PA 66
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.
[ad#aryshi1]
