Isocyanate Content Research Articles

Identification of Isocyanate Number on IPDI and TDI Due to Storage Period and Their Impact on Composite Solid Propellant

Isocyanate compounds serve as a curing agent in the production of composite propellants, playing a crucial role in determining their characteristics. Due to its high reactivity with moisture, it is essential to determine the isocyanate number of stored samples after a specific duration. The investigation identified the isocyanate number of IPDI and TDI that had been held for 84 and 90 months. The FTIR analysis identified the presence of the NCO group at 2240 cm-1 for TDI and 2243 cm-1 for IPDI samples held for 84 and 90 months. The isocyanate number identification decreased by 1-2% after being stored for six months. This reduction in isocyanate quantity undoubtedly impacts the development of propellant compositions. The reduction in isocyanate content will alter the propellant formulation, leading to propellants with different characteristics due to its impact on the curing ratio (NCO/OH). The curing ratio greatly impacts the mechanical properties of composite propellant.

Preparation and systematic study of mechanical properties and suspension stability of polyurethane-polyurea double-shell perfume oil microcapsules

The perfume oil microcapsules are of considerable significance in personal care and household products due to their ability to control the targeted release of perfume oil under specific conditions, which enhances the longevity and overall performance of the product. In this study, double-shell perfume oil microcapsules were prepared using polyurea (PUA) and polyurethane (PU) as wall materials. The first PUA shell was formed at room temperature to encapsulate the perfume oil by adjusting the agitation speed, protective colloid concentration, and polymethylene polyphenyl isocyanate (PAPI) content. Subsequently, the second PU shell was formulated. Mechanical property analysis revealed that PU/PUA microcapsules exhibited superior characteristics, showing a threefold increase in mechanical strength compared to PUA microcapsules. Thermogravimetric analysis and dynamic release experiments in ethanol demonstrated that the PU/PUA microcapsules possessed higher thermal stability and greater compactness compared to the PUA microcapsules. Finally, by incorporating bacterial cellulose (BC) microgels and hydroxypropyl methylcellulose (HPMC) as stabilizing agents into the laundry detergent matrix (JJ), a stable suspension of PU/PUA microcapsules was achieved for over 180 days.

High-performance and economic biodegradable composites based on polybutylene adipate terephthalate and modified lignin

Polybutylene adipate terephthalate (PBAT) is a biodegradable polymer with promising properties, but its wide-spread application is limited by factors such as high cost, suboptimal mechanical characteristics, and slow biodegradation rate. In this study, these limitations are addressed by incorporating low-cost lignin, modified with an isocyanate compound, into PBAT via an extrusion process. By investigating the effects of varying isocyanate content on the lignin surface, the optimal modification conditions at room temperature are identified over 48 h. Additionally, the maximum lignin content (5 wt%) is determined within the composite to achieve a balance between mechanical, thermal, and biodegradation properties while also considering the economic and environmental feasibility of the polymer composite. The research delved into the examination of both the minimum selling price (MSP) and global warming potential (GWP) for all PBAT-mLigA composites. The optimized PBAT-mLigA composites exhibited a notable reduction of up to 7% in both MSP and GWP when compared to pure PBAT. The predominant determinants influencing the economic and environmental potentials of these composites are the pricing of PBAT and the ratio in which it is incorporated into the proposed composites.

Optimization of Isocyanate Content in PF/pMDI Adhesive for the Production of High-Performing Particleboards.

Due to the fact that impregnation with fire retardant usually reduces the strength of the produced particleboards, this research was carried out to investigate whether it is possible to use phenol-formaldehyde (PF) resin modified using various amounts (0%, 5%, 10%, 15%, and 20%) of polymeric 4,4'-methylene diphenyl diisocyanate (pMDI) for this purpose. The need to optimize the addition of pMDI is particularly important due to health and environmental aspects and high price. Furthermore, the curing process of hybrid resins is still not fully explained, especially in the case of small loadings. Manufactured particleboards differed in the share of impregnated particles (50% and 100%). The mixture of potassium carbonate and urea was used as the impregnating solution. Based on the outcomes of hybrid resins properties, it was found that the addition of pMDI leads to the increase in solid content, pH, and viscosity of the mixtures, to the improvement in resin reactivity determined using differential scanning calorimetry and to the decrease in thermal stability in the cured state evaluated using thermogravimetric analysis. Moreover, particleboard property results have shown that using impregnated particles (both 50% and 100%) decreased the strength of manufactured boards bonded using neat PF resin. However, the introduction of pMDI allowed us to compensate for the negative impact of fire-retardant-treated wood and it was found that the optimal loading of pMDI for the board containing 50% of impregnated particles is 5% and for board made entirely of treated wood it is 10%.

