Polyolefin pulp and process for producing same

Abstract

A polyolefin pulp suitable for papermaking is described which is formed of a mass of discrete fibers formed of convoluted strands, the convoluted strands being twisted or wound-up film or sheet-like elements, the pulp having a drainage factor greater than 1.0 seconds/gram and a compressability constant (N) between about 0.3 and 0.4. Further, a process of manufacturing such fibers by forming a dispersion (mixture) of a solvent, a polyolefin, a water dispersing agent for the pololefin fibers to be formed and water and flashing the mixture through a nozzle. Water is present as a continuous phase in the mixture. The polyolefin is crystalline, or partially crystalline, preferably polyethylene, polypropylene, copolymers of ethylene and propylene, and mixtures thereof. The fibers thus formed can be easily refined and used for making paper webs.

Description

BACKGROUND OF THE INVENTION

It is known to make fibers of synthetic polymers by extruding melts or solutions thereof through spinnerets and chopping the continuous filaments thus formed into short lengths called staple fibers. Such fibers are characterized by having a very low gas absorption surface area (less than 1.0 m.sup.2 /gm) which causes a fast drainage rate and poor formation, and are generally not useful in making synthetic paper webs.

A method is described in U.S. Pat. No. 2,999,778 for preparing polymeric particles called "fibrids" which have utility in synthetic paper webs as a binder. This process comprises injecting a solution of polymer into a precipitant for the polymer under conditions of shear to thereby precipitate the fibrid particles. One difficulty with this process is that large volumes of solvent and precipitant must be employed, and where the solvent and precipitant are different materials, a solvent separation problem is presented. Also, the particles thus formed are of such a size that while useful as binder particles they have little utility as a substantial replacement for cellulose fibers in papermaking.

In U.S. Pat. No. 3,081,519 a process for forming continuous "plexifiliments" which can be chopped into staple lengths is described. Briefly, the process comprises forming a solution of the polymer and flashing the solution through an orifice under conditions such that all the solvent is vaporized and a continuous filament having an integral three-dimensional netowrk is formed. This process requires that sufficient energy be supplied to the solution to effect complete vaporization of the solvent and precipitation of the polymer. The product obtained can be spunbonded into a non-woven product; however, it requires chopping into staple lengths before it can be used for papermaking by conventional techniques. The fibrous network structure thus obtained is difficult to separate into discrete fibers by refining, and discrete fibers that are obtained upon refining are not very satisfactory for papermaking purposes because of the presence of unrefinable chunks of polymer material caused by the fused intersections in the integral three dimensional network.

A process similar to that of U.S. Pat. No. 3,081,519 is described in German Offenlegungsschrift No. 1,958,609. This latter patent states that at lower polymer concentrations discontinuous fibrous material may be obtained although there is no specific teaching of forming such fibers. Also, the economics of going to a very low polymer concentration is not satisfactory.

A process is described in British Pat. No. 1,262,531 which is similar to that of U.S. Pat. No. 3,081,519 but wherein a surface-active agent is employed in the polymer solution in order to form a product called a "microflake aggregation" which can be separated into individual microflakes by beating. The use of a surface-active agent even in small amounts becomes quite expensive and during formation of a paper web on a paper machine presents a foaming problem. Also, surface-active agents weaken paper sheets formed from blends with cellulose fibers, probably due to degradation of the cellulose bonds by the surface-active agent.

Furthermore, the energy requirements of the process are high as sufficient energy must be imparted to the polymer solution to effect complete vaporization of the solvent, and at the temperatures thus required there is a tendency for a plexifilamentary product to form.

A process of forming fibers is described in German Offenlegungsschrift No. 2,121,512 wherein an emulsion of a polymer solution in water is formed and flashed at a relatively high temperature and pressure through an orifice to form fibers. Water is present as a continuous phase and is preferably present in amounts greater than 200 volume percent of the polymer solution. Surface-active agents are preferably employed in order to form a stable emulsion. The emulsion particle size is a critical factor. This latter process requires the presence of a large amount of water which must be heated thus increasing the energy requirements of the system and at the high polymer concentration and high temperatures described there is a tendency toward plexifilament formation or the formation of fibrous materials which cannot be easily refined into fibers suitable for papermaking by the waterlaying technique. The presence of surface-active agents causes the same deficiencies previously mentioned. Also controlling the emulsion particle size within the critical limits set forth is a difficult task.

A process similar to that of German Offenlegungsschrift No. 2,121,512 is described in German Offenlegungsschrift No. 2,144,409. In this latter process, a smaller amount of water can be employed, generally approximately equal in volume to the volume of the polymer solution. Relatively high temperatures and pressures are described which tend to form a plexifilament or a fibrous product that is difficult to refine or separate into discrete fibers. The process requires the presence of a material which will infiltrate a portion of the water within the polymer solution particles. The present of such material in the large quantities described may detrimentally affect the strength properties of the resulting fibers, and the process generally suffers from the same deficiencies discussed previously with regard to German Offenlegungsschrift No. 2,121,512.

Processes are described in German Offenlegungsschrift No. 2,147,462 and U.S. Pat. No. 3,402,231 wherein a dispersion of molten polymer in water is prepared and flashed to fibers. In U.S. Pat. No. 3,402,231 no solvent is present and in German Offenlegungsschrift No. 2,147,461 either no solvent is present or a small amount of solvent is present. These processes suffer from the large difficulty of forming a dispersion of molten polymer in water, and also must generally start with a preformed polymer rather than a polymer solution or slurry in an organic solvent which is the usual direct product of a polymerization procedure.

Finally, in French Pat. No. 1,350,931 a process for preparing fibers is described wherein a dispersion of water in molten polymer is flashed. No solvent is present in the process, and the process suffers from the same deficiencies as previously described for German Offenlegunsschrift No. 2,147,461 and U.S. Pat. No. 3,402,231.

BRIEF DESCRIPTION OF PRESENT INVENTION

The present invention has as one of its objects providing a pulp of discrete polyolefin fibers that are especially suitable for making synthetic papers webs by the water-laying technique.

The present invention has as another of its objects providing a process for forming such a pulp.

Briefly, the pulp produced by the present invention can be easily formed into paper webs by the conventional water laying technique or other conventional techniques, either by itself or in admixture with conventional cellulose papermaking pulp. The pulp has a drainage factor greater than 1.0 second / gram and a compressability constant between 0.3 and 0.4.

The present process comprises forming a mixture of a polyolefin, a solvent for the polyolefin at elevated temperatures, a polymeric water dispersing agents for the polyolefin fibers to be formed and water at a temperature above the melt dissolution temperature of the polyolefin in the solvent and at a substantially autogeneous pressure, the water being present in a sufficient amount to form the continuous phase in the mixture, passing the mixture through a nozzle into a zone of lower pressure to form an aqueous slurry of fibrous polyolefin, and refining the aqueous slurry of fibrous polyolefin into a pulp of discrete fibers.

Various aspects of the invention are illustrated in the drawing herein:

FIG. 1 is a diagrammatic view of examplary apparatus which may be employed in the present invention to produce fibers.

FIG. 2 is a diagrammatical view of the apparatus for receiving such fibers as they are produced, separating the vaporized solvent therefrom and for beating or refining such fibers.

FIG. 3 is a cross-sectional view of a preferred precipitation nozzle as illustrated generally at 6 in FIG. 1.

FIG. 4 is a 500 times magnification of typical fibers of the invention; and

FIG. 5 is a 10,000 times magnification of the same typical fibers.

