Amendment 1 - Measurement procedures for materials used in photovoltaic modules - Part 1-6: Encapsulants - Test methods for determining the degree of cure in Ethylene-Vinyl Acetate

Amendement 1 - Procédures de mesure des matériaux utilisés dans les modules photovoltaïques - Partie 1-6: Encapsulants - Méthodes d'essai pour déterminer le degré de durcissement dans l'éthylène-acétate de vinyle

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Publication Date
25-May-2020
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26-May-2020
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IEC 62788-1-6:2017/AMD1:2020 - Amendment 1 - Measurement procedures for materials used in photovoltaic modules - Part 1-6: Encapsulants - Test methods for determining the degree of cure in Ethylene-Vinyl Acetate
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IEC 62788-1-6
Edition 1.0 2020-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
A MENDMENT 1
AM ENDEMENT 1
Measurement procedures for materials used in photovoltaic modules –
Part 1-6: Encapsulants – Test methods for determining the degree of cure
in Ethylene-Vinyl Acetate
Procédures de mesure des matériaux utilisés dans les modules
photovoltaïques –
Partie 1-6: Encapsulants – Méthodes d'essai pour déterminer le degré
de durcissement dans l'éthylène-acétate de vinyle
IEC 62788-1-6:2017-01/AMD1:2020-05(en-fr)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC 62788-1-6
Edition 1.0 2020-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
A MENDMENT 1
AM ENDEMENT 1
Measurement procedures for materials used in photovoltaic modules –
Part 1-6: Encapsulants – Test methods for determining the degree of cure
in Ethylene-Vinyl Acetate
Procédures de mesure des matériaux utilisés dans les modules
photovoltaïques –
Partie 1-6: Encapsulants – Méthodes d'essai pour déterminer le degré
de durcissement dans l'éthylène-acétate de vinyl
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.160 ISBN 978-2-8322-8209-0

Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 3 ----------------------
– 2 – IEC 62788-1-6:2017/AMD1:2020
© IEC 2020
FOREWORD

This amendment has been prepared by IEC technical committee 82:Solar photovoltaic energy

systems.
The text of this amendment is based on the following documents:
FDIS Report on voting
82/1691/FDIS 82/1720/RVD

Full information on the voting for the approval of this amendment can be found in the report

on voting indicated in the above table.

The committee has decided that the contents of this amendment and the base publication will

remain unchanged until the stability date indicated on the IEC website under

"http://webstore.iec.ch" in the data related to the specific publication. At this date, the

publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
_____________
2 Normative references
Add the following new references:

ISO 6721-1, Plastics – Determination of dynamic mechanical properties – Part 1: General

principles

ISO 14577-1, Metallic materials – Instrumented indentation test for hardness and materials

parameters – Part 1: Test method
3 Terms and definitions
Add the following new term:
3.6
degree of cure

parameter that correlates with the extent of cross-linking within the EVA

using the indentation method
Note 1 to entry: Unit: dimensionless.
5.2.1 Sampling and storage
Replace the existing subclause 5.2.1.1 with the following:
---------------------- Page: 4 ----------------------
IEC 62788-1-6:2017/AMD1:2020 – 3 –
© IEC 2020
5.2.1.1

Because the results for the secondary method may depend on the make of EVA, test results

may only be directly compared for the same formulation of EVA. Therefore, test specimens

should come from the same manufacturer(s) for the same fabrication lay-up configuration

(backsheet/EVA combination). Changes in the encapsulant that affect the curing process,

including but not limited to a change of the material supplier, would require validating the

correlation between G and the degree of cure. For example, if the percentage vinyl acetate

and the degree of cure (from
content in the EVA resin changes, a new correlation between G

a secondary method) should be obtained because the percentage vinyl acetate content is

known to significantly affect the viscoelastic-dependent cure characteristics of the

encapsulant.
7 Test report
Replace the existing item h) with the following:

h) identification of test method used and test instrument and other equipment used, including

the laminator and the temperature, pressure, and time settings used, when applicable. In

the case of the indentation secondary method (per correlation or usual use of method), the

test temperature, tip material, tip geometry, and tip size, maximum indentation load,

maximum indentation depth, and frequency of modulation (if applicable) shall also be

reported;
Replace the existing item i) with the following:

i) reference to sampling procedure, where relevant, including the number of tests per

specimen;
Replace the existing item k) with the following:

k) measurements (associated and their uncertainty), examinations and derived results

supported by tables, graphs, sketches and photographs as appropriate including degree of

cure, specimen mass, measured enthalpy, graphs of the enthalpy/temperature data,
graphs of the crystallization peaks, and gel content;
Add the following new clause:
8 Indentation secondary method
8.1 General

An alternative secondary method, using indentation to characterize the degree of cure of EVA,

has emerged from the PV industry. The method is presently being used by module

manufacturers and has been demonstrated in in-line application. A general description of the

principle, related equipment, and use of the method is given herein.

G is obtained using an indentation instrument, where the result may follow from the analysis

of the applied load, penetration depth, use of a modulated applied load (such as harmonic

stiffness, storage modulus, loss modulus, or tan[δ]), and/or the specimen viscoelastic

response (including the relaxation or recovery response). G may be obtained from a more

complicated dimensionless fit, including a fit applied to the specimen's viscoelastic response

(e.g., using a Maxwell model) or a combination of characteristics monitored during

indentation. G may be obtained from a dimensionless fit of the response of the specimen (for

example
CC−
G=
CC−
---------------------- Page: 5 ----------------------
– 4 – IEC 62788-1-6:2017/AMD1:2020
© IEC 2020
where
C is the characteristic of interest,
C is the test specimen;

C is the reference specimen with the greatest thermal history (“maximum correlated”, e.g.,

most-cross-linked); and

C is the reference specimen with minimal or no thermal history (“minimum correlated”, i.e.,

not-laminated or not-cured).

