PROCESS FOR CROSSLINKING POLYPROPYLENE

Abstract

Process for crosslinking polypropylene comprising the steps of (a) treating a mixture of (i) polypropylene, (ii) a maleimide-functionalized mono-azide and/or a citraconimide-functionalized mono-azide, and (iii) a radical scavenger selected from the group consisting of hydroquinone, hydroquinone derivatives, benzoquinone, benzoquinone derivatives, catechol, catechol derivatives, 2,2,6,6-tetramethylpiperidinooxy (TEMPO), TEMPO derivatives, and combinations thereof, at a temperature in the range 120-250 C. to form a functionalized polypropylene, and (b) reacting the functionalized polypropylene with a peroxide at a temperature in the range 150-350 C.

Claims

1. Process for crosslinking polypropylene comprising the steps of a. treating a mixture of (i) polypropylene, (ii) a maleimide-functionalized mono-azide and/or a citraconimide-functionalized mono-azide, and (iii) a radical scavenger selected from the group consisting of hydroquinone, hydroquinone derivatives, benzoquinone, benzoquinone derivatives, catechol, catechol derivatives, 2,2,6,6-tetramethylpiperidinooxy (TEMPO), TEMPO derivatives, and combinations thereof, at a temperature in the range 120-250 C. to form a functionalized polypropylene, and b. reacting the functionalized polypropylene with a peroxide at a temperature in the range 150-350 C.

2. Process according to claim 1 wherein the radical scavenger is selected from the group consisting of quinone, t-butyl hydroquinone (TBHQ), 2,5-ditertiary-butylhydroquinone (DTBHQ), 2-methylhydroquinone (toluhydroquinone, THQ), p-benzoquinone, catechol, 4-methoxyphenol 4-tert-butylcatechol, 2,2,6,6-tetramethylpiperidinooxy (TEMPO), and 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (OH-TEMPO) 4-methoxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-methoxy-TEMPO), 4-carboxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-carboxy-TEMPO), 4-oxo-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPONE), and combinations thereof.

3. Process according to claim 2 wherein the radical scavenger is selected from the group consisting of t-butyl hydroquinone (TBHQ), 2,2,6,6-tetramethylpiperidinooxy (TEMPO), 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (OH-TEMPO), and combinations thereof.

4. Process according to claim 1 wherein the peroxide is selected from the group consisting of t-butyl cumyl peroxide, 3,6,9-triethyl-3,6,9,-trimethyl-1,4,7-triperoxonane, dicumyl peroxide, di(t-butylperoxyisopropyl) benzene, and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.

5. Process according to claim 1 wherein a maleimide-functionalized mono-azide is used, said maleimide-functionalized azide having the following structure: ##STR00008## wherein Y is ##STR00009## m is 0 or 1, n is 0 or 1, n+m=1 or 2, R is selected from the group consisting of hydrogen, linear and branched alkyl groups with 1-6 carbon atoms optionally substituted with O, S, P, Si, or N-containing functional groups, alkoxy groups with 1-6 carbon atoms, and halogens, and X is a linear or branched, aliphatic or aromatic hydrocarbon moiety with 1-12 carbon atoms, optionally containing heteroatoms.

6. Process according to claim 1 wherein a citraconimide-functionalized azide is used, said citraconimide-functionalized azide having the following structure: ##STR00010## wherein Y is either ##STR00011## m is 0 or 1, n is 0 or 1, n+m=1 or 2, R is selected from the group consisting of hydrogen, linear and branched alkyl groups with 1-6 carbon atoms optionally substituted with O, S, P, Si, or N-containing functional groups, alkoxy groups with 1-6 carbon atoms, and halogens, and X is a linear or branched, aliphatic or aromatic hydrocarbon moiety with 1-12 carbon atoms, optionally containing heteroatoms.

7. Process for recycling crosslinked polypropylene, comprising the step of treating a crosslinked polypropylene obtainable by the process of claim 1 with a peroxide at a temperature in the range 150-350 C.

8. Process according to claim 7 wherein the peroxide is selected from the group consisting of t-butyl cumyl peroxide, 3,6,9-triethyl-3,6,9,-trimethyl-1,4,7-triperoxonane, dicumyl peroxide, di(t-butylperoxyisopropyl) benzene, and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.

