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     

Colloidal aggregation of aminoplastic polycondensation resins: Urea–formaldehyde versus melamine–formaldehyde and melamine–urea–formaldehyde resins

Colloidal particles formation followed by their clustering have been shown to be the normal way of ageing of aminoplastic resins, namely urea–formaldehyde (UF) resins, melamine–formaldehyde (MF) resins, and melamine–urea–formaldehyde (MUF) resins. Ageing or further advancement of the resin by other means such as longer condensation times causes whitening of the resin. This is a macroscopic indication of both the formation of colloidal particles and of their clustering. It eventually progresses to resins, which are mostly in colloidal, clustered state, followed much later on by a supercluster formation starting to involve the whole resin. The initial, filament‐like colloidal aggregates formed by UF resins have different appearance than the globular ones formed by MF resins. MUF resins present a short rod‐like appearance hybrid between the two. GPC has been shown to detect the existence of colloidal superaggregates in a UF resin, while smaller aggregates might not be detected at all. The star‐like structures visible in the colloidal globules of MF resins are likely to be light interference patterns of the early colloidal structures in the resins. These star‐like interference patterns become more complex with resin ageing or advancement due to the advancement of the resin to more complex aggregates, to eventually reach the stage in which filament‐like and rod‐like structures start to appear. The next step is formation of globular masses that are representative of the true start of physical gelation. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1406–1412, 2006     

Modification of acetophenone–formaldehyde and cyclohexanone–formaldehyde resins

Cyclohexanone–formaldehyde and acetophenone–formaldehyde resin were in situ modified with phenol, Bisphenols, and substituted acetophenones. Furthermore, acetophenone–formaldehyde, cyclohexanon–formaldehyde, and in situ-modified resins were modified with anhydrides such as acetic anhydride, maleic anhydride, dodecenylsuccinic anhydride, 3,4,3′,4′-biphenyltetracarboxylic dianhydride, and 4,4′-oxydiphtalic anhydride. Modification of these resins with hydroxyl amine, semicarbazide, and phenyl hydrazine were also studied. Melting points, solubilities in organic solvents, FTIR, and NMR spectrum of the modified resins were determined. © 1996 John Wiley & Sons, Inc.     

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     

Pyrolysis of melamine–formaldehyde and urea–formaldehyde resins

Pyrolysis of melamine–formaldehyde and urea–formaldehyde resins in helium and air was investigated by means of TG and gravimetry with isothermal heating, as well as elemental and HCN analyses. Weight loss curves suggest three kinds of reactions involved in the pyrolysis, namely, initiation reactions, reactions splitting off volatile fragments, and reactions forming stabilized structures. In TG, in both helium and air atmospheres, the active weight loss of the melamine resin was completed in two steps, and that of the urea resin was completed in one step, which, however, consisted of a few small successive steps. The isothermal heating weight losses progressed through a few stages of first- and zeroth-order reactions. Arrhenius parameters were obtained for the weight losses in TG and with isothermal heating. The residue from the melamine resin is rich in carbon and nitrogen, and poor in oxygen and hydrogen, while that from the urea resin is rich in carbon, and poor in nitrogen, oxygen, and hydrogen. The effects of temperature on HCN yield changed, depending on the amount of air fed. The melamine resin evolved much more HCN than the urea resin because of the more stable C—N linkages in the resin.     

Infrared spectral study on the heat curing reaction of glycerin-terminated urethane prepolymers

