Design, construction, and selection of adhesives requires a thorough understanding of the properties of the materials involved.
Adhesive tape applications range from the obvious, such as wound care and bandages, to the more obscure, such as diagnostic strips used in pregnancy tests and other point-of-care diagnostic devices. Selecting the appropriate adhesive tape for any application is critical to a new-product development project's success. It requires a detailed understanding of performance requirements, analysis of available options and their performance attributes, and insight into the manufacturability of various tape, adhesive, and substrate combinations.
A number of variables—such as the suitability of the adhesive tape for the application, material compatibility, adhesive performance properties, environmental factors, sterilization methods, and material cost targets—must be investigated and taken into consideration. See the sidebar for a detailed list of products that use adhesive tape.
Composition and Construction
The specific composition of adhesive tapes can vary widely, but understanding the common components of construction provides an excellent framework for identifying the optimal product for any application.
Nearly every adhesive tape used in a medical application comprises three primary components: a face stock, a release liner, and an adhesive. Whether the final product is a simple short-term-use bandage or a complex multimillion-dollar medical device, the adhesive tape requirements must be assessed in terms of the same three elements.
Face Stock. The face stock is the base carrier material onto which an adhesive is applied. Face stock can consist of a number of materials, including film, foam, foil, paper, and woven and nonwoven materials. Face stock can also vary in thickness and weight to improve conformability and manufacturability, multiplying the available configuration possibilities significantly. The composition of the face stock helps determine the elongation or stretching characteristics of the adhesive tape, the tape's conformability to a structure such as a part of the human body, and the tape's anchorage, tear strength, moisture-vapor transmission rate (MVTR), and cost.
Adhesives. The composition of adhesives varies as well, with each variation offering unique characteristics. Cost-effective rubber-based adhesives deliver a high initial tack, but are limited by a relatively low maximum temperature. Rubber-based adhesives can be unreliable at temperatures above 140°F and could fail at high temperatures.
Acrylic polymer–based adhesives, on the other hand, can be used at relatively high temperatures (up to approximately 250°F) and resist environmental factors. Acrylic polymer adhesives are limited by low initial-tack characteristics, but have excellent long-term aging characteristics. Although these adhesives can be modified to increase tack, doing so reduces the adhesive's resistance to solvents and high temperatures.
Hydrocolloid adhesives are specifically designed for wound-care applications. Hydrocolloid is hypoallergenic and has a high MVTR. The material is engineered to maintain moisture in the wound bed and absorb exudates, making it ideal for long-term wear of up to 7 days. Most hydrocolloids combine with wound exudate to form a gel-like covering, which protects the wound bed and maintains a moist wound environment. In addition, hydrocolloid is comfortable on the skin, conformable to the body, and easy to remove, all desired characteristics for wound-care applications.
The thickness of an adhesive also affects its strength and performance. If the substrate to be bonded is not uniform or has a deep cellular structure, a thick adhesive may be desirable. A thick adhesive more readily adapts to the irregularities of a substrate, effectively seeping into the “nooks and crannies” and adhering to the cell structure, making a strong and durable bond. A thick adhesive also tends to be very flexible. Such flexibility is desired for applications in which expansion and contraction are a concern, such as if temperature fluctuations are prevalent.
Very-high-strength super-cross-linked adhesives (e.g., VHB and UHA) are also available. These have very high bond strength for particular applications but are not typically used for wound-care applications. They are, however, suitable for box builds and other applications that do not require skin contact.
On occasion, an adhesive is coated onto both sides of a carrier (or scrim) in a double-coated-tape format. This can involve the use of the same adhesive on both sides, or two different adhesives with different and distinct properties for each particular application. The carrier in the middle gives the final product greater structural integrity, reduces stretching, and aids in lamination, slitting, and die-cutting. Typical materials for the scrim in medical tape are paper, film, tissue, or a nonwoven component. Hydrogel sheet materials often use a scrim to provide structure and integrity.
Release Liners. Coated release liners are used to improve the handling of the adhesive material and allow for slitting, precision die-cutting, and protection of the adhesive. Liners are typically made of paper or film and can be coated on one or two sides. Liner selection is often based on the cost and design requirements of the application, as follows:
Adhesive Tape Construction
Tape construction varies and the correct tape for the task depends on a number of variables, including the application, environmental factors, and material compatibility. Adhesive tape construction (see Figure 1) can include any of the following configurations, as follows:
Figure 1. (click to enlarge) Tape configurations are determined by the application, material compatibility, and the environment of the application. Shown here (from top to bottom) are unsupported, single-sided self-wound, single-sided, and double-sided tape constructions.
The composition of the adhesive tape best suited for any given application and how it is constructed are determined by a number of factors. These factors include the application suitability, the material compatibility, and the environment in which it will be used. No one tape is suitable for all applications, which explains why the number of adhesive tapes available is in the thousands and why new ones are created and manufactured every week.
Suitability for Application
The performance requirements of an application are the key drivers of material selection. If the application is a wound-care device, the MVTR (the rate at which moisture transfers through a material measured in grams per square meter per day) is the critical consideration. Typically, the higher the MVTR, the better a material will perform for wound-care applications, especially for applications in which products are applied for 3 days or more. In addition, very-high-MVTR materials tend to be ultraflexible and can conform to a wide variety of shapes. These materials often must be supplemented with a release liner or frame that provides greater structural rigidity to aid in die-cutting and product application.
Wound-care applications also require evaluation of the tape's skin sensitivity, fluid resistance, shelf life, color characteristics, and expected duration on the skin. The tapes are also compared based on adhesion, tack, and shear parameters.
In vitro diagnostic applications require an inert adhesive and material that will not introduce chemicals into the test. Typically, hydrophilic materials that promote moisture absorption and flow are desirable for such applications.
A major and often overlooked consideration for adhesive tape selection is the bonding surface. For example, if the adhesive will be used to bond to the skin of an elderly person or a newborn, a very gentle, easy-release adhesive is preferred because it reduces the risk of skin tear. If the adhesive is for the general population, a more aggressive adhesive can be used and would be more desirable because of the increased activity, motion, and flexing of the skin relative to the elderly or newborns. A high-surface-energy substrate is the easiest to bond to, because the adhesive will form a good cohesive bond.
A silo creates adhesives based on user specifications.
For non-wound-care applications in which the substrate being bonded to is not skin, the substrate composition must be taken into consideration. If the material outgases, it could react with the adhesive and cause it to break down and fail prematurely. A low-surface-energy substrate (e.g., polypropylene, powder-coated metal) requires a specific high-tack adhesive to form a good bond. On low-surface-energy surfaces, an adhesive tends to resist flowing, or wetting, onto the surface, making poor interface contact and adhesion strength. Corona treating (applying voltage buildup to ionize the air around the surface) can significantly improve adhesion onto low-surface-energy surfaces. Other methods that can be used are to increase the thickness or tackify the adhesive. Such methods have varying levels of effectiveness depending on the materials involved.
The environment in which the part will be used is a primary concern. For example, exposure to ultraviolet light can cause many adhesives to dry out or degrade over time. Because of this, designers should think about whether the application will be in direct contact with sunlight.
In addition, exposure to high temperatures can take an adhesive close to its melting point and reduce its cohesive strength, while exposure to solvents, in either a liquid or a gaseous state, that are not compatible with the adhesive selected can cause the adhesive to dissolve and fail.
The sterilization method chosen also needs to be considered when selecting an adhesive tape. Some adhesives are adversely effected by gamma sterilization, but have no reaction when ethylene oxide is used, or vice versa. Other adhesives may react to both sterilization methods. Medical-grade adhesive tapes typically have information regarding sterilization compatibility available.
Current materials and converting technology enable the design and formulation of medical tapes that can satisfy the requirements of nearly any medical application. Successful selection requires a disciplined review that begins with a thoughtful analysis of an application's demands and matches those demands with available adhesive chemistries, substrate materials, and release-liner technology. Creative approaches to overall construction can yield a tape design that delivers robust product performance and manufacturability that satisfies the economic and quality demands of an increasingly challenging marketplace.
Successful adhesive tape selection requires forethought and planning and is critical to a project's success or failure. Knowledge of the application, environmental factors, and material compatibility are essential to a project's success. In many cases, the correct adhesive tape for an application is found only after experimenting with several materials and design configurations. A good material converter can prove to be invaluable during this process and can provide the tools and knowledge essential to the success of the project.
The most successful companies form partnerships with converters and adhesive tape manufacturers from the outset to create a team with seamless lines of communication and transfer of information. By doing so, the trial-and-error process can be shortened and the time to market can be reduced substantially.
Special thanks to John Alfini of Baril Corp., who made significant contributions to this article.
Bill Hanna is vice president of sales and marketing at Baril Corp. (Haverhill, MA). He can be reached at email@example.com.