Friday, March 5, 2021

Choosing The Right Coating For The Right Application

 


The annual cost of corrosion worldwide is equivalent to 3 to 4% of the global gross domestic product, or more than $3.0 trillion. Historically, in non-critical industries, corrosion was treated as a repair and upkeep issue. More just recently, nevertheless, preventive measures significantly decrease corrosion expenses. These procedures consist of proper product choice, cautious component design, and corrosion control.


Active corrosion control includes utilizing a sacrificial product (typically zinc) that takes part in corrosion responses instead of the metal substrate. Passive protection involves applying a barrier product that prevents corrosive reagents and water from reaching the surface of the metal substrate. These coatings and films likewise frequently offer extra protection versus impact, abrasion, and other mechanical damage.


Provided the very broad use of metals and the vast array of metal types, the performance expectations vary substantially, as do the appropriate cost/balance ratios. Various coating technologies meet the varied requirements; picking the very best coating technology for an application can be challenging. Here are numerous guidelines.
Traditional technology: epoxies
There are 2 significant coating technologies recognized for protective properties: epoxies and polyurethane-type systems, that include polyurethanes, polyureas, and hybrids of these two chemistries.


Epoxies are commonly utilized as corrosion security coatings for factory-applied metal applications since they exhibit exceptional adhesion to metals and use high moisture-, chemical-, and impact-resistance. They continue to be widely used as primers (in some cases zinc-rich) in multi-coat systems, consisting of those with different topcoat chemistries (acrylics for light-duty applications, epoxies, silicones, polyurethanes, and polyureas for medium- to heavy-duty applications). The majority of epoxy coatings utilized today are high-solids or 100%- solids formulas that fulfill strict ecological regulations concerning the emission of unstable organic compounds (VOCs).


There are, however, constraints to epoxy coatings that have actually driven interest in alternative technologies for corrosion control. In particular, epoxy coatings are not very flexible and can split in applications that involve substrate motion, high wear, or heavy impacts. They likewise do not perform well at low temperature levels (ended up being brittle) and yellow gradually in exterior applications due to destruction upon UV radiation exposure.
For these factors, polyurethane and derivative coating technologies are increasingly utilized as corrosion control coatings for OEM metal applications due to their greater flexibility integrated with high adhesion and high resistance to wetness, chemical attack, and impact.



Chemistry of polyurethanes and polyureas
Isocyanates are used to synthesize both polyurethane and polyurea resins. Polyurethanes are acquired when diisocyanates (or polyisocyanates) respond with polyols, while polyureas are generated when they respond with amines. In hybrid systems, isocyanates are responded with a mix of amines and polyols. For numerous polyurethanes (other than moisture-cured systems, for instance), a driver needs to guarantee quick reaction of the isocyanate and polyol elements. On the other hand, isocyanates react rapidly with amines, and therefore no catalyst is needed for the development of polyureas.
A range of isocyanate, polyol and amine reactants are available for the synthesis of polyurethanes, polyureas, and hybrids. Isocyanates can be aliphatic or fragrant. Fragrant compounds (such as diphenylmethane diisocyanate (MDI) and toluene diisocyanate (TDI)) include bonds that can take in UV radiation, which leads to their breakdown and the unwanted yellowing of the coatings. As a result, aliphatic isocyanates (such as hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI)), which do not have these bonds, are typically chosen for the synthesis of polyurethane/polyurea binders planned for the formula of exterior coatings.
Polyethers, polyesters, and polycarbonates are the polyol types most extensively utilized for polyurethane and hybrid polymer production. In many cases, the polyol includes more than one kind of linkage. Specialized polyols, such as polycaprolactones, are chosen for some applications. The length of the polyol chain has a substantial effect on the firmness
( short) and flexibility (long) of the coating, while the type of polyol effects homes such as chemical and wetness resistance.
The diamines used for the preparation
of pure polyureas and hybrids are normally polyamines. Typically 2 various types are used: amine-terminated polymer resins, normally polyether amines, and amine-terminated chain extenders, usually pure polyamines. Both main and secondary amines can be used, with secondary amine reacting more gradually. Hybrids can be formed by responding isocyanates with a physical blend of polyols and diamines or integrating hydroxyl groups into the polyamine (often the chain extender).


