Waterborne plastic sealer additives cannot replace correct polymer identification, mold-release removal or surface treatment. First prove the plastic grade, surface age, cleanliness, wetting tension and adhesion of the full stack. Then separate wetting from flow and cure chemistry.
Grade, additives, texture and release.
Clean, treat and verify wetting.
Cure, adhesion and appearance.
Define the substrate precisely
Record polymer family and grade, filler, recycled content, molded or film form, texture, mold-release agent, cleaning, corona/plasma/flame treatment, time after treatment and storage. PP, PE, ABS, PC, PET and blends cannot share one assumed surface-energy or solvent-resistance rule.
Measure treatment, then adhesion
ASTM D2578 measures wetting tension of polyethylene and polypropylene films using specified solutions. Its own scope notes that wetting tension is not a complete adhesion measure. Use it only for applicable films and correlate the result with the actual sealer and recoat.
Separate the three additive routes
| Itinerario | Published facts | First justified use | Boundary |
|---|---|---|---|
| 3071 | 0.1–1%; water/solvent/UV; surface tension <25 mN/m | Verified spreading deficit after treatment | May stabilize foam; no actual-plastic adhesion result |
| 3345 | 100% active; 0.1–1%; water/solvent/UV | Measured film-flow or leveling defect | Must preserve recoat adhesion |
| 3467 | 100% active; waterborne 0.5–5%; OH ≤85 | Compatibility/leveling after chemistry review | Upper-limit OH cannot support exact reaction calculation |
Use 3071 for wetting, not as adhesion proof
3071 is a polyether-modified polysiloxane recommended at 0.1–1.0% and may be added in grinding, stirring or later. Its page describes wetting on polyethylene, polypropylene and polyester, but that application statement is not a quantified adhesion result for the actual grade, treatment and coating stack.
The page warns that 3071 may stabilize foam in waterborne systems. Track wet density, foam persistence, microbubbles and pinholes along with coverage. Reject a wetting gain if the cured sealer or basecoat adhesion declines.
Use 3345 for a controlled flow defect
3345 is a 100% active polyether-modified polysiloxane with a 0.1–1.0% range. Screen it only after application viscosity, atomization or roll settings, flash and film build are stable. Measure craters, orange peel, gloss and edge coverage at equal thickness.
If co-solvent is used to enhance effect, include it in plastic stress-cracking, swelling, dry, VOC and adhesion evaluation. Better leveling cannot compensate for an incompatible plastic or untreated surface.
Gate 3467 by exact chemistry
3467’s page reports OH ≤85 mg KOH/g, an upper limit rather than an exact batch value. A polyurethane reaction calculation needs batch OH, resin OH, isocyanate equivalent, solids and target NCO/OH. Other cure systems need their own reaction and bake evidence.
Without those inputs, treat 3467 as a non-stoichiometric compatibility/leveling trial. Verify cure, plastic stress response, sealer adhesion, intercoat adhesion and service performance.
Use a substrate-by-treatment matrix
| Stage | Variants | Primary measure | Stop gate |
|---|---|---|---|
| Surface | Actual grade × cleaning/treatment | Wetting tension/contact angle and coverage | Surface drifts before coating |
| Wetting | 3071 low/mid/high | Spreading, foam, pinholes | Adhesion declines |
| Flow | 3345 low/mid/high | Leveling, craters, gloss | Stress cracking or recoat loss |
| Chemistry | 3467 after data review | Cure and compatibility | Stoichiometry unresolved |
Test the complete coating stack
ASTM D3359 was developed for coatings on metal; if used on plastic, agree the limitation and method before testing. Record film thickness, conditioning, cut spacing, tape and failure plane. Add crosshatch, peel or pull methods appropriate to the component and customer specification.
Age before release
Repeat adhesion after humidity, heat, water, cleaning chemicals, UV or thermal cycling as the end use requires. Check untreated controls and parts at different times after surface treatment. Plastic additives and mold-release residues can migrate, so an immediate pass may not remain a pass.
Control application and cure
Measure wet and dry film, part temperature, flash, oven or ambient cure and the interval before the next coat. Thin spots on texture peaks and pooled film in recesses can produce different failure modes on the same molded part. Compare flat plaques with representative geometry, ribs, edges and gates.
Use the same plastic lot for the first screen, then repeat on additional production lots. Track recycled-content and color-package changes because fillers, pigments and internal lubricants can change surface behavior. A candidate should pass the expected manufacturing variation, not only a specially prepared plaque.
Verify recoat and assembly operations
Apply the actual basecoat or topcoat at minimum and maximum recoat intervals. Test masking, tape, adhesive, printing, heat staking or assembly operations where relevant. Inspect for stress cracking after cleaners and co-solvents. A sealer is successful only if it improves the entire stack without weakening the plastic component.
Decision rules
- Identify and prepare the plastic before additives.
- Use 3071 for measured wetting while monitoring foam.
- Use 3345 for flow, not as an adhesion substitute.
- Use 3467 reactively only with exact chemistry.
Information to send
Provide polymer grade, filler/recycle content, molding and release agent, cleaning and treatment, treatment-to-coat delay, resin/crosslinker, pH, viscosity, application, film build, first defect and adhesion/aging limits.
Ask Longchang for a plastic-sealer trial matrix built around your exact substrate.