Recent Advances in the Production of Bio-Based Phenol Formaldehyde Resins
Phenolic resins which are synthesized with the consendation reaction of phenol and formaldehyde maintain their importance in the thermoset
polymer industry since their first commercial production in 1909.
Thanks to their advanced mechanical properties, flame retardancy, flexibility, low cost, thermal stabilty, resistance to water and chemicals they have wide range of applications in various sectors such as aerospace, automotive, wood-based panel, electricity, paint and coating [1-3].
Research and development studies of phenolic resins have held since the past and new modifications are continually being developed, allowing the creation of new valuable products made from phenolic resin. An important share of the researches focus on the partial or complete substitution of petroleum-based phenol and formaldehyde raw materials with biobased ones.
The main motiviations of these studies are; evaluation of sustainable bio-based resources in formulations, reducing VOC exposure during production and end-use and increasing the resin performance. It’s known that the petroleum based chemicals will not last long and many researchers are aimed the use of renewable resources in existing chemical industries.
Recent advances on aforementioned studies are expected to make it possible to convert
biomass into valuable chemicals and biofuels in biorefineries [4]. For example, lignocellulose, which has advantages such as accessibility, low cost, being a waste of another sector, is the primary source in the production of bio-based materials. Phenolic
compounds, alcohols, furan based structures can be listed as the lignocellulose biorefinery products [5- 7].
Lignin has the higher potential in industrial use among its biobased rivals–such as cardanol, tannin, gallic acid and bio-oils- [8-13]. Apart from phenol, it is possible to substitute formaldehyde with natural resources.
Hydroxymethylfurfural (5-HMF), furfural, terephthalaldehyde, glyoxal are candidates for
formaldehyde substitution in the literature [14-16].
1. Phenol Substitutes for PF Resin
1.1. Lignin Based PF Resins
Lignin which is a product of lignocellulose has gained a noteworthy attention due to its potential in phenol substitute.
It consists of three polymers; lignin, cellulose, hemicellulose and composition of these three varies with natural origin. Various processes such as high temperature and pressure extraction of lignin, lignocellulose depolymerization in the presence of mineral acids at high temperatures, dissolution and precipitation of cellulose, steam/ ethanol process, hydrolysis and extraction, organosolv and ultrafiltration process, catalytic conversion and enzymatic process have been developed to separate lignocellulose into its components.
Lignin derivatives, such as kraft lignin and lignosulfonate, have high hydroxyl numbers and are promising precursors of phenolic resins. Lignins can be used as raw lignin, purified lignin and chemically modified lignin in phenolic resin formulations. Methylolation, phenolation and demethylation are the main techniques to optimize reactivity of the lignin towards formaldehyde to achieve resins with satisfying performance.
Just like other biopolymers, lignin derivatives have different reactivity and therefore different substitution rates in the formulation due to their irregular chemical structure and unstable chemical composition [17].
1.2. Tannin Based PF Resins
Another green source used in the synthesis of phenolic resin is tannin, a polyphenolic macromolecule obtained from tree bark. Tannins isolated from tree bark with water at different temperatures are generally used in the production of heavy leather.
Tannins are classified into two subcategories, hydrolysable and condensable tannins based on their chemical properties. Generally, tannins can be hydrolysed to phenolic acid and carbohydrate derivatives in the presence of weak acids and bases. Although they have low reactivity due to their complex structure, they are preferred in phenolic resin formulations less often than condensable tannins.
1.3. Cardanol Based PF Resins
Cardanol is obtained from cashew nut shell liquid (CNSL) which is a byproduct of the cashew nut processing industry. CNSL is a mixture of cardanol (4,7%), anacardic acid (71,7%), cardol (%18,7) and contains phenolic functions.
Thanks to its phenolic structures it has been used as a biobased alternative of phenol in the phenolic resol and novolac formulations.
2. Formaldehyde Substitutes for PF Resin
As we mentioned in our previous articles, formaldehyde is widely used as an important raw material in a wide variety of industries and production areas. Its main areas of use are the wood industry, flooring materials, lubricants, cosmetics, pharmaceuticals, insulation materials, disinfectants and cleaning products, preservatives, paper and photo processing.
It is widely used in indoor and outdoor applications and people are somehow exposed to formaldehyde. Due to the environmental and occupational concerns of formaldehyde, a number of regulations and recommendations have been established to limit the level of exposure. The occupational exposure limit of formaldehyde varies between countries [18].
