Hemp’s central woody portion contains 36% cellulose and 27% lignin,whereas bast fiber contains 72% cellulose and 4% lignin.The whole hemp stem contains 47% cellulose and 18% lignin,which is more favorable than pine and birch wood.Hemp stalk contains the highest percentage of cellulose,with the lowest lignin content over almost all non-woody stalks.However,hemp bast fiber secures second for alpha-cellulose after cotton.The lignin and cellulose content in hemp stalks considerably vary among cultivars and growing seasons.The morphology and low lignin content of hemp bast fiber allow easy penetration of processing chemicals resulting faster pulping process with less harsh chemical use.Hemicellulose enhances swelling of the pulp and bonding capacity among the fibers and provides the necessary strength to paper.The hemp core has a higher ability to supply alpha-cellulose to fibrous suspensions and a similar amount of hemicellulose than that of kenaf and hardwood.Hemp fiber exhibited high yield and high tear strength of paper obtained by organosolv pulping process due to having long fibers in pulp.It allows a safer oxygen delignification process with a higher yield than that of birch or pine pulps.The organosolv pulping uses organic solvents to solubilize lignin and hemicellulose,which avoids environmentally harmful sulfur used in conventional pulping techniques,offers high-quality lignin recovery,and easy removal of solvents with less water consumption compared to kraft pulping process.The organosolv process,trim tray for weed which utilizes ethanolamine,produces high quality hemp core pulp.
Moreover,hemp paper processing does not require harmful chlorine bleach; instead,it can use peroxide,which is safer for the environment.In searching for a sustainable pulping process from non-wood feedstock,BioRegional MiniMill technology has been claimed as a promising zero-emission environmentally friendly process for small-scale production.Alkaline pulping of woody hemp core enhances fibrillation and forms inter-fiber bonding which imparts strength in paper and can produce smooth printing grade paper as produced from straw and hardwood pulp.A combination of hemp hurd pulp with hardwood exhibited improved tensile index,bursting strength,softness with identical water absorbency of hand tissue.Blending hemp fiber or woody core with pine and eucalyptus demonstrates higher fiber length in pulp,hence the sufficient paper strength.In most cases,only hemp bast fibers are used for paper-making purposes because of outstanding tearing strength,although lower tensile strength.The successful use of hemp as paper-making materialdepends on the technical viability of exploitation of both fiber and woody core,not simply one since their differences in morphology and chemical compositions offer suitability for paper-making raw materials.The bleached kraft pulps from hemp stalk were found useful raw material than hemp bast fiber or woody core alone .Organosolv pulping process of whole hemp stalks exhibited the strength between commercial hardwood and softwood pulps.Hemp with core and sheath showed a slightly higher kappa number than eucalyptus,indicating that it is more difficult to delignify in kraft cooking process due to lower degree of core fiber polymerization.Strengths are affected due to low hemicellulose content and a lower degree of internal fibrillation.Hemicellulose helps the pulp sheet by strengthening its tensile,bursting,tear strength,specific surface,folding endurance and opacity.The elementary chlorine-free bleaching method with enzyme treatment is not suitable for hemp stalks since it deteriorates the physical properties.It was suggested that separate pulping of fibers and cores followed by blending could yield a pulp with desired properties for multipurpose applications.
