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Water borne Polyurethane vs Solvent based Polyurethane
What are the application differences of a Polyurethane dispersion coating compared to a solvent based polyurethane and what are the advantages upon each other?
A polyurethane coating is a versatile product with many advantages upon other coating systems. A major disadvantage of classical, solvent based polyurethane coatings, are the volatile organic compounds (VOCs) present in the wet state. New regulations force formulators to keep the VOC content below 350 g/l. Recent developments dealt with this problem. Developers have succeeded in making a polyurethane dispersion in water, eliminating most of the volatile organic solvents.
However, the costs of a so called water borne polyurethane are higher than that of a solvent based polyurethane. A question that may arise is whether or not the dispersed polyurethane performs the same way as the classical, solvent based, polyurethanes and whether or not it is worth the money.
Besides possible differences in performance, processing techniques may differ. The goal of this investigation is to give an overview of the differences between the performance, processing and cost between a polyurethane dispersion and solvent based polyurethanes. The major formulations of both types will be summarized and compared to find the best coating.
The chemistry of PU
A polyurethane (PU) is a polycondensation reaction product of an isocyanate with a monomer. The isocyanate must have at least two functional groups and the monomer at least two alcohol groups. The catalyst for the reaction can be a tertiary amine like dimethylcyclohexylamine or organic metallic materials like dibutyltin dilaturate. The condensation of a cyanate with a hydroxyl end-group results in a urethane linkage. Both the isocyanate and the hydroxyl alcohol (diol) need to be bi-functional to form polyurethane. The reaction mechanism of the formation of PU catalyzed by a tertiary amine is given by:
Figure 1: reaction mechanism of the catalyzed condensation reaction of PU by a tertiary amine
Many isocyanates can be used but MDI, aliphatics such as H12MDI, HDI, IPDI and TDI are the most widely used among others. MDI consumption exceed 45% of the total amount of isocyanates used, closely followed by aliphatics (35-40%) and TDI (15%). Whilst the reactivity of the isocyanate determents the rate of the reaction, the main properties of the PU is devised from the diol.
As with the choice of an isocyanate, a wide variety of diols can be used. EG, BDO, DEG, glycerin and TMP are all useable. For hard and weatherable coatings acrylic and polyester polyols tend to be preferred. Polyols with a low molecular weight as the main reactant produces polymer chains with more urethane groups hence a harder and stiff polymer is formed. High molecular weight polyols however produces a more flexible polymer. Also a low functionality long-chain polyol produces soft and flexible PU while short-chain polyols with high functionality makes more cross-linked products which are more rigid.
Different types of PU formulations
PU coatings can be divided into two main groups, namely into 1 and 2 pack systems (1k and 2k). The 1k system basically contains a dissolved, fully reacted PU whilst a 2k system can contain partially reacted PU and unreacted monomers. Both systems can be solvent-based or waterborne. Furthermore there are several curing (or drying) systems known, each resulting in different performing coatings.
Two-component or 2k
As mentioned briefly 2k systems are reactive and the primary reaction is of isocyanate with polyols. The main disadvantage of a 2k system is the pot-life. When the isocyanate is added the mixture begins to react and hardens. However, the main advantage of a reactive system is the outstanding mechanical performance. Because the PU particles react and crosslink, an endless polymer forms which is hard and chemical resistant. Two-component systems include solvent-based and waterborne formulations.
Solvent-based 2kcoatings are obtained by mixing aliphatic isocyanates with polyester polyols or blends of polyester with acrylic grades. Formulations like these cure by partially physical drying and cross-linking with the isocyanate. Solvent-based 2k formulations are mostly used in the automotive and aviation industry as a finish coating.
Waterborne 2k coatings are formulated with dispersible isocyanates and water-dispersible polyols such as polyacrylates or emulsifiable polyesters. The most commonly used isocyanate is a HDI trimer but IPDI trimers can also be used. Aromatic isocyanates cannot be used in waterborne formulations because they react dangerously with water. Dispersible isocyanates can be used as such or can, by partial reaction with a dispersible polyol, be emulsified, making it easier to mix.
However some waterborne systems still need up to 10% co-solvent to form a homogeneous finish. These formulations cure by partially physical drying and by cross-linking but can also be thermally cured at temperatures ranging from 20 ˚C to 80 ˚C. Waterborne 2k systems are frequently used as protective coatings in the transportation, machinery and furniture industry. Their high flexibility also makes it possible for use on polymeric and wooden surfaces.
