Multilayer white polyester film method for manufacturing said film and use of this film as part of a back sheet for photovoltaic cells

Abstract

The invention concerns a multilayer biaxially oriented white polyester film (adhesion, absence of chalking, opacity whiteness, reflectance, hydrolysis resistance & light stability) comprising three polyester layers: a core layer and two outer layers and contains TiO.sub.2 particles. In this film: at least one layer comprises a PET whose: number average molecular weight is within [18500-40000]; intrinsic viscosity IV is 0.70 dL/g; and carboxyl group content is 30 eq/T. Additionally, the core layer comprises TiO.sub.2 particles in a range of [0.1-40]% w/w; the intrinsic viscosity IV is between [0.5-0.85] dL/g; a small endothermic peak temperature is between 180-230 C.; and at least one light stabilizer is added in at least one of the outer layers, in a total concentration between [0.1-35]% w/w. The invention also includes the method for manufacturing such film and the laminate which is part of the back sheet of a solar cell.

Claims

1. A multilayer biaxially oriented white polyester film comprising at least three polyester layers, respectively a core layer and two outer layers that is the same or different, and containing TiO.sub.2 particles, wherein: (i) at least one layer includes a PolyEthyleneTerephthalate (PET) whose: a. number average molecular weight comprises a range of 18500-40000; b. intrinsic viscosity IV is included within a range of 0.70 in dL/g; c. carboxyl group content comprises a range of 30 in eq/T; (ii) the core layer includes TiO.sub.2 particles at a concentration comprising a range of 0.1-40 in % w/w; (iii) an intrinsic viscosity IV of the film comprises a range of 0.5-0.85 in dl/g; (iv) a small endothermic peak temperature, Tmeta measured on the film comprises a range of 180230 C.; (v) at least one light stabilizer is added, in at least one of the outer layers, in a total concentration comprising a range of 0.1-35 in % w/w; (vi) at least one of the outer layers contains TiO.sub.2 particles in a concentration comprising a range of 0.01% w/w to <4% w/w.

2. The film according to claim 1, wherein a global planar SST modulus of the film (GPSM) comprises a range of 440-465 in kgf/mm.sup.2.

3. The film according to claim 1, wherein no chalking phenomenon represented by a powdering on the surface of the film, is observed before or after natural accelerated ageing.

4. The film according to claim 1, having a total thickness comprised within the a range of 10-500 in m.

5. The film according to claim 1, wherein the film comprises a flame retardant.

6. The film according to claim 1, wherein the film comprises a radical scavenger.

7. A method for manufacturing a multilayer biaxially oriented white polyester film for controlling the hydrolysis resistance of said polyester film according to claim 1, said method consisting of comprising: a. Synthesizing or implementing one or several different polyesters; b. Adding TiO.sub.2 and at least one light stabilizer in polyester(s); c. Optionally pre-drying the polyester(s); d. Heating the polyester(s) to melt it and make it malleable; e. Extruding the melted polyester(s) and processing into a multilayered film; f. Quenching and solidifying the multilayered film; g. Subjecting the multilayered film to biaxial stretching in the longitudinal and transverse directions at a given stretching temperature Ts; h. Heating the stretched film at a given heating temperature Th; wherein Ts, Th, or both Ts and Th are selected so that the endothermic peak temperature, Tmeta, be maintained below 240 C., in order to control the hydrolysis resistance of said polyester film.

8. The method according to claim 7, wherein in step b, TiO.sub.2 is added via PET-based master batches; TiO.sub.2 particles being preferably introduced from a white master batch WMB2 containing at least 40% of TiO.sub.2, said white master batch WMB2 being obtained by performing solid phase polymerization under vacuum and high temperature using a white master batch starting raw material WBM1.

9. A laminate comprising the film according to claim 1 and at least one adjacent film.

10. The laminate according to claim 9 wherein a peel strength at an interface between the film of claim 1 and said at least one adjacent film is greater than or equal to 5N.

11. A solar or photovoltaic cell or battery having a back sheet comprising the film according to claim 1.

12. The solar or photovoltaic cell or battery according to claim 11, wherein said back sheet is made by the process of claim 7.

13. A method for avoiding chalking or powdering phenomenon after ageing in a multilayer biaxially oriented white polyester film comprising at least three polyester layers, respectively a core layer and two outer layers that is the same or different, and containing TiO.sub.2 particles, wherein: (i) at least one layer includes a PolyEthyleneTerephthalate (PET) whose: a. number average molecular weight comprises a range of 18500-40000; b. intrinsic viscosity IV is included within a range of 0.70 in dL/g; c. carboxyl group content comprises a range of 30 in eq/T; (ii) the core layer includes TiO.sub.2 particles at a concentration comprising a range of 0.1-40 in % w/w; (iii) an intrinsic viscosity IV of the film comprises a range of 0.5-0.85 in dl/g; (iv) a small endothermic peak temperature, Tmeta measured on the film comprises a range of 180230 C.; (v) at least one light stabilizer is added, in at least one of the outer layers, in a total concentration comprising a range of 0.1-35 in % w/w; (vi) at least one of the outer layers contains TiO.sub.2 particles in a concentration comprising a range of 0.01% w/w to <4% w/w.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional view illustrating a photovoltaic battery whose backsheet includes the multilayered white polyester film according to the invention.

(2) FIG. 2 is a cross-sectional view illustrating an example of the multilayered white polyester film according to the invention.

(3) FIG. 3 illustrates the Beer-Lambert law where I.sub.0 is the light energy of monochromatic incident source; I is the light energy transmitted through the material; l is optical length (m); c is the concentration of absorbing species (mole/L); c is the molar extinction coefficient of absorbing species (L/mole*m); and (log I/I.sub.0) is absorbance or optical density.

