Polymers, burning protection

The char layer from a burning polymer, while it exerts protective action, is itself vulnerable to oxidation. This can manifest itself either during flaming combustion as a constant destruction of the char as it forms, or as afterglow. Means for prevention of this undesired char destruction have been reported. In studies on preventing combustion of carbon fibers, incorporation of borates, phosphates, or low melting glasses has been shown to be effective (12, 13). 

In order for a solid to burn 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 solid phase and may continue in the liquid Illicit) 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 comhusiion. Thus Ihe bunting of a solid is like a chain reaction. For a compound to function as a flame retardant it must interrupt this cycle in sonic way. There are several mechanistic descriptions by which flame retardants modify flammability inen gas dilution, thermal quenching, protective coatings, physical dilution, and chemical interaction. 

With the exception of glass and mineral fiber products, and, like rubbers and plastics, textiles are largely hydrocarbon polymers and as such have a strong tendency to ignite and burn from a small flame. Textiles are essentially sheets of woven, knitted, or sometimes randomly orientated fibers and may be directly used on their own or in combination with other materials, e.g., coated fabrics, or as reinforcement, e.g., in rubber hoses. Other examples of textile products are upholstered furniture and protective clothing. 

Metal hydrates such as aluminium trihydrate or magnesium hydroxide remove heat by using it to evaporate water in their structures, thus protecting polymers. Bromine or chlorine-containing fire retardants interfere with the reactions in flames and quench them. Mixtures of flame retardants antimony trioxide and organic bromine compounds are more effective at slowing the rate of burning than the individual flame retardants alone. 

Fire-retardant treatment Hammability of acrylic textiles can be reduced by using (meth)acrylates containing phosphorus monomers known to be effective as flame-retardant compounds (Price et al., 2002). Tsafack et al. (2004) smdied the plasma-grafted thin layer of phosphorus polymer by plasma-induced graft-polymerization. The formation of a characteristic protective char layer during the burning test was observed for the treated compounds whereas the untreated ones burned without residuals (Tsafack et al., 2004). 

An UV detection window was prepared on the capillary by removing 0.5 cm of the protecting polymer layer from one end of the capillary by burning. The detection window was covered with aluminum foil so that photopolymerization would not occur in this area. Plastic tubing was then placed on both ends of the capillary and it was filled by syringe with a polymerization mixture consisting of monomer (0.72 mol/L MAA), cross-linker (0.72 mol/LTRIM), radical initiator (0.022 mol/L... 

Some of the biomedical application of nanofibers is to cure for wound and burnings in human skin. It can be designed for especially hemostatic tools. Electrospun biodegradable polymers can be spun onto the wound skin. They form a thin web onto the skin. This web protects skin from microbes. Moreover, it helps to heal the wound quickly. Finally, it minimizes the possibility of scars. Electrospun nanofiber equipment used in wound healing is shown in Fig. 2.2. 

In addition to polymers, other natural molecules have been associated with polyesters in order to improve their performance. Spirulina is a microalgae which exhibits antibacterial and anti-inflammatory properties and is interesting for use in skin tissue engineering, mainly in burn patients who need a greater protection barrier. Because of this, spirulina biomass was associated with poly-D,L-lactic acid (PDLLA) and electrospinning was performed. The biocompatibility in vitro assays showed that the presence of spirulina improved the biological performance of the scaffolds. These tests found that PDLLA/spirulina scaffolds exhibited more adhered stem cells on their surface and greater cellular viability than PDLLA scaffolds without spirulina [30]. 

Active fire protection is provided by appl5dng agents as liquids, gases, solid powders, or foams to the flame and/or to the surface of the burning polymers. 

R D efforts in halogen-fiee FRs are often aimed at designing a protective closed barrier on the burning polymer surface to reduce heat and mass transfer to the combustion zone. In some polymer applications this can be achieved by the use of intumescent systems. However, these are not always suitable in other apphcations for reasons of water uptake, thermal stability or actual FR performance. Therefore, it is of interest to the material developer to have options available to control the structure of the burning polymer surface layer. In this layer no cracks should appear which could allow for the escape of volatile, ignitable gases and so sustain the combustion process. 

Zylon or PBO is a more recently developed fibre than PBI and has outstanding tensile properties, as well as thermal and fire properties superior to any of the polymer-based fibres mentioned in this chapter (see Table 4.2). While there are at least two variants of the fibre, Zylon-AS and Zylon-HM, of which the latter has the higher modulus, both have the same thermal and burning parameter values. Principal examples of thermally protective textiles include heat protective clothing and aircraft fragment/heat barriers, where its price, similar to that of PBI, restricts its use to applications where strength, modulus and fire resistance are at a premium. 

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