Thermal Curing Behavior Research Articles

Thermal stability and thermal degradation kinetics of urea-formaldehyde resin modified by sodium lignosulfonate

ABSTRACT In this paper, an environmentally friendly urea-formaldehyde resin (UF) was prepared by using sodium lignosulfonate (SL) as a modifier. According to the basic properties of the resin, the strength of the plywood and the formaldehyde emission, the optimum addition amount of SL was determined. At the same time, the thermal curing and thermal degradation behavior of the resin were studied. The results show that the addition of SL can significantly reduce the formaldehyde emission of plywood. When the addition amount of SL was 1%, the wet bonding strength and formaldehyde emission of SL-modified UF (SL-UF) resin were 1.28 MPa and 0.35 mg·L−1, respectively, which were 23% higher and 71% lower than those of UF resin. Moreover, the initial curing temperature of SL-UF resin is lower, but the thermal curing activation energy is higher. SL also has a significant effect on the thermal degradation performance of UF resin. SL is a modifier that can effectively improve the performance of UF.

Effects of wheat flour levels on the thermal curing behavior of Urea–Formaldehyde and Phenol–Formaldehyde resins

Urea–formaldehyde (UF) and phenol–formaldehyde (PF) resins are the most prevalent adhesives in the manufacturing of wood composites, and they are cured by hot pressing to build a three-dimensional network structure in the composites. UF and PF resins are typically formulated with organic fillers like wheat flour to control viscosity and reduce costs in the commercial production of plywood. Although this formulation is well established, research on the effect of filler on the curing behavior of UF and PF resins has been insufficient. Therefore, this study used differential scanning calorimetry (DSC) to examine the influence of wheat flour as a filler in varying amounts on the thermal curing behavior of UF and PF resins. The results demonstrated that the viscosity and solid content of the UF and PF resins increased as the filler amount increased. In contrast, the gelation time, which reflected the curing speed of the resins, decreased as the filler content increased. Similarly, DSC results demonstrated that with increased filler amounts, the curing reaction of UF resins accelerated, as evidenced by a decrease in the apparent activation energy (Ea) determined using the Kissinger–Akahira–Sunose and Vyazovkin methods. However, in the case of PF resins, the average Ea increased and subsequently declined when 10% and 20% fillers were added. This demonstrated the complicated interaction between wheat flour and resins, particularly PF resins. Furthermore, the isothermal DSC curves reveal that the highest exothermic peaks of UF resins are observed at different isothermal temperatures as the filler amount increases. In contrast, PF resins maintain an isothermal temperature for the highest exothermic peaks across all filler levels. In addition, FTIR analysis suggested that wheat flour might contribute to the curing reaction of UF and PF resins.

Relationship between thermal curing behavior and bond strength development in formaldehyde-based resins with different molecular weights

Urea–formaldehyde (UF) and phenol–formaldehyde (PF) resins are the most widely used adhesives in the production of wood composites, and are cured through hot-pressing to provide adhesion strength with the composites. This study investigates the relationship between thermal curing behavior monitored by differential scanning calorimetry (DSC) and adhesive bond strength development measured by an automated bonding evaluation system (ABES) for UF and PF resins with different molecular weights (M w). DSC results demonstrated that UF resins cured faster than PF resins, as indicated by a lower apparent activation energy (E a). In addition, UF and PF resins with lower M w cured faster in the early stages of curing and slower in the latter stages of curing as compared to higher-M w resins. Furthermore, according to the ABES results, when increasing the molecular weight, both UF and PF resins exhibited greater adhesive bond strength, irrespective of pressing parameters (temperature and time). The isothermal DSC and ABES results demonstrated that the adhesive bond strength increased with an increase in the degree of cure of the resins, indicating a correlation between them. It also showed that UF resins attained optimal adhesive bond strength at low temperature (110–130 °C) in roughly 180 s while PF resins attained it at high temperature (120–130 °C) in about 300 s. However, the degree of cure under isothermal scans was inconsistent with temperatures. These results indicate that the degree of cure is closely related to the adhesion strength development of UF and PF resins which are curing at low and high temperatures to obtain the optimum adhesion strength.

