Reference · adhesives, sealants and adhesion

Bonding

That nothing sticks to polypropylene is not the adhesive's fault. It is a property of the surface, and it can be measured — which means it can also be changed. Almost every failed joint comes down to one of two numbers: the surface energy of the material, or the surface tension of the adhesive.

Know which materials you are joining but not which adhesive? The material matrix answers directly.

Version 1.0 Adhesive types 10 Sections 12 SEEN Units mN/m, MPa, °C

Surface energy — why anything sticks at all

Tap a material · logarithmic scale, mN/m

An adhesive must have a lower surface tension than the material's surface energy in order to wet the surface. Most adhesives sit between 30 and 45 mN/m. Everything to the left of the line is therefore a problem, and everything to the right takes care of itself.

01

First principles

Five ideas. The first explains most failures, the fourth explains the rest.

1 · The adhesive has to wet the surface

Adhesion requires the adhesive to lie as an unbroken film in microscopic contact with the material. That happens only when its surface tension is lower than the material's surface energy. If it is higher the adhesive pulls itself into droplets, exactly like water on a waxed bonnet, and the contact area becomes a fraction of what it looks like.

Ordinary adhesives sit around 30–45 mN/m. Metal, glass and ceramic are in the hundreds and are therefore easy. PP, PE, PTFE and silicone are below, and there no brand name helps — the surface has to be changed first, or you choose a mechanical fixing.

2 · Adhesion and cohesion are two different failures

Look at the fracture surface when a joint lets go. If the adhesive stays on one side and the other is bare, that is adhesive failure — the adhesive released from the material, so a surface energy or cleanliness problem. If the adhesive has split down the middle and sits on both sides, that is cohesive failure — the adhesive itself was too weak, and you need a stronger type or a thicker joint. The diagnosis takes three seconds and decides everything you do next.

3 · The joint should be loaded in shear, never in peel

An adhesive bond can be stronger than the material in shear and still be torn apart with your fingers in peel. The difference is a factor of ten or more, and it is a matter of design rather than of adhesive.

  • A lap joint rather than a butt joint. A butt joint in wood or plastic is nearly always the wrong design.
  • The overlap should be at least three times the material thickness. Longer than ten times adds almost nothing — the stress concentrates at the ends anyway.
  • Relieve peel with a rivet, screw or rebate that takes that load.

4 · The right bond line is thinner than you think — except when it is not

Rigid adhesives — cyanoacrylate, epoxy, anaerobics — are strongest in a thin film, 0.05 to 0.2 mm. More adhesive gives a weaker joint, which is the opposite of most people's instinct. Elastic adhesives — MS, PU, silicone — need 2 to 5 mm instead, so they can move. An elastic joint squeezed thin behaves like a rigid one and cracks.

This is also the answer to why things let go after a season: two materials with different thermal expansion move relative to each other, and a rigid adhesive between them has nowhere to put that movement.

5 · The curing mechanism decides everything practical

Open time, gap filling, how deep it cures and what happens in the cold all follow from how the adhesive sets, not from which brand it is.

Five ways to set
MechanismAdhesive typesConsequence in practice
Moisture-curingCyanoacrylate, PU, MS, silicone Cures from the surface inwards. Deep joints take days. Dry winter air slows it sharply.
Two-componentEpoxy, PU 2K, methacrylate Cures everywhere at once, independent of humidity. The mixing ratio is critical.
AnaerobicThreadlocking, flange sealing Cures only in the absence of oxygen and close to metal. Excess outside the joint stays wet.
Radiation-curingUV adhesive Seconds, but only where the light reaches. At least one part must be transparent.
Physical dryingPVAc, contact adhesive, hot melt Solvent or water has to leave, so at least one porous part is required. Reversible by heat or moisture.
02

Preparation

Three steps that take five minutes and decide whether the joint holds for twenty years or three weeks.

  1. Abrade. 120–240 grit paper or a blasted finish. The point is not that the adhesive keys mechanically — that is a myth, and an overstated one — but that the contaminated top layer a few molecules thick comes away, and that the area grows.
  2. Degrease. Isopropanol on a lint-free cloth, in one direction, change the cloth. Acetone is more effective but attacks many plastics. Never wipe back and forth with the same cloth — you are only moving the grease around.
  3. Bond within minutes. A freshly abraded, degreased surface starts collecting dust and airborne contamination immediately. Aluminium builds new oxide in seconds.