DETAILED DESCRIPTION OF THE INVENTION

(4) It should be noted that in this text, every singular shall be construed as a plural and vice versa.

(5) The Multilayered White Polyester Film

(6) As an example, the multilayered white polyester film according to the invention can be part of the back sheet of a photovoltaic battery. A photovoltaic battery (or solar cell) is a system that converts sunlight into electricity. Preferably, the structure of the photovoltaic battery is based on a structure of a high light transmission material, a photovoltaic battery module, a filled resin layer and a backside protection laminate as in the structure shown in FIG. 1. The photovoltaic battery can be mounted onto domestic or industrial building roof tops, or used in solar farms or used for electric/electronic parts.

(7) The photovoltaic battery shown in FIG. 1 comprises a total light transmission material 1 (for instance glass), a photovoltaic battery cell 2, a filled resin 3 (also called encapsulant), a photovoltaic battery backside protection laminate 4 and a lead wire 5.

(8) The film of the invention, part of the photovoltaic battery backside protection laminate 4 is shown on FIG. 2. This is a three layer polyester film made preferably of PET (outer layers 4.1 & 4.3) and PET-TiO.sub.2 (core layer 4.2), which possesses the above mentioned features (i) to (vi).

(9) The present invention encompasses laminates, notably the backsheet laminates for photovoltaic batteries. The laminates according to the invention have a peel strength (90) greater than or equal to 5N, which corresponds to improved adhesion properties at the interface between film of the present invention and other films of the back sheet laminate.

(10) Said peel strength (90) is measured according to the test hereinafter described.

(11) Polyester Resins

(12) The polyester layers of the films according to the invention are for example essentially linear aromatic polyester, obtained from an aromatic dibasic acid or from an ester derived from said acid, and from a diol or from an ester derived from said diol.

(13) The polyester constituting each layer of the multilayer film can be selected from the polyesters that are usually used in order to obtain biaxially oriented semi-crystalline films. They are film-forming linear polyesters, crystallisable by orientation and obtained in the usual way from one or more aromatic dicarboxylic acids or derivatives thereof (esters of lower aliphatic alcohols or halides for example) and one or more aliphatic diols (glycols).

(14) As examples of aromatic acids, there may be mentioned the phthalic, terephthalic, isophthalic, 2,5-naphthalenedicarboxylic, and 2,6-naphthalenedicarboxylic acids. These acids can be combined with a smaller quantity of one or more aliphatic or cycloaliphatic dicarboxylic acids, such as adipic, azelaic, tetra- or hexahydroterephthalic acids.

(15) As non-limitative examples of aliphatic diols, there may be mentioned ethylene glycol, 1,3-propanediol, 1,4-butanediol, cycloaliphatic diols (cyclohexanedimethanol), neopentylglycol. These diols can be used separately or as a combined mixture thereof.

(16) Preferably, the film-forming crystallisable polyesters are polyterephthalates or alkylenediol polynaphthalenedicarboxylates and, in particular, polyethylene terephthalate (PET) or 1,4-butanediol or copolyesters having at least 80 mol. % of ethylene glycol terephthalate units.

(17) The biaxially oriented multilayer polyester films are, for example, containing: either polyethylene terephthalate, or mixtures, or not, of polyethylene terephthalate copolyesters containing cyclohexyl dimethylol units instead of the ethylene units (see patent U.S. Pat. No. 4,041,206 or EP-A-0408042), or mixtures, or not, of polyethylene terephthalate copolyesters with a polyester portion having isophthalate units (see patent EP-B-0515096),
or are constituted by several layers of polyesters of different chemical natures, as described previously, obtained by coextrusion.

(18) Specific examples of aromatic polyesters are in particular polyethylene terephthalate, polyethylene isophthalate, polybutylene terephthalate, poly-(dimethyl-1,4-cyclohexylene terephthalate) and polyethylene-2,6-naphthalenedicarboxylate. The aromatic polyester can be a copolymer of these polymers or a mixture of these polymers with a small quantity of other resins. Among these polyesters, polyethylene terephthalate (PET) and polyethylene-2,6-naphthalenedicarboxylate (PEN) and polybutylene terephthalate (PBT) are particularly preferred as they offer a good balance between physical properties, mechanical properties and optical properties.

(19) Advantageously, the polyethylene terephthalate resin with improved hydrolysis resistance (HRPET) which is used for the present invention presents a high intrinsic viscosity (IV>0.78 dL/g) and a low carboxylic groups content ([COOH]<15 eq/T). The intrinsic viscosity is directly related to the molecular chains length. The carboxylic groups content is representative of polyethylene terephthlate chain ends. The lower the carboxylic groups content is, the longer the molecular chains are. A low carboxylic groups content in combination with a high intrinsic viscosity give to the biaxially oriented film an excellent hydrolysis resistance in comparison with the one that can be achieved with standard polyethylene terephthalate polymers for the production of biaxially oriented PET films (0.62<standard IV<0.70 dL/g and 25<standard [COOH]<40 eq/T). The above described polyester resin has preferably a number average molecular weight of 18500 to 40000.

(20) White Pigments

(21) Suitable white pigments are especially titanium dioxide, barium sulphate, calcium carbonate or incompatible polymers such as polypropylene, polyethylene or cycloolefin copolymers (COC), or combinations thereof. Particular preference is given to titanium dioxide (TiO.sub.2). Preferably, the TiO.sub.2 particles which are used in the core layer of the present invention are composed of the rutile crystalline form. The TiO.sub.2 particles are obtained by the chloride process and they have a surface post treatment (inorganic and/or organic) to reduce photocatalytic effect and to improve dispersability. More preferably, the TiO2 particles which are used in the film of the invention can be Sachtleben RD3 from Sachtleben, Tronox CR-826 from Tronox, Kronos 2220 from Kronos.