Preparation and environmental analysis of biodegradable polylactic acid and modified cellulose nanofiber composites

Polylactic acid (PLA) and cellulose nanofibers (CNFs) show promise as bio-based alternatives to non-degradable plastics, which pose environmental challenges. Yet, CNFs' hydrophilicity hampers their dispersion in hydrophobic PLA. This study modifies CNF surfaces with diisocyanate compounds to create hydrophobic isocyanate-modified CNFs (mCNFs), enhancing their compatibility with PLA. One isocyanate of the diisocyanate compound is grafted onto the surface of CNF and the isocyanate content varied by adjusting grafting reaction time. PLA-mCNF composites are produced by a twin-screw extruder and show improved mechanical properties and biodegradability compared to pristine PLA and PLA-unmodified CNF composite. The environmental impacts of PLA-mCNF composites are evaluated using a life-cycle assessment method and compared to corn grain-based PLA. The optimized PLA-mCNF composite (6 h of grafting reaction) has lower environmental impacts than corn grain-based PLA, with global warming and fossil resource scarcity of 3.021 kg eq and 0.813 kg oil eq, respectively. This study underscores the feasibility of optimizing PLA composite properties by employing a small proportion of toxic diisocyanate on CNF surfaces. The findings highlight the potential of such modified composites to offer improved sustainability and performance characteristics, contributing to the development of eco-friendly materials.

Polyaspartic polyurea/graphene nanocomposites for multifunctionality: Self-healing, mechanical resilience, electrical and thermal conductivities, and resistance to corrosion and impact

Polyurea elastomers have attracted increasingly more attention due to excellent mechanical performance, and their wide industrial applications are in need of multifunctionality such as thermal conductivity. We herein prepared polyaspartic polyurea elastomers with optimal tensile strength and fracture strain by adjusting the isocyanate index and free isocyanate content. As a class of nanofiller, isocyanate-modified graphene nanoplatelets (IP-GNPs) were developed and they formed a stable, robust interface with the polyurea matrix, resulting in mechanical reinforcement and multifunctionality. The nanocomposite at 0.05 vol% of IP-GNPs revealed a tensile strength of 15.72 ± 0.67 MPa, representing an increment of 108.21% over pure polyurea, with excellent resistance to acidic and alkali corrosion. After 9 h of post-maintenance at 60 °C, the nanocomposite reached a healing efficiency up to 80.10% as driven by hydrogen bonds. An electrical percolation threshold was observed at 3.61 vol% of IP-GNP for the nanocomposites. The thermal conductivity reached 38.49 W/m K at 7.00 vol% due to the formation of the IP-GNP network in polyurea, where the electron thermal conductivity plays a dominant role. In comparison with those high thermal conductivity values obtained by back-filling polymers into a network established beforehand, this facile approach would be highly favored in industry for the development of elastomer nanocomposites with multifunctionality for many applications, e.g. smart sensors, protective coatings, energy harvesting, etc.

Influence of Isocyanate Content and Hot-Pressing Temperatures on the Physical–Mechanical Properties of Particleboard Bonded with a Hybrid Urea–Formaldehyde/Isocyanate Adhesive

