Cold seal coatings Handling and conversion guideline

Transcription

1 Brian Stephenson, Dr. Roberto Pela, Dr. Dennis Bankmann May 2018 Cold seal coatings Handling and conversion guideline Globally relevant content

2 02/11 Coldseal coatings Handling and conversion guideline Points to consider before starting a new conversion To run an efficient conversion, ensure that the structure of the final packaging material is well understood, ideally write it down with release lacquers (in case of monoweb structures), release films (in case of laminated structures), corona treatments, primers, cold seals, all in the proposed position. Ensure upfront that the cold seal is suitable for the surface of the films and the use intended and check that all components are compatible. Trials should be performed before any changes in the structure are made later on. Many types of films are available in the market which can be printed with cold seals, the majority being coex OPP (clear, white, metalized, or coated). The versatility of OPP films makes them suitable for a wide range of packaging applications. Other types of films are plain polypropylene, polyester (clear, coated, or metalized), polyethylene (clear, coated, white or PE coated paper), paper, and aluminum foil. It should be understood that the properties of the base material strongly affect the performance of the cold seal. In case of monofilm applications, release lacquers should be used to avoid transfer of cold seal to the other side of the film during storage. The choice of the right release lacquer can depend on its solvent blend, required heat resistance, gloss, release and slip properties. On monoweb applications, release lacquers should have always a minimum dry application weight of 1.5 g/m². In case of applications on laminated structures, only plain BOPP with release surfaces suitable for cold seal applications should be used. In some cases, matt release films are utilized, these having poorer release properties than glossy release films due to a greater surface area (embossed surface). The treatment level of a release film is also a key parameter affecting the performance of the printed cold seal. With treated release films, the use of harder cold seal grades is recommended to provide release from these films. Storage and handling of cold seal Cold seal products are water based and must not be allowed to freeze in storage or transport. Freeze watches on containers allow to verify correct delivery conditions. Cold seals are preferably stored between C, away from direct sunlight. If the temperature of the cold seal falls below 5 C (without freezing occurring), the material should be allowed to recover for 2-3 days at temperatures above 15 C before use. Cold seals can also be damaged by excessive temperatures: under such conditions, they can separate and latex can concentrate, causing skinning in the containers. Antifoam additives can become ineffective, or separate. Apply a first-in-first-out principle as much as possible. Copper, even in trace amounts can contaminate the cold seal both in the wet and dry state. Levels as low as 5 ppm act as a catalyst that can accelerate oxidative degradation of the natural latex in the cold seal. All substrates, machine parts and materials that can come into contact with cold seal must be of low copper content. Other metals like Iron, Manganese or Chromium, in levels as low as 100 ppm, can have a similar effect as copper in cold seal products.

3 03/11 Coldseal coatings Handling and conversion guideline Pre-press Efficient stirring of the cold seal is very important to ensure a completely homogeneous mix before printing. Ensure the cold seal is stirred before use. This can be done with propeller stirrer for around 5 to 10 minutes in Mauser containers, or for a period of 30 minutes in 1-ton containers (IBC). Suitable agitation is indicated by the surface of the liquid being in motion. After the initial stirring, the stirrer should be left on at a gentle stir during the conversion (especially if the feeding of cold seal to the Figure 1 Stirrer connected to an IBC press is applied in the feed on demand mode). 100kg and smaller containers require less time to stir, typically in the order of 10 minutes. If the cold seal has been left to stand for a period of longer than 24 hours, a further 30 minutes of vigorous stirring is recommended. The stirrer assembly should be attached to IBC tanks or drums as shown in Fig 1 and 2. Figure 2 Stirrer connected to a drum Stirrer Stirrer LIOFOL CS A recommended stirrer geometry for 1000kg IBC and 110kg Mauser containers is shown in Fig 3. The orientation of the impeller head should be in such a way that it forces down the liquid away from the impeller ensuring maximum mixing efficiency. Figure 3 Typical stirrer setup for 110kg Mauser, 1000kg IBC Recommended propeller, esp. for high viscosity products Air outlet/exhaust Air in HP25A stirrer monitor rotation Stirrer mounting screws into inner opening of IBC lid Shaft should be cut so that impeller is mm from base of tank A310 impeller Stir vigorously for 30min before use (surface of liquid should be in motion) The temperature of the cold seal should be greater than 15 C before introduction to the press. Cold seals can be printed with viscosities between 20 and 40 seconds on a BS4 cup. Occasionally, cold seal viscosity can fall out of this range due to age or storage conditions. In such cases, water can be added up to a maximum amount of 5% to dilute the cold seal to a suitable viscosity. Henkel cold seals contain long lasting and effective antifoam additives. Should excessive foaming occur, additional defoamer can be added to cold seal in the tray. Suitable antifoam additives and defoamers are mentioned on the technical data sheet. Read the other chapters for further information on foam avoidance. Cold seal containers should be used up in a single campaign.

