CA1216118A - Process for producing stretched articles of ultrahigh- molecular-weight polyethylene - Google Patents

Process for producing stretched articles of ultrahigh- molecular-weight polyethylene

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Publication number
CA1216118A
CA1216118A CA000455728A CA455728A CA1216118A CA 1216118 A CA1216118 A CA 1216118A CA 000455728 A CA000455728 A CA 000455728A CA 455728 A CA455728 A CA 455728A CA 1216118 A CA1216118 A CA 1216118A
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Prior art keywords
weight
molecular
ultrahigh
melt
polyethylene
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CA000455728A
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French (fr)
Inventor
Masanori Motooka
Takao Ohno
Hitoshi Mantoku
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Mitsui Chemicals Inc
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Mitsui Petrochemical Industries Ltd
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Priority to CA000455728A priority Critical patent/CA1216118A/en
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Abstract

Abstract of the Disclosure A process for producing a stretched article of ultrahigh-molecular-weight polyethylene, which com-prises (1) melt-kneading a mixture composed of (A) 15 to 80 parts by weight, per 100 parts by weight of the components (A) and (B) combined, of ultrahigh-molecular-weight polyethylene having an inherent viscosity [n], determined at 135°C in decalin, of at least 5 d1/g and (B) 85 to 20 parts by weight, per 100 parts by weight of the components (A) and (B) combined, of a paraffinic wax having a melting point, determined by the DSC method, of 40 to 120 °C and a weight average molecular weight (Mw), determined by the GPC
method, of at least 230 but less than 2,000 in a screw extruder while maintaining the temperature of the mixture at 180 to 280°C, (2) melt-extruding the molten mixture through a die kept at a temperature of 180 to 300°C, (3) cooling the resulting unstretched extru-date to solidify it, and (4) subjecting the unstretched solidified extrudate to a stretching treatment at a temperature of 60 to 140°C at a stretch ratio of at least about 3:1 when step (2) is carried out while a draft is applied to the unstretched extrudate or at a stretch ratio of at least about 10:1 when step (2) is carried out in the absence of a draft.