Polyurethanes
While traditionally solvent-based,
two-component (2K) coating systems, polyurethanes are also available as 1K water-based PU dispersions, 2K waterborne systems, and high- and 100%- solids 2K solutions. Preliminary water-based systems did not carry out as well as their solvent-based equivalents and also struggled with application concerns. Nevertheless, advances in PU technology have actually caused the development of numerous PU dispersions that have application residential or commercial properties closer to those of solvent-based systems. The 100% solids systems also initially provided application problems, but here again, advances in application strategies and devices have conquered these obstacles.


The crosslinking that happens throughout PU movie development imparts particular residential or commercial properties to these coatings. They show excellent gloss and color retention (for pigmented formulas; clear overcoats are likewise possible), integrated with good chemical and moisture resistance, even for thin movie develops, which is vital for applications where weight is an issue (such as in automobile and aerospace industries). Aliphatic PUs likewise withstands UV light for outside applications. PU coating's mechanical properties include high impact, abrasion, and scratch resistance.
Because polyurethanes make up two distinct parts-- the polyisocyanate and polyol portions-- their properties are tunable by picking various isocyanate and polyol building blocks. As a result, it is possible to accomplish PU coatings that range from extremely flexible (elastomeric) to really rigid. Also, PU coatings can be formulated with a distinct mix of flexibility/elongation and harness that can not be achieved with epoxy acrylic systems. They likewise have outstanding adhesions to different substrates, consisting of metal. Therefore, one line of product can often be utilized for several applications, lowering bring expenses in inventory. In addition, for light- to medium-duty applications, polyurethanes can be used as single, direct-to-metal coatings, getting rid of the requirement for a primer, reducing material and labor costs.


Polyurethane coatings cure relatively rapidly, even at lower temperature levels, however the majority of require a catalyst. The exception is moisture-cured systems, in which the water in the air serves as the driver. These systems are suitable for usage in damp conditions. On the other hand, a lot of 100%- solids PU coatings are more conscious moisture
( prone to blistering) than other technologies, consisting of epoxies, polyureas, and PU/polyurea hybrids to the need for a catalyst. Solvent-based PU coatings are usually applied as thin movies (< 5 mils dry movie density) utilizing conventional airless sprayers. In contrast, 100% solids systems can be used at thicker film builds (> 20 mils dry film thickness) however require plural component spray technology, which immediately mixes the resin and catalyst components prior to spraying. Operation of this complex devices needs trained/licensed applicators.


OEM applications for polyurethane coatings cover a broad swath of industries. Factory-applied, solvent-based systems
( including high-solids formulas) are commonly utilized in the furniture, kitchen cabinetry, and floor covering industries. PU coatings are also used to some extent in the vehicle industry in the underbody, interior, and outside (guide, base, topcoat) applications and truck-bed liners (both factory-applied and aftermarket). All types of PU coatings find use in general commercial metal, heavy equipment and plastic primer, overcoat, and clear-coat applications. Direct-to-metal rigid PU systems are used for OEM pipeline coating and steel tank applications, while elastomeric systems used as a foam hard-coat are for waterproofing and durability in architectural trim, themed home entertainment applications, and occasionally building panels.



Polyurea elastomers
Polyurea coatings are 100% solids, zero-VOC formulas that treat rapidly (just 30 seconds) without the requirement for a driver or heat, even at low temperatures (down to - ° 20 C). Due to the nature of the urea linkage, they are not conscious wetness; no blistering takes place, even when polyureas are used on substrates in the presence of liquid water. Similar to polyurethanes,
the formation of crosslinked networks in polyurea films imparts excellent mechanical residential or commercial properties, but the enhanced hard block/softback division in polyureas lead to enhanced hardness/stiffness, tear and abrasion resistance and weathering, thermal shock, and effect resistance. The urea linkages also contribute to enhanced chemical and water resistance. The combination of isocyanate and amine-terminated polyol sectors provides an attractive mix of flexibility and solidity.
Unlike polyurethanes, polyureas can be applied at very high movie develops. As a result, they can work as protective coating layers and contribute to the structural integrity of a substrate. They follow a series of substrates, including concrete, metals, wood, composites, foam, and so on. There are obstacles to dealing with polyureas, nevertheless. When first introduced, polyureas frequently experienced substrate wetting, inter-coat adhesion, and surface area defect issues. Developments in both basic materials and application devices have assisted get rid of these shortcomings. Nevertheless, correct mixing is crucial for optimum movie development and adhesion. Similar to 100%- solids PUs, high-pressure, plural component sprayers are needed to use polyureas, and applicator training is essential to ensure that operators understand how to determine maximum mixing and spray conditions. Polyurea elastomers are normally inappropriate for applications needing thin (< 5 mils) coatings.


