Introduction & Literature Review

1. Introduction 1.1 Polymeric

In recent years, polymeric foams continue to grow at a rapid pace throughout the world, because of their light weight, excellent strength to weight ratio, superior thermal and acoustic insulating capabilities, energy absorption ability and their good cushioning and comfort features.1 Polymeric is dispersion of a gas in a matrix. It generally consists of a minimum of two phases, a solid polymer matrix and a gaseous phase (blowing agent). Other solid phases may also be present in the foams in the form of fillers. Polymeric foams may be either expanded rubbers or cellular elastomers or sponges. It may be either or thermosets. The physical and mechanical properties of the foam differ significantly from the solid matrix material. For example, foams can have much better heat and sound insulation properties compared to solid polymer. In addition, foams can have the ability to absorb an enormous energy, which makes them more useful in cushioning and packaging applications compared to the solid polymer.2 Another advantage of polymeric foams is the small amount of polymer mass is needed to obtain high volume, because of cellular structure with entrapped gas.

Polymeric foams may be prepared with varying densities ranging from as low as 1.6 to as high as 960 kg/m3. Approximately, 70−80% of all commercially produced polymeric foams are based on , and .3 Polymeric foams can be classified as flexible, semi−flexible, or semi−rigid, and rigid, depending upon the rigidity of the polymer backbone, which in turn depends on chemical composition as well as matrix polymer characteristics like the degree of crystallinity and the degree of cross−linking. Various method of foam manufacturing can be adopted and tailor made hardness and other properties can be achieved for the foam to suit different application. Typical processing methods include continuous Copyrightslabstock produced by pouring, foaming−in−place, molding, extrusion, spraying, rotational casting, frothing, precipitation, composites and . The polymeric foams may be prepared in any shape and forms such as blocks, boards, slabs, sheets, tubing, molded shapes, or in composite forms as laminates, with facing materials such as solidIIT , metals, fabrics,Kharagpur paper, wood, etc.

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1.1.1 Historical Development of Polymeric Foams

The first cellular polymer to be placed on the market was sponge rubber which was developed as early as 1914. It was produced by the addition of gas generating chemicals like sodium and ammonium carbonate or sodium polysulfide to natural rubber latex.4 The oldest rigid cellular was cellular ebonite, which was produced in the early 1920. The Dunlop latex foam process originated at the end of 1928 and was based on a combination of foaming and delayed action gelling. Several other processes were subsequently developed for the production of latex , but the only major competitive process to reach commercial importance was the Talalay process, which had its origin in about 1935. The Swedish engineers Munters and Tandberg invented the extrusion of foamed polystyrene in 1931 and simultaneously the Dow Chemical Company independently developed “Styrofoam” by extrusion process and commercial production in the U.S. started in 1943. The introduction of commercial phenolic foams occurred in 1945, while the use of phenolic “microballoons” (hollow microspheres based upon phenolic resins and filled with an inert gas, e.g. nitrogen) for use in specialty type “syntactic” foams developed in 1953. Epoxy foams were first introduced in 1949 as light weight materials for the encapsulation of electronic components. Urea formaldehyde foams in the form of slabs used for thermal insulation whereas vinyl foam was first manufactured in Germany prior to World War II.

The technology of urethane foam originated in Germany in the late 1930’s and Prof. Otto Bayer and his co−workers first developed rigid polyurethane foams based on based polyol and toluene diisocyanate in the laboratories of the German I.G. Farbenindustrie. Preparation of flexible urethane foams were first reported in the year 1952. foams for use as a low−loss insulation for wire and cables was introduced in 1944. foams, both thermoplastics and cross−linked types were introduced due to their relatively high service temperature and good Copyrightabrasion resistance property. The development of silicone foams started in 1950, in order to meet the need for a light weight material that could withstand long−term exposure to temperatures in the range of 200°−375 °C. A number of other types of high−temperatureIIT resistant foams Kharagpur have been developed recently. These include foamed fluorocarbons, cellular aromatic , and syntactic polybenzimidazole foams. In addition, many other types of flexible and rigid foams have been developed

2 Introduction & Literature Review

based on both natural as well as synthetic polymeric materials. These include foams based on butadiene−styrene, butadiene−acrylonitrile, , acrylonitrile butadiene styrene, acrylics, cellulose acetate, ionomers, and many others. It can be mentioned that foams can be made from almost any polymer, employing one or more processing techniques.

1.1.2 Basic Principles in the Formation of Polymeric Foams

In general, most of the polymeric foams are formed by a process involving nucleation and growth of gas bubbles in a polymer matrix, except in the syntactic foam where micro−beads of encapsulated gas are compounded into a polymer system

or latex. According to the nucleation mechanism, the fundamental principle for the formation of polymeric foam involves three different important stages such as, bubble formation, bubble growth and bubble stability. The foam is expanded by increasing the bubble size before stabilizing the system. As the bubbles grow, the foam structure changes through number of stages.5 These are the following characteristics observed during the formation of foam.

ƒ Initially, small dispersed spherical bubbles are produced in a liquid polymer matrix, with a small reduction in density. The further growth of cells leads to lower foam density, which involves distortion of cells to form polyhedral structures, sometimes idealized as pentagonal dodecahedrons.

ƒ Effects of viscosity and surface tension subsequently cause materials to flow towards the uniform cell formation.

ƒ Extensive rupture before the foam is stabilized may lead to foam collapse. Cooling of closed cell foam before stabilization may lead to shrinkage, because of the reduced pressure in cells.

The foaming of polymeric materials can be carried out by mechanical, Copyrightchemical, or physical methods.4 Some of the most commonly used methods are; ¾ Thermal decomposition of a chemical blowing agent, generating either nitrogen or carbon dioxide or both, by application of heat or as a result of the exothermic reaction during polymerization. Chemical blowing agents are IITeither inorganic materialsKharagpur such as carbonates, bicarbonates, borohydrides, etc., or organic materials such as, hydrazides, azides, and nitroso compounds, etc.

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¾ Mechanical whipping (frothing) of gases into a fluid polymer system (melt, solution, or suspension), then it hardens either by catalytic action or heat or both, thus entrapping the gas bubbles in the polymeric matrix.

¾ Volatilization of low−boiling liquids (fluorocarbons or methylene chloride) within the polymer mass as a result of the exothermic reaction or by application of heat.

¾ Chemical blowing action via in-situ reaction during polymerization. (In this in-situ reaction water reacts with isocyanate to form carbon dioxide which is responsible for polyurethane foam formation).

¾ Expansion of dissolved gas in a polymer mass upon reduction of pressure in the system.

¾ Incorporation of tiny beads or microspheres into a polymer mass. The hollow microspheres may consist of either glass or plastic beads, expandable by heat.

1.1.3 Application of Polymeric Foams

Applications of polymeric foams depend on the nature of polymer and their types. For example, the main applications of flexible foams are for cushioning, packaging, automotive safety, footwear, etc. The rigid foams are used for insulation in building, transportation, appliance (refrigerator and freezers), buoyancy and in−fill and packaging.6 Four main areas in which polymeric foams find wide applications are listed in Table 1.1.

