Stability of nitrile and vinyl latex gloves under repeated disinfection cycles

Abstract

As severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission by fomites is one of the main concerns of coronavirus disease 2019, the World Health Organization advised on the use of protective gloves for handling contaminated surfaces and fomites. The shortage in the supply of personal protective equipment (PPE) due to the surging demand in conjuncture with the disposal of an unprecedented quantity of contaminated PPE into the landfill led to an interest for alternative platforms for the management of PPE. In this study, we evaluated the potential of reusing gloves after repeated disinfection cycles using six readily available and common sterilization methods: UV, ethanol, heat, steam, bleach, and quaternary ammonium compounds (quats) for the inactivation of SARS-CoV-2. For this, two commercially available medical-grade gloves, i.e. nitrile and vinyl (polyvinyl chloride) gloves were tested. Both types of gloves showed deterioration in mechanical and thermal performance with the use of quats as sterilization treatment while no remarkable change in properties was observed up to 20 cycles of disinfection for the other sterilization methods. The exceptions were that the vinyl and nitrile gloves did not tolerate steam/dry heat and UV treatment over 10 cycles due to likely dehydrochlorination and thermal degradation, respectively. Subsequent rounds of sterilization caused no significant change in the glass transition temperature (T_g) of either medical gloves; however, quats caused a slight reduction in T_g due to its plasticizing effect. Overall, the physical sterilization treatments including steam, dry heat, and UV allowed the gloves to retain their thermomechanical performance up to ten cycles of sterilization.

1. Introduction

On March 11, 2020, the World Health Organization designated the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) disease, coronavirus disease 2019 (COVID-19), a global pandemic [1,2]. The continuous growth in the number of cases worldwide has induced an unprecedented shortage of personal protective equipment (PPE) such as masks, gloves, gowns, and aprons for frontline health care workers and the general public in domestic situations [[3], [4], [5]]. The PPE shortage can also impact other industries that rely on the use of PPE for their regular operation, such as manufacturing, research laboratories, petrochemical industries, food production, etc. On the other hand, a rapid accumulation of potentially infectious PPE in domestic solid waste streams, landfills, and water bodies is already detected [6]. To address this challenge, triggered by surging demand and supply chain issues and waste disposal concerns, urgent solutions are desired that can include efforts to conserve PPE use [7] through the extended use or safe reuse via effective disinfection protocols. Recently, it was revealed that the filtration efficiency preservation of N95 respirators can be affected by sterilization processes [8,9].

Bioaerosol contact (e.g. respiratory expelled droplets) [10], as well as indirect physical contact by touching the contaminated surface (e.g. fomites) [11] are the main potential disease transmission pathways that introduce pathogens in the respiratory tract of the host [12]. While populations generally protect themselves from visible droplet transmissions, a fomite-related transmission has been overlooked both in hospitals and communities [13]. Fomites play a key role among health care workers that may find themselves exposed to contact with contaminated surfaces from a large number of undiagnosed but infected patients [14]. Recent studies on surface persistence of SARS-CoV-2 showed that fomites made of stainless-steel, polymers, and cardboard paper can transmit infection for extended periods (i.e. viable virus can be detected up to 72, 48, and 24 h after application to these surfaces, respectively) [15,16]. This highlights the elevated importance of gloving as part of the protection process to reduce the risk of viral transmission [17]. To prevent any accidental risk, the protective gloves need to be made of smooth materials with a high tear strength and tight fit to hands to be effective [18]. Traditionally, latex gloves are used for single or short-term use in the medical, manufacturing or domestic environments and discarded. The disinfection and reuse of gloves are not typically practiced because of the concern that the repeated exposure of gloves to various chemical or physical disinfection processes could compromise their barrier properties against viruses or other pathogens. Scientific consensus on the best practice ensuring the retention of gloves’ quality after sterilization cycles is scarce [[19], [20], [21]].

