NO141619B - PROCEDURE FOR PREPARING A POLYOLEFINE FIBER MASS SUITABLE FOR PAPER MAKING - Google Patents

Description

The present invention relates to a method for the production of a polyolefin fiber mass suitable for the production of paper, whereby a hot solution of a thermoplastic polymer, containing 0.5-5% by weight of polymer is pressed out into a zone of lower pressure, whereby the solvent flash- evaporates and the polymer in the form of fibers is precipitated, after which the precipitated polymer is ground.

Paper is traditionally made from a pulp consisting of cellulose fibers (primarily wood fibers; an exemplary form of a cellulose fiber in the papermaking pulp consists of a longitudinal

nal direction extending body of approximately circular cross-section, with a typical length in the range 0.1 mm - 4 mm, and an average typical diameter in the range QQ1 mm - 0.03 mm,

and from whose body fine hair-like "fibrils" protrude. The fiber body can be imagined to consist of tightly spun fibrils, some of which are forced to protrude during pulp production.

Many attempts have previously been made to use it

synthetic polymers as a substitute for wood in papermaking. Among the first attempts was the use of staple fibres. Polymer stack is the product obtained by adding

forming a filament of the polymer (either by extruding a melt of the polymer through a nozzle so that it cools as a continuous solid filament, or by extruding a discharge of the polymer into an atmosphere hot enough to vaporize the discharge agent and leave a continuous filament), and cutting the filamentÆ into pieces of the desired length. Staple fibers are produced for use mainly as textile fibers, and do not produce good paper. Staple fibers are essentially cylinders with a smooth surface.

It has been proposed to produce synthetic fibers with a structure that is more developed than the staple structure, and some of these structures are shown to produce a better paper than staple. E.g. are the so-called "net filaments", which are described in

British Patent No. 891,943 and German Patent No. 1,292,301, made of very thin film (less than of rib-like structure units interconnected to form a uniform three-dimensional network. This filament structure is a network consisting of film-like or rib-like elements which are joined and separated arbitrarily. Another structure for fibers for papermaking is described in German Publication No. 21 21 512. This structure is described as consisting of inner fibrils that run together, while the number of outer fibrils is little This is alternatively described as a bundle of fibrils, ribbons or threads.

Both net filament and fibers in German publication no. 21 21 512 are produced by flash evaporation of a heated solution, which is under pressure, and which contains dissolved polymer. In the case of net filament, a hot release under pressure expands through a nozzle into a lower pressure zone so that all release agent flash evaporates with bubble formation, whereas according to German publication No. 21 21 512 a hot aqueous emulsion, containing a polymer solution as dispersed phase,

similarly flash-evaporate, whereby the solvent evaporates.

The polymer that precipitates from the solution by flash evaporation

of the solvent turns out to be mainly in thin plate form in the aforementioned methods. Other processes involving flash evaporation of the solvent from a polymer coating are described in German Patent No. 1,958,609

and 1,951,609. Although the products from these processes are less clearly described, the basic unit appears to be plate-shaped. The product according to German patent no. 1,951,609 appears to consist of micro-flakes, which are loosely aggregated, so that the micro-flakes can be de-aggregated in an organic solvent, whereby a suspension of flakes in a solvent is obtained, and from which suspension you can make paper. According to the product

German Patent No. 1,958,609 contains fiber-like elements that are fused at the ends like net filament.

None of the preceding products are commercially satisfactory as papermaking material and as a substitute for wood pulp. The purpose of the present invention is to provide a polymer seal for cellulose pulp.

In our pulp there are fiber-like elements, but we do not strive to produce a pulp with a morphology that mimics

the morphology of the cellulose fibers. The aim is instead to produce a pulp which, in the case of paper production, is at least as good as the cellulose pulp, Qg the method is characterized by what is stated in the characterizing part of claim 1.

A mass of polyolefin polymer, based on sheets, has been found

of polymer, obtained by flash evaporation of a solution of the polymer, which gives a very good paper. In the mass obtained there are rolled or folded sheets in the form of tubes, and these tubes are separated from each other, so that instead of extending in bundles in the same general longitudinal direction, they extend in arbitrary directions relative to each other. This arbitrary orientation of rolls extending

in the longitudinal direction, and which consists of polymer plates is best seen at a magnification of 500x or lOOOx, and to more easily assess this structure, which we have found to give uniquely satisfactory properties, then refer to figures 4 and 6, which constitute electron -micrographs of products obtained in accordance with example 1 and example 3, 5.. experiment respectively, and which appears from the following description.

From Figure 4, you can see in the foreground a number of rolls that extend from side to side in the picture, whose rolls do not connect with each other, and behind these you can clearly see other rolls that extend from above and downwards in the picture. The ends of some of these rolls are clearly visible. In the same way, figure 6 shows rolls of polyolefin film, and the ends of some of the rolls are visible, and as in figure 4

is a characteristic feature that while each roll has its own longitudinal orientation, the rolls form a network.

1 Although the plate is referred to in general terms

as an olefin polymer, then with regard to what has been mentioned regarding the advantages of this structure, the preferred polymer is a crystalline olefin polymer, and then preferably polyethylene or polypropylene.

Rolls of polyolefin sheets may be rolled up in the mass, whereby it is desirable that they are rolled up, but the fact that they are rolled up indicates that they are separate rolls. direction as loose spirals around each other.)

Accordingly, the present invention provides a synthetic

pulp for papermaking, whereby the pulp consists of an olefin polymer, preferably a crystalline olefin polymer in the form of a film produced by flash evaporation of the solvent from a solution of the polymer,

and the film is in the form of longitudinally extended rolls which are separated from each other, whereby the longitudinal direction orientation of rolls lying next to each other is different.

