LITERATURE REVIEW
Can coating
Many different can coatings are commercially available, but most of them are based on a limited number of chemical functionalities The epoxy-based coating has the highest market share of more than 90% However, can manufacturers and food companies have started to replace BPA-based epoxy coating with alternatives as a consequence of toxicological evidence, public discussions, and recent regulatory decisions Acrylic and polyester coating are currently used as first-generation alternatives to epoxy coating and, more recently, polyolefin and non-BPA epoxy coating were developed Further inventions include BPA capturing systems and top coating Most of the alternative coating is more expensive than epoxy coating and may not display the same array of characteristics with respect to their stability and universal applicability yet.[3]
Most cans produce annually worldwide for packing food People normally use the interior to prevent reacting with the ingredients of the filling goods and use the exterior to prevent corrosion and decoration The exterior can coating based on combinations of polyester or acrylic resins with melamine resins/phenolic resins Interior can coating based on combinations of epoxy resins with phenolic resins or PVC organosols.[4]
For many years, can coating used solvent-borne paint, but nowadays, waterborne can coating developed to reduce solvent In waterborne can coating, binders are modified epoxy resins (after partially neutralizing with amines, the resins can be dispersed in water).[5]
Environmental sustainability, cost reduction, and increased performance are key drivers in the coating industry Environmental sustainability requires a decrease in VOC emissions Cost reduction depends on reducing material usage as well as prep and application hours Increased performance results in a longer-lasting coating that can withstand greater external stressors In many markets, decreasing the VOC emissions of coating has been the highlighted area of focus and a progressive effort for scientists in this industry Replacing solvent-borne systems with waterborne dispersions is an effective strategy for reducing VOC emissions Dispersions, however, often contain processing aids such as initiator fragments and surfactants that can decrease coating performance This puts the goal of environmental sustainability in competition with the goals of cost reduction and increased performance.[6]
Tinplate consists of sheets of steel, coated with a thin layer of tin Before the advent of cheap milled steel, the backing metal was iron While once more widely used, the primary use of tinplate now is the manufacture of tin cans.[7]
Tinplate is the most common metal material used for food cans It consists of a low-carbon, mild steel sheet or strip, 0.50-0.15 mm thick, coated on both sides with a layer of tin This coating seldom exceeds 1% of the total thickness of the tinplate The mechanical strength and fabrication characteristics of tinplate depend on the type of steel and its thickness The minor constituents of steel are carbon, manganese, phosphorous, silicon, sulfur, and copper At least four types of steel, with different levels of these constituents, are used for food cans The corrosion resistance and appearance of tinplate depend on the tin coating.[8]
Steel is used primarily to make rigid cans, whereas aluminum is used to make cans as well as thin aluminum foils and coatings Nearly all steel used for cans was coated with a thin layer of tin to inhibit corrosion, and called a “tin can” The reason for using tin was to protect the metal can from corrosion by the food Tin is not completely resistant to corrosion, but its rate of reaction with many food materials is considerably slower than that of steel [9]
The strength of the steel plate is another important consideration, especially in larger cans that must withstand the pressure stresses of retorting, vacuum canning, and other processes Can strength is determined by the temper given the steel, the thickness of the plate, the size and the geometry of the can, and certain construction features such as horizontal ribbing to increase rigidity This ribbing is known as beading The user of cans will find it necessary to consult frequently with the manufacturer on specific applications since metal containers like all other materials of packaging are undergoing constant change [9]
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
Entry: Bare coils of metal are placed on an unwinder or decoiler The front end of the coil is fed up to a joiner, which will link this to the back end of the previous coil (coil splicing) Whilst most coil coating lines use a mechanical stitch to join coils, there are alternatives:
- 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
Clear: Before applying any coating, it is essential that the surface of the coil is free from impurities such as grease, oil, carbon or abraded metal particles Cleaning can consist of more than one stage and in some cases includes a rotary abrasive brush section to remove any localized corrosion products 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
In general, roller coaters are automated machines that coat one or both sides of a flat substrate with precisely controlled thicknesses of coating material Many types of formulations can be applied with roller coating but must have good flow properties (to avoid ribbing), flexibility, adhesion, and high opacity Low surface tension is also necessary for wetting the rolls
