The Fundamentals of Selecting Pressure-Sensitive Adhesives

The Fundamentals of Selecting Pressure-Sensitive Adhesives (no template)

Medical Plastics and Biomaterials
Magazine

MPB Article Index

Originally published January 1998

ADHESIVES

Pressure-sensitive adhesives (PSAs) represent a system that actually dates back to the invention of the self-adhesive label in 1935, when R. Stanton Avery produced the first coating unit using a wooden cigar box with two holes cut in the bottom. The box was filled with adhesive, which dripped through the holes onto a narrow roll of paper that was

run below. The thickness of the adhesive coating was regulated by narrowing the holes with pieces of tape, producing pressure-sensitive labels that could be applied without moistening or glue. Although the technology may seem primitive, the multimillion-dollar, sophisticated coaters currently used to fabricate a range of PSA medical devices of uncommon versatility and design flexibility operate according to the same basic principles.

Pressure-sensitive adhesives can be defined as "a distinct category of adhesive tapes that in dry form are aggressive and permanently tacky at room temperature." PSAs will adhere to a variety of substrates when applied with pressure; do not require activation by water, heat, or solvents; and have sufficient cohesive strength to be handled with the fingers. The primary mode of bonding for a PSA is not chemical or mechanical but rather a polar attraction to the substrate, and always requires pressure to achieve sufficient wet-out onto the surface to provide adequate adhesion.

PSAs VERSUS ALTERNATIVE ADHESIVE TYPES

An excellent way of gaining an understanding of PSAs is to contrast them with some of the prominent alternative adhesive types such as hot melts or spray, liquid, or light-cure adhesives. For example, hot melts are polymeric thermoplastic compounds that are applied molten and form a bond upon cooling. Hot melts require specialized equipment capable of melting and delivering the adhesive. Spray adhesives are generally solvent-borne systems applied to the substrate via air-pressure mist. Overspray and solvent emissions are among the application issues that can be of concern with sprays. Liquid adhesives are available in emulsion or solvent varieties, and require brush or roller coating to apply, whereas light-cured adhesives are products that cure when exposed to UV or visible light. Other common adhesive formulations include urethanes and cyanoacrylates.

It is important to compare total applied cost and unit cost in order to understand the true impact of using PSAs versus these alternatives. Hot-melt, spray, liquid, epoxy, and cyanoacrylate adhesives all demand the attention of the user to ongoing quality inspection, necessitating an in-house adhesives expert to keep abreast of application-specific requirements, thus adding costs in excess of the base cost of the adhesive. These systems also reduce the ability to tailor adhesives to converting specifications involved in die-cutting and release mechanisms.

Some of the inherent advantages of using PSAs compared with other adhesive types include:

  • The ability to employ lighter, thinner materials.

  • The ability to bond dissimilar materials with fewer concerns about incompatibility.

  • Shortened assembly times and improvements in productivity.

  • Elimination of visible mechanical fasteners.

  • The ability to adhere to difficult surfaces such as human skin.

  • Uniform thickness throughout the product.

  • Elimination of the need to bond both substrates together at the same time and location.

  • No cure cycle, since solvents are removed as the PSA product is coated.

  • Minimal issues of health, safety, and disposal.

ADHESIVE PERFORMANCE FACTORS

There are many properties that need to be taken into account when manufacturing a PSA, such as chemistry, viscosity, and coatability. However, when one considers the performance of the end product, there are three distinct performance factors—adhesion, tack, and shear—that can be used to characterize adhesive types and differentiate one adhesive from another. The relationship of these characteristics is critical to the understanding of PSA performance and selection.

Adhesion. The measure of bond strength between an adhesive and a substrate is defined as adhesion. It is important to note that this is the measure of bond strength after pressure is applied to the adhesive, allowing for wet-out onto the substrate. Adhesion properties are typically measured using the 180° or 90° peel adhesion test method (see Figure 1a).

Figure 1. PSA performance characteristics such as adhesion, loop, and shear can be tested via procedures that include (a) 180° or 90° peel adhesion tests, (b) the loop tack test, and (c) the static shear test.


Tack. Tack is the initial attraction of an adhesive to a substrate—the instant grab of the adhesive, with no external pressure applied. Sometimes tack is referred to as "quick stick" or "thumb appeal." Tack properties are typically measured using the loop tack test (see Figure 1b).

Shear. The internal or cohesive strength of an adhesive film is known as shear. This is not a measure of the bond between the adhesive and a substrate, but rather a measure of the internal strength of the adhesive itself. Shear properties are typically quantified using the static shear test method (see Figure 1c).

