1 Scope
This document describes a method for determining the plasticity retention index (PRI) of natural rubber.
PRI is a measure of the thermal and oxygen ageing resistance of natural rubber, with higher values indicating better thermal and oxygen ageing resistance.
This document applies to the determination of the PRI of natural raw rubber.
2 Normative references
The contents of the following documents constitute essential provisions of this document by means of normative references in the text. Where a reference is dated, only the version corresponding to that date applies to this document; where a reference is not dated, the latest version (including all amendment sheets) applies to this document.
GB/T 2941-2006 Rubber - Physical Test Methods - General Procedures for Preparation and Conditioning of Test Specimens (ISO 23529:2004, IDT)
GB/T 3510 Determination of Plasticity of Unvulcanized Rubber - Rapid Method (GB/T 3510-2006, ISO 2007:1997, IDT)
GB/T 6038 Rubber Testing - Rubber Batching and Mixing Equipment and Operating Procedures (GB/T 6038-2006, ISO 2393; 1994, MOD)
GB/T 15340 Sampling and Sample Preparation of Raw Natural Synthetic Rubber (GB/T 15340-2008, ISO 1795; 2000, IDT)
3 Terms and definitions
The following terms and definitions apply to this document.
3.1
Plasticity number
The thickness of a specimen after deformation under specified conditions of pressure, time and temperature as a measure of plasticity. 3.2
Plasticity retention index; PRI
The ratio of the plasticity value (3.1) of a specimen aged in hot air at 140 °C for 30 min to the plasticity value (3.1) before ageing.
4 Principle
Using a parallel plate plastometer with a 10 mm diameter indenter, the rapid plasticity of unaged specimens and specimens aged for 30 min in a 140 °C oven is determined according to the procedure specified in GB/T3510.
The PRI is the ratio of the rapid plasticity value (P,) to the unaged rapid plasticity value (P.) after heating and ageing multiplied by 100.
5 Apparatus
5.1 Parallel plate plasticity meter, according to GB/T 3510, with a 10 mm diameter indenter.
5.2 Cutting machine, in accordance with the provisions of GB/T 3510, capable of punching a film of about 3 mm thickness out of a disc of about 13 mm diameter for the preparation of specimens.
5.3 Thickness gauges with dials in 0.01 mm divisions, with flat contacts of approx. 10 mm diameter and operating pressures of (20sh3 >kPax.
5.4 Laboratory open refiner, in accordance with GB/T6038, with the following characteristics;
5.5 Aging chamber, meeting the following requirements at 140 °C;
--The temperature in the vicinity of the specimen should be controlled within 0.5 °C during a period of 30 min.
--The temperature of the trays and discs should be brought back to within 1 °C of the set temperature within 5 min after the trays and discs have been placed in the chamber.
--Air should be changed 10 times per hour, or the air flap of the chamber should be set to half open, note, see Appendix A for additional information on the number of air changes.
5.6 Lightweight aluminium discs and trays with low heat capacity.
The size of the aluminium discs and trays should be suitable for the size of the oven.
5.7 Thin paper, in accordance with GB/T 3510, or 22 x/yd ~ 1 26 g/m' cigarette paper, cut into two pieces of equal size (approximately 30 mm x 45 mm).
6 Procedure
6.1 Preparation of specimens
Homogenize the raw glue according to GB/T 15340. From the homogenised film, take (20sh2) g of test material and pass it through the refiner (5.4) twice at (27sh3) °C (fold the film in half when passing the spokes). The film is then immediately folded in half with no holes in the texture and the two parts are pressed tightly together to form a smooth film to avoid the formation of air bubbles. The spacing should be adjusted so that the final thickness of the folded film is (3.4 o.2) mm.
In order to obtain a smooth film from the harder rubber, it is necessary to overspray the film three times. If this is the case, it should be stated in the test report. In accordance with GB/T 3510, the specimens are cut from the folded film with a cutter (5.2) and their thickness is measured with a thickness gauge (5.3). 6 specimens of (3.4±0.2)mm thickness are taken. One group was used to determine the plasticity value before ageing and the other group was used in the post-ageing test to determine the plasticity value after ageing.
