Adhesive Tape

Chapter One – What is an Adhesive Tape?

Pressure-sensitive adhesive tapes consist of a backing material film coated with an adhesive intended for relatively low-stress applications. Light pressure, usually done by the fingertips, is applied to initiate the binding. In the sticking process, the fluid properties of the pressure-sensitive adhesive change rapidly to flow and set in the surface of the substrate.

Adhesive tapes first appeared in the mid-19th century for medical applications. In 1845, Dr. Horace Day created bandages from strips of fabric with rubber adhesive tapes. It was the inspiration behind Johnson and Johnson’s Band-Aid. In 1923, the 3M Company formulated the first masking tapes. Adhesive tapes kept on evolving to materialize the present varieties of adhesive tapes (e.g., packing tape, painter’s tape, and electrical tape) which serve their unique application. Nowadays, adhesive tapes are certainly found in most offices, households, shops, and industries which undoubtedly makes them one of the most useful tools ever invented.

Adhesive tapes are efficient and readily available to perform different functions such as joining, masking, sealing, splicing, bundling, and surface protection, which does not require machinery or any other special tools. Adhesive tapes are lightweight and efficient to store; it usually comes in rolls and is only unwound if there is a need for adhesive. Unlike the traditional mechanical fastening objects such as screws and bolts, it eliminates the need to pierce or punch the substrate which induces a stress area.

Chapter Two – The Science of Adhesive Tapes

The principles behind the bonding of adhesives, which may be overlooked because of the simplicity of its process, will be discussed in this section.

Parts of the Bond

The bonding layer made by an adhesive tape, shown macroscopically in the image below, presents a cross-section of a bond. The layer consists of an adhesion zone, cohesion zone, and a transition layer.

The adhesion zone is the interfacial layer between the adhesive and the substrate. The cohesion zone is the pure adhesive layer that holds the adhesive and the backing material, or another substrate if the adhesive is unsupported. The transition layer is intermediate between the adhesion and cohesion zones.

Mechanism of Action

The mechanism of adhesive tape bonding occurs as follows. These steps can be accomplished instantly after the activation of the pressure-sensitive adhesive.

1. The pressure-sensitive adhesive on the tape is made to contact with the substrate by minimal pressure.
2. Adhesive increases its surface area and penetrates through the surface of the substrate.
3. Adhesive sets on the substrate, producing a strong bond.

The three elements of a pressure-sensitive adhesive tape to achieve a successful bond are adhesion, cohesion, and tackiness.

• Adhesion: Adhesion refers to the ability of an adhesive to stick on the surface of the substrate through adhesive forces. Adhesive forces refer to the attraction of two different materials. When the substrate and the adhesive molecules come in close proximity, adhesion forces occur at the microscopic level through intermolecular forces (e.g., van der Waals forces, dispersion forces) between them.

  Surface energy is one of the properties which determines the wettability of the substrate when it comes in contact with the adhesive. Wettability is important to penetrate the surface of the substrate, thus making a continuous bond.

  Surface energy is defined as the sum of the intermolecular forces, and the attraction and repulsion energies which a liquid exerts on the surface of a solid. If the substrate has high surface energy, the adhesive will easily flow on its surface and more surface area will be covered. Some of the high-energy substrates include polycarbonate, polyvinyl chloride, and zinc. On the other hand, if the substrate has low surface energy, the adhesive will build up as “small beads” and only a small area will be covered. Low surface energy substrates include Teflon, rubber, powder coatings, etc.

  

  
  

  Surface contaminants prevent the fusion of the adhesive into the substrate. Therefore, it is important to keep the surface free from grease, dirt, and moisture before application of the adhesive tape.

• Cohesion: Cohesion is the inner strength of the adhesive. It refers to the bonding of the adhesive within itself. Cohesion holds the adhesive layer intact and prevents it from splitting.

  Cohesive forces attract the neighboring molecules of a liquid by pulling it inward. Molecules at the surface of the liquid have greater attractive forces that bind them together. This phenomenon is responsible for the property of a liquid called surface tension. Surface tension is the ability of the adhesive to resist deformation on the surface of a solid, thus reducing its surface area. In the case of adhesive tapes, the molecules of the adhesive must be strong cohesive forces to hold and maintain the bond over time.

  Silicone is an example of a liquid with high surface tension. If silicone is present as a coating on the surface of the substrate, it will be difficult to be wetted by the adhesive. If used as an adhesive, on the other hand, it will produce a durable bond.

  Adhesion and cohesion must be considered when formulating or choosing the right adhesives. An ideal bond consists of a combination of a high surface energy substrate and a low surface tension adhesive. To achieve good wetting of the substrate, adhesive forces must be greater compared to cohesive forces and the contact angle must be less than 90°.

• Tackiness: Tackiness refers to the characteristic of a pressure-sensitive tape by which the adhesive sticks to the surface of the substrate under minimal pressure. All pressure-sensitive adhesives are activated by an initial finger pressure applied to the tape, usually from 14.5 to 29 psi. The required pressure and contact time for the adhesive tape to stick varies according to the type of adhesive and substrate material. Less pressure and contact time are required to stick for adhesive tapes with higher tack.

  The phenomenon described is attributed to the viscoelasticity of the adhesive at room temperature. Viscoelastic objects are materials that possess both viscous and elastic characteristics. When gentle pressure is applied to the tape, it decreases its viscosity thus encouraging flow on the substrate at the microscopic level. Due to its elastic property, the original viscosity is regained with stronger intermolecular force with the substrate.

  The Rolling Ball Test and Loop Tack Test are the most common quality control checks used to assess the tackiness of the adhesive tapes being produced.

  

  
  
  • Rolling Ball Test: The rolling ball test directly gauges the adhesive behavior of the adhesive tape. A steel ball with a standard weight and diameter is made to roll from the top of an inclined track which consists of the sticky side of the tape. The tackiness is measured by the distance traveled by the ball on the sticky tape track; the shorter the distance, the greater the tack.
    
    

    

    
    
  • Loop Tack Test: The loop tack test is a quantitative and repeatable method to assess adhesive tape tackiness. A loop of adhesive tape is attached to the probe of a tensile tester machine. The loop is made to contact with a horizontal surface for a short time, then the tool pulls it away. Numerical values of the bond’s tensile strength are recorded and subjected to evaluation.

Chapter Three – Components and Types of Adhesive Tapes

Adhesive tapes consist of a carrier, adhesive, and release liner. Not all types of adhesive tapes are required to have all the components but are still dependent on the properties of the material and the intended storage of the tape itself. A more careful materials selection must be done on the carrier and adhesive especially when downstream processes, such as cutting, are involved. The adhesive tape may contain the following:

Carrier

The carrier, or the backing material, is a thin flexible film that contains the adhesive. The carrier can have the following materials, depending on the end application of the adhesive tape:

• Plastic Films: Plastic films are one of the widely used carriers for adhesive tapes. It offers flexibility, high tensile strength, and good weathering resistance. Its color and transparency can be customized, and it can be easily printed. The following polymers used in plastic carriers are:
  • Polyester-polyethylene Films: Polyester-polyethylene films offer heat and electrical insulating properties and anti-scratch proof surfaces.
  • Polyvinyl Chloride (PVC) Films: PVC adhesive tapes are valued for their chemical, moisture, and flame resistance, high toughness, and high tensile strength. It is commonly used for household and electrical applications which require the adhesive tape to be durable and to give a semi-permanent bond.

    

    
    
  • Polyimide Films: Polyimide carriers are used in adhesive tapes which are intended to be exposed at high temperatures. It is widely used in the manufacture of flexible printed circuit materials The Kapton tape is the most popular brand of adhesive tape with polyimide carriers.
  • Acetate Films: Acetate carriers are lightweight with good heat, chemical, and electrical resistance.
  • Fluoropolymer Films: Fluoropolymers such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) are used as carriers for adhesive tape due to their excellent chemical resistance, low coefficient of friction, and good dielectric properties. It also has an anti-scratch and anti-stick surface on the exposed side of the tape.
• Metal Foil: Metal foil carriers are valued for their resistance to severe temperature extremes and high humidity, and their excellent electrical conductivity. Metal foil carriers are usually made of aluminum, copper or zinc, and are used to bond electrical components. It may be reinforced to a plastic or paper film to increase its strength.

  

  
  
• Paper: Paper carriers are suitable for lightweight and decorative applications. It is easily customizable and printed. It can be laminated with other materials to improve their tensile strength as paper films are naturally brittle. The most common adhesive tapes with paper carriers are masking tapes and paper tapes, which are used to seal boxes and packaging materials, and fragile surgical tapes. Zinc gives the white color on all medical tapes and enables its antimicrobial properties.

