For many small-scale enterprises, although the continuous production line of polyurethane flexible foam offers high output, the costs are also very high, and the target market may not require such large quantities. As a result, non-continuous production lines for polyurethane flexible foam have become their preferable choice. The following is an introduction to the non-continuous production line for polyurethane flexible foam:

1. Box Foaming Process Equipment

The box foaming process and equipment have been developed as a new technology to accommodate the needs of small-scale polyurethane foam production facilities. It builds upon laboratory and manual foam production techniques, essentially an upscaled version of laboratory foam methods. This process has gone through three development stages. Initially, all component materials were sequentially weighed and added to a larger container, followed by the addition of TDI. After rapid mixing, the mixture was immediately poured into a large box mold. This method had high labor intensity, emitted high concentrations of toxic gases, and posed significant health risks to operators. Additionally, the splattering of materials during pouring would entrain a large amount of air, leading to the formation of large air bubbles within the foam structure and even causing foam cracking. Furthermore, there was a significant amount of leftover waste, resulting in substantial material waste and high production costs. 

Later on, the process incorporated metering pumps to convey materials to a mixing barrel with an automatically opening bottom. After high-speed mixing, the bottom plate of the mixing barrel would open, and compressed air would swiftly expel the material into the mold for foam expansion. However, this approach suffered from uneven foam pore structures due to the rapid material flow, leading to swirling foam structures and quality issues like crescent-shaped cracks. The third stage of process improvement is the box foaming device that is mostly adopted today. Its fundamental foaming principle is illustrated in Picture

(a) Raw Material Metering and Mixing      (b) Foaming     (c) Foam Rises to Limit Height

1 - Elevatable Material Mixing Barrel; 2 - Assemblable Box Mold; 3 - Floating Box Top Plate; 4 - Foam Body

Picture 1: Schematic Diagram of Box Foaming Principle

The industrial production equipment for box foaming primarily consists of raw material tanks, metering pump units, elevatable mixing barrels, and assemblable wooden box molds. As depicted in the schematic diagram of the box foaming equipment manufactured by Hennecke (Picture 2), the foaming raw materials are stored in tanks and regulated by control devices to attain the required processing temperature range, typically maintained at 23°C ± 3°C. Sequentially, the metering pump injects polyether polyols, catalyst, surfactants, foaming agents, etc., into the mixing barrel for a stirring duration of 30 to 60 minutes. Next, according to the formulation, TDI is introduced, either directly or through an intermediate container with a bottom switch. Immediate mixing follows TDI addition. Depending on the materials and formulation, the stirring speed is usually controlled at 900 to 1000 revolutions per minute (r/min), with a stirring time of 3 to 8 seconds. After stirring, the mixing barrel is swiftly lifted. The lower part of the barrel lacks a bottom and is placed on the mold box's bottom plate upon lowering, utilizing a sealing ring at the barrel's bottom edge to prevent material leakage.

When lifted, the well-mixed slurry can be directly spread and dispersed on the bottom plate of the box mold, allowing natural foam rise. To prevent the formation of a domed surface on the upper part during foaming, an upper mold plate that matches the mold area and allows for upward limit movement is equipped. The mold box primarily comprises rigid wooden panels, with the bottom plate fixed on a movable mold transport carriage. All four side panels are assemblable, featuring quick-opening and closing locking mechanisms. The inner sides of the panels are coated with silicone-based release agents or lined with polyethylene film material to prevent adhesion. After 8 to 10 minutes of forced maturation within the box, the side panels of the mold box are opened, allowing the removal of block-shaped flexible foam. Following an additional 24 hours of maturation, these foam blocks can undergo cutting and other post-processing procedures.