In the first stage, the metal strip passes through a roller coater machine that applies a primer paint coat to both sides of the strip In the second step, once the primer is dried, the strip passes through a second roller coater machine which applies the finish paint coat to either one or both surfaces There are a number of coater designs depending on the configuration of the coil line, the types of coating being used, and the types of metal being coated In almost every case, however, the coater is equipped with: a pan, steel or ceramic pick-up roll, rubber covered coating roll
Ingredients of can coating
Printing inks are a complex mixture of pigments, solvents, binders, and additives Different types of printers and print jobs require different types of ink As a result, there are several types of printing inks available, for example, solvent- based, water-based, and digital as well as UV inks-just to name a few of them All of them contain similar ingredients, but the types of raw material and also the quantity vary depending on the print method.[12]
Pigments are used in order to create visual effects and color impressions Solvents enhance the flow characteristics of the ink to ensure optimum processability Binders, on the other hand, keep the pigments equally dispersed and bind them to the substrate’s surface Finally, additives change the ink’s physical properties to suit different situations, e.g promoting adhesion or increasing scrub resistance.[13] Overprint varnish includes resins (binders), solvents, and additives
Material, either solid or semi-solid, uses as a printing-ink binder in ink vehicles to assist the ink pigment in adhering to the substrate Resins also determine the ink qualities of gloss, hardness, and adhesion.[14]
A polymer is a large molecule composed of repeating units called monomers, linked via covalent bonds The process which forms a polymer is called polymerization Polymerization results in polymer chains formed from monomers These polymer chains can be linked to each other via Van Der Waal forces This makes a 3D structure of a polymer Thus, they are called macromolecules.[15]
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
According to the polymerization process, polymers are divided into addition polymers and condensation polymers Polymers can be either amorphous or semi- crystalline Amorphous polymers have no ordered structure whereas crystalline polymers have well-organized structures Amorphous polymers produce transparent polymer structures whereas semi-crystalline polymers are opaque.[15]
Molecular weight and molecular weight distribution determine the properties of the polymers In general, as molecular weight increases the strength, toughness, and chemical stress crack resistance increase Lower molecular weight will typically flow easier.[16]
Epoxide resins are characterized by the presence of more than one ethylene oxide or epoxide group per molecule They are linear polymers of low to medium molecular weight requiring a co-reactant to obtain three-dimensional cross-linking
Prepared by the reaction of epichlorohydrin with diphenylolpropane (bisphenol in the presence of sodium hydroxide The degree of polymerization of the product will depend on the molar ratio of the reactants and the physical conditions prevailing.[17] The general formula is:
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 used in solvent-less coating systems and are cross- linked through the epoxide groups with aromatic and cycloaliphatic amines.[17] The solid resins are pale in color and without appreciable odor Their brittleness increases with increasing molecular weight Epoxide films are fairly resistant to alkalis and promote good adhesion to many substrates They are soluble in methyl ethyl ketone, methyl isobutyl ketone, methyl cyclohexanone, diacetone alcohol, glycol ethers, and many co-solvent systems They are compatible with certain pure phenolic, urea-formaldehyde, melamine-formaldehyde, polyamides, and vinyl resins, but incompatible with vegetable oils, alkyd resins, and cellulose and rosin derivatives.[18]
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 often protected with overprint varnish based on epoxy esters Besides conferring good abrasion and processing resistance in such things as food and chemical containers, they protect the print from any contents spilled on the outside
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 are linear if bifunctional molecules are used for the synthesis and are branched if at least one of the starting materials is at least trifunctional Linear high molecular weight polyesters are rarely used in paints and coatings because the viscosity of high molecular weight polyesters in solution is very high and, moreover, they require solvents that cannot be used in the paint industry Very high and, moreover, they require solvents that cannot be used in the paint industry Some polyester with a number average molecular weight MW>10000 are useful for coil coating combined with lower-weight reactive polyesters These polyesters are frequently based on terephthalic acid and short-chain diols such as ethylene glycol They have high viscosity, and excellent mechanical properties yet low reactivity due to two terminal hydroxyl functions for a high molecular weight polyester of M>