(22) The titanium dioxide particles can be introduced in the film by any conventional method. For example, the introduction can be done during the esterification stage of the polymerisation following which a polymer condensation reaction can be performed.

(23) Advantageously, TiO.sub.2 particles are introduced from a white master batch (WMB2) which is obtained by performing solid phase polymerization under vacuum and high temperature using white master batch WMB1 as the starting raw material. WMB1 is obtained by melting and mixing 40% weight parts of a high IV polyethylene terephthalate (IV>0.80 dL/g) to 60% weight part of titanium dioxide (TiO.sub.2) particles. The white master batch WMB1 can also contain an antioxidant (so-called radical scavenger or thermal stabilizer) from the phenolic family (Irganox 1010 from BASF with RNCas.6683-19-8) in order to prevent the PET resin from thermo-oxydative degradation. The final intrinsic viscosity of WMB2 is above 0.80 dL/g and the concentration of carboxylic groups lower than 35 eq/T.

(24) In another variant, the radical scavenger is incorporated for instance through an additional master batch.

(25) Light Stabilizer

(26) According to the invention, at least one of the layers of the film, more preferably the outer layers of the film include preferably at least one light stabilizer in above mentioned preferred concentrations. Light stabilizers are used for countering the effects of light (UV) and oxygen on the polyester film.

(27) The light stabilizers can be selected from several groups of known products such as those described in the work Additives for plastics on book, John Murphy, 2.sup.nd Edition 2001, Elsevier Advanced Technology and more specifically for PET films such as those described in patent U.S. Pat. No. 6,593,406.

(28) As examples of light stabilizers described in patent WO 2009/083552 A1, there may be mentioned: the family of antioxidants or UV-absorbers such as benzophenones, benzotriazoles, benzoxazinones and triazines; the family of Hindered Amine Light Stabilizers (HALS), alone or in combination with antioxidants.
The polyester film will preferably comprise at least one light stabilizer additive, and even more preferably an UV-absorber of the triazine type with two phenyl or two biphenyl groups (Tinuvin1577 from BASF with RNCas 147315-50-2, CyasoreUV-1164 from Cytec, CGX-006 from BASF).

(29) The light stabilizer can be advantageously introduced in the film from a Master Batch (UVMB), which is obtained by melting and mixing 85% weight parts of a super high IV polyethylene terephthalate (IV>1 dL/g) to 15% weight part of a light stabilizer in order to maintain a final IV above 0.80 dL/g and a concentration of carboxylic groups lower than 28 eq/T.

(30) Other Additives

(31) Some fillers other than TiO.sub.2 can be included in the polyester in order to modify its properties. These fillers can be included alone or as mixtures in the film. As examples of commonly known fillers for polyester films, there is calcium carbonate, calcium oxide, aluminium oxide, kaolin, silica, zinc oxide, carbon black, silicon carbide, tin oxide, particles of cross-linked acrylic resin, particles of cross-linked polystyrene resin, particles of cross-linked melamine resin, particles of cross-linked silicone resin or similar may be mentioned. Fillers of the silica and/or carbonate type are preferably used.

(32) Moreover, if necessary, the polyester film can further contain at least one other additive, preferably selected from the following group: radical scavenger, flame retardant, dye, antistatic agent, antioxidant, organic lubricant, catalyst or any other similar additive.

(33) Surface Treatment

(34) The film of the invention can have a surface treatment on at least one side, to improve adhesion, antistatic performance, slip and winding performances and/or processing performances. The surface treatment can be a physical surface treatment (for example UV, corona treatment under ambient air or gases, vacuum evaporation, plasma treatment or physicochemical vacuum deposition) or a chemical surface treatment (for example coating of acrylic, copolyester, polyester or polyurethane based formulations). The chemical surface treatment can be obtained by coextrusion, extrusion coating, in-line coating done prior to transverse stretching during the film making process or off-line coating.

(35) The Method for Manufacturing a Biaxially Oriented Polyester Film

(36) The aromatic polyester film can be obtained by melt extrusion through a slot die, which produces an aromatic polyester molten web, which is then subsequently cooled and solidified on a forming drum to obtain an unstretched film. The unstretched film is then stretched at a temperature between the glass transition temperature Tg and a temperature equal to Tg+60 C. in the longitudinal direction, one or more times (for example from 3 to 6 times), then the film is stretched at a temperature between Tg and Tg+60 C. in the transverse direction with a stretching ratio of 3 to 5 times. The biaxially stretched film is then heat-treated at a temperature between 180 and 250 C. for 1 to 60 seconds for example and then at a lower temperature in order to stabilize the film.

(37) The polyester film can be of simple structure or coextruded ABA or ABC (with the symbols A, B and C corresponding to layers with different or same nature and/or composition).

(38) The method for manufacturing a multilayer biaxially oriented white polyester film, notably the above mentioned one according to the invention is also a method for controlling the hydrolysis resistance of said polyester film. Said method consists essentially of the steps a,b,c,d,e,f. The selection of stretching temperature Ts and/or heating temperature Th so that a small endothermic peak temperature, Tmeta, be maintained at inventive values, in order to control the hydrolysis resistance of said polyester film, is particularly remarkable. It makes it possible to reach the outstanding properties in terms of % retention elongation MD after 48 h of pressure cooker test notably.

