Aromatic Polyester Polyols

Polyester Polyols for Rigid Foam

Polyester polyols used for rigid foams are typically made from aromatic dicarboxylic acids (or their anhydrides or esters) and diols (or polyols). Common raw materials for polyester production include terephthalic acid, phthalic anhydride, and various polyols such as ethylene glycol and propylene glycol. Considering viscosity, cost, and performance, aromatic polyester polyols often use a mixture of adipic acid and phthalic anhydride in specific proportions as dicarboxylic acid raw materials. Adding small amounts of triols introduces branched structures into the polyester polyol molecules.

In the early 1980s, polyester polyols derived from polyester waste and residues (e.g., polyethylene terephthalate or PET) developed rapidly due to their low cost and competitive pricing. Rigid foams based on these polyols exhibit better flame resistance compared to those using polyether polyols. Early applications of polyester polyols for polyurethane (PU) rigid foams involved high-functionality polyols, but their inherently higher viscosity compared to polyether polyols posed operational challenges at room temperature. Subsequently, lower-functionality polyester polyols, with relatively lower viscosities, were found to deliver excellent performance in applications like PIR (polyisocyanurate) foams.

Application in Rigid Foam Formulations

In the industry, aromatic polyester polyols often replace part or all of the polyether polyols in polyurethane and polyisocyanurate rigid foam formulations. Aromatic polyester polyols are particularly suitable for polyisocyanurate foams. The abundant benzene rings in the foam structure improve both heat resistance and flame retardancy. Internationally, aromatic polyester polyols are widely used in the production of sandwich foam panels for construction and on-site spray foam applications in the building industry.

These polyurethane rigid foams containing polyester exhibit not only the properties of polyether-based rigid foams but also enhanced toughness, excellent flame resistance, and cost advantages. Additionally, the high primary hydroxyl group content of polyester polyols increases their reactivity, allowing for reduced catalyst usage.

Mixing with High-Functionality Polyether Polyols

Aromatic polyester polyols can be mixed with high-functionality polyether polyols for the preparation of rigid polyurethane foams. For example, Stepan Company in the U.S. produces aromatic polyester diols for polyurethane and polyisocyanurate rigid foams. Using high-purity raw materials, these polyester polyols feature low color, low viscosity, and consistent properties, with a moisture content ≤0.15%. The high aromatic content enhances foam hardness and heat resistance, while the low viscosity facilitates easy mixing.

These polyester diols serve as chain extenders in producing rigid polyisocyanurate (PIR) panels, low-density spray-applied foams, high-density rigid foams, and packaging foams.

Recycling and Innovations

Since the early 1980s, polyester polyols produced from PET waste and residues have seen rapid growth due to their cost-effectiveness. For instance, an aromatic polyester polyol made from PET residues and diethylene glycol has a hydroxyl value of (400±50) mg KOH/g, viscosity of 2.0–4.0 Pa·s, acid value ≤5.0 mg KOH/g, and moisture content ≤0.2%.

Recently, France's TBI Company launched a new plant with a production capacity of 15,000 tons per year for aromatic polyester polyols (APP) using PET waste as raw material. Flattened PET waste bottles are directly processed via alcoholysis to produce low-viscosity aromatic polyester polyols. These polyols have a viscosity of 700 mPa·s, a free diol content below 5%, and excellent storage stability. Applications include flame-retardant PIR foam boards for construction, thermal insulation products, and polyurethane soundproofing materials.