Use of a mixture of organic peroxides for crosslinking a polyolefin elastomer

Abstract

The present invention concerns the use of a mixture of organic peroxides for crosslinking a polyolefin elastomer (POE), in particular intended to be used in photovoltaic applications. The invention also relates to a crosslinkable composition comprising at least one polyolefin elastomer (POE) and at least one mixture of organic peroxides. The present invention also concerns a method for preparing a material made from polyolefin elastomer (POE), preferably an encapsulating material or a sealing agent, in particular for photovoltaic cells, comprising a step of crosslinking a crosslinkable composition as defined previously.

Claims

1. A mixed organic peroxide composition for use in crosslinking a polyolefin elastomer, the composition comprising: at least one monoperoxycarbonate conforming to the formula (I): ##STR00004## wherein in formula (I) R.sub.1 represents an alkyl radical comprising a number of carbon atoms of less than or equal to 6, and R.sub.2 represents an alkyl radical, and at least one monoperoxycarbonate, conforming to the formula (II): ##STR00005## wherein in formula (II) R.sub.1 represents an alkyl radical comprising a number of carbon atoms of greater than or equal to 7, and R.sub.2 represents an alkyl radical, wherein the polyolefin elastomer is a copolymer of ethylene and at least one alpha-olefin.

2. The composition as claimed in claim 1, wherein the composition comprises strictly less than 0.4% by weight of a tert-alkyl hydroperoxide, calculated relative to 100 parts by weight of a mixture of monoperoxycarbonate of formula (I) and monoperoxycarbonate of formula (II).

3. The composition as claimed in claim 1, wherein the polyolefin elastomer is selected from the group consisting of ethylene/propylene copolymers, ethylene/1-butene copolymers, ethylene/1-hexene copolymers, ethylene/1-octene copolymers, ethylene/styrene copolymers, ethylene/propylene/1-octene copolymers, ethylene/propylene/1-butene copolymers, ethylene/1-butene/1-octene copolymers, and ethylene/1-butene/styrene copolymers.

4. The composition as claimed in claim 1, wherein the polyolefin elastomer is selected from the group consisting of linear and uniformly branched copolymers of ethylene and alpha-olefin.

5. The composition as claimed in claim 1, wherein, in the formula (I), R.sub.1 is a C.sub.2-C.sub.5 alkyl radical.

6. The composition as claimed in claim 1, wherein, in the formula (I), R.sub.2 is a C.sub.1-C.sub.10 alkyl radical.

7. The composition as claimed in claim 1, wherein the monoperoxycarbonate of formula (I) is selected from the group consisting of tert-amylperoxy isopropyl monocarbonate (TAIC), tert-butylperoxy isopropyl monocarbonate (TBIC), tert-octylperoxy isopropyl monocarbonate (TOIC), and tert-hexylperoxy isopropyl monocarbonate (THIC).

8. The composition as claimed in claim 1, wherein, in the formula (II), R.sub.1 is a C.sub.7-C.sub.10alkyl radical.

9. The composition as claimed in claim 1, wherein, in the formula (II), R.sub.2 is a C.sub.1-C.sub.10 alkyl radical.

10. The composition as claimed in claim 1, wherein the monoperoxycarbonate of formula (II) is selected from the group consisting of OO-tert-amyl O-(2-ethylhexyl) monoperoxycarbonate (TAEC), OO-tert-butyl O-(2-ethylhexyl) monoperoxycarbonate (TBEC), OO-tert-octyl O-(2-ethylhexyl) monoperoxycarbonate (TOEC), and OO-tert-hexyl O-(2-ethylhexyl) monoperoxycarbonate (THEC).

11. The composition as claimed in claim 1, wherein the mass ratio between the monoperoxycarbonate of formula (I) and the monoperoxycarbonate of formula (II) varies in the range from 1:99 to 70:30.

12. A crosslinkable composition comprising: at least one polyolefin elastomer, which is a copolymer of ethylene and at least one alpha-olefin, at least one monoperoxycarbonate of formula (I) as defined in claim 1, at least one monoperoxycarbonate of formula (II) as defined in claim 1; the composition comprising preferably strictly less than 0.4% by weight of a tert-alkyl hydroperoxide, calculated relative to 100 parts by weight of the mixture of monoperoxycarbonate of formula (I) and monoperoxycarbonate of formula (II).

13. The crosslinkable composition as claimed in claim 12, wherein the amount of monoperoxycarbonates of formulae (I) and (II) is less than or equal to 3 parts by weight per 100 parts by weight of the polyolefin elastomer.

14. The crosslinkable composition as claimed in claim 12, wherein it further comprises at least one coagent other than an organic peroxide.

15. The composition as claimed in claim 1, wherein the composition is devoid of a tert-alkyl hydroperoxide present in an amount of from 0.4 to less than 4% by weight, calculated relative to 100 parts by weight of a mixture of monoperoxycarbonate of formula (I) and monoperoxycarbonate of formula (II), particularly calculated relative to 100 parts by weight of the total monoperoxycarbonate content of the composition.

16. The composition as claimed in claim 1, wherein the composition is devoid of a tert-alkyl hydroperoxide.

17. A method for producing a material, comprising (a) at least one step of crosslinking a crosslinkable composition as defined in claim 12.

18. The method as claimed in claim 17, wherein the material is selected from the group consisting of an encapsulating material.

19. The method as claimed in claim 17, wherein the crosslinking step (a) is implemented at a temperature of from 130 to 250? C.