1 - Raw Material Tank; 2 - Metering Pump Unit; 3 - Control Cabinet; 4 - Mixing Barrel with Elevating Device; 5 - Foaming Box; 6 - Foam Finished Product; 7 - Floating Plate

Picture 2: Box Foaming Equipment Manufactured by Hennecke (BFM100/BFM150)

Box foaming process and equipment exhibit characteristics such as simple operation, compact and straightforward equipment structure, low investment, small footprint, and convenient maintenance. These features make it particularly suitable for small enterprises engaged in intermittent production of low-density block foam. However, its drawbacks are also quite evident: lower production efficiency, less favorable production environment, high concentration of emitted toxic gases on-site, necessitating the use of highly effective exhaust and toxic gas purification systems.

To enhance mixing efficiency, some companies have added several vertical and equidistant baffles to the inner walls of the mixing barrel. These baffles, combined with high-speed spiral-type agitators, facilitate high-speed mixing. This approach can to a certain extent reduce laminar flow effects in the mixing liquid and improve mixing efficiency. An example of this is our product, the SAB-BF3302. For the product's appearance and technical specifications, please refer to Picture 3.

Picture 3: Fully Automatic Box Foaming Machine (Sabtech Technology Limited)

This production line comes with both fully automatic computer control and manual control modes. It's suitable for producing flexible polyurethane foam with densities ranging from 10 to 60 kg/cm. Maximum foam output: 180L. Foam height: 1200mm. Mixing power: 7.5kW. Total power: 35kW.

2. Equipment for Open-Cell Foam Preparation

Open-cell polyurethane foam is a functional foam product developed in the 1980s. It possesses a high porosity, a distinct network structure, softness, breathability, and good mechanical strength. It finds wide application as excellent filtration and shock-absorption material in transportation, instrumentation, medical material filtration membranes, and as catalyst carriers in the chemical industry. Filling it into aircraft fuel tanks can suppress oil agitation and reduce the risk of explosions. Impregnating it with ceramic slurry and high-temperature sintering results in a novel open-cell ceramic filter material used in the metallurgical industry.

The preparation of open-cell polyurethane foam involves methods such as steam hydrolysis, alkaline soaking, and explosion. In industrial production, the explosion method is predominantly used. Initially, polyurethane foam of a specific pore size is prepared using the box foaming process. Subsequently, it's placed in dedicated explosion network equipment, filled with explosive gas, and ignited after completely filling the foam body. By utilizing the impact energy and high-temperature heat generated by the explosion parameters, the cell walls of the polyurethane foam are ruptured and fused onto the cell walls, forming a distinct network structure, as shown in Picture 4.

Picture 4: Clearly Networked Open-Cell Foam

Methods like steam hydrolysis or alkaline soaking are used to prepare open-cell foam. However, there are issues of low efficiency, poor quality, and environmental pollution with these methods. They are mainly employed for small-scale production such as laboratory sample testing. Large-scale production primarily uses the explosion method.

ATL Schubs GmbH, a German company, specializes in the research and development of polyurethane reticulated foam and manufactures the ReticulatusTM foam explosion machinery. The explosion chamber of the reticulated foam explosion equipment comes in two forms: cylindrical and rectangular. The former is suitable for cylindrical foam, while the latter is more versatile. It can be used not only for square foam but also for processing reticulated foam from cylindrical foam, as shown in Picture 5. The explosion chamber is constructed from high-quality 100mm-thick steel plates. Operation is controlled by a computer modem, offering features like automatic opening and closing, automatic locking, automatic operation, and automatic alerts. Additionally, remote program design and modification can be facilitated through data transmission sensors.

Picture 5: Polyurethane Foam Reticulation Processing Equipment (ATL Schubs)

During production, foam bodies measuring 3 to 6 meters in length that are intended for reticulation are pushed into the explosion chamber. The chamber's door is closed hydraulically, and the air inside the chamber is evacuated using a vacuum pump. Under computer control, a precise proportion of oxygen and hydrogen gases is introduced, and the gas mixture's ratio is mechanically adjusted based on factors such as foam sample type and network size requirements. 

