REACTIVE COMPOUNDING OF ETHYLENE VINYL ACETATE

Abstract

The present invention provides a method for producing an at least partially crosslinked polymer composition, having a first melt index (MI) value and a first tensile strength, comprising the steps of: a. providing an ethylene vinyl acetate (EVA) copolymer, the EVA copolymer having a second MI value and a second tensile strength and containing at least 30 wt % units derived from vinyl acetate, b. adding from 0.01 to 0.03 wt % of an organic peroxide, wherein the organic peroxide is diluted in 0.001 to 0.05 wt % of white oil, and c. blending the EVA copolymer and the organic peroxide at a temperature sufficient to initiate crosslinking. The first MI value of the at least partially crosslinked polymer composition is less than 5 g/10 min (190° C., 2.16 kg).

Claims

1. A method for producing an at least partially crosslinked polymer composition, having a first melt index (MI) value and a first tensile strength, comprising the steps of: a. providing an ethylene vinyl acetate (EVA) copolymer, said EVA copolymer having a second MI value and a second tensile strength and containing at least 30 wt % units derived from vinyl acetate, b. adding from 0.01 to 0.03 wt % of an organic peroxide, wherein the organic peroxide is diluted in 0.001 to 0.05 wt % white oil, c. blending the EVA copolymer and the organic peroxide at a temperature sufficient to initiate crosslinking, wherein: the first MI value of the at least partially crosslinked polymer composition is less than 5 g/10 min (190° C., 2.16 kg).

2. The method according to claim 1, wherein the peroxide is blended with the copolymer in a continuous mixing process.

3. The method according to claim 2, wherein the continuous mixing process comprises an extruder.

4. The method according to claim 3, wherein the extruder is a twin-screw extruder.

5. The method according to claim 3, wherein the extruder is a single-screw extruder.

6. The method according to claim 1, wherein the peroxide is blended with the copolymer in a batch mixing process.

7. The method according to claim 6, wherein the batch mixing process comprises an internal mixer.

8. The method according to claim 3, wherein the residence time in the extruder is from 30 seconds to 5 min, preferably from 30 seconds to 2 min.

9. The method according to claim 3, wherein the extruder is maintained at a temperature sufficient to initiate peroxide crosslinking.

10. The method according to claim 3, the diluted peroxide is injected into an extruder or mixer.

11. The method according to claim 1, wherein the mixture of peroxide and white oil comprises from 5 to 50 wt % peroxide, preferably from 5 to 20 wt % peroxide.

12. The method according to claim 1, wherein the first melt index of the polymer composition is from 0.05 to 2.0 g/10 min, preferably from 0.05 to 1.0 g/10 min.

13. At least partially cross-linked polymer composition comprising ethylene vinyl acetate (EVA) containing at least 30 wt % units derived from vinyl acetate, 0.001 to 0.05 wt % of white oil and having MI of below 5 g/10 min.

14. A polymer composition according to claim 13, further comprising carbon black.

15. A wire or cable comprising the polymer composition according to claim 13.

16. A wire or cable according to claim 15, wherein said polymer composition constitutes a strippable semiconductive layer, and wherein strip force of said strippable semiconductive layer is below 8 kN/m.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0042] Materials

[0043] Luperox D-16, commercially available from Arkema, is t-butyl cumyl peroxide, having structural formula:

##STR00001##

[0044] Levapren 400, commercially available from Lanxess, is an ethylene-vinyl acetate copolymer, having 40 wt % vinyl acetate, and MFR2=3 g/10 min.

[0045] Evatane 40-55, commercially available from Arkema, is an ethylene-vinyl acetate copolymer, having 38-41 wt % vinyl acetate, and melt index (190° C./2.16 kg) of 48-62 g/10 min (ISO 1133/ASTM D1238).

[0046] Mineral oil, commercially available from Eki-Chem, is a light mineral oil, CAS 8042-47-5.

[0047] Methods

[0048] The MFR2 was measured with 2.16 kg load at 190° C. according to ISO 1133.

[0049] Strippability is defined in cable standards, e.g ANSI/ICEA S-93-639. The insulation shield is notched with two cuts ⅕ inch apart. A tensile tester is used to pull the semiconductive layer from the insulation layer and measure the strip force in lbs/½ inch.

[0050] Strip Force 90°

[0051] Cable samples of 10 cm up to 13.5 cm of length and 10 mm width were cut in cross sectional direction from a test cable which had an inner semiconductive layer with a thickness of 0.8+0.05 mm, an insulation layer with a thickness of 5.5+0.1 mm, and an outer semiconductive layer with a thickness of 1+0.1 mm. The test cables were prepared according to the method as described below under “Production of test cables”. The strip force test can be made for test cable wherein said sample is in non-crosslinked or crosslinked form. The samples were conditioned for 16 hours to 2 weeks at 23° C. and 50% relative humidity. The separation of the outer semiconductive layer from the insulation was initiated manually. The cable was fixed to Alwetron TCT 25 tensile testing instrument (commercially available from Alwetron). The manually separated part was clamped onto a wheel assembly which is fixed to a moveable jaw of said instrument. The movement of the tensile testing machine causes the separation of said semiconductive layer from said insulation layer to occur. The peeling was carried out using a peeling angle of 90° and peeling speed of 500 mm/min. The force required to peel said outer semiconductive layer from the insulation was recorded and the test was repeated at least six times for each test layer sample. The average force divided by the width (10 mm) of the sample was taken as said strip force and the given values (kN/m at 90°) represent the average strip force of the test samples, obtained from at least six samples.

