Process of making polyolefin fibers

Abstract

An improved process of manufacturing fibers by the technique of forming a mixture of polymer, solvent for such polymer and, optionally, water or other flashing aids, at a temperature (flash temperature) which is high enough to bring the polymer to a plastic state and which will permit substantially complete vaporization of the solvent when the mixture is flashed, flashing the mixture into a flash zone to produce a fibrous product and subsequently refining the fibrous product characterized in that the fibrous product is first cooled prior to passing it through a primary refining zone. The primary refining is effected under conditions that impart a vigorous fibrillating action to the fibrous product and provides a pulp having a drainage factor in excess of 1.0 seconds/gram.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of application Serial No. 295,339 filed October 5, 1972.

Description

BACKGROUND OF THE INVENTION

Numerous process have been proposed for preparing synthetic fibrous materials by flashing polymer solutions or dispersions held at high temperature and pressure into a zone of reduced pressure. In various patent literature, such as German Offenlegungsschrift No. 1,958,609 and Japanese patent application No. 71-34921, processes are proposed in which a polymer is dissolved in a solvent therefor and heated under at least autogenous pressure and then flashed into a zone of lower pressure to thereby vaporize the solvent and form fibrous materials. In the latter mentioned patent, the fibrous material thus formed is quenched with a water spray at a temperature between 60.degree.C and 80.degree.C.

Similar processes are presented in U.S. patent application Ser. No. 295,339 filed Oct. 5, 1972, (assigned to the assignee of the present application) and as well in German OLS No. 2,121,512 and German OLS No. 2,144,409. In these processes a polymer dissolved in a solvent is mixed with water or other liquid non-solvents for the polymer to form an emulsion of the polymer solution in a continuous water phase and this emulsion is heated and flashed to reduced pressure zone to produce fibers.

Another approach is described in U.S. patent application Ser. No. 285,386, filed Aug. 30, 1973, and now abandoned (assigned to the assignee of the present application) wherein a polymer solution in which water is dispersed as a discontinuous phase is flashed to form fibers. In this process, the water concentration is held between 30% and 70% of the entire mixture and in forming the mixture is preferable to add the water to the preformed polymer solution to insure that the water forms the discontinuous phase.

OLS No. 2,117,461 describes a process in which molten polymer is emulsified with water (optionally, with a minor amount of solvent in the polymer phase) and then heated and flashed to a reduced pressure zone to form fibers.

While many variations are evident, it is seen that the common features of all these fiber-making processes is the flashing into a zone of reduced pressure of a heated mixture containing a polymer and a solvent for such polymer. Various conditions of flashing are suggested in the referenced processes including temperature and pressure ranges for the mixture to be flashed and various solvents, flashing aids, etc. Most of the references contemplate flashing into a zone held at atmospheric pressure. Others suggest the possibility of flashing into a zone which is above or below atmospheric pressure, in some case with the application of heat in such zone. All of the processes may lead to the production of fibrous material. However, each suffers from the shortcoming that a specific set of conditions of flashing, particularly with regard to temperature and pressure, and inter-relationship of such conditions are not provided which will permit manufacture, in a practical manner, of fibers having the optimum properties desirable for their use as a synthetic pulp in the manufacture of paper by conventional techniques.

Fibrous materials produced under the general process conditions described in these references tend to be interconnected or bundled together to an undesirable degree and the paper produced therefrom is undesirably low in strength (e.g., tensile strength). Such fibrous material is more difficult to separate, cut or refine in preparation for papermaking and contains a high content of gels and chunks of polymer which cause undesirable "fish eyes" or transparent spots in paper manufactured therefrom.

It has recently been suggested (in a U.S. patent application Ser. No. 340,140 of H. Yonemori filed by the present assignee on Mar. 12, 1973 and entitled "Process of Making Fibers") to overcome these problems by providing a specific interrelated set of flashing conditions to produce a product capable of being readily separated and refined into an improved pulp for papermaking which has a low content of gels and chunks and excellent drainage factor characteristics. Briefly, that process comprises a flashing into a zone at subatmospheric pressure to provide a fibrous product having a temperature less than 70.degree.C and subsequently refining the pulp at this low temperature, preferably while maintaining the pulp under such subatmospheric conditions in order to prevent condensation of solvent vapor. While this approach is completely satisfactory from a technical point of view, it is expensive to practice commercially because of the vacuum requirements imposed on the system.

It has been suggested (in a U.S. patent application of J. Kozlowski recently filed by the present assignee entitled "Process of Making Fibers") to obtain a fibrous pulp having high drainage characteriztics by maintaining the flashed product at a temperature above the boiling point of the solvent (preferably between 70.degree.C and 100.degree.C) subjecting the fibrous produce to a primary refining that comprises a light defibering or shredding action, and subjecting the pulp to a vigorous fibrilating action in a secondary refining zone.

In Netherlands application No. 72/10567 published Feb. 3, 1973 it is disclosed that a fibrous gel material containing large amounts of solvent is first subjected to disc refining in the presence of the solvent to develop the fibers, followed by steam stripping the solvent from the fibers in an aqueous slurry in the presence of polyvinyl alcohol.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

The purpose of the present invention is to provide an economical technique applicable to all of the flashing processes previously described which will produce a pulp useful for producing synthetic paper by conventional papermaking techniques which has surprisingly improved properties and a low content of gels and chunks. The present process obviates the need for refining under subatmospheric pressure.

Briefly the present process comprises taking the flashed fibrous product produced by any of the foregoing processes, forming an aqueous slurry of the fibrous product at a temperature between about 10.degree.C and 60.degree.C, and passing the fibrous product at such a temperature through a primary refining zone operated under conditions such that the fibrous product is subjected to a vigorous refining or fibrillating action. The resulting pulp has a drainage factor in excess of 1.0 second/gram and as high as 200 seconds/gram or higher.

