Performance of improved bacterial cellulose application in the production of functional paper

Authors


Altaf H. Basta, Cellulose & Paper Department, National Research Centre, Dokki-12622, Cairo, Egypt. E-mail: Altaf_Basta@yahoo.com

Abstract

Aim:  The purpose of this work was to study the feasibility of producing economic flame retardant bacterial cellulose (BC) and evaluating its behaviour in paper production.

Methods and Results:  This type of BC was prepared by Gluconacetobacter subsp. xylinus and substituting the glucose in the cultivation medium by glucose phosphate as a carbon source; as well as using corn steep liquor as a nitrogen source. The investigated processing technique did not dispose any toxic chemicals that pollute the surroundings or cause unacceptable effluents, making the process environmentally safe. The fire retardant behaviour of the investigated BC has been studied by non-isothermal thermogravimetric analysis (TGA & DTGA). The activation energy of each degradation stage and the order of degradation were estimated using the Coats–Redfern equation and the least square method. Strength, optical properties, and thermogravimetric analysis of BC-phosphate added paper sheets were also tested.

Conclusions:  The study confirmed that the use of glucose phosphate along with glucose was significant in the high yield production of phosphate containing bacterial cellulose (PCBC1); more so than the use of glucose phosphate alone (PCBC2). Incorporating 5% of the PCBC with wood pulp during paper sheet formation was found to significantly improve kaolin retention, strength, and fire resistance properties as compared to paper sheets produced from incorporating bacterial cellulose (BC).

Significance and Impact of the Study:  This modified BC is a valuable product for the preparation of specialized paper, in addition to its function as a fillers aid.

Introduction

Environmental pollution has increased rapidly in the last two decades, increasing the pressure on the governments to adopt laws that force industries to limit the environmental hazards they are responsible for. Burning of lignocellulosic products (e.g. paper and wood) releases serious environmental pollutants like emission of many greenhouse gases, i.e. carbon monoxide and dioxide, as well as sulfur dioxide, in the case of alum-rosin sized paper, which are hazardous to their surroundings.

Flame-retardant compounds add value to traditional characteristics of paper, as they control smoke generation and toxicity. They mainly operate by minimizing the formation of levoglucosan by lowering the decomposition temperature of cellulose and enhancing char formation by catalysing the dehydration and decomposition reactions (Serebrenikov 1934; Schuyten et al. 1955; Shafizadeh et al., 1978). Further study reveals that flame retardation is results from the presence of elements such as phosphorous, halogens, nitrogen or boron in the retardant, as well as sodium silicate (Gordon 1990; Singh et al. 1996; Nassar et al. 1999). Compounds containing phosphorous strongly affect pyrolysis and char formation, while halogens considerably influence polymer breakdown and combustion. Dehydration reactions of polysaccharides, catalysed by many inorganic salts, lead to a higher portion of charring with a reduced amount of evolved levoglucosan.

Introduction of a phosphorous element in the cellulose chain is considered a known procedure and is used for the derivatization of cellulose for the preparation of a cationic exchanger. Generally, phosphorous oxychloride and phosphoric acid were used to prepare phosphorylated carbohydrates (Lehrfeld 1996; Gong et al. 2007). Recently, Oshima et al. (2008) prepared the phosphorylated bacterial cellulose (PCBC) using phosphoric acid and they pointed out that the degree of phosphorylation of bacterial cellulose (BC) is higher than that produced from plant cellulose, under the same phosphorylation condition. This PCBC is suitable as adsorbent of some metal ions. Removal of the unreacted phosphorylation agents was found to cause various environmental hazards. Thus, the exploration of new approaches is not only of need but urgent.

Earlier work in this field demonstrates that paper sheets have flame-retardant behaviour and high strength properties when produced by incorporating cellulose ether–metal complexes as beater additives and removing sodium silicate by magnesium chloride or chitosan by silicate (Basta and El-Saied 2001; Abd El-Sayed and Basta 2003; Basta et al. 2004).

The use of biotechnological techniques in paper production is considered a benefit for economic and environmental purposes. BC produced from Gluconacetobacter subsp. xylinus (ATCC 10 245) is different from plant-derived cellulose with respect to its unique physical and chemical properties, i.e. high crystallinity, degree of polymerization, tensile strength, thermal stability and high purity (Yoshinaga et al. 1997; El-Saied et al. 2008). These properties have attracted much attention to the use of BC as a new functional material in applications like paper and nonwoven fabric-like products (Jonas and Farah 1998; El-Saied et al. 2004). BC has also been investigated as a binder in paper because it consists of extremely small clusters of cellulose microfibrils, a property that greatly enhances the strength and durability of pulp when integrated into paper. Ajinomoto Co. along with Mitsubishi Paper Mills in Japan is currently active in the development of microbial cellulose for paper products (JP patent 63295793) (Hioki et al. 1995).

