On – site Experience and Knowledge of Polyurethane Rigid Foam Foaming

Polyurethane rigid foam has become an indispensable material in numerous fields, from construction insulation to refrigeration equipment. Mastering the on – site foaming experience and knowledge is vital to produce high – quality products. In this blog, we will explore the key points of polyurethane rigid foam foaming in detail.

1. About Calculation

The core is to determine the ratio of black and white materials (isocyanate index) to ensure the complete reaction between white and black materials. It is necessary to calculate the molar amount of – NCO consumed by each component:

1. Main materials (polyether, polyester, silicone oil, etc.): Multiply the formula amount by the hydroxyl value and sum to get Q, then S1 = Q ÷ 56100.
2. Water: S2 = formula amount of water ÷ 9.
3. Small molecules: S3 = (formula amount × functionality) ÷ molecular weight.
4. Total consumption S = S1 + S2 + S3. The amount of crude MDI required for the basic formula is [(S × 42) – 0.30] × 1.05 (with an isocyanate index of 1.0).

2. Determination of Catalytic System

1. Pre – requirements: The best whitening time after the material is ejected from the gun is 6 – 8 seconds. This can avoid problems such as material leakage, poor cell structure and increased thermal conductivity. If the whitening time is too long, the liquid material is easy to leak from the gaps of the box, causing dirt to stick to the mold. It will also affect the fineness and overall structure of the cells, thus increasing the thermal conductivity of the foam. On the contrary, if the whitening time is too short, it may affect the filling effect.
2. Medium – term flow: The longer the time period from wire drawing to whitening, the better. The ideal state is that the foam material is filled in place 3 – 5 seconds before wire drawing, and there is obvious material escaping from the farthest exhaust hole. This can ensure that the foam fills every corner of the box without serious deformation of the foam pores.
3. Post – curing: The requirements are not strict. During continuous production, the mold is not cold, and if there is a heat – preservation tunnel in the production line, there is no need to worry about the curing effect.
4. Recommended combination: Am – 1 + cyclohexylamine.
5. Process confirmation: Clarify the foaming system (141B or cyclopentane), the details of the production line, the ambient temperature and humidity, and consult on – site personnel about the defects of the current process and raw materials.

3. Specific Experiments

1. Compatibility:
   • Take a 100 – mL small beaker, add a short glass rod, tare it, and then pour in the main polyether (polyester) in sequence and stir well to check if it is transparent.
   • Add silicone oil, catalyst, and water, stir well, and check if it is transparent.
   • Add the physical foaming agent and stir well to check if it is transparent (note that the volatilized physical foaming agent after stirring should be replenished).
   • The pre – prepared composite material sample should be stored for at least 3 days. It is better if it remains transparent and does not separate into layers.
   • Place the composite material samples at 35℃ and 15℃ respectively for 24 hours to check if they are transparent.
   • When conditions permit, measure the viscosity of the composite material in the design (at 25℃ and the temperature under normal production conditions) to see if the viscosity fluctuates significantly with temperature.
2. Anti – shrinkage: The freely foamed sample is cut into a regular cube after 1 hour, its fixed – side dimensions are measured, and then it is placed in a – 20°C freezer for 24 hours to observe the dimensional change. A linear shrinkage within 2% is acceptable.
3. Free foaming: Carry out free foaming according to the designed ratio of black and white materials and temperature control, and pay attention to the material speed, core density and wire drawing marks. The quality of fluidity can be roughly judged by the shape of the remaining foam in the cup: the more the foam looks like a mushroom after being taken out of the cup, the better; the more it looks like a straight stick, the worse.
4. Liquidity: After the density and velocity of the free foam are determined, a fluidity test must be carried out. A simple method is as follows: After uniformly mixing a certain amount of foaming material (usually 200g), immediately put a slightly larger long plastic bag over the mouth of the foaming cup. Then, hold the bag straight and vertically upward, allowing the foaming material to grow upward until it sets (this requires two people to operate). The ratio of the height L from the cup mouth to the top to the weight G of the material will be used as an important parameter to evaluate the fluidity of the combined material. The larger the L/G value, the better the fluidity. After that, the core density should also be measured in sections as an auxiliary reference (the density difference from the bottom to the top should not be too large; otherwise, the fluidity cannot be considered good, especially for the section at the highest point).
5. Process condition tolerance:
   • Carry out foaming with a preset temperature control of + 3℃ and – 3℃ to check whether good fluidity and cell structure are still maintained (the cell structure of “fast material” and “slow material” should not have a large difference).
   • Conduct a free foaming test with a constant amount of white material and a black material amount of + 10% and – 10%. If the foam does not show significant shrinkage at room temperature after 30 minutes, it can be considered acceptable.