DESCRIPTION OF PREFERRED EMBODIMENTS

In practicing the process of the present invention, any polymer or copolymer may be employed which is capable of forming fibers by conventional spinning techniques. It is preferred to employ crystalline or partially crystalline polyolefins such as low pressure polyethylene, isotactic or partially isotactic polypropylene, and ethylene-propylene copolymers. Additionally, polybutenes and polymethyl pentenes may be employed in the practice of this invention. Crystalline or partially crystalline polyamides and polyesters may also be used. Noncrystalline polymers such as polycarbonates, polysulfones, polyvinyl chloride, polymethyl-methacrylate, polyacrylonitrile and polystyrene may be used. Mixtures of the foregoing with each other or other polymers may also be employed.

The preferred polyolefins employed are those having an intrinsic viscosity above about 0.7 dl/g., which for high density polyethylene corresponds to a viscosity average molecular weight of about 30,000 to 40,000.

The polymers employed in practicing the present process may be in the form of dried powder or pellets or, prefereably, as a wet cake, slurry or solution of polyolefin in the reaction solvent as obtained after polymerization.

Generally, any substituted or unsubstituted aliphatic, aromatic or cyclic hydrocarbon which is a solvent for the polymer at elevated temperatures and pressures, which is relatively inert under the conditions of operation and which has a boiling point at atmospheric pressure that is between 20.degree.C and 130.degree.C, preferably between 50.degree.C and 100.degree.C, and at the flash zone pressure (substantially atmospheric) that is less than the softening point of the polymer may be employed in practicing the present process. Illustrative of the solvents which may be utilized are aromatic hydrocarbons, e.g., benzene; aliphatic hydrocarbons, e.g., pentane, hexane, heptane and their isomers and homologues; alicyclic hydrocarbons, e.g. cyclohexame; chlorinated hydrocarbons, e.g. methylene chloride, carbon tetrachloride and chloroform; higher alcohols; esters; ethers; kestones; nitriles, amides; fluorinated compounds, e.g. fluoro-hydrocarbons; nitro-methane; and mixturee of the above solvents and other solvents having a boiling point between about 20.degree.C and 100.degree.C at atmospheric pressure. The preferred solvent is hexane which has a boiling point of 68.7.degree.C at atmospheric pressure.

The polymer-solvent mixture may be formed by any one of several methods. One may start with a solution of polymer in solvent as it comes from a solution polymerization process, either at the same concentration, diluted or concentrated. Alternatively, one may start with a slurry of polymer particles in the solvent such as is produced by a slurry of polymerization procedure and the appropriate amount of water is added to the slurry of vice versa. A further alternative would be to start with a dry polymer powder, or granules, or a wet cake such as might be produced at some stage of solvent removal in the polymer plant and the appropriate amount of solvent is admixed therewith.

The polymer concentration relative to the solvent is not critical, the solvent being present in an amount that is greater than 100% by weight of the polymer and sufficient to give a viscosity at the flash temperature employed that can be easily handled. Frequently this viscosity will be up to 3,500 centipoises. Generally the polymer concentration will vary from about 2% to about 30% by weight of the solvent plus polymer, and preferably is in the range of about 5% to about 15%.

In one preferred embodiment water is employed as a flashing aid. In this embodiment the water may in a continuous phase or discontinuous phase, depending upon the amount of water added to the polymer-solvent mixture and the manner of addition. If the water is to form the discontinuous phase it should be present in an amount less than 70%. To form a continuous water phase the water should be present in an amount greater than 30% by volume of the mixture and preferably between 50% and 70%. The particular method of mixing is not critical, but if it is desired to have the water form a discontinuous phase it has been found to be advantageous to have the solvent present prior to water addition since the solvent or polymer solution will form the continuous phase of the mixture to be formed. This latter approach is particularly desirable when one employs an amount of water which is neat the borderline of an inversion occurring, i.e., at the point where the amount of water is approaching that level where it would form the continuous phase. Conversely, if the water is added with or before the solvent, it will tend to form the continuous phase.

A primary function of the water is to provide energy to aid the vaporization of the solvent during flashing since it is not desirable to have the temperature so high that there is sufficient energy imparted to the solvent alone to effect its complete vaporization. However, the amount of water should not be so great as to require the expenditure of unnecessary heat values in attaining the desired flashing temperature, i.e., once that amount of water required to form an aqueous solution or dispersion of the agent having a suitable viscosity is determined, additional water may be employed to a certain extent since it helps to lower the mixture viscosity and aid solvent vaporization but the additional amount need not be great.

Another function of the water is to reduce the temperature of the fibrous mass in the zone immediately following the nozzle (flash zone). The addition of water increases the total vapor pressure of the system at the moment of flashing, thus reducing the boiling point of the flashing mixture. This is independent of the amount of water employed, and very small quantities may thus be employed for this purpose. As a practical matter, however, water would be employed in the amount of at least about 1% by volume of the solvent-water mixture. Lowering the boiling point of the mixture in this fashion will assist in establishing proper temperature conditions in the fibrous mass formed upon the flashing in accordance with this invention, as discussed in detail at a later point.

Another function of the water is to act as the carrier for a hydrophilic water-dispersing agent for the fibers to be formed. It has been found that it is most advantageous to have the water-dispersing agent present during flashing and precipitation of the fibrous polymer. An equivalent amount of the same agent added at a later stage to the already formed fibers does not give the same degree of dispersibility and the presence of the agent enhances the refinability of the fibers. Therefore, ,the water should be present in an amount sufficient to carry that amount of the hydrophilic agent employed to impart to the fibrous polymer the desired level of water dispersibility, preferably as a solution thereof. Additional water above such minimum amount required to carry the agent may be employed to impart a suitable viscosity to the aqueous solution or dispersion agent, i.e., the aqueous solution of the water-dispersion agent should not be so viscous as to present problems of handling or incorporation into the polymer solution as a dispersed phase. Also, the water can aid in reducing the viscosity of the mixture to a level less than that of the polymer solution alone, thus permitting higher polymer concentrations.

