Journal of Occupational and Environmental Hygiene
ISSN: 1545-9624 (Print) 1545-9632 (Online) Journal homepage: http://www.tandfonline.com/loi/uoeh20
Exposure Assessment of a High-energy Tensile Test With Large Carbon Fiber Reinforced Polymer Cables Lukas Schlagenhauf, Yu-Ying Kuo, Silvain Michel, Giovanni Terrasi & Jing Wang To cite this article: Lukas Schlagenhauf, Yu-Ying Kuo, Silvain Michel, Giovanni Terrasi & Jing Wang (2015) Exposure Assessment of a High-energy Tensile Test With Large Carbon Fiber Reinforced Polymer Cables, Journal of Occupational and Environmental Hygiene, 12:8, D178D183, DOI: 10.1080/15459624.2015.1029614 To link to this article: http://dx.doi.org/10.1080/15459624.2015.1029614
Accepted online: 19 Mar 2015.
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Date: 03 October 2015, At: 18:41
Journal of Occupational and Environmental Hygiene, 12: D178–D183 ISSN: 1545-9624 print / 1545-9632 online c 2015 JOEH, LLC Copyright DOI: 10.1080/15459624.2015.1029614
Case Study
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Exposure Assessment of a High-energy Tensile Test With Large Carbon Fiber Reinforced Polymer Cables
This study investigated the particle and fiber release from two carbon fiber reinforced polymer cables that underwent high-energy tensile tests until rupture. The failing event was the source of a large amount of dust whereof a part was suspected to be containing possibly respirable fibers that could cause adverse health effects. The released fibers were suspected to migrate through small openings to the experiment control room and also to an adjacent machine hall where workers were active. To investigate the fiber release and exposure risk of the affected workers, the generated particles were measured with aerosol devices to obtain the particle size and particle concentrations. Furthermore, particles were collected on filter samples to investigate the particle shape and the fiber concentration. Three situations were monitored for the control room and the machine hall: the background concentrations, the impact of the cable failure, and the venting of the exposed rooms afterward. The results showed four important findings: The cable failure caused the release of respirable fibers with diameters below 3 µm and an average length of 13.9 µm; the released particles did migrate to the control room and to the machine hall; the measured peak fiber concentration of 0.76 fibers/cm3 and the overall fiber concentration of 0.07 fibers/cm3 in the control room were below the Permissible Exposure Limit (PEL) for fibers without indication of carcinogenicity; and the venting of the rooms was fast and effective. Even though respirable fibers were released, the low fiber concentration and effective venting indicated that the suspected health risks from the experiment on the affected workers was low. However, the effect of long-term exposure is not known therefore additional control measures are recommended. Keywords
airborne exposures, carbon fiber reinforced polymer, fiber, occupational health
INTRODUCTION
L
arge Carbon Fiber Reinforced Polymer (CFRP) composite cables are finding increasing applications, e.g., in the sailing boat industry. One example is the emerging use of noncorrosive, high-strength, and lightweight CFRP cables as riggings to stabilize the mast of regatta sailing boats and of large one- or two-mast yachts. These tendons are highly prestressed in tension (with a mechanical service stress of approximately 1,000 MPa) and are subjected to variable stress amplitudes depending on the sailing conditions. The tendons are reinforced by high-strength carbon fibers in the direction of their main axis which leads to a high stiffness and therefore reduces wind-induced cable and mast vibrations, which increases the sailing speed, comfort, and safety.
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Journal of Occupational and Environmental Hygiene
Column Editor Steven Lacey Reported by Lukas Schlagenhauf1,2,3 Yu-Ying Kuo2,3 Silvain Michel4 Giovanni Terrasi,4 and Jing Wang2,3 1
Laboratory for Functional Polymers, Empa - Swiss Federal Laboratories for Materials Science and Technology, Dubendorf, Switzerland 2 Laboratory for Analytical Chemistry, Empa - Swiss Federal Laboratories for Materials Science and Technology, Dubendorf, Switzerland 3 Institute of Environmental Engineering, ETH Zurich, Zurich, Switzerland 4 Mechanical Systems Engineering, Empa - Swiss Federal Laboratories for Materials Science and Technology, Dubendorf, Switzerland Address correspondence to: Jing Wang, Institute of Environmental Engineering, ETH Zurich; Zurich, Switzerland; e-mail:jing.wang@ ifu.baug.ethz.ch Color versions of one or more of the figures in the article can be found online at www. tandfonline.com/uoeh.
