Evaluating the Use of Alternatives Assessment To Compare Bulk

Research Article pubs.acs.org/journal/ascecg

Evaluating the Use of Alternatives Assessment To Compare Bulk Organic Chemical and Nanomaterial Alternatives to Brominated Flame Retardants Leanne M. Gilbertson*,† and Carla A. Ng*,† †

Department of Civil and Environmental Engineering, University of Pittsburgh, 3700 O’Hara Street, Pittsburgh, Pennsylvania 15261, United States S Supporting Information *

ABSTRACT: Alternatives assessment (AA) provides a framework for selection of safer substitutes for problematic chemicals. This study assesses alternatives for flame retardants (FR) in electrical and electronic equipment (EEE), including two common brominated FR, decabromodiphenyl ether (deca-BDE) and tetrabromobisphenol A (TBBPA). Although deca-BDE is restricted in the EU and undergoing phase-out in the US, TBBPA is still widely used. However, concerns about potential hazards are driving a search for halogen-free alternatives. Nonhalogenated organic chemical alternatives (e.g., phosphorus-based FRs) as well as minerals (e.g., montmorillonite) and nanomaterials (e.g., carbon nanotubes) have been proposed, yet it is unclear whether current frameworks can be used to systematically compare such heterogeneous alternatives. This study aims to (i) identify technologically and economically viable alternative FRs and (ii) evaluate each under the current AA frameworks, to (iii) elucidate challenges and shortcomings to adopting proposed alternatives. Uncertainties persist regarding the hazards of both novel nanomaterials and traditional chemicals. Historically, problematic chemicals undergoing restriction have been substituted with another chemical providing, at best, marginally reduced hazard, a problem that AA was, in part, developed to solve. Its successful implementation will depend on our ability to reduce hazard during the design stage, which is currently precluded by the “commercially available and economically viable” emphasis of AA. Methods are needed to bridge AA with sustainable chemical design to prevent it from becoming a tool of only incremental improvement. KEYWORDS: Alternatives assessment, Flame retardant, Engineered nanomaterials, Carbon nanotubes, Montmorillonite, Nanoclay, Design for the environment

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INTRODUCTION

However, existing AA frameworks are largely reactive and, therefore, are not in themselves a means to achieve chemical and material innovation. GreenBlue’s CleanGredients15 and the EU’s SUBSPORT16 provide databases and tools to select safer alternatives. However, many of the alternatives presented in these databases are (often structurally similar) drop-in chemical substitutes rather than functional use substitutes with substantially different attributes.17 For example, the BPA alternatives assessment case study provided by SUBSPORT still retains bisphenol S (BPS) as a viable substitute for BPA in thermal papers. 18 Recent work has revealed that the replacement of BPA by BPS may be a prime example of a regrettable substitution.19

Alternatives assessment (AA) is a systematic process for identifying and comparing safer substitutes for chemicals of concern. Increasing public and regulatory pressures in recent years have driven industries to seek alternatives to chemicals such as bisphenol A (BPA),1 phthalates,2 and nonylphenol ethoxylates.3 Although AA is not a new concept (see, e.g., Sances and Ingham 1997 or Wick and Nissenbaum 19984,5), these pressures have led to a greater awareness and wider use of AA. It is now incorporated in both regulatory and private sector frameworks that seek to minimize the human and environmental health impacts of such hazardous substances.6−12 Ideally, application of AA should be “action-oriented” and lead to the adoption of inherently safer substances.6,7,13 Successful implementation of AA, then, should be congruent with and complementary to the goals of Green Chemistry, which aims to reduce risk across the life cycle by designing inherently safer chemicals and processes, guided by the Twelve Principles.14 © XXXX American Chemical Society

Special Issue: Building on 25 Years of Green Chemistry and Engineering for a Sustainable Future Received: June 13, 2016 Revised: October 3, 2016

A

DOI: 10.1021/acssuschemeng.6b01318 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Given their proposed technical feasibility, the question becomes how to evaluate ENM and drop-in alternatives in comparison to current FRs. Here, we conduct a heterogeneous AA (one that involves bulk organic chemical drop-in replacements, minerals, and ENMs) as a first attempt to evaluate comprehensively the potential to substitute ENMs for currentuse FRs. The assessment is carried out following a procedure that combines the features of two commonly used, comprehensive alternatives assessment frameworks: the US EPA’s DfE Program8 and BizNGO’s Chemical Alternatives Assessment Protocol (CAAP).40 The stages include (1) identify chemical of concern, (2) characterize end uses and function, (3) identify and prioritize alternatives, (4) assess hazards of alternatives, (5) evaluate technical and economic performance, (6) consider life cycle impacts and exposure, (7) select preferred alternative, and (8) implement safer alternative. Stages 1−6 are discussed in turn according to the established frameworks followed by identified challenges that prevent moving forward to Stage 7, selecting the preferred alternative. The outcome from this AA is intended to elucidate challenges to the current approach and shortcomings in the ability to compare bulk organic chemical, mineral, and nanomaterial alternatives in the face of persistent uncertainty.

