MEASUREMENT OF HARDNESS

IN RUBBER COMPOUNDS

Luis Tormento

October/2017

Introduction

• A question we often ask: what is the meaning of hardness in an artifact? However, in itself, the hardness represents little to determine the use of an artifact: we must add other characteristics such as tensile strength and elongation.

• But what actually represents hardness, be it Shore A or IRHD? They are defined by standardized methods.

• Standard methods for measuring hardness of rubber and plastics use such nomenclatures as Shore A and D or IRHD methods (N, H, L and M); these methods are specified in ISO 868 and BS903 / ISO 48, respectively. Through the use of these techniques we have the best accuracy of the measurement process.

Shore Hardness

• Shore hardness specifies methods for determining the hardness of materials by means of two types of hardness: type A for soft materials and type D for hard materials, even if the ASTM standard has 8 types of scales.

• The method allows the initial penetration on the artifact surface to be measured after a short period of time. The measure of hardness is inversely proportional to this penetration; that is, the higher the penetration, the softer the product (the lower the hardness). It is a property that depends on the elastic modulus and the viscoelasticity of the material.

• The shape of the penetrator, the force applied, and the duration of the test influence the results.

• The Shore A durometer consists of a support base, a penetrator, an indicating device, and a calibrated spring that applies force to the penetrator. The difference between the Shore A hardness and the D hardness is in the form of the penetrator and the calibrated spring.

Shore Hardness

Shore Durometer Penetrator Force applied, F / mN

Tipe AHardened steel cylinder 1.10 mm - 1.14 mm dia, with

a truncated cone at 35 ° cone, 0.79 mm dia.F = 550 + 75 HA

Tipe DHardened steel cylinder 1,10 mm - 1,14 mm dia,?

With a conical point at 30º, 0,79 mm diaF = 445 HD

Table 1. Scales of Shore A hardness

Note 1: For hardness above 90 Shore A, use the D scale; for hardness

below 20 Shore D use the A scale

Shore Hardness

• The hardness units range from 0 (with total cylinder penetration from 2.50 mm) to 100 without penetration.

• The force is applied as soon as possible without shock, and the hardness is measured after 15 +/-1 s. If the instant reading is specified, this time will be 1s. The relationship between the different results obtained by a durometer may be different from those obtained by other measuring equipment.

IRHD Hardness

• In fact, the IRHD (International Rubber Hardness Degrees) has four different degrees of measurement for the hardness measurement in vulcanized or thermoplastic rubbers (N, H, L and M), and four methods for determining the apparent hardness of curved surfaces (CN, CH, CL and CM).

• The test consists of measuring the difference between the penetration depth of a sphere on the rubber surface under an initial contact force followed by the total application of the contact force. The methods differ in the diameter of the penetration sphere and the magnitude of the penetration force, selected according to the hardness of the rubber.

IRHD Hardness

Test MethodPenetrator diameter,

/ mm

Contact Force,

/ NAdditional force/ N

Total Force,

/ NAplication

N 2,50 ± 0,01 0,30 ± 0,02 5,40 ± 0,01 5,70 ± 0,03

Thickness: = 4 mm,

Range: 35 - 85 IRHD,

or 30 - 95 IRHD

H 1,00 ± 0,01 0,30 ± 0,02 5,40 ± 0,01 5,70 ± 0,03Thickness: = 4 mm,

Range: 85 - 100 IRHD

L 5,00 ± 0,01 0,30 ± 0,02 5,40 ± 0,01 5,70 ± 0,03Thickness: = 6 mm,

Range: 10 - 35 IRHD

M 0,395 ± 0,005 0,008 3 ± 0,000 5 0,145 ± 0,000 5 0,153 3 ± 0,001

Thickness: < 4 mm,

Range: 35 - 85 IRHD,

ou 30 - 95 IRHD

Table 2 . IRHD hardness scales and force of test.

Note 2: The hardnesses obtained in the N method in the range of 85-95 IRHD and 30-35 IRHD do not correspond to the values obtained in methods H or L

Contact

LT Químicos

Av. Pedro Severino Jr., 366 Cjto 35

04310-060 – São Paulo – SP – Brasil

Luis Tormento

NPD Director

Luis.tormento@ltquimicos.com.br

Tel: +55 (11) 5581-0708