**3.3 Optical properties**

PE can vary between almost transparent as in LDPE, translucent as in LLDPE, and opaque as in HDPE. The ability of polyethylene films to scatter light is


**Table 3.**

*Important PE grades and properties.*

determined by the quenching conditions and sample's thermal history. Based on recent quantitative studies, the light scattered by a thin polyethylene sample quenched to 0°C from 125°C is much lower than that scattered by a heat-treated sample [27]. Because the refractive index along the tangent to the PE spherulites is lower than that along the radius of the spherulites, extruded polyethylene films have a slight positive birefringence. As a result, the clarity of polyethylene film is determined by the light scattered by it [28].

#### **3.4 Thermal properties**

Polyethylene exhibits a low thermal conductivity. It has a thermal expansion coefficient of 0.26 mm/°C and thermal conductivity of 0.4 W/m per °C. The specific heat of PE depends on temperature. Its low-density form has a specific heat capacity of 2.3 J/g at room temperature and 2.9 J/g at 120–140°C. The higher the molar mass and the more the branching, the lower the brittle point. Polyethylene is sensitive to surface imperfections. The standard commercial grade of high-density polyethylene's melting point is between 120 and 180°C, and LDPE is 105–115°C [29]. The zero-shear rate apparent viscosity of linear PE is related to the weightaverage molar mass by Eq. (1) for polymers with a molar mass over 5 kg/mol.

$$\mathbf{u}\left(\mathfrak{n}\mathfrak{a};\mathbf{0}\right) = K\mathbf{M}\mathfrak{w}^{3,4} \tag{1}$$

Polymers with long branches do not fit in the above equation, and Eq. (2) represents a relationship between polymers of different degrees of long branching. In many cases

$$\log\left(\eta \mathbf{a}; \mathbf{0}\right) = A + B M n^{1/2} \tag{2}$$

where log a;0 (η ) = zero shear rate apparent viscosity, K and A are constants, Mw = weight average molecular weight, and Mn = number average molecular weight [30].

Generally, as the branching and molecular weight distribution increases, the viscosity of the PE increases, and its shear rate decreases. The increase in the molecular weight of the polymer increases its viscosity, decreases its melt flow index, and decreases its critical shear rate. Finally, an increase in the molecular weight distribution decreases the PE flow behavior index [11, 13, 31, 32].
