Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Relative permittivity estimation of wheat starch: A critical property for understanding electrostatic hazards J.V. Seidel a , O.A. Castañeda-Uribe b , S. Arevalo c , F. Muñoz d , W. Proud e , A. Avila c, ⁎ a Imperial College London, London, SW7 2BP, United Kingdom b Vicerrectoría de Investigaciones, Grupo de Investigación en Ingeniería Biomédica (GIIB), Universidad Manuela Beltrán, Bogotá, DC 110231, Colombia c Department of Electrical and Electronic Engineering and Centro de Microelectrónica (CMUA), Universidad de los Andes, Bogotá, 111711, Colombia d Department of Chemical Engineering, Universidad de los Andes, Bogotá, 111711, Colombia e Institute of Shock Physics, Imperial College London, London, SW7 2BP, United Kingdom ARTICLE INFO Keywords: Wheat starch AFM characterization Relative permittivity Impedance measurements KPFM EFM ABSTRACT Wheat starch is a widely used material in the food, pharmaceutical and entertainment industry not considered hazard but recently associated to dust explosions during processing and handling. How an insulating starch grain is charged and how its ability to be polarized is affected by environmental conditions such as temperature, humidity and frequency? are fundamental questions that must be explored in order to understand the dust explosion phenomena. Here we investigate the dependence of temperature, humidity and low-frequency on the relative permittivity of wheat starch. We characterized starch at the micro and macro scales using atomic force microscopy-based techniques and capacitive planar sensor-based measurements respectively. The results show high values of permittivity (˜80) at the microscale (single starch grains) compared to the low values (10–20) at the macroscale (20 mg of wheat starch). The differences are attributed to the Maxwell–Wagner–Sillars interfacial polarization process on individual grains and potential charge exchange between grains. Permittivity is a critical property to investigate starch electrostatic hazards. 1. Introduction Starch, a widely used food ingredient composed of amylopectin polymers, in its pulverized and dispersed form has been responsible for explosions both in the food processing and entertainment industry, causing damage, injuries and loss of life, either in factories or during the Holi festival and related color festivals; on June 6, 2015, flammable starch-based powder exploded on stage during a Colour Play Asia party at Formosa Fun Coast Water Park in Taiwan killing 29 people while injuring 508 with 199 in critical condition [1–5]. In the scenarios where materials that exhibit low ignition energies, changes on environmental conditions modify the material´s ability to resist an electric field or their ability to be polarized. Charge displacement or accumulation results from frictional contact or impact between grains and imposes a po- tential static electrical hazard for the material manipulation and sto- rage. Therefore, there is a need for quantitative values of permittivities and their dependence on environmental conditions including electro- magnetic radiation. In this paper we characterized wheat starch electrical properties and its dependence on humidity, temperature and frequency. Wheat starch exhibit a low ignition energies and different phenomena have been identified as triggers of their ignition including heating and shock waves, electrical, frictional and impact sparks from the equipment’s movement or impact between grains that characterized the movement of large volumes of granular material, as well as static charge of the transported material [2,6,7]. In particular, static charge and discharge promoted by the impact or friction (contact electrification – tri- bocharging) or contact of particles in the dust cloud can change the ignition dynamics and reduce even more the starch activation energies [8]. Several practical methods have been investigated to characterize particle charging and morphology: single particle impact test, cascade, vibration, centrifugal, isolation (based Faraday cages) and fluidization methods [9,10]. Characterizing the electrical properties of wheat starch represent a fundamental stage in order to understand the electrical charge influ- ence on ignition and explosions. In particular, the charge density in insulating grains is directly related to changes on polarization re- presented in the relative permittivity. The relative permittivity allows to understand where and how much charge is trapped on the starch grains becoming in a fundamental parameter necessary for any https://doi.org/10.1016/j.jhazmat.2019.01.047 Received 23 July 2018; Received in revised form 12 December 2018; Accepted 15 January 2019 ⁎ Corresponding author at: Department of Electrical and Electronic Engineering, Universidad de los Andes, Cra 1 # 19A-40, Bogotá, 111711, Colombia. E-mail address: a-avila@uniandes.edu.co (A. Avila). Journal of Hazardous Materials 368 (2019) 228–233 Available online 16 January 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved. T