| Recently, the using of plant fibers abundant in nature such as flax fibers to replace glass fiber and synthetic fibers as the reinforcement of resin composites has gained more and more attention. The plant fiber-reinforced composites have the merits of light weight, high strength and good mechanical performances. However, because of the strong hydrophility and polarity of plant fibers, the interfacial adhesion between natural fibers and nonpolar resins is bad, which deteriorates the mechanical properties of the composites. In order to obtain composites with high performance, the surface modification of natural fibers has been proved to be an important and practical method.In this study, the characteristics, abundance of lignin on jute fiber surface and laccase-catalyzed oxidation reaction of phenolic hydroxyl groups in the lignin structures to free radicals, were utilized to initiate the grafting of foreign functional monomers with hydrophobic groups(dodecyl gallate and octadecylamine) to lignins on the jute fiber surface, which realized the surface hydrophobic modifaction of jute fibers and established a kind of novel and efficient method to functionalize jute fibers via the laccase-mediated bio-grafting in a heterogeneous system. Then, the bio-modified jute fibers were applied to reinforce nonpolar polypropylene(PP) resins to prepare composites. The interfacial compatibility and adhesion between jute fibers and resins in fiber-reinforced composites as well as the whole properties of the composites were enhanced through the investigations on the effective enzymatic establishment and control of the hydrophobic jute fiber surface in the heterogeneous system composing of jute fibers, laccases and functional monomers as well as on the relationship between the interfacial structure characteristics, mechanical properties of the fiber/resin composites and the surface properties of the reinforcing fibers. The main research contents and conclusions are below:Firstly, the effects of laccase treatment on the content and chemical structure of lignin in jute fibers as well as the properties of jute fabrics were investigated. After the laccase treatment, only a little lignins(ca. 4% of the total lignin) in jute fibers were degraded and dissolved in the reaction liquid; most lignins still stayed on the fibers and were polymerized, which led to an increase in the molecular weight and the width of the molecular weight distribution. Meanwhile, lignins were oxidized by laccase with decreased phenolic hydroxyl group, methoxyl group, ?-O-4, ?-? structures and increased carbonyl group, ?-5, ?-1 structures. The concentration of lignin on jute fiber surface(mainly the middle lamella) is higher than that in fiber body, which is in favor of the lignin-based surface modification by laccase. The content of phenolic hydroxyl groups in lignins on the laccase-treated jute fabric surface was decreased and the content of ether structures was increased, which enhanced the surface hydrophobicity of the jute fabrics slightly. The static contact angle of the laccase-treated jute fabrics was 109.2o,and the wetting time by water was 79.4 s; The surface of the laccase-treated jute fibers became neat, smooth and regular with lignin etc. attached tightly, as the reversible reaction of lignin catalyzed by laccase(degradation in the early stage and polymerization in the latter stage) allowed lignins jutting on the jute fiber surface to dislodge and redistribute on the fiber surface; Because lignins on jute fibers were coupled and polymerized by laccase, the tensile breaking strength of the laccase-treated jute fabrics was increased by 4.9% in broad-wise and by 2.6% in warp-wise. The breaking elongation of the jute fabrics was also increased after the laccase treatment by 173% in broad-wise and by 151.7% in warp-wise probably owing to decreased cohesive forces between smoother fibers in yarns.Secondly, the characteristics, abundance of lignin on jute fabric surface and coupling of two kinds of free radicals produced by the laccase-catalyzed oxidation of phenolic hydroxyl groups in lignins and the phenolic monomers-dodecyl gallates(DG), were utilized to graft foreign hydrophobic monomers with long alkyl chains in the molecular structure, dodecyl gallate, onto the jute fabric surface, mediated by laccase in the heterogeneous system. Initially, the jute fabric surface was characterized by ATR-IR, XPS, SEM and AFM; Then, the quantitative evaluation of the laccase-mediated grafting reaction of jute fabrics with DG was measured by the weighting and saponification methods. The grafting percentage of DG determined by weighting was 4.16%, and the grafting