The silicone coating is extremely thin, typically of the order of one micron thick. It is applied as a liquid and then transformed to a silicone elastomer or rubber. (Figure 3.3).
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PAPER SUBSTRATES
Both surface and mechanical properties are critical when specifying paper-based release liners.
Paper substrates for use as release liner base are selected to be as smooth as possible, and with a ‘closed’ surface to significantly reduce any silicone from penetrating inside the paper, minimizing the amount of coating required. The surface treatment of paper is also important as some chemicals used in certain paper grades can ‘poison’ the platinum catalyst – which is often used as a critical component of many silicone release coatings – preventing the silicone curing or cross-linking as it is transformed from a liquid into an elastomer.
The paper surface properties must also be optimized to encourage robust anchorage of silicone to the paper surface – especially important where high coating speeds are being used.
In terms of mechanical properties of the paper, the release liner has to carry the weight of the laminate, act as the base for die-cutting, and carry the die-cut label through a high-speed label applicator, as well as being able to withstand the stresses of the silicone coating process itself. All of these requirements mean that the paper must have a high degree of mechanical strength and tear resistance to prevent snapping or tearing during coating, die-cutting or label application processes.
In addition to this the caliper (or thickness) must be closely controlled to a high degree of consistency, or problems will be encountered in the die-cutting process.
Paper stiffness is also important since die-cutting problems can arise if the material is too soft, as the cutting blade will tend to penetrate into and deform the paper rather than actually cutting the face material.
By far the most commonly used paper materials are glassines or super-calendar krafts (SCK). Whilst there are some differences between the two types in terms of how they are made, super-calendared Kraft has become primarily the paper of choice in the US, while glassine is the choice in in Europe and Asia. (The reason for this is historical rather than technical – US manufacturers simply carried on making super-calendar kraft rather than changing to glassine).
Both glassine and SCK are characterized by their very smooth surfaces and high level of surface refinement (closed), along with their excellent mechanical and chemical properties.
The other commonly used paper substrates are the clay coated krafts (CCKs). These are essentially standard kraft papers on which a combination of clay and a latex is applied to form a sealed surface. As well as a highly closed surface they have excellent lay flat properties.
Other variants include polyethylene-coated kraft (or PEK), which is more common in Asia than Europe or the US. This has a very smooth and highly closed surface, as well as excellent mechanical properties.
FILM SUBSTRATES
A major trend in recent years has been the move towards filmic release liners, with PET (polyethylene terephthalate) the film most commonly used, mainly due to its mechanical properties. PET is very ‘hard’ and can survive relatively high temperatures, which is why it tends to be used in preference to other films. Its very smooth surface means lower silicone consumption is possible, and a high degree of transparency makes it ideal for ‘no-label-look’ labels.
There is some limited use of BOPP and HDPE substrates for special applications.
Overall, in terms of percentage useage in the label industry, glassine and super-calendar kraft account for roughly 50 percent of all substrates used. Clay coated, polyethylene coated and PET account for roughly equal shares of the remaining 50 percent (Figure 3.3).
RELEASE LINER PROPERTIES;
Very smooth surface, High level of surface
refinement (closed)
Excellent mechanical and chemical
properties.
Highly closed surface, Excellent lay-flat
properties
Very smooth surface, High level of surface
refinement (closed)
Excellent mechanical properties.
Very smooth surface (lower silicone
consumption), ideal for ‘no-label look’ labels
Excellent mechanical properties (and
transparent)
Some limited use of BOPP substrate.
HDPE for special applications
SILICONE RELEASE TECHNOLOGY
The function of the thin layer of silicone release coating is to release something that is ‘sticky’, meaning it has to have anti-adhesion properties. In PS labels this means protecting the surface of the base substrate from a pressure-sensitive adhesive.
To understand how silicone works as a release coating, it is necessary to know how a pressure-sensitive adhesive works and then what it is about silicones that enable them to stop the PSA from doing its job.
The function of pressure-sensitive adhesives is to bond two surfaces together and stop them from separating (Figure 3.4).
When trying to peel apart such a laminate which has been bonded together, we are essentially trying to get a crack to propagate between the two surfaces. To prevent or slow down the propagation of this crack we either need to form chemical bonds between the two surfaces – an adhesive force that must be overcome before separation – or we need to absorb/dissipate energy within the layers to prevent their separation.
