Surface Damage of Polymer Nanocomposites Project

By 2014, this project will develop and implement measurement science to characterize, model, and predict surface damage and nanofiller release as a function of environmental and mechanical stresses for polymer nanocomposites used in infrastructure and manufacturing applications. The results will be embodied in ASTM and ISO standards, effectively transferring new knowledge to end-users and manufacturers for measuring surface damage and nanofiller release in polymer nanocomposites.

Objective: By 2014, this project will develop and implement measurement science to characterize, model, and predict surface damage and nanofiller release as a function of environmental and mechanical stresses for polymer nanocomposites used in infrastructure and manufacturing.

What is the new technical idea? It is well-documented that the surface properties of polymeric systems differ greatly from their bulk properties, and that the surface is the first point of attack in any degradation process initiated by ultraviolet (UV) radiation, mechanical stress, temperature, and/or moisture. Currently, polymer nanocomposites, which are either a polymer matrix containing nanofillers or are sophisticated nano-enabled fiber-reinforced polymer (NeFRP) composites (conventional fiber-reinforced polymer composites containing nanofillers), are increasingly used or being explored for use in manufacturing (e.g., automotive, aerospace, and electronics) and infrastructure. Surface damage caused by environmental and/or mechanical stresses can lead to the release of nanofillers incorporated within the polymer nanocomposites, changes in optical, morphological, and mechanical properties, pathways for ingress of moisture and corrosive agents, and/or cracks as stress concentrators. The surface damage not only affects the long-term performance of these complex materials but also potentially poses risks to environment, health, and safety (EHS). This project will develop advanced methods for characterization and modeling of surface damage to enable a comprehensive understanding of how nanofillers impact surface properties, and how surface properties influence the long-term performance (including EHS) of complex polymer nanocomposites.^[1],[2] This project will develop scientifically-based performance protocols that will be incorporated into standard test methods for characterization and quantification of surface damage and ultimately serve as inputs into models to accurately predict the long-term performance of polymer nanocomposites. The results will be embodied in ASTM and ISO standards, effectively transferring new knowledge to end-users and manufacturers with a main goal to minimize surface damage and nanofiller release for all polymer nanocomposites.

What is the research plan? Measurement science research will address two critical problems that have hindered the innovation and commercialization of polymer nanocomposites:

• The role of nanofiller dispersion on surface damage resistance and long-term performance, and
• The fate of nanofillers during the life cycle of nanocomposites. 

The research plan consists of three major components:

• Characterization of material structure and properties as a function of nanofiller type, degree of dispersion, and interface properties,
• Exposure of materials to relevant environmental (e.g., UV, temperature, relative humidity) and mechanical stresses (e.g., scratch, abrasion, and impact) and assessing long-term performance and nanofiller release kinetics, and
• Development of models to describe surface damage and nanofiller release, and use of these models to predict polymer nanocomposite performance and safety as a function of environmental and mechanical stresses.

Because the structure of NeFRP and related composites is very complex and has not been well investigated, initial research of this project will focus on two model matrix systems containing spherical nanoparticles: an acrylic polymer matrix containing nano-titanium dioxide (nanoTiO₂) and an epoxy matrix containing nanosilica (nanoSiO₂). These two model materials actually represent two of the most common polymer nanocomposites used in infrastructure and manufacturing, so this choice is practical as well as expedient for research. Data and knowledge gained from these nanocomposites will be directly applicable to nano-enabled fiber reinforced polymer composites and other types of nano-filled polymer composites. With inputs from industrial partners to improve/refine the measurement protocols, more complex nanocomposite systems will be identified and investigated.

Task 1:  Characterization – Quantitative measurement of both the degree of dispersion of nanofillers in a polymer matrix and the release of nanofillers during the life cycle of nanocomposites is very challenging and so requires sensitive methods to be developed for these measurements. Scattering, microscopy, and chemical spectroscopy-based technologies will be developed/adopted for quantifying nanofiller dispersion, polymer/filler adhesion, and the resulting structure/morphology of the nanocomposites.  Additionally, advanced techniques such as inductively-coupled plasma-optical emission spectroscopy (ICP-OES) will be investigated for quantitative measurement of nanofiller release. To characterize surface damage due to applied mechanical and environmental stresses, critical surface mechanical, morphological, and chemical properties will be measured using a number of sensitive surface analytical techniques (indentation, spectroscopy, and optical scattering).  The data obtained from these measurements will be used for understanding the mechanism of surface damage resulting from environmental exposures in Task 2, and for validating the models developed in Task 3.

Task 2:  Exposure – Polymer nanocomposite samples prepared with different levels of filler dispersion will be exposed to specified UV radiation/temperature/relative humidity conditions using the NIST SPHERE and special designed sample holders. Varying environmental and mechanical stresses will be applied to provide essential data for understanding the surface damage mechanism and for validating prediction models. Nanofiller release during environmental exposures will be captured using a novel sample holder and quantified using advanced techniques described in Task 1.  To assess mechanically-induced surface damages, the NIST-developed scratch test method will be used under various force conditions.

Task 3: Modeling – Linkages between surface mechanical, morphological and chemical properties will be established. Mechanically-induced surface damage will be modeled using first principle polymer physics, and environmentally-induced nanofiller release will be modeled using the laws of chemical reaction kinetics. The models will be developed through collaborations with researchers from the Structural Systems Group and the Statistical Division at NIST and from academic institutes. Data obtained in Task 1 and Task 2 will be used as inputs and verification for the models that will be developed in Task 3.

