## 1 Introduction  
The disposal of plastic waste material ranging from the nano- to the macro-scale has become a serious contemporary environmental issue [1; 2; 3]. This arises partly from the sheer volume of plastics produced (over \(100\times 10^{9}\) kg per annum [2]) and the lack of a 'cradle to grave' approach to the synthesis, manufacture, use, and disposal of plastics [4]. Many approaches have been proposed to address this issue, including the recent launching of the Alliance to End Plastic Waste (AEPW; www.endplasticwaste.org) by numerous plastics manufacturing companies to address the issue of plastic pollution of the environment.  
Approaches to the disposal of plastic waste vary and include: 1) Generating polymers that will decompose with time via built-in chemical procedures [5; 6], 2) The development of enzymes/bacteria that will decompose polymers [7], 3) Landfill options, and 4) Thermal decomposition by combustion [8]. In this latter option, the possibility exists to convert these waste plastics into high-value materials that could be re-used by society [9; 10]. For example, plastic waste can be converted into spherical carbons and these spherical carbons can be solid or hollow and can be made with a range of sizes and porosities. These spherical carbons have been used in areas such as energy storage (batteries and capacitors), adsorption, catalysis, drug delivery etc. [11; 12; 13]. This paper provides an example of this, describing how a common waste polymer, that is, polypropylene (PP) was converted into higher value spherical carbon nanomaterials.  
PP is a common polymer found in society and is used in making films, fibers, tapes, etc. for use in the packaging of consumer products, in the automotive industry, and in textiles [14]. PP has also been used in more sophisticated items, such as centrifuge tubes--the material that was used in this study. Previous studies to convert waste polymers into high-grade chemicals have been reported. For example, the conversion of polystyrene and polyethylene into carbon nanotubes (CNTs), carbon nanofibers (CNFs) and carbon spheres (CSs) has been well documented [15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. Less work has been reported on the use of PP in similar studies.  
Thermal studies on the conversion of PP to CNTs/CNFs have been successful, but the process required the use of a catalyst [24; 25; 26]. Attempts to produce spherical PP derived materials have been less successful. For example, the reaction of PP with Co(acetate)\({}_{2}\) as a catalyst produced a mixture of hollow carbon spheres (HCSs) and solid carbon spheres (SCSs) [25], while thermal plasma treatment of PP gave a mixture of poorly formed SCSs and CNTs [27].  
The synthesis of SCSs [28; 29] and HCSs [30; 31] from hydrocarbons is well documented and their synthesis does not require the use of a catalyst. Thus, a simple process for the conversion of PP waste to produce a pure carbon material with a morphology dependent on the reaction conditions and choice of chemical reagents should be straightforward. Herein, we report on the use of used PP centrifuge tubes as a carbon source to make SCSs, HCSs, and nitrogen-doped HCSs (NHCSs).