Lightweight Design worldwide

, Volume 10, Issue 1, pp 40–43 | Cite as

Assured Use of Release Agents and Adhesives in the Same Production Process

  • André Kraft
  • Kai Brune
Production Surfaces


Adhesive Bonding Release Agent Cohesive Fracture Wetting Pattern Technical Quality Assurance 
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Structural components made of CFK have proved most effective in motor sport. The implementation of lightweight concepts for all road vehicles using composite materials is possible by intelligent design and the management of reliable joining methods. The use of adhesive bonding processes in the car manufacturing industry is paving the way here, with organisational matters regulated by standards and advanced methods for extended quality assurance testing in the production.

Lightweight design in the automotive industry would today be unimaginable without the use of composite materials. The BMW Group in particular is employing ever more carbon fibre-reinforced plastics (CFRP) for the manufacture of structural components. An example is the passenger cell, the so-called Life module, of a BMW i8 shown in Figure 1. Production processes involving hybrid design, which combine different materials and allow increased freedom of design, require the effective use of reliable joining technologies at an increasing number of different interfaces. It is here where adhesive bonding processes are becoming ever more important. Indeed, adhesive bonding technology is not without reason often lauded as being the joining technique of choice for the 21st century.
Figure 1

Life-Module of a BMW i8 (© BMW)

Adhesive bonding is a special process, and quality assurance for the production of bonded joints is a vital aspect for the manufacture of safe and effective products. Organisational aspects relating to quality assurance are, for example, specified in the new DIN 2304 standard “Adhesive bonding technology — Quality requirements for bonding processes”. Technical aspects cover pre-process, in-process, and post-process quality assurance to verify that there are no technical shortcomings with the joining process [1] and there are an increasing number of new and efficient automated solutions [2] available for this.

In-process Quality Assurance

The realisation of technical quality assurance during a production process in the car manufacturing industry involves cycle times of just minutes, multiple shift operations with different equipment operators, complex geometries involving curved surfaces and different materials, clear and known tolerances, and finally very high requirements on comprehensive documentation of test results and the decisions based thereon. Understandably, non-destructive testing (NDT) is preferred to destructive tests.

Traditional non-destructive tests aim, for example, to demonstrate that the materials and joints are free of undesired delamination or free of three-dimensional defects. However, applying so-called extended NDT for monitoring the bonding process facilitates an improved documentation for the properties of the adherent surfaces.

The result of the bonding process substantially depends on the state of the substrate surfaces prior to the application of the adhesive. For this reason, the car and aircraft manufacturing industries use information acquired from extended non-destructive tests, for example automated wetting tests or laser based methods. Further work in this area is the topic of ongoing Europe-wide collaborative research, one example of which is the project entitled “Quality Assurance Concepts for Adhesive Bonding of Aircraft Composite Structures by Extended NDT ComBoNDT” [3] having scientific coordinators from Fraunhofer IFAM.

Quality Assurance for Bonding CFRP Materials

Various joining processes have been qualified for the production of lightweight CFRP structures using adhesives, also including manual bonding processes used for repairs [4]. The bonding of CFRP materials in the production of composites often involves substrates originating from a shaping process which uses molds and release agents. Release agent residues on the CFRP surface are generally removed via pretreatment prior to the application of the adhesive. This allows effective and durable material-fit bonded joints to be manufactured. From a technical point of view, the quality assurance task prior to the adhesive application involves documentation of the procedure to use for successful pretreatment of the whole surface within the required cycle time.

In-line process monitoring is a fundamental aspect of a complex production environment.

In respect of extended NDT, the BMW Group explores an approach based on an automated wetting test [5]. The commercially available system used is called BonNDTinspect [6]. A fine aerosol of uniformly sized micro-droplets of ultra pure water is sprayed onto the substrate surface. The wetting pattern in a function of the position is accurately imaged by a camera. As the droplets evaporate, the wetting pattern is observed and evaluated using image analysis algorithms. The resulting parameters are compared to the tolerance ranges determined during process qualification based on mechanical testing of bonded substrates.

