1 октября, 2015 
Malyar During gelling and hardening of the condensation resins in the hot press one can distinguish between the chemical advancement of the condensation reaction during curing of the thermosetting resin (build up of the three-dimensional network) and the progressive development of the mechanical strength of the joint (increase in cohesive bond strength). The two quantities do not progress at the same rate. The test methods that are used to follow the progression of the hardening of the resin are shown in Table 20.
The extent of chemical curing can be monitored using DTA and DSC. The exothermic behavior of the curing process is then measured as a temperature difference or directly as heat flow. Figure 5 shows a DSC plot of a PF resin [2]. The DSC run was done with pressure sealed capsules at a heating rate of 10°C/min.
|
Test method |
Description |
References |
|
Differential thermal analysis (DTA) |
Measures the difference in temperature between two cells, these two cells are heated at a certain heating rate; one ofthe two cells contains the sample under investigation. |
357-359 |
|
Differential scanning calorimetry (DSC) |
Uses a similar type of instrument as DTA, but measures directly the heat flow of the exothermic and endothermic reactions occurring. The data obtained that are of interest are: shape of the curve, temperatures of the onset and the top of an exothermic or an endothermic peak, slope of the upcurve, width of the peak. |
15,360-363 |
|
Differential mechanical analysis (DMA) |
DMA uses a small sheet of glass fiber mats as a substrate, which is impregnated with the resin. This sample then undergoes periodic oscillations, at the same time the sample is heated following a special temperature program. The curing of the resin leads to an increase in the strength of the sample which then can be correlated with the increase of the cohesive bonding strength. |
364-367 |
|
Thermomechanical analysis (TMA) |
Similar to DMA but follows the adhesive hardening in situ on the real wood substrate (rather than on glass fiber). Thin wood strips are used to sandwich a liquid glue line which is then hardened. The curing of the resin leads to an increase in the strength of the sample which can then be correlated with the increase of the cohesive bonding strength as well as with the internal bond strength of wood particleboard using the same adhesive. It has been used both at constant heating rate and in isothermal mode. |
368-377 |
|
Torsional braid analysis (TBA) |
The damping behavior of the torsion of a glass fiber probe impregnated with the resin is characteristic for the increase of stiffness. |
169,171,378 |
|
Automatic Bonding Evaluation System (ABES) |
The ABES consists of a small press and a tiny testing machine in a single unit. It enables bonds to be formed under highly controlled conditions; the joints that contain the bonds which are to be measured are pressed against heated blocks for a certain time, cooled within a few seconds, and pulled immediately thereafter in shear mode. Repetition of this procedure at different curing times and temperatures yields the points (a point for each specimen) of a near-isothermal strength development curve. |
96,97 |
|
Figure 5 DSC plot of a PF resin. Pressure sealed capsules, heating rate 10°C/min. (From Ref. 2.) |
During the curing of the resin the cohesive bonding strength develops step by step. Monitoring the effective strength increase (defined as the degree of mechanical curing) enables conclusions to be drawn about the suitability or not of a resin for a certain application. The best methods to use for this purpose are DMA (Fig. 6), TMA (Fig. 7) and ABES [96] (Fig. 1).
In the TMA plot in Fig. 7 it is possible to note the interactive nature of the substrate on the curing of the PF adhesive. For example, the modulus of elasticity (MOE) increase curve shows two sections (and a two peak first derivative curve). This indicates formation of entanglement networks of the resin in wood which is not possible on noninteractive substrates such as glass as in Fig. 6. Of course DMA and TMA give equally good results when used on the same wood substrate [379,380]. The ABES technique is also linearly correlated with TMA and DMA results as has been demonstrated by the linear relationship that has been found for both MUF and tannin-formaldehyde adhesives in the results of TMA and ABES [381].
In board manufacturing, when the press opens, a certain level of mechanical hardening and with this a certain bond strength is necessary to withstand the internal steam pressure in the pressed board. The full chemical curing, however, can be attained outside of the press during hot stacking. Advanced formation of the bond strength already at the same degree of chemical curing will enable shorter press times and will, therefore, increase the production capacity and reduce production costs. Plotting the chemical and mechanical degrees of curing in an x-y diagram shows the different hardening behaviors of various resins; such a correlation plot of the degree of chemical cure (e. g., measured by DSC) and the increase of mechanical strength (e. g., measured by TMA, DMA, or ABES) can be regarded as a fingerprint of the curing behavior of a resin [154].
Опубликовано в рубрике 