Producing polyols in a sustainable way

If polyurethane is to be more sustainable, then more sustainable feedstocks will be required. Several key papers at the Automotive and Sustainability Congress held in Amsterdam late last year addressed this important topic.

The ideal sustainable polyol will be odour-free, easy to process on existing equipment with minimum adjustments, and will give additional advantages in the finished product, according to many of the speakers in the sustainability session of the UTECH Congress. They all also agreed that polyols must not contribute to fogging, VOC or staining.

Peter Groome of China’s Jiahua Chemical used the congress to outline his company’s capacity and range of sustainable polyurethane additives, and to explain their use in automotive applications.

Jiahua developed its own natural oil polyols at its R&D labs in Shanghai. ‘We looked to see where the new polyols would fit in to replace existing polyether polyols,’ he explained. ‘We have commercialised two processes from this work, and these are running on our plants. Our polyols have found use in slabstock polyurethane foam, moulded foam, insulation, glass mat reinforcement and truck bed liners.’

The aim was to use high-volume raw materials, and produce the polyols on a plant with a positive eco-balance. ‘Additionally, it should not affect the food chain,’ Groome said.

No odours, please

‘We have a capacity of 700kT/year of polyols and five manufacturing sites in Fushan, near the Korean border, Guangzhou, Shandong, Shanghai, and Maoming, China,’ he told delegates. Conventional polyurethane polyols are dominated by polyether polyols because of their flexibility and compatibility. ‘These can be used with a range of initiators to give, for example, autocatalytic polyol, and to modify their functionality, which ranges from 2 to 8,’ Groome said. Polyester polyols account for around 25% of the total used and, again, have great flexibility in the acids and alcohols that can be used to produce them. Natural oil polyol properties vary widely because of the chemical composition of natural oils. A typical chemical composition for natural oils is shown in Table 1. ‘We looked at soya, sunflower and palm oil,’ Groome explained. In  this Table, the first number in brackets gives the number of carbons in the backbone, and the second gives the amount of unsaturation within the fatty acid. The differences in unsaturation give the different levels of functionality. Castor oil contains an unsaturated fatty acid bearing a hydroxyl group. ‘This makes it one of nature’s natural polyols,’ Groome said.

Table 1:Compostion of Natural Oils
Natural Oil	Oleic	Linoleic	Linoleic	Ricinoleic (b)	Stearic	Palmitic	Other
	18:1 (a)	18:2	18:3	18:1	18:0	16:0
Soya Bean	24	54.5	6.8	--	3.2	10.9	0.6
Castor	6.09	1	--	89.5	1	2	0.5
Sunflower	13.1	77.7	--	--	2.4	6.5	0.3
Palm	45.2	7.9	--	--	3.6	40.8	2.5
Notes:	(a) first number = No Carbon bonds; Second No=unsaturation sites; (b) Ricinoleic acid residue contains hydroxyl

But castor oil has the drawback of its hydroxyl number. ‘This is 160 mgKOH/g, and so it sits between a rigid and a flexible polyol,’ Groome said. ‘This can be modified to make it more useful by reacting the hydroxyl group with ethylene or propylene oxide, which can dramatically change the hydroxyl value without breaking the molecule down.’ Fatty acids can also be dimerised to achieve these types of polyols, he added, but there is a problem when they are used in low density insulation foams. ‘Where there is low functionality, the polyols tend to have problems with short- and long-term dimensional stability,’ he explained. The routes Jiahua chose were direct addition to the double bonds in a range of raw materials: sunflower oil, rape seed oil, jatropha, palm oil (both virgin and re-processed); and by co-initiation of a mixture of sugar, vegetable oil and catalysts. Both processes essentially follow normal polyether polyol production steps. The other advantage of these processes is that they give a great deal of flexibility with hydroxyl value, viscosity and functionality of the polyol. ‘The polyols have high bio-content, the economics are attractive, and it was easy to introduce a new product into the plant in terms of operator understanding,’ he said.