1.2 (PUs)

Polyurethane (PU) is one of the most important classes of specialty polymeric material. It was first synthesized as a fiber−forming polymer by Bayer in 1937 to compete with . Besides Bayer, Hoechtlen, Hoppe and Weinbrenner had combined the description of the PUs on the scientific basis and their analysis with Copyrightpotential application areas and corresponding market volumes for this new material developed in Leverkusen. Rinke and collaborators were successful to prepare the from a low viscosity melt which resulted in what now called as polyurethane.IIT Rinke and associates Kharagpur were awarded the first US Patent on PUs in 1938. The first commercial products were Igamid U for synthetic fabrics and Perlon U for producing artificial silk or bristles. Initially, all commercial applications of PUs were

4 Introduction & Literature Review

based almost exclusively on polyester based polyol. Polyether polyols were first introduced in 1957. These polyols have several technical and commercial advantages, so they rapidly gained the preferred role in PUs. In 1995, polyurethane was ranked 5th with a share of just over 5% of total worldwide plastic production.7 In recent years the PU is one of the most versatile polymers in the modern cellular . The majority of PUs is used in the production of flexible foams (48%), followed by rigid foams (28%), elastomers (7.8%) and also used for specialty applications to an extent of 16.2% in the form of coatings, fibers, adhesives, caulks, sealants, etc.

Table 1.1: Application of Polymeric Foams S.No. Area of Application Polymer Types Uses 1 Cushioning Slabstock flexible public transport seats, carpet underlay, PUF furniture, bedding Molded flexible footwear, furniture, auto−motive PUF seating, auto−bumper systems 2 Insulation − Polystyrene boardstock Construction Polyolefins Pipe insulation Rigid PUF Boardstock/laminates, sandwich panels, spray/ pour−in−place, slabstock/pipe section, pipe−in−place. Phenolic Boardstock/laminates, pipe section. Appliance Rigid PUF Refrigerators/freezers, picnic boxes/others. Transport Rigid PUF Sandwich panels, reefer boxes. Polystyrene Sandwich panels. 3 Packaging − Sheet Polystyrene Single service uses, food packaging, miscellaneous packaging. Polyolefins Furniture, cushion packaging. Non−insulation Polyurethane Miscellaneous packaging. Molded Polyolefins Cushion packaging Boardstock Polyolefins Cushion packaging Copyright4 Safety − Molded Polyolefins Auto−bumper systems Integral skin Polyurethane Steering wheels, etc. Sheet Polyolefins Flotation, life vests. Board Polyolefins Flotation/buoyancy IIT KharagpurPolystyrene Flotation/buoyancy Molded/Injected Polyurethane Flotation/buoyancy

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The chemistry of PU makes use of the poly−addition reactions of organic isocyanates with compounds containing at least two active hydrogen atoms.8 An isocyanate group reacts with the hydroxyl group of a polyol/diol to form the repeating urethane linkages. The polymer is usually formed into the final article during this polymerization reaction. There are a wide variety of PUs with quite different compositions and correspondingly with different properties. It has a wide variety of properties and applications such as soft to hard, plastic, elastic, or thermoset, compact or foamed. They are in different forms such as molded articles, film or fibers, in solutions or dispersions. The properties of PU can be tailored to fulfill the requirements of different applications; e.g. flexible foam for upholstery, semi−rigid integral skin foam for automotive parts, rigid foam for insulation, high performance rigid coatings for a diversity of substrates and requirements, flexible coatings for textiles and leather, elastomers for elastic fibers and adhesive, etc.9 The reaction between isocyanates and polyol is exothermic in nature and is catalyzed by organo−tin compounds (eq. 1.1). The rate of the reaction depends upon the structure of both the isocyanate and the polyol.

The isocyanates also react with water to form an unstable intermediate of

carbamic acid which readily decomposes to generate carbon dioxide (CO2) and the

corresponding amine (eq. 1.2). This CO2 also acts as a blowing agent during the formation of polyurethane foams. This important isocyanate−water reaction is catalyzed by tertiary amine compounds. The amine formed during the isocyanate− water reaction, reacts with additional isocyanate to form disubstituted urea (eq. 1.3). In addition, a number of cross−linking side reactions may also take place, depending upon the stoichiometry and the reaction conditions such as temperature and the type of catalyst used. The reaction of isocyanates with urea groups (formation of biuret, eq. 1.4) is significantly faster and occurs at a lower temperature than that of allophanate formation (reaction between urethane−isocyanate, eq. 1.5). Normally this cross− Copyrightlinking or secondary reaction does not occur readily without a catalyst at temperatures below 120 °C.10 Isocyanate also undergoes self−addition reaction in presence of basic catalysts to form isocyanurate (commonly called trimers, eq 1.6), and carbodiimide (dimers, eq. 1.7)IIT and uretidione (eq.Kharagpur 1.8).

6 Introduction & Literature Review

HO (1.1) R N C O + ROH R N C O R urethane

HO

R N C O + H O 2 R N C OH Carbamic acid (1.2)

R NH + 2 CO2 amine

HOH

R N (1.3) C O + R NH2 R N CRN urea

H O H O H

R N C N R + R N CO R N C N R urea (1.4) C N R O H Biuret H O O

R N C O R + R N CO R N C OR (1.5) urethane C N R O H Allophanate R O N (1.6) 3 R NCO O N R N R O Isocyanurate (1.7) 2 R N CO R N C N R + CO Copyright2 carbodimide 2 R N CO R N C O (1.8) O C N R IIT Kharagpururetidione

Scheme 1: Different Possible Side Reactions in the Preparation of Polyurethane

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Dimer formation arises only from aromatic isocyanates and it is inhibited by ortho substituents. Thus, 2,4 and 2,6−TDI do not form dimers at normal temperatures, but 4,4−diphenylmethane diisocyanates (MDI) dimerise slowly when left standing at room temperature (RT). Isocyanurates are formed on heating both aliphatic and aromatic isocyanates. Isocyanurate and carbodiimide linkages provide improved thermal stability and flame retardant (FR) properties to PU materials to a certain extent. Thus, a typical PU structure may contain aliphatic and aromatic hydrocarbons, esters, ethers, amides, urea, biuret, allophanate, isocyanurate, uretidione and carbodiimide groups, in addition to the urethane linkages.11 The versatility of compounds is even more extensive on the diol or polyol side. The polyols are available in different form differing in functionality, chain length, and reactivity.

1.2.1 Polyurethane Foams (PUFs)

Unlike most systems designed to give cellular plastic, polyurethane foams are formed in a single rapidly occurring step. The foaming and formation of the urethane polymer take place simultaneously by a rapid and controllable release of gas in system undergoing fast exothermic polymerization. Polyurethane foams having microcellular structures are produced by gas bubbles formed during the polymerization reaction. The process of bubble formation in polyurethane foams is called blowing. The chemical ingredient in the formulation that provides additional gas formation is called the blowing agent. Low−boiling liquids added to physically assist in the foaming process are auxiliary or physical blowing agents. Polyurethane foam is also formed by the reaction between isocyanate and water through release of

CO2, which acts as a chemical blowing agent (eq. 1.2 in Scheme 1). In general, the polyurethane foams are prepared by three different processes viz: i) one−shot, ii) quasi−or semi−prepolymer and iii) prepolymer process. In one−shot process, all the components are mixed together to produce foam in a single step. In the quasi− Copyrightprepolymer or semi−prepolymer approach, part of the polyol to be used in the formulation is pre−reacted with all of the isocyanate. The resultant product is isocyanate terminated prepolymer having free isocyanate content between 16 and 32% by weight. Foams are prepared by adding rest of the foam ingredients along with remaining polyol.IIT In the prepolymer Kharagpur process, the hydroxyl compound is reacted with an excess of isocyanate to form an isocyanate−terminated prepolymer with free

8 Introduction & Literature Review

isocyanate content of 1 to 15% by weight. Among these three methods, the one−shot process is widely used, because it is the fastest, simplest and the most economical technique for manufacturing PUF. Since most of the one−shot processes are carried out at room temperature, it is necessary that all the ingredients should be liquid at the reaction temperature. It is also highly desirable that the ingredients should have moderate degree of compatibility with each other, thereby facilitating the uniform mixing.12

Structurally, there are two different types of polyurethane foams, viz: flexible and rigid foam. The cell geometry (such as cell size, shape, and type of cell) is also of considerable importance in controlling the foam properties.13 The scanning electron microphotographs (SEM) of the two extreme cases, flexible or interconnected/open cell foam and rigid or closed cell foam are shown in Figure 1.1 (a) and in Figure 1.1 (b), respectively. In general, open cell foams have high gas permeability and high compression modulus, making them very suitable for packaging. In closed cell foams the macroscopic flow of gases is extremely low. The gas permeability in the closed cell foams is governed by the permeation through cell walls, resulting in very low permeability.

