Question 2 of 7: PVC–PE Copolymer, Plastic Processing Methods, and Chloroprene Rubber Crosslinking

Question 2: PVC–PE Copolymer, Plastic Processing Methods, and Chloroprene Rubber Crosslinking (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. (a) A PVC–PE random copolymer, normally syndiotactic in its vinyl-chloride segments. (b) Four finished articles: PE squeeze bottle, melamine dish, nylon fishing line, fibreglass boat hull. (c) Rubber: $94\%$ by weight polymerized chloroprene (repeat unit $\text{CH}_2\text{CCl}{=}\text{CHCH}_2$, i.e. C$_4$H$_5$Cl), $6\%$ by weight sulphur; all sulphur reacts to form crosslinks.

Find. (a) Rationale for copolymerizing, expected crystallinity, and thermoplastic/thermosetting behaviour. (b) The shaping process for each article. (c) The fraction of chloroprene mer units that are crosslinked.

Approach

(a)–(b) are structure–property and processing-selection questions answered from polymer-chemistry reasoning. (c) is a stoichiometric mass-balance: convert the given weight fractions to moles of chloroprene mer and moles of sulphur on a 100 g basis, then use the assumption that one sulphur atom bridges two chloroprene mer units per crosslink (a mono-sulfidic bridge, the simplest crosslink consistent with "all the sulphur is utilized in crosslinking" and no additional information on chain length per crosslink).

1. (a) Why copolymerize PVC with PE? Homopolymer PVC is normally atactic, glassy at room temperature (T$_g\approx80^\circ$C), and inherently rigid/brittle without added plasticizer; it also has poor melt flow and is thermally sensitive (dehydrochlorination/HCl release) during processing. Copolymerizing with the flexible, non-polar ethylene mer lowers the effective $T_g$, improves impact toughness and melt processability, and reduces raw-material cost — without needing a large fraction of small-molecule plasticizer that can migrate out and embrittle the part over time.
2. (a) Crystallinity of the syndiotactic copolymer. Tacticity is the datum the question supplies, and it is the controlling one here: syndiotactic means the chlorine substituents alternate regularly from side to side along the backbone, so the chains have the stereochemical regularity that ordered packing into a crystal lattice requires. Commercial PVC homopolymer is atactic (random substituent placement) and is therefore essentially amorphous; a syndiotactic chain of the same chemistry crystallizes far more readily. So yes — expect the copolymer to be crystalline, though only partially, with a low-to-moderate degree of crystallinity rather than the high crystallinity of linear PE. Two things hold it down: the bulky C–Cl side group hinders close chain packing, and the interspersed ethylene mers break up the chemical-sequence regularity along the backbone (a semicrystalline copolymer crystallizes only over its regular runs — sequence irregularity, if it were severe enough or the copolymer fully random, would suppress crystallinity altogether regardless of tacticity). The expected answer is therefore a semicrystalline (partially crystalline) material, much more crystalline than atactic PVC but well short of PE.
3. (a) Thermoplastic or thermosetting? Copolymerization does not introduce any covalent crosslinks between chains — the PVC–PE copolymer remains a linear/branched chain held together only by secondary (van der Waals) interactions, so it softens and flows on heating and can be melt-processed and reprocessed repeatedly: it is thermoplastic.
4. (b) PE squeeze bottle — blow molding. A thermoplastic parison (a short, thick-walled tube of molten PE) is extruded or injection-moulded, clamped inside a two-piece hollow mould, then inflated with compressed air so it expands to take the shape of the mould cavity and cools against the mould wall — the standard route for hollow thermoplastic containers.
5. (b) Melamine dish — compression moulding. Melamine–formaldehyde is a thermosetting resin. A measured charge of resin (often with a cellulose filler) is placed in an open, heated mould cavity; the mould is closed under pressure, and heat plus pressure drive the irreversible cross-linking (curing) reaction that locks the part into its final rigid shape. Once cured it cannot be remelted — only the still-uncured charge can be shaped.
6. (b) Nylon fishing line — melt spinning and drawing. Molten nylon (a thermoplastic polyamide) is extruded through the small round holes of a spinneret to form continuous filaments, which are then cold-drawn (stretched well beyond the yield point at a temperature below $T_m$) to align and straighten the polymer chains along the fibre axis — this drawing step is what gives the fishing line its high tensile strength and stiffness relative to the undrawn extrudate.
7. (b) Fibreglass boat hull — hand lay-up (open-mould contact moulding). A thermosetting resin (typically unsaturated polyester, sometimes epoxy) is applied in alternating layers with continuous or chopped glass-fibre reinforcement (mat or woven roving) over an open, hull-shaped mould, rolled to remove air/wet out the fibres, and cured (cross-linked) at or near room temperature. Hand lay-up (or the mechanized spray-up variant) suits large, complex, low-to-moderate production volumes like boat hulls, where a closed-mould, high-pressure process would be uneconomical.
8. (c) Moles of chloroprene mer and sulphur (100 g basis). Repeat-unit mass $M_{\text{Cl}}=4(12.01)+5(1.0)+35.45=88.49$ g/mol. On a $100$ g basis ($94$ g chloroprene, $6$ g S; $M_S=32.06$ g/mol): $$n_{\text{Cl}}=\frac{94}{88.49}\approx1.0623\ \text{mol},\qquad n_S=\frac{6}{32.06}\approx0.1871\ \text{mol}$$
9. (c) Fraction crosslinked. With one sulphur atom bridging two chloroprene mer units per crosslink, the number of mer units tied into a crosslink is $2n_S$: $$f_{\text{crosslinked}}=\frac{2n_S}{n_{\text{Cl}}}=\frac{2(0.1871)}{1.0623}$$ $$\boxed{f_{\text{crosslinked}}\approx0.352\ \ (35.2\%\ \text{of the chloroprene mer units})}$$