Siliconized Polyester for Release Liner Applications

Transcription

1 Siliconized Polyester for Release Liner Applications By Richard G. Nicholson & Kyle Rhodes Elkem Silicones USA

2 The choice of substrates for siliconized release liners spans a range from glassine, to SCK, to CCK, PE/PP, and PET. Selecting the release liner is usually dependent on the application requirements. This paper will explore the needs and uses of applying silicone post-stretching onto polyester film due to it s greater dimensional stability, more consistent caliper tolerance, lower silicone coating weights, higher tensile and tear properties, and overall more consistent processing. This paper will also evaluate balancing the challenges versus the technical benefits for the different applications using siliconized polyester liner. Introduction Polyester (PET) has for many years been used as a base for silicone release liners. In-line siliconizing of PET is typically accomplished with water based (emulsion) silicone systems. These silicone release coatings can also be coated onto PET post-stretched into a wide web format (I.e. off-line) using conventional solvent or water based coating methods; such as meyer rod, direct gravure, and reverse gravure. PET offers some advantages over typical paper base materials, including: Consistent gauge, or caliper control Dust/debris free; for electronic and clean room applications Strength and tear resistance Dimensional stability No remoisturizing required Provides an ultra-smooth surface for clear-on-clear labels Good silicone coverage at reduced coat weights Off-line coating a solventless silicone system has been a challenge to converters because of the cost of higher gauge films, higher coat weights from the process, wetting, good coating quality, tension control (unwinding and rewinding), silicone backside transfer, and thermal shrinkage. Today, silicone suppliers, liner producers, PET manufacturers, and equipment manufacturers are working together to overcome these challenges; and to increase the options of PET as a more versatile silicone liner base. Caliper Control Strength & Resistance Stability Coatweight Reduction 1

3 Market Drivers and Base Paper Challenges In recent years the supply of paper as a base sheet for silicone release liners has been volatile. Closures of paper machines and paper mills, conversions from silicone base sheets to other grades, and price fluctuations have become common. Consolidations through mergers and acquisitions has reduced the number of suppliers. Less super calendered papers are being produced, with an increase in clay coated papers and clay washed base sheets. These factors, along with variations in pulp and paper processing, and surface treatments, has led to insecurity of some base sheets for use as a viable silicone release liner. With the increase in home delivery of goods and products, label use is increasing; but it is more profitable for paper producers to use their paper machines to produce the higher basis weight packaging, rather than the release liner base sheet for the relatively small label. End users (label converters and printers) continue to demand reduced costs in a highly competitive market. Paper liners are now offered in the 2mm caliper range but die retooling is expensive for the slightly lower savings due to basis weight reduction. Paper suppliers are also limited on caliper reduction. Reducing thickness of a typical SCK 2.5 mil base liner results in lower density; which in turn can lead to a reduction in tear strength, and more silicone soak in. Paper suppliers are limited in reducing caliper, and when paper companies reduce basis weights their ton per hour output is reduced and the cost per hour to run the machine is more expensive than the fiber cost reduction. As converters scramble to qualify new base sheets, volatile paper prices have led to the use of multiple base sheets for the same release liner grades and applications. This increases complexity in the manufacturing and supply chain processes. Silicone formulations may require modifications to meet these changes in base papers and surface treatments. These modifications are needed to address inhibition, hold out, anchorage, etc. According to Alexander Watson Associates North American Release Liner 2018 Market Study, Film liners continue to grow at a faster rate than the overall North American release liner market, and will slowly gain share as a result. This is largely driven by high growth within the pressure sensitive labelstock, tape, and medical markets. PET usage for liners was 14%, as compared to glassine/sck at 40%, CCK at 16%, and PCK at 12% of the release liner market in 2017, but expected to continue to grow. 2

4 Solutions As we have seen a reduction to base paper capacity in recent years, there has been an increase in polyester film capacity. Paper costs have fluctuated in recent years due to changes in pulp and energy costs; and paper machine and mill shutdowns. PET pricing has remained relatively stable as the raw material costs are less variable; and with increased global capacity. As silicone coating equipment becomes more sophisticated, lower caliper films are becoming more of an option. Lower caliper of the PET, such as 0.9 or 1.2 mil, allows more linear feet per roll, when compared to a 40# SCK at 2.4 mil; which can help in logistics and storage. As freight costs increase, coupled with a current shortage of trucks and drivers, getting more linear feet in a truckload or railcar offers significant cost reduction. A lower caliper also will reduce the MSI base cost; and can lead to less scrap in waste liner. Another economic advantage is lowering silicone consumption by reduced silicone coat weight and eliminating soak in. PET s smoother surface can lead to fewer adhesive defects, specifically for clear-on-clear labels. Caliper Caliper Tolerance Die Cuttability Web Handling Tensile/ Tear Moisture Intensivity Relative Cost 0.9 mil PET 0.9 mil Low 1.2 mil PET 1.2 mil Med 1.5 mil PET 1.5 mil High 40# SCK 2.4 mil Low/Med 40# Glassine 2.1 mil Low 44# PCK 3.4 mil High 3

5 Keys to Success Coating PET and other films can be challenging, but putting the right steps in place to make a troublefree process requires a good understanding of the chemistry and the process. The silicone formulation may need to be adjusted to match the process and the PET. Important criteria to be considered include selecting the proper silicone application method, run speed, gravure cell size, heat control, tension control during unwinding and rewinding (to reduce telescoping, especially with low caliper films), to name just a few. Ensuring a good coating quality, and a good cure is only part of the process to ensure a uniform and consistent release performance. Selecting the silicone base polymer will help shape the release profile at different release speeds, from low speeds of 3-30 meters/min for hand apply applications, to higher speeds of meters/min for automated matrix stripping and high speed automated labeling applications. A low silicone crosslink density will have a higher release force at high speed, where as a high silicone crosslink density will demonstrate a lower release force at high speed. It is dependent on the adhesive which will ultimately be in contact with the silicone, as this has the biggest impact on release force. The PET formulation and process is also important to create a uniform and consistent surface, without bagginess in lanes, or skew within the film itself. It is also important to understand if there are additives which may lead to poor anchorage, as some additives or migratory stabilizers can bloom to the surface, creating a weak boundary layer. In today s efforts to be more environmentally friendly, some percentage of recycled content may be used in the PET in some cases, and should be well understood by all parties to determine what impact this will have to the performance of the PET, anchorage of the silicone, and any transference of unwanted chemicals by contact onto printable surfaces. Post usage recycling is also something to consider in your downstream application, as we strive for sustainability with our raw materials. Pilot coaters can be a handy tool when evaluating all of these options and variables. Most major silicone suppliers have a small pilot coater available to help screen and evaluate silicone formulations. These can be evaluated with various test tapes, transfer tapes, or used as the base for wet casting adhesive drawdowns. Testing is an integral part of the evaluation. Release testing results should be evaluated at a minimum of 1 day aging before making a decision on the data. Off-machine testing should be used as a gross screening test to look for large problems (like forgetting to apply the silicone, or using the wrong paper), but most interactions need to sit for 1 day for the interface to stabilize. Accelerated aging under medium temperature (50-70 C is common), or under heat/humidity conditions for various lengths of time to replicate actual aging conditions is helpful to look for chemical interactions which may develop over time. Placing samples under kiel weights to simulate the weight of the roll is sometimes used in the accelerated aging conditions. 4

6 Case Studies A label manufacturer can utilize a number of different constructions, with many different options for both the face and the liner. Balancing the performance requirements with the look, feel, image, and cost of a label can often lead to the choice of a specific face stock and liner material. In one such case, a label manufacturer was developing a new label construct, utilizing a BOPP film facestock, with an emulsion adhesive, and siliconized liner. One of the key requirements was to have very good layflat at mild and low temperatures. Initially, multiple CCK liners were evaluated, ranging from single side coated, 2 side clay coated liners (equal clay coat weight), 2 side clay coated liners (differential clay coat weight). Due to some paper and machine remoisturization limitations, the dimensional stability was not repeatable and reproducible within a tight quality specification. Poly coated kraft (PCK) liners were considered, but were generally too thick in this particular converting application. A low/medium caliper PET was evaluated. Application of a silicone release coating was able to be coated in tandem, and still provide a good economic model. The end product met the application requirements, coated easily and quickly, and had good dimensional stability. In another case, the development of a clear on clear label required the use of a siliconized polyester liner. Clear on clear labels are designed to have maximum clarity of a clear film face matched to a clear bottle, allowing the brand owner to showcase the contents of the container. Other requirements were good moisture resistance, low or no curl during dispensing, and required a higher adhesion that provided superior tack and quick stick. For high gloss applications like these, the surface of the filmic face stock is wound under pressure, and comes into contact with the backside of the liner. A paper liner or clay coated liner will emboss a slight pattern under this high pressure, and detract from the high optical clarity required for these applications. The smoothness of the front side allows the adhesive coating to remain smooth as well, as texture in the adhesive will also cause haze or cloudiness. The selection of a PET liner allowed for the face and backside contact surfaces to remain glossy, and gave the best visual performance of the clear label. 5

