Hygrothermal Ageing of Bio Based Composites

Thread by Brooke Jenkins on 18 Mar 2010 at 14:51:30 
To All:

Nowadays bio-derived composite resins - like polylactide and furan based resins - combined with natural flax (obtained from bast) fibres - rather than petrochemical-based products - is a hot topic. It is stated that material properties are rather promising, not only because of environmental friendly disposal, but also because of intrinsic flax performance. For example: it is told that natural flax fibres have a comparable strength and equal fire-resistance properties and that they are 10-30% lighter than glass fibres (no doubt about that). Assuming that limitations in processing technologies and molded part performance will be overcome within a relative short term (also with support from the European Union, see the recently commenced Biocomp project), my interest is the mechanical and corrosion resistant properties of the following materials when exposed to water vapour (moisture humidity 0 to 100% at ambient conditions):

- Ciba Geig LY556 Epoxy Resin reinforced with 50 wt% natural flax fiber;
- Polylactide based resin with 20 wt% natural flax fibre;
- Furan based resin with 20 wt% natural flax fibre.

The thing that worries me most is swelling behaviour of the natural reinforcements when exposed to diffused Water and temperature gradients, also in relation to probable weak wetting when compared to sized glass fibre material. The long term interfacial strength is probably insufficient / not corrosion resistant? Looking for good suggestions.

Regards,
Brooke Jenkins

p.s. what about the long term water permeation rates of these bio composites; are they comparable to the WVTR of say a 35 volume percent glass reinforced commonly used GRP, say polyester, bisphenol A or phenolic epoxy material?

    Comment by Composite Agency on 18 Mar 2010 at 17:35:35  | |responses: 0|
    Hi Brooke,

    Natural fibres, like flax and hemp, have a relative high strength and stiffness to weight ratio, relative high toughness, good thermal properties and are biodegradable. However there are some challenges in the practical use, which is an intrinsic poor compatibility between the fibre and several matrices, including hdpe, pp, vinyl ester, epoxy and polyurethane resin materials and inherent high moisture uptake and subsequent swelling of the flax (or hemp which is also used).

    At 23 degrees Celsius, the 100% moisture absorption of most low or medium hydrophobic polymers - including pe, peek, polyimide and epoxy resins - is below 1%, whereas the water solubility in flax is approx. 7% (hemp approx. 8%).

    Whether this behaviour is a problem depends - i.e. leads to cracks and undesired dimensional composite change - on many factors. Mentioning a few: the laminate build-up, the chemical treatment of the fibres (alkali / acid), the resin used, the Gibbs Free restraint coupled to the Water diffusion, and the dimensional shape of the final application.

    The best option is to simulate the hygrothermial ageing characteristics of the bio derived composite material using the CheFEM software. Also before commencing any laboratory experiment. Be warned that accelerated testing (high temperature to make use of time-temperature superposition) will produce incorrect results.

    Regards,
    Composite Agency