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Selecting the Right Release Liner: A Step-By-Step Approach

1.  Define the adhesive chemistry being used.

  • Is it an acrylic adhesive or a rubber-based adhesive? Some UV silicones do not work well with rubber-based adhesives.
  • Is it an aggressive acrylic adhesive? When combined with an electron-beam-cured silicone, acrylic “lock-up” can result.
  • Is it a permanent or removable adhesive? Removable adhesives, normally low-tack, generally require a tighter-release silicone.
  • Does the adhesive chemistry involve any additives? Some additives can “hop-up” the curing of the adhesive formula, but have detrimental effects on the release of the liner.

The liner manufacturer has a head start when samples of the adhesives are provided for testing. At the very least, the name of a comparable adhesive can help in the initial stages.

2.  Discuss conversion processing conditions for the product.

  1. What is the temperature range and how long is the product processed? High process temperatures can affect the substrate, since some films and polycoated substrates have lower melt points than paper.
  2. Under what tension will the product be processed? A 2-mil (50 micron) low density film cannot take the same tension as a board.
  3. Does moisture play a role? Again, substrate selection can depend on the moisture involved. If an emulsion coating is put directly on paper, there may be too much cockling and curl. If the environment is humid or arid, certain liners can pick up or lose moisture easily along the edges and exposed surfaces.
  4. What does the equipment configuration look like? A long draw between the coating head and the oven can create difficulties in keeping the adhesive evenly spread out on the liner.

In general, the more the release liner manufacturer knows about your processing conditions, the more likely both parties are to solve the hidden problems that can affect the final product. Surprises late in the development process can lengthen cycle times.

3.  Define the performance required of the liner.

  1. What is the expected release level of the liner? Certain formulas are inherently tighter than others. The level of release is also impacted by the adhesive being used.
  2. At what speed will the liner be stripped? The actual stripping speed is what is important, rather than a lab testing speed. Stripping speeds can affect the level of release of the product.
  3. Will elongation be an issue? Print registration can be poor on a substrate that does not lay flat or that stretches during processing.
  4. Will the product be die-cut? If you are kiss cutting, you should take into account the compressibility of the substrate. Also, when using polycoated release liners, different poly resins provide a harder surface than others for die-cutting.
  5. What about other issues like expected shelf life, storage conditions, and final application? Whether the liner will be applied by hand or machine can substantially affect the liner recommendation, since the release level has higher tolerances when hand-applied.

The more knowledgeable the release liner manufacturer is regarding how the liner is actually used and what stresses it encounters in your process, as well as your customer’s process, the better the line recommendation.

4.  Implement the release liner manufacturer’s recommendations and evaluate submitted samples in your converting application.

Once a liner is chosen, it is important to get samples in a real manufacturing environment. That’s because things can happen in your manufacturing environment that are not apparent in the lab. Quick and accurate feedback to the liner supplier is essential in reducing the time to market for your products.

5.  Conduct realistic aging studies and performance testing.

The ideal test predicts performance in the field. Some tests are easy to do and you get results quickly, but they may not reflect what will happen in the “real world”. The test method and results should be shared with the liner vendor, so that they understand your critical requirements and make sure the material sent to you meets them.

6.  Agree on the final product specifications.

Specify the attributes that are critical for your end use, but do not over- or under-specify the product. For example, if a release level of 100-300 is requested, but 120 will cause a failure, the spec needs to be narrowed. On the other hand, unnecessary specs drive up costs and create needless waste.

Follow these steps and you’re on your way to a successful scale-up. Cycle times can be reduced and performance enhanced through open communication and early involvement.


Common Problems

Combining silicones, adhesives and face stocks is complex. Regardless of how much information is available and how much communication there is between the manufacturer and converter, problems can occur.

Several common problems have been occurring for many years, so there are relatively reliable means for preventing and combating them. This section is devoted to explaining the causes of these common problems. As well as providing suggestions to avoid them or correct them.

Adhesive Confusion

Adhesive confusion occurs when the adhesive does not stay on the proper side of the substrate and unwinding defects occur due to the lifting or tearing of the adhesive. Adhesive confusion can be caused by adhesive ooze, non-uniform moisture levels, improper winding tensions, inadequate cure, or not a wide enough release differential on the release liner.

