Migration of phthalate plasticisers in heritage objects made of poly(vinyl chloride): mechanical and environmental aspects

This paper investigates the migration of ortho-phthalate plasticisers in heritage PVC objects to establish that surface cleaning is generally safe and beneficial, while proposing a step-by-step protocol for non-destructive assessment to guide conservation decisions.

The Gist

The Big Question: To Clean or Not to Clean?

Imagine you have a vintage plastic toy or a piece of modern art made from a soft, flexible plastic called PVC (the same stuff old shower curtains or vinyl records are made of). Over time, these objects start to feel sticky, look greasy, or attract a lot of dust. This happens because the "softener" inside the plastic is slowly leaking out.

Conservators (the people who take care of museum treasures) face a tough dilemma: Should they wipe off this sticky goo?

• If they wipe it, will they make the object break faster?
• If they leave it, will the sticky surface ruin the object's look and fill the room with bad air?

This paper tries to answer that question by looking at how the "softener" (called a plasticiser) moves inside the plastic.

The Two Ways Plastic Moves

Think of the plastic object like a sponge filled with water (the softener). There are two ways the water can leave the sponge:

1. The "Evaporation" Mode (Surface Emission): Imagine the water is already sitting right on the surface of the sponge, ready to evaporate into the air. If you wipe the surface, you just remove what was already about to leave. It doesn't change how fast the rest of the water leaves the sponge.
2. The "Diffusion" Mode (Inner Migration): Imagine the water is trapped deep inside the sponge. It has to slowly crawl through the sponge's holes to get to the surface. If you wipe the surface clean, you create a "dry" zone. This makes the water inside rush to the surface faster to fill the gap, potentially causing the sponge to dry out and crack unevenly.

The paper asks: Which mode is happening in our museum objects?

The Science Detectives: Simulations and "Magnetic Scans"

To figure this out, the researchers didn't just wait 50 years to see what happens. They used two high-tech tools:

• Molecular Dynamics (The Virtual Lab): They built a computer model of the plastic and the softener molecules. It's like running a super-fast movie of billions of tiny molecules bouncing around. They watched how fast the softener molecules could wiggle through the plastic chains.
• NMR Diffusometry (The Magnetic Scan): They used a powerful magnetic scanner (like a very specific MRI for materials) to measure how fast the molecules actually move in real plastic samples. This confirmed their computer models were accurate.

The Key Discovery: Size Matters

The researchers found that the answer depends heavily on how big the softener molecule is and how much of it is in the plastic.

They looked at different types of softeners (called ortho-phthalates). Some are small and light (like DBP), and some are large and heavy (like DEHP).

• The Heavy Hitters (DEHP, DINP): These are the most common softeners in museum collections (about 90% of them). The paper found that for these large molecules, the process is usually "Evaporation-Controlled."

  • The Analogy: Think of a crowd of heavy people trying to leave a room through a door. They are so big and slow that they can't move through the crowd (the plastic) very fast. The bottleneck is just getting out the door. Wiping the door (cleaning the surface) doesn't make them move faster through the crowd.
  • The Result: For most museum objects, it is safe to gently clean the surface. It won't make the object brittle or crack. It actually helps by removing the sticky dust trap and stopping the plastic smell from spreading to other objects.

• The Lightweights (DBP, DEP): These are smaller molecules found in fewer objects (about 10%). For these, the process is often "Diffusion-Controlled."

  • The Analogy: These are like tiny mice running through the walls of the house. They can move through the plastic very easily. If you wipe the surface clean, the mice inside rush to the surface immediately, which might dry out the outer layer of the plastic and make it brittle or crack under stress.
  • The Result: For these specific, rarer objects, you have to be very careful about cleaning, as it might speed up damage.

The "Glassy" vs. "Rubbery" State

The paper also explains that plastic can be in two states, like butter:

• Rubbery: Soft and flexible (like butter at room temperature).
• Glassy: Hard and brittle (like butter straight out of the fridge).

Most museum objects are in the "Glassy" state because they have lost some softener over time. However, the researchers found that even in this hard state, the large softeners (the 90% majority) still behave like the "heavy people" at the door. They don't rush to the surface just because you wiped it.

The Practical Guide for Museums

Based on this, the authors propose a simple 3-step checklist for museum workers to decide if they can clean an object:

1. Identify: Use a handheld scanner (like a high-tech flashlight) to see what kind of softener is inside.
2. Estimate: Calculate if the object is "rubbery" or "glassy" based on how much softener is left.
3. Decide:
   • If it's a large softener (which is true for 90% of objects), go ahead and gently clean it. It won't hurt the object, and it will stop the air pollution caused by the sticky smell.
   • If it's a small softener (rare), be very careful, as cleaning might speed up damage.

The Bottom Line

The paper concludes that for the vast majority of plastic heritage objects, cleaning is safe. The fear that wiping off the sticky surface will cause the object to crack is mostly unfounded for the most common types of plastic. Cleaning actually helps the museum environment by reducing the "plastic smell" (air pollution) and keeping dust off the art.

The researchers also proved that using computer simulations and magnetic scans is a much faster and more accurate way to predict how plastic ages than the old methods of waiting for objects to rot or soaking them in chemicals.

