Performance Analysis of Polyurethane Thermal Insulation Materials for Construction

Polyurethane rigid foam insulation material for the construction industry is an important branch of the polyurethane industry. It is characterized by multi-functionality, simultaneously providing thermal insulation and waterproofing capabilities. Since its application in European construction in the 1960s, it has a history of over 40 years. Some countries have even legislated to designate polyurethane as the specified thermal insulation and waterproofing material for construction. In recent years, with the rapid development of China's building energy efficiency market, polyurethane rigid foam insulation products have been widely used in the field of building thermal insulation and waterproofing, becoming one of the leading thermal insulation and energy-saving products in the market.

Polyurethane (PU, short for Polyurethane) is a polymer compound containing many repeating -NHCOO- groups on its main chain. Polyurethane products are generally polymerized from di- or polyfunctional organic isocyanates and polyols with the aid of various additives. If both main raw materials contain only two or fewer hydroxyl functional groups, the polymerization results in a linear structure. If one or both main materials partially or fully possess three or more functional groups, a cross-linked (network) structure polymer can be obtained. Due to different polymer structures, their properties vary significantly. This is similar to steel: although all are iron-carbon alloys, different ratios and heat treatment processes result in vastly different properties.

Polyurethane raw materials are widely used across various industries. Due to the different characteristics of each industry and the products produced, the performance requirements for composite materials based on polyurethane also differ. The polyurethane rigid foam (SPF) that serves the dual functions of thermal insulation and waterproofing is completely different from the polyurethane (PUR) used in refrigerators and cold storage, which only provides insulation. Although they may look similar in appearance and structure, as an integrated building thermal insulation and waterproofing material, polyurethane rigid foam breaks the common problem of traditional building materials with single functions—waterproof materials do not insulate, and insulating materials do not waterproof. Once the waterproof layer leaks, the insulation layer immediately loses its function.

Compared with other single-function thermal insulation or waterproof materials, polyurethane rigid foam has obvious advantages:

It is multi-functional, simultaneously providing thermal insulation, waterproofing, sound insulation, vibration absorption, and other functions.
It has excellent thermal insulation performance. It is currently the thermal insulation material with the lowest thermal conductivity (≤0.024, using CFC-free foaming technology for environmental protection) and the highest thermal resistance among all domestic building materials. Its thermal conductivity is only half that of EPS (expanded polystyrene board).
The continuously dense skin and nearly 100% high-strength interconnected closed cells of polyurethane rigid foam provide ideal water impermeability. Constructed by spraying, it forms a continuous, seamless waterproofing and insulation layer, creating a seamless roof and an integral external wall insulation shell with excellent waterproofing and anti-seepage performance.
It has super self-adhesive performance (without any intermediate bonding materials), firmly adhering to roofs and external walls, with good resistance to wind uplift and negative wind pressure.
Integral spray construction completely eliminates 'thermal bridges' and 'cold bridges'.
Flexible gradient technology can effectively prevent the waterproof layer from cracking.
Mechanized operation with automatic batching ensures uniform quality, fast construction, and a short cycle.
It has stable chemical properties, a long service life, and does not pollute the surrounding environment.
It is self-extinguishing away from an open flame. When burning, it only carbonizes without dripping. The size and shape of the carbonized layer remain basically unchanged, effectively blocking air intake and preventing the spread of fire, providing good fire safety performance.

The quality of polyurethane rigid foam material itself has a significant impact on the safety, thermal insulation function, and waterproofing function of buildings, and must be taken seriously during material selection.

The significant performance differences between polyurethane rigid foam waterproofing and insulation materials for construction and those for cold storage and refrigerators are mainly reflected in four performance indicators: elongation at break, closed cell ratio, dimensional change rate, and bonding strength.

Difference 1: Elongation at Break

Elongation at break is an important performance indicator measuring the ability of polyurethane rigid foam to resist stress without permanent deformation. However, polyurethane rigid foam used in cold storage and refrigerators, which only provides insulation without waterproofing, has no requirements for this key performance indicator.

The National Building Materials Industry Standard (JC/T 998-2006) clearly stipulates that if polyurethane rigid foam is used as an integrated waterproofing and insulation material for roofs and walls, it must meet the basic requirement of elongation greater than 10%. This ensures that the waterproof layer does not break, bulge, or crack due to changes in ambient temperature, drying shrinkage and wet expansion of roofs and walls, freeze-thaw damage in severe winters, and uneven settlement of building foundations. Only when the elongation meets the above national industry standards can polyurethane rigid foam fully demonstrate its reliable waterproofing function while exerting its excellent thermal insulation performance, ensuring that buildings are cool in summer and warm in winter, roofs are completely watertight, and external walls are free from dampness.

Difference 2: Dimensional Change Rate

Polyurethane rigid foam undergoes certain changes in size and volume under the influence of ambient temperature changes. The magnitude of the dimensional change rate is related to many factors such as the type of raw materials, foam structure, core density, molding process, and type of foaming agent. Polyurethane rigid foam with good temperature resistance should not undergo significant dimensional changes in an ambient temperature range of -20°C to +80°C.