10000 should be noted They have high melting points, but they are essentially thermoplastic resins.[21]
The molecular weights of most polyesters used for industrial coatings range from 2000 to 6000 and they are normally cross-linked (with a crosslinking agent) or used for plasticizing various systems such as nitrocellulose Crosslinking agents would typically include aminoplasts and polyisocyanates or modified polyisocyanates Polyesters cross-linked with melamine-formaldehyde resins or blocked isocyanates are used in finishes Polyester resins are suitable for a large number of applications The anticorrosive properties of polyester are not as good as those of other types of primer, but their mechanical properties are far superior
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’-COCl → HO-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
A melamine-formaldehyde resin is a member of the aminoplast family, which encompasses urea-formaldehyde and benzoguanamine formaldehyde resins that act as cross-linking agents At a temperature above 120°C, the following reaction is observed: b With an 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 functionality in a polyester resin can react with the ether linkages of the melamine resins through transesterification reactions that lead to the formation of volatile alcohols This reaction is principally used in thermosetting liquid paints and is catalyzed by acids such as para-toluene-sulphonic acid b With a blocked polyisocyanate
(iii) With a polyisocyanate (two-component system)
The diversity of the monomers that can be used for the preparation of polyesters remains a prime asset for the progression of these products and guarantees their presence for many years to come on the paint market They can be found in water-reducible coatings, powder coatings, and UV crosslinking systems.[20]
Future of can coating
Organised retail industry has witnessed a significant growth in the recent past, which in turn has driven the demand for various packaging options in the food industry, and metal cans are no exception The proliferation of canned food and beverages in organized retail stores is backed by prolonged shelf life and preservation of the nutritional value of food products linked with canned packaging
In addition, the increasing dependence of vendors on organized retailers for their product sales and market penetration continues to underpin demand for metal cans in organized retail stores A wider range of options to choose from has led to consumer preference to shop from modern trade channels, which in turn will continue to create demand for metal cans in the near future
The increasing demand from the industry is expected to foster the growth of the can coating market Improving consumer lifestyle has led to a rise in mobile food products with longer shelf life, limited migration, thermal stability, and corrosion resistance.
The recent work on can coating
Nowadays, governmental regulations on overcoating materials and VOC emission may result in health problems More and more studies about the migration of bisphenol A from can coating led to reducing using epoxy resin on food and drink products.[33] Consumers and Health Experts say about Bisphenol A was the Bisphenol A molecule has a shape similar to estrogen’s and thus may act as an endocrine disrupter which can mimic the body's own hormones and may lead to negative health effects.[29] Epoxy can coatings are extensively used in a wide scope of applications among beverage can manufacturers In recent years, manufacturers have started substituting epoxy-based coatings with other replacements owing to toxicological presence and regulatory policies put forth by various governing agencies In 2010, Canada was one of the 1st countries to officially declare BPA as a toxic chemical Under the Canada Consumer Product Safety Act, it is illegal to manufacture, import, advertise, or sell polycarbonate baby bottles that contain BPA Acrylic and polyester coatings are currently being used as first-generation substitutes for epoxy coatings and, more recently, polyolefin and non-BPA epoxy coatings were developed Acrylic-based can coatings offer a clean appearance coupled with corrosion and sulfide stain resistance However, these coatings are brittle, with a possibility of a change in the odor and taste of foods Thus, these coatings are mostly ideal for applications in general line cans.[34]
The bio-based and biodegradable system replaces harmful artificial chemicals with safer green alternatives for corrosion protection in metal industries Some research was the development of natural lacquer to be applied on the internal and external surfaces of metal cans for foodstuffs such as the lacquer obtained from industrial tomato processing by-products (skins), bio-based coatings for food metal packaging inspired in biopolyester plant cutin [35] [36] Additionally, water-based coatings are used as an environmental solution to replace standard coatings [37] The water-based coatings products have can coating formulations that include grease resistance, mineral oil barrier, and moisture and water resistance These coatings have applications in fast food packaging, frozen food packaging, and metal packaging products and increase the shelf-life of coatings.