In the mass, it is preferred that a mainly part of the surfaces of the plates, which are made up of the above-mentioned separate rolls, have a "shark skin" texture. A sharkskin texture is a series of wrinkles parallel to each other and transverse to the longitudinal direction of the roll. A pulp with little or no sharkskin texture can be a good pulp for papermaking, but we have found that a pulp in which there are many sharkskin surface-like fiber elements is particularly good. We prefer a special form of sharkskin surface in which there are grooves or valleys that extend in the direction of the roll, as well as wrinkles that extend transversely to the roll between the grooves. However, the sharkskin texture can degenerate into a nubbly texture. Both the sharkskin surface and the nupret surface appear to contribute to the valuable properties of the fibres, and constitute a preferred secondary property of our pulp. The preferred shark skin surface is shown in Figure 5, which shows the central area of the micrograph from Figure 4 enlarged twenty times, and in Figure 7 which is again a similar enlargement of Figure 6.

Consequently, the mass produced according to the invention can be recognized due to its relatively coarse morphology of separate, arbitrarily oriented rolls of film visible at a magnification of 500x, and the fine morphology of the mass, which is constituted by the shark inner surface, is visible at a magnification of 10,000x.

Possibly as a result of the structure described above, the pulp according to the present invention has measurable properties with the help of which said pulp can be recognized, and which account for the importance of the pulp as a paper-making material.

One of these properties is the surface area, which is high for the mass according to the present invention. By means of gas absorption (ordinary BET technique) it turns out that the surface area of the mass according to the present invention is in every case greater than 1.0 m 2 /g. The fibers of this pulp, freed from solvent by steam stripping or solvent exchange for water as described below, have a surface area as high as 30 m 2 /g, and when the fibers are freed from solvent by freeze-drying, they have a surface area of

area as high as 200 m 2/g.

The hydrodynamic surface area of the manufactured fibers is greater than approx. 0.6 m 2/g, and a typical area will be between 0.6 and 1.0 m 2/g. The latter characteristic is more closely related to the drainage properties of fibers from an aqueous slurry to be used for paper production than the gas absorption surface area. Determination of hydrodynamic specific surface area is done in accordance with methods 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 in this paper were used to determine the "hydrodynamic" specific surface area (S) discussed here.

Because of. that it is difficult and time-consuming to measure the hydrodynamic surface area of the fibres, so their drainage characteristics can be determined more easily.. •„-; using the drainage time. The draining time of the fibres, which are produced according to the present method, is greater than approx. 5 seconds, and the typical time is between 5 and 6 seconds. The determination of the drainage time is finally described in the present description.

A characterization of the drainage properties, which is more precise than the drainage time and which is closely related to the hydrodynamic surface area, is the drainage factor. The determination of the drainage factor is also described at the end

in the present description. The drainage factor of the fibers in the pulp of the present invention is greater than 0.2, preferably greater than 0.5, and can be up to 1.0 second per gram.

A further measure of the drainage properties of the fibres

is the compressibility constant ^N), which is determined from the slope of the curve obtained by logarithmically plotting c against p in the connection C = Mp<N>, where c is the apparent filling density in g/cm, p is the compression pressure in grams per square centimeter, and M and N are compression constants. One refers to equation {&) on page 5 25 of the above-cited TAPPI article by Ingmanson et al. in TAPPI 37, No. 11, pp. ^23-534.

The compression constant N for cellulose fibers is typically between

0.3 and 0.^1. Compression constant (N) for the manufactured fibers is greater than approx. 0.2, and typically between 0.2 and 0.3.

According to the invention, a method for the production of a polymer mass suitable for the production of paper has been provided. The method of the present invention, like previously known methods discussed above, depends on flash evaporation of solvents from a solution of the polymer. However, the method requires special evaporation conditions, which enable a further mechanical processing refinement step, which produces a mass containing separate fiber elements as described above. According to our method, a solution of polymer, which is at a temperature sufficient to cause dissolution of the polymer in the solvent and which is at a pressure at least sufficient to keep the solution in the liquid phase at said temperature, is subject to pressure relief, so that some but not all of the solvent for-

vapors so that the polymer precipitates from the solution. A gel is thus formed which consists of precipitated polymer in the remainder of the solvent in liquid phase. The gel is available in the form of long noodles consisting of intertwined fiber elements. This gel is then subjected to refining, which is a non-destructive grinding action, whereby the noodles are straightened. By "refining" is meant the mechanical treatment of the pulp which in the paper manufacturing industry is called refining or grinding. The preferred apparatus for refining is the disc refiner, and thus the preferred form of the refining step according to the method according to the present invention consists in: letting

the polymer gel in the solution continuously passes through the raffinbr space

which is bounded by coaxial, in relation to each other opposing and in relation to each other rotating surfaces.

Disc refiners are of two main types; "single disc" refiners having two coaxial discs, one of which is stationary while the other rotates on its axis with a short axial distance relative to the stationary disc, samfdouble-

disc" ("double-disc") refiners having two coaxial discs rotating in opposite directions on the axis, and with a short axial h space between them.

Disc refiners are built in such a way that the clearance between the discs

can be varied, and when it comes to the refining stage according to the present method, a clarification of 100-15 µm is preferred. After an initial refinement with relatively extensive clarification,

then a further refinement can take place at less clear-

ing, e.g. 25-7^um.

An alternative and less preferred raffinor apparatus is a "mill", of which the Jordan type is best known in the paper industry. Generally, refining the gel means making the gel subject to grinding between two surfaces that move relative to each other and are in close proximity to each other.

In flash evaporation of solvent there are many operating parameters that can affect the properties of the product, and in the first flash evaporation step in the present process, this step is regulated mainly to give only partial evaporation,

and so that in this way you get an approx. 60% or maybe 70%

of the solvent evaporated (at least 20% of the solvent

the agent evaporated), whereby a gel is formed.