In general, as the shear strength of an adhesive system is increased, tack and adhesion performance decrease.

PSA VARIETIES AND CHARACTERISTICS

Several common technologies are used for manufacturing PSAs, including solvent-based, hot-melt, and emulsion processes. In solvent-based fabrication, the adhesive ingredients are polymerized in solvent and cast onto a web. After the web is coated, the solvent is dried off, leaving behind the functional adhesive. Hot-melt processing features thermoplastic rubbers formulated with tackifying resins, oils, and antioxidants. The adhesive is coated onto a web at very high temperatures (>300°F) and cools in the roll. Emulsion techniques use adhesive ingredients that are polymerized in water, applied to a web, and then dried.

The four main varieties of PSAs are derived from rubber-based, acrylic, modified acrylic, and silicone formulations. Each of these types exhibits distinct performance characteristic (see Table I).

Characteristic Rubber-Based Acrylic Modified Acrylic Silicone
Cost Lowest Medium/high Medium/high Very high
Tack Medium/high Medium/low High Low
Temperature resistance Low High Low/moderate Very high
Adhesion Medium/high Moderate/high High Medium/low
Shear Medium/high Moderate/high Low Moderate
Solvent resistance Poor Good Low/moderate Excellent
Ultraviolet resistance Poor Excellent Poor Excellent
Plasticizer resistance Poor Moderate/good Poor/moderate Excellent
Low-surface-energy materials Excellent Poor/moderate Excellent Poor
High-surface-energy materials Excellent Excellent Excellent Moderate


Table I. PSA adhesive characteristics.

Rubber-Based Adhesives. Rubber-based PSAs are synthetic, nonlatex rubbers (styrene block copolymers) formulated with tackifying resins, oils, and antioxidants. These adhesives provide good to excellent initial tack and adhesion along with excellent adhesion to low-surface-energy materials such as plastics. They do not demonstrate good temperature resistance (typically <150°F) or resistance to environmental stresses such as solvents, sterilization, chemicals, or ultraviolet rays. Some rubber-based products are specially formulated to achieve exceptional adhesion in high-moisture applications, such as adhering to human skin under extreme conditions.

Acrylic Adhesives. Formulated from acrylic polymers, acrylic PSAs offer resistance to solvents, UV light, elevated temperatures, plasticizers, chemical reagents, and sterilization methods. Acrylic adhesives tend to be more costly than rubber-based varieties, but provide better long-term aging and environmental resistance. They demonstrate low to moderate initial tack and adhesion, and generally do not adhere well to low-surface-energy materials. Clear in color, acrylic PSAs can be made porous (breathable) and constitute the majority of adhesives suitable for use on human skin.

Modified Acrylic Adhesives. Fabricated from acrylic polymers but incorporating additional components (like tackifiers) found in rubber-based systems, modified acrylics offer improved initial tack and adhesion to low-surface-energy materials compared with nontackified acrylic formulations. The additives, however, will decrease resistance to solvents, plasticizers, UV light, and sterilization. Shear properties and temperature resistance are also reduced. Thus, although modified acrylics gain tack and adhesion, the trade-off is a loss of internal strength and environmental stability.

Silicone Adhesives. Compounded from silicone polymers, these adhesives are expensive relative to other types of systems. Silicone PSAs generally demonstrate low initial tack and adhesion but provide outstanding temperature performance (to 700°F) and resistance to chemicals. These are the only PSAs that will consistently bond to silicone substrates.

PSA CONSTRUCTION

PSAs are manufactured and delivered to the user as some form of webstock or tape. The construction type and components of the tape can be considered a system, and are as important to end-use performance as is the specific type of adhesive used.

Single-Coated Systems. Single-coated systems are constructed with an adhesive coated on one side of a facestock. This facestock can be any one of a variety of materials, ranging from films to cross-linked polyethylene (PE) foams. The adhesive is protected by a silicone-coated release liner. Typical uses for single-coated PSAs include wound-care products, electromedical devices, and ostomy applications.


Application Factors for PSAs

  • Price target
  • How the product is applied
  • Expected usage (volume)
  • Condition of substrates
  • Timing of project
  • Function the tape must fulfill
  • Materials to be joined
  • Texture of substrates
  • Manner of processing
  • Temperature at application
  • Migratory components of substrate
  • Temperature during use
  • Product life expectancy
  • Exposure to UV/light
  • Existing specifications
  • Exposure to solvents
  • Previous problems encountered
  • Sterilization methods


Transfer Tapes. An unsupported adhesive film coated on a release liner that is silicone-coated on both sides constitutes a transfer tape. This construction does not include a facestock or carrier, but is merely a free film of adhesive. Transfer tapes offer superior conformability to irregular surfaces for applications such as test strips, surgical drapes, and medical packaging. If desired, these tapes can be pattern-coated for ease of slitting and processing.