(39) An example of a method for producing the PET multilayer white film for photovoltaic batteries of the present invention can be described as follows:

(40) Step a

(41) For example, terephthalic acid or derivative thereof and ethylene glycol are subjected to ester interchange reaction by a well-known method. Examples of a reaction catalyst include alkali metal compounds, alkaline earth metal compounds, zinc compounds, lead compounds, manganese compounds, cobalt compounds, aluminium compounds, antimony compounds, and titanium compounds. Examples of a colouring agent include phosphorus compounds. It is preferable that an antimony compound or a germanium compound and a titanium compound be added as polymerization catalysts. For such a method, in the case of adding, for example, a germanium compound, it is preferable to add germanium compound powders.

(42) A preferable example of the method for controlling the number average molecular weight of the polyester resin of the present invention to be 18500 to 40000 includes a so-called method of solid phase polymerization which comprises the steps of polymerizing a usual polyester resin having a number average molecular weight of 18000 by the above-described method and heating at a temperature ranging from 190 C. to a temperature being less than a melting point of polyester resin under reduced pressure or circulation of inert gas such as nitrogen gas. The method can increase the number average molecular weight without increasing the amount of the terminal carboxyl group of polyester resin.

(43) Step b

(44) TiO.sub.2 and additives such as light stabilizer are preferably added in polyester(s), via PET-based master batches with high viscosity. Advantageously, TiO.sub.2 particles are introduced from a white master batch (WMB2) which is obtained by performing solid phase polymerization under vacuum and high temperature using white master batch WMB1 as the starting raw material.

(45) WMB1 contains preferably at least 40% by weight of TiO.sub.2 particles.

(46) WMB2 has a final IV above 0.80 dL/g and a concentration of carboxylic groups lower than 35 eq/T.

(47) The light stabilizer can be advantageously introduced in the film from a Master Batch (UVMB), with a final IV above 0.80 dL/g and a concentration of carboxylic groups lower than 28 eq/T.

(48) Step c & d

(49) Subsequently, when the PET multilayer film for photovoltaic batteries is formed from the previously described polymer, the method can include the step c of drying the polymer to a pre-determined moisture content limit, if necessary (notably when single screw extruders are used), and the step d of making a multilayer film with the polyester resin delivered from different passages in two or more extruders with the use of a static mixer or similar device in each channel leading to a feed-block or a multimanifold die, or the like. Alternatively, these steps c & d may be combined.

(50) Steps e & f

(51) The multilayer sheet that is discharged from a slot die is extruded onto a cooling body such as a casting drum, which is then cooled and solidified to form a casting sheet. In this case, it is preferable that the sheet be adhered tightly to the cooling body such as a casting drum by an electrostatic force by using an electrode with shapes such as a wire-shape, a tape-shape, a needle-shape, or a knife edge-shape, which is then quenched and solidified.

(52) In another variant, the sheet can be adhered tightly to the cooling body by other well known means, such as an air blade, water meniscus, suction box, glycol moisture, and all methods well-known from the art. Alternatively, all these methods may be optionally combined.

(53) Step g

(54) The casting film thus obtained may be optionally subjected to biaxial stretching. The term biaxial stretching means stretching in the longitudinal and transverse directions. The stretching may be sequential biaxial stretching, whatever is the order of the sequences, or simultaneous biaxial stretching. Further, the re-stretching may be performed in a longitudinal and/or transverse direction.

(55) The term stretching in the longitudinal direction means stretching to produce a molecular orientation in the longitudinal direction of the film. It is usually given by a roll peripheral speed difference. The stretching may be performed in one step. Alternatively, the stretching may be performed at multiple stages with a plurality of roll pairs. The stretching ratio depends on the type of resin. The method of stretching can be a combination of contact roll heating and other means of heating, like, but, without limitation, radiation heaters of short or medium wavelength, heating tapes, hot air blown from nozzles, and, in a non limitative way can follow all the possibilities of the previous known art. Alternatively, all these methods may be optionally combined. Usually, the combined stretch ratio (longitudinal ratiotransverse ratio) is preferably 2 to 25 times. For example, for PET, the stretching ratio in the longitudinal direction is preferably 2 to 6 times.

(56) If the biaxial stretching of the film is generally carried out sequentially with the first orientation longitudinally, it is also possible to first orient transversally then longitudinally, or to use simultaneous stretching.

(57) Then, in order to stretch the film in the transverse direction, the film is passed through a diverging rail set to the stretching ratio of 2 to 5 times. The clips hold the edges of the film, and stretched in the transverse direction (machine cross-direction). The stretching temperature Ts of the film is 85 to 140 C., and the film is stretched in the transverse direction.

(58) Step h

(59) In a subsequent step, heat treatment is performed at Th of 180 to 245 C. in order to obtain dimensional stability, and the polyester resin sheet for photovoltaic batteries of the present invention is produced.

(60) Examples

(61) The invention will be better understood and its advantages will become apparent from the examples given below. Hereinafter, physical properties, evaluation method thereof, and criteria of evaluation which are used in the present invention will be described.

(62) Physical Properties of Polymer and Film

(63) Thickness

(64) The preferred total thickness has been given above. The Beer-Lambert law describes that the capacity to absorb UV radiation is proportional to the sample thickness and to the concentration of the UV absorber which is present in the sample
Log (I/I.sub.0)=.Math.l.Math.c
(I/I.sub.0)=e.sup..Math.l.Math.c With: Io=Light energy of momochromatic incident source I=Light energy transmitted through the material l=optical length (m) c=concentration of absorbing species (mole/L) =Molar extinction coefficient of absorbing species (L/mole-m) (Log I/Io) is also called Absorbance or optical density
Mechanical Properties

(65) In this invention it is an advantage that the said preferably biaxially stretched polyester white film has a high planar orientation. A suitable non-destructive method for the characterisation of such a property is the determination of the speed of propagation of ultrasound waves within the plane of the said film and measured successively in all directions to characterise the global planar orientation by calculating the Global Planar Sonic sheet tester (SST) Modulus of the film (GPSM).

