Combustion is initiated by heating a plastic material to its decomposition point. The three critical sources required to sustain combustion process are ignition source, fuel and oxygen. The combustion cycle can be stopped by isolating any of the above mentioned sources. Numerous combustible decomposition products like hydrocarbons, hydrogen and carbon monoxide are formed.
Flame retardance is chemical that added in product as to prevent or protect the product from being damaged by fire accident. Wood is being used in the building and furniture industries because of its unique properties as well as cheap cost (Grexa et al., 2003). However, the flammability of wood limits its capability because of easily being damaged by fire accident. Thus, modification on wood as to improve the capability of withstanding fire is of importance. The surface of wood could be treated by applying layers of flame retardance coating as to protect it against fire.
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There are two types of flame retardance; active or conventional flame retardance and intumescent flame retardance. Active flame retardant mostly are halogen based which leads to emission of toxic to environment. It also produces large amount of smokes, corrosive and irritating gases such as Hydrobromic acid and Hydrochloric acid upon burning.
Meanwhile, intumescent flame retardance (IFR) system promotes a fire suppression mechanism which is toxic free, light smoke density, low corrosion, easy and safe handling compare to active flame retardance system.
Intumescent flame retardance system
IFR system is not only simple, efficient, and economical method to protect substrate from fire but also does not affecting the intrinsic properties of the substrate (Ju-wei Gu et al., 2007). It is also one of the easiest the most efficient ways to protect material against fire without disturbing the mechanical properties of material.
Intumescent can be defined as the foaming and swelling of a plastic when exposed to high surface temperature or flames. While intumescent coating is coating of plastic type formulated with and intumescent action to protect an object from intense heat or flames by decomposing into a foam barrier.
IFRC consist of several components; which are
Polymeric binder such as epoxy, polyurethane, acrylics and polyesters which will bind all other components together.
Carbonizing agent which normally of polyols such as glycerol and pentaerythritol.
Acid source such as phosphate which will react with carbonizing agent to form char layer.
Foaming agent such as melamine which act the expander of the char layer to provide thicker protection layer.
Mechanism of IFRC system
Following are the mechanism involved in IFRC (Chuen-Shii Chou et al., 2009):
As the plywood with a layer of IFRC is exposed to the heat source for a certain period, the heat source starts to melt the solidified resin binder in the IFRC and an amorphous layer is formed and sandwiched between plywood and the char layer.
The dehydrate agent produces the non flammable gases which can dilute the concentration of oxygen and delay the spreading of flame.
The foaming agent produces non flammable gases which not only delay the spreading of flame but also puff out char layer so that its strength may be strengthened by the flame retardant filler.
Epoxy is used as the binder for intumescent flame retardant coating which binds all the compounds in this formulation.
Figure 2.3: Chemical structure of epoxy (bisphenol-A based)
2.3.1 Raw material (K. J. Saunders, 1988)
220.127.116.11 Bisphenol A
Bisphenol A is one of the raw materials in producing epoxy. Bisphenol A is formed from phenol (2 moles) and acetone (1 mole). In atypical process, the phenol and acetone is mixed and warmed to 50°C. Hydrogen chloride which act as catalyst is passed through the mixture for about 8 hours, and during that period of time, temperature is kept below 70°C to suppress the formation of isomeric products. The precipitates is then washed with toluene to remove any unreacted phenol. The product is then recrystallized from aqueous ethanol.
Epichlorohydrin is a colourless liquid with an irritating odour and boiling point of 115°C. The first step in attaining epichlorohydrin is the 'hot' chlorinatin of propylene. A mixture of propylene and chlorine is heated at about 500°C and 0.2 MPa. Then it is treated with pre-formed hypochlorous acid at about 30°C to give the addition product, dichlorhydrin. The reaction mixture gives two layers; the aqueous layer is removed to leave dichlorhydrin. This dichlorhydrin is stirred with a lime slurry to obtain epichlorohydrin.
