KH-792

What is KH-792?

KH-792, chemically named N-(β-Aminoethyl)-γ-aminopropyltrimethoxysilane, is a diamine-functional silane coupling agent with CAS number 1760-24-3 and molecular formula C₈H₂₂N₂O₃Si. It is the Chinese designation for a molecule sold globally as Dow Corning Z-6020, Momentive Silquest A-1120, Shin-Etsu KBM-603, and Evonik Dynasylan DAMO. All of these trade names refer to the same compound with identical CAS and chemistry.

What distinguishes KH-792 from KH-550 (the monoamine equivalent) is the presence of two amine groups in the molecular structure: a primary amine at the end of the chain and a secondary amine in the middle of the alkyl spacer. This diamine architecture provides two reactive sites for epoxy or other electrophilic functionalities, resulting in higher crosslink density when incorporated into epoxy networks. The secondary amine in the middle of the chain is more reactive toward epoxy groups than a primary amine at equivalent temperature, making KH-792 the preferred choice in applications where fast cure, high crosslink density, and maximum adhesion are required.

The methoxy hydrolysis groups in KH-792 hydrolyze faster than the ethoxy groups in KH-550. This provides advantages in aqueous application systems (faster surface bonding) but also means the prepared aqueous solution has a shorter pot life. Another practical difference: methoxy hydrolysis releases methanol as a byproduct versus the ethanol released by KH-550. In enclosed manufacturing environments where air quality monitoring is in place, the methanol byproduct requires consideration of occupational exposure limits (OEL), which are more restrictive for methanol than ethanol.

Key Properties and Performance

KH-792 is a clear, pale yellow to amber liquid at room temperature, with a characteristic amine odor stronger than KH-550 due to the second amine group. Boiling point is approximately 259 °C at 1 atm; density 1.030 g/cm³ at 25 °C; flash point approximately 116 °C (closed cup). GC purity of commercial KH-792 is typically ≥97.0%.

The amine equivalent weight of KH-792 (approximately 54 g/eq for the combined primary and secondary amine) is lower than KH-550 (73 g/eq for the single primary amine). Lower amine equivalent weight means that per gram of silane added to an epoxy formulation, KH-792 contributes more amine hardener equivalents. This must be accounted for in stoichiometric calculations when using KH-792 as an adhesion promoter in amine-cured epoxy adhesives — if the silane is included at more than about 1 wt%, the stoichiometry shift may need compensation.

In lap shear adhesion tests of epoxy adhesives on glass substrates, KH-792 typically gives 10–20% higher dry adhesion and 15–30% better wet adhesion retention than KH-550 at equivalent loading. The improvement is due to the higher crosslink density at the silane-matrix interface created by the diamine structure. In aerospace structural bonding specifications, KH-792 (or its equivalent Z-6020/A-1120) is often explicitly called out because the improved wet adhesion retention is critical for structures exposed to humidity and temperature cycling.

Applications in Industry

Structural Epoxy Adhesives

KH-792 is the amino silane of choice in structural epoxy adhesives used for aerospace, automotive body-in-white bonding, and wind turbine blade manufacturing. In these systems, the adhesive must retain high lap shear strength not just dry but after immersion in water, humidity cycling, and thermal cycling between extreme temperatures. The diamine structure of KH-792 achieves this by forming a denser, more moisture-resistant crosslinked interface layer between the adhesive and the glass or metal substrate.

Application is as direct addition to the resin component of the adhesive at 0.5–2.0 phr. The silane migrates to the substrate interface during bond line assembly and reacts with both the substrate hydroxyl groups (via silanol condensation) and the epoxy/amine network (via the amine groups) during cure.

Glass Fiber Sizing for High-Performance Composites

In filament winding, pultrusion, and pre-preg lay-up processes producing structural composites for aerospace, defense, and high-end sporting goods, glass fiber sized with KH-792 provides higher interfacial adhesion than KH-550-sized fiber in the same epoxy matrix. The measurable improvement is in interlaminar shear strength (ILSS) and in cyclic fatigue resistance. For applications where composite components are weight-critical and must operate near their design limit under variable loads, this improvement justifies the premium over KH-550.

The KH-792 sizing is applied from dilute aqueous solution (0.3–0.6 wt%, pH 3.5–5.0 adjusted with acetic acid) during fiber drawing, the same process as KH-550 sizing.

Nylon and Polyamide Adhesion Systems

KH-792 performs significantly better than KH-550 in adhesion systems involving nylon (polyamide) substrates. The secondary amine in KH-792 undergoes transamidation reactions with nylon surface amide linkages, forming covalent bonds between the silane layer and the nylon surface. KH-550's primary amine does not undergo this transamidation under normal processing conditions. This makes KH-792 the preferred silane for priming nylon parts before bonding with epoxy adhesives or applying protective coatings.

Corrosion Inhibiting Primers on Aluminum Alloys

For aluminum alloys used in aerospace and automotive applications, KH-792 is used as a corrosion-inhibiting silane pre-treatment applied from dilute aqueous solution (0.1–0.5 wt%) before primer application. The silane forms a dense, cross-linked layer on the aluminum oxide surface through both Si–O–Al bonds and hydrogen bonding, providing a hydrophobic barrier against water access to the metal-oxide interface. The diamine structure of KH-792 creates a denser barrier layer than KH-550, giving improved salt spray resistance (ASTM B117) in panel tests.

Handling, Dosage, and Storage

KH-792 is an alkaline material due to the two amine groups. Wear appropriate gloves and eye protection — skin and mucous membrane irritation is possible on prolonged contact. Ensure adequate ventilation when handling in bulk.

For aqueous pre-treatment: dissolve 0.3–1.0 wt% in deionized water. The solution pH will be alkaline (pH 9–10); adjust to pH 3.5–5.0 with dilute acetic acid to promote silanol formation and surface bonding. Use freshly prepared solutions; pot life is 4–6 hours at pH 4–5.

For direct addition to epoxy adhesives or coatings: add 0.5–2.0 phr to the epoxy resin component before combining with hardener. Adjust hardener stoichiometry if using above 1.5 phr.

Storage: sealed containers below 25 °C, away from moisture. Shelf life is 12 months.

Frequently Asked Questions

When should I choose KH-792 over KH-550? Choose KH-792 when maximum epoxy adhesion, wet adhesion retention, or nylon substrate bonding is required, and the cost premium and methanol byproduct are acceptable. Choose KH-550 for general-purpose epoxy applications where moderate adhesion is sufficient and lower cost or ethanol byproduct (regulatory preference in enclosed environments) is a priority.

Does the methanol byproduct from KH-792 cause regulatory problems? In well-ventilated industrial settings the methanol released during silane hydrolysis is typically well below occupational exposure limits. In confined environments with poor ventilation, industrial hygienists should evaluate the potential methanol exposure and implement controls if needed. This is rarely a practical limiting factor for normal industrial usage.

Can KH-792 be used as a curative in epoxy adhesives? At very high loadings (above 5 phr), KH-792's amine content is large enough to act as a partial co-curative. At typical adhesion promoter loadings of 0.5–2.0 phr, it functions as an adhesion promoter only and does not significantly affect cure stoichiometry.

Is KH-792 the same as DAMO? Yes. Evonik's Dynasylan DAMO is the same chemical (CAS 1760-24-3) with the same structure and the same applications. KH-792 from Chinese suppliers is a cost-effective equivalent with the same specification.