## 1 Introduction
### Liner in High Pressure Composite Vessel: Materials and Manufacturing Process  
From the perspective of the exploitation of a type 4 high-pressure vessel, the most critical element is the liner. It is directly responsible for the accumulation of gaseous hydrogen; thus, its tightness is of paramount importance. In accordance with binding standards, hydrogen permeation through container walls is permitted as long as it does not exceed 6 Ncm\({}^{3}\) per hour of hydrogen, per liter of the internal volume of the container [2]  
Figure 1: Scheme of a high-pressure hydrogen storage vessel. (**a**) A cross section and (**b**) a construction scheme: 1—inlet boss, 2—composite load carrying shell, 3—liner, 4—permeation reducing sol–gel coating, 5—cylindrical part, 6—dome with visible PE liner inside. or is <46 mL/h/L water capacity of the storage system in the case of a container meeting type-approval requirements [3]. In addition to this, the current regulations, codes and standards (RC&S) specify that the materials used to make a container, including the liner, must exhibit so-called hydrogen compatibility pursuant to [4]. For more information on the influence of hydrogen on polymers, see the literature studies below. Simultaneously, the liner performs the role of a core in the composite layer winding process, which determines the final strength of the construction. Most frequently, the liner has the shape of a cylinder with non-spherical geometry domes. The liner also contains metal bosses that allow fixing pressure valves and a connection between the container and the gas installation. A boss can also be used as a mounting grip (in the so-called neck mounting system). The liner can be made of metal (appropriate steel types or aluminum alloys). However, in the most technologically advanced containers, liners are only made of polymers (Figure 2). Then, the dominant materials used to make the liner are: high-density polyethylene (HDPE), polyamide (PA6) or, not so frequently, PET, polyoxymethylene (POM), and ethylene vinyl alcohol copolymer (EVOH) [5; 6]. Despite the fact that the manufacturing cost of a liner, in comparison with that of the whole container, is relatively low and under series production conditions, it does not exceed 10% of the cost to make a high-quality liner; one needs an expensive machine park, as well as specialist knowledge on the materials and their processing technology. Usually, a liner is produced by extrusion, rotomolding or blow molding, or some combination of these, through the use of the plastic jointing technique, i.e., bonding of extruded domes with an integrated boss with extruded tubes [7]. Metallic bosses can also be integrated within the domes by gluing them directly to the liner in a second step [8]. Below, short descriptions of two manufacturing methods are given. One of the most commonly used technologies in liner production is rotomolding, in which a final product is formed in a mold by simultaneously rotating and raising the temperature to about 200 \({}^{\circ}\)C. First, metal bosses and polymer (e.g., HDPE, PA) in the form of a granulate are placed in a mold that is used to make the liners. During the process, the input material settles on the mold walls, which make the external geometry of the liner. The thickness of the liner walls is controlled by the amount of granulate put into the mold and the program controlling the machine motion. During the cooling stage, which lasts until ambient temperature is reached inside the mold, a massive pressure pushes the walls of the newly made liner against the mold walls, thus preventing collapse during a contraction. After the cooling process, the finished product is removed from the mold and requires only quality control and mechanical processing of the sealing surfaces near the boss. The fundamental advantages of rotomolding are the lack of connections and welds on the liner and no internal tensions in the construction, regulated wall thickness, and easy formation of additional elements (threaded elements, bosses, etc.).