LITERATURE REVIEW
Can coating
The market for can coatings features a variety of options, predominantly dominated by epoxy-based coatings, which account for over 90% of market share Due to growing concerns over toxicity and regulatory changes, manufacturers are increasingly shifting away from BPA-based epoxy coatings Current alternatives include acrylic and polyester coatings, with newer developments in polyolefin and non-BPA epoxy coatings Innovations such as BPA capturing systems and top coatings are also emerging However, these alternative coatings tend to be more expensive than traditional epoxy and may not yet match its stability and versatility.
Cans are widely produced globally for food packaging, featuring interior coatings that prevent chemical reactions with the contents and exterior coatings that protect against corrosion and enhance aesthetics The exterior coatings typically consist of blends of polyester or acrylic resins with melamine or phenolic resins, while the interior coatings are primarily made from combinations of epoxy resins with phenolic resins or PVC organosols.
For years, can coatings primarily utilized solvent-borne paints; however, the industry has shifted towards waterborne can coatings to minimize solvent usage These waterborne coatings feature binders made from modified epoxy resins, which can be effectively dispersed in water after partial neutralization with amines.
The coating industry is driven by the need for environmental sustainability, cost reduction, and enhanced performance A critical aspect of environmental sustainability is the reduction of VOC emissions, which can be achieved by transitioning from solvent-borne systems to waterborne dispersions While this shift helps lower VOC emissions, it often introduces processing aids like initiator fragments and surfactants that may compromise coating performance Consequently, the pursuit of environmental sustainability can conflict with the objectives of reducing costs and improving the longevity and resilience of coatings against external stressors.
Tinplate is made from steel sheets coated with a thin layer of tin, originally using iron as the backing metal before the availability of affordable milled steel Today, its primary application is in the production of tin cans.
Tinplate, the primary metal used for food cans, is made from low-carbon, mild steel sheets or strips that are 0.50-0.15 mm thick and coated with tin on both sides This tin coating typically constitutes less than 1% of the total thickness of the tinplate The mechanical strength and fabrication properties of tinplate vary based on the steel type and its thickness, with at least four different steel types utilized for food cans, each containing varying levels of carbon, manganese, phosphorous, silicon, sulfur, and copper Additionally, the corrosion resistance and visual appeal of tinplate are influenced by the quality of the tin coating.
Steel is mainly utilized for the production of rigid cans, while aluminum is employed for both cans and thin foils or coatings Most steel cans are coated with a thin layer of tin to prevent corrosion, known as a "tin can." This tin layer protects the steel from food-related corrosion, as tin reacts with food materials at a significantly slower rate than steel, although it is not entirely corrosion-resistant.
The strength of steel plates is crucial for larger cans that endure pressure during processes like retorting and vacuum canning Can strength is influenced by factors such as the steel's temper, plate thickness, can size and geometry, and construction features like horizontal ribbing, known as beading, which enhances rigidity Users should regularly consult manufacturers regarding specific applications, as metal packaging materials are continually evolving.
According to EN-10169:2012-06, the coil coating process is a method of applying an organic coating material on rolled metal strip substrate in a continuous process This process includes:
- Unwinding the coil and cleaning, if necessary
- Chemical pre-treatment of a metal surface (either one side or two sides)
- Single or multiple application of (liquid) paints or coating powders which are subsequently cured or laminated with plastic films
- Cooling and rewinding the same coil for shipment to a sheeter, a slitter, or a fabricator
All coil coating lines from the oldest and most basic line to the newest and most modern line have a number of common steps or processes
Bare metal coils are positioned on an unwinder or decoiler, with the front end of the coil connected to a joiner that links it to the back end of the previous coil through a process known as coil splicing While the majority of coil coating lines utilize a mechanical stitch for this connection, there are alternative methods available.
- Adhesive or adhesive tape can be used to stick one end of the coil to the other
- Welding is commonly used on other types of continuous processing lines, such as high-speed annealing and galvanizing lines
Before applying any coating, it is crucial to ensure that the coil surface is free from impurities like grease, oil, carbon, or metal particles The cleaning process may involve multiple stages, including the use of a rotary abrasive brush to eliminate localized corrosion from the substrate.
