Foamable ethylene polymer

Abstract

The invention relates to a foamable ethylene polymer composition comprising at least one antioxidant, at least one process aid and at least 80 wt % of a peroxide-treated ethylene polymer composition. The foamable ethylene polymer composition has melt strength of at least 2 cN, a density of 940 to 970 kg/m3, and dissipation factor measured at 1.9 GHz of 50-80−10.sup.−6. The invention further relates to a process for making such a foamable ethylene polymer composition, and use of the foamable ethylene polymer composition in a foamed cable insulation.

Claims

1. A foamable ethylene polymer composition, wherein said foamable ethylene polymer composition comprises a. at least one antioxidant, b. at least one process aid, c. at least 80 wt % of a peroxide-treated ethylene polymer composition, wherein said foamable ethylene polymer composition has a melt strength of at least 2 cN, a density of 940 to 970 kg/m.sup.3, and dissipation factor measured at 1.9 GHz of 50-80.Math.10.sup.−6, and wherein the amount of peroxide with which said peroxide-treated ethylene polymer composition was treated is 0.1 to 2 wt %.

2. The foamable ethylene polymer composition according to claim 1, wherein said foamable ethylene polymer composition has MFR.sub.2 (2.16 kg, 190° C.) of 0.1 to 10 g/10 min.

3. The foamable ethylene polymer composition according to claim 1, wherein said foamable ethylene polymer composition has crystallinity of 70 to 90%.

4. The foamable ethylene polymer composition according to claim 1, wherein said foamable ethylene polymer composition has crystallisation temperature of 110 to 130° C.

5. The foamable ethylene polymer composition according to claim 1, wherein the process aid is a stearate.

6. The foamable ethylene polymer composition according to claim 1, wherein said peroxide-treated ethylene polymer composition is derived from an ethylene polymer with a density of 940 to 970 kg/m.sup.3 and an MFR.sub.2 of 3 to 50 g/10 min prior to peroxide treatment.

7. The foamable ethylene polymer composition according to claim 1, wherein said peroxide-treated ethylene polymer composition is derived from an ethylene polymer with a density of 950 to 970 kg/m.sup.3 and an MFR.sub.2 of 5 to 20 g/10 min prior to peroxide treatment.

8. The foamable ethylene polymer composition according to claim 1, wherein the peroxide with which said peroxide-treated ethylene polymer composition is treated has a half-life temperature T½ at 0.1 h above 133° C.

9. The foamable ethylene polymer composition of claim 1, wherein the at least one antioxidant comprises a phenolic antioxidant.

10. The foamable ethylene polymer composition of claim 9, wherein the phenolic antioxidant is sterically hindered.

11. The foamable ethylene polymer composition of claim 9, wherein the phenolic antioxidant is pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate).

12. The foamable ethylene polymer composition of claim 5, wherein the stearate is zinc stearate.

13. The foamable ethylene polymer composition of claim 1, wherein the melt strength is in the range of 3 cN to 15 cN and the density is in the range of 950 kg/m.sup.3 to 970 kg/m.sup.3.

14. The foamable ethylene polymer composition of claim 1, wherein the melt strength is in the range of 4 cN to 15 cN and the density is in the range of 960 kg/m.sup.3 to 970 kg/m.sup.3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The FIGURE is a graph showing melt strength versus take up for the Examples.

DETAILED DESCRIPTION OF THE INVENTION

(2) Test Methods

(3) Melt Flow Rate

(4) MFR.sub.2 (190° C.) is measured according to ISO 1133 (190° C., 2.16 kg load).

(5) MFR.sub.5 (140° C.) is measured according to ISO 1133 (140° C., 5.0 kg load).

(6) Density

(7) Density is measured according to ISO 1183.

(8) Melting Temperature Tm and Crystallisation Temperature Tc

(9) Melting temperature Tm was measured with a TA Instruments 02000 differential scanning calorimetry device (DSC) according to ISO 11357/3 on 5 to 10 mg samples. Melting temperatures were obtained in a heat/cool/heat cycle with a scan rate of 10° C./min between 30° C. and 180° C. Melting and crystallisation temperatures were taken as the peaks of the endotherms and exotherms in the cooling cycle and the second heating cycle respectively.

