CA1118165A - High strength composites and a method for forming - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
High strength composites prepared by (A) mixing (1) an aqueous slurry of negatively charged, water--insoluble natural or synthetic fiber such as cellulose fiber, (2) a latex having a high density of pH independent cationic charges, such as sulfonium, bound at or near the particle surface, (3) a water-soluble, high molecular weight anionic polymer co-additive, such as a hydrolyzed polyacrylamide and optionally (4) conventional wet end additives; (B) draining water from the resulting aqueous suspension and (c) drying by heating.

18,430A-F

Description

l~lB1~5 HIGH STRENGTH CO~?OSITES AND A METHOD FOR FORMING

This inventiorl is concerned with binder systems for use in the manufacture of high strength composites and the composites produced thereby, especially paper, paperkoaxd, hardboard and in ulation board.

The use of a latex in the manufacture of paper by wet-end addition, or as a beatex 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-eIId addi-tives 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 densi~y of pH
independent bound charge at or near the particle surface has not been disclo~ed or suggested in the prior art.
This nvention provides a method for preparing high ~trengt}i compo3~. tes by mixing an aqueous slurry of negatively chargfd wa~er-insolll~le natural or synthetic fibers with a latex contc.ining a cationic watf~-in~oluble 18~430A-~
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copolymer and a co-addi-tive characterized in th~t the copolymer is present as particles having pH independent bound charges at or near the particle surface in an amount from o.a7 to 0.6 nQilli-eq~livalent per gram of copolymer and the co-additive is a water-soluble anionic polymer having a degree of poly~erization above 3000, having an available charge of from 0.3 to 8 milli-er~ivalents per gram of pol~ner, 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 fonn an aqueous suspension of components, and fur~ler characterized in that the latex is added in an amount greater than that required to cause charge reversal on the fiber but less than an amount ~Lich wculd exceed the capacity of the fiber to hold a wet mat, together during processing, and the co-additive is added in 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 aqueous suspension; rem~ving 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 pro oe ss.
m e 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 frcm 10 to 100 percent, solids basis calculated on the dry weight of the fiber.
m e cationic latex particles preferably have a charge density of from 0.1 to 0.6 milli-equivalent, and st preferably from 0.15 to 0.5 milli~
equivalent, per gram of copolymer. Latexes with bound charge densities less than 0.1 meq/g tend to be insufficiently stable for some applications.
me 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 efective to cause substantial redispersion of ~ - 2 -components of ~h~ aqueous suspension. The optimum amount provides good strength with ve.ry little redispersion.
That amount is usually from 0.05 to 16Q percent by weight based on the dry weight of the fiber.

The fiber is any kind of negatively charged, water insoluble, natural or synthetic fiber or blend of fibers which can be dispersed in aqucous slurry and includes crude, low quality "screenings," i.e., coarse by-product pulp from unbleached chemical pulp. Either long or short fihers, or mixtures thereof are useful.
Suitable zlso are glass f.ibers, 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--heated mechanical pulp, chemimechanical pulp, semi-chemical pulp and chemi.cal pulp. Specific exzmples are groundwood pulp, unbleached sulfite pulp, b].eached 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-addit.ive in an amount of 0.15 to 160 weight percent based on dry fiber weight, one obtains fibrous webs havi.ng good formation.

The invention also provides a fibrous web com-prising 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 structllred-particle latex having z 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;

18,430A-F

the latex having from 0.15 to 0.6 milliequivalent of bound char~e 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.

For operability in the process, the latex component is rather insensitive to the copoiymer compo-sition thereof provided that the glass transition temperature (Tg) of ~he copolymer is less than the temperature ~hich will be used in the processing steps. Preferably the Tg will be room temperature or lower, even as low as -80C, although polvmers having Tg values up to about 100C 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 ls used, e.g~, a copolymer having a Tg value greater than 0C. Neverthe-less, the copolymer in the latex must be deformable at the temperature to be used in the process.

The latexes are represented by but not restricted to struc~ured particle latexes having a non-ionic pclymer core, such as, for example, a copolymer of a monovinylidene aromatic mono~Rr, 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 sur ace.

