POLYMER CHEMISTRY SEM-6, DSE-B3 PART-7, PPT-7

POLYMER CHEMISTRY

SEM-6, DSE-B3 PART-7, PPT-7

Dr. Kalyan Kumar Mandal

Associate Professor

St. Paul’s C. M. College

Kolkata

Polymer ChemistryPart-7

Contents

• Aromatic Polyamides

• Phenolic Resins

1. Novolac

2. Bakelite

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

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.


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.


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.


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.


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).


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.


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.



• 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.



• 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.


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.


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.


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.


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.


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.


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POLYMER CHEMISTRY SEM-6, DSE-B3 PART-7, PPT-7