Exploiting sugarcane waste molasses and dephenolized cottonseed protein as the promising component for eco-friendly wood-based panel adhesive formulation

ABSTRACT In this study, sugarcane waste molasses (SWM) was utilized as a raw component to prepare an eco-friendly plywood adhesive. The optimization of preparation conditions, examination of thermal curing behavior, and exploration of the adhesive's curing mechanism were investigated. Based on the results, the optimal preparation conditions were determined as follows: the SWM is mixed with dephenolized cottonseed protein (DCP), a mass ratio of 2:1 between SWM and DCP, with a concentration of 75% and the addition of 7.5 wt% ammonium dihydrogen phosphate (ADP). Additionally, the optimal hot-pressing condition involved a temperature of 190°C for 10 min. Under these optimized conditions, the adhesive exhibited a maximum wet shear strength of 1.14 MPa, successfully meeting the China National Standard GB/T9846-2015. The curing behavior of the adhesive was investigated by analyzing thermal analysis and measuring the insoluble mass proportion. As the heating temperature and heating time increased, the insoluble mass proportion gradually rose. Thermogravimetric analysis (TG) showed that the thermal decomposition reaction temperature was 146.4°C and 202.05°C, differential scanning calorimetry (DSC) showed that endothermic reaction occurred at 135.8°C, exothermic reaction occurred at 227.8 °C. Through Fourier-transform infrared spectroscopy (FTIR) and pyrolysis gas chromatography-mass spectrometry (Py-GC/MS), it was observed that the addition of ADP promotes the conversion of sucrose hydrolyzed products into furan compounds and reacts with proteins. Moreover, the amino and carboxyl groups of proteins react with the phenol hydroxyl groups of lignin to form C–N and C–O bonds, a polymer with a cross-linked network is formed.

Synthesis and characterization of low dielectric constant polybenzoxazines with different fluorine contents

AbstractThe rapid development of integrated circuits puts forward higher requirements for low dielectric constant polymers. In this study, two fluorinated benzoxazine monomers, bis‐(3,4‐dihydro‐3‐(4‐fluoro‐3‐(trifluoromethyl)phenyl)‐2H‐1,3‐benzoxazine)isopropane (BA‐ftfa) and bis‐(3,4‐dihydro‐3‐(4‐fluoro‐3‐(trifluoromethyl)phenyl)‐2H‐1,3‐benzoxazine)isoperfluoropropane (BAF‐ftfa), were successfully synthesized. Differential scanning calorimeter (DSC) was used to monitor the thermal curing behaviors of the monomers and their mixtures. Five groups of polybenzoxazine samples with different fluorine content (25% ~ 35%) were obtained by copolymerizing the two monomers with different mass ratios. The effects of fluorine content on dielectric properties were investigated and the results show that PBAF‐ftfa with the highest fluorine content has the lowest dielectric constant of 2.53 at 1 MHz. Furthermore, the results of thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) show that all polymers possess excellent thermal stability and exhibit high glass transition temperatures over than 180°C.

Synthesis of acetone-fractionated hardwood kraft lignin-based adhesive crosslinked with epichlorohydrin

Studies on lignin have garnered great interest for increasing the value of lignin-added products, especially lignin-based adhesives. However, even with the advantage of abundant aromatic materials in lignin, its aromaticity and heterogeneity require its fractionation with various solvents. This study used hardwood kraft lignin (HKL) extracted from black liquor and was acetone-fractionated to produce acetone-soluble HKL (AS-HKL) and acetone-insoluble HKL (AI-HKL). The lignins were crosslinked with different amounts of epichlorohydrin (ECH; 5%, 10%, and 15%) for the synthesis of lignin-based adhesives with a three-dimensional network. The crosslinked HKLs were characterized with gel permeation chromatography (GPC), Fourier transform infrared (FT-IR), nuclear magnetic resonance (13C-NMR), and differential scanning calorimetry (DSC) to investigate their molecular weight, chemical structure, and thermal curing behavior. AI-HKL exhibited higher molecular weight than that of AS-HKL. As expected, the FT-IR and 13C-NMR spectra showed successful crosslinking of HKL by ECH with the intensity reduction of − OH, increased C–O stretching, and new peak appearance. From the DSC results, the exothermic peak temperature for crosslinking was 100 °C–130 °C. HKL and AI-HKL plywoods showed good tensile shear strength with minimal variations when they were crosslinked with 10 and 15% ECH. These results suggest the potential of high molecular weight HKL as lignin-based adhesives for wood bonding.

