Foam Rebonding Machine: Things You May Want to Know

Polyurethane foam(PU foam) primarily consist of polyurethane as their main component. The raw materials primarily include polyisocyanates and polyols, with the addition of various additives, the most crucial of which are a series of foaming agents related to the foaming process. These additives lead to the production of a significant amount of foam within the reaction product, resulting in polyurethane foam products. This article provides a brief overview of the raw materials used in producing PU foam and the foaming agents.



1.Polyisocyanates

The most commonly used polyisocyanates in the industrial production of polyurethane foams include toluene diisocyanate (TDI), polymethylene polyphenyl isocyanate (PAPI), diphenylmethane diisocyanate (MDI), and liquid MDI (L-MDI).



TDI

TDI is mainly used in the production of polyurethane flexible foams. MDI has higher reactivity than TDI, lower volatility, and some modified forms of MDI can be used as substitutes for TDI in the production of polyurethane flexible foams, including high-density polyurethane foam and the manufacturing of semi-rigid or microcellular polyurethane elastomers.

PAPI, also known as crude MDI or polymerized MDI, typically has an average molecular weight ranging from 30 to 400, with an NCO content of 31% to 32%. In the field of foam plastics, PAPI and modified PAPI are primarily used to produce various polyurethane rigid foams, with some also used in the production of high-rebound flexible foams, integral skin foams, and semi-rigid foams. PAPI can be mixed with TDI to manufacture cold-cure, high-rebound foam plastics.



2.Polyether and Polyester Polyols

2.1Polyether Polyols

Polyether polyols used for producing polyurethane flexible foams are generally long-chain, low-functionality polyethers. In the formulation of flexible foams, the functionality of polyether polyols is usually between 2 and 3, with an average molecular weight ranging from 2000 to 6500. Polyether triols are most commonly used in flexible foams, typically initiated with glycerol (propane-1,2,3-triol) and obtained through ring-opening polymerization with 1,2-epoxy propane or copolymerization with a small amount of ethylene oxide, with a molecular weight generally falling within the range of 3000 to 7000.



Polyether Polyols

High-activity polyether polyols are mainly used for high-rebound flexible foams and can be used in the production of semi-rigid foams and other foam products. Some polyether diols can be used as auxiliary materials, mixed with polyether triols in flexible foam formulations. Low unsaturation and high molecular weight polyether polyols are used for the production of soft foams, reducing the amount of TDI required.

Polyether polyols used in rigid foam formulations are generally high-functionality, high hydroxyl value polyether polyols to achieve sufficient cross-linking and rigidity. The hydroxyl value of polyether polyols for rigid foam formulations is typically in the range of 350 to 650 mg KOH/g, with an average functionality of 3 or higher. Rigid foam formulations often use a combination of two types of polyether polyols, with an average hydroxyl value of around 4000 mg KOH/g.

Semi-rigid foam formulations often use some high molecular weight polyethers, especially high-activity polyether triols, and some high-functionality, low molecular weight polyether polyols from rigid foam formulations.

2.2Polyester Polyols

Low-viscosity aliphatic polyester polyols, such as hexanediol adipate diols with a hydroxyl value of approximately 56 mg KOH/g, or slightly branched polyester polyols, can be used for producing polyester-based polyurethane flexible foams. Polyester polyols have high reactivity. Currently, block polyurethane foam made from polyester is only used in a few fields such as auxiliary materials for clothing.



Polyester Polyols

Aromatic polyester polyols, synthesized from dicarboxylic acids (such as phthalic anhydride, terephthalic acid, etc.) and small-molecule diols (such as ethylene glycol, etc.) or polyols, are used to produce polyurethane rigid foams and polyisocyanurate rigid foams. Lower hydroxyl value polyester polyols derived from phthalic anhydride can also be used for high-rebound flexible foams, integral skin foams, semi-rigid foams, and non-foam polyurethane materials.

2.3Polymer Polyols

Polymer polyols, including rigid styrene, acrylonitrile homopolymers, copolymers, and grafted polymers, act as organic "fillers" to enhance load-bearing performance. Polymer polyols are used in the production of high-hardness flexible block foams, high-rebound foams, thermoplastic flexible foams, semi-rigid foams, self-skinning foams, and reaction injection molded (RIM) products. They can reduce product thickness, lower foam density to reduce costs, increase foam plastic cell opening, and impart flame retardant properties to the products.



