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Cashew Nutshell Liquid (CNSL) Diols Bring Sustainability in PU

Saud Sami – Sep 30, 2019

TAGS:  Natural-based Adhesives      Polyurethane Adhesives      Reactive Adhesives    

Cashew Nutshell Liquid (CNSL) Diols Bring Sustainability in PUSustainable chemistry aims to substitute completely, or at least partially, petroleum-based raw materials with natural alternatives to develop products with equivalent or enhanced performance properties. In this regard, cashew nutshell liquid (CNSL) has proven itself to be an invaluable resource for the development of a wide variety of products including:

  • Friction linings
  • Surfactants
  • Synthetic rubber & wax compounds
  • Foundry resins
  • Paints and varnishes
  • Epoxy resins, Epoxy curing agents
  • Adhesives, sealants and composites
  • Oil-soluble resins
  • Laminating resins
  • Rubber compounding resins, and
  • Polyurethanes

How can you benefit from this valuable resource? Explore in detail…



What are Cashew Nutshell Liquid Diols?


Produced as a byproduct during the processing of cashew nuts, CNSL is a viscous reddish-brown oil found in the honeycomb structure of the cashew nutshell.

Technical grade CNSL, due to the anacardic acid decarboxylation to cardanol under heating conditions, is composed of approximately:

  • 60-65% cardanol
  • 15-20% cardol and 2-methyl cardol
  • 10% polymeric material

By distilling CNSL under proper temperature and vacuum conditions, high purity cardanol is obtained.

Cardanol is a highly desirable alkylphenolic compound, useful for many polymerization reactions, due to its meta-substituted unsaturated aliphatic chain and a reactive phenolic structure. Products derived from cardanol offer an environment-friendly alternative to petroleum-based raw materials for the polymer and adhesive industries.

Cardanol is used to develop a large number of useful monomers and additives. Polyols derived from cardanol show superior properties, when used for the production of polyurethanes, as compared to the generally used polyester, polyether, and other petro-polyols. The special structure of cardanol gives the derived polyols a host of useful properties, such as:

  • Fast cure
  • Excellent adhesion
  • Water resistance
  • Flexibility
  • Low viscosity
  • Thermal and fire resistance
  • Chemical resistance
Molecular Structure of Cardanol Gives CNSL polyols Superior Properties
The molecular structure of cardanol gives CNSL polyols superior properties as compared to polyester, polyether, and other petroleum-based polyols


CNSL Diols Properties for PU Systems


Reduced Moisture Sensitivity of CNSL Diols Extends the Durability of Polyurethane Materials


  • The long aliphatic chain makes the polyols very hydrophobic
  • The use of cardanol as building block makes the diols stable to hydrolysis
  • Fewer ether-oxygen atoms make it less hydrophilic as compared to typical polyether polyols

The hydrophobic nature of CNSL polyols leads to excellent water resistance and reduces moisture sensitivity during cure with isocyanate in a polyurethane system, thus increasing the durability of the final product. In addition, CNSL diols cure rapidly with isocyanates, minimizing the need for a catalyst. The diols show greater reactivity to aliphatic and aromatic isocyanates compared to PPG diols and similar reactivity to aromatic isocyanates compared to Poly BD diols. The aromatic structure of CNSL polyols provides superior thermal resistance and excellent chemical resistance to acidic and alkaline solutions.

The combination of aromaticity and a long aliphatic chain further boosts hydrolytic stability and increases the mechanical strength of materials based on CNSL diols and polyols.

The NCO group in isocyanates can react with water, during cure, to release CO2. Apart from moisture-cured systems, this reaction is undesirable as the goal is to react the isocyanate group with a hydroxyl in the polyol to form the polyurethane linkage. The hydrophobic nature of different polyols can be quantified by pouring a mixture of polyol and isocyanate over the same amount of water. The amount of CO2 released is proportional to the amount of isocyanate which reacts with water. Results show that CNSL diols and polyols are significantly more hydrophobic than polypropylene glycol (PPG) diols and castor-based polyols. CNSL diols can show hydrophobicity very close to that of Poly BD.

100g of polyol/MDI + 100g of water

CO2 (ml) after 24 hours

 

100g of polyol/p-MDI + 100g of water

CO2 (ml) after 24 hours

CNSL diol/MDI

19

 

CNSL polyol/pMDI

6.0

PPG/MDI

> 75

 

Castor oil based/pMDI

19.2

PolyBD/MDI

18

 

 

 


CNSL diols and polyols show significantly lower moisture sensitivity as compared to PPG and Castor oil-based polyol


CNSL Diols are Compatible with a Broad Range of Diols, Hydrocarbons, and Additives Used in Polyurethane Formulations


CNSL polyols are available as diols, and medium and high functionality polyols. They can be tailored for varied applications and functionalized to achieve a wide range of desirable properties.

