NO750652L - - Google Patents

Description

The present invention relates to the production of twisted cellulose fiber strips which have been reshaped and stiffened by substantially internal impregnation with wet strength resins. The present invention also relates to a method for the production of twisted cellulose strips, resin-impregnated fibers and the use of these fiber materials in fiber products to contribute to increased absorbency, smoother feel, increased absorbency, greater bulk, increased porosity and resistance to compression or elasticity in wet or dry conditions . The resin impregnated fibers of this invention do not develop conventional fiber to fiber hydrogen bonds when formed into sheets from unpainted fiber slurries and they do not form sheets with observable wet strength characteristics. If, however, the present wet strength

sheets of impregnated fibers are formed into sheets with paper-like structures and then dried, the structure expands to form a downy mat that resembles loose cushions made by air drying dry cellulose fibers. This expansion can be limited and a degree of paper-like bonding achieved if the resin-impregnated fibers are thoroughly tamped or refined in papermaking equipment.

It has recently been discovered that the cellulose fibers, when these are

internally impregnated with wet strength resins and then transformed into a lost down for drying the fibers and treated with the resin, a highly desirable and surprising modification of the physical and chemical properties of the fibers is achieved for many applications. The wet strength resin impregnated fibers produced according to this invention thus achieve a

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unique combination of physical and chemical properties [indicated by increased stiffness of the fibers, increased fiber and fiber structure elasticity, characteristic twisted strip shape, reduced swelling ability in water, increased solubility in common cellulose solvents, inability to form typical fiber to fiber bonds in papermaking devices, desired wetting ability and increased biodegradability. Although the moist resin-strengthened impregnated fibers are resistant to hydration during papermaking stamping and refining,

they withstand this treatment and are not significantly reduced by this.

The various characteristics and properties of the present wet strength resin impregnated fibers make them particularly useful for many applications involving such diverse operations as absorption of liquids, filtration of liquids, cushioning, insulation and swelling. The useful area for special applications can be adapted by the initial selection of cellulose fibers,

selection and degree of wet-strength resin impregnation, and in formulating mixtures of resin-impregnated fibers with untreated fibers and other materials.

The present invention relates to wet strength impregnation of dried and dried cellulose fibers from any source. Further, the advantage of the present method is achieved by treating,

in the present impregnation method, unbleached, semi-bleached or fully bleached fibers regardless of the method by which the fibers are isolated. Bleached and unbleached fibers obtained from cotton or cottonseed are particularly suitable for wet strength hair dye impregnation according to the present method.

Within the state of the art, there is of course considerable knowledge regarding the use of wet strength resins to improve the dry and wet strength of the paper sheet product, the purpose of the polymerized resin being to increase the degree of bonding between the fibers in the sheet. For example, the method for obtaining wet-strength paper by adding melamine resin^ is described. colloid to the pulp as described by C.G. Landes and CA. Maxwell in Paper Trade Journal, Volume 121, Page 51,

11945. Furthermore, in US Patent No. 2,595,935 in

described a breakthrough for the production of wet and dry strength paper using thermosetting resins prepared by condensing an alkaline polyamine with a halohydrin.

In this reference, the improved wet strength resin is applied to it. conventional way, the resin being added to the fiber suspension after which this is directly converted into a sheet and dried into paper or by application to a wet, preformed web which is then dried to give a paper with improved wet strength.

In the paper industry, it is a common practice to use or recycle small amounts of wet-strength resin-coated fibers derived from wet-strength resin-treated waste paper or scrap. Such application has been fraught with difficulties when folding the waste and methods for carrying out this folding of wet-strength treated waste paper have been proposed and used. Mechanical' defibration processes are also generally regarded as an unwanted cost factor in the production of wet strength paper. Conventional mechanical defibration processes have also

a tendency to cause fiber destruction and fiber shortening, which further reduces the value of the recovered fibers. Furthermore, the recognition of the disadvantages of mechanical methods for defibrating wet-strength paper has stimulated research to find wet-strength resins that can be rendered inactive by chemical methods to thereby promote the recovery of fibers from wet-strength resin-treated paper for the reuse of fibers unaffected by the wet-strength resin. Such attempts have not been particularly successful.

Chemical cross-linking of the cellulose fibers' internal elements

is well known, for example from US patent no. 3,224,926,

US Patent No. 3,241,553 and No. 3,658,612, all of which show and

requires methods for the production of cross-linked cellulose fiber products and their use in a number of different structures. In the cross-linking methods, compounds with a relatively small molecular size are used to penetrate the small voids in the cellulose fibers and cross-linking will thereby cause short, inelastic links between the cellulose molecules by forming covalent bonds with nearby hydroxyl groups.

In US patei t no'. * 3 . 756. 913 is shown and requires a special process sequence which results in an external coating of the cellulose fibers with substantive polymeric compounds applied as wet strength resins. Such external coating of the fibers differs from the present invention,

where the fibers are impregnated Internally with wet strength resins.

In contrast to the above-mentioned methods for the manufacture and use of modified and wet-strength-treated cellulosic fibers and structures containing the same, the present searcher has found that internally wet-strength resin-impregnated fibers are produced in such a way that essentially no wet-strength resin bonds are formed between the fibers during their manufacture and initial drying,

exhibits unique absorption and elasticity properties.

