High-performance Resin Research Articles

Material genome approach-based design of multi-functional self-curing epoxy resin with intrinsic flame retardancy and ultra-high modulus

Data-driven methods or computational techniques have opened new paradigms for the rational design of materials. The introduction of specific groups could largely enhance anti-flammable and mechanical properties for epoxy resins. In this work, two high-performance epoxy resins (mABAEP and SAAEP) containing assisted flame-retardant “genes” were designed adopting a material genome approach combined with DFT calculations and synthesized through a one-pot method. The resin can be automatically molded in the presence of a heat source without mixing with hardener, called “self-curing”, which considerably increases the density of assisted flame-retardant “genes”. The presence of such “genes” in epoxy resin could promote char layer formation and release nonflammable gases during combustion, further improving flame retardancy. On the other hand, those “genes” with strong polarity enhance intermolecular forces thus reducing the motility of chain segments. The bending test shows that SAASC (SAAEP after self-curing) has higher flexural modulus and flexural strength (5155 MPa, 125.7 MPa) compared to E51/DDM (2564 MPa, 103.7 MPa), with 101% and 21% improvement respectively. In addition, SAASC has a higher glass transition temperature (217 °C) and char yield (45.0%), as well as superior antibacterial properties, which can satisfy multifunctional applications.

Protein engineering of a nanoCLAMP antibody mimetic scaffold as a platform for producing bioprocess-compatible affinity capture ligands

Protein A affinity chromatography is widely used for the large-scale purification of antibodies because of its high yield, selectivity, and compatibility with NaOH sanitation. A general platform to produce robust affinity capture ligands for proteins beyond antibodies would improve bioprocessing efficiency. We previously developed nanoCLAMPs (nano Clostridial Antibody Mimetic Proteins), a class of antibody mimetic proteins useful as lab-scale affinity capture reagents. This work describes a protein engineering campaign to develop a more robust nanoCLAMP scaffold compatible with harsh bioprocessing conditions. The campaign generated an improved scaffold with dramatically improved resistance to heat, proteases, and NaOH. To isolate additional nanoCLAMPs based on this scaffold, we constructed a randomized library of 1× 1010 clones and isolated binders to several targets. We then performed an in-depth characterization of nanoCLAMPs recognizing yeast SUMO, a fusion partner used for the purification of recombinant proteins. These second-generation nanoCLAMPs typically had a Kd of <80nM, a Tm of >70 °C, and a t1/2 in 0.1mg/ml trypsin of >20h. Affinity chromatography resins bearing these next-generation nanoCLAMPs enabled single-step purifications of SUMO fusions. Bound target proteins could be eluted at neutral or acidic pH. These affinity resins maintained binding capacity and selectivity over 20 purification cycles, each including 10min of cleaning-in-place with 0.1M NaOH, and remained functional after exposure to 100% DMF and autoclaving. The improved nanoCLAMP scaffold will enable the development of robust, high-performance affinity chromatography resins against a wide range of protein targets.

Computation-based design of multifunctional self-curing epoxy resin containing intensive hydrogen bond network: A novel super-strong, high reactivity and flame-retardant coating

The crosslink network of epoxy resin has a decisive influence on the overall performance, and the design of special network topologies through computer-aided methods is gaining much more attention. Based on molecular dynamics (MD) simulation and DFT calculation, a novel high-performance epoxy resin containing an amide bond has been ingeniously designed, which can be molded under heating without the addition of curing agent, thus greatly optimizes the fabrication process. The absence of curing agent significantly increases type and number of hydrogen bonds and forming an intensive hydrogen bond network, which leads to ultra-high flexural modulus up to 7026 MPa, 106 % higher than the traditional high-performance resin (AG80/DDM). The catalytic effect of the amide bond greatly improves the curing efficiency, endowing lower post-curing temperature (150 °C), and the rich catalytic sites enables rapid crosslinking of the resin to form more compact network. In addition, the abundant amide bonds impart intrinsic flame retardancy as well as increased char yield to the resin. The resin with ultra-high modulus, high curing efficiency and flame retardancy has good potential for application as protective coating and provides new ideas for the design of high-performance epoxy resins.

