Effect of surface treatment in cork reinforced composites

The Components' Roles in Thermal Stability and Flammability of Cork Powder

In this study, an analysis of the influence of extractives, suberin and lignocellulosic components on the pyrolysis decomposition and fire reaction mechanisms of a cork oak powder from Quercus suber L. is presented. The summative chemical composition of cork powder was determined. Suberin was the main component at 40% of the total weight, followed by 24% of lignin, 19% of polysaccharides and 14% of extractives. The absorbance peaks of cork and its individual components were further analyzed by means of ATR-FTIR spectrometry. Thermogravimetric analysis (TGA) showed that the removal of extractives from cork slightly increased the thermal stability between 200 °C and 300 °C and led to the formation of a more thermally stable residue at the end of the cork decomposition. Moreover, by removing suberin, a shift of the onset decomposition temperature to a lower temperature was noticed, indicating that suberin plays a major role in enhancing the thermal stability of cork. Furthermore, non-polar extractives showed the highest flammability with a peak of heat release rate (pHRR) of 365 W/g analyzed by means of micro-scale combustion calorimetry (MCC). Above 300 °C, the heat release rate (HRR) of suberin was lower than that of polysaccharides or lignin. However, below that temperature it released more flammable gases with a pHRR of 180 W/g, without significant charring ability, contrary to the mentioned components that showed lower HRR due to their prominent condensed mode of action that slowed down the mass and heat transfer processes during the combustion process.

Development of Cork Biocomposites Enriched with Chitosan Targeting Antibacterial and Antifouling Properties

The demand for bio-based and safer composite materials is increasing due to the growth of the industry, human population, and environmental concerns. In this framework, sustainable and safer cork-polymer composites (CPC), based on green low-density polyethylene (LDPE) were developed using melt-based technologies. Chitosan and polyethylene-graft-maleic anhydride (PE-g-MA) were employed to enhance the CPC's properties. The morphology, wettability, mechanical, thermal, and antibacterial properties of the CPC against Pseudomonas putida (P. putida) and Staphylococcus aureus (S. aureus) were examined. The CPC showed improved stiffness when compared with that of the LDPE matrix, preferably when combined with chitosan and PE-g-MA (5 wt. %), reinforcing the stiffness (58.8%) and the strength (66.7%). Chitosan also increased the composite stiffness and strength, as well as reduced the surface hydrophilicity. The CPCs' antibacterial activity revealed that cork significantly reduces the biofilm on the polymer matrix. The highest biofilm reduction was found with CPC containing cork and 5 wt. % chitosan for both P. putida (54% reduction) and S. aureus (36% reduction), confirming their potential to extend the lifespan of products for packaging and healthcare, among other applications. This work leads to the understanding of the factors that influence biofilm formation in cork composites and provides a strategy to reinforce their behavior using chitosan.

Unraveling the Complex Interfacial Properties of Cork-Based Materials in Their Use as Wine Stoppers

This study investigates the surface and interfacial properties of the different components of a system composed of an agglomerated cork stopper in a glass bottleneck. Each constituting element has carefully been examined to unveil its underlying complexity. First, there was no effect of supercritical CO₂ pretreatment or particle size on the surface properties of cork particles. The wettability of the binder was also evaluated, showing that the binder can spread relatively well on the surface of cork particles. Second, capillary rise measurements carried out on three different agglomerated corks indicate that the formulation of the agglomerates has no effect on its surface properties. The binder represents only a small fraction of the total stopper volume and is therefore not the major contributor to the surface tension. Third, the two coating agents studied display different behaviors. The first one, composed of a paraffin emulsion, exhibits poorer wettability than the second one, composed of a paraffin and silicone emulsion. However, once the coating agent has solidified on the surface of the stopper, both coatings display similar adhesion with the glass of the bottleneck. Starting with fundamental considerations, and then progressing to a more applicative aspect, has led to a better understanding of the properties of cork-based materials in their use as wine stoppers.

