January 17, 2022
Polyurethanes are a modern wonder polymer that can take on many different properties depending on how they’re made. First created in the 1930s, Polyurethane rose to prominence during World War 2 as a replacement for natural rubber, which grew increasingly scarce during the war. Since then, different kinds of polyurethane have exploded in number, ranging from clear polyurethane varnish that you can brush on to protect an item from the effects of sunlight and weather, to hard plastic liquid resin like our Urethane Casting Resin, or even a rigid foam that can be poured in place like our 2# Density Pourable Urethane Foam. Let’s take a quick look at how polyurethane materials get made, what they get used for, and how to figure out what you need for your project.
Polyurethane is a polymer created by a reaction between two types of chemicals called Polyols and Isocyanates. When these two chemicals combine, they react vigorously and form polyurethane. Controlling the weight of the polyol molecules can change a polyurethane from being rigid to become a soft rubber, then different additives can be added to change other characteristics of the material. For example, adding a Blowing Agent to the mix will cause the combined Polyols and Isocyanates to create a foam as the hydrogen gas formed in the reaction gets caught up in the soup instead of escaping.
Now that we know what makes up a Polyurethane, let’s look at what you can use them for. To start, the original usage is as a synthetic rubber- using high molecular weight Polyols in the reaction produces a softer, more flexible polyurethane polymer that can be used for most things that natural latex rubber can be used for, and countless more to boot. This includes applications like isolation mounts for motors, insoles and outsoles for shoes, concrete molds, car tires, and more. The specific properties of the urethane rubber can be further modified to make a firmer, more durable rubber or a softer and more pliable rubber depending on the final user’s needs.
Using lower molecular weight polyols in the blend produces a rigid material once reacted and cured. This type of polyurethane sees use as a liquid casting resin to produce rigid plastics, including fishing lures, enclosures for electronic parts, parts for tools, and more. On an industrial scale, there are variations of this type of polyurethane that are used for countless types of plastic goods – materials made with thermoset plastic frequently use polyurethane.
First let’s look AeroMarine Products 75A Urethane Rubber that, when properly mixed and cured, has the feel and texture of a car tire. It is slightly flexible, extremely durable, and most importantly very resistant to tearing which makes it ideal for use with cement and concrete. The 75A Urethane Concrete Molding and Stamping Rubber excels at creating block molds for cement counters and pavers, as well as creating stamps for adding a unique flair to your walkways and walls. The catch to this material is it is highly adhesive, so using it to create molds requires the use of a good silicone-based mold release paste to create a barrier between the urethane and the item you’re using to make your mold.
This adhesive characteristic does make for an excellent potting compound if you have electronics that you’re sending underwater or that will be frequently exposed to wet areas. The durability and adhesion presented by 75A Urethane Potting Compound helps it to withstand deep atmospheric water pressure for extended periods of time.
Our next products to look at will be our AeroMarine Products White Casting Resin and Jet Black Casting Resin. These products are extremely similar in characteristic. The Jet Black Casting Resin has a very fast setting time while the White Casting Resin takes a bit longer giving you time to roto-cast a part if desired. The White Casting Resin can easily be colored with urethane pigments. Keep in mind since you are starting with a white base your colors will be more muted in tone. These rigid casting resins form a hard, solid plastic that captures fine details with minimal shrinkage and are ideal for creating many things ranging from finely detailed figures and models to rigid parts and components.
Lastly we come to Polyurethane Foam. This type of urethane can be soft like the padding in your office chair or rigid like a wooden plank, and almost as light as the air around it or as heavy as needed. The insulating properties of urethane foams are also quite impressive with the insulating value generally increasing as the density decreases, finding applications as a sprayable home insulation (when proper additives are used to make it sticky and quick-curing enough that it will stay where it is sprayed). In maritime applications, rigid close-celled foam is used to prevent water from entering open spaces in boats. These products are fantastically useful for keeping water out of places it shouldn’t be in your boat, protecting different components, or even for repairing rotted decks and transoms (8# Foam). On top of that, they can even be used for both audio and thermal insulation and can be shaped into decorative objects (4# Foam). Using a low-density closed-cell rigid foam such as AeroMarine Products #2 Density Rigid Pourable Foam allows users to fill bilges without significantly affecting their buoyancy or center of gravity and exclude water from entering if they should strike an obstacle or get swamped.
There are a couple things that our pourable rigid urethanes can’t be used for though. First, they cannot be used for spray applications. These urethanes are designed to flow like a thin oil and won’t really start expanding until it’s stopped flowing. This is a fantastic property if you’re trying to fill a void under the deck of your boat without taking everything apart, but it does also mean that putting it in a sprayer will result in a puddle of urethane foam on your floor and not on the wall where you wanted it.
The second place that our rigid urethane foams can’t be used in is mixing applicators, especially for industrial processes. This is because the foam kicks off very quickly and will clog the tips making them useless for the application. You’ll need a specialized foam for that type of use that we don’t currently provide.
We’ll go into depth on when to use each type of Rigid Urethane Foam in a future blog, but before I go, I wanted to share a couple general urethane tips. These tips apply to all our urethane products, whether it’s a rubber, rigid plastic, or foam.
Urethanes are sensitive to moisture until they have cured. Anything porous can hold moisture, and that moisture can transfer to the urethane while mixing. This includes wooden stir sticks, paper cups for measuring and mixing, even concrete can retain moisture which can get into urethane. We recommend always using plastic containers for measuring and mixing, and plastic stirring paddles for stirring. Wood and paper, even waxed paper, should always be avoided.
Urethanes are very sensitive to moisture. This is important enough to repeat. Humid days can result in undesirable cured results. You might have a foam that is weakened in areas because there are large voids in the structure instead of tiny uniform bubbles, or unsightly bubbles forming on the surface of a cast part, or even crumbly rubber that can’t hold its shape. Moisture is the number one reason we see resulting in failed projects. If you’re not working in an environmentally controlled area with under 60% relative humidity (lower being better), then it may be best to wait for a less humid day to work on your urethane project.
Finally, temperature matters. Our urethanes are all designed to work around room temperature (approximately 70f/22c). Temperatures below this can cause increased cure times while temperatures above this can accelerate the cure time. This is not good news – in cold temperatures a urethane foam may cure more slowly, but this will also result in increased density of foam. If you’re using it in your boat, a 2# foam may cure at 3# or 4# per cubic foot density and could throw off your ballast calculations leaving you with a list or risking capsizing if it’s bad enough. Likewise, a foam curing at higher temperatures may cure faster but can leave you with a lower density that is not as strong as you wanted. As an example, if you’re using an 8# density per cubic foot foam on your transom but it’s too hot, it could cure at something closer to 5# or 6# pound per cubic foot that crushes as soon as you tighten down your motor mounts instead of holding firm like the original wood or 8# density foam would have.
As ever, if you have any other questions, we’re more than happy to help you out. You can email us at info@aeromarineproducts.com or use the contact form here, or give us a call at 1-877-342-8860 between 9 and 2 PM Pacific time (12 to 5 Eastern time) Monday through Friday, or even catch us on Live Chat in the lower right corner of the screen here, from 10 to 2 Pacific time (1 to 5 PM Eastern Time) Monday through Friday.
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