PROCESS FOR PRODUCING A POLYETHYLENE COMPOSITION USING MOLECULAR WEIGHT ENLARGEMENT

Abstract

The present invention provides a process for producing a polyethylene composition by treating a cross-linkable ethylene copolymer containing monomer units with hydrolysable silane-groups and polar monomer units. The invention further provides a treated cross-linkable polyethylene composition 5 obtained by the process and a silane-crosslinked polyethylene composition obtained by the process. The invention further provides articles comprising the treated cross-linkable polyethylene composition or comprising the silane-crosslinked polyethylene composition.

Claims

1. Process for producing a polyethylene composition, comprising the steps of a) providing a cross-linkable ethylene copolymer composition (A) having a first MFR.sub.2, comprising (A1) monomer units with hydrolysable silane-groups, and (A2) polar monomer units, b) adding a free radical generator (B) in an amount of from 0.01 to 1 wt. % to the cross-linkable ethylene copolymer composition (A), c) treating the cross-linkable ethylene copolymer composition (A) with the free radical generator (B) to obtain a treated cross-linkable ethylene copolymer composition (A) having a second MFR.sub.2, wherein the second MFR.sub.2 is lower than the first MFR.sub.2, and wherein the first MFR.sub.2 of the cross-linkable ethylene copolymer composition (A) is from 1 to 100 g/10 min and/or the second MFR.sub.2 of the treated cross-linkable ethylene copolymer (A) composition is from 0.1 to 10 g/10 min.

2. The process according to claim 1, wherein the first MFR.sub.2 of the cross-linkable ethylene copolymer composition (A) is from 1 to 50 g/10 min and/or the second MFR.sub.2 of the treated cross-linkable ethylene copolymer (A) composition is from 0.5 to 5 g/10 min.

3. The process according to claim 1, wherein the monomer units with hydrolysable silane-groups (A1) are present in the ethylene copolymer composition (A) in an amount of 0.1 to 6 wt. % and/or wherein the polar monomer units (A2) are present in the ethylene copolymer composition (A) in an amount of 10 to 35 wt. %.

4. The process according to claim 1, wherein the monomer units with hydrolysable silane-groups (A1) are represented by formula (I):
R.sup.1SiR.sup.2.sub.qY.sub.3-q   (I) wherein R.sup.1 is an ethylenically unsaturated hydrocarbyl, hydrocarbyloxy or (meth)acryloxy hydrocarbyl group, each R.sup.2 is independently an aliphatic saturated hydrocarbyl group, Y which may be the same or different, is a hydrolysable organic group and q is 0, 1 or 2.

5. The process according to claim 1, wherein the polar monomer units (A2) are selected from (C1-C6)-alkyl acrylate and (C1-C6)-alkyl (C1-C6)-alkylacrylate.

6. The process according to claim 1, wherein the free radical generator (B) comprises a peroxide, preferably an organic peroxide.

7. The process according to claim 1, wherein the treatment of step c) is conducted in the reactor extruder.

8. The process according to claim 1, further comprising the steps of a) adding a silanol condensation catalyst (D) to a polyethylene composition comprising the treated cross-linkable ethylene copolymer composition (A), and b) silane-crosslinking the polyethylene composition in the presence of the silanol condensation catalyst (D) to obtain a silane-crosslinked polyethylene composition.

9. The process according to claim 8, wherein the silanol condensation catalyst (D) is present in an amount of 0.01 to 3 wt. % based on the total amount of a polyethylene composition comprising the treated cross-linkable ethylene copolymer composition (A).

10. A polyethylene composition comprising the treated cross-linkable ethylene copolymer composition (A) obtainable by a process according to claim 7.

11. A polyethylene composition according to claim 10 wherein the treated cross-linkable ethylene copolymer composition (A) has an MFR.sub.2 of from 0.1 to 5 g/10 min, and/or an MFR.sub.5 of from 1 to 20 g/10 min, and/or a Shore A value of 50 to 90, and/or an SHI.sub.eta(0.05/300) value of from 85 to 150.

