By Kathryn Tomiello, Staff Editor
Here, top adhesives manufacturers give some insights into how to move past some of the most common pitfalls and get the most out of your adhesive.
Keep the manufacturing
facility’s needs in mind when specifying an adhesive into your design.
If the plant engineer has little room for downtime, consider using a
light cure adhesive. They remain in a liquid state indefinitely until
exposed to UV light, at which time they cure in seconds.
Today’s adhesives have many advantages over their traditional competitors – nuts and bolts, fasteners, rivets, welding, and soldering. They distribute stress load evenly across the entire bonded surface, resist flex and vibration stresses, and thermal cycling; and insulate against corrosion in metal assemblies. Adhesives also eliminate unsightly protrusions, punctures, and attachments commonly seen with other types of fastening and joining materials. What’s more, some adhesives can bond dissimilar materials. Recent improvements to epoxies, for example, have helped adhesives gain popularity in applications where, at one time, they might not have been a good fit. Newer epoxies have better toughness and peel strength, increased high-temperature service capabilities (up to 500° F), and higher resistance to water.
Adhesives can be an ideal fastening and joining method, but, as Walter Brenner, Technical Director, Masterbond, Inc., explains, “Working with adhesives is as much of an art as it is a science. Many users think adhesive work is a very simple subject, but nothing is simple.” Working with adhesives may not always be easy, but adhesive manufacturers do suggest several ways that can help make the process run more smoothly. Adequate substrate surface preparation, correct joint design, appropriate curing and dispensing techniques, and proper selection of an adhesive for its final use condition are critical to getting the job done correctly. It is also wise to involve manufacturing facilities up-front and work closely with the adhesives manufacturer throughout the design process. The adhesive manufacturer’s technical team can help in the testing process and can give you recommendations, as well as information about particular adhesives that may be unknown to you.
Step 1. Preparing the substrate
“Improper surface preparation is probably the most common cause of problems,” said Brenner. “Unless you properly pre-treat the surface of the materials you are bonding, you will not get satisfactory results.” Pre-treatments remove contaminants from the surface of the substrates or modify their chemistry, surface structure, or surface features to promote adhesion and make them more durable. Pre-treatments for metals split into two categories: mechanical and chemical. Mechanical methods include abrasion, grit blasting, and shot blasting. Chemical methods include degreasing, etching, and anodizing; the use of adhesion promoters – and for difficult surfaces – flame, corona, and plasma treatments.
The method of pre-treatment always depends upon which materials are being bonded. Some surfaces, such as aluminum, are easy to pre-treat, and others, like titanium and stainless, require a very strong etch because they are inert. The more inert the material, the more difficult it is to pre-treat.
Additionally, the more unequal in chemistry the two materials are, the more difficult it is to establish a good bond. Brenner notes, “For example, say you want to bond polyethylene to steel. Polyethylene doesn’t like water. It is hydrophobic. Steel and metals are hydrophilic. You need to pre-treat the polyethylene with acid to oxidize the top layer to make it more hydrophilic.” According to Ed Fisher, Henkel Corp., clean, dry parts are essential to getting a good bond. However, he says, “There is always a level of contamination in any manufacturing facility. We do everything in a laboratory environment, and 99% of the time we clean the substrates with an alcohol wipe. However, sometimes, in the field, even though designers have specked it into their design notes, manufacturers say there’s no way they can do any type of cleaning.”
Step 2. The triangle of product, parts, and process
Fisher said that any bonded assembly can be described using three words that start with the letter “p.” They include the product – the glue you are going to use, the parts – what you are going to make, and the process – how you are going to put those parts together. “From this triangle, you can build a robust assembly using adhesives as your joining method,” said Fisher.
He claims that the most often overlooked variable is the process. It is important to involve the manufacturers up-front.
A typical process is to send a complete widget to a distant manufacturing facility. However, even though the plant engineer has instructions on how to build the widget, quality may not be his number one concern. “He’s trying to make his production numbers,” said Fisher. “He may have 10,000 parts to process and must squeeze in 300 per day without adding shifts or manpower. The plant engineer could miss a critical step in the manufacturing process that causes the widget to fall apart. It is crucial that you and the manufacturer communicate and plan the production of that widget.”
Step 3. Joint design
Joint design is another important factor in determining how well an assembly will survive service loads. Adhesives should be part of the initial design of a component – not an after-thought. Bond failures are most likely to occur when adhesives are simply substituted into existing assemblies that were designed for use with traditional fixing methods.
Proper joint design requires that loading stresses be directed along the lines of the adhesive’s greatest strengths. Adhesives are strongest in shear and tension and weakest under peel and cleavage loading. An adhesive’s strength is usually strongest when stressed in shear because the entire surfaces of both substrates are making contact. Bond-line thickness only moderately affects bond strength. Uniform joint thickness and a void-free adhesive layer are more important considerations.
When specifying a joint design, Brenner notes that the bigger the area, the better the adhesion. “I see many people who try to use butt joints, sometimes referred to as point-to-point. The only way to use this type of joint is to put a third part on top of the butt joint or to use a scarf butt, where the two substrates are angled and mated.” Otherwise, some type of overlap is necessary for a joint to hold a bond.
Joint stress affects joint design
Adhesive joints are subject to four specific types of stress, including tensile, shear, cleavage, and peel. Tensile stress happens when forces acting on either side of the material pull them apart or elongate them. Shear stress occurs when forces acting parallel to the joint are distributed across the entire bonded area. The two materials then slide across each other. Cleavage stress occurs when forces at one end of a bonded assembly pull the substrates away from each other. Peel stress is similar to cleavage stress, but it applies to a joint with flexible substrates.
Step 4. Curing
For best results, choose an adhesive with temperature requirements and cure schedule you can properly follow. “If you deviate from the suggested directions, you will most likely have failure, and it will most likely show up as a product failure. We may recommend a 2-hour cure at 150° C, but somewhere in the line someone might want to cut 15 minutes off of the process,” said Fisher. In this case, since time is a factor, you may need to select an adhesive with a different temperature and cure schedule. The cure schedule starts when the adhesive reaches the recommended cure temperature.
Step 5. Dispensing
Use equipment specified for adhesive use. Otherwise, you run the risk of premature curing in the equipment itself. In pressure/time systems, a tank filled with pressurized air pushes adhesive through feed lines to a valve. A timer opens and closes the valve to release the product. If this is the type of system being used, make sure it is free of contaminants from outside influences. This can also cure adhesives inside feed lines and valves. “It will not show up as a gel or solid inside your wetting components, but you’re increasing your viscosity. The glue that you start with will be different from what is dispensed because its characteristics will change as it goes through the feed line and valve, ” said Fisher.
Additionally, specify clean, dry air and a filter. “Make sure you have an extra air dryer right before your dispensing equipment,” notes Fisher. Air in most manufacturing facilities is filled with moisture, and adhesives cure with moisture.
Proper surface preparation is necessary to bond dissimilar materials. Additionally, this type of application requires special adhesives. Light cure adhesives, for example, are not substrate
specific, and are suitable for bonding unlike materials.
Step 6. Testing
Perhaps the most important factor to consider when specifying an adhesive for an assembly are the types of stress and environmental conditions it will undergo during its lifetime. “The bond in the device needs to be tested in its intended environment to make sure the adhesive is appropriate for the application. Engineers also want to do accelerated age testing. What will the part look like ten years from now?” said Ken Morton, Manager, Applications Engineering, Dymax Corp.
Other tests include heat, humidity, cold, impact, UV radiation, salt fog, and more. “Engineers will want to guarantee that certain liquids, gases, or oils will not interfere with the bond. For example, you may want to run a bleach vapor test on a printed circuit board in a washing machine to check for resistance in a chlorine environment. To test a bond between two pieces of plastic in an outdoor application, it would be important to expose it to more ultraviolet radiation than it would actually encounter. You want to see whether it will turn yellow,” Morton said. Brenner finds that users often believe the claims the manufacturer makes on the product’s datasheet. Be skeptical of these claims, Brenner said, unless the adhesive has been tested under its final use application. Testing can often be long and drawn-out, and it can become a big expense. Also, because of time constraints, appropriate testing is short-circuited so make sure you know about all of the required tests for your application.
Partnering with a reputable adhesives manufacturer for testing can save you time and money. Its engineers can help you run the tests to gather data that might be missing from the product’s datasheet.
If you use an automated system to dispense your adhesive, proper maintenance is critical. Contaminants from the environment can cure adhesives inside both the feed lines and valve.
Step 7. Don’t go it alone.
Working with an adhesives manufacturer who has a technical staff guiding you through the process is key to getting the most out of your design. Good adhesives companies will have application engineers who can listen to your assembly challenges, provide recommendations for design, and recommend an appropriate adhesive. “We can help them with dispensing equipment, troubleshooting any type of accelerated age testing, or quality quirks they might uncover along the way,” Morton said. He also suggested that the company have an applications engineering lab with engineers who understand that no two applications are alike. “We won’t have data for every unique application out there, but we’re willing to go out and get it or run the test to obtain it. We furnish the critical data that engineers need to know to feel comfortable using adhesive products,” said Morton.
The adhesive manufacturer can also offer recommendations up-front about the rheology of the adhesive and how it may wick and flow into the parts bond gap. “All adhesives should have an established viscosity range, referred to as a viscosity specification. For example, an adhesive may have a viscosity range from 8,000-14,000 centipoise. For high volume automated production or intricate part geometry, we may recommend they perform their up-front preliminary and machine dispense work with the adhesive at the normal viscosity. In rare cases, we custom formulated two batch’s, one at the high end and one at the low end of the viscosity range so our customers can run tests to make sure both work at the correct rate and the coverage is good.”
It can also be helpful to find a supplier who will do custom formulations, if necessary. For example, when engineers at Dymax tried to use standard light-cure adhesives to join the plastic parts of disposable inflatable tracheotomy devices, they found the adhesives did not have enough flexibility.
Get the most out of your adhesive by educating yourself on the basics of surface preparation, joint design, cure schedules, and dispensing techniques. Also, become familiar with the tests required for your application.
So, on behalf of our customer, our Applications Engineering Lab submitted a customization request into Dymax Research and Development. In addition to requesting a more flexible adhesive, we requested excellent adhesion to PVC, low water absorbing, water clear and a viscosity of 8,000 centipoise. In the end they made a commercial product meeting all the targets and have a very pleased customer.
Cap joints are probablly the most commonly used types of joints. Here are some examples.
Each industry has a governing body that sets up specifications an adhesive must meet to obtain certification. It helps to understand the specifications that govern the industry within which you are working. This is another area where a solid relationship with a reputable adhesives manufacturer pays-off. Fisher noted that governing bodies do not always know the properties of the glue well enough and base their specifications on limited data.
Work closely with a reputable adhesives manufacturer during the design process. Its technical team can provide you with information, help you perform necessary testing, as well as design and specify equipment to dispense your adhesive, and in some cases, move the parts.
For example, Quartia, a certification body for fiber optics applications, requires adhesives to have a certain glass transition temperature (the temperature at which a product goes from hard and glassy to soft and rubbery). Heat introduces moisture, which can adversely affect adhesives. By Quartia’s standards, if the Tg is below 95° C, moisture will get into the adhesive, which will degrade the bond-line, and the assembly will fall apart. “I have reams of data on subsets of adhesives that love hot-moist environments. They have Tg’s way below 85° C,” said Fisher. He also noted that he must reassure his customers that he is guiding them in the right direction. Often, they believe the certification body knows all. Fisher cautions, “Don’t let people who don’t know much about glue dictate what you’re using.”
Use your resources. Become acquainted with adhesives and be fully versed and knowledgeable of the working instructions for the adhesives you want to specify into your device. Understand the specifications that govern the industry that suits your application. Follow the manufacturer’s directions, and finally, work with a reputable adhesives manufacturer who can help educate you, provide recommendations, and do the necessary testing to ensure success. You will have a better bond, and you can guarantee that today’s bond-line will hold for five to ten to
fifteen years from now.
: Design World :