adhesive-selection

name	adhesive-selection
description	Adhesive selection — epoxy/acrylic/polyurethane/silicone, lap shear strength, peel strength, surface preparation, ASTM D1002/D1876/D903, bonded joint design, thermal limits, aerospace/structural bonding.
metadata	{"priority":7,"promptSignals":{"phrases":["adhesive selection","adhesive bonding","structural adhesive","lap shear adhesive","epoxy adhesive","adhesive joint design"],"minScore":3}}

Adhesive Selection — Complete Skill

Adhesive Chemistry Types

Epoxy (Structural — Highest Strength)

1K (single component): heat cure 120–180°C; shelf stable; aerospace qualification 2K (two component): room temperature cure or heat cure; mix ratio critical; immediate use Key properties:

• Lap shear strength: 15–35 MPa (aluminum); 20–45 MPa (steel)
• Tensile strength: 35–60 MPa (bulk)
• Peel strength: 0.5–2.0 kN/m (lower than acrylics; brittle peel failure)
• Service T: -55°C to +120°C (standard); up to +200°C (high-T grades)
• Chemical resistance: excellent (acids, bases, solvents)
• Elongation: 1–5% (rigid; brittle without toughener)
• Toughened epoxy: +rubber particles or CTBN; elongation 5–15%; peel ↑↑

Applications: aerospace primary structure, tooling, structural assemblies

Structural Acrylic (MMA — Methyl Methacrylate)

Two-component (bead-on-bead): activator on one surface; adhesive on other Properties:

• Lap shear: 15–25 MPa
• Elongation: 50–200% (much more flexible than epoxy → better peel)
• Peel: 4–10 kN/m
• T range: -55°C to +120°C
• Cure time: 5–90 min (fast cure; less prep required)
• Gap filling: fills 0.1–5 mm gaps (forgiving)

Applications: marine, truck body, assembly where thermal cycling + peel important

Polyurethane (PU)

Flexible structural adhesive:

• Lap shear: 5–20 MPa
• Elongation: 100–400% (very flexible)
• Peel: 5–15 kN/m
• T range: -40°C to +90°C
• Moisture sensitive during cure; CO₂ bubbles if substrate wet
• Good for dissimilar materials (CTE mismatch absorption)

Applications: automotive windshield (ELV), trim, flexible assemblies

Silicone

Not structural — primarily sealing:

• Shear strength: 0.5–3 MPa (very low)
• Elongation: 200–600% (excellent flexibility)
• T range: -60°C to +260°C (silicone) or to +300°C (high-T)
• UV/weather resistant; excellent for outdoor sealing

Use only for: sealing, electronics potting, gasket replacement, vibration damping

Cyanoacrylate (Instant Adhesive)

• Rapid bond (1–60 sec); requires close fit (gap < 0.1 mm)
• Lap shear: 5–25 MPa (brittle)
• T limit: 70–120°C
• Poor chemical resistance, moisture sensitivity long term
• Use for: fast assembly, small parts, difficult-to-clamp

Anaerobic

• Cures in absence of oxygen in metal contact
• Threadlocking (Loctite 243), retaining compounds (Loctite 638)
• Shear: 5–25 MPa; T: 150°C max (Loctite 272: 230°C)

Surface Preparation (Critical)

Rule: adhesive strength is only as good as weakest boundary layer

Bare aluminum (etching):

1. Degrease (MEK, acetone, or IPA wipe)
2. Etch: chromic/sulfuric acid (FPL etch) or phosphoric acid anodize (PAA) for aerospace
3. Bond within 4 hr of prep (prevent oxide regrowth) Lap shear improvement: clean → 8 MPa; PAA prep → 35 MPa

Steel:

1. Degrease + abrade (80-grit), or grit blast (Ra 6–10 μm)
2. Acid pickle or phosphate conversion coating

CFRP/GRP composites:

1. Abrade (80–120 grit); avoid exposing fibers → generates loose debris
2. Solvent wipe (acetone)
3. Peel ply removal (preferred over sanding; produces fresh, clean surface)

Primer: epoxy primer for elevated-T service; promotes adhesion + prevents corrosion

Joint Design Principles

Lap joint geometry: Shear stress in overlap: τ_avg = F / (b × L) [Pa; b = width; L = overlap length] End stress concentration: τ_max / τ_avg = k_s (depending on overlap L, adhesive modulus, substrates)

Design approach: Overlap L ≥ 20–30 t_adherend (substrate thickness) to reduce peel Chamfer or thin substrate at overlap ends to reduce peel stress Avoid purely peel loading (adhesive peel strength ≪ shear strength)

Tapered (scarf) joint: Shear stress uniform along bond length; no peel stress concentration Preferred for high-performance structural applications

Hartsmith design approach (structural adhesive): τ_design = τ_fail / 3 (factor of safety = 3) Overlap length: L = 3 × F / (τ_fail × b)

Failure Modes

Cohesive failure: fracture within adhesive layer → adhesive not over-designed Adhesive failure: fracture at adhesive-substrate interface → surface prep inadequate Substrate failure: substrate fails before adhesive → optimal (adhesive over-designed)

Target: cohesive failure at > 80% of joint area → good surface prep

Standard Test Methods

Test	Standard	Measures
Lap shear	ASTM D1002	Tensile shear strength [MPa]
T-peel	ASTM D1876	Peel flexibility [N/m]
Climbing drum peel	ASTM D1781	Peel for thick laminates
180° peel	ASTM D903	Peel strength [N/m]
Tensile (butt joint)	ASTM D897	Normal tensile strength [MPa]
Wedge test	ASTM D3762	Environmental durability
Mode I	ASTM D5528	G_IC for composites

Environmental Durability

Moisture: most significant degrader; water weakens adhesive-metal interface Testing: 1000 hr salt spray (ASTM B117); 1000 hr humidity (ASTM D2247: 100% RH, 38°C) Requirement: retain > 60% of dry lap shear after exposure

Applications and Selection Guide

Application	Recommended adhesive	Reason
Aerospace primary structure	1K epoxy	Highest strength; qualified
Automotive body	Structural acrylic MMA	Fast; flexible; peel
Marine	Polyurethane or modified epoxy	Flexibility; UV resistance
Electronics	Epoxy or silicone	Potting; T range
Threadlocking	Anaerobic	Self-locking; correct Loctite grade
Fast assembly (light load)	Cyanoacrylate	Speed

Output

Provide: adhesive type and specific product grade, lap shear strength [MPa] at operating T, peel strength [kN/m], service temperature range [°C], elongation [%], recommended surface preparation procedure, joint geometry (overlap L [mm], width b [mm]), design shear stress [MPa] with safety factor, expected failure mode (cohesive/adhesive), environmental test requirement (ASTM B117/D2247), and applicable standard (ASTM D1002/D1876/D903).