In the specific case of PSAs, their performance as adhesives is largely based on their ability to absorb/dissipate energy when they are being deformed, rather than any chemical bonding. This unique rheology is the reason, for example, that PSA labels can adhere to a polyethylene bottle despite its very low surface energy and the difficulty of chemically bonding with the surface.
SILICONE CHEMISTRY
Although silicones are typically found in liquid form, they can be modified by crosslinking polymer chains to form silicone elastomers (rubber), which is the basis of label release coatings (Figure 3.5).
In terms of their architecture, silicones are quite unusual for polymer structures. They are made up of polymers based on a backbone of Silicon and Oxygen surrounded by ‘organic’ groups (typically methyl groups).
This gives silicones both a very low surface energy – which means they are difficult to ‘wet’ – and a very stable backbone, which means they are quite unreactive. In terms of their surface energy (referred to as PDMS), silicones are typically in the range of 22-23 dyn/cm. This is significantly lower that many ‘organic’ polymers such as PE, although there are a few specialized polymers, such as PTFE, with an even lower surface energy (see Figure 3.6).
The low surface energy of silicone will already mean that an adhesive coated onto this surface will not easily ‘wet out’ on the surface, and so will not easily bond with the silicone surface.
But low surface energy is not enough to explain why silicones work so well as release coatings, otherwise PTFE ought to perform even better than silicone, which is not the case in practice. The other aspect of silicones which is important for their performance as release coatings is the way that their surface still behaves like a liquid even when the silicone polymers are crosslinked together to form an elastomer. So even when cured into an elastomer, the silicone polymer chains are actually still mobile.
This has the effect that an adhesive coated onto this surface will still be able to ‘slide’ across the silicone surface (if we look at it at the ‘nano-scale’). This effect of allowing surface ‘slippage’ of the PSA on the surface of the release coating means that it will stop the PSA from absorbing energy as we try to separate the two layers – essentially stopping the PSA from doing what it is designed to do.
As an analogy, imagine you are having a ‘tug-of-war’ with somebody (the PSA), much stronger than you, but who is standing on ice.
Normally, their strength would mean they should win the contest, but because they are standing on ice and you are not, it doesn’t matter how strong they are: the slippery nature of the ice means that they cannot make use of their strength. This is effectively what the silicone release coating is doing: stopping the adhesive from using its built-in strength to absorb energy and stick to a surface.
The combination of low surface energy and highly flexible polymer chains means that the force needed to remove a PSA from the surface of a silicone release coating is low enough to make them ideal for use in label manufacture.
MANUFACTURING PROCESS
The manufacturing process for release liners consists of taking the base paper or film and applying the silicone in liquid form, which can be as an emulsion, a solvent dispersion or solvent-free silicone. The liquid silicone coating is then transformed through the action either of heat – the most common technique in the pressure-sensitive label industry – or UV radiation, to create a cross-linked silicone elastomer (Figure 3.7).
Regardless of the nature of the liquid or the type of crosslinking (heat or UV), the final silicone release coating is always in the form of a silicone elastomer.
The choice and design of the silicone release coating is very much influenced by the process requirements of the equipment and the materials making up the label laminate.
For cost reasons the silicone is coated as a very thin layer, typically just one micron thick, and at high speeds of up to 1,000 m/min. It is critical to completely cover the surface of the substrate with silicone, because wherever there is no silicone, the PSA will be able to ‘stick’ to the substrate underneath.
At these very high line speeds, the time allowed for the silicone to be transformed from a liquid to an elastomer is typically no more than 1-2 seconds. In this short space of time not only does the silicone need to ‘crosslink’ but it must also ‘stick’ to, or react with, the surface of the film or paper. If not, the silicone coating can be easily abraded from the surface of the substrate. This is why it is so important that the surface of the substrate is of sufficient quality.
The most common silicone technology used for labels today is thermally cured solventless. While this is generally the most cost-effective process, it does require an expensive precious metal catalyst based on platinum to provide the very fast cure speeds.
As a result, there is a lot of focus in the industry on reducing the amount of platinum required as far as possible. There are still a few applications where emulsion-based and solvent-based are used, most typically in Asia, but these are quite small and specific to unusual combinations of materials. An example is PVC release liners, where solvent based systems are still used. There is also a portion of self-adhesive labels where UV-cured solventless silicones are used.
