Flammability resistance

Nevertheless the euphoric optimism where these materials were allowed a huge potential in material applications has given way to a more realistic view. Nanocomposites are not a universal solution for reinforced materials. Their full potential can only be realised if every step in the added value chain is taken into account during the whole development process. From todays perspective nanocomposite materials with an improved thermal flammability resistance or improved barrier properties have the best chances to fulfil these requirements. 

Furthermore, another advantage of nanofillers is not only to reinforce the rubber matrix but also to impart a number of other properties such as barrier properties, flammability resistance, electrical/electronic and membrane properties, and polymer blend compatibility. In spite of tremendous research activities in the field of polymer nanocomposites during the last two decades, elastomeric nanocomposites... 

There is evidence to show that the particle size of the filler also plays a significant role in flammability resistance. For example, below a certain particle size (about 1-2 pm), in many tests, including oxygen index, aluminum hydroxide shows enhanced fire-retarding performance,34 which may be associated with the rate of filler decomposition and/or with the formation of a more stable ash. However, it has been found that the particle size effect is absent, or less evident, in the cone calorimeter test.35 Similarly, particle size reduction has been shown to enhance fire retardancy in magnesium hydroxide-filled PP in this case, samples were characterized by the UL94 test.36 This raises the question as to whether further reductions in particle size to the nanoscale will lead to an additional increase in flammability performance, and perhaps enable filler overall levels to be significantly reduced. This aspect is considered in a later section. 

Reduced overall filler level/ lower smoke Enhanced flammability resistance at low co-additive additions... 

Colored formulations Low co-additive additions Enhanced flammability resistance/reduced smoke Improved processibility and physical properties Handling issues Char promotion Reduced filler levels Can be pigmented Reduced overall filler levels Color limitations Possible adverse toxicity effects... 

Figure 36. Limiting oxygen index (LOI) of flammability resistance for carbon/carbon composites and related hybrid composites with epoxy resin impregnant, as functions of bulk density and resin content (64).

Fillers are used in these products to improve mechanical properties or impart flammability resistance. Fillers are frequently silane-treated to further improve mechanical properties. Fillers must have a low moisture (below 0.1%), a low absorption of resin, and are expected to impart thixotropic properties. There are special cases. For example, if peroxyketals are used as initiators, basic fillers have to be used because acidic fillers interfere with cure times and the shelf-life of the composition. Shape and particle size distribution must be considered in filler selection to impart the desired rheological properties. Calcium carbonate is the most popular filler but aluminum trihydrate, anhydrous calcium sulfate, and silica are also frequently used. Barite is well suited to this application, especially if acid... 

The materials shown in Table I are available in varying molecular weights and filler compositions. For molded circuit applications, the most commonly used fillers comprise chopped or milled glass fibers and/or mineral talcs. In addition to these fillers, additional components may be incorporated to impart flammability resistance or to promote electroless plating. The latter is achieved by the addition of a proprietary mineral filler which renders the polymer "catalytic" to plating. 

Diluents or organic liquids with low solvent power for the vinyl resin are frequently used to lower cost, alter evaporation rate, change rheological properties of the solution, or to achieve special characteristics such as flow out, odor, flammability, resistance to blushing, etc. A more soluble resin and a high concentration of good solvent in the system permits wider latitude in using the diluent. 

Specifically, PVC blends with polyethylene, polypropylene, or polystyrene could offer significant potential. PVC offers rigidity combined with flammability resistance. In essence, PVC offers the promise to be the lowest cost method to flame retard these polymers. The processing temperatures for the polyolefins and polystyrene are within the critical range for PVC. In fact, addition of the polyolefins to PVC should enhance its ability to be extruded and injected molded. PVC has been utilized in blends with functional styrenics (ABS and styrene-maleic anhydride co-and terpolymers) as well as PMMA offering the key advantage of improved flame resistance. Reactive extrusion concepts applied to PVC blends with polyolefins and polystyrene appear to be a facile method for compatibilization should the proper chemical modifications be found. He et al. [1997] noted the use of solid-state chlorinated polyethylene as a compatibilizer for PVC/LLDPE blends with a significant improvement in mechanical properties. A recent treatise [Datta and Lohse,... 

The term aramid is used to describe aromatic polyamides, which were developed to improve the heat and flammability resistance of nylon. Nomex is a highly heat-resistance nylon introduced in 1961 by DuPont. It is produced by the solution or interfacial polymerization of isophthaloyl chloride and 7M-phe-nylenediamine ... 

