POLYOLEFIN COMPOSITIONS WITH REDUCED SHRINKAGE

Abstract

The present invention concerns the use of carbon black for reducing the shrinkage of a polyolefin composition to below 0.9%, wherein the polyolefin composition comprises a polyolefin polymer, and carbon black in an amount of from 0.05 to 2 wt. % based on the total weight of the polyolefin composition.

Claims

1. A method for reducing the shrinkage of a polyolefin composition to below 0.9%, comprising adding carbon black to the polyolefin composition, so that the polyolefin composition comprises a) a polyolefin polymer, b) carbon black in an amount of from 0.05 to 2 wt. % based on the total weight of the polyolefin composition.

2. The method according to claim 1, wherein the polyolefin polymer has an MFR2 of from 0.1 to 10 g/10 min measured according to ISO 1133.

3. The method according to claim 1, wherein the polyolefin polymer is present in an amount of from 95 to 99.9 wt. %, based on the total weight of the polyolefin composition.

4. The method according to claim 1, wherein the carbon black has a BET surface of 20 to 550 m.sup.2/g measured according to ASTM 6556.

5. The method according to claim 1, wherein the amount of carbon black is from 0.07 to 1.9 wt. %, based on the total weight of the polyolefin composition.

6. The method according to claim 1, wherein the polyolefin composition has an MFR2 of from 0.1 to 10 g/10 min, measured according to ISO 1133.

7. The method according to claim 1, wherein the polyolefin polymer is a multimodal polyolefin polymer.

8. The method according to claim 1, wherein the polyolefin polymer has a density of from 920 to 970 kg/m.sup.3, measured according to ISO 1183.

9. The method according to claim 1, wherein the polyolefin polymer is an ethylene homo- or copolymer.

10. The method according to claim 1, wherein the polyolefin composition further comprises an UV stabiliser and/or an antioxidant.

11. The method according to claim 1, wherein the shrinkage of the polyolefin composition is reduced to below 0.8%.

12. The method according to claim 1, wherein the polyolefin composition is used to form one or more layers of a cable.

13. The method according to claim 12, wherein the cable is a power cable or a communication cable.

14. The method according to claim 13, wherein the communication cable is a fiber optic cable or a coaxial cable.

15. The method according to any claim 12, wherein the layer is the outermost layer of the cable.

Description

EXAMPLE SECTION

1. Materials

[0039] Polymer A is a bimodal high density ethylene 1-butene copolymer having a density of 944 kg/m.sup.3 and a MFR2 of 1.7 g/10 min, commercially available from Borealis.

[0040] The CB-MB (carbon black master batch) is a composition of 60.39 wt. % HDPE, 0.11 wt. % pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) and 39.5 wt. % carbon black (CB).

[0041] The carbon black (CB) is “Printex alpha A” carbon black having a BET (NSA) surface area of 105 m.sup.2/g and an average particle size of 20 nm, commercially available from Orion Engineered Carbons GmbH.

2. Test Methods

a) Melt Flow Rate (MFR)

[0042] The melt flow rate (MFR) is determined according to ISO1133—Determination of the melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of thermoplastics—Part 1: Standard method, and is indicated in g/10 min. The MFR is an indication of flowability, and hence processability, of the polymer. The higher the melt flow rate, the lower the viscosity of the polymer.

[0043] The MFR2 of polyethylene is determined at a temperature of 190° C. and a load of 2.16 kg.

b) Density

[0044] The density of the polymer was measured according to ISO1183-1/method A.

c) Carbon Black Content (Ash)

[0045] The sample is weighed in a porcelain crucible that is placed in an oven with nitrogen flow. Nitrogen flow shall be 100±15 l/min. Oven is heated to 550° C. where polyethylene is burnt. The temperature is kept at 550° C. for 15 minutes. After decreasing the temperature to 350° C. the sample is taken out and allowed to cool down before being weighed again. The remains consist of carbon black and ash. Ash content was considered so small that it is insignificant to the result.

[0046] The calculation is

Percentage (carbon black+ash)=((w.sub.crucible+sample after oven−w.sub.empty crucible)/(w.sub.crucible+original sample−w.sub.empty crucible))*100

d) Creep and Recovery (Compliance Vs. Time)

[0047] To determine the creep and recovery behaviour, measurements were conducted using a Paar Physica MCR 501 rotational rheometer. A parallel-plate geometry with a diameter of 25 mm and a gap of 1.8 mm was chosen as measuring system. The tests were conducted at a set temperature of 190° C. The test starts with a period of imposed constant stress—creep phase. After a certain time the imposed shear stress is removed and the recovery phase starts.

[0048] During the creep phase, a constant shear stress of 50 Pa is applied to the sample and the accumulated shear strain (deformation) is measured as a function of time during 100 seconds, each measurement point having a duration of 1 second. In the recovery phase, the shear stress applied to the sample is set to zero pascals (0 Pa) and the recovery shear strain is measured as a function of time during 500 seconds, each measurement point having a duration of 1 second. The recovery shear strain is given by the absolute difference between the actual total strain and the total strain at the end of the creep phase. From the respective shear strains, the compliance (deformation(γ)/constant stress (τ) is determined as a function of time. Using the software Rheoplus from Anton Paar, several parameters are extracted from the respective compliance data, namely:

η.sub.0=Zero-shear viscosity
J.sub.e.sup.0=Steady state compliance
Je=Elastic compliance
Jv=Viscous compliance
Jmax=Max. creep compliance
Je/Jmax=Elastic share of max. creep compliance Jmax
Jv/Jmax=Viscous share of max. creep compliance Jmax
e) Stress Relaxation (Relaxation Vs. Time)

[0049] To determine the relaxation behaviour, stress relaxation tests were conducted using a Paar Physica MCR 501 rotational rheometer. A parallel-plate geometry with a diameter of 25 mm and a gap of 1.8 mm was chosen as measuring system. The tests were conducted at a set temperature of 190° C. using a strain step of 40%. The test specimens can be prepared in a disk shape by compression moulding with a thickness of about 2 mm, directly on a frame mould or by stamping out from a plaque using a cutting die, with the required diameter. The specimen was loaded between the plates of the pre-heated rheometer and the heating chamber was closed to allow for the sample to melt. Before the application of the strain step, and after loading the sample onto the plates, a waiting time for thermal equilibration inside the heating chamber of about 5 to 10 minutes was applied. The heating chamber was continuously purged with nitrogen during the tests to avoid degradation of the sample. After the step strain is applied, the test geometry was kept on a fixed angular position and the decaying (relaxation) stress (in Pascal, Pa) was determined as a function of time. The relaxation modulus (in Pascal, Pa) as a function of time is then determined by dividing the stress by the applied strain (in dimensionless units). The relaxation behaviour is characterized by the parameter Time (G(t)=100 Pa), which is the time (in seconds) at which the relaxation modulus attains an arbitrary value of 100 Pascal (Pa). Lower values is an indication of lower shrinkage. Materials with high elastic share have taken longer time to relax and consequently most of its elastic stresses would be frozen into the resultant jacket after the drawing operation (Shrinkage).

f) Cable Extrusion

[0050] The cable extrusion is done on a Nokia-Maillefer cable line. The extruder has five temperature zones with temperatures of 170/175/180/190/190° C. and the extruder head has three zones with temperatures of 210/210/210° C. The extruder screw is a barrier screw of the design Elise. The die is a semi-tube on type with 5.9 mm diameter and the outer diameter of the cable is 5 mm. The compound is extruded on a 3 mm in diameter, solid aluminum conductor to investigate the extrusion properties. Line speed is 75 m/min. The pressure at the screen and the current consumption of the extruder is recorded for each material.

g) Shrinkage

[0051] The shrinkage of the composition is determined with the cable samples obtained from the cable extrusion. The cables are conditioned in the constant room for one week before the cutting of the samples. The conditions in the constant room are 23±2° C. and 50±5% humidity. Samples are cut to 500 mm at least 2 m away from the cable ends after which they are placed in an oven on a talcum bed at 100° C. for 24 hours. After removal of the sample from the oven they are allowed to cool down to room temperature and then measured. The shrinkage is calculated according to formula below:

[(L.sub.Before−L.sub.After)/L.sub.Before]×100%

wherein L is length.

3. Results

[0052] Polymer A is compounded with pure carbon black (CB) or with the carbon black masterbatch (CB-MB) and extruded. Details about the comparative examples (CE) and inventive examples (IE) are given in Table 1 below.

TABLE-US-00001 TABLE 1 CE1 CE2 IE1 IE2 IE3 Polymer A, wt.% 93.4 100 99.5 99 97.5 CB-MB, wt.% 6.6 2.5 CB, wt. % 0.5 1.0 CB content (ash), wt.% 2.19 0.01 0.19 1.02 1.05 MFR2, g/10min 1.68 1.52 1.64 1.62 1.66 Shrinkage, % 0.74 0.43 0.57 0.66 0.68 Creep (Je/Jmax) 6.95 6.45 7.21 8.44 6.45 Stress relaxation 7.6 6.85 7.8 8.2 7.3 Extruder amps 50 55 52 50 50 Screw speed 57.8 60.5 58.1 58.2 58.2

[0053] As can be seen from Table 1, small amounts of carbon black reduce the shrinkage while maintaining the MFR of the polyolefin composition.