Description

I

This invention relates to a process for product in stretched articles of ultrahigh-molecular-weight polyethylene, such as stretched filaments, strands, fibers films, sheets, tapes and the like. Portico-laxly, it relates to a process by which stretched shaped articles of ultrahigh-molecular-weight polyp ethylene having high tensile strength and modulus of elasticity and a high quality of excellent uniformity and reproducibility and being substantially free from stretching unevenness can be produced easily with industrial advantage and excellent productivity by using a screw extruderO
More specifically, this invention relates to a process for producing a stretched article of ultrahigh-molecular-weight polyethylene, which comprises (1) melt-kneading a mixture composed of PA) 15 to 80 parts by weight, per 100 parts by weight of the components (~) and By combined, of ultrahigh-molecular-weight polyethylene having an inherent viscosity I
determined at 135C in decline, of at least 5 dug preferably at least 7 dug and By 85 to 20 parts by weight, per 100 parts by weight of the components (A) and (B) combined, of a paraffinic wax having a melting point, determined by the DISC method, of 40 to 120C
and a weight average molecular weight (My), deter-mined by the GPC method, of at least 230 but less than
2,000, preferably less than 1,000, in a screw extrude while maintaining the temperature of the mixture at 180 to 280C, I melt-extruding the molten mixture through a die wept at a temperature of 180 to 300C,
(3) cooling the resulting unstretched extradite to solidify it, and Jo , Lo
(4) subjecting the unstretched solidified extradite to a stretching treatment at a temperature of 60 to 140C at a stretch ratio of at least about 3:1 when step (2) is carried out while a draft is applied to the unstretched extradite or at a stretch ratio of at least about 10:1 when step (2) is carried out in the absence of a draft.
ltrahigh-molecular-weight polyethylene is known and commercially available, and has superior impact strength, abrasion resistance, chemical resistance and tensile strength to general-purpose polyethylene. It has found wider applications as engineering plastics. Since, however, it has a much higher melt viscosity than general-purpose polyethylene and poor flyability, it is quite difficult, both in respect of the shaping operation and the quality and properties of the resulting shaped articles, to provide practical shaped articles by extrusion molding or in-section molding. In the prior art, therefore, ultrahigh-molecular-weight polyethylene is shaped exclusively by compression molding means such as ram extrusion, and extrusion molding means having high productivity cannot be employed. As an exception, only rod-like shaped articles are formed -from it by extrusion molding at very low speeds.
For the stretching of monofilaments of high-density polyethylene at high stretch ratios, there were proposed, for example, a method which comprises preparing a dispersion of polyp ethylene in a high concentration by incorporating 20 to 150%, based on the weight of polyethylene, of an additive having a high boiling point higher than the melting point of polyethylene, forming a primary fibrous material from the dispersion, and hot-stretching it to 3 to 15 times its original length while leaving 5 to 25%, based on the as-spun fibrous material, of the additive therein (Japanese Lo Patent Publication No 9765/1962) and a method which comprises spinning a solution of linear polyethylene having a molecular weight of at least 400,000, and stretching the filaments at a temperature at which the
5 modulus of the filaments reaches at least 20 Gap (Japanese Laid-Open Patent Publication No. 15408/1981 corresponding to British Patent No. 2,051,667).
In these previously proposed methods, ultrahigh-molecular-weight polyethylene is dispersed or dissolved 10 in a solvent such as dichlorobenzene, chlorobenzene, nitrobenzene, zillion, solvent naphtha, octane, nonage, decant, Tulane, naphthalene, decline, or tetralin, and spun by specified methods.
When an attempt is made to use such a liquid 15 solvent as a stretchability improver for ultrahigh-mole-cular-weight polyethylene in continuous extrusion spinning from a screw extrude, the solvent and the powdery polyethylene cannot at all be mixed because of an excessively great difference in viscosity between 20 the solvent and the powder Moreover, since the sol-vent acts as a lubricant between the powder and the screw, the powder and the screw revolve together and extrusion becomes practically impossible. Even if extrusion can be effected, the extradite cannot be 25 stretched at all because it is not a uniform mixture.
furthermore, it is impossible to perform melt extrusion spinning continuously by a screw extrude. These solvents have relatively low boiling points and high flammability, and are dangerous to use in a screw 30 extrude adapted to be electrically heated.
As a different technique, Japanese Laid-Open Patent Publication No. 17736/1982 (published October 30, l9B21 discloses the use of a relatively low-mole-cular-weight polyethylene as a moldability improver.
35 This patent document proposes a composition composed of 100 parts by weight of ultrahigh-molecular-weight polyethylene having a molecular weight of at least 1,000,000 and 10 to 60 parts by weight of low molecular weight polyethylene having a molecular weight of 5,000 to 20,000 in order to improve the moldability of the ultrahigh-molecular-weight polyethylene. The molecular weight of the low-molecular-weight polyethylene in this proposal, however, is too high to be suitable for melt-kneading and extrusion molding by a screw extrude.
In fact, a ram-extrusion type compression molding means is employed in this patent document. Naturally, this patent document does not at all refer to melt extrusion under a draft nor the stretching of the melt-extrudate.
A technique relying on extrusion molding is known from Japanese Laid Open Patent Publication No.
193319/1982 (published on November 27, 19~2). This patent document states that a compression molding method has been employed industrially for the molding of ultrahigh-molecular-weight polyethylene; extrusion molding having high productivity is scarcely employed for the molding of ultrahigh-molecular-weight polyp ethylene; and that attempts are made to perform ox-trusion molding by using a special extrude, but only thick sheets having a narrow width can be obtained at very slow molding speeds and the molded articles have a rough surface and much reduced abrasion resistance. In order to overcome these defects, this patent document proposes a process for producing a sheet of ultrahigh-molecular-weight polyethylene, which comprises extrusion-molding a composition composed of 100 parts by weight of ultrahigh-molecular-weight polyethylene having a molecular weight of at least 1,000,000 and 20 to 70 parts by weight of at least one normality solid flow-ability improver selected from the group consisting of aliphatic compounds and alicyclic compounds and having a shear stress, determined by a Koka-type flow tester Lo (with a nozzle having a diameter of 1 mm and a length/
diameter ratio of 5) at a temperature of 180 C and a shear speed of 30 sea 1, of 8 x 105 dyne/cm2 to 4 x 106 dyne/cm2 at a resin temperature of 160 to 250C
and a shear speed of 1 to 30 sea 1, thereby forming a sheet having a width (W) of at least 200 mm and a width width thickness (T) ratio (W/T) of at least 150.
Examples of the normally solid flyability improver shown in this patent document include aliphatic hydrocarbons such as paraffin waxes and low-molecular-weight polyethylene having a molecular weight of about 1,000 to 10,000, alic~clic hydrocarbons such as cycle-pontoon, cyclopentadiene, cyclohexene-type polymers and polymers of C2 fractions in petroleum refining, higher alcohols having at least 15 carbon atoms such as Seattle alcohol and stroll alcohol, and aliphatic esters such as bottle owlet.
This proposal, however, does not include any technical idea of providing stretched articles such as stretch films or filaments having high tensile strength and modulus of elasticity and being free from stretching unevenness. On the contrary, it is quite silent on the stretching of the molten extradite of ultrahigh-mole-cular-weight polyethylene. In its working examples, polyethylene wax or a mixture of polyethylene wax with a DCPD-type petroleum resin or stroll alcohol is used as the flyability improver and the polyethylene used has a molecLIlar weight of 2,000 to 9,000~ Furthermore, this patent document does not at all refer to extrusion molding under a draft.
The present inventors made investigations in order to develop a process for producing stretched articles such as stretched filaments, films or the like of ultrahigh-molecular-weight polyethylene by using an industrially advantageous screw extrude.
These investigations have led to the discovery that stretched shaped articles of ultrahigh-molecular-weight polyethylene having high tensile strength and modulus of elasticity and a high quality of excellent uniformity and reproducibility and being substantially free from stretching unevenness can be produced easily with industrial advantage and excellent productivity by a process comprising a combination of steps (1) to (4) described above using a screw extrude in which the aforesaid mixture composed of (A) and (B) is melt-kneaded and extruded from the screw extrude under adrift or without applying a draft, the resulting us-stretched extradite is solidified by cooling and then stretched at a specified temperature at a stretch ratio above the minimum stretch ratio varying depending upon whether a draft has been applied in the extruding step.
It has also been found that stretched articles of ultrahigh-molecular-weight polyethylene having more improved properties can be provided by melt-kneading and extruding the aforesaid mixture in the screw ox-truer to form the unstretched extradite under a craft of more than 1, preferably at least 2, and/or using a paraffinic wax (B) having the melting point specified in (B) above and a My of at least 230 but less than 1, 000 .
It is an object of this invention therefore to provide a process for producing stretched articles of ultrahigh-molecular-weight polyethylene which have excellent properties and have not been provided here-tougher.
The above and other objects and advantages of this invention will become apparent from the following description.
The ultrahigh-molecular-weight polyethylene and a method for its production are known, and such polyethylene is commercially available.
The ultrahigh-molecular-weight polyethylene ~Z~6~B

