Cork: Current Technological Developments and Future Perspectives

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Cork – Current Technological Developments and Future Perspectives for this Natural, Renewable and Sustainable Material Ivo M. Aroso, Ana R. Araújo, Ricardo A. Pires, and Rui L. Reis ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b00751 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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ACS Sustainable Chemistry & Engineering

Cork – Current Technological Developments and Future Perspectives for this Natural, Renewable and Sustainable Material

Ivo M. Aroso 1,2,*, Ana R. Araújo 1,2, Ricardo A. Pires 1,2, Rui L. Reis 1,2

1

3B’s Research Group - Biomaterials, Biodegradables and Biomimetics, University of Minho,

Headquarters of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine, AvePark-Parque de Ciência e Tecnologia, Zona Industrial da Gandra, 4805-017 Barco GMR, Portugal 2

ICVS/3B’s - PT Government Associate Laboratory, Braga/Guimarães, Portugal

* Corresponding author: Ivo M. Aroso ([email protected])

1

ACS Paragon Plus Environment


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ABSTRACT Cork is the bark of Quercus suber L., the cork oak tree. It is currently explored for different industrial applications, of which, stoppers for the wine industry is the most representative and economically important. During the processing stages, up to 30% of cork is transformed into powder, which is mainly used for energy production by the industry. This underexploited natural resource stream constitutes an opportunity for the development of new products. In this review we discuss cork as a potential source of chemicals for alternative applications. Special emphasis is dedicated to a) suberin; b) the extractives fraction; and c) the use of cork in non-traditional applications. Suberin constitutes a source of long chain hydroxyacids which can serve as building blocks for new macromolecules and materials. The structure and composition are briefly addressed, while the advances in suberin depolymerization, extraction methodologies and the proposed applications for this material are thoroughly discussed. The extractives fraction is constituted by lipophilic and phenolic compounds that present strong antioxidant and biological activities. The extractives composition and its properties are addressed. Finally, the use of cork for recently proposed applications, such as, the preparation of activated carbons and as templates for the adsorption of pollutants, are also presented. This review is intended to summarize the current knowledge and technological development state and to push for the progress towards an integrated cork economy.

Keywords: cork; suberin; secondary metabolites; antioxidants; natural resources; sustainable resources.

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Cork, production and economic importance The cork oak tree is a slow developing tree that grows predominantly along the Mediterranean basin. It requires a combination of hot and arid conditions, and has good tolerance to extended periods of drought, making it well adapted to poor soils and desertification prone regions

1-2

. The existing cork forests under commercial exploitation are managed by direct

human intervention, however, they constitute a unique and diversified ecosystem of fauna and flora where a large number of animals and plants coexist

3-4

. This ecosystem receives praises

from a number of international environment organizations, such as, the World Wildlife Fund (WWF)

5-6

. The cork oak forests occupy an estimated 2.1 million hectares worldwide, which

translates to an annual production of 201 ktons of cork 7. Portugal accounts for 55% of the world production, with 90 % for export. The cork stoppers are the most important product and constitute 70 % of the revenue generated for the industry 7-8.

Figure 1 – Quercus suber tree after debarking, cork products and alternative routes for the valorization of residues.

The harvesting of cork is performed periodically, through a manual and labor intensive process, without severe damage to the tree, after which it naturally regrows. The first extraction occurs when the tree reaches 25 cm of diameter, which happens at around 20 - 25 years. The first material extracted is thin, corrugated, with low suberin content, high density and with many morphological defects. It is designated “virgin cork” and doesn’t have sufficient quality for most of the common applications, consequently, it is only used on the production of agglomerated particle boards for non-technical applications. The subsequent extractions are usually performed with a minimum interval of 9 years, allowing for the cork to develop the 3



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necessary thickness for the production of natural stoppers. The second extraction cork, designated “first reproduction cork”, is composed of material of higher quality than virgin cork; however, it still does not present the necessary properties for the production of wine stoppers. It is mainly used on less technically demanding applications such as agglomerated particle boards for thermal insulation, panels for sound and vibration dampening and, in some cases, for agglomerated stoppers (Figure 1). Only on the third extraction the harvested material has the necessary quality for the production of natural wine stoppers. These are constituted of one single piece of cork and are obtained from the best quality cork, also constituting the highest revenue item. Therefore, there is a span of 40 to 45 years between the seeding and the cork stopper 1-2, 9. The macroscopic cellular structure of cork is well known and recognizable; however, its microscopic structure is still poorly understood. After growth, the cork cells lose their cytoplasm content and become void. It is accepted that the cell wall is a complex structure, constituted by the primary wall, mainly composed of cellulose, the secondary wall, assembled from lamellar depositions of suberin and also lignin to some extent, and the tertiary wall which is constituted by polysaccharides

2, 10-13

. The resulting structure is constituted by an

arrangement of hollow cells. The cross section of the radial direction (surface perpendicular to the bark growth direction) resembles that of a honeycomb, with cells with 4 to 8 sides. The cross sections of the tangential and axial directions are morphologically equivalents, and the structure reminds that of a brick wall (Figure 2) 8, 14.

Figure 2 – Piece of cork after extraction and cork cellular structure: cross section on radial (A) and axial (B) directions (scale bar: 50µm).

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Cork compiles a remarkable set of properties that are unique within natural occurring materials. These stem from its peculiar morphological arrangement and chemical composition and are the reason for the numerous applications were cork has been successfully used 15. It presents near-zero Poisson coefficient, complete recovery of shape after compression, low density, high thermal and acoustic insulation, damping properties, imperviousness to liquids and resistance to most solvents including mild acidic and basic solutions

16-22

. Its chemical

composition is highly variable and is affected by different factors, such as, geographical origin, climate or age of the trees 23. The typical composition account for 30 - 50 % suberin, 15 – 30 % lignin, 6 – 25 % polysaccharides and 8 – 20 % extractives 24-25. The industrial exploitation of natural origin materials or chemicals is often hampered by the absence of a logistic chain that guarantees raw material sourcing in sufficient volumes, its year-round availability, cost of transportation, storage, decomposition and preservation. Cork is already industrially produced and transformed, thus, these requirements are satisfied. Moreover, if the newly developed products, chemicals or materials are obtained from existing, currently unused or sub-used material streams, it will positively impact the business economy of raw material production and processing industries.