In our previous study, we tested methods that may be suitable for the reuse or extended use of medical-grade protective gloves, so that no change in the chemical structure of the gloves was assured without compromising the barrier properties [22]. Here, we have taken this work one step further to study the retention of mechanical and thermal properties after repeated sterilization cycles to avoid the risk of infection due to sudden failure. An understanding of the property retention may have profound impacts on risk-avoidant decision-making for implementation of appropriate disinfection protocols where the reuse of the gloves is practiced. This may also be of interest to the PPE producing industries to design stable medical gloves with extended life span.

2. Experimental

2.1. Sterilization procedures

Two powder-free medical examination gloves, vinyl-based (Pro-Medix, Canada) and nitrile-based (The Safety Zone, USA), were chosen to investigate property retention after repeated sterilization cycles [23]. While SARS-CoV-2 has been found to persist on gloves for a number of days [15,24], it can be rapidly inactivated by surface sterilization procedures recommended by the CDC guidelines [25]. Here, two categories of readily deployed clinical sterilization methods, 3 physical and 3 chemical procedures, were used for inactivation of SARS-CoV-2 on contaminated medical-grade gloves in accordance with the guidelines [25]. The former includes ultraviolet (UV) sterilizer cabinet (15 min) [26,27], dry heat (85 °C, 30 min) [24,26], steam (10 min) [24], and the latter includes commercial grade 75% alcohol spray (Purple Frog Products, Canada) [9,24], 2% household bleach solution spray (Clorox®, USA) [24], and quaternary ammonium compounds (0.15% benzalkonium chloride/0.15 alkyl ethylbenzyl ammonium chloride) spray (Lloyds Laboratories Inc, Canada) [24]. Scheme 1 presented the six disinfection methods used in this study i.e. 1) UV radiation, 2) dry heat treatment, 3) steam treatment, 4) alcohol, 5) chlorine compounds, and 6) quaternary ammonium compounds. The sample codes for sterilized gloves were defined as per the glove type, sterilization cycle, and sterilization method, e.g. the nitrile gloves sterilized for 10 cycles by steam were defined as nitrile-10-steam. The effect of sterilization repetition was evaluated for 1, 5, 10, and 20 cycles.

Scheme 1.

Six disinfection methods that applied to disinfect the medical gloves in this study for reuse purpose.

2.2. Characterizations methods

2.2.1. Toluene swelling studies

Rubber vulcanizate swelling in toluene was used to estimate the changes in crosslink density of nitrile gloves. Samples weighing approximately 0.02 g were cut from each glove specimen and immersed in toluene at room temperature until equilibrium swelling was attained for all the samples (72 h). The weight of each sample was recorded every 24 h by blotting the excess surface toluene with wipes and immediately weighing the sample. The volume fraction of the rubber, $V_r$, in the swollen network was calculated using (Equation (1) [28,29]:

$$V_r=\frac{\raisebox{1ex}{$m_1$}\!\left/ \!\raisebox{-1ex}{${\rho }_1$}\right.}{\frac{m_1}{{\rho }_1}+\frac{m_2}{{\rho }_2}}$$ (1)

where, $m_1$, $m_2(m_2=m_3-m_1)$, and $m_3$ are the weight of dry rubber sample, weight of solvent in swollen sample, and weight of the swollen sample, respectively. ${\rho }_1$ is the density of the dry rubber sample and ${\rho }_2$ is the solvent density. After $V_r$ is determined, the crosslink density ($\upsilon$) was estimated using the Flory-Rehner equation (Equation (2)).

$$\upsilon =-\frac{1}{2V_s}\frac{\ln \left(1-V_r\right)+V_r+\chi V_r^2}{V_r^{1/3}-\frac{V_r}{2}}$$ (2)

where, $\upsilon$ is crosslink density in mol per unit volume ($mol/c{m}^3$), $V_r$ is volume fraction of rubber in equilibrium swollen vulcanized rubber sample, $V_s$ is molar volume of used solvent at room temperature in $c{m}^3/mol$ and $\chi$ ($=0.34$) is Flory-Huggins polymer-solvent interaction parameter.