These unique properties are measured by results obtained by forming wet and dry sheets or by felt (pad) renewal processes and are shown in the absorbent cellulose fiber structure formed in this way.

Consequently, it is an aim of the present invention to provide separated cellulose fibers which are internally impregnated with wet strength resins of all types.

It is a further purpose of the invention to provide a method by which such internally wet-strength resin-impregnated fibers can be produced advantageously and economically for use in absorbent, elastic, compression-resistant fiber structures.

It is now a further purpose of this invention to produce internally wet-strength impregnated fibers which can be wet-formed and dried into sheets with low density.

According to the present invention, cellulose fibers are produced which are essentially internally impregnated with wet strength resin. These fibers are characterized by improved properties, such as increased absorption rate in the fiber structures, improved structural elasticity and bulk, generation of

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increased wet structure swelling and lack of hydrogen bonding I in fiber structures formed from these fibers. In the present method, cellulose fibers are suspended in water and the pH of the suspension is increased to promote fiber swelling, the wet strength resin is added to the fiber suspension while mixing and drawing is continued at the increased fiber swelling pH to promote impregnation of the fibers' internal surface rag cavities by the wet strength resins. The increased fiber-swelling pH is then lowered to a lower one

pH which promotes fiber contraction and polymerization of the wet strength resin during the subsequent mixing and drawing. The fiber suspension is dewatered to a consistency whereby the impregnated fibers can be separated or defibrated, the fibers are separated and heat curing of the wet strength resin takes place together with drying. the fibers. Additional processing steps may be used to remove any remaining fiber bundles, add flame retardants or dyes, and otherwise mechanically or chemically condition the fiber products for special applications. The present method provides a unique twisted cellulose ribbon fiber, . which are essentially internally provided with or impregnated with additions of polymerised wet strength resins. The fiber products can provide high airiness, improved

bulk and moisture absorption, as well as elastic non-hydrogen binding properties for webs or sheets produced by wet or dry processes from them.

According to the present invention, cellulose fibers which are internally impregnated with wet-strength resin are produced by a method which includes as necessary steps a first wet-strength resin impregnation carried out in a fiber suspension at a higher and cellulose fiber-swelling pH and a second step at a fiber-contracting and wet-strength resin polymerizing lower pH. The manufacturing steps are discussed in more detail below:

(1) Preparation of the fiber suspension.

It has been found that cellulose fibers of varied natural origin, preferably unbleached and non-dried fibers in the unbleached, partially bleached and fully bleached state from conifers and hardwoods, as well as Jbomull linters are particularly suitable for the method according to I

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present invention. Dried fibers from the above-mentioned Skilders as well as non-dried and dried fibers derived from Esparto grass, bagasse, hemp, flax and other lignin-containing

and cellulose fiber sources can be used as raw materials.

Such fibers are internally impregnated with wet strength resin

by first suspending them in water at a consistency of approx.

1.5 - 2.5%, preferably 2%, in order to swell, disperse and condition the fibers for the internal impregnation into the wet strength resins. It has been found advantageous to set the fiber consistency of the aqueous suspension to approx. 2% to promote the transfer of the wet strength resin in the fibers. Higher consistencies of the fiber suspension will promote wet strength resin transfer into the fibers, provided effective agitation conditions are maintained.

(2) Addition of wet strength resin dry matter by fiber-

swelling pH.

After fiber suspension preparation step 1 is added

an aqueous solution of wet strength resin solids to the original fiber suspension in the form of a dilute solution. Aqueous solution is preferred, although solutions of wet strength resins in suitable organic solvents can be used. The fiber suspension with the added wet strength resin dry matter is carefully mixed to ensure an initial contact with the individual fibers in the suspension for a drawing period to ensure penetration and impregnation of the fibers with the wet strength resin dry matter.

In addition to the diluted wet strength resin solution,

the pH of the suspension is adjusted to a pH in the range of 5 to approx. 11, depending on the pH of the particular wet strength resin used. Suitable pH value for use in this wet strength resin-dry substance f-t oxygenation step is e.g. a pH of 9 for neutral curing resins, a pH of 7 for urea formaldehyde wet strength resins and a pH of 5 for melamine formaldehyde wet strength resins.

The pH is adjusted with sodium hydroxide or other pH adjusters

In chemicals to the extent necessary to maintain the highest possible pH without a simultaneous precipitation of the added high-strength resin solids, to promote fiber swelling and

In availability of the wet strength resin resins for penetration and deposition on the interior cavity surfaces of the cellulose fibers. The present method is intended for the use of all wet strength resin types, including urea formaldehyde, melamine formaldehyde, polyamide-polyamine-epichlorohydrin, polyethylene imine and dialdehyde starch wet strength resins.

About 8 - 20% of the wet strength resin solids, calculated on the weight of the fibers in the suspension, is added to the cellulose fiber suspension in the form of a diluted aqueous solution of about 1 - 15%, preferably approx. 5% wet strength resin consistency. Furthermore, the addition of wet strength resins is not detrimental to the process but can lead to difficulties in recovering the resin solids and uneconomical operation.