9. A power cable comprising the crosslinked polypropylene of claim 1 extruded onto a conductor.

10. A method of making an electrical cable comprising the step of shaping the crosslinked polypropylene of claim 1 with an extruder onto a conductor.

11. A method of making a tube comprising the step of shaping the crosslinked polypropylene of claim 1 with an extruder.

Description

EXAMPLES

Example 1

[0055] Polypropylene (50 g; Ineos 100GA12) was mixed with a maleimide sulfonyl azide (4-(2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)benzenesulfonyl azide), optionally TBHQ, and optionally Irganox 1010 (tetrakis-(methylene-(3,5-di-(tert)-butyl-4-hydrocinnamate))methane) in a pre-heated 50 ml Banbury internal mixer. Mixing was conducted at a temperature ranging from 180 to 190 C., a rotor speed of 50 rpm, for 10-12 minutes.

[0056] The torque in the Banbury mixer was recorded as a function of time. Premature crosslinking leads to an increase in torque, which means that the torque increase should as low as possible.

[0057] Table 1 reports the torque increase, relative to the torque of the starting polymer. It shows that, in the absence of TBHQ (Experiment 1A), significant premature crosslinking occurred. This premature crosslinking was reduced by the additional presence of TBHQ (experiments 1C-1F). The (additional) presence of Irganox 1010 did not reduce premature crosslinking.

TABLE-US-00001 TABLE 1 1A 1B 1C 1D 1E 1F PP Ineos 100GA12 phr 100 100 100 100 100 100 Irganox 1010 phr 0 1 0 0.5 1 2 Maleimide phr 2 2 2 2 2 2 sulfonylazide TBHQ phr 0 0 0.2 0.2 0.2 0.2 Mixer torque Nm 10.6 10.5 4.6 5.5 4.9 4.9 increase

Example 2

[0058] The functionalized polypropylenes of Example 1 (experiments 1A-1F) were cooled to room temperature by ambient exposure and ground to 3 mm sized pieces on a granulator (Colortronic M102L), using a 3 mm screen.

[0059] T-butyl cumyl peroxide (Trigonox T) was subsequently added to the granulated polypropylene and mixed for 4 hours in a tumbler mixer to allow proper homogenization.

[0060] All samples were cured for 30 minutes at 180 C. in a rheometer (MDR2000 ex Alpha Technologies).

[0061] The results are listed in Table 2, which indicates:

[0062] T5: time to 5% of maximal torque

[0063] T50: time to 50% of maximal torque

[0064] t90: time to 90% of maximal torque,

[0065] ML: minimum torque level,

[0066] MH: maximum torque level,

[0067] delta S=MH-ML.

[0068] ML is an indicator for the processability of the compound.

[0069] The functionalized polypropylenes of experiments 1A and 1B showed a significant torque increase in the mixer (see Example 1) and the highest ML values. Functionalization in the presence of TBHQ (experiment 2C) led to a significant reduction in the ML value, indicating an improved processability by reduced premature crosslinking.

[0070] The t90 data indicate that crosslinking was delayed by the presence of TBHQ (experiments 2C-2F).

[0071] In the absence of this radical scavenger (2A) or in the presence of Irganox 1010 (2B) not all maleimide groups are available for crosslinking the polypropylene and the effective result is degradation of the polypropylene initiated by the peroxide.

TABLE-US-00002 TABLE 2 2A 2B 2C 2D 2E 2F PP ex Example 1 1A 1B 1C 1D 1E IF Trigonox T phr 0.7 0.7 0.7 0.7 0.7 0.7 Rheometer cure [ C.] 180 180 180 180 180 180 temperature t5 [Min] 0.8 0.7 0.9 1.0 0.9 0.9 t50 [Min] 1.0 1.0 1.7 1.7 2.0 1.7 t90 [Min] 1.3 1.4 3.8 3.6 4.6 3.4 ML [dNm] 0.5 0.5 0.3 0.2 0.2 0.2 MH [dNm] 0.8 1.0 0.9 1.0 1.0 1.1 Delta S [dNm] 0.3 0.5 0.7 0.8 0.8 1.0

Example 3

[0072] Polypropylene was functionalized and crosslinked in accordance with experiments 2C-2F by compression moulding into sheets of 1 mm thickness at a temperature of 180 C., for the time periods indicated in Table 3. The crosslinked polypropylene was subjected to refluxing xylene (140 C.) for 16 hours. The gel content of the crosslinked polypropylene is defined as the sample weight after extraction, relative to the sample weight prior to extraction.