Particleboard (PB) is mainly produced using urea–formaldehyde (UF) adhesive. However, the low hydrolytic stability of UF leads to poor water resistance by the PB. This research aimed to analyze the effect of hot-pressing temperatures and the addition of methylene diphenyl diisocyanate (MDI) in UF adhesive on the physical and mechanical properties of PB. The first experiment focused on pressing temperature treatments including 130, 140, 150, and 160 °C. The particles were bonded using a combination of UF and MDI resin at a ratio of 70/30 (%w/w). Furthermore, the second experiment focused on UF/MDI ratio treatment, including 100/0, 85/15, 70/30, and 55/45 (%w/w), and the particles were pressed at 140°C. All of the single-layer particleboard in this research were produced in 250 × 250 mm, with a target thickness and density of 10 mm and 750 kg/m3, respectively. This research used 12% resin content based on oven-dry weight wood shaving. The pressing time and pressing pressure were determined to be 10 min and 2.5 N/mm2, respectively. Before the tests, the board was conditioned for 7 days. When studying the effect of treatment temperature, good physical properties (thickness swelling and water absorption) and mechanical properties (MOR and MOE) were obtained at 140 °C. However, no significant difference was observed in the UF/MDI ratio between 85/15 and 55/45 using the same temperature. The increase in the MDI adhesive ratio improves the MOE and MOR values. However, the internal bond was the contrary. This study suggests that a combination of UF/MDI at a ratio of 85/15 and hot-pressing temperature at 140 °C could produce a PB panel that meets a type 8 particleboard according to the JIS A5908-2003 standard and type P2 according to the EN 312-2010 standard.

Study on Mechanical Properties of Two-Component Polyurethane Based on Multi-Scale Molecular Simulation.

Mechanical properties determine the use of two-component polyurethane materials. The compatibility of two components in the polyether polyol-MDI molecular system greatly influences the formation of mechanical properties in polyurethane materials. In this paper, we studied and evaluated the compatibility and mechanical properties of two-component polyurethane at multiple scales by combining molecular dynamics simulation with macroscopic experiments, which is an important guideline for synthesizing and preparing two-component polyurethanes. We evaluated the stability of the two-component polyurethane system by calculating the solubility parameter, binding energy, and diffusion coefficient at four temperatures with three isocyanate contents. The Perl scripting language obtained the mechanical properties of the MDI-polyether polyol system. The MD calculation results show that the solubility parameter of two-component polyurethane negatively correlated with temperature, and the intermolecular binding energy and MDI diffusion coefficient positively correlated with temperature. When the mass ratio of polyether polyol to isocyanate was 1:0.6, the solubility parameter difference between the two was 1.43 (J/cm3)1/2, the intermolecular binding energy was 531.68 kcal/mol, and the two-component system was more stable. A macroscopic direct tensile test was employed to assess the polyurethane elastomers' tensile properties. Our results show that the tensile strength of polyurethane elastomers increased with the increase in isocyanate content and decrease in temperature. Furthermore, the elongation at the break decreased, and the modulus increased, which is consistent with the law of molecular simulation.

Efficient encapsulation of isocyanates in PCL/PLA biodegradable microcapsules for adhesives

Microencapsulation of isocyanates eliminates the risks associated with their direct handling, protects the isocyanate species from air moisture, and increases the storage life. In light of this, we present a straightforward and efficient process to encapsulate isophorone diisocyanate (IPDI) using biodegradable polymers by an emulsion system combined with the solvent evaporation method. We produced spherical, disaggregated, and core–shell microcapsules (MCs) using poly(ε-caprolactone) (PCL), polylactic acid (PLA), or PCL/PLA blends as shell materials. As far as we know, it is the first time that isocyanate species are encapsulated by PLA or PCL/PLA blends. Our process leads to a production yield of 70–74% and an encapsulated isocyanate content up to 73 wt% of the MCs. All MCs showed very good isocyanate protection, especially when stored in low-moist environments. Their use as cross-linking agents for harmless, eco-innovative, and high-performance adhesive formulations for footwear was tested, for the first time, according to the specifications of industry standards. The peel tests of the adhesive’s joints revealed peeling strength values between 3.45 and 6.09 N/mm which is higher than the minimum required. The creep tests demonstrated that the use of MCs with encapsulated IPDI improves the heat resistance of the adhesive joints. Therefore, the MCs revealed an effective isocyanate release and very capable adhesive joints with no negative effects from the polymeric shell, validating the employment of these cross-linking agents in the footwear industry. Although PCL MCs were the ones with the highest IPDI content, the best adhesive formulations were produced with MCs with a high amount of PLA, due to their smaller size, good dispersibility and better distribution at the substrate.Graphical abstract

Novolac-based microcapsules containing isocyanate reagents for self-healing applications

