Composition for preparing paperboard container for liquids

Disclosed is a composition useful for preparing paperboard containers comprising wood pulp, polyolefin pulp containing carboxylic functionality and a water-soluble cationic wet-strength resin. The paperboard is used for preparing containers for liquids such as juices and dairy products. The filled containers have improved durability when exposed to high humidity.

FIELD OF INVENTION 
This invention relates to compositions useful for preparing containers for 
liquids. This invention especially relates to the incorporation of 
polyolefin pulp containing carboxylic fuctionality and a water-soluble 
cationic wet-strength resin into wood pulp to produce a composition useful 
for the manufacture of paperboard. The paperboard is used for preparing 
containers for liquids such as, for example, juice and dairy products. The 
filled containers have improved durability and therefore a longer shelf 
life than currently available containers for liquids. 
BACKGROUND OF THE INVENTION 
Currently available containers for liquids such as orange juice, milk, sour 
cream and liquid quiche generally are multi-ply structures of metal foil, 
such as aluminum foil, and polyethylene-coated bleached paperboard. The 
shelf life of the filled container is normally less than 14 days. During 
this time period the paperboard tends to absorb moisture from the contents 
of the container and from the surrounding atmosphere, causing the 
container to bulge and to feel soft to the hand. This bulging results in 
product returns from the retailer even though the quality of the liquid in 
the container is still satisfactory. 
The problem of bulging is especially severe when the containers are stored 
in a non-frost free environment, e.g. in a walk-in refrigerator, where the 
relative humidity is above 90%. Increasing the thickness of the 
polyethylene coating does not solve the problem of moisture absorption by 
the paperboard, and in some cases may have a negative effect because more 
moisture is trapped in the board. 
SUMMARY OF THE INVENTION 
It has now been found that incorporation of polyolefin fibers and a 
water-soluble cationic wet-strength resin into the wood pulp used to 
prepare paperboard containers for liquids will improve the strength, 
toughness and rigidity of the container not only under ambient conditions, 
but also after exposure to high humidity. The increased durability of the 
container extends the shelf life of the packaged product and results in a 
lower product return from retailers. 
The composition of this invention comprises (1) wood pulp, (2) from about 
1% to about 10%, based on the weight of the wood pulp on a dry basis, of a 
polyolefin pulp containing carboxylic functionality, and (3) from about 
0.05% to about 0.6%, based on the weight of the wood pulp on a dry basis, 
of a cationic water-soluble wet-strength resin. Papermaking additives such 
as sizing agents, retention aids, etc. may optionally be added to the 
composition.

DETAILED DESCRIPTION OF THE INVENTION 
Polyolefin pulps are very fine, highly-branched, discontinuous fibers made 
from polyolefins such as polyethylene, polypropylene, an 
ethylene-propylene copolymer or a mixture of any of these polyolefin 
materials. Such pulps are known in the art, as are their methods of 
manufacture. See, e.g., "Pulp, Synthetic", Kirk-Othmer, Encyclopedia of 
Chemical Technology, 3rd. ed. (New York, 1982) Vol. 19, pp. 420-435, which 
is incorporated herein by reference. 
The fibers (fibrids) of which polyolefin pulp is composed have a high 
surface area, in general at least 1 square meter per gram, lengths which 
will be in the range of from about 0.5 millimeter to about 10 millimeters, 
and diameters of about 1 micron to about 100 microns. 
The polyolefin pulp containing carboxylic functionality used in the present 
invention can be a polyolefin polymer pulp containing carboxyl groups 
which have been introduced into the polymer molecule by grafting the 
polyolefin with a monomer containing carboxylic functionality or by 
oxidizing the polyolefin with oxygen or ozone, or a polyolefin may be 
cospurted with an anionic polymer containing carboxylic functionality. The 
anionic polymer may be a polyolefin containing carboxyl groups directly 
attached to the polymer backbone; a polyolefin grafted with acrylic acid, 
methacrylic acid, maleic anhydride or mixtures thereof; a copolymer of any 
of ethylene, propylene, styrene, alpha-methylstyrene or mixtures thereof 
with any one of acrylic acid, methacrylic acid, maleic anhydride or 
mixtures thereof; as well as mixtures of any these anionic polymer 
components. Bonding between the carboxyl groups of the spurted polyolefin 
pulp and the --OH groups of the cellulose in the wood pulp is believed to 
contribute to the increased durability of the paperboard. 
In this specification, all parts and percentages are by weight unless 
otherwise specified. 
The amount of polyolefin pulp (dry basis) to be used in the composition is 
from about 1% to about 10% based on the weight of the wood pulp. An amount 
of from about 4% to about 5% is preferred. The optimum amount will depend 
on the polyolefin pulp chosen and the properties desired in the final 
paperboard sheet. Generally it has been found that as the amount of 
polyolefin pulp increases, the durability of the paperboard sheet also 
increases. 