2.2.2. Plasticizer migration studies

The plasticizer migration rate was estimated in accordance with ASTM D552. Dried PVC films weighing approximately 0.02 g were immersed in n-hexane at 50 °C for 2 h. The PVC films were dried and reweighed, and the migration rate was calculated in accordance with Equation (3).

$$Degreeofmigration=\left[\frac{\left(W_1-W_2\right)}{W_1}\right]\times 100$$ (3)

where, $W_1$ and $W_2$ are the sample weight before and after immersion in the solvent, respectively.

2.2.3. Mechanical properties

An AGS-X Shimadzu universal testing machine with a load cell of 0.5 kN was used for the tensile testing of the glove samples before and after treatment. Before testing, all samples were conditioned at room temperature and 50% relative humidity in accordance with ASTM D618. The tensile testing was carried out in accordance with ASTM D882-18. Samples with dimensions of 1 × 7 cm (width × length) were prepared and tested at a strain rate of 50 mm/min. The tensile strength, modulus at 100% strain, toughness, and elongation at break values were recorded after which their mean values and standard deviations from five specimens of each treatment group were reported.

2.2.4. Polarized optical microscopy

The surface morphology of the different glove types and treatments were investigated using an Olympus BX53 M polarized light microscopy (Melville, NY, USA). The optical microscope equipped with a 50 × objective lens and a polarized light filter was used. All samples were prepared by cutting rectangular pieces (10 × 50 mm) out of the glove specimens.

2.2.5. Thermogravimetric analyses (TGA)

TGA analysis of the samples was carried out on a TA instrument Q500 analyzer both under nitrogen and air atmosphere with a 40 ml/min flow in a dynamic non-isothermal mode at a heating rate of 20 °C/min from ambient temperature to 700 °C.

2.2.6. Dynamic mechanical analysis (DMA)

The loss factor (tan δ) of the gloves was analyzed using a Dynamic Mechanical Thermal Analyzer model Q800 from TA Instruments, USA, to investigate the change in the glass transition temperature of the gloves as a result of the cyclic sterilization processes. The glove test specimens from each treatment groups were prepared in a rectangular shape with dimensions ~25 mm × 8.5 mm × 0.06 mm (length × width × thickness). The test was conducted in a tensile mode, at a fixed frequency of 1 Hz, strain amplitude of 10 μm, and an applied force of 0.01 N. The samples were equilibrated at −75 °C (using liquid nitrogen) and heated at a rate of 3 °C/min to a final temperature of 50 °C to acquire the tan δ curves.

2.3. Statistical analysis

To determine the normal distribution of the replicates (per cycle per glove and per treatment), a Ryan-Joiner (RJ) normality test was performed on the crosslinking density (CD) and mechanical property test results. After which, a one-way analysis of variance (ANOVA) test with Tukey error of 5% for comparison was conducted to evaluate significant difference between the means for the different treatments from cycle 0 to cycle 20 for both the nitrile and vinyl gloves. The difference in means was considered to be statistically significant when p<0.05.

2.4. Odor intensity test

To evaluate whether the disinfection techniques imparted any significant odor to the gloves, protocols from odor test standards were adapted and conducted herein where a human testing panel of five participants were used to rank the disinfected glove samples. Disinfected gloves (20 cycles) were placed in a 35 mm diameter cylindrical glass jars, covered with aluminum foil to minimize visual bias, and sealed with lids for a period of 24 h. Untreated nitrile and vinyl gloves were also placed in the jars as a control samples. Five panelists were recruited, whereby each panelist was required to quickly unseal the cups and smell the contents provide ranking and seal back. A ranking system relative to the control samples odor tests were conducted in accordance with ASTM E2609. An adjusted odor level range was used here between −2 and 2 to capture feedback that found the odor that perform better than the reference. An odor level of 2 was designated a foul or unpleasant odor, and the reference gloves were given a score of 0. Furthermore, roasted coffee beans and the reference glove samples were provided to allow panelists to reset their sense of smell.