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2.3.2 Preparation of epoxy
A mixture of bisphenol A and epichlorohydrin is heated to about 60°C while stirring. Solid sodium hydroxide is added slowly. The reaction is exothermic and cooling is applied to keep temperature maintain at 60°C. Excess of epichlorohydrin is then removed by distillation under reduced pressure. The residue is mixed with sodium chloride, then filter off with toluene added as to facilitate filtration. Toluene is removed by distillation. Finally resin is clarified by passing it through a fine filter.
Melamine in this intumescent flame retardant system will act as foaming agent which will puff out and expanding char out from the coating (Chuen-Shii Chou et al.). Melamine with molar mass of 126.12 g/mol.
Figure 2.4: Chemical structure of melamine
Melamine is used in the production of melamine resins, typically by reaction with formaldehyde. It has many industrial uses, including in the production of laminates, glues, adhesives, moulding compounds, coatings and flame retardants. In the US melamine is an indirect food additive for use only as a component of adhesives.
Melamine is not metabolized and is rapidly eliminated in the urine with a half life in plasma of around 3 hours. The compound has a low acute toxicity, with an oral LD50 in the rat of 3161 mg/kg body weight (World Health Organization, 25 September 2008).
Nitrogen containing FR such as melamine is environmental friendly because there are less toxic and do not have additional elements. There are no dioxin and halogen acids by product and low evolution of smoke during combustion. Moreover, nitrogen based FR is suitable for recycling.
Glycerol or also known as propane-1,2,3-triol has a structure as below. It consists of a chain of three carbon atoms with each of the end carbon atoms bonded to two hydrogen atoms (C-H) and a hydroxyl group (-OH) and the central carbon atom is bonded to a hydrogen atom (C-H) and a hydroxyl group (-OH). Glycerol with molar mass of 92.09 g/mol and density of 1.261 g/cm3. Glycerol which the melting and boiling point is 17.8°C and 290°C respectively, and acts as carbon source in the intumescent flame retardant coating formulation.
Figure 2.5: Chemical structure of glycerol
One important property of glycerol or glycerin is that is not poisonous to humans. Therefore it is used in foods, syrups, ointments, medicines, and cosmetics.
Phosphorous compounds are widely used in conferring flame retardancy in polymers. There are types of phosphorous-containing flame retardant (I. Reshetnikov et al., 1996):
Inorganic phosphates, polyphosphates and their derivative with pentaerythritol and/or melamine are applied as intumescent system for development of fire retarded polymer based on cellulosic, acrylics, epoxy resins, polyurethanes and polypropylene as well as creating IFRC.
Organic phosphate, phosphites and phosphates are employed for creating fire retarded polymers such as PVC and engineering thermoplastics.
Additives having phosphorous and nitrogen, for example are polyphosphazines and phosphoramines.
Polyammonium phosphate is an inorganic type of phosphate which is suitable for flame retardant based on epoxy (I. Reshetnikov et al.). Polyammonium phosphate with molar mass of 149.09 g/mol is used upon receiving from Taiwan Chemical, Taiwan. Phosphate is added into the formulation as acid source which will promoting char formation on coating surface (Chuen-Shii Chou et al.).
Figure 2.6.1: Chemical structure of ammonium phosphate
2.6.1 Glycidyl Silane
Glycidyl silane is a type of coupling agent that is used to bind glycerol (polyol) to the resin system. This silane has a boiling point of more than 250°C and is used as received. Silane type of coupling agent is suitable to be formulated with epoxy due to its ability to increase the strength of thermoset system (ShinEtsu).
Figure 2.7.1: Chemical structure of glycidyl silane
Coupling agent can be defined as any chemical substance designed to react with both filler and plastic such as in compounds and particularly in reinforced plastic. They are applied to the reinforcement phase from aqueous or organic solution, from the gas phase or added to the matrix as an integral blend (Dominick V. Rosato, 1993).