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Halogen-Free Flame-Retarded Polybutylene Terephthalate and its Processing into Polymer Foams

Polybutylene terephthalate (PBT) as an engineering thermoplastic is used among other things in the communication and automotive industries. In these applications flame retardants are required that do not show a negative influence on other material properties like the processability or the mechanical properties. Halogen based flame retardants have to be replaced because of their environmental and health hazards. Therefore, currently halogen-free phosphorous based additives are developed.

Apart from the usage of engineering thermoplastics as compact material the trend is moving also into foaming of such materials. PBT foams have so far not been available commercially, because different from amorphous thermoplastics no broad temperature window for foam growing and stabilisation exists.

Comparing two commercial low-molecular additives a marked correlation of mechanical properties and flow behaviour with particle size and shape becomes apparent. By application of phosphorous based polyesters as polymeric flame retardants it was tried to reduce the negative influence on the mechanical properties. The embrittlement of the blends is comparable to the compounds with low-molecular additives, but can be improved by using a copolymer with a better phase adhesion in the blend system. While doing so the relationship between the rheological properties of matrix and flame retardant as well as the resulting structure has been determined.

The rheological properties of the melt under process conditions and blowing agent loading is essential for understanding the foaming process. The influence of the process parameters temperature, pressure, and blowing agent concentration on the rheological properties has been quantified by means of an in-line rheometer in foam extrusion and compared to a foamable polypropylene.

Despite the problematic foam stabilisation it was possible to produce PBT foams with a density reduction of at least 50 % and suitable production methods as well as material and process parameters could be identified. The production of extruded PBT foams is possible in principle. By using an underwater pelletizer the foam stabilisation after the die can considerably be improved. By choosing the right granulation and material parameters spherical expanded E-PBT beads were produced. PBT structural foams with a density reduction of up to 80 % can be realised by foam injection moulding with a breathing mould. These foams were analysed regarding structure-properties relationships of flame-retarded PBT. The influence of a low-molecular flame retardant on the foaming behaviour, foam structure, the mechanical properties, and the burning behaviour were considered.

Lesen Sie die deutsche Zusammenfassung auf Kunststoffe.de
Author
 Thomas Köppl

Thomas Köppl
Lehrstuhl für Polymere Werkstoffe
Universität Bayreuth

Information

Free keywords: flame retardants, polymer blends, rheology, foam extrusion, foam injection moulding, bead foams
Institute / chair: Fakultät für Ingenieurwissenschaften der Universität Bayreuth
Language: German
Technical consultant for expert services: Prof. Dr.-Ing. Volker Altstädt (Betreuer), Prof. Dr. rer. nat. habil. Manfred Döring
Publication year: 2014
Provider: Wissenschaftlicher Arbeitskreis Kunststofftechnik (WAK) / Kunststoffe.de

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