RH Cycling, Vapor Pressure, and Why “Microclimate Fatigue” Is Real
This article explains why repeated temperature and humidity swings can cause fatigue-style structural damage in animation cels. The core idea is simple: the cel is a composite (CTA base + paint layers). When moisture content or temperature changes, each layer wants to expand/contract differently. That mismatch creates interfacial shear stress. Do it enough times, and you can drive curling, edge lift, microcracking, and paint delamination — even when no single swing looks “extreme.”
2) The “drivers”: thermal strain vs moisture strain
A useful mental model is total strain as the sum of two components:
| Strain type | Simple model | What it means physically |
|---|---|---|
| Thermal strain | εT = α · ΔT |
Expansion/contraction from temperature change (α = CTE). |
| Moisture strain | εM ≈ β · Δ(RH) (conceptual) |
Expansion/contraction from moisture sorption and water activity changes (β is “hygroexpansion sensitivity”). |
The reason RH cycling matters so much: moisture-driven strain can be surprisingly large compared to thermal strain, and it also changes the material properties (softening/plasticization), which increases creep and interfacial damage.
3) Quick thermal expansion example (6°F swing)
Thermal expansion by itself is usually not the dominant driver in typical indoor swings — but it still matters because it sets a clearance requirement whenever the cel is constrained (tight mats, clamped frames, edge binding, compressed sleeves).
- Cel length L = 11 in (typical display width scale)
- CTA coefficient of thermal expansion αCTA ≈ 70 × 10−6 / °F (plastic range)
- Temperature drift ΔT = +6°F (framework anchor)
| Quantity | Expression | Value | Interpretation |
|---|---|---|---|
| Free expansion (CTA) | ΔL = L · αCTA · ΔT |
≈ 0.0046 in (≈ 4.6 mil) | This is the extra “room” the film wants when warmed by 6°F. |
| Percent of length | ΔL / L |
≈ 0.042% | Small as a % — which is why thermal alone often isn’t catastrophic. |
| Clearance requirement | ΔL (if constrained) |
≈ 0.0046 in | If edges are constrained, this expansion turns into compressive stress instead of harmless growth. |
Why this usually isn’t “the big one”: a few mil of free expansion is often tolerated if the cel is not mechanically constrained and the sleeve/matting geometry has slack. Thermal becomes dangerous when it is repeated and constrained, or when it occurs out of phase with moisture-driven strain.
4) Why mismatch matters (CTA vs paint) — interfacial shear, not total growth
The structural risk is not “the cel grew 4.6 mil.” The risk is that the system is a bonded laminate: a base film (CTA) with paint regions that have different stiffness and different expansion behavior. When CTA tries to move and paint resists, the result is interfacial shear stress — especially at paint edges and thickness transitions (stress concentrators).
εmis ≈ (αCTA − αpaint) · ΔTEven if total expansion is small, mismatch drives shear at the interface.
- Micro-slip at the paint/CTA interface
- Fatigue cracking at paint edges / stiff islands
- Edge lifting / delamination over long timelines
- Curl / creep-set when combined with moisture-driven softening
Why RH Cycling Is a Cumulative Fatigue Mechanism
RH-driven deformation is not an instantaneous failure mode. It is a cumulative mechanical fatigue process acting on a bonded composite (CTA base + paint layers) over long time horizons.
Each RH cycle induces a small amount of interfacial mismatch strain between the CTA film and the paint. Individually, these cycles are often harmless. However, like mechanical fatigue in metals, the damage is banked.
5) Why RH changes during temperature drift (even when AH is constant)
This is the subtle trap: at lower temperatures, the air holds less water at saturation — so people assume RH swings should “matter less.” But if the air’s absolute humidity (AH) stays constant, then the actual vapor pressure does not change much, while saturation vapor pressure changes strongly with temperature.
RH = e / esat(T)where
e is the actual water vapor partial pressure (set by AH), and esat(T) increases with temperature.
Translation: a temperature drift creates an RH swing even if you did not add/remove any water. That RH swing creates a repeated water activity (aw) swing at surfaces, which drives sorption/desorption cycling.
5.1) Example A — 70°F / 50% RH with +6°F drift (AH constant)
Anchor the drift with reality: this is typical HVAC behavior (day/night load, sun, occupancy, thermostat bands). Start at 70°F and 50% RH, then drift to 76°F without changing the amount of water in the air.
| Condition | T (°F) | RH (%) | AH (g/m³) | e (hPa) | esat(T) (hPa) | What changed? |
|---|---|---|---|---|---|---|
| Baseline (home) | 70 | 50 | ≈ 9.2 | ≈ 12.6 | ≈ 25.2 | Reference state |
| Warm drift (HVAC) | 76 | ≈ 41 | ≈ 9.2 | ≈ 12.6 | ≈ 31.0 | AH and e stayed ~constant; only esat rose, so RH dropped. |
The RH change is not hand-wavy here: 50% → ~41% purely from a +6°F drift at constant AH. That’s a real cyclic strain input if it repeats daily.