Glycerin, toluene diisocyanate (TDI), and polyglycol (PG) were reacted at various molar ratios to produce glycerin-terminated urethane prepolymers of different molecular weights. The prepolymers were mixed with equivalent phenol-blocked trimethylol propane–TDI–urethane triisocyanate in m-cresol to give a coating solution. The solution was coated and baked to give polyurethane crosslinked films. The changes of the functional groups during the crosslinking reaction and the mechanical properties of the polyurethane crosslinked films were studies. Experimental results show that the phenol-blocked urethane triisocyanate will deblock phenol to regenerate free isocyanate groups above 120°C and then react with the hydroxyl groups of urethane prepolymers. At 220°C, the rate of deblocking phenol to regenerate isocyanate groups is faster than that of the reaction of urethane prepolymers with isocyanate groups. The deblocking reaction is contemporaneous with the reaction of isocyanate groups with hydroxyl groups, so that the characteristic absorption peaks of isocyanate groups can be observed from IR spectra during the crosslinking reaction. The absorption peak of isocyanate groups gradually decreased with the crosslinking reaction, but the absorption peak increased after curing for about 50–60 min. This feature is caused by the reactivity of the secondary hydroxyl groups of glycerin which is slower than that of the primary hydroxyl groups of glycerin.     

Multifunctional formaldehyde resins as curing agent for epoxy resins

Formaldehyde resins (FR) at 1/1/2 molar ratios of monomers (Cl‐phenol/amino monomers/p‐formaldehyde) were synthesized under acid catalysis. The obtained resins were characterized using elemental analysis, FTIR and RMN spectroscopic methods, being used as crosslinking agents for epoxy resin formulations. The curing of epoxy resins with FR were investigated. The glass transition temperature (T_g) and decomposition behavior of crosslinked resins were studied by differential scanning calorimetry (DSC) and thermogravimetric (TGA) techniques. All DSC scans show two exothermic peaks, which implied the occurrence of cure reactions between epoxy ring and amine or carboxylic protons, in function of chemical structures of FR. The crosslinked products showed good thermal properties, high glass transitions, and low water absorption. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Studies on applications of natural polyphenol–phenol–formaldehyde copolymer based cation exchange resins

Naturally occurring polyphenol(condensed tannin)-phenol-formaldehyde copolymer matrix based cation exchange resins have been successfully used as a solid catalyst (in H⁺ ion form) in the inversion of sucrose and hydrolysis of methyl acetate. Apart from the normal advantages of the solid catalyst over the acid catalysts in the solution phase, the values of the activation energy, entropy, and enthalpy of activation of the reactions involved reveal that the resins prepared acted as a catalyst with characteristics comparable to those of the commercially available resins based on a styrene–DVB matrix. The resins have also been successfully used for softening of hard water and enrichment of metal ion concentration from very dilute solutions. © 1995 John Wiley & Sons, Inc.     

Synthesis and structure–property relationships of UV-curable urethane prepolymers with hard–soft–hard blocks

UV-curable urethane prepolymers derived from diisocyanates, rigid diols, and polypropylene glycols of different molecular weights and end-capped with 2-hydroxylethyl methacrylate were prepared by stepwise addition reactions. These prepolymers have the common structure, HEMA-hard segment-soft segment-hard segment-HEMA. As the weight ratio of soft segment to hard segment and thus the rigidity varied, the cured films from these prepolymers exhibited a wide range of mechanical and other key properties such as oxygen permeability.     

Fibers from urea–formaldehyde resins

The preparation of fibers from aqueous urea–formaldehyde resins has been investigated; a dry spinning process has been developed based on the extrusion of catalyzed resin into a drying chamber at 180–220°C, producing a multifilament yarn at spinning speeds of up to 600 m/min. A range of UF filaments was produced with diameters between 10–70 μm; the tenacities of spun filaments were 6–10 cN/tex, initial moduli were 220–340 cN/tex, and elongation at break was 4–10%. The best tensile properties resulted from conditions that produced the smallest diameter fibers. Postspin heat treatment improved the tenacity to 14 cN/tex and the elongation to 20%. Spinnability improved with increased viscosity of the spinning solution and increased cell temperature, while tenacity and elongation increased with increasing cell temperature and spinning stretch. A correlation was found between TGA weight loss (between 105 and 200°C) and fiber tenacity. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 64–74, 2000     

Modification of cyclohexanone–formaldehyde resins with silicone tegomers

Cyclohexanone–formaldehyde resins were modified in situ with α,ω‐diamine polydimethylsiloxanes and α,ω‐dihydroxy polydimethylsiloxanes. Melting points, solubilities in organic solvents, gel permeation chromatographs, Fourier transform infrared spectra, and NMR spectra of the modified resin were determined, and the surface properties of the resins were investigated by contact angle measurements. A small amount of silicon compounds seemed to effect the physical properties of the cyclohexanone–formaldehyde resins significantly. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 97–101, 2005     

NMR studies of urea–formaldehyde resins

A procedure used for analyzing urea–formaldehyde resins by NMR spectroscopy was developed. Using this procedure, the condensation of dimethylolurea under acidic and alkaline conditions was sudied. It was confirmed that polymerization under acidic conditions proceeds via the formation of methylene linkages and under alkaline conditions, via the formation of dimethylene ether groups. The more highly condensed water-soluble urea resins were found to contain hemiformal groups, which could be quantitatively determined. Two butylated urea–formaldehyde resins with varying degrees of butylation were also studied and were found to differ in the number of methylene and dimethylene ether groups and degree of butoxylation. Both were found to contain hemiformal groups. The resin with lower degree of butylation was also found to contain dimethyl ether linkages. None of the resins studied showed any detectable amounts of fully substituted amide groups.     

Preparation of cured urea–formaldehyde resins of low formaldehyde emission

Several polymers containing amino or amido groups and biuret were tested as additives to ureaformaldehyde (UF) resin in order to neutralize its inherent acidity and combine free formaldehyde released upon hydrolysis of cured UF polycondensate. Each modifier was incorporated to liquid methylolureas at weight ratios of 1:100, 2:100, and 3:100 prior to curing with the aid of acetic acid. Over 10 days of maintaining aqueous suspensions of the ground-up resultant solid resins at ambient temperature, a neutralizing effect was exhibited most visibly by polyacrylamide, polymethacrylamide, and biuret, the test with chitosan and casein giving results slightly different from those obtained for the control nonmodified cured UF polymer. Polyacrylamide, biuret, and casein proved to be excellent inhibitors of formaldehyde release from the hardened resins which were suspended in water at ambient temperature. On the other hand, chitosan did not reduce the evolution of HCHO but, instead, augmented it when its content was 1 g/100 g of the original liquid resin before cure.     

Moisture curing kinetics of isocyanate ended urethane quasi‐prepolymers monitored by IR spectroscopy and DSC

The study of the kinetics of the curing of isocyanate quasi‐prepolymers with water was performed by infrared spectroscopy and differential scanning calorimetry. The influence of the free isocyanate content, polyol functionality, and of the addition of an amine catalyst (2,2′‐dimorpholinediethylether) in the reaction kinetics and morphology of the final poly(urethane urea) was analyzed. A second‐order autocatalyzed model was successfully applied to reproduce the curing process under isothermal curing conditions, until gelation occurred. A kinetic model‐free approach was used to find the dependence of the effective activation energy (E_a) with the extent of cure, when the reaction was performed under nonisothermal conditions. The dependence of E_a with the reaction progress was different depending on the initial composition of the quasi‐prepolymer, which reveals the complexity of the curing process. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Uron and uron–urea‐formaldehyde resins

The favored pH ranges for the formation of urons in urea‐formaldehyde (UF) resins preparation were determined, these being at pH's higher than 6 and lower than 4 at which the equilibrium urons ↔ N,N′‐dimethylol ureas are shifted in favor of the cyclic uron species. Shifting the pH slowly during the preparation from one favorable range to the other causes shift in the equilibrium and formation of a majority of methylol ureas species, whereas a rapid change in pH does not cause this to any great extent. UF resins in which uron constituted as much as 60% of the resin were prepared and the procedure to maximize the proportion of uron present at the end of the reaction is described. Uron was found to be present in these resins also as linked by methylene bridges to urea and other urons and also as methylol urons, the reactivity of the methylol group of this latter having been shown to be much lower than that of the same group in methylol ureas. Thermomechanical analysis (TMA) tests and tests on wood particleboard prepared with uron resins to which relatively small proportions of urea were added at the end of the reaction were capable of gelling and yielding bonds of considerable strength. Equally, mixing a uron‐rich resin with a low F/U molar ratio UF resin yielded resins of greater strength than a simple UF of corresponding molar ratio indicating that UF resins of lower formaldehyde emission with still acceptable strength could be prepared with these resins. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 277–289, 1999     

