NL9000941A - PROCESS FOR PREPARING A REINFORCED POLYMER MASS INCLUDING FIBRILS OF A CRYSTALLINE POLYETHENE - Google Patents

Description

Inventors: Jozef M.A. Jansen in Geleen Hendrikus Oostra in Susteren Hendrikus J.J. Rutten in Maastricht

METHOD FOR PREPARING A REINFORCED POLYMER MASS INCLUDING FIBRILS OF A CRYSTALLINE POLYETHENE

The invention relates to a process for preparing a reinforced polymer mass comprising fibrils of a crystalline polyethylene having a weight average molecular weight (Mw) of at least 5 * 10 ^ kg / kmol, wherein particles of the crystalline polyethylene in a matrix material are immiscible is deformed into fibrils with the crystalline polyethylene.

Such a method is known from DE-A-2418803. According to this "Offenlegungsschrift", a reinforced polymer mass is obtained in which fibrils of the crystalline high-molecular polyethylene give the polymer mass an increased tensile strength at break. Preferably, the fibrils have a diameter of less than 5 * 10 m and a length -3 -3 of 2 * 10 to 20 * 10 m. The polymer mass has a homogeneous appearance on a macroscopic scale. The polymer mass can be processed into films, plates, strips, profiles and the like.

The temperature at which fibrils are formed in the polymer mass is near the melting point and preferably slightly above the melting point of the crystalline polyethylene. According to the examples, this temperature is about 150 ° C. When no special measures are taken, relaxation occurs after the formation of the fibrils, the fibrils losing their elongated shape. As an example of such a measure, mention is made of cooling the polymer mass under tension to a temperature below the melting point of the crystalline polyethylene. A drawback of this measure is that it requires careful control of the processing conditions. Moreover, there is always some relaxation of the fibrils.

The object of the invention is therefore to prepare a polymeric mass reinforced by fibrils, which has a high tensile strength at break, wherein the formation of fibrils takes place in an easy to control manner and relaxation of formed fibrils is prevented.

This object is achieved in that in the method according to the invention the crystalline polyethylene has a maximum degree of stretching of at least 20 and the deformation into fibrils takes place at a temperature below the melting point of the crystalline polyethylene.

Surprisingly, polymer masses prepared according to the invention have a considerably higher tensile strength at break and modulus of elasticity than the known polymer masses. The fibrils in the polymer mass have a tensile breaking strength (σ) of at least 1 GPa and a modulus (E) of at least 20 GPa.

Crystalline polyethylene with a maximum degree of stretching of at least 20 is already known per se, for example from US-A-4769433. This patent describes the provision of very high molecular weight polyethylene ("virgin PE"), which has never been dissolved or melted, which has been prepared by polymerization at relatively low temperatures. Such a crystalline polyethylene with a high maximum stretching degree has a low entanglement density.

The "virgin" polyethylene as prepared according to US-a-4769433 can be further characterized by the difference between the initial melting enthalpy of a sample and the melting enthalpy obtained when the sample is completely melted and then solidified again. The difference (decrease) in melting enthalpies is preferably at least 10% of the initial melting enthalpy, especially at least 20%. In addition, the crystallinity of the polyethylene just mentioned is high, at least 77%, preferably more than 80%.

Polyethylene with a high maximum degree of stretching can also be obtained from polyethylene with a normal or low maximum degree of stretching via the so-called 'gel route'.

Here, the polyethylene of normal or low maximum stretching degree is dissolved at an elevated temperature in a suitable solvent, such as, for example, decalin, paraffin oil or paraffin wax, the solvent being removed after cooling, for example by evaporation or extraction. The resulting material can be ground, preferably cryogenically, after which further processing can take place in the method according to the invention. Polyethylene with a high maximum stretch degree prepared in this manner does not always have to meet the above criteria of crystallinity and difference in melting enthalpies.

The maximum degree of stretching of the crystalline polyethylene is at least 20, preferably at least 40 and in particular at least 80.