Addition Reaction

Direct addition to a double bond made it possible to produce polyols with a functionality of 2 to 2.2 and a hydroxyl value between 47 mgKOH/g and 320 mgKOH/g. ‘With the co-initiation process, we achieved functionalities between 2 and 5 and hydroxyl values between 200 and 400 mgKOH/g,’ he explained. Groome also spoke about work in automotive applications that has been done in Jiahua’s Shanghai labs, and also some carried out in the field. This concentrated on seating, glass-mat reinforced PU, truck bed liners and thermal insulation. In slabstock formulations, they used a natural polyol with hydroxyl number of 52mg KOH/g, with a 10% SAN solids slabstock polymer polyol. The NOP was used at between 0 and 30%. The SAN polyol on its own had good stability, and the product with between 12 and 25% NOP in the polyol had the same properties as the standard. ‘At much higher levels of NOP, we ran into significant problems with the product,’ he said. In moulded foam applications, they concentrated on automotive seating using MDI. A 20% NOP loading was compared with conventional polyol at a density of 35 kg/m3 and compression set, indentation, tear, tensile strength, elongation and resilience were all found to be similar for both conventional and NOP polyols in the laboratory. The product was used in truck-body insulation using various production processes. ‘We looked at producing discontinuous PU panels, continuous PU/PIR panels, PIR discontinuous blocks and spray foam,’ Groome said. ‘At 30% NOP from the co-initiation process, we found the results for compression strength, lambda value, specific density and dimensional stability after ageing were identical to formulations made with the 100% conventional polyol.’

Using yeast

Reverdia’s sustainable approach is slightly different. Instead of modifying polyols directly, it uses succinic acid produced by GM yeast to produce the feedstock for polyols, according to Lawrence Theunissen, the company’s global director for application development. ‘Reverdia is a joint venture between DSM and Roquette Freres,’ he said. ‘We produce Biosuccinium, a bio-based succinic acid that is made via fermentation. In addition to supplying our direct customers, we also try to collaborate with people further along the chain.’ The company’s Italian factory has 10 kt/year capacity. ‘Succinic acid is converted into polyester polyols and then into polyurethanes,’ he said. It is chemically related to adipic acid – it is a four-carbon dicarboxylic acid, whereas adipic acid has a six-carbon chain – and thus can often be used in a similar way. ‘We sometimes also offer our product as an alternative to phthalic anhydride or isophthalic acid.’ Moving to a biotechnological production route leads to significant savings in carbon dioxide, Theunissen explained. ‘According to work at the Copernicus Institute of Utrecht University, the Netherlands, there is a 53% reduction in carbon dioxide emitted in producing succinic acid using Reverdia’s biological product compared to the traditional petrochemical solution,’ he claimed. ‘The petrochemical option leads to emissions of 1.9 kg CO₂/kg succinic acid produced, whereas Reverdia’s process leads to 0.9 kg/CO₂/kg. There could be scope to capture more carbon dioxide in the process than is generated by it.’ The carbon dioxide savings are even more notable when compared to petrochemically derived adipic acid. ‘Typically, Reverdia’s approach produces a 90% saving in CO₂ emissions, compared with the petrochemical adipic acid process,’ he said. ‘If the Reverdia material is used in a pair of running shoes, for example, then it is possible to reduce carbon emissions from 15kg/pair to 12.5kg/pair. That could be a significant saving for a brand owner.’

Cut back on CO₂

Theunissen explained that, most of the time, the decision to use a bio-based material comes from the other end of the value chain – the brand owners. ‘This group is driven by its own targets and goals, and that is where we find traction,’ he said. ‘We work with people along the chain to help all the stakeholders understand what can be done. The furthest, brand owners, typically talk up the chain to the polyurethane and polyol suppliers, their direct suppliers. Brand owners indicate it is quite uncommon to be visited by monomer suppliers to discuss brand owner’s needs and the opportunities for biobased raw materials.’ In the polyurethane sector, shoe soles are a key market, while in the automotive sector, textiles in flame lamination applications are important, he said. ‘TPU elastomers, PU dispersions and adhesives are also interesting.’ Polyols built from fossil-based succinic acid are specified in applications because they perform better in the application than alternatives. ‘It has nothing to do with being bio-based, it is about functionality,’ Theunissen said. ‘Looking at bio-based succinic acid, the starting point is often different: our customers want to replace oil-based products, for example adipic acid, and they try our material. However, once they’ve started to experiment, they see new properties, some of which are better than the one they are used to.’ He cited the use of bio-based succinic acid in Vaude’s 2018 summer collection of Skarvan trekking shoes. ‘Here, bio-based TPU is used in the toe-cap because the brand wanted to use sustainable, bio-based resources,’ he said. ‘In a separate footwear-related project in Asia, Reverdia’s material is used in a high-density (500-600 kg/m3) micro-cellular foam slipper sole. Here again, the main drive is to replace fossil-based raw materials with bio-based materials.’ Theunissen gave the example of an industrial squeegee used in the printing industry. Here, PU strips are used to spread the ink in the screen printing process, and the strips are exposed to solvents and other types of chemicals during use. TPU based on succinic acid is a well-established product, but he said that companies have switched from fossil to bio-based materials because of production economics. ‘Along with flame lamination products, bio-succinic acid can be used in the automotive sector for acoustic foam,’ said Theunissen. He outlined the work done at the Institute of Polymers, Composites and Bio Materials in Italy, a research institute working with local company Adler Plastics. This looked at Dow’s SpecFlex system, where part of the fossil-based polyether polyol is replaced with a polyester polyol made by COIM using Reverdia’s bio-succinic acid.

