Effects of Different Interfacial Modiers on the Properties of Digital Printing Waste Paper Fiber/Nano-Crystalline Cellulose/Poly (Lactic Acid) Composites*

： Digital printing waste paper fiber/nano-crystalline cellulose/poly (lactic acid) 8 (DPF/NCC/PLA) composites, modified through γ-methacryloxy propyl trimethoxy silane (KH570), 9 isopropyl tri (dioctylpyrophosphate) titanate (TMC201), sodium hydroxide (NaOH), polyethylene 10 glycol 6000 (PEG6000), and a composite silane coupling agent (KH570/PEG6000), were fabricated 11 by melt blending and injection molding and the effects of different modifiers on the properties of 12 composites were studied. Results showed that mechanical properties of the modified composites 13 generally improved , and the best mechanical properties, including flexural, tensile and impact strength, 14 were achieved PEG6000, KH570/PEG6000, and KH570 modification, respectively. Thermal 15 performance analysis showed improved thermal properties of composites treated by KH570, but the 16 crystallinity of the modified materials was increased. Both water absorption and degradation properties 17 showed a decreasing trend, and water absorption performance was obviously improved after 18 KH570/PEG6000 modification. Under the action of several modifiers, the diffusion coefficient, 19 thermodynamic solubility and permeability of composites were reduced to varying degrees. 20 Furthermore, scanning electron microscopy (SEM) demonstrated that interfacial adhesion and 21 composite compatibility were improved with significantly fewer and smaller pores, as well as a fuzzy 22 boundary among the three phases.

With the continuous development of an industrial society, non-renewable resources such as 3 petroleum resources and plastic products have been consumed in enormous quantities, and their non-4 degradability has brought severe environmental problems. In recent years, the public's awareness of 5 environmental protection has gradually strengthened, and research on composite materials technology 6 has made progress. Therefore, environmentally friendly composite materials have attracted the 7 attention of researchers worldwide. 8 Poly (lactic acid) (PLA), derived from renewable resources, such as starch, is an ideal 9 biodegradable polymer material [1] , which can be completely decomposed to obtain carbon dioxide and 10 water under specific conditions, such as composting and combustion, to realize an ecological carbon 11 cycle originated from nature and attributed to nature. PLA has preferable mechanical strength and 12 excellent biocompatibility, degradation, and sustainable utilization [2] , but at the same time, it is limited 13 by its chemical structure, resulting in poor toughness, poor hydrophilicity, slow crystallization rate and 14 great brittleness, thus limiting widespread applications. Nano-crystalline cellulose (NCC) is a new type 15 of biodegradable and renewable nanomaterial, which has good thermal stability, high strength, high 16 crystallinity and other properties, and can improve wide variety of composite materials. Existing 17 studies have shown that NCC can strengthen the characteristic defects of PLA materials [3][4][5][6] . Similarly, 18 strengthening resin with plant fiber can also effectively improve its performance defects [7][8][9][10] . 19 In the printing industry, digitally printed papers are becoming a larger proportion of the incoming 20 waste paper stream to the recycling industry and increasing amounts of digital prints from offices 21 accounts for a certain proportion of the recovered paper [11] . For traditional recycling methods, waste 22 paper is mostly used as raw material for recycled paper. However, deinking of digital prints involves 23 a deeper understanding of the ink and its interactions with various types of substrates [12] . Consequently, 24 even though digital printing waste paper has excellent fiber quality, it is rarely used in the production 25 of recycled paper and provides little added value. Therefore, the efficient utilization of digital printing 26 waste paper attracts much attention. Blending filled digital printing waste paper fiber (DPF) can reduce 27 the production cost of composite materials, and the utilization of DPF is economical and environment-28 friendly, has ample sources, and easily obtained, which can realize the idea of "turning waste into 29 treasure" [13] . Therefore, in this paper, we will study the performance of a ternary DPF/NCC/PLA 1 degradable composite material with DPF as the blend filler material, PLA as the matrix and NCC as 2 the reinforcement, which is not only of academic significance, but also of great practical importance. 3 In this study, DPF was used as the blending filler material to prepare NCC/PLA composites, and 4 the modifier was used to react with PLA and cellulose to improve the interfacial conditions and the 5 composite properties [14][15][16][17] . Modifiers, such as γ-methacryloxy propyl trimethoxy silane (KH570), 6 isopropyl tri (dioctylpyrophosphate) titanate (TMC201), sodium hydroxide (NaOH), polyethylene 7 glycol 6000 (PEG6000) and composite silane coupling agent (KH570/PEG6000), were added to the 8 composite material. DPF/NCC/PLA composites were prepared by melt blending and injection molding 9 process, and the influence of different modifiers on DPF/NCC/PLA composite material was studied.

