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Instructions for Elastomer Materials Hand-Mixing

 

For Room-Temperature-Curable Polyurethane Elastomer

 

This is generic information for many of our room-temperature-curable polyurethane materials.  This document does not contain the mixing ratio or other specifics information of the particular material(s) you may be interested.  Please see the information provided by Northstar Polymers for details on the specific formulation.  If your application is to make soft-molds for concrete casting, prototyping, and other molding application, please also refer to Mold-Making Procedure page. 

 

 

Safety

 

Be sure to operate in an open area with a good air circulation or well-ventilated area.  Wear rubber groves, long sleeves, and protective eyeglasses to avoid skin/eye contact of the material. A washing facility should be readily available in case of skin/eye contact.  No eating, drinking, or smoking, should be allowed at the working site.  Please read the MSDS (Material Safety Data Sheets) for more details. 

 

Basic Terminology

 

The polyurethane elastomer is made from mixing two components; we call those two components prepolymer (or part-A) and curative (or Part-B).  The name for the combination to make a product (solid, elastomer, or foam polyurethane) is called system.  (Some other companies may call part-A and B in the opposite way. Please do not be confused. )

 

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, large empty coffee can, lab beakers or any other round-shaped dry/clean container. Make sure the inside of container is clean, dry, and free of any solvent. Some polyurethane materials stick to the surface and hard to come off.  You may have to through away the container.  Consider a disposable container.

 

2. Scale

 

A regular kitchen scale suffices.  However, a digital scale that shows integer values and first and second decimal numbers is convenient. A lab scale may be very handy.  You can also use measuring cups using volume ratio.  Please note, for the same formulation, the volume and weight ratios are different.  Please refer to the ratio information provided by Northstar Polymers.

 

3. Gloves         

 

To handle polyurethane component materials, please wear safety gloves to avoid skin contact of the material.  Latex, Nitrile, (or similar rubber material) disposable safety gloves are 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

 

They have many other items for chemical handling.  You may want to get catalogues from them.

 

4. Protection   

 

Make sure the whole operation is done in an open area with a good air circulation or well-ventilated area .  Dynamic means of ventilation (fans) are strongly recommended if you operate in a small enclosed area.  Aside from the gloves, you should also wear long sleeves and safety glasses.  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.  The material is relatively safe for its kind, but still can cause some skin irritation etc.  Read the Material Safety Data Sheet (MSDS) for more information.  MSDS is sent together with the each component.

 

5. A Stir Stick

 

Plastic or stainless steel flat-end spatula, table knife, or any stick clean, dry and free of solvent.  We do not recommend paint sticks as they often carry some moisture.  You can use a hand mixer if you are making a large batch.  The blades can be ones that are used to mix paint, which prevents air enclosure into the material.

 

6.  Mold Release

 

Typically, you need to use a mold release for ease of demolding.  We recommend pure silicone brushable mold release, because you can apply it right before pouring.  You can also use silicone dissolved in a solvent.   If you use a release with 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 time to dry.  Solvent is highly volatile and very often flammable, so you need to be careful avoiding fire and inhalation accident of solvent.  Please follow the instruction on the mold release for safety. (Our polyurethane components do not contain solvents.) 

 

Wax mold release can also be used.  However, wax release may require more care due to the build-up problem.  Wax mold release may have a narrower operating temperature range for effective release. 

 

Mold release can be obtained at your local industrial supply distributors.  

 

7. Oven (Optional)

 

Some materials are very thick (high viscosity), and difficult to handle.  Heating makes it thinner.  To make it easy to handle and mix well, you can heat the material within the range of 100 to 180 F (38 to 82 C).  Please consult Northstar Polymers for the appropriate temperature range for a particular formulation.  Be careful when you handle heated materials.  It can send a fume that is heath hazardous in some degree.  Use of a half-face, or full-face respirator is recommended when you need to heat the component materials.

