Polyurethane material is a block polymer containing carbamate characteristic groups produced by the reaction of polyisocyanate and hydrogen donor. Because of the various appearance forms of the generated products, its application has entered various fields of the world economy. The following is an overview of the horizontal continuous production line for polyurethane flexible block foam.

1. Hennecke flat top method

The continuous production line equipment for large-scale flexible polyurethane foam blocks was designed and put into production by Hennecke Company in Germany in 1952, which is the basis for continuous production of polyurethane foam blocks. Many companies have successively designed and manufactured continuous production lines for various forms of block bubbles, but only the basic principles designed by Hennecke have been used to this day. The production equipment is shown in Picture 1.



Pic 1 Schematic diagram of Hennecke polyurethane soft foam flat top continuous foaming production line

The polyurethane flexible foam continuous production line produced by Hennecke consists of several main parts: raw material supply section, mixing and pouring section, foaming and curing section, cutting section, post-curing section, and post-processing of the product. This production line has high production efficiency and requires a large supply of raw materials. Therefore, in addition to equipping tanks for polyols and isocyanates, separate systems for raw material storage, process parameters, condition control, and preparation are necessary to ensure a continuous supply of prepared raw materials to the production line during continuous operation (Picture 2).





Pic 2: Metering supply systems and mixing head input systems for 22 components

Temperature has a significant impact on the foaming reaction, and strict control of raw material temperature is necessary during foaming. Typically, the temperature is controlled within the range of 18 to 25°C, with a temperature fluctuation range of around 1°C. High-precision metering pumps are used for the metering and delivery of raw material components, with a general viscosity range of less than 2000 mPas. For high viscosity components such as colorants and flame retardants, gear pumps can be used. To prevent leakage of isocyanate components, the use of magnetic couplings is recommended. For convenience of operation and improved metering accuracy, some additives are now combined to reduce the number of metering pumps. However, it's important to note that certain additives, such as organic tin catalysts, are sensitive to other components and prone to degradation.

The mixing device used in this production line typically employs a low-pressure mixing head, with the agitator driven by a variable-speed motor at a rotational speed of 3000 to 6000 r/min. In modern continuous block foam production enterprises, high-pressure metering, mixing, and foaming equipment have also been adopted, allowing for adjustments in the mixing head's stirring form, flow rate, and nozzle size to enhance product quality. An air input device can also be configured at the mixing head to create gas nuclei and generate a fine and dense cell structure.

Well-mixed material is continuously discharged from the mixing head under certain pressure. To prevent material splashing and the entrapment of a large amount of air causing large voids within the foam body, various measures are taken during the foaming process. Apart from reducing the distance between the mixing head and the bottom plate and minimizing impact force, special designed baffles, horn-shaped or duckbill-shaped deflection tubes, and metal meshes are installed in the front part of the mixing head's outlet to reduce the impact energy of the material.

Meanwhile, the distance from the material outlet pipe to the bottom plate needs to be decreased to around 10 mm. To ensure the uniformly distributed material on the bottom plate, cross beams are set up on the production line. The mixing head can be adjusted to move left and right in coordination with the bottom plate conveyor belt's movement speed. Alternatively, the material can be divided into multiple conduits to enter distribution slots arranged laterally in the direction of the bottom plate's movement, ensuring the material is evenly distributed on the conveyor belt, as shown in Picture 3.



Pic 3 In order to prevent the spit out material from splashing, the mixing head is equipped with some deflectors

The material ejected from the mixing head exhibits good flowability before the emulsification time. As the reaction progresses, the mixed material gradually initiates and expands. At the front end of the conveyor belt in the ejection section, the conveyor belt should be inclined at an angle of 3° to 9° and equipped with hydraulic or manual adjustment devices. This allows for appropriate adjustments of the incline angle according to process requirements, ensuring the material flows and initiates uniformly in one direction. If the incline angle is too small or the conveyor belt's movement speed is too slow, the foam thickness increases, and initiating the foam becomes difficult. If the incline angle is too large, the ejected material will flow too quickly, reaching the lower part of the foam layer that has already started to rise, causing cracks in the foam body.