The maximum degree of stretching is determined as follows: A layer of polyethylene powder with a thickness of 2 mm is pressed in a round mold with a diameter of 5 cm for 5 minutes at room temperature under a weight of 50000 kg. The round film obtained is then pressed in a flat press at 130 ° C for 10 minutes under a weight of 100000 kg. A dumbbell-shaped test piece with a length between the shoulders of 10 mm is punched from the film thus obtained. This test piece is stretched in a Zwick 1445 Tensile Tester at a temperature of 130 ° C / at a rate of 10 mm / min until breakage in the test piece occurs. The maximum degree of stretching is determined as the quotient of the length of the part of the test piece between the shoulders when fracture occurs in the test piece and its length before drawing (10 mm).

Melting temperatures Tm and melting enthalpies ΔΗ are determined by differential scanning calorimetry (DSC) at a scan rate of 5 ° C / min, and an initial temperature of 40 ° C. The DSC measurements were performed with a Perkin Elmer DSC-7, on samples weighing 4 * 10 ~ 3 g. Calibration of the DSC measurements was done with Indium (Tm = 156 ° C and ΔΗ = 28/45 J / g). The melting enthalpy ΔΗ is determined from the area under the melting curve. Crystallinities (%) are calculated by dividing the melting enthalpy ΔΗ of the sample by the melting enthalpy of 100% crystalline polymer. For polyethylene, the ΔΗ for 100% crystalline polymer is assumed to be 293 kj / kg.

The weight average molecular weight Mw is determined by the previously known methods such as Gel Permeation Chromatography (GPC) and Light Scattering. For polyethylene with a weight average molecular weight Mw of at least 5 5 * 10 kg / kmol / Mw is calculated from the Intrinsic Viscosity (IV) / determined in decalin at 135 ° C. The stated weight average molecular weights of 0.5 and 1.0 * 10 ^ kg / kmol correspond to an IV in decalin at 135 ° C of 5.1 and 8.5 dl / g according to the empirical relationship:

Mw = 5.37 x [4 [IV] 1.37.

Preferably, the weight average molecular weight Mw of the crystalline polyethylene is between 1 * 10 ^ and 10 * 10 ^ kg / kmol.

According to the invention, crystalline polyethylene is understood to mean a homo- or copolymer of a polyethylene with a DSC crystallinity of at least 30%. Preferably, the DSC crystallinity is at least 50%.

The process according to the invention uses very high molecular linear polyethylene (ultra high molecular weight polyethylene or UHMWPE). UHMWPE is here understood to mean linear polyethylene with less than 10 side chains per 1000 carbon atoms and preferably with less than 3 side chains per 1000 carbon atoms or a similar polyethylene which also contains minor amounts, preferably less than 5 mol%, of one or more other copolymerized therewith olefins such as propylene, butene, pentene, hexene, 4-methyl-pentene, octene, etc., which ethylene or ethylene copolymer has a weight average molecular weight of at least 5 * 10 kg / kmol. The polyethylene may further contain minor amounts, preferably up to 25% by weight, of one or more other polymers, in particular an olefin 1 polymer, such as polypropylene, polybutadiene or a copolymer of propylene with a minor amount of ethylene.

Such UHMWPE can be produced, for example, using a Ziegler or Phillips process using suitable catalysts and under known polymerization conditions.

The polyethylene can also contain non-polymeric materials, such as solvents, waxes and fillers. The amount of these materials can be up to 60% by volume relative to the polyethylene.

The matrix material used in accordance with the invention and the crystalline polyethylene should be immiscible on a molecular scale under the conditions of deforming the crystalline polymer into fibrils.

The processing temperature of the matrix material should be below the melting point of the crystalline polyethylene. Preferably, the matrix material is selected so that the adhesion between the matrix material and the crystalline polyethylene is good. For this purpose, an adhesion improver can be added to the matrix material and / or to the crystalline polyethylene.