3. Results and discussion

3.1. Crosslink density change in nitrile gloves

The CD of rubber gloves is an important parameter for the performance of gloves, which is commonly determined by swelling the rubber in solvents. Swelling is initiated by the diffusion of small solvent molecules into the rubber chain segments. In the beginning, the surface of the rubber has a high solvent concentration while the concentration of the bulk is zero. The diffusion proceeds until equilibrium swelling is achieved, which can be correlated with the CD by the Flory-Rehner equation. The lower the swelling, the higher the CD [28].

The change in the CD of the nitrile gloves with sterilization treatments is presented in Fig. 1 . In comparison with the untreated glove (nitrile control), the CD of the treated gloves either remained unchanged or decreased. Dry heat (Fig. 1f) sterilization caused a statistically significant reduction (p<0.05) in the CD for all the sterilization cycles (Fig. 1) as compared with the control. The decrease in CD could be a result of reversion, a phenomenon that occurs when degradation of the polymer chain and/or desulphurization takes place in rubber, which leads to a loss in the CD accompanied by a decline in physical properties [30]. The CD of nitrile gloves sterilized by dry heat decreased up to 5 treatment cycles and increased thereafter. When rubber is subjected to thermal aging, polysulfide linkages are susceptible to disassociation, and hence a decrease in the CD. On the other hand, free sulfur present in the rubber gloves could create new crosslinks, resulting in an increase in CD [31]. This explains the initial decrease and subsequent increase in the CD up to 5 treatment cycles and after 10 dry heat treatment cycles, respectively (Fig. 1f).

Fig. 1.

Toluene swelling crosslink density (CD) of rubber gloves for the various sterilization cycles by (a) ethanol, (b) UV, (c) steam, (d) bleach, (e) quats, and (f) dry heat treatment.

3.2. Plasticizer migration (for vinyl gloves)

Vinyls are mostly plasticized with phthalate esters to improve chain mobility and lower the glass transition temperature (T _g) of the plastic. However, the ease of migration of the plasticizer into the environment with aging and treatment renders the vinyl gloves less flexible and reduces their physical properties [32]. In this study, the leaching stability of the plasticizers from vinyl as a result of the sterilization methods and cycles was studied by soaking the glove specimens in n-hexane, which is a good solvent for phthalates [33].

Fig. 2 presents the degree of plasticizer migration of vinyl gloves sterilized by various methods. A significant increase in the degree of migration was observed in the samples treated by UV, bleach, and quats (Fig. 2b, d, and e), respectively, while no significant changes were observed in samples sterilized by ethanol, steam, and dry heat (Fig. 2a, c, and f), which increased with the number of treatment cycles. As a result of the plasticizer leaching, all tested specimens showed a reduction in flexibility compared with the original specimen. However, sterilization with bleach and quats increased the degree of leaching compared with the control. This increase in the degree of migration can be attributed to the solubility of phthalate plasticizer in alcohol, bleach, and quats, thereby leading to an increase in the degree of leaching compared with the non-chemical treatments. These results can also be correlated with the mechanical property tests in which a decrease in the elongation at break attributed to the leaching out of the plasticizers was noted (Fig. 3 ). Similarly, Rabek et al. [34], in their study of the effect of plasticizers erosion on the mechanical properties of phthalate plasticized polymeric films, observed a significant decrease in the elongation of samples after incubation for 2 h attributed to the migration of plasticizer from the films.

Fig. 2.

Degree of plasticizer migration from vinyl gloves sterilized by (a) ethanol, (b) UV, (c) steam, (d) bleach, (e) quats, and, (f) dry heat treatment.

Fig. 3.

Mechanical properties (tensile strength, modulus, elongation at break, and toughness), of the examined nitrile and vinyl gloves with the six sanitization treatments.