These parameters are:

The Losninqs concentration. A relatively diluted solution is preferred, and in particular a solution that contains less than 5% by weight of polymer. Such dilution is particularly important in connection with the high molecular weight polyolefins to allow easy handling, as a concentrated solution of high molecular weight polyolefin may have a viscosity at the solution temperature in excess of 500 cp. Even for low molecular weight polyolefins, however, it is advantageous to keep the polymer concentration below 5% by weight, thereby having a target to ensure the formation of the desired gel.

Discharge temperature. The solvent must of course be hot enough to effect dissolution of the polymer, i.e. above the melt-dissolution temperature, which depends on the solvent. Solvent types and the determination of melt dissolution temperature are discussed below. Furthermore, the discharge temperature must not be so high that the free heat of the discharge is sufficient to evaporate more than 70% of the discharge agent when the pressure above the discharge is relieved. The determination of the free heat is also discussed later.

The pressure above the discharge. The pressure above the solution immediately before the flash evaporation is usually at least the self-pressure of the solution at the relevant temperature, in order to have the solvent in the liquid phase until the time for evaporation. A pressure higher than the self-pressure can be achieved by injecting gas, or by means of pumps as described below with reference to the attached drawings. Also, a minor proportion of solvent may occur as vapor even for the adiabatic expansion, as this appears to cause "vapor cutoff". By "steam cutting" is meant that the steam bubbles cut the fibrous gel-nodes to a more suitable length.

Pressure and viscosity determine the flow rate, which in turn affects the shear stress discussed later.

Pressure loss during evaporation. Normally the flash evaporation is carried out

by allowing the discharge, which is under pressure, to flow into a zone which is under atmospheric pressure. The absolute pressure of the zone, to which the solution is flashed, is not critical according to our method, but a pressure slightly above atmospheric pressure can be advantageous for processing the noodles.

The expansion of the discharge into a zone of lower pressure is an adiabatic expansion, whereby the heat of vaporization is taken from the discharge mass, which is in contrast to evaporation whereby the heat is supplied from outside. It is not necessary to maintain strictly adiabatic conditions by isolating the apparatus to avoid

all heat exchange with the surroundings.

Strarrring speed. The easiest and most practical way to

cause sudden evaporation of part of the solvent,

with accompanying temperature reduction in the solution and precipitation of polymer, is to allow the solution to flow continuously through an adiabatic expansion nozzle. The size of the nozzle together with the viscosity of the solution (which in turn is concentration and temperature dependent) determines the flow rate, which should usually exceed 10 meters/second, and preferably be in the range of 50-150 meters per second. The flow rate must be sufficient to cause turbulence at the exit from the nozzle. Flow that is unstable, but below the speed required to cause melt fracture (elastic turbulence), can produce the desired shark-skfim effect.

It is desirable that the flow velocity is in the range 60-127 m/sec., and a flow velocity of 70-90 m/sec. is found to

be commercially usable. The operating parameters are chosen on the basis of the above-mentioned principles to give the fibers the described characteristics, and also taking into account the polyolefin and the solvent used.

The polyolefin should be a crystalline homopolymer of an olefin or copolymer containing mainly olefin derivative units. Polyethylene, and particularly high-density polyethylene, isotactic polypropylene and crystalline copolymers of ethylene and propylene constitute the preferred polyolefin. A pulp made from a polyolefin with a high molecular weight can be imparted more easily than fiber structure according to the invention, and a better paper will be obtained. Thus, according to the invention, polyolefin with a high molecular weight is preferred. By high molecular weight is meant a polyolefin with a melt index (determined according to ASTM Standard Test No. D-1238) of less than o.5

and preferably substantially zero. Alternatively, high molecular weight means some crystalline polyolefin with a limiting viscosity number (n) greater than 2.0 dl/g. A melt index of zero for high density polyethylene corresponds to an average viscosity molecular weight greater than 100,000, and a melt index of zero (or melt flow index) for substantially isotactic polypropylene corresponds to an average viscosity molecular weight greater than 300,000. average viscosity molecular weight, as referred to, is determined by first measuring the specific viscosity of polyolefin in decalin at 135°C, and using "Ubbelohde" No. 50 or 75 viscometers. The average viscosity-molecular weight (Mv) is then determined using the relationship

where = limit viscosity number, determined from specific viscosity using the Schulz and Blaschke equation.

K = constant from the literature (5.45 x 10-x for polypropylene 2.74 x 10<->x for polyethylene)

a = constant, from the literature (0.65 for polypropylene 0.81 for polyethylene).

The polyolefin used can be preformed, i.e. previously produced in the form of dried powder or pellets or it can preferably be produced in a polymerization step which is connected to the present process, and thereby in an integrated process which extends from monomer to paper pulp. It is preferred to prepare the polyolefin solution by means of a solution polymerization process. Alternatively, a slurry polymerization process can be used, and the slurry is heated above the melt-dissolution temperature to thereby cause dissolution.

In general, one can use a substituted or unsubstituted aliphatic, aromatic or cyclic hydrocarbon, which is a solvent for the polyolefin used at eight temperature and pressure. The solvent must be relatively inert under the operating conditions.

The solvent preferably has a boiling point at the pressure after the flash evaporation (normal atmospheric pressure) which is less than the softening point of the polyolefin. The solvent can be a liquid or a gas at room temperature and atmospheric pressure, although it is preferred that the solvent is in liquid form under the conditions where otherwise the system must be pressurized after the precipitation zone. Among the solvents that can be used are aromatic hydrocarbons, e.g. benzene, toluene; aliphatic hydrocarbons, e.g. butane, pentane, hexane, heptane, and octane, and their isomers and and homologues; alicyclic hydrocarbons e.g. cyclohexane and chlorinated hydrocarbons, e.g. methylene chloride, carbon tetrachloride, chloroform, ethyl chloride, methyl chloridej alcohols! esters; etherej ketonesj nitriles; amides; fluorinated compounds; e.g. fluorocarbons; sulfur dioxide; nitromethane; and mixtures of the above-mentioned solvents.