efficiency was 49%. The DG content on the grafted jute fabrics measured by saponification was 3.19%; The optimal conditions of the enzymatic grafting process were buffer p H 3, laccase dosage 1.0 U/m L, DG dosage 5 m M, incubation temperature 50 oC and incubation time 4 h; The surface hydrophobicity of jute fabrics was increased after the enzymatic grafting of dodecyl gallate. The static contact angle and wetting time of control jute fabrics were 29.8o and 3.6 s, respectively. However, those of DG-grafted jute fabrics were 111.5o and ?30 min, respectively.Thirdly, the characteristics, abundance of lignin on jute fiber surface and coupling of amine compounds with free radicals produced by the laccase-catalyzed oxidation of phenolic hydroxyl groups in lignins, were utilized to graft foreign hydrophobic monomers with long alkyl chains in the molecular structure, octadecylamine(OA) to lignins on the jute fiber surface, mediated by laccase in the heterogeneous system. Initially, the jute fabric surface was characterized by ATR-IR, XPS and SEM; Then, lignins in the OA-grafted jute fabrics were separated and characterized by MALDI-TOF MS and 1H-NMR, 13C-NMR, HSQC-NMR to prove the OA monomers were covalently grafted to the lignin structural units catalyzed by lcaase; Afterwards, the quantitative evaluation of the laccase-mediated grafting reaction of jute fabrics with OA was measured by the micro Kjeldahl method. The grafting percentage of OA was 0.712%, and the grafting efficiency was 10.571%. As the amine monomers cannot be polymerized by laccase, they can only couple with the free radicals of lignin and be grafted to lignin molecules or the surface of lignocellulosic materials. The residue monomers can be collected to use in the next time. However, the phenolic monomers will be homopolymerized inevitably at the catalysis of laccase, which decreases the utilization of monomers. Therefore, amine monomers have more economic advantages in practical applications compared with phenolic monomers. The surface hydrophobicity of jute fabrics was increased after the enzymatic grafting of octadecylamine. The static contact angle, advancing angle, receding angle and wetting time of control jute fabrics were 0o, 43.5o, 0o and 4 s, respectively. However, those of grafted jute fabrics were 112.5o, 116.4o, 42.7o and 1110 s, respectively. The processing parameters in the laccase-mediated grafting reaction, concentration of the monomers and incubation time were optimized. With the increasing of OA concentration, the grafting percentage of OA on jute fabrics was increased gradually, while the grafting efficiency was decreased. With the increasing of incubation time, both the grafting percentage and grafting efficiency were increased gradually, but went up slowly after 12 h. The contact angle and wetting time of the grafted jute fabrics showed different increasing trends. With the increasing of OA concentration or incubation time, the wetting time increased gradually. However, the contact angle increased dramatically after the grafting treatment of jute fabrics, but remained almost the same with the increasing of OA concentration or incubation time.Finally, the modified jute fabrics by laccase were applied to reinforce PP resins to prepare composites. The whole properties of the laccase-treated jute fabric/PP composites were increased. The tensile breaking strength, tensile modulus, shear breaking strength and shear modulus increased by 4.5%, 18.2%, 14.7% and 32.6%, respectively. However, the tensile and shear breaking elongations decreased by 2.9% and 12.8%, respectively. For the dynamic thermo-mechanical properties, the storage modulus of the laccase-treated jute fabric/PP composites increased and the melting temperature of the resin matrix slightly enhanced. The tensile breaking strength of DG-grafted jute fabric/PP composites was increased by 58.3% when compared with the control samples. The fracture section was neat and regular and the fibers were broken together with resins, showing good interfacial compatibility between the modified jute fibers and PP matrix. The tensile breaking strength and modulus of OA-grafted jute fabric/PP composites was increased by 82.0% and 22.2%, respectively compared with the control samples. The reinforcing fibers were broken together with the resin matrix when pulling the composites, which made the fracture section neat and regular. Meanwhile, the storage modulus and loss modulus of the OA-grafted jute fabric/PP composites were increased, indicating enhanced interfacial compatibility and adhesion between the modified jute fabrics and PP matrix. |
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