| The use of fillers in paper products can provide cost and energy savings, improved paper properties, and increased productivities. Also, fillers can be used in the production of papers with special/functional properties. There are certain limitations associated with the use of fillers, such as unsuitability of cost-effective and high-performance calcium carbonate fillers in acid and pseudo-neutral papermaking, negative effects of increased filler loading on paper strength, sizing, and retention, tendencies of fillers to cause wire abrasion, and limited filler dispersability. Therefore, the research work regarding the avoidance or elimination of these limitations is much needed. In this work, in order to improve the acid-resistant property of papermaking grade precipitated calcium carbonate filler, to enhance the critical properties of filled papers, and to alleviate the negative effects of filler loading on paper strength, dissolution-inhibiting modification and organic encapsulation modification of precipitated calcium carbonate filler were studied, and the use of modified fillers in papermaking was explored.1. Dissolution-inhibiting modification of precipitated calcium carbonate filler and use of the modified fillers in papermakingThe dissolution behavior of precipitated calcium carbonate filler was measured and analyzed. Several methods of filler modification were used to obtain modified fillers in powder form, and the dissolution-inhibiting effects of modifiers were preliminarily explored. The dissolution behavior of calcium carbonate filler was found to be very striking, and the use of modifiers including alum, alum/sodium silicate, sodium silicate/hydrochloric acid, zinc chloride, and sodium silicate/zinc chloride all lowered the dissolution tendency of calcium carbonate filler to a certain degree.Phosphoric acid, phosphoric acid/cofactor, silicate-based modifiers, calcium chloride, and calcium chloride/cofactor were used in the dissolution-inhibiting modification of precipitated calcium carbonate filler to obtain modified fillers in slurry form, and the effects of the relevant modifiers were studied. For the use of phosphoric acid and phosphoric acid/cofactor in filler modification, phosphoric acid, phosphoric acid/sodium hexametaphosphate, phosphoric acid/disodium ethylenediamine tetraacetate, phosphoric acid/aluminum sulfate, and phosphoric acid/aluminum chloride were all found to be effective in the dissolution-inhibiting of filler, and the effect of phosphoric acid/sodium hexametaphosphate was the most striking, and the optimum dosages of phosphoric acid and sodium hexametaphosphate were found to be 4% and 1%, respectively. For the use of sodium-silicate-based modifiers in filler modification, sodium silicate/aluminum sulfate, sodium silicate/aluminum chloride, sodium silicate/phosphoric acid, sodium silicate/phosphoric acid/sodium hexametaphosphate, and sodium silicate/phosphoric acid/sodium polyacrylate were all proved to be effective in dissolution-inhibiting of the filler, and the effect of sodium silicate/phosphoric acid/sodium hexametaphosphate was the most striking, and the optimum dosages of sodium silicate, phosphoric acid, and sodium hexametaphosphate were found to be 4% and 1%, respectively. For the use of calcium chloride and calcium chloride/cofactor in filler modification, calcium chloride, calcium chloride/sodium hexametaphosphate, calcium chloride/disodium ethylenediamine tetraacetate, and calcium chloride/phosphoric acid were all proved to be effective in dissolution-inhibiting of the filler, and the effect of calcium chloride/phosphoric acid was the most striking, and the optimum dosages of calcium chloride and phosphoric acid were found to be 1% and 4%, respectively. The surface characteristics of phosphoric acid/sodium hexametaphosphate modified filler, sodium silicate/phosphoric acid/sodium hexametaphosphate, and calcium chloride/phosphoric acid modified filler was slightly different from that of unmodified filler, as shown from EDAX analysis and SEM observations.Sodium silicate/zinc chloride based modifiers, titanium sulfate/sodium hydroxide, and nanostructured titanium dioxide were used in the dissolution-inhibiting modification of precipitated calcium carbonate filler to obtain modified fillers in powder form, and the effects of the relevant modifiers were studied. For the modifiers including sodium silicate/zinc chloride, sodium silicate/zinc chloride/sodium hexametaphosphate, sodium silicate/zinc chloride/sodium hexametaphosphate/phosphoric acid, and sodium silicate/zinc chloride/phosphoric acid, the dissolution-inhibiting effect of sodium silicate/zinc chloride/sodium hexametaphosphate/phosphoric acid was the most striking, and the optimum dosages of sodium silicate, zinc chloride, sodium hexametaphosphate and phosphoric acid were 10%,3%,1%, and 0.2%, respectively. The use of titanium sulfate/sodium hydroxide decreased filler dissolution to a certain degree, and the optimum dosages of titanium sulfate and sodium hydroxide were found to be 20% and 2%, respectively; however, the effect of nanostructured titanium dioxide (P25) was found to be very limited. The surface encapsulation effect of sodium silicate/zinc chloride/sodium hexametaphosphate/phosphoric acid on precipitated calcium carbonate filler was confirmed by XPS analysis and SEM observations, and the enhancement in acid-resistant property of the filler was assumed to be presumably caused by the surface encapsulation and precipitation of acid-stable SiO2 on filler surfaces, which might act as barriers and block the dissolution sites.The use of acid-tolerant modified precipitated calcium carbonate fillers including phosphoric acid/sodium hexametaphosphate modified filler, sodium silicate/phosphoric acid/sodium hexametaphosphate modified filler, calcium chloride/phosphoric acid modified filler, sodium silicate/zinc chloride/sodium hexametaphosphate/phosphoric acid modified filler, and titanium sulfate/sodium hydroxide modified filler in papermaking of deinked pulp was explored. Compared with the unmodified precipitated calcium carbonate filler, the use of acid-tolerant modified precipitated calcium carbonate fillers