The closed cell ratio of polyurethane rigid foam is as high as over 95%. The gas pressure enclosed in the cells changes with ambient temperature. If the structural strength of the cell walls is low, the foam will deform due to low-temperature shrinkage or high-temperature expansion caused by changes in gas pressure in the closed cells. The greater the relative dimensional deformation (i.e., the dimensional change rate), the higher the probability of cracking or opening of the polyurethane waterproofing and insulation layer.

According to the provisions of the national industry standard JC/T 998-2006, the dimensional change rate of polyurethane rigid foam used in construction should be ≤1% to adapt to the excessive shrinkage and expansion of the external wall finishing system caused by the rapid changes in day-night temperature differences during hot summer and cold winter seasons. Dimensional stability is obviously closely related to the safe use of the external wall finishing system. The larger the value, the worse the safety. Polyurethane rigid foam with a dimensional change rate greater than 1% does not meet the requirements of the national building materials industry standard.

Difference 3: Closed Cell Ratio

The closed cell ratio is an important indicator for measuring material water absorption and thermal conductivity. Rigid foam with a low closed cell ratio has higher water absorption and thermal conductivity, which have a crucial impact on the material's thermal insulation function and freeze-thaw resistance.

When polyurethane rigid foam for construction is used as a roof waterproofing and insulation material, its foam closed cell ratio should be at least greater than 95%. When the closed cell ratio is lower than 70%, short-term heavy rain may not cause roof leakage. However, during continuous rainy seasons, the rigid foam absorbs a large amount of water in the open cells due to long-term immersion in rainwater. The rainwater infiltrating the cells will penetrate through the interconnected pores into the roof base under gravity and be trapped between the base and the rigid foam. Even after the rain stops and the sun shines, it is difficult for the water to drain upward through the rigid foam insulation layer and protective layer in a short time. On the contrary, because the roof is exposed to sunlight, the upper layer temperature is high and the lower layer temperature is low, moisture migrates downward instead, causing the abnormal phenomenon of 'no leakage on rainy days, but leakage on sunny days'. This phenomenon is particularly prominent during the plum rain season in southern China.

Difference 4: Influence of Tensile Strength and Self-Adhesive Strength

The overall strength of the polyurethane rigid foam external wall external insulation system depends on the tensile strength of the polyurethane base material (the weakest link of the system strength). Therefore, the tensile strength of the polyurethane rigid foam itself is actually the pull-out strength of the entire external wall finishing system. For example, if the tensile strength of polyurethane rigid foam is 200 kPa, the pull-out strength of the entire external wall system is 200 kPa. If this indicator drops to 100 kPa, the pull-out strength of the entire external wall system will be reduced to half of the original, and the safety factor will inevitably drop sharply to half of the original.

In addition, when formulating sprayed polyurethane rigid foam for construction, the bonding strength requirements of the external wall external insulation system during use should be fully considered. Therefore, it is required to have excellent self-adhesive properties to building materials such as metal, concrete, masonry, wood, and glass.

The National Building Materials Industry Standard JC/T 998-2006 and the recently adopted 'Technical Guidelines for Polyurethane Rigid Foam External Wall External Insulation Engineering' both stipulate mandatory indicators for the self-adhesive strength of polyurethane rigid foam. This is also an important indicator that gives sprayed polyurethane rigid foam a significant safety advantage over EPS (expanded polystyrene board) and XPS (extruded polystyrene board) external wall external insulation systems.

It is worth mentioning that strength is the most important mechanical property of polyurethane rigid foam. Its magnitude directly determines the wind pressure resistance, impact resistance, strain resistance, and total load-bearing capacity of the external wall finishing system, and is the most important and direct performance indicator for evaluating the safety of the external wall external insulation system.

In summary, when users select polyurethane rigid foam as a dual-function building thermal insulation and waterproofing material, they must strictly identify the material's density, strength, elongation at break, dimensional stability, and closed cell ratio in accordance with the requirements of the National Building Materials Industry Standard JC/T 998-2006.

As the only new type of integrated thermal insulation and waterproofing building material at present, the application of polyurethane rigid foam insulation materials in the domestic construction industry is still in the initial stage. Fortunately, to accelerate the innovation of building thermal insulation materials and promote the application of polyurethane rigid foam in the field of building energy efficiency, the Ministry of Housing and Urban-Rural Development has specially established the 'Polyurethane Building Energy Efficiency Application and Promotion Working Group'. Under the high attention and full promotion of the Ministry of Housing and Urban-Rural Development, the application of polyurethane rigid foam waterproofing and insulation materials in the domestic building energy efficiency industry has achieved substantial progress.

Performance Analysis of Polyurethane Insulation Materials for Construction

Performance Analysis of Polyurethane Thermal Insulation Materials for Construction

Difference 1: Elongation at Break

Difference 2: Dimensional Change Rate

Difference 3: Closed Cell Ratio

Difference 4: Influence of Tensile Strength and Self-Adhesive Strength