Such addition of wet strength resin results in a finished internal impregnation of the cellulose fibers of approx. 2 - 6%

or higher wet strength resin solids. Typical internal impregnation of the fibers with approx. 2.6 - 3% wet strength resin solids are obtained. As stated above, the addition of additional amounts of wet strength resin and impregnation can lead to somewhat improved fiber products, but is considered uneconomical with respect to the improved fiber properties obtained.

The wet strength resins are supplied in the form of a dilute aqueous solution or in an organic solvent to

promote the fiber penetration and, although it is preferred that the wet strength resin solutions in the process are used in the freshly prepared state to ensure a small collodial size of the wet strength resin solids in the solution to promote the fiber penetration, penetration is achieved by the present two-sided pH process when the wet strength resins are used within their normal shelf life.

When the fiber suspension is added to the wet strength resin solids,

is a mixing and drawing time sufficient to ensure an intimate contact with the wet strength resin solids and fibers necessary and a mixing time of 15 - 6o min. together with an [additional drawing time from 3o min. to approx. 15 hours have been found

to provide the desired fiber impregnation. It is especially noted that slow stirring can be continued during the draw time and total times ' of approx. 45 min. to 16 hours are effective during the addition of the wet strength resin solids. During the mixing and

during the draw, the fiber suspension is preferably maintained at the ambient or mixing-induced temperature, as elevated temperatures decrease fiber swelling and reduce the desired wet strength resin solids penetration for the internal fiber impregnation. A slightly reduced fiber suspension temperature of lo - 21°C can be used to further strengthen the fiber swelling. (3) Wet strength resin solid mix and draw at deposition pH.

After impregnation of the cellulose fiber suspensions with wet-strength resin dry matter at the elevated pH in step 2,

the fiber suspension's pH is lowered by adding a pH-controlling chemical compound, preferably diluted sulphur-

acid (lo% aqueous solutions of sulfuric acid are used to

avoid fiber burning) and the suspension is carefully mixed in lo min. to approx. 1 hour for the final draw time on

about. 2o min. to approx. 15 hours, preferably one hour. In this mixing and extraction step at low pH it must be noted that stirring can be continued during the extraction period and total times of 15 min. to approx. 16 hours are effective. In this low pH step, the pH is adjusted to 7 - 2.5, again depending on the pH characteristics of the particular wet strength resin used. It has been found that effective pH values for use in this step 3 are exemplary

pH 7 for neutral curing resins, pH 4, 5 for urea formaldehyde wet strength resins and pH 3.2 for melamine formaldehyde wet strength resins. This mixing at low pH and subsequent extraction is carried out at ambient temperatures. The pH lowering leads to fiber contraction and deposition of the impregnated wet strength resin solid inside and on the surface of the inner cavities of the cellulose fibers. The pH lowering provides

also brings effective conditions for curing wet strength resin solids. Although it is preferred to use dilute sulfuric acid solutions as pH lowering agents,

other mineral acids such as hydrochloric acid, organic acids such as acetic acid and acid salts can be used.

(4) Dewatering the fiber suspension.

After impregnation of the fibers with wet strength resins and

in the cellulose fibers in steps 2 and 3, the in-

terntly impregnated fibers from the fiber suspension and dewatered to a preferred fiber consistency of approx. 5o - 55% in favor of

achieve defibrating and fiber fluffing conditions while maintaining the fiber mass in a contracted state.

Such dewatering can be accomplished by forming an uncontracted

sheet, which has a thickness of approx. o.25 - o.38 cm and a basic weight of approx. 35 g/ 75o cm 2, on a Fourdrinier wire and then the sheet thus formed is dried on a drying drum to

produce the predetermined fiber consistency. Although the specified sheet has been found effective in achieving defibration, the present method expects defibration from any pulp sheet with which such defibration can be accomplished.

The fibers can also be dried in a drying tower as an alternative

method of the Fourdrinier sheet formation and the trommel4:6rke method. The mechanical devices which are used to carry out the defibration can also effect the necessary fiber consistency.

stones to achieve the fiber separation.

(5) Defibrating and fluffing.

The unshrunk and partially dried sheet formed in step (4) is dissolved into fibers in a moist state by mechanical treatment.

In a preferred form of operation, the defibration and fluffing of the moist fiber sheet, consisting of internally wet-strength resin-impregnated fibers, is carried out by passing the sheet tangentially to a rotating steel brush having a diameter of

3o cm and rotates at 342o rev/min. to tear apart the sheet using an air current to assist the defibration and remove the wet strength resin impregnated fibers from the steel brush.

Another equipment that has been found effective to

for defibrating and fluffing partially dewatered and wet strength resin-impregnated fibers, is a wet pulp fluffer supplied by Improved Machinery Co., Inc., Nashya, New Hampshire. This mechanical device operates by removal

of the moist fibers through a series of rotating fingers that operate in grooves and avoid fiber impact, densification and shortening.

Such equipment is more fully described in US Patent |3,725,193. in

(6) Curing of wet strength resin solids and dry fibers.