18,430A-F

111&~

One me~hod of obtaining such latexes is by copolymerizing und~r 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 oE
adsorbed cationic surfactantO The resulting latex is reacted with a non-ionic nucleophile to form a latex suit-able for use in the practice of this invention.

Ordinarily, the particle size will range from 500 to 5000 Angstroms, preferably from 800 to 3000 Angstroms.

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.

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. Rep-resentative of such groups are sulfonium, sulfoxonium, isothiouronium, pyridinium and quaternary ammoniumgroups.

By "available charge" is meant the amount of charge an ion zable group would provide to a polymer when completely lonized.

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 ~)ecome ionic by a simple change in pH. For illustratiorl, w~!ile a monomer containing an amine ~roup is non-ionic at hig}l pEl, the addition of a water--soluble acid reduces ~he pH alld forms a water-soluble 18,~3G~-F

salt; hence, such a monomer is not included. The non--ionic nucleophil~5, however, are not similarly restricted, i.e., ~Inon-ionic~ as used with nucleophiles applies to such compounds which are non ionic under conditions of use and tertiary amines, for example, are included.

The co additive utilized in this invention preferably has a degree of polymerization above 5000, and an available charye of from 0.7 to 4.5 milliec~uivalents per gram of polymer. Such water-soluble polymers may be natural or synthetic. The upper limit of the degree of polymerization is not critical provlded that the co-additive has the requisite solubility. In some cases, such as a par~ially cross-linked polymer, the DP value is indeter-minate. Representative examples of the co-additive are polymers such as wa-ter-soluble high molecular weight aerylamide polymers having pendant anionic groups repre-sented ~y carbo~yl, sulfate, sulfonate and the li~e, sodium polystyrene sulfonate, partially hydrolyzed copolymers of viny] acetate and acrylic acid, sulfated polyvinyi alcohols, polyvinyl acetate polymers having pendnt anionic groups represented by carboxyl, sulfate and sulfonate and copolymers of hydrox~7ethyl acrylate and sulfoethyl methacrylate. ~arious known methods can be ~tSed 'LO obtain these anionic acrylamide polymers. For example, polyacrylamide can be hydrolyzed to various levels. Other ~thods include direct copolymerization of substituted acrylamide monom~rs such as 2-acrylamido-2--methylpropane sulfonic acid with other hydrophilic monomers such as the ~ ethylenically unsaturatea carboxylic acids represented by acrylic acid, r,lethacrylic acid, fumar c acid, maleic acid and itaconic acid.

The preferred polymsr co-additive has a moiecular weight high enough to flocculate the fines but low enough to avoid poor tormatjon. If the dec~ree of polymeri2ation 18,430A-E

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.

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.

The non-ioniC copolymer core o~ the latexes operable in the practice of this inventlon 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 insol~le.

The monovinylidene aromatic compounds are represen~ed by styrene, substituted styrencs (e.g., styrene having halogen, alkoxy, cyano or alkyl substituents), vinyl naphthalene and the like.
Specific examples are styrene, ~-methylstyrene, ar-methylstyxene, 2r-ethylstyrene, ~-ar~dimethyl~
styrene, ar,ar-dimethylstyrene, ar-t-butylstyrene, 18,430A-F

methoxystyrene, cyanostyrene, acetylstyrene, mono-chlorostyrene, dichlorostyrenes, other halostyrenes and vinylnaphthalene.

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 radlcal of the formula, R
CH2=C-wherein R is hydrogen or a lower alkyl such as an alkyl havin~ from 1 to 4 carbon atoms.

The aliphatic conjugated dienes operable in -~he 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-dimethy1butadiene-1,3, methylpentadiene, and especially l,3-butadiene (often abbreviated butadiene).

The polar, non-ionic, ethylenically un~
sat-~rated monomerC are repxesented by the acrylamides such as acrylamide and methacrylamide; the hydroxyl--containing esters of ~,~-ethylenically unsaturated, aliphatic monocar~oxylic acids such as ~-hydroxyethyl acrylate, ~ hydroxyethyl ~lethacîylate, ~-hydroxypropyl ~5 methacrylate, 4-hydroxybutyl acrylate and 5-hydroxypentyl methacrylate.