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For the characterization and testing of the crucial tensile loadcarrying capacity of large CFRP cables, tensile test facilities are needed with forces above several hundred tons. In strength testing, the cables are first slowly loaded with elastic energy, which is then, during the failure event, dissipated in a very short time. This high-energy release rate can cause severe damage of the cable and thus induce a vast amount of dust into the air. It is known that during the machining of CFRPs, freestanding fibers are released into the air.(1–4) Beside fibers with the same diameter as the embedded fibers in the composite, respirable fibers with smaller diameters were also generated. It is hypothesized that during the tensile tests of CFRP cables, similar particles are released into the air and thus can pose a risk to the operating staff. Respirable fibers are of great interest because of their toxic properties when inhaled. The toxicity of fibers is generally determined by the three “D’s”: Dose, Dimension, and Durability.(5) A correlation between biopersistence and adverse pulmonary effects has been demonstrated,(6) while the fiber material is of minor importance.(7) Fibers with good biopersistence produce chronic pulmonary inflammation and interstitial fibrosis; if very biopersistent, fibrosis is followed by lung cancer and/or pleural mesothelioma.(5) Defined by the World Health Organization (WHO), respirable fibers have a length above 5 µm, a diameter below 3 µm, and an aspect ratio (length/diameter) above or equal to 3.(8) Fibers have been found in lungs with lengths up to 200 µm.(9) The recommended PEL by the Scientific Committee on Occupational Exposure Limits (SCOEL) is 1 respirable fiber cm−3 for an 8 hr time-weighted average.(10) This limit applies to fibers without indication of carcinogenicity. A summary of the toxicity of respirable carbon fibers has been published by Warheit et al.,(5) but only a few studies were available at that time. The authors explained this with the facts, that the occupational exposures generally were low and that the dust in workplaces was assumed to be nonrespirable. They reported two short term in vivo studies. The results showed no toxicity for polyacrylonitrile- (PAN) based fibers, mainly because usually they had a diameter above 5 µm. Another study showed an inflammation reaction for pitch-based carbon fibers. Those fibers usually had a diameter below 2 µm. Martin et al.(11) investigated the cytotoxicity of particles generated during machining of CFRP composites (characterized by Boatman et al.(2)). They found marginal toxicity for two samples, but as the particles consisted of fibers and matrix materials, it was not clear what caused the effect. The number of release and toxicity literature for carbon fibers from the machining of CFRPs is surprisingly low, given that in the past years a larger number of studies dealt with the same subjects for composites with carbon nanofibers (CNFs) and carbon nanotubes (CNTs).(12,13) For nanomaterials, the weight percentage in composites lies in the low single-digit range causing no or only a low number of released fibers when machined.(14–16) In contrast, the fiber content of CFRPs is above 50 vol% which means that the produced dust during machining consists mainly of material from the fibers.
FIGURE 1. Sketch of the experimental set-up in the facility. A: Machine hall; B: ELS testing cavern, covered with a shutter (represented with a grid); and C: ELS control room.
In this study, the fiber release and possible risk for the operating staff in two CFRP cable tensile tests at the Swiss Federal Laboratories for Materials Science and Technology (Empa) is reported. The experiments have been carried out using the ELS, a tensile test device with a maximum force of 30 MN.(17) The particle concentration measurements were focused not only on the exposure of the directly involved operating staff but also on the exposure of people, who might be present in the machine hall above the testing cavern during such tests. The second location was chosen because it was separated from the testing cavern only with a mechanical shutter, which is not air-tight. The particle concentrations of the respirable dust in the nano to sub-micro range (10–500 nm) and in the micro range (0.5–10 µm) were recorded. Further, particles were collected on filters and transmission electron microscopy (TEM) grids in order to determine the shape, size, and concentration. Three situations were investigated, the particle background concentrations before the experiments, the cable failure event, and also the opening of the shutter on top of the testing cavern as well as the hall gates and windows in order to vent the facility. MATERIALS AND METHODS Facility The ELS testing facility is located in the basement of a large machine hall (volume = 15,600 m3, location A in Figure 1) and consists of two compartments: (1) the test setup cavern (volume = 210 m3, location B), in which the mechanical testing device stands, and (2) a control room (volume = 285 m3, location C), where both the hydraulic infrastructure and the control desk for the ELS machine are located. The cavern is open to the machine hall above but can be closed with a moving shutter. From the control room, the cavern is accessible via two large entrances. They can be closed
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with moving protective panels only. The shutter and the panels have not been designed for being air-tight, and air can easily flow from the cavern to the adjacent control room as well as to the machine hall above. Air Quality Measurements Air sampling was conducted at two sites: in the control room and in the machine hall close to the shutter. For the characterization of particles in the micro range, an aerosol particle sizer (APS) (Model 3321, TSI, Shoreview, MN) and an optical particle sizer (OPS) (Model 3330, TSI) were used. The particles in the nano to sub-micro range were measured with a fast mobility particle sizer (FMPS) (Model 3091, TSI) and with a scanning mobility particle sizer (SMPS) (DMA Model 3080, TSI; CPC Model 3775, TSI). For particle collection, a constant flow sampler was used (Model Gilian AirCon2, Sensidyne, St. Petersburg, FL) and the particles were collected on Nuclepore track-etch membrane filters (111106, pore size 0.4 µm, Whatman, Little Chalfont, UK). The sampling flow rate was 5 L/min for all filter samples. For the TEM investigation (CM30, Philipps, Amsterdam, Netherlands) of collected particles, TEM grids were taped on the Nuclepore filters according to the work of Tsai et al.