Microcapsules (MCs) containing isocyanate compounds for use as self-healing materials in waterborne polyurethane coatings have been synthesized, in the presence of modified Novolac resins. With modification of Novolac resin, it is succeeded partial or total protection (Benzylation and Acetylation) of its hydroxyl groups. The idea here is to use a protected Novolac resin as the organic substrate for the encapsulation of the less reactive Isophorone isocyanate (IPDI) while the more reactive one, Methylene diphenyl diisocyanate (MDI), is used for the shell formation. Based on that strategy microcapsules of different morphologies and sizes were obtained, depending on the agitation conditions, as revealed using SEM and optical microscopy. Selective extraction was performed to determine the amount of the less reactive isocyanate (IPDI) stored inside the capsules through FTIR-ATR spectroscopy and isocyanate titration as well as the stability of IPDI inside the capsules over time. As determined, microcapsules based on Acetyl-modified Novolac resin encapsulated 96 wt% of IPDI monomer; this amount is about five and ten times higher than that encapsulated in MCs based on by Benzyl-modified Novolac resin or unprotected Novolac resin, respectively. At the same time, MCs based on Acetyl-modified Novolac resin were stable, maintaining approximately 80 % of the initial isocyanate content after two months of storage under inert conditions. Finally, the self-healing ability of the microcapsules was tested by adding selected IPDI-loaded microcapsules in waterborne polyurethane dispersions. It was proven that the Acetyl-modified Novolac-based MCs showed efficient healing behavior, in the absence of any catalyst, on the polyurethanes’ surfaces when scratched artificially.

Development of thermoplastic vulcanizates based on in situ synthesized thermoplastic polyurethane and acrylonitrile-butadiene rubber: the influence of the curing system

We prepared different thermoplastic elastomers (TPEs) using an internal mixer. We investigated the properties of thermoplastic polyurethanes (TPUs) produced from prepolymers with various isocyanate content and two different chain extenders to select the most appropriate TPU matrix for preparing thermoplastic vulcanizates (TPVs). Based on the results, we selected a prepolymer with moderate isocyanate content (10%) and the more flexible 1,6-hexanediol as a chain extender and prepared TPVs by dynamic vulcanization with in situ produced TPU. The rubber phase was acrylonitrile-butadiene rubber (NBR) with peroxidic and sulfuric curing with different accelerators. Dynamical mechanical analysis (DMA) results show that the room temperature storage modulus of the TPVs decreases due to the rubber phase. The TPU/NBR-CBS TPV with delayed action accelerator yields the highest tensile and tear strength and elongation at break. TPU/NBR-DCP made with a peroxide-based rubber blend shows similar tensile strength with significantly lower elongation at break and tear strength. The good mechanical properties of TPU/NBR-CBS can be explained by the long induction period, which allowed the rubber phase to disperse before vulcanization took place. In the TPU/NBR-DCP, the peroxide formed bonds not only in the rubber phase but also between the rubber and TPU phases, which is manifested in the high mixing torque and the result of the thermogravimetric analysis (TGA).

Laboratory investigation on the cracking performance of paving polyurethane mixture using semi-circular bending test

Polyurethane mixture (PUM) consisting of polyurethane (PU) binder and aggregates has been attracting great interest due to its high-temperature stability and mechanical strength. However, the understanding of crack resistance under ultimate load and fatigue fracture performance under repeated load of PUM is limited. Recent studies have demonstrated that the isocyanate content is strongly associated with the properties of polyurethane (PU) obtained from the chemical reaction between isocyanate and polyol. Few studies have quantified the effects of PU binder with different isocyanate contents on the crack resistance and fatigue performance of PUM. To fill this gap, this study prepared PU binders of different elongations and tensile strengths by varying the isocyanate content. The crack resistance and fatigue fracture performance of PUM were evaluated by semi-circular bending test (SCB) monotonic test at different temperatures and SCB fatigue test based on the number of cycles to fatigue failure and dissipated energy. As a comparison, the crack resistance and fatigue fracture performance of SBS modified asphalt mixture (SMAM) were also investigated. The results indicate that with the increase of isocyanate content, the elongation of PU binder decreases and tensile strength increases. The PU binder with a higher isocyanate content enhanced fracture energy, fracture toughness, and stiffness of PUM. According to the fracture energy, fracture toughness, stiffness and the number of cycles to fatigue failure, PUM outperforms the SMAM counterpart in both crack resistance and fatigue performance. As isocyanate content increases, under the same stress level, the number of cycles to fatigue failure increases, and the fatigue failure of PUM also requires more dissipated energy. Finally, the elongation correlated well with crack resistance, as they affected the fracture energy, fracture toughness, and stiffness. The number of cycles to fatigue failure correlates well with the load for different elongations.

