The Interface Behavior And Application Of Chitosan And Its Ionic Derivative As Wet-end Additives In Papermaking Systems

Chitosan, a biodegradable, nontoxic, antibacterial, as well as renewable resource commodity, is the second most widespread natural polysaccharide, and has exhibit basic rather than acidic characteristics. This basicity gives chitosan in particular its unique properties. Further, chitosan is easy to be modified to become the cationic polyelectrolyte, which pretend strongly to interact with cellulosic substrates or mineral fillers presenting in wet-end of papermaking systems, and is expected to enhance the physicomechanical characteristics of paper and improve the papermaking process. Therefore, chitosan and its derivatives are used as wet-end potential functional additives in papermaking. Since functions of chitosan are closely associated with its molecular structure parameters, elucidation of the relationships between function and structure of chitosan is important not only for the acadmic interest but also for the industrial applications. The recent advances on the application of chitosan as a wet-end additive in papermaking were reviewed in this paper. Based on these previous works, chitosan and N-(2-hydroxy-3-trimethyl -ammonio)-propyl chitosan chloride (C-CS) with various molecular structure parameters such as molecular weight (Mw), degree of deacetylation (DD), and degree of substitution (DS) were successfully prepared. The interactions between these samples and the main components of reed kraft pulp suspensions(Unwashed pulp, Fines suspension, Washed pulp, Microcellulose and Alkaline-treated pulp) were investigated with gel permeation chromatography (GPC), transmission electron microphotograph (TEM), scanning electron microphotograph (SEM) , phenol/sulfuric acid spectrophotometric, florescence microscope, microelctrophoresis, photon correlation spectroscopy (PCS) and retention/drainage (DDJ) methods. The retention of natural occuring CaCO₃ by C-CS and the flocculation of cellulose induced by C-CS—nanoparticle (SiO₂) were also evaluated. For comparision, the properties of commercial cationic starch were tested too. The results were shown as follows:1. Adsorption of Chitosan Wet-end Additives and Polyelectrolyte Complexes inPapermaking System: The peroxide bleached reed kraft pulp was firstly treated to give five pulp with different surface morphology, and then the interface behaviors (adsorption, flocculation, complexes) on these substrates surfaces were investaged. The results shown that chitosan additive was almost completely adsorbed onto the surfaces of the cellulosic fibers, especially onto the surfaces of the fines in a variety of cellulosic systems at low dosages corresponding to those used in industrial operation. This adsorption was increased when the degree of deacetylation of chitosan was increased. Moreover, the aggregation of the fine cellulosic particles was maximum with dosage about 12 mg/kg of chitosan. The uptake of hemicellulose always occurred in the cellulose suspensions upon addition of chitosan, whereas glucose was averse to forming a complex with chitosan. These phenomena were relative to the maximum fines retention and drainage performance of the pulp suspensions; the optimum drainage dosage was found to be about 5-8 mg of chitosan per gram of oven dried (o.d.) pulp and °SR of bleached reed kraft pulp was decreased about 26%. The interactions between chitosan and the cellulosic substrates were dominated by hydrogen bonding at the pH^7 for these experiments, whereas elctrostatic forces did play a contributory role in the destabilization of the suspensions.2. The interface interaction mechnism between water-solube chitosan (DD=50%) and peroxide bleached reed kraft pulp: The zeta potential of peroxide bleached reed kraft pulp transformed from negative to zero and then to positive due to the adsorption of water-soluble chitosan (DD=50%).This adsorption was founded to relate with the surface physicochemical preperties of cellulosic substrates: non-electrostastic forces (hydrogen bonding, Van der Waals forces) existed between the fibers and water-soluble chitosan(DD=50%), but the interactions between fines and water-soluble chitosan were electrostatic attractives. Furthermore, the adsorption was an endothermic process for the Fines suspension, but the adsorption heat (AH) was positive for the Unwashed pulp or Washed pulp. The experimental results showed the fines presenting in pulp suspension would be aggregated upon the addition of water-soluble chitosan, and the degree of flocculation was