«CURE KINETICS OF WOOD PHENOL-FORMALDEHYDE SYSTEMS By JINWU WANG A dissertation submitted in partial fulfillment of the requirements for the degree of ...»
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Chapter 8 Kinetic Analysis and Correlation of Mechanical and Chemical Cure Development for Phenol-formaldehyde Resin Bonded
ABSTRACT The kinetics of phenol-formaldehyde (PF) resin cure governs both the duration of hot-pressing for wood-based composites and the properties of final panels.
Chemical advancement of the forming polymer does not always produce a linear relation to mechanical development. The objective of this research is to relate the chemical and mechanical manifestation cure so that a cure model for hot-pressing can encompass information on both. Dynamic three-point bending tests were conducted on a foil-wrapped sandwich specimen of two wood adherends bonded with a PF adhesive layer using dynamic mechanical analysis (DMA). The specimen was cured using various isothermal and linear heating regimes. A small disc trimmed from the DMA specimen was scanned with differential scanning calorimetry (DSC) at the same linear heating rate as DMA. Assuming that the curing conditions in the foil-wrapped specimen were similar to that in the high pressure DSC pan, the relationship between chemical and mechanical degree of cure (i.e. α and β, respectively) was thus correlated by an equation analog to a two-parameter Weibull cumulative distribution function. From this relationship, it was found that the α at gelation was independent of the heating regime while the α at vitrification increased with cure temperature. The maximum rate, dβ/dα, was found to occur at the vitrification points. Model-free kinetics was used to model mechanical cure development and an algorithm was obtained for describing the mechanical degree of cure during curing process.
Key words: Dynamical mechanical analysis (DMA); kinetic models; phenol formaldehyde resins; mechanical degree of cure; chemical degree of cure.
INTRODUCTIONModeling and optimizing of wood-based composite manufacture is playing a larger role in design of processes and manufacturing equipment. In these models, internal temperature and moisture conditions are computed with an aim towards predicting when polymeric cure is sufficient to avoid delamination at the time of press opening. In order to incorporate cure kinetics into a comprehensive hot-pressing model for fully describing thermodynamic, adhesive, and rheological processes, it is necessary to find a suitable model for kinetics of cure development for the wood/phenol-formaldehyde (PF) bondline. PF resols are most commonly used adhesives in the manufacture of wood-based panels. Curing is intended to advance the PF molecules into a crosslinked network to achieve a high durability of the products;
curing is also meant to develop physical and mechanical strength in the products.
Most kinetics of PF resins are based on data from differential scanning calotrimrtry (DSC) (Wang et al. 2005, 2006). Heat evolution in DSC is typically assumed to be proportional to the formation of a chemical network during polymerization (Prime 1997), but how other mechanical and electrical properties relate to chemical bond formation remains unknown. The kinetics of chemical advancement of the adhesive layer is not necessarily linearly related with developing rate of mechanical properties.
It was observed that cure development derived from storage modulus E’ (defined as mechanical cure) was not in agreement with that derived from reaction heat by DSC (defined as chemical cure) (Malkin et al. 2005). Dynamic mechanical analysis (DMA) detected that mechanical cure completes earlier than chemical cure under same cure conditions (Christiansen et al. 1993; Laborie 2002; Vazquez et al.
2005). However, Steiner and Warren (1981) reported that the dramatic stiffness increase in a torsional braid analysis was in agreement with the thermal event by DSC for an advanced plywood PF resin. Yet the relationship between the mechanical cure and chemical cure has not been expressed explicitly.
From an empirical point of view, kinetics may represent the rate of development of physical, mechanical or electrical properties. Therefore, mechanical cure kinetics can be modeled in a similar way that the chemical cure kinetics is determined using DSC. The difference is found in defining the degree of cure, β. The value for β is commonly defined as a fraction of E’, with the minimum value set as zero and the maximum E’ as unity (Vazquez et al. 2005). The commonly used nth order and autocatalytic models have been applied (Toffey and Glasser 1997) previously. There model-fitting approaches can describe a shape for a sigmoidal curve and have been used for kinetics of mechanical development using DMA data for PF resins (Wang et al. 2007).
OBJECTIVES Understanding cure kinetics and mechanical property development in wood/adhesive systems is important for evaluating adhesive performance, formulating new resins, and optimizing process parameters. DMA is a commonly used analytical technique for evaluating cure development of polymer systems but has not been standardized in wood adhesion research. Most cure kinetics models are focused on either predicting the chemical state of the resin (Wang et al. 2005) or the mechanical properties (Wang et al. 2007). The relationship of the chemical and mechanical advancements remains unclear; however, both are needed to describe bondline development completely. To date, all model-free kinetics such as the Kissinger-Akhira-Sunnose (KAS), Friedman and Vyazovkin methods are exclusively used for modeling DSC data (Wang et al. 2005). In this perspective, the objectives of
this research are to:
5. Explore improved techniques for directly evaluating wood-adhesive systems and the relationship between mechanical cure and chemical cure, and
6. Validate the application of model-free kinetics to DMA data.
EXPERIMENTALPF resin A PF resole resin, tailored as an adhesive for the core layer of oriented strand boards, was obtained from Georgia-Pacific Company, frozen and stored at -20°C until use. The resin had a high molecular weight with an Mw = 6576 g/mol and Mw/Mn =1.72. The resin solid content was 45.0% with 3.7 wt % nitrogen, indicating the presence of urea in the resin (Wang et al. 2005).
Specimen preparations Planed basswood strips (Midwest Products, Inc.) with nominal dimensions of 50x12x1 mm were oven-dried at 103 °C and stored in a desiccator over anhydrous calcium sulfate until use. Sandwich-type DMA specimens were produced from a layer of PF resin between two pieces of wood adherends. Care was taken to match the grain, thickness, and weight of the two wood adherends within the specimen to maintain a balanced composite design. The bonding surfaces were lightly hand sanded along the grain with 220-grit sandpaper and cleaned with a paper towel immediately prior to resin application. The resin was uniformly applied to the prepared surface of both wood adherends using a small airbrush (BADGER Model 350). The amount of resin solid applied to each surface was set at ca. 50 g/m2, which equates to ca. 12% of dried wood mass.
Maintaining a consistent resin content was deemed important to repeated cure analysis. He & Yan (2005) demonstrated that the degree of resin loading can influence the cure development. They concluded that this influence occurred primarily through water absorption and evaporation during the DMA test. Therefore, the specimens were wrapped with aluminum foil to maintain moisture content during the tests for the DMA analysis in order to evaluate the cure kinetics to compare to DSC data.
DMA DMA measurements were conducted on the sandwich specimens in three-point bending mode at a span 25 mm with a Tritec 2000 analyzer (Triton Technology). The frequency was fixed at 1 Hz. Strain sweep tests have been conducted to establish the linear viscoelastic ranges at working temperature. Oscillation displacement amplitude of 0.03 mm was thus chosen. DMA was performed isothermally at 90, 100, 110, 120, 130, 140, and 160 °C with three replicates in each temperature. In each test, the DMA oven was preheated to predetermined isothermal temperature, and then the specimen was installed quickly and held at the cure temperature until both modulus and damping approached a constant value signifying the completion of detectable mechanical cure. In addition, ramp experiments were performed at heating rates of 2, 3, 4, and 5 °C/min from room temperature to 250 °C with three replicates in each heating rate. Low heating rates were selected to make sure that the effect of thermal lag was minimal.
DSC To determine the relationship of cure development by DMA and DSC, a Mettler-Toledo DSC 822e was used to scan a sample in a small disk shape (sandwiched a layer of resin between two pieces of wood), which could fit in a 30µl high pressure gold-plated crucible, trimmed from the DMA specimens immediately following DMA sample preparation for foil-wrapped PF bonded wood joints. Ramp temperature scans were conducted at 6 heating rates 2, 3, 4, 5, 10, and 15 °C/min from 25 to 240 °C. The chemical cure development was obtained under linear heating rates and isothermal chemical cure development was predicted with ramping data as described by Wang et al. (2005).
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.