As the thickness of the specimen has an effect on the PRI, the specimens should be prepared as specified above. The required spacing should be determined from pre-tests. The required spacing may vary from rubber to rubber and from refiner to refiner. If six specimens of the required thickness cannot be obtained, a new folded film should be prepared.
6.2 Ageing
Before ageing begins, check the temperature of the ageing chamber (5.5) to ensure that the temperature is stable for at least 5 minutes.
To ensure that all specimens are aged at the exact temperature, the chamber should not be overloaded, as this will cause a significant drop in the chamber temperature and disrupt the uniformity of the chamber temperature (5.5).
Place the set of specimens ready for the ageing test in an aluminium dish on a tray. Quickly place the tray with the specimens (5.6) into the ageing chamber and close the chamber door. Start timing when the chamber temperature reaches (140±0.5)°C. Care should be taken to ensure that the aluminium discs and trays are placed in the calibrated temperature zone of the ageing chamber. Check that the ageing chamber recovers quickly and maintains its specified temperature (5.5). After (30±0.25) min, remove the tray from the ageing chamber, remove the aluminium disc from the tray and allow to cool naturally to standard laboratory temperature. 6.3 Determination of plasticity values The rapid plasticity values of the specimens are determined according to GB/T 3510 using a parallel plate plasticity meter with a 10 mm indenter as specified in 5.1. Three parallel measurements were carried out. The rapid plasticity (P.) of the unaged specimen is measured first, followed by the rapid plasticity (Ps) of the aged specimen.
Under normal circumstances, the plasticity value should be measured at least 0.5 h after ageing, but not more than 2 h, provided that the specimen is cooled to standard laboratory temperature before the test. The rapid plasticity value (P.) of unaged specimens and the rapid plasticity value (Ps) of aged specimens should be determined using the same type of paper. Rapid plasticity readings should be accurate to 0.5 units (1 unit corresponds to 10 u.m).
7 Presentation of results
The median of the rapid plasticity values of the three unaged specimens and the three aged specimens is used to calculate the plasticity retention rate (PRI) according to equation (1);
8 Precision
See Appendix B.
9 Test report
The test report shall include the following information:
a) The number of this document;
b) all the details required to identify the sample
c) the median of the rapid plasticity values of the unaged and aged specimens for each sample
d) PRI for each sample;
e) date of test;
f) any operations not included in this document or in the normative references, and any operations considered to be optional.
Appendix A (Informative) PRI Determination of Air Exchange in the Aging Chamber
A.1 Background
Ventilation of the ageing chamber is one of the conditions to be controlled in PRI determination. Optimal flow of hot air ensures good heat distribution and uniform sample ageing. 2016, a study was carried out using the procedures described in ISO 2930 to identify the available options for air flow requirements. The main objective of the study was to provide the user with a practical option when controlling the air exchange operation of the ageing chamber.
Four materials with different plasticity retention rates were used in the study. The test results were averaged over 10 replicate measurements over a 2 d period. For each replicate, the median of the results of three unaged and three aged specimens was taken.
As an alternative to 10 air changes per hour, three conditions were studied. The openings of the aging chamber air baffle were positioned as follows:
a) fully closed;
b) fully open
c) half open.
The results obtained for each of these conditions were then compared with the results obtained for 10 air changes. The results obtained were analysed using a t-test to determine if there was a significant difference between these conditions and their mean PRI values.
A.2 Results
A.2.1 General
Table A.1 gives the p-values for each of the four test materials, for each baffle condition, based on a significance level of α = 0.05 for the results of 10 air changes per hour and the mean PRI for each oven air change condition.
The results for the half-open condition of the oven air baffle were comparable to the results for 10 air changes per hour. There was no significant difference between the results for these two conditions and the mean PRI values.
General descriptions of the use of p-values and alpha levels are given in A.2.2 and A.2.3.
Appendix B (informative) notes on the precision of plasticity retention
Bibliography
1 Scope
2 Normative references
3 Terms and definitions
4 Principle
5 Apparatus
6 Procedure
7 Presentation of results
8 Precision
9 Test report
Appendix A (Informative) PRI Determination of Air Exchange in the Aging Chamber
Appendix B (informative) notes on the precision of plasticity retention
Bibliography
