Thomas Kuhlbusch, Richard Canady1,
Elyse Lee1, Libby Tsytsikova1
Comparison of existing
studies of release
measurement for MWCNTpolymer composites:
Report to Steering
Committee
IUTA e.V.,
“Air Quality &
Sustainable
Nanotechnology“
Germany
1 Center
for Risk Science
Innovation and
Application, ILSI
Research Foundation,
USA
Aim of the Report
Provide a clear path forward for methods development for measuring
release of multi-walled carbon nanotubes (MWCNTs) from polymer
matrices.
through the review of the specific strengths and limitations of existing test
methodologies by
1. reviewing “all” published studies of MWCNT release from polymer
composites,
2. Interviews of experts in laboratories that had conducted such studies
regarding (focus on unpublished analyses, details of the studies
relevant to selecting materials and methods),
3. comparing the results to the draft white papers prepared by the
NanoRelease project, and
4. compilation of a report to support NanoRelease Consumer Products
Steering Committee deliberations for selecting material(s) and
method(s) to be carried forth for Phase 3: Inter-Laboratory Studies.
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Content
- Summary information on release mechanisms
(from the literature review and interviews)
- Results of the Interviews and Existing Studies
- Summary of findings in the Task Group White
Papers
(not presented)
- Expert comments and points to consider
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Schlagenhauf et al. (2012)
• Mixing and Sonication
• mechanical process where the nanomaterial is brought into or already in a
liquid phase
• no standard simulation machines available
• Sanding
• higher energy type of mechanical stress where
shear forces of a rough surface act on the matrix
• e.g. orbital sanding, belt sanding, disc sanding
• no standard simulation machines available
• Abrasion
• mechanical process describing the dynamic
friction between two surfaces
• e.g. Taber Abraser Degradation
• many national and international standards
• Grinding
• mixed process of milling and cutting
• no standard simulation machines available
Göhler et al. (2010)
Mechanical processes (mainly)
• Drilling
• mechanical process where high speed mechanical shear forces are used
often to produce a hole
• e.g. automated drill press
• no standard simulation machines available
• Cutting/Sawing
• low speed mechanical process with a limited contact area to the material
• e.g. band-saw, rotary cutting wheel, wet saw cutting
• no standard simulation machines available
• Scratching
• special case of low speed mechanical process with a limited contact area
• e.g. linear taber with metal tool
• no standard simulation machines available
• Mechanical shock
• special case of low energy mechanical process
• e.g. vibrating engraver tool
• no standard simulation machines available
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Golanski et al. (2012)
Mechanical processes (mainly)
• UV Weathering
• process of degradation and release that can be simulated using UV
radiation
• e.g. weathering apparatus, Suntest™ XLS+, SPHERE (Simulated
Photodegradation via High Energy Radiant Exposure)
• ISO 4892-2:2006
• Wet Weathering
• extends on the dry weathering
method by adding simulated rain
• e.g. weathering apparatus,
SunTest XLS+ or XXL,
real-time precipitation
• ISO 4892/06
SunTest XLS+
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Wohlleben et al. (2013)
Chemical and physical processes
• Thermal Degradation
• mechanism selectively removing the matrix from a CNT containing
polymer
• e.g. thermogravimetric analyses
• Combustion
• exothermal process not entirely different from
thermal degradation
• main difference: high energy release in the form
of heat during explosion or fire
• Incineration
• aims at high temperatures in an oxygen sufficient
system
PerkinElmer
Pyris 1 TGA
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www.perkinelmer.de
Thermal processes
Literature about CNT release
Release mechanism
Number of studies
Number of interviews
Mixing/Sonication
2
0
Sanding
15
4
Abrasion
10
3
Grinding
3
1
Drilling
2
1
Cutting/Sawing
3
1
UV Weathering
5
5
Wet Weathering
3
4
Thermal degradation
2
0
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Interviews - CNT & polymers
• MWCNT (e.g. Baytubes C 150P, Nanocyl NC 7000)
• Mainly non-functionalized CNT
• Commonly used polymer materials:
• Epoxy (most common)
• Polyamide
• Polycarbonate
• Polyethylene
• Polyurethane
• Testing of rubbers containing CNT was mentioned in several
interviews as being of relevance for possible future use
• Typical load used: < 10 wt% CNT in polymer (average 2-3 wt%)
• Compounds added to the CNT-polymer formulation included:
curing agents, carbon fiber, organophosphorus flame-retardants
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Interviews - Release scenario
• Most release scenarios modeled occupational situation, with a
focus on various aspects of the machining process (drilling,
cutting/sawing, sanding, grinding, etc.) and abrasion. A few studies
evaluated weathering (UV and wet).
