Sustainable PSA Release Liners and Coatings

TAGS:  Sustainability / Natural Adhesives    Reactive Adhesives    

• What are Release Liners?
• Function of Release Liners & Coatings
• Requirements for Release Liner Selection
• Types of Release Liners & Coatings
• Sustainability in Release Liners – Going with the Trend
• Recent Innovations in Release Coatings

What are Release Liners?

A release liner is a critical element in a pressure sensitive adhesive (PSA) product – perhaps as important as the adhesive itself. A release liner is a paper or polymeric film that carries and protects the adhesive film as well as preventing it from unwanted sticking. When necessary the release liner is coated on one or both sides with a release agent.

Release liners protect pressure sensitive tapes, labels, and transfer films from accidental contact with other surfaces. They allow the PSA to be wound into rolls for the purpose of packaging and dispensing. Release liners must also provide necessary unwind characteristics to allow for flawless application. Without a release liner many pressures sensitive adhesive (PSA) products could not be produced.

Because PSA release liners will be eventually thrown away, end-users rarely give these materials much consideration. Although end-users may prefer PSA products due to their environmental benefits (e.g., avoidance of VOCs, safety, non-flammability) and avoidance of working with a liquid adhesive), they may actually be damaging the environment due to the waste generated by release liners.

As a result, new innovative release products have been developed not only to provide the necessary release function but also to be environmentally favored. Today’s prime market drivers – sustainability, cost control, and new applications – are impacting the release liner industry as much as any other industry.

Understanding the role of the release liner and the types of construction available is becoming increasingly more important as PSAs gain broad acceptance in diverse industries and applications. This article provides an update of release technology to provide a guide for their selection and application. However, the primary focus is to identify sustainable trends in this important technology.

Let’s explore the function & requirements of release liners & coatings along with the current sustainability trends.

Function of Release Liners and Coatings

Release liners are simply carriers for the PSA that prevent it from sticking to itself and provide a method of dispensing and application. PSA products (tapes, labels, transfer films) generally require a release liner of some type. Figure below illustrates the construction of a PSA label.


PSA products may be constructed as either one-sided or two-sided products.

• With a one-sided product such as pressure sensitive tape, the adhesive backing may also serve as the release mechanism (e.g., linerless labels). This is environmentally ideal in that a separate throw-away liner is not required.
• With two-sided products such as transfer film or double-sided tape, a separate liner is required.

Release liners serve several very useful functions. They are used as a carrier sheet onto which the adhesive can be cast. They protect the adhesive during storage and transit and also during various converting and assembly processes so that unintended blocking does not occur. They provide functional support during die cutting and printing. The liner also provides a lay-flat characteristic that prevents curling. Most importantly, liners must perform these functions without damaging the adhesive or compromising its subsequent performance properties.

The optimal selection of a release liner can be a significant factor that impacts greatly on bond quality, assembly productivity, and environmental waste. Poorly chosen release liners can result in the following problems:

• The release liner fails to protect the adhesive
• The liner releases prematurely
• Machines must be set to a slower speed because of poor liner reliability and performance
• Waste disposal and janitorial costs for gathering and discarding the spent liners
• Unsafe workplace areas where slip-and-fall accidents can be caused by discarded liners.

Requirements for Release Liner Selection

The selection of a proper liner material will depend on the application and the type of PSA product being constructed. A number of essential questions will require answering before proper selection can be made. The table below summarizes the type of forethought that is required.

Should the liner offer easy release, or will a higher release strength ensure that it will stay in place during all processing conditions? Is a heavyweight liner required to give the product stiffness, or would a lightweight liner be better to allow conformation to the substrate surface? How important is dimensional stability and the liner having a lay-flat characteristic? Will the liner be die-cut or printed? If so, what are the particulars of these processes (e.g., thermal printing, type of die-cut operation, etc.)? Is product identification or color important? Will the liner be removed during the initial assembly procedure or remain intact until removed by the end-user? What is the price vs. performance limit and which properties can be sacrificed for a lower price? Is environmentally acceptable disposal of the liner after its use a requirement?

A relatively new type of release liner has been introduced for bonding substrates of two differing materials. The differential release liner can carry two adhesives with a carrier between them. Each adhesive is optimized for the particular substrate that it will bond. A differential liner can be used with two-sided PSA tape where the liner will have a tight release value on one side and a lower release value on the opposite side. This enables the tape to be easily rolled, unrolled, and applied with maximum efficiency.

