[0001] This invention is concerned with binder systems for use in the manufacture of high
strength composites and the composites produced thereby, especially paper, paperboard,
hardboard and insulation board.
[0002] The use of a latex in the manufacture of paper by wet-end addition, or as a beater
additive, is well known. Commonly, the latex has been an anionic latex but a water-soluble
cationic deposition aid has been used therewith. Because of the slightly anionic nature
of pulp, it has been suggested that a low-charge density cationic latex should be
used in order to get good deposition on the fibers without the use of a deposition
aid. Combination of anionic and cationic wet-end additives in which both species are
water soluble are known. However, the combination as wet-end additives of a cationic
latex with a water soluble anionic polymer, particularly latexes having particles
with a high density of pH independent bound charge at or near the particle surface
has not been disclosed or suggested in the prior art.
[0003] This invention provides a method for preparing high strength composites by mixing
an aqueous slurry of negatively charged, water-insoluble natural or synthetic fibers
with a latex Containing a cationic water-insoluble copolymer and a co-additive characterized
in that the copolymer is present as particles having pH independent bound charges
at or near the particle surface in an amount from 0.07 to 0.6 milliequivalent per
gram of copolymer and the co-additive is a water-soluble anionic polymer having a
degree of polymerization above 3000, having an available charge of from 0.3 to 8 milliequivalents
per gram of polymer, having an acyclic carbon-carbon chain backbone and having the
capability of retaining its solubility in the presence of polyvalent metal ions at
a pH from 4 to 7 to form an aqueous suspension of components; removing water from
the aqueous suspension to form a wet mat; and drying the mat by heating; the copolymer
particles of the latex being deformable at the temperature of the process.
[0004] The latex is added in an amount greater than that required to cause charge reversal
on the fiber but less than the amount which would exceed the capacity of the fiber
to hold a wet mat together during processing. That amount is usually from 0.5 to 2000
percent, preferably from 10 to 100 percent, solids basis calculated on the dry weight
of the fiber.
[0005] The cationic latex particles preferably have a charge density of from 0.1 to 0.6
milliequivalent, and most preferably from 0.15 to 0.5 milliequivalent, per gram of
copolymer. Latexes with bound charge densities less than 0.1 meq/g tend to be insufficiently
stable for some applications.
[0006] The amount of co-additive is an amount greater than that required to cause essentially
complete retention of the latex on the fiber but less than the amount which would
be effective to cause substantial redispersion of components of the aqueous suspension.
The optimum amount provides good strength with very little redispersion. That amount
is usually from 0.05 to 160 percent by weight based on the dry weight of the fiber.
[0007] The fiber is any kind of negatively charged, water insoluble, natural or synthetic
fiber or blend of fibers which can be dispersed in aqueous slurry and includes crude,
low quality "screenings," i.e., coarse by-product pulp from unbleached chemical pulp.
Either long or short fibers, or mixtures thereof are useful. Suitable also are glass
fibers, reclaimed waste papers, cellulose from cotton and linen rags, straws and similar
materials. Particularly useful fibers are the cellulosic and lignocellulosic fibers
commonly known as wood pulp of the various kinds such as mechanical pulp, steam- cheated
mechanical pulp, chemimechanical pulp, semi- chemical pulp and chemical pulp. Specific
examples are groundwood pulp, unbleached sulfite pulp, bleached sulfite pulp, unbleached
sulfate pulp and bleached sulfate pulp. When employing these paper-making pulps, latex
in an amount of 5-2000 percent solids basis, calculated on the dry fiber weight, and
co-additive in an amount of 0.15 to 160 weight percent based on dry fiber weight,
one obtains fibrous webs having good formation.
[0008] The invention also provides a fibrous web comprising a dried composite containing
(a) a paper-making grade of fiber having an anionic charge, (b) from 5 to 2000 percent,
solids basis calculated on the weight of the fiber, of a structured-particle latex
having a non-ionic copolymer core; the non-ionic core being encapsulated by a thin
layer of a water-insoluble organic copolymer having bound charges of pH independent
cationic groups; the latex having from 0.15 to 0.6 milliequivalent of bound charge
per gram of polymer in the latex and (c) from 0.15 to 160 percent, based on the weight
of the fiber, of a co-additive which is a water-soluble anionic polymer of an acrylamide
having a degree of polymerization of from 3000 to 10,000 and having an available charge
of from 0.3 to 8 milliequivalents per gram of co-additive wherein the co-additive
retains its water solubility in the presence of metal ions at a pH from 4 to 7; all
percentages being by weight.
