Cork-polymer Composites Research Articles

Development of bio-based expanded cork polymer composites (eCPC) with poly(lactic acid) (PLA)

Expanded cork polymer composites (eCPC) using a PLA matrix and expandable microspheres (EMS) were developed and characterized. A blowing agent was introduced to evaluate its effect on cork structure in eCPC injected pieces. A chemical and thermal characterization was conducted using Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared-Attenuated Total Reflection (FTIR-ATR) to investigate the effect of cork and EMS in eCPC. The results have shown that the introduction of cork and EMS led to composites with up to 8% less density and with lower temperature of cold crystallization. The presence of cork in CPC and eCPC allowed to lower the cold crystallization temperature by almost 13 ºC. It was found an increase on the crystallinity degree in the presence of cork and EMS. Scanning Electron Microscopy (SEM) was used to perform morphological studies. A recovered alveolar structure was found for cork cells in eCPC injected pieces.

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.

Processing–Structure–Properties of Cork Polymer Composites

This work aims at presenting a new strategy for the use of cork by-products on the development of sustainable composites with potential applications in rotational molding. In this study different cork granulometries (0.15 to 3 mm) and matrix/cork ratios (90/10, 85/15, 80/20) are analyzed to access the processability of medium density polyethylene (PE) with cork by rotational molding. The influence of processing parameters, such as the mold peak internal air temperature (200-240oC) is analyzed and correlated to the aesthetics, morphological and mechanical properties of the parts. The aim is to obtain a complete understanding of the processing-structure-properties relationships. Defect free parts are obtained with thin granulometric cork powders at maximum of 10% by weight in PE/cork composites. The increase of cork content reduces the sintering capability, increases porosity, wall thickness and surface defects and, simultaneously, weakening the mechanical properties. The increase of peak internal air temperature (PIAT) inside the mold favors a more compact structure and reveals better impact properties. All the parts are soft on touch and transmit the comfort and sensation of warm feeling of the cork. Moreover, lightweight parts and hydrophobic surface is achieved from the cork intrinsic properties. As a natural material, cork darkens its color with temperature which is attributed to the reactions of the extractives within the components of cork, with no degradation associated. The work shows that polymer cork composites (CPC) are suitable for rotational molding within the processing window characteristic of PE, to achieve innovative and sustainable products with unique aesthetics and functionalities given by cork material.

Non-isothermal cold crystallization kinetics of cork–polymer biocomposites based on polylactic acid for fused filament fabrication

Cork–polymer composites (CPC) based on polylactic acid (PLA) matrix were prepared for the development of filaments for fused filament fabrication. The non-isothermal cold crystallization behaviours of PLA and CPC were investigated by differential scanning calorimetry. Cold crystallization kinetic behaviours of PLA and CPC with 15 mass/% of cork powder residues at different heating rates (1.25, 2.5, 5 and 7.5 K min−1) were studied. Results showed that cold crystallization temperature (Tcc) of PLA matrix decreased with the addition of cork. Crystallization kinetic behaviour was studied by Avrami and Tobin models. It was shown that cork powder surface acts as a nucleating agent during non-isothermal cold crystallization, by accelerating the crystallization rate and, therefore, by reducing the half-time crystallization (t1/2) values. Polarized optical microscopy and X-ray diffraction were used to evaluate the crystalline structure of PLA and CPC. Kissinger and Friedman methods were employed to determine the crystallization activation energy (Ec).

Cork-like filaments for Additive Manufacturing

A cork-like filament fully biodegradable and filled with low granulometry cork powder residues was developed. Cork-polymer composites (CPC) were prepared using a Brabender type mixer incorporating 15% (w/w) of cork powder (corresponding to 55% (v/v)) and having polylactic acid (PLA) as matrix. In order to promote a chemical adhesion between cork particles and PLA, the effect of maleic anhydride grafted PLA (MAgPLA) was studied. Fourier Transform Infrared – Attenuated Total Reflection (FTIR-ATR) analysis was used to evaluate the functionalization of MAgPLA onto the polymeric chain. The addition of MAgPLA enhanced the mechanical behavior by increasing tensile properties while improving the dispersion of cork particles within PLA matrix. In addition, cork particles and MAgPLA acted as nucleating agents during PLA melting process. To evaluate the printability of the developed CPC filament, specimens were printed by Fused Filament Fabrication (FFF) and compared to those obtained by injection molding (IM). FFF allowed to preserve the cork alveolar structure in the specimens, benefiting CPC mechanical behavior. 3D parts could be printed with the CPC filament thereby demonstrating the usefulness of the fully biodegradable cork-based filament here developed. 3D printed parts exhibit unique characteristics, such as a nonplastic and warm touch, a natural colour and the release of a pleasant odour during the printing process.