Typically, for high-flow-rate units, the conveyor belt's movement speed is controlled at 3 to 10 m/min, while for medium-sized units, it's controlled at 1.5 to 3 m/min. During operation, it's crucial to carefully adjust process parameters such as the ejection rate, conveyor belt angle, and movement speed to maintain a suitable distance of 300 to 600 mm between the ejected distribution line and the milky line formed during foam initiation.

The mixed material ejected from the mixing head is directly distributed onto the pre-laid liner paper on the conveyor belt. In the foaming section, a conveying and recovery device is assembled, including conveyor belts, a drying tunnel, side guards, and foam liners. In the past, a three-liner system was commonly used, with liner paper on the left and right sides moving synchronously with the foam body along the exhaust duct, while the bottom liner paper moved forward in sync with the conveyor belt. In the past, the upper part of the foam body was not restricted, resulting in a wasteful arched shape. Subsequently, the Hennecke-Planidiock method (see Picture 4) and the Hennecke flat-top foaming method (see Picture 8-5) were invented. The improved Hennecke flat-top method is now widely used.



Pic 4 Hennecke-Planidiock method





Pic 5 Schematic diagram of Hennecke flat-top foaming process

Both of the aforementioned production methods are equipped with mechanical balance pressure plates on the upper part of the rising foam body to reduce the volume of arched waste generated at the top of the foam body. Currently, the equipment for Hennecke's flat-top foaming often employs four liner papers synchronized to move in the upward, downward, leftward, and rightward directions along with the conveyor belt.

The lining materials for the foam body include specialized liner paper and plastic film. The base material of the liner paper is strong and durable kraft paper, treated with release agents such as polydimethylsiloxane or paraffin, or coated with non-adhesive chemicals like polyethylene. In recent years, some production facilities have begun using cost-effective plastic films like polyethylene, but it's important to ensure that the film does not crease during operation. Regardless of the lining material, it must remain flat and free of folds during operation.

In the foaming section's drying tunnel, the foam body expands and foams on the liner paper of the conveyor belt. Depending on the specific production formulation, the heat generated by the material's reaction or external heat sources are utilized to expedite the foam body's reaction, curing, and solidification, achieving the desired strength and performance for the subsequent process. The drying tunnel is equipped with multiple exhaust devices to remove various harmful gases produced by the foam body. After purification, these gases are released into the atmosphere.

The conveyor belt system for the foam body requires an extremely smooth surface and operates very steadily without any vibrations. The spacing between the side guards can be adjusted within a certain range as needed, allowing the production of rectangular foam bodies of different widths. The width can reach up to 2.2 meters, and the height of the produced foam bodies generally exceeds 1 meter.

After passing through the drying tunnel, although the foam body has not yet reached its maximum performance, it has been shaped. To facilitate subsequent stages of work, an online assembly cutting machine is used to cut the foam body into desired lengths. Following this, post-curing is performed to ensure complete reaction before further processing.

2. Maxfoam down-moving foaming method

The Maxfoam method, also known as the downward foaming method, was invented by Norwegian scientist Leader Berg in 1959. It employs a distinctive approach, where the foam foaming bottom plate moves downward. The fundamental principle involves raising the front end of a movable bottom plate to a position approximately 70% of the anticipated final foam height. This allows the entire bottom plate to be inclined downward. As the poured material rises to around 30% of its foam height, the lower bottom plate moves downward at the foam's rate of expansion. This causes the remaining 70% of the foam's height to expand downward, resulting in a foam body with a rectangular cross-section. The principle and equipment can be seen in Picture 6. Leader Berg used this principle to design and develop the renowned Maxfoam downward foaming process, depicted in Picture 7.



