*3.2. Thermal*

Significant efforts have been made to enhance the thermal properties of various polymers, as well as to produce heat tolerant polymers, in order to qualify them for use in applications requiring efficient serviceability at high temperatures. The production of heat-resistant polymers began in the late 1950s with the goal of producing heat-resistant polymers that could satisfy the needs of the aerospace and electronics industries. Heat resistant polymers differ from ordinary polymers in that they can retain their desired properties at high temperatures. While there is no universal consensus about how to classify heat resistant polymers, the following is a helpful general classification [84]: (a) Heterocyclic polymers such as polyoxadiazoles, polybenzimidazoles; (b) condensation polymers such as polyhydrazides, polyesters, polyazomethines, and polyamides; (c) Condensation heterocyclic copolymers such as poly(amide-imide)s, poly(ether-imide)s, poly(amidehydrazide) and poly(ester-imide)s, and (d) Ladder polymers including poly(benzimidazo benzophenanthroline) and polyquinoxaline.

#### 3.2.1. Mold Stress-Relief Distortion

Products from injected molded polymers have residual stresses that may unfavorably affect the product performance, directly related to other processing conditions such as holding pressure, molding, and melting temperature [85]. Injection modeling in polymers is a common phenomenon. It has several merits such as low-cost mass production and good finishing's. However, due to the tricky deformations/bends or twist and pressure, residual stresses build up. The viscoelastic properties of polymers coupled with their high pressure during molding leads to complex situations such as orientation, stretching, and chain relaxation [85].

For most polymeric parts, residual stresses are quantified by destructive methods such as layer removal and hole drilling [86]. Although these techniques have problems related to their accuracy, their major drawback is reusing the measured parts. To overcome these limitations, new methods such as indentation are used. This technique involves applying higher force at the stressed location to align the same depth and add residual stresses at the indented locations.

#### 3.2.2. Generic Thermal Indices

UL746B [87] and IEC60216 [88] were widely applied to determine relative thermal indices of polymers suitable for applications related to electrical applications. This is important to decide on the highest operating temperature considering the lifetime of the products. For polymeric materials, three essential properties can lead to their thermal categorization. These include electrical, mechanical/tensile, and impact properties.

The thermal index of polymers relies on the base polymers' property due to their deterioration, decomposition, and degradation caused by chemical reactions. In general, polymers' relative thermal index is vital in determining base polymers' long-standing thermal resistance [89].

#### 3.2.3. Relative Thermal Capability

Nowadays, synthesis polymers are readily available and produced through the extrusion process. Due to the broader application of polymers, it is essential to know the relative thermal capability of polymers. It is vital to understand their thermal properties at molten temperatures, assembly, and the final product.

The thermal conductivity of polymer composites refers to both polymers and their fillers [90]. However, the importance of polymers' thermal conductivity has not been overemphasized concerning that of the fill. It is generally known that polymers' thermal conductivity is vital at low filler loadings due to the separation of the polymer matrix's thermally conductive fillers. To obtain high thermal conductivity in composite polymers, there is the need to create a continuous filler network. However, one major issue with network formation is high cost, reduced mechanical properties. The fundamental factors influencing polymers' thermal conductivity are their chain structure, crystallinity, and orientation domains [90]. On the other hand, polymers have thermal conductivities that are lower than ceramic or metals. Table 4 shows the thermal conductivities of renowned polymers, usually with low thermal conductivity [91].


**Table 4.** Thermal conductivities of polymers.

#### 3.2.4. Relative Thermal Index

The performance of two materials (developed new and reference) are compared to their thermal stability. Underwriters Laboratories (UL) standards are involved in projects with thermal aging of polymeric material. Properties of polymeric material can be changed by increasing the temperature, which will be considered the main factor in determining the particular material's thermal aging as per the procedure mentioned in UL 746B. The mathematical equation will give a relation between time and temperature, as presented in Equation (7):

$$\ln(\mathbf{t}) = \mathbf{A} + \mathbf{B}/\mathbf{T} \tag{7}$$

where, A = Constant, frequency factor; B = Energy constant for activation; T = Absolute temperature in Kelvin.

A comparison between newly produced polymers with control material will be made. The results established a relative thermal index based on the material's specific property, color, and thickness [87,88].

#### 3.2.5. Temperature Excursions beyond the Maximum Temperature

The long-term aging data related to the product's stability, developed with a polymeric material, can establish a better thermal index. The temperature extrusions with short-term accepted for specific application for the material have enough thermal indices. The material will go to transition temperature immediately or anytime during the operation with electrical equipment. The heating appliance, initially thermostat will come automatically after highest temperature excursion but later on-air ambient temperature increases, the circuit will clear by thermostat and peak temperature decreases [92].
