ORI GIN AL

Variability of the compression properties of cork

Vanda Oliveira • M. Emılia Rosa • Helena Pereira

Received: 26 November 2013 / Published online: 19 June 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract The variability of the compression properties of cork was determined

after field sampling covering the main production area in Portugal (10 sites) with

samples taken from 20 trees per site at the time of cork stripping. In all cases, cork

showed compressive stress–strain curves typical for cellular materials, which are

characterized by an approximately ‘‘elastic’’ region up to a 5 % strain, followed by a

large plateau up to 70 % strain caused by progressive buckling of cell walls, and a

steep stress increase for higher strains corresponding to cell collapse. The radial

direction of compression offered higher strength. The Young’s moduli averaged

10.4 and 9.2 MPa for radial and non-radial directions, respectively, spreading from

3.5 to 22.5 MPa for the non-radial and 4.2 to 21.5 MPa for the radial directions. The

geographical location of cork production was the major factor of variability. Den-

sity, annual growth ring width and chemical composition influenced compression.

Cork samples with higher density showed overall larger resistance to compression.

The energy absorbed per unit volume to achieve the maximal deformation with full

densification of cork is higher when the average annual ring width is smaller. Cork

samples with relatively higher suberin content required less stress for deformation.

The results encompass the natural variability of cork and are the most extensive to

characterize cork. They allow a better insight into the differences that may explain

the variation in cork properties and strengthen its use either in the known appli-

cations, i.e. as a sealant, or in novel applications.

V. Oliveira (&) � H. Pereira

Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade de Lisboa,

Tapada da Ajuda, 1349-017 Lisbon, Portugal

e-mail: vandaoliveira@isa.ulisboa.pt

M. E. Rosa

ICEMS-Instituto de Ciencia e Engenharia de Materiais e Superfıcies, Instituto Superior Tecnico,

Universidade de Lisboa, 1049-001 Lisbon, Portugal

123

Wood Sci Technol (2014) 48:937–948

DOI 10.1007/s00226-014-0651-2

Introduction

Cork is a natural cellular material characterized by an interesting combination of

properties that have been valued in research and in practical utilizations, e.g. low

density, reduced permeability to liquids and gases, chemical and biological inertia,

mechanical elasticity, and insulation properties (Pereira 2007). Of all the

mechanical properties, it is the compression behaviour that has attracted most

attention, triggered by the role of the compression properties in the world-renown

use of cork, which is sealing of wine bottles. The cork used for wine stoppers comes

from the cork oak (Quercus suber L.), a species distributed around the western

Mediterranean basin, with most cork-producing forests located in Portugal, followed

by Spain.

The compression behaviour of cork is largely explained by the features of its

cellular structure (Pereira et al. 1987). The three-dimensional structure of cork is

compact and may be described as an array of closed prismatic, on average

hexagonal cells stacked base-to-base making rows oriented in the radial direction of

the tree and assembled side by side, forming a honeycomb-type structure (Fig. 1). In

adjacent rows, the prism bases of neighbour cells most often lay in staggered

positions. The volume occupied by the solid cell walls is about 10 %. The cork cell

walls, especially those that constitute the lateral prism faces, show ab initio some

bending and undulations of varying intensity that can attain strong corrugation

derived from constraints during cork growth in the tree (Pereira et al. 1987; Fortes

and Rosa 1992). Cork also shows a layered structure, corresponding to the

biological annual rhythm of formation of cork rings: the cells formed in the main

growth period (earlycork cells) have more height and thinner walls than those

formed at the end of the growth period, which are smaller and thicker-walled

(latecork cells) (Pereira et al. 1987).

The compression of cork has first been characterized in general terms by Gibson

et al. (1981) and detailed later in several studies (e.g. Rosa and Fortes 1988a, b;

Fortes and Nogueira 1989; Rosa et al. 1990; Pereira et al. 1992; Mano 2002; Anjos

et al. 2008). The compressive stress–strain curves of cork have the characteristics of

Fig. 1 Schematicrepresentation of the cellularstructure of cork

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those of cellular foams: an approximately ‘‘elastic’’ region up to a strain of 5–7 %,

followed by a broad plateau with strong dimensional reductions for small stress

increases, and a sharp increase in stress starting at a strain of about 70 %

corresponding to the densification of the material. Full densification should occur at

about 85 % strain. Under compression, cork does not fracture neither across cells

nor across cell walls. Fracture only occurs when cork is submitted to tensile or

torsion stresses overcoming the material’s strength (Anjos et al. 2010, 2011a, b;

Rosa and Fortes 1991).

