Polyurethane - Simplified

It's in the cars we drive, the shoes on our feet, and the furnishings in our homes and businesses. It can be found in many forms such as a molded part, a foam cushion, a spray coating, or an adhesive. In other words, polyurethane is everywhere.

So where does polyurethane come from? Simply put, it is the result of a chemical reaction usually consisting of two components - isocyanate and polyol. What's exciting is the unlimited possibilities for creating new formulations - or "recipes" - that can manipulate the polyurethane to become hard or soft, flexible or rigid, light weight or gel-like.

It's a Piece of Cake

A Specialty Polyurethane Systems Manufacturer

For the non-technical person, it’s kind of like baking. With the two main ingredients – flour and sugar – you can make a huge variety of foods such as pie crusts, cookies, bread or angel food cake, by adjusting the quantities of flour and sugar and adding other ingredients such as baking soda and seasonings. Or, you can buy premade mixes that already have the right proportions of everything you need. Just mix in the liquids, pour the batter into a mold, or spoon it onto a cookie sheet, heat at a specific temperature for a certain amount of time, and you have a finished product.

Addition of a Catalyst

A catalyst is used to lower the activation energy required for a reaction to occur thus, increasing the speed of a chemical reaction. While air, water or heat may be enough to trigger the reaction, chemical catalysts may also be used – usually in small amounts.
catalyst icons When baking a cake, you could mix the ingredients and leave them on the counter, but you’ll probably never have a cake. The heat in the oven acts as the catalyst to turn the reactants (ingredients) into the desired product – your cake!

Polyurethane

Cake! Mmm.

A Specialty Polyurethane Systems Manufacturer

Polyurethane systems are like liquid cake mixes. In one-component systems, the ‘batter’ is a prepolymer that is already mixed. Once the container is opened, the mixture will start reacting with the moisture in the air and the pot life clock will start ticking. In a two-component system, the ISO and POLYOL components are kept separated so the chemical reaction will not start until the two components are mixed together in quantities defined by the mix ratio. For example, a mix ratio of 100:25 indicates 100 parts of POLY to 25 parts of ISO.

Caution – Handle polyurethane systems with care. They should never be ingested.The cake terminology is only a decorative way to explain how the ingredients combine and do not constitute the outcome as being edible.

When introduced to ISOs or prepolymers, even a tiny amount of water is enough to start the chemical reaction that can create enough pressure to deform a steel drum or turn it into a projectile. For this reason, it is extremely important to keep ISOs and prepolymers dry. Nitrogen is heavier than oxygen, so before sealing the drum, nitrogen gas is added – often called a nitrogen blanket – into the head space between the product and the top of the drum. This serves as an additional layer of protection against moisture.

Always be sure to consult Safety Data Sheets (SDS) and Technical Data Sheets (TDS) to learn about the precautions that must be taken when handling any chemicals.

Glossary

We might use some intimidating language, but the meanings are simple - here are some definitions in case you forget

Additives – A substance that is added to a polyol component to alter the properties, usually in a relatively small percentage. Examples are: degassing agents, moisture scavengers, surfactants, adhesion promoters and etc.
Adhesive – A substance used to join two or more materials.
Aliphatic – A type of polyurethane that does not contain carbon atoms arranged in aromatic (benzene) ring structures. As compared to aromatic, products based on aliphatic polyurethane have superior resistance to UV weathering, and better color and gloss retention.
Ambient Temperature – The surrounding air temperature.
Aromatic – A type of polyurethane that contains some carbon atoms arranged in aromatic (benzene) ring structures. As compared to aliphatic, products based on aromatic polyurethane are usually tougher, but have less resistance to UV weathering, and lesser color and gloss retention.
Catalyst – An ingredient used to increase the speed of a chemical reaction.
Compression Set – Measures the resistance of material to permanent deformation.
Curative – A compound that completes a polymer reaction.
Cure – A material bond by permanently cross-linking its molecules.
Cure Time – A term which refers to the completeness of the chemical reaction. At 100% completion the polyurethane should have reached 100% of its maximum physical properties attainable.
Demold Time – The fasted time a cured part can be removed from the mold.
Density – Density equals the mass of the substance divided by its volume.
Discoloration – Any change from the initial color.
Durometer – One of several measures of the hardness of a material.
Elasticity – The measure of a material’s ability to reshape itself after it has been stretched or deformed.
Elastomer – Polyurethane that has “rubber” like characteristics which resist and recover from deformation produced by force.
Elongation – The percent of its original length to which a sample will stretch before breaking.
Filled System – Polyurethane with filler
Flammability – Relative ability of a material to support combustion as expressed by its flash point.
Flash Point – The lowest temperature of a material at which it gives off vapors sufficient to form an ignitable mixture with air near its surface.
Gel – The initial semisolid phase that develops during the formation of a resin from a liquid.
Gel Time – The “working time” from the initial mixing of the urethane raw material components to the solidification of the material.
Glass Transition Temperature – The temperature region where the polymer transitions from a hard, glassy material to a soft, rubbery material.
Green Strength – The ability to be handled before it has completely cured.
Hydrophilic – An affinity for water
Hydrophobic – Water repellent
Ignition Temperature – The minimum temperature to which a solid, liquid, or gas must be heated in order to initiate or cause self-sustained combustion independent of the heating element.
Isocyanate (ISO) – Chemical compounds having one or more reactive NCO groups.
MDI – An acronym for 4,4′ diphenylmethane diisocyanate, a common raw material used in the production of polyurethane elastomers.
Mix Ratio – Expresses the proper proportion (either by weight or volume) of POLY and ISO to be combined before pouring.
Moisture Cure – Prepolymers that are formulated to cure on exposure to moisture either in the substrate or atmosphere.
NCO% – A measure of the isocyanate content of a prepolymer or other isocyanate-containing compound measured as the weight percent of unreacted isocyanate groups in the material. Value is used to determine proper mix ratio of polyurethane system components.
One-Component System – Systems that cure either by water vapour reacting.
Open Time – The time after adhesive is applied during which a serviceable bond can be made.
Plasticizers – Chemicals used as additives that generally serve to increase the flexibility and lower the viscosity and hardness.
Polyol – An alcohol with more than two reactive hydroxyl groups per molecule.
Polyurea – A type of elastomer that is derived from the reaction product of an isocyanate component and a synthetic resin blend component. The isocyanate can be aromatic or aliphatic in nature.
Polyurethane – A compound based on the reaction of various isocyanates and polyol resins.
Post Curing – A secondary period of curing a cast part at elevated temperature, once it has reached its “green strength” which promotes the completion of the chemical reaction.
Pot Life – The amount of time available to work with a product before it begins to gel.
Potting – The process of encapsulating a device by pouring a casting compound into a cavity in which the device has been fixed and curing the compound in place.
Prepolymer – Formed by combining an excess of diisocyanate with polyol.
Resilience – The ability of a substance or object to spring back into shape.
Resin – Any natural or synthetic organic compound consisting of a noncrystalline or viscous liquid substance.
Shelf Life – The period of time during which a product can be stored under specified temperature conditions and remain suitable for use.
Shore Hardness – Measure of hardness based on the shore scale.
Specific Gravity – The ratio of the weight on any volume of a substance to the weight of an equal volume of another substance taken as standard at a constant or stated temperature.
Surfactants – An additive that either helps or hinders the formation and stabilization of bubbles.
TDI – An acronym for toluene diisocyanate, a common raw material used in the production of polyurethane elastomers.
Tear Strength – The maximum force required to tear a specimen, the force acting substantially parallel to the major axis of the test specimen.
Tensile Strength – The force necessary to rupture a material sample divided by the sample’s original cross sectional area.
Thermal Conductivity – Ability of a material to conduct heat.
Thermoplastic – A type of polymer that can be melted and reformed by application of heat.
Thermoset – A type of polymer that cannot be melted and reformed by application of heat.
Thixotropic – Having the property of decreasing viscosity with increasing shear stress. A coating is thixotropic if it thins with stirring or pumping but thickens back up when movement ceases.
TrioI – A polyol characterized by having three reactive hydroxyl (OH) groups.
Two-Component System – Polyurethane formed by the mixing and the reaction of two different materials.
Unfilled System – Polyurethane without filler.
Viscosity – The thickness or resistance to flow of a liquid. Viscosity generally decreases as temperature increases.
Working Time – The time period during which an adhesive must be used after its container has been opened or, in the case of two-part curing adhesives, after it has been mixed.