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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.48 no.4 Concepción Dec. 2003

http://dx.doi.org/10.4067/S0717-97072003000400009 

J. Chil. Chem. Soc., 48, N 4 (2003) ISSN 0717-9324

SUGAR-LIGNOCELLULOSIC COMPOSITES: THE INCORPORATION OF TWO SIMPLE SACCHARIDES INTO MOULDING AS ADDITIVES

Cristian Moya Villablanca1 and Nazmy Reyes Velásquez2

1Faculty of Forest Sciences, Universidad de Concepción. Concepción, Chile. e.mail: cmoya@udec.cl
2 Faculty of Chemistry Sciences. Department of Organic Chemistry, Universidad de Concepción, Concepción, Chile. e-mail: nreyes@udec.cl
(Received: December 30, 2002 - Accepted: August 11, 2003)


ABSTRACT

Wood veneer of pine (Pinus radiata D. Don) and eucalyptus (Eucaliptus globulus ssp. globulus Labill) were analyzed by means of Dynamic Mechanical Analysis (DMA). Previously, they were saturated with buffer phosphate (pH=4), fructose and maltose (2%) solutions. The viscoelastic behavior and temperatures of relaxation of wood in both species were differents, the effect was attributed to its chemical difference. This behavior is changed in both species by sugars presence generating plasticizing effects, being more marked in presence of maltose than fructose. Between species, the pine wood presents the most important changes of its viscoelastic properties. With this information, molding samples from both species, made with mixture of particles (0.25 mm) of bark (49%), wood (49%) and sugars 2%, were obtained using pressure and temperature controlled. They were essayed for determinate the internal bond considering the Modulus of rupture (MOR). According to Multiple Linear Regression Analyses (MLR), with a 95% of confidence, the variation of the MOR is explained in 68% of the cases by the particular specie, wood and bark, moisture content, time of impregnating and type of sugar. The large values of the MOR are produced by a greater amount of hydroxyls groups contributed by maltose producing the greater plasticizing effect and the greater union between particled lignocellulosic materials. The minor water content generated the greater MOR, this is attributed to its plastizicing effect, it means that dependency of the water content under the saturation point of the fiber is not directly proportional to the plastizicing effect.

Keywords: DMA, sugars, pine, eucalyptus, MOR.

INTRODUCTION

Nowadays, exists an increasing interest in using chemical additives like filler or modifiers to obtain compounds that allow to develop different types of products. These additives tend to be of natural origin and their current use is due to their low costs, environmental requirements and the need for sustainability of the resources. Johnsons S.1 and his colleagues made tests with carbohydrates derived from cereals as additives for the production of urea formaldehyde or phenol formaldehyde (U-f or F-f ) adhesives. They were using carbohydrates to obtain plywood through pressure and temperature. The internal bond (IB) of these panels is improved with an important reduction of costs, replacing up to 20% in weight the resins used. Moreover, in the European Union, other investigators2 have designed composites from wood particles (50% approx.), mixed with carbohydrates as such starch and sugars to obtain useful moldings. In these processes of transformation, the viscoelastic behavior of the materials is the most important factor for its suitable transformations. This behavior depends on the compositional and morphological characteristics of the components and of the molecular interactions generated under particular experimental conditions3, 4, 5.

The chemical difference between the species studied in this work is marked, mainly when related with the amorphous components. Considering similar quantity of semi-amorphous and crystalline cellulose, late pine wood presents a greater amount of lignin than early pine wood and eucalyptus wood. Both pine woods are composed by residues type guayacil, while the eucalyptus wood presents residues type guayacil-syringil in the same proportions and eucalyptus wood presents the greater amount of hemicellulose than early and late pine wood, it is mainly composed of carbohydrates of C5, while pine wood is composed mainly by carbohydrates of C66, 7. On the other hand, the bark presents the same differences as in both species of wood, but they present a minor molecular mass. Also, the bark presents tannins which are differentiated by type and amount. Eucalyptus bark presents a minor amount of tannins than pine bark, and mainly tannins of the type hydrolyzable, while pine bark presents condensed tannins8, 9, 10.