Unlike wood paper,hemp papers survive hundreds of years and do not get into decomposition and yellowing due to aging.Hemp paper’s high tear strength and wet strength make it ideal for currency paper,art paper,cigarette paper,tea bags,specialty non-wovens,carbon tissue,grease-proof paper,and other applications.The paper made from hurds can also be used as low-grade printing paper without compromising the quality of the printing surface.Hemp paper showed better oil/air filtration performance than cotton paper in practical application.As a non-wood fiber,hemp also has considerable shortcomings for industrial-scale paper making.Existing paper-making technology is not well capable of handling non-woody crops,and it significantly decreases productivity.In most cases,fibers are separated from the woody core and go through the process separately.The processing technology of hemp woody core and even the separation method of bast fibers must be introduced with state-of-the-art knowledge to reduce costings.Technological advancement is also required to process the whole stalk without separation of fibers and woody core for achieving more yield.Since hemp is an annual crop,and its stalks are very bulky,some difficulties are associated with transportation,storage capacity,and smooth handles.Long-time storage can damage the fibers and cause color change.Pulp quality and yield is related to morphology and chemical composition; they can be affected by harvesting time,geographic location,plant breeding,and even the variation are observed within the biomass,which may lead to a possible lack of consistency to paper quality.The globally produced agricultural residue could produce five times more paper than demand,but technology fails to exploit those in the paper industry.According to the study on different pulping methods of hemp bast fiber,it is found quite acceptable in place of wood from the technical point of view,and even woody cores can also be a promising raw material for paper.Hemp pulps are generally used by mixing with wood pulp,and currently,100% hemp paper production is not seen.Hemp is speculated to take the opportunity to outvie the other non-wood plants by its yield and grow ability in moderate or boreal climates.Easy pulping,good quality bleached paper and specialty papers are the key advantages of hemp.Hemp,the second largest grown bast fiber after jute,has gained considerable attention as a reinforcement in the polymer matrices in the last decade due to its renewable,biodegradable,and recycling properties.
Several composite processing methods have been investigated,including hand lay-up,film stacking,vacuum infusion,manual winding,filament winding,resin transfer molding,pultrusion and injection molding,to name a few.Hemp fiber composites with thermoset,thermoplastic and biodegradable matrices have demonstrated strong mechanical properties due to higher tensile strength of fiber up to 1110 MPa.Table 2 summarizes some important physical and mechanical properties of vegetable-based natural fibers.Thermoplastic matrix composites perform better over thermoset matrix composites concerning high specific strength,corrosion resistance,cost efficiency,recyclability and design versatility.But the main drawback of natural fiber-reinforced thermoplastic composite is that their processing temperature needs to be kept below 230 C to protect them from thermal degradation .Hemp fiber begins to degrade at a temperature above 150 C,hemicellulose and pectin decompose at around 260 C,while cellulose decomposes at about 360 C .Hemp fiber-reinforced thermoplastic composites are typically made of polyethylene,polyurethane,or polypropylene matrices.It shows the potential to replace synthetic fiber composites in many lightweight and low-cost applications .The frequently used thermoset matrices for hemp fiber-reinforced composites are epoxy resin,phenolic,vinyl ester,or unsaturated polyester resin.Composites made from hemp fiber with thermoset matrices are creep resistant,solvent resistant,and tough.W€ otzel et al.investigated a life cycle study on materials reinforced with hemp fiber against ABS for car parts.Their study revealed that cumulative energy demand for producing hemp composite was half compared to the ABS basic component and hemp composite for an inner lining of cars showed more ecological benefits.Green composites are referred as bio-composites,the combination of natural fibers with biodegradable polymeric materials.Researchers focused on green composite due to dwindling fossil fuel resources and their negative impacts on environment.The biodegradable polymer matrices such as epoxidized soy oil ,acrylated epoxidized soybean oil,starch-based emulsion,cashew nutshell,euphorbia oil,cellulose acetate and polylactic acid were used to develop hemp fiber-reinforced green composites in number of studies and outstanding mechanical properties for primary structural applications were observed.Though cost and service longevity due to decomposition in nature are associated as drawbacks with green composites further study may open new door to overcoming these barriers.
