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.

14. Poor Rebound

A. Raw materials: high activity polyether polyols, low molecular weight, highly active silicone oil.

B. Process formulation: high silicone oil content, excessive tin, high water content with the same tin usage, high TDI index, large amount of white oil and powder.

15. Poor Tensile Strength

A. Raw materials: excessive low molecular weight polyether polyols, low functionality hydroxyl value.

B. Process formulation: insufficient tin, poor gelation reaction, high TDI index with the same tin usage, low water content, low crosslinking.

16. Smoke during Foaming

Excessive amine release a large amount of heat during water and TDI reaction, causing evaporation of low-boiling substances and smoke. If not core scorching, smoke mostly consists of TDI, low-boiling substances, and monomer cycloalkanes in polyether polyols.

17. Foam with White Streaks

Fast foaming and gelation reaction but slow transfer in continuous foaming, resulting in a dense layer due to localized compression, causing white streaks. Increase transfer speed or lower material temperature, reduce catalyst usage.

18. Brittle Foam

Excessive water in the formulation leads to excess biuret formation, which does not dissolve in silicone oil. Poor use of tin catalyst, insufficient cross-linking reaction, high content of low molecular weight polyether polyols, high reaction temperature causing ether bond breakage and decreased foam strength.

19. Foam Density Lower than Set Value

Foam index is too high due to inaccurate measurement, high temperature, low pressure.

20. Foaming with Skin, Edge Skin, and Bottom Air

Excessive tin, insufficient amine, slow foaming rate, fast gelation rate, low temperature during continuous foaming.

21. Elongation Rate Too High

A. Raw materials: high activity polyether polyols, low functionality.

B. Process formulation: low TDI index, insufficient cross-linking, high tin.

22. Uncontrolled Foam (Small bubbles rapidly moving beneath surface)

A. Low-pressure foaming machine: increase mixing head speed, decrease gas injection.

B. High-pressure foaming machine: increase mixing head pressure.

23.Milky Moving Lines

A. Increase conveyor speed

B. Adjust cushion plate inclination.

C. Reduce amine catalyst usage

24.Inserted Material Backflow

A. Increase conveyor speed.

B. Adjust cushion plate inclination.

C. Increase amine catalyst usage.

25.Moon Pits

A. Low-pressure foaming machine: reduce mixing head speed and gas injection.

B. High-pressure foaming machine: increase mixing head pressure.

C. Silicone oil quality issue.

D. Increase amine amount while reducing tin amount to ensure adequate cell opening.

26.Slow Curing,Sticky Surface

Polymer strength increases too slowly, resulting in soft, sticky foam that is difficult to cut.

Foam blocks appear unstable when exiting channels.

Increase catalyst usage, check polyol, water, and TDl measurement accuracy.

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)

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.

Understanding the principles behind foam reactions is crucial. To master foaming, we must strive to establish a foam reaction model in our minds using the following four reaction equations. Through familiarity with the variations within the model, we cultivate sensitivity that allows us to comprehend the entire foam reaction process. This approach helps structure our knowledge base and professional skills in polyurethane foam. Whether actively studying foam reaction principles or passively exploring them during the foaming process, it serves as a vital means for us to deepen our understanding of formulations and enhance our skills.

Reaction 1

TDI + Polyether → Urethane

Reaction 2

TDI + Urethane → Isocyanurate

Reaction 3

TDI + Water → Urea + Carbon Dioxide

Reaction 4

TDI + Urea → Biuret (Polyurea)

01: Reactions 1 and 2 are chain-growth reactions, forming the main chain of the foam. Before the foam reaches two-thirds of its maximum height, the main chain rapidly elongates, with chain-growth reactions predominating inside the foam. At this stage, due to relatively low internal temperatures, reactions 3 and 4 are not prominent.

02: Reactions 3 and 4 are cross-linking reactions, forming the branches of the foam. Once the foam reaches two-thirds of its maximum height, the internal temperature rises, and reactions 3 and 4 intensify rapidly. During this stage, reactions 1 to 4 are vigorous, marking a critical period for the formation of foam properties. Reactions 3 and 4 provide stability and support to the foam system. Reaction 1 contributes to foam elasticity, while reactions 3 and 4 contribute to foam tensile strength and hardness.

03: Gas-producing reactions are termed foaming reactions. The generation of carbon dioxide is a foaming reaction and the primary exothermic reaction in polyurethane foam. In reaction systems containing methane, the vaporization of methane constitutes a foaming reaction and an endothermic process.

04: Reactions leading to the formation of foam constituents are known as gelation reactions, encompassing all reactions except for gas-producing reactions. This includes the formation of urethane, urea, isocyanurate, and biuret (polyurea) from reactions 1 to 4.