(18) The fiber concentration was measured following the procedure of NIOSH method 7400, but the fibers were imaged and counted by the usage of scanning electron microscopy (SEM) (Nova NanoSEM 230, FEI, Hillsboro, OR) images (image area of approximately 0.076 mm2) instead by phase contrast microscopy and Walton-Beckett graticule. The fibers were identified and counted according to “Rule B” (minimum length of 5 µm, maximum diameter of 3 µm, and aspect ratio > 5). For one experiment, the shutter, the roof windows, and gates were completely opened 15 min after the cable failure. The air exchange rates between the testing cavern and the machine hall, and between the control room and the machine hall were calculated by fitting the measured particle concentrations with Equation (1):(19) Q (1) C(t) = C0 + C1 · exp − · t , V where C(t) is the time dependent total particle concentration, C0 is the background concentration, C1 is the dust concentration just before the shutter, windows, and gates were opened, Q is the air exchange rate, V is the room volume, and t is the time. CFRP Cable Materials and Cable Failure Tests The cylindrical cables had two eye-shaped terminations on each side to anchor them to the ELS testing machine. They consisted of longitudinally loop-wise wound carbon fiber layers in the core (as an “endless race-track loop”) wrapped with a confining orthogonally oriented carbon fiber bandage. The carbon fibers had a filament diameter of 5 µm (Tenax IMS60, Teijin, Tokyo, Japan), a minimum experimentally certified tensile strength of 5,600 MPa with a longitudinal elastic modulus of 290 GPa and a minimum certified ultimate tensile strain R
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FIGURE 2.
Failure mode of the primary damage location
of 1.9 %.(20) The cables had a fiber volume fraction of 60% and the fibers were pre-impregnated by an epoxy polymer resin matrix (a proprietary epoxy hot melt by CarboLink, Fehraltorf, Switzerland) and were cured in an oven for 2 hr at 140◦ C. In a quasi-static tensile test, the cables were loaded to failure at room temperature. The load was increased continuously with a deformation controlled ramp of 8 mm/min. The stored elastic energy was released very abruptly, heard as an acoustic shock and a cloud of dust grew in the testing cavern. Both ends were heavily damaged. The primary failure occurred in the arch of the eye (Figure 2). On the opposite side a secondary damage was found caused by the load release wave that propagated through the cable and left damage as severe as the primary one, thus giving two comparable dust sources. RESULTS Background Prior to the tensile tests, the background particle concentrations and size distributions were measured. Further, filter samples were collected to identify other particle sources. The background concentrations, measured by SMPS and FMPS, varied between 9,000 and 10,000 cm−3 in the machine hall and also in the control room. The filter samples from the machine hall of the background air showed one single fiber. It had a diameter below 1 µm and a length of 230 µm. It was distinguishable from released carbon fibers due to the small diameter and the undamaged structure. Tensile Test Emissions Machine Hall The aerosol in the machine hall was monitored by FMPS, OPS, and a filter. The evolution of the particle number concentration without venting is shown in Figure 3a. Immediately after the cable failure at 16:15, the particle concentration increased from 9,000 cm−3 to 27,000 cm−3 in the nano to sub-micro range. In the micro range, the OPS also measured
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an increase in the particle concentration simultaneously with the FMPS, but this could be attributed to other sources since during the measurement, the detected particle concentration by OPS was rather inconsistent. The SEM and TEM images from the filter in the machine hall showed a low number of collected fibers. While some still had the original diameter of 5 µm and thus were not considered to be respirable, there were also a few fibers collected that were split during the cable failure and could be respirable due to diameters clearly below 3 µm (Figure 3e). On the TEM grid, several particles with rough surfaces were detected (Figure 3f). Those most probably were formed during the cable failure at locations in the cable where the tensile energy was converted into heat, similar to the micro- and nanoparticles formed during thermal cutting of polymer foams.(21) Control Room The development of the number and mass concentrations in the control room, measured by SMPS and APS and without venting after the experiment, is shown in Figure 3b; they increased to 110,000 cm−3 and to 260 µg/m3, respectively. Afterward, agglomeration resulted in a decrease of the particle number concentration while the mass concentration remained stable until the end of the measurement. The SEM images showed a large number of different fibers of which were all split and probably respirable due to the small diameter. Further, fragmented particles from the composite matrix and fibers, whereof most had a diameter below 10 µm, were also collected. The TEM images showed few fibers with a diameter below 1 µm. For the calculation of the fiber concentration in the control room after the cable failure, 20 SEM images were analyzed and a total of 180 fibers were detected. The histogram with the fiber length distribution is shown in Figure 3d The average fiber length was 13.9 µm and the longest detected fiber had a length of 53 µm. The measured fiber concentration was 0.76 fibers/cm3. Shutter Opening The shutter opening after the experiment allowed a calculation of the air exchange rates for the testing cavern and control room by Equation (1). The normalized concentrations are shown in Figure 3d. The fitting analysis shows that in the testing cavern the air can be assumed to be clean (i.e., removal of 99% of the dust) after 413 s and in the control room after 1855 s. The calculated air exchange rates for the testing cavern and the control room are 2.35 m3/s and 0.72 m3/s, respectively.
