This paper deals with cationic modified cellulose fibrils obtained by reacting the cellulose fibrils with 2,3-epoxypropyltrimethylammonium chloride (EPTMAC). The physical and chemical properties of unmodified cellulose fibrils (UMCF) and cationic modified cellulose fibrils (CMCF) were characterized by SEM, FTIR, degree of substitution, colloid titration, zeta potential, and thermogravimetric analysis. The experimental results showed that, after cationization, surface charge density and zeta potential reversed, thermal stability decreased, and new functional groups appeared, while the surface morphology did not show much difference from the UMCF. With the addition of three kinds of additives (UMCF, CMCF, and cationic starch (CS)) to BCTMP, the addition of UMCF and CMCF had little effect on zeta potential, while the addition of CS changed zeta potential obviously. With the increasing of additive amount, the bulk of paper sheets added CMCF did not change obviously, while the bulk of paper sheets added UMCF and CS decreased rapidly. With regard to physical strength, all the three kinds of additives could improve the tensile index and tear index; the tensile index of paper sheets added CS was higher than that of added UMCF and CMCF, while the tear index of paper sheets added CMCF was the highest among the three additives.
Cellulose is the most abundant polysaccharide in nature; its renewability, sustainability, and biodegradability have brought cellulose more and more attentions all over the world [
As an important cellulose derivative, cationic cellulose is widely applied in various fields, such as papermaking process, cosmetic, adsorbents, and antibacterial agents [
The cationic modification of cellulose fibrils consists of an etherification reaction between the alkali activated hydroxyl group of cellulose fibrils and epoxy group of EPTMAC (Figure
Competitive reactions during the cationic modification of cellulose fibrils using EPTMAC/H2O/NaOH System.
Desired reaction: cationization of cellulose fibrils
Side reaction: hydrolysis of EPTMAC
Side reaction: degradation of cationic cellulose fibrils
In the field of pulp and paper making, the concept of cationic fiber was introduced to serve as a substitute for the traditional cationic polymer used in papermaking industry [
The requirements for paper bulk and cost control have made papermaking enterprises pour more and more interests into high yield pulp (HYP), especially BCTMP (bleached chemithermomechanical pulp); due to their particular characteristics [
Many investigations have aimed at the improvement and utilization of BCTMP, while cationic modified cellulose fibrils has hardly been considered. Therefore, the aim of this study is to investigate the effects on the physical properties of BCTMP paper sheets by adding three kinds of additives (CMCF, UMCF, and CS); differences may be detected on the strength properties of BCTMP paper sheets between the addition of CMCF and the other two additives.
Cellulose fibrils were produced from HBKP (hardwood bleach kraft pulp) by mechanical treatment in PFI mill (FRANK-PTI P40110.E000), with a beating gap of 0 mm in accordance with the TAPPI standard method (T 248 sp-00), refining at 10 wt.% total solids at 20,000 total revolutions; the shearing and compression forces produced by the impacts of the rotor bars cause intrafiber bond breaking, external fibrillation, and fiber cutting [
EPTMAC was used as a cationization agent and obtained from Aoerte Chemical Ltd. in Zibo, Shandong province, with a purity of 60 wt.%. Polydiallyl dimethyl ammonium chloride (PDADMAC) and polyethene sodium sulfonate (PES-Na) were provided by BTG Instruments GmbH, Germany, applied without further purification. Sodium hydroxide, acetic acid, and ethanol were all analytical grade chemicals from Sinopharm Chemical Reagent Beijing Co., Ltd. Cationic starch (Cato. 304, DS 0.033-0.035) was received from National Starch Inc.
In previous studies, the 3-chloro-2-hydroxypropyl trimethylammonium chloride (CHMAC) and 2,3-(epoxy propyl-)trimethylammonium chloride (EPTMAC) were widely applied in the etherification field; the latter was found to be more active for cationization efficiency and easy to produce cationic fiber under similar conditions [
The cationization of cellulose fibrils has been carried out in a laboratory kneading machine (TYPE: SH-O); during the process, the staff of the cellulose fibrils was firstly diluted to 10 wt.%; then a certain amount of sodium hydroxide was added to the staff and kneaded for half an hour in the kneading machine; then the cationization reagent was slowly added to the mixing systems kneading for 150 minutes; and the temperature was kept at 30°C; when reached the reaction time, the mixture was neutralized with a certain amount of acetic acid and then washed with a large amount of deionized water to remove the excess chemical reagents; also, for a blank test, none of the chemicals was added, which was called unmodified cellulose fibrils (UMCF). Cationic modified cellulose fibrils (CMCF) were prepared under the conditions of EPTMAC to cellulose molar ratio of 0.4, NaOH dosage of 30 wt.% of cellulose fibrils, and water content of 90 wt.%.
The fiber quality analysis of cellulose fibrils before and after cationic modification was detected by L&W fiber tester; the test items included fiber length, fiber width, and kink index.
The surface morphology of cellulose fibrils was observed by SEM (SU1510, HITACHI, Japan) at the acceleration voltages of 10.0 kV after being freeze-dried and sputtered with gold-palladium.
