Ulla Hippi

Graphene/cellulose nanocomposite paper with high electrical and mechanical performances

Graphene/cellulose nanocomposite paper with high mechanical and electrical performances was reported in this study by combining reduced graphene oxide sheets (RGO) and amine-modified nanofibrillated cellulose (A-NFC) in a well-controlled manner. By adjusting the GO content, various graphene/cellulose nanocomposites with 0.1–10 wt% content of graphene were obtained. The RGO/A-NFC nanocomposite synthesized by the developed method exhibits an electrical percolation threshold of 0.3 wt% with an electrical conductivity of 4.79 × 10−4 S m−1, which is well above the antistatic value. Furthermore, with 10 wt% of graphene, a high conductivity of 71.8 S m−1 was measured for the nanocomposite. Moreover, it was found that on addition of only 0.3 wt% of graphene, the tensile strength increased by 1.2 fold and 2.3 folds compared to that of the neat cellulose and graphene oxide paper, respectively, revealing an excellent reinforcement of graphene sheets. Moreover, the elongation at break of the composite with graphene content was 8.5%, which is similar to that of A-NFC paper and much higher than that of GO paper. It is noteworthy to mention that with 5 wt% of graphene, the RGO/A-NFC composite paper showed a significantly enhanced tensile strength of 273 MPa that is 1.4 fold and 2.8 folds higher than that of the cellulose papers and graphene oxide paper, respectively. Such a high enhancement of electrical and mechanical properties in cellulose paper by graphene has never been reported before for any carbon-based material/cellulose composite paper.

Compatibilization of polyethylene/aluminum hydroxide (PE/ATH) and polyethylene/magnesium hydroxide (PE/MH) composites with functionalized polyethylenes

Study was made of the compatibilization of polyethylene/aluminum hydroxide (PE/ATH) and polyethylene/magnesium hydroxide (PE/MH) composites with composition 60/40 wt%. Compatibilizers were hydroxyl or carboxylic acid functionalized copolymers prepared in our laboratory with metallocene catalysts and commercial butyl acrylate, maleic anhydride, epoxy, and acrylic acid functionalized polyethylenes. Comparison was made with stearic acid treatment of the composites. The effect of polymeric compatibilizers on the properties of the composites was studied by tensile and impact tests, dynamic mechanical analysis, differential scanning calorimetry, morphology studies, and flammability tests. Adhesion of the PE/ATH and PE/MH composites was significantly improved, as seen in morphology studies in the changed fracture mechanism. The improved adhesion was particularly reflected in the mechanical properties: an improvement in both stiffness and toughness was achieved with use of the functionalized copolymers prepared with metallocene catalysts. According to cone calorimetry, flame retardancy properties of the composites did not deteriorate upon addition of compatibilizers.

A Fast Method to Produce Strong NFC Films as a Platform for Barrier and Functional Materials

In this study, we present a rapid method to prepare robust, solvent-resistant, nanofibrillated cellulose (NFC) films that can be further surface-modified for functionality. The oxygen, water vapor, and grease barrier properties of the films were measured, and in addition, mechanical properties in the dry and wet state and solvent resistance were evaluated. The pure unmodified NFC films were good barriers for oxygen gas and grease. At a relative humidity below 65%, oxygen permeability of the pure and unmodified NFC films was below 0.6 cm(3) μm m(-2) d(-1) kPa(-1), and no grease penetrated the film. However, the largest advantage of these films was their resistance to various solvents, such as water, methanol, toluene, and dimethylacetamide. Although they absorbed a substantial amount of solvent, the films could still be handled after 24 h of solvent soaking. Hot-pressing was introduced as a convenient method to not only increase the drying speed of the films but also enhance the robustness of the films. The wet strength of the films increased due to the pressing. Thus, they can be chemically or physically modified through adsorption or direct chemical reaction in both aqueous and organic solvents. Through these modifications, the properties of the film can be enhanced, introducing, for example, functionality, hydrophobicity, or bioactivity. Herein, a simple method using surface coating with wax to improve hydrophobicity and oxygen barrier properties at very high humidity is described. Through this modification, the oxygen permeability decreased further and was below 17 cm(3) μm m(-2) d(-1) kPa(-1) even at 97.4% RH, and the water vapor transmission rate decreased from 600 to 40 g/m(2) day. The wax treatment did not deteriorate the dry strength of the film. Possible reasons for the unique properties are discussed. The developed robust NFC films can be used as a generic, environmentally sustainable platform for functional materials.

Network formation of nanofibrillated cellulose in solution blended poly(methyl methacrylate) composites

Composites of poly(methyl methacrylate) (PMMA) and nanofibrillated cellulose (NFC) were prepared by solution blending and further processed by injection and compression molding. To improve adhesion at the PMMA/NFC interface, the nanofibrils were covalently grafted with PMMA. Formation of a percolating nanofibril network was observed between 1 and 5 wt.% of NFC by dynamic rotational rheometry in molten state. This observation was further supported by the behavior of glass transition temperature which decreased at low NFC concentrations but recovered above the percolation threshold, indicating a decreased mobility of the matrix polymer. This effect was more pronounced with ungrafted NFC, possibly due to a stronger network. The unmodified NFC induced a minor degradation of the molar mass of PMMA. As thin plates, the composites were transparent at low NFC concentrations but became partially aggregated at the highest NFC concentrations. Despite the continuous NFC network, tensile testing showed no improvement of the mechanical properties.

Erratum to: Surface functionalization of nanofibrillated cellulose using click-chemistry approach in aqueous media (Cellulose, (2011), 18, 5, (1201), 10.1007/s10570-011-9573-4)

The article “Surface functionalization of nanofibrillated cellulose using click-chemistry approach in aqueous media”, written by Nikolaos Pahimanolis, Ulla Hippi, Leena-Sisko Johansson, Tapio Saarinen, Nikolay Houbenov, Janne Ruokolainen and Jukka Seppälä, was originally published Online First without open access.