3.3. Mechanical properties

Understanding fundamental material properties such as tensile strength, modulus, elongation at break, and tensile toughness is important for the manufacturing, testing, and validation of gloves used in various industries. In accordance with ASTM D412 and ASTM D5250, the mechanical properties of gloves must not decrease or increase more than 25% caused by various photochemical and/or thermochemical degradation mechanisms to ensure their suitability for the intended use. All the mechanical data sets were examined via the RJ statistical analysis test and concluded to have a normal distribution due to the values being close to one, ranging from 0.90 to 0.99 (refer Fig. S1 and Table S1). Before examining the effect of various sterilization treatments on nitrile and vinyl gloves, the non-treated samples were tested to compare them with each other and to generate a baseline. As shown in Fig. 3 (cycle 0), the vinyl gloves are in general stiffer with higher tensile strength and modulus by 25 and 68%, respectively. However, the nitrile gloves were more ductile allowing for these gloves to have high elasticity and toughness as compared with vinyl gloves by about 45 and 159%, respectively. Similar property values are reported in the literature for vinyl gloves. However, the nitrile gloves are on the lower end of the reported values for this type of product in the literature as it ranges from 300 to 600% strains [[35], [36], [37]].

The six different treatments resulted in a range of effects on the tensile strength, elongation at break, and the modulus at 100% strain as shown in Fig. 3 and Tables S2 and S3. Under UV light treatment, the nitrile gloves did not experience property degradation until after cycle 10, whereas the vinyl gloves do not experience any significant degradation after performing a one-way ANOVA for each treatment and test, as shown in Tables S2 and S3. Unlike vinyl gloves, the nitrile gloves exhibited significant (p<0.05) property degradation in tensile strength, modulus, toughness, and elongation at break by 33, 32, 46, and 22%, respectively, after the tenth (10th) cycle. However, the vinyl gloves displayed an overall increase in their modulus, which was likely due to self-crosslinking [38]. The property deterioration in the nitrile gloves was likely due to sulfur cure reversion or devulcanization as noted in the crosslinking study.

The dry heat, steam, ethanol, and bleach treatment yielded similar results to one another for both the nitrile and vinyl gloves. On examining the tensile strength, modulus, toughness, and elongation at break, there is no significant (p>0.05) change from the baseline value. This means after 20 consecutive cycles of sterilization, the gloves did not experience catastrophic failures. However, one exception is that the vinyl gloves under dry heat and steam treatments had a significant increase in their modulus after the 20 cycles which could be a result of dehydrochlorination. Fonseca et al. [39] has reported that dry heat and steam treatments of vinyl may lead to a dehydrochlorination by a quasi-ionic mechanism at low temperatures (<200 °C). The dehydrochlorination typically occurs at thermally labile sites such as allylic chloride structures present on the polymer backbone. As the dehydrochlorination process increases, the polymer structure begins to form a crosslinked structure [39].

The quats treatment had noticeable effects on both the nitrile and vinyl gloves. Under the quats conditioning, both types of gloves experienced a decrease in tensile strength and toughness right after the first treatment cycle. Moreover, the nitrile gloves experienced a decrease in modulus cycle over cycle. Typical commercial quats disinfection solutions are composed of a mixture of high molecular weight compounds in which the alkyl group represents a series of homologous radicals derived from the fatty acids and commonly impart a slippery feel to skin [40]. The decrease in tensile strength, modulus, and toughness of gloves was likely as a result of the plasticizing effect of those fatty acids in conjuncture with other effects. In nitrile gloves, there was a noticeable increase in the elasticity as a result of quats treatment that could have been as a result of the plasticizing effect of the fatty acids. Contrarily, the vinyl gloves that normally contain phthalate plasticizers displayed reduction in elasticity as a result of quats treatment. Because the plasticizer migration study as a result of quats treatment displayed increase in the n-hexane soluble plasticizers in vinyl gloves (Fig. 2), the observed reduction in elongation at break could also be a direct result of leaching out of plasticizers.

3.4. Surface morphology

Fig. 4, Fig. 5 depict the visual observation and optical microscope images of the glove surfaces after 20 times of disinfection cycles for nitrile and vinyl gloves, respectively. From the visual inspection, no significant changes were found for the disinfected nitrile gloves (Fig. 4a). The glove color remains similar as the untreated control samples. For the vinyl gloves, no significant changes after disinfection up to 20 cycles were observed except for quats disinfection method. The vinyl glove became clear and transparent after treatment as compared with the control vinyl glove sample (Fig. 5a). This might be due to the leaching out of the plasticizer as evidenced from the CD and mechanical properties’ results discussed previously.

