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Adhesives Ingredients
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Adhesives Ingredients
Additives for Pressure Sensitive Adhesives

Polymers and Additives for Pressure Sensitive Adhesives

Pressure sensitive adhesives, when in the dry state, are aggressively tacky at room temperature and possess a high degree of "quick stick" or initial grab. They can adhere to a variety of dissimilar materials without the need for much more than slight pressure. Often the pressure is applied by hand, but rollers, presses, and other mechanical devices can also be used.

There’s always a scope for tailored performance for a particular high-end application. But, how to select the right additive for your requirement?

Learn here, the additives and polymers that are used in pressure sensitive adhesives and how overall performance properties can be adjusted with respect to composition.

Overview

What are Pressure Sensitive Adhesives?

What are Pressure Sensitive Adhesives?

Pressure sensitive adhesives are generally defined as being permanently tacky. Thus, they require no activation by water, solvent, or heat although certain reactive and contact adhesives do show pressure sensitive properties at certain times during their processing.

In addition to being tacky, pressure sensitive adhesive compositions are also inherently soft materials. Thus, a balance of adhesive properties and cohesive strength must be made depending on the specific application. This balance will depend on the viscoelastic nature of the adhesive as well as the additives used in its formulation.

Let's take a look at the additives that are used in pressure sensitive adhesives and how overall performance properties can be adjusted with respect to composition.


Bonding in PSA

Bonding in PSA

The primary mode of bonding for a pressure sensitive adhesive is not chemical or mechanical but rather a polar attraction of the adhesive to the substrate. With these adhesives, the pressure is required to achieve sufficient wet-out onto the substrate surface to provide adequate adhesion. The adhesion or tack must be sufficient to adhere with no more than finger pressure. The cohesion or internal strength of the adhesive film must be high enough to be able to be removed cleanly from the adherend and sufficiently high to resist creep and shear stresses in the end-use application.

Pressure sensitive adhesives are usually based on elastomeric or thermoplastic resins in a solvent or waterborne system. They are coated on the substrate or on a film that is used as a carrier. Once the solvent evaporates, the adhesive is ready to be applied, or it is packaged with a carrier so that it can be dispensed at the point of assembly. The more obvious examples of pressure sensitive adhesives are:

  • Tape (duct tape, office tape, etc.)
  • Laminating resins
  • Medical bandages, and
  • Double backed tape for mounting bumper strips
  • Wall hangers, etc.

Certain pressure sensitive adhesives can also be applied as a hot melt or as a radiation curable product either by ultraviolet light or electron beam.


PSA Advantages and Disadvantages

PSA Advantages and Disadvantages

Some of the inherent advantages of using pressure sensitive adhesives compared with other adhesive types include:

  • The ability to employ lighter, thinner materials
  • The ability to bond dissimilar materials with few concerns about incompatibility
  • Shortened assembly times; improvements in productivity; no mixing or activation required
  • Elimination of visible mechanical fasteners
  • The ability to adhere to difficult surfaces such as human skin
  • Uniform thickness throughout the product
  • The ability to transfer assembly from the factory to the field
  • The bonded substrates can generally be separated without damage
  • Minimal issues regarding health, safety, and disposal
Advantages of PSA


The major disadvantages are that the adhesive strength (shear and peel) is low and generally sensitive to high temperatures and solvent. Thus, most pressure sensitive adhesives are not well suited for high strength or structural applications. They are often used with relatively weak substrates such as paper or film. Pressure sensitive adhesives are also not well suited for rough surfaces, and they are relatively expensive in terms of cost per bond area.


Adhesive Material Characteristics

Adhesive Material Characteristics

The adhesive and cohesive characteristics of the pressure sensitive adhesive as well as its processing characteristics can be correlated with three basic properties of the base polymer used in the formulation:

  • Molecular weight
  • Glass transition temperature (Tg)
  • Polarity

Molecular weight and molecular weight distribution will influence polymer viscosity which in turn influences how readily the polymer can be applied and how well it wets a substrate to form a bond. When the viscosity is lower, the mobility of the polymer molecules is greater and wetting is improved. However, as the molecular weight is decreased, so is the cohesive strength of the adhesive?