4 04/11 Coldseal coatings Handling and conversion guideline On Press Feeding options The cold seal user can choose between two feeding options: Feed on demand (Fig 4) Recirculation (Fig 5) Figure 4 Feed on demand - For long and fast jobs Figure 5 Recirculation - For short runs Stirrer 1000kg IBC or 110kg Mauser Gravure Cylinder Gravure Cylinder & tray Level sensor Inlet Outlet Cylinder Tray Low shear peristatic or diaphragm pump Covered Cold seal reservoir Manually filled Low shear peristatic or diaphragm pump There are several advantages to use feed-on-demand, particularly when using high speed printing presses: Fresh cold seal is continually available Minimum foam build-up Reduction of manual handling Reduced packaging waste The system involves cold seal supplied in 1ton IBC containers from which the cold seal is pumped directly into the application tray via a low shear pump, such as a peristaltic or diaphragm pump. It is also possible to feed by gravity by placing the IBC at an elevated position and replacing the pump with a solenoid valve. The pump or solenoid needs to be controlled by a level sensor on the cylinder tray which activates when the level of adhesive falls to a certain point, and turns off again when the desired filling level is reached. Due to the possibility of different levels forming between running and stopped state of the press and the possibility of foam affecting level sensor control, it is recommended to modify the tray with an external tube for the sensor (Fig 6). Figure 6 External tube connected to the tray CS Sensor

5 05/11 Coldseal coatings Handling and conversion guideline The second feed option, recirculation, encompasses the use of a reservoir tank (Fig 7). Cold seal is initially filled into this reservoir and continuously circulated between reservoir and cylinder tray. The same recommendations apply with regards to the pump selection as for feed on demand. The reservoir should be large enough to give time for air bubbles to rise to the surface, typically litres. It is strongly recommended that the reservoir has a lid, ideally removable with a lip, holes cut for hoses and double hinged so that the cold seal can be readily inspected. To avoid excessive foam creation, the recirculation of cold seal into the reservoir should be set up such that the inlet Figure 7 Cold seal inlet under liquid surface is located below the liquid s surface or that the returning cold seal it led onto a gentle slope (see Fig 7 and 8). Baffles in the reservoir can help reduce foaming further. The use of a stirrer to agitate the cold seal in the reservoir can help break foam formed in the tank and to prevent skin formation. A crude inline filter on outlet of the reservoir is recommended. It must further be ensured that tubes and hoses are not natural rubber based. The suggested material for tubes is silicone rubber or polychloroprene. A good internal bore size avoids blockages. Figure 8 Cold seal inlet via ramp CS entering the reservoir from the tray Lid Baffle CS going back to the tray Reservoir Inlet Foam Baffles trap excess foam Space for the CS to move from one side to another Outlet Cylinder tray design A certain amount of foam is acceptable and does not lead to technical issues to the printed cold seal. It is important to highlight, however, that an excessive foam can be also influenced by the machine design. The size of the cylinder, as well as the dimension of the tray are two machine factors, which can determine the formation of the foam. More details regarding the design of the tray and the cylinder are summarized in Fig 9. Cylinder tray with large surface area allows foam to dissipate Large diameter cylinder reduces rotational speed and causes less foam Low liquid level can cause excessive foam because of air being drawn in and excessive shear Figure 9 Schematic diagram of cylinder tray design 45 Large gravure cylinder. Low surface speed Large area allows foam to dissipate Cylindrical tray optimizes distance and reduce volume Good filling level, contact angle ca. 45 Poor filling level Extra volume caused by straight sided tray

