1. Polymers Department of Polymer & Process Engineering, UET, Lahore
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

2. Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains.
In more straightforward polymerization, alkenes, which are relatively stable due to σ bonding between carbon atoms form polymers through relatively simple radical reactions.
In contrast, more complex reactions such as those that involve substitution at the carbonyl group require more complex synthesis.
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

3. Types Of Polymer Synthesis
There are two major classes of polymer formation mechanisms
Addition Polymerization
Step-Growth ( Condensation) Polymerization
a)-Addition Polymerization:
The polymer grows by sequential addition of monomers to a reactive site
Chain growth is linear
Maximum molecular weight is obtained early in the reaction
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

4. General Mechanism
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

5. a)-Addition Polymerization:
It isn't difficult to form addition polymers from monomers containing C=C double bonds; many of these compounds polymerize spontaneously unless polymerization is actively inhibited.
There are 3 mechanisms through which the addition polymerization can proceed
Free-Radical Polymerization Reactions
Anionic Polymerization
Cationic Polymerization
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

6. I-Free-Radical Polymerization:
The simplest way to catalyze the polymerization reaction that leads to an addition polymer is to add a source of a free radical to the monomer.
The term free radical is used to describe a family of very reactive, short-lived components of a reaction that contain one or more unpaired electrons.
The presence of a free radical, addition polymers form by a chain-reaction mechanism.
There are 3 main steps involved
Chain-initiation
Chain-propagation
Chain- termination
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

7. II-Anionic Polymerization:
When the chain reaction is initiated and carried by negatively charged intermediates, the reaction is known as anionic polymerization.
Mechanism:The reaction is initiated by a Grignard reagent or alkyl lithium reagent, which can be thought of a source of a negatively charged CH3- or CH3CH2- ion.
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

8. III-Cationic Polymerization
The intermediate that carries the chain reaction during polymerization can also be a positive ion, or cation. In this case, the cationic polymerization  reaction is initiated by adding a strong acid to an alkene to form a carbocation.
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

9. b)- Step-Growth Polymerization:
Monomers react together to make small oligomers.
Small oligomers make bigger ones, and big oligomers react to give polymers.
Chain growth is exponential
Maximum molecular weight is obtained late in the reaction
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

10. General Mechanism
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

11. Chain Termination mechanisms

12. Formation of Nylon

13. Polymerization Methods
A number of different techniques are applied to carry on the polymerization process, important of them are:
Bulk polymerization
Suspension polymerization
Solution polymerization
Emulsion polymerization
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

14. 1-Bulk polymerization Neat monomer (and initiator)
Simplest formulation and equipment
Most difficult in control, when polymerization is very exothermic
If polymer is insoluble in monomer ⇒ polymer precipitate
Common problems:
Heat transfer
Increase in viscosity
Commercial uses:
1. Casting formulations
2. Low MW polymers for adhesives, plasticizers, and lubricant additives
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

15. 2-Suspension polymerization:
Disperse monomer droplets in a non compatible liquid (e.g. H2O)
Polymerize the monomer by an initiator
Stabilize the dispersion with a stabilizer (e.g. poly(vinyl alcohol) or methyl cellulose)
Heat transfer is efficient and reaction is easily controlled
Similar to bulk polymerization in kinetics and mechanism (”micro-bulk” polymerization)
Not applicable for tacky polymers (e.g. elastomers) due to the tendency of agglomeration
Commercial uses:
For making granular polymers, e.g. PS, PVC, PMMA
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

16. 3-Solution polymerization:
Use monomer solution
Heat transfer is very efficient
MW may be severely limited by chain transfer reaction (probably caused by the solvent molecule or its impurities).
Solvent residues ⎯ difficult to remove completely
Environmental concerns ⎯ organic solvent waste
Product sometimes directly usable
Commercial uses:
Acrylic coating, fiber spinning, film casting.
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

17. 4-Emulsion polymerization:
Widely used for large-scale preparation
For water-based (latex) paints or adhesives
Emulsion ⎯ oil-in-water.
Inverse emulsion ⎯ water-in-oil (less stable)
Extremely high MW are obtainable, but often too high to be useful
Chain transfer reagents are often added to control the MW
Also suitable for the polymers having small particles .
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

18. POLYMER COMPOUNDING
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

19. Basic Concept of Compounding
“Polymer compounding, basically is an intensive mixing of two or more types of polymers along with various additives like plasticizers, lubricants, color pigments, fillers etc, to incorporate various desired properties in the compounded polymers as well as to reduce its cost.”
Why Compounding is required?
Limited properties of pure resins
Polymers are used as materials of design and construction.
Competes with steel, aluminum and many other materials ( including itself).
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

20. Overview: Compounding is a very important market.
The composition of compound is dictated by the requirements of the converter’s application.
Methods used to compound these plastics can be
dry blending,
melt mixing,
or both using batch or continuous methods.
Batch systems tend to be more labor-intensive
and less complicated to operate and control.
Continuous systems are more consistent and easier to instrument for statistical process control.
Extrusion equipment, predominantly twin and other multi-screw types, provides the major source for compounding.

21. 11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

22. 11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

23. Plastic Materials
In compounding, a plastic is modified physically and/or chemically by additives that change its properties.
Wide range of grades of plastics are available.
Better the mixing/compounding, the better the materials performances.
The plastic being used is usually in the form of powder just as it comes from the plastic manufacturer.
Why Compounding is interesting and complicated?
Color requirements are important. Concentrate or master batch compounds are used.

24. Blend
Mixing of two compatible Plastics having different properties in order to produce an intermediate compound.
Two factors are considered during mixing;
Plastics are ultimately miscible
Plastics are ultimately immiscible
Minor Component (Dispersed)
Major Component (Matrix)
Other factors including:
Order of Mixing
Thermal Properties
Rheological Properties
Nature of Feed streams ( Solids or melt)
Average stress level
Heating History of Extruders
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

25. Important features of Blending:
Medium-intensity mixers usually consist of low-intensity mixers fitted with attachments called intensifiers. The intensifiers are normally a high-speed mixing head added to the low-intensity mixer.
The high-intensity mixer is a high-speed unit with specialized blade designs.
Mixing times are typically measured in minutes, High-intensity mixers are used most often in the areas of pigment dispersion and premixing of the compound.
Single and Twin screw extruders are used in continuous compounding.
Re-active Compounding:
Extruder is not only used as melt processor but also as a chemical reactor.
The most common reactants are plastic or pre-plastic melts and gaseous, liquid, or molten low molecular weight compounds.

26. Pellets
The primary form of materials to be processed are compounds in pellet form.
Advantages:
Simpler feeding system
Dust-free handling
Greater extrusion capacity
Lower shipping costs as a result of higher feed-bulk density

27. The plastics additional heating cycle The additional cost
Disadvantages:
The plastics additional heating cycle
The additional cost
Pellets are produced by using Pelletizer
Two methods are used to produce pellets
Strand Palletizing (3.2mm dia.)
Underwater Palletizing

28. Strand Pelletizer
11/11/2018
Department of Polymer & Process Engineering, UET, Lahore

29. Process Optimization
Contamination problems with changing the material in a single extrusion line.
Control
Die
Troubleshooting
Flecks formation
Non-uniform distribution of stabilizers
Bubble formation due to decomposition of additives
Using wrong additives