International Journal of Theoretical and Applied Nanotechnology (IJTAN)

ISSN: 1929-1248

1. Introduction

Titanium dioxide nanoparticles (nTiO₂) are increasingly present in commercial products such as paints, varnishes, sunscreens, etc. [1, 2]. The global production of nTiO₂ was 50,400 tons in 2010 and is expected to reach 201,500 tons in 2015 (Future Markets 2011). As a result, the exposure of workers and researchers to nTiO₂ is expected to rapidly increase. At the same time, an increasing number of studies are expressing concern about the likely harmful effects of nTiO₂ on human health. For example, a small increase in the number of cancers among workers who were in contact with nTiO₂ has been reported [3]. Moreover, for studies conducted by intratracheal instillation of rats and mice to 250 nm TiO₂ pigments and 20 nm nTiO₂, inflammation of the lungs was higher for the nanoparticulate form of TiO₂ [4-6]. Following these results, the International Agency for Research on Cancer (IARC) classified nanosized titanium dioxide as being possibly carcinogenic to humans (i.e. 2B group, [7]). In response to this classification, several government agencies responsible for the health and safety of workers hav recommended application of he precautionary principle [8, 9]. One of their recommendations is the use of protective gloves and clothing when working with nanoparticles, even though no scientific validation of their efficiency has yet been made.

A limited number of groups have reported research on the use of protective gloves for nanoparticles. Much of the research has involved nanoaerosols and, in some cases, conflicting results have been found. Diffusion of 30 and 80 nm graphite nanoparticles through nitrile, vinyl, latex and neoprene commercial glove samples has been reported [10], while no penetration was later measured for the same gloves when exposed to 40 nm graphite, 10 nm TiO₂ or 10 nm Pt particles [11, 12]. Nonetheless, under occupational settings, nanoparticles may be found as colloidal suspensions. This situation is especially relevant to protective gloves. For example, Vinches et al. exposed samples of nitrile, latex and neoprene gloves to nTiO₂ solutions in water while deforming the gloves by simulating flexing [13, 14].Analyses by inductively coupled plasma mass spectrometry (ICP-MS) suggested that nTiO₂ was able to penetrate through the protective gloves when they were subjected to dynamic mechanical deformations for a period of 5 hours or more. In addition to mechanical deformation, the swelling of gloves materials might be facilitate the penetration of nTiO₂ solutions through protective gloves [15].

This paper reports analysis of some physical and mechanical phenomena which may facilitate the penetration of nTiO₂ solutions through nitrile rubber and latex protective gloves submitted to biaxial deformations. These results were obtained using commercial colloidal solutions of 15 nm nTiO₂ in water and in 1,2-propanediol.

2. Materials

2. 1. Protective Gloves

Three models of protective gloves corresponding to two types of elastomers were selected for this study: disposable 100 and 200 µm thick nitrile rubber gloves (identified as NBR-100 and NBR-200 respectively), and disposable 100 µm thick latex gloves. All samples were taken from back and palm sections of the gloves.

2. 2. Nanoparticles

TiO₂ nanoparticles have been chosen in this study because of their widespread use. The nTiO₂ (15 nm, anatase) was suspended in two solutions: water (15 wt%, Nanostructured & Amorphous Materials, Inc., Houston, TX) and in 1,2-propanediol, PG (20 wt%, MK Impex, Mississauga, ON).

3. Methods

3. 1. Characterization of the nTiO₂ Solutions

A series of experiments were performed to characterize the nanoparticle solutions. First, the analysis of the nTiO₂ stock solution following dilution to 10 mg L^-1 was performed by fluorescence correlation spectroscopy (FCS) in order to give the hydrodynamic diameter [16]. Second, thermogravimetric analysis (TGA, Diamond TGA/DTA Perkin Elmer) was used to evaluate the mass ratio of the nTiO₂ and identify the presence of additives in the colloidal solutions, aside from the liquid carrier. Gradual evaporation of the liquid carrier occurred between 25 and 150 ºC at a heating rate of 5°C/min. Finally, comparisons were made among the nTiO₂ carrier solutions and the technical grade and ultra-high purity solvents that were used as the liquid carriers by analysing spectra obtained using Fourier transform infrared spectroscopy (FT-IR, Nicolet Continuum XL). Measurements were made in attenuated total reflectance (ATR) mode, between 500 and 4000 cm^-1, on drops of solutions after near total solvent evaporation.

