Forming a strong bond between materials

  • 14-Feb-2014 10:39 EST
Lead art-BMW i3.jpg

Adhesives are used extensively in the BMW i3, which features a CFRP passenger cell and aluminum structures. Dow Automotive, for example, optimized its Betaforce structural adhesives for the program. (This topic and others will be discussed as part of a May 6, 2014, SAE webcast on Lightweighting; go to www.sae.org/webcasts.) (BMW)

Lightweighting in automotive design is currently a major trend. This initiative is driven by the need to improve the fuel efficiency of vehicles to meet stricter upcoming regulations. Even so, the challenge remains to ensure optimum mechanical, NVH, and crash-safety performance while designing lightweight components.

When designing lightweight components and assemblies, it has become more common to design them as “hybrid structures” that make use of multi-materials. Use of structural adhesives for joining different materials is beneficial for several reasons.

Conventional joining methods for sheet metal parts and assemblies such as spot welding, riveting, and brazing are not as effective when joining dissimilar materials such as metal and plastics. Conventional joining has certain limitations of lower endurance life and lower impact strength.

While addressing the drawbacks of conventional joining techniques, adhesive bonding enables applications ranging from flexible sealings to high-performance structural bonding. The smooth appearance of the joints produced using adhesives results in lower stress concentrations at the joint edges. Thus, the load is more evenly distributed and stress concentrations are minimized. As a result, a more effective dynamic-fatigue resistance of the component or structure can be obtained. A good joint design will be energy-absorbing, and tend to have good noise and vibration damping properties.

The major factors that determine the integrity of an adhesive bond are selection of the most appropriate adhesive, joint design, preparation of the bonding surfaces, quality control in production, and condition monitoring in service. Selection of adhesive for a particular application is based on substrate type, surface condition of substrate, curing condition (corrosion protection coating, painting) open time, strength requirements, end temperature conditions, gaps filling, vibration damping, etc.

Researchers from Tata Motors examined different aspects of the adhesive bonding process, emphasizing the performance verification of different adhesive joints to meet end product requirements. Samples were prepared with a variety of adhesive and adherend combinations. These combinations were tested for tensile, single lap shear, T-peel, flexural, and fatigue tests according to standard testing methods. Since the performance of the adhesive depends upon weathering parameters, the test samples were also subjected to mechanical testing after conditioning them under extreme temperatures, exposing samples to fuels (diesel, petrol) and oils (gear oil, axle oil) to simulate actual field conditions.

This material level data generated in lab is used for 1) selection of adhesive, 2) optimization of adhesive curing parameters based on manufacturing practice, 3) carrying out design modification to get desired level of adhesive strength, and 4) inputs for carrying out crash/NVH CAE at vehicle level.

Types of adhesives

An adhesive is a polymeric material that, when applied to the surfaces of materials, can join them together and resist separation. In a conventional car body there is extensive use of adhesives. Depending on the application, the adhesive should satisfy a wide range of requirements like compatibility with the substrate, open time, curing time, strength, chemical resistance, temperature resistance, fatigue, weldability (for spot weld adhesive), and emissions during baking.

Adhesives can be categorized based on several parameters, including base materials and their associated properties (see table), as well as number of components: for example, single component, which does not need homogenized and can be used directly on the substrate; and two component, which consists of a resin and a curing agent and requires homogeneous mixing of the two components.

Adhesives also can be categorized based on curing conditions. Anaerobic adhesives cure in the absence of air and can be used as sealing compound and locking compound for screws. Pressure-sensitive adhesives are often used in tape form. They have relatively low strength and are used for applications like fastening decor strips and emblems. Instant-curing adhesives cure within a few seconds of application at room temperature and have a very small open time, while delayed-curing adhesives usually take more than 24 h to cure and may require a week to attain full strength at room temperature. Heat-curing adhesives require high temperature to cure, generally provide very high strength, and are used for structural bonding applications. The substrate should be able to withstand the curing temperature.

Structural adhesives provide high strength in the range of 10 to 30 MPa (1450 to 4350 psi) and low elongation. They are generally epoxy- or acrylate-based. Mostly based on rubber or silicone, nonstructural adhesives provide very low strength ranging from 0.5 to 5 MPa (72.5 to 725 psi) but high elongation.

