Failure detection and monitoring in polymer matrix composites subjected to static and dynamic loads using carbon nanotube networks

In this work, multiwall carbon nanotubes (MWCNTs) have been used as a network of sensors to predict the failure region and to monitor the degradation of mechanical properties in laminated composites subjected to tensile and cyclic fatigue loadings. This is achieved by measuring the electrical resistance change in the semi-conductive MWCNT-fiber glass-epoxy polymer matrix composites. By partitioning the tensile and fatigue samples with electrically conductive probes, it has been shown that with both increasing tensile load and number of cycles, different resistance changes are detected in different regions and failure happens in the part in which higher resistance change was detected. In cyclic loading, in which the maximum load is higher than the elastic limit of the laminate, a sharp increase in resistance occurs within the first several cycles. There is also a change in resistance during long term cyclic loading. In cyclic loading, when compared to strain gauge readings, resistance change measurements show more sensitivity in identifying the crack initiation site, which gives this technique a good potential for monitoring strength degradation during fatigue. Keywords: composites; Carbon nanotubes; Electrical properties; Sensors

Nanoparticle Interactions and Molecular Relaxation in PLA/PBAT/Nanoclay Blends

ABSTRACT: Organo-modified clay nanoparticles were mixed at 1 and 5 wt% concentrations with a molten blend of 75 wt% of polylactide (PLA) and 25 wt% poly[(butylene adipate)-co-terephthalate] (PBAT). Three mixing strategies were used to control the localization of nanoclay. Small amplitude oscillatory shear (SAOS) and stress growth tests were conducted to clarify the nanoclay interactions with the blend components and its effect on the molecular relaxation behavior. SAOS and weighted relaxation spectra properties were determined before and after pre-shearing at a rate of 0.01 s−1. Molecular relaxation and its characteristics were influenced by PLA degradation, PBAT droplet coalescence, and nanoclay localization

Expanded PLA Bead Foaming: Analysis of Crystallization Kinetics and Development of a Novel Technology

Bead foam technology with a double crystal-melting peak structure has been well established for polyolefins. The double crystal melting peak structure, which is required in the molding stage of the bead foams, generates a strong sintering among the foamed beads and maintains the overall foam structure. In this research, despite the PLA’s poor foaming behavior and its slow crystallization kinetics, we successfully developed expanded PLA (EPLA) bead foams with double crystal melting peak structure and the inter-bead sintering behavior was verified through steam chest molding. For this purpose, the generation and evolution of double crystal melting peak structure in different PLA materials is simulated in a high-pressure differential scanning calorimeter (HP-DSC). The simulation results shows that the formation of double crystal melting peak with different peak ratios can be controlled by varying the processing parameters (i.e., saturation pressure, temperature, and time) during the saturation. The PLA bead foams characterization showed that the high melting temperature crystals generated during the saturation and the low melting temperature crystals formed during the cooling and foaming can significantly affect the foaming behavior of PLA bead foams. Moreover, the crystallization kinetics of different PLA materials are systematically investigated in presence of dissolved gas. It is shown that the different crystallization kinetics (i.e., crystal nucleation and growth rate) that can be induced at various gas pressures can significantly influence the PLA’s foaming behavior (i.e., cell nucleation and expansion behavior).Ph

Influence of nanoparticles and their selective localization on the structure and properties of polylactide-based blend nanocomposites

This article critically reviews the influence of different nanoparticle localization on the final structure-property relationships of polylactide (PLA)-based blend nanocomposites. The effects of kinetics and thermodynamic parameters on the final localization of the nanoparticles are discussed. The different mechanisms of the stabilizing effect of nanoparticles are reviewed with respect to their final localization as well as their size/shape characteristics. Alternatively, the effects of localization of various types of nanoparticles on the morphological, rheological, electrical, and mechanical properties of PLA-based blend nanocomposites are elaborately discussed. The sensitivity of the final performance of the PLA-based blend nanocomposites is explored with regard to the different localizations of different nanoparticles towards specific applications such as packaging and functional and sensory polymers. The recent progress in computer simulation on this topic is also addressed. In summary, this review provides new insight into the design and formulation of advanced PLA-based blend nanocomposites for a wide range of applications where the use of bioplastics and sustainability are critically considered