Improving corrosion resistance of BFRP bars by coating CNTs modified resin in simulated pore solution of seawater sea sand concrete

The corrosion of basalt fiber-reinforced polymer (BFRP) bars restricts their use in alkaline concrete structures. However, developing high-performance resin can improve the anti-alkaline corrosion of BFRP bars. This work investigates the alkaline resistance of BFRP bars by coating carbon nanotubes (CNTs) modified epoxy resin incorporating varying CNTs contents (0, 0.1, 0.3, and 0.5 wt%). The durability of untreated and treated BFRP bars with CNT-modified resin was evaluated using water uptake, transverse shear, and interlaminar shear tests. Scanning electron microscopy (SEM) and X-ray micro-computed tomography (micro-CT) were also used to observe the surface corrosion morphology and interior deterioration of BFRP bars, respectively, while Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) were applied to analyze the resin deterioration of BFRP bars. It was found that CNTs modified resin can significantly improve the alkaline resistance of BFRP bars. After 180 d of immersion, the interlaminar shear strength (ILSS) retention of untreated BFRP bars is 31.8%, while it is 63.3, 75.0, 91.3, and 93.7% for BFRP bars treated with 0, 0.1, 0.3, and 0.5 wt% CNTs modified resin, respectively. The improvements in ILSS retention of BFRP bars treated with 0.5 wt% CNTs modified resin are 61.9% and 30.4% more than untreated and pure resin-treated BFRP bars, respectively. BFRP bars treated with 0.3 and 0.5 wt% CNTs modified resin exhibit insignificant resin hydrolysis and less porosity than untreated BFRP bars. The effective prevention of hydroxyl ions into the interior of BFRP bars and the enhancement of the anti-crack behavior of resin by CNTs are the leading causes of corrosion resistance improvement in BFRP bars. The outcomes of this study provide insights into using CNTs modified resin to improve the durability of BFRP bars in marine concrete structures.

Low-Temperature Terpolymerizable Benzoxazine Monomer Bearing Norbornene and Furan Groups: Synthesis, Characterization, Polymerization, and Properties of Its Polymer.

There is an urgency to produce novel high-performance resins to support the rapid development of the aerospace field and the electronic industry. In the present work, we designed and consequently synthesized a benzoxazine monomer (oHPNI-fa) bearing both norbornene and furan groups through the flexible benzoxazine structural design capability. The molecular structure of oHPNI-fa was verified by the combination characterization of nuclear magnetic resonance spectrum, FT-IR technology, and high-resolution mass spectrum. The thermally activated terpolymerization was monitored by in situ FT-IR as well as differential scanning calorimetry (DSC). Moreover, the low-temperature-curing characteristics of oHPNI-fa have also been revealed and discussed in the current study. Furthermore, the curing kinetics of the oHPNI-fa were investigated by the Kissinger and Ozawa methods. The resulting highly cross-linked thermoset based on oHPNI-fa showed excellent thermal stability as well as flame retardancy (Td10 of 425 °C, THR of 4.9 KJg-1). The strategy for molecular design utilized in the current work gives a guide to the development of high-performance resins which can potentially be applied in the aerospace and electronics industries.

Preparation of a novel high-performance lignin-based anionic adsorption resin for efficient removal of Cr(VI) in aqueous solutions

To remove Cr(VI) in water, a novel lignin-based anionic adsorption resin epichlorohydrin cross-linking lignin grafted by diethylenetriamine (E-DAL) was prepared by the direct cross-linking and curing of alkali lignin grafted by diethylenetriamine in aqueous solutions. Under the determined optimum synthesis conditions, E-DAL with higher nitrogen (7.46%) and lignin (32.78%) contents was obtained. Structural and physicochemical measurement results showed E-DAL was a macroporous resin, having a regular connected pore structure with porosity of 67.48% and pore size of 615.6 nm, also strong hydrophilicity, excellent acid-alkaline resistance and thermal stability. Adsorption experiment results indicated the adsorption isotherm of E-DAL to Cr(VI) was well-described by Langmuir model. E-DAL showed a higher adsorption capacity to Cr(VI), and the fitted maximum adsorption capacity reached up to 775.2 mg/g. Pseudo-second-order model could fit this adsorption process well, suggesting its chemisorption characteristics. The calculated thermodynamic parameters indicated a spontaneous and endothermic adsorption of E-DAL to Cr(VI). Dynamic column adsorption results showed E-DAL could completely remove Cr(VI) in wastewater. This whole preparation process only used a very small amount of cheaper amination agent and was relatively low-cost. This work provided efficient technical supports for the structural design of high-performance lignin-based anionic adsorption materials in water treatment field.

