From building blocks to plastics, fibres and detergents: How polymers are made

2.1 Chains grow either one monomer at a time or by joining fragments

There are two ways to build a polymer. In chain growth, the older name for which is addition polymerisation, an initiator turns one monomer into a reactive species, usually a free radical (a fragment with an unpaired electron). That species adds monomers one after another in a fraction of a second until something stops it. The three stages are initiation, propagation and termination.

Chain length depends on the race between propagation and termination. If many radicals are started at once, they meet and kill each other early, giving short chains; long chains need slow initiation and fast propagation, thousands of times faster than termination. A growing radical can also steal a hydrogen from the middle of a finished chain, which then grows a branch. In high-pressure polyethylene, the radical end bends back into a ring-like loop and moves its radical to a carbon a few atoms down the chain; growth restarts there, leaving a four-carbon branch, which is why that polymer is full of short butyl branches.

In step growth, or condensation, molecules with reactive groups at each end join repeatedly, often giving off water. A glycol and a dibasic acid make a polyester; a diamine and a diacid make a nylon. The monomer disappears early into short pieces called oligomers, which then join each other, so chain length keeps rising the longer the reaction runs.

Functionality, the number of links each monomer can form, sets the overall shape. Two-ended monomers give linear chains that melt and can be reshaped: thermoplastics. Monomers with three or more links give cross-linked networks, thermosets, that never melt. Strength rises quickly between about 50 and 500 repeat units and changes little beyond that.

• Wittcoff (2004), Ch 15 How polymers are made · pdf pp. 486, 490-500, 538
• Kent (2012), Ch 12 Manufactured textile fibers · pdf p. 447

2.2 Catalysts decide what shape the chain takes

Until the 1950s ethylene could only be polymerised by free radicals at around 2,000 bar and 200 C, giving branched low-density polyethylene. Then, in 1954, Karl Ziegler announced that titanium tetrachloride combined with an aluminium alkyl made linear, crystalline polyethylene at about atmospheric pressure and 60 C. A titanium atom with one empty position grabs a monomer and inserts it into the growing chain, then the vacancy reappears for the next one.

Giulio Natta tried the catalyst on propylene. Free radicals had turned propylene into a useless, low-molecular-weight tangle, but the Ziegler catalyst gave long chains in which every methyl group pointed the same way. Such isotactic polymer packs into crystals and is stiff and high-melting; random, atactic polymer is a soft gum. Phillips developed a rival, chromium oxide on silica, which gives very long chains that are tamed with hydrogen as a chain stopper and about 1% butene; it works at around 100 C and 40 bar. By 2004 Ziegler-type catalysts made about 60% of HDPE in the United States and Western Europe.

Catalyst productivity has changed the economics of whole plants. The first polypropylene catalysts gave 1,000-2,000 lb of polymer per lb of catalyst. Modern ones give up to 75,000 lb, with 97% isotactic content, so neither the catalyst residue nor the atactic fraction has to be removed.

Metallocenes, commercialised by Exxon in 1991 and Dow in 1993, are single-site catalysts: every active site is identical, unlike the many different sites on a Ziegler-Natta solid. They give a very narrow spread of chain lengths and spread comonomer evenly, which means clearer, tougher films with fewer of the short chains that add taste and odour. The trade-off is poorer melt processing and an expensive methylaluminoxane co-catalyst, so in 2000 metallocene polypropylene had under 1% of its market.

• Wittcoff (2004), Ch 3 Ethylene polymers · pdf pp. 131-133
• Wittcoff (2004), Ch 4 Propylene polymers · pdf pp. 199-200
• Wittcoff (2004), Ch 15 Metal complex and metallocene catalysts · pdf pp. 515-526
• Kent (2012), Ch 12 Polypropylene fibre synthesis · pdf p. 462

2.3 Reactor design is mostly about getting the heat out

Polymerisation releases a lot of heat, and as chains grow the mix turns viscous, so heat cannot easily reach a cooling surface. Each industrial process is largely an answer to that problem. Wittcoff lists four basic procedures: bulk (monomer and initiator alone), solution (a solvent soaks up heat and thins the mix), suspension (monomer droplets stirred in water) and emulsion (monomer held in soap micelles in water).