In practice, the application of AA can be hampered by lack of knowledge (few empirical data available for alternative chemicals, mixture or materials) or lack of suitable comparisons among substances with very different properties or methods of application.6 In case studies provided by the Design for Environment (DfE) Program of the US Environmental Protection Agency (EPA),20,21 functional use substitutes, such as nanomaterials, are excluded from the analysis due to a lack of expertise on how to compare them to more traditional drop-in replacements. Thus, even when AA is applied under several frameworks to a class of chemicals that has been extensively studied, the selection of a suitable “safer substitute” can be unclear or appear restricted to drop-in replacements that may not represent a substantial step toward greener chemistry. A systematic approach for proactive safer design was introduced through the 12 Principles of Green Chemistry and Green Engineering.14,22 Since their inception, the Principles have provided a guiding framework for the chemical, pharmaceutical, and related industries to meet the triple bottom line.23−25 Furthermore, these principles have been adapted for their application to ENMs.26−29 At the same time, critical components of AA, including the consideration of inherent hazard, technical performance, and life cycle exposure, could inform the design principles needed to achieve a greener chemical industry. Here, we consider the case of brominated flame retardants (BFRs), a class of chemicals receiving attention due to their negative impacts on human health and the environment,30,31 and for which better alternatives are being actively sought by both regulators and industry.32 We focus our analysis on the specific functional use of BFRs for electrical and electronic equipment (EEE). EEE applications consume a large portion of BFR production volume, yet there has been limited success in finding suitable replacements, especially for printed circuit boards, where tetrabromobisphenol A (TBBPA), a problematic BFR, still dominates the industry.33 In this study, the suitability of AA to identify and select safer alternatives to FRs like TBBPA in electrical and electronic equipment (EEE) is examined. A conventional AA based on currently used frameworks is used and applied to three classes of substances: BFRs in need of substitution, “drop-in” replacement chemicals that are already commercially available, minerals that can serve as functional alternatives, and novel substancesspecifically nanomaterialsthat offer equivalent functionality to BFRs but have no current commercial application in EEE. Engineered nanomaterials (ENMs) have been and continue to be proposed as potential alternatives to currently used chemicals, promising functional novelty and/or enhancements. This is also the case for applications as FRs, where carbon-based nanomaterials (CNMs) and nanoclays (i.e., montmorillonite, MMT) are being tested to meet flammability standards (e.g., UL-94).34−37 Given the number of examples in which unintended consequences of a chemical were realized only after widespread adoption−halogenated flame retardants being just one−it is important to exercise vigilance in considering this replacement. The science and engineering communities have been engaged in research for the past decade to understand better the potential adverse environmental and human health implications of ENMs to preclude future unintended consequences.38,39 Yet, the incorporation of data from this body of literature has not yet been compiled in a formal AA, specifically for their use as alternative FRs in EEE applications.

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MATERIALS AND METHODS

Identification and Consolidation of Alternatives Assessment Frameworks. Jacobs et al. provide an excellent review of the types and features of available alternatives assessment frameworks, which covers 20 different frameworks developed by regulatory agencies (e.g., US EPA, European Chemicals Agency, Ontario Toxics Use Reduction Program), NGOs and IGOs (BizNGO, United Nations Environment Programme), and academia (UCLA Sustainable Policy & Technology Program, Lowell Center for Sustainable Production, National Academy of Sciences).6 The approach herein is based on the US EPA’s DfE Program and BizNGO’s CAAP. The former was chosen primarily because it has produced two recent and comprehensive AAs for decaBDE and flame retardants in printed circuit boards.20,21 The latter was chosen in order to explicitly consider life cycle exposure related to identified alternatives as well as their cost. These two frameworks are also identified as being particularly accessible and transparent.9 Identification of Drop-In Organic Chemical, Mineral and Nanomaterial Alternatives. Four classes of substances were chosen for this assessment. First, decaBDE, a PBDE, and TBBPA, a nonPBDE BFR, were selected as the FRs used in EEE applications for which safer alternatives are being actively sought. PBDEs are globally distributed contaminants, and can be found in the tissues of most organisms due to accumulation from the surrounding environment.30,41,42 DecaBDE was banned in Europe in 2008 under the EU Restriction of Hazardous Substances (RoHS) and entered a three-year voluntary phase out in the US in December 2009.43,44 In 2001, decaBDE was the most widely used FR for plastic enclosures in electronic equipment in the US, accounting for 80% of the 24 500 t demand for decaBDE,45 nearly half of the global demand. Since that time, many of the major electronics manufacturers, including Dell, HP, Sony, IBM, Apple, and Intel, among others, have phased out decaBDE in their products.45 In response to this substitution pressure, several AAs have been conducted by the US EPA, independent NGOs, individual states and European Authorities to identify possible replacements.21,45−47 However, lack of data on the physicochemical properties, human and ecological health impacts, and environmental fate of alternative FRs has hampered the substitution process. Production infrastructure, product formulation, and even regulation relating to performance metrics can be heavily driven by the existing dominant chemical class, making it difficult to find viable alternatives outside the class. Deca-BDE has therefore primarily been replaced with either phosphorus-based nonhalogenated FRs or with alternative B


