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     

Method for time–temperature–transformation diagrams using DSC data: Linseed aliphatic epoxy resin

A novel method to generate time–temperature–transformation (TTT) diagrams from Differential Scanning Calorimetry (DSC) data is presented. The methodology starts with dynamical DSC information to obtain the total transformation heat, followed by an isothermal‐dynamic temperature ramp that allows the inclusion of diffusion‐controlled reaction kinetic. The cure kinetics is modeled using an auto‐catalytic Kamal–Sourour model, complemented with a Kissinger model that allows the direct prediction of one energy of activation, DiBenedetto's equation for the glass transition temperature as a function of the cure degree and adjusted reaction constants to include diffusion mechanisms. The methodology uses a nonlinear least‐squares regression method following J.P. Hernández‐Ortiz and T.A. Osswald's methodology (J. Polym. Eng. 2004, 25, 23). A typical linseed epoxy resin (EP) presents two different kinetics control mechanisms, thereby providing a good model to validate the proposed experimental and theoretical method. TTT diagrams for EPs at two different accelerator concentrations are calculated from direct integration of the kinetic model. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40566.     

Evaluation of isomeric composition of resol‐type phenol formaldehyde matrix resins for silica–phenolic composites and its effect on cure characteristics of the resin

The isomeric composition of several samples of resol‐type phenol formaldehyde resins used for silica–phenolic composites was evaluated by ¹H‐ and ¹³C‐NMR spectroscopy and reverse‐phase HPLC techniques. The variations in the isomeric compositions were attributed to inadvertent variations in the process parameters. A mathematical relation was determined for calculation of free phenol from ¹H‐NMR measurements. The samples were cured at 160°C for 8 h in an inert atmosphere of N₂. The extent of cure in the hardened samples was measured by FTIR analysis. The effect of isomeric composition on the extent of cure was studied. Free phenol and p‐hydroxymethyl phenol, exhibiting a linear correlation, were found to have a pronounced effect on the extent of cure. The cure kinetics were derived by dynamic DSC measurements. Activation energy (E) for curing exhibited a near linear correlation independently with free phenol content and the extent of cure. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2517–2524, 2003     

Comparisons of the cure of phenol–formaldehyde novolac and resol systems by differential scanning calorimetry

Comparisons were made of differential scanning-calorimetric (DSC) thermograms of both liquid and powdered commercial phenol–formaldehyde resins. By a combination of the results from analyses under a variety of conditions, such as ambient pressure, high pressure, using freeze-dried samples, and also by direct observation of the resin-curing process in wood-veneer assemblies, the curing reactions of phenol–formaldehyde resins were found to differ for resol and novolac systems. At a heating rate of 10°C/min, the resol resin showed endothermic curing reactions at temperatures of about 150°C, while the novolac-type resin showed an exothermic peak maximum at about 160°C. Results are presented to show how DSC can be used to differentiate between a resol and novolac system.     

Effect of thermal exposure on the properties of phenolic composites: Dynamic mechanical analysis

Changes in the dynamic response of glass‐reinforced phenolic composites following thermal exposure at 180^oC for periods of time up to 28 days were monitored using dynamic mechanical analysis. Four phenolic resins were investigated: a resol/novolac blend, a phenolic–furan novolac/resol graft copolymer, a novolac, and a resol. Reactive blending and copolymerization of phenolic resins are currently being investigated to determine if these techniques will produce phenolic resins (and composites) that have improved impact properties and retain the excellent high‐temperature properties of resol and novolac phenolic resins. The results indicate that thermal aging at 180^oC for 1 day led to a more complete cure of all four phenolic resins as indicated by an increase in the temperature of the maximum of plots of both loss modulus (E″) and tan δ versus temperature. The storage modulus (E′) of the composites at 40^oC varied little following thermal aging at 180^oC for 1 day but decreased with increasing exposure time for samples aged 2, 7, and 28 days. Thermal aging led to an increase in E′ at higher temperatures and the magnitude of E′ at a given temperature decreased with increasing exposure time. The magnitude of E″ and tan δ decreased with aging time for all resins, although E″ and tan δ were larger for the blend and copolymer composites than for the novolac and resol composites. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 385–395, 2001     