4. Requirements for White Material System and Products

1. Requirements for white material system:
   • Viscosity: It should not be higher than 2000mPa.S (25℃) in principle, except for high – density ones (with a free – foam density of over 130kg/m³). This is related to the initial fluidity after the gun is fired, the stirring and mixing effect, the gun discharge flow rate and the ratio of black and white materials.
   • Compatibility: Unless it is prepared and used immediately or the white material is put into the machine in a whole barrel and mixed while being injected, the white material should be homogeneous, transparent and not stratified. (If a silicone oil white material with a lower cloud point is selected, it may also be opaque at low temperatures, but it must not be stratified), especially for manually mixed materials.
   • Fluidity: For products with special dimensions and closed – mold casting, the material is required to have good liquidity during the foaming process, as it will at least affect the density distribution of the product.
   • Adaptability: The white material system should preferably be able to switch to different black material varieties (such as M20S, 44V20, 5005, etc.) with slight adjustments to process parameters (such as material temperature, pipe pressure, etc.) or ensure that the products meet the requirements within the reasonable fluctuation range of the ratio of white to black material.
2. Product requirements:
   • The surface skin of imitation wood products should have a certain thickness and hardness to ensure the surface is “hard enough”.
   • The surface of the product should be smooth and free of air bubbles, pinholes, and hidden bubbles (for some products, the requirements for the back can be reduced, such as picture frames and wall hangings).
   • Try to avoid the “white line” defect (irregular white patches and long linear patches appear on the product surface, and the area is significantly softer). At least the white patches should be hard enough.
   • The internal pores of the product should be dense and uniform, and the fewer large pores (with a diameter of over 0.2mm) that stand out, the better.
   • The products should not peel or blister after painting or applying gold leaf.
   • The product is stable. After the long – sized semi – finished product is demolded, it should not “shrink upon cooling”, and the final product should not deform even after traveling across the ocean and experiencing extreme cold and heat.
   • Have sufficient toughness (usually able to withstand flat drops and toss drops from a height of 1 meter).
   • Before painting and foiling, products often need to be washed with solvents to remove the release agent and undergo roughening treatment, so the product surface should be resistant to corrosion.