The figure below illustrates the effect of molecular weight on peel strength. To improve the cohesive strength of pressure sensitive adhesives without adversely affecting peel, light crosslinking is often advantageous employed.

Effect of Molecular Weight on Adhesive Peel Strength
The Effect of Molecular Weight on Adhesive Peel Strength


In designing a pressure sensitive adhesive, the glass transition temperature (Tg) should ideally be near the use temperature since the energy dissipation is maximized at the Tg. Such internal energy dissipation provides stronger bonds and higher peel strength. Thus, polymers with relatively low Tg (near room temperature or lower) are commonly used in pressure sensitive adhesive formulations.

Often a polymer with a Tg below room temperature is employed so that the adhesive will spread and wet a surface when applied and given slight pressure at room temperature. However, there must be a balance between polymer mobility and cohesive properties. Achieving this balance is a key goal in all pressure sensitive adhesive formulations.

The surface energy of the intended substrate will often dictate the type of base polymer to be used. Polar polymers have a tendency to adhere to high surface energy substrates (e.g., metals, glasses, etc.) by virtue of their dipole-dipole interaction and/or hydrogen bonding. Polymers with lower polarity tend to adhere better to low surface energy substrates such as most plastics.


Basic Polymers Used in Pressure Sensitive Adhesives

Basic Polymers Used in Pressure Sensitive Adhesives

Pressure sensitive adhesives normally are composed of elastomeric or thermoplastic base polymers, resinous tackifiers, and a series of additives such as plasticizers, fillers, and antioxidants that are designed to improve performance and processing properties.

Table below provides examples of components commonly found in pressure sensitive adhesives.

Polymers Tackifiers Plasticizers Fillers Antioxidants
  • Natural rubber
  • Reclaimed rubber
  • Styrene butadiene rubber
  • Block copolymers
  • Polyvinyl ethers
  • Polyisobutylene
  • Polybutadiene
  • Polyacrylate esters
  • Polyterpene resins
  • Rosin esters
  • Petroleum hydrocarbons
  • Mineral oil
  • Lanolin
  • Lecithin
  • Zinc oxide
  • Titanium dioxide
  • Clay
  • Silica
  • Various Pigments
  • Common rubber antioxidants
  • Metal chelating agents
Common Components in Pressure Sensitive Adhesive Formulations


Elastomer-based Pressure Sensitive Adhesives


These typically have synthetic rubber as the base polymer although natural rubber is also used in a significant number of products. Natural rubber is expensive, and it becomes brittle during aging because of a tendency of the unsaturation in the polymer backbone to crosslink. Styrene block copolymers or styrene butadiene rubbers are commonly used as the synthetic base resin in these adhesive formulations.

Elastomeric adhesives are generally non-polar and adhere well to non-polar substrates such as:
  • Polyolefins
  • EPDM rubber, and
  • Other low energy surfaces
Elastomer based pressure sensitive adhesives are very thermoplastic with low Tg. The addition of heat will soften the adhesive and directly affect its properties. Generally, elastomeric adhesives require the addition of a tackifier or a blend of tackifiers to achieve aggressive grab.


Acrylic Pressure Sensitive Adhesives


These are made with several types of acrylic monomers that are polymerized to high molecular weight polymers. The adhesive can be made from one monomer or a mixture of several types. The two primary acrylates used in pressure sensitive adhesives are 2-ethylhexyl acrylate and iso-octyl acrylate.

Acrylic adhesives are polar in nature and tend to stick well to polar substrates such as:
  • Metals
  • Glass, and
  • High-energy polymeric surfaces
Acrylic adhesives, like the synthetic rubber based adhesives, are thermoplastic by nature and soften when exposed to heat. They are formulated with polymers of relatively low molecular weight and Tg so that they are inherently soft at ambient temperatures.

Acrylic adhesives do not necessarily require tackifiers for pressure sensitive adhesive properties. The adhesive properties, however, can be varied and fine-tuned by compounding with numerous ingredients as in the case of elastomeric pressure sensitive adhesives. Modified acrylic adhesives commonly incorporate tackifiers and other resinous components to improve adhesion. These modified acrylics offer improved initial tack and adhesion to low surface energy materials compared with standard acrylic formulation.