 

This is an optional, you can omit heating if you do not want to.   Consult Northstar Polymers for an alternative methods if high viscosity may be an issue.   

 

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. 

 

May room-temperature-curable polyurethane casting materials have lower viscosity and slower pot-life (open time) to release major bubbles from the liquid.  However, with hand mixing, you cannot completely avoid bubbles.  If it is necessary to eliminate bubbles, you can vacuum the blended material in a cup or the mold into a vacuum chamber to degas.

 

For your production, using an airless mixing/dispensing machine will allow you to produce bubble free products.

 

9. Cleaning agent

 

This is for cleaning the tools.  You can use MEK, DBE,  acetone, n-pyloridone, isopropanol, or other industrial solvents. Mineral spirit or paint thinner does not work well with this material.  Please consult your supplier for the safety and handling of the solvent or cleaning agents.

 

10. Paper Towels

 

For cleaning.  If you use cloth towel or rag, expect you cannot re-use.  Cured polyurethane does not come off by washing.  Follow your local rules when disposing.

 

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 storage.  Praxair 800-772-9247 can provide you information on how to obtain nitrogen gas supply. 

 

The part-A has chemical called isocyanate, which is highly reactive to water/moisture.  It is so reactive that it reacts to any form of water including the moisture in the air very quickly. 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 loose isocyanate content and will not cure properly.  In a worse case, it hardens itself and will be deposited at the bottom of the container or make a skin on top.   Nitrogen gas will not react to isocyanate, so it will keep the quality for longer time.

 

Minus 40 F ( - 40 C) dewpoint dry air can also be used instead to blanket the components.

 

If you are using the entire content of part-A at a one time only or using it up in very short time, you may not have to use nitrogen gas.

 

 

Procedure

 

All materials and tooling should be in room temperature (about 70-100 F), when you do not need to heat the material. 

 

1. Heat the material (100 to 180 F), if needed. Consult Northstar Polymers for the appropriate temperature range.

 

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

 

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

           

4. Open the can of part-A or B, pour the necessary amount into the container as you weigh.  It is a good idea to pour thicker (higher viscosity) material first.  If the viscosity is the same, pour part-A or B whichever the smaller ratio component first.  (The order of part-A and part-B to be poured into the mixing container is not critical.  Pour whichever is more convenient first.)

 

5. Open the can of the other component, pour into the container as you weigh necessary amount.

 

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

 

This part-B material is not as much moisture sensitive as part-A, so no 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.

 

7. *** omitted

 

8. Mix

 

When mixing by hands, use a stir-stick and agitate it vigorously, but try not to fold in air. Scrape the side and bottom of the mixing container as you agitate.  If you do not heat thicker component, it may take a longer time to mix. Mix carefully so the components are mixed homogeneously.

 

If the material is less than a half gallon, mix at least one to two minutes.  Even if the material appears to be mixed, they are often not enough. Thorough mixing is very important.

 

If you use a hand-held powered mixer, mix at a lower speed.  High speed can fold the air into the material. Also, scrape the side and bottom of the container for thorough mixing even with a powered mixer.

 

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

 

Put 29" Hg of vacuum until you see most of the bubbles are gone.  If your vacuum does not reach 29" Hg, it can take longer time to degas.  Do not leave the material too long in the vacuum as it many start quick reaction.

 

10. Pour it in the mold

 

If the part is small, you can pour in one paddle.  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.

 

For your information, 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".  The typical pot life of this type material is about 8 to 25 minutes.  Curing time pattern can be controlled by catalyst level.  Consult us for your specific curing time requirements.

 

11. Clean up the tools with isopropanol and paper towels

 

 

12. Cure

 

Curing is the process that the components of polyurethane react to each other and harden.  First they are both liquid. Then the mixture becomes thicker.  It will be too hard to pour as liquid, it will be a gel that is do not move itself, and finally becomes solid. 

 

Observe the change in hardness of the material.  It will be useful later to know the shortest time you can demold without damaging the part. For testing, cure over night before de-mold.

