40.1: Phenol Formaldehyde
Polymer Chemistry : The formation of phenol formaldehyde follows step – growth polymerization. The process can be either acid – catalysed or base –
Base-catalysed phenol formaldehyde resins are made with formaldehyde to phenol ratio of around 1.5. Such resins are sometimes called resols, and are found in varied industrial products. Laminates are made by impregnating one or more layers of base materials like paper or cotton with phenolic resin with the application of temperature and pressure. Moreover the resins act as binding agents in brake pads, brake shoes and various other applications.
Technology (Figure 40.1) :
The polymerization process for the manufacture of phenol formaldehyde is an exothermic one and is controlled by a batch reactor as the viscosity of
the material changes rapidly.
The raw materials for the process – phenol, formaldehyde and the catalyst are mixed in a jacketed autoclave , which is also termed as a resin kettle. Inside the autoclave, the mixture is heated with steam. Water cooling and refluxing remove the excess heat of reaction. During the initial stages of the reaction , the heavy viscous resins settle as the bottom layer, with an aqueous layer on top. A combination of heat and vacuum in the resin kettle (autoclave) enables the dehydration of the reaction mixture.
At around 130-150oC, the resin fuses and is removed from the kettle . Then product is then cooled and is ground to form fine powders. This molding powder which forms can be mixed with fillers, coloringagents , lubricants and catalysts in a ball mill. Inorder to prepare the commercial phenolic molding powder, the final product may be further heated on a pair of differential heating rolls.
Figure 40.1Manufacture of Phenol Formaldehyde.
Polymer Chemistry: The raw material for the production of Viscose rayon is wood pulp. However, the manufacturing process emits toxic effluents which have adverse environmental effects. Because of its cellulosic base , it has some properties which are similar to those of cotton and any other natural cellulosic fibres. It has moderate resistance to acids and alkalis. However, it may be damaged by relatively weak acids. Moreover, due to the inelasticity of fibers, it has poor abrasion resistance.
Technology (Figure 40.2):
The raw materials used in this process are cellulose wood pulp sheets or cotton linters. These are placed in a steeping press with contact in aqueous NaOH Solution for a period of 2 – 4 hours at normal room temperature. A hydraulic ram presses out the excess alkali and the sheets are shredded to crumbs and aged for 2 – 3 days. The aging process has its direct consequence on the viscosity of the solution.
Later CS2 is added in a rotating drum mixer over a period of 3 hours. The orange cellulose xanthate which forms is transferred to a solubilizer , wherein it is mixed into dilute caustic . Themixing of cellulose xanthate and dilute caustic yields a orange colored viscous solution which contains 7 – 8 % cellulose and 6.5 – 7 % NaOH. It is digested at room temperature for 4-5 days. Thereafter , the solution is filtered and fed to spinning machines.
The spinning is carried out in extrusion spinnerettes , which are made up of platinum or gold alloys. The orifices of these spinnerets have a diameter of 0.1 – 0.2 mm. for continuous filament yarns and of diameter 0.05 – 0.1 mm for short fibre shapes. The solution extruded from the spinnerette is contacted with an acid bath which precipitates the filaments without causing them to break or stick together. The processing treatments of washing, desulphurizing, bleaching and conditioning takes place continuously and in order after the filaments are wound on a series of plastic rolls.
Polymer Chemistry: Nylon 66 (or 6,6 - Nylon) is a condensation polymer formed by the condensation polymerization of Adipic acid (a dibasic acid) and hexamethylenediamine (a diamine). In the nomenclature, the first number refers to the number of carbon atoms in the diamine and the second number designates the number of carbon atoms in the dibasic acid. Nylon – 6 is a polymer of caprolactum . The number refers to the total carbon atoms in the ring, which can vary from 5 to 12.
Both Nylon – 6 and Nylon – 66 have similar properties and can be used as thin films and fibres; and for extrusion and injection molding in plastics. The major difference in the physical properties of these two nylons is that the melting point of Nylon – 66 is 40 – 45oC higher than Nylon – 6. Moreover, Nylon – 66 is harder, rigid and its abrasive – resistance is lower than that of Nylon – 6.
Technology (Figure 40.3):
In the production of Nylon – 66, first, the amine and acid is mixed in an aqueous solution , with the pH adjusted at 7.8 to form Nylon salt. The salt is then concentrated under vacuum. It is thereafter charged to an autoclave where the rest of the water is removed. The temperature is gradually increased to 280oC in the autoclave to complete the polymerization and the water of condensation is removed.
