«CURE KINETICS OF WOOD PHENOL-FORMALDEHYDE SYSTEMS By JINWU WANG A dissertation submitted in partial fulfillment of the requirements for the degree of ...»
Insert in (a) highlights the influence of heating rate (2, 5, 10, and 20 oC/min) on the cure of PF-high.
Figure 3.2 Comparison of the test data at 10 oC/min and MF predictions of PF-low and PF-high for reaction rate (dα/dt) and degree of cure (α).
Figure 3.3 Experimental degree of cure (α) at 120°C and MF predictions for (a) PF-low and (b) PF-high.
Figure 4.1 DSC thermograms at 2oC/min for the PF-low and PF-high resins.
Insert highlights the influence of heating rate (2, 5, 10, and 20 oC/min) on the cure of the
Figure 4.2 Activation energies change with the degree of cure by the Friedman, Vyazovkin and KAS methods for the PF-low and PF-high.
Figure 4.3 Combined parameters of the Friedman, Vyazovkin and KAS methods for PF-high
Figure 4.4 Comparisons of experimental data and KAS predictions for dynamic conditions at 2, 5, 10, 20 and 25 oC/min for (a) the reaction rate of PF-high and (b) the degree of cure of PF-low
Figure 4.5 Comparison of experimental data with the Friedman, Vyazovkin and KAS predictions of degree of cure of PF-low and PF-high during isothermal cure at 120°C
Figure 5.1 Baselines of southern yellow pine (SYP) at two moisture contents scanned at 10 oC/min in a high pressure crucible.
Figure 5.2 DSC thermograms of PF/southern yellow pine at various contents.
.........98 Figure 5.3 Experimental cure development of PF/southern yellow pine at various wood contents.
Figure 5.4 Activation energy changing patterns of PF/southern yellow pine mixtures at various wood contents by Vyazovkin method
Figure 5.5 Experimental cure development of pure PF at 120 oC and model-free predictions at 120 oC and 80 oC by Vyazovkin method for pure PF and 70% PF/southern yellow pine.
Figure 5.6 Comparison of DSC thermograms for wood particle size
mixtures at 35% wood content
Figure 5.8 Comparison of DSC thermograms for cellulose, xylan, lignin, and SYP extractives mixtures at 35% substrate content at a linear heating rate of 5 oC/min.
..108 Figure 5.9 Comparison of activation energies with theVyazovkin method for southern yellow pine (SYP), extracted SYP, SYP extractives, and lignin/PF mixtures at 35% substrate content
Figure 6.1 The three point bending sandwich beam, the gray adhesive layer between two wood adherends.
Figure 6.2 A typical of DMA output.
Effective storage modulus (E’) and storage stiffness (C’) changes with temperature during curing at 3 °C/min for a foil wrapped
joints at isothermal temperature 120 °C
Figure 6.5 The effects of thickness of the adhesive layer on the item M.
.................134 Figure 6.6 Summary of effective storage modulus (E’) development with all three kinds of samples together.
(a) Typical E’ development for PF-low and PF-high bonded
bonded wood joints at 2 °C /min. Wood was scanned at oven-dried
Figure 7.1 The DMA cure profiles of the aluminum foil-wrapped PF-high bonded wood joints: a) storage modulus development (E’) and three peaks in tan δ showing glass transition temperature of the uncured and dehydrated PF resin (Peak 1), gelation (Peak 2), and vitrification (Peak 3) events at 2 °C /min; b) gelation and vitrification temperature dependence on heating rates; c) modulus development and gelation (small shoulder 2) and vitrification (Peak 3) in tan δ curve at 120 °C; d) tan δ at different isothermal temperatures.
Figure 7.2 DMA tan δ traces for PF-low bonded wood joints at different heating rates designated on the curves.
Only vitrification has been recorded at all heating rates..156 Figure 7.3 The effect of preparations on cure development of PF-low bonded wood joints scanned at 2 °C/min: 1.
