Bio-Based Polyester Polyols for Performance Polyurethane Elastomers

The rising demand for “green,” bio-sourced raw materials and intermediates originates from the concept of sustainability. In our industry, sustainability is about employing materials that are safe, efficient to use and derived from renewable resources. 

At Gantrade, we also consider sustainability, with an improved environment and reduced CO2 footprint, as a component of our corporate social responsibility. As consumer requests for green products have continued to grow, Gantrade has introduced a line of polyester polyols derived from bio-renewable sourced raw materials for application in the performance segment of polyurethane elastomers.  Our emphasis has been on both utilizing raw materials from bio-renewables and marketing polyols that produce durable and high-strength polyurethane elastomers with superior performance attributes and optimized processing characteristics. Additionally, the raw material production technologies are all energy-efficient and environmentally-efficient.  With this product program, Gantrade has combined polyurethane performance with concern for sustainability.  

In this article, we will assist polyurethane formulators in exploring the advantages of the Gantrade product line of bio-based polyester polyols.

Our portfolio of bio-based linear polyester polyols uses bio-succinic acid and bio-sebacic acid as the aliphatic diacids and bio-1,3-propane diol and bio-1,4-butane diol to produce polyester polyols with 100 percent renewable content.  Formulators can use these polyols to make polyurethane elastomers with a high renewable content, especially in conjunction with bio-1,4-butanediol as the chain extender. The bio-PDO, bio-BDO and succinic acid are all produced by fermentation of corn sugars.  Sebacic acid is a natural C10 linear fatty acid, directly produced by saponification of castor oil with caustic soda.

While bio-sourced adipic acid, the standard diacids used in performance polyester polyols, is not available for consideration, bio-based sebacic and succinic acids are structurally similar and can produce polyols that afford high performance polyurethanes. The chemical structures below show the similarities between the linear, even-number C6 adipic acid and the C10 sebacic and C4 succinic acids.

Applications for bio-based polyester polyols include cast and thermoplastic polyurethane elastomers, coatings and adhesives, inks, artificial leather, footwear, and polyurethane foam products.

Succinic Acid (SuA) Based Polyester Polyols – Cast Polyurethane Properties

Formulators can produce polyurethanes from 100 percent bio-based polyester polyols derived from bio-succinic acid and bio-1,3-propane diol or bio-1,4-butanediol.  However, the rigid nature of the C4 succinic acid, as well as its  restricted degrees of rotational freedom, can lead to polyester polyols that have high melting points, as well as a higher Tgs and a high degree of crystallinity.  This dynamic is evident with BDO/SuA polyester polyols, which reveal dense crystal packing in models and exhibit a very high melting point of about 110 °C This attribute makes conversion of  the BDO/SuA polyol into polyurethanes difficult, due to the high temperatures required to melt the polyol. The melting point of the BDO/SuA polyester polyol can be reduced by incorporating branched diols such as 2-methyl-1,3-propanediol (MPO) or other glycols like 3-methyl-1,5-pentanediol (MPD).  However, the methyl branched diol components in MPO and MPD disrupt chain packing to such a degree that polyurethanes show lower tensile strength and modulus values, tear and abrasion resistance, and rebound properties.

1,3-propanediol SuA polyester polyols do not exhibit the high crystallinity which is characteristic of the BDO/SuA polyester polyols.  This has been explained by an odd-even carbon atom spacial effect of the 1.3-PDO copolymer with succinic acid. The odd-even carbon sequences in the PDO/SuA copolyester leads to a reduction in the intermolecular bonding forces and chain packaging. This linear polyester polyol is a waxy solid at room temperature, exhibits a broad melting range of 36-56 °C, and can remain liquid at room temperature after melting-out.  Thus, the odd-even carbon atom effect observed in polyurethanes can be used to advantage in the design of polyols derived from succinic acid.

The table below compares the polyurethane properties on the 100 percent-bio-based 1,3-PDO/SuA polyol against the homologous polyurethanes derived from bio-based polyols using adipic acid as the diacid component.  With adipic acid, the bio contents would be less than 50 percent. Adipic acid polyester polyols were selected as the comparative systems because they represent the standard for high performance polyester-polyurethanes.

The pre-polymer method used  in casting the polyurethanes is a two-step process.  MDI was added stepwise with mechanically stirring in the molten polyol under nitrogen to prepare the pre-polymer, controlling the exotherms to 80 °C. The molecular weight of the polyester polyol was 2000.  In the second step, 1,4-butanediol was added with stirring as the chain extender to the pre-polymer. The homogeneous mixture was poured onto a metal sheet and cured for 16 hours at 110 °C. The NCO/OH index used was 1.02. 

