Viscoelasticity. It’s a big word and might sound a bit intimidating but the concept is not difficult to grasp. The pressure-sensitive adhesives used to make tapes are “sticky” or “tacky”. It feels this way to your thumb because these adhesives are really very high viscosity (“visco”) liquids that also exhibit elastic (rubber band like) properties. It’s the combination of these physical properties that allow the tape to “wet out” or develop intimate contact with the microscopic hills and valleys of a surface and then demonstrate resistance when you try to remove them.
Why Viscoelastic Behavior Matters
Adhesive bonding can be achieved through chemical mechanisms as in the case of structural adhesives (epoxies, acrylics, and urethanes) or primarily via mechanical means such as pressure-sensitive tapes. The mechanical “flow” or “wet-out” of the adhesive tape onto a surface with low to moderate application pressure is the driving force for adhesion (smaller forces such as electrostatic and Van der Waals forces can also be present). On the other hand, the pressure-sensitive adhesive must also exhibit cohesion or intramolecular binding forces that give it elastic properties. These two contradictory requirements need to be balanced depending on the intended application.
In more technical terms, at low shear rates, the PSA must flow (or wet out) and at high peeling or debonding rates, it must exhibit an elastic response as seen in the graph above.
Temperature Dependency
Polymeric materials like tapes are also temperature dependent. This means the viscoelastic properties described above can vary with temperature. An easy way to visualize this is with your common rubber garden hose. If your hose sits in the sun on a hot summer day, it’s soft, compliant, and easy to roll up. If you try the same thing on a cold winter day, it’s much stiffer and harder to coil. This temperature dependency is why you see tapes come with a recommended application temperature (usually between 50°F and 120°F. The warmer the surface and surrounding air, the easier the tape will flow or wet out on the surface (like the hose example). Once the tape is applied and proper wet out has occurred, it can then be exposed to much wider temperature ranges (referred to as service temperature) and the full viscoelastic response can be observed.Applications
Three applications where the viscoelastic behavior of tapes really shine:- Glazing insulated glass units (IGU) onto aluminum framing systems in curtain wall and window wall
- Stiffener attachment on metal façade panels
- Side panel attachment on a truck or utility trailer
Summary
Polymeric materials are time/temperature dependent. Pressure-sensitive adhesives are viscoelastic which allows them to flow or wet out onto a surface (“Visco”) and then absorb and dissipate energy through elastic response. By selecting the right tape, the resulting bonds are highly durable and will hold up for decades in indoor and outdoor applications. Want to know more about how tapes can solve your bonding and assembly challenges? Contact Tom Brown, Inc, today at www.tombrowninc.com
I’m stating the obvious here, I know, but the pandemic put the brakes on nearly all in-person trade shows during the past 14 months. There were certainly some innovative attempts at holding “virtual” trade shows and conferences, but I’m old school. I think nothing beats going to a show looking at new materials, machinery, talking with the technical and sales folks, and generating new ideas.
From July 13-15, 2021 the Foam Expo and Adhesives and Bonding Expo will take place at the Suburban Collection Showplace in Novi, MI. It is the premier event for “tape nerds” like me, and my fellow industry colleagues.
Why Attend?
It’s impossible to imagine all the applications and industries where foams, adhesives, and tapes are used as engineering materials. Here is where you can truly grasp how many types of foams there are, where they are used, and how they’re processed. Same with the adhesives and tapes section of the expo. One of the best things about this show is the free conference sessions that take place on the show floor. This year there are three tracks: Day 1: Sustainability. These are sessions on bio content, meeting consumer and regulatory requirements, and overcoming product fatigue through manufacturing methods and material selection. Day 2: Applications and Manufacturing. These are sessions on acoustic, thermal, and shock absorption, adhesive selection and surface preparation. Day 3: Regulation and Testing. These are sessions on regulation reviews, preventing adhesive failure using data analysis, and certifications. Click here to view all the speakers and session times. Be sure to visit this blog after the trade show. We’ll provide a post-show blog to let you know about the more interesting things we see at the show. If you’d like to meet up with us, contact me at ralexander@tombrowninc.com.
Many of us have a roll of masking or duct tape that’s been sitting in our garage or basement for quite a while. It probably doesn’t unwind as easily as it did when it was new and it might look a little rough.