[0009] For operability in the process, the latex component is rather insensitive to the
copolymer composition thereof provided that the glass transition temperature (Tg)
of the copolymer is less than the temperature which will be used in the processing
steps. Preferably the Tg will be room temperature or lower, even as low as -80°C,
although polymers having Tg values up to about 100°C may be used. However, when the
latex loading exceeds about 100 percent, based on the fiber, the wet mats are easier
to handle when a hard latex copolymer is used, e.g., a copolymer having a Tg value
greater than 0°C. Nevertheless, the copolymer in the latex must be deformable at the
temperature to be used in the process.
[0010] The latexes are represented by but not restricted to structured particle latexes
having a non-ionic polymer core, such as, for example, a copolymer of a monovinylidene
aromatic monomer, an aliphatic conjugated diene and optionally other non-ionic monomers,
encapsulated by a thin layer of a water-insoluble organic copolymer having bound charges
as pH independent cationic groups at or near the particle surface.
[0011] One method of obtaining such latexes is by copolymerizing under emulsion polymerization
conditions an ethylenically unsaturated, activated-halogen monomer onto the particle
surface of a non-ionic, organic polymer which is slightly cationic through the presence
of adsorbed cationic surfactant. The resulting latex is reacted with a non-ionic nucleophile
to form a latex suitable for use in the practice of this invention.
[0012] Ordinarily, the particle size will range from 500 to 5000 Angstroms, preferably from
800 to 3000 Angstroms.
[0013] By "bound" as applied to groups or charges is meant that they are not desorbable
under the conditions of processing. A convenient test is by dialysis against deionized
water.
[0014] By the term "pH independent groups" as applied to ionic groups is meant that the
groups are predominantly in ionized form over a wide range in pH, e.g., 2-12. Representative
of such groups are sulfonium, sulfoxonium, isothiouronium, pyridinium and quaternary
ammonium groups.
[0015] By "available charge" is meant the amount of charge an ionizable group would provide
to a polymer when completely ionized.
[0016] By the term "non-ionic" as applied to the monomers in this specification is meant
that the monomers are not ionic per se nor do not become ionic by a simple change
in pH. For illustration, while a monomer containing an amine group is non-ionic at
high pH, the addition of a water--soluble acid reduces the pH and forms a water-soluble
salt; hence, such a monomer is not included. The non--ionic nucleophiles, however,
are not similarly restricted, i.e., "non-ionic" as used with nucleophiles applies
to such compounds which are non-ionic under conditions of use and tertiary amines,
for example, are included.
[0017] The co-additive utilized in this invention preferably has a degree of polymerization
above 5000, and an available charge of from 0.7 to 4.5 milliequivalents per gram of
polymer. Such water-soluble polymers may be natural or synthetic. The upper limit
of the degree of polymerization is not critical provided that the co-additive has
the requisite solubility. In some cases, such as a partially cross-linked polymer,
the DP value is indeterminate. Representative examples of the co-additive are polymers
such as water-soluble high molecular weight acrylamide polymers having pendant anionic
groups represented by carboxyl, sulfate, sulfonate and the like, sodium polystyrene
sulfonate, partially hydrolyzed copolymers of vinyl acetate and acrylic acid, sulfated
polyvinyl alcohols, polyvinyl acetate polymers having pendant anionic groups represented
by carboxyl, sulfate and sulfonate and copolymers of hydroxyethyl acrylate and sulfoethyl
methacrylate. Various known methods can be used to obtain these anionic acrylamide
polymers. For example, polyacrylamide can be hydrolyzed to various levels. Other methods
include direct copolymerization of substituted acrylamide monomers such as 2-acrylamido-2--methylpropane
sulfonic acid with other hydrophilic monomers such as the α,8-ethylenically unsaturated
carboxylic acids represented by acrylic acid, methacrylic acid, fumaric acid, maleic
acid and itaconic acid.
[0018] The preferred polymer co-additive has a molecular weight high enough to flocculate
the fines but low enough to avoid poor formation. If the degree of polymerization
is less than 3000, flocculation is inadequate and drainage and retention are poor.
If the degree of polymerization is over 10,000, the flocculation is excellent but
paper formation is unsatisfactory for some grades of paper.
[0019] The optimum charge on the co-additive depends somewhat on the hardness of the water
used, i.e., the concentration of multivalent cations such as Ca
++ in the water. Generally polymers of low available charge content, such as less than
about one milliequivalent of available charge per gram (meq/g) of polymer, work best
in hard water. However, in soft water, better results are obtained when the anionic
co-additive has greater than one milliequivalent of available charge per gram of polymer.