The other ethylenically unsaturated non-ionic monomers which when in the form of homopolymers are water-insol~le are represented by the lower alkyl 18,-~30A F

- 9 - ~

acrylate and me~lacrylate esters such as methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, isopropyl methacrylate; and the unsaturated nitriles such as acrylonitrile and methacrylonitrile.

In one method for the prepara-tion of latexes usable in the practice of this invention, a known starting latex of a non-ionic polymer of a monGvinylidene aromatic compound and an aliphatic conjugated diene is encapsulated with a thin layer of a copolymer of an ethylenically unsaturated activated-halogen monomer eithex by adding the activated-halogen monomer or a mixture of such monomers to the reactlon 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 late~ containing essentially no residual monomers, and initiating and continuing polymerlzation of the thus-added monomers to substan'cially ccmplete ~onversion. The resulting latex, havir.g a particle 20 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 mono-molecular 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.

Representative specific nucleophilic com-pounds are pyridine, quinoline, isoquinoline, tetra-methyl thiourea, tetraethyl thiourea, hydroxye-Lhyl-methyl sulfide, hydroxyethyletllyl sulfide, dimethyl 1~,430A-F

sulfide, dieth~yl sulfide, di-n-propyl sulfide, methyl--n-propyl sulfide, methylbutyl sulfide, dibutyl sulfide, trimethylene sulfide, thiacyclohexane, tetrahydro-thiophene, N-methylpiperidine, N-ethylpyrrolidine, N-hydroxyethylpyrrolidine, trimethylphosphine, tri-ethylphosphine, tri-n-butylphosphine, trimethylamine, triethylamine, tri-n-propylamine, tri~isobutylamine, hydroxyethyldimethylamine, butyldimethylamine, tri--hydroxyethylamine, triphenylphosphorus, and N,N,N--dimethylphenethylamine.

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 reactlon, or optionally after dispersion of the compound in the latex, the stirring ma~ be discontinued. The reaction is conveniently carried oui at an~ient temperature although temperatures from 0C to 60C 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 wltll a predomin3nt proportion of the colloidal stability or 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.
l~hen a desired degree of reaction is reached, any excess nucleophile commonly is removed by standard methods, e.g., dialysis, vacuum stripping and steam distillation~

30~-F

Other p~l independent cationic groups can be substituted for cationic groups which are chemically hound to the latex particles acccrding to the fore-going description by carrying out a further reaction S with such cationic groups. For example, a cationic structured-particle latex having sulfoni~n groups chemically bound to the structured par-ticles at or near the particle surface can be reacted with hydrogen peroxide at a temperature of from 20C to 80C, 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., frorn 2 to 10 moles of hydrogen peroxide for each mole of sulfonium groups.

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 milli-~0 equivalent per gram can be tolerated.

The process to prepare the product of thisinvention preferably is carried out as follows: A dilute aqueous suspension of the fiber is formed in the normal manner often iII a concentration of from 0.5 to 6 percent.
The latex is added at any convenient concentration, often ~n the concentration as supplied and the resulting mixture is stirred, usually for at least two minutes depen~ing somewhat on the equipment available. The aqueous suspension usually is then diluted further, often with white water froln 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 ~inimum time to obtain thorough Inixing. While the co-additive 18,~30~-E

1~8~ 5 12~

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.

Other optional constituents of the composiie--forming cor,lposition 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.

A composite is formed by flowing the resulting suspension over a porous support such as a scre~n to form a wet mat, dewatering the wet mat and completing drying by heating. The dewatering step includcs drain-ing 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 s~eps may be included~ The temperatures used for hot pressing, curing and tempexing or other heating steps often are from 100C to 250C, although higher or lower temperatures 2C are operable.

For making many of the composites, paper machines such as a Fourdrinicr machine, a cylinder machine or a laboratory sheet-forming apparatus are useful.

The product of tlle 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 tharl to properties of the 18,q30~-F

polymer composition. At high levels of latex, if suffi-cient heat is applied to fuse the latex particles, the properties of ~he resulting procluct depena markedly on the properties of the polymer phase.

