Enhancing the Thermal and Fungal Resistance of Wood Treated with

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Enhancing the thermal and fungal resistance of wood treated with natural and synthetic derived epoxy resins Liliana Rosu, Cristian-Dragos Varganici, Fanica R. Mustata, Teodora Rusu, Dan Rosu, Irina Rosca, Nita Tudorachi, and Carmen-Alice Teaca ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00331 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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ACS Sustainable Chemistry & Engineering

1

Enhancing the thermal and fungal resistance of wood treated with natural

2

and synthetic derived epoxy resins

3 4

Liliana Rosua, Cristian–Dragos Varganicia*, Fanica Mustatab, Teodora Rusua, Dan Rosua,

5

Irina Roscaa, Nita Tudorachib, Carmen–Alice Teacăa a

6 7 8

Centre of Advanced Research in Bionanoconjugates and Biopolymers, ”Petru Poni” Institute of Macromolecular Chemistry, 41A Gr. Ghica-Voda Alley, 700487 Iasi, Romania

b

”Petru Poni” Institute of Macromolecular Chemistry, 41A Gr. Ghica–Voda Alley, 700487 Iasi,

9

Romania

10 11 12

1

ABSTRACT

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The paper describes the obtaining of new epoxy derivatives and their influence on wood

14

thermal and fungi stability. Epoxy derivatives were thermally and photochemically crosslinked.

15

The softwood samples, modified with succinic anhydride, were impregnated individually or in

16

the mixture with glycidyl methacrylate, diglycidyl ether of bisphenol A, epoxidized grapeseed

17

oil and crosslinked afterwards. The samples impregnated with glycidyl methacrylate were

18

firstly exposed to UV at 254 nm 30 minutes on all surfaces and thermal crosslinking 5 hours at

19

130 oC and 30 minutes at 150 oC. The samples were characterized by thermogravimetric

20

analysis and decay resistance to fungi. The untreated samples exhibited the most significant

21

mass losses. Evolved gases analysis was undertaken. The sample containing epoxy grapeseed

22

oil exhibited a superior thermal stability and decay resistance to Penicillium chrysogenum and

23

Cladosporium cladosporioides.

24 25 26

KEYWORDS: wood; epoxy derivatives; thermal behavior; fungi resistance; anti decay

27 28 29 *

To whom correspondence should be adressed: Phone: +40 232 217 454 Fax: +40 232 211 299 E-mail address: [email protected]

ACS Paragon Plus Environment

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INTRODUCTION

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Wood is a natural renewable material, mainly composed of cellulose, hemicelluloses,

3

lignin and resins, used as both raw and processed material in industrial and domestic fields.

4

Since the ancient times wood has been used in tool crafting, construction, energy and

5

manufacturing of art objects. Besides the positive and useful mechanical, electrical and thermal

6

properties, wood also possesses some undesired properties (dimensional instability, degradation

7

under temperature, UV light, insects, humidity or microorganisms, etc.) which limit its

8

applications. Since wood represents a source of limited renewable raw materials, its quality

9

preservation and lifetime increase under environmental factors is strictly necessary. Several

10

methods have been implied in wood preservation, especially chemical modification or coating

11

with films from natural or synthetic materials.1-7 Vegetable oils are biodegradable and non–

12

toxic compounds mainly constituted from triglyceride esters of saturated and unsaturated fatty

13

acids. Depending on their chemical composition, the oils can be edible or non–edible. Based on

14

the degree of unsaturation of fatty acid moieties, these oils are non–drying, semi–drying and

15

drying.8 Oils possess in their chemical structure different functional groups (hydroxyl, epoxy

16

etc.) and up to three double bonds in fatty acid moieties, which can be transformed into

17

monomers.9 The most encountered chemical transformations of vegetable oils are the synthesis

18

of polyalcohols, polyacids or epoxy resins. The obtained epoxidized oils were used without

19

chemical modification or functionalized by opening the epoxy ring with anhydrides, α,β

20

unsaturated acids, allylic alcohols, amines, thiols etc.10-16 The use of fungicides and insecticides

21

to increase wood durability is limited due to the high toxicity to humans and the environment

22

and requires new substitution alternatives.17,18 Due to their hydrophobic properties, the

23

vegetable oils have been used to increase wood durability.19,20 The single oils provide sufficient

24

protection against fungi only coupled with various fungicides (boric acid, bio oil, copper

25

derivatives etc.).21,22


2

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1

In the present study, some novel chemical combinations based on polyfunctional

2

monomers, i.e. glycidyl methacrylate (GMA), diglycidyl ether of bisphenol A (DGEBA),

3

epoxidized grapeseed oil (EGSO), succinic anhydride (SA) as crosslinking agent, and salicylic

4

acid (SAc) as anti–decay agent, were used in the protection of wood. Firstly, the softwood was

5

modified with SA. Then the monomer mixture, containing hardener and photoinitiator, was

6

impregnated in the samples, followed by photo and thermal crosslinking. The modified wood

7

samples were tested from the physico–chemical, thermal behavior and decay resistance point of

8

view. In this sense, there were used the following characterizations: ATR–FTIR, solid state

9

CP/MAS 13C NMR, WAXD, SEM/EDAX and coupled TGA/DTG/DTA/FT–IR/MS.

