HIGH-SPEED CROSS-LINKING SYSTEM
Field of the invention
The subject of the invention is a crosslinking 5 g system
comprising an organic peroxide and a crosslinking
coagent. The invention relates more particularly to a
composition comprising a polyolefin and the
crosslinking system as well as the use of this
10 composition as encapsulant for photovoltaic cells.
State of the art
Organic peroxides are commonly used for the
15 crosslinking of thermoplastic resins or elastomers,
these resins and elastomers being grouped together in
the present description under the term “polymers”. In
order to crosslink a polymer, a peroxide is generally
mixed with the polymer to be crosslinked in a first
20 step, and then a second step of shaping the polymer is
carried out and then a third step of crosslinking is
carried out, for example by a heat treatment.
At room temperature, peroxides may be in liquid or
solid form. When peroxides are mixed with these
25 polymers, they are mixed at high temperature, that is
to say a temperature greater than the softening point
of the polymer, for example by extrusion or kneading;
the peroxides are then generally in a liquid form.
30 Moreover, photovoltaic modules comprise light-sensitive
cells, called “photovoltaic cells”, which are capable
of converting light to current. These cells are
protected from their surroundings (moisture, oxygen,
and the like) by bottom and top protective layers.
35 These layers are generally made of glass or of polymer.
One or more layers of encapsulating composition, which
is often applied in the form of a film, make it
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possible to assemble the cells and the protective
layers. The encapsulating composition must perfectly
take the shape of the space existing between the cells
and the other protective layers of the module, this
being in order to avoid the presence 5 e of air which
limits the yield of the photovoltaic module.
It is also advantageous for the composition to be
sufficiently transparent to visible light in order to
allow a good yield of the photovoltaic cells.
10 Furthermore, the top protective layer made of
transparent glass or plastic is placed on top of the
layer of encapsulating composition. Now, the weight of
this protective layer on this film may be great: it is
therefore also preferable for the composition to have
15 good creep resistance so that the thickness of the
layer is preserved over time, this being in order to
improve the shelf life of the module.
This encapsulating composition is generally a
composition comprising a polymer, generally an ethylene
20 and vinyl acetate copolymer, which is crosslinked by an
organic peroxide. The various constituent layers of the
panel are assembled (cells, layer(s) of encapsulant
comprising peroxide, protective layers) and the panel
thus assembled is subjected to a curing step allowing
25 the crosslinking of the layer of encapsulant.
Reference may be made for example to the Handbook of
Photovoltaic Science and Engineering, Wiley, 2003,
which describes the operation and the composition of
photovoltaic modules.
30 Moreover, one of the problems encountered by industrial
manufacturers of photovoltaic modules is that this
curing step reduces the yield of the process for the
manufacture of the modules. Another problem is also
that it is advantageous to reduce the temperature of
35 the curing step in order to reduce the consumption of
energy necessary for the manufacture of photovoltaic
modules.
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In order to crosslink the polymer, the composition
comprising the peroxide is heated in order to activate
the peroxide. Bubbles may then appear during this
activation. The large presence of these bubbles may be
a disadvantage since they can reduce the 5 he transparency
of the composition and prevent good encapsulation of
the cells.
It is therefore necessary to find novel solutions which
10 make it possible to improve the yields of the processes
for the manufacture of photovoltaic modules. This is
precisely one of the objects of the present invention
which consists in the use of specific peroxides in
photovoltaic modules.
15
Summary of the invention
The subject of the invention is more particularly a
system for the crosslinking of an ethylene copolymer
20 and an ethylene monomer bearing a polar functional
group comprising at least:
􀂃 one organic peroxide whose half-life at a given
temperature chosen in the range from 80°C to 115°C
is equal to one hour;
25 􀂃 and one crosslinking coagent;
the half-life of said peroxide being measured by
dissolving it in n-dodecane at a concentration of
0.2 mol/L.
30 The use of this system is particularly advantageous
because it allows, when it is added to a polyolefin
comprising a polar comonomer, rapid crosslinking of
this polymer. When this polymer is used as encapsulant
for photovoltaic modules with their system according to
35 the invention, the yields for the processes for the
manufacture of said modules are excellent and the
polymer thus crosslinked has final properties which are
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completely suited to the encapsulation of the modules.