Suberin The presence of suberin is ubiquitous across the vegetable kingdom and it can be found with different extent on barks, roots and peels of some tubers. It constitutes a protective barrier and participates on the wound healing process of external tissues. Suberin presents an aliphatic-aromatic macrostructure with a highly variable composition between plant species and even from different structures from the same plant. Therefore, the term suberin is loosely used to define such structures 26. Moreover, despite the monomeric qualitative composition of the extracted material is usually well characterized, because the limits of the true suberin macrostructure are not well defined, its quantification is difficult and depends of the extraction methods employed. Also, the macrostructure arrangement and its interconnection with the remaining cell wall structural elements are still poorly understood. The suberin in cork constitutes one of the few natural, sustainable and direct sources of αhydroxyfatty acids, α,ω-dicarboxilyc acids and homologous mid-chain dihydroxy or epoxy derivatives in sufficient amounts to justify a commercial exploitation

25, 27

. Due to the high

content of suberin in cork, it is expectable that less raw material processing would be required when compared to other potential sources which will impact favorably the economics of the extraction process.

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In this section, we summarily review the physical structure and chemical composition of suberin, and focus on the most recent progresses in suberin depolymerization and its applications.

Physical structure and chemical composition The chemical composition of cork suberin has been proposed to be constituted by a polyester structure of long chain fatty acids, hydroxyl fatty and phenolic acids, linked by ester groups and crosslinked by glycerol units

26, 28-29

. The aliphatic domain is bridged to the aromatic one by

hydroxycinnamates, mainly ferulic acid derivatives, feruloylamides and caffeates

26, 30

.

However, the true limit of the suberin aromatic domain, and whether it is part of the suberin structure, is still a matter of debate between researchers 26, 28, 31. Nevertheless, the consensus among most researchers is to consider suberin as the fraction obtained after cleavage of the ester crosslinks present in cork. Based on this view, Graça

26

has proposed that the term

suberin should be reserved for the aliphatic polyester fraction structure, thus classifying suberin as a poly(acylglycerol) macromolecule 26. Along the years, several methodologies have been employed to study the chemical composition of suberin. Due to its tridimensional and insoluble nature, most methods are based on the depolymerization of the macrostructure through chemical treatment, i.e. hydrolysis of the ester linkages. The process results in the release and dissolution of the constitutive monomers, which are then identified and quantified by gas chromatography coupled with mass spectrometry (GC-MS) detection, usually after derivatization

32-38

. The in

situ characterization of suberin, using attenuated total reflectance infrared spectroscopy (ATRFTIR) and solid state carbon-13 nuclear magnetic resonance (13C-NMR), has also been performed

39-42

. However, due to the complexity of the molecule and limitations of these

techniques, the insight to its structural organization is limited. The depolymerization followed by GC-MS analysis is by far the most used technique for qualitative analysis of suberin, as demonstrated by the large number of reports comprising the use of this technique 25-26. The depolymerization with sodium methoxide in methanol under reflux conditions has been the most used methodology. It produces an extensive cleavage of the suberin molecule, with the release and dissolution of the monomers, mainly as methyl esters 33, 36, 38, 43. The monomers and small mass fragments (mainly dimers and trimers) have been used to devise the suberin’s chemical composition, as wells as the local organization of the different components and the overall macromolecular structure of suberin. Other depolymerization methods have been also employed; these include alkaline hydrolysis (NaOH and KOH) with other alcohols 44, containing

6


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variable water percentages 25, 45; hydrogenolysis with LiBH4 46; permanganate oxidation 32; ionic liquids 47; and even enzymatic degradation 48. The depolymerization process results in the release of fatty acids, methyl esters of fatty acids and fatty alcohols, glycerol and ferulic acid derivatives. However, irrespective of the method, the depolymerization is always incomplete, resulting in mixtures of monomers, dimers and other small mass fragments in combination with oligomeric fragments (Figure 3). Consequently, the quantification of suberin and the relative abundance of its monomers can fluctuate significantly across different studies. Despite this, the qualitative composition is not significantly altered, implying that only the extension of the depolymerization is affected, but not the selectivity. The percentage of detectable oligomeric structures is higher when mild depolymerization processes are used, decreasing for harsher conditions. However, even after such procedures are employed, a significant fraction of suberin mass is still not detected by GC-MS, due to its high molecular mass and low volatility 25, 38. Gandini et al. 25 have speculated that this fraction is actually composed by structures analogues to suberan, a non-hydrolysable highly aliphatic macromolecule, found in the periderm of some Angiosperm species

25, 49

. But

to date, no other authors have reported on the presence of such fraction in cork suberin, or have completely excluded it. These suberan-like compounds are not considered in the currently proposed models of suberin 50-51. Therefore, these are most certainly incomplete and do not represent the cork suberin molecule on its entirety.

Figure 3 – Typical size gel permeation chromatogram (GPC) for suberin obtained after alkaline depolymerization; average MW = 6306 g/mol (unpublished results).

The depolymerized suberin is usually comprised of linear long chain hydrocarbons, possessing from 16 to 26 carbons (with particular relevance to species with 18 and 22), and include αacids, α,ω-diacids and ω-hydroxyacids with or without extra functionalization at mid-chain by unsaturation, vicinal di-hydroxy and epoxy groups

25-26, 52

. Glycerol can amount up to 40% of

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the molar content, reflecting the importance of this component on the overall structure 28-29, 53. By

mass,

the

most

abundant

monomers

are

the

9-epoxyoctadecanodioic,

22-

hydroxydocosanoic, 9,10-dihydroxyoctadecanodioic and 9-epoxy-18-hydroxyoctadecanoic acids 25-26. The structures of the most relevant suberin monomers are summarized in Table 1. Table 1 – Structures of the most common suberin monomers.

Typical structures

Substitutions n1:

14; 16; 18; 20

n2:

-CH3: fatty acid -CH2OH: α,ω – hydroxyacid -COOH: dicarboxylic acid

n1

14; 16; 18; 20

n2

-CH3: aliphatic alcohol -OH: dialcohol

n:

-CH2OH: α,ω-hydroxyacid -COOH: dicarboxylic acid

-CH3: octadecen-9-oic acid n:

-CH2OH: α,ω-hydroxydecenoic acid -COOH: octadecenodioic acid

Depolymerization methods Cork suberin presents a complex molecular architecture and is intricately connected to other structural components. It is insoluble in most common solvents, requiring prior chemical cleavage whether for analysis, extraction or conversion. Therefore, any attempt to valorize suberin will require the development of suitable depolymerization processes for its conversion 8


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to a usable format. However, until recently, the exploitation of suberin for practical applications was scarcely reported and the depolymerization processes relied mostly on suberin obtained through the same procedures as for characterization studies. In the last few years, this paradigm appears to be changing with an increasing number of potential applications for suberin being reported in the literature. Moreover, it is also accompanied by a renewed interest in the depolymerization process, including optimization of the traditional ones or through the development of new methods. As stated before, the depolymerization of suberin is usually achieved by chemical cleavage of the ester crosslinks and the newly proposed methods also follow this strategy. The approach seems to be to rely on technologies that have been proven for the conversion of other biomass feedstocks and to adapt them to the specificities of suberin and cork. The ideal depolymerization technology would be one that produces high yields of depolymerized suberin with the desired physical and chemical properties for the envisaged application. To facilitate the industrial implementation, it should also: i) comprise the minimum number of steps as possible; (ii) to avoid the use of harsh chemicals and or conditions; (iii) and, ideally, to use well established operations in the chemical industry for scalability simplicity. In this section, we discuss the technologies that have been used to obtain suberin monomers and that, in our opinion, have the potential to answer the challenge of yielding suberin and suberin sub-products for further use (summarized in Table 2). Namely, the working conditions of the proposed methods and its limitations and advantages are reviewed, while the yield and physical and chemical properties of the depolymerized products are also discussed.

