RESEARCH ON ELECTROKINETIC, RHEOLOGICAL, HYDRODYNAMIC AND PHYSICO-MECHANICAL BEHAVIOUR OF PAPERSTOCKS FROM RECYCLED FIBRES

Transcription

1 RESEARCH ON ELECTROKINETIC, RHEOLOGICAL, HYDRODYNAMIC AND PHYSICO-MECHANICAL BEHAVIOUR OF PAPERSTOCKS FROM RECYCLED FIBRES Em. Poppel, Z. Lado*, Corina Malutan, I. Mamaliga, Th. Malutan, Irina Obrocea, I. Abstract Ibanescu U.T. Gh. Asachi Iasi, *) S.C. CEPROHART S.A. Suceava In literature very few data are available concerning the electrokinetic, rheological and hydrodynamic behaviour of fibre suspensions from recycled materials. The present study is trying to elucidate some aspects of this problem and to find correlation between the papermaking defining process parameters. In this way specific tests have been accomplished on recovered write/print papers, recycled once with or without deinking. Good correletions between the paperstocks electrokinetic and rheological characteristics were found, but to preestablish the paper characteristics as final product further investigation requires. 1. OBJECTIVES The wastepaper represents an important raw material for the paper industry /1/, but few data there exist concerning the behaviour of fibre suspensions from recycled materials in different paper processing operations. Therefore, the objective of this study is to approach the following aspects of paper recycling: to measure the electrokinetic, rheological and hydrodynamic characteristics for paperstocks from recycled fibres and virgin pulp; to determine the physico-mechanical properties of papers from these materials; to correlate the fibre suspension rheological, hydrodynamic and electrokinetic properties with the physico-mechanical characteristics of the paper as a final product.. EXPERIMENTAL PART.1. Raw materials Four types of fibrous materials were taken in consideration in this study //: Sample 0 paperstock obtained in laboratory from 4% bleached softwood pulp and 58% bleached hardwood pulp beated separately up to SR and charged with 18.5% CaCO 3, 0.14% Aquapel, 0.7% cationic starch and 0.05% Cartaretin. Sample 01 paperstock obtained from unprinted write/print paper, produced on paper machine, having the same composition with the laboratory paper sheet. Sample 10 wastepaper stock resulted from offset printed sample 01 and conventional alkaline deinked on a pilot plant. The following treatment sequences were used: slushing for 45 minutes at 5% consistency and temperature of 45 C; flotation for 10 minutes at 0.5%

2 consistency and at the same temperature; washing with dewatering-dilution to 1% consistencydewatering steps. In conventional deinking 1.3% sodium silicate, 0.6% NaOH, 0.7% DTPA, 1% H O and 1% soup flakes were introduced. Sample 0 wastepaper stock resulted in the same conditions, but deinked in neutral medium. In this case an unique nonionic surface active agent based on nonyl-phenol, ethylene oxide and propylene oxide (EO/PO) was used at an addition rate of 0.1% and at 50 C temperature... Measurements and apparatus The electrokinetical potential? by measurements of streaming potential was determined /3/. The total retention and the dynamic drainage rate on a modified Britt apparatus were performed /4/. The rheological tests for stocks subjected to shear stress on a rheoviscosimeter type Rheotest provided with coaxial cylinders were performed, adapted for high consistency domain for cellulose fibrous suspensions /5/ and the rheological tests for stocks subjected to tensile stress, respectively for determination of tensile strength of suspensions (TSS) an apparatus conceived at the TU Iassy have been used /6/. For hydrodynamic experiments an installation for specific flow tests was utilized, put in operation at the TU Iassy /7/. 3. RESULTS AND DISCUSSIONS 3.1. Concerning the electrokinetic and retention-drainage tests Table 1 presents zeta potential and dewatering properties for the fibrous materials analyzed in study. Table 1. Electrokinetic and drainage results Sample no. Characteristics Sample?, mv?.10 6,? -1 cm - 1? v.10 6,? s.10 6, R total? -1 cm -1? -1 cm -1 % R dew. 0 Laboratory sheet from virgin pulp Recycled write/print paper Charged conventional alkaline deinked pulp Charged neutral deinked pulp R tot - total retention; R dew - rate dewatering These data point out that no significant differences there exist between virgin pulp and unprinted recycled paperstock concerning zeta potential and drainage properties, but for deinked materials a slightly decrease for both parameters have been registered, probably due to deinking chemical, ml