The peroxide may for example be chosen from tert-butyl
2-ethylperhexanoate, tert-amyl 2-ethylperhexanoate,
1,1-di-(tert-butylperoxy)-3,3,5-5 -trimethylcyclohexane,
bisdecanoyl peroxide and dilauroyl peroxide as
crosslinking agent. Advantageously, the peroxide is
tert-butyl 2-ethylperhexanoate.
10 The coagent, which is different from the organic
peroxides, advantageously bears at least one carbamate,
maleimide, acrylate, methacrylate or allyl functional
group. Allyl carboxylates may be used. The coagents may
be compounds of the allyl, diallyl and triallyl type.
15 Advantageously, the crosslinking coagent is chosen from
triallyl cyanurate, triallyl isocyanurate, N,N’-mphenylenedimaleimide,
triallyl trimellitate and
trimethylolpropane trimethacrylate, preferably triallyl
cyanurate.
20
The coagent is used here not to accelerate the
crosslinking of the polymer but to increase the level
of crosslinking. This coagent also makes it possible to
reduce residual gas emission during the decomposition
25 of these same peroxides, and ultimately reduce the
number of bubbles in the encapsulating film.
Preferably, the ratio by mass of organic peroxide and
crosslinking coagent is within the range from 1:10 to
30 10:1, most preferably from 1:3 to 3:1.
The system according to the invention may be used for
the crosslinking of an ethylene copolymer and an
ethylene comonomer bearing a polar functional group.
35
The subject of the invention is also a composition
comprising at least one polymer and the crosslinking
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system according to the invention.
The polymer is advantageously a polyolefin, preferably
a copolymer of ethylene and an ethylene monomer bearing
a polar functional group, this monomer 5 r preferably
comprising from 3 to 20 carbon atoms, preferably from 4
to 8 atoms. The ethylene monomer may be chosen from
vinyl acetate and methyl, ethyl or butyl
(meth)acrylates. The ethylene copolymer of the
10 composition according to the invention preferably
comprises from 10 to 60% by mass of ethylene monomer
bearing a polar functional group relative to the total
mass of the copolymer, preferably from 25 to 45% by
mass.
15
The composition according to the invention has a
quantity of crosslinking system preferably within,
relative to the total weight of the composition, the
range from 0.1 to 10%, preferably from 1 to 5%.
20
Another subject of the invention is a film comprising a
composition according to the invention.
A subject of the invention is also the use of the
25 composition or of a film according to the invention, as
encapsulant in a photovoltaic module.
The invention also relates to a process for the
manufacture of a composition according to the invention
30 comprising a stage for mixing organic peroxide,
crosslinking coagent and polymer, it being possible for
this stage to be carried out in one or more steps.
According to a very advantageous version of the process
35 for the manufacture of the composition according to the
invention, it comprises a step for mixing a master
batch comprising the organic peroxide with the polymer
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and the crosslinking coagent.
The invention thus also relates to a master batch
comprising:
􀂃 a polymer, preferably a copolymer of ethylene and
an ethylene monomer bearing a polar function5 al
group;
􀂃 and at least one organic peroxide chosen from
peroxides having an organic peroxide whose halflife,
measured by dissolving it in n-dodecane at a
10 concentration of 0.2 mol/L, is equal to one hour
for any temperature chosen from the range from
80°C to 115°C, this peroxide being preferably
chosen from tert-butyl 2-ethylperhexanoate, tertamyl
2-ethylperhexanoate, 1,1-di-(tert15
butylperoxy)-3,3,5-trimethylcyclohexane,
bisdecanoyl peroxide and dilauroyl peroxide;
the quantity by mass of peroxide being within the range
from 5 to 30% relative to the total weight of the
master batch, preferably from 7 to 16%.
20
This master batch may be advantageously manufactured by
a manufacturing process comprising:
a. a step for contacting said peroxide;
b. a step for the absorption of the peroxide by the
25 polymer in order to form a master batch;
c. a step for isolating said master batch.
Other advantages of the invention will appear in the
detailed description which follows.
30
Detailed description of the invention
The subject of the invention is a crosslinking system
comprising at least:
35 􀂃 one organic peroxide whose half-life at a given
temperature chosen in the range from 80°C to
115°C, preferably from 90 to 105°C, is equal to
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one hour;
􀂃 and one crosslinking coagent;
the half-life of said peroxide being measured by
dissolving it in n-dodecane at a concentration of
0.2 5 .2 mol/L (HL).
The half-life of an organic peroxide makes it possible
to determine its rate of disintegration. It is given,
for a concentration in a solvent, at a given
10 temperature and time.
By using a peroxide having this half-life, a rapid
crosslinking system is obtained which makes it possible
to solve at least one of the problems of the invention.
15 Preferably, the peroxide is chosen from tert-butyl 2-
ethylperhexanoate (HL=1h at 94°C), tert-amyl 2-
ethylperhexanoate (HL=1h at 92°C), 1,1-di-(tertbutylperoxy)-
3,3,5-trimethylcyclohexane (HL=1h at
115°C), bisdecanoyl peroxide and dilauroyl peroxide,
20 which constitute, in combination with a coagent,
crosslinking systems which are particularly efficient
in terms of speed and level of crosslinking.
A peroxide forms during its activation free radicals on
25 the polymer, which allows the crosslinking of the
polymer chains, without the peroxide becoming
integrated into these chains. A crosslinking coagent
has a function that is different from a peroxide:
indeed, it is activated with the aid of a free radical
30 initiator such as organic peroxides. Thus activated
during the decomposition of the peroxide, it then forms
crosslinking bridges with the polymer and is therefore
integrated into the chain of the crosslinked polymer,
unlike peroxides.
35 The coagent may be monofunctional or polyfunctional. It
advantageously bears at least one carbamate, maleimide,
acrylate, methacrylate or allyl functional group. They
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are substances which advantageously have a molar mass
of less than or equal to 1000 g/mol, preferably less
than or equal to 400 g/mol. Allyl carboxylates may be
used. The coagents may be compounds of the allyl,
diallyl and triallyl type. Advantageously, th5 e
crosslinking coagent is chosen from triallyl cyanurate,
triallyl isocyanurate, N,N’-m-phenylenedimaleimide,
triallyl trimellitate and trimethylolpropane
trimethacrylate, preferably triallyl cyanurate.
10 The system may optionally comprise an organic solvent.
Any type of solvent may be used. For example, solvents
of the alkane, aromatic, alkene, halogenated or alcohol
type are used. Preferably, the solvent molecules
comprise from 1 to 12 carbon atoms. By way of example
15 of solvent, mention may be made of decane, n-dodecane,
2,4,4-trimethylpentene, α-methylstyrene, trichloroethylene,
toluene, benzene, ethylbenzene, (1-
methylethenyl)benzene, 2-ethylhexanol, isopropanol, tbutyl
alcohol or acetone. Use may also be made of a
20 mixture of solvents, for example a mixture of the
solvents listed above. Preferably, the quantity of
solvent is less than or equal to 25% of the total mass
of the system, or even less than or equal to 10%.
25 The system may be used to crosslink a polymer. Another
subject of the invention is a composition comprising a
polymer and the crosslinking system according to the
invention. If the crosslinking system comprises a
solvent, the solvent used is preferably not a solvent
30 for the polymer to be crosslinked. The expression
solvent for the copolymer is understood to mean a
polymer concentration greater than or equal to
0.05 g/ml of solvent when 1 g of copolymer per ml of
solvent are brought into contact for one hour at 23°C.
35
The polymer contained in the composition according to
the invention may be any type of polymer. It is for
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example a polyolefin, that is to say a polymer
comprising an olefin. Preferably, the olefin is an
alpha-olefin having for example from 2 to 10 carbon
atoms, such as for example ethylene, propylene, butene-
1, hexene-1, 5 , octene-1 or decene-1.
