The question

A PBT compound contains an impact modifier, but molded parts or test bars are more brittle than expected. Should the buyer immediately increase the modifier dosage or change suppliers?

Quick answer

No. PBT is a thermoplastic polyester, and excessive moisture during melt processing can hydrolyze the polymer chains and reduce molecular weight, impact resistance and elasticity. Excessive residence time or melt temperature, contamination, weak dispersion, an unsuitable modifier grade, recipe drift and unmatched test specimens can produce similar symptoms. First lock the batch, drying, transfer, extrusion, molding, conditioning and test method; then compare moisture and melt-flow or viscosity evidence with a retained control. Change modifier chemistry or dosage only after the process and measurement causes have been separated.

Check first

Information that changes the recommendation

  • Exact PBT base grade, modifier commercial grade and dosage, glass fiber or mineral level, recycled content and complete additive recipe
  • Incoming packaging condition, opening time, storage humidity, regrind handling and measured pellet water content before processing
  • Dryer type, set and actual temperature, dew point, airflow, residence time, hopper loading and closed transfer to the machine
  • Extruder and molding melt-temperature evidence, screw speed, back pressure, residence time, shutdown or restart history and purge practice
  • Pellet appearance and dispersion plus any contamination, fines, color concentrate, lubricant or flame-retardant change
  • Impact method, notch, specimen thickness, molding direction, conditioning, test temperature and actual failure mode
  • Retained raw material, pellets and parts from a known-good lot and the suspect lot for side-by-side verification
Practical guidance

Treat moisture as a measurable process variable. BASF's Ultradur processing guide states that thermoplastic polyesters such as PBT are susceptible to hydrolysis: too much moisture during melting cleaves molecular chains, reduces average molecular weight and can appear as loss of impact resistance and elasticity. The same guide gives a moisture limit for its Ultradur materials, while Celanese publishes a different acceptable level for its Celanex PBT and Vandar PBT alloys. Those brand-specific figures are useful evidence that drying matters, but they are not universal specifications. Use the current processing sheet for the exact base compound or finished grade and measure water content with an appropriate method rather than relying only on dryer settings.

Audit the whole drying path, not just the temperature display. Celanese's polyester manual explains that reduced airflow from a clogged return filter can leave resin inadequately dried and that both virgin and reground polyester must reach the recommended moisture level. Check dew point, airflow, bed residence time, hopper loading, leaks, open-container exposure and the delay between drying and melting. A good dryer setting does not prove that pellets entering the screw were dry.

Separate processing hydrolysis from later service hydrolysis. Wet pellets can degrade while molten during compounding or molding; a finished PBT part can also degrade during hot and humid service. BASF's hydrolysis-resistant PBT information describes water and elevated temperature as a service-aging mechanism and offers specially stabilized grades for demanding environments. If toughness is already low immediately after molding, investigate pellet moisture and heat history first. If initially acceptable parts deteriorate only after humidity and heat exposure, reproduce the actual aging requirement and review whether the exact compound is designed for it.

Only evaluate the modifier after the process baseline is controlled. Arkema lists CLEARSTRENGTH E920 as an MBS impact modifier for PBT and other engineering plastics, which confirms that a documented PBT-compatible grade can be a valid candidate; it does not show that every MBS, acrylic modifier or dosage will work in every filled, flame-retardant or recycled PBT recipe. Compare dispersion, interface, dosage and all formulation changes against an unmodified control and a known-good lot. Check flow, stiffness, heat, dimensions, surface and any electrical or flame requirement together with impact.

Use evidence that can distinguish chain damage from a test artifact. ISO 15512:2019 defines methods for determining water content in plastics. BASF notes that PBT degradation can be demonstrated through viscosity number or melt-volume index, while ISO 1133-1:2022 defines MFR and MVR measurement and ISO 180:2023 defines Izod impact testing. Agree the exact methods, conditioning and decision limits with the customer before interpreting results. For a China manufacturer or source-factory offer, require the exact grade, batch documents, test methods, order quantity, Incoterm and quotation validity; factory-gate price, performance and delivery must be confirmed for the specific batch and destination.

Practical procedure

Step-by-step evaluation

  1. 01Quarantine the suspect pellets and parts, record the lot identity and preserve unopened bags, compounded pellets, molded specimens and a known-good reference.
  2. 02Write the complete recipe and actual batch record, including base PBT, modifier, reinforcement, regrind, color and every additive; do not use the target recipe as proof of what was fed.
  3. 03Measure pellet water content with an agreed suitable method and audit dryer dew point, airflow, time, temperature, loading and dry transfer against the exact supplier instruction.
  4. 04Review compounding and molding heat history from actual evidence, including melt temperature, pressure or torque trends, residence time, interruptions, restarts and purge condition.
  5. 05Prepare controlled specimens from a known-good lot and the suspect lot using one frozen drying, molding, conditioning, notch and test schedule.
  6. 06Compare impact and fracture mode with water content, MFR/MVR or the agreed viscosity indicator, dispersion and the application's flow, stiffness, heat, appearance and dimensional guardrails.
  7. 07Change one variable at a time: first correct drying or heat history when evidence supports it; then screen exact documented modifier grades or dosages and repeat the leading condition in a production-representative run.
Validation tips

What to record before approval

  • Do not accept a dryer setpoint, drying certificate or bag label as a substitute for measured pellet condition at the machine feed.
  • Do not compare impact numbers unless resin recipe, specimen geometry, notch, conditioning, test temperature and molding direction match.
  • A higher MFR or MVR can support a degradation investigation, but interpret it with the exact grade, method and reference lot; it is not by itself proof of one root cause.
  • Confirm whether the requested solution concerns immediate processing damage or long-term hot-humid service because the material and validation plan may differ.
  • Approve only the exact compound and modifier grades with a documented lot-control and change-notification plan; reconfirm compliance, price and delivery by batch, quantity, Incoterm and destination.
Authoritative sources

Primary sources and official test methods