In general, glucose has been used as a carbon source in the cultivation medium for bacterial cellulose production from Gluconacetobacter subsp. xylinus. If we can modify the cultivation medium of Gluconacetobacter subsp. xylinus for the production of phosphorous-containing bacterial cellulose, the phosphorylated BC can substantially be extended as an economical and environment-friendly product. The objective of this work is to study the possibility of the synthesis of a fire-retardant paper additive through substituting glucose in the cultivation medium with glucose phosphate. Based on the previous study of the production of economic BC (El-Saied et al. 2008), corn steep liquor will be used as a nitrogen source in the cultivation medium.

Experimental

Materials

  • i stock: Bleached spruce sulfite paper pulp of 87·05%α-cellulose (Merkblatt IV/29 Zellcheming; German Association of Cellulose Chemistry & Engineering), 7·8% pentosans (Jayme and Sarten 1940), trace lignin (The Institute of Paper Chemistry 1951) and 0·201% ash, as a paper substrate, was delivered from RAKTA Paper Mill, Alexandria, Egypt.
  • ii Additives: Bacterial cellulose (BC) and BC phosphate (PCBC) were used as paper additives. Gluconacetobacter subsp. xylinus (ATCC 10245) was used in this study in the production of BC and was purchased from the American Type Culture Collection (ATCC), Manassas, VA, USA. The composition of the main cultivation medium used in the production of the investigated BC was glucose-corn steep liquor-based medium (CSL) (Hwang et al. 1999), with the following compositions: 80 ml CSL (total solid content 32·49%), 20·0 g glucose, 2·7 g Na2HPO4 and 2·25 g citric acid monohydrate.

The CSL used in this study was obtained from the Company of Starch and Glucose Manufacture, Torah, Cairo, Egypt. For BC phosphate production, glucose was substituted by glucose phosphate. The pH of these media was adjusted between 5 and 6. The sugars and organic acid were autoclaved separately before their addition to the media under aseptic conditions. The fermentation media were divided into 40-ml triplicates in 250-ml Erlenmeyer flasks, and each flask was inoculated using a 48-h old slant of Gluconacetobacter subsp. xylinus (ATCC 10245).

The inoculated flasks were incubated at about 30°C for 3 days under static conditions, followed by 8 days under agitated conditions. At the end of the fermentation period, the BC gels formed on the surface of the fermentation media were carefully recovered, washed and immersed overnight in a 1 N NaOH solution at room temperature in order to remove bacterial cells and media components. The BC gels were then thoroughly washed with distilled water until the pH of the washing water reached ∼7·0 (Toda et al. 1997). Afterwards, the BC gels were immersed in a 96% alcohol solution followed by a diethyl ether solvent for a few hours. Finally, the BC gels were dried by gentle heating (40°C) and weighed in order to compare the BC production yield of the producing organism using traditional and investigated carbon sources in the fermentation medium.

The phosphorous content of BC derivative was estimated by the method reported by Hassouna and Hassan (1994), based on the formation of yellow vanadiphosphomolybdate complex, and the absorbance was measured at 350 nm against reagent blank (10 ml 1 : 10 H2SO4, 5 ml ammonium vanadate and 5 ml ammonium molybdate solutions).

Handsheets papermaking and strength properties measurement

Papermaking: The wood pulp (WP) was beaten up to the degree of the Schoper Reigler (SR°) 45, using a valley beater. 5% never-dried gel of BC or PCBC was added while stirring the beaten pulp, and then subjected to handsheet formation according to the Swedish Standard method (S.C.A.). The prepared unloaded paper sheets and BC-loaded paper sheets, in the absence and presence of 2% kaolin (based on O.D. pulp), were placed for conditioning at a relative humidity of 50% and a temperature of 23°C (International Standard, ISO 187 1990). Then, they were tested for breaking length, burst and tear factors (The Institute of Paper Chemistry 1952). For each test, at least five measurements were taken, and the arithmetic mean of the results was calculated.

Thermal analysis measurements

Thermogravimetric analyses (TGA and DTGA) were performed using a Perkin-Elmer Thermogravimetric Analyzer, TGA7. The analyses were performed with a heating rate of 10°C per min and nitrogen flow rate of 50 cc per min, under nonisothermal conditions. This test was carried out to examine the fire-retardation behaviour of the investigated BCs and handsheets produced.