The fibers produced in accordance with this invention have average lengths between 0.5 mm and 10 mm (as measured by TAPPI Test T 232 SU 68) when prepared for use as a substitute for normal cellulose fibers. For speciality uses they may be prepared in average length longer than 10 mm and for some uses fibers having average lengths up to 100 mm or longer may be prepared. They have an average coarseness (as measured by TAPPI Test 234 SU 67) of between about 1 and 10 decigrex (mg/100 m).

After refining, the fibers of the present invention are of such a size that less than about 10 percent by weight of the fibers are retained on a 20 mesh Tyler Standard screen but at least about 25 percent by weight are retained on a 65 mesh screen and preferably at least about 25 percent by weight are retained on a 35 mesh screen. Typically, the weight average length of the fibers after refining is between about 1.4 and 3.0 mm, the weight average coarseness is typically between about 3.3 and 8.0 decigrex, and the weight average length to coarseness is typically 0.37 to 0.51:1.

The fibers of this invention are predominantly made up of sheet or film like elements which are rolled or twisted into convoluted strands (visible at 500 times magnification) having a diameter betwen about 0.5 and 30.mu. and having lenths similar to the fiber length. When congregated together in a mass or pulp these convoluted strands are mechanically entertwined with substantially no inter-strand bonds being present in distinction to the integral three dimensional network of plexifilaments and other products. There may however be some intra-strand interconnection as in the case where the orginal film or sheet was torn and the dangling portions rolled or twisted into separate strands with the untorn portion of the orignal film or sheet still connecting the strands.

Morphologically, the fibers of the present invention appear quite similar to those described in U.S. pat. application No. 257,609 (filed May 30, 1972). In addition to the convoluted strands or rolls visible at 500 times magnification, the fibers have a characteristic "shark-skin" or "pebble" texture at 10.000 times magnification. A large number of fibers from any given sample will exhibit grooves or valleys which extend in the direction of the roll or strand with wrinkles extending transversely thereto between the grooves.

The convoluted strand or roll structure may be seen in FIG. 4 which is a 500 times magnification of the fibers produced in Example 10, run 1. The shark-skin and pebbled appearance may be seen in FIG. 5 which is a 10,000 times magnification of the same fibers.

The surface area of these fibers may range from 2 to 150 m.sup.2 /g as measured by gas adsorption technique on freeze dried samples.

Paper-like sheets may be produced from these fibers having a tensile strength, both wet and dry, between 0.2 and 5 grams per denier.

While the fibers produced under this invention are produced as an entangled mass or pulp of the convoluted strands just described, these strands can be separated from one another since they are not bonded together, and in such cases these individual convoluted strands may be considered as "fibers" themselves.

While the fibers produced by the present process are similar morphologically to the fibers produced by the process described in aformentioned U.S. pat. application Ser. No. 257,609 they have several unique features which make them even more suitable for use in manufacturing paper. Many of these unique properties are related to the drainage or filtration resistance characteristics of these fibers.

While the surface area of the fibers of the present process, as measured by gas adsorption techniques, is quite similar to the surface area of the fibers produced in the aforementioned U.S. patent application, the present fibers have a more favorable hydrodynamic specific surface area. This latter parameter is more closely related to the drainage characteristic of fibers to be used in papermaking. The larger the hydrodynamic specific surface are the more fibrillated the fibers which leads to improved strength properties of the web or sheet produced therefrom due to the larger interfiber contact area which can be bonded. Unbeaten or unrefined cellulose fibers will typically have a hydrodynamic specific surface area of about 1.0 m.sup.2 /gm and, upon refining can be as high as 10 to 25 m.sup.2 /gm. The polyolefin fibers produced by the process of the aforementioned U.S. patent application Ser. No. 257,609 have a hydrodynamic specific surface area less than 1.0 m.sup.2 /gm, typically between 0.7 and 0.9 m.sup.2 /gm. The polyolefin fibers of the present invention have a hydrodynamic specific surface area much more like unbeaten cellulose fibers, i.e., greater than 1.0 m.sup.2 /gm and typically between 1.0 and 2.0 m.sup.2 /gm.

Determination of hydrodynamic specific surface area is determined in accordance with the procedures described in the article "The Filtration Resistance of Pulp Slurries". W. L. Ingmanson et. al., TAPPI 37, No. 11 : pp. 523-534 (1954). Equations 9 and 10 on pages 515 and 526 of this article were employed in determining the "hydrodynamic" specific surface area (S) discussed herein.

A further measure of the drainage characteristics of fibers is compressability constant (N) as determined from the slope of the curve obtained in making a logarithmic plot of c versus p in the relationship.

c = Mp.sup.N

wherein c is the apparent pad density in grams/cubic centimeters, p is the compacting pressure in grams per square centimeter, and M and N are compressibility constants. Reference is made to equation (8) on page 525 of the above cited TAPPI article.

The compressibility constant of N of cellulosic fibers is typically between 0.3 and 0.4 whereas the compressibility constant N of dacron, orlon and nylon staple fibers is typically between 0.2 and 0.3. What this means is that cellulose fibers are more compressible than synthetic staple fibers which provides improved bonding potential and stronger paper webs. The fibers produced by the aforementioned U.S. patent application Ser. No. 257,609 have a compressibility constant N similar to staple fibers (i.e., between about 0.2 and 0.3), whereas the polyolefin fibers produced by the present process have a compressibility constant N between 0.3 and 0.4 which is similar to cellulose fibers.

The drainage time of the fibers produced by the present process is also more favorable for papermaking than the drainage time of the fibers produced by the process described in the aforementioned U.S. pat. application Ser. No. 257,609. Drainage time is measured by introducing 400 ml of a 0.5 percent consistency slurry of fibers into the standard sheet mold described in TAPPI Test T 205 M-58 having a 150 mesh stainless steel wire screen in the bottom thereof and having water covering the screen prior to introduction of the fiber slurry. Water is added up to the mark in the sheet mold. The slurry is agitated by four up and down strokes of the standard stirrer. The valve on the sheet mold is opened and the water drained from the mold. The time between opening the valve and the first sound of air suction through the handsheet mat deposited on the forming screen is recorded on a stop watch and is reported as the drainage time in seconds.

The polyolefin fibers produced by the process of the aformention U.S. patent application Ser. No. 257,609 typically have drainage times of 5 to 6 seconds, whereas the fibers of the present invention have a higher drainage time of between 6 and 8 seconds. This slower drainage is desirable since it permits better formation to occur on a paper machine and is indicative of better fibrillation and hence better bondability.

A more accurate characterization of drainage characteristics than drainage time, and one that is highly correlated to the hydrodynamic surface area, is the drainage factor. The drainage factor for the present fibers is greater than about 1.0 and typically ranges up to about 3.0 seconds per gram or higher. The drainage factor of the fibers produced by U.S. patent application Ser. No. 257,609 is generally between about 0.2 and 0.9 seconds/gram. The fibers produced by the examples of German Offenlegungsschrift No. 2,121,512, even when treated to render them water dispersible, exhibit a drainage factor less than about 0.8 second/gram and as low as about 0.02 second/gram. Drainage factor is determined substantially in accordance with TAPPI Test T221 OS-63 with a slight modification in the method of calculation. Briefly, approximately ten grams of a fiber sample is weighed and dispersed in water. The slurry is then added to the standard sheet mold and water added to the mark. The slurry is stirred by four up and down strokes of the standard stirrer, which is then removed. The water temperature in the mold is measured and the drainage valve opened. The time between the opening of the valve and the first sound of suction noted. The procedure is repeated with water only (no fiber) in the sheet mold and the temperature and drainage time noted. The drainage factor in seconds per gram is then calculated as follows: ##EQU1## where DF = drainage factor, seconds/gram

D = drainage time with pulp in mold, seconds

d = drainage time without pulp in mold, seconds

V.sub.T = viscosity of water at temperature T

w = weight of fibers employed in test, grams

The quantity (1/V.sub.T - 1) is tabulated in the aforementioned TAPPI Test T221 OS-63. This quantity is multiplied by 0.3 which has been empirically determined for the present fibers.