15
DPF was obtained from the local digital printing waste paper of Xi'an, Shaanxi, China. NCC was 16 prepared from DPF using a sodium hydroxide (NaOH) treatment and sulfuric acid (H2SO4) solution 17 with a mass fraction of 66% in our laboratory. PLA (3052D) was purchased from Nature Works China) at 170 °C and 9 MPa injection pressure. Besides that, the overall processing techniques of NCC 4 and DPF/NCC/PLA modified composite preparations are schematically shown in Figure 1.  3.2 Fourier transform infrared spectroscopy testing 14 Using a tablet press, the NCC and DPF modified samples were separately cold pressed with 15 potassium bromide (KBr) powder to prepare test samples, which were then scanned and analyzed by 16 the 8400S Fourier transform infrared spectrometer (FTIR) (SHIMADZU, Japan), including the test 17 band range of 500~4000 cm -1 with 32 scanning times and a resolution of 4.0. 18 3.3 Thermogravimetric testing 19 The thermogravimetric (TG) curves of the modified composites were drawn using a STA 449F3 20 thermogravimetric analyzer (NETZSCH, Germany) over a temperature range of 30 to 600°C in an 21 argon (Ar) atmosphere with a 40 mL/min flow rate, and at a constant heating rate of 20°C/min. instrument (NETZSCH, Germany) with two cycles (with a 5 min interval between them) at 200℃ to 25 eliminate trace of thermal history [18] . The first cycle was carried out from 20℃ to 200℃ under a 26 nitrogen flow of 60 mL/min followed by cooling from 200℃ to 20℃. Then, a second heating was 27 performed from 20℃ to 200℃. All heating scans were performed at a rate of 10℃/min. The composite 28 crystallinity ( ) was calculated using following equation (1) [19] : where , Φ and 0 are the melting enthalpy of the composite, mass fraction of PLA in the 2 composite, and the theoretical enthalpy of fully crystallized PLA, which is 93.7 J/g [17,20] , respectively where and 0 are the weight after and before water absorption, respectively.

10
The diffusion coefficient (D) was calculated based on the initial slope of the water absorption curve: where, ℎ is the specimen thickness, is the balanced water absorption, 2 and 1 are the 13 water absorption at 2 and 1 , respectively.
14 Furthermore, the thermodynamic solubility (S) can be calculated by the following formula: 15 = (4) 16 where and are the mass of absorbed water at equilibrium and composite materials mass, 17 respectively. 18 Permeability was obtained by the product of diffusion coefficient and solubility: where 0 is the quality before degradation, and is the quality after degradation. among which PEG6000 showed the highest improvement with a 6% increase in flexural strength. The 10 flexural modulus increased by 7%, 13%, 2%, 12%, 1%, respectively, which means that the rigidity of 11 the PLA matrix has increased with the addition of modifier [21] .