Preparation of biphenyl-containing polybenzoxazines with low dielectric constants and high thermal stability

In this work, two biphenyl-containing benzoxazine monomers (C-tfmb and C-dmb) have been synthesized using 2,2′-bis(trifluoromethyl)benzidine/2,2′-dimethylbenzidine, 4-cyclohexylphenol and paraformaldehyde as raw materials. The thermal curing behaviors of the benzoxazine monomers were investigated by Fourier transform infrared spectroscopy and non-isothermal differential scanning calorimetry. After curing, the biphenyl-containing polybenzoxazines (PC-tfmb and PC-dmb) exhibited high thermal stability with the 5% weight loss temperatures at 335 °C and 367 °C, respectively. Moreover, both polybenzoxazine films showed high hydrophobicity and low dielectric properties. Due to the presence of low-polarity and bulky cyclohexyl and trifluoromethyl groups, PC-tfmb had a dielectric constant of 2.78 and a dielectric loss of 0.0035 at 1 MHz.

Synthesis of eugenol-modified epoxy resin and application on wood flame retardant coating

In this paper, a novel epoxy flame retardant PPDEG-EP (P-E) was synthesized from eugenol by nucleophilic substitution reaction and Prilezhaev epoxidation. The chemical structure was identified by nuclear magnetic resonance (NMR). The synthetic P-E was combined with curing agent diaminodiphenylmethane (DDM) and applied to wood surface coating decoration. To evaluate the thermal curing behavior of P-E, the P-E/DDM system was characterized by DSC. The results show that P-E/DDM has a lower reaction activation energy (Ea), and the curing temperature of the system is reduced with the incorporation of P-E. As a wood coating, P-E/DDM retains the excellent physical and chemical properties of epoxy resins. Thermogravimetric (TG) showed the char formation of P-E/DDM was improved, that the char residue of P-E/DDM+W was higher than both that of pure wood and P-E/DDM. And it produced significant flame retardancy that the UL-94 test reached V-0 rating and the value of LOI test was 32.1%. As expected, SEM shows that the internal structure of the burned wood was still intact. This eugenol-modified coating has been proved to be a good flame retardant for wood.

A Novel One‐Step Synthesis of Silicon‐Containing Arylacetylene Resin Catalyzed by Et2Zn with Excellent Processability and Thermal Properties

AbstractA novel one‐step synthesis of silicon‐containing arylacetylene resin (PSA) catalyzed by Et2Zn was reported. PSA‐Et2Zn was prepared by the reaction between diethynylbenzene and dichlorodimethylsilane in the presence of diethylzinc. The structure of PSA‐Et2Zn was determined and the reaction mechanism was proposed. In addition, the processability, thermal curing behavior, and thermal properties were studied. When compared with the zinc powder catalysis method, the Et2Zn catalysis method demonstrates a greater effectiveness in avoiding the formation of olefinic products at the ends of the molecular chain, which can improve thermal capabilities. The results show that PSA‐Et2Zn possesses excellent processability and thermal properties. PSA‐Et2Zn has a wide processing window of 35∼182 °C and the thermal crosslinking reactions of PSA‐Et2Zn resin are mild. The cured PSA‐Et2Zn shows high thermal stability with a residue yield up to 87.9 % at 800 °C. Furthermore, the temperature at 5 % weight loss in nitrogen and air atmospheres is 584 °C and 539 °C, respectively.

