WO2024256444A2

WO2024256444A2 - Titanium dioxide-containing polypropylene compositions for external elements of bifacial photovoltaic modules

Info

Publication number: WO2024256444A2
Application number: PCT/EP2024/066187
Authority: WO
Inventors: Minna Kaarina AARNIO-WINTERHOF; Francis Reny COSTA; Dietrich Gloger; Jeroen Oderkerk; Jessica HESSELGREN; Qizheng Dou
Original assignee: Borealis Ag
Priority date: 2023-06-13
Filing date: 2024-06-12
Publication date: 2024-12-19
Also published as: WO2024256444A3

Abstract

The present invention relates to a photovoltaic module having a protective back layer element that comprises a polypropylene composition comprising low levels of one or more white pigments, as well as to the use of titanium dioxide for maintaining elongation at break after UV treatment.

Description

Titanium dioxide-containing polypropylene compositions for external elements of bifacial photovoltaic modules

Field of the Invention

Background to the Invention

In certain end use applications, e.g. outdoor end use, the mechanical properties of polymeric articles have particular requirements. The polymeric material must, for example, withstand UV light that can be severe in some geographical regions. Moreover, at outdoor end use the temperature can vary within wide range. Therefore, long-term thermal stability, especially at high temperatures, is also often required.

Usually articles produced using polymer material, such as polypropylene (PP), require additives to provide the UV stability and long-term temperature stability.

Whilst a wide range of additives are known for improving the UV stability of polypropylenes, it is far from straightforward to predict which combination of additives may be the most effective at providing the UV protection, as well as also providing long term thermal protection.

For polypropylenes used as external elements in photovoltaic modules, in particular in bifacial photovoltaic modules, it is important to combine good UV and thermal stability with excellent optical properties to ensure that the polypropylene external elements do not reduce the amount of light arriving at the photovoltaic elements of the photovoltaic module.

Bifacial PV modules produce solar power from both sides of the panel. Whereas traditional opaque-backsheeted panels are monofacial, bifacial modules expose both the front and rear side of the solar cells. By producing solar power also from the rear side an increase of power output from bifacial PV modules of up to 30% compared to monofacial PV modules can be expected. Bifacial modules come in many designs. Some are framed while others are frameless. Some are dual-glass, and others use clear backsheets. Most use monocrystalline cells, but there are polycrystalline designs. The one thing that is constant is that power is produced from both sides. There are frameless, dual-glass modules that expose the rear side of cells but are not bifacial. True bifacial modules have contacts/busbars on both the front and rear sides of their cells. Prerequisite for using a PV module as a bifacial PV module is a high transparency of the layer elements on the rear side of the solar cells for increasing the power output from the rear side of the solar cells. Nevertheless, the backsheet layer element also needs to show good mechanical stability in its function as protective back layer element Therefore, most bifacial PV modules are dual glass modules with both protective elements on front and rear side being glass elements.

The main drawback of glass-glass modules with bifacial solar cells is the weight, which makes the handling and installation tedious, also the logistic costs might be negatively affected.

Glass being a great source of Na+ ion and bifacial solar cells (especially the rear side) being sensitive to potential induced degradation (PID), the bi facial modules have tendency to show high PID degradation. Currently, industry is solving it by having PID resistant encapsulant and/or Na free glass. One alternative and cheaper solution will be replacement of rear glass by a PP based transparent backsheet.

Another problem with glass-glass module is the lamination process takes longer time and also process optimization is very tedious with conventional membrane based laminators. Therefore, plate-plate laminator or autoclave based lamination are ideal to produce good quality glass-glass module. However, more than 95% of solar laminators are based on membrane based laminator and hence many module producers just cannot simply switch to bi-facial module due to this limitation. A transparent polymeric backsheet will solve this problem fully.

The problems with competitive polymeric transparent backsheet are also manifold, like high cost, week interlayer adhesion, non compatibility with different types of encapsulant and limited hydrolytic stability (especially for PET based backsheets), and environmental aspects (presence of fluorinated polymers).

Furthermore, the polymeric elements of bifacial photovoltaic modules are exposed to even more UV light than for monofacial photovoltaic module, thus the choice of UV (and thermal) stabilizers are even more critical. WO 2021/239446 describes a typical bifacial photovoltaic module.

Accordingly, there is a continuous need for polypropylene compositions for demanding end applications wherein UV light stability and long-term thermal stability are required, without sacrificing the transmittance of the material.

Summary of the Invention

The present invention is based on the finding that very low amounts of white pigments, such as titanium dioxide, help to provide polypropylene compositions with long term UV and oven-aging protection without impacting the optical properties of the polypropylene compositions too severely, making such polypropylene compositions an excellent choice for use as an external layer of a photovoltaic module, especially a bifacial photovoltaic module.

Therefore, the present invention is directed to a photovoltaic (PV) module, comprising, in the given order, a protective front layer element (1), a front encapsulation layer element (2), a photovoltaic element (3), a rear encapsulation layer element (4) and a protective back layer element (5), wherein the protective back layer element (5) comprises a polypropylene composition (PC) that comprises: a) 95.0 to 99.995 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); b) 0.005 to 0.065 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P).

In a second aspect, the present invention is directed to the use of titanium dioxide for maintaining elongation at break in the machine direction (MD), determined according to ISO 527-3 on monolayer film specimens having a thickness of 400 pm, of a polypropylene composition comprising said titanium dioxide after UV aging for 1000 h as described in the determination methods, wherein maintaining the elongation at break in the machine direction (MD) is achieved when the value of the elongation at break in the machine direction (MD) after the UV aging is at least 20% of the value determined before the UV aging, without reducing the total luminous transmittance, determined according to determined according to ASTM DI 003, by more than 22%, relative to an analogous polypropylene composition that is free from titanium dioxide.

In a third aspect, the present invention is directed to the use of titanium dioxide for maintaining elongation at break in the machine direction (MD), determined according to ISO 527-3 on monolayer film specimens having a thickness of 400 pm, of a polypropylene composition comprising said titanium dioxide after oven aging at 120°C for 1000 h, wherein maintaining the elongation at break in the machine direction (MD) is achieved when the value of the elongation at break in the machine direction (MD) after the oven aging is at least 40%, more preferably at least 50%, most preferably at least 60%, of the value determined before the oven aging, without reducing the total luminous transmittance, determined according to ASTM DI 003, by more than 22%, more preferably more than 20%, most preferably more than 15%, relative to an analogous polypropylene composition that is free from titanium dioxide.

