High Performance Water Based Polyurethane Topcoat

There are several techniques by Jeff Jewitt of Homestead Finishing Products:

You can find more information on our web, so please take a look.

DRY BUFF WITH MIRKA ROYAL MICRO PAPER

Using Mirka Royal Micro sandpaper, always start with -grit sandpaper. Begin with a small area, make 2-3 passes, then evaluate results. Go to a heavier grit if surface irregularities are not fully removed. Finish with -grit paper.

DRY/WET BUFF WITH ABRALON POLISHING DISCS

Start by dry sanding with Abralon -grit, and then -grit. Start slowly, monitor to see if buildup is occurring. A large sized table might take 5-25 sheets of paper.

If needed, lubricate by moistening with General Finishes Satin Finishing Wax, mineral spirits, or naphtha. Never use water as a lubricant.

If lubricated, one sheet of -grit or -grit should do the entire table.

BUFFING/POLISHING WITH PRESTA POLISHING KITS

Spray Presta polish on buffing pad stuff to lubricate the pad. Apply a quarter-sized dab of polish to sand one section. Spread with pad. Continue section by section. Mist buffing pad with Presta lubricating material when it gets too dry. You should be able to use a buffing pad on 10-20 tables before washing and reusing.
 
When done, mist entire surface with and buff with a micro-cloth to remove splatter

NOTE: Never wet sand a water-based finish.

It is normal to see a bit of stain on the brush when applying the first coat of topcoat. Topcoats often pull a bit of color on the first pass, but good preparation will minimize this.

To prepare open grains woods such as raw Oak for a water-based stain, we recommend sanding with 180-grit followed by no more than 220-grit sandpaper.

300-grit or 400-grit sandpaper is too fine for preparing raw wood. Too fine of a grit changes the wood from a porous surface to one that is too smooth to absorb the stain, which causes the first top coat application to pull excessive color (it is like trying to apply stain to glass).

There is always a small amount of color pull when using water-based stains, but the smooth surface escalates this condition. This was evident in the areas that you used the brush to remove excess topcoat.

See recommended sanding schedules here.

Instructional Video: How to Prep Sand New or Raw Wood

Here are some options to try. Always test a small area before proceeding with your entire project.

1. TONING: Lightly sand the light areas with a 220-grit sanding foam pad to open up the pores of the wood.
   Create a toning mix of 10-20% Dye Stain to 80-90% topcoat. Using a small brush, apply this mix over the light areas to blend with the darker areas.
   Let this dry 3-4 hours. Then apply another coat of the mix over the entire surface. If this is successful, then apply 2-3 coats of the topcoat.
2. GLAZING: Glaze the light areas. This will change the look of your doors but is an easier remedy.
3. START OVER: Optimally, you should sand down to bare wood and start over with this prep sanding schedule indicated able.
   You can apply the Dye Stain directly to the wood, or mix in 10% topcoat to help lock in the color.

All bright white paint will yellow slightly with time, even without topcoat. You have probably tried to touch up white woodwork in your home after several years and noticed that the new paint is brighter.

But white and light paints can react if clear coated with a waterbased finish; water-based topcoats are reactive and may draw out substances in the wood such as tannins, dyes or unknown substances in existing finishes causing the topcoat to yellow. This is an industry-wide issue and can happen right away, years later or never.

There is no way to reliably predict yellowing ahead of time. Every existing finish is different and we rarely know the finishing provenance on a previously finished piece. Every tree is different; every piece of wood is unique. Wood can bleed tannins immediately after the topcoat dries or months later with a change in temperature that comes with a change in seasons. Oak, pine, mahogany, and Douglas Fir are particularly prone to bleed-through.

Throughout all of our testing, General Finishes water-based topcoats are clear drying over a non-reactive substrate such as plastic or metal but when applied to something as unpredictable as wood, it is best to play it safe.

