Jari Vartiainen

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.

Comprehensive elucidation of the effect of residual lignin on the physical, barrier, mechanical and surface properties of nanocellulose films

We elucidate the effect of residual lignin on the interfacial, physical and mechanical properties of lignocellulose nanofibrils (LCNF) and respective nanopapers. Fibers containing ∼0, 2, 4, and 14 wt% residual lignin were microfluidized into LCNF aqueous suspensions and were processed into dry films (nanopapers). A systematic decrease in fibril diameter with increasing residual lignin was observed upon fibrillation, consistent with the radical scavenging ability of the lignin that results in better cell wall deconstruction. The stiff nature of the lignin-containing fibrils made them less able to conform during filtration and improved extensively dewatering, owing to a more open structure. However, the softening of the lignin during hot-pressing of the nanopapers and its amorphous nature enabled a binding effect, filling the voids between the nanofibers (thus reducing the number of micropores) and making the surface of the nanopapers smoother. The interfacial free energy of interaction changed drastically with the increased lignin content: the corresponding water contact angles were 35° and 78° for the lignin-free and for the (14%) lignin-containing nanopaper, respectively, revealing the increase in hydrophobicity. Together with the significantly less porous structure of LCNF nanopapers, lower water absorbency was observed with increased lignin content. Lignin in the nanopapers reduced the oxygen permeability by up to 200-fold. Water vapor permeability, in turn, did not correlate linearly with lignin content but depended most significantly on material density. The tensile strength, modulus, and strain for the LCNF nanopapers were found to be in the range 116–164 MPa, 10.5–14.3 GPa, and 1.7–3.5%, respectively. To a good degree of approximation, these mechanical properties were rather insensitive to lignin content and comparable to those of nanopapers derived from fully bleached CNF. Whilst it might be expected that lignin interferes in hydrogen bonding between fibrils, this was apparently counteracted by the uniform distribution of lignin seemingly aiding stress-transfer between fibrils and thus preserving mechanical properties. Overall, LCNF is demonstrated to be a suitable precursor of nanopaper, especially when reduced polarity and low hydrophilicity are desirable in related bio-products.

Antimicrobial and Barrier Properties of LDPE Films Containing Imazalil and EDTA

Imazalil and ethylenediamine-tetraacetic acid (EDTA) were incorporated into low-density polyethylene (LDPE) aimed at producing antimicrobial packaging films for foodstuffs. Moulded plates (thickness 2mm) containing 5% of EDTA inhibited Bacillus subtilis, whereas 0.05–0.25% of imazalil had strong activity against Aspergillus niger. Further tests for antimicrobial activity, migration and oxygen and water vapor barrier properties were carried out using biaxially stretched LDPE films containing different combinations of both substances. The addition of imazalil and EDTA increased the oxygen transmission rates and water vapor permeabilities, although the effects with imazalil films were not as significant. Transparency of the EDTA containing films decreased rapidly as a function of added EDTA, whereas imazalil films were optically faultless. Total migration into 3% acetic acid and 10% ethanol was below 4 mg/dm2. Although imazalil retained its activity against A. niger on a high level (inhibition zones >30 mm), the activity of EDTA was gone. None of the samples inhibited Escherichia coli.

Modification of nanofibrillated cellulose using amphiphilic block-structured galactoglucomannans

Nanofibrillated cellulose (NFC) and hemicelluloses have shown to be highly promising renewable components both as barrier materials and in novel biocomposites. However, the hydrophilic nature of these materials restricts their use in some applications. In this work, the usability of modified O-acetyl galactoglucomannan (GGM) for modification of NFC surface properties was studied. Four GGM-block-structured, amphiphilic derivatives were synthesized using either fatty acids or polydimethylsiloxane as hydrophobic tails. The adsorption of these GGM derivatives was consecutively examined in aqueous solution using a quartz crystal microbalance with dissipation monitoring (QCM-D). It was found that the hydrophobic tails did not hinder adsorption of the GGM derivatives to cellulose, which was concluded to be due to the presence of the native GGM-block with high affinity to cellulose. The layer properties of the adsorbed block-co-polymers were discussed and evaluated. Self-standing NFC films were further prepared and coated with the GGM derivatives and the effect of the surface modification on wetting properties and oxygen permeability (OP) of the modified films was assessed.

Influence of High-Temperature Heat Treatment on Barrier and Functional Properties of Polyolefin-Coated Papers

In this study, the effect of heat treatment on barrier and functionality of polyolefin-coated papers was investigated. The aim was to find the optimal improvements on barrier without losing the applicability of the materials due to physical damages. The results of the study proved considerable improvement in barrier characteristics of the structures. Both water vapor and oxygen transmission rates of LDPE-coatings decreased linearly following the set-temperature until 200°C. At this point, the treatment caused a continuing decrease in oxygen transmission achieving 10 × lower transmission levels than the untreated structure, whereas moisture transport faced corresponding but lower increase. This was considered to be caused by the difference in diameter of the H2O and O2 molecules; the smaller water molecules are able to penetrate between spherulites, whose size increased due to higher treatment temperature followed by cooling.

