The drive towards sustainability, even in materials technologies, has fuelled an increasing interest in bio-based composites. Cellulosic fibres, such as flax and jute, are being considered as alternatives to technical synthetic fibres, such as glass, as reinforcements in fibre reinforced polymer composites for a wide range of applications. A critical bottleneck in the advancement of plant fibre composites (PFRPs) is our current inability to predict PFRP properties from data on fibre properties. This is highly desirable in the cost-and time-effective development and design of optimised PFRP materials with reliable behaviour. This study, alongside limited other studies in literature, have found that the experimentally determined (through single fibre tests) fibre properties are significantly different from the predicted ('backcalculated' using the popular rule-of-mixtures) fibre properties for plant fibres. In this note, we explore potential sources of the observed discrepancy and identify the more likely origins relating to both measurement and errors in predictions based on the rule-of-mixtures. The explored content in this discussion facilitates the design of a future investigation to (1) identify the sensitivity of the discrepancy between measured and predicted fibre properties to the various potential origins, (2) form a unified hypothesis on the observed phenomenon, and (3) determine whether the rule-of-mixtures model (in specific cases) can be improved and may be able to predict properties precisely.

Jute yarns were treated by tap water with and without tension at room temperature for 20 minutes and then dried. Fibre and yarn strength were measured before and after treatment. Unidirectional (UD) composites were made by both treated and untreated yarns with and without applying hydroxyethyl cellulose (HEC) as size material. Water-treated jute yarns without tension and composites made of those yarns showed decreased strength, and water treated jute yarns with tension and composites made of those yarns showed increased strength with respect to raw yarns and composites made of raw yarns. However, no specific trend was noticed for fibre tensile strength and tensile modulus. HEC sized yarns showed up to 12% higher failure load with respect to unsized yarns, and composites made of HEC sized yarns showed up to 17% and 12% increase in tensile strength and tensile modulus, respectively, compared to composites made of similar types of unsized yarns.