To obtain statistically significant data, triplicate measurements were performed for all analyses.

3. 2. Experimental Setup for the Mechanical Deformations

The test setup used for this study is illustrated in Figure 1. It includes an exposure chamber and a sampling chamber, which are separated by the glove sample. Both chambers and all elements that come in contact with the nTiO₂ are made of ultrahigh molecular weight polyethylene in order to limit the adsorption of nTiO₂ as documented by FCS. nTiO₂ are introduced into the exposure chamber, in direct contact with the external surface of the glove samples.  The setup has been designed in order to expose the glove samples to colloidal solutions of nTiO₂, while simultaneously subjecting them to static or dynamic mechanical constraints. Indeed, the test setup was equipped with a probe linked to an electronic system for applying static or dynamic mechanical stress to the sample (Figure 1).The system is computer controlled and includes a 200-N load cell and a position detector. The whole system is enclosed in a glove box to ensure operator safety during the assembly, dismounting and clean-up operations as well as during the tests. The time profile of sample deformations is illustrated in Figure 2 a and involved applying a 50% deformation every minute. The probe head used in this study is shown in Figure 2b. It corresponded to a conical-spherical geometry that simulates biaxial deformations (BD) similar to that produced in gloves when flexing the hand [17].

3. 3. Quantification of the Characteristic Features on the Glove Surface

The surface morphology of five samples for each elastomer, was analysed by scanning electron microscopy (SEM, Hitachi S3600N – Vacc = 15 kV – magnification × 1000). Quantification of the surface area of the primary features (pores for nitrile and cracks for latex) was performed using image processing software (ImageJ).

3. 4. Strain Energy

Stress-strain curves, corresponding to the applied 50% dynamic deformation during tests, were plotted in order to follow the variation of the strain energy. An estimation of the relative strain energy was carried out by computing the area under the curves using the trapezoidal method.

3. 5. Determination of the Degree of Crystallinity

A semi-crystalline polymer is composed of crystallites dispersed in an amorphous matrix. Each of these two phases gives a particular diffraction signal [18]. X-ray diffraction profiles were obtained using a Pananalytical X'Pert Pro diffractometer in the scanning range of 2-50º and a Cu Ka radiation (45 kV, 40 mA). The degree of crystallinity (χ) was calculated as the ratio of the area under the crystalline peaks divided by the total area under all peaks in the diffraction profile [19], Eq. (1):-

(1)

with I_T being the area under the total intensity profile; I_a the intensity of the amorphous phase and I_B the area under the baseline profile.

3. 6. Length Change Measurements

Length change measurements were performed on 5 × 60 mm rectangular samples taken from the back or the palm section of the gloves. To obtain statistically significant data, three replicates were evaluated for each test. The measurements were performed by immersing the samples in the commercial nTiO₂ solutions. At regular intervals, samples were removed from the liquid, their surface gently wiped with a paper towel and lengths measured using a calliper (±0.01 mm). Length change data were computed using Eq. (2):

(2)

with L_t being the length at time t and L₀ the corresponding length prior to immersion.

4. Results and Discussion

4. 1. Characterization of the nTiO₂ Solutions

FCS analyses were performed to measure the hydrodynamic diameter of the nTiO₂ in the two solutions. In water, the hydrodynamic diameter ranged from 19 nm to 23 nm, while in PG, the analysis was not possible due to an incompatibility between the FCS cells and the solvent.

The relative mass loss was measured as a function of temperature for the PG and for the PG containing nTiO₂ (Figure 3). In the presence of nTiO₂, the curve was slightly shifted towards higher temperatures as compared to its absence. This observation can be attributed to the adsorption of solvent molecules by the nanoparticles, thus increasing the vapour pressure of the system [15]. In the case of nTiO₂ in water, the effect was also observed, but with a smaller amplitude. The calculated mass fraction obtained for nTiO₂ was 14.3 ± 0.8 % in water and 25.0 ± 3.7 % in PG. These results are in agreement with the manufacturer's indications.