10 11

RESULTS AND DISCUSSION

12

FT–IR characterization of SAW

13

As it may be observed from Figure S1a, the main FT–IR signals of W sample are

14

located at 3343 cm–1 (assigned to the OH groups), at 2922 and 2852 cm–1 (specific to CH, CH2

15

and CH3 groups), at 1730 cm–1 (assigned to carbonyl groups from lignin moieties), at 1660 cm–1

16

(attributed to water from wood), at 1507 cm–1 (assigned to the aromatic ring from lignin) and at

17

1022 cm–1 (assigned to C–O–C in carbohydrates).1 There are some differences between FT–IR

18

wood spectrum and that of sample treated with SA. Following the esterification of wood with

19

SA, the intensity absorbance band for hydroxyl groups (3343 cm–1) decreases and, based on the

20

chemical reaction presented in Scheme S1, a number of carboxyl groups appear with the signal

21

located at 1719 cm–1. This confirms that ester groups appeared and the hydroxyl groups will be

22

replaced with carboxyl groups. The absence of signals in the range 1300 – 1880 cm–1 for treated

23

samples also confirms the absence of free SA in the composite structure. On the other hand, the

24

new signal located at 1150 cm–1 may be attributed to C–O–C bridges in esters which appear

25

after succinylation reaction.

26


3


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1

13

2

Figure S2 shows the

3

(Figure S2c). In the W spectrum the most important signals are located between 20 and 120

4

ppm and are specific to cellulose and hemicellulose moieties. The signals located at 21.1 ppm

5

are specific to CH3–COO– groups from hemicelullose, while the signals located at 57.3 ppm are

6

assigned to –OCH3 from lignin. The signals between 60 and 80 ppm correspond to C2, C3, C5,

7

and C6 carbons from glucopyranose nuclei, and the ones from 84.5 and 89.5 ppm to the C4

8

carbon from the amorphous and crystalline areas. The signal at 105.5 ppm is attributed to C1

9

carbon. In addition, the weak signal from 172.5 ppm is due to carbonyl groups from

10

hemicellulose. In the spectrum of SAW (Figure S2b), new signals appear as a result of the

11

esterification reaction between OH groups from W with SA. The signals located at 63 and 65.8

12

ppm and at 75.2, 72.5 ppm are specific to C6 and C2, C3 carbons to which the hydroxyl groups

13

susceptible to esterification are attached. As a result of the esterification reaction, two new

14

signals appear at 29.6 and 175 ppm, specific to the C=O group from ester and –COOH groups

15

from succinic anhydride moieties.

C CP/MAS NMR characterization of SAW 13

C CP/MAS spectra of SA (Figure S2a), W (Figure S2b) and SAW

16 17

FT–IR and 1H–NMR characterization of EGSO

18

The EGSO was obtained by the reaction between GSO, glacial acetic acid and H2O2 in

19

the presence of H2SO4 as catalyst (Scheme S2). The resulted EGSO was a pale yellowish color

20

liquid with low viscosity. The chemical structure of the EGSO was confirmed by FT–IR and

21

1

22

located at 3010 cm–1 and at 1652 cm–1 specific to the olefin double bond from GSO and the

23

presence of the new signals at 845 cm–1 and 823 cm–1, attributed to the epoxide groups,

24

demonstrates the successful occurrence of the epoxidation reaction.

H–NMR analyses. In the FT–IR spectrum of EGSO (Figure S3b), the absence of the signals


4

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1

1

H–NMR spectrum of EGSO is presented in Figure S4. The main difference appeared between

2

1

H–NMR spectra of GSO (Figure S4a) and EGSO (Figure S4b) is the presence of the adjacent

3

protons from the oxiranic ring located at 2.83 and 3.011 ppm. The intensity of the olefin protons

4

from EGSO is lower in comparison with those from GSO.

