Thomas Kuhlbusch, Richard Canady 1, Elyse Lee 1, Libby Tsytsikova 1 Comparison of existing studies...

Thomas Kuhlbusch, Richard Canady1, Elyse Lee1, Libby Tsytsikova1

Comparison of existing studies of release

measurement for MWCNT-polymer 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

NanoRelease Consumer Products: MWCNT in Polymer

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 by1. 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regarding (focus on unpublished analyses, details of the studies relevant to selecting materials and methods),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.

Thomas Kuhlbusch et al. 2


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



Mechanical processes (mainly)

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• 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



• 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 G

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Mechanical processes (mainly)


Chemical and physical processes

• 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

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Thermal 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

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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



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



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



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



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)


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)


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



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



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




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.




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)



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.



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



CNT material

• Nearly all release related studies were conducted for non-functionalized 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


Thomas Kuhlbusch, Richard Canady 1, Elyse Lee 1, Libby Tsytsikova 1 Comparison of existing studies...

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