Ethanol or chlorine for case disinfection?

Answer: Match the material. PP shells and metal parts take 75% ethanol, which flashes off in 30 s; 0.5% chlorine is for visible contamination, with 10 min contact and a water wipe after. Neither belongs on a PC lens.

A gasket went chalky after disinfection — can it stay in service?

Answer: Chalkiness is the anti-ageing skin oxidised by chlorine, usually under 0.1 mm deep. If the rubber still rebounds within 2 s, keep it; if cracks appear or compression set passes 25%, replace the strip.

How is quaternary ammonium residue measured?

Answer: Swab 50 cm², elute into 10 mL of pure water, read by spectrophotometry, and pass at ≤10 μg/cm². Without a lab, spray pure water and watch: a film that holds 30 s is clean.

Do the two wipes need a drying step between them?

Answer: No full dry, but the detergent must be wiped off with no foam left. The 10 min after the second pass is dwell time, not drying time — wiping early wastes the round.

Chlorine left the latch spring stiff — what now?

Answer: Two clean water wipes to clear the salt, air dry, then 0.1 g of silicone grease on the pin and 30 open-close cycles. If opening force stays above 45 N, the spring is pitted and the latch is worth replacing.

Case Disinfection: Disinfecting Cases in Medical Use

Quick answer: Case disinfection is set by shell compatibility, not kill rate: 75% ethanol leaves a PP shell intact, 0.5% chlorine chalks a gasket in 24 h.

Three chemical classes, three jobs
75% ethanol and PC lens cracking
Chlorine at 500 mg/L and 10 min
Quaternary ammonium and the residue line
Gasket, latches and the two-wipe order
FAQ

1. Three chemical classes, three jobs

The same medical case tells two stories. Wiped three times with 75% ethanol, its shell keeps colour for three years; soaked once in 0.5% sodium hypochlorite, the gasket turns chalky inside 24 h. Case disinfection is decided by what the chemical does to shell, lens and seal, not by the kill rate on the label.

Only three classes reach the bedside. Alcohol flashes off in 30 s and carries soil away. Chlorine holds 500 mg/L available chlorine for 10 min. Quaternary ammonium sits at 1000 mg/L for the same 10 min. Each has a weak spot, and none covers everything.

Chemical	Working strength	Contact	Main risk to the case
Ethanol	75%	flashes off in 30 s	stress cracking of PC lens
Sodium hypochlorite	500 mg/L chlorine, 0.5% for spills	10 min	rust on metal, chalky gasket
Quaternary ammonium	1000 mg/L	10 min	residue film, tacky and dusty

Kill spectra differ by half a log across the three classes; damage to a case surface differs by a factor of several. That is why two cases in one department can be five years old and still tight, or eighteen months old and no longer sealing.

2. 75% ethanol and PC lens cracking

At 25 ℃, 75% ethanol is gone within 30 s, so the case is dry when the wipe finishes. On PP shells, TPE gaskets and nylon latches it swells almost nothing: 200 consecutive wipes cut gloss by 5%.

Case disinfection pause: a black case on a stainless trolley in a hospital corridor, two latches and a round valve closed, handle facing camera

The risk sits in the polycarbonate lens. A 3 mm lens under repeated ethanol contact grows fine cracks that start at the screw boss and travel 8 mm within 40 min. Ethanol penetrates the skin layer and releases orientation stress frozen in during moulding, so the most stressed zone cracks first.

A side-by-side run made it visible: neutral detergent left lenses clean after 300 wipes, ethanol cracked them at wipe 60. The variable was annealing — a lens held at 80 ℃ for 2 h survived 200 wipes.

On a case with a lens, ethanol stays on the outer shell, the handle and the lid rim; the lens takes 1000 mg/L quaternary ammonium, wiped once and wiped dry. That boundary costs far less than a replacement lens.

3. Chlorine at 500 mg/L and 10 min

Chlorine runs at two strengths: 500 mg/L available chlorine for 10 min on routine rounds, 0.5% sodium hypochlorite for visible contamination. Stronger mixes add under one log of kill and roughly double corrosion on metal.

