Plasticizer migration is the movement of plasticizer out of a flexible PVC compound into whatever is on the other side of its surface — the air, a solid it is pressed against, or a liquid it is immersed in. It happens because a conventional plasticizer is held in the resin by physical mixing alone: nothing bonds the ester to the polymer chain, so any gradient from the compound to a place the ester can evaporate or dissolve will move it.
How fast it happens is set by the ester, the temperature, the medium and the loading, and those four are also the levers for controlling it.
How plasticizer migration happens
Plasticizer migrates in two stages: it diffuses through the PVC matrix to the surface, then leaves the surface by whichever route is open — evaporation into air, partition into a solid it touches, or dissolution into a liquid.
- Volatilization: the ester reaches the surface and evaporates. The common esters are close to nonvolatile at room temperature; loss by this route becomes significant at processing temperatures and in hot service — cable in an engine bay, plastisol under a heat lamp.
- Exudation into a contacting solid: a lacquer, adhesive, print layer, rubber or amorphous plastic in contact with the PVC dissolves the ester at the interface, and the ester partitions across with no evaporation step at all.
- Extraction by a liquid: oil, fuel, solvent or a surfactant solution dissolves ester at the surface while diffusion from the bulk replenishes it. The rate follows the ester’s solubility in that particular liquid.
For loss to air, the step that limits the rate is not the one most formulators assume. A published ageing study of PVC-DEHP found diffusion through the polymer to be rate-limiting only above 110–120 °C; below that — every real service condition — evaporation from the surface controls the loss.
That is why ester vapour pressure and air movement matter as much as the matrix: loss in stagnant air runs at roughly a third of the rate in ventilated air.
The three routes are also three different tests. Loss to activated carbon (ISO 176, ASTM D1203) measures volatility; loss into absorbent sheets pressed against the specimen (ISO 177) measures exudation; weight loss after immersion (ASTM D1239) measures extraction. When a customer spec says “migration”, the standard it names tells you which route the customer is worried about.
What Causes Plasticizer Migration
Nothing has to go wrong for plasticizer to migrate: an unbonded ester and a gradient to somewhere it can go are cause enough. What decides whether it matters within a product’s life is the rate, and four factors set that — the ester’s molecular size and structure, temperature, the contact medium, and the loading.
Ester structure. Larger, heavier esters diffuse more slowly, have lower vapour pressure and dissolve less readily in most media. Phthalate or non-phthalate status has nothing to do with it. In a 2018 infusion-set study — 24 h at 25 °C, 50/50 ethanol/water — DINP gave up 1.10–1.67 % of its initial content and DINCH, its hydrogenated counterpart, 0.82–1.50 %; the study grouped the two together as the high migrators.
DEHT (DOTP) released 0.40–0.70 % and TOTM 0.02–0.14 %. The para-substituted terephthalate and the larger trimellitate molecule did the work; removing the aromatic ring, as DINCH does, bought nothing.
Temperature. Diffusion and evaporation both speed up with temperature, evaporation more steeply, so a compound that is stable on the shelf can lose plasticizer quickly once the part runs hot. Into a liquid the effect is just as sharp: in a blood-bag study, n-hexane extracted about 60 % of the bag’s DEHP in the first hour at 40 °C and almost 95 % at the solvent’s boiling point of 69 °C.
Contact medium. The material on the other side is a sink, and how good a sink depends on how well it dissolves the ester. For DEHP the extraction ranking was n-hexane > methanol > t-butanol > isopropanol > ethanol: the non-polar solvent took the most, and loss fell as the liquid became more polar. Plain water is a poor sink for the common esters; oils, fats, fuels and surfactant solutions are not.
Solids behave the same way. Polystyrene, ABS, polycarbonate, acrylic and nitrocellulose lacquers and many adhesives dissolve esters readily; polyolefins hardly at all.
Loading. More plasticizer means more free volume, so the ester moves faster through the matrix, and a steeper gradient to the sink. Exceeding a secondary plasticizer’s or extender’s compatibility limit is a different case: the excess is not slowly migrating, it is being rejected by the resin, and it sweats out on its own schedule whichever primary ester you chose.