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Table 1. Hazard Summary Table, Including Screening-Level Hazard Summary Based on US EPA’s Design for Environment (DfE) Hazard Criteria57a aquatic toxicity

standard FR mineral/ENM alternative

deca-BDE TBBPA DOPO melamine poly phosphate single-walled carbon nanotube (SWNT) multiwalled carbon nanotube (MWNT) nanomontmorillonite (nano-MMT) bulk-montmorillonite (bulk-MMT)

environmental fate

CAS no.

acute

chronic

persistence

bioaccumulation

1163-19-5 79-94-7 35948-25-5 15541-60-3 308068-56-6b 308068-56-6b N/A 1318-93-0

L VH L L VH H M H

L H M L

VH H H H N-RB N-RB

H M L L L L

M

a

L = low, M = moderate, H = high, VH = very high, N-RB = not readily biodegradable designations for aquatic toxicity and environmental fate. Italics and bold indicate hazard designations based estimate values. bCAS no. for pristine carbon nanotubes (functionalized CNTs not included).

BFRs. Because in-place chemicals are usually those with the most known information, alternatives that are very different can be assumed riskier due to lack of knowledge proving otherwise. This “devil you know” argument has been used to justify the continued dominance of TBBPA as a FR in printed circuit boards.48 The use of TBBPA as a reactive FR in circuit boards is not currently regulated and it is a highproduction-volume chemical in both the US and China. It is the most widely used FR in printed circuit boards, used as a reactive FR in 90% of circuit board materials, and is also used as an additive FR in a number of other electronics applications, especially in Asia.49−52 Recent monitoring surveys of TBBPA, particularly in areas in China where e-waste recycling occurs, have found high levels in both indoor and outdoor environments and concomitant high exposure in local populations.41,42 TBBPA’s degradation to form BPA under anaerobic conditions54 is an additional source of concern.55,56 The second class of substances considered is commercially available drop-in organic chemical alternatives to decaBDE and TBBPA: melamine polyphosphate and 9,10-dihydro-9-oxy-10-phosphaphenanthrene-10-oxide (DOPO). Both are nonhalogenated, phosphorusbased substitutes: one additive (melamine polyphosphate) and one reactive (DOPO). The third class of substances comprises viable nanoenabled FR alternatives identified based on the literature,34−37 CNMs and nano-MMT. Both are demonstrated effective FRs due to inherent properties achieved by these materials at the nanoscale. Additional properties, such as mechanical strength and electrical conductivity of CNMs, offer the potential for multifunctional product enhancements. As a point of comparison, we also include bulk MMT. Hazard Assessment. Hazard assessment of the alternatives are based on the US EPA’s DfE hazard assessment evaluation criteria.57 Here, the assessment is limited to environmental hazard metrics (persistence, bioaccumulation potential, and ecotoxicity). Although human health assessment is an integral component of AA, environmental impacts are where many of the problematic features of BFRs were first identified and also where challenges in data availability and assessment methods remain. Although a number of AAs for decaBDE and TBBPA in EEE exist,45−47,51,52,58,59 data for drop-in organic chemical alternatives were primarily sourced from the two EPA assessments,20,21 because they were comprehensive in nature and the most recently compiled. In addition, a literature review was conducted to identify new data sources published since 2014, restricted to those that provided experimental data for persistence, bioaccumulation potential, or ecotoxicity. One study was found that had not yet been included in the two EPA assessments, which reported on in vitro cytotoxicity and neurotoxicity of MPP, DOPO, and nano-MMT.60 However, these experiments were performed with rat neural cells, and thus are not compatible with the ecotoxicological data. Data for the mineral and CNM alternatives were collected and compiled based on a literature review of empirical studies, including many that followed standard methodology (e.g., as outlined by OECD or the US EPA). The hazard designation was determined from the most sensitive organism and study (i.e., the lowest LC50 or EC50

