«CURE KINETICS OF WOOD PHENOL-FORMALDEHYDE SYSTEMS By JINWU WANG A dissertation submitted in partial fulfillment of the requirements for the degree of ...»
determine the quality of adhesive cure and the effectiveness of the wood-adhesive interaction. Since DMA can record thermal and viscoelastic properties simultaneously during the curing process, it has advantages to provide not only cure kinetics but also
Emin, ∆E ', and R values for specimens cured at heating rates from 2 to 5 °C /min and ' isothermal temperatures 90-130 °C are presented. In each category, the coefficient of variation (CV) for resin load is larger than those for mechanical properties, which suggests that E’ is relatively insensitive to the resin load. Among the derived E’
preparations as other parameters. DMA was used as an analytical tool for evaluating the effectiveness of formulations or the performance of wood-adhesive systems (Garcia and Pizzi1998; He and Yan 2005a). In these cases, one parameter is prefered
and loss factor tan δ reflect the final product performance, substantiating their use for evaluating product quality. In contrast, the other parameters are mainly related to the process and might be used for optimizing the cure process. There was no doubt that the modulus of the bonded wood joints determined by DMA was related to the modulus of wood substrate. If the effects of wood species on adhesive performance need to be evaluated, the ratio R is likely to be a good choice since the parameter corrected on wood modulus and has a theoretical value approaching 4 if sandwich structure is used.
Comparison of PF-high and PF-low performance The results of an analysis of variance (ANOVA) demonstrated a significant
tan δ than those produced with the PF-High resin. Two types of DMA (Tritec 2000 and Rheometrics RSA II) were used and similar conclusions were obtained (Table 6.1).
Conducting a second scan of the cured specimens further confirmed that PF-low bonded joints possessed a higher E’ and lower damping tan δ than PF-high bonded joints, whereas the joints with both resins displayed a lower E’ and higher tan δ as compared with the solid wood (Figure 6.7). There was no significant difference among levels of resin load for the PF-high or PF-low bonded wood joints. However, the average thickness of adhesive layer for PF-low bonded wood joints is 0.02 mm while it is 0.06 mm for PF-high bonded wood joints. This observation suggests that PF-low resin penetrates significantly into the wood structrue as compared with PF-high. Laborie et al. (2006) has hypothesized that low molecular weight PF can penetrate into cell wall to possibly form interpenetrating network with lignin and increase intermolecular cooperativity and relaxation time. This concept is in agreement with the higher E’ and lower damping for the PF-low found in this research.
It was also observed during the application of resin by the air brush where the PF-Low was easily air-atomized and uniformly formed a layer on the wood surface as compared with PF-high. It appears that the high molecular weight of the PF-High detracted from good wetting and distribution on the wood surface as compared with PF-Low. It was noted that PF-Low bonded wood joints maintained a minimum E’ plateau longer than PF-high either in linear or isothermal heating rate, which was beneficial for wetting the wood surface and forming good interphase. Wrapping with aluminum foil also increased slightly the E’ and tan δ since wrapping film did not integrate with wood perfectly.
4 0.02 2 0.01
Figure 6.7 Comparison of E’ and tan δ changing with temperature for cured PF bonded wood joints at 2 °C /min.
Wood was scanned at oven-dried.
CONCLUSION The experiment involved measuring the bending stiffness of the sandwich beam using DMA under linear and isothermal heating regimes, and combining an analytical solution to determine the in situ shear modulus of the adhesive. The in situ shear modulus of the PF resins changed from 0.01 to around 16 MPa during curing process and was typical of the rubber range for polymers. The measured value was in general agreement with results independently produced by parallel-plate rheometry. In practice, the DMA test was much easier to execute than the rheology test for PF resols since water vaporization induced a large shrinkage and non-uniform curing under the parallel plate, especially in later stage of curing process. Although the combined use of a sandwich specimen and an analytical solution to measure shear modulus development during curing of an adhesive layer is simple for both sample preparation and measurement procedure, the sensitivity of the technique is unknown. Further research should be conducted to explore the effects of experimental variables on the calculation of shear modulus.
During a DMA test of a wood-PF sandwich beam, the effective modulus E’ was calculated by the instrument under assumption of a homogeneous beam throughout the curing process. Therefore the ratio of the maximum to minimum E’ (R) approaches a value of 4 in two extreme cases when the shear modulus is negligible before curing and is comparable to the adherends after curing. Experimentally, a large portion of measured values for R was near 4. The fact that the modulus of wood adherend increased from moisture lost during the test accounted for the large values of R in excess of 4. The difficulty of accurately maintaining or measuring sample dimensions during curing process further affected accuracy of determining R. Despite these uncertainties, the ratio, R, can be used as a parameter to direct evaluation of wood adhesive system performance. Further, variation for the maximum E’ was smallest among all possible parameters derived from DMA E’ curves. Hence, this parameter was deemed best for direct evaluation of wood-adhesive systems as compared with ∆E’ since minimum E’ was a parameter of cure process and subjected to a variety of source of variations. It was noted that maximum E’ was highly related to the modulus of the wood substrate. Hence, for evaluating the effects of wood species on wood-adhesive system, the ratio of maximum to minimum was recommended which had a theoretical value of 4.
DMA curves showed that the PF-low bonded wood joints cured slower and achieved a higher maximum E’ and a lower magnitude of tan δ than those of PF-high bonded wood joints. With similar resin load and small applied force, the PF-low formed a very thin bond layer as compared with PF-high, suggesting a good interphase responding for good stiffness and low viscoelasticity for cured PF-low wood joints.
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Chapter 7 Model-fitting Kinetic Analysis of Phenol-formaldehyde
ABSTRACT Cure development of phenol-formaldehyde (PF) resins has been extensively modeled based on chemical advancement. However, it is in situ mechanical development of wood-adhesive systems that is most relevant with process optimization such as hot-pressing of wood-based panels. The objective of this research is to examine the feasibility of applying common model-fitting kinetic analysis to describe cure development based on storage modulus development recorded with dynamic mechanical analysis (DMA). Dynamic three-point bending tests were conducted on a sandwich specimens composed of two wood adherends bonded with an adhesive layer. Two commercial PF resins of different molecular weights distributions (labeled as PF-high and -low respectively) were used as adhesive. In addition, PF-high bonded wood joints were also wrapped by aluminum foil to investigate the effect of moisture loss. The specimen curing process was monitored using various isothermal and linear heating regimes. The results showed that the PF-low joints cured more slowly than the PF-high joints. The foil-wrapped PF-high joints displayed slower curing process than the unwrapped joints and rendered two peaks in the tan δ curves. These peaks were attributed to gelation and vitrification with an activation energy of 40 and 48 kJ/mol; respectively. The activation energy from three model-fitting models of autocatalytic, Prout-Tompkins, and Avrami-Erofeev was in agreement with that from vitrification. Overall, model-fitting kinetic analyses were effective to describe the mechanical development of wood-adhesive systems.
Key words: Dynamical mechanical analysis (DMA); kinetic models; phenol formaldehyde resins; gelation; vitrification; activation energy.