Table 2 – Summary of suberin depolymerization methods, conditions and major product properties to obtain high yields.

Method

Depolymerization conditions

Product properties

Chemical dep.

Methanolysis or hydrolysis;

Variable

with

acid/base From room T to reflux with

yield

References and [34] [54] [55]

chemical composition;

[56] [57] [58]

Limited selectivity

[59]

catalyst.

NaOH and H2SO4

Chemical dep.

200 °C and 40 bar of H2; Pd/C 11.5% Ph/C and 7.2% for

with metal catalysts

and Rh/C catalysts

Pd/C of cork dry weight

Cork powder + PO + KOH

Complete

Oxypropylation

cork

at 145 °C for 1 h (240 °C liquefaction; during reaction)

Mixture

[60]

[68] of

all

9



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components Cork powder + Gly and NaOH; Selective

for

suberin

Liquefaction

[64] [65] [66] Gly + PEG400 at 200 °C for 3h

with the use of NaOH Aliphatic

Pyrolysis

bio-oil

800-900 °C

from [87]

cork liquefaction Ionic Liquids

Cholinium alkanoates

Selective

for

suberin; [47] [80] [81]

100 °C and variable time

mainly oligomers

[82] [84]

NaOH: sodium hydroxide; H2SO4: sulfuric acid; KOH: potassium oxide; Pd/C: palladium in carbon support catalyst; Rh/C: ruthenium in carbon support catalyst; PO: propylene oxide; Gly: glycerol; PEG400: polyethylene glycol.

Traditionally, the depolymerization methods of election have been alkaline hydrolysis and alcoholysis, with particular emphasis on the later one. Several authors have reported the use of different homogeneous catalyzed based reactive systems, such as NaOH or sodium methoxide in methanol or water, under different conditions

34, 54-58

. Coquet et al

59

have

performed a comparative evaluation of these different methods and reaction conditions, namely, by comparing acidic and basic methanolysis, acidic hydrolysis and basic hydrolysis with water and ethanol. They found that, in general, as described in previous studies, the alkaline conditions produce higher yields. However, they also found that the processes conducted at room temperature produced only marginally lower yields than when carried at reflux temperature, 22 % of total cork weight in opposition to 25 %. The chemical composition was also different with higher yields of dihydroxy and tryhydroxy acids and lower on ω-hydroxy fatty acids. Therefore, the use of optimized methodologies for suberin depolymerization can provide a basis to obtain specific monomers or enriched mixtures for future valorization. However, these homogeneous catalyzed methods require the neutralization of reactants, the separation of solids and the recovery of suberin is performed through solvent extraction with organic solvents. All of these steps add complexity, decrease the economic value and put in question the environmental credentials of suberin. In this sense, Garrett et al

60

have proposed the heterogeneous catalytic hydrogenolysis of

several tree barks (including cork) with palladium (Pd/C) and ruthenium (Rh/C) in carbon supports catalysts. The process was carried out at 200 °C and 40 bar of hydrogen pressure for 4 hours resulting in the formation of suberin-like liquid that can be filtered out. Despite the total depolymerization of cork only reaching 11.5 % (Rh/C) and 7.2 % (Pd/C) per dry weight of cork;

10


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the study is presented by the authors as a proof of concept for the catalytic depolymerization of suberin using heterogeneous catalysts. This process presents the advantage of producing a suberin based extract while avoiding the neutralization and separation steps that are necessary when homogeneous catalysis with acids or bases is used. The liquefaction of cork is an alternative process to the traditional chemical depolymerization through methanolysis or hydrolysis that seems to have gained recent interest. The liquefaction of biomass offers the prospect of its rapid and efficient conversion to hydroxylated products and liquid polyols. When the reactive medium is integrated on the final product, the conversion process can be performed in one step, reducing the production of residues and increasing the economic and environmental interest of the process

61-63

. In particular, the

liquefaction of cork on polyhydric alcohols has been recently suggested 64-66. Yona et al 65 have proposed the liquefaction of cork in glycerol and glycerol/polyethylene glycol (PEG400) mixtures. The process is catalyzed by H2SO4 or NaOH and proceeds at fairly moderate temperatures (150 to 200 °C) for this methodology. The acid catalyzed liquefaction in glycerol resulted in the liquefaction of the lignocellulosic fraction (lignin and cellulose fractions). However, when PEG400 is added, an increasing amount of suberin is also liquefied, with up to 95 % of cork mass being liquefied for the glycerol/PEG400 50:50, 200 °C and 3h process. On the other hand, when NaOH is used, irrespective of the process conditions or solvent mixture, only the suberin fraction is obtained. Soares et al 64 only evaluated the acid catalyzed process and reached the same level of conversion (95 %) after 60 min of liquefaction time, at 150 °C and in the presence of 4 % of H2SO4. Margarida et al

66

have used ultrasound irradiation to

increase the rate of the liquefaction process. The reaction was carried out on a mixture containing 10 % w/w of cork powder in 1:2 % w/w of 2-ethylhexanol and diethylene glycol containing 3 % of p-toluene sulfonic acid. High frequency sound waves (24 kHz) with amplitudes from 60 % to 100 % at 400 W were applied for 3 min at the start of the reaction. The accelerated process could achieve 90% conversion in less than 3 min while the nonaccelerated only achieved the same level of conversion after 60 min. The oxypropylation refers to a particular liquefaction methodology where a solid material is converted into a liquid polyol by means of grafting of OH groups with propylene oxide (PO). This process has been successfully applied to different natural materials, including cork 67. The oxypropylation of cork was proposed by Evtiouguina et al 68, who submitted a mixture of cork powder and PO to high temperatures (up to 250 °C) and pressure (15 atm), in the presence of KOH. The product is a liquid polyol of significant viscosity at room temperature, composed by a mixture of all cork components and some polymerized PO.