Mechanical properties of fiber-reinforced composites strongly depend on fiber length,diameter,orientation,degree of dispersion,aggregate formation and fiber-matrix compatibility.Hemp is susceptible to thermal and oxidative degradation during processing.Another key impediment to manufacturing hemp-reinforced composites is the inability of hydrophilic lignocellulose fibers to adhere to hydrophobic matrices.Due to their polar surface character,natural fiber reinforcement shows lower compatibility with strongly apolar thermoplastic matrices.Again,vapor and void creation during processing due to moisture content,photodegradation due to UV radiation,and poor resistance to moisture can significantly affect the performance of the composite for outdoor application.A summary of the performance of hemp composite against moisture can be seen in Table 3.Hemp composites absorb moisture,and when dry,the constituent fibers shrink rapidly.Thus interfacial bonding of composites no longer can retain due to cyclic absorption and desorption of moisture and eventually debonding cracks occur inside the structure which allows room for further water penetration.The schematic diagram of the mechanism is illustrated in Figure 2.Alongside the formation of cracks,moisture uptake also encourages colonial fungal growth that further accelerates the degradation of the composite.Several researchers studied the relationship between fiber fraction and amount of moisture absorption and concluded that for natural fiber,moisture absorption increases with increasing fiber loading.For solving these inherent limitations,many studies were carried out on physical and chemical modification of natural fibers to enhance surface characteristics and effectiveness as a reinforcement material.Coupling agents were used in some experiments in chemical modification and better compatibility was achieved between cellulose fibers and hydrophobic polymers.Treating hemp fibers with methacrylic anhydride increases interfacial adhesion between hemp and polyester matrices.Mwaikambo and Ansell found that hemp fibers with a 4% & 6% alkalized treatment had the highest modulus and tensile strength.Propionylation and acetylation treatments on hemp fibers resulted in a reduced hydrophilicity but at the same time,decreased crystallinity slightly.The Scanning electron microscope results showed that the esterified materials’ surfaces were smoother than the untreated hemp fibers.Dayo et al.studied the influence of different chemical treatments on hemp composite and recorded the lowest water absorption for silane treated fiber.The same author also reported a similar result from another experiment where only washed,trimming tray weed alkaline treated and silane treated hemp fibers were compared.
Among the resins,Polybenzoxazine polymers showed better resistance against moisture absorption with additional advantages like good mechanical and thermal properties.Oza and Lu investigated the effects of silane and NaOH treatment on the thermal and thermomechanical properties of hemp fiber-reinforced high-density polyethylene composites.They observed that thermal stability decreases as fiber loading increases,and treated fiber composites show higher thermal stability than untreated fiber composites.The storage modulus of treated composites was higher than that of untreated composites in dynamic mechanical analysis.Up to 40% fiber loading,the storage modulus value increased,while it dramatically decreased at 50% fiber loading.It was found that the storage modulus of Silane-treated composites was higher than NaOH-treated fiber composites.A similar achievement was also reported by Dayo et al..A summary of hemp composites’ mechanical,thermal and thermomechanical performance can be seen in Tables 4 and,respectively.Hemp fibers have significant advantages over synthetic fibers in reinforcing composites and can be used efficiently for a variety of applications because of their high specific strength,low density,low production cost,bio-renewable nature and eco-friendly behavior.The applications of hemp-reinforced composites had been traced in the automotive industry in the 1940s,where Henry Ford produced car components from hemp fiber with soybeans-based bio-matrices.It is predominantly used in the automotive sector to reinforce door panels,passenger rear decks,pillars,and boot linings .Compared to other natural fibers,its uses have remarkably increased in the German and Austrian automotive industries.Due to the higher vibration damping capacity of hemp fibers,researchers also focused on hemp composites in manufacturing sporting goods and musical instruments.Claudio and Marco developed electronic racks for the helicopter by utilizing hemp fabric/epoxy composite materials.The study revealed that this electronic rack from hemp composite was 55.6% lighter than existing steel electronic racks.Hemp fiber reinforced with polycaprolactone composites proved their potential application in fabricating orthotic devices .Hemp chair was developed from hemp yarns with epoxy resin,and Xia et al.innovated a hybrid composite from hemp fiber mats and aluminum sheet with epoxy resin which offered excellent electromagnetic interference shielding performances.The term “plastic” refers to a material’s flflexibility or ability to deform into any shape without breaking.Plastic is a carbon chained polymer allowing it to be molded into any shape; that is why they are the most adaptable material.The majority of monomers used to make plastics like ethylene and polypropylene are derived from fossil fuel hydrocarbon.As a result,they are neither biodegradable nor easily decomposable; instead,they accumulate in the landfill and the natural environment.According to literature,roughly 9% of all plastics produced are recyclable,while the remaining 79% end up in landfills and the atmosphere.Petroleum-based plastics and its by-product have a devastating effect on the land,water,and wildlife.For this reason,increasing demand has been started in the world for the usage of high-performance bio-based plastics capable of being environmentally friendly and compensating depleting of petroleum resources.Hemp plastic which is 100% biodegradable,can be a better alternative to synthetic plastic.The cellulose of the hemp plant is rated 60–70%,which can be extracted for making a different range of plastics,including rayon,celluloid and cellophane.