It has been found that fibers having the desirable characteristics for papermaking just described can be made by the selection of certain specific process parameters. If these parameters are not observed, discrete fibers satisfactory for papermaking by waterlaying may not be obtained and, in many instances, only continuous filaments which are difficult or impossible to refine will be obtained.

Turning now to the process for producing the abovedescribed fibers a description of one suitable form of apparatus will first be described.

Autoclave 1, illustrated in FIG. 1, is equipped with stirrer 2 and valve 3 to supply inert gas or water for preparing the dispersion. The autoclave is jacketed whereby heating fluid may be used to heat the contents thereof. Autoclave 1 is also provided with tubular conduit 4 having an open end inside of the autoclave near its bottom, and extending therefrom to the exterior of the autoclave. At the outside end of conduit 4 is shut-off valve 5, constituting a ball cock valve, to which is connected precipitation nozzle 6. As seen more clearly from FIG. 3, precipitation nozzle 6 constitutes a section of tubular conduit 4 and being connected therewith through valve 5. Precipitation nozzle 6 in turn is connected through post-precipitation transfer conduit 7 to vaporization vessel 8 as depicted in FIG. 3. Vaporization vessel 8 constitutes a cyclone having a conduit 9 for removal of vaporized solvent and a conduit 10 through which fibers may drop to disc refiner 11; alternatively, other commonly available attrition or beating mills may be used. Vaporization vessel 8 is also equipped with spray means 12 for spraying water onto the fibers discharged into such vessel. The sprayed water is desirably at a temperature sufficiently high enough not to cause condensation of the solvent vapors moving upwards to conduit 9.

Autoclave 1 is also provided with a discharge valve 13, constituting a ball cock valve, at the bottom thereof to which is connected a short precipitation nozzle 14.

In general operation of this invention using the exemplified apparatus the polymer and the solvent therefor may be introduced into autoclave 1 and the polymer dissolved in the solvent by heating and stirring. Water may then be introduced with stirring to form a dispersion with the polymer solution as the discontinuous phase and the water as the continuous phase. A water dispersing agent for the fibers to be formed is also added to the contents of the autoclave; most advantageously the agent is added with the water. Alternatively, the water may be added first followed by addition of the solution or solvent and polymer thereto.

The pressure maintained in the vessel is substantially autogeneous. If substantially higher pressures are employed poor fiber formation results, i.e., fibrous material is formed which is difficult to refine into satisfactory papermaking fibers.

The dispersion thus formed is maintained under pressure tight conditions in the autoclave and heated to a temperature sufficiently high to maintain the polymer dissolved in the solvent, but no higher than about 160.degree.C. If the temperature exceeds about 160.degree.C there is a tendency for a fibrous product to be formed which cannot be easily refined into fibers suitable for papermaking. Preferably, the temperature is between about 130.degree.C and 160.degree.C. The solvent selected must be stable at these temperatures. Thereupon shut-off valve 5 may be opened and by the pressure head inside the autocalve the dispersion therein will be forced rapidly through conduit 4 and thence through precipitation nozzle 5. During passage of the dispersion through conduit 4 and precipitation nozzle 5 the pressure on the dispersion becomes reduced which thus causes violent vaporization of the solvent. The loss of heat effected by this vaporization causes the temperature of the dispersion to drop. This drop in temperature lowers the solubility of the polymer in the solvent and the loss of solvent through vaporization also decreases the amount of polymer that may remain dissolved. Consequently the polymer precipitates as fibers as the dispersion passes through conduit 4 and out precipitation nozzle 6. Agitation of the dispersion is maintained throughout the operation as otherwise the solution phase and water would rapidly separate.

Upon discharge from precipitation nozzle 6 through conduit 7 into vaporization vessel 8, which is maintained at a pressure substantially lower than that existing in conduit 4 and preferably at atmospheric or subatmospheric pressure, substantially all of the solvent vaporizes leaving the fibers dispersed as a pulp in the water. The pulp may be in the form of a "noodle" of fibers losely aggregated together. Free solvent vapor is removed via conduit 9 and may thereafter be condensed for reuse. Desirably a spray of water is introduced through spray means 12 onto the dispersed fibers to inhibit agglomeration of the fibers and to facilitate refining thereof. The temperature of the spray should be high enough to avoid condensation of the vaporized solvent.

The fibers are then beaten or refined to reduce or adjust the fiber length of the material as to desired to give an appropriate fiber length distribution and degree of fibrillation for the particular desired end use.

Following formation of the fibers they may be treated with an additional amount of water dispersing agent. This optional treatment is supplemental to the addition of the water dispersing agent to the dispersion prior to flashing. This may be conveniently done by incorporating the agent in the water which may be sprayed on the fibers by spray means 12 in the vaporization vessel 8, or by directly adding the agent to the fibers in the refiner.

For some uses of the fibers of this invention, such as their use in non-wovens, textile thread manufacture, insulation material, oil absorption material, etc., it may not be necessary to beat or otherwise cut the fibers.

Any crystaline polyolefin may be used in accordance with the present invention to form fibers provided that a suitable solvent may be found to dissolve the polymer. Of particular importance are polyethylene and crystalline or predominantly crystalline polypropylene, ethylene propylene copolymers, and mixtures thereof. Additionally, polybutenes, polymethyl pentenes, may be desirable polymers in the practice of this invention.

Polyethylene is the preferred polyolefin employed in the present invention, and desirably is a low pressure polyethylene having a viscosity average molecular weight range of 20,000 to 2,000,000. The most advantageous molecular weight range for polyethylene is found to be between about 25,000 and 200,000 as this material has the viscosity and other properties which permit the most economical manufacture of good quality fibers under this invention. The preferred polyethylene has an intrinsic viscosity between about 0.85 and 35 and most desirably between about 1.0 and 5.3.

For polypropylene, the viscosity average molecular weight is preferably between 100,000 and 4,000,000 and most desirably betwen 140,000 and 650,000. The preferred polypropylene has an intrinsic viscosity between about 1 and 20 and most disirably between about 1.3 and 4.3 for the same reasons indicated for the most desirable polyethylene range.

The viscosity average molecular weights referred to herein are determined by first measuring the specific viscosity of the polyolefin in decolin at 135.degree.C, using Ubbelohde No. 50 or 75 viscometers. The viscosity average molecular weight is then determined by the relationship:

(.eta.) = K M.sub.v.sup.a

(.eta.) = intrinsic viscosity, and is determined from specific viscosity by the Schultz and the Blaschke equation.

