Page 42 - New Cloth Market December 2022 Digital Edition

Industrial Hemp Fibers: An Overview

MAPP on the mechanical properties or decreasing its diameter increase the the composite properties are controlled
of hemp fiber-reinforced PP compos- fiber aspect ratio (length/diameter), by the matrix properties rather than the
ites, with 15 wt % and 30 wt % hemp which induces a positive effect on the fiber. Other reported works on natural
fiber. It was found that the surface treat- mechanical properties of polymer com- fiber-reinforced polymeric composites
ments resulted in composites with bet- posites [85]. Decreasing the fiber length reached the same conclusion, such as
ter mechanical properties compared to more than the critical length over which in Brahim and Cheikh [87].
the composites without any treat- the fiber stress is decreased from the
The increase in fiber volume fraction
ments, and the composites with 5 wt maximum value reduces the stress
leads to high mechanical properties of
% MAPP showed the best mechani- transfer between matrix and fiber [2].
composite material; however, over a
cal properties.
Ensuring a good fiber dispersion in the specific value the fiber volume frac-
Physical methods such as plasma dis- matrix results in a better wettability of tion deteriorates the composite’s me-
charges have been comprehensively the fibers with the matrix, reduces the chanical properties, due to poor fiber
reviewed by Baghaei et al. [78] and void content by ensuring that fibers are distribution and wettability with the
studied by Mwaikambo and Ansell [79]. surrounded by the matrix, and also matrix [88]. Additionally, water absorp-
Plasma discharge can be generated by leads to an increase in interfacial bond- tion is increased with an increase in
either corona treatment or cold plas- ing between matrix and fibers. The use fiber content, leading to swelling of the
ma treatment. The distinguishing fea- of a twin-screw extruder with thermo- natural fibers and debonding of the
ture between the two categories of plastic matrices leads to better fiber matrix–fiber interface as well as dete-
plasmas is the frequency of the elec- dispersion; however, this can cause riorating the mechanical properties of
tric discharge. High-frequency cold fiber damage and fiber length reduc- the composite [89,90]. Dhakal et al.
plasma can be produced by microwave tion [20,24]. [89] studied the effect of water absorp-
energy, whereas a lower frequency al- tion on the mechanical properties of
ternating current discharge at atmo- In injection molding, the fibers are nonwoven hemp fiber-reinforced unsat-
spheric pressure produces corona aligned along the flow direction. In this urated polyester composites, contain-
plasma. Plasma treatment can result process, the fibers’ alignment depends ing 0 wt %, 10 wt %, 15 wt %, 21 wt
in changes of the fiber’s surface char- on matrix viscosity, mold design, fiber %, and 26 wt % of fiber. It was shown
acteristics as chemical and mechani- volume fraction, and fiber length [20]. that moisture absorption increased with
cal as well as increase the polarity, the The best mechanical properties of com- fiber volume fraction increases, due to
surface roughness, and dimensional posite materials are obtained when the a higher void and cellulose content.
stability of the fibers. Plasma treatment fiber is aligned parallel to the direction Furthermore, the tensile and flexural
is an attractive method and has many of the applied load [86]. Large reduc- properties were found to decrease due
advantages compared with chemical tions in tensile strength and elastic to the debonding of the matrix–fiber
treatments, such as simplicity, low modulus have been observed with an interface.
energy consumption, short treatment increasing fiber orientation angle rela- Extensive studies have been conduct-
times, and low cost, and it does not tive to the principal loading direction ed to find the optimum relation be-
require water or any potentially hazard- (on-axis direction). Baghaei et al. [78] tween volume fraction and optimal me-
ous chemicals [80,81,83]. Corona-treat- investigated the influences of different chanical properties [8]. For injection-
ed fibers were studied by Ragoubi et orientation off-axial directions of molded thermoplastic matrix compos-
al. [84]. It was shown that the corona aligned hemp fiber-reinforced PLA ites reinforced with natural fibers, the
treatment allowed a greater improve- composites.
maximum fiber content is usually be-
ment in the hemp fiber-reinforced PP
The results showed that the mechani- tween 40% and 55 wt % of fiber con-
composites’ properties with an en-
cal properties of the composites were tent [19]. Hargitai et al. [90] found that
hancement of 32% and 30% of tensile
affected by the fiber direction. Tensile, the optimum volume fraction of hemp
strength and Young’s modulus, respec-
flexural, and impact values of the com- fiber-reinforced PP composites was
tively, to those of untreated corona fi-
posites demonstrated a decreasing between 40% and 50 wt % of fiber, and
bers.
trend for off-axial composites (fiber Hu and Lim [91] showed that the hemp
Fiber Dispersion, Length, orientation angles of 45° and 90°) com- fiber-reinforced PLA composites with
Orientation, and Volume Fraction pared to the principal fiber direction (on- 40 wt % volume fraction of alkali-treat-
axis direction). The authors’ justifica- ed fiber had the best mechanical prop-
Fiber dispersion, length, orientation, tion for the results was based on the erties.
and volume fraction in the matrix have fact that, along the on-axis direction,
Industrial Applications
a significant effect on the mechanical the composite properties are strongly
properties of the composites [2,77]. dependent on fiber and matrix proper- Industrial hemp is being used in a num-
Increasing the hemp fiber length and/ ties, whereas in the off-axis direction ber of industries such as textiles, au-

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