Polymer Technology

The utility of nylons rests upon their combination of properties and upon their susceptibility to modification. Key properties are resistance to oils and solvents; toughness; fatigue and abrasion resistance; low friction and creep; stability at elevated temperatures; fire resistance; drawability; good appearance and good processability.

Additives are often used to improve properties of polyamides. Processing additives include coloration inhibitors, lubricants, moldrelease agents, nucleating agents, and viscosity modifiers. Nucleating

agents increase rate of crystallization to shorten molding cycles and to modify mechanical properties. Additives used to alter properties or appearances are antioxidants, antistatic agents, biodegradative agents, biopreservatives, blowing agents, colorants, fragrances, and stabilizers against hydrolysis, thermal degradation, or UV degradation.

Mineral fillers, glass or carbon fibers are used to improve rigidity and strength. Lubricants are used to improve wear and friction. Plasticizers, fire retardants, electrically conductive materials, and other polymers are used for respective property improvements.

Alternative techniques of modification include copolymerization, adjusting molecular mass and post treatments such as annealing, conditioning to some moisture level, dyeing, metallizing, painting, irradiation, or chemical reaction.

Nylons are processed into useful articles by variety of techniques extrusion, injection molding, blow molding, monomer casting, solution coating, fluidized-bed or electrostatic coating, or forming.
Nylons are used in many diverse ways. They are found in appliances, business equipments, consumer products, electrical/electronic devices, furniture, hardware, machinery, packaging, and transportation. Transportation is the largest market for nylons Electrical and electronic applications comprise another major market.[ad#aryshi1]

Continuous processes are used by the major manufacturers of PA 6. the so called VK tube (Vereinfacht Kontinuierlich = simplified continuous) was developed in Germany. It is a vertical tube operated at atmospheric pressure wherein heating and prepolymerization take place in the upper part and polymer is formed in the lower section. A schematic of BASF process shows a VK tube feeding a pelletizer followed by a water extraction unit.

Production of P A 6 by BASF

a) Feed Tank
b) VK Tube
c) Pourer
d) Pelletizer
e) Water bath
f) Extractor
g) Dryer

An Allied Chemical (now with BASF) involving vacuum extraction is shown below. Improved processes include a baffled hydrolyser operating in plug flow and at a carefully controlled temperature.

Production of nylon 6 by Allied Chemical
a) Pump; b) Stirrer; c) Holding tank; d) Filter; e) Flowmeter; f) Prehe,ter; g) Hydrolyzer; h) Metering pump; i) Polyaddition reactor; j) Vent; k) Vacuum flasher; I) Finisher; m) Spinning heads

The first step is preparation of pure, balanced salt in aqueous solution Nylon salt is a stoichiometric mixture of dibasic acid and diamine. Stoichiometric equivalence is determined by pH measurement. Some excess diamine is normally added to compensate for losses due to its relative volatility The equivalence pH for aliphatic salts approximates to 7.6. Charcoal decolorization of the salt solution is required unless the diamine has been carefully refined. A two-stage, recycling method of salt preparation is used in a continuous process. Two-step addition to diamine with intermediate evaporation of water yields a concentrated salt solution. To avoid precipitation when stored at about 25°C, a 50% aqueous salt solution is normally prepared.
PA 66 is produced by batch as well as continuous process.

Batch Production of PA 66

The stored solution is concentrated under pressure to 65 – 80% before charging to autoclave (reactor). The salt solution is then heated to 210°C under autogeneous pressure to reach a pressure of 1.75 MPa (17.5 barg, 250 psig). Temperature is further increased gradually to about 275°C while releasing steam at a rate which maintains the pressure. Further the pressure is reduced at a rate which avoids cooling, finally holding the batch at atmospheric or reduces pressure to obtain target molecular mass before extruding the polymer under inert gas pressure.

The extrudate is a wide ribbon which is quenched with water that is subsequently removed by jet blowers. The ribbon is cut into chips which are blended and packaged.

A Typical autoclave cycle useful for batch preparation of nylon 66 (DuPont Technical Laboratory, Seaford, Delaware)

Continuous Production of PA 66

In a continuous process, first stage involves evaporation/reaction with controlled loss of water to form a prepolymer and minimize loss of diamine. Further reaction occurs in subsequent stages with controlled evaporation devices known as ’separators’ and ‘flashers’. The desired molecular mass and water content are obtained in a ‘finisher’.

Continuous polymerizer for PA 66
a) Evaporator/reactor; b) Vent; c) Pump; d) Finisher; e) Flash tubes

The polymerizations yielding PA 69, PA 610, PA 612, PA 1212 are subject to the same concerns as that yielding PA 66. These polyamides are made in much the same way with differences due to different melt viscosities and melting points. Polyamides from monomers with aromatic or cycloaliphatic rings present special problems due to their high viscosity.

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Polyamides are polymers that contain an amide group, -CONH-, as a recurring part of the chain. Synthesis of polyhexamethylene adipamide, the original “nylon”, in the DuPont laboratory in 1935 provided a material whose properties were appropriate for apparel use and resulted in its commercial introduction in 1938 The other principal polyamide or nylon, polycaprolactum, was made in an 18 Farben laboratory by Schlack in 1938.

Although use as fiber dominated the interest in nylon, application as plastic for variety of purposes such as brush filaments, wire coating, coil forms, and gears occurred very soon thereafter. The use of nylons as plastics has increased steadily against that of fiber.

Nomenclature

The nylons (polyamides) are most often made from diamines and dibasic acids(1), D)-amana acids(2), or ladums(3).

The polyamides (PA) or nylons are identified by numbers corresponding to the number of carbon atoms in the monomers (diamine first).

For example for the nylons showed in (1):

Nylon 46 or PA 46 is polymerized from
H2N(CH2)4NH2 and HOOC(CH2)4COOH

Nylon 66 or PA 66 is polymerized from
H2N(CH2)6NH2 and HOOC(CH2)4COOH

Nylon 612 or PA 612 is polymerized from
H2N(CH2)6NH2 and HOOC(CH2)lOCOOH

And for the nylons showed in (2 and 3)
Nylon 6 or PA 6 is polymerized from H2N(CH2)5COOH
Nylon 12 or P A 12 is polymerized from H2N(CH2)11COOH