CURABLE COMPOSITION COMPRISING AN ETHYLENE POLYMER, A
MONOPEROXYCARBONATE AND A t-ALKYL HYDROPEROXIDE
The present invention pertains to a curable composition comprising an ethylene
polymer, such as an ethylene / vinyl acetate copolymer. It is also directed to a
method for preventing scorching of a curable composition comprising such an
ethylene polymer.
BACKGROUND OF THE INVENTION
Ethylene / vinyl acetate (EVA) is a copolymer widely used, inter alia, in solar
panels, as an encapsulating material to protect solar cells (and especially the
semiconductors contained therein) from outdoor environmental elements, especially
moisture and UV radiation, and to provide electrical insulation. EVA indeed provides
a good transparency and adhesion to the substrates of the photovoltaic (PV) module,
together with a high resistivity and good moisture-barrier effect. These properties
may alternatively be used in the manufacture of laminated glass.
It is common practice to crosslink these EVA copolymers in order to improve their
thermal stability, especially their creep strength, their adhesion to the substrates and
their weathering degradation resistance. For this purpose, various crosslinking
agents have been used to date in the EVA formulation, which are typically peroxides
such as dicumyl peroxide (DCP), peroxyesters, peroxyketals, peroxycarbonates and
mixtures thereof. An example of monoperoxycarbonate used for this purpose is 00-
t-butyl-O-2-ethylhexyl-monoperoxycarbonate (TBEC). This peroxide has proven to
require a lower operating temperature than DCP, without providing the resulting
product with a yellowish colour. It is thus used in the manufacture of photovoltaic
modules (see for instance K. Thaworn et al, Open Journal of Polymer Chemistry,
2012, 2, 77-85). The Applicant has further shown that the addition of OO-t-amyl-0-
2-ethylhexyl-monoperoxycarbonate (TAEC) to TBEC shortened the reaction time
and improved the crosslinking density of EVA, which resulted in an increased tensile
strength and a high modulus of the products made from the crosslinked EVA (WO
2010/007315).
During the laminating process of the PV module, the EVA composition is first
deposited onto the frontsheet, then covered by the solar cells and deposited again
thereon, before applying the backsheet, so as to obtain a PV module which is then
heated at a certain high temperature for some time and pressed into place, whereby
the EVA composition is cured.
It has been found that the EVA composition was susceptible to premature
crosslinking in the barrel or die head of the extruder in which it is processed, prior to
the formation of the above laminate. This phenomenon, which is called "scorching",
results in irregularities in the EVA sheet thus formed, which in turn impairs the
appearance and properties of the PV module. In some cases, pressure may also build
up in the extruder, which requires discontinuing the extrusion process. This has
especially been observed in the case of EVA having a relatively low melt-flow index
and/or a relatively narrow molecular weight distribution {source : US-4,015,058).
However, on the other hand, in order to achieve commercially feasible process
speeds, it is necessary that once the EVA sheets have been shaped and then heated
above the thermal decomposition temperature of the peroxides, crosslinking proceeds
as rapidly as possible in order to increase the economics of the process and also
minimize possible side reactions.
Various solutions have been proposed to date to prevent scorching of EVA
compositions. For instance, it has been suggested to add polymerization inhibitors to
the EVA composition. However, unwanted yellowing has been noted. Alternatively,
it has been proposed in US-4,01 5,058 to add at least 1 wt. % of cumene
hydroperoxide and/or tertiary butyl hydroperoxide to dicumyl peroxide (DCP).
However, the crosslinking rate obtained with this system is not high enough for
industrial applications, in particular in the manufacture of PV modules, due to the
presence of DCP. Because of the aromatic structure of DCP, the yellowing problems
also still remain.
Another solution has been provided in JP201 1-140588, which is said to be
appropriate for the manufacture of PV modules. It consists in adding from 4 to 50
parts by weight of a hydroperoxide, such as t-butyl hydroperoxide, to 100 parts by
weight of an organic peroxide selected from a monoperoxycarbonate, a dialkyl
peroxide, a peroxyketal and a peroxyester. Although this solution allows overcoming
the drawbacks associated with the use of DCP, it has been found in JP201 1-140588
that the above amounts of hydroperoxide negatively affected EVA crosslinking
density. In this respect, it was suggested in this document to increase the total
amount of monoperoxycarbonate and hydroperoxide while keeping the ratio of
hydroperoxide to monoperoxycarbonate constant, i.e. around 20% (see Table 1).