As determined, the temperature of the polymer solution, which

flashes to evaporate the solvent, be above the solution's melting temperature, which is specific to any selected combination of polymer and solvent.

The dissolution-melting temperature of any particular polyolefin in a solvent is easily determined. Low concentrations of the polyolefin (e.g., 0.1 and 1.0 percent by weight) are added in the solvent to a bottle that is sealed and placed in an oil bath. The temperature of the oil bath is increased slowly

(10°C/hour) until the polymer dissolves. This temperature is the dissolution-melting temperature. For polyethylene with ultra-high molecular weight (approx. 10 million) at low concentration

(0.1% by weight) in cyclohexane, the dissolution-melting temperature is 118.5 - 1.9°C. For a dilute solution of high molecular weight polypropylene in cyclohexane, the dissolution-melting temperature is 130°C. At higher concentrations, the dissolution temperature approaches the melting point of the polyolefin. Lowering the molecular weight lowers the dissolution-melting temperature at a given concentration.

The solution cannot be hotter than the dissolution-melting temperature

to provide the heat which converts a part of the solution into vapor phase when the pressure of the hot solution is relieved. A suitable temperature can be calculated using an enthalpy diagram.

Figure 3 shows an enthalpy diagram for hexane, from which it can be seen that the temperature required to evaporate 20 percent of the hexane volume at atmospheric pressure is 120°, while the maximum temperature, which causes 70% of the hexane to evaporate, is 160°c. The boiling point of hexane at atmospheric pressure is 68°C.

In summary, it can be said that the method according to the present invention for the production of synthetic pulp for papermaking by allowing a hot solution of the polymer to flow from a zone of high pressure to a zone of low pressure, whereby flash evaporation of the solvent is effected and whereby polymer is precipitated, is characterized in that (i) the flow from one zone to another is carried out under parameters such as solution concentration, solution temperature and the pressure in the zone with lower pressure, so that only part of the solvent evaporates, and so that a gel of precipitated polymer is formed in the solvent; and (ii) the gel of the dissolved polymer is subjected to mechanical refining.

The concentration of the hot solution resides, to achieve de

best results, be in the range of 0.5 to 5 percent polymer by weight. The flow velocity of the polymer discharge into the zone of reduced pressure can be 50-150 m/sec., advantageously 60-127 m/sec. The proportion of evaporated solvent from the total amount of solvent is from 20 to 70%,

in other words, that means that the proportion is thus

that the ratio of evaporated solvent to the residual solvent liquid is in the range of 0.3:1, calculated on the weight, to 1.5:1, calculated on the weight (more precisely, an evaporation of 23 to 66%).

The preferred polymer is a crystalline polyolefin, preferably of high molecular weight as mentioned above. It is desirable that the gel, which is formed by partial flash evaporation of the solvent, mostly contains approx. 7.5% polymer (or less than 8% polymer), and preferably no more than 5% polymer. When the gel is fed to the refining step, the gel should contain no more than 5% polymer, preferably less than 2% polymer, so that it may be necessary

and diluting the lightly flashed gel with a solvent before it is refined, and it is therefore advantageous to flash the solution to a polymer content low enough to minimize the need to add more solvent for the refining. The gel, after being refined to straighten the noodles, still contains release agent, although some release agent may be squeezed out during refining. A typical refined gel contains 80-98% by weight solvent, and this is still a gelatinous mass from which the solvent does not drip when the mass suspends. . After removing the release agent, care must be taken not to press the fibers tightly together as this can result in release agent binding of the fibers.

Before giving special examples of the mass and the method

according to the invention, a preferred general method will be described in the following together with any process steps that can be used with advantage. Suitable equipment is illustrated in figure 1 in the attached drawing.

In pressure vessel 10, which is shown in Figure 1, a solution is heated

of the polymer to the desired temperature. If a suspension of polymer shavings or a slurry of polymer grains, which is produced by a low-temperature polymerization process, is used, then this suspension or slurry can be dissolved in container 10. Container 10 is equipped with a stirrer 11 and a mantle, in which heating -liquid can be supplied via rudder 12 and flow out via rudder 13. Solvent can be supplied to the container via rudder 14, and polyolefin

with high molecular weight can be supplied via rudder 15. An inert gas can be supplied via rudder 16 to maintain the desired pressure in the container. Alternatively, vessel 10 may be a polymerization reactor vessel in which an olefin or mixture of olefins, which may form a crystalline polyolefin, may be polymerized. As a further alternative, the polymerization can be carried out in

another container, and the polyolefin solution or slurry is fed into container 10. While a solution process would be the preferred method of carrying out such polymerization due to the fact that the polymer will already be in solution, one may; slurry polymerization method is used, and the resulting polyote fine suspension or slurry is heated above the dissolution-melting temperature of the polyolefin in container 10 to form a solution. The actual polymerization method used does not form any part of the present invention, as any conventional polymerization method in the field can be used. However, it should be noted that the preferred method

is a method capable of forming a polyolefin with a relatively high molecular weight as mentioned above.

Once the polyolefin solution has been formed in container 10, then

it is supplied through shut-off valve 17 to transfer rudder 18. Transfer rudder 18 is surrounded by mantle 19, to which the heating medium, such as steam, is supplied via rudder 20 and flows out via

ror 21. The heating liquid ensures that the polyolefin solution is kept at a temperature above the solution-melting temperature.