K = constant, from literature (2.74 .times. 10.sup.-.sup.4 for polyethylene)

a = constant, from literature (0.81 for polyethylene)

The polyolefin or other polymer employed in practicing the present process may have been preformed, i.e., previously prepared in the form of dried powders or pellets, or, preferably, is prepared as an integral part of the present process. It is preferred to prepare the polyolefin solution by a solution polymerization process. Alternatively, a slurry process may be employed and the slurry heated above the melt dissolution temperature to effect solution.

Generally, the solvent may be selected from any substituted or unsubstituted aliphatic, aromatic or cyclic hydrocarbon which is a solvent for the polymer employed at the temperatures utilized in the process and which does not decompose at the temperatures utilized, which is relatively inert under the conditions of operation and which is substantially immiscible in water or forms a polymer solution which is substantially immiscible in water. The solvent should have a boiling point at atmopsheric pressure less than the softening point of the polyolefin and deisrably in the range of about 30.degree.C to 120.degree.C. for polyethylene and polypropylene. The solvent may be liquid or gaseous at room temperature and atmospheric pressure but preferably is liquid. Among the solvents which may be utilized are aromatic solvents, e.g., benzene and toluene; aliphatic hydrocarbons, e.g., pentane, hexane, heptane, octane and their isomers and homologues, alicyclic hydrocarbons, e.g., cyclohexane, cyclohexene and methycyclohexane; halogenated hydrocarbons, e.g., chlorobenzene, carbon tetrachloride, chloform, ethyl chloride, methyl chloride; alcohols; esters; ethers; ketones; nitriles; amides; fluorinated compounds e.g., fluorohydrocarbons; sulfur dioxide; nitromethane; and mixtures of the above solvents.

One of the features of the present invention is that it has been found that it is not necessary to form a stable "emulsion" thereby eliminating the necessity for emulsifiers to be used. However, the present invention does contemplate the employment of agents in the mixture to impart water dispersibility to the fibrous polymer. These agents are preferably water-soluble or partially water-soluble polyhydroxylated, polymeric materials which are substantially non-foaming in aqueous slurries at the concentrations employed. By "polymeric" we mean polymers preferably having a molecular weight in excess of about 1000. By "polyhydroxylated" we mean polymers having numerous hydroxyl groups distributed along or pendent from the polymer chain rather than merely terminal hydroxyl groups. However, polymers having other hydrophilic moieties such as amine groups, acid groups and salts and esters thereof distributed along or pendent from the polymer chain may be employed. Some of these agents may be technically classified as "emulsifiers", but they are employed in an amount sufficient to impart the requisite degree of water dispersiblity to the fibers to be formed and not in the amounts generally required to form a stable emulsion. The agents are desirably substantially non-foaming in aqueous slurries at the concentration employed while most emulsifiers cause foaming if employed in an amount sufficient to impart a satisfactory degree of water dispersibility to the fibers, and they are preferably polymeric materials since such materials resist removal from the fibers in squeous slurries.

The amount of water-dispersing agent employed may range from about 0.2 to about 15 percent by weight preferably from about 0.1 to about 5 percent by weight, and most preferably between about 0.7 and about 2.5 percent as shown by the examples below.

The preferred water-dispersing agent is a water soluble polyvinyl alcohol having a degree of hydrolysis greater than about 77 percent and preferably greater than about 85 percent, and having a viscosity (in a 4 percent aqueous solution at 20.degree.C) greater than about 2 centipoises. The polyvinyl alcohol is preferably added with the water as a solution therein at the time the mixture is formed. Illustrative of other water-dispersing agents that may be employed are polyacrylic acid, polyacrylates, gelatin, casein, cationic guar gum, cationic starch, potato starch, cellulose derivatives such as carboxyethyl and carboxymethyl polymeric amines and Lytron 820 (a stryrene-maleic acid copolymer)

The phase "water-dispersing agent", as stated above, means an agent which renders the polyolefin fibers water-dispersible. Since polyolefins are normally hydrophobic, fibers made therefrom cannot be employed in papermaking by the water-laying techniques unless they are rendered water-dispersible by the agent.

To measure the extent to which fibers are dispersible in water, a "dispersibility index" may be measured. To obtain a numerical value for the dispersibility index, 2 grams of fiber (dry weight) is dispersed in 400 ml water (total volume) in a Waring Blender at top speed for 5 seconds. The resulting fiber slurry is placed in a 500 ml graduated cylinder, inverted four times and placed on a flat table top. The volume of clear water under the fiber slurry is recorded after 10, 20, 30, 40, 50, 60, 80, and 120 seconds. The values are summed, and the sum divided by 4 to give the dispersibility index. The lower the number, the better dispersibility. To form an adequate sheet from the fiber by conventional papermaking water laying techniques, it is desirable that the dispersibility index number be below 350, and, preferably, below 300. The fibers of the present invention typically have a dispersibility index well below 300.

While it is not necessary to the present invention to add a surfactant to the dispersion to form a `stable` emulsion, a surfactant may optionally be added in addition to the water dispersing agent previously discussed. Such surfactants may be nonionic, cationic or amphoteric and are preferably of the type that enchance an oil-in-water type emulsion i.e., ones having a relatively high (greater than about 7.0) HLB (hydrophilic-lipophilic balance) value. If such a surfactant is employed and the fibers produced are to be used for making paper webs by the water laying technique, it is desirable to remove the surfactant from the fibers prior to web formation by washing in order to prevent foaming.

The polymer, solvent and water dispersing agent, selected as described above, should provide, upon sufficient agitation, a uniformly dispersed solution phase (solvent plus dissolved polymer) in the water phase (preferably containing the water-dispersing agent dissolved therein) at the temperatures and pressures of operation.

For practical operation utilizing preformed solid polymer the polymer is first dissolved in the solvent utilizing heat and agitation where necessary. The water is then added with agitation to form the dispersion. The water-dispersing agent is conveniently incorporated into the water prior to its addition to the dissolved polymer. Alternatively, the solution may be added to the water.

However, notwithstanding the batch process embodiment previously exemplified, it is preferred and advantageous to carry out this invention in a continuous process together with the polymerization process for the polymer. Polymerization of the polymer may be carried out by conventional means in a separate vessel. If a solution polymerization process such as that used for polyolefins is employed, then the resulting polymer solution may be fed directly to a dispersion forming essel or autoclave, with the adjustment of the solution concentration and treatment of the residual polymerization catalyst as may be necessary. The water may be added to the dispersion forming vessel, in the manner previously described, on a continuous basis with stirring to maintain the dispersion. The dispersion forming vessel may be heated to maintain the dispersion at the desired temperature and pressure for discharge to a zone of lower pressure to form fibers in the same general manner as described for the batch process embodiment.

If a slurry polymerization process, such as that used for polyolefins, is employed, the above described process may be employed with the desirable modification that the polymer suspension formed is heated to form a polymer solution prior to its introduction into the dispersion forming vessel, preferably by feeding the slurry from the polymerization vessel to an intermediate heated vessel prior to feeding to the dispersion forming vessel.

As a further modification, the dispersion ingredients may be mixed by in-line mixing devices prior to the nozzle; preferably the polyolefin solution and water containing the water dispersing agent would be the two streams brought together.

The viscosity of the polymer solution is a variable which has an important influence on character and quality of the fibers produced, such as their length, thickness and degree of entanglement. The viscosity of the polymer solution is related to both the concentration of polymer and its molecular weight. The viscosity of the polymer solution increases with the concentration and the molecular weight (or intrinsic viscosity) of the polymer selected; therefore the viscosity can be adjusted by appropriate selection of polymer molecular weight and by adjusting the concentration of the polymer solution.