(66) The speed of the supersonic pulse is measured in a given direction over a given distance following the standard ASTM F89-68 for the determination of the Sonic Modulus of a flexible barrier material by Sonic Method, using an SST-250 Sonic sheet tester manufactured by Nomura Shoji. Co. ltd, Japan. A modulus is calculated using an empirical correlation:
E=92*v.sup.2 With: E=modulus in kgf/mm.sup.2 v=speed of ultrasound propagation in sample in km/s
By rotating the sample progressively by 5 angle increments from 0 (MD) to 175 (TD+85), a global planar SST modulus (GPSM) can be calculated from the average of the 36 individual measurements within the plane.

(67) Number Average Molecular Weight

(68) Molecular weight calibration is performed using polystyrene (standard reference). Measurement on the polyester resin film for back sheet laminates of photovoltaic modules is made at room temperature (23 C.) with 244 type gel permeation chromatograph GCP-244 (manufactured by WATERS), two columns: Shodex K 80M (manufactured by Showa Denko K.K.) and one column: TSKGEL-G2000Hx1 (manufactured by TOSOH CORPORATION).

(69) Coefficient (A1) of the third approximation is calculated using elution volume (V) and molecular weight (M) and calibration curves are established:
log(M)=A.sub.0+A.sub.1*V+A.sub.2*V.sup.2+A.sub.3*V.sup.3
After calibration, a sample of the polyester resin film for back sheet laminate of photovoltaic modules is dissolved in a solvent of orthochlorophenol/chloroform (volume ratio ) so as to be 0.2% (wt/vol). The quantity injected to the chromatograph is 0.400 mL and the injection is performed at a flow rate of 0.8 mL/min. As a detector, R-401 differential-refractive index detector (WATERS) is used, and the number average molecular weight is calculated by the following equation:
Mn=sum(Ni*Mi)/sum(Ni)
Molar fraction Ni and molecular weight Mi corresponding to each retention volume Vi.

(70) Intrinsic Viscosity

(71) A sample solution is obtained by dissolution of a given quantity of the sample (polymer or film) at least at 120 C. for 30 min in 100 mL of a solvent mixture of 1,2-dichlorobenzene/phenol 50/50. After cooling down, the elution time of the sample solution is measured with an Ubbelhode viscosimeter. The intrinsic viscosity value IV of the sample is calculated according to the standard ISO 1628/5 using the following correlations.

(72) The viscosity of the pure solvent mixture .sub.0 is compared to the viscosity of the sample solution . The relative viscosity .sub.r is given by:
.sub.r=/.sub.0=t*/t.sub.0*.sub.0 With: t.sub.0 and .sub.0 are the elution time and density of the solvent mixture t and are the elution time and density of the sample solution Since .sub.0 in our case of study, the following equation for specific viscosity .sub.sp is then obtained: .sub.sp=.sub.r1=(tt.sub.0)/t.sub.0 The correlation between .sub.sp and intrinsic viscosity IV is given by:
(.sub.sp/C)=IV+k*IV.sup.2*C with: (.sub.sp/C) viscosity number. C: concentration of polymer in solution k: constant
The intrinsic viscosity IV can be determined experimentally by measuring the viscosity number (.sub.sp/C) as function of concentration C as the Y-axis intercept.

(73) COOH Content

(74) Concentration fo terminal carboxyl groups is measured by the so-called titration method. Specifically, the sample is dissolved at 100 C. for 30 min of 0.5 g of sample in 10 g of a solvent mixture o-cresol/H.sub.2O 17/1. After dissolution and cooling, add 3 mL of CH.sub.2Cl.sub.2. Titration is performed with a KOH 0.02N solution.

(75) A blank sample of solvent mixture o-cresol/H.sub.2O 17/1 is titrated by the same method.

(76) The carboxyl group content [COOH] is calculated by the following equation:
(VVo)t/P Vo (L) KOH volume used for titration of blank sample P (g): sample weight V (L): KOH volume used for titration of sample t: KOH solution concentration (0.02N or 0.02 mol/l)

(77) Thermal Properties

(78) The small endothermic peak temperature, Tmeta mentioned previously, is measured by DSC differential scanning calorimetry (DSC Diamond from Perkin-Elmer). This method is used to study thermal transitions of polymers. The polymer is heated up to 250 C. (10 C./min) under nitrogen flow.

(79) Criteria of Film Evaluation

(80) Hydrolysis Resistance

(81) To characterize the hydrolysis resistance, the polyester film is subjected to ageing in a Pressure Cooker Test (PCT) (SYSTEC VE-100) with the following conditions: temperature 125 C., relative humidity 100%, pressure 2.3 bars. The retention ratio of elongation to break is measured after 48 h of the ageing test. (cf. table of examples p. 17)

(82) Light Stability

(83) To characterize the light stability, the polyester film is subjected to an outdoor exposure acceleration test in a UVCON (Atlas) device with test conditions according to ISO 4892-3 Method A, cycle 1: Type of lamp: 1A (UVA340) (fluorescent lamp) Exposition cycle: After performing the ultraviolet irradiation (0.76 W.m-.sup.2.nm-1 at 340 nm) at a temperature of 60 C. (+1-3 C.) in dry conditions for 8 hours, aging for 4 hours is performed in a dew condensation state at temperature 50 C. (+/3 C.), in an atmosphere with relative humidity of 100% (light off). The exposition cycle is performed up to 1000 hours minimum (cf. table of examples p. 17)
After the ageing test, the retention ratio of elongation to break is evaluated and the delta b* is measured to characterise the light stability.

