Life Cycle Assessment of Biobased Fibre-Reinforced Polymer Composites (Levenscyclusanalyse van biogebaseerde, vezelversterkte polymeercomposieten)

Today, global environmental issues, such as global warming and fossil depletion, drive a paradigm shift in material applications from conventional fossil sources to renewable sources. Following this trend, the topic of this thesis is to analyse the use of biobased resources for fibre reinforced composite fabrication. Currently the most widely used fibre reinforced composites are composed of glass fibre reinforcements and polymeric matrices. In this thesis, the biobased alternative, i.e. flax fibre, which is one of the most widely used natural fibres, is studied as a glass fibre substitute from an environmental impact point of view. Moreover, a newly emerging biobased polymer, the wheat gluten, is analysed in the study; and it is compared to a conventional polymer and to a commercially available biobased polymer from environmental impact perspective. The main question addressed in this thesis is whether there are general environmental advantages for the use of flax fibres over glass fibres, and for the wheat gluten polymer over petrochemical polymers. Although biobased materials are often perceived to be environmentally friendly, an in-depth analysis is still needed to answer this question. Life cycle assessment (LCA) is considered to be an appropriate tool for environmental impact quantification. The thesis addresses the following research objectives: (i) to deliver life cycle inventory data for the flax fibre and wheat gluten; (ii) to perform LCA studies on flax fibres compared to glass fibres in composite reinforcement applications and on wheat gluten polymer compared to the conventional low density polyethylene (LDPE) and commercially available biobased polymer polylactic acid (PLA); (iii) to conduct an in-depth analysis concerning sensitivity and uncertainty to obtain information on the robustness of the obtained results. As the initial step, a detailed life cycle inventory for flax cultivation in Northern France is documented. Field emissions associated with the nitrogen cycle are estimated both from a process-oriented DeNitrification-DeComposition (DNDC) method and the generic Intergovernmental Panel on Climate Change (IPCC) method. Since the IPCC method is derived from field measurements at sites with various soil types, climate conditions, and crops, it is subject to relatively large uncertainty. In contrast, the outputs from the DNDC method are believed to be more accurate because this method is constructed based on complex biochemical models from soil science and makes use of site specific data. The comparison of the results lead to the conclusion that the emission factors from the DNDC method and the recommended values from the IPCC method for flax cultivation in Northern France are quite different. The DNDC based emission factor for direct N2O emission, which is a strong greenhouse gas, is by a quarter to half lower than the recommended emission factor according to the IPCC method. The DNDC method leads to a reduction of 17% in the impact category of climate change per kg retted flax straw production in comparison to the level obtained from the IPCC method. Different co-product accounting principles: mass allocation, economic allocation, and system expansion are investigated for the flax fibre extraction process. The impact results per kg flax hackled long fibre show that economic allocation and system expansion lead to similar values, while the mass allocation approach leads to significantly lower values among all impact categories. Furthermore, methods including the statistical fitting, qualitative judgement, and expert-judgement are integrated to parameterise the important inventory data to run a Monte-Carlo simulation for uncertainty analysis. In the case of the climate change impact category, the uncertainty levels through the mass allocation, economic allocation, and system expansion are compared for the results. It is found that the system expansion approach implies the highest level of uncertainty due to inclusion of additional external processes, while the mass allocation leads to the lowest uncertainty. The uncertainty level related to economic allocation is significantly higher than the mass allocation, which can be caused by fluctuating prices of flax (co-)products. With a reliable impact assessment on flax fibre production and with the economic allocation method is used, the study further extends to a comparison of flax fibres and glass fibres in composite applications. The Ashby method is used to ensure functionally equivalent designs. A so-called life cycle environmental impact indicator (LEI) has been introduced. Such LEI is derived from the material mass indices illustrated in the Ashy method. The material mass indices contain only material intrinsic properties and are proportional to the product mass while ensuring equal functionality (e.g. stiffness, strength, etc.) for a specified structure and loading type. Then the LEI can be constructed by using unit environmental impact values (e.g. climate change, fossil depletion, and etc.) in production, use, and EoL, and multiplying these with the mass indices. The LEI highlights three important variables severely influencing the outcome of comparative studies based on this indicator method: the replacement ratio, the specific fuel saving (in case of transport