Cyanoacrylate / super glue

Plain-English definition

Cyanoacrylate (CA) is the chemistry behind most super glues: a single liquid that hardens in seconds when it meets trace moisture on the surface and in the air. There is no mixing. You apply a thin film, press close-fitting parts together, and the bond forms as long polymer chains lock up.

For DIY, CA is the fast fix for small, clean, rigid joints—chipped ceramic, hard plastic trim, metal pins, and tight cracks. It is not a structural wood adhesive, gap filler, or flexible joint solution. Cured CA is hard and brittle; it resists pulling apart in tension but fails easily under peel, impact, vibration, and flex.

Why it matters

Viscosity grades (what the bottle thickness means)

CA is sold from water-thin to thick gel. Viscosity controls how the glue spreads, wicks, and stays put:

Grade	Typical feel	Best for	Watch out for
Thin / “penetrating”	Like water	Hairline cracks, tight capillary gaps, model work	Runs everywhere; sets almost instantly on fingers
Medium	Syrup-like	General small repairs, plastics, ceramics	Still needs a close fit; limited gap fill
Thick	Slow-flowing	Slightly larger contact areas, controlled beads	Slower to wick; can leave a visible ridge
Gel	Non-drip paste	Vertical surfaces, overhead spots, porous or uneven faces	Does not wick deep into tight cracks

Practical rule: Use the thinnest grade that stays where you put it. Thin CA on a loose joint runs out and leaves almost no bond; gel on a hairline crack may not penetrate.

Gel vs liquid

• Liquid / thin CA wets smooth surfaces and pulls into gaps by capillary action—excellent for tight-fitting breaks you can clamp shut.
• Gel CA has fillers or thickeners so it does not run on vertical or overhead work. It tolerates slightly rougher or more porous surfaces but still will not bridge large voids reliably.

Neither replaces epoxy for filling missing material or building up a load-bearing fillet.

Accelerators (speed vs strength)

CA accelerator (activator) spray or mist forces very fast cure on the surface—useful when humidity is low, the part is awkward to hold, or you are doing many small tacks.

Trade-offs:

• Can whiten or frost the bond line (blooming from vaporized monomer).
• May reduce final bond strength compared with a natural moisture cure—test on scrap if strength matters.
• A DIY trick is a light dusting of baking soda on one side, then CA on the other: the powder speeds cure and can fill a tiny gap, but the joint is still brittle and cosmetic.

Use accelerators sparingly on visible or stressed repairs.

Shelf life and storage

CA polymerizes in the bottle once opened. Humidity and heat age it faster than most DIY adhesives.

State	Typical shelf life (varies by brand)	Storage tips
Unopened	Often 12–24 months from manufacture if cool and dry	Buy fresh for important jobs; check date codes
Opened	Often 1–3 months of reliable performance; can last longer if stored well	Wipe nozzle clean; cap tight; store upright
Extended storage	Refrigeration (~2–8°C) can slow aging unopened and opened	Let the bottle warm to room temperature before opening so condensation does not spoil the glue

If the glue is stringy, gels in the nozzle, or won’t wet the surface, replace it—old CA is a common cause of “super glue doesn’t work.”

What it bonds well — and poorly

Usually works well on (with clean, dry, close-fitting surfaces):

• Hard, non-porous plastics: ABS, acrylic (PMMA), many rigid hobby and appliance plastics
• Ceramics, glass, stone (smooth glass benefits from light abrasion)
• Metals (aluminium, steel, brass) after degreasing; not a substitute for load-rated engineering bonds
• Rubber and leather — only some types; silicone rubber and many flexible plastics resist CA

Weak or unreliable on:

• Polyethylene (PE), polypropylene (PP), PTFE (“Teflon”), silicone — low surface energy; CA sits on the surface or peels off
• Flexible or oily plastics (some soft PVC, flexible polyurethanes) — joint opens as the material moves
• Wet, greasy, dusty, or release-agent-coated surfaces
• Porous wood as a structural joint — CA soaks in and leaves a brittle skin; PVA wood glue is stronger for lumber
• Large gaps — CA is not a gap-filling adhesive unless the gap is very small (fractions of a millimetre for thin grades)

When in doubt, test on scrap of the same plastic. Plastics are often unlabeled; a failed bond is cheaper on offcuts than on the project.

