The invention relates to crosslinked oriented high molecular weight polyethylene and to a process for preparin articles from such polyethylene. Oriented high molecular weight polyethylene is known, inter alia, from US-A-4,344,908, in which is described the production of polyethylene fibres with a high tensile strength at break and a high modulus.

The disadvantage of these polyethylenes however, i that their resistance against high temperatures is poor. If a clamped article from the known oriented high molecular weight polyethylene is exposed for some time to a temperature of 145°C, the tensile strength and the elastic modulus will show a very strong decrease. At a temperature higher than 155°C, the article will break. Attempts have been made to improve the resistance against high temperatures by crosslinking the polyethylene. For instance from D.J. Dijkstra, A.J. Pennings, Polymer Bulletin ___, 507 (1987) it is known to crosslink oriented high molecular weight polyethylene fibres by radiation with high-energy electrons.

However, the fibres thus obtained do not have sufficient resistance against high temperatures, while the tensile strength at break decreases in consequence of the radiation.

An object of the invention is oriented high molecular weight polyethylene with great resistance against high temperatures, a high tensile strength at break (σ) and a high elastic modulus (E).

This object is achieved in that the polyethylene contains up to 30% (wt) crosslinked poly-l,4-butadiene. Articles from polyethylene according to the invention do not break when they are exposed under stress to a temperature even as high as 200°C. The articles have been found to have a high initial tensile strength at break (σ) and a high elastic modulus (E), which continues to be high after exposure to a temperature of 200°C.

The poly-l,4-butadiene preferably consists for at least 90% (mole) of poly-trans-l,4-butadiene.

Poly-trans-l,4-butadiene is known per se from G. Natta, M. Pegoraro and P. Cremonesi, Chim. e Industrie, 47, No. 7, 722, (1965).

Articles with a high tensile strength and E-modulus cannot be obtained from it, because articles from poly-trans-l,4-butadiene cannot be properly oriented. For one thing, the maximum drawability of an article from poly-trans-l,4-butadiene is only about 5 (see S. Iwayanagi, I. Sakurai, T. Sakurai and T. Seto in Journal of Macromolecular Science-Physics, B2, 163, (1968).

An article according to the invention, containing high molecular weight polyethylene and up to 30% (wt) poly-l,4-butadiene, has a high drawability at temperatures ranging from 70-140°C. At 100°C this drawability is at least 10 (for instance 100).

High molecular weight polyethylene is understood to mean, according to the invention, a polyethylene having a average molecular weight of at least 5*10 kg/mole. The weight-average molecular weight (M ) is determined by applying the methods known for this purpose, such as Gel Permeation Chromatography (GPC) and Light Scattering. The number-average molecular weight (M ), too, can be determined

by applying GPC. In the case of polyethylene with a c weight-average molecular weight (M ) of at least 5*10 kg/kmole, the M is calculated from the Intrinsic Viscosity

(IV) determined in decalin at 135°C. The said weight-average g molecular weights of 0.5 and 1.0*10 kg/kmole correspond with an IV in decalin at 135°C of 5.1 resp. 8.5 dl/g according to the empirical relation:

M _ω w « 5.37 x 10 ⁴ [IV] ¹ ' ³⁷ .

Preference is given to the use of a very high molecular weight polyethylene (ultra-high molecular weight polyethylene, or UHMWPE).'Of such a polyethylene the weight-average molecular weight M is, for instance, between 1*10 ⁶ and 10*10 ⁶ kg/kmole.

High molecular weight polyethylene is in this connection further understood to mean linear polyethylene with fewer than 10 side chains per 1000 carbon atoms and preferably with fewer than 3 side chains per 1000 carbon atoms, or such a polyethylene containing also minor amounts, preferably smaller than 5% (mole), of one or more other alkenes copolymerized therewith, such as propylene, butylene, pentene, hexene, 4-methyl-pentene, octene, etc. The polyethylene may further contain minor amounts, preferably 25% (wt) at most, of one or more other polymers, particularly an alkene-1-polymer, such as polypropylene, polybutylene or a copolymer of propylene with a minor amount of ethylene. The poly-l,4-butadiene used according to the invention is prepared according to processes known in the art. In this connection see, for instance, G. Natta, M. Pegoraro and P. Cremones.i, Chim. e Industrie, ___ , No. 7, 722 (1965). Generally, poly-1,4-butadiene with a viscosity-average molecular weight (M ) of at least 1 x 10 is used. The M preferably amounts to at least 3 x 10 , ^v 4 particularly at least 6 x 10 . In the polymerization process a high poly-trans-l,4-butadiene content is preferably aimed

at. The poly-l,4-butadiene consists of, for instance, at least 90% (mole) poly-trans-l,4-butadiene. The poly-trans-l,4-butadiene content is particularly at least 95% (mole), more particularly at least 98% (mole).