The agents which may be added to the mixture to impart water dispersibility to the fibrous polymer are preferably water-soluble or partially water-soluble high molecular weight materials. However, they may also be materials which are soluble or partically soluble in the solvent so long as they are somewhat hydrophilic and impart water dispersibility to the fibers. The amount of water-dispersing agent employed may range from about 0.1% to about 15% by weight of the polymer, preferably from about 0.1% to about 5% by weight. The preferred water-dispersing agent is an at least partially water-soluble polyvinyl alcohol (PVA) having a degree of hydolysis greater than about 77% and, preferably, greater than about 88 mol.% having a viscosity (in a 4% aqueous solution at 20.degree.C) between about 2 and 50 centipoises. Desirably the PVA has a degree of polymerization in the range of 200 and 4000 and preferably between 300 and 1500. If desired, the PVA may be chemically modified to enhance its adhesion to the polymer, dispersion and other properties. The polyvinyl alcohol is preferably added with the water at the time the mixture is formed. Illustrative of other water-dispersing agents that may be employed are cationic guar, cationic starch, potato starch, methyl cellulose and Lytron 820 (a styrene-maleic acid copolymer).

The use of polyvinyl alcohol is important in developing the fiber properties during refining of the flashed product in accordance with this invention as described at a later point. For this purpose, the polyvinyl alcohol may be added subsequent to flashing, such as with the dilution water for refining. It is particularly advantageous to add at least 0.75% and preferably between 11/2% and 5% by weight of the flashed polymer of polyvinyl alcohol, either before and/or after flashing (but prior to refining). If polyvinyl alcohol is not present during refining, the refining can be carried out only with difficulty, if at all, and the properties of the resulting pulp and sheets made therefrom are very poor.

The ingredients of the mixtures can be placed in any suitable vessel which is capable of being heated to an elevated temperature and pressure. Generally, an autoclave is employed. However, when water is added as a flashing aid it is important that the vessel employed be equipped with a mixing or stirring device capable of keeping the mixture in a constant state of agitation since a stable emulsion is not formed and upon standing the mixture will quickly separate into two distinct and separate phases.

The ingredients are then heated to a suitable temperature and preferably agitated if water is present to form a uniform mixture wherein water is present as a discontinuous or dispersed phase within a continuous phase of polymer solution or as a continuous phase with polymer solution dispersed uniformly therein, depending upon the water concentration and mode of addition as previously discussed. The temperature employed is preferably above the melt dissolution temperature of the polymer in the solvent employed. The melt dissolution temperature of any particular solvent is determined by placing low concentrations of the polymer (e.g., 0.1 and 1.0% by weight) into the solvent in a vial which is then sealed and placed in an oil bath. The temperature of the oil bath is raised slowly (e.g., 10.degree.C/hour) until the last trace of polymer disappears This temperature is the melt dissolution temperature. In some instances, it may be desirable to operate at a temperature below the melt dissolution temperature. In this case the temperature sould be high enough under the operating conditions so that the polymer is dissolved in the solvent or at least is in a swollen state with sufficient fluidity to be discharged from the nozzle, i.e. in a plastic state.

Flashing is preferably carried out substantially adiabatically, utilizing the heat (enthalpy) in the heated mixture to provide the heat of vaporization for vaporizing substantially all of the solvent when the mixture is discharged to the flash zone held at a suitable lower pressure. Accordingly, for adiabatic flashing the temperature of mixture prior to flashing should be high enough to provide sufficient heat content or enthalpy for vaporization adiabatically of substantially all solvent upon flashing to the flash zone. However, the maximum temperature employed should be less than the critical temperature of the solvent and/or the decomposition temperature of the polymer.

It is also possible to carry out the flashing to some extent in a non-adiabatic fashion, for example, by the addition of heat to the material as it is flashed from the nozzle. For instance, low pressure steam (e.g., below 20 psig) or water at about 100.degree.C may be added to the fibrous noodle in the flashing zone as by injection thereof in a conduit immediately following the flash nozzle into which the flashed noodle is also injected. In this case, the flashing temperature should be chosen so that the heat content in the mixture to be flashed plus the heat added to the flashed material is sufficient to vaporize substantially all of the solvent in the flash zone.

The pressure employed in the vessel containing the heated mixture is preferably substantially autogenous although pressures higher than autogenous may be employed. It may be desirable, particularly in batch operations, to employ an inert gas such as nitrogen during the flashing operation to maintain substantially autogeneous pressure in the vessel and thus maintain the velocity of the mixture through the nozzle at a fairly constant level.

Flashing is preferably effected through a nozzle which has a substantial longitudinal dimension in order to efficiently impart shear to the mixture (particularly the polymer component thereof) immediately prior to flashing. Such shearing action aids fiber formation and enhances fiber properties for papermaking purposes. The nozzle may be circular or noncircular in cross-section and may be an annulus.

The flash zone is maintained at a pressure which, in conjunction with the other conditions of flashing are selected so that the temperature of the flashed product is almost immediately lowered in the flash zone, by evaporation of substantially all of the solvent (and a portion of the flashing aids, if employed), to a temperature below the softening temperature of the polymer, preferably below about 100.degree.C.

In a typical flashing procedure in accordance with this invention vaporization may be substantially complete 10 to 100 cm downstream of the nozzle. However, this can vary widely depending on the flow velocity, flash zone pressure, flash temperature, solvent, etc.

