In the present study natural fiber composites,containing 10%,20% and 30% hemp fibers as reinforcement and mixture of virgin and recycled high density polyethylene as matrix,have been fabricated.The tensile and flexural properties of the composites are evaluated and compared with samples made of 100% virgin HDPE and 50-50 mixture of virgin and recycled HDPE.In the last years,considering the increasing environmental awareness and the introduction of new rules and regulations to reduce the environmental impact and reach sustainability,the new products must minimize the environmental impact,so the interest and the subsequent use of eco-friendly materials is grooving more and more in different application fields.This aspect is particularly relevant in the composite manufacturing field,where both synthetic fibers and petroleum polymers are still widely used.In this contest,the use of natural fibers as reinforcement is reaching an increasing attention,indeed a lot of researches have aimed on the study of a range of recyclable materials based on natural fibers such as flax,ramie,sisal,hemp and many more in order to study their possible use as interesting substitute over the conventional ones.Among various kinds of natural fibres,the hemp one is one of the most promising because of its interesting properties such as the low density,low cost and high specific mechanical properties.In addition,the hemp plant is characterized by the ability of extracting heavy metals from the soil makes and the environmental conditions required from its cultivation allow the easy growth of this plant around the world.Regarding the matrix used for the natural fibre composites manufacturing,commercial greenhouse supplies its selection is limited by the temperature at which natural fibres degrade.Both thermoset and thermoplastic polymers are coupled with natural fibres and each one highlights its peculiarity characteristics: thermoplastics are capable of being repeatedly softened by the application of heat and hardened by cooling and have the potential to be the most easily recycled,on the other hand a better emphasis of the fibres mechanical properties are generally achieved by using thermosets as matrix.

In fact,thermoset polymers are particularly attractive as matrix materials for natural fibre reinforced composite production as they generally have reactive functional groups that make them compatible with hydrophilic fibre surfaces.Among these,the epoxy is one of the most interesting polymer resins that used as matrix for the natural fibre composites shows very high mechanical properties of the final product.Several applications as interior and insulation components of hemp/epoxy composites was largely found in literature in particular in the automotive and building sectors,whilst there are few applications as structural components and few works aimed on the study of manufacturing processes to produce hemp composites characterized by low density and high specific mechanical properties.Based on this overview,this work is focused on the manufacturing of lightweight hemp/epoxy composites through a new process easily implemented in industrial production by using hemp fabrics characterized by a large mesh size.To assess their mechanical performances,tensile and bending tests were performed and coupled with FE simulations.Therefore,due to the no optimum control in the fabric superimposition,even though the fibre weight content is the same for each sample typology,the difference in the thickness vales is directly connected with the difference in the density and in the fibre volume fraction values.Ideally,considering for the B type a good alignment able to obtain the same thickness of the A type sample and also considering that the B type samples contained half number of tows then the A type,the ideal values of the density and of the fabric volume fraction should be half than the ones of the A type.Figs.8 and 9 shows the tensile and bending stress-strain curves for the specimens under investigation.The composites with density of around 0.74 g/cm3 are characterized by a tensile strength of 40 MPa and a tensile modulus of 3000 MPa,whilst the composites with a density of around 0.47 g/cm3 showed a tensile strength of 24 MPa and a tensile modulus of 1650 MPa; the tensile properties of the B type are approximately half of the ones of the A type.Looking at Fig.9,it is possible to note that all curves show the same behavior up to the failure,but when the maximum stress value was reached and then the specimens started to fail,the behavior changes.

This is observed by comparing the stress-strain curves and the type of failure of the specimens show in Fig.10.The curves of the A type show a significant drop in the stress and then a brittle failure of the specimens was observed,whilst the curves of the B type show a slighter decrease in the stress coupled with a sliding failure.This is due to the higher presence of resin on the bottom surface of the A type specimens.The composites with a density of 0.77 g/cm3 showed a bending strength of 40 MPa and a bending modulus of 3100 MPa,instead of the samples with a density of around 0.47 g/cm3 that was characterized by a bending strength of 22 MPa and a bending modulus of 1200 MPa.It is also possible to observe that the flexural mechanical response variability is in any case quite limited.However,a slightly greater value was reached for the B type specimens due to above said misalignment problems that mainly occurs for this specimen typology.This is also corroborated from the results of the FE simulations of the three-point bending tests where the eventual misalignment error was considered.Illicit marijuana use in the United States has been a longstanding public health concern for both adolescents and adults.As many as 44% of college-aged individuals endorse having used marijuana at some point in their life,and 21% of college-aged individuals report marijuana use in the past 30 days.Marijuana intoxication is associated with motor coordination deficits,euphoria,impaired temporal estimation,and a variety of other psychological phenomena.Marijuana use has also been associated with more specific cognitive deficits,even after acute intoxication has subsided,and with the development of severe psychopathology.Furthermore,chronic marijuana use has been related to adverse physiological consequences in the cardiovascular and respiratory systems.Adolescence and young adulthood represent periods of the lifespan when increased risk-taking occurs,including the use of illicit substances,such as marijuana.The combination of an innate propensity for risk-taking and use of a judgment-altering substance is a striking example of the immediate public health concern over marijuana use in young-adults.This concern is particularly pertinent in light of recent efforts in support of marijuana’s legalization in the United States.A challenge for the field is to identify which chemical systems and associated information processing networks are most affected by chronic marijuana use.