Adhesive Films Based on Benzoxazine Resins and the Photoreactive Epoxyacrylate Copolymer

UV-cross-linkable and thermally curable self-adhesive structural tapes (SATs) were compounded using solid commercial benzoxazine resins (Araldite MT 35700 and Araldite MT 35910) and a photoreactive epoxyacrylate copolymer (EAC). As initiators of benzoxazine resin polymerization and epoxy component cationic polymerization, two kinds of latent curing agents (LCAs) were tested, i.e., amine type and ionic liquid type. The influence of the benzoxazine resin and the LCA type on the UV-cross-linking process, the self-adhesive features and thermal curing behavior of UV-cross-linked tapes, as well as the shear strength of cured aluminum/SAT/aluminum joints and thermal stability of adhesives were investigated. It was found that the amine additive and the benzoxazine resin take part in the UV-cross-linking process of the EAC as hydrogen donors, which is confirmed by an increase in cohesion (+86%) and a decrease in adhesion (−25%) of SATs. The highest results of adhesion to steel (47 N/25 mm) and overlap shear strength (11.1 MPa) values were registered for SATs based on Araldite MT 35910 and contained 7.5 wt. parts of the amine-type hardener. The formation of a polyacrylate-benzoxazine network has a significant impact on the course of the thermal curing process and the thermomechanical properties of adhesive joints, which was also confirmed by the Cure Index calculation.

Facile thiol-epoxy click chemistry for transparent and aging-resistant silicone/epoxy composite as LED encapsulant

The thiol-epoxy click chemistry is promising for microelectronic packaging due to its outstanding properties, while there is little knowledge on silicone/epoxy composite fabricated via this approach. Herein, a thiol-functionalized silicone resin (MDT-SH) is synthesized and compounded with epoxy resin to form silicone/epoxy composite. The thermal-curing behaviors, mechanical properties and LED encapsulation of the developed composite are studied detailedly. The thermal-curing kinetics indicate that the thiol-epoxy polymerization in silicone/epoxy composite tends more to the first-order reaction and its active energy is about 60.3 kJ/mol. The mechanical properties of the composite are varied with the molecular structure of MDT-SH and thiol/epoxy ratio. Moreover, the composite has excellent transparency in visible region, and its transparency is little affected by thermal and UV ageing. More importantly, the transparency is only slightly decreased after the composite is serviced at normal temperature and pressure for 2 years. The light output power of LEDs encapsulated by this composite is comparable to the commercial counterparts like Dow Corning OE-6550, but its thermal-curing time and temperature are much lower than the latter, contributing to cost reduction. Therefore, this silicone/epoxy composite based on thiol–epoxy click chemistry would greatly improve the microelectronic packaging.

Properties of bio-based thermosetting composites synthesized from epoxidized soybean oil and azo-cardanol benzoxazine

In this study, a new type of bio-based azobenzene-containing benzoxazine (Bza) was synthesized using p-aminoazobenzene, paraformaldehyde, and cardanol the raw materials, and the resulting Bza was mixed with bio-based epoxy soybean oil (ESO) in different composition ratios in the presence of adipic acid as a reaction curing agent, affording a bio-based thermosetting composite thermosets. The chemical structure of the synthesized Bza was characterized by Fourier transform infrared spectroscopy and 1H nuclear magnetic resonance spectroscopy. The thermal curing behavior in the absence of any catalyst was investigated by differential scanning calorimetry, indicating that the synthesized Bza exhibits certain liquid crystalline characteristics during heating. The existence of liquid crystal state was further confirmed by the polarizing microscope studies. The mechanical and thermal properties of the composite thermosets with different proportions of Bza were studied. The results show that the addition of increasing amount of Bza to ESO improved its performance to a certain extent; however, further addition beyond a certain range deteriorated the performance. Attributing to excellent processability and high performance of soybean oil-based thermosetting materials and photochromism of azobenzene compounds, the resulting composite materials endowing the characteristics of their components will have a wide range of applications in many fields.

Improvement of the oxidation resistance of silicon‐containing arylacetylene resins upon the introduction of carbazoles

AbstractSilicon‐containing arylacetylene resins (PSAs) can be used in high temperature environment due to their excellent thermal stability. However, their high temperature oxidation is still bottle‐neck for further application. Herein, Materials Genome Initiative (MGI) was utilized to identify a target monomer, 3,6‐diethynylcarbazole (DEC), to design new PSAs with enhanced antioxidant properties and heat resistance. After incorporation of DEC, the thermal curing behavior observed using differential scanning calorimetry (DSC), fourier transform infrared (FTIR) and thermogravimetric analysis (TGA) revealed that obtained silicon‐containing carbazolylacetylene resins (PSA‐VBC) can exhibit hydroamination reaction to decrease the initial curing temperature. And the temperature at 5% weight loss (Td5) occurred in cured copolymers ranged from 665°C to 691°C, which showed excellent heat resistance. Moreover, the oxidation behavior of cured resins was investigated by thermogravimetric/derivative thermogravimetry (TG/DTG), X‐ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM). Upon the incorporation of DEC, the thermal oxidation decomposition temperature of PSA‐VBC were 100°C higher than those observed for PSAs, which also proved by XPS analysis results that the oxygen content of PSA‐VBC solidified oxide was lower than that of PSAs. In addition, the surface morphology of cured PSA‐VBC resins still maintained integrity after oxidation at 400°C for 2 hr, which showed excellent oxidation resistance.