Sensors continuously monitor the process, ensuring that all process parameters are within the specified conditions before controlled detonation is initiated. The explosive force and flame intensity generated by the explosion penetrate through the entire foam body, creating a distinct network structure. After forming, the foam body is cooled, residual materials and waste gases are purged using nitrogen, and the pressure chamber can then be opened to retrieve the reticulated foam. The entire process takes approximately 8 to 10 minutes. The pore diameter of the reticulated foam falls within the range of 10 to 100 pores per inch (ppi) (Note: ppi refers to the number of pores within one inch).

The above provides some insight into the non-continuous production process of polyurethane flexible foam. I hope this information proves helpful to you.

In the production of polyurethane products, it is necessary to use release agents to help the molded products detach from the molds. Among the release agents that can be used directly on the production line, the substance responsible for the release performance is called the effective release component. This component is usually wax, organosilicon, organic fluorine, etc., and generally accounts for less than 10% of the total. The majority, over 90%, consists of solvent components, which help the effective release components form a uniform film on the mold surface. In this article, let's explore the applications of solvents in polyurethane release agents.

During the molding process of filled materials in molds, the materials come into contact with the mold surface, which may have minor defects due to the mold's working surface being uneven, leading to some friction resistance when the molded products detach from the mold. During the injection or extrusion process of filled materials, negative pressure may form between the filling material and the mold, or they may bond due to physical adsorption or chemical bonding, making it difficult to remove the molded material from the mold after forming. To weaken the adhesion or bonding between the product and the mold, additives (release agents) that can form an effective isolation film are often used. A release agent is an interface film layer used on the surfaces of two objects that are prone to bonding, forming isolation between the bonded substances and the releasing material, making it easier and more convenient for the product to detach from the mold.

Broadly speaking, release agents include many types, applied in various fields such as chemical engineering, metallurgy, building materials, etc. This discussion will focus only on the use of release agents in the chemical engineering field, specifically discussing the application changes of solvents in the release agents used for materials such as polyurethane.

Phase One:

In the initial stages of industrial production, environmental and safety considerations for raw materials were not a concern, and the focus was solely on high solubility and easy volatility. The requirements for solvent use were limited to considering them as diluents, requiring them to quickly evaporate after spraying on the mold to avoid foam surface defoaming. Therefore, some organic solvents became widely used due to their high solubility and easy volatility, such as:

1. Dichloromethane: Strong solvency, easy volatility at room temperature, chemically stable, and due to its lack of a flash point, it does not lead to combustion, and it is also inexpensive. It was extensively used in the early days. However, the high concentration of dichloromethane vapor can cause poisoning, and large emissions can also lead to the greenhouse effect. Before the widespread use of environmentally friendly release agents in the production of shoe materials in Jinjiang, which used dichloromethane, thousands of dichloromethane molecules were present in the air.

2. Petroleum Ether: It is a low-boiling fraction of petroleum, a mixture of low-level alkanes, usually used as a solvent with a boiling range of 30-60°C, highly volatile. It was commonly used in release agents in the early stages. Due to its low boiling point, it is easy to mix benzene and ketone compounds into the product, resulting in poor overall VOC (Volatile Organic Compounds) and odor performance.

3. Naphtha: Also known as crude gasoline, mainly composed of C5-C11 components of alkanes, flash point -2°C, extremely volatile and flammable. The vapor is irritating to the eyes and upper respiratory tract and can also cause environmental pollution.

4. Xylene: C8H10, isomers, boiling range around 140°C, high flash point, flammable, with a pungent odor, classified as a Group 3 carcinogen.

5. Quick-drying water: Cyclohexanone, a colorless or light yellow transparent liquid with a strong irritating odor, a Group 3 carcinogen. As we can see, the above products only focused on their usage characteristics but ignored their long-term effects on human health and the environment. Therefore, in today's environmentally themed society, their market share is getting smaller and they are sinking into low-end product usage. Some of them have been strictly prohibited from use as they are listed as carcinogens.

