Pressure Sensitive Adhesives (PSA): Know How for Beginners As Well As Professional Formulators
What is pressure sensitive adhesive? Anything that bonds without pressure would not be considered PSA. A pressure sensitive adhesive is a material that will hold two surfaces together solely by surface contact. This bond is accelerated by the application of external pressure to the adhesive. PSA’s are able to bond a wide variety of materials such as papers, plastics, metals, wood and glass. A PSA is a dry solid that does not require activation by water, solvent, or heat. Pressure sensitive adhesives are permanently tacky at room temperature. Pressure sensitive adhesives can be used as replacement to screws, rivets, nuts and bolts, clips, and any other form of attachment. Ease of application makes PSA’s worthy of consideration in many manufacturing processes and makes pressure sensitive media an excellent choice for point of purchase applications and general purpose graphics.
Pressure-sensitive adhesives are intended for applications that are either permanent or removable. Protection labels for power equipment foil tape for HVAC ductwork, vehicle interior trim assembly, and sound/vibration damping films are examples of permanent applications. Some permanent high-performance PSAs have high adhesion values and can accommodate kilograms of weight per square centimeter of contact.
Many chemical materials, (polymers), have been considered for use as a pressure sensitive adhesive over the years. Some of the more common pressure sensitive polymers are:
ACRYLIC: Acrylic monomers are polymerized in a reactor under heat, pressure, specific time and catalysts to produce an acrylic pressure sensitive polymer. The advantages acrylic PSA’s offer are: excellent clarity, good color stability, better resistance to oxidization, higher resistance to heat and ultra violet radiation. Acrylics bond well to polar surfaces such as metals, glass, polyester and polycarbonate plastic films.
RUBBER: A rubber based polymer does not have pressure sensitive properties at room temperature. It has to be formulated with tackifying resins, oils, plasticizers, antioxidants and possibly pigments to produce a functional pressure sensitive adhesive. Rubber based PSA’s provide high adhesion to a variety of substrates and face stocks including low energy plastics such as polyethylene and polypropylene. Due to some factors such as poor chemical resistance, degradation at temperatures over 150 F, and poor UV resistance limit the useful life of a rubber based PSA’s for exterior applications.
SILICONE: Silicone PSA’s offer both high temperature resistance (up to 500ºF) along with the ability to bond at extremely low temperatures (-100ºF). Silicone PSA’s have excellent chemical resistance along with the ability to bond to low energy substrates. Silicone polymers are also the most expensive PSA resins by far limiting their applicability to only the highest performance tapes and media where rubber based and acrylic resins cannot meet the performance requirements.
MANUFACTURE & TYPES OF PRESSURE SENSITIVE ADHESIVES: There are four popular chemistries resulting in a pressure-sensitive adhesive.
SOLVENT: Solvent borne PSA’s can be considered high performance adhesives.
EMULSION OR WATER BASED: Emulsion PSA are very competitive for removable, short term and general purpose applications.
HOT MELT: Hot melt adhesives are generally used for interior applications where they are not exposed to high temperatures or UV radiation.
ENERGY CURABLE: Most UV curable PSA’s are 100% solids and do not require emission control equipment
Pressure-sensitive adhesives are made in 100 percent solid form or with a liquid carrier. By coating the adhesive on support and evaporating the organic solvent or water carrier, usually in a hot air dryer, items such as tapes and labels are made from liquid PSAs. To initiate a cross- linking reaction and increase molecular weight, the dry adhesive can be further heated. Low viscosity polymers that are coated and then reacted with radiation to increase molecular weight and form the adhesive (radiation cured PSA) maybe 100 percent solid PSAs; or they may be high viscosity materials that are heated sufficiently to decrease viscosity to allow coating and then cooled to their final shape (hot melt PSA, HMPSA). A major group of adhesive tapes, Pressure Sensitive Adhesive Tape (also known as PSA, self- adhesive, self-adhesive) is a relatively thin flexible film with a single or double-sided coating.
When applied to most clean and dry surfaces with pressure, PSA can bind to a variety of substrates. Pressure Sensitive adhesives do not require the adhesive to be activated by solvent, water, or heat. The bond is directly affected by the amount of pressure used for the surface application of the adhesive.
APPLICATION: There are various ways to apply a pressure sensitive adhesive to the substrate. Some techniques are common with a certain adhesive type but generally any method of adhesive application can be considered for any type of pressure sensitive adhesive.
GRAVURE: Gravure coating is ideal for lower viscosity adhesives making it a common application method for acrylic emulsion PSA’s. It involves etching a pattern into a smooth roll that contains the adhesive. As the roll rotates, the coated substrate comes in contact with the gravure roll and the coating is transferred from the gravure cylinder to the substrate.
COMMA COATER: It produces very smooth coatings of adhesive that can compensate for irregularities in the coated substrate. It is ideal for high viscosity adhesives and should be considered when applying solvent acrylic and rubber based PSA’s.
REVERSE ROLL: This is a very versatile adhesive application procedure and is widely used for solvent and to some extent water based adhesives.
KNIFE OVER ROLL: This technique is similar to reverse roll with the metering roll being replaced by a rigid smoothly ground blade. The coated substrate is tight to the bottom roll with a gap between the substrate and blade to meter the thickness of the adhesive. It is ideal for higher viscosity coatings such as solvent borne acrylics and rubber based PSA’s.
PSA TAPES: SOME COMMON ADVANTAGES
- Bonds dissimilar materials without incompatibility problems
- Thinner and lighter materials
- Provides vibration dampening and reduction in noise4
- Reduces time for assembly
- Eliminates the need to refinish surfaces
- Eliminates visible mechanical fasteners for dominance in cosmetics
- Provides uniform properties of thickness and void filling
PSA GLUE DOT: Glue dots are pressure-sensitive adhesive dots, used without the need for a hot glue gun in various applications, such as sticking credit cards to paper, arts and crafts, and as a stable adhesive for children to use. Glue dots are adhesive globules, allowing attachments to float above a page. They have a clean and immediate bond and are also a more fitting solution than glues or sticky tapes that are hot or liquid. Glue dots leave far less mess, residue, and smell that help to better display a product, while increasing efficiency and reducing costs. Removable glue dots also allow you to build a bond that, if necessary, can be removed. Glue dots quickly build bonds, leave no mess, cause less waste and prevent painful burns that some people can experience while using a hot glue gun.
PSA ADHESIVE LABEL: An adhesive label or sticky label is a small piece of paper designed to be affixed to any surface such as paper, plastic, wood, glass, or metal, typically by the action of a layer of adhesive on the front or back of the label. The term adhesive refers to a sticky substance, while something that is self-adhesive implies that it will stick without wetting or the application of glue to the product.
MAIN REASONS FOR SUCCESS OF PSA: Versatility, durability, cost-effective, wide range of functionalities, comes in many shapes, sizes and colors and in fact it has in some way revolutionised the concept of adhesive and adhesion.
To get an upper hand in PSA formulations, you can access this highly recommended training: Pressure Sensitive Adhesives (PSA) Formulation Optimization & Key Ingredients Selection For Specialized Applications