PC/ABS Alloy or Straight Polycarbonate? A Cost, Flow and Weathering Comparison

Evaluating straight polycarbonate against a PC/ABS alloy requires moving beyond single-point data sheets. On production injection moulding lines, the comparison is resolved through melt rheology at the operating shear rate, part mass derived from density, and long-term weathering response under ISO 4892-2 or ASTM G155. General-purpose polycarbonate provides higher low-temperature ductility and better outdoor colour retention, while PC/ABS lowers melt temperature, improves mould-filling length, and reduces material cost per cubic centimetre.

Melt Rheology, Drying Boundaries and Injection Moulding Economics

Straight polycarbonate is processed as a high-temperature amorphous melt with a glass transition near 147 °C. Injection moulding machine barrel settings typically range from 270 °C in the feed zone to 300 °C–320 °C at the nozzle, and the mould is held at 80 °C–120 °C. If the mould surface is below 70 °C, frozen-in orientation increases the risk of environmental stress cracking in alcohol or solvent contact. Polycarbonate must be dried to less than 0.02% moisture with a desiccant dryer delivering a dew point of -40 °C or lower. Residual moisture above 0.03% causes splay at the gate and hydrolytic molecular weight loss, which can lower notched Izod impact from above 600 J/m to below 400 J/m after repeated regrind cycles.

PC/ABS is moulded at 240 °C–270 °C with mould temperatures from 50 °C to 80 °C. The butadiene phase limits the upper melt temperature: above 280 °C, rubber degradation generates brown streaks, black specks, and acidic volatile products. On production lines with hot-runner systems, PC/ABS requires open-flow channels and low shear at the gate; dead spots in valve-gate manifolds can produce black speck contamination within 10 min to 20 min of hold time. Capillary rheometry under ISO 11443 shows that straight polycarbonate at 280 °C and 1000 s⁻¹ may have a shear viscosity near 350 Pa·s to 500 Pa·s, whereas PC/ABS under identical conditions may fall to 150 Pa·s to 250 Pa·s. The difference is more pronounced at lower shear rates, so large gates and short flow paths are less critical for PC/ABS.

Spiral flow tests at a nominal 2 mm wall thickness and 900 bar injection pressure typically show 300 mm–450 mm for straight polycarbonate and 450 mm–650 mm for PC/ABS. The lower melt viscosity allows thinner-wall parts and multi-cavity tools with lower clamp force, but it also reduces melt strength in extrusion blow moulding or profile extrusion. When PC/ABS is compounded on a co-rotating twin-screw extruder with an L/D of 40:1, barrel temperatures are maintained between 220 °C and 260 °C; shear heating at the kneading blocks must be controlled because a melt temperature excursion above 270 °C can degrade the butadiene phase and release styrene vapour.

On a cost-per-part basis, the price-per-kilogram difference is amplified by density. Straight polycarbonate has a density of 1.20 g/cm³ per ISO 1183-1; unfilled PC/ABS typically ranges from 1.13 g/cm³ to 1.17 g/cm³. At a fixed cavity volume, the PC/ABS shot therefore consumes 3%–5% less mass. Regional spot pricing varies, but when general-purpose PC/ABS is 10%–25% lower per kilogram than straight polycarbonate, the combined mass and price effect can reduce part material cost by 15%–30%. Flame-retardant grades do not follow the same model because halogen-free phosphate additive systems raise PC/ABS density to approximately 1.20 g/cm³ and reduce flow, narrowing the gap. In addition, straight polycarbonate involves higher drying energy at 120 °C, while PC/ABS dries at 80 °C–90 °C; the energy difference is measurable on high-volume lines but is usually secondary to raw material cost.

What Does Xenon-Arc Exposure Reveal About Impact Retention After 2000 Hours?

Accelerated weathering separates the two materials at the molecular level. Straight polycarbonate undergoes photo-Fries rearrangement at the surface, producing chromophoric species that raise yellowness index. Under ISO 4892-2 xenon-arc exposure at 0.51 W/m² at 340 nm and a black-panel temperature of 65 °C, unstabilized polycarbonate may shift yellowness index by more than 10 delta YI after 2000 h. A UV-stabilized polycarbonate grade containing a benzotriazole or triazine absorber typically holds delta YI below 5 and retains more than 80% of initial tensile strength. The weathering signature is surface yellowing and haze rather than deep embrittlement.

PC/ABS weathers primarily through unsaturated butadiene oxidation. The rubber phase loses elongation and impact before the SAN/PC matrix shows tensile strength loss. Under the same xenon-arc conditions, unpigmented PC/ABS may lose more than 50% of notched Izod impact after 1000 h–2000 h, with pronounced gloss loss and chalking. Carbon black or opaque pigments reduce UV penetration and slow the effect, but the material is still not considered intrinsically weatherable. A straight polycarbonate grade with UV stabilization is therefore preferred for outdoor electrical enclosures requiring a UL 746C f1 listing; PC/ABS grades with f1 ratings are far less common.

While Straight PC Retains Low-Temperature Ductility, PC/ABS Lowers Peak Melt Temperature

Mechanical property differences are not uniform across temperature and strain rate. At 23 °C, general-purpose straight polycarbonate shows tensile stress at yield between 60 MPa and 70 MPa and notched Izod impact from 600 J/m to 900 J/m under ASTM D256. PC/ABS ranges from 45 MPa to 58 MPa tensile yield and 300 J/m to 550 J/m Izod. At -30 °C, straight polycarbonate retains a substantial ductile plateau, while standard PC/ABS can transition toward brittle fracture unless a low-temperature ABS modifier is used. This is why straight polycarbonate is specified for security glazing, medical reservoirs, and outdoor sensor housings subjected to impact.

Heat deflection temperature follows the same pattern. Straight polycarbonate under 1.82 MPa load typically exhibits 120 °C–130 °C according to ISO 75-2/A, whereas PC/ABS falls at 95 °C–110 °C. The lower heat deflection temperature of PC/ABS is acceptable in automotive interior components such as instrument panel retainers and centre console substrates, where peak air temperature rarely exceeds 100 °C. PC/ABS also offers lower gloss and easier paint adhesion in visible interior applications, but painting and adhesive bonding must account for the unsaturated rubber phase; adhesion promoters or primers are commonly required.

In transparent applications, PC/ABS is excluded. The blend is opaque even at thin sections, while straight polycarbonate offers light transmission above 88% at 3 mm thickness per ISO 13468-1. Chemical exposure also creates a distinction. Straight polycarbonate is sensitive to stress cracking in aromatic solvents, ketones, and certain amines; field failures occur when moulded-in stress is high. PC/ABS improves resistance to aliphatic hydrocarbons and dilute acids but remains incompatible with esters, ketones, and aromatic hydrocarbons. For outdoor electrical enclosures requiring a UL 746C f1 listing, the choice usually defaults to UV-stabilized straight polycarbonate with a mould temperature of at least 90 °C to reduce residual stress. For indoor electronic housings with long thin-wall flow paths, PC/ABS often outperforms straight polycarbonate in cycle time and dimensional stability.