Pic 6 Schematic diagram of the principle of bottom plate moving down method





Pic 7 Schematic diagram of Maxfoam down-moving foaming process

In the development of the Maxfoam production apparatus, Leader Berg initially placed a baffle at the discharge point of the mixed material. This gradually evolved into an elongated downward foaming trough, and the flat plate where the material flowed was transformed into a downward-inclined bottom plate. This alteration changed the foam body's upward expansion during initiation to a downward expansion, leading to the creation of the renowned Maxfoam foaming process. Leader Berg's company has been dedicated to the research, development, production, and sales of flexible polyurethane block foam production processes and equipment, becoming one of the most prominent companies in this field. The basic process flow can be seen in Picture 8.



Pic 8 Maxfoam equipment produced by Hennecke

(1)The cross-section of the produced foam body is in a regular rectangular shape, leading to a significant reduction in waste rate and a high yield of finished products. In traditional processes, waste from edge and corner cuts is approximately 15%. In the Draka edge sliding method, it's around 12%. However, the waste generated by the Maxfoam process is less than 8%. With further improvements, such as using rotating forks, traction, and flattening devices covered with polyethylene film to fully envelop the foam body (see Picture 9), and utilizing the heat generated by the reactants to heat the bottom plate to make the lower skin of the foam thinner, waste can be lowered to 1% to 2%.



Pic 9 Laying polyethylene thin turning fork (a) device (b) and flattening device (c)

(2) The equipment is well-designed, precisely manufactured, accurately controlled, with a long lifespan, low production costs, and typically requires only 3 to 4 personnel for operation, with low maintenance costs.

(3) The unique foaming process ensures that the produced foam body has a uniform and consistent density, fine cell structure, and excellent quality.

(4) A typical control panel or an enhanced computer system monitors the entire production process with precision.

(5) The range of applicable raw materials is extensive, including both polyether and polyester types. Various types of foam bodies can be produced, including standard flexible foam as well as high-resilience foam, flame-retardant foam, filled foam, viscoelastic foam, and foam produced using carbon dioxide foaming.

In 1960, Leader Berg established his own company, Laader Berg AS, dedicated to the research and production of continuous polyurethane foam production equipment. The key components of the basic MaxformTM foaming machine are the Multi Trough (Picture 10) and the drop plate. As shown in the equipment schematic in Picture 11, the mixed materials are conveyed through multiple pipes to the bottom entry of the multi-trough. The material begins to react in the multi-trough and flows onto the bottom liner paper sliding on the inclined drop plate just before the emulsification of the mixed liquid. The foam from the multi-trough evenly overflows and spreads between the two side walls of the drop plate. The overflow volume of the multi-trough can be adjusted based on the foaming formula and production volume, and its outlet height is set at 70% of the final foam height.

Simultaneously, the angle, quantity, length, and width of the inclined drop plate can be adjusted according to the formula and production volume, ensuring that the foam body completes its full expansion process when it reaches the horizontal conveyor belt. During the downward flow of the foam body in the foaming channel of the drop plate, the friction between the foam body and the side walls is eliminated by downward gravity, resulting in a more uniform and smooth foam structure on both sides of the foam body. The foam body discharges the waste gases produced during production in the foaming channel, completes the maturation of the foam body, and can then proceed to the cutting process.



Pic 10 Multiple slots for Maxfoam foam machine multi trough



Pic 11 Basic MaxfoamTM Schematic

Our company also produces this kind of production line on the basis of this foaming method. The introduction reference is as follows (see Picture 12)

Technical parameters of SAB-CF02 automatic horizontal continuous foaming production line produced by Sabtech Technology

1. Main machine specification: total length 42m × width 6m × 4m

2. Foam sponge width: 915mm ~ 2350mm

3. Foaming height: below 1300mm

4. Foaming speed: 1500rpm ~ 7000rpm

5. Maximum output: 350kg / min

6. Spraying mode: The way of trough device, with inverter controlling

7. Foam box specification: L21m * W4.5m * H3m

8. Oven inner conveyer line (standard): L27m * W2.6m * H0.8m

9. Oven side links(standard) L21m * H1.3m

10. Drop frame: 7 sections of electric adjustment height / 0.2KW deceleration motor chain is used to drive rack adjustment between each section of plate

11. Side paper lifting device: front and rear electric movement, lift lever height electric adjustment, left and right-side independent control.