The main psychoactive component of marijuana,Δ9 -tetrahydrocannabinol,acts as an agonist in central nervous system cannabinoid receptors and in other peripheral cell types,primarily immune cells.In the CNS,CB1 receptor density is high in the basal ganglia,particularly in the dorsal striatum.Cannabinoid receptor signaling acts on multiple neurotransmitters through a variety of biochemical cascades,including inhibition of voltage-dependent calcium channels and by directly inhibiting vesicle release.Both excitatory and inhibitory neurotransmitters,including glutamate,γ-aminobutyric acid and dopamine,are either directly or indirectly affected by CB1 receptor activation.For marijuana and other drugs of abuse and dependence,the dorsal striatum has been hypothesized to play a key role in the transition from intermittent drug use to compulsive habit-based drug-taking via mechanisms that underlie long-term synaptic plasticity.Exogenous activation of CB1 receptors,as occurs with marijuana intoxication,inhibits the release of glutamate as well as GABA in both the dorsal and ventral striatum.This inhibition facilitates the development of long-term depression in the striatum,which is a critical component in the altered synaptic plasticity that accompanies drug addiction.Thus,the manner in which corticostriatal functional connectivity is altered in the context of marijuana use is of interest,as is metabolic activity within the chemical systems that contribute to those alterations.Magnetic resonance spectroscopy is a widely used tool,allowing for in vivo characterizations of various brain metabolites.MRS data is acquired either from single voxel or multiple voxels.The SVS method typically benefits from high spectral resolution and signal-to-noise ratio.MRSI has better spatial resolution compared to SVS,but typically has a much more limited spectral resolution.To the best of our knowledge,only four other studies utilizing some form of MRS to examine marijuana users have been published,and the methods of these studies are relatively heterogeneous.The existing studies are summarized in Table 1.

Individuals ages 16-to-42 years were studied with either SVS or MRSI.In two of the studies,only males were examined.In most cases,marijuana use was reported at 20 or more days per month.Lower levels of Glu,N-acetylaspartate,and myo-inositol were observed in marijuana users compared to controls in regions known to be associated with substance use,cannabis dry rack including the basal ganglia,thalamus,cingulate cortex,dorsolateral prefrontal cortex,and the striatum as well as posterior cortical regions.The methods,ages of subjects,and extent of current marijuana use in the samples tested vary considerably across studies as summarized in Table 1.As disruptions in glutamate activity have been implicated in the development of addiction,we hypothesized disruptions in glutamate concentrations in marijuana users compared to controls.Several lines of evidence suggest inhibition of glutamate excitotoxicity by marijuana.In addition,based on the MRS literature described above related to the basal ganglia of adult marijuana users and literature describing the inhibitory effects of CB1 receptors on glutamate release,we specifically hypothesized that young-adult MJU subjects would show lower levels of Glu + glutamine in the basal ganglia compared to their non-using counterparts.We did not have a specific hypothesis regarding concentrations of other metabolites given that other researchers have not concentrated their assessments on the striatum.However,the limited available literature suggested the possibility of altered mIns as well as NAA levels in users versus controls.Twenty-seven marijuana users were recruited into the study through local advertisements on the University of Minnesota-Twin Cities campus.Marijuana users’ ages ranged from 18-to-21 years,with a mean and standard deviation of 19.5 ± 0.6 years.Exclusion criteria are described below.Twenty-six healthy young adult non-users,who were participants in a large,longitudinal study of normal brain development,served as a control sample.Control participants’ ages ranged from 13-to-24 years,with a mean and standard deviation of 19.3 ± 3.1 years.The recruitment strategy for the control sample has been described elsewhere.Briefly,participants younger than 18 years of age were recruited through a database of research volunteers throughout the Metro community,through post-cards mailed to University of Minnesota civil service employees,and through local advertisements.Participants over the age of 18 years were recruited using on-campus advertisements.During the controls’ third longitudinal follow-up visit,MRS was added to the protocol as time allowed.Thus,the control sample described in this study has a broader age range than the MJU sample,a feature that was considered in the statistical approach described below.A description of the study was initially given to both the MJU and control participants over the phone.Interested participants were then invited to complete a brief phone screening to ascertain study eligibility.Exclusion criteria included major physical,neurological or psychiatric illness,substance use disorders,head injuries resulting in loss of consciousness >20 min,mental retardation,learning disabilities,current use of psychoactive medications,non-native English speaking,vision or hearing that was not normal or corrected to normal,complications at birth,current pregnancy,and MRI contraindications.Inclusion criteria for MJU participants included current use of marijuana at least five times per week for at least one year,and an age of onset of use prior to the age of 17 years.Marijuana users were also excluded if they were daily cigarette smokers,or if their alcohol use exceeded four drinks for females and five drinks for males on more than two occasions per week.Marijuana users were asked to refrain from drug use for at least 12 h prior to their visit to avoid acute intoxication during study procedures.Participants provided written informed consent and all study procedures were approved by the University of Minnesota’s Institutional Review Board.After the phone interview,eligible participants were invited to the University of Minnesota’s Center for Neurobehavioral Development for an in-person screening session to further ascertain eligibility and to verify information given over the phone.The Kiddie Schedule for Affective Disorders and Schizophrenia,Present and Lifetime version was used to assess for current or past Diagnostic and Statistical Manual,Fourth Edition axis I disorders,including childhood disorders given the relative youth of the sample.The presence or absence of DSM-IV disorders was confirmed by case consensus meetings with staff members including a license-eligible clinical psychologist.In addition,a two-subtest version of the Wechsler Abbreviated Scale of Intelligence was administered to yield estimated full scale IQ.Participants who met all inclusion criteria after the in-person interview were invited back for a comprehensive neuropsychological testing battery and an MRI scan.