SUMMARY
Surprisingly, the present inventors have found that the crosslinking density of EVA
can be improved by lowering the weight ratio of hydroperoxide to
monoperoxycarbonate to less than 0.4%. Moreover, they found a marked increase in
the scorch preventing effect of the hydroperoxide at these low weight ratios, contrary
to what was expected in JP201 1-140588, without impairing the rate of the
crosslinking reaction. Further, it was observed that the film homogeneity was
satisfactory with substantially no bubbles. In this respect, it should be noted that
bubbles formed by the evaporation of water entrapped within the film at extrusion
temperatures above 100°C are responsible for surface defects which negatively affect
the resistivity of the film. This is particularly damaging in the case where the film is
to be used as an encapsulating material in PV modules.
The compositions comprising an ethylene polymer such as EVA and the above
peroxides may thus be processed in extruding devices at fast throughput rates
without experiencing scorching.
It should be noted that monoperoxycarbonates are also useful for crosslinking other
ethylene polymers such as polydiene elastomers, including ethylene-propylene-diene
(EPDM) elastomers and also polyethylene, including low density and high-density
polyethylene, which are used, inter alia, in the manufacture of wire and cable
insulation, pipes and hoses (including pipes for automobile radiators, drinkable water
and under-floor heating, for instance), roller coverings, rotational moldings and
foamed articles. The composition of this invention is thus also useful in these
applications, e.g. to prevent scorching while extruding the composition as an
insulation sheath onto an electrical conductor.
More specifically, this invention is directed to a curable composition comprising:
(a) at least one ethylene polymer,
(b) at least one monoperoxycarbonate,
(c) from 0.05 to less than 0.4 part by weight of at least one t-alkyl hydroperoxide for
100 parts by weight of constituent (b).
Some further features of the present invention are listed below :
- advantageously, the ethylene copolymer is an ethylene / vinyl acetate copolymer.
- preferably, the monoperoxycarbonate is selected from the group consisting of OOt-
alkyl-O-alkyl monoperoxycarbonates such as OO-t-butyl-O-2-ethylhexylmonoperoxycarbonate
(TBEC), OO-t-butyl-O-2-isopropyl-monoperoxycarbonate
(TBIC), OO-t-amyl-O-2-ethylhexyl-monoperoxycarbonate (TAEC), OO-t-amyl-O-2-
isopropyl-monoperoxycarbonate (TAIC) and mixtures thereof, and more preferably
TAEC and/or TBEC.
- preferably, the composition according to the invention comprises from 0.1 to 0.2
part by weight of at least one t-alkyl hydroperoxide for 100 parts by weight of
constituent (b).
It also pertains to the use of a t-alkyl hydroperoxide to prevent scorching of a curable
composition comprising (a) at least one ethylene polymer, and (b) at least one
monoperoxycarbonate, wherein the t-alkyl hydroperoxide represents from 0.05 to
less than 0.4 part by weight for 100 parts by weight of constituent (b).
This invention is further directed to a method for preventing scorching of a curable
composition comprising (a) at least one ethylene polymer, and (b) at least one
monoperoxycarbonate, comprising the step of adding at least one t-alkyl
hydroperoxide into the composition, in an amount of from 0.05 to less than 0.4 part
by weight for 100 parts by weight of constituent (b).
It is also directed to a method for manufacturing a solar cell encapsulating material
or sealant, comprising the step of extruding the above composition at a temperature
between 80 and 150°C.
DETAILED DESCRIPTION
The ethylene polymer used as constituent (a) in this invention may be an ethylene
homopolymer or preferably an ethylene copolymer. Examples of ethylene
copolymers are those made from both ethylene monomers and at least one other
monomer selected from hydrocarbons having at least one unsaturation such as
propylene, butadiene, isoprene and styrene; acryl monomers such as acrylic acid,
methacrylic acid, alkyl methacrylate and alkyl acrylate, wherein the alkyl group may
be selected from methyl, ethyl, propyl or butyl, for instance; and vinyl monomers
such as vinyl acetate. Usually, these copolymers comprise at 30 weight percent of
ethylene and at most 70 weight percent of the other monomer(s).
According to a preferred embodiment, the ethylene copolymer is an ethylene / vinyl
acetate (EVA) copolymer. The EVA copolymer may comprise from 15 to 60 wt.%,
and preferably from 25 to 45 wt. %, of VA monomer. Examples of such EVA
copolymers are available under the trade name "Evatane® 18-150" and "Evatane® 40-
55" from ARKEMA.
Other ethylene polymers that may be used in this invention have been provided, e.g.,
in EP 2 242 647. They comprise a functionalized polyolefm, such as a homopolymer
of ethylene or a copolymer of ethylene with an alkyl(meth)acrylate or vinyl acetate,
which may be functionalized either by grafting of by copolymerization with maleic
anhydride or glycidyl methacrylate. This functionalized polyolefm may optionally be
mixed with a copolymer of ethylene / carboxylic acid vinyl ester such as EVA.
