponding non-reinforced web. Hence, with products made from a
polyurea-reinforced web produced according to the present invention, lesser amounts
of fibrous web are required to obtain a crush resistance equal to the crush resistance
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of similar products made from non-reinforced web, which can yield considerable
savings in cost and weight while adding other benefits such as wet strength.
It has been found that fiber-based materials reinforced with at least one
polyurea-impregnated stratum according to the present invention actually have greater
S crush strength at lower loading levels than similar fiber-based materials impregnated
with prior-art reinforcing resins. lt has also been found that the flexibility of
reinforced fiber-based materials according to the present invention is the same as or
better than prior-art materials having the same loading level. Hence, at a givencrush strength, reinforced fiber-based materials according to thepresent invention are
10 more flexible than prior-art reinforced fiber-based materials.
A key benefit of greater flexibility at equal strength is that it is now
possible for the first time, for example, to apply a polyisocyanate resin to paperboard
according to the present invention and fully cure the resin to a polyurea beforepassing the paperboard through a corrugator. In contrast with hwwn prior-art
15 reinforced paperboards, corrugating a reinforced paperboard made according to the
- present invention will not cause the paperboard to crack along the corrugations.
Hence, for the first time, paperboard destined to become corrugated medium can
receive a fully cured polyurea stratum at a first location, be rolled and shipped to a
second location remote from the first location, and be made into a corrugated
material at the second location. Similarly, cartons and the like can now be madefrom fully cured polyurea-reinforced corrugated paperboard produced according to, the present invention, including such operations as cutting and folding, without the
paperboard breaking along cut and fold lines.
It has also been found that polyurea-reinforced fiber-based materials
according to the present invention can be adhered together using conventional
adhesives. ~n part, this is because the polyurea impregnant is not present through
the entire thich~ess dimension of the web, as described above. For example,
reinforced corrugated paperboards can be assembled from a corrugated medium and
at least one linerboard (wherein at least one of the medium and linerboards are
-; 30 polyurea-reinforced according to the present invendon) using convendonal water-
soluble adhesives such as starch-based adhesives, latex-based adhesives, or latex-
,
Wo 93/07337 2 1 2 ~ 5 2 I Pcr/uss2/08236
starch adhesives to adhere nonimpregnated surfaces together. Alternatively, if
desired, conventional non-aqueous adhesives can also be used on either non-
impregnated or impregnated surfaces. Such non-aqueous adhesives include, but are- not limited to, hot-melt adhesives, polyurethanes, isocyanates, epoxies, rubber-based
5 adhesives, various solvent-borne polymers, mastics, and silicones.
Additional benefits of polyurea-reinforced fiber-based materials
according to the present invention include:
(a) Wet resistance: the materials maintain some crush resistance
even when wet, which is of considerable benefit when the materials are employed
10 in making shipping cartons.
(b) Printability: the materials can be printed on either impregnated
surfaces or non-impregnated surfaces using conventional printing inks and methods.
(c) Resistance to fracture, even after being folded a number of
times. Such resistance is due in part to the better flexibility of polyurea as a15 reinforcing agent and in part to the fact that the polyurea impregnant does not extend
entirely through the thichless dimension of the web. Hence, the non-impregnated
portion of the web can serve as a hinge during folding, even after a iengthy series
of folds.
A polyurea-reinforced fiber-based material according to the present
20 invention also has potential uses other than packaging and storage containersincluding~ but not limited to, various laminates, skins, and facings for paneling,
plywood, and other construction materials; wall coverings; and analogous uses.
In order to further illustrate the invention, the following examples are
provided.
` `
Examples 1-i6
In these examples, the ring-crush strength (edgewise compression
resistance) of a paperboard material treated according to the present invention (i.e.,
containing a polyurea-impregnated stratum) was compa~ed to the ring~rush strength
30 of similar paperboard material treated with various other resins h~own in the art for
use as reinforcing agents. Ring-crush strength is an accepted measure of the crush
wo 93/07337 Pcr/uss2/o8236
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resistance of objects made from the respective material. The tests comprising these
examples were performed according to the TAPPl T81~ OM-87 standard test
procedure.