Common signs of plasticizer migration
The common signs are tack, rising hardness and shrinkage on the PVC, and softening, crazing or fogging on whatever it touches — they come in pairs, what the PVC has lost and what the material next to it has gained.
On the PVC itself:
- Surface tack or an oily film: ester reaching the surface faster than it can evaporate — typical of heavy, low-volatility esters and usually the first sign in warm storage. A dry, waxy or powdery bloom is more often a lubricant or stabilizer than plasticizer.
- Rising hardness and lost cold flexibility: Shore A creeps up, and a jacket or sheet that was pliable cracks on flexing or in the cold.
- Shrinkage and curl: the lost ester was volume; thin films and calendered sheet shrink and curl as it leaves.
- Weight loss and odour: the direct signature of volatile loss in hot service.
On the neighbouring material:
- Softened, tacky or dulled coatings: lacquers, paints and printed inks against the PVC pick up the ester and lose hardness.
- Crazing of rigid plastics: polystyrene, ABS, polycarbonate and acrylic parts in contact craze or stress-crack as the ester plasticizes them.
- Adhesive failure and staining: bonds soften and let go, and migration marks appear on the substrate under flooring or leather.
- Fogging: the volatile fraction condenses as a haze on cooler surfaces — the inside of a windscreen above a PVC dashboard skin is the classic case.
Timing separates the causes. Tack that appears within days of processing points to an over-limit secondary or extender; hardness and embrittlement that develop over months of warm service are migration proper.
Can you prevent plasticizer migration
No. In a compounded PVC it can be slowed until it no longer matters within the product’s life, but it cannot be stopped, because nothing holds the ester in place except its compatibility with the resin. The only true prevention is bonding the plasticizer to the chain, and that is a multi-step synthesis with no place on a compounding line.
So the useful question is never “does it migrate” but “how much, by which route, into what, over how long” — and acceptable is whatever limit the customer’s test sets. Every measure that lowers the rate costs something in efficiency, process or surface integrity, so reduction is a trade made deliberately, not a defect removed.
How manufacturers reduce it
By pulling four levers, in the order a compounder reaches for them: a larger or better-suited ester, loading held to what the hardness spec needs, a barrier between the PVC and the sink, or a treated surface.
Ester choice. The biggest single lever. Moving up in molecular size within the plasticizer range — DOP to DINP to DIDP, DOP to DOTP, and for hot or fluid-contact service to a trimellitate — lowers loss by every route. In heart-lung machine tubing, TOTM migrated into blood at roughly 1/350 of the rate of DEHP.
Polymeric plasticizers go further still against lacquers and rigid plastics, at a price: more phr for the same Shore A, higher plastisol viscosity, poorer cold flexibility. Match the ester to the medium the part will actually face — DOA’s low release into aqueous media is no comfort for a part that sees oil.
Loading. Migration rate scales with plasticizer content, so every phr above what the hardness spec needs adds migration and nothing you can sell. The catch is that the esters which reach a hardness with the fewest phr are the small, efficient ones that migrate fastest, so loading is trimmed within an ester family, not by switching to a more efficient one.
Barrier layers. Put something the ester will not cross between the PVC and the sink. A coextruded polyethylene inner layer cut TOTM release from infusion extension lines by about ten times; a lacquer topcoat on synthetic leather or flooring, a laminated film, or simply specifying a polyolefin rather than polystyrene for the part that touches the PVC does the same job at a different scale.
Surface treatment. Crosslinking the skin by plasma, UV or gamma irradiation, grafting the surface, or depositing a thin barrier or sol-gel coating all reduce free volume at the surface and slow the flux. Each has a failure mode — an over-dosed irradiated skin embrittles and microcracks, a deposited film crazes on bends, a coating can trap solvent — and each is an extra process step, so they belong to medical tubing and high-value parts rather than commodity compound.
Conclusion
Plasticizer migration is the unbonded ester leaving the compound for wherever it evaporates or dissolves more readily, and it is a rate to be managed, not a fault to be eliminated. That rate is set by the ester’s size and structure, the temperature, the medium on the other side and the loading — and the medium is the one most often left out of the conversation. Name it first, choose the ester and the construction against it, and verify with the test the spec actually calls for.