The gel point was taken as the time when the sol did not show any movement on turning the container upside down.For analysis of the binder,the left-over sol contaminated with leached out hemp shiv extractives was aged in a container until the gel point was reached and the specimen was termed the “binding matrix”.A schematic illustration of silica glass has been presented in Fig.1.The EDX analysis shows the surface composition of the silica specimens.The percentage of carbon is significantly higher in the binding matrix than the silica glass.The presence of carbon in the silica glass is due to the alkyl groups providing functionalisation.The chemical composition of the silica glass and the binding matrix was determined by X-ray photoelectron spectroscopy.The atomic percentage of various elements present at the sample surface was determined by a low-resolution survey scan.The relative elemental composition for the specimens is listed in Table 2.The main elements detected for both the silica specimens were carbon,oxygen and silicon.The binding matrix showed higher content of carbon as seen in Fig.3.A high-resolution scan was performed on the C1s region for the silica glass and the binding matrix to determine the type of oxygen-carbon bonds present.The chemical bond analysis of carbon was performed by curve-fitting the C1s peak and deconvoluting it into four sub peaks corresponding to unoxidized carbon C1,and various oxidised carbons C2,C3 and C4.The binding energy,corresponding bond type and their relative percentage are listed in Table 3.The silica based binder shows additional oxidised carbon sub peaks,C3 and C4.The C1s high resolution spectra with the deconvoluted peaks for silica glass and binding matrix are represented in Fig.4.The C1 peak is related carbon-carbon or carbon-hydrogen bonds whereas C2,C3,and C4 peaks are associated with carbon-oxygen bonds.The identification of the extracted compounds was performed using GCMS.The polar components of the extractives were analysed for identification of the lipophilic extractives which are responsible for their tacky nature and would contribute to the adhesive properties of the binding matrix.
The yield of total extractives in hemp shiv was 6.23%.The hexane yield and methylene chloride yield in the total extract was 9.05% and 5.00% respectively.The chromatographs for hexane extract and methylene chloride extract are presented in Figs.6 and 7 respectively.All the compounds identified by GCMS are listed in Tables 4 and 5.The individual compounds were identified based on a comparison with GC retention times and mass spectra from the NIST library.Over twenty compounds were identified in the hexane extract and twelve compounds were identified in the methylene chloride extract.For the analysis of the GCMS data,cannabis square pot peaks lower than 30000 counts were rejected.From the chromatograms,it was determined that fatty acids esters,mainly lauric acid and phthalic acid,gave the highest peaks.The compression testing of the composite samples prepared with hemp shiv and binding matrix is imaged in Fig.8 and stress versus strain curves for the before and after immersion samples are presented in Fig.9.The moisture sensitivity of the composite was determined by comparing the mechanical properties of the hemp shiv composite before and after immersion in water for 24 h.Preparation of composite samples using hemp shiv and ethanol-water solution was unsuccessful as the hemp shiv particles were not able to bind.From Fig.9,the results from three test samples before immersion reveal that the composite reaches an average compressive stress of 0.48 ± 0.02 MPa at 30% strain.After the immersion test,a slight reduction in compressive stress by 15% was observed for the three samples and the average reading was 0.41 ± 0.01 MPa at 30% strain.It was noted that further compression led to densification of the sample.After compression,the sample showed some elastic behaviour as seen in Fig.8.In the present study,hemp shiv based composites have been manufactured by using silica sol as a binder.The binding matrix has been characterised and its morphology,chemical composition and physical properties have been studied in comparison with silica glass.
The binder is prepared by the hydrolysis and condensation of TEOS in water in the presence of ethanol as the mediator solvent.HDTMS is added for functionalisation thereby providing hydrophobic alkyl groups in the silica network.The formulation has been used earlier for treatment of hemp shiv particles for imparting hydrophobicity to the material.Here we report the binding properties of silica when mixed with hemp shiv.The silica sol interacts with hemp shiv leaching out extractives and waxes which leads to visual changes turning the silica matrix fromcolourless transparent to yellowish opaque.The silica is able to covalently bond to hemp shiv through the hydroxyl groups of cellulose.During the drying process,the gel starts condensing,releasing ethanol and water and develops a silica network.The extracts from the shiv that are entrapped in the silica network alter the characteristics of the silica.From the SEM analysis,it was seen that the silica morphology is modified.The structure of the new modified silica with incorporated extracts is less brittle when compared to the pure silica glass.The chemical composition of the silica specimens is mainly composed of carbon,oxygen and silicon.Chemical characterisation using EDX reveals that the modified silica has a higher carbon content than the pure silica.Detailed XPS analysis indicates that due to sol interaction with hemp shiv,the silica chemistry has been significantly altered.The surface carbon content of the binding matrix increased by 27%.On the other hand,the oxygen content decreased by 21%.This change in C/O ratio and increase in the surface carbon content can be attributed to the additional extracts that have been identified in the modified network of the binding matrix.The decrease in surface oxygen content can be related to the masking effect of the hemp shiv extracts reducing the detectability of the oxygen bonds in the silica network.The C1s high resolution XPS spectra reveal that the hemp shiv extracts have modified the silica network leading to the appearance of C3 and C4 peaks which are not present in the pure silica glass.Furthermore,the increase in the intensity of the C1 component for the binding matrix from 68% to 84% indicates the presence of CeC and CeH bonds from the incorporated extracts.To analyse the extracts that were leaching out from hemp shiv during the silica based treatment,the process was simplified by using a solution of ethanol and water for the extraction process.