 

The material hardens gradually and it becomes solid.  De-molding time of parts differs depending on the formulation as well as the part sizes, curing pattern of the material, and complexity of the part.  Heating (up to 180 F) will accelerate curing and ensure the part integrity when demolding.

 

When you are evaluating the mechanical properties of the material for your specific application, please conduct at least for 3 days as it gets the final hardness. This material continuously hardens for several days until it comes to the final hardness.  If you need to conduct physical test sooner, you can post cure at 180 F for 12 to 16 hours and cool to room temperature. 

 

Cold tooling (molds, frame, or model) could hinder the curing process of polyurethane.  If you are using steel or other metal for mold or tooling, especially in wintertime, you may need to warm up the tooling at around 100 to 120 F.  Do not heat it too hot (exceeding 180 F) as it could affect the quality of urethane.  When you are using a natural stone for mold, the same situation may be.

 

Heating up the material would help curing; however, it also shortens the pot life.   

 

13.  De-mold

 

At overnight de-molding, the material may be solid but still soft. Pull away from the mold 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 can also be controlled by level of catalyst.

 

The curing pattern varies depending on the formulation, catalyst level, and temperature. Consult Northstar Polymers for information on curing pattern of the particular formulation.

 

Trouble Shooting

 

 

Problems

Suggestions

The material does not cure.

 

      Temperature may be too cold?  Heat the mold, model, inserts, or frame and cure for longer hours.  Cold tooling hinders the curing process. Heat your tooling to 100 - 120 F.  If you are using natural stone or any heat-absorbing material for your model, it has a greater effect and it requires heat.

         Check the mixing ratio.  Make sure you are using the right ratio of the right combination.

         Contamination of material?  If part-A is badly contaminated with moisture, material may not cure.

         Catalyst level may be too low?  Contact Northstar Polymers

         Separation of the contents of the container for the components? Agitate the contents before dispensing.

 

Bubble Problems

(If you hand mix, there will be some bubbles.  Use a vacuum chamber to degas.  However, if there are excessive amount of bubbles, it could be a problem.)

         Wet tooling? If you are using wood tooling, dry it very well by kiln or some other dynamic methods. Plaster tooling must be coated after baked and completely dried. Wood surface may need to be sealed or coated with plastic coating or paint. 

         Avoid using water to clean the tooling. 

         Condensation on the material could cause bubbles as well.  Bring the components to warm room and leave it there for overnight before opening the container to avoid condensation problem.

         Wrong mold release? Do not use water-base mold release.  Hydrocarbon-base mold release could cause bubbles sometimes as well. If you are using solvent base wax or other type mold release, dry the solvent completely before pouring polyurethane.  

         Material contamination?  If part-B material is contaminated with moisture, often a bubble problem occurs.  To de-contaminate, Put the material in a vacuum chamber, and run the vacuum at 29 Hg or above for more than a half-hour to dry the material. When storing components in drums, cans, or pails, make sure the cover is tightly shut. 

         Whipping in air when mixing? Use slow speed mixer or mix carefully with a spatula.

Parts sticks to the mold

         Use right type of mold release.  Silicone mold release is recommended. If you are using wax mold release, do not heat it too hot as it could loose effectiveness of the mold release. 

         You might have missed some the spot when you are applying the mold release.  Ensure the mold release is applied to the entire surface that polyurethane material is in contact.

         Not enough mold release? Try increasing the amount of mold release agent.

         Mold has a pattern, which may be difficult to demold?  It may be that the urethane is too hard to demold.  If possible, choose softer polyurethane material so it flexes when demold.

         Using porous surface mold material? Seal the surface and make it smooth.

         Damage in the mold?  If the mold or frame is leaking, the material could get into the leak holes and solidify.  Examine the mold for leakage.   