Polymer Polyols

Polyurea polyols (PHD dispersions) are a special class of polymer-modified polyols used in high-rebound flexible foams, semi-rigid foams, and soft foams, but their presence in the market is limited.

There are also some special polyols used for the production of polyurethane foams, such as vegetable oil-based polyols, rosin-based polyester polyols, and polymer polyesters. These are not described in detail in this article.

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In modern industrial production, polyurethane flexible foam play an important role in various fields such as furniture, automotive seats, and shoe insoles. However, the key technical control points for producing high-quality polyurethane flexible foam plastic products cannot be overlooked. Here are several key technical points in the production process:



Control of Toluene Diisocyanate (TDI):

The optimal isomeric ratio of TDI is 80/20. If this ratio is exceeded, it can lead to the formation of large and closed cells in the foam, prolonging the curing time. Particularly in the production of large block low-density foam products, an excessive isomeric ratio can delay heat release, potentially causing the foam center temperature to remain high for a long time, leading to carbonization and even ignition. If the isomeric ratio is too low, the foam product's density and resilience will decrease, and fine cracks may appear on the foam surface, resulting in poor process repeatability.



Addition of External Blowing Agents:

External blowing agents (water) not only reduce the density of the foam but also improve the softness of the product and help remove reaction heat. To prevent center carbonization in the foaming process of large block foam products, a certain amount of water is usually added. However, as the amount of water increases, the amount of catalyst should also increase correspondingly; otherwise, it may prolong the post-curing time of the foam. Generally, for every 5 parts increase in water, 0.2 to 0.5 parts of silicone oil should be added.



Catalyst Ratio:

Organic tin and tertiary amine catalysts are commonly used to control the NCO-OH and NCO-H2O reactions. By adjusting the ratio of different catalysts, the growth of polymer chains and the foaming reaction can be controlled. Under certain product densities, choosing the appropriate catalyst ratio can control the foam's open-cell rate, cell size, and void load value. Increasing the amount of organic tin catalyst can generally produce foams with smaller cell sizes, but excessive use may increase the closed-cell rate. It is necessary to determine the optimal catalyst dosage through experiments to achieve the best performance of foam products.



Foam Stabilizers:

The role of foam stabilizers is to reduce surface tension of the material, making the foam film wall elastic and preventing foam wall rupture until the molecular chain growth and cross-linking reactions lead to material solidification. Therefore, foam stabilizers play a critical role in the production of one-step polyether sponge and must be strictly controlled in usage.



Temperature Control:

The foam generation reaction is highly sensitive to temperature, and changes in material and foaming temperature will affect foaming operations and physical properties. Therefore, temperature control is one of the important conditions to ensure stable foaming processes. The material temperature is generally controlled at 20-25°C.



Stirring Speed and Time:

The stirring speed and time affect the amount of energy input during the foaming process. If stirring is uneven, a large number of bubbles may appear on the foam surface, leading to defects such as cracking. During mixing of Component A, the speed is 1000r/min; after Component B is added to Component A, the high-speed stirring speed is 2800-3500r/min for 5-8 seconds.



In summary, the key technologies for producing polyurethane flexible foam include controlling TDI, adding external blowing agents, adjusting catalyst ratios, using foam stabilizers, temperature control, and controlling stirring speed and time. Proper control of these technical parameters can ensure the production of stable quality and high-performance polyurethane flexible foam plastic products.

Polyurethane soft foam plastic is one of the important products in the polyurethane industry. Its production necessarily involves the use of organic amine catalysts, especially organic tertiary amine catalysts. This is because organic tertiary amine catalysts play a significant role in the main reactions of polyurethane foam formation: the reactions of carbon dioxide and molecular polymerization, promoting rapid expansion of reaction mixtures, increased viscosity, and sharp increase in polymer molecular weight. These conditions are essential for the formation of foam bodies, ensuring that soft foam plastics have advantages such as low density, high strength-to-weight ratio, high resilience, and comfort for sitting and lying. There are many types of organic amine catalysts that can be used for polyurethane soft foam plastics. Among them, the highly efficient catalysts recognized by various manufacturers are: triethylene diamine (TDEA) and bis(dimethylaminoethyl) ether (referred to as A1). These are also the most widely used organic amine catalysts in the world today, with the highest consumption among various catalysts.