Importantly, in the mixture to be flashed, all of the components of the mixture and the concentration of each in the mixtures are chosen with respect to the heat capacity of each, with respect to the heat vaporization of each component which will be volatized in flashing, and with respect to the flash temperature chosen so as to produce in the flashed product a temperature above the boiling point of the solvent but preferably less than about 100.degree.C upon flashing of the mixture into the flash zone. Expressed in another way, the heat content of the mixture to be flashed and the heat to be removed through vaporization of the vaporizable components (solvent and any flashing aids vaporized, if employed) should be adjusted so that the residual heat in the flashed product, after removal of the heat of vaporization of the vaporized components, will impart a temperature in the flashed product which is between the boiling point of the solvent and about 100.degree.C. Selection of the appropriate flashing temperature and of the components of the mixture and their concentrations for this purpose will depend upon the pressure selected for the flash zone and the amount of heat added to the flash material during flashing if the flashing is performed non-adiabatically. Preferably the flash zone is at substantially atmospheric pressure. However, it can be at subatmospheric pressure, (i.e., below about 600 mm Hg) as disclosed in copending application Ser. No. 340140. If the flash temperature is too high or if the components of the flash mixture and their concentrations with respect to their heat capacity and heat of vaporization (for the vaporizable components) are improperly chosen, the temperature of the flashed product, upon substantially complete evaporation of the solvent, will remain above 100.degree.C. If the flash temperature is too low, or, again, the components are improperly chosen with respect to heat capacity and heat of vaporization, there will be incomplete evaporation of the solvent. For the purpose of this invention appropriate selection of these variable parameter, namely, the flash temperature, components and concentrations thereof in the flash mixture, may be determined for a given pressure condition in the flash zone by making a heat balance for the flashing operation which will produce the desired noodle temperature below 100.degree.C. Advantageously, these variable parameters may be selected so as to satisfy the following equation: ##EQU1## Q.sub.p = Enthalpy of polymer(s) in flash mixture Q.sub.s = Enthalpy of solvent(s) in flash mixture

Q.sub.f = Enthalpy of flashing aid(s) in flash mixture

Q.sub.na = Heat added to flashing mixture (if non-adiabatic)

V.sub.s = Enthalpy of vaporization of solvent vaporized

V.sub.f = Enthalpy of vaporization of flashing aids vaporized (if any).

W.sub.p = Weight of polymer in the flashed product

C'.sub.p = Heat capacity of polymer in flashed product

W'.sub.f = Weight respectively, of the flashing aids, adjuvants, and/or non-volatile components other than polymer in the flashed product (if any).

C'.sub.f = Heat capacity of the respective flashing aids, adjuvants, and/or non-volatile components other than polymer in the flashed product (if any).

T.sub.b.p. = Boiling temperature of the solvent at the flash zone pressure.

and where such enthalpy values are basaed on the same temperature datum plane and the heat capacities are those applicable between such datum plane and the temperature of the flashed product.

In using this formula, the heat capacity values and weights of the flash components may be substituted into this formula, for example, as follows for the specific case where a single polymer, solvent and flashing aid are present in the flash mixture: ##EQU2## where the additional terms are: W.sub.s = Weight of the solvent in the flash mixture

C.sub.s = Heat capacity of the solvent in the flash mixture

W.sub.f = Weight of the flashing aid in the flash mixture

C.sub.f = Heat capacity of the flashing aid in the flash mixture

C.sub.p = Heat capacity of the polymer in the flash mixture

H.sub.f = Heat of vaporization of solvent under flash conditions.

H.sub.s = Heat of vaporization of flashing aids under flash conditions.

T.sub.f = Flash temperature, .degree.C

and where the indicated heat capacities are those applicable between the flash temperature and the previously mentioned temperature datum plane.

In practice, the heat capacity and enthalpy of vaporization values for the desired components can be substituted into this equation. Then, a flash temperature and the concentrations of the flash mixture components may be selected relative to each other to satisfy the equation for a desired flashed product temperature. For ease of calculation it can be useful to program these variables for computer analysis to select the desired components and flash temperature.

It is not necessary to actually measure the noodle temperature (which is a cumbersome procedure under the flash zone conditions). All that is necessary for control purposes is to maintain the indicated parameters for the flash procedure at values that satisfy this equation for the noodle temperature to be above the boiling point of the solvent but below 100.degree.C. Of course, the various parameters should also satisfy the other conditions for proper flashing as previously discussed, e.g., the flash temperature should be above the melt dissolution temperature, substantially all of the solvent should be vaporized, etc.

During flashing, the polymer is precipitated as a fibrous product or "noodle", which is a loose aggregation of fibers which is sometimes continuous. The fibrous product is collected in a suitable receiving vessel, preferably one which permits the vaporized solvent to be separated therefrom.

The fibrous product in the receiving vessel may be either substantially "dry" as flashed from a polyolefin solution or as an aqueous slurry if flashed from a dispersion or emulsion employing water as a flashing aid. However, in both cases a certain amount of unflashed residual solvent will remain with the fibrous product which can be subsequently removed.

In the copending application Ser. No. 340140 of H. Yonemori referred to above, it was discovered that improved products were obtained by refining the fibrous product at relatively low temperatures, and to promote continued vaporization of the solvent the vapor separation zone and the refining zone had to be maintained under subatmospheric pressure.

The present invention resides in the discovery that similarly improved products can be obtained by carrying out the refining of the cooled fibrous product while it is maintained at substantially atmospheric pressure, i.e., a pressure between about 600 and 800 mm Hg.

The fibrous product is cooled to a temperature between about 10.degree.C and 60.degree.C prior to subjecting it to primary refining it at substantially atmospheric pressure. If the temperature employed is higher than about 60.degree.C, the drainage factor of the pulp is detrimentally effected. Temperatures below about 10.degree.C do not give any substantial improvement and become uneconomical to use. Preferably the temperature is between 10.degree. and 40.degree.C.