[0020] The non-ionic copolymer core of the latexes operable in the practice of this invention
for making papers having good formation preferably contains from 20 to 50 percent
of an aliphatic, conjugated diene (preferably 1,3-butadiene), from 20 to 80 percent
of a monovinylidene aromatic compound (preferably styrene), from 0 to 5 percent of
polar, non-ionic ethylenically unsaturated monomers and from 0 to 25 percent of other
ethylenically unsaturated non-ionic monomers which when in the form of homopolymers
are water insoluble.
[0021] The monovinylidene aromatic compounds are represented by styrene, substituted styrenes
(e.g., styrene having halogen, alkoxy, cyano or alkyl substituents), vinyl naphthalene
and the like.
[0022] Specific examples are styrene, a-methylstyrene, ar-methylstyrene, ar-ethylstyrene,
a-ar-dimethylstyrene, ar,ar-dimethylstyrene, ar-t-butylstyrene, methoxystyrene, cyanostyrene,
acetylstyrene, monochlorostyrene, dichlorostyrenes, other halostyrenes and vinylnaphthalene.
[0023] By the term "monovinylidene aromatic" monomer or compound is meant that to an aromatic
ring in each molecule of the monomer or compound is attached one radical of the formula,
wherein R is hydrogen or a lower alkyl such as an alkyl having from 1 to 4 carbon
atoms.
[0024] The aliphatic conjugated dienes operable in the practice of this invention include
butadiene and substituted butadiene and other acyclic compounds having at least two
sites of ethylenic unsaturation separated from each other by a single carbon-to-carbon
bond. Specific examples are isoprene, chloroprene, 2,3-dimethylbutadiene-l,3, methylpentadiene,
and especially 1,3-butadiene (often abbreviated butadiene).
[0025] The polar, non-ionic, ethylenically unsaturated monomers are represented by the acrylamides
such as acrylamide and methacrylamide; the hydroxyl--containing esters of α,β-ethylenically
unsaturated, aliphatic monocarboxylic acids such as B-hydroxyethyl acrylate, β-hydroxyethyl
methacrylate, B-hydroxypropyl methacrylate, 4-hydroxybutyl acrylate and 5-hydroxypentyl
methacrylate.
[0026] The other ethylenically unsaturated non-ionic monomers which when in the form of
homopolymers are water-insoluble are represented by the lower alkyl acrylate and methacrylate
esters such as methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate,
isopropyl methacrylate; and the unsaturated nitriles such as acrylonitrile and methacrylonitrile.
[0027] In one method for the preparation of latexes usable in the practice of this invention,
a known starting latex of a non-ionic polymer of a monovinylidene aromatic compound
and an aliphatic conjugated diene is encapsulated with a thin layer of a copolymer
of an ethylenically unsaturated activated-halogen monomer either by adding the activated-halogen
monomer or a mixture of such monomers to the reaction mixture of the starting latex
before all of the monomers are converted to polymer or by adding the activated-halogen
monomer together with one or more hydrophobic monomers to a starting latex containing
essentially no residual monomers, and initiating and continuing polymerization of
the thus-added monomers to substantially complete conversion. The resulting latex,
having a particle size (diameter) of from 800 Angstroms to 3,000 Angstroms consists
of the starting latex particle now encapsulated with a bound layer having a thickness
of from a monomolecular layer of the copolymer to about 100 Angstroms. The latex according
to the foregoing description can then be reacted with a low molecular weight, non-ionic,
water-stable, nucleophilic compound which can diffuse through an aqueous phase, to
form particles of polymer having pH independent cationic groups, i.e., onium ions,
chemically attached at or near the particle surface.
[0028] Representative specific nucleophilic compounds are pyridine, quinoline, isoquinoline,
tetramethyl thiourea, tetraethyl thiourea, hydroxyethylmethyl sulfide, hydroxyethylethyl
sulfide, dimethyl sulfide, diethyl sulfide, di-n-propyl sulfide, methyl--n-propyl
sulfide, methylbutyl sulfide, dibutyl sulfide, trimethylene sulfide, thiacyclohexane,
tetrahydrothiophene, N-methylpiperidine, N-ethylpyrrolidine, N-hydroxyethylpyrrolidine,
trimethylphosphine, triethylphosphine, tri-n-butylphosphine, trimethylamine, triethylamine,
tri-n-propylamine, tri-isobutylamine, hydroxyethyldimethylamine, butyldimethylamine,
tri--hydroxyethylamine, triphenylphosphorus, and N,N,N--dimethylphenethylamine.