lied in the range of outdoor air and was not influenced by other conducted experiments. Directly after the failure of a CFRP cable, particles in the nano and micro range were released and transported to both the control room and the machine hall. The particle concentration in the control room increased to 105 particles/cm3. Compared to the particle exposure of a worker who is dry cutting a CFRP composite, the measured concentration was about one order of magnitude lower.(3) In the machine hall, the particle concentration increased to 27,000 particles/cm3 for the nanoto sub micro particles and the impact of the micro particles was barely measurable. Furthermore, for the machine hall, the particle concentration decreased back to background values within short time because of the large room volume. The filter samples proved the diffusion of a low number of respirable fibers into the machine hall. The staff in the control room was much more exposed to respirable fibers. The measured fiber concentration of 0.76 fiber/ cm3 lies close, but below the PEL given by the SCOEL. But this concentration was only valid for the time directly after the failure experiments. Assuming a staff member is performing 2 tensile tests per day in the control room and he is only exposed to fibers during the 15 min after the cable failure and during the venting, an average 8 hr fiber concentration of approximately 0.07 fibers/cm3 can be calculated. This is clearly below the PEL and thus the risk posed by the released fibers to the operating staff is low. For long-term exposure, not much data about the toxicity of fibers are available. Therefore, during the experiment, the use of nondisposable dust respirators with sealing gaskets (rating N95 or higher) is recommended. There is no carcinogenic classification by the International Agency for Research on Cancer (IARC) neither for carbon fibers nor graphite, their basic material.(22) Therefore, the use of a PEL of 1 fibers/cm3 for the investigated exposure is reasonable. But compared to neat carbon fibers, the released fibers from the CFRP cables have two additional features: a rough surface and agglomeration with epoxy particles that were exposed to heat. The effect on toxicity of these features has to be assessed. The venting by opening the shutter, gates, windows, and the shelter panels showed sufficient air exchange rates to clean the air in both the testing cavern and the control room. Due to settling of fibers on the floor and walls, a regular cleaning of the test cavern and the control room is recommended to avoid re-aerosilization and new exposure to fibers. Further, the diffusion of particles into other rooms of the facility and the environment could be avoided with the installation of a venting system that creates a partial vacuum inside the testing cavern.
DISCUSSION
T
o ensure workers’ health, the exposure to particles and fibers was monitored for all measurement steps. Generally, the background air prior to the experiments was clean with respect to the fiber concentration, only one single fiber from an unknown source was found in the machine hall. The background particle concentration of about 10,000 particles/cm3
CONCLUSION
T
his study investigated the particle release from two CFRP cables that underwent a high-energy tensile test until rupture. Measurements with aerosol devices and collection on filter samples showed that the cable failure caused a release
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FIGURE 3. Measurement results from the tensile test emissions in the machine hall and the control room: a) particle number concentration measured by FMPS and OPS (size range: 1–10 µm) in the machine hall; b) particle number and mass concentrations in the control room measured by SMPS and APS; c) histogram of the fiber length distribution for respirable fibers in the control room according to NIOSH 7400; d) effect of shutter opening and air exchange on the normalized particle concentrations in the control room and the testing cavern; e) SEM image of captured fibers; and f) TEM image of captured particles from the CFRP matrix material
of particles as well as free-standing fibers whereof a fraction had diameters below 3 µm and thus were respirable. Even though the tensile test was carried out in a separated room, particles and fibers did migrate through small openings into the control room and the adjacent machine hall. Both the particle mass concentration and the fiber concentration did remain below the PEL. A venting by fresh air of all rooms by opening the doors, gates, and windows of the building and the shutter above the testing cavern caused a reduction of the particle concentration back to the background concentration within approximately 30 min. Under the given circumstances, including the total amount of released particles and fibers, the room sizes, and the air exchange rates during the venting, in combination with the knowledge of toxicity tests, it can be assumed that the health D182
risk of the operating staff in the control room and of the workers in the machine hall was low. To avoid health effects due to longterm exposure, several suggestions were made to improve the occupational health. The cable failure also caused the release of heat induced epoxy particles for which the exposure was not assessed.
ACKNOWLEDGEMENTS
T
he authors would like to thank Dr. Matthias Nagel for the organization of the measurement, the company CarboLink for allowing the publication of the measured data and the operating staff of the tensile test for the smooth collaboration.
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FUNDING
T
his study was financed by the Swiss National Science Foundation (NFP 64), “Evaluation platform for safety and environment risks of carbon nanotube reinforced nanocomposites,” 406440 131286.