(a) (b)

Figure 1.1: SEM Microphotograph of Polyurethane Foams (a) Flexible and (b) Rigid 1.2.1.1 Flexible Polyurethane Foam

The basic technology for the commercial production of flexible polyester foams was first developed in Leverkusen, Germany, from 1952 to 1954. The Copyrightformation of flexible PUFs depends on two basic reactions such as foaming and gelling. The addition of water to toluene diisocyanate (TDI) results in urea, along with

generation of carbon dioxide (CO2). The CO2 expands the reaction mixture to foam and is then released during cell opening. Depending upon the choice of polyol component,IIT flexible PUFs canKharagpur be chemically classified as ester or ether foams. They can be distinguished according to their elastic properties as standard or high resilience

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foams. The semi−rigid or semi flexible foams are classified as special foam types. This distinction comes from the foam production methods such as continuous slabstock process and discontinuous molding process. Flexible PUFs show relatively low load−bearing properties coupled with high recovery, while the rigid foam displays high load−bearing (but with a definite yield point) and subsequent cellular collapse and lack of recovery. It is primarily used for the different applications such as upholstery, mattresses, transportation, packaging, automotive seating and crash pads, carpet underlays and cushions, bedding and furniture. The polyols are dominated by the polyether based polyols. Polyester polyols are used in special applications where their specific properties are required.14 Today, about 90% of all flexible PUFs are produced using polyethers and TDI via one−shot process.

1.2.1.2 Rigid Polyurethane Foam

Rigid polyurethane foams (PUFs) are microcellular three dimensional cross− linked thermosetting plastics and have mainly closed cell structure. Initially, they were used as a sandwich structure core material in aircraft construction and flotation materials for temporary bridges during 1940’s. The earliest PUFs were made using TDI in combination of polyester polyols as raw materials and processing was done by the one shot method. However, it was difficult to control the exotherm of the foaming process. This led to the development of TDI prepolymers where crude TDI was used. This generated a lower exotherm system with easy processing. But, the crude TDI had several impurities and had handling problem because of the higher vapor pressure. Since 1960’s low cost and low viscosity polyether polyols and polymeric MDI (PMDI) as the isocyanate component are being used in the large scale production of PUF. PMDI is easy to handle in the one shot process because of its low vapor pressure. The PMDI and the polyether polyols met the technical and economical requirements for a fast expansion of PUF. Like flexible foams, rigid PUFs are made in Copyrighta wide range of densities from less than 10 to about 1100 kg/m3. Three approaches are used for the preparation of rigid PUF in the form of slabstock, molded and sprayed foam. Rigid foams with density lower than 24 kg/m3 are not dimensionally stable. However, the major proportion of PUF production consists of lightIIT weight foam for thermalKharagpur insulation with density ranging from 28 to about 50 kg/m3. During foaming process because of high exothermic nature of

10 Introduction & Literature Review

different reactions occurring in the system, the non−reactive liquid (physical blowing agent like chlorofluorocarbon, CFC) gets vaporized due to heat of the reaction, leading to foam formation. Water is less commonly used as a chemical blowing agent in PUF systems for insulation applications. The CFC gas has much lower thermal conductivity than air. So, the PUFs having CFC gas in their closed cells have lower thermal conductivity than foams containing air or other gases.15 Sometimes, the use of CFC in combination with water resulted in closed cell PUFs with required densities more than 80 kg/m3. These foams exhibit high mechanical properties and have extremely low thermal conductivity, which cannot be achieved with other plastic foams. However it is well established that CFCs contribute to the destruction of the earth’s stratospheric ozone layer, which shields the planet from ultraviolet (UV) radiation. In addition, CFCs are good absorbers of infrared (IR) radiation, their presence in the upper atmosphere contributes to the green−house effect, causing a rise in the earth’s temperature. In response, the environmental organization of the United Nations (UNEP) ratified the Montreal Protocol signed by 24 nations was issued in 1987. This agreement restricts the production as well as the use of CFC.16 The percentage of closed cells in PUF depends on the degree of cross−linking and the surfactant used during foaming, as well as on the equivalent weight of polyol. For most PUF with high closed cell content (85−95%) is desirable. For example, it is necessary for the foam to have low water absorption, low moisture permeability and solvent retention in solvent−blown foams. However, for certain specialty uses, such as air filters, a low closed cell content is required. The chemical nature of the polymer matrix phase is dominant factor in determining the physico−mechanical properties and the composition of the gaseous phase is important for thermal insulation properties of foams.17 The versatility of raw materials and reactivity have led to a great variety of processes. The basic raw materials for the preparation of rigid PUFs are polyols, isocyanates, catalysis, blowing agents, surfactants, etc. Copyright1.2.1.2.1 Polyols The choice of polyol has a major influence on the physical properties of the foam. The most important characteristics of the polyol are its hydroxyl number, equivalentIIT weight, functionality Kharagpur and rigidity or flexibility of chain units. The source of hydroxyl groups for almost all commercial uses of urethane foams are polyether and polyester based polyols. Both types of polyols are branched and have low molecular

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weight with relatively high viscosities. Sometimes the polyols based on naturally occurring oils bearing hydroxyl group, such as caster oils and its derivatives are also used. Worldwide about 90% of the polyols used for the preparation of PUF are based on polyethers, because of their low cost, easy processability (lower viscosity), very low levels of contaminants, uniform narrow molecular weight distribution and compatibility with most formulation additives. Polyester polyols have higher cost and are difficult to process because of higher viscosity. So they are used in very specific applications such as better thermal stability and flammability properties. The polyester polyols are generally the condensation products of dicarboxylic acids such as adipic acid or phthalic anhydride and saturated polyfunctional alcohols such as 1, 2, 6−hexane triol, trimethylolpropane and diethylene glycol. The polyether polyols are produced by reacting alkylene oxides (ethylene/propylene) with polyfunctional alcohols such as glycerin, trimethylolpropane, pentaerythritol, sorbitol, α− methylglucoside, sucrose and polyamines (ethylenediamine, diethylenetriamine and piperazine) or oxyphosphorus acids.10 The choice of polyfunctional alcohols depend on their average functionality or degree of branching. It has also a profound influence on its final viscosity.