Gap-filling limits

CA needs intimate contact. Strength comes from a thin bond line (often cited around 0.05–0.5 mm for many formulations—not a precise DIY measurement, but the idea is “almost touching”).

• Too thick a bead cures slowly, traps liquid inside, and is weaker than a thin film (and can get uncomfortably hot from exotherm).
• For missing chunks, loose fits, or structural gaps, use epoxy or mechanical fixings.
• Gel can bridge a slight mismatch; it will not rebuild a broken corner like filler.

Brittleness, peel, and impact

Cured CA is a hard, glassy plastic. It handles direct pull along the bond better than side load:

• Poor peel strength — prying with a screwdriver or flexing the part pops the bond.
• Poor impact and vibration — dropped tools, cabinet doors, and flexing plastics are common failure modes.
• Not for flexible joints (living hinges, shoe soles, tarps) — use a flexible adhesive or mechanical fix.

Treat CA repairs as light-duty and inspectable, not permanent structural engineering.

Heat and chemical resistance

Hobby CA is not high-temperature:

• Many consumer grades are rated for continuous service roughly up to 80–120°C depending on formulation—always read the label.
• Near boiling water, in an oven, on engine parts, or in a hot car, bonds soften or fail.
• Water can weaken some bonds over time; acetone and many solvents attack cured CA (useful for debonding, harmful if you need chemical resistance).

For heat, chemicals, or outdoor weathering, choose epoxy or a product specified for that environment.

Safety: skin, fumes, eyes, ventilation

• Skin bonding happens instantly. Do not pull bonded skin apart—you can tear skin. Soak in warm soapy water, then work apart gently; acetone (nail polish remover) on cotton helps unstick fingers but irritates skin and is unsuitable near eyes.
• Eyes are a medical emergency if CA gets in them—flush with water per first-aid guidance and seek urgent care; do not try to force lids open.
• Fumes (and vapor from curing) irritate eyes, nose, and lungs. Work ventilated; avoid breathing over the bottle; some “low odor” / ethyl formulations smell less but still need ventilation.
• Exotherm: A large puddle on cotton, leather, or tissue can heat enough to smoke or burn—never use CA to close fabric wicks or big blobs.
• Gloves: Thin nitrile can help; CA often finds pinholes. Keep a bottle of remover or soapy water nearby.

When to use CA vs epoxy, PVA, or hot-melt

Job	Better choice
Small rigid chip on ceramic or hard plastic	CA (thin for tight cracks, gel if vertical)
Wood furniture joint, drawer, frame	PVA wood glue + clamp
Metal bracket, gap fill, high strength	Epoxy (correct metal/plastic type)
Craft tack, cardboard, fabric (low stress)	Hot-melt or fabric glue
Unknown plastic, flexing part, outdoor load	Test or use a plastic-specific or polyurethane adhesive; not default CA

Surface prep (often matters more than brand):

1. Dry-fit parts—CA has almost no open time (seconds).
2. Degrease metals and plastics with isopropyl alcohol; avoid oily shop rags.
3. Lightly abrade smooth glass or metal if the label allows; blow off dust.
4. Apply a thin film—less is stronger for CA.
5. Hold still for 30–60 seconds; full strength may take hours (check label).

Debonding and cleanup:

• Uncured: acetone on a cloth (test plastic first—acetone melts some).
• Cured: acetone soak, mechanical scraping, or patience with warm soapy water on skin—forced peeling damages substrates.
• On glass or finished surfaces, solvent can haze coatings—mask and test.

Common mistake

The most costly error is using CA as “universal super glue” on every plastic, gap, or stressed joint. It fails quietly on PE/PP/silicone, in gaps, and under flex or impact—while looking fine at first.

Other typical slip-ups: old or half-empty bottles; too much glue (weak, slow, hot blob); oily or dusty surfaces; moving parts before set; expecting wood-joint strength without clamped PVA; and forcing bonded skin instead of soaking apart.

For anything load-bearing, overhead, safety-critical, high-heat, or flexible, use the right adhesive from the table above—or a mechanical fixing—not default super glue.