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| ADVANCED HEAT SEAL COATINGS FOR MEDICAL GRADE TYVEK® If, 10 years ago someone asked, "What's new in heat seal coated Tyvek®?", the answer probably would have been, "Not much, what else do you need? What we have is good enough; why change?" In 1989, Tyvek® had already been in use for sterile device packaging for about 20 years. Device manufacturers had become familiar with the benefits of Tyvek®, particularly when heat seal coated. CFC/Ethylene Oxide (88:12) was still the predominant sterilization medium, and process conditions were designed to not destroy seals or otherwise compromise the sterility of the package. Cycle times were generally slow and relatively gentle, so that seals would not be subjected to undue stresses of pressure or vacuum, that might cause seal fatigue or sterilizer creep.Actually in the early '80's change was happening in the marketplace, which began to exert pressure on packaging manufacturers and converters to improve quality and functionality of materials. Packaging processes speeded up. Sealing temperatures had to be increased and dwell times decreased. Coatings of that time had narrow windows of sealability, which were often different for each tray or pouch material. This made it difficult to establish reliable seal parameters when changing from one packaging material to another. Seal failure on package resulted when coatings lacked sufficient hot tack. Sterilization methods were also undergoing change as sterilizers began to move from CFC/EtO mixtures to different gas blends. These changes often required higher pressure or vacuum cycles to achieve comparable results. High humidity and pressure also permeated adhesive bonds, causing the seal to breakdown and weaken. Print quality was fair at best, and prone to ink-offset when wound into rolls, or stacked in lid form. Typically, H-S coatings that performed well as adhesives also had an affinity to most printing when wrapped tightly. Since medical grade Tyvek® was and is not treated for ink adhesion, it was fairly common to experience varying degrees of transfer or offset of ink to the adhesive surface. Most importantly however, package integrity served as the principal driver to change the performance of disposable device packaging. The trend toward higher process speeds and shorter delivery time resulted in many devices being transported by air and long distance trucks under a wide range of environmental conditions. Packages exposed to sub-freezing conditions, often at reduced atmospheric pressure, tend to embrittle. When subjected to shock or vibration such as what might occur in a plane or truck, seals may fracture, thereby losing sterile integrity. The examples of seal failure mentioned above describe the following ways in which package failure can be caused through adhesive limitations: Hot Creep - Poor hot adhesive strength immediately after sealing before bond sets OR Slow seal failure at high storage or sterilization temperatures due to low stress resistance. Adhesive separates cohesively, leaving residue in sealed areas. Sterilizer Creep (EtO) - Softening of adhesive by temperature, humidity and sterilant gases resulting in seal failure. Clear, smooth rounded seal areas, loss of adhesive split. Cold Creep - Embrittlement of adhesive layer at cold temperatures during transport, resulting in seal fracture. Straight, sharp line, ratchety type of seal failure. As the various modes of seal integrity failure became more evident and predictable, the needs for improvements became more clearly defined. A table of desired adhesive characteristics set the stage for a new generation of heat seal coatings. An early version of '90's adhesive technology was introduced by TOLAS as a high performance lidding material in 1988, to solve the growing need for improved creep resistance during EtO sterilization and in cold temperature transport. This product uses a high molecular weight polymer in aqueous dispersion to provide the hot strength and low temperature flexibility to satisfy both of these objectives. In its first high volume application this "material" has successfully overcome sterilizer creep in a number of high stress EtO sterilization cycles. It also provides significant improvement over conventional coatings under stress at low (sub-zero) temperature conditions. While this product represented a breakthrough in seal integrity under high stress exposure, it was still somewhat limited in a few other key performance characteristics: seal range and porosity control. Because of its high polymer content, the material required higher than normal seal conditions to achieve total adhesive fusion and 100% visual seal indication upon opening packages. The optimum seal characteristics of this coating began to encroach upon the melting range of Tyvek®, and the thermal softening range of typical tray plastics. The result was a fairly narrow seal window to achieve maximum seal quality. Sealing at too low