Double-Coated Systems. Double-coated systems feature a carrier coated on both sides with a PSA. The adhesive is in turn protected by a release liner coated with silicone on both sides (see Figure 2). Carrier materials can include plastic films, tissue, nonwovens, PE foams, and so forth. The carrier offers ease of handling and slitting, and can serve to reduce overlamination on porous materials. Another advantage of a carrier is the ability to use different adhesives on each side, which enhances product flexibility and allows for permanent/removable-type designs for such medical applications as suture-clip holders and instrument-holding devices used in the operating room. Typical uses of double-coated constructions also include surgical drapes, medical packaging, wound-care products, and electromedical devices.

Figure 2. Typical construction of double-coated PSA tape.


Self-Wound Systems. Self-wound PSAs comprise a facestock coated on one side with PSA and on the other with a silicone release coating. There is no release liner in this type of product. Among the common applications are diaper tapes and tapes provided in short roll lengths for catheter holders or wound dressings.

Release Liners. The purpose of release liners is to protect the adhesive, transport the web, and provide a surface for silicone or adhesive coating. The type and quality of release liner can have a significant impact on PSA end-product performance. Liner varieties include PET films, densified kraft paper, polycoated kraft, and polyethylene films.

Facestock and Carriers. An extensive range of materials—such as films, foams, tissues, nonwovens, and papers—can be used as facestock or carriers. The selection of a particular material depends on the needs of the application, such as requirements for conformability and elongation, color, stiffness, solvent and temperature resistance, and dimensional stability. Nonpermeable films such as polyester and PE can be used as effective bacterial barriers, whereas such materials as polypropylene (PP) nonwovens, PE films, polyester films, closed-cell PE foam, and taffeta can offer good moisture resistance. PE films—because they are stiffer and less porous—are well suited for surgical-drape tape applications and as a bacterial barrier used in concert with drape material. Films can provide visibility to the wound site when used for wound-care applications. For example, polyurethane films can provide clarity as well as breathability when used in concert with porous adhesive systems.

SELECTION AND APPLICATION FACTORS

Selecting a PSA product for an application requires as much knowledge as can be gathered about the needs of both the converter and end-user (see box below). For example, attempting to satisfy end-use requirements without addressing processability concerns will most likely not lead to a successful product. The following information is critical for informed product selection, as
it takes into account both the characteristics of the PSAs and the conditions under which they will be used.

PSAs are used in electrodes, suture strips, and nasal dilators.


Surface Contour. The contour of the substrate will influence product selection (see Figure 3). For uses requiring conformability around irregular angles, more flexible materials should be employed. Regardless of the strength of the formulation, it is virtually impossible for an adhesive to overcome a continuous stress placed on
it by a rigid material trying to return to its original form (memory). For such applications, one should choose a conformable product with a tissue or nonwoven carrier facestock or consider adding stress relief to the converted part via scoring or perforation.

Figure 3. The contour of the substrate can influence adhesive selection.


Surface Energy. The ability of an adhesive to wet out over the surface of a material is related to its surface energy. Low-surface-energy (LSE) materials do not allow the adhesive to wet out, whereas high-surface-energy materials allow excellent wet-out and provide the best adhesion. Rubber-based adhesives generally offer better adhesion to LSE substrates—a softer, better flow. Some materials require special treatment—corona treatment, primers, topcoats—to promote better adhesion. On some LSE materials, adhesion levels will improve with longer adhesive dwell times.

Surface Contamination. The presence of surface contamination such as skin oils and bodily fluids can prevent contact of the adhesive with the substrate. There are many different types of surface contamination, most of which are not visible to the eye but can be identified analytically. It may not be possible to obtain an acceptable bond without cleaning the surface (e.g., through washing or flame treating). Surface contamination may be present if one can detect loose material on the surface of a substrate or the material feels slippery, greasy, or slimy. Contamination may also be suspected if testing indicates poor bond strength and the adhesive feels "dead" after removal from the substrate.

Surface Texture. The texture of a substrate can also have an impact on the PSA bond. For example, elderly patients may have rough skin, whereas babies may have very smooth skin. Textured materials do not allow complete contact of the adhesive with the substrate—the less contact, the smaller the bonding area and the lower the adhesion (see Figure 4). In such cases, either a more aggressive adhesive or a PSA construction offering greater adhesive mass will be required.