DMA scan beginning at room temperature immediately followed sample preparation; 2. DMA scanning immediately followed sample preparation from low temperature; 3. closed assembly at 30 °C for16 hours; 4. closed assembly at 50 °C for 16 hours; 5. closed assembly at 60 °C for 16 hours; 6. closed assembly at 70 °C for 8 hour
Figure 7.4 The effects of foil-wrapping on the mechanical cure development at linear heating rate for the PF-high bonded wood joints
Figure 7.5 The effects of foil-wrapping on the mechanical cure development under isothermal regime for the PF-high bonded wood joints
Figure 7.6 Comparison of the mechanical cure development at linear heating rate
Figure 7.7 An example of cure development under isothermal temperatures for PF-low bonded wood joints.
Figure 7.8 Kinetic parameter m of autocatalytic model changes with isothermal temperature for the PF-low bonded wood joints
Figure 7.9 Kinetic parameter n of Avrami-Erofeev model changes with isothermal temperature for PF-low bonded wood joints.
Figure 8.1 Typical DMA traces at 2 °C/min for flexural storage modulus E’, loss modulus E”, and loss factor tan δ.
Numbers (1, 2, and 3 on tan δ; 1’, 2’ and 3’ on E”) indicate the glass transition temperature of uncured PF resin, cure transition of gelation and vitrification points; respectively.
Figure 8.2 The evolution of degree of cure at different cure conditions: (a) Chemical cure by DSC at 2, 3, 4, 5, 10, and 15 °C/min from left to right, (b) Predicted chemical cure at isothermal temperature 90, 100, 110, 120, 130, 140, and 160 °C bottom up by Vyazovkin model-free kinetics from DSC ramp data in (a), (c) Mechanical cure by DMA at 2, 3, 4, and 5 °C/min from left to right for aluminum foil-wrapped PF bonded wood joints, and (d) Mechanical cure by DMA at 90, 100, 110, 120, 130, 140, and 160 °C bottom up for aluminum foil-wrapped PF bonded wood joints.
Figure 8.3 Relationship between mechanical cure (β) and chemical cure (α) from aluminum foil-wrapped PF bonded wood joints under isothermal temperature (a) and linear heating rate (b).
Figure 8.4 DSC thermgram at 10 °C/min showing glass transition temperature for a
DMA specimen after scanned from room temperature to 240 °C at 5 °C/min..........191 Figure 8.5 Temperature dependence of parameters for the relationship equation of the mechanical and chemical degree of cure under isothermal temperatures, T in Celsius degree
Figure 8.6 Sensitivity of mechanical property development to chemical advancement at designated isothermal temperature computed from experimental data.
................196 Figure 8.7 Sensitivity of mechanical property development to chemical advancement at designated linear heating rates.
Figure 8.8 Activation energy dependence of mechanical cure (a) and combined parameters (b) obtained by KAS, time event, and Vyazovkin methods for aluminum foil-wrapped PF bonded wood joints
Figure 8.9 Comparison of experimental mechanical degree of cure at 1, 2, and 3 °C /min for aluminum foil-wrapped PF bonded wood joints and KAS predictions from ramp data
Figure 8.10 Comparison of experimental mechanical degree of cure at 120 °C for aluminum foil-wrapped PF bonded wood joints and predictions with parameters Eβ and C(β) from ramp data
Figure 8.11 Comparison of experimental mechanical degree of cure at 120 °C for aluminum foil-wrapped PF bonded wood joints and predictions with the parameters from isothermal data.