  

  
  
• Cloth: Cloth carriers provide adhesive tape flexibility and additional heat resistance. It is often combined with other carrier materials. A variation of surgical tape has a cloth carrier which allows permeability of air for breathability.
• Fiberglass: Fiberglass carriers in adhesive tapes offer dimensional and thermal stability. It does not shrink in a severe environment. It may be reinforced with other material types to produce a laminate that has greater plasticity and tensile strength. Typical examples of laminates are polyester-glass fiber, Teflon-glass, and fiberglass-aluminum laminates.

Adhesive

This is the main component of every adhesive tape which primarily serves its purpose. Adhesives are the polymeric compounds that are applied to the surface of two items to connect them and resist their separation. The commonly used adhesives are enumerated below.

• Rubber Adhesives: These adhesives are inexpensive and intended for low stress and room temperature applications. It provides a quick stick to substrates and does not require long cure times. Modifiers are added to rubber adhesives to increase their tackiness. However, heat and UV radiation have adverse effects on the performance of the rubber adhesive.

  Rubber adhesives are commonly used in duct tapes, masking tapes, and carton sealing tapes. It may be derived from natural rubber or synthetic rubber.

  

  
  
  • Natural Rubber Adhesives: This type has higher tack and shear strength. It also exhibits good removability after the tape has been removed.
  • Synthetic Rubber Adhesives: These are formulated by manufacturers and provide better adhesion and higher shear resistance. Types of synthetic rubber adhesives are:
    • Solvent Rubber Adhesives have better adhesion than natural rubber when exposed to heat and extreme environments but still have poor UV resistance.
    • Hot Melt Rubber Adhesives are only suitable for low duty applications. Its low melting temperature causes a change in phase which makes it underperform at lower temperatures.
    • Butyl Rubber Adhesives are resistant to both UV and extreme environmental conditions but have a lower mechanical strength.
• Acrylic Adhesives: Acrylics are high-performance adhesives that are based on polymerized acrylic monomers. It overcomes the weaknesses of rubber adhesives. Acrylics are resistant to UV radiation, high temperature, chemical attack, and oxidation. The bonds made by acrylic adhesives are initially strong from the time of application, and become much stronger as it sets. These adhesives offer heavy-duty and long-term bonding solutions.
  
  

  

  
  

  Acrylic adhesives are available as water-based or solvent-based adhesives:

  • Water-based Acrylic Adhesives: This type is also referred to as emulsion acrylic adhesives. They have polymeric compounds dispersed in water and surrounded by a surfactant. Surfactant lowers the surface energy of the acrylic polymer, hence increasing the wettability of the surface to be bonded. Water-based acrylics are inexpensive compared to their solvent-based counterpart. However, being water-based, these adhesives are less resistant to moisture and heat.
  • Solvent-based Acrylic Adhesives: These have acrylic polymers dissolved in a solvent. It produces a stronger bond which is better retained over time and is more resistant to extreme conditions compared to its water-based counterparts.
• Silicone Adhesives: They are regarded as the most expensive adhesives used in tapes. Among the adhesive tapes, it can withstand the highest temperatures of up to 475°F. It also has excellent UV radiation and chemical resistance. It has low initial bond strength, which becomes significantly tough as the adhesive sets from the time of application.
  
  

  

  
  

  Silicone adhesives are found in tapes which are mainly used in the electronics industry, where significant amounts of heat are dissipated in electronic components. These adhesives are also found in splicing tapes and Teflon tapes.

  Release Liner

  A release liner is one of the components that must be present in double-coated tapes and adhesive transfer tapes. This layer is peeled-off before the bond is made to the substrate. It preserves the tackiness of the adhesive until it is ready to be used, and protects it from sticking from the carrier.

Chapter Four – Different Types of Adhesive Tapes

Adhesive tapes are categorized according to the arrangement of the carrier, adhesive, and release liner:

• Single Coated Tape: Single coated tapes are the most basic configuration of adhesive tapes. The adhesive film is only found on one side of the carrier. The exterior side of the carrier may be coated with a release agent to help in unwinding the tape, or a release liner may be used. Typical examples of a single coated tape are electrical tapes, duct tapes, masking tapes, etc.
  