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
In case the resin is solid, make the varnish from solid resin before doing experiments To make varnish, we mix solid resin and suitable solvent After that, we stir at suitable speed until the solution is homogenerous
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
After making the topcoat, apply the film to the tinplate, and dry it in the oven at a suitable temperature and time The sample after drying is tested 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 applicable to the coating of substrates consisting of smooth, rigid materials such as metal or glass A uniform film of coating material is produced on a test panel by the means of 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 coating materials under test are applied at a uniform thickness to panels of sheet metal or rubber-type materials After drying or curing the coated panels are bent over a mandrel and the resistance to cracking of the coating is determined
The purpose of the test is to rate the coated material for resistance to cracking, the substrate may be any type of sheet metal or rubber-type material (for example, steel, aluminum, tinplate, or synthetic rubber) The thickness of the sheet metal may be less than 1⁄32 in (0.8 mm) and the thickness of the rubber-type materials may be as great as 1⁄2 in (13 mm) The recommended panel size is 4 in
(100 mm) in width and 6 in (150 mm) in length The maximum size that the conical mandrel can accommodate is 41⁄2 in (115mm) wide and 71/2 in (190mm) long The surface preparation of the substrate shall be agreed upon between the purchaser and the seller Prior to the application of the coating, round slightly the edges of metal panels to remove burrs in order to eliminate anomalous edge effects
- Coated panels (sample) apply uniform coatings of the materials
2.2.2.3 Conditioning and number of tests
At least 24h at 73.5 ± 3.5°F (23 ± 2°C) and 50 ± 5% relative humidity, and test in the same environment or immediately on removal therefrom unless otherwise specified by the purchaser and seller Test at least three replicate specimens
- With the operating lever of the apparatus in a horizontal position, slip between the mandrel and the drawbar with the finish side towards the drawbar Rigidly clamp the specimen in a vertical position adjacent to the mandrel by placing the long edge behind the clamping bar in such a manner that the panel is always set up to the narrow end of the mandrel Slip two sheets of No.1 brown kraft wrapping paper, substrate 30, thoroughly lubricated on each side with talc, between the specimen and the drawbar and hold in position only by the pressure of 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
- Examine the bent surface of the specimen immediately with the unaided eye for cracking Having determined and suitably marked the end of the crack farthest from the small end of the mandrel, which shall be considered as the endpoint, bring the drawbar to the starting position and 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
Apply the surface coating by appropriate means to a smooth rigid substrate and cure properly, or use representative panels cut from coated stock The panels used, the curing conditions, and the age of the coating prior to the test shall be within the limits agreed upon between the purchaser and the seller The film thickness of the coating shall be as specified or as agreed upon between the purchaser and the seller Conduct the test at 23 ± 2°C (73.5 ± 3.5°F) and 50-65% relative humidity
- For wood pencils, remove approximately 5 to 6 mm (3/16 to 1/4 in.) of wood from the point of each pencil using a draftsman-type mechanical sharpener, being careful to leave an undisturbed, unmarked, smooth cylinder of lead Holding the pencil holder (when using drawing leads) at an angle of 90° to the abrasive paper, rub the lead against the paper maintaining an exact angle of 90° to the abrasive paper until a flat, smooth, and circular cross-section is obtained, free of chips or nicks in the edge of the cross-section
- Holding the pencil holder (when using drawing leads) at an angle of 90° to the abrasive paper, rub the lead against the paper maintaining an exact angle of 90° to the abrasive paper until a flat, smooth, and circular cross-section is obtained, free of chips or nicks in the edge of the cross-section The desired edge may be obtained by cementing the abrasive paper to a flat motor-driven disk By supporting the pencil at 90° to the rotating disk a uniform flat lead end may be obtained more reproducibly
- Place the coated panel on a level, firm, horizontal surface Starting with the hardest lead, hold the pencil or lead holder firmly with the lead against the film at a 45° angle (point away from the operator) and push away from the operator Exert sufficient uniform pressure downward and forward either to cut or scratch the film or to crumble the edge of the lead It is suggested that the length of the stroke be 6.5 mm (1⁄4 in.)