The wood shows a low termoplasticity, due to its structural characteristics and a great molecular mass5, 6. Nevertheless, one of the factors that allow to modify its viscoelastic behavior is pH. In both conditions, acid or alkaline, processes of relaxation due to the change of interactions between the structural components of the material are favored7. Also the water content modifies behavior by acting like a plasticizing agent, decreasing the interactions between the present macromolecular components in the wood 11, 12, 13. In addition, the moisture content improves the mass transference and acts like means promoting chemical changes in the matrix14. The bark as well show phenomenon related with the pH, moisture content and temperature. The cure phenomenon observed is attributed to tannins present15, 16, 17.

Besides, simple saccharides can interact with some lignocellulosic components, promoting chemical modification in situ. A. Hamilton18 and G. Allan19, 20, 21 studied the behavior of these compounds in pulps delignificated and pulp non-dried, under certain humidity and temperature conditions. They showed a great capacity of hydrogen bonds with present carbohydrates into material. The nature of these interactions is governed by the stereochemistry and structural similarity between these carbohydrates and the present into the matrix. In addition, interactions or formation of hydrogen bonds, due to the high number of hydroxyls groups, can generate calculated enthalpy of the bonds being of the same order of magnitude as of the enthalpy of the C-C and C=O covalent bonds, which form the backbone of cellulose polymer. This behavior is related to the molecular mass, because a greater amount of hydrogen bonds is obtained with saccharides from high molecular mass.

In solution, the monosacharides are presented in equal proportions of open chain and cyclical forms. Without regard of their conformation, they can participate in reactions with alcohol, ketones and aldehydes. The disaccharides need initially, to break the glycosidic bond to obtain the corresponding monomer and the later ring opening or form cyclic22, 23. In general, temperature (120ºC) and pH (acid or alkaline) favor the reaction knowledge like caramelization reactions, they are formed by enolization, dehydration by b -elimination, dicarboxylic cleavage, retro-aldol, condensation aldol and radical reactions and they depend on type of sugar. Nevertheless, the sugars in alkaline conditions generate undesirable reactions for their analysis22. Bean and Osman24 determined the influence of the structure of several simple saccharides on the viscous behavior through the time, under the same conditions of temperature and pH. The disaccharides needed a greater time than the monosacharides to obtain their maximal viscosity. Also the affinity with water depends on the type of sugar, being more soluble fructose than maltose. These differences are related mainly to the molecular mass, structures and sterical hinderings between involved molecules25, 26. Due to this, sugars with differences in structural and molecular mass can interact differently with the same lignocellulosic specie, influencing in their elasticity and temperature of relaxation.

In this work, the influence of the structural difference and molecular mass of fructose and maltose on the viscoelastics properties of the wood of pine late and early wood and eucalyptus have been evaluated by means of DMA. The parameters analized were considering the changes of elastic component (Storage Modulus, E') and Tan Delta (E''/E'), where E’’ corresponds to the Loss Modulus who represents the viscous component3, 27. With the collected information, molding samples from particles of both wood species, including bark and sugars, were obtained by pressure and temperature and submitted to mechanical test to determinate the internal bond (IB) considering the MOR. By mean MLR a general model is generated to determine the influence of the type of species, both bark and wood, water content, time of impregnating and presence of type of sugars, on the MOR.

EXPERIMENTAL

Materials

- DMA: Veneers wood of Pinus radiata D. Don (radiata pine) separated into earlywood and latewood, and Eucaliptus globulus ssp globulus Labill (eucalyptus) were obtained from Tulsa S.A. and Colcura S.A., VIII Region, Chile. The eucalyptus veneers dimensions used in DMA, obtained by foliation, were of 0.8 X 13 X 35 mm. The pine veneers dimensions, obtained by shear roller, were of 2.4 X 13 X 35 mm.