  

  

  
  
• Adhesive Transfer Tape: These are intended for a discrete, seamless bonding of two substrates. The unsupported adhesive film is protected with a release liner, which is peeled off after bonding to the first substrate. The second substrate is then attached to the other side of the adhesive film. The release liner is used on both sides of the adhesive to aid its separation. Unlike double coated tapes, the temperature resistance of an adhesive transfer tape is limited only by the properties of the adhesive itself.
  
  

  

  
  
• Double-coated Tape: This type consists of backing material with adhesive film applied on both sides. A release liner is used to separate the adhesive layers.
  
  

  

  
  

  Double-coated tapes are used to bond substrates with different surface properties which require different bonding requirements. It is highly customizable and the thickness of the adhesive can be varied on the opposite sides. The thermal resistance of a double-coated tape depends on the properties of its carrier.

Chapter Five – Advantages of Adhesive Tapes

Benefits of pressure-sensitive adhesive tapes over glues, adhesives, and other fastening components are the following:

• Curing is not necessary. Adhesives undergo an irreversible phase change (from liquid to solid) in order for them to make strong adhesive and cohesive forces. For pressure-sensitive adhesive tapes, it can readily flow and set on the substrate without heat, or long curing or drying times. The fluid properties of the pressure-sensitive adhesive change rapidly due to its viscoelasticity.
• A stress-free bond is produced. Traditional mechanical fastening components such as screws, bolts, nuts, and staples require piercing and punching the material which creates stress points when subjected to several forces.
• Uniform adhesive thickness. The adhesive layer on pressure-sensitive adhesive tapes is designed to be uniform in thickness, which gives a precise bond on flat surfaces. Manual application of adhesives tends to be wasteful if in excess, or a weak bonding is produced if insufficient.
  
  

  

  
  
• Highly versatile. The functions of adhesive tapes are not limited to joining two or more substrates. Adhesive tapes can also serve as:
  • Protective covering for valued objects from scratch, impact, moisture, chemicals, and dirt
  • Mend holes present in an object
  • Seal openings
  • Labels and decorative purposes, as adhesive tapes are customizable
• A clean bond is produced. Pressure-sensitive adhesive tapes offer a mess-free bonding unlike glues and liquid adhesives. This is advantageous if one wants to preserve the aesthetic value of the object.
• Convenient to use. No special tools are needed when using adhesive tapes during application. Cutting and sticking it on a surface can be accomplished by hand. Adhesive tapes are also easy to store and do not occupy large spaces.
  
  

  

  
  

  Pressure-sensitive adhesive tapes may not be suitable to use under some circumstances. They are not suitable for bonding some joint types and materials which are intended for high-stress applications due to their limited bonding strength. Pressure-sensitive adhesives have weaker adhesive forces produced compared to glues and sealants.

  They also underperform at high and low temperatures. Tackiness is dependent on the temperature of the substrate. A poor bond is created when the temperature of the substrate is low. At high temperatures, the viscosity of the adhesives decreases causing the weakening of the bond. Glues and sealants are preferred when objects are exposed to thermal cycling. Pressure-sensitive adhesives are also sensitive to UV radiation and oxidative agents.

Conclusion

• Adhesive tapes consist of a backing material coated with pressure-sensitive adhesive. Pressure-sensitive adhesives are viscoelastic materials by which flow and bonding are initiated by minimal pressure.
• The components of pressure-sensitive adhesives that explain their bonding mechanism are adhesion, cohesion, and tackiness. Adhesion is the ability of the adhesive to stick on the surface of the substrate. Cohesion is the inner strength of the adhesive which keeps the bond intact. Tackiness refers to the ability of the adhesive to flow after the initial pressure is applied.
• Typical methods used to test tackiness are Rolling Ball Test and Loop Tack Test.
• Components of adhesive tapes are the carrier, adhesive, and release liner. Carrier is the thin film that supports the adhesive, the primary component for adhesive tapes. The release liner protects the adhesive from sticking on the carrier and is peeled-off before application.
• Types of adhesive tapes according to the configuration of the components are single coated tapes, adhesive transfer tapes, and double-coated tapes.
• Adhesive tapes are advantageous over glues and other fastening tools because no curing is necessary, is stress-free and a uniform bond is produced. It is versatile, mess-free, and convenient.
• Pressure-sensitive adhesives may not be suitable for some joint types and materials which are intended for high-stress applications. Bonding is poor at high and low temperatures. Also, they are sensitive to UV radiation and oxidation.