Polymers and Plastics - Comprehensive Notes

Petrochemicals

• Petrochemicals are chemical products derived from petroleum, but can also be obtained from other fossil fuels like coal and natural gas, or renewable sources like corn or sugar cane.
• The two most common petrochemical classes are:
  • Olefins (including ethylene and propylene)
  • Aromatics (including benzene, toluene, and xylene isomers).

Primary Petrochemicals

• Primary petrochemicals are divided into three groups based on their chemical structure:
  • Olefins:
    • Include ethylene, propylene, and butadiene.
    • Ethylene and propylene are important sources for industrial chemicals and plastics.
    • Butadiene is used in making synthetic rubber.
  • Aromatics:
    • Include benzene, toluene, and xylenes.
    • Benzene is a raw material for dyes and synthetic detergents. Benzene and toluene are used for isocyanates in polyurethanes.
    • Xylenes are used to produce plastics and synthetic fibers.
  • Synthesis Gas:
    • A mixture of carbon monoxide and hydrogen.
    • Used to make ammonia and methanol.
    • Ammonia is used to produce urea fertilizer, and methanol is used as a solvent and chemical intermediate.

Petrochemical Global Production

• Global ethylene production is approximately 115 million tonnes per annum.
• Global propylene production is approximately 70 million tonnes per annum.
• Aromatics production is approximately 70 million tonnes per annum.
• The largest petrochemical industries are located in the USA and Western Europe.
• Major growth in new production capacity is in the Middle East and Asia.
• There is substantial inter-regional petrochemical industries growth.

Production of Olefins by Steam Cracking

• Oil refineries produce olefins and aromatics by fluid catalytic cracking of petroleum fractions.
• Chemical plants produce olefins by steam cracking of natural gas liquids like ethane and propane.
• Aromatics are produced by catalytic reforming of naphtha.
• Olefins and aromatics are building blocks for a wide range of materials such as solvents, detergents, and adhesives.
• Olefins are the basis for polymers and oligomers used in plastics, resins, fibers, elastomers, lubricants, and gels.

Feedstocks for Petrochemical Industries

• Methane and BTX (Benzene, Toluene, Xylenes) are used directly as feedstocks.
• Ethane, propane, butanes, naphtha, and gas oil serve as optional feedstocks for steam crackers.
• Steam crackers produce the following intermediate petrochemical feedstocks:
  • Ethylene
  • Propylene
  • Butenes and butadiene
  • Benzene

Steam Cracking

• Steam cracking is a petrochemical process where saturated hydrocarbons are broken down into smaller, often unsaturated, hydrocarbons.
• It is the principal industrial method for producing the lighter alkenes (olefins), including ethene (ethylene) and propene (propylene).

Steam Crackers

• Steam cracker units are facilities in which a feedstock such as naphtha, liquefied petroleum gas (LPG), ethane, propane, or butane is thermally cracked using steam in pyrolysis furnaces to produce lighter hydrocarbons.
• The products obtained depend on:
  • The composition of the feed
  • The hydrocarbon-to-steam ratio
  • The cracking temperature
  • Furnace residence time

Operating Conditions of Steam Crackers

• A gaseous or liquid hydrocarbon feed, like naphtha, LPG, or ethane, is diluted with steam and briefly heated in a furnace without oxygen.
• The reaction temperature is very high, around 850°C, but the reaction is only allowed to take place briefly.
• In modern cracking furnaces, the residence time is reduced to milliseconds to improve yield, resulting in gas velocities faster than the speed of sound.
• After the cracking temperature has been reached, the gas is quickly quenched to stop the reaction in a transfer line heat exchanger or inside a quenching header using quench oil.