To reduce the occurrence of yellowing, we formulated Stain Blocker, a chemical stain and tannin blocking primer, and Brushable White Enamel. But the safest route is no use of topcoat over whites and light paint. As all of our white pigmented paints are self-sealing and do not require a topcoat, we warn users as follows:

CAUTION: Do NOT use any water-based clear coat over bright whites such as GF Snow White Milk Paint, or GF White Poly as it may cause yellowing. Lighter-colored paints may alter slightly with a topcoat application, but the color shift is less noticeable. Any clear coat can become reactive over wood substrates or existing finishes, causing tannin or dye bleed-though regardless of priming. All of GF's white paints do NOT require a topcoat.

TIPS FOR PROTECTING YOURSELF AND PREVENTING YELLOWING

1. Don't apply any clear coat when using light or white paints. Topcoat is perfectly fine over darker colors.
2. Always prime white paint with a paint system: Gf recommends either 2 coats of Stain Blocker or White Undercoat.
   • Over raw wood: Apply two coats of Stain Blocker or Enduro White Undercoat, then 1-2 coats of Brushable White Enamel, White Poly (spray only), or Milk Paint.
   • Over an existing finish: Apply two coats of Stain Blocker, then 1-2 coats of Brushable White Enamel, White Poly (spray only), or Milk Paint. Always let any primer dry overnight. Many primers suggest a 3-hour dry time and that is not enough.
3. We offer three types of self-sealing pigmented paint.
   • Milk Paint: A clear topcoat is not required on our Milk Paint for increased durability. It is a self-sealing, exterior rated coating with very high-performance properties. However, higher sheen topcoats provide a smoother surface that is easier to clean for high use projects such as tabletops and kitchen cabinets. Topcoat is fine for darker colors, but not for light or white Milk Paint. In those cases, use Brushable White Enamel (available in Satin or Semigloss) or spray our pigmented White Poly (available in Flat Satin or Semigloss.)
   • Enduro White Poly: Use a professional sprayable "white coat" such as our professional Enduro White Poly. It is a white paint with "increased topcoat properties", is a stand-alone finish when 3 coats are applied and does not require sealing with a topcoat.
   • Brushable White Enamel was developed as a stand-alone finish for those who want a bright white but do not use spray equipment.
4. Use disclaimers in your contracts. Upcharge for the extra coats needed and never guarantee a white finish over a piece when the customer wants a topcoat. Here is a suggestion for your contracts: 
   
   Terms of Agreement and Warranties: ________ (Initials) I have been informed that more coats are required when painting with bright whites, reds, greens or yellows. I understand that white paint can yellow over time and water-based topcoats can react with the substrate or existing finish under white paints causing yellowing, even if a stain-blocking primer is used. I have been informed that topcoat over white or light paints should NOT be used.
5. Always test your project's ENTIRE finishing schedule (from cleaning to topcoat) on an inside door or a more hidden area of the piece. This does not help if the yellowing occurs later but you will at least know if there is an immediate problem.
6. If you are working on period pieces such as a 's serpentine mahogany desk which were often finished in stain containing aniline dyes that cast a pinkish bleed-through under light paint, stay away from light colors. Not every piece of furniture is suitable for up-cycling with a light paint color. Pine, Mahogany, and furniture of the 's and 50's are a red flag.
7. Last, not all manufacturer's topcoats are compatible with other finishes and may react with a color change. Always follow best practices by not rushing, and testing to your satisfaction first.

WHAT YOU NEED TO KNOW ABOUT WHITE PAINT:

• Bright white paints can yellow over time with or without topcoat. It is the nature of oxidation.
• The underlying finish or wood species can affect the final color of light paint.
• Details and inside corners are difficult to cover with any paint color, but this property tends to be more noticeable with whites. This is a naturally occurring phenomenon in paint application and does not necessarily constitute a defect in the paint finish or your technique.
• The more porous the paint (chalk paint vs an acrylic paint), the more likely that yellowing will occur. The topcoat is actually seeping through the spaces caused by the larger particles of filler that give chalk style paints their texture.
• Whites have a lower “hide” quality and are more transparent than other colors. Most bright whites require additional coats to achieve the desired color and minimize color variation. This can increase the cost of paint finishing. Always include a clause in your contracts addressing the need for additional coats to achieve coverage.