temperatures result in spotty seal transfer, and lower stress resistance. Sealing at too high temperatures causes tray flange distortion and Tyvek® transparentization (melting). Porosity control requires very tight process conditions in the coating process. However, for its time, it provided a solution for two serious causes of seal integrity failure. This set the stage for development of a new generation of Tyvek® HS coatings, to meet the needs of the new millennium. In order to meet this challenge, we surveyed the characteristics which were considered optimum or desirable in an all-purpose Tyvek® coating. Key adhesive performance criteria, Heat Seal Coated Tyvek® Seal Versatility - Seals to most commonly used tray plastics and pouch films. Seal Range (temp.) - Wide enough to seal rapidly and achieve optimum seal strengths (1.5-3.0 lb./in width). Minimize tray or film distortion during sealing. Minimize Tyvek® melting. Peel Consistency - Provides comparable peel results regardless of material sealed to. Visual Seal Quality Indication - Solid white adhesive "split" when peeled. High gas porosity (low Gurley values) - Low as possible with an overall coating. Below 50 sec. Gurley. EtO & Radiation Sterilizable - Chemically stable. Withstand stresses of vacuum and pressure in high vacuum cycles. Humidity/moisture resistant. Low temperature flexible - Withstands impact and vibration forces during shipping by air or cold weather ground transport. From the above criteria and through extensive formulation work, the level of seal functionality has been raised well above that of conventional coatings used in the industry for several years. General purpose lidding and Form-Fill-Seal materials are now capable of satisfying these criteria better than ever. Following charts and graphics show relative performance of the new generation coatings vs. a conventional aqueous coating. Seal Range Comparison - Expanded Seal Window. See graph Humidity Creep Resistance, at elevated temperature See graph Improved Cold Flex and impact resistance. See graph Porosity Comparison. See graph Enhance visual seal quality indication - Degree of transfer. See graph Water Resistant Seals. No graph attached      Form-Fill-Seal Lidding - In response to introduction of Tyvek® 2FSTM for FFS and pouch packaging, improvements have been made in aqueous adhesive coatings, to provide a more user-friendly companion web to various films.  Lower Temperature Seals - will run on rotary sealers, at less than ½ sec. contact time with typical pouch films. Strong Seals - to flexible forming webs. Higher seal strength than typical aqueous coatings. Excellent Porosity - superior performance for pouch and header bags. High Hot Tack - ideal for high speed packaging machinery. Easy Peel Lidding - Specially developed coating for small kit packaging for use in home dispensed applications. Strong enough for sealing process, shipping and handling. Soft peel characteristics aid in ease-of-use by elderly or arthritic patients. Excellent porosity suitable for EtO sterilization without compromising seal integrity. Low static generating - Coating for sheet-fed and die cut lidding. Special aqueous coating exhibits static dissipative properties which makes sheet handling easier without sticking together. Tyvek® Lidding for Dry Heat and Steam Sterilization. - This represents a small, but growing area of medical packaging. Heat sealable Tyvek® lidding has been used for several years, primarily for packaging of implants, and other devices that can tolerate the temperatures encountered in high temperature sterilization. Special coatings, applied in zone patterns or overall, have enabled the use of Tyvek® in processes in the 121-125°C (250-260°F) range. Advantages of zone coating and resulting benefits:
In summary, I believe the answer to our opening question, "What's new in heat seal coated Tyvek®?," is, "a lot of exciting new developments." The range of specialized coatings has expanded considerably. The new "general purpose" adhesives offer the promise of superior performance throughout the life cycle of the medical package. We have raised the bar in heat seal coating! I believe we are well positioned to move into the new millennium with high-volume materials. Where do we see coatings heading as we move into the 21st century? A lot of progress has been made with films that provide excellent seal-peel performance to plain Tyvek®. These offer the potential for significant cost savings, and will consume a growing share of flexible packaging business in the next decade. However, I believe that needs for special adhesive functionality will continue to emerge and feed the demand for coated materials. As it has in the past, changes in the marketplace will determine the needs. Adhesive formulators and coaters will be there to fulfill those needs. Tyvek® is a DuPont registered trademark for its brand of spunbonded olefin. 2FSTM is a DuPont trademark. |
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