One-component or 1k
A 1k PU coating consists of partially reacted polymers (prepolymers) which are liquid at room temperature. These prepolymers are synthesized by reacting MDI, HDI or TDI with a polyester or polyester polyols. The main advantage of a one-component system is that no mixing is required and pot life is no issue. 1k systems are storage stable with a shelf life of up to six months. However a disadvantage is that most 1k formulations are not cross-linked making them less hard and vulnerable to solvents.
1k formulations are broadly used as maintenance and repair coatings for their ease in application and mechanical behavior. They are used for painting steel constructions such as bridges and other large steel structures where corrosion protection is needed.
Solvent-based 1k coatingsare obtained through reacting aromatic or aliphatic isocyanates (MDI and IPDI) with polyesters or polyether polyols. This reaction forms high molecular weight linear PUs. Commonly used additives are chain extenders. Curing occurs by evaporation of the solvent but 1k systems can be formulated so they cure by oxidation, with moisture and even by UV-radiation.
Waterborne 1k or better known as PUDs are fully reacted polyurethane systems. The PU particles have hydrophilic groups in their backbone and are maximum one tenth of a micrometer in length, dispersed in water. This makes a both chemically and colloidal stable mixture (Figure 2). A PUD can also be formed by incorporating a surfactant.
PUDs are currently very popular because they are environmentally friendly, but still being able to perform reasonably. Because PUDs are relatively expensive they are mixed with acrylic grades to lower the material costs. However more acrylic means less hardness. 1k waterborne formulations can cure physical, by oxidation and by UV-radiation.
There are formulations at the market containing no solvent. These coatings find their application in the building sector. To obtain solvent-free formulations MDI is reacted with polyether or oil-modified polyester polyols. To obtain higher hardness chain extenders and catalysts are added to the formulation.
As mentioned above the way a coating cures strongly effects the final performance of the coating. The way a coating dries is dependant of its formulation. A 2k system can cure on air, by heat and under influence of UV. One-component systems can cure physically, with moisture, by oxidation, under influence of UV-radiation and by heat.
Physical drying basically means that the solvent containing the PU evaporates, leaving the PU to form a film. A major disadvantage of this way of drying is that there is no cross-linking between the PU particles. This drying mechanism affects some one component systems.
UV curing coatings can be formulated as solvent-based or waterborne and both 1k and 2k. In a UV-curing formulation the catalyst is inactive in absence of UV-radiation; this behavior is seen with a photo-initiator. When UV-radiation hits the catalyst it unblocks and becomes active and initiates the curing. A schematic representation of this process is shown in Figure 3. UV-curing coatings are predominantly used as automobile finishes as it has unmatchable hardness and gloss.
Oxidative drying is a process which is used with a special type of 1k PU coatings. So called oil-modified PUs (OMU) are synthesized through an addition reaction of isocyanate with a hydroxyl bearing, fatty acid modified ester (TDI). To obtain higher densities more isocyanate can be added but this means that more solvent is needed, as much as 550 g/l (not VOC-compliant).
Natural oils like linseed oil can be used as the diol and a mineral spirit can be used as the solvent. An OMU can be solvent- or water-based and cures by reacting with air surrounding the coating. The fatty acid groups of the oil (attached to the PU) form cross-links with each other by mean of oxidation. OMUs have better mechanical and weathering properties than unmodified, non-reactive alkyds, but reactive PU coatings are superior. OMUs are predominantly used as wood finishes for their distinctive yellowing/aging which some formulators prefer.
Moisture curing PU (MCPU) coatings are formulated with NCO-terminated PU prepolymers. The NCO groups react with atmospheric moisture which produces a amine-group. This further reacts with remaining isocyanate to form highly cross-linked urea-networks. MCPU coatings have superior hardness, strength and stiffness. Even though the coating is cross-linked a MCPU has a relatively high flexibility. Because a MCPU cures with moisture this strongly affects the storage stability.
Thermal curing formulations are based on deactivated isocyanate mixed with a polyol. This semi-one-component formulation is stable at room temperature but when heated (100-200 ˚C) the deactivated isocyanate unblocks and reacts with the polyol, the same way as a reactive 2k coating. The isocyanates (aromatic or aliphatic) all have one active hydrogen. For blocking the isocyanate caprolactam is mostly used. A different way of blocking the isocyanate is creating uretidinedione or dimer links. Thermally cured coatings find their main usage on surfaces which need to withstand excessive heating and cooling cycles.
An overview of all the PU coatings with their distinctive curing system is shown in Figure 5.