- Repeat the process down the hardness scale until a pencil is found that will not cut through the film to the substrate (either metal or a previous coat) for a distance of at least 3 mm (1⁄8 in) Continue the process until a pencil is found that will neither cut through nor scratch the surface of the film Any defacement of the film other than a cut (gouge) is considered a scratch Record each endpoint (if applicable) for gouge and scratch hardness Make a minimum of two determinations for gouge hardness and scratch hardness for each pencil or lead
Figure 2.7 View of the mechanical holder with sharpened drawing lead 2.2.4 Measuring adhesion by tape test (ASTM D 3359) [41]
These test methods cover procedures for assessing the adhesion of coating films to metallic substrates by applying and removing pressure-sensitive tape over cuts made in the film If a coating is to fulfill its function of protecting or decorating a substrate, it must adhere to it for the expected service life Because the substrate and its surface preparation (or lack of it) have a drastic effect on the adhesion of coatings, a method of evaluating the adhesion of a coating to different substrates or surface treatments, or of different coatings to the same substrate and treatment, is of considerable usefulness 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
When this test method is used in the field, the specimen is the coated structure or article on which the adhesion is to be evaluated For laboratory use apply the materials to be tested to panels of the composition and surface conditions on which it is desired to determine the adhesion
- Select an area free of blemishes and minor surface imperfections, place on a firm base, and under the illuminated magnifier, make parallel cuts For coatings having a dry film thickness up to and including 2.0 mils (50 àm) space the cuts 1 mm apart and make eleven cuts unless otherwise agreed upon
RESULTS AND DISCUSSION
External clear lacquer
There are some types of resin used in metal coating, such as epoxy resin, acrylic resin, polyester resin… Epoxy resins are comparatively expensive and are only used in coating formulations when their superior properties are required These properties include good chemical resistance, outstanding adhesion to a variety of substrates, excellent toughness, hardness and flexibility, and outstanding water resistance Epoxy resins are suitable for this application because of their good exterior performance (good chemical resistance, outstanding adhesion on tinplate, excellent hardness, and flexibility)
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 resin with EEW’s of 180-475 is used mainly in 2K low temperature cure systems Epoxy resins with EEW’s in the range of 1,500-3,200 are used in stoving finishes We choose E19 because the use of epoxy resins with high MW show chemical resistance coupled with extreme flexibility
Because E19 is solid, we need to make a varnish before making a solution of topcoat We make the varnish at different solid content, by the way, stirring at 1,500 rpm until the solution is transparent Solvent viscosity varnish is influenced by the concentration of resin, the softening point, the molecular weight distribution, the chemical composition of the resin, and the type of solvent Epoxy resins are usually soluble only in highly polar solvents especially ketones, esters, and ethers Ketone solvents are considered to have good solvating power thanks to their carbonyl group, a hydrogen acceptor Like ketones, esters are also hydrogen acceptors and thus have similar solvating power Glycol ethers are usually divided into two categories: the ones based on ethylene, E-series, and the ones based on propylene, P-series P-series are considered less toxic than E-series Glycol Ether solvents have usually a slow evaporation rate, which can limit their use to some specific applications However, due to their good solvating properties, these solvents have the advantage of improving the flow and surface quality of the paint film
Table 3.2 Relationship between solid content and viscosity
Figure 3.1 Relationship between solid content and viscosity
The relation between the relative viscosity of a polymer solution and the concentration of polymer in solution shows a more gradual increase with increasing concentration In low viscosity area, the viscosity and solid content is almost linear relation, with the solid content increasing; the viscosity grows more and more rapidly The resistance to gradual deformation by shear stress increases because more space between molecules reduces In the dilute region, the polymer molecules are widely separated and not able to interact hydrodynamically In the semi-dilute region, the relative viscosity starts to change more rapidly with concentration, molecules are closer together and interact with each other In the concentrated region, further contraction of effective volume is obtained by overlap of the molecule volumes, the polymer chains are now forced close together, and intermolecular forces between them are increased
When selecting a solvent system the other components of the system must be taken into consideration, the use of esters in an amine cure system results in solvent- amine interaction which can considerably inhibit the curing reaction The viscosity of E19 in MPA is lower than in BGE and PM at different solid content, but ester solvents inhibit the reaction between epoxy and amine The way to choose the solid content is a solution with high concentration and low viscosity; high concentration helps to decrease the cost of storage Among them, 40% E19 in PM is most suitable
