Promotion And Application of Water-Blown Polyester Polyols

Comparison of the Properties of Various Blowing Agents for Polyurethane Foam

The blowing agents currently used in polyurethane foam primarily include chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), pentane and carbon dioxide.

1. Chlorofluorocarbons (CFCs)

a CFC-11

CFC-11 has a boiling point of 23.7 °C and is liquid at room temperature. It is easy to handle, has a low heat of vapourisation, and vaporises readily during the foaming process. It has low thermal conductivity, and the resulting foam exhibits good thermal insulation properties; it is non-flammable and non-toxic, and has minimal corrosive effect on polyurethane substrates. Although it is an excellent blowing agent, it has been phased out due to its significant damage to the atmospheric ozone layer, which affects the ecological environment on which human survival depends.

b HCF-141b

HCF-141b has a boiling point of 32.0 °C. As a transitional substitute for CFC-11, it possesses similar properties to CFC-11 but still has a certain destructive effect on the atmospheric ozone layer, with an ODP of 0.15; its use will also be phased out after a certain period.

2. Hydrofluorocarbons (HFCs)

Hydrofluorocarbons contain no chlorine atoms and have an ODP of 0; they are a highly competitive class of CFC-11 replacement blowing agents. Notable compounds in this class include: HFC-245fa, HFC-365mfc and HFC-356mffm. Their boiling points are 15.3 °C, 40 °C and 24.9 °C respectively. As their boiling points are similar to that of CFC-11, existing equipment requires little modification during use; however, their high cost results in polyurethane foam products being prohibitively expensive, which hinders their promotion and adoption.

3. Pentane

Pentane exists in three structural forms: n-pentane, isopentane and cyclopentane, with boiling points of 36 °C, 28 °C and 49 °C respectively. It has an ODP of 0 and a higher thermal conductivity than CFC-11. However, as pentane is flammable and forms explosive gas mixtures with air, production equipment and the working environment must be modified to ensure safe production. Furthermore, due to the escape of pentane during use, the product is unsuitable for use in enclosed spaces, which severely limits the scope of application for polyurethane foam.

4. Carbon Dioxide

Carbon dioxide foaming generally utilises the reaction between water and isocyanate to generate carbon dioxide for foaming, also known as water-blown foaming. It is typically used in combination with physical blowing agents to reduce the quantity of the latter required. Furthermore, the reaction between water and isocyanate to form polyurea alters certain properties of polyurethane foam, thereby enhancing the foam’s strength. However, as the thermal conductivity of carbon dioxide is far greater than that of CFC-11, the thermal insulation performance of the product is somewhat reduced. Furthermore, because carbon dioxide escapes faster than air can enter, the dimensional stability of the foam is affected to a certain extent. To ensure dimensional stability, the foam density is generally increased. Currently, liquid carbon dioxide foaming processes and equipment have been successfully developed; however, their use is somewhat limited due to the high capital investment required for the equipment.

 

Foaming Methods for Polyurethane Foam

1. Physical foaming: This involves utilising the exothermic reaction between -OH and -NCO to cause a low-boiling-point physical blowing agent to vapourise, thereby producing foam. Examples include pentane and HFC-141b.

2. Chemical foaming: This involves foaming using gases generated by chemical reactions, primarily through the production of carbon dioxide from the reaction between water and isocyanates, or through the decomposition of bicarbonates to produce carbon dioxide. Foaming via the reaction of isocyanates with water, combined with a physical blowing agent, is currently the primary foaming method; it improves foam performance whilst reducing production costs. Bicarbonate foaming is generally used for microcellular structures, which have higher hardness and greater density, such as wood-effect products and certain moulded profiles.

