Elevated Temp Processing Info
Instructions for Urethane Elastomer Hand-Batching
For Elevated-Temperature-Cure Polyurethane Elastomers
Safety
Make sure the whole operation is done in a well-ventilated area. Dynamic ventilation (overhead hooded fans) is recommended. Aside from the gloves, you should also wear long sleeves and safety goggles/glasses. For the operators who handle heated materials regularly, we also recommend OSHA approved half-face respirator.
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. These materials are relatively safe for an industrial chemical, but still can cause some skin irritation etc. Read the Material Safety Data Sheet (MSDS) for more information. MSDS will be sent together with the each component.
Basic Terminology
You make this polyurethane elastomer 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 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 poly-bucket, large empty paint can or lab beaker. Make sure the inside of container is clean, dry, and free of any solvent. This polyurethane material sticks to the surface and may be hard to come off. You may have to through away the container.
2. Scale
Use something that you can read in decimal units. The amounts of material needs to be calculated in ratio. Using metric unit could avoid the confusion in calculation; you can avoid conversion between ounce (1/16 pounds), pounds, and grams. A digital scale with decimal values is easy to use. You want to weigh within 2 % accuracy.
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 attached.
3. Gloves
To handle any polyurethane material, you need to wear safety gloves. Please consult your supplier for recommendations.
4. Protection
Aside from the gloves, you should also wear long sleeves and safety goggles/glasses. For the operators who handle heated materials regularly, we also recommend OSHA approved half-face respirator. Read the Material Safety Data Sheet (MSDS) for the detail information.
5. A Stir Stick
Plastic or stainless steel flat-end spatula or any stick clean, dry and free of solvent. Do not use wood sticks as it contains moisture, which contaminates the material. You can use a hand mixer if you are making a large batch. Electric hand mixers with paint mix blades show a good success rate. You should use a type of blades that prevents air enclosure.
6. Mold Release
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. This silicone mold release is available from Northstar Polymers through its distribution. We recommend this for ease of use and safety.
You can also use a solvent-based silicone release. Those are usually in an aerosol can. If you need a very fine surface resolution, you may need to use spray. However, spray silicone mold release tends to become airborne and will be on surface of all items in the plant. If you have other operations that require adhesion of material, this will become a problem. If you are using aerosol type, be sure to contain the airborne release. Also, if you are using any solvent base release, make sure the solvent is dry before pouring.
Do not use water-base mold release as it increases the chance of water contamination. Mold release for rubber processing often has water content. 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. However, wax release may require more care due to the build-up problem and its effective operating temperature.
Aerosol type mold release can be obtained at the following supplier or your local industrial supply distributors.
Slide Products: 800-323-6433
7. Industrial Oven
For all elevated-temperature-cure systems, you need an industrial quality oven. Oven is used to thaw materials, heat molds and inserts, cost-cure products, and so on. The range for common processing temperatures for polyurethane materials is between 100 and 200 ºF.
8. Vacuum equipment
If you do not want bubbles in your parts, you can use a vacuum chamber to de-gas. This is an option. With hand mixing, you cannot completely avoid bubbles. You can put the mixed material into a vacuum chamber to eliminate the bubbles up to a certain degree. You can also put your mold into the vacuum chamber and degas as you cure.
Often, the high-performance polyurethane formulations do not have a long pot-life, and it is difficult to degas manually. For your future production, using an airless casting machine or some vacuum capability will allow you to produce bubble free products.
9. Cleaning Solvent
You need to clean you tools and equipment. We recommend DBE (dibasic ester) for your cleaning operation. DBE is least volatile and reasonably priced. You can save this in a paint can and use it many times to clean your tools. This material does not dissolve cured urethane; however it softens. If you soak your tools in it, it makes it easier to scrape off cured urethane stuck on your tools.
You can also use MEK or acetone if you need a stronger solvents to clean; however, these are more flammable items. Isopropanol can also be used and is commonly available. Be sure to dry all solvents in your mold before you cast urethane into it. Mineral spirit or paint thinner does not work with this material.
Handling of solvent may need special care; many of them are flammable and/or hazardous. Please read the MSDS of the solvent carefully and consult your solvent supplier for safety recommendations.
10. Paper Towels
For cleaning. You can also use cloth towel or rag, but expect it to be disposed rather than reused. Cured polyurethane does not come off by washing. Clean the items with paper towels and the cleaning solvent, dispose of it as industrial solid waste. Make sure that your rag is not wet with water.