The viscosity (h) of the matrix material is an important parameter in the method according to the invention. Depending on the conditions under which the crystalline polyethylene is deformed into fibrils, the viscosity under processing conditions must be at least a minimum value hm ^ n, which is determined according to: h · = τ. / y where Tm ^ n is the minimum shear stress for deformation to fibrils and y is the shear rate. The minimum shear stress * rmin depends on the crystalline polyethylene used. For UHMWPE at 120 ° C it is about 20-30 * 10 ”^ N / m ^. The shear rate y depends on the equipment in which the deformation to fibrils takes place and on the setting of this equipment. The shear rates to be achieved in different equipment are known to the skilled person. The shear rate y depends, among other things, on the geometry of this equipment. Therefore, in equipment in which high shear rates occur, the viscosity of the matrix material may be less than in equipment in which the shear rates remain low.

As examples for the matrix material can be mentioned: polyethylene, polypropylene, plasticized polyvinyl chloride or copolymers based on these polymers and rubbers; particularly suitable as matrix materials are low density polyethylene (LDPE), linear low density polyethylene (LLDPE), very low density polyethylene (VLDPE), ethylene-vinyl acetate copolymers, ethylene-propylene (third monomer) rubbers (EP (D) M- rubbers) and styrene-butadiene-rubber (SBR).

Reactive substances, such as vulcanizing agents or cross-linking agents, can be included in the matrix material. These reactive substances preferably have an activation temperature above the processing temperature of the matrix material. However, the activation temperature must be below the melting point of the crystalline polyethylene.

The polymeric masses can be used in many applications known for reinforced polymeric masses, for example as construction material (moldings, plates, profiles, pipes or hoses) in laminates or as foils. Both rigid and flexible reinforced polymer masses are available according to the invention.

A special embodiment of the invention is that in which the matrix material, after the formation of the fibrils, is separated from the fibrils. By selecting a suitable matrix material, the fibrils can be separated therefrom, whereby loose fibrils with high tensile strength and stiffness are obtained. The loose fibrils can be used in various applications, such as staple fibers. The separation of matrix material and fibrils can take place, for example, by dissolving or melting the matrix material.

According to the invention, fibrils are intended to be bodies whose length (L) is large relative to the width and the thickness or the diameter (D). The L / D ratio for a fibril is at least 4, preferably at least 10, and in particular at least 25. Fibrils of different diameter and length exist in a polymer mass prepared according to the invention. Usually the dimensions of the fibrils are such that they have a diameter less than 5 * 10- ^ m and a length from 2 * 10- ^ to 100 * 10 ”^ m. Preferably, the fibrils have a diameter of -4 -4 less than 3 * 10 m, in particular less than 1 * 10 m.

The particle size of the crystalline polyethylene from which the fibrils are prepared is important for the dimensions of the fibrils.

The temperature Tff at which the crystalline polyethylene is deformed into fibrils should be higher than the processing temperature of the matrix material, but lower than the melting temperature of the crystalline polyethylene.

The mixing of the crystalline polyethylene and the matrix material and the deformation into fibrils can take place in equipment suitable for applying shear stresses and generating strain flow. Preferably, large tensile stresses can be imposed in this equipment on the polymer mass contained in the equipment. Examples of such equipment are extruders and injection molding machines, provided with conically tapered screw parts and or Couette parts, rollers, and Brabender mixers.

Depending on the conditions during the formation of the fibrils, anisotropic polymer masses can be prepared by the method according to the invention, which are oriented in a certain direction and therefore exhibit a greater tensile strength at break and modulus in this direction than in other directions.

The invention is explained below with reference to the following exemplary embodiments.

Example I:

The matrix material used is ethylene-vinyl acetate copolymer (EVA) with a melt index (determined according to the standard ASTM D1238 at 190 ° C and a load of 2.16 kg) of 0.2, of the type ELVAXr 670 of the company DuPont. Crystalline high molecular weight polyethylene is prepared according to Example 5 of US-A-4769433. The crystalline polyethylene thus obtained has an intrinsic viscosity g (I.V.) of 30, and a molecular weight (M) of 5.6 * 10. It

W.

melting point is 142 ° C. The average particle size of the crystalline polyethylene particles is 900 µm. The maximum degree of stretching of the polyethylene is 100. Using a roller of the Collin 150x450 type (Walzwerk Dr Collin GmbH), a polymer mass is prepared from 90 g of matrix material and 10 g of crystalline polyethylene.

