The production and properties of polylactide composites filled with expanded graphite
Composites have been produced by melt-blending biodegradable polylactide (PLA) with commercially available expanded graphite (EG). Using different techniques of addition, the manifold effects of EG on PLA molecular, thermo-mechanical and fire-retardant properties were evaluated. The EG nanofiller provides PLA composites with competitive functional properties. They have a high rigidity, with Young's modulus and storage modulus increasing with EG content. They also have excellent thermal stability while preserving the glass transition and melting temperature of the original PLA matrix. Purification and pre-dispersion of EG nanofiller proved beneficial for preserving PLA molecular weights and led to improved mechanical performance. The presence of dispersed graphene nanolayers in PLA significantly accelerated the polyester crystallization process. The flame retardant properties also displayed improvements with a large decrease in the maximum rate of heat release as recorded by cone calorimetry, whereas the horizontal burning test (UL94 HB) was successfully passed revealing non-dripping and char formation.
New trends in polylactide (PLA)-based materials : "Green" PLA-Calcium sulfonate (nano) composites tailored with flame retarant properties
Starting from gypsum, a by-product of lactic acid fabrication process, novel “green” composites have been produced by melt-blending polylactide (PLA) and this filler after a previous specific dehydration to obtain anhydrite II (AII). Such a material is potentially interesting in biodegradable/rigid packaging and in technical applications requiring rigidity, heat resistance and dimensional stability. In order to obtain PLA–AII composites characterized by specific end-use flame retardant properties, the addition of selected organo-modified layered silicates (OMLS) was considered. Co-addition of AII and OMLS leads to PLA (nano)composites characterized by good (nano)filler dispersion, thermal stability and adequate mechanical resistance. The flame retardant properties as shown by cone calorimetry showed significant increase in the ignition time compared to neat PLA and a substantial decrease, i.e., ca. 40%, of the maximum rate of heat release, whereas the UL94 HB test was successfully passed revealing non-dripping effect and extensive char formation. The study represents a new approach in formulating novel PLA grades with improved characteristic features.
Effect of magnesiumdihydroxide nanoparticles on thermal degradation and flame resistance of PMMA nanocomposites
The influence of magnesium dihydroxide (MDH) nanoparticles on thermal degradation and fire behavior of poly(methyl methacrylate) (PMMA) has been investigated by thermogravimetric analysis (TGA), cone calorimeter, and pyrolysis-combustion flow calorimeter (PCFC) tests, respectively. MDH nanoparticles with either lamellar or fibrous shape have been synthesized via a sol-gel technique and characterized by transmission electron microscopy (TEM) and X-ray diffraction analysis. PMMA-MDH nanocomposites have been prepared by melt blending the recovered MDH nanoparticles within PMMA at different loadings (5, 10, and 20 wt% MDH). According to TGA results, the incorporation of lamellar or fibrous MDH nanoparticles into PMMA leads to a significant improvement in PMMA thermal stability, both under air and inert atmosphere. The results obtained by PCFC and cone calorimeter tests show an important decrease in the peak of heat release rate (pHRR) concomitant with charring during the combustion. Lamellar MDH nanoparticles were found to be more efficient than fibrous MDH nanoparticles
Calcium Sulfate as High-Performance Filler for Polylactide (PLA) or How to Recycle Gypsum as By-product of Lactic acid Fermentation Process
Reinforcing of polylactide (PLA) with fillers can be an interesting solution to reduce its global price and to improve specific properties. Starting from calcium sulfate (gypsum) as by-product of the lactic acid fermentation process, novel high performance composites have been produced by melt-blending PLA and this filler after a previous specific dehydration performed at 500°C for min. 1 h. Due to PLA sensitivity towards hydrolysis, it has first been demonstrated that formation of β-anhydrite II (AII) by adequate thermal treatment of calcium sulfate hemihydrate is a prerequisite. Then, the modification of filler interfacial properties with different coating agents such as stearic acid (SA) and stearate salts has been considered. The effect of surface treatment on molecular, thermal and mechanical properties has been examined together with the morphology of the resulting composites. To take advantage of the improved lubricity and better wetting characteristics, the filler was coated by up to 2% (by weight) SA. The coating of the filler leads to PLA-AII composites that surprisingly exhibit thermal stability, cold crystallization and enhanced impact properties. Such remarkable performances can be accounted for by the good filler dispersion as evidenced by SEM-BSE imaging of fractured surfaces. As far as tensile proprieties are concerned, notable utilization of uncoated filler or filler coated by stearate salts leads to PLA-AII composites characterized by higher tensile strength and Young's modulus values. The study represents a new approach in formulating new melt-processable grades with improved characteristic features by using PLA as polymer matrix.
Influence of the stress generated during an ageing cycle on the barrier properties of cataphoretic coatings
The ageing of organic coatings is generally associated with an increase of stresses leading to a coating degradation: loss of adherence, increase of the porosity and irreversible changes during exposure to climatic parameters such as humidity and temperature. In this work, electrochemical impedance spectroscopy (EIS) and the cantilever method (stressmeter) were used to establish a correlation between the loss of barrier properties and the increase of the stress of electrocoated paints. The stress was generated during hygrothermal ageing cycles in a climatic chamber (20 h at 55 C-84% RH and 4h at 23 C-40% RH). Phosphatised steel plates were used for the electrochemical measurements and calibrated steel strips for the stress measurements. Two electrocoated paints differing by their composition were tested. For both electrocoatings, the plates and the calibrated steel strips were submitted to 0, 1, 2 and 4 hygrothermal cycles in the climatic chamber. The generated stress was determined after each cycle. The strong influence of humidity on the stress was underlined during humidity cycles at ambient temperature. The sensitivity of the organic coatings to humidity increases with the number of humidity cycles and finally induces an irreversibility of the system. The barrier properties were evaluated after the different hygrothermal cycles and after 0, 2, 7, 14 and 21 days of immersion in 0.5 M NaCI electrolyte solution by EIS on the electrocoated steel samples. The stress generated during ageing increases with the number of cycles and at the same time a decrease of the barrier properties is observed. For the electrocoating whose stress evolution is more sensitive to humidity, a good correlation is observed between the increase of the internal stress and the increase of the coating capacitance due to water uptake by the coating.
Nickel–boron electrochemical properties investigations
Electroless nickel–boron (Ni–B) was synthesized on mild steel. Coating thickness was approximately 30 µm. Some of the coatings were submitted to a hardening heat treatment at 400 °C for 1 h in an atmosphere containing 95% Ar and 5% H2 to improve their mechanical performance.
Heat treated and untreated samples were submitted to the Taber abrasion test to assess their wear resistance. The wear track was then examined by SEM and roughness measurement. The Taber Wear Index of untreated samples was slightly better than that of steel but heat treated samples attained TWI as small as 13.
The corrosion resistance of the Ni–B coatings was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy. The EIS results showed diffusion phenomena in 0.1M NaCl solution. Electroless Ni–B coating increases the corrosion resistance of steel and heat treatments allow a further enhancement. Wear decreases that resistance but the worn product keeps a better behaviour than uncoated parts.