CNSL diols and polyols show good compatibility with other CNSL derivatives, with polyether polyols such as PPG, with hydroxyl-terminated polybutadiene (Poly BD) and various additives, allowing them to be used in formulations containing a mix of diols and polyols. CNSL diols can be used directly in polyurethane formulations or as a building block for HDI/MDI/IPDI/TDI prepolymers.


CNSL Diols Provide Excellent Hydrolytic Stability to Polyurethane Materials in Aqueous Solutions


Polyurethanes based on CNSL diols offer excellent hydrolytic stability. The figure below shows the hardness variation during a hydrolytic stability test of various cured prepolymers prepared using:

  • PPG
  • Aliphatic polycarbonate diols
  • Poly BD
  • CNSL diols

Cured prepolymers were immersed in 3 different solutions, water, sodium hydroxide, and sulfuric acid for 21 days at room temperature. The hardness was measured before and after exposure and the variation recorded.

Cured prepolymers prepared with suitable CNSL diols can show a hardness variation of less than 1%. Under similar conditions, cured prepolymers based on PPG, aliphatic polycarbonate diols and Poly BD may show hardness variation of up to 20%.
Hydrolytic Stability Test Results
Hydrolytic Stability Test Results, Showing Hardness Variation for Various Cured Prepolymers


CNSL Diols Show Superior Resistance to Acid and Alkali


CNSL polyols also show vastly superior alkaline resistance when compared to castor oil polyols. Coatings produced with castor oil polyols can form blisters after few days of immersion in 10% NaOH, while a CNSL polyol-based coating can remain virtually blister-free after several weeks of immersion.

CNSL diols also offer superior acid resistance compared to polyester and polyether diols. Hardness variation of polyurethanes, due to immersion in 50% H2SO4, can be greatly reduced by combining CNSL diols with Poly BD in product formulation.


What are the typical applications of CNSL diols?


CNSL diols can be used to combine the benefits of CNSL and polyester technologies providing outstanding hydrophobicity, good chemical resistance in acidic and alkaline solutions, excellent hydrolytic stability, compatibility with other diols and additives, and balanced mechanical properties. In addition, CNSL diols are suitable for prepolymer synthesis and some grades are safe for use in food contact materials.

Typical applications of CNSL polyols and diols include:

  • 1K/2K PU adhesives and sealants
  • Thermoplastic polyurethanes (TPU)
  • Polyurethane dispersions (PUD)
  • Polyurethane reactive hot-melts (PUR Hot-melt)
  • Polyurethane prepolymers
  • Binders, co-reactants and flexible/rigid foams


Where can I find CNSL diols?


As leader in CNSL technology, Cardolite offers the widest variety of CNSL based diols and polyols in the market functionalized to provide specific performance properties. A wide range of viscosities, hydroxyl values, molecular weights, and functionalities are available to meet different application requirements. Cardolite also offers CNSL Mannich polyols, with medium to high functionality, which provide fast reactivity, high compression strength and improved fire and thermal resistance properties. Other available medium to high functionality polyols include CNSL novolacs and specially modified polyols that deliver high overall strength and hydrophobicity. Cardolite has also developed light-colored CNSL polyols based on its new Ultra LITE process technology.



References

  1. https://cardanol.vn/cashew-nut-shell-liquid-cnsl.html
  2. https://www.researchgate.net/publication/263685055_CNSL_An_environment_friendly_alternative_for_the_modern_coating_industry
  3. https://www.researchgate.net/publication/257593445_Polyols_and_Rigid_Polyurethane_Foams_from_Cashew_Nut_Shell_Liquid
  4. https://www.cardolite.com/products/polyols_diols/
  5. https://www.cardolite.com/technology/
  6. https://www.cardolite.com/wp-content/uploads/2017/11/Polyols-for-Rigid-Foams-March2017-EN.pdf
  7. Cashew Nut Shell Liquid. "A Goldfield for Functional Materials", A. Parambath, Springer Editions, 2017.
  8. PU Magazine, 2017, 14, 3, 214-220
  9. https://www.sciencedirect.com/science/article/pii/S030094401300310X

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