Durometer / Hardness

Hardness, as applied to elastomers, is defined as the relative resistance of a surface to indention by an indenter of specified dimension under a specified load. The most commonly used measuring instrument is a durometer. Numerical hardness values are derived from the depth of penetration. The harder the sample, the further it will push back the indenter point and the higher the readings. On the durometer A scale, 0 is very soft, and 100 is very hard. Values are usually read immediately after firm contact has been established. The hardness range of elastomers is so broad that a single durometer cannot indicate practical measureable differences of hardness. For this reason durometers are available in more than one scale model, (eg., A and D scale durometers). The A scale durometer is widely used throughout the rubber industry. The durometer D model, which has a stiffer spring and a more pointed indenter, is used to measure the hardness of hard rubbers.

Mechanical limitation of durometers and the way people use them cause hardness measurements to be inconsistent. Also, indenter penetration is reduced immediately since elastomer deformation occurs and all elastomers inherently try to yield. It is not uncommon to find a difference of 5 points in individual hardness readings of an elastomer specimen. Tabletop durometers can measure hardness more accurately, but they are not as convenient and are not used routinely.

Frequently, hardness is assumed to correlate with stiffness (modulus), but this is not always true. Variations of a few points in hardness can show a marked difference in compression-deflection.

A statistical determination has been made of the relationship between hardness and 100% modulus of Pentathane measured with durometers on the A and D scales. As expected as shown in Table II, the A scale is more reliable for predicting the modulus of the softer stock; the D scale should be used with the harder stocks.

Hardness A	95% Confidence Limits	Average 100% Modulus psi	Hardness D	95% Confidence Limits
55	105-205	150	--	--
60	140-265	200	10	175-275
64	180-330	250	14	195-330
68	210-400	300	18	240-400
73	295-530	400	23	315-520
77	365-650	500	27	390-630
80	430-795	600	31	480-760
83	500-910	700	34	560-880
86	580-1060	800	36	630-1000
88	645-1200	900	38	700-1120
90	700-1350	1000	40	760-1250
92	800-1550	1150	43	900-1400
		1500	49	1200-1900
		2000	55	1650-2500
		2500	59	2000-3100
		3000	63	2400-3800
		4000	69	3200-5100
		5000	73	3900-6400

The reliability of predicting modulus from either A or D scale is approximately +/-33%, for 95% confidence limits. In the low modulus range of less than 500 psi, predictability is 25%.

The A scale should be used with moduli of 500 psi and under. The D scale is more useful for predicting moduli of more than 1000 psi. Either scale may be used for the intermediate areas as in Table II.

One hundred percent modulus can be predicted to within +/- 115 at a level of 400 psi, ranging up to +/- 315 at 1150 psi using the A scale; using the D scale predictability ranges from +/-100 at 400 psi to +/- 1200 at 5000 psi.

A linear relationship between durometer A and D does not exist. Approximate equivalent readings for durometer A and durometer D are shown in Table III.

Durometer A	Durometer D
50	--
60	--
70	18
75	25
80	30
85	30
90	40
95	48

Hardnesses above 95 on the A scale should not be specified because the upper accuracy limit of the instrument is being approached. Accuracy at the lower end of the D scale is also limiting and values below 25D are questionable.