Testing the coatings
To make a comparison of different types of PU coatings the performance of a coating need to be tested. Because of the broad application possibilities of PU coatings and because of the need of a wide variety of different characteristics, the comparison will be narrowed down to floor coatings. Floor coatings are tested on mar and scuff resistance, taber abrasion, chemical resistance, color and König hardness. Because thermal cured coatings are not applicable as flour coatings these formulations will not be used in the comparison. Moisture cured…
Mar and scuff resistance
Mar and scuff resistance or simply put resistance to marking can be measured by several methods. One of them is the pendulum method which consists of a pendulum arm with a hard-wood block attached to the end. The weighted block hits the coated panel four times and the average 20˚ gloss of the coating is measured before and after the test. The results are expressed as percentage 20˚ gloss retained and visual assessment of the panel (scratching and scuffing).
To test taber abrasion can be described as wear resistance. To measure abrasion resistance an arm is weighted with 1000 gram weights and attached to abrasive wheels (mostly consist of minerals). The arm makes 1000 cycles over the substrate. The initial weight of the coated substrate is compared with the weight after the test. The results are expressed in milligrams removed.
Chemical resistance is determined of dry films using eight household stains and chemicals. Test chemicals include MEK, olive oil, several cleaning chemicals, ethanol, white vinegar, water and 7% of ammonia solution. The chemicals are applied on a two-ply square towel on the test film, completely saturating the towel. The towel with the liquid is immediately covered with a watch glass. After a period of two hours the stains are removed and the panel is rinsed and dried. The impact of the chemicals on the coating is investigated immediately after the rinsing. The surface is investigated on discoloration, blistering and softening. Each chemical is rated on a scale of 1 to 10 with 10 being “no effect”.
An important property to investigate of coatings is color, especially when evaluating PU coatings, because PUs tend to yellow, especially OMUs. The initial yellowness index of a coating is measured and after a period of lighting. The difference between initial and final yellowness index is also measured as the Delta E. When this value is below 1.0 the color difference is insignificant. The higher the value the more yellow the coating has become in a period of time.
König hardness is a method used for measuring the hardness of a coating. With this method a pendulum rocks back and forth over the coated substrate. The coating will dampen the rocking motion, slowing the pendulum down. The results with the König hardness are expressed in seconds; the longer the pendulum rocks, the harder the coating.
Solvent-based vs. waterborne
For this comparison different formulations of each type are reviewed. Solvent-based 2k, solvent-based OMU and 2k UV are compared with waterborne OMU, 2k and a PUD/acrylic mixture. Data from sb OMU, PUD/acrylic, wb OMU and wb 2k is obtained from the article “Oil-modified urethanes for clear wood finishes: Distinction or extinction” by Richard A. Caldwell from Reichhold. Data from sb 2k, 2k UV and 100% PUD formulations are obtained from several commercially available coatings […]. Some values may differ because of the objective opinion of the investigator and the formulation.
Some values are projected as expected where data was missing. These projections include chemical resistance of sb 2k, 2k UV and 100% PUD formulations. The taber abrasion resistance – at 500 cycles – of the wb OMU coating is multiplied by a statistically calculated value, using know data from other formulations, to obtain a value with 1000 cycles.
As shown in Graph 1 a 2k UV coating has superior mechanical properties (König hardness and taber abrasion). This is because a 2k UV coating has a high amount of cross-linking. This can be related to the 20˚ gloss of the dried film. High gloss usually means high cross-linkage. The solvent-based 2k formulations perform comparable with 2k UV coatings but are slightly less cross-linked as shown in the gloss and the hardness.
The waterborne formulations are all significantly softer but tend to be slightly more mar and scuff resistant. However the initial gloss values of these waterborne coatings are somewhat lower. It can also be found that the 1k OMU formulations perform better than most non-reactive coatings. However they tend to yellow and perform worse than reactive (2k) coatings. This shows that the oxidative cross-linking cannot be compared with the reactive cross-linking of 2k formulations. It is somewhat surprising that 100% PUD performs comparable with a wb 2k formulation. However the chemical resistance and the scuff resistance are lower showing the benefits of cross-linking.
If a hard coating with high gloss is wishful UV cured 2k coatings are the best choice. The best mar and scuff resistance is obtained with waterborne formulations but these show less hardness and chemical resistance. Solvent-based systems have an overall better performance than waterborne systems but VOC regulations restrict the amount of solvents used, causing a lower amount of solids possible.
This results in less cross-linking hence less hardness and chemical resistance. Even though high VOC content solvent-based coatings perform better, VOC regulations cause a shift to waterborne formulations which are increasingly performing better. During the investigation it became clear that a good comparison between different PU formulations is a nearly impossible task because of the large amount of different possible formulations of each class.
Waterborne PU coatings, when properly formulated, can meet the performance of solvent-based coatings, especially when compared with VOC-compatible solvent-based coatings but with a higher price. Eventually VOC regulations are further sharpened causing a market shift towards waterborne formulations, making them worth the money.