This tends to be the technology of choice where UV silicones are coated on narrow web presses.
COATING TECHNOLOGY
Solventless silicones, the most commonly used release coatings for pressure-sensitive applications, are usually applied in one of two ways: either a multi-roll coating head or a 3-roll offset gravure system (Figures 3.8 and 3.9).
The difference between the two types of coating head is a combination of cost vs desired line speed.
The most expensive system is multi-roll coating head. This consists of either five or six rolls pressed together under high pressure in a stack, with the individual rolls turning at different speeds relative to one other.
They are run up to 1,000m/min, although trials have shown that the technology can reach speeds of 1,600m/min and still provide an even silicone coating. An efficient cooling system is required to prevent heat build-up, and this adds even more to the cost.
The older, and cheaper, offset-gravure technology consists of a gravure cylinder that transfers silicone to an applicator roll (which is turning at a different speed), and then onto the substrate when pressed against a backing roller. This achieves the same effect but is limited to speeds of around 300 m/min. At such slow speeds, though, there are fewer challenges in terms of heat build-up.
Faster coating speeds means more output from the coating line in the same production time, but at very high speeds there is also the challenge of ‘misting’.
This is an unfortunate side-effect of trying to coat a liquid at high speed where transferring a coating from one surface to another. A mist of small droplets is formed (in this case silicone), as the film is transferred from one roll to another, and especially from the final roll onto the paper or film surface (Figure 3.10).
At lower speeds this is not a major issue. But at speeds in excess of 600-800m/min it needs to be dealt with in order to prevent a ‘fog’ or ’mist’ of silicone droplets appearing around the coating hall and covering every surface as well as getting into the drying ovens.
Misting can be reduced by mechanical modifications to the coating head, but there are also chemical solutions through the use of additives in the silicone coating.
RELEASE FORCE
The whole purpose of a silicone release coating is that it should ‘release’ the PSA. The release performance of a release coating is characterized in terms of its release force – the force required to peel a self-adhesive label away from the surface of the release liner.
The way the release force is measured is essentially a simulation of the way in which the label is dispensed using a label dispensing head, and the force required is related to the angle at which this happens.
Typically, the industry performs tests at 180 degrees (Figure 3.11), but it could equally be 90 degrees or another angle if needed.
The measurement of the release force using the classic ‘peel adhesion test’ is not just a measurement of the ease of removing the adhesive from the silicone surface, but also a measurement of the flexibility of the PSA layer, the face stock and even, to some extent, the base substrate.
This is important since the strength of the release force is not only related to the silicone release coating, but also to the characteristics of the PSA (thickness and type), and the stiffness of the face stock and base substrate.
In terms of the silicone release coating, the main factors that influence the release performance are the quality and coverage of the coating – how completely the surface of the base paper is covered by the silicone – and the silicone cure, which is determined by how well the silicone is crosslinked.
A silicone that has not been fully crosslinked can potentially interact with the PSA surface it is in contact with, giving unstable release force and even a release force that rises over time as the label laminate is aged. In extreme cases, if the level of silicone cure is poor, then there can potentially be un-reacted silicone present which may migrate to other surfaces and impact the performance of other materials.
This could include migration to the PSA surface, leading to a loss of ‘tack’ or adhesion. It could also include migration to the surface of the label where it could affect the printing performance of the label laminate. It is therefore of key importance that the silicone has been completely transformed/cured to a silicone elastomer.
What is also important with the release force measurement is the speed at which the peel test is performed: the release force will actually vary depending on the peel speed being used. This is important as there may be different processes where the laminate needs to be peeled apart (de-lamination for example), and these processes may be run at very different speeds.
As a simple example, hand-applying a label would be done at a relatively low peed speed, while machine-applying a label on a high speed bottling line would be at a higher peel speed, and die-cutting/converting a label – where the matrix needs to be removed – would be at an even higher speed.
The change in release force with different peel speeds is known as the ‘release profile’ of the laminate, and is an important factor in the choice of the silicone release coating being used.
The graph at Figure 3.12 shows a typical release profile of a label laminate and how much the release force can change depending on the peel speed being used. It shows how the different processes handling the laminate can equate to different peel speeds.
In the past, when the materials used within the label industry were relatively thick, and labeling and converting processes were slow, this change in release force with changing release speed (the release profile) was not so important.
