Butyric Anhydride O-Ring Compatibility: Which Compounds Hold Up

Butyric anhydride (n-butyric anhydride) is a clear, pungent acid anhydride used as an acylating intermediate for cellulose acetate butyrate, fragrance esters, pharmaceuticals, agrochemicals, and specialty dyes. Seals on transfer pumps, reactor nozzles, distillation takeoff lines, and drum or tote fill heads see a reactive organic anhydride that hydrolyzes with moisture and attacks the wrong elastomer families. Pick poorly and you get swell, softening, or early weep.

This guide maps the handbook Chemical Compatibility Chart row for butyric anhydride to practical compound choices.

Where it shows up

• Cellulose esters and coatings: primary feedstock for cellulose acetate butyrate (CAB) and related ketobutyrate chemistry
• Flavors and fragrances: acylating agent for butyrate esters and aroma intermediates
• Pharmaceuticals and agrochemicals: intermediate in drug and crop-protection synthesis
• Dyes and specialty organics: acylating agent in dye and fine-chemical manufacture

Service is usually neat or solvent-cut organic anhydride in stainless process equipment, near ambient to moderately warm. Moisture, alcohol, and amine co-streams change the aggressiveness. Chart ratings assume a generic commercial compound near room temperature.

How butyric anhydride stresses an O-Ring

Acid anhydrides are reactive, polar organics. They prefer polymers that tolerate polar intermediates and do poorly in many oil-service nitriles and FKMs that shine in petroleum duty. Moisture turns anhydride into butyric acid at the seal face, so aqueous-friendly families (EPDM, butyl, neoprene in many grades) are the usual starting point. Continuous immersion and vapor-phase seals both matter on reactors and vent lines. See Chemical Compatibility and Failure Analysis for swell and softening modes.

Handbook chart ratings for butyric anhydride

Same rating badges and layout as the interactive chart. Search butyric anhydride on the Chemical Compatibility Chart to pin the live row.

Best family on the chart row: EPDM (E). Neoprene, fluorosilicone, butyl, FFKM, and PTFE also rate excellent (1). Silicone rates good (2). Nitrile, HNBR, and FKM rate poor (3). Polyurethane is not recommended (4). Aflas lacks data (X) on the handbook matrix.

Recommended O-Ring materials

First choice: EPDM

EPDM is rated excellent (1) and is the chart’s preferred letter for butyric anhydride. It prefers polar and aqueous-leaning service over petroleum oils, which matches CAB, fragrance, and fine-chemical anhydride duty. Use 70 Shore A for most static covers and transfer seals; step hardness or add a back-up ring if pressure and gap climb.

Also usually serviceable

• Neoprene (CR) 1Excellent Strong polar / anhydride rating; confirm grade for continuous immersion.
• Fluorosilicone (FVMQ) 1Excellent Useful when cold flexibility or mixed hydrocarbon traces matter on the same plant.
• Butyl (IIR) 1Excellent Good polar-fluid resistance; watch heat and dynamic wear on pumps.
• FFKM 1Excellent Overkill for many transfer skids; use for mixed aggressive anhydride streams.
• PTFE 1Excellent Chemically inert but not an elastomer. Consider PTFE-encapsulated when you need rebound.
• Silicone (VMQ) 2Good Usually OK for static service; watch tear and abrasion in dynamic glands.

Materials to use with caution or avoid

• Nitrile (Buna-N) 3Poor Petroleum workhorse, weak here. Prefer EPDM for anhydride intermediates.
• HNBR 3Poor Same polarity mismatch as standard nitrile for this fluid.
• Viton® / FKM 3Poor Not the first pick for butyric anhydride; save FKM for fuels and hot oils.
• Polyurethane 4Not recommended Avoid in continuous butyric anhydride service.
• Aflas® (FEPM) XNo data Insufficient chart data. Do not assume steam or amine grades are automatic fits here.

Physical properties to match (after chemistry)

A chart 1 means the polymer family usually survives the fluid. It does not finish the spec. After you shortlist EPDM (or another excellent family above), lock the physical properties the gland and process actually need.

Hardness (Shore A / durometer)

• 70 Shore A is the everyday default for static covers, flanges, and low-to-moderate pressure transfer and fill lines.
• 90 Shore A (or 70 plus a back-up ring) when pressure and clearance gap push extrusion risk on pumps and filter housings.
• Soft grades (50–60) seal rough faces better but give up extrusion margin.

See Shore A Hardness for the extrusion-gap chart and feel references.

Temperature

Handbook chemical ratings are near room temperature. Anhydride transfer is often ambient, but reactors, distillation takeoffs, and heat-traced lines run warmer. As a starting frame:

Confirm process temperature and any heat tracing as well as the elastomer’s window. Hot anhydride service ages seals faster than a room-temperature chart letter implies.

Static vs dynamic duty

• Static face and cover seals: chemical rating + hardness + squeeze usually dominate.
• Dynamic shafts, rods, and reciprocating glands: add abrasion, extrusion, and friction. A harder EPDM often outlasts a soft general-purpose grade in frequent cycling. Check groove design and seal types.

Size and squeeze

Lock ID × CS (or the AS568 dash) on the Master O-Ring Size Chart before you argue compound. Wrong size fails faster than a second-choice polymer that fits the groove.

Moisture and hydrolysis notes

Butyric anhydride reacts with water (and with alcohols / amines). Seal choice does not replace dry handling, materials of construction, or vapor control. If the loop also sees strong acids, free butyric acid, or mixed anhydrides, say so when you specify the ring. Related chart rows to compare: other organic anhydrides and butyric acid on the Chemical Compatibility Chart.

Practical selection checklist

1. Confirm the fluid. Neat anhydride vs solvent cut, moisture, and co-reactants.
2. Temperature. Ambient drum transfer is milder than warm reactor or distillation duty.
3. Hardness and pressure. Default 70 Shore A; step to 90 or add a back-up ring when gap and psi demand it.
4. Static or dynamic. Metering pumps and valves need abrasion and extrusion resistance beyond a chemical rating alone.
5. Confirm on the chart, then test. Pin Butyric Anhydride on the Chemical Compatibility Chart. For critical service, contact The O-Ring Store with fluid grade, temperature, hardness, and duty cycle.

Quick answers

Which O-Ring for butyric anhydride?
Start with EPDM at 70 Shore A for general static duty, or 90 Shore A / back-up rings when pressure and gap climb. Neoprene, fluorosilicone, butyl, FFKM, and PTFE also rate excellent on the handbook row. Silicone is usually good for static service. Avoid polyurethane; treat nitrile, HNBR, and standard FKM as poor fits; treat Aflas as unproven here until you test.

Is this the same as butyric acid or acetic anhydride?
Related acid / anhydride chemistry shows up in the same plants. Always match the exact fluid name on the chart and on the SDS.

Chart ratings are guidance for generic commercial compounds near room temperature. Concentration, temperature, and co-fluids change outcomes. Evaluate specific parts before use in critical applications.