[0029] In carrying out the reaction between the nucleophilic compound and the particles
of latex having activated halogens chemically bound to the surface thereof, the latex
is stirred gently while the nucleophilic compound is added thereto, as the compound
per se or in the form of a solution. Gentle stirring may continue throughout the ensuing
reaction, or optionally after dispersion of the compound in the latex, the stirring
may be discontinued. The reaction is conveniently carried out at ambient temperature
although temperatures from 0°C to 80°C can be used. The reaction occurs spontaneously
at a rate which depends upon the reactivity of the activated halogen and of the nucleophile.
It is preferred to carry out the reaction until a predominant proportion of the colloidal
stability of the product is provided by the resulting chemically bound cationic groups.
Usually a catalyst is not required although with the less reactive materials, a small
amount of iodide ion may be used to facilitate the reaction. When a desired degree
of reaction is reached, any excess nucleophile commonly is removed by standard methods,
e.g., dialysis, vacuum stripping and steam distillation.
[0030] Other pH independent cationic groups can be substituted for cationic groups which
are chemically bound to the latex particles according to the foregoing description
by carrying out a further reaction with such cationic groups. For example, a cationic
structured-particle latex having sulfonium groups chemically bound to the structured-particles
at or near the particle surface can be reacted with hydrogen peroxide at a temperature
of from 20°C to 80°C, preferably at ambient temperature, for a sufficient time to
oxidize part or all of the sulfonium groups to sulfoxonium groups. Such treatment
also reduces the odor of the latex. For best results in such oxidation reaction, the
hydrogen peroxide is used in excess, e.g., from 2 to 10 moles of hydrogen peroxide
for each mole of sulfonium groups.
[0031] The latexes can contain usual additives such as antifoamers, coalescing solvents,
pigments, and pH adjusting agents. It is preferred that the latexes are soap-free
but quantities up to about 0.1 milliequivalent per gram can be tolerated.
[0032] The process to prepare the product of this invention preferably is carried out as
follows: A dilute aqueous suspension of the fiber is formed in the normal manner often
in a concentration of from 0.5 to 6 percent. The latex is added at any convenient
concentration, often in the concentration as supplied and the resulting mixture is
stirred, usually for at least two minutes depending somewhat on the equipment available.
The aqueous suspension usually is then diluted further, often with white water from
the process. The co-additive is added as an aqueous solution at a concentration usually
less than 1 percent solids and the mixture is stirred generally for near the minimum
time to obtain thorough mixing. While the co-additive is usually the last component
added at the wet-end of the process, it may be added at any time. Optional wet-end
additives can be added at a suitable time.
[0033] Other optional constituents of the composite--forming composition at the wet-end
in the present process include pigments, fillers, curing agents, waxes, oils and other
common additives well known in the paper-making art.
[0034] A composite is formed by flowing the resulting suspension over a porous support such
as a screen to form a wet mat, dewatering the wet mat and completing drying by heating.
The dewatering step includes draining and may include wet pressing. Pressing and heating
may be carried out simultaneously to form a composite. Alternatively, ambient temperature
pressing followed by heating to complete drying may be employed. Optionally, other
compacting, shaping, tempering and curing steps may be included. The temperatures
used for hot pressing, curing and tempering or other heating steps often are from
100°C to 250°C, although higher or lower temperatures are operable.
[0035] For making many of the composites, paper machines such as a Fourdrinier machine,
a cylinder machine or a laboratory sheet-forming apparatus are useful.
[0036] The product of the process of this invention has improved internal bond strength
compared to prior art methods. Within the range of permissible variables in carrying
out the invention at low levels of latex, the properties are more sensitive to degree
of bonding of polymer particles to fiber than to properties of the polymer composition.
At high levels of latex, if sufficient heat is applied to fuse the latex particles,
the properties of the resulting product depend markedly on the properties of the polymer
phase.
[0037] The process of the invention also is advantageous compared to prior art processes
in that there is better retention; i.e., more of the suspended solids are removed
from the aqueous suspension. During carrying out of the process, the shear stability
of the system allows mechanical working without redispersion of solids. Hence, the
effluent from the process is lower in solids which allows the use of a higher level
of recycle and minimizes discharge of pollutants to the environment.