In today’s industry, however, there is a never-ending drive to become more efficient in terms of materials and processes, which means that label materials are constantly being downgauged to save on material and costs, and production processes are constantly being speeded up.
The effect of these changes is that the release profile of a self-adhesive label laminate is now very important in determining how well a given laminate will perform across the different processes. Controlling the release force at a specific peel speed is very important in how that laminate will perform.
If we look at label dispensing in a machine-applied bottle labeling line as an example, if the release force is too high at the point the label should be dispensed, there is a risk that the label will simply remain on the release liner.
If it is too low, then the labels may fly off the liner within the labeling machine before they reach the bottle. Only if the release force is within a narrow range will the labels properly dispense onto the bottle.
The release profile of a given label laminate need not be a ‘fixed’ set of values that cannot be changed. By modifying the silicone – specifically be modifying the rheology of the cured silicone rubber – it is possible to change the way in which it ‘releases’ the PSA and thus change the release profile.
This makes it possible to modify the release profile to suit the requirements of where the labels are to be used, as well as to suit the characteristics of different types of PSA, such as hotmelt vs water-based, acrylics vs rubber based and so on.
Typically the target is to reduce the release force at high peel speeds, making it easier to convert the laminate, and this can be achieved by ‘flattening’ the release profile. Note that when the release forces at higher peel speeds are reduced, this often coincides with an increase in release force at lower peel speeds.
This effect is shown in Figure 3.13, where modification of the silicone release coating has led to a change in the release profile of the laminate.
SILICONE TESTING – COVERAGE AND CURE
A. Coverage: As mentioned earlier, an important factor in determining how a silicone coating may influence the release performance of a given laminate is the silicone coverage. This is simply a measure of how well covered the paper or film surface is by silicone. The reason that silicone coverage is so critical is simple: wherever there is no silicone, the PSA will come into contact with the base substrate and will happily stick to it.
In the case of a paper base substrate this can be particularly challenging since the PSA may be mobile enough not only to come into contact with the base paper but even to penetrate into the paper structure and bond even better to the paper. This can even lead to situations where there is no longer any ‘release’ at all and only by tearing the face or base paper can we separate the laminate, meaning a sticky mess which will no longer release at all.
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Since the release force of a laminate can be so sensitive to silicone coverage, it is an important quality check during the production of silicone release liner to make sure that the surface is fully covered by silicone.
The simplest solution would be of course to coat much more silicone, but this would be an expensive solution, so the focus is always on trying to optimize the coating process as far as possible to use the least amount of silicone and still maintain excellent coverage.
FINAT testing methods for silicone coverage include stain tests and optical measurements (Figure 3.14).
The stain test is one where a colored stain or dye solution is applied to the silicone coated surface, designed to color the substrate but not the silicone. These stain/dye solutions are ideal for SCK and Glassines which are easily stained, but have only very limited use for clay coated papers (CCK), and are not suitable at all for filmic substrates or even PEKs.
Specialist optical techniques are available which use polarized light to differentiate between silicone and substrate to help identify defects in the silicone coating and variations in silicone coat weight. This method is better suited to filmic substrates and PEKs, due to their specific optical properties. These systems have the advantage that they can be set up for in-line process measurements on a moving web.
B. Cure: The other important silicone-based factor that can affect release force is how well cured the silicone release coating is. Not fully cured silicone can mean problems with release stability over time as well as migration of unreacted silicone polymers into other materials such as the PSA or the label surface prior to printing.
Measurement of silicone release coating cure can be achieved either directly or indirectly.
Direct testing involves submerging the silicone coated substrate into a solvent (MIBK) and extracting any of the silicone polymers that are not cross-linked. If the level of extract is below around five percent, this normally indicates a stable coating in terms of silicone cure.
Indirect testing is where we measure the impact of migration of unreacted silicone from the release coating into a PSA that has been in contact with the silicone surface. This is referred to as the ‘Subsequent Adhesive Strength’ test (SAS test).
According to the FINAT test methods the Subsequent Adhesive Strength test involves taking a self-adhesive tape and applying it to the silicone release liner for a certain period. The strip of PSA-tape is then peeled off the silicone surface and applied to another, standardized surface such as glass, steel or PET.