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     

Chain extended cyclohexanone-formaldehyde and acetophenone–formaldehyde resins

The preparation of chain extended cyclohexanone–formaldehyde and acetophenone–formaldehyde resins and their physical properties were studied. The chain extension was regulated by the ratio of the hydroxyl groups of the ketonic resin/reactive reagents. Both resins were chain extended with dimethyl dichlorosilane, phosphorus oxychloride, phenylphosphonic dichloride, toluene-2,4-diisocyanate, prepolymers (prepared from trimethylolpropane and toluene-2,4-diisocyanate), phthalic anhydride, tetrahydrophthalic anhydride, trimellitic anhydride, 4,4′-oxydiphthalic anhydride, and maleic anhydride. Solubilities, melting point, molecular weight, and flammability of the chain extended resins were affected by the extender reagent. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 655–663, 1998     

Two-step synthesis and characterization of urea–isobutyraldehyde–formaldehyde resins

Urea–isobutyraldehyde–formaldehyde (UIF) resins were synthesized from urea, isobutyraldehyde and formaldehyde using sulfuric acid as catalyst by two-step method. The effect of molar ratio of isobutyraldehyde to formaldehyde (n(I)/n(F)), molar ratio of aldehyde to urea (n(A)/n(U)), catalyst concentration and reaction time on the yield, hydroxyl value and softening point of UIF resins were investigated. The UIF resins were characterized by Fourier transform infrared spectroscopy (FT-IR), ¹H-nuclear magnetic resonance (¹H NMR), gel permeation chromatography (GPC) and thermogravimetric (TG). The results showed that the yield, hydroxyl value and softening point of the UIF resin were 76.5%, 90 °C and 32 mgKOH/g, respectively, when the molar ratio of urea to isobutyraldehyde to formaldehyde (n(U)/n(I)/n(F)) was 1.0/3.6/2.4, catalyst concentration was 6.0%, and reaction time in the second step reaction was 3.0 h. FT-IR and ¹H NMR results showed that α-H in isobutyraldehyde participated in the synthesis reaction of UIF resins, and the reaction was Mannich reaction. The amount of aldehyde groups in UIF resins increased with the increase of the amount of isobutyraldehyde. GPC results showed that the UIF resins had narrow molecular weight distribution and TG results indicated that the UIF resin had excellent heat resistance.     

Toughening of vinylester–urethane hybrid resins through functionalized polymers

Liquid nitrile rubber, hyperbranched polyester, and core/shell rubber particles of various functionality, namely, vinyl, carboxyl, and epoxy, were added up to 20 wt % to a bisphenol‐A‐based vinylester–urethane hybrid (VEUH) resin to improve its toughness. The toughness was characterized by the fracture toughness (K_c) and energy (G_c) determined on compact tensile (CT) specimens at ambient temperature. Toughness improvement in VEUH was mostly achieved when the modifiers reacted with the secondary hydroxyl groups of the bismethacryloxy vinyl ester resin and with the isocyanate of the polyisocyanate compound, instead of participating in the free‐radical crosslinking via styrene copolymerization. Thus, incorporation of carboxyl‐terminated liquid nitrile rubber (CTBN) yielded the highest toughness upgrade with at least a 20 wt % modifier content. It was, however, accompanied by a reduction in both the stiffness and glass transition temperature (T_g) of the VEUH resin. Albeit functionalized (epoxy and vinyl, respectively) hyperbranched polymers were less efficient toughness modifiers than was CTBN, they showed no adverse effect on the stiffness and T_g. Use of core/shell modifiers did not result in toughness improvement. The above changes in the toughness response were traced to the morphology assessed by dynamic mechanical thermal analysis (DMTA) and fractographic inspection of the fracture surface of broken CT specimens. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 672–680, 2002; DOI 10.1002/app.10392