Material Properties of Plastics (一)

- Feb 12, 2019-

1.1 Formation and Structure 

The basic structure of plastics (or polymers) is given by macromolecule chains, 

formulated from monomer units by chemical reactions. Typical reactions for chain assembling

are polyaddition (continuous or stepwise) and a condensation polymerization (polycondensation).

.   Polyaddition as a chain reaction: Process by chemical combination of a large number of monomer molecules, in which the monomers will be combined to a chain either by an orientation of the double bond or by ring splitting. No byproducts will be separated and no hydrogen atoms will be moved within the chain during the reaction. The process will be started by energy consumption (by light, heat or radiation) or by use of catalysts. 

.   Polyaddition as step reaction: Process by the combination of monomer units without a reaction of double bonds or separation of low molecular compounds. Hydrogen atoms can change position during the process. 

.   Polycondensation: Generation of plastics by the build-up of polyfunctional compounds. Typical small molecules like water or ammonia can be set free during the reaction. The reaction can occur as a step reaction.

   The monomer units are organic carbon-based molecules.  Beside carbon and hydrogen atoms as main components elements like oxygen, nitrogen, sulfur, fluorine or chlorine can be contained in the monomer unit.  The type of elements,  their proportion and placed in the monomer molecule gives the basis for generating different plastics, as shown in Table 1.1. 

   The coupling between the atoms of a macromolecular chain happens by primary valence bonding [2].  The backbone of the chain is built by carbon atoms linked together by single or double bonding. Given by the electron configuration of carbon atoms, the link between the carbon atoms occurs at a certain angle, for example, for single bonding at an angle of 109.5 . Atoms like hydrogen, which are linked to the carbon atoms, hinder the free rotation of the carbon atoms around the linking axis. 

Figure 1.1   Processes for generating plastics and examples.

    The “cis”-link of carbon atoms has the highest bonding energy while the "trans"-link 

    has the lowest (Figure 1.2) [3]. 

       Depending  on  the  type  of  bonding  partners  several  chain  conformations  are 

    possible. Examples of such conformations are zig-zag conformation (e.g., PE or PVC) 

    or helix conformation (e.g., PP, POM or PTFE) (Figure 1.3) [2]. 

    Table 1.1   Examples of some common plastics and their monomers. 

                          Monomer                                             Polymer 

    Ethylene                                                Polyethylene (PE) 

    Propylene                                               Polypropylene (PP) 

    Vinylchloride                                           Polyvinylchloride (PVC) 

    Caprolactame                                            Poly(E-Caprolactame)  (PA6) 

    Tetraflourethylene           Polytetraflourethylene  (PTFE)