The effect of side-chain length of cellulose fatty acid esters on their thermal, barrier and mechanical properties

Abstract Currently, long-chain cellulose esters are not produced commercially because of high price, and since their preparation typically requires a large quantity of chemicals. To reduce the chemical consumption, cellulose reactivity needs to be increased without losing its quality. One way to increase the reactivity of cellulose is to decrease its molar mass in a controlled manner. In this study, we have synthesized cellulose esters with different side-chain length (C6–C18) in a homogeneous system using ozone molar mass-controlled cellulose. The target was to keep the degree of substitution as low as possible while still ensuring the suitability of cellulose esters for solvent casting. Thermal, barrier and mechanical properties were studied depending on cellulose fatty acid ester side-chain length. All our molar mass-controlled cellulose esters form optically transparent, flexible and heat-sealable films with good water barrier properties and are processable without the addition of an external plasticizer. Furthermore, the films have mechanical properties comparable to some generally used plastics. These good properties suggest that our molar mass-controlled cellulose esters could be potential candidates for various applications such as films and composites.

Understanding the mechanisms of oxygen diffusion through surface functionalized nanocellulose films

A concept for direct surface modification on self-standing films of cellulose nanofibrils (CNF) is demonstrated using an aminosilane group in cellulose compatible solvent (dimethyl acetamide, DMA). The chemically modified structure efficiently prevents the oxygen molecules from interacting with the nanocellulose film in the presence of water molecules. Oxygen permeability values lower than 1mLmmm-2day-1atm-1 were achieved at extremely high levels of relative humidity (RH95%). The aminosilane reaction is compared to conventional hydrophobization reaction using hexamethyldisilazane. The differences with respect to interactions between cellulosic nanofibrils, water and oxygen molecules taking place with aminated and silylated CNF films correlated with the degree of surface substitution, surface hydrophilicity and permeability of the formed layer. The self-condensation reactions taking place on the film surface during aminosilane-mediated bonding were decisive for low oxygen permeability. Experimental evidence on the importance of interfacial processes that hinder the water-cellulose interactions while keeping films low affinity towards oxygen is demonstrated.

Hydrophobization and smoothing of cellulose nanofibril films by cellulose ester coatings

The Cellulose nanofibrils (CNF), also referred to as nanocellulose, is one of the most studied bio-based material in recent year, which has good potential in the future for packaging applications due to its excellent mechanical strength and oxygen barrier properties. In the future, CNF films may also find new applications for example in printed electronics, if the surface smoothness of CNF films can be improved. One way to improve surface smoothness is to use thin coating solutions with zero porosity, such as molar mass controlled cellulose ester coatings. In this study, we have coated CNF films using molar mass controlled cellulose esters with different side chain lengths forming 3-layer film (ester-CNF-ester). These coatings improved significantly the smoothness of CNF films. The 3-layer films have also good water vapor barrier and mechanical properties and the films are heat-sealable, which enable various new applications in the future.

Improving the water resistance of nanocellulose-based films with polyhydroxyalkanoates processed by the electrospinning coating technique

Polyhydroxyalkanoates (PHAs) comprise a family of biodegradable aliphatic polyesters with enhanced sustainable profile and high water vapor barrier. As environmentally friendly materials, nanostructured cellulose-based films, also called nanopapers, such as films made of cellulose nanofibrils (CNFs) and lignocellulose nanofibrils (LCNFs), are also of growing interest due to their high mechanical strength and outstanding oxygen barrier properties at dry conditions. Unfortunately, nanopapers are highly hydrophilic, lacking of sufficient moisture resistance for uses in, for instance, food packaging. The present study reports, for the first time, on the effect of electrospun poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) double side coatings on the morphology, water contact angle, mechanical properties, and barrier performance of CNF and LCNF films. The resultant multilayer structures showed significantly improved water contact resistance, more balanced mechanical properties, and higher barrier performance against water vapor in comparison to the neat nanopapers. Although the PHA-coated nanopapers presented slightly lower aroma barrier due to the intrinsic affinity of PHA for limonene uptake, these sustainable multilayer films further improved the oxygen performance of the nanopapers, showing significant potential as barrier materials even at high humidity conditions. As a result, the here-developed novel films, based on nanopapers double side coated with electrospun PHB and PHBV layers, appear as a very promising fully bio-based material concept for food packaging applications due to their outstanding water vapor and oxygen barrier performance.

Low-temperature atomic layer deposition of SiO2/Al2O3 multilayer structures constructed on self-standing films of cellulose nanofibrils

In this paper, we have optimized a low-temperature atomic layer deposition (ALD) of SiO2 using AP-LTO® 330 and ozone (O3) as precursors, and demonstrated its suitability to surface-modify temperature-sensitive bio-based films of cellulose nanofibrils (CNFs). The lowest temperature for the thermal ALD process was 80°C when the silicon precursor residence time was increased by the stop-flow mode. The SiO2 film deposition rate was dependent on the temperature varying within 1.5–2.2 Å cycle−1 in the temperature range of 80–350°C, respectively. The low-temperature SiO2 process that resulted was combined with the conventional trimethyl aluminium + H2O process in order to prepare thin multilayer nanolaminates on self-standing CNF films. One to six stacks of SiO2/Al2O3 were deposited on the CNF films, with individual layer thicknesses of 3.7 nm and 2.6 nm, respectively, combined with a 5 nm protective SiO2 layer as the top layer. The performance of the multilayer hybrid nanolaminate structures was evaluated with respect to the oxygen and water vapour transmission rates. Six stacks of SiO2/Al2O with a total thickness of approximately 35 nm efficiently prevented oxygen and water molecules from interacting with the CNF film. The oxygen transmission rates analysed at 80% RH decreased from the value for plain CNF film of 130 ml m−2 d−1 to 0.15 ml m−2 d−1, whereas the water transmission rates lowered from 630 ± 50 g m−2 d−1 down to 90 ± 40 g m−2 d−1. This article is part of a discussion meeting issue ‘New horizons for cellulose nanotechnology’.