Phase Two:

People with health awareness began looking for new solvent alternatives with lower toxicity. Heptane, a colorless and easily volatile liquid, gradually became widely used as a solvent for release agents due to its high fat solubility, high volatility, and strong degreasing ability. Commonly known as white gasoline, its chemical name is n-heptane, with a flash point of -4°C, making it a highly flammable organic solvent. Pure n-heptane is relatively expensive, so industrial-grade heptane emerged, which contains more impurities such as aromatic hydrocarbons, sulfur, nitrogen, etc., resulting in poor odor and VOC performance of the product. Also, in terms of toxicity, although heptane has lower toxicity than the initial solvents, with long-term use, there is still a risk of chronic poisoning, mainly manifested as damage to the nerves around the body. At this point, heptane gradually sank into the usage of low-end products.

Phase Three:

The overwhelming momentum of environmental and safety inspections forced manufacturers to pay attention to environmental and safety issues. Relatively safe and environmentally friendly solvent products became the direction sought by release agent manufacturers. This is where the de-aromatized solvent oil for release agents came into play. De-aromatized solvent oil is mainly produced by using aviation kerosene through high-pressure hydrogenation at both ends, deep desulfurization, and then distillation. Desulfurization aims to eliminate odors and corrosiveness, while de-aromatization reduces harmful benzene-like substances and improves VOC performance. Currently, on the market, there are several types of de-aromatized solvent oils based on solvent flash points (such as D40, 60, etc., referred to as the D series solvent oils). Since pentanal has a low boiling point, the higher the distillation range, the lower the harmful substances in the solvent product itself.

De-aromatized solvent oil has low toxicity, which has almost no impact on personal safety and the production environment. Additionally, its higher flash point brings higher safety to production. However, high-boiling products evaporate relatively slowly, which can lead to foam defoaming on the surface of polyurethane products. Therefore, the initial scope of use is mostly limited to solvents that are relatively easy to volatilize.

Phase Four:

Environmental regulations are demanding higher safety for products, storage, and logistics in enterprises. Hazardous chemicals are finding less space to exist, and operating costs are rising. Transforming release agents from hazardous chemicals into non-hazardous chemicals has become a new challenge. Water-based release agents with water as the main solvent have long been considered by manufacturers. They are currently commonly used in fields such as shoe materials, metal die-casting, carpets, front-end sound insulation pads, steering wheels, etc. However, water evaporates relatively slowly, and even though water-based release agents can solve all environmental and safety issues brought by organic solvents once and for all, they face extremely stringent and inconvenient application processes due to the complex structures of seat foam production line molds, high-speed production, and the high rebound foam's sensitivity to water. Therefore, manufacturers tend to choose high-flash-point release agents with organic solvents as carriers for their convenience. According to the national classification standards for flammable liquids, liquids with flash points above 60°C are not hazardous chemicals and can be transported and stored as ordinary chemicals. Although high-flash-point release agents also evaporate slowly, through adjustment in the spraying process, they can still meet production needs. Therefore, in this phase, high-flash-point solvent oils entered the scope of use.

Phase Five:

Odor and VOC reduction become the main focus. High-end cars have extremely strict requirements for the interior environment, but changes are driven downward through a chain of suppliers. Therefore, higher requirements are placed on release agents, with the ideal goal being no odor and no VOC. While de-aromatized solvent oil already belongs to the trace VOC category, it still has a distinct solvent odor. Therefore, until the application of water-based release agents is fully mature, release agent manufacturers can only continue to search for alternatives in organic solvents. Eventually, isoalkanes suitable for personal care and cosmetics entered the application range of release agents. They are colorless, odorless, high-purity, single-component, free of aromatic hydrocarbons and sulfur, truly low-toxic, and have good solvency. These characteristics make isoalkanes the highest-end product in environmentally friendly solvent oils.