Most rigid foams are prepared from formulated blends or copolymer polyols. The blends consist of one or several polyols, surfactant and flame retardants. Sometimes they also contain catalysts and blowing agents. The formulation may also contain cross−linking agents such as trimethylolpropane, glycerine which improve curing and strength of the foam. Recently, there is an increasing interest to use raw materials based on renewable resources, like vegetable oil, which are biodegradable as well as good alternative to petroleum products. Some of the works have been carried out to identify the polyols from natural sources and recycled polyethylene terephthalate (PET) waste used as alternative source to prepare the rigid PUF. The rigid PUFs made from starch based polyols have properties comparable with the Copyrightproperties of PUF based on conventional polyol.18, 19 The natural oils such as jet cooked starch oil,20 defatted soy flours,21 soybean oil,22 palm oil,23 linseed and rapeseed oil,24−26 sunflower and flaxseed oil,26 and biomass materials27 (soy protein isolate, soy fiber, corn starch) and also rosin28, 29 and modified rosin,30 lactitol,31 reduced sweetIIT whey permeate 32 haveKharagpur been used as natural resources for polyols. These oily materials do not impart sufficient rigidity to make a stable PUF when they

12 Introduction & Literature Review

are used alone. This is due to their high equivalent weights and more flexible aliphatic structures. But when they are used in combination with conventional polyols the properties of PUF formed are found to be better. For example, The PUF from sodium lignosulfonate mixed with diethylene, triethylene and polyethylene glycols were prepared and thermal properties were reported.33 The PUF from recycled PET waste are found to be better in flame retardant properties compared to PUF made from conventional polyols.34

1.2.1.2.2 Isocyanates

The aromatic isocyanates these used for preparation of rigid PUF are generally toluene diisocyanate (TDI) and polymeric methane diphenyl diisocyanate (PMDI). TDI plays a minor role in rigid PUF production, because of unfavorable physico− mechanical properties of TDI based rigid PUF. TDI is also very difficult to handle because of its higher vapor pressure. PMDI is currently the predominant isocyanate component for rigid PUFs. It has significantly lower cost than crude TDI and also has lower vapor pressure. Therefore this minimizes the toxicity problems usually encountered with general isocyanates. PMDI is prepared by the phosgenation of aniline formaldehyde condensed products. The different types of PMDI are available for various applications, for example, when priority is placed on flowability, the expanding reaction mixture has to fill narrow gaps, low viscosity PMDI with low functionality are preferred. The viscosity range of PMDI may vary from 50 to 2000 mPa·s at 25 °C, the functionality varies from 2.5 to 3.2 and the NCO content varies between 25% and 29%. Sometimes the mixture of TDI and PMDI is used as isocyanate component to prepare rigid PUF.

1.2.1.2.3 Blowing agents

Most rigid PUF formulations use inert liquids as blowing agent since they can be easily volatilized during the polymerization. In 1958, halocarbons were first used Copyrightas blowing agent for PUFs, made for the application in thermal and electrical insulator.35 The most widely used blowing agents for the production of PUF and PUF−PIR are monofluorotrichloromethane (CFC−11) and difluorodichloromethane (CFC−12). Both of these chemicals are suspected to contribute to the depletion of the stratosphericIIT ozone layer. DifferentKharagpur hydrochlorofluorocarbons (HCFC) are used as the alternatives for conventional CFC, 36 for example, HCFC−123 and HCFC−141b for

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CFC−11 and HCFC−124 and HCFC−142b for CFC−12. In recent years, water is being used as a co−blowing agent in order to decrease the volume of HCFC−141b. As a result of the Montreal Protocol (1993), the U.S. Environmental Protection Agency scheduled the phase−out of these blowing agents by the year 2003.37, 38 After 2003, manufacturers must use other blowing agents with zero Ozone Depletion Potential (ODP) for PUF insulation applications. However blowing agents like hydrofluorocarbon (HFC) blowing agents such as HFC−R356, HFC−134a, HFC− 245fa 39−43 and HFC−365mcf 44, 45 can all be considered as possible alternatives. All these chemicals are inert and do not produce additional cross−linking in the foam and also non−flammable and have a very low order of toxicity and in addition, their thermal conductivity is higher than CFC blowing agent. Furthermore, processing of a gas blowing agent is more desirable in the rigid foam industry. 46

Methylene chloride is of interest as blowing agent, because of its low cost.47 It softens urethane polymer and causes cracks in rigid foams expanded with it. Hence, it is not used in any practical process. Hydrocarbon blowing agents such as propane, butane, and pentane (n−, cyclo− and iso−) have been used to prepare rigid PUF and PUF−PIR foams. 48−54 These blowing agents, some of which have to be added under pressure in liquid form, vaporize either when pressure is released from the reaction mix, or as a result of the exothermic heat from the urethane reaction and are retained in the closed cells of the foam. However, these hydrocarbon blowing agents have not gained wide acceptance because of their flammability and their value as fuels. Some of the other physical blowing agents such as fluorinated ethers, 55 trans 1, 2− dichloroethylene, 56 and formic acid 57 are used for the production of PUF. In addition to physical blowing agents, the chemical blowing agents are also used in PUF. 58−61 In case of chemical blowing agent, it reacts with some of the raw materials for rigid PUF and evolves gases which act as actual blowing agent. For example, water reacts with Copyrightisocyanates to form unstable carbamic acid which later decomposes into amine and CO2. This CO2 gas acts as the actual blowing agent in the preparation of rigid PUF. With the efforts that now are taking place to reduce the usage of CFCs, the water/isocyanate reaction is gaining an increased importance. Industrially, the rigid foams are producedIIT by using blendedKharagpur blowing agents such as combination of hydrofluorocarbons with hydrocarbons, 62, 63 different hydrocarbons 64 and or physical

14 Introduction & Literature Review

blowing agents mixed with chemical blowing agents 65 (such as water) to get the optimum properties of PUF.

1.2.1.2.4 Catalysts

The rigid foam formation reaction basically consists of three different types of chemical reactions viz; blowing reaction (which is the reaction of isocyanate with water), gelling or network formation reaction (resulting from the reaction between isocyanate with hydroxyl) and trimerization reaction (the isocyanurate formation).66, 67 Many other complex reactions also occur simultaneously during the production of PUFs. The catalysts play an important role in the control and balance between the gelling and blowing reactions. In addition to primary reactions, it is considered that the catalytic activity of cross−linking reactions such as allophanate, biuret, and isocyanurate formation have a certain effect on the foaming behavior as well as foam properties.68 The catalysts most commonly used are tertiary amines and organometallic compounds. Tertiary amines such as 1,4−diazabicyclo 2.2.2 octane (DABCO) or triethylene diamine,69 tertiary alkylamines, substituted morpholines, piperazines, guanidines, and substituted hydroxy amines are catalysts for both the blowing and network formation, whereas organometallic compounds (dibutyltin dilaurate and stannous octoate) catalyze mainly isocyanate−hydroxyl reaction and sometimes metal salts are also used.70 In rigid polyisocyanurate (PIR) foam production, alkali metal catalysts of organic carboxylic acids, potassium salts and hydroxyalkyl trimethyl quarternary ammonium salts are used as polyisocyanurate foam catalysts.71

The choice of catalyst for the preparation of PUF depends on many factors, such as method of foam preparation, the types of polyol or isocyanates used, and the activity of the catalyst itself in promoting urethane or gas formation. Thus, the selectivity towards one or the other reaction strongly depends on various formulations Copyrightrelated factors such as component miscibility, reaction advancement or relative concentration of the various active species. For example, the catalyst is mandatory where secondary hydroxyl groups containing polyoxypropylene triol and also sterically hindered aromatic isocyanates are used in the preparation of PUF. However, the lowIIT activity of the aliphaticKharagpur isocyanates has made it impractical to use them in most cases.72 The reactivity of the hydroxyl group was found to be independent of the

15 Chapter 1

chain length of the polyol, and of the difference in the structure of the oligomethylene and oligooxyethylene chains.73 Sometimes combination of different types of catalysts may be used to balance the reactions. However, the catalyst is not necessary in the preparation of PUF made from mannich based polyether polyol. This is due to the inherent properties of the mannich polyols include aromatic character, increased compatibility and catalytic capabilities.74