- Also, alkali cleaning is commonly used on aluminum surfaces but here, acidic cleaning can also be utilized
- Electrolytic cleaning is occasionally used on lines that process aluminum
Pretreatment: In order to ensure organic coating adheres properly to metallic surfaces, chemical conversion or pre-treatment must be applied Pre- treatment solutions are often applied by:
- Spray or with a simple roller-coaster, called a chemical coater (chemcoater)
- Immersion, which require subsequent rinsing
- The chemcoater contains a roller for the top and bottom surface of the coil that takes the solution from a tray and applies it to the coil surface
- The two most popular substrates are hot dip galvanize (HDG) and galvalume (GLUM) and both continue to demonstrate superior performance when treated with conversion treatments
Figure 1.3 Structure of coating in metal
After pre-treatment process, the painting process comprises of two stages, namely: primer application and finish coat application There are several general classes of coil coating lines:
1 One class has a single coater and oven These lines serve markets that are predominately consumer products orientated and have products constructed with cold-rolled steel (CRS), tin mill black plate (TMBP), and/or tin-free steel (TFS)
2 The lines with two or more coaters are called tandem lines They have the ability to apply a primer and a topcoat in a single pass These lines predominately serve the construction and appliance markets
3 There also exist several lines that have three coaters and three ovens They serve a highly specialized market for construction products that require a primer, a top coat, and a barrier coat As might be expected, these products are extremely weather-resistant and are generally found in highly corrosive environments
Roller coaters are automated machines designed to apply uniform coatings to one or both sides of flat substrates with precise thickness control To achieve optimal results, the coating formulations used must possess excellent flow properties to prevent ribbing, along with flexibility, strong adhesion, and high opacity Additionally, low surface tension is essential for effective wetting of the rolls during the coating process.
The metal strip undergoes a two-stage coating process, starting with a roller coater machine that applies a primer paint coat to both sides After the primer dries, the strip moves to a second roller coater, which applies a finish paint coat to one or both surfaces Various coater designs exist based on the coil line configuration, coating types, and metal types, but they typically include essential components such as a pan, steel or ceramic pick-up roll, and a rubber-covered coating roll.
Ingredients of can coating
Printing inks consist of a complex blend of pigments, solvents, binders, and additives, tailored for various printers and print jobs There are several types of inks available, including solvent-based, water-based, digital, and UV inks, each formulated with similar ingredients but differing in raw materials and quantities based on the printing method used.
Pigments are essential for producing visual effects and color impressions in inks Solvents improve the flow characteristics, ensuring optimal processability, while binders maintain uniform dispersion of pigments and adhere them to the substrate's surface Additionally, additives modify the ink's physical properties for various applications, such as enhancing adhesion or boosting scrub resistance Overprint varnish is composed of resins (binders), solvents, and additives, contributing to its functionality.
Materials, whether solid or semi-solid, serve as binders in printing inks, helping pigments adhere to surfaces Resins play a crucial role in defining the ink's characteristics, including gloss, hardness, and adhesion.
Polymers are large molecules made up of repeating units known as monomers, which are connected by covalent bonds through a process called polymerization This process leads to the formation of polymer chains that can interact with one another via Van Der Waals forces, resulting in a three-dimensional structure Due to their size and complexity, polymers are classified as macromolecules.
According to the type of monomers present, polymers are of two types: homopolymers and copolymers Depending on the physical properties of polymers, there are 3 major classes:[15]
- Thermoplastics-one-dimensional chains that can be melted and reformed
- Elastomers-polymers have elastic properties
- Thermosets-three-dimensional structures that do not melt once they are formed and degrade upon heating
Polymers are categorized into addition and condensation types based on the polymerization process They can be classified as either amorphous or semi-crystalline; amorphous polymers lack an ordered structure, resulting in transparent materials, while semi-crystalline polymers possess well-organized structures, making them opaque.
Molecular weight and its distribution are crucial factors influencing polymer properties Generally, higher molecular weight enhances strength, toughness, and resistance to chemical stress cracking, while lower molecular weight allows for easier flow.
Epoxide resins consist of multiple epoxide groups within each molecule, forming linear polymers with low to medium molecular weight To achieve three-dimensional cross-linking, these resins require a co-reactant.
The reaction of epichlorohydrin with diphenylolpropane (bisphenol) in the presence of sodium hydroxide leads to the formation of a polymer, with the degree of polymerization influenced by the molar ratio of the reactants and the prevailing physical conditions The general formula for this process is established.