Getting at low-energy plastics

PP, PE and POM cannot be bonded by ordinary means. There are four ways around it, in order of how well they work:

  • Polyolefin primer. A solution that leaves a thin, high-energy layer on the surface. Sold alongside cyanoacrylate and works remarkably well.
  • Flame treatment. A quick pass with a propane torch oxidises the surface and raises the surface energy dramatically. Must be bonded within minutes.
  • Plasma or corona. The industrial method, same principle.
  • Structural methacrylate adhesives formulated for polyolefins. Expensive, they smell foul, but they are the only ones that bond PP to PP with real strength.
A test that reveals everything

Drop water on the prepared surface. If it spreads into a thin film the surface energy is high and the adhesive will wet it. If it draws up into a bead the surface is either greasy or too low in energy, and the joint will fail. The same test is used industrially.

03

Cyanoacrylate

Superglue. Fastest, most brittle, and the only adhesive that cures because you breathed on it.

Cyanoacrylate Superglue · CA · ethyl, methyl or alkoxyethyl ester

Anionic polymerisation, started by moisture
Fixture time5–60 s
Full strength24 h
Gap< 0.15 mm
Shear strength15–25 MPa
Temperature−30 to +80 °C
Shelf life1 year, 2–3 refrigerated

Why it is so fast and so brittle

The monomer polymerises on trace amounts of water — the humidity on the surface is enough. The chains grow long and highly ordered, which gives high tensile strength and almost no toughness.

Use for

  • Small, well-fitting joints in plastic, rubber, metal and ceramic.
  • Wood and leather — the porous variety soaks in and becomes strong.
  • Tacking ahead of a proper bond or a mechanical fixing.
  • With bicarbonate or coffee grounds as filler it becomes a fast-curing, sandable mass. Pour powder into the gap and drop adhesive on — it sets instantly and goes as hard as plastic. One of the most useful workshop tricks there is.
  • Crack stopping in plastic — drill a hole at the end of the crack and fill it.

Varieties

  • Thin grades creep into hairline gaps by capillary action. Apply after assembly.
  • Gel does not run and fills up to a couple of tenths of a millimetre. On vertical and porous work it is the only sensible choice.
  • Rubber-modified (black or grey) has considerably better impact and peel strength. Dearer, and worth every krona on anything that will be handled.
  • Activator in spray form cures excess in seconds, but lowers final strength markedly. Use it sparingly and only outside the joint.
The activator is the dangerous half of the kit

The adhesive itself is a skin irritant and nothing more. The activator spray that comes with the fast-cure kits is another matter: it is typically 20–40 % n-hexane with under one per cent of N,N-dimethyl-p-toluidine to start the polymerisation, in an LPG propellant.

The hexane carries H372 — damage to organs through repeated exposure — and the damage in question is nerve injury in the hands and feet. The toluidine is acutely toxic by all three routes: swallowed, on skin and inhaled. That means outdoors or under extraction, gloves on, and never a generous spray in a closed room to hurry a joint along. The dosing advice above is not only about strength.

Four weaknesses
  • Water and heat break it down. A CA joint in a damp place loses strength over months. Not for outdoor use.
  • Blooming. A white deposit around the joint where monomer vapour condenses and cures on the surface. Worst on dark and glossy materials.
  • It does not attack the plastic, but stress cracks still appear in polycarbonate and acrylic, because of the heat of curing.
  • Cotton and wool give a violent exotherm. Superglue on a cotton glove or rag can become scalding hot in seconds and smoke. Not a curiosity — it burns.
If you get it on your fingers

Ease apart gently, roll — do not tear. Ethyl acetate dissolves cured CA without destroying the substrate, and is the best remover on plastic and paint as well. Acetone works but attacks more.

FastBrittleThin jointNot damp
04

Epoxy

The strongest structural adhesive, and the only one that fills large gaps without losing strength.

Epoxy Two-component · bisphenol A diglycidyl ether plus amine

Addition polymerisation, resin + hardener
Pot life5 min – 2 h
Full strength24–72 h
GapUp to 5 mm+
Shear strength20–40 MPa
Temperature−40 to +120 °C
ShrinkageVery low

The mixing ratio is not a guideline

Resin and hardener react in fixed molar proportions. An excess of either does not take part — it stays behind as a plasticiser in the cured mass and lowers both strength and heat resistance. So no, you cannot "add a bit more hardener to speed it up". Weigh it or use the syringe, and mix for at least a minute, edges and bottom included.