The ethylene polymer is mixed with a specific peroxide used as constituent (b),
which is at least one monoperoxycarbonate. This peroxide compound may be
selected from the group consisting of OO-t-alkyl-O-alkyl monoperoxycarbonates
such as OO-t-butyl-O-2-ethylhexyl-monoperoxycarbonate (TBEC), OO-t-butyl-O-2-
isopropyl-monoperoxycarbonate (TBIC), OO-t-amyl-O-2-ethylhexylmonoperoxycarbonate
(TAEC), OO-t-amyl-O-2-isopropyl-monoperoxycarbonate
(TAIC) and mixtures thereof. Preferred monoperoxycarbonates are TAEC and
TBEC. According to an embodiment of this invention, a mixture of TBEC and
TAEC is used as constituent (b), wherein TAEC represents from 0.001 to 99.9 wt %,
relative to the total weight of the mixture. These monoperoxycarbonates are available
under the trade name Luperox ® or Lupersol ® supplied by ARKEMA.
The amount of constituent (b) in the composition of this invention may range from
0.1 to 5 parts by weight, and preferably from 0.3 to 3 parts by weight, for 100 parts
by weight of constituent (a).
The third component of the composition according to this invention is a t-alkyl
hydroperoxide, which may be selected from the group consisting of t-butyl
hydroperoxide (TBHP), t-amyl hydroperoxide (TAHP), t-hexyl hydroperoxide
(THHP), 1,1,3,3-tetramethylbutyl hydroperoxide (TOHP), paramenthane
hydroperoxide (PMHP), 2,5-dimethyl-2,5-di-hydroperoxide (2,5-2,5) and their
mixtures, for instance. Preferably, the t-alkyl hydroperoxide is TAHP.
The amount of constituent (c) in the composition of this invention ranges from 0.05
to less than 0.4 part by weight, and preferably from 0.1 to 0.2 part by weight, for 100
parts by weight of constituent (b).
The composition of this invention may further include additives such as coupling
agents, UV stabilizers, UV absorbers, fillers, plasticizers, flame retardants, anti
oxidants, dyes and their mixtures. Examples of coupling agents are monoalkyl
titanates, (vinyl)trichlorosilanes and (vinyl)trialkoxysilanes. They may represent
from 0.01 to 5 wt. % relative to the weight of ethylene polymer. UV stabilizers may
be chosen among hindered amine light stabilizers (HALS), whereas UV absorbers
may be selected, for instance, from benzophenones, triazines and benzotriazoles.
These compounds may represent from 0.01 to 3 wt. % relative to the weight of
ethylene polymer. Inorganic fillers such as silicon dioxide, alumina, talc, calcium
carbonate may be added to increase mechanical strength, although nanometric clays
are preferred because of the transparency they provide. Examples of plasticizers are
paraffinic or aromatic mineral oils, phthalates, azelates, adipates and the like.
Antioxidants may be phenolic, phosphate or sulfur antioxidants. Alternatively,
quinolines such as l,2-dihydro-2,2,4-trimethylquinoline, may be used as an
antioxidant.
According to a preferred embodiment, the composition of this invention does not
include any aromatic peroxide such as dicumyl peroxide. Still preferably, this
composition consists of constituents (a), (b) and (c) above and optionally at least one
of the following additives: a coupling agent, a UV stabilizer, a UV absorber, a filler,
a plasticizer, a flame retardant, an anti-oxidant, a dye and mixtures thereof.
The composition according to this invention may be prepared by mixing the above
constituents (a) to (c) in conventional devices such as continuous mixers and
compound extruders, below the degradation temperature of the peroxides.
It may then be used for the manufacture of various articles and more specifically in a
method for manufacturing a solar cell encapsulating material or a solar panel sealant,
comprising the step of extruding said composition at a temperature between 80 and
150°C, preferably from 90 to 120°C. The extrusion step may be performed in such a
way as to obtain a sheet having a thickness of from 50 to 2000 mih, preferably from
100 to 1000 mih, for instance. It is thus possible to use a T-die extruder or
alternatively a twin-screw extruder coupled with a two-roll mill. Preferably, a
photovoltaic module will be built, comprising successively: a front sheet (such as a
glass sheet or PMMA sheet), an encapsulating material sheet, solar cells (made from
crystalline silicon or organic photovoltaics), another encapsulating material sheet,
and a backsheet (such as a multilayer PVDF/PET film or a glass sheet or a PMMA
sheet). This laminate may then be pressed by conventional techniques while heating
and/or under vacuum, for instance at a temperature of from 130 to 180°C, more
preferably from 140 to 155°C under vacuum, for a curing time that may range from 1
to 20 minutes, for instance from 3 to 10 minutes. The encapsulating material sheet
may be crosslinked during this pressing step of afterwards. Preferably, the process
includes a single step of pressing and curing.