Rheology of poly (lactic acid)-based systems

Being commercialized in 1992, poly (lactic acid) (PLA) has been considered for biomedical applications and as a reliable substitute for a wide range of commodity and engineering applications where noncompostable petroleum-based polymers are currently being used. However, PLA suffers from series of drawbacks and it would not be applicable unless these shortcomings resolve somewhat. Besides the PLA’s brittleness and low toughness which originate from its higher glass transition temperature, the major shortcomings which negatively influence the other features of PLA are its low melt strength and slow crystallization kinetics. These weaknesses limit the processability, formability and foamability of PLA, and hence, the manufacturing of PLA based products. In this context, the improvement of rheological and viscoelastic properties of PLA is of a great importance as it enhances the melt strength. To control the PLA’s rheological and viscoelastic properties, various attempts such as varying the D-lactide content in PLA molecules, increasing the PLA’s molecular weight, the use of chain extender and branching, controlling the PLA’s crystallization, compounding with micro-/nano-sized fillers and blending with other polymers have been considered. This article critically reviews these studies that have been conducted so far on rheological investigations of various PLA-based systems

Synergistic Enhancement of Flame Retardancy Behavior of Glass-Fiber Reinforced Polylactide Composites through Using Phosphorus-Based Flame Retardants and Chain Modifiers

Flame retardancy properties of neat PLA can be improved with different phosphorus-based flame retardants (FRs), however, developing flame retardant PLA-based engineering composites with maintained mechanical performance is still a challenge. This study proposes symbiosis approaches to enhance the flame retardancy behavior of polylactide (PLA) composites with 20 wt% short glass fibers (GF). This was first implemented by exploring the effects of various phosphorus-based FRs up to 5 wt% in neat PLA samples. Among the used phosphorus-based FRs, the use of only 3 wt% of diphosphoric acid-based FR (P/N), melamine coated ammonium polyphosphate (APPcoated), and APP with melamine synergist (APP/Mel) resulted in achieving the V0 value in a vertical burning test in the neat PLA samples. In addition to their superior efficiency in improving the flame retardancy of neat PLA, P/N had the least negative effect on the final mechanical performance of PLA samples. When incorporated in PLA composites with 20 wt% GF, however, even with the use of 30 wt% P/N, the V0 value could not be obtained due to the candlewick effect. To resolve this issue, the synergistic effect of P/N and aromatic polycarbodiimide (PCDI) cross-linker or Joncryl epoxy-based chain-extender (CE) on the flame retardancy characteristics of composites was examined. Due to the further chain modification, which also enhances the melt strength of PLA, the dripping of composites in the vertical burning test terminated and the V0 value could be reached when using only 1 wt% PCDI or CE. According to the scanning electron microscopic analysis, the use of noted chain modifiers further homogenized the distribution and refined the particle size of P/N within the PLA matrix. Hence this could synergistically contribute to the enhancements of the fire resistance performance of the PLA composites. Such incorporation of P/N and chain modifiers further leads to the enhancement of the mechanical performance of PLA composites and hence the resultant product can be proposed as a promising durable bioplastic engineering product where fire risk exists

Isothermal and non-isothermal cold crystallization kinetics of polylactide/cellulose nanocrystal (PLA/CNC) nanocomposites

The effect of cellulose nanocrystal (CNC) content on the crystallization and melting behaviors of polylactide (PLA)/CNC nanocomposites prepared by solution mixing was investigated. Isothermal and non-isothermal cold crystallization kinetics of specimens were quantified using Ozawa, Avrami, and Liu-Mo methods. Overall and conversion dependent crystallization activation energy values were also determined through the Kissinger and Flynn-Wall-Ozawa equations. It was found that although CNC acted as a nucleating agent for cold crystallization of PLA under isothermal and non-isothermal conditions, it differently affected the crystal growth behavior. Kinetic calculations revealed that the increase in CNC amount decreased the non-isothermal cold crystallization rate of PLA possibly due to the reduced interaction among PLA molecules and formation of strong hydrogen bonding between the carboxyl groups of PLA and CNC surfaces. CNC addition also increased the overall cold crystallization activation energy whereas progress in crystallization yielded a significant reduce in the activation energy. This was because the relative crystallinity and temperature simultaneously increase during cold crystallization under non-isothermal conditions. Avrami analysis implied that CNC addition improved the crystallization rate of PLA possibly following athermal nucleation and two-dimensional discotic growth