Written by: Marek Bernaciak
Structural Bonding of Thermoplastics
New MA Adhesives for Bonding Plastics and Metals
Introduction
Structural bonding of components during assembly using adhesives is a rapidly developing technology in industry. This article provides examples of the use of adhesives in the automotive and shipbuilding industries, where it has been adopted by the world’s leading manufacturers. Critical design and control criteria for the assembly process to ensure technical success are identified. The results of today’s adhesive applications are also discussed.
The trend toward part consolidation is leading to the bonding of large parts. While this simplifies the assembly process, it also increases the risk of failure. After all, almost every structural bonding operation involves parts that are large, expensive and responsible. If something goes wrong, the result can be costly or dangerous. Therefore, this article includes some considerations that are usually overlooked in other bonding situations.
From an industrial manufacturer’s point of view, the introduction of adhesive bonding into aerospace and all structural bonding has led to a general interest in the true capabilities of adhesives. In structural bonding, an adhesive is used that carries the full load of the structure. Very often the joints are also subjected to severe working conditions. Adhesives used in such cases, regardless of chemistry, must have the following properties:
- Strength in the range of 10-35 MPa,
- very high impact and peel strength, and
- operate in a temperature range of -50 to 170°C.
Considering the above working conditions, special attention should be given to the selection of the right adhesive and durability tests.
Adhesive selection: a job for the engineer-designer
An engineer’s task of selecting an adhesive is like an awkward gauntlet of products and technologies. When engineers approach their first adhesive application, they typically try to gather as much information as possible about each type of adhesive and each chemistry. This is the wrong path and the wrong approach. It often leads to delayed production starts and process control problems.
A better way to design an assembly operation with adhesives is to review the expectations of the finished, assembled product and the operational constraints in the factory environment. This approach leads to defining joint durability expectations and manufacturing process requirements prior to substrate processing and adhesive selection. Defining these parameters in advance will clearly guide adhesive selection and limit the choice to a few chemistries (Figure 1).
Thinking about the sustainability of the connection
The most important issue in structural bonding is the durability of the bonded part. A single specimen does not answer the question of whether the bonded part will last 10 or 20 years in a working assembly. It is often true that adhesives that cause substrate failure in shear or peel tests fail completely in cyclic fatigue tests. Experience shows that simply testing overlap bonded specimens is a very poor indicator of long-term durability. In critical applications, it is better to perform repeated tests under expected loads, possibly with thermal cycling. Such a methodology is a far better indicator of the true durability of bonded joints, and a reputable adhesive supplier will be happy to assist with such testing.
When it comes to structural bonding, a critical requirement is to properly determine the requirements of the finished joint. Failure to fully understand the durability requirements is the number one cause of field failures observed by the author in the field. Here’s what to consider and how to properly investigate the following factors:
- Working loads on the joint,
- Type of stresses to be expected, i.e.: shear, impact, bending, etc,
- Operating temperature range,
- Cyclic thermal loads,
- Impact and shock resistance,
- Chemical resistance, and
- Expected product life.
Technological parameters
Having defined the requirements for the durability of the bonded joint, the next step is to determine the process parameters for the bonding operation. Some points to consider are
- The desired fixation time,
- The required life time (mounting)
- Bonding position, e.g. vertical or horizontal
- Folding position and time,
- Reaction method, i.e. heat, room temperature cure, etc,
- Solvent system – aqueous, acetone, 100% solids, etc,
- Adhesive rheology, i.e. should it be more liquid or more viscous?
- Manual or automatic assembly,
- Adhesive packaging – small, or feeding from bulk packaging (e.g., drum)
- Dispensing equipment, one, two, three component,
- Non-destructive testing of bond quality and adhesion, and
- Transportation of parts.
Substrate Selection
Because the surface chemistry of the substrates to be bonded can affect the curing properties of many adhesives, it is best to leave the final selection of substrates to a time when the parameters of joint durability and manufacturing technology have already been defined. This is consistent with the philosophy of “design for manufacturing. A responsible engineer should make the selection of substrates and adhesives the result of defining prior requirements for joint durability and manufacturability. However, because such reasoning goes against the “common sense” favored by many engineers, the process too often is to select substrates and then build the concept of the bonding process around them.
Adhesive Selection: The Role of the Manufacturer (Supplier)
Once the durability requirements, process parameters, and substrate selection have been determined, it’s time to bring the adhesive manufacturer into the equation. The technical service department or a good distributor of almost any major adhesive manufacturer can usually provide a great deal of assistance at this stage. This can lead to the establishment of an industry-accepted test standard. Pitfalls in process parameters can also be identified. Because structural bonding depends so much on the condition of the substrate, the adhesive supplier usually (but not always – MB) can’t help with surface preparation, finishing and debris removal. Ultimately, any reputable company will be able to assist with bonding and test samples to demonstrate that the bonded joint meets the requirements.
Performance Standard
The next step in bonding implementation is to develop a written manual, a “production standard” that links process parameters, substrate specifications, and adhesive requirements. This step is often overlooked and is the key to continuous technology improvement. Problems can be solved by going back to this point where some parameters can be adjusted or changed and then written into an updated production standard.
Experience shows that serious problems arise when there is no production standard for the structural bonding process.
Case Study: Ford Car Bumpers
In 1986, Ford introduced a new model of the Taurus / Sable series. This series went down in history as the best-selling cars. The bumper design of this model used a completely new solution. The entire bumper consisted of a thermoplastic face and reinforcement beam bonded with reinforced methacrylate adhesive (Photo 1 and Figure 2). Determining the durability requirements and process parameters was critical to the choice of substrate and adhesive. Obviously, the bumper had to meet low-temperature impact requirements comparable to its metal counterparts. Ford’s long experience in defining bumper durability requirements added additional requirements, as did Federal Motor Vehicle Safety Standards (FMVSS) No. 215 for bumpers.
Durability requirements
Based on this experience, Ford determined that a structure using adhesive bonded substrates must achieve 80% of the control values after the following durability tests:
- 1000 hours at 88°C,
- 1000 hours at 38°C / 100% relative humidity
- 1000 hours at 54°C / water immersion
- 240 neutral salt spray cycles
- 80 Ford thermal cycles
Ford’s new bumper is made from the new Xenoy plastic manufactured by GE Plastic. This plastic is designed to be impact resistant, weather resistant, easy to paint and fully recyclable. It met all the durability requirements and was chosen for the job. One critical requirement was obvious: the bumper had to withstand a crash at -40°C at 8 km/h without failure. It was important for both the plastic and the adhesive to retain their energy absorption properties at this temperature without cracking. As a result, the Xenoy bumpers performed admirably, sustaining even less damage than their metal counterparts. To date, Ford has bonded more than 4 million thermoplastic bumpers with the reinforced methacrylate adhesive without a single failure during their service life.
Field Repair and Recycling
The history of Ford bumpers has highlighted two other issues that have led to increased interest and use of structural bonding. The first is the need for the bonding system to facilitate field repairs. Because structural bonding applications involve parts that are typically very large and valuable, the ability to repair them in the field is fundamental.
Second, more and more manufacturers, especially in the automotive industry, are choosing adhesive systems that can be recycled. This includes most thermoplastic adhesives and excludes most chemically cured adhesives. When an adhesive is recyclable, it means that the bonded part can be completely cut up and reshaped without costly and time-consuming cutting of the adhesive line from the recycled part. The methacrylate adhesives Ford chose to bond its thermoplastics meet both requirements.
Case Study: Vanguard's Racing Sailboats
Vanguard manufactures high-end fiberglass racing sailboats for club and collegiate programs. The company’s most popular boat, the Club 420, went into production more than 20 years ago. Since then, more than 1,800 of these boats have been sold. They are used by every major school, university and yacht club in the United States. The boats are designed to withstand all the difficult situations caused by inexperienced boaters, such as collisions with a kaya or other boat, stranding (coming ashore – MB), and capsizing and sinking.
Patrick Muglia, production manager at Vanguard, explains: “We sell to the institutional market, our customers demand durability, some of these schools like Coast Guard and marine schools keep these boats in the water seven days a week. So they are constantly colliding, including with buoys during maneuvers.”
An analysis of customer observations and warranty repairs led to the conclusion that a disproportionate number were caused by installation with putty used to bond the hull to the deck and other fiberglass components. Over time, these putties became brittle and cracked, causing leaks and weakening the boat’s structure.
