Measure of IRHD Hardness relaxation and IRHD hardness hysteresis to predict the cure level of rubber parts Relator: Eng. Mauro Belloni

Aim

The study analizes the possibility to use information obtained with Hardness tests to predict the level of curing of a rubber part

The measures which have been analized are:

• IRHD hardness after 30 seconds (ISO 48)

• Regression curve of hardness relaxation curve

• Hysteresis reading after the removal of the main force after 30 seconds

Why Hardness ?

• Low cost test

• Non distructive test

• Evolution of technology improves the quality of the measures

• Easy to calibrate

• Completely described by international standards

Why IRHD-Micro Hardness ?

• High Repeatability of the method

• Most of technical part have small thickness

• Ease of preparation of samples by splitting of bigger parts

• Short time between curing and testing required

• Instruments with automatic sample loader permit to perform the test automatically

Description of the test

Several rubber compounds have been tested

For each product a range of samples with different cure level have been produced

For each sample several hardness measurements have been done

Tested Compounds Compound Code Polymer Curing Curing details. Cure Temperature

EWRC73 EPDM Peroxide 1.4% Dicumile 180°C

EPTO65 EPDM Sulphur 0.45 Sulphur 180°C

NEI014 NBR Sulphur 0.35% Sulphur 160°C

Silicon Silicon Peroxide 2,5% Luperox 180°C

Different types of polymer and different curing systems have been tested in order to check if the conclusions are valid for generally valid.

Preparing samples with different curing level Samples with different curing levels have been prepared to investigate

the capability of hardness test to estimate the curing.

The prepartion of the samples have been done using a specially prepared MD Rheometer.

Gibitre Rheometer permits to set the distance between the dies using a unique distance-regulation ring.

Preparing samples with different curing level

Preparing samples with different curing level

For the specific need of this research, the gap between the dies has been set to 2 mm to obtain samples conforming to ISO 48 specification for IRHD-M test

The samples for each product have been tested using the MD Rheometer with the special setup in order to know the curing behaviour of each product

Preparing samples with different curing level

The samples for each product have been tested using the MD Rheometer with the special setup in order to know the curing behaviour of each product

Preparing samples with different curing level

A new test is performed for each curing level to be tested.

The first test is stopped when the test time corresponds to t’10 time to get a sample with 10% curing level.

The sample is immediately removed from the instrument and cooled down in cold water to stop the curing.

Preparing samples with different curing level

The sample is immediately removed from the instrument and cooled down in cold water to stop the curing.

Preparing samples with different curing level

The same operation is performed for each curing step to obtain samples cured at t’10, t’30, t’50, t’70, t’90

t’10

t’30

t’50

t’70

t’90

The sample obtained from the rheometer is removed from the film used for the curing and the central part is cut using a cutting tool

Preparing samples with different curing level

5 samples for each compound are obtained and identified separately

Preparing samples with different curing level

Measurement of the hardness The Hardness is measured using

the Gibitre Micro-IRHD automatic hardness tester

Constant forces are applied to the sample:

• Contact Force: 5 s,

• Indenting Force for 30 s

Indentor have Spherical Shape

Reference Standards: ISO 48, ASTM D 1415

IRHD Hardness Testers: Construction

Gibitre Technology for irhd

The hardness units produced by Gibitre have some unique features

The sample holder is connected to a ball-recirculation screw and moved up and down using a micro-motor.

The position of the sample holder is controlled with 0.1 Micron resolution

Gibitre Technology for irhd

The indentor is fixed

to a load cell.

Gibitre Technology for irhd

The use of a load cell permits to obtain a perfect control of the force applied to the sample and to increase the Repeatability and Reproducibility of the test results

The hardness result calculated according to ISO 48 standard is the recording of the hardness reading after 30 seconds from test start

Data Analysis - Hardness at set time

The shape of the curve can provide a lot of information about the visco-elastic properties of the product under test

Data Analysis: Hardness vs. Time

The software automatically:

• Plots the curve of Hardness versus time in logarithmic time exe.

• Calculates the Regression line of the curve and the relative Angle Coefficient

The Angle coefficient is an indicator of visco-elastic behaviour of the product

Data Analysis – Angle Coefficient of hardness regression line

a

• After the standard irhd test the main load can be removed from the sample (only pre-load is kept on the sample) to check the hysteretic behaviour of the sample.

• The % of the position recovery of the indentor is calculated at the set test time. Positive correlation has been found between % final deformation and compression set results

Data Analysis - Hysteresis Curve (IRHD)

Each sample is tested 3 times in different points

Each sample is tested after 24, 48 and 72 hours after curing to analyse the effect of sample conditioning

Measurement of the samples

For each test the following results are calculated:

• IRHD hardness at 30 seconds

• Hysteresis after 20 seconds

• Hysteresis after 30 seconds

• Regression angle of hardness relaxation curve

Measurement of the samples

Tests performed:

For each product and for each cure level:

- 3 tests are performed after 24 hours,

- 3 tests are performed after 48 hours,

- 3 tests are performed after 72 hours.

