«CURE KINETICS OF WOOD PHENOL-FORMALDEHYDE SYSTEMS By JINWU WANG A dissertation submitted in partial fulfillment of the requirements for the degree of ...»
However, both are needed to describe bondline development completely so that a cure model for hot-pressing can encompass information on both. Usually, the phenolic has developed enough stiffness and durability in use before it has sufficient strength at pressing temperature to permit pressing opening. So pressing time is determined such that developed interface adhesion is enough to withstand internal pressure or stresses imposed by changes in wood thickness spring back at the time of press opening. The relationship of degree of cure and mechanical properties can assist to determine exactly when this occurs, thus stop pressing in time to short the press cycle. Curing process continues to achieve higher crosslinking density by stacking the boards together by virtue of residual heat long after pressing. For this purpose, it is worthwhile to investigate the relationship between mechanical development and chemical advancement.
OBJECTIVES In this perspective, the overall goal of this research is to develop a protocol to obtain cure kinetic data, provide a practical kinetic analysis and model methodology, and recommend appropriate models for predicting cure behavior of PF resins in terms of mechanical cure and chemical cure. The chemical advancement in reaction heat evolution will be monitored with differential scanning calorimetry (DSC); and the mechanical property evolution will be monitored by dynamical mechanic analysis (DMA) through a sandwich beam with an adhesive layer between two wood substrates. The chemical advancement and mechanical develop rate will be correlated so that a cure model for PF resins can encompass information on both. These objectives fit in the broader aim to incorporate a cure kinetic model into a hot-pressing model for wood-based composites. The specific objectives of this
(1) To evaluate and compare the ability of most common kinetic models for
ORGANIZATION OF THE DISSERTATIONThe dissertation is divided into nine chapters, including this introductory chapter. Chapters two through eight are written as a stand-alone paper each consisting of an introduction, objectives, methodology, results, and a conclusion. All chapters are connected by using the same two PF resins for addressing one aspect of the objectives presented in this introduction chapter. In chapter two the chemical structures and molecular weights of the two PF resins are characterized with 1H- and C-nuclear magnetic resonance as well as gel permeation chromatography to provide a basic knowledge of the materials used in all chapters. Chapter three introduces and compares the applications and limitations of model-fitting kinetics to characterize the chemical cure of the PF resoles. Chapter four examines the potential of model-free kinetics for gaining insight on the cure mechanisms of PF resoles and for predicting chemical cure development under isothermal and constant heating rate regimes.
Chapter five investigates the effects of wood content and wood species on chemical cure development of PF resins. Chapter six demonstrates a DMA technique for directly evaluating wood-adhesive system, by estimating the the in situ shear modulus of a PF adhesive layer in a sandwich beam Chapter seven examines the feasibility of characterizing cure development events such as gelation and vitrification points and obtaining mechanical cure kinetic data using DMA. The cure kinetic data are modeled with three model-fitting equations. In chapter eight the mechanical cure modeled by DMA and the chemical cure modeled by DSC are correlated. The cure kinetics is modeled with model-free kinetics. Finally, Chapter nine summarizes the conclusions of this project. Scheme 1 is a schematic diagram showing connections of various topics.
Scheme 1 A schematic diagram of linkage between various topics presented in this research.
REFERENCES Chow, S.-Z. A kinetic study of the polymerization of phenol-formaldehyde resin in the presence of cellulosic materials. Wood Science (1969), 1(4), 215-221.
Dai, C.; Yu, C.; Zhou, X. Heat and mass transfer in wood composite panels during hot pressing. part II. Modeling void formation and mat permeability. Wood and Fiber Science (2005), 37(2) 242-257 Furuno, T.; Imamura, Y.; Kajita, H. The modification of wood by treatment with low molecular weight phenol-formaldehyde resin: a properties enhancement with neutralized phenolic-resin and resin penetration into wood cell walls. Wood Science and Technology (2004), 37(5), 349-361.
He, G.; Riedl, B.; Ait-kadi, A. Model-free kinetics: curing behavior of phenol formaldehyde resins by Differential Scanning Calorimetry. Journal of Applied Polymer Science (2003), 87, 433 -440.
