Surface Damage of Polymer Nanocomposites Project

Summary

Surface damage caused by environmental and/or mechanical stresses can lead to the release of nanofillers incorporated within polymer nanocomposites, leading to changes in optical, morphological, and mechanical properties, creating pathways for ingress of moisture, corrosive agents and/or cracks acting 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). 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 engineering applications. The results will be presented to the relevant ASTM and ISO committees, with the goal of being embodied in ASTM and ISO standards. Once adopted, these standards will effectively transfer new knowledge to end-users and manufacturers for measuring surface damage and nanofiller release in polymer nanocomposites.

Description

Objective: Currently, it is impossible to accurately measurethe release of nanofillers from surface-damaged polymer nanocomposites. 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 stress for polymer nanocomposites used in infrastructure and manufacturing.

What is the new technical idea?

The surface properties of polymer nanocomposites differ greatly from their bulk properties, and the surface is the first point of attack in any degradation process initiated by ultraviolet (UV) radiation, mechanical stress, temperature, and/or moisture. Current polymer nanocomposites are either a polymer matrix containing nanofillers or are sophisticated nano-enabled fiber-reinforced polymer (NeFRP) composites, which are conventional fiber-reinforced polymer composites containing nanofillers. These polymer nanocomposites are increasingly used in manufacturing (e.g., automotive, aerospace, and electronics) and infrastructure. Surface damage can lead to the release of nanofillers incorporated within the polymer nanocomposites, leading to changes in optical, morphological, and mechanical properties, and creating pathways for ingress of moisture, corrosive agents, and/or cracks acting as stress concentrators. Surface damage also potentially poses risks to environment, health, and safety (EHS). This project will develop 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 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 presented to the relevant ASTM and ISO committees, to be embodied in standards that will effectively transfer new knowledge to end-users and manufacturers.

What is the research plan?  

Initial research has focused 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.

Using model epoxy and acrylic nanocomposites, NIST has gained a good understanding in this project of the mechanisms of these nanocomposite degrade and release nanofillers during exposure to UV radiation. NIST has developed protocols and methods to expose samples, measured released nanofillers, and developed a kinetic model to predict the release rate of nanosilica as a function of UV dose. In FY14, in addition to completing the factorial experiment data analysis to provide kinetic parameters for further validation of the model, NIST will study surface damage and nanoparticle release in a polyurethane (PU) nanosilica composite. Selection of a PU nanocomposite has been recommended by industrial partners, and from inputs provided by the recent NanoProject workshop and Nanoparticle release symposia. PU matrix composites have many desirable commercial attributes and are used extensively in infrastructure, engineering, textiles, and consumer products.  However, this matrix is also susceptible to UV attack, with potential release of the embedded nanofillers, but the mechanisms may differ from epoxy and acrylic polymer matrices. These PU nanocomposites studies will broaden our knowledge of surface damage and nanoparticle mechanisms, so that the conclusions of this project are more general.   

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 are producers or users of nanofillers and polymer nanocomposites who are engaging with NIST in characterizing and optimizing the properties of nanomaterial systems.

The detailed research plan has three major tasks.

Task 1: Characterization – Quantitative measurement of the degree of dispersion of nanofillers in a nanocomposite and the release of nanofillers during surface damage 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.  Inductively-coupled plasma-optical emission spectroscopy (ICP-OES) will be used to measure nanofiller release. Surface mechanical, morphological, and chemical properties will be measured using indentation, spectroscopy, and optical scattering.  The data obtained from these measurements will be used to analyze 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 will be exposed to specified UV radiation/temperature/relative humidity conditions using the NIST SPHERE and specially designed sample holders. 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 the techniques described in Task 1. To assess mechanically-induced surface damage, 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 are developed in Task 3.     

^[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

 

Major Accomplishments

Recent Results:

Technology Transfer Outcomes in FY13

• Design and fabrication of a new UV exposure cell to improve sampling and data collection for nanoparticle release measurement. This device will be included in the proposed ASTM photo-induced particle release test methods.
• Development of a laser scanning confocal microscopy measurement protocol for quantifying released metal-oxide nanoparticles on the surface of polymer nanocomposites after mechanical scratching or abrasion.
• Completion of a round robin experiment to develop methodology and standard tests for quantifying the release of nanoparticles by weathering, which was a joint project with the European Union NanoGEM and Polynanotox projects.
• Completion of a work plan for methods development and a state of the science analysis of nanofiller release in the Inter-laboratory Testing Group of the NanoRelease Project, which is a multi-stakeholder project led by government (NIST, CPSC, EPA), industry, and consumer and labor organizations under the non-profit International Life Sciences Institute (ILSI).

Major Product: 

• An experimental-validated reaction kinetic-based model to predict the rate of nanosilica release from epoxy and other selected nanocomposites exposed to UV radiation.
• New knowledge/technology, draft test method and measurement protocols for characterizing surface damage resistance of selected polymer nanocomposite systems; reviewed and validated by relevant industrial users and submitted for consideration as ASTM guidelines and standard test methods.
• The data, mechanisms, and models on surface accumulation and release of nanofillers during environmental exposures of nano-filled polymer composites generated by this project will help the public become aware of the health and environment risks, and will help international regulatory agencies to accurately assess and mitigate the risks, associated with surface accumulation and release of nanofillers, during the life cycle of these advanced composites.