Rolling procedure:

The matrix material is processed on the roller at a rolling temperature of 110 ° C until a roller sheet is formed. The speed of both rollers of the roller is 10 revolutions per minute (no friction, RPM = 10). After 2 minutes, the crystalline polyethylene is spread evenly over the roll sheet. After 1 minute of waiting, the revolution speed of one of the rollers of the roller is increased to RPM = 20. The roll gap is adjusted to 0.1 mm. After 3 minutes of rolling, the roller is stopped and the resulting polymer mass is taken from the roller.

Presses:

Of the polymer mass obtained after the rolling procedure, 22 g are pressed for 30 s at a temperature of 90 ° C and under a weight of 1000 kg in a mold for eight plates of 150 x 50 x 3.2 mm. The amount (22 g) is calculated from the total volume of the mold and the density of the polymer mass, maintaining an excess of 5%. After this, the polymer mass is pressed at a temperature of 120 ° C and under a weight of 50000 kg for 7 minutes into the steel mold to a thickness of 3.2 mm, after which the shaped polymer mass is cooled at a speed of 40 ° C / minute.

The tensile strength at break (σ), the modulus (E) and the elongation at break (ε) of the polymer mass are determined in accordance with standard ISO 37 type 2, on a test rod with a thickness of 3.2 mm, at a drawing speed of 20 * 10 "^ m / minute.

A fiber with a length of approximately 40 mm is taken from the polymer mass, the tensile strength of which is determined using a Zwick 1445 tensile testing machine. A fiber with a length of 40 mm is clamped in the clamp of the tensile testing machine over a clamping length (gauge length) of 20 mm. The drawing speed is set at 0.5 times the clamping length per minute. The diameter of the fiber is calculated from measured values for the length and density of the fiber. The properties of the polymer mass are given in Table 1.

When the matrix material without crystalline polyethylene is processed as described above, a material with a tensile strength at break σ of 20 MPa, a modulus E of 52 MPa and an elongation at break ε of 928% is obtained.

Example II:

As in Example 1, a polymer mass is prepared, but now from 80 g of matrix material and 20 g of crystalline polyethylene. The results are shown in Table 1.

Example III:

As in Example 1, a polymer mass is prepared, but now from 70 g of matrix material and 30 g of crystalline polyethylene. The results are shown in Table 1.

Example IV:

As in Example 1, a polymer mass is prepared, but now from 60 g of matrix material and 40 g of crystalline polyethylene. The results are shown in Table 1.

Table 1

Results of Examples I-IV: Influence of the amount of UHMWPE (matrix material is EVA, roll temperature 110 ° C)

Pre- Qty. Fibril- Fibrils Polymer mass image UHMWPE formation σ E σ E ε (wt.%) (GPa) (GPa) (MPa) (MPa) (%) I 10 yes 1,8 31 10 80 140 II 20 yes 1,0 21 12 132 47 III 30 yes 1.5 31 12 130 39 IV 40 yes 1.3 20 17 184 27

Examples V, VI, and comparative examples A and B:

In Examples V, VI, and Comparative Examples A and B, the influence of the processing temperature on the properties of the obtained polymer mass is investigated. As in Example 1, a polymer mass is prepared, but now from 80 g of matrix material and 20 g of crystalline polyethylene. The rolling temperature is varied from 120-150 ° C for the various examples. The results are shown in Table 2.

This series of examples shows that when highly stretchable UHMWPE is processed above its melting point, no fibril formation occurs.

Table 2:

Results of Examples V and VI and Comparative Examples A and B: Influence of rolling temperature (matrix material is EVA; amount of UHMWPE 20% by weight).