Preferably, the polyolefin is a copolymer of ethylene
and an ethylene monomer bearing a polar functional
group. The expression ethylene monomer is understood to
mean a monomer comprising an unsaturation liable to
10 react with ethylene in a process by the free radical
route. The expression polar functional group is
understood to mean a functional group exhibiting a
dipole moment, such as the amine, alcohol, urethane,
acid, ester or acid or diacid anhydride functional
15 groups. Preferably, the polar functional group is an
acid, ester or acid or diacid anhydride functional
group.
The ethylene monomer bearing a polar functional group
preferably comprises from 3 to 20 carbon atoms,
20 preferably from 4 to 8 carbon atoms.
By way of example of copolymer, mention may be made of
copolymers of ethylene and a carboxylic acid vinyl
ester, the copolymers of ethylene and an unsaturated
carboxylic acid or alternatively the copolymers of
25 ethylene and an alkyl acrylate and/or methacrylate,
grouped together in the present application under the
term alkyl (meth)acrylate. Advantageously, the ethylene
monomer may be chosen from vinyl acetate and methyl,
ethyl or butyl (meth)acrylates.
30 The quantity by mass of ethylene monomer relative to
the total mass of the copolymer (a) may be within the
range from 1 to 70%, advantageously from 10 to 60% and
preferably from 20 to 45%.
According to the invention, the quantities of the
35 various monomers present in the copolymer (a) may be
measured by infrared spectroscopy using the standard
ISO8985 (1998).
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In order to manufacture the polyolefins, use may be
made of the known so-called free radical polymerization
processes usually operating at pressures between 200
and 2500 bar. These processes are carried out
industrially using two main types of reactor: 5 r: an
autoclave type reactor or a tubular type reactor. These
polymerization processes are known to a person skilled
in the art and use may be made for example of the
processes described in the documents FR2498609,
10 FR2569411 and FR2569412. Persons skilled in the art
know the proportions in which each of the monomers are
used in order to obtain the copolymer (a) used in the
invention.
These copolymers are marketed by the applicant under
15 the trade marks EVATANE® and LOTRYL®.
The level of crosslinking of the polymer is generally
quantified by measuring the gel content. This gel
content may be measured using the method A of the
20 standard ASTM D2765-01 (2006). Advantageously, the gel
content of the polymer is greater than or equal to 10,
preferably greater than or equal to 20, for example
greater than or equal to 50.
25 The composition may also comprise additives or
inorganic fillers. By way of example of additive,
mention may be made of plasticizers, antioxidants,
antiozone agents, antistatic agents, coloring
materials, pigments, optical brighteners, heat
30 stabilizers, light stabilizers and flame retardants.
Coupling agents may be advantageously added in order to
improve adhesiveness on another support of the
composition (I) or of the polymer to be crosslinked. It
35 may be organic, inorganic and more preferably semiinorganic
semi-organic. Among these, mention may be
made of titanates or organic silanes, such as for
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example monoalkyl titanates, trichlorosilanes and
trialkoxysilanes. Preferably, the quantity of coupling
agent is within the range from 0.05 to 5% by mass of
the composition.
As fillers, mention may be made of clay, silica, 5 ca, talc,
carbonates such as calcium carbonate and silicates such
as sodium silicate.
The composition according to the invention may take the
10 form of a film. The film according to the invention
which comprises the composition preferably has a
thickness ranging from 50 to 2000 microns, for example
from 100 to 1000 microns. It may be manufactured by any
of the methods known for film manufacturing. The film
15 may be manufactured for example by film extrusion,
calendering or by pressing.
One advantage of the composition or of the film
according to the invention is that the polymer of this
20 composition or of this film may be crosslinked by a
process at a lower temperature and/or more rapidly than
the compositions of the prior art, the crosslinked
polymer also having a very good appearance, in
particular a small number of bubbles and good creep
25 resistance.
All these properties make it possible for the
composition and the film according to the invention to
be very advantageously used as encapsulant in the
photovoltaic modules.
