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Polymeric coatings are often applied in the inner surface of food metal cans acting as a barrier between food and the metal surface. They play an essential role in the preservation of food maintaining its quality as well as protecting the metal substrate from corrosion. The main components present in can coatings include resins, cross-linking agents, additives and solvents.
There are different types of can coatings, each with its unique properties and advantages. For food contact applications in metal cans, the most commonly resins used include epoxy, phenolic, polyester, acrylic, vinyl and oleoresins, and variations of them. There is no single type of resin that serves for all foods, each one is suitable for a certain type of food. The most common types include:
Initially, before the introduction of synthetic resins, oleoresins were extensively used to coat the food contact surface of metal cans. These resins of natural origin are obtained from natural gums and rosins, and then are blended with drying oils such as linseed or tung oil. The oleoresinous-based coatings present a poor adhesion to metal surfaces and require long times of curing. With the exception of specific applications, particularly nonaggressive foods (e.g., dried beans), nowadays their use is very limited due to their poor resistance to corrosion.
Vinyl coatings are widely used in the packaging industry because of their excellent flexibility and resistance to chemicals. They are known to provide excellent adhesion to metal substrates, which makes them suitable for use in can coatings. The vinyl-based coatings are commonly mixed with other type of resins such as epoxy and phenolic in order to improve their properties. With the exception of vinyl organosol coatings, generally, they are not applied directly on the metal surface because of their limited adhesion to metal substrates.
Vinylic organosols are manufactured by mixing several types of resins including poly(vinyl chloride) (PVC), epoxy, and phenolic and epoxy-phenolic resins. PVC is produced by polymerization of vinyl chloride monomer (PVM). The production of PVM involves the reaction between chlorine and ethylene to yield ethylene dichloride, then the dichloride is dehydrochlorinated to give the monomer. Organosols present very good flexibility and pack resistance. They can be found in two-piece cans (e.g., canned fish). Vinyl coatings are typically used in metal cans that will contain foods and beverages.
Polyester coatings are widely used in the packaging industry because of their excellent durability and scratch resistance. They are known to provide an excellent balance between hardness and flexibility, which makes them ideal for use in cans that will undergo frequent handling, such as beverage cans. Polyester coatings are also known to have good adhesion to metal substrates, which makes them suitable for use in can coatings. Due to the uncertainties regarding the potential adverse effects associated to BPA, polyester-based coatings are being increasingly used as a promising alternative to epoxy resins.
Polyesters are synthetized by a condensation reaction between polycarboxylic acids and polyols. Different monomers are employed in their manufacture; some of the more widely used carboxylic acids include phthalic acid, terephthalic acid, isophthalic acid, adipic acid, and trimellitic acid; and among the polyols, some of the most frequent are ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 2-methyl-1,3-propanediol, neopentyl glycol, 1,6-hexanediol, 2,2,4-trimethylpentane-1,3-diol, trimethylolpropane, and cyclohexyldimethanol.
This type of resins can be cross-linked with amino resins, phenolic resins, or polyisocyanates. In past years, polyester-polyurethane coatings are being developed; these coatings contain in their formulations polyisocyanates to aid the cross-linking of the polymer network during the curing process, resulting in a more inert coating with lower migration characteristics.
From the mechanical properties’ standpoint, polyester resins exhibit very good flexibility. In terms of food contact applications, the polyester-based coatings are not suitable for aggressive and acidic foods since they are sensitive to hydrolytic attack of the ester bond under low-pH conditions.
Polyurethane coatings are known for their excellent resistance to abrasion, impact, and chemicals. They are widely used in the packaging industry, particularly in can coatings for metal cans that will contain chemicals, adhesives, and coatings. Polyurethane coatings are known to provide excellent adhesion to metal substrates, which makes them suitable for use in can coatings.
Phenolic coatings are known for their excellent resistance to high temperatures, chemicals, and solvents. They are widely used in the packaging industry, particularly in can coatings for metal cans that will contain paints, solvents, and other chemical products. Phenolic resins (called also resoles) are very complex mixtures derived from the reaction products of phenols (phenol, cresol, tert-butylphenol) with formaldehyde and low-molecular-weight aliphatic alcohols such as butanol. They are commonly used as cross-linker in combination with other resins such as epoxy resins. These coatings are often used in food cans and are resistant to aggressive foodstuffs. They have good acid and sulfur resistance, but they impart flavor to some foods. Phenolic coatings are known to provide an excellent barrier against oxygen, water vapor, and other gases.
Acrylic coatings are known for their excellent clarity, gloss, and resistance to water and chemicals. Acrylic polymers have been used in different applications such as adhesives for food packaging, kitchenware, and coatings on different substrates. These types of coatings result from the polymerization of acrylic acid and its derivatives. The main monomer used for the production of acrylic resins is ethylacrylate. These coatings are commonly applied in the external surface; however, these coatings also have applications in the easy open ends and bodies of cans intended to contain beverages. These materials offer heat stability. Acrylic coatings are known to provide good barrier properties against oxygen and water vapor, which helps to protect the contents of the can from degradation.
Hybrid coatings are a combination of two or more different types of coatings. They are used to provide improved performance and properties compared to single coatings. Because of their good mechanical properties, high adhesiveness, and chemical resistance, these hybrid coatings are widely employed in internal coatings for metal cans for food and beverages. For example, a hybrid coating may combine the excellent adhesion and barrier properties of an epoxy coating with the excellent durability and scratch resistance of a polyester coating.
