CNSL for green rigid polyols

The cashew nut tree Anacardium occidentale not only provides edible cashew nuts, but has shells the contain the extractable aromatic cashew nutshell liquid, or CNSL. This substituted phenol makes up approximately 25% of the shell, and is the basis of a series of speciality phenolic polyols with desirable characteristics developed by Anacarda. Paul Jones, the company’s managing director, explains.

Refined cashew nutshell liquid (CNSL) is an interesting source of sustainably derived polyols for rigid polyurethane formulations. It is also used commercially to produce a range of niche polymers, marketed by Anacarda under the Cardamine brand.

The move towards a circular economy and the use of sustainable feedstocks is of great importance, and this phenolic polyol is based on a renewable non-food chain waste. Products made from CNSL have huge potential because they fit with society’s increasing desire for green chemistry that comes from sustainable, non-petrochemical feedstocks.

It is also possible to use these bio-based raw materials to reduce or eliminate the hazardous rating of the products. Conventional phenolic polyols use substituted phenols, which are suspected to cause genetic disorders. This is one aspect that is eliminated by the use of CNSL.

Additionally, bio-based products such as this can help fill a market niche for products that do not rely on petrochemicals. The Cardamine range of CNSL-derived polyols is already finding applications in a broad number of areas.

Cashew nuts are grown on trees in the tropics. Several million tonnes are harvested every year, with the major output coming from Vietnam, Nigeria and India. Although the nuts are harvested in these countries and others, India is the largest source of raw and refined oils. These oils are extracted from the shells and are inedible, and therefore do not feature in either human or animal food chains. This makes it a good raw material.

Crude CNSL is used in some phenolic polymer applications, but the major market is for refined oils. Refining yields four major components, but the structures of most interest are in the cardanol family. Cardanol’s empirical formula is C21H30O and, as well as the phenol group, it includes a 15-carbon unsaturated hydrocarbon chain containing three double bonds. This material is a feedstock used to make epoxy curing agents.

Classic rigid polyurethanes are made using phenolic polyols. These are mixed with flame retardant additives, surfactants and blowing agents into formulations widely used for thermal insulation. Cardanol homologues have properties that are comparable with these petrochemically derived chemicals.

There are numerous polyols designed for rigid foam systems, and there is extensive use of Mannich bases derived from p-nonyl phenol and its alkoxylated derivatives.

These products are synthesised as shown in Scheme 1. In this example, nonyl phenol, formaldehyde and diethanolamine are synthesised in a 1:2:2 ratio. It yields a mixture of oligomers.

As a result of the condensation of the phenol with formaldehyde, methylene-bridged species capped by the Mannich structure will occur. This reaction adversely affects viscosity, and if process times are extended and molecular weight builds significantly, very high viscosity material may result.

A good way to control molecular mass and avoid this viscosity problem is to use an oxazolidine route. Scheme 2 This is an intermediate process technique which can result in the same product, with a slightly lower molecular weight. This can yi eld products such as the CNSL-derived Curaphen 22-211, a nonyl phenol-derived phenolic polyol produced with a Mannich base. It has Newtonian flow and is supplied without either solvents or inert diluents. Its physical properties are shown in Table 1, along with BDP 4133, another grade that is under development.

Formulation and testing

We supplied the polyols to Barrie Colvin, technical director of Advanced Thermosets, who formulated them into foams and tested them. Each polyol was incorporated into a standard, B2-type flame retarded rigid foam system.

Our CNSL-derived materials were the only polyol used in each formulation. The formulations also included typical catalysts, silicone surfactants, flame retardants and blowing agents, and these were used to produce a foam with suitable reactivity and density when the polyols and MDI were combined at a 1:1 volume ratio.

Each polyol mixture was maintained at 5°C, and equal volumes of were then placed in cups. Both polyols were perfectly miscible with other ingredients. The picture also shows the formulations produced foams with a very fine cell structure. However, the formulation with a cashew nut oil-derived product (BDP 4133) produced foam with a reddish tinge.

The first tests were to measure the cream, rise and tack free gelation times. The cream time was taken as the point at which the blowing agents initiate bubble generation making the liquid increase in volume and rise up the cup. Once this increase in volume reached a peak the rise time was recorded. The final gel-time in this evaluation was the tack free gel- time. The results are shown in Table 2.

There were no appreciable differences in these characteristics between the two formulations. Both foams displayed similar core density, closed cell content and compression strength.

The percentage of closed cells in a rigid foam influences the thermal insulation capacity of the foam. In both systems, more than 90% of the cells were closed. The heat transfer value of the foams is also shown in Table 2, and expressed by the Lambda values. As expected, the low results for both foams is very good. The tests show that these polyol samples reacted in a very similar manner, with both producing good quality, closed-celled foams with acceptable lambda values.

As well as carrying out tests on these formulations, we also reviewed some of the blowing agents, catalysts, fire retardant and other additives that are commonly used in rigid polyurethane formulations. This review suggests that there are no issues from a compatibility perspective. However, there are some slight question marks over some of polyester polyols.

As with their petrochemical forerunners, the tertiary amine generated when manufacturing the Mannich polyols from refined CNSL conveys some auto-catalytic acceleration that can reduce the level of acceleration that may be needed in the formulation.

Product stability

Mannich-base phenolic polyols can increase in viscosity and molecular weight over time especially after prolonged storage at high temperatures. There are no appreciable differences in viscosity or chain extension observed between the standard petrochemical grade and the new bio-based grade. The BDP 4133 contains 67% carbon derived from renewable sources, compared with 0% for petrochemically derived Curaphen 20-211.

The new series of Cardamine bio-based phenolic polyols are derived from sustainable feedstocks, and offer an excellent alternative to conventional petrochemical-based Mannich base polyols for rigid polyurethane systems. They are compliant with Reach and other regulatory requirements, and may be used in standard formulations that can be readily adjusted to match to any target requirements.

With comparable cell structure, comparable density, and good insulation properties, we believe these bio-based products are a suitable alternative to current industry-standard materials. Additionally, it is possible to alter the functionality and aromaticity of these bio-based products to meet market demands. The bio-based grades offer comparable performance to the established petrochemical grades.

Bio

Paul H Jones FRSC is the owner and managing director of Bitrez and Anacarda. Paul has been responsible for the development of new products that have been recognise with two separate Queen’s Awards for Enterprise for innovation, and an IChemE global award. He is also 2021 Chemistry World Entrepreneur of the Year.

Acknowledgements

I would like to express special thanks to Dr Barrie Colvin for providing the independent screening and analysis of the conventional and bio-based system foams.