In the image you can see the results of the tests performed on:

EPDM-Peroxide

30% cure level

Results of 3 days overlapped

Measurement of the samples

Effect of Storage Time on the Results EPDM PEROX EPTO 65 MF 275 LZ rosso NBR SULPHUR_180°C

cure 1 2 3 1 2 3 1 2 3 1 2 3

10 60,27 60,40 60,43 55,10 55,57 57,63 37,23 40,63 41,50 60,87 62,27 62,20

30 66,08 65,50 65,57 56,77 58,00 59,13 54,63 55,60 56,77 65,93 65,23 66,07

50 68,10 67,43 68,10 58,17 57,47 60,10 63,50 64,03 64,57 65,37 65,47 65,87

70 69,57 69,93 70,93 58,90 60,40 60,70 68,03 67,90 70,37 65,03 65,60 65,87

90 70,43 71,23 72,93 60,60 61,43 61,27 72,50 71,70 72,57 64,87 65,07 65,50

Storage time of the samples (24, 48 or 72 hours) does not affect the results

In the following the results obtained during 3 testing days will be considered together.

IRHD Hardness Vs. Cure Level

10 30 50 70 90

EPDM PEROX 60.37 65.78 67.88 70.14 71.53

EPTO 65 56.10 57.97 58.58 60.00 61.10

MF 275 LZ rosso 39.79 55.67 64.03 68.77 72.26

NBR SULPHUR_160°C 54.43 65.07 67.54 67.38 68.24

35.00

40.00

45.00

50.00

55.00

60.00

65.00

70.00

75.00

IRHD - Hardness

Cure Level Correlation Coefficient

0,96

0,95

0,99

0,82

Hysteresis at 20 sec. Vs. Cure Level

10 30 50 70 90

EPDM PEROX 95.32 98.23 98.42 98.99 99.17

EPTO 65 93.71 94.24 95.44 95.77 95.68

MF 275 LZ rosso 78.73 94.06 96.74 98.21 98.44

NBR SULPHUR_160°C 94.54 99.18 99.68 99.44 99.53

75.00

80.00

85.00

90.00

95.00

100.00

105.00

Hysteresis at 20 seconds

Cure Level Correlation Coefficient

0,98

0,94

0,85

0,50

Hysteresis at 30 sec. Vs. Cure Level

10 30 50 70 90

EPDM PEROX 96.18 98.58 98.66 99.10 99.27

EPTO 65 94.21 94.72 95.86 96.11 95.97

MF 275 LZ rosso 79.84 94.38 96.90 98.27 98.51

NBR SULPHUR_160°C 95.76 99.38 99.77 99.56 99.64

75.00

80.00

85.00

90.00

95.00

100.00

105.00

Hysteresis al 30 seconds

Cure Level Correlation Coefficient

0,96

0,94

0,81

0,45

Hardness Regression angle Vs. Cure Level

10 30 50 70 90

EPDM PEROX -3.27 -2.53 -2.08 -1.76 -1.55

EPTO 65 -2.39 -1.91 -2.10 -1.63 -1.70

MF 275 LZ rosso -2.54 -1.58 -1.20 -1.14 -1.05

NBR SULPHUR_160°C -4.82 -2.25 -1.90 -1.81 -1.76

-5.50

-5.00

-4.50

-4.00

-3.50

-3.00

-2.50

-2.00

-1.50

-1.00

-0.50

Hardness Regression Angle

Cure Level Correlation Coefficient

0,97

0,88

0,85

0,79

Comparison between batches - Silicon rubber

10 30 50 70 90

MF 275 LZ rosso -2.54 -1.58 -1.20 -1.14 -1.05

MF 280 LZ rosso -2.61 -1.66 -1.30 -1.12 -0.90

-3.00

-2.50

-2.00

-1.50

-1.00

-0.50

0.00

Silicon - Comparison between batches - Regression Angle

10 30 50 70 90

MF 275 LZ rosso 39.79 55.67 64.03 68.77 72.26

MF 280 LZ rosso 48.08 64.61 69.88 74.13 76.14

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

Silicon - Comparison between batches - IRHD Hardness

Cure Level Cure Level

Conclusions • The measure of the hardness after 30 seconds has strong correlation with

the level of curing

• The Hysteresis after 20 or after 30 seconds has correlation with the level of curing but the correlation coefficient is not very high

• The Angle Coefficient of Hardnss Regression Curve has high correlation with the level of curing

• The advantage in the use of Angle coefficient of regression curve is that the results, for a specific formulation, are not related with the hardness of the compound and can be successfully used for detecting cure variations even if different batches have different hardness.

• The results are significant for different products with different curing systems

Thank you for your attention