The viscosity of the solution should be low enough so that the solution may be conveniently formed into a dispersion with the water. If the viscosity is too high a good dispersion is difficult to form, flow problems are encountered and the product may be a thick ropy mass rather than the desired fibrous material. For satisfactory operation the viscosity of the polymer solution under the temperature and pressure of operation with polyethylene as the polymer is desirably below 3500 centipoises for longer nozzles and desirably below 500 to 1000 centipoises for shorter nozzles. The viscosity of the solution should be greater than about 100 centipoises in order to form thin, well fibrillated fibers of polyethylene.

Fibers may be produced when the polymer employed is 0.5 percent by weight of the solvent or even lower. However it is generally desirable to utilize concentrations higher than 0.5 percent by weight because the properties of the fibers are generally better when a more concentrated solution is employed. Additionally, higher polymer solution concentrations are more economical because the amount of solvent required for production of a given amount of fiber is less and the amount of heat required for vaporization thereof is correspondingly decreased. In general the concentration may be selected from the range of 0.5 to 15 percent by weight and preferably between 3.5 and 15 percent by weight. The practical upper limit of concentration is determined based upon solution viscosity as previously mentioned. The preferred concentration range for polyethylene and polypropylene is between 25 and 100 grams per liter of solvent, with a concentration between around 50 and 100 grams per liter being especially useful.

Another variable which greatly influences the character of fibers, particularly the length, thickness and strength of the fibers and the extent to which they are entangled, is the ratio of solvent to water which is employed. For a given polymer, polymer concentration in the solvent and conditions of precipitation, the higher the ratio of solvent to water, the longer, thicker and more entangled are the fibers produced. In some cases if the ratio of solvent:water is too high and at the same time where the polymer solution is above certain concentrations, then the product which is produced may be so thick and entangled that it is difficult to refine or otherwise treat them in order to produce fibers having desirable properties. If the ratio of solvent to water is too high, the fibers produced may be undesirably weak utilizing certain polymer concentrations, and the process also becomes uneconomical.

In general the ratio of solvent to water on a volume basis may be selected from the range of 0.5 : 1 to 2 : 1, preferably between about 0.5 : 1 and 1 : 1. The ratio of solvent to water is desirably low enough so that the water contributes sufficient sensible heat (enthalpy) to the dispersion so that the total sensible heat of the dispersion is adequate to vaporize substantially all of the good solvent upon flashing, at the temperature and pressure differentials employed in the flashing.

Yet another variable which affects the character of the fibers produced relates to the conditions under which precipitation of the fibers takes place. The nozzle through which the dispersion is discharged must provide a constriction on the flow of the dispersion therethrough to establish adequate shear stress in the dispersion so as to aid orientation of the polyolefin molecules. The minimum shear stress required to produce adequate fibers is dependent upon a number of variables, including the type and molecular weight of the polymer, concentration of the polymer solution and the ratio of solvent to water, as previously discussed. The shear stress can be adjusted by appropriate selection of the therethrough. size, e.g., diameter if a circular configuration is used, and length of the precipitation nozzle and any associated conduit communicating therewith which imparts shear action on the suspension discharged therfethrough.

For polyolefins which crystallize more rapidly, such as high density polyethylene, a relatively shorter period of shear stress may be employed. In the case of polyethylene, for example, fibers may be produced simply by discharging the dispersion directly through a circular nozzle 2 millimeters in diameter and 2 millimeters long, as for example through the precipitation nozzle at the bottom of autoclave 1 in the exemplified apparatus. In fact sufficient shear stress may be produced simply by throttling the dispersion through a partially opened valve having an annular port.

On the other hand for a polymer which crystallizes more slowly, such as polypropylene, it may be necessary in order to produce desirable fibers to discharge the dispersion through a fairly long shear zone such as small, long conduit 4 in the exemplified apparatus and then through a yet smaller shear zone such as precipitation nozzle 6 in order to creat sufficient shear force or turbulence on the polymer prior to and during its precipitation. In the depicted apparatus there is a substantial pressure drop along conduit 4 during discharge. However there should still be a substantial pressure drop across nozzle 6, perphas 5 atmospheres or higher, in order to maintain an adequately high temperature in conduit 4 to prevent premature precipitation of the polymer on the walls of the conduit or of nozzle 6.

The temperature of the dispersion in the vessel should be maintained high enough so that when it is discharged rapidly through the precipitation nozzle into the zone of reduced pressure substantially all of the solvent will vaporize but the temperature employed should not be so high as to cause any substantial vaporization of the water. Additionally the pressure in the vessel should be high enough to force the dispersion through the precipitation zone with sufficient velocity to create adequate turbulence and shear action to form desirable fibers. The autogeneous pressure of the confined vapor of the dispersion is the preferred pressure employed. Preferably the pressure is between 6 and 15 kg/cm.sup.2 .

In the case of polyethylene and polypropylene in hexane as the solvent and with water as the continuous phase, the temperature in the vessel is desirably maintained between 130.degree.C and 160.degree.C, preferably about 140.degree. to 150.degree.C, prior to discharge of the dispersion through the precipitation zone. The autogeneous pressure developed by the dispersion at this temperature range creates the most desirable pressure to force the dispersion through the precipitation zone, such as through conduit 4 and nozzle 6, at a velocity and residence time in the nozzle sufficient to create adequate shear stress for good fiber production.

If a pressure substantially higher than autogeneous (say, greater than about 20 Kg/cm.sup.2) is employed the fibers are not as suitable for papermaking probably due to the high velocity and low residence time in the nozzle. It should be noted that the "autogeneous" pressure may include a small partial pressure developed by residual monomer if the process is integrated with the polymerization process. The sensible heat in the dispersion at this temperature range is also adequate to vaporize substantially all of the good solvent when the dispersion is discharged into atmospheric pressure. The pressure is desirably maintained constant during flashing by introduction of an inert gas such as nitrogen into the vapor space over the dispersion in the vessel.

Where a precipitation nozzle of relatively small cross-section is employed at the end of a larger cross-section shear zone conduit, it is desirable that the pressure of the dispersion just prior to entry into the nozzle is maintained high enough so that the temperature of the dispersion is above the dissolution temperature of the polymer so that it does not precipitate prematurely on the walls of the conduit. In practice the pressure is usually 5 atmospheres and higher for polyethylene and polypropylene.

The pressure in the zone of reduced pressure (e.g., varporization vessel 8) should be low enough so that the temperature of the dispersion upon flashing falls below the melting or softening point of the polymer. This pressure is usually atmospheric pressure or lower, preferably about 1 kg/cm.sup.2 . The dispersion may be flashed into the atmosphere or into a gas, preferably an inert gas such as nitrogen. Optionally but less desirably if the pressure gradient is great enough the dispersion could be flashed directly into a liquid maintained under low pressure and at a temperature above the boiling temperature for the solvent but below the boiling temperature for water and the softening temperature of the polymer.

Thus in the exemplified embodiment the fibers are discharged or flashed into vaporization vessel 8 which is at atmospheric pressure and substantially all of the solvent vaporizes and passes out through conduit 9 leaving the fibers dispersed in the water. The water intimately contacts the surfaces of the fibers and advantageously prevents fusing or sticking together of the fibers so that they remain dissociated in a loose mass. The fibers thus remain in discrete form as contrasted to the tangled and fused product which results from prior art melt spinning or solvent spinning (e.g., plexifilaments) of the prior art.