5. Brief Introduction to Raw Material Selection

1. Main polyether (polyester):
   • “4110” is the main component in most cases (with a majority share of over 60%). It has an ideal rigid framework and flexible long chains, and its price is reasonable. The hydroxyl value is preferably in the range of 380 – 420 mgKOH/g.
   • It is recommended to add 3 – 12% of 403. It can make the cell structure finer, reduce bubbles, pinholes and hidden bubbles on the product surface, and improve the overall hardness of the product. However, it is too sticky, which is not conducive to the control of the viscosity of the white material.
   • Sorbitol type, represented by the 635 series (10 – 20%), can significantly improve the overall hardness of the product and the surface effect, but it has high viscosity and price.
   • Soft foam polyether, such as 220, 210, 330N, is necessary for high – end products and flower pots, which can improve the flexibility of the product and the surface skin thickness. But except for 210, others have poor compatibility with the main polyether.
   • Aromatic polyesters (5 – 20% depending on the quality and properties of the raw materials) are not encouraged to be used. Adding them is mostly for cost consideration, but if the small materials are not properly combined, there will be many troubles.
   • Within 8%, various natural vegetable oils or their modified derivatives can be used to improve the flexibility of the product and slightly reduce the cost, but there are certain risks.
   • For high – value – added products, similar imported polyethers can be considered, especially those for the all – water foaming system.
2. Auxiliary small molecule substances (0.5 – 3%): There are glycerol and the small – molecule polyethers starting from it, N – ethanolamines, small – molecule diols, etc. Their functions include pre – reaction, overall hardening, increasing the thickness and hardness of the surface layer, and shortening the production cycle. It is necessary to select the appropriate model and use a reasonable amount.
3. Silicone oil (1.5 – 2.5%): It follows the usage of other raw materials. To improve some properties of the product (such as fine cell structure, white lines, and fluidity), careful selection is required. It is advisable not to use those with a price below 24 yuan/kg. Pay special attention that the silicone oil model may need to be changed after the main component of the polyether (ester) changes.
4. Catalyst:
   • Pre – stage type: A – 1, Am – 1 function to control the starting time, activate the efficacy of other catalysts, improve the fineness of cell structure, and enhance the fluidity during the foaming process. The dosage should be within 0.5%.
   • Steady type: The dosage of cyclohexylamine is 0.3 – 0.6%. It is currently the rigid foam catalyst with the highest cost performance.
   • Main catalyst: A – 33 is an essential catalyst in wood – like materials, directly affecting the skinning effect and product hardness, with a minimum dosage of 0.4%.
   • Organotin: The imported T – 12 is good, and a very small amount can significantly improve the skinning effect and shorten the demoulding time. Pay attention to the storage period as ordinary organotins are not resistant to hydrolysis.
   • For three – cluster types: It is best to use PC – 41 (0.3 – 0.5%) to harden without affecting the cell structure.
5. Foaming agent combination: Most are 141B/water foaming systems. The water content is 0.2 – 0.7%. For ordinary wood – like materials, the amount of 141B generally should not exceed 10 parts. The usage amount of 141B will directly affect the skin thickness and hardness.
6. Other additives: Some non – reactive substances can be added to reduce the viscosity of the white material, such as adding 3% of DBP. Colorants, antioxidants, etc. can be added directly.
7. Filler: For manual foaming, inert fine powder – like substances can be added to enhance rigidity. Do not add substances with excessive water content.

Mastering these on – site experience and knowledge of polyurethane rigid foam foaming is crucial for improving product quality and production efficiency. By strictly following the calculation methods, optimizing the catalytic system, conducting various tests, meeting the requirements of the white material system and products, and selecting appropriate raw materials, high – quality polyurethane rigid foam products can be produced.

About calculation

What needs to be calculated most in the rigid foam compound is whether the ratio of the white and black materials (by weight) is reasonable. Another formal term seems to be the “isocyanate index” being reasonable. In layman’s terms, it means “the white and black materials mixed in proportion should completely react.” Therefore, all the substances in the white material that participate in the reaction with -NCO should be taken into account.

The calculation of the molar amount of -NCO consumed by each component of the theory is as follows

(1) Main ingredients: Polyether, polyester, silicone oil (ordinary rigid foam silicone oils all have hydroxyl values, allegedly due to the addition of diethylene glycol, etc.). Multiply the formulation quantity of each by its respective hydroxyl value, and then add them together to obtain the result Q.

S1=Q÷56100

(2) Water: The formulated amount of water, w

S2=W÷9

(III) Small molecules participating in the consumption of -NCO: The formulation amount is K, its molecular weight is M, and the functionality is N

S3=(K×N)/M

S=S1+S2+S3

The amount of crude MDI required for the basic formula: [(S×42) – 0.30]×1.05 (the so – called isocyanate index of 1.0)

Determination of catalytic system

Pre – requirements

Previously, many friends thought that the whitening of the material would lead to a slower rise, and the material could be directly made to rise after it flowed noisily to the bottom of each “position”. However, this is not the case. Firstly, the liquid material is very likely to leak out from the gaps in the box, causing dirt to stick to the mold. Secondly, it affects the cell fineness and the overall structure, thus increasing the thermal conductivity of the foam. Thirdly, accelerating the rising speed will instead accelerate the advancing speed of the foaming material. Generally speaking, the best whitening time after the material is ejected from the gun is 6 – 8 seconds.

Medium-term flow

During the foaming and shaping period, the longer the middle-stage flow time period (the time from stringing to cream time) is, the better. This can ensure that the foam fills every corner of the box without causing serious deformation of the foam pores. The most ideal state is that the foam material has filled in place 3 – 5 seconds before the start of stringing, and there is obvious material escaping from the farthest exhaust hole.

Post-curing

This requirement doesn’t need to be too strict. Anyway, the mold isn’t cold during continuous production. If there is a heat – retaining tunnel in the production line, there is no need to worry about not being able to “harvest the crops” on time.