To reduce the likelihood that a system will experience fire, many Companies write UL VO listings into contract requirements when purchasing electrical enclosures. The UL VO listing is a stringent flammability test for elastomers. But just because a material can serve in high temperature applications, does not automatically guarantee that it has adequate flammability resistance to qualify for the UL 94 VO rating. A case in point is the various brands of silicone. 

This chapter reviews efforts to include organoantimony into polymers. This is not an exhaustive review but will contain most of the reports. Polymers containing organoantimony moieties have been reported to exhibit some potentially useful properties including an unusually low solution viscosity relative to molecular weight, antimicrobial activity, semiconducting properties, and flammability resistance. While antimony is typically found in both the plus 3 and plus 5 state, essentially all polymers are of the plus 5 state. 

The flammability behaviour of clay-polymer nanocomposites could be restricted by incorporating the nano-clay as reinforcement in limited volume fraction. The heat release rates also are found to be diminished substantially by nano-clay incorporation. The flammability resistance can be enhanced by the incorporation of nano-clay platelets without compromising other properties [114]. This improvement in flammability resulted in development of Wire Cable jacket material [115]. 

The Federal Aviation Authority in the USA commissioned NASA Ames Research Laboratory to examine the problems of aircraft interior flammability and they initially concentrated on limiting the effects of post crash fuel fire. The intense radiated heat ignites curtains, seats and decorative panels and it was reasoned that since the seats contained urethane foam, which will burn and generate asphyxiating gases, any improvement in upgrading the flammability resistance of the seats would provide a significant improvement in safety performance in the event of a post crash fire. 

PMMA/PVC blends provide heat resistance and chemical and flammability resistance into materials for injection moulding and extrusion applications. The major applications of these blends are interior panelling, trim and seat backs in mass transit vehicles. Commercially available PMMA is miscible with PVC [28, 31]. However, its phase behaviour is considered to be only partially miscible [29]. Chlorinated polyethylene blends with PVC have been used as impact modifiers and as secondary plasticisers [32], Chlorine contents of 42% and 30% by weight lead to miscible and immiscible properties, respectively. Impact modification will generally require phase separation, and plasticisation will require miscibility. 

Flammability resistance. An increased flammability resistance has been noticed as an important property enhancement involving nano-platelets incorporated into polymers involving exfoliated clay this involves the formation of a stable carbon/nano-platelet or nanofibre surface. This surface exhibits analogous characteristics to intumescent coatings, whereby the resultant char provides protection to the interior of the specimen by preventing continual surface regeneration of available fuel to continue the combustion process. The primary advantage noted with nano-filler incorporation is the reduction in the maximum heat release rate. 

Other types of fillers are often used to give the polymer other properties, such as flammability resistance. Nonhalogen flame-retardant materials, such as magnesium hydroxide, are often added to polypropylene to produce low-smoke, nonhalogen, flame-retardant compounds. 

It offered essentially the same 220°C Tg as that of PES but significantly superior performance attributes relative to both PSF and PES in terms of mechanical toughness, chemical resistance, and flammability resistance. 

Polysulfones are used in quite a few electrical and electronic applications. They offer a very good fit in such applications because of their good dielectric strength, low dielectric constant and low dissipation, and excellent dimensional stability. PSF and PSF blends are used in a variety of electronic connectors because of flammability resistance, dimensional stability and warp resistance, and heat resistance. Good processability is also critical in this application in that tight dimensional tolerances are required for success in connector applications. For similar reasons, PES... 

PP copolymers are used for the insulation of telephone singles for twisted pairs, in the form of solid or cellular insulation. Here again, PE is the usual resin for this application, except in the USA where PP is preferred for its higher mechanical properties and better thermal and flammability resistance. PP copolymers are preferred to homopolymers for their flexibility. 

Catalysis is usually accomplished through the use of tertiary amines such as triethylenediamine. Other catalysts such as 2,4,6-tm-(A, A -dimethylaminomethyl)phenol are used in the presence of high levels of crude MDI to promote trimerization of the isocyanate and thus form isocyanu-rate ring structures. These groups are more thermally stable than the urethane structure and hence are desirable for improved flammability resistance (248). Some urethane content is desirable for improved physical properties such as abrasion resistance. 

Carbon-Fiber Composites. Cured laminates of phenolic resins and carbon-fiber reinforcement provide superior flammability resistance and thermal resistance compared to unsaturated polyester and epoxy. Table 14 shows the dependence of flexural strength and modulus on phenolic-carbon-fiber composites at 30-40% phenolic resin (112). These composites also exhibit long-term elevated temperature stability up to 230°C. 

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