The polyolefin 16s is then allowed to pass through the nozzle

22 to precipitation zone (low pressure zone) 23. During the passage of the polyolefin solution through nozzle 22, the pressure and temperature of the solution are reduced adiabatically, and part of the solvent is thereby evaporated. The loss of heat caused by this evaporation cools the residual solution, so that polyolefin is precipitated as a very swollen fibrous mass or fibrous gel. By "fibrous" gel is meant a fibril structure of separate crystalline polyolefin fibers, containing interconnected spaces which are filled with the solvent. The polyolefin content in such a part must not exceed approx. 5 percent by weight of the gel, and in all cases one is forced to dilute the gel with a lot of solvent to refine the gel.

The fibrous gel and free solvent vapor are then allowed to pass through a circular shear tube 24. It is believed that some molecular orientation occurs as the gel passes through the precipitation zone and tube 24.

The fibrous gel is allowed to pass from the round shear tube 24

via follow-up transport pipe 25 to separation vessel 26^ which is maintained preferably at atmospheric or subatmospheric pressure.

Free solvent vapor, which is evaporated as the polyolefin vapor passes through precipitation zone 26, is separated from the

fibrous gel via rudder 27. The solvent vapor thus removed can then be condensed and recycled back to container 10.

The fibrous polyolefin gel, which is in the form of relatively short "noodles" caused by solvent vapor pockets formed during precipitation, and which still contains a substantial amount of the solvent, falls to the bottom of the vapor separation vessel 26, and from there into in disc refiner 28 via rudder 27. While the process is illustrated know? that preferably a disc refiner is used in this step, then one can alternatively use equipment for mechanical processing of the fibrous gel, which is known from the paper manufacturing industry and mentioned above.

The polyolefin gel is fed to disc raffinor 28, and some solvent is squeezed out as the gel passes through it. The fibers and extruded release agent fall from the bottom of the disc refiner 28 into a receiving container (not shown) via the rudder 30.

As it is highly desirable to have a polymer concentration below

5% by weight in the case of the fibrous gel when it reaches the mechanical refining step, then it is preferred to work at a discharge temperature at which when the gel:.: the type of solvent and the polymer concentration used in the discharge, the evaporation becomes so limited that the polymer concentration in the gel does not exceeds 5%.

Transfer pipe 18 is not important for the present process, and it is only used to transport the polyolefin solution from discharge container 10 to precipitation zone 22. The transfer pipe can be completely removed and precipitation zone 22 can be directly connected to container 10. Transfer pipe 18 is heated to keep the polyolefin in discharge. The dimensions of the transfer pipe 18 are not critical, but it is however very desirable to use a diameter so that little or no pressure drop is obtained along the pipe.

It has been found useful to effect some solvent evaporation as the polymer solution passes through precipitation nozzle 22, thereby causing the aforementioned "vapor cutting". Such evaporation supports the development of a suitable length of fibrous gel nuclei, which appear in the evaporation container 24. The greater the evaporation, the shorter the noodles. The partial evaporation can be effected by careful heating of transfer rudder 18. The temperature of the discharge

does not increase with such heating due to maintaining constant pressure conditions. Alternatively, a valve (not shown) can

is inserted into the transfer tube 18 in one place or another for the nozzle 22. However, such partial evaporation should not be sufficient to cause precipitation of the polymer.

The length of the precipitation nozzle 22 must be such that a sufficient pressure drop is produced to effect rapid precipitation of the polymer and partial evaporation of the solvent. The pressure drop must be such that it causes strong evaporation of the part of the solvent that evaporates. Such strong evaporation creates turbulence and causes the precipitated fibrous gel to break up into relatively short noodles, which are

a very desirable form of the subsequent treatment. Such a turbulence strain ratio is important when developing fiber properties. The degree of turbulence necessary to

provide good fiber properties depends at least in part on the molecular weight of the polyolefin used. The lower the molecular weight, the greater the turbulence must be in order to subsequently produce fibers of good quality. If low turbulence and low molecular weight polyolefin (e.g. polyethylene with plane 40,000) are used, the polymer will precipitate more as a powder than as a fiber. It is difficult to determine the minimum degree of turbulence required to produce fibers due to that many variable factors are important.

In practice, the precipitated polymer is given minimal or optimal turbulence by regulating the dimension of the nozzle 22, which

in a preferred embodiment is an adjustable throttle ball valve to increase the degree of turbulence by narrowing the nozzle

and make the opening narrower. This adjustment can continue until the gel product becomes fibrous, and can be optimized to produce fibers with the best properties for the particular application desired.

Part of the pressure drop, which occurs in connection with precipitation and evaporation, occurs in the precipitation pipe 23, which is located immediately after the precipitation nozzle 22. The precipitation pipe 23 has been found to be important for continuously imparting shear stress to the precipitated polymer and to avoid formation of continuous threads, which are difficult to treat. The length of precipitation tube 23 seems to regulate to a certain extent the length of the obtained fibrous gel noodle product. If the deposition tube is too long, unwanted long noodles can be obtained. The length of the deposition tube 23 is preferably adjusted so that

noodles are produced with a length between 5 and 15 cm.

If the noodle is continuous, there is a tendency for this to follow to the drain line to the steam separation vessel 26, and braiding and rope formation may occur in the disc refiner 28. The diameter of the precipitation pipe 23 is preferably not significantly larger than the full opening of the nozzle 22.

After precipitation tube 23, the fibrous gel is transported to steam separation vessel 26 via post-precipitation transport tube 25. The dimension of tube 25 is not critical, except that its diameter should be sufficiently large so that a small pressure drop is obtained during transport of fibrous gel. For this reason, very little additional solvent will evaporate,

and the solvent, which is removed in evaporation-separation vessel 26, is the solvent that was evaporated in precipitation zone 23.

From the vapor separation vessel 26, the fibrous gel falls into the disk raffinor 28 via the paddle 29.

