13C‐NMR study on cure‐accelerated phenol‐formaldehyde resins with carbonates

Both liquid‐ and solid‐state carbon‐13–nuclear magnetic resonance (¹³C‐NMR) spectroscopies were used to investigate the cure acceleration effects of three carbonates (propylene carbonate, sodium carbonate, and potassium carbonate) on liquid and cured phenol‐formaldehyde (PF) resins. The liquid‐phase ¹³C‐NMR spectra showed that the cure acceleration mechanism in the propylene carbonate‐added PF resin seemed to be involved in increasing reactivity of the phenol rings, whereas the addition of both sodium carbonate and potassium carbonate into PF resin apparently resulted in the presence of ortho–ortho methylene linkages. Proton spin‐lattice rotating frame relaxation time (T_1ρH) measured by solid‐state ¹³C cross polarization/magic‐angle spinning NMR spectroscopy was smaller for the cure‐accelerated PF resins than that of the control PF resin. The result indicated that the cure‐accelerated PF resins are less rigid than the control PF resin. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1284–1293, 2000     

13C‐NMR study on cure‐accelerated phenol–formaldehyde resins with carbonates

Both liquid‐ and solid‐state ¹³C‐NMR spectroscopies were employed to investigate the cure‐acceleration effects of three carbonates [propylene carbonate (PC), sodium carbonate (NC), and potassium carbonate (KC)] on liquid and cured phenol–formaldehyde (PF) resins. The liquid‐phase ¹³C‐NMR spectra showed that the cure‐acceleration mechanism in the PC‐added PF resin seemed to be involved in increasing reactivity of the phenol rings, while the addition of both NC and KC into PF resin apparently resulted in the presence of ortho–ortho methylene linkages. Proton spin‐lattice rotating frame relaxation time (T_1ρH) measured by solid‐state ¹³C‐CP/MAS‐NMR spectroscopy was smaller for the cure‐accelerated PF resins than for that of the control PF resin. The result indicated that cure‐accelerated PF resins are less rigid than the control PF resin. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 841–851, 2000     

Cure rate of Phenol–Formaldehyde (PF) resol resins catalyzed with MgO

One of the main drawbacks that has prevented a wider use of phenol–formaldehyde (PF) resins in the manufacture of impregnated paper and wood composite panels is their relatively slow cure rate. In this study, the curing characteristic of PF resol resins catalyzed with MgO was studied with various formaldehyde (F)/MgO/phenol (P) ratios at various pH values. The results indicated that the pH value, nature of pH regulator and synthesis parameters, including the F/P ratio and MgO content, all influence the rate of cure. The pH value played an important role in affecting both the cure rate and cure time. The cure rate was fast when pH was below 7.5. The cure time decreased as the pH value decreased at all F/MgO/P ratios. The MgO/P ratio had a definite influence on the cure rate, the cure time decreased with the increase of MgO/P molar ratio, and the F/P ratio had no significant influence on the cure rate. Differential scanning calorimetry (DSC) results showed that MgO catalyzed PF resin can cure at a low temperature.     

Synthesis and cure kinetics of liquefied wood/phenol/formaldehyde resins

Wood liquefaction was conducted at a 2/1 phenol/wood ratio in two different reactors: (1) an atmospheric three‐necked flask reactor and (2) a sealed Parr reactor. The liquefied wood mixture (liquefied wood, unreacted phenol, and wood residue) was further condensed with formaldehyde under acidic conditions to synthesize two novolac‐type liquefied wood/phenol/formaldehyde (LWPF) resins: LWPF1 (the atmospheric reactor) and LWPF2 (the sealed reactor). The LWPF1 resin had a higher solid content and higher molecular weight than the LWPF2 resin. The cure kinetic mechanisms of the LWPF resins were investigated with dynamic and isothermal differential scanning calorimetry (DSC). The isothermal DSC data indicated that the cure reactions of both resins followed an autocatalytic mechanism. The activation energies of the liquefied wood resins were close to that of a reported lignin–phenol–formaldehyde resin but were higher than that of a typical phenol formaldehyde resin. The two liquefied wood resins followed similar cure kinetics; however, the LWPF1 resin had a higher activation energy for rate constant k₁ and a lower activation energy for rate constant k₂ than LWPF2. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Cure kinetics of aqueous phenol–formaldehyde resins used for oriented strandboard manufacturing: Analytical technique