(A) used in this invention has an inherent viscosity I], determined at 135C in decline, of at least 5 dug preferably at least 7 dug particularly 7 to 30 dug When the inherent viscosity of the polyethylene is less than 5 dug a stretched article having excel-lent tensile strength cannot be obtained by stretching the unstretched solidified extradite. There is no particular restriction on the upper limit of the in-hornet viscosity, but the upper limit is preferably 30 dug as exemplified above. If the inherent viscosity of the polyethylene is too high beyond 30 dug the melt-spinnability of a mixture of such polyethylene and the paraffinic wax (B) in a screw extrude at the melt-kneading and extruding temperatures specified by the process of this invention tends greatly to be reduced The use of ultrahigh-molecular-weight polyp ethylene having an inherent viscosity of up to 30 dug is preferred.
The term "ultrahigh-molecular-weight polyp ethylene", as used in the present application, denote snot only a homopolymer of ethylene but also a copolymer of ethylene with up to 5% by weight of an alpha-olefin having at least 3 carbon atoms, preferably an alpha-olefin having 3 to 8 carbon atoms, such as propylene, battalion, pontoon, Helene, 4-methyl-1-pentene, or octane The paraffinic wax (B) used in this invent lion has a melting point, measured by the DISC method, of 40 to 120C, preferably 45 to 110C, and a weight average molecular weight (My), measured by the GPC
method, of at least 230 but less than 2,000, prefer-ably less than 1,000, more preferably not more than 900, especially not more than 800. Paraffinic waxes having a melting point of less than 40C, such as liquid paraffins, cause the ultrahigh-molecular-weight polyethylene (A) and the screw to revolve together during melt kneading and extruding by a screw extrude, and make it impossible to perform melt shape in stably. forced rnelt-shaping would not be able to give extradites of practical quality. On the other 5 hand, when the melting point of the paraffinic wax is too high above 120C, there is a restriction on the stretch ratio of the solidified unstretched extradite obtained by the screw extrude, and the stretched article cannot have high tensile strength and modulus of elasticity. Moreover, since the melt-extrusion is carried out under a draft, molding imperfections such as yarn breakage or film breakage occur, and it is difficult to extract and remove the excess of the component (B) from the stretched article as will be described below.
When the weight average molecular weight of the para~finic wax is 2,000 or above, the same defects as in the case of the melting point being higher than 120C arise. On the other hand, when the weight average molecular weight of the wax (B) is less than 230/ the same trouble as in the case of its melting point being lower than 40C occurs.
The melting point determined by the DISC
method, as referred to in this application, denotes the melting point measured by a differential scanning calorimeter (DISC) in accordance with ASTM D34170 The weight average molecular weight (My) determined by the GPC method, as referred to in this application, denotes the weight average molecular weight measured by GPC (gel-permeation chromatography) under the following conditions.
Device: Model 150C, made by Waters Co.
Column: TUSK GMH-6 (6 mm x 600 mm) jade by Toy Soda Co., Ltd.
Solvent: ortho-dichloroben~ene (ODCB) Temperature: 135 C

I

Flow rate: 1.0 ml/min.
Injecting concentration: 30 mg/20 ml ODCB
(the amount injected 400 micro liters) The column elusion volume is corrected by the universal method using standard polystyrene made by Toy Soda Co., Ltd. and Pressure Chemical Co.
The paraffinic wax (By used in this invention may be any paraffinic wax which meets the above melting point and weight average molecular weight requirements specified above, and needs not to be composed only of carbon and hydrogen. For example, it may have a minor amount of oxygen or other elements.
The paraffinic wax (B) may be a variety of substances containing as a main component saturated aliphatic hydrocarbon compounds having the aforesaid melting points and weight average molecular weights.
Specific examples include n-alkanes having at least 22 carbon atoms such as tricosane, tetracosane and in-acontane or mixtures of a major proportion of thesen-alkanes with a minor proportion of lower n-alkanes;
paraffin waxes separated from petroleum and purified;
low pressure method, medium pressure method or high pressure method polyethylene waxes or ethylene co-polymer waxes having a relatively low molecular weight which are obtained by polymerizing or copolymerizing ethylene or ethylene and another alpha-olefin and/or a dine; polyethylene or ethylene cop~lymer waxes ox-twined by reducing the molecular weights of polyp ethylene or ethylene coplymers having a relatively high molecular weight by such means as heat degradation; and oxidized waxes or alpha beta-unsaturated acid-modified waxes such as the oxidation products or maleinized products of the above-exemplified waxes. From these paraffinic waxes, those having the above-specified melting points and weight average molecular weights are selected and used in the present invention.
Naphthalene (melting point about 80C; mole-cuter weight 128), for example, exemplified as an additive in Japanese Laid-Open Patent Publication No.
15408/1981 is a hydrocarbon compound not included within the scope of the paraffinic wax By specified in the present invention As shown in Comparative Example 5 shown hereinbelow, when naphthalene is used together with the ultrahigh-molecular-weight polyethylene (A) specified in this invention and melt-kneaded and melt-extruded by a scow extrude, a strand of uniform quality cannot be formed because of the poor compute-ability of naphthalene with the polyethylene (A) and therefore it cannot be stretched uniformly nor at such a high stretch ratio as can give a satisfactory stretched article. Likewise, as shown in comparative Example 4 given hereinbelow, n-hexadecane (melting point 18.14C; molecular weight 226), a C16 saturated hydrocarbon not included within the scope of the purify-phonic wax (B) specified in the present invention, cannot be used in the process of this invention.
In step (1) of the process of this invention, a mixture composed of the aforesaid ultrahigh-molecular-weight polyethylene having an inherent viscosity [I], determined at 135C in decline, of at least 5 dug preferably at least 7 dug and the paraffinic wax (B) having a melting point, determined by the DISC method, of 40 to 120C and a weight average molecular weight (My), determined by the GPC method, of at least 230 but less than 2,000, preferably less than 1,000, in the proportions specified above is melt-kneaded in a screw extrude while the temperature of the mixture is main-twined at 180 to 280C~
Consequently, there is obtained a molten mixture of (A) and (B) which can be subsequently melt-extrude with or without applying a draft and after if -cooling, subjected to a stretching treatment, with little or no unevenness in extrusion and stretching to provide a product having a high quality of excellent uniformity.
The mixture of the polyethylene (A) and the paraffinic wax (B) to be melt-kneade~ can be prepared by properly selecting known mixing means such as a Herschel mixer and a V-blender at a temperature of, for example, room temperature to 120C. It is also possible to prepare the mixture by such means, melt-knead it in a single or multiple screw extrude in the same way as in step I then granulate the mixture to form molding pellets and use the molding pellets in step (1) of the process of this invention.
The proportion of the ultrahigh-molecular-weight polyethylene (A) is 15 to 80 parts by weight, preferably 20 to 60 parts by weight, and the proportion of the paraffinic wax (B) is 85 to 20 parts by weight, preferably 80 to I parts by weight, both per 100 parts of the components (A) and (B) combined.
If the amount of the ultrahigh-molecular-weight polyethylene (A) is less than 15 parts by weight, the resulting mixture is difficult to melt-knead unit firmly in the screw extrude and obtain a uniform molten mixture. Furthermore, when the resulting mix-lure is melt extruded in step (2) under a draft, a trouble of breakage of the molded article occurs. The same trouble occurs in the stretching treatment in step (3) to be carried out after the unstretched extradite has been solidified by cooling. On the other hand, when the proportion of the polyethylene (A) exceeds 80 parts by weight (namely, the proportion of the purify-phonic wax (B) is too small), melt-kneading in the screw extrude becomes difficult, and the melt viscosity of the molten mixture becomes unduly high so that melt-extrusion in step (2) becomes difficult. Furthermore, there is a marked roughening of the surface of the resulting unstretched extradite, and a trouble of breakage of the article occurs during the stretching treatment after solidification and during the melt extrusion under a draft.
In step (1), the melt-kneading in the screw extrude is carried out while the temperature of the mixture of (A) and (B) is maintained at 180 to 280 C, preferably 180 to 250C. If the temperature is lower than the lower limit specified above the melt vise costly of the molten mixture is too high so that unit form kneading is difficult. If the temperature is too high, the ultrahigh-molecular-weight polyethylene PA) itself is degraded and unduly reduced in molecular weight, and it is difficult to provide a stretched article having the desired high tensile strength and modulus of elasticity. The temperature of the mixture of (A) and (B) can be measured and determined by the method described in Chapter 10: Experimental Temperature and Pressure Measurement of "Engineering Principles of Plasticating Extrusion" written by Zehev Tadmor and Imrich Klein and published by Robert E. Krueger Publishing Company, Huntington New York (1978).
There is no particular restriction on the screw extrude used. For example, a single screw extrude, a multiple screw extrude and other various known-types of screw extrudes can be used.
In step (2) of the process of this invention, the molten mixture of (A) and (B) obtained in step (A) is melt-extruded from a die provided at the extrusion end of the extrude and kept at a temperature of 180 to 300C, preferably 180 to 270C.
If the temperature of the die is lower than the specified lower limit, the melt-extruding operation becomes difficult, and an attempt to extrude the molten mixture forcibly results in non-uniform extrusion. If ~2~6~