• Sanding: accomplished manually, mechanically or through an
automated system
• Grinding: grinder, double grinder, or mill
• Abrasion: most commonly Taber equipment
• Weathering: weathering chamber or real-time outdoor
conditions
• The majority of studies were simulation, although a few involved
actual workplace measurement.
• Most studied release into the air and some also into liquids (water)
• Scenarios are intended to represent “real world” conditions
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Interviews - Sampling & Analytical method
• Methods based on existing standards were used for abrasion tests
and weathering
• No standard sampling methods exists specific for sampling of
released particles. General international/national standards were
sometimes applied.
• More than one analytical technique was used to characterize and
quantify the released material (mainly particle counting methods
and microscopy)
• Few CNT specific release amounts were reported to be
quantitative
• Standard analytical methods were applied when available,
otherwise internal SOPs were used
• In many cases overall particle mass, mass distribution, diameter,
and count were compared to negative control without CNT
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Interviews - Instrumentation
A wide range of instrumentation was used. However, most
instruments were only used to describe particle counts that did not
distinguish CNT from other particles.
Aerosol analysis
• Aerodynamic Particle Sizer (APS)
• Condensation Particle Counter (CPC)
• Electrostatic Precipitator (ESP)
• Electrical Low Pressure Impactor (ELPI)
• Fast Mobility Particle Sizer (FMPS)
• Nano-Aerosol Sampler (NAS)
• Optical Particle Counter (OPC)
• Particle Surface Sensitive Device (e.g. NSAM, DiscMini, Nanocheck)
• Scanning Mobility Particle Sizer (SMPS)
• Thermophoretic Precipitator (TP)
• Universal Nano Particle Analyzer (UNPA)
• Wide-Range Aerosol Particle Sampling System (WRASS)
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Interviews - Instrumentation
Suspension analysis
• Analytical Ultracentifugation (AUC)
• Dynamic Light Scattering (DLS)
• Laser Diffraction Particle Size Analyzer (LDPSA)
Chemical and morphological analysis
• Energy Dispersive X-Ray Analysis (EDX)
• Field Emission Scanning Electron Microscope (FE-SEM)
• Fourier transform infrared spectroscopy (FTIR)
• Inductively coupled plasma mass spectrometry (ICP-MS)
• Laser-Induced Breakdown Spectroscopy (LIBS)
• Photoelectric aerosol sensor (PAS)
• Scanning electron microscopy (SEM)
• Thermogravimetric Analyzer (TGA)
• Time-Of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)
• Transmission electron microscopy (TEM)
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Interviews - Existing collaboration and networks
• So far no participation in an interlaboratory CNT release study
• Most interview partners are part of a consortium or collective
with regard to the measurement of nanoparticle release
• Several interview partners use shared facilities or collaborate
with other laboratories
• General willingness to participate in an interlaboratory study or
a pilot project for developing standard methods
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Expert comments and points to consider
• Basis for selecting which method(s) and material(s) to be further
developed in pilot methods development and inter-laboratory
approach
• Identify the material-method-scenario conditions which are the
most feasible and offer the greatest utility to improving
measurements needed today
• Release via three main mechanisms: 1) mechanical stress,
2) thermal stress and 3) complex stress (e.g. weathering)
• Most studies published to date are related to mechanical stress.