Kay, R.; Westwood, A. R. Differential scanning calorimetry (DSC) investigations on condensation polymers. I. Curing. European Polymer Journal (1975), 11(1), 25-30 Lei, Y.; Wu, Q. Cure kinetics of aqueous phenol-formaldehyde resins used for oriented strandboard manufacturing: effect of wood flour. Journal of Applied Polymer Science (2006), 102(4), 3774-3781.
Park, B.-D.; Riedl, B.; Bae, H.-J.; Kim, Y. S. Differential scanning calorimetry of phenol - formaldehyde (PF) adhesives. Journal of Wood Chemistry and Technology (1999), 19(3), 265-286 Pizzi, A.; Mtsweni, B.; Parsons, W. Wood-induced catalytic activation of PF adhesives.
Auto-polymerization vs. PF/wood covalent bonding. Journal of Applied Polymer Science (1994), 52(13), 1847-1856.
Thoemen, H.; Humphrey, P. E. Modeling the continuous pressing process for wood-based composites. Wood and Fiber Science (2003), 35(3), 456-468.
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Chapter 2 Quantitative Analysis of Phenol-formaldehyde Resins with C-, 1H-NMR and Gel Permeation Chromatography ABSTRACT The chemical structure and molecular weight distributions of phenol-formaldehyde (PF) resol resins have been correlated with resin performance and have provided useful information for resin formulations and applications. In this research, the chemical structure and molecular weights of two commercial PF resins with different molecular weights were characterized by 1H- and quantitative 13C-NMR spectroscopy and gel permeation chromatography (GPC) in their neat and acetylated states. Results from all three techniques showed that the two PF resins were distinctive in their relative quantities of methylene bridge structures, methylol groups, and molecular weight distributions and that one PF resin was more advanced than the other. The average molecular weight of the acetylated lower molecular weight resin determined by GPC corresponded with those obtained with 1H and 13C-NMR spectra.
However, GPC analysis of the higher molecular weight resin (on the acetylated) tended to over-estimate the molecular weight.
Key words: 1H-NMR, quantitative 13C-NMR spectroscopy; gel permeation chromatography; phenol–formaldehyde resin; molecular weight.
INTRODUCTIONPhenol-formaldehyde (PF) resins are widely used in the manufacture of wood-based composites. A measure of the molecular weights of PF resins is important for quality control during synthesis, for predicting properties of the cured resin and following reaction kinetics (Dargaville et al. 1997). Molecular weight distributions have been correlated with composite performance (Wilson et al. 1979, Nieh and Sellers 1991). It is therefore critical to accurately characterize resin structure and molecular weight. Gel permeation chromatography (GPC) is the most widely used method for characterizing molecular weight distributions of PF resins.
Tetrahydrofuran (THF), the solvent most frequently used as an eluent, dissolves only lower molecular weight fractions of PF resins and not higher ones (Wellons and Gollob 1980). High molecular weight PF resins have been traditionally acetylated in order to become soluble in THF and amenable to GPC analysis. PF resins can also be treated with an ion-exchange resin to remove sodium ions and become soluble in THF.
Even after treatment with an ion-exchange resin (Yazaki et al. 1994), the advanced PF resins are not completely soluble in THF or other organic solvents.
Furthermore, it has been shown that the average molecular weights of ion-exchange treated PF resin are lower than those measured on acetylated PF resins (Yazaki et al. 1994). Riedl et al. (1988) identified binary solvents that were suitable for GPC analysis of low molecular weight PF resins. In particular, THF modified with a small amount of trichloroacetic acid system produced good results, with minimal association and high solubility. Therefore, although GPC analysis can be effectively used for low molecular weight PF resins, resins with high molecular weight and high alkalinity must be acetylated prior to dissolution in THF for GPC analysis. However, intermolecular associations of acetylated PF species in THF result in an overestimation of the actual molecular weights (Wellons and Gollob 1980; Yazaki et al. 1994).
In addition to the GPC technique, researchers can use nuclear magnetic resonance (NMR) in solution to estimate molecular weights. The number average molecular weights for PF resins can be calculated using Woodbrey’s formulae (Woodbrey et al. 1965) on the basis of 1H-NMR spectra. Yazaki et al (1994) have confirmed the validity of the formulae by applying it to a model compound, which has a molecular weight of 320 g/mol. On the other hand, 13C-NMR spectroscopy provides a qualitative evaluation of the chemical structures of PF resins (Holopainen et al.