Definitions

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although, any methods and materials similar or equivalent to those described herein can be used in practice for testing of the present invention, the preferred materials and methods are described herein. In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below.

Unless clearly indicated otherwise, use of the terms “a,” “an,” and the like refers to one or more. In the following, amounts are given in % by weight (wt.-%) unless it is stated otherwise.

A propylene homopolymer is a polymer that essentially consists of propylene monomer units. Due to impurities especially during commercial polymerization processes, a propylene homopolymer can comprise up to 0.1 mol% comonomer units, preferably up to 0.05 mol% comonomer units and most preferably up to 0.01 mol% comonomer units.

A propylene copolymer is a copolymer of propylene monomer units and comonomer units, preferably selected from ethylene and C4-C8 alpha-olefins. A propylene random copolymer is a propylene copolymer wherein the comonomer units are randomly distributed along the polymer chain, whilst a propylene block copolymer comprises blocks of propylene monomer units and blocks of comonomer units. Propylene random copolymers can comprise comonomer units from one or more comonomers different in their amounts of carbon atoms.

Heterophasic propylene copolymers typically comprise: a) a crystalline propylene homopolymer or copolymer matrix (M); and b) an elastomeric rubber, preferably a propylene -ethylene copolymer (E).

In case of a random heterophasic propylene copolymer, said crystalline matrix phase is a random copolymer of propylene and at least one alpha-olefin comonomer.

A plastomer is a polymer that combines the qualities of elastomers and plastics, such as rubber-like properties with the processing abilities of plastic.

An ethylene-based plastomer is a plastomer with a molar majority of ethylene monomer units.

A bifacial photovoltaic module is a photovoltaic module that produces solar power from the front and the rear side of the solar cells of the photovoltaic element.

Different in the context of the present invention means that two polymers differ in at least one property or structural element. The present invention will now be described in more detail.

Detailed Description

Photovoltaic (PV) module

In a first aspect, the present invention is directed to a photovoltaic (PV) module, comprising, in the given order, a protective front layer element (1), a front encapsulation layer element (2), a photovoltaic element (3), a rear encapsulation layer element (4) and a protective back layer element (5).

The protective back layer element (5) comprises, more preferably consists of, the polypropylene composition (PC) as defined below.

The “photovoltaic element” means that the element has photovoltaic activity. The photovoltaic element can be e.g. an element of photovoltaic cell(s), which has a well-known meaning in the art. Silicon based material, e.g. crystalline silicon, is a non-limiting example of materials used in photovoltaic cell(s). Crystalline silicon material can vary with respect to crystallinity and crystal size, as well known to a skilled person. Alternatively, the photovoltaic element can be a substrate layer on one surface of which a further layer or deposit with photovoltaic activity is subjected, for example a glass layer, wherein on one side thereof an ink material with photovoltaic activity is printed, or a substrate layer on one side thereof a material with photovoltaic activity is deposited. For instance, in well-known thin film solutions of photovoltaic elements e.g. an ink with photovoltaic activity is printed on one side of a substrate, which is typically a glass substrate.

The photovoltaic element is most preferably an element of photovoltaic cell(s).

“Photovoltaic cell(s)” means herein a layer element(s) of photovoltaic cells, as explained above, together with connectors. The materials of the above elements other than the protective back layer element (5) are well known in the prior art and can be chosen by a skilled person depending on the desired PV module.

The front and rear encapsulation layers (2 and 4) may be selected from any known encapsulation layers known in the art. Suitable materials for encapsulation layers include ethylene vinyl acetate (EVA) and silane-functionalized polyethylenes, such as those disclosed in WO 2017/076629 Al.

The photovoltaic (PV) module of the present invention is a bifacial photovoltaic module, meaning that light may enter from either the front side or the rear (back) side.

As such, it is preferred that the protective back layer element (5) has a total luminous transmittance, determined according to ASTM D1003, in the range from 75 to 95%, more preferably in the range from 77 to 90%, most preferably in the range from 79 to 87%.

It is also preferred that the protective back layer element (5) has a haze value, determined according to ASTM D1003, in the range from 30 to 75%, more preferably in the range from 40 to 73%, most preferably in the range from 50 to 70%.

It is also preferred that the protective back layer element (5) has a clarity value, determined according to ASTM D1003, in the range from 90.0 to 99.5%, more preferably in the range from 92.0 to 99.0%, most preferably in the range from 94.0 to 98.0%.

The protective front layer element (1) and the protective back layer element (5) may be rigid or flexible. Preferably, the protective front layer element (1) and the protective back layer element (5) are flexible.

The protective back layer element is preferably a fdm, more preferably a monolayer fdm, having a thickness in the range from 100 to 1000 pm, more preferably in the range from 200 to 800 pm, most preferably in the range from 300 to 600 pm. Due to the presence of the blend (B) of UV stabilizers and hindered amine light stabilizers in the polypropylene composition (PC) of the protective back layer element, the back layer element has exceptionally good UV protection. This may be evaluated by comparing the elongation at break before and after aging treatments, such as UV aging and thermal (i.e. oven) aging.

As such, it is preferred that the protective back layer element (5) has an elongation at break, determined according to ISO 527-3, after UV aging for 1000 h as described in the determination methods, is at least 20%, more preferably at least 25%, most preferably at least 30% of the elongation at break, determined according to ISO 527-3, before the UV aging.

It is also preferred that the protective back layer element (5) has an elongation at break, determined according to ISO 527-3, after oven aging at 120 °C for 2000 h, is at least 40%, more preferably at least 50%, most preferably at least 60% of the elongation at break, determined according to ISO 527-3, before the oven aging.

It is especially preferred that the protective back layer element (5) has an elongation at break, determined according to ISO 527-3, in the range from 600 to 800%.

Polypropylene composition (PC)

The protective back layer element (5) of the photovoltaic (PV) module comprises a polypropylene composition (PC).

