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.

Polyether Polyol: Hydroxyl Value 36, Primary Hydroxyl > 65%, 60%.

Polymeric Polyol: Hydroxyl Value 28, Copolymer 20%, 40%.

Water: 3%.

80TDI and Polymeric MDI (Viscosity 300mpa): 80:20.

T12: 0.025%.

A33: 0.4%.

HR-3 Silicone Oil: 1%.

HA-1 Crosslinker: 6%.

Di(b-dimethylaminoethyl) Ether: 0.15%.

Aluminum Hydroxide

Also known as hydrated alumina. The aluminum hydroxide used as a fire retardant is mainly @-tri-hydrated alumina. It appears as a white fine crystalline powder with an average particle size of 1-20 micrometers. Its relative density is 2.42, refractive index is 1.57, and 30% slurry pH is 9.5-10.5. The dehydration initiation temperature is 200 degrees Celsius, with an absorption heat of 2.0 KJ/G.

During combustion, it releases a large amount of chemically combined water, absorbs a considerable amount of heat, slows down the polymer's thermal degradation rate, reduces the material surface temperature, delays and suppresses the combustion of the substrate. It will generate a large amount of steam on the substrate surface, diluting the oxygen in the combustion zone, reducing the concentration of smoke and toxic flammable gases. The aluminum oxide generated during combustion can promote the formation of a carbonized protective layer on the polymer surface.



Melamine

Commonly known as melamine, it is a white monoclinic crystal with low toxicity, non-flammable, and a melting point of 354 degrees Celsius. It undergoes endothermic sublimation and rapid decomposition under high heat. At temperatures between 250-450 degrees Celsius, it can absorb a large amount of heat and release nitrogen during decomposition, slowing down the material's combustion rate. At the same time, it forms a carbonized barrier layer on the substrate surface, acting as a fire retardant. However, there are some dispersion problems, so it needs to be used in combination. When used as a fire retardant, high-temperature decomposition can produce toxic cyanide gas.



Organophosphorus Flame Retardant

Tris(1,3-dichloro-2-propyl)phosphate (TDCPP)

A pale yellow transparent viscous liquid. It contains 7.2% phosphorus and 49.4% chlorine, with a flash point of 251.7 degrees Celsius, ignition point of 282 degrees Celsius, and spontaneous combustion temperature of 514 degrees Celsius. It starts to decompose at 230 degrees Celsius and is soluble in alcohols, benzene, carbon tetrachloride, etc. When used at 5%, it can achieve self-extinguishing properties, and at 10%, it can make the material self-extinguish or non-flammable, while also having water resistance, light resistance, and antistatic properties.



Fire Retardant Polyether Polyol

1. Formula Ingredients:

Polyether polyol 3050: Mn3000;

Flame-retardant polyether polyol: Hydroxyl value 28, flame-retardant solid mass fraction 23%;

Silicone oil: L580

Triethylene diamine solution: Mass fraction 33%;

Tin octoate solution: Mass fraction 33%;

TDI: Industrial grade

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)

Density:

A. PU flexible foams with high density have many small pores, appearing fuller. However, the higher the density, the poorer the water absorption.

B. Generally, PU flexible foams with high density also have high hardness, but it is not ruled out that some high-density foams may add super soft additives, making the sponge very soft. Therefore, PU flexible foams of the same density may have different degrees of softness or hardness.

C. High-density foams are often used for sound-absorbing cotton, sofa cushions, soft packaging materials, etc. Medium and low-density foams are generally used for protective materials.



Resilience:

A. Slow rebound foams, also known as inert foams, memory foams, zero-pressure feeling foams, etc. Slow rebound foams have a "honeycomb" structure, so they cannot quickly return to their original shape after being compressed.

B. Characteristics of slow rebound foams: good water absorption, sound insulation performance; strong toughness, high tensile strength, good shock absorption and cushioning performance; good heat insulation and thermal insulation, can withstand severe cold and heat.

C. High rebound foams have a mixture of pore sizes, different skeleton thicknesses, and a high open hole rate. When compressed, they produce rebound forces with different supporting forces in different deformation states.

D. High rebound foams have super strong resilience and breathability; excellent anti-fatigue performance and flame retardancy; the feel is similar to latex surface.