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Introduction to Rigid Polyurethane Foam Raw Materials

Rigid polyurethane foam (PU foam) is widely used for its lightweight nature, excellent thermal insulation, ease of installation, soundproofing, shock resistance, electrical insulation, and resistance to heat, cold, and solvents. It finds applications in insulating storage tanks and pipelines in sectors like petroleum and chemicals, enhancing efficiency in refrigeration equipment, etc.

What gives rigid polyurethane foam such excellent performance? Today, we will uncover rigid polyurethane foam raw materials and how they affect the performance of rigid polyurethane foam.

What is Rigid Polyurethane Foam?

Rigid polyurethane foam is a new type of low-temperature insulation material formed by the chemical reaction of polyether or polyester with polyisocyanates as the main raw materials, along with the addition of flame retardants, stabilizers, and foaming agents. The resulting foam has a high closed-cell rate and low water absorption. Due to the presence of foaming agent gases with low thermal conductivity within its cells, it has a certain degree of self-extinguishing properties and is commonly used for cold insulation and insulation within low-temperature ranges.

Rigid Polyurethane Raw Materials

Isocyanates and polyols are the two main components of the rigid polyurethane foam system.

Isocyanates

Isocyanates primarily include compounds such as toluene diisocyanate (TDI) and diphenylmethane diisocyanate (MDI). Their chemical structure contains multiple isocyanate groups (-NCO), which enables them to react with polyols to form polyurethane. Isocyanates have high reactivity, allowing them to quickly react with polyols to produce polyurethane foam. Different types of isocyanates can be used to adjust reaction conditions and formulations to produce polyurethane products with varying properties.

Polyols

Polyurethane polyols mainly include polyether polyols and polyester polyols, with the former being produced in larger quantities. The molecular structure of polyols contains multiple hydroxyl groups (-OH), which can react with polyisocyanates to form polyurethane.

Polyether polyols are widely used in the production of both flexible and rigid polyurethane foams due to their good flexibility and water resistance. Polyester polyols, on the other hand, are suitable for applications requiring higher performance due to their higher strength and chemical resistance.

By selecting different types and molecular weights of polyols, the hardness, elasticity, and other physical properties of polyurethane foam can be adjusted.

Characteristics of Raw Materials for Rigid Polyurethane

The characteristics of raw materials for rigid polyurethane foam are mainly reflected in the molecular structure of polyols and their reaction properties with isocyanates.

The molecular chains of polyols are usually highly branched with high functionality (approximately 3-8 hydroxyl groups per mole), and the chain segments extending from the hydroxyl groups are very short (low relative molecular weight). These structural characteristics allow polyols to react with aromatic diisocyanates (such as crude MDI or polymeric MDI/PAPI) to form a highly cross-linked rigid polyurethane structure. Due to the low relative molecular weight and high density of polyurethane groups, the hydrogen bonding force between molecular chains is very strong, which imparts high rigidity to the material.

Therefore, the hydroxyl value of polyols used in rigid polyurethane is much higher than that in flexible foam, mostly ranging from 300-600 mg KOH/g (some special polyols may fall outside this range, such as 200-300 mg KOH/g and 600-800 mg KOH/g). The high density of hydroxyl groups results in a high density of hydrogen bonds between them, directly leading to strong intermolecular forces between polyol chains and high system viscosity. Generally, the polyols used in rigid polyurethane have much higher viscosity than those used in viscoelastic PU, ranging from 2,000 to 50,000 mPa-s at 25 °C.

Function of Aromatic Polyols in Rigid Polyurethane

With the development and use of aromatic polyols in PU rigid foam, the presence of aromatic ring structures (which have low mobility and high rigidity) has been shown to significantly contribute to the rigidity of polyurethane. For example, low-functionality aromatic polyols (f = 2.3-3 hydroxyl groups/mol; such as Mannich polyols, aromatic polyester polyols, and phenolic polyols) can react with crude MDI to produce highly rigid polyurethane structures.

Function of Polyols in Rigid Polyurethane

The most important polyols for rigid foam raw materials are polyether and aromatic polyester polyols. Aromatic polyethers, based on the polycondensation of aromatic compounds with aldehydes, are also increasingly becoming important polyols, especially with the introduction of new blowing agents.

The structure of polyols has a significant impact on the mechanical properties, heat resistance, and fire retardancy of rigid foam. Higher functionality leads to higher compressive strength, better dimensional stability, and improved heat resistance, but results in decreased tensile strength and elongation. When functionality and hydroxyl value are the same, rigid foams synthesized from cycloaliphatic or aromatic polyols (with low molecular mobility) exhibit better mechanical and thermal properties than those made from aliphatic polyols (with high molecular mobility).

Generally, rigid foams made from polyether are softer and have better hydrolysis resistance compared to those made from polyester. Meanwhile, polyester-based rigid foams offer better temperature resistance and fire retardancy. The use of plant-based polyols in rigid foam can provide better hydrophobicity and excellent compatibility with pentane (a blowing agent).

Role of Microcellular Structure in Rigid Polyurethane

In addition to raw materials, other factors also affect the performance of rigid polyurethane, and one that must be mentioned is the microcellular structure.

The microcellular structure of rigid foam is formed through two methods: reactive and non-reactive. In the reactive method, water is used as a blowing agent, reacting with -NCO to generate CO2. In the non-reactive method, low-boiling-point blowing agents (such as pentane and hydrofluorocarbons) are used, which evaporate due to the exothermic reaction between hydroxyl groups and polyols with -NCO, simultaneously forming polyurethane polymers and a microcellular structure.

While soft foam primarily features an open-cell structure, rigid foam predominantly has a closed-cell structure (over 90% closed cells). This structure provides rigid PU foam with excellent thermal insulation properties. Therefore, the primary application of rigid foam is insulation—used in low-temperature refrigeration, building and home insulation, chemical storage tanks, the food industry, pipelines, and construction components (such as sandwich panels).

Rigid PU foam is currently known as the best insulation material, possessing the lowest thermal conductivity (K-factor) among all materials. Using PU rigid foam as an insulating material can significantly reduce energy consumption, by up to 90% in crude oil storage tanks and 50% in building homes.

Conclusion

The exceptional performance of rigid polyurethane foam as an insulating material is a result of its carefully selected raw materials and the unique microcellular structure it forms. The combination of high-functionality polyols and reactive isocyanates results in excellent thermal insulation of rigid polyurethane foam. This makes rigid polyurethane foam an indispensable component in a wide range of applications, from refrigeration and construction to industrial storage and beyond.

As improvement in raw material, the potential for even greater efficiency and sustainability in rigid polyurethane foam production and application remains promising, paving the way for more energy-efficient solutions across industries.

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We provide detailed product information covering various aspects, including technical specifications, performance indicators, application cases, etc. These materials will help you better understand the characteristics and advantages of our products in order to make wise decisions.

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