Projects

Structural bonding of textile-reinforced thermoplastic compounds

Double-overlapping bonds of spacer fabrics with and without metal joining partners
© Fraunhofer IWS Dresden

Double-overlapping bonds of spacer fabrics with and without metal joining partners

To use thermoplastic fiber-reinforced composites in large series production, suitable reproducible bonding techniques are also needed. The adhesive bonding technology is of significant value, since it can realize a surface-based and homogeneous load transfer in the complex fiber-reinforced composite structure. Thermoplastic polymers - polyamides, polyethylene and polypropylene – outperform the thermoset-based matrix materials in producing and processing fiber-reinforced polymers when making large quantities. For this reason, the joining process has to fulfill high initial strength and good ageing stability, as well as short cycle times in series production.

Solution

Functionally integrated vehicle ramp
© Fraunhofer IWS Dresden

Functionally integrated vehicle ramp

Driver’s cab with automated pre-processing and adhesive bonding
© Fraunhofer IWS Dresden

Driver’s cab with automated pre-processing and adhesive bonding

Consequently, the group “Adhesive bonding and fiber composite technology“ at the Fraunhofer IWS Dresden engineered and implemented easy-to-automate process steps enabling the bonding of planar and sculptured fiber-reinforced thermoplastic compounds. The following sub-process steps were analyzed and refined:

  • Surface pre-processing,
  • Selection of the adhesive,
  • Adhesive application and curing,
  • Documentation of transfer strength and ageing resistance.

These tasks were performed in close cooperation with the TU Dresden within the scope of the Collaborative Research Centre (SFB) 639 »Textile-reinforced Composite Components for Function-integrating Multi-material Design in Complex Lightweight Applications«.

Surface pre-processing:

Thermoplastics with low surface energy, such as polypropylene, cannot be bonded without additional effort due to their poor adhesion. As a result, they are rarely used for structural applications. Making use of physical methods for surface pre-processing or modified adhesive systems can improve adhesion, however. Atmospheric pressure plasma, as well as laser irradiation, are preferable methods to be used for flexible surface pre-processing. Both methods are primarily employed for surface cleaning (removal of plasticizers, release agents etc.) and to a certain extent for the functionalization of the non-polar plastic surface. In automated plasma pre-processing with low heat input, we succeeded in detecting the formation of functional groups. An increase in surface area can be achieved by laser surface structuring, in addition to surface cleaning and activating. Here the adhesive not only chemically interacts with the surface, but also anchors mechanically .

Selection, application and curing of adhesive:

To establish heavy-loaded structural bonded joints for fiber-reinforced thermoplastic compounds, modified adhesive systems based on polyolefins, epoxy resin, polyurethane, and acrylate were analyzed and their quasi-static adherence strength values were compared. To meet the process requirements of series production, research also considered the option of accelerated adhesive curing for the example of thermally sensitive fiberglass- reinforced polypropylene compounds. The purpose of this activity is to allow for larger-sized structures to be handled quickly and made available within a few minutes for the next process steps.

High frequency induction heating and the addition of ferromagnetic particles into industrial 1K- and 2K epoxy resin-based adhesive systems contributed to accelerated curing of the adhesive. We succeeded in reducing curing times of 60-90 minutes in the drying oven to 3-5 minutes. In the abovementioned processes, the fiberglass-reinforced thermoplastic parts were not subjected to any significant thermal impact, as it is typically the case during conventional curing in drying ovens, since the adhesive layer is heated locally in the targeted manner.

Results

Pyrometer-controlled, inductively accelerated adhesion curing with industrial robot
© Fraunhofer IWS Dresden

Pyrometer-controlled, inductively accelerated adhesion curing with industrial robot

In structural bonding of fiber-reinforced thermoplastic compounds, significant increases in the adherence strength were achieved by means of atmospheric pressure plasma processing, as well as pulsed solid state laser radiation even after ageing tests. In comparison with the initial state (only cleaned by means of solvent), with pure adhesive failure of the bonded joints and peeling forces of 15 N/m, an obvious increase in the peeling forces of 3800 N/m and a cohesive or delaminating failure of the samples could be achieved by laser structuring of the samples. The various surface states of the initial sample and the laser structured sample are shown in Figure 4. Another advantage was the achievement of accelerated curing of the adhesive by means of high frequency induction. This was made possible by mixing nanoscaled super-paramagnetic iron oxide particles, embedded with silicon oxide, into different adhesive systems. A motion-controlled inductor excites the nanoferrites and causes the adhesive to cure at temperatures from 130 °C to 180 °C.

In mechanical testing, the adherence strengths of the samples cured by induction achieved the same level as those cured conventionally, basing on curing at room temperature or in the drying oven. When using fiberglass-reinforced polypropylene, this parameter amounted to 8 MPa …12 MPa, depending on pre-processing and the adhesive system used. Within the scope of the Collaborative Research Centre SFB 639, automation of the bonding processes was implemented by connecting the pre-processing units (atmospheric plasma head) as well as the adhesive application unit (2K dosing and application unit) or the induction unit to industrial robots with a cooperative working principle.