Due to the molecular structural differences between TDEA and A1 catalysts, there are significant differences in their catalytic performance, particularly in their reactions to carbon dioxide gas and molecular polymerization. If the user does not pay attention to these differences in production, not only will they fail to produce qualified foam products, but it will also be difficult for foam bodies to form. Therefore, understanding and mastering the performance differences between these two catalysts in polyurethane foam production is of great significance. TDEA exists in a solid state under normal conditions, making its application less convenient. In actual production, low molecular weight alcohol compounds are commonly used as solvents, formulated into 33% solutions for ease of use, commonly referred to as A33. On the other hand, A1 is a low-viscosity liquid that can be directly applied. Below is a comparison of the catalytic performance differences between A1 and A33 in the production of polyurethane soft foam plastics.



A33 has a 60% catalytic function for the reaction with carbon dioxide gas and a 40% catalytic function for molecular polymerization. It has a low effective utilization rate of carbon dioxide gas, resulting in lower foaming height and higher foam density. Since most of the catalytic function is used for molecular polymerization reactions, it is easy to produce closed-cell foam bodies, which are stiff with low rebound, and the adjustable range of tin catalysts becomes narrower. To achieve the same catalytic function, the amount used is 33% more than A1. Both the bottom skin and outer skin of the foam body are thicker. Increasing the amount can increase the reaction speed, but the amount of tin catalyst must be reduced accordingly, otherwise closed-cell foam bodies will be produced.



A1 has an 80% catalytic function for the reaction with carbon dioxide gas and a 20% catalytic function for molecular polymerization. It has a high effective utilization rate of carbon dioxide gas, resulting in higher foaming height and lower foam density. Since most of the catalytic function is used for gas generation reactions, it is easy to produce open-cell foam bodies, which are soft with high rebound, and the adjustable range of tin catalysts becomes wider. To achieve the same catalytic function, the amount used is less than A33. Both the bottom skin and outer skin of the foam body are thinner. Increasing the amount can increase the reaction speed, but the amount of tin catalyst must be increased accordingly, otherwise over-foaming and cracking may occur.



In terms of overall performance between TDEA and A1, A1 has a higher comprehensive catalytic performance than triethylene diamine. Its actual application effects are also better, although not as convenient as triethylene diamine in terms of transportation and storage. Currently, the vast majority of mechanical continuous foam production facilities almost exclusively use A1, while all box-type foam production facilities use TDEA. However, this is not absolute. With a clear understanding of the differences between the two and appropriate formulation adjustments, they can be interchangeable and both can produce excellent foam products.

Surfactants used for foam stabilization, also known as foam stabilizers, are a crucial additive in the production of polyurethane foam plastics. When selecting surfactants, what factors should be considered?



Surfactants can be divided into two types: soft foam surfactants and hard foam surfactants. Let's examine each:



Soft Foam Surfactants:

During the initial foaming stage, soft foam surfactants can dissolve to form polyurea, and in the later stages, they can assist in cell opening and bursting. Therefore, when selecting soft foam surfactants, the following factors need to be considered:



1. The activity of the surfactant;

2. The relationship between the operating range of the surfactant and the required dosage range;

3. The impact of the various components of the surfactant on the operating range.



Hard Foam Surfactants:

Hard foam surfactants have a higher degree of cross-linking, making bubbles more stable. They are commonly used for molded products or foaming in restricted cavities. When selecting hard foam surfactants, the following factors need to be considered:



1. Strong emulsification capability of the raw material components;

2. Ability to control the size of the foam cells effectively;

3. Ability to improve material flowability and ensure uniform density distribution.



Surfactants not only play a role in foam stabilization but also affect foam density, elasticity, tensile strength, and other properties. Therefore, when choosing surfactants, factors such as the activity of the raw materials, operating conditions, and foaming process need to be considered.

What is Polyurethane Rigid Foam?