Wet spots on surface

         The components may not be mixed well.  Even if it appears to be mixed, the material may not be mixed enough.  If you are mixing less than one gallon, agitate for at least one minute.  Increase the mixing time according to the amount for homogeneous mixture. Scrape the bottom and side of the mixing container as you agitate the components.  If you do not scrape the sides, a last few drops out of the mixing container may not be mixed and make wet spots.

         Wrong mixing ratio?  Check the ratio again.

         Cold mold, inserts, or model? Operating temperature too cold?  Warm the tooling to 100 - 120 F.

         Contamination of the material?  The mixing container or tooling may not be clean.  It could be contaminated with incompatible substance, which appeared to the surface?

Pot-life too short

         Processing temperature too high?  Mix at room temperature.

         Catalyst level too high?  Contact Northstar Polymers.

  

Does not cure fast enough

         Heat the material in the mold after it is poured. The maximum temperature is 180 F.

         Catalyst level too low?  Contact Northstar Polymers

The parts not as hard as expected.

       It may not have been cured completely yet.  Often it takes longer time for the material to cure when the room is cool.  You may need to move the part to warmer area.  Some material can take 7 days to get to the final hardness. Use auxiliary heat if you need it to cure faster.

       It may need to post cure. Thinner sections of the molded part may need auxiliary heat to cure.  Put the demolded parts into an oven at 180 F and post cure for overnight. 

         Inaccurate mixing ratio?  Often, if the part-B is put in more than the suggested ratio, the outcome becomes softer. Try to be more accurate on the ratio.  Slightly higher ratio of part-A can turn the material harder.

Voids in parts

      The material may be loosing the flow too soon? Specially for larger part, the material needs time to flow throughout the mold.  If the material gels too soon, it cannot flow to fill the mold.  Consult Northstar Polymers for a material with lower reactivity.

      Too much mold release? If you are using silicone mold release, a very thin layer of mold release should be efficient.

       If you are making a very small part or need to cast into a very narrow section, the liquid consistency of the mixed component material may be too thick to pass.  Heat the component materials to about 120 F may lower the viscosity and give a better flow. Or consult Northstar Polymers for an alternative formulation.

         Dirty mold? Clean the mold.

         Employing the right casting technique? Air pockets made from the mold pattern may cause voids.  Tilt the mold when you pour halfway, or vibrate the mold to avoid air pockets. Avoid the turbulence of the polyurethane mixture in the mold.  Pour in one paddle and try not make layers.  If you are pouring a large part, pour from one side to the other without coming back.  Pour from the deeper end to the shallow end if applicable.

         Exothermic reaction may be too fast or material is out-gassing?   Contact Northstar Polymers.

High shrinkage

         Exothermic reaction may be too fast.  Use lower processing temperature.  Contact Northstar Polymers.

 

 

Additional Information

 

Modification of the system

 

Curing Pattern (Catalyst Modification)

The cure pattern can be modified.  We can change pot life, demolding time, and complete cure time; however they are all related.   Shorter one element gets, also shorter the other elements to be. We can control it by modifying the formulation to accommodate your production requirement. 

 

Pigmenting

You can use polyurethane grade colorants/pigments to add colors to the material.  Particulate dispersion type colorants are commonly used with this type of polyurethane casting materials.  They are typically dispersed into polypropylene glycol ether.  We recommend using colorant paste dispersed in polypropylene glycol ether of above 3000 molecular weigh.  You can add this type of colorant at concentration of 0.1% to 2 % of the total weight without having major concern of the effect to the properties of polyurethane materials. 

 

You can obtain those colorants from the distributors of the following manufacturer:

 

Plasticolors:  http://www.plasticolors.com/  (440) 997-5137

 

 

Fillers

Fillers can be added to obtain certain properties.  However, addition of solid filler would deplete strength of the polyurethane material.  Also, microscopic surfaces of the filler would introduce a large amount of air into the polyurethane, and will cause bubbles to form.  If the bubbles are a concern, degassing by vacuum is needed.  In general, fillers cannot contain moisture as polyurethane components react with moisture and will have adverse effects.  High moisture filler materials such as wood powder, pulp, cannot be used.  Commonly used fillers are dry clay, silica granule, wollastnite, milled glass, and microsphere glass bubbles.  Low moisture calcium carbonate (lime stone powder) may be used at lower concentration with vacuum.   Fumed silica is often used to develop thixotropic effect to the liquid material.