IF YOU ARE A PAINTING PROFESSIONAL, DO NOT CARRY THE COST OF WHITE PAINT YOURSELF

Include the cost of the increased labor for applying white paints with a fair, reasonable up-charge. White paints, even if they did not yellow, require more coats and high-quality primers to achieve coverage. See our video How to Achieve a Bright White Finish that Lasts for more information.

If your white cabinets have yellowed, we recommend prep cleaning and scuff sanding, applying a coat of General Finishes Stain Blocker primer, finish sand and re-coat with another coat of paint.

Watch "How to Get a Bright White Finish" to learn more.

General Finishes water-based products can be sprayed through compressed air, HVLP, airless or C.A.S. units.

DOWNLOAD SPRAY TIP CHART PDF 

SURFACE PREPARATION:

All surfaces should be clean and free from dirt and oil and sanded.

Indicated below are instructional videos we have for preparing a surface for a new coat of paint or finish:

• Prep sand raw wood.
• Prep sand an existing finish.
• Power prep an existing high-use finish such as kitchen cabinets.

SPRAY APPLICATION OF GENERAL FINISHES WATER-BASED FINISHES:

All General Finishes water-based topcoats and wood stains are ready to spray from the container without additives with the exception of Conversion Varnish. Always strain material through a medium to fine mesh filter before spraying. If necessary, in hot or dry climates, reduce 10 to 15% with General Finishes Extender to extend the open time.

PRE SEALING:

Softwoods such as Pine absorb stain at an uneven rate and may respond better to staining if the wood is pre-sealed. General Finishes Pre-Stain Conditioner Natural can be applied to raw wood to condition the surface for uniform penetration of the stain. Pre-sealing will cause the final stain to be lighter so you may need to use a darker color. Always test your color on a hidden part of the furniture. Allow the Pre-Stain Conditioner to dry 30 minutes before applying your final stain color.

If you are using a sprayer that has been used for oil-based or lacquers, clean the unit thoroughly with acetone followed with hot water to purge the fluid passage. Apply a thin coat first that will dry and harden faster. Sand this first coat down to a smooth base on which to build your finish coats with a 220-320 grade foam sanding pad or #400-grit sandpaper. It is better to spray 2 thin coats rather than 1 heavy coat.

Contact your supplier to verify proper tip sizes for your specific equipment. GF's general recommended fluid tips for Wood Stains and Top Coats are Compressed air - .040 and Airless - .009.

DOWNLOAD SPRAY TIP CHART PDF

This PDF lists general guidelines for several different types of sprayers with specific General Finishes product recommendations for HVLP listed below.

RECOMMENDED TIPS FOR GENERAL FINISHES PRODUCTS USING HVLP:

• Milk Paint: 1.8mm-2.0mm
• Brushable White Enamel 1.8mm-2.0mm
• Stain Blocker: 1.8mm-2.0mm
• Pearl Effects: 1.8mm-2.0mm
• Glaze Effects: 1.8mm-2.0mm
• High-Performance Topcoat: 1.1mm-1.3mm
• Flat Out Flat Top Coat: 1.1mm-1.3mm
• Enduro-Var 1.1mm-1.3mm
• Water-Based Wood Stain: 1.1mm-1.3mm
• Water-Based Dye Stain: 1.1mm-1.3mm
• Exterior 450 Topcoat:1.1mm-1.3mm
• Exterior 450 Stain: 1.1mm-1.3mm
• Enduro Sanding Sealer: 1.3-1.4mm
• Enduro White Poly: 1.3mm-1.5mm
• Enduro Black Poly: 1.3mm-1.5mm
• Enduro White Under Coat 1.5mm-1.8mm
• Enduro Clear Poly: 1.1mm-1.3mm
• Enduro Pre Cat Lacquer: 1.1mm-1.3mm
• Enduro Conversion Varnish: 1.1mm-1.3mm

Air caps should be medium size. Contact your supplier to verify proper tip sizes for your specific equipment.

Break your work into sections such as dresser top or drawer fronts. Spraying too large of an area can result in a textured grainy surface. A correctly sprayed finish should appear even and glossy. It is important to spray enough material to allow proper flow and leveling of the finish.