The cure kinetics of commercial phenol–formaldehyde (PF), used as oriented strandboard face and core resins, were studied using isothermal and dynamic differential scanning calorimetry (DSC). The cure of the face resin completely followed an nth‐order reaction mechanism. The reaction order was nearly 1 with activation energy of 79.29 kJ mol^?1. The core resin showed a more complicated cure mechanism, including both nth‐order and autocatalytic reactions. The nth‐order part, with reaction order of 2.38, began at lower temperatures, but the reaction rate of the autocatalytic part increased much faster with increase in curing temperature. The total reaction order for the autocatalytic part was about 5. Cure kinetic models, for both face and core resins, were developed. It is shown that the models fitted experimental data well, and that the isothermal DSC was much more reliable than the dynamic DSC in studying the cure kinetics. Furthermore, the relationships among cure reaction conversion (curing degree), cure temperature, and cure time were predicted for both resin systems. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1642–1650, 2006     

Catalytic and ortho-directing effect of Zn2+, Mg2+, Ba2+, and Ca2+ metal hydroxides on the preparation of phenolic-formaldehyde resin

A one step method was employed to prepare PF (phenol-formaldehyde) resin adhesive from phenol and formaldehyde with Zn²⁺, Mg²⁺, Ba²⁺, and Ca²⁺ hydroxides as catalysts, and the physicochemical properties of the resulting polymeric material were analyzed. The viscosity of the liquid adhesive decreased at intermediate catalyst concentrations, while the polymerization rate decreased at the highest concentrations; no clear pattern could be observed between catalyst concentration and solid content. Differential scanning calorimetry (DSC) performed on the PF resin catalyzed by Ba²⁺ showed that the polymer had a curing exothermic peak at around 110?°C, while the initial curing temperatures of PF resins were affected by the type of metal hydroxide used, and were in the order Ca²⁺?<?Mg²⁺ <control sample. FTIR spectroscopy was used to monitor the degree of ortho/para substitution during the synthesis of PF resin with Ba²⁺, Ca²⁺, and Mg²⁺ hydroxides. The highest ortho/para ratio was achieved with Ba²⁺ at a concentration of 0.45?wt.%, which also resulted in the most significant increase in the polymerization rate for the formation of PF resin.     

Curing properties of high-Ortho phenol-formaldehyde resins with co-catalysis

In this study, sodium carbonate (Na₂CO₃) was used as a catalyst to prepare high-ortho phenol-formaldehyde (HOPF) resin, and ester and carbonate curing accelerators were used to increase its curing rate. The physicochemical properties of the prepared resins and the mechanism of curing acceleration were investigated. The results showed that, with the addition of Na₂CO₃, the ortho/para ratio of methylol groups increased from 7.257 to 27.800. The gel time of the cure-accelerated HOPF resins decreased from 620 to 240 s as compared with PF resin. The bonding strength of plywood bonded with the cure-accelerated HOPF resins were all above 0.70 MPa. The curing acceleration was caused by the carbonate ions rather than the metal ions, and a temporary incorporation mechanism apparently occurred for the ester accelerators. The prepared phenolic resin had fast curing rate, low curing temperature, high thermal stability, and favorable mechanical performance, which has potential for industry applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47229.     

Characterization of epoxy resins by photochemical hole burning (PHB) spectroscopy

The phonon frequencies, E_s, measured as the energy differences between zerophonon hole peak and pseudophonon side hole peak in photochemical hole burning (PHB) spectra, were studied for tetraphenylporphin (TPP) in epoxy resin films under various conditions of sample preparation and annealing. The values of phonon frequency, E_s, reflecting low energy excitation modes of the matrix epoxy resins were about 13 to 16 cm^?1. They decreased when the epoxy resin was cured at relatively low temperatures and vice versa. At the same cure temperatures the sample quenched to liquid nitrogen showed a lower value than that of the annealed sample. These results are in agreement with the results of qualitative free volume measurements. The thermal stability of holes burned at 20 K was also tested, and the results were compared with the extent of cure and glass transition temperature of the resins.     