Fig. 4.

(a) Digital photo of nitrile gloves before and after disinfection (visual inspection), optical microscope images of, (b) untreated, (c) bleach, (d) UV, (e) ethanol, (f) heat, (g) quats, and, (h) steam after 20 cycles of disinfections.

Fig. 5.

(a) Digital photo of vinyl gloves before and after disinfection (visual inspection), optical microscope images of (b) untreated, (c) bleach, (d) UV, (e) ethanol, (f) heat, (g) quats, and (h) steam after 20 cycles of disinfections.

The surface morphology of the disinfected gloves was further inspected with optical microscope, and the optical images are showed in Figs. 4b–h and 5b–h for the different disinfection methods of nitrile and vinyl gloves, respectively. Surface microcracks and voids were observed for the nitrile gloves after disinfections. Bleach-, UV-, heat-, and quats-disinfected nitrile gloves showed noticeable microcracks as compared with other samples (Fig. 4c, d, f, and g).

Fig. 5b–h shows the optical images of the vinyl gloves' surface after exposed to the various disinfection methods. As compared with untreated sample, it can be seen that the vinyl gloves experienced some extent of swelling after 20 cycles of disinfections. UV, ethanol, heat, and quats-disinfected vinyl gloves showed higher extent of swelling as compared with other samples (Fig. 5d–g). Overall, both nitrile and vinyl glove surfaces indicated minor to mild surface changes after the physical and chemical disinfection treatments up to 20 repeated cycles of disinfection.

3.5. Thermal stability

TGA/DTG analysis was performed to evaluate the effect of various sterilization treatments on the thermal stability of the gloves. The effect of twenty cycles of various sterilization methods on TGA and DTG thermograph of nitrile gloves under an inert atmosphere is shown in Fig. 6 (a and a′). The results confirmed that untreated nitrile gloves underwent one major degradation stage (300–600 °C). Any possible weight loss below that can be attributed to the evaporation of surfactants, stearic acid, and wax that are typically used in glove formulations [41]. The main weight loss step was initiated at around 300 °C, indicating that the gloves were thermostable up to 300 °C and contain negligible amount of moisture. This degradation step could be related to the decomposition of the other heavy carbonaceous organic compounds [42]. Fig. 6 (b and b') displays TGA/DGT data of sterilized nitrile gloves under air. Oxygen as a degrading media creates a different mechanism and affects the overall thermal degradation by producing a two-step degradation.

Fig. 6.

TG/DTG curves of sterilized nitrile gloves after 20 sterilization cycles (a, a') thermal degradation, (b, b′) oxidative degradation.

As noted from Fig. 6, sterilization of nitrile gloves by quats, ethanol, and bleach led to a shift in the degradation thermograms toward lower temperatures under both oxidative and non-oxidative environments. This shows that the involvement of chemical sterilizations has caused a minor reduction in the thermal stability of nitrile gloves. The emergence of a small degradation peak with quats- and ethanol-treated nitrile gloves at temperatures below the main degradation suggests the generation of small molecular weight materials. Contrarily, the use of physical sterilization treatments such as steam, dry heat, and UV did not change the degradation behavior of nitrile gloves as compared with the non-treated baseline gloves. Overall, the impact of the sterilization treatments on the thermal degradation behavior is in agreement with the mechanical property changes (Fig. 3 and Tables S2 and S3) and barrier performance [22] of nitrile gloves.

The effect of sterilization treatments on the thermal stability of vinyl gloves is presented in Fig. 7 . It was noted that all untreated and treated vinyl gloves showed two thermal degradation stages. The first stage, in the temperature range of 200–350 °C, was attributed to dehydrochlorination of vinyl. Cyclization of conjugated polyene sequences occurred at the second stage (400–500 °C) [43]. It can be observed from the insert that the decomposition of ethanol-, bleach-, and quats-sterilized vinyl gloves took place at lower temperatures compared with the control (untreated vinyl gloves) under both inert and air atmospheres. Besides, the degradative effect of chemical sterilization measures is evidently lower in vinyl gloves as compared with nitrile ones. This revealed that vinyl gloves were less susceptible to the cyclic chemical sterilization treatments used in this work. The other sterilization measures did not influence the initial thermal stability of vinyl, which means that no degradation initiation sites were induced in the vinyl main chain.

