A rheological study of the gelling of UF polycondensates

Halász, László; Vorster, O.C.; Pizzi, A.; Alphen, J. van

doi:10.1002/(sici)1097-4628(20000307)75:10<1296::aid-app11>3.3.co;2-6

Cited by 10 publications

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“…The gel times of resins were found in analogy to ref. [14] using the dependence of storage time on the reciprocal of zero shear viscosity , extrapolating a linear graph to zero value of 0 η 0 η 1 . It is not surprising that the studied resins have different gel times (53 and 38 days at 25°C, respectively).…”

Section: Resultsmentioning

confidence: 99%

“…Hence, in the rheokinetic studies the processes before and after the gel point can be characterized by power law advancements of viscosity or viscoelastic shear stiffness. Dynamic thermomechanical analysis (DTMA) was successfully used for the rheological study of gelling and vitrification of urea-formaldehyde [14] and melamineformaldehyde [15] resins. The gel time was determined by extrapolating the reciprocal of zero shear viscosity and by values of storage and loss moduli.…”

Section: Introductionmentioning

confidence: 99%

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Rheological behaviour of urea-formaldehyde adhesive resins

2004

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An attempt is made to describe the complicated rheokinetic behaviour of urea-formaldehyde resins during storage, in close connection with the changes in their chemical structure. The alkali-promoted non-linear polycondensation during resin storage is evaluated by pre-gel viscosity measurements. Viscosity changes are described by a power law function using extrapolation to zero shear and infinitely high shear rates. During most of the period of viscosity increase, the rheological behaviour of studied resins is similar. The deviation from the power law dependence occurring in the initial low-rate growth of viscosity takes place because of formaldehyde redistribution, which does not lead to covalent network build-up. In the presence of methanol this period is extended due to the formation of stable methoxymethylene groups.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Rheological behaviour of urea-formaldehyde adhesive resins

2004

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“…Halasz et al [49] reported two different values for the apparent activation energy in gel time tests using ammonium chloride as hardener: 104.6 kJ/mol for a molar ratio F/U = 1.3 and 66.9 kJ/mol for a UF resin with a molar ratio F/U = 1.5 as also confirmed by low-resolution-pulsed 1 H-NMR [37]. Halasz et al [49] conclude that the differences in the results show that the curing processes might be different and might depend on the F/U molar ratio or on other factors. From viscosity measurements reported by Starkopf [50], an activation energy of approximately 52 kJ/mol with oxalic acid as catalyst was obtained; the time to reach infinite viscosity under the given test methods was used as a measure for the gel time in the curing reaction.…”

Section: Arrhenius Plots and Apparent Activation Energy For The Gelatmentioning

confidence: 98%

“…At higher molar ratios lower activation energies have been determined; surprisingly, there was no difference in the activation energies using pure UF resin or resins containing melamine. Halasz et al [49] reported two different values for the apparent activation energy in gel time tests using ammonium chloride as hardener: 104.6 kJ/mol for a molar ratio F/U = 1.3 and 66.9 kJ/mol for a UF resin with a molar ratio F/U = 1.5 as also confirmed by low-resolution-pulsed 1 H-NMR [37]. Halasz et al [49] conclude that the differences in the results show that the curing processes might be different and might depend on the F/U molar ratio or on other factors.…”

Section: Arrhenius Plots and Apparent Activation Energy For The Gelatmentioning

confidence: 99%

Catalytic influence of wood on the hardening behavior of formaldehyde-based resin adhesives used for wood-based panels

Stefke¹,

Dunky²

2006

Journal of Adhesion Science and Technology

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The aim of this work was to monitor the hardening behavior of two different ureaformaldehyde (UF) resins and to investigate the possible catalytic effect of cold water extractives of different wood species on the hardening characteristic. A pure UF resin and a melamine-modified UF resin were used for this investigation. For the experiments, ammonium chloride, ammonium sulphate, acetic acid and oxalic acid at different addition levels were used as hardeners. The gel times determined at three temperatures, the pH drop of the resins after the hardener addition, the buffering behavior and changes in the complex viscosity η * with time, as well as the development of the bond strength were chosen as parameters for characterizing the systems both with and without wood extractives. As expected, the buffering capacity of the melamine fortified resin was found to be much greater than for the pure UF resin. The gel times of the investigated resins were strongly dependent on the applied temperature, decreasing steeply at higher temperatures. Based on these data the apparent activation energies for various hardener additions have been calculated. The complex viscosity and the development of the internal bond strength indicated that cold water wood extractives had little or no effect on the curing progress of the resins.

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“…10 In this article we instead determined the gel point, the values of the critical exponents, and the gel stiffness of alkaline-setting PF resols. …”

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confidence: 99%

Rheology study of gelling of phenol-formaldehyde resins

Halász¹,

Vorster²,

Pizzi³

et al. 2001

J. Appl. Polym. Sci.

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Recently, this approach was taken to determine the values of the relevant parameters for the even more common, but acid-setting, urea-formaldehyde (UF) resins. 10 In this article we instead determined the gel point, the values of the critical exponents, and the gel stiffness of alkaline-setting PF resols. EXPERIMENTAL MaterialsTwo commercial PF resins (PR 141 and PR 240) produced by a South African wood processing company, one used for paper impregnation for high pressure lam-

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A rheological study of the gelling of UF polycondensates

Cited by 10 publications

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Rheological behaviour of urea-formaldehyde adhesive resins

Rheological behaviour of urea-formaldehyde adhesive resins

Catalytic influence of wood on the hardening behavior of formaldehyde-based resin adhesives used for wood-based panels

Rheology study of gelling of phenol-formaldehyde resins

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