Non‐isothermal crystallization kinetics of cork‐polymer composites for injection molding

ABSTRACTThe non‐isothermal crystallization behavior of cork–polymer composites (CPC) based on polypropylene (PP) matrix was studied. Using differential scanning calorimetry (DSC), the crystallization behavior of CPC with 15 wt % cork powder at different cooling rates (5, 10, 15, and 20 °C/min) was studied. The effect of a coupling agent based on maleic anhydride was also analyzed. A composite (PPg) containing polypropylene grafted maleic anhydride (PPgMA) and PP was prepared for comparison purposes. Crystallization kinetic behavior was studied by Avrami, Ozawa, Liu, and Kissinger methods. The Ozawa method fails to describe the behavior of these composites. Results show that cork powder surface acts as a nucleating agent during non‐isothermal crystallization, while the addition of PPgMA decreases the crystallization rate. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44124.

Rheological behaviour of cork-polymer composites for injection moulding

The incorporation of cork in synthetic polymers has become an effective approach to develop new sustainable materials. Cork-polypropylene composites (CPC) filled with three different cork granulometries were studied. Rheological analyses were performed to assess the processability of these CPC and a set of experiments was conducted keeping the same matrix/cork weight ratio, varying the cork granulometric distribution. The effect of three different cork granulometries, temperature and the effect of a coupling agent, polypropylene graft maleic anhydride (PPgMA), were analysed. All composites exhibited non-Newtonian, pseudoplastic behaviour. Related to neat PP, cork incorporation led to a viscosity increase. This increase is more significant in the CPC with the lowest powder cork granulometry used. On the other side, the addition of PPgMA resulted on a decrease of CPC viscosity. The experimental results were fitted to Cross-WLF Model through a viscosity master curve obtained by the time–temperature superposition principle (TTSP). This study showed that cork can be considered on the development of sustainable materials for injection moulding technology.

Functionalized cork-polymer composites (CPC) by reactive extrusion using suberin and lignin from cork as coupling agents

High density polyethylene (HDPE) and cork powder were compounded in a co-rotating twin-screw extruder to obtain cork-polymer composites (CPC) with improved properties. Benzoyl peroxide (BPO) was used as initiator agent, and suberin or lignin isolated from cork enhanced filler-matrix bonding and promoted mechanical reinforcement with environmental benefits. The novel composites were characterised in terms of dimensional stability, evolution of morphology, thermal and mechanical properties and their performance was compared with that of composites containing polyethylene-grafted maleic anhydride (PE-g-MA) as coupling agent. As expected, composites with coupling agent present higher mechanical properties, lower water uptake and thickness swelling variation. Suberin acts as plasticizer with antioxidant benefits, while lignin works as a coupling agent, improving tensile modulus and maximum strength. Increasing lignin content does not improve the mechanical properties but improves thermal stability.

Polypropylene-based cork–polymer composites: Processing parameters and properties

Cork–polymer composites are one of the most promising fields of cork technology to create materials for a sustainable development. In this work, cork particles and powder were compounded with polypropylene (PP) by twin-screw extrusion and further processed by injection or compression moulding. The obtained PP/cork injection moulded composites were used to investigate the tensile properties before and after water absorption (WA) tests. The addition of polypropylene–graft–maleic anhydride (PP–g–MA) and stearic acid was evaluated. The composites show good distribution and dispersion of cork with interesting aesthetic characteristics. Low cork content (5wt.%) reinforces the stiffness of the PP. The addition of PP–g–MA up to 8wt.% promotes an increase on the tensile strength while the stearic acid contributes only with a slight increase on the tensile strain. The thermo-physical properties were investigated and the composites present low WA. In general the tensile properties of the composites were not significantly affected after the WA tests. Moreover, the WA was found to increase with an increase in cork content and was reduced with the PP–g–MA and stearic acid. It was also found, that cork act as nucleating agent and promotes antioxidant protection to PP. Both properties ameliorate with the increase of the cork content. Low cork content improves the thermal resistance while increasing the bio-based component increases the composite density and the thermal conductivity. The maximum incorporation of cork on the PP matrix through extrusion process was 93% in volume.