What you are observing is the interplay between shelf life and storage conditions. Tapes and die cut parts made from tape products come with a shelf life and a set of recommended storage conditions. It’s easy to overlook this seemingly mundane information but it’s important to understand them both so that you get the best performance from your products.
Shelf Life
All reputable tape manufacturers list the recommended shelf life for their products. The most common time frames listed are anywhere from 6 months to 2 years. The primary reason for the difference in shelf life is the composition of the tape itself; the adhesive system, the backing, and release liner if it has one. Certain adhesive systems such as acrylics and silicones are very stable chemically and resist aging. Other adhesives such as rubber/resin systems often have antioxidant and UV stabilizer packages added to improve aging characteristics.
Another factor in determining shelf life is an aging study. Aging studies can be conducted using natural or real time aging, accelerated aging, or a combination of both. Real time aging is very reliable but is not always practical as this process can require too much time to get to market.
Accelerated aging is a procedure that utilizes elevated temperature as a way of exposing the tape or die cut part to stresses that will simulate real time aging in a compressed time frame. Physical testing on the aged tape specimens (peel, tack, shear, and liner release) are performed after exposure and this data can be compared to room temperature control samples and real time aged samples to understand which properties might have been negatively affected by the accelerated aging conditions. The stability of the physical properties can then be used as evidence to establish a shelf life.
The last factor is determining shelf life is risk management. Risk management is where product managers, business leaders, and legal tend to get involved. They typically consult with the technical group to get a recommendation but they will decide how much risk they are willing to incur as a business and this will influence the stated shelf life. Some companies are more risk averse than others and their shelf life statements often reflect this bias.
Storage Conditions
There tends to be strong agreement among tape manufacturers that optimum storage conditions are 70°F (21°C), 50% relative humidity, out of direct sunlight.
Some will expand this by quoting a range; 70°F +/- 20°F and 50% RH +/- 20% as an example. There is little doubt that optimum storage conditions will maximize (and even prolong in many cases) shelf life.
Tapes and die cut parts are stored in warehouses most of the time. Some of these warehouses are temperature and even humidity controlled. Many of them are not. Warehouses can experience large swings in temperature and humidity as the seasons change. If your warehouse sees large swings in temperature; particularly high temperatures over 95°F, the tape is experiencing accelerated aging in a similar way to a formal aging study.
Extending Shelf Life
We are often asked if a tape or die cut part is “still good” after it may have been at the customer’s facility for an extended time; usually beyond stated shelf life. The good news is that unlike the fairy tale “Cinderella”, tapes and die cut parts don’t turn into a pumpkin at midnight of their shelf life. The performance of most tapes is not projected to change significantly even after the shelf life expires.
In most cases, questions will be asked about storage conditions and then the tape or die cut part can be inspected to check appearance and tack and compared against unaged material. Samples can also be sent for physical testing either to the tape manufacturer or an outside lab.
If the material has not been adversely affected and especially if it has been stored correctly, the shelf life might be able to be extended for a short period of time.
Summary
The shelf life and storage conditions are often the most overlooked items on a technical data sheet. Most tape products list a 6 month to 2 year shelf life. Shelf life is determined by the composition of the tape, backing, and release liner along with aging studies that document the performance of the tape in real time and/or accelerated aging conditions.
Optimum tape and die cut part storage conditions are usually listed at 70°F (21°C) and 50% relative humidity but warehouse conditions are usually more variable than the ideal. The best way to understand how to store your tapes or die cut parts is to talk with a knowledgeable converter. Contact us at www.tombrowninc.com
Pressure sensitive adhesive (PSA) tapes and die cut parts provide efficient methods of assembly in many applications. If die cut parts or tape failure occurs, the entire assembly or subassembly is at risk.
In most cases, failure can be anticipated and avoided. This post will provide some insight into the types of failure and success factors to be considered so failures can be avoided.
Types of Adhesive Failure
There are four basic types of failure modes with tapes and associated die cut parts: adhesive failure, cohesive failure, mixed mode and substrate failure.
