Understanding the Properties of Urethane Hard Coats

Understanding the properties of urethane hard coats is essential to picking the best material for theming projects. They are designed to harden surfaces like Styrofoam or EPS foam to protect architectural shapes and forms, custom signs, and props. If you’re new to hard coatings, you may not know what properties are essential to look at compared to other coatings. VFI has put together a comprehensive guide to help you learn more about them and how they function due to their properties.

What Physical Properties of Urethane Hard Coats Are Important?

Physical properties will tell you how the material will perform when cured. Most users look at these properties to help them decide if the coating will withstand the demands of their unique project. These properties are tested using standardized methods from the American Society for Testing and Materials (ASTM). The most prominent ones listed can include:

Shore Hardness

Test method: ASTM D2240

Definition: Shore hardness is a material’s resistance to indentation or compressive forces. It’s measured using a tool called a durometer and rated on various scales depending on the characteristics of the material. The most common scales for polymer materials are Shore A, which rates softer materials, and Shore D, which rates harder materials.

Importance: Hardness is a property that will tell you a lot about a urethane hard coat’s wear resistance, strength, and service life. It directly impacts the material’s ability to protect an underlying surface from scratches, abrasion, and other physical damage. A harder material will give more strength, while a softer material will give more flexibility.

Polyurethane hard coats most often use the Shore D hardness scale. They will typically be in the 65-75 D range, which has a similar feeling to a plastic hard hat. When they drop below this range, they feel more flexible or rubbery. When they go above this range, they will have high resistance to deformation but can also be brittle unless they reach a certain thickness.

Achieving a good balance of hardness and flexibility while also maintaining good weathering characteristics is important. Coatings with a higher hardness have less flex and can be more susceptible to cracking when applied to surfaces that expand and contract as temperatures change. Because they have less flex, they have a limit on substrate usage and are best applied to EPS foams for theming projects. Hard coats are not recommended for surfaces softer than themselves. However, other properties can contribute to them having more flex, even at a higher hardness.

Note: All VFI EPS theming hard coats list their hardness in the product name (ex: VFI-6170 70 D Spray Hard Coat) to help you find what you’re looking for.

Tensile Strength

Test method: ASTM D638

Definition: Tensile strength is the maximum amount of stress that a material can withstand before it fails when being stretched. It subjects a test specimen to an applied force or load until it reaches its breaking point. That force is then measured and expressed in pounds per square inch (psi).

Importance: Tensile strength is an important property that determines if a urethane hard coat is suitable for your application. It can tell you a lot about structural integrity and durability. Coatings with high tensile strength should be capable of withstanding significant force without breaking or deforming. They are often used to protect and support delicate substrates from heavy loads. Demanding applications that would require high tensile strength include EPS foam projects within touching distance of people.

The strength of a hard coat helps it maintain its integrity under stresses such as wind, vibrations, and pressure to prevent deformation. Even small impacts can cause stress, but high tensile strength helps it absorb those forces, preventing damage from occurring.

Elongation

Test method: ASTM D638

Definition: Elongation is a property that measures the percentage (%) increase in length of a material before it breaks when stretched. It has an inverse relation with tensile strength and uses the same test method (stretching force).

Importance: Elongation is a very important property for urethane hard coats due to their rigidity. Their job is not necessarily to resist stretching force, which is why they will have high tensile strength and low elongation. However, this property ensures some flexibility and complements the strength offered by hardness and tensile strength. It is the balance of all of these properties that allows hard coats to perform the way they are designed.

Elongation will provide some necessary give to the hard coat to resist cracking even with high tensile strength and hardness. It will absorb some energy from impacts and distribute stress more evenly. Elongation with a high tensile strength also provides thermocycling capability that would not be possible with high tensile alone.

Tear Strength

Test method: ASTM D624

Definition: Tear strength or tear resistance is measured by assessing the maximum force required to tear a material in a direction perpendicular to the direction of the stress. More simply put, it is a measure of how well a material can resist tearing. It is expressed in pounds per linear inch (pli).

Importance: Tear strength is another property that can tell you about the durability and lifespan of lower durometer urethane hard coats. In a hardcoat, tear strength is a hard property to obtain unless the coating is under 65 D, but it is a good sign for thermocycling if the tear strength is high with good elongation.

After damage has been endured, the tear strength will tell you how the material will hold up. It will also tell you about its resistance to rips, punctures, and cracks. Minor nicks, scratches, and cracks can be a starting point for bigger issues if tear strength is too low. Tear strength makes the coating more resistant to damage, preventing a problem from occurring that compromises its ability to protect the underlying substrate.

Elastic Modulus

Definition: Elastic modulus is a material’s resistance to elastic deformation when stress is applied. Elastic deformation is the temporary change a material goes through when under stress, so the material will return to its original shape or size once the stress is removed. This property is expressed in pounds per square inch (psi).

Importance: Elastic modulus is a material’s ability to bear loads without significant deformation (temporary bending or indentation), which can be very important for urethane hard coats. The stiffer the coating, the higher its elastic modulus will be.

A high elastic modulus will allow it to perform effectively by handling impacts and other forces without cracking, scratching, or denting. This is because these coatings can store more elastic energy before deforming. So, the hard coat will absorb the impact energy from a blow and prevent it from transferring to the substrate.

A lower elastic modulus will allow a coating to deform rather than crack. This is useful when an object might be kicked or something sharp will hit it. It will also allow the coating to deform and not break to protect the foam. This is also important if water and outdoor elements are a concern.

Impact Resistance Unnotched/Notched Izod

Test method: ASTM D256

Definition: Impact resistance is a test that measures a material’s resistance to impact from a swinging pendulum. Unnotched Izod tests do not make a premade notch in the test specimen, so the impact energy is focused on the entire test piece. Not creating a notch will test the overall toughness of a material. The value, in pounds per inch (lb/in), will be a less accurate representation of real-world impact situations and give an elevated number. An alternative test method is a notched Izod impact resistance test with a preset notch to accurately direct the force.

Importance: Urethane hard coated projects, especially ones placed outdoors, may experience sudden forces, shocks, or blows throughout their lifespan. Impact resistance provides an understanding of whether the material has the toughness needed to protect theming applications long-term. High impact resistance will help shield the substrate from damage by absorbing the blow, preventing the coating from cracking or chipping.

Softer coatings are better at taking impacts because they have more flexibility but have a low impact strength. Usually, softer coatings will not have an impact strength tested because they will flex out of the way and provide a low number as a result. However, impact resistance is only one property that will help determine the material’s toughness. Consider the tensile strength, hardness, and elongation for a better understanding of the material’s strength.

What Liquid Properties of Urethane Hard Coats Are Important?

Not all manufacturers divide up their properties between liquid and physical. VFI does this so our urethane styrofoam hard coating users know which properties apply to the material when it’s in a liquid state versus when it has fully cured. The following are liquid properties we typically list:

Solids by Volume

Test method: ASTM D2697

Definition: Solids by volume or volume solids is the percentage (%) of the total volume of a material that remains once cured. In the context of coatings, it’s how much material will remain on the substrate and how much will evaporate into the air.

Importance: The amount of volume solids lets applicators know how much material they’re actually putting down, which is important when trying to build it to a certain thickness. Some manufacturers specify a millage that must be applied to achieve optimal protection, so making sure you know what you’re putting down is that much more important.

Luckily, most urethane hard coats are 100% solids, meaning no material evaporates during cure. The amount of material you apply to a surface is guaranteed to be the amount that remains once cured. Because of this, you also don’t have to worry about calculating wet and dry film thickness using the solids by volume.

Note: Be careful of shrinkage on extremely long parts as it can change depending on your thickness causing your part to warp.

Mix ratio

Definition: A mix ratio is a ratio that entails the exact measurements needed from multiple components to be mixed to produce the needed chemical reaction for a material to cure (ex: 1:1 or 2:1). A mix ratio can be expressed in two ways:

• By weight: Uses an accurate scale to measure the amounts of each component needed to cure. This property will typically only be listed for hard coats that can be applied by brush or roller.
• By volume: Uses same-sized mixing containers to measure the amounts of each component needed to cure. This property will typically be listed for sprayable and brushable hard coats.

Importance: Urethane is a very touchy material. If you don’t follow the mix ratio properly, then it might not cure to the desired effect. Adding too much or too little of one component can make the coating feel sticky or goopy rather than plastic-like when it sets. Typically, when this happens, it won’t develop physical properties, so it cannot effectively protect the substrate it is applied to. Affected physical properties can include hardness, adhesion, and resistance to abrasion or impact. The coating might be softer and not adhere to the substrate, leading to peeling or flaking.

Incorrect mixing of the components can also lead to material waste. Not only are you wasting material from each mixed component, but you may also have to scrap the entire project you’re working on and start fresh. This problem can be costly as these foam pieces take time to carve, so you don’t want to mess up.

Off ratio material will also affect the final finish of the project. When you try to paint over a hard coat that has iso-rich spots, you might be able to get away with it. However, the paint might peel off more easily when placed in the sun. When you try to paint over a hard coat that has poly-rich spots, the paint might react more with the sun and cause the paint to bubble because it is not breathable.

Viscosity

Test method: ASTM D2196

Definition: Viscosity is a fluid’s resistance to flow or change in shape and describes the internal friction of a moving fluid. It is often referred to as the thickness of a liquid and is measured in centipoise (cps). Viscosity can be listed for each component (A and B sides) and the material when combined. To understand viscosity, here is a list of household items and their relative viscosities:

Material	Viscosity
Water	1-5 cps
Corn syrup	50-100 cps
Maple syrup	150-200 cps
Castor oil	250-500 cps
Honey	2000-3000 cps
Molasses	5000-10000 cps
Chocolate syrup	10000-25000 cps

 

Importance: Viscosity will tell you a lot about how a urethane hard coat will perform. It determines how easily the coating can be brushed, rolled, or sprayed onto a surface. With a very high viscosity, they can be hard to spread evenly but prevent dripping or sagging. On the other hand, a material with a very low viscosity might not provide adequate coverage because it is too runny.

