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Understanding hardness

Aug. 21, 2003

A better understanding of hardness lets designers match the best soft-touch elastomer to their application.

Associate Editor

A gellike 30 Shore 00 Versaflex TPE is engineered to serve as ultrasoft cushions in bicycle seats, shoes, and furniture as well as nontacky grips on personal care products and sports equipment.

Relative relationship between Shore hardness scales.

Hardness is often one of the first criteria considered when choosing thermoplastic elastomers (TPEs). But confusion can arise when discussing hardness because of the variety of ways to measure it.

Hardness measurement

Hardness is defined as a material's resistance to indentation when a static load is applied. The most common instrument used for measuring hardness is Shore Durometer. It measures the depth of penetration of an indenter on a scale from zero to a tenth of an inch (0.1 in.). A zero reading indicates that penetration depth was at its maximum; a reading of 100 specifies there was no penetration.

The Shore A scale is the most common scale for TPEs, and the Shore A Durometer consists of a blunt indenter with moderate spring force. Shore A instruments are less accurate when readings are above 90. Shore D Durometers are more appropriate when hardnesses exceed 90 Shore A. This instrument uses a sharper indenter and higher force. Softer TPEs (below 5 Shore A) use a Shore 00 scale. Most soft gels and foam rubbers are measured using this scale.

Methods

Most materials resist initial indenture but eventually yield over time due to creep or relaxation, so Durometer readings can either be taken instantaneously or after a specified delay - typically on the order of 5 to 10 sec. Instantaneous readings typically give higher (or harder) results than delayed readings. But delayed readings are more representative of hardness and resiliency. Weak, less-elastomeric materials creep more than higher strength, more resilient materials.

Hardness is often confused with other properties such as flexural modulus or Coefficient of Friction (COF). Flexural modulus measures the materials' resistance to bending, and COF measures the resistance an object experiences as it slides along the TPE surface. Although flexural modulus and COF also affect the overall feel and flexibility of the TPE, they are different properties than hardness which measures a materials' resistance to penetration.

TPEs, TPUs, and TPVs for softer designs

From ultrasoft thermoplastic elastomers (TPEs) and high-performance rubberized thermoplastic polyurethanes (TPUs) to alloyed thermoplastic vulcanizates (TPVs) GLS Corp., McHenry, Ill., has expanded its array of options for putting chemical-resistant, sure-grip surfaces on new designs.

Versaflex CL2003 is reportedly one of the softest commercially available TPEs with a gellike hardness of 30 Shore 00. It is injection moldable and extrudable, and comes in free-flowing pellets. The clear TPE is engineered to meet the needs of ultrasoft applications including gel bicycle seats, wrist pads, shoe sole inserts, and furniture arm rests, as well as grips for personal-care products.

Versollan TPUs are second-generation high-performance rubberized TPU elastomer alloys that rank as some of the softest in their class at 45 Shore A. Made from specialty TPUs from BASF Corp., Wyandotte, Mich., these alloys serve as handles and grips for hand and power tools, lawn and garden equipment, and recreational gear. They offer a balance of softness and high performance, and fast set up rates relative to nonalloyed TPUs.

Versalloy 8000 TPV alloys come from the alliance between GLS and Netherlands-based DSM. These alloys have good tear and tensile properties like heat and chemical resistance, while offering good flow for easy processing. Hardness ranges from 45 to 70 Shore A.