value). Studies included 10−14 d, 48 and 96 h behavioral (immobilization, swimming velocity) and survival of Daphnia magna,61−64,64,65 24, 48 h behavioral (reproduction, immobilization) and survival of Ceriodaphnia dubia,66−68 respiratory toxicity in Oncorhynchus mykiss (rainbow trout),69 hatching delay of embryonic Danio rerio (zebrafish)70 and 10-day mortality of amphipods, Leptocheirus plumulosus and Hyalella azteca.66 Chronic toxicity studies are limited with data available for MWNTs only. The hazard designation for MWNTs was determined based on a LOEC value for a study using Daphnia magna, which measured survival, reproduction and feeding end points.65 Persistence values are based on the degradation half-life in water or soil (units of days).71 Although there are many degradation studies for CNTs (see Flores-Cervantes et al.72), only one was conducted using a standard protocol to evaluate whether MWNTs and SWNTs were readily biodegradable.73 Bioaccumulation of CNTs was determined based on the bioaccumulation factor (BAF), from uptake and depuration studies with Daphnia magna and Eisenia fetida (earthworm).74,75 There are relatively few studies on bulk and nano-MMT. LC50 values from 5 and 7 d survival studies were used to determine the acute aquatic toxicity.76,77 In addition to the compiled hazard designations in Table 1 and Figure 1, Table S1 includes the complete compiled data. There are several identified challenges that arise when standard assays are applied to the evaluation of ENMs. These are outlined and discussed in several recent reviews, and their relevance to the outcome of hazard assessment is discussed in detail in the SI.78−82 Data Availability and Quality. The EPA uses a “Hierarchy of Data Adequacy” to rank the quality of available data.57 This hierarchy assigns the highest confidence to (i) information from studies conducted according to established guidelines, (ii) studies that are “experimentally valid” but do not follow a standard guideline, (iii) reported data that do not give experimental details, (iv) data estimated using structure−activity relationships or expert judgment based on similar substances and (v) expert judgment based on knowledge about mechanisms or chemical structure. As such, a data reliability score was applied based on this hierarchy to all available data. We consolidated the last two categories (estimated data and expert judgment) and ranked values from most reliable (score = 1) for empirical studies that follow established guidelines (e.g., from OECD), to least reliable (score = 4) for estimated data/expert judgment. Performance Assessment. The technical performance of decaBDE and TBBPA are discussed in a number of AA reports. Here, the focus is on the two FR applications: as an additive to plastics in electronics enclosures (primarily televisions) for decaBDE, and as a reactive FR in printed circuit boards for TBBPA. The relevant standard for both is the UL-94 V0 flammability rating.45,83 The technical performance for each mineral and nanomaterial alternative was determined from literature empirical studies. Although several metrics (e.g., after flame time, burning rate, flame spread, smoke production)84 are used to evaluate the relative performance of a given FR, the peak heat release rate (PHRR) was the most ubiquitous. Therefore, the percent reduction of the PHRR relative to the host C


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Figure 1. Summary of available acute and chronic ecotoxicity data for standard FRs (TBBPA and deca-BDE), chemical alternatives (DOPO and MPP) and mineral/ENM alternative substances (MMT, nano-MMT, SWNT and MWNT). Data are classified according to species type (aquatic invertebrates, fish or terrestrial organisms), and quality of data source is indicated by the shading and shape of individual data points.

intensifying. In some cases, FRs can extinguish a flame. The mechanism by which the different alternatives function as a FR varies and is discussed in the section on technical performance below, including their use in combination to impart synergistic effects that enhance the extinguishing potential. While flame retardant chemicals are used in many applications in order to meet consumer regulations and fire codes, the focus of this study is on electronics applications (e.g., electric wire casing, circuit boards, enclosures for appliances), as previously described. The use scenario is therefore operation of EEE in which the housing, wires or circuit boards contain an additive or reactive FR. An additive FR like decaBDE is not chemically bonded to the plastic resin. Thus, there is the potential to diffuse from the treated material and enter the environment, partitioning to the surrounding air, dust and solid surfaces, where exposure may occur. Reactive FR, like TBBPA, are chemically bonded (for example to the epoxy resin used in circuit boards) and as such, do not leach during the use phase. However, the final product can retain a substantial amount of unreacted FR following manufacturing, and this fraction can be subject to similar emissions and exposure processes as the additive FR. Identify and Prioritize Alternatives. Fortunately, there are a number of alternatives to BFRs. Some are chemicals, both similar and distinct in chemical composition to existing halogenated flame-retardants. Here, we focus our discussion on DOPO and melamine polyphosphate, both nonhalogenated FRs. DOPO in particular was recognized by BizNGO’s assessment of alternatives to BFRs with “no end points of high concern”.94 In addition, there has been significant effort over the past decade to develop alternatives to bulk organic FRs. Examples discussed herein include SWNTs and MWNTs, and bulk and nano-MMT. There is significant evidence from empirical studies suggesting these alternatives exhibit FR qualities necessary to meet standards when combined with

material (no FR) is used to compare the technical performance of the alternatives. The economic assessment for chemicals was based on current market value of commercial grade decaBDE, TBBPA, DOPO and melamine polyphosphate, as well as the total cost of materials (e.g., enclosures or printed circuit boards) containing the FR and any necessary synergists at the concentrations needed to meet current flammability standards (e.g., UL-94 V0). Much of this information was extracted from the existing AA reports,20,21,45−47,51,52,58 and from additional sources on FR properties and market data.85−89 The economic assessment for mineral and nanomaterial alternatives was determined based on current market value, which was determined from multiple sources of CNMs (single- and multiwalled carbon nanotubes, SWNTs and MWNTs, respectively).90−93 There is a significant range in CNT cost, which is primarily driven by the level of purity (wt % as CNTs versus amorphous carbon or metal catalyst residual). For SWNTs, additional cost is incurred to attain specific chirality tubes, which modulates the electronic properties. These properties are not considered essential to the FR function of CNTs. Additional properties, such as aspect ratio, vary based on the method and conditions employed during synthesis and are included in the summary table found in the SI (Table S2). The cost ($) per gram is averaged for low purity SWNTs and MWNTs from the surveyed vendors. The average cost per gram was then multiplied by the wt % of active mineral or nanomaterial additive to the best performing formulation to get a value for the cost of the active FR. These values, in cost active FR/gram of material, are reported in Table 2.