[0038] The "formation" of a sheet of paper refers to the uniformity of distribution of fibers
in the sheet. Poor formation occurs when the fibers flocculate or clump together causing
alternating heavy and light spots in the sheet. Besides diminishing the aesthetic
appeal of the paper, poor formation tends to decrease in-plane strength properties
such as tensile strength. Poor formation causes uneven surfaces which contribute to
poor printability.
[0039] In this specification and claims, all references to degree of polymerization (DP)
are weight average unless otherwise indicated.
[0040] The following examples illustrate ways in which the present invention may be carried
out, but should not be construed as limiting the invention. All parts and percentages
are by weight unless otherwise expressly indicated.
[0041] Many of the latexes used in the examples are identified in Table I. The monomers
shown for the base latex were polymerized under emulsion polymerization conditions
using dodecylbenzyldimethylsulfonium chloride as emulsifier in amounts varying from
1.8 to 2.5 percent, 0.2 percent of dodecanethiol as chain transfer agent and 0.5 percent
of a
la
l-azobisisobutyronitrile as catalyst, the percentages being based on the total weight
of monomers. The base latex particles were then capped (encapsulated) by adding the
cap monomers of the kind and in the amount shown in Table I in a continuous manner
over a period of about one hour for each 100 grams of the total monomeric components.
Additional catalyst of the same kind, stirring and elevated temperature, usually 70°C,
were used for the capping reaction. The resulting latex products were then reacted
at a temperature of 70°C with an excess of a nucleophile which was 2-(dimethylamino)ethanol
(for the quaternary ammonium bound charge) and the excess nucleophile was removed
by steam distillation after the desired amount of bound charge was reached. For the
sulfonium bound charge, the nucleophile was dimethyl sulfide, the reaction temperature
was 50°C and excess nucleophile was removed by vacuum distillation. The structured
particle latexes thus produced had the properties shown in Table I.
[0042] Tests referred to in the examples were carried out as follows:
Tensile:
[0043] Tensile values are recorded as breaking length, in meters, and are determined according
to TAPPI Standard T 494-os-70 except the values are the average of 3 samples rather
than 10 and the jaw gap is 5.08 cm rather than 20.32 cm.
Canadian Standard Freeness (CSF):
[0044] The values are determined according to TAPPI Standard T 227-M-58 except where variations
in the procedure are indicated.
Formation
[0045] A common way to measure formation is to compare visually the sheet to be measured
with a set of ten standard sheets specifically made with decreasing levels of uniformity
of fiber distribution (formation) and ranked from 1 to 10 with 1 being the best and
10 the worst. Alternatively, optical instruments, which are available commercially
can be used for measuring formation.
Delamination Resistance:
[0046] The internal bond strength of the products is measured by the delamination resistance
test. In this test, a strip one inch in width of the product to be tested is placed
between two strips of adhesive tape having sufficient adhesiveness that failure will
occur in the paper when the two pieces of tape are pulled apart. Delamination is started
by hand, then continued and measured by an Instron Tensile Tester using a jaw separation
rate of 30.48 cm per minute. The average force resisting delamination over a length
of about 10.16 cm is determined for each of two samples. The average of the two samples
is recorded in ounces per inch of width, abbreviated oz/in. Those values are followed
in parenthesis by conversion to metric units, i.e., grams per 2.54 centimeters (g/2.54
cm). When a different testing tape is used, the new tape is calibrated according to
the initial tape and values are reported in values according to the initial tape.
Example 1
[0047] A steam-heated, mechanically-defibered pulp having a Canadian Standard Freeness (CSF)
of 785 milliliters and a solids content of 24% was diluted to 1% solids with water
having a hardness of 10.6 (as CaCO
3, ppm) and an alkalinity of 48 (as CaC0
3, ppm). The components shown in Table II were added to the resulting fiber suspension
in the order shown and stirring was continued for the time indicated before the next
step. About 3% of Latex A would be required to reach the charge reversal point of
the pulp.
[0048] The co-additive is a hydrolyzed polyacrylamide having a degree of polymerization
of 25,000 and an available charge of 1.94 milliequivalents per gram of co-additive
(meq/g).
[0049] A sheet was formed on filter paper (12.5 cm in diameter) by filtering the resulting
suspension through a Buchner funnel with vacuum from a water aspirator. The resulting
wet sheet was removed from the funnel, placed between two clean filter papers and
4 blotters and pressed on a Williams press at 1715 psig (120 kg/sq cm). After the
blotters and filter papers were removed, the resulting damp sheet was dried on a hot
plate at l65°C for 5 minutes. The dried sheet was conditioned by being kept in a room
maintained at 50% humidity and 23°C for at least 2 hours, generally overnight, before
testing.