It is recommended to be used in combination with

Am-1 + cyclohexylamine.

Process Validation

1. Determination of the foaming system: Whether it is 141B or cyclopentane. The range of water volume/physical foaming agent dosage is pre-determined.

2. First, clarify the process details of the target production line: type of foaming machine, pouring flow rate, temperature control value before pouring, temperature control value and heat preservation time of the heat preservation tunnel, location of the box pouring port, travel of the foaming material flow route, and the operation process of closing the mold and sealing the holes after pouring.

3. Current changes in the temperature and humidity of the (working) environment.

4. Consult the on – site operators and quality inspectors about the defects of the current process and raw materials, and ask them to put forward any other specific requirements.

Specific experiments

Compatibility:

① Take a 100 – mL small beaker, add a short glass rod, tare it, and then pour in the main polyether (polyester) in sequence and stir well to check if it is transparent. ② Add silicone oil, catalyst, and water, stir well, and check if it is transparent. ③ Add the physical foaming agent and stir well to check if it is transparent (note that the volatilized physical foaming agent after stirring should be replenished). ④ The pre – prepared composite material sample should be stored for at least 3 days. It is better if it remains transparent and does not separate into layers. ⑤ Place the composite material samples at 35℃ and 15℃ respectively for 24 hours to check if they are transparent. ⑥ When conditions permit, it is necessary to measure the viscosity of the composite material in the design (at 25℃ and the temperature under normal production conditions) to see if the viscosity fluctuates significantly with temperature.

Anti-shrinkage:

The freely foamed sample is cut into a regular cube after 1 hour, its fixed – side dimensions are measured, and then it is placed in a -20°C freezer for 24 hours to observe the dimensional change. A linear shrinkage within 2% is acceptable.

Free foaming:

Carry out free foaming according to the designed ratio of black and white materials and temperature control, and pay attention to the material speed, core density and wire drawing marks.

Liquidity:

After the density and velocity of the free foam are determined, a fluidity test must be carried out. A simple method is as follows: After uniformly mixing a certain amount of foaming material (usually 200g), immediately put a slightly larger long plastic bag over the mouth of the foaming cup. Then, hold the bag straight and vertically upward, allowing the foaming material to grow upward until it sets (this requires two people to operate). The ratio of the height L from the cup mouth to the top to the weight G of the material will be used as an important parameter to evaluate the fluidity of the combined material. The larger the L/G value, the better the fluidity. After that, the core density should also be measured in sections as an auxiliary reference (the density difference from the bottom to the top should not be too large; otherwise, the fluidity cannot be considered good, especially for the section at the highest point).

When foaming in normal free foaming, the quality of fluidity can be roughly judged by the shape of the remaining foam in the cup: the more the foam looks like a mushroom after being taken out of the cup, the better; the more it looks like a straight stick, the worse.

Process condition tolerance:

①Carry out foaming with a preset temperature control of +3℃ and -3℃ to check whether good fluidity and cell structure are still maintained (the cell structure of “fast material” and “slow material” should not have a large difference). ②Conduct a free foaming test with a constant amount of white material and a black material amount of +10% and -10%. If the foam does not show significant shrinkage at room temperature after 30 minutes, it can be considered acceptable.

Requirements for White Material System and Products

White material viscosity:

It involves the initial fluidity after the gun is fired (machine foaming type), the stirring and mixing effect (including the manual foaming type), as well as the gun discharge flow rate and the ratio of black and white materials. In principle, it should not be higher than 2000mPa.S (25℃, the same below), except for high – density ones (with a free – foam density of over 130kg/m3).

Compatibility:

Unless it is prepared and used immediately or the white material is put into the machine in a whole barrel and mixed while being injected, the white material should be homogeneous, transparent and not stratified. (If a silicone oil white material with a lower cloud point is selected, it may also be opaque at low temperatures, but it must not be stratified), especially for manually mixed materials.

Liquidity:

Generally, the liquidity requirement for imitation wood materials is not high, which depends on the reasonable arrangement of its own production process. For products with special dimensions and closed – mold casting, the material is required to have good liquidity during the foaming process, as it will at least affect the density distribution of the product.

