Curing heat released

Phenolic coatings Phenolic is reacted with formaldehyde under heat to form a completely insoluble material. They are usually applied in an alcohol solution by spray, dip or roller. During their curing, they release water, which must be removed. They have maximum chemical and solvent resistance but poor alkaline resistance. 

Furthermore, the wide range of polymerization rates is controlled by the photoinitiation conditions. Specifically, the initiator concentration and incident light intensity control the rate of polymerization and, therefore, the rate of heat released upon curing. These conditions can be conveniently altered for in vivo applications to minimize local tissue necrosis from the... 

The second important aspect in thermodynamic studies is the determination of the enthalpy. A knowledge of the thermochemistry of epoxy-amine interactions is important also as a prerequisite for rational curing processes as manufacturing methods. The solution of this problem is also important for the application of the calorimetric method to the kinetic investigations. In fact, in the case of reactions with continuously varying concentrations of the donors and acceptors, the observed heat release (Q) may depend nonlinearly on conversion (a) as of the general case... 

The uncured binder material should be a liquid which cures with minimum heat release and shrinkage and without evolution of gases. 

DSC data is commonly fitted to semi-empirical models that accurately describe the curing reaction. Hence, the rate of cure can be described by the exotherm, Q, and the total heat released during the curing reaction, Qt, as... 

Total heat released of a cured thermoset polymer Qt As we saw in Chapter 2, the heat released by a thermoset or an elastomer during curing can be directly related to the degree of cure and the curing rate by... 

In order to obtain the degree of cure and rate of curing, we must first measure the reaction. This is typically done using a differential scanning calorimeter (DSC) as explained in Chapter 2. Typically, several dynamic tests are performed, where the temperature is increased at a constant rate and heat release rate (Q) is measured until the conversion is finished. To obtain Qt we must calculate the area under the curve Q versus t. Figure 7.17 shows four dynamic tests for a liquid silicone rubber at heating rates of 10, 5, 2.5 and 1 K/min. The trapezoidal rule was used to integrate the four curves. As expected, the total heat Qt is the same (more or less) for all four tests. This is to be expected, since each curve was represented with approximately 400 data points. 

Cotton Ammonium phosphates are the most effective FRs for cotton as first identified by Gay-Lussac in 1821 and still widely used. All phosphates on heating release phosphoric acid, which catalyses dehydration reactions of cellulose to yield char at the expense of volatiles formation reactions.50 However, ammonium phosphates like mono- or diammonium phosphates are water soluble, hence applicable as nondurable treatments only. Ammonium bromide can be used in combination with ammonium phosphates to provide some vapor-phase FR action. Other examples include borax and boric acid, ammonium sulfamate, and sulfates. These nondurable finishes are useful for disposable fabrics, insulation, wall boards, theatrical scenery, packaging material, paper, etc. Ammonium polyphosphates (APPs) are used in combination with urea to provide semidurable finishes and by curing at 160°C, when some phosphorylation can occur. Semidurable finishes are very useful for materials that may not need frequent washings, e.g., mattresses, drapes, upholstery, carpets, etc. Some commercial examples of semidurable finishes include Flammentin FMB (Thor Specialities), Pyrovatim PBS (Ciba, now marketed by Huntsman), etc.26... 

Apart for exothermic curing reactions, another thermal effect has to be considered, due to the interaction of ionizing radiation with matter. The temperature profile depends on the balance among (on one hand) the rate of heat production, due both to curing reactions and radiation absorption, and (on the other hand) the heat released, in unit of time, by the system toward the environment. Taking constant the geometry of the reacting system, the heat released toward the environment is constant, while the heat production increases with the pulse frequency. 

Table 2 Cone calorimeter data for modified bisphenol A vinyl ester (Mod-Bis-A Vinyl Ester), bisphenol A novolac vinyl ester (Bis-/Novolac Vinyl Ester) and methylenedianiline and benzyldimaine (BDMA) cured epoxy resins and their intercalated nanocomposites ( ) containing 6% dimethyl dioctadecylammonium-exchanged montmorillonite. Heat flux = 35 kW/m, HRR = heat release rate, MLR = mass loss rate. He = heat of combustion, SEA = specific extinction area [121]...

Fig. 22 Heat release rate data for DGEBA epoxy resin cured by methylenedianiline (MDA with and without nanocomposite (6 wt % clay). The clay was a montmorillonite treated with dimethyl ditallow ammonium ions. The cone calorimeter was run at a heat flux of 35kW/m2...

From the temperature integral of the thermograms, both conversion, a and the change of conversion with time, a(t) = da/dt, were determined at a specific cure time (t). The a (t)-value was determined from dynamic runs as the ratio between the heat released until the time t and the total heat of the reaction according to equations (1) and (2) ... 

When the curing reaction is performed in a state in which the drying process is insufficient, a residual solvent or residual moisture is foamed by the heat release during pol5merization or is extruded from a cured material, bled out and left on the surface of the cured film, thereby causing a surface tackiness. 

Differential scanning calorimetry (DSC) measures heat uptake or heat release of a compound as the temperature is raised and also the heat effects associated with material transitions such as melting and curing. 

There are fluorocarbon liquids available that can be compounded and blended to have boiling points in the range of the cure temperatures of many adhesives. The vapors of these materials are very heavy and can, therefore, be readily contained in the proper vessel. If objects are immersed in the vapors of these fluids, they will rapidly be raised to the boiling point of the particular liquid. As in the case of steam cooking, heat transfer is quite rapid because the heat released to the object is a function of the heat of vaporization of the liquid. 

---