Accelerated cure of phenol–formaldehyde resins: Studies with model compounds

2‐Hydroxymethylphenol (2‐HMP) and 4‐hydroxymethylphenol (4‐HMP) were used as model compounds to study the reactions that occur during cure of phenol–formaldehyde (PF) resin to which cure accelerators (ethyl formate, propylene carbonate, γ‐butyrolactone, and triacetin) have been added. The addition of cure accelerators significantly increased the rate of condensation reactions. The cure accelerators were consumed during the reaction, indicating that they do not act as true catalysts. Major dimeric and trimeric reaction products were isolated and their structures determined. The results are consistent with a mechanism in which the hydroxymethyl group of 2‐HMP (or 4‐HMP) is first transesterified by the cure accelerator. The ester group is then displaced by reaction with the negatively charged ortho or para position of a second molecule (S_N2 mechanism) or is converted to a reactive quinone methide intermediate, which subsequently reacts with the negatively charged ortho or para position of a second molecule (quinone methide mechanism). When accelerators were added to the reaction mixture, the self‐condensation of 2‐HMP was faster than that of 4‐HMP. As is well documented in the literature, the exact opposite is true without added accelerators. This result would seem to indicate that the phenolic oxygen helps activate the esterified ortho‐hydroxymethyl group. The number and nature of crosslinks in a PF resin cured with added cure accelerator might be different than those in a PF resin cured without an added cure accelerator. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3256–3263, 2002     

Rheology Study of Sugar Cane Bagasse Lignin-Added Phenol–Formaldehyde Adhesives

The rheological properties of sugar cane bagasse lignin–phenol formaldehyde (PF) (30% lignin – PF) resins were studied using oscillation tests. The bagasse lignin was introduced in the classic adhesive formulation in order to supply a part of PF. Rheological qualities of optimal lignin–PF (30% lignin – PF) resins and commercial PF resin were assessed by using a rotary rheometer (ARES). Dynamic rheological measurements, performed at low strain in the linear range, are useful to characterize the network properties of resins.

The results obtained showed that the time sweep indicates excellent structural stability of optimal lignin–PF (30% lignin–PF) resins and commercial PF resin. The elastic modulus is greater than the viscous one showing a remarkable elastic character of the resins, and the performed frequency sweeps show a typical spectrum of a “weak gels” structure. The time dependence at 125°C shows that the optimum cure time is 7.5 min.     

Properties of phenol‐formaldehyde resins prepared from phenol‐liquefied lignin

In this study, alkaline lignin (AL), dealkaline lignin (DAL), and lignin sulfonate (SL) were liquefied in phenol with sulfuric acid (H₂SO₄) or hydrochloric acid (HCl) as the catalyst. The phenol‐liquefied lignins were used as raw materials to prepare resol‐type phenol‐formaldehyde resins (PF) by reacting with formalin under alkaline conditions. The results show that phenol‐liquefied lignin‐based PF resins had shorter gel time at 135°C and had lower exothermic peak temperature during DSC heat‐scanning than that of normal PF resin. The thermo‐degradation of cured phenol‐liquefied lignin‐based PF resins was divided into four temperature regions, similar to the normal PF resin. When phenol‐liquefied lignin‐based PF resins were used for manufacturing plywood, most of them had the dry, warm water soaked, and repetitive boiling water soaked bonding strength fitting in the request of CNS 1349 standard for Type 1 plywood. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Bark extractives-based phenol–formaldehyde resins from beetle-infested lodgepole pine

In this study, phenol–formaldehyde (PF) resins derived from the bark extractives were synthesized and characterized. Bark of lodgepole pine (Pinus contorta Dougl.) infested by mountain pine beetle (Dendroctonus ponderosae Hopkins) was first extracted with 1% NaOH. The bark extractives with and without acid-neutralization were then dried to the solid state. The neutralized and non-neutralized extractives were used to partially replace petroleum-based phenol for synthesizing the bark extractives-PF resins. In comparison with a commercial PF resin and a laboratory made PF resin (Lab PF), the bark extractive-PF resins were found to have higher molecular weights, higher viscosities, and shorter gel times. Acid neutralization of the bark extractives increased the molecular weight of the extractives and modified the performance and curing behavior of the resulting bark extractive-PF resins. Bark extractive-PF resins (BEPF) showed a similar level of post-cured thermal stability to that of the lab PF at higher temperatures, but they differed significantly from that of the commercial PF resin. The bark extractive-PF resins made from both neutralized and non-neutralized extractives at 30% replacement of phenol (by weight) exhibited similar dry and wet bond strengths to the commercial PF resin. At 50% substitution level, BEPF had dry and wet bond strengths similar to the lab PF resin. Our findings suggest that alkaline extractives from mountain pine beetle-infested lodgepole pine bark are suitable for partially substituting phenol in the synthesis of phenolic resin for use in wood adhesives.     