Development of polyurethane-based solid-solid phase change materials for cooling asphalt pavements

Effective cool pavement technologies with using phase change materials (PCMs) are of great significance to alleviate Urban Heat Island (UHI) effect and improve the functionality, safety and service life of asphalt pavements. This study focuses on the development of polyurethanes (PUs) as solid–solid PCMs (s-s PCMs) that are suitable for asphalt pavement cooling utilizations. First, a range of PUs were synthesized through two-step polymerization reactions. Successful synthesis was confirmed by the Fourier transform infrared (FTIR) spectroscopy test. Next, characterizations were performed on the synthesized PUs. It is demonstrated that a PU with a larger isocyanate content usually sustains a higher temperature without liquid leakage. The applicable s-s PCM among the synthesized PUs is M_PU5 with 4,4′-diphenylmethane diisocyanate (MDI) as the isocyanate and the molar ratio of MDI and polyethylene glycol (PEG) being 5, due to its desirable latent heat storage capacity of 93.5 J·g−1 and phase change temperatures of 36–48.3 °C, excellent thermal stability and reliability, and its strong ability to withstand harsh conditions of asphalt pavements. Finally, the feasibility of M_PU5 for actively cooling asphalt pavements has been verified by outdoor test. The investigated results provide a basis and reference for building cool asphalt pavement systems with using s-s PCMs.

Polyurethane coatings from formulations with low isocyanate content using a transurethane polycondensation route

A strategy aiming to decrease the amount of isocyanate in the PU industries without changing their current facilities is proposed. Hydroxyl terminated non-isocyanate polyurethane (NIPU) oligomers (H-Ol) with controlled molecular weights and chemical structures have been prepared as precursors. They were prepared by transurethane polycondensation between bis(methylcarbamate) (BMC), hydroxyterminated poly(tetramethyleneoxide) (PTMO) and butanediol at several molar ratios. H-Ol were then reacted with a polyisocyanurate as crosslinking agent to afford several partially NIPU coatings. Polyurethanes based on the PTMO polyols and the polyisocyanurate were also prepared as controls to highlight the importance of the urethane function in the H-Ol. The influence of the chemical structures on the thermal and mechanical properties of the final coatings has been investigated. The prepared coatings displayed thermal stabilities above 200 °C, Young modulus ranging from 3 to 64 MPa, tensile strength values from 0.9 to 17 MPa and elongation at break varying from 25 to 530%. H-Ol with the highest urethane content gave a coating with the best adhesion properties on a metal surface. All the obtained properties were within the characteristic range of two commercial PU reference coatings.

Eco-Friendly Ether and Ester-Urethane Prepolymer: Structure, Processing and Properties.

This study concerns bio-based urethane prepolymers. The relationship between the chemical structure and the thermal and processing parameters of bio-based isocyanate-terminated ether and ester-urethane prepolymers was investigated. Bio-based prepolymers were obtained with the use of bio-monomers such as bio-based diisocyanate, bio-based polyether polyol or polyester polyols. In addition to their composition, the bio-based prepolymers were different in the content of iso-cyanate groups content (ca. 6 and 8%). The process of pre-polymerization and the obtained bio-based prepolymers were analyzed by determining the content of unreacted NCO groups, Fourier transform infrared spectroscopy, proton nuclear magnetic resonance, thermogravimetry, and rheological measurements. The research conducted facilitated the evaluation of the properties and processability of urethane prepolymers based on natural components. The results indicate that a significant impact on the processability has the origin the polyol ingredient as well as the NCO content. The thermal stability of all of the prepolymers is similar. A prepolymer based on a poly-ether polyol is characterized by a lower viscosity at a lower temperature than the prepolymer based on a polyester polyol. The viscosity value depends on the NCO content.