The disc refiner 28 can be any conventional disc refiner used in the paper industry. However, it has been found desirable to use coarse-grained plates in the refiner to avoid clogging, which seems to occur more often with fine-grained olates. The distance between the disks should be small, preferably in the order of 100 - 15()u. A portion of the solvent is pressed out of the gel in the disk refinery, and

the thus free solvent can be separated from the residual gel

by filtration or decantation. More solvent can be removed by centrifuging the gel, which results in a more concentrated suspension of mass in the solvent. The mass produced according to the present method is a suspension of fibers in the solvent. It is possible to make paper directly from this pulp, but due to solvent economy and using conventional papermaking equipment, so

it is absolutely preferred to convert the mass into a suspension consisting of fibers in water. This can be achieved by solvent exchange, i.e. by replacing hexane or a similar organic solvent with another organic liquid ^e.g.

with hexanol), which in turn can be replaced with water. A similar technique is to strip the solvent with steam, whereby the solvent evaporates, but this can lead to a loss of the fine surface structure.

In the following examples, polyolefins with a high molecular weight were used, and although polymers with a high molecular weight are difficult to handle, they are usually better as pulp according to the present invention. When using the product for something other than paper production, but also for paper production, you can use

a polyolefin with an average viscosity molecular weight as low as 30,000 - 40,000, i.e. with a limiting viscosity number (?)) as low as 1.0 dl/g.

Pulp should contain fibrous elements (i.e., rolls of thin felt) having a length comparable to natural cellulose fibers and suitable for papermaking. In the mass,

produced by the method according to the present invention, the fibers are of suitable length, which will be apparent from the data determined in connection with the individual examples.

The fibers are treated with a water dispersant, preferably before or during steam stripping, to prevent fiber agglomeration, preferably with poly(vinyl alcohol) as this material is not subsequently washed away from the fibers to a large extent.

A major part (calculated by weight) of the fibers, which can be produced in accordance with the present invention and for use in papermaking, have average lengths (determined according to TAPPI Test 232 SU68) between 0.3 mm and 10 mm, and preferably between approx. 1.0 mm and 5.0 mm. If the fibers are longer than approx. 5.0 mi then increases the risk of flocculation during paper production. In other words, less than 10% remains, e.g. at most 8.3% by weight, on a 20 Mesh Tyler Standard sieve, but at least 14%

and preferably approx. 28% by weight remains on a 65 mesh sieve, and preferably at least 25% by weight remains on

a 35 mesh sieve.

A main part (calculated on the weight) of the fibres, which are produced

in accordance with the present invention, has a diameter between 0.01 mm - 0.10 mm, and preferably between approx. 0.01mm and 0.03mm.

Because of. that the diameter of the individual fibers varies along the length of the fibers, it is sometimes easier to characterize the dimensions of the fibers by referring to their fineness.

A major part (calculated by weight) of the fibers from the present invention has a medium fineness (determined according to TAPPI

Test T234 SU67) between 10 and 20 decigrex (mg/100m).

Fibers with a fineness greater than approx. 20 decigrex has increasingly poor drainage properties, which makes it less suitable for paper production.

See the attached drawings, where:

Figure 1 shows a schematic drawing of the apparatus described in detail above; Figure 2 shows a cross-section of the precipitation zone, which is shown in figure 1; Figure 3 shows the enthalpy diagram for hexane;

Figure 4 shows an electron micrograph of the product from example

1 and with a magnification of 500x; Figure 5 shows an electron micrograph of the central area from Figure 4, and with a magnification of 10,000x; Figure 6 shows an electron micrograph in the sar product from example 3 trial 5, and with magnification 500x; Fig. 7 shows an electron micrograph of the central area of fig. 6, magnification 10,000x.

Example 1

Twenty kilograms of high density polyethylene with a viscosity average molecular weight of 200,000 was dissolved in 2,000 liters of n-hexane at a temperature of 138°C in a 7,000 liter glass-lined container, forming a solution in which the polyethylene concentration was 10.0 g/liter The solution was agitated at

about. 100 revolutions per minute. The pressure in the discharge vessel was maintained at 5.0 kg/cm o. The polyethylene discharge was fed through a transfer tube, which was gently heated to maintain the temperature at 138°C and the pressure at 5.0 kg/cm<2>. The overlining rudder was 30 meters long and had an external diameter of 25 cm. This discharge was then adiabatically directed at a rate of 800 liters per hour through a 12.7 mm nozzle to a throttle ball valve, which was held 60-80% open during the passage to atmospheric pressure. The material was then led through a precipitation tube with a diameter of 12.7 mm and a length of approx. 270 mm. From the precipitation tube, the fibrous gel, which has a fiber content of approx. 3 percent by weight and a temperature of approx. 70°C, conducted at atmospheric pressure to a post-precipitation transfer tube, which was 50.8 mm in diameter. The fibrous gel was then passed into a vapor separation vessel (cyclone), whereby vaporized hexane departed from the top. The fibrous gel nodules (which were less than about 10 cm long) were collected at the bottom, diluted to about 1% fiber content by addition of hexane, and then this was passed through a simple disc refiner with 30.5 cm discs as medium coarse plates. The refiner was run at 2,000 revolutions per minute. The refined fibrous slurry was then collected in a glass-lined container and recycled through the refiner a total of four times. The fiber slurry of 1.0% consistency was pumped through a filter, and the resulting wet cake of 13% consistency was added to a 2,000 liter glass-lined steam-stripped container together with 1,500 liters of water and (to facilitate dispersion) 150 grams of polyvinyl alcohol with a molecular weight of 10,000, after which 94% was hydrolysed. The remaining hexane was steam stripped from the fibers at a temperature of 60°C. The resulting aqueous slurry of polyethylene fibers with a

consistency of 1% was then centrifuged to remove water. This resulted in a wet fiber cake with approx. 50% oven-dry consistency.

The fibers produced in this way have a gas absorption surface area of 4.8 m 2 /g.