The polypropylene composition (PC) comprises: a) 95.0 to 99.995 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); and b) 0.005 to 0.065 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P). More preferably, the polypropylene composition (PC) comprises: a) 96.0 to 99.990 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); and b) 0.010 to 0.060 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P).

Yet more preferably, the polypropylene composition (PC) comprises: a) 97.0 to 99.985 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); and b) 0.015 to 0.055 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P).

Even more preferably, the polypropylene composition (PC) comprises: a) 98.0 to 99.980 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); and b) 0.020 to 0.050 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P).

It is further preferred that the polypropylene composition (PC) comprises: a) 98.0 to 99.980 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); and b) 0.020 to 0.045 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P).

In one embodiment, the polypropylene composition (PC) further comprises: c) 0.050 to 3.0 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more hindered amine light stabilizers (HALS) and/or one or more UV stabilizers (UV).

The skilled practitioner would be able to select suitable hindered amine light stabilizers and/or UV stabilizers that are well known in the art. In this embodiment, the polypropylene composition (PC) thus comprises: a) 95.0 to 99.945 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); b) 0.005 to 0.065 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P); and c) 0.050 to 3.0 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more hindered amine light stabilizers (HALS) and/or one or more UV stabilizers (UV).

More preferably, the polypropylene composition (PC) comprises: a) 96.0 to 99.890 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); b) 0.010 to 0.060 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P); and c) 0.10 to 2.5 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more hindered amine light stabilizers (HALS) and/or one or more UV stabilizers (UV).

Yet more preferably, the polypropylene composition (PC) comprises: a) 97.0 to 99.785 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); b) 0.015 to 0.055 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P); and c) 0.20 to 2.0 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more hindered amine light stabilizers (HALS) and/or one or more UV stabilizers (UV).

Even more preferably, the polypropylene composition (PC) comprises: a) 98.0 to 99.480 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); b) 0.020 to 0.050 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P); and c) 0.50 to 1.5 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more hindered amine light stabilizers (HALS) and/or one or more UV stabilizers (UV).

It is further preferred that the polypropylene composition (PC) comprises: a) 98.0 to 99.480 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); b) 0.020 to 0.045 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P); and c) 0.50 to 1.5 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more hindered amine light stabilizers (HALS) and/or one or more UV stabilizers (UV).

In this embodiment, it is further preferred that the remaining amount of the polymer composition (PC) that is not the polypropylene or mixture of polypropylenes (PP), the one or more white pigments (P), the one or more hindered amine light stabilizers (HALS) and/or one or more UV stabilizers (UV), is made up of one or more further additives (A) other than pigments, hindered amine light stabilizers and UV stabilizers.

The skilled practitioner would be able to select suitable additives that are well known in the art.

Preferably, the one or more further additives (A) are selected from the group consisting of antioxidants, nucleating agents, clarifiers, brighteners, acid scavengers, slip agents, processing aids, release agents and mixtures thereof.

It is understood that the content of additives includes any carrier polymers used to introduce the additives to the polypropylene composition (PC), i.e. masterbatch carrier polymers. An example of such a carrier polymer would be a polypropylene homopolymer in the form of powder.

As such, it is preferred that the polypropylene composition (PC) consists of: a) 95.0 to 99.945 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); b) 0.005 to 0.065 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P); c) 0.050 to 3.0 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more hindered amine light stabilizers (HALS) and/or one or more UV stabilizers (UV); and d) 0.0 to 4.945 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more further additives (A) other than pigments, hindered amine light stabilizers and UV stabilizers.

More preferably, the polypropylene composition (PC) consists of: a) 96.0 to 99.890 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); b) 0.010 to 0.060 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P); c) 0.10 to 2.5 wt.-% of one or more hindered amine light stabilizers (HALS) and/or one or more UV stabilizers (UV); and d) 0.0 to 3.89 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more further additives (A) other than pigments, hindered amine light stabilizers and UV stabilizers.

Yet more preferably, the polypropylene composition (PC) consists of: a) 97.0 to 99.785 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); b) 0.015 to 0.055 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P); c) 0.20 to 2.0 wt.-% of one or more hindered amine light stabilizers (HALS) and/or one or more UV stabilizers (UV); and d) 0.0 to 2.785 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more further additives (A) other than pigments, hindered amine light stabilizers and UV stabilizers. Even more preferably, the polypropylene composition (PC) consists of: a) 98.0 to 99.480 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); b) 0.020 to 0.050 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P); c) 0.50 to 1.5 wt.-% of one or more hindered amine light stabilizers (HALS) and/or one or more UV stabilizers (UV); and d) 0.0 to 1.48 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more further additives (A) other than pigments, hindered amine light stabilizers and UV stabilizers.

It is further preferred that the polypropylene composition (PC) consists of: a) 98.0 to 99.480 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); b) 0.020 to 0.045 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P); c) 0.50 to 1.5 wt.-% of one or more hindered amine light stabilizers (HALS) and/or one or more UV stabilizers (UV); and d) 0.0 to 1.48 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more further additives (A) other than pigments, hindered amine light stabilizers and UV stabilizers.

Polypropylene or mixture of polypropylenes (PP)

One of the essential components of the polypropylene composition (PC) is a polypropylene or mixture of polypropylenes (PP).

In the broadest sense, this component may be any polypropylene or mixture of polypropylenes. Suitable polypropylenes for use in back layer elements of photovoltaic (PV) modules are well known in the art. In the context of the present disclosure, it is preferred that the polypropylene or mixture of polypropylenes (PP) is a heterophasic propylene -ethylene copolymer (HECO), comprising: a) a crystalline matrix (M) being a propylene homopolymer; and b) an amorphous propylene -ethylene elastomer (E) that is dispersed in said crystalline matrix (M).

It is preferred that the crystalline matrix (M) has a melt flow rate (MFR2), determined according to ISO 1133 at 230 °C and 2.16 kg, in the range from 0.5 to 15.0 g/10 min, more preferably in the range from 1.0 to 10.0 g/10 min, most preferably in the range from 1.5 to 5.0 g/10 min.

It is preferred that the polypropylene or mixture of polypropylenes (PP), more preferably the heterophasic propylene-ethylene copolymer (HECO), has a melt flow rate (MFR2), determined according to ISO 1133 at 230 °C and 2.16 kg, in the range from 0.5 to 20.0 g/10 min, more preferably in the range from 1.0 to 15.0 g/10 min, most preferably in the range from 2.0 to 10.0 g/10 min.

