Foam is a fantastic cushioning material, used in industries from automotive to medical for providing comfort and protection. It's not the easiest material to work with though. Cutting accurate, high-quality shapes takes specialized equipment and more than a little skill, while securing it in place can be even more of a challenge. Conventional fasteners can pull through, especially with low density foams, and adhesives applied as a bead tend to run into the open cells, reducing flexibility rather than bonding to adjacent material--many times, the psa is used as a temporary fastener until a mechanical fastener can be put in place.
In many cases the solution is to laminate an adhesive onto the foam. Merryweather offers an adhesive lamination service which makes foam fabrications easier to handle and put in-place. Adhesives come in many forms; the type that works best with foam is known as "Pressure Sensitive Adhesive" (PSA). These come in many forms with the most popular being two sided (also known in the industry as double coated adhesive) with a carrier membrane (typically a paper, film, foil or cloth) or a transfer adhesive which peels directly off the release liner when it is removed to adhere the foam to the substrate. Double coated adhesives add stability to the foam so that it cannot be stretched out of shape. They can also be used to prevent plasticizer migration. Since transfer tapes do not have a carrier, the adhesive is extensible and more conformable, and sometimes at a cost savings.
Specifying a PSA lamination is a good first step towards simplifying foam assembly, but it's not enough. Adhesives have many different properties and the relative importance of each depends on the application. A deeper understanding of PSA's helps with selecting the best adhesive for any given application.
What is a PSA?
Some adhesives need a chemical reaction to create a bond, others use heat or exposure to UV light. In the case of a PSA the activation method is pressure. Bringing adhesive-coated surfaces together with just light pressure is enough to create a bond. (In chemical terms, the adhesive "wets" the surface, allowing a bond to form.) Increasing the pressure doesn't automatically increase the strength of the bond, although it may do so if it increases the area "wetted" by the adhesive.
Tack, Peel and Shear: The Key Adhesive Properties
Tack indicates the initial bond strength. In the lab it's usually measured by the "loop" test. A loop of adhesive-coated tape is briefly brought into contact with a surface. The force needed to separate tape and surface is the tack strength.
A high tack number shows a bond forms quickly. This can be a problem if it might be necessary to separate and reposition two surfaces, which is why "Post-It" notes have low tack. Conversely, shipping labels are secured with a high tack adhesive, which is why repositioning them is never a good idea!
As a measure of the force needed to separate two adhesive-bonded surfaces, peel indicates bond strength. It's determined by pulling the two surfaces in opposing directions, but only after the bond has had time to build strength.
Shear is measured by applying a force parallel to the bonded surfaces. It's really an indicator of bond durability.
Specification sheets for adhesives typically list all three of these parameters, usually with a note about the test procedures followed. Most often, these are ASTM standards, although Pressure Sensitive Tape Council (PSTC) testing methods are sometimes used. However, since both temperature and humidity are factors in the bond, both organizations standardize temperature and humidity during testing.
PSA's consist of an adhesive, mixed with an elastomeric base material and a tackifier. The tackifier, as the word suggests, increases the initial tack, while the adhesive creates the actual bond, (which may take time to build.) Forming the foundation of the PSA, the elastomer provides properties like flexibility and temperature range.
Three elastomer chemistries are used in PSA's: rubber, acrylate and silicone. Rubber is the least expensive and provides good peel and shear strength plus a high level of flexibility. Rubber-based adhesives tend to yellow over time and lack strength at elevated temperatures.
Acrylates Acrylics stand up well to UV and solvent attack and will work over a temperature range of -45 to 121 degrees Celsius (C). Their downsides are poor creep resistance and a higher price.They also require a 72 hour dwell time to build up to their full bond strength.
More expensive still, silicone-based PSA's have a broader temperature range, (-73 to 260 degrees C,) and good resistance to chemical and solvent attack.
While relative importance depends on what the application needs, these points should always be considered:
- Need for repositioning or removal – this would indicate use of a low-tack PSA.
- Lowest temperature expected – PSA's can lose flexibility at low temperatures.
- Highest temperature expected – elevated temperatures reduce shear strength.
- Humidity – moisture-laden atmospheres will significantly reduce the bond strength achieved.
- Vibration – especially if combined with high temperatures, as this can lead to premature shear failure.
- Presence of chemicals and/or solvents – these will attack many PSA's, especially those using rubber elastomers.
- Substrates being bonded together are very important in determining the type of PSA needed
Typical Applications and Benefits
Applying PSA to cut foam shapes simplifies assembly operations and results in higher quality products. An appropriate level of tack lets workers position foam pieces before securing them in place. Fasteners are eliminated, saving money as well as space in stores and at assembly. Perhaps most importantly, with the right PSA for the application, foam pieces will stay in place for the life of the product, avoiding warranty problems and improving quality. Whether the application is automotive interior trim, under-hood sound deadening or cushioning, acoustic control, packaging, transit cases, medical or something else, pre-laminating a PSA onto foam results in a better product.
Pick up the Phone
PSA's come in many different forms and matching properties to the application is essential. Start that discussion today by calling or emailing a product specialist at Merryweather Foam.