Joint testing and validation

To have a robust design guideline for the selection of adhesives, laboratory experiments covering different adhesive materials were performed using two substrates: EDD-513, non-coated steel with thickness of 1 to 1.2 mm (0.039 to 0.047 in); and polypropylene (PP) with 20% talc filled, MFI (melt flow index) of 20 g/10 min at 230°C (446°F) with 2.16 kg (4.76 lb) load and thickness of 3 mm (0.118 in).

Tensile strength, lap shear strength, peel strength, fatigue, and flexural strength tests are designed to evaluate various mechanical properties of the adhesive joints. Glass beads having 0.5 mm (0.02 in) diameter were used to control the thickness of adhesive. In all the cases thickness of the adhesives was maintained at 0.5 mm. Once the joint was prepared, it was cured according to the conditions mentioned in the technical data sheet.

All five tests are possible for steel + steel combinations, but for the other two combinations—PP + PP and PP + steel—only lap shear tests could be done. For every combination of substrate and adhesive, five samples were prepared and tested.

To determine the effectiveness of different adhesive systems, processing variables, and surface pretreatments, it is necessary to expose adhesively bonded joints to various environmental and loading conditions that can simulate actual service conditions. The predominant factors in climatic exposure are solvent, moisture, and temperature. Only a single lap shear test was carried out to evaluate the effect of different environmental conditions on adhesive joints.

It is clear from the test results that adhesive performances are substrate-specific—i.e., all the types of adhesives are not suitable for every type of substrate. Each type of adhesive has some advantages and disadvantages—i.e., even if an adhesive is compatible to a particular substrate, it may not perform well in all the environmental conditions.

For example, the lap shear strength of five different types of adhesives were determined with steel, PP, and steel + PP substrate combination. With steel substrates, epoxy-based adhesives provide the highest strength, followed by cyanoacrylate (CA) and acrylic-based adhesives. One of the problems associated with CA is that after curing, it becomes very hard and brittle. Polyurethane (PU) and silicone-based adhesives, though compatible with steel substrates, can’t be used as a structural adhesive because of their low strength but can be used for applications where high elongation and gap filling is required.

It is very difficult to find a suitable adhesive for bonding PP because of its very low surface energy. However, CA can provide good bond strength for PP to PP joint. Most of the time it was observed that the adhesive did not fail but the substrate itself failed.

Epoxy-based adhesives can also provide good strength but the failure mechanism is adhesive type and not cohesive type—i.e., the joint does not fail through the adhesive but separated out from the PP surface, which is not acceptable. The other three adhesives—i.e., acrylic, PU, and silicone-based—are not compatible with PP, and therefore a PP to PP combination for these three adhesives was not considered for future tests.

For the steel and PP combination, again CA is the best possible solution available. Though epoxy provides moderate strength, it is not acceptable due to adhesive failure. Again, the other three adhesives were not compatible with PP.

Research conclusions

A proven substitute for conventional mechanical joining, adhesive bonding in the automotive industry has led to a new direction of producing lightweight and energy-efficient cars. The most important aspect in designing an adhesive joint is the appropriate choice of adhesive, and surface preparation plays a vital role in achieving the optimum performance of the joint.

Based on their study, the Tata Motors research team discovered that epoxy-based adhesives are superior in structural bonding of steels in all conditions; however, they showed very low adhesive strength for PP substrates.

Cyanoacrylate-based adhesives were found suitable for PP substrate with very good bond strength. However, CA-based adhesives have low resistance for high and low temperatures and hence are not suitable for extreme temperature conditions. They also have very poor resistance to petrol and exhibit very low fatigue life.

Acrylic-based adhesives have lower lap shear strength as compared with epoxy-based adhesives; however, these adhesives exhibit the best fatigue life. Acrylic adhesive is recommended when components experience fatigue cycles and moderate shear strength.

PU-based adhesives provide very low strength, but their performance remains unchanged in most of the environmental conditions, except high temperature ageing and gear oil resistance.

Silicone-based adhesives are suitable for high-temperature applications. They are resistant to oil and hence are suitable for powertrain applications.

This article is based on SAE International technical paper 2014-01-0788 written by Debabrata Ghosh, Lokesh Pancholi, and Asmita Sathaye of Tata Motors Ltd. The paper is part of the Welding, Joining, and Fastening technical session taking place April 9 at the SAE 2014 World Congress in Detroit.

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