1.2.1.2.5 Surfactants

Surfactants (surface active agent) are substances that adsorb on the surfaces or in the interfaces of systems and make a remarkable change in the surface or interfacial free−energies of the adhered surfaces. For the production of PUFs the use of surfactants is essential. Their action during the foam formation process is manifold: (i) to stabilize the dispersion of any immiscible reaction components, (ii) to promote nucleation of bubbles during mixing, (iii) to stabilize gas bubbles in the rising foam, and (iv) to prevent the foam from collapsing by reducing stress concentrations until the ongoing polymerization reactions develops sufficient mechanical strength in foam. The surfactants determine the technical performance of the final foam to a great extent.75

The choice of surfactant is governed primarily by the type of polyol used and by the method of foam preparation. It determines the flowability of the reaction mixture, the cell structure, and the closed cell content. The commonly known surfactants used earlier for foam preparation were organic non−ionic such as nonyl phenol ethoxylates, fatty acid ethoxylates and alkylene oxide block copolymers. Later silicone oils and poly (dialkyl siloxanes) are widely used.76 Nowadays, graft copolymers which consist of polysiloxane backbone with polyether side chains are also used as surfactants.77−79 Semi−fluorinated diblock copolymers have also been used as a surfactant in preparation of PUF.80 Many water−soluble polymers, because Copyrightof their ampiphilic structure and surface activity similar to the traditional surfactants, are used as surfactants. For example the polysorbates exhibit excellent emulsifying properties for oil in water systems, and the polyoxyethylene−polyoxypropylene block copolymers, known by their generic name as poloxomers, exhibit surface properties, such as emulsionIIT stabilization, dispersion,Kharagpur and demulsification.81 Polyester based systems are often used without added foaming stabilizers, because of their high initial

16 Introduction & Literature Review

viscosities. In these systems the silicones can have an effect opposite to stabilizing the foaming mass and in fact can cause bubble instability. Stabilizers used for are usually of the ionic or nonionic type and include sulfonated castor oils, amine esters of fatty acids, and polyoxyalkylene derivatives of acids or alcohols.10 Additionally, some of the other raw materials such as fillers, flame retardants, and extenders are also used in order to comply with industrial requirements. Some of the research works have also been carried out to identify the change in properties of the PUF with respect to foam density, 82 fillers 83−90 and nano−fillers, such as nanoclay 91− 101 and nanosilica.102

1.3 Flame Retardants (FRs)

Flame retardants (FRs) are the chemicals that are incorporated in organic polymers (both natural and synthetic) by blending in compounding stage or in−situ preparation stage, in order to increase their fire safety. The idea of using FR additives is known from the ancient time. Historically, vinegar and alum (potassium or ammonium aluminum sulfate) were used as FR additives in paints or coatings for wood to impart flame resistance by Egyptians as early as 450 BC and by Romans in 200 years BC. Over the years, different types of clays, gypsum, borax, and asbestos have been used to make canvas and clothing impervious to flame. Later 17 to 19th century, inorganic salts such as ferrous sulphate and ammonium phosphate were used as FRs.103 It was not until the early part of the 20th century that William Henry Perkins developed the ground work for the FR theory. He was the first person to study the mechanism of flame retardancy on wood, cotton, paper and plastics. Inorganic salts, acids and clays were the main ingredients of his FR mixtures. World War II was another dominant milestone in the creation of FRs. In 1950’s, the polymer industry was started to grow into a big market. During this period the majorities of plastics (e.g. polystyrene, polyurethane, and polyethylene) were developed on a widespread Copyrightand economically favorable scale and were preferred over other materials such as wood and metal alloys. Since mid−1960s there have been several regulations to mandate that plastics should be made less flammable. There have been established guidelines for fire safe materials in fabrics and transportation materials, particularly in airplanes.IIT Kharagpur

17 Chapter 1

Flame retardants can be divided into two types, viz; additive and reactive.104 The additives are various organic and/or inorganic compounds that are added to commercial polymers during compounding stage and in conjunction with a synergist to make a fire resistance polymers. They are used especially in thermoplastics and elastomers. Additives work well for most FR applications but they also have a few drawbacks. FR additives leach out of the polymer over time, making the polymer less flame resistant and polluting the environment. Another drawback to the approach of using FRs additives is their tendency to degrade the mechanical properties of pristine polymer. There are many examples of reactive FRs in which FR moieties are incorporated either into the backbone of the polymer or remain as a pendant group. This prevents them from leaving the polymer and keeps the FR properties intact over time with very low emissions to the environment. Reactive FRs are mainly used in thermosets (epoxy resins and polyurethanes) in which they can be easily incorporated.105 Around 350 different FR substances (both organic and inorganic) are described in literature and are available in the market, with no specific reference to national or international fire regulations. But the FRs most abundantly used at the present time is based largely on six elements: chlorine, bromine, phosphorus, aluminum, boron, antimony. In addition, nitrogen and silicon can also confer some degree of flame retardance. Other elements and their compounds have proved to be less effective. The total annual quantity of FRs produced in 2007 was approximately 1.8 million metric tons.

40%

11%

11% 7% 8% Copyright23% Brominated Antimony oxides Chlorinated Organo phosphorus Aluminium hydroxides Others

Figure1.2:IIT Total Annual Production Kharagpur of the Major Fire Retardant Chemicals

18 Introduction & Literature Review

The hydrated alumina/aluminum hydroxide or alumina trihydrate (ATH) production has been reasonably higher, due to the consumption as one of the superior FR additives in USA and EU region based on volume (Figure 1.2). One of the main advantages in this additive is that it does not produce any toxic gases, but produces a good amount of char. The production of brominated FRs differ by region (Figure 1.3), is mostly produced in Asia.106Flame−retardants can act by a variety of mechanisms, mainly in the condensed or the gas phase.107, 108 They can terminate the free−radical reactions in the condensed phase, act as heat sinks due to their heat capacity, form a non−flammable protective coating or char to insulate the flammable polymer from the source of the heat and oxidant, and interrupt the flame combustion in the gas phase.

Figure 1.3: Consumption of the Major Fire Retardants in Different Regions (bar-chart in unit of 1000 tons)

Many FRs appear to be capable of functioning simultaneously by several different mechanisms, often depending on the nature of polymers. Combinations of FRs often have synergistic, additives and adjuvants or antagonistic effects. Sometimes a hetero atom already present in the polymer backbone may interact with an FR additive and thus, exhibits synergism or antagonism characteristics.

1.3.1 Selection and Requirements for Flame Retardants CopyrightThe primary concern when selecting an FR for a given application is that it should be effective to the extent required for the application. The end−use application often determines the selection of FR. Assessment is made by exposing the samples of the final formulation to a series of tests. The thermal decomposition of effective FR compoundsIIT should start beforeKharagpur or during the thermal decomposition of the polymer. The FR material should not generate smoke and any toxic gases beyond those

19 Chapter 1

produced by the degrading polymer itself, should not significantly alter the mechanical properties of the polymer, and should be compatible and easy to incorporate into the host polymer. The FR materials should not react with any other additives present in the formulation and should be resistant towards aging, hydrolysis and light. Finally, FR should be less expensive and should be commercially easily available.