Adapt to changes in the type of black material and the ratio of black and white materials:

The white material system should preferably be able to switch to different black material varieties (such as M20S, 44V20, 5005, etc.) with slight adjustments to process parameters (such as material temperature, pipe pressure, etc.) or ensure that the products meet the requirements within the reasonable fluctuation range of the ratio of white to black material.

Thickness and hardness of the product’s outer skin:

Imitation wood products should at least ensure that the surface is “hard enough”. Generally, the surface skin is required to have a certain thickness to ensure surface hardness. Specific indicators are difficult to quantify due to factors such as product density requirements and changes in temperature and humidity of the working environment.

Surface Finish and Bubbles of the Product:

The surface of the product should be smooth and free of air bubbles, pinholes, and hidden bubbles (for some products, the requirements for the back can be reduced, such as picture frames and wall hangings).

White line:

This is the most likely defect (irregular white patches and long linear patches appear on the product surface, and the area is significantly softer). Try to avoid it, and at least the white patches should be hard enough.

Cell fineness:

The internal pores of the product should be dense and uniform, and the fewer large pores (with a diameter of over 0.2mm) that stand out, the better.

Painting and Gilding:

The products should not peel or blister after painting or applying gold leaf.

Resistance to deformation:

The product is stable. After the long-sized semi-finished product is demolded, it should not “shrink upon cooling”, and the final product should not deform even after traveling across the ocean and experiencing extreme cold and heat.

Impact resistance:

Have sufficient toughness (usually able to withstand flat drops and toss drops from a height of 1 meter).

Surface corrosion resistance of the product:

Before painting and foiling, products often need to be washed with solvents to remove the release agent and undergo roughening treatment.

Brief Introduction to Raw Material Selection

Main polyether (polyester):

① In most cases, “4110” is the main component (with a majority share of over 60%). As a widely used type of rigid foam polyether, it has an ideal rigid framework and flexible long chains (with sucrose and diol as the starting materials for grafting propylene oxide), and its price is also reasonable. There are not many products suitable for wood imitation in the market. At least, the quality should be stable. It is best not to choose those products that are adulterated, whose hydroxyl value/viscosity fluctuates with the price of PO, or whose cell structure is not fine enough. Regarding the production formula of 4110, the most suitable for wood imitation is the [sucrose + glycerol] starting type (unfortunately, I have only used it twice so far, and it seems to be out of stock now), followed by the [sucrose + propylene glycol] starting type, and the remaining is the [sucrose + ethylene glycol or diethylene glycol] starting type. Other blended types are really not easy to use. The specification indicators of 4110 also have some requirements. Viscosity: Generally, it is 2500 – 3500 mPa·S. For the all-water foaming system, the viscosity should be even lower. If the hydroxyl value is higher than 430 mgKOH/g, the product made will be only hard but not tough, and it needs to be blended with other polyethers with lower hydroxyl values. It is best to be in the range of 380 – 420 mgKOH/g. Color and appearance: If the color is too dark, it indicates that the control during the polyether production process is not good. Moreover, the product made from it will have a darker appearance, making the contrast of the white lines more dazzling.

②403. It is recommended to add some (3 – 12%). The cell structure will be finer, and there will be far fewer bubbles, pinholes, and hidden bubbles on the product surface. The overall hardness of the product will also increase. The drawback is that it is too sticky, which is not conducive to the control of the viscosity of the white material. Currently in the market

The 403 products in the market are also of uneven quality. The genuine 403 should start with ethylenediamine. Since the raw materials are indeed a bit expensive (seemingly over 20 yuan/kg), some people try to cut corners. They add urea or glycerin to the initiator, or increase the amount of PO to reduce costs. The 403 with urea added has a strong ammonia smell and coarser cell structure. The hardness of the 403 with glycerin added is affected. The 403 with more PO grafted has a lower viscosity and insufficient hardness.

③ Sorbitol type. Represented by the 635 series (10 – 20%). It can be omitted considering cost, but using it can significantly improve the overall hardness of the product and the surface effect (smoothness, pinholes, etc.). There are also drawbacks: this type of polyether has a relatively high viscosity and a relatively high price.

