**2. High-density polyethylene (HDPE)**

#### **2.1 Properties of HDPE**

Polyethylene is used commonly as several types that grouped by density. These are high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and very low-density polyethylene (VLDPE) [33]. The mechanical properties of PE are highly dependent on variables like its crystal structure, molecular weight, and extent and type of branching. Physical and thermal properties of HDPE are given in **Table 1**; mechanical and electrical properties are given in **Table 2**. Although the O2 permeability of HDPE is higher than most plastics such as PLA, PVC, PET, and PVDC, its water permeability is quite low [50]. The thermal conductivity of HDPE is about 0.5 W/mK at 23°C, while that of PLA, ABS, and PP is 0.2 W/mK, and PA is 0.3 W/mK [51]. However, its Tm temperature is 134°C, which is low compared to other plastics (>200°C) [52]. Although this limits HDPE's use at high temperatures, it is still a thermoplastic with adequate mechanical and thermal properties for many applications. Although its tensile strength is as good as PA, PC, and PS, it is one of the polymers with the highest impact strength value [53]. Due to HDPE's low water absorption (<0.4), high glass transition temperature (102°C) and hardness (94), satisfactory tensile strength (18 MPa), antibacterial properties, and chemical and UV resistance, it is widely used in the chemical and food packaging industry.

#### **2.2 HDPE applications**

Polymers, which are used as one of the materials in the manufacture of implants, are one of the most popular synthetic materials [54]. They are preferred because of their low price, good electrical properties, chemical inertness, and easy processing [55]. PE is also used in the medical sector, both in the production of medical equipment and in the production of implants. HDPEs are commonly used for catheters, facial restoration, acetabular endoprosthesis, ear reconstruction, nasal dorsal augmentation, mandibular contours, facial contouring, orbital floor, and socket reconstruction (**Figure 2a**) [58–63]. High-density polyethylene (HDPE) is also preferred as a low-cost alternative material to replace the lost tissue in living tissue (**Figure 2b**) [64].


#### **Table 1.**

*Physical and thermal properties of HDPE.*


#### **Table 2.**

*Mechanical and electrical properties of HDPE.*

Thermoplastic matrix composites are used in semi-structural and engineering applications. HDPE-based hollow particle-filled composite materials can be produced using the polymer injection molding process (**Figure 3a**) [70, 71]. In addition, HDPE is used in products such as storage boxes, beverage bottles, consumer electronics

*Mechanical and Thermal Properties of HDPE/PET Microplastics, Applications, and Impact… DOI:http://dx.doi.org/10.5772/intechopen.110390*

#### **Figure 2.**

*Implant application of HDPE (a) MedPor implant (left: before, right: after) [56] (b) Microtia repair (left: before, right: after) [57].*