1.3.2 Thermal Decomposition Mechanism

The thermal decomposition of polymers involves either chemical or physical processes.109 The chemical processes are responsible for the generation of flammable volatiles and physical changes, such as melting and charring, can alter the decomposition and burning characteristics of a material. In most cases, a solid polymer, when heated to a certain temperature, will decompose to give varying amounts of volatile products and solid residues. These residues can be either carbonaceous char or inorganic (originating from heteroatoms contained in the original polymer, either within the structure or as a result of additive incorporation), or a combination of both. Many fire tests have shown that char formation is an important route to achieve flame retardancy, but little is understood about the detailed structure of char or how it forms. van Krevelen has proposed a two−step model for charring.110 Below 550 °C, the polymers decompose to fuel gases, tars, and a primary char. On further heating above 550 °C, the primary char is slowly converted to a conglomerate of loosely linked small graphitic regions, which is virtually independent of the structure of original polymers. Levchik and Wilkie also proposed that the char formation of polymers includes the following steps: cross−linking, aromatization, fusion of aromatics, turbostratic char formation (an incomplete process of graphitization), and graphitization.111 The formation of char can be promoted through many chemical reactions, including graft copolymerization, Lewis acid catalysis, Friedel−Crafts chemistry, redox reactions, and the use of various additives. As to the Copyrightstructure of char, it is believed that char is composed of polynuclear aromatic compounds with heteroatoms (O, N, P, S), consisting of crystalline and/or amorphous regions. The thermalIIT decomposition Kharagpur processes for different polymers are complex and can vary from system to system. The rate, mechanism and product composition of

20 Introduction & Literature Review

these thermal decomposition processes depend on both the physical properties and chemical composition of the original material. In the thermal decomposition of organic polymers, four general mechanisms can be identified (1) random−chain scission; for example, in polyethylene (2) end−chain scission or unzipping; for example, in polymethylmethacrylate (3) chain stripping; for example, in polyvinyl chloride and (4) cross−linking (high−charring polymers). Some polymers undergo a reaction that exclusively belongs to one category. Others exhibit mixed behavior, depending on the structures of polymers.112 Many of the addition polymers, such as vinyl polymers, seem to decompose through a reverse polymerization (initiation, propagation, chain transfer, and termination) or random chain scission. Polymers prepared by a condensation process, such as polyesters, polyurethanes and , decompose according to random chain scission followed by cross− linking into carbonaceous chars. However, the detailed decomposition mechanisms of different polymers are greatly dependent on their chemical structure and composition.

Polymer - Q Pyrolysis (Endothermic)

Non-combustible Combustible Liquid Solid charred gases gases products residue Air Thermal Gas mixture Ignites Air feedback Flame + Q Embers (Exothermic)

Combustion products

Figure 1.4: Schematic Diagram of Polymer Combustion Process

1.3.3 Polymer Combustion Process CopyrightWhen polymers are subjected to some of the ignition sources (matches, cigarettes, torches, or electric arcs) will undergo self−sustained combustion and are most frequently responsible for the propagation of a fire in air or oxygen. A burning polymer constitutes a highly complex combustion system as shown in Figure 1.4. ThereIIT are three steps involvedKharagpur in the combustion of polymers such as heating, decomposition and ignition. 113 The polymer is first heated to a temperature at which it

21 Chapter 1

starts to decompose and gives gaseous, liquid and solid products. The gases and liquid products are usually combustible, which will undergo ignites with presence of oxygen/air to form flame and finally combustion products are formed. The solid charred residue may react with air to form embers. Under steady−state burning conditions, some of the heat is transferred back to the polymer surface, producing more volatile polymer fragments to sustain the combustion cycle.

There are two types of combustion involved when polymers are burned: flaming and non−flaming combustion.114 Flames are self−propagating combustion reactions in which both the fuel and the oxidant are present in the gas phase. Since most polymers are hydrocarbon based, the flame above burning polymers is usually a hydrocarbon flame. The principal reactions in the flames are free−radical reactions. The most important radicals in hydrocarbon flames are simple species such as H•, •O•, • • • • and OH, and a small amount of HO2 , HCO , and CH3 . The chain−branching • • • • reactions in the combustion process are H + O2 → HO + O ; it can accelerate the burning of polymers by generating more radicals. Non−flaming combustion, including smoldering and glowing combustion, propagates through the polymer by a thermal front or wave, involving the surface oxidation of the pyrolysis products.112 From a practical point of view, it is also important to consider the associated fire hazards. The effects resulting from polymer combustion, which can threaten human life, include oxygen depletion, flame, heat, smoke, hot and toxic combustion gases, and structural failure. The two major causes of fire−related deaths are inhalation of toxic gases or smokes.115 Smoke formation in flames is highly dependent on the structure of the gaseous fuel and on the fuel−to−oxidant ratio. Normally, polymers containing purely aliphatic structural units produce relatively little smoke, while polymers with aromatic groups in the main chain produce intermediate amounts of smoke. Copyright1.3.4 Inhibition of Polymer Combustion In order to stop polymeric materials from burning, the combustion cycle should be stopped. In the condensed phase, two methods were utilized to stop the combustion: IITformation of char, whichKharagpur adds a protective layer between the flame front and the polymer fuel, and dilution of solid fuel with inorganic fillers that decompose to dilute the flame. In the vapor phase, the combustion cycle can be stopped by

22 Introduction & Literature Review

physically diluting the flame with non−combustible gases and chemically capping the high energy free radicals with halogens or other retardant species. Attacking the flame in both the condensed and vapor phase has historically proven to be the best strategy in stopping a fire.116 From the industrial and manufacturing point of view, the introduction of FR additives undoubtedly constitutes the easiest way of making a polymer less flammable. There is no universal FR that is applicable in all formulations. The FR materials which are effective in a solid molded item may be completely ineffective in foam. There are several criteria which are used to assess the fire retardant characteristic of a material. These include polymer ignitability, rate of flame spread, rate of heat release, formation of smoke and toxic gases and also corrosivity of acidic gases.

1.3.4.1 Halogenated Flame Retardant Additives

Halogen containing FR materials are very important class of FR chemicals. These chemicals are used primarily in polymers for the electronic and building industries and are known particularly for their performance in styrenic copolymers, engineering thermoplastics, and epoxy resins. The order of the thermal stability for the different halogen compounds is F > Cl > Br > I. Fluorine compounds have not been as effective because C−F bond energy is so high that energy needed to break the C−F bond is not produced at the temperature at which these halogenated FRs work. Conversely, the C−I bond is so weak that it is easily cleaved by light, and can leach out of the polymers, therefore making it not suitable for this application. Bromine or chlorine compounds are the most widely used halogen containing FR. As reactive FRs, halogen−containing alkenes, cycloalkanes, and styrene can be copolymerized directly with the corresponding non−halogenated .117 In many cases the organic−halogenated compounds are used in conjunction with phosphorus compounds or with metal oxides, especially antimony oxide.104 The flame retardant function of Copyrighthalogen−containing FRs can occur mainly in the vapor phase.118 The action of the FR depends on the structure of the additive and of the polymer. Generally, the radicals produced by thermal decomposition of a halogenated FR can interact with the polymer to form hydrogen halide. Hydrogen halides inhibit the radical propagation reactionsIIT that take place in theKharagpur flame by reacting with the most active radicals, H• and •OH. It also should be noted that aromatic−brominated compounds and antimony

23 Chapter 1

chlorine mixture can produce char. Although halogen compounds are widely used as FRs, their effectiveness is sometimes considerably increased by a free−radical initiator or peroxide and antimony trioxide. The group of halogenated FRs represents around 30% by volume of the global production, where the brominated FRs dominates the international market. Although the use of brominated FRs is still growing by around 5% per year, their less use is strongly questioned due to their potentially harmful environmental and health characteristics. Halogenated compounds can have negative environmental and toxicological impacts which have deterred many countries from using them in commercial products. The European Union is trying to remove halogenated compounds from all plastics due to environmental concerns. Leaching of additives can contaminate drinking water. Additionally, halogenated organic compounds are considered persistent organic pollutants (POPs) which are not easily broken down or oxidized by the environment. Heavy metal cause severe environmental problem. For example, antimony oxide may have a possible link to sudden infant death syndrome (SIDS).119 In recent years there has been special interest in halogen−free FRs, because of they are potential for providing reduced toxic emissions and less smoke generation.120

