Curing chamber, 5.18

Advantages are similar to the epoxy system, in that these can be solventless and do not require thermal energy. Disadvantages unique to this system, however, include the need to inert the cure chamber to avoid air-inhibition of cure as well as some release instability with acrylate adhesives [72]. 

Single plates are usually cured in special devices (curing ovens or curing chambers, cf., e.g., [25] ) that control humidity as well as temperature. In continuous plate production, the drying of the pasted ribbon is correspondingly controlled. Furthermore, in continuous manufacture final curing can occur after the plates are separated and inserted into the containers. 

Full Scale Measurements in a Curing Chamber for Hardboard... 

The time to tQ is the time for the wood-monomer mass to reach oven or curing temperature at T5. During the period of constant temperature, the induction period, the inhibitor is being removed by reaction with the free radicals. Once the inhibitor is eliminated from the monomer and wood, the temperature rises to a maximum which corresponds to the peak of the exothermic polymerization reaction. Polymerization continues to completion although at a decreased rate and the temperature returns to that of the curing chamber. The time to the peak temperature depends upon the amount of catalyst present, the type of monomer, the type of crosslinker, and the ratio of the mass of monomer to that of the wood. The wood mass acts as a heat sink. Figure 4 illustrates the effect of increased Vazo catalyst on the decrease in time to the peak temperature, and the increase in the peak temperature(10)... 

If energy savings is to be the prime consideration, this will be best achieved by drying the cloth at low temperatures and high speeds and then curing the cloth in a separate oven where there is little or no air flow through the curing chamber. 

The compressive strength of the set sealant is measured with ASTM standard specimens of a (2 X 2 X 2 in. ) cube [6]. The slurry from the consistometer is poured in molds and is allowed to set in a curing chamber. It is then taken out after a desired period, and the strength is measured by applying a load on it in a uniaxial press. Knowing the total load and the area of the face on which the load is applied, one can calculate the compressive strength. 

Transportation of pasted plates in the tunnel dryer results not only in removal of excess moisture from the paste, but also in drying the plate surfaces. Thus sticking of plates to one another is avoided when they are arranged in piles (stacked). At the outlet of the tunnel dryer the plates have a temperature of 40—45 °C and moisture content 9.5%. They are then piled (stacked) on palettes to be transported to and loaded into the curing chamber. The controlled parameters of the plates after drying are plate weight, moisture content of the paste and paste density. 

Dried plates are then arranged in piles, stacked in palettes and transferred to the curing chambers. 

Figure 8.2c presents the X-ray diffraction patterns for the paste cured at 90 °C for 1 h and then steam treated for another hour. This paste contains a well pronounced 4BS crystal phase. Hence, introduction of steam into the curing chamber facilitates the conversion of 3BS particles into 4BS ones. With this technology, a small quantity of Pb9(C03)6(OH)6 is also formed in the paste. 

Figure 8.6 illustrates the changes in phase composition of the paste during curing at 93 °C for 8 h under continuous flow of water steam through the curing chamber [6]. 

The above processes require water steam and high temperamre (above 90 °C). To accelerate the process of 3BS conversion into 4BS steam, it should he purged through the curing chamber. 

By introducing curing chambers with independent temperature and humidity control in the battery manufacturing practice, the problems related to moisture content of the pastes after tunnel drying have been solved. Humidity and temperature in the curing chamber are continuously monitored and controlled, and the moisture content of the paste varies in accordance with the selected humidity profile. 

This dependence has been established through measuring the residual unoxidized lead on curing pastes at 25 °C in curing chambers with various humidity levels. The obtained results of these experiments are presented in Fig. 8.15 [14]. 

Amount of residual unoxidized lead as a function of curing time for plates cured at 25 °C and different relative humidity in the curing chamber [14]. 

With increase of temperature in the curing chamber the rate of lead oxidation increases, irrespective of the RH. However, at RH >81% this increase is insignificant, whereas at RH = 55—75% it is substantial. For 1 h of curing at 50 °C in air atmosphere with RH between 55 and 75%, the content of residual free lead in the paste diminishes from 21 to 11—12%. 

The rate of lead oxidation is influenced also by the moisture content of the paste prior to curing. The amount of water in the paste pores should be reduced so as to open the pores for access of oxygen from the air needed to oxidize the residual free lead in the paste. The rate of water evaporation from the paste pores depends on the RH in the curing chamber. This dependence is illustrated in Fig. 8.17 for plate curing at 25 °C and various relative humidities [14]. 

Water evaporates from the paste pores quickly when the initial moisture content of the paste is less than 15% and the RH in the curing chamber is between 40 and 75%. 

Besides by reactions (8.13) and (8.15), water is also lost through evaporation. Consequently, water content falls rapidly below the critical value and oxidation of Pb stops. Water evaporation can be reduced by keeping relatively high humidity levels in the curing chamber (e.g. 55—75%). Thus, the reaction of Pb oxidation will proceed at a high rate for a longer period of time and the moisture content of the paste will be maintained above the critical value of 5%. 

The above reactions proceed in the solid state, i.e. at slow rate. It is known that the duration of the curing process at 30—40 °C is between 48 h and 72 h, including the drying procedure. Water loss, due to formation and growth of corrosion layer, should be compensated for. That is why a RH from 40 to 75% should be maintained and air should be introduced in the curing chamber. 

Figure 8.23 shows the interface paste/CL2 layer of a plate prepared with 3BS paste cured at 90 °C for 3 h and then treated with steam for another 2 h. Carbon dioxide is introduced into the curing chamber with the water steam [16]. The shape of the crystals formed under these conditions is typical of plumbonacrite and cerussite crystals. Obviously, the technology of 3BS conversion into 4BS through purging with water steam, i.e. introduction of CO2 from the air, leads to formation of hydrocarbonates at some sites of the paste/CL2 interface. 

Sometimes, water evaporation from the surface plate layers may proceed at a much higher rate than the diffusion of capillary water from plate interior to its surface. In these cases, the radii of the pores in the plate surface layers will diminish more quickly than those in the interior of the plate. Hence, inner tensions will be created in the cured paste and the latter will crack. To avoid this, the air humidity in the curing chamber should be reduced slowly accounting for the speed of capillary water movement through the plate cross-section. 

Plate curing can be conducted under optimum conditions in a curing chamber with independent (autonomous) temperature and humidity control. In this case it is essential to devise the optimum curing algorithm (profile) for each particular plate type and size. The pre-set parameters are temperature and RH for each of the two stages curing and drying. 

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