Page 41 - New Cloth Market December 2022 Digital Edition

Industrial Hemp Fibers: An Overview

water aging on the mechanical proper- creases the fiber surface roughness
Hemp Fiber
ties of pultruded hemp and hemp/glass topographies as well as modifies cel-
Surface Modifications
fiber hybrid reinforced polyester com- lulose structure by increasing its crys-
posites. It was observed that the addi- Significant research has been carried tallinity [40]. Different studies on alkali
tion of glass fibers reduced water ab- out on improving the interfacial bond- treatments have reported both im-
sorption, thus delaying the aging deg- ing between the natural fibers and provements in interfacial shear
radation of the hybrid composite ma- matrix, which can be divided into three strength, thermal stability, and mois-
terial. Also, the hybrid composites different approaches—hybridization, ture absorption reduction as well as
showed a superior strength retention. physical treatment, and chemical treat- improvements in the composites’ final
ment [2,4,7]. Hemp fibers contain large properties, such as the tensile and
Natural and Hemp
amounts of cellulose, hemicelluloses, impact strengths, elastic modulus, and
Fiber-Reinforced Composites
lignin, and pectin. Consequently, they fracture toughness [19,78,79].
have hydroxyl groups in their structure
Natural fibers can be blended with oth- Beckermann and Pickering [80] ob-
and tend to behave as polar hydrophilic
er natural fibers and be embedded in a served improvements in tensile
material, while polymeric materials are
polymeric matrix to form a hybrid com- strength and elastic modulus, lignin
polar due to their hydrophobic, long
posite. Many researchers have inves- and pectin reduction, and higher ther-
aliphatic primary chain [2]. The combi-
tigated the mechanical properties of mal stability for hemp fibers treated
nation of both hydrophilic and hydro-
natural with hemp fiber hybrid compos- with a solution of 5 wt % NaOH/2 wt %
phobic materials in the composite pro-
ites [11,68,72]. Chaudhary et al. [73] NA2SO3. Furthermore, the crystallini-
cessing results in poor matrix–fiber
studied and compared the mechanical ty index increased by 4.4% when com-
interfacial adhesion, leading to ineffec-
properties of natural fiber-reinforced pared with untreated hemp fibers. The
tive stress transfer between the ma-
epoxy composites (jute fiber/epoxy increase in crystallinity index is thought
trix and fiber and loss of the compos-
matrix, hemp fiber/epoxy matrix, and to be due to the removal of lignin and
ite materials’ final properties [4].
flax fiber/epoxy matrix) and hybrid com- pectin, which allowed a better packing
posites (jute and hemp fiber/epoxy Chemical and physical treatments can of the cellulose chains [24].
matrix, hemp and flax/epoxy matrix, improve the matrix–fiber interfacial
as well as jute, hemp, and flax fiber/ bonding, by changing the surface po- The properties of untreated and alka-
epoxy matrix). Jute fiber-reinforced larity and increasing fiber roughness, line-treated hemp fiber/carbon fiber
epoxy composites have better flexur- allowing a better wettability of the fi- hybrid reinforced polyester composites
al strength (85.59 MPa), and hybrid bers in the matrix. In the literature, were investigated by Ramesh et al.
composites also showed improve- more research has focused on chemi- [81]. The mechanical properties of the
ments in mechanical properties, where cal treatments than on physical ap- alkali-treated hemp/carbon fiber-rein-
hemp and flax fiber hybrid reinforced proaches with better improvements forced hybrid polyester composites
epoxy composites had the highest ten- [19]. Chemical treatment involves the were found to be superior as well as
sile strength, elastic modulus, and modification of the fiber’s hydroxyl and have long-term moisture resistance as
impact strength of 58.59 MPa, 1.88 carbonyl groups, introducing other in- compared with the untreated fibers.
GPa, and 10.19, kJ/m2, respectively, teracting groups that effectively inter- However, at high concentrations of
while jute and hemp fiber hybrid rein- lock with the polymeric matrix at the NaOH, Mwaikambo and Ansell [82] and
forced epoxy composites achieved the interface [75]. Chemical treatments Pickering et al. [24] observed a reduc-
maximum flexural strength of 86.6 include alkali, acetyl, silane, benzyl, tion in thermal resistance and a de-
MPa. acryl, permanganate, peroxide, isocy- crease in strength in the hemp fibers,
anate, titanate, zirconate, and acryloni- which were attributed to the degrada-
Similar results were found by Maslind
trile treatments, whereas physical tion of cellulose. Sawpan et al. [83]
et al. [74] in their research, in which
treatments include corona, cold plas- demonstrated that alkali and silane
they investigated the effect of water
ma, ultraviolet (UV), and heat treatment treatments applied to hemp fiber-rein-
absorption on tensile and flexural prop-
electron radiation [19,76]. forced PLA composites improved ten-
erties of hybrid composites consisting
sile strength and Charpy impact prop-
of interwoven kenaf/jute and kenaf/ Alkali treatment, also known as mer-
erties as the result of better wettabili-
hemp yarn-reinforced epoxy matrix. It cerization, is one of the most used
ty of the fibers in the matrix and an
was observed that the mechanical chemical treatments for natural fibers.
increased matrix crystallinity index.
(tensile and flexural strengths) and KOH, LiOH, or NaOH is usually used
water-resistant properties of the inter- in the process of mercerization [6,77]. Sullins et al. [54] studied the effects
woven hybrid composites were supe- Alkali treatment removes a fiber’s of 5 wt % MAPP (maleated polypropy-
rior to those of the individual woven chemical constituents, such as hemi- lene), 5% and 10% NaOH-treated
composites. cellulose, lignin, and pectin, which in- hemp fiber, and 5% NaOH + 5 wt %
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