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The works here described demonstrate that the liquefaction process can be applied to convert the cork solid material to a more workable liquid polyol in one step. On particular conditions, it is possible to adapt the methodology to present selectivity and yield a suberin fraction of tunable properties, through the simple selection of solvent/reactant and/or reaction conditions. On trade-off, the polyols produced are complex mixtures containing a large percentage of oligomeric structures, which can be a hurdle for the development of industrial applications. The use of ionic liquids (ILs) for the processing of biomass is a somewhat recent proposition but has shown great promise

69-70

. ILs present exceptional solvent properties and versatility,

which makes them excellent choices to process natural origin polymers, such as, cellulose, lignin and suberin (all insoluble in common solvents without prior extensive chemical cleavage) 71-72

. The number of all possible ILs has been estimated to be a staggering 1018 and its

properties are easily tuned based on the components combination, justifying its classification of “designer solvents”

73-75

. However, the development of processes for the processing of

natural polymers has been hampered by its high cost and high toxicity

76-77

. With the

introduction of biocompatible and biodegradable ILs based on cheap, readily available and, in some cases, natural occurring molecules derived from biomass, can change the present situation 78-79. Following this view, the extraction of cork suberin with ILs based on the cations cholinium and 1-ethyl-3-methylimidazolium combined with different alkanoate anions (hexanoate, octanoate and decanoate) has been explored 80-83. The best results were obtained with cholinium based ILs, which were able to solubilize the suberinic fraction of cork, while the polysaccharide and lignin domains remained insoluble 80-81. The extracted material was mainly composed by oligomers and polymeric fragments of suberin composed by the ester crosslinked monomers

81, 84

. However, the challenge of recovering the suberin fraction from

the mixture and the effective and economically feasible regeneration of the IL wasn’t addressed and will constitute a major hurdle for the process. Nevertheless, this methodology comprises a simple and direct procedure to solubilize and to separate suberin from the solid cork material. Attending to the tunability of the ILs’ properties, it is expectable that further refinement of this strategy will result in higher extension of the ester cleavage and even to the selective extraction of suberin monomers. Pyrolysis constitutes yet another alternative route for conversion of biomass to chemicals. This technology is based on the thermal degradation, in absence of oxygen, of complex molecules into a pyrolytic liquid, the bio-oil. It has been extensively researched for the conversion of different biomass feed stocks, such as, agricultural wastes (e.g. straw, olive pits, nut shells) and forestry wastes, such as, bark and wood leftovers 85. The bio-oils properties are derived from 12


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the feedstock materials and processing conditions, and are typically constituted by complex mixtures with hundreds of different compounds

85-86

. Exploring this route, Marques et al

87

tested the pyrolysis of cork material producing different bio-oil products. They found that the composition is severely influenced by process temperature. At 500 – 650 °C, the cork bio-oil presents a composition similar to that of wood bio-oil, with high composition on aromatic lignin-like compounds. On the other hand, at higher temperature (800 to 900 °C), a cork specific aliphatic rich hydrophobic bio-oil is produced which is associated to higher depolymerization of suberin. The cork bio-oil is mainly composed of simple molecules (alkenes, alkadienes and alkanes) with lower molecular weight than is observed for other depolymerization methods. Therefore, the cork bio-oil shows prospect to be explored as basis for bio-fuel production or raw material for the synthesis of olefins. In this section, we discussed several methods that have been developed to depolymerize the cork’s suberin, under the prospect of valorization of the cork sub-products. The final product is usually a complex mixture of the numerous constitutive monomers and often includes variable percentages of oligomers and higher molecular weight structures of crosslinked material. Due to the structure of the suberin molecule, it is expectable that the depolymerization products will always be complex mixtures requiring additional processing steps for refinement and or purification. However, due to the chemical similarity between the different monomeric structures, the development of an effective separation process will be technically challenging and economically questionable. Therefore, the suberin valorization route should be concentrated on the development of technical solutions where the depolymerized suberin can be used as whole, avoiding further processing steps.

Applications for depolymerized suberin Despite the extensive literature on the physical structure and chemical composition of suberin, the reports on the development of applications for this material or based on its components are far scarcer. Nevertheless, the use of depolymerized suberin and its monomers for different applications has been the focus of some studies that were reported in last few years (Table 3). It includes the use of suberin monomers as off-set ink additives 88, as starting macromonomers for the preparation of polyurethane pre-polymers polyester pre-polymers

93-94

57, 89-91

, rigid polyurethane materials

92

,

, rigid polyester films 84, grafting agent for composite materials 44,

and as ingredient for cosmetic products 95. In this section, we discuss the most relevant studies that report promising applications for suberin.

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The performance of depolymerized suberin as additive on waterless and vegetable-oil printing ink was proposed and evaluated by Cordeiro et al

88

. The suberin material was obtained by

alkaline depolymerization in methanol and included in the ink formulation up to 10 % w/w. The authors measured the impact in the ink properties and determined that, with exception of reduction in viscosity, no other significant alteration was observable. Despite the conclusion that the suberin material could be used as additive in the ink, no further developments have been reported and, to the best of our knowledge, there is no commercial use regarding this application. The first truly consolidated effort to use depolymerized suberin as starting macromonomers in polymeric materials synthesis was reported by Cordeiro et al

57, 91

in the mid-90s. Namely,

suberin monomers, obtained through the alkaline (NaOH) methanolysis and after chloroform extraction, were used on the preparation of polyurethane materials via polycondensation reactions. The reactions were carried out with different isocyanates producing a solid material comprised of linear, branched and crosslinked structures. However, the presence of low molecular weight and non-crosslinked fractions that are easily extracted with organic solvents was still observed. The typical physical aspect of depolymerized suberin (from alkaline depolymerization) and a rigid polyurethane membrane after crosslinking with hexamethylene diisocyanete are show in Figure 4.

Figure 4 – Depolymerized suberin as polyol raw material to prepare rigid polyurethane materials physical aspect of (A) depolymerized suberin and (B) rigid polyurethane membrane after polymerization with hexamethylene diisocyanate. (Unpublished results.)

A similar approach was followed by Evtigouguina et al

89

who have explored the used of the

oxypropylated cork polyol to obtain polyurethane materials (in reaction with aliphatic and aromatic mono- and di-isocyanates). The reaction was performed in dichloromethane and the products recovered after separation with solvents. The reaction kinetics and products were similar to what was observed for depolymerized suberin obtained using alkaline methanolysis. However, in both cases, the authors have focused on understanding the type of reaction and product formation, failing to further develop practical applications for these polymers. 14


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Recently, the preparation of rigid polyurethane foams (PUFs) has been proposed by Gama et al 96

. The authors have relied on the liquefied cork extract obtained by acid catalyzed liquefaction

process performed on a Glycerol/PEG400 1:9 mixture as starting materials

64

. The produced

foams were rigid and presented suitable properties to be applied as thermal insulators (low thermal conductivity). The properties of these foams are strongly influenced by the properties of the starting materials (which can be optimized through modification of the liquefaction process conditions), the reaction and foaming process conditions. Therefore, it is a versatile process that allows the fine tuning of the PUF formulations and properties for specific applications. Following, or perhaps, inspired by the polyester nature of the suberin molecule, several studies have focused on the exploitation of suberin monomers to prepare polyester polymeric materials. However, despite previous reports with suberin from different sources, the use of cork suberin with this purpose has only recently received interest