It is preferred that the polypropylene or mixture of polypropylenes (PP), more preferably the heterophasic propylene-ethylene copolymer (HECO), has a xylene cold soluble (XCS) content, determined according to ISO 16152, in the range from 2.0 to 30.0 wt.-%, more preferably in the range from 5.0 to 25.0 wt.-%, most preferably in the range from 10.0 to 20.0 wt.-%.

It is preferred that the xylene cold soluble content of the polypropylene or mixture of polypropylenes (PP), more preferably of the heterophasic propylene-ethylene copolymer (HECO), has an ethylene content (C2(XCS)), determined by FT-IR spectroscopy, calibrated using quantitative ¹³C-NMR spectroscopy, in the range from 20.0 to 60.0 wt.-%, more preferably in the range from 25.0 to 50.0 wt.-%, most preferably in the range from 30.0 to 45.0 wt.-%.

It is preferred that the polypropylene or mixture of polypropylenes (PP), more preferably the heterophasic propylene-ethylene copolymer (HECO), has a melting temperature (Tm), determined by differential scanning calorimetry (DSC), in the range from 155 to 171 °C, more preferably in the range from 159 to 170 °C, most preferably in the range from 163 to 169 °C.

It is preferred that the polypropylene or mixture of polypropylenes (PP), more preferably the heterophasic propylene-ethylene copolymer (HECO), has an ethylene content (C2(total)), determined by FT-IR spectroscopy, calibrated using quantitative ¹³C-NMR spectroscopy, in the range from 0.5 to 20.0 wt.-%, more preferably in the range from 1.0 to 10.0 wt.-%, most preferably in the range from 2.0 to 5.0 wt.-%.

It is preferred that the polypropylene or mixture of polypropylenes (PP), more preferably the heterophasic propylene-ethylene copolymer (HECO), has a Vicat softening temperature, determined according to ASTM D 1525 method A, in the range from 125 to 170 °C, more preferably in the range from 135 to 165 °C, most preferably in the range from 145 to 160 °C.

It is preferred that the polypropylene or mixture of polypropylenes (PP), more preferably the heterophasic propylene-ethylene copolymer (HECO), has a flexural modulus, determined according to ISO 178 using 80x 10x4 mm³ test bars injection-moulded in line with ISO 19069-2, in the range from 1000 to 2500 MPa, more preferably in the range from 1200 to 2000 MPa, most preferably in the range from 1300 to 1600 MPa.

It is preferred that the polypropylene or mixture of polypropylenes (PP), more preferably the heterophasic propylene-ethylene copolymer (HECO), has a Charpy Notched Impact Strength (NIS), determined according to ISO 178 using 80x 10x4 mm³ test bars injection-moulded in line with ISO 19069-2, in the range from 20 to 100 kJ/m², more preferably in the range from 30 to 80 kJ/m², most preferably in the range from 40 to 60 kJ/m². One or more white pigments (P)

The other essential component of the polypropylene composition is the one or more white pigments (P).

In the broadest sense, the one or more white pigments may be any white pigments known in the art.

It is preferred that the one or more white pigments (P) are selected from inorganic white pigments.

Even more preferably, at least one of the one or more white pigments (P) is titanium dioxide.

It is particularly preferred that only one white pigment is present, wherein this white pigment is titanium dioxide.

The titanium dioxide is preferably in a form of rutile. Rutile is a mineral that is primarily based on titanium dioxide and has a tetragonal unit cell structure as well known in the art.

Since very small amounts of the one or more white pigments (P) are present in the polypropylene composition (PC), it is generally advisable to add the one or more white pigments (P) via a pigment-containing masterbatch. Such a pigment-containing masterbatch comprises a carrier polymer, such as a propylene homopolymer or a propylene copolymer, the one or more white pigments (P) and optional further additives, such as the hindered amine light stabilizers (HALS), UV stabilizers (UV) and the further additives (A) discussed above. If the one or more white pigments (P) is added via the use of a pigment-containing masterbatch, the amount of the masterbatch carrier polymer is not counted towards the total amount of one or more white pigments (P). In the embodiment provided above wherein the polypropylene composition consists of a polypropylene or mixture of polypropylenes (PP), one or more white pigments (P) one or more hindered amine light stabilizers (HALS) and/or one or more UV stabilizers (UV), and one or more further additives (A), the masterbatch carrier polymer is counted as one of the one or more further additives (A).

Use

In a second aspect, the present invention is directed to the use of titanium dioxide for maintaining elongation at break in the machine direction (MD), determined according to ISO 527-3 on monolayer film specimens having a thickness of 400 pm, of a polypropylene composition comprising said titanium dioxide after UV aging for 1000 h as described in the determination methods, wherein maintaining the elongation at break in the machine direction (MD) is achieved when the value of the elongation at break in the machine direction (MD) after the UV aging is at least 20%, more preferably at least 25%, most preferably at least 30%, of the value determined before the UV aging, without reducing the total luminous transmittance, determined according to ASTM DI 003, by more than 22%, more preferably more than 20%, most preferably more than 15%, relative to an analogous polypropylene composition that is free from titanium dioxide.

All fallback positions for the first aspect apply mutatis mutandis for the uses of the second and third aspects. E X A M P L E S

1. Measurement methods

The following definitions of terms and determination methods apply for the above general description of the invention including the claims as well as to the below examples unless otherwise defined.

Comonomer content

The comonomer content was determined by quantitative Fourier transform infrared spectroscopy (FTIR) after basic assignment calibrated via quantitative ¹³C nuclear magnetic resonance (NMR) spectroscopy in a manner well known in the art. Thin films are pressed to a thickness of between 100-500 micrometer and spectra recorded in transmission mode. Specifically, the ethylene content of a polypropylene-co-ethylene copolymer is determined using the baseline corrected peak area of the quantitative bands found at 720-722 and 730- 733 cm ¹. Specifically, the butene or hexene content of a polypropylene copolymer is determined using the baseline corrected peak area of the quantitative bands found at 1377- 1379 cm ¹. Quantitative results are obtained based upon reference to the film thickness.

