Composite Bonding Technology: The Invisible Force Behind Lightweighting

Against the backdrop of rapid development in strategic industries such as aerospace and new energy, composite materials have become core materials driving industrial upgrading due to their excellent properties including high strength-to- weight ratio and corrosion resistance. Adhesive bonding technology, as a key process for joining composite materials, directly determines the structural performance and service life of components.

I.Industry Overview

Currently, global composite adhesive technology has entered a stage of parallel development characterized by large-scale application and continuous technological iteration. According to the “Global Composite Adhesives Market Research Report 2025,” the global composite adhesives market was valued at $4.522 billion in 2024 and is projected to grow to $7.077 billion by 2031, with a compound annual growth rate of 7.0%.

Adhesive bonding technology has been widely applied in fields such as aerospace, automotive manufacturing, rail transit, and wind power equipment. In the aerospace industry, secondary bonding and co-curing bonding are used for critical components like aircraft skins; in new energy vehicles, polyurethane adhesives have become the preferred solution for lightweight parts, in wind power applications,adhesive bonding is employed for blade connections, significantly enhancing component fatigue resistance.

The industry still faces challenges: weak bonding at the interface of heterogeneous composite materials; secondary bonding processes prone to defects such as air entrapment and porosity; stricter environmental regulations driving the development of solvent-free adhesives; and higher demands for precision in quality inspection.

II. Core Technologies

(1) Adhesive Technology

Adhesives are the core support for bonding performance. Currently, mainstream adhesives are divided into epoxy resin adhesives (accounting for 55%) and polyurethane adhesives (accounting for 25%). Epoxy resins dominate the aerospace sector due to their excellent thermal stability, while polyurethanes are experiencing rapid growth in automotive lightweighting applications thanks to their outstanding flexibility.

The focus of technological innovation lies in modification optimization and environmental enhancement: improving the brittleness of adhesives by incorporating nanofillers such as graphene and carbon nanotubes, while developing solvent-free, low-VOC products that balance performance with sustainability.

(2) Surface Treatment Technology

Weak boundary layers exist on the surface of composite materials, making surface treatment a critical preliminary step. The main methods include: mechanical treatments (such as sanding and sandblasting to increase surface roughness), chemical treatments (using acid or alkaline solutions to form an active layer), and plasma treatments (effectively removing weak boundary layers and enhancing interfacial bonding), which are particularly suitable for bonding heterogeneous composites.

(3) Adhesive Bonding Technology

Mainstream processes are divided into three categories: co-cure bonding (high quality but complex molds), co-bonding (good forming quality but higher risk), and secondary bonding (high part yield, the dominant method for complex components, but prone to air entrapment defects). To address the challenges of secondary bonding,the industry has developed a step-by-step approach involving “inspection, encapsulation, and curing.” effectively resolving defect issues.

(4) Quality Inspection Technology

Commonly used inspection techniques include digital image correlation (DIC) and scanning electron microscopy (SEM), which can accurately identify minute defects such as bubbles and cracks. Machine learning technologies are gradually being applied to predict bonding performance, driving the advancement of inspection toward intelligent upgrading.

III. Detailed Preparation Method

(1) Preparation before bonding

The key is to ensure that the surfaces of the bonded parts are clean. Select appropriate surface preparation methods based on material type, and design joints according to the principles of “reasonable load distribution and sufficient bonding area.” When necessary, use composite joining methods such as adhesive-bolt or adhesive-rivet combinations.

(2) Adhesive Preparation and Application

Two-component adhesives must be accurately measured in precise proportions, with a weight error not exceeding 2% to 5%. The application speed should be controlled between 2 and 4 cm/s, and the adhesive layer thickness maintained at 0.08 to 0.15 mm. Solvent-based adhesives require 2 to 3 passes for application.

(3) Sealing and Encapsulation

The secondary bonding process requires two - stage encapsulation using vacuum bag components: the first stage verifies the compatibility between the adhesive film and the part, while the second ensures tight adhesion of the film to the part, removing air to prevent void defects.

(4) Solidification

Taking modified epoxy resin film as an example, the process is: vacuum leak testing → heating to 55 °C and releasing vacuum → pressurizing to 0.7 MPa → heating to 150 °C and holding for 180 min → cooling and removing. Moderate pressure promotes adhesive wetting, while appropriate temperature ensures complete curing.

(5) Quality Inspection

Including visual inspection (bubbles, cracks), microscopic analysis (SEM observation of interfaces), performance testing (tensile, shear, peel tests), and full-process traceability to ensure non - conforming products are not put into use.

IV. Outlook

With continuous breakthroughs in adhesive modification, surface treatment, process optimization, and testing technologies, bonding technology is evolving toward greater intelligence, environmental sustainability, and efficiency. In the future, the industry will focus on core challenges such as bonding heterogeneous materials, developing eco-friendly adhesives, and intelligent process control, providing strong technical support for the high-quality development of China’s advanced manufacturing sector.