DE2204637A1 - Process for the production of high density polyethylenes with good processability - Google Patents

Description

dr. W. Schalk dipl.-ing. P.Wirth dipl.-ing. G. Dan ν en berg DR. V. SCHMIED-KOWARZIK · DR. P. WEI N HOLD · DR. D. G U DE L

6 FRANKFURT AM MAIN

CR. ESCHENHEIMER STRASSE 39 qix / cir

P. 1501

Montecatini Edison S.p.A. Foro Buonaparte 31 Milan / Italy

Process for the production of polyethylenes with high density and good processability

The present invention relates to a method of manufacture of polyethylenes with high density and good processability. The present ~ The invention further relates to those obtained by this process

The difficulties in processing linear polyethylene with high Density, especially in the extrusion process, are known where in many Cases the extruded products show surface damage or deformation. For the well-known linear, high-density polyethylene there is actually a certain critical melt flow rate above which the surface of the extruded product becomes irregular and takes on the appearance of "shark skin", with sufficient flow speeds the phenomenon of melt fracture occurs.

209834/1078

In order to avoid the occurrence of extrusion damage, must be at relatively low Extrusion speeds are worked.

A well-known method of improving the processability of polyethylene consisted in expanding the molecular weight distribution (MWD). This method however, its limitations were due to the fact that an excessive widening of the molecular weight distribution resulted in a marked decrease in the physical-mechanical properties of polyethylene showed. Has also been the expansion of the molecular weight distribution achieved by mixing polyethylenes with very different molecular weights, so occurred encountered considerable difficulties during their homogenization.

There has therefore been a demand for a high density polyethylene and not too broad molecular weight distribution, that at higher extrusion speeds are processed without occurrence of shark skin and melt fracture phenomena and retain good physical-mechanical properties could. This demand was particularly evident in blow molding processes of bottles and hollow bodies in general.

A process for the production of high-density polyethylenes has now been found which, compared to known high-density polyethylenes with a similar melt index and molecular weight distribution, showed better processability and, in particular, the possibility of being able to be extruded at higher speeds.

209834/1078

The inventive method is characterized in that one or more known linear polyethylenes with densities greater than 0.94 g / cc, a melting point above 120 ⁰ C. and a solubility in tetrahydronaphthalene at 135 ° C. treated with small amounts of organic peroxides in a device with strong homogenizing power. Due to the above procedure, there is no noticeable change in density, melting point and solubility in tetralin at 135 ° C. while the melt index decreases proportionally to the amount of peroxide used.

The products obtained according to the invention differ from the known, high-density polyethylenes in the special ratio that exists between the intrinsic viscosity /% J and the melt index m, which differs from the ratio existing in conventional, high-density polyethylenes.

For the known linear polyethylene with a density of over 0.94 g / ccm and a melting point of over 12C ° C, the structural viscosity number [4 * J interferes with the equation: ζ / Λ J = Am with the melt index ra, where A \ 1.5 and B = -O _f 164. If this equation is therefore represented in a bilogarithmic system for A = 1.5, the straight line shown in FIG. 1 is obtained. The known linear polyethylenes with a density above 0.94 g / ccm and a melting point above 120 ° C. have values for /, - w J and m such that each point identified in the diagram of FIG. 1 is on this straight line or above it lies.

The polyethylenes obtained by the process according to the invention differ from the above-mentioned known, linear polyethylenes with a density above 0.94 g / ccm, since even at a density above 0.94 g / ccm they have such an intrinsic viscosity value // ή J and a Have melt index m,

209834/1078

that each point identified in the diagram of Fig. 1 lies below the straight line there. This means that the polyethylenes obtained according to the invention have such an intrinsic viscosity / 4 « J and a value of m _f that the following equation can be used:

M - Am ^B where A is 0.5 and B = -0.164.

The aim of the present invention is therefore to provide a process for the production of high-density polyethylenes with good processability, which is characterized in that one or more linear polyethylenes with a density above 0.94 g / ccm, a melting point above 120 C _{1 and} a solubility in tetrahydronaphthalene at 135 C ₁ with an intrinsic viscosity / jy \ J and a melt index m, which coincide with the straight line of the equation l; ij = 1.5 m 'in the diagram of FIG. 1 or above, with an organic peroxide Treated thermomechanically in such amounts that density, melting point, intrinsic viscosity / _-% and solubility in tetrahydronaphthalene at 135 ° C. remain practically unchanged, but a decrease in the melt index is brought about to values that are shown in the diagram of Fig. 1 below the specified straight line.