5 6

FT–IR characterization of crosslinked polymers

7

The FT–IR spectrum of the crosslinked SAW/GMA (Figure S5a) shows the absence of any

8

signal at 915 cm–1 (specific to the epoxy ring) and presents one small shoulder located at 1640

9

cm–1, specific to methacrylic moieties. The disappearance of the signal at 915 cm–1 confirms the

10

addition reaction between epoxy and carboxylic groups. The absence of the peaks at 3010 cm–1

11

and 1640 cm–1 confirms the polymerization of methacrylic double bonds.5 The only difference

12

that occurs between the FT–IR spectrum of the crosslinked SAW/DGEBA/SA/SAc/EGSO

13

(Figure S5b) and that of the crosslinked SAW/GMA (Figure S5a) is the presence of a reduced

14

intensity signal near 915 cm–1. This can be attributed to traces of unreacted epoxy groups from

15

EGSO, a component in which some epoxy groups undergo steric hindrance in the chemical

16

structure of the oil.

17 18 19

Thermal analyses

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It is generally known that wood treating leads to a modification of its properties. The

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thermal analysis of the treated wood samples is aimed at establishing the thermal stability

22

changes generated through treatment. Figure 1 shows the typical TG and DTG thermograms for

23

the studied samples recorded at three heating rates under nitrogen atmosphere.


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1 2

Figure 1. TG and DTG experimental and regenerated curves (the symbols represent the

3

experimental values and the lines represent the calculated) recorded at () 5, ()10 and ()

4

20 oC min–1 for: (a) W, (b) SAW, (c) SAW/GMA and (d) SAW/DGEBA/SA/SAc/EGSO.

5 6

The domain up to 150 oC has not been shown. The shift of the TG curves towards higher

7

temperature values with increasing heating rates can be observed. This phenomenon was

8

explained by the fact that with the increase of heating rate the sample temperature is exceeded

9

by the furnace temperature, due to the thermal inertia.23,24 Some data extracted from TG

10

measurements, such as: T5% (temperature for 5% mass loss) Tpeak (temperature corresponding to

11

the maximum degradation rate), T30 (temperature for 30 % mass loss), T50 (temperature for

12

50 % mass loss), W (residue at 500 oC) and TGS (temperature for the maximum of Gram–

13

Schmidt profile in the IR spectrum) are shown in Table S1. The statistic heat resistant index (Ts)

14

was used to estimate the thermal stability of the samples using the following equation:25


6

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Ts = 0.49[T5% + 0.6(T30% − T5% )]

(1)

1

For the all samples, the main decomposition domain is located between 300 oC and 400 oC.

2

Based on the Ts values, it can be concluded that samples W, SAW and SAW/GMA have

3

comparable thermal stability, ranging between 108 oC and 114 oC. The highest thermal stability

4

was recorded for SAW/DGEBA/SA/SAc/EGSO sample (with values between 137 oC and 147

5

o

6

the residual mass measured at 500 oC (25.79 %).

7

The kinetic parameters of the thermal degradation reactions were evaluated firstly with the

8

Friedman and OFW isoconversional methods. The dependence of the activation energy of the

9

degree of conversion, were shown in Figure S6.

C), more than 30 oC higher than the other samples. All analyzed samples have close values for

10

The changes in the TG and DTG curve shapes and of the activation energy values are an

11

indication that the thermal degradation takes place following a complex reaction path, in at least

12

two steps.26,27 Taking into account the above conclusion, the multivariate non–linear regression

13

method was used to determine the reaction mechanisms and kinetic parameters specific to each

14

reaction step. In order to do this, the initial kinetic parameters obtained with the Friedman

15

equation were used. The differential equations associated with each kinetic model were

16

numerically solved in the range α = 0.1 – 0.9. The obtained results with the data extracted from

17

experimental curves were compared. Finally the optimum kinetic parameters were found.

18

Models with best values for F–test, Fit–Quality, and maximum correlation coefficient were

19

chosen. The optimal models for each sample consist in the following reaction sequences: A-1 → B-2 → C

(2)

A-1 → B-2 → C-3 → D

(3)

20

where: the reaction code type d:f; An, Fn is for W and SAW/GMA, d:f; An, An is for

21

SAW/DGEBA/SA/SAc/EGSO and reaction code type t:f,f; An, D3, Fn, is for SAW. In the

22

above codification d:,f; and t:f:f; are notations specific to “Thermokinetics 3“ software. The


7


Page 8 of 25

1

codification d:f; represents the two–step successive reaction mechanism and t:f,f; represent the

2

three–step successive reaction mechanism, while 1, 2, 3 denote the reaction steps. For the

3

process mechanisms d:f; A is the initial reactant, B is the intermediate products and C is

4

residue. For t:f,f; process mechanism A is the initial reactant, B and C are the intermediate

5

products and D is the final residue.