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.

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 causiny alternating heavy and light spots in the sheet. Besides diminishing the aesthetic appeal of the paper t poor formation tends to decrease in-plane strength properties such as tensile strength. Poor formation causes uneven ~urfaces which contribute to poor printability.

In this specification and claims, all references to degree of polymerization (DP) are weight average unless other~ise indicated.

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 e~pressly indicated.

18,43OA-F

J
-~4-Many of the latexes us~d 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 ~,~'-azobisisobutyronitrile as catalyst, the percen-taqes bein~ based on the total weight of monomers. The base latex particles were then capped (encapsulated) by adding the cap monomers o-f 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 tOtâl monomeric components. ~dditional catalyst of the same kind, stirring and elevated temperature, usually 70C, were used for the cappin~ reaction. The resulting latex products were then reacted at a temperature of 70C with an excess of a nucleophile which was 2-(dimethyl-amino)ethanol (for the quaternary ammonium bound charge) and the excess nucleophile was removed by steam distilla~
tion after the desired amount of bound charge was reached.
For the sulfonium bound charge, the nucleophlle was dimetnyl sulfide, the reaction temperature was 50C
and excess nucleophile was removed by vacuum dis,illation.
The structured particle latexes thus produced had the properties shown in Table I~

18,430~-F

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Tests xeferred to in the examples were carried out as follows:

Tensile:
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 lO and the jaw gap is 5.0S cm rather than 20.32 cnl.

Canadian Standard Freeness (CSF):
The values are determined according to TAPPI
Standard r 227-M-58 except where variations in the pro-cedure are indicated.

Formation ___ A common way to measure formation is to compare visually the sheet to be measured with a set of ten standard sheets speclfica]ly made with decreasing levels of uniformity of fiber distri-bution (formation) z.nd ran~ed from l to lO with l being the best and lO the worst. Alternatively, ~ optical instruments, which are available commercially can be used for measuring formation.

Delaminatio_ Resistance:
The internal bond strength of the products is measured by the delamination resistance test. In ~5 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 ~-hen 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 separa-tion rate of 30.1S cm per minute. The average force 18,430A-F

,5 -lB-resisting delal~rlation over a length of about 10.16 cm is determined for edch of two samples. The average of the two samples is recorde~ in ounces per inch of width, abbreviated oz/in. Those values are followed in paren-S thesis 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 A steam heated, m~chanically-defibered pulp having a Canadian Standard Ereeness ~CSF) of 785 milli-liters and a solids content of 24~ was diluted to 1%
solids with water having a hardness of 10.6 (as CaCO3, ppm) and an alkalinity of 48 (as CaCO3, ppm). The com-ponents shown in Table II were added to the resulting fiber suspension in the order shown and stirring was continued for tihe time indicated before -the next step.
About 3% of Latex A would be required to reach the charse reversal point of 'che pulp.

TABLE II
Stirring Amount Time Step Component ~ (a) Min 1 Latex A 5 1.5 2Co-additive (b) 0.8 0O5

3 Alum 2 2.0 (a) = Dry basis, calculated on the dry weight of flber (b) = ~Ivd;oLvze~ polyacrylamide as described below 18,430A-Y

~ ~L~ 5 The co additive is a hydrolyzed polyacryl-amide having a degree of polymerization of 25,000 and an available char~e of 1.94 milliequivalents per gram of co-additive ~meq/g).
.

A sheet was formed on filter paper (12.5 cm in diameter) by filtering the resulting sus~ension through a Buchner funnel with vacuum from a water as-pirator. The resulting wet sheet was removed from the funnel, placed between two clean filter papers and ~ blotters and pressed on a Williams press at 1715 psiy (12¢ kg/sq cm). After the blotters and filter papers were removed, the resulting damp sheet was dried on a hot plate at 165C for 5 minutes. The dried sheet was conditioned by being kept in a room maintained at 50%
humidity and 23C for at least 2 hours, generally overnight, before testing.

The effluent from the filiration was ana--ly~ed for turbidity with a spectrophotometer as a measurement of the white water clari-ty. The wavelength 20 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 ~iltraticn are shown in Table III.

