Lignocellulosic substrates influence on TTT and CHT curing diagrams of polycondensation resins

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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“…(11) and (12). 12,14,15 (12) the obtained reaction rates were similar to the values already reported in the literature.…”

Section: Resultssupporting

confidence: 88%

Rheology study of gelling of phenol-formaldehyde resins

Halász¹,

Vorster²,

Pizzi³

et al. 2001

J. Appl. Polym. Sci.

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“…The trend is the same as noticed for other resins in previously reported work with the max modulus being higher for the slower‐cured resin. Although clearly evident from Figure 1, this trend is clearly less extreme here than that noticed for UF resins alone 7. Figure 2(a–c) shows, respectively, the upper‐temperature details of the total vitrification curve, gel curve, and pseudogel curve of the CHT diagrams of top of the range MUF resins of an M:U weight ratio of 47:53 and of (M+U):F molar ratios of, respectively, 1:1.5, 1:1.7, and 1:1.9.…”

Section: Discussionmentioning

confidence: 67%

“…The values of the Young's modulus for the resin/substrate systems were then calculated according to already‐reported methods based on the equation f = km /α E and connected regression equations 3. The CHT diagrams showing the total vitrification, gel, and pseudogel curves and their variations as a function of the manufacturing parameters were built according to methods already reported 6, 7…”

Section: Methodsmentioning

confidence: 99%

Influence of preparation parameters on the CHT curing diagrams of MUF polycondensation resins

Properzi¹,

Pizzi²,

Uzielli³

2001

J of Applied Polymer Sci

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Lignocellulosic substrates such as wood have been found to have a marked modifying influence on both lower-temperature and higher-temperature zones of TTT and CHT diagrams during hardening of formaldehyde-based polycondensates. While the modifying influence of the substrate has been described, the modifying influence of some of the most important manufacturing parameters of the resins on the CHT diagram, not having been previously investigated, are explored here and clear trends are shown. In the case of melamine-urea-formaldehyde (MUF) resins for wood adhesives, the molar ratio (MϩU):F appear to be the dominant parameter influencing the relative position of gel and vitrification curves in relation to each other. The ratio of melamine to urea does not appear to have any effect on the relative position of the curve, lacking any clear trend, at least at the higher (MϩU) molar ratio of 1:1.9 used for this series of resins. In the case presented for the first time, the influence of resin manufacturing parameters on CHT curing diagrams was studied in combination with the modifications introduced by the substrate.

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“…As the deflections Af obtained were proven to be constant and reproducible (Kamoun et al 1997;Pizzi et al 1997;Pizzi 1997;Garcia and Pizzi 1997 a, b;Lu and Pizzi 1998;Probst et al 1997), and as they are proportional to the flexibility of the assembly, the Young's moduli of the different assemblies under the different treatments can be calculated through the relationship EI/E2 = Af2/Afl. As the deflections Af obtained were proven to be constant and reproducible (Kamoun et al 1997;Pizzi et al 1997;Pizzi 1997;Garcia and Pizzi 1997 a, b;Lu and Pizzi 1998;Probst et al 1997), and as they are proportional to the flexibility of the assembly, the Young's moduli of the different assemblies under the different treatments can be calculated through the relationship EI/E2 = Af2/Afl.…”

Section: Tween Force and Deflection E = [L3/(4bh3)][af/af]mentioning

confidence: 98%

“…The IB performance improvements, for instance, are introduced by the series of reactions pertaining to internal methylene ether bridges rearrangements to a tighter methylene bridges network which have already been observed and extensively discussed in TMA analysis of aminoplastic and phenolic resins (Garcia and Pizzi 1997;Kamoun et al 1997;Pizzi et al 1997;Lu and Pizzi 1998). These are well able to counterbalance the degradative trend to which the aminoplastic resin should be subjected.…”

Section: Particleboard Experiments Parameter Trends and Molecular Lementioning

confidence: 99%

Curing conditions effects on the characteristics of thermosetting adhesives-bonded wood joints

Pizzi²

1998

Holz als Roh- und Werkstoff

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Thermomechanical analysis (TMA) results indicate that simple post-treatment (such as some postcuring) conditions capable of improving the mechanical performance of aminoplastic resins-bonded particleboard without any degradation do indeed exist. A model to describe the decrease in temperature under different conditions of a particleboard after hot pressing has been developed and shown to correlate well with experimental results of board temperature variation after pressing, both on cooling and during postcuring under different conditions. From this, conditions of temperature and time favourable to improve panel performance by postcuring treatments were also determined. The validity of the improvements forecasted under such conditions was then confirmed at molecular level for urea-formaldehyde (UF) adhesive/wood joints by TMA testing, and finally confirmed by testing the mechanical performance of laboratory UF-bonded boards prepared under the postcuring treatment conditions identified. Noticeable improvements in several UF-bonded board properties such as internal bond (IB) strength, thickness swelling, MOE and MOR were observed by implementing short postcuring periods at temperatures much lower than pressing temperature. The panels performance improvements observed were explained on the basis of the shifts in relative importance of already described and well-known molecular level rearrangements of the cured adhesive network in modern, lower formaldehyde content UF adhesives. The conclusion was that modern, lower formaldehyde content UF adhesives can considerably benefit from short, hot postcuring periods as regards board performance, a trend in clear contrast with the degradation and loss of performance this practice was known to induce in the older, very much higher formaldehyde content aminoplastic resins of the past. The coupling of the simple and very rapid TMA technique with the developed model allows the rapid scan of many other posttreatment schedules and thus to forecast other still possible improvements in the performance of UF-bonded and other-adhesives-bonded particleboard. EinfluB der Aush~rtebedingungen ouf dos Verholten von Holzverbindungen mit hitzeh&tenden Klebern. Tell 2 : Verbesserung von UF-Spanplatten (lurch Nachhiirten und Vorhersage des TemperoturverlaufsErgebnisse der TMA zeigen, dal~ es tats~ichlich einfache Bedingungen zum Nachh~irten yon Spanplatten gibt, womit die mechanischen Eigenschaften yon Aminoplastgebundenen Platten ohne Abbauerscheinungen verbessert werden k6nnen. Es wurde ein Modell entwickelt, das den Temperaturverlauf in Spanplatten unter verschiedenen Bedingungen nach dem Heiflpressen beschreibt. Das Modell korreliert gut mit experimentellen Temperaturmessungen nach dem Pressen, und zwar sowohl w/ihrend des Abkfihlens als auch beim Nachh~irten unter verschiedenen Bedingungen. Daraus wurden g~nstige Temperatur-und Zeitbedingungen abgeleitet. Die Gfihigkeit der vorhergesagten Verbesserungen wurde zun~ichst auf molekularem Niveau an Holz/Leim-Verbindungen (UF-Harz) fiberprfift, ...

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Lignocellulosic substrates influence on TTT and CHT curing diagrams of polycondensation resins

Cited by 31 publications

References 18 publications

Rheology study of gelling of phenol-formaldehyde resins

Rheology study of gelling of phenol-formaldehyde resins

Influence of preparation parameters on the CHT curing diagrams of MUF polycondensation resins

Curing conditions effects on the characteristics of thermosetting adhesives-bonded wood joints

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