Crosslinking agents improve properties of coatings systems with creating a network into the internal polymer structure with a consequent increasing in washing and rubbing fastness. Polymer chains are linked and a network is created by crosslinking.
Our high molecular butylated phenolic resins are used as crosslinkers for epoxy and polyester systems. These products offer good compatibility with different resin systems. According to the formulation, the produced can coatings provide high elasticity and excellent chemical resistance.
Melamine formaldehyde (MF) resins are used to crosslink many industrially applied thermosetting coatings. These amino resins readily react with primary and secondary hydroxyl functional polymers, resulting in three-dimensional thermoset polymer networks. By far the most significant discovery in this resin chemistry was the unique properties, as a crosslinking agent, of hexakis(methoxymethyl)melamine (HMMM), the fully methylolated amine and methylated monomeric derivative of melamine. It is a versatile crosslinking agent for a wide range of polymeric materials, both water- and solventborne, containing hydroxyl, carboxyl amide groups. The relative reaction rate for these functional groups follows the order of – SH > – OH > – CONH2 > – COOH. A product of NewLifeChem Industries Inc is supplied in liquid form at > 98% non-volatile. It is composed predominantly of monomeric HMMM and lower levels of dimeric and trimeric analogs, which are linked either through methylene, -N-CH2-N-, or methyleneoxy, -N-CH2-O-CH2-, bridges. HMMM is also used as a crosslinking agent with polyurethanes.
The Isocyanate Crosslinker is a reactive crosslinker typically used in topcoats where high-performance coatings are needed. The Isocyanate Crosslinker will greatly increase the wear, scuff, flexibility, and chemical resistance of the coating. Recommend adding 5% Crosslinker to coating.
Blocked isocyanates, normally hexamethylene diisocyanates, unblock at around 120°C to form a strong urethane linkage. These resins have been introduced to improve stone-chip resistance, film toughness, and impact resistance without leading to a loss in hardness. Surfacers formulated in this way are designated PU-modified.
During synthesis, the reactive group is rendered inert through the formation of a relatively weak urea bond. This bond can be cleaved upon heating starting at 170 °C. NewLifeChem provides a full range of externally blocked crosslinkers to allow formulators to meet various demands of end customers.
Water based NewLifeChem crosslinkers are blocked polyisocyanate, stable and easily dispersible in water-based systems. The products can be added during production and can be used in coating and printing applications as well as booster for water-repellent treatments.
Waterborne coating systems offer manufacturing a more sustainable alternative to solvent-borne products, which means a cut in harmful emissions through gases and vapours. Waterborne coatings use water as a solvent to disperse a resin, thus making these coatings eco-friendly and easy to apply. These coatings are also known to be environment-friendly as US and European regulations require waterborne coatings to have a VOC content of less than 3.5 pounds per gallon of water. Below are some of the various types of waterborne coatings that are available in the market today:
Water-soluble paints – contain water-soluble resins whose individual molecules dissolve completely in water. The resins are usually produced via polycondensation or polymerization reactions in an organic medium, hence they mostly contain organic co-solvents like alcohols, glycol ethers or other oxygen-containing solvents that are soluble or miscible with water. The resins used include polyesters, polyacrylates, alkyds, epoxies and epoxy esters. These paints provide high gloss, a high level of corrosion protection, good pigment, wetting and stabilization.
Water-dispersible paints or colloidal coatings – contain small clusters of insoluble resin particles that are suspended in water using mechanical agitation. Small quantities of organic solvents are used as coalescing agents, which evaporate on drying. The resins used in these types of dispersion paints include vinyl propionate copolymers, vinyl acetate copolymers, acrylate-methacrylate copolymers, and styrene-butadiene copolymers and polymers. Colloidal dispersions are used mainly to coat porous materials.
Emulsions/latex paints – are quite similar to water-dispersible paints. The main difference is that the resin clusters in emulsions tend to be larger, and an emulsifier is required to keep the clusters in suspension. The resins used include styrene-butadiene copolymers, acrylics, alkyds, polyvinyl acetate, and polystyrene. These paints posses increased permeability which allows them to “breathe,” thus reducing blistering or peeling.
The key advantages to using waterborne coatings are listed below:
UV-Cured coatings speed up the beverage can production process. UV curing is a specialized process by which high intensity ultraviolet light to cure paint to a hard, durable, and chemical resistant finish in just seconds. Available in 100% solids, waterborne, and solvent based formulations, UV curing has the ability to dramatically reduce the footprint of your paint line while reducing energy costs and improving quality. UV cure coatings are unique because they use light rather than heat energy to cure and depending on the formulation can dry in less than a second upon exposure to UV light. UV cure coatings can be made non flammable and low VOC or VOC free to meet your environmental and safety needs.
NewLifeChem UV Cure coatings have notable performance features including haptic “soft feel” properties, outstanding abrasion and mar resistance, exterior durability, and adhesion to difficult plastics, glass, and metal. We have formulations suitable for both Mercury Vapor Lamp and LED Curing.
In summary, there are many different types of resins at can coatings available, each with its unique set of properties and advantages. The choice of resins will depend on the intended use of the can, the type of contents that will be stored in the can and the specific properties required for the coating. Can manufacturers and food producers must carefully consider the type of coating they use to ensure that the safety and quality of the products inside the can are not compromised.
The can coatings industry is constantly evolving, with new materials and technologies being developed to improve the safety, quality, and sustainability of can coatings. One emerging trend in can coatings is the use of water-based materials and UV-Cured coatings, which are more environmentally friendly than traditional coatings.