Although anisotropic, the difference in the compressive behaviour of cork in the

different directions is not very large, although strength in the radial direction is

higher than that in axial and tangential directions, which are more similar; for this

reason, frequently only two compression directions, radial and non-radial, are

considered as an approximation to a perfect hexagonal prismatic structure where

there is symmetry around the radial direction, as in Fig. 1.

The cell wall chemical composition of cork should also play a role in its

properties in compression. The distinctive features of cork are the presence of

suberin as the main cell wall structural component, amounting on average to 43 %,

and of lignin with 22 % (Pereira 1988, 2013). Therefore, the mechanical properties

of cork should be related mostly to the combined presence of these two polymers.

Their mechanical role should be different as a result of their contrasting spatial

development, as discussed by Pereira (2007, 2013): (a) lignin, a networked three-

dimensional aromatic polymer with rather isotropically distributed inter-monomeric

bonds, should be responsible for the resistance values under compression (Pereira

and Marques 1988); (b) suberin, a polymer with glyceridic-linked long-chain

aliphatic acids forming rather flexible planar ribbon-like structures, is the

preferential contributor to the elasticity and relaxation properties.

The natural variability of the compressive properties of cork has only been very

partially addressed, and most studies were made on only a few samples. The

variation in the cork properties between samples of known origin from different

production areas has not been investigated yet.

This is addressed in the present study where the compression properties of cork

were determined on a large number of samples that were collected in a field

sampling covering a large diversity of locations in the main production area in

Portugal. A total of 200 cork boards were studied from ten locations (20 corks per

location), allowing to cover between-tree and between-site variation.

The results in this study cover the natural variability of cork’s compressive

properties and correspond to the most extensive work so far undertaken to explain

and characterize the variation in cork compressive properties.

Materials and methods

Sampling

Cork samples were taken directly in the cork oak forests at different locations in the

major regions of cork production in Portugal. At each location, mature cork oak

Wood Sci Technol (2014) 48:937–948 939

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trees under production of reproduction cork were randomly selected (20 trees per

site) at the time of cork stripping excluding trees with visible phytosanitary damage.

A total of ten sites were sampled for the study of the compressive properties of cork.

The location of the sampling sites is shown in Fig. 2, superposed on a schematic

representation of the cork oak area.

The cork samples were taken at breast height (1.3 m above ground) from the cork

plank (reproduction cork) at the time of cork stripping and taken to the laboratory

where they were allowed to air-dry under well-ventilated conditions.

Sample preparation

A sample with approximately 20 9 20 cm2 was cut from the raw cork plank and

boiled in water at atmospheric pressure for 1 h and left to air-dry. This is a post-

harvest procedure that is made in the industry for all the raw reproduction cork

boards.

After equilibration in the laboratorial environment at about 20 �C and 80 % RH

to approximately 7 % mean moisture content, the cork thickness of the board was

measured in four positions in the radial direction, the total number of complete

annual rings was counted, and their radial width was determined, and a mean ring

width was calculated. The values obtained are indicated in Table 1.

Fig. 2 Schematicrepresentation of continentalPortugal including the cork oakarea distribution and the locationof the 10 cork sampling points

940 Wood Sci Technol (2014) 48:937–948

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The test specimens were cut from each cork sample as cubes with 18 mm edge,

oriented with the edges parallel to the radial, axial and tangential directions of cork.

Care was taken not to include non-cork tissues such as the outer layer of phloemic

lignocellulosic material (the so-called back of the cork plank) or conspicuous

defects, i.e. fractures or holes. The specimens were weighed, and density was

calculated. The moisture content of the specimens was confirmed to be of a mean

value of 7 %. The mechanical properties vary with moisture content (as compiled in

Pereira 2007), but most compression tests of cork use air-dried samples at 6–7 %

moisture content (e.g. Anjos et al. 2008; Pereira et al. 1992).

