# Novel Hybrid PETG Composites for 3D Printing
## 1 Introduction  
The beginnings of 3D printing date back to 1984, when [PERSON] applied for patenting of a method based on hardening of plastics by ultraviolet (UV) radiation [1]. The principle of the technology was the irradiation of UV-sensitive polymer material in the places to be hardened. Non-irradiated unhardened material was removed, and a final 3D product was obtained. Later on, a method where polymer powder was sintered by laser to obtain a 3-dimensional object was developed [2]. Currently, there are several 3D printing methods in use, and in recent years there has been a huge boom in 3D printing development as the original patents gradually expire. Many specialized companies selling 3D printers also focused on research activities, and they want to provide a competitive advantage. They try to improve the printing technique and prepare new materials that could be used for the most demanding professional applications. The improvement of the printing technology also increases the requirements for the used materials. Initially, 3D printing was to be used only for prototyping products and generating a low number of products. Current professional 3D printers with enhanced materials for 3D printing are gradually being inserted into manufacturing plants as well. In the case of a machine failure during the production, products from a 3D printer are printed out of suitable materials capable of replacing damaged parts of the device for at least a short while until they have the original accessories [3]. The large variability of printers and the ability to print various shapes are used in medical applications, bionics, and also in the food industry [4]. The 3D printer is able to print artificial teeth [5], dental fillings [6], endoprostheses [7], implants [8], and prosthetic limbs [9], and recently printing tissues by proteins and living cells has been considered [10]. The most ambitious visions are about printing complete organs from patient cells [11]. The most common non-filled thermoplastic materials used in fused filament fabrication (FFF) are acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA). However, other examples of non-filled thermoplastic filaments commercially available include acrylonitrile styrene acrylate (ASA), polyamide (PA), polycarbonate (PC), polyphenylsulfone (PPSF, PPS, or PPSU), polyetherimide (PEI), thermoplastic polyurethane (TPU), polyethylene terephthalate (PET), thermoplastic elastomer (TPE), high impact polystyrene (HIPS), polyvinyl alcohol (PVA), polyether ether ketone (PEEK), polyvinylidene fluoride (PVDF), polyoxymethylene (POM), polyhydroxyalkanoate (PHA) blended with PLA, and some other blends of the previously mentioned polymers. To improve strength, appearance, conductivity, and temperature resistance, different fillers are added to the polymers. New composite materials are gradually emerging to expand the possibilities of 3D printing [12]. To eliminate crystallization limits of the PET polymer, a glycol-modified PET copolymer has been prepared. Poly(-ethylene glycol-co-1,_A_-cyclohexanedmethanol terephthalate) (PETG) is synthesized by partially replacing the ethylene glycol (EtGly) units of PET with 1,4-cyclohexanedimethanol (CHDM) units [13]. The PETG copolymer is amorphous when the CHDM content is in the range of 32%-62% [14]. Mechanical properties of the PETG copolymer are close to those of PET. PETG copolymer has noticeable tensile toughness, transparency, flexibility, high processability, and excellent chemical resistance [15]. It can be used for many applications and in recent years PETG has also become popular in 3D printing [16]. In this work the preparation of composite materials of PETG with carbon fillers--expanded graphite (EG), carbon fibers (CF), and combinations thereof--is described.