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Journal of Occupational and Environmental Hygiene
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ISSN: 1545-9624 (Print) 1545-9632 (Online) Journal homepage: http://www.tandfonline.com/loi/uoeh20
Exposure Assessment of a High-energy Tensile Test With Large Carbon Fiber Reinforced Polymer Cables Lukas Schlagenhauf, Yu-Ying Kuo, Silvain Michel, Giovanni Terrasi & Jing Wang To cite this article: Lukas Schlagenhauf, Yu-Ying Kuo, Silvain Michel, Giovanni Terrasi & Jing Wang (2015) Exposure Assessment of a High-energy Tensile Test With Large Carbon Fiber Reinforced Polymer Cables, Journal of Occupational and Environmental Hygiene, 12:8, D178D183, DOI: 10.1080/15459624.2015.1029614 To link to this article: http://dx.doi.org/10.1080/15459624.2015.1029614
Accepted online: 19 Mar 2015.
Submit your article to this journal
Article views: 100
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=uoeh20 Download by: [University of Windsor]
Date: 03 October 2015, At: 18:41
Journal of Occupational and Environmental Hygiene, 12: D178–D183 ISSN: 1545-9624 print / 1545-9632 online c 2015 JOEH, LLC Copyright DOI: 10.1080/15459624.2015.1029614
Case Study
Downloaded by [University of Windsor] at 18:41 03 October 2015
Exposure Assessment of a High-energy Tensile Test With Large Carbon Fiber Reinforced Polymer Cables
This study investigated the particle and fiber release from two carbon fiber reinforced polymer cables that underwent high-energy tensile tests until rupture. The failing event was the source of a large amount of dust whereof a part was suspected to be containing possibly respirable fibers that could cause adverse health effects. The released fibers were suspected to migrate through small openings to the experiment control room and also to an adjacent machine hall where workers were active. To investigate the fiber release and exposure risk of the affected workers, the generated particles were measured with aerosol devices to obtain the particle size and particle concentrations. Furthermore, particles were collected on filter samples to investigate the particle shape and the fiber concentration. Three situations were monitored for the control room and the machine hall: the background concentrations, the impact of the cable failure, and the venting of the exposed rooms afterward. The results showed four important findings: The cable failure caused the release of respirable fibers with diameters below 3 µm and an average length of 13.9 µm; the released particles did migrate to the control room and to the machine hall; the measured peak fiber concentration of 0.76 fibers/cm3 and the overall fiber concentration of 0.07 fibers/cm3 in the control room were below the Permissible Exposure Limit (PEL) for fibers without indication of carcinogenicity; and the venting of the rooms was fast and effective. Even though respirable fibers were released, the low fiber concentration and effective venting indicated that the suspected health risks from the experiment on the affected workers was low. However, the effect of long-term exposure is not known therefore additional control measures are recommended. Keywords
airborne exposures, carbon fiber reinforced polymer, fiber, occupational health
INTRODUCTION
L
arge Carbon Fiber Reinforced Polymer (CFRP) composite cables are finding increasing applications, e.g., in the sailing boat industry. One example is the emerging use of noncorrosive, high-strength, and lightweight CFRP cables as riggings to stabilize the mast of regatta sailing boats and of large one- or two-mast yachts. These tendons are highly prestressed in tension (with a mechanical service stress of approximately 1,000 MPa) and are subjected to variable stress amplitudes depending on the sailing conditions. The tendons are reinforced by high-strength carbon fibers in the direction of their main axis which leads to a high stiffness and therefore reduces wind-induced cable and mast vibrations, which increases the sailing speed, comfort, and safety.
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Journal of Occupational and Environmental Hygiene
Column Editor Steven Lacey Reported by Lukas Schlagenhauf1,2,3 Yu-Ying Kuo2,3 Silvain Michel4 Giovanni Terrasi,4 and Jing Wang2,3 1
Laboratory for Functional Polymers, Empa - Swiss Federal Laboratories for Materials Science and Technology, Dubendorf, Switzerland 2 Laboratory for Analytical Chemistry, Empa - Swiss Federal Laboratories for Materials Science and Technology, Dubendorf, Switzerland 3 Institute of Environmental Engineering, ETH Zurich, Zurich, Switzerland 4 Mechanical Systems Engineering, Empa - Swiss Federal Laboratories for Materials Science and Technology, Dubendorf, Switzerland Address correspondence to: Jing Wang, Institute of Environmental Engineering, ETH Zurich; Zurich, Switzerland; e-mail:jing.wang@ ifu.baug.ethz.ch Color versions of one or more of the figures in the article can be found online at www. tandfonline.com/uoeh.