TG curve analysis

Kinetic studies based on the weight loss data were obtained by TG curve analysis. The activation energy was evaluated by applying Coat–Redfern method of analysis (Coat and Redfern 1964). For pseudo-homogeneous kinetics, the irreversible rate of conversion of the weight fraction of reactant may be expressed by the following equation:

image(1)

where α is the fraction of material decomposed at time t, k is the specific rate constant and n is the order of reaction. The temperature dependence of k is generally expressed by the Arrhenius equation:

image(2)

where A is the frequency factor (s−1) and T is the absolute temperature.

The linear heating rate a (deg min−1):

image(3)

For calculating the activation energy, Ea, of thermal decomposition when = 1, we use:

image(4)

when ≠ 1, we use:

image(5)

Plotting the left-hand-side value of the equation i.e., inline imageagainst 1/T using various values of n should give a straight line with the most appropriate value of n (Basta1999). Least square method was applied for the equation, using values of n ranging from 0·0 to 3·0 in increments of 0·5 and calculating the correlation coefficient (r) and standard error (SE) for each value of n. The n value that corresponds to the maximum r and minimum SE is the order of the degradation process. The activation energies and frequency factors were calculated from the slope and intercept, respectively, of the Coat–Redfern equation with the most appropriate value of n.

Results

Evaluation of bacterial cellulose derivative as a fire-retardant compound

To clarify the possibility of producing phosphate-containing bacterial cellulose (PCBC) using corn steep liquor (CSL) as a cheaper nitrogen source for the cultivation medium of Gluconacetobacter subsp. xylinus and using it as a fire-retardant compound, the following studies were carried out to compare it with unmodified bacterial cellulose (BC):

  • iEvidence the presence of the phosphate group, as well as the yield of PCBC to be used in a commercial scale.
  • iiNonisothermal TGA analysis, based on the theory that a flame-retardant minimizes the formation of levoglucosan by lowering the decomposition temperature of cellulose and enhancing char formation (Serebrenikov 1934; Schuyten et al. 1955; Shhafizadeh et al., 1978).

The results of yield and phosphorous content show that a relatively high yield of BC is produced by using glucose phosphate in conjunction with glucose, in a ratio of 1 : 1, as a carbon source in the CSL cultivation medium [PCBC(1)], when compared to using just glucose phosphate [PCBC(2)]. In the former case, the yield and phosphorous content are 6·4955 g l−1 and 8·8 mg g−1 BC, respectively, in contrast with the latter case, with a yield and phosphorous content of 4·5 g l−1 and 10·9 mg g−1, respectively. BC produced by using a conventional carbon source (glucose) has a yield of 10·905 g l−1.

Figure 1 shows the TGA and DTGA curves for dried PCBC samples in comparison to curves of BC. The corresponding thermal measurements (temperature ranges, maximum weight loss temperature and activation energy) of each degradation stage are given in Table 1.

Figure 1.

 Thermogravimetric analysis (TGA and DTGA) curves of bacterial cellulose and phosphate-containing bacterial cellulose produced from glucose and glucose phosphate as carbon sources of cultivation medium. (inline image, Weight % and inline image, Deriv,a,tive weight %).

Table 1.   Thermal degradation measurements of bacterial cellulose and phosphorous-containing bacterial cellulose samples
Cellulose originMain degree stage Temperature range (°C)Maximum weight loss temperature (°C)Order ‘nrSe Ea kJ mole−1Weight remain at ∼500°C
BC1st50–98·9178·64 0·0
2nd138·14–261·3182·691·50·95840·209885·6824
3rd280·6–362·3308·31·50·95540·2377219·85
4th375·8–419·01398·991·50·97160·18 201516·309
ΣEa = 821·84
PCBC (1)1st50–88·770·5229·83
2nd88·75–169·03111·831·50·96470·173284·9172
3rd190·1–300·9237·031·50·98680·1411155·33
4th306·8–449·7353·31·50·97750·1528100·923
ΣEa = 341·17
PCBC (2)1st50–8872·4732·61
2nd88–160·35104·861·50·96250·167584·9965
3rd160·35–244·8232·980·50·96560·1501100·132
4th250–378·9344·041·00·98060·1546137·1618
ΣEa = 322·205

Evaluation of bacterial cellulose derivative as additive for functional paper production

This study was undertaken to examine the effect of adding BC and PCBC on the strength, optical and thermal behaviour of wood pulp paper sheets, especially those loaded by kaolin filler. The results of breaking length, burst and tear factors, which represent the mechanical properties, as well as of brightness, which represents the optical properties, are shown in Fig. 2. For the purposes of comparison, the properties of strength were expressed using a modified formula known as the quality number, Qz (Basta 1999). The Qz was calculated by summing the values of strength properties and dividing by the number of properties minus one (Fig. 2). SEM graphs of the investigated paper sheets are illustrated in Fig. 3. The TGA and DTGA curves of PCBC-added WP paper sheets, when compared to the BC-added WP and WP sheets (control samples), are shown in Fig 4 and 5.