New type of phenolic resin: Curing reaction of phenol‐novolac based benzoxazine with bisoxazoline or epoxy resin using latent curing agent and the properties of the cured resin

In this study, we aimed to reduce the cure time, and to lower the cure temperature of the benzoxazine compound. Therefore, curing reaction of benzoxazine with bisoxazoline or epoxy resin using the latent curing agent and the properties of the cured resins were investigated. The cure behavior of benzoxazine with bisoxazoline or epoxy resin using the latent curing agent was monitored by differential scanning calorimetry and measurements for storage modulus (G′). The properties of the cured resin were estimated by mechanical properties, electrical insulation, water resistance, heat resistance, and flame resistance. As a result, it was confirmed that by using the latent curing agent, cure time of benzoxazine and bisoxazoline or epoxy resin was reduced, and cure temperature was lowered. And it was found that the curing reaction using phenol‐novolac based benzoxazine (Na) as the benzoxazine compound could proceed more rapidly than that using bisphenol‐A based benzoxazine (Ba) as the benzoxazine compound. However, the cured resins from Ba and bisoxazoline or epoxy resin using the latent curing agent showed good heat resistance, flame resistance, and mechanical properties compared with those from Na and bisoxazoline or epoxy resin using the latent curing agent. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effects of urea and curing catalysts added to the strand board core‐layer binder phenol–formaldehyde resin

Effects of adding urea to the strand board core‐layer phenol–formaldehyde (PF) resin were investigated in conjunction with cure‐accelerating catalysts. Ten percent urea based on the liquid resin weight was added at the beginning, at three different middle stages of polymerization, and at the end of PF resin synthesis. No significant cocondensation between the urea and PF resin components occurred as identified by ¹³C NMR analyses, which corroborated well with the curing and strand board bonding performance test results. The various urea addition methods resulted in resins that slightly differ in the various tests due to the urea's temporary holding capacity of formaldehyde. The preferred method of urea addition was found to do it in the later part of PF resin synthesis for convenience, consistency, and slightly better overall performance. Some cure‐accelerating catalysts were shown to reduce the thickness swelling of strand boards. This study showed the usefulness of adding some urea to strand board core‐layer binder PF resins of replacing higher cost phenolic components with lower cost urea. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

On the Cure Acceleration of Oil-Phenol-Formaldehyde Resins with Different Catalysts

Oil-phenol-formaldehyde (Oil-PF) resins containing 50 wt% replacement of petroleum phenol with bio-oil were prepared and different catalysts [sodium carbonate (Na₂CO₃), urea, and magnesium oxide (MgO)] were added in the synthesis process of resins to accelerate the cure. The cure-acceleration effects of catalysts on cure characteristics of oil-PF resins were investigated by using differential scanning calorimetry (DSC), gel time, and a plywood panels test. The results indicated that catalysts presented different accelerating effects on the cure of the oil-PF resin. Both Na₂CO₃ and MgO can accelerate the oil-PF resin cure at a low temperature; however, urea seemed to have no significant effect on the cure of the resin. The application of Na₂CO₃- and MgO-accelerated oil-PF resins reduced hot pressing time for the manufacture of three-layer plywood panels. Compared with MgO, Na₂CO₃ had more significant accelerating effect on the cure of the oil-PF resin.     

Comparison of model‐fitting kinetics for predicting the cure behavior of commercial phenol–formaldehyde resins

Phenol–formaldehyde (PF) resins have been the subject of many model‐fitting cure kinetic studies, yet the best model for predicting PF dynamic and isothermal cure has not been established. The objective of this research is to compare and contrast several commonly used kinetic models for predicting degree of cure and cure rate of PF resins. Toward this objective, the nth‐order Borchardt–Daniels (nth‐BD), ASTM E698 (E698), autocatalytic Borchardt–Daniels (Auto‐BD), and modified autocatalytic methods (M‐Auto) are evaluated on two commercial PF resins containing different molecular weight distributions and thus cure behaviors. The nth‐BD, E698, and M‐Auto methods all produce comparable values of activation energies, while Auto‐BD method yields aberrant values. For dynamic cure prediction, all models fail to predict reaction rate, while degree of cure is reasonably well predicted with all three methods. As a whole, the nth‐BD method best predicts degree of cure for both resins as assessed by mean squared error of prediction. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Time–temperature–transformation cure diagrams of phenol–formaldehyde and lignin–phenol–formaldehyde novolac resins