The manner in which the fibrous product is cooled is not critical. The fibrous product can be produced at the proper temperature by employing a flashing zone at subatmospheric pressure (as described in copending application Ser. No. 340140) or by employing a low boiling solvent, i.e. one having a boiling point less than about 60.degree.C, or by a combination of a subatmospheric pressure flash zone and low boiling solvent.

Alternativey, the fibrous product can be produced at an elevated temperature up to about 100.degree.C, and cooled to the appropriate temperature by use of cold dilution water, external cooling means, or a combination of these or other conventional cooling methods.

Prior to passing the fibrous product through such primary refining zone, dilution water is added in the receiving vessel to provide an aqueous slurry of the fibrous product having an appropriate consistency. Typically, the consistency of the squeous slurry is between about 1 to 10% by weight of the fibrous material although high consistencies between about 10% and 60% by weight may be employed by using screw feeding or similar feeding means. The dilution water temperature should desirably be such as to provide an aqueous slurry having a temperature between about 15.degree.C and 40.degree.C, although external cooling can be employed to lower the slurry temperature to within this range.

The aqueous slurry of fibrous product at the appropriate consistency and temperature is then passed through the primary refining zone to effect the desired vigorous fibrillating action on the material. In its preferred form, the primary refining zone is a disc refiner of the type conventionally employed in the papermaking art and may employ either a single or double rotating disc. The refiner is operated to impart a vigorous fibrillating action to the fibrous product passing therethrough. This is accomplished by spacing the discs relatively close together, that is, by a distance between about 0 and 2500 microns, preferably between about 0 and 200 microns. The moving disc or discs are rotated at speeds to provide relative peripheral velocities between about 4000 and 8000 feet per minute. The size of the discs and the plate design may be any of those normally employed in the papermaking art. The primary refining may be carried out by passing the fibrous material one or more times through the refining zone. The "vigorous fibrillating" action may be characterized by the amount of work done on the fibers in the primary refining zone. The work imparted to the fibrous product passing through the primary refining zone is desirably greater than about 0.2 Kilowatt-hour/Kilogram.

The refining action just described may also be characterized by the fiber fractionation as carried out in accordance with TAPPI Standard Test T.-233- SU64, wherein 0.5% by weight of the fiber in an aqueous slurry is fractionated for 20 minutes in a Bauer-McNett classifier equipped with 20, 35, 65, 150 and 270 Tyler mesh screens. After refining, the amount of fiber left on the 20 plus 35 mesh Tyler screens should be less than 90% by weight and preferably less than about 60% by weight of the sample.

The aqueous slurry of fibrous product reslting from the primary refining operation drops from the bottom of the disc refiner into a receiving vessel which may be purged with an inert gas such as nitrogen to remove any solvent vaporized during the primary refining step.

The resulting pulp may be subjected to further or "secondary" refining if desired.

By employing the refining sequence just described, i.e., primary refining of the fibrous material as a reduced temperature a pulp is produced having superior qualities. In this respect, one indication of the improvement of the fibers made by the process of the present invention is the drainage factor of such fibers as compared to fibers of similar classified fiber length which are made using parameters outside the present invention. The drainage factor is a measure of the drainage characteristics of a fiber when a slurry thereof is placed on a foraminous surfce. For synthetic fibers of similar fiber length made by the flashing process, important strength properties thereof correlate generally with their drainage factor. For a fiber pulp having the same classified fiber length, various strength properties of paper made therefrom increase with the drainage factor of such fibers.

Another indication of improvement of the fibers made in accordance with this invention is their slenderness relative to fibers prepared by typical conventional technique. It is desirable to produce fibers which are relatively thin (or of low coarseness) as these fibers will impart higher capcity, density and better formation to the paper prepared therefrom.

Generally, the set of operating parameters of the present invention will result in improved fiber properaties such as thinness and drainage factor compared with fibers prepared using typical conventional parameters. Because other process variables in addition to the specific parameters of this invention also influence fiber properties (e.g., flash nozzle size and configuration, polymer type and molecular weight, solvent, flashing aids, dispersants, etc.), such resulting fiber comparasons are appropriately made with the other process variables held constant.

For the same reason, i.e., the influence of other process conditions besides the parameters specific to this invention on the resulting fiber properties, no absolute fiber property values can be assigned to the fibers which may be prepared by the process of this invention. However, with appropriate selection of all parameters, it has been found that pulps can be produced in accordance with this invention which have a drainage factor in excess of 1 often in excess of 10 and as high as 100 to 200 seconds per gram or higher and which have an average coarseness below about 15 decigrex (m/100 m). For papermaking use, a drainage factor between 2 and 10 may be the preferred range as an appropriate balance between increased strength and ease of water removal from the fibers. Higher drainage factors can be obtained and may be useful where the enhanced strength is more important than rapid water removal.

In the accompanying drawing,

FIG. 1 represents a schematic representation of apparaus suitable for use in carrying out the process of this invention, and

FIG. 2 is a detailed representation of the flash nozzle schematically. shown in FIG. 1.