The defibrated and fluffed wet strength resin impregnated

the fibers obtained in . step 5 is heated over a period of time. to cure the internally deposited wet strength resin solids and to dry the fibers. When the wet strength resin is a urea formaldehyde wet strength resin, e.g. "Casco PR 335" supplied by Borden Chemical Co., Columbus, Ohio, a suitable drying and curing temperature is 115°C and the temperature is maintained for a period of 2o - 3o min. Other wet strength resins have optimum curing and drying times, the information of which is available from the manufacturer. The neutral curing resins of the polyamide-polyamine-epichiorhydrin type supplied as "Kymene 557" by Hercules, Inc., Wilmington, Delaware, may, for example, cured and dried to wet strength resin-treated fibers at a temperature of 12o°C held for 45 min. The person skilled in the art will also understand that a number of temperature-time relationships exist for the curing of wet strength resin, where higher temperatures generally reduce the necessary curing time,

while normal paper drying temperatures and times followed,

of storage for use may result in wet strength resin curing.

It has also been shown that within the available time-temperature relationship, the optimum wet strength resin cure occurs at a time-temperature relationship that can be provided from the manufacturer or literature information.

Although wet strength resin-impregnated fiber production in stages

(6) constitute a finished product with the benefits of the process

and the product that is hereby encompassed and required, the present invention has found that a certain amount, up to approx. 5%, based on fiber weight, of fiber bundles may remain in the dried and cured wet strength impregnated fibers due to incomplete defibration in step 5. For certain beneficial uses of the wet strength impregnated fibers contemplated by the seeker, e.g. tissues, towels and non-woven materials possessing the ultimate in feel, shaping and porosity, where the advantages of internal wet strength resin impregnated fiber producing low jdensity, absorbency, porous sheets and webs are achieved, such fiber-

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bundles are present even in a small proportion and are considered Jodelegende. The damages are spots, lowered porosity and !■ formation of relative unevenness. In order to avoid such negative results, the additional process steps 7, 8 and 9 for the removal of fiber bundles have been considered within the scope of the present invention and described below.

(7) Resuspension.

The dried and cured internally impregnated wet strength resin-

in the fibers from step (6) is reslurried in water at fiber consistencies of about 0.2% to 3%, preferably 2%.

or lower. Such lower consistencies of fibers contribute to loosening and provide separable, e.g. by sieving, fiber bundles as discussed above. The reslurried fibers are stirred thoroughly for dispersion in the aqueous slurry.

(8) Separation of the fiber bundles.

The slurried dispersed internally impregnated and cured wet strength resin fibers and fiber bundles are separated by allowing the slurry as dispersed in runoff to contact a sieve. Such sieves can be of the usual flat bed or rotating drum type equipped with grooved sieves. One such rotating drum type screen that has been found to be effective in performing the separation according to the present process is a vibrator model L-20 screen supplied by Bird Machinery Co., South Tfalpole, Massachusetts. In such sieving operations sieves of the grooved blade type can be used effectively, and sieves with 0.038 cm grooves have been found to be effective in performing the fibre-bundle separation, which allows the fibers to pass and holds the fiber bundles back.

(9) Retorting.

The internally impregnated wet strength resin fibers, with fiber bundles removed as prepared in step (8), are retoriced.

It has been found that a temperature of 115°C for 15 min. want

complete such drying. The heat and time required for drying can be supplied in a tunnel dryer, oven dryer, drying tower or other conventional apparatus for drying separated fibers. • A currently preferred form of Itorking makes use of the formation of a loose fiber web at a

Foudrinier-silf., Such loose webs of internally impregnated wet strength resin fibers have no cohesiveness due to the aforementioned lack of hydrogen bond formation and are held for drying purposes between enclosing sheets of thin tissue, sheets of cotton canvas or netting. Such wrapping cloths, cotton canvas or netting can be made endless to wrap the loose internally wet strength resin impregnated fibers during drying and then return to wrap for drying further quantities of such fibers.

The internally wet strength resin impregnated fibers prepared

in the described method according to the invention has a twisted, curled ribbon shape different from the relatively straight cellulose fiber ribbons that appear in conventional paper sheets treated with wet strength resin for developing wet strength, where the fiber is retained during drying and exhibits a characteristic substantially recovered ribbon configuration. The fibers according to the present invention are also unique in that they have wet strength resins with which they are impregnated substantially deposited within their fiber walls. The internally wet-strength resin-impregnated fibers thus produced are non-hydrogen-bonding in nature and are substantially insoluble, less than 10%, in the standard copper ethylenediamine solution used in the fiber testing industry. The present fiber product also exhibits

a surprising suction velocity of o.7 - 1.1 cm/cm in bulk or fiber sheet structures, have low swelling characteristics in water and are beatable or refineable in the papermaking operations without suffering significant fiber damage and degradation.

The sampling procedures used to determine the unique properties

of present fibers and to obtain the test results obtained in the examples below were determined by the following procedures: Determination of absorption capacity and wet and dry densities.

Fluffed internally wet strength resin impregnated fibers are conditioned at 22° 1°C and 50° 2% relative humidity for the test. lo x lo cm mats that have two approximately the same

(layers placed 18o° apart are produced so that they have weights|

of 2 and 4 g each for the lo x lo cm mats. The mats are pressed j between metal surfaces to give a lo x lo cm mat with 2-o,o2 g and 4^ o,o2 g at each of the densities, o,lo, o,15,

o.2o and o.25 g/cm .

After such mat production, the mats are saturated with water

a sloping platform and non-absorbed water. dry up carefully. The weight of saturated mat is measured. After such a measurement, the mat is placed on a surface and a weight plate which gives a 1 psig pressure is slowly lowered. The water is squeezed out and sucked up.

