Structural adhesives selection
Structural adhesives are polymeric formulations capable of transmitting loads between components without excessive deformation, replacing or supplementing mechanical fasteners. The technical market concentrates on epoxies, acrylics, cyanoacrylates, and ceramic cements, selected according to the specific strength required — from 0.055 MPa·m³/kg (approximately 7 800 psi·in³/lb) for epoxy to the high-temperature resistance of ceramics (980 °C / 1800 °F). The following table summarizes the key properties of epoxy adhesive as a representative class of structural adhesives.
| Property | Value |
|---|---|
| Density | 1.25 × 10³ kg/m³ / 78 lb/ft³ |
| Tensile modulus | 3.5 GPa / 508 ksi |
| Tensile strength | 69 MPa / 10 000 psi |
| Specific modulus (E/ρ) | 2.8 GPa·m³/kg / 0.101 Mpsi·in³/lb |
| Specific strength (σ/ρ) | 0.055 MPa·m³/kg / 7 800 psi·in³/lb |
| Maximum service temperature | 80–215 °C / 176–419 °F |
Data extracted from Engineering Toolbox (2008).
Operating parameters
Section titled “Operating parameters”Each adhesive family requires particular processing conditions to develop optimal structural properties. The following table gathers the verified parameters from the consulted sources.
| Adhesive | Cure temperature | Cure time | Clamping pressure | Maximum service temperature |
|---|---|---|---|---|
| Cyanoacrylate (e.g., Eastman 910 SL) | Room temperature (20–25 °C / 68–77 °F) | 10 min (ready to use) | 1–2 min light hand pressure | Not specified; short-term use |
| Epoxy (heat cure) | 120 °C / 250 °F (several hours) | 2–6 h (depending on formulation) | 35–140 kPa / 5–20 psi (general); 350 kPa / 50 psi for thin bond lines | Depending on formulation: 80–215 °C / 176–419 °F |
| Ceramic cement (e.g., NBS-x-142) | Air drying and subsequent sintering | Variable | No specific pressure | Up to 980 °C / 1800 °F |
| Cellulose nitrate cement (for paper-backed gages) | 54 °C / 130 °F (hot air to remove solvents) | Until solvent dries | Light contact pressure | Dry room temperature; moisture sensitive |
Process parameters according to eFunda (strain gage adhesives) and Engineering Toolbox.
Typical tolerances
Section titled “Typical tolerances”The dimensional tolerances of the parts to be joined and the adhesive bond line thickness have a direct impact on joint strength. The values given are recommended ranges for general structural applications; always verify with the manufacturer’s data sheet.
| Parameter | Typical tolerance |
|---|---|
| Adhesive bond line thickness – epoxy | 0.05–0.2 mm / 0.002–0.008 in |
| Adhesive bond line thickness – cyanoacrylate | 0.025–0.1 mm / 0.001–0.004 in |
| Maximum clearance between substrates for structural bonding | ≤ 0.25 mm / 0.010 in |
| Recommended surface roughness (Ra) | 3.2–12.5 µm / 126–492 µin |
Thickness tolerances depend on viscosity and application method; the values collected here are typical of industrial practice [VERIFY].
Applicable materials
Section titled “Applicable materials”Structural bonding works on a wide range of engineering materials. Adhesive selection must consider the surface energy and service loads of the substrate.
| Substrate | Recommended adhesives |
|---|---|
| Steel (e.g., AISI 1045) | Epoxy, acrylic, cyanoacrylate |
| Aluminum (e.g., 6061-T6) | Epoxy (suitable after anodizing or chemical treatment) |
| Cast iron (grade 20) | Epoxy (good wetting) |
| Carbon fiber/epoxy composites | Epoxy, methacrylate acrylic |
| Plastics (nylon, polypropylene) | Cyanoacrylate, modified polyurethane adhesives |
| Ceramics (alumina) | Ceramic cement, modified epoxy |
| Glass | Epoxy, UV-curable acrylic |
The adhesive-substrate pairs are based on the compatibilities described in the consulted sources and standard industrial practice.