Get inspired: Design successful PSA bond by learning how to select appropriate test methods for specific applications »

Types of Release Liners and Coatings

Carrier Materials

A variety of release liners are used in PSA products. They range from coated paper to plastic films. They all have certain advantages and disadvantages and must be chosen in concert with the needs of the specific application. The various types of liners that are generally used are described in table below.

Type	Description	Characteristics
Kraft paper	The release coating is applied on one or both sides of densified kraft paper	An economical option, suited for general-purpose applications and rotary die-cutting.  Not for kiss cutting.  Kraft liners can exhibit problems with dimensional stability under changing temperature or humidity conditions.
Board	Made of paper with a heavy weight and a caliper of 12-14 mils	The large caliper maximizes kiss-cutting performance and allows easy removal of small parts and waste pieces.
Non-wovens	Nonwovens are manufactured by putting small fibers together in the form of a sheet or web, and then binding them either mechanically or with an adhesive.	These liners are breathable and often used in the healthcare (e.g., bandages) or consumer product industries. They provide stretch and softness that are often needed for flexible PSA joints.
Clay-coated paper	Paper coated on one or both sides with clay	Provides high temperature performance and better humidity resistance (dimensional stability).  Lower tear strength than kraft papers due to the brittle nature of the clay.
Poly-coated paper	Base paper with extruded polyethylene or polypropylene film on one or both sides and sometimes coated with a silicone release agent	A versatile option.  Resistant to tearing and to wrinkling or “cockling” when exposed to humidity.  Can be used for all forms of die-cutting.
Film	Produced from low- or high-density polyethylene, polypropylene, and polyester	Most expensive liner.  Provides excellent humidity resistance (dimension stability). The liner will provide the smoothest coating of adhesive possible. Do not produce “paper dust” during slitting and conversion.  Offers high tear strength, high-speed rotary cutting and hot-wire cutting.  The production of static electricity from the liner is a concern.

Types of Liners Commonly Employed in PSA Products
The surface to which the release coating is applied should be reasonably non-porous to hold the release coating on the surface and to prevent the flow of soft pressure sensitive adhesive into the pores and roughness of the surface, causing an increase in the unwinding force. An additional coating, which may or may not have the release characteristics, can be used on fabric and paper surfaces to provide a smooth and impermeable surface. If necessary, the release coating is applied over the non-porous coating.

The release liner must be specifically designed to perform in a particular product and production process to which it will be subjected. For example, masking tape requires that the release coating have a good solvent resistance and that the paint adheres reasonably well to the release coating. Otherwise, the paint chips may fall off and remain adhered to the freshly painted surface.

Film liners represent the greatest potential for growth, especially in high-volume, fast-moving applications. Release liners on automated assembly lines are usually made of film to avoid tearing during the removal. Film liners are also preferred for most electronics applications because the fibers introduced with paper liners can cause contamination. However, paper liners will continue to enjoy the majority of the market due to their lower cost, caliper, and technology enhancements via improved release coatings.

Coating Materials

When a liner consists of a release coating applied to a substrate (e.g., kraft liners), the release coating is applied from a dilute solution or dispersion. Many types of release coatings have been used on liners over the years.

The first release coatings for pressure sensitive tapes were polymers such as:

• Shellac
• Starch
• Casein, and
• Nitrocellulose

These older release coatings did not exhibit good release characteristics but functioned primarily by preventing the adhesive from penetrating the pores of the liner.

In more recent times, silicones, vinyl acetate copolymers, acrylic resins, fluorocarbons, and PVC resins, have all been applied as release coatings for PSA products. Several of the more widely used release coatings are summarized in the table below. Many modern release liner manufacturers now use waterborne or UV curing coatings, which have no VOCs or environmental constraints during manufacturing. Thin coatings are generally sufficient and environmentally preferred.