Technical Summary

Technical Summary: Migration of Phthalate Plasticisers in Heritage PVC Objects

Problem Statement
The preservation of heritage objects made of plasticised poly(vinyl chloride) (PVC) is complicated by the migration of plasticisers, primarily ortho-phthalates. This migration leads to aesthetic degradation (surface exudates, stickiness, dust accumulation) and mechanical instability (embrittlement, cracking). A critical dilemma for conservators is determining whether the removal of surface plasticiser exudates accelerates the overall deterioration rate. Theoretical models suggest that if migration is diffusion-controlled, cleaning the surface increases the concentration gradient and accelerates loss; conversely, if migration is evaporation-controlled, cleaning has minimal impact on the migration rate. Furthermore, the release of plasticiser vapours poses environmental risks to other objects in the same enclosure and raises safety concerns regarding hazardous substances like DEHP. Current literature lacks a unified, evidence-based framework to predict the rate-limiting step (diffusion vs. evaporation) for various plasticisers under typical museum conditions, and existing methods for determining diffusion coefficients often suffer from overestimation due to solvent-induced swelling.

Methodology
The study employs a multi-faceted approach combining experimental characterization, computational modeling, and theoretical analysis:

1. Sample Preparation: Reference PVC films were prepared via solvent casting with varying concentrations of di-n-butyl phthalate (DBP) to create both glassy and rubbery states.
2. Thermal Analysis: Dynamic Mechanical Thermal Analysis (DMTA) was used to determine the glass transition temperature ($T_g$) as a function of plasticiser content.
3. Molecular Dynamics (MD) Simulations: Simulations were conducted on a PVC-DBP system (30 wt%) at 25°C to model plasticiser motion. The study analyzed Mean Square Displacement (MSD) to distinguish between short-time, anomalous, and long-term diffusion regimes. A logistic function was fitted to short-time simulation data (up to 120 ns) to extrapolate long-term diffusion coefficients ($D$).
4. NMR Diffusometry: Pulsed-field gradient spin-echo (PGSE) NMR spectroscopy was performed on the same model system at temperatures ranging from 45°C to 110°C to experimentally validate the MD-predicted diffusion coefficients.
5. Activation Energy Correlation: The study compiled literature data and new experimental results to correlate the activation energy of diffusion ($E_{a,diff}$) and evaporation ($E_{a,vap}$) with the molar mass ($M$) of various ortho-phthalates. This allowed for the determination of the rate-limiting step based on the difference in activation energies ($\Delta E_a$).

Key Results

• Physical State of Heritage Objects: Statistical analysis of historical PVC collections indicates that the average plasticiser content (approx. 18 wt% for DEHP and 5 wt% for DBP) places the majority of objects in a glassy state at typical display and storage temperatures (below $T_g$).
• Diffusion Coefficient Prediction: MD simulations combined with logistic fitting successfully predicted long-term diffusion coefficients that aligned with PGSE NMR data and literature values. This approach was found to be more accurate than classical extraction methods, which overestimated $D$ by approximately one order of magnitude due to material swelling.
• Rate-Limiting Step Determination:
  • Rubbery State: For highly plasticised PVC, the activation energy of diffusion is generally lower than that of evaporation for larger plasticisers, suggesting an evaporation-controlled process.
  • Glassy State: For objects in the glassy state, the activation energy of diffusion is significantly higher than evaporation for smaller plasticisers (e.g., DEP, DBP), indicating a diffusion-controlled process.
  • Molar Mass Threshold: A critical molar mass ($M_{critical}$) was identified (approx. 290–330 g/mol). Plasticisers with $M > M_{critical}$ (e.g., DEHP, DINP) generally exhibit evaporation-controlled migration, while smaller plasticisers (e.g., DBP, DEP) in glassy objects exhibit diffusion-controlled migration.
• Statistical Distribution: Approximately 90% of PVC heritage objects contain larger ortho-phthalates (DEHP, DOTP, DINP), which are predominantly in the evaporation-controlled mode regardless of their specific content.

Significance and Claims
The paper proposes an evidence-based, step-by-step protocol to assist conservators in decision-making without requiring destructive testing or complex modeling for every object:

1. Identification: Use non-destructive spectroscopy (FTIR/Raman) to identify plasticiser type and content.
2. State Estimation: Apply the proposed $T_g$ vs. concentration correlations to determine if the object is in a rubbery or glassy state.
3. Risk Assessment: Utilize the derived correlations between molar mass and activation energy to identify the rate-limiting step.

Key Conclusions:

• Cleaning Safety: Since the vast majority (approx. 90%) of heritage PVC objects contain larger plasticisers and are likely in an evaporation-controlled mode, the removal of surface exudates via mild cleaning techniques is unlikely to accelerate plasticiser migration or cause mechanical damage.
• Environmental Benefit: Surface cleaning can reduce dust deposition and lower the concentration of plasticiser vapours in the museum environment, thereby mitigating cross-contamination risks to other artefacts.
• Methodological Advancement: The study validates short-time MD simulations extrapolated via logistic fitting as a reliable, fast, and non-destructive alternative to traditional extraction methods for determining diffusion coefficients in polymer matrices.

The authors emphasize that while this protocol offers a robust general guideline, specific artefacts with small plasticisers in a glassy state may still require careful assessment due to the potential for diffusion-controlled migration and associated mechanical risks.