Of these, the majority of studies are related to sanding and
abrasion
• Sanding, abrasion, and weathering qualify among the most
practical to pursue on the basis of published work and expertise
in academic, industry, and government laboratories
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Mechanisms of MWCNT Release
Sanding
• Frequently studied by research groups all over the world
• Viewed as a relevant source of personal exposure
• Housed facilities have been developed allowing easier identification and
quantification of the emission and ensure safety for the worker
• First interlaboratory test will start soon
• No standard testing conditions have been applied yet
• Some basic information is missing such as influence of grit size and weight of
sanding paper
• Information on heat production during sanding is limited
Abrasion
• Frequently studied by research groups all over the world
• Viewed as relevant source of personal exposure
• Housed facilities have been developed allowing easier identification and
quantification of the emission and ensure safety for the worker
• Standards for this testing procedure exist and were employed in some studies
• Relevant process parameters have yet to be harmonized
Recommendation: Start with testing of sanding and abrasion
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Mechanisms of MWCNT Release
Weathering
• Frequently studied from research groups all over the world
• Viewed as relevant source of environmental release and exposure
• Housed facilities are available and exposure to test personnel can be limited
• Particle release can be discriminated from background by using enclosures
• Standards for artificial weathering exists and were employed in most weathering
studies
• Standardization is seen as a straight forward approach but specific analytical
methods for the liquid phase may require development
Recommendation: Weathering is viewed as important, following abrasion and
sanding.
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Mechanisms of MWCNT Release
Thermal degradation
• Heat stress, combustion and incineration processes
• Studied in a limited context using TGA techniques, simulation of combustion
conditions in incinerators, or general combustion conditions
• Heat stress (TGA) is sometimes viewed as a simulation of aging
• The number of researchers involved is limited to two groups
• Full combustion of tested CNTs when temperatures exceed 700°C for an
adequate time period. Consequently, combustion is viewed as of minor
importance in this evaluation.
• MWCNT may be more stable at high temperatures, especially in polymers
formulated with metals and metal-oxides, which may make thermal degradation
a more important pathway
Recommendation: Define conditions to ensure complete incineration of CNT (see
dioxines); some basic evaluation studies needed for accidental combustion and
heat stress (e.g. TGA)
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Detection methods
• Electron microscopy is the only specific method identified to
characterize MWCNT and MWCNT-polymer fragments
• Some methods have been used to estimate upper limits of what
amount of exposure-relevant MWCNT could be present in
released material such as those measuring particle number,
surface area or mass concentrations
• Concentration of catalysts (e.g. Ni, Co) used for the production
of the specific MWCNT to estimate MWCNT concentrations
• Thermal-optical carbon analysis of the sample for CNT
quantification
Recommendation: Combined measurement strategy for
quantitative MWCNT detection has to be established, e.g. based
on number size distribution and electron microscopy.
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Polymer material
• Primary criteria: commercial relevance and relevance to release
scenarios
• Wide variety of polymer composite materials were used in
existing studies
• No specific recommendation on material selection can be
derived from the studies in the literature
• Choosing materials based on the range of methods that are
being developed is to be considered in first standardization
tests to define realistic test conditions
Recommendation: A mix of different polymer materials with
specific varying properties should be chosen, e.g. brittle and soft
material, hard combustible to easy inflammable, depending on
the specific release scenario of interest
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CNT material
• Nearly all release related studies were conducted for nonfunctionalized MWCNTs
• The type of CNT (e.g. long straight, bent, multi-walled, and
single-walled) can lead to significant differences in the way
CNTs are embedded into the polymer
• However, information on this are very limited in the published
reports
• Therefore, MWCNT functionalization (e.g. carboxyl, amine, and
hydroxyl) should be considered
Recommendation: No specific source or type of CNT can be
recommended based on the studies reviewed
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