Polyurethane rigid foam, often abbreviated as PU rigid foam, is one of the most commonly used polyurethane products, second only to polyurethane soft foam, in polyurethane applications. Polyurethane rigid foam is mostly a closed-cell structure, known for its excellent insulation, lightweight, high strength-to-weight ratio, ease of construction, as well as soundproofing, shock absorption, electrical insulation, heat resistance, cold resistance, solvent resistance, and more. It is widely used in the insulation layers of refrigerator and freezer boxes, cold storage rooms, refrigerated trucks, as well as insulation materials for buildings, tanks, and pipelines. A small amount is used in non-insulation applications such as wood imitation and packaging materials. Generally, lower-density polyurethane rigid foam is primarily used as thermal insulation material, while higher-density polyurethane rigid foam can be used as structural material (wood imitation).



Polyurethane rigid foam is typically foamed at room temperature, with a relatively simple molding process. It can be categorized into manual foaming and mechanical foaming based on the degree of construction mechanization; high-pressure foaming and low-pressure foaming based on the foaming pressure; and casting foaming and spraying foaming based on the molding method.



What is Polyurethane Soft Foam?



Polyurethane soft foam, also known as PU soft foam, is a type of flexible polyurethane foam with a certain degree of elasticity. It is the most extensively used polyurethane product among all polyurethane products.



Polyurethane soft foam is mostly an open-cell structure, characterized by low density, good elastic recovery, sound absorption, breathability, and insulation properties. It is primarily used as cushioning materials for furniture, mattresses, vehicle seat cushions, and also finds industrial and domestic applications as filter materials, soundproofing materials, shock absorption materials, decorative materials, packaging materials, and insulation materials. Based on the degree of softness and load-bearing capacity, polyurethane soft foam can be divided into ordinary soft foam, super soft foam, high load-bearing soft foam, high resilience soft foam, etc. High resilience and high load-bearing soft foams are generally used for manufacturing seat cushions and mattresses. According to the production process, polyurethane soft foam can be divided into block foam and molded foam. Block foam is produced by continuous process to form large volume foam which is then cut into required shapes, while molded foam is produced by direct injection of the mixture into molds to form foam products of desired shapes.



After understanding polyurethane rigid foam and polyurethane soft foam, the question arises: how do we differentiate between the two?



In fact, the classification can be based on the degree of hardness, dividing them into soft foam plastics and hard plastics. Soft foam plastics have a matrix polymer component above the crystalline melting point, or, if it is an amorphous polymer, it is above the glass transition temperature; hard foam, on the other hand, has its matrix polymer in a crystalline state or an amorphous state but below the glass transition temperature. Semi-rigid foam is a foam plastic between soft and hard foam. It is similar to soft foam, with an open cell rate above 90℃, but semi-rigid foam has higher density and higher compression strength. After compression deformation, semi-rigid foam takes much longer to recover, and its crosslinking density is much higher than soft foam but lower than hard foam.



Based on this classification of softness and hardness, most polyolefin foams, unplasticized polyvinyl chloride (PVC) foams, phenolic foams, polycarbonate foams, and polyphenylene ether foams are all classified as hard foams, while elastic polyurethane foams and some polyolefin foams and plasticized PVC foams are classified as soft foams.



According to the national standard, soft foam plastics are those that are flexible, have low compression hardness, return to their original state after stress is relieved, and have minimal residual deformation. On the other hand, hard foam plastics are inflexible, have high compression hardness, deform when stress reaches a certain level, and do not return to their original state after stress is relieved.



The American Society for Testing and Materials (ASTM) specifies that to differentiate between soft and hard foam plastics, at a temperature of 18-29℃, a rod with a diameter of 2.5 cm is rotated around it for one full rotation within 5 seconds. If it does not fracture, it is classified as soft foam plastic; otherwise, it is classified as hard foam plastic.



According to ISO standards, when the compression deformation reaches 50% and is then released, if the thickness decreases by no more than 2% compared to the original thickness, it is classified as soft foam plastic. If it decreases by more than 10%, it is classified as hard foam plastic. If the decrease is between 2-10%, it is classified as semi-rigid foam plastic.



If we use elastic modulus as the criterion, in a standard environment of 23℃ and 50% relative humidity, a foam plastic with an elastic modulus greater than 686 MPa is classified as hard foam, less than 68.6 MPa is classified as soft foam, and between 68.6-686 MPa is classified as semi-rigid foam. Although the elastic modulus of semi-rigid foam is higher than that of soft foam, its stress-strain behavior is closer to soft foam and significantly different from hard foam. Generally, soft foam plastics mostly have an open-cell structure, while hard foam plastics mostly have a closed-cell structure, but there are exceptions.



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