ENZYMATIC REMOVAL OF STICKIE CONTAMINANTS

Transcription

1 ENZYMATIC REMOVAL OF STICKIE CONTAMINANTS Marguerite S. Sykes John H. Klungness Forest Products Technologist Chemical Engineer Freya Tan Said Abubakr Chemical Engineer Supervisory Chemical Engineer USDA Forest Service USDA Forest Service Forest Products Laboratory 1 Forest Products Laboratory Madison, WI Madison, WI ABSTRACT Stickie contaminants are a costly problem for both recyclers of market pulp and the papermakers using it. Increased use of pressure sensitive adhesives (PSAs) has compounded this problem in recycled furnishes. Preliminary laboratory experiments using commercially available enzyme preparations at the ambient ph of the paper furnish (neutral ph range) have resulted in a significant reduction of all residual contaminants, including stickies. We found in recent laboratory trials that pulping with either cellulase, lipase, or a mixture of both enzymes at the ambient ph of mixed office paper removed the adhesives more effectively than did conventional alkaline pulping at ph 10. To confirm these preliminary results, a pilot plant scale-up, including pressure screening, will be run at the USDA Forest Service, Forest Products Laboratory, and results will be presented at the 1997 TAPPI Pulping Conference. Comparative process water quality will be monitored and residual stickies quantified at various stages of recycling. Process water will also be analyzed for colloidal and dissolved micro-stickies. INTRODUCTION Stickies, previously a pitch problem associated primarily with mechanical pulping of softwoods, have emerged as the primary problem in deinking mills and in papermaking with recycled content furnishes (1-3). Pressure sensitive adhesives (PSAs), hot melts, waxes, latex coatings, synthetic toner ink The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service. binders, and high lignin-containing fibers are some sources of stickies that appear during reprocessing wastepapers. Postal stamps, address labels, post-it notes, and self-seal envelopes are among the most common sources of PSAs in office papers. The use of PSAs is increasing rapidly; correspondingly, the stickie problem is escalating. Stickies precipitate on process equipment, redeposit on pulp fibers, or pass into process water, only to agglomerate unexpectedly and deposit when least desired or expected. In all cases, stickies contaminate pulp, cause mill shut-downs as a result of fouling of machinery, and cause breaks in the paper during papermaking or printing. Control of stickies is a costly mill problem (4) that is growing to epidemic proportions. Problems associated with stickies are limiting the use of recycled fiber now that there is little competitive economic edge over using virgin fiber. Contaminants, including stickies, are removed most effectively when they are large enough to be screened out of pulp slurries, have a density suitable for removal in cleaners, or are hydrophobic enough and of the appropriate size to be separated by flotation (5). Unfortunately, many PSAs are hard to separate by cleaners because their density is close to 1.0. Flotation is also problematic because the tackiness of stickies is often increased during the hot, alkaline repulping conditions that cause stickies to be too large for good removal. Current technology for stickie removal depends upon pressure screening through screens with decreasing slot sizes for the optimal removal. However, the elastic nature of stickies paired with their low glass transition temperature (Tg) enables them to pass through screen openings as much as a magnitude smaller than their actual diameter (6). Perhaps a more critical concern is the problem of colloidal or soluble micro-stickies. Many stickies have some degree of alkaline solubility (7,8); therefore, alkaline pulping can create micro-stickies that remain in process water. Recirculation of process water spreads the stickie contamination to other areas of the mill where process conditions alter the solubility of these colloidal or dissolved stickies. These undetected stickies, which resurface upon temperature or ph shock or agglomerate with other contaminants, are referred to as secondary stickies (9,10). The trend to system closure and reduced water use will only exacerbate the build-up of stickie concentration in process water. Enzyme preparations are proving to be excellent replacements for some conventional pulping, bleaching, and deinking chemicals. When used under neutral ph conditions and low sheer pulping, enzymes could prove to be an attractive alternative for stickie removal as well. Our previous studies have demonstrated that during enzymatic deinking of office 1997 Pulping Conference /687