Ethanol is able to dissolve waxes and isolate lipophilic extractives.These ethanol-soluble extractives were analysed using GCMS and it was found that the extract was mainly composed of lauric acid and phthalic acid with many other fatty acids.The majority of the compounds identified using GCMS belong to the group of lipophilic extractives which are hydrophobic in nature.This could possibly be one of the factors for the compatibility between the lipophilic extractives and the sol-gel chemistry due to their hydrophobic nature.The thermal decomposition patterns of the silica specimens were studied by TGA.The binding matrix had a higher weight loss below 100 °C and a greater endothermic peak that can be attributed to the presence of fatty acids in addition to the physically adsorbed water.The embedded extracts in the silica network changed the decomposition range of the organic fragments of the silane corresponding to the temperature range of 270–600 °C.Due to the higher percentage of the organic compounds in the binding matrix,the weight loss was greater and a peak shift was observed in the first derivative of the weight loss thermogram.The maximum decomposition rate in the DTG curve for silica glass was at 520 °C attributed to the loss of silanol groups.The modification of silica network with hemp shiv extracts lowered the thermal stability of the binding matrix.Composites were prepared using hemp shiv and silica sol and their mechanical performance was evaluated.The composites were light weight with a density of 175 kg/m3 and the compressive stress of 0.48 MPa attained at 30% strain is relatively good when compared to other hemp shiv based composites such as hemp-lime,hemp-starch and hemp-clay.Higher strains corresponded with higher compressive stresses leading to densification of the sample without reaching a failure point.This suggests that the interfacial adhesion between the shiv and binding matrix is good and the shear forces are low.After the immersion test,the decrease in mechanical strength can be related to the swelling of the shiv when placed in water for 24 h.Since the binder also provides hydrophobicity to the hemp shiv,the compressive stress versus strain characteristics are not compromised to a great extent.However,the swelling could be related to the slow penetration of water through micro-cracks on the coated surface or due to the presence of small uncovered pores within the hemp shiv.The binder can provide hydrophobicity to the hemp shiv but it cannot fully protect the hemp shiv against long-term water interaction.The slight decrease in compressive stress reached at 30% strain can be attributed to the weakening of the interfacial bonding between the hemp shiv and the binding matrix.However,composites produced using an ethanol-water mixture instead of silica sol was unsuccessful as the hemp shiv fell apart on demoulding.The ethanol is responsible for isolation of the extractives and waxes from hemp shiv but the extractives cannot bind hemp shiv on their own.
The extractives modify the silica chemistry and the binding matrix holds the hemp particles together resulting in the production of coherent composite blocks.When compared to conventional hemp-lime composites,it is evident that the production costs of the hemp-silica composites would be higher due to the hydrophobic treatment on hemp shiv.However,this cost could be off-set by savings elsewhere,both in production ingredients as well as an extension in service life,potentially reducing the whole life cost.Moreover,trim tray the commercial availability of sol-gel solution on an industrial scale would significantly lower the cost of this novel composite.The preparation of hemp-silica composite results in the reduction of 2 L of mixing water per 1 kg of hemp shiv when compared to a conventional hemp-lime composite.The thermal performance of the new composite is expected to be better due to their significantly lower density than hemp-lime.Overall early indications are that the global warming potential of this composite would be approximately 5% lower than that of a conventional composite.The life span is expected to increase by 50% due the improved resistance to water that is responsible for degradation of the composite.Hemp is a genetically diverse and variable crop that produces raw products in three distinct categories: seed/oil,fiber,and metabolites.Within each category,hemp can be improved by multiple avenues of research.We highlight key research areas which increase grower yield or product quality for processors.These topics are not exhaustive,but are intended to guide research to areas which are of the highest priority.Notably,due to the diverse nature of raw products produced from hemp,research targeting hemp yield traits will improve our understanding of basic plant biology.Seed and oil research will enrich our comprehension of grain yield and composition.Research into hemp fiber will enhance our knowledge of stem development and composition,genetic regulation of fiber traits,and biofuel production.Studies targeting metabolite yield will expand insights into both Cannabis-specific and shared plant chemistries,interaction with biotic stresses,and trichome development.Investigations into the plasticity of hemp’s sexual phenotype will contribute to identifying mechanisms underlying plant sex determination.Importantly,unlike previously domesticated crops,selection for increased hemp yield provides a unique opportunity to study plant domestication for grain,fiber,and chemistry traits.With hemp,unlike most other crops,these valuable characteristics can be studied within a single species for which they are essential to sustainable and profitable production.As a semi-domesticated crop,many traits for hemp seed and oil yield require improvement; these include seed size consistency and improved shattering resistance.Significant advances in hemp seed production occurred with the development of FIN-314 ,an auto flowering grain variety with a short stature,adaptation to high latitudes,and high yield,resulting in it presently being the most popular cultivar grown in Canada.However,seed size is highly variable among hemp cultivars and ‘Finola’ seeds are 50% the size of many commercial varieties.During domestication,hemp has retained little resistance to shattering.However,hemp field trials have revealed that significant grain is lost due to shattering prior to and during harvesting as a result of inconsistent inflorescence maturity ,particularly if collected outside of the optimal harvest time windows.To mitigate this problem,growers harvest seeds at 70% maturity ii.Hemp inflorescences are large multi-seeded heads in which each individual seed is partially surrounded by a bract,and an abscission zone connects the hull to the pedicle.Selection for a stronger-walled abscission zone or the prevention of bracts releasing seeds are possible physiological traits to target to reduce hemp seed loss due to shattering.Furthermore,immature seeds are similar in size,but weighed only half that of mature seeds due to incomplete embryo development.