Degradation of flax PLA biocomposites during and after manufacture

Abstract

With rapidly increasing interest in green composites, natural fibre reinforced plastic (NFRP) biocomposites are becoming a very real alternative composite to the synthetic/man-made composites in indoor applications. Sustainability concerns of the public and recently approved regulations made by governments, in order to reduce the landfill materials, are among the factors that have intensified interest in natural fibre composites. Natural fibres can be used as renewable, sustainable reinforcement due to their small (perceived) environmental footprint. Apart from the environmental concerns, natural fibres possess some desirable engineering related characteristics such as low cost, low density, high stiffness- and strength-to-weight ratio. Among various natural fibres, flax is one of the oldest cultivated and used fibres. It has been utilized with success in a variety of applications, and can be used as a replacement for synthetic fibres in some applications. Thermal stability and degradation, which are common drawbacks of natural fibres, are constraints for further extending the range of applications for flax fibre reinforced biocomposites. This problem can be significantly more critical for manufacturers and users when the matrix is also intended to be a sustainable material such as poly lactic acid (PLA). Since exposing flax/PLA biocomposites to high temperatures or outdoor environments leads to degradation, more research studies are necessary in order to increase our understanding about the mechanisms for degradation. In particular, a key point of interest is thermal stability when flax/PLA biocomposites are subjected to high temperatures during the manufacturing process. Degradation is a key issue and occurs in both flax fibre and PLA matrix of a biocomposite. At high temperatures, flax components such as cellulose, hemicellulose and lignin degrade. The thermo-chemical degradation of these components subsequently limits the mechanical properties of flax fibre as reinforcement. In the case of PLA, in addition to low stability at manufacture temperatures, its biodeterioration can become a significant drawback in terms of durability when exposed to moisture related environments. The degradation process during and after manufacturing of flax/PLA biocomposites can also cause separation in the bonding of flax and PLA, and further reduction in mechanical properties. Important gaps in the existing studies in this field are a lack of: (i) a methodology to construct a map or window of degradation during compression moulding for setting practical upper and lower limits to the consolidation temperature and process time, (ii) an analytical model to calculate the deterioration of PLA due to the chain-scission degradation process, (iii) a demonstrated procedure to calculate the effect of transient temperature on the degradation of flax/PLA biocomposites, (iv) study on the degradation rate of flax/PLA biocomposites at high temperatures in association with molar mass changes or degree of polymerisation, (v) a complete study on the performance of flax/PLA biocomposites in moisture related environments such as wet, freezing and humid conditions. Thus, the key objectives of this study were designed to fulfil these gaps/requirements in the existing body of knowledge. The research was divided into four stages which were carried out and the findings were published in four journal papers and one book chapter. In order to set practical upper and lower limits to the consolidation temperature and processing time (in the form of a processing window), an in-depth investigation of the degradation processes of natural fibre biocomposites during thermal processing was done. The properties and processes considered in defining the processing window were the melting temperature, thermal penetration, impregnation of matrix into fibres, pyrolysis of matrix and fibre, and thermochemical degradation of matrix and fibre. To set lower limits to the processing time, critical studies and calculations were performed on the thermal penetration as a result of heating the platens of the compression moulding machine to higher than the melting point of the PLA, and on the impregnation of matrix into flax fibres. Upper limits to the processing temperature were achieved by comparing reaction process in terms of pyrolysis and thermochemical degradation. Evaluations on the pyrolysis degradation of PLA, flax fibres and their main compositions (including cellulose, hemicellulose and lignin) were conducted based on TGA data from the literature. Moreover, assessments on the thermo-chemical (chain-scission) degradation of PLA, flax fibres and natural fibres’ main compositions were conducted; and by bringing them all together, an optimum processing window for a biocomposite was constructed. The proposed processing window was tested experimentally. Several tests measuring changes in the tensile properties of a flax/PLA biocomposite were performed to examine the validity of the concept within and outside the borders for the optimized window. Thus, the key considerations were highlighted and a quantitative guide was proposed for moulding time limits based on the available literature, along with a practical study of a representative flax/PLA biocomposite. Since calculation of the extent of the chain-scission degradation of biopolymers is the key consideration for monitoring degradation of mechanical properties during compression moulding, it is of value to develop accurate calculation procedures that can be readily implemented. Models in the current literature for biopolymer degradation require a simultaneous solution of at least three chemical rate equations making the analysis somewhat complex and cumbersome. To facilitate the calculation, a simplified and revised model was proposed which no longer requires the solution of simultaneous differential equations and, for isothermal conditions; an analytical solution is readily available. The model was examined and validated against