After 45 sec. the wet thickness is measured and the charging plate

removed. The weight of the mat at this point is measured as the 1 psig weight. Then the mat is re-moistened with water, unabsorbed water is sucked up and the mat is weighed and the weight is measured as the final re-moistened, unloaded state of the plate. The measured data is used to calculate wet and dry density together with an absorption capacity equation versus dry density. The absorption capacity at o.lo, o.15, o.2o and o.25 g/cm dry density is calculated.

Determination of the suction speed.

In the determination of the suction rate of wet strength resin impregnated fibers, lo x lo cm mats are prepared in a similar manner as in the method above indicated in the determination of the absorption capacity and wet and dry density. The mats are produced so that they have a 3.3o<i>o.lo g weight and a 0.25o-o.o!5 g/cm density. The mats produced in this way are placed vertically in a sample holder with at least 2 electrodes in contact with the mat near the bottom and near the top edges. The time period for suction begins when the contact is formed between 2 bottom electrodes and ends when the contact is formed between 2 top electrodes. The time for suction in a vertical mat is measured for 5 samples and the individual mat weights, densities and times and suction speeds in cm/sec. is determined and the average suction speed is determined as cm/sec.

Drip Capacitance,

In the drip capacity test, test mats are prepared as follows

as described above. The mats are arranged on a support comprising a wire cloth with square mesh openings of ~1A25cm. Water with an outflow rate of 1 ml/sec. dripped onto the center of the mat. The time required for the mat to emit the first drop of water from its underside is determined and the dripping capacity,, is indicated in g of water per g. cellulose in the mat.

Wet compressibility and dry density.

The wet compressibility of fibers internally impregnated with wet strength resin is determined by breaking them up in a TAPPI disintegrator, after which a laboratory sheet is produced in a TAPPI sheet former from which sheet samples (lo x lo cm surface weight of approx. 29o g/m 2 ) are cut out. The wet test pieces are subjected to compression and the thickness of the test pieces is measured under increasing loads of o kg/cm 2 , o.ol7 kg/cm 2 , o.o35 kg/cm 2 , o.o52 kg/cm 2 and o.o7 kg/cm 2. The wet mat is then dried at lo5°C, and the dry weight is determined. The thickness of the dry mat is measured at o kg/cm 2 and o.o7 kg/cm<2>. The apparent wet density (g/cm 3) of the mat

is determined from the wet thickness for each load level

and the dry weight of the mat. The dry density (g/cm 3) is determined from the dry thickness and the dry weight of the mat.

To determine the effects on the dry density of hard shaking, 6 g of air-dried samples are shaken in 600 ml of water with a kitchen whisk for 2 min. lo x lo cm samples are prepared as for the wet compressibility and dry density samples above. The samples are compressed at o.o7 kg/cm 2 for at least 1 min. and

then dried at lo5°C. The dry weight is determined and the thickness of the mat is measured at o kg/cm 2 and o.o7 kg/cm 2 during the equilibrium setting for 1.5 min. under o.o7 kg/cm 2 load. Based on these data, dry densities of o kg/cm 2 and o.o7 kg/cm 2 for the samples are calculated.

CED insolubility.

Copper ethylenediamine (CED) insolubility samples, which measure

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the insolubility of the cellulose samples indicated in the examples/was carried out by weighing a 0.5 - 1.0 g dry fiber sample in a

tared 15o ml beaker and determine the weight to 4 decimal places on an analytical balance. The beaker was placed in an ice bath and 28 ml of 0.3 molar CED solution was added to allow the fibers to swell. The solution and the fibers were stirred and the sample

was left in my fur. After the 10 minute period, 22 ml of 1.0 molar CED solution was added and stirred to give a 0.5

molar CED solution that was able to dissolve unmodified cellulose. The sample was left in the beaker and stirred with periodic stirring for 3o min. At the end of the 30 minute period, 50 ml of cold water was added and mixed in the beaker to stop the CED dissolution action. The liquid in the beaker and the residue (undissolved cellulose) were filtered on a tared coarse glass filter and the recovered residue was washed with cold 0.2 molar CED to remove any -dissolved cellulose. The CED solution was then washed away from the residue and. any remaining CED was neutralized with dilute hydrochloric acid until the characteristic deep blue CED color disappeared. The neutralized residue was returned to the tared filter and washed acid-free with water. The denatured filter containing the undissolved cellulose residue was oven dried at 105°C, cooled in a desiccator and reweighed. The weight gain was taken as the weight of CED insoluble material and was calculated as a percentage of the original sample weight.

Moisture equilibrium test.

The moisture equilibrium samples specified here are determined by samples taken in a wet state. In this test, an air-dried fiber sample of approx. 3 g slurry 5 min. in 2oo ml of water in a TAPPI disintegrator. After the slurry, one sheet of the sample is formed in a TAPPI sheet former. The sheet sample is conditioned by aeration at 24^1°C and a relative humidity of 54^2% until a constant weight is obtained for 1.5 to 2.5 hours. The constant weight of the sample is determined and the sheet is oven-baked at lo5°C for 2 hours, weighed again; the equilibrated percentage moisture content is calculated from the difference, of the weight of the sample.

Water retention value test.