The amount of peroxide used in the thermomechanical treatment depends on the chemical nature of the peroxide, the melt index of the starting polyethylene (s) and the melt index desired in the end product. In any case, the Peroxydmenge r.uß be such that it causes the melt index of the m o ^ he the polyethylenes because of the thermomechanical treatment, a decrease at least 20 corresponds ° _{/ 0} of the output value. On the other hand, the amount must not be so high that the polyethylene (s) are only partially converted into tetrahydro

ο
naphthalene can become insoluble at 135 C.

209834/1078

The thermomechanical treatment takes place at temperatures between 140-25 ° C.

When carrying out the method according to the invention, it is essential that the treatment of the polyethylene or the polyethylene mixture takes place in a device with a strong homogenizing force. Incomplete homogenization actually leads to a decrease in certain physical and mechanical properties, especially impact resistance and tensile strength.

In a preferred embodiment, the polyethylene or the polyethylene mixture is thermomechanical in an extruder with a strong homogenizing force Subjected to treatment, the peroxide being added in the form of a basic batch with polyethylene through a lateral introduction device which is mounted perpendicular to the major axis of the extruder.

The polyethylene or polyethylene blend to be treated is preferred homogenized before and after the introduction of peroxide. This double homogenization is important because the polyethylene releases when it comes into contact with the peroxide must be of high molecular weight agglomerates caused by the peroxide could be made insoluble! and the peroxide continues to be dispersed to avoid having too localized an effect on the formation of insoluble Gels leads.

The basic polyethylene / peroxide approach is obtained by separately at Room temperature the peroxide with the polyethylene in powder or grain form

with the same or different melt index as the poly- »·

ethylene mixes. The Gruncian approach can also be obtained by adding the peroxide at temperatures at which this is not very reactive, incorporated into the molten polymer.

209834/1078

The processability of polyethylene, shown as behavior during extrusion, was determined using a capillary rheometer (e.g. Instron-type) evaluated. Each polymer sample was at 160 ° C. in the gradient area the flow rate (V-) between 3 and 3000 sec ~ examined. the The data obtained made it possible to set up the flow curves of the various products, namely, depending on the gradient of the flow velocity

¥ "the apparent viscosity ^ ₃ " T ^ t represented by the

Relationship between the load tangential to the wall of the capillary

2L

where Δ stands for the pressure drop occurring in the capillary; R for the

Radius of the capillary, L is the length of the capillary and Y * is the gradient of the flow velocity on the capillary wall, calculated on the basis of

• 4Q
of the formula V "« »- and applies to Newtonian fluids, where

0 ^p TTr ³
Q is the volumetric capacity through the capillary. It was considered expedient to use the definition of viscosity and not make any changes due to the non-Newton ¹ nature of the material in order to comply with the evaluation data in the literature. The high value for the ratio of length: diameter L: D ^ 60 of the capillary used also made it possible to ignore the influence on the measurement of the entry effects.

Furthermore, the surface effect of the extruded products. The values for 1 * were determined at where the melt fracture was found j in a normal microscope with 10x magnification could make the "shark skin phenomenon" which has so far been described in detail in the art, can be observed.

209834/1078

In general, the values of V ^ both in the case of normal extrusion press as well as during blow molding can be related to the processing speed. It can therefore be seen that the increase in critical fourth of JjV, i.e. of values at which in the according to the invention Process obtained product the two types of surface irregularities Beginning to occur, an improvement in the processability of the material by allowing higher extrusion speeds allow. ..

The following examples illustrate the present invention without it to restrict.

Example 1 to 3

Three samples of linear polyethylene obtained by the low pressure method with high density and different melt index were according to the invention Process treated thermomechanically with different amounts of peroxide .

The initial properties of these polyethylenes are given in Table 1, Examples 1 to 3. These properties were:
Density at 23 ° C. in g / ccm, determined according to ASTM D-1505-63T. Melting point: temperature in C. according to the apex of the top of the melting point curve obtained in a Perkin Elmer 1B differential colorimeter at a heating rate of C./min.

Intrinsic viscosity in dl / g, determined in tetralin at 135 C. Sehmelz index in g / 10 min, determined according to ASTM D-1238-62T, condition E.