Figure 2.

 Effect of adding bacterial cellulose and phosphate-containing bacterial cellulose on properties of paper produced. ( inline image ), without kaolin and ( inline image ), with kaolin.

Figure 3.

 SEM of un- and kaolin-loaded wood pulp paper sheets. ×500.

Figure 4.

 Thermogravimetric analysis (TGA and DTGA) curves of unloaded- and kaolin- and/or bacterial cellulose (BC)-loaded wood pulp paper sheets. ( inline image ), Weight % and ( inline image ), derivative wt. %.

Figure 5.

 Thermogravimetric analysis (TGA and DTGA) curves of unloaded- and kaolin-loaded phosphate-containing bacterial cellulose 1 (PCBC1)- and PCBC2-treated wood pulp paper sheets. inline image, Weight %; inline image, derivative wt. %.

Analysis of TGA curve can also be helpful in studying the fire-retardation property, which exerts the WP sheets by adding PCBC during sheet formation. Because the main theory behind flame retardation is minimizing the formation of levoglucosan by lowering the decomposition temperature of cellulose and enhancing char formation by catalysing the dehydration and decomposition reactions (Serebrenikov 1934; Schuyten et al. 1955; Shafizadeh et al., 1978). Therefore, the TGA measurements are taken (Coat and Redfern 1964 and Basta 1999) and registered in Table 2.

Table 2.   Thermal degradation measurements of bacterial cellulose and phosphorous-containing bacterial cellulose containing wood pulp paper sheets
FillerAdditiveMain degree stage Temperature range (°C)Maximum weight. loss temperature (°C)Order ‘n’rSe Ea kJ mole−1Ash %
In absence of kaolin1st50–104·758·080·261
2nd228·7–340·6303·31·00·98840·1299129·988
3rd348·3–448·4428·31·00·96760·1507173·021
ΣEa = 303·01
BC1st50–92·0774·090·1
2nd236·08–317·7298·60·50·98060·1489184·014
3rd375·07–454·8429·351·00·96930·1514232·429
ΣEa = 416·443
PCBC (1)1st50–123·3363·40·521
2nd158·74–488·08255·61·00·98620·1443146·034
3rd351·99–427·88398·381·00·96540·1565250·784
ΣEa = 396·818
PCBC (2)1st50–138·870·621·281
2nd138·8–309·9272·560·50·99490·002677·343
3rd417·1–427·4426·31·00·99830·0228143·833
ΣEa = 221·176
In presence of kaolin1st50–91·0459·080·471
2nd228·5–334299·61·00·98730·1405147·83
3rd388·5–422·7422·71·00·96770·1521166·941
ΣEa = 314·771
BC1st50–91·0459·080·9238
2nd228·5–334299·61·00·98730·1405162·752
3rd388·5–422·7422·71·00·96770·1521161·054
ΣEa = 323·806
PCBC (1)1st50–88·961·851·1957
2nd216·49–317·7303·571·00·98890·1391169·33
3rd351·23–454·81433·391·00·96370·1707173·335
ΣEa = 342·665
PCBC (2)1st50–103·1582·651·589
2nd195·95–331·5300·291·00·98480·1498147·104
3rd363·1–442·7427·341·00·96980·1472279·09
ΣEa = 426·194

Discussion

Thermogravimetric (TG and DTG) degradation curves for a dried BC sample are represented by Fig. 1, in which four stages can be observed. The first stage, from 50–90·9°C, represents the evolution of residually absorbed water that corresponds to 9·8% of the total weight. The second and third stages start at 138·14°C and 280·6°C and end at 261·3°C and 362·3°C, respectively, and occur because of the depolymerization of cellulose to levoglucosan competing with dehydration (Walker 1970; Shafizadeh et al. 1979; Yuki and Shafizadeh 1984). These stages represent a prominent thermal degradation of weight loss of the BC, with peaks maxima at 182·7°C and 308·3°C. The fourth stage occurs in the region of 373·8°C–419°C and has a peak maximum at 398·99°C, and occurs because of rapid volatilization accompanied by the formation of carbonaceous residue (Table 1).

Substitution of glucose in the cultivation medium with glucose phosphate leads to a decrease in the onset temperature of degradation, with a relatively small weight loss in the depolymerization stages and shifting the peak maxima to lower temperatures during the decomposition of the BC derivative (PCBC 1 and PCBC 2).