the temperature of the die is higher than the above-specified upper limit, the ultrahigh-molecular-weight polyethylene (A) itself is degraded and reduced unduly in molecular weight, and a stretched article having the desired high tensile strength and modulus of elasticity is difficult to obtain.
The die used can be properly selected accord-in to the desired shape of the unstretched extradite.
For example, when a die of the spinnerets type is used, filaments or strands can be obtained by melt extrusion.
Or a tape, film or sheet can be molded by melt extra-soon by using a lip die or a T-die for tapes, films and sheets.
In the process of this invention, the us-stretched extradite formed as in step (3) is cooled and solidified.
At this time, the as-formed unstretched extra-date before cooling can be melt-extruded as described above under a draft, and this gives better results.
Means for applying a draft to the as-formed unstretched extradite are known, and can be utilized in the process of this invention. A draft can be applied by taking up the extradite at a higher take-up linear speed than the extrusion linear speed in step (2). according to one embodiment, in subjecting the unstretched extradite formed by melt extrusion in step I to the cooling and solidification treatment in step (3), a drafting action can be exerted on the as-formed unstretched extradite by taking up its cooled product solidified to such an extent as can be taken up, at a larger take-up linear speed than the melt-extrusion linear speed For example, as shown in Examples given hereinbelow, an air gap of a suitable desired distance is provided between the melt-extrusion die and a cooling medium, for example, the water surface of a cooling tank containing cold water, for cooling and solidifying the unstretched extradite extruded from the die, and the unstretched extradite is taken up by a take-up roll or bar disposed in the cold water and cooled and solidified. By taking up the unstretched extradite at a larger take-up linear speed than the extrusion linear speed of the molten mixture of (A) and (B) from the die, a drafting action can be exerted on the unstretched extradite.
When a draft is to be exerted on the us-stretched extradite by melt extrusion in the process of this invention, the draft ratio is more than 1, prefer-ably not less than 2.
In the present invention, the draft ratio denotes the ratio of the diameter Al of a die orifice (the diameter of an as-spun filament) to the diameter r2 of the solidified filament (rl/r2), or the ratio of the die lip clearance do (the thickness of the as-formed film) to the thickness d of the solidified film (dl/d2).
Cooling and solidification in step (3) of the process of this invention can be carried out by any desired means by which the unstretched extradite formed by step (2) on which a draft has been, or has not been, exerted as above, can be cooled and solidified. For example, it can be carried out by contacting the extra-date with a gaseous cooling medium such as cooled air or a cooled inert gas, a liquid cooling medium such as cold water, or other suitable cooling media. The cooling temperature is, for example, about -20C to about 60C.
In step (4) of the process of this invention, the solidified product obtained in step (3) is sub-jetted to stretching treatment The solidified us-stretched extradite is subjected to a stretching treat-mint at a temperature of 60 to 1~0C at a stretch ratio of at least about 3:1, for example from 3:1 to 50:1, when the above melt-extrusion is effected while applying a draft to the resulting unstretched extradite, and at a stretch ratio of at least about l0:1, for example from 10:1 to 100:1 when the melt-extrusion is effected without application of a draft. The stretch ratio can be varied properly depending upon not only the presence or absence of drafting but also the draft ratio, the weight average molecular weight (My) of the paraffinic wax (B). Let us assume, for example, that an unstretched extradite formed without drafting is cooled and solidified and then subjected to a stretching treatment. If the My of the paraffinic wax (B) is less than about 800, a satisfactory stretched article having a high modulus of elasticity can be obtained. But when the It of the paraffinic wax (B) is more than about 800 and up to about 2,000, better results are obtained by employing a stretch ratio of at least about 15:1, preferably at least about 17 lo On the other hand, let us assume that an us-stretched extradite formed under a draft is cooled and solidified and then subjected to a stretching treat-mint. In this case, a satisfactory stretched article of a high modulus of elasticity can be obtained at a stretch ratio of at least about 3:1 if the My of the paraffinic wax (B) is not more than about 800.
When the My of the paraffinic wax (B) is more than about 800 and up to about 2,000l better results are obtained by employing a stretch ratio of at least about 5:1, preferably at least about 10:1.
The stretching treatment is carried out at a temperature of 60 to 140C, preferably 100 to 135C.
If the stretching temperature is below the above-specified lower limit, it is difficult to stretch the extradite it the desired stretch ratio. If, on the other hand, it is higher than the specified upper limit, the ultrahigh-molecular-weight polyethylene (A) becomes too soft and a stretched article having a high ~;2~6~