The article according to the invention contains up to 30% (wt) crosslinked poly-l,4-butadiene. It preferably contains up to 0.5-10% (wt) poly-l,4-butadiene, more particularly 0.5-5% (wt). The degree of crosslinking of the poly-l,4-butadiene must at least be 50%, particularly at least 90%, more particularly at least 95%.

The article according to the invention may also contain non-polymeric materials, such as solvents, colourants, stabilizers, anti-oxydants, waxes and fillers. The amount of these materials may total up to 60% (vol) in respect of the polymer.

The preparation of articles according to the invention is carried out according to processes known in the art for the preparation of articles from oriented high molecular weight polyethylene. Preferably a process can be applied in which a solution of high molecular weight polyethylene and poly-l,4-butadiene in a suitable solvent is converted into a gel article by thermally reversible gellation, upon which the resulting gel article is drawn, with orientation of the polymer molecules. This last-mentioned process, the so-called gel route, will be elucidated extensively below. In applying the gel route according to the present invention various solvents can be used. Suitable solvents include halogenated or non-halogenated hydrocarbons, such as paraffins, paraffinic waxes, toluene, xylene, tetraline, decalin, monochlorobenzene, nonane, decane or petroleum fractions. Of course, mixtures of solvents can be used also. The polyethylene and poly-l,4-butadiene concentrations in the solution may vary. Of importance in this connection are, inter alia, the nature of the solvent and the molecular weight of the polyethylene. Solutions in

which the total concentration of polymer with a very high molecular weight (M for instance higher than 1 x 10 ) is higher than 50% (wt) are difficult to handle on account of the prevailing high viscosity. For the viscosity of the solution the molecular weight of the poly-l,4-butadiene is of minor significance, because it is relatively low molecular in respect of the polyethylene. The disadvantages of the use of solutions with a total concentration of, for instance, less than 0.5% (wt) are a loss of yield and an increase in the costs for the separation and recovery of solvent. Therefore, a solution will generally be started from having a total high molecular weight polymer concentration of between 1 and 40% (wt), particularly 5-30% (wt).

The solutions to be applied can be prepared in various ways, for instance by suspension of solid particle-shaped poly-l,4-butadiene and polyethylene in a solvent, followed by stirring at elevated temperature, or by converting the suspension into a solution in an extruder, for instance a twin-screw extruder provided with mixing and conveying devices. The conversion of the solution into a shaped, solvent-containing article can be effected in the present invention in different ways, for instance by spinning, via a spinning head with a circular die or a slit die, into a filament or ribbon, or by extrusion via an extruder, usually with a profiled extruder head.

The temperature during the shaping must be chosen above the dissolving point. This dissolving point is determined by the solvent, the polyethylene and poly-l,4-butadiene concentrations, the molecular weights of these polymers and the pressure applied.

This temperature is preferably at least 90°C, particularly at least 100°C. Of course, this temperature must be chosen below the decomposition temperature of the poly-l,4-butadiene and of the high-molecular polyethylene.

The shaped, solvent-containing article is subsequently cooled to below the gelation temperature in such a manner that a gel article with a homogeneous gel structure is obtained, in which process the article is cooled rapidly using air and/or a liquid cooling medium, for instance water. The gelation temperature depends on, inter alia, the solvent and generally virtually corresponds with the said dissolving temperature. The article is preferably cooled to about ambient temperature.

The gel article thus obtained can successively be drawn. It is also possible for at least part of the solvent to be removed, before the drawing, by, for instance, extraction. The drawing can be carried out also under such conditions that the solvent still present is wholly or partly removed, for instance by means of a gas, or by drawing in an extraction bath. The drawing must be carried out according to the invention at a temperature higher than the temperature of the first order solid transition of poly-trans-1,4- butadiene. This is the temperature at which a change occurs in the crystal structure (monoclinic to pseudo-hexagonal) of the poly-trans-l,4-butadiene.

