Driven vigorously by the "dual carbon" strategy, demands for structural lightweighting have reached an unprecedented level across transportation, aerospace and high-end equipment manufacturing sectors. As a new-generation primary load-bearing material, carbon fiber-reinforced polymer (CFRP) boasts a density only one-quarter that of steel and a specific strength over five times higher than steel, which has enabled its widespread application in Boeing 787 fuselages, battery pack housings for new-energy vehicles, and airframe structures of electric vertical takeoff and landing (eVTOL) aircraft. Statistics indicate that each 10% reduction in the weight of transport vehicles can improve fuel efficiency by 6%–8%, delivering remarkable benefits for energy conservation and emission reduction. Nevertheless, with expanding engineering applications, the high manufacturing cost of monolithic CFRP, coupled with its inherent drawbacks in wear resistance, fluid barrier performance and food-grade safety, have gradually become core bottlenecks restricting its full-scale deployment. Against this backdrop, heterogeneous integration of CFRP with high-performance thermoplastics based on the "multi-material hybrid design" concept has become an inevitable pathway to realize integrated structural-functional performance and optimized cost efficiency. Among various thermoplastic matrices, polypropylene (PP) stands out as an ideal candidate for lightweight composite pairing with CFRP owing to its ultra-low density (~0.9 g/cm³), excellent chemical resistance and superior cost performance.

Despite prominent engineering value of PP/CFRP composite structures, drastic disparities in physicochemical properties between the two materials render their interfacial bonding a formidable challenge in materials science. In accordance with the thermodynamic wetting theory, favorable interfacial adhesion originates from sufficient spreading of adhesives over adherend surfaces. However, PP is a typical nonpolar crystalline polymer whose molecular backbone consists entirely of chemically inert C–H and C–C bonds, leading to an extremely low surface energy (~30 mN/m). This characteristic triggers strong hydrophobicity of liquid resins on PP surfaces with contact angles exceeding 90°, which prevents effective physical wetting. Conventional modification approaches including mechanical sanding, acid-base etching and flame treatment can moderately improve bonding performance, yet their intrinsic downsides such as stress concentration, environmental pollution and poor processing stability fail to meet stringent process quality requirements imposed by high-end manufacturing fields such as aerospace.

As an advanced dry surface modification technology, low-temperature plasma treatment has gained growing traction in polymer surface engineering in recent years due to its high efficiency and low cost. Abundant electrons, ions and high-energy metastable particles contained in plasma can conduct nanoscale surface modification without damaging the thermal sensitivity of substrate materials. Figure 1 illustrates the working mechanism of plasma treatment: a particle generator ionizes working gas to produce high-energy active species, which clean, etch and modify substrate surfaces via synergistic physical bombardment and chemical activation, so as to elevate the interfacial bonding performance of materials. The modification mechanism is governed by coupled physical and chemical effects. On the physical front, sputtering induced by high-energy ion bombardment removes weak boundary layers and surface contaminants, meanwhile generating nano-to-microscale pits and grooves to drastically enlarge the specific surface area and form abundant mechanical interlocking sites for adhesive infiltration.

Analysis on Surface Modification Effect of Plasma-Treated PP

To further clarify how plasma modification alters physicochemical properties of PP surfaces, contact angles of deionized water and ethylene glycol on PP surfaces treated with 14 distinct processing schemes were measured in this study (Figure1). The total surface free energy (SFE) along with its dispersive and polar components were subsequently calculated via the OWRK model, as summarized in Table 1.

Figure 1 Contact angle test results of treated surfaces

Table 1 Contact angle and surface energy analysis for specimens under each treatment scheme

Test results reveal that untreated PP (Scheme 14) exhibits strong hydrophobicity with a water contact angle of 90.3° and a total SFE merely of 31.8 mN/m. Plasma treatment drastically reduces surface contact angles and raises total surface free energy of PP.

For instance, Scheme 9 witnesses a sharp rise in total surface free energy and its polar component; the increment of polar component dominates the elevation of total SFE, while physical etching facilitates mechanical interlocking to effectively eliminate wetting barriers of nonpolar polymers.

Such modification effects stem fundamentally from chemical functionalization on PP surfaces induced by plasma active species. High-energy oxygen free radicals can effectively cleave C–C and C–H bonds within PP molecular chains, grafting polar oxygen-containing groups including hydroxyl (–OH), carbonyl (C=O) and carboxyl (–COOH) onto chain terminals to enhance the overall surface polarity of PP. Quantitative verification of polar functional groups is further supported by surface energy test data: the polar component of pristine PP surface, initially close to zero, surges to 62.7 mN/m. This quantitative evidence directly validates successful grafting of the aforementioned hydroxyl, carbonyl, carboxyl and other oxygen-containing polar functional groups.

Collectively, plasma treatment constructs stable chemical bonding and mechanical interlocking at PP/CFRP interfaces by fabricating micro rough textures via physical etching and introducing polar functional groups through chemical grafting, thereby significantly enhancing the interfacial bonding strength of composite joints.