Kristiina Oksman

Natural fibres as reinforcement in polylactic acid (PLA) composites

The focus in this work has been to study if natural fibres can be used as reinforcement in polymers based on renewable raw materials. The materials have been flax fibres and polylactic acid (PLA). PLA is a thermoplastic polymer made from lactic acid and has mainly been used for biodegradable products, such as plastic bags and planting cups, but in principle PLA can also be used as a matrix material in composites. Because of the brittle nature of PLA triacetin was tested as plasticizer for PLA and PLA/flax composites in order to improve the impact properties. The studied composite materials were manufactured with a twin-screw extruder having a flax fibre content of 30 and 40 wt.%. The extruded compound was compression moulded to test samples. The processing and material properties have been studied and compared to the more commonly used polypropylene flax fibre composites (PP/flax). Preliminary results show that the mechanical properties of PLA and flax fibre composites are promising. The composite strength is about 50% better compared to similar PP/flax fibre composites, which are used today in many automotive panels. The addition of plasticizer does not show any positive effect on the impact strength of the composites. The study of interfacial adhesion shows that adhesion needs to be improved to optimise the mechanical properties of the PLA/flax composites. The PLA/flax composites did not show any difficulties in the extrusion and compression moulding processes and they can be processed in a similar way as PP based composites.

Mechanical properties and morphology of impact modified polypropylene-wood flour composites

The mechanical properties and morphology of polypropylene/wood flour (PP/WF) composites with different impact modifiers and maleated polypropylene (MAPP) as a compatibilizer have been studied. Two different ethylene/propylene/diene terpolymers (EPDM) and one maleated styrene–ethylene/butylene–styrene triblock copolymer (SEBS–MA) have been used as impact modifiers in the PP/WF systems. All three elastomers increased the impact strength of the PP/WF composites but the addition of maleated EPDM and SEBS gave the greatest improvements in impact strength. Addition of MAPP did not affect the impact properties of the composites but had a positive effect on the composite unnotched impact strength when used together with elastomers. Tensile tests showed that MAPP had a negative effect on the elongation at break and a positive effect on tensile strength. The impact modifiers were found to decrease the stiffness of the composites. Scanning electron microscopy showed that maleated EPDM and SEBS had a stronger affinity for the wood surfaces than did the unmodified EPDM. The maleated elastomers are, therefore, expected to form a flexible interphase around the wood particles giving the composites better impact strength. MAPP further enhanced adhesion between WF and impact-modified PP systems. EPDM and EPDM–MA rubber domains were homogeneously dispersed in the PP matrix, the diameter of domains being between 0.1–1 μm.

Dispersion and characteristics of surfactant modified cellulose whiskers nanocomposites

Biodegradable nanocomposites based on 5 wt% cellulose nanowhiskers (CNW) and polylactic acid (PLA) were prepared using an extrusion process. An anionic surfactant (5, 10 and 20 wt%) was used to improve the dispersion of the CNW in the PLA matrix. The results showed that increased surfactant content resulted in improved dispersion but at the same time degraded the PLA matrix. The results from mechanical testing showed a maximum modulus for the composite with 5 wt% surfactant and as the surfactant content increased, the CNW dispersion improved and the tensile strength and elongation at break was improved compared to its unreinforced counterpart.

Different preparation methods and properties of nanostructured cellulose from various natural resources and residues: a review

The main goal of this article is to provide an overview of recent research in the area of cellulose nanomaterial production from different sources. Due to their abundance, renewability, high strength and stiffness, eco-friendliness and low weight, numerous studies have been reported on the isolation of cellulose nanomaterials from different cellulosic sources and their use in high-performance applications. This report covers an introduction to the definition of nanocellulose as well as the methods used for isolation of nanomaterials (including nanocrystals and nanofibers, CNCs and CNFs, respectively) from various sources. The web-like network structure (CNFs) can be extracted from natural sources using mechanical processes, which include high-pressure homogenization, grinding and refining treatments. Also, rod-like CNCs can be isolated from sources such as wood, plant fibers, agricultural and industrial bioresidues, tunicates and bacterial cellulose using an acid hydrolysis process. Following this, the article focuses on the characterization methods, material properties and structures. Encyclopedic characteristics of CNFs and CNCs obtained from different source materials and/or studies are also included. The current report is a comprehensive review of the literature regarding nanocellulose isolation and demonstrates the potential of cellulose nanomaterials for a wide range of high-tech applications.

High quality flax fibre composites manufactured by the resin transfer moulding process

In this work the use of high quality natural fibres as reinforcements was studied using the resin transfer moulding (RTM) processing technique. The fibres were unidirectional high quality Arctic Flax and the matrix was an epoxy resin. The mechanical properties of the composites were compared to conventional RTM manufactured glass fibre composites, traditionally retted UD-flax fibre composites and to the pure epoxy. The results from mechanical testing showed that the (50/50) high quality ArcticFlax/epoxy composite has a stiffness of about 40 GPa compared to the stiffness in pure epoxy of 3.2 GPa. The same composite has a tensile strength of 280 MPa compared to 80 MPa of the epoxy. RTM showed to be a suitable processing technique for natural fibre composites when high quality laminates are preferred.