30
Another subject of the invention is a process for the
manufacture of said composition which comprises a stage
for mixing the various constituents. The composition
may be manufactured by any of the techniques suitable
35 for the manufacture of thermoplastic compositions. In
particular, mention may be made of the conventional
techniques for mixing thermoplastics such as kneading
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or extrusion. The mixing is advantageously carried out
at a temperature greater than the softening temperature
of the polymer, measured according to the standard
ASTM E 28-99(2004). Preferably, the temperature is also
less than the decomposition temperature of th5 e
peroxide. The stage for mixing the organic peroxide,
the crosslinking coagent, the optional additives and
the polymer may be carried out in one or more steps,
that is to say that the peroxide, the crosslinking
10 coagent and the optional additives may be mixed
simultaneously or successively with the polymer of the
composition.
According to a preferred process for the manufacture of
the composition, a step is carried out for mixing a
15 master batch comprising the organic peroxide, the
polymer and the crosslinking coagent. This master batch
according to the invention may comprise:
􀂃 a polymer;
􀂃 and at least one organic peroxide chosen from
20 peroxides having an organic peroxide whose halflife,
measured by dissolving it in n-dodecane at a
concentration of 0.2 mol/L, is equal to one hour
for any temperature chosen from the range from
80°C to 115°C, preferably chosen from tert-butyl
25 2-ethylperhexanoate, tert-amyl 2-ethylperhexanoate,
1,1-di-(tert-butylperoxy)-3,3,5-trimethylcyclohexane,
bisdecanoyl peroxide and
dilauroyl peroxide;
the quantity by mass of peroxide being within the range
30 from 5 to 30% relative to the total weight of the
master batch, preferably from 7 to 16%.
This master batch has a very particular importance:
these particular peroxides being particularly unstable,
35 it is advantageous to be able to transport, store and
handle them in this master batch form. This master
batch allows rapid and risk-free crosslinking of
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polymers, in particular of polyolefins.
As polymer useful for the manufacture of the master
batch according to the invention, use may be made of
the same polymers as those already mentioned for the
manufacture of the composition according 5 to the
invention, including in its preferred versions.
To manufacture this master batch, use may also be made
of the same manufacturing techniques already mentioned
for the manufacture of the composition, that is to say
10 the conventional techniques for mixing thermoplastics.
According to an advantageous mode of the process for
the manufacture of the master batch according to the
invention, the process comprises the following steps:
a. a step for contacting said peroxide with the
15 polymer at a temperature less than the softening
temperature of the polymer measured according to
the standard ASTM E 28-99(2004);
b. a step for the absorption of the peroxide by the
polymer in order to form a master batch;
20 c. a step for isolating said master batch.
One advantage of this process is that it may be carried
out at a temperature below the softening temperature of
the polymer, unlike the processes in the molten state.
When a master batch comprising these particular
25 peroxides is manufactured by the techniques for mixing
thermoplastics, a premature crosslinking phenomenon may
be observed. Thus, as the temperature is lower with
this preferred process, the phenomenon of premature
crosslinking of the master batch is limited. The master
30 batch thus obtained by this preferred process exhibits
a greater transparency than a master batch obtained by
the conventional techniques for thermoplastics. The
invention therefore also relates to the master batch
obtained by the process according to the invention.
35 The use of this master batch allows rapid crosslinking
of polymers already mentioned and allows the
manufacture of a composition having final properties
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which are completely suited to the encapsulation of
photovoltaic cells. Furthermore, the introduction of
organic peroxide by a master batch is easier than the
direct introduction of liquid or solid peroxide into
the 5 he encapsulating polymer.
According to an advantageous mode of the process
according to the invention, the polymer used for the
manufacture of the master batch is in the form of
10 particles having a mean volume of 1 to 1000 mm3,
preferably of 3 to 120 mm3. The expression “particles”
is understood to mean polymer pieces which may have any
type of geometry, for example spherical, spheroidal or
cylindrical. Preferably, at least 90% by mass of these
15 particles have a volume within these preferred volume
ranges.
In this case, the master batch is directly obtained in
the form of particles. It can then be easily used as
master batch. On using particles having this particular
20 volume, the absorption of the peroxide by the copolymer
is excellent and little agglomeration is observed
between the particles.
According to one version of the preferred process for
25 the manufacture of the master batch of the invention,
there are several contacts or there is a continuous
contact between the peroxide and the polymer, that is
to say that there are several injections or there is a
continuous injection of the peroxide solution during
30 the process.
