Ford’s current model Ranger and Everest use weld bonding extensively throughout their construction. The use of weld bonding and rivet bonding methods on various makes and models is now common practice. The increased use of adhesives is being combined with the use of different types of materials such as alloys, composites, plastics, and a wide range of steels in varying strengths.
This article is for information purposes only and is general in nature. For more detailed instructions on the use of adhesives in collision repairs, repair procedures are available from Ford in vehicle-specific Service Manuals, Body Repair Manuals, Technical Service Bulletins and Instruction Sheets. Independent collision repairers can obtain subscription based repair methods at www.motorcraftservice.com.
The heat sensitive nature of many of the higher strength steels is a contributing factor when OEMs introduce adhesives into the vehicle structure. Traditional thermal joining techniques can negatively influence the various metal types during vehicle repair. Cold joining techniques used when bonding surfaces together remove high temperatures from the process ensuring the substrate is not compromised in any way, with the additional benefit of increased corrosion protection providing greater longevity and strength.
When carrying out collision repairs the use of mechanical and chemical bonding is rapidly becoming the norm rather than the exception. The most common example of mechanical bonding in collision repair is welding, but can also include nuts, bolts, clips, rivets, and screws to mention a few. Adhesives are products used to create a chemical bond. However, to ensure a repair is carried out to meet the strict strength and safety requirements designed into all Ford vehicles, understanding how adhesives work and the pitfalls that can affect an adhesively bonded joint is critical.
Many adhesives are now structural elements for vehicle manufacture and repair. Failure to use the correct adhesive and follow workshop manual instructions will adversely affect the structural integrity and crash safety performance of the vehicle, which could result in serious injury to the vehicle occupants in a collision. Adhesives are developed to create a specific strength and bond to a substrate.Innocently mixing brands or types of adhesives may result in ineffective bonding. Adhesive properties are chosen with care by the engineers for very specific purposes, so it is vital that the properties required for the task are appropriate.
The following are the most common adhesives found in collision repairs.
Epoxy is a two-component product. One component is a reactive agent also known as an epoxide. The second component is a hardening agent.
Epoxies provide a very strong bond that is resistant to moisture, chemicals, and corrosion. The product hardens when both components are mixed.
Polyurethane needs moisture for it to dry. As it dries, it will usually expand making it a good product to use where high gap fill is required, and it is less brittle than other adhesives.
Acrylic based products can be used for several different construction applications. They are more prone to cracking due to the bond being more brittle.
Silicon is an example of single compound adhesive but is more likely to be used as a sealant. The bond strength from silicon is usually lower than other products. Silicon provides good flexibility and some gap filling properties. It can also provide for waterproof joints.
What is an adhesive and how does it work?
An adhesive resists the separation of two surfaces which may or may not be the same material. There are many factors that affect adhesive force including:
Absorption is when a number of smaller forces work together like electrostatic forces. For an adhesive to work, it needs to wet the surface and spread thinly over the surface of the material.
Chemisorption is when an adhesive creates a chemical bond with the material it is in contact with. An example is using a glue on certain plastics where the glue mixes with the plastic to create a new chemical compound.
Mechanical is when the surface of the material is porous. A porous material has lots of tiny holes that allow the glue to go into the holes. Once the glue dries, the glue holds the material like it’s grabbing it.
Diffusion is when molecules from the material surface and glue swap molecules. This mixture of molecules holds the two items together once the glue dries.
The strength achieved when using adhesives should not be underestimated. Structural strength adhesives can reach a minimum of a 1000psi overlap shear strength withnon-structural adhesives varying in strength from repositionable to strength that’s equal to or greater than thesubstrate being bonded and typically have less than a 1000psi overlap shear strength.
Windscreens are a key component in the structural integrity of a vehicle. They provide a significant percentage of the overall strength across the front of the vehicle structure as well as playing a vital role in the correct deployment of the passenger airbag and collision energy management. If you have ever watched ANCAP crash videos, you will clearly see the windscreen flex and bend during impact to absorb and distribute collision forces around the vehicle structure. The correct use of adhesive is vital in these scenarios.
This image is of a crash test between a 1998 Ford Fiesta and the 2018 model. The strength of the front windscreen can clearly be seen, not only supporting, and adding strength to the structure, but also as a main component in the SRS system by deflecting the passenger airbag into the correct position for the occupants. This scenario is the ultimate test of the adhesive. Incorrect application, or use of an adhesive that is out of date or not suitable can critically alter the crash performance of the vehicle.