12. Silane-crosslinked polyethylene composition obtainable by a process according to claim 9.

13. The silane-crosslinked polyethylene composition according to claim 12, which has a Shore A value of 55 to 97, and/or a compression set of 8 to 35% at 23° C., and/or a compression set of 10 to 80% at 70° C.

14. Article comprising the polyethylene composition comprising the treated cross-linkable ethylene copolymer composition (A) according to claim 10.

15. Article according to claim 14, wherein the article is a profile, a seal or a gasket.

16. Article comprising the polyethylene composition comprising the silane-crosslinked polyethylene composition according to claim 12.

Description

EXAMPLES

[0111] 1. Determination Methods

[0112] a) Melt Flow Rate

[0113] The melt flow rate (MFR) is determined according to ISO 1133 and is indicated in g/10 min. The MFR is an indication of the flowability, and hence the processability, of the polymer. The higher the melt flow rate, the lower the viscosity of the polymer.

[0114] The MFR.sub.2 of ethylene (co-)polymers is measured at a temperature 190° C. and at a load of 2.16 kg.

[0115] The MFR.sub.5 of ethylene (co-)polymers is measured at a temperature 190° C. and at a load of 5 kg.

[0116] The FRR.sub.5/2.16 is the ratio of MFR.sub.5/MFR.sub.2.

[0117] b) Shore A

[0118] Shore A measurements are performed according to according IS0868: 2003.

[0119] The measurement was done at room temperature, sample thickness 4 mm compression moulded samples, prepared like samples for compression set but with 4 mm.

[0120] The compression moulding is normally done according EN ISO 1872-2-2007. The plaques as used herein were prepared as described hereinafter in item f).

[0121] c) Rheological Parameters

[0122] The characterization of polymer melts by dynamic shear measurements complies with ISO standards 6721-1 and 6721-10. The measurements were performed on an Anton Paar MCR501 stress controlled rotational rheometer, equipped with a 25 mm parallel plate geometry. Measurements were undertaken on compression moulded plates, using nitrogen atmosphere and setting a strain within the linear viscoelastic regime. The oscillatory shear tests were done at T 190° C. applying a frequency range between 0.01 and 600 rad/s and setting a gap of 1.3 mm.

γ(t)=γ.sub.0 sin(ωt)   (1)

[0123] If the applied strain is within the linear viscoelastic regime, the resulting sinusoidal stress response can be given by

σ(t)=σ.sub.0 sin(ωt+δ)   (2)

[0124] where

[0125] σ.sub.0 and γ.sub.0 are the stress and strain amplitudes, respectively

[0126] ω frequency is the angular

[0127] δ is the phase shift (loss angle between applied strain and stress response)

[0128] t is the time

[0129] Dynamic test results are typically expressed by means of several different rheological functions, namely the shear storage modulus G′, the shear loss modulus, G″, the complex shear modulus, G*, the complex shear viscosity, η*, the dynamic shear viscosity, η′, the out-of-phase component of the shear viscosity η″ and the loss tangent, tan δ which can be expressed as follows:

[00001]$\begin{array}{cc}{G}^{\prime }=\frac{\sigma 0}{\mathrm{\gamma 0}}\cos \delta \left[\mathrm{Pa}\right]& \left(3\right)\end{array}$$\begin{array}{cc}{G}^{\prime }=\frac{\sigma 0}{\mathrm{\gamma 0}}\sin \delta \left[\mathrm{Pa}\right]& \left(4\right)\end{array}$$\begin{array}{cc}{G}^{*}=G^{\prime }+{\mathrm{iG}}^{\mathrm{\prime \prime }}\left[\mathrm{Pa}\right]& \left(5\right)\end{array}$$\begin{array}{cc}{\eta }^{*}={\eta }^{\prime }-i{\eta }^{\mathrm{\prime \prime }}\left[\mathrm{Pa}.Math.s\right]& \left(6\right)\end{array}$$\begin{array}{cc}{\eta }^{\prime }=\frac{G^{\mathrm{\prime \prime }}}{\omega }\left[\mathrm{Pa}.Math.s\right]& \left(7\right)\end{array}$$\begin{array}{cc}{\eta }^{\mathrm{\prime \prime }}=\frac{G^{\prime }}{\omega }\left[\mathrm{Pa}.Math.s\right]& \left(8\right)\end{array}$