As the most common, adhesive failure occurs when the adhesive delaminates or de-bonds from the intended surface(s) without leaving any residue. However, there are certain applications where adhesive failure is actually desirable. Think of the protective film on an appliance or paint masking tape.
Cohesive failure occurs when the adhesive “splits” or is sheared through the bulk of the coating. This usually leaves adhesive residue on both the tape backing and the intended substrate.
Mixed mode failures occur as a combination of both adhesive and cohesive failures. It is not uncommon for external factors such as plasticizer migration or elevated temperatures to have occurred when encountering a mixed mode failure.
Substrate failures are technically not adhesive failures but a tape design problem. These failures happen less frequently but point to a tape backing delaminating or otherwise destructing in some fashion. This can often be the result of environmental or mechanical forces that were unforeseen or misunderstood.
Success Factors
We’ve examined the four types of adhesive failures, now let’s explore the four categories of factors that increase the chances for success.
- Surface – this includes surface energy, geometry, texture, and preparation.
- Environment – what chemicals will the tape or die cut part encounter? What about UV exposure and high/low temperatures?
- Joint Stresses – understanding the physical forces that will be acting on the bond line during service
- Tape or Part Application – understanding best practices for application success
Surface Energy
Knowing the chemical makeup of the surface you want a tape or die cut part to adhere to is a key step in proper product selection. In our last blog post, we discussed surface energy and its influence in depth.
Simply put, surfaces such as metals and glass exhibit high surface energy, making it easier to bond to them. Plastics such as polyethylene and polypropylene exhibit low surface energy similar to the surface of a freshly waxed car. This makes them more difficult for bonding. Also, there are many plastics in between both extremes.
Tape manufacturers offer a variety of tapes that feature bonding capabilities with the wide range of surfaces available. Your converter can help you in selecting the best candidates.
Surface Geometry
Not every bonding surface is flat. Often, you’ll find tapes and die cut parts required to adhere to concave or convex curves and over sharp angles. These curves or angles create tensile, cleavage, and other forces on the bond line that will impact performance over time. The example below shows how using a thicker and slightly softer adhesive can overcome the forces that would otherwise cause lifting or “flagging.”
Surface Texture or Roughness
Bonding surfaces might come embossed, debossed, stippled or feature any number of functional coatings. These treatments can add aesthetic beauty but also add performance benefits such as anti-slip properties, water resistance, etc.
The adhesive systems on tapes behave like both a solid and a liquid—called viscoelastic behavior. The diagram above shows the benefit of selecting the right adhesive thickness to maximize the flow or “wet out” of the adhesive into the microscopic valleys in the surface. This increased wet out translates directly into improved bond strength.
Tape manufacturers offer a variety of tapes with varying adhesive coat weights or thicknesses to allow the end user to account for any surface texture and achieve the best possible bond.
Surface Preparation
As silly as this may sound, tapes are not particularly smart. They can’t delineate between dust, oils, mold release and other contaminants that might be on a bonding surface from upstream operations. Instead, they adhere to the first thing they encounter. If that’s contaminants, you’re headed for trouble.
In most cases, a simple wipe with a 50/50 or 70/30 blend of isopropyl alcohol and water (common rubbing alcohol you can buy in a drug store) is enough to remove most contaminants.
However, some situations require more aggressive surface prep. Some metals have cutting fluids or oils that might require a degreaser or a more aggressive solvent than IPA to cut through and remove the oil.
Some plastics might require a surface treatment such as corona discharge, plasma or flame treatment to gain sufficient surface energy to achieve high bond strength. In some high performance architectural applications such as curtainwall and window wall bonding, a primer might be needed on the metal extrusion and a silane coupling agent on the glass or infill panel to achieve bond strengths capable of withstanding wind loads.
Operating Environment
Another critical success factor is understanding the environment the bonded assembly will be exposed to during service life. This includes (but not limited to) chemical/solvent resistance, UV resistance and temperature.
Chemical Resistance
Contact with solvents and chemicals are an innate part of many tape applications. This can be as simple as common household window cleaners used on residential or commercial windows; the grease, oil and gasoline encountered under a car hood or exposure to jet fuel in aerospace applications.