In general, polyurea coatings are chosen as overcoats for rapid curing applications ( quickly turnaround times), the covered substrate will be exposed to severe conditions, and appearance is not an essential issue. Also, they are used in applications where a thinner overcoat (PU or otherwise) might be damaged, resulting in prospective problems with corrosion and degradation. Polyureas are likewise often selected to replace epoxies in applications where elongation and impact resistance are very important since epoxies frequently crack and delaminate under such conditions. Examples include OEM waterproofing applications and rail and barge coatings. In general, polyureas are frequently utilized in the field due to their insensitivity to moisture and temperature. On-site applications consist of roof, pipeline and tank coatings, truck-bed liners, liners for large tanks (freight ship lines, bulk transportation wagons), automobile parking decks, bridges, and offshore protection.
Polyaspartic ester-based polyurea coatings represent a more recent technology based upon the reaction of isocyanates with aliphatic polyaspartic esters (aliphatic diamines). These coatings normally treat more gradually than polyureas and can be used at thinner film constructs. Like polyurethanes, they are applied using conventional airless sprayers. Therefore, they are frequently utilized in the exact same applications that can utilize PUs.


Hybrids-- the best of both the use of blends of amine- and hydroxyl-terminated polyols creates much more opportunities for fine-tuning the residential or commercial properties of hybrid coatings. The look, hardness/flexibility, and mechanical homes, and reactivity of these coatings can be changed by selecting different isocyanates, polyether amines, and polyols. Controlling the treating time lets you develop smooth to textured movies with the desired surface area appearance integrated with the higher efficiency of polyureas.
Hybrids are usually 100%- solids solutions that treat quickly (catalyst needed) and can be applied in high movie builds. They have great elongation and flexibility integrated with outstanding chemical and solvent resistance and resistance to abrasion and impact. Like polyureas, they can be applied at low-temperature levels. However, the application of hybrid coatings is less complex (simpler impingement mixing) than polyureas, although plural component spray equipment and applicator training are still required.
The customizability of polyurethane/polyurea hybrid coatings has actually led to their use in various applications where the high performance of polyureas and an appealing surface are both wanted. Subsequently, they are typically chosen over polyureas since hybrids can fulfill these requirements and still offer quick turn-around times and do so at a lower cost than pure polyurea coatings. The most common OEM usages are corrosion control and waterproofing applications. The high performance of polyureas is not needed, and hybrids offer more appealing solutions on a cost-performance basis. Sometimes, hybrids are chosen since they offer a distinct set of homes that can not be attained with a pure PU or polyurea system. Hybrids are likewise finding use in secondary containment applications over concrete and geotextiles.


Making the right option
There are polyurethane, polyurea, and PU/polyurea hybrid coating technologies for virtually every OEM coating application possible. These chemistries provide a variety of properties ideal for various application conditions and efficiency requirements. The aspects to consider when picking a protective coating for an offered application consist of the type of substrate, the application technology, the conditions under which the coating must carry out, the remedy time, the wanted film density, and the efficiency requirements (adhesion, appearance, and mechanical and resistance properties). Expense is also plainly a crucial motorist in the choice of coating technology. Polyurethane coatings are the least expensive, polyureas the most costly, and hybrid coatings fall in between. Polyurethanes are often thought about to offer the very best compromise in between cost and performance. Hybrids, on the other, offer 80% of the benefits of polyureas at roughly 50% of the additional cost compared to polyurethanes.


Thin-film polyurethanes fit applications where efficiency and a top quality finish are needed. In less requiring applications, they can be applied as a single coat (for instance, direct-to-metal), however are typically utilized as the overcoat when the layered substrate must be protected from more extreme conditions.


Polyureas, which can be sprayed in poor conditions, consisting of extreme temperature levels and high humidity, are typically utilized for outside or on-site applications. Polyurethanes and hybrids, which need catalysts for curing, are not suitable here. Less-expensive hybrids are preferred for OEM applications where the higher-performance curing homes of polyureas are not required, but comparable applied-film properties are wanted.



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