In FIG. 1, 1 is a steam jacketed vessel, provided with an agitator 1a, which may be charged with solvent, polymer (or polymer solution) and, if desired, flashing aids such as water. Conduit 2 is provided at the bottom of the vessel 1 in communication with flash nozzle 3 through shut-off valve 4 (preferably a ball valve). As seen in FIG. 2, flash nozzle 3 has a substantially smaller diameter than conduit 2. After the mixture is heated to the desired temperature and agitated, if necessary, to dissolve the polymer and/or disperse the flashing aids, valve 4 is opened and the mixture thus formed is forwarded from vessel 1 through flashing nozzle 3 under autogenous pressure of the heated mixture. As the mixture discharged from vessel 1, nitrogen or other inert gas may be introduced into the head space thereof through line 5 in order to maintain the pressure in the vessel at autogenous or higher pressure. The mixture is flashed through nozzle 3 into the flash zone which is comprised of a vapor separation vessel such as cyclone 6, and connecting conduit 7. Conduit 7 is a pipe having an internal cross-sectional area larger than that of flash nozzle 3 and of sufficient internal diameter to permit rapid, unrestricted passage of the flashed noodle to cyclone 6 and preferably has an internal cross-sectional area many times larger than that of the nozzle 3. Optionally, low pressure steam may be introduced into conduit 7 through line 8 located shortly beyond flash nozzle 3 to aid in the vaporization of solvent from the precipitated noodle.

Vaporized solvent and water leaving cyclone 6 pass through line 21 to a solvent recovery unit (not shown).

The flashed fibrous noodle entering cyclone 6 through conduit 7, together with the unvaporized portion of water or other flashing aid present, passes downward through conduit 9 into primary disc refiner 10. Conduit 11 is provided for introduction of dilution water at an appropriate temperature to form a mixture or slurry with the fibrous material having an appropriate temperature and consistency for disc refining.

The material leaving the primary refiner 10 passes through conduit 12 to first receiving rank 13. Nitrogen is introduced into tank 13 through line 14 which sweeps out solvent vapors which are removed via suitable means not shown.

The slurry may be withdrawn from receiving tank 13 through line 15 utilizing pump 16, for secondary refining in disc refiner 17. The fibrous slurry leaving secondary refiner 17 passes through conduit 18 to second receiving tank 19 where it is collected prior to further processing for shipment or for papermaking use. If desired, the fibrous slurry leaving refiner 17 may be recycled through line 20 for one or more additional passes through refiner 17.

The fibers after refining may be diluted to a suitable consistency and made into synthetic paper webs either alone or blended with normal cellulose papermaking fibers. Alternatively, the fibers can be dewatered, pressed into bales, stored and shipped to the ultimate user.

The illustrated apparatus may be operated on a batch basis as described or on a continuous basis by continuously feeding vessel 1 with polymer solution and any flashing aids desired at flow rates to maintain the appropriate mixture in the vessel for flashing, while heating the vessel to maintain the mixture at the appropriate flash temperature. In order to insure uniform dispersion of flashing aids, it may be desirable to place an in-line mixing device in line 2 between vessel 1 and flash nozzle 3.

Optionally, instead of using a stirred heated vessel such as vessel 1, it is also possible to prepare the mixture on a continuous basis by blending a polymer solution with any flashing aids desired (as by adding superheated water) continuously in an in-line mixing device just prior to flashing through a nozzle. It is also possible to utilize an in-line mixer.

The following examples will illustrate the invention.

EXAMPLE 1

The apparatus employed in this example is that illustrated in the drawing and previously described. The dissolution vessel 1 was a 2 gallon stirred autoclave equipped with five 4 inch impellers on a single shaft rotated at 1000 r.p.m.

The vessel was charged with 0.32 Kg of polyethylene (Mitsui 2200P) having an intrinsic viscosity ( .eta.) of 1.4 dl/gram. 4 liters of n-hexane, 4 liters of water and 2% by weight of 88% hydrolyzed polyvinyl alcohol (Gelvatol 20/30 made by Monsanto) based on the polyethylene. The vessel was sealed and the contents raised to a temperature of 145.degree.C .+-. 2.5.degree.C. The vessel was pressurized with nitrogen to a pressure of 160 psig. The heated contents were then flashed through a nozzle 3 having a diameter of 2.5 mm and a length of 28.5 mm into a receiving vessel 6. Dilution water at a temperature of 93.degree.C was added to the fibrous material to provide a slurry having a cosistency of about 1% by weight. The aqueous fibrous slurry at a temperature of about 40.degree.C was then passed through a primary refining zone consisting of a Sprout Waldron single moving disc refiner having 12 inch plates (Pattern C29 -79B) at a plate clearance of 0.005 inch. The peripheral velocity of the refiner disc was 5495 feet per minute (1750 r.p.m.). The resulting pulp had a drainage factor of 12.6 seconds/gram, a surface area (gas adsorption) of 12.7 m.sup.2 /gram, and a fiber fraction on the 20 plus 35 mesh screen of 16%. Handsheets were made from the pulp in accordance with TAPPI Standard Test T-205M-58, with modified wet processing (400 psig) and a heat bonding treatment (121.degree.C at minimum pressure). The handsheets were tested and had the following properties:

Basis weight, g/m.sup.2 60.5 Tensile, Kg/15 mm 1.19 Breaking length, m 1309 Tensile Energy Absorption, Kg-cm/cm.sup.2 0.037 Scott Internal Bond, Kg-cm/cm.sup.2 .times. 10.sup.-.sup.3 155 Opacity, % TAPPI corrected) 96.8

Handsheets were also prepared by blending the above-described polyethylene fibers 50/50 with bleached alder kraft pulp having a Canadian Standard Freeness of 150 c.c. The properties of the resulting handsheets were as follows:

Basis weight, g/m.sup.2 59.9 Tensile, Kg/15 mm 2.40 Breaking length, m 2677 Tensile Energy Absorption Kg - cm/cm.sup.2 0.032 Scott Internal Bond C, Kg-cm/cm.sup.2 .times. 10.sup.-.sup.3 319 Opacity, % (TAPPI corrected) 94.6

In the foregoing example and subsequent examples, the sheet density, basis weight and caliper were determined by TAPPI Standard T-220, tear strength by TAPPI Standard T-414, opacity by TAPPI Standard T-425 m-60, coarseness by TAPPI Standard T-234 SU 67, and tensile strength, stretch, tensile energy adsorption and breaking length were determined by TAPPI Standard T-494.

