Adhesion science explains why an adhesive wets a surface, holds to it and fails, and this section splits that science into eight pages that follow one bond from the surface to the break. The same principles govern every one of the adhesive chemicals that industrial adhesives are formulated from, so the adhesive technology of a hot melt and of an epoxy starts at the same interface.
Adhesion acts through adsorption, chemical bonding, mechanical interlocking, diffusion and electrostatic forces, never through one mechanism alone. Wetting decides whether contact forms: untreated polypropylene carries 29.6 mN/m of total surface free energy against 64.6 mN/m for glass. A pressure-sensitive adhesive tacks below a storage modulus of 3 × 10^5 Pa at 25 °C and 1 Hz, and in one fast epoxy study a 60 °C cure gave a glass transition temperature of 66.3 °C against 116.9 °C after a 120 °C cure. The topic table assigns each question its page, and the bond sequence ties them together.
Sources and checks: identities and hazard classifications are read at ECHA and the issuing regulator, standards at the body that publishes them, and grade values at the data sheet named next to them; methodology and fact-checking explains each step.
What Is Adhesion?#
Adhesion is the attraction between two different bodies across an interface, such as an adhesive and a substrate, while cohesion is the internal strength of one material, the adhesive or the substrate itself. Practical adhesion combines both: interfacial forces create the contact, drying, cooling or curing builds cohesion, and defects such as weak boundary layers, poor wetting and under-cure subtract from the sum.
Which forces act at the interface? A review of adhesion mechanisms in PubMed Central (PMC8707418) groups them into four main theories: adsorption or chemical bonding, mechanical adhesion, diffusion and electrostatic adhesion. Adsorption rests on van der Waals forces (Debye, Keesom and London) and on hydrogen bonds, and London dispersion forces are present in every bond. A second mechanism review (PMC12853151) concludes that "adhesion theories are complex, making the attribution of bonding to a single mechanism hard".
Adhesive Science Topics: One Page per Question#
Eight adhesive science pages each answer one question about a bond, from why it sticks to what a bonding term means. The table below compares their questions, governing concepts and key values, in the order a bond meets them.
| Page | Question it answers | Governing concept | One value or rule |
|---|---|---|---|
| how adhesion works | Why does an adhesive stick? | Adsorption or chemical bonding, mechanical adhesion, diffusion and electrostatic theories | London dispersion forces are present in every bond |
| surface energy and wetting | Why do low-energy plastics resist bonding? | An adhesive wets a solid when its surface tension is at or below the solid's surface free energy | OWRK total surface free energy: PP 29.6 mN/m vs glass 64.6 mN/m (Krüss application report AR272) |
| surface preparation for bonding | How is a surface made bondable, and for how long? | Cleaning, roughness and polar groups; treatment decays by hydrophobic recovery | Treated BOPP at a 68° water contact angle stabilized at 75° after one year (PMC12526541) |
| adhesive failure modes | Where did the joint break? | Failure in the substrate, in the adhesive or at the interface, designated by ISO 10365:2022 | ISO 10365:2022 is the second edition and replaced ISO 10365:1992 |
| tack in adhesives | Why does a pressure-sensitive adhesive (PSA) grab on light contact? | Dahlquist criterion | Storage modulus G' below 3 × 10^5 Pa (0.3 MPa, about 43.5 psi) at 25 °C (77 °F) and 1 Hz (Dahlquist 1969) |
| how adhesives cure | How does a liquid adhesive become a solid bond? | Physical set (cooling, evaporation) or chemical cure | Chemical routes: polyaddition, moisture cure, free-radical, cationic UV, anionic, hydrosilylation |
| glass transition temperature in adhesives | At what temperature does a bond soften? | Tg is a method-dependent region; it rises with degree of cure and falls when absorbed water plasticizes the network | Fast epoxy: Tg 66.3 °C (151.3 °F) after a 60 °C cure, 116.9 °C (242.4 °F) after a 120 °C cure (PMC13120039) |
| adhesive terminology glossary | What does a bonding term mean? | Working definitions of core terms, built from cited sources | Standard definitions are quoted only from the standards themselves |
A topic earns its own page where one question has its own governing quantity: surface free energy for wetting, the Dahlquist modulus for tack, Tg for softening and the failure pattern for the break. The surface free energy values are total OWRK values, never mixed with Zisman critical surface tension; 1 mN/m equals 1 dyn/cm and 1 mJ/m².
How a Bond Forms and Fails: Surface, Contact, Set and Service#
A bond forms in five steps, from a surface the adhesive can wet to a joint that keeps its strength in service, and each step has its own page in this section. The five steps are listed below in the order a joint passes through them.
- Wet the surface: the adhesive's surface tension must sit at or below the substrate's surface free energy, with matching polar and dispersive parts.
- Prepare the surface, removing weak boundary layers, and bond before the treatment decays.
- Make intimate contact, so that interfacial forces act and the adhesive keys into pores and asperities.
- Build cohesion by cooling, drying or curing.
- Keep the joint below its softening region in service, and read any break by its location.
Interfacial failure on a polyolefin or a fluoropolymer points first to low surface free energy or to a treatment that has decayed. Beside the PP and glass values in the topic table, Krüss application report AR272 measured OWRK total surface free energy of 32.2 mN/m for untreated PE, 55.9 mN/m for aluminum and 16.1 mN/m for PTFE. Treatment raises the polar share, but the gain fades: in data reported in PMC12526541, BOPP treated to a 68° water contact angle stabilized at 75° after one year, and extruded PP treated to 50° stabilized at 80°, with most of the change in the first month.
Cure temperature caps the glass transition temperature (Tg) of a reactive adhesive, because Tg rises with degree of cure. The fast epoxy study behind the topic table (PMC13120039) also measured Tg 101.7 °C (215.1 °F) after an 80 °C cure and 109.3 °C (228.7 °F) after a 100 °C cure, between its 60 °C and 120 °C results. Absorbed water lowers Tg by plasticizing the network. When a joint finally breaks, ISO 10365:2022 designates the failure pattern by its location: in the substrate, in the adhesive or at the interface.
How Adhesive Science Connects to Adhesive Test Methods and Adhesive Chemistries#
Adhesive science explains a bond, adhesive test methods measure it, and adhesive chemistries apply it, so this section sits between the raw materials and the test pages. Peel, shear and tack values come from adhesive test methods, which measure what this section explains.
Each test page names its method with edition and status, and the glossary does the same for terminology. The science pages describe the quantity a test reads, and a peel or shear value is read together with the failure pattern beside it.
A tack value from a loop or probe test is read against the Dahlquist modulus on the tack page. Its frequency logic, a tan delta of 1 or more at low frequency for wetting and below 1 at high frequency for debonding, explains how one PSA wets on light contact and still resists separation.
Raw Materials That Raise Adhesion and Tack#
Silane adhesion promoters form covalent bonds across the interface to primed metals and glass, while tackifiers raise tack and lower modulus in PSAs and hot melts. Chemical bonding across the interface is the job of adhesion promoters and silane coupling agents, which form links such as silane Si-O-M bonds, isocyanate reacting with surface hydroxyl groups and epoxy bonding to oxide. A tackifier is a low-molecular-weight resin that works on the adhesive side of the bond.
A primer is a film-forming material applied before the adhesive to raise adhesion, the definition used in SCAQMD Rule 1168 (amended November 4, 2022; amendments proposed for a 4 December 2026 hearing). It adds a separate layer between the substrate and the adhesive.
No standard sets a threshold for a low-surface-energy (LSE) substrate. PE, PP and PTFE count as LSE plastics by their measured surface free energy, not by a cutoff.
Adhesive Chemistries Grouped by How They Set and Cure#
Hot melts set by cooling, water-based adhesives by evaporation, epoxy and polyurethane by polyaddition, cyanoacrylates by anionic cure and anaerobics by redox radical cure. Each set or cure route marks out one of the types of adhesives by chemistry, and moisture cure (isocyanate, silane), cationic UV cure and hydrosilylation complete the list.
Physical set against chemical cure decides whether Tg moves after application. A physically set adhesive builds cohesion by cooling, drying or coalescence without a reaction, while a chemically cured network raises its Tg with degree of cure and gains more with a post-cure.
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