[0052] Production of Test Cables

[0053] The test cables were prepared using a so-called “1 plus 2 extruder set-up”, in a Maillefer extruder, supplied by Maillefer. Thus, the inner semiconductive layer was extruded on the conductor first in a separate extruder head, and then the insulation and outer semiconductive layer are jointly extruded together on the inner semiconductive in a double extruder head. The inner and outer semiconductive extruder screw had a diameter of 45 mm/24 D and the insulation screw had a diameter of 60 mm/24 D.

EXAMPLES

Example 1

[0054] The polymer compositions in Table 1 are prepared in bench scale using Banbury mixer. The polymer base resin is added to the mixer, followed by the rubber components. Then, carbon black is added. The components are mixed at 146° C.

[0055] Comparative example 1 (CE1) presented in Table 1 below is a semiconductive composition comprising commercially available Levapren, which is a 40% EVA with MFR2=3 g/10 min. When a medium voltage cable is extruded using the composition of the comparative example as a semiconductive layer and then tested for strip forces, a value around 2.1 kN/m is obtained.

[0056] If a semiconductive cable composition is made of only EVA having high MFR, such as Evatane 40-55 with 40% EVA and MFR2=55 g/10 min, then the corresponding semiconductive layer will be bounded. Therefore, an EVA resin having low MFR is needed to achieve acceptable strip forces.

[0057] As may be seen in Table 1, inventive examples 1 and 2 (1E1 and 1E2) describe a semiconductive composition comprising EVA 40% with MFR2=2.3 g/10 min and MFR2=2.2 g/10 min, respectively. These resins are obtained by subjecting commercially available EVA having MFR2=55 g/10 min,

[0058] i.e. Evatane 40-55, to reactive compounding using different levels of peroxide dispersed in mineral oil. Strip forces evaluated on medium voltage cables using above compositions show values of 2.6 and 3.0 kN/m respectively.

[0059] This indicates that both the inventive compositions meet the strippability demands (<3.3 kN/m).

TABLE-US-00001 TABLE 1 Description CE1 IE1 IE2 Levapren 400 40% EVA MFR2 = 24.15 — — 3 g/10 min EVA w 0.02% 40% EVA MFR2 = — 24.15 — peroxide/mineral oil 2.3 g/10 min EVA w 0.01% 40% EVA MFR2 = — — 24.15 peroxide/mineral oil 2.2 g/10 min Evatane 40-55 40% EVA, MFR2 = 20.29 20.29 20.29 55 g/10 min Perbunan 3430F NBR rubber 13.77 13.77 13.77 Antilux 654 Lubricant 3.04 3.04 3.04 Columbian CD7060U CB 36.34 36.34 36.34 Vulkanox HS/LG AO 1.86 1.86 1.86 Struktol Zn stearate Lubricant 0.5 0.5 0.5 pellets Struktol Zn stearate Lubricant 0.1 0.1 0.1 powder Strip forces (kN/m) 2.1 2.6 3.0

Example 2

[0060] Preparation of the Tapes

[0061] The tapes are prepared in a single screw extruder applying temperature profile of 110, 110, 115° C. The tapes are extruded through a slot cast die and have a thickness of 0.25 mm.

[0062] Comparative Sample 1 (CS1)

[0063] The tape of comparative sample 1 was extruded using commercially available Levapren 400.

[0064] Comparative Sample 2 (CS2)

[0065] 1 kg of Evatane 40-55 was loaded into a Banbury mixer, followed by addition of 0.0002 kg of t-butyl cumyl peroxide (D-16). The mixture was allowed to react at 150° C. for 3 minutes at a rotor speed of 100 rpm. The reaction mixture was dropped at a temperature of 150° C. The piece was chopped into flakes, which were extruded to tape according to the procedure above.

[0066] Inventive Sample 1 (IS1)

[0067] 0.0002 kg of t-butyl cumyl peroxide (D16) was dissolved in 0.5 kg of mineral oil. 25 kg of Evatane 40-55 was loaded into a Henschel mixer followed by 0.25 kg of the mineral oil solution of peroxide. The mixture was allowed to react at at least 150° C. for 3 minutes.

[0068] Analysis and Results

[0069] Three different tape samples obtained above were analyzed by visual inspection of a human being. The tapes were assessed using the grades from A to C, wherein A is the best grade showing a visually and tactilely smooth tape with less than 1 defect per 64.5 cm2, while grade C is the poorest having more than 50 defects per 64.5 cm2. The results are summarized in Table 2.

TABLE-US-00002 TABLE 2 Sample Components Grade CS1 Levapren 400 C CS2 Evatane 40-55 + D16 C IS1 Evatane 40-55 + D16 + mineral oil A

[0070] As may be seen from Table 2, mineral oil is needed in order to have a good processing. When mineral oil is not added, tapes of poor grades were obtained.

[0071] Viscosity of the compositions CS1, CS2 and IS1 were measured using CEAST piston rheometer. Pellets were fed into the throat, the plunger was inserted, and the measurement was started when the preset level of the plunger was reached. The results are summarized in Table 3, and also represented in FIG. 1, wherein the viscosity of the samples is plotted as a function of shear rate.

TABLE-US-00003 TABLE 3 Shear rate Viscosity (Pa .Math. s) (1/s) CS1 CS2 IS1 6.9 8155.7953 2085.1449 1769.9275 27.9 3627.6882 1172.491 1110.2151 97.7 1683.3417 736.694 687.564 195.5 1082.289 537.7877 513.2353 893.6 391.0586 241.6629 232.3887 1500 268.7333 176.8333 169.85 2000 213.3312 147.8937 142.2187

[0072] As may be seen from FIG. 1, the viscosity, which is inversely proportional to melt flow rate, of the compositions of CS2 and IS2 are almost identical.

[0073] Tensile strength of the samples was measured using Instron tensile tester. It was found that none of the samples broke. When elongation was well over 1000%, the stop position was reached.

[0074] 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.