12. Side film collect and release system: the side film and lifting film releasing device is equipped with motor drive, the side film adopts magnetic powder clutch device to automatically reel in.

13. Bottom paper storage system.

14. Exhaust fan: 3kw * 2 sets (excluding exhaust pipe).

15. Constant temperature system: 20HP air-cooled cold and hot thermostat. Proportional valve is installed at the front inlet of tank coil, and raw material temperature is controlled and set.

16. Powersupply:3phrase 380V 50HZ



Figure 12 Sabtech Technology Limited horizontal continuous foaming unit3

3. Vertical Foam Method

In 1971, the UK-based company Hyman Development Corporation developed a unique vertical foam process technology and equipment. The apparatus mainly consists of a material storage tank system, metering conveying system, mixing injection system, barrel-shaped foaming device, heating and foam lifting device, as well as a cutting mechanism (see Picture 13).



Pic 13 Schematic diagram of vertical foaming equipment

The material storage tank system consists of five main components: raw material tanks (equipped with temperature control and stirring devices) for PPG, with TDI as the primary raw material, mixed with water, oil, amine catalyst, additives, MC foaming agent, and organic tin catalyst. Their metering and conveying systems generally use gear pumps driven by stepless speed-regulated motors, and flow meters can also be added to enhance metering accuracy. Low-pressure, agitating-type mixing heads are typically chosen. Once the materials are mixed, they are injected through pipelines from the bottom into the conical foam bucket. The foam bucket is pre-fitted with continuous sheets of polyethylene film. As the mixed materials react and foam, they initially move horizontally, filling the conical cross-section and gradually rising as the cross-section expands, eventually filling the polyethylene film-lined bucket and moving upward into the heating section. An electric heating system surrounds the heating section to expedite the foam maturation process.

The foam's ascent is facilitated by vertical conveyors equipped with fine needles (10-15mm in length). Multiple such conveyors are arranged around the entire foam body, with their fine needles embedded in the foam of a certain strength. As the conveyor belt rotates, the foam is gradually lifted. The upper part of the equipment is equipped with a cutting machine and a linked clutch mechanism that activates the cutting machine when the foam body reaches the designated height. The cut foam pieces are transported along an inclined slide to the post-maturation chamber.

This process can produce foam bodies with either square or circular cross-sections, simply by changing the shape of the foam bucket. During continuous production, the foam's color can be changed online, with a transition zone of only 150mm. This not only facilitates easy color changes but also maintains a high yield of finished foam products. The density and hardness performance on the foam's cross-section are consistent, and the foam's skin thickness at the edges is thin, resulting in low waste rates. Importantly, vertical foaming equipment occupies a smaller footprint, only a quarter of that of traditional horizontal foaming equipment, making it suitable for small and medium-sized enterprises. The products are not only suitable for general soft foam products but also the sliced circular foam bodies are particularly suitable for use as clothing lining materials.

The vertical foaming process imposes stricter requirements on aspects such as raw materials, formulations, and production process adjustment and control, compared to the production process of horizontal block foams. Precise control of various process parameters such as raw material temperature, formulation ratios, foam discharge rate, air injection rate, mixing speed, maturation section temperature, and traction speed is necessary to produce high-quality foam. In actual production, the following issues are prone to occur and must be addressed:

1.High Foam Closed Cell Rate or Shrinkage:

This can result from excessive use of organic tin catalyst, leading to rapid gelation during foaming and excessive growth of pore wall strength. Additionally, an excess of foam stabilizer can hinder the formation of open-cell foam structure due to its excessive stability.