Compression tests were undertaken using a mechanical testing machine

Shimadzu AG-5000 at a constant crosshead speed of 2 mm min-1 (strain

rate = 2 9 10-3 s-1) up to a maximum load of 5,000 N. The compression axis

was, in different tests, parallel to the radial and non-radial (axial and tangential)

directions. Stress–strain curves and data were recorded. Young’s modulus (E, MPa)

was calculated from the average slope of the stress–strain curve between 1 % and

2 % strain. The strain for the maximal load (emax, %) was recorded as well as the

energy per unit volume absorbed at that strain (M, J cm-3). The stresses required for

strains of 20 and 30 % (r20 and r30, respectively) were measured. These strain

values correspond approximately to those occurring when a cork stopper is inserted

in a wine bottle and when it is compressed by the bottling machine prior to insertion.

Three replicate cubes were tested for each sample and compression direction.

Statistical analysis

To assess the main factors of variation in compression parameters, the results were

statistically analysed using a nested-factorial design analysis of variance with

direction (radial and non-radial) and site as fixed effects and trees as random effects

and nested within site. The analysis of variance was followed by a Tukey HSD

multiple comparison test to provide specific information on which means were

Table 1 Location of cork sampling sites (municipality and nearest town or village), cork oak perimeter

over cork at 1.3 m of height and annual growth ring width (mean of 20 trees and standard deviation)

Site Location Rainfall Temperature Over cork tree

perimeter

Cork mean

ring width

(municipality, town) (mm) (�C) (cm) (mm)

5A Abrantes, Alvega 688 15.5 150.0 (39.9) 3.8

5B Chamusca, Pinheiro 813 15.6 144.2 (23.7) 3.3

5C Benavente, Samora Correia 610 16.4 122.4 (27.6) 2.6

7A Ponte de Sor, Montargil 618 16.0 136.2 (34.8) 4.0

7B Avis, Cabecao 618 16.0 120.3 (19.2) 2.3

7C Coruche, Santana do Mar 618 16.0 123.2 (19.1) 3.0

9A Grandola, Canal Caveira 557 15.8 150.2 (37.8) 4.1

11B Santiago Cacem, S. Bartol. Serra 736 15.6 155.1 (49.8) 3.2

11C Odemira, Luzianes 614 15.0 135.6 (52.6) 3.4

14A Silves, Sao Marcos da Serra 1,138 16.2 148.9 (38.1) 3.2

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significantly different from each other. A variance components procedure for

mixed-effects nested model was applied to estimate the contribution of each effect

to the variance of the dependent variable.

The normality assumption for all independent variables was investigated by the

application of Kolmogorov–Smirnov test with Lilliefors significance correction and

analysis of skewness and kurtosis. Despite a few deviations from normality, it was

considered that the analysis of variance is robust and only slightly affected by them.

The equality of variances was validated with the Levene test.

It was considered that effects were statistically significant when the p value was

less than or equal to 0.05. All the statistical analysis were performed using SPSS�

statistical software (version 20.0; SPSS Inc., Chicago, IL).

Results and discussion

Cork compression properties

The compression behaviour of the cork samples is shown in Fig. 3 by the average

stress–strain curves for compression parallel to the radial and non-radial directions

for all the samples tested. In all cases, the curves followed the known pattern of an

approximately ‘‘elastic’’ region up to a strain of approximately 5 %, followed by a

large plateau with a small positive slope for strains between about 5–70 % and a

subsequent steep increase in stress for higher strains.

The large number of cork samples and compression tests that were made in this

work allow for firmly establishing the behaviour of cork in compression: a short

elastic region, a broad plateau and a densification region.

This behaviour has previously been described in studies involving the compres-

sion of cork and corresponds to the following phenomena for the cells: the ‘‘elastic’’

region of compression curve corresponds to the bending and compression of the cell

walls and is practically fully reversible; after this region, the plateau corresponds to

the buckling of cell walls that starts in a localized region of weaker cells and further

extends throughout the material; during densification, there is the collapse of cells

and compaction of the successive cell walls (Rosa et al. 1990; Rosa and Fortes

1988b; Gibson et al. 1981).

The overall mean, maximal and minimal values, standard deviation, and

coefficient of variation in the mean for the compression parameters of cork are

given in Table 2. Cork strength in the radial direction is higher with average stresses

of 1.2 and 1.5 MPa for strains of 20–30 %, respectively, in comparison with the 1.0

and 1.3 MPa for the non-radial directions. The Young’s moduli averaged 10.4 and

9.2 MPa for the radial and non-radial directions, respectively, with individual values

covering a large range spreading from 3.5 to 22.5 MPa for the non-radial direction

and 4.2 to 21.5 MPa for the radial direction.