Flame lamination

The formulations are shown in Table 2  and the cell structure of a typical polyurethane foam produced is shown in Figure 1.

Table 2: PU formulations
Sample	%SP	%BP	%H2O a	% TEP a	% CB a
PU	100		1	1
PU-C	100		1	4	5
PU-B	87.5	12.5	1	1
PU-CB	87.5	87.5	1	4	5
PU-BW	87.5	87.5	1.2	1
PU-CBW	87.5	12.5	1.2	4	5
Source: Reverdia

All of the formulations produced open-cell foam with different sized pores. The foam’s morphology and density were influenced by the viscosity of the polyol, and the amount of water used in the formulation. ‘However,’ Theunissen said, ‘there was little difference between the interconnectivity of the cells or the size of the cells between foam samples produced with 100% fossil-polyol and foam samples where 12.5% petro-polyol had been replaced with bio-polyol.’

There were differences in sound absorption at frequencies > 1 kHz. The foam made with biopolyol and carbon black had the greatest improvement in absorption. Similarly, using the biopolyol resulted in foams with greater transmission loss than standard foams.

He concluded that bio-PU foams have well-balanced mechanical and acoustic properties, and that they could be used to increase comfort levels in automobiles.

Bottle brush polyols

Phillippe Lodefier, European R&D manager of Total Cray Valley, proposed a biological route for producing low viscosity diols based on beta-farnesene that could be used in the automotive sector.

Beta-farnesene is a sesquiterpene. It occurs naturally in small amounts in natural products, he said, and now industrial quantities of the material can be produced by Amyris. With increased production comes increased opportunities, Lodefier said.

The Amyris approach is to feed natural sugar to a strain of genetically modified yeast which then produces trans-beta-farnesene, C15H24, which has a molecular weight of 204.3.

He said that anionic polymerisation and functionalisation allow this raw material to be used to make well-defined diols.

The diols have low molecular weight, currently about 3,000. This can be tightly controlled, as can the chain ends which can contain primary or secondary hydroxyl groups.

The polyols have a bottle-brush structure (see Figure left) with a rigid central chain surrounded by flexible branches, Lodefier said. This structure gives polymers with glass transition temperatures that remain close to -75°C, irrespective of the vinyl content of the backbone.

He gave details of the polyurethanes that can be produced using three different mixtures of his company’s Krasol LBH P3000 and LBH F3000 polyols with 42.5% 2,4 and 4,4 MDI, and also with 21.4% 2-ethyl 1,3-hexanediol. The formulations contained 20% dibutyltin dilaurate catalyst.

The LBH-P3000 and LBH-F3000 materials have molecular weight of 3,000 and a polydispersity of 1.1 to 1.3. They show the same APHA colour of 20, and KOH number of between 0.6 and 0.7.

The P3000 material has a viscosity of 13,000 cP at 25°C, compared with the F3000 material with 1700 cP at 25°C. The glass transition temperature (T_g) of P3000 -44°C compares with -65 °C for the F3000 material. About 65% weight percent of the P3000 grade comprises 1,2 vinyl groups, with the balance made up of 1,4-cis and 1,4-trans.

Krasol F3000, however, is 40% by weight 3,4 vinyl, and 60% 1,4-cis and 1,4-trans structures. Both materials have a functionality of between 1.9 and 2, Lodefier said.

The formulation containing 100% P3000 had T_g of -38°C, dielectric strength of 516 V/mm and transmitted water vapour at the rate of 0.0173 g/(h.m²) and 85 Shore A hardness.

The F3000 material hadT_g of -59°C, dielectric strength of 555 V/mm and transmitted water vapour at the rate of 0.0122 g/(h.m²) and 80 Shore A hardness.

The materials are suited for applications where water resistance and electrical insulation would be useful properties, Lodefier said.

Longer hydrophobic chains

Angela Smits, technology development manager at Croda, outlined her company’s approach to producing more sustainable bio-based polyols for automotive applications. ‘We produce and develop specialty additives for plastics and elastomers and rank our new products according to the 12 principles of green chemistry,’ she said. ‘This ensures that we keep sustainability as a focus area.’ Smits added that these polyols offer durability as they are based on fatty acids that are dimerised to C36 chain length. ‘This helps make the materials hydrophobic and water-resistant,’ she said. ‘These are used as the starting point for polyols which can contain up to 100% bio-material.’ Structurally, she explained, the polyols produced from the diacids are not like ethers or esters. ‘Because they are not ethers they’re not sensitive to oxidation,’ she claimed. ‘And because of their highly hydrophobic nature, despite being polyesters they are not sensitive to hydrolysis or water absorption.’ She said they can be used to produce flexible polyurethanes because of the branched nature of the dimers. ‘There is no strain hardening – you can keep bending them and they stay flexible,’ she said. Much of Croda’s work in this area has concentrated on adhesives and coatings, where adhesion to the substrate is very important if polyurethanes are used to over-mould other materials such as thermoplastics.

