Thermal structure of carbonized residue. One of

Thermal
degradation can present an upper limit to the service temperature
of plastics as much as the possibility of mechanical property loss 11.

With regard to the nature of thermal
degradation, several types of polymers should be differentiated:

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(1) Polymers in which scissions occur
primarily in the backbone of the chain. These polymers tend to vaporize
completely at sufficiently high temperatures.

(2) Polymers in which the scissions occur
primarily between the carbons of the backbone and the side groups. Such
scissions result in formation of double bonds in the channel and, perhaps, also
crosslinkages between the chains. On prolonged heating such polymers become
more or less stabilized in the form of a partially carbonized residue.

(3) Polymers that are highly crosslinked
polymers are converted on heating into a combed structure of carbonized
residue.

One
of major drawback is the limited thermal stability of these fibers, where the
first degradation occurs at temperatures above 180°C. Because of this, the typically
used thermoplastics as matrix are polyvinylchloride, polypropylene, and polyethylene,
with melting temperatures below or equal to the degradation temperature.
Another issue is that the incompatibility of the fibers and poor resistance to moisture often
reduce the potential of natural fibers and these draw backs become critical
issue 12. Furthermore
the reason of disadvantages is that the presence of
surface impurities and the large amount of hydroxyl groups make plant ?bers
less attractive for reinforcement of polymeric materials. The  alkalization modi?es plant ?bers promoting
the development of ?ber resin adhesion, which then will result in increased
interfacial energy and, hence, improvement in the mechanical and thermal
stability of the composites.  Natural fibers from agricultural residues and
forest product’s processing mainly consist of natural lingo cellulosic
polymers. As a result, they are subjected to thermal degradation during
composite processing 13. Cellulose
is the most abundant polymer on the earth among other natural textile polymers.
Understanding the cellulose thermal degradation is of great importance in a
vast array of areas such as generation of energy from biomass and the improvement
of thermal resistance of cellulosic ?bers. Thermal decomposition of cellulose
has been widely studied for the past several years. It has been reported that
the source of cellulose and its composition greatly affect its pyrolysis. Thermogravimetric
(TG) analysis (Thermogravimetric Analysis is
a method used was based on continuous measurement of weight on a sensitive
balance as sample temperature was increased in an inert atmosphere. This is
referred to as non isothermal TGA. Data were recorded as a thermogram of weight
versus temperature 14 is a widely used technique in this area. It is useful for
the thermal characterization of both inorganic and organic materials, including
polymers (such as cellulose). It provides quantitative results regarding the
loss of mass of a sample as a function of increasing temperature or time 15.
This thermal degradation of cellulose-based
fibers is greatly influenced by their structure and chemical composition 16. Due to the complexity of thermal decomposition reactions of
natural ?bers, extensive researches have been done in determining individual
behaviors of the main components (or pseudo-components) of natural ?bers (e.g.,
pure cellulose, lignin, and hemicelluloses). In this case, improvement of
classic ”Broidoe Sha?zadeh” model and calculation of decomposition activation
energy of pure cellulose is the primary focus. Antal and Varhegyi and Antal et
al 17 reviewed the pyrolysis of pure, ash-free cellulose and
described it with a single step, irreversible, ?rst-order rate law containing
high activation energies (238 or 228 kJ/mol). Milosavljevic and Suuberg reviewed
global cellulose pyrolysis kinetics and found that low activation energies
(140-155 kJ/mol) were obtained when cellulose was rapidly heated above 600 K. Capart
et al. calculated kinetics parameters of microgranular cellulose using dynamic
and isothermal methods in nitrogen atmosphere and described it with two
reactions with activation energies of 202 and 255 kJ/mol respectively. Besides
pure cellulose, the pyrolysis of lignin and hemicelluloses (i.e., xylan) has
also been studied. In developing ?ber decomposition kinetics, different
reaction schemes have also been considered for a better interpretation for ?ber
decomposition process. Koufopanos et al. established multi-step reaction mechanisms for wood and
wood components. Diebold later developed an elaborate seven-step global
kinetics scheme for cellulose pyrolysis. Orfao et al. introduced three
independent reactions’
model to pyrolysis kinetics of some lignocellulosic materials, while Di
Blasi 18 used three parallel reactions to describe the fast pyrolysis process of wood. More recently, Chen et
al. 19 employed a two-step consecutive reaction model for some
forest fuels. For practical engineering applications, however, it may be suf?cient to consider only the basic
characteristics of the thermal decomposition process with some simpli?ed
mechanisms. For natural ?ber reinforced polymer composite processing, it is of more practical
relevance to understand and predict the thermal
decomposition of the reinforcing ?bers based
on the simpli?ed kinetic scheme and parameters under speci?c process temperature of
polymer/natural ?ber composite. However, there have been few fundamental studies in this ?eld. The another study shows that degradation is
followed by the orders in thermal stability (in
absence of oxygen): lignin, alpha-cellulose, hemi cellulose as published by
Nasser 20. Opposite data were published by Ramiah with decomposition
temperatures in the order: hemicellulose, lignin, alpha cellulose by using the
dilatometric method 21. It is well known that, when cellulose-based materials are
heated in the range of 100 to 250°C some of the changes in physical properties
of the fibers can be explained in terms of alterations in either physical or
chemical structures such as depolymerization (Depolymerization is the conversion of a polymer into its component monomers 22.),
hydrolysis (Hydrolysis is a
reaction involving the breaking of a bond in a molecule using water.