The cationic water-soluble wet-strength resin used in the composition of 
this invention is a cationic starch, such as cationic potato starch, or a 
water-soluble aminopolyamide-epichlorohydrin resin. 
Suitable aminopolyamide-epichlorohydrin resins are disclosed and described 
in U.S. Pat. Nos. 2,926,116 and 2,926,154, the disclosures of which are 
incorporated herein by reference. The aminopolyamide is derived by 
reaction of a dicarboxylic acid and a polyalkylene polyamine in a mole 
ratio of polyalkylene polyamine to dicarboxylic acid of from about 0.8:1 
to about 1.4:1. 
Particularly suitable dicarboxylic acids are diglycolic acid and saturated 
aliphatic dicarboxylic acids containing from 3 through 10 carbon atoms 
such as malonic acid, succinic acid, glutaric acid, adipic acid, pimelic 
acid, suberic acid, azelaic acid, and sebacic acid. 
Other suitable dicarboxylic acids include terephthalic acid, isophthalic 
acid, phthalic acid, maleic acid, fumaric acid, itaconic acid, glutaconic 
acid, citraconic acid, and mesaconic acid. 
The available anhydrides of the above acids can be used in preparing the 
water-soluble aminopolyamide as well as the esters of the acids. Mixtures 
of two or more dicarboxylic acids, their anhydrides, and their esters can 
be used to prepare the water-soluble aminopolyamides, if desired. 
A number of polyalkylene polyamines, including polyethylene polyamines, 
polypropylene polyamines, polybutylene polyamines and the like can be 
employed. Polyalkylene polyamines can be represented as polyamines in 
which the nitrogen atoms are linked together by groups of the formula 
--C.sub.n H.sub.2n --where n is a small integer greater than unity and the 
number of such groups in the molecule ranges from two up to about eight. 
The nitrogen atoms can be attached to adjacent carbon atoms in the 
groups--C.sub.n H.sub.2n --or to carbon atoms farther apart, but not to 
the same carbon atom. Polyamines such as diethylenetriamine, 
triethylenetetramine, tetraethylenepentamine, and dipropylenetriamine, 
which can be obtained in reasonably pure form are suitable for preparing 
water-soluble aminopolyamides. Other polyalkylene polyamines that can be 
used include methyl bis-(3-aminopropyl)amine; methyl 
bis-(2-aminoethyl)amine; and 4,7-dimethyltriethylenetetramine. Mixtures of 
polyalkylene polyamines can be used, if desired. 
The spacing of an amino group on the aminopolyamide can be increased if 
desired. This can be accomplished by substituting a diamine such as 
ethylenediamine, propylenediamine, hexamethylenediamine and the like for a 
portion of the polyalkylene polyamine. For this purpose, up to about 80% 
of the polyalkylene polyamine can be replaced by a molecular equivalent of 
diamine. Usually, a replacement of about 50% or less will be adequate. 
Temperatures employed for carrying out the reaction between the 
dicarboxylic acid and the polyalkylene polyamine can vary from about 
110.degree. C. to about 250.degree. C. or higher at atmospheric pressure. 
For most purposes temperatures between about 160.degree. C. and 
210.degree. C. are preferred. The time of reaction will usually vary from 
about 1/2 hour to 2 hours. Reaction time varies inversely with reaction 
temperatures employed. 
In carrying out the reaction, it is preferred to use an amount of 
dicarboxylic acid sufficient to react substantially completely with the 
primary amine groups of the polyalkylene polyamine but insufficient to 
react with the secondary amine groups and/or tertiary amine groups to any 
substantial extent. This will usually require a mole ratio of polyalkylene 
polyamine to dicarboxylic acid of from about 0.9:1 to about 1.2:1. 
However, mole ratios of from about 0.8:1 to about 1.4:1 can be used. The 
aminopolyamide, derived as above described, is reacted with 
epichlorohydrin at a temperature of from about 45.degree. C. to about 
100.degree. C., and preferably between about 45.degree. C. and 70.degree. 
C., until the viscosity of a 20% solids solution in water at 25.degree. C. 
has reached about C or higher on the Gardner-Holdt scale. This reaction is 
preferably carried out in aqueous solution to moderate the reaction. pH 
adjustment is usually not necessary. However, since the pH decreases 
during the polymerization phase of the reaction, it may be desirable, in 
some cases, to add alkali to combine with at least some of the acid 
formed. When the desired viscosity is reached, water can be added to 
adjust the solids content of the resin solution to a desired amount, 
usually from about 2% to about 50%. 