applications), and the fibre volume fraction. The first parameter addresses the concern that flax FRPs are likely to fail earlier than conventional FRPs. Therefore, more flax FRP components are needed for the expected lifespan. The specific fuel saving denotes how much fuel can be saved by a certain weight reduction over a total transport distance. Since flax fibres have a much lower density than conventional glass fibres, flax FRPs can facilitate structures with reduced weight. Therefore flax FPRs are likely to contribute to lower fuel consumption rates. The third parameter, the flax volume fraction, influences the mechanical properties of flax FRPs. Constructed on the polypropylene (PP) matrix, two types of flax FRP products: the flax mat-PP (panel) and short flax fibre-PP (strut), produced via compression moulding and injection moulding, respectively, have been evaluated for multiple impact categories using the ReCiPe method for fixed values of the fibre volume fraction and specific fuel reduction. Compared to a 9 kg glass mat-PP with 20 vol% fibre content, with the equal stiffness criterion under bending, the flax mat-PP with 20 vol% leads to a reduced mass design (7.8 kg). In contrast, with an equal stiffness criterion under tension, the short flax fibre-PP is similarily heavy (9.2kg) as the corresponding GFRP (9kg). The LCA study compared the environmental impact of flax FRPs and GFRPs within Europe in non-transport use (production plus EoL phases) and transport use. Over the production and EoL phases, the flax mat-PP exhibits lower values for most environmental impact categories than its corresponding glass fibre based counterpart. For most categories, the reductions can reach 20%~50%. In particular, a 23% reduction in the climate change category and a 24% reduction in fossil depletion are documented. Moreover, for the impact categories of ozone depletion and human toxicity reductions of up to 70% can be achieved. On the other hand, replacing the short glass fibre-PP by short flax fibre-PP only results in modest environmental impact reductions in multiple impact categories, including the impact categories of climate change and fossil depletion where only reductions of around 10% are recorded. Coming to the full life cycle impact, the saved fuel consumption during the use phase could be very significant depending on the lifetime distance travelled. In 14 out of 17 impact categories, lower life cycle impact results can be observed for the flax mat-PP. However, in case of the short flax fibre-PP compared to short glass fibre-PP, the two material systems lead to very similar environmental impact values in multiple categories due to their very similar mass. Subsequently, a consequential life cycle analysis (CLCA) is conducted to capture how overall environmental impact will change when shifting from glass fibres to flax fibres as reinforcements in composite fabrication. With certain assumptions, the marginal flax fibre supply is identified to be a combination of Chinese flax fibre (70%) and French flax fibre (30%). Due to inferior cultivars and coal-fired electricity in Chinese flax cultivation, the CLCA study reveals that flax mat-PP has 0.8~2 times higher environmental impact values than the glass mat-PP in most environmental impact categories over the production and end-of-life (EoL) phases. For the purpose of providing potential trajectories of marginal flax fibre supply, additional all French fibre, and all Chinese fibre scenarios are also evaluated, formulating the lower and upper boundaries in terms of environmental impact change, respectively. The attributional fibre supply mix scenario is supplied as well. The all French fibre scenario leads to better scores than glass fibre composites in most impact categories; while the break-even mix lies at attributional supply mix scenario since it presents a close eco-profile to that of glass mat-PP. All of these scenarios are useful for policy analysis. Finally, the study quantifies the life cycle impact for wheat gluten based materials. The contribution serves as a base assessment which can feed into the evaluation of any future wheat-gluten based product. The study evaluates wheat gluten based packaging film and compares it with low density polyethylene LDPE and PLA film over the life cycle of these products. Scenarios including extrusion and casting for film production; incineration with energy recovery and composting for end-of-life treatments are evaluated. The comparison offers insight into the environmental benefits of using wheat gluten over conventional plastic film as well as its biobased alternative and identifies its optimal production and disposal methods. For wheat gluten production, the LCA results show that the impacts of the wheat cultivation and gluten drying phase are dominant in the majority of the impact categories in the ReCiPe midpoint assessment method. The LCA results also exhibit that the scenario with wheat gluten film produced by extrusion and incinerated to recover embodied energy is favourable from an environmental perspective. It offers great benefits in climate change and fossil depletion over LDPE film and in 14 impact categories over PLA film. Although wheat gluten film suffers from common problems for biobased materials (e.g. land occupation), the overall environmental performance indicates that wheat gluten represents a promising source for biobased polymer production

Publication year:2014

Accessibility:Open