Adhesion

Adhesion is the interaction that develops between two dissimilar bodies when they are in contact. At the molecular level, adhesion is based on physical and in some cases chemical bonding. The strength of adhesion depends on the type of adhesive.

Pressure sensitive adhesives build up adhesion under light pressure. The ultimate bonding strength is reached after 24 ~ 72 hours.

Adhesive failure

Adhesive failure describes the separation of adhesive either from the carrier or from the substrate. The other basic failure mechanism of an adhesive bond is “cohesive failure” which refers to a fracture in the middle of the bulk adhesive.

Adhesive

Adhesives are polymer materials that are used to join dissimilar materials. Adhesives may be classified in many ways, eg. by mode of application and setting, chemical composition, cost and suitability for various adherents and end products.

The term “pressure sensitive adhesive” (PSA) is used to describe adhesives that are permanently tacky in dry form at room temperature.

The most common pressure sensitive adhesives are acrylics, natural rubber/resin and synthetic rubber/thermoplastic rubber.

Ageing resistance

Degree of reliable performance of the tape over time, under certain conditions.

Depending on the adhesive system being used, PSA tapes are often usable for permanent applications. This permanence is reflected by the resistance of the adhesive against:

• Ozone (O3), Oxygen (O2)
• UV light : relevant for transparent substrates such as glass or PC ; under direct exposure, yellowing or discoloration of the adhesive or backing (e.g. window bars).
• Temperature
• Humidity, water
• Different kinds of chemicals…

Generally acrylic adhesives are much better suited to withstand these environmental influences than rubber adhesives and can maintain their permanent, reliable functionality over many years.

Bobbin

The term where the adhesive tape is wound onto a spool in narrow widths as a continuous length. Dependant on the material a bobbin may have over 10,000 metres of tape to it.

Carrier

Materials which “carry” the adhesive. The carrier also reinforces the
PSA tape and improves handling and processing properties.
Most commonly used carrier types are:

1. Film backings (e.g. PET, PP, PVC, PE)
2.Paper based backings (e.g. non-woven, tissue)
3.Foam backings (e.g. PU-, PE- PVC-foams)

A specialty PSA tape type is a transfer PSA tape which has no carrier.
The adhesive is directly coated on the liner.

Closed Side

The surface of the adhesive which remains in contact with the release liner.

Cohesion

Cohesion describes the inner strength of the adhesive. It mainly determines the
holding power (shear resistance) of the tape.

Cohesive failure

Cohesive failure leaves adhesive residue on both the PSA tape backing and the
laminated surface, showing that the adhesive broke internally.

Conformability

The carrier of a PSA tape influences its ability to adhere to curved, rough or irregular surfaces. Conformable carriers increase the contact area of adhesive and substrate. Foam carriers are inherently conformable and can therefore compensate for surface irregularities between the two bonding substrates.

Corona treatment

Method to modify the surface of a substrate to provide better anchorage of the adhesive. Corona treatment is an atmospheric plasma treatment. It is applied to non polar/low energy surfaces to facilitate the anchorage of the adhesive.

Delamination

The liner separates from the tape.

Dimensional stability

Correlates with humidity of liners. Dimensional stability prevents the liner from
showing an irregular surface or dimensional change due to absorption of moisture.
Dimensionally stable liners are mainly

• PE coated paper liners
• Film liners

Double Sided Tape

Comprised of a carrier material coated with adhesive on both sides. Usually one adhesive layer is covered with a release liner (closed side) in order to wind the PSA tape in roll form. In d/s tape production the carrier is often pre-treated with a primer to enable a maximum anchorage between the carrier and adhesive.

Edge picking

Negative effect on the unwinding behaviour of a roll of tape caused by oozing of soft adhesives.

The adhesive flows out at the roll edges so that neighbouring layers of tape stick together. This problem occurs usually with transfer tapes and/or thicker layers of soft adhesives.

Elongation at break

Ratio of length of max. stretched tape at break to original length. Expressed in percentage of original length.

Fish eyes

Optical effect caused by entrapment of air between different tape layers on a roll.

Hand tearability

Property of tapes which allows manual cutting or tearing without the use of additional equipment such as a knife, scissors or a dispenser. Both the carrier and the release liner must be tearable.

Hard Adhesive

Term usually used to characterize highly cohesive PSA tapes.

Advantages (compared to soft adhesives):

• higher holding power
• withstands higher sustained loads
• good temperature resistance
• less edge picking
• improved die cuttability

Disadvantages:
• low initial tack and adhesion; requires higher contact pressure
• not suited for rough surfaces

Holding power

See shear resistance.