[0050] The effluent from the filtration was analyzed for turbidity with a spectrophotometer
as a measurement of the white water clarity. The wavelength of light used was 425
nm and a cuette diameter of 19 mm. Tensile values for the composite, drainage time,
and clarity (percent transmission) of the effluent from the filtration are shown in
Table III.
Comparative Example 1C
[0051] A sheet was prepared in the same manner and with the same components except the latex
and the co-additive were omitted. The drainage time, clarity and tensile results are
shown in Table III.
[0052] Example 1 illustrates the improvement in drainage time and clarity of the effluent
(waste water) from the process and improvement in strength of the product provided
by the invention.
Examples 2-10
[0053] For each example, an aqueous dispersion containing 1393 parts of water having a hardness
of 106 ppm (calculated as calcium carbonate) and an alkalinity of 48 ppm (calculated
as calcium carbonate) and 7 parts (dry basis) of unbleached Canadian softwood kraft
having a Canadian Standard Freeness (CSF) of 400 ml was stirred at such a rate that
the kraft was just turning over gently. To the moving kraft suspension was added 1.4
parts (dry weight basis) of Latex A. After stirring at the same rate for an additional
two minutes, a dilute aqueous solution (0.2% solids) of the specified co-additive
was added in the amount shown and stirring was continued for an additional 30 seconds.
The resulting furnish (pH 7-8) was made into a handsheet 30.48 x 30.48 cm on an M/K
systems "Mini-Mill" handsheet machine using water for dilution of the description
given above. The handsheet was pressed to a solids content of from 37 to 38% by placing
the sheet and couching blotter between two pieces of wool felt and running the resulting
sandwich through the press at medium speed using a press pressure of 80 psig (5.6
kg/sq cm). The pressed sheet was removed from the wool felts, and stripped from the
couching blotter, then dried in a drier maintained at 220°F (104°C). The product was
just cockle free, and contained about 95 percent solids.
[0054] The co-additive used in the examples is a hydrolyzed polyacrylamide having 1.94 milliequivalent
of available charge (carboxyl) per gram and a degree of polymerization of 25000. Data
are shown in Table IV.
Comparative Examples 2Cl-2C5, 5C and 8C
[0055] Comparative Examples 2C2 and 2C4 were prepared in the same manner from the same components
as described for Examples 2-10. The amounts of the latex were below that required
(5.5%) to cause charge reversal on the fiber used.
[0056] Comparative Examples 2Cl, 2C3, 2C5, 5C and 8C were prepared in the same manner except
no co-additive was included. Data for the comparative examples are included in Table
IV.
Example 11
[0057] A sheet was prepared using the materials in the same amounts and by the process described
in Examples 2-10 except the latex was Latex B (see Table I), the amount of the same
co-additive was 0.6%, dry basis calculated on the weight of the fiber. The sheet was
tested for delamination resistance (see Table V).
Comparative Example 11C
[0058] A sheet was prepared in the same manner with the same materials in the same amounts
as in Example 11 except Latex X was used instead of Latex B. Latex B and Latex X have
the same average polymer composition but Latex B is a structured particle latex whereas
Latex X has rather uniform composition throughout the particle as described above.
For comparison with Example 11, data are shown in Table V.
[0059] From these delamination resistance values shown in Table V, it is seen that Example
11 (an example of the invention) provides considerable improvement over a process
using a latex having the bound charges throughout the particle rather than only near
the particle surface even though the total charge was greater for Comparative Example
11C.
Examples 12-14
[0060] Sheets were prepared in the same manner as described for Example 11 except that for
Latex B there were substituted Latex C, Latex D and Latex E, respectively. The latter
three latexes differ from each other in the amount of nucleophile (dimethylaminoethanol)
reacted with the capped latex and thus differ in the amount of bound charge. Data
are shown in Table VI.
[0061] Examples 12-14 show that the internal bond strength of the product increases as the
bound charge on the latex increases.
Examples 15-22
[0062] Sheets were prepared as described in Example 11 except that different latexes were
used but in the same proportions. The major differences among the latexes relate to
the composition in the base latex from which the structured particle latex was made
(see Table I). Data are shown in Table VII.
Comparative Example 22C
[0063] A sheet was prepared in the same manner as for Examples 15-22 except the latex and
the co-additives were not included. Results are shown in Table VII.
