Charring, thermosetting resins

Incorporation of modified clays into thermosetting resins, and particularly in epoxy35 or unsaturated polyester resins, in order to improve thermal stability or flame retardancy, has been reported.36 A thermogravimetric study of polyester-clay nanocomposites has shown that addition of nanoclays lowers the decomposition temperature and thermal stability of a standard resin up to 600°C. But, above this temperature, the trend is reversed in a region where a charring residue is formed. Char formation seems not as important as compared with other polymer-clay nanocomposite structures. Nazare et al.37 have studied the combination of APP and ammonium-modified MMT (Cloisite 10A, 15A, 25A, and 30B). The diluent used for polyester resin was methyl methacrylate (MMA). The... 

Thermosetting resins are usually used lubricated, often with water. When they run in dry situations they benefit considerably from the addition of PTFE, graphite or molybdenum disulphide. Their wear resistance is still not as good as that of the better PTFE composites , but they can be used with higher loads because they are stronger and are not so catastrophically affected by heat. The effect of heat on the polymer is to cause surface charring, which does not seriously degrade performance. 

Classification Thermosetting resin Definition Plastic derived from furfuryl alcohol or from furfural or reaction prods, of furfural and a ketone cured resins char, by heat/fire/chem. resist. 

The investigation on the mechanisms of thermal stabilisation and flammability reduction in nanocomposites reveals that an addition of organoclays into a thermoset resin can substantially aid flame retardancy by encouraging the formation of a carbonaceous char in the condensed phase [118, 119]. The nanoscale dispersed lamellae of clay, either intercalated or exfoliated in polymer matrix, all enhance the... 

Pitches are mainly used as binders (i. e. as precursors for binder cokes) but the term binder should include any carbonaceous binder material, for example thermosetting resins such as polylfutfuryl alcohol) or phenolics and similar compounds which may form a char during carbonization. 

The time to failure of the steel plate covered with thermoset resin (curve B) is close to the time to failure of the steel plate alone (curve A). When the APP derivative is added to the thermoset resin (cnrve C), an improvement in performance is observed (time to failure of 11.3 min compared to 5 min for the uncoated steel). Intumescence and charring take place, but the char falls off the plate before the end of the experiment (change of slope at 610°C). Addition of boric acid (curve D) to the resin also leads to improved performance the time of failnre is increased to 18.2 min. Development of intnmescence is also observed ... 

Thermosets consist of a network of interconnected chains whose positions are fixed relative to their neighbors. Such polymers do not flow when heated. Instead, when exposed to high temperatures, thermosets degrade into char. Examples of thermosets include some polyurethanes and epoxy resins. 

Research on the pyrolysis of thermoset plastics is less common than thermoplastic pyrolysis research. Thermosets are most often used in composite materials which contain many different components, mainly fibre reinforcement, fillers and the thermoset or polymer, which is the matrix or continuous phase. There has been interest in the application of the technology of pyrolysis to recycle composite plastics [25, 26]. Product yields of gas, oil/wax and char are complicated and misleading because of the wide variety of formulations used in the production of the composite. For example, a high amount of filler and fibre reinforcement results in a high solid residue and inevitably a reduced gas and oiFwax yield. Similarly, in many cases, the polymeric resin is a mixture of different thermosets and thermoplastics and for real-world samples, the formulation is proprietary information. Table 11.4 shows the product yield for the pyrolysis of polyurethane, polyester, polyamide and polycarbonate in a fluidized-bed pyrolysis reactor [9]. 

As result of chemical corrosion, the polymer itself may be affected in one or more ways. For example, the polymer may be embrittled, softened, charred, crazed, delaminated, discolored, blistered, or swollen. All thermosets will be attacked in essentially the same manner. However, certain chemically resistant types suffer negligible attack or exhibit significantly lower rates of attack under a wide range of severely corrosive conditions. This is the result of the unique molecular structure of the resins, including built-in properties of ester groups. 

Thermoset polymers are not capable of handling concentrated sulfuric acid (93%) and concentrated nitric acid. P5rrolysis or charring of the resin quickly occurs so that within a few hours, the laminate is destroyed. Tests show that polyesters and vinyl esters can handle up to 70% sulfuric acid for long periods of time. 

As a result of their three-dimensional crosslinked stmctures, thermosets do not soften or flow when burning. The tendency to form gaseous decomposition products is also less than with thermoplastics. Heat may cause surface charring that can prevent ignition. Unsaturated polyester resins and epoxy resin systems require flame retardants to meet the fire protection standards in the constmction, transport and electrical industries where such resins are mainly used. The flammability of epoxy resins is greater than comparable thermosets since they have a reduced tendency to carbonise. After removal of an ignition source, once alight they continue to bum on their own. 

Several mechanisms are needed to explain the action of the many different phosphorus compoimds used as FRs. Some of these compounds decompose in the condensed phase to form phosphoric acid or polyphosphoric acid. They can promote charring. Char formation is further enhanced by cellulosics, polyurethanes, phenolics, epoxy resins and EVA copolymers, and there are catalysts that promote it. Phenol-formaldehyde polymers can be used as flame retardants themselves when combined with a more flammable thermosetting polymer to form an interpenetrating network. 

Resins are of two general types thermoplastic and thermoset plastic. The former melts every time its temperature is raised above a certain limit. This gives rise to coti-cems over its performance in elevated temperatures, because while it can be melted several times during the manufacturing process, it can also be inadvertently melted in service. Thermoset plastic covers a range of plastic resins that only melt (or flow) Mice. Typically they are manufactured in two parts (a base and an accelerator or catalyst), and when mixed and heated, they commingle and flow as a plastic. Once set in this form, they do not melt again, but on the addition of excessive heat, they will soften then char. Thus, like thermoplastics, they are also sensitive to extreme heat. 

However, densely cross-linked plastics (thermosets) are sometimes deliberately used. Reasons tend to be nonmechanical increased solvent and swelling resistance, high-temperature resistance, and reduced flammability are cited. The last arises because the polymer chars rather than degrades with concomitant reduction of volatile (and flammable) components. Examples include epoxies, phenol-formaldehyde resins, and some polyurethanes see Section 14.2. 

In general purpose applications competitively priced thermosets are used for the printed circuit board base material which is usually FR4 (Flame Retardant) . One of the main flame retardants used in America is to have tetrabromobisphenol-A reacted into the epoxy resin. Non-halogen systems include additives such as alumina trihydrate, alumina trihydrate/red phosphorus and aromatic phosphates. Flame retardant epoxy coatings are reported to use ammonium polyphosphate with char-forming additives. 

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