1,4-Butanediol is the chain extender of choice in cast PURs and TPUs, because it consistently creates a hard-block segment with the highest melt enthalpy in all cases of the polyester polyols used.  The high-melt enthalpy indicates greater crystallinity, which is the driving force for phase separation of the hard and soft block segments.

Succinic Acid: A Bio-Based Building Block for Succinate Polyester Polyols in Modified Thermoplastic Urethanes using a Cast Molding Urethane Pre-polymer Process

Comparison of the properties of the three cast PURs above show that the 1.3-PDO succinate/MDI/BDO system exhibits a performance profile which is very comparable to the two adipate based systems, with no performance deficiencies. The solvent resistance and hydrolytic stability of the PDO/SuA polyurethane was also comparable to the adipate based polyester PURs. This is significant, since the BDO/adipate based PUR are considered to be in the high range of performance of polyester-polyurethane systems.

Sebacic Acid (SuA) Based Polyester Polyols – Cast Polyurethane Properties

The table below shows that the general performance properties of polyurethanes based on 2000 MW polyols from bio-1,3-PDO and sebacic acid are very comparable to the properties of PURs based on 2000 MW PDO adipates and BDO adipates.  The isocyanate is MDI and the chain extender is 1,4-BDO. Related studies on these systems showed the PDO sebacate exhibited better hydrolysis resistance. As TPUs, the PDO sebacate-2000 required a lower processing temperature versus. the other two adipates and a shorter demold time.

Polyols and PURs based on sebacic acid are more hydrophobic than their adipate counterparts.  For this reason, they exhibit better hydrolytic stability and acid and alkali resistance.

1,4-BDO/SA polyols (2000 mw) exhibit a high Tm of 100-110 °C compared to 1.3-PDO/SA polyols having a Tm at 48 °C.  The BDO/SA polyol also exhibits a high-melt viscosity; the corresponding MDI prepolymers were too high in viscosity to process.  Hence, the BDO/SA polyols are best converted to PURs using a quasi-prepolymer (3:1 NCO:OH ratio) or a one-shot method. The BDO/SA-based polyurethanes exhibit an elite combination of properties, including excellent heat resistance, solvent and hydrolysis resistance, good abrasion characteristics, and flexural strength. Their very high hardness values, 55 Shore D, are attributable to the high crystallinity of the BDO/SA soft-block.  See the table below.

PURs based on BDO/SA at 1000 MW have lower viscosities and lower crystallinity, facilitating their usefulness in the preparation and processing of PURs.

A waterborne polyurethane dispersion (PUD) coatings study compared 1,3-PDO sebacate (1000 MW) with a series of other polyols based on adipates, The mechanical properties, hardness, adhesive strength, impact resistance, solvent resistance, and hydrolytic stability of the PDO sebacate were similar to those of the other PUDs studied.  The PDO-1000 sebacate coatings were transparent and clear, and showed a Tg at – 45 °C, indicating the lack of soft-segment crystallinity. The PDO sebacate PURs are effective in a wide range of industrial coating applications.

In a film study, TDI-polyurethanes based on sebacic acid-diol polyester were evaluated in moisture-cured films.  Only the 1,4-BDO/SA polyester polyol with a molecular weight of 1000 afforded a hard, semi-flexible film exhibiting a tensile strength of 540 psi and an elongation at break of 30 percent.  The film was opaque, based on the crystallinity. All other diol combinations with SA gave brittle films. Note that 1,3-PDO/AA was not evaluated in this study (R.N Mukherjea and K.K. Saha, Chemical Engineering Department, Jadavpur University, Calcutta, India).

Gantrade Sales Specifications for Bio-based Polyester Polyol

Gantrade’s line of 100 percent bio-based polyester polyols is delineated below.  These grades are based on the combination of succinic acid (SuA), sebacic acid (SA), bio-1,3-PDO or bio-1,4-BDO.  The catalyst used in the production of these polyols is either a titanate catalyst or a tin catalyst.

There are also polyol options where non-bio-based acids like adipic acid or other diols can be incorporated in the polyester polyol.

Please note that there are three BDO sebacate polyester polyols at molecular weights of 1000 to 3000.  These are unusual polyols, in that they afford high hardness polyurethane elastomers with durometer readings of up to 57 on the Shore D-scale, with excellent hydrolytic and chemical resistance, and high mechanical properties.  Because of the higher melting points and high viscosities of the BDO subacate polyols, PURs are best produced using a one-shot or a quasi-prepolymer process.

To explore how our bio-based polyester polyols can address your unique polyurethane elastomer application, partner with the expert teams at Gantrade Corporation. Our teams, armed with a wealth of technical knowledge and expertise, can guide you to the best solutions for your applications.  Contact Gantrade today to get started.  We offer a wide portfolio of products to achieve your high-performance requirements.