Step a) for contacting may be carried out in any type
of container. The container may be left open or may be
closed after the contacting. The container may be
closed in an airtight or non-airtight manner.
35 Preferably, the container is closed in an airtight
manner and is equipped with a valve.
Step b) is a step for the absorption, with stirring, of
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the peroxide solution by the polymer. Preferably, this
is a complete absorption. The expression “complete
absorption” is understood to mean that the remaining
volume of nonabsorbed peroxide in the container after
the absorption step is less than 5%, preferably 5 ably less
than 2%, most preferably less than 1%.
The period of absorption is generally within the range
from 10 to 600 minutes, preferably from 20 to
240 minutes.
10 The absorption step is carried out with stirring. This
stirring may be carried out by any stirring system,
such as for example a paddle, propeller, screw or
ultrasound system or in a device of the rotary or drum
type, such as a drier.
15
During the third step of the process, the master batch
is isolated, which master batch may be in the form of
particles of copolymer comprising the peroxide.
Optionally, a step for drying the particles recovered
20 during the third step may be carried out, for example
in an oven or any other type of drier. This is
preferably carried out at a temperature below the
decomposition temperature of the peroxide of the
composition.
25
The master batch may be used to manufacture the
composition according to the invention.
According to one embodiment, the polymer of the master
batch is a copolymer of ethylene and vinyl acetate and
30 the polymer of the composition to be crosslinked is a
copolymer of ethylene and vinyl acetate.
The composition or the film according to the invention
may be used to encapsulate photovoltaic cells.
35 The photovoltaic cells which may be encapsulated may be
of any type. These cells may be for example based on
doped, monocrystalline or polycrystalline silicon, in a
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thin layer formed for example of amorphous silicon,
cadmium telluride, copper-indium disilenide or
alternatively based on organic materials.
The photovoltaic modules thus formed preferably
comprise a front protective 5 tive layer and a back protective
layer.
The front protective layer preferably has abrasion and
impact resistance properties, is transparent and
protects the photovoltaic sensors from external
10 moisture. To form this layer, mention may be made of
glass, polymethyl methacrylate (PMMA) or any other
polymer composition combining these characteristics.
The back protective layer essentially protects the
module from moisture. It may comprise glass or
15 alternatively fluorinated polymers such as polyvinyl
fluoride (PVF) or polyvinylidene fluoride (PVDF).
To assemble the various layers, use may be made of all
types of pressing technique such as for example hot
20 pressing, vacuum pressing or laminating, in particular
hot laminating. The manufacturing conditions will be
easily determined by a person skilled in the art by
adjusting the temperature and the pressure to the flow
temperature of the composition.
25
The invention also relates to a process for the
manufacture of a photovoltaic module comprising the
steps:
􀂃 of assembling the layers of photovoltaic cells,
30 encapsulating film(s) and protective layers, at
least one of the encapsulating films being a film
according to the invention;
􀂃 of curing the module, preferably at a temperature
greater than or equal to the decomposition
35 temperature of the peroxide.
To manufacture the photovoltaic modules with the
composition or the film according to the invention,
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persons skilled in the art may also refer for example
to the Handbook of Photovoltaic Science and
Engineering, Wiley, 2003.
E5 xamples
Products used:
To formulate the examples of master batches according
to the invention, the following products are used:
Copolymer: copolymer of ethylene and vinyl acetate
10 comprising 33% by mass of vinyl acetate relative to its
total mass.
Peroxide 1: tert-butyl 2-ethylperhexanoate.
Peroxide 2: OO-tert-butyl and O-(2-ethylhexyl)
peroxycarbonate.
15 Crosslinking coagent: triallyl cyanurate.
Composition of the master batches:
The master batches according to the invention (MB 1)
and comparative master batches (MB 2) have, relative to
the total mass of the master batch, the following
20 compositions:
Products MB1 MB2
Copolymer (%) 90 90
Peroxide 1 (%) 10 0
Peroxide 2 (%) 0 10
Preparation of the master batches:
Absorption is carried out on the granules of copolymer
25 for each of the solutions of peroxide.