Effect of the initial F/U molar ratio in urea-formaldehyde resins synthesis and its influence on the performance of medium density fiberboard bonded with them

The formaldehyde to urea molar ratio (F/U) plays an important role on the properties of urea formaldehyde (UF) resins and of the medium density fiber board (MDFs) bonded with them. This work presents a hypothesis that besides the final F/U molar ratio in UF resin preparation, the initial F/U affects both the formaldehyde emission levels and the physico-mechanical properties of the boards. Three initial molar ratios F/U of 1.9, 2.1 and 2.3 in resin synthesis have been examined in this work. The structural changes and thermal curing behavior of UF resins were tracked by 13C NMR and DSC, respectively. At parity of final F/U molar ratio, the resin with initial F/U = 2.1 yielded the highest proportion of linear methylol groups, resulted in the best internal bond strength and lowest thickness swelling of the board. A resin synthesized with the initial F/U = 1.9 provided the highest proportion of total methylene linkages, most methylene ether linkage and lowest free formaldehyde, consistently yielding the lowest formaldehyde emission. These results partially explained why UF resins with lower F/U molar ratios showed relatively poor adhesion when used to manufacture wood-based composites. MDFs bonded with a UF resin having the initial F/U = 2.3 had a relatively good performance with the exception of free formaldehyde, but also showed the best storage stability compared to the other resins having lower initial F/U molar ratios.

Cardanol-based polybenzoxazine superhydrophobic coating with improved corrosion resistance on mild steel

Cardanol-based polybenzoxazine and amino-modified silica nanoparticles were combined to develop a superhydrophobic coating (PC-a/SiO2) on mild steel for anti-corrosion coating application. The thermal curing behavior of the PC-a/SiO2 coating was studied by Fourier transform infrared spectroscopy. The durable PC-a/SiO2 coating with a static water contact angle of 162.8 ± 2.9° could retain its superhydrophobic property even after heat treatment at 250 °C for 1 h or immersion in 3.5 wt% NaCl aqueous solution for 10 days. The electrochemical measurement results indicate that the PC-a/SiO2 coating exhibits superior corrosion resistance during prolonged immersion in 3.5 wt% NaCl aqueous solution. It is expected that the cardanol-based polybenzoxazine superhydrophobic coating is promising for practical applications of corrosion resistance.

Thermal Kinetics and Post Curing Thermal Stability of Bio-Based Phenol Formaldehyde Resins for Its Potential in Particleboard Production

The thermal degradation kinetics and post curing behavior of phenol-formaldehyde and Bio-phenol formaldehyde (BPF) resole resins were investigated with thermogravimetry analysis at four different heating rates (5, 10, 15 and 20°C/min) from 25 to 700°C. The curing reaction for all the resole resins proved to be approximately 1st order, and the activation energies of the BPF resins at 25 wt% and 50 wt% bio-phenol replacement were 132.2 kJ/mol and 131.6 kJ/mol. These were comparable to that of the laboratory synthesized pure PF resin 131.0 kJ/mol but also remarkably lower than the commercial PF resin (138.1 kJ/mol) respectively. The result of the thermal degradation kinetics showed that the post-curing thermal stability of the 25 and 50 wt% BPF resins were similar to that of the commercial and laboratory synthesized pure PF resin but, that of 75 wt% differed from the commercial or laboratory synthesized PF resin.