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RESULTS AND DISCUSSION The following discussion is structured around the stages of AA, as described above, and includes bulk chemical and nanomaterial alternatives to BFRs. Identify Chemical of Concern. As outlined above, decaBDE and TBBPA were selected as chemicals for which alternatives are sought. Characterize End Uses and Function. The primary function of a FR is to dissipate heat to prevent a fire from igniting and if a fire does ignite, prevent it from spreading or D


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Table 2. Performance Comparison of Proposed Drop-In Chemical, Mineral or Nanomaterial Formulations for Electrical and Electronic Applicationsa % reduction in PHRR compared to raw material standard FR deca-BDE: 10−15 wt % in HIPS TBBPA: 15 wt % in epoxy bulk organic chemical alternative DOPO: 10−20% in epoxy resin MPP: 8% in epoxy resin (25% needed to achieve V0) mineral/ENM alternative carbon-based single-walled 0.5 wt % SWNT-PMMA multiwalled 2 wt % MWNT150-PS 5.0 wt % MWNT-PP-FR 2 wt % MWNT-ABS 1 wt % MWNT, 1 wt % MMT-ABS montmorillonite nano-MMT 2 wt % nMMT-MH-EVA bulk-MMT 5 wt % OMMT-PP 5 wt % clay-30 wt % RDP-PS 2 wt % NaMMT-ABS 5 wt % NaMMT-C30B-EVA

cost FR/g materialb ($)

ref

27 51

0.0013 0.00038

127 88,89

16−20 33

0.0026 0.00081

85,87,128 129,130

low purity avg/lowest cost 62

0.15/0.12

131

56 46 55 57

0.25/0.012 0.62/0.03 0.25/0.012 0.12/0.006

132 96 99 99

46

0.004

133

50 92 24 57

0.01 0.01 0.004 0.01

134 135 99 98

a

When studies reported multiple wt % loadings of the active FR, the best performing formulation is reported. Complete performance data is included in Table S2. HIPS = high-impact polystyrene, PMMA = poly(methyl methacrylate), PS = polystyrene, MWNT150 = MWNT with aspect ratio of 150, PP = polypropylene, PP-FR = PP with combination of tetrabromobisphenol A bis(2,3-dibromopropyl ether) (BDDP) and Sb2O3, ABS = acrylonitrile-butadiene-styrene resin, EVA = ethylene-vinyl copolymer, MH = magnesium hydroxide, OMMT = organic clay, NaMMT = sodium montmorillonite, RDP = resorcinoldiphosphate, C30B = quaternary ammonium chloride bCost is calculated per gram of material considering the FR cost only (cost of FR multiplied per weight % used in treated material).

weigh uncertainty in estimated hazard metrics relative to one another remain unclear. In their 2012 assessment of novel and emerging brominated FRs, the European Food Safety Authority (EFSA) concluded that a risk characterization of most of the substances considered was not possible due to a lack of experimental data on physicochemical properties, environmental fate, toxicity and current production volume and use.101 The lack of data on persistence, bioaccumulation and toxicity is a recurring theme in the assessment of industrial chemicals102−104 and significant uncertainty remains in the field of FRs, despite recent immense pressure for enhanced evaluation and/or substance replacement. Mineral and Nanomaterial Alternatives. Hazard data from empirical studies on mineral and nanomaterial substances considered here were compiled as previously described and are summarized in Table 1 (see Table S1 for compiled data). The results suggest the following:

conventional polymers (e.g., poly(methyl methacrylate), polypropylene, polystyrene, ethylene-vinyl acetate).35,95−100 Assess Hazards of Alternatives. Drop-In Organic Chemical Alternatives. Table 1 summarizes the hazard profiles of decaBDE, TBBPA, and two nonhalogenated alternatives, DOPO and melamine polyphosphate, extracted from the two EPA studies.20,21 Hazard levels (L, low; M, moderate; H, high; VH, very high) are based on the US EPA hazard criteria. The original assessments indicate that, for end points with no experimental data available, values were estimated using quantitative structure−activity relationships (QSARs) or expert judgment (indicated by bold italic font). From these results, the chemical with the most information, TBBPA, is also the most hazardous. For decaBDE, experimental data were only available for persistence; bioaccumulation potential and aquatic toxicity are estimated. For the alternatives, DOPO only had one experimentally determined hazard metric, a low acute aquatic toxicity. Chronic toxicity is estimated to be moderate, bioaccumulation potential low and persistence high. For melamine polyphosphate, both toxicity metrics are experimentally derived and low, whereas persistence and bioaccumulation potential are estimated to be high and low, respectively. Comparing these chemicals, the clearest conclusion based on existing data is that TBBPA poses the greatest potential environmental hazard. Both DOPO and melamine polyphosphate are estimated to be persistent. DOPO is estimated to have moderate chronic toxicity in aquatic systems, but all other hazard metrics are low, and thus may represent the best choice between the drop-in alternatives. However, mechanisms to