New thermosets obtained by cationic copolymerization of DGEBA with γ‐caprolactone with improvement in the shrinkage. II. Time–temperature–transformation (TTT) cure diagram

Mixtures of diglycidylether of bisphenol A (DGEBA) with different proportions of γ‐caprolactone (γ‐CL) were cured with ytterbium triflate as initiator. The curing was studied with differential scanning calorimetry (DSC) and thermo mechanical analysis (TMA). The results are presented in the form of a time–temperature–transformation diagram. The kinetic analysis was performed by means of the isoconversional integral procedure and the kinetic model was also determined using the Coats–Redfern method. Gelation was determined by means of combined experiences of DSC and TMA. The relationship between the glass transition temperature (T_g) and the degree of conversion α was determined by DSC. Using the isoconversional lines and the T_g‐α relationship, the vitrificacion curve was obtained. The methodology developed makes it possible to obtain the TTT diagram using only no‐isothermal experiments with equivalent results to those using classical isothermal procedures. The addition of γ‐CL accelerates the curing and reduces the shrinkage after gelation and consequently the internal stresses in the material. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Activation energy and curing behavior of resol‐ and novolac‐type phenolic resins by differential scanning calorimetry and thermogravimetric analysis

The thermal behavior, thermal degradation kinetics, and pyrolysis of resol and novolac phenolic resins with different curing conditions, as a function of the formaldehyde/phenol (F/P) molar ratio (1.3, 1.9, and 2.5 for the resol resins and 0.5, 0.7, and 0.9 for the novolac resins) were investigated. The activation energy of the thermal reaction was studied with differential scanning calorimetry at five different heating rates (2, 5, 10, 20, and 40°C/min) between 50 and 300°C. The activation energy of the thermal decomposition was investigated with thermogravimetric analysis at five different heating rates (2, 5, 10, 20, and 40°C/min) from 30 to 800°C. The low molar ratio resins exhibited a higher activation energy than the high molar ratio resins in the curing process. This meant that less heat was needed to cure the high molar ratio resins. Therefore, the higher the molar ratio was, the lower the activation energy was of the reaction. As the thermal decomposition of the resol resins proceeded, the activation energy sharply decreased at first and then remained almost constant. The activation energy of the thermal decomposition for novolac resins with F/P = 0.5 or F/P = 0.7 was almost identical in all regions, whereas that for novolac resins with F/P = 0.9 gradually decreased as the reaction proceeded. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2589–2596, 2003     

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     

Synthesis of phenolic resol resins using cornstalk-derived bio-oil produced by direct liquefaction in hot-compressed phenol–water

For the synthesis of biomass-based resol resins, cornstalk powders were liquefied in a hot-compressed phenol–water (1:4, wt./wt.) medium at 300–350 °C. It was observed that essentially no phenol was reacted with the cornstalk degradation intermediates during the liquefaction process. The cornstalk-derived bio-oils contained oligomers of phenol and substituted phenols, originated primarily from the lignin component of the cornstalk feedstock. Using the cornstalk-derived bio-oils, resol resins were readily synthesized under the catalysis of sodium hydroxide. The biomass-derived resol resins were brown viscous liquids, possessing broad molecular weight distributions. In comparison with those of a conventional phenol resol resin, the properties of the bio-based resins were characterized by GPC, FTIR, DSC and TGA. The as-synthesized bio-oil resol resin exhibited typical properties of a thermosetting phenol–formaldehyde resin, e.g., exothermic curing temperatures at about 150–160 °C, and an acceptable residual carbon yield of ca 56% at 700 °C for the cured material.     

Phenol–formaldehyde resin curing and bonding in steam-injection pressing. I. Resin synthesis,characterization, and cure behavior