The fibers were fractionated on Tyler standard sieves in accordance with TAPPI Test T 233 SU 64, and the following fiber fractions were obtained:

The residue on the 20 mesh sieve has a minimum average length of 1.18 mm and a roughness equivalent to 62 decigrex (the residue on the 35 mesh sieve has an average length of 1.5 4 mm and a roughness equivalent to 32 decigreks, ' while that which

remains on 65 mesh sieve has a length of 1.18 mm and a roughness corresponding to 26 decigrex.

Two sets of standard handsheets were prepared, one set from 100% fibers and one set from a mixture consisting of 50% by weight fibers and 50% by weight bleached kraft fibers with a C.S.F. of 400.

C.S.F. = Canadian Standard Freeness.

The hand sheets, which have a base weight of approx. 60.44 g/m 2 , was formed on a British sheet machine using 150 mesh stainless steel screen in accordance with TAPPI Test T 205 M-58. The sheets were gusseted from the screen or wire in the standard manner and subjected to a 15 second long cold press (21°C), at 7.0 kg/cm 2 against a polished mesh. The sheets were pressed in the same way a second time after turning them on the web, so that both sides of the hand sheet were given a smooth finish. The final drying was carried out using a rotary dryer at 104.5°C. The sheets were then tested for strength and optical properties according to the standard TAPPI testing methods:

. The fibers were further mixed with bleached Orkraft paper 400 C.S.F. as 40 and 80% by weight blends, and the blends were formed into paper on a small paper machine, and the resulting paper webs were tested for strength and optical properties as mentioned above. These samples, which almost entirely correspond to current industrial paper production, show that the strength properties are higher than for hand sheets:

Example 2

This example shows the production of pulp in accordance with the present invention of polyethylenes with high density and with different average viscosity-molecular weights. The method, which is shown in example 1, was essentially used again for the different polyethylenes. Process details are shown in Table 2A below, the fiber fraction results are shown in Table 2B, and the handsheet properties are shown in Table 2C below: It can be seen from the preceding example 2 that an increase in molecular weight usually results in improved handsheet properties.

Example 3

This example illustrates the operation of the present method under different temperature conditions. The solvent used was n-hexane. The general procedure from Example 1 was followed. Table 3A indicates special process conditions. Table 3B provides the fiber fraction results, and Table 3C provides handsheet properties. The gel was diluted to a concentration of

1% polyethylene by adding hexane for refining.

The meaning of the maximum temperature limit, as before

is mentioned, can probably be best illustrated by reference to figure 3, which is an enthalpy diagram for hexane, which is the solvent used in previous example 3. As can be seen from this diagram, if the solution temperature exceeds approx. 188°c (point A on the saturated liquidus curve), and after precipitation of polymer, the solvent is in vapor phase and not in liquid phase, as the temperature of the precipitated polymer will essentially correspond to the boiling temperature of hexane (68.3°C) at atmospheric pressure, and which corresponds to point B on the saturated steam curve in the diagram. Therefore, the used discharge temperature must be less than the temperature of the saturated liquidus curve (A) and which corresponds (at constant temperature) to the point on the saturated

vapor curve (point B), which represents the boiling point of the solvent. The chosen temperature should therefore be less than that at point A, so that part of the solvent will be in liquid phase, and the chosen temperature will depend on the polymer concentration in the starting solution, taking into account that the polymer concentration in the fibrous gel should not exceed approx. 5 percent by weight.

When it e.g. It is desirable that the polymer concentration in the gel does not exceed approx. 5 percent by weight, and since the maximum polymer concentration in the used solution with polymer with a high molecular weight, as previously described, is 2%, the evaporation of this solvent should not exceed 76%

at this concentration level. Referring to Figure 3,

and provided that hexane is used as the solvent,

then you see that the maximum evaporation corresponds to a temperature of approx. 160°C (see line C-D) as the line segment C-D on the left side of the hexane boiling point ordinate represents the liquid phase portion

(approx. 1/3 of the line C-D), and the line segment C-D on the right side of the boiling point ordinate represents the vapor phase (approx. 2/3 of the line

C-D).

A similar analysis will give maximum release temperatures, which are applicable to the present method for other solvent and/or other pressure conditions.

Measurement of properties

This example indicates typical values for hydrodynamic surface area, compressibility constant and drainage time for fibers produced according to the present invention. Some of the fibers are produced according to previous examples. All the fibers were treated with 2% PVA, which was added during the steam stripping described in Example I.

The polyethylene fibers specified in example 4, i.e. production series 1-4 in table 4 above, were produced using the method indicated in example 1, but with the following changes with regard to special conditions:

Example 5

This example illustrates the production of a fiber-shaped

gel suitable for subsequent refining into separate fibers. A 3% by weight polymer solution was prepared by dissolving polyethylene, which has an average viscosity molecular weight of 400,000 (n = 9.4) and was prepared by the Ziegler method, in hexane at 135°C. The solution vessel, containing this polymer solution, was connected to a pressure-reduced vessel (receiver vessel), which was at atmospheric pressure, by means of a rudder 5 cm in diameter. A valve was mounted in this rudder, and at a short distance from the dissolution container.

The polymer discharge was then transferred to the depressurized vessel by opening the valve. The flow rate of the discharge between the valve and the pressure-reduced container was 80 m/sec. at equilibrium, and the vapor-liquid ratio of hexane was 1/1.

The produced fibrous material had a noodle length

of 5cm, a diameter or thickness of 50pm, a gas absorption surface area of . 80 m 2/g, and a strength of 2 g/d.

Example 6

This example shows the production of a polypropylene gel which

is suitable for subsequent refining into separated fibres.