Currently, there are three main production processes for isoalkanes: straight-chain alkanes aromaticization, isobutene synthesis, and coal-to-liquid (CTL) Fischer-Tropsch synthesis of isoalkanes. Most imported products from abroad are produced by isobutene synthesis, where isobutene is cracked from naphtha, then undergoes isomerization, distillation, and hydrogenation to obtain high-purity synthetic isoalkanes that are completely odorless at room temperature and do not produce any odor when heated to a certain temperature. However, due to the high comprehensive cost of this process, the solvent prices are high, more than twice that of de-aromatized solvent oil. Currently, it can only be limited to the use of high-end products. Since China has abundant coal resources, coal-to-liquid (CTL) Fischer-Tropsch synthesis of isoalkanes has become the mainstream research route. With the gradual maturation of the later-stage process, it is believed that the price of domestically produced isoalkanes will significantly decrease, and the usage will see explosive growth.

In conclusion, in the release agent industry, you get what you pay for. Prices to some extent determine the quality of the products. Low prices cannot meet high-end demands, and high prices are also used to offset the upfront research and development costs.

I. Advantages of Polyurethane On-site Foaming Technology:

The method of on-site foaming, spraying (or pouring) polyurethane foam insulation layer, has the surface as a whole without seams, reducing heat loss, with high construction efficiency, easy to meet quality requirements, reducing construction procedures, and eliminating the need for anti-corrosive coatings on pipe surfaces.

II. Polyurethane On-site Foaming Construction Process Principle:

The principle of polyurethane foam plastic foaming and spraying, pouring process is that polyether isocyanate can undergo a polycondensation reaction to form amine methacrylate, which can generate the required polyaminomethyl ethyl, commonly known as polyurethane foam plastic. Catalysts, crosslinking agents, foaming agents, foam stabilizers, etc., are simultaneously added during the reaction to promote and perfect the chemical reaction.

These raw materials are divided into two groups, fully mixed, and then pumped into a special spray gun by metering pumps in proportion. They are fully mixed and sprayed on the surface of pipelines or equipment in the spray gun or pouring mixer, react, foam, and form foam plastic within 5-10 seconds, which then cures and solidifies.

III. Polyurethane On-site Foaming Construction Methods:

Spraying Method: According to this formula, two groups of solutions are stored in two barrels respectively. The materials are filtered to the metering pump, driven by a pneumatic motor, and input into the gun body through the material tube. Compressed air regulates the material into the mixing chamber, mixed, and then sprayed onto the pipeline or equipment to foam and form.

Pouring Method: The prepared two groups of solutions are stored in barrels, filtered to the metering pump, driven by a pneumatic motor, and input into the pouring mixer through the material tube. Compressed air is introduced into the pouring motor, driving the stirring shaft to mix the two groups of materials, which are then injected into the mold for foaming and forming.

Precautions for Polyurethane On-site Foaming Construction:

Stir the material at room temperature to mix and react, then quickly pour it into the space that needs to be formed. During construction, control the reaction foaming time so that the mixed material after stirring is in a liquid state when poured into the gap. During the foaming process, significant expansion forces will be generated, so proper reinforcement should be made to the pouring interlayer or mold.

The amount of foam stabilizer determines the size of the foam structure's cells. More stabilizer leads to finer cells, but too much can cause shrinkage. Finding the right balance is crucial; too little stabilizer and the cells won't support each other, resulting in collapse during forming. Both are catalysts in action.

Polyurethane (Soft Foam) refers to a type of flexible polyurethane foam plastic with a certain elasticity, mostly having open-cell structures.

Polyurethane (Hard Foam) refers to foam plastics that do not undergo significant deformation under certain loads and cannot recover to their initial state after excessive loads. Mostly closed-cell.

Hard Foam Silicone Oil

Hard foam silicone oil is a type of highly active non-hydrolyzable foam stabilizer with a silicon-carbon bond, belonging to a broad-spectrum silicone oil category. It has excellent comprehensive performance and is suitable for HCFC-141b and water foaming systems, used in applications such as boards, solar energy, pipelines, etc.