7 Case Studies between web breaks. superior die cut. In some cases, having a film facestock and a paper liner work extremely well, with millions of feet manufactured each year. However, for applications requiring high release levels, or a soft oozing adhesive, the stress on the paper liner can cause a tear, web break, or a matrix break. If changing the silicone is not an option, or in combination with a silicone change to lower release force at high speed, it may become necessary to increase the toughness and durability of the liner to match. This also benefits the converter by providing a harder cutting surface, and a tighter caliper tolerance. Balancing the properties versus cost versus technical benefit versus converting ability and converting speed can be a challenge but can provide a longer run time with more labels per roll, and a longer up time for the converter A 0.9 mil PET may give an acceptable die cut, whereas a 1.2 mil PET may give a A tape manufacturer required a 1.0 mil PET to give superior tensile strength required when peeling the liner off of an aggressive solvent based adhesive. During the development of the construction, evaluations were done on an untreated polyester, a chemical treated polyester, and a corona treated polyester. The silicone formulation will often give some additional adhesion to the polyester, either thru the choice of the polymer or the choice of the crosslinker (-H groups can help react onto the surface of the polyester). A specific anchorage additive (with some version of an epoxy functional silane) can also be added to the formulation to increase anchorage if needed. Corona treatment just prior to silicone application onto the film can help, even if only a low kv bump. This will help increase the surface energy, and improve the wet out of the silicone onto the surface. The general recommendation is to start with the simplest system, and then start adding in treatments or additives only if needed for the application. 6

8 Silicones Solutions The silicone solution should be flexible and have products to address the release liners requirements for the final construction. Consideration should be made for the market of the final product. This could be labels, graphic films, tapes, hygiene, medical, industrial, envelopes, food contact, or another applications. Each of these markets has specific needs and requirements. The polymer type, ranging from vinyl end blocked to vinyl on the side chain, can vary from low to high vinyl, and give a variable level of cross link density within the cured silicone. Polymer choices can also include thermal cured systems such as platinum addition or tin condensation cures, as well as radiation curable systems with either acrylate or epoxy functionality. Each cure chemistry offers various benefits, which can be discussed with the silicone vendor. The crosslinker choice will also be important, as this selction can be a tradeoff between anchorage and reactivity/cure speed, depending if a homopolymer or copolymer crosslinker is selected, as well as length of the crosslinker molecule. Various additives can be employed, ranging from anchorage additives to improve adhesion to the polyester surface, antimist additives to improve run speeds, controlled release additives to help increase the release forces as needed, and other processing aids to improve rewinding, especially for two side coated products. The silicone vendors technical service groups can provide guidance to help navigate the various polymer and chemistry choices. Experience within various markets, applications, processes, and anticipating customer needs helps make the silicone selection easier. A strong R&D team will be able to work with you if custom silicones are required, and to stay at the forefront of developing technology. A strong customer service and logistics team will help ensure on-time delivery, or to smooth out any wrinkles along the way. Conclusion The selection and choice of the liner for a silicone release system will often be dictated by the requirements and needs of the application. Polyester base materials will give better dimensional stability in various environments, maintain a tighter caliper tolerance, higher durability, and can lead to a more consistent product. The silicone choices can be tailored to match the application, and still provide an economical option for manufacturers. To talk with one of our technical specialists about your application, call Elkem Silicones at (866) , or visit us at or 7

9 Elkem Silicones Elkem Silicones is one of the world s leading fully integrated silicone manufacturers with applications and research laboratories, production sites and sales offices located around the globe. At Elkem Silicones, we re more than just high quality silicone products and associated services. We are a team of professionals located around the globe ready to provide you with the service and performance you deserve with a personal touch. From technical support to customized formulations and regulatory support, Elkem Silicones has the people in place when and where you need them, committed to help you deliver your potential. Because we care! The information contained in this document is given in good faith based on our current knowledge. It is only an indication and is in no way binding, particularly as regards infringement of or prejudice to third party rights through the use of our products. Elkem Silicones guarantees that its products comply with its sales specifications. This information must on no account be used as a substitute for necessary prior tests which alone can ensure that a product is suitable for given use. Determination of the suitability of product for the uses and applications contemplated by users and others shall be the sole responsibility of users. Users are responsible for ensuring compliance with local legislation and for obtaining the necessary certifications and authorizations. Users are requested to check that they are in possession of the latest version of this document and Elkem Silicones is at their disposal to supply any additional information. For detailed commercial contacts please visit our website :