polymers Review How Green is Your Plasticizer? Roya Jamarani 1, Hanno C. Erythropel 1,2 ID , James A. Nicell 3 ID , Richard L. Leask 1 and Milan Mari´c 1,* ID 1 Department of Chemical Engineering, McGill University, 3610 University St, Montréal, QC H3A 0C5, Canada; [email protected] (R.J.); [email protected] (H.C.E.); [email protected] (R.L.L.) 2 Center for Green Chemistry and Green Engineering, Yale University, 370 Prospect St, New Haven, CT 06511, USA 3 Department of Civil Engineering & Applied Mechanics, McGill University, 817 Sherbrooke Street West, Montreal, QC H3A 0C3, Canada; [email protected] * Correspondence: [email protected]; Tel.: +1-514-398-4272 Received: 5 July 2018; Accepted: 25 July 2018; Published: 28 July 2018 Abstract: Plasticizers are additives that are used to impart flexibility to polymer blends and improve their processability. Plasticizers are typically not covalently bound to the polymers, allowing them to leach out over time, which results in human exposure and environmental contamination. Phthalates, in particular, have been the subject of increasing concern due to their established ubiquity in the environment and their suspected negative health effects, including endocrine disrupting and anti-androgenic effects. As there is mounting pressure to find safe replacement compounds, this review addresses the design and experimental elements that should be considered in order for a new or existing plasticizer to be considered green. Specifically, a multi-disciplinary and holistic approach should be taken which includes toxicity testing (both in vitro and in vivo), biodegradation testing (with attention to metabolites), as well as leaching studies. Special consideration should also be given to the design stages of producing a new molecule and the synthetic and scale-up processes should also be optimized. Only by taking a multi-faceted approach can a plasticizer be considered truly green. Keywords: additive; plasticizer; phthalate; toxicity; biodegradation; leaching; metabolites 1. Introduction Plasticizers are additives, typically small organic molecules, that decrease the glass transition temperature (Tg) of the polymer they are blended with, creating flexible or semi-rigid products with improved processing characteristics [1]. Approximately 90% of all globally produced plasticizers are used to make flexible poly(vinyl chloride) (PVC), with di(2-ethylhexyl) phthalate (DEHP) being the most frequently used plasticizer [2]. Plasticizers can be classified as either internal or external. Internal plasticizers achieve flexibility by lowering Tg through grafting or copolymerization of softer monomer units to the polymer chain, while external plasticizers, such as DEHP, are simply blended with the polymer at elevated temperatures and do not form covalent bonds [3]. Internal plasticizers are less commonly used, often for specific purposes, because the fixed chemical bonds offer less freedom and limited properties compared to external plasticizers. External plasticizers offer higher flexibility to adjust the final polymer properties, given that the plasticizer is added after polymerization [1,3]. Additionally, the type and amount of plasticizer can be carefully tailored to produce a wide variety of formulations and product properties and impart different levels of flexibility depending on the desired application. Furthermore, because no chemical reaction is involved, external plasticization Polymers 2018, 10, 834; doi:10.3390/polym10080834 www.mdpi.com/journal/polymers Polymers 2018, 10, 834 2 of 17 Polymers 2018, 10, x FOR PEER REVIEW 2 of 16 also tendsThe to belack more of a cost-effective, chemical bond and between are thus external used to plasticizers a greater extent. and polymers Therefore, allows this the review plasticizer will to focusdiffuse exclusively within on and external out of plasticizers. the blend over time. Once plasticizer molecules reach the surface of the blend,The lack leaching of a chemical into their bond surroundings between external occurs plasticizers that results and in polymers human allowsexposure the plasticizerand entry toof the diffusecompounds within and into out the of the envi blendronment over time.[4,5]. For Once example, plasticizer DEHP molecules and its reach metabolites the surface have of thebeen blend, found to leachingbe ubiquitous into their surroundingsenvironmental occurs contaminants, that results likely in humandue to their exposure slow anddegradation entry of therates compounds combined with into thehigh environment rates of entry [4,5 ].into For the example, environment DEHP and[6,7]. its Phthalate metabolites plasticizers have been including found to beDEHP ubiquitous have been environmentaldetected in contaminants, a wide variety likely of environmental due to their samples, slow degradation including rates house combined dust [8–10], with air high [11], rates soil [12], of entrywatersheds into the [4], environment and animals [6, 7[6].]. Phthalate This is especially plasticizers problematic including given DEHP that have many been studies detected have in linked a wideDEHP variety and of environmentalits metabolite, samples,mono(2-ethylhexyl) including house phthalate dust [(MEHP),8–10], air to [11 endocrine], soil [12 ],disruption watersheds in [human4], and animalsand animal [6]. models, This is especiallyand negative problematic effects on male given reproductive that many studies development have linked (anti-androgenic DEHP and itseffects) metabolite,[13–17]. mono(2-ethylhexyl) As a result of these phthalatefindings, (MEHP),the use of to DEHP endocrine and other disruption phthalates in human has been and r animalegulated in models,consumer and negative items such effects as on children’s male reproductive toys in many development countries,(anti-androgenic including Canada effects) [18], [the13– United17]. As States a result[19], of these the European findings, Union the use [20], of DEHP and Japan and other[21]. 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