The maximum strain was similar for compression in radial and non-radial

directions (at & 83 %), but the corresponding absorbed energy per unit volume was

slightly higher for the radial compression (2.4 vs. 2.2 J cm-3) as a consequence of

the higher strength of cork under radial compression, as seen in Fig. 3.

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The data reported in the literature for Young’s modulus are in the range of

10–20 MPa, and the same type of anisotropy between radial and non-radial

directions was also reported (Rosa et al. 1990; Rosa and Pereira 1994; Pereira et al.

1992; Anjos et al. 2008). The present work firmly establishes the mean Young’s

modulus of cork at the lower end of the reported values, although the individual

values cover a larger range (Table 2).

Cork strength in the radial direction is higher, especially for strains above 10 %,

as previously described (Rosa et al. 1990). It is noteworthy that the range of

variation is wide, especially regarding the energy consumed for the maximal strain

(M, J cm-3). However, in the case of the Young’s modulus, emax, r20 and r30

values, the distribution of values is rather concentrated, and the coefficients of

variation of the mean values are moderate (Table 2).

Variation in cork compression properties

The mean compression parameters of the cork samples collected from the ten

production sites are summarized in Table 3. The variation in cork properties

between samples of known origin from different production areas is reported here

for the first time.

The analysis of variance showed that the direction of compression is a

statistically significant factor of variation in all the compression variables, but there

was no interaction between production site and the direction of compression. The

anisotropic behaviour in the compression of cork is thus clearly revealed and is

independent of the cork origin.

The average Young’s modulus is higher for compression in the radial direction

independently of site: it ranged between 9.1 MPa (site 11B) and 12.3 MPa (site 7A)

in the radial direction and 8.0 MPa (site 5C) and 10.6 MPa (site 7C) in the non-

radial direction. The strain obtained for the maximal load (emax) was slightly higher,

while the energy consumed for that strain (M) was slightly lower for compression in

non-radial direction for all sites. The strain obtained for the maximal load for radial

compression varied from 81.8 % (site 5A) to 87.5 % (site 5C) and for non-radial

compression between 82.1 % (site 5A) and 87.8 % (site 5C). The highest M value

0

2

4

6

8

10

12

0 10 20 30 40 50 60 70 80

σ(M

Pa)

ε (%)

radial

non-radial

Fig. 3 Average stress–strain curves for the compression of cork in the radial and non-radial directions

Wood Sci Technol (2014) 48:937–948 943

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of 3.1 J cm-3 was found for radial compression of corks from site 5A, while the

lowest was 1.8 J cm-3 for non-radial compression of corks from site 9A.

The variation in cork compression properties was analysed separately for each

compression direction between corks from different trees in each production site

and between corks from different production sites. The analysis of variance showed

that the variation found for the compression parameters is largely explained by the

production site, although the tree within production site was statistically significant.

Table 2 Mean, maximal and minimal values, standard deviation (SD), and coefficient of variation (CV)

of the mean for the compression properties of cork: Young’s modulus (MPa), stresses required for 20 and

30 % strains (r20 and r30 respectively, MPa), strain for the maximal load (emax, %) and energy absorbed

per unit volume at that strain (M, J cm-3) (n = 600)

Mean Min–max SD CV (%)

Radial

E (MPa) 10.4 4.2–21.5 3.0 28.8

r20 (MPa) 1.2 0.7–1.9 0.3 25.3

r30 (MPa) 1.5 0.8–2.5 0.4 26.6

emax (%) 83.4 69.9–100.0 4.0 4.8

M (J cm-3) 2.4 1.3–13.2 1.0 42.2

Non-radial

E (MPa) 9.2 3.5–22.5 2.6 28.2

r20 (MPa) 1.0 0.6–2.0 0.3 28.6

r30 (MPa) 1.3 0.7–2.3 0.3 23.5

emax (%) 83.8 72.0–92.6 3.9 4.7

M (J cm-3) 2.2 1.2–12.0 1.1 50.1

Table 3 Mean compression parameters and standard deviation (in parentheses) for radial and non-radial

direction of the cork samples collected from the 10 sites of production: Young’s modulus (E, MPa), strain

for the maximal load (emax, %) and energy absorbed per unit volume at that strain (M, J cm-3) (n = 60)

Site E (MPa) M (J cm-3) emax (%)

Radial Non-radial Radial Non-radial Radial Non-radial

5A 9.6 (3.1) 9.1 (2.4) 3.1 (2.2) 3.0 (2.4) 81.8 (3.9) 82.1 (3.1)

5B 9.5 (2.7) 8.2 (2.6) 2.2 (0.6) 2.0 (0.8) 83.8 (3.7) 83.9 (5.0)

5C 9.7 (2.4) 8.0 (2.1) 2.2 (0.3) 2.0 (0.3) 87.5 (3.4) 87.8 (2.9)

7A 12.3 (3.6) 10.2 (3.7) 2.5 (0.7) 2.2 (0.6) 82.5 (4.4) 83.3 (3.6)

7B 9.7 (2.4) 9.9 (2.7) 2.8 (1.1) 2.8 (1.4) 83.5 (3.7) 83.9 (3.5)

7C 12.2 (2.2) 10.6 (1.9) 2.4 (0.5) 2.3 (0.7) 81.9 (3.8) 83.2 (3.4)

9A 11.5 (2.1) 9.3 (2.1) 2.0 (0.3) 1.8 (0.3) 82.2 (3.2) 82.3 (2.9)

11B 9.1 (2.3) 8.8 (2.1) 2.1 (0.3) 1.8 (0.3) 85.2 (2.5) 85.8 (2.5)

11C 10.0 (3.8) 7.9 (2.5) 2.2 (0.5) 1.9 (0.6) 83.4 (3.0) 83.7 (3.9)

14A 10.6 (2.7) 10.0 (2.2) 2.3 (1.0) 2.1 (0.7) 81.9 (4.2) 82.5 (4.1)

944 Wood Sci Technol (2014) 48:937–948

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Radial and non-radial Young’s moduli were explained in 93 % by the production

site. The group of production sites 5A, 5B, 5C, 11B and 11C presented non-radial

Young’s modulus lower than 9.1 MPa, while production sites 7A, 7C and 9A could

be grouped together and had a radial Young’s modulus higher than 11.5 MPa.

The production site also largely explained the variation in the energy absorbed

per unit volume at the strain obtained for the maximal load: 82.2 and 86.0 % for

non-radial and radial compression, respectively.

The differences between corks from the same production site can be perceived by

their corresponding coefficient of variation in the site mean (Table 3): it was very

small for the strain obtained for the maximal load (coefficient of variation of 4.2 %),

but higher for the Young’s modulus (26.6 %) and energy consumed (34.3 and

29.8 % for non-radial and radial compression, respectively).

Based on these results, it seems possible to have a provenance differentiation

using compression properties, i.e. a geographical differentiation of cork origin. The

major role of genetics in cork oak growth and cork formation has been repeatedly

stressed leading to large variation in cork features (Pereira 2007) that have so far

hindered origin differentiations using other characteristics, i.e. chemical composi-

tion (Pereira 2013).

Influence of physical and chemical characteristics

In the present study, the densities of the cork samples ranged from 0.13 to

0.38 g cm-3 and a correlation analysis was made with the compression properties.

The Young’s modulus had a significant and strong positive correlation with density

with Pearson’s coefficient of 0.34 and 0.46 for non-radial and radial compression,

respectively. The strain obtained for the maximal load (emax) has a significant and

strong negative correlation with density (-0.78 and -0.67 for non-radial and radial

compression), while between M and density, this correlation was strongly positive

(0.73 and 0.65 for non-radial and radial compression).

A linear model between emax and density could be adjusted (Fig. 4). Using non-

radial direction data, more than 60 % of the variation in the strain obtained for the

maximal load is explained by the model (R2 = 0.61).

The results showed that cork density influenced compression and the cork

samples with higher density showed overall larger resistance to compression, e.g.

Young’s modulus and the energy consumed to densification had an increasing trend

with density while densification tended to occur earlier (&75 %) (Fig. 4). A similar

trend of compression with density has previously been described by Anjos et al.

(2008).

Another factor of variation regards the thickness of the original cork plank or its

mean annual growth ring width, since it is related to cork density. The influence of

the mean annual ring width of the original cork plank on the compression was

therefore also investigated. It has previously been reported that cork samples from

planks with 2 to 4 mm ring width showed higher strength in compression than cork

samples with a mean 6 mm annual ring width (Pereira et al. 1992).

The mean ring width ranged in these samples from 1.63 to 6.15 mm, with an

average value of 3.37 mm and a coefficient of variation of 25 %. There was no

Wood Sci Technol (2014) 48:937–948 945

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correlation between the ring width with E and emax. Nonetheless, the energy

absorbed per unit volume at emax in the non-radial direction showed a significant and

negative linear correlation (r = -0.42, p = 0.000) with the annual ring width

(Fig. 5). Cork samples with the thinner annual growth rings require more energy to

achieve emax, while for ring widths above 3 mm M stays constant.

This could be explained by the fact that thinner cork planks, i.e. with small mean

ring width, comprise a higher proportion of small and thick-walled cells that will

require more energy for wall buckling and cell collapse (Pereira et al. 1987, 1992).

The number of the thick-walled cells (corresponding to the latecork cells formed at

the end of the annual growth season) is rather constant, so their effect loses

relevance for wide rings above 3 mm width.

As regards chemical composition, the relation between compression properties

and the combined presence of the two main cell wall structural components suberin

and lignin was investigated. The ratio of suberin-to-lignin (Sub:Lig) content in

extractive-free cork was calculated for 36 trees, for which the chemical composition

had been analysed in the lab and reported in Pereira (2013): this ratio had an average

value of 2.0, a standard deviation of 0.4 and a coefficient of variation of 18 %,

ranging from 3.2 to 1.0.

This chemical parameter is important since it relates the differing contribution of

suberin and lignin to the physical behaviour of the cells, i.e. compression strength

(Pereira 2013): lignin should be responsible for the mechanical strength under

compression, while suberin should be the preferential contributor to the elasticity

and relaxation properties (Pereira 2007).

A scatter plot of the stress required for deformations of 20 % (r20 %) in relation

to the ratio of suberin-to-lignin content (Fig. 6) shows a trend towards decreasing

stress required for deformation with a relatively increase in the relative suberin

content e.g. with the increase of Sub:Lig, even if a linear correlation was not

statistically significant (r = -0.39, p = 0.093). Scatter plots for the other

compression variables failed to show clear trends in data.

R² = 0.61

R² = 0.46

60

65

70

75

80

85

90

95

100

0.1 0.15 0.2 0.25 0.3 0.35 0.4

ε(%

)

Density (gcm-3)

non-radial

radial

Fig. 4 Effect of cork density on the strain obtained for the maximal load (emax, %) for compression innon-radial and radial directions

946 Wood Sci Technol (2014) 48:937–948

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These results suggest that the cellular characteristics of the cork certainly

superpose on the chemical features of the cells in complex interactions that are

difficult to single out.

Conclusion

1. The behaviour of cork in compression was firmly established in this work by

encompassing its natural variability: an elastic region up to a yield point of

about 5 % strain, a plateau with a small positive slope until about 70 % strain

and a densification with steeply increasing stress.

2. Cork shows higher strength in compression along the radial direction, which is

independent of the cork origin.

3. The geographical location of cork production was the major factor of variation

in compression features, allowing for considering them for cork provenance

differentiation.

R² = 0.55

0

2

4

6

8

10

0 1 2 3 4 5 6 7

M (

Jcm

-3)

Ring width (mm)

Fig. 5 Effect of the mean annual growth ring width on the energy absorbed per unit volume at the strainobtained for the maximal load (M, J cm-3) for compression in the non-radial direction

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0.8 1.3 1.8 2.3 2.8

σ20

%

suberin-to-lignin content

Fig. 6 Effect of suberin-to-lignin ratio of extractive-free cork on the stress required for strain 20 %(r20 %, MPa)

Wood Sci Technol (2014) 48:937–948 947

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4. Density, annual growth ring width and chemical composition influenced

compression. Cork samples with high density and small ring width showed

overall larger resistance to compression, while cork samples with a high

suberin-to-lignin ratio required less stress for deformation.

Acknowledgments The research was carried out under the framework of Centro de Estudos Florestais,

a research unit funded by Fundacao para a Ciencia e Tecnologia, Portugal (Pest OE/AGR/UI0239/2011)

and within project PAMAF 4053. Funding from Fundacao para a Ciencia e Tecnologia is acknowledged

by Vanda Oliveira as doctoral student (SFRH/BD/77550/2011). Thanks are due to Fernando Lopes who

designed and coordinated the sampling and to Alexandra Lauw for sample handling.

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