The 0.5% solution is safe on PP and unforgiving on latch springs, hinge pins and rivets. A carbon steel pin that passed 48 h of salt spray shows a rust film after 10 min in chlorine, and the latch feels gritty after 30 open-close cycles.

Mixed chlorine must be spent the same shift. A 500 mg/L batch left open loses 40% of its chlorine in 24 h and holds 30% by day three, and colour never changes, so only a strip reveals it. On a 20 L drum opened eight times a day, strength fell to 300 mg/L by the second evening; mix 2 L at a time and check with a 0–2000 mg/L strip.

Chlorine also cannot be left to dry. Residual salts crystallise in shell gaps, swell, and lift the gasket by 0.2 mm, which costs a whole IP class. A clean water wipe inside 10 min removes most of that risk.

4. Quaternary ammonium and the residue line

Quaternary ammonium at 1000 mg/L is the gentlest of the three: no metal corrosion, no EPDM swelling, little odour. Its trouble is residue: surfactant on the shell forms a tacky film that collects dust.

Case disinfection route for metal shells: an open stainless instrument case with perforated trays in lid and base, three spring latches along the front

A stainless instrument case follows another path. Its metal shell tolerates 0.5% chlorine and a 10 min soak without harm, yet its hinges and latches are metal too, and chloride creeps along the rivets until rust shows inside.

The acceptance line is ≤10 μg/cm². Above it the surface beads water, the next disinfectant is blocked by the film, and measured kill rate drops 30%.

Check	Limit	Method	If over limit
Quaternary residue	≤10 μg/cm²	swab over a 50 cm² template	two water wipes, retest
Water break	film holds 30 s	spray pure water, watch	wipe clean again
Appearance	no white film	45° side light	rework

Sampling covers 50 cm², eluted in 10 mL of pure water and read by spectrophotometry; three points per case decide the result. Without a lab, a water-break test screens the batch in 3 min. Never chase residue with ethanol: alcohol drives the film into the micro-porosity of the shell, where no wipe reaches it.

5. Gasket, latches and the two-wipe order

Two wipes, in a fixed order. Detergent lifts soil first; the disinfectant then holds 10 min before the surface is wiped clean. A single pass merely spreads soil and chemical evenly across the shell.

Dwell time decides the second pass. After 1000 mg/L quaternary ammonium goes on, nothing touches the surface for 10 min; below 20 ℃ the hold stretches to 15 min, or contact falls short and kill rate loses nearly half.

The gasket groove is 0.8 mm deep, too narrow for cloth, so a soft brush runs along it once. Latch spring cavities take one lightly loaded swab each, because pooled chemical reacts with the grease on the spring.

Drying is mandatory. A gap holding 0.3 mL of liquid will not dry in 20 min at RH 60%; 15 s of air, or 15 min upside down with the lid open, brings gap humidity below 30%. Skip it and a misty white ring appears under the gasket a week later.

Conclusion

Case disinfection reduces to three decisions. Set the material: PP shells and stainless parts go to alcohol, EPDM gaskets stay clear of strong chlorine, and PC lenses only ever see quaternary ammonium. Set the time: ethanol needs 30 s to flash off, while chlorine and quaternary ammonium need a full 10 min, stretched to 15 min below 20 ℃. Set the acceptance: residue ≤10 μg/cm², a water film that holds 30 s, gap humidity under 30%. The third step is the one that gets skipped and the one that sends batches back for rewashing. Order matters as much as the numbers: material, then time, then acceptance, and the lid stays open until the third step is signed off. No chemical is better in the abstract; the only question is whether it matches this shell, this lens and this gasket.

Case part	75% ethanol	500 mg/L chlorine	1000 mg/L quaternary
PP shell	fine, 5% gloss loss at 200 wipes	fine, water wipe after	fine, wipe residue off
PC lens	avoid, cracks from wipe 60	avoid, hazes	suitable
EPDM gasket	suitable	cautious, chalks in 24 h	suitable
Carbon steel spring	suitable	avoid, rusts in 10 min	suitable

This article discusses container-side specifications and design implications only, and does not cover any handling, loading or use of the contents.