1.3.4.2 Flame Retardant Additives based on Phosphorus

Both organic and inorganic phosphorus compounds are useful for imparting flame retardance to many polymers. Phosphorus FRs include elemental red phosphorus, water−soluble inorganic phosphates, insoluble ammonium polyphosphate, and phosphonates, phosphine oxides, and chloroaliphatic and bromoaromatic phosphates.121 Both additive and reactive FRs are commercially available. Phosphorus based FRs are used for thermoplastics, thermosets, textiles, paper, coatings and mastics. These highly diverse chemicals with different chemical properties function differently in their FR mechanism. These include the formation of a charred layer on the surface to protect the substrate from Copyrightoxygen and flame and also free−radical inhibition. The FR mechanism depends on the type of phosphorus compound and the chemical structure of the polymer. Phosphorus FRs containing halogen or nitrogen is often stated to exhibit synergistic behavior due to the formationIIT of phosphorus halidesKharagpur or oxyhalides, as postulated to support a synergistic mechanism by analogy with antimony/ halogen or P−N bonds on decomposition.122, 123

24 Introduction & Literature Review

1.3.4.3 Inorganic Hydroxide Flame Retardant Additive

Inorganic hydroxides (aluminum or magnesium) are a very important class of FRs, accounting for more than 50% of the volume (by mass) of FRs sold on a global basis. This is due to their relatively low cost, ease of handling, and low toxicity as compared to other FR system.105 These classes of materials provide FR formulations that meet appropriate standard tests for many applications. Alumina trihydrate (ATH) is the most widely used FR and smoke suppressant since the 1960’s and also the largest selling inorganic hydroxide to this market because of its low cost and easy availability.124 It is used in elastomers, thermoset, and thermoplastics that are processed below 200 °C. It is normally introduced into polymers in large quantities (>50% by weight) in order to attain a significant FR effect. When heated, it decomposes to form anhydrous alumina and releases water, which is an endothermic reaction. This energy consumption can remove the heat from the substrate, slow the decomposition of the substrate, and keep it below its ignition temperature. Also, water released into the vapor phase dilutes the concentration of the combustible gases. The oxide residue generated during decomposition has a relatively high heat capacity and can reduce the heat transfer to the substrates.125 Another advantage of using inorganic hydroxides is that they can reduce the amount of smoke generated during combustion. The particle size and surface treatment are some of the important parameters of ATH that determine the FR effectiveness of the material selected for a given application. Surface treatments include a variety of materials including carboxylic acids, silanes, zirconates, and titanates. There are two issues that limit the increased use of inorganic hydroxides as a FR additive for thermoplastics. One is the relatively low temperature of decomposition for materials like ATH. The other is the relatively high level of addition required to achieve adequate levels of flame retardation, which led to inferior mechanical properties of pristine polymers. Many approaches are being evaluated as a means to overcome these deficiencies. In addition to the FR compounds mentioned Copyrightabove, there are several other inorganic compounds such as zinc hydroxystannate (ZHS), zinc stannate (ZS), etc. have been tried over the years, but have limited use in specialty applications. 1.3.4.4IIT Flame Retardant AdditivesKharagpur based on Nitrogen The presence of nitrogen in natural polymers appears to exert some degree of flame retardance, as shown by the relatively low flammability of wool, silk, and

25 Chapter 1

leather.126 Synthetic polymers that contain nitrogen are not so resistant to combustion. A number of nitrogen−containing organic compounds are used as reactive FRs for certain polymers. These include triazines, isocyanates, urea, guanidine, and cyanuric acid derivatives. Some of these compounds are also employed as additive FRs, often in conjunction with phosphorus compounds, to reduce the flammability of polymeric materials. In the latter cases, the nitrogen appears to act to a considerable extent by strengthening the attachment of phosphorus to the polymer, but nothing is yet certain about the mechanisms of action. One possible explanation is that the release of non− combustible gases such as ammonia dilutes the volatile polymer decomposition products and, hence, makes them less flammable. Ammonium salts and metal−amine complexes have also been quite widely used as FRs for certain applications, such as ammonium phosphates for wood.

1.3.4.5 Intumescent Char Flame Retardant Additives

There is a constant requirement for new FR systems for polymeric materials. The reexamination of fire−retarded systems reveals that the conventional FR additives produce corrosive and/or toxic gases which are detrimental to the environment and ecology. This has led to new type of intumescent FR systems. These systems on heating generate a swollen multi−cellular char, which acts as physical barrier against heat transfer and diffusion of oxygen toward the polymer. Thus, the rate of pyrolysis of the polymer is expected to decrease below flame feeding requirement, which leads to flame extinguishment. Intumescence occurs with a limited evolution of volatile products, this condensed phase mechanism of the fire retardance should minimize the undesired secondary effects (smoke and toxicity) of halogen based FR systems. Furthermore, the intumescent char adheres to the molten zone of the burning polymer and prevents it from dripping, thus eliminating a possible source of propagation of fire.127 CopyrightFire protective coatings with intumescent properties are being used since last 50 years, whereas incorporation of intumescent additives in solid polymeric materials is a relatively recent approach.128, 129 The first literature report on an FR intumescent coating is revealed in a patent in 1938 by Tramm et al.130 Wang et al.131 discussed the composition IITof intumescent systemsKharagpur in terms of functions performed, defined ‘carbonifics’ the compounds acting as a source of carbon to build up the char, and

26 Introduction & Literature Review

‘spumifics’ those which induce the foaming agent. Vandersall has listed for categories of components that are necessary to provide intumescence: (a) an inorganic acid or a compound which generates an acid on heating between 100 °C and 250 °C; (b) a polyhydric compound, rich in carbon atoms; (c) an amine or amide; and (d) a halogenated organic compound.132 Because there are very few published literature on this systems. It is difficult to know exactly the function of each component of the mixture during burning. Different other intumescent systems have also been applied in polymers such as expandable graphite (EG), inorganic intumescent silicates or glass forming systems.133 Whatever the detailed mechanisms for intumescents systems are, the formation of a thick char layer, high carbon concentration, high viscosity of pyrolyzing melt and low penetration capability for propagation of heat, makes intumescent systems efficient to reduce flammability and the exposure of toxic gases.

1.3.4.6 Flame Retardant Additives based on Nanoclay

Polymer layered silicate nanocomposites (PLNC) are a new class of composite materials in which one of their constituents has a dimension in the nanometer (10-9 m)

range. Use of nanofiller such as clay, silica, CaCO3, ZnO, and metallic oxides greatly enhance the contribution of the contact area/interphase with the matrix materials and thus improve the overall properties of the nanocomposites.134−138 It was first reported in 1961, when Blumstein demonstrated polymerization of vinyl monomers intercalated into montmorillonite (MMT) clay.139 In general, these methods achieve molecular−level incorporation of the layered silicates (e.g. montmorillonite) into the polymer by the addition of a modified silicates; either during the polymerization (in− situ method), or to a solvent−swollen polymer (solution blending) or to a polymer melt (melt blending). Two terms (intercalated and exfoliated or delaminated) are used to describe the two general classes of nano−morphologies that can be prepared. CopyrightIntercalated structures are well−ordered multilayered structures and contain self− assembled, where only a one or two layers of extended polymer chains are inserted into the gallery space (2−3 nm) between parallel individual silicate layers. The delaminated/exfoliated structures result when the individual silicate layers are no longerIIT close enough to interactKharagpur with gallery cations of the adjacent layers and are well dispersed in the organic polymer. In this case the interlayer spacing (2−200 nm) can

27 Chapter 1

be of the order of the radius of gyration of the polymer. Therefore, the silicate layers may be considered well dispersed in the polymer. The delaminated structure may not be as well ordered as in an intercalated structure and are also referred to as exfoliated or disordered nanocomposites. Both of these hybrid structures can coexist in the polymer matrix, this mixed nano−morphology is very common for composites based on smectite silicates and clay minerals.140−146 Recent research describes extended nanoparticles of clay as promising char−forming fillers for good fire protection.147, 148 These applications are, however, still on a research level and detailed studies are in progress, to make them commercial potential.