Two different phenol–formaldehyde (PF) resole resins are serving as models in a study aimed at establishing the effects of moisture, temperature, pressure, and time on resin cure and bonding during the pressing of wood flakeboard. This phase of the program had two goals: first, to characterize the two resins in terms of their structure and chemistry during synthesis, aging, and cure—using viscosity measurement, gel permeation chromatography (GPC), nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and dynamic mechanical analysis (DMA); second, to make a preliminary evaluation of the utility of DSC, FTIR, and DMA for measuring the degree of resin cure. The two resins differed significantly in relative amounts of hydroxymethyl groups and methylene linkages (NMR), in molecular weight and its distribution (GPC), and in reaction rate (as measured by viscosity, DSC, FTIR, or DMA). The degree of cure developed during constant heating rate DSC scans was calculated for a series of maximum DSC temperatures from both the loss in hydroxymethyl groups (FTIR) and the decrease in available exothermic heat (DSC). Agreement between the two methods was quite good, considering the inherent difficulties in quantifying infrared data. For comparison, the degree of cure developed during constant heating rate DMA scans was calculated for a series of maximum DMA temperatures from both the increase in storage modulus (DMA) and the decrease in exothermic heat (DSC after rewetting). Samples that apparently achieved complete cure in the DMA still exhibited significant residual cure potential in the DSC. We attribute the lower apparent cure in the DMA to loss of moisture from samples during the DMA scan, with consequent loss in plasticization and molecular mobility.     

Time–temperature–transformation (TTT) and TTT–viscosity diagrams of a typical benzoxazine resin

Benzoxazine resins have attracted much attention because of their excellent properties. As a new kind of thermosetting resin, their gelation and vitrification behaviors during the curing process are worth studying for promoting their development, but few research works have been done on this subject. In this work, an ordinary diamine-type benzoxazine resin (PH-ddm) was chosen as a research object. Its curing kinetics were studied by differential scanning calorimetry (DSC) and a phenomenological model was used to get equal curing degree curves reflecting the relationship among curing degree, curing temperature and time. Moreover, a gelation curve and a vitrification curve of PH-ddm based on an Arrhenius equation and a DiBenedetto equation were obtained by parallel plate rheometer and modulated DSC (MDSC), respectively. Then, a well-known time–temperature–transformation (TTT) diagram was plotted. Besides, the rheological behavior of PH-ddm was analyzed through rotatory viscometer testing and modified double Arrhenius equation to obtain the equal viscosity curves. A TTT–viscosity diagram was obtained through combining the equal viscosity curves with the TTT diagram. This is the first time for using the TTT diagram to investigate the curing process of benzoxazine resins. Our results provide an effective method to optimizing molding conditions of the benzoxazine resins.     

Effect of synthesis parameters on thermal behavior of phenol–formaldehyde resol resin

Differential scanning calorimetry (DSC) was used to investigate the influence of resin synthesis parameters on the thermal behavior of low molecular weight phenol–formaldehyde (PF) resol resins prepared with different formaldehyde/phenol (F/P) molar ratios, different sodium hydroxide/phenol (NaOH/P) molar ratios, and different catalysts. As the F/P molar ratio increased, the molecular weight and activation energy increased while the gel time, peak temperature, resin pH, and nonvolatile solids content decreased. By contrast, the molecular weight, gel time, resin pH, resin solids content, and peak temperature increased with an increasing NaOH/P molar ratio. However, the activation energy decreased with an increasing NaOH/P molar ratio. The polydispersity increased with both F/P and NaOH/P ratios. Calcium hydroxide gave a faster curing resin compared to sodium and potassium hydroxides. All DSC thermograms of this study showed just a single exothermic peak for the resins that were used. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1415–1424, 2002     

Thermally resistant carbazole‐modified phenol–formaldehyde resins

Phenol–formaldehyde resins were modified with carbazole in order to improve their thermal resistance. Attempts to incorporate carbazole rings into novolac and resol resins were made using three methods: (1) the addition of N‐(hydroxymethyl)carbazole (HMC) into a phenol–formaldehyde mixture, (2) the addition of carbazole into a phenol–hydroxymethyl derivative of acetone mixture, where the hydroxymethyl derivative of acetone was used as formaldehyde donor, and (3) by prolonging the time of high‐temperature reaction between phenol, carbazole and formaldehyde. The temperature and time of reaction were critical for incorporation of carbazole, which successfully led to highly temperature‐resistant carbazole‐modified novolacs for the latter procedure. When carbazole was incorporated into novolac structure at a level of 8 mol%, the thermal resistance increased by 118 °C measured as 5% mass loss temperature. Other procedures led to solids containing carbazole or HMC as physical admixtures. The obtained composites revealed variable thermal resistance effects; the carbazole‐modified resol containing 9 mol% of carbazole showed 47 °C increase of thermal resistance in comparison with non‐modified resol, measured as 5% mass loss temperature. © 2015 Society of Chemical Industry     