The paperboard selected for these examples was a 42-pound basis
S weight kraft linerboard. separate sheets of the linerboard measuring 12 inches by
12 inches were treated individually on one face with the following resins and at the
following loading levels:
Table I
Example_ _Resin* Loadin~Level
PMDI 3 5b
2 PMDl 4 %
3 PMDl 8%
4 PMDI 20%
S Urea-formaldehyde (UF) 3%
6 Urea-formaldehyde (UF) 4%
7 Urea-forrnaldehyde (UF) 8~o
8 Urea-formaldehyde (UF) 20%
9 Phenol-fonnaldehyde (PF) 3%
Phenol-formaldehyde (P~) 4%
11 Phenol-fonnaldehyde (PP) 8%
12 Phenol-formaldehyde (PF) 20%
13 Polyester-styrene (PE/S) 3%
14 Polyester-styrene (PE/S) 4%
Polyester-styrene (PE/S) 8~
16 Polyester-styrene (PE/S~ 20%
*The PMDI was "PAPI 2027" from the Dow Chemical Company,
Midland, Michigan; the urea-formaldehyde was "SR 398BN from Borden Chemical,
Columbus, Ohio; the phenol-formaldehyde was "RPLS 5460" from Georgia-Pacific,
Atlanta, Georgia; the polyester-styrene was "33~02" from Reichold Chemicals, Inc.,
Pensacola, Florida, catalyzed with 0.6% methylethylketone peroxide.
The UF, P~, and PE/S resins were selected for comparison because they represent
resins typically used in the art for reinforcing fiber-based materials such as
paperboard~
The resins were applied to the sheets using a gravure coater. The
PMDl resin was cured by heating the treated sheets at 204C, 800 psig, for about
Wo 93/07337 ~ 1 2 0 ~ 2 4 P~r/uss2/08236
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40 seconds. The urea-formaldehyde, phenol-formaldehyde, and polyester-styrene
resins were cured by heating at 150C and 800 psig for about five minutes.
After curing, the treated linerboards were cut into strips 1/2 inch wide
- and 6 inches long using a precision cutter. For each example, a representative strip
from ea~h example was rolled end-t~end into a cylinder and placed into a specimen
holder manufactured by Sumitomo Corp., Chicago, Illinois. The holder with the test
"cylinder" was then mounted on the lower platen of a conventional machine adapted
for applying a compressive force. A progressively increasing axially compressiveload was applied by the machine undl the cylinder experienced compressive failure.
The compressive force in pounds was recorded at time of failure. All tests were
performed at 50% relative humidity. Experimental controls were similar
compressive tests performed using the same but unereated linerboard.
Results of the ring-crush tests are shown in FIG. 1 wherein, at loading
levels less than or equal to about eight percent w/w, the linerboard comprising a
polyur~a-impregnated stratum (treated with PMDI) exhibited substantially greatercrush resistance than linerboards treated with UF, PP, or PE/S at the same loading
levels. As can be seen, the ring-crush strength of the control was about 87 pounds.
Of the L;nerboards treated with a three percent loading level, the PMDI-treated
linerboard exhibited the greatest ring-crush strength (128 pounds for PMDI compared
~- 20 to 111 pounds for PE/S, 96 pounds for UP, and 94 pounds for PP). At a four-
-~ ~pcent loading level~ the PMDI-treated linerboard again exhibited the greatest ring-
~ ~ . crush strength (139 pounds for PMDI compared to 113 pounds for PE/S, 108 pounds
-
for PF, and 100 pounds for UF). At eight-percent loading, the PMDl-treated
- linerboard was again substantially better (1$4 pounds for PMDI versus 133 pounds
, ~ ~
` 25 for PP,~131 pounds for PEIS, and 124 pounds for UP).
FIG. 1 also indicates that the PMDI-treated linerboard at a three-
; ~ ~; percent loading level had a ring-crush strength after curing that was about equal to
-- ~ the ring-crush strengths of linerboards treated with eight percent UF, PF, or PE/S.
Hence, the polyurea-reinforced fiber-based material prepared at a loading level
~ ,
30 than half the loading level of the prior-art reinforced fiber-based materials exhibited
about the same ring-crush strength as the prior-art fiber-based materials.
:
WO 93/07337 PCr/US92/08236
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It is also evident from FIG. 1 that the ring-crush strength of linerboard
treated with four-percent PMDI is even greater than the ring-crush strengths of
linerboards treated with eight percent of PF, UF, or PE/S. Thus, again, polyureawill impart the same crush resistance to a fiber-based material at half (or less) the
S loading level of prior-art reinforcing agents. While it would be expected thatincreasing the loading level of any thermosettable resin in a fibrous web would yield
corresponding increases in ring-crush strength for virtually any fibrous web treated
as described with these resins, it was unexpected that polyurea at such low loading
levels (particularly five percent w/w or less) would produce such dramatically
10 improved compression strength over other reinforcing agents commonly known in the art.
Since compression strengths of fibrous materials treated with polyurea
according to the present invention are substantially equal to such strengths of prior-
art materials containing a loading level of reinforcing impregnant at least twice the
15 loading level of polyurea, a brittleness (more flexibility) than fiber-based materials
reinforced with prior-art agents such as PP, UP, or PE/S.