1.4 Flame Retardant Rigid Polyurethane Foam

Rigid Polyurethane foams (PUFs) have cellular structures, which are easily ignitable and highly flammable. In the event of fire the flame spread is very fast and wide spread. This is a serious problem and it severely limits PUF applications. Recently, fire safety concerns put even more stringent requirements for the materials to be used in enclosed and inescapable areas, such as electronic enclosures, high−rise buildings, ships, and aircraft cabins, etc. A variety of FR chemicals have been developed over the past 40 years. They mainly consist of inorganic and organic compounds based on halogens, phosphorus, aluminum, magnesium, etc. Rigid PUF undergoes thermal degradation, when exposed to heat and flame. The thermal energy is absorbed until there is breakage of main chain in PUF, leading to formation of low molecular weight volatile gases. Although the mechanism is somewhat different in the presence of oxygen, giving rise to some new products, a gaseous mixture is still formed which is inherently flammable. The precise nature of the degradation products is determined by the chemical composition of the polymer, the additives present and the degradation conditions. The burning process of PUF is considered to occur mainly in four stages: Copyrighti) Heat from external source is applied to foam progressively raising its temperature. The thermal insulation properties of foam promote a rapid temperature rise at the exposed surface because heat cannot be easily transmitted to the lower layer of foam. ii) The polyurethaneIIT foam reaches itsKharagpur temperature of initial decomposition and begins to form combustible gases, non−combustible gases, entrained solid particles and carbonaceous char.

28 Introduction & Literature Review

iii) The resulting combustible gases ignite in the presence of sufficient oxygen and further combustion begins. The condition of ignition depends on the presence of an external source of ignition, temperature and composition of the gas phase.

iv) The heat of combustion raises the temperature of the gaseous products of combustion and of the noncombustible gases, which results in increase in heat transfer by conduction.149

Flame resistant PUF are produced by using the different FR additives, like FR additives based on phosphorus, 150−164 nitrogen, 165, 166 inorganic hydroxides, expandable graphite167−172 and nanoclays.173−175 These FR compounds are used alone or in combination with other compounds to impart the synergistic behavior. It has also been produced by incorporation of flame retarding atoms in the base polyol, which react with foam components to become an integral part of the polymer backbone. Few such reactive FR compounds are; phosphorus containing diol/polyol,176−180 phosphorus and halogen containing diols and additives,181−184 and phosphorus and nitrogen containing polyol/diol and additives.185−190 They are used along with base polyol to prepare the flame retardant PUF. Many other factors of great importance are involved in the flammability of PUFs, such as smoke evolution, which often causes greater danger to people than the fire itself. More people die from smoke inhalation (primarily carbon monoxide) than by fire. In assessing the relative performance of different FR additives in PUF a number of small scale tests have been developed such as rate of burning, ignition time, limiting oxygen index (LOI), etc. These are carried out under well−defined conditions and under certain existing standards like ASTM, FVMSS, etc. It should be noted that the performance of an FR−PUF in small scale tests does not necessarily reflect the hazards present under actual fire conditions.

1.5 Applications of Rigid Polyurethane Foam

One of the major uses of rigid PUF is in domestic and commercial Copyrightrefrigerators and freezers, because of their superior insulation characteristics of the fluorocarbon blown foam. All types of refrigerated trucks such as milk trucks, ice cream trucks, trailers and industrial plants such as tanks, piping, water heaters, heat exchangers,IIT flasks, air conditioningKharagpur units, cool boxes, liquefied natural gas (LNG), etc. are insulated with rigid PUF. Besides having good insulation properties, rigid PUF contribute to the structural strength and adhesion to the bodies of the trucks,

29 Chapter 1

have low moisture pickup, and can withstand gasoline and temperature up to 100 °C. Potentially, the biggest market for rigid PUF is in the building and aircraft industry. The areas of utilization in this filed encompass curtain wall construction, lining boards, pipe section, cold store panels, pipe−in−pipe, food processing enclosures, spray applied wall construction, and roofing insulation, either sprayed or in preformed panels. This market includes residential homes and commercial and industrial buildings such as large refrigerated warehouses. The use of rigid PUF in marine applications such as flotation equipment for temporary bridge is growing. Many modern boats utilize rigid PUF to support the boat to float in the water. Larger ships have used rigid PUF as void fillers, and it is also used in lifeboats and refrigerator ships. The major applications for rigid foams are building insulation, appliance insulation and packaging.7−10 A wide ranges of foam properties and different foaming techniques allowed the application of rigid PUF in many important areas.

1.6 Scope and Objective of the Present Work

Rigid polyurethane foams (PUF) are easily flammable due to more surface area and rapid flame spread. This leads to an increase in fire hazard (smoke and toxic gas generation). It has been widely used in variety of commercial applications such as insulation, cushioning and packaging. Thus, it is necessary to induce fire retardancy in PUF by adding suitable FR chemicals. The recent restrictive legislation, codes and specifications for the use of FR−PUF in different applications such as construction, automotive, rail−road transportation, aircraft etc. have brought forth an entirely new approach to develop new types of FR−PUF which yields low smoke/toxic gas during burning in addition to flame retardancy. The literature search reveals that the conventional FR−PUF systems containing halogens and/or phosphorus are unsatisfactory, when both flame resistance and smoke development are taken into account. This has led to the development of new classes of FRs, which are quite Copyrighteffective in flame retardancy as well as hardly pollute the environment. In recent years the attention has been paid World−wide towards the development of new halogen−free FR systems such as based on phosphorus and nitrogenous compounds, intumescent technology, as well as nanotechnology. The intumescent technology has been well documentedIIT in the literature Kharagpur based on coating materials. But, the use of this technology and also nanocomposites in rigid polyurethane foams are scanty.

30 Introduction & Literature Review

The objective of this thesis is to prepare the rigid polyurethane foam (PUF) which can have good fire retardant properties and release less amount of smoke and toxic gases during combustion. These new rigid PUFs were characterized and their flammability properties were evaluated with the help of modern sophisticated tools. The objectives of this thesis are the followings,

• Preparation of rigid polyurethane foam (PUF) with wide range of densities (30−500 kg/m3) by one shot free−rise as well as molding method.

• To study the effect of density and different raw materials on the properties of rigid PUF.

• To study the influence of various fillers and their concentration on the properties of water blown rigid PUF.

• To study the flame−retardant properties and thermal properties of these PUFs.

• To study the effect of various non−halogenated FR additives on the fire retardant (FR) property of the water blown rigid PUF.

• To explore the use of different intumescent FR additives such as expandable graphite in the rigid PUF and to study its FR property.

• To study the effect of nanoclay as an additive in water blown rigid PUF.

Different experimental techniques, i.e., universal testing machines (UTM), thermal conductivity meter, FT−IR, SEM, WAXD, TEM, DSC, TGA, limiting oxygen index (LOI), cone calorimeter, smoke density and toxicity analysis were used to measure the physico−mechanical, thermal, flammability, smoke and toxic gas (carbon monoxide) evolution of water blown rigid PUF. Through this investigation the relationships between polyurethane structures, composition, morphology and their FR behavior have been established. The objective of identifying the best combination Copyrightnon−halogenated FR additives and their optimum change to produce PUF having adequate flame retardancy with low smoke and toxic gas generation could also be attained.IIT Kharagpur

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