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For the characterization and testing of the crucial tensile loadcarrying capacity of large CFRP cables, tensile test facilities are needed with forces above several hundred tons. In strength testing, the cables are first slowly loaded with elastic energy, which is then, during the failure event, dissipated in a very short time. This high-energy release rate can cause severe damage of the cable and thus induce a vast amount of dust into the air. It is known that during the machining of CFRPs, freestanding fibers are released into the air.(1–4) Beside fibers with the same diameter as the embedded fibers in the composite, respirable fibers with smaller diameters were also generated. It is hypothesized that during the tensile tests of CFRP cables, similar particles are released into the air and thus can pose a risk to the operating staff. Respirable fibers are of great interest because of their toxic properties when inhaled. The toxicity of fibers is generally determined by the three “D’s”: Dose, Dimension, and Durability.(5) A correlation between biopersistence and adverse pulmonary effects has been demonstrated,(6) while the fiber material is of minor importance.(7) Fibers with good biopersistence produce chronic pulmonary inflammation and interstitial fibrosis; if very biopersistent, fibrosis is followed by lung cancer and/or pleural mesothelioma.(5) Defined by the World Health Organization (WHO), respirable fibers have a length above 5 µm, a diameter below 3 µm, and an aspect ratio (length/diameter) above or equal to 3.(8) Fibers have been found in lungs with lengths up to 200 µm.(9) The recommended PEL by the Scientific Committee on Occupational Exposure Limits (SCOEL) is 1 respirable fiber cm−3 for an 8 hr time-weighted average.(10) This limit applies to fibers without indication of carcinogenicity. A summary of the toxicity of respirable carbon fibers has been published by Warheit et al.,(5) but only a few studies were available at that time. The authors explained this with the facts, that the occupational exposures generally were low and that the dust in workplaces was assumed to be nonrespirable. They reported two short term in vivo studies. The results showed no toxicity for polyacrylonitrile- (PAN) based fibers, mainly because usually they had a diameter above 5 µm. Another study showed an inflammation reaction for pitch-based carbon fibers. Those fibers usually had a diameter below 2 µm. Martin et al.(11) investigated the cytotoxicity of particles generated during machining of CFRP composites (characterized by Boatman et al.(2)). They found marginal toxicity for two samples, but as the particles consisted of fibers and matrix materials, it was not clear what caused the effect. The number of release and toxicity literature for carbon fibers from the machining of CFRPs is surprisingly low, given that in the past years a larger number of studies dealt with the same subjects for composites with carbon nanofibers (CNFs) and carbon nanotubes (CNTs).(12,13) For nanomaterials, the weight percentage in composites lies in the low single-digit range causing no or only a low number of released fibers when machined.(14–16) In contrast, the fiber content of CFRPs is above 50 vol% which means that the produced dust during machining consists mainly of material from the fibers.
FIGURE 1. Sketch of the experimental set-up in the facility. A: Machine hall; B: ELS testing cavern, covered with a shutter (represented with a grid); and C: ELS control room.
In this study, the fiber release and possible risk for the operating staff in two CFRP cable tensile tests at the Swiss Federal Laboratories for Materials Science and Technology (Empa) is reported. The experiments have been carried out using the ELS, a tensile test device with a maximum force of 30 MN.(17) The particle concentration measurements were focused not only on the exposure of the directly involved operating staff but also on the exposure of people, who might be present in the machine hall above the testing cavern during such tests. The second location was chosen because it was separated from the testing cavern only with a mechanical shutter, which is not air-tight. The particle concentrations of the respirable dust in the nano to sub-micro range (10–500 nm) and in the micro range (0.5–10 µm) were recorded. Further, particles were collected on filters and transmission electron microscopy (TEM) grids in order to determine the shape, size, and concentration. Three situations were investigated, the particle background concentrations before the experiments, the cable failure event, and also the opening of the shutter on top of the testing cavern as well as the hall gates and windows in order to vent the facility. MATERIALS AND METHODS Facility The ELS testing facility is located in the basement of a large machine hall (volume = 15,600 m3, location A in Figure 1) and consists of two compartments: (1) the test setup cavern (volume = 210 m3, location B), in which the mechanical testing device stands, and (2) a control room (volume = 285 m3, location C), where both the hydraulic infrastructure and the control desk for the ELS machine are located. The cavern is open to the machine hall above but can be closed with a moving shutter. From the control room, the cavern is accessible via two large entrances. They can be closed
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with moving protective panels only. The shutter and the panels have not been designed for being air-tight, and air can easily flow from the cavern to the adjacent control room as well as to the machine hall above. Air Quality Measurements Air sampling was conducted at two sites: in the control room and in the machine hall close to the shutter. For the characterization of particles in the micro range, an aerosol particle sizer (APS) (Model 3321, TSI, Shoreview, MN) and an optical particle sizer (OPS) (Model 3330, TSI) were used. The particles in the nano to sub-micro range were measured with a fast mobility particle sizer (FMPS) (Model 3091, TSI) and with a scanning mobility particle sizer (SMPS) (DMA Model 3080, TSI; CPC Model 3775, TSI). For particle collection, a constant flow sampler was used (Model Gilian AirCon2, Sensidyne, St. Petersburg, FL) and the particles were collected on Nuclepore track-etch membrane filters (111106, pore size 0.4 µm, Whatman, Little Chalfont, UK). The sampling flow rate was 5 L/min for all filter samples. For the TEM investigation (CM30, Philipps, Amsterdam, Netherlands) of collected particles, TEM grids were taped on the Nuclepore filters according to the work of Tsai et al.