Examples 17 and 18
In these examples, webs made of thermoplastic fibers were treated
20 with PMDI ("PAPI 2027") to produce a reinforced fiber-based material. The webs
were a ~SSfS0~ RayonlPET (polye*yleneterphthalate) spun lace web, 2 oz/yd2
, ~e~mple 17) and a spun-bonded po!ypropylene web, 1 oz/yd2 (example 8). These
webs are paper-like in appearance but have "hand" characteristics similar to fabrics.
-~ ~The fibers comprising these webs are long compared to, for example, the fibers in
.
25 a paperboard. Without reinforcement, these webs have no edgewise cmsh resistance
at all. The~ PMM loading level in each web was eight percent.
After curing, the treated webs were cut into strips and subjected to
ring-crush tests at 50 percent relative humidity as described hereinabove.
~:~Thepolyurea-reinforced "55:50~ rayon/polyethyleneterphthalate spun
30 lace exhibited a ring crush strength of 4.4 pounds and the polyurea-reinforced
~ ~ .
~polypropylene spun bonded web exhibited a ring crush strength of 1.9 pounds.
WO 93/07337 2 1 2 3 ~ 2 'i Pcr/US~2/08236
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These results indicate that hydrophobic fiber-based materials can be reinforced with
polyurea so as to substantially increase the crush resistance of these materials.
- Examples 19-~7
These examples comprise various paperboards treated on one side with
PMDI according to the present invention. Also included are eontrols which received
either no treatment (zero-present loading level) or PMDI loading levels greater than
about 5 % w/w.
The PMDI was diluted with 10% w/w propylene carbonate. The
paperboards included 20 lb/1000 ft2 kraft bag paper (examples 19-22~9 26 lbllO00ft~ kraft linerboard (examples 23-25), 58 Ib/1000 ft2 kraft linerboard (examples26-31), 26 lb/1000 ft2 medium paperboard (examples 32-34), and 33 Ib/1000 ~t2
medium paperboard (examples 35-37). Untreated controls were examples 29, 23,
26, 32, and 35. After the resin was applied to the paperboards, the boards were
cured as described in Examples 1^16. In examples 21, 22, 25, and 35, the resin was
applied at loading levels greater than five percent. Ring~rush and foldability tests
were performed as described in ~xamples 1-16. Data are presented in Table II.
Ta~le II
~0
Example We~ ading ~n~sh Count
19 10 lb/1000 ft2 0% 29 lbs 956
kraft bag paper
" " 4.7% 33 Ibs 876
21 " " 9% 33 lbs 904
22 " " 13% 42 Ibs 674
23 26 lb/1000 ft2 0% 47 lbs 997
k~aft linerboard
24 " " 3.9% 71 lbs 616
" " 9% 83 l~s 311
26 58 lb/1000 ft2 0% 113 lbs> lO00
- kraft linerboard
27 " " 1% 163 lbs 967
28 " " 2.3% 196 lbs 937
29 " " 3.4% 212 Ibs 416
" " 4.6% 218 lbs 421
WO 93/07337 PCl /US92/08236
21~
-20-
31 " " 5% 219 lbs 269
32 26 lb/1000 ft2 0% 41 Ibs 125
medium paperboard
33 " " 3% 47 lbs 60
34 ~ 5.7% 51 lbs 80
33 lbll000 ft2 0% 58 Ibs 143
medium paperboard
36 " " 0.4% 65 lbs 60
37 ~ 4.3% 72 lbs 50
Referring to the untreated contr~ls of Table II (examples 19, 23, 26,
32 and 35), it can be seen that each of ~e paperboards is quite different. The 2~1b
kraft bag paper is a very flexible paperboard. Kraft linerboards are more rigid than
bag paper (compare the ring-crush strength of example 23 to the ring-cn~sh strength
of example 19). Kraft medium paperboard is stiffer and more brittle than either bag
paper or linerboard (compare, for example, the fold count of example 32 to the fold
count of examples 19, 23, and 26). The data of Table II alss~ show that, as PMDIloading increases, foldability (a measure of brittleness) decreases. Also, as loading
increases, ring-crush strength increases. Pinally, with lower basis-weight
- 20 paperboards, the increase in ring-crush stren~th expenenced with increased PMDI
loading is less than the increase in ring-crush strength seen with increased PMD~
loading of higher basis-weight paperboards.
While the invention has been described in connection with preferred
embodiments and several examples, it will be understood that it is not limited to
2,5 those embodiments. On the contrary, it is intended to c~ver all ~ltematives,
modifications, and equivalents as may be included within the spirit and scope of the
invention as defined by the claims.
