Instructions to Test-Pour Elastomer Material by Hand Mixing

 

For Gel Polyurethane Materials

 

Polyurethane gel material is used in many applications for cushioning, vibration damping, and other effects.  Applications for polyurethane gel elastomer includes shoe insoles, bicycle sheets, computer keyboard and mouse pad wrist rests, motorcycle seating, axially seat cushion for automobiles, wheel chair sheets, shock absorbing parts for sports equipment, medical and personal care parts, horse-back riding saddles, vibration damping for machines, and many other applications.  

 

Room temperature curable polyurethane materials in general are relatively easy to process with minimum investment into tools and equipment.  However, there are some techniques you may want to learn before you start manufacturing your parts.  We recommend testing with small quantities to learn the properties and behaviors of the material. 

 

Most of the polyurethane gel formulations are made in such way that the user or the parts manufacturer can control the softness of the gel product by changing the mixing ratio of part-A and part-B components.  Through your tests, you may want to determine many processing aspects as well as the optimum softness of the gel part for your application.  Generally, increasing part-B ratio will give softer products.  The mixing ratio for your formulation is not shown in this document as each formulation has different mixing ratio.  Please refer to other document provided by, or ask, Northstar Polymers for mixing ratio recommendation for your application.

 

Basic Terminology

 

Polyurethane elastomer gels are made by mixing the two components; we call those two components prepolymer and curative.  The prepolymer is also called part-A in short.  The curative is called part-B.  The name for the combination to make a product (solid, elastomer, or foam polyurethane) is called a system.  To achieve your desired softness, you may also incorporate plasticizer.  Plasticizers are inert liquid materials widely used in many synthetic materials. They are added for a few different purposes.   

 

Before you open the containers, you should prepare the following items.

 

Required Items

 

1. Containers to weigh and mix            

 

The mixing container can be a regular poly-bucket, paint cans, cups, or lab beaker. Make sure the inside of container is clean, dry, and free of any solvent.  Some polyurethane materials are very sticky.  It is not easy to clean the container after use.  Use a disposable container.   

 

2. Scale

 

A digital scale that shows integer values and first and second decimal numbers is convenient.  A lab scale may be very handy because you can reset to zero after you measure one part with container.  You can also use measuring cups.  Please note, for the same formulation, the volume and weight ratios are different.   Keep your mixing ratio record uniform in terms of weight or volume.  Please refer to the ratio information provided by Northstar Polymers. 

 

3. Gloves         

 

To handle any polyurethane material, you need to wear safety gloves. Disposable safety gloves made of Latex, Nitrile, Neoprene, or other similar material will be appropriate.  If you need a source for the gloves and other paraphernalia, here are some web pages you can browse and telephone numbers.

 

            www.labsafety.com      1-800-356-0783

            www.wwrsp.com          1-800-932-5000

            www.coleparmer.com   1-800-323-4340

 

 

4. Protection   

 

Make sure the entire operation is done in a well-ventilated area or large room with air circulation.  Dynamic means of ventilation (fans) are recommended.  Aside from the gloves, you should also wear long sleeves and safety glasses to prevent skin/eye contact of the material.  If the chemical gets on to your skin/eye, wash it off with soap and water as soon as possible.  A washing facility should be readily available in the working area.  The material is relatively safe for an industrial chemical, but it is still harmful to health if it enters a human body through inhalation, skin/eye contact, and ingestion.  Read the Material Safety Data Sheet (MSDS) for more information.  The MSDS are shipped together with each component material.

 

5. A stir stick

 

Plastic or stainless steel flat-end spatula, paint stick, table knife, or any stick clean, dry and free of solvent.  Avoid using wood stir sticks.  You can use a hand mixer if you are making a large batch.  If you are to use hand-held power mixer for mixing, we recommend a paint mixer that is made to prevent folding air into the material. 

 

6.  Mold Release

 

You need to use a mold release for demolding. We recommend pure silicone brushable mold release, because you can apply it right anytime pouring and easy maintenance.  You can also use silicone dissolved within a solvent.   If it is mixed with a solvent, make sure the solvent is dry before pouring.  Do not use water base mold release.  Hydrocarbon base (mineral spirit, kerosene, etc.) mold release takes longer to dry.  Many solvents are flammable.  Please follow the instruction on the mold release for safety.  Wax mold release can also be used. 

 

7. Oven (Optional)

 

The part-A material is very thick (high viscosity) and heating makes it thinner.  To make it easy to handle and mix well, you can heat the material between 100 and 180º F (38 to 82 ºC.  This is an optional, and if you do not have an oven, you can omit this, but it may be difficult to handle the part-A material and to degas. 

 

Heating material increases chance of the materials to vaporize.  Be sure to wear protective glasses when handling heated materials.  Do not put your face directly over the heated material.    

 

8. Vacuum Chamber

 

If you do not want bubbles in the material, you can use a vacuum chamber to de-gas. This is also an option.  With hand mixing, you cannot completely avoid bubbles.  For your future production, using casting machine with vacuum capability will allow you to produce bubble free products. 

 

9. Cleaning solvent

 

This is for cleaning the tools.  You can use Isopropanol, dibasic ester, MEK, acetone, or other ketones.  Isopropanol is easier to obtain.  You may obtain it in a local drug store.  Always dry solvent completely before reusing your tools.  Mineral spirit or paint thinner does not dissolve this material well.  Aromatic hydrocarbon solvent has some solubility to this material.  The most of solvents are flammable with high VOC rate.  Follow the safety instructions of the solvent material when using. 

 

10. Paper Towels

 

For cleaning.  If you use cloth towel or rag, expect you cannot re-use.  Clean the items with paper towels and isopropanol, dispose of it as industrial solid waste.  Do not drain any of this material down into the public drain system or unlawful area. 

 

11. Nitrogen gas

 

When you are done with mixing, the part-A material needs to be blanketed with nitrogen gas and the lid has to be closed tight for future use.  You will need the nitrogen gas in future for your production, too.  If you need, call Praxair 800-772-9247 for your local distributor.  We use a 50 LBS cylinder at our lab.  It is portable if you use a cart. 

 

The reason for doing this is that the part-A has isocyanate, which is very reactive.  It is so reactive that it reacts to any form of water in contact.  If you leave this material out, it will react with the moisture in the air, which will spoil the material. When part-A is reacted with moisture in the air, it will harden by it self and you cannot use anymore.  Nitrogen will not react to isocyanate, so it will keep the quality for longer time. - 40 ºF dew point dry air can also be used to blanket the components.

 

If you are using a small amount at a time, you may want to repackage part-A into smaller size paint cans.  This will minimize opening of the can and in case it is contaminated, the damage will be minimum.  If repackaging, fill it closer to the top and minimize the space between the inside top of the can and the material.

 

Procedure

 

1. Heat the part-A material (100 to 140 ºF) in an oven (if needed).

 

2. Prepare the mold. Apply the mold release lightly. Be sure to dry the solvent (if needed).

*If you are casting right into a film encapsulation, you may not need the release agent.

 

3. Calculate the correct amounts of each part-A and part-B material.

           

4. Open the can of part-A pour the necessary amount into the container as you weigh.  Be careful if you are handling a heated material.

 

5. Close the can of part-A.  When you close the can of part-A, be sure to blanket the material with nitrogen gas and close the lid tight. 

 

6. Open the can of part-B, pour into the container as you weigh necessary amount. Close the lid of part-B.  The order of part-A and part-B to be poured into the mixing container is not critical.  Pour whichever convenient first.

 

Generally, part-B materials are not as much moisture-sensitive as part-A, so you do not need to blanket with nitrogen gas.  However, it is a good idea to close the lid tight to avoid moisture and other contamination, when you store.

 

8. Mix

 

When mixing by hands, use plastic or steel stir-stick and agitate vigorously, but try not to be enclosing or folding-in air. In other words, "do not whip" like beating egg.  If you do not heat part-A, it may take longer time to mix. Mix carefully and thoroughly so the components are mixed homogeneously without any inconsistency. 

 

If the material is less than a half gallon, mix at least for one minute.  Scrape the side and bottom to make sure it is mixed very well.  Even if the material appears to be mixed, they are often not enough. Thorough mixing is very important.

 

If you use a hand-held electric mixer, mix at the low speed.  High speed can ruin the material by creating folding-in the air or inducing heat.  Mechanically scrape the sides of the container is still needed for thorough mixing since the mixture is very thick.  

 

9. Place the container in a vacuum chamber (Option)

 

Ideally, put 29" Hg or more vacuum until you see most of the bubbles are gone.  Do not leave the material too long as it may induce a quick reaction with heat.

 

10. Pour into the mold

 

If the part is small, you can pour in one puddle.  If you pour bigger parts, avoid pouring layers; pour straight from one end to the other and try not to come back.  If you are pouring a flat part with different depth, pour the deeper end first and move to the shallower end.

 

The time between you mix the part-A and part-B and the time the material becomes too thick to pour is called "pot life". Usually our standard gel materials are made for 15 to 20 minutes pot-life, unless specified other wise.  Curing time pattern varies depending on the system, and it can be controlled by catalysts to meet your production requirement.  Consult Northstar Polymers for your specific curing time requirements.

 

11. Clean up the tools with isopropanol and paper towels

 

12. Cure

 

For testing, cure over night before de-mold.  The material hardens gradually.  De-molding of parts differs depending on the part sizes, curing pattern of the material, and complexity of the part.  Generally, you can demold the part in about 5 to 6 hours if you are cuing at a room temperature.  At this point material should be cured enough so that you can handle it as a solid piece.  However, the material is not completely cured at this point.  The hardness of the gel gradually increases over 7 days or so. Usually this is not a problem at the production level since often your part will not be in service for about one week for packaging and shipping etc.

 

Heating (up to 180 ºF) will accelerate curing and ensure the part integrity when demolding.  When you are evaluating the mechanical properties of the material for your specific application, conduct your tests at least 7 days after the date you make the part.  If you need to test sooner, you can post cure at 180 ºF for 16 to 24 hours and cool to room temperature. 