Novel liquid phthalonitrile monomers towards high performance resin

Phthalonitrile resins are one of the high temperature resistant polymers which can be potentially used for harsh environments in both military and domestic fields. The high melting points of the monomers led to short processing windows and made processing procedure challenging and energy intensive. To solve this problem, liquid phthalonitrile monomers containing flexible siloxane segments were designed and synthesized. The long bond length and large bond angle of Si-O-Si chain segments introduced flexibility into the monomers and effectively reduced the melting point to −35.9 °C. Meanwhile, the high bond energy preserved the thermal stability of the resulting cured resin. The high fluidity (viscosity of ∼ 2 Pa·s at 30 °C) allows the monomers to be processed at room temperature without additional heating. In addition, the monomers can be dissolved in common organic solvents, such as ethyl acetate and ethanol. The resins cured at 250 °C demonstrated good thermal and thermal oxygen stability. The weight loss of 5 % in argon and air at temperatures of 428.6 °C and 434.9 °C, respectively. Furthermore, blending this liquid monomer blended with powdered monomers improved both the processibility of the high melting monomers and the overall temperature resistance of the cured resin. The resin showed a weight loss of 5 % in argon and air of 534.4 °C and 532.3 °C, respectively.

Synthesis of new biobased benzoxazine through green strategy and its polybenzoxazine resin with high thermal resistance and mechanical strength

AbstractBenzoxazine resins from benzoxazine monomers synthesized with green strategies often do not have high thermal resistance and mechanical strength. Herein, starting from renewable resources citraconic anhydride, furfurylamine and tyramine, a new benzoxazine monomer (CT‐fa) was synthesized and purified in green solvents (ethanol and ethyl acetate), which has excellent molding processability reflected by low melting point (84.9 °C) and low curing temperature (207–245 °C). More attractively, the curedCT‐faresin (poly(CT‐fa)) has both high glass transition temperature (281 °C) and high initial thermal decomposition temperature (407 °C), the former is higher than those of all monofunctional benzoxazine resins synthesized though green strategies in literature, and the latter is even higher than those of all benzoxazine resins synthesized by green strategies (SCI database). In addition,poly(CT‐fa)also has high storage modulus (3955 MPa) and good tensile strength (51.60 MPa). These attractive properties demonstrate thatpoly(CT‐fa)is a promising green high performance resins.

Furfural-based P/N/S flame retardant towards high-performance epoxy resins with flame retardancy, toughness, low dielectric properties and UV resistance

The development of high-performance EP composites with high glass transition temperature (Tg), excellent flame retardancy, and UV resistance is a challenging topic in the flame retardancy community. In this work, a bio-based phosphorus/nitrogen/sulfur (P/N/S) containing flame retardant (FDI) was prepared from furfural, 2-amino-5-nitrothiazole, and DOPO. It was applied to improve the flame retardant, dielectric, and UV-resistant properties of EP. Thanks to the combination of phosphorus, nitrogen, and sulfur in FDI, the EP/FDI composites showed superior flame retardancy. Only adding 5 wt% FDI made EP achieve UL-94 V-0 flammability rating with a limiting oxygen index (LOI) of 27%. Meanwhile, EP/FDI-5% composite showed significant reduction of 65.1% and 33.6% in total smoke production (TSP) and the peak heat release rate (pHRR), respectively. The flame-retardant mechanisms of EP/FDI thermosets were investigated by residual char photographs, SEM, XPS, and Py/GC-MS. Moreover, compared to pure EP, EP/FDI composites showed excellent UV resistance, low dielectric properties, and high glass transition temperature, which was expected to be extensively used in multiple areas of production.

High-Branched Organosilicon Epoxy Resin with Low Viscosity, Excellent Toughness, Hydrophobicity, and Dielectric Property

Rapidly developing technology places higher demands on materials, thus the simultaneous improvement of materials' multiple properties is a hot research topic. In this work, a high-branched silicone epoxy resin (QSiE) was synthesized and applied to the curing system of bisphenol A epoxy resin (DGEBA) for modification investigations. When 6 wt% QSiE was added to the system, the viscosity dropped by 51.8%. The mechanical property testing results indicated that QSiE could significantly enhance the material's toughness while preserving good rigidity. The impact strength was enhanced by 1.31 times when 6wt% of QSiE was introduced. Additionally, the silicon skeleton in QSiE has low surface energy and low polarizability, which could endow the material with good hydrophobic and dielectric properties. This work provided a new idea for the preparation of high-performance epoxy resin additives, and provided a broad prospect for cutting-edge applications of epoxy resins.

