The Silent Killer
What is Vinegar Syndrome?
Vinegar Syndrome is not the smell of vinegar. It’s the point where cellulose acetate enters a self-accelerating chemical collapse. Once enough ester bonds have been hydrolyzed, two destructive feedback loops take over: acid autocatalysis (acid creates more acid) and hydrophilicity amplification (water absorption accelerates further breakdown). From this point, deterioration no longer progresses slowly—it accelerates and becomes difficult to stop.
Key insight: Vinegar odor occurs when acetic acid builds up in the air. If diffusion is limited, even healthy cels can smell — so confirm with AD strips or acidity testing, not smell alone.
Why it happens
Hydrolysis (the slow beginning)
Cellulose acetate contains acetyl groups that make the film or cel clear and processable. Over time, moisture in the air slowly breaks these acetyl ester bonds in a reaction called hydrolysis. Each time a bond breaks, one molecule of acetic acid is produced and one hydrophilic hydroxyl (–OH) is restored on the polymer backbone. Early on—especially in cool, dry storage—this process is very slow.
Two feedback loops (the accelerating phase)
As more bonds cleave, two self-reinforcing loops begin to amplify decay:
- Acid autocatalysis: Hydrolysis releases acetic acid → pH drops → hydrolysis speeds up → even more acid is released.
- Hydrophilicity amplification: Hydrolysis restores –OH groups → the polymer absorbs more moisture (higher EMC) → hydrolysis speeds up.
Together these loops explain why Vinegar Syndrome starts slowly and then suddenly accelerates—especially in sealed containers or tightly packed storage.
When does hydrolysis become “Vinegar Syndrome”?
Hydrolysis is always happening in the background, but Vinegar Syndrome begins when acid and hydrophilicity reach a critical threshold and the reaction becomes self-sustaining. We identify this transition with A–D strips, which measure acetic acid vapor (free acid) in a sealed environment. Consistent test volume and timing are essential for trends to be meaningful.
| A–D Strip Level | Meaning | Interpretation |
|---|---|---|
| < 1.5 | Pre-VS | Hydrolysis is slow; feedback weak. Keep conditions cold/dry and monitor. |
| ≈ 1.5 | Onset of VS | Feedback loops begin. Watch the trend closely and take preventive action. |
| ≥ 2.0 | Active VS | Self-accelerating breakdown likely. Separate, reset, and move to cold/dry storage. |
Key idea: Vinegar Syndrome isn’t defined by smell—it’s defined by feedback. The A–D strip shows when that is happening.
Free vs. bound acid
When hydrolysis occurs, the acetic acid that forms appears in two states:
- Bound acid — trapped inside the polymer (dissolved or hydrogen-bonded). It lowers internal pH and accelerates decay but is not detectable by smell.
- Free acid — vapor that escapes into the air. This is what you can smell and what A–D strips detect.
Bound acid appears first and drives both feedback loops long before odor. Free acid appears later—when internal acid is already high. This is why VS cannot be “ventilated away” and why control strategies must slow the reaction itself, not just remove odor.
How Vinegar Syndrome shows up (physical signs)
Chemical change begins long before major physical damage is visible. Each symptom has a direct molecular cause:
| Sign | What’s happening | Why it matters |
|---|---|---|
| Vinegar odor | Free acetic acid escapes into the air (late compared to bound acid buildup). | Warning signal—isolate and test with A–D strips immediately. |
| Warping / waviness | Hydroxyl regeneration increases moisture uptake; layers expand differently (differential expansion). | Dimensional instability—handle carefully; move to cold/dry conditions. |
| Tackiness / blocking | Plasticizer migration and surface moisture create a tacky surface. | Layers may stick or fuse—interleave or rehouse. |
| Shrinkage | Loss of acetyl groups and plasticizers plus chain reorganization pull the film inward. | Permanent size change—distortion and cracking risk increase. |
| Embrittlement | Plasticizers leave; DS drops; the polymer becomes rigid and fracture-prone. | High break risk—prioritize cold storage. |
| Opacity / haze | Crystallinity and microstructure change as hydrolysis progresses. | Loss of clarity—visual quality and readability degrade. |
| Paint adhesion loss | Surface becomes more polar and acidic; anchoring weakens. | Flaking/lifting—stabilize environment; avoid flexing. |
Important: By the time shrinkage or brittleness appears, deterioration is advanced.
“If I smell vinegar, the cel has Vinegar Syndrome.”
Not necessarily. A vinegar odor comes from acetic acid concentration in the air, not directly from the severity of film degradation. Even healthy, stable cels release a small amount of acetic acid—and in small, sealed spaces this builds up quickly.
Real example (rate inferred from AD strip):
- Stable cel (AD blue, ~0.5 ppmv in 48 h within 1 L)
- Emission rate ≈ 0.010 ppmv per hour
Time to reach ~1 ppmv (lowest odor threshold for many):
- From 0 → 1 ppmv: ≈ 4 days
- From 0.5 → 1 ppmv: ≈ 2 more days
- Small volume → faster buildup
- No ventilation → accumulation
- No sorbents/paper → nothing removes it
- Human nose can detect ~0.5–1 ppmv
Bottom line: Odor ≠ diagnosis. Use AD strips, film acidity/pH, diffusion context, and environment—not smell alone.
| Container Volume | Time to ~1 ppmv |
|---|---|
| 1 L | ~4 days |
| 5 L | ~20 days |
| 10 L | ~40 days |
Assumes same cel and emission rate; larger volumes take proportionally longer to reach the same headspace ppmv.
Why resets help — but are not enough
What a reset does
“Resetting” (ventilation, isolation, or scavengers) removes free acid from the air and surfaces, temporarily raising pH and slowing the acid-catalyzed loop.
What a reset cannot do
- Remove bound acid inside the polymer.
- Reverse hydroxyl regeneration (hydrophilicity remains high).
- Restore lost acetyl groups (DS never increases) or plasticizers.
- Undo chain scission, shrinkage, or embrittlement.
Bottom line: A reset buys time. It is not a cure.
Summary & Next Steps
Vinegar Syndrome is the accelerated stage of cellulose acetate decay, driven by two feedback loops: acid autocatalysis and hydrophilicity amplification. It begins slowly through hydrolysis, then crosses a threshold where decay becomes self-sustaining.
- VS begins when feedback accelerates decay (often around A–D 1.5–2.0).
- A–D strips reveal rising free acid long before smell is reliable.
- Bound acid drives damage internally; free acid produces odor.
- Physical signs follow chemical change.
- Resets help, but they don’t reverse chemistry.
Most effective actions:
- Lower temperature (cold or frozen storage slows all reactions).
- Limit humidity (less water = slower hydrolysis).
- Use ventilation or scavengers to limit acid buildup.
- Isolate affected items to protect others.
- Monitor with A–D strips—trends matter more than one result.
Learn more:
- Chemistry of Hydrolysis — where VS begins
- How to Read A–D Strips (and Actually Understand Them)