polymer chemistry sem-6, dse-b3 part-7, ppt-7
TRANSCRIPT
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POLYMER CHEMISTRY
SEM-6, DSE-B3 PART-7, PPT-7
Dr. Kalyan Kumar Mandal
Associate Professor
St. Paul’s C. M. College
Kolkata
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Polymer ChemistryPart-7
Contents
• Aromatic Polyamides
• Phenolic Resins
1. Novolac
2. Bakelite
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Aromatic Polyamides• A number of aromatic polyamides are known to form a mesophase (a mesophase is a state of
matter intermediate between liquid and solid. Gelatin is a common example of a partially
ordered structure in a mesophase.) in concentrated solutions. Aromatic polyamide fibres, are
commonly known as aramid fibres. Aramid fibres are a class of heat-resistant and strong
synthetic fibres. They are used in aerospace and military applications, for ballistic-rated body
armor fabric and ballistic composites, in marine cordage, marine hull.
• Polycondensation between appropriate
diamines (m or p-phenylenediamine)
and acid chlorides (isophthaloyl or
terephthaloyl chloride) leads to the
production of industrial aromatic
polyamides.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata
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Aromatic Polyamides
• The reaction product between p-phenylenediamine and terephthaloyl chloride is known by
the name Kevlar (Dupont) in the market. Kevlar fibres are nearly one-fifth in density
compared to steel but are nearly equal to steel in tensile strength. The high tenacity, high
modulus (The modulus of a material describes how well it resists deformation.) aramid fibres
are excellent in dimensional stability. Kevlar has a Tg > 300 °C and a melting point of 500 °C
which is above its decomposition temperature in air.
• It is wet spun from a solution of concentrated sulphuric acid, methyl pyridine or a mixture of
hexamethylene phosphoramide (HMPA), N,N-dimethyl acetamide and LiCl. The aromatic
polyamide solutions initially show a rise in viscosity with increase in volume fraction of
polymer followed by a reversal in the trend and an abrupt decrease in viscosity indicative of
mesophase formation when a critical volume fraction of polymer is exceeded.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata
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Aromatic Polyamides
• The aromatic polyamides have important practical applications. The exceptionally high
degree of orientation, achieved when the lyotropic mesophase is spun or extruded, imparts
exceptional high strength and moduli to aramid fibres and films.
• They are useful in cables for stiffness, corrosion resistance and light weight. They are in
demand in fibre reinforced composites where their price and properties lie between those of
glass fibre and carbon fibre. Bullet resistant fabrics are another important area of use of the
aramid fibres.
• A mesogen is a compound that displays liquid crystal properties. Mesogens can be described
as disordered solids or ordered liquids because they arise from a unique state of matter that
exhibits both solid- and liquid-like properties called the liquid crystalline state. This liquid
crystalline state (LC) is called the mesophase and occurs between the crystalline solid state
and the isotropic liquid state at distinct temperature ranges.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata
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Phenolic Resins• Resin is a solid or highly viscous substance of plant or synthetic origin that is typically
convertible into polymers. Resins are usually mixtures of organic compounds. The reaction
between different phenols and aldehydes leads to the formation of what is known as phenol-
aldehyde resins or simply phenolic resins or the phenolics. The simplest of these reactants,
viz. common phenol and formaldehyde are most commonly employed in the making of
different grads of phenolic (phenol-formaldehyde) resins. Resins from phenol and
formaldehyde are also commonly known as p-f resins. Other phenols used are cresols,
resorcinol and p-tertbutyl phenol. Among aldehydes other than formaldehyde, only furfural
has been used commercially to a limited degree.
• Phenol-formaldehyde resins, as a group, are formed by a step-growth polymerization (Step-
growth polymerization refers to a type of polymerization mechanism in which bi-functional
or multifunctional monomers react to form first dimers, then trimers, longer oligomers and
eventually long chain polymers.) reaction that can be either acid- or base-catalyzed.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata
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Phenolic Resins
• Since formaldehyde exists predominantly in solution as a dynamic equilibrium of methylene
glycol oligomers, the concentration of the reactive form of formaldehyde depends on
temperature and pH.
• For resinification with aldehydes, common phenol is recognized to function as a trifunctional
compound (the 2 ortho-positions and the para-position are the three reactive sites). In most
resinification reactions involving formaldehyde, formalin solution (nearly 37% aqueous
formaldehyde) is used and the aqueous formaldehyde is considered to remain in this solution
as methylene glycol (H2C=O + H2O ⇄ HO-CH2-OH).
• Thus, phenol-formaldehyde resinification system is a tri-bi functional system and the
resinification that proceeds by a polycondensation process invariably leads to the ultimate
formation of a space network, i.e. three dimensional crosslinked polymer. The first step in the
resinification reaction producing monomethylol phenols as shown in Figure 2.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata
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Chemistry of Phenolic Resin Formation
• The reaction is catalyzed by both acids and bases. Basic catalysts (Na2CO3, NaOH, NH4OH,
etc.) and higher molar proportions of formaldehyde favour formation of di- and trimethylol
phenols in the subsequent steps (Figure 3).
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata
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Chemistry of Phenolic Resin Formation
• The initial reaction products or the resinoids and the reactants further react among them
leading to chain extension and resin formation giving progressively increasing molecular
weight. Acids are, however, more powerful catalysts.