Epoxide resins are used extensively in the electronics, paint, and lacquer industries In printing ink manufacture, they use two-pot lacquers and inks where extreme product resistance is essential.[17]
Liquid epoxy resins are integral to solvent-less coating systems, cross-linking with aromatic and cycloaliphatic amines These solid resins exhibit a pale color and minimal odor, with increased brittleness corresponding to higher molecular weight Epoxide films demonstrate notable resistance to alkalis and ensure strong adhesion to various substrates They are soluble in solvents such as methyl ethyl ketone and glycol ethers, while being compatible with certain resins like phenolic and melamine-formaldehyde However, they are not compatible with vegetable oils, alkyd resins, and cellulose or rosin derivatives.
Epoxide resins can be cross-linked to form inert flexible films with excellent adhesion by reaction with:[19]
- Amines or reactive polyamides, when the reaction takes place with the epoxide group
- Organic acids, when esters are formed at the hydroxyl and epoxide groups;
- Isocyanates, when the reaction takes place at the hydroxyl group; phenol, urea, and melamine-formaldehyde resins
Metal-decorating inks are typically safeguarded with an overprint varnish made from epoxy esters, which enhances their durability and resistance to abrasion This protective layer is particularly effective for food and chemical containers, as it not only shields the print from spills but also ensures the integrity of the decoration during processing.
Polyesters can be defined as condensation polymers obtained by the esterification of polyols with polyacids Generally, polyester is a polymer that contains ester functionality repeated in the polymer chain.[20]
Polyesters can be categorized as linear or branched based on their molecular structure, with linear polyesters being synthesized from bifunctional molecules and branched polyesters incorporating at least one trifunctional starting material High molecular weight linear polyesters are seldom utilized in paints and coatings due to their high solution viscosity and the requirement for solvents unsuitable for the paint industry However, polyesters with a number average molecular weight (MW) greater than 10,000 can be effective in coil coating when combined with lower-weight reactive polyesters Typically derived from terephthalic acid and short-chain diols like ethylene glycol, these polyesters exhibit high viscosity and excellent mechanical properties, yet they possess low reactivity due to their two terminal hydroxyl groups.
10000 should be noted They have high melting points, but they are essentially thermoplastic resins.[21]
Most industrial coatings utilize polyesters with molecular weights ranging from 2000 to 6000, which are typically cross-linked using agents such as aminoplasts, polyisocyanates, or modified polyisocyanates These cross-linked polyesters, often combined with melamine-formaldehyde resins or blocked isocyanates, are ideal for various finishes While polyesters may not offer the same level of anticorrosive protection as other primers, they excel in mechanical properties, making them suitable for a wide array of applications.
Polyesters, principally used with melamine-formaldehyde resins, are particularly suited for stoving finishes on industrial articles, with curing temperatures range from 120°C to 180°C.[22]
* Important considerations for polyester resin:[22]
- General properties: hydroxyl value, molecular weights, aliphatic constituents.[22]
- Diols and acid chiorides: the reaction of acid chlorides with diols:
OH-R-OH + ClOC-R’-COClă ăHO-R-O-CO-R’-COCl + HCl
- Self-esterification: this reaction requires a high temperature, up to 250 o C, and is often carried out at reduced pressure
The different chemical functions available within the polyester resin can be used as discussed below:
(i) Reactions with the acid function a With an etherified melamine-formaldehyde resin
Melamine-formaldehyde resin, part of the aminoplast family alongside urea-formaldehyde and benzoguanamine formaldehyde resins, serves as an effective cross-linking agent When exposed to temperatures exceeding 120°C, it undergoes a notable reaction with epoxide resin.
This reaction is extensively used in powder coatings the following reaction takes place:
(ii) Reactions with the hydroxyl function a With an aminoplast (melamine-formaldehyde or urea-formaldehyde resins)
The hydroxyl groups in polyester resins can engage in transesterification reactions with the ether linkages of melamine resins, resulting in the release of volatile alcohols This process is primarily utilized in thermosetting liquid paints and is catalyzed by acids like para-toluene-sulphonic acid, particularly in conjunction with a blocked polyisocyanate.
(iii) With a polyisocyanate (two-component system)
The wide variety of monomers available for polyester production is a key advantage that ensures the longevity and relevance of these materials in the paint industry Polyesters are utilized in various applications, including water-reducible coatings, powder coatings, and UV crosslinking systems.
Future of can coating
The organized retail industry has experienced substantial growth recently, leading to an increased demand for diverse packaging solutions in the food sector, including metal cans The rise of canned food and beverages in retail stores is supported by their extended shelf life and the ability to preserve the nutritional value of food products, making canned packaging a popular choice among consumers.