As can be noticed, PCBC 2 (P content: 10·9 mg g−1 BC), produced by using glucose phosphate, has a relatively higher fire-redardant property than that produced by the combined glucose phosphate and glucose (PCBC1; P content: 8·8 mg g−1 BC). The total activation energies of the depolymerization stages and residual char in the cases of glucose, glucose–glucose phosphate and glucose phosphate are 821·84 kJ mol−1 and 0%, 341·17 kJ mol−1 and 29·83%, and 322·29 kJ mol−1 and 32·609%, respectively (Table 1). The collected data indicate that the presence of phosphate groups in the investigated BC derivatives has increased its resistivity to thermal decomposition and minimized the formation of levoglucosan.

These results are promising enough to warrant the evaluation of these compounds as antifiring paper additives and the examination of their effect on the properties of the paper produced.

In the cases of strength and brightness, Fig. 2 shows that adding either BC or PCBC leads to an improvement in the breaking length and burst factor of the paper. PCBC1 and PCBC2 display a relatively higher improvement on the paper strength of wood pulp sheets (WP sheets) when compared to the addition of bacterial cellulose (BC). PCBC synthesized with a CSL glucose phosphate and glucose medium (PCBC1) provides the highest paper strength and brightness when compared with PCBC synthesized by using a CSL glucose phosphate medium (PCBC2) and a CSL glucose medium (BC).

The aforementioned improvement can probably be attributed to the small clusters of cellulose microfibriles containing BC (Hioki et al. 1995). This structure enhances kaolin retention in pulp fibres. The addition of PCBC1 leads to a higher paper quality, Qz, when compared to the addition of PCBC2 and can be ascribed to the presence of the phosphate group in the vicinity of a hydroxyl group. This situation is more probable in the addition of PCBC1 than PCBC2 and tends to enhance the attraction between pulp fibres and fillers. This reduces the gap between adjacent chains, as manifested by the SEM graphs (Fig. 3). The ultrafine fibrils of BC-based additives trap the kaolin filler granules and retain them in the paper, making the formation of polymer bridges between the fibres and filler particles more probable, while the relatively low filler retention of kaolin in case of BC- and PBC-free WP sheets is ascribed to the repulsion between the negative charges of fibres and kaolin base.

As can be shown, the tear factor property of the investigated paper sheets behaves in the reverse trend (decreases), compared to breaking length and burst factor (Fig. 2). This is ascribed to the assumption that the presence of binder (BC) with pulp fibres tends to increase fibre stiffness and hence reduce tearing resistance [Casey 1981].

Figure 3 leads to the observations that adding PCBC to wood pulp tends to change the macroscopic visual aspect of technical WP paper sheet fibres, forming weblike features.

The TG curves exhibit similar decomposition stages to the dried BC samples examined earlier, with the difference that only three stages are present in contrast to the earlier four stages. PCBC1-WP and PCBC2-WP sheets began to depredate at relatively lower temperatures (138·8 and 195·95°C, respectively), with rapid volatilization stages peaking at 272·6 and 295°C, respectively, in comparison to the WP sheet, which began to degrade at 209·9°C and peaking at 303·3°C (Table 2, Fig. 4).

The weight loss observed in the volatilization stage of BC-added sheets is lower than that of the WP sheets. BC-added sheets began to depredate at a higher temperature of 228·5°C and peaked at 298·6°C. Also, its activation energy for the main depolymerization stage is higher than that of the WP sheets and the PCBC-added sheets.

The data obtained further indicate that the incorporation of kaolin to the BC and PCBC sheets led to an increase in the onset temperature, the DTG peak and the activation energy of degradation (Table 2, Fig. 5). This can be explained by the interaction between the positive charges of base kaolin granules with the negatively charged phosphate and wood pulp fibres, as well as the increase in polymer bridges between WP, PCBC and Kaolin.

As can be observed, in the absence of kaolin, there is a substantial improvement in the fire-retardant properties of WP sheets with a PCBC2 addition.

Conclusion

It is possible to synthesize PCBC by using glucose phosphate alone or as a combination of glucose phosphate and glucose, as a carbon source for the cultivation medium for Gluconacetobacter subsp. xylinus along with a very cheap nitrogen source like CSL. PCBC is used as a new, environmentally friendly paper additive that is successful in the production of functionalized paper sheets characterized by high strength, fire retardation and high filler load.

Acknowledgements

The authors thank the Egyptian Academy of Scientific Research and Technology for financing this work. The authors are grateful to Prof. Dr H. Gobran, Drexal University, USA, for providing the strain.

Ancillary