Copyright IIT Kharagpur Introduction & Literature Review 1 1. Introduction 1.1 Polymeric Foams 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 foam is dispersion of a gas in a polymer 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 thermoplastics 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/m 3 . Approximately, 70−80% of all commercially produced polymeric foams are based on polyurethane, polystyrene and polyvinyl chloride. 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 slabstock produced by pouring, foaming−in−place, molding, extrusion, spraying, rotational casting, frothing, precipitation, composites and lamination. 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 solid plastics, metals, fabrics, paper, wood, etc.
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1. Introduction 1.1 Polymeric Foams
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 foam is dispersion of a gas in a polymer
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 thermoplastics 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 polyurethane, polystyrene and polyvinyl chloride.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
slabstock produced by pouring, foaming−in−place, molding, extrusion, spraying,
rotational casting, frothing, precipitation, composites and lamination. 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 solid plastics, metals, fabrics, 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 plastic 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 foam rubber,
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 polyester 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. Polyethylene foams for use as a low−loss insulation for wire and cables
was introduced in 1944. Polypropylene foams, both thermoplastics and cross−linked
types were introduced due to their relatively high service temperature and good
abrasion 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−temperature resistant foams have been developed recently. These include
foamed fluorocarbons, cellular aromatic polyimides, and syntactic polybenzimidazole
foams. In addition, many other types of flexible and rigid foams have been developed
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based on both natural as well as synthetic polymeric materials. These include foams
based on butadiene−styrene, butadiene−acrylonitrile, neoprene, 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.
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.
Effects of viscosity and surface tension subsequently cause materials to flow
towards the uniform cell formation.
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,
chemical, or physical methods.4 Some of the most commonly used methods are;
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
either inorganic materials such as carbonates, bicarbonates, borohydrides, etc.,
or organic materials such as, hydrazides, azides, and nitroso compounds, etc.
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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.
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.
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).
Expansion of dissolved gas in a polymer mass upon reduction of pressure in
the system.
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 Polyurethanes (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 nylon. Besides Bayer, Hoechtlen, Hoppe and Weinbrenner had
combined the description of the PUs on the scientific basis and their analysis with
potential application areas and corresponding market volumes for this new material
developed in Leverkusen. Rinke and collaborators were successful to prepare the
polymers from a low viscosity melt which resulted in what now called as
polyurethane. Rinke and associates 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
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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 plastics industry. 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 Slabstock flexible
Figure1.2: Total Annual Production of the Major Fire Retardant Chemicals
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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
The 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
compounds should start before or during the thermal decomposition of the polymer.
The FR material should not generate smoke and any toxic gases beyond those
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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
structure 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 thermal decomposition processes for different polymers are complex and
can vary from system to system. The rate, mechanism and product composition of
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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
polyamides, 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.
Figure 1.4: Schematic Diagram of Polymer Combustion Process
1.3.3 Polymer Combustion Process
When 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.
There are three steps involved in the combustion of polymers such as heating,
decomposition and ignition. 113 The polymer is first heated to a temperature at which it
Polymer
Pyrolysis
Non-combustible gases
Combustible gases
Liquid products
Solid charred residue
AirIgnites Gas mixture
+ Q (Exothermic)
- Q (Endothermic)
Combustion products
Air
Embers
Thermal feedback
Flame
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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.
1.3.4 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: formation of char, which 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
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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 monomers.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
halogen−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
reactions that take place in the flame by reacting with the most active radicals, H• and •OH. It also should be noted that aromatic−brominated compounds and antimony
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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