Type	Characteristics
Silicones	Most widely used release coating of PSA products.  Exhibit easy release at low peel rates.  Can be applied as solventless, solvent borne, or waterborne systems.  Low surface energy makes silicone coatings ideal for all types of liners including polymeric films.
Waxes	Generally used as an additive to film forming polymer coatings to improve release.  Effectiveness is based on their incompatibility and ability to migrate to the surface of the coating.  Could affect the performance of the pressure sensitive adhesive if the coating attaches to the adhesive.
Long chain branched polymers	Polymers with long side chains are waxy compounds exhibiting good coating performance and release characteristics. Generally, a copolymer of alkyl acrylate and acrylic acid, nitrocellulose, and vinyl chloride.
Polyvinyl carbamates	Generally used as a release agent on film backings (e.g., cellophane film).
Chromium complexes	Provides good release characteristics and water repellency to paper.  Chromium complexes are attached to C14-C18 fatty acids.
Fluorocarbon copolymers	Have very low surface tension to provide for easy release characteristics.  Expensive.
Amines	Long chain alkyl substituted amines have good release properties especially on paper substrates.  Used similarly to the chromium complexes.

Commonly Applied Release Coatings for Pressure Sensitive Products

Sustainability in Release Liners – Going with the Trend

The major sustainable innovations in release liners include recyclable liners, thinner film liners, and linerless PSA products. Leaders in the liner industry are continually looking for ways to provide a sustainable product without sacrificing performance or cost. However, it is the end-user’s responsibility for recycling the liner waste.

Recyclable Liners

A recent condition placed on release liners is that they be recyclable once used. Most liner carriers can be recycled and taken back to its near-original form, either fiber or resin. However, there are issues regarding the effective recycling of polymeric release coatings such as segregation of material by type, sufficient quantities, and contaminants.

Although silicone-coated paper release liners have been one of the most popular types, it is difficult to remove the silicone coating from the carrier to allow the carrier to be recycled. The silicone coating can be mechanically skimmed off the carrier or separated from the recycled melt. Residual silicone that is not separated from the carrier can cause a variety of problems in the repulping process.

Siliconized label liner is waste as it can’t be easily recycled. As a result, more sustainable release liners are being developed.

• At Western Michigan University a method of making a repulpable silicone release liner paper has been developed which enables better separation of silicone from the liner paper during recycling.² The technology involves coating the release liner base paper with a water-soluble material before coating it with silicone release. The water-soluble “sacrificial” layer readily dissolves in the water and heat-based repulping process, releasing the silicone layer for separation.


• Avery Dennison has begun a program to educate customers about which liners are most recyclable. They have also developed EarthFirst® PLA, a corn-based release film and a tree-free paper face stock made from bamboo, bagasse, and cotton linters.


• Polyethylene terephthalate (PET) films offer liners that are thinner than paper, and PET can be recycled and even converted into energy pellets. The pellets can be considered a clean alternative to fossil fuel.


• Dow Corning has entered a partnership with RecuLiner, a Belgian company. The partnership has developed a recycling technology for converting paper release liner waste from self-adhesive labels into a range of end products including thermal and sound insulation.


• A unique product called Zero Waste Label Release Liner (EcoEnclose) is claimed to be curbside recyclable. The release liner is made from 100% recycled materials and is 100% recyclable. EcoEnclose has also developed a recycle compatible adhesive for a complete environmentally disposable product.³
  
  Zero Waste Label Release Liner by EcoEnclose

Thin Liner Carriers

Release liners are now being constructed out of thinner materials for certain applications such as clear-on-clear labels. They provide a sustainable release liner that has lower energy costs to produce and transport as well as less material going into landfills. Thinner release liners also provide more material to be wound on the same core and fewer web breaks compared to paper.

As noted above, PET provides a release carrier that is thinner than paper. But other thin film release liners include synthetic films such as polypropylene and polyethylene. These new liners are 60 percent thinner than traditional paper options.⁴

Linerless PSA Products

A disruptive way to reduce liner material is to eliminate the release liner entirely. Linerless technology is primarily used within the label industry. A special release coating (topcoat in figure below) is applied to the face of the label that allows it to be wound on a roll without the adhesive sticking to the label below it.

Linerless PSA technology not only reduces the amount of waste but also provides additional advantages. Eliminating the liner reduces freight and storage costs. It is estimated that by eliminating the liner, savings of up to 40% of the overall weight can be made. Shipping costs can be reduced because there are twice as many labels per box compared to standard labels.

A disadvantage is that linerless labels tend to be more expensive. Another issue label converters have with linerless labels is that they have limited shapes. For some manufacturers, the lack of release liners is a downside as the label will not have something to hold it in place when die-cut.⁵

Get Inspired: What circular economy in adhesives & sealants means today?