Characterization of a novel silicon‐containing hybrid polymer by thermal curing, pyrolysis behavior, and fluorescence analysis

ABSTRACTA novel linear silicon‐containing hybrid polymer with C  C and SiH linkages, diphenyl‐dihydroethynyl‐silane (DPHES), had been conveniently synthesized and well characterized. The thermal curing, crosslinked structure and pyrolysis behavior were studied by DSC, Fourier transform infrared (FTIR), and Pyrolysis‐gas chromatography/mass spectrometry (Py‐GC/MS). The results suggest the crosslinked structures consist mainly of phenyl and aromatic fused rings, which are formed through cyclotrimerization, addition reaction, and hydrosilylation reaction. The thermal properties indicate that the crosslinked DPHES exhibits excellent thermal stability with the temperature at 5% weight loss are higher than 572 and 509 °C under nitrogen and air atmosphere, respectively. The fluorescence emission spectra show DPHES and its crosslinked product also have moderate fluorescence properties, emitting purple light, and visible blue light, respectively, which further verified the structure of DPHES. These results suggest that DPHES has the potential to be used as light‐emitting materials with excellent thermal properties. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47403.

Catalytic effect of sulfonated poly (styrene-divinylbenzene) microspheres on the thermal curing behavior of benzoxazine

In this study, the effects of sulfonated poly (styrene-divinylbenzene) (SP(St-DVB)) microspheres on the thermal curing of bisphenol A-based benzoxazine (Ba) was studied by preparing resin systems consisting of Ba and SP(St-DVB) microspheres. The thermal curing behavior and mechanism of Ba/SP(St-DVB) systems were investigated by differential scanning calorimetry and in situ Fourier transform infrared spectroscopy. The curing kinetics of Ba and Ba/SP(St-DVB) systems were investigated by non-isothermal DSC. The apparent activation energy of Ba and Ba/SP(St-DVB) systems gradually increased with the reaction extent (α). The dynamic mechanical analysis indicated that the glass transition temperatures of PBa/SP(St-DVB) composites did not decrease. The improved thermal stability of PBa/SP(St-DVB) composites was investigated by thermogravimetric analysis in the air and nitrogen atmosphere, respectively. The images of the fracture surfaces for PBa and PBa/SP(St-DVB) composites were investigated by scanning electron microscopy, demonstrating improved toughness of PBa/SP(St-DVB) composites.

Investigation of cure advancement in dual-cure polyurethane-acrylate coatings over metal substrates

In the present study, a polyurethane acrylate (PUA) system cured via a thermal–UV (dual-cure process) was developed. The system selected for this work was a two-pack polyurethane acrylate with polyester polyol as the main component and urethane monoacrylate (UMA) as hardener. The polyester polyol was synthesized in a way to provide a final film coating containing both a suitable flexibility and high surface hardness. The thermal and photochemical curing behavior of the resin was studied via the chemorheology technique and the real-time FTIR. The Boltzmann sigmoidal model was implemented and well-fitted to the data obtained from the chemorheology measurements. The comparison between two reactive diluents, butanediol diacrylate (BDDA) and trimethylolpropane triacrylate (TMPTA) showed that BDDA reacts faster than TMPTA in the thermal curing condition. Nevertheless, the network buildup is stronger when TMPTA is used. The photopolymerization is also faster for the case of TMPTA. However, its final double bond conversion is restricted to a lower amount due to steric hindrance and higher viscosity of the system.

Synthesis and Characterization of Bio-Oil Phenol Formaldehyde Resin Used to Fabricate Phenolic Based Materials.

In this study, bio-oil from the fast pyrolysis of renewable biomass was used as the raw material to synthesize bio-oil phenol formaldehyde (BPF) resin—a desirable resin for fabricating phenolic-based material. During the synthesis process, paraformaldehyde was used to achieve the requirement of high solid content and low viscosity. The properties of BPF resins were tested. Results indicated that BPF resin with the bio-oil addition of 20% had good performance on oxygen index and bending strength, indicating that adding bio-oil could modify the fire resistance and brittleness of PF resin. The thermal curing behavior and heat resistance of BPF resins were investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Results showed that adding bio-oil had an impact on curing characteristics and thermal degradation process of PF resin, but the influence was insignificant when the addition was relatively low. The chemical structure and surface characteristics of BPF resins were determined by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The analysis demonstrated that adding bio-oil in the amount of 20% was able to improve the crosslinking degree and form more hydrocarbon chains in PF resin.