[0130] The values of storage modulus (G′), loss modulus (G″), complex modulus (G*) and complex viscosity (eta*) were obtained as a function of frequency.

[0131] Thereby, e.g. eta*0.05 rad/s is used as abbreviation for the complex viscosity at the frequency of 0.05 rad/s or eta*300 rad/s is used as abbreviation for the complex viscosity at the frequency of 300 rad/s.

[0132] Besides the above mentioned rheological functions one can also determine other rheological parameters such as the so-called elasticity index EI(x). The elasticity index EI(x) is the value of the storage modulus, G′ determined for a value of the loss modulus, G″ of x kPa and can be described by equation 10.

EI(x)=G′ for (G″=x kPa)[Pa]  (9)

[0133] For example, the EI(5 kPa) is the defined by the value of the storage modulus G′, determined for a value of G″ equal to 5 kPa.

[0134] The determination of the so-called Shear Thinning Indexes is done, as described in equation 10.

[00002]$\begin{array}{cc}\mathrm{SHI}\left(x/y\right)=\frac{{\mathrm{Eta}}^{*}\mathrm{for}\left(G^{*}=xk\mathrm{Pa}\right)}{{\mathrm{Eta}}^{*}\mathrm{for}\left(G^{*}=yk\mathrm{Pa}\right)}\left[\mathrm{Pa}\right]& \left(10\right)\end{array}$

[0135] For example, the SHI(2.7/210) is defined by the value of the complex viscosity, in Pa.Math.s, determined for a value of G* equal to 2.7 kPa, divided by the value of the complex viscosity, in Pa.Math.s, determined for a value of G* equal to 210 kPa.

[0136] The values are determined by means of a single point interpolation procedure, as defined by Rheoplus software. In situations for which a given G* value is not experimentally reached, the value is determined by means of an extrapolation, using 15 the same procedure as before. In both cases (interpolation or extrapolation), the option from Rheoplus “-Interpolate y-values to x-values from parameter” and the “logarithmic interpolation type” were applied.

[0137] d) Content of monomer units

[0138] The content of the polar monomer units was determined based on Fourier Transform Infrared Spectroscopy (FTIR) as known in the art. Details are given in WO 2019/086948.

[0139] The content of monomer units having hydrolysable silane-groups was determined by quantitative nuclear-magnetic resonance (NMR) spectroscopy as known in the art. Details are given in WO 2019/086948. Reference is made to A. J. Brandolini, D. D. Hills, “NMR spectra of polymers and polymer additives”, Marcel Dekker Inc., 2000.

[0140] e) Gel content

[0141] The gel content was calculated according to ASTM D 2765-01. The gel content was measured from the plaques samples for compression set measurements, see “Sample preparation and compression set” below.

[0142] d) Sample preparation and compression set

[0143] The sample preparation for compression set measurement was done as follows.

[0144] The tested materials, optionally dry blended with a silanol condensation catalyst, were extruded into tapes. Tape samples were produced on a Collin extruder (Teach-Line E20T) with a temperature profile of 120-130-140° C. The tape samples had a thickness of 2mm and a width of 40mm.

[0145] Plaques samples for compression set were made by compression moulding the tapes to get a thickness of 6.3 mm for the compression set test and also into 4 mm and 1 mm thickness plaques for Shore A and DMTA measurements, respectively.

[0146] The plaques were compression moulded as follows:

[0147] The press was preheated to set temperature, above the melting temperature of used polymer (herein at 180° C.).