209834/1078

-B-

Ratio Mw / Mn: parameter that defines the breadth of the molecular weight distribution and increases as the molecular weight distribution becomes broad; it was determined by gel permeation chromatography using the Waters Inc. Device, Model 200, under the following conditions: Solvent = 2,4,6-trichlorobenzene; Temperature = 135 C; Elution rate «1 cc / min; Polymer concentration = 0.25 g / 100 ecm; five analytical

3 4 5 column, the crosslinked polystyrene with a porosity of 10, 10, 10,

10 and 10 K contained.

The device used for the treatment consisted of a twin screw extruder (e.g. of the Werner and Pfleiderer type, described in Enc. of Poly.Sei. and Tech., Vol. 6, p. 544 (1968); Interscience Publishers], in which sections of screws with strong homogenizing force alternate with screw sections, which mainly have a transport effect exercise.

The. Amounts of peroxide used in each treatment and the properties of the treated polyethylenes are given in Table 1. The processability of the treated polyethylene was then evaluated by determining the relative values of Y * bsi at the beginning of the "shark skin phenomenon" and at the beginning of melt fracture in the manner described above. These values are also given in Table 1.

The processability of the treated polyethylene can be compared with that of the untreated polyethylene with a similar melt index. For this purpose, give ^ -Table 1 the properties of untreated polyethylene A and B ₁ which at the beginning of the "shark skin phenomenon" and the beginning of melt fracture have too low a value for K * and are therefore not suitable for processing at high extrusion speeds will.

209834/1078

- 9 The comparison can be made between:

A) the treated polyethylene of Example 1 and the untreated polyethylene A with a similar melt index and density;

B) the treated polyethylene of Example 2 and the untreated polyethylene B with a similar melting index and density (these two polyethylene differ from A) by lower densities) and

C) the treated polyethylene from Example 3 and the untreated polyethylene B with a much higher melt index (0.3 compared to Q ₉ 03).

From the comparisons A) and B) it can be seen how with the same melt index and density, the polyethylenes treated according to the invention have a clear

*
Show increase in critical values of W; therefore they can be processed at higher speeds than the known, untreated polyethylene.

Comparison C) also shows that - although the speed index of the treated Polyethylene is significantly lower (0.03) than that of the untreated Comparative polyethylene (0.3) - the processability of the treated according to the invention Polyethylene is noticeably higher, with the processability of the Polyethylene normally decreases with a decrease in melt index.

Example 4

Two high-density polyethylenes with different melt indices (0.05 and 20), produced by the low-pressure process and with the properties listed in Table 1, were mixed in a weight ratio of 26:74. The resulting melt index of the mixture was 2 g / 10 min.

209834/1078

The polyethylene mixture was then treated thermomechanically as in Examples 1 to 3 with 0.20 wt .- ^ 2 _f 5-0imethyl-2,5-di-tert-butylperoxyhexane. The properties of the treated products are shown in Table 1.

209834/1078

Table 1

Example 12 3 Mixture · Comparative Samples

I. f AB

Properties of the starting polyethylenes

Density; g / cc 0.962 0.943 0.945. "0.959 0.961 0.958 0.943

Melting point; ⁰ C. 139 132 132 138 140 139 132

Intrinsic viscosity number; dl / g 1.3 1.4 1.2 2.5 0.90 2.1 1.9

Melt index; g / 10 min 5.2 2.0 3.6 0.05 20 '0.26 0.30

; Beginning of the "shark skin phenomenon ¹¹ ; see""- - - - - 59 117

Start of melt fracture; see "" - - - ^ - -y 147 176

CU _n I

The composition of the mixture; Gew.-JJb cd melt index of the mixture; g / 10 min

u> Mw / Mn. 5.6 5.8 5.3 10.2 5.9 6.2

CD-P-CD

Table 1 continued
Example 12 3 4

treatment

* 2,5-dimethyl-2,5-di-tert-butyl-

peroxyhexane; Weight J ^ 0.2 0.066-0.20

2,5-dimethyl-2,5-di.-tert.-butyl-

peroxydhexine; Wt .- ^ - - 0.21 -

Properties of the treated polyethylene

ro density; g / ccm

ο ο

J ₀ melting point; C.

