From building blocks to plastics, fibres and detergents: The big bulk plastics
Understanding Chemicals / Module 6: From building blocks to plastics, fibres and detergents Part 3 of 5The big bulk plastics
Polyethylene, polypropylene, PVC, polystyrene and PET make up most plastic tonnage. They are commodities: many producers sell near-identical grades, so profit depends on cheap monomer, efficient conversion, large plants and integration with the cracker or refinery upstream. This part takes the processes and uses in turn and then looks at the cost structure.
3.1 Polyethylene and polypropylene are the cheapest and largest polymers
Polyethylene is cheap and is the largest-volume polymer. US consumption in 2002 was about 28 billion lb (12.7 Mt), 46% of it HDPE and 54% LDPE and LLDPE. LDPE's biggest use is film; its long branches make it easy to blow into film bubbles, and it is still essential for coating paperboard for milk and juice cartons. LLDPE took much of LDPE's film market and went on into stretch wrap, injection moulding and rotational moulding. HDPE's largest use is blow-moulded bottles for milk, water, detergent, antifreeze and motor oil, followed by large containers, pipe and conduit, and carrier bags.
Polypropylene is made with Ziegler-Natta catalysts at 40-80 C. Kent describes isotactic polypropylene as about 60% crystalline and melting near 165 C: rigid, strong and cheap, able to compete with costlier polystyrene and ABS. It has two weaknesses. Its tertiary hydrogens make it prone to oxidation in heat, which antioxidants slow; and its stiff chains turn brittle below room temperature. The fix for the cold is a block copolymer: polypropylene is made first, then ethylene and more propylene are added to grow a rubbery ethylene-propylene block on the same chains. About 40% of polypropylene is such copolymer. The homopolymer goes largely into fibre: carpet backing, nonwovens, rope and strapping.
Shreve's early-1980s split of polypropylene uses was injection moulding of toys, car parts and appliances 40%, fibres 35% and film 10%. The IEA's estimate, based on 2002-14 data, puts packaging at 36% of world plastics demand and construction at 16%, with polyethylene and polypropylene the largest resins: PP about 18% of plastics, LDPE and LLDPE 17%, HDPE 14%.
| Polymer | Typical process | Density (g/cm3) | Crystallinity | Largest uses |
|---|---|---|---|---|
| LDPE | High-pressure tubular or autoclave, free radical | 0.915-0.925 | About 55% (Kent: 30-40%) | Film, carton coating |
| LLDPE | Gas phase, slurry or solution with 6-8% comonomer | 0.915-0.925 | About 55% | Stretch and packaging film, liners |
| HDPE | Gas phase, slurry or solution, about 1% comonomer | 0.945-0.965 | 85-95% (Kent: 50-90%) | Bottles, pipe, large containers, bags |
| Polypropylene homopolymer | Liquid propylene loop or gas phase | About 0.92 (as fibre) | About 60% | Fibre, film, injection moulding |
| PP impact copolymer | Second reactor adds ethylene-propylene rubber block | Slightly lower | Lower | Uses needing low-temperature impact strength |
- Wittcoff (2004), Ch 3 Ethylene polymers · pdf pp. 131-134, 165
- Wittcoff (2004), Ch 4 Propylene polymers and copolymers · pdf pp. 199-201
- Kent (2012), Ch 15 Synthetic resins and plastics · pdf pp. 596-598
- Shreve (1984), Ch 34 Plastic industries · pdf p. 675
- IEA (2018), Ch 1 · pdf pp. 19-20
3.2 PVC and polystyrene hang on chlorine and benzene chains
PVC starts with a clever piece of chlorine bookkeeping. Ethylene and chlorine react in liquid ethylene dichloride over dissolved iron chloride at 40-70 C and 4-5 bar to give more EDC. EDC is then cracked to vinyl chloride monomer (VCM) and hydrogen chloride. HCl is corrosive and hard to ship, so it is fed to an oxychlorination reactor where ethylene, two moles of HCl and air pass over copper chloride with potassium chloride at about 225 C and 2-4 bar to make more EDC. With half the EDC from each route, no chlorine is wasted. HCl piped from MDI and TDI plants supplied about 30% of US oxychlorination needs in 2000. VCM used about 20% of US chlorine.
Kent says about 90% of PVC is made by suspension polymerisation, giving a porous powder that soaks up additives, and about 10% by emulsion and spray drying for pastes. Rigid PVC is polymerised at 55-70 C and compounded with tin stabilisers, lubricants and impact modifiers; it goes into pipe, siding and window frames. Flexible PVC is polymerised at 40-55 C and mixed with 20-80 parts of plasticiser per hundred of resin for flooring, cable insulation and hose. PVC holds around 70% of the plastics construction market.
Polystyrene starts with ethylbenzene, now made over ZSM-5 zeolite in the vapour phase at about 400 C and 18 bar with 99.5% selectivity. Dehydrogenation to styrene is endothermic and needs 550-600 C and low pressure; superheated steam supplies heat, dilutes the feed and limits coking. Styrene is polymerised by continuous bulk polymerisation. Plain polystyrene is clear but brittle, so 2-10% of dissolved rubber gives high-impact polystyrene, over half the market. Expanded polystyrene, swollen with about 10% pentane, foams to densities as low as 0.01.