Vat Photopolymerization of Nanocellulose-Reinforced Vegetable Oil-Based Resins: Synergy in Morphology and Functionalization

With the advancement of additive manufacturing (AM) and the mass adoption of 3D printing technology, it is essential to shift focus to environmentally and economically sustainable materials. As the utilization of renewable feedstocks is quite limited in this context, the utilization of more bio-based raw materials in the ongoing development of AM represents an essential means of achieving this shift. In this work, vat photopolymerization 3D printing has been used to process vegetable oil-based (VO) resins with an ultralow concentration of 0.07 vol % nanocellulose fibrils (NFC) and crystals (NCC). The developed nanocellulose containing bio-based vat photopolymerization resin shows excellent shelf stability, enabling high-resolution printing. Compatibilization of the nanocellulose with the polymer matrix was achieved through the introduction of isocyanate or acrylate groups via reactions of acryloyl chloride (AC) and hexamethylene diisocyanate (HMDI) with cellulose surface hydroxyls. Surface functionalization results in ∼20–30% increases in interfacial adhesion and stress transfer, yielding significant improvements in mechanical performance (4× higher toughness, 2.4× higher tensile strength, and 2× higher tensile strain) in 3D-printed specimens. Fourier-transformation infrared (FTIR) spectroscopy complemented by solid-state nuclear magnetic resonance (NMR) techniques enabled a more detailed study of the chemical structure of these materials as well. Tensile performance comparison with literature data on VO-based natural fiber-reinforced resins showed that this work brought bio-based resins one step closer to competing with petroleum-based resins. The prepared VO/nanocellulose resins are promising candidates for high-performance bio-based resins derived from completely renewable feedstocks.

A facile strategy and mechanism to achieve biobased bismaleimide resins with high thermal-resistance and strength through copolymerizing with unique propargyl ether-functionalized allyl compound

Preparing biobased thermosetting resins with high mechanical properties and superior heat resistance is an important issue for the sustainable development and applications of high performance resins. Herein, a novel biobased allyl compound (DBB) with aryl propargyl ether groups was synthesized from renewable honokiol, which was then used to copolymerize with 4,4′-bismaleimidodiphenylmethane (BDM) to develop new biobased resins (BDM/DBB). The effects of the molar ratio of BDM to DBB on the thermal and mechanical properties were studied. Results show that all BDM/DBB resins possess both high thermal and mechanical properties. Specifically, their glass transition temperatures (Tg > 440 °C) and storage moduli (> 1.3 GPa at 400 °C) are higher than those of biobased allyl compounds modified BDM resins reported so far (SCI database); moreover, their initial thermal decomposition temperatures are about 448 °C and flexural strengths range from 106 to 117 MPa. These excellent comprehensive performances are attributed to the unique crosslinked network resulted from the complex curing mechanism of BDM/DBB, which including the copolymerization between imide groups and allyl groups as well as the additional thermal crosslinking of aryl propargyl ether. This work provides a sustainable and facile strategy to construct high performance biobased resins.

Cover Image, Volume 140, Issue 15

This cover by Miaotian Long describes a facile strategy to construct microencapsulated urea ammonium polyphosphate (PPy-UAPP) with a polypyrrole shell. The good compatibility between epoxy matrix and PPy-UAPP endows the resulting EP composites with a satisfactory balance between fire safety and mechanical properties. This work may open a new route for developing a super flame retardant and corresponding high-performance flame-retarded resins. DOI: 10.1002/app.53675

Boron Nitride Nanotubes: Force Field Parameterization, Epoxy Interactions, and Comparison with Carbon Nanotubes for High-Performance Composite Materials

