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Material Properties of Plastics (三)
- Feb 14, 2019 -

1.2.1.1  Amorphous Thermoplastics 

     Amorphous thermoplastic resins consist of statistical oriented macromolecules without any near order. Such resins are in general optically transparent and most brittle. Typical amorphous thermoplastic resins are polycarbonate (PC), polymethyl-methacrylate (PMMA), polystyrene (PS) or polyvinylchloride (PVC). 

        Table 1.2 shows examples of amorphous thermoplastic resins with typical material properties. 

        Temperature state for application of amorphous thermoplastic resins is the so-called glass condition below the glass temperature T. The molecular structure is frozen in a definite shape and the mechanical properties are barely flexible and brittle (Figure 1.8). 

        On exceeding the glass temperature, the mechanical strength will decrease by increased molecular mobility and the resin will become soft elastic. On reaching the flow temperature Tf the resin will come into the molten phase. Within the molten phase, the decomposition of the molecular structure begins by reaching the decomposition temperature  T.d 


      1.2.1.2  Semicrystalline Thermoplastics 

     Semicrystalline thermoplastic resins consist of statistically oriented macromolecule chains as an amorphous phase with embedded crystalline phases, built by near-order forces.  Such resins are usually opaque and tough elastic.  Typical  semicrystalline thermoplastic resins are polyamide (PA), polypropylene (PP) or B (POM) (Table 1.3). 

        The crystallization grade of semicrystalline thermoplastic depends on the regularity of the chain structure, the molecular weight and the mobility of the molecule chains, which can be hindered by loop formation [6]. Due to the statistical chain structure of plastics complete crystallization is not feasible on a technical scale. Maximum technical crystallization grades are of the order of approximately 80% (see Table 1.3). 

        The process of crystallization can be controlled by the processing conditions. Quick cooling of the melt hinders crystallization. Slowly cooling or tempering at the crystallization temperature will generate an increased crystallization grade. Semi-crystalline thermoplastics with low crystallization grade and small crystallite phases will be more optically transparent than materials of high crystallization grade and large crystallite phases. 

   Below the glass temperature T, the amorphous phase of semicrystalline thermo-plastics is frozen and the material is brittle (Figure 1.9). Above the glass temperature, usually the state of application  [1],  the amorphous phase thaws and the macro-molecules of the amorphous phase gain more mobility. The crystalline phase still exists and the mechanical behavior of the material is tough elastic to hard. Above the 


Table 1.3   Examples for semicrystalline thermoplastic resins with typical material properties according to [1]. 


Resin       Temperature of          Crystallization         Specific weight         Tensile strength 

            use [                                                3                         2 

                  C]                grade [%]               [g/cm ]                 [N/mm ] 


PA 6        40–100                 20–45                   1.12–1.15               38–70 

HDPE        50–90                  65–80                   0.95–0.97               19–39 

PETP        40–110                 0–40                    1.33–1.38               37–80 

PP          5–100                  55–70                   0.90–0.91               21–37 

PPS         <230                    30–60                   1.35                    65–85 

PVDF        30–150                 52                     1.77                    30–50 


crystal melt temperature T        the crystalline phase also starts to melt and the material becomes malleable. As for amorphous thermoplastics, the flow ability of semicrystalline thermoplastics in the molten phase is characterized by the melt-flow index MFI. 

        The melt temperature of semicrystalline thermoplastics depends among other things on the size of the crystallites and the ratiobetween the amorphous and crystalline phases. The larger size and a higher proportion of crystallites will increase the melt temperature (Figure 1.10) [12]. As with amorphous thermoplastics, degradation of semicrystalline thermoplastics will start in the molten phase by exceeding the decomposition temperature T. d 


1.2.2 

Elastomers 


Elastomers are plastics with wide netlike crosslinking between the molecules. Usually, they cannot be melted without degradation of the molecule structure. Above the glass temperature T, as the state of the application (Figure 1.11), elastomers are soft elastic. Below T     they are hard elastic to brittle. The value of the glass temperature increases with increasing number of crosslinks. Examples of elastomers are butadiene resin (BR), styrene butadiene resin (SBR) or polyurethane resin (PUR) [13]. 

   Raising temperature affects an increase of elasticity, caused by reducing the stiffening effects of the crosslinks and increasing the mobility of the molecule chains. On exceeding the decomposition temperature T, the atom bonding within and between the molecule chains will be broken and the material will be chemical decomposed. 


1.2.3 

Thermosets 


Thermosets are plastic resins with narrow crosslinked molecule chains [1]. Examples of thermosets are epoxy resin (EP), phenolic resin (PF) or polyester resin (UP). 

   In the state of the application (Figure 1.12) thermosets are hard and brittle. Because of the strong resistance of molecular movement caused by the crosslinking, mechanical strength and elasticity are not temperature dependent, as with thermoplastics or elastomers. 

   Thermosets cannot be melted and joining by thermal processes like ultrasonic welding or laser welding is not possible. On exceeding the decomposition temperature T, the material will be chemical decomposed.