Here is the conclusion first: a chemical load is not a single action but a combination — which reagents, applied at what temperature, for how long, and judged against what afterwards. Get any one of the four wrong and the test is meaningless however neatly it was run. The same plastic part has to survive cleaners, sunscreen and sweat when mounted in the cabin, engine oil, coolant and brake fluid in the engine bay, and de-icing salt as well near the underbody. Copying a peer's reagent list is where this type of test most often falls over.
1. Chemical load tests material compatibility, not corrosion resistance
Automotive parts meet things they should never meet at three stages — assembly, use and service: cleaning agents and release agents on the assembly line, fuel and urea solution at refuelling, brake fluid and antifreeze during service, sunscreen and insect repellent on the user's hands.
These reagents attack polymers and platings along four paths:
- Swell and softening: plastic absorbs solvent and expands while losing stiffness, and the clips and sealing features fail first;
- Stress cracking: moulding residual stress combined with reagent attack cracks the part at stress concentrations, often with no visible warning beforehand;
- Plating and coating damage: acids, alkalis and salts dull, blister and lift the plating, and contact resistance drifts with it;
- Marking and bonding failure: screen printing, labels and adhesive layers dissolve, and traceability information simply disappears.
This is not the same category of test as salt spray or damp heat. Salt spray looks for electrochemical corrosion in a chloride environment; damp heat looks for insulation degradation and electrochemical migration caused by moisture absorption (conditions and duration classes for the double-85 test); a chemical load asks whether this material is damaged by this reagent. Their failure signatures overlap little, and one conclusion cannot stand in for another.
2. The reagent list is set by mounting location, not by material grade
Choosing reagents by material is a common mistake. The supplier knows it uses PC/ABS, finds a "chemical resistance table for PC/ABS" online, picks a few reagents, runs them and calls it done. The correct order is to fix the location first and then ask what that location is exposed to.
| Mounting location | Typical reagent categories | What to focus on |
|---|---|---|
| Cabin interior parts | Cleaners, sunscreen, sweat, beverages | Appearance and feel; on light-coloured parts check discolouration and dulling first |
| Engine bay | Engine oil, coolant, brake fluid, fuel | High temperature plus reagent; seals and harness sheaths first |
| Underbody and wheel area | De-icing salt, muddy water, high-pressure cleaner | Overlaps with corrosion testing; plated parts and fasteners dominate |
| High-voltage compartment (EV) | Coolant, electrolyte leakage scenarios | Beyond appearance, insulation performance is the key criterion |
A note on sourcing: the reagent categories follow the published clause framework of GB/T 28046.5 and ISO 16750-5; for the specific reagents, concentrations, application temperatures and durations, defer to the official clauses and the OEM technical protocol rather than picking a class by experience.
German OEM projects add a layer: corporate standards such as LV 124 have their own media resistance lists and severity classes that do not map exactly onto the national standard (the differences between LV 124 and ISO 16750). Export parts follow the customer protocol; do not simply substitute the national standard's list.
3. Application method is easier to overlook than reagent type
For the same reagent, immersing and wiping differ by orders of magnitude in damage.
| Application method | Suitable for | Points to watch |
|---|---|---|
| Full immersion | Small plastic parts, O-rings, coated test panels | Use with care on parts with cavities or electronics; residual fluid that cannot be flushed out introduces false failures |
| Surface wipe / coating | Large interior and exterior trim, display covers | Wipe force, number of passes and contact time must be fixed, otherwise it cannot be reproduced |
| Vapour / atmosphere exposure | Fuel vapour, evaporating cleaner scenarios | Requires dedicated vessels; results are not interchangeable with immersion data |
Harnesses and connectors are the classic case of parts that must not be immersed whole. Once reagent gets into the terminal cavity, the only cleanup available is drying, and the residue changes contact resistance; the drift measured afterwards cannot be separated into reagent attack and residue contamination.
4. A chemical load is often pre-treatment, not the end point
In automotive validation flow, a chemical load frequently sits at the front of a sequence: let the material meet the reagent first, then stack temperature cycling, damp heat or mechanical load, and what you are looking at is whether the attacked part still withstands subsequent stress.
Reverse the order and the conclusion changes. Run the climatic test first and the surface microcracks and moisture state have already changed, which changes the reagent penetration path, so the two data sets cannot be compared. The test sequence therefore has to be fixed on the order form; do not let scheduling decide it (condition differences for mechanical loads).
5. Without clear acceptance items, the report has no conclusion
Acceptance items normally fall into four directions:
- Appearance: discolouration, dulling, blistering, cracking, tackiness, compared under a standard light source and colour chart;
- Dimensions and mass: change in critical dimensions and gain or loss of mass, used to quantify swelling and leaching;
- Function and electrical performance: functional status class, insulation resistance, contact resistance drift;
- Material property retention: retention ratios such as tensile strength and elongation at break.
When TMC LAB takes on a chemical load commission, the reagent list, the application method and the acceptance basis all go into the test plan first and are confirmed with the customer before work starts. Who supplies the acceptance basis — drawing, technical protocol or internal specification — must be written on the order form: at OEM review, acceptance items without a stated basis are not accepted.
6. A few straight answers
Chemical load is easily reduced to a formality: pick a few common reagents, soak the part, take a photo, write "no significant change", and the report is done. Such a report may survive internal review, but one question about where the reagent list came from exposes it at customer audit.
The time-saving approach is to spend the effort before submitting samples: align the mounting location, the reagents involved, the application method, the subsequent test sequence and the acceptance basis in one go. Write all five on the order form and there is normally no rework afterwards.
To work through the clause framework first, see the GB/T 28046 environmental testing and ISO 16750 standard pages; the matching material services are restricted substance screening and plating thickness and mechanical properties.
Turnaround time is arranged according to the actual project. You are welcome to state your target date when submitting samples; we will check it against the current schedule, confirm a feasible plan with you, and update you at each milestone.