Double mixing

Mix thoroughly in one pot, pour into a clean pot, and mix again. It takes ten seconds and eliminates the commonest epoxy failure: unmixed material from the wall of the pot.

Use for

  • Load-bearing joints in metal, composite, stone and glass.
  • Gap filling where the fit is poor. The only adhesive type that keeps its strength in a thick joint.
  • Potting of electronics and magnets.
  • Laminating of glass and carbon fibre.
  • Filler by stirring in fumed silica to a mayonnaise consistency. With microballoons it becomes light and sandable, with carbon powder it becomes stiff and black.

Fast or slow

Five-minute epoxy is convenient and considerably weaker — typically half the strength, and clearly worse heat and moisture resistance. It is a fixing adhesive, not a structural one. If the joint has to carry load, use a grade with at least an hour's pot life.

Heat speeds it up and improves it. A day at 40 °C gives higher final strength and heat resistance than a week at room temperature. Below 15 °C it cures incompletely and stays soft for good.

Steel epoxy contains no steel

The tubes are called steel epoxy and metal epoxy and they are grey, which is the entire marketing idea. Grey is the colour of the filler. Biltema's Stål-epoxi 26+26 g (art. 364013) declares, in the safety data sheet for the hardener component, quartz at 50–70 % and barium sulfate at 10–30 % — mineral, in other words — plus benzyl alcohol, an amine hardener and an accelerator. No iron anywhere, and the same holds for most competitors.

The filler still earns its place, just not in the way the name suggests: it reduces shrinkage, damps the exotherm in a thick batch, and makes the cured joint stiffer and easier to file, drill and sand. What it does not do is raise tensile strength, conduct heat or current the way metal does, or make the joint weldable. Choose epoxy by pot life and by what the joint has to do, not by the word on the tube.

Allergy

Uncured epoxy is strongly sensitising, and once the allergy has developed it is lifelong. Amine hardeners are the worst. Nitrile gloves, and change them — epoxy penetrates them within about twenty minutes. Wash off with soap and water or acetone before curing — afterwards nothing dissolves it.

StrongestGap-fillingLoad-bearingAllergen
05

Polyurethane

Tough, moisture-resistant, and the only common adhesive that expands while it cures.

Polyurethane PU · one-component moisture-curing · ”Gorilla”, D4 adhesive

Isocyanate + moisture → polyurethane + CO₂
Fixture time30–120 min
Full strength24 h
Expansion2–4× the volume
Shear strength10–20 MPa
MoistureD4, boil-proof
Shelf life6–12 months once opened

The isocyanate groups react with water — from the air, from the wood, or from a dampened surface — and give off carbon dioxide, which is what makes it foam. The foam itself has no strength: what carries the load is the thin film in the joint itself.

The foam deceives

A joint swollen full does not mean it is full of adhesive. A PU bond must be forced together under pressure for the whole cure time, otherwise the gas pushes the parts apart and you get a joint made of foam. That is the commonest PU failure by far.

Use for

  • Wood outdoors — D4 rated, withstands boiling water. Better than PVAc when wet.
  • Dissimilar materials: wood to metal, stone to plastic.
  • Damp substrates, where most other adhesives refuse.
  • Foam for insulation and filling in a can, the same chemistry.
Isocyanates

Uncured isocyanates sensitise the airways and can cause occupational asthma. Ventilate, wear gloves, and avoid skin contact. Since 24 August 2023 REACH Annex XVII entry 74 requires training for professional handling of products containing more than 0.1 % monomeric diisocyanates. Private use is exempt — but the reason behind the requirement, an estimated 5000 new cases of work-related asthma a year in the EU, applies just as much in a garage.

Wood outdoorsToughExpandsNeeds clamping
06

MS polymer

The modern all-rounder. It bonds and seals, sticks to nearly everything, can be painted over and has no smell.

MS polymer Silane-modified polymer · SMP · hybrid adhesive · ”assembly adhesive”

Silane crosslinking with atmospheric moisture
Skin time10–30 min
Cure rate2–3 mm/day
Joint thickness2–5 mm
Elongation150–300 %
OverpaintableYes
Temperature−40 to +100 °C

Why it has replaced so much

MS polymer is in practice silicone's adhesion and elasticity without silicone's two great problems: it can be painted over, and it does not contaminate the surface for future bonding or painting. It contains neither isocyanate nor solvent and hardly smells. Wherever something has to stay put and move, it is the first choice. Odourless is not the same as unlabelled, though: the cross-linking and the adhesion are done by vinyl and amino silanes at tenths of a per cent, catalysed by a trace of a tin compound, and the silanes are why the tube still carries an allergy warning.