Comparative _ ample lC
A sheet was prepared in the same manner and with the same components ~xcept the latex and the co-additive were omitted. The drainage time, clarity and tensile results are shown in Table III.

18,430A-F

6~i T~BLE III
Draillage Example Time Clarity Tensile No Min % a 1 1.2 94 762 *1~ 8.0 60 127 * = Not an example of the invention a = Breaking length, meters Example 1 illustrates ~he 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 For each example, an aqueous dispersion con-taining 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 weigh~ 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 result-ing 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 handshee~ was pressed to a solids content of from 37 to 38~ by placing the sheet and couching blotter b2tween two pieces of wool felt and running the resulting 18,430A-F

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 220F
(104C). The product was just cockle free, and contained about 95 percent solids.

The co-additive used in the examples is a hydrolyzed polyacrylamide having 1.94 milliequivalellt of available charge (carboxyl) per gram and a degree of polymerization of 25000. Data are sho~n in Table IV.

Comparative Examples ~Cl-2C5, 5C and 8C
CGmparative Examples 2C2 and 2C4 were pre-pared in the same manner from the same components as de~cril~ed for Examples 2-lOo The amounts of the ].atex weîe below that required (5.5%) to cause charge reversal on the ~iber vsed.

Comparati.ve 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.

18,430~F

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18,430A-F

Exam~ e 11 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 hasis cal-culated on the weight of the fiber. The sheet was tested for delamination resistance (see Table V).

Comparative Exa~mple ll' 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 samc average polymer composit.on but Latex B is a struc~ured 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.

TAB LE V
Example Bound Charge ~elaminatiol Resistance No.Location me~_g oz/in(g/2.S4 cm 11Cap 0.300 21.6 (612) *].lCThrough 0.400 14.6 (414) Particle * Not an example of the invention.

From these delamination resistance values shown in Table V, it is seen that Example 11 (an ex~mple of the invention) provi.des considerable improvement over a process using a latex having the bound charges throughout the particle rather than only near tlle particle surface even though the total charge was greater for Comparative Example llC.

18,430~--F

E ~
Sheets were prepared in the same manner as described for Example 11 except that for Latex B there were substituted Latex C, I,atex D and hatex E, respec-tively. The la-tter 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.

TABLE VI
Bound Example Charge Delaminaticn Resistance No. Latex meq/g oz/in (~/2.54 cm) 12 C 0.223 1~.9 (536 13 D 0.294 21.8 (618) 14 E 0.415 25,0 (709) Examples 12-14 show that the internal bond strength of the product increases as the bound charg~
on the latex .increases.

~xamples 15-22 Shee-ts were prepared as described in Example 11 except that diff~rent latexes were used but in the same proportions. The major d.i:Eferences among the latexes relate to the composition in the base latex ~rom which the structured particle latex was made (see Table I).
Data are sho~.~n in Table VII.

Comparative Example 22C
A sheet was prepared in the same manner as for Examples 15-22 except the lat~x and the co-additives were not included. Results are shown in Table VII.

18,~30~ F

.6~

TABLE VII
Example Core ~elamination Resistance No Latex C oz in g 2.54 cm) __ _ F -15 23.9 (678) 16 G - 8 23.8 (675) 17 H 2 24.6 (697) 18 I 20 25.5 (723) 19 J 2 22.6 (641) K 35 19.7 (558) 21 L 61 17.8 ~505) 22 ~ 80 14.6 (414) *22C ~~~ ~-~ 5.8 (164) * Not an example of the invention.

Examples 15-]8 illustrate the process of the invention where the sulfonlum group provides the bound charge rather than the quaternary ammonium group of Examples 1-14 and shows operability of thc 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.

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 ~han one minute, the delamination resistance was 22.4 oz,t;n (635 g/2.54 cm); 18.5 oz/in (524 y/2.54 cm) ~ld 14.8 oz/in (420 g/2.54 cm), for 18,430A-F

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, hlgher tempera-tures and/or longer times shoulcl be selected or converse].y at a given time/temperature condi~ion in the process a latex with sufficiently low Ty value shou].d be used.