Table 3:Bond strength with different polyols
Sample	Adhesion (MPa)
HDO-Adipate	0.7
PPG	1.5
PTMEG	0.8
Priplast 1838	1.95
Priplast 3192	2.5
Priplast 3238	2.5
Priplast 3293	2.5
Source: Croda

‘Lap shear tests for polyurethane made with Croda’s Priplast polyols show that their adhesive strength is greater than 2.5 MPa for both amorphous and semi-crystalline products,’ Smits said. ‘Standard polyols based on a adipate and PTMEG in the same formulation are much weaker.’ Her data showed that these products typically failed at between 0.75 MPa for adipate samples, and 1.5 MPa in the shear test for PPG-based materials. It is a similar story for adhesion to polyamide 6 and polystyrene substrates. ‘We see similar mechanical performance behaviour to polyether in elastomers, and when compared with a polyester polyol that yields before it breaks,’ she said. ‘Changing from an amorphous to a semi-crystalline Priplast grade, there is an increase in modulus, higher elongation, and no strain hardening. This means that such grades can be used in applications where there is a lot of movement or as an adhesive for metal to plastic.’ Typically, HDO-adipate- and PTMEG based grades had bond strength to polyamide 6 of under 0.5 MPa. Formulations made with the Croda materials and bonded to polystyrene substrates show a similar performance. The substrate, rather than the polyurethane, fails at around 2 MPa. Polyurethanes based on HDO-adipate formulations failed at around 0.75 MPa, according to the presentation. The PTMEG materials failed at around 0.2 MPa. None of the polyurethane made using Priplast and bonded to polycarbonate failed at below 2.5 MPa, while adhesives based on HDO-adipate failed at around 0.75 MPa. PTMEG failed at around 0.8 MPa, and PPG-based formulations failed at around 1.5 MPa, see Table 3. Thermo-oxidative stability is known to be a problem for polyurethanes made with polyether polyols. Smits’ work examined this by testing a number of formulations based on polyols that had been stored at 140°C for four weeks. This, she said. was ‘quite a severe test’. The polyether-based systems using aged PPG- or PTMEG-based polyols lost all of their tensile strength and elongation properties. Polyurethane made with polybutadiene-diol, a moisture resistant polyol that is sensitive to oxidation, retained about 37% of its tensile strength and around 3% of the elongation compared with formulations made with unaged polyols. Formulations made with aged Croda polyols were not affected, and all maintained their strength and elongation.

Hydrophobicity

Water plays an important part in the durability and processing of many polyurethane adhesives and sealants. Smits suggested that the hydrophobicity of the bio-based Priplast polyols gives very good moisture resistance to the system, and in the case of coatings, give much faster drying because the water is repelled from the polyurethane dispersion. Her work showed that a formulation based on Priplast 3192, a semi-crystalline polyol, will typically dry in just over 3.5 min, compared with an equivalent HDO-based formulation, which will take about 7 min. Another common problem for polyester-based polyurethanes is moisture uptake. Smits tested this by weighing samples, immersing them in distilled water at 23 °C for one week, before drying and weighing them. According to Smits’ work, after a week in water, samples made using Priplast 3196 and 1838 had gained less than 0.4% extra weight compared with polyurethane elastomers produced with butane-diol chain-extenders, while PPG-based samples gained more than 1.8% extra weight. To test the hydrolysis resistance, a number of samples were immersed in boiling water for 15 days, and their tensile strength compared before and after. Her work showed that a formulation made with polyesters based on hexanediol has poor hydrolysis resistance, and lost all of its tensile strength by day 10. The ether formulation retained 20% of its original strength after 15 days, while the Priplast material retained more than 90% of its original tensile strength when the test was complete. The branched nature of the polyols helps to reduce the glass transition temperature. The C36 materials typically have greater flexibility than similar C18 materials, she said. ‘We found glass transition temperatures of -20 to -30°C and the bio-based semi-crystalline polyol was stiffer with T_g of -10°C, but by modifying the recipe, it is possible to reduce this,’ she explained. Overall, the results so far have been based on systems using bio-based polyols as single polyols, but this could change. ‘We have been looking into blends,’ she said. ‘Our polyols are not compatible with everything because of their low polarity. But some combinations – PTMEG with an amorphous polyol, for example – can give compatibility between 10 and 30%, this gives flexibility and a good moisture barrier.’