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The conversion functions chosen for one step are:

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– Avrami–Erofeev reaction model An: f(α) = n(1 – α)[ – ln(1 – α)]

8

– three–dimension diffusion (Jander’s type) D3: f(α) = 1.5(1 – α)0.333/[(1– α-0.333)

(5)

9

– reaction order nth model, Fn: f(α) = (1 – α)n

(6)

n −1 n

(4)

10

where n is the reaction order and α is the conversion degree. The non–isothermal kinetic and

11

regression parameters are presented in Table 1. Wood

SAW

SAW/GMA

SAW/DGEBA/ SA/SAc/ EGSO

d:f; An,Fn

t:f,f; An,D3,Fn

d:f; An,Fn

d:f; An,An

mechanism scheme

mechanism scheme

mechanism scheme

mechanism scheme

A-1 → B-2 → C

A-1 → B-2 → C -3 → D 297 26.21 0.258 323 24.51 293 23.31 3.11 0.341 0.285 1.00 1.20 1.958

A-1 → B-2 → C

A-1 → B-2 → C

204 15.86 0.281 244 18.12 0.188 0.563 1.00 1.20 1.958

173 12.88 0.505 135 7.71 0.183 0.329 1.00 1.19 1.958

parameters

E1 /kJ mol–1 log A1/s–1 n1 E2 / kJ mol–1 log A2/s–1 n2 E3 /kJ mol–1 log A3/s–1 n3 follReact 1 follReact 2 Fexp Fcrit–0.95. t–critical(0.95; 343)

213 16.54 0.463 209 15.23 1.489 0.439 1.00 1.20 1.958


8

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correl.–coeff.

0.999676

0.99961

0.999746

0.999091

E1, E2, E3 – activation energy of degradation for each step; log (A1, A2, A3) – pre–exponential factor for each step; n1, n2,n3 – reaction orders; follReact 1 share of reaction step 1 (A→B), follReact 2 share of reaction step 2 (B→C) and share of reaction step 3 (C→D) in the total mass loss, is given by 1−

∑ (follReact)

______________________________________________________________________________ 1

Table 1. Kinetic and statistic parameters determined after non–linear regression for the most

2

probable mechanism of thermal degradation process by applying a kinetic model in two and

3

three–steps, with consecutive reactions on the temperature interval of 150 – 450 oC.

4 5

The activation energy of thermal degradation obtained for W has values similar to those

6

reported in the literature.28 In case of the SAW/GEBA/SA/SAc/EGSO sample, which consists

7

of a polymer and wood mixture, this value is situated between the thermal decomposition

8

activation energy obtained for W and the values obtained for a similar polymer blend.13 Based

9

on the parameters from Table 1 and on the reaction mechanisms presented above, the

10

regenerated curves were calculated and shown in Figure 1. From Figure 1 it may be seen that

11

for the temperatures ranging from 150 to 450 oC, the regenerated curves fit very well the

12

experimental data, suggesting that the theoretical model most accurately approximates the real

13

thermal degradation process.

14 15 16

Evolved gases analysis The evolved gases identification is an additional method referring to the thermal

17

characterization of the modified wood samples.

FT–IR and MS techniques were used to

18

analyze the volatile products generated during the thermal degradation of the studied structures.

19

As an exemplification, Figures S7 a, b, c and d represent the 3D and 2D FT–IR spectra of the

20

volatiles evolved during thermal degradation of W and SAW/DGEBA/SA/SAc/EGSO. The 3D

21

spectra were recorded in the temperature range 100 – 650 oC while the 2D spectra correspond to

22

the peak temperatures from DTG curves. In the 2D spectrum of W sample (Figure S7c) the

23

most important signals are located at: 3729 cm–1, 3236 cm–1, 2972 cm–1, 2867 cm–1, 2806 cm–1, ACS Paragon Plus Environment

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2372 cm–1, 2291 cm–1, 1755 cm–1, 1666 cm–1, 1533 cm–1, 1471 cm–1, 1328 cm–1, 1263 cm–1,

2

1197 cm–1, 784 and 730 cm–1, while for sample SAW/DGEBA/SA/SAc/EGSO (Figure S7d)

3

these signals are situated at: 3720 cm–1, 3236 cm–1, 2943 cm–1, 2834 cm–1, 2345 cm–1, 1726 cm–

4

1

5

according to literature data.29 Based on the chemical structure of wood, it can be considered that

6

1–4 bonds between D–glucopyranose nuclei and CH2–OH groups attached to the glycosidic

7

ring are the most susceptible to degradation.30 Thus, in a first instance, in nitrogen atmosphere,