The water not only has the ability to favorably affect the resulting product as just mentioned but it provides additional benefits as well. It is believed that during discharge of the dispersion the water assists in crystal orientation and fiber development during and prior to precipitation of the polymer. This occurs possibly by the separation of the dissolved polymer into individual drops or globules which may more readily permit formation of separate, independent fibers when they are subjected to shear stress. The use of water, because of its high density in comparison with most solvents, may also enhance the turbulence and shear forces acting on the drops of dissolved polymer during discharge, thereby enchancing orientation and fiber formation.

Additionally, the sensible heat energy or enthalpy of the water at the elevated temperatures of operation is available to assist in the evaporation of the solvent during discharge. Therefore the water, which has high enthalpy, permits the use of lower temperatures prior to discharge or flashing while still effecting vaporization of substantially all of the solvent upon discharge. The ability to use lower pressures provides the corresponding ability to use lower pressures prior to discharge with the attendent economies of lower pressure operation. Also, the water lowers the temperature of the fibers after flashing due to the phenomenon of the water-solvent mixture having a lower boiling point than the solvent or water alone.

The lower the ratio of solvent to water employed, the more pronounced is this effect and even lower temperatures of operation may be possible. Lower solvent to water ratios are therefore preferred, e.g., below 1 : 1 for polyethylene, where this is consistent with other variables affecting the fiber properties desired. Also, the water has a "steam stripping" type of effect during vaporization of the solvent which tends to decrease the temperature of the fiber mass during flashing so that it may more readily be decreased below the softening point of the polymer.

One of the most important variables in controlling the fiber length is the ratio of solvent to water. For a given polymer and polymer solution concentration, the higher the ratio of solvent to water, the longer are the fibers that result. The lower such ratio is, the shorter are the fibers.

The viscosity of the polymer solution is another variable affecting fiber length and this is related to the nature of the polymer and its concentration in the solvent. The higher the molecular weight and the higher the polymer concentration in the solvent of the polymer, the longer the resulting fibers. Polymers that crystallize more rapidly, such as polyethylene, tend to produce shorter fibers under similar conditions of viscosity. In fact, for polypropylene under most operating conditions for rpoducing fibers of desirable properties, the fibrous product from flashing is a substantially continuous complex of fibrous material. On the other hand, polyethylene produced under similar conditions results in much shorter fibers.

Also, the higher the shear forces during discharge the longer the fibers that result. Therefore the factors controlling shear force, i.e., the pressure gradient, temperature of operation, the size and configuration of the discharge conduit and nozzle, may also be adjusted as previously discussed to assist in fiber length control.

The fibers, dispersed in the water after flashing from the zone of reduced pressure, may be dried to a lower moisture content and used without further treatment for uses not requiring carefully controlled fiber length. Such uses include molding pulp use, use as non-wovens, as an absorption or insulation material or the like.

It is particularly advantageous for manufacture of a paper pulp under this invention to adjust operating conditions to produce a pulp having a fiber length somewhat longer than the actual fiber length desired, for example average length of 5 to 10 mm or even up to 100 mm or longer. This product can then be beaten or cut to the exact fiber length and fiber length distribution to correspond to natural cellulose pulp or to such other length as may be desired. In this manner the properties of the final product can be more exactly controlled. In some cases, as with polypropylene, it may be desirable even to produce a fiber product of continuous nature and beat or cut this product to a length resembling cellulose fiber length in order to provide the strongest fibers. In such instances it is most important to carry out the refining operation upon the fibrous product immediately after flashing and while it is still at an elevated temperature, preferably above 50.degree.C, initially, and usually around 60.degree.-70.degree., initially. This is because once the material has cooled it is impractically difficult to refine or cut because it becomes tough and fused together. The initial refining may be followed by additional refining at room temperature.

The invention will be illustrated by specific embodiments set forth in the following examples, but the scope of the invention to be protected is not limited thereto.

EXAMPLE 1

To a SUS-made 5 liter autoclave of the general type depicted in FIG. 1, equipped with a stirrer and a jacekt through which steam at 10 kg/cm.sup.2 is introduced to heat the autoclave was added with stirring to dissolve the polymer prior to addition of the water and water dispersing agent.

______________________________________ polyethylene (molecular weight 28,000 and melt index 14). (Trade name: HiZex 1300J) 100 grams water 2 liters n-hexane 1 liter polyvinyl alcohol, (degree of saponifi- cation 86.5-89 mol %, degree of poly- merization above 1500 and viscosity at 4% water solution of 20.degree.C is 30 centipoise). (Trade name: GH-17) 2.5 grams non-ionic surfactant (alkyl phenol ethylene ether). (Trade name: Nissan Nonion NS 210) 3 grams ______________________________________

The mixture was stirred and heated to 140.degree.C after the atmosphere within the system was replaced with nitrogen. Polyethylene was dissolved completely by maintaining the above temperature for 30 minutes. The viscosity of the solution was 100 centipoises. Thereafter, water, PVA, and surfactant were introduced and the mixture was kept at this temperature for 30 minutes resulting in a homogenous suspension. This suspension was flashed to the atmospheric pressure through conduit 4, a copper pipe, and cock valve 5. However, no nozzle, such as nozzle 6 was employed. Flashing was continued for about 15 seconds until the pressure inside the autoclave dropped from 11 kg/cm.sup.2 -g to 1 kg/cm.sup.2 -g. The resulting polymer fibrous materials were separated from vaporous n-hexane in the vaporization vessel. The pressure inside the vaporization vessel was maintained at the atmospheric pressure and its temperature was 80.degree.C at the end of the flashing. The polymer fibrous materials thus produced contained residual hexane of less than 0.5 percent and was refined for 15 minutes by a Waring blender, resulting in beaten pulp. This pulp fiber showed a strength of 3 g./d.

COMPARATIVE EXAMPLE 1

To the same apparatus as used in Example 1, were added only

polyethylene (Trade name: Hi-Zex 1300J) 105 grams and n-hexane 3 liters

The content was stirred and the atmosphere inside the system was replaced with nitrogen followed by heating at 140.degree.C. The viscosity of the solution was less than 100 centipoises. After maintaining at the same temperature for 30 minutes, the content was transferred by being flashed to the atmospheric pressure into a 10 liter autoclave through conduit 4 and cock valve 5 (but without nozzle 6). Hexane was then filtered out by centrifugal separation and the polymer was washed with four portions of 1 liter acetone, then with four portions of 1 liter water and centrifuged, resulting in an aqueous slurry substantially free of hexane. It was then beaten for 15 minutes by the Waring blender, resulting in beaten pulp. This synthetic pulp had the strength of only 0.6 g./d and was incapable of being hand-sheeted as it was.

EXAMPLE 2

______________________________________ polypropylene (molecular weight 240,000, melt index 12.0 isotacticity index 96.5 and intrinsic viscosity = 1.9) (Trade name : Mitsui Sekiyu Kagaku, Polypro F 707) 100 grams Water 2 liters n-hexane 1 liter polyvinyl alcohol (Trade name: Gosenol GH - 17) 2.5 grams and non-ionic surfactant (Trade name: Nissan Nonion NS 210) 3 grams ______________________________________

were processed as in Example 1 except that nozzle 6 was employed, and flashed at 140.degree.C resulting polymer fibrous material was refined for 15 minutes by the Waring blender. The synthetic pulp revealed a strength of 2.5 g./d.