Hot melt pressure-sensitive adhesive

Pressure sensitive is a term commonly used to designate a distinct category of adhesive tapes and adhesives which in dry form (solvent / water free) are permanently tacky at room temperature

Humidity resistance

Moisture or even humidity can affect the performance of an adhesive especially if the tape is applied under wet or very humid conditions. The adhesive absorbs the humidity which then leads to reduced adhesion performance. This effect occurs especially with water-based acrylics tapes meaning they should not be used under these conditions

Liner

Anti-adhesive material which covers the adhesive on a d/s tape and prevents the adhesive from sticking to itself. The liner is used as a protection aid during handling/processing and storage.
Most commonly used liner types are

1. Paper liners (e.g. Glassine paper, PE-coated paper, Clay-coated paper)
2. Film liners (e.g. PP, PET, PE)

Silicone is used as release system to avoid adhesion between liner material and adhesive.

Natural Rubber

Component of adhesives, not inherently self-adhesive. Resins, so called “tackifiers” need to be added to achieve self adhesive properties.

Non-polar substrates

Critical surfaces to adhere to due to low surface energy. The lower the surface energy the lower the molecular attraction to the adhesive (adhesion). Typical materials are polyolefins such as PP and PE, but also PS, EVA and many powder painted surfaces. By surface pretreatment (e.g. corona treatment) the polarity can be modified to achieve higher surface energy and improved adhesion. Use of primers will also act as adhesion promoters.

Non-woven

Paper and polymer fibre based carrier material for PSA tapes.

Oozing

A ”squeezing out“ of the adhesive at the edge of the tape, caused by “cold flow” of
a soft adhesive.

Peel adhesion

The force required to remove a pressure sensitive adhesive-coated flexible material from a surface.

PE (Polyethylene)

Non-polar substrate; critical material to adhere to, which requires high performance PSA tapes. Corona pre-treatment can increase the surface polarity and therefore the bond to the adhesive. As foamed material PE is also used as a carrier for PSA foam tapes.

PET (Polyester)

Polar substrate which is easy to adhere to. Often used as a carrier and liner material of PSA tapes due to excellent mechanical characteristics and high resistance to moisture, solvents, oil and various chemicals.

Plasticizer resistance

The ability of a PSA tape to maintain its properties under influence of plasticizers. In contact with plasticizers (e. g. a component of foam materials or Soft-PVC) the adhesive performance of PSA tapes can be affected, especially with rubber based adhesives. PSA tapes with a film carrier sometimes perform better since the film layer acts as a barrier against migration of plasticizers. Important for applications on EPDM or soft-PVC profiles for instance.

PP (Polypropylene)

Non-polar substrate (similar to Polyethylene), critical material to adhere to, which requires high performance PSA tapes. Corona treatment can increase the surface polarity and therefore the bond with the adhesive. As foamed material PE is also used as backing for PSA foam tapes.

Primer

A coating applied to a surface (e. g. backing or a substrate) prior to the application of an adhesive, improving bond performance between backing and adhesive or between tape and substrate.

PVC

Polar substrate which is easy to adhere to. Often used as a carrier and available in varying degrees of hardness.

Release force

Force needed to remove the liner from the covered adhesive layer of the d/s PSA tape. The release force can be adjusted by release agents, such as silicones.

Repositioning

Short term repositioning can be important to correct the first mounting of a substrate. It depends on the initial adhesion level, the cohesion of the adhesive and the carrier type. This is important e.g for mounting of displays or front panels.

Residue free removability

Relevant when adhesive materials have to be removed at the conclusion of the application. An example of this are PAL’s glass transportation pads (RAS78161)

Shear resistance

Shear resistance is measured as a force required to pull the pressure sensitive material parallel to the surface to which it was affixed under specific conditions. The shear resistance of PSA´s may be measured statically or dynamically. Static shear test methods use a constant load of longer test times. Dynamic shear tests measure the cohesion of the sample in a tensile tester under increasing load (force).

Soft adhesive

Contrary to “hard adhesives”, “soft adhesives” flow easier into rough and textured surfaces and substrates.

Advantages (compared to hard adhesives):
• good wetting – adhesive flows into rough,
uneven substrates
• high initial tack (immediate bond)

Disadvantages:
• lower temperature resistance
• lower shear resistance (holding power)

The example of honey shows very clearly how a very soft mass behaves. Honey has “supertack” – it sticks to the finger immediately, but its internal strength is so low that a bond made with honey withstands only little load.

Substrate

Material that the PSA tape is applied to.

Tack

Tack describes the contact behaviour of the adhesive to the substrate under a minimum of time and pressure. Also referred to as “wet-grab” and “quick-stick”.

Temperature resistance

Ability of a d /s tape to remain unaffected (adhesion, cohesion) when exposed to higher temperatures. We distinguish between “short term” temperature resistance (seconds to minutes) and “long term” temperature resistance (hours to years)

Test method: “temperature resistance”

Tensile strength

Tensile strength is the force required to break the material.

Transfer tape

PSA tape without carrier reinforcement. The adhesive is directly coated on to the release liner.

Unwinding force

Force required to unwind a d/s PSA tape. The unwinding force is a result of the interaction between adhesive and release liner. A low and constant unwinding force is an important property for the processing of an adhesive tape.

Weather resistance

The ability of a material (tape or adhesive) after application to resist exposure to such conditions as light (daylight, UV-light) and humidity. Generally acrylic adhesive tapes display good weathering resistance.