2.Foam Body Cracking:

Foam body cracking is often due to errors in formulation or metering. Insufficient amounts of organic tin catalyst and foam stabilizer can lead to decreased reactivity. Mechanical factors, such as the presence of impurities, oil contamination within the foam body, and fluctuations in traction speed, can also contribute to extensive foam body cracking.

3. Large Bubble Cavities in Foam Body:

When large bubble cavities appear in the foam body, it is important to thoroughly inspect the following aspects: When there is a regular distribution of air bubbles, check whether there are any air leakage issues in the mixing chamber, feed pipes, and other equipment. If there are a few conical large bubbles present, it could be due to excessively high raw material temperature, causing the foaming agent to vaporize more easily. When the foam body exhibits irregularly distributed large air bubbles, the main cause could be excessive mixing speed resulting in a higher amount of entrapped air. Typically, with a well-sealed mixing head, the mixing speed should be controlled within the range of 2500 to 3000 rpm. If large perforations or interconnected bubbles appear in the foam sheet without a clear network structure, it might be due to excessive air input into the mixing head.

4.The Foam Body Sliding Downward：

This issue should be considered from several aspects, including formulation errors, excessive foaming time, insufficient foaming, excessively low maturation temperature, and improper coordination of the traction conveyor. It is a problem that can easily occur in the initial stages of equipm



Pic 14 Schematic diagram of the production process of the polyurethane flexible foam pressure-swing foaming continuous production line device

(1) Open the middle chamber door 3a and close the exit chamber door 3b. Activate the pressure control system 4a4b to bring the pressure in the entire channel to the set pressure value. The typical pressure range is 50 to 150 kPa (0.5 to 1.5 atm).

(2) Start the foaming machine, and the mixed material enters the overflow trough in the enclosed channel and flows to the drop plate for foaming under the set pressure environment.

(3) After the foam body is preliminarily cured and shaped to a certain length, the cutting machine operates to cut it.

(4) The cut foam body enters the post-area of the channel. Close the middle chamber door, adjust the pressure in the rear area to be equal to the ambient pressure, open the exit chamber door, and transport the foam body to the curing area to complete the curing. At the same time, the exit chamber door should be closed immediately, and the pressure regulation device should be activated immediately to equalize its pressure with the pressure in the entire channel. Then, open the middle chamber door to accommodate the next cut foam body.

This production line is monitored by highly automated computers, with segment control of the channel, cycle switching, and pressure adjustment system. Depending on the sealed channel, whether it's a vacuum or pressure vessel, it can produce foam bodies with rectangular or circular cross-sections. Based on this continuous production line, intermittent production lines with box-type variable pressure foaming have also been developed. Although the production efficiency is high, the control system is complex, and the equipment is bulky, with sealed channel lengths often exceeding hundreds of meters, resulting in significant investment.

The above provides an introduction to the horizontal continuous production line for flexible polyurethane foam blocks. Hope it can help you how to choose polyurethane flexible foam continuous production line. Welcome to leave a comment and discuss with me more about polyurethane foam.

When using a batch foam machine for polyurethane soft foam foaming, have you encountered the following situations?

1.Uneven and numerous foam pores,

2. Rough foam texture.

3. Chaotic pore sizes across the entire foam surface, with slight signs of large pores.

Issues like these are quite common. The main reason for the first issue is that the distance between the mixing impeller of the foam machine and the bottom of the mixing barrel is too great; the second issue is that the mixing blades are too short and narrow: the third issue is that the angle of the mixing blades is too large.

Many manufacturers who design and produce foam machines only understand the principles during the design process, without understanding the significant relationship between a different design in foam production and product quality. A reasonable and perfect mechanical design can only be gradually improved in actual work, and only experienced foamers can achieve this.