8

water and alcohols are released by cleavage of hydroxyl groups, accompanied by cellulose

9

depolymerization through scission of glycosidic bonds as a result of heating. The signals

10

assigned to water vapours and alcohols are located at 3729 cm–1, while the band from 3236 cm–1

11

may be assigned to secondary alcohols and phenols derived from lignin moieties.31 The signals

12

from 2972 cm–1, 2867 cm–1, 2806 cm–1 and 1328 cm–1 are specific to the valence vibration of

13

CH, CH2 and CH3 groups from methane, ethane and propane. The absortion bands from 2372

14

cm–1 and 2291 cm–1 are characteristic to carbon monoxide and dioxide.32,33 By cleavage of the

15

1–4–glycosidic linkages from the D–glucopyranose rings and the recombination of the obtained

16

glucosan moieties, levoglucosan is generated. With temperature increase, in the presence of OH

17

radicals and water, levoglucosan is transformed in aldehydes, ketones and carboxylic acids.30

18

The signal specific to the C=O groups can be identified at 1755 cm–1. The absorption bands

19

located in the range of 1470 – 1540 cm–1 can be assigned to aromatic compounds which appear

20

as a result of lignin moieties degradation. Also, the peaks at 1263 and 1197 cm–1 are assigned to

21

the volatile substances containing etheric bonds (C–O–C). The notable difference between FT–

22

IR spectra of W and SAW/DGEBA/SA/SAc/EGSO samples consists in the fact that in the latter

23

case a much larger quantity of carbon dioxide has evolved. A possible explanation would be

24

that the sample containing polymer leads to an increase in the amount of carbon oxides during

25

degradation.

, 1446 cm–1, 1232 cm–1, 1027 cm–1 and 667 cm–1. The signals significance was established


10

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1

At high temperatures there occur condensation reactions, resulting tars containing highly

2

condensed aromatic nuclei. Finally, a carbonization phenomenon occurs.

3

The mass spectra interpretation was correlated with the registered FT–IR spectra. Figure S8

4

shows the MS spectra of the gases obtained during the thermal degradation of W and

5

SAW/DGEBA/SA/SAc/EGSO. The main signals are located up to 100 amu values. The ionic

6

fragments specific to water traces are located at m/z values from 18, 17 and 16. The m/z signals

7

from 44, 28, 16 and 12 were assigned to carbon dioxide, while the m/z values of 28 and 12

8

belong to carbon monoxide. Some signals with m/z values up to 44 amu may be assigned to

9

saturated aliphatic hydrocarbons, such as methane (m/z = 16, 15, 14, 13, 12), ethane (m/z = 30,

10

29, 28, 27, 26, 15), propane (m/z = 44, 43, 39, 29). The fragments resulted by breaking of 1–

11

4–glycosidic bonds are: formaldehyde (m/z = 30, 29, 28, 15, 12), 2–propenal (m/z = 56, 55, 37,

12

29, 28, 27, 26, 25), acetone (m/z = 58, 43, 42, 27, 15), formic acid (m/z = 46, 45, 42, 29, 28, 17)

13

and acetic acid (m/z = 60, 45, 43, 42, 29, 15). For both samples, and taking into account the

14

registered FT–IR spectra, the signals greater than 60 amu can be assigned to the aromatic

15

structures resulting from thermal degradation of lignin [benzene (m/z = 78, 77, 50, 39), toluene

16

(m/z = 92, 91, 65, 64, 51, 39), phenol (m/z = 94, 66, 65, 39), furan (m/z = 68, 49, 39, 28, 29), 2–

17

methylfuran (m/z = 82, 81, 53, 39, 27), 2–furanone (m/z = 84, 55, 54, 39, 29, 27)]. Such

18

gaseous products have also been identified at the degradation of other thermosetting resins.13,34

19 20

Resistance to decay

21

After 8 weeks of exposure to C. cladosporioides and P. chrysogenum the control samples (W)

22

were completely covered by the fungi, while the treated samples were less or not covered at all.

23

(Figure 2).


11


Page 12 of 25

1 2

Figure

3

SAW/DGEBA/SA/SAc/EGSO

4

SAW/DGEBA/SA/SAc/EGSO exposed to P. chrysogenum; (c) W and SAW/GMA exposed to

5

C. cladosporioides, (d) W and SAW/GMA exposed to P. chrysogenum; (e) W and

6


7

SAW/DGEBA/SA/SAc/EGSO before the exposure to P. chrysogenum; (g) W and SAW/GMA

8

before the exposure to C. cladosporioides and (h) W and SAW/GMA before the exposure to P.

9

chrysogenum

2.