(10) Experiments and Results

(11) Sample Preparation

(12) The compositions were compression-moulded at 200° C. in a frame to yield plates of 4 mm thickness, 80 mm width and 80 mm length. The pressure was adjusted to a level providing a smooth surface of the plates. Visual inspection of the plates showed no inclusions such as trapped air or any other visible contamination.

(13) Sample Characterization

(14) Dielectric Constant and Dielectric Loss Tangent

(15) For measurement of dielectric constant and dielectric loss tangent (tan δ) of the materials, a split-post dielectric resonator (SPDR) was used. SPDR was developed by Krupka and his collaborators [see: J Krupka, R G Geyer, J Baker-Jarvis and J Ceremuga, ‘Measurements of the complex permittivity of microwave circuit board substrates using a split dielectric resonator and reentrant cavity techniques’, Proceedings of the Conference on Dielectric Materials, Measurements and Applications—DMMA '96, Bath, UK, published by the IEE, London, 1996.]. A comprehensive review of the method is found in J Krupka, R N Clarke, O C Rochard and A P Gregory, ‘Split-Post Dielectric Resonator technique for precise measurements of laminar dielectric specimens—measurement uncertainties’, Proceedings of the XIII Int. Conference MIKON '2000, Wroclaw, Poland, pp 305-308, 2000. The technique measures complex permittivity of dielectric laminar specimen (plates) in the frequency range from 1-10 GHz. The test is conducted at 23° C.

(16) Two identical dielectric resonators were placed coaxially along z-axis such that a small laminar gap is formed between them into which the specimen can be placed. By choosing suitable dielectric materials, the resonant frequency and Q-factor of SPDR can be made to be temperature stable. Once a resonator is fully characterized, only three parameters need to be measured to determine the complex permittivity of the specimen: its thickness, changes in resonant frequency Δf, and changes in Q-factor ΔQ, wherein the changes occur when the specimen is placed in the resonator.

(17) Specimens of 4 mm thickness were prepared by compression-moulding as described above and measured at high frequency of 1.9 GHz.

(18) Attenuation

(19) For pair cables the dependence of attenuation a on dielectric loss factor tan δ is:

(20) $a=A\left(\frac{1}{d\log \left(\frac{2s}{d}\right)}\right)\sqrt{f}\sqrt{\mathrm{.Math.}}+\mathrm{Bf}\tan \delta \sqrt{\mathrm{.Math.}}$
wherein A and B are constants, 2 s is the distance between the wires in a pair, d is conductor diameter, f is frequency, and E is dielectric constant.

(21) A foamed insulation layer generally has a lower dielectric constant compared to a corresponding solid material. Density of a foam is dependent on density of the pure unfoamed starting material and degree of expansion. The dielectric constant ε.sub.Foam can be derived from the density of the foam ρ.sub.Foam according to:
ε.sub.Foam=a.Math.ρ.sub.Foam+b
wherein a and b are constants. From the equation follows that the higher degree of expansion, the lower the foam density, thus the lower the dielectric constant. Further information on the concept of attenuation can be found in Standard IEC 61156-7 which specifies a calculation method for the attenuation.
Permittivity

(22) Permittivity is measured by the same method as described for dielectric loss tangent.