The comonomer content is herein assumed to follow the mixing rule (equation 2):

C_b = w, * C, + w₂ * C₂ (eq. 2)

Where C is the content of comonomer in weight-%, w is the weight fraction of the component in the mixture and subscripts b, 1 and 2 refer to the overall mixture, component 1 and component 2, respectively.

As it is well known to the person skilled in the art the comonomer content in weight basis in a binary copolymer can be converted to the comonomer content in mole basis by using the following equation

where c_m is the mole fraction of comonomer units in the copolymer, c_w is the weight fraction of comonomer units in the copolymer, MW_C is the molecular weight of the comonomer (such as ethylene) and MW_m is the molecular weight of the main monomer (i.e., propylene). CRYSTEX QC analysis

Crystalline and soluble fractions method

The crystalline (CF) and soluble fractions (SF) of the polypropylene (PP) compositions as well as the comonomer content and intrinsic viscosities of the respective fractions were analyzed by use of the CRYSTEX instrument, Polymer Char (Valencia, Spain). Details of the technique and the method can be found in literature (Ljiljana Jeremie, Andreas Albrecht, Martina Sandholzer & Markus Gahleitner (2020) Rapid characterization of high-impact ethylenepropylene copolymer composition by crystallization extraction separation: comparability to standard separation methods, International Journal of Polymer Analysis and Characterization, 25:8, 581-596)

The crystalline and amorphous fractions are separated through temperature cycles of dissolution at 160 °C, crystallization at 40 °C and re-dissolution in 1 ,2,4-trichlorobenzene at 160 °C. Quantification of SF and CF and determination of ethylene content (C2) are achieved by means of an integrated infrared detector (IR4) and for the determination of the intrinsic viscosity (IV) an online 2-capillary viscometer is used.

The IR4 detector is a multiple wavelength detector measuring IR absorbance at two different bands (CH₃ stretching vibration (centred at app. 2960 cm ¹) and the CH stretching vibration (2700-3000 cm ¹) that are serving for the determination of the concentration and the Ethylene content in Ethylene-Propylene copolymers. The IR4 detector is calibrated with series of 8 EP copolymers with known Ethylene content in the range of 2 wt.-% to 69 wt.-% (determined by ¹³C-NMR) and each at various concentrations, in the range of 2 and 13mg/ml. To encounter for both features, concentration and ethylene content at the same time for various polymer concentrations expected during Crystex analyses the following calibration equations were applied:

Cone = a + b*Abs(CH) + c*(Abs(CH))² + d*Abs(CH₃) + e*(Abs(CH₃)² + f*Abs(CH)*Abs(CH₃) (Equation 1)

CH₃/1000C = a + b*Abs(CH) + c* Abs(CH₃) + d * (Abs(CH₃)/Abs(CH)) + e * (Abs(CH₃)/Abs(CH))² (Equation 2) The constants a to e for equation 1 and a to f for equation 2 were determined by using least square regression analysis.

The CH3/IOOOC is converted to the ethylene content in wt.-% using following relationship:

Wt.-% (Ethylene in EP Copolymers) = 100 - CH3/IOOOTC * 0.3 (Equation 3)

Amounts of Soluble Fraction (SF) and Crystalline Fraction (CF) are correlated through the XS calibration to the “Xylene Cold Soluble” (XCS) quantity and respectively Xylene Cold Insoluble (XCI) fractions, determined according to standard gravimetric method as per ISO16152. XS calibration is achieved by testing various EP copolymers with XS content in the range 2-31 wt.-%. The determined XS calibration is linear:

Wt.-% XS = 1 ,01 * Wt.-% SF (Equation 4)

Intrinsic viscosity (IV) of the parent EP copolymer and its soluble and crystalline fractions are determined with a use of an online 2-capillary viscometer and are correlated to corresponding IV’s determined by standard method in decalin according to ISO 1628-3. Calibration is achieved with various EP PP copolymers with IV = 2-4 dL/g. The determined calibration curve is linear:

IV (dL/g) = a* Vsp/c (equation 5)

The samples to be analyzed are weighed out in concentrations of lOmg/ml to 20mg/ml.

After automated filling of the vial with 1,2,4-TCB containing 250 mg/1 2,6-tert-butyl-4- methylphenol (BHT) as antioxidant, the sample is dissolved at 160 °C until complete dissolution is achieved, usually for 60 min, with constant stirring of 400rpm. To avoid sample degradation, the polymer solution is blanketed with the N2 atmosphere during dissolution.

A defined volume of the sample solution is injected into the column filled with inert support where the crystallization of the sample and separation of the soluble fraction from the crystalline part is taking place. This process is repeated two times. During the first injection the whole sample is measured at high temperature, determining the IV[dl/g] and the C2[wt.%] of the PP composition. During the second injection the soluble fraction (at low temperature) and the crystalline fraction (at high temperature) with the crystallization cycle are measured

(wt.-% SF, wt.-% C2, IV).

Intrinsic viscosity

The intrinsic viscosity (iV) is measured according to DIN ISO 1628/1, October 1999, in Decalin at 135 °C.

Melt Flow Rate

The melt flow rate (MFR) is determined according to ISO 1133 and is indicated in g/10 min. The MFR is an indication of the flowability, and hence the processability, of the polymer. The higher the melt flow rate, the lower the viscosity of the polymer. The MFR2 of polypropylene is determined at a temperature of 230 °C and a load of 2.16 kg.

Density:

The density is measured according to ISO 1183-187. Sample preparation is done by compression moulding in accordance with ISO 1872-2:2007.

The xylene soluble fraction at room temperature (XCS, wt.-%): The amount of the polymer soluble in xylene is determined at 25 °C according to ISO 16152; 5^th edition; 2005- 07-01.

Vicat softening temperature

The Vicat softening temperature was measured according to ASTM D 1525 method A (50°C/h, 10N).

DSC analysis, melting temperature (T_m) and heat of fusion (Hf), crystallization temperature (T_c) and heat of crystallization (H_c): measured with a TA Instrument Q200 differential scanning calorimetry (DSC) on 5 to 7 mg samples. DSC is run according to ISO 11357 / part 3 /method C2 in a heat / cool / heat cycle with a scan rate of 10 °C/min in the temperature range of -30 to +225 °C. Crystallization temperature (T_c) and crystallization enthalpy (H_c) are determined from the cooling step, while melting temperature (T_m) and melting enthalpy (H_m) are determined from the second heating step.

