Automotive adhesion

Materials World magazine
1 Feb 2015

Dr Jon-Paul Griffiths, Technology Manager at Oxford Advanced Surfaces, looks at the role of plastics and composites in the future of automotive lightweighting. 

New laws to reduce the emissions of vehicles, coupled with ongoing uncertainty around fuel prices, mean that designers and engineers have to look at new ways to build vehicles and structures. In the automotive sector, new fuel-efficient powertrains that could reduce the fuel required to power the vehicle are in development. However, while more fuel-efficient than the incumbent designs, they are also heavier. In order to maximise fuel efficiency and lower emissions, lightweighting of structures is a key area of development and design in new products.

This drive to create lighter structures has propelled the use of engineering plastics and composites in the automotive and transportation markets, with predictions that, by 2020, plastics will comprise 18% of the average vehicle’s weight.
Semi-crystalline thermoplastic materials show some of the most exciting combinations of properties for automotive applications, with excellent resistance to chemicals and oils, the ability to be modelled and formed, the potential for high thermal operating limits and the ability to be recycled. When using a reinforcement fabric or fibre in combination with these thermoplastics, the mechanical properties of the materials allow them to be used for structural or semi-structural applications as a replacement for metals such as aluminium.

Like for like

Crucially, to use thermoplastics in commercial products, they must be able to be bonded, painted or coated in a similar manner to the materials they are replacing. This is a significant problem, as the physical and chemical bulk properties of these materials (such as chemical inertness and high temperature resistance) mean that they are generally incompatible with most standard paints or adhesives, which have been designed for metals or amorphous plastics. To overcome this, surface preparations or adhesion promoters have been developed to improve the adhesion between metals and semi-crystalline thermoplastic materials.

There are five general mechanisms for achieving adhesion between a coating, paint or adhesive and a substrate, and enhancing one or more of these mechanisms will result in adhesion promotion of the system. Ionic bonding and Van der Waals interactions are of little use in the adhesion promotion of semi-crystalline thermoplastics in practical terms, due to the weak nature of the Van der Waals interactions under peel force and the lack of ionic functionality in plastics.
Mechanical interlocking can be achieved by chemically or mechanically etching the substrate to create gaps in which the coating, paint or adhesive can cure and set, locking the coating and substrate together. Etching can be achieved with aggressive chemical agents or mechanical techniques, such as shot/grit blasting or sanding.

While relatively cheap, both techniques suffer from issues with homogeneity of treatment as well as potential damage to the underlying mechanical or optical properties of the substrate. This is especially relevant in the case of reinforced thermoplastics, where etching can damage the reinforcement agent comprising the visual and mechanical properties of the material.

Entanglement is one favoured method of achieving substantial levels of adhesion between a substrate and a coating and this is the reason why amorphous substrates such as polycarbonate, acrylonitrile butadiene styrene (ABS) and PVC show few issues with adhesion of paints or adhesives.

The reason why entanglement is such a powerful method for achieving adhesion can be compared to having a bowl of cooked spaghetti. To remove one strand of spaghetti, you have to either slowly unwind (a process that is rarely encountered in the materials testing community) or use enough force to break through the knotted mess. In a bonding application, this means the polymer chains of the substrate and those of the applied coating intermingle and, on drying/setting of the coating, the structure is locked in place.

Amorphous substrates can be swelled or melted to achieve this effect but semicrystalline substrates cannot. In these cases, the application of corona or plasma treatment can break up the semicrystalline surface allowing entanglement to occur. However, these treatments do not work effectively on all substrate-coating chemistry combinations. 

The nature of the reactive head depends on the type of substrate that is to be modified. For metallic, ceramic or glass substrates, chemical treatments such as silanes and phosphonates are widely used. The reactive head in these systems is designed to react with the oxide surface present on the surface of these materials.

On thermoplastic substrates, neither silane nor phosphonate treatments work, because of the chemically inert nature of the plastic. In this instance, highly reactive chemistries, such as carbene modification, which have the ability to chemically react with the carbon backbone of the thermoplastic, must be employed.

The highly reactive and unselective nature of carbene-based surface treatments mean they are capable of modifying most types of thermoplastic material, including semi-crystalline thermoplastics such as polyolefins, polyesters and even high-end materials such as polyether ether ketone.

Because chemical adhesion promotion only reacts with the top layer of a material, the bulk properties (both physical and mechanical) are unaffected. Furthermore, as the process of modification is a chemical reaction, there is a time limit to the adhesion promotion effect after application and curing.

The last hurdles

The use of reinforced semi-crystalline thermoplastics as semi-structural or decorative trim that would previously have used aluminium is a prime example of a lightweighting opportunity in vehicle design. However, unlike an aluminium trim, which can be glued with an epoxy adhesive, many designers have to rely on mechanical fasteners to adhere a
semi-crystalline thermoplastic trim. In addition to being unsightly, the fasteners add extra and unnecessary weight to the design, and could potentially introduce weak spots into the composite structure itself.
By treating the thermoplastic substrate with a chemical adhesion promoter, the panels could be adhered using commercial epoxy adhesive in a manner just like the aluminium trim they are replacing.

Similar challenges exist in the coating and painting of polyolefin or reinforced polyolefin panels, where chemical adhesion promotion is a crucial and powerful tool to obtain good bonding with commercial paints.

Further applications of chemical adhesion promotion lie within the composite material itself, where chemical adhesion promoters have the ability to act as a chemical sizing on thermoplastic reinforcement fibres or fabrics (such as ultra-high molecular weight polyethylene or polyester) with thermoset resins. Because the chemical treatment does not affect the bulk properties improvements in interlaminar, sheer strength can be obtained without impairing the physical properties of the fibre.

In summary, adhesion promotion of materials is crucial to their use in real-world applications and, with the rise in the use of semi-crystalline thermoplastics, this need will only grow. Chemical adhesion promotion best allows improvement in bonding properties without affecting the visual or bulk mechanical or physical properties of the substrate. 

Acquiring adhesion

Chemical adhesion is the formation of chemical bonds between the coating and the substrate, leading to an interface that is chemically (rather than physically, as in the previous examples) bonded at a molecular level. Although, conceptually, it is one of the easiest adhesion mechanisms to visualise, it is also one of the most often misinterpreted or misrepresented.

A chemical adhesion promoter is a wet coating that is applied to the substrate with a layer thickness of around 5–50nm. The adhesion promoter itself contains two parts:

  • A reactive head(s)
  • A functional tail(s)

Once applied, the reactive head undergoes a chemical reaction with the substrate surface to form permanent non-reversible bonding, leaving the functional tail free to interact and bond with the corresponding coating or adhesive that is applied.