Use for

  • Mounting of trim, panels, mirrors and fittings.
  • Elastic joints between materials that move differently.
  • Vehicles and boats — resists water, salt and UV.
  • Sealing joints that are to be painted, unlike silicone.
The joint has to be able to stretch

An elastic joint should adhere to two surfaces, not three. If it also grips the bottom it cannot stretch and it cracks. Lay backer rod of closed-cell foam in the bottom, and keep the depth to about half the width.

ElasticPaintableOdourlessAll-round
07

Silicone

Unmatched in heat, wet and movement. And the material that ruins the surface for everything else, for good.

Silicone Polysiloxane · acetoxy or neutral cure

Condensation curing with atmospheric moisture
Skin time5–20 min
Cure rate2–3 mm/day
ElongationUp to 600 %
Temperature−60 to +200 °C
OverpaintableNo
Surface energy24 mN/m

Two systems that are not interchangeable

  • Acetoxy silicone smells of vinegar while curing and gives off acetic acid. Cheapest, and the strongest adhesion on glass and tile. It corrodes metal, copper, brass and concrete in particular. Never in electronics.
  • Neutral silicone (oxime or alkoxy) smells faintly or not at all and does not attack metal. Necessary on natural stone, metal and anything electrical.
Read which type the tube is before you open it

The smell settles the question only once it is already too late. The ingredient list settles it in the shop: an acetoxy system declares acetoxysilanes, and the two common ones are methylsilanetriyl triacetate (CAS 4253-34-3) and triacetoxyethylsilane (CAS 17689-77-9). Biltema's Silikon Transparent (art. 3624052) contains both, which makes it the cheap kind that gives off acetic acid and attacks limestone, concrete and base metals. A neutral system declares oxime or alkoxy silanes instead.

Silicone contaminates everything it touches

Silicone oil migrates, spreads through the air as dust and settles as an invisible film with a surface energy of 24 mN/m. A surface that has had silicone on it can afterwards be neither painted, bonded nor lacquered — the coating draws up into what are called fish eyes. It cannot be washed off completely. Never keep silicone in the same room as painting, and use separate tools and cloths. This is the commonest cause of inexplicable adhesion failures in a workshop.

Getting a bead out is a different problem, and that one is solvable. Cured silicone cannot be dissolved — the network is cross-linked — but it can be swollen with d-limonene until its grip fails: Chemicals recipe 32. The contamination above stays behind regardless.

It barely works as an adhesive

Silicone has low strength and is used for sealing and taking up movement, not for carrying load. Judge it as a sealant.

HeatWetMovementContaminates
08

Anaerobics

Threadlocking and flange sealing. Cures only where it is shut in between two metal surfaces — which is exactly where you want it.

Anaerobics Threadlocking, thread sealing, bearing retention · ”Loctite”

Radical polymerisation, inhibited by oxygen, catalysed by metal ions
Fixture time10–30 min
Full strength24 h
Gap0.05–0.5 mm
Temperature−55 to +150 °C
SubstrateMetal
Shelf life1–2 years

The mechanism is elegant: oxygen in the air prevents polymerisation, and iron and copper ions start it. As long as the adhesive sits in the bottle — polyethylene, which lets oxygen through — nothing happens. The moment it is shut in between two metal surfaces it begins to cure. The excess squeezed out stays wet and is wiped away.

The colour codes, roughly
TypeStrengthUsed for
PurpleLowSmall screws, adjusting screws. Releases with hand tools.
BlueMediumThe standard threadlocker. Releases with ordinary tools.
RedHighPermanent. Needs heat, 250 °C, to release.
GreenWickingThin, drawn into threads and bearings already assembled.
White or yellowSealingPipe threads, replaces hemp and PTFE tape.
Passive metals need an activator

Stainless, aluminium, titanium and zinc-plated give off too few ions, and the cure becomes very slow or fails entirely. Use an activator, or count on several days. On plastic threads nothing happens at all — and several plastics stress-crack from uncured anaerobic adhesive into the bargain.

ThreadlockingMetalVibration
09

UV-curing

Unlimited open time and then three seconds to finished. The price is that the light has to get through.