Advantages and limitations
Section titled “Advantages and limitations”Each type of structural adhesive presents a unique balance of performance. The qualitative criteria are summarized below, based on verified data.
| Adhesive | Advantages | Limitations |
|---|---|---|
| Cyanoacrylate | – Cures in 10 min without mixing – High tensile strength on plastics – Easy application |
– Limited short-term durability (months) – Poor moisture resistance – Poor fatigue behavior |
| Epoxy | – High tensile strength (69 MPa / 10 000 psi) – Excellent thermal and chemical resistance – Can be loaded with reinforcements (glass fiber, carbon) |
– Slow cure (several hours at 120 °C) – Requires cleaning and precise mixing – Brittle if not formulated with elastomers |
| Ceramic cement | – Withstands up to 980 °C / 1800 °F – Low electrical conductivity – Good compressive strength |
– Brittle and sensitive to thermal shock – Not suitable for high tensile loads – Often requires high-temperature cure |
| Cellulose nitrate | – Compatible with paper backings – Low temperature cure (54 °C / 130 °F) – Easy removal |
– Very moisture sensitive – Low mechanical strength – Only for temporary instrumentation |
The advantages and limitations have been extracted directly from the eFunda selection guides and the epoxy properties listed in Engineering Toolbox.
Selection guide
Section titled “Selection guide”The choice of structural adhesive must be guided by service conditions and mechanical requirements. The following table guides the decision based on critical variables.
| Criterion | Cyanoacrylate | Epoxy | Ceramic cement |
|---|---|---|---|
| Intended use | Quick non-structural permanent bonds | Permanent structural bonds with high load | Rigid bonds in very high temperature environments |
| Tensile strength | Medium-low (depends on substrate) | High (69 MPa / 10 000 psi in neat resin) | Low in pure tension, high in compression |
| Service temperature range | −20 to 80 °C / −4 to 176 °F (estimated) | 80–215 °C / 176–419 °F (depending on formulation) | Up to 980 °C / 1800 °F |
| Assembly time | < 2 min | 2–6 h + cooling | Variable, often with subsequent sintering |
| Required surface preparation | Solvent cleaning | Degreasing, abrasion, optional chemical treatment | Clean, dry surface |
| Moisture resistance | Low | Excellent (when fully cured) | Good (unless porous) |
| Relative cost | Low | Medium–high | High |
Source of criteria: eFunda (Strain Gage: Materials) and epoxy properties from Engineering Toolbox.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the typical tensile strength of a structural epoxy adhesive?
Section titled “What is the typical tensile strength of a structural epoxy adhesive?”Neat epoxy adhesive reports a tensile strength of 69 MPa / 10 000 psi, a value that can be increased with glass fiber up to 300 MPa / 43 500 psi.
Up to what temperature can a ceramic adhesive be used?
Section titled “Up to what temperature can a ceramic adhesive be used?”Ceramic cements of the NBS-x-142 type maintain their integrity up to 980 °C / 1800 °F, according to specifications validated by eFunda.
How long does a cyanoacrylate adhesive require to reach handling strength?
Section titled “How long does a cyanoacrylate adhesive require to reach handling strength?”Conventional cyanoacrylate allows handling of the part within 10 min, with only 1–2 min of light pressure during the initial fixing phase.
What clamping pressure is recommended when applying an epoxy adhesive?
Section titled “What clamping pressure is recommended when applying an epoxy adhesive?”A clamping pressure of 35–140 kPa / 5–20 psi is common; for thin bond lines 350 kPa / 50 psi is recommended, always according to the manufacturer’s instructions.
What density does a typical epoxy adhesive have and how does it affect joint weight?
Section titled “What density does a typical epoxy adhesive have and how does it affect joint weight?”The density of neat epoxy is 1.25 × 10³ kg/m³ / 78 lb/ft³, making it a lightweight option compared to equivalent metallic mechanical fasteners.
What is the specific strength (strength‑density ratio) of an epoxy adhesive?
Section titled “What is the specific strength (strength‑density ratio) of an epoxy adhesive?”Epoxy presents a specific strength of 0.055 MPa·m³/kg (≈ 7 800 psi·in³/lb), a value that justifies its use in aerospace and automotive applications where the strength-weight ratio is critical.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/engineering-materials-properties-d_1225.html
- efunda.com: https://www.efunda.com/designstandards/measurements/strain_gages/strain_gage_selection_matl.cfm
- manufacturingguide.com: https://www.manufacturingguide.com/en/search/process?field_process_parents_is=1031