Study Of All-cellulose Composites Based On Dissolution Effect Of Cellulose

With the increasing interest in the development of biodegradable materials based on biomass due to the increasingly serious ecological problems, more and more materials are being designed and produced from the perspective of sustainable development or eco-design. All-cellulose composites (ACC) are completely renewable "green" composites, which could improve the interfacial compatibility of composite materials and reduce energy consumption in preparing them.In this study, ACC were prepared by the acid system of phosphoric/polyphosphoric acid and the alkali system of NaOH/urea which based on dissolution effect of cellulose, and response surface methodology was used to optimize the factors affecting the tensile strength of ACC. Experimental results showed that the optimum conditions of ACC in the phosphoric/polyphosphoric acid were as follows:solvent concentration;74.08%, the dissolution time:50.82min, solution temperature:19.35℃and predicted tensile strength:60.59MPa. Optimum experiments were performed after modification and the tensile strength was58MPa, indicating small prediction error. Meanwhile, in optimum process of NaOH/urea method, when the freezing time amount of solvent and frozen times were1.9h,12.02g,2.88, respectively, the predicted tensile strength would reach50.14MPa. Under optimum experimental conditions, the tensile strength of ACC was47MPa.The tensile strength of the ACC prepared with phosphoric/polyphosphoric acid and NaOH/urea under optimum conditions was58MPa and47MPa, whose intensity increased by452.3%and347.6%, respectively, compared with the raw material of paper. Experimental results showed that the structure of ACC prepared by acid method became denser with the increase of the solvent concentration within a certain range. When the concentration was74%, more dense structures emerged. With the solvent concentration increasing, the surface appearance of the cellulose presented network-like structure by the electron microscope. Meanwhile, the structure of ACC prepared by alkali method became denser as the amount of the solvent increased. When the amount of solvent was15g, more dense structure appeared. With the amount of solvent increasing, the surface of the cellulose presented a small amount of loss and the sample surface showed "gully" structure by electron microscopy. Under the optimum conditions, the surface and the internal structure of ACC prepared by acid method is denser than that by alkali method. Moreover, the tensile strength of ACC is related to its structure, that is, the denser the structure of ACC is, the greater its tensile strength is.The crystal structure of ACC was analyzed by using X-ray diffraction (XRD). The results showed that the crystal form of cellulose changed from ⅠtoⅡin the preparation process, with the crystallinity decreased. Compared with the raw material of paper, the crystallinity of ACC with acid was lower than that with alkali under the optimum conditions whose value was75.3%,21.5%and29.4%, respectively. The chemical structure and crystal structure of ACC were investigated by Fourier transformation infrared spectroscopy (FTIR). In the FTIR spectra, a peak at3381cm-1in the preparation process shifted around3431cm-1, which further indicated the change of crystal structure of cellulose occurred. The lack of an absorption band in the range of1740-1745cm-1indicated oxidative reactions on the chain segments of cellulose did not occur in the process due to the absence of carbonyl groups. Compared with the raw material of paper, characteristic absorption peak of ACC did not present significant change, which shows that the ACC prepared by the two methods still have the basic chemical structure of cellulose. The thermal properties of ACC were investigated by thermal gravimetric analyzer (TGA). The results showed that the thermal stability of the ACC by the two methods declined with the increase of concentration of acid and alkali, although the effect by the alkali method was not so obvious. And the thermal stability of ACC prepared by alkali method was better than that prepared by acid. The hydrophobicity of ACC was investigated by contact angle (CA). The results showed that the contact angle of the sample prepared by acid method was66.8°and the alkaline method was65.5°. Moreover, the structure of the ACC was denser than filter paper and ACC had strong hydrophobicity. And the contact angle of the samples prepared by acid method was slightly larger than alkali, which further proved that the structure of the samples prepared by acid method was denser.On the other hand, a preliminary exploration of the cyclodextrin immobilized on ACC was conducted, namely, the cyclodextrin immobilized on ACC matrix by one-step method. When the concentration of cyclodextrin was small, it did not affect the tensile strength of ACC, but when the concentration of cyclodextrin increased to7%, the tensile strength of composites decreased rapidly. ESEM showed that cyclodextrin particles could be observed on the surface and cross section of the composite. The contact angle of the samples which were not immobilized by cyclodextrin was66.8°, and the sample with7%NaOH/urea12%/5%cyclodextrin decreased to34.8°. Furthermore, the hydrophobicity of ACC decreased significantly after being immobilized by cyclodextrin and the interface properties of ACC could be significantly altered by adding a small amount of cyclodextrin.