FTIR analysis of the UMCF and CMCF was performed using a Fourier Infrared Spectrometer (TENSOR 22, BRUKER, GmbH) in transmission mode. For transmission mode, a sample of KBr powder (IR grade, Aldrich) was used as the background; the sample was first dried through a vacuum freeze dryer (FD-1D-50) provided by Beijing Boyikang Experimental Instrument Co., Ltd.; the dried sample was then grinded with a mortar and pestle in KBr, with a 2 wt.% of KBr. The spectrum was obtained at a nominal resolution of 2 cm−1, and the spectrum region was recorded between 600 cm−1 and 4,000 cm−1.
The degree of substitution of quaternary ammonium groups on the CMCF was calculated through the nitrogen content, according to formula (
Surface charge density of UMCF and CMCF was measured using a colloidal titration method with a Mutek
The zeta potential of UMCF and CMCF without electrolyte was measured at 25°C using a zeta potential analysis (Mutek SZP 06). The samples were diluted to 5 g/L with deionized water and dispersed by homogenizer, IKA T18 basic (ULTRA TURRAX), the PH value of the samples was kept at 7, and the set pressure was given to be 200 mbar.
The TGA of all samples was operated using Thermogravimetric Analyzer (TGA/DSC1) provided by Mettler Toledo; the TG and DTG curves were obtained under the nitrogen gas with flowing rate of 35 mL·min−1 and heating rate of 10°C·min−1 from 25°C to 450°C; the added sample mass was about 6.0 mg in a ceramic pan.
The bleached chemithermomechanical pulp (BCTMP) with a beating degree of about 40°SR was obtained through a Valley beater; then the BCTMP was screened with a 0.35 mm sieve according to TAPPI standard methods T275 sp-02; after screening, the obtained BCTMP was collected and the beating degree was measured to be 41°SR. This pulp was used to make paper sheets (grammage 60 g/m2) containing various proportions of additives (UMCF, CMCF, and CS); according to TAPPI standard methods T205 sp-02, ten paper sheets were made of each additive amount, while five paper sheets were picked out for mechanical testing; the rest of pulp suspensions were retained for zeta potential analysis according to Section
Physical properties of the paper sheets were measured in an atmosphere of 50% relative humidity at 23°C for 24 h in accordance with TAPPI standard method (T402-sp-03); the main physical properties (grammage, thickness, bulk, tensile strength, and tear strength) were measured according to TAPPI standard T220 sp-01 (physical testing of pulp handsheets).
Table
The fiber quality analysis of UMCF and CMCF.
Sample | Mean length [mm] | Mean width [ |
Kink index |
---|---|---|---|
UMCF | 0.656 | 17.5 | 1.309 |
CMCF | 0.672 | 18.0 | 2.564 |
Surface morphology of UMCF (Figure
SEM micrographs of cellulose fibrils. (a) UMCF and (b) CMCF.
The FTIR spectra of the UMCF and CMCF are shown in Figure
FTIR spectrum of UMCF (Spectrum a) and CMCF (Spectrum b).
The TG and DTG curves of UMCF (a) and CMCF (b).
Compared to Spectrum a, Spectrum b of the CMCF gives clear evidence of quaternization in cellulose fibrils. An increase in intensity of the major ether bonds in the spectrum region between 1030 cm−1 and 1163 cm−1 provides evidence of grafting of EPTMAC onto the fibril surface [
Generally speaking, there are no molecular containing nitrogen in cellulose chains; experimental data showed that the UMCF had no nitrogen, while the obtained nitrogen content of CMCF was 1.16%; according to formula (
The cationic modification of cellulose fibrils consists of nucleophilic reaction between the alkali activated hydroxyl group of cellulose fiber and epoxy group of EPTMAC (Figure
The analysis of surface charge density and zeta potential.
Sample | Surface charge density [ |
Zeta potential [mV] |
---|---|---|
UMCF | −14.29 ± 0.3 | −23.7 ± 0.5 |
CMCF | +18.68 ± 0.5 | +15.7 ± 0.5 |
CS | +505 ± 5 | ND |
The TG and DTG curves of UMCF and CMCF under the same conditions are shown in Figure
Figure
Zeta potentials of pulp suspensions with various additive amounts of three kinds of additives (UMCF, CMCF, and CS).
As mentioned, bulk is one of the most important features for BCTMP and should be paid more attention. Figure
Bulk of paper sheets with various additive amounts of three additives (UMCF, CMCF, and CS).
Results in Figure
The tensile index of paper sheets with various additive amounts of three additives (UMCF, CMCF, and CS).
The results of tear index from the paper sheets added three kinds of additives are shown in Figure
Tear index of paper sheets with various additive amounts of three additives (UMCF, CMCF, and CS).
In this study, CMCF was prepared by cationization of cellulose fibrils with EPTMAC, with the cellulose fibrils produced from mechanical treatment in a PFI mill at a high revolution. Due to the quaternary ammonium connecting to the fibril surface, the chemical properties of the cellulose fibrils were thus extremely changed after cationic modification, the inversion of electrical property, reduction of thermal stability, and appearance of new functional groups. While the surface morphology of cellulose fibrils before and after cationization showed no obvious difference. The addition of UMCF and CMCF had little effect on zeta potential compared with CS. The tensile index and tear index could be improved with the addition of CMCF, UMCF, and CS; at the same time, CMCF could preserve the BCTMP’s high bulk property to some extent.
The authors declare that there are no competing interests regarding the publication of this paper.
The authors thank China National Pulp & Paper Research Institute and Tianjin Key Laboratory of Pulp & Paper for the technical support.