The growing reliance of vendors on organized retailers for product sales is driving the demand for metal cans in these retail environments As consumers increasingly prefer the diverse options available through modern trade channels, this trend is expected to sustain and enhance the demand for metal cans in the foreseeable future.
The can coating market is poised for growth due to rising industrial demand Enhanced consumer lifestyles are driving the popularity of mobile food products, which require coatings that offer longer shelf life, limited migration, thermal stability, and corrosion resistance.
The recent work on can coating
Government regulations on overcoating materials and VOC emissions are raising health concerns, particularly regarding bisphenol A (BPA) migration from can coatings Studies indicate that BPA, which resembles estrogen, may act as an endocrine disruptor, potentially leading to negative health effects As a result, beverage can manufacturers are increasingly replacing epoxy-based coatings due to toxicity concerns and regulatory pressures Canada was among the first countries to classify BPA as a toxic chemical, prohibiting its use in polycarbonate baby bottles Currently, acrylic and polyester coatings serve as first-generation substitutes, while newer polyolefin and non-BPA epoxy coatings have been developed Although acrylic coatings provide a clean appearance and resist corrosion, they are brittle and may alter the taste and odor of food, making them more suitable for general line can applications.
The bio-based and biodegradable systems are revolutionizing corrosion protection in the metal industry by replacing harmful artificial chemicals with safer, environmentally friendly alternatives Research has focused on developing natural lacquers derived from industrial tomato processing by-products for use on metal cans that store food Additionally, bio-based coatings inspired by plant cutin and water-based coatings serve as sustainable solutions, offering formulations that provide grease resistance, mineral oil barriers, and moisture protection These innovative coatings are particularly beneficial for fast food, frozen food packaging, and various metal packaging products, effectively extending their shelf life.
EXPERIMENT AND MATERIALS
Experiment procedure
No Name Source Non volatile(%)
3 C1419 blocked DNNSA (Dinonylnaphthalenesulfonic acid) United States 47.8
Spherical, micronized polyolefin wax, coated with PTFE Germany 20
7 MPA Methoxy Propyl Acetate China 0
8 BGE Butyl Glycol Ether China 0
To create varnish from solid resin, first ensure the resin is in a solid state Combine the solid resin with an appropriate solvent, then stir the mixture at the correct speed until a homogeneous solution is achieved.
Figure 2.1 Process of making varnish from solid resin
After making varnish, do a mixture of topcoats Mixing varnish, solvents and additives and stirring at suitable speed
Figure 2.2 Process of making topcoat 2.1.3 Testing procedure
Once the topcoat is prepared, apply the film to the tinplate and dry it in the oven at the appropriate temperature and duration After drying, the sample undergoes testing for pencil hardness, gloss, chemical resistance, and flexibility.
Testing methods
2.2.1 Producing films of uniform thickness (ASTM D 823) [38]
This test method is designed for applying coatings to smooth, rigid substrates like metal or glass A consistent film of coating material is achieved on a test panel using a hand-held applicator blade.
- Test panels, any clean, smooth, rigid substrates, or maybe paper charts or other similar materials
- Place substrate to be coated on the smooth surface
- Place film applicator with desired gap depth on the substrate
- Pour coating in front of the gap in the pulling direction
- Pull at uniform speed (approx 25 mm/s)
- Put the applicator immediately into diluted cleaning solvent and clean with a brush
2.2.2 Resistance to cracking (flexibility) (ASTM D 522) [39]
The tested coating materials are uniformly applied to panels made of sheet metal or rubber-like substances Following the drying or curing process, these coated panels are bent over a mandrel to assess the coating's resistance to cracking.
The test evaluates the resistance of coated materials to cracking, applicable to various substrates such as steel, aluminum, tinplate, or synthetic rubber The sheet metal can have a thickness of less than 1/32 in (0.8 mm), while rubber-type materials may be as thick as 1/2 in (13 mm) A panel size of 4 in is recommended for the test.
The conical mandrel can accommodate a maximum size of 4.5 inches (115 mm) in width and 7.5 inches (190 mm) in length Surface preparation of the substrate must be mutually agreed upon by the purchaser and the seller Before applying the coating, it is essential to slightly round the edges of metal panels to remove burrs and prevent anomalous edge effects.
- Coated panels (sample) apply uniform coatings of the materials
2.2.2.3 Conditioning and number of tests
For accurate testing, specimens should be maintained for a minimum of 24 hours at a temperature of 73.5 ± 3.5°F (23 ± 2°C) and a relative humidity of 50 ± 5% Testing must occur in the same environment or immediately after removal, unless specified otherwise by the buyer or seller It is essential to conduct tests on at least three replicate specimens to ensure reliability.
To prepare the apparatus, position the operating lever horizontally and insert a slip between the mandrel and the drawbar, ensuring the finish side faces the drawbar Secure the specimen vertically next to the mandrel, aligning the long edge behind the clamping bar so that the panel is oriented towards the narrow end of the mandrel Place two sheets of thoroughly lubricated No.1 brown kraft wrapping paper, coated with talc on both sides, between the specimen and the drawbar, maintaining their position solely through the pressure exerted by the drawbar against the paper.
- Move the lever through about 180° at uniform velocity to bend the specimen approximately 135° If the purpose of the test is to measure elongation, the bend should be 15s
Inspect the bent surface of the specimen closely for any visible cracks Once you identify and mark the endpoint of the crack, which is the farthest point from the small end of the mandrel, reposition the drawbar to its starting position and carefully remove the panel from the mandrel.
2.2.3 Film hardness by pencil test (ASTM D 3363) [40]
- A set of calibrated drawing leads or equivalent calibrated wood pencils meeting the following scale of hardness:
- Mechanical lead holder, for drawing leads if used
Figure 2.4 Pencil hardness tester 2.2.3.2 Test specimens and conditions
To ensure optimal results, apply the surface coating to a smooth, rigid substrate using appropriate methods and allow for proper curing, or utilize representative panels from coated stock It is essential that the panels, curing conditions, and the coating's age before testing align with the agreed-upon parameters between the purchaser and seller The film thickness of the coating must also meet specified requirements as determined by both parties Testing should be conducted at a controlled temperature of 23 ± 2°C (73.5 ± 3.5°F) and a relative humidity of 50-65%.
To sharpen wood pencils effectively, remove about 5 to 6 mm (3/16 to 1/4 in.) of wood from the tip using a draftsman-type mechanical sharpener, ensuring the lead remains a smooth, undisturbed cylinder When sharpening drawing leads, hold the pencil holder at a precise 90° angle to the abrasive paper and rub the lead against it until a flat, smooth, and circular cross-section is achieved, free from chips or nicks.
To achieve a flat, smooth, and circular cross-section on drawing leads, hold the pencil holder at a precise 90° angle to the abrasive paper and rub the lead against it until the desired edge is obtained, free of chips or nicks For improved consistency, consider cementing the abrasive paper to a flat motor-driven disk, allowing you to support the pencil at a 90° angle to the rotating disk for a more reproducible flat lead end.
To effectively use a coated panel, position it on a stable, flat surface Begin with the hardest lead, holding the pencil or lead holder securely at a 45° angle away from yourself Apply consistent downward and forward pressure to either cut or scratch the film, or to break the lead's edge It is recommended to keep each stroke approximately 6.5 mm (1/4 inch) in length for optimal results.
To determine the hardness of a film, systematically test pencils down the hardness scale until you find one that does not cut through the film to the substrate for at least 3mm (1/8 inch) Continue testing until identifying a pencil that neither cuts nor scratches the film's surface, noting that any damage other than a cut is classified as a scratch Document the results for gouge and scratch hardness, ensuring to make at least two assessments for each pencil or lead used.
Figure 2.7 View of the mechanical holder with sharpened drawing lead 2.2.4 Measuring adhesion by tape test (ASTM D 3359) [41]
The test methods for assessing the adhesion of coating films to metallic substrates involve applying and removing pressure-sensitive tape over cuts in the film For a coating to effectively protect or enhance a substrate, it must maintain strong adhesion throughout its expected service life The adhesion quality is significantly influenced by the substrate and its surface preparation, making it essential to evaluate the adhesion of various coatings to different substrates or treatments This evaluation process is highly valuable in the industry.
Figure 2.8 Cross-cut adhesion tester
- Tape, one-inch (25-mm) wide semitransparent pressure-sensitive tape with an adhesion strength agreed upon by the supplier and the user is needed
In field applications, the test method evaluates the adhesion of coatings on the specific structure or article For laboratory assessments, the materials should be applied to panels that match the desired composition and surface conditions to accurately determine adhesion levels.
To ensure accurate testing, choose a smooth area devoid of blemishes, position it on a stable surface, and utilize an illuminated magnifier to create parallel cuts For coatings with a dry film thickness of 2.0 mils (50 µm) or less, space the cuts 1 mm apart and perform a total of eleven cuts, unless otherwise specified.
RESULTS AND DISCUSSION
External clear lacquer
Various types of resin are utilized in metal coatings, including epoxy, acrylic, and polyester resins Epoxy resins, although more expensive, are preferred for specific applications due to their superior properties These include exceptional chemical resistance, strong adhesion to diverse substrates, and remarkable toughness, hardness, and flexibility Their excellent water resistance and performance make epoxy resins particularly suitable for metal coating applications, especially on tinplate surfaces.
The most epoxy resin used in surface coating systems has EEW (Epoxide equivalent weight) between 180 and 3,200 There are some types of commercial epoxy resins:
Table 3.1 Type of commercial epoxy
No Name EEW (g/eq) MW Number of repeat unit (n)
Epoxy resins with an equivalent weight (EEW) of 180-475 are primarily utilized in 2K low-temperature cure systems, while those with EEW values ranging from 1,500 to 3,200 are ideal for stoving finishes We prefer E19 due to its high molecular weight, which provides excellent chemical resistance and exceptional flexibility.
To prepare a topcoat solution for E19, a solid varnish must first be created, which involves stirring at 1,500 rpm until the mixture becomes transparent The viscosity of the varnish is affected by several factors, including resin concentration, softening point, molecular weight distribution, chemical composition, and solvent type Epoxy resins typically dissolve in highly polar solvents like ketones, esters, and ethers, with ketones offering excellent solvating power due to their carbonyl groups Esters, sharing similar properties as hydrogen acceptors, also demonstrate good solvating capabilities Glycol ethers are categorized into ethylene-based E-series and propylene-based P-series, with the latter being less toxic While glycol ether solvents have a slower evaporation rate, limiting their applications, their effective solvating properties enhance the flow and surface quality of paint films.
Table 3.2 Relationship between solid content and viscosity
Figure 3.1 Relationship between solid content and viscosity
The relationship between the relative viscosity of a polymer solution and its concentration exhibits a gradual increase as the concentration rises In the low viscosity range, viscosity shows a nearly linear correlation with solid content; however, as solid content increases, viscosity escalates more rapidly This is due to the reduced space between molecules, which heightens resistance to shear stress In the dilute region, polymer molecules are widely spaced and lack hydrodynamic interaction As concentration enters the semi-dilute region, relative viscosity begins to change more significantly as molecules come closer together and start to interact In the concentrated region, the effective volume contracts further due to the overlap of molecular volumes, forcing polymer chains closer and enhancing intermolecular forces.
When selecting a solvent system, it is crucial to consider the interactions between components, as esters in an amine cure system can significantly inhibit the curing reaction due to solvent-amine interactions Although E19 in MPA exhibits lower viscosity compared to BGE and PM at varying solid contents, ester solvents can hinder the epoxy-amine reaction The optimal choice for solid content is a solution with high concentration and low viscosity, which not only facilitates processing but also reduces storage costs Among the options, a 40% E19 solution in PM is identified as the most suitable.
Epoxy resin acts as a reactive intermediate that requires curing to transform into durable coatings These coatings achieve their superior properties through reactions with curing agents, which can be categorized into those that cure at ambient temperature and those needing elevated temperatures By selecting the appropriate cross-linking agent, resin, and modifiers, the performance characteristics of epoxy resin films can be customized Heat curing involves the interaction of hydroxyl and epoxide groups in the resin, often facilitated by suitable catalysts, enabling the resins to bond with each other or with other polymers Commercial heat cure systems typically utilize amino resins, aromatic amines, and phenolic resins.
We can use amino resins and aromatic amines for this application There are some types of commercial hardeners for epoxy
Table 3.3 Types of commercial hardener for epoxy
Amino resins, also known as aminoplasts, are thermosetting polymers formed by the condensation of formaldehyde with urea or melamine, with melamine being derived from three urea molecules Epoxy resins that are cross-linked with amine hardeners exhibit high glass transition temperatures ranging from 150°C to 250°C, along with excellent thermal stability and strong chemical resistance The viscosity changes during curing are closely linked to the reaction between epoxy groups and amine groups, both primary and secondary Aromatic amines, particularly those containing the phenyl group on the triazine ring of benzoguanamine, are known for their superior chemical resistance, making N659 a preferred choice in applications.