In the aminopolyamide-epichlorohydrin reaction, satisfactory results can be 
obtained utilizing from about 0.1 mole to about 2 moles of epichlorohydrin 
for each secondary or tertiary amine group of the aminopolyamide, and 
preferably from about 1 mole to about 1.5 moles of epichlorohydrin. 
The aminopolyamide-epichlorohydrin resin comprises from about 0.05% to 
about 0.6% of the composition of this invention, based on the weight of 
the wood pulp on a dry basis. From about 0.2% to about 0.3% is preferred. 
The wood pulp used in the composition may be any of the pulps commonly used 
in the manufacture of paperboard. Softwood kraft or a mixture of softwood 
kraft and hardwood kraft are preferred. 
Conventional additives such as, for example, sizing agents and dry strength 
agents may be added to the pulp before formation of the paperboard sheet. 
External sizing and strength agents may also be added at the surface of 
the formed sheet. 
EXAMPLE A 
The preferred polyolefin pulp containing carboxylic functionality is a 
mixture of high density polyethylene coflashed with an ethylene/acrylic 
acid copolymer. The polyolefin pulp is prepared as follows. 
Ninety parts of high density polyethylene (DuPont, melt index 5.5-6.5 at 
190.degree. C.) and 10 parts of an ethylene-acrylic acid copolymer (Dow, 
92:8 ethylene:acrylic acid, melt index 5.3) are charged to a closed 
autoclave along with 400 parts of pentane as the solvent. The contents of 
the autoclave are stirred and heated to 193.degree. C. at which point the 
vapor pressure in the autoclave is raised to 1650 psi by the introduction 
of nitrogen. The resulting solution is spurted from the autoclave into the 
atmosphere through an orifice having a diameter of one millimeter and a 
length of one millimeter, resulting in evaporation of the pentane solvent 
and formation of the desired fiber product. This fiber product is then 
disc refined for six minutes in a Sprout Waldron disc refiner at 1.5% 
consistency in water. 
EXAMPLE B 
This example illustrates the preparation of a cationic, water-soluble 
wet-strength resin from diethylenetriamine, adipic acid and 
epichlorohydrin. 0.97 Mole diethylenetriamine is added to a reaction 
vessel equipped with a mechanical stirrer, a thermometer and a reflux 
condenser. One mole of adipic acid is then gradually added to the reaction 
vessel with stirring. After the acid dissolves in the amine, the reaction 
mixture is heated to 170.degree.-175.degree. C. and held at that 
temperature for about one and one-half hours until the reaction mixture 
becomes very viscous. The reaction mixture is then cooled to 140.degree. 
C., and sufficient water is added to produce a polyamide solution with a 
solids content of about 50%. A sample of the polyamide isolated from this 
solution has a reduced specific viscosity of 0.155 deciliters per gram 
when measured at a concentration of two percent in a one molar aqueous 
solution of ammonium chloride. The polyamide solution is diluted to 13.5% 
solids and heated to 40.degree. C., and epichlorohydrin is slowly added in 
an amount corresponding to 1.32 moles per mole of secondary amine in the 
polyamide. The reaction mixture is then heated at a temperature between 
70.degree. and 75.degree. C. until it attains a Gardner Holdt viscosity of 
E-F. Sufficient water is added to provide a solids content of about 12.5%, 
and the solution is cooled to 25.degree. C. The pH of the solution is then 
adjusted to 4.7 with concentrated sulfuric acid. The final product 
contains 12.5% solids and has a Gardner Holdt viscosity of B-C. 
EXAMPLES 1-3 
Polyethylene pulp having carboxylic functionality is prepared as described 
in Example A. Varying amounts of the polyethylene pulp, as shown in Table 
1, are blended with bleached softwood kraft by dispersing both in water at 
2% consistency. 0.25% by weight, based on the weight of the wood pulp on a 
dry basis, of alkylketene dimer sizing agent and 0.25% by weight, based on 
the weight of the wood pulp on a dry basis, of cationic wet-strength resin 
prepared as described in Example B are added to the pulp. The pulp is then 
formed into sheets, dewatered and dried on conventional papermaking 
equipment. 10% total solids enzyme-converted pearl starch is added at the 
size press with an add on of approximately 6 lbs./ream of paper. A 
solution of 2% unconverted pearl starch containing colloidal silica is 
applied to the surface of the sheet at the wet stack. 
Paperboard containing varying amounts of polyethylene pulp, prepared as 
described above, and a control containing no polyolefin pulp are subjected 
to tensile energy absorption (TEA) and Mullen plybond (burst) tests under 
ambient conditions. The results are given in Table 1. The data are an 
average of five determinations for weight, ten for caliper, five for TEA 
and four for Mullen plybond. 