11. Nitrogen gas
For storage, the part-A material needs to be blanketed with nitrogen gas and the cover for the container has to be closed tight. For pails, you can send the nitrogen gas for 5 seconds and quickly close the cover. Cylinders should be available from your local supplier. You can obtain a shut-off valve and connect a flexible hose (about 10 feet). It will be convenient if you set this on a dolly and make it portable.
The reason for doing this is that the part-A has chemical called isocyanate, which is very reactive. It is so reactive that it reacts to any form of water in contact including the moisture in the air. 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, in the beginning your product starting to come out softer than expected, then eventually it will harden by itself, and you cannot use it anymore. Nitrogen will not react to isocyanate, so it will keep the quality for longer time.
Minus 40 ºF ( - 40 ºC) dewpoint dry air can also be used instead to blanket the components.
If you are using a whole container of part-A at one time only and do not need to store, you may not have to worry about the nitrogen gas for now.
Nitrogen gas can be obtained through the following supplier.
Praxair: 800-772-9247
12. Thermometers. Ones you can measure up to about 250 ºF. The material is very much temperature sensitive. Use thermometers all time to control operation temperature and do not rely on "feeling" of the operators.
13. A log book. This is to keep track of important parameters of each batch for quality control.
Mold or Frame
Mold or frame can be made out of metal such as aluminum die, sheet metal, or stainless steel as well as resins such as epoxy, polyurethane, and silicone rubber. Each material has its own advantages and disadvantages. Cost is a large concern in many cases. In general, resin molds are less expensive. But, it may not retain heat as good as metal molds. Some resin molds deform when the heated urethane is cast into. This type of material has shrinkage of 1 to 2%. The molders usually make the mold slightly larger than the actual part size. Because of the shrinkage factor, the dimensional tolerance of molded products made of this material is limited. If tighter tolerance is required, machining may be needed after the part is cured.
Inserts
Some parts have inserts, which are molded together with the cast urethane. The insert could be steel, plastic, fabric, or other materials. In general, cast polyurethane material has limited adhesion property to many substrates. Surface preparation may be required to ensure adhesion of the inserts to the cast urethane part. In case of metal, sandblasting to roughen the surface and used of primer are common methods to ensure adhesion. Creating a mechanical bond structure such as steel mesh welded on the substrate is another effective method to ensure adhesion. For plastic substrates, there are primer products available for each plastic substrate. Please consult the following or your supplier for primer recommendations.
Lord Chemicals: 800-243-6565
Procedure
All materials and toolings and molds should be at 180 ºF.
1. Heat the part-B material (180ºF) in an oven. If you have a material in a plastic pail, be sure to keep the pail off the wall of oven as it may melt the plastic. If you are taking small amounts in paint cans to heat, use locks on the cover as the heated air pressure inside may pop the cover.
2. Prepare the mold. Apply the mold release lightly. Be sure to dry the solvent (if solvent base release is used).
3. Calculate the correct amounts of each part-A and part-B material. (See the appendix for calculation.)
4. Open the container of part-A, pour the necessary amount into the container as you weigh. Be careful handling a hot material. If you are doing this by scale, pour one part-A into the mixing container on the scale. Read the weight and write it down. Calculate the amount of part-B and cast part-B. If you are measuring it by volume, take a part-A into a measuring cup and write down the amount. Calculate the amount of part-B. Take part-B into a different measuring cup to measure the correct amount, then cast into part-A in the mixing cup. Be as accurate as possible.
5. Close the containers of the material quickly as it may be contaminated by moisture in the air. When you close the can of part-A, be sure to blanket the material with nitrogen gas and close the lid tight. To assure the quality of your raw material, you need to do this as soon as you can.
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.
If drum feeder is used, you can install desiccant filter to your drum vent.
6. Record the temperatures and precise amounts of part-A and part-B, as well as time you mix the material into your batch log book. This is important for your product quality control.
By doing all the operations manually by hands, you would see some range in your results. The record helps you to link the quality of the products and actual operations took place to make the product. Depending on your product specification, you can also set acceptable range of "off-ratio" and other parameters. This will help trouble-shooting for any problem may occur in future as well.
7. Mix
When mixing with hands, use stir-stick and agitate it vigorously, but try not to fold in air (do not whip). Scrape the side and bottom to make sure it is mixed very well. Mix carefully so the components are mixed homogeneously without spill. If the material is less than a half gallon, mix at least 30 to 40 seconds. Even if the material appears to be mixed, they are often not mixed enough. Thorough mixing is very important. Do not try to mix hard, but try to mix quickly and thoroughly.
