Question

What is the Adhesive (Pressure Sensitive) - What is a pressure sensitive adhesive??

Accepted Answer

A type of adhesive that creates an adhesive bond through the application of light pressure only  while other adhesives  sometimes collectively known as    structural adhesives     form an adhesive bond once they have hardened  via a chemical or physical process   pressure sensitive adhesives or PSAs are tacky  sticky  in their dry form at room temperature and form a bond through a combination of flow and resistance to flow         PSAs create bonds through interfacial forces between two surfaces  e g  the adhesive and the adherend or substrate   which means that they need to be soft enough to be able to flow  creating the maximum area of contact between the two surfaces  and hard enough to resist flow when stress is applied to the bond  The three key properties of all PSAs  therefore  are the initial tack  ultimate adhesion  and shear resistance  Initial tack describes the strength of the bond that is created immediately upon contact between an adhesive and an adherend  while ultimate adhesion describes the maximum strength of the bond once the adhesive has fully set  Shear is a type of stress  whereby the external force acting on an object or surface is parallel to the surface  while resistance to shear stress involves both the adhesive and cohesive strength of an adhesive  shear resistance is most closely associated with cohesive strength  shear resistance is also known as cohesive strength  as well as shear strength  shear adhesion  and holding power   PSAs with low shear resistance  soft  have more tendency to flow  creating higher initial tack  and are more likely to split apart under stress  while adhesives with high shear resistance  firm  have less tendency to flow  which may lower their initial tack  and are less likely to split under stress because they have better cohesion         Generally speaking  PSAs are made up of an elastomer  the base  and a tackifier  to improve the initial tack of the adhesive   although additives may be used to introduce or enhance key properties that are required for a specific label application  Elastomers are polymers that have weak inter-molecular forces and have the property of viscoelasticity  which means that they have both viscosity  the ability to resist flow  and elasticity  the ability to return to their original size and shape after being deformed   Common examples of elastomers used to make PSAs include acrylics  rubbers  and silicone rubbers         PSAs are made as emulsions  solutions  hot melts  ultraviolets  or 100  solids  In emulsions and solutions  the adhesive components are suspended in a liquid  emulsion  or dissolved in a solvent  solution  so that they can be coated in liquid form before the liquid or solvent evaporates to leave behind a layer of pressure sensitive adhesive  In hot melts and ultraviolets  the adhesive is also coated in a liquid form  molten form  before hardening into their final form when they cool  hot melts  or are cured with ultraviolet light  ultraviolets   100  solids are sometimes known as    warm melts    and may react with radiation or may be heated and then cooled to produce their final form  Emulsion and solvent based adhesives are the most common types of PSAs  while solvent adhesives offer greater strength and higher moisture resistance  emulsion adhesives are the most popular type of PSA used to create adhesives for labels  packaging  and tapes         Most PSAs are manufactured from rubbers  rubber-based adhesives  or acrylics  acrylic-based adhesives   although some adhesives may be made with a different base material  such as silicone adhesives  which offer high resistance to moisture  chemicals  and other environmental conditions  and are often used for highly specialised applications such as those that involve skin contact  including medical applications  e g  wound dressings  EKG electrodes  athletic tape  analgesic and transdermal drug patches  or drama production applications  e g  make up and prosthetics    Rubber-based adhesives can be made using natural rubber or synthetic rubber and tend to be softer adhesives  which produce good initial tack  although they are more prone to adhesive bleed and can be affected by oxidation and UV light  Acrylic adhesives are made from acrylic polymers and are more expensive than rubber-based adhesives  although they offer greater resistance to heat  oxidation  UV light  and solvents         While adhesives can be grouped according to manufacturing method or material base  they are most commonly classified as either    permanent adhesives    or    removable adhesives     permanent adhesives are designed to create a strong adhesive bond that will result in the label and or the substrate being damaged when a label is removed  while removable adhesives are designed to create a good adhesive bond that will hold the label in place but that will also allow the label to be removed cleanly and easily without damaging the label and or the substrate when the label is no longer needed  labels with a removable adhesive are therefore also known as temporary labels   Some removable adhesives may also be repositionable adhesives  which allow a label to be removed  without damaging the label and or substrate  but also retain enough tack to allow the label to be reapplied onto the same surface or a different surface  PSAs may be further classified according to the intended label application or key properties of the adhesive  common examples include marine adhesives  VOID adhesives  deep freeze adhesives  glass removable adhesives  and opaque adhesives         The efficiency of PSAs can be influenced during and after the application of a label by a number of factors  including the strength of the adhesive itself  both adhesive and cohesive   the length of time a label has been stuck onto a surface  the surface type  and environmental conditions  Obviously  an adhesive must have enough adhesive and cohesive strength to allow an efficient bond to be formed and sustained  while the passage of time is vital to allow a PSA to develop into its ultimate adhesion  The surface can influence the efficacy of an adhesive bond according to its smoothness  if the surface is completely smooth  no troughs  or very rough  too many troughs OR very deep troughs  the surface area that the adhesive can reach will be too small for a successful bond     the best surface type has some shallow troughs   its surface energy  high energy surfaces have greater interfacial interactions with the adhesive  creating a stronger adhesive bond   and the presence of any contaminants  particles of dust  oils  or moisture on the surface reduce the amount of surface area available and the adhesive may actually bond with these particles instead of the surface  resulting in a weaker bond   If the surface changes over time  this can also result in the failure of an adhesive bond         The main environmental factor that can influence a PSA is temperature  exposure to extreme temperatures or fluctuating temperatures can result in the failure of an adhesive bond  At higher temperatures  PSAs will become more fluid and mobile  which can result in adhesive bleed or reduced adhesion  If the temperature then suddenly drops  the adhesive may experience additional stress because it has stretched more than it could initially and so loses some of its contact area as it cools  which reduces its adhesion  Likewise  if a PSA is exposed to lower temperatures  the adhesive may become harder and stiffer  which reduces its elasticity and makes it harder for the adhesive to maintain surface contact  resulting in the adhesive becoming less adhesive and brittle like glass  Other environmental factors that can cause issues are humidity  UV light  and exposure to other substances  such as chemicals         Pressure sensitive adhesives are used for a wide variety of applications because of their ability to create a bond between a wide range of different materials  e g  paper  wood  metals  plastics  and ceramics  and surface types  Common applications include pressure sensitive labels  packaging  tapes  glue dots  note pads  post-it notes  automobile trim  protective films  sound vibration damping films  masking tapes  price marking labels  promotional materials  and skin contact applications         At Label Planet  all of our labels are made with   pressure sensitive adhesives    we have a range of permanent adhesives and removable adhesives  including a number of    special adhesives    that have been designed for a specific type of label application

Cohesive Failure - What is cohesive failure?

Definition of COHESIVE FAILURE: