Understanding the Properties of Urethane Foam

Understanding the properties of expanding urethane foam is important to optimize performance and success when using the material. They are typically used for part-making, rotocasting, or backfilling purposes. If you’re new to using polyurethane materials, you may not know which properties are most important for your desired application. This comprehensive guide will direct your attention to the essential properties and why you should know what they mean.

What Physical Properties Are Important?

The physical properties of urethane foam refer to the properties of the material when it is in its cured state. As opposed to most other urethane products, urethane foams have few and very different physical properties that matter to the user. They are tested using standard methods from the American Society for Testing and Materials (ASTM) and other in-house lab tests. The following properties are the most important to consider before purchasing urethane foam.

Foam Density / Approx. Poundage

Definition: Foam density is a measurement that indicates the compactness of the foam upon expansion in a given space. In other words, it indicates the weight a cube (12” x 12” x 12”) of foam holds. It is expressed in pounds per cubic foot (PCF). Density can be calculated by dividing the total weight of the foam by the total number of cubic feet. Ex: 100 lb. / 20 cubic feet = 5 PCF.

Importance: Foam density is one of the most critical aspects of choosing the right material for your application. The density selected will depend on the intended use for the foam. The application will also determine if you need a more rigid or flexible foam. There are rigid, semi-flexible, and flexible foams available with similar densities. However, you should only compare densities of like materials (ex. rigid foam to rigid or flexible to flexible).

It is a key indicator of the overall feel and performance of the foam. It determines the durability, strength, and support of the foam as well. Expanding foam densities vary, but common ranges include 3-24 lb. Each will be unique in its characteristics and uses.

The denser the foam, the more compact the material is. Foams at a higher density will typically be more durable and supportive because of their ability to withstand more pressure and compression without losing shape. They will also generally last longer because they are less prone to deformation over time.

Lower-density foams are best for non-structural applications such as taxidermy, lightweight parts, props, and backfilling. They’re typically easier to sand and carve.

Higher-density foams are suitable for similar projects or larger, heavy-duty applications like manufactured and cast stone projects or dunnage because of their durable, load-bearing characteristics. These foams are also more capable of holding objects like nails, screws, and more if needed.

You can also overpack a mold to produce a higher-density foam or a denser skin on the foam. You need to be sure that the mold can withstand the pressure. Overpacking is also a way to save money since you’ll need more dense foam to fill the same space as a less dense foam.

Foam density also directly correlates with expansion rate. The higher the density of the foam, the less the foam expands, and vice versa.

Color

Definition: The color that the material cures to.

Importance: Most urethane expanding foams will be beige/yellow or white, which provides a good base for top coating, if needed. The color doesn’t typically matter to a lot of users due to it being a common backfilling material, so no one will see the material anyway. Painting is recommended if appearance is a concern. For a UV-stable part, we recommend using an in-mold coating that is applied to the mold before the foam is poured or injected.

Some foams can even be formulated to cure to spintoecific colors. They can be pigmented if a solid color is desired for applications such as archery targets. Pigments are added to the polyol (B side) to achieve the desired color before the pot life begins. Too much pigment, however, can affect the cell structure and overall quality of the foam.

Compression at Yield (Rigid Only)

Test Method: ASTM D1621

Definition: Compression at yield is a measurement that determines the specific amount of stress the foam can withstand under a compressive load before it begins to permanently deform. When compression exceeds the yield point, the material cannot return to its original shape, even after the load is removed. This property is for rigid foams and is expressed in units of pressure like pounds per square inch (psi).

Importance: This property will indicate how well a rigid foam withstands pressure before losing shape. It is not typically relevant for semi-flexible or flexible foams because they are made to compress. Foam with a higher compression at yield will support more weight and handle stress better before it deforms. If you need your foam to be more structural, you’ll typically want to use a high-density foam with a high compression at yield.

Compression at yield is also a factor that determines how firm the foam may be, since it is a measurement of the foam’s ability to resist compression. The higher the compression strength, the harder the foam will be.

What Liquid Properties Are Important?

Liquid properties are the bulk of what foam users want to consider when choosing a urethane foam. These properties apply to the material as a liquid, which are most important when you’re actively working with the material.

Mix ratio

Definition: Mix ratio is a property that tells users of materials with two or more components the exact amount they need of each to produce a solid final product, like urethane foam. The property is expressed as a ratio of the A and B side materials (A:B). Mix ratio can be listed in two ways:

• By weight – The exact proportions of Part A and Part B that must be combined to elicit the correct reaction, measured using an accurate scale. (Ex: VFI-5104 has a mix ratio by weight of 100A:85.12B)
• By volume – The exact proportions of Part A and Part B that must be combined to elicit the correct reaction, measured using mixing containers with volume measurements. (Ex: VFI-5104 has a mix ratio by volume of 1A:1B)

Importance: Accurately measuring the two parts for expanding foam is critical to achieving a proper cure, expansion, and full properties. Not mixing the material correctly can result in a failed or uneven cure. Staying on ratio is important to elicit the best expansion reaction; otherwise, the foam may be brittle, shrink, or not expand fully, making it unusable.

Most urethane expanding foam users choose to measure their material by volume. This is because these foams have simple mix ratios by volume of 1A:1B or 1A:2B, which makes processing convenient and relatively easy. Once measured and added to the same cup, the expansion process will start shortly after mixing. If you don’t stick to the correct mix ratio, you might be at risk of wasting material.

Mixed Viscosity

Test Method: ASTM D2393

Definition: Mixed viscosity is the combined material’s (A and B side) resistance to flow. It is expressed in centipoise (cps). The chart below lists the viscosity of common household items to help you better compare the mixed viscosity of urethane foam.

Material	Viscosity
Water	1-5 cps
Corn syrup	50-100 cps
Maple syrup	150-200 cps
Castor oil	250-500 cps
Urethane expanding foam	200-2200 cps
Honey	2000-3000 cps
Molasses	5000-10000 cps
Chocolate syrup	10000-25000 cps

Importance: Viscosity is a property that affects the workability of the foam when it is in a liquid state. Foams with a lower viscosity will be much easier to mix, which is essential because of their limited pot life. The easier it is to mix quickly, the better the foaming reaction will be.

Foam with a lower viscosity will also be easier to pour and let flow into a mold. This is nice if you have a complex mold. However, once the foaming reaction starts to occur, the flow of the material will change, and the foam will get thicker. It will also go in the path of least resistance until it is done expanding. This viscosity increase is essential for the foam to reach its final density.

Mixed Liquid Density

Test Method: ASTM D1475

Definition: Mixed liquid density is a measure of the mass per unit volume of the mixed foam components when it is still a liquid. It is typically expressed in pounds per gallon (lb/gal).

Importance: The liquid density of the foam will not be nearly as important as the density after the foam cures. It can help with calculations to determine how much material you need to fill a specific volume. Most rigid and semi-flexible foams are around 9 lb/gal.

If you determine how much combined material you will be using in weight per gallon and divide that by the mixed liquid density, it will tell you what the starting volume will be. Multiplying the starting volume by the volumetric expansion will tell you the free-rise volume in gallons.

Cream Time (Pot Life)

Definition: Cream time, also called pot life, is the time after combining the two components that you will have before the foam starts to expand. This time frame is usually expressed in seconds to minutes when working at room temperature (77°F).

Importance: Urethane foams are sometimes harder to work with compared to other urethane materials because of their short pot life. It is important to take note of the pot life to successfully use these materials without generating waste. The second the two components touch is when the pot life starts. Some foams only have 30 seconds for you to mix and pour them before the rising process begins. Having everything laid out before mixing the material will make it easier to stay within the cream time.

Consider the size of your project when looking for an expanding foam to use. If you’re working on a large project, you’ll want to make sure you have enough time to mix up the amount of foam you need before the rising reaction occurs.

Several things will impact the pot life, including temperature. If you use the material in really warm working conditions (above room temperature), that will speed up the pot life, which isn’t ideal when the pot life is so short.

Rise Time

Definition: Rise time is the time between the initial foaming reaction and the time at which the foam stops expanding, reaching its highest volume. During this time, the foam will grow and fill whatever cavity or structure it is poured into. This property will usually be expressed in minutes since the foam’s viscosity changes, which causes it to slow down as it rises.

Importance: Rise time is important to know because it will determine how quickly the foam will expand and then begin curing. Having a controlled rise time also allows for proper filling of the mold to prevent defects in the foam, like voids.

A fast rise time may result in an incomplete fill, while a slow rise time gives you more time to work with the material before it fully sets. This makes it nicer for filling more complex-shaped molds. It can also make it easier to sufficiently clamp a mold shut before the foam rises too much. Foams with a longer pot life will also have a longer rise time, handling time, and demold time.

Like pot life, temperature can also affect the rise time. If you’re working in a warmer environment or your mold or material is too warm, your rise time might decrease. If you need the foam to be more pliable for a longer period, you’ll want to work in room temperature conditions or conditions that are slightly cooler.

Tack Free Time (Handling)

Definition: Tack free time, also known as handling time, is the point at which the foam forms an outer skin and loses stickiness. It can be touched and then demolded shortly after. Depending on the foam formula and its pot life and rise time, this property can be expressed in seconds or minutes.

Importance: Tack free time indicates when the foam will no longer be able to pick up dust or debris in the work area. These contaminants are undesirable because they may compromise the appearance and performance of the foam.

Knowing when you can handle the material can be important for certain molding applications, especially when the foam is used as a backfilling material. You’ll be less concerned about when you can remove it from a mold and more concerned about when you can post work and move it. When the foam has lost its tackiness, it will soon be ready for further processing, like sanding, machining, and painting, without compromising the final properties.

Demold Time

Definition: Demold time is the time the material should be left in the mold or form at room temperature (77°F) before it is removed. It is usually expressed in minutes for urethane foams specifically, but other urethane products can take hours.

Importance: All demold times are typically based on a room temperature cure unless otherwise noted. Demold time will vary based on volume, temperature, mold type, and more. It’s important to wait for the foam to cure fully before demolding to prevent deformation. Most foam demold times are pretty fast (same day) for a quick return to service.

If you are making a foam part, you’ll want to know how soon you can remove it from the mold and begin post-work. Shorter demold times are desirable for processing parts quickly. You also want to be careful when demolding so you don’t tear the foam. Most rigid foams should not have a problem, though.

However, demold time is not always an important factor for foam, depending on the application. If the foam is used for backing material, you’re probably less concerned about demolding. In this application, the foam is meant to stick to a thin film of plastic or rubber to make the interior lightweight with a durable outer shell.

Cure Time

Definition: Cure time is the amount of time it takes for the foam to reach its full physical properties. Most cure times are listed as hours.
Importance: On initial cure or when you are able to handle the foam, it will not have developed full properties. Once the foam has cured, it will perform as per the properties listed on the technical data sheet. The cure time can be affected by many factors, with temperature being the most common.

Approximate Volumetric Expansion

Definition: The approximate volumetric expansion of a foam determines how much it will expand by its original liquid volume once fully risen.
Importance: Foams that are lighter in density will typically fill more volume and expand at faster rates. For example, a 3 PCF foam like VFI-5103 will expand nearly 23 times its original liquid volume, while a 20 PCF foam like VFI-5120 will rise under 4 times that.

The expansion rate can be affected by multiple factors, including temperature, mixing technique, humidity, and mold shape. Higher temperatures and humidity will increase the reaction. If you rapidly mix the material, you may also see a greater foaming reaction. However, if your mold is thin and tall, you may see less reaction.

Where to Find Material Properties?

VFI urethane foam properties can be found on all product pages and technical data sheets. Navigate to the product page of any foam product, and you will be able to find physical and liquid properties listed on the right-hand side of the page or the TDS listed under resources. For mobile users, these tabs are at the bottom of the product page. The VFI-5100 Series has a special family sheet, which lists all our rigid foams and their properties in one place.

We pride ourselves on providing accurate properties for all our products, so various test methods are used to determine these properties before they are posted.

Contact VFI if you need further assistance finding foam properties or determining a foam that’s perfect for your project.