Question 4 of 7: PVC/PE Copolymer; Polymer Processing; Rubber Vulcanization

Question 4: PVC/PE Copolymer; Polymer Processing; Rubber Vulcanization (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. (c) Rubber composition: 94 wt% polymerized chloroprene ($\text{C}_4\text{H}_5\text{Cl}$), 6 wt% sulphur; all sulphur consumed in crosslinking.

Find. (a) Why PVC/PE copolymer, crystallinity, thermoplastic/thermosetting. (b) Processing method for each of four named articles. (c) Fraction of chloroprene mer units crosslinked.

Approach

Parts (a) and (b) are conceptual polymer-processing questions; part (c) is a vulcanization stoichiometry problem — convert both the rubber mer and the sulphur to moles per 100 g, then relate moles of sulphur crosslinks to moles of mer units through an explicit, stated crosslink assumption (since the paper does not specify how many sulphur atoms form each crosslink).

1. (a) Why a copolymer, and crystallinity/thermal behaviour. Pure PVC homopolymer has a high glass transition temperature, is comparatively brittle, and its C–Cl bonds make it thermally sensitive (prone to HCl evolution/degradation) during hot processing; copolymerizing with PE improves flexibility, lowers processing viscosity, and improves impact toughness while retaining PVC's chemical/fire resistance. A syndiotactic arrangement (Cl substituents regularly alternating sides of the backbone) is a highly regular stereochemistry, so the chains can pack into an ordered lattice — the copolymer would be expected to show some (low-to-moderate) crystallinity, far more than an atactic (random) arrangement would allow, though the bulky Cl substituent and the PE comonomer segments still limit it well below PE's own crystallinity. Both PVC and PE are linear-chain molecules with no covalent crosslinks between chains, so the copolymer is thermoplastic (softens and reflows on reheating) — it is not thermosetting, which requires a covalently crosslinked, infusible network.
2. (b) Processing methods.
   • Polyethylene squeeze bottle — blow molding: a thermoplastic parison is extruded, clamped in a split mold, and inflated with air against the mold walls to form a hollow part.
   • Melamine dish — compression molding: melamine-formaldehyde resin (with filler) is loaded into a heated mold cavity and cured under heat and pressure; because it is a thermoset, it cannot be injection molded or reprocessed once cured.
   • Nylon fishing line — melt spinning followed by cold drawing: molten nylon is extruded through a spinneret to form continuous filaments, which are then drawn (stretched) to align and orient the polymer chains along the fibre axis, greatly increasing tensile strength.
   • Fibreglass boat hull — hand lay-up (open-mold contact molding): layers of glass-fibre mat/cloth are placed in an open mold and impregnated with a thermosetting resin (polyester or epoxy) by hand rolling; suited to large, low-volume, complex-shaped parts.
3. (c) Chloroprene mer and sulphur moles (per 100 g rubber). The chloroprene mer $\text{C}_4\text{H}_5\text{Cl}$ has molar mass (page-1 atomic masses: C=12.01, H=1.01, Cl=35.5) $$M_{mer}=4(12.01)+5(1.01)+35.5=48.04+5.05+35.5=88.59\ \text{g/mol}.$$ Per 100 g rubber: moles of mer $=94/88.59=1.061$ mol; moles of sulphur (S=32.1) $=6/32.1=0.1869$ mol.
4. Crosslink stoichiometry. With no crosslink-sulphur ratio stated, the standard simplifying assumption is a monosulfide crosslink (one S atom bridges two mer units). Each such crosslink consumes 1 S atom and links 2 mer sites, so $$\%\text{crosslinked}=\frac{2\times(\text{mol S})}{\text{mol mer}}\times100 =\frac{2\times0.1869}{1.061}\times100=\boxed{35.2\%}.$$
   Check

   Assumed a monosulfide (single-S-atom) crosslink since the paper does not state the average number of S atoms per crosslink; real vulcanizates typically use polysulfide bridges (2–6 S atoms), which would scale this result down proportionally (e.g. ≈17.6% for a disulfide, n_S=2). The 35.2% figure is reported under the stated, explicit monosulfide assumption.