• Acid catalysts (mineral or organic acids) and higher molar proportions of phenol favour
immediate reaction involving two phenol molecules for each formaldehyde molecule
resulting in the production of 2,4′-dihydroxydiphenylmethane as the primary isolable product
in the early stages (Figure 4). The early stage resinoids readily react with additional
formaldehyde to form larger monomethylol derivatives which immediately react further with
an additional phenol molecule.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata
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Chemistry of Phenolic Resin Formation: Novolacs
• Repetition of this reaction sequence leads to the formation of practically linear polymers with
average molecular weight of nearly 600-700, which are commonly known as novolacs. A
typical novolac contains about 6 aromatic rings linked through methylene linkages and
having practically no methylol group in the resin molecule (A; Figure 5). Novolacs are thus
permanently soluble and fusible.
• Novolacs resemble the polymer except that they are of much lower molecular weight and are
still thermoplastic. Curing to network polymer is accomplished by the addition of more
formaldehyde or, more commonly, of compounds that decompose to formaldehyde on
heating.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata
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Chemistry of Phenolic Resin Formation: Novolacs
• If additional formaldehyde is made available and the resin is thermally treated, cross-linking
reactions between novolac chains via the formation of methylol groups along with chain
extensions take place ultimately rendering the resins insoluble and infusible. A segment of a
typical cross-linked structure is shown in Figure 6.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata
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Chemistry of Phenolic Resin Formation: Novolacs
• The cross-linking of the novolacs in the second step in presence of additional formaldehyde
is better accomplished using a basic catalyst. The overall technology of the novolacs is a two
step process; the first step corresponds to the formation of the linear, soluble and fusible
resins using phenol and formaldehyde, usually in 1 : 0.8 molar proportions and the second
step corresponds to heating the linear resin in presence of what is known as a hardener or
cross-linking agent which decomposes under heat and pressure to liberate formaldehyde and
bring about cross-linking through establishment of methylene bridges.
• Hexamethylene tetramine or simply hexa, most commonly used as the cross-linking agent,
decomposes under heat and using the moisture present to generate formaldehyde (the curing
agent) and ammonia (the curing catalyst) [(CH2)6N4 + 6H2O → 4NH3 + 6CH2O].
• A curing agent can be defined as a substance able to participate in the chemical reaction
between the oligomer, pre-polymer and polymer, to achieve the polymerization process and
provide the final film.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata
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Chemistry of Phenolic Resin Formation: Bakelite
• By contrast, the resinification using
basic or alkaline catalysts in presence
of excess formaldehyde slowly form
a three-dimensional, insoluble,
infusible, cross-linked polymer
known as bakelite (A; Figure 7).
• Bakelite was the first plastic made
from synthetic components. It is a
thermosetting phenol formaldehyde
resin, formed from a condensation
reaction of phenol with
formaldehyde.
• Bakelite is obtained when novolac is allowed to undergo cross-linking in the presence of a
cross-linking agent. In general, phenol taken in excess acts as the cross-linking agent.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata
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Commercial Production
• Commercial resinification reactions for the production of novolac, using appropriate
proportions of phenol and formaldehyde and an appropriate catalyst are done at nearly 100°C
in stainless steel jacketed (to permit heating or cooling) kettles each fitted with an efficient
stirrer and a condenser with provisions for reflux.
• After adequate progress of the reaction, water is removed usually under reduced pressure and
under controlled temperature conditions and the molten or high viscous resin is discharged by
opening the main discharge outlet valve at the bottom of the kettle.
• Uses of resins: Foamed objects based on phenolic resins have gained importance, particularly
in structural applications in the building industry. Resins based on para substituted phenols
are used in surface coating formulations; p-tertbutyl phenol and p-phenyl phenol are reacted
with formaldehyde to produce oil-soluble grade phenolics.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata
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Applications of Phenolic Resins
• Resorcinol-formaldehyde resins are extensively used as suitable bonding or coupling agents
aimed at achieving good adhesion between reinforcing cords and rubber in the tyre, hose and
belting industry. Other applications, taking advantage of the binding or adhesion capacity of
the phenolics include brake linings, abrasives, sand core bonding and shell moulds for metal
castings, grinding wheels, high performance refractory items etc.
• Another important application of the phenolics is in the area of making ion-exchange resins
via sulphonation (cation exchanger) and chloromethylation followed by quaternization by
treatment with tert-amines followed by alkali treatment (anion exchanger).
• Uses of Novolac: They are available in solid and liquid form and is used in binders, articles,
paints, molded pieces, laminated plastics and adhesives. They are used as chemically
unmodified synthetic resins based on their capability to undergo cross-linking with
hexamethylenetetramine.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata
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Properties of Bakelite
• It can be quickly molded.
• It can be molded very quickly, decreasing production time.
• Bakelite Moldings are smooth, retain their shape and are resistant to heat, scratches, etc.
• They are also resistant to several destructive solvents.
• Owing to its low electrical conductivity, bakelite is resistant to electricity.
• Uses of Bakelite: Since this element has a low electrical conductivity and high heat
resistance it can be used in manufacturing electrical switches and machine parts of electrical
systems. It is a thermosetting polymer and has high strength meaning it basically retains its
form even after extensive molding. Further, Bakelite has been used for making the handles
of a variety of utensils. It is one of the most common and important polymers that are used
to make different parts of many objects.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata
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Novolac, Resole and Bakelite
• When the phenol is taken in excess and the reaction medium is made acidic, the product of
the condensation reaction obtained is acidic. Whereas, when the quantity of formaldehyde
taken is more than that of phenol in the reacting mixture, and the reaction occurs in a basic
medium, the condensation product is known as Resole.
• These intermediate condensation products are used as resins in different industries. Bakelite
is obtained when Novolac is allowed to undergo cross-linking in the presence of a cross-
linking agent. In general, phenol taken in excess acts as the cross-linking agent.
This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata