Substandard Properties and Inaccurate Packaging Information
Abstract
The durability and mechanical properties of synthetic medical gloves, such as those made from nitrile, vary drastically depending on the manufacturer. This study reports the chemical composition of several brands of nitrile gloves via FTIR and solid-state NMR analysis and relates composition to glove durability (found via GAD), mechanical performance (found via Instron), and whether the gloves meet or fail ASTM International standards. Out of the four nitrile examination glove brands tested, American Nitrile Slate brand had superior durability results and was found to be made of acrylonitrile butadiene rubber, as expected. The U.S. Medical glove brand, which was also found to be pure nitrile, had the superior tensile results, consistently reaching over 800% elongation before breaking. Although Restore Touch brand exam gloves were made of nitrile, they exhibited substandard tensile strength and durability due to the thinness of the glove, which barely met the ASTM minimum thickness value. The Vglove brand glove had the overall worst mechanical properties, did not meet ASTM requirements, and had an NMR spectrum consistent with that of a polyvinyl chloride glove, rather than nitrile. Gloves that fail to meet the minimum performance requirements should not be used for medical purposes to protect the health and safety of consumers.
Introduction
Protective gloves are a first line of defense for healthcare workers and their patients, protecting against the transmission of pathogens and toxins. Acrylonitrile butadiene rubber (NBR) is a petroleum-based synthetic polymer that is widely used to manufacture examination gloves. Although not as durable or as comfortable as natural latex gloves [1–5] nitrile and other synthetic gloves do not induce Type I latex allergic reactions, as may occur when using improperly leached natural latex protects [6]. Residual chemicals may be present at different levels in the various glove lots which could cause contact reactions. However, it is not the purview of this paper to ensure compliance with all the FDA requirements for nitrile gloves. Because nitrile is derived from petroleum, it is also a non-biodegradable material. The COVID-19 pandemic caused an unprecedented surge in demand for nitrile gloves. Consequently, low inspection rates of these gloves resulted in the U.S. market being flooded with poorly made products in recent years [7].
Among synthetic elastomers, nitrile has gained wide acceptance as a glove that has much better performance (strength, softness and elasticity) than PVC or PE gloves, but is much cheaper than the better performing polyisoprene and chloroprene gloves. However, although nitrile gloves have improved in mechanical properties since their introduction, they are not generally viewed as a preferred glove material overall because they lack the properties of natural latex gloves and high-cost synthetics and have minimal tear resistance.
Previous durability studies have demonstrated that nitrile gloves have vastly different in-use times to failure depending on the manufacturer, and many brands failed to meet the minimum requirements specified by ASTM International (ASTM D-6319) and the U.S. Food and Drug Administration (FDA) [1, 8, 9]. This has raised serious concerns as to whether these gloves are simply poorly made, or if the nitrile has been partially or completely substituted by a cheaper, less durable alternative, such as polyvinyl chloride (PVC), or by diluent fillers like calcium carbonate. Even a tear the size of a pinhole can allow pathogenic viruses and bacteria, some of which are deadly, to transfer through the medical glove.
The purpose of this study is to evaluate a range of gloves imported into the United States as nitrile examination, emergency response technician (EMT) and industrial-grade gloves with respect to their polymer composition, durability, and mechanical properties.
Methods
Glove samples
This study is not intended to survey all brands and manufacturers of nitrile gloves but is an in-depth evaluation of a selection of readily available gloves in use at our institution. All gloves tested were brand new in-package and unused. The gloves that were tested had no visible holes, tears, or physical defects prior to testing. Because glove properties can vary depending on temperature, all gloves were stored and tested at room temperature (22°C). Four brands of nitrile examination gloves (Restore Touch, U.S. Medical Glove, Vglove, and American Nitrile Slate), two brands of nitrile EMT gloves (Curaplex and Ansell Life Star), and one brand of industrial-grade nitrile gloves (N-Dex Plus) were evaluated. One brand of polyvinyl chloride (PVC) gloves (Safeko) was used as a positive control for FTIR tests. One brand of glove made from natural Hevea latex (Aloe Touch) was included in the FTIR testing as an additional reference. A solidified sample of pure nitrile latex was used as a negative control for FTIR tests. All gloves tested were a size large, with the exceptions of Safeko and Aloe Touch brands, which were both size medium. Manufacturer information: Restore Touch (Medline), $0.09/glove, Northfield, IL, USA, U.S. Medical Glove, $ 0.09/glove, Montgomery, IL, USA, American Nitrile Slate, $0.15/glove, Grove City, Ohio, USA, Curaplex, $0.25/glove, Dublin, OH, USA, Ansell Life Star, $0.29/glove, Iselin, NJ, USA, 8005PF N-Dex Plus (Showa Gloves), $0.17/glove, Menlo, GA, USA, Safeko, $0.05/glove, Brooklyn, New York, USA, Aloe Touch (Medline), $0.10/glove, Northfield, IL, USA. Vglove brand does not provide manufacturing information for their gloves, either on their packaging or online.