Spray medium wet films at 3-5 wet ml thickness. Practice makes perfect. If you have never sprayed finishes before, take a large piece of cardboard and practice your technique first. Spray water on the cardboard to learn how the gun works. Check your fluid settings and adjust the controls to get comfortable with the spray angles and to develop your technique.

SPRAYING TOPCOATS:

Keep your gun at a 90* angle, 6-8" from the surface. On large flat areas, use wet, even patterns 6 to 8 wide. For narrow surfaces, reduce the fan pattern to 2-3" to reduce overspray. For topcoats, overlap each pass 25% to conceal lines.

Watch our video how to spray water-based topcoats here.

SPRAYING WATER BASED WOOD STAINS:

Keep your gun at a 90* angle, 6-8" from the surface. On large flat areas, use wet, even patterns 6 to 8 wide. For narrow surfaces, reduce the fan pattern to 2-3" to reduce overspray. Overlap each pass of stain 50% for even coverage and wipe back the excess with an absorbent cloth. For narrow surfaces, reduce the fan pattern to 2-3" to reduce overspray.

SPRAYING WATER BASED DYE STAINS:

Keep your gun at a 90* angle, 6-8" from the surface. On large flat areas, use wet, even patterns 6 to 8 wide. For narrow surfaces, reduce the fan pattern to 2-3" to reduce overspray. Overlap each pass of stain 50% for even coverage. If coverage is even, there is no need to wipe. If you have issues with uneven coverage, wipe back the excess with an absorbent cloth. For narrow surfaces, reduce the fan pattern to 2-3" to reduce overspray.

The term for a cloudy, milky appearance in the finish is Blush. There are three causes:

     1. The most common reason is an incompatible stain. For example, using a water-based topcoat over an oil-based based stain. When the top coat is applied, the oil in the stain seeps up through the finish and reacts with the acrylic causing a chemical blush.

To prevent this, either use a quick-drying water-based stain or allow the oil-based stain to dry for 72 hours. You can also seal an oil-based stain with a coat of shellac or lacquer sealer to provide a barrier between the oil and the acrylic.

     2. Another cause for blushing is high humidity. Applying a water-based finish in humidities of over 75% may cause blushing because moisture becomes trapped beneath the finish and cannot evaporate.

You can prevent this condition by increasing air movement in the finishing area with a fan. All water needs to evaporate is sufficient air movement. You can also improve drying conditions by increasing the temperature in the drying area.

     3. Another common cause is a wet object or a water spill left two long on the surface. In most cases, the white mark will recover and fade within an hour if the spill is cleaned up in a timely manner. If the water has been sitting on that area for an extended period of time then it might not be able to bounce back.

To fix this, the surface needs to be scuffed and sanded first. Second, we recommend applying a sanding sealer to protect the top. Sanding sealer has a larger resin particle, so it will provide a better build for the next sanding step. Then finish with your topcoat.

This is a guesstimate as GF cannot assist with the performance of another manufacturer's product used in conjunction with our own finishes.

It looks like Annie Sloan Chalk Paint is absorbing some of the topcoat because it is so porous. High Performance does not streak over a sealed surface when properly applied. Most likely the chalk paint is so dry it is pulling some of the solvents away from the High Performance, causing an uneven finish or the appearance of streaking. This happens on the original powder Chalk or Milk Paints also.

Possible solution: Porous paints such as chalk paint may require additional coats of High Performance Topcoat to seal off the paint. If the first coat absorbs into the paint it will require multiple coats of High Performance to build a film of sealer above the paint. Using a foam brush will accent the streaking due to the number of strokes needed. We suggest a fairly liberal application with a large applicator such as a foam roller or a pad applicator.

Always test before proceeding with the entire project when using General Finishes projects with other manufacturer's products.

GF recommends using the least expensive product that will provide the needed performance characteristics. We offer a range of products designed for professional spray applications and brush-on finishes for consumer use.