[0148] 1 mm plaque: Step 1—the polymer tapes were put in the press and kept for 5 min without pressure. Step 2—the pressure was increased to 187 bar and kept at this pressure for 5 min. Step 3—the plaque was cooled down to room temperature at rate of 15° C./min at 187 bar pressure.

[0149] 4 mm plaque: same procedure as for 1 mm plaque, except that tapes were kept in Step 1 for 10 min.

[0150] 6.3 mm plaque: same procedure as for 1 mm plaque, except that tapes were kept in Step 1 for 15 min.

[0151] After compression moulding the plaques were merged in hot water (50° C.) for 24 hours to fully crosslink the material before measuring the compression set. The actual specimen is then cut from the plaque and fixed between two metal plates at room temperature.

[0152] The compression is set to be 25% of the thickness of the specimen by utilizing different spacers. The compressed samples are then stored at the selected temperature (23° C. or 70° C.) for 24 hours. Thereafter, the samples are moved to room temperature and released from compression. After 30 minutes of recovery at room temperature, the samples are measured to determine the compression set. The compression set is measured according to DIN ISO 815-1, method A, specimen B.

[0153] g) DMTA measurements

[0154] The samples had a thickness of 1 mm and where prepared as described above, item f).

[0155] The characterization of dynamic-mechanic properties complies with ISO standards 6721-1, 6721-4, 6721-11. The measurements were performed on a “Netzsch DMA 242E Artemis” strain/stress-controlled dynamic mechanical Analyzer, equipped with a tensional-sample holder for rectangular specimen geometry. Measurements were undertaken on compression moulded plates under nitrogen atmosphere, using liquid nitrogen for cooling. The dynamic mechanic thermal analysis were performed in the temperature range from −170° C. to +130° C. with a heating rate of 2° C./min, a frequency of 1 Hz, in strain-stress controlled modus with a maximum dynamic applied stress of 1.80 MPa, a static load on the specimen of 0.20 MPa and a maximum strain of 0.20%. The clambing of the specimens were performed in the temperature range from −100° C. to −130° C. with a torque of 2.5 cNm per screw. The conditioning at the start-temperature of −170° C. was carried out with an isothermal section of 30 minutes, wherein the last 5 min had a change in E′ equal or less than 5 MPa per minute. The evaluation was performed with the software “Proteus Thermal Analysis—Version 6.1.0”

[0156] 2. Materials

[0157] a) Ethylene Copolymers

[0158] The ethylene copolymers with the type and amount of comonomer(s) indicated used in the present invention are given in Table 1 below.

[0159] For Polymer A, Polymer B, Polymer C: see Table 1.

[0160] Polymer D: Ethylene-methyl acrylate copolymer comprising 24 wt. % methyl acrylate monomer units, MFR.sub.2=50 g/10 min.

[0161] Polymer E: Ethylene-methyl acrylate copolymer comprising 25 wt. % methyl acrylate monomer units, MFR.sub.2=0.5 g/10 min.

[0162] Polymer F: Ethylene-methyl acrylate-vinyl trimethoxysilane terpolymer comprising 21 wt. % methyl acrylate and 1.8 wt. % vinyl trimethoxysilane monomer units, MFR.sub.2=1.8 g/10 min.

[0163] b) Additives

[0164] Crodamide ER and Crodamide 212 are commercially available from Company Croda.

[0165] 3. Results

[0166] The following Table 1 provides an overview of the inventive (IE) and comparative (CE) examples used. IE4, IE5 and CE7 are blends of two polymers as indicated in Table 1 below.