The Scott Internal Bond was determined by employing the Scott instrument under the standard procedure.

The drainage factor in this and following examples was 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 is 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: ##EQU3## 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.

COMPARATIVE EXAMPLE 1

In order to show the improvement in fiber properties obtained by subjecting the fibrous noodle to a rigorous fibrillating action at a reduced temperature, example 1 was repeated except that the primary refining was accomplished by passing the fibrous noodle at a temperature of 93.degree.C through the refiner with the plates set at 0.005 inch apart. The resulting pulp had a drainage factor of 0.17 seconds/ gram. The resulting pulp was then cooled to about 40.degree.C as in example 1 and passed once through a secondary refiner having a plate clearance of 0.005 inch. The resulting pulp had a drainage factor of only 0.57 seconds/gram compared to 12.6 seconds/gram for the pulp of example 1. The lower drainage factor is indicative of a fiber which will form a much weaker sheet.

EXAMPLE 2

Example 1 was repeated except that the polyvinyl alcohol was present in the amount of 5 % by weight of the polyethylene. It took 76 seconds to pass all the material through the nozzle. After cooling to about 40.degree.C and one pass primary refining at a plate clearance of 0.005 inch the resulting pulp had a drainage factor of 108 seconds/gram, a surface area of 14.4 m.sup.2 /gram and a fiber fraction on the 20 plus 35 mech screens of 55% by weight. This example illustrates that increasing the amount of polyvinyl alcohol during refining further enhances the drainage factor of the resulting pulp. Handsheets (both 100% and a 50/50 blend) were prepared as in example 1. The properties were as follows:

Property 100% 50/50 ______________________________________ Basis weight g/m.sup.2 61.5 60.2 Tensile, Kg/15 mm 1.83 3.0 Breaking length, m 1991 3321 Tensile Energy Absorption, Kg-cm/cm.sup.2 0.359 0.061 Scott Internal Bond Kg-cm/cm.sup.2 (.times. 10.sup.-3) 304 352 Opacity % (TAPPI corrected) 96.1 93.1 ______________________________________

EXAMPLE 3

Example 1 was repeated except that the temperature of the contents of the dissolution vessel was 149.degree.C, and low pressure steam (10 psig) was introduced into line 7 via line 8 which entered line 7 3 inches downstream of nozzle 3. Two different kinds of polyvinyl alcohol were employed. In the first run, the polyvinyl alcohol employed was Hoechst Movial 30-88 which is 88% hydrolyzed and has a viscosity (4% by weight aqueous solution at 20.degree.C) of 3-5 centipoises and in the second run the polyvinyl alcohol was Hoechst 30-98 which is 98% hydolysed and has a viscosity of 3-5 centipoises. Primary refining was effected at a temperature of 40.degree.C and a plate clearance of 0.005 inch.

The resulting pulps had the following properties:

Run PVA Drainage Factor Classified No. % Hydrolyzed sec/g. Fiber length, mm ______________________________________ 1 88 14.1 1.15 2 98 5.0 1.09 ______________________________________

Handsheets made from the pulps, both 100% and 50/50 blends with bleached alder kraft, as in example 1. The handhseet properties were:

Property 100% Handsheets 50/50 Handsheets PVA % Hydrolysis PVA % Hydrolysis 88% 98% 88% 98% ______________________________________ Basis weight g/m.sup.2 63.3 62.5 58.5 57.4 Caliper, mm 0.15 0.16 0.11 0.11 Density, g/cc 0.41 0.40 0.53 0.52 Tear, g/sheet 35.2 25.6 35.2 32.2 Tensile, Kg/15 mm 1.2 0.97 2.6 2.3 Breaking length, m 1.26 1.04 2.98 2.68 Stretch % 7.1 5.6 3.7 3.2 Tensile Energy Absorption, Kg-cm/cm.sup.2 0.05 0.03 0.05 0.03 Scott Internal Bond cm/cm.sup.2 .times.(10.sup.-.sup.3) 175 200 184 183 Opacity, % (TAPPI corrected) 97.6 96.6 94.7 92.6 ______________________________________

This example illustrates that improved strength properties may be obtained with the present process by employing polyvinyl alcohols of various degrees hydrolysis, hydrlysis, and that the less hydrolyzed polyvinyl alcohols are preferable.

EXAMPLE 4

Example 3 was repeated using polyvinyl alcohols having higher molecular weights. The polyvinyl alcohol employed in run 1 was Hoechst Movia 1 75-88 (viscosity of 20-25 centipoises) in 4% aqueous solution at 20.degree.C, and is 88% hydrolyzed, and the polyvinyl alcohol employed in run 2 was Hoechst Movial 75-98 (viscosity of 25-30 centipoises and is 98% hydrolyzed). The primary refining was effected at a temperature of 40.degree.C and at a plate clearance of 0.005 inch. The resulting pulps had the following properties:

Run PVA % Drainage Factor Classified Fiber No. Hydrolyzed sec/g. length, mm ______________________________________ 1 88 4.9 1.03 2 98 3.3 1.08 ______________________________________

Handsheets made from the pulps, both 100% and 50/50 blends with bleached alder kraft, as in example 1 The handsheet properties were:

Property 100% Handsheets 50/50 Handsheets PVA % Hydrolysis PVA % Hydrolysis 88% 98% 88% 98% ______________________________________ Basis weight g/m.sup.2 60.4 58.6 60.2 58.7 Caliper, mm 0.16 0.15 0.12 0.11 Density, g/cc 0.38 0.37 0.51 0.53 Tear, g/sheet 22.4 12.8 32 32 Tensile, Kg/15 mm 0.87 0.68 2.7 2.5 Breaking length, m 0.96 0.77 3.01 2.84 Stretch, % 7.0 2.6 3.4 3.3 Tensile Energy Absorption Kg-cm/cm.sup.2 0.03 0.008 0.04 0.04 Scott Internal Bond cm/cm.sup.2 .times. (10.sup.-.sup.3) 183 120 249 164 Opacity, % (TAPPI corrected) 97.7 96.9 93.7 93.1 ______________________________________

This example illustrates that improved strength properties may be obtained with the present process with polyvinyl alcohols of varying molecular weights.

