Selecting Polymers & Modern Developments
Selection Criteria for Polymers
Choosing the right polymer for a product requires designers to balance several competing factors. No single polymer is perfect for every application, so the selection process involves a structured evaluation of function, aesthetics, cost, availability and environmental impact. In an exam, you are expected to justify a polymer choice by linking specific properties to specific functions — not simply listing generic advantages of 'plastic'.
- Function — the polymer must withstand the forces and conditions it will encounter in use. A structural bracket needs tensile and compressive strength; a flexible hose needs elasticity; a saucepan handle needs heat resistance. Thermosets are chosen where permanent rigidity at high temperature is critical; thermoforming polymers where flexibility, impact resistance or recyclability matters.
- Aesthetics — many thermoforming polymers are naturally transparent (acrylic, PET) or easily pigmented to any colour (HIPS, ABS, PP). Surface finish can range from high gloss to matte. Self-finishing polymers (acrylic, ABS) require no secondary paint stage, reducing cost and production time.
- Cost — commodity polymers such as LDPE and HIPS are very cheap per kilogram; engineering polymers such as PEEK or ABS cost more but offer superior performance. High-volume injection moulding dramatically reduces unit cost, making polymers economical for mass production even when tooling costs are high.
- Availability — most commodity polymers are widely available in standard stock forms from plastics suppliers. Specialist polymers may need to be ordered with longer lead times, affecting production schedules.
- Environmental impact — designers consider whether the polymer is recyclable (all thermoforming polymers carry a recycling code), whether it contains harmful additives such as plasticisers, and whether lower-impact alternatives such as biopolymers are suitable for the application.
Stock Forms of Polymers
Polymers are supplied in a range of stock forms that suit different manufacturing processes:
- Sheet — flat panels of a set thickness; used in vacuum forming, line bending and fabrication. Acrylic, HIPS and ABS are commonly bought as sheet.
- Rod and tube — used for turning on a lathe or for structural/fluid-carrying applications. Nylon and acrylic rods are machined in schools and workshops.
- Granules and powder — the raw feedstock for injection moulding and extrusion. Polymer granules (also called pellets) are fed into the hopper of a moulding machine and melted before forming.
- Film and tape — very thin sheet used for packaging, electrical insulation and adhesive tapes. LDPE and PET are commonly used as film.
Polymers are supplied as sheet, rod, tube, granules/powder and film. The stock form dictates which manufacturing process is used: sheet suits vacuum forming and line bending; granules suit injection moulding and extrusion.
Additives
Raw polymers are rarely used in their pure form. A range of additives are incorporated during compounding to tailor the final material's properties:
- Plasticisers — small molecules that insert between polymer chains, reducing the forces between them and making the material more flexible. PVC is inherently rigid (uPVC) but becomes the flexible vinyl used in electrical cable insulation when plasticisers are added. Some plasticisers (phthalates) have raised health concerns, driving development of safer alternatives.
- Stabilisers — prevent degradation caused by heat during processing or UV light in service. Without UV stabilisers, outdoor PVC would become brittle and yellow within a few years. Stabilisers are critical in window frame extrusions.
- Pigments — disperse evenly through the polymer melt to provide consistent colour throughout the material's cross-section, unlike paint which only coats the surface. ABS and PP products are pigmented to eliminate painting entirely.
- Flame retardants — added to polymers used in electrical products and construction to slow the spread of fire. Important in ABS used in computer housings and in cable insulation.
When selecting a polymer, a designer must consider the type of stress the product will experience in service. Tension (pulling apart) — PET bottles must withstand internal pressure. Compression (squeezing) — HDPE cutting boards resist the compressive force of knife blades. Torsion (twisting) — a PP living hinge experiences repeated torsion at its fold point. Bending — an acrylic shop sign bracket must resist the bending force of its own weight and wind loading. Shear (forces acting in opposite directions across a cross-section) — snap-fit joints in ABS products are designed to withstand shear. Selecting a polymer without considering these stresses can lead to premature product failure and costly recalls.
Modern and Smart Polymers
The polymer industry continues to develop new materials with properties that respond to environmental stimuli or significantly reduce environmental impact:
- Photochromic polymers — change colour in response to UV light. Used in eyeglass lenses that darken outdoors and lighten indoors. The photochromic molecules are either mixed into the polymer or applied as a coating.
- Shape-memory polymers — can be deformed and then recover their original shape when heated to a trigger temperature. Used in medical stents and self-healing aerospace components.
- Polylactic acid (PLA) — a biopolymer derived from fermented plant starch (typically corn or sugarcane). PLA has similar clarity and rigidity to PET but is industrially compostable under the right conditions. Widely used for 3D printing filament, disposable cups and food packaging. Critically, PLA does not biodegrade in home compost or landfill — it requires industrial composting at 58°C or above.
- Biodegradable polymers — a broader category designed to break down through microbial action. Oxo-degradable plastics (conventional polymer with an additive) fragment into microplastics rather than truly biodegrading — they are now banned in the EU. True biodegradable polymers such as PHA (polyhydroxyalkanoate) are produced by bacteria and break down in soil and marine environments.
PLA (polylactic acid) is made from plant starch, not crude oil, making it a renewable biopolymer. It is industrially compostable but does not biodegrade in home compost or landfill — a critical distinction the exam tests. Photochromic polymers darken in UV light and are used in self-tinting lenses.
Biopolymers such as PLA are often marketed as environmentally friendly, but their benefits depend heavily on end-of-life infrastructure. In many countries, industrial composting facilities are not widely available, so PLA ends up in landfill alongside conventional plastics — where it does not biodegrade. Furthermore, growing crops for PLA requires land, water, pesticides and fertilisers, all of which have their own environmental costs. PLA also contaminates PET recycling streams if incorrectly sorted. A designer choosing PLA must therefore consider whether the product will actually enter an industrial composting system, and whether a recycled conventional polymer might have a lower overall carbon footprint in a given context. The concept of a complete lifecycle assessment (LCA) is central to evaluating biopolymer claims honestly.