Steam Cracking Process Description

1. Naphtha/LPG saturates are mixed with superheated steam and fed to a furnace, which uses fuel gas and fuel oil to generate heat. The superheated steam is generated from heat recovered from the furnace itself.
2. C2-C4 saturates are fed to a separate furnace fueled by fuel gas and fuel oil to generate heat.
3. In the furnace, in addition to steam cracking, steam is also generated using waste heat recovery from combustion gases.
4. After pyrolysis, the products are sent to another heat recovery steam boiler to cool the product streams (from about 700 – 800°C) and generate steam from water.
5. The produced vapors enter a scrubber fed with gas oil as an absorbent. The gas oil removes solids and heavy hydrocarbons.
6. Separate waste heat recovery boilers and scrubbers are used for the LPG furnace and Naphtha steam cracking furnaces.
7. After scrubbing, both product gases are mixed and fed to a compressor, increasing the system pressure to 35 atms.
8. The compressed vapor is fed to a phase separator, dividing the feed into a vapor phase stream and a liquid phase stream.
   • The vapor phase stream consists of H2, CO, CO2, and C1-C3+ components in excess.
   • The liquid phase stream consists of C3 and C4 compounds in excess.
9. The vapor and liquid phase streams are then subjected to separate processing.

Properties of Polymers: Polymerization - From Olefins to Polymers

Ancient polymers

• Naturally occurring polymers (derived from plants and animals) have been used for centuries, such as:
  • Wood
  • Rubber
  • Cotton
  • Wool
  • Leather
  • Silk
• Oldest known uses include rubber balls used by Incas.

Cellulose

• Cellulose is a highly abundant organic compound.
  • Extensive hydrogen bonding between the chains causes native cellulose to be roughly 70\% crystalline.
  • This hydrogen bonding also raises the melting point (>280°C) to above its combustion temperature.
• Cellulose serves as the principal structural component of green plants and wood.
• Cotton is one of the purest forms of cellulose and has been cultivated since ancient times.
• Cotton, along with treated wood pulp, serves as the source for the industrial production of cellulose-derived materials, which were the first "plastic" materials of commercial importance.

Rubber

• A variety of plants produce a sap consisting of a colloidal dispersion of cis-polyisoprene; this milky fluid is especially abundant in the rubber tree (Hevea).
• Natural rubber is thermoplastic, with a glass transition temperature of -70°C.
• Raw natural rubber tends to be sticky when warm and brittle when cold.
• It did not become generally useful until the mid-nineteenth century when Charles Goodyear found that heating it with sulfur—a process called vulcanization—could greatly improve its properties.

Hydrocarbon Molecules

• Many organic materials are hydrocarbons, composed of hydrogen and carbon.
• Most polymers are made up of hydrogen and carbon.
• The bonds between the hydrocarbon molecules are covalent.
• Each carbon atom has 4 electrons that may be covalently bonded, and the hydrogen atom has 1 electron for bonding.
• A single covalent bond exists when each of the 2 bonding atoms contributes one electron (e.g., methane, CH_4).

Saturated Hydrocarbons

• Each carbon has a single bond to 4 other atoms; the 4 valence electrons are bonded, making the molecule stable.
• The covalent bonds within each molecule are strong, but only weak hydrogen and van der Waals bonds exist between the molecules.
• Most of these hydrocarbons have relatively low melting and boiling points.
• However, boiling temperatures rise with increasing molecular weight.

Unsaturated Hydrocarbons

• Double and triple bonds are somewhat unstable – involve sharing 2 or 3 pairs of electrons, respectively. They can also form new bonds.
  • Double bond found in ethylene - C2H4.
  • Triple bond found in acetylene - C2H2.

Ethylene

• Lightest olefinic hydrocarbon with formula C2H4.
• Does not occur freely in nature.
• Largest building block for a variety of petrochemicals such as plastics, resins, fibers, solvents, etc.
• Produced primarily from the thermal cracking of hydrocarbon feedstocks derived from natural gas and crude oil.

Products from Ethylene

• Polyethylene
• Engine coolant
• Ethanol
• Ethylene glycol
• Polyesters
• Ethylene oxide
• Glycol ethers
• Vinyl acetate
• Ethoxylates
• Tetrachloroethylene
• 1,2-dichloroethane
• Trichloroethylene
• Vinyl chloride
• Polyvinyl chloride

Propylene

• Propene, also known as propylene or methyl-ethylene, is an unsaturated organic compound with the chemical formula C3H6.
• It has one double bond and is the second simplest member of the alkene class of hydrocarbons.
• Propene is found in nature and is a byproduct of vegetation and fermentation processes.
• Propane dehydrogenation converts propane into propylene and by-product hydrogen. The propylene from propane yield is about 85 wt\%. Reaction by-products (mainly hydrogen) are usually used as fuel for the propane dehydrogenation reaction.

Products from Propylene

• Isopropyl alcohol
• Acrylonitrile
• Polypropylene
• Polyol
• Propylene oxide
• Propylene glycol
• Glycol ethers
• Acrylic acid
• Acrylic polymers
• Allyl chloride
• Epichlorohydrin
• Epoxy resins

Benzene

• Benzene is an organic chemical compound with the molecular formula C6H6. Its molecule is composed of 6 carbon atoms joined in a ring.
• Benzene is a natural constituent of crude oil and is one of the most elementary petrochemicals.
• Four chemical processes contribute to industrial benzene production: catalytic reforming, toluene hydrodealkylation, toluene disproportionation, and steam cracking.

Products from Benzene

• Ethyl benzene
• Styrene
• Polystyrenes
• Phenol
• Acetone
• Cumene
• Epoxy resins
• Bisphenol A
• Polycarbonate
• Solvents
• Adipic acid
• Cyclohexane
• Nylons
• Caprolactum
• Nitrobenzene
• Aniline methylene diphenyl diisocyanate
• Alkyl benzene detergents
• Chlorobenzene
• Polyurethanes

Toluene

• Toluene is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from a group of six atoms from the benzene molecule has been replaced by a univalent group, in this case CH_3. As such, its IUPAC systematic name is methylbenzene.
• Toluene occurs naturally at low levels in crude oil and is usually produced in the processes of gasoline via a catalytic reformer, in an ethylene cracker, or making coke from coal.
• Toluene is mainly used as a precursor to benzene. The process involves hydrodealkylation: C6H5CH3 + H2 → C6H6 + CH_4

Products from Toluene

• Benzene
• Toluene diisocyanate
• Polyurethanes
• Benzoic acid
• Caprolactum
• Nylons

Xylene

• Xylene is an aromatic hydrocarbon consisting of a benzene ring with two methyl substituents.
• Xylenes encompass three isomers of dimethylbenzene. The isomers are distinguished by the designations ortho- (o-), meta- (m-), and para- (p-), which specify to which carbon atoms (of the benzene ring) the two methyl groups are attached.
• The three isomeric xylenes each have a molecular formula of C8H{10}.

Products from Xylene

• Alkyd resins
• Isophthalic acid (from meta-xylene)
• Polyamide resins
• Unsaturated polyesters
• Dimethyl terephthalate (from para-xylene)
• Terephthalic acid
• Phthalic anhydride (from ortho-xylene)
• Polyesters

Polymers

• Polymer: many repeat unit (building blocks)
  • Polyethylene (PE)
  • Poly(vinyl chloride) (PVC)
  • Polypropylene (PP)

Synthetic Polymers

• Polymers: Macromolecules formed by the covalent attachment of a set of small molecules termed monomers.
• Polymers are classified as:
  • Man-made or synthetic polymers that are synthesized in the laboratory.
  • Biological polymers that are found in nature.
• Examples:
  • Synthetic polymers: nylon, poly-ethylene, poly-styrene
  • Biological polymers: DNA, proteins, carbohydrates

Chemistry and Structure of Polyethylene

• Polyethylene is a long-chain hydrocarbon.
• It has repeat unit and chain structures with a zigzag backbone structure due to Tetrahedral arrangement of C-H bonds.

Isomerism

• Two compounds with the same chemical formula can have different structures (atomic arrangements).
  • Example: C8H{18}
    • Normal-octane
    • 2,4-dimethylhexane

Methods for Making Synthetic Polymers

1. Addition polymerization
2. Condensation polymerization

Addition Polymerization

• Monomers react to form a polymer without net loss of atoms. Most common form: free radical chain reaction of n ethylene molecules
  n \space monomers \rightarrow one \space polymer \space molecule

Condensation Polymerization

• The polymer grows from monomers by splitting off a small molecule such as water or carbon dioxide.
• Example: formation of amide links and loss of water

Monomers \rightarrow First \space unit \space of \space polymer + H_2O

Topics

• What are the general structural and chemical charactersitics of polymer molecules?
• What are some of the common polymeric materials, and how to they differ chemically?
• How is the crystalline state in polymers different from that in metals and ceramics?

Important Constitutions for Synthetic Polymers

• Simple polymer
• Alternating copolymer
• Block copolymer
• Graft copolymer

Supramolecular Structure of Polymers

• They are polymeric arrays of monomer units and are large molecules formed by grouping or bonding smaller molecules together.
• These structures are a result of various noncovalent interactions, including van der Waals interaction, electrostatic interaction, hydrogen bonding, hydrophobic interaction, coordination.
• The resulting materials therefore maintain their polymeric properties in solution.

Structural Properties of Linear Polymers: Conformational Flexibility and Strength

• Linear polymers resemble 'spaghetti' with long chains.
• The long chains are typically held together by the weaker van der Waals or hydrogen bonding.
• Since these bonding types are relatively easy to break with heat, linear polymers are typically thermoplastic.

Polymerization

• Free radical polymerization: ethylene gas reacts with the initiator (catalyst). (“R.” is the unpaired electron)
• Monomer refers to the small molecule from which a polymer is synthesized.

Additional (3 steps) Polymerization

• Initiation
• Propagation
• Termination
  • Disproportionation
  • Combination

Some Common Addition Polymer

Some Condensation Polymers

Molecular Weight

• Molecular weight, M: Mass of a mole of chains. Low M -> high M
• Polymers can have various lengths depending on the number of repeat units.
• During the polymerization process not all chains in a polymer grow to the same length, so there is a distribution of molecular weights. There are several ways of defining an average molecular weight.
• The molecular weight distribution in a polymer describes the relationship between the number of moles of each polymer species and the molar mass of that species.

Molecular Weight Distribution

• x_i = Number fraction of chains in size range i
• w_i = weight fraction of chains in size range i
• M_i = molecular weight of size range i
• Mn = the number average molecular weight Mn = \sum xi Mi
  Mw = \sum wi M_i

Molecular Weight Distribution

• Since polymer samples contain molecules of different sizes, we cannot give the exact molecular weight of the polymer. Therefore, we use averages of different parameters to indicate the molecular weight of the polymer.
• The number average and weight average molecular weight are such two forms.
• The key difference between number average and weight average molecular weight is that the number average molecular weight refers to the mole fraction of molecules in a polymer sample whereas the weight average molecular weight is the weight fraction of molecules in a polymer sample.
• Another difference between number average and weight average molecular weight is that we can determine the number average molecular weight using gel permeation chromatography, viscometer and colligative methods such as vapor pressure osmometric while we can determine the weight average molecular weight using static light scattering, small angle neutron scattering, X-ray scattering, and sedimentation velocity.

Degree of Polymerization, DP

• DP = \frac{M_n}{m}, where m = repeat unit molecular weight
• DP = average number of repeat units per chain
• Example: for PVC:
  • m = 2(carbon) + 3(hydrogen) + 1(Clorine)
  • = 2(12.011) + 3(1.008) + 1(35.45) = 62.496 g/mol
  • DP = 21,150 / 62.496 = 338.42

Polymer Chain Lengths

• Many polymer properties are affected by the length of the polymer chains. For example, the melting temperature increases with increasing molecular weight.
• At room temperature, polymers with very short chains (roughly 100 g/mol) will exist as liquids.
• Those with weights of 1000 g/mol are typically waxy solids and soft resins.
• Solid polymers range between 10,000 and several million g/mol.
• The molecular weight affects the polymer’s properties (examples: elastic modulus & strength).

Polymers – Molecular Shape

• Straight and twisted chain segments are generated when the backbone carbon atoms are oriented.
• Chain bending and twisting are possible by rotation of carbon atoms around their chain bonds.
• Some of the polymer mechanical and thermal characteristics are a function of the chain segment rotation in response to applied stresses or thermal vibrations.

Chain End-to-End Distance, r

• Representation of a single polymer chain molecule that has numerous random kinks and coils produced by chain bond rotations; it is very similar to a heavily tangled fishing line.
• “r” is the end to end distance of the polymer chain which is much smaller than the total chain length.

Molecular Structures for Polymers

• The physical characteristics of a polymer also depend on differences in the structure of the molecular chains (other variables are shape and weight).
• Linear polymers have repeat units joined end to end in single chains. There may be extensive van der Waals and hydrogen bonding between the chains. Examples: polyethylene, PVC, nylon.