UV adhesive Acrylate with photoinitiator · ”5-second glue” with a lamp

Radical polymerisation started by UV or visible blue light
Open timeUnlimited in the dark
Cure time3–30 s
Wavelength365 or 405 nm
Gap< 1 mm
RequirementA transparent part
Shear10–20 MPa

Being able to position at leisure and then cure on command is a unique combination. Used for glass, optics, jewellery, and for tacking components down on circuit boards. The same chemistry as 3D printer resin, which means a bottle of SLA resin works very well as UV adhesive, often at a tenth of the price.

Two limitations

Shaded areas never cure. What lies under a metal part stays wet for years. Oxygen inhibition also leaves the free surface against air tacky — cover it with film or post-cure under water.

UV light and uncured resin

365 nm damages the cornea and causes snow blindness within minutes. Use UV-blocking glasses, not sunglasses. Uncured acrylate is sensitising in the same way as epoxy.

Glass and opticsPositioningNeeds light
10

Others

Three adhesive types that dry rather than cure, and therefore follow entirely different rules.

Wood glue, PVAc White glue · polyvinyl acetate · D2, D3, D4

The water leaves, the dispersion coalesces
Clamping time30–60 min
Full strength24 h
Minimum temp+10 °C
Moisture classD2 indoors – D4 outdoors
RequirementPorous substrate
CreepUnder load

A properly made PVAc joint in wood is stronger than the wood itself — the break happens in the fibre beside the glue. It needs clamping, a flat fit and at least one porous part, because the water has to go somewhere.

It creeps under constant load and softens with heat and moisture. For load-bearing structures outdoors: PU or epoxy instead.

WoodStronger than the fibreNeeds clamping

Contact adhesive Neoprene or polychloroprene in solvent

The solvent evaporates, the surfaces weld under pressure
Flash-off10–20 min
Strength immediatelyYes
AdjustableNo
SurfaceLarge, flat
Heat resistanceLow, 60–70 °C
Fire riskHigh

Applied to both surfaces and left to dry until it no longer transfers to a finger. Only then are the parts brought together, and there is no second chance — it grips instantly and permanently.

For laminate, rubber matting, upholstery and trim. Not for anything that has to hold in heat.

Large areasInstant grabNo adjustment

Hot melt Glue gun · EVA or polyamide

Melts and sets again, no chemical reaction
Open time10–60 s
Working temp120 or 190 °C
Heat resistance50–80 °C
GapFills well
StrengthLow
ReversibleYes, with heat

Underrated for temporary fixturing, strain relief and cable management, and unbeatable for holding something in place quickly. That it can be removed with heat is a feature, not a weakness.

The high-temperature grade at 190 °C gives noticeably better adhesion and strength than the 120-degree kind. Polyamide hot melt withstands 120 °C and is used in electronics.

It creeps in heat

A hot melt joint in a car in summer or next to a power supply slides apart. Do not use it where the temperature can go above 50 °C, and not for anything that has to carry load over time.

TackingReversibleNot heat
11

Material matrix

Look up the material, read across the column. Valid after correct preparation — without it every box is a cross.

Key

✓ works well  ·  ! works with reservations, see the note  ·  ✗ does not work, or damages the material

Material against adhesive type
MaterialSurface energy
mN/m		Cyano­acrylateEpoxy PUMSSiliconeAnaerobicUVNote
Steel and cast iron700+ ✓✓✓ ✓!✓ ✗Acetoxy silicone corrodes. Abrade and degrease.
Aluminium500 ✓✓✓ ✓!! ✗The oxide reforms in seconds — bond immediately. Anaerobics need activator.
Stainless700+ ✓✓✓ ✓!! ✗Passive — anaerobics need activator.
Copper and brass1100 ✓✓✓ ✓✗✓ ✗Acetoxy silicone corrodes badly. Catalyses anaerobics best of all.
Glass250–500 !✓✓ ✓✓✗ ✓UV adhesive is the first choice. CA blooms visibly on glass.
Ceramic and stonehigh ✓✓✓ ✓!✗ !Natural stone needs neutral silicone — acetoxy stains it.
Wood40–60 !✓✓ ✓✗✗ ✗PVAc is usually best indoors, PU outdoors. CA soaks into end grain.
ABS42 ✓✓✓ ✓!✗ !Best of all: solvent-weld with acetone.
PETG44 ✓✓✓ ✓!✗ ✓The commonest 3D printing plastic after PLA. Bonds well.
PLA40 ✓✓✓ ✓!✗ ✓Takes no heat — avoid hot melt and exothermic fast epoxy in a thick joint.
Polycarbonate46 !✓✓ ✓!✗ ✓Stress-cracks from CA, anaerobics and IPA. Test somewhere hidden.
Acrylic, PMMA41 !✓✓ ✓!✗ ✓Weld with dichloromethane for an invisible joint. CA blooms.
PVC, rigid40 ✓✓✓ ✓!✗ !Pipe cement is solvent welding and beats every adhesive.
PVC, flexible39 !!! ✓!✗ ✗The plasticiser migrates out and breaks the joint within months. An elastic adhesive is required.
Nylon, PA42 !!✓ ✓!✗ !Takes up moisture that disturbs curing. Dry it in an oven first.
POM, acetal37 ✗✗✗ !✗✗ ✗Needs primer or a mechanical fixing. Notoriously difficult.
PP polypropylene29 ✗✗✗ ✗✗✗ ✗Primer, flame treatment or methacrylate adhesive. Otherwise: plastic welding.
PE and HDPE polyethylene31 ✗✗✗ ✗✗✗ ✗The same. It is why glue bottles are made of PE.
PTFE18 ✗✗✗ ✗✗✗ ✗Needs etching with sodium naphthalenide. In practice: a mechanical fixing.
Silicone rubber24 ✗✗✗ ✗!✗ ✗Only silicone sticks to silicone, and even that badly. Primer required.
EPDM and nitrile rubber33 ✓✗! ✓!✗ ✗CA is surprisingly good on rubber. Epoxy is too rigid.
Carbon and glass fibrehigh !✓✓ ✓✗✗ ✗Sand off surface resin and release agent. Epoxy to epoxy is unbeatable.
12

Troubleshooting

Arranged by what happened, not by cause. Look at the fracture surface first — it nearly always gives the answer.

When the joint does not hold
SymptomLikely causeRemedy
The adhesive stays on one side, the other is bareAdhesive failure — the surface was greasy, too low in energy, or oxidisedAbrade and degrease again. Check the surface energy on the scale at the top.
The adhesive split down the middle and stayed on bothCohesive failure — the adhesive itself was too weakChange to a stronger type, or reduce the bond line if the adhesive is rigid.
The joint let go after a few weeks, not immediatelyThermal movement between dissimilar materials, or moistureChange to an elastic adhesive, MS or PU. A rigid adhesive between aluminium and plastic does not last.
It never cured, still tacky after a dayWrong mixing ratio, too cold, or poorly stirredWeigh it rather than judging by eye. Below 15 °C epoxy cures incompletely.
Tacky patches in otherwise cured epoxyUnmixed material from the wall of the potMix in two pots. It cannot be saved — scrape it off and start again.
White deposit around the superglue jointBlooming — monomer vapour cured on the surfaceLow-odour grade, more ventilation, less adhesive. Wipe off with acetone on anything delicate.
The plastic crazed in a fine network after bondingStress cracking in polycarbonate or acrylicAnneal the part with heat before bonding, or change to epoxy or UV adhesive.
The paint pulls into fisheyes when sprayingSilicone contaminationSilicone in the room. Wash with degreaser, change cloths, and never spray in the same room.
The PU joint swelled but carries nothingThe parts were pushed apart by the gasForce them together under pressure for the whole cure time.
The anaerobic did not cure in the threadA passive metal — stainless, aluminium, zinc-platedActivator, or allow several days.
The UV adhesive is wet under the partShadowing — the light does not reachAt least one transparent part, or change adhesive type.
The UV adhesive is tacky on top but hard insideOxygen inhibition at the free surfaceCover with film during curing, or post-cure under water.
The joint cracked lengthwise down the middleThree-point adhesion — the joint also grips the bottomBacker rod. Joint depth about half the width.
Flexible PVC let go after a few monthsThe plasticiser migrates into the bond lineAn elastic adhesive intended for plasticised PVC, or a mechanical fixing.
The wood glue joint slid apart under loadPVAc creeps under constant loadPU or epoxy for load-bearing joints.
The hot melt let go in heatA thermoplastic adhesive softensNo fixturing above 50 °C. Change adhesive type.
Nothing sticks at all, whatever the adhesivePP (polypropylene), PE (polyethylene), POM, PTFE or siliconePrimer, flame treatment, methacrylate adhesive, plastic welding or a screw.
The tube is solid although it is half fullA moisture-curing adhesive that has had airKeep cyanoacrylate refrigerated. MS and PU: squeeze the air out, seal the nozzle.