So how do any abnormalities manifest themselves in the test? Local heterogeneities in the substrate wetting were only detectable on the images after applying the water aerosol to improve the contrast. This is clearly shown in Figure 2 for a fingerprint on the surface a CFRP substrate. With the relevant color representation, the water droplets of a few tenths of a millimeter in width appeared dark on a white background.
Figure 2

Fingerprint on a CFRP surface made visible with the aerosol wetting test (© BMW)

Regarding the approximately 1 cm wide fingertip, the density, size, shape, and alignment of the water droplets differed compared to the surrounding region. Conclusions can also be drawn by comparing different images. This is demonstrated in Figure 3 for an example application. Homogeneously wettable substrate surfaces are also being characterised and the wetting patterns are being compared over their full area with target specifications.
Figure 3

Images of the wetting pattern after spraying an aerosol of water onto CFRP surfaces purposefully contaminated with release agents (actual image size 3 cm x 3 cm) (© BMW)

Using wetting patterns, the determination of target values for appropriate parameters with tolerance ranges must be implemented as part of the technical quality assurance process. To complete the correlation between the measurements from the wetting test and the mechanical properties of the bonded joints and fracture patterns, the bonded joints must be subjected to destructive testing. This was done using substrates with known, customised surface properties. Based on these results, subsequent substrates that are tested can then either be released for adhesive application or returned for further finishing.

Using the laboratory wetting test equipment shown in Figure 4, several iterative steps were performed for a test head on a robot arm. This allows the testing of complete components of complex geometry. With a test speed of currently 150 mm/s and a test width of 30 mm, the testing of relevant areas is possible within the specified cycle times. Figure 5 shows the automated robot testing a specimen CFRP component.
Figure 4

Representation of the BonNDTinspect system with test head, test specimen, and positioning table (© BMW)

Figure 5

Robot-aided BonNDTinspect system in the research laboratory at Fraunhofer IFAM (© IFAM)

Example Application

To highlight the comprehensive quality monitoring that is required along the production chain, the planning and implementation of quality assurance technology for the bonding of CFRP substrates is described below.

The CFRP substrates were manufactured from an amine-curing epoxy resin matrix system (Hexion Epikote Resin PAT, Hexion Epikote Curing Agent PAT). PAT 657 BW was used as the internal release agent. The manufacturing process employed an external mold release agent (PAT 608 FP/50) supplied by Würtz. This was applied to the surfaces of the molds.

After contact with the molds the CFRP surfaces were analysed using the aerosol test method and also using HPLC (High Performance Liquid Chromatography) for comparative purposes. In addition, release agent aerosols were applied to the cleaned surfaces in differing concentration (between 0.125 and 1.1 wt. %). The sample labels here are inversely proportional to the concentration of release agent applied to the surface. To manufacture single lap shear specimens, a cold curing two-component polyurethane adhesive was used (Betaforce 2816S from Dow Automotive). The second substrate was 2.5 mm sheet steel with e-coat. Regarding destructive testing for joints from a production process, the goal is to achieve a uniform cohesive fracture pattern for structural requirements the goal is to achieve the relevant joint strengths.

Figure 3 shows that the wetting patterns for the contaminated surfaces have significantly larger droplets than the cleaned surface. The reason for this is the hydrophilic nature of the release agent that was employed. With the highest degree of contamination, the release agent on the surface resulted in film formation. The inhomogeneous distribution of the water droplets on sample 1, Figure 3, is due to spinodal dewetting. The fracture patterns for the cleaned sample and the samples with low contamination (samples 6 to 20) all showed cohesive fracture, and a lap shear strength of circa 10 MPa. Adhesive fracture was observed in the samples with medium and high contamination with release agent, and the lap shear strength was at least 40 % lower. For example, sample 1 had a lap shear strength of 3 MPa.

Figure 6 shows the relationship between the average droplet size, extent of wetting, and roundness (of the droplets) and the fracture patterns of the lap shear specimens. The detection limit of the HPLC-MS test method for the external mold release agent is also shown.
Figure 6

Images of the wetting pattern after spraying an aerosol of water onto CFRP surfaces purposefully contaminated with release agents (actual image size 3 cm x 3 cm) (© BMW)

It is clear that thresholds can be defined for all three parameters, so allowing surface states to be differentiated into those that give undesired adhesive fracture and those that give the desired cohesive fracture. In the production process the attainment or not of such a threshold determines the course of action: Forward to the bonding step or back to the cleaning process.

Summary and Outlook

The new DIN 2304 standard regulates organisational matters for adhesive bonding. The use of a robot-aided aerosol wetting test (in-process) allows reliable characterisation of the state of substrate surfaces. This is necessary for evaluating the preparation of fibre-reinforced plastic materials for adhesive bonding.

The automated evaluation of the wetting pattern determines whether the substrate can proceed to the next step (bonding) or must return for further pretreatment.

An increasing number of systems for quality assurance will become available in the near term. The focus here will be on the digitalisation of analysis data. Indeed, DIN 2304 and new efficient test methods for extended non-destructive testing represent a further milestone on the path to Industry 4.0.


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Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • André Kraft
    • 1
  • Kai Brune
    • 2
  1. 1.BMW AGLandshutGermany
  2. 2.Fraunhofer IFAMBremenGermany

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