Here are some experiences we have had with machine modifications and upgrades, hoping they will be helpful:

First, the installation position of the mixing wheel should be as low as possible, closer to the bottom of the mixing barrel is better. In general, the distance between the lowest point of the mixing blade and the bottom of the mixing barrel should be around two centimeters

Second, the shape of the mixing blade should be fan-shaped, with a moderately wide edge. The advantage of being wide is that it increases the contact area with the liquid material, providing sufficient power and also balances the liquid material.

Third, the length of the mixing blade should also be as long as possible, leaving about three to four centimeters from the baffle inside the mixing barrel.

Fourth, the two edges of the mixing blade should be sloped, with the angle of inclination based on the width of one end and two centimeters difference on both sides. After the mixing blade is modified, proper operation is also crucial, especially the mixing speed. Most batch foam machines nowadays are equipped with high-speed timing frequency conversion devices. However, in actual production, this device is often unnecessary. The operating speed mainly depends on the amount of material in the mixing barrel. If there is a lot of material, the speed should be appropriately faster, and if there is less material, then the speed should be lower.

Testing Conditions:

1. Fast foaming is taken from the center of the foam, while molded foam samples are taken from the central part or for whole sample testing.

2. Newly made foam should be matured for 72 hours in its natural state before sampling. Samples should be placed in a constant temperature and humidity environment (as per GB/T2918: 23±2℃, relative humidity 50±5%).



Density: Density = Mass (kg) / Volume (m3)



Hardness: Indentation Load Deflection (ILD), Compression Load Deflection (CLD)

The main difference between these two test methods is the loading area of the foam plastic. In the ILD test, the sample is subjected to a compressed area of 323 cm2, while in CLD the entire sample is compressed. Here, we will only discuss the ILD test method.

In the ILD test, the sample size is 38*38*50mm, with a test head diameter of 200mm (with a round corner of R=10 on the bottom edge), and a support plate with 6mm holes spaced 20mm apart. The test head loading speed is (100±20) mm/min. Initially, a pressure of 5N is applied as the zero point, then the sample is compressed to 70% of its thickness at the zero point, and unloaded at the same speed. This loading and unloading is repeated three times as pre-loading, then immediately compressed at the same speed. The compression thicknesses are 25±1% and 65±1%. After reaching the deformation, hold for 30±1s and record the relative indentation value. The recorded value is the indentation hardness at that compression level.

Additionally, 65% ILD / 25% ILD = Compression Ratio, which is a measure of foam comfort.



Tensile Strength, Elongation at Break: Refers to the maximum tensile stress applied during the tensile test until fracture, and the percentage elongation of the sample at fracture.

Tensile Strength = Load at Fracture / Original Cross-sectional Area of Sample

Elongation at Break = (Fracture Distance - Original Distance) / Original Distance * 100%



Tear Strength: Measures the material's resistance to tearing by applying specified tearing force on a sample of defined shape.

Sample size: 150*25*25mm (GB/T 10808), with the sample thickness direction as the foam rise direction. A 40mm long incision is made along the thickness direction (foam rise direction) at the center of one end of the sample. Measure the thickness along the sample thickness direction, then open the sample and clamp it in the test machine fixture. Apply load at a speed of 50-20mm/min, using a blade to cut the sample, keeping the blade at the center position. Record the maximum value when the sample breaks or tears at 50mm.

Tear Strength = Maximum Force Value (N) / Average Thickness of Sample (cm)

Usually, three samples are tested, and the arithmetic mean is taken.



Resilience: Measures the foam's rebound performance by allowing a given diameter, weight steel ball to freely fall onto the surface of the foam plastic sample from a specified height. The ratio of the rebound height to the steel ball's drop height indicates the foam's resilience.

Test Requirements: Sample size 100*100*50mm, the ball drop direction should be consistent with the foam usage direction. The steel ball size is ∮164mm, weight 16.3g, and it drops from a height of 460mm.

Resilience Rate = Steel Ball Rebound Height / Steel Ball Drop Height * 100%

Note: Samples should be horizontal, steel ball should be fixed before dropping (static), each sample is tested three times with 20s intervals, and the maximum value is recorded.



Compression Permanent Deformation: In a constant environment, the foam material sample is maintained under constant deformation for a certain period, then allowed to recover for a period of time, observing the effect of the deformation on the sample's thickness. The ratio of the difference between the initial thickness and final thickness of the sample to the initial thickness represents the foam plastic's permanent compression deformation.

Compression Permanent Deformation = (Initial Thickness of Sample - Final Thickness of Sample) / Initial Thickness of Sample * 100



Fire Resistance



VOC (Volatile Organic Compounds)

When establishing a polyurethane foam factory, careful consideration of the site selection and construction conditions is crucial for its success. Several principles guide the selection of the factory site:

Firstly, the principle of optimizing and reorganizing existing resources of project units is essential. This ensures that the factory can make the best use of available resources without unnecessary duplication.

Secondly, the principle of saving land and reducing investment is vital. By selecting a site that is efficient in its land use, the factory can minimize costs and maximize efficiency.

Thirdly, the principle of facilitating transportation and reducing product production costs is significant. A location that allows for easy transportation of raw materials and finished products helps in lowering overall production costs.

Lastly, the principle of preventing urban pollution and protecting the environment is paramount. Choosing a site away from densely populated areas helps in reducing the impact of factory operations on the city's environment.



In addition to these site selection principles, various factors related to construction conditions must also be considered:

Geographic location and transportation conditions play a crucial role. An ideal location would have good access to transportation networks, such as highways or railways, facilitating the movement of goods.

Resource status and social conditions are important factors. This includes assessing local service supporting facilities, availability of labor resources, and government policies that might affect the factory's operations.

Natural conditions, such as climate, geological factors, and seismic considerations, should not be overlooked. Understanding these factors helps in planning for any potential risks or challenges during construction and operation.

Factory construction conditions such as water supply, drainage, power supply, and heating are essential for the smooth functioning of the facility. Adequate provisions for these utilities must be ensured during the planning stages.

In conclusion, the successful establishment of a polyurethane foam factory hinges on a thoughtful analysis of both site selection principles and construction conditions. By adhering to these considerations, the factory can be set up in an optimal location with the necessary infrastructure for efficient and sustainable operations.

1. Polyether

Polyether, as the main raw material, reacts with isocyanate to form urethane, which is the skeletal reaction of foam products. When the molecular weight increases with the same functionality, the tensile strength, elongation, and resilience of the foam increase, while the reaction activity of similar polyethers decreases. With the same equivalent value (molecular weight/functionality), an increase in functionality accelerates the reaction, increases the cross-linking degree of polyurethane, raises foam hardness, and reduces elongation. The average functionality of polyols should be above 2.5; if it is too low, the recovery of the foam body after compression is poor.



If the amount of polyether used is high, equivalent to a reduction in other materials (TDI, water, catalysts, etc.), it is easy to cause foam products to crack or collapse. If the amount of polyether used is low, the foam product tends to be hard, with reduced elasticity and poor touch.



2. Foaming Agent

Generally, when producing polyurethane blocks with a density greater than 21, only water (chemical foaming agent) is used as the foaming agent. Low-boiling compounds such as methylene chloride (MC) are used as auxiliary foaming agents in low-density formulas or ultra-soft formulas.



Auxiliary foaming agents reduce the density and hardness of the foam. Since they absorb some of the reaction heat, curing is slowed down, requiring an increase in the amount of catalyst. By absorbing heat, the danger of core burning is avoided.