- Samples of Composites Processing: Wood and bark particles were obtained from radiata pine and eucalyptus sawdust generated in processes of sawmill of Tulsa S.A. and Colcura S.A., companies of the VIII Region, Chile. The sawdust and the pieces of bark were dried for five days under atmosphere no forced, until reaching a equilibrium humidity. Late they were cut to obtain their final sizes, 0.25 mm. From this waste, they are not separated into late and early wood.

Equipment

- DMA: DMA 2980 of the TA Instruments, interfaced to computer through OS2/WARP V.3.0 program was used to obtain the data on the changes in the viscoelastics behavior of veneers of both species, with and without treatment of wood.

- Sizing of particled material: To diminish the size of the particles of the pieces of bark and sawdust, was used a three knives RestchMühle laboratory mill at 1390 r.p.m. The generated particles were separated using a Janke & Kunkel (IKA) shaker model KS 500, using 300 oscillations per minutes during 60 minutes. The particles separated through an Endecotts mesh of 0.25 mm.

- Obtaining of molding samples: The samples were obtained by hot pressing, using a plate press Blümel of 10 X 10 cm.

- Mechanical tests: MOR in parallel traction to the longitudinal axis of the test tubes by means Frank Universal tester Model 305001723 was determined. The samples obtained corresponded to make the one by hot pressing which were of dimension 10 X 10 X 0.5 cm.

Methods

- DMA: The early and late pine wood veneers and eucalyptus wood veneers were analyzed in single cantilever mode to frequency of 1 Hz, 20m m of oscillation amplitude and heating of 5ºC/min from room temperature to 200º C, according to ASTM D 4065-9528. These veneers were impregnated previously by 24 hours to Pa using water, fructose and maltose diluted to 2% in phosphate buffer solutions (pH = 4.0).

- Preparation of particled material: Initially, pieces of bark and sawdust were dried through air conditions without direct light during 5 days until obtaining a equilibrium humidity of 10-12 % base dry weight (bdw), to avoid desfibration. Later, they were re-dimensioned in the mill obtaining particles that were sifted and separated to 0.25 mm. The particle size was determined in a previous optimization29.

- Preparation of mixture: 32 samples were obtained from mixing particles of bark and wood considering each specie and mixture between species, and each sugar, fructose or maltose (49:49:2), with 10 or 20% of moisture content in acidic conditions. Half of the total samples were impregnated during 24hours and the other half, during 48hours, see Table 1.


Table 1.- Mixture of bark and wood treated with the different processes.


*Mixture

Time of
Impregnating (h)

Moisture Con
tent (%pbs)

Sugar (2%)


CEME-1

24

10

Fructose

CEME-2

48

10

Fructose

CEME-3

24

10

Maltose

CEME-4

48

10

Maltose

CEME-5

24

20

Fructose

CEME-6

48

20

Fructose

CEME-7

24

20

Maltose

CEME-8

48

20

Maltose

"

"

"

"

CPMP-32
48
20
Maltose

* Code Combination (50% c/u) CEME Bark of eucalyptus + Wood of eucalyptus CPMP Bark of pine + Wood of pine CEMP Bark of eucalyptus + Wood of pine CPME Bark of pine + Wood of eucalyptus.-

Obtaining the samples for mechanical test: The samples obtained previously were prepared to a final density of 800 Kg/cm3, with dimensions of 10 X 10 X 0.5 cm. A pressing cycle of two stages with a total time of 9 minutes was used. In the first stage was used 25 Kg/cm2 and 140º C for 8 min, in the second stage 10 Kg/cm2 and 140ºC were applied to release the internal steam pressure generated by evaporation of water. These parameters were determinated in previous investigations16, 17, 29.

- Mechanical Test: The total samples were submitted by means of the test of resistance to the rupture in parallel traction to the longitudinal axis of the test body, according to ASTM D1037-9330, but with another size of the test body, determining the Modulus of rupture (MOR).