 

Other Modification

Other modifications including adding colorant, UV absorbing agent, anti-bacteria agent, and other additives as well as change in mixing ratio are also available.  Northstar Polymers customizes the formulations for each customer. 

 

Seasonal Concerns

 

Some materials are sensitive to cold weather.  If the material has been in a cold temperature during the shipping etc., separation within the material could occur.  If you see crystallization or gelatin of the component, use a drum heater, or put the container into an oven and heat it to 160-180 F, then agitate the material before use to ensure homogeneous re-mixing.  Generally, part-A materials are more sensitive to cold weather. 

 

In cold weather, cold toolings (molds, frame, or model) could hinder the curing process of polyurethane. 

 

In hot weather, condensation of moisture could cause a problem. If you store the material in cooler place then bring it to hotter/humid place just before the operation, the material itself and the container could create dew on the surface, which causes bubbling problem.  Be sure to leave the materials and toolings in the temperature that is what you are going to operate.  If the work place is very humid, it could also cause a problem related to moisture contamination.

 

 

Handling Information for the Component Materials

 

Storage:

 

Part-A component (prepolymer) contains isocyanate component, which is very much sensitive to moisture.  If it is left in air, part-A will react with atmospheric moisture and will be ruined.  This reaction is non-reversible.  Soon after opening a can and dispensing the content, nitrogen gas or negative-40-degree-due-point dry air needs to be injected to the can to blanket the material.  Silica gel or calcium chloride desiccant filter should be installed to 55 gallon drum-vent for your drum feeding system.  The storage temperature should be at a room temperature between 65 and 80 F.

 

Part-B component is hygroscopic.  If the material is exposed to ambient air, it may absorb moisture. Moisture contaminated part-B material may become source of degradation or excessive bubbles in the product. Avoid exposure of the material to air.  Purging the empty space in the container with nitrogen gas or negative-40-degree-due-point dry air is also recommended to prevent moisture contamination of part-B as well; however most of the cases, keeping in an airtight container will be sufficient. Store it in a dry indoor storage at a room temperature between 65 and 80 F. The moisture contamination of part-B material is reversible.  By heating material to 160 - 180 F and vacuuming it at about 29" Hg negative pressure for several hours will reduce the moisture level.

 

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 and/or using an appropriate type of respirator to prevent inhalation of the fume.  

 

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 and follow the other procedures of the Standard Industrial Hygiene Practices.  Please refer to the MSDS for each component for the detailed health information.

 

Person 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 can also be used for extra protection.

 

For any questions, please contact Northstar Polymers.

 

 

 

General Processing Instructions for Room-Temperature-Curable Systems

Mold-Making Instructions

Step-By-Step Mold Making Images (Open-Top Mold)

Step-By-Step Mold Making Images (Block Mold)

 

 

 

Northstar Polymers, LLC

3444 Dight Avenue South

Minneapolis, MN 55406

Tel: 612.721.2911

Fax: 612.721.1009

E-Mail: info@northstarpolymers.com

 

Notice: All of the statements, recommendations, suggestions, and data concerning the subject material are based on our laboratory results, and although we believe the same to be reliable, we expressly do not represent, warrant, or guarantee the accuracy, completeness, or reliability of same, or the material or the results to be obtained from the use thereof, neither do we warrant that any such use, either alone or in combination with other materials, shall be free of the rightful claim of any third party by way of INFRINGEMENT or the like, and NORTHSTAR POLYMERS DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, OF MERCHANTABILITY and FITNESS FOR A PARTICULAR PURPOSE.