6 06/11 Coldseal coatings Handling and conversion guideline The critical issue with a cylinder in a tray is the point at which the outside surface of the cylinder is in the closest proximity to the tray. It is at this point that any effect due to the movement of the cylinder through the static cold seal will have the greatest influence due to this being the area where the viscous drag of the cold seal against the cylinder surface is highest and the movement of the cold seal will be greatest. In effect, the cold seal is almost pumped from one side of the tray to the other. To reduce the influence of viscous drag and to thus ensure an optimum level of cold seal in the tray it is necessary to have the maximum distance possible between the tray and the cylinder. The best way to achieve this whilst still keeping the volume of the tray to a minimum is to have a cylindrical tray: this provide greater distance between the cylinder and the tray and may actually result in a smaller volume that an equivalent straight sided design. Compared to a straight side tray, a cylindrical tray can optimize the distance between the rotating cylinder and the inner side of the tray, thus reducing the volume of the cold seal inside. As a result, the used cold seal will be fresher as the dwell time in the cylindrical tray is shorter. that there is sufficient space for the cold seal to flow under the cylinder. With this adjustment, the cylinder can still rotate in the liquid without causing excessive viscous friction in the liquid. It should also be noted that the level of the cold seal in the tray should be changed depending upon the size of the cylinder used. This will require the level sensor to have different settings for each cylinder used. The angle of entry of the cylinder into the liquid cold seal should be kept to as near to 45 degrees as possible. Other options to consider are the fitting of an overflow device so that if cold seal level rises, the material do not flow out of the tray onto the machine and can be diverted back to the tank. In addition to the fitting of the overflow, the pump should be set up so that when the press is stopped the pump no longer runs automatically, this again helps to prevent any over-filling of the tray that can then result in overflow when the press restarts. Ensure that the position of the cylinder in the tray is such Gravure cylinder design and correct use General recommendations Quadrangular etch lines per cm, 37-50µ depth A specific roughness (Rz) is desirable to reduce frictional heat build up of doctor blade on cylinder and to prevent doctor blade wear. It also aids rewetting of cold seal on cylinder Typical Rz value µ (most commonly 0.4µ) Ensure the cylinder is always turning (idling) in the cold seal when the press stops. If the cylinder stops, the cold seal can dry quickly in the cells and block the cylinder. Remember: cold seal cannot be re-dissolved. If the cylinder is blocked, it will have to be scrubbed and cleaned with suitable solvents. Figure 10 Gravure cylinder design Cylinder manufacturers can provide best guidance Diagonal Screen quadrangular cell

7 07/11 Coldseal coatings Handling and conversion guideline Doctor blade use and settings Typical doctor blade angle 60 at tangent from cylinder. Gives optimum wipe and reduces risk of blade lifting. Use correct pressure of the blade. Blade should be lifted when machine id idling. Use correct doctor blade angle (typically 60 ). Avoid extreme shear on cold seal by forcing it into too narrow angles between the blade and the cylinder. Only sufficient pressure must be used on the blade to get a good wipe. Excessive pressure can cause poor wipe. The blade must be lifted up when the press stops. Worn blades must be replaced immediately. Different blade thickness, angle and pressure can all result in different wipe and coating weight. Figure 11 Doctor blade setting A precisely adjusted Doctor Blade angle is crucial for an optimal contact zone Recommended Doctor Blade angle is between 55 and 65 Foam control and problems caused by foam Foam control is essential to ensure good printing properties and performance of all water based coatings. Foam is generated by shear and mixing actions of the machine when surfactants and/or emulsifiers are present in the liquid phase. Foam can be controlled by incorporation of antifoam both during manufacture (in the product formulation) and on press, if required. Foam can also be minimized by reducing shear and mixing action on press. The level of foam should be always checked in the tray when running at full speed. Normally, only a small level of foam should be present which does not increase over time due to air entrapment. However, if foam occurs in high amounts, it can result in: Skin and particles in the liquid, which can cause poor printing. An increase in viscosity of liquid if microfoam is created: this can affect cold seal rheology, cold seal coating weight, and cold seal transfer. Foam can mask actual levels of liquid in printing tray and can result in missing cold seal on printed film. Foam can result in ineffective level sensor operation in feed on demand systems. If foam does increase during the printing process and cold seal overflows or the coating weight is affected, an antifoam should be utilized, either in a dilute form (as a defoamer), or in its pure form (Fig. 12).