EXAMPLES
This invention will be better understood in light of the following examples which are
given for illustrative purposes only and do not intend to limit the scope of the
invention, which is defined by the attached claims.
Example 1 : Scorch protection effect
Compositions according to this invention were prepared by mixing an ethylene /
vinyl acetate (EVA) copolymer (Cosmothene® EVA KA-40 containing 28% VA,
supplied by SUMITOMO) with OO-t-amyl-O-2-ethylhexyl-monoperoxycarbonate
(Luperox® TAEC available from Arkema) and t-amyl hydroperoxide (Luperox®
TAHP available from Arkema) in a Haake internal mixer at 35°C for 12 minutes,
using a stirring rate of 50 rpm/min. The polymeric mixture was then passed through
an open mill set at 60°C to produce sheets of about 2 mm thickness.
Samples of about 2 to 3 g of the above compositions were deposited in plate on a
moving die rheometer (MDR) supplied by GOTECH, which is able to measure the
cure properties of the samples and includes a DFT software for analyzing the results.
Each of the samples is placed in a temperature-controlled cavity between two dies,
the lower of which oscillates to apply a cyclic stress or strain to the sample while the
upper die is connected to a torque sensor to measure the torque response of the
sample at the deformation. The stiffness is recorded continuously as a function of
time. The stiffness of the sample increases as vulcanization proceeds.
This apparatus is able to provide, inter alia, calculated values of ML (minimum
torque), MH (maximum torque), tclO (time to 10% state of cure) and tc90 (time to
90% state of cure) as defined by International Standards (ASTM D5289 and ISO
6502).
The MDR was operated at 105°C with an oscillation amplitude (deformation degree)
of 0.5° applied to the sample for 30 min. The scorch time was defined as the time
necessary to reach 10% of the total cure, i.e. tclO.
This experiment was conducted on the following samples, wherein the amounts of
TAEC and TAHP are indicated as parts per hundred parts of EVA resin (phr):
From this table, it can be seen that TAHP acts as a scorch-protection agent since
scorch time (tclO) increases with the amount of TAHP. However, there is a dramatic
decrease in scorch time, and thus a lower scorch protection effect of TAHP, when the
latter reaches 0.75 wt% relative to TAEC. At lower values of TAHP, scorch is
effectively prevented while maintaining a high crosslinking rate (tc90) and a good
crosslinking density (MH-ML).

CLAIMS
1. A curable composition comprising:
(a) at least one ethylene polymer,
(b) at least one monoperoxycarbonate,
(c) from 0.05 to less than 0.4 part by weight of at least one t-alkyl hydroperoxide for
100 parts by weight of constituent (b).
2. The composition of claim 1, wherein the ethylene copolymer is an ethylene / vinyl
acetate copolymer.
3. The composition according to Claim 1 or 2, wherein the monoperoxycarbonate is
selected from the group consisting of OO-t-alkyl-O-alkyl monoperoxycarbonates
such as OO-t-butyl-O-2-ethylhexyl-monoperoxycarbonate (TBEC), OO-t-butyl-O-2-
isopropyl-monoperoxycarbonate (TBIC), OO-t-amyl-O-2-ethylhexylmonoperoxycarbonate
(TAEC), OO-t-amyl-O-2-isopropyl-monoperoxycarbonate
(TAIC) and mixtures thereof, preferably TAEC and/or TBEC.
4. The composition according to any of Claims 1 to 3, wherein the t-alkyl
hydroperoxide is selected from the group consisting of t-butyl hydroperoxide
(TBHP), t-amyl hydroperoxide (TAHP), t-hexyl hydroperoxide (THHP), 1,1,3,3-
tetramethylbutyl hydroperoxide (TOHP), paramenthane hydroperoxide (PMHP), 2,5-
dimethyl-2,5-di-hydroperoxide (2,5-2,5) and their mixtures, preferably TAHP.
5. The composition according to any of Claims 1 to 4, which comprises from 0.1 to
0.2 part by weight of at least one t-alkyl hydroperoxide for 100 parts by weight of
constituent (b).
6. Use of a t-alkyl hydroperoxide to prevent scorching of a curable composition
comprising (a) at least one ethylene polymer, and (b) at least one
monoperoxycarbonate, wherein the t-alkyl hydroperoxide represents from 0.05 to
less than 0.4 part by weight for 100 parts by weight of constituent (b).
7. A method for preventing scorching of a curable composition comprising (a) at
least one ethylene polymer, and (b) at least one monoperoxycarbonate, comprising
the step of adding at least one t-alkyl hydroperoxide into the composition, in an
amount of from 0.05 to less than 0.4 part by weight for 100 parts by weight of
constituent (b).
8. A method for manufacturing a solar cell encapsulating material or sealant,
comprising the step of extruding a composition according to any of Claims 1 to 5 at a
temperature between 80 and 150°C.