Environmental Engineering Reference
In-Depth Information
requirements of performance. For instance, metal parts in cars were replaced by
reinforced plastic during the period of a steel crisis because of their lower price
and lower weight.
A third approach to be considered is the ecological and sustainable aspects of a
material. For instance, the fossil resources and energy used for the production of
plastics led to undesirable carbon dioxide emissions to the atmosphere increasing
global warming. Moreover, the long persistence of some plastic materials in the
environment causes visual pollution and suffocation of animals if their post-use
disposal is not adequate.
The choice of a material and its source has therefore become a new challenge
in our modern world. There is no reliable warranty on the supply of raw materials
in our planet and we grow with more and more ecological consciousness. A
replacement strategy has already begun to be implemented and this includes bio-
materials. In this chapter we define biomaterials as those materials obtained from
biomass (also called bio-based materials). A particular emphasis will be given to
agricultural biomass.
Plant biomass sources can grow fast. Plants naturally convert the carbon dioxide
from atmosphere into polymers (the so-called biopolymers) and other compounds
such as sugars and lipids, easily convertible into materials. Biomaterials are there-
fore the best choice to regulate the carbon cycle in the lithosphere provided that
their life cycle analysis is positive, which is not a general rule.
6.2 Wood and Natural Fibres
Nature has always given to mankind a large palette of biomaterials from plants. The
fact that they are natural does not mean that their performances are poor, as might
be wrongly assumed. For instance, wood combines excellent elasticity, insulating
and toughness properties. Linen provides a fresh and elegant textile in summer.
Stubbles in thatched roofs are still used in rural houses or even in ancient Japanese
temples; its strength and insulating properties account for this preference.
The common element in the cited examples is the mechanical resistance and the
insulating properties. These advantages come not only from the macroscopic con-
figuration of these materials (the hollow cylindrical structure of stubble, for instance)
but mainly from their microscopic structure. Most vegetal fibres can be described
through two models: wood fibres and cotton fibres. In order to better understand the
mechanical properties of these fibres, let us first consider their molecular constitu-
tion (Section 6.2.1) then their hierarchical structure (Section 6.2.2).
6.2.1
Molecular Constitution
All natural fibres essentially comprise three groups of components: polysaccha-
rides, including cellulose, hemicelluloses (xylan, etc.), pectin, and so on; lignin;
and water- and solvent-soluble compounds (waxes, minerals, etc.).