The PSA tape is then peeled away from the ‘standard’ surface and the peel force compared, as a percentage, to that of a freshly applied strip of PSA-tape that has not been in contact with silicone. If the ‘SAS’ value falls below a level of 85 percent, it indicates that there has been some contamination of the adhesive by unreacted silicone, showing that the silicone cure was insufficient.
Release liners are a key component in medical products’ overall development and performance, for example, transdermal drug delivery systems, medical devices, advanced wound care dressings and pharmaceutical packaging products.
Release liners are required to offer consistent release performance and inertness in the end-use application while simultaneously adhering to increasingly strict regulatory criteria around performance, quality, purity and cleanliness.
It is imperative that careful evaluation and analysis be conducted to ensure the most appropriate release liner is chosen for a specific drug or medical product under development.
Image Credit: Shutterstock
Adhesive formulations may contain a range of wetting aids, viscosity modifiers, tackifiers, fillers and other additives, any of which could affect release performance.
The drug loaded into the adhesive adds further unknowns and complications, meaning that it is important to determine the compatibility of the release liner materials with the pharmaceutical ingredients as early as possible.
The sheer range of factors involved in this process means that it is advisable to evaluate several candidate release liners as part of the medical liner selection process.
Figure 2. Schematic views of liners holding (a) a drug-loaded adhesive product, and (b) drug-filled reservoir. Image Credit: IEEE Globalspec.
Release liners fulfill several key roles in medical device manufacturing and pharmaceutical packaging – release liners package and protect a diverse array of products (Table 1).
Table 1. Release liner applications in drug delivery, medical device and pharmaceutical packaging products. Source: IEEE Global Spec
Medical / Pharmaceutical Applications Product Examples Diagnostic & Point of Care Diagnostic Test Strips (blood glucose, cholesterol, urinary tract infections)They often act as process carriers, allowing easier handling, conversion and assembly by ensuring that adhesive medical products do not stick together.
Release liners are comprised of a release coating on a paper or cloth substrate or a plastic film (Figure 4) – the latter being the most commonly used in medical release liners.
Figure 4. Sketch of medical release liner construction. Image Credit: IEEE GlobalSpec
These medical release liners are manufactured to precise coating weights on high-speed web coating or roll-to-roll machines in cleanroom environments. Once it has been deposited onto the substrate, UV light is used to set or dry the release liner coating.
The plastic films and substrates utilized in medical release liners are generally polyvinyl chloride (PVC), polyethylene (PE), polyester (PET), polypropylene (PP) or made from various fabrics.
It is important that the liner substrate offers appropriate mechanical properties such as elongation percentage, tensile strength and tear resistance in order to avoid breaks during handling and conversion processes. The film must be cut cleanly during any slitting, die-cutting and related converting steps.
Polyethylene and polyester substrates typically offer improved tear resistance. Polyethylene and polyester carrier films tend to be available in wider gauge ranges (100 to and 92 to gauge, respectively) than polystyrene and polyvinyl chloride films (~800 to gauge).
The release liner substrate must be rigid enough that healthcare professionals or patients can easily peel off the liner. Polyester typically offers improved die-cutting and kiss-cutting performance over PP, PVC and PE.
Some applications may require consideration to be given to other properties such as thermoformability, optical transmission or optical clarity.
For example, the optical clarity of the release may impact a medicine-filled dressing’s ability to be properly positioned over a wound while still allowing this to be monitored for any allergenic response.
Other designs may require the creation of a reservoir well by thermoforming the substrate material. Both polystyrene and polyvinyl chloride films can be thermoformed, while polyester substrates offer exceptional smoothness and optical clarity.
Table 2 details thicknesses, key properties and application suitability for a range of films employed in VERSIV ™ medical grade release liners.
Table 2. VERSIV ™ medical release liner film types, liner gauge availability and their key properties for drug delivery, medical device and pharmaceutical packaging applications. Source: Versiv Composites Limited
Film Gauge Application Key Properties Polyester (PET) 92 - Pharmaceutical, Toiletries, Health & Beauty* Represents typical performance properties and should not be used for specification purposes.
A release liner’s functionality greatly depends on the release coating, which provides its non-stick or release characteristics. Silicone and fluorosilicone are the most used release coatings for medical release liners.
Release liners must possess consistent coating thickness levels to avoid silicone skip (too thin) and over-coating of silicone (too thick). Inconsistencies in coating thickness may result in issues with dispensing, patch or device movement and conversion.