Hybrid cork–polymer composites containing sisal fibre: Morphology, effect of the fibre treatment on the mechanical properties and tensile failure prediction

In this study, we investigated the use of short sisal fibre with and without polyethylene-graft-maleic anhydride (PE-g-MA) as a strategy to reinforce cork–polymer composite (CPC) materials. The use of alkali treatment of sisal to improve fibre–matrix adhesion was evaluated. High density polyethylene (HDPE) was used as matrix and the composites were produced in a two-step process using twin-screw extruder followed by compression moulding. FTIR, TGA and XRD were used to confirm the sisal fibre modification. Additionally, morphology, density, diameter and tensile properties of the fibres were evaluated before processing. The hybrid composites containing cork powder (40wt.%) and randomly distributed sisal fibres were evaluated in terms of morphology and mechanical properties. The use of a 10wt.% sisal fibre in the presence of a 2wt.% coupling agent based on maleic anhydride, has shown to improve the tensile and flexural properties of the composites. The higher mechanical properties were achieved by using alkali treated sisal fibres and PE-g-MA. In the presence of the coupling agent the composite morphology revealed good interfacial adhesion between the natural components and the polypropylene matrix, being in accordance with the mechanical results. Weibull cumulative distribution was successfully used to accurately predict the tensile strength failure of the hybrid CPC materials.

Novel cork–polymer composites reinforced with short natural coconut fibres: Effect of fibre loading and coupling agent addition

Composites from high density polyethylene filled with cork powder and coconut short fibres, in two different ratios, were prepared in a twin-screw extruder followed by compression moulding process. The main motivation of this work was to improve the mechanical performance without compromising the use of high weight percentage of natural component used in the preparation of cork-based composites. The morphology of the hybrid composites were more homogeneous in the presence of the coupling agent (CA) displaying enhanced fibre–matrix adhesion. Moreover, the use of CA based on maleic anhydride promotes a mechanical reinforcement effect on the tensile properties , including the elongation at break . The addition of coconut fibre resulted on an increase of 27% in elastic modulus and 47% in the tensile strength when compared with the unreinforced cork-based (50–50) wt.% composite. This work clearly shows that the addition of 10 wt.% of short-coconut fibres, randomly distributed, can be effectively used as reinforcing strategy of cork-based composite materials, preferably in the presence of 2 wt.% of CA.

Natural Fibres as Reinforcement Strategy on Cork-Polymer Composites

Cork powder, the most important sub-product of cork processing, combined with thermoplastic matrixes like, high density polyethylene (HDPE), offer a new class of cork-polymer composite (CPC) materials with high added-value. Therefore, reinforcing strategies must be considered to increase the mechanical performance, especially when high content of cork powder is added to the formulation. Coconut fibres have several advantages, such as, low density, renewable source, low cost and biodegradability. The use of these fibres on the reinforcement of CPC materials will not only contribute to improve the mechanical performance but also for increasing the amount of natural component present on the final composition. The main goal of this work was to prepare HDPE/cork (50-50 wt.%) composites reinforced with discontinuous coconut fibres (5 and 10 wt.%) with and without the addition of coupling agent (2 wt.%) by extrusion. The developed reinforced cork based composites were characterized regarding its morphology and mechanical performance. Optical micrographs have shown a homogeneous distribution of the fibres. The coupling agent effect on CPC performance was also investigated. The tensile strength and tensile modulus of the reinforced composites were significantly improved with the addition of coupling agent. The use of 10 wt.% of coconut fibres in the presence of coupling agent promote an increase on maximum tensile strength of around 41 % comparing with the HDPE/cork (50-50 wt.%) composites. Scanning electron microscopy (SEM) micrographs of the tensile fractured specimens confirmed that the use of coupling agent promoted the interfacial adhesion between the fibres and the thermoplastic matrix. Since, like cork powder, coconut fibres have good thermal and acoustic properties, we consider that the novel reinforced CPC herein described have high potential to be used in building and construction systems and other structural 3D applications.

Properties of new cork–polymer composites: Advantages and drawbacks as compared with commercially available fibreboard materials

Cork powder (50 wt.%) was mixed with polypropylene (PP) or polyethylene (PE) by pultrusion aiming to prepare cork-based composites. In a further step, samples were produced by compression moulding using the compounded composites. Bending strength, impact resistance, hardness, dimensional stability, thermal and acoustic properties of the developed cork–polymer composites (CPC) were determined and compared with commercially available products namely medium density fibreboard (MDF) and high density fibreboard (HDF). It was found that the CPC have good dimensional stability, lower water uptake, a better acoustic insulation performance and similar behaviour in terms of hardness and fire resistance when compared with both MDF and HDF. However, the mechanical strength is inferior comparing with both commercial materials based on fibres. It was also observed that addition of cork improved the flexural modulus, impact resistance and hardness on the developed CPC. Thus, the herein described CPC materials showed important characteristics to be considered as good candidates to be applied in the design of flooring and construction systems.