Compare all GF Topcoats Here

Watch Video: Comparison of All GF Water Based Finishes Here

PROFESSIONAL TOPCOATS

ENDURO PRE CAT LACQUER (Spray only)

Looking for hardness, clarity and fast stack time? Pre Cat Lacquer is the choice. Use in applications where there is little risk of chemical contamination issues from substances such as ketchup, soap, chemical cleaners, or ammonia. This is the perfect spray finish for interior doors, casing, moldings, door jambs and furniture that receives light use. When you think about it, interior doors don't take a lot of wear. You might wipe them down or dust them once in a while but that is about it. PreCat Lacquer is a hard finish that provides excellent build and superior clarity at a phenomenal price point. If you are not finishing a surface such as a tabletop where you anticipate chemical contamination during use, why pay for a more expensive finish?

Dries clear.

Watch Pre Cat Lacquer Product Overview Video Here

ENDURO CLEAR POLY (Spray only)

Poly is the go-to cabinet finish. It has all the durability you need at a lower price point than Conversion Varnish. It is similar to High Performance in test measurements for chemical, water and wear resistance, but HP can be brushed and Clear Poly can't - it is already thinned for spraying. Clear Poly has a bit of an edge over High Performance in overall chemical resistance, a 98 rating vs 92.

Dries clear.

Watch Clear Poly product overview video here

ENDURO CONVERSION VARNISH (2K-Spray only)

This is our top of the line. Conversion Varnish is a post catalyzed high solids, two component urethane formulated for professionals who need the highest level of durability and chemical resistance. Varnish will cure the fastest. Requires NCO Catalyst. It is recommended for commercial applications or extremely high-wear surfaces such as countertops and bar tops

Dries clear.

Watch Conversion Varnish Product Overview Video Here

Enduro 2K Clear Poly (Spray Only) 

For today's professional, industrial and OEM users, water-based Tintable 2K Clear Polyurethane is formulated to stop your biggest frustrations and provide ultimate durability for interior/exterior use. Key features are easy application, fast film build & production time, excellent adhesion, plus optimum water & chemical resistance.

BGP Product Page

Dries clear. 

Watch 2K Clear Poly Product Overview Video Here

Enduro Pro Series Clear Poly 

General Finishes Enduro Pro Series Clear Polyurethane is a single-component, self-cross linking polyurethane/acrylic topcoat designed for fast build with excellent flow and leveling. Priced competitively for the professional and industrial markets, Pro Series Clear is tintable and meets or exceeds KCMA standards. Recommended use is interior cabinets, trim, doors, and furniture. Use over raw, stained, or primed wood substrates.

Dries clear.

RETAIL TOPCOATS

HIGH PERFORMANCE (Brush or Spray)

GF's best performing brush on finish with very similar performance testing as our Poly. (High Performance can be thinned for spraying).

Dries clear.

Watch High Performance Product Overview Video Here

FLAT OUT FLAT (Brush or Spray)

Flat Out Flat is formulated with a self cross-linking acrylic, and is designed to mimic the look and feel of a wax finish. Because of the increased matting agents used to create the "Flat look", this finish will have less clarity. Like wax, it dries softer to the touch than High Performance when completely cured, and will show more fingerprints. FOF finish will mark up more easily because of increased matting agents used to reduce the sheen, so it is not recommended for high-use tabletops.

Dries clear.

Watch Flat Out Flat Product Overview Video Here

BOTH PROFESSIONAL AND RETAIL TOPCOATS

ENDURO-VAR II (Spray or brush)

Enduro-Var II is an oil modified water-based alternative for customers that want the warm look of an oil finish in a water based coating. It looks more like an oil finish than a water coating, and can be sprayed or brushed. The original Enduro-Var adheres well ONLY over WATER STAINS, DYE STAINS, and RAW WOOD. Use over other finishes at your own risk. The new formula of Enduro-Var II is compatible with all GF products and dries to a lighter amber than the previous formula.

Dries Amber

Watch Enduro-Var II Product Overview Video Here

EXTERIOR 45O TOPCOAT

Exterior 450 provides a clear drying tough, protective finish that will withstand the rigors of the sun, rain, and wind. It is suitable for vertical surfaces such as fences, patio furniture, garage doors, entrance doors, outdoor kitchens and interior windows. It is not recommended as a deck finish. Fortified with UV absorbers to stabilize the finish, Exterior 450's built-in mildewcide retards mold and mildew growth.

Dries Clear

Watch Exterior 450 Topcoat Overview Video Here

Toning is the process of adding colorant, either dye or pigmented stain into a top coat and then applying over an entire piece in order to subtly deepen the color. All of our stains can be used: Stock base colors of RTM, Water-Based Wood Stains or Dye stains. If you are new to toning, we recommend starting with Dyes stains, which can be easier to use as they only contain dyes and no pigments. 

All of General Finishes water-based topcoats with the exception of Conversion Vanish can be tinted to create lightly colored finishes for toning.  It is YOUR RESPONSIBILITY TO TEST to your satisfaction before application to your entire project. 

• https://gf.video/How-To-Darken-Cabinets-By-Toning-Water-Based

There are limits anytime you add pigments or dyes to water-based topcoats. Toning should always be done in small increments to achieve the desired color. If you are starting with a light honey tone door and you want to create an Espresso color this will take multiple coats of color including base coat of stain followed by several toning coats. It is impossible to achieve a radical color change with just 1 coat of toner.

A good rule to follow when toning is starting with a minimal amount of stain or dye, 5-10% by volume. Some colors work better than others. Normally 1-2 ounces or stain per quart of clear topcoat is sufficient to create a toner. Always test color first and strain material through a fine mesh filter to remove any undissolved particle of pigment. Once you have achieved the desired color tone apply a clear topcoat to lock in the color. Toner made with water based wood stain should be used at point of use only. Do not store for extended periods of time. 

Until now, Matte sheens always came with a compromise in durability. Consumers have been asking for an ultra-flat topcoat that is as durable as our other sheens, and we are finally able to deliver!

Thanks to brand new patented polymer technology, we are now able to create a Matte sheen without waxes and matting agents that decrease durability.

Defining Durability
All of our testing shows High Performance Dead Flat is as durable as High Performance Satin, which is a drastic improvement over Flat Out Flat topcoat.

Flat: (degree sheen)
Flat Out Flat: <5
High Performance Dead Flat: <5

Water Resistance: (max 5)
Flat Out Flat: 2
High Performance, Dead Flat: 4

Chemical Resistance: (0-100)
Flat Out Flat: 82
High Performance, Dead Flat: 92

Scratch Resistance and Marring:
Scratch Resistance is strong with both products.
Marring is better than Flat Out Flat and comparable to the Satin and Flat.

High Performance vs. Clear Poly
The main differentiator between these products is that Clear Poly is a spray-only product and High Performance can be applied with a brush or sprayed. Clear Poly is popular among high production contractors but both products produce professional results.

Upgraded Features

• Patented self-matting polyurethane
• Uses renewable resources, with no Prop 65 chemicals
• Meets KCMA standards
• VOC compliant nationwide

Warning: Do Not Mix!
Topcoat with a Dead Flat sheen CAN NOT be mixed with another sheen because a completely different polymer is used and the combined formulas will not perform well as one.

Leaders in Water-Based Finishing Innovation
General Finishes actively works to improve our products on an ongoing basis. As new finishing technology is released, we test and experiment to stay on the leading edge. Our chemist travels around the world and is sent new ingredients when they are discovered on a regular basis. We are one of the rare companies capable of incorporating new innovations and ingredients into our products, no expense spared, because of our dedication to staying on the leading edge and providing the highest quality product on the market. 

Ordering soon?
High Performance Dead Flat is available in pints, quarts and gallons. Clear Poly Dead Flat is available in gallons. 5-gallon pails may be delayed due to label production and Canadian labels are pending translations.

When using cast polyurethane systems, the polyurethane casting process requires the right temperature for polyurethane components that are used in the process. Cast polyurethanes are processed in a liquid state. That is to say that the individual components of the system are liquified prior to mixing and then the mixed compound is cast into a mold or poured onto a surface to allow the cure to advance to a solid state.

Processing conditions can have a significant impact on the final properties of the cast polyurethane, regardless of the system used. Two of the processing parameters that have the most critical impact on the performance of the polyurethane/urea systems are the polyurethane melting temperature and the ratio of the prepolymer to the curative. 

The amount of curative required to match the stoichiometry of the prepolymer is generally selected to improve a specific set of properties, including but not limited to tensile strength, or tear and abrasion resistance. For this reason, the ratio needs to be adhered to in the metering and mixing process.

However, the PU melting temperature also has a strong impact on the finished product, both during the mold curing process as well as in post-curing processing. Upon completion of the initial cure in the mold, the cast articles can be transferred to a post-curing stage to complete the reaction at an appropriate polyurethane temperature range to achieve the properties your project or operation desires. 

The figure below outlines the cast polyurethane process steps:

In this blog, we'll address polyurethane temperature range requirements required to effectively work with numerous curatives such as diols, diamines, and formulated products that are used as chain extenders and curatives in polyurethane/urea systems.

Solid Curatives Used in Cast Polyurethane Systems

Many cast polyurethane systems utilize solid curatives including MOCA, MCDEA, HQEE, and HER to create a strong final product. These chemical products are melted and processed in a liquid state to be incorporated into the polyurethane. 

Temperature requirements for these chain extenders are shown below, including how quickly they will set up and the hardness they are able to achieve.

MOCA-Cured Systems

MOCA is a widely used curative in the hot cast process, especially with TDI prepolymers heated to an appropriate PU melting temperature. It delivers polymers that feature excellent high-temperature property retention and good dynamic performance in the polyurethane casting process. Relative to other diamine curatives such as MBOEA, MCDEA, and DETDA, MOCA exhibits lower reactivity and a longer working life.

The melting point of MOCA is 98–102°C (208–216°F). Accordingly, the nominal operating temperature for a MOCA melter should be from 110 to 120°C (230–248°F). However, one limitation of MOCA in finding a compatible temperature for polyurethane is its susceptibility to higher temperatures, as MOCA will decompose. For this reason, avoid heating MOCA above 125°C (257°F).

MCDEA-Cured Systems

MCDEA is an outstanding solid aromatic diamine curative for high-performance cast polyurethane applications. It affords improved dynamic mechanical properties and low hysteresis. MCDEA-based formulations generally exhibit good abrasion resistance, good resilience and tear strength properties, and good processing characteristics. Typically, these properties are valuable in industrial wheels and tires, sport and amusement park wheels and rollers, industrial rolls, and conveyor belts.

The melting point of MCDEA is 87–90°C (189–194°F). Accordingly, the nominal processing temperature for MCDEA should be from 95–110°C (203–230°F), typically 100°C (212°F). MCDEA exhibits higher reactivity than MOCA. Consequently, the pot life of MCDEA/TDI systems is shorter compared to MOCA/TDI systems, and polyurethane melting temperature must be watched carefully.

HQEE-Cured Systems

HQEE is an aromatic diol chain extender for MDI prepolymer systems, creating polyurethane elastomers with excellent dynamic properties, good split tear resistance, and high resiliency. The HQEE/MDI elastomer series can parallel many of the performance attributes of MOCA/TDI systems.

Applications for MDI/HQEE elastomers include industrial wheels and tires, sport and amusement park wheels, pipe linings and coatings, rollers, industrial roll covers, gaskets, oil-field seals, and other high-performance end uses.

However, HQEE's high melting point at ~98°C (~208°F), combined with the fact that HQEE does not supercool, crystallizing quickly below its melting point, can create difficulties. It's important to uniformly heat and mildly agitate HQEE in the melting tank and preheat the prepolymer polyurethane temperature range to 90°C (194°F) or above before mixing HQEE in. 

In addition, all HQEE transfer lines should be heated to 110°C (230°F) and insulated to prevent cold spots that might cause HQEE to crystallize in the lines and result in an inaccurate mix ratio or blockage. The mold temperature should be 110°C (230°F) or higher to prevent defects in the cast parts.