Boron nitride nanotubes (BNNTs) are a very promising reinforcement for future high-performance composites because of their excellent thermo-mechanical properties. To take full advantage of BNNTs in composite materials, it is necessary to have a comprehensive understanding of the wetting characteristics of various high-performance resins. Molecular dynamics (MD) simulations provide an accurate and efficient approach to establish the contact angle values of engineering polymers on reinforcement surfaces, which offers a measure for the interaction between the polymer and reinforcement. In this research, MD simulations and experiments are used to determine the wettability of various epoxy systems on BNNT surfaces. The reactive interface force field (IFF-R) is parameterized and utilized in the simulations to accurately describe the interaction of the epoxy monomers with the BNNT surface. The effect of the epoxy monomer type, hardener type, local atomic charges, and temperature on the contact angle is established. The results show that contact angles decrease with increases in temperature for all the epoxy/hardener systems. The bisphenol-A-based epoxy system demonstrates better wettability with the BNNT surface than the bisphenol-F based epoxy system. Furthermore, the MD predictions demonstrate that these observations are validated with experimental results, wherein the same contact angle trends are observed for macroscopic epoxy drops on nonwoven nanotube papers. As wetting properties drive the resin infusion in the reinforcement materials, these results are important for the future manufacturing of high-quality BNNT/epoxy nanocomposites for high-performance applications such as aerospace and aeronautical vehicles.

Design and synthesis of anhydride-terminated imide oligomer containing phosphorus and fluorine for high-performance flame-retarded epoxy resins

Challenges remain in the design and construction of robust epoxy thermosets with concurrently improved flame retardancy, thermal, mechanical and dielectric properties.In this work, a novel method has been exploited to synthesize anhydride-terminated imide oligomer containing phosphorus and fluorine (FPI) in the mixed solvents consisting of benzoic acid and acetic acid by single-step reaction. Compared to the ones synthesized by the conventional methods, FPI possessed a much higher content of anhydride groups, and was more suitable to be used as co-curing and flame-retarded agents for epoxy resins. With multifunctional groups, FPI can endow epoxy thermosets with a series of excellent properties. At the loading of 28.5 wt% FPI (only 0.87 wt% phosphorus), the epoxy thermoset (EP/FPI-3) passed the UL-94 V-0 test with the limiting oxygen index (LOI) value of 31.1%. Instead of the reduction in other properties of most polymeric materials after being flame-retarded, the overall performance of EP/FPI-3 was improved, hereinto, the glass transition temperature, heat deflection temperature, 5 wt% mass loss temperature (under N2) and Young’s modulus were increased by 37.2 °C, 42.4 °C, 10.2 °C, 50.6%, respectively, and the dielectric constant, dielectric loss and water adsorption were decreased by 9.9% (100 MHz), 33.3% (100 MHz), 16.2%, respectively, compared to those of the original epoxy thermoset. This work provides a simple and practical way to prepare the flame-retarded epoxy thermosets with improved overall properties by the bespoke multifunctional imide oligomers used as co-curing and flame-retarded agents, and the resulting epoxy thermosets are highly competitive as the advanced electrical materials.

Determination of cure kinetic parameters and thermal stability of benzoxazine coating modified by poly (bisphenol-A borate)

Benzoxazine resin is a high-performance thermosetting resin that limits its use in ablative materials due to limitations such as toughness and high curing temperature. This work intends to investigate the effect of the introduction of poly (bisphenol-A borate) (PBAB) into benzoxazine coating on the curing kinetics and thermal stability. The curing reaction of benzoxazine coating in the presence of poly (bisphenol-A borate) was catalyzed by facilitating the oxazine ring opening. The kinetic of the cure was investigated by differential scanning calorimetry (DSC); then, the data were fitted and compared utilizing the Kissinger, Ozawa, and Borchardt-Daniels methods, which allow for determining the kinetic parameters of this system. Also, the results of thermo-gravimetric analysis (TGA) showed that with increasing the percentage of PBAB, thermal stability and char yield of composite increase significantly.