8 08/11 Coldseal coatings Handling and conversion guideline Figure 12 Correct use of foam control additives Gravure cylinder & tray Inlet Outlet Low shear peristaltic or diaphragm pump Covered cold seal reservoir manually filled Defoamer Addition to cylinder tray Defoaming action Used to knock down foam which has built up Mix typically 1 part of antifoam with 10 parts of water Spray diluted antifoam directly into the tray, then mix with a stirrer (avoid wood sticks!). Antifoam Addition to reservoir Antifoam action Used to prevent formatin of foam Add undiluted antifoam into reservoir Concentration according to technical data sheet. Cold seal application weight & print quality As in all adhesive processes, close molecular contact of adhesive films and interfaces is necessary for optimum bond performance. In cold seal technology, smooth and even printing facilitates efficient low-pressure sealing on high-speed packaging lines. The resultant cold seal film/ film coalescence promotes optimum peel strength and reduces stringing if anchorage to base film is good. It has been shown that peel strength of a cold seal bond is directly proportional to the coat weight up to ca. 6.0 g/m² if anchorage to base film is good. Figure 13 Cold seal application weight Uneven cold seal surfaces require higher seal pressures and due to uneven seal can have lower seal strength than uniform coatings of the same product. The typical application weight on monofilms is above 3.5 g/m² dry, with a release lacquer previously applied on the other side of the monofilm (the minimum application weight required to a release lacquer is 1.5 g/m²). Cold seal on laminated structures should have a coating weight above 4 g/m². Surface treatment Seal Strength, g / 25 mm Coating Weight g/m 2 Certain cold seal adhesives bond more effectively to base film surfaces rendered more polar by surface treatment (e.g. corona discharge) or to polar surface coatings (e.g. PVDC or acrylic). Other cold seals will give better adhesion to the non-polar surface of certain types of film i.e. co-extruded polypropylene. It is paramount to always choose a cold seal that matches the substrates surface properties, as the surface chemistry of the packaging films affects cold seal adhesion.

9 09/11 Coldseal coatings Handling and conversion guideline Drying Precise drying of cold seal is important to obtain full performance. If the cold seal is under dried, too much moisture remains in the coating and adhesion to base film is impaired, thus reducing overall seal strength, with excessive stringing and easy removal of the cold seal coating from the base film. Rub resistance is a good indicator of cold seal drying as poor drying will result in poor cold seal adhesion. If a cold seal is under dried, it can also potentially have blocking and transfer problems within the reel. If the cold seal coating is overdried, seal ability can be impaired and seal strength reduced by poor coalescence. It is believed that the effect is a result of reduction in cold seal film flexibility. This phenomenon is rare and is only seen with very hard cold seal formulations. Generally maximum possible drying temperatures should be used, whilst avoiding film stretching. Typical temperature settings are 70, 75, and 80 C. In a 4-zones 10 m dryer running at 200 m per minute, temperatures can be increased to 80, 85, 90 C if necessary or even higher depending upon tunnel length and speed of printing. Quite often, in a 4-zones dryer, the last zone may be lower than 3rd zone. Figure 14 Common temperature settings in drying process Drying in gradually increasing temperature zones is recommended, with temperatures depending on printing speed, size of drying tunnel and air flow, atmospheric conditions, coating weight and film choice amongst other factors. It is important that the cold seal does not form a skin when drying at a too high temperature in the first oven zone. When skinning occurs, complete drying is virtually impossible and a very uneven cold seal surface results from holes formed by steam eruptions. Such uneven cold seal surfaces dramatically reduce the initial seals. Drying efficiency can be affected by extreme climatic conditions, which influences the moisture content of the external air being drawn into the dryers and temperatures may need adjusting depending upon climatic conditions for example to overcome air saturation in driers in humid conditions. Machine adjustments may be required in extreme conditions, i.e., steam injection to overcome low humidity and improved airflow and increase temperatures to overcome high humidity. Air should not be recirculated but should be 100% fresh air. Rewind C C C Rewinding should be performed at the minimum temperature possible. The risk of blocking can be reduced throughout the shelf life of the conversion by the correct control of film tension on rewind. High film tension is a primary cause of blocking in the reel.