[0073] Table 3 shows that polypropylene modified with 2 phr of the maleimidobenzene sulfonylazide in the presence of TBHQ can be crosslinked to a gel fraction above 80% using 0.7 phr of Trigonox T.

[0074] Polypropylene functionalized according to experiment 1F, absent of peroxide treatment (exp. 3ref), dissolved completely in refluxing xylene. This indicates that functionalization with the maleimide sulfonylazide was not sufficient for obtaining crosslinks. A subsequent peroxide treatment was required for crosslinking to occur.

[0075] The hot set value is an indicator for the creep resistance at high temperature under a fixed load. This value was determined by subjecting test species (1 mm thick dumbbell shaped sheets) to a temperature of 200 C. and a load of 20 N/mm.sup.2 for 15 minutes and recording the change in elongation. A low hot set value means a proper resistance to this load, indicating a high temperature resistance. Uncrosslinked polypropylene (exp. 3ref) failed this test as it was unable to withstand this temperature and load.

TABLE-US-00003 TABLE 3 3C 3D 3E 3F 3ref PP ex Example 1 1C 1D 1E IF IF Trigonox T phr 0.7 0.7 0.7 0.7 0 Curetime in mould [min] 10 10 10 8 Gel [%] 82 82 83 83 0 Hotset (200 C.) [%] 72 73 59 84 FAIL

Example 4

[0076] Experiment 1F was repeated, using another type of polypropylene and the radical scavengers and amounts listed in Table 4.

[0077] The torque increase was measured and the results are displayed in Table 4.

TABLE-US-00004 TABLE 4 modification of polypropylene 4A 4B 4C PP Polychim MF7 phr 100 100 100 Irganox 1010 phr 2 2 2 Maleimide sulfonylazide phr 1 1 1 TBHQ phr 0.1 OH-TEMPO phr 0.08 Mixer torque increase Nm 7.5 0 0.7

[0078] Irganox 1010 had a positive effect on the color of the sample: it reduced the discoloration affected by the azide.

[0079] Example 4B shows that premature crosslinking can be completely halted by the addition of TBHQ. OH-TEMPO also showed a strong effect on premature crosslinking.

Example 5

[0080] The functionalized polypropylenes of Example 4 (experiments 4A-4C) were cooled to room temperature by ambient exposure and ground to 3 mm sized pieces on a granulator (Colortronic M102L) using a 3 mm screen.

[0081] 3,6,9-Triethyl-3,6,9,-trimethyl-1,4,7-triperoxonane (Trigonox 301) was subsequently added to the granulated polypropylene and mixed for 4 hours in a tumbler mixer to allow proper homogenization.

[0082] The samples were cured as described in Example 2.

[0083] The gel content was determined according to the method described in Example 3 and the results are displayed in Table 5. The use of OH-TEMPO gave a slightly improved gel content when compared to the use of TBHQ.

TABLE-US-00005 TABLE 5 PP ex Example 4 4A 4B 4C Trigonox 301 phr 0 1 1 Gel [%] 0 76 83

Example 6

[0084] Polypropylene was functionalized and crosslinked in accordance with Example 2C by compression moulding into sheets of 1 mm thickness at a temperature of 180 C., for 10 minutes. The crosslinked material was cut into strips and heated in an internal mixer for 3 minutes at 160 C. in the presence of 2 phr Trigonox T and optionally 0.1 wt % OH-TEMPO. The gel content (determined according Example 3) of 82 for the crosslinked polypropylene was reduced to 49 (treatment with Trigonox T only) and 35 (treatment with Trigonox T and OH-TEMPO), indicating de-crosslinking.