The water retention value samples (rtRVs) which measure the capacity of absorption and retention of water stated here, were determined by a method where oven-dried samples weighing approximately o.15 g were dipped in 100 ml of water in a covered container for at least 16 hours at room temperature. After the dipping period, the submerged fibers that made up the sample were placed in funnel tubes, where the funnel tubes were placed in centrifuge tubes. The centrifuge tubes were placed in a centrifuge and the sample was centrifuged with a relative centrifugal force of 3ooo g. The required rotation speed for 3ooo g was achieved in one minute and maintained for a total of 10 minutes. The .. centrifuged samples were quickly removed from the tubes using tweezers, placed in tared weighing bottles and weighed. The weighed samples were then dried to constant weight at loo°C and weighed again. The WRV was then calculated based on the following formula:

in which

tf = wet weight of centrifuged sample

D = dry weight of centrifuged sample

vi - D = weight of absorbed water.

In order to fully describe the present invention, the following examples are presented. It is to be understood that the examples are only cited as illustrative of the invention, and that the details of the method and the product therein are not to be interpreted as limiting the inventive concept for the invention within the scope of the claims appended to this application.

EXAMPLE 1

Internally wet-strength resin-impregnated twisted cellulose tape fibers are first produced by slurping in water 4o2 kg, calculated on the basis of dry pulp weight, of undried, fully bleached Southern pine Kraft pulp. The fiber consistency of the thus formed fiber slurry is adjusted to 2.0%. Next, 92 kg of "Casco PR 335" (a urea formaldehyde wet strength resin) supplied by The Borden Co., Columbus, Ohio, as an aqueous solution containing 35% wet strength resin solids is diluted with 363 kg of water and added to the preformed fiber slurry while stirring over a lo minute period. The pH of the slurry is 7.1. The fiber slurry with added wet strength resins is stirred after the addition of wet strength resin for one hour and the fiber slurry is left to stand and infuse for a further hour.

After drawing, the fiber slurry is adjusted to pH 4.1

by adding lo% sulfuric acid while stirring the fiber slurry, and the stirring and pulling continue for 6o min after the acid addition or the pH reduction. After such drawing at a lower pH, the slurry of internally wet-strength resin-impregnated fibers is brought. on a Fourdrinier paper machine to form an unpressed wet piece having a 35% fiber consistency. After forming on Fourdrinier wires, the wet piece is run over drum dryers for drying to an increased fiber consistency of 5o%.

After dewatering, the 50% fiber consistency sheet of internally wet strength impregnated fibers is run through a brush type disintegrator employing - clean 30 cm diameter wire brush rotating at 3420 rpm. The wet sheet is defibrated by brush contact to give a fibrous pulp containing 95% by weight of separated fibers and 5% small fiber bundles. The thus formed fibrous mass of internally impregnated wet strength resin fibers is dried and hardened in a curing tower by passing hot air while moving. The temperature of the curing tower is allowed to rise to 12o°C, and the fiber mass is held at this temperature for 3o min. to dry the internally wet strength resin impregnated fibers and cure the impregnated wet strength resin.i.

The dried fibers are removed from the drying tower. They thus

the dried fibers are a wet-strength resin-impregnated fiber product produced according to the present invention. As stated above, however, ■ the fiber pulp product in this manufacturing step contains approximately 5% fiber bundles which in some applications are considered to be separated and scrapped.

Should removal of the fiber bundles be considered undesirable,

the internally wet-strength resin-impregnated fibers slurried in . water at a consistency of 1% and the resulting slurry is fed to a Vibrotor screen using a screen with 0.0375 cm grooves. The waste that remains on the sieve during this screening mainly consists of fiber bundles and can be thrown away or used as fibers in products that have less stringent requirements.

The fibers that pass through the sieve are fibre-bundle-free internally wet-strength impregnated fibres. This fiber yield from the Vibrotor sieve is slurried in water at a fiber consistency of

2% and run out on a Fourdrinier machine to form a wet piece of sheet. This wet piece is non-adherent and is retained between cotton cloth shroud layers to enable its passage over drum dryers used to dry the internally wet strength impregnated fibers to a moisture content of 10%.

Internally wet strength impregnated fibers produced by the method in this Example I exhibit the physical properties given in Table I below.

The test results, indicated in Table I above, express

the obtained properties of the wet strength resin-impregnated fibers produced according to the method in example I. Fibers produced according to the method in example I and others produced according to the method of this invention are suitable and advantageous for the production of tissue sheets, nappy covers, nappy cores and paper towels.' Fibers prepared according to the process of Example I without removal of fiber bundles; and using either a polyacrylamide-polyamine-epichlorohydrin or dialdehyde-starch wet strength resin, will show similar enhanced characteristics.

EXAMPLE II

Internally wet strength resin impregnated twisted cellulosic fiber strips are prepared by first slurping Southern softwood bleached kraft fibers in water at a fiber consistency of 2%.

8%, based on the fiber weight in the slurry, "Casco PR 335"

(ureaformaldehyde) wet strength resin is added to the fibrous slurry in the form of a dilute aqueous solution containing 2% by weight resin solids diluted from a stock solution containing 35% resin solids. The slurry is stirred while adding wet strength resin and the stirring is then continued for 15 min. The pH of the slurry during the above-mentioned stirring period and a further 30 minute extraction period is 7.5.