modulus of elasticity cannot be obtained although the extradite can be stretched.
The treatment can be performed at the above temperature by stretching means known per so. For example, in the case of a filament or strand, a pair of godet rolls may be used for example, and the relative linear speed of the godet rolls is properly changed and selected so that stretching is effected at the desired stretch ratio. A film or tape, on the other hand, is stretched by using a pair of snap rolls, for example.
Hot stretching may be carried out in an atmosphere of a heat medium, for example in an atmosphere of heated air, steam, a heated liquid, etch Or it can be carried out by using heat waves or a hot plate. These means may be used in combination.
Preferably, the hot stretching is carried out in an atmosphere of a heat medium. It is especially preferred to use as the heat medium a solvent (liquid medium) which can dissolve the paraffinic wax (B) or remove it by leaching and has a boiling point higher than the stretching temperature employed, preferably a boiling point at least about 10C higher than the stretching temperature employed. examples of the liquid medium are decline, decant and kerosene. By employing this preferred embodiment, the excess of the paraffinic wax (B) can be removed by extraction or leaching simultaneously with the stretching treatment, and it becomes possible to reduce unevenness in stretching and perform stretching at a high stretch ratio. Of course, the excess of the paraffinic wax tub) may be removed by other means. For example, it can be achieved by treating the solidified unstretched extra-date with such a solvent as hexane or Hutton prior to the stretching treatment. Or the stretched product may be subjected to a similar solvent treatment.
Fibers having fine pores can be obtained by ~2~6~

removing the paraffinic wax (B) such that the amount of the wax (B) remaining in the stretched article is not more than about 10% by weight. The modulus of elastic city and tensile strength of such fibers which are determined by a method involving measuring the true cross-sectional area of the fibers on a weight basis do not fall below those of the fibers before extraction of the paraffinic wax (B). This embodiment is therefore preferred.
If in step (4), the stretch ratio in the aforesaid solvent is less than 3:1 in stretching the solidified product of the unstretched extradite obtained under a draft, it is frequently the case that the tensile strength and the modulus of elasticity of the stretched article increase only to a small extent, and stretching unevenness occurs in the stretched article to degrade its appearance On the other hand, if in step I the stretch ratio in the aforesaid solvent is less than 10:1 in stretching the solidified product of the unstretched extradite obtained without applying a draft, it is frequently the case that the tensile strength and the modulus of elasticity of the stretched product increase only to a small extent and the polyp ethylene in the stretched article is whitened to degrade the appearance of the stretched article.
The stretching treatment in step (4) needs not to be performed in one stage. If desired, it can be performed in a multiplicity of stages. When the latter is employed, the stretch ratio specified in step (4) of the process of this invention denotes the total of stretch ratios in the individual stages. The final stretching speed in the stretching treatment is not particularly restricted. But from the viewpoint of productivity, it is preferably at least 3 m/min., more preferably at least 5 m/min. usually, the stretching is carried out monoaxially in the extruding direction (machine direction). In the case of a film or sheet, the stretching may further be carried out in the trays-verse direction (biaxial stretching). the stretching in the transverse direction can be carried out under the same conditions as described above except that the stretch ratio is set at 1.5:1 or higher, preferably at 2:1 or higher.
In the unstretched extradite formed by steps I and to) of the process of this invention, the compatibility between the component (A) and the come potent (B) is excellent and they form a very homogene-out mixture. This state can be ascertained, for ox-ample, by observing the sectional surface of the us-stretched filament or film with a high magnification scanning electron microscope. Taking up a filament as an example, a blend of the ultrahigh-molecular-weight polyethylene (A) and the paraffinic wax (B) in equal proportions is melt-kneaded and melt-spun by using a screw extrude. The resulting unstretched filament is cut by a sharp blade such as a micro tome in a direction at right angles to its longitudinal direction to form a sample (a). By the same method, another sample (b) is cut out from the unstretched filament and then immersed at room temperature in a non-polar solvent such as hexane or Hutton for at least 1 hour to remove the paraffinic wax (B) by extraction. The sectional sun-face of the sample (a) and the sectional surface of the sample (b) after extraction are both observed compare-lively with a scanning electron microscope at a magnify-cation of at least 3,000. The same procedure can bused for a film.
Since the paraffinic wax (B) of this invention has good compatibility with the ultrahigh-molecular-weight polyethylene I depressions of a size above 0.1 micron are scarcely observed. On the other hand, when, for example, naphthalene mentioned hereinabove is used instead of the paraffinic wax (B), its dispersion in the polyethylene it poor and innumerable depressions of a size above 0.1 micron are observed in the final stretched article.
If desired, the stretched articles of ultrahigh-molecular-weight polyethylene obtained by the process of this invention may contain other additives. These additives are preferably blended with the polyethylene (~) or the mixture of the polyethylene (A) and the paraffinic wax (B) in step (1). If desired, however, they may be added and mixed during melt-kneading in the screw extrude.
These additives may be various additives conventionally used for polyolefins, for example heat stabilizers, weather ability stabilizers, coloring agents and fillers. The amounts of these additives blended may be properly selected within the ranges which do not impair the objects of this invention For example, they are about 0.01 to about I by weight for the heat stabilizers, about 0.01 to about 2% by weight for the weather ability stabilizers, and about 0.01 to about I by weight for the coloring agents Examples of the heat stabilizers are finlike compounds such as 2,6-di-tert-butyl-4-methylphenol and 2,2-thiobis-(6-tert-butyl-4-methylphenol) and amine compounds such as phenyl-l-naphthylamine.
An example of the weatherabili~y stabilizers is 2-(2-hydroxyphenyl)benzotriazole.
Examples of the coloring agent or fillers are phthalocyanine pigments, nutrias lake pigments, titanium oxide, zinc oxide, precipitated silica and carbon black.
The stretched articles of ultrahigh-mole~ular-weight polyethylene obtained by the process of this invention have high tensile strength and modulus ox elasticity which cannot be obtained with conventional stretched articles of polyethylene. Accordingly, they can find use as fibers having high tensile strength and modulus of elasticity as well as in conventional applications of stretched articles such as manful-5 metes and tapes. They can be used as various reinforce in materials which require light weight. Furthermore, by utilizing a high degree of crystal orientation due to stretching at very high ratios and micro pores formed secondarily by the extraction of the excess of the paraffinic wax (B), these stretched articles can find applications as various functional materials such as selective membranes and electorates The following examples illustrate the present invention more specifically. It should be noted that the invention is not limited to these examples alone unless it departs from the scope of the present invent lion.
Example 1 A 39:61 blend of ultrahigh-molecular-weight polyethylene ([~]=8.20 dug and paraffin wax (melting point=69C, molecular White) was melt-spun and stretched under the following conditions.
A powder of ultrahigh-molecular-weight polyp ethylene and pulverized paraffin wax were mixed and then melt-kneaded by using a screw extrude (20 mm in diameter; L/D=20) while maintaining the temperature of the mixture at 190C. The molten mixture was extruded from a die with an orifice diameter of 1 mm in the absence of a draft, and solidified in cold water at 20C with an air gap of 10 cm. Subsequently, the extradite was stretched in a stretching vessel (the temperature of the inside of the vessel 130C; the length of the vessel 40 cm) containing n-decane as a heat medium by means of a pair of godet rolls. The stretch ratio was determined by calculation from the rotation ratio of the godet rolls. Table 1 summarizes ISLE