Epoxy resin may be considered a reactive intermediate Although the solution of very high molecular weight epoxy resins is used on its own, in this case, epoxy resin requires curing (crosslinking into a three-dimensional network) to form coatings Epoxy resin coatings obtain their excellent properties through reaction with curing agents The epoxy resins have a wide range of reactivity with multiple- choice type hardeners Curing agents for epoxies can be divided into two categories, those that give cured film at ambient temperature and those that require elevated temperature With the correct choice of cross-linking agent, resin, and modifier, the properties of epoxy resin films can be tailored to cover a wide variety of performance characteristics Heat cure involves the hydroxyl and epoxide groups of epoxy resin In the presence of a suitable catalyst, epoxy resins can be made to link with each other or with other polymers or high molecular Amino resins, aromatic amines, and phenolic resins are used in commercially available heat cure systems
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 (aminoplasts) are condensation thermosetting polymers of formaldehyde with either urea or melamine, melamine is a condensation product of three urea molecules In particular, epoxy resins cross-linked with amines hardeners have high glass transition temperatures of 150–250°C They also have excellent thermal stability and good chemical resistance The variation in viscosity during their curing is closely related to the reaction mechanism between the epoxy groups and primary and secondary amine groups In general, aromatic amines are good chemical resistance because of the presence of the phenyl group on the triazine ring of benzoguanamine Therefore, we use N659
3.1.3 The ratio of hardener to main resin
The amount of curing agent required is determined by the type and stoichiometry of the chemical reaction Where a cross-linking reaction is required, there is a calculable level of curing agent, which is necessary for the complete reaction of all available functional groups in the epoxy resin In practice, the actual level optimizes for the desired performance properties of the film, and small variations in the quantity of curing agent used can have major effects on film properties When the curing agent is catalytic in its action (e.g tertiary amines), the exact amount required must be arrived at empirically for each system and may be varied widely without significant effects on film properties When calculating amounts of curing agent the following may be taken as a guide: 1 amino hydrogen per EEW of epoxy resin Because TDS of N569 does not mention amino value, so we need to test to find the best ratio of hardener to main resin
Preparing liquid sample at different ratios of hardener to main resin (add 0.5% catalyst/total resin solid); applying the sample with 20àm thickness on substrates and drying at 200 o C in 15 minutes, and testing performance (MEK resistance, hardness by tape, flexibility by conical mandrel tester) Epoxy-aromatic amines require temperatures of up to 200 o C for complete cure and impart excellent physical and chemical resistance properties to the cured film We dry this film at
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)
Flexibility and adhesion on tinplate are stable and fit for customer’s standards when changing hardener amount However, pencil hardness and MEK resistance dramatically fluctuate
Figure 3.2 The influence of hardener amount on coating properties
The effect of polyamide/epoxy ratio on final film properties summarizes by considering the properties required Hardness and degree of cure increase with increasing epoxy content of the coating as aromatic solvent resistance at this drying condition High polyamide content increases flexibility and resistance to alcoholic solvents but the film becomes softer According to figure 3.2, the optimum hardener amount required for any formulation and application is 15% solid hardener/total solid resin (by weight) At this ratio, the pencil hardness of lacquer is 2H and MEK resistance meets the customer’s standard
Wt% solid hardener/total solid resin
Catalyst can reduce the drying time and decrease the curing temperature are possible ways to optimize productivity and costs, enhance the crosslinking or polymerization of the resins may strengthen the dry film, and offer better quality coatings The reaction between epoxy–amine system is catalyzed by weak acids which promote ring-opening by proton complication with the epoxide oxygen There are some types of commercial catalysts used 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 most widely used catalyst has been p-TSA, but some grades of p-TSA contain significant quantities of sulfuric acid, which can lead o pronounced yellowing of coating films during baking When a DDBSA catalyzed coating is used directly on metal, the adhesion of the coating is likely poor It seems probable that the sulfonic acid group on DDBSA is strongly adsorbed on the metal surface, leading to the surface becoming covered with dodecyl groups The low surface tension of the long hydrocarbon dodecyl groups on DDBSA may cause dewetting by the r of the coating or formation of a weak boundary layer that reduces adhesion The better adhesion imparted by DNNSA might be related to the presence of two sulfonic acid groups Therefore, we use C1419
To optimize the catalyst amount, we prepare a liquid sample (15% solid hardener/total solid resin) at different catalyst amounts; apply the sample with 20àm thickness on substrates and dry it at 200 o C in 15 minutes, and test performance (MEK resistance, hardness by tape, flexibility by conical mandrel tester) to find The result is below:
Table 3.6 The influence of catalyst amount on coating properties
Catalyst amount (wt%/total solid resin)
