The question

A modified PC/ABS compound or molded housing passes the room-temperature impact test but fractures at the specified cold temperature. How should a processor or buyer identify the cause before adding more modifier or changing suppliers?

Quick answer

A room-temperature pass does not predict cold performance. PC/ABS can move rapidly from ductile to brittle behavior over a grade- and condition-specific temperature range, while notch preparation, specimen thickness, conditioning, temperature soak, impact method and loading rate can change the result. First prove that the two tests and samples are comparable. Then isolate material identity, water content and thermal history, modifier chemistry and dispersion, recycled content or contamination, weld lines, orientation and residual stress. Use one controlled baseline and change one variable at a time. There is no universal critical temperature, modifier dosage, drying schedule or impact target: confirm each for the exact grade, lot, part, method and destination requirement.

Check first

Information that changes the recommendation

  • Exact impact method and edition, apparatus, striker, specimen geometry, notch preparation, thickness, orientation, test speed and reported units
  • Room and cold test temperatures, conditioning atmosphere, cold-soak time, transfer time, specimen count, failure mode and individual results rather than an average alone
  • Whether plaques, machined bars, molded test pieces or production parts were compared, including gate position, flow direction, weld lines and wall thickness
  • Exact PC/ABS grade, formulation revision, color, flame retardant, filler, lubricant, recycled-content source and percentage, batch and manufacturing site
  • Incoming water content measured with a suitable method, drying record, hopper exposure, residence time, melt-temperature history, purge appearance, odor and color change
  • Impact-modifier chemistry, exact grade, dosage, feeding route, dispersion evidence, compatibility with the matrix and any supplier or process change
  • MFR or MVR comparison under one method and condition, plus retained samples or molecular-weight evidence where degradation is suspected
  • Failure location and surface: ductile deformation or a clean brittle break, notch root, weld line, gate, corner, boss, knit line or contamination feature
  • China source-factory identity, sample-to-order traceability, retained lot, change notification and quotation tied to the exact approved grade and production site
Practical guidance

Verify the test signal before reformulating. ISO 180 defines Izod testing for specified specimen and notch configurations, while ISO 291 addresses controlled conditioning atmospheres. Record the full method, specimen source, notch, dimensions, orientation, conditioning, cold-soak and transfer procedure. Compare individual values and fracture appearance. A cold result from a machined production part cannot be ranked directly against a room-temperature result from an injection-molded bar unless the buyer has established that comparison.

Treat temperature as a material-response variable, not a correction factor. Covestro's PC/ABS product literature shows that mechanical response changes with temperature and that a rapid impact and fracture-mode change can occur near a critical, ductile-to-brittle transition range. The location of that range depends on the exact blend, rubber phase, additives, specimen and loading conditions. Do not copy a transition temperature from another grade or a family brochure into a purchase specification.

Confirm material identity before blaming the impact modifier. PC-to-ABS balance, polymer molecular weight, rubber phase, flame retardant, pigment, filler, lubricant and recycled content can all move the property balance. Freeze one exact formulation and compare the failed lot with an approved retained lot under the same molding and test conditions. If recycled PC/ABS is used, segregate feedstock source, contamination and repeated heat history; a room-temperature pass can mask a cold-temperature margin that has narrowed.

Check water content and thermal history with evidence. Polycarbonate-containing blends can be damaged by inappropriate moisture and excessive heat or residence time, but a hopper setting is not proof that pellets were dry and a universal drying number is not safe for every grade. Use the exact supplier's current processing guidance, measure water content with a suitable method such as ISO 15512 where applicable, and compare MFR or MVR, color, odor, purge and fracture behavior against a controlled reference. Confirm the acceptable limits and method for the exact commercial grade.

Evaluate modifier chemistry, dosage and dispersion together. Dow identifies PARALOID EXL-2620 as an MBS core-shell modifier for PC and PC/ABS with low-temperature impact as a design objective; Arkema similarly positions CLEARSTRENGTH E920 for PC/ABS applications requiring low-temperature impact. These are exact-grade examples, not evidence that every MBS or acrylic powder is interchangeable. Check the candidate's shell compatibility, thermal stability, particle dispersion, dosage ladder and interaction with flame retardant, pigment, filler and recycled content. Agglomerates or a poor interface cannot be corrected reliably by increasing dosage alone.

Separate plaque performance from part performance. Weld lines, flow orientation, sharp notches, wall-thickness transitions, gate stress, packing, cooling and assembly preload can localize a brittle failure. Covestro recommends practical loading tests on actual molded parts because ideal specimens do not capture all geometry, processing and media effects. Mold a controlled plaque plus representative flow-end and weld-line specimens, map the production-part failure, and change one process or geometry variable at a time before concluding that the resin is deficient.

For a Chinese producer or source factory, make technical approval and the factory-gate quotation reference the same controlled material. Verify the producer's legal entity, plant, exact grade, formulation revision, batch, bag label, TDS, COA and retained sample. A Chinese factory-gate price or a claim of equivalent low-temperature impact is not test evidence. Require the quotation to state grade, quantity, package, Incoterm, named place and validity, and reconfirm data, capacity, lead time and compliance for the actual batch and destination.

Practical procedure

Step-by-step evaluation

  1. 01Freeze the requirement: actual service temperature, impact method, specimen or part, conditioning, acceptance rule and required fracture mode.
  2. 02Repeat room and cold tests on one retained lot with identical specimen preparation, notch, orientation, conditioning, temperature control and documented transfer time.
  3. 03Compare the failed lot with an approved retained lot and an unmodified or current-control formulation on the same molding machine and tool.
  4. 04Audit exact formulation identity, recycled feedstock, water-content measurement, drying, hopper exposure, melt history, residence time and purge evidence.
  5. 05Inspect fracture location and surface, then isolate weld line, orientation, residual stress, thickness, gate and assembly effects with controlled molding or part trials.
  6. 06Only after the baseline is stable, run a small exact-grade modifier dosage or chemistry ladder while holding matrix, additives and process constant.
  7. 07Confirm the selected balance with multiple commercial lots and representative parts at every required temperature, then approve the factory, grade, control plan and change-notification triggers.
Validation tips

What to record before approval

  • Report individual impact results and fracture mode; an average can conceal a mixed ductile and brittle population near a transition range.
  • Keep notch preparation, thickness, orientation, cold soak and transfer time identical when comparing suppliers or lots.
  • Use water-content measurement and process records rather than assuming the dryer setting proves material condition.
  • Do not infer equivalence from the labels MBS, ACR or PC/ABS; validate the exact modifier, matrix, additives and finished compound.
  • Retest more than one commercial lot and require advance notice for changes to resin source, recycled feedstock, modifier, formulation, process or site.
Authoritative sources

Primary sources and official test methods