Product Features:

1. Good emulsification performance: The excellent emulsification performance allows for good dispersion and mixing of the composite materials during the reaction with isocyanate, resulting in good flowability. The produced product has uniform cells and a very high closed-cell rate.

2. Good stability: The special molecular structure effectively controls the surface tension of the cells, stabilizing the cell structure and providing the product with excellent mechanical properties.

Soft Foam Silicone Oil:

A general-purpose siloxane surfactant for polyether-type flexible polyurethane foam plastics, it is a non-hydrolyzable polydimethylsiloxane-polyethylene copolymer, a high-activity stabilizer. It is used as a foam stabilizer in the production of polyurethane soft foam (sponge). It can provide a thin skin. In very low-density foam, it provides strong stability with fine and uniform cells. In medium-depth foam, compared to similar silicone oils, it has better foam opening properties and breathability.

1. Adjust Formulation:

Control the amount of water to not exceed 4.5 parts, and if necessary, use low-boiling-point liquid compounds as auxiliary foaming agents to replace some water. Pay attention to the amount of water in the formulation, which must not exceed 5 parts. The highest safe temperature rise point for low-density foam is 160°C, and it must not exceed 170°C.

2. Strictly Control the Accuracy of Component Measurement:

During continuous block foam production, adjust the discharge speed of the mixing head material and the conveyor belt speed to coordinate them. Avoid phenomena such as under-foaming materials flowing into the bottom of already foaming materials due to slow conveyor belt speed or excessive discharge, which can prevent normal foaming, resulting in collapse. Collapsed materials are not easily able to produce localized "gas species," leading to localized heat accumulation and increased risk of scorching. In actual production, poor process parameters may result in small yellow scorching lines appearing at the bottom of foam blocks.

3. Avoid Compressing the Newly Produced Foam:

This is because compressing the foam before it is fully cured affects the foam network and structure. It also prevents heat accumulation due to compression, increasing the risk of self-ignition of new foam. Especially during the most sensitive stage of foam rising, any operational errors and vibrations, such as sudden movements caused by tight conveyor belt chains or excessive folding of isolation paper and belt shaking, can cause compression of immature foam, leading to scorching.

4. Strictly Observe the Curing and Storage Process of Foam:

For the production of polyurethane soft block foam, the curing process of new foam is a high-risk period for fire accidents. Due to the high internal temperature and long duration of heat dissipation in large block foams, the time to reach the highest internal temperature is usually about 30 to 60 minutes, and it takes 3 to 4 hours or longer for it to slowly decrease. During this time, the new foams have left the production line and entered the curing and storage phase, which is easily overlooked. Without proper monitoring measures, it can easily lead to fires. There have been reports that when producing block soft foam with a density of 22kg/? using a polyol with a molecular weight of over 5000, 4.7 parts of water, and 8 parts of F-11 with a TDI index of 1.07, a small amount of light yellow smoke was observed 2 hours later. Although the external temperature of the foam was not high, the interior was in a very dangerous initial stage of decomposition, with a temperature of around 200-250°C, already beginning to self-ignite.

5. To Prevent Self-Ignition of Foam:

Newly produced foam should be cured and stored, not exceeding 3 layers when stacked, with a spacing of more than 100mm between layers, preferably placed separately. The curing and storage phase should have dedicated personnel for enhanced monitoring, such as measuring the internal temperature of the foam every 15 minutes for at least 12 hours, or even longer, before normal storage. For foams that may generate high temperatures, large foam blocks should be cut horizontally (e.g., with a thickness of 200mm) to facilitate heat dissipation. When smoke or self-ignition is detected, use water spray or fire extinguishers, and do not move the foam or open doors and windows indiscriminately to prevent increasing airflow and exacerbating the fire.