affect by the type of cellulosic substrates, the concentration of the background electrolyte and pH: forUnwashed pulp, the flocculation performance of water-soluble chitosan was decreased insignificantly with the increasing NaCl; for Fines suspension, the flocculation efficiency of water-soluble chitosan was decreased significantly as the concentration of NaCl was increased. These adsorption and flocculation resulted in good drainage of reed pulp, for example, the °SR of cellulosic pulp reduced about 39% or 18% at CNacrOmol/L, pH=5 or pH=7, respectively.3. The interface interactions between N-(2-hydroxy-3-trimethylammonio) -propyl chitosan chloride and cellulosic substrates: A variety of quaternary chitosan with different DS and Mw and a carboxymethyl chitosan sample were prepared, and then elucidated the relationshops between surface functions and molecular structures of these polyelectrolytes, It was found that the adsorption and aggregation behaviors which occured in pulp suspension were not only affected by the surface physicochemical characteristics of the cellulosic substrates but were also strongly affected by the nature (charge density, charge type and molar mass) of the polysaccharide additives, that is, C-CS additives were almost completely adsorbed onto the surfaces of the cellulosic fibers and aggregated the fines at low dosages, corresponding to those used in industrial operation, but the optimum polymer concentration was increased with the reducing of the charge density of the C-CS additives, and was decreased with reducing of the Mw of C-CS. Just like chitosan, C-CS was easy to complex with hemicelluloses in the adsorption processing, and these complexes then became adsorbed or deposited onto the cellulosic fibers, and this also correlated to the maximum fines retention and drainage. The experimental results also suggested the dominant interactions between C-CS and cellulosic substrates are of an electrostatic nature and the fine flocculation was a charge patch mechanism.However, the adsorption of carboxymethyl chitosan promoted the stabilization of the fines and colloidal carbohydrates rather than their aggregation due to its anionic nature. It was found the adsorption of commercial cationic starch onto the surfaces of cellulosic substrates was weaker than C-CS and showed higher optimum polymer concentration. The phenol/sulfuric acid spectrophotometric experiment demonstrated that optimum polymer concentration was 60mg/L for Unwashed pulp system.4. Retention-aid properties of quaternary chitosan for natural occurring CaC(>3 filler: Other main component is mineral filler in wet-end of papermaking systems. Calcite has already been extensitively used in alkaline papermaking due to excellent quality and low cost. Therefore, the adsorption of C-CS onto the CaCO3 surfaces and the floccultion of CaCO3 suspension had been assessed at different ionic strength, pH and temperature, and the retention-aid properties of C-CS for natural occurring CaCC>3 filler had been discussed by simulating the industial operation.It was found that C-CS was almost completely adsorbed onto the surfaces of CaCO3 particles at the experimental dosage level. With same DS of 93%, the complete surface coverage, which was determined by zeta potential measurements, of CaCO3 particles used in this work were 0.735mg/g, 0.826mg/L and 0.953mg/g for the quaternary salt with Mw=4.8xlO4, llxlO4and 19xlO4, respectively. The electrostatic attractive was expected to be the dominant driving forces for the adsorption. Compared with a commercial cationic starch, C-CS had a lower flocculation concentration and a higher flocculation performance. Further, the flocculation of CaCCb dispersion was significantly affected not olny by the molecular structure parameters ( Mw, DS) of C-CS but also by the surrounding conditions (the type of electrolyte, ionic strength, pH ). The optimum flocculation concentrations were about 4mg/l, 5mg/l and 6mg/l for the C-CS with Mw=19xlO4, llxlO4 and 4.8xlO4, respectively. It was also shown that the CaCCb flocculation induced by the adsorption of quaternary chitosan was dominated by a charge patch mechanism.The results demonstrated that the retentions of CaCO3 fillers were almost same (ca. 80%) for highly charged C-CS93 with Mw=19xlO4, llxlO4 and 4.8xlO4 at the dosage of lmg/L,1.7mg/L and 3mg/L, respectively. The retention of CaCO3 fillers were about 62%, 54% and 40% for C-CS 69 with Mw=17.5xlO4, C-CS43 with Mw=18.6xlO4 and C-St. SEM microgragh comfirmed these conclusion. The results also shown that the total amounts of saccharide could be reduced 40%, 24% and 12% by C-CS69, C-CS43 and C-St, respectively. The mechanical measurements indicated that the tensile index of handsheet decreased in a maximum about 8.4% due to thesignificant increasing of retention of CaCCb and fines. It can be concluded that C-CS was more promising retention-aid in papermaking.5. Flocculation of cellulosic fiber suspensions by a microparticulate retention aid system consisting of quaternary chitosan and nano-SiO2.