TABLE-US-00001 TABLE 1 Comonomer(s) MFR.sub.2, MFR.sub.5, Content, g/ g/ Example Polymer(s) Type(s) wt. % Additive 10 min 10 min IE1 Polymer A MA/VTMS   30/2.2 Crodamide 29 99 ER 5000 ppm IE2 Polymer B MA/VTMS   26/2.5 16.3 56.4 IE3 Polymer C MA/VTMS 22.7/4.5 Crodamide 24.6 84 212 2033 ppm IE4 22 wt. % MA/VTMS 23.7/1 47 Polymer C + 78 wt. % Polymer D IE5 45 wt. % MA/VTMS 26.7/1 38 Polymer A + 55 wt. % Polymer D CE6 Polymer F MA/VTMS   21/1.8 1.8 6.9 CE7 45 wt. % MA/VTMS 23.2/1 1 4.3 Polymer B + 55 wt. % Polymer E MA-methyl acrylate, VTMS-vinyl trimethoxy silane

[0167] Table 1-1 below shows rheological properties of inventive examples IE1 to IE3 from Table 1.

TABLE-US-00002 TABLE 1-1 Rheological properties of inventive examples IE1 to IE3 IE1 IE2 IE3 eta(0.05 rad/s), Pa .Math. s 2048 2323 3997 eta(300 rad/s), Pa .Math. s 104 144 91 SHI eta(1 kPa), Pa .Math. s 559 906 615 eta(2.7 kPa), Pa .Math. s 378 641 353 eta(5 kPa), Pa .Math. s 288 501 254 eta(1 kPa)/eta(5 kPa) 1.94 1.81 2.42

[0168] 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane (POX) was mixed to inventive examples IE1 to IE5 in the amounts indicated in Table 2 below.

[0169] The POX was not directly added, but base material was soaked with liquid POX and the soaked material was added as a masterbatch during the compounding. The compounding was done on a twin screw extruder ZSK18 Coperion, screw speed 300 rpm, temperature zones 1-8 20/170/190/210/230/240/230/220° C. The soaked material was added via side feeder.

[0170] The temperatures in the extruder were such that resulted in decomposition of the POX and causing a molecular weight enlargement of the inventive copolymers. Properties of the obtained molecular weight enlarged inventive examples IE1-1 to IE5-1 are given in Table 2 below.

TABLE-US-00003 TABLE 2 Samples after molecular weight enlargement, i.e. after treatment with peroxide IE1-1 IE2-1 IE3-1 IE4-1 IE5-1 POX, wt. % 0.39 0.36 0.30 0.60 0.44 MFR.sub.2, g/10 min 3.3 1.9 2.65 1.4 2.55 MFR.sub.5, g/10 min 12.9 9.25 13.4 8.86 11.9 FRR5/2.16 3.9 4.9 5.1 6.3 4.7 Gel content 0.13 0.21 0.51 0.34 0.21 Shore A 59 59.6 72.6 77.1 70.5 eta(0.05 rad/s), Pa .Math. s 14122 20011 18942 20755 19384 eta(300 rad/s), Pa .Math. s 142 208 141 159 156 SHI 99.5 96.2 134.3 130.5 124.3 eta(1 kPa), Pa .Math. s 7470 19220 53285 eta(2.7 kPa), Pa .Math. s 2335 5549 2585 eta(5 kPa), Pa .Math. s 1281 3044 1326 eta(1 kPa)/eta(5k Pa) 5.83 6.31 40.18 G′, −100° C. 2000 2600 2385 2743 2769 G′, −50° C. 1305 1673 1611 1783 1754 G′, 23° C. 8.5 13.5 17.4 23.6 17.3 G′, 50° C. 2.4 4.2 5.1 6.9 5.1

[0171] As can be seen from comparing Table 1-1 (examples before POX treatment) with Table 2 (POX treated examples), POX treatment causes molecular weight enlargement which results in increased shear thinning performance for all the POX treated inventive examples IE1 to IE3 compared to the non-treated, as demonstrated by SHI, FRR.sub.5/2.16 and eta(1 kPa/5 kPa).

[0172] After extrusion, inventive examples IE1-1 to IE5-1 as well as comparative examples CE6 and CE7 have been formed into plaques of 6.3 mm thickness as described above. The properties of the thus obtained plaques of inventive examples IE1-2 to IE5-2 and CE6-2 and CE7-3 are given in Table 3 below.