Effects of diffusion on the kinetic study and TTT cure diagram for an epoxy/diamine system

The curing reactions of an epoxy system consisting of a diglycidyl ether of bisphenol A (BADGE n = 0) and 1,2-diamine cyclohexane (DCH) were studied to determine a time–temperature–transformation (TTT) isothermal cure diagram for this system. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and a solubility test were used to obtain the different experimental data reported. Two models, one based solely on chemical kinetics and the other accounting for diffusion, were used and compared to the experimental data. The inclusion of a diffusion factor in the second model allowed for the cure kinetics to be predicted over the whole range of conversion covering both pre- and post-vitrification stages. The investigation was made in the temperature range 60–100°C, which is considered optimum for the isothermal curing of the epoxy system studied. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1931–1938, 1998     

New soybean oil–styrene–divinylbenzene thermosetting copolymers. VI. Time–temperature–transformation cure diagram and the effect of curing conditions on the thermoset properties

A new class of thermosetting resins has been developed that is based on the cationic copolymerization of regular soybean oil (SOY), low saturation soybean oil (LSS), or conjugated LSS (CLS) with various alkene comonomers initiated by boron trifluoride diethyl etherate (BFE) or related modified initiators. The activation energy for the gelation process for these thermosets ranges from 95 to 122 kJ/mol. A time‐temperature‐transformation (TTT) isothermal cure diagram has been established for the model system LSS45–ST32–DVB15–(NFO5–BFE3) ie 45 wt% low saturation soybean oil, 32 wt% styrene, 15 wt% divinylbenzene, and 5 wt% Norway fish oil ethyl ester plus 3 wt% boron trifluoride diethyl etherate. The effect of curing conditions on the thermophysical and mechanical properties, including the mechanical damping and shape memory properties, has been subsequently investigated using this model thermoset. These findings allow the efficient optimization of desired properties for specific applications. © 2003 Society of Chemical Industry     

Influence of the synthesis conditions on the curing behavior of phenol–urea–formaldehyde resol resins

The curing behavior of synthesized phenol–urea–formaldehyde (PUF) resol resins with various formaldehyde/urea/phenol ratios was studied with differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The results indicated that the synthesis parameters, including the urea content, formaldehyde/phenol ratio, and pH value, had a combined effect on the curing behavior. The pH value played an important role in affecting the shape of the DSC curing curves, the activation energy, and the reaction rate constant. Depending on the pH value, one or two peaks could appear in the DSC curve. The activation energy was lower when pH was below 11. The reaction rate constant increased with an increase in the pH value at both low and high temperatures. The urea content and formaldehyde/phenol ratio had no significant influence on the activation energy and rate constant. DMA showed that both the gel point and tan δ peak temperature (T_tanδ) had the lowest values in the mid‐pH range for the PUF resins. A different trend was observed for the phenol–formaldehyde resin without the urea component. Instead, the gel point and T_tanδ decreased monotonically with an increase in the pH value. For the PUF resins, a high urea content or a low formaldehyde/phenol ratio resulted in a high gel point. The effect of the urea content on T_tanδ was bigger than that on the gel point because of the reversible reaction associated with the urea component. Too much formaldehyde could lead to more reversible reactions and a higher T_tanδ value. The effects of the synthesis conditions on the rigidity of the cured network were complex for the PUF resins. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 1368–1375, 2005     

Jute felt composite from lignin modified phenolic resin

Raw and dewaxed jute felt composites were prepared with resol and lignin modified phenol formaldehyde resin. Four different types of lignin modified resins were used by replacing phenol with lignin. The lignin modified resins were prepared from purified lignin obtained from paper industry waste black liquor. To investigate bonding between jute and resin, IR spectroscopy of jute felts and composites was carried out. The thermal stability of the composites was assessed by DSC and TGA. It was found that the lignin resin jute composite is thermally more stable than resol composite. XRD of jute felt and composite shows that the crystallinity of the jute fiber increases after composite preparation. The lignin resin composites were tested for water absorption and thickness swelling, and it was found that the results are comparable with those of resol jute composite. Composites prepared from lignin phenol formaldehyde resin with 50% phenol replacement has shown 75% tensile strength retention to that of pure resol jute composite.     

