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The role of adhesives in safe food packaging

Henkel : 03 July, 2014  (Technical Article)
More than 77 billion litres of milk, juice and other beverages – even some wines – were sold to customers last year in a simple box, the famous liquid carton. Around 173 billion of these cartons were filled by one of the 8700 filling machines in use around the world, many of which were built using adhesives and sealants from Henkel.
The role of adhesives in safe food packaging
For instance, safety and inspection windows can be simultaneously assembled and sealed using fast-cure flexible adhesives that do not require a primer and are free from toxic chemicals. By using new designs that allow more balanced and efficient load transfer, lighter panels and doors can also be manufactured without compromising their mechanical strength. These methods are well known in other industries, for example elevator and even aircraft manufacturing, which benefit from significant production and transportation cost savings.
Fast-cure flexible sealants allow increased output and improved product quality. Their natural adhesion surpasses standard requirements, as they are capable of bonding materials such as painted and unpainted steel, stainless steel, cast metal and glass. In addition, Henkel’s high-performance sealants comply with food regulations, making them ideal for all food processing sectors. Manufacturers of liquid cartons benefit from Henkel’s accelerated flexible adhesives and sealants, which reduce manufacturing costs and comply with the most rigorous machinery standards.
Launched recently, Loctite SI 5616 is a flexible adhesive and sealant. This two-component silicone permits fast elastic joints and seals on most materials, including glass, PMMA, stainless steel, carbon steel and painted steel. It is offered in different formats, including a new coaxial 300ml cartridge compatible with most standard manual silicone guns. The product can be easily applied in the same way as any conventional one-component flexible adhesive and sealant; however, its cure rate is much faster and significantly reduces process times.
Machinery mechanics are increasingly reliant on automated systems. The faster and more reliable the mechanisms, the higher the manufacturing output in automated lines. The most modern assembly and sealing technologies afford new design opportunities for automated systems. Widely used materials such as stainless steel can be bonded using modern engineering adhesives. Compared to welding, structural adhesives that cure at room temperature – the preferred assembly system today – do not modify material properties, nor do they create residual stresses. This leads to more durable, fatigue-resistant joints. Moreover, the bonding manufacturing processes consume less energy, the equipment used is less costly and fewer safety precautions are required in the factory.
Lightweight materials such as plastics and composites can be incorporated and joined with other materials using structural adhesives. Flanged joints in casting parts can be sealed and reinforced using anaerobic liquid gaskets, which enhances their load-transfer capability. These materials prevent fretting corrosion between the mating flanges, thus guaranteeing long-term machinery reliability.
Rollers used in automated line conveyors, paper mills, paper converting and laminating and packaging equipment can be manufactured using efficient assembly systems that operate at room temperature, thereby reducing manufacturing costs and scrap without compromising mechanical performance.
Henkel offers a comprehensive portfolio of thread sealants and liquid gaskets to solve all fluid sealing challenges. These range from low-pressure fluid-drainage and air-recirculation systems that can be sealed using high-performance yarns, to high-pressure hydraulic, lubricant and pneumatic supply connections, the threads of which require high-performance anaerobic thread sealants.
One of the latest innovations in Henkel’s thread sealant family is Loctite 55 yarn. This grade has all the advantages of PTFE tape and hemp but none of their disadvantages. Threaded connections are easily and instantly sealed, allowing part repositioning. One Loctite 55 ergonomic application container allows workers to seal around 50 times the number of threaded connections that would be feasible with one PTFE tape reel – without mixing sticky, dirty grease and hemp. Loctite 55 also complies with food-contact regulations.
Loctite anaerobics have evolved dramatically since those first early formulae. High temperature resistance of around 200C in many grades, compatibility with oily surfaces and low surface sensitivity, mean that most grades cure even on inactive surfaces such as stainless or passivated metals.
Threadlockers are well known by all machinery manufacturers, although their use is still associated with heavily loaded bolts. In terms of cost, threadlockers are as cheap as washers and significantly cheaper than other mechanical solutions such as higher-quality bolts or tab washers. They are also more efficient in ensuring bolt clamping force. Vibration is not only a source of bolt loosening; it also causes temperature changes, pressure loads, part surface settling, etc.
Fretting corrosion is another major problem that can be prevented using anaerobics. Flanges sealed with liquid gaskets not only prevent leakages and the ingress of fluids, but also reinforce joint versus local shear stresses, thereby preventing any micromovement. As transversal forces are prevented, load-transfer capability is actually enhanced without any design modifications and with no need for surface-finishing improvements.
Cylindrical joints can also be resolved using anaerobics. In this case, stress distribution is more even, preventing all typical fatigue-related failures that occur when keyways and other mechanical systems are used. Compared with welding, the metal properties are not modified by high temperatures and no residual stresses are generated. Balancing and turning processes are dramatically minimized, as parts can be aligned while the adhesive is curing. Metal removal during turning is minimal.
As one example, rollers can now be manufactured using this cheaper and more sustainable system. Anaerobic retainers make transportation rollers more reliable and simpler to manufacture. The bondline evenly transfers the dynamic loads expected during roller operation. Fatigue-related failures are thus prevented.
In addition, rollers of all sizes and functions including conveyors, lamination systems, wrapping and thermoforming rollers, converting equipment and calendering machines can be manufactured using this system. The same principle can be applied to the bearings and driving gears of such rollers, which are often subject to deterioration. Part slippage is not unusual, but can be easily prevented by simply applying a fluid anaerobic retainer with no need for any design modifications. This is a straightforward measure that makes the mechanism more reliable and avoids costly malfunctions.
Anaerobic retainers are not restricted to heavily loaded parts such as shafts, gears and rollers; bearings and static joints can also be assembled using these adhesives. Bearings that are assembled using anaerobics only are better aligned and exhibit no residual stresses. As a result of these benefits, the life time and durability of these mechanical elements increase dramatically.
Not long after anaerobics were introduced, another adhesive technology was added to the Loctite range: cyanoacrylates, or superglues. These adhesives were soon used by machinery manufacturers to bond materials such as plastics and rubbers that are used in door seals, tags and other parts. Today, Henkel cyanoacrylates are unrivalled in terms of performance and manufacturing speed, as the latest developments have increased temperature resistance by 50 percent up to 120ºC. The fixture speed is just a few seconds in most working conditions. No other commercial grade is able to equal this.
Temperature resistance is not only noticeable at close to the upper limit of 120C; these adhesives exhibit higher strength than competitor products even at temperatures of around 50 and 60C, where the resistance of most grades falls dramatically. Several food-compliant grades are available in addition to medically approved grades that have been used for decades to manufacture disposable medical devices.
Machinery often generates heavy-duty dynamic forces, heavy vibration and noise. The use of flexible adhesives in the construction of panels reduces noise and vibration transfer. However, only the most state-of-the-art structure-borne sound-deadening coatings can efficiently absorb heavy vibration. These materials can be sprayed on the metal surfaces at room temperature, which reduces manufacturing costs and manual operations.
Improved machine-construction components are not the only parts that can be manufactured at lower cost. Certain food processing and packaging machinery subcomponents such as cutting blades and stirrers also benefit from Henkel’s products. Henkel’s surface-treatment solutions reduce subcomponent manufacturing costs because they require less cutting fluid and added bactericide and at the same time offer a longer cutting-tool life. Furthermore, these subcomponents often use adhesive and sealant solutions that are subject to stringent food requirements.
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