Photographs

of

raw

wood

exposed

before

to

exposure

and

treated

C.

cladosporioides;

to

C.

samples:

(a) (b)

cladosporioides;

(f)

W

and

W

and

W

and

10

The least covered sample by C. cladosporioides is SAW/DGEBA/SA/SAc/EGSO

11

(Figure 2a), while the same sample inoculated with P. chrysogenum is the most covered (Figure

12

2b). It may also be seen that SAW/GMA (Figures 2c, d) exhibits good resistance to both fungi.

13

The mass loss after fungi action was, given in Table 2, are higher for undecayed wood, while

14

for the composites, this loss is 28.5 % lower for P. chrysogenum treated samples and 35 %

15

lower for C. cladosporioides, depending on the coating type. The moisture content is also

16

affected. For example, the SAW/GMA sample presented a 13 % dropping under P.

17

chrysogenum action and 50 % under C. cladosporioides.


12

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1

mean weight lossa humiditya, sample (%) (%) P. C. P. C. chrysogenum cladosporioides chrysogenum cladosporioides W 9.17 9.65 37 45.5 SAW/GMA 6.4 6.3 33 23 SAW/DGEBA/EGSO/SA/SAc 6.6 6.2 32 41 a) Means were for 5 samples Table 2. Some characteristic of untreated and treated sample after 8 week exposure against C.

2

cladosporioides and P. chrysogenum fungi.

3

FT–IR investigation of the samples exposed to fungi attack

4

The FT–IR spectra for the undecayed and decayed wood samples accompanied by the

5

difference between them, recorded after 8 weeks exposure time, are presented in Figure S9. All

6

FT–IR spectra were normalized at the area of the whole spectrum. Positive signals in the

7

difference spectra show the functional groups consumed, while the negative signals indicate the

8

new structures resulting under the action of the fungi. Unmodified functional groups do not

9

appear in the difference spectra. From Figure S9 significant differences can be observed

10

between decayed and undecayed samples. Under the fungi action it may be noticed the increase

11

in the content of hydroxyl groups, possibly due to the depolymerization of cellulose and

12

hemicelluloses. The negative signals from 3334 cm–1 (sample exposed to C. cladosporioides)

13

and 3257 cm–1 (sample exposed to P. chrysogenum) confirm this aspect. The increase in the

14

–OH groups content is accompanied by the enhancement in the amount of absorbed water,

15

confirmed by the negative peaks located at 3787 cm–1 (sample exposed to C. cladosporioides)

16

and 3730 cm-1 (sample exposed to P. chrysogenum).29 The new signals from 1103 cm–1 and

17

1030 cm–1 (sample exposed to C. cladosporioides) and 1032 cm–1 (sample exposed to P.

18

chrysogenum) confirm the presence of the new OH groups, specific to the deformation

19

vibrations of C–OH bonds and the stretching of >C–OH and –CH2–OH linkages from cellulose.

20

35

21

cellulose, hemicellulose and lignin by the fungi. Thus, peaks located around 2926 cm–1, 2855

The positive signals in the difference spectra reflect the consumption of some quantities of


13


Page 14 of 25

1

cm–1, 1434 cm–1 confirm the consumption of CH2 and CH3 groups from wood components. The

2

signals around the 1739 cm–1, specific to C=O groups from lignin (acetyl groups) and carbonyl

3

groups from hemicelluloses, also show a content decrease in these components. The

4

consumption of lignin is highlighted by the signal from 1546 cm–1, specific to bonds vibration

5

between carbon atoms from aromatic ring. The degradation of the ether bonds specific to

6

cellulosic rings and hemicellulose is confirmed by the peak located around 1164 cm–1.

7

From Figure S10 (SAW/DGEBA/SA/SAc/EGSO sample) one may observe fewer modifications

8

in the FT–IR difference spectra of impregnated samples as compared those of initial non–

9

treated wood samples. The peaks, specific to OH groups, at 3344 cm–1 , 1058 cm–1 and 1011

10

cm–1 (decayed by C. cladosporioides) and at 3349 cm–1 , 1103 cm–1 and 1011 cm–1 (decayed by

11

P. chrysogenum) decreased. This is a consequence of the protective coating effect. The negative

12

signals in the difference spectrum suggest a slight contamination of the surface samples with

13

fungus.

14

In Figure S11 the difference spectra for decayed and undecayed SAW/GMA sample under the

15

action of the two fungi are presented. Very small values of the signals in the difference spectra

16

are an indication of the coating effectiveness against fungi action.