After the 30 minute draw period, the pH of the fibrous wet strength resin slurry is adjusted to 4.2 by adding lo% sulfuric acid while stirring for lo min. After stirring, the acidic fibrous wet strength slurry is drawn for a further 2o min.

The wet strength resin impregnated fibers are recovered by leaving

deposit them on one sieve and let the aqueous medium drain away. After deposition on the screen, the resulting fibrous mat is dewatered on a drum drier to a consistency of 45% and then split into individual fibers using a mechanical rotary knife pulp beater and air stream.

The separated internally wet strength resin impregnated fibers are then dried and cured in an oven at 105°C for 16 hours.

The thus dried and cured separated wet strength resin impregnated fibers exhibit the low density or high "sheet loft" properties tabulated in Table II below, when tested as hand sheets with a basis weight of 3 g/loo cm 2 .

The results shown in Table II clearly demonstrate the improvement

in compression resistance, low density and resilience of the hand sheets made from fibers produced according to the method in Example II compared to hand sheets made from similar untreated fibers, wet strength resin impregnated fibers made according to the method in this example are suitable and advantageous for use in the manufacture of diaper paper, towels and tissue products. Wet strength resin-impregnated fibers made from hardwood

fibers according to the method in example II exhibit outstanding felting characteristics and are particularly suitable for the production of packaging materials and furniture fillings.

EXAMPLE III

Internally impregnated wet strength resin twisted cellulose fiber strips are prepared by first slurping undried Southern softwood bleached kraft fibers in water at a fiber consistency of 2%. 8%, based on the weight of fiber in the slurry, "Parez 6o7" wet strength resin solids is added to the slurry with stirring to an aqueous solution containing 5% by weight of wet strength resin solids. "Parez 6o7" is a melamine formaldehyde wet strength resin supplied by<!>American Cyanamid Co., tfayne, New Jersey. The slurry is stirred during the addition of wet strength resin and the stirring is then continued for 15 min. The pH of the slurry during the aforementioned 15 minute stirring period and during the subsequent 3o minute drawing period is 5.

After the 30 minute drawing period, the pH of the fibrous wet strength resin slurry is adjusted to 3.2 by additions of lo% sulfuric acid with stirring for lo minutes. After stirring, the acidic fibrous wet strength slurry is drawn for a further 2o min.

The internally wet-strength resin-impregnated fibers are recovered by deposition on a sieve, and the aqueous medium is allowed to drain away. After deposition on the sieve, the resulting fibrous pulp is dewatered on a drum drier to a consistency of 45% and separated into individual fibers using a mechanical rotary knife pulp beater and air current. The separated internally wet strength resin impregnated fibers are dried and cured in an oven at 105°C for 16 hours.

In order to form the dried product produced by the method in this Example III for testing, the thus wet-strength resin-impregnated fibers are redispersed in water in a TAPPI disintegrator in a mixture with 10% by weight of untreated cellulose fibers. The internally wet strength resin impregnated fiber-cellulosic fiber blend is formed into a wet handsheet with a basis weight of 1.952 g/loo cm . These wet hand sheets are wet pressed under varying loads and dried. The dry density measured under o.o56 kg/cm 2 load is tabulated in table III below. The results set forth in Table III above clearly show the low sheet densities even under pressure obtained by the fibers made according to the process of Example III compared to the sheet densities of sheets made with fibers having no wet strength resin impregnation.

EXAMPLE IV

Internally wet-strength resin-impregnated twisted cellulosic fiber tapes are prepared by first slurping undried Southern Softwood bleached kraft fibers in water to a fiber consistency of 2%. pH

of the fibrous slurry is adjusted to lo.3 by additions of lo% sodium hydroxide solution. 8%, based on fiber weight in the slurry, "Kymene 557" wet strength resin solids is added to the fibrous slurry with stirring as an aqueous solution containing 5% by weight of the wet strength resin solids. "Kymene 557" is a neutral curing, polyamide-polyamine-epichlorohydrin wet strength resin supplied by Hercules Powder Co., Wilmington, Delaware. The fibrous slurry is stirred during wet strength resin addition and stirring is then continued for 15 minutes. The pH in the fibrous slurry during the above-mentioned 15-minute stirring period and during a further 1-hour drawing period is kept at lo,3 as adjusted.

After the 1-hour drawing period, the pH in it is adjusted

fibrose the wet strength resin slurry to 7.5 by adding lo% aqueous acetic acid solution, with stirring for lo minutes. After rowing, it is pulled. fibrose the wet strength slurry for a further 2o minutes at a lower pH.

The internally wet-strength resin-impregnated fibers are recovered by deposition on a sieve; and the aqueous medium is allowed to drain away. After deposition on the sieve, the resulting fibrous mat is dewatered on a drum drier to a consistency of 45%

and is then split into individual fibers using a mechanical rotating knife pulp beater and air current which is operated so that the split fibers are carried away. The separated internally wet strength resin impregnated fibers are dried and cured in an oven at 105°C for 16 hours to develop fibers according to the present invention.

To form the dried fiber product according to this example

IV for testing, the internally wet strength resin-impregnated fibers thus produced are slurried in water in a TAPPI disintegrator to a fiber consistency of 0.1%, and handsheets are produced in a TAPPI handsheet mold with a basis weight of 3 g/loo cm 2 . The hand sheets are wet pressed under a 0.07 kg/cm<2> load and dried at 105°C for 2 hours.