TABLE-US-00004 TABLE 3 Properties of plaques after blending with silanol condensation catalyst before silane-crosslinking IE1-2 IE2-2 IE3-2 IE4-2 IE5-2 CE6-2 CE7-2 Shore A 59.5 70.2 73.3 77.3 70.5 81 82 G′, −50° C., 1377 1798 1673 1783 1679 1792 1776 MPa G′, 23° C., MPa 9.9 15.7 19.7 26 19.5 36 33 G′, 50° C., MPa 3.3 5.3 6.2 7.9 6.1 12 11 Compression 23.6 23.2 25.2 30.6 28.7 25.8 26.9 set, 23° C., % Compression 97.2 90.7 89 85.8 84.3 88 89 set, 70° C., %

[0173] IE1-2 to IE5-2 as well as comparative examples CE6-2 and CE7-2 were crosslinked in the presence of 2 wt. % of silanol condensation catalyst masterbatch Ambicat LE4476 (corresponding to 0.03 wt. % of pure catalyst) which is commercially available from Borealis. Cross-linking was performed at constant room temperature at 23° C. and 50% RH (relative humidity) for 168 hrs. All examples have been formed into plaques of 6.3 mm thickness as described above.

[0174] The properties of the silane-crosslinked examples are given in Table 4 below.

TABLE-US-00005 TABLE 4 Properties of plaques after silane-crosslinking IE1-3 IE2-3 IE3-3 IE4-3 IE5-3 CE6-3 CE7-3 Shore A 74.1 80.6 86.6 81.5 76.6 87 84 G′, −50° C., MPa 1556 1746 1745 1818 1811 1575 1689 G′, 23° C., MPa 13.7 20.5 30.6 27.6 20.4 40 31 G′, 50° C., MPa 7.2 10.4 18.5 10.3 8 18 12 G′, 70° C., MPa 4 5.78 10.85 4.72 3.95 9.44 6.01 G″, −50° C., MPa 170.15 119.3 113.89 110.14 110.62 111.39 90.88 G″, 23° C., MPa 1.31 2.2 2.8 3.64 2.59 5.3 4.12 G″, 50° C., MPa 0.38 0.5 0.81 0.9 0.69 1.18 0.93 G″, 70° C., MPa 0.14 0.19 0.24 0.3 0.24 0.32 0.32 tan δ, −50° C. 0.109 0.068 0.065 0.061 0.061 0.071 0.054 tan δ, 23° C. 0.096 0.107 0.092 0.132 0.127 0.133 0.133 tan δ, 50° C. 0.053 0.048 0.044 0.087 0.086 0.066 0.078 tan δ, 70° C. 0.035 0.048 0.044 0.087 0.086 0.066 0.078 J, −50° C., 1/MPa 0.05 0.12 0.13 0.15 0.15 0.13 0.20 J, 23° C., 1/MPa 7.98 4.24 3.90 2.08 3.04 1.42 1.83 J, 50° C., 1/MPa 49.86 41.60 28.20 12.72 16.80 12.93 13.87 J, 70° C., 1/MPa 204.08 160.11 188.37 52.44 68.58 92.19 34.07 Compression 15.4 16.8 16.6 25 22.4 20.6 25.6 set, 23° C., % Compression 32.3 31.2 27.6 63.5 59.4 37.2 60.8 set, 70° C., % Gel content 88 92 94 52 65 97 63

[0175] As can be seen from Table 4, the compression set properties have improved with the molecular weight enlargement treatment. All crosslinked samples IE1 to IE3 with VTMS level of around 2% and above show lower compression set versus CE6 at 70° C. and at 23° C. All crosslinked samples IE4 and IE5 with VTMS level of around 1% show lower compression set versus CE7 at 23° C.

[0176] Also, the Shore A values of IE4 is lower than that of CE7, and the Shore A value of IE3 lower than that of CE6.