COMPARATIVE EXAMPLE 2

Polypropylene (Trade Name: Misui Sekiyu Kagaku Polypro F 707) 30 grams and n-hexane 3 liters

were treated as in Comparative Example 1, flashed, centrifuged and the hexane slurry was replaced with water slurry according to the procedure of Comparative Example 1. After refined for 15 minutes by the Waring blender, a synthetic pulp was obtained. The resulting pulp was fragile like glass fibers having a strength of only 0.2 g./d. and unable to be hand-sheeted as it is.

EXAMPLE 3

______________________________________ 4-methyl-1-pentene homopolymer (isotacticity index in heptane 98.6, .eta. = 12) 100 grams water 1 liter benzene 2 liters polyvinyl alcohol (Trade name: Gosenol GH-17) 2.5 liters and non-ionic surfactant (Trade name: Nissan Nonion NS 210) 2.5 grams ______________________________________

were processed as in Example 1 and flashed at 140.degree.C. The resulting polymer fibrous materials were refined for 5 minutes by the Waring blender. The resulting product revealed a strength of 1.0 g./d.

EXAMPLE 4

______________________________________ Polypropylene (Trade name: Mitsui Kagaku Polypro F 707) 70 grams polystyrene (Trade name: Styron 666) 30 grams and hexane 1 liter. ______________________________________

were heated to and maintained at 150.degree.C for 30 minutes to dissolve. Then 3 grams of a surfactant (Trade name: Nissan Nonion NS 210) and 2.5 grams of polyvinyl alcohol (Trade name : Gosenol GH-17) dissolved in 2 liters of water were introduced to the solution with pressure and stirred for 30 minutes to obtain a suspension which is then flashed to the atmospheric pressure. The resulting fibrous material was refined for 10 minutes by the Waring blender to produce synthetic pulp. The pulp showed a strength of 2.5 g./d.

EXAMPLE 5

______________________________________ Polyethylene (Trade name: HiZex 1300J) 100 grams water 2 liters n-hexane 1 liter and polyvinyl alcohol (Trade name : Gosenol GH-17) 1 gram ______________________________________

were treated as in Example 1 and flashed at 140.degree.C. The viscosity of the solution was about 100 centipoises at 140.degree.C. The resulting fibrous material formed a collection of fibers slightly harder than the product of Example 1. After refining for 30 minutes by the Waring blender, it showed a strength of 2.1 g./d.

EXAMPLE 6

Polyethylene (Trade name: HiZex 1300 J) 100 grams water 2 liters n-hexane 1 liter and gelatine (animal gelatin from Nitta gelatin) 3 grams

were treated as in Example 1 and flashed at 140.degree.C. The viscosity of the solution was about 100 centipoise at 140.degree.C. The resulting fibrous material was refined by the Waring blender. The resulting fibers showed a strength of 3.0 g./d.

EXAMPLE 7

Polyethylene (Trade name: Hi-Zex 1300 J) 70 grams Calcium carbonate powder (Trade name: Homocal-D) 30 grams n-hexane 1 liter water 2 liters Polyvinyl alcohol (Trade name: Gosenol GH-17) 2.5 grams non-ionic surfactant (Trade name: Nissan Nonion NS 210) 3 grams

were treated as in Example 1 and flashed at 140.degree.C. The viscosity of the solution was about 100 centipoises at 140.degree.C. The fibrous material thus obtained was refined by the Waring blender, resulting in synthetic pulp containing about 27 percent by weight of calcium carbonate. The pulp revealed a stength of 1.0 g./d. This example illustrates that modifying agents such as pigment may be added to the dispersion to alter the character of the resulting fibers.

COMPARATIVE EXAMPLE 3

______________________________________ Polyethylene (Trade name: Hi-Zex 1300 J) 100 grams water, and 2 liters n-hexane 1 liter ______________________________________

were processed as in Example 1 and flashed at 140.degree.C. through a cock valve with an opening diameter of 6 mm as shown at 13 in FIG. 1 but without nozzle 14. After flashing was completed, the autoclave was checked to find the side wall of the autoclave and the stirrer were covered by hard polyethylene adhered to the surfaces. The resulting fibrous materials were refined in the Waring blender for 30 minutes but the product contained many particles and the product was not practical for use as a synthetic pulp. This example illustrates the production of fibers without use of water dispersing agent prior to flashing.

EXAMPLE 8

Polyethylene (120,000 mol.weight) 50 grams n-hexane 1 liter water 2 liters polyvinyl alcohol (Gosenol GH-17) 0.5 grams

To the 5 liter autoclave as depicted in FIG. 1 the polyethylene and n-hexane were added with stirring and heat to dissolve the polymer. The viscosity of this solution at 140.degree.C was 200 centipoise. Water containing PVA was then added and the system flushed with nitrogen. These materials were heated with stirring for 30 minutes to form a uniform dispersion at a temperature of 140.degree.C. The dispersion was then discharged through conduit 4 and flash nozzle 6. Conduit 4 had an internal diameter of 7 mm and a length of approximately 6 meters and nozzle 6 had a diameter of 3 mm and a length of 21 mm.

The fibers were collected without refining and inspected. These fibers had an average length greater than 100 mm and some were substantially continuous. They constituted very strong small hollow tubes having average diameters ranging from 30 to 180 and the walls of the tubes were composed of a thin film having an average thickness less than 2.

EXAMPLE 9

Example 8 was repeated utilizing 70 grams of the polyethylene instead of 50 grams. The viscosity of the solution of polymer in the hexane was 400 centipoises at 140.degree.C. The structural character and dimensions of the fibers were substantially the same except the fibers produced were longer and stronger. These fibers are particularly suitable for use in manufacture of non-wovens and the like or they can be refined to be used as a cellulose pulp substitute.

EXAMPLE 10

Polyethylene fibers were prepared utilizing the procedures of Example 1, by flashing through a cock valve 6 mm in diameter as shown at 13 in FIG. 1 but without nozzle 14. Also, the fibers were refined in a Beloit single disc refiner having 12 inch discs, rather than in a Waring blender.

The resulting fibers were tested for various drainage resistance characteristics in accordance with the procedures described in TAPPI 37, No. 11 : pp. 523-534. They were also tested for drainage time and drainage factor in accordance with the procedure described previously. The results are as follows:

Sample Mv .times. S V M N DT DF 10.sup..sup.-3 ______________________________________ 1 65 1.54 2.93 .00296 .340 8.0 1.0 2 28 1.17 2.65 .00434 .309 6.2 -- 3 41 1.26 2.16 .00434 .309 7.1 1.1 where M.sub.v = viscosity average molecular weight S = hydrodynamic specific surface area m.sup.2 /g V = hydrodynamic volume, cc/g M & N = compressibility constants DT = drainage time, seconds DF = drainage factor, seconds/g Process Conditions Sample Mv Concentration Hexane/Water PVA/ Hexane Vol/Vol Polymer ______________________________________ 1 65,000 50 1/1 1 2 23,000 100 1/2 1 3 41,000 75 1/2 1 Refining Conditions Sample Number of Passes Number of Passes Temperature With Refiner Disk With Refiner Disk Clearance at Clearance at 10 0 Micron. Micron ______________________________________ 1 8 4 65 - 70.degree.C 2 4 5 70 - 80.degree.C 3 8 5 60.degree.C ______________________________________

COMPARATIVE EXAMPLE 4

Polyethylene fibers prepared in accordance with the process described in U.S. patent application Ser. No. 257,609 were tested as in Example 10. The results are as follows:

Drainage Resistence Characteristics ______________________________________ Sample Mv .times. 10.sup..sup.-3 S V M N DT DF ______________________________________ 1 100 0.88 1.95 .0072 .266 5.5 0.78 2 100 0.83 1.88 .00695 .270 -- -- 3 65 0.70 1.83 .0101 .245 5.1 -- 4 300 0.93 2.03 .00635 .274 5.7 0.89 5 44 0.81 2.24 .0124 .218 5.3 0.65 ______________________________________

It is seen that the fibers prepared in accordance with the present invention have a more favorable hydrodynamic surface area, compressibility constant N and drainage time.