Formulation and Characterization of a Composite Coating Formulation Based on Acrylic Foam and Cork Granules

Cork, the bark of Quercus suber L., in addition to presenting several notable physical-mechanical properties, possesses a distinctive look and feel that make it attractive for interior surfaces, such as in furniture, wall paneling, or flooring. This work envisaged the development of a coating based on cork granules, a subproduct from the wine stopper industry, capable of creating a smooth surface similar to natural cork. In order to avoid the high rugosity that characterizes surfaces coated with paints that incorporate cork granules, a new solution was developed, based on a foamed acrylic binder, applied by knife coating. The foam formulation was successfully optimized, using appropriate additives and resorting to mechanical agitation to promote the generation of air bubbles. The addition of cork granules did not hinder foam stability, and the final coating displayed the intended visual and sensory characteristics. Dynamic Mechanical Analysis was performed on the pristine acrylic foam and on the composite foam showed a stiffening effect associated with the presence of cork granules, and a thermal transition centered at around −10 °C, associated with the acrylic binder’s glass transition. The surface has hardness slightly lower than cork, depending on the amount of particles incorporated. Pull-off testing consistently resulted in substrate failure, indicating that the coating’s cohesion and adhesion are excellent. The developed coating showed to have the intended functionality while being easily applicable on flat panel surfaces. The fact that a foam is used as a binder system allows for a smooth and soft surface, having excellent opacity with minimal usage of cork.

Surface properties of cork: Is cork a hydrophobic material?

Knowledge of the surface tension of cork and its hydrophobicity is of critical importance in many applications of this material at the interface with solid or liquid phases. The conventional technique based on contact angle measurement by sessile drop is not adapted to this naturally textured material and does not allow to accurately determine its hydrophobic character. A study based on capillary rise measurement is reported. A statistical distribution of the surface tension of cork is obtained, based on experiments performed on cork powder with various liquids and using a specific data processing to take into account the intrinsic heterogeneity of cork. This gives a surface tension of 22.6 (±1.2) mN·m^-1, with a polar component at 5.2 (±0.5) mN·m^-1 and a dispersive component at 17.4 (±1.6) mN·m^-1. With a water contact angle of around 90°, cork shows an intermediate hydrophobic/hydrophilic behaviour. Locally, the specific surface texture and chemical composition can reinforce either the hydrophobic or the hydrophilic character. This critical analysis invites reflection on the notion of surface hydrophobicity as it can be determined macroscopically by a contact angle measurement and as defined at the molecular level by the free enthalpy of sorption of water.

Effect of Silver Nanopowder on Mechanical, Thermal and Antimicrobial Properties of Kenaf/HDPE Composites

This study aims to investigate the effect of AgNPs on the mechanical, thermal and antimicrobial activity of kenaf/HDPE composites. AgNP material was prepared at different contents, from 0, 2, 4, 6, 8 to 10 wt%, by an internal mixer and hot compression at a temperature of 150 °C. Mechanical (tensile, modulus and elongation at break), thermal (TGA and DSC) and antimicrobial tests were performed to analyze behavior and inhibitory effects. The obtained results indicate that the effect of AgNP content displays improved tensile and modulus properties, as well as thermal and antimicrobial properties. The highest tensile stress is 5.07 MPa and was obtained at 10wt, TGA showed 10 wt% and had improved thermal stability and DSC showed improved stability with increased AgNP content. The findings of this study show the potential of incorporating AgNP concentrations as a secondary substitute to improve the performance in terms of mechanical, thermal and antimicrobial properties without treatment. The addition of AgNP content in polymer composite can be used as a secondary filler to improve the properties.