(1) Acute toxicity is a concern for CNTs and MMT. The moderate hazard designation of nano-MMT presents the lowest hazard potential. (2) There are very few chronic toxicity studies for any mineral or nanomaterial alternative. The data available for MWNTs suggests moderate chronic toxicity. (3) There were no studies on the environmental fate of MMT. (4) The environmental persistence of CNTs is of moderate to high concern. (5) The bioaccumulation potential is low for CNTs. E


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Degradation or Transformation Half-Life? ENMs rapidly transform when exposed to environmental and biological conditions. When not intentionally modified (e.g., through surface functionalization) to inhibit strong attractive forces (e.g., van der Waals), ENMs will aggregate rapidly as well as sorb to other suspended constituents. Through this process, ENMs can cease to be “nano” and, rather, form micrometer (or larger) agglomerates. Additional transformations may occur through biological and chemical processes, which, again, will transform the parent nanomaterial. In such cases, it is important to consider the hazard potential of the transformed or degraded forms rather than the parent ENM, and whether these new potentially non-nano materials are more or less hazardous. It is generally assumed that ENMs will be more inherently hazardous than their bulk counterparts due to increased reactivity, resulting from the increased active surface area and/or rapid release of ions, as well as size-induced novel pathways through traditional biological barriers (e.g., cell membranes). In most cases, this assumption is true. Yet, the MMT study included in the hazard assessment76 shows that micrometer-sized MMT is more toxic to daphnia (i.e., lower LC50) than the nanosized MMT. This suggests that fabricating a material at the nanoscale does not guarantee enhanced toxicity. Furthermore, we have learned from chemicals (e.g., deca-BDE, endosulfan and DDT) that degradation products can be significantly more persistent, bioaccumulative, and/or toxic than the parent chemical.112−115 CNM degradation products and their potential hazards are not comprehensively understood. Combined, this suggests that the first step toward enhanced heterogeneous AA hazard evaluation should be to (i) identify transformation and/or degradation products in the environmental compartment of the most likely release pathway, and (ii) consider the relevant transformation half-lives rather than typical degradation half-lives as a more accurate representation of bulk organic chemical, mineral or nanomaterial environmental persistence. Evaluate Technical and Economic Performance. BFRs impart resistance to burning primarily through the action of bromine in the gas phase, which interferes with gas-phase reactions, whereas phosphorus-based FRs primarily work by promoting the formation of a char layer. The combination of CNT or MMT FR additives functions in a similar manner to phosphorus-based FRs by forming an organic−inorganic residual on the surface of the burning composite, serving as (i) a protective layer to the underlying combustible material, (ii) a barrier against transport of volatiles into flame and (iii) a barrier against heat transport.95,116,117 This results in a slower increase in the temperature of the nanocomposite and unfilled polymer, reducing the heat release rate (HRR) and peak HRR (PHRR). In several studies, CNTs and MMT FRs are used in combination with a metal oxide and/or conventional chemical FR, reducing the total amount (wt %) of additive necessary to achieve a specific FR level. The displacement of metal hydroxides, for example, is desired because they often require very high loading to meet FR standards and can significantly impair the mechanical and electrical properties of the polymer.35 Formulations (i.e., wt % active FR) and relative performance values (as % reduction of PHRR) are compiled in Table 2, and a comprehensive table of all formulations can be found in Table S2. In addition to comparable or enhanced performance of MMT or CNT alternatives, there are further benefits to be realized, particularly for CNTs, which can increase the mechanical properties, electrical and thermal

On the basis of these results, mineral and nanomaterial alternative FRs present a level of concern that must be considered in the ultimate decision-making. The data for all these alternatives receive a reliability score57 ranging from 1 (highest quality, empirical study conducted according to established guidelines) to 2 (experimentally valid study, but does not follow a standard guideline). As previously mentioned, however, there is significant controversy over toxicity data collected for nanomaterials following current standard assay procedures. Significant literature exists discussing these challenges, putting into question the validity of data collected from aquatic toxicity studies.78,82,105−111 These challenges, as they relate the hazard evaluation herein, are discussed in detail in the SI. Comparing Toxicity for Drop-In, Mineral and Nanomaterial FR Alternatives. Comparing the compiled hazard data for alternatives (Table 1), there is no clear “winner”. First, the reliability of the data varies significantly. Of the bulk organic chemicals, TBBPA has the most reliable data (i.e., all data are based on experimental studies) and is designated as the most hazardous chemical overall. DOPO exhibits the lowest acute and chronic toxicity, when only experimental data are considered, but reliability of the persistence and bioaccumulation potential is low given only estimated values are available. The hazard designations of the nanomaterial alternatives, on the other hand, are all based on experimental studies that either follow an established guideline or provide experimental details (reliability level 1 and 2, respectively). Yet, the amount of available data varies significantly among alternatives. These discrepancies in data availability and reliability that make choosing a “best” alternative challenging is illustrated in Figure 1, which includes all of the aquatic ecotoxicity data (LC50 and EC50 values), categorized by model organism and level of data reliability. Interestingly, TBBPA and MWNTs have the most available data points, suggesting the hazard designation (H) is more reliable for these two alternatives. According to the DfE framework, the hazard designation for toxicity is determined by the most sensitive (i.e., lowest concentration) study. It is important to consider not only this data point but also the spread in available data. For TBBPA, there are approximately 3 orders of magnitude between the most reliable (score = 1) data points (as well as the median concentration) and the most sensitive LC50. This is also true for deca-BDE, where there are almost 5 orders of magnitude spread in data points, with the highest concentration (i.e., least toxic) being the most reliable (score = 3), and lowest (i.e., most toxic) being the least reliable (score = 4). However, it is unclear whether an experimental data point with no supporting study details should always be considered more reliable than an estimated data point, as per the EPA criteria,57 particularly when the estimate is based on sound knowledge (e.g., a large training set of data) for a particular chemical class. Finally, the compiled data in Figure 1 suggests that the situation for mineral and nanomaterial alternatives, in terms of reliability and amount of data, is perhaps not as dire as generally conceived, particularly when the lack of data for existing chemicals, like decaBDE and DOPO, is also considered. Still, there is significant concern over the applicability of current testing methodologies for insoluble substances (e.g., nanomaterials), which is not reflected under the current AA framework (see discussion in SI). F