- Statistical Analysis: The data obtained were analyzed by MLR using statistical software Statistica/w 5.0, to propose a general model to determine the influence of bark and wood species, sugar presence, water and time of impregnating (independents variables), and the MOR (dependent variable).

RESULTS AND DISCUSSION

DMA of wood

The storage modulus (E') that represents the elastic component for veneers treated with aqueous medium, fructose and maltose to pH=4.0 is showed in Figure 1. In general, differences in elastic behavior between species and a similar pattern for both types pine wood, early and late wood, were observed. Between 25-90º C to eucalyptus specie and 25-100º C to pine specie, a constant decreasing of E’ in all the samples is observed, attributed mainly to the water presence8. In this interval of temperature, the wood of eucalyptus presents a greater decrease of the E’ when it is comparing with both pine wood, it is more elastic than pine, while both type of pine wood present a elastic behavior similar to each other. This behavior can be attributed chemical difference of the amorphous and semi amorphous components between species and different molecular mass, which generate different interactions with water present inside of the matrix when the temperature increase. Considering the veneers with both sugar impregnated, the plasticizing is favored mainly in pine specie, the values of E' are smaller when temperature increase. This is more marked with maltose presence than with fructose. However, between wood treated and no treated of eucalyptus samples, the E' are not significantly different, and fructose generate more plasticizing effect than maltose. This behavior is related with the structural and stereo chemical difference between both sugar19, 20, 21 and its different interactions generated into different matrixes of both species.


Fig.1.- E' of veneers saturated during 24 hours with water, fructose and maltose to pH = 4: a) Eucalyptus; b) Pine earlywood and c) Pine latewood.

From 90-125ºC to eucalyptus specie and from 100-125º to pine specie, changes in the line bases in all the samples are presented, independent of the treatment. Both pinewoods show a sudden fall of E', being it more pronounced than the fall of E' of eucalyptus wood when the last one occur. In this interval of temperature, both pine woods are more elastic than eucalyptus wood, while eucalyptus wood tends to keep its elasticity constant. These changes occurring at low temperature with sugar presence that only water, it means that the elasticity is favored when this additives are present. However, exist differences between both sugar types on the elastic component, probably it is not significant. Over 120ºC, E' tend to keep constant, indicating that the elasticity stay relatively constant.

Figure 2 shows Tan Delta temperature plot which indicate the motional changes of the amorphous and/or semi amorphous molecules of the wood, when the temperature increases. The temperature in which maximums are observed (Tg), corresponds to the most greater molecular motion given by the free volume fraction of the any given relaxing phase27, It is important to consider that to these temperatures the material show the greater capacity of associated plastic deformation to each particular phase. If this temperature coincides with the temperature where the material shows the greater elasticity, see Figure 1, the material will present the greatest ductility3. In the samples treated with buffer phosphate to pH = 4.0, eucalyptus wood presents maximums to lower a temperatures (83ºC) than pine wood (145-150ºC). Moreover, the behavior of the eucalyptus wood is homogenous due to the presence of a single maximum. However, for both pine wood, this behavior is heterogeneous, appear several maximums on 140ºC, reflecting several phenomena. This behavior is attributed to the chemical difference and the different intermolecular interaction between lignins and hemicelluloses presents into matrix, when the temperature and water evaporation are increasing. It is clear that the compositional difference of the amorphous and semi-amorphous components is influencing the viscoelastic properties.


Fig. 2.- Tan Delta of wood veneer saturated by 24 hours with water, fructose and maltose to pH=4: a) Eucalyptus; b) Pine latewood c) Pine early wood.