5.2) Example B — 40°F / 50% RH with +6°F drift (AH constant)
Anchor the drift with reality: this is typical compressor cycling in a wine fridge / beverage fridge or small enclosure. We assume the microclimate sits at the low setpoint and drifts upward during compressor cycling. Start at 40°F and 50% RH, drift to 46°F, with no moisture added/removed.
| Condition | T (°F) | RH (%) | AH (g/m³) | e (hPa) | esat(T) (hPa) | What changed? |
|---|---|---|---|---|---|---|
| Baseline (cold) | 40 | 50 | ≈ 3.2 | ≈ 3.7 | ≈ 7.4 | Reference state |
| Warm drift (cycling) | 46 | ≈ 38 | ≈ 3.2 | ≈ 3.7 | ≈ 9.6 | AH and e stayed ~constant; esat rose → RH dropped more sharply. |
This directly answers the “lower temperature = less available water” intuition: yes, cold air has less AH here (3.2 vs 9.2 g/m³), but a 12% RH swing still happens because the driver is e / esat. The vapor pressure doesn’t have to change for RH — and surface water activity — to cycle.
5.3) Example C — Temperature steady, but moisture is added (cooking/shower/door opening)
Separate the mechanisms: here temperature is steady, and AH increases from life events (cooking, showering, door opening). At constant temperature, increased AH means increased vapor pressure e, which increases RH directly.
| Scenario | T (°F) | RH (%) | What changed? | Meaning for fatigue |
|---|---|---|---|---|
| Baseline | 70 | 50 | Reference | Baseline sorption strain |
| Moisture added | 70 | 60 | AH ↑ → e ↑ | Higher aw → more absorption/swelling |
| More moisture added | 70 | 70 | AH ↑↑ → e ↑↑ | Larger cyclic amplitude; stronger fatigue + creep coupling |
Real homes often have both: temperature drift changes RH even at constant AH, and daily life changes AH directly. That’s why “the thermostat says 70” does not mean the cel sees stable water activity at its surface.
6) The “Cold Storage Trap”: Slow Chemistry vs. Fast Physics
Storing cels in a fridge slows hydrolysis (Vinegar Syndrome), but "unintentional" cold storage introduces a mechanical risk: High-Frequency Fatigue.
- The Cycle Count ($N$): A home HVAC might cycle 2–4 times a day. A small fridge compressor may cycle every 15–20 minutes.
- The Fatigue Math: Damage is a result of Cycles ($N$) × Amplitude. High-frequency cycling can accelerate mechanical failure faster than room-temperature storage.
- The Phase Lag: CTA reacts to temperature changes instantly, but moisture absorption is slow. The material stresses are constantly "chasing" each other, creating a chaotic internal environment.
Hysteresis and Phase Lag in Moisture Cycling
Moisture absorption and desorption do not occur symmetrically. CTA and paint layers exhibit hysteresis: they absorb water and release it at different rates and along different paths.
In environments with frequent cycling (e.g., small refrigerators or unbuffered enclosures), the cel may never reach equilibrium before the next cycle begins.
- Strain inputs are out of phase
- Stress never fully relaxes
- Fatigue damage accelerates even at modest ΔRH
7) Why CTA–paint stress exists (what’s physically happening)
- Composite laminate behavior: paint islands + clear CTA regions behave like a bonded laminate with non-uniform stiffness.
- Mismatch strain inputs: temperature strain and moisture strain do not match between layers (and are rarely in phase).
- Transient gradients: moisture does not equilibrate instantly; gradients through thickness create bending moments.
- Softening / creep: moisture and acidity plasticize/soften materials, increasing time-dependent deformation per cycle.
8) Anchoring the Framework: The ΔRH Likelihood Bands
In our framework, we anchor risk to Amplitude (ΔRH)—the primary driver of interfacial fatigue.
- The 10% Baseline (Neutral): Standard indoor HVAC drift. This involves measurable loading but is the historical "survivable" status quo.
- The ≤5% Band (Lowest Likelihood): Keeps moisture-driven strain minimal and within the elastic limit. This is Active Preservation.
- The >12% Band (High Likelihood): Found in unbuffered fridge environments. High frequency + high amplitude = high risk of interfacial failure.
| Lstruct | Relative Structural / Mechanical Risk (50 yrs) | Examples (handling + RH cycling) |
|---|---|---|
| 1 – Very Low | RH cycling **ΔRH < 5%** per typical cycle (much tighter than a good room). | Microclimate buffered so the effective aw band is extremely narrow (e.g., engineered cold box, well-buffered small volumes). |
| 2 – Low | RH cycling **5% ≤ ΔRH ≤ ~10%**, slightly better than or equal to a best-case room. | Room or microclimate held near a stable setpoint with modest drift, generally as good as (or a bit tighter than) a well-behaved 70 °F / 50 % room. |
| 3 – Neutral | RH cycling **≈10–12%** per typical cycle (our reference case). | RH swings equivalent to a typical interior room drifting **67–73 °F** around 70 °F with constant AH, giving an aw band of roughly 0.47–0.54 (~47–54% RH equivalent). No extremes, but no deliberate buffering either. |
| 4 – High | RH cycling **~12–20%**, but still without sustained condensation. | Environment has large day–night RH shifts, seasonal spikes, or repeated excursions above ~60% or below ~30% RH, yet does not routinely hit saturation. |
| 5 – Very High | RH cycling **> 20%**, including condensation or very low RH episodes. | Environments that push aw toward 1.0 (condensation, damp basements) or toward very low values for long periods, combined with strong repeated T/RH shocks (fridge-box cycling, attic or garage extremes). |
9) Mitigation: Dampening the Physics
To move into a safer Likelihood Band, you must use Mass to fight Frequency:
- Thermal Mass: "Ballast" water bottles inside a fridge to slow down how often the compressor kicks in.
- Hygroscopic Buffering: Archival board or paper inside a sealed microclimate to absorb RH swings before they reach the cel.
- High-Barrier Packaging: Use Ziplocs or laminate bags to decouple the cel's microclimate from the outside air.