Fig. 7.

TG/DTG curves of sterilized vinyl gloves after 20 sterilization cycles (a, a') thermal degradation, (b, b') oxidative degradation.

To study the effect of multiple sterilization cycles on the thermal stability of the nitrile and vinyl gloves, the evolution of TGA/DTG curves is shown in Fig. 8, Fig. 9 for ten and twenty sterilization cycles. An interesting phenomenon noticed from these graphs was that the degradation initiation temperatures for the samples treated by chemical sterilization decreased gradually with increasing sterilization cycles. It can be seen that the degradation of the nitrile gloves was more pronounced as compared with vinyl ones indicating higher stability of the vinyl-based gloves against chemical sterilization.

Fig. 8.

Effect of sterilization cycle on the thermal degradation of nitrile gloves.

Fig. 9.

Effect of sterilization cycle on the thermal degradation of vinyl gloves.

3.6. Dynamic mechanical analysis

The changes in the dynamic mechanical properties of the nitrile and vinyl gloves after different sterilization methods and cycles were examined and the loss factor, tan δ curves are presented in Fig. 10, Fig. 11 , respectively. The tan δ curves of the nitrile glove display one peak in the range of −75 to 50 °C, due to the maximum chain relaxation i.e. glass transition temperature (T _g). A minor shift in the T _g toward higher temperatures (~1–2 °C) was observed for the UV, steam, and ethanol sterilization methods with increasing sterilization cycles (Fig. 10b–d). This suggests that the treatments caused minor changes to the CD of the nitrile rubber after sterilization. The UV, steam, and ethanol sterilization methods cause slight degradation overtime. No significant changes in T _g for the heat and bleach sterilizations were observed (Fig. 10a and e). The T _g of the nitrile glove shifted to a lower temperature after treatment with quats solution (Fig. 10f), which indicates the plasticizing effect of quats compounds to the nitrile glove. The shifting of T _g toward lower temperature is mainly attributed to the plasticizing effects [44,45]. The chain scission after different sterilization methods and cycles causes an increase in the tan δ magnitude, except for the bleach method. The exposure of moisture from different sterilization methods causes the hydrolysis process which attracts the hydroxyl group in the chemical compounds by the hydrophobic polymer matrix that weakens the polymer chain [46].

Fig. 10.

Tan δ curves for different sterilization methods and cycles as compared with the untreated nitrile gloves. (a) Dry heat, (b) UV, (c) steam, (d) ethanol, (e) bleach, and (f) quats.

Fig. 11.

Tan δ curves for different sterilization methods and cycles as compared with the untreated vinyl gloves. (a) Dry heat, (b) UV, (c) steam, (d) ethanol, (e) bleach, and (f) quats.

For the vinyl gloves, a single maximum chain relaxation T _g was detected in the tested temperature range. However, the tan δ curve of the vinyl glove is much broader with lower amplitude than the nitrile gloves. This shows that nitrile gloves exhibited greater molecular mobility as compared with vinyl gloves [47]. This is in agreement with the higher elongation result for nitrile as compared with the vinyl gloves (Fig. 3). There was no noticeable shift in the T _g of the vinyl gloves after disinfection with dry heat, UV, and steam (Fig. 11a–c). However, additional small shoulder peaks on the tan δ curves was observed when treated with ethanol and bleach (Fig. 11d–e). This might be due to the additives leach out and residue formation (e.g. plasticizers) cause by the etching effect from the treatment. This is corroborated with the plasticizer migration studies discussed previously where a higher degree of migration percentage was observed due to the interaction of the polymer with the treated disinfection solutions. In addition, the bleach treatment reduces the tan δ peak magnitude of both the nitrile and vinyl gloves, which indicates the restriction in chain mobility. A similar plasticizing effect (reduction in T _g) as observed in nitrile glove was also noticed for the vinyl glove when treated with quats (Fig. 11f).