EXAMPLE 5

The apparatus employed in this example is that illustrated in the drawing and previously described except that ball valve 4 and nozzle 3 were replaced with a valtek angle central valve with a 9.5 mm diameter orifice. The dissolution vessel 1 was an 800 gallon stirred autoclave.

The stirred vessel was charged with 300 gallons of hexane, 91 Kg of polyethylene (Mitsui 2200P) having an intrinsic viscosity (.eta.) of 1.4 dl/gram, and 60 gallons of water and 3% by weight of 88% hydrolyzed polyvinyl alcohol (Gelvatol 20/30 made by Monsanto) based on the polyethylene (2730 grams). The vessel was sealed and the contents raised to a temperature of 140.degree.C. The heated contents were then flashed through the control valve into receiving vessel 6 at a rate of about 200-250 grams polyethylene per minute. Low pressure (10 psig) steam was introduced into line 7 via line 8 located about 6 inches downstream of the control valve. Dilution water at a temperature of 85.degree.C was added to the fibrous material at a rate of about 51/2 gallons/minute. Part of the aqueous fibrous slurry at a consistency of 1 % and at a temperature of 85.degree.C was then passed through a primary refining zone consisting of a Jones double disc refiner having 12 inch plates (Pattern 1,1,1,1/2 + 10.degree.) at a plate clearance of 0.002 inch (0.05 mm). The peripheral velocity of the moving refiner disc was 6688 feet per minute (2130 r.p.m.).

The slurry was passed through the refiner at a flow rate of about 15 liters per minute. A second portion of the fibrous material was collected and cooled to a temperature of about 20.degree.C prior to refining, and was then subjected to primary refining under the same conditions as the first sample. Both samples were subjected to five passes of secondary refining at 20.degree.C, the other refining conditions being the same as for the primary refining. Handsheets (both 100% and 50/50 blends) were made and tested as in example 1. The fiber and handsheet properties were as follows:

Property Run 1 Run 2 85.degree.C Primary 20.degree.C Primary Refining. Refining ______________________________________ Drainage factor sec/g. 18.4 114.1 Fiber Fractionation % on 20 mesh screen 2.3 6.7 35 mesh screen 38.2 25.4 65 mesh screen 23.5 28.2 150 mesh screen 17.7 20.1 270 mesh screen 5.2 10.7 Trhough 13.1 8.9 Classified fiber Length 1.01 0.97 100% Handsheet Density, g/cc 0.44 0.44 Corr. Opacity % 96.7 95.8 Scattering Coefficient cm.sup.2 /g 90.8 91.7 Brightness % 90.8 91.7 Breaking length 1154 1855 TEA, kg-cm/cm.sup.2 0.052 0.163 Internal Bond, Scott units 64 167 Tear, g/sheet 19 32 Basis weight, g/m.sup.2 59.9 62.2 50/50 Handsheet Density g/cc 0.58 0.57 Corr. Opacity % 91.2 91.5 Scattering Coefficient cm.sup.2 /g 744 763 Brightness, % 85.1 86.0 Breaking length, m 3332 3545 TEA, kg-cm/cm.sup.2 0.051 0.066 Internal Bond, Scott Units 119 162 Tear g/sheet 30 33 Basis weight g/m.sup.2 61.2 63.1 ______________________________________

This example clearly shows the marked improvement of properties, such as drainage factor, obtained by employing the cold primary refining concept of the present invention.

Claims

I claim:

1. In a method of producing a pulp of polyolefin fibers wherein an aqueous dispersion of a polyolefin in a solvent at an elevated temperature and pressure is flashed through a nozzle into a zone of reduced pressure to form a fibrous product, the improvement comprising dissolving in the aqueous phase of the dispersion from 0.75 to 10% by weight polyvinyl alcohol based on the weight of the polyolefin flashing said dispersion to form the fibrous product and passing the fibrous product at a temperature between about 10.degree.C and 60.degree.C through a primary refining zone under substantially atmospheric pressure in the presence of the polyvinyl alcohol under conditions such that a vigorous fibrillating action is imparted to the fibrous product.

2. The process of claim 1 wherein the fibrous product is diluted with water to a consistency between about 1% and 10% by weight prior to refining.

3. The process of claim 1 wherein refining is carried out by passing the fibrous material through a disc refiner with the spacing between the discs set at between about 0 and 2500 microns.

4. The process of claim 3 wherein the discs are rotated at a relative velocity between about 4000 and 8000 feet/minute.

5. The process of claim 1 wherein the fibrous product is subjected to a total power greater than about 0.2 kilowatt-hour/kilogram during refining.

6. The process of claim 1 wherein the polymer is an at least partially crystalline polyolefin.

7. The process of claim 1 wherein the polyolefin is polyethylene.

8. The process of claim 1 wherein the primary refining is carried out under conditions to impart a vigorous fibrillating action to the fibrous product effective to produce a pulp having a drainage factor greater than 1.0 second/gram.

9. The process of claim 1 wherein the primary refining is carried out under conditions to provide a pulp less than 90% by weight of which is retained on a 20 plus 35 mesh Tyler screens when passed therethrough at a consistency of 0.5% by weight.