* In recent years, the microparticle retention aid system has focused on the studies of papermaking chemicals. C-CS—nanoparticle SiC?2 dual component system was investigated in this work. The adsorpton kinetic experiments indicated that there was a very fast (< 1 min) polymer adsorption under good mixing conditions. Adsorption of C-CS onto the particles was followed by a rearrangement to reach an equilibrium conformation. At the same time, C-CS, existing on the surface of fiber, could disperse into fiber inside, resulting in the reducing of zeta potential of the cellulosic fiber. In addition, the flocculation would be increased with the increasing of SiCh, the reasons for this was like to be the reducing of the interaction sites between C-CS and substrates due to the strong attractive of C-CS onto high negatively charged SiC>2, the chain of C-CS in the solution was so long that it could reach the surface of other particule, which induced the bridge flocculation to oceur between the particules. But the fiber substrates surfaces should be net positively charged by an adsorbed C-CS layer.It was found that the flocculation efficiency of C-CS—SiCh was decreased as the concentration of NaCl was increased. The reason for this was the effciniency of screening by NaCl was higher for the interactions between C-CS and SiC>2 than that for the interactions of C-CS with substrates. In addition, it was also shown shear led to floe breakage and reduced the flocculation. These phenomena were very obvious for one-component system (C-CS or C-St), but the microparticle system (C-CS—SiC>2 or C-St—SiCh ) was shear-resistance. This is because that a transient network in the gap between the surfaces was formed by polymer chains and nanoparticles, and thus the reflocculation occured between the broken floes at lower shear level.The main creative ideal:1. The wet-end cellulosic fiber suspensions are consist of coarse fibers and fine.particles as will as dissolved and colliodal carbohydrates, but the fundamantal studies of interface interactions between additives and these substrates are still not reported. In this work, a novel experimental method was presented, this is, the peroxide bleached reed krafit pulp was treated to give six substrates (Unwashed Pulp, Wash Pulp, Fines Suspension, Alkali-treated Pulp, B-cellulose, y-cellulose) with different surface morphology. The interface phenomena(adsorption; flocculation, carbohydrate uptake)occuring in above pulp system, which was induced by chitosan or quaternary chitosan, were fistly systematically investigataed. The interface interaction mechanism was put forward according to the different system. In this way, we could obtain a better understanding of the physicochemical properties that take place at the concentrations representative of those found in industrial papermaking system.2. The interface behaviors of novel retention-aid system(C-CS—SiC^) were systematically studied, and a flocculation mode of C-CS—SiC?2 system was firstly proposed, that is, adsorbed C-CS on the surface of substrates, presenting in a flatter conformation, was rearrangement to reach a random coil due to the attractions between nanoparticle SiC>2 and C-CS, and the molecular chains of C-CS bridge between suspension substrates upon the addition of S1O2. Further, SiC>2 particle was able to attract several chains of polyelectrolyte simultaneously, and the the floes formed by fines were shear-resistance.3. The adsorption phenomena and the interface interactions between C-CS and natural occurring CaCOs filler were systematically investigataed. The results showed that the flocculation mechnism of CaCC<3 suspension upon the addition C-CS was found to be a charge patch. In addition, the relationships between interface functions and molecular structures of the additives and the effects of the surrounding conditions such as electrolyte, pH, temperature on the adsorption and flocculation behaviors were also assessed. Compared to cationic starch, C-CS was more effective for retention of CaCO3 filler and drainage of the pulp, and thus was a potential founctional additives in alkaline papermaking system.