Glove Testing Demystified – Tensile Strength and Ultimate Elongation

Glove Testing Demystified - Tensile Strength and Ultimate Elongation

Glove material strength and stretch are important factors in a glove’s quality and performance. Medical and cleanroom gloves undergo testing according to ASTM D412 to meet International standards. This ensures you get a glove that is not prone to tear or fail, and has adequate stretch.

What is ASTM D412?

ASTM D412 is one of over 12,000 standards that operate globally under ASTM International. The current version of the standard is D412-06a(2013) Tensile Properties of Vulcanized Rubber and Thermoplastic Elastomers – Tension.

In this standard, glove material is measured for tensile stress (or tension,) tensile strength, yield point and ultimate elongation.

The Tests

InstromIn Test Method A, a die cut “dumbbell” shaped piece of glove material is stretched from either end by a machine at a constant speed until it breaks. The computer on the machine collects the data as the piece is stretched. Once the piece breaks, the sample’s elongation is measured and recorded.

The test is repeated on samples that have been put through the accelerated aging process in heating chambers according to ASTM D573. This test simulates the glove’s physical properties at the end of its shelf life. This is a “worst case” scenario. Many gloves that have been stored properly, in a cool, dry place away from sunlight, heat and humidity, will have better strength and elongation than the “after aging” test results.

Tensile strength is measured in MPa, or megapascals. The minimum tensile strength for latex exam gloves before aging is 18 MPa, and 14 MPa after aging. The minimum tensile strength for nitrile exam gloves is 14 MPa both before and after aging.

The minimum elongation for latex gloves before aging is 650%, and 500% after aging. Nitrile gloves have a minimum elongation of 500% before aging, and 400% after aging.

Typical Performance vs. ASTM Minimums

Most gloves, however, are manufactured with much higher tensile strength and elongation properties than the ASTM minimums. The typical performance of HandPRO® RoyalTouch300™ Nitrile Exam Gloves during its shelf life, for example, is a tensile strength of 28-32 MPa and an ultimate elongation of 520 – 560%.

HandPRO® Cleanroom gloves are also manufactured to exceed ASTM minimums. HandPRO® Series 9100 Clean Class 100 Nitrile Gloves have a typical tensile strength of 27.1 – 31.0 MPa after aging, and a 540-560% ultimate elongation after aging.

Tensile Strength and Elongation in Use

So what do these numbers mean for you when selecting and wearing gloves?

While high tensile strength is preferred, the “highest” number does not necessarily mean “best” performance. Glove manufacturers must carefully balance strength with stretch and comfort. Too high tensile strength will make a glove stiff or difficult to don.

Elongation is important to the performance of the glove. High elongation helps the glove material stretch instead of tear when snagged. A glove with good elongation will stretch and conform to your hand, improving comfort and performance.

Are Your Cleanroom Nitrile Gloves Full of Pinholes?

Cleanroom Gloves and Pinholes

Cleanroom and controlled environments have unique glove requirements.  While healthcare workers primarily wear gloves to protect themselves from bloodborne pathogens, a person working in a clean manufacturing environment primarily wears gloves to protect the product or process from contamination.

Pinholes in gloves provide a path for human-borne contaminants in the clean manufacturing environment.   Just one square inch of the surface of a person’s hand can contain 10,000 microorganisms!

So how common are pinholes in cleanroom gloves?

A study published in May 2011, Integrity of Disposable Nitrile Exam Gloves Exposed to Simulated Movement,  compared cleanroom nitrile gloves to medical-grade, low-modulus and general duty nitrile gloves.  A total of thirty different glove products were tested, including six cleanroom and nine medical-grade nitrile glove products.  A modified water-leak test was used to detect a 0.15 to 0.05 mm hole in different areas of the glove, including the thumb and pinky.  What did they discover?

“The cleanroom gloves, on average, had the highest percentage of leaks, and 50% failed the water-leak test.”

Two of the cleanroom nitrile gloves tested had an out of the box failure rate of 6.25%.  The medical grade and low modulus gloves had the lowest percentage of leaks.  Even the general duty gloves performed better than the cleanroom gloves tested.   However, two of the six cleanroom gloves tested had a failure rate of zero percent.  Why such a significant difference?

The study authors indicate that it could be due to differences in nitrile material formulation.  A low-modulus nitrile glove with a higher percentage of plasticizer, which is more water resistant, was expected to perform better in the water leak test.

What was not discussed in the paper is the AQL, or Acceptable Quality Level, of the gloves tested.  The AQL for the water leak test, a test that indicates what percentage of the gloves can fail a water test for pinholes, is set by ASTM at 2.5% for cleanroom gloves.  Many manufacturers, however, use the medical grade requirement of 1.5%.  This means that less than 1.5% of the gloves from any given lot could fail due to pinholes – substantially less than 2.5%.

Clearly, there are significant differences in the quality of cleanroom gloves on the market.  To protect your cleanroom environment, select cleanroom gloves made from consistently high quality low-modulus NBR materials, and an AQL of 1.5% or less.

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