The ideal viscosity will depend on the application method and desired film thickness. When a material is applied by brush, the viscosity will be higher to allow easy application without excessive dripping or running. The higher viscosity also allows for better control when using this application method. When using spray equipment, lower viscosity coatings are required for even distribution, fine atomization, and a desirable finish. A similar viscosity A and B side is also highly critical to maintaining a nice even spray. Not considering the viscosity when choosing your application method can result in surface defects, such as brush marks, orange peel texture, or uneven coverage.

Viscosity affects the final thickness of cured coatings. Higher viscosity materials will generate a thicker film build, while lower viscosity materials will have thinner builds. A lower viscosity also ensures that the material can level and flow effectively to minimize imperfections in the surface finish.

Air bubbles are also typically generated in the application process. High viscosity materials are more prone to trapping air bubbles, which can create imperfections in the cured finish. Low viscosity materials allow more of the trapped air to come to the surface and escape before cure. Because the bubbles leave easier, you’ll achieve a smooth, uniform finish.

Pot Life

Definition: Pot life is the length of time a material can be used. Depending on the material, pot life can be as quick as seconds or as long as hours.

Importance: Pot life is not a property that is listed for every urethane hard coat. Since a chemical reaction happens much faster for spray coatings, you’ll see it as a property for brushable ones. It’s specific to these coatings because you need to know how much time you have to brush or roll the material onto the surface before it becomes unworkable.

Within the pot life window, your mixture maintains a viscosity for smooth and efficient application. Once the pot life ends, viscosity increases, making it hard to spread. Ensuring you have adequate time to apply the coating will minimize defects in the finish, such as orange-peel texture, air bubbles, and incomplete coverage. Also, mixing more material than can be used within the pot life leads to material waste. In some cases, mixing less material can even extend the pot life. Finding a coating with an acceptable pot life for your project is crucial for optimal protection over your theming project.

Tack Free

Definition: Tack free determines the amount of time after mixing that a material will no longer feel tacky (sticky). Depending on the material, this can happen in seconds or minutes.

Importance: Unlike pot life, this property is usually listed on sprayable coatings because it tends to happen shortly after the pot life. After a hard coat is sprayed, it needs some time to cure and become tack free so it will no longer adhere to dust, debris, or other objects that come into contact with it.

This property is essential to know for handling and assembling a coated project. If it is still tacky, it can pick up contaminants that compromise its appearance and performance. These contaminants can come from airborne dust or someone touching the surface before it has solidified enough. The material then traps these particles, marks, or smudges once cured.

Most users will want a fast tack free time. The speed will allow further processing, such as sanding or painting, to occur much sooner. It also helps if the coated component needs to be assembled with other parts. The faster an applicator can post-work their project, the faster their production cycles are. However, a fast tack free time can limit the working window, especially for larger projects.

Note: A long tack free time also creates a great window for recoating and will help with better adhesion. Always check your technical data sheet (TDS) for recoat windows to prevent bad adhesion or additional sanding.

Cure to Handle

Definition: Cure to handle is the time until a material can be handled after application. It usually comes between the tack free time and the recoat window since the material has not completely cured. Most cure to handle times happen minutes after application.

Importance: Cure to handle time directly affects how soon you can handle a coated object after application. It can also tell you how soon it is ready to be sanded, painted, or assembled without compromising the final properties.

It needs sufficient time to cure and develop its properties. If you handle the coated project too early, the surface can crack or become damaged by touch. Handling it too soon can also lead to uneven curing, weak spots and inadequate protection of the underlying surface. Different urethane hard coats will have different cure to handle requirements.

Recoat Window

Definition: The recoat window is a property specific to coatings as it is the time frame between which a previous application can receive a subsequent coat or topcoat. Depending on the coating, a recoat window can extend from minutes to hours and may have different requirements based on the specific formula.

Importance: Some urethane hard coats must be applied in layers to build the thickness. Adding layers is usually done for more adequate protection of the underlying surface. During the recoat time, you can be sure the subsequent coat will have excellent adhesion with the prior one. This property is also important if you must paint over the coated surface for theming purposes. You can be sure that the paint will stick to the hard coat.

If you apply a new coat outside the recoat window, there will be weak adhesion between the coats. That weak bond can compromise the overall integrity of the coating, making it more likely to peel, crack, or delaminate. Poor adhesion can also compromise scratch resistance, impact protection, and other properties. In some cases, the initial coating can be sanded or scuffed to create better mechanical adhesion for the second coat. A strong bond between them will allow them to perform consistently across the entire project.

How to Find Urethane Hard Coat Properties

Because material properties are so important for users to know before they purchase a product, we display them accurately after meticulous testing and review. They can be found on technical data sheets or product pages of any product. Technical data sheets can be found under the resources tab on any product page.

Contact VFI if you would like more information on properties to determine the best product for your project.