17 18

WAXD characterization

19

Figure 3 shows the WAXD diffractograms of the treated and untreated samples recorded

20

before and after the decay tests. The undecayed samples present a major diffraction plane,

21

namely 002 in the range 22.6 – 22.9 degree, characteristic to the crystalline cellulose. If one

22

compares curve 3a (undecayed wood) with the curves 3e (decayed in the presence of C.

23

cladosporioides) and 3f (decayed in the presence of P. chrysogenum), it can be seen their

24

flattening in the range of 10 – 20 degree, as well as the fact that the decayed samples present a

25

decreasing in the intensity of the plane 002. This is caused by the exposure to fungi action when


14

Page 15 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

the cellulose and hemicelluloses depolymerization takes place and the thickness of the

2

crystalline cell decreases and crystalline packaging defects occur (Table 3).36 As one may

3

observe from Table 3, the decrease in the crystallinity index, as a result of fungal exposure, is

4

more pronounced in unprotected wood samples (from 75.4 % to 62.8 %) compared to

5

SAW/GMA sample (from 62.6 % to 61.5 %) and with SAW/DGEBA/SA/SAc/EGSO sample

6

(from 62.4 % to 61.4 %) as a result of blocking the access of the fungi to wood.

7 8 9 10 11 12 13 14

Figure 3. WAXD curves of undecayed: (a) W; (b) SAW; (c) SAW/GMA; (d) SAW/DGEBA/SA/SAc/EGSO and decayed 8 weeks: (e) W in the presence of C. cladosporioides; (f) W in the presence of P. chrysogenum; (g) SAW/GMA in the presence of C. cladosporioides; (h) SAW/GMA in the presence of P. chrysogenum; (i) SAW/DGEBA/SA/SAc/EGSO in the presence of C. cladosporioides; (j) SAW/DGEBA/SA/SAc/EGSO in the presence of P. chrysogenum. crystalline index (%) sample undecayed decayed with C.

W

SAW

SAW/GMA


75.4

61.5

62.6

62.4

69

-

62.6

62.3

62.8

-

61.5

61.4

cladosporioides decayed with P. chrysogenum 15 16

Table 3. Crystalline index


15


Page 16 of 25

1 2

SEM and EDAX measurements

3

The SEM measurements are presented in Figure 4 where samples surfaces were shown

4

in section. The SEM examination was performed on specimens of 1 mm thickness cut from the

5

sample. In Figure 4 the samples decayed with C. cladosporioides [(a) W, (b)

6

SAW/DGEBA/SA/SAc/EGSO, (c) SAW/GMA] and decayed with P. chrysogenum [(d) W, (e)

7

SAW/DGEBA/SA/SAc/EGSO, (f) SAW/GMA] are presented. From the micrographs

8

examination it can be seen that the structure of the unprotected wood samples is strongly

9

affected by the existence of the fungi hives, while the protected porous wood structure (m) is

10

preserved. The changes of the elemental chemical composition on the untreated and treated

11

surface samples before and after biological decay were identified using EDX analysis. As it

12

may be seen from Table S2, the percentage of carbon for the W decayed sample decreases

13

compared to the undecayed control sample, while the percentage of oxygen increases. As it may

14

be seen from Table S2, the percentage of carbon for the SAW/GMA sample increases compared

15

to the control sample, while the percentage of oxygen also increases, since GMA brings extra

16

quantities of oxygen. In the case of SAW/DGEBA/SA/SAc/EGSO sample, the chemical

17

composition of the coating is less rich in oxygen but richer in carbon, fact confirmed by the

18

percentages of measured carbon and oxygen. Also, the justification of the decrease in the other

19

elements can be taken into account considering the increasing of the quantities of coating

20

products. It can be observed an increase in the amounts of Cl in the case of coated samples,

21

which may be a consequence of the presence of residual Cl quantities in the composition of

22

GMA and DGEBA monomers.

23

As a result of the fungi attack, a significant decrease in the percentage of carbon (11 % under

24

the action of C. cladosporioides and 5.7 % under the action of P. chrysogenum) occurs for the

25

blank sample (W). Sample SAW/GMA does not exhibit significant mass increase (2.4 % under


16

Page 17 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

the action of C. cladosporioides and 1.6 % under the action of P. chrysogenum), while the

2

sample SAW/DGEBA/SA/SAc/EGSO shows an increase of 6.4 % under the action of C.

3

cladosporioides and 7.6 % under the action of P. chrysogenum. These changes are confirmed by

4

the FT–IR spectra (Figures S9 – S11).