(18) The fiber concentration was measured following the procedure of NIOSH method 7400, but the fibers were imaged and counted by the usage of scanning electron microscopy (SEM) (Nova NanoSEM 230, FEI, Hillsboro, OR) images (image area of approximately 0.076 mm2) instead by phase contrast microscopy and Walton-Beckett graticule. The fibers were identified and counted according to “Rule B” (minimum length of 5 µm, maximum diameter of 3 µm, and aspect ratio > 5). For one experiment, the shutter, the roof windows, and gates were completely opened 15 min after the cable failure. The air exchange rates between the testing cavern and the machine hall, and between the control room and the machine hall were calculated by fitting the measured particle concentrations with Equation (1):(19) Q (1) C(t) = C0 + C1 · exp − · t , V where C(t) is the time dependent total particle concentration, C0 is the background concentration, C1 is the dust concentration just before the shutter, windows, and gates were opened, Q is the air exchange rate, V is the room volume, and t is the time. CFRP Cable Materials and Cable Failure Tests The cylindrical cables had two eye-shaped terminations on each side to anchor them to the ELS testing machine. They consisted of longitudinally loop-wise wound carbon fiber layers in the core (as an “endless race-track loop”) wrapped with a confining orthogonally oriented carbon fiber bandage. The carbon fibers had a filament diameter of 5 µm (Tenax IMS60, Teijin, Tokyo, Japan), a minimum experimentally certified tensile strength of 5,600 MPa with a longitudinal elastic modulus of 290 GPa and a minimum certified ultimate tensile strain R
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FIGURE 2.
Failure mode of the primary damage location
of 1.9 %.(20) The cables had a fiber volume fraction of 60% and the fibers were pre-impregnated by an epoxy polymer resin matrix (a proprietary epoxy hot melt by CarboLink, Fehraltorf, Switzerland) and were cured in an oven for 2 hr at 140◦ C. In a quasi-static tensile test, the cables were loaded to failure at room temperature. The load was increased continuously with a deformation controlled ramp of 8 mm/min. The stored elastic energy was released very abruptly, heard as an acoustic shock and a cloud of dust grew in the testing cavern. Both ends were heavily damaged. The primary failure occurred in the arch of the eye (Figure 2). On the opposite side a secondary damage was found caused by the load release wave that propagated through the cable and left damage as severe as the primary one, thus giving two comparable dust sources. RESULTS Background Prior to the tensile tests, the background particle concentrations and size distributions were measured. Further, filter samples were collected to identify other particle sources. The background concentrations, measured by SMPS and FMPS, varied between 9,000 and 10,000 cm−3 in the machine hall and also in the control room. The filter samples from the machine hall of the background air showed one single fiber. It had a diameter below 1 µm and a length of 230 µm. It was distinguishable from released carbon fibers due to the small diameter and the undamaged structure. Tensile Test Emissions Machine Hall The aerosol in the machine hall was monitored by FMPS, OPS, and a filter. The evolution of the particle number concentration without venting is shown in Figure 3a. Immediately after the cable failure at 16:15, the particle concentration increased from 9,000 cm−3 to 27,000 cm−3 in the nano to sub-micro range. In the micro range, the OPS also measured
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an increase in the particle concentration simultaneously with the FMPS, but this could be attributed to other sources since during the measurement, the detected particle concentration by OPS was rather inconsistent. The SEM and TEM images from the filter in the machine hall showed a low number of collected fibers. While some still had the original diameter of 5 µm and thus were not considered to be respirable, there were also a few fibers collected that were split during the cable failure and could be respirable due to diameters clearly below 3 µm (Figure 3e). On the TEM grid, several particles with rough surfaces were detected (Figure 3f). Those most probably were formed during the cable failure at locations in the cable where the tensile energy was converted into heat, similar to the micro- and nanoparticles formed during thermal cutting of polymer foams.(21) Control Room The development of the number and mass concentrations in the control room, measured by SMPS and APS and without venting after the experiment, is shown in Figure 3b; they increased to 110,000 cm−3 and to 260 µg/m3, respectively. Afterward, agglomeration resulted in a decrease of the particle number concentration while the mass concentration remained stable until the end of the measurement. The SEM images showed a large number of different fibers of which were all split and probably respirable due to the small diameter. Further, fragmented particles from the composite matrix and fibers, whereof most had a diameter below 10 µm, were also collected. The TEM images showed few fibers with a diameter below 1 µm. For the calculation of the fiber concentration in the control room after the cable failure, 20 SEM images were analyzed and a total of 180 fibers were detected. The histogram with the fiber length distribution is shown in Figure 3d The average fiber length was 13.9 µm and the longest detected fiber had a length of 53 µm. The measured fiber concentration was 0.76 fibers/cm3. Shutter Opening The shutter opening after the experiment allowed a calculation of the air exchange rates for the testing cavern and control room by Equation (1). The normalized concentrations are shown in Figure 3d. The fitting analysis shows that in the testing cavern the air can be assumed to be clean (i.e., removal of 99% of the dust) after 413 s and in the control room after 1855 s. The calculated air exchange rates for the testing cavern and the control room are 2.35 m3/s and 0.72 m3/s, respectively.