It is important that the medical release liner avoid pre-dispensing – the liner must retain the drug delivery product or medical device until the liner is manually peeled off.
Release coatings may be UV-cured or thermal-cured. An optimum release coating will exhibit slow degradation and long shelf life, coupled with a high degree of chemical stability. It must also maintain consistent release properties throughout the medical product’s lifetime.
UV-cured release coatings offer a warranty and shelf life of 12 months, while thermal-cured coatings offer a 6-month warranty and shelf life.
Both UV- and thermal-release coatings offer good optical clarity, while release coatings may be water-based and solvent-free, solvent-based or completely solid.
It is possible to adjust a release liner’s release properties from easy, to moderate, to tight – while an easy release liner may be removed using minimal low peel force. A tight release liner necessitates the use of a higher peel force in order to remove the adhesive medical product.
Numeric peel or release force values are provided in force per length (g/in, cN/m or N/m). These values can be determined through the measurement of peel force at a controlled peel rate (in/min, cm/min).
Medical release liners can be provided with peel release levels between 5 to 200 g/in. Differential release liners feature release coatings on either side, which possess varying levels of release – a potentially useful feature for more sophisticated multilayer medical product constructions.
Single-coated release liners are suitable for the majority of medical applications, such as wound care dressings, transdermal drug delivery patches, surgical tape and electrodes.
The easy release is ideal for applications that use a soft silicone gel adhesive or delicate drug-loaded adhesive, as this helps reduce any risk of damage to the gel or adhesive.
Adhesives are viscoelastic materials, meaning that the peel or stripping rate can influence the effective release force.
A release profile can be used to plot the release force for a range of peel rates, though it is recommended to ensure a flat release profile so that release force is consistent when working with changing manufacturing processes or differing end-use peel forces.
Release properties can also be impacted by peel speeds, the adhesive used, temperature, time at temperature (Keil aging) and surface roughness.
Each manufacturer employs its own distinct release-level designations and test methods, so working with the same supplier throughout the selection process is recommended.
The adhesive must not transfer to the release coating or otherwise be retained, and the release coating must not transfer to the adhesive. The release coating is required to keep the adhesive tacky because any transfer could inhibit the adhesive’s stickiness or impair drug delivery.
The release liner is discarded in the majority of applications, but specific medical applications may necessitate its reuse, repositioning or reapplication, for example, sensors or TENS electrodes. In these applications, it may be necessary to peel an adhesive-coated medical device on and off the release liner.
A range of release coatings can be provided to accommodate specific medical adhesives or drug-loaded adhesives.
For example, silicone release coatings are suitable for use with polyisobutane (PIB), acrylic (polyacrylate) and polyisoprene (synthetic rubber) adhesives.
Silicone and soft silicone gel adhesives are seeing increased use in wound dressing applications and medical devices due to their gentler action, which results in less trauma on injured tissue or other delicate skin. Silicone adhesives do require the use of a fluorosilicone release coating, however.
Table 3. Suitability of silicone and fluorosilicone release coatings for several adhesives commonly used in medical applications. Source: IEEE GlobalSpec.
Medical Release Liner Coating Compatible Adhesive SiliconeA medical liner with appropriate film type, thickness, release level and other relevant properties can ensure a product’s operational and technical success, but regulatory success is a more complex issue.
A medical product is not permitted to go to market without appropriate regulatory compliance and approval.
Release liners employed in a medical product are required to meet relevant food and drug contact regulations and standards; for example, FDA CFR 177.160, USP, Ph.Eu. and EDQM.
There are also regulations like USP 467 in place that legislate a liner’s purity or levels of contaminants (including metal, monomer and solvent residuals) to ppm concentrations (Table 4).
Table 4. UPS 467 Class 2 residual solvents ppm levels. Source: Table 2 in USP 467
Solvent PDE (mg / day) Concentration Limit (ppm) Acetonitrile 4.1 410 Chlorobenzene 3.6 360 Chloroform 0.6 60 Cumene 0.7 70 Cyclohexane 38.8 1,2-Dichloroethene 18.7 1,2-Dimethoxyethane 1.0 100 N,N-Dimethylacetamide 10.9 N,N-Dimethylformamide 8.8 880 1,4-Dioxane 3.8 380 2-Ethoxyethanol 1.6 160 Ethylene glycol 6.2 620 Formamide 2.2 220 Hexane 2.9 290 Methanol 30.0 2-Methoxyethanol 0.5 50 Methylbutylketone 0.5 50 Methylcyclohexane 11.8 Methylene chloride 6.0 600 N-Methylpyrrolidone 5.3 530 Nitromethane 0.5 50 Pyridine 2.0 200 Sulfolane 1.6 160 Tetrahydrofuran 7.2 720 Tetralin 1.0 100 Toluene 8.9 890 Trichloroethylene 0.8 80 Xylene* 21.7* Usually 60% m-xylene, 14% p-xylene, 9% o-xylene with 17% ethyl benzene.