3.7. Odor intensity test

Fig. 12a and b presents the odor test ranking setup and the average rankings of sample odors caused by the various disinfection methods based on a group of five panelists, respectively. The gloves used in these tests were conditioned after disinfection to ensure the gloves were dry, resulting in a diminished odor as opposed to when still wet in the case of the chemical disinfection methods. It must be noted that each set of data for the nitrile and vinyl gloves were tested against their own reference sample, as the magnitude of odor intensity differed between nitrile and vinyl gloves. Overall, the intensity of odor from the nitrile gloves was higher than the vinyl gloves. Despite the difference in intensity of the odor level, the results indicate that each disinfection technique produced the same directional effect, either increasing or decreasing, on both the nitrile and vinyl gloves. The techniques involving the use of wet chemicals resulted in distinct increase in unpleasant odor by imparting a scent characteristic of the respective chemical. Bleach and ethanol in particular scored the lowest and were found the most unpleasant of the odors. Dry disinfection techniques on the other hand demonstrated to either slightly reduce the original odor of the glove to even remove some of the original odor present. As evidenced by the large standard deviation, the subjectivity of odor tests was a challenge when conducting odor tests and further indicates that the odor is not statistically significant. Odors caused by the use of wet disinfection chemicals can be mitigated by conditioning or drying of the gloves before use. As odors are only detectable in close proximity of the disinfected gloves, it did not present itself as a significant obstacle in the reuse of nitrile and vinyl rubber gloves.

Fig. 12.

a) Containers wrapped in aluminum foil for odor testing. (1) for nitrile (2) and vinyl (3) rubber gloves. b) Average rankings of sample odors caused by different disinfection methods.

4. Conclusions

Several sterilization methods including physical and chemical procedures were used for the sterilization and reuse of medical-grade gloves to simultaneously mitigate the shortage and reduce the waste generated from single-use gloves in the current time of COVID-19 pandemic. Our study showed that medical-grade gloves retain their mechanical, thermal, and thermomechanical capability after sterilization via dry heat, steam, UV, and ethanol for up to 20 cycles in a laboratory environment. The different disinfections cause slight changes on the surface morphology, including microcracking, voids, and swelling of the gloves’ surfaces. The incorporation of quats in both nitrile- and vinyl-based gloves led to a significant deterioration in mechanical and thermal properties. Thus, quats are not recommended as a disinfectant of gloves if disinfection and reuse of gloves is to be considered. Considering the thermal stability of the gloves, bleach and ethanol are not recommended as disinfection treatments for nitrile glove reuse and to a marginally lower extent, vinyl gloves. The use of physical sterilization measures including steam, dry heat, and UV to disinfect gloves allowed to retain the mechanical and thermal properties of both types of gloves to a satisfactory extent. However, these methods can be considered non-destructive for up to sterilization cycles and after that, not all the gloves could maintain acceptable performance. After appropriate drying procedure, there was no significant unpleasant odor detected on the gloves even when subjected to 20 cycles of the disinfection process. This study provided important information on the sterilization and reuse of medical gloves which is critical given the time of global shortages such as the current situation of COVID-19 pandemic. World health policymakers, FDA, CDC, and OSHA can use our results together with the real barrier properties against bodily fluids contaminated with viruses (e.g. COVID-19) to choose appropriate treatment and durations to ensure the proper functioning of the medical gloves.

CRediT authorship contribution statement

E. Esmizadeh: Methodology, Investigation, Formal analysis, Writing – original draft, Writing – review & editing. B.P. Chang: Methodology, Investigation, Formal analysis, Writing – original draft, Writing – review & editing. D. Jubinville: Methodology, Investigation, Formal analysis, Writing – original draft, Writing – review & editing, Methodology, Investigation, Writing – review & editing. C. Seto: Investigation, Writing – review & editing. E. Ojogbo: Investigation, Writing – review & editing. C. Tzoganakis: Conceptualization, Project administration, Supervision, Writing – review & editing. T.H. Mekonnen: Conceptualization, Project administration, Supervision, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following is the Supplementary data to this article: