lesson5.pptx

High-performance fibres Liquid crystalline polymer-2

High-performance fibresLiquid crystalline polymer-2Handbook of tensile properties of textile and technical fibres, Woodhead Publishing, 2009Mechanical propertiesVectran properties depend on the molar ratio of the two monomers and heat treatment, as shown in the table.A molar ratio of 3/1 HBA/HNA represents the best balance between mechanical properties and melt processability.

16/10/201440Prof. Ada Ferri MSc Textile Engineering A.Y. 2012/2013

LC aromatic heterocycles fibresHeterocyclic polymers with a lyotropic LC behaviour are characterized by wholly aromatic molecular structures with fused heterocyclic rings along the main chains.

16/10/20143Prof. Ada Ferri MSc Textile Engineering A.Y. 2012/2013Polybenzazole PBO (Zylon by Toyobo) and PBT Aromatic heterocyclic are characterised by high chain rigidity. They display mechanical properties superior to para-aramid fibres. These fibers and filaments are characterized by high tensile strength (10 times higher than steel, 1.6 that of Kevlar), excellent impact energy absorption (twice that of para-aramid), and exceptional thermal stability (limiting oxygen index of 68). 16/10/20144Prof. Ada Ferri MSc Textile Engineering A.Y. 2012/2013The synthetis of PBO and PBT is typically conducted via polycondensation of aromatic tetramines and terephalic acid (TA) or terephtalic acid derivatives (terephtaloyl chloride).In the scheme below the monomer synthesis of PBT is shown:

DABDT 2,5-diamino-1,4-benzenedithiol dihydrochloride16/10/20145Prof. Ada Ferri MSc Textile Engineering A.Y. 2012/2013PBO is manufactured via the monomer synthesis of 4,6-diamino-1,3-benzenediol dihydrochloride (DABDO) and terephtalic acid. The reaction is carried out in poly-phosphoric acid (PPA).

16/10/20146Prof. Ada Ferri MSc Textile Engineering A.Y. 2012/2013Before the polymerization, amino hydrochloride and PPA are heated at 60-130C for 3-24 hours (under vacuum or inert gas) to allow the dehydrochlorination that is necessary for the complete activation of the amino monomer. TA can be added before or after the dehydrochlorination; moreover, after the process, an additional amount of phosphorus pentoxide (P2O5) and/or PPA is required to obtain a stirrable mixture.Afterwards, the reactant mixture is heated at 100-150C for a few hours to remove the last traces of hydrogen chloride, to wet the terephtalic acid, and to initiate the polymerization reation.Subsequently, the reactant mixture is heated at a temperature higher than 150C (typically between 190 and 200C) for several hours (up to 48 hours) until the completion of heterocyclization is reached.16/10/20147Prof. Ada Ferri MSc Textile Engineering A.Y. 2012/2013The main problem during the early development stage was related to the difficulty of obtaining a molecular weight sufficiently high to guarantee elevated mechanical properties.The purity of reactants and solvent, especially the amine reactant, is a critical issue: for this reason methods of production an purification of the amine are of great importance and very expensive.Moreover, another critical issue to enhance the molecular weight is the dimension of the TA aggregates: they must be small enough (less than 10 mm) to be completely dissolved in the reaction mixture.16/10/20148Prof. Ada Ferri MSc Textile Engineering A.Y. 2012/2013Fibre spinningPBO and PBT manifest lyotropic behaviour like polyaramides. They have no melting point (i.e. they decompose before melting) and form an LC phase in strong acids such as PPA, methanesulphonic acid (MSA), chlorosulphonic acid, 100% sulphuric acid and trifluoroacetic acid.

MSA

Chlorosulphonic acid

Trifluoroacetic acid16/10/20149Prof. Ada Ferri MSc Textile Engineering A.Y. 2012/2013PBO molecules form a liquid crystal phase in 100% sulphuric acid at about 5.5%.The different solvents do not induce significant differences in terms of mechanical properties of the resulting fibres. Dry-jet wet spinning is used to produce PBO and PBT. 16/10/201410Prof. Ada Ferri MSc Textile Engineering A.Y. 2012/2013

Heat treatmentDry jet-wet spinning is usually followed by a heat treatment to enhance the molecular orientation and, consequently, the mechanical properties of the fibres.In a typical treatment, PBO fibres are drawn under tension at temperature of 500-700C in nitrogen for a few seconds.16/10/201411Prof. Ada Ferri MSc Textile Engineering A.Y. 2012/2013

16/10/201412Prof. Ada Ferri MSc Textile Engineering A.Y. 2012/2013MorphologyA fibre is envisioned to comprise macrofibrils, fibrils, and microfibrils, with typical diameters of 5, 0.5 and 0.05 mm, respectively. The fibrils run parallel to the fibre axis with voids between them. A skin-core differentiation is evident, being the surface region (