20. The method as claimed in claim 17, further comprising a step (a) prior to and/or simultaneous with step (a), wherein step (a) is selected from the group consisting of molding, extrusion, and injection-molding of the crosslinkable composition.

21. A material comprising at least one polyolefin elastomer, obtainable by the method as defined in claim 17.

22. A method for producing a photovoltaic module, comprising: (i) at least one step of laminating an assembly comprising in succession: a first, transparent layer forming the front face of a photovoltaic module, a layer obtained from the crosslinkable composition as defined in claim 12, a plurality of solar cells disposed side by side and connected electrically to one another, a layer obtained from the crosslinkable composition as defined in claim 12, a second layer or a multilayer assembly forming the rear face (or backing) of the module; and (ii) at least one step of pressing the layers laminated together during step (i).

Description

EXAMPLE

Example 1

(1) The following compositions were prepared by mixing: a polyolefin elastomer (KST340T from Japan Polyethylene Corporation, MFI 12-16 g/10 min with a load of 5 kg; melting point of 55-70? C., measured by DSC), tert-amylperoxy isopropyl monocarbonate (Luperox? TAIC sold by Arkema), the same polyolefin elastomer, but with OO-tert-butyl O-isopropyl monoperoxycarbonate (Luperox? TBIC sold by Arkema), the same polyolefin elastomer, but with tert-amylperoxy isopropyl monocarbonate (TAIC) and OO-tert-amyl O-(2-ethylhexyl) monoperoxycarbonate (TAEC) in a mass ratio of 60:40, the same polyolefin elastomer, but with tert-amylperoxy isopropyl monocarbonate (TAIC) and OO-tert-butyl O-(2-ethylhexyl) monoperoxycarbonate (TBEC) in a mass ratio of 60:40, the same polyolefin elastomer, but with OO-tert-amyl O-(2-ethylhexyl) monoperoxycarbonate (TAEC), the same polyolefin elastomer, but with OO-tert-butyl O-(2-ethylhexyl) monoperoxycarbonate (TBEC).

(2) The compositions were thus prepared in a Haake internal mixer at a temperature of 35? C. for a time of 12 minutes, using a rotary speed of 50 revs/min. The polymer mixture is subsequently passed through an open mill regulated to a temperature of 50? C., to produce sheets approximately 2 mm in thickness.

(3) Samples of approximately 2 to 3 grams of the compositions above are applied to a plate on a moving die rheometer (MDR) supplied by GOTECH, which is capable of measuring the curing properties of the samples and includes software for analyzing the results. Each of the samples is placed in a temperature-controlled cavity between two dies, the lower of which oscillates so as to apply a cyclical stress or deformation to the sample, while the upper die is connected to a torque sensor for measuring the torque response of the sample to the deformation.

(4) The stiffness is recorded continuously as a function of the time. The stiffness of the sample increases in line with the vulcanization which occurs.

(5) This instrument is capable of providing, among other data, calculated values of ML (minimum torque), MH (maximum torque, which, when it is attained, also defines the time required for the attainment of the cure state), tc10 (time before 10% of cure state), and tc90 (time before 90% of cure state), as defined by the international standards (ASTM D5289 and ISO 6502).

(6) The MDR is activated at a temperature of 115? C. and of 145? C. with an amplitude of oscillation (degree of deformation) of 0.5? applied to the sample for 30 minutes. The scorch time is defined as the time needed to reach 10% of the total curing, i.e., tc10.

(7) This test is conducted on the following samples, in which the amounts of monoperoxycarbonate are indicated in parts per hundred parts of POE (phr):

(8) TABLE-US-00001 MH MH-ML tc10 tc90 (dN,m) (dN,m) (m:s) (m:s) Compo- at at at at sition Content 115? C. 145? C. 145? C. 145? C. TAIC 0.5 phr 0.38 1.73 01:50 14:20 TBIC 0.5 phr 0.31 1.66 02:45 20:00 60% 0.5 phr 0.32 1.7 02:02 16:10 TAIC + 40% TAEC 60% 0.5 phr 0.29 1.68 02:15 17:50 TAIC + 40% TBEC TAEC 0.5 phr 0.3 1.35 02:20 17:30 TBEC 0.5 phr 0.29 1.1 03:00 22:00

(9) The use of TBIC leads to a POE crosslinking rate which is too slow, given that the crosslinking time (tc90) is 20 min, which is unsatisfactory industrially.

(10) The use of TAIC allows an increase in the POE crosslinking rate relative to TBIC and a slight improvement in the crosslinking density (MH-ML). However, the scorch time (tc10) causes problems and may give rise to risks from an industrial standpoint, especially by creating roughenings on the surface of the encapsulating material.

(11) The use of TBEC leads to polymer crosslinking times (tc90) which are too great. Moreover, the use of TAEC and TBEC leads to an insufficient crosslinking density (MH-ML).

(12) The use of a mixture of TAIC and TAEC makes it possible both to accelerate the crosslinking of the POE, relative to the formulation containing only TBIC or only TBEC (tc90), and to retain a good crosslinking density, better than with TAEC or TBEC, while providing protection from the risks of premature crosslinking by virtue of an extension to the scorch time (tc10) relative to the formulation containing only TAIC.

(13) Similarly, the use of a mixture of TAIC and TBEC makes it possible both to accelerate the crosslinking of the POE relative to the formulation containing only TBIC or only TBEC (tc90), and to retain a good crosslinking density, while providing protection from the risks of premature crosslinking, by virtue of an extension to the scorch time (tc10) relative to the formulation containing only TAIC.