In this connection see M. Moller, Makromol. Chem. Rapid Comm. , 9, 107 (1988). This temperature, like other phase transition temperatures mentioned herein, such as melting temperatures, is determined by means of Differential Scanning Calori etry (DSC). Using this technique an endothermic peak corresponding with the first order solid transition temperature of poly-trans-l,4-butadiene is found at about 65-70°C and an endothermic peak corresponding with the melting temperature of the high molecular weight polyethylene is found at about 140-145°C. The endothermic peak corresponding with the melting point of trans-1,4- polybutadiene also occurs at 140-145°C and can therefore not be observed separately from the endothermic peak of the polyethylene.

According to the invention DSC measurements are carried out in the following manner. Thermograms are made using a calorimeter of the DSC-7 type of the firm of Perkin-Elmer. The heating rate applied is 10°C/min. The standard for temperature calibration is Indium, which has a melting temperature (T _m ) of 156.6°C and a melt enthalpy ( ^ΔH _m' ^{1 of 28} « ^{4 J} 9* ^τhe samples weigh 10 mg. A drop of silicone oil is added to the samples for proper heat conduction.

In the drawing process, high draw ratios can be applied according to the invention. Generally, a draw ratio of at least 10 is applied, preferably at least 20, and particularly at least 40.

Otherwise, in addition to the gelling process mentioned hereinbefore, other processes for the preparation of oriented polyethylene articles can be used also, for instance the processes known in the art for the processing of virgin high molecular weight polyethylene. In this connection see WO-87/03288. Instead of pure virgin polyethylene, a mixture of virgin polyethylene and poly-l,4-butadiene is used in the process according to the invention.

In order to obtain a good resistance against high temperatures, an article according to the invention must be crosslinked at least in part. The crosslinking can take place before, during or after the orientation of the molecules in the article. So, if the 'gel route' is used, the gel, for instance, can be crosslinked, or crosslinking may take place during or after the drawing. Crosslinking preferably takes place after the orientation of the polyethylene and poly-l,4-butadiene molecules. The crosslinking takes place according to processes known in the art for the crosslinking of materials. This can be done by radiation with γ-rays or high-energy electrons, or by the addition of crosslinking agents. Preference is given to the use of electron radiation. The radiation dose

is preferably 1-150 kGy (Kilo Gray), particularly 10-100 kGy. The temperature at which radiation takes place is important. At higher temperatures a higher degree of crosslinking takes place. This temperature is -10 to 150°C, preferably this temperature is 80-140°C.

As the polyethylene (fibres, films, profiles, etc.) according to the invention is crosslinked, it can not only be used in known applications of oriented polyethylene, as well as in such applications where resistance against creep, compression strength, resistance against high temperatures and fibrillation resistance are important. This is the case, for instance, in load-bearing composites, for which fibres according to the invention are highly suited as reinforcement.

The invention will be elucidated below by means of the following examples. In examples I to III inclusive and in comparative example A, the effect of the degree of radiation is examined.

Example I; The high molecular weight polyethylene used in this example has a M of 1.5 x 10 g/mole and a M of 2 x 10 g/mole and is of the Hostalen Gur-412 type of the firm of Hoechst Ruhrchemie. Poly-trans-l,4-butadiene is prepared according to G. Natta, M. Pegoraro and P. Cremonesi, Chim. e Industria, 47, No. 7, 722 (1965). The poly-l,4-butadiene thus prepared has a viscosity-average molecular weight M of

4 ^v

7.5 x 10 g/mole and a vinyl content of 0.9-1.2% (mole) and a trans-l,4-butadiene content of 99.1-98.8% (mole).

12 g high molecular weight polyethylene and 3 g poly-l,4-butadiene are suspended in 1 dm xylene, with di-t-butyl-p-cresol (DBPC) added as stabilizer in an amount of 0.5% (wt) calculated on the high-molecular weight polyethylene. The -resulting suspension is devolatilized in vacuum, subsequently saturated with nitrogen gas and heated

in a εilicone bath to about 120°C. During the heating the dispersion is stirred and a homogeneous dispersion is formed. After some time the stirring is discontinued, upon which the dispersion is kept at 130°C for about 4 hours until a homogeneous solution is obtained. This solution is poured out into an aluminium dish, upon which the solution is cooled to room temperature, at which gelling takes place, The resulting film is air-dried and the stabilizer is extracted at 23°C using n-hexane. Subsequently the film obtained is compressed for 1 hour at a pressure of 3 x 10 7

Pa.