FT-IR analyses were performed to see if additives, such as stabilizing agents, were present in the nTiO₂ solutions (Figure 4).  A peak appearing at 1070 cm^-1 in the water used to suspend the nTiO₂ was not observed in Milli-Q water (18.2 MΩ.cm of resistivity and Total Organic Carbon less than 1 µg.L^-1). This peak was associated with the elongation of a CO bond, likely indicative of a surfactant additive in the nTiO₂ solution. In contrast, the FTIR spectra of the PG used to suspend the nTiO₂ did not differ significantly from reagent grade PG. Nonetheless, spectra were admittedly complex, making the detection of additional peaks difficult.

4. 2. Characterization of the Surface of the Protective Gloves

Micrometer-size pores could be observed on the surface of the gloves (Figure 5), as already reported in the literature [20]. Indeed, micrometer-size pores were observed on both the inner and outer surfaces of both models of the nitrile glove. For the latex, micrometer-size cracks were seen on both surfaces.

4. 3. Swelling of Glove Materials in nTiO₂ Solutions

In order to investigate the cause of the nTiO₂ penetration through protective gloves, the swelling of nitrile rubber and latex was studied by recording the length change of samples after immersion in nTiO₂ solutions. Figure 6 displays a gradual length change increase for the two nitriles immersed in the aqueous nTiO₂, indicating significant diffusion of the nTiO₂ solution into the elastomer. A larger swelling ratio was recorded for NBR-100. For example, after 3 hours of immersion, the length change ratio for the NBR-100 was 4% whereas it was 2% for the NBR-200 (Table 1). It should be noted that maximum swelling was not attained for either of the two nitrile rubber gloves after 3 hours of immersion. For the NBR-100, a plateau was reached after 20 hours of immersion, with a maximum swelling of 9%. For the NBR-200, a maximum swelling of 12% was observed after 8 days of immersion. The difference in behaviour between the two models of nitrile rubber can be attributed to possible differences in the penetration of water, which may induce different affinities to the colloidal solution. According to the Table 1, the difference in swelling between NBR and latex glove may well the primary reasons that the aqueous nTiO₂ was able to more easily penetrate the NBR [21].

The values of the length change ratio after 3 hours of immersion are provided in Table 1. The same tendency was observed for each of the nTiO₂ solutions: NBR-100 swelled more than NBR-200 and latex. A large difference was also observed between the nTiO₂ solutions: nTiO₂ in water induced swelling faster than the nTiO₂ in PG for all of the studied glove materials. This difference in swelling behavior produced by the two solutions can be attributed to differences in their ability to penetrate (diffuse) into the elastomer.

Table 1. Values of the length change ratio after 3 hours of immersion of the glove materials in the nTiO₂ solutions.

	Length change ratio after 3 hours (%)
	nTiO2 in water	nTiO2 in PG
NBR-100	4.0	2.8
NBR-200	2.0	0.2
Latex	1.7	0.3

4. 4. Effect of the Mechanical Deformations and the Ntio₂ Solutions on the Gloves

The sample glove surface morphology was first characterized following exposure to the dynamic biaxial deformations (BD) in the absence of the nTiO₂ solutions. Figure 7 displays the variation of the surface features as a function of the number of deformations for NBR-100. While no significant effect was recorded on the inner surface, the outer surface appeared to be strongly affected by the mechanical deformations. Indeed, an increase in the surface area attributed to the pores was observed after only 30 BD, increasing by more than three-fold after 180 BD. The same observations could be made for the outer surface of the latex glove. In contrast, NBR-200 was much less affected than either the NBR-100 or latex. In those cases, the deterioration of the materials could be attributed to an abrasion of the sample surface by the probe [22]. This hypothesis was supported by the observation of a reduction in surface features when the mechanical deformations were performed in the presence of the nTiO₂ solutions (see Figure 8),which might have played the role of a lubricating agent.