High-pressure LDPE is a bulk process run in a tubular reactor or an autoclave at 1,300-2,600 bar and 200-300 C. Only about 30-35% of the ethylene reacts on each pass; going further would add too many branches. The rest is separated at high pressure and recycled, so the plant is built around high-pressure compression and recycle.

The gas-phase fluidised bed, first Union Carbide's Unipol process, has been licensed extensively for HDPE and LLDPE, and Wittcoff calls it the most important process in use. Small polymer particles are kept floating by a rising stream of ethylene and comonomer at about 85-105 C and 20 bar. Catalyst is sprayed in, polymer grows on the particles, and the circulating gas carries away the heat. Only 2-3% of the gas reacts per pass, so it is cooled and recompressed. Particles grow over three to five hours, then leave through a gas lock as dry powder.

Slurry processes disperse catalyst and growing polymer in a light hydrocarbon such as hexane. For polypropylene the slurry can be in liquid propylene itself, pumped round a loop of large-diameter pipe; such bulk plants were over half of all PP plants in 2000, and gas-phase plants rose from about 15% in 1990 to almost 30%. Solution processes dissolve the polymer; they fell out of use for polypropylene because of solvent losses and the difficulty of removing the last traces.

• Wittcoff (2004), Ch 3 Ethylene polymers · pdf pp. 132-134
• Wittcoff (2004), Ch 4 Propylene polymers · pdf p. 200
• Wittcoff (2004), Ch 15 Polymerisation procedures · pdf pp. 504-506
• Shreve (1984), Ch 34 Plastic industries · pdf pp. 674-676

2.4 Grades are set by chain length, branching and regularity

Producers sell dozens of grades of each polymer, and three molecular features do most of the tuning.

The first is chain length. Because chains in a sample vary, it is quoted as averages: the number average and the weight average, which leans towards the heavier chains. Their ratio measures the spread. A broad spread helps processing, since short chains act as a built-in lubricant in the melt; a narrow one, as from metallocenes, gives more uniform properties but harder processing.

The second is branching. Branches stop chains packing side by side, so they lower crystallinity, density, melting point and stiffness. Wittcoff gives HDPE 85-95% crystallinity and LDPE about 55%, with densities ranging from 0.965 down to 0.915 g/cm3. LLDPE reaches LDPE's density with short, regular branches from 6-8% butene, hexene or octene, against about 1% in HDPE; regular branches let the crystalline parts pack better, so its film is stronger and can be thinner.

The third is regularity of structure, such as tacticity in polypropylene or the cis or trans form of polybutadiene. Regular chains crystallise; irregular ones stay amorphous or rubbery.

Two temperatures follow from these features. The glass transition temperature, Tg, is where amorphous regions turn from glassy to rubbery; the crystalline melting point, Tm, is where crystals melt. A plastic used above its Tg but below its Tm is tough; below Tg it is brittle. Additives then finish the grade: Kent notes that HDPE for detergent bottles is made tougher against stress cracking by raising molecular weight or adding 1-2% butene, and that flexible PVC carries 20-80 parts of plasticiser per hundred of resin.

• Wittcoff (2004), Ch 3 Ethylene polymers, Table 3.2 · pdf pp. 132-134
• Wittcoff (2004), Ch 15 Molecular weight; Polymer properties · pdf pp. 503-504, 538-547
• Kent (2012), Ch 15 Synthetic resins and plastics, Tables 15.2, 15.5 · pdf pp. 596, 598

See it in the table

Check your understanding

In free-radical polymerisation, what happens if the initiation rate is set very high?

Why does the gas-phase polyethylene reactor convert only 2-3% of the ethylene per pass?

LLDPE and LDPE can have the same density. What gives LLDPE its higher film strength?