If you use hand held electric mixer, mix at the low speed. High speed can fold-in air. Scrape the side and bottom of the container also with a stir stick for thorough mixing.
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". Pot life for this material is about 2-1/2 minutes. This means you need to mix and finish casting within this period. The operator needs to be trained to a certain proficiency level in order to do this.
8. Place the container in a vacuum chamber (Option)
Put at least 29" Hg until you see most of the bubbles are gone. Do not leave the material too long as vacuum often invokes a quick reaction.
For this material, the pot life is very short. You probably do not have enough time to degas. You would probably have some bubbles on top. This bubble will be more sever if you cast the mixture too late. The material gradually, but quickly increases the viscosity after components are mixed. Casting it while it still has lower viscosity will alleviate the bubbles enclosed during your cast operation. You may need to machine off the bubbles later. The molds need to me designed so that the bubbles are at inconspicuous places.
9. Pour it in the mold
Make sure your mold is at the right temperature. If you are open-casting, be sure the mold or frame is perfectly level.
If the part is small flat part, 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.
If you are casting this into an enclosure, pour into the designated pour-hole of the closed mold. If the mold has air-trapping pattern, cast to a half of the mold and slash to let out air, then cast the rest.
The mixture increases its viscosity quickly towards the end of pot-life. Try to finish casting while the viscosity of the material is still low.
10. Record the mold temperature and time to your logbook for quality control.
A Useful Tip:
One way to find out the concurrent state of the material inside is what's left in the mixing container. Keep the mixing container together with the mold. Doing this will give you information of the concurrent state of the material in the mold without damaging your part. You can see how hard the material is and so on. Also, after it is cured, peel off the left over and feel the quality. If the material surface in contact with the mixing container has a lot of stickiness, this means you have not scrape the side enough to ensure good mixture when you are mixing. This indicates the insufficient mix may be a cause of the defective product.
11. Clean up the tools with DBE (or other cleaning agent) and paper towels
12. Cure
Curing is the process that the components of polyurethane react to each other and harden. First, the mixture is liquid when cast in. Then the mixture becomes thicker. It will become too hard to pour as liquid; then it will be a gel that is do not move it self. It continues to cure to solid. It then will be hard enough to demold.
Observe the change in hardness of the material. The standard demolding time for this material at 180 ºF mold temperature is 30 to 40 minutes. It will be useful later to know the shortest time you can demold without damaging the part to improve your productivity. Shorter the demolding time, your per-mold production rate will be better.
13. De-mold
Observe the strength of the material after 30 minutes (counting from the time you started agitate the mixture). At 180 ºF mold temperature, this should be strong enough to demold between 30 and 40 minutes point.
If mold release is not working well, demolding will be very difficult. Some manufacturer uses molds with a structure to let compressed air go in between the mold and the cured product to help demolding. Using a knife or spatula to ply may damage the part and mold. If the material does not have enough strength for demolding, cure longer until it has enough strength for demolding.
The strength of the material at de-molding time is called "green strength." This material is made to give a good green strength at a de-molding time. However, this stage the material is not fully cured. The material needs to be post-cured in an oven at 180 ºF for 16 hours before it comes to final cure.
14. Post Cure
After the part is demolded, place it in an oven for post cure. Standard post cure for this material is at 180 ºF for 16 hours.
If you are testing the properties of the material or the part, be sure to cool to ambient temperature before test. It often take several hours to completely cool the part.
15. Machine/Finish the part
You can use slow speed hand-tools to machine and finish. The urethane powder or dust coming from machining is considered to be health hazardous if inhaled. Be sure to wear an adequate protection. Do not use high speed circular saw or other high speed tools as it could create heat to melt urethane. The gas from heat-decomposed urethane is highly toxic.
Trouble Shooting
Problems
Suggestions
The material does not cure.
· Check the mixing ratio. Make sure you are using the right ratio of the right combination.
· Temperature may be too low? Heat the mold, insert, and/or frame to 180 ºF. Cold tooling hinders the curing process. If you are using some filled resin mold/inserts, you may want to raise the mold temperature to 200 ºF. Some filler materials tend to be more difficult to retain heat.
· Contamination of material? If part-A is badly contaminated with moisture, material may not cure.