Silicone Pressure Sensitive Adhesives


These are compounded from silicone polymers. They are expensive compared to the other types of pressure sensitive adhesives. Silicone pressure sensitive adhesives have relatively low tack and adhesion, but they have outstanding resistance to heat and many chemicals. They are capable of end-use applications up to 700F and are used in applications such an adhering gasketing to oven doorframes. Silicone elastomers must be tackified as well as crosslinked to get optimal pressure sensitive properties and cohesive strength. Silicone adhesives are also hypoallergenic and are used in many medical applications.

Polyisobutylene, polyvinyl ethers, and polybutadiene are also used in pressure sensitive adhesive formulations. However, they serve a smaller portion of the overall market than those polymers identified above.

PSA Formulation Strategies


The advantages and disadvantages of the more common types of pressure sensitive adhesives are shown in table below.

Base Polymer Advantages Disadvantages
Elastomer (natural and synthetic)
  • Adhere to a wide range of materials
  • Economical
  • High degree of tack or "grab"
  • Good adherence to both low and high energy surfaces
  • Excellent adhesion build-up
  • Poor resistance to elevated temperatures
  • Fair to poor resistance to chemicals
  • Generally poor resistance to UV and oxidation
Acrylic
  • Crosslinkable
  • Good resistance to varying temperature (-45 to +121°C)
  • Good resistance to chemicals, UV, and oxidation
  • Color stable
  • Good shear strength
  • Good hydrolysis resistance
  • Good hydrolysis resistance
  • Generally poor adhesion to low surface energy polymers (e.g., polyolefins)
  • Moderate cost
  • Initial tack is low
  • Poor creep resistance compared to elastomer based adhesives
Silicone
  • Excellent chemical and solvent resistance
  • Wide temperatures use range (-73 to +260C)
  • Good oxidation resistance
  • Good adhesion to high and low energy surfaces
  • Highest cost
  • Lack of aggressive tack
  • Lack of cohesive strength without crosslinking
Polyvinyl Ether
  • Performance almost equivalent to the acrylics
  • Feeling of "dry tack"
  • Not as aggressive a tack as with the acrylics
Polyisobutylene
  • Resistant to oil
  • Low moisture permeability
  • Good weatherability
  • Difficult to crosslink
  • Poor shear characteristics
  • Initial tack is low without tackifiers
Polybutadiene
  • Extremely crosslinkable
  • Excellent electrical properties
  • Low Tg
  • Bonds well to low energy surfaces
  • Poor oxidation and UV resistance
Advantages and Disadvantages of Various Pressure Sensitive Adhesives


Additives Used in Pressure Sensitive Adhesives

Additives Used in Pressure Sensitive Adhesives

Additives in pressure sensitive adhesive formulations include tackifiers, antioxidants, flame retardants, pigments, and fillers. These compounds are added to achieve very specific end-use or processing properties.

Generally, tackifiers or blends of tackifiers are added to the base polymer in pressure sensitive formulations. There are hundreds of tackifiers available commercially, each providing different properties. Rosin ester tackifier resins were used in early pressure sensitive adhesive formulations. They have been largely replaced by better aging terpene resins, which provide excellent performance characteristics. The terpene resins are now seeing competition by less expensive tackifying resins obtained by the cationic polymerization of petroleum fractions (C4, C5, and C6).

Commercial Resin Supplier Type Glass Transition Temperature, °C Softening Point, °C
Regalrez™ 1094 Eastman Hydrogenated aromatic 33 85.5
Foral™ 85-E Eastman Glycerine rosin ester 86
Foral™ 105-E Eastman Hydrogenated pentaerythritol 108
Escorez™ 2101 ExxonMobil
Mixed aliphatic / aromatic 44 185
Escorez™ 1310 ExxonMobil C-5 49 85.5
 Listing of Selected Commercially Available Tackifying Resins and Their Properties1


Tackifiers


Tackifiers generally raise the glass transition temperature of elastomers so that the adhesive mixture has a higher modulus at high strain rates and ambient temperature. Elastomers that increase in modulus greatly at some particular elongation make high tack adhesives. A high degree of tack is associated with adhesives that have high elongation-to-break in simple tensile strength tests. Tacky adhesives form long "legs" during peel testing (see Figure below).