3.1.3 The ratio of hardener to main resin
The required amount of curing agent in epoxy resin applications is influenced by the type of chemical reaction and its stoichiometry For cross-linking reactions, a precise quantity of curing agent is essential to fully react with all functional groups in the resin However, practical adjustments are often made to optimize the film's performance properties, as even minor changes in the curing agent's quantity can significantly impact film characteristics In cases where the curing agent acts catalytically, such as with tertiary amines, the required amount must be determined empirically for each specific system, allowing for greater flexibility without adversely affecting film properties A general guideline for calculating curing agent amounts is to use 1 amino hydrogen per equivalent weight (EEW) of epoxy resin Since the technical data sheet (TDS) for N569 does not specify the amino value, testing is necessary to establish the optimal hardener-to-resin ratio.
To prepare a liquid sample, mix different ratios of hardener to main resin, adding 0.5% catalyst based on the total resin solid Apply the mixture to substrates at a thickness of 20 µm and cure by drying at 200 °C for 15 minutes The performance of the cured film is then tested for MEK resistance, hardness using tape, and flexibility with a conical mandrel tester Epoxy-aromatic amines require high curing temperatures, up to 200 °C, to achieve optimal physical and chemical resistance properties in the finished film.
200 o C for 15 minutes The result is below:
Table 3.4 The influence of hardener amount on coating properties
Wt% hardener solid/total solid resin
Drying time to achieve MEK resistance (hour)
Flexibilityăandăadhesionăonătinplateăareăstableăandăfităforăcustomer’săstandardsă when changing hardener amount However, pencil hardness and MEK resistance dramatically fluctuate
Figure 3.2 The influence of hardener amount on coating properties
The polyamide/epoxy ratio significantly influences the final properties of coatings, with higher epoxy content enhancing hardness and cure degree while improving aromatic solvent resistance Conversely, increased polyamide content boosts flexibility and resistance to alcoholic solvents, though it softens the film Optimal performance is achieved with a hardener ratio of 15% solid hardener to total solid resin by weight, resulting in a pencil hardness of 2H and meeting customer standards for MEK resistance.
Wt% solid hardener/total solid resin
Using catalysts can significantly reduce drying times and lower curing temperatures, optimizing productivity and costs Additionally, enhancing the crosslinking or polymerization of resins can strengthen dry films and improve the quality of coatings In epoxy-amine systems, weak acids act as catalysts, promoting ring-opening through proton interaction with epoxide oxygen Various commercial catalysts are available for use in this system.
Table 3.5 Type of commercial catalyst for epoxy-amine system
3 C2500 blocked p-TSA (p-Toluenesulfonic acid) Isopropanol/n-propanol
The commonly used catalyst, p-TSA, can cause significant yellowing in coating films due to its sulfuric acid content When using DDBSA as a catalyst on metal surfaces, poor adhesion is often observed, likely because the sulfonic acid group adsorbs strongly to the metal, leaving dodecyl groups that lower surface tension This can lead to dewetting or a weak boundary layer, further reducing adhesion In contrast, DNNSA offers improved adhesion, potentially due to its two sulfonic acid groups, which is why we prefer using C1419.
To optimize the catalyst amount, we prepared a liquid sample consisting of 15% solid hardener relative to the total solid resin, varying the catalyst amounts The sample was applied at a thickness of 20 µm on substrates and dried at 200°C for 15 minutes We then tested its performance in terms of MEK resistance, hardness using tape, and flexibility with a conical mandrel tester The results are detailed below.
Table 3.6 The influence of catalyst amount on coating properties
Catalyst amount (wt%/total solid resin)
Flexibilityăandăadhesionăonătinplateăareăstableăandăfităforăcustomer’săstandards when changing catalyst amount However, pencil hardness and MEK resistance dramatically fluctuate
Figure 3.3 The influence of catalyst amount on MEK resistance and pencil hardness
Increasing the catalyst concentration from 0% to 0.2% in total solid resin enhances both MEK resistance and pencil hardness However, further increasing the catalyst from 0.2% to 2% maintains stable pencil hardness levels Due to cost-effectiveness and quality considerations, a catalyst concentration of 0.2% is preferred for optimal performance.
Catalyst amount (wt%/total solid resin)
To enhance hardness in formulations, wax is incorporated, which can be categorized into various forms such as emulsions, granules, flakes, and micronized powders Waxes are further classified by type into pure natural, modified natural, semi-synthetic, and synthetic varieties, as well as by chemical composition, including polyethylene, polypropylene, polytetrafluoroethylene, and polyamide waxes Wax additives play crucial roles in paint applications, providing benefits like wear resistance, anti-scratch and anti-scuff properties, friction control, chemical stability, prevention of lamination, and modulation of gloss and matting effects based on the quantity used For specific purposes and application methods, PTFE wax can be selected from a range of commercial options available.