Solvents and chemicals can have different effects on the adhesive bond line. Some chemicals can swell or soften the adhesive affecting bond strength while some solvents can partially dissolve the adhesive. Tapes with rubber based adhesive systems typically don’t do very well with solvents or chemicals. However, they perform quite well in aqueous environments. Acrylic adhesives, particularly those that are highly cross-linked, tend to exhibit good solvent and chemical resistance.
UV Resistance
UV exposure is another environmental factor that should must considered. Prolonged UV exposure can affect the adhesive system and even the backing or carrier the adhesive is coated on. UV light can cause the backing or adhesive to embrittle, discolor and degrade over time. Tape portfolios abound with products that can easily handle UV exposure and certain products like protective films will even state how long they can resist UV exposure.
Service Temperature
The last critical factor is service temperature. Low temperature exposure can cause some adhesives to become very firm and lose tack and adhesion as the glass transition temperature is approached (the adhesive becomes more “glass like” rather than more “rubber like”).
High temperatures can cause the adhesive system to soften and even flow in some cases. The associated drop in cohesive strength can result in failure if there are any shear, tensile or cleavage forces acting on the bond line. Tape manufacturers are aware of the range of temperatures that are encountered in industrial bonding applications and have a host of products that can handle a variety of temperature extremes.
Joint Stresses
After any tape is applied, there will likely be a force (or forces) acting on it—even if very minimal. These forces typically consist of peel, shear, tensile, cleavage and compression.
Peel
Some tape applications are very “peel intensive.” When you apply a paint masking tape for a painting project, apply a Post-It® Note to your office wall or buy a new appliance covered with protective film, you have a peel intensive application. In these cases, you know that the application of the tape is temporary and it needs to remove cleanly after some period of time.
Shear
Shear forces are present in applications where a tape is asked to support a load. This can be a static load such as using an adhesive backed hook to hang a picture in your home. Also, it can be a dynamic load such as a strong wind gust hitting a side wall panel on a truck or utility trailer.
Tensile
Tensile forces are forces that act in the ‘Z” direction of a bond uniformly over the entire bond area. A tensile force can act on the bond line or the tape carrier itself. The tensile strength of tapes, particularly self-wound tapes is often included in technical data sheets to make comparisons easier.
Cleavage
Cleavage forces are a type of pull force that acts on the leading edge of a bond line. It is analogous to using a crow bar on the end of a board that is nailed down to pry it loose. This leverage effect on the small area of the leading edge causes failure at lower levels than what might be observed in shear or tensile tests.
Compression
Compressive forces are basically the opposite of tensile. This type of force “crushes” the bond line by applying a downward force over the entire bond area. The negative effect of compressive forces can be “squeezed out” along with the edge of the bond line unless the proper adhesive is selected.
Tape and Die Cut Part Application
Tapes and associated die cut parts are made to attach quickly. That’s a big part of their attraction as an assembly method. Application can be by hand, (which is still the way many tapes are applied) by a taping machine or automatic applicator.
Time, application temperature and pressure are the three variables that should be controlled at the application stage.
Application Temperature
Most tapes are best applied at room temperature and most tapes will specify an application temperature range of 60-100°F. Higher application temperature will enable faster wet out of the adhesive and a corresponding build in adhesion levels. Colder application slows the wet out process considerably. If an application requires low temperature application, there are specific tapes designed to accommodate application temperatures down to 0°F.
Pressure
Good application pressure can accelerate the wet out process and increase adhesion. In many cases, tapes or die cut parts might be applied using only finger pressure. This is not necessarily a bad thing. Smaller parts or narrow tapes can be easily applied using finger pressure in many instances. Larger parts and wider tape can benefit from using a rubber roller or squeegee to help expel air from the bond line. Air bubbles are areas where the adhesive is not in contact with the intended substrate and a reduction in adhesion will result.
Time
Dwell time is also a factor when looking at tape and die cut part application. It is particularly critical when testing these products. Different tapes have different build times with adhesion. Rubber based adhesives tend to build quickly and can reach peak adhesion levels very quickly—often in leas than 24 hours. Acrylic and silicone adhesives require build slowly and can require up to 72 hours to reach their peak adhesion levels.
Summary
Tapes and die cut parts typically have four types of failure modes. These failures can be attributable to four categories of factors: surface, environment, joint stress and application.