(84) Retention Ratio of Elongation

(85) The elongation to break (%) is measured according to ASTM D-882, with dynamometer INSTRON 5543 with detector of 1 kN. The samples are normalized 100*15 mm. The elongation to break without ageing is considered 100% and the retention ratio of the elongation to break without ageing to the elongation to break after ageing is calculated.

(86) The retention ratio is measured after 48 h of Pressure Cooker Test in SYSTEC VE-100 and after 1000 h of outdoor exposure acceleration test in UVCON.

(87) The retention ratio is determined by the following ranking: A: the retention ratio is 70% or more. Considered as good. B: the retention ratio is 50-70%. Considered as medium. C: the retention ratio is lower than 50%. Considered as bad.

(88) Delta b*

(89) After performing the outdoor exposure acceleration test, the coloration of the film is measured in terms of b* value by transmission measurement with MINOLTA CM-508d Spectrocolorimeter.

(90) Colour measurements are done in accordance with ISO 7724 (03/1988) (specular reflection includedunder D65 illuminant) and the difference between the b* value after the outdoor exposure acceleration test (1000 h in UVCON) and b* value before the outdoor exposure acceleration test is calculated:
Delta b*=b*after ageingb*before ageing and evaluated according to the following ranking: A: Delta b*<2.5. Significantly good light stability B: Delta b* comprised between 2.5 and 4. Improved light stability C: Delta b*>4. Poor light stability

(91) Total Light Transmittance

(92) The film according to the invention has special optical properties given by Total Light Transmittance TLT of the film. It is measured using a hazemeter BYK-Gardner HazeGuard Plus in accordance with ASTM D-1003.

(93) The targeted film aims to be white with sufficient opacity according to following TLT ranking: A: TLT is less than 40%. Considered as good. B: TLT is 40-60%. Considered as medium. C: TLT is more than 60%. Considered as bad.

(94) White Index

(95) The white index of the film is measured using a spectrophotometer Minolta CM508i in accordance with ASTM E-313.

(96) Evaluation is determined according to the following ranking: A: white index is more than 88. Considered as good. B: white index is 84-88. Considered as medium. C: white index is less than 84. Considered as bad.

(97) Relative Reflectance

(98) A spectrophotometer (U-3310, manufactured by Hitachi Ltd) is used. Alumina oxide is used as a standard white plate. At 560 nm, the angle of gradient of the opening portion of the specimen is 10. The diffuse reflectance is measured and designated as (T.sub.0). The reflectance of the standard plate is 100%. Then the opening portion of the specimen is replaced by the specimen and diffuse reflectance is measured at 560 nm.

(99) Then the diffuse reflectance is converted into the relative reflectance (R) by using the following equation:
R (%)=T.sub.1/T.sub.0*100

(100) T.sub.0: reflectance of a standard white plate and T.sub.1: reflectance of specimen.

(101) Evaluation is determined according to the following ranking: A: relative reflectance at 560 nm is more than 70%. Considered as good. B: relative reflectance at 560 nm is 40-70%. Considered as medium. C: relative reflectance at 560 nm is less than 40%. Considered as bad.

(102) Peel Strength

(103) The behaviour of film when used in a laminate was tested by means of an adhesion test. The film side intended to be dry laminated is corona treated before adhesive deposit (5 m). Urethane adhesive (Takerakku A-1106/Takeneto A-23, 6/1) is dried for 45 sec, 45 C. An ageing of 96 hours at 50 C. is performed on sample (15 mm wide).

(104) The peeling strength is measured at 200 mm/min, 90 peeling angle in the machine direction (MD).

(105) Evaluation is determined according to the following ranking: A: Peel strength >5 N: good adhesion B: Peel strength 4-5 N: medium adhesion C: Peel strength <4 N: bad adhesion

(106) Chalking

(107) The chalking phenomenon is represented by a powdering on the surface of the film that can occur before or after natural or accelerated ageing. A tape test is done to evaluate the chalking phenomenon. A transparent tape is positioned on surface of the film and removed. The presence or absence of powder on the tape gives an evaluation of the chalking phenomenon.

(108) Evaluation is determined according to the following ranking: A: No powder on tape. Film presenting no chalking is considered as good. C: Powder on tape. Film presenting chalking is considered as bad.

Examples

(109) One hundred parts by weight (hereinafter simply referred to as parts) of dimethyl terephtalate was mixed with 64 parts of ethylene glycol, to this, 0.1 parts of zinc acetate and 0.03 parts of antimony trioxide were added as catalysts. Ester interchange was performed with a circulation temperature of ethylene glycol. Trimethylphosphate 0.08 parts were added to the resulting product, which was gradually heated up and polymerized under reduced pressure at a temperature of 271 C. for 5 hours.

(110) The inherent viscosity of the obtained polyethylene terephthalate was 0.55. The polymer was cut into a chip with a cylindrical shape with a length 5.95-8.05 mm, width 3.20-4.80 mm, and height 1.70-2.30 mm. The specific gravity was 1.3 g/cm3. The PET was placed in a rotary vacuum dryer (high polymerization temperature: 190-230 C., degree of vacuum 0.5 mmHg) and heated while stirring for 10 to 23 hours. Thus a HRPET, polyethylene terephthalate polymer with high intrinsic viscosity (IV=0.82 dL/g), a low carboxylic groups content [COOH]=14 eq/T and a number average molecular >20000 was obtained.

