Material Properties of Plastics(二)

- Feb 13, 2019-

Types of Plastics 

Caused by the macromolecular structure and the temperature-dependent physical properties plastic materials are distinguished into different classes. Figure 1.7 gives an overview of the classification of plastics with some typical examples. Thermoplastics are in the application range of hard or tough elasticity and can be melted by energy input (mechanical, thermal or radiation energy). Elastomers are of soft elasticity and usually cannot be melted. Thermosets are in the application range of hard elasticity and also cannot be melted. Plastics as polymer mixtures are composed of two or more polymers with homogeneous or heterogeneous structure. Homogeneous structures are for example copolymers or thermoplastic elastomers, built by chemical composition of two or more different monomer units in macromolecules. When using thermoplastic monomers such plastic material can be melted by thermal processes. Heterogeneous structures are for example polymer blends or thermoplastic elastomers, built by physical composition of separate phases from different polymers. Polymer blends with thermoplastic components also can be melted by thermal processes. Plastic composites consist of a polymeric matrix with integrated particles or fibers. 

When using thermoplastics as the matrix, such composites can be melted. If thermosets are used as matrix the composite cannot be melted. Characteristic of the different classes of plastics are the phase transitions that occur in contrast to metallic materials in temperature intervals. 

       Phase-transition temperatures are dependent on the molecular structure of the plastic. Limited mobility of the molecule chains, for example, by loop forming, long side chains or high molecular weight cause an increased phase-transition temperature [6]. A large variance of the molecule chain length or number and length of side chains also have an effect on the spreading of the phase-transition ranges. 


Thermoplastic Resins 

Thermoplastic resins consist of macromolecular chains with no crosslinks between the chains. The macromolecular chains themselves can have statistically oriented side chains or can build statistical distributed crystalline phases.  The chemistry and structure of thermoplastic resins have an influence on the chemical resistance and resistance against environmental effects like UV radiation. Naturally, thermoplastic resins can vary from optical transparency to opaque,  depending on the type and structure of the material. In an opaque material, the light is internally scattered by the molecular structure and direct transmission of light is very poor with increasing material thickness. Thermoplastic resins can be reversibly melted by heating and resolidified by cooling without significant changing of mechanical and optical properties. Thus, typical industrial processes for part manufacturing are the extrusion of films, sheets and profiles or molding of components. 

      The viscosity of the melt is dependent on the inner structure, like average molecular weight and spreading of the molecular weight around the average value. According to DIN  EN  ISO  1133:2005–2009  [10], the melt-flow index  (MFI) is a measure for the melt viscosity. The MFI gives the amount of material that will be extruded in 10 min through a standardized nozzle diameter by using a determined force. 

      Low MFI values signify high viscosity with glutinous flow behavior of the melt (materials for extrusion). Increasing MFI values result in decreasing viscosity and lighter melt flow behavior (materials for molding). It has to be noted that MFI values are only a rough estimation for the melt flow behavior because the structure viscosity of thermoplastics strongly depends on the loading [11]. 

        The macromolecular structure of thermoplastics is given by the chemical structure of the monomer units, the order of the monomer units in the molecule chain and the existing side chains. A pure statistical distribution of the macromolecules results in an amorphous material structure, but also semicrystalline structures can occur depending on the material. Therefore, thermoplastic resins are differentiated into amorphous and semicrystalline types [1, 6].  Amorphous Thermoplastics 

     Amorphous thermoplastic resins consist of statistically 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    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. 

   Above the glass temperature, usually the state of application [1], the amorphous phase thaws and the macromolecules of the amorphous phase gain more mobility. The crystalline phase still exists

and the mechanical behavior of the material is tough elastic to hard.