Endothermic reaction, polymer

The first step in Eq. (32) involves desolvation. In this step, AH, may be positive (endothermic reaction) and its absolute value is related to the strength of the interaction between solvent and each polymer component. The following step is considered to involve complex formation between desolvated polymer components. In this step, hydrogen bonds are formed between polymers so that AH2 may be negative (exothermic reaction). The final step consists in the conformational change involving complex formation and other factors (AH3). The total heat of mixing as shown in Table 15 is the sum of the increments, AH, AH2 and AH3 ... 

There are a number of flame-retarding mechanisms that operate in the solid phase of polymers. One is to use additives that absorb some of the heat of combustion by endothermic reactions this was mentioned in the previous section in connection with metal hydroxides. 

Liquid hydrocarbons are considered to be the most valuable products of a potential recycling process as they can be used as blends for motor engine fuels. In such a process short-chain hydrocarbons in the gas phase are also produced and they are crucial to provide the heat needed for an endothermic reaction such as polymer cracking, bnt their value is considered low due to their transportation cost. 

Where RH - the monomer units of polymer. Reaction can be triggered by physical factors such as ultraviolet and ionizing radiation, heat, ultrasound, or mechanical treatment chemical factors, such as catalysis, a direct reaction with molecular, singlet or atomic oxygen and ozone. However, initiation by direct interaction of molecular oxygen with the polymer, leads to detachment of a hydrogen atom, was unlikely, because it is endothermic reaction, enthalpy is 126-189 kj/mol (Chan J.H., Balke S.T., 1997). Often, the birth of the chain portrayed as the bimolecular interaction of oxygen with the monomer units of polymer... 

Reduction of heat of combustion. This type of flame-retardant decomposes via endothermic processes or their decomposition radicals react with those from the polymer in endothermic reactions. By means of these additives, the over-aU heat of combustion can be reduced to such an extent that the burning process ceases to be self-supporting. 

Borates with zinc, calcium, magnesium, melamine or barium as the cation, combine char formation with endothermic decomposition as a dual mechanism that provides an efficient flame retardant system. Although the exotherm reduction is not as great as that achieved with ATH, there is enough endotherm to delay the initiation of the polymer exotherm by as much as 100 °C in some systems. It is this endothermic reaction in the polymer condensed phase and the formation of a glassy char that makes borate products good s mergists with the fire retardant antimony trioxide. 80% of the boron from zinc borate remains in the char after PVC combustion. 

Ethylene-vinyl acetate copolymers, usually known as EVA, are used in many applications, but especially for low voltage cables. These polymers are easily flammable and flame retardants are added to reduce their flammability. The classic solution is to incorporate aluminium hydroxide or magnesium hydroxide that develop endothermic reactions when heated. Nevertheless, large amounts have to be incorporated, often around 60% and this can lead to a loss of mechanical properties in the compound. Intumescent technology that works well with polypropylene has also been tried for EVA polymer systems. 

Magnesium hydroxide decomposes at 300-330 °C also as a result of an endothermic reaction with the evaporation of 31% of water in the form of steam [Ij. Endothermic effect for this reaction is valued between 1244 and 1450 J/g according to various sources [1, 2, 4, 5]. The high decomposition temperature of magnesium hydroxide makes it more suitable for polymers which require high temperatures of processing. According to Hornsby, the effectiveness of hydroxide as a fire retardant means is caused by [6-8]... 

In order for a soHd to bum it must be volatilized, because combustion is almost exclusively a gas-phase phenomenon. In the case of a polymer, this means that decomposition must occur. The decomposition begins in the soHd phase and may continue in the Hquid (melt) and gas phases. Decomposition produces low molecular weight chemical compounds that eventually enter the gas phase. Heat from combustion causes further decomposition and volatilization and, therefore, further combustion. Thus the burning of a soHd is like a chain reaction. For a compound to function as a flame retardant it must intermpt this cycle in some way. There are several mechanistic descriptions by which flame retardants modify flammabiUty. Each flame retardant actually functions by a combination of mechanisms. For example, metal hydroxides such as Al(OH)2 decompose endothermically (thermal quenching) to give water (inert gas dilution). In addition, in cases where up to 60 wt % of Al(OH)2 may be used, such as in polyolefins, the physical dilution effect cannot be ignored. 

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