By asking good qualification questions up front, these factors can be understood and managed. The right tapes and materials can be selected that will handle the specific requirements of each application. This understanding can eliminate most die cut parts and tape failures before any significant cost is incurred. This not only saves money and time in the long run but also protects the end user’s credibility by providing robust assemblies that utilize the best materials for the job.
Need help determining which tapes or die cut parts will work for you? Contact Tom Brown, Inc. today.
We’re all familiar with duct tape. It’s been around since it was first developed for the military in 1942 and most of us have a roll or two in our toolbox, garage, or kitchen drawer. Duct tape is popular because the adhesive has good “quick stick” properties and easily bonds to many surfaces while the backing is strong and can be torn by hand. (more…)
Adhesive bonded joints, whether made using pressure-sensitive tapes or liquid adhesives, can experience failure. Oftentimes, the comments we hear are, “It’s not sticking,” “It’s coming off,” or “It’s just not working.”
Fortunately, there is a method for understanding adhesive failures, and that knowledge can lead to an effective solution.
Types of Failure
Adhesive failure occurs when the adhesive system debonds or separates prematurely from one of the surfaces or substrates. In some cases, you want adhesive failure; particularly if you want the tape to peel cleanly away from the substrate (think of a Post-It Note). In most cases, however, the reality is a bit more complicated. Sometimes, the adhesive selected incapable of developing a strong bond, due to its chemistry and limited ability to wet out a surface with a lower surface energy than the adhesive can handle. Other times, there can be contamination left on the surface, preventing the adhesive from bonding properly. Cleaning and proper surface preparation (including primers in some cases) can resolve this issue. Environmental factors, like temperature, water, chemical contact and radiation are common culprits that can also deteriorate the adhesive bond to a substrate. Still, other times, there are migratory species in substrates. Even the adhesive itself, such as plasticizers or other additives, can migrate to the substrate interface and disrupt the bond. Lastly, the release liners that protect a tape product can sometimes leave behind uncured silicone or fluorocarbons residue that can inhibit the formation of a good bond.Cohesive Failure
This is the breakdown of the intermolecular forces within the adhesive itself, and its occurrences in the bulk layer of the adhesive. This breakdown can be caused by shear, tensile or cleavage forces that are acting on the bond line. Often, there are the environmental factors already listed that are acting simultaneously with these forces that exacerbate the effects and cause a failure.
Just as the name implies, mixed mode typically exhibits both adhesive and cohesive failures. This failure is not uncommon when plasticizer migration is involved, particularly after exposure to elevated temperatures.
Substrate Failure
This is not the only failure where the adhesive is not the culprit. The implication is that the strength of the adhesive bond exceeds the strength of the substrate itself. In some foam materials, you can observe this type of failure and, frankly, it is not always a bad thing. Have more questions about adhesive bond failures? Call Tom Brown, Inc. today. We’re here to help!
What Is Spooling?
Spooling is a process where rolls of tape products can be spliced together and wound onto a common core, similarly to fishing line or thread wound onto a reel. This traverse winding process allows significant footage to be wound onto a single roll without becoming excessively large and difficult to handle.How Spooling Benefits Your Process
Spooling allows your machines to keep running longer and thereby reducing costs associated with downtime, labor and changeovers.- More material per spool as compared to pancake rolls
- Fewer changeovers per shift
- More up-time on automated equipment
- Decreases downtime
- Saves warehouse space
- Quicker run-times
What Tape Products Can Be Spooled?
The more common types of tapes typically spooled are:
- Double coated foam tapes
- Single coated foam tapes
- Double Coated Film Tapes
- Nonwovens and tissue supported tapes
Specialized Spooling Services
There are some tapes that are difficult to spool, namely acrylic foam tapes (VHB type tapes) and unsupported transfer adhesives. These products are challenging because they tend to “knit” back together along the edges. Tom Brown has the right spooler to solve this issue and successfully spool these products. A slightly wider secondary liner can be introduced just prior to application and the tape will spool beautifully. There are some tape products that need a finger lift, or dry edge, along one side for the tape to facilitate easy release liner removal. Tom Brown has the right equipment to add a dry edge and wind a functional, attractive spool package.
Critical Spooling Parameters
It’s important to discuss the “put up” or spool configuration with your supplier to make sure the spools work as intended.
The first thing you typically discuss is the required width of the tape and the requested length of the spool. This discussion usually goes hand-in-hand with the core diameter and spool width. The most common core diameters for spooling jobs are 3” or 6”. But spool widths can vary greatly, with 6”-12” wide spool packages as the most common.
The thickness of the tape, the spool width, and the machine’s capabilities are three factors that most influence how long the spool can be. The thinner the material, the more you can put on the spool. A 3” ID core can usually be wound with additional material (up to a point), but is a little harder to handle. With a 6” core, you sacrifice a little length, but the spool package is a little easier to maneuver.
For example, a 1/16” double coated foam tape can be spooled on a 7”-10” width spool with anywhere from 2,500- 4,700 ft depending on the three factors mentioned above.
By adjusting the “pitch” – the distance between the adjacent winds of tape and the “dwell” (how long the traversing head stays on the edge prior to making the next pass or layer) – you can create different types of spools:
- Normal
- Pineapple
- Pancake
- Fixed Lobing
- Cyclic
- Step Pack
Want to know more about spools and how they can help your process? Contact Tom Brown, Inc. today.
Glass is known to be a “water loving” (the technical term is hydrophilic) surface and this makes adhesive bonds, particularly with acrylic adhesive systems, susceptible to change in high humidity conditions. The good news is that a simple treatment of the glass surface with a silane coupling agent reduces this “water loving” behavior and improves the bond strength of the tape when subjected to high humidity and moisture.
What are Silane Coupling Agents?
The chemical name for the class of silane coupling agents found to be most effective as a glass pretreatment is 3-Glycidoxypropyl trimethoxysilane resin (commercially known as Dow Corning Z6040). The important thing to understand is that this class of silanes possess both organic and inorganic reactivity that allows them to “couple” organic polymers (tape) and inorganic surfaces (glass).
The trimethoxysilyl group is subject to hydrolysis (reacts with water) and that is why the pretreatment is offered in a water/isopropyl alcohol blend (like the rubbing alcohol you can buy at the local pharmacy). The alcohol helps to stabilize the liquid and the alcohol flashes off easily at room temperature and makes it easy and safe to use.
These silanol groups condense with the hydroxyl groups on the glass surface. After this condensation reaction, the coupling agent is bonded to the glass and is now reactive with the organic material (the tape). These properties allow these materials to withstand physical, chemical, weather, and thermal degradation.
Where are they used?
Silane pretreatment is essential when fabricating curtain wall and window wall systems that relay on acrylic structural glazing tapes. The added bond stability afforded by the use of the silane coupling agent reduces the risk of de-bonding in high humidity and moist conditions.
The silane pretreatment is also used in attaching muntin bars for creating simulated divided lites on residential and commercial window systems.
How to Apply Silane Coupling Agents
- The glass surface should be clean and dry. If it is contaminated, it should be cleaned with a 50/50 mixture of isopropyl alcohol and water using a clean, lint-free cloth.
- Moisten the clean, absorbent, lint-free cloth with the silane solution and wipe over the area to be bonded with the tape in one direction. Alternately, the silane solution can be sprayed onto the perimeter to be bonded and wiped with a clean cloth around the perimeter. More is NOT better! Experience shows that a thin layer (monolayer) is much more effective and gives the best performance.
- To get that monolayer, the primed area should be re-wiped in one direction with a new lint-free cloth after the initial application. Under normal room temperature conditions, the tape should be applied within 2 minutes so that any residual moisture can evaporate.
Final Notes
Sometimes we’re asked how a silane coupling agent works if it’s such a small concentration (less than1%) in the liquid. We remind them to think of baking muffins or biscuits at home where you use baking powder. You use a lot of flour and other ingredients but only a very small amount of baking powder because it is highly reactive. (in case you’re wondering, it works when the dry acid and base in the powder hits water and reacts to make carbon dioxide). You don’t need a lot to get a very big result.
Tom Brown, Inc. offers premixed silence coupling agents under the 3M AP115 brand in convenient 4 oz. spray bottles. Contact us today if you’d like to learn more.
Intumescent Foam Applications