(23) Melt Strength

(24) The strain hardening behaviour (melt strength) is determined by the method described in the article “Rheotens-Mastercurves and Drawability of Polymer Melts”, M. H. Wagner, Polymer Engineering and Science, Vol. 36, pages 925 to 935. The content of the document is included by reference. The strain hardening behaviour of polymers is analysed by Rheotens apparatus (product of Göttfert, Siemensstr.2, 74711 Buchen, Germany) in which a melt strand is elongated by drawing it down with a defined acceleration. The haul-off force F in dependence of draw-down velocity v is recorded. The test procedure is performed in a standard climate-controlled room with controlled room temperature of 23° C. and 50% RH. The Rheotens apparatus is combined with an extruder/melt pump for continuous feeding of the melt strand. The extrusion temperature is 200° C.; a capillary die with a diameter of 2 mm and a length of 6 mm is used. The strand length between the capillary die and the Rheotens wheels is 80 mm. At the beginning of the experiment, the take-up speed of the Rheotens wheels is adjusted to the velocity of the extruded polymer strand (tensile force zero). The experiment starts by slowly increasing the take-up speed of the Rheotens wheels until the polymer strand breaks. The acceleration of the wheels was small enough so that the tensile force was measured under quasi-steady conditions. The acceleration of the melt strand drawn down is 120 mm/sec.sup.2. The Rheotens was operated in combination with the PC program EXTENS, a real-time data-acquisition program, which displays and stores the measured data of tensile force and drawdown speed. The schematic diagram in the FIGURE shows in an exemplary fashion the measured increase in haul-off force F (i.e. “melt strength”) versus the increase in draw-down velocity v (i.e. “drawability”).

(25) Shear Rheology

(26) Dynamic rheological measurements were carried out with an Anton Paar MCR501 rheometer on compression-moulded samples under nitrogen atmosphere at 230° C. using 25 mm diameter plate and plate geometry. The oscillatory shear experiments were done within the linear viscoelastic range of strain at frequencies from 0.01 to 500 rad/s in line with ISO 6721-1. The values of storage modulus (G′), loss modulus (G″), complex modulus (G*) and complex viscosity (η*) were obtained as a function of frequency (w).

(27) Extruder Settings

(28) RPM: 200

(29) Melting temperature: 206° C.

(30) Extruder pressure: 13-21 bar

(31) Productivity: 10 kg/h

(32) Materials

(33) HE1123 is a unimodal Ziegler-Natta catalysed HDPE with MFR.sub.2 of 8 g/10 min and density of 963 kg/m.sup.3. It contains one antioxidant, one process stabiliser and one process aid. The grade is commercially available from Borealis AG.

(34) HE 3465 is a unimodal Ziegler-Natta catalysed HDPE grades with MFR.sub.2 of 12 g/10 min and density of 965 kg/m.sup.3. It contains one antioxidant, one process stabiliser and one process aid. The grade is commercially available from Borealis AG.

(35) LE1120 is an autoclave LDPE with MFR.sub.2 of 4.5 g/10 min and density of 923 kg/m.sup.3. The grade is commercially available from Borealis AG.

EXAMPLES

(36) Foamable ethylene polymer composition samples with different MFR and high melt strength have been produced on grafting line by adding the peroxide, 2,5-Bis(tert-butylperoxy)-2,5-dimethylhexane, during extrusion.

(37) As mentioned above, MFR.sub.2 of a composition may be changed by varying the amount of peroxide added to the composition. Peroxide treatment method is reliable and stable process for changing MFR.sub.2. Table 1 disclose how MFR.sub.2 can be controlled by adding different amounts of peroxide.

(38) TABLE-US-00001 TABLE 1 MFR change after addition of peroxide MFR.sub.2 after peroxide addition Material Peroxide addition (wt %) (g/10 min) HE1123 ref — — HE1123 0.14 4.5 HE1123 0.40 2.09 HE1123 0.69 1.0 HE3465 ref — — HE3465 0.32 4.6 HE3465 0.63 2.1 HE3465 1.3 0.97
Differential Scanning Calorimetry

(39) The results from DSC analysis of peroxide materials and reference materials are presented in Table 2. There is only a slight decrease in crystallinity for the peroxide treated materials compared to the reference materials. This is advantageous since a high crystallinity is desired by the invention. The effect on melting and crystallisation temperatures is small meaning that the material is keeping the original properties which is advantageous. A decrease in crystallinity would negatively affect dielectric loss factor and decrease the crystallisation temperature, which in turn would negatively affect the foaming process.