1997). The large chemical shift range of 13C-NMR allows the identification of many functional groups of PF resins that overlap in the 1H-NMR spectra (McGraw et al.
1989). Owing to the nuclear overhauser effect (NOE), 13C-NMR spectra are not quantitative and may not be used for molecular weight determination. However, Luukko et al. (1998) and Rego et al. (2004) demonstrated that the quantitative C-NMR analyses of PF resols are possible using inverse gated-decoupling and a shiftless paramagnetic relaxation reagent. This suggests that molecular weight distribution may be computed from both 13C-NMR and 1H-NMR.
The objectives of this work are to 1) characterize the chemical structures and molecular weights of two commercial PF resins with GPC, 1H- and quantitative C-NMR, and to 2) compare the estimates of molecular structure and molecular weights as determined by 1H-NMR, quantitative 13C-NMR, and GPC. For these purposes, 1H-NMR and 13C-NMR spectral analyses were made on both neat resins and acetylated resins, while GPC analysis was only made on acetylated resins.
EXPERIMENTALMaterials Face and core resol PF resins used for the face and core layers of oriented strand boards were obtained from a commercial company and stored in a freezer at
-20 oC. The core PF resin has higher viscosity, pH value and lower solid content than the face PF resin (Figure 2.1). Resins were characterized as received and also after acetylation.
Table 2.1 Features of the core PF and face PF resins.
Acetylation of PF resins Prior to GPC analysis, the PF resins were acetylated in order to afford solubility in THF (Yazaki et al. 1994). Approximately 15 g of liquid PF resin was acetylated in a 1:1 (79.10 g and 102.09 g) molar mixture of pyridine: acetic anhydride placed in an ice-bath for an initial 2-hour period, followed by a 72-hour period at room temperature. After 74 hours of reaction time, the reaction mixture was poured into ice-water (400mL) and stirred for 20 minutes. The white precipitate was collected, washed with water (4x400mL), and dissolved in chloroform. The organic solvent layer was washed with water (4 x200mL). The organic phase was dried with anhydrous sodium sulfate and then filtered. The chloroform was removed by rotational evaporation in vacuum at 40 °C. The dried residue was dissolved in a small amount of acetone, to which water (30mL) was added, and evaporated in vacuum at 40 °C by the rotavapour. The residuals were further dried under high vacuum overnight to yield approximately 5g acetylated PF resins.
GPC analysis of the acetylated PF resins Viscotek 270 was coupled with a Waters HPLC unit and Jordi Gel polydivinylbenzene mixed bed column with a right angle, refractive index and differential viscometer detectors. Polystyrene standards were used for calibration.
Acetylated PF resins were dissolved in THF at a concentration of 3.6 mg/ml. Aliquots, 200µl in size were injected in the column that had THF as the mobile phase with a flow rate of 0.5 ml/min. 4 replicates have been conducted.
H- and quantitative 13C-NMR analyses of the acetylated resins Based on solubility criteria and solvent chemical shifts, chloroform-d was selected as a solvent for solution NMR analysis of the acetylated PF resins.
Chloroform also served as a chemical shift reference. In order to reduce the testing time with 13C-NMR, Chromium (III) acetylacetonate (Cr(C5H7O2)3 ) of 97% purity from Aldrich was added in 20 mM concentration in the NMR tube as a relaxation reagent (Lukko et al. 1998). 1H-NMR and 13C-NMR spectra were recorded on a Bruker AMX-300 spectrometer operating at 75.5 MHz. The operating parameters for H-NMR were as follows: acquisition time: 4.95 s; pulse width: 9.7 s; pulse delay: 2 s;
number of scans: 16. The inverse gated-decoupling technique was used to eliminate the nuclear overhauser effect (NOE) for quantitative 13C-NMR. Typical spectra of resins were run with a 45o pulse. Preliminary tests were run to optimize the delay time, acquisition time, and the number of scans. Specific times and numbers of scans were then selected (8 s delay, 0.6 s acquisition, 2560 scans) to measure the spectra of the acetylated and neat resins for quantitative analysis with three sample repetitions.