12
The tensile strength and elongation at break of the modified DPF/NCC/PLA composites given in 13 Figure 3 show that the tensile strengths of the composites modified with KH570, NaOH, PEG6000 14 and KH570/PEG6000 improved to 65.6, 65.1, 66.8 and 68.6 MPa, respectively, but a decrease is 15 observed with the addition of TMC201. For elongation at break, the addition of NaOH shows no 16 changes before and after modification, and KH570, TMC201, PEG 6000, KH570/PEG6000 increased 17 by 12%, 7%, 5%, 12% respectively. Therefore, with the addition of modifiers, the toughness of the 18 composites has been improved.

24
In general, the mechanical properties of the modified composites have been improved to varying improvements of 6%, 8%, and 19% respectively, when compared with the values prior to modification.

28
The reason is that the -Si-OH produced after the hydrolysis of KH570 is dehydrated to bond with the 29 polar hydroxyl (-OH) on the surface of DPF and NCC, which can then be grafted with PLA through 1 hydrogen bonding to make the interfacial bonding of the composites more compact [22] , as shown in 2 Figure 5 (a). At the same time, the surface of the reinforced materials becomes more wrinkled after 3 modification, and a strong mechanical interlocking with the molecular chain of PLA is generated, so 4 that DPF, NCC and PLA matrix have better compatibility and stronger adhesion. PEG6000 with 5 multiple hydroxyl-terminated groups not only increases the contact between the enhancer and matrix 6 through chemical bonding with the hydroxyl groups on the surface of DPF and NCC [23] , but also 7 conducts an esterification reaction, which is compatible with PLA and able to increase toughness [24][25] . 8 The composite silane coupling agent, KH570/PEG6000, provides a synergistic effect of the above two. The molecular structure of cellulose provides a basis for its surface modification due to the 11 multitude of hydroxyl groups [26] . The FTIR spectra of NCC and DPF after interfacial modification are 12 shown in Figures 6 and 7, respectively. Near 3370 cm -1 is the stretching vibration peak of -OH, which 13 decreases after modification, and is caused by the reduction of -OH groups after reacting with the 14 modifier. The FTIR spectra also shows peaks at 1720 and 1634 cm -1 , which correspond to the 15 characteristic absorptions of C=O and C=C after modification with KH570 [27] , further indicating that 16 KH570 was been successfully grafted. However, TMC201 is chemically and directly coupled by its 17 alkoxy group to the hydroxyl group on the cellulose surface, but no corresponding characteristic groups 18 are generated in the coupling reaction, and the connection is still the -O-Ti bond, as shown in Figure 5 19 (b). Therefore, the grafting effect can only be judged by the -OH and -CH3 groups on the cellulose 20 surface. Thus, in addition to the weakened stretching vibration peak of -OH at 3370 cm -1 , the slight 21 stretching vibration peak of -CH3 and -CH2 at 2961 cm -1 also indicate that TMC201 has been 22 successfully grafted to the cellulose surface [28] .  improved by absorbing some energy to destroy the interaction force and then pyrolyzing itself. The 5 onset temperature of the composites modified by KH570 remains unchanged, while the maximum 6 decomposition temperature increases, which is due to the fact that the KH570 long-chain alkanes were 7 introduced onto the surface of DPF and NCC successfully, forming a coating layer and providing better 8 cross-linking. In the thermal decomposition process, it is necessary to absorb a certain amount of 9 energy to destroy the force between the two, and then pyrolysis takes place, so the thermal stability of 10 the modified composites is improved [29] . Meanwhile, Table 1 shows that the residual amount of the 11 modified composites increases, which is likely related to the residual organic modifier added to the 12 composite material.  shows a decreased from 122.9 to 109.3°C after modification, indicating that the modifiers can promote 20 the movement of the PLA chain segment [30] , and provide the ability to block the crystallization process 21 of the composites. This may be attributed to more restricted molecular movements of PLA chains due 22 to the formation of hydrogen bonds [31] . As the DSC heating curves demonstrate, bimodal endothermic 23 melting peaks could be seen for all samples except the material modified with KH570. Among which, 24 the lower one corresponds to crystalline melting with low perfection and thinner lamella and the higher 25 peak is related to the melting of more perfect recrystallized crystals [32] . It can be seen from Table 2 that 26 the crystallinity of the modified composites is improved, because the addition of modifier promotes 27 the movement of the PLA molecular chain segment, which enables the molecular chain in the crystal 28 region of the composite to regularly arrange [17] . The cellulose molecules contain hydroxyl groups that attract water molecules through hydrogen 2 bonding, causing the fiber to expand and absorb water [33][34][35] . Interfacial bonding between fiber and 3 resin is also an important factor affecting the water absorption properties of composites. Figure 11   4 illustrates the effect of modifier on the water absorption behavior of DPF/NCC/PLA composites. It 5 shows that water absorption increases with increasing immersion time for all modified samples. The 6 water uptake rate is linear and very rapid in the beginning of the exposure, but then slows and reaches 7 a saturation level. In the initial stage, the unmodified DPF/NCC/PLA showed a faster rate of water 8 absorption, which was due to the hydrophilic properties of cellulose fibers, and the abundant hydroxyl 9 groups that attract water molecules through hydrogen bonds, which in turn lead to fiber swelling and 10 water absorption. At the same time, the fiber has a hollow cavity in the center, allowing a large amount 11 of water to be absorbed by the capillary effect [36] . In addition, the small amount of water absorption of 12 the PLA matrix and the gap water absorption existing in the combination of PLA, NCC and DPF are 13 also important reasons for the high water absorption rate. After modification, the water absorption of 14 the composites decreased to some extent. Among them, KH570/PEG6000 decreased the most, 15 followed by PEG6000, NaOH, KH570, and TMC201, and the reduction reached 23%, 21%, 20%, 19% 16 and 18% after seven weeks, respectively. This can be attributed to the reaction of modifiers with -OH 17 on the cellulose surface, which reduced the amount of -OH, resulting in decreased of polarity and 18 hydrophilicity. Furthermore, the interfacial affinity between reinforcement and matrix was improved 19 with a more compact internal structure which reduced the water absorption rate of the gap after 20 modification, so that the water absorption rate of the composites was improved. Due to the synergistic 21 effect of the two modifiers, the water absorption properties of the composite modified by 22 KH570/PEG6000 significantly improved.