3 conveyance to the finally pulp. That indicates higher cationic demand for paperstocks from deinked pulps. 3.. Concerning the rheological tests a. In tensile stress measurements Tensile strength values for pulp suspensions in 0.5% to 1.5% consistency domain reveal the following aspects:? Independent of the recycling method was used, maximum values for TSS generally at a consistency of 0.8% was registered for all paperstocks analyzed;? A slightly diminished TSS values for recycled pulps could be evidenced, especially for neutral flotation deinked wastepaper. In the latest case, however, the maximum value for this characteristic is preserved over a large domain of consistency, indicating higher structural stability into % consistency domain for this material. That structural stability of the deinked pulps correlates well with the electrokinetic characteristics, which play an essential role in the subsequently paperstock preparation processes (sizing, filling, dewatering, retention etc.). c, % m n Fig. 1. The influence of stock consistency and recycling method upon the TSS Fig.. Evolution of flow index m and n with consistency for laboratory paperstock (sample 0) b. In shear stress measurements Shear strength parameters indicate as follow:? Each fibrous material, irrespective of method and recycling technology utilized, has non-newtonian pseudoplastic character, indicated by flow index n<1.? The non-newtonian pseudoplastic character increases in high consistency range (fig.), but differs from virgin pulp to recycled material. This is evidenced pregnantly for sample 0 at a consistency over 1% and for sample 10 at c<1%, which is again in correlation with the zeta potential measurements.? The hysteresis curves? = f (?) do not show a clear non-newtonian, tixotropic, timedependent character for fibrous suspensions (fig. 3).

4 cons = 1,5 % y = x x x R = ???Pa y = x x x R = ???s -1 Fig. 3. Evolution of shear stress (?) as a function of shear rate (?) for recycled write/print paperstock (sample 01)? The phenomenon of tixotropy appears however very clear in curves? = f (?) (fig. 4 a and 4 b). The greatest changes for viscosity in the consistency range of % was observed. However, over this domain the pulp viscosity continue to increase, but somewhat in less extent for recycled samples 10 and 0, which is explained by the presence of retained deinking additives in the secondary fibre structures. Fig. 4. Viscosity as a function of shear rate for recycled write/print paperstock (sample 01) 3.3. Concerning the hydrodynamic tests During the stock transport in pipeline the fibrous suspension is subjected to structuringdestructuring effects /8/. Evidently, the moment of destructuring, i.e. the critical value for? depends on the stock composition and on the type and grinding finish of the pipeline, as well as on the stock consistency. Analyze of the? = f (?) curves confirm clearly the non-newtonian pseudoplastic and tixotropic character of all paperstocks taken in study. At a certain value for? (? critic ) the pulp enters in newtonian domain, when the fibre-fibre bonds practically disappear. Those critical values are indicated in table for all fibrous suspensions analyzed here.