(40) TABLE-US-00002 TABLE 2 Melting temperature, crystallisation temperature and crystallinity obtained from DSC analysis Melting Crystallisation temperature temp. Crystallinity Material (° C.) (° C.) (%) CE1 (HE1123 ref) 132.6 120.6 78.8 IE1 (HE1123 MFR.sub.2 132.2 120.4 76.8 4.5 g/10 min) IE2 (HE1123 MFR.sub.2 132.4 120.9 78.3 2.1 g/10 min) IE3 (HE1123 MFR.sub.2 132.3 121.0 76.2 1.0 g/10 min) CE2 (HE3465 ref) 131.7 119.9 78.4 IE4 (HE3465 MFR.sub.2 131.3 120.1 75.6 4.6 g/10 min) IE5 (HE3465 MFR.sub.2 132.0 120.3 77.0 2.1 g/10 min) IE6 (HE3465 MFR.sub.2 132.1 120.7 76.2 1.0 g/10 min)
Shear Rheology

(41) For both materials the peroxide treatment has a large impact on the shear complex viscosity η* and it increases dramatically with increased peroxide dosing. The Newtonian plateau where complex viscosity is constant and not affected by increased angular frequencies decreases with peroxide addition, and for the materials treated with peroxide down to an MFR.sub.2 of 1 g/10 min the curve gets a LDPE like form with no plateau at all. This means that the materials become very shear thinning, and at high angular frequencies it can be seen that the difference in complex viscosity between reference material and the high melt strength ethylene polymer composition becomes very small. This is positive as it indicates that the peroxide treatment has a low impact on process viscosity, meaning processability is not highly affected. The peroxide treated materials behave more like LDPE when subjected to high angular frequencies, indicating long chain branched structures.

(42) TABLE-US-00003 TABLE 3 Complex viscosity η* (Pa .Math. s) for HE1123 based materials Angular frequency (rad/s) CE1 IE1 IE2 IE3 0.05 1215.5 3565 9001 20015 300 356 407.4 447.8 497.9

(43) TABLE-US-00004 TABLE 4 Complex viscosity η* (Pa .Math. s) for HE3465 based materials Angular frequency (rad/s) CE2 IE4 IE5 IE6 0.05 808.5 4378.5 10285 20375 300 279.5 370 417 466.5
Dissipation Factor

(44) Dissipation factor at 1.9 GHz was measured after drying the material for 72 hours at 90° C. HE1106 is a mixture of 70% of the HDPE base resin used in HE1123 and 30% of the LDPE in LE1120. The dissipation factor increases with peroxide addition, but rather slowly. All the HE1123 based materials have lower dissipation factors than HE1106 after drying.

(45) Table 5 contains examples showing that the foamable ethylene polymer composition according to the present invention provides a better balance of high melt strength and low dissipation factor than the two comparative examples.

(46) CE1 is a HDPE material which has a very low dissipation factor, but too low melt strength for foaming. Commercial ref is a commercial product for high frequency communication cables where 70% of HE1123 ref is blended with 30% autoclave LDPE in order to improve melt strength. Commercial ref has high melt strength, but at the same time much higher dissipation factor than for HE1123 ref. As mentioned above, low dissipation factor is extremely important for high frequency communication insulation. IE1 and IE3 are inventive examples of HDPE extruded with peroxide addition in order to yield long chain branched polymer structures. All the inventive examples have higher melt strength than CE1, while dissipation factors of the inventive examples are lower than for CE3. This proves that the invention can provide a foamable ethylene polymer composition having a better balance of high melt strength and low dissipation factor compared to the comparative examples.

(47) TABLE-US-00005 TABLE 5 Material properties Peroxide MFR.sub.2 Melt Dissipation addition (190° C., strength factor Material (ml/kg) g/10 min) (cN) (.Math.10.sup.−6) CE1 — 8 0.6 54.3 CE3 (Commercial ref, — 7.0 3.4 88.2 70% HE1123 + 30% LE1120) IE1 0.18 4.5 2.7 65 IE3 0.9 1 7.7 76.9

(48) Although the present invention has been described with reference to various embodiments, those skilled in the art will recognize that changes may be made without departing from the scope of the invention. It is intended that the detailed description be regarded as illustrative, and that the appended claims including all the equivalents are intended to define the scope of the invention.