Drawing

The film is cut into ribbons measuring 25 x 8 mm and drawn at 100°C over a hot plate. The draw ratio is determined by marking a non-drawn sample at every other millimeter into the direction of drawing and measuring the distances between the marks before and after the drawing. The draw ratio (λ) is the quotient of the distances between the marks after the drawing and the distances between these marks before the drawing. If the drawing is to be homogeneous, the distances between all marks, after the drawing, must be about the same. In example I, drawing takes place up to a draw ratio (λ) of 40. The tensile strength at break (σ) of the (unradiated) ribbon obtained is 1.0 GPa and the elastic modulus (E) is 41 GPa.

Radiation

Electron radiation is carried out using a Van de Graaf generator. The drawn sample is radiated with a bundle of electrons with an energy of 3 MeV, with an amperage of 150 tA, resulting in a radiation dose of 0.855 kGy for every passage through the bundle. The sample is radiated with a total dose of 60 kGy (70 passages) in a ₂ atmosphere at a temperature of 30°C.

Determination of the high-temperature resistance The following temperature treatment is carried out.

The radiated sample is clamped in a framework at constant length, upon which the framework is placed in an oven with a temperature of 200°C. After 30 seconds, the framework is removed from the oven. The sample does not break during this treatment and no change in the outward appearance of the sample can be observed. The results are shown in table 1.

Example II In the same way as in example I a film is prepared, but this time starting from 15 g high molecular weight polyethylene as used in example I and 0.75 g poly-l,4-butadiene. The film is cut into ribbons, the ribbons are drawn and radiated as in example I. The results are shown in table I.

Comparative example A

In the same way as in example I a film is prepared, but this time starting only from 15 g high molecular weight polyethylene. The film is cut into ribbons and the ribbons are drawn as in example I. The tensile strength at break (σ) of the resulting (unradiated) ribbons is 1.5 GPa and the modulus of elasticity (E) is 62 GPa. The ribbons are subsequently radiated as in example I. After radiation, the tensile strength at break (σ) is 0.9 GPa and the elastic modulus (E) 62 GPa. In the temperature treatment as described under example I, the ribbon shrinks and breaks within a few seconds. The results are shown in table 1.

Table 1: Effect of the amount of poly-l,4-butadiene

(1,4-PB) on the tensile strength at break (σ) and elastic modulus (E) of radiated ribbons before and after the temperature treatment (Ttr).

Example 1,4-PB before Ttr after Ttr percentage a E σ E

(%) (GPa) (GPa) (GPa) (GPa)

I 20 II 5

A 0

* The article breaks.

In examples III, IV and comparative example B the effect of the radiation dose on the mechanical properties of the ribbons is examined (no temperature treatment).

Example III In the same way as in example I a film is prepared.

The film is cut into ribbons, the ribbons are drawn and radiated as in example I, but the radiation dose is 20 kGy.

The mechanical properties of the ribbons obtained are shown in table 2.

Example IV

In the same way as in example I a film is prepared.

The film is cut into ribbons, the ribbons are drawn and radiated as in example I, but the radiation dose is 100 kGy. The mechanical properties of the ribbons obtained are shown in table 2.

Comparative example B In the same way as in example I a film is prepared.

The film is cut into ribbons and drawn as in example I, but no radiation takes place. The mechanical properties of the ribbons obtained are shown in table 2.

Table 2: Effect of the radiation dose on the mechanical properties of the ribbons obtained. The amount of 1,4-PB is 20% (wt), the draw ratio (λ) is 40.

No real effect of the radiation dose on the mechanical properties can be demonstrated. In examples V to VIII inclusive and in comparative example C the effect of the amount of poly-l,4-butadiene on the mechanical properties of the ribbons is examined. The samples are not radiated.

Example V

In the same way as in example I a film is prepared, but starting from 14.85 g polyethylene and 0.15 g poly-l,4-butadiene. The film is cut into ribbons, the ribbons are drawn as in example I. The mechanical properties of the ribbons obtained are shown in table 3.

Example VI

In the same way as in example I a film is prepared, but starting from 14.25 g polyethylene and 0.75 g poly-l,4-butadiene. The film is cut into ribbons, the ribbons are drawn as in example I. The mechanical properties of the ribbons obtained are shown in table 3.

Example VII

In the same way as in example I a film is prepared, but starting from 13.5 g polyethylene and 1.5 g poly-l,4-butadiene. The film is cut into ribbons, the ribbons are drawn as in example I. The mechanical properties of the ribbons obtained are shown in table 3.

Example VIII

In the same way as in example I a film is prepared, but starting from 10.05 g polyethylene and 4.95 g poly-l,4-butadiene. The film is cut into ribbons, the ribbons are drawn as in example I to a ratio (λ) of 40.

