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International
Standard
ISO 2930
Sixth edition
Raw, natural rubber —
2024-07
Determination of the plasticity
retention index (PRI)
Caoutchouc naturel brut — Détermination de l'indice de
rétention de plasticité (PRI)
Reference number
© ISO 2024
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ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Apparatus . 2
6 Procedure . 3
6.1 Preparation of test pieces .3
6.2 Ageing .3
6.3 Determination of plasticity .3
7 Expression of results . 3
8 Precision . 4
9 Test report . 4
Annex A (informative) Air change in an oven for PRI determination . 5
Annex B (informative) Precision statement for plasticity retention index . 7
Bibliography . 9
iii
Foreword
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This document was prepared by Technical Committee ISO/TC 45, Rubber and rubber products, Subcommittee
SC 3, Raw materials (including latex) for use in the rubber industry.
This sixth edition cancels and replaces the fifth edition (ISO 2930:2017), which has been technically revised.
The main changes are as follows:
— subclause 5.4, “Laboratory mixing mill”, has been updated;
— subclause 5.7 has been updated, replacing “Tissue paper, as described in ISO 2007” by “Bleached, acid-
free tissue paper, of approximately 27 g/m ”.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
International Standard ISO 2930:2024(en)
Raw, natural rubber — Determination of the plasticity
retention index (PRI)
WARNING — Persons using this document should be familiar with normal laboratory practice. This
document does not purport to address all of the safety problems, if any, associated with its use. It is
the responsibility of the user to establish appropriate safety and health practices and to determine
any national regulatory conditions applicable before use.
1 Scope
This document specifies a method to determine the plasticity retention index (PRI) of raw natural rubber.
The PRI is a measure of the resistance of raw natural rubber to thermal oxidation. A high resistance to
thermal oxidation is shown as a high value of the index. PRI is not an absolute value and cannot give an
absolute classification of plasticity number of different natural rubber after oxidation.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 1795, Rubber, raw natural and raw synthetic — Sampling and further preparative procedures
ISO 2007, Rubber, unvulcanized — Determination of plasticity — Rapid-plastimeter method
ISO 2393, Rubber test mixes — Preparation, mixing and vulcanization — Equipment and procedures
ISO 23529:2016, Rubber — General procedures for preparing and conditioning test pieces for physical test methods
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
plasticity number
measure of plasticity, based upon the height of a test piece after being subjected to deformation under
specified conditions of compressive force, time and temperature
[SOURCE: ISO 1382:2020, 3.359]
3.2
plasticity retention index
PRI
ratio of the plasticity number (3.1) measured after air-oven ageing for 30 min at 140 °C to the plasticity
number before oven ageing
[SOURCE: ISO 1382:2020, 3.360]
4 Principle
The rapid plasticity numbers of unaged test pieces and test pieces aged by heating in an oven at 140 °C for
30 min are determined using a parallel-plate plastimeter with a platen of diameter 10 mm, following the
procedure specified in ISO 2007.
The PRI is the ratio of the rapid plasticity numbers before and after the heating multiplied by 100.
5 Apparatus
The usual laboratory apparatus and, in particular, the following shall be used.
5.1 Parallel-plate plastimeter, with a platen of 10 mm diameter, as specified in ISO 2007.
5.2 Punch, capable of compressing a portion of the material being tested to a thickness of approximately
3 mm and cutting out a disc of approximately 13 mm in diameter for the preparation of test pieces, as
specified in ISO 2007.
5.3 Thickness gauge, having a scale graduated in unit divisions of 0,01 mm, fitted with a flat contact of
10 mm diameter and operating with a pressure of (20 ± 3) kPa.
5.4 Laboratory mixing mill, priority to the following characteristics, or minimally in conformity with
the requirements of ISO 2393.
— roll diameter: 150 mm to 250 mm;
— linear speed of back (fast) roll: (14,6 ± 0,5) m/min;
— roll speed ratio: 1:1,4;
— temperature: (27 ± 3) °C;
— roll length between guides: (265 ± 15) mm.
5.5 Oven, meeting the following requirements at 140 °C.
— The temperature in the vicinity of the test pieces shall be controllable to within ±0,5 °C over a period
of 30 min.
— Following insertion of the tray plus dishes into the oven, the temperature of the oven shall recover, and
that of the tray plus dishes increase, to within 1 °C of the set temperature within 5 min.
— The air shall be changed 10 times per hour. Alternatively, the air flap of the oven should be set as semi-open.
NOTE Additional information on the air change in an oven is given in Annex A.
5.6 Lightweight aluminium dishes and tray, with a low thermal capacity.
A suitable size of either a tray or dishes, or both, should be used depending on the size of the oven.
2 2
5.7 Bleached, acid-free tissue paper, of approximately 27 g/m , or cigarette paper of 22 g/m to 26 g/
m cut into two equal pieces (approximately 30 mm × 45 mm).
6 Procedure
6.1 Preparation of test pieces
Homogenize the raw rubber as specified in ISO 1795. Take a test portion of (20 ± 2) g from the homogenized
piece and pass twice (doubling the sheet between passes) between the rolls of the mill (5.4) at (27 ± 3) °C.
Then, immediately double the sheet, which shall be uniform in texture and free from holes, and press the two
halves smoothly together by hand, avoiding the formation of air bubbles and running with the nip adjusted
so that the final sheet thickness is (3,4 ± 0,2) mm.
In order to obtain a smooth sheet from an old rubber, three passes can be necessary. In which case, this shall
be stated in the test report.
Cut the test pieces, as specified in ISO 2007, from the doubled sheet with the punch (5.2), and measure their
thickness with the gauge (5.3) until six pieces are obtained with a thickness of (3,4 ± 0,2) mm. Randomly divide
these test pieces into sets of three: one set for testing before ageing and the other for testing after ageing.
The preparation of test pieces, as described in this subclause, shall be carried out with care, since the PRI is
affected by the sheet thickness. The required nip setting shall be ascertained by a preliminary trial. It varies
with the rubber and with the mill. If six test pieces of the required thickness as described in this subclause
are not obtained, a fresh doubled sheet shall be prepared.
6.2 Ageing
Before starting the ageing process, check the temperature of the oven (5.5) to ensure that it has been stable
for at least 5 min.
To ensure that all test pieces are aged at the correct temperature, the oven shall not be overloaded, as this
would cause a marked and prolonged decrease in temperature and upset temperature uniformity (see 5.5).
Place the test pieces for ageing test on the dishes in the tray. Quickly insert the tray (5.6), close the oven door
and start timing once the oven temperature has reached 140 °C ± 0,5 °C. Care shall be taken to ensure that
the dishes and tray are arranged within the calibrated region of the oven. Check if the correct temperature
is quickly regained and maintained (5.5).
After (30 ± 0,25) min, remove the tray from the oven and the dishes from the tray. Allow them to cool to
standard laboratory temperature.
6.3 Determination of plasticity
Carry out in triplicate the rapid plasticity determination, as specified in ISO 2007, using the equipment with a
platen of 10 mm diameter, as specified in 5.1, first on the unaged test piec
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