 

The above curing pattern is all under the standard catalyst level and can be modified to meet your requirement.  Please consult Northstar Polymers for modification.  

 

13.  De-mold

 

Demold slowly and carefully.  It may be difficult to demold if you have not put enough mold release.  The time between when you mix and the material becomes hard enough to demold is called "demolding Time".  Demolding time is also variable by concentration level of catalyst.

 

The surface of gel parts may be very sticky.  You may need to develop your method to handle your demolded gel parts.  Use of talc powder may alleviate the problem.

 

Encapsulation

 

Because of the stickiness, this gel material is often used with an encapsulation material.  Thin and flexible polyurethane films are often used because of their strength and flexibility.  Coated Lycra fabric is also used for covering. Coated fabric can be used as encapsulation as well as covering material. If any fabric is used to contain the material when it is liquid, the fabric material needs to be coated for leak-proof.  Vacuum forming technique is often used to shape films and coated fabrics.  Dispersion coating (silicone rubber, polyurethane, or other flexible coating materials) can also be used to cover the surface of gel.

 

 

 

Additional Information

 

Modification of the system

 

The time needed to completely cure the material is called "Complete Cure Cycle".  This is an important element to your productivity.  Shorter the complete cycle, better the productivity will be.  However, the pot life, demolding time, and complete cure time are all related.   If you make one element shorter, other elements also get shorter.  We can control the cure pattern by modifying the formulation, but we cannot shorten one element significantly faster or slower without changing others. 

 

Other modifications including change of mixing ratio and adding colorant, UV absorbing agent, anti-bacteria agent, and other additives may also be available.

 

Plasticizer

 

You can add a plasticizer to modify the properties of the product.  Plasticizers are widely used in plastic and elastomer industry for softening the products and other purpose.  In many years, plasticizer could gradually leak or vaporize from gel parts over time.  It is generally true for most of plasticizers when they are incorporated into plastic or elastomer materials. We recommend testing and study the long term effect if you are using plasticizers.

 

Storage of materials

 

Part-A contains reactive isocyanate and needs to be blanketed by nitrogen gas or minus 40 ºF (40 ºC) du-point dry-air.  Keep the lids shut tight.  Store the components at 70 - 77 ºF (21 to 25 ºC) degrees. Under the right condition, the material should last at least six mouths.  Repackaging into one-shot size can prevent having a large amount of material contaminated from air/moisture leak into the container.

 

Disposal

 

Do not dispose of the materials into any public sewage system or unlawful area.  Generally, you can dispose of mixed and cured material as a solid industrial waste. Consult your local authorities for the disposal information.  Please read the enclosed Material Safety Data Sheet or detail information.

 

Prepolymer free NCO %

 

Free NCO % is level of NCO sites available for reaction within the prepolymer.  Prepolymer is produced under our specification and the quality stays consistent within the specification.  However, even within the specification, some sensitive application may be effected by the slight fluctuation of free NCO % within our specification.  In general, higher NCO % results harder products.  If your application is very sensitive to the hardness, you may need to adjust the mixing ratio for a particular lot of material.  If that is the case, please consult with Northstar Polymers for methods of calculation for ratio adjustment.

 

Fire retardant properties

 

Our standard gel materials are not classified as a fire retardant material.  Some applications including automotive, and airplane parts require fire retardant property.  This material cannot be used for those applications.  Please check the relevant regulations for your application.  Modification may be done to our standard formulation to enhance fire retardant properties.  Please consult Northstar Polymers for details.

 

Persons with isocyanate hypersensitivity

 

In normal operations, this material should have MDI in vapor under the threshold limit value of 0.02 ppm.  However, an individual who has been sensitized by an isocyanate (TDI/MDI/IPDI etc.) may have hypersensitivity reactions such as irritation of the eyes and respiratory tract (similar to asthma-like responses) even under this threshold value.  To minimize the risk due to the exposure to the materials containing an isocyanate, operate under well-ventilated condition (dynamic ventilation), wear protective cloths and gloves should be worn.  A half-faced respirator is also recommended for extra protection.

 

 

Safety:

 

The component materials are industrial-grade chemicals.  Please keep them in a secure place and prevent access from any unauthorized individual.  The personnel who handles these materials needs to read the Material Safety Data Sheet  (MSDS) for detail information on safety and handling of the material.  The MSDS for each component is sent with the shipment of the material. 

 

When using this material, be sure to operate in a wide-open area with good air movement, or in a well-ventilated area. Wear rubber gloves, long sleeves, and protective eyeglasses to prevent skin/eye contact of the material.  When your operation involves heating or spraying of the material, we recommend, in addition to the above, installation of a proper ventilation system such as a overhead hood with a dynamic ventilation system, and/or using a proper half-face respirator recommended for the use to prevent inhalation of the fume.  Extreme heat should be avoided when storing.  

 

Direct contact of polyurethane raw materials to skin/eye, as well as ingestion may lead to health problems.  No eating or smoking should be permitted at the working area.  The operator should wash hands well with soap and water after handling the materials.  Please refer to the MSDS for each component for the detailed health information.

 

For any questions, please contact Northstar Polymers.