The foaming capacity can be expressed by the foaming index (the number of parts of water or equivalent number of water used for 100 parts of polyether):

IF = m (water) + m (F-11) / 10 + m (MC) / 9 (100 parts of polyether)



Water, as a foaming agent, reacts with isocyanate to form urea bonds and releases a large amount of CO2 and heat. It is a chain-growth reaction. Excess water reduces foam density and increases hardness. However, it also reduces the size and strength of foam pores, reducing their load-bearing capacity, making them prone to collapse or cracking. Increased TDI consumption leads to more heat release and a higher risk of core burning. If the water amount exceeds 5.0 parts, physical foaming agents must be added to absorb some of the heat and prevent core burning. Less water means a corresponding reduction in the amount of catalyst used, but it increases density.



3. Catalyst

Amine: A33 is generally used, which promotes the reaction between isocyanate and water, adjusting foam density, bubble opening rate, etc., mainly promoting foaming reactions.

Too much amine: The foam product cracks, and there are holes or bubbles in the foam; Too little amine: The foam shrinks, closes pores, and the bottom of the foam product becomes thick.



Tin: Typically, Tin(II) octoate (T-9) is used; Tin(IV) oxide (T-19) is a highly active gel reaction catalyst, mainly promoting the gel reaction, i.e., the later stage reaction.

Too much tin: Fast gelation, increased viscosity, poor resilience, poor air permeability, leading to closed-cell phenomenon. Properly increasing its dosage can obtain good open-cell foam plastics with relaxation, further increasing the dosage makes the foam gradually become denser, leading to shrinkage and closed cells.

Too little tin: Insufficient gelation, resulting in cracking during foaming. There may be cracks on the edges or tops, with burrs and poor consolidation. Reducing amine or increasing tin can increase the strength of the polymer foam film when a large amount of gas is generated, thereby reducing hollow or cracking phenomena.



Whether polyurethane foam plastics have an ideal open or closed cell structure mainly depends on whether the gel reaction rate and gas expansion rate are balanced during foam formation. This balance can be achieved by adjusting the types and amounts of tertiary amine catalysts and foam stabilizers in the formula.



4. Foam Stabilizer (Silicone Oil)

Foam stabilizers are a type of surfactant that disperses polyurea well in the foaming system, acting as "physical cross-linking points" and significantly increasing the early viscosity of the foam mixture, preventing cracking. On the one hand, it has an emulsifying effect, enhancing the miscibility between foam material components. On the other hand, the addition of organic silicon surfactants can reduce the liquid's surface tension, reduce the free energy required for gas dispersion, make the dispersed air in the raw materials nucleate more easily during stirring and mixing, facilitate the production of fine bubbles, adjust the size of foam pores, control the foam cell structure, and improve foaming stability. It prevents collapsed or burst bubbles, makes the foam walls elastic, controls the pore size and uniformity of the foam. Generally, the more foaming agent and POP used, the greater the amount of silicone oil used.



High usage: Increases the elasticity of foam walls in the later stage, making them less likely to burst, resulting in smaller pores and closed cells.

Low usage: Foam bursts, collapses after foaming, larger pore size, and easy co-foaming.



5. Temperature Influence

The foaming reaction of polyurethane accelerates with an increase in material temperature, which can pose a risk of core burning and ignition in sensitive formulations. Generally, the temperatures of the polyol and isocyanate components are kept constant. When foaming, the foam density decreases as the temperature of the material increases. With the same formula, if the temperature remains the same but the ambient temperature is high in summer, the reaction speed increases, leading to a decrease in foam density and hardness, an increase in elongation, and an increase in mechanical strength. In summer, the TDI index can be appropriately increased to correct the decrease in hardness.



6. Influence of Air Humidity

With increasing humidity, the isocyanate in the foam reacts with the moisture in the air, causing a decrease in hardness. Therefore, when foaming, the TDI amount can be appropriately increased. Excessive humidity can cause the curing temperature to rise too high, leading to core burning.



7. Influence of Atmospheric Pressure

With the same formula, foaming in high-altitude areas results in lower foam product density.