Durability tests
Durability testing was performed using the Glove Durability Assessment device (GAD), which was developed to allow the user to objectively compare the durability of medical gloves without the need for manual inspection [1, 10]. Previous reports refer to this device as the New Glove Durability Assessment Device (N-GAD). Five trials were performed for each glove brand and type, with the exception of Ansell Life Star brand EMT gloves, for which four trials were performed due to limited quantities available. The 120-grit sandpaper used to create a rough glove contact surface was replaced between each trial. Although 120-grit sandpaper was selected, any grit of sandpaper could be used with the GAD, as long as the selection remains consistent for the entire study. This is because the GAD provides data on relative durability via the order of failure of glove samples, and therefore, the relative values should remain consistent regardless of sandpaper selection. It should be noted that the GAD simply presses the glove against the sandpaper–it does not drag the glove across the rough surface, which would cause abrasion of a type not encountered in normal glove use. The default settings for roller force (15 N) and speed (3.5 mm/s) were utilized for these durability tests. Following durability testing, the middle finger of each glove was removed, the thickness of the fingertip (mm) was measured three times using electronic calipers, and the median value was recorded. The average of these values was then reported for each glove variety. The thick N-Dex industrial glove was too stiff to fit onto the mandrel, and so durability could not be assessed.
Tensile tests
Tensile data were collected according to a modified version of ASTM D412 [11]. Five dumbbells were cut out of the glove samples using Die C (CCSI, Akron, OH, USA). Tensile properties were determined using a tensiometer (model 5542, Instron, Norwood, MA, USA). The thick N-Dex glove (0.39 mm average) usually failed to break during the tensiometery tests, and so was not included in the dataset. The stress and strain data collected also provides information on the modulus of elasticity, or stiffness of the glove. The modulus of each glove type is not explicitly stated, however, for brevity.
Fourier Transform Infrared Tests (FTIR)
A square-shaped sample with a side length of approximately 12.7 mm was cut from each glove and placed on the crystal. For the solidified pure nitrile, a thin layer was placed so that it completely covered the crystal. The pressure clamp was then used to press the sample. Spectral data were collected using an FTIR spectrometer (model 4500a, Agilent Technologies Inc., Danbury, CT, USA) equipped with a diamond-ATR accessory, ZnSe beamsplitter and DTGS detector. MIR spectra were collected over the range of 4000–700 cm-1 with a resolution of 4 cm-1, and 64 spectra were co-added to improve the signal to noise ratio. The infrared spectra of background and samples were recorded on a personal computer using Agilent MicroLab PC software (Agilent Technologies Inc., Danbury, CT, USA). The device was thoroughly cleansed with ethanol and wiped clean between samples to prevent cross contamination.
Solid state Nuclear Magnetic Resonance Tests (NMR)
Glove samples were cut up and packed into 3.2 mm zirconium rotors and spun at 10 kHz MAS at 300 K. An aqueous dispersion of butadiene & acrylonitrile was packed wet into the solid-state rotor via tabletop centrifugation. Solid-state quantitative multi-cross-polarization (CP) experiments were acquired with a Bruker Avance IIIHD 600MHz (14.1T) NMR spectrometer equipped with a 3.2 mm triple-resonance (HXY) DNP probe tuned in 1H-13C double mode [12]. Quantitative multi-CP experiments with 11 ms total CP duration and 10,240 scans were calibrated on an external standard (N-acetyl-valine) for a total experimental time of 19.5 hrs. Recycle delays were set to 3.0 s. Spectra widths were 394 ppm with an acquisition time of 34.4 ms. Spectra were processed in Bruker TopSpin with no additional linebroadening and with a polynomial baseline correction. The acquisition settings and parameters for all spectra were identical. Note that these were solid samples. They were not dissolved in any solvent and therefore do not have a concentration to reference. No solvent suppression was used.
Statistical analysis
Statistical analyses were preformed using the software JMP 16 and included a one-way analysis of variance test and a Tukey-Kramer HSD test.
Results
Durability
Curaplex and Ansell Life Star brand EMT gloves (n = 5) withstood the most sandpaper touches before rupturing, with averages of 246 and 245 touches, respectively (Fig 1). Restore Touch and Vglove branded examination gloves were the least durable, withstanding averages of 24.2 and 7.2 touches, respectively (Fig 1). A one-way ANOVA test (α = 0.5) revealed that at least one glove brand had a significantly different average number of touches to failure (P = 0.009). The subsequent Tukey-Kramer HSD test showed that the means from the following brands were significantly different from each other: Curaplex and Vglove (P = 0.0289), Ansell Life Star and Vglove (P = 0.0453), and Curaplex and Restore Touch (P = 0.0487).
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