BACKGROUND During hot-pressing of wood-based composites, the press schedule is predominantly controlled by two parameters, bond strength development and internal gas pressure. Short pressing times are desirable from an economical standpoint but can result in low bond strength and panel delamination upon press opening. In order to assist in understanding and optimizing hot-pressing conditions for wood –based composites, several researchers have developed hot-pressing models that predict mat temperature, moisture, internal pressure, and density developments as a function of the input hot-pressing parameters (Zombori et al. 2003; Thoemen & Humphrey 2003;
Dai et al. 2005). These hot-pressing models have either ignored bond strength development or have used an arbitrary cure kinetic model to portray bond strength development. In order to improve the accuracy of hot-pressing models, bond strength development models need to be incorporated. In the literature, the cure kinetics of thermosets resins has been mainly modeled based on chemical advancement (He et al.
2003; Lei and Wu 2006). However, both chemical and mechanical cure progression are needed to describe bond development. Indeed, the kinetics of chemical cure is not linearly related to the bond strength development. Therefore this research proposes to consider both chemical and mechanical cure in designing kinetic models of bond strength development that are suitable for hot-pressing models.
Phenol-formaldehyde (PF) resins are widely used in wood-based composites manufacture. It is well established that wood has an influence on the cure kinetics of PF resins (Chow 1969; Pizzi et al. 1994). Therefore the models for bond strength development need to account for the influence of wood. The research is proposed for PF as the adhesive system.
Generally, there are three methods to investigate the cure kinetics of PF resins:
in neat state, in mixture with wood flour, or in thin film between wood substrate. The cure kinetic of PF resins in a neat state have been the subject of many cure kinetic studies (Kay and Westwood 1975; Park et al. 1999; He et al. 2003), yet the best model for predicting PF dynamic and isothermal cure has not been established. Clearly there is an need for researchers to compare and contrast several commonly used kinetic models for predicting degree of cure and cure rate of PF resins so that models that are most adequate for incorporation in hot-pressing models be determined.
Although cure development of neat PF resins can provide useful information for resin formulators, it is in situ cure development in the presence of wood can assist to disclose the cure mechanism and investigate the effects of wood presence on cure kinetics. There are substantial physical and chemical interaction between wood and PF resins (Pizzi et al. 1994). The porous structure of wood may preferentially absorb some PF components or the resin with a specific molecular weight range (Furuno et al.
2004); the covalent bond may also form between wood and PF molecular chains (Chow 1969). This presents a question whether the kinetic models appropriate for neat resins are also working for the wood-adhesive systems.
As in literature, wood-PF interactions are maximized and conveniently researched with the mixtures of wood flour and PF (Chow 1969; Pizzi et al. 1994; He et al. 2005; Lei an Wu 2006). However, PF resin is used as adhesives and is usually applied as a thin film between two wood substrates, thus it does not have same intricate contact with wood as in the form of powder. Questions are raised whether the adhesive is ‘same material’ in neat bulk form and in powder mixtures as when presents as a thin adhesive layer between wood substrates. Besides wood-PF interactions and preferential absorption along the interface, residual internal stresses are more likely to be present in the adhesive when it is cured between substrates.
These factors might obviously alter the kinetics of PF chemical reaction by which the adhesive hardens and influence the mechanical properties of the in situ cured resin compared to the bulk adhesive. Cure kinetics and in situ property formation processes are important for setting up optimum process parameters. These aspects are undoubtedly worthy of further investigation. A sandwiched structure with an adhesive layer between two wood substrates was favored in investigating the wood-adhesive bondline development since the sandwiched geometry more closely resembles the practical application of the adhesives when compared to the mixture in PF/wood flour.
The in situ shear and flexural modulus development of the adhesive layer during cure is different from bulk property formation process and is one of basic material parameters needed to construct an useful hot-pressing model.
Another question is how physical and mechanical properties change with the chemical degree of cure. Curing is intended to advance the PF molecules into a highly crosslinked network to achieve a high durability of the products; curing is also meant to develop physical and mechanical strength in the products. The kinetics of chemical advancement of the adhesive layer is not necessarily linearly related with developing rate of mechanical properties. In literature, the cure kinetics of thermosets such as PF resin is most modeled based on chemical advancement (He et al. 2003; Lei and Wu 2006). The relationship of chemical and mechanical advancements remains unclear.