Pre- TWals Fibril- Fibrils Polymer mass imaging σ E σ E ε (° C) (GPa) (GPa) (MPa) (MPa) (%) V 120 yes 1.9 23 11 153 54 VI 130 yes 1.7 26 12 165 54 A 140 no 7 60 129 B 150 no 7 62 74

In comparative examples C to G the influence of the type of crystalline polyethylene on the properties of the polymer mass obtained is investigated. It appears that when UHMWPE is used with a maximum draw degree less than 20, no fibril formation occurs below the melting point and relaxation of formed fibrils occurs above the melting point (example G), so that ultimately no fibrils are present in the polymer mass.

Comparative Example C:

As in Example 1, a polymer mass is prepared, but starting from 80 g ethylene vinyl acetate type Elvaxr 670 as matrix material and 20 g crystalline polyethylene type Stamylan UH510 from DSM. This crystalline polyethylene has a maximum degree of stretching of 1. The melting point is 140 ° C. The shore temperature is 120 ° C. The results are shown in Table 3.

Comparative Example D;

As in Example 1, a polymer mass is prepared, but starting from 80 g ethylene vinyl acetate type Elvaxr 670 as matrix material and 20 g crystalline polyethylene type HIFAX 1900r from Hercules. This crystalline polyethylene has a maximum degree of stretching of 16. The melting point is 142 ° C. The shore temperature is 120 ° C. The results are shown in Table 3.

Comparative Example E:

As in Example D, a polymer mass is prepared, however, 20 weight percent decalin calculated to the crystalline polyethylene is added to the crystalline polyethylene of type HIFAX 1900. The shore temperature is 120 ° C. The results are shown in Table 3.

Comparative Example F:

A polymer mass is prepared as in Example E, however, the roll temperature is 130 ° C. The results are shown in Table 3.

Comparative Example G:

As in Example 1, a polymer mass is prepared, however, starting from 80 g of ethylene vinyl acetate type Elvaxr 670 as matrix material and 20 g of crystalline polyethylene type Stamylan UH210 from DSM. This crystalline polyethylene has a maximum degree of stretching of 1. The melting point is 142 ° C. The shore temperature is 150 ° C. The results are shown in Table 3.

Table 3.

Results of Comparative Examples C-G: Influence of UHMWPE type. (* = with 20% w / w decalin)

Pre-type Twals F ^ r ^ ·· * · Polymer mass image UHMWPE (° C) formation σ E ε (MPa) (MPa) (%) C UH510 120 no 7.4 64 575 D HIFAX1900 120 no 6.5 64 414 E HIFAX1900 * 120 no 6.6 54 369 F HIFAX1900 * 130 no 7.7 57. 424 G UH210 150 no 9.5 54 483

Examples VII-IX:

In Examples VII to IX, the matrix material used is LDPE with a density of 0.921 kg / m of the type Stamylan 2101 TN04 from DSM. As in Example I, a polymer mass is prepared, however, starting from 80 g LDPE as matrix material and 20 g crystalline polyethylene of the type from Example I. In Examples VII to IX, the rolling temperature is varied. The results are shown in Table 4.

When the LDPE matrix material without crystalline polyethylene is processed as described above, a material with a tensile strength at break (σ) of 12 MPa, a modulus (E) of 121 MPa and an elongation at break (ε) of 915% is obtained.

Table 4

Results of Examples VII-IX: Influence of the rolling temperature (Matrix material = LDPE; 20 wt.% UHMWPE)

Pre-Twins Fibril- Fibrils Polymer mass imaging σ E σ E ε (° c) (GPa) (GPa) (MPa) (MPa) (%) VII 130 yes 0.7 13 10.2 252 31 VIII 120 yes 1, 2 25 19.6 335 27 IX 110 yes 1.2 24 7.9 94 92

Examples X, XI and Comparative Examples H, J and K relate to mixtures in which the matrix material is a rubber. The results are shown in Table 5.

Example X: (A) Via a rotating twin-screw extruder, a suspension of 15 wt.% UHMWPE of the type HB 312 from Himont, with a weight average molecular weight of 1.5x10 ^ kg / kmol in decalin, is converted at about 180 ° C into a homogeneous solution. The solution is extruded as a 1.0 mm thick flat sheet and cooled to form a gel. After evaporation of the decalin, the resulting plate is ground into a fine powder with an average particle size of 900 µm. The crystalline polyethylene powder obtained has a maximum draw ratio of 43.