High-Strength, Degradable and Recyclable Epoxy Resin Based on Imine Bonds for Its Carbon-Fiber-Reinforced Composites

Carbon fiber (CF) is widely used in the preparation of carbon-fiber-reinforced polymer composites (CFRP) in which it is combined with epoxy resin due to its good mechanical properties. Thermosetting bisphenol A epoxy resin, as one of the most common polymer materials, is a non-renewable resource, leading to a heavy environmental burden and resource waste. To solve the above problems and achieve high mechanical and thermal properties comparable to those of bisphenol A, herein, a high-performance, degradable and recyclable bio-based epoxy resin was developed by reacting the lignin derivative vanillin with 4-amino cyclohexanol via Schiff base. This bio-based epoxy resin showed a Young's modulus of 2.68 GPa and tensile strength of 44 MPa, 36.8% and 15.8% higher than those of bisphenol A epoxy, respectively. Based on the reversible exchange reaction of the imine bond, the resin exhibited good degradation in an acidic environment and was recoverable by heat treatment. Moreover, the prepared epoxy resin could be used to prepare carbon fiber (CF)-reinforced composites. By washing off the epoxy resin, the carbon fiber could be completely recycled. The recovered carbon fiber was well preserved and could be used again for the preparation of composite materials to realize the complete recovery and utilization of carbon fiber. This study opens a way for the preparation of high-performance epoxy resin and the effective recycling of carbon fiber.

Pultruded carbon fibre reinforced polymer strips produced with a novel bio-based thermoset polyester for structural strengthening

This paper presents the development and industrial manufacturing of a pultruded high-performance carbon fibre (CF) reinforced polymer (CFRP) composite strip using a bio-based unsaturated polyester (UP) resin containing more than 50% of its weight derived from renewable raw materials. The bio-based CF/UP strengthening strip, comprising unidirectional CF strands, was successfully produced and its mechanical and thermomechanical behaviours were assessed, and compared to an equivalent carbon fibre reinforced epoxy-vinyl ester (CF/VE) strip produced with conventional petroleum-based high-performance epoxy-vinyl ester resin and the same fibre architecture. The bio-based CF/UP composite strip presented 2095 MPa, 167 GPa, and 1.4% of tensile strength, modulus of elasticity, and strain at failure, respectively. These figures met or exceeded the mechanical properties of its equivalent conventional CF/VE counterpart tested under the same conditions. Furthermore, the bio-based CF/UP composite strip presented a glass transition temperature of 78 °C (defined from the onset of the storage modulus decay), enabling a maximum service temperature of ∼60 °C, which will not likely be exceeded in most climates, highlighting its potential for structural strengthening applications, and offering a key advantage in terms of sustainability.

Depolymerization of robust polyetheretherketone to regenerate monomer units using sulfur reagents

Super engineering plastics, high-performance thermoplastic resins such as polyetheretherketone, and polyphenylene sulfide have been utilized in industries, owing to their high thermal stability and mechanical strength. However, their robustness hinders their depolymerization to produce monomers and low-weight molecules. Presently, chemical recycling for most super engineering plastics remains relatively unexplored. Herein, we report the depolymerization of insoluble polyetheretherketone using sulfur nucleophiles via carbon–oxygen bond cleavages to form benzophenone dithiolate and hydroquinone. Treatment with organic halides converted only the former products to afford various dithiofunctionalized benzophenones. The depolymerization proceeded as a solid–liquid reaction in the initial phase. Therefore, this method was not affected by the shape of polyetheretherketone, e.g., pellets or films. Moreover, this depolymerization method was applicable to carbon- or glass fiber-enforced polyetheretherketone material. The depolymerized product, dithiofunctionalized benzophenones, could be converted into diiodobenzophenone, which was applicable to the polymerization.

Evaluation of the Equilibrium Melting Temperature and Melting Behavior of Poly(Phenylene Sulfide) Crystals.

Focusing on the melting process of poly(phenylene sulfide) (PPS) crystals, one of the high-performance resins with a very wide range of applications, we attempted to clarify the melting process and derive the equilibrium melting temperature with structural analysis using X-ray scattering and thermal measurements. Two melting points were observed in the crystallized PPS films by DSC measurements. We focused on the low-temperature melting point, in situ small-angle and wide-angle X-ray scattering measurements using synchrotron radiation and DSC thermograms indicate the melting of microcrystals because of thickening of the amorphous part from small-angle X-ray scattering results. In addition, the Hoffman-Weeks plot based on the results of fast scanning calorimetry showed that the melting temperature is almost parallel to the straight line where melting temperature and crystallization temperature are equal, indicating that the melting is of a metastable structure. The melting point on the high temperature side was attributed to the melting of lamellar crystals. The equilibrium melting point was derived from the Hoffman-Week and Gibbs-Thomson plots as 318 °C.