the module ox elasticity, tensile strengths and the amounts of the residual paraffin wax at the various stretch ratios.
The modulus of elasticity was measured by 5 means of a dynamic viscoe~asticity measuring device (Vibron DDV-II, made by Toy Baldwin Co., Ltd.) at zoom temperature (23C) and a vibration frequency of 110 Ho, The tensile strength was measured by an Instron universal tester model 1123, made by Instron Company) at room temperature (23C). At this time, the length of the sample between the clamps was adjusted to 100 mm, and the pulling speed, to 100 mm/min. The cross sectional area of the filament required for measurement was determined by measuring the weight and length of the filament with the density of polyethylene taken as 0.96 g/cm3. The amount of the residual paraffin wax was measured after the filament was immersed for 24 hours in Nixon and the paraffin wax was removed from the filament.
It is seen from Table 1 that when the stretch ratio is not more than 10:1, stretched articles having a high modulus of elasticity cannot be obtained.

Table 1 _ Stretch ratio 10.0 20.0 25.5 30.235.5 Run No. 1 2 2 _ 5 Modulus of Tensile 18.6 59.2 68.~ 89.3106.0 strength 0.66 1.02 1.21 1.34 1.

Amount of residual 10.7 5.8 6.9 OWE 6.2 pa r a f i n __ _ , (*) Measured by a dynamic viscoelasticity measuring device (Vibron DDV-II).
Example 2 A 50:50 blend of ultrahigh-molecular-weight polyethylene ([~1=8.20 dug and paraffin wax (melting point=109C, molecular White) was melt-spun and stretched under the same conditions as in Example 1.
Table 2 shows the module of elasticity and tensile strengths of the resulting products at the various stretch ratios. It is seen from Table 2 that at a stretch ratio of 10:1 or less, a stretched article having a high modulus of elasticity cannot be obtained, and when stretching is carried out at a stretch ratio of more than 17:1, a stretched article having a higher modulus of elasticity can be obtained.

aye T b 1 e 2 \
\ Sire tech rail o \ 10.0 15.3 17.4 20.0 25.6 Run No. 6 7 8 9 10 Modulus of elasticity 16.0 26.5 31.2 50.5 71.2 I*) (Gap) strength 0.70 0.92 1.0 a 1 . 24 1.32 (*) Measure by Vibron DDV-IIo Example 3 A 50:50 blend of ultrahigh-molecular-weight polyethylene (~]=8.20 dug and paraffin wax (melting point=42-44C, molecular White) was melt-spun and stretched under the same conditions as in Example 1. Table 3 shows the module of elasticity, tensile strengths and the amounts of residual paraffin wax at the various stretch ratios.
6~L8 - I -Table 3 \
\ Sire Shea rail o , \ 12.8 15.3 17.5 19.9 24.0 Run No. 11 _ 12 13 14 _15 Modulus of elasticity 30.0 42.4 41.6 49.5 64.3 Tensile strength 0.83 1,02 1.12 1.23 1.29 mount of residual 28.6 35.9 32.9 28.1 25.3 paraffin (White (*) Measured by Vibron DDV-II.
Example 4 A 50:50 blend of ultrahigh-molecular-weight polyethylene (1~]=8.20 dug and paraffin wax (melting point=52-54C, molecular White) was melt-spun and stretched under the same conditions as in Example 1.
Table 4 summarizes the module of elasticity and the amounts of residual paraffin at the various stretch ratios.

Table 4 \ _ Stretch ratio \ 12.9 15.3 17.0 20.0 25.7 29.7 Run No. 16 17 18 19 _ 20 21 Modulus of elasticity 25.2 40.2 45.2 49.8 84.1 85.4 (*) (Gap) Amount of residual 10.7 12.9 14.9 15.8 15.4 16.3 paraffin it %
(*) Measured by Vibron DDV~
Example 5 A 17:83 blend of ultrahigh-molecular-weight polyethylene ([n~=19.6 dug and paraffin wax (melting point=69C, molecular White) was melt-spun and stretched under the same conditions as in Example 1.
Table 5 summarizes the module of elasticity at the various stretch ratios. It is seen from Table 5 that when the stretch ratio is not more than 10:1, a stretched article having a high modulus of elasticity cannot be obtained.
Table 5 Sir etch ratio , \ 10.0 12.9 15.3 17.0 20.3 Run No. 22 23 24 25 26 Modulus of elasticity 17.0 42.1 60.9 71.5 83.0 (*) (Gap) (*) Measured by Vibron DDV-II.

