Structure And Properties Of Alkali Lignin Modified By Bacterial Laccase And Its Application In 3D Printing

In recent decades,inadequate plastic disposal practices have produced a massive environmental plastic pollution;an evident by the state of many bodies of water worldwide.The vast majority of plastic produced is petroleum-based and most of it is almost entirely non-biodegradable.Plastic offers competitive advantages compared to other materials due to its characteristics of lightweight,mechanical resistance,acceptable aesthetical appearance,abundance and low cost.Alongside more efficient plastic waste separation and collection techniques,a bio-based and biodegradable approach to plastic production represents a more promising strategy than relying on the old production methods.In addition,recent technologies such as additive manufacturing(also known as 3D printing)have the potential for avoiding and reducing the generation of plastic waste,allowing for customized solutions by changing the logistics of thermoplastics supply and transportation.Poly(lactic acid)is one of the most promising bio-based thermoplastics for 3D printing due to its industrial maturity and its capacity for biodegradation.However,its mechanical and thermal performances require an upgrade by blending it with copolymers to provide higher strength,and greater elasticity.Lignin is an abundant and undervalued by-product of the paper and pulp mills.Recent efforts to increase industrial lignin's value have been made,it has been used as a potential nucleating agent with poly(lactic acid).Researchers have hypothesized that industrial lignin could render improved mechanical,thermal and rheological properties to poly(lactic acid)by modifying its crystallization rate,and by the inclusion of aromatic functionalities via hydrogen bonding.Extant results have called this hypothesis into question;mainly due to lignin' s relatively low molar mass,and the relatively low abundance of key functional groups such as hydroxyl,ether and carbonyl.Furthermore,the dark color conferred by adding lignin to poly(lactic acid)limits its acceptance in thermoplastic markets.In response to the present challenges of utilizing industrial lignin as filler of bio-based thermoplastic blends,this dissertation asks the question whether it is possible to improve the mechanical and thermal performance of a 3D printed poly(lactic acid)/thermoplastic polyurethane blend by adding alkali lignin as a nucleating agent modified by a ligninolytic enzyme from a bacterial mutant strain.Additionally,this hypothesis aims to prove that this particular method will not be detrimental to the aesthetic value of the final printed specimens.The scientific objectives of this study have been achieved through the development of three research steps.The first step consisted of an investigation of the chemical,morphological and thermal modifications of alkali lignin by bacterial laccase(laccase was prepared using a mutant E.coli BL21(Lacc),which harboring laccase encoding gene from Bacillus ligniniphilus L1).In a second research step,the enzymatically-modified lignin was pretreated and incorporated as a nucleating agent within a thermoplastic matrix blend composed of 90%poly(lactic acid)(PLA)and 10%thermoplastic polyurethane(TPU)in terms of weight(wt%).A third research step consisted of evaluating the mechanical and rheological performance of 3D printed specimens from lignin modified by bacterial laccase.The specific results are summarized as follows:(1)The thermogravimetric analysis showed that enzymatically-modified lignin has a good thermal stability,with a decreased mass loss from 12.26%to 2.00%after heat pretreatment.Further analysis indicates that the specific surface area of the enzymatically-modified lignin sample significantly increased;the ultraviolet absorption at the wavelength of 280 nm decreased,the absorption signal at the wavelength of 340 nm increased,and the appearance of the lignin changed from dark brown to milky-white color.In addition,nuclear magnetic resonance and Fourier-transformed infrared spectroscopy analyses confirmed the preponderance of guaiacyl-type lignin.It also proved that laccase may be involved in several modifications of lignin chemical structure,weakening hydrogen bond interactions and producing deeper signals in key functional groups such as aliphatic and aromatic OH,and C-O-C ether bonds.XRD spectral analysis found that a series of peaks appeared throughout the diffraction angle' s spectrum in the enzymatic lignin sample,and the approximate grain sizes were estimated in 4.33 ?m,3.68 ?m,4.04 ?m,and 3.60?m,indicating the presence of amorphous crystals in lignin's molecular structure.The findings mentioned above indicate that the chemical structure,morphology,thermal behavior and response of lignin to X-ray diffraction have changed correspondingly after laccase treatment,favoring the improvement of lignin's performance as a nucleating agent in the thermoplastic polymers of this study.