Structure * viscosity number; dl / g

* "* Melt index; g / 10 min

_a Ay at the beginning of the "shark skin phenomenon"; lake""

^ j · at the beginning of melt fracture; sec I

oo C ^P

Mw / Mn 5.8 5.9 5.3 10.0

0.958 0.943 0.944 0.959 138 133 133 138 M. 1.4 1.3 1.5 0.24 0.27 0.03 0.36 587 587 587 > 3000 3000 1056 1174 > 3000

CT5 U)

Example 5

A polyethylene with a density of 0.953 g / ccm, a melting point of 136 ° C. _t jfnj 1.4 dl / g, a melt index of 2.0 g / 10 min and Mw / Mn = 5.8 with 0.05 wt. -tfa of a mixture of ρ and m-ot, d '-Bis- (tert-butylperoxy) -diisopropylbenzene treated. The treatment was carried out in an extruder with a strong homogenizing force as in Example 1-3. When so obtained the characteristics indicated in Example 1-3, and the tensile strength were measured according to ASTM 636-6B (tensile speed of 50 mm per minute; sample type C) and notched Izod impact strength at 23 ⁰ C. according to ASTM D 255- 56 (procedure A). The following values were obtained: density = 0.951 g / ccm; Melting point = 135 ° C; JmJ »1.4 dl / g; Melt index = 0.37 g / 10 "in; Mw / Mm = 5.6; W at the beginning of the "shark skin phenomenon" = 587 see;

—1 · 2

J ^ _ at the beginning of the melt fracture J 3000 see; Tensile strength = 230 kg / cm;

Notched Izod impact strength at 23 ° C. = 24 kgem / cm.

The same starting polyethylene was treated with the same amount of the same Perpxyds in a conventional twin screw extruder with limited homogenizing power and gave a product with the following properties: density - 0.950 g / ccm; Melting point = 135 ° C; {/ ft J = 1.4 dl / g; Melting agent, = 0.34 g / 10 min; Mw / Ün = 5.5; V " at the beginning of the" shark skin phenomenon ¹¹ = 587 see; r at the beginning of the melt fracture = ^ 300Q see; Tensile strenght; 180 kg / cm; Notched Izod impact strength at 23 ° C. = »6.7 kg / cm.

209834/1078

Claims (1)

Claims J ~ i ~ Process for the production of polyethylenes with high density and good processability, characterized in that one or more linear polyethylenes with a density above 0.94 g / ccm _f a melting point above 120 C, one in a device with strong Homagenisierungskraft one solubility in tetrahydronaphthalene at 135 C, an intrinsic viscosity number £ / yj J and a melt index m, so that in the diagram of Fig. 1 * to coincide the points with the straight line of "equation // YLJ" 1.5 ra or above, with an organic peroxide in such amounts thermo-technically treated that density, melting point, intrinsic viscosity £ / rijf and solubility in tetrahydronaphthalene at 135 G. remain unchanged, but a decrease in the melt index ra is caused to values that in the curve of FIG. 1 below the straight line are listed. 2. - Method according to claim 1 _f, characterized in that the amount of peroxide is such that a decrease in the melt index of the polyethylene (s) is caused, which corresponds to at least 20% of the initial value. 3.- Process according to claims 1 and 2, characterized in that the organic peroxide is 2,5-dimethyl-2 _f 5-di-tert-butylperoxyhexane, 2,5-dimethyl-2,5-di-tert-butylperoxyhexine or a mixture of p- and mc ^, p ('-Bis ·· (tert-butylperoxy) -diisopropyliazole is used. 4, - Method according to claim 1 foiä 3, characterized in that the peroxide in the form of a basic approach (Kunzentrafces) is used with polyethylene. 209834/1078 5.- The method according to claim 1 to 4, characterized in that the or the Polyethylenes are homogenized before or after the addition of the peroxide. 6. - Method according to claim 1 to 5 _f, characterized in that the thermomechanical treatment with peroxide at temperatures between 140-250 ° C. is carried out. 7.- polyethylene with a melting point above 120 C _1, a density above 0.94 g / ccm, a solubility ih tetrahydronaphthalene at 135 C, characterized in that it has such an intrinsic viscosity Dft 3 and such a melt index m that in the diagram of Fig. 1 the points lie below the straight line of the equation £ / ttJ = 1.5 m '. 8.- polyethylene according to claim 7 in the form of molded articles, Fibers or films. The patent attorney: 209834/1078 Jb Blank page