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.

5 Good Glove Fit Factors

There are many factors to consider when selecting a disposable glove.  After choosing the right glove type and glove material for the task, good fit ranks high on the list.  A properly fitted glove is critical to performing procedures correctly.   A glove that fits properly conforms to the hand, and does not feel too tight or too stiff.   Consider some glove physical characteristics that influence fit.

1.  Glove Length

Glove length is measured from the tip of the middle finger to the cuff.  Exam gloves are typically a minimum of 9” in length, while surgical gloves and some cleanroom gloves are 12” in length to provide more protection.

2.  Glove Width

The width of a glove is one of the most important considerations for proper glove fit.  Gloves are manufactured to specific ASTM glove standards, including palm width.  But there is a fairly wide range within each size.  A medium exam glove, for instance, can vary from 85mm to 105mm in width.   And there is room for overlap, so one manufacturer’s “medium” exam glove could be another manufacturer’s “large” glove.

3.  Finger Length and Width

Finger lengths and widths also vary between product lines from one manufacturer to another.  The finger length and width is largely determined by the formers used on the manufacturing line.  However, the glove material affects the amount of stretch, which affects finger fit.  A super stretchy, low protein latex glove works with the widest range of finger sizes, and could be a good choice for individuals with shorter fingers.  When it comes to nitrile gloves, the formulations range from “some stretch” to “very stretchy,” which affects finger fit.

4.  Glove Material

Glove thickness and stretchability also influences overall glove fit when donned.  If you are accustomed to wearing a “medium” glove in one brand, you may need to try a “small” or a “large” glove in another brand.  If you are trying a very thin glove, you may need to try a smaller size in order to get the proper fit.

5.  Finger Contour

Hand specific gloves with contoured fingers and a natural thumb position are most commonly found in surgical glove lines.  While offering the best fit, these gloves cost more than an examination glove.  Fortunately, ambidextrous nitrile exam gloves (gloves that can be worn on the left or right hand) made from newer, thinner nitrile material, allows the glove to warm and follow the contours of the hand, fingers and thumb to enhance comfort, improve tactile sensitivity, and reduce hand fatigue.

Finding a good fitting glove is essential.  A tight fitting glove can negatively affect fine motor skills, irritate the skin, and lead to hand fatigue.  On the other hand, a glove that fits too loose can cause a person to execute movements awkwardly, and potentially lead to exposure to bloodborne pathogens or hazardous chemicals.  Evaluating a glove’s ability to contour to the hand, and properly fit the width of the hand is key to finding a good fitting glove that you will enjoy wearing.

[note]Each of our HandPRO Exam Gloves offers a unique fit and stretch due to their different compounds.
Click on the glove below to request your sample pack:

AirSoft900 Nitrile Exam Gloves
Cirrus800 White Nitrile Exam Gloves
FreeStyle1100 Nitrile Exam Gloves with Low Dermatitis Potential
HandPRO 400 Latex Exam Gloves[/note]

Disposable Glove Quality Testing

The FDA sets high standards to ensure the disposable gloves you purchase here in the U.S. perform as expected and provide an adequate barrier for their intended use.  To meet these standards, glove manufacturers have tight quality controls and manufacture gloves according to ASTM specifications and testing requirements.

A key quality measurement that glove manufacturers publish on glove boxes, bags and product literature is AQL, or Acceptable Quality Level.

Stated as a percentage, the AQL is a statistical measurement of the quality of the gloves.  An AQL of 2.5% means that statistically, only 2.5 gloves for every hundred gloves will fail a quality test.

How AQL is Determined

Let’s say a glove manufacturer produces 10,000 gloves from the same material, settings and processes.  Two hundred gloves would be pulled randomly from the line, throughout the batch, to be tested.  To meet an AQL of 2.5%, no more than 10 gloves can fail the quality tests.  If more than 10 gloves fail, the entire batch fails, and each glove must be tested individually for quality, or else the whole batch is discarded.  An AQL of 1.5% would mean that no more than 7 gloves could fail.

ASTM D5151 Test for Detection of Holes in Medical Gloves

Disposable gloves are subjected to numerous ASTM tests throughout the manufacturing process.  One test that medical and cleanroom gloves have in common is a test for pinholes.

ASTM D5151 is the Standard Test Method for Detection of Holes in Medical Gloves, often referred to as the “watertight” test or “water leak” test.   In this test, the gloves are each filled with 1000 ml of water at room temperature, secured at the cuff and hung vertically for two minutes to check for pinholes.  If water does not leak from the glove, it gets a “pass.”

The current FDA mandated maximum AQL for examination and cleanroom gloves on this test is 2.5%, down from the previous 4.0% prior to December 2008.  Some gloves, however, are manufactured and tested to meet the lower AQL of 1.5% required for surgical gloves.  This means higher quality and fewer pinholes.

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.

NOTICE: Beware of large offers of gloves through unauthorized distributors. Email us at request@hourglass-intl.com for verification.