Flexural Modulus

Flexural Modulus is determined according to ISO 178 method A (3-point bending test) on 80 mm x 10 mm x 4 mm specimens. Following the standard, a test speed of 2 mm/min and a span length of 16 times the thickness was used. The testing temperature was 23±2 ° C. Injection moulding was carried out according to ISO 19069-2 using a melt temperature of 230 °C for all materials irrespective of material melt flow rate.

Impact Strength

The Charpy notched impact strength (NIS) is measured according to ISO 179 leA at +23 °C or -20 °C, using injection moulded bar test specimens of 80x 10x4 mm³ prepared in accordance with ISO 19069-2 using a melt temperature of 230 °C for all materials irrespective of material melt flow rate.

Tensile properties of films

Tensile properties (Elongation at Break) in machine and transverse direction were determined according to ISO 527-3 at 23 °C on the monolayer films produced in the experimental section. Testing was performed at a cross-head speed of 1 mm/min.

Optical properties

The total luminous transmittance, diffuse luminous transmittance and haze were measured according to ASTM DI 003 -21 (Method A-Hazemeter). The clarity is measured using the same machine and principle as haze but for angle less than 2.5° from normal. For clarity measurements, the specimens are positioned in the “clarity-port“. The measurement was performed as follows:

• Device: Haze gard plus

• Manufacturer: BYK-Gardner GmbH

• Type: 4725

• Illuminant C Conditions:

• Conditioning time: > 96 h

• Temperature: 23 °C

• Test procedure: A - Hazemeter

Oven aging

The monolayer films produced in the experimental section were heated at 120°C for the specified amount of time. No protective atmosphere was employed, the samples simply being exposed to the air present in the oven during the test.

UV aging

UV aging was carried out according to IEC 62788-2-2, conditions A5.

Specimens are mounted in aluminium frames such that the surface faces a xenon arc and are rotated around the irradiation source. The rotation and the air-flow is put such that the defined reference black standard (defined black surface without enhanced heat dissipation) or black panel (defined black surface with enhanced heat dissipation by metal back surface) is at a defined temperature.

A5 conditions:

Irradiation energy :

82 W/m² (300-400nm)

0.8 W/m³ (340 nm)

Temperature: 110 °C

Humidity: 20 %

Filter: Quarz

The irradiation of the xenon arc is filtered by a suprax filter as to accurately reflect spectral power distribution of sunlight especially in the ultraviolet region of radiation. The humidity of the environment is defined. Conditioning is performed for a defined time interval without any testing. Short circuit current (Isc)

Flash test Method

Current- voltage (IV) characteristics of the 1-cell glass-backsheet modules were obtained using a HALM cetisPV-Celltest3 flash tester. Prior to the measurements, the system was calibrated using a reference cell with known IV response. The 1-cell modules were flashed using a 30 ms light pulse from a xenon source. All results from the IV-measurements were automatically converted to standard test conditions (STC) at 25°C by the software PV Control, available from HALM. Every sample setup was flashed three times, on both the glass side and the backsheet side of the module, and given IV parameters are calculated average values of these three individual measurements. All modules were flash tested with a black mask.

The black mask was made out of standard black coloured paper and had a square-shaped opening of 163* 163 mm. During flash test, the black mask was positioned in such way that the solar cell in the solar module was totally exposed to the flash pulse, and that there was 2 mm gap between the solar cell edges and the black mask. All IV characterisation was done in accordance with the IEC 60904 standard.

2. Examples

2.1 Compounding of polypropylene compositions

The inventive and comparative compositions were prepared in a co-rotating twin-screw extruder (Coperion ZSK 57 for IE1 to IE4 and CE1 or Coperion ZSK 40 for IE5, IE6, CE2, and CE3) at 220 °C according to the recipes in Table 1.

Table 1 Recipes of Inventive and Comparative Examples

The titanium dioxide-containing examples consequently have the following titanium dioxide contents:

IE1 0.007 wt.-%

IE2 0.018 wt.-%

IE3 0.035 wt.-%

IE4 0.053 wt.-%

CE1 0.070 wt.-%

IE5 0.035 wt.-%

IE6 0.035 wt.-%

The following components were employed in Table 1 :

PP Ziegler-Natta catalysed heterophasic propylene copolymer composition with a propylene homopolymer matrix phase having a MFR₂ of 2.5 g/ 10 min and an ethylene-propylene elastomeric phase in an amount of 14 wt.- % (measured as XCS content), and an ethylene content in the elastomeric phase of 37 wt.-%. The heterophasic propylene copolymer composition has a MFR₂ of 3.6 g/10 min, atotal ethylene content of 4.2 wt.-%, a melting temperature of 168 °C, a Vicat A softening temperature of 153°C, a flexural modulus of 1400 MPa and a Charpy notched impact strength at 23 °C of 45 kJ/m² and has been produced using the process as described for the heterophasic propylene copolymer Inv. 2 in the example section of WO 2015/173175. Contains an additive package of 1500 ppm ADK- STAB A-612 (supplied by Adeka Corporation) and 300 ppm Synthetic hydrotalcite (ADK STAB HT supplied by Adeka Corporation).

TiCF MB A titanium dioxide masterbatch composition, containing 7.0 wt.-% of titanium dioxide, produced by blending 10 wt.-% of Polywhite P 8377 CL (a titanium dioxide masterbatch containing 70 wt.-% of titanium dioxide, commercially available from LyondellBasell (Italy)) with 90 wt.-% of a propylene ethylene random copolymer having an MFR2 of 2.0 g/10 min, a total ethylene content of 1.5 wt%, a xylene cold solubles (XCS) content of 0.7 wt%,a melting temperature of 142°C, a Vicat A softening temperature of 133°C, a flexural modulus of 1050 MPa and a Charpy notched impact strength at 23°C of 4.7 kJ/m², which has been produced using the process as described for the random polypropylene copolymer Poly 1 in PCT/EP2023/058435

HALS Hindered amine light stabilizer (CAS No. 192268-64-7, commercially available from BASF AG (Germany) under the trade name Chimassorb 2020).