Recent Innovations in Release Coatings

Silicones are, by far, the most widely used materials for pressure-sensitive release applications. They give label and tape makers an almost limitless array of substrate, processing, performance, and application options.

Silicone resins provide uniform, thin coatings that exhibit easy release at low peel rates. This provides sufficient adhesion between the silicone and the PSA to keep the adhesive in place yet prevent easy parting of the liner from the PSA during application.

The three basic types of silicone release coatings used in the PSA industry can be classified by the form in which they are applied to the liner: solvent-borne, waterborne, and solvent-free. These are characterized in the table below.

Form	Cure Rate	Release	Comments
Solvent borne	Fast to slow	Easy to tight	Dull to glossy, good anchorage
Waterborne	Medium to slow	Easy to medium	Some coatings require post cure
Solventless (100% solids)	Fast to medium	Easy to tight	Glossy coatings, premium release

Comparison of Silicone Release Coating Systems⁶
Solventless silicone release coatings have been developed to address environmental pollution/health problems, solvent cost, and productivity issues. This class of coatings includes thermal, ultraviolet (UV), and electron beam (EB) curing.

UV and EB curing of silicone release coatings appear to be a major driver in release coatings today. This is due to their:

• Very fast cure
• Low energy consumption, and
• Ability to be easily modified to meet specific release requirements.

These materials can be applied to both paper and polymeric film-based liners. A benefit of UV and EB cured release coatings is that significant heat is not required for the cure, and as a result, they can be applied to heat-sensitive substrates such as plastic film.

The major component of a cured silicone release coating is polydimethylsiloxane. In the environment, polydimethylsiloxane breaks down into water, carbon dioxide, and minerals already found in the earth’s crust. Crosslinking of the silicone coating is necessary for it to resist penetration by the PSA. Without crosslinking, the adhesive could diffuse into the flexible siloxane polymer chain. However, crosslinking also provides a coating that is more difficult in repulping processes.

In most paper recycling processes, it is a challenge to separate the silicone coated paper. Sufficiently energetic pulping conditions are required. Most recent innovations have been directed toward the pulping process rather than modifying the silicone resin.

Many tape and label manufacturers are seeking solvent-free and silicone-free alternatives to reduce landfilled waste, potential emissions, and worker exposure. The goal is to find environmentally preferred release coatings that match the performance of the current standard.⁷

Ideally, these release coatings are waterborne, recyclable, and repulpable. They may also be candidates for a wide range of applications other than release liners (e.g., baking and food packaging). Examples of such release agents include:

• Long-chain alkyl carboxylic acids and esters,
• (Meth)acrylates with long-chain side groups
• Polyurethanes
• Special silicone polymers (particularly polydialkylsiloxane homo- and copolymers), and
• Fluorinated polymers

Several prominent examples pertain to PSA release liners.

• OMNOVA Solutions has launched Suncryl® HP 114 acrylic release coating, a new waterborne product that delivers excellent adhesion to paper as well as film substrates. It also promises environmentally preferred attributes (free of alkyl phenol ethoxylate (APE)-containing surfactants, free of formaldehyde, VOCs, silicone-free) that enable manufacturers to meet market expectations for green practices and products. When applied to paper, the finished release product is repulpable.


• Mayzo’s waterborne release coatings represent a “greener” alternative to traditional solvent-based products. These are based on acrylate, polyvinyl acetate/acrylate, fluoro-acrylate, fluoro-phosphate ester, modified polyurethane, polyester/acrylic, and modified starch chemistries. These release coatings are claimed to be recyclable and repulpable when applied to paper. Their benefits include wide release levels and printability.

Conclusion

World consumption of pressure-sensitive labels alone is running at 1.4 million tons annually, and only 12 percent of the release liners are recycled. Consequently, this accounts for approximately 616,000 tons of liner waste material that ends in landfills.

Stringent environmental regulations imposed by various industries result in challenges for the growth of the release liner market. These regulations not only pertain to the manufacture of PSA release liners and coatings but also to their eventual disposal by end-users.

A large number of release liner producers are focusing their development efforts on producing sustainable products for PSA tapes and labels. These innovations include new materials and constructions for release layers as well as repulpable and environmentally safe release coatings.

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