This distributes the stress over a large area and increases the energy absorption capability of the adhesive joint.

Stress Distribution by Flexible Adhesive
Tough, Flexible Adhesives Distribute Peel Stress Over a Larger Area than Brittle, Low Modulus Adhesives


Tack can sometimes be significantly affected by small amounts of other additives that modify the surface (interface) of the adhesive. Stearic acid, lanolin, or paraffin wax added to an adhesive formulation can greatly reduce its tack. These additives do not greatly change the other rheological properties of the adhesive.


Antioxidants


Many of the base polymers used in pressure sensitive adhesive formulations contain unsaturated double bonds along the chain and, therefore, are subject to oxidative attack. This is usually aggravated by exposure to elevated temperature or ultraviolet light. These degradative factors can occur during processing or end-use. Consequently, antioxidants are used to stabilize the adhesive against oxidation and heat and light degradation.

Antioxidants are especially important in natural rubber based adhesives. Often blends of antioxidants are used to achieve optimum protection. Most of the antioxidants and stabilizers found effective in standard rubber formulations are also used in elastomeric pressure sensitive adhesive formulations.


Plasticizers


Plasticizers are mainly used in rubber based adhesives to reduce cost. The effect of plasticizer addition on tack is quite variable. However, they generally reduce peel and cohesive strength. This can be useful in certain applications such as reducing peel adhesion to unwind pressure sensitive tapes.

Plasticizers are also often used to improve processing properties of the adhesive such as reducing melt temperature and solution viscosity. Plasticizers generally used in pressure sensitive adhesives include mineral oil, lanolin, and lecithin. The properties of a plasticized formulation are highly dependent on the plasticizer composition, its solubility parameter, and its molecular weight.


Pigments


Pigments or fillers are added to pressure sensitive adhesive formulations for coloring and to improve performance - notably cohesive strength and tack retention. Titanium dioxide, zinc oxide, and silica have been used for these purposes. Pigments have also been used to reduce the material cost of the adhesive.

Non-reinforcing fillers such as clay, talc, whiting, etc., are generally used for this purpose. Carbon black is used as a pigment and a screening UV stabilizer. The amount of filler used in pressure sensitive adhesive formulations is limited because fillers tend to increase stiffness and reduce aggressive tack.


Reinforcing Fillers or Crosslinking


In many applications, such as heavy-duty strapping tape, creep resistance is very important. The creep resistance of pressure sensitive adhesive can be improved by using reinforcing fillers or crosslinking. Crosslinking can be either chemical or by exposure to radiant energy such as electron beams. Natural rubber adhesives, for example, can be crosslinked with poly or diisocyanates. The addition of titanate esters, di- or polyamines, and hydrazides have also been shown to increase cohesive strength of natural rubber pressure sensitive adhesives.

Pressure sensitive adhesives for high-temperature applications (e.g., electrical insulation, masking tape used in a paint-baking oven, etc.) sometimes contain a minor amount of heat curing, oil-soluble phenolic resin. An alkaline filler or zinc resinate is added as a catalyst to promote the reaction between the phenolic resin and the base elastomer. Sometimes an oil-soluble, heat curing phenol-formaldehyde resin is also applied and cured to the backing as a primer.

PSA Testing: Standard Tests When There Is No Standard


Conclusion

Conclusion

Optimization of pressure sensitive adhesive formulations depends on the proper balancing of adhesive and cohesive properties. Adhesive properties, such as tack and peel strength, provide the pressure sensitive adhesive with many of its processing advantages. However, high cohesive properties such as shear strength and creep resistance are required in certain applications. The proper molecular design of the base polymer, as well as appropriate selection of additives, achieves this balance of seemingly exclusive properties.


References:

  1. Pocius, A. V., "The Chemistry and Physical Properties of Elastomer-Based Adhesives", Adhesion and Adhesives Technology, Hanser Publisher, New York, 1997.
  2. Varanese, D. V., The Fundamentals of Selecting Pressure Sensitive Adhesives, Medical Plastics and Biomaterials, January 1998.

Key Applications

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1 Comments on "Polymers and Additives for Pressure Sensitive Adhesives"
David W May 15, 2019
Useful review. Is there a PDF link for reading in-hand off-line? Thanks

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