5 6

Figure 4. SEM micrographs in cross–section of wood undecayed sample (m) and decayed in

7

the presence of C. cladosporioides: (a) W; (b) SAW/DGEBA/SA/SAc/EGSO; (c) SAW/GMA

8

and decayed in the presence of P. chrysogenum: (d) W; (e) SAW/DGEBA/SA/SAc/EGSO; (f)

9

SAW/GMA

10

All the studied samples show increases in the percentage of nitrogen. These increases range

11

from 122 % for the wood specimen exposed to C. cladosporioides and 79 % for the wood

12

exposed to P. chrysogenum. The SAW/GMA sample shows a 91 % increase in the presence of

13

C.

cladosporioides

and

24

%

in

the

presence

of

P.

chrysogenum.

The


17


Page 18 of 25

1

SAW/DGEBA/SA/SAc/EGSO sample shows an increase of 92 % for C. cladosporioides and

2

59 % for P. chrysogenum. Calculus was performed using the carbon and nitrogen initial values

3

(Table S2).

4 5

CONCLUSIONS

6

New protection materials based on epoxy derivatives from natural and synthetic materials have

7

been obtained, applied on wood and tested in terms of the influence on two independent

8

properties: the thermal stability and fungi resistance. The coatings obtained by immersion of the

9

wood or wood samples modified with SA, in formulations based on DGEBA, EGSO or GMA,

10

together with a potential antifungal agent (SAc) were photo–crosslinked under UV light action

11

and thermally cured. The most probable mechanisms of the thermal degradation process for raw

12

and coated wood samples occurred in two or three steps. The decay resistance was tested by

13

exposure to P. chrysogenum fungi and brown rot fungus C. cladosporioides and monitored

14

using WAXD, FT–IR and SEM/EDAX methods. The mass loss after fungi action was higher

15

for undecayed wood, while for the composites, this loss was 28.5 % lower for P. chrysogenum

16

treated samples and 35 % lower for C. cladosporioides, depending on the coating type. The

17

moisture was also affected, for example, SAW/GMA sample presented a 13 % dropping under

18

P. chrysogenum action and 50 % under C. cladosporioides. The modifications in relative

19

crystallinity, chemical composition of wood and protected wood after decay indicated

20

significant changes to unprotected wood as compared to the protected one. From the

21

SEM/EDAX observations resulted that the coatings made with the two compositions protect the

22

wood lumen by blocking the access of the fungi to the wood components. The control samples

23

were strongly affected, the fungi attacking all wood components.

24 25

MATERIALS AND METHODS

26

Materials and methods are presented in Supporting Information.


18

Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

ASSOCIATED CONTENT

3

Supporting Information

4

The Supporting Information is available free of charge on the ACS Publications website.

5

Description of the used materials and methods, wood esterification with succinic anhydride

6

(SAW), synthesis of epoxidized grape seed oil (EGSO), samples preparation, testing against

7

decay, schemes indicating the obtaining of SAW, EGSO and probable crosslinking reactions,

8

13

9

crosslinked SAW/GMA and SAW/DGEBA/SA/SAc/EGSO, 1H–NMR spectra of GSO and

10

EGSO, description of the dependence of Ea and logA on the conversion degree for W, SAW,

11

SAW/GMA and SAW/DGEBA/SA/SAc/EGSO, 3D and 2D FT–IR and MS spectra of the gases

12

that appear at the degradation of W and SAW/DGEBA/SA/SAc/EGSO, FT–IR spectra of

13

undecayed W, SAW/DGEBA/SA/SAc/EGSO and SAW/GMA samples and decayed 8 weeks

14

with C. cladosporioides and P. chrysogenum and comparative difference spectra, table

15

containing data extracted from TG curves recorded at three heating rates, table describing

16

SEM/EDAX composition of samples.

C CP/MAS spectra of SA, W and SAW samples, FT–IR spectra of W, SAW, GSO, EGSO,

17 18

ACKNOWLEDGMENTS

19

This work was supported by a grant of the Romanian National Authority for Scientific Research

20

and Innovation, CCCDI–UEFISCDI project number ERANET-ERA IB 2–ProWood, within

21

PNCDI III.

22

Authors are grateful to Dr. Xenia Filip of the National Institute for Research and Development

23

of Isotopic and Molecular Technologies in Cluj–Napoca, Romania, for the

24

and to Chem. Elena Marlica of the ”Petru Poni” Institute of Macromolecular Chemistry in Iasi,

25

Romania, for the FTIR and WAXD measuremets.

13

C NMR spectra


19


Page 20 of 25

1 2

CONFLICT OF INTEREST

3

The authors declare no conflict of interest.

4 5

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New epoxidized grapeseed oil based wood coatings exhibit greater resistance to C. cladosporioides by comparison to the untreated wood 129x78mm (300 x 300 DPI)


Enhancing the Thermal and Fungal Resistance of Wood Treated with

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