Thermosetting polymers are widely used in bonding and adhesives for the automotive and aircraft industry, in repairing products for civil infrastructures, electronic circuit boards, maintenance coatings for marine and other industries, and as binders in laminates and composites.
Due to their chemical resistance and thermo-mechanical properties, epoxy resins are one of the most widely used thermosets, perhaps surprisingly constituting as much as 14% of the overall production of polymers, including thermoplastics and elastomers.
Recently, bio-based thermoset resin systems have attracted significant attention, given their environmental benefits related to the wide variety of available natural resources, as well as the resulting reduction in the use of petroleum feedstocks. However, the use of bio-based epoxy systems is not yet significant in many fields, including aviation.
Curing agents are used to promote polymerisation of epoxy resins by catalytic action. Most of the curing agents currently used with epoxy resins cure by polyaddition reactions that result in both the coupling as well as cross-linking of the epoxy resin molecules.
The type of epoxy curing agent influences both the curing characteristics of an epoxy resin system as well as the final thermoset performance. Curing characteristics include items such as pot life and thin film cure speed, while thermoset performance includes items such as hardness, glass transition temperature, and tensile strength. Curing agents include:
- Polymercaptan
- Polyamide and Amidoamine
- Amine (Aliphatic, Cycloaliphatic or Aromatic)
- Mannich-base, including Phenalkamine
A good comparison of the various characteristics and applications of these different classes is provided here.
A Mannich reaction consists of an amino alkylation of an acidic proton placed next to a carbonyl functional group by formaldehyde and a primary or secondary amine or ammonia. The final product is a β-amino-carbonyl compound, known as a Mannich-base (Fig 1).
The Mannich reaction of cardanol, formaldehyde, and certain amines is called a phenalkamine. Phenalkamines exhibit features characteristic of the Mannich-base class: extremely fast cure, low temperature cure (even below 0°C), good chemical resistance, good surface appearance, good moisture tolerance, and non-blushing properties. However, due to the long aliphatic side chain of cardanol, phenalkamines also have very good pot life, good flexibility, surface tolerance, and excellent water and salt water resistance. As a result of this, common applications of Mannich-base curing agents include marine coatings, industrial primers, and moisture barrier coatings.
However, a drawback of some Mannich-base curing agents is the low molecular weight, with free amine and free phenol (highly toxic and a carcinogen). In Europe, resin manufacturers have recently been forced to withdraw or rationalise some conventional Mannich base curing agents from their product range because of REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) compliance.
Now, Chemical Process Services (CPS), a specialist polymer design and development company, has researched and developed a new series of ‘green’ epoxy curing agents for the composites industry, still within the class of Mannich-based curing agents. A patent is pending for this new chemistry and CPS is developing the products further.
Derived from biomass rich in pentosane, the Furalkamine Mannich base (Fig 2), is REACH compliant and has free amine but no free monomer – phenol or otherwise. Subsequent reaction with various amines influences the processing characteristics and offers a new means of curing under low temperature or other adverse conditions, whilst maintaining regulatory compliance.
The new agents are expected to find use in solvent-free or high solids marine coatings in the low VOC maintenance category, as well as in flooring and as adhesives for bio-based composites matrix systems. These epoxy hardeners are also especially useful for formulating systems for protective coatings and civil engineering applications.
Established in 1980 as a specialist manufacturer of synthetic resin, catalysts and curing agents, Bitrez will be manufacturing the Furalkamine grades under license from CPS. The inventor of the Furalkamines is Paul Jones, managing director of both Bitrez and CPS.
At JEC, Bitrez launched a family of REACH-compliant bio-based resins, including bio-epoxy systems and PFA (Polyfurfuryl Alcohol), designed especially for composite applications.
PFA is a thermosetting bio-resin derived from biomass crop waste with similar qualities to phenolic, but with lower VOC emissions. In addition, PFA has equivalent fire-retardant properties, plus excellent temperature and chemical resistance.
By employing creative chemistry, Bitrez offers epoxy products based on renewable substitute feedstock which when combined with the Furalkamine curing agents provide formulated systems with high bio-content.
Reference
Ramon et al. A Review of Recent Research on Bio-Based Epoxy Systems for Engineering Applications and Potentialities in the Aviation Sector. Sept 2018. Pasrtt of the ECO-COMPASS project.
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