The final product has a molecular weight of 12000 – 16000. It is extruded as ribbons onto chilling rolls. Then, it is sent to a chipper which produces small chips, which possesses the convenience of storage and rehandling. These chips are melted, metered through high pressure pumps. After filtration, these are passed via a melt spinnerette to produce nylon fibers. Upon cooling , these filaments harden and are wound on bobbins at a rate of 750 metres per minute or even higher. Inorder to give the fibre desirable textile properties, the threads are stretched to about 4 times its original
The production of Nylon – 6 is similar to that of Nylon – 66 , the only difference being the nature of polymerization. Nylon – 6 is manufactured by the step – wise condensation of caprolactum with no net water removal. However, to maintain the thermal equilibrium between monomer and polymer at the melting and spinning temperatures is the only difficult task. Monomer retained on the fibres accounts upto 10% , so the fibres are water – washed to remove the soluble caprolactum retained.
40.4 .Technical Questions
Q. 1. What is the Spherilene process ? Explain how spherilines are developed.
Ans .1. The Spherilene process is a new, gas – phase Polyethylene ‘swing’ technology. It is based on a well – established technology, known as the Reactor Granule Technology (RGT), which is basically a refinement of gas – phase and bulk – polymerization reactors.
In the Spherilene process, Zeigler spherical – form – supported catalyst particles act as microreactors. In the first and second gas – phase reactors, the polymerizing particles grows on itself in a gradual manner. However, during the whole growing process , the particles retain their original spherical form, and allows for perfect control of the final morphology of the
Q.2. What are General – purpose Polystyrene (GPPS) and High – impact Polystyrene (HIPS) ? How are they different ?
Ans.2. GPPS is also known as crystal polystyrene. It has good clarity and fairly good mechanical properties. Although GPPS can be processed into most types of objects, it is relatively brittle. It is used in applications where light transmission properties are important.
On the other hand, HIPS contains an elastomer like polybutadiene rubber, which improves its mechanical properties. It has appreciable impact strength and hence, is milky or opaque in appearance. HIPS is tougher than GPPS, and also has good processing properties.
Q.3. In high pressure technology, which one is more advantageous – Centrifugal Compressors or Reciprocating Compressors? Why?
Ans.3. In high pressure technologies, centrifugal compressors are more advantageous than reciprocating compressors. This is because centrifugal compressors have low investment and maintenance costs, and ensures more reliability than the formerly used reciprocating compressors. Moreover, they require less space and provide high efficiencies even at high flow rates.
Q. 4. What are the problems that may be encountered when the reactor walls are contaminated with polymer materials? How can this problem
be mitigated ?
Ans. 4. If the polymer material builds up on the reactor walls, then it can lower the rate of heat transfer considerably. Moreover, it can also have detrimental effects on the quality of the product. This problem is very much encountered in the Poly vinyl chloride manufacture process.
The reactor walls can be kept free of the polymer material by using certain build – up suppressing agents or anti – fouling agents. In addition, cleaning of the reactors with high pressure water after a certain number of batches ensures the reactor walls free from polymer materials.
Q. 5. In conventional drying processes, why is the method of selective drying so important ?
Ans. 5. Selective drying is important in most drying processes, because , it ensures the prevention of thermal degradation of the material being processed. Moreover, the overdrying of the drying goods is avoided using this method.
Q. 6. Match the product in group 1 with the nature of reaction in group
Group 1 Group 2
a. Polyethylene 1. Condensation Polymerization
b. Nylon 2. Addition Polymerization
Q.7. What kind of catalyst in the Low Pressure Zeigler Process for the manufacture of polyethylene ?
Ans . 7. Aluminium triethyl combined with titanium tetrachloride.
Q. 8. Match the processes used for producing various products from PVC.
Group 1 Group 2
a. Extrusion 1. Rexin / Leather cloth
b. Injection Moulding 2. Rigid containers / bottles
c. Blow Moulding 3. Pipe fittings
d. Calendering 4. Rigid pipes
Q.9 What does the Ziegler – Natta catalyst consist of ?
Ans9. The Ziegler Natta catalyst consists of a combination of HF and BF3
Q.10 Which two polymerization methods are commonly used for the manufacture of polyvinyl chloride ? Mention some of the disadvantages of using PVC as a polymer.
Ans10. Two methods for manufacture of PVC are Emulsion Polymerization and Suspension Polymerization.Disadvantage of PVC is that it has low solvent solubility. In addition to this, the heat stability of PVC is poor.
Q11. What is the main criteria based upon which Polyethylene is classified ?
Ans11. Based upon its density and molecular bonding.
Q12. Which of the following has a higher melting point – Nylon -6,6 or Nylon -6 ?
Ans12. Nylon – 6,6 has a higher melting point than Nylon -6 ( about 40 – 45 o higher).