25, 93-94

. Sousa et al

93-94

reported the preparation of several novel biopolyesters from suberin. By using suberin monomer mixtures of tailored composition, they were able to obtain linear and crosslinked polymers. However, to achieve the desired reactive group balance, synthetic co-monomers bearing hydroxyl and carboxyl groups were added. On a subsequent work, the authors have also reported the polycondensation of long-chain suberin model co-monomers under different catalyzing systems. Namely, using bismuth(III) trifluoromethane sulfonate, p-dodecylbenzene sulfonic acid and enzymatic catalysis with lipase B from Candida antartica, linear polyesters of up to 7,300 Da were obtained. These results consist of the first systematic report on the use of cork suberin monomers as starting materials for the preparation of polyester materials. Garcia et al 84 developed a method to obtain a suberin based film, from the reconstitution of ILs depolymerized suberin monomers and oligomers. The distinctive characteristic of this process is that it proceeds through self-association without chemical additives or previous purification. This has only been observed for IL extracts and is attributed to the macrostructure being preserved and the catalytic effect exerted by the solvent. Following this procedure, the authors were able to prepare solid suberin films which mimic the original hydrophobicity with antimicrobial properties. From its simplicity and absence of harsh reaction conditions or use of toxic chemicals, this method opens interesting possibilities to develop films, membranes and coatings based on natural polyesters, which can find a broad range of applications. The production of cork composite materials is currently one of the hot topics in cork research with several reports and applications being presented recently. The common strategy is to combine the polymeric material (containing additives and compatibilizing agents) directly with cork granules. However, Fernandes et al

44

followed a somewhat different approach: they 15



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Page 16 of 41

proposed the use of depolymerized cork suberin (obtained from alkaline alcoholysis in ethanol) as compatibilizing agent for composites. The depolymerized suberin was grafted to high density polyethylene (HDPE) during the reactive extrusion and combined with the cork particles to prepare the composites in one step. The suberin acted as plasticizing agent, improving the cork particle dispersion, reducing the composite stiffness and increasing its flexibility. The composites present higher mechanical properties, dimensional stability and thermal resistance than the non-functionalized material. These effects are attributed to the compatibilizing effect of suberin between the functionalized polymer and the surface of the cork particles. Table 3 - Summary of potential applications proposed for molecules and/or materials derived from cork.

Molecules sourced from cork

From suberin Ink additives Polyurethane materials: pre-polymers and rigid materials Polyester materials: pre-polymers and rigid films Polymer grafting additivies Additives for cosmetic products

From extractives fraction Bioactive molecules UV protection agents Polymer antioxidant additives Cork or cork modified materials Biosorbents for oil spillages Biosorbents for heavy metals Activated carbons

Cork extractives fraction The cork extractives fraction is composed by low molecular weight molecules that are not covalently bonded to (or have weak interaction with) the cell wall structural elements (i.e. suberin, lignin and cellulose). This fraction can account up to 24 % w/w of the cork total mass; however, values of around 15 % w/w are more commonly found 97. The quantity and quality of the extractives fraction is dependent on the geographical origin, climate, soil, genetic origin, the age of the tree or even the different tree part from which the cork was obtained 97-98.

16


Page 17 of 41

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The presence of these unbounded low molecular weight molecules is ubiquitous to all plants. They do not participate in the primary plant functions (growth, photosynthesis and reproduction) and are commonly classified as secondary metabolites

99

. These secondary

metabolites are evolutionary markers and an adaptive characteristic of plants; they have functions related to defense against pathogens, signaling pathways, protection against ultraviolet radiation and oxidative stress, among others. These secondary metabolite have diverse structures and present different properties, such as, antioxidant, anti-mutagenic, anticarcinogenic, anti-inflammatory, and anti-microbial effects 100. These properties are beneficial for plants, since they help to prevent diseases and to protect plants of toxic compounds, from bacteria and fungi, and to maintain the stability of the genome

101-103

. Therefore, these

molecules constitute prime candidates to transfer these functions to new systems and are inspiration for new molecular structures for further modification toward increased activity. The potential impacts of these molecules on human health have long been recognized and plant metabolites have received renewed interest in the last few years, namely for therapeutic and biotechnological applications, for food and pharmaceutical uses

104-105

. Some of the

chemicals extracted and purified from plant materials, such as, alkaloids, anthocyanins, flavonoids, quinones, lignans, steroids, and terpenoids have found commercial application as drugs, dyes, flavours, fragrances, insecticides, among others 106-107. The cork extractives are of no exception. The extractives fraction has been the focus of several studies, which helped to clarify its composition; however, the presence of new molecules has been recently reported

108-109

. It comprises very distinct molecules, such as, phenols and

polyphenols, triterpenes, sterols, fatty acids and linear chain alcohols. These are mainly classified as secondary metabolites, but some are also regarded as precursors or intermediaries of other components

110

. Most of the earlier studies focused in the extractives

obtained with aqueous or polar solvents, probably due to the interest in the molecules that can migrate from stoppers to the wine 111. The molecules that compose the cork’s extractives fraction are usually obtained through simple solvent extraction, with the properties of the extracting solvent largely influencing the nature of the extract obtained. In this context, dichloromethane, chloroform and hexane have been the solvents of choice to obtain the lipophilic components; while the more polar components are usually obtained with methanol, ethanol, water and its mixtures 98, 112-114. The use of supercritical carbon dioxide has also been reported

115-117

; these extracts are usually

obtained as complex mixtures, still comprising many compounds. Therefore, the qualitative and quantitative analytical methods of choice are based on chromatographic techniques, such as, GC-MS 59, 108, 113, 116, 118 and HPLC-MS 119-121. 17



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Page 18 of 41

The cork extractable fraction is typically divided in two main subgroups: i) waxes (aliphatic and triterpenic compounds), obtained using non-polar or low polarity solvents, such as, dichloromethane, chloroform and supercritical fluids; and ii) phenolic compounds typically obtained with polar solvents, such as, water, methanol and ethanol.

Waxes The term waxes describes the primary cuticle of vascular plants. They comprise a diverse mixture of aliphatic, triterpenoids, flavonoids and/or phenolic lipids. Moreover, waxes contribute to the barrier function by reducing the loss of water, and also reduce the absorption of chemicals (including pollutants and agrochemicals) through the aerial surface of plants

122

. In cork, the waxes are found in the secondary cell wall in close association with

suberin, constituting the wax lamella

123

. This fraction is mainly composed by triterpenes and

n-alkanes (from 16 to 34 carbons), n-alkanols (from 20 to 26 carbons) and fatty acids similar to those constituting suberin

59, 108, 124-127

. A comparison between the triterpene composition and

sterols obtained with different extracting techniques is presented in Table 4. The differences reflect not only the natural variability, but also the use of different analytical techniques, making direct comparisons very difficult.

Table 4 - Comparative composition of main triterpenes and sterols extracted under different procedure conditions (the yields are presented in mg of mass of compound per kg of cork dry weight).

Extracting

Supercritical Chloroform

Dichloromethane (DCM)

DCM

solvent

CO2

Reference

Coquet al,

et Castola et Touati et Sousa et Touati et Castola et al,

2008 al,

2005 al, 2015* al, 2006 al, 2015**

2005

Compound

[58]

[116]

[107]

[112]

[107]

[116]

Friedelin

19.5

12.6

2.50

2.31

4.70

12.2

Cerin

-

-

1.26

4.63

7.28

-

Friedelanol

7.81

-

-

-

-

-

Betulin

6.52

1.62

0.26

0.32

1.65

0.84

Betulinic acid

-

4.38

1.30

2.19

2.80

1.26

β-Sitosterol

-

-

0.30

0.59

0.51

1.74

-

1.74

-

-

-

9.0

6.5

6.0

5.6

3.6

5.6

7.0

Sitost-4-en-3one Total

extract

yield (%) *** 18


Page 19 of 41

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The extract composition was determined before (*) and after (**) alkaline hydrolysis; (***) total extract yield in mass of fraction per mass of dry cork.

The triterpenic fraction is mainly constituted by friedelin, cerin, betulin, betulinic acid, smaller amounts of sterols, and a minor fraction of other friedelane and lupane type components (Figure 5)

45, 97-98, 127

. Cork is the main natural source of friedelin and friedelanol, from which

several derivatives (e.g. 2α-hydroxyfriedelan-3-one, 2,3-secofriedelan-2-al-3-oic acid, 2α,3βdihydroxy-friedelane, synthesized.

128-130

3,4-secofriedelan-4-oxo-3-oic

acid,

among

others)

have

been

.

Important bioactive properties have been attributed to different triterpenic molecules, including the capacity to inhibit the growth of several types of cancer, such as, breast, renal, lung, central nervous system and melanoma

120, 131-134

. For example, pentacyclic lupane-type

triterpenes plant extracts demonstrate cytostatic activity on various in vivo cancer model systems

135

. Fujiokta et al have reported on the inhibitory effect betulinic acid in HIV-1

replication 136. Betulinic acid and betulin were also found to have anti-malarial, anti-microbial, and spasmogenic activities 137. However, despite the presence of several triterpene molecules in cork, to the best of our knowledge, these are not being currently commercially explored and other cork triterpenes have not yet been sufficiently studied.

Figure 5 – Structure of several triterpenes and one sterol (β-sitosterol) identified in cork lipophilic extracts. 19



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Page 20 of 41

Saturated and unsaturated fatty acids are the second major group of lipophilic compounds. The most abundant are the docosanoic and hexadecanoic acids and the C18-acids, 9octadecenoic, 9,12-octadecadienoic and octadecanoic acids. A number of aliphatic alcohols in free and esterified form (e.g. docosan-1-ol and tetracosan-1-ol) and some aromatic compounds (vanillic and ferulic acids) have also been identified within the extracts of nonpolar solvents. Docosanol is currently used in cosmetic industry as a emulsifier 138 and in 2000 it was approved by the Food and Drug Administration as a pharmaceutical antiviral agent for herpes under the commercial name Abreva®

139

. Ferulic acid is used as a precursor in the

manufacturing of vanillin, a synthetic flavoring agent often used in place of natural vanilla extract

140

or a stabilizer of Vitamins C and E in skin photo-protection cosmetic products

141

.

However, due to their limited availability, it is doubtful that cork will constitute a viable economic source.

Phenolic compounds The cork extractive fractions are also rich in phenolic and polyphenolic compounds with very diverse structures (the structures of the most common are shown in Figure 6). These are usually obtained through extractions with polar solvents. Tannins or plant polyphenols can be monomeric or polymeric and can be classified into condensed tannins (or proanthocyanidins) and hydrolysable tannins. While the central structure units of condensed tannins are flavan-3ol units, linked together through carbon-carbon bond

142

ellagitannins and gallotannins (glucose and gallic acid)

143-145

, hydrolysable tannins comprise the . Proanthocyanidins are the most

widely studied polyphenols, since they are the most common flavonoids present in the human diet 146. Other phenolic compounds or extracts that have received particular interest are green tea extracts (epigallocatechin gallate - EGCG), curcumin, resveratrol, isoflavones, black raspberry powder (anthocyanins and ellagitannins), bilberry extract (anthocyanins), ginger extracts (gingerol derivatives), and pomegranate extracts (ellagitannins and ellagic acid) 147-149. Despite the huge progress in conventional chemistry and pharmacology in producing effective drugs, the practical result about the use of phenolic compounds in human health is still under debate 150. Some of these compounds or similar structures can be found in the phenolic fraction of cork. Several studies have shown that ellagic acid is the predominant phenolic compound, followed by gallic and protocatechuic acids, and the ellagitannins vescalagin and castalagin 119-121, 151-152. The phenolic extracts, its fractions or compounds have been directly associated with antitumoral

153-154

, anti-bacterial 84, insecticide

130, 155

, UV radiation protection

120

and antioxidant 20


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120, 156-157

effects. In particular, several recent studies have focused in specific potential

applications for this fraction. Santos et al

152

performed an in-depth characterization of the

cork phenolic fraction using multi-stage HPLC-MS, and have identified five previously unknown molecules: methyl gallate, brevifolin carboxylic acid, caffeic acid isoprenyl ester, isorhamnetinrhamnoside and isorhamnetin. They also characterized its antioxidant potential and determined to be higher than that of commonly used antioxidants, such as, butylated hydroxytoluene (BHT). Fernandes et al 121 also reported significant antioxidant activity of cork phenolic extracts, while detecting inhibition of the proliferation of human breast cancer cell line (ER+) MACF-7 and of the colon cancer cell lines Caco-2 and HT-29. Araújo et al 120 showed in an in vitro study that the presence of the cork phenolic extracts significantly decreases the DNA fragmentation and cell death after exposure to UV radiation, therefore exerting protective action. The authors concluded that this effect is correlated with the extracts’ antioxidant capacity, while the antioxidant capacity was stronger for the extracts or its fractions with higher vescalagin and castalagin content. The use of cork extracts as processing additives, namely as thermal oxidative protection in polymer processing has been also explored by Fernandes et al

44

and Aroso et al

156

. Cork-polypropylene(PP) composites were

shown to present “natural” thermal oxidative protection and after direct processing of PP with different fractions of cork fractions, it was shown that this effect derives from the extractives’ ethanol soluble fraction. The effect is attributed to the antioxidant capacity and good mixing of this fraction in this polymer. Therefore, for other polymers, it is possible that different fractions will also exert this effect. These results give hints for future developments that can lead to commercial products based on cork phenolic extracts. However, at present, further studies are necessary to completely understand the full potential of these fractions.

21



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Page 22 of 41

Figure 6 – Structure of some of the phenolic and polyphenolic compounds identified in the cork’s extractive fraction.

Other applications of cork Apart from the traditional uses of cork, and the attempts to valorize its distinctive chemical constituents (suberin and extractives), cork has also been explored as a biosorbent for traditional and emerging pollutants. Emerging pollutants are synthetic or naturally occurring chemicals that are not yet monitored but that have the potential to negatively impact the environment and human health

158

. The removal of these contaminants often requires costly

operations and new techniques will be necessary for an economic decontamination process. In this sense, the use of biomass to remove pollutants constitutes an alternative and less expensive process. Different sources have been tested including bacteria, fungi, sea weeds and agricultural and forestry residues. The use of living organisms is based on their capacity to grow on the polluted environment, incorporating the pollutant molecules through accumulation within their bodies, or their ability for its metabolization. However, these are very specific and time consuming processes, while the biomass residues are prospective options for bio-based sorbent materials for the rapid and unspecific removal of contaminants. The sorption of the pollutants molecules occurs on the surface of the biosorbent material due 22


Page 23 of 41

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to chemical or physical compatibility. Within this approach, cork’s industrial residues have been proposed for the extraction of pollutants. Unmodified or surface modified cork particles have been used as biosorbent with some commercial success, and also as raw materials for the preparation of activated carbons. On the next sections, we discuss cork as a biosorbent material and as precursor for activated carbons.

Biorsorbents from cork Biosorbents for organic pollutants One of the applications where cork has been proposed as a biosorbent is for the adsorption of organic pollutants from aqueous streams, namely for polycyclic aromatic hydrocarbons (PAHs). PAHs are molecules composed of multiple aromatic rings without further substituents. Despite some natural sources, such as, volcanic activity and wildfires, most of what is found in the environment has origin from human activities. They are primarily produced from the incomplete combustion of organic matter, namely fossil fuels, and also from industrial processes, such as, the plastics industry. These molecules are chemically and biologically very stable and are, therefore, persistent in the environment. Because they are lipophilic, they can bioaccumulate along the food chain. The limits for surface water have been enforced by the European Directive 2006/0129 EC 159. Its removal is usually performed using activated carbon; however, this is an expensive procedure, namely for large volumes of water containing low levels of pollutants. Therefore, there is a strong motivation for using low cost adsorbents that can significantly decrease the cost of the process. Olivella et al 160 proposed the use of granulated cork from processing leftovers for the removal of PAHs from water streams. The adsorption-desorption behavior of a mixture containing thirteen PAHs was evaluated. The PAHs were found to adsorb rapidly on the surface of cork particles, with 80 % of the adsorption occurring on the first 2 minutes of contact time. The best results were achieved for pyrene, anthracene and phenanthracene 160. The adsorption capacity of cork for PAHs is directly related with the content on aliphatic extractives (waxes), rather than with the presence of suberin. Also, the adsorption is higher for lower molecular weight PAHs 161-162. On contrary, the adsorption of aromatic pesticides on cork occurs primarily at the lignin and phenolic extractives level. These interact with cork components by establishing πinteractions and hydrogen bonding 163-164.

23



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Page 24 of 41

Biosorbents for oil spillages Another field of application of biosorbents is the sorption of oil and grease from water. The oil is more difficult to achieve as it implies the competitive adsorption between oil and water molecules and requires the ability of the biosorbent to hold the adsorbed material. The presence of oil in water can originate from effluents of industrial processes or accidental spillages. In either situation the rapid and efficient removal of the oil is required to limit contamination. The sorption process of oil on cork’s surface depends on the properties of the oil and oil-inwater emulsion properties. The sorption occurs on the surface with almost no oil reaching the interior cellular layers and the sorption capacity is proportional to the granules’ specific surface area

165

and is usually well described by the Freundlich or linear partition models

166

.

The sorption is favored for lower pH due to the protonation of carboxylic groups, which decreases the surface charge and for higher ionic strength solutions. However, the presence of surfactants decreases the sorption rate due to emulsion stabilization effects 167. The possibility of desorption and regeneration of the biosorbent material can further decrease the material requirements and the operational costs, constituting an interesting prospect. In this sense, Pintor et al 168 evaluated several strategies for the desorption of sunflower oil from cork granules, including the use of acidic and alkaline solutions, anionic and cationic surfactants and mechanical compression. The best results were obtained following a step mechanical compression protocol achieving a maximum oil recovery of 67 % after 10 cycles 168. The use of mechanical compression desorption is preferable due to its simplicity and also because it does not uses chemicals or extraction solutions, which would require subsequent treatments. Moreover, cork is highly elastic with near complete shape recovery after compression and presents near zero Poisson coefficient

18, 22

. Therefore, cork can withstand

repetitive compression cycles without reduction of its properties due to its intrinsic mechanical properties. To our knowledge, the only cork biobased sorbent currently commercially available are the ones sold under the tradename Corksorb®, produced by Amorim Isolamentos SA. These are sold under different formats (bulk, pillows, booms, etc.) and are designed to be applied for the absorption of oils, solvents and chemical compounds across a number of industries169.

Biosorption of heavy metal ions Heavy metals are common pollutants originating from the industrial activity. Due to their toxicity, they must be removed from the water waste streams. The most common method 24


Page 25 of 41

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employed is the precipitation as sulphides or hydroxides. However, this method produces sludgers that have to be deposited on landfills. Therefore, alternative methods, such as, those based on the adsorption on activated carbon, activated alumina or polymer resins are preferred. In this view, the use of inexpensive bio-based sorbents is desirable and cork wastes offer the prospect of cheap adsorbent materials. Cork wastes have been studied for the adsorption of copper 170-171, nickel 170-171, zinc 171, trivalent chromium 172, hexavalent chromium 173-176

, cadmium 177, mercury 178 and lead 177. However, despite the technological feasibility, the

somewhat lower adsorption capacity has hindered the development of commercial applications.

Activated carbons from cork Activated carbons are extensively exploited in a wide variety of adsorption processes due to their chemical resistance and form adaptability. They are most commonly prepared from charcoal and are therefore sometimes also designated by activated charcoal. However, the use of natural carbon sources, such as, biomass, represents a prospective alternative for obtaining low cost carbons 179. Therefore, the use cork industrial wastes as source for the production of activated carbon has been proposed and investigated in several studies

180-186

. Cork may also

present advantages over other carbonaceous materials due to its cellular structure, leading to high surface carbon materials and low water content (energy savings). The preparation of activated carbons from cork has been performed without pre-activation and with physical and chemical pre-activation 182. The chemical pre-activation is preferred as it usually leads to final materials with higher surface area, and has been performed with phosphoric acid, potassium hydroxide or potassium carbonate

180, 183-184, 186

. The use of cork activated carbons has been

demonstrated for the adsorption of volatile organic compounds (VOCs) (hexane, cyclo-hexane, methyl ethyl ketone and 1,1,1-trichloroethane) clofibric 191

188

acetaminophen

180, 189

, amitriptyline

186

190

, emerging pollutants (ibuprofen

, paracetamol

180

180, 187-188

,

), p-nitrophenol and phenol

). However, regardless of the good performance of the cork’s activated carbons, these

studies remain at the laboratory scale and no commercial application has been reported.

Conclusions and perspectives Cork is a natural material with a long history of human use. Correct forestry practices have also made it one of the most environmentally sustainable. Cork stoppers for wine bottles are the most recognizable cork product, continuing to be the most important in terms of industry 25



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Page 26 of 41

revenue. The revenues generated from cork stoppers support the entire production chain, from the forestry activities to the industrial processing of the material. But not all cork can be used in the production of cork stoppers. As with most natural materials, cork is a heterogeneous material, with parts that do not meet the standards for the production of stoppers, whether due to insufficient thickness or the presence of structure defects. Additionally, during stoppers production, part of the cork is also rejected as leftovers. Moreover, during tree growth, the initial two harvests are also not usable. Therefore, there is a significant percentage of cork material that cannot be used for the production of stoppers. This stream of unused material has prompted the industry to develop alternative applications. The leftovers of good quality cork are granulated to smaller pieces and used to produce agglomerated cork stoppers. The remaining granulated material, usually of lower quality, are used to obtain different products, such as, composite materials, insulation panels, cork pavement or fashion products, among others. However, despite the development of these alternative applications, one grade of cork by-product remains to which no significant use has been developed – cork powder. Cork powder is defined by the Portuguese standards NP-114 and NP-273 as being composed of particles bellow 0.25 mm. Currently, lacking better application and due to its high calorific value (18.9 to 29.3 MJ/kg) and low moisture content (10 to 15 %), it is typically burned for energy production

192

. However, when burned, the

commercial value of this cork material is limited by (and equal to) the cost of the fuel it is replacing, usually natural gas. Cork powder has recently been proposed and commercially explored as biosorbent, though this constitutes a low added-value solution that doesn’t address the true potential of cork. Therefore, within the industry, cork powder continues to be treated as a residue of low economic value. Due to cork powder’s small particles composition, it’s not expectable that this material can find application within the traditional cork uses, requiring innovative approaches. Until recently, the innovations in cork applications have focused in the macroscopic material, overlooking the potential as source of natural and renewable chemicals. In our opinion, cork is a notorious example of a natural material that possesses all the requirements to assume an ever-growing role in the green chemistry of the future. The full potential of cork and its components is still underexplored. In this report we have reviewed the current knowledge in cork chemical composition and discussed its potential applications, with particular focus in the industrial by-products and its chemical components. From the different valorization opportunities identified, we consider cork suberin to constitute the most promising towards future commercial exploration. It is abundant and its depolymerization has already been well described by several methodologies. 26


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However, its exploration in the preparation of new bio-based materials has not received the same level of effort. Nevertheless, this situation seems to be changing in the recent years with the number of publications growing substantially. Several authors have demonstrated different routes from depolymerized suberin to new functional materials. Notable examples are the preparation of polymeric materials such as polyester and polyurethanes in different physical formats. Others have explored the use of cork suberin in composites production to obtain better properties, with apparent success. Others still have demonstrated cork extractives as additives in polymer processing. The latest research results provide strong hints that with the continuous effort from the industrial and scientific community, the development of commercial products based on cork suberin may be a reality in the foreseeable future.

Acknowledgments Ivo Aroso acknowledges the financial support from FCT through grant SFRH/BD/42273/2007. The authors acknowledge the financial support from Projeto “NORTE-08-5369-FSE-000037”, financed by Programa Operacional Norte 2020. This work was also supported by the European Research Council, grant agreement ERC-2012-ADG-20120216-321266 for the project ComplexiTE and the European Union H2020, grant number H2020-TWINN-2015-692333 – CHEM2NATURE.

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For Table of Contents Use Only

Synopsis This Perspective discusses developments and challenges in exploring cork as renewable source for chemicals and materials towards new sustainable products.

Graphical Abstract

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Ivo M. Aroso graduated in chemistry from the Faculty of Sciences of the University of Porto in 2003 and concluded post-grad studies in Processing and Characterization of Materials in the University of Minho in 2007. In 2005, he joined 3B’s Research Group (University of Minho, Portugal), where he has participated in several industry-funded and academic research projects. His main research interests are focused on the valorization of natural materials for different applications, including biomaterials for medical applications, active pharmaceutical ingredients (APIs) development, natural antioxidants for polymer processing, and deep eutectic solvents (DES).

Ana R. Araújo graduated in Chemical Engineering from the Instituto Superior de Engenharia do Porto in 2011. Starting in October of 2011, she has developed research work in the 3B’s research group, participating in several projects funded by the cork industry, with the focus on the development of new applications for cork sub-products and their potential application in the biomedical field. In 2015, she initiated her studies towards obtaining a Ph.D., under the scope development of biomedical applications from natural polyphenols extracted from cork. She aims to prove the capacity of these molecules as inhibitors of protein aggregation, a common cause of Neurodegenerative Disorders.

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Dr. Ricardo A. Pires graduated in Technological Chemistry from the Faculty of Sciences of the University of Lisbon (12/1998) and was awarded a Ph.D. in Materials Science and Engineering from the Technical Superior Institute from the Technical University of Lisbon (12/2004). He is currently an Assistant Researcher at the 3B’s Research Group, University of Minho, Braga, Portugal, where he been developing research activities for more than 10 years. His research is focused on the development of natural-based composite and self-assembling hydrogel systems for biomedical applications, as well as the exploitation of anti-oxidant activity of natural-based compounds, e.g. polyphenols, in the development of therapeutical strategies to tackle neurodegenerative disorders.

Professor Rui L. Reis, Ph.D., DSc, Hon. Causa MD, FBSE, FTERM, member of NAE, is the Vice-President for R&D of the University of Minho, Portugal, Director of the 3B’s Research Group and of the ICVS/3B´s Associate Laboratory of UMinho. He is also the CEO of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine, the Global President of the Tissue Engineering and Regenerative Medicine International Society (TERMIS) and the Editor-in-chief of the Journal of Tissue Engineering and Regenerative Medicine. He is a recognized World expert, with almost 1000 published works and more than 36 000 (h=94 according to Google Scholar) citations to his work, on biomaterials, natural origin polymers, tissue engineering and regenerative medicine. He has been awarded many international prizes and is the PI of projects with a budget totalizing more than 40 million Euros.

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Cork: Current Technological Developments and Future Perspectives

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