The organic peroxide (2.2 kg) is brought into contact
in a roller mixer with the copolymer (19.8 kg)
optionally with the coupling agent and the crosslinking
coagent in a closed container at 50°C, the axis of
30 rotation of the cylinder being horizontal, and stirred
by rotation of the container at a speed of 10
revolutions per minute.
A first half of the peroxide solution is injected at
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the start of the absorption and a second half is added
after absorbing for 30 minutes.
The polymer particles are recovered after 120 minutes.
The absorption of the peroxide solution into the
particles is 5 complete.
The particles were assayed after washing for one hour
in n-heptane: the quantity of peroxide in the copolymer
is 10% by total mass of the composition for MB1 and
MB2.
10 Preparation of the test pieces
In order to evaluate the invention, films of a mixture
of the constituents are prepared, that is to say the
copolymer with the master batch MB1 or MB2 and
optionally a crosslinking coagent. A premixing in a bag
15 of the various constituents is carried out before
mixing in an extruder. The films are then made by
introducing this premixture into a counter-rotating
twin screw Haake 1 extruder equipped with a film die.
The temperature profile of the extruder is: hopper 20°C
20 – Zone 1 : 75 – Zone 2 : 75 – film die: 75°C, the speed
of the screw 80 rpm. Films 8 cm wide are obtained.
Measurement of the properties
The elastic modulus is measured with the aid of a
controlled stress plate-plate type rotational rheometer
25 of the Anton Paar brand, model Physica MCR301.
A sample of a film having a thickness of 0.5 mm
approximately and a 25 mm diameter is placed between
the two parallel plates which are heated by conduction.
The oscillatory rotation of the top plate around the
30 longitudinal axis applies a deformation to the sample
placed between the two plates. The latter responds to
this deformation by exerting a stress. The couple to be
provided in order to maintain the deformation is then
measured.
35 The experiment is started at 70°C with a rise in
temperature of 5°C.min-1 up to 150°C, and then a plateau
temperature is applied at 150°C for 30 minutes. During
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the entire test, the elastic deformation modulus G’ is
measured by applying a deformation of 0.2% at a
frequency of 1 Hz (6.28 rad.s-1), this modulus
decreasing during the melting of the copolymer and then
increasing during 5 its crosslinking.
The criterion t90 is defined, which represents the time
taken to reach 90% of the final value of G’. Comparison
of the t90 values thus makes it possible to
10 qualitatively classify the crosslinking speed of the
films formulated.
The final value of G’ is also noted.
The same heat treatment is again carried out as before
without applying stress and the number of bubbles/cm2
15 (N) formed during the crosslinking is noted. The
compositions of the films and the results obtained for
these various compositions according to the invention
(EX1 to EX7) or the comparative compositions (CP1 to
CP3) are grouped together in the following table:
WO 2011/067504 PCT/FR2010/052498
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Examples Peroxide
1
Peroxide
2
Copolymer Coagent t90
(s)
Final G’
(MPa)
N Presence
of
bubbles
EX1 1.6 0 98.4 0 7.1 bubbles
EX2 1.6 0 97.9 0.5 780 2.30E+05 few
bubbles
EX3 1.6 0 97.4 1 804 2.87E+05 no
bubble
EX4 1.6 0 96.9 1.5 824 3.19E+05 1.2 no
bubble
EX5 1.6 0 96.4 2 828 3.37E+05 no
bubble
EX6 1.6 0 95.9 2.5 840 3.74E+05 no
bubble
EX7 3 0 95 2 776 3.74E+05 no
bubble
CP1 0 1.5 98.5 0 1404 3.20E+05 1.6 no
bubble
CP2 0 2 98 0 1392 4.46E+05 no
bubble
CP3 0 3 97 0 1380 4.65E+05 no
bubble
The tests show that the peroxide according to the
invention (peroxide 1) allows a more rapid crosslinking
of the composition. This is particularly advantageo5 us
for its use as encapsulating composition in
photovoltaic modules in order to increase the
productivity of the photovoltaic modules. The tests
also show that it is possible to reduce the number of
10 bubbles formed during the crosslinking of the
composition while further increasing the mechanical
resistance of the crosslinked composition. This is also
an advantage for its use as encapsulant for
photovoltaic cells.
