Ploymerization

A Ziegler–Natta catalyst: is a catalyst used in the synthesis of polymers of 1-alkenes (alpha-olefins). Two broad classes of Ziegler–Natta catalysts are employed, distinguished by their solubility:

• Heterogeneous supported catalysts based on titanium compounds are used in polymerization reactions in combination with cocatalysts, organoaluminum compounds such as triethylaluminium, Al(C2H5)3. This class of catalyst dominates the industry.[1]
• Homogeneous catalysts usually based on complexes of the group 4 metals titanium, zirconium or hafnium. They are usually used in combination with a different organoaluminum cocatalyst, methylaluminoxane (or methylalumoxane, MAO). They are soluble in the reaction medium

Angel hair problem:

The causes of which lie in the presence of angel hair, impurities or dust

1. Addition or chain growth polymerization: monomers, unsaturated alkenes, add on together on large scale to form a polymer. The repeating unit has the same atoms as the monomer. This is because no other molecule is formed in the reaction. The following mechanisms can be named:

• Radical polymerization: through radicals that contain an unpaired electron whereas ionic polymerization, which is the most common mechanism.
• Ionic polymerization: via ionic species that has no unpaired electrons: a)anionic /b) cationic

2. Condensation or step growth polymerization: In organic chemistry a condensation reaction is one where two molecules join together with the loss of a small molecule when that happens. In this case, every time you create a new ester link, a water molecule is lost. That’s different from addition polymerisation where you join molecules up with nothing being lost.

Thermoplastic

1. formed by addition polymerisation
2. usually have a linear structure
3. chains are held by weak forces (Van der Walts
4. soften on heating and harden on cooling
5. usually soft, weak and less brittle

Thermoset

1. formed by condensation polymerisation
2. usually have a 3 dimensional cross/linked structure
3. chains are held by cross links which are stronger than covalent bonds
4. don not soften on heating but harden.
5. usually hard, strong and more brittle

Amorphous polymers are polymers that are composed of amorphous regions where molecules are randomly arranged. Polymers can be either completely amorphous or mixed with both amorphous and crystalline regions. Amorphous polymers possess widely different mechanical and physical properties owing to their structure and temperature. Below glass transition temperature (Tg), amorphous polymers exhibit glassy, hard and brittle properties. As the temperature is increased, while it passes the Tg, amorphous polymers form cross-links and show elastic properties.

Crystalline polymers: Not a single polymer is crystalline because all the crystalline polymers contain considerable amounts of amorphous material. Thus, crystalline polymers are generally called semicrystalline polymers.

Degree of crystallinity is directly related to whether a polymer melts like a typical solid or whether it transitions between glassy and rubbery states. Highly crystalline polymers have a more traditional melting point, so when they are heated, they reach a certain temperature at which the orderly arrangement of their long-chain structure transitions to a random and disorganized arrangement. This value is usually a specific number, designated as the melting point, or Tm.

Amorphous solids don’t melt suddenly when they’re heated. Instead, they reach a range of temperatures over which the material becomes less glassy and more rubber-like or vice versa. As a result, amorphous polymers don’t have a melting point — they have a glass transition temperature, or Tg. The glass transition temperature of a specific polymer may be listed as a single temperature, but this number is a representative value representing a range of temperatures.

To explain glass transition temperature in terms of molecular motion, we would say that, at temperatures below Tg, the amorphous polymer chains cannot rotate or move in space (the cooked spaghetti is frozen and cannot move). This produces the glassy state, which is hard, rigid, and brittle. When the temperature rises above Tg, the entangled chains can move (small portions of the spaghetti noodles can move around). This produces a rubbery state, when an amorphous polymer is soft and flexible.

Chewing gum is a mixture of polymers. Above Tg, it is rubbery, while it is brittle under Tg.

Polypropylene (Isotactic*): Crystalline Polymers, Tm: 174C

Polypropylene (Atactic*): Amorphous Polymers, Tg: -17C

Extruder:

• Nucleating agent: are inorganic materials added to polymers to increase the crystallinity and shorten the cycle times resulting in more transparency due to fine dense network. They speed up the transition from melted to solid material. Changing the crystallinity of plastics changes properties like density and clarity. Nucleating agents are compositions, compounds, etc., that induce the formation of polymer crystals (i.e., regulate and control crystallinity)
• The amount of crystallization and the type of crystals formed are controlled by thermal history, cooling rate, and additives.
• Extruder needs a high torque. If the derive is blocked, the derive will be destroyed. The torque limiter can protect the derive from damaging. It joins the derive and reduce the torque on the shaft derive. For having torque limiter, some air would be used for rotating the torque limiter.
• Torque limiter (Rutschkupplung):is an automatic device that protects mechanical equipment, or its work, from damage by mechanical overload. A torque limiter may limit the torque by slipping , or uncouple the load entirely. If the torque on the derive side will be too high, the static friction will be exceeded. The clutch slipps and the gear will be disconnected.

An energy balance over an extruder:

The power of the motor+ the power of heating= the enthalpy for melting * powder amount + hydrodynamic power (powder amount*density of the melt* pressure prior to the sieve)

Power of the motor: torque*rotation number

Enthalpy for melting: enthalpy curve depending on the melting temp.

• Shear stress (tangential spannung) is the reason of melting in the extruder.
• The force on the medium comes from the integral of the shear stress over the surface
• The mechanical power comes from the force * rotation of the screw in the extruder