4. 5. Variation in the Strain Energy

A second mechanical test involved measuring the relative strain energy during each deformation. As shown in, an important decrease in the relative strain energy was observed during the first deformations.  Thereafter, values reached a plateau, which can be attributed to the Mullins effect. Indeed, some polymer chains reach their limit of extensibility and break during the first deformations, which reduces the energy required to deform the sample during the next cycles [23]. In the absence of the nTiO₂ solutions, the variation of the relative strain energy as a function of the number of deformations was similar for the NBR-100 and NBR-200 samples. In the presence of the nTiO₂ solutions, the energy required to deform the sample was less important because the elastomer membrane was weakened by its swelling [15]. As it will be shown in section 4.3, NBR-100 swells more than NBR-200 for a similar immersion time, which can explain the difference between plateaus obtained for the two materials, i.e. relative strain energy is low when the swelling of the sample is important. For the NBR-100, results were similar for both solvents (data not shown). On the other hand, for the NBR-200, the variation of the relative strain energy was similar for the nTiO₂ in PG and those generated in the absence of the nanoparticles, i.e. the swelling of the NBR-200 that was in contact with the nTiO₂ in PG was insignificant. Moreover, only the nTiO₂ in water affected the variation of the relative strain energy of latex samples after 80 BD (Figure 10).

4. 6. Effect of Mechanical Deformations and the nTiO₂ Solutions on the Degree of Crystallinity

Mechanical deformations can lead to a modification in the degree of crystallinity of elastomers [18]. Indeed, the degree of crystallinity (DOC) is a function of the elongation [24]. To highlight this phenomenon, X-ray diffraction was used in order to evaluate the DOC of glove materials that were subjected to mechanical strains in the presence of the nTiO₂ solutions. In Figure 11, an X-ray diffraction profile of a native NBR-100 samples and one exposed to 180 BD and aqueous nTiO₂ in water. An important decrease in the intensity can be observed after 180 biaxial deformations in presence of the nTiO₂ in water.

For the NBR-100, the decrease in the degree of crystallinity reached 35% after 180 BD (Figure 12). Complementary XRD analyses showed that the DOC reached this value after only 30 BD (data not shown). As was observed with the strain energy measurements, this decrystallization could be attributed to a polymer chain rupture during the initial deformations, i.e. Mullins effect. The observed decrystallization under stress appeared to be amplified as a result of the contact with the nTiO₂ solutions. For example, in the presence of the aqueous nTiO₂, the DOC decreased by 44% for the NBR-100 and a similar trend was observed for NBR-200. This increase in the decrystallization could be attributed to plasticisers having leached out of the elastomers when they were swollen by the liquid carrier [25]. This would induce a fragilization of the elastomer chain network after solvent evaporation, which would lead the polymer to become more sensitive to decrystallization under stress. On the other hand, for latex, the DOC measured appeared to be unaffected by the application of BD.

5. Conclusion

Based upon the above results, penetration of nanoparticles through protective glove materials, under conditions simulating occupational use, can attributed in part to a degradation of the physical and mechanical properties of the elastomer that is accelerated by repetitive mechanical deformations and by contact with the nTiO₂ solutions. This paper has investigated the physical phenomena behind this facilitated penetration of nTiO₂ solutions. To simulate realistic work conditions encountered by protective gloves, one 50% biaxial deformation per minute was applied over 3 hours. It was observed that these dynamical deformations increased the physical degradation of the glove surfaces and decreased the strain energy. Moreover, mechanical deformations with or without the additional contact of the nTiO₂ solutions could affect the degree of crystallinity of the elastomer materials. The colloidal solutions had a significant effect on the swelling of both the nitrile rubber and latex glove materials. In conclusion, the results show that great care must be taken in selecting protective gloves for the handling of nanoparticles in colloidal solutions. It is already possible to recommend the frequent replacement of disposable gloves when exposed to nTiO₂in a liquid carrier.

Acknowledgements

The authors would like to acknowledge the contribution of M. Ben Salah, G. Perron, and K. Inaekyan (École de technologie supérieure) to the project.

References