The dried sheets are found to have a bulk density of 0.025 g/cm 3 , must be under a load of 0 kg/cm 2 . Under a o.o7 kg/cm 2 load, the dried sheets have a bulk density of o.o61 g/cm 3 .

In contrast, . the hand sheets produced by the same method from bulk fibers identical in all respects, but without internal wet strength resin impregnation found to have a bulk density of

o.o82 measured under o kg/cm 2 load/and exhibits a o.135 g/cm<2>bulk density under o.o7 kg/cm .

The experimental results obtained from hand sheets produced from fibres

made according to the method in this example IV shows, once again, the surprising ability of the present fibers to

induce bulk, resilience and compression resistance under load.

Since it will be obvious to those skilled in the art that modifications

of the methods described above and the new fiber products resulting from these can be carried out, it is not hereby intended to limit the thought and scope of the invention to the particular shown or described methods. In particular, the person skilled in the art will clearly understand that although the invention is described within the framework of cellulose fibers and wet strength resins of all types, other fibers with the ability to internally deposit resins can be produced with similar enhanced properties within the broad scope of the invention. Likewise, the fibrous products according to the invention can be used to produce pure or mixed cushions, mats, tissues, sheets and fibrous structures with a large range of physical properties according to the designer's wishes. The conditions used in the description of the invention are therefore used in their descriptive sense and not as limiting conditions, as it is intended that all<1>

equivalents thereof are covered by the framework for added requirements.

Claims (19)

Process for the production of internally wet-strength resin-impregnated cellulose fibers, characterized in that the steps consist of: (1) slurrying the cellulose fibers in water at a dispersed consistency to form a fibrous slurry; (2) adjusting the pH of the fibrous slurry to a higher cellulose fiber swelling inducing pH, (3) addition of wet strength resin during stirring and drawing to the fibrous slurry at higher pH, (4) reducing the pH of the fibrous slurry to a lower cellulosic fiber shrinking and wet strength resin polymerizing pH; (5) dewatering the fibrous slurry to a fiber separation promoting consistency and separating the fibers; (6) drying the separated fibers and curing the internally impregnated wet strength resin solids. 2. The method according to claim 1, characterized in that the dried and hardened internally wet strength resin-impregnated fibers are reslurried and sieved to remove fiber bundles. 3. The method according to claim 1, characterized in that the cellulose fibers are wood fibers. 4. Method according to claim 1, characterized in that the cellulose fibers are cotton fibers. 5. Method according to claim 1, characterized in that the wet strength resin solids are added in amounts of approximately 8 - 20% based on the weight of the fibers in the fibrous slurry. 6. The method according to claim 1, characterized in that the wet strength resin solids are urea formaldehyde resins. 7. Method according to claim 1, characterized in that the wet strength solids are melamine formaldehyde resins. 8. Method according to claim 1, characterized in that the wet strength resin solids are polyamide-polyamine-epichlorohydrin resins. 9. Method according to claim 1, characterized in that the cellulose fibers are slurried at a fiber consistency of around 1.5 - 2.5%. 10 . Method according to claim 1, characterized in that the total time for adding and drawing the wet strength resin at higher fiber swelling-inducing pH is about 45 min. to about 16 hours. 11. Method according to claim 1, characterized in that the total time for adding and drawing wet-strength resin at the lower cellulose fiber-contracting and wet-strength resin polymerizing pH is about 30 min. to about 16 hours. 12. Method according to claim 1, characterized in that the higher cellulose fiber swelling inducing pH is 7, the lower cellulose fiber contracting and wet strength resin polymerising pH is 4.5 and the wet strength resin is urea formaldehyde wet strength resin. 13. The method according to claim 1, characterized in that the higher cellulose fiber swelling inducing The pH is 5, the lower cellulose fiber shrinking and wet strength resin polymerizing pH is 3.2 and the wet strength resin is melamine formaldehyde wet strength resin. 14. Method according to claim 1, characterized in that the higher cellulose fiber swelling inducing pH is lo, the lower cellulose fiber contracting and wet strength resin polymerizing pH is 7 and the wet strength resin is polyacrylamide-polyamine-epichlorohydrin wet strength resin. 15. Internally wet-strength resin-impregnated cellulose fibers, characterized by a twisted cellulose tape fiber form, where the fibers lack hydrogen bond-forming ability, exhibit at least 80% copper ethylenediamine solubility, are refineable in conventional papermaking equipment and induce increased bulk absorption rate in fiber structures. 16. The internally wet-strength resin-impregnated cellulose fibers according to claim 15, characterized in that they exhibit bulk absorption velocities of o.7 - 1.1 cm/sec. in fiber structures. 17. The internally wet strength resin impregnated cellulose fibers according to claim 15, characterized in that the wet strength resins are urea formaldehyde wet strength resins. 18. The internally wet strength resin impregnated cellulose fibers according to claim 15, characterized in that the wet strength resins are melamine formaldehyde wet strength resins. 19. The internally wet-strength resin-impregnated cellulose fibers according to claim 15, characterized in that the wet-strength resin is polyamide-polyamine-epichlorohydrin wet-strength resins.