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Figure 2. Life cycle stages specific to electronics applications of flame retardants. Exposures can include occupational (during production phase), consumer (during use phase) and environmental throughout the life cycle. The arrow color highlights differences in exposure pathways between drop-in alternatives and minerals/engineered nanomaterials.

conductivity.96 In this way, the required FR property and added functionality may be realized simultaneously, offering a multifunctional component. This offers exciting opportunities for FR applications where weight reduction is desired (e.g., vehicles, planes, military equipment) and electronic and mechanical properties of CNTs are also advantages. Economic feasibility was considered for DOPO, MPP, SWNTs, MWNTs and MMT, compiled in Table 2 as cost per mass of the active material (refer to Table S3 for a full list of costs). Increased costs can impede the adoption of alternatives on industrial scales. For example, the dominance of deca-BDE as a FR in electronics enclosures was driven primarily by its low cost. Its technical performance as an additive to plastics was inferior to other FR-plastic combinations, but nonhalogenated FRs could cost more than twice as much.45 Similarly, replacing TBBPA in printed circuit boards with DOPO could substantially increase the cost; this is largely responsible for the small market share (∼1%) of DOPO in this sector.20 MMT, both bulk and nano form, is cheap and therefore, is competitive with current and proposed chemical alternatives. The cost of CNTs, on the other hand, varies significantly (e.g., $595,000/g to $0.60/g). The range in cost is dictated by the level of purity; in terms of the presence of amorphous carbon and, more importantly, specific chirality tubes. In the studies conducted so far, there is limited information on the type of CNTs employed, but all are commercially available and there is no indication that high purity is necessary for the intended function of the CNT as a FR. Overall, MWNTs tend to be lower cost than SWNTs, and offer an economically competitive alternative to conventional chemicals. Consider Life Cycle Impacts and Exposure. There is potential for release and exposure of FR from electronics applications across the life cycle (Figure 2). Occupational risk during upstream production of both bulk organic chemicals and nanomaterials is a reality. CNMs are of particular concern given the potential to induce serious adverse human health impacts (e.g., fibrosis, mesothelioma).29 While this exposure can be minimized through mitigation practices for known or inherently hazardous substances, a preferred approach is through the design of inherently safer substances; a movement that has gained momentum and traction over the past decade and is further motivated by the Principles of Green Chemistry.26−29,118,119 There are also numerous release and exposure pathways during the “use phase”, for the parent and

byproducts of the FR. For electronic applications, the FR chemical or ENM is associated with a substrate or support material (e.g., polymer composite). Additive chemical FR, such as decaBDE, have been shown to volatilize from treated surfaces, entering indoor air and subsequently depositing to dust,120 leading to human exposure during both use and disposal phases.50,53,120,121 This can also occur with reactive FRs like TBBPA when a portion of the chemical in the matrix remains unreacted during production.122 Although ENMs can be either chemically bound to the parent polymers or embedded through weaker associative bonds, there is no evidence suggesting volatilization. Still, there is the potential for the ENMs to be released during wear-and-tear, most likely remaining incorporated with the support material (rather than as the pristine CNM). Differences between bulk organic chemicals and ENMs are also apparent when considering actual use of FRs (i.e., when fire is introduced). The ENMs discussed in this study are most likely not released, but rather oxidize during the combustion process as discussed above. Bulk organic chemicals, on the other hand, have the potential to release hazardous byproducts during combustion such as brominated dioxins and furans during incineration, when the temperature is not high enough.123,124 Given the current practices of electronics recycling, the end of life treatment of electronics applications presents significant potential for exposure. This is particularly true for recycling conducted in nonregulated scenarios, and exposure pathways are similar to those identified for general electronics recycling.125,126 Considered together, there are many release and exposure pathways that are common to both chemical and mineral or nanomaterial alternatives. Yet, there are specific pathways of release across the life cycle that are unique to organic chemicals, where no analogous unique mineral and ENM release pathways occur. As such, exposure should be an important consideration for these heterogeneous alternatives.10 Select Preferred Alternative. Following the standard assessment steps for this heterogeneous AA, selecting the preferred alternative is the next stage, and the decision becomes very challenging. It appears that based on the hazard properties of bulk chemical, mineral and nanomaterial alternatives to decaBDE and TBBPA, all present trade-offs and there is no clear front-runner. Early evaluation of the technical and economic performance suggests MMT and CNT alternatives have the potential to be equally efficacious and cost-effective. G