WO 2011/067504 PCT/FR2010/052498
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CLAIMS
1. A system for the crosslinking of an ethylene
copolymer and an ethylene monomer bearing a polar
functional group 5 comprising at least:
􀂃 one organic peroxide whose half-life is equal to
one hour for any temperature chosen in the range
from 80°C to 115°C;
􀂃 and one crosslinking coagent;
10 the half-life of said peroxide being measured by
dissolving it in n-dodecane at a concentration of
0.2 mol/L.
2. The crosslinking system as claimed in claim 1, in
15 which the organic peroxide is chosen from tertbutyl
2-ethylperhexanoate, tert-amyl 2-ethylperhexanoate,
1,1-di-(tert-butylperoxy)-3,3,5-trimethylcyclohexane,
bisdecanoyl peroxide and
dilauroyl peroxide.
20
3. The system as claimed in one of the preceding
claims, in which the crosslinking coagent is
chosen from triallyl cyanurate, triallyl isocyanurate,
triallyl trimellitate and
25 trimethylolpropane trimethacrylate, preferably
triallyl cyanurate.
4. The system as claimed in one of the preceding
claims, in which the ratio by mass of organic
30 peroxide and crosslinking coagent is within the
range from 1:10 to 10:1, preferably from 1:3 to
3:1.
5. The use of the system as claimed in one of claims
35 1 to 4, for the crosslinking of an ethylene
copolymer and an ethylene comonomer bearing a
polar functional group.
WO 2011/067504 PCT/FR2010/052498
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6. A composition comprising at least one polymer and
the system as claimed in one of the preceding
claims.
5
7. The composition as claimed in claim 6, in which
the polymer is a polyolefin, preferably a
copolymer of ethylene and an ethylene monomer
bearing a polar functional group, this monomer
10 preferably comprising from 3 to 20 carbon atoms,
preferably from 4 to 8 atoms.
8. The composition as claimed in claim 7, in which
the ethylene monomer is chosen from vinyl acetate
15 and methyl, ethyl or butyl (meth)acrylates.
9. The composition as claimed in either of claims 7
and 8, in which the copolymer comprises from 10 to
60% by mass of ethylene monomer bearing a polar
20 functional group relative to the total mass of the
copolymer, preferably from 25 to 45% by mass.
10. The composition as claimed in one of claims 6 to
9, in which the quantity of crosslinking system is
25 preferably within, relative to the total weight of
the composition, the range from 0.1 to 10%,
preferably from 1 to 5%.
11. A film comprising a composition as claimed in one
30 of claims 6 to 10.
12. The use of the composition as claimed in one of
claims 6 to 10 or of a film as claimed in claim
11, as encapsulant in a photovoltaic module.
35
13. A process for the manufacture of a composition as
claimed in one of claims 6 to 10, comprising a
WO 2011/067504 PCT/FR2010/052498
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stage for mixing organic peroxide, crosslinking
coagent and polymer, it being possible for this
stage to be carried out in one or more steps.
14. The process for the manufacture 5 e of a composition
as claimed in claim 13, comprising a step for
mixing a master batch comprising the organic
peroxide, polymer and the crosslinking coagent.
10 15. A master batch comprising:
􀂃 a copolymer of ethylene and an ethylene monomer
bearing a polar functional group;
􀂃 and at least one organic peroxide chosen from
peroxides having an organic peroxide whose half15
life, measured by dissolving it in n-dodecane at
a concentration of 0.2 mol/L, is equal to one
hour for any temperature chosen from the range
from 80°C to 115°C, preferably chosen from tertbutyl
2-ethylperhexanoate, tert-amyl 2-ethyl20
perhexanoate, 1,1-di-(tert-butylperoxy)-3,3,5-
trimethylcyclohexane, bisdecanoyl peroxide and
dilauroyl peroxide;
the quantity by mass of peroxide being within the
range from 5 to 30% relative to the total weight
25 of the master batch, preferably from 7 to 16%.
16. A process for the manufacture of the master batch
as claimed in claim 15,
a. a step for contacting the peroxide;
30 b. a step for the absorption of the peroxide by
the polymer in order to form a master batch;
c. a step for isolating said master batch.