· Catalyst level may be too low? Add more catalyst. For recommendation on catalysts, contact Northstar Polymers
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 there is any residual moisture on tooling it causes bubbles. Plaster toolings must be coated after kiln dried for a few days. Avoid using water or water base cleaning agent to clean the toolings.
· Condensation on to the material could cause bubbles as well. Storing material in a cold storage room and bring it into a worm and moist room could cause condensation.
· 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, heat the part-B material to 180 ºF, put the material in a vacuum chamber, and run the vacuum at 29 Hg or above for more than an hour to two hours 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.
· Casting material near the expiration of pot-life will trap air and it becomes bubbles. Cast while the material has a good flow.
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. Choose release agents that are effective at the temperature range of near 180 ºF.
· You might have missed some spot when you are applying the mold release. Ensure enough mold release is applied to the entire inner surface of the mold.
· 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 choice of urethane material is too hard to demold. If possible, choose softer polyurethane material so it flexes when demold. Or, consider using a flexible molds such as urethane or silicone rubber.
· 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 40 to 60 seconds. 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 toolings to 100 - 120 ºF.
· 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?
Streaks or delamination of the cured part
· The material may be curing as you pour. Your part may be too large to pour manually by hands. Consider using casting machine or try to finish mixing and casting while the viscosity of the mixture is low.
Skin forms on the mold contact surface
· The mold temperature is not high enough or the mold is not retaining the heat well. Use a solid metal mold (aluminum, steel etc.) and elevate the mold temperature between 180 and 200 °F.
Green strength does not form in time
· The mold temperature or/and material temperature is not high enough. Modification in catalyst package may be needed.
Pot-life too short
· The material temperature too high? Mix at the designated temperature. If it is still too fast, use part-A at room temperature. This should give you additional 30 to 45 seconds pot life.
· Catalyst level too high? Contact Northstar Polymers.
Does not cure fast enough
· Mold temperature too low. Heat to the designated temperature.
· Material temperature too low. Heat to the designated temperature.
· Catalyst level too low. Contact Northstar Polymers
The parts not as hard as expected.
· Mold temperature too low. Heat to the designated temperature.
· Wrong mixing ratio. Check the log. If mixed with off-ratio, the material will be softer. Mix at the right ratio.
· It may need to cure longer. Some mold/ insert has heat properties, which hinder retaining heat within the mold. Use higher temperature and cure for a longer time.
Voids in parts
· Mold has leak? If mold has leak and it slowly allows air to come in, you will see bubble or air void near the leak. Repair the leak.
· Too much mold release? If you are using silicone mold release, a very thin layer of mold release should be efficient. Slightly cut back the mount of the release agent and see the effect.
· The temperature of the material and/or mold may not be right. It could be too high. Lower the mold temperature and try again. Do not heat the material above 200 ºF.
· Dirty mold? Residual solvent, water, or other volatile material could create void. 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 shake the mold to avoid air pockets. Avoid the turbulence of the polyurethane mixture in the mold specially near the expiration of the pot life. Pour in one paddle and try not to 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.
· Exothermic reaction may be too fast. Contact Northstar Polymers.
High shrinkage
· Wrong ratio? Try to be accurate on ratio.
· Material is too hot. Use lower temperature.
· Contamination of part-B? Vacuum to de-contaminate the part-B.
· Exothermic reaction may be too fast. Contact Northstar Polymers.
Additional Information
Modification of the system
The cure pattern can be modified. The time needed to completely cure the material is called "Complete Cure Cycle". The pot life, demolding time, and complete cure time 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.
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. Customizing can be done at smaller quantities than most companies in the formulation business.
Storage of materials
Part-A contains reactive isocyanate and needs to be blanketed by nitrogen gas or minus 40 ºF (40 ºC) dew-point 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 for six mouths.
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. Please read the enclosed Material Safety Data Sheet or detail information. Also, consult your local authority for the disposal regulations.
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/pail heater, or put the container into an oven and heat it to 160-180 ºF, then agitate the material before use to ensure homogeneous re-mixing.
In cold weather, cold toolings (molds, frame, or model) could hinder the curing process of polyurethane.
In hot weather, moisture in the air 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. We recommend humidity control of the work place.
This Material is Not Fire Retardant Grade
This material is not fire-retardant grade and cannot be used in applications requiring fire retardant grade materials including some building structures and automotive interior parts. Please consult Northstar Polymers for availability of fire-retardant materials to your application.
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 ºF.
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 ºF. The moisture contamination of part-B material is reversible. By heating material to 160 - 180 ºF 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.
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.