5 Table. Values for? critic as a function of consistency C, % 0.1 % OE/OP Sample These data show an easier destructuring effect for recycled stocks over 0.75% consistency, which is in good concordance with both the TSS and the rheoviscosimetry tests. Relatively high destructuring strength was found for sample 10 (? critic = 80 s -1 ) and sample 0 (? critic = 0 s -1 ), at c = 1.5%, showing enhenced colloidal stability for these dispers systems. The friction coefficient? decreases with increasing the shear rate? and flow rate V (fig. 6 and 7). We have found very low? values for recycled paperstocks (from 0.01 to 0.10), comparatively to those for virgin pulps (from to 0.60) /9,10/. That matter of fact is explained both by the secondary fibre fractional and morphological structure and by the retained chemical content of the deinked stocks. Fig. 6. Variation of the friction coefficient? as Fig. 7. Variation of the friction coefficient? a function of shear rate? for sample 10 as a function of flow rate V for samples 01, 10 and 0, at c = 0.75% 3.4. Concerning the papermaking characteristics of the recycled fibres Table 3 evidences the following aspects:? Recovered write/print papers, recycled once with or without deinking, an increased tear strength and reduced interfibre bonding ability show, reflected in 8% loss for breaking length and 13% for burst index.? The diminished number of fibre-fibre contact an increased opacity has resulted for papers from secondary fibres.? A deinking step alone, especially in neutral medium without bleaching chemicals, could not completely recover the initial brightness of virgin bleached pulp. For high-grade papers additional process operation stages are recommended.? Some correlations between the paperstock rheological characteristics and the finished

6 paper physico-mechanical properties were found, but this problem further investigation needs. No Table 3. The papermaking characteristics of recycled fibres. Sample Martor Paperstock Wastepaper paperstock (Sample P01) stock Characteristic (Sample 0) (Sample Degree of beating, SR Grammage, g/m Thickness, mm Density, g/cm 3 Breaking length, m Elongation, % Burst index, kpa.m /g Tear index, mn.m /g Brightness, % Opacity, % Ash content, % Sizing, mm P10) Wastepaper stock (Sample P10) CONCLUSIONS? Deinked pulps higher negative zeta potential values show than virgin pulps do. This aspect has to be retained to choose the proper type and dose of additives when papers from secondary fibres are produced.? Rheological tests confirm that all recovered fibre suspensions a clear nonnewtonian, pseudoplastic and tixotropic character present, the time-dependent properties being more pregnantly evidenced in low consistency domain for deinked pulps.? For deinked pulps low friction coefficient and relatively high destructuring strength were found, which good colloidal stability show for these dispers systems.? Recycling reduces the number of interfibre bondings, which probably with an appropriate chemical dosage could be compensated.? This study useful data provides both for process designers and for optimizing the papermaking process parameters /11/.

7 REFERENCES 1. SUDAN, J - Waste Paper-Fibre Resources of our Cities, in World Pulp and Paper Technology, Roberts, F. eds. Strerling Publication Ltd., London, p.31-3, (1988). LADO, Z.,Contributii în domeniul reciclarii materialelor fibroase utilizate la fabricarea hârtiei. Cercetari privind descernelizarea maculaturii, Iasi, p.80, (1998) 3. POPPEL, E., PETROVAN, S., SIMIONESCU, C., Zum Studium von elektrokinetischen Erscheinungen in der Technologie de Papierfabrikation, Bull.Instit.Polit. Iasi, seria noua, t. X (XIV), fasc.1-, p , (1964) 4. BRITT, K.W., UNBEHEND,J.E., HOLMON,J.C. In Tapi Journal, 11, p.64-66, (198) 5. POPPEL, E., CIOVICA, S., GHIURCO, N., Zelstoff u. Papier, 39, 5-6, p.1-8, (1990) 6. POPPEL, E., Rheologie und elektrokinetische Vorgänge in der Papiertecrheologie, VEB Fachbuch Verlag, Leipzig, p.9, (1977) 7. POPPEL, E., BOBU, E., Cell.Chem. and Tehnol., 4, 1, p , (1990) 8. TERENTIEV, O.A., Zellstoff u. Papier, 37, 5, p , (1988) 9. POPPEL, E., TERENTIEV, O.A., Der Nutzen der Papierrheologie fur die Praxis, Vorgetragen anlasslich der APV + Tagung, Graz, 9 mai- 1 iunie, 1990, Veroflentlicht in der Ap. V., , (1990) 10. TERENTIEV, O.A.,Transportul de pasta si uniformitatea foii de hârtie (lb. rusa), Ed. Lesn.Promâslenostî, Moscova, p , (1980) 11. POPPEL, E., Applied Rheology, p.69-75, (1996)