I

Comparative Example 1 A 50:50 blend of ultrahigh-molecular-weight polyethylene ([ no .20 dug and high-density polyp ethylene (melting point=130C, molecular weight=
40,000) was melt-spun and stretched under the same conditions as in Example 1. Table 6 summarizes the module of elasticity at the various stretch ratios.
With the above blend, a high stretch ratio could not be achieved, and filaments having a high modulus of elasticity could not be obtained.
table 6 . \ Stretch h ratio 5.4 7.7 10.5 16.1 Hun No. _ 27 28 29 30 Modulus of Breakage elasticity 2.0 7.0 14.2 occurred (*) (Gap) during stretching _ _ _ (*) Measured by Vibron DDV-II.
Comparative Example 2 A 95:5 blend of ultrahigh-molecular-weight polyethylene ([~=8.20 dug and paraffin wax (melting point-69C, molecular White) WAS melt-spun under the same conditions as in Example 1. With this blend, the molten blend could not be extruded from the die orifice of the screw extrude.

Comparative Example 3 A 5:95 blend of ultrahigh-molecular-weight polyethylene ([~]=~.20 dug and paraffin wax (melting point=69C, molecular White) was melt-spun under 5 the same conditions as in Example 1. the resulting strand cooled by an air gap was fragile, and could not be stretched by godet rolls.
Comparative example 4 A 50:50 blend of ultrahigh-molecular-weight polyethylene (~]=19.6 dug and n-hexadecane was melt-spun and stretched under the same conditions as in Example lo Since a uniform strand could not be obtained, unevenness in stretching occurred, and uniform filaments could not be obtained.
Comparative Example 5 A 50:50 blend of ultrahiyh-molecular-weight polyethylene ([~]=8.20 dug and naphthalene was melt-spun and stretched under the same conditions as in Example 1. Since a uniform strand could not be obtained, unevenness in stretching occurred, and uniform filaments could not be obtained.
Example 6 A 25:75 blend of ultrahigh-molecular-weight polyethylene ([~]=8.20 dug and paraffin wax (melting point=69C, molecular White) was melt-spun and stretched under the following conditions. A powder of the polyethylene and pulverized paraffin wax were mixed and melt-kneaded by a screw extrude (20 mm in diameter, L/D=20) while the temperature of the mixture was main-twined at 190C. The molten mixture was then extruded from a die having an orifice diameter of 1 my and solidified in cold water at 20C with an air gap of 10 cm. The melt-extrusion was performed under a draft so that the diameter of the solidified filament became 0.50 mm (specifically, at a draft ratio of 2). Sub-sequently, the resulting unstretched filament was stretched in a stretching vessel (the temperature of the inside of the viselike, the length of the visual cm) containing n-decane as a heat medium by means of a pair of godet rolls While the rotating speed of a first godet roll was adjusted to 0.5 m/min., filaments of different stretch ratios were produced by changing the rotating speed of a second godet roll. The stretch ratios were calculated from the rotating ratios of the godet rolls.
Table 7 summarizes the module of elasticity and strengths at the various stretch ratios. It is seen from Table 7 that a stretched article of high strength can be obtained when the stretch ratio is set at 10:1 or higher.
The module of elasticity and tensile strengths were measured by an Instron universal tester (Model 1123, made by Instron Company) at room temperature (23C), The length of the sample between the clamps was adjusted to 100 mm, and the pulling speed, to 100 mm/min. The modulus of elasticity was calculated by using a stress at 2% distortion. The sectional area of the filament required for calculation was determined by measuring the weight and length of the filament with the density of polyethylene taken as 0.96 g/cm3.

go - 2') -Table 7 \ = Stretch ratio =
_ \ 10.0 12.0 _ 14.0 16.3 18.0 20.0 Run No. 31 32 33 34 35 36 Modulus of elasticity 9.84 15.0 19.0 28.2 31.7 36.3 (*) (Gap) Tensile (Gap) 0.99 1.22 1.30 1.~6 I.47 1.53 (*) Measured by an Instron Universal tester (Model 1123).
Example 7 A 25:75 blend of ultrahigh-molecular-weight polyethylene ([~=8.20 dug and paraffin wax (melting point=69C, molecular White) was melt-spun and stretched under the same conditions as in example 6 except as noted below. The molten mixture was extruded from a die having an orifice diameter of 1 mm, and solidified in cold water at 20C with an air gap of 10 cm. The melt-extrusion was carried out under a draft so that the diameter of the solidified filament became 0.20 mm (specifically at a draft ratio of 5).
lo Table 8 summarizes the module of elasticity and tensile strengths at the various stretch ratios.
It is seen from Table 8 that a stretched article having a high tensile strength can be obtained even at a stretch ratio of about 8:1.

Table 8 _ _ _ \ Stretch ratio \ _ 8.0 Lowe 14.0 _ .
Run No. 37 _ 38 39 40 modulus of elasticity 10.3 19.432.5 33.5 (*) (Gap) Tensile strength 1.511.54 1.72 (*) Measured by Instron Ludlow 1123.
Example 8 A 25:75 blend of ultrahigh-molecular-weight polyethylene (~l=8.20 dug and paraffin wax (melting point=69C, molecular White) was melt-spun and stretched under the same conditions as in Example 6 except as noted below. The molten mixture was extruded from a die having an orifice diameter of 2 mm, and solidified in coxed water at 20C with an air gap of 10 cm The melt-extrusion was carried out under a draft so that the diameter of the solidified filament became 0.04 mm (specifically at a draft ratio of 50).
Table 9 summarizes the module of elasticity and tensile strengths at the various stretch ratios.
It is seen that by increasing the draft ratio from that used in Example 7, a stretched article having a high tensile strength can be obtained even at a stretch ratio of about 6.