 

Problems with all-water foaming and their solutions

As all-water foaming does not use physical foaming agents, and due to the inherent flaws of the carbon dioxide foaming system, the use of ordinary polyether and polyester polyols presents a number of problems, primarily as follows:

1. The exothermic reaction is too intense, making it prone to core burning and difficult to carry out in a single operation. By reducing the hydroxyl value and thereby limiting the reaction between -OH and -NCO groups, the heat of reaction can be minimised. Concurrently, the use of all-polyester foaming increases the thermal decomposition temperature of the molecular chains; as the thermal decomposition temperature of ester groups far exceeds that of ether bonds, the product is less prone to decomposition, vapourisation and combustion at high temperatures. This prevents core burning and enables the production of foam in a single-pass operation. Following numerous experiments, it has been found that controlling the hydroxyl value of the polyester polyol within the range of 180–200 mg KOH/g generally meets the requirements.

2. Poor dimensional stability of the foam Poor dimensional stability is primarily caused by the excessive rate of carbon dioxide escape. To address this, a multi-branched structure is introduced into the polyester molecular chain to increase the airtightness of the cells and reduce the rate of carbon dioxide escape. At the same time, by increasing the functional group density and introducing -NH₂ groups into the molecular chain, the rigidity of the cells is enhanced, improving their pressure-bearing capacity and ensuring their dimensional stability.

3. The high viscosity of the blend is detrimental to the mixing and atomisation of the black and white components. As the absence of a physical blowing agent in the system is equivalent to a reduction in solvent content, the viscosity of the system inevitably increases. This can only be addressed by reducing the viscosity of the polyester polyol; therefore, alkyl side chains with higher molecular weights are introduced into the polyester structure to lower the molar ratio of ester groups, thereby reducing the viscosity of the polyester. In practice, the viscosity of the system can also be reduced by appropriately increasing the amount of flame retardant, thereby improving the flowability of the blend and facilitating mixing and atomisation with the isocyanate.

4. Reduced thermal insulation performance and increased thermal conductivity. Although CO₂ has a higher thermal conductivity than CFC-11, we can improve the foam’s thermal insulation performance by increasing the density of the cells and reducing thermal convection between CO₂ and air.

5. Poor storage stability. The water consumption in all-water foaming is higher than in existing foaming methods; and contact between water and various additives and flame retardants inevitably leads to some degree of hydrolysis, slowing down the reaction rate during foaming. By introducing hydrophilic groups at the polyester terminal ends, water molecules are firmly locked at the ends of the polyester molecular chains via hydrogen bonding. Using a water-in-oil emulsion reduces the likelihood of water molecules coming into contact with additives and flame retardants, thereby minimising hydrolysis and improving the storage stability of the raw materials.

 

Technical Specifications for Water-Based Foamed Polyester

As water-based foamed polyester requires consideration of hydrophilicity, cell density, cell rigidity and the viscosity of the polyester itself, it is difficult to operate using a single polyester polyol. Currently, we utilise a blend of two polyesters: XF-2003 and XF-2006. XF-2003 is a water-based polyester with a certain degree of miscibility with water. XF-2006 contains a multi-branched structure and a certain amount of -NH₂, and its technical specifications are as follows:

 

 

 

Foaming Process for All-Water Polyester and Product Performance

Full water blowing foam currently mostly employs an open-cell foaming process. Firstly, this ensures dimensional stability; secondly, it reduces the risk of core burning. However, as the open-cell structure results in reduced thermal insulation properties of the foam plastic, limiting its applications, this process utilises a novel polyester polyol comprising a combination of XF-2003 and XF-2006, which perfectly overcomes the shortcomings of existing processes. Closed-cell foaming is employed, and the process is as follows:

 

Conclusions and Summary

All-water foaming offers the advantages of simple operation, safety and environmental friendliness; it requires no modifications to existing equipment and, being solvent-free, poses no health risks to operators. Following the phase-out of chlorofluorocarbons (CFCs), it offers significant advantages in reducing the cost of finished products.

Through improvements to their molecular structure, XF-2006 and XF-2003 confer excellent dimensional stability and thermal insulation properties on polyurethane foam. Compared with open-cell all-water foaming, they offer greater advantages and superior performance; compared with hydrofluoroalkane foaming, they provide better value for money. With continued efforts and the ongoing development of new materials, all-water foaming is bound to become a new trend.