A 3% by weight solution was prepared by dissolving polypropylene, which has an intrinsic viscosity number of 5.2, in benzene at 135°C. The resulting polymer solution was allowed to flow out at atmospheric pressure according to the same method as described in Example 5. The flow rate of the mixture was 85 m/sec, and the vapor-liquid ratio of benzene was 1.5/1.

The fibrous material obtained was on average 7cm,

. in length, 3^u in diameter or thickness, and the gas absorption surface area was 30 rn^/g, and the strength was 1.5 g/d.

Example 7

A fibrous polypropylene gel, which was prepared according to Example 6, was diluted to a consistency corresponding to 2% by weight. This gel was refined by passing it once through a disc refiner with a plate spacing of 75 microns.

The refined fibers were then fed to a mixer, the water is exchanged with isopropanol, which is carried out by first adding 100% isopropanol, filtering, adding 50/50 isopropanol/water, filtering and finally dispersing in 100% water. Potato starch (5% by weight of the fibers) was added to the water to make the fibers water-dispersible.

Standard hand sheets were made from 100% polypropylene fibers, which

was produced in accordance with TAPPI test 205 m-58, and which is described in example 1.

The sheets were then tested in accordance with the standard

TAPPI testB methods, and the following characteristics were noted:

According to the method according to the present invention, the polymer solution flows out under at least self-pressure as a continuous stream at a constant speed through a nozzle to thereby cause partial flash evaporation of the solvent, and again becomes a polymer solution gel which is then ground. The paint releases separate fibers from the gel, which have those above. mentioned measurable properties. These properties were measured on fibers containing polyvinyl alcohol, as we have found it advantageous to add PVA to the pulp to facilitate dispersion. The polyvinyl alcohol must be at least 77% hydrolysed, preferably 80%

or more hydrolysed, and may have a relatively low molecular weight, e.g. 12,000-20,000. The best method of application is when the mass amount is made up of at least 1% by weight of polymer, e.g. it can be added in a ratio of 2% by weight of polymer, even if the 2% cannot necessarily be detected.

Dispersants (surfactants) other than PVA can be used, and on the whole the use of a dispersant is not an important feature of the method according to the present invention. However, we have found that the result is best when using PVA.

The morphology of all the masses prepared according to the above examples is studied under the electron microscope, and they all contain large quantities of separate arbitrarily oriented rolls of film whose thickness was approx. 1 m and up to perhaps 10 V m* Most of the film showed, as previously described, a wrinkled or knotted texture.

The properties drainage time and drainage factor were measured as follows: Drainage time was measured by introducing 400 ml of 0.5 percent fiber slurry in the standard sheet form, which is described in TAPPI test T205 M-58, whereby a 150 mesh stainless steel wire was used. sieve at the bottom of this, and whereby the water covered the sieve for the fiber slurry was introduced. Water was added up to the mark in the sheet form. The slurry was agitated using four up and down strokes caused by the standard stirrer. The valve on the sheet mold was opened, and the water was drained from the mold. The time between opening of the valve and the first sound of air suction through the handsheet material, which was deposited on the screen, was recorded with a stopwatch and given as drainage time in seconds.

A more precise characterization of the drainage characteristics, and then a characterization that is most closely related to the hydrodynamic surface area, is the drainage factor. The drainage factor for the relevant fibers is greater than

about. 0.2, preferably greater than 0.5/ and it is typical that it amounts to 1.0 second/gram. This characteristic is mainly determined in connection with TAPPI test T221 OS-63 following a minor modification regarding the calculation method. In short, it weighs approx. 10 grams of the fiber sample that is dispersed in water. The slurry is added to the standard sheet form, and water is added up to the mark. The slurry is stirred using four upward and downward movements

blows from the standard stirrer, which is then removed. The water temperature in the mold is measured, and the drain valve is opened. The time between opening the valve and the first sound of suction is noted. The method is repeated with just water (not fibres) in the sheet form, whereby the temperature and drainage time are noted. The drainage factor in seconds per gram is calculated as follows:

where

DF = Drainage factor, second/gram

D = Drainage time with mass in form, seconds

d = Drainage time without mass in form, seconds VT = Viscosity of water at temperature T

W = Weight of the fibers used in the experiment, grams. The amount (i - 1) appears in tabular form in the aforementioned TAPPI test TT221 OS-63. This amount is multiplied by 0.3, which is empirically determined for the fibers produced.

Claims (8)

1. Process for the production of a polyolefin fiber mass, suitable for the production of paper, whereby a hot solution of a thermoplastic polymer, containing 0.5-5 wt. % polymer, is extruded into a zone with lower pressure, whereby the solvent flash evaporates and the polymer is precipitated in the form of fibers, after which the precipitated polymer is ground, characterized in that the extrusion is carried out under such conditions with regard to solvent temperature and pressure in low- the pressure zone that 20 - 70% of the solvent is evaporated from the solution, whereby a gel is formed which contains the remaining solvent and where the polymer makes up less than 8% by weight of the gel/and that the polymer is subjected to a grinding treatment while it is a gel in the solvent. 2. Method according to claim 1, characterized in that 23 - 60% of the solvent is evaporated from the polymer solution. 3. Method according to claim 1, characterized in that the gel is subjected to grinding treatment when the amount of polymer in the gel is less than 5% by weight of the gel. 4. Method according to claim 1, characterized in that the gel is subjected to the painting treatment when the amount of polymer in the gel is less than 2% by weight of the gel. 5. Method according to claim 4, characterized in that the refining is carried out by passing the gel through a disc refiner. 6. Method according to claim 5, characterized in that a solution of a crystalline olefin polymer is used. 7. Method according to claim 5, characterized in that a polyolefin with a high molecular weight is used, and which has a melt flow index of less than 0.5. 8. Method according to claim 7, characterized in that polyethylene is used with a medium viscosity molecular weight above 100,000, or polypropylene with an average viscosity molecular weight above 300,000.