Examples 23-28 Sheets were prepared as described in Example 11 except a different latex was used in all these examples and the identity of the co-addi~tive was changed but not the amount in all these examples except No. 25, Data are shown in Table VIII.

18,430~-F

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LÇ; a~

le 29 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 ]craft suspension was added 90 parts (dry weig11t basis) of Latex I. After stirring a-t the same rate for an additional two minutes, 3.3 parts of the specified co-additive as a dilute aqueous solu~ion (0.2% solids) was added and stirring was con-tinued for an additional 30 seconds. The resulting 15 furnish was made into a handsheet (30.48 x 30.48 cm) on an M/K Systems "Mini-Mill" handsheet machine using water or dilution of the description given above.
The resulting wet sheet was placed between two pieces of clean filter paper and 4 blo-tters and pressed on a 20 ~illiams 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 165C for 5 minutes.

The resulting dry sheet was so strong that the delamination resistance could not be determined by the method discLosed in this specification.

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.

_xam~le 30 A sheet was prepared using the same materials in ~le same amounts and by the process described ill Examples 2-10 except the amount of the same co-additive 18.430~-~

ÇP~

was 0.1~ and the latex was a latcx prepared according to United States Patent No. 3,873,488 in a batch process from 65 parts of styrene/ 30 parts of butadi~ne, 5 parts of acrylonitrile and 4 parts of vinylbenzylmethyl-dodecylsulfonium chloride with no added nonpolymerizablesu~face active material. The latex had a solids content of 23.9%, a particle size of 1090 angstroms (determined by light scatterin~) and 0.072 milliequivalent of bound charge per gram of polymer. The estimated Tg was 25C.
The delamination resistance of the sheet was 19.3 oz/in (547 g/2.5~ cm).

This example illustrates the practice of the invent:ion uslng a latex having a level of bo~md charge near the rninimum suitable for this invention and also illus~rates the use of a latex which is not prepared according to the method for making structured particle latexes.

Examples 31 and 32 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 dif~erent co-additive was used in ~he amount shown in Table IX and deionized water was substituted for the water.

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 =
-10C) capped by 10 parts of a copolymer of 33% of ~inylbenzyl chloride and 67~ of butyl acrylate which was 18,430A-F

~30-subsequently reacted with trimethylamine to provide 0.121 milliequi valent or bound charge per gram of latext solids basis~

The co-additive used in these examp]es was 5 a hydrolyzed polyacrylamide having a degree c f poly-merization of 20,800 and an available charge of 3.48 milliequivalent (from carhoxyl groups) per gramO Data are shown in Table IX.

o~parative Examples 31Cl and 31C2 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 31Cl and the co-additive was omitted in 31C2. Data are included in Table IX.

TABIE IX
Example Latex Co-Additive Delamination Resistance No. ~ a % a oz/in (g/2.54 cm) -*31C1 0 0 5.8 (164) *31C2 20 0 9.Q (250) 31 20 0.1 13.1 (371) 32 20 0.2519.1 (541) * Not an example of the invention.
a = Solids basis, calcui ated on the fiber.

Examples 31 and 32 illustrate the invention 15 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 arid 34 An aqueous dispersion containing 1393 parts 20 of water having a hardness of 105 ppm (calculated as 18,430A~F

calci~n carbonate) and an alkalinity of 48 ppm (calculated as calcium carbonate! and 7 parts (dry basis) of unbleached Canadian sof wood 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 parks (dry weight basis) of a latex of the kind described below. After stirring at the same rate for an a~ditional two minutes a dilute aqueous solution (0.2% solids) of the specifled co-additive was added in the amount shown in Table X and sti~ring 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" handshee~ 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 fel~ and running the resulting sandwich through the press at medium speed using a press pressure o $0 psig (5.6 kg/sq cm). The pressed sheet was removed Erom the wool felts, and stripped from the couching blot~er, then dried in a drier maintained at 220F (104C). The product was ~ust cockle free, and contained about 95 percent solids.