C2H4 + Cl2 → C2H4Cl2 in liquid EDC, FeCl3 catalyst, 40-70 C, 4-5 bar
2 EDC crackingC2H4Cl2 → C2H3Cl + HCl; HCl sent to oxychlorination
3 OxychlorinationC2H4 + 2HCl + 0.5O2 → C2H4Cl2 + H2O over CuCl2/KCl, about 225 C, 2-4 bar, often fluid bed
4 VCM purificationDistil VCM; recycle uncracked EDC
5 Suspension polymerisationVCM droplets stirred in water with initiator; 55-70 C for rigid grades, 40-55 C for flexible
6 Strip, dry and compoundRecover unreacted VCM; dry porous powder; add stabiliser, plasticiser, fillers
Outputs PVC resin1 t Water from oxychlorinationby-productOverall: C2H4 + 0.5Cl2 + 0.25O2 → C2H3Cl + 0.5H2O. Per-tonne figures are stoichiometric arithmetic, not plant data.
Source: Wittcoff (2004), Ch 3; Kent (2012), Ch 15, pdf pp. 145-147; pdf pp. 598, 624- Wittcoff (2004), Ch 3 Vinyl chloride; Styrene · pdf pp. 145-147, 158-161
- Kent (2012), Ch 15 Synthetic resins and plastics · pdf pp. 598-599, 624
- Wittcoff (2004), Ch 15 Graft copolymers · pdf p. 515
- Brennan (2020), Ch 7 Economies of scale · pdf pp. 188-189
3.3 Commodity polymer economics come down to monomer, scale and integration
In a commodity polymer the monomer is most of the cost and conversion is a thin layer on top. Wittcoff gives the pieces. About 75% of EO's cost is ethylene. Variable costs, meaning feedstock and utilities less co-product credits, are about half of a cracker's total cost. Labour is tiny: about 2.5% of the cash cost of HDPE, 3.5% of benzene, 5.7% of PTA and 7.3% of acrylonitrile.
Scale is the second lever. A vessel's volume grows faster than its surface, so cost rises more slowly than capacity; for whole petrochemical plants Wittcoff quotes an exponent of about 0.6, and doubling a unit raises labour by only 10-20%. Plant sizes grew accordingly: typical ethylene crackers went from 70 million lb a year in 1951 to 1 billion lb in 1972 and 2 billion lb (about 0.9 Mt) at Port Arthur in 2001; Brennan notes Dow's 1.5 Mt-a-year cracker of 2017. A single polypropylene line could reach 770 million lb a year (about 350 kt) by 2004.
Integration is the third. The IEA reports that more than three-quarters of China's capacity for ethylene, propylene and aromatics sits in or near refineries, and that for integrated refiners the path to ethylene and polyethylene can earn higher margins than fuels. It adds that combining refinery and cracker saves feedstock transport, overheads and staff, offers synergies in utilities and logistics, and above all lets energy supply and demand be matched across the site.
The IEA expects key thermoplastics to grow from about 350 Mt in 2017 by nearly 70% to 2050, and average production of these plastics to rise from about 47 to over 60 kg per person. Rich countries use 55-80 kg each and some appear to level off near 60 kg; some developing regions use as little as 4 kg. India's share of world plastic use is projected to more than double.
| Item | Figure | Year and source |
|---|---|---|
| Ethylene share of EO cost | About 75% | Wittcoff 2004 |
| Labour share of cash cost: HDPE / PTA / acrylonitrile | 2.5% / 5.7% / 7.3% | Wittcoff 2004 |
| Capital cost scaling exponent, whole plant | About 0.6 | Wittcoff 2004 |
| Polypropylene catalyst yield | 1,000-2,000 lb to 75,000 lb polymer per lb catalyst | Wittcoff 2004 |
| Largest single PP line | 770 million lb a year (about 350 kt) | Wittcoff 2004 |
| Ethylene and BTX per t PET | About 0.2 t ethylene and 0.6 t BTX | IEA 2018 |
| Ethylene, BTX and propylene per t PVC (with plasticiser) | About 0.4 t, under 0.1 t, 0.1 t | IEA 2018 |
| World key thermoplastics | About 350 Mt (2017), nearly +70% by 2050 | IEA 2018 |
- Wittcoff (2004), Ch 1 Capital intensity and economies of scale · pdf pp. 58-59
- Wittcoff (2004), Ch 2 Economics of steam cracking · pdf p. 98
- Wittcoff (2004), Ch 4 Propylene polymers · pdf p. 199
- Brennan (2020), Ch 2; Ch 7 · pdf pp. 56, 188
- IEA (2018), Ch 1, Ch 2, Ch 4, Ch 5 · pdf pp. 18-20, 39-40, 72-75, 99
See it in the table
H₂O WaterCu CopperCl2 ChlorineH2 HydrogenC2H4 EthyleneC3H6 PropyleneBz BenzeneSM StyreneVCM Vinyl chloride monomerACN AcrylonitrilePP PolypropylenePVC PVCABS ABSCheck your understanding
About 40% of polypropylene is made as a block copolymer with an ethylene-propylene segment. Why?
The rubbery ethylene-propylene block lowers crystallinity and toughens polypropylene, whose homopolymer becomes brittle below room temperature.What is the purpose of running direct chlorination and oxychlorination side by side in a VCM plant?
Each mole of EDC cracked gives one mole of HCl. Oxychlorination uses two moles of HCl per mole of EDC, so making half the EDC each way balances the chlorine.A plant doubles in capacity. With a scaling exponent of 0.6, roughly how does capital cost per tonne change?
Total capital rises by 2^0.6 = 1.52, so capital per tonne is 1.52/2 = 0.76 of the original, about 24% lower.