(B) 10% by weight of the powder obtained in the above manner is added to an ethylene-propylene-diene-rubber (EPDM) compound of the Keltan 512x50 type from DSM. A mixture according to ISO 4097-1980 (E) is prepared from this compound, which is then rolled for 5 minutes at 120 ° C according to the rolling procedure as described in Example I, whereby UHMWPE fibers are formed. After rolling 1 wt. part of Rhenocure AT vulcanizing agent from Rheinau GmbH quickly mixed with 272 wt. parts of the compound, after which it is vulcanized for 30 minutes at 120 ° C.

Example XI:

As in Example X, a compound containing 20 wt% UHMWPE in EPDM is prepared.

Comparative Example H;

As in Example X under (B), a compound is prepared directly from UHMWPE of the type HB 312 from Himont, without pretreatment via a solution. The UHMWPE has a maximum degree of stretching of 1.

Comparative Example J:

As in Example X, a compound is prepared, however vulcanization takes place at 160 ° C.

Comparative Example K;

As in Example X under (B), a compound is prepared, but without UHMWPE.

Table 5

Results of Examples X and XI and Comparative Examples H, J and K (matrix material is ethylene propylene diene rubber):

For- Via Quantity Vulcanization Polymer Mass

image solution UHHWPE temperature σ E

(wt%) (° C) (MPa) (MPa) X yes 10 120 78 5.4 XI yes 20 120 95 7.2 H no 10 120 45 3.1 Y yes 10 160 50 7.8 K - 0 120 42 1.9

From the results it follows that polyethylene processed with solution, with a maximum degree of stretching of 43, gives a great improvement in the tensile strength at break and E-modulus. When the polymer mass is vulcanized at a temperature of 160 ° C, i.e. above the melting point of the polyethylene, the tensile strength at break remains much lower.

Claims (11)

A method for preparing a reinforced polymer mass comprising fibrils of a crystalline polyethylene having a weight average molecular weight Mw 5 of at least 5 * 10 kg / kmol, wherein particles of the crystalline polyethylene, in a matrix material that is immiscible with the crystalline polyethylene, are deformed into fibrils, characterized in that the crystalline polyethylene has a maximum draw degree of at least 20 and the deformation into fibrils takes place at a temperature below the melting point of the crystalline polyethylene. Process according to claim 1, characterized in that the crystalline polyethylene has a weight average molecular weight Mw of 1 * 106 to 10 * 10 kg / kmol. Process according to claim 1 or 2, characterized in that the crystalline polyethylene has a maximum degree of stretching of at least 40. Method according to claim 1 or 2, characterized in that the crystalline polyethylene has a maximum degree of stretching of at least 80. A method according to any one of claims 1-4, characterized in that the crystalline polyethylene has a DSC melting temperature of at least 140 ° C. Process according to any one of claims 1 to 5, characterized in that the crystalline polyethylene has a crystallinity of at least 80%. A method according to any one of claims 1-6, characterized in that the matrix material consists of a thermoplastic rubber. A method according to any one of claims 1-6, characterized in that the matrix material consists of a low density polyethylene (LDPE). Reinforced polymer mass obtainable by the method according to any one of claims 1-8. Fibrils obtainable by the method according to any one of claims 1-8, after separation of the matrix material. 11. Method or object as substantially described in the description and / or the examples. EXTRACT The invention relates to a process for preparing a reinforced polymer mass comprising fibrils of a crystalline polyethylene having a weight average molecular weight (Mw) of at least 5 * 10 kg / kmol, wherein particles of the crystalline polyethylene in a matrix material that do not miscible with the crystalline polyethylene are deformed into fibrils, the crystalline polyethylene having a maximum draw degree of at least 20 and the deformation into fibrils occurring at a temperature below the melting point of the crystalline polyethylene. In addition, the invention relates to reinforced polymer masses obtainable by this method.