Studies and characterization of RESOL/VAc–EHA/HMMM IPN systems in aqueous medium

Resol was solution‐blended with vinyl acetate‐2‐ethylhexyl acrylate (VAc–EHA) resin in an aqueous medium, in varying weight fractions, with hexamethoxymethylmelamine (HMMM) as a crosslinker and the data were compared with a control. The present work was aimed to obtain an optimum combination of high‐temperature resistance by synthesis of an interpenetrating network (IPN) of the resins. The control gave a semi‐IPN system, in which the resol crosslinked, while the acrylic did not, whereas the blend, where HMMM was the crosslinker, gave a full‐IPN system. FTIR spectra of the blends of resol/VAc–EHA/HMMM indicated the formation of new stretching, which was generated due to crosslinking reactions among VAc–EHA and the crosslinker HMMM. TGA showed that, with an increase in the VAc–EHA percent in semi‐IPNs, the decomposition temperature decreased gradually, whereas in case of full‐IPNs, the decomposition temperature increased with increase in the VAc–EHA percent. However, the full‐IPNs had a higher decomposition temperature than that of the semi‐IPNs, at the same resol/(VAc–EHA) ratio. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3581–3588, 2002     

TTT‐diagram for epoxy film adhesives using quasi‐isothermal scans with initial fast ramps

The characterization of film adhesives is challenging because they required freezer storage, contain an inseparable filler—thermoplastic knit or fiber‐reinforcement, and are heat activated systems with a pre‐cure and unknown chemistry. A testing protocol that eliminates these sources of error is proposed. This study presents a method to generate time–temperature‐transformation (TTT) diagrams of epoxy film adhesives via differential scanning calorimetry (DSC). Non‐isothermal and isothermal DSC scans are used to capture the reaction and the glass transition temperature. The use of an initial fast ramp—up to 500 K/min—in the isothermal scans is explored for the first time. This technique shows the potential to produce a quasi‐isothermal cycle, eliminating the loss of data in the initial stage of the reaction. The total heat released, the activation energy, and the fractional kinetic parameter, are estimated via model‐free methods. The Kamal–Sourour model and the formal kinetic model are fit to model the rate of cure. The simplest model that accurately captures the reaction, a parallel two‐step model, A , is outlined. The glass transition temperature is modeled via DiBenedetto's equation to include the diffusion‐controlled mechanism. The TTT‐diagrams of two commercial adhesives, DA 408 and DA 409, are shown with an analysis of processing optimization. The use of quasi‐isothermal scans with initial fast ramps combined with the correction for filler, moisture, and pre‐curing history can be applied to characterize fast curing thermosets, complex B‐stage resins, and thermosetting composites. The modeling results can also be used in numerical studies of residual stresses and dimensional stability in the manufacturing of thermosetting composites. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45791.     

CHT and TTT curing diagrams of polyflavonoid tannin resins

TTT and CHT curing diagrams for tannin-based adhesives were built by thermomechanical analysis (TMA) by following the in situ hardening directly in a wood joint, and the curve trends observed were similar to those previously observed for synthetic polycondensation resins on lignocellulosic substrates. Of the parameters that most influence the relative position of vitrification and gel curves on the diagrams (i.e., where the influence has been quantified), chief among them is the reactivity of the tannin with formaldehyde and any factor influencing it: thus, the inherent higher reactivity of the A-ring of the tannin (such as in procyanidins versus prorobinetinidins) and the pH of the tannin solution. The percentage formaldehyde hardener has some influence in CHT diagrams, especially for the slower-reacting tannins, but practically no influence in TTT diagrams within the 4–10% formaldehyde range used. As in the case of synthetic polycondensation adhesive resins, regression equations relating the internal bond strength of a wood particleboard, prepared under controlled conditions, with the inverse of the minimum deflection, obtained by constant heating rate TMA of a wood joint during resin cure, have been obtained for the two types of tannins of lower reactivity (profisetinidins/prorobinetinidins) but not for the faster-reacting procyanidin and prodelphinidin tannins. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 3220–3230, 2001