Table 3.7 Types of commercial wax
No Name Chemical description Particle size
Spherical, micronized polyolefin wax, coated with PTFE
Spherical, micronized polyolefin wax, coated with PTFE
External gold lacquer
Gold topcoats commonly utilize various types of resins, including epoxy-phenolic and epoxy-amine The epoxy-phenolic resin offers exceptional resistance to solvents, chemicals, acids, and bases, making it ideal for protective coatings on steel structures that require durability and heat resistance up to 180°C; however, it is not suitable for outdoor applications In contrast, epoxy-amine is the preferred choice for external gold lacquer finishes.
3.2.1 The ratio of hardener to main resin
We selected E19 as the primary resin, N659 as the hardener, and C1419 as the catalyst, resembling an external clear lacquer To determine the optimal hardener-to-resin ratio at 180°C for 15 minutes, we prepared liquid samples with varying hardener ratios, adding 0.5% catalyst based on the total resin solid The samples were applied at a thickness of 20 µm on substrates and cured at 180°C for 15 minutes, followed by performance testing for MEK resistance, hardness via tape test, and flexibility using a conical mandrel tester The results indicated that the ideal hardener content for any formulation is 15% solid hardener relative to the total solid resin by weight.
Table 3.13 The influence of hardener amount on coating properties
Wt% hardener solid/total solid resin
Drying time to achieve MEK resistance (hour)
Flexibilityăandăadhesionăonătinplateăareăstableăandăfităforăcustomer’săstandardsă when changing hardener amount However, pencil hardness and MEK resistance dramatically fluctuate
Figure 3.7 The influence of hardener amount on coating properties
According to result, the optimum hardener amount required for any formulation and application is 15% solid hardener/total solid resin (by weight)
Wt% solid hardener/total solid resin
Using a catalyst like C1419 can significantly reduce drying time and lower curing temperatures, optimizing productivity and costs This approach enhances the crosslinking or polymerization of resins, resulting in stronger dry films and improved quality coatings, similar to the effects of external clear lacquer.
To optimize the catalyst amount, we prepared a liquid sample consisting of 15% solid hardener relative to the total solid resin, applying varying catalyst amounts The sample was applied to substrates at a thickness of 16 µm and dried at 180°C for 15 minutes Performance tests were conducted to evaluate MEK resistance, hardness using tape, and flexibility with a conical mandrel tester, with results detailed below.
Table 3.14 The influence of catalyst amount on coating properties
Catalyst amount (wt%/total solid resin)
Flexibilityăandăadhesionăonătinplateăareăstableăandăfităforăcustomer’săstandards when changing catalyst amount However, pencil hardness and MEK resistance dramatically fluctuate
Figure 3.8 The influence of catalyst amount on MEK resistance and pencil hardness
Increasing the catalyst concentration from 0% to 0.2% in total solid resin enhances both MEK resistance and pencil hardness However, further increasing the catalyst from 0.2% to 2% maintains stable pencil hardness Due to cost-effectiveness and performance, a catalyst level of 0.2% in total solid resin is preferred.
Catalyst amount (wt%/total solid resin)
After high-temperature drying, the epoxy-amine system becomes crystal-clear, prompting the addition of colorants such as dyes and pigments The key distinction lies in their particle size; dyes are finer and lack UV stability, while pigments are typically UV stable and consist of finely ground coloring matter suspended in liquid, creating a paint film that bonds to surfaces Pigments yield more opaque paints, enhancing the ability to cover underlying colors, which is why dyes are selected for the epoxy-amine system For optimal results, we choose D57 (C.I Solvent Yellow 82), known for its high light fastness (6-7) and heat stability (5), making it ideal for this application.
To optimize dye concentration, we prepared a liquid sample using the same epoxy-amine system as the external clear lacquer, consisting of 15% solid hardener, 0.2% catalyst, and 0.5% wax based on total solid resin, applied at various ratios The sample was coated onto substrates at a thickness of 16 µm and cured at 180°C for 15 minutes Performance tests, including MEK resistance, tape hardness, and flexibility using a conical mandrel tester, were conducted to evaluate the results.
Table 3.15 The influence of solvent on coating properties of external gold lacquer
E represents the difference between a given color and a different color A lower E means better color accuracy Normally, E