For those still questioning the use of adhesives in vehicle production and repair, it is understandable when you consider the traditional alternative of welding which initially would seem the stronger and more suitable attachment method. However, OEMs have invested a significant amount of time into the research and testing of adhesively bonded joints and the benefits they can provide.
Ford highlight the importance of adhesives and sealers throughout their workshop manuals.
The correct adhesive bonding is essential to repairing the vehicle correctly. Adhesives are used in many areas of the body structure in place of welding. In addition to providing a structural bond between component, adhesives can also help prevent wind noise, water leaks, exhaust fumes and dust from entering the vehicle. They alsoprovide anti-corrosion barriers.
Important considerations when using adhesives
Always ensure the adhesive being used is within the expiration date. Failure to observe the expiry date can lead to significant issues with the quality and effectiveness of the adhesive. The composition of adhesives will change over time, this will have a negative impact on the efficiency and effectiveness of the adhesive. The use by date should never be ignored as any use after that date can affect the strength and adhesion of the bond.
From storage to application and curing, temperature is one of the most important considerations when using adhesives. Storing adhesive outside of the recommended temperature range will permanently damage the adhesive by negatively affecting the molecular structure. Once damaged, the adhesive may still appear to be ready to use, but due to the molecules being altered, it will not perform in the same way as expected.
Preparation is another area where temperature should be carefully considered. Many OEMs stress that all vehicle components, the adhesive, and the ambient temperature are all within the recommended processing temperature range for a certain period of time, normally 20-30 minutes before beginning the bonding process. This is to ensure that any risk of moisture between the adhesive and the substrate is removed. If the adhesive is warmed to room temperature (20 degrees Celsius) then applied to a surface that is significantly cooler, moisture will form between the materials as the warm and cold surfaces come together. This will not be immediately noticeable, but the long-term damage this can lead to can be severe, with corrosion and surface separation becoming a very serious reality.
Adhesives are incredibly strong but rely heavily on the correct surface preparation. Without the correct preparation, the adhesive can become totally ineffective and separate under even the smallest of load pressures. It is important to read the information to determine how the surface should be prepared. Some adhesives can be applied directly to the manufacturer e-coat, while others must be applied directly to the material surface. Primers can be required for certain adhesives, and understanding how to apply these correctly is vitally important. If abrasives are required to prepare the surface, ensure not only the correct grit size is used, but also the correct type of abrasive. If the adhesive is being used with aluminium, the abrasive must be rated for that material. Using a standard abrasive belt or disc can contaminate the material surface leading to a weakened bond.
Mixing and Application
Always follow the application instructions by the vehicle OEM and the adhesive manufacturer which is critically important when using two-part products for correct mixing ratios. The correct mixing nozzle that is supplied with the adhesive should always be used and never replaced with an alternative. The nozzles may appear the same, but the augers within can be significantly different, varying is number and size. These are developed to ensure they provide the correct mixing ratio for the adhesive they are supplied with. Replacing them can lead to incorrect mixing and curing.
Two beads should always be squeezed from the tube prior to use. Firstly, a mixing bead the length of the nozzle and then a second test bead with supporting information, such as temperature, use by date etc which can be kept as a reference and verification.
Once the panel is placed into position on the vehicle, the various anchoring methods must then be applied to work in conjunction with the adhesive. Rivets and weld bonding being some of the most common.
Whenever thermal joining is required in conjunction with adhesives, careful consideration of the heat input must be taken. Excessive or prolonged heat input can seriously affect the adhesive, causing weakness and potential corrosion issues.
Health and Safety should always be considered during any type of vehicle repair, but with the increasing use of adhesives, awareness of the health risks becomes paramount. Weld bonding is commonly required by a number of OEMs, but failure to follow OH&S guidelines can place the technician at increased risk of long-term health problems. When heat is applied directly through the adhesive, it will burn and emit smoke and gases into the atmosphere. These emissions can be very harmful when inhaled, with carcinogens and toxins being released. Using fume extraction and additional breathing protection should always be standard practice.
The use of adhesives has increased dramatically in recent times, and it has changed vehicle construction dramatically, improving the strength and durability of modern vehicle structures. With the correct understanding and application during vehicle repair, it is a relatively easy material to work with, but, as noted throughout this article, there are many considerations to be taken into account to ensure that the initial bonding qualities are reproduced and the vehicle is returned to its pre-accident condition.