Isocyanate toughened pCBT: Reactive blending and tensile properties

Abstract

The in situ ring-opening polymerization of cyclic butylene terephthalate oligomers is a promising route to replace thermosetting polymers in composites and has become of great interest. CBT oligomers melt at low temperatures (120-160ºC) and exhibit a water-like viscosity prior to polymerization. Due to this very low initial viscosity, fiber reinforcements can be readily impregnated. After impregnation, the molten CBT oligomers quickly polymerize in an entropically driven, athermal ring-opening polymerization (ROP) without releasing volatile organic compounds. This process yields pCBT, a thermoplastic polyester with high molecular weight [1][2][3]. So far, continuous fiber reinforced composites made from CBT and glass fibers [3][4][5][6][7][8], basalt fibers [9], and carbon fibers [10][11][12][13] have been reported. Moreover, a hydrogenated nitrile rubber (HNBR) was modified with CBT and the HNBR/CBT hybrids exhibited improved mechanical and triblogical properties compared to pristine HNBR [14]. CBT has also been extensively used in nanocomposites due to the low melt viscosity prior to ROP which is advantageous for a good dispersion of the reinforcement. Nanocomposites with nanosilica [15], multiwalled carbon nanotubes [16][17][18][19] Abstract. Cyclic butylene terephthalate oligomers (CBT) were reacted in a ring-opening polymerization with three types of isocyanates: a bifunctional aromatic type, a bifunctional aliphatic type and a polymeric aromatic isocyanate. All reactions took place in a batch mixer. The use of 0.5 to 1 wt% isocyanate led to a dramatic increase in elongation at break of polymerized cyclic butylene terephthalate (pCBT), from 8 to above 100%. The stiffness and strength of the modified pCBT, however, were found to slightly decrease. Proton nuclear magnetic resonance (NMR) analysis shows that the formation of thermally stable amide groups is the dominant chain extension reaction mechanism. Gel content measurements suggest a linear structure for samples containing bifunctional isocyanates while pCBT modified with polyfunctional isocyanate exhibited some gel formation at higher isocyanate content. Melting and crystallization temperatures as well as degree of crystallinity were found to decrease with increasing isocyanate content. No phase separation was detected by scanning electron microscopy (SEM) analysis. Moreover, a high degree of polymerization is deduced due to the absence of CBT oligomer crystals.

Isocyanate toughening of pCBT/organoclay nanocomposites with exfoliated structure and enhanced mechanical properties

“…A possible explanation is that bound crystal water of the clay may have affected the CBT catalyst activity [35] and also caused pCBT hydrolysis. In comparison, the ternary blend exhibited less delay in the torque onset, reaching a maximum level of 43 Nm after 5 min due to the fast reaction between PMDI and growing pCBT chains [11]. The torque decreased and reached a stable plateau value of 20 Nm which is considerably higher than that of neat CBT, suggesting a relatively higher molecular weight.…”

“…This allows for isothermal processing below the T m where crystallization and polymerization occur simultaneously, hence demoulding is possible without further cooling [7][8][9]. Nevertheless, pCBT is inherently brittle which has been ascribed to the formation of large perfect crystals with a lack of intercrystalline tie molecules [8], different crystalline morphologies [10] as well as to a low molecular weight [11,12]. An increasing amount of publications dealing with pCBT/organoclay nanocomposites can be found in the literature [7,[13][14][15][16][17][18][19][20][21]; however, despite this considerable body of papers, little is known about the mechanical properties of these materials [15,18,20] and information about the deformation behaviour is rare.…”

“…The method described herein involves a coupling reaction of the hydroxyl groups of the organic surfactants with -N=C=O functional groups of a polyfunctional isocyanate in order to intercalate the organoclay. Since the tethered isocyanate is also highly reactive toward the pCBT functional groups, specifically toward carboxyl end groups as shown in [11], pCBT is expected to be grafted onto the isocyanate-modified organoclay. To the best of our knowledge, this has not been reported in literature to date.…”

A Review of the Recent Advances in Cyclic Butylene Terephthalate Technology and its Composites

“…108 The used isocyanates were a bifunctional aromatic type (hexamethylene diisocyanate; HDI), a bifunctional aliphatic type (4,4'-methylenebis(phenyl isocyanate); MDI) and a polymeric methylene diphenyl isocyanate (PMDI). All reactions took place in a Brabender batch mixer equipped with a torque measuring system at 230 °C, 60 rpm and under nitrogen atmosphere to minimise hydrolysis.…”

Effect of Cyclic Butylene Terephthalate Oligomer on the Properties of Styrene-Butadiene and Acrylonitrile-Butadiene Rubbers

“…Several techniques can be found in the literature for the compensation of this embrittlement, for example co-polymerization with ε-caprolactone [7], compounding with polycaprolactone, polycarbonate, polytetrahydrofurane, multiwalled carbon nanotubes [8], chain extending by a bifunctional epoxy resin during the polymerization [9], or reactive blending with isocyanates [10,11]. Most of these toughening techniques lead to a drop in the crystallinity of the resulting pCBT, which decreases the brittleness of it.…”