Table 9 Stretch ratio ```\ 5.6 6.7 8.0 9.2 Rut No 41 42 43 I
Modulus of elasticity 23.9 31.2 36.5 50.6 (*) (Gap) Tensile strength 1.49 l.B0 1.81 2.43 (*) Measured by Instron Model 1123.
Example 9 A 25:75 blend of ultrahigh-molecular-weight polyethylene ([~]=8.2 dug and paraffin wax (melting point=69C, molecular White) was molded into a film by a T-die and stretched under the following conditions.
powder of the polyethylene and pulverized paraffin wax were mixed, and melt-kneaded and pot-latticed by a screw extrude (20 mm in diameter, L/D=20) while maintaining the temperature of the mixture at 190C. The pellets were molded into a film by a screw extrude (20 mm in diameter, L/D=20) equipped with a coat hanger-type die (lip length mm, lip thickness mm) at 220C. The width of the film was adjusted to 300 rum by using a roll cooled with cold water at 20C. Subsequently, the film was stretched in a stretching vessel (the temperature of the inside of the viselike, the length of the visual cm) containing n-decane as a heat medium by using a pair of snap rolls.
While the rotating speed of a first snap roll 5 was set at 0.5 m/min., stretched tapes of different stretch ratios were obtained by varying the rotating speed of a second snap roll. The stretch ratios were calculated from the rotating ratios of the snap rolls.
Table 10 summarizes the module of elasticity, tensile strengths and tape widths at the various stretch ratios.
Table 10 _ \ Stretch ratio \ , 15.4 24.0 27.0 33.9 46.5 53.0 Run No. 45 46 47 48 49 50 ladles tf(G)aYls- 13.3 22.6 26.2 34.6 49.1 59.4 Tensile strength 0.91 1.27 1.40 1.55 1.73 1.81 to 75.8 60 5 57 0 50.~ 43 3 39 a (*) Measured by Insrton Model 1123.
Example 10 I A 25:75 blend of ultrahigh-molecular-weight polyethylene (l~=8.2 dug and paraffin wax (melting point=84C, molecular White) was molded into a film by a T-die and then stretched under the same conditions as in Example 9 except that a draft was applied to the unstretched extradite during melting from the lip having a width of 300 mm so that the width of the film became 43 mm.
Table 11 summarizes the module of elasticity, tensile strengths and tape widths at the various stretch ratios.

Table 11 - Stretch ratio Jo 2~0 4~0 5~0 7~0 _ Hun No 51 52 53 54 Modulus of elasticity 8 ox 16 I 29 I 62 I
(*) (Gap) Tensile strength 0 ~93 1~60 2~01 2023 Width of (mm) 30.1 20.8 19.3 15.8 (*) Measured by Instron Model 1123 Comparative Example 6 A 50:50 blend of ultrahigh-molecular-weight polyethylene tl~]=8~2 dug and high-density polyp ethylene (melting point=130C, molecular weight=
40,000) was molded into a film by a T-die and then stretched under the same conditions as in Example 9 Since with this blend application of a draft to the unstretched extradite during melting would cause break-age during stretching, the film was molded in a width of 300 mm. Table 12 summarizes the module of elastic city, tensile strengths and tape widths. It is seen that with this mixture, a high stretch ratio could not be achieved, and tapes having a high modulus of elastic city and a high tensile strength could not be obtained.

- I -Table 12 Strut ah ratio _``` \ I 8.2 12.~ 17.5 Run No. _55 _ 56 57 58 Modulus of elasticity 3.8 5.1 11.0 15.2 (*I (Gap) Tensile strength 0.23 0.34 0.41 0.48 Width of the tape I27.5 I03.0 85.2 70.3 (*) Measured by Instron Model 1123.

Claims (5)

What is claimed is:
1. A process for producing a stretched article of ultrahigh-molecular-weight polyethylene, which com-prises (1) melt-kneading a mixture composed of (A) 15 to 80 parts by weight, per 100 parts by weight of the components (A) and (B) combined, of ultrahigh-molecular-weight polyethylene having an inherent viscosity [n], determined at 135°C in decalin, of at least 5 dl/g and (B) 85 to 20 parts by weight, per 100 parts by weight of the components (A) and (B) combined, of a paraffinic wax having a melting point, determined by the DSC method, of 40 to 120 °C and a weight average molecular weight (Nw), determined by the GPC
method, of at least 230 but less than 2,000 in a screw extruder while maintaining the temperature of the mixture at 180 to 280°C, (2) melt-extruding the molten mixture through a die kept at a temperature of 180 to 300°C, (3) cooling the resulting unstretched extru-date to solidify it, and (4) subjecting the unstretched solidified extrudate to a stretching treatment at a temperature of 60 to 140°C at a stretch ratio of at least about 3:1 when step (2) is carried out while a draft is applied to the unstretched extrudate or at a stretch ratio of at least about 10:1 when step (2) is carried out in the absence of a draft.
2. The proces of claim 1 wherein the weight average moleculr weight of the paraffinic wax (B) is at least 230 but less than 1,000.
3. The process of claim 1 wherein the inherent viscosity of the ultrahigh-molecular-weight poly-ethylene (A) is 5 to 30 dl/g.
4. The process of claim 1 wherein the melting point of the paraffinic wax (B) is 45 to 110°C.
5. The process of claim 1 wherein the paraffinic wax (B) is a member selected from the group consisting of n-alkanes having at least 22 carbon atoms, mixtures of a major proportion of the n-alkanes and a minor proportion of lower n-alkanes, paraffin wax, poly-ethylene waxes, ethylene copolymer waxes, heat-degraded polyethylene waxes, heat-degraded ethylene copolymer waxes, oxidation products of these waxes, and alpha,beta-unsaturated acid-modified products of the foregoing waxes.
CA000455728A 1984-06-01 1984-06-01 Process for producing stretched articles of ultrahigh- molecular-weight polyethylene Expired CA1216118A (en)

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