Changes on the maximums of the Tan Delta (Tg) in the samples treated with sugars to pH = 4.0 were observed. This characteristic depends of the species and of the sugar presence which generate different interactions. For all the samples treated with any sugar, the maximums of Tan Delta appear to smaller temperatures (71-73ºC eucalyptus, 123-140ºC pine wood) than the samples treated with water. This behavior is attributed to interactions between sugars and macromolecular components of the material, generating plasticizing effect in both species. Considering the presence of both sugars, the plasticizing effects are most notorious in wood veneer with maltose than fructose, effect observed by the highest peak and the smaller Tg. This behavior could be attributed to the greater amount of hydroxyls groups and the large molecular mass presents in maltose than fructose and its structural differences, generating the former more interactions than the later18,19,20,21, favoring the plasticizing effects. Between species, both type of wood pine are more influenced than wood eucalyptus, the former present the greater reduction of Tg than the later. Nevertheless, late wood pine presents the peak of Tan Delta more high than peak of early wood pine, means late wood presents the greater molecular motional in presence of both sugars. The shoulders observed in late pine wood and eucalyptus wood are attributed to the heterogeneity in interactions between components, perhaps due to the presence of products of caramelization or policondensation reactions. However, in early wood pine this phenomenon is not observed because it presents a behavior homogeneous in phase, attributed probably to the interactions between the sugars and the smaller molecular mass of its components amorphous and/or semi amorphous, which favor the diffusions process. The Tg of early pine wood is the more small with fructose presence than maltose presence, however, the peak is a small, meaning the fraction of free volume relaxed is smaller than the fraction presented in the late pine wood, it can be attributed to the difference in the proportions of the chemical components between both pines and to the structural and molecular mass differences between both sugars, generating different interactions.

MOR of samples composites

The Figure 3 presents the collected original dates of the MOR obtained from molding samples submissive to traction test. Combinations of bark and wood between different species and mixtures of species are considerate. In general, the greater values of the MOR are observed in mixtures with pine bark and pine wood or eucalyptus wood, within of 10% of moisture content, time of impregnating of 48h and maltose presence.


Fig. 3. - Original data ordered by combination of bark, wood and sugar (49:49:2).

The Figure IV presents average values and its standard deviations of the MOR for the several treatments. According to these data, samples with maltose impregnated present a greater values, effect attributed to the greater molecular mass of maltose and its structure. The greater amount of hydroxyls groups presents are favoring a greater interaction between the macromolecular components of the wood, reflected by greater MOR values. The impregnating time favors the diffusion of the sugar in the lignocellulosic matrix of the particled material.


Fig. 4. Predictions of MOR obtained from the general model. LI = Inferior limit; LS = Superior limit; VP = value predicted by the model. White, mixtures with fructose, in black mixtures with maltose.

Generation of the General Model

This model considers interactions between continuous variables like moisture content, amount of hydroxyls groups present into simple saccharides and impregnating time. As discreet variable, bark and wood were considered (Eucalyptus = 0; Pine = 1). These model have the form:

Y = b0 + b1 * X1 +b2 * X2 +. . . . . . . + bn * Xn + e
(1)

General model which it considering interactions between all variables, see p-level in Table 3.


Table 3.- p-level of bn from general model.


  Intercept
B
W
MC*B
MC*S*B
S*T
 

p-level*

1.342E-06

0.00087

0.03895

0.01685

0.04475

0.014481

* this values indicate that the coefficients are significant to a < or = to 0.05.


MOR= 5. 4 + 10. 0 * B - 5 . 9 * B * W - 0 . 76 * M C * B

 

0.004*MC*W*T+0.09*MC*S*B*W+0.01*S*T+e

(2)

Where:
MOR = Modulus of rupture in parallel traction to longitudinal axis (Kg/cm2)
B = Type of bark species
W = Type of wood species
MC = Moisture Content (%)
S = Number of hydroxyls groups present into sugar
T = Impregnating time (hours)

With a 95% of confidence, the general model indicates that the MOR of molding samples obtained on the basis of particled lignocellulosic material of 0.25 mm is explained in 68 % of the cases by: the species of bark, of wood, moisture content, impregnating time and type of sugar. In addition, the values of the coefficient of the model bn of the bark indicate that it is the variable that more influences MOR. From of the general model, the particular models are made of: For samples made with particles of pine bark (Cp) and pine wood (Mp), (Cp=1 and Mp=1) the particular model is:

MOR = 9,535 - 0,76*MC - 0,004*MC*T + 0,09*MC*S + 0,01 S*T

(3)

Samples made with particles of eucalyptus bark (Ce) and eucalyptus wood (Me), (Ce=0 and Me=0), the particular model are:

MOR = 5,39 + 0,01*S*T

(4)

The values of the coefficients b0 indicates that the variables including in the models (3) and (4) influence more in the mixture composed by the pine species than eucalyptus species. Both models reflect the influence of sugars, when the absolute values of b3 and b4 of the model (3) and b1 of the model (4) are compared. The sugar presence influences more in the mixture with pine bark and wood than eucalyptus. Moreover, the time of impregnating and the water content influences more in the pine specie than eucalyptus specie.

For samples made with particles pine bark (Cp) and eucalyptus wood (Me), (Cp=1 and Me=0) the particular model is:

MOR = 15,41 - 0,76*MC + 0,01*S*T
(5)

For Samples made with particles of eucalyptus bark (Ce) and pine wood (Mp), (Ce=0 and Mp=1) the particular model is:

MOR = 15,41 - 0,76*MC + 0,01*S*T
(6)

According to these models (5) and (6), when appears pine bark in the mixture the value of b0 is greater than in mixtures that contain eucalyptus bark, independent of the wood species. Also the moisture content influences more in the presence pine bark than eucalyptus bark, due to the greater values of the coefficients b1. The sugars influence at the similar form in both mixtures. Nevertheless, impregnating time affects more when mixtures content pine wood, due by the bn values.

According to the models (3)-(6) it is observed that for all the combinations of bark or wood, interacting sugar type with impregnating time generates positives influences. Maltose contributes more than fructose to the value of MOR, it considering the time of impregnating of 48 hours. The coefficients b of the moisture content are negative, generating greater detrimental value when exist a high level of water, with a 10% moisture content generate the greater values of the MOR. Mixtures that contain pine bark generate MOR, under the previous conditions indicated.

In the Figure 4, the dates obtained from the general model are observed, with interval of confidence. They are ordered considering mixtures obtained from each specie and mixtures of species, bark and wood, with 10 and 20% of MC, 24 and 48 hours impregnating time and fructose or maltose presence. In general, mixtures composed by the same specie, the MOR values tends to be higher with maltose than fructose, considering the greater time of impregnating and the low moisture content. The mixtures composed by eucalyptus and pine bark or wood, maltose and 10% of MC, present significantly the greater values of MOR than the rest of mixtures with 20%of MC, independent of the wood specie However, the mixtures composed only with bark and wood of eucalyptus, the variation of the MOR does not changes when the MC variations. Does not exist significant differences between maltose and fructose into the samples composed by pine specie, as well as bark and wood.

CONCLUSIONS

- Both pine and eucalyptus wood present different viscoelastic behavior and both type of pine wood present a similar behavior. The chemical composition of specie influence the viscoelastic behavior, the elasticity and Tg are different when the temperature increase.

- The main effect of sugars in the viscoelastic behavior of all wood samples analyzed is the plastizicing effect.

- Pine wood is more influenced than eucalyptus wood when any sugar is present. Between both types of pine wood, the latewood is more influenced than early wood.

- Maltose exerts more plastificizing effect than fructose, attributed to their structure configuration and its greater molecular mass.

- In the all mixtures, maltose generates a greater MOR than fructose, effect attributed to the greater amount of hydroxyls groups present in maltose, exerting a greater plasticizing effect and a greater internal bond.

- Only when pine bark is present in the composite sample, the presence of maltose and 10% of moisture content generate significant difference, independent of the wood species. It means that dependency of the water content under the saturation point of the fiber is not directly proportional to the plastizicing effect in this conditions.

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ACKNOWLEDGEMENTS

The authors acknowledge for the support to the Renewable Resources Laboratory of the Faculty of Chemistry, University of Concepción.

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