The latex used in the foresoing 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 encapsu]ating layer of 35 percent of butadiene, 15 percent of styrene and 50 pelcent of vinylbenzyl chloride which was reacted subsequent to polymerization with 2-(dimethylamino) ethanol to provide a bound quaternary ammonium charge of O r 355 millie~ui~aLent per gram of polymer in the latex.

18,430A-F

The co-additive (A) used in the example is a hydrolyzed polyacrylamide having 1.94 milliequivalent of available charge per gram ancl a degree of polymerization of 5500.

For cor,~parison 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 omittedO Data are shown in Table X below.

18,~130A-F

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The turhidity measurements are made on the effluent from the freeness (CSF) test and are indi-cative of the "white water" characteristics which are obtained in the paper making process.

The sheets from Examples 33 and 34 were acceptable in all the properties measured. Com-parative examples 33A and 34B were deficient in formation and therefore are unacceptable. While comparative example 33B indicated good formatio~, the transmission was low indicating poor flocculation onto the fibers and additionally the delamination resistance was low.

Examples 33-38 -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 strustured particle latexes containing 70~ of a core copolymer con-sisting of 65% of styrene and 35% of butadiene modified with 0.2~ of dodecanethiol and the core was encapsula-ted (capped) with 30% of a copolymer of 50% of vinylbenzyl-chloride, 35~ of butadiene and 15~ of styrene which wassubsequently reacted with an excess of dimethylsulfide.
The latter reaction was stopped for Latex P by vacuum distilling tlle excess dimethyl sulfide when the bound charge of sulfonium group was O.lS5 milliequival~nt per gram and for Latex Q when the bound charge was 0.388 milliequivalent per gram.

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 polymeri zation of 4100. Data are shown in Table XI.

18,430~-F

C ~ ~amples 35C and 37C
.
Sheets were prepared as described for Examp~es 35 and 37 except no co-additive was used. Data are included in Table XX.

18,430A-F

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18, A 30A-F

.6~

Examples 35-38 illustrate the practice of this invention using a different kind of cationic bound chaxge than in Examples 33 and 34 and also illustrate the use of widely differing amounts of bound charge.

18,A30A-F

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. 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 milli-equivalent per gram of copolymer and the co-additive is a water-soluble anionic polymer having a degree of polymerization about 3000, having an available charge of from 0.3 to 8 milli-equivalents 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, and further characterized in that the latex is added in an amount greater than that required to cause charge reversal on the fiber but less than an amount which would exceed the capacity of the fiber to hold a wet mat together during processing, and the co-additive is added in 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 re-dispersion of components of aqueous suspension; removing water from the aqueous suspension to form a wet mat; and drying the mat by heating; the copolymer parti-cles of the latex being deformable at the temperature of the process.

2. The method of claim 1 characterised in that the cationic polymer particles are structured particles consisting of a non-ionic organic polymer core encapsulated by a thin polymer layer having bound charges of pH independent cat-ionic groups at or near the particle surface.

3. The method of claim 1 characterised in that the amount of latex, solids basis, is from 0.5 to 2000 percent based on the weight of the fiber.

4. The method of claim 1 characterized in that the amount of bound charges in the latex is greater than 0.1 milliequivalent per gram.

5. The method of Claim 1 characterized in that the co-additive is an acrylamide polymer.

6. The method of Claim 5 characterized in that the anionic charge of the co-additive is provided by a carboxyl group.

7. The method of Claim 1 characterized in that the co-additive is used in an amount of from 0.05 to 160 percent by weight, based on the dry weight of the fiber.

80 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 polyvalent metal ions at a pH from 4 to 7; all percentages being by weight.

9. Web of Claim 8 characterized in that the core comprises a copolymer of from 20 to 50 percent of an aliphatic, conjugated diene, from 20 to 30 percent of a monovinylidene aromatic compound, from 0 to 5 percent of polar, non-ionic ethylenically unsaturated monomers and from 0 to 25 percent of other ethylenically unsaturated, nonionic monomers which when in the form of homopolymers are water-insoluble.

18,430A-F

10. The fibrous web of Claims 8 or 9 characterized in that the amount of available charge of the co-additive is from 0.15 to 0.5 milliequivalent per gram and the degree of polymerization of the co-additive is from 5000 to 10,000.

18,430A-F