BSE/TSE requirements state that the release liner must be free of animal-based raw materials. It should also be able to withstand EtO, UV or thermal sterilization regimes designed to meet microbial limits without suffering any property degradations.
It is also important that the release coating does not transfer, leach out additives or otherwise contaminate medicine in a reservoir, a drug-loaded adhesive or monolithic matrix.
The release liner should also be able to avert medicine leakage from a drug reservoir, and it should not extract or react with any of said medicine in a reservoir or a monolithic matrix.
Manufacturers and users of medical liners must be aware of additional compliance restrictions around REACH and the liner being free of allergens, latex, phthalates and melamine.
A specific medical packaging product may be awarded permitted status or a broader approval status. FDA approval affirms testing of the raw materials for compliance. Permitted materials have been reviewed and are considered to be free of harmful effects for certain FDA categories.
Medical release liners should be able to demonstrate an FDA DMF (Type III Packaging) status before being incorporated into pharmaceutical packaging or a medical device.
A Drug Master File (DMF) includes comprehensive documentation on the raw materials, formulation, processing, testing and analysis of a medical release liner, confirming that this meets all required FDA regulations and standards.
The Drug Master File often features proprietary information, so its contents remain confidential between the FDA and the liner OEM. Any medical manufacturer OEM incorporating a release liner into a medical product will refer to the DMF for the liner in its drug application to the FDA or other notified body.
Within this reference, the release liner OEM will provide a Letter of Authorization, allowing the medical manufacturer OEM to reference the DMF numbered product as part of the drug application.
Manufacturers of medical release liners frequently partake in thorough audits of quality management systems and facilities to ensure compliant processes.
Full GMP manufacturing is not required because release liners are deemed an inactive packaging product by the FDA. Despite this, medical release liners must be manufactured in cleanrooms that are free of pests and microbial contamination.
There are several factors that must be considered when selecting a medical release liner. The release liner must be strong enough to withstand conversion, handling and assembly throughout the various stages of medical device manufacturing.
The correct degree of rigidity allows the release liner to be peeled off the pressure-sensitive adhesive with ease, while the optimum release coating type will be influenced by the desired release level, the adhesives in use and the medicine being released.
All medical release liners are required to have FDA DMF status. The use of a release liner without DMF status is not recommended, as this can impede or disrupt the regulatory approval process for a medical product.
Figure 6. Factors for medical release liner selection. Image Credit: IEEE GlobalSpec.
Medical liner selection should begin with the sampling and testing of several release liner samples in product assemblies. This should be completed using the actual adhesive or drug-loaded adhesive formulations.
Once the range of challenges around design, approval and implementation of products using a medical liner has been fully considered, a liner can be selected.
It is advisable to work with a competent, experienced release liner OEM such as Versiv Composites Limited from the beginning of the development process in order to maximize performance, reduce costs and better troubleshoot problems.
There are several recommended steps for selecting a medical release liner. These include:
The selection of a widely-recognized, reliable and robust release liner supplier is essential. The supplier should ideally offer a broad product portfolio and custom engineering capability and should be able to rapidly respond and provide a solution to any medical liner implementation issues throughout the product development process.
When a suitable, reliable, high performance medical release liner system has been selected for an application, it is also important to maintain product quality and minimize product variability by working with a single release liner OEM, such as Versiv Composites Limited.
Versiv™ is a recognized expert in high-performance, technology-driven, composite films and fabric solutions. Through our ability to provide proven products and customized solutions we serve customers across a diverse array of sectors.
Our composite materials encompass a broad range of high-performance material solutions on the market, designed and manufactured with the most demanding applications in mind. At the core of our offering is our expertise in films and fabrics composites and true versatility of the products that we offer.
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