Flexibility and adhesion on tinplate are stable and fit for customer’s standards 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
When increasing the amount of catalyst from 0 to 0.2% catalyst/total solid resin, the MEK resistance and pencil hardness are higher and higher However, when increasing the amount of catalyst from 0.2% to 2% catalyst/total solid resin, pencil hardness is stable Because of cost and duality, we choose 0.2% catalyst/total solid resin
Catalyst amount (wt%/total solid resin)
To optimize the hardness, we add wax to the formulation Waxes are classified by form as emulsions, granules, flakes, micronized powders, etc By type, there are pure natural waxes, modified natural waxes, semi-synthetic waxes, synthetic waxes According to chemical composition, there are polyethylene, polypropylene, polytetrafluoroethylene, polyamide wax… Wax additives have a lot of roles in paint: wear resistance, anti-scratch, anti-scuff; control the coefficient of friction; chemical resistance due to the stability of the wax; prevent lamination; control gloss depending on the amount added and have different matting effects We can choose PTFE to use in this system depending on the purpose and application method There are some types of commercial PTFE wax
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
There are some types of resin used in gold topcoats such as epoxy-phenolic resin, and epoxy-amino The epoxy-phenolic coating is a polyamine-cured paint that has excellent resistance against solvents, chemicals, acids, and bases and can be used as a protective coating for steel structures where chemical resistance and heat resistance up to 180 o C is required However, it is not suitable for the exterior Epoxy-amine is chosen to make external gold lacquer
3.2.1 The ratio of hardener to main resin
We choose the main resin E19, hardener N659 and catalyst C1419 similar to external clear lacquer After that, to find the ratio of hardener to main resin at 180 o C for 15 minutes, we prepare liquid sample at different ratios of hardener to main resin (add 0.5% catalyst/total resin solid); apply the sample with 20àm thickness on substrates and dry at 180 o C in 15 minutes, and test performance (MEK resistance, hardness by tape, flexibility by conical mandrel tester) According to result, the optimum hardener amount required for any formulation and application is 15% solid hardener/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)
Flexibility and adhesion on tinplate are stable and fit for customer’s standards 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
Catalyst can reduce the drying time and decrease the curing temperature are possible ways to optimize productivity and costs, enhance the crosslinking or polymerization of the resins may strengthen the dry film, and offer better quality coatings Same as external clear lacquer, we use C1419 as a catalyst for this reaction
To optimize the catalyst amount, we prepare a liquid sample (15% solid hardener/total solid resin) at different catalyst amounts; apply the sample with 16àm thickness on substrates and dry it at 180 o C in 15 minutes, and test performance (MEK resistance, hardness by tape, flexibility by conical mandrel tester) to find The result is below:
Table 3.14 The influence of catalyst amount on coating properties
Catalyst amount (wt%/total solid resin)
Flexibility and adhesion on tinplate are stable and fit for customer’s standards 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
When increasing the amount of catalyst from 0 to 0.2% catalyst/total solid resin, the MEK resistance and pencil hardness are higher and higher However, when increasing the amount of catalyst from 0.2% to 2% catalyst/total solid resin, pencil hardness is stable Because of cost and duality, we choose 0.2% catalyst/total solid resin
Catalyst amount (wt%/total solid resin)
After drying at high temperature, the epoxy-amine system is crystal-clear, so we add colorants to impart color to the solution Colorants include dyes and pigments The major difference between dyes and pigments is the particle size Dyes are much finer than pigments, so dyes are not UV stable whereas pigments are usually UV stable Pigments, on the other hand, consist of extremely fine particles of ground coloring matter suspended in liquid which forms a paint film that actually bonds to the surface it applies Pigments produce paints that are more opaque than dyes Opacity is the ability for paint to cover and hide another dried color that it has been applied over Because of the opacity, we choose dyes to add to the epoxy– amine system Based on the color, light fastness, heat stability, and type of solvents, we choose D57 (C.I Solvent Yellow 82) According to TDS, D57 has high light fastness (6-7) and heat stability (5), so it is suitable for this application
To optimize the amount of dye, we prepare a liquid sample with the same epoxy-amine system as external clear lacquer (15% solid hardener/total solid resin, 0.2% catalyst/total solid resin, 0.5% wax/total solid resin) at different ratio; apply the sample with 16àm thickness on substrates and drying at 180 o C in 15 minutes, and test performance (MEK resistance, hardness by tape, flexibility by conical mandrel tester) to find The result is below:
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 200) and the viscosity of the mixture slightly fluctuates for one month Therefore, this formulation adapts the standard from the customer and current coating product as well as the source of ingredients
In the future, the cost and properties of the coating layer can be optimized by different solvents or ingredients The effect of dye on the coating properties and durability is needed to be concerned