In this study, the time–temperature– transformation (TTT) cure diagrams of the curing processes of several novolac resins were determined. Each diagram corresponded to a mixture of commercial phenol–formaldehyde novolac, lignin–phenol–formaldehyde novolac, and methylolated lignin–phenol–formaldehyde novolac resins with hexamethylenetetramine as a curing agent. Thermomechanical analysis and differential scanning calorimetry techniques were applied to study the resin gelation and the kinetics of the curing process to obtain the isoconversional curves. The temperature at which the material gelled and vitrified [the glass‐transition temperature at the gel point (_gelT_g)], the glass‐transition temperature of the uncured material (without crosslinking; T_g0), and the glass‐transition temperature with full crosslinking were also obtained. On the basis of the measured of conversion degree at gelation, the approximate glass‐transition temperature/conversion relationship, and the thermokinetic results of the curing process of the resins, TTT cure diagrams of the novolac samples were constructed. The TTT diagrams showed that the lignin–novolac and methylolated lignin–novolac resins presented lower T_g0 and _gelT_g values than the commercial resin. The TTT diagram is a suitable tool for understanding novolac resin behavior during the isothermal curing process. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Model‐free kinetics: Curing behavior of phenol formaldehyde resins by differential scanning calorimetry

Isoconversional analysis was used to treat nonisothermal DSC data and yield the dependence of activation energy on conversion during the curing process of PF resins. The shape of the dependence revealed that the curing process of PF resins displayed a change in the reaction mechanism from a kinetic to a diffusion regime. In the kinetic regime a comparative DSC experimental analysis between monomer mixtures and PF resins showed that the addition reactions between phenol and formaldehyde had been mostly completed during the synthesis of PF resins and that the main kinetic reactions contained parallel condensations in the curing process. For the diffusion regime a modified equation for the diffusion rate constant, k_D = D₀ exp(?E_D /RT + K₁α + K₂α²), is proposed. This equation is in good agreement with the experimental dependence of E_α on α in the diffusion regime, which shows the effect of both temperature and conversion on diffusion. A prediction of the conversion advancement with the reaction time under isothermal condition for PF resin has been made. This prediction can be useful in practical applications for evaluating isothermal behavior of thermosetting systems from nonisothermal experimental data. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 433–440, 2003     

Water vapor transport in an epoxy resin based on TGMDA and DICY

Water uptake has been measured in an epoxy resin based on tetraglycidylmethylenedianiline curved with dicyanidamide. The curing behavior of this system as elucidated by differential scanning calorimetry and Fourier transform infrared is complex. Based upon this information we selected curing temperatures and times in addition to the “standard” cure. The kinetics of the sorption of water by the materials which have undergone the standard cure indicate that the two modes of sorption are involved at high humidity and only a single mode at lower humidity (as seen by changes in the slope of the log M_t vs log t plots). The kinetics of the sorption in the resins which have undergone post cure at higher temperatures also indicate two or more modes of sorption at high humidities. However the slopes of the log M_t vs log t plots differ from those for the resin with standard cure. Subsequent sorption/desorption cycles on the standard cure resin showed marked increases in the initial sorption rate as well as changes in mode, suggesting that irreversible changes in the resin had occurred.     

Physicomechanical and thermal properties of moldings made from liquefied wood‐based novolak PF resins under various hot‐pressing conditions

The wood powder of Cryptomeria japonica (Japanese cedar) was liquefied in phenol, with H₂SO₄ and HCl as a catalyst. The liquefied wood was used to prepare the liquefied wood‐based novolak phenol formaldehyde (PF) resins by reacting with formalin. Furthermore, novolak PF resins were mixed with wood flour, hexamethylenetetramine, zinc stearate as filler, curing agent, and lubricating agent, respectively, and hot‐pressed under 180 or 200°C for 5 or 10 min to manufacture moldings. The results showed that physicomechanical properties of moldings were influenced by the hot‐pressing condition. The molding made with hot‐pressing temperature of 200°C for 10 min had a higher curing degree, dimensional stability, and internal bonding strength. The thermal analysis indicated that using a hot‐pressing temperature of 180°C was not sufficient for the liquefied wood‐based novolak PF resins to completely cure. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009