EXAMPLE 11

Polypropylene powder (molecular weight 160,000, melt index 55, intrinsic viscosity 1.5 and isotactivity index 94.7) is utilized in this example together with 1% PVA based on the polypropylene. The PVA used was supplied from Nippon Gosei (Grade NCO 5) and had a viscosity of 4 percent in water at 20.degree.C of 5.3-0.7, a degree of saponification of 98.5-100 percent and a degree of polymerization of under 1000. The polymer was dissolved with n-hexane in the autoclave of FIG. 1 and then the PVA and water were added and the autoclave purged with nitrogen. The mixture was agitated for 30 minutes to form and heat the dispersion to 160.degree.C with a pressure of 15 kg/cm.sup.2 . The dispersion was then discharged to atmospheric pressure through conduit 4 and nozzle having the dimensions described in Example 8 into vaporization vessel 8 which was at atmospheric pressure and contained nitrogen gas.

The continuous fibrous product was immediately refined at 10,500 rpm for 20 minutes in a Waring blender with a beginning temperature of approximately 10.degree.C and a temperature at the end of refining of about 40.degree.C with a concentration in the blender of 10 grams fiber in 1 liter water. A number of runs were made using a progression of hexane/water ratios of 5/10, 7.5/10 and 10/10 on a volume basis. For each indicated ratio of runs were made with the following series of polymer concentrations 25, 50, 75, 100 and 200 grams/liter of solvent.

The handsheet properties of the refined product were tested and the results are shown in following Tables I and II. Also the influence of the temperature of beating was studied and is shown in Table III.

Table I __________________________________________________________________________ Properties of Handsheet __________________________________________________________________________ Hexane/Water Basis = 5/10 Weight Caliper Apparent Opacity Tear g/ Zero Span Tensile Test gm/m.sup.2 10.sup..sup.-3 mm Density % Sheet Km lb/in % __________________________________________________________________________ 25.sup.G/L-C6 62.8 418 0.150 97.2 3 0.22 0.07 1.6 50 50.7 312 0.162 95.3 5 0.31 0.19 2.5 75 60.4 346 0.174 97.3 5 0.30 0.21 2.8 100 59.5 357 0.166 96.3 11 0.39 0.62 3.1 200 60.9 301 0.202 97.0 11 1.10 1.0 2.7 Hexane/Water = 7.5/10 (At 25 G/L-C6 100% SWP handsheet could not be obtained) 50 60.0 386 0.155 95.9 2 0.38 0.15 2.2 75 55.7 336 0.165 95.8 3 0.55 0.23 3.0 100 59.2 330 0.179 96.6 3 0.77 0.44 2.4 Hexane/Water = 10/10 25.sup.G/L-C6 100% SWP handsheet could not be obtained 50 73.2 468 0.158 94.1 1 0.31 0.06 2.0 75 61.7 367 0.168 95.9 3 0.44 0.20 2.3 100 60.2 306 0.196 95.9 3 0.62 0.43 2.2 __________________________________________________________________________ Note: .sup.G/L-C6 =grams polypropylene/liter hexane

Table II ______________________________________ Freeness ______________________________________ Polypropylene grams/liter 5/10 7.5/10 10/10 15/10 ______________________________________ 25 426 559 -- -- 50 416 497 520 566 75 -- 483 -- -- 100 350 417 373 -- ______________________________________

Basis Weight Caliper Apparent Opacity Tear g/ Zero g/m.sup.2 10.sup..sup.-3 mm Density % Sheet span Tensile Test g/cc Km Lb/in % __________________________________________________________________________ (start of refining) ( -end of refining) (16-41.degree.C) 60.5 246 0.245 96.1 8 1.01 0.70 1.6 (16-41.degree.C) 60.5 260 0.232 96.7 14 1.07 0.45 0.8 (60-70.degree.C) 57.5 258 0.222 95.6 1 1.03 0.24 0.8 (74-78.degree.C) 54.7 234 0.233 94.8 1 1.03 0.14 0.6 (11-43.degree.C) 52.3 262 0.199 96.0 5 1.03 1.1 4.6 (63-66.degree.C) 49.6 277 0.179 95.2 2 0.83 0.37 1.5 (77-78.degree.C) 46.8 260 0.180 95.0 2 0.85 0.13 0.6 __________________________________________________________________________ Note: Higher temperature of refining always gives longer fibers.

Claims

I claim:

1. A process for producing a polyolefin pulp comprising forming a mixture of a polyolefin, a solvent for the polyolefin at elevated temperatures, water as the continuous phase, and polyvinyl alcohol dissolved in the water, the mixture being at a temperature above the melt dissolution temperature of the polyolefin in the solvent and at substantially autogeneous pressure, the polyolefin being present in an amount of from about 0.5 to 15 percent by weight of the solvent, the polyvinyl alcohol being present in an amount from about 0.1 to 5 percent by weight of the polyolefin, the solvent being inert and stable at the mixture temperature and substantially immiscible in water or forming a polymer solution which is substantially immiscible in water; passing the mixture through a nozzle into a zone of lower pressure to vaporize the solvent and form an aqueous slurry of fibrous polyolefin, the pressure in the zone being such that the temperature of the mixture falls below the softening point of the polyolefin, and; refining the aqueous slurry of fibrous polyolefin into a pulp of discrete fibers.

2. The process of claim 1 wherein the polyolefin is a crystalline polyolefin selected from the group consisting of polyethylene, polypropylene, ethylene-propylene copolymers and mixtures thereof.

3. The process of claim 2 wherein the polyolefin is polyethylene.

4. The process of claim 3 wherein the molecular weight and concentration of the polyethylene in the solvent is selected to provide a solution viscosity at the temperature of the mixture between about 100 and about 3500 centipoises.

5. The process of claim 1 wherein the solvent to water volume ratio is between about 0.5 : 1 to about 2 : 1.

6. The process of claim 1 wherein the solvent has a boiling point lower than the melting range of the polyolefin.

7. A process for producing a polyolefin pulp comprising forming a mixture at a temperature between about 130.degree.C and about 160.degree.C and under substantially autogeneous pressure of a crystalline polyolefin, a solvent for the polyolefin at elevated temperatures, polyvinyl alcohol and water; the polyolefin being present in an amount up to 15 percent by weight of the solvent, the solvent to water ratio being between about 0.5 : 1 and about 2 : 1, the water bieng present as the continuous phase of the mixture the polyvinyl alcohol being present in an amount up to about 15 percent by weight of the polyolefin, the solvent having a boiling point lower than the melting range of the polyolefin and stable at the mixture temperature; passing the mixture through a nozzle into a zone of lower pressure to vaporize the solvent and form an aqueous slurry of fibrous polyolefin, and refining the firbrous polyolefin slurry into a pulp of discrete fibers.