the more complex model and experimental results for PLA degradation reported in the literature. The effect of the temperature history on the degradation progress and effects on the tensile strength was studied. A thermal degradation model which accounts for the effect of time-dependent process temperature variation during manufacture of green composites was proposed. Kinetic data was used to calculate the degradation progress parameters, defining experiment process maps for identifying the effect of the temperature history on the degradation progress and effects on the tensile strength. Thus it was demonstrated that the present model is a useful tool for predicting the degradation effect of any temperature history to which the composite is subjected during manufacture. Molar mass degradation or reduction in degree of polymerisation was taken into account as a critical indicator of the extent of thermo-chemical degradation, making it possible to determine the rate of deterioration of mechanical properties for both matrix and fibre. A link between the chemical degradation of NFRP biocomposite during thermal processing and their mechanical properties was introduced. For the first time, this study has brought the thermochemical degradation concepts together with the models which have been used for composites, to predict the tensile strength of NFRP biocomposites after thermal processing. The following processes were taken into account: (i) mechanical properties of the biocomposite can be calculated by estimating their relationship with the changes of degree of polymerization over temperature and time. This relationship has separately been proposed for both matrix and fibre, (ii) the modulus of elasticity for both the flax fibre and for PLA may be assumed to be independent of the thermal processing, (iii) a linear relationship between strength and (degree of polymerization)-1, as proposed in the literature, was used to calculate the tensile strength of matrix and fibre, and for the first time used to predict the mechanical properties of NFRP biocomposites, (iv) to predict the tensile strength of NFRP biocomposite, the linear model was found to be unreliable for extended periods of time and subsequently a new exponential model was proposed which is realistic within 10% uncertainty. The sensitivity of properties of flax/PLA biocomposites to different moisture-related environments was studied. Composites were exposed to the following conditions: water immersion, warm humid, and ‘freeze-and-thaw’ cycling environments. The mechanical performance (tensile and flexural properties), moisture content and physical changes (dimensional stability) of the composites during the exposure to the different environments were analysed. The findings were also compared with those of previous works which had been produced for various applications, and are as follows for each condition: (i) when the flax/PLA composites were immersed in water, water absorption followed Fick’s law. Tensile and flexural modulus and strength decreased significantly due to the quantity of water absorbed by the composites, which led to the development of different degradation mechanisms, such as the weakening of the flax/PLA interface and plasticisation. However, the tensile strain value found for the saturated specimens almost doubled that of “as manufactured” specimens due to the plasticising effect of water in the flax/PLA biocomposites. Physical changes were relatively large, as the thickness of the samples increased considerably during the test, (ii) after the saturation moisture content was reached in the immersion tests, some samples were completely dried to analyse the residual properties of the composites. The drying process proved to be effective in partially restoring the mechanical properties. However, the “as manufactured” properties were not reached, inferring that some permanent damage was caused after the immersion tests, which was attributed to the degradation of the fibre-matrix interface. Nevertheless, the results suggest that it is advantageous to completely dry biocomposite prior to use in structural applications if the fibres have high water content, (iii) when exposed to a warm humid environment, both water absorption and physical changes were much lower than for water immersion, leading to less significant reductions in mechanical properties. In addition, the hydrolysis process can be involved in the PLA degradation, decreasing the properties of the matrix and degrading the interfacial bonding between flax and PLA by molar mass degradation, (iv) Freeze and thaw cycling has small negative impacts on tensile and flexural properties owing to small water absorption and physical changes, causing internal stresses, (v) Freeze and thaw cycling of water-saturated specimens shows further deterioration of properties in comparison with the water saturated only specimens. Water saturated and freeze/thaw cycling damages the material because of the negative synergy caused by water trapped in the microstructure and freeze/thaw cycles, which leads to the development of internal stresses, Altogether, based on the measurements and analysis, direct contact with liquid water is the most deteriorating environment for biocomposites, and therefore underwater applications of these materials are strongly discouraged. In such cases, a drying process can restore partially the mechanical performance of these materials. On the other hand, biocomposites can endure reliably in warm humid environments and in those that could create freeze-and-thaw cycles for short-term outdoor applications. Finally, a discussion was provided for practitioners who are considering using natural fibre reinforced biomatrix products. Over all, the degradation of flax/PLA biocomposites during and after manufacturing process was studied and the findings were published in four journal papers and one book chapter. A summary of the research contributions, suggestions and recommendations for the future research in the field of NFRP biocomposites are also provided in this thesis.