Figure 4. Substrate texture has an impact on the strength of the adhesive bond.


Of particular concern for medical PSAs is the fact that human skin is one of the most variable substrates encountered. No two persons' skins are identical. Human skin is dynamic: it breathes, stretches, perspires, absorbs, and will even react differently depending on which soap a person uses. Acrylic adhesives tend to be the most suitable for use on the skin because of their resistance to moisture and to body chemicals. In general, most rubber-based systems do not normally seem to hold up as well on the skin over time, although rubber-based adhesives can be specially formulated to provide exceptional adhesion and stability in skin applications.

Biocompatibility Testing. An adhesive selected for use on human skin requires safety testing in accordance with FDA guidelines and ISO 10993 standards. Any adhesive placed in direct contact with the skin should be safety tested using RIPT (repeat-insult patch testing), the Draize test (dermal irritation in albino rabbits), or other tests that can be performed depending on application requirements. Performance results for medical adhesives should remain with FDA in drug master files and be available to customers on request. More detailed information on biocompatibility testing can be found in the ISO 10993/EN 30993 standard and the FDA blue book memorandum G95-1.

Moisture-Vapor Transmission. The moisture-vapor transmission rate (MVTR) is the rate at which moisture permeates a dressing, measured in grams per square meter per day. Different methods for measuring MVTR include the upright method for low moisture contact (indirect liquid contact with dressing), the inverted method for high moisture contact (direct liquid contact with dressing), and the gravimetric method, which follows ASTM E 96 and measures water loss after 24 hours.

Effects of Sterilization. Ethylene oxide (EtO) gas and gamma irradiation are low-temperature processes that can be used to render plastic devices sterile without deformation. These techniques are also suitable for devices employing PSAs as a bonding component.

EtO sterilization normally does not affect the properties or performance of PSA constructions. At the higher end of the typical 2.5—5.0-Mrd dose range, gamma processing may result in slightly lower peel adhesion values, with the potential for release-liner adhesion to the adhesive. Although this trend may be seen with some adhesive systems, its impact on short- and long-term acrylic adhesive performance is negligible and should not be considered a reason to avoid gamma radiation as a viable sterilization technique for PSAs.

Because of the high temperatures and high moisture concentrations involved in steam sterilization, it is the least desirable method for products incorporating PSAs.

CONVERTING

Regardless of its acceptability for end-use performance, a PSA product must process efficiently to meet application requirements. Product selection can have a substantial impact on the abilitiy to convert the laminated construction into the end product. It is important to always include processing considerations in the product decision matrix (see Table II).

Process Requirement Recommendation
Laminate stretchable materials Use product with film carrier
Rotary die-cut Hard kraft liners are best, also PET
Heavy die-cut part Need heavy liner for stability
Hand laminating Aggressive unwind adhesive will benefit
Flat sheet needed Consider moisture-stable liner
Small die-cut part Tighter release will hold part to liner
Difficult die configuration May need lower release to remove waste
No curl at roll end 6-in. cores will reduce curl due to memory


Table II. Processability is an important factor in meeting application requirements.

CONCLUSION

The proper selection of PSA systems requires the consideration of variables such as adhesive type, required product construction, cost targets, end-use environmental conditions, and processing factors. Although typical performance data will assist the product selection process, it is important to note that there is no substitute for testing under actual end-use conditions. Such testing is always recommended to ensure that all requirements are satisfied. It is hoped that this article has provided a basis for understanding the PSA selection process and conveyed some appreciation of the potential role that PSAs can play in today's medical marketplace.

ACKNOWLEDGMENTS

The author wishes to thank Lori Weinstein, Diane Ewanko, and David G. Anderson for their contributions in the preparation of this article.

BIBLIOGRAPHY

Booth A, "Industrial Sterilization Technologies: New and Old Trends Shape Manufacturer Choices," Med Dev Diag Ind, 17(2):65—72, 1995.

Satas D (ed), Handbook of Pressure Sensitive Technology, 2nd ed, New York, Van Nostrand Reinhold, 1989.

Test Methods for Pressure Sensitive Tapes, 8th ed., Glenview, IL, Specifications and Technical Committee of the Pressure Sensitive Tape Council, 1985.

As national technical marketing manager for Avery Dennison Specialty Tape Division (Painesville, OH), Donald V. Varanese heads the technical marketing department serving the fabricator, automotive, and medical markets. His responsibilities include overseeing new product development, custom product redesign, application engineering, converting engineering, field sales technical support, and product training. Prior to joining Avery Dennison in 1990, he served as technical division manager for ASM International.


Copyright ©1998 Medical Plastics and Biomaterials

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