Grafting Poly(Methyl Methacrylate) (PMMA) from Cork via Atom Transfer Radical Polymerization (ATRP) towards Higher Quality of Three-Dimensional (3D) Printed PMMA/Cork-g-PMMA Materials

Cork is a unique material and its by-products are attracting an ever-growing interest for preparing new materials in an attempt to extend the outstanding properties of cork toward innovative and high value applications. Yet, the miscibility of cork particles with thermoplastic matrices is not easy due to its low density and surface properties. Here, cork is functionalized with poly(methyl methacrylate) (PMMA) via atom transfer radical polymerization (ATRP) to yield cork grafted with PMMA chains particles (cork-g-PMMA). Both the ATRP macroinitiator and the cork-g-PMMA obtained are fully characterized by Fourier-transform infrared spectroscopy (FT-IR), ¹³C cross-polarized magic-angle spinning solid-state nuclear magnetic resonance (¹³C CP/MAS solid state NMR), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD) and thermogravimetric analyses (TGA). The functionalized cork particles are then blended with commercial PMMA to afford cork-g-PMMA/PMMA. To compare, cork also is mixed with PMMA and the ensuing cork/PMMA sample and its morphology, thermal, and mechanical properties are compared with those of cork-g-PMMA/PMMA and commercial PMMA. The cork surface modification via ATRP of the methyl methacrylate (MMA) yields better dispersion in the matrix. Consequently, a blend with enhanced mechanical performance, higher thermal stability, and a higher melt flow index (MFI) is obtained when compared to the blend prepared using unmodified particles. The similarity of the MFI of cork-g-PMMA/PMMA to that of PMMA suggests good printability. Indeed, a three-dimensional (3D) printed specimen is obtained confirming that grafting using ATRP is a promising route for the preparation of high quality 3D printed products.

Preparation and Characterization of a Biocomposite Based on Cork Microparticles in Poly(β-hydroxybutyrate)-co-Poly(β-hydroxyvalerate) Matrix

Poly(β-hydroxybutyrate)-co-poly(β-hydroxyvalerate) (PHBV) loaded with 1 to 10 wt.% of cork microparticles (CM) were manufactured via compression molding technique. CM were isolated by alkali, bleaching and acid hydrolysis treatments. They were then added to the PHBV matrices via solution casting technique. The filler microparticles showed typical diameters of 10 to 55 μm. Mechanical tests in tension showed that in PHBV containing 5 wt.% CM the Young's modulus and the tensile strength increased by 50% and 10% respectively. Meanwhile, the elongation at break decreased by 70% for the same CM content. The mechanical behavior of the reinforced composites was discussed in conjunction with the morphology of the samples studied by optical and electron microscopy. The temperature of initial degradation Tid, the temperature at 5% weight loss Td5% and the temperature of maximum degradation Tdmax increase with increasing CM content by up to 13.6°C, 10.1°C and 12.3°C respectively for the composite of 5 wt.%. The composite with 10 wt.% CM content showed the best thermal stability with temperature increments of 20°C, 14.1°C and 21.5°C respectively. DSC results showed that the presence of CM has a very weak influence on the melting/crystallization behavior of the composites.

Recycling the Cork Powder in a PVC-Based Composite Material: Combined Effect on Physico-Mechanical and Thermal Properties

A large amount of cork powder (CP) produced by cork industry ended up in landfills, they are the result of the crushing of natural floor (bark) cork oak, hence the need to recycle these wastes to prevent environmental problems. To this end, the work is to combine the powdered cork recovered with a thermoplastic material to provide a cork based composite (CPC) having physico-mechanical characteristics and thermal very interesting. Several formulations of composite materials based on PVC (as matrix), cork powder (as a charge) and a PVC-g-MA coupling agent, have been the subject of experimental study to evaluate the physical-mechanical and thermal properties. To investigate the combined effect of cork powder and PVC-g-MA on composite properties, the samples were manufactured by varying the content of PC (30 %, 40 % and 50 %) with two PVC-g-MA concentrations. Improving the stability of PVC which is accentuated in the presence of the coupling agent. In addition, it was found that the incorporation of the coupling agent has enabled the elimination of the adverse effect of cork powder, improving the tensile strength. The composite obtained in this study yielded some promising properties, even if building strategies to achieve greater strength and rigidity could beings needed for specific applications.