Ribbons with a draw ratio (λ) of 1 and 11 are prepared also.

The degree of orientation of poly-l,4-butadiene and polyethylene molecules in these ribbons (λ = 1, 11 and 40) is determined by means of X-ray diffraction.

X-ray diffraction measurements (Wide Angle x-ray

Scattering (WAXS)) are carried out using a camera of the

^• D

Statton type. Ni-filtered CuK -radiation was generated at a voltage of 50 kv and an amperage of 30 mA. The distance between sample and film was 50 mm. The WAXS photos are shown in figure 1. A draw ratio λ of 1 is marked (a), a draw ratio λ of 11 is marked (b) and a draw ratio λ of 40 is marked (c). A point pattern as in figures lb and lc is indicative of a high degree of orientation of both polyethylene and poly-l,4-butadiene molecules. Without drawing, there is hardly any orientation (figure la). The mechanical properties of the -ribbons obtained (at λ = 40) are shown in table 3.

Comparative example C In the same way as in example I a film is prepared, but starting from 7.5 g polyethylene and 7.5 g poly-l,4-butadiene. The film is cut into ribbons, the ribbons are drawn as in example I. However, with a draw ratio (λ) of 25, the article breaks. The results are shown in table 3.

Table 3: The effect of the amount of poly-l,4-butadiene (1,4-PB) on the properties (σ and E) of the non-radiated ribbons; the draw ratio (λ) is 40.

Example Amount of σ E

1,4-PB (% (wt)) (GPa) (GPa)

A 0 1.5 52

V 1 1.4 52

VI 5 1.2 49

VII 10 1.1 45 I 20 1.0 41 VIII 33 0.8 30

C 50 - -*

* At λ = 25, the article breaks.

Comparative example D and examples IX-XI

Preparation of filaments

The high molecular weight polyethylene used in this example has an intrinsic viscosity determined in decaline at 135°C of 17 dl/g. The poly-l,4-tubadienen is the same as used in example 1.

A blend of 10 parts by weight poly-l,4-butadiene and 90 parts by weight polyethylene was suspended as decalin to a

10 weight % suspension. 0,25 weight % of di-t-butyl-p-cresol (DBPC) was added to the suspension as a stabilizer. The suspension was kneaded in a twin screw extruder having a temperature of 210°C producing a uniform and clear solution. This solution was spun through an orifice having a diameter of 0.8 mm. The spun filaments were cooled to 65°C and subsequently drawn in two stages in an air heated oven. During the first stage the filaments were drawn 20 times (by length) at 110°C, during the second stage 4 times at 147°C. The decalin evaporated during the drawing procedure. The filaments obtained have an elastic modulus E of 76,1 GPa and a tensile strength σ of 2,4 GPa at an elongation at break ε of 3.5%.

Radiation

Electron radiation is carried out as in example 1.

In comparative example D the filaments are not radiated (0 kGy) . In example IX the radiation dose is 20 kGy, in example

X 50 kGy and in example XI 100 kGy. The mechanical properties of the irradiated filaments are shown in table 4.

Table 4: mechanical properties of irradiated filaments

Example Pa) σ (GPa) ε (%)

D

IX X XI

Determination of the high-temperature resistance The filaments were clamped in a framework upon which the framework was placed in an oven with a temperature of 170°C. After 90 seconds, the framework is removed from the oven. The filaments of comparative example D shrink and break within a few seconds. The filaments of examples IX to XI do not break and no change in appearance occured during the heating treatment. The mechanical properties of the heat treated filaments are shown in table 5.

Table 5: mechanical properties of inadiated filaments after heating treatment

Example

D IX X XI

* filaments were broken during the heating treatment

It is found that the elongation at break increased by the heating treatment.

Retractive stress measurement

The retractive stress upon heating was determined by clamping the filaments at constant length in a framework of an apparatus for measuring tensile strength. The framework was placed in an oven. The temperature was increased from 20°C to 200°C. During the increase of temperature the stress in the filaments was measured.

The results are shown in figure 2. On the horizontal (I) axis the temperature (°C) is shown, on the vertical (II) axis the retractive stress (MPa). Comparative example D is marked Δ, Example IX is marked 0, Example X is marked D and Example XI is marked Δ in this figure. Comparative example D shows a sharp increase in retractive stress. The filaments break at about 145°C. The irradiated filaments (examples IX-XI) show a moderate increase in retractive stress and show a retractive stress at temperatures of 150-200°C that is comparable to the retractive stress at 20-110°C.