1 Material Properties of Plastics 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 step wise) and condensation polymer- ization (polycondensation) [1] (Figure 1.1). . Polyaddition as 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 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 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 build up of polyfunctional com- pounds. 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 placing 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. Laser Welding of Plastics, First Edition. Rolf Klein. Ó 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA. j3
Material Properties of Plastics
Jun 17, 2023
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