[0064] Examples 15-18 illustrate the process of the invention where the sulfonium group
provides the bound charge rather than the quaternary ammonium group of Examples 1-14
and shows operability of the process over a broad composition of latex polymer (broad
range of Tg values) at approximately equal bound charge. Within this series, there
is little variation in internal bond strength in the products as shown by the delamination
test.
[0065] Examples 19-22 illustrate a higher level of bound charge in the latex and variations
in the base latex (core composition) to provide increasing Tg values. Within the series,
as the polymers become harder (higher Tg values), the measured delamination resistance
of the product sheets decreased under the conditions of these experiments. However,
when the heating time was increased to 12 minutes rather than one minute, the delamination
resistance was 22.4 oz/in (635 g/2.54 cm); 18.5 oz/in (524 g/2.54 cm) and 14.8 oz/in
(420 g/2.54 cm), for Examples 20, 21 and 22, respectively. These examples support
the position that at the higher Tg values in the disclosed range for the latex used,
higher temperatures and/or longer times should be selected or conversely at a given
time/temperature condition in the process a latex with sufficiently low Tg value should
be used.
Examples 23-28
[0066] Sheets were prepared as described in Example 11 except a different latex was used
in all these examples and the identity of the co-additive was changed but not the
amount in all these examples except No. 25. Data are shown in Table VIII.
Example 29
[0067] An aqueous dispersion containing 1990 parts of water having a hardness of 106 ppm
(calculated as calcium carbonate) and an alkalinity of 48 ppm (calculated as calcium
carbonate) and 10 parts (dry basis) of unbleached Canadian softwood kraft having a
Canadian Standard Freeness (CSF) of 400 ml was stirred at such a rate that the kraft
was just turning over gently. To the moving kraft suspension was added 90 parts (dry
weight basis) of Latex I. After stirring at the same rate for an additional two minutes,
3.3 parts of the specified co-additive as a dilute aqueous solution (0.2% solids)
was added and stirring was continued for an additional 30 seconds. The resulting furnish
was made into a handsheet (30.48 x 30.48 cm) on an M/K Systems "Mini-Mill" handsheet
machine using water for dilution of the description given above. The resulting wet
sheet was placed between two pieces of clean filter paper and 4 blotters and pressed
on a Williams press at 136 psig (9.52 kg/sq cm). After the blotters and filter papers
were removed, the resulting damp sheet was dried on a hot plate at 165°C for 5 minutes.
[0068] The resulting dry sheet was so strong that the delamination resistance could not
be determined by the method disclosed in this specification.
[0069] The co-additive used in the example was a hydrolyzed polyacrylamide having 1.94 milliequivalents
of anionic group (carboxyl) per gram and a degree of polymerization of 5500.
Example 30
[0070] A sheet was prepared using the same materials in the same amounts and by the process
described in Examples 2-10 except the amount of the same co-additive was 0.1% and
the latex was a latex prepared according to United States Patent No. 3,873,488 in
a batch process from 65 parts of styrene, 30 parts of butadiene, 5 parts of acrylonitrile
and 4 parts of vinylbenzylmethyl- dodecylsulfonium chloride with no added nonpolymerizable
surface active material. The latex had a solids content of 23.9%, a particle size
of 1090 angstroms (determined by light scattering) and 0.072 milliequivalent of bound
charge per gram of polymer. The estimated Tg was 25°C. The delamination resistance
of the sheet was 19.3 oz/in (547 g/2.54 cm).
[0071] This example illustrates the practice of the invention using a latex having a level
of bound charge near the minimum suitable for this invention and also illustrates
the use of a latex which is not prepared according to the method for making structured
particle latexes.
Examples 31 and 32
[0072] Sheets were prepared using the same materials in the same amounts and by the process
described for Examples 2-10 except the latex was as described below, a different co-additive
was used in the amount shown in Table IX and deionized water was substituted for the
water.
[0073] The latex as described, at 23% solids and having a particle size of 880 angstroms,
was a structured particle latex having 100 parts of a core copolymer of 40% of styrene,
and 60% of butyl acrylate (core Tg = -10°C) capped by 10 parts of a copolymer of 33%
of vinylbenzyl chloride and 67% of butyl acrylate which was subsequently reacted with
trimethylamine to provide 0.121 milliequivalent of bound charge per gram of latex,
solids basis
[0074] The co-additive used in these examples was a hydrolyzed polyacrylamide having a degree
of polymerization of 20,800 and an available charge of 3.48 milliequivalent (from
carboxyl groups) per gram. Data are shown in Table IX.