23
The parameters related to water absorption of composites, including diffusion coefficient, 24 thermodynamic solubility and permeability, are shown in Table 3. The water absorption behavior of 25 composite materials (water diffusion behavior in composite materials) is usually represented by 26 diffusion mechanism, which basically conforms to Fick water absorption model [37] . Compared with 27 the unmodified DPF/NCC/PLA, the diffusion coefficients of the composites with the addition of the 28 modifiers decreased, that is, the diffusion velocity of water molecules in the micro-gaps between the 29 polymer chains decreased [38] . It may be that the interface binding between cellulose molecular chains 1 and matrix was improved under the action of modifiers. Through the reaction between the modifiers 2 and the hydroxyl group on the surface of the cellulose, the hydrophilicity of the cellulose molecular 3 chains was weakened, resulting in the decrease of the gap between the cellulose molecular chains and 4 the hydrophobic PLA molecular chains, and therefore the diffusion coefficient reduced. In addition, 5 the parameters of thermodynamic solubility and permeability can also help to further clarify the kinetic 6 behavior of water absorption. The solubility is related to the absorption of the penetrant to a certain 7 extent [37] . With the addition of the modifiers, the diffusion channel of water was inhibited, and the  followed by PEG6000, NaOH, KH570, and TMC201, showing decreases from 5% to 3%, 3%, 4%, 4% 19 and 4%, respectively. The lower degradation rate obtained for modified DPF/NCC/PLA composites is 20 attributed to the modifier introduction which increases interfacial compactness, affects the contact area 21 with phosphoric acid and, consequently, lowers the acid degradation rate [39] .