UV1 UV stabilizer (CAS No. 4221-80-1, commercially available from BASF

AG (Germany) under the trade name Tinuvin 120).

UV2 UV stabilizer (CAS No. 70321-86-7, commercially available from BASF

AG (Germany) under the trade name Tinuvin 234).

A An additive package containing 1200 ppm of an antioxidant blend, commercially available from BASF AG (Germany) under the trade name Irgastab FS301 FF, 1500 ppm of a hindered light stabilizer (CAS No. 192268-64-7), commercially available from BASF AG (Germany) under the trade name Chimassorb 2020, 5000 ppm of a hindered light stabilizer (CAS No. 65447-77-0), commercially available from BASF AG (Germany) under the trade name Tinuvin 622 SF, and 500 ppm of calcium stearate (CAS No. 1591-23-0), commercially available from Baerlocher (Germany).

Monolayer films having a thickness of 400 pm were produced on a Collin line with calendaring on both sides, chill roll temperature 25 °C.

2.2 Evaluation of titanium dioxide without UV stabilizers

The UV-protection performance of the titanium dioxide additive was evaluated by comparing the elongation at break in the machine direction of the monolayer films before UV aging and after 1000 hours of UV aging as described in the determination methods section.

Table 2 UV-protection afforded by titanium dioxide alone

*n/a - not applicable, since sample was destroyed after 1000 h of UV aging, preventing a measurement from being carried out.

As can be seen from the data presented in Table 2, the addition of titanium dioxide to polypropylene films results in improved UV protection, which improves with increasing titanium dioxide content.

Furthermore, the optical properties of the titanium -dioxide containing films were evaluated, with the results presented in Table 3. Table 3 Optical properties of the titanium dioxide-containing examples

As can be seen, the clarity of the inventive examples is increased relative to the titanium dioxide-free PP, with the effect decreasing with increasing titanium dioxide content. Diffuse transmittance increases with increasing titanium dioxide content, as does haze, whilst the total luminous transmittance decreases with increasing titanium dioxide content.

Finally, the titanium dioxide-containing films were employed as protective back layer elements in photovoltaic modules, produced according to the following procedure:

The cell used was a mono crystalline bifacial cell with five bus-bars and having a dimension of 158.75x158.75x0.2 mm and with a cell efficiency of 22.6%. The cell was supplied by Lightway with a part number of 158B5M-BiFi-2260A-0220315.

All modules laminated were of type glass/encapsulant/cell with connectors/encapsulant/TBS (Transparent BackSheet). The encapsulant, both front and back, is a monolayer film made of EVA F406P from supplier Hangzhou (ethyl vinylacetate with 28% vinylacetate and MFR₂ = ca. 35 g/10 min).

For the PV modules comprising the layer elements as described above as transparent backsheet elements, 300 mm x 200 mm laminates consisting of Glass/Encapsulant/Cell with connectors/Encapsulant/Layer element as described above were prepared using a PEnergy L036LAB vacuum laminator. Glass layer, structured solar glass, low iron glass, supplied by InterFloat, length: 300 mm and width: 200 mm, total thickness of 3.2 mm. The same type of structured solar glass having a thickness of 3.2 mm was used for all modules.

The front protective glass element was cleaned with isopropanol before putting the first encapsulation layer element film on the solar glass. The front and back encapsulation layer element was cut in the same dimension as the solar glass element. After, the front encapsulation layer element was put on the front protective glass element. Then the soldered solar cell was put on the front encapsulation layer element. Further the back encapsulation layer element was put on the obtained PV cell element. The layer element of the invention was cut in the same dimension as the solar glass element. Further the layer element of the invention was put on top of the back encapsulation layer element.

No soldering wire was used during soldering. During soldering a soldering flux “952-S Soldering Flux”, product code 4060041, supplied by Kester was used.

The obtained PV module assembly was then subjected to a lamination process as described below in Table 4.

Table 4 Lamination settings (time profile) for photovoltaic module production

* Step 1 = heating step, Step 2 = evacuation step, Step 3 = pressure build up step, Step 4= pressure holding step.

The vacuum lamination occurred at 150°C using a lamination program of 5 minutes evacuation time (i.e. Step 2), followed by 10 minutes pressing time with an upper chamber pressure of 800 mbar (i.e. Step 4).

The effect of the titanium dioxide on the current generating capabilities of the PV modules was evaluated by exposing said PV modules to light from either the front or the rear side, with the effect characterized as the percentage change in short circuit current (Isc), relative to the neat cell (i.e. the photovoltaic element without any protective front layer element, front encapsulation layer element, rear encapsulation layer element or protective back layer element)

Table 5 Titanium dioxide -containing films as protective back layer elements

As can be seen from the data presented in Table 5, the addition of small amounts of titanium dioxide to the protective back layer element results in an increase in the Isc (front), due to reflectance/scattering of light that has passed through/past the photovoltaic element. The Isc (rear) decreases with increasing titanium dioxide content, due to the reduced total luminous transmittance of the protective back layer element.

As such, the inventive examples IE1 to IE4 offer the best balance between UV-protection (which improves with increased TiO? content) and Isc properties (which improve with decreased TiCfi content).

2.3 Combinations of titanium dioxide with UV stabilizers

The thermal-protection performance of the titanium dioxide additive (in combination with UV stabilizers) was evaluated by comparing the elongation at break in the machine direction before oven aging and after 2000 hours of oven aging as described in the determination methods section. Table 6 Thermal-protection afforded by titanium dioxide in combination with UV stabilizers

As can be seen from the data presented in Table 6, exceptional thermal protection can be achieved when titanium dioxide is combined with hindered amine light stabilizers (HALS) and UV stabilizers, with these values being considerably higher than for analogous compositions that are free from titanium dioxide (IE5 vs CE2 and IE6 vs CE3).

Claims

C L A I M S

1. A photovoltaic (PV) module, comprising, in the given order, a protective front layer element (1), a front encapsulation layer element (2), a photovoltaic element (3), a rear encapsulation layer element (4) and a protective back layer element (5), wherein the protective back layer element (5) comprises a polypropylene composition (PC) that comprises: a) 95.0 to 99.995 wt.-%, relative to the total weight of the polypropylene composition (PC), of a polypropylene or mixture of polypropylenes (PP); and b) 0.005 to 0.065 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P).