This project involves collaborations with various partners. The Consumer Product Safety Commission (CPSC), Environmental Protection Agency (EPA), the National Institute for Occupational Safety and Health (NIOSH), and the National Institutes of Health are partners with a mission to understand and control the exposure of humans and the environment to nanomaterials. Boeing, BYK-USA, CSM International, and Eastman Chemical are all members of the NIST/Industry Polymer Surface and Interface (PSI) consortium. These companies, along with DuPont, Dow Chemical Inc., PPG, Eastman Chemical, Arkema, International Automotive Component (IAC) Group- North America, and CEA (France) are producers of nanofillers and polymers, formulators, or users of polymer nanocomposites who are engaged with NIST in characterizing and optimizing the properties of nanomaterial systems. The Federal Highway Administration (FHWA), Department of Defense (DoD) and Sandia National Laboratory are users of nano-filled and NeFRP composites who undertake joint research in the area.

[1] “The New Steel?  Enabling the Carbon Nanomaterials Revolution:  Markets, Metrology and Scale-Up,” http://www.nist.gov/cnst/thenewsteel.cfm

[2] A Research Strategy for Environmental, Health, and Safety Aspects of Engineered Nanomaterials (2012),
Committee to Develop a Research Strategy for Environmental, Health, and Safety Aspects of Engineered Nanomaterials, National Research Council, http://www.nap.edu/catalog.php?record_id=13347

Recent Results:

Recognition:

• NIST Bronze awards, 12/2011, for development and delivery of measurement science to relate optical, mechanical and chemical properties of polymeric coatings to their scratch and damage resistance.
• NIST work on photo-induced release of carbon nanotubes and nanosilica from epoxy nanocomposites exposed to UV is cited in almost every current publication and repeatedly shown, with credit to NIST, in workshops related to nanoparticle release and nano EHS.

Outputs:

• Workshops on “Polymer Surface and Interface Characterization: Metrology Development and Applications” with invited speakers from universities, industry, and other agencies (Nov. 2, 2011 at The Boeing Co, Seattle, WA and March 6, 2012 at NIST, Gaithersburg, MD), provided opportunities for researchers to discuss industrial practice and exchange technical expertise/experiences.
• Organized symposia on “Nanoparticle release during the life cycle of polymer nanocomposites and “EHS of silver nanoparticles” (silver nanoparticles are used as antibacterial agent in paints and textiles), June 18-21, Santa Clara, CA, which provided opportunities for researchers, manufacturers, and policy makers to learn state-of-the-art research in nanofiller release and receive technical feedbacks from international experts.
• Seven publications through WERB in FY2012, providing the technical foundation for understanding of nanofiller dispersion and release effects, which will be referenced in the ASTM and ISO standards being developed.
• Seven invited presentations, disseminating NIST latest research efforts on surface damage and nanofiller release of polymer nanocomposites, characterization techniques and protocols, and providing opportunities for collaboration with researchers from industry, academic, and other agencies.
• Drafted a measurement protocol/methodology to link surface damage to optical diffuse scattering –working with industrial partners to develop a standardized nanoindenter scratch test methodology to predict optical performance after field scratch tests for multi-layer composites.

Outcomes:

• Established a three-year new Phase for NIST-Industry Polymer Surface/Interface (PSI) Consortium, starting May 1, 2012 with four companies (CSM Instrument, BYK-USA, Eastman Chemical Co., and The Boeing Co.). The research focus includes development of measurement science for characterizing surface damage and interfacial adhesion of multi-layer, polymeric systems and composites.
• NIST serves in the Inter-laboratory Testing Group of the NanoRelease Project, which is charged with drafting a work plan for methods development and a state of the science analysis of nanofiller release. The NanoRelease Project is a multi-stakeholder project led by a Steering Committee of experts from government, industry, consumer and labor organizations under the  non-profit International Life Sciences Institute (ILSI), seeking to develop standard methods to measure release of engineered nanomaterials from articles relevant to commerce.
• Program personnel joined the EU NanoGEM and Polynanotox projects to develop methodology and standard tests to quantify release of nanoparticles by weathering.
• Developed a protocol to expose polymer nanocomposites to UV radiation, collect, and measure (at sub-nanogram resolution) photo-induced quantity of nanoparticles released from nanocomposites as a function of UV dose.  These data are used to verify kinetic-based model for nanoparticle release.
• Developed a reaction kinetic-based model to predict the rate of nanosilica release from epoxy nanocomposites exposed to UV radiation.

Standards and Codes:

PIs working with ISO TC 229 Nanotechnologies, and are participating in ASTM E56.03 Nanotechnologies-Environment, Health and Safety on a measurement protocol for quantifying nanofiller release from polymer nanocomposites. Draft on standard practice and protocols for measuring nanosilica release from polymer nanocomposites exposed to UV radiation will be submitted to ASTM E56.03.

PIs working with ASTM D01 Paint and Related Coatings, Materials and Applications and  subcommittees D01.23 Physical Properties of Applied Paint Films, D01.26 Optical Properties, and D01.24 Dispersion and Particle Size, to develop measurement protocols for quantifying nanofiller dispersion, related surface properties (mechanical and optical properties), and surface damage assessment for selected composites with the support of industrial users.