(2)The scanning electron microscopy analyses in the filaments' cross-section areas showed lower pore formation and better uniformity in the thermoplastic filament's macrostructure at a concentration of enzymatically-modified lignin of 5 wt%than the thermoplastic filaments with untreated lignin at the same lignin concentration.The results showed that the degree of dispersion of enzymatically-modified lignin into the PLA/TPU thermoplastic matrix was significantly higher than that of untreated lignin.The thermogravimetric analysis results showed that the weight loss of the enzymatically-modified lignin-based thermoplastic filament at the 3D printing temperature was significantly lower than that of the alkali lignin-based thermoplastic filaments,and the weight loss was only 0.197%when the lignin concentration is 2.50 wt%(the weight loss of alkali ligninbased thermoplastic filament was 0.446%).The thermal flow response analysis showed in the cooling scans an increase in the crystallization rate and addition of intermediate crystallization phases of the thermoplastic filaments based on enzymatically-modified lignin,confirming the nucleating effect of lignin modified by laccase.Contrasting with the relatively high degree of crystallinity of alkali lignin-based thermoplastic filaments,X-ray diffraction analysis results confirmed a significantly lower degree of crystallinity of the enzymatically-modified lignin thermoplastic filaments,avoiding excessive brittleness.The above results indicate that the laccase treatment improved the thermal properties of lignin as nucleating agent added to the thermoplastic blend matrix,composed of 90 wt%PLA and 10 wt%TPU,preserving filaments' flexibility.(3)The tensile failure test results showed that the ultimate tensile stress of the best enzymatically-modified lignin 3D printed specimens was increased by 8.35%compared with the control,while the higher brittleness of the untreated lignin 3D printed specimens made difficult to perform additional tensile experiments beyond a lignin concentration of 5 wt%.The enzymatically-modified lignin 3D printed specimens significantly improved the elastic response and the mechanical properties of PLA/TPU thermoplastic copolymer blends.In terms of ultimate axial stress and the elastic modulus,when the enzymatically-modified lignin concentration is between 1.25 wt%and 5 wt%,the mechanical properties of the 3D printed samples are significantly better than the PLA/TPU matrix.The results of nuclear magnetic resonance and Fourier-transformed infrared spectroscopy confirmed the beneficial effect of the steric hindrance produced by the addition of enzymatically-modified lignin into the PLA/TPU thermoplastic matrix at lignin concentrations lower than 15 wt%.The filaments produced with enzymatically-modified lignin displayed lower values in the melt volume rate test than the filaments from unmodified lignin,suggesting a better rheological performance of the filaments produced from enzymaticallymodified lignin.Finally,the 3D printed objects at enzymatically-modified lignin concentrations of 1.25 wt%and 2.50 wt%by the fused deposition method have adjustable fading performance and acceptable printing resolution.These results indicate that the performance of the 3D printed samples of enzymatically-modified lignin is significantly better than that of unmodified alkali lignin.The results demonstrated the superior performance of the specimens printed with enzymatically-modified lignin filaments compared with the equivalent specimens printed with alkali lignin without enzymatic modification.The mechanical properties of the 3D printed specimens from enzymatically-modified lignin outperformed the mechanical properties of the poly(lactic acid)/thermoplastic polyurethane matrix at modified lignin concentrations of 1.25 wt%to 10 wt%in the parameters of ultimate axial stress and elasticity modulus.Last,two objects were 3D printed with the fused deposition method from filaments at modified lignin concentrations of 1.25 wt%and 2.50 wt%.The results demonstrated the tunable discoloring properties and acceptable printing resolution.The scientific breakthroughs discussed in this dissertation open the door for the use of industrial lignin modified by bacterial laccase as a suitable bio-based scaffold with the potential to confer tunable and improved mechanical,thermal,rheological and discolored properties to combine with PLA thermoplastic blends in 3D printing applications.The resulting improved thermoplastic blends pave the way for use of more environmentally-friendly plastics.