④ Soft foam polyether. 220, 210, 330N, and even grafted polyether (such as 36/28). It may not be added for ordinary hard imitation wood materials. It must be added for high – end products and flower pots. This type of polyether has a low hydroxyl value, good product flexibility, and can improve the surface skin thickness of the product. Disadvantages: Except for 210, the others have poor compatibility with the main polyether and are prone to layering and separation.

⑤ Polyester types. Aromatic polyesters (5 – 20% depending on the quality and properties of the raw materials). In principle, their use is not encouraged. The main purpose of adding them is to be “cost-effective”, but if the small ingredients are not properly combined, there will be many troubles. A high acid value can cause coarse cell structure, numerous pinholes, and even a “white fog” to appear on the surface of the product.

⑥ Other polyols (within 8%). There are various natural vegetable oils or their modified derivatives that can be used. They can improve the flexibility of the product and slightly reduce the cost. The risks are the same as those of item ⑤. You shall bear the consequences if used improperly.

⑦ Other ideas: If the product is of high added value, similar imported polyethers can be considered, especially those for the all-water foaming system. It is difficult for domestic polyether raw materials to reach the level of Dow.

Auxiliary small molecule substances: (0.5 – 3%)

There are glycerol and the small – molecule polyethers starting from it, N – ethanolamines (N: mono -, di -, tri -), small – molecule diols, and even MOCA (this is carcinogenic, it’s better not to touch it). Their functions include pre – reaction, overall hardening, increasing the thickness and hardness of the surface layer, and shortening the production cycle. It is also necessary to select the appropriate model and use a reasonable amount.

Silicone oil: (1.5 – 2.5%)

This follows the usage of other raw materials (raw material combination). Generally, any regular rigid foam silicone oil can be used. However, to improve some properties of the product (such as fine cell structure, white lines, and fluidity), careful selection is required. It is advisable not to use those with a price below 24 yuan/kg. Pay special attention that the silicone oil model may need to be changed after the main component of the polyether (ester) changes.

Catalyst:

① Pre-stage type: A-1, Am-1 function to control the starting time, activate the efficacy of other catalysts, improve the fineness of cell structure, and enhance the fluidity during the foaming process. The dosage should be within 0.5%, depending on the amount of “water” used. When the weather conditions are stable, it is best to maintain a “fixed amount”.

② Steady type: The dosage of cyclohexylamine is 0.3 – 0.6%. It is currently the rigid foam catalyst with the highest cost performance, and its catalytic ability is very average in each stage.

③ Main catalyst: A-33 is an essential catalyst in wood-like materials, directly affecting the skinning effect and product hardness, with a minimum dosage of 0.4%. Here, A-33 refers to solid amine (triethylenediamine) dissolved in small-molecule diols, not the products for sponges sold on the market (there are many counterfeits).

④ Organotin: The imported T-12 is the best. A very small amount (one ten-thousandth) can significantly improve the skinning effect and shorten the demoulding time. It should be noted that: Pay attention to the storage period of the white material. Because ordinary organotins are not resistant to hydrolysis and will gradually become ineffective. For example, for T-12, the signs of ineffectiveness become very obvious after one week when used in the composite material. It is recommended to use the hydrolysis-resistant T-120/T-6.

⑤ For three-cluster types: It is best to use PC-41 (0.3 – 0.5%) to harden without affecting the cell structure. Using 0.5% of DMP-30 did not yield very satisfactory results. Dimethylethanolamine was said to be quite useful, but upon testing, it was found that the cells showed signs of coarsening.

⑥ Others: Dimethylbenzylamine (0.5%) does not improve the product quality significantly. However, it can improve the fluidity of the material when used in low-density, relatively long-sized, and closed-mold cast decorative cornice boards.

Foaming agent combination

Now, most of them are 141B/water foaming systems. The water content is 0.2 – 0.7%. For ordinary wood – like materials, the amount of 141B generally should not exceed 10 parts. The usage amount of 141B will directly affect the skin thickness and hardness. Using too much of it will also cause hidden bubbles on the surface.

Other additives for viscosity reduction, colorants, anti-yellowing, etc.

You can add some non-reactive substances to reduce the viscosity of the white material, such as adding 3% of DBP. Just add the colorant, antioxidant, etc. directly without worry.

Filler

This is only for manual foaming. Inert fine powder-like substances can be added to enhance rigidity. Note: Do not add substances with excessive water content.