10 10/11 Coldseal coatings Handling and conversion guideline Post-press On the packaging line Parent reels (before slitting) should be stored in suspended form to avoid excessive pressure on any part of the reels which could lead to intermittent blocking on unwind prior to slitting. Finished slit reels should be wrapped well in polyethylene film (ideally black) and stored between 15 and 30 C at 45-60%rH, away from direct light and dust. Every effort should be made to store and transport cold seal converted reels in controlled conditions of temperature and humidity. Converted reels may be stored at least up to 6 months from production when correct printing of cold seal was performed, as well as when all the parameters like compatibility between cold seal and films, and cold seal and release lacquer in monofilm applications (or release films, in laminates) were evaluated prior to the conversion. If stored or transported at lower temperatures or humidity, cold seal coated reels are required to equilibrate to factory climate for 24 hours before use on a packaging line. On high speed packaging lines, it is recommended that films are brought to at least 15 C for 24 hours before use. The stiffness and pressure distortion properties of the packaging film (i.e. monofilm or laminate) can affect cold seal ability. Close contact and coalescence of the cold seal coating is more difficult to achieve especially at high packaging machine speeds if film stiffness is high. Low temperatures in storage and in use of such conversions can increase film rigidity and aggravate seal problems. Seal tolerance can be improved by increasing coat weight of cold seal, thus producing a thicker more easily deformable cushion to facilitate low pressure, high-speed sealing. Beside the phenomenon of contamination with copper and other metals (see Storage and handling of cold seal on page 2), other factors should be taken into consideration, which can influence the final performance of cold seal products. The performance of converted cold seal can be seriously affected and at worst destroyed by contaminants. Such contaminants include residual solvents and plasticizers from inks and release lacquers, slip additives from films and lacquers as well as excessive addition of certain deformers on-press. Migration of base film additives such as slip agents can also cause cold seal adhesion problems, as can offset of oxidized aluminum in metallized film reels before applying the cold seal. Seal ability can also be affected as a result of cross contamination from the reverse side of the packaging when stored in reel form. Gradual migration can reduce cold seal performance during the shelf life of the converted reels. These factors need to be considered carefully when choosing the packaging films and components to be converted.

11 11/11 Coldseal coatings Handling and conversion guideline Meet Your Experts Brian Stephenson, is the leader of the water-based technical team responsible for the development of LOCTITE LIOFOL water-based products in Europe. He has over 30 years experience in both the technical support and development of cold seal adhesives, lamination adhesives and other coatings used in the flexible packaging industry. Dr. Roberto Pela, an organic chemist by training, joined Henkel in 2010 and worked for a number of years as a Technical Manager in the R&D department, developing new laminating adhesives for the food packaging market with a strong focus on improved safety. In 2016, he became a Technical Service Manager for LOCTITE LIOFOL in Flexible Laminates Europe. In his current role, he supports customers by providing extensive knowledge in laminating adhesives, lamination techniques, and general processes in the food packaging business. Dr. Dennis Bankmann, a physico-organic chemist by training, is responsible for the development of LOCTITE LIOFOL products in Europe. Since 2007, he has worked on research projects for a number of different adhesive sectors before focusing exclusively on adhesives for flexible packaging and food contact materials. He and his team bring deep chemical knowledge to the interface between raw material suppliers, production, analytics and product safety. Further information Contact us! Many options available. Visit the Food Safe Packaging Premium Area offering webinars, white papers, FAQs, videos and more: Henkel AG & Co. KGaA Düsseldorf, Germany The information provided herein, especially recommendations for the usage and the application of our products, is based upon our knowledge and experience. Due to different materials used as well as to varying working conditions beyond our control we strictly recommend to carry out intensive trials to test the suitability of our products with regard to the required processes and applications. We do not accept any liability with regard to the above information or with regard to any verbal recommendation, except for cases where we are liable of gross negligence or false intention. The information is protected by copyright. In particular, any reproductions, adaptations, translations, storage and processing in other media, including storage or processing by electronic means, enjoy copyright protection. Any exploitation in whole or in part thereof shall require the prior written consent of Henkel AG & Co. KGaA. Except as otherwise noted, all marks used in this document are trademarks and/or registered trademarks of Henkel and/or its affiliates in the US, Germany, and elsewhere. Henkel AG & Co. KGaA, , 03/2018