Another processing challenge encountered with HQEE as a chain extender is a phenomenon called “starring” on the surface of parts. This phenomenon has been attributed to the phaseout of the reaction product of HQEE and free MDI monomer, especially on mold surfaces temperatures below 110°C (230°F). It has been reported that low free MDI prepolymers exhibit a much lower tendency to starring because of the lower free MDI monomer content.

In some cases, other diols like HER or PTMEG can be added in small amounts to HQEE to facilitate maintaining a molten state, giving you a better PU melting temperature for the polyurethane casting process.

HER-Cured Systems

HER is an isomeric counterpart of HQEE and another aromatic diol chain extender for MDI prepolymer systems. The properties of HER extended MDI prepolymers are comparable to those of HQEE, but HER has a lower melting point of 89°C (192°F) and the ability to supercool. This offers significant processing advantages versus HQEE: lower processing temperatures, a broader processing temperature window, the absence of “starring,” and a lower tendency to crystallize in processing equipment.

After melting at 90–100°C (194–212°F), the HER temperature can be reduced to about 80°C (176°F) without crystallization, allowing a lower polyurethane melting temperature compared to HQEE without sacrificing cast quality and part durability.

In urethane elastomers, HER affords excellent mechanical properties; toughness; very good tear, cut, and abrasion resistance; and good resiliency, while providing a wider polyurethane temperature range in production. 

Applications are the same as those of HQEE, including industrial wheels and tires, sport wheels, die-forming pads and coatings, rollers, industrial roll covers, gaskets, oil-field seals, conveyor parts, and other high-performance end uses.

Liquid Curatives Used in Cast Polyurethane Systems

Some industrial chemical curatives for cast polyurethane are in a liquid state at room temperature, but their viscosity may be too high for efficient mixing with the prepolymer. In such cases, the curative temperature usually needs to be increased to reduce viscosity. Below are the temperature requirements for several liquid diamine and diol curatives used in cast polyurethane systems.

Diamines

Some commonly used liquid aromatic diamine curatives are DETDA, DMTDA, MBOEA, and P-. These diamines allow processing at a lower temperature for polyurethane casting. However, it is known that higher temperature cures can provide improved tensile strength as well as tear and abrasion resistance. 

DETDA and MBOEA exhibit very fast cure rates and short pot lives with TDI prepolymers. DMTDA has a lower reactivity and exhibits a workable pot life at 100°C (212°F) comparable to that of MOCA/TDI systems.

Blends of diamine curatives, such as MBOEA with DETDA, can reduce the overall viscosity and improve flow at a lower PU melting temperature, making it a superior option.

P- is a polymeric diamine curative that’s a viscous liquid at room temperature and exhibits a much lower reactivity in TDI systems. It can also be used in MDI systems. P- is a waxy solid at ambient temperatures with a lower melting point in the 18–23°C range. Both curatives benefit from processing at a higher polyurethane melting temperature to reduce viscosity.

Diols

Liquid diol chain extenders such as 1,4-butanediol (BDO) and 2-methyl-1,3-propanediol (MPO) can be used in MDI-based cast polyurethane systems at room temperature. Processing temperature and catalyst loading levels impact pot life of the diol/MDI systems. Uncatalyzed systems can have a pot life of 10–20 minutes at 70°C; however, processing at higher temperatures or using catalysts will significantly reduce the pot life.

When processing cast polyurethane systems, it’s important to note that BDO freezes at 20°C (68°F). Therefore, it needs to be maintained above this freezing point to avoid crystallization in the equipment lines and phase separation in a B-side formulation of the polyurethane casting process.

MPO is another liquid primary-diol chain extender with a freezing point of -54°C. As a BDO isomer, it has identical molecular and equivalent weights as BDO with better compatibility for polyester- and polyether-based prepolymers, providing a much lower temperature for polyurethane casting. 

Note that the resulting elastomer will be softer compared to the one produced with BDO and will exhibit better hydrolytic stability. BDO reactivity is only slightly faster than MPO, and blends of BDO with MPO are commonly used very successfully.

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