(111) The aromatic polyester film is obtained by melt extrusion through a slot die, which produces an aromatic polyester molten web, which is then cooled and solidified on a forming drum to obtain an unstretched sheet. The sheet thus obtained by quenching-solidification is guided to a roll-group heated at 80-130 C., and is stretched 3-5 times in the longitudinal direction and then cooled by a roll-group of 21-25 C. Subsequently, the longitudinally stretched film is guided into a tenter while both edges are held by clips. Then, the film is stretched 3-4.5 times in the transverse direction at 100-135 C. Thereafter, the resulting film is subjected to heat fixing in the tenter and uniformly slowly cooled in order to be thermally stabilized. The film is cooled to room temperature and wound to obtain a roll of film having a thickness of 50 m.

(112) The films in the examples and comparative examples below are all mono- or multilayer biaxially oriented films of 50 m, produced on an extrusion line. Unless otherwise stated, the polyethylene terephthalate polymer from which the films were produced is an HRPET.

(113) The films were tested for TLT, white index, reflectance, adhesion, chalking, hydrolysis resistance and/or light resistance. Therefore, the films were possibly subjected to an outdoor exposure acceleration test in device UVCON (Atlas) with test conditions according to ISO 4892-3, and to ageing in a Pressure Cooker Test (PCT) (SYSTEC VE-100). Results are presented in Table 1.

Example 1

(114) Pellets of hydrolysis resistant polyethylene terephthalate (HRPET), pellets of white master batch (WMB2) and pellets of UV-absorber Master Batch (UVMB) were dried, melted and extruded through a slit die onto a rotating cooling drum to provide an unstretched 3-layer white film. The unstretched 3-layer white film was submitted to longitudinal and transversal stretching at a given stretching temperature Ts and finally exposed to a heating temperature Th for the purpose of thermal fixation and hydrolysis resistance control. The corresponding small endothermic peak Tmeta measured at 215 C.

(115) The White Master Batch (WMB2) is obtained by performing solid phase polymerization under vacuum and high temperature using WMB1 as the starting raw material which is obtained by melting and mixing 40% weight parts of a high IV polyethylene terephthalate to 60% weight part of titanium dioxide (TiO.sub.2) particle. The final intrinsic viscosity of WMB2 is above 0.80 dL/g and the concentration of carboxylic groups lower than 35 eq/T.

(116) The light stabilizer Master Batch (UVMB) is composed of 20% UV-absorber Tinuvin 1577 and 80% polyethylene terephthalate with an intrinsic viscosity above 0.80 dL/g and a concentration of carboxylic groups lower than 28 eq/T.

(117) The 3-layer white film, comprising 8% of rutile type titanium dioxide (Kronos 2220 from Kronos) and 1.5% of UV absorber (Tinuvin 1577 BASF), exhibits IV of 0.685 dL/g and GPSM of 451 kgf/mm.sup.2.

(118) This film is rated A for all performances presented in Table 1.

Comparative Example 2

(119) A method based on example 1 was used to produce a monolayer biaxially oriented white film of 50 m, comprising standard polyethylene terephthalate (IV=0.640 dL/g, high [COOH]=35 eq/T and Mn=19000) as main constituent and 5% titanium dioxide. The small endothermic peak Tmeta is measured at 216 C. The film exhibits IV of 0.615 dL/g and GPSM of 450 kgf/mm.sup.2. Results are presented in Table 1.

Comparative Example 3

(120) A method based on example 1 was used to produce a 3-layer biaxially oriented transparent film of thickness 50 m comprising as main constituent an hydrolysis resistant PET (HRPET) and 1.5% of UV absorber (Tinuvin 1577 BASF). The small endothermic peak Tmeta is measured at 214 C. The film exhibits IV of 0.695 dL/g and GPSM of 456 kgf/mm.sup.2. Results are presented in Table 1.

Comparative Example 4

(121) A method based on example 1 was used to produce a 3-layer biaxially oriented white film of thickness 50 mm comprising standard polyethylene terephthalate (IV=0.640 dL/g, high [COOH]=35 eq/T and Mn=19000) as main constituent and 14% of rutile-type titanium dioxide WMB2 in outer layers. The small endothermic peak Tmeta is measured at 213 C. The film exhibits IV of 0.618 dL/g and GPSM of 451 kgf/mm.sup.2. Results are presented in Table 1.

Comparative Example 5

(122) A method based on example 1 was used to produce a monolayer biaxially oriented transparent film of 50 m comprising polyethylene terephthalate (IV=0.705 dL/g, [COOH]=30 eq/T and Mn=21000) as main constituent. The small endothermic peak Tmeta is measured at 215 C. The film exhibits IV of 0.650 dL/g and GPSM of 452 kgf/mm.sup.2. Results are presented in Table 1.

Comparative Example 6

(123) A method based on example 1 was used to produce a monolayer biaxially oriented transparent film of 50 m comprising polyethylene terephthalate (IV=0.800 dL/g, [COOH]=40 eq/T and Mn=23000) as main constituent. The small endothermic peak Tmeta is measured at 214 C. The film exhibits IV of 0.740 dL/g and GPSM of 451 kgf/mm.sup.2. Results are presented in Table 1.

Comparative Example 7

(124) A method based on example 1 was used to produce a monolayer biaxially oriented transparent film of 50 m comprising HRPET as main constituent. The small endothermic peak Tmeta is measured at 215 C. The film exhibits IV of 0.698 dL/g and GPSM of 456 kgf/mm.sup.2. Results are presented in Table 1.

