Hair is a keratin filament having three major compartments; cortex medulla and cuticle. It is the characteristics feature of the mammals and usually grows to cover a body part or the whole. Cuticle is the outer covering of hair fiber composed of overlapping flattened cells [1, 2]. Cortex having cortical cell, comprises middle layer of the hair which contain macro fibrils of the keratin and matrix proteins. Medulla, the innermost layer has cells with air spaces and amorphous materials. This layer is responsible for hardness of hair filaments. Overall, hair fibers are keratinized dead cells [3, 4]. Biomolecules of the hair includes keratin (alpha and beta) protein, matrix protein, lipid, melanin and water. In addition to basic organic elements carbon, oxygen, hydrogen, nitrogen and sulphur these biomolecules of hair contain trace metals like zinc, magnesium, copper, cobalt, chromium, and nickel [5]. Concentration of some of these metals may vary with age and sex as well as color [6]. Matrix protein contains 20 to 50 amino acids which are structured in helical form. Serine, cystine, methanoine, theorine, glutamic acid, citrulline, cysteine, proline, tryptophan, glycine, alanine, valine etc. are the major amino acids. Functional groups like carboxylic acid (-COOH) and amino (-NH2) group are present as peptide (CONH) group along with carbonyl (= CO) and imino (NH) group in amino acids. Among these, cystine is the most abundant amino acid in the hair filaments. Cystine imparts disulphide linkage between two nearby helical chain of protein. The matrix proteins having helical chains that are responsible for disulphide bonding are termed as keratin associated proteins. Although other binding structure like salt bond and hydrogen bonding are also present between two helices, but disulphide bonding has the most remarkable effect [7, 8].
Despite having same composition, thickness and morphology of hair can vary with geographical origin of the people. Human hair with Asian, African and Caucasian origin has diameter 100 µm, 80 µm, and 50 µm respectively. Also, the outer cuticle of hair from an Asian has more folded structure than of others [1]. Moreover, even a single strand of hair has different structure on its tip, middle and near scalp region. Diameter and cuticle deposition gradually decreases from root to tip. Cuticle may even be absent in the tip due to mechanical stress, friction and combing [9].
Human hair is not bio-degradable and is considered as a waste. Burning of hair in open environment generates harmful gases like ammonia, carbonyl sulphide, hydrogen sulphide, sulphur dioxide, phenols, nitriles, pyrrole and pyridines. This also imparts unpleasant and foul odor on locality [10, 11, 12]. However, controlled heat treatment of such municipal waste provides an alternative way to minimize the pollution. Heating rate, temperature and quenching phenomenon decides the resultant products. Pyrolysis and gasification are used to produce carbon product from waste such as fuels, chemicals, and solvents [13, 14]. Pyrolysis is a method of changing any organic matter to obtain an array of solid, liquid and gas products by controlled heating. Based on the temperature range, pyrolysis are of three types; low temperature (< 550 °C) pyrolysis, moderate temperature (550 °C to 800 °C) pyrolysis and high temperature (> 800 °C) pyrolysis. Char, gas and tar are the final product of the pyrolysis. Char from the waste has large surface area and porosity which can be used for the manufacturing of active carbon [15, 16, 17].
Pyrolysis of human hair yields a black, shiny solid product at low and moderate temperature. Use of pyrolyzed hair was first reported by a Chinese herbalist Li Shi-Zhen in his book Ben Cao Gang in 16th century as a medicine. It was named as Xue Yu Tan and termed as Crinis Carbonisatus in English [6, 18]. This is still used in some parts of South Asian countries for fast relief as well as long term recovery of wound [19, 20, 21, 22].
Chemically modified pyrolyzed hair has been reported in literature. Guo, Y. et.al. (2016) prepared sample at 200 °C by hydrothermal treatment and reported the presence of carbon, nitrogen and oxygen by X-ray photoelectron spectroscopy. Fourier Transform Infrared (FTIR) result of the same work revealed the presence of carboxylic, alkyl, aryl, and amino functional group [23, 24, 25]. Qian, W. et.al (2014) and Chaudhari, K.N. et.al (2014) reported the crystalline nature of the pyrolyzed sample at 800 °C as confirmed by XRD. Crystalline behavior of sample increased with increase in carbonization temperature [19, 20, 26].
However, Altuntaş, D.B. et.al (2019) found that the crystalline nature decreases with increase in pyrolysis temperature when the hair sample is activated by ZnCl2 [24]. Transmission Electron Microscopy (TEM) images of hydrothermally prepared carbonized sample showed the quantum dot of size 2–10 nm. However, TEM images of the pyrolyzed sample (800 °C) revealed the presence of the micro/meso porous channel like texture. While investigating the same sample by Scanning Electron Microscopy (SEM), images displayed graphite like carbon flakes structure with high level of disorder. This heteroatom doped carbon material had dimension in nanometer range with high surface area and better porosity [23]. Due to these properties, it has been found to be useful for intensifying oxygen reduction reaction in fuel cells and to increase the electrochemical performance in super capacitor [20, 21].
The works so far reported have been carried out on chemically modified pyrolyzed human hair. However, works related to structure and properties of the Crinis Carbonisatus have not been reported in literature so far. This work is focused on the pyrolysis of hair under different heating conditions to obtain the Crinis Carbonisatus without any chemical modification and hence on investigation of physicochemical properties of the substance.