Although formaldehyde is an important resource in many industries, its negative impact on human health has led industries and researchers to seek safer and more environmentally friendly alternatives. Bio-based alternatives will be the most important substitution option
of formaldehyde.
Hydroxymethylfurfural, furfural, furfuryl alcohol, glyoxal and vanillin are some of the biobased molecules that have been researched to replace formaldehyde in PF resin synthesis.
2.1. 5-HMF Based PF Resins
Hydroxymethylfurfural (HMF) is considered by many to be the best alternative to formaldehyde, as it can be a source in the production of another chemicals and used as a liquid fuel as well. It is an organic compound consisting of a furan ring with an aldehyde and alcohol functional group.
HMF is highly reactive and soluble in aqueous media due to the presence of aldehyde and alcohol functional groups. It can be used for the production of dimethylfuran (DMF) and ethoxymethylfurfural (EMF), which are biofuel candidates for automated vehicles [19, 20].
Some other chemicals, including levulinic acid (LA), succinic acid, and 2,5-furandicarboxylic acid can also be synthesized from HMF by various chemical reactions [21-23]. HMF can be synthesized from lignocellulose and cellulose using organic solvents and ionic liquids [24].
It is an aromatic aldehyde found in dried fruits, honey, coffee and flavorings. Catalytic dehydration is widely used to take HMF from biological sources [25]. The main biological sources used in HMF production are carbohydrates such as simple sugars and their derivatives or carbohydrate polymers such as lignocellulose, cellulose, lignin, inulin and starch.
Zhang et al. developed a series of PF resins by replacing formaldehyde with HMF. The group converted glucose to HMF using CrCl2 / CrCl3 and tetraethylammonium chloride (TEAC) catalysts. HMF reacted with phenol was cured using hexamethylenetetramine (HMTA) and used to prepare glass fiber reinforced composite. The resulting resin showed thermal stability up to 315°C [26].
2.2. Furfural Based PF Resins
Furfural is an organic compound containing a furan ring with an alhehit functional group. Furfural has industrial value as a lubricant in resin production and in the synthesis of organic compounds such as tetrahydrofuran, furfuryl alcohol (FFA), tetrahydrofurfuryl alcohol (THFA), methyltetrahydrofuran (MTHF), furfurylamine, furoic acid and methylfuran.
Furfural is industrially produced from the non-food residues of wood waste. Companies
in China, Australia and the Netherlands produced furfural from lignocellulose biomass with good yields. Ionic liquids have also been used to produce furfural from lignocellulose materials [7].
Pizzi and his team have developed a phenol-furfural resin and investigated its flow and electrical properties [27,28]. Ahuja et al. studied the kinetics of phenol-furfural novalac resins in the presence of potassium carbonate catalyst with a wide range of furfural-phenol (F/P) mole ratios [29].
2.3. Glyoxal Based PF Resins
Glyoxal is a non-toxic, reactive organic compound with two aldehyde groups in its structure. The non-volatility, low cost, and easy biodegradability make it a suitable
candidate for formaldehyde replacement in the production of phenolic adhesive. Glyoxal is found in products such as wine, beer, tea, coffee, yogurt, bread, rice, soybeans, soy sauce, and oil.
It can be prepared directly from glucose by retroaldol condensation and from a glycoaldehyde intermediate by autooxidation [30]. Formaldehyde substititutes are summarized in the Table 4.
Conclusion
For the last 20 years, extensive studies have been carried out on phenolic resins obtained using bioresources and few versions are commercialized. Despite all these studies, their industrial production and application areas are at a relatively early stage.
Although they show promise as a substitute for chemicals that have already taken their place in the industry, technical and economic challenges must be overcome for the industrial application of bio-based materials to achieve versatile use areas [18].
Bio-based materials containing phenolic groups such as lignin, tannin, cardanol must be competitive in terms of performance in order to replace their petroleum-based competitors. In addition, major commercialization barriers of the technology will be removed when the high production costs of purification of biobased materials are overcomed.
Fluctuations in oil prices, potential energy crises all over the world, and health and environmental concerns during PF resin production and application have been the driving force for safe and sustainable resource use for phenolic resin synthesis.
The literature information presented in the article has been compiled from current articles on the production of PF resins using bio-based raw materials, and has been created in order to convey the current state of technology to the limited Turkish literature in the sectors where PF resins are used, especially in the wood-based board sector.
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Tolga Kaptı - R&D Manager
Polisan Kimya R&D Center
Berkay Demiralp - R&D Supervisor
Polisan Kimya R&D Center