Comparative Example 8

(125) Like the film of example 7 except that the film was processed with Ts and Th suitably chosen to obtain a standard Tmeta of 225 C. The film exhibits IV of 0.698 dL/g and GPSM of 456 kgf/mm.sup.2. Results are presented in Table 1.

Comparative Example 9

(126) Like the film of example 7 but the film was processed with suitably chosen process parameter to obtain a low global planar SST modulus (GPSM) of 440. The small endothermic peak Tmeta is measured at 214 C. The film exhibits IV of 0.698 dL/g. Results are presented in Table 1.

(127) TABLE-US-00001 TABLE 1 Example Comparative examples 1 2 3 4 5 6 7 8 9 white index A A C A C C C C C TLT A A C A C C C C C relative reflectance A A C A C C C C C retention ratio elongation A A B C A B C MD (48 h PCT) retention ratio elongation A A C C C MD (1000 h UV ageing) delta b* (1000 h UV ageing) A C A B C C C peel strength A C A C A A A A A chalking A C A C A A A A A

(128) The invention will be further described by the following numbered paragraphs:

(129) 1A multilayer biaxially oriented white polyester film comprising at least three polyester layers, respectively a core layer and two outer layers that can be same or different, and containing TiO.sub.2 particles, wherein: (i) at least one layer includes a PolyEthyleneTerephthalate PET whose: number average molecular weight is comprised within the following ranges given herein in an increasing order of preference 18500-40000]; [19000-35000]; [20000-30000]; intrinsic viscosity IV is included within the following ranges given herein in an increasing order of preference and in dL/g: [0.70]; [0.75]; [0.78]; [0.78-1.20]; carboxyl group content is comprised between the following ranges given herein in an increasing order of preference and in eq/T: [30]; [25]; [21]; [15]; [5-15]; (ii) the core layer includes TiO.sub.2 particles in the following concentration ranges given herein in an increasing order of preference and in % w/w: [0.1-40]; [0.5-30]; [1-20]; [2-10]; (iii) the intrinsic viscosity IV of the film is comprised between the following ranges given herein in an increasing order of preference and in dl/g: [0.5-0.85]; [0.55-0.8]; [0.6-0.75]; [0.65-0.75]; (iv) a small endothermic peak temperature, Tmeta measured on the film is comprised between 180-230 C., preferably between 180-220 C.; (v) at least one light stabilizer is added, preferably in at least one of the outer layers, in a total concentration comprised between the following ranges given herein in an increasing order of preference and in % w/w: [0.1-35]; [0.2-25]; [0.3-20]; [0.4-10]; [0.5-5].

(130) 2Film according to paragraph 1, wherein at least one of the outer layers contains TiO.sub.2 particles in a concentration comprised between the following ranges given herein in an increasing order of preference and in % w/w: [<5]; [<4]; [0.01-3].

(131) 3Film according to paragraphs 1 or 2, wherein the global planar SST modulus of the film (GPSM) is comprised between the following ranges given herein in an increasing order of preference and in kgf/mm.sup.2: [440-465]; [445-465]; [450-465].

(132) 4Film according to paragraph 1, wherein no chalking phenomenon, represented by a powdering on the surface of the film, is observed before or after natural or accelerated ageing.

(133) 5Film according to paragraph 1, having a total thickness comprised within the following ranges given herein in an increasing order of preference and in m: [10-500]; [20-300]; [35-250].

(134) 6Film according to paragraph 1, wherein the film comprises a flame retardant

(135) 7Film according to paragraph 1, wherein the film comprises a radical scavenger.

(136) 8Film according to paragraph 1, wherein it comprises at least one coating on at least one side, said coating being obtained by coextrusion, coating, extrusion coating, corona treatment under ambient air or gases, vacuum evaporation, plasma treatment or physicochemical vacuum deposition.

(137) 9Method for manufacturing a multilayer biaxially oriented white polyester film according to paragraph 1 and for controlling the hydrolysis resistance of said polyester film, said method consisting of: a. Synthesizing or implementing one or several different polyesters, preferably PET(s); b. Adding TiO.sub.2 and at least one light stabilizer in polyester(s), c. Possibly Pre-drying the polyester(s); d. Heating the polyester(s) to melt it and make it malleable; e. Extruding the melted polyester(s) and processing into a multilayered film; f. Quenching and solidifying the multilayered film; g. Subjecting the multilayered film to biaxial stretching in the longitudinal and transverse directions at a given stretching temperature Ts; h. Heating the stretched film at a given heating temperature Th;
wherein Ts and/or Th are selected so that the endothermic peak temperature, Tmeta, be maintained below 240 C., preferably between 180-230 C., more preferably between 180-220 C., in order to control the hydrolysis resistance of said polyester film.

(138) 10Method according to paragraph 9, wherein in step b. TiO.sub.2 is added via PET-based master batches; TiO.sub.2 particles being preferably introduced from a white master batch WMB2 containing at least 40% of TiO.sub.2, said white master batch WMB2 being obtained by performing solid phase polymerization under vacuum and high temperature using white master batch WMB1 as the starting raw material.

(139) 11Laminate comprising the film according to paragraph 1 or obtained by the method according to paragraph 9.

(140) 12Laminate according to paragraph 11 wherein the peel strength (90) at interface between film of the present invention and other films of the back sheet laminate is greater than or equal to 5N.

(141) 13Solar or photovoltaic cell or battery whose back sheet comprises the film according to paragraph 1 or obtained by the method according to paragraph 9.

(142) It is to be understood that the invention is not limited to the particular embodiments of the invention described above, as variations of the particular embodiments may be made and still fall within the scope of the appended claims.