Molecular Structure - Branched

• Side-branch chains connected to main chains are termed branched polymers. Linear structures may have side-branching.
• HDPE (High Density Polyethylene) is primarily a linear polymer with minor branching, while LDPE (Low Density Polyethylene) contains numerous short chain branches.
• Greater chain linearity and chain length tend to increase the melting point and improve the physical and mechanical properties of the polymer due to greater crystallinity.

Molecular Structures – Cross-linked, Network

• In cross-linked polymers, adjacent linear chains are joined to one another at various positions by covalent bonding of atoms. Examples are the rubber elastic materials.
• Small molecules that form 3 or more active covalent bonds create structures called network polymers. Examples are the epoxies and polyurethanes.

Thermoplastics and Thermosets

• The response of a polymer to mechanical forces at elevated temperature is related to its dominant molecular structure.
• One classification of polymers is according to its behavior and rising temperature. Thermoplastics and Thermosets are the 2 categories.
• A thermoplastic is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently.
• Most thermoplastics are high-molecular-weight polymers whose chains associate through weak Van der Waals forces (polyethylene); stronger dipole-dipole interactions and hydrogen bonding (nylon).
• Thermoplastic polymers differ from thermosetting polymers (Bakelite, vulcanized rubber) since thermoplastics can be melted again and remolded.
• Thermosetting plastics when heated, will chemically decompose, so they cannot be recycled. Yet, once a thermoset is cured it tends to be stronger than a thermoplastic.
• Typically, linear polymers with minor branched structures (and flexible chains) are thermoplastics. The networked structures are thermosets.

Examples of Thermoplastics

Specific Thermoplastic Properties

Thermoset Data

Thermoset Properties

Specific Elastomeric Properties

• Elastomers, often referred to as rubber, can be a thermoplastic or a thermoset depending on the structure. They are excellent for parts requiring flexiblity, strength and durability: such as automotive and industrial seals, gaskets and molded goods, roofing and belting, aircraft and chemical processing seals, food, pharmaceutical and semiconductor seals, and wire and cable coatings.

Thermoplastic vs Thermoset

Copolymers

• Two or more monomers polymerized together
  • Random – A and B randomly positioned along chain
  • Alternating – A and B alternate in polymer chain
  • Block – large blocks of A units alternate with large blocks of B units
  • Graft – chains of B units grafted onto A backbone

Crystallinity in Polymers

• The crystalline state may exist in polymeric materials.
• However, since it involves molecules instead of just atoms or ions, as with metals or ceramics, the atomic arrangement will be more complex for polymers.
• There are ordered atomic arrangements involving molecular chains.
• Example shown is a polyethylene unit cell (orthorhombic).

Crystal Structures

Polymer Crystallinity

• Polymers are rarely 100\% crystalline
  • Difficult for all regions of all chains to become aligned
  • Degree of crystallinity expressed as \% crystallinity
  • Some physical properties depend on \% crystallinity
  • Heat treating causes crystalline regions to grow and \% crystallinity to increase

Paper or Plastic?

• We live in a plastic society.
• Everything around us is plastic.
• Could you go for a day without plastic?
  • Toothbrush, clothing, food containers, cooking spatulas, pans, bottled water, automobile parts, bicycle parts, eyeglasses, iPod, calculator, mouse, computer parts, printer, stapler, headphones, TV, clock, flash memory housing, USB connector, keyboard, shoes, backpack parts, cell phone, credit cards.

Plastic Recycling Symbols

• In 1988 the Society of the Plastics Industry developed a numeric code to provide a uniform convention for different types of plastic containers. These numbers can be found on the underside of containers.
  • 1. PET; PETE (polyethylene terephthalate): plastic water and soda bottles.
  • 2. HDPE (high density polyethylene): laundry/dish detergent
  • 3. V (Vinyl) or PVC: Pipes, shower curtains
  • 4. LDPE (low density polyethylene): grocery bags, sandwich bags
  • 5. PP (polypropylene): Tupperware®, syrup bottles, yogurt cups,
  • 6. PS (polystyrene): Coffee cups, disposable cutlery
  • 7. Miscellaneous: any combination of 1-6 plastics