Polystyrene vs Polyurethane: Understanding the Differences

When it comes to insulation materials, polystyrene and polyurethane are frequently mentioned due to their excellent thermal insulation properties. Both materials are commonly used in building cold rooms. However, many people are not familiar with what these materials actually are, nor do they understand their properties and characteristics. This article will explain the differences between polystyrene and polyurethane in terms of chemical composition, thermal insulation performance, fire resistance, mechanical properties, and price, helping you make a more informed choice based on your needs.

 

Polystyrene and Polyurethane

Chemical Composition

Polystyrene 

Polystyrene

Specifically, this article refers to extruded polystyrene (XPS). XPS is a foam material made by mixing polystyrene resin with a foaming agent and then continuously extruding the mixture. The resulting foam has a closed-cell structure that looks like a honeycomb.

Polyurethane 

Polyurethane

Polyurethane is a foam material formed by the reaction of isocyanates with polyols, followed by high-pressure spraying and on-site foaming. There are two types of polyurethane foam: rigid foam, which has a closed-cell structure, and flexible foam, which has an open-cell structure. Rigid polyurethane foam is typically used in cold rooms.

 

Thermal Insulation Performance

The thermal insulation performance of materials is commonly evaluated using the R-value and K-value. Below, we compare the R-value and K-value of extruded polystyrene foam and rigid polyurethane foam to illustrate the differences in their insulation performance.

R-Value 

Compressive Strenght / PSIDensityR-Value/ Per inchR-Value for 2″R-Value for 3″R-Value for 4″
101.00 PCFR 4.21R 8.45R 12.65R 16.89
151.25 PCFR 4.26R 8.56R 12.85R 17.12
201.50 PCFR 4.58R 9.15R 13.69R 18.26
252.00 PCFR 4.78R 9.55R 14.29R 19.05
402.50 PCFR 4.86R 9.72R 14.58R 19.42
603.00 PCFR 5.06R 10.15R 15.16R 20.25

The R-value, or thermal resistance, is an important indicator of a material’s insulation performance. It represents how well a material resists the flow of heat. In simple terms, the higher the R-value, the better the material’s insulation performance.

According to relevant data, extruded polystyrene foam has an R-value of about R-5 per inch of thickness, while rigid polyurethane foam has an R-value of about R-6 per inch of thickness. This indicates that rigid polyurethane foam has a slightly higher R-value than extruded polystyrene foam, meaning it provides better insulation at the same thickness.

K-Value 

The K-value, or thermal conductivity, measures a material’s ability to conduct heat. It represents the amount of heat that passes through a unit area of a material of unit thickness over a unit time period. The lower the K-value, the better the material’s insulation performance.

Relevant data shows that the K-value of extruded polystyrene foam typically ranges from 0.029 to 0.034 W/m·K, while the K-value of rigid polyurethane foam usually ranges from 0.022 to 0.028 W/m·K. This indicates that rigid polyurethane foam has a lower K-value than extruded polystyrene foam, meaning it provides better insulation at the same thickness.

Fire Resistance

Fire Resistance

Untreated extruded polystyrene foam and polyurethane foam are both flammable and release toxic gases when burned. To improve their fire resistance, appropriate fire retardants are added during the production process. Polyurethane foam typically achieves a higher fire resistance rating compared to extruded polystyrene foam.

 

Mechanical Properties

Density 

Extruded polystyrene foam (XPS) has a relatively low density, typically ranging from 28 to 45 kg/m³. In contrast, rigid polyurethane foam has a higher density, usually between 30 and 60 kg/m³. The lower density of XPS means it is lighter, making it easier to handle and install. The slightly higher density of rigid polyurethane foam contributes to its enhanced mechanical strength and insulation performance.

Compressive Strength 

The compressive strength of a material depends on its density and manufacturing process. XPS typically has a compressive strength ranging from 150 to 500 kPa, while rigid polyurethane foam usually falls between 150 and 300 kPa. The higher compressive strength of XPS makes it suitable for load-bearing applications, such as foundation and floor insulation. Although rigid polyurethane foam has a slightly lower compressive strength, it is still suitable for load-bearing applications and is often used for flooring in small cold rooms.

Water Absorption 

Water Absorption

XPS and polyurethane foam have significant differences in water absorption, primarily due to their chemical structures. As mentioned earlier, XPS has a uniform, closed-cell structure with low porosity, effectively preventing water from entering the foam. Consequently, XPS has a very low water absorption rate, typically between 0.1% and 0.7%. This makes XPS ideal for moisture- and water-resistant applications, such as floor insulation in cold rooms.

Due to different manufacturing processes, polyurethane foam can be either open-cell or closed-cell. The open-cell structure allows water to easily penetrate the material, leading to a higher water absorption rate, which can reach up to 30% or more. While rigid polyurethane foam also has a closed-cell structure, its water absorption rate is generally higher than that of XPS, usually ranging from 2% to 5%.

Price 

The price of materials is typically influenced by factors such as density, thickness, performance, and manufacturing processes. In the current market, XPS is generally less expensive than polyurethane foam of the same thickness. However, since XPS and rigid polyurethane foam are often used in different applications, price is usually not the primary consideration when choosing between the two materials.

 

Conclusion

Conclusion

XPS boasts excellent compressive strength and water resistance, while polyurethane foam offers superior insulation and fire resistance. When deciding which material to use, you should carefully consider your specific requirements and the characteristics of both XPS and polyurethane foam. Understanding the differences between these two materials will help you choose the most suitable option to achieve the best results for your intended application.

 

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