Realizing the enhancement of interfacial interaction in semicrystalline polymer/filler composites via interfacial crystallization

On transcrystallinity in semi-crystalline polymer composites

Cellulose fibres have long been used in the plastics industry as cost-cutting materials. Nowadays they are recognised as a potential replacement for glass fibres for use as reinforcing agents in composite materials. They have a number of certain advantages over glass fibres, such as low cost, high strength-to-weight ratio, biodegradability and ease of processing. In this study crystallisation from the melt of two different isotactic polypropylene matrices (iPP) in the presence of flax ( Linum usitatissinum ) fibres of four different types (green flax, dew retted flax, Duralin ® treated flax and stearic acid sized flax) was examined. The effect of processing parameters such as crystallisation temperature and cooling rate was investigated using hot stage optical microscopy. Differential scanning calorimetry (DSC) was used to investigate the inner morphology of the transcrystalline (TC) layer. Scanning electron microscopy (SEM) and X-ray diffraction were used in an attempt to identify the origin of the TC layer in connection with the structural characteristics of the fibres. The effect of transcrystallinity upon the mechanical properties of the interface was assessed using the single fibre fragmentation test. It was found that the interfacial adhesion is improved by the presence of a TC layer.

A study of transcrystallinity and its effect on the interface in flax fibre reinforced composite materials

Institut f(Jr Verbundwerkstoffe GmbH, Universit#t Kaiserslautern, Pf.3049, D-67663 Kaiserslautern, Germany It was recognized early on that the fibre/matrix interphase (or interface) strongly influences the mechanical properties of composite materials. One of the current research directions in the field of composites with semicrystalline thermoplastic ma- trices is devoted to the question of whether or not particular supermolecular arrangements of the ma- trix in the vicinity of reinforcing fibres can improve the overall mechanical performance. Interest is focused particularly on the potential benefits of a transcrystalline interphase [1-4], since it is believed that this kind of "physical coupling" would enhance the stress transfer between the fibre and the matrix. On the effects of the transcrystalline interlayer, however, rather contradictory reports have appeared. The formation of the transcrystalline interphase layer, originated by fibre surface induced heterogeneous nucleation, is generally studied by single fibre model composites [2, 3]. This has also been the case for glass fibre (GF) reinforced polypropylene (PP) [5-8], where a transcrystaUine- like supermolecular structure could only be pro- duced when the crystallizing PP melt was sheared by pulling the embedded GF. In our recent work [9, 10], it was demonstrated that the columnar structure developed by melt shearing is not tran- scrystalline, but row-nucleated cylindritic. We have shown further [9, 10] that when both the crystalliza- tion (Tc) and pulling temperature (Tpull) are set in the range of the formation of the/3-PP, i.e. between T,~ ~ 100 °C and T~, ~ 140 °C [11, 12], a /3-rich columnar structure appears. Partial melting of this supermolecular formation revealed a layer attached to the pulled GF. This sheared layer of oi-PP contained ol-row nuclei and originated/3-crystalliza- tion (c~-fl bifurcation of growth, [11]). So under the conditions To~ < To, Tpun < T~o: melt shearing by pulling of GF resulted in a complex polymorphous structure containing both the o~- (row-nuclei along the GF) and fl-form of PP (grown up onto the oL-row nuclei). Due to this "/3-overgrowth" the oc-layer along the fibre surface can be resolved on the optical level only after separate melting of the/3-phase [10]. In the cited works on the shear induced crystalliza- tion of PP by pulling the fibre [5, 6, 9, 10] the GF proved to be "inert", i.e. it did not show any o-nucleation ability. Therefore it seemed that it would be very interesting to study how the above scenario changes when ol-nucleating fibres are used. 0261-8028 © 1994 Chapman & Hall In this case the conditions of both transcrystalliza- tion (due to heterogeneous 0l-nucleation) and row- induced cylindritic crystallization (due to melt shear- ing) are met. One can easily differentiate between them by taking into account the polymorphism of the PP and thus choosing the following crystalliza- tion conditions: T~ < To, Tpull < T~. Under these conditions the development of an o~-transcrystalline front is evidence of transcrystallization, while the formation of a fl-PP rich columnar structure indic- ates cylindritic crystallization [9, 10]. As oL-nucleating fibre poly(ethylene-terephtha- late) (PET) fibre taken from a commingled PET/PP yarn supplied by Toyobo Co. (Japan) was used. The oL-nucleating ability of PET has been reported in several works (cf. [7, 13, 14]). The isotactic PP used in this study was a general-purpose injection-mould- ing grade (Tipplen H-523, Tisza Chemical Works, Hungary). The crystallization and melting behaviour of the GF/PP model composites were studied with a Leitz polarizing optical microscope equipped with a Mettler hot stage. All further experimental details can be found in our previous work [9, 10]. Figs 1-3 show the effect of PET fibre on the isothermal crystallization of PP in its quiescent melt. Fig. 1 demonstrates the supermolecular structures formed in 2-step isothermal crystallization. At higher T0 (Td = 134 °C, t~ = 30 min) only spherul- ites were grown sporadically (Fig. la). Reducing T~ after 30 min crystallization time (t~) to To2 -- 124 °C gives rise to o:-transcrystallization of PP along the PET fibre (Fig. lb and c). A well developed transcrystalline layer can be obtained in 1-step isothermal crystallization, provided Tc is low enough (To = 124 °C, Fig. 2). This indicates that for the transcrystallization growth necessary high surface nucleation density on the PET fibre is guaranteed only at Tc ~< 130 °C in the given PP/PET composi- tion. The strong ol-nucleating ability of the PET fibre can be demonstrated by using a/3-nucleated PP matrix (Fig. 3). Fig. 3 shows that the PET fibre preserves its c~-nucleating ability even in fl-PP (achieved by using a proprietary selective fl-nucleat- ing agent (in 0.1 wt %). The oL-transcrystalline layer on the PET fibre is obvious, whereas/3-spherulites characterize the bulk (Fig. 3a). This structure becomes even more striking after selective melting (cf. later) of the /3-phase at Tf = 158 °C (Fig. 3b). Figs 1-3 show that the PET fibre acts as an

The difference between transcrystallization and shear-induced cylindritic crystallization in fibre-reinforced polypropylene composites

Isothermal crystallization behavior and mechanical properties of ionomer-treated sisal fibres reinforced high-density polyethylene (HDPE) composites have been investigated. Avrami analysis of the crystallization kinetics showed high crystallization rate (Kn) and short crystallization half time (t0.5) for composites compared to neat HDPE, indicating strong nucleating ability of sisal fibres. The crystallization activation energy (Ea) of the composites decreased with increasing fibre content. The polarizing optical microscopy showed the occurrence of transcrystallization at the fibre–matrix interface. The HDPE/sisal composites displayed superior tensile and flexural properties compared to neat HDPE. For these composites, predictions of Young's modulus using the Modified Rule of Mixtures and Cox model correlated well with the experimental data.

Silva Incardona

Papers

The mechanical role of the fibre/matrix transcrystalline interphase in carbon fibre reinforced J-polymer microcomposites