provided considerably more brightness improvement in papers. The pH values of furnish slurries containing acid-tolerant modified precipitated calcium carbonate fillers were all lower than that of furnish slurry containing unmodified precipitated calcium carbonate filler. The retention performace of sodium silicate/phosphoric acid/sodium hexametaphosphate modified precipitated calcium carbonate filler was superior to that of the unmodified precipitated calcium carbonate filler, and the retention performace of sodium silicate/zinc chloride/sodium hexametaphosphate/phosphoric acid modified precipitated calcium carbonate filler was generally equivalent to that of the unmodified precipitated calcium carbonate filler. The effects of acid-tolerant modified precipitated calcium carbonate fillers on the strength and air permeability of the papers were closely related to the specific filler categories. Sodium silicate/phosphoric acid/sodium hexametaphosphate modified precipitated calcium carbonate filler and sodium silicate/zinc chloride/sodium hexametaphosphate/phosphoric acid modified precipitated calcium carbonate filler were proved to be comparatively competitive, as far as their use in papermaking is taken into consideration.On balance, in addition to the improvement in acid-resistance of the filler, the use of sodium silicate/phosphoric acid/sodium hexametaphosphate or sodium silicate/zinc chloride/sodium hexametaphosphate/phosphoric acid in the dissolution-inhibiting modification of papermaking grade precipitated calcium carbonate was found to be capable of improving the use of precipitated calcium carbonate filler in papermaking of deinked pulp.2. Organic encapsulation modification of precipitated calcium carbonate filler and use of the modified fillers in papermakingThe use of starch/sodium oleate/alum in organic encapsulation modification of precipitated calcium carbonate filler was studied, and the encapsulated amount values of modified fillers were measured, and the use of modified fillers in papermaking was explored. In the presence of alum, the use of starch/sodium oleate was effective in filler modification, and starch/fatty acid complex can presumably be effectively precipitated and anchored on the filler surfaces. The encapsulated amount of the modified filler increased with the increasing alum dosage in the range of 16% to 20%. When the loading level was 20%, filler modification with starch/sodium oleate/alum only had negligible effect on furnish drainage, however, the strength properties of the filled papers were strikingly improved, and filler retention was also improved at certain conditions. Additionally, filler modification with starch/sodium oleate/alum had certain effect on the optical properties of the filled papers, brightness was negatively influenced to a minor degree; however, opacity and light scattering coefficient increased slightly at certain alum dosages. The surface encapsulation of the filler was confirmed by XPS and XRD analysis as well as SEM observations.The use of chitosan/acetic acid/sodium hydroxide in organic encapsulation modification of precipitated calcium carbonate filler was studied, and the encapsulated amount values of modified fillers were measured, and the use of modified fillers in papermaking was explored. The modifiers were proved to be effective in filler modification, and there was almost no chitosan loss during the modification process. After modification, zeta potential of the filler increased, and the drainage of the furnish slurry was slightly improved, and the dissolution behavior and brightness of the filler were also changed. The modified precipitated calcium carbonate fillers were found to be quite effective in strength improvement of the filled papers as compared with unmodified precipitated calcium carbonate filler. The organic encapsulation modification of precipitated calcium carbonate filler lowered the brightness of the filled paper, and its effect on the opacity and light scattering coefficient of the filled paper was not striking. Additionally, the retention performances of modified precipitated calcium carbonate fillers were found to be superior to unmodified precipitated calcium carbonate filler when the encapsulated amount was below 5%.The use of carboxymethyl cellulose and alum in organic encapsulation modification of precipitated calcium carbonate filler was studied, and the role of alum in filler modification was identified, and the use of modified fillers in papermaking was explored. In the presence of alum, carboxymethyl cellulose was capable of being effectively encapsulated and achored on the filler surfaces at certain conditions. The alum dosage was found to be much critical to the effective precipitation and encapsulation of carboxymethyl cellulose on filler surfaces. When the dosages of carboxymethyl cellulose and alum were 4% and 12%(based on the dry weight of precipitated calcium carbonate), respectively, filler modification significantly improved filler retention by as much as 71.5%, and the brightness and opacity of the filled paper were strikingly enhanced; however, paper strength was practically unchanged. XPS analysis of the modified filler confirmed the encapsulating effect of carboxymethyl cellulose and alum on the filler. SEM observations of the handsheets (filled and unfilled) indicated that modified filler particles were more effectively adhered and bonded to the pulp fibers, in comparison to unmodified filler particles, and the encapsulating effect of the modifiers on the filler was further confirmed.On balance, the above mentioned organic encapsulation modification methods were all proved to be capable of improving the use of precipitated calcium carbonate filler in papermaking, and the negative effect of filler loading on paper strength can all evidently be alleviated as indicated from the experimental results, and the commercialization of these filler modification methods can potentally be beneficial to the papermaking industry. Filler modification with carboxymethyl cellulose and alum is possibly of greater simplicity in comparision to the other two methods.
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