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In addition to necessary updates to the hazard evaluation components of AA, there is the opportunity for AA to work in concert with proactive design frameworks, such as the 12 Principles of Green Chemistry. Ultimately, the goals of AA and chemical design are the same: to provide safer alternatives to current problematic substances in a way that precludes adverse impacts across the entire life cycle. Yet, AA does not address inherent design, rather is dependent upon existing substances. By combining AA with the 12 Principles in an iterative assessdesign-select cycle, application-specific chemistry can begin to evolve toward inherently safe, and therefore better, alternatives. In the case of FRs for EEE applications, such a substitution cycle might take the following form: (I) Perform AA for the replacement of a problematic chemical (e.g., TBBPA) to identify possible existing alternatives. (II) If “incremental” alternatives exist, use 12 Principles to redesign for greener performance. If no alternative with sufficient potential exists, design from the ground up according to 12 Principles. (III) Perform AA for newly (re)designed alternatives to confirm suitability. If successful, select and implement. Finding a path forward in replacing hazardous substances with alternatives that take advantage of paradigm shifts in structure and function will thus require an integration of hazard into the design process in a way that ensures we no longer face the problem of regrettable substitution. Yet, such an endeavor will rely on our ability to accurately assess hazards for a wide variety of heterogeneous substances and, as such, calls for increased efforts in understanding what the fourth Principle of Green Chemistry“Designing Safer Chemicals”means at a fundamental level across the range of relevant environmental and biotic systems.

The data used to inform the ENM AA is fairly reliable (all based on empirical studies and the majority following standard assay procedures). There are some data gaps, particularly for MMT, and lack of studies to inform potential chronic ecotoxicity. Bioaccumulation studies of CNTs indicate very low bioaccumulation potential. Decisions have been made with similar levels of data uncertainty or limitations for drop-in chemical alternatives. Moving forward with choosing a functional alternative, it is difficult to assess how lack of data in different categories as well as a lack of confidence in the suitability of a hazard assessment (e.g., reliability of standard toxicity tests applied to ENMs) should be weighted. For example, one may assume that since MMT is a naturally occurring mineral, perhaps this is the best option (despite the lack of available ecotoxicity data). Yet, asbestos is also a naturally occurring mineral, and unintended consequences of its use had an immense impact and continue to plague society. To compare objectively these alternatives, a systematic approach to dealing with these uncertainties is needed. There are fundamental challenges when it comes to comparing heterogeneous alternatives under the current AA framework. One of the primary differences is their behavior in aqueous systems, namely that many material alternatives are not soluble. This poses a challenge to the current toxicity and environmental fate evaluations and, thus, has motivated a shift in ENM toxicology paradigms. Rather than single standard toxicological assays, perhaps independent frameworks will eventually exist that are comparable and suitable for soluble and insoluble substances. This hybrid evaluation system would present a new set of hazard designations that combines individual rankings for such alternatives to be considered together. Finally, unlike addative organic FRs, such as deca-BDE, mineral and nanomaterial FRs are not volatile, and thus the exposure during the use phase will be significantly lower. This suggests that exposure should be a primary consideration in selecting the best alternative.10

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01318. Comprehensive tables of the hazard evaluation (Table S1), performance (Table S2), and cost (Table S3) data; discussion on challenges faced in applying standard assays to the evaluation of ENM hazard. (PDF)

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CONCLUSIONS Flame retardant alternatives for use in electronic equipment are compared here, including bulk organic chemicals, naturally occurring minerals, and engineered nanomaterials, as a way to evaluate the applicability of the current framework to a “heterogeneous” AA. There is a movement toward replacing conventional FRs with ENMs, yet change is slow primarily due to perceived hazard of those materials and lack of sufficient data to inform a conclusive decision. Although there is merit to the perceived hazard, it is shown here that similar amounts of (if not more) data exist for ENMs as for drop-in chemicals. For example, the ENM toxicity data is, overall, more reliable when considering the EPA standard guidelines for data quality and reliability (Figure 1). One critical component of the hazard data that is not captured in the current AA framework is the appropriateness of the standard assays for insoluble substances. This is a concern not only for ENMs but also insoluble organic chemical alternatives. Furthermore, the heterogeneity of nanomaterials (e.g., purity, structural parameters) may mean that available hazard data for highly pure homogeneous nanomaterials may not be representative of the actual materials used as FRs. Finally, the released ENM is likely to remain associated with the composite material rather than as pure ENM, which is typically assessed.

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AUTHOR INFORMATION

Corresponding Authors

*L. M. Gilbertson. Phone: (412) 624-1683, e-mail: LMG110@ pitt.edu. *C. A. Ng. Phone: (412) 383-4075, e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the generous funding support from the Department of Civil and Environmental Engineering at the Swanson School of Engineering of the University of Pittsburgh.

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