Comparative Examples 3lCl and 31C2
[0075] Sheets were prepared in the same manner as for Examples 31 and 32 except that both
the latex and the co-additive were omitted in 31C1 and the co-additive was omitted
in 31C2. Data are included in Table IX.
[0076] Examples 31 and 32 illustrate the invention using a softer water, i.e., deionized
water, a different co-additive and a latex having different composition than in the
other examples of the invention.
Examples 33 and 34
[0077] An aqueous dispersion containing 1393 parts of water having a hardness of 106 ppm
(calculated as calcium carbonate) and an alkalinity of 48 ppm (calculated as calcium
carbonate) and 7 parts (dry basis) of unbleached Canadian softwood kraft having a
Canadian Standard Freeness (CSF) of 400 ml. was stirred at such a rate that the kraft
was just turning over gently. To the moving kraft suspension was added 1.4 parts (dry
weight basis) of a latex of the kind described below. After stirring at the same rate
for an additional two minutes, a dilute aqueous solution (0.2% solids) of the specified
co-additive was added in the amount shown in Table X and stirring was continued for
an additional 30 seconds. The resulting furnish was made into a handsheet (30.48 x
30.48 cm) on an M/K Systems "Mini-Mill" handsheet machine using water for dilution
of the description given above. The handsheet was pressed to a solids content of from
37 to 38% by placing the sheet and couching blotter between two pieces of wool felt
and running the resulting sandwich through the press at medium speed using a press
pressure of 80 psig (5.6 kg/sq cm). The pressed sheet was removed from the wool felts,
and stripped from the couching blotter, then dried in a drier maintained at 220°F
(l04°C). The product was just cockle free, and contained about 95 percent solids.
[0078] The latex used in the foregoing experiment was a structured-particle latex having
80 percent of
(a) a core copolymer of 35 percent of butadiene, 65 percent of styrene and 20 percent
of (b) an encapsulating layer of 35 percent of butadiene, 15 percent of styrene and
50 percent of vinylbenzyl chloride which was reacted subsequent to polymerization
with 2-(dimethylamino) ethanol to provide a bound quaternary ammonium charge of 0.365
milliequivalent per gram of polymer in the latex.
[0079] The co-additive (A) used in the example is a hydrolyzed polyacrylamide having 1.94
milliequivalent of available charge per gram and a degree of polymerization of 5500.
[0080] For comparison with the above example of the invention, a sheet was prepared in the
same manner except for co-additive (A) there was substituted co-additive (B), a hydroylzed
polyacrylamide having the same available charge but having a degree of polymerization
of 25,000, i.e., outside the range required for this invention. A further comparison
(33B) was made in the same manner and with the same components except the co-additive
was omitted. Data are shown in Table X below.
[0081] The turbidity measurements are made on the effluent from the freeness (CSF) test
and are indicative of the "white water" characteristics which are obtained in the
paper making process.
[0082] The sheets from Examples 33 and 34 were acceptable in all the properties measured.
Comparative examples 33A and 34B were deficient in formation and therefore are unacceptable.
While comparative example 33B indicated good formation, the transmission was low indicating
poor flocculation onto the fibers and additionally the delamination resistance was
low.
Examples 33-38
[0083] Sheets were prepared as described in Examples 33 and 34 except different latexes
and a different co-additive in two different amounts were used as shown in Table XI.
The latexes used (Latex P and Latex Q) were structured particle latexes containing
70% of a core copolymer consisting of 65% of styrene and 35% of butadiene modified
with 0.2% of dodecanethiol and the core was encapsulated (capped) with 30% of a copolymer
of 50% of vinylbenzylchloride, 35% of butadiene and 15% of styrene which was subsequently
reacted with an excess of dimethylsulfide. The latter reaction was stopped for Latex
P by vacuum distilling the excess dimethyl sulfide when the bound charge of sulfonium
group was 0.195 milliequivalent per gram and for Latex Q when the bound charge was
0.388 milliequivalent per gram.
[0084] The co-additive was a hydrolyzed polyacrylamide having an available charge of 3.3
milliequivalents per gram as carboxyl groups and having a degree of polymerization
of 4100. Data are shown in Table XI.
Comparative Examples 35C and 37C
[0085] Sheets were prepared as described for Examples 35 and 37 except no co-additive was
used. Data are included in Table XI.
[0086] Examples 35-38 illustrate the practice of this invention using a different kind of
cationic bound charge than in Examples 33 and 34 and also illustrate the use of widely
differing amounts of bound charge.