Nanoporous membranes with cellulose nanocrystals as functional entity in chitosan: Removal of dyes from water

Fully biobased composite membranes for water purification were fabricated with cellulose nanocrystals (CNCs) as functional entities in chitosan matrix via freeze-drying process followed by compacting. The chitosan (10 wt%) bound the CNCs in a stable and nanoporous membrane structure with thickness of 250-270 μm, which was further stabilized by cross-linking with gluteraldehyde vapors. Scanning electron microscopy (SEM) studies revealed well-individualized CNCs embedded in a matrix of chitosan. Brunauer, Emmett and Teller (BET) measurements showed that the membranes were nanoporous with pores in the range of 13-10nm. In spite of the low water flux (64 Lm(-2) h(-1)), the membranes successfully removed 98%, 84% and 70% respectively of positively charged dyes like Victoria Blue 2B, Methyl Violet 2B and Rhodamine 6G, after a contact time of 24h. The removal of dyes was expected to be driven by the electrostatic attraction between negatively charged CNCs and the positively charged dyes.

Mechanical properties of natural fibre mat reinforced thermoplastic

The use of natural fibres instead of man made fibres, as reinforcements in thermoplastics, gives interesting alternatives for production of low cost and ecologically friendly composites. In this work different commercially available semi-finished natural fibre mat reinforced thermoplastics (NMT) composites have been studied. Mechanical properties and microstructure of different NMT composites were investigated and compared to conventional GMT (glass fibre mat reinforced thermoplastic) composites and pure polypropylene (PP). The study included also NMT composites manufacturing processing parameters as processing temperatures and pressure during compression moulding. The results showed that NMT composites have a high stiffness compared to pure polymer and the NMT with a high fibre content (50% by weight) showed even better stiffness than the GMT. The GMT composites had superior strength and impact properties compared to the NMT which might be due to the relatively low strength of the natural fibres but also to poor adhesion to the PP matrix. The NMT materials showed a large dependence on direction and are therefore believed to have more fibres oriented in one direction. The stronger direction (0°) of the NMT was in some cases as much as 45% better than the 90° direction.

The nature and location of SEBS-MA compatibilizer in polyethylene-wood flour composites

A maleic-anhydride-grafted styrene-ethylene - butylene-styrene (SEBS-MA) triblock copolymer has been used as a compatibilizer in low-density polyethylene-wood flour (LDPE-WF) composite system. The ...

Electrospun chitosan-based nanocomposite mats reinforced with chitin nanocrystals for wound dressing

The aim of this study was to develop electrospun chitosan/polyethylene oxide-based randomly oriented fiber mats reinforced with chitin nanocrystals (ChNC) for wound dressing. Microscopy studies showed porous mats of smooth and beadless fibers with diameters between 223 and 966 nm. The addition of chitin nanocrystals as well as crosslinking had a positive impact on the mechanical properties of the mats, and the crosslinked nanocomposite mats with a tensile strength of 64.9 MPa and modulus of 10.2 GPa were considered the best candidate for wound dressing application. The high surface area of the mats (35 m(2)g(-1)) was also considered beneficial for wound healing. The water vapor transmission rate of the prepared mats was between 1290 and 1,548 gm(-2)day(-1), and was in the range for injured skin or wounds. The electrospun fiber mats showed compatibility toward adipose derived stem cells, further confirming their potential use as wound dressing materials.

Biocomposite Hydrogels with Carboxymethylated, Nanofibrillated Cellulose Powder for Replacement of the Nucleus Pulposus

Biocomposite hydrogels with carboxymethylated, nanofibrillated cellulose (c-NFC) powder were prepared by UV polymerization of N-vinyl-2-pyrrolidone with Tween 20 trimethacrylate as a cross-linking agent for replacement of the native, human nucleus pulposus (NP) in intervertebral disks. The swelling ratios and the moduli of elasticity in compression of neat and biocomposite hydrogels were evaluated in dependence of c-NFC concentration (ranging from 0 to 1.6% v/v) and degree of substitution (DS, ranging from 0 to 0.23). The viscoelastic properties in shear and the material relaxation behavior in compression were measured for neat and biocomposite hydrogels containing 0.4% v/v of fibrils (DS ranging from 0 to 0.23), and their morphologies were characterized by cryo-scanning electron microscopy (cryo-SEM). The obtained results show that the biocomposite hydrogels can successfully mimic the mechanical and swelling behavior of the NP. In addition, the presence of the c-NFC shows lower strain values after cyclic compression tests and consequently creates improved material relaxation properties compared with neat hydrogels. Among the tested samples, the biocomposite hydrogel containing 0.4% v/v of c-NFC with a DS of 0.17 shows the closest behavior to native NP. Further investigation should focus on evaluation and improvement of the long-term relaxation behavior.