Lecture 2: Polyethylene (PE) and Polypropylene (PP)

Polyethylene has a low Tg (Tg= -120°C) but is still hard at room temperature. Why?

Polyethylene is a semi-crystalline thermoplastic polymer. It consists of:

• Crystalline regions (ordered chain packing)

• Amorphous regions (disordered chains)

The crystalline regions behave like physical crosslinks and give the material stiffness and hardness.

But the amorphous regions are flexible at room temperature (since Tg is so low) that gives polyethylene its ductility and it is therefore NOT brittle.

Draw the structure of Polyethylene (PE)

(C2H4)n

Mention the density, Tm and Tg for Polyethylene

Density: 0.905 -0.965 g/cm3 , will float in water! Most plastics sinks

Tm= 105-140°C

Tg= -120°C

Where does the majority of the monomer of PE (ethylene) come from?

Large majority of ethylene comes from petroleum sources such as Naphta

Explain one synthesis method for LDPE

Synthesis method: High-pressure polymerization

Polymerization technique: Free Radical Polymerization

Monomer: Ethylene

Initiator: Oxygen

• The process occurs under high temperature and pressure in a tube reactor

• The degree of branching is controlled by the temperature, pressure, and reactor type. High pressure gives a higher molecular mass of polymer.

Risk: The reaction is highly exothermic which can lead to uncontrolled temperature increase. To avoid this, powerful stirring and cooling systems are used.

How is branches formed in LDPE?

Branches are usually formed during Free Radical Polymerization of LDPE via chain transfer reactions.

Explain one synthesis method for HDPE and LLDPE

Polymerization technique: Coordination polymerization

Catalyst: Ziegler-Natta Catalysts

What is LLDPE?

Linear low density polyethylene (LLDPE) is a copolymer of ethylene and alpha-olefins (such as 1-butene or 1-hexene)

Compare the properties between LDPE, HDPE and LLDPE

LDPE has a low density and a high degree of branching. The degree of crystallinity is about 40%. Melting temperature is 105°C.

HDPE has a high density and a low degree of branching. The degree of crystallinity is about 95% (because of the low degree of branching). Melting temperature is 140°C.

LLDPE has a medium density, but more toward the lower end. The degree of crystallinity is higher than 50%. LLDPE has intermediate properties between LDPE and HDPE. Melting temperature is 130°C.

Compare the structures between LDPE, HDPE and LLDPE

LDPE has a highly branched structure

HDPE has a linear structure with minimal branching

LLDPE has a linear structure with short branches

Mention some application of Polyethylene

• Toys

• Pipes

• Bags

• Food containers

Mention the crystallinity and monomer of Polypropylene (PP)

Polypropylene is a semicrystalline thermoplastic.

Monomer: Propylene

Mention the density, Tg and Tm for Polypropylene.

Density: 0.9 g/cm3

Tg= -10°C

Tm= 160-170°C

Where does the monomer of Polypropylene come from?

Polypropylene comes from petroleum based sources such as Naphta

Explain one polymerization technique of Polypropylene

Spheripol-process:

• Ziegler-Natta catalyst

• No atactic polypropylene is formed

• 97% of PP produced is isotactic

Explain some important structure - properties for PP regarding chain stiffness and thermal stability.

• Polypropylene has a methyl group on every other carbon atom in the chain. The methyl group will give steric hindrance and therefore, the chain can not rotate as freely. This will give a higher stiffness and Tg compared to polyethylene.

• The tertiary carbon in the chain is more reactive and sensitive towards oxidation. Therefore, Polypropylene is more sensitive towards heat and UV radiation compared to polyethylene.

Describe the structure-properties for Isotactic PP, Syndiotactic PP and Atactic PP

• Isotactic PP: Highly crystalline - stiff and strong material because all methyl groups is on the same side

• Syndiotactic PP: Lower crystallinity because of alternating placement of methyl group, less rigid and higher impact resistance (because it doesnt crack easily)

• Atactic PP: Amorphous, rubbery like because of random placement of methyl groups

Mention 3 applications for Polypropylene

• Bottles

• Packaging

• Instrument panel

Name one important additive for polypropylene (PP)

Antioxidants is an important additive because it hinders thermal degradation by oxidation of the tertiary carbon.