Before choosing an adhesive to replace the polyester putty, Vanguard asked us to compare the methacrylate polyurethane adhesives commonly used in yacht construction, as well as how they compare to a commonly used method such as bonding with a glass mat saturated with polyester resin (Figure 4).
Figure 4 shows a comparison of cyclic fatigue tests comparing both a marine polyurethane adhesive/sealant and a well-known methacrylate adhesive. The tests were performed under two different loading conditions. It is clear that polyurethane is not up to the task. It performs better as a sealant than as a structural adhesive.
Conversely, the methacrylate adhesive gives results that exceed the capabilities of polyester resin. How is this possible? The lightweight, flexible adhesive allows slight deformation, unlike a rigid resin, and the energy of the deformation is converted to heat.
Upon closer inspection, Vanguard found other problems with the previous production techniques using putty. Before applying putty, the entire surface to be bonded had to be thoroughly sanded or abraded to improve adhesion. This task is time-consuming, labor-intensive, and results in a high degree of scattering, which is also considered a normal feature of the technology.
Sanding produces fiberglass dust, which is harmful to health and causes illness by inhalation. For this reason, workers must wear safety goggles, dust masks, appropriate protective clothing and gloves while performing this operation. Note from Mr. Muglia: “People really hate this job. They have to change clothes, they sweat in those clothes, the goggles fog up… to tell you the truth, no one wants to do this job”.
Vanguard’s initial comparison yielded a financial result that they say will add $60 to the cost of building Club 420 boats. Mr. Chip Johns, President of Vanguard: “At first we just compared material costs. But that didn’t give us the whole picture. We gained labor savings, eliminated the sanding process and materials like sandpaper, and significantly reduced the cost of claims.
When they finished comparing costs, production efficiencies and warranty repairs, Vanguard was truly amazed. Despite the fact that methacrylate adhesives are much more expensive than polyester putties, overall production costs have definitely dropped.
Pat Muglia summed up his experience with adhesives this way: “Since we’ve been using structural adhesives in production, we’ve produced better quality boats, we’ve cut costs, people are happier, and I don’t have to worry about warranty repairs.
Chip John: “Our boats had a definite weakness – it was the bonding of the deck to the hull. The putty was the weak link in the whole system. Now that same joint is the strongest point in our design. The boat is as if it was made in one piece, which takes our product to a higher level of quality.
ISO 9000 and adhesive dispensing equipment
The philosophy of the ISO 9000 system is to continually build and improve the written standard that defines the entire structural assembly process. Describing each step in the actual assembly process, as well as specifying breaks for equipment maintenance, helps ensure that the job is done correctly. Many bonding operations also extensively involve the adhesive dispensing and application system. Inadequate equipment maintenance can result in a lack of repeatable bonding. One major boat manufacturer has taken the ISO 9000 philosophy to heart. Here, every adhesive dispensing machine has a maintenance action plan and a work card permanently attached to the machine. The benefits of ISO 9000 are tremendous when fully implemented in a bonding shop.
Adhesives Reduce Fugitive Emissions
Federal, state, and local laws regarding fugitive emissions are another challenge for manufacturers. Often, new adhesive technology can replace existing methods and reduce emissions. In another case study, a boat manufacturer reduced fugitive emissions by approximately 60% by introducing bonding of composite stringers in the assembly of liners. In this case, this represented a reduction in emissions of approximately two thousand kilograms per year.
Summary
The Ford bumper example illustrates the benefits of properly defining the required durability of an assembly based on structural bonding at the beginning of the manufacturing process. More than four million bumpers have been bonded without a single complaint or failure. The Vanguard example shows how durability requirements evolve over time. The use of polyester putty to bond the hull to the deck was appropriate 10 years ago. However, what was acceptable in the previous decade is a lower standard today. That’s why Vanguard re-evaluated the durability requirements for its boats and found that reinforced structural adhesives significantly improved this parameter. As Chip Jones noted, the project was successful because it focused on the durability expectations of the end product, not just the cost of materials.
Based on a paper presented at the Adhesives ’97 conference in Rosemont, M.Sc. Marek Bernaciak – Technical Consultant