2. The photovoltaic (PV) module according to claim 1, wherein the one or more white pigments (P) are selected from inorganic white pigments, more preferably the one or more white pigments is/are titanium dioxide.

3. The photovoltaic (PV) module according to claim 1 or claim 2, wherein the polypropylene composition (PC) comprises: a) 96.0 to 99.990 wt.-%, more preferably 97.0 to 99.985 wt.-%, most preferably 98.0 to 99.980 wt.-%, relative to the total weight of the polypropylene composition (PC), of the polypropylene or mixture of polypropylenes (PP); and b) 0.010 to 0.060 wt.-%, more preferably 0.015 to 0.055 wt.-%, most preferably 0.020 to 0.050 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more white pigments (P).

4. The photovoltaic (PV) module according to any one of the preceding claims, wherein the polypropylene composition (PC) further comprises: c) 0.050 to 3.0 wt.-%, relative to the total weight of the polypropylene composition (PC), of one or more hindered amine light stabilizers (HALS) and/or one or more UV stabilizers (UV).

5. The photovoltaic (PV) module according to claim 4 wherein the remaining amount of the polypropylene composition (PC) that is not the polypropylene or mixture of polypropylenes (PP), the one or more white pigments (P), the one or more hindered amine light stabilizers (HALS) and/or one or more UV stabilizers (UV), is made up of one or more further additives (A) other than pigments, hindered amine light stabilizers and UV stabilizers, preferably selected from the group consisting of antioxidants, nucleating agents, clarifiers, brighteners, acid scavengers, slip agents, processing aids, release agents, and mixtures thereof.

6. The photovoltaic (PV) module according to any one of the preceding claims, wherein the polypropylene or mixture of polypropylenes (PP) is a heterophasic propylene -ethylene copolymer (HECO), comprising: a) a crystalline matrix (M) being a propylene homopolymer; and b) an amorphous propylene -ethylene elastomer (E) that is dispersed in said crystalline matrix (M).

7. The photovoltaic (PV) module according to any one of the preceding claims, wherein polypropylene or mixture of polypropylenes (PP), more preferably the heterophasic propylene-ethylene copolymer (HECO), has a melt flow rate (MFR2), determined according to ISO 1133 at 230 °C and 2.16 kg, in the range from 0.5 to 20.0 g/10 min, more preferably in the range from 1.0 to 15.0 g/10 min, most preferably in the range from 2.0 to 10.0 g/10 min.

8. The photovoltaic (PV) module according to any one of the preceding claims, wherein polypropylene or mixture of polypropylenes (PP), more preferably the heterophasic propylene-ethylene copolymer (HECO), has a xylene cold soluble (XCS) content, determined according to ISO 16152, in the range from 2.0 to 30.0 wt.-%, more preferably in the range from 5.0 to 25.0 wt.-%, most preferably in the range from 10.0 to 20.0 wt.-%.

9. The photovoltaic (PV) module according to any one of the preceding claims, wherein polypropylene or mixture of polypropylenes (PP), more preferably the heterophasic propylene-ethylene copolymer (HECO), has a melting temperature (Tm), determined by differential scanning calorimetry (DSC), in the range from 155 to 171 °C, more preferably in the range from 159 to 170 °C, most preferably in the range from 163 to 169 °C.

10. The photovoltaic (PV) module according to any one of the preceding claims, wherein polypropylene or mixture of polypropylenes (PP), more preferably the heterophasic propylene-ethylene copolymer (HECO), has an ethylene content (C2(total)), determined by FT-IR spectroscopy, calibrated using quantitative ¹³C- NMR spectroscopy, in the range from 0.5 to 20.0 wt.-%, more preferably in the range from 1.0 to 10.0 wt.-%, most preferably in the range from 2.0 to 5.0 wt.-%.

11 . The photovoltaic (PV) module according to any one of the preceding claims, wherein polypropylene or mixture of polypropylenes (PP), more preferably the heterophasic propylene-ethylene copolymer (HECO), has a Vicat softening temperature, determined according to ASTM D 1525 method A, in the range from 125 to 170 °C, more preferably in the range from 135 to 165 °C, most preferably in the range from 145 to 160 °C, and/or a flexural modulus, determined according to ISO 178, in the range from 1000 to 2500 MPa, more preferably in the range from 1200 to 2000 MPa, most preferably in the range from 1300 to 1600 MPa.

12. The photovoltaic (PV) module according to any one of the preceding claims, wherein the protective back layer element (5) has a total luminous transmittance, determined according to ASTM DI 003, in the range from 75 to 95%, more preferably in the range from 77 to 90%, most preferably in the range from 79 to 87%.

13. The photovoltaic (PV) module according to any one of the preceding claims, wherein the protective back layer element (5) has an elongation at break, determined according to ISO 527-3 after UV aging for 1000 h as described in the determination methods, of at least 20% of the elongation at break, determined according to ISO 527-3, before the UV aging and/or wherein the protective back layer element (5) has an elongation at break, determined according to ISO 527-3 after oven aging at 120 °C for 2000 h, that is at least 40% of the elongation at break, determined according to ISO 527-3, before the oven aging.

14. Use of titanium dioxide for maintaining elongation at break in the machine direction (MD), determined according to ISO 527-3 on monolayer film specimens having a thickness of 400 pm, of a polypropylene composition comprising said titanium dioxide after UV aging for 1000 h as described in the determination methods, wherein maintaining the elongation at break is achieved when the value of the elongation at break in the machine direction (MD) after the UV aging is at least 20% of the value determined before the UV aging, without reducing the total luminous transmittance, determined according to ASTM D1003, by more than 22%, relative to an analogous polypropylene composition that is free from titanium dioxide.

15. Use of titanium dioxide for maintaining elongation at break in the machine direction (MD), determined according to ISO 527-3 on monolayer film specimens having a thickness of 400 pm, of a polypropylene composition comprising said titanium dioxide after oven aging at 120 °C for 2000 h, wherein maintaining the elongation at break is achieved when the value of the elongation at break in the machine direction (MD) after the oven aging is at least 40% of the value determined before the oven aging, without reducing the total luminous transmittance, determined according to ASTM DI 003, by more than 22%, relative to an analogous polypropylene composition that is free from titanium dioxide.