lied in the range of outdoor air and was not influenced by other conducted experiments. Directly after the failure of a CFRP cable, particles in the nano and micro range were released and transported to both the control room and the machine hall. The particle concentration in the control room increased to 105 particles/cm3. Compared to the particle exposure of a worker who is dry cutting a CFRP composite, the measured concentration was about one order of magnitude lower.(3) In the machine hall, the particle concentration increased to 27,000 particles/cm3 for the nanoto sub micro particles and the impact of the micro particles was barely measurable. Furthermore, for the machine hall, the particle concentration decreased back to background values within short time because of the large room volume. The filter samples proved the diffusion of a low number of respirable fibers into the machine hall. The staff in the control room was much more exposed to respirable fibers. The measured fiber concentration of 0.76 fiber/ cm3 lies close, but below the PEL given by the SCOEL. But this concentration was only valid for the time directly after the failure experiments. Assuming a staff member is performing 2 tensile tests per day in the control room and he is only exposed to fibers during the 15 min after the cable failure and during the venting, an average 8 hr fiber concentration of approximately 0.07 fibers/cm3 can be calculated. This is clearly below the PEL and thus the risk posed by the released fibers to the operating staff is low. For long-term exposure, not much data about the toxicity of fibers are available. Therefore, during the experiment, the use of nondisposable dust respirators with sealing gaskets (rating N95 or higher) is recommended. There is no carcinogenic classification by the International Agency for Research on Cancer (IARC) neither for carbon fibers nor graphite, their basic material.(22) Therefore, the use of a PEL of 1 fibers/cm3 for the investigated exposure is reasonable. But compared to neat carbon fibers, the released fibers from the CFRP cables have two additional features: a rough surface and agglomeration with epoxy particles that were exposed to heat. The effect on toxicity of these features has to be assessed. The venting by opening the shutter, gates, windows, and the shelter panels showed sufficient air exchange rates to clean the air in both the testing cavern and the control room. Due to settling of fibers on the floor and walls, a regular cleaning of the test cavern and the control room is recommended to avoid re-aerosilization and new exposure to fibers. Further, the diffusion of particles into other rooms of the facility and the environment could be avoided with the installation of a venting system that creates a partial vacuum inside the testing cavern.
DISCUSSION
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o ensure workers’ health, the exposure to particles and fibers was monitored for all measurement steps. Generally, the background air prior to the experiments was clean with respect to the fiber concentration, only one single fiber from an unknown source was found in the machine hall. The background particle concentration of about 10,000 particles/cm3
CONCLUSION
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his study investigated the particle release from two CFRP cables that underwent a high-energy tensile test until rupture. Measurements with aerosol devices and collection on filter samples showed that the cable failure caused a release
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FIGURE 3. Measurement results from the tensile test emissions in the machine hall and the control room: a) particle number concentration measured by FMPS and OPS (size range: 1–10 µm) in the machine hall; b) particle number and mass concentrations in the control room measured by SMPS and APS; c) histogram of the fiber length distribution for respirable fibers in the control room according to NIOSH 7400; d) effect of shutter opening and air exchange on the normalized particle concentrations in the control room and the testing cavern; e) SEM image of captured fibers; and f) TEM image of captured particles from the CFRP matrix material
of particles as well as free-standing fibers whereof a fraction had diameters below 3 µm and thus were respirable. Even though the tensile test was carried out in a separated room, particles and fibers did migrate through small openings into the control room and the adjacent machine hall. Both the particle mass concentration and the fiber concentration did remain below the PEL. A venting by fresh air of all rooms by opening the doors, gates, and windows of the building and the shutter above the testing cavern caused a reduction of the particle concentration back to the background concentration within approximately 30 min. Under the given circumstances, including the total amount of released particles and fibers, the room sizes, and the air exchange rates during the venting, in combination with the knowledge of toxicity tests, it can be assumed that the health D182
risk of the operating staff in the control room and of the workers in the machine hall was low. To avoid health effects due to longterm exposure, several suggestions were made to improve the occupational health. The cable failure also caused the release of heat induced epoxy particles for which the exposure was not assessed.
ACKNOWLEDGEMENTS
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he authors would like to thank Dr. Matthias Nagel for the organization of the measurement, the company CarboLink for allowing the publication of the measured data and the operating staff of the tensile test for the smooth collaboration.
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FUNDING
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his study was financed by the Swiss National Science Foundation (NFP 64), “Evaluation platform for safety and environment risks of carbon nanotube reinforced nanocomposites,” 406440 131286.
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