Shape memory characteristics of thermadapt polyurethane incorporated with two structurally distinctive aliphatic isocyanates

Shape memory polymers (SMPs) have the ability to fix a temporarily deformed shape and recover its original shape. Owing to the shape memory effect, SMPs have been widely investigated for applications in various industries. Shape memory polyurethane (SMPU) is a common SMP material which possesses good shape memory and mechanical properties, and it is mostly focused on the thermoplastic SMPU. In this study, we introduced two structurally different aliphatic isocyanates, hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI), to fabricate thermadapt SMPUs. The formation of the hard segment in the SMPU matrix was tunable by varying the IPDI mol% of the total isocyanate content. The shape memory characteristics were strongly related to the contents of the hard segments and the crystallinity of the thermadapt SMPU. Owing to the transcarbamoylation reaction between the carbamate moieties, the network of our thermadapt SMPU can be rearranged to redefine its permanent shape, and the redefined shape can also be recovered from the temporarily deformed state.

Water-dispersible isocyanate modified using plant-based castor oil: Synthesis and application as crosslinking agent

To improve the performance of waterborne materials, this study attempts to prepare a novel water-dispersible isocyanate crosslinking agent based on plant-based castor oil. The intermediate maleated castor oil and water-dispersible isocyanates with different R values [n(NCO)/n(OH)] are prepared and characterized by FTIR and 1H NMR. The viscosity test, isocyanate content titration, particle size analysis, and the morphologies indicate that the optimal condition for the water-dispersible isocyanates (e.g., best storage stability and particle size distribution) is at R value = 40. The mechanical test proves that the water-dispersible isocyanates can greatly improve the bonding strength and water resistance of wood adhesives, showing an increase of wet strength to be as high as 146 % and 506 % for urea-formaldehyde resin and white latex, respectively. The castor oil-mediated water-dispersible isocyanate system opens a novel avenue to synthesize waterborne crosslinking agent for applications in adhesives, coatings, and structural materials.

Integrated preparation and properties of polyurethane‐based sandwich structure composites with foamed core layer

AbstractThe polyurethane‐based sandwich structure composite was integrally prepared through using glass fiber reinforced polyurethane composite (GFRPUC) as the skin layer and rigid polyurethane foam (RPUF) as the core layer with the application of a prepolymer infiltrating‐foaming integrated manufacturing process. The effect of isocyanate content on the mechanical properties of polyurethane (PU) matrix was explored, and the influences of n‐pentane as the foaming agent on the mechanical properties of RPUF and sandwich structure composite were investigated. The results showed that PU prepolymer synthesized with 10% isocyanate group had the excellent comprehensive performances including the tensile strength of 77 MPa, bending strength of 110 MPa and impact toughness of 59 kJ/m2. Under the optimal molding process, the tensile strength, bending, and impact strengths of the GFRPUC surface are 794 MPa, 846 MPa, and 228 kJ/m2, respectively. Using the PU prepolymer method, the surface layer and foam core are integrally fabricated as the sandwich structure composites, whose structure is characterized by strong surface layer and porous core with energy absorption effect. The integrated SSC plate with foamed core layer has excellent mechanical properties especially the core layer containing 2 wt% foaming agent, such as shear property.

Pyrolysis Kinetics and Flammability Evaluation of Rigid Polyurethane with Different Isocyanate Content

Polyurethane (PU) is a typical product of the reaction between isocyanate and polyol, whose ratio would greatly influence material properties. In this paper, to investigate the influence of isocyanate on PU thermal stability and flammability, three kinds of rigid polyurethanes (RPUs) with different isocyanate ratio (1.05, 1.1, and 2.0) were manufactured in a laboratory and employed to have a series of TG (thermogravimetry), DSC (differential scanning calorimetry), and cone calorimetry tests. Kissinger’s method was used to calculate the activation energy and judge their stabilities. However, for such a complex degradation which consists of five reactions, it does not make sense by Kissinger method to obtain only two peak active energies. Considering complexity of PU degradation in air, genetic algorithm (GA) was employed to calculate kinetic triplets of five sub-reactions. The effects of isocyanate contents on each sub-reaction stability were obtained and then analyzed. By cone calorimeter testing, we found that great differences in heat release rate data. However, DSC analysis showed a complete opposite changed trend. Such difference is caused by DSC and calorimeter’s sample morphology, the former using grinded polyurethane powders but the latter polyurethane foam block.

Effects of Transmitted Light on Growth and Allyl isocyanate Content of Wasabi Rhizome Grown under Shading Nets Capable of Cutting Near-infrared Rays

Effects of Transmitted Light on Growth and Allyl isocyanate Content of Wasabi Rhizome Grown under Shading Nets Capable of Cutting Near-infrared Rays