NOTE A suitable apparatus is supplied by LayTec AG in Germany. For this presently available commercial

equipment, G can range from 0,3 to 2,5..

The terminology and definitions related to dynamic indentation, including harmonic stiffness,

storage modulus, loss modulus may be found in ISO 14577-1. The terminology and definitions

related to tan[δ] may be found in ISO 6721-1.
8.2 Principle

The indentation method probes the mechanical response (viscoelastic characteristics) of

cured EVA layers, which change due to the cross-linking of the EVA during the curing

process. In the method, a probe tip (connected to a force transducer) is pressed against the

flexible back-sheet affecting the EVA. The corresponding reaction force acting on the tip

during indentation and subsequent relaxation is recorded by the force transducer and

analyzed, giving a figure of merit (G ) describing the viscoelastic properties of the material.

The principle of the indentation method is described in the related references in the

bibliography.

A correlation between the degree of cure and the gel content can be established by a series

of measurements on samples, each with a different degree of cure. Indentation can thus be

used as a secondary method to quantify the degree of cure. To enable more widespread

comparison, the same set of sample materials shall be characterized using the gel content

method (Clause 6 of IEC 62788-1-6) after indentation to establish a correlation between gel

content and the degree of cure. The indentation method is valid for the specific combination of

EVA and back-sheet examined.

This test procedure may also be applied to cross-linking ethylenic co-polymers other than

EVA. The maximum tip displacement, maximum applied load, and temperatures identified for

the indentation measurements in this procedure have been optimized for EVA. For other

materials, the optimum temperature depends on the stack of materials subject to indentation,

including the encapsulant and backsheet.

The method and instrumentation is designed for non-destructive examination of PV modules

with a flexible back-sheet for the purpose of manufacturing process control. The method may

not be applied to modules with a glass/glass laminated construction. The indentation method

may be used for quality control in production lines. The variation of the method (one standard

deviation) is typically ≤ ±3 % (see Kunath et al.).
8.3 Instrument and equipment for the indentation method

Equivalent instruments that contain the following components and can be shown to provide

repeatability and reproducibility of ≤ ±5 % for two standard deviations for correlation of G as

defined in Section 3.6 may be used.
The instrument consists of:
__________

This information is given for the convenience of users of this International Standard and does not constitute an

endorsement by IEC of the product named.
---------------------- Page: 6 ----------------------
IEC 62788-1-6:2017/AMD1:2020 – 5 –
© IEC 2020
– a rigid probe;

– a force transducer connected to the probe, measuring the reaction force acting on the

probe during indentation;

– a temperature regulation system, to maintain the test specimen (module) at a designated

temperature;

– a mechanically rigid linear stage that may be used to move the probe and force transducer

for site-specific indentation.
8.4 Instrument calibration

The tip, force transducer, and temperature regulation system shall be verified and calibrated

regularly according to the specifications of the equipment vendor.
8.5 Correlation of the degree of cure
) given by the instrument
For EVA, to establish a correlation between the figure of merit (G

and the gel content, a set of specimens with different cross-linked gel content (G ) including

the range of the manufacturer’s specification limits (e.g., between 70 % and 93 %) shall be

used. Specimens for correlation, usually prepared by varying the lamination time or

temperature, shall first be examined using the indentation tester. The correlation specimens

shall be subsequently analysed using the gel content method (Clause 6 of IEC 62788-1-6) so

that an empirical correlation (best fit or series of best fits through the range of correlation)

between G and G is established. Figure 5 (similar to Lux et al.) shows an example of a

% i

correlation for an EVA material. The figure shows an example of correlation data for a single

material rather than the application of indentation to multiple module specimens, therefore no

error bars are given. Because of the nonlinear nature of the curing process, the results of the

secondary methods (including indentation) should not be extrapolated beyond the range of

G and G of the correlation specimens.
% i

An example of the empirical correlation (dashed line for a first order exponential fit) is shown for the correlation

data (circles) for a representative combination of encapsulant and backsheet.
Figure 5 – Example of the correlation applied between G (indentation)
and G (gel content)
---------------------- Page: 7 ----------------------
– 6 – IEC 62788-1-6:2017/AMD1:2020
© IEC 2020
8.6 Specimen preparation for the indentation method

The indentation method does not require sample specific preparation. Test specimens may

consist of full-size modules or mini-modules. Specimens shall be placed in the test device

with the backsheet-side facing the probe tip. Measurements shall only be conducted at

locations within the specimen where just a cell is present (i.e., not between cells or at a

location where the interconnect ribbon is present) and at least 10 mm away from the edge of a

cell. The measurement locations should be clean and without defects or damage in the

specimen materials.

If measurements are performed for the purpose of correlation between G and G , subsequent

% i
preparation requirements for the gel content method may apply.
8.7 Test procedure for the indentation method

A module specimen is placed under the indentation test probe, with both the specimen and

probe being held rigidly in place. The specimen is heated to a temperature above the melt

transition temperature of the encapsulant (for example, within the range of 85 °C and 95 °C

for EVA) and maintained constant at the designated temperature ±1 °C for all measurements.

For example, heating can be achieved from below with a heater, such as a halogen light

source that irradiates the front (glass side) of the PV module. When the designated test

temperature is achieved and stabilized, the probe is pressed into the backsheet/encapsulant

stack and the measurement is performed. For contemporary backsheet materials laminated on

EVA encapsulant, the stabilization time of at least 45 s is recommended; however, shorter

stabilization times may be used if validated for indentation. For contemporary backsheet

materials laminated on EVA encapsulant, the maximum indentation depth may be in the order

of 120 µm and the applied load may be in the order of 15 N. The optimum measurement

temperature rang
...

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