Blistering/Bubbling

A blister is a disruption in the surface of a substrate, polyolefin coating, adhesive coating, etc. Blisters are generally caused when liquids, such as water or organic solvents, or gases such as air, expand at a rate faster than the overall system is able to dissipate the increased volume and pressure. For instance, if an adhesive has skinned over, blister or bubbles may form in the adhesive as the remaining solvent or water, unable to diffuse rapidly enough through the skin, vaporizes and forms bubbles in the adhesive as a means of pressure relief. In another case, if a two-side polyolefin-coated release liner is heated to an excessively high temperature, the polyolefin will soften, and when the vapor pressure of the water in the paper is high enough, a blister will form in the polyolefin – again as a means of pressure relief.

Blister can be caused by:

-          Excessive heat

-          Restricted permeation/dissipation of the liquid/vapor components of a system

-          Vaporization of solvents or water

Blister can be minimized by:

-          Reducing maximum temperatures

-          Reducing heating rates

-          Perforating the release liner

-          Using a more porous liner

Blocking

When layers of the release liner are fused together making it difficult to unwind the liner without damaging the sheet or using high unwind tension, blocking is usually occurring. Blocking may occur only in parts of the web, such as the edges, or across the entire web and can be caused by a number of different mechanisms.

-          Inadequate cure of the coating

-          Non-uniform moisture levels

-          Excessive winding tensions

-          Chemical contamination

-          Inadequate release characteristics

Blocking can be prevented by a number of different methods

-          Increase the degree of cure

-          Reduce ambient moisture to improve storage conditions

-          Reduce tension settings by decreasing lay-on roll pressure, adding or increasing winding taper, or reducing torque

Many people also use noise as an indicator for blocking. This method is very subjective and does not necessarily identify blocking because some coatings are inherently “grippy” or ”grabby” by touch.

Delamination

Delamination is the separation of layers, at the interface, that have been bonded together by mechanical or chemical means. This effect occurs when internal bonding forces are exceeded, frequently causing an expected failure. Delamination can also occur at the interface of polyethylene to paper or adhesive to silicone.

Usually delamination is a function of the material and interface. For example, a polyethylene coated paper could delaminate due to poor surface-treatment before polyethylene extrusion coating. Adhesive delamination could be due to easy release of the silicone.

Delamination is caused by:

-          Improper flame-, corona- or chemical surface treatment.

-          Poor internal bond

-          Poor interply bond between fibers

-          Improper release properties of the silicone coating (too easy)

Delamination can be corrected by the following:

-          Increasing surface treatment

-          Improving internal bond of base sheet

-          Increasing release force of the silicone coating

Dimensional Stability

Dimensional stability relates to the ability of a material to resist change when exposed to external forces such as moisture, heat or physical stress. All material exhibits some dimensional change when exposed to these forces and the challenge is to control the degree of change so that the quality of the product is not compromised during processing and use.

For paper, the change due to moisture gain and loss (hygroexpansivity) is the major concern. Changes due to moisture show up as curl, edge waviness or poor layflat characteristics. The dimensional stability of paper is controlled by many factors. Among them are fiber types and percentages, degree of refining, interfiber bonding, filler content, sizing type and amount, fiber orientation, drying process, sheet density and formation. Subsequent converting processes also affect the dimensional stability of the release liner.

Dimensional stability is also a factor with film release liners. However, in this case it is due to thermal stresses. Exposing a film to an excessive temperature, especially under tension, can cause shrinkage, stretching or even severe distortion.

Dimensional stability is influenced by:

-          Changes in humidity

-          Initial moisture of the paper when produced

-          Percentage and types of pulp

-          Drying process of paper

-          Fiber freeness

-          Filler content

-          Sheet density and formation

Film and Paper Bagginess

Baggy film/paper occurs when a substrate does not have uniform tension across the width of the web. These areas of low and tight tension are present throughout the sheet because the lengths of incremental bands across the web are not equal.

Bagginess is a property of film or paper that is created in the processing step of the base material. Film can be further processed by annealing to relax and/or minimize the bagginess.

Causes of bagginess are as follows:

-          Non-uniform web thickness

-          Winding too tight

-          Surface winding on a non-uniform thickness

-          Improper spreading

-          Improper drying

Corrective remedies include the following:

-          Reducing caliper variation

-          Using a slip shaft mode for rewinding

-          Adjusting spreading device to properly obtain uniform web tensions

-          Reducing oven temperature gradient

Layflat

Layflat is generally considered to be the ability of a release liner or a finished construction to resist developing curvature or curl when exposed to humidity changes. This is fundamentally related to the dimensional stability of the release liner. However, other factors such as improperly matched tensions during lamination, shrinkage of the face stock, exposure to excessive temperatures or roll set can cause curl or layflat problems.

Layflat can be optimized by:

-          Using release liners of high dimensional stability

-          Properly matching tensions during laminating

-          Minimizing face stock shrinkage

-          Avoiding exposure to excessive temperature

Moisture Wrinkles

Most paper is produced from a slurry of fiber dispersed in water. Most of the water is removed during manufacture to make paper as we know it. However, paper retains the ability to pick up or lose large amounts of water during additional processing or storage. The absorption of water frequently results in moisture wrinkles, especially on the outer plies of a roll of paper or paper based products.

Clay-coated papers, supercalendered krafts, and polyolefin-coated papers will lose or gain moisture content upon thermal processing or during storage, if packaged improperly. Low moisture-content papers will absorb moisture very quickly and moisture wrinkles on the outer layers are usually the result. Remoisturizing the paper after a thermal processing step, which dries the sheet out, will minimize the tendency to develop moisture wrinkles due to subsequent moisture pickup. Proper packaging with moisture barrier wrapping and proper storage conditions are also very important.

Moisture wrinkles can be caused by absorption of ambient moisture following:

-          Excessive drying of the paper during processing

-          Exposure to low humidity for extended periods

-          Exposure to very high humidity of extended periods

These effects can be minimized or eliminated by the following:

-          Remoisturizing rolls after thermal processing

-          Controlling storage environment. Wrap rolls in packaging that provides a moisture barrier.

Scratches

Scratches are physical abrasion of the surface of a release liner caused by contact with a non-uniform area of a material which is harder than the material being damaged. Scratching occurs when line speeds and surface finishes are not matched properly during processing. Consequently, damage occurs to the softer surface which is usually the coating or the substrate. Scratching can also occur when slippage occurs in the machine due to non-uniform tensions throughout the process in the machine or cross direction. Proper roll alignments and tension control is a necessity when processing.

Scratches are caused by:

-          Differential in line speed to web speed

-          Large rapid changes in surface topography

-          Misalignment of idlers and tension setting

Scratches are eliminated by:

-          Web speed matching idler speed

-          Uniform surface topography

-          Uniform web tensions throughout the process

-          Good machine alignment

Silicone Transfer

Silicone transfer is the transferring of the coating from one surface to another. This transfer is usually caused by either un-reactive components in the silicone coating or inadequate cure of the silicone. Silicone transfer will normally manifest itself on the back side of a substrate or on the adhesive surface. Transfer can be detected by the change in surface energies across the back side of the substrate. Analytical testing (ESCA) and subsequent adhesion testing against a metal plate are additional ways to identify silicone transfer on the adhesive.

Silicone transfer is caused by:

-          Inadequate cure levels of the silicone

-          Excessive or inadequate surface treatment levels

-          Excessive winding pressure

-          Unreactive silicone polymer

The transfer of silicone can be reduced by:

-          Increasing cure levels of the silicone

-          Determining adequate surface energy levels for coating. Surface energy levels should not be excessive

-          Reducing winding tensions in the roll

Static Electricity

Static electricity refers to stationary charges that are built up on the surface of a substrate. These changes can be positive or negative.

Static electricity is generated when two different surfaces are separated (triboelectric effect), thereby transferring charges from one surface to the other. Pressure, friction, line speed, humidity, and surface smoothness influence the magnitude of electron generation and transfer.

Roll potentials can routinely exceed 50,000 volt when no static control devices are utilized in the converting process. Corona discharges can occur when the potential level is 4,000 volts or less depending on the distance between charges. Consequently, physical danger exists to operators and equipment if in-line potential is not reduced to acceptable levels during processing. Corona discharges will also affect the wettability of the coating and continuity of the coating on the substrate. In the case of silicone coating, tight release areas will be created.

Static electricity can be controlled by:

-          Matching surfaces as closely as possible on the triboelectric series

-          Increasing humidity levels (surface moisture)

-          Using passive- and air-assisted ionization bars and blowers

-          Draping grounded tinsel loosely on the substrate. This is also a very efficient and cost effective method of removing static from rotating rollers.

Wrinkles

A wrinkle is defined as a fold over of the web in the transverse or cross direction. Fold over occurs because of poor tension profile and cross direction edge tracking (traction loss). Normally, wrinkles are categorized as hard or soft. The effect of the wrinkle is application dependent.

Major reasons for wrinkles are:

-          Insufficient cross or transverse machine tension across the web.

-          Loss of traction between the web and idlers

-          Poor machine alignment

-          Bagginess in base substrates

Corrective actions to remedy the above causes are:

-          Applying a spreader roll

-          Improving edge guide equipment to keep the web uniform

-          Checking machine alignment. Optical alignment may be necessary

Silicone Chemistry Explained to Understand the Complexities of Release

THE INTRICACIES OF RELEASE
 
In order to develop a basic understanding of the complexities of release, we will examine the interaction of the silicone chemistry, substrate and process (which is controlled by the liner producer), and then comment on the interaction of the user’s product process with the release liner.
 
I.  CHEMISTRIES
 
The release chemistries available will be categorized in order to develop a better understanding.  There are two different variables that identify the type of silicone chemistries available.  They are catalyst and energy.  The catalyst refers to the chemical that initiates the reaction.  The energy refers to the type of cure mechanism.  There are also non-silicone chemistries that will be discussed.
 
  A.  Thermal Reaction Silicone Systems cure in the presence of heat.  There are three different thermal activated silicone systems currently available:  
  1. Tin Catalyzed Systems are the oldest commercially available silicone chemistries.  They come in emulsion and solvent diluted forms.  These systems are relatively slow in cure time and require a higher heat and/or longer dwell time to get good cure.  Tin silicone should have a low potential for silicone transfer when cured properly on paper substrates.  Temperature restrictions on heat sensitive substrates have the potential of being more of an issue.

  2. Platinum Catalyzed Systems are faster in cure time and have very little post cure requirements.  These systems are available in emulsion, solvent diluted and 100% solids.

  3. Rhodium Catalyzed Systems are not as common as the tin and platinum systems.  This system was the original 100% solids offering and to a large extent it has been replaced by platinum systems.


  B.  Ultra Violet Reactive Systems were developed for thermally sensitive substrates like polyethylene.  These systems require a photo initiator to catalyze the cationic reaction in the epoxy systems and free radical reaction of the acrylate systems.
 
  C.  Electron Beam Cure Systems are not as common as the thermal and U.V. systems.  The process equipment is very expensive. The product consistency can be adversely affected by inadequate inerting.  The electron beam is the energy source for this type of curing.
 
  D.  Non-silicone Release Systems lack the ability to yield a consistent premium release.  In most cases, the release level is too high to meet the needs of release liner customers.
 
  Some of the commercially available non-silicone chemistries are Chromium III Chloride Complexes of Myristic and Stearic Acid.  At Loparex, we are seeking a premium non-silicone release system that could meet the release requirements of the Automotive, Composite, Pressure Sensitive, Medical and Electronic Markets.
 
II.  SUBSTRATE
 
The substrate requirements are end use dependent.  There are two basic categories of available release substrates:
 
  A.  Paper is a generic term used to describe different styles of substrates:
  1. Kraft is the machine grade paper that uses basic wet-end chemistries.  There are many different paper machines that have unique fiber formations.  The silicone holdout of these substrates are usually poor due to openness of the sheet.  The dimensional stability of this liner is paper machine specific.

  2. SCK or Densified Kraft is the term used to describe paper that has been densified through a specific calendering system.  These substrates usually have fair dimensional stability.  Densification increases the effects of moisture on stability.

  3. Clay Coated substrates utilize clays and other natural fillers to enhance the surface characteristics. The addition of the clay also decreases the amount of fiber which decreases the effect of moisture and improves dimensional stability.

  4. Saturated Grades utilize latex binders in the wet-end chemistries to enhance the physical properties of a paper substrate.  Latex saturated base stocks generally have good dimensional stability.

  5. PlastiKraft is a Loparex trademark name for a series of barrier coatings that improve the oil and moisture resistance of release liners.  PlastiKraft is available in 1 or 2 side coated materials.

  6. Polykraft is the term used to describe polyolefin extruded paper.  The polyolefin is extruded onto the surface of a kraft stock.  These substrates are relatively expensive and thermally sensitive.  They have very good dimensional stability but poor heat resistance.  This temperature sensitivity can increase the potential for silicone transfer unless carefully produced.  These stocks come in either 1 or 2 side poly-extruded.

Here are some of the physical properties associated with the fiber formation and wet end chemistries on the paper machine:  
  1. Tensile Strength is the force at break when a substrate is pulled apart.

  2. Tear Resistance is the force required to tear a single sheet of paper through a specific distance after the tear has been started.

  3. Basis Weight and Caliper are the weight/unit area and thickness.

  4. Hygroexpansivity is usually expressed in a percent and is a measure of the growth of paper due to moisture absorption or loss after changing the humidity in the paper storage environment.

Sizing treatments and other process conditions can influence the physical properties listed above.  The majority of these process treatments are designed to enhance the surface of the sheet.  Here are some of the physical properties that are influenced by these treatments.
  1. Smoothness is the level of surface consistency of a substrate.

  2. Silicone Holdout is the ability to coat a thin film of the release formulation on a surface of a substrate.  The silicone coat weight is dramatically influenced by the substrate holdout ability.  The less the silicone soaks into the paper the more consistent the silicone surface at less cost.

  3. Silicone Cure is the level of cure that can be achieved on a substrate.  The level of cure can be quantified by extracting the residual non-crosslinked silicone and expressing it in terms of the original silicone coat weight.  This terminology is known as the Percent Silicone Extractable.

  B.  The Film category of substrates is just as complex as the paper substrates.  Here are some of the commercially available film liner substrates:
  1. Polyethylene is available in different densities and gauges.  Polyethylene is very temperature sensitive and this gives it a high potential for silicone transfer in thermally cured processes because of the balance between silicone cure and substrate deformation.
  2. Polyester is the least thermal sensitive film substrate.  The anchorage of silicone can be addressed with surface enhancements or prime coatings.
  3. Polypropylene has some shrinkage issues that have to be considered, but this film is not as thermally sensitive as the polyethylene.  There are some process enhancements that allow the polypropylene suppliers to orient the film in both the machine direction and the cross-direction.  This process minimizes the shrinkage.
III.  PROCESS

The surface of the release coating has a mechanical influence on the level of release that you are able to achieve.  Most of the surface smoothness is dictated by the substrate. The application of the release coating also has an influence on the release surface.  Therefore, it is important to understand the distinct differences in silicone applications.  Here are some of the most common silicone applicators:
 
  A.  Metering Rod coating utilizes a wire spun rod that allows a certain volume flow of a solution.  The volume of the solution is metered by the gauge of the wire.  The speed of the solution delivery and the speed of the rod also influence the volume delivery.  By maintaining solids levels and process, the metering rod systems yield a uniform consistent coating.  This application system is designed for emulsion and solvent diluted release systems.  

  B.  Gravure coating heads utilize an etched roll for coating delivery.  
  1.   Direct Gravure is application from etched roll to substrate. 
  2.   Offset Gravure uses an applicator roll to transfer from the etched roll to the substrate.
  C.  Multi-Roll coating heads utilize a stack of rolls to meter the coating.  In general, the greater the number of rolls or film splits the more consistent and uniform the coating deposition.  These coating heads need to be combined with paper substrate selection to optimize the coverage of the coating.  The rolls on these coating heads usually alternate between compressible (rubber) and non-compressible (metal).  When the rolls are nipped together, the surface area of contact between the compressible and non-compressible roll is called a “footprint”.  Roll nip footprints and roll speed differentials not only affect the amount of coating deposited on the substrate, but the coverage as well.
 
IV.  END USES
 
We have looked at the intricacies of release liners from a release converters point of view.  The proper development chronology would be to look at the end use first.  This is where the specific parameters are dictated.  The end use of a release liner can be as simple as a carrier for bubble gum to the liner needs of the complex epoxy systems in the aerospace market.  The relationship between the silicone converter to the end use will influence the time for product development.  The end user of the release liner is our customer’s customer.  This makes it difficult to get exact parameters of the release requirements.  At Loparex, we want to be a resource to our customers and assist them in meeting their customer’s needs. 
 

Loparex Dimension Conversion Table

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