A. Ameli

Lightweight Polypropylene/Stainless-Steel Fiber Composite Foams with Low Percolation for Efficient Electromagnetic Interference Shielding

Lightweight polypropylene/stainless-steel fiber (PP-SSF) composites with 15-35% density reduction were fabricated using foam injection molding. The electrical percolation threshold, through-plane electrical conductivity, and electromagnetic interference (EMI) shielding effectiveness (SE) of the PP-SSF composite foams were characterized and compared against the solid counterparts. With 3 wt % CO2 dissolved in PP as a temporary plasticizer and lubricant, the fiber breakage was significantly decreased during injection molding, and well-dispersed fibers with unprecedentedly large aspect ratios of over 100 were achieved. The percolation threshold was dramatically decreased from 0.85 to 0.21 vol %, accounting for 75% reduction, which is highly superior, compared to 28% reduction of the previous PP-carbon fiber composite foam.1 Unlike the case of carbon fiber,1 SSFs were much longer than the cell size, and the percolation threshold reduction of PP-SSF composite foams was thus primarily governed by the decreased fiber breakage instead of fiber orientation. The specific EMI SE was also significantly enhanced. A maximum specific EMI SE of 75 dB·g(-1)·cm(3) was achieved in PP-1.1 vol % SSF composite foams, which was much higher than that of the solid counterpart. Also, the relationships between the microstructure and properties were discussed. The mechanism of EMI shielding enhancement was also studied.

Processing and characterization of solid and foamed injection-molded polylactide with talc:

Polylactide (PLA) suffers from poor processability due to its low melt strength and slow crystallization kinetics and it is thus very challenging to achieve uniformly distributed fine-celled PLA foams with high void fractions in injection molding process. In this work, the low-pressure structural foam molding of linear PLA with a relatively high void fraction of ∼30% was conducted and by fine tuning the talc content and the foaming and processing parameters, a relatively uniform fine-celled structure with improved cell size and cell density was successfully produced. The effects of twin-screw compounding and the addition of talc on the foaming behavior, structural uniformity, crystallinity, and mechanical properties of the solid and foamed PLA samples were investigated. The results showed that the addition of 5 wt.% talc significantly improved the foaming properties such as cell density, cell size, structural uniformity, and consequently improved the mechanical properties of foams. The twin-screw compounding before injection molding did not significantly change the foaming behavior, but adversely affected the mechanical properties of the solid and foamed PLA samples due to mechanical and thermal degradation. The changes in the mechanical properties were discussed in terms of the crystallinity, talc toughening effect, and foam quality.

Poly(lactic acid)-Based in Situ Microfibrillar Composites with Enhanced Crystallization Kinetics, Mechanical Properties, Rheological Behavior, and Foaming Ability.

Melt blending is one of the most promising techniques for eliminating poly(lactic acid)s (PLA) numerous drawbacks. However, success in a typical melt blending process is usually achieved through the inclusion of high concentrations of a second polymeric phase which can compromise PLAs green nature. In a pioneering study, we introduce the production of in situ microfibrillar PLA/polyamide-6 (PA6) blends as a cost-effective and efficient technique for improving PLAs properties while minimizing the required PA6 content. Predominantly biobased products, with only 3 wt % of in situ generated PA6 microfibrils (diameter ≈200 nm), were shown to have dramatically improved crystallization kinetics, mechanical properties, melt elasticity and strength, and foaming-ability compared with PLA. Crucially, the microfibrillar blends were produced using an environmentally friendly and cost-effective process. Both of these qualities are essential in guarantying the viability of the proposed technique for overcoming the obstacles associated with the vast commercialization of PLA.

Hygrothermal Properties of Highly Toughened Epoxy Adhesives

The absorption and desorption of water in two different rubber-toughened epoxy adhesives was measured gravimetrically over a relatively wide range of temperature and relative humidity (RH). The data were fitted to a new diffusion model in which Ficks law was assumed to act in two sequential stages, each with its own diffusion coefficient and saturated water concentration. This “sequential dual Fickian” (SDF) model and a Langmuir-type diffusion model were both able to model the absorption behaviour. The dependence of the five SDF model parameters on temperature and RH was investigated in detail. The two diffusion coefficients were found to be largely independent of RH, while the fractional mass uptake values for each stage increased with RH. The absorption temperature only had a significant effect on the diffusion coefficient of the first stage and the fractional mass uptake of the second stage. Water desorption from the two epoxies was modeled accurately using Ficks law. A significant difference was observed between the amounts of retained water in the two adhesives after drying. The results can be used to predict the water concentration distribution in adhesive joints exposed to environments of changing temperature and RH.

Characterization and prediction of fracture properties in hygrothermally degraded adhesive joints: an open-faced approach

One of the challenges in the application of structural adhesive joints is the prediction of their long-term durability. During the service life, moisture diffuses into the adhesive layer and eventually degrades its fracture properties. Environmental degradation should thus be taken into consideration in the design and analysis of adhesive joints. This work first provides an overview, summarizing the recent efforts regarding the hygrothermal exposure of adhesive joints, accelerated aging methods, water diffusion modeling, and characterization of fracture properties in adhesively bonded joints. The second part presents a recent degradation methodology by which the fracture toughness evolution of adhesive joints can be predicted using fracture test data obtained using the accelerated open-faced degradation method.

Analysis and design of adhesively bonded joints for fatigue and fracture loading: a fracture-mechanics approach

An experimental–computational fracture-mechanics approach for the analysis and design of structural adhesive joints under static loading is demonstrated by predicting the ultimate fracture load of cracked lap shear and single lap shear aluminum and steel joints bonded using a highly toughened epoxy adhesive. The predictions are then compared with measured values. The effects of spew fillet, adhesive thickness, and surface roughness on the quasi-static strength of the joints are also discussed. This fracture-mechanics approach is extended to characterize the fatigue threshold and crack growth behavior of a toughened epoxy adhesive system for design purposes. The effects of the mode ratio of loading, adhesive thickness, substrate modulus, spew fillet, and surface roughness on the fatigue threshold and crack growth rates are considered. A finite element model is developed to both explain the experimental results and to predict how a change in an adhesive system affects the fatigue performance of the bonded joint.