TABLE 1 
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% Poly- Caliper TEA Mullen 
ethylene Weight 1/1000 ft. lbs./ft.sup.2 
Plybond 
Example 
Pulp lbs./ream 
inch MD CD psi 
______________________________________ 
Control 
0 210.2 16.2 12.4 22.2 123 
1 5.0 212.5 16.3 13.7 26.0 141 
2 6.0 211.8 16.5 16.3 27.6 153 
3 7.5 206.3 16.0 12.2 25.9 164 
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Samples of the same paperboard sheets are brought to an equilibrium 
moisture content at 25% relative humidity (RH) and are then conditioned 
and tested at 45% and 93% RH for STFI short span compression in both the 
machine direction (MD) and the cross direction (CD), and for Z tensile. 
The results are given in Table 2. The data are an average of two 
determinations for moisture content, five for Z tensile and twenty for 
STFI compression. 
TABLE 2 
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% 
Poly- Moisture MD STFI CD STFI 
Ex- ethyl- Content Z Short Span 
Short Span 
am- ene of Tensile, 
Compression, 
Compression, 
ple Pulp Board % psi lbs./in. lbs./in. 
______________________________________ 
45% Humidity 
Con- 0 5.56 56 .+-. 0.9 
59.6 .+-. 2.7 
40.0 .+-. 2.6 
trol 
1 5.0 4.56 64 .+-. 1.5 
62.9 .+-. 2.8 
40.7 .+-. 2.4 
2 6.0 5.29 69 .+-. 1.9 
62.8 .+-. 3.2 
41.4 .+-. 1.9 
3 7.5 5.14 77 .+-. 1.8 
62.6 .+-. 2.7 
40.5 .+-. 1.6 
93% Humidity 
Con- 0 13.6 38 .+-. 1.9 
22.6 .+-. 1.6 
16.2 .+-. 1.2 
trol 
1 5.0 12.4 43 .+-. 1.5 
27.3 .+-. 2.2 
19.5 .+-. 1.7 
2 6.0 12.5 47 .+-. 1.5 
27.0 .+-. 1.4 
20.1 .+-. 1.5 
3 7.5 12.8 53 .+-. 1.3 
26.8 .+-. 1.7 
19.7 .+-. 1.6 
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EXAMPLE 4 
Polyethylene pulp having carboxylic functionality is prepared as described 
in Example A. 3% Polyethylene pulp, based on the weight of the wood pulp 
on a dry basis, is blended with Manitoba unbleached kraft by dispersing 
both in water. The pulp is then refined at 1.8% consistency in a double 
disc refiner to a Canadian Standard Freeness (CSF) of 503 at pH 7.1. 
0.175%, based on the weight of the wood pulp on a dry basis, of alkylketene 
dimer sizing agent, and 0.25%, based on the weight of the wood pulp on a 
dry basis, of quaternary amine -modified cationic potato starch (A. E. 
Staley Co.) are added to the pulp. 
A control comprising Manitoba unbleached kraft and the same amounts of 
alkylketene dimer and cationic potato starch is prepared in the manner 
described above except that the pulp is refined to 505 CSF at pH 6.9. 
Both the control and the polyolefin containing pulp are then formed into 
sheets, dewatered and dried at 149.degree. C. on conventional papermaking 
equipment. The paperboard sheets are subjected to tensile energy 
absorption (TEA), dry tensile, wet tensile and percent stretch tests under 
ambient conditions. The results are given in Table 3. The data are an 
average of five determinations for TEA, five for dry tensile, five for wet 
tensile, five for percent stretch and ten for the base weight. 
TABLE 3 
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Dry Wet 
Weight Tensile Tensile % TEA 
lbs./ lbs./in. lbs./in. Stretch ft. lbs./ft..sup.2 
ream MD CD MD CD MD CD MD CD 
______________________________________ 
Con- 165.1 76.0 48.8 4.6 3.5 1.1 3.2 5.3 12.5 
trol 
Poly- 168.5 83.1 54.2 6.1 4.5 1.4 3.4 7.2 15.4 
ethyl- 
ene 
Pulp 
______________________________________ 
Samples of the same paperboard sheets are brought to an equilibrium 
moisture content at 25% relative humidity (RH) and are then conditioned 
and tested at 45% and 95% RH for STFI short span compression in the cross 
direction and for Z tensile. The results are given to Table 4. The data 
are an average of twenty determinations for STFI compression and five for 
Z tensile. 
TABLE 4 
______________________________________ 
CD STFI 
Z Tensile Short Span Compression, 
psi lbs./in. 
45% RH 95% RH 45% RH 95% RH 
______________________________________ 
Control 59.8 35.0 25.1 12.0 
Polyethylene 
65.0 37.6 28.0 13.1 
Pulp 
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