Question 8 of 8: ABS versus hardened PVC for an engine-mounted box

Question 8: ABS versus hardened PVC for an engine-mounted box (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 box mounted on a bracket attached directly to an automobile engine, with two candidate thermoplastics: ABS (acrylonitrile–butadiene–styrene) and hardened, that is unplasticised or rigid, PVC (polyvinyl chloride).

Find. (a) a comparison of the two against the six stated criteria, and (b) a reasoned selection.

The service environment is the specification, and it must be written down first. A box bolted to an engine is not a general-purpose enclosure. Under the hood of a car the ambient air reaches 90 to 105 °C in traffic on a hot day, and a component clamped to the engine itself, within radiant view of the exhaust manifold, routinely sees 120 °C and can exceed it during a post-shutdown heat soak. In Canadian service the same part must also survive a cold start at −40 °C, so the low-temperature end of the range matters as much as the high. The part is bolted to a running engine, so it is vibrated continuously over a broad frequency band, which makes fatigue crack initiation at stress concentrations — boss roots, snap features, mounting holes — a real failure mode rather than a theoretical one. It will be splashed with engine oil, coolant, washer fluid, road salt and, in a fuel-system bay, petrol vapour. And it will be handled during service work, so it must tolerate being knocked with a spanner at −20 °C without shattering.

(a) The comparison, criterion by criterion.

Strength. The two are closer than intuition suggests. Rigid PVC is the stronger and considerably the stiffer of the pair, with a tensile strength of roughly 45 to 55 MPa and a modulus of 2.4 to 4.1 GPa; ABS runs 40 to 50 MPa at 2.0 to 2.6 GPa. Neither is a structural material in the metallic sense, and for a box carrying its own contents both are ample at room temperature. The distinction that matters is that PVC's advantage evaporates with temperature far sooner than ABS's does, and that both creep under sustained load, so a bolted joint in either must be designed with a metal insert or a compression limiter rather than clamping the polymer directly.

Impact resistance. This is ABS's decisive advantage and the reason the material exists. ABS is a two-phase polymer: a rigid styrene–acrylonitrile matrix with a dispersed butadiene rubber phase whose particles craze and cavitate ahead of a crack tip and absorb energy. Notched Izod values of 200 to 400 J/m are routine, and useful toughness is retained down to about −40 °C. Unmodified rigid PVC — which is what “hardened” PVC means — is notch-sensitive and brittle, typically 30 to 150 J/m, and its own glass transition sits near 80 °C so that below about 5 °C it becomes markedly more brittle still. Impact-modified PVC grades exist and close much of this gap, but they do so by adding exactly the kind of rubbery phase that hardened PVC is defined as omitting.

Manufacturing methods. ABS is one of the easiest engineering thermoplastics to process. It injection moulds over a wide window at 200 to 260 °C with low shrinkage and good dimensional reproducibility, extrudes, thermoforms, machines cleanly, solvent- or ultrasonically-welds, takes threaded inserts and moulded-in snap features, and is uniquely easy to electroplate — so a ribbed box with bosses, a sealing lip and integral clips is a single tool. Rigid PVC is a more difficult moulding proposition. It begins to dehydrochlorinate not far above its processing temperature, releasing hydrogen chloride that corrodes tooling and screws, so it needs heat stabilisers, stainless or chrome-plated flow paths, tight temperature control and generous, well-radiused gating; its natural home is continuous extrusion of constant sections such as pipe and window profile rather than complex injection mouldings. It does, however, solvent-cement beautifully and machines well.

Chemical resistance. Here the ranking reverses, and it is PVC's one clear win. Rigid PVC is highly resistant to acids, alkalis, salt solutions, alcohols, oils, greases and aliphatic hydrocarbons including petrol, which is precisely why it dominates chemical pipework. ABS is good in aqueous acids, alkalis and salts but is attacked by aromatic and chlorinated hydrocarbons, esters, ketones and many brake fluids, and it swells and softens in prolonged contact with petrol and some engine oils. ABS is also poor in sunlight: the butadiene phase oxidises and the surface chalks and embrittles unless the grade is UV-stabilised or painted. PVC is inherently flame-retardant, with a limiting oxygen index near 45 against about 18 for ABS — a genuine consideration in an engine bay, though one usually met by grade selection rather than by base-polymer choice.

Heat resistance. This is the criterion that decides the question. Rigid PVC has a deflection temperature under load of roughly 60 to 70 °C at 1.8 MPa and a continuous service ceiling near 60 °C; ABS reaches 85 to 100 °C depending on grade, with heat-resistant grades at the upper end and a continuous service ceiling of 80 to 85 °C. Set those numbers against an underhood environment of 90 to 125 °C and the conclusion is stark: PVC softens well inside the ordinary operating range, will creep under its own mounting loads, and will progressively lose hydrogen chloride, which then attacks nearby metal. ABS does not clear the range either, but it reaches the bottom of it, which means a cooler mounting position or a heat shield can bring it into specification.

Cost. Rigid PVC is among the cheapest of all thermoplastics on a per-kilogram basis, typically 30 to 50 per cent below ABS. That advantage narrows once the stabiliser package, the corrosion-resistant tooling and the tighter process control are costed in, and for a moderate-volume moulded box the difference in finished part cost is small — and irrelevant if the part will not survive.

(b) Selection. Of the two candidates offered, ABS is the correct choice, on three grounds that are not close. It has the higher deflection temperature by some 25 to 30 °C, which is the only criterion that can disqualify a material outright here. It is far tougher, both at room temperature and at the −40 °C cold start, which matters for a part that is permanently vibrated and occasionally struck. And it is the far easier material from which to injection mould a ribbed, bossed, sealed box in one tool. PVC's advantages — chemical resistance, flame retardancy and raw-material price — are real, but they are secondary to a heat-resistance shortfall that cannot be designed around at the temperature in question.

The selection must nevertheless be qualified, and a professional answer says so. A heat-resistant ABS grade should be specified rather than a general-purpose one; the bracket should place the box away from the exhaust manifold and turbocharger, with a radiant shield if the layout forces proximity; the box should be kept out of direct oil and fuel wetting, or a coating or liner used where splash is unavoidable; and metal compression limiters should carry the bolt loads so that creep in the polymer cannot relax the joint. If the survey of the actual installation shows sustained temperatures above about 85 °C or unavoidable fuel contact, then neither shortlisted material is appropriate and the short list itself is wrong: the correct family is a semi-crystalline engineering thermoplastic, typically 30 per cent glass-filled PA66, PBT or PPA, or PPS for the hottest positions, all of which hold their properties to 150 °C and above and are indifferent to hydrocarbons.