Abstract
In order to explore the reinforcing capabilities of cellulose nanofibrils, composites containing high contents of cellulose nanofibrils were prepared through a combination of water-assisted mixing and compression moulding, the components being a cellulose nanofibril suspension and an aqueous dispersion of the polyolefin copolymer poly(ethylene-co-acrylic acid). The composite samples had dry cellulose nanofibril contents from 10 to 70 vol%. Computed tomography revealed well dispersed cellulose fibril/fibres in the polymer matrix. The highest content of 70 vol% cellulose nanofibrils increased the strength and stiffness of the composites by factors of 3.5 and 21, respectively, while maintaining an elongation at break of about 5%. The strength and strain-at-break of cellulose nanofibril composites were superior to the pulp composites at cellulose contents greater than 20 vol%. The stiffness of the composites reinforced with cellulose nanofibrils was not higher than for that of composites reinforced with cellulose pulp fibres.Graphical
Highlights
Natural cellulosic fibres such as wood pulps are well known reinforcing agents in composites and the high strength and large aspect ratio of nano-scale cellulose fibrils make them suitable as reinforcement in composite materials (Berglund and Peijs 2010; Oksman et al 2016)
It can be argued that dewatering and drying of cellulose nanomaterials promotes irreversible bonding between nanofibrils, as discussed in Fernandes Diniz et al (2004). This incompatibility can affect the mixing of the cellulose in the matrix and can lead to aggregation of cellulose fibre/fibrils resulting in a nonuniform thermoplastic composite (Arino and Boldizar 2012)
Common matrices used in melt extrusion of cellulose nanofibrils (CNF) are polyesters such as poly(lactic acid) (PLA) (Jonoobi et al 2010), due to its renewable origin and biodegradability
Summary
Natural cellulosic fibres such as wood pulps are well known reinforcing agents in composites and the high strength and large aspect ratio of nano-scale cellulose fibrils make them suitable as reinforcement in composite materials (Berglund and Peijs 2010; Oksman et al 2016). It can be argued that dewatering and drying of cellulose nanomaterials promotes irreversible bonding between nanofibrils, as discussed in Fernandes Diniz et al (2004) This incompatibility can affect the mixing of the cellulose in the matrix and can lead to aggregation of cellulose fibre/fibrils resulting in a nonuniform thermoplastic composite (Arino and Boldizar 2012). Surfactants or the surface adsorption of amphiphilic co-polymers have been used to improve the dispersion and compatibility of the CNF in the polymer matrix (Volk et al 2015) These methods have been effective, but the use of non-polar solvents and multiple reaction steps may limit their industrial relevance. The EAA polymer was selected mainly due to it being insoluble in water and having the ability to form a stable aqueous dispersion This modified polyethylene had previously shown good adhesion and compatibility with regenerated cellulose (Saarikoski et al 2012). The cellulose pulp selected for comparison consisted mainly of spruce sulphite and was of a highly beaten type with a similar surface structure as the CNF used
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