2 paper and subsequent bleaching with pressurized peroxide, stickie contaminants were also significantly reduced (11). The enzyme-deinked sample contained more than 60% less stickie contaminants after bleaching than did conventionally deinked pulp (12). The urgency of the problem of stickie contaminants has brought us back to reexamine the possibilities of substituting alkaline repulping with enzyme treatment for stickie removal PSAs in particular. In the study reported here, two commercially available enzymes were used singly and in combination. Our previous experience with cellulase preparations for removing toner inks from office papers made the best performing commercially available product the logical choice for our stickie removal trials. Because some commercially available lipases are effective for pitch control, we also selected a lipase for our evaluations in this study. The mechanism by which enzyme preparations facilitate stickie removal is unclear. It is possible that cellulase mixtures release stickie particles from fiber surfaces by the same mechanism used to describe enzymatic toner ink removal (13,14). Gentle neutral ph pulping should keep stickie particles large so that many can be screened out of the slurry. Fibrils attached to other stickie particles could be severed by cellulase, thus facilitating removal of the additional stickies by flotation (15). Although the lipase preparation was designed to cleave ester linkages in naturally occurring stickie compounds extracted from wood during pulping and bleaching, it may work on this synthetic stickie substrate in a very different way when stickies become intermingled with printing inks and papermaking additives. Of equal importance is increased tackiness and increased solubility of adhesives during conventional alkaline treatment. Neutral pulping used with enzyme preparations should also improve the quality of the process water, Process water from enzyme-treated pulp should contain lower chemical oxygen demand, be less toxic, and contain fewer micro-stickies than that produced from alkaline repulping as a result of less solubilized material and alkalinity. Quantification of stickies is not as straight forward as residual ink measurement. Speck check scanners give a fairly accurate measurement of residual ink specks. However, many stickies are more three dimensional than are ink particles; therefore, measurements based on weight of stickies collected on screens or adsorbed on polyfoam (16,17) may give a more accurate indication of stickie removal. We used both methods for a complementary examination of micro-stickies. This study examined the effectiveness of several commercially available enzymes for stickie removal from mixed office paper by flotation and screening. Process water was analyzed for micro-stickie content. Results will be confirmed by scaleups at the USDA Forest Service, Forest Products Laboratory (FPL), pilot plant, hopefully in time to report the results at the 1997 TAPPI Pulping Conference. EXPERIMENTAL This study parallels an extended study of the U.S. Postal Service (USPS) that is evaluating recyclability of various PSAs in the FPL pilot plant. Unprinted copy paper is being used for the USPS evaluation to eliminate variables in paper stock and interactions of adhesives with inks or other contaminants. However, in mill recycling a heterogeneous paper furnish containing multiple printing inks and sources of stickie contaminants is the norm. We selected a representative post-consumer paper furnish to observe resulting interactions of stickies with printing inks and other stickie contaminants while measuring the relative effectiveness of enzymes for stickie removal. Paper Furnish Mixed office paper containing approximately 50% paper printed with noncontact toner inks was used in this study. Four kilograms of this paper was shredded and fiberized in a Hobart mixer for 5 in at 12% consistency and 45 C. The fiberized pulp was watered and shredded to ensure a uniform mix and divided into twenty 200-g (ovendry basis) samples. Each sample spiked with 50 g of an adhesive/paper mixture. The adhesive mix was made similarly in a Hobart mixer by fiberizing 1 kg additional office paper on which 18 g of postal stamps were attached. Adhesives Two PSAs used in this study an acrylate and an isoprene rubber-based latex-were select because they are especially difficult to remove. These experimental adhesives compounded for recyclability had been applied by the adhesive manufacturers on postage stamp paper for the USPS evaluations. Enzyme Preparations The enzyme preparation used in this study was a liquid concentrate also containing some hemicellulase activities. The lipase preparation, also a liquid concentrate, contains virtubefore use to ensure good distribution in the pulp. In our ally no side activities. Both enzyme preparations were diluted 1:100 from the commercial concentrates with deionized water experimental use of these enzymes, we added the appropriate amount of diluted enzyme to give 0.02% to 0.2% enzyme concentrate/kg of ovendried pulp (on a weight/weight basis) to optimize the dosage level appropriate for stickie removal. 688/ TAPPI Proceedings

3 Pulp Processing Three trial batches of paper and two different PSAs were used for comparison of enzymatic repulping with conventional alkaline repulping. In all instances, we found that pulping with enzymes at either the ambient ph of the recovered paper or when adjusted to 7.8 was more effective than the alkaline repulping at ph 10. Table I summarizes the results of the acrylate PSA on unprinted office paper experiments. The initial pulper batch contained approximately 900 ppm (parts per million) adhesive particles. After flotation and screening, the ph 10 sample contained 705 ppm residual stickies com- pared with 531, 439, and 402 ppm for the batches pulped with a lipase, cellulase, or a mixture of both enzymes, respectively. All the enzyme treatments used ambient ph of the furnish approximately 8.5. Flotation. Eight liters (containing 80 g ovendried fiber) of the pulped slurry were transferred to an 8-L capacity flotation cell and floated for 3 min. at 1% consistency, Rejects were scraped off manually with the stable foam, Accepts were drained over an 80-mesh laboratory screen to approximately 8% solids. Some of the floated pulp was screened as described in the following. In addition, handsheets were made for contaminant analysis. Screening. Thirty grams of ovendried previously floated pulp was screened through the 6-cut (0.15-mm) screen in the Pulmac Analyzer at low consistency. Screen rejects were collected and transferred to filter paper for examinations. Accepts were made into handsheets according to TAPPI test method T-205-om88 and later used for relative stickie quantification. Pulping was done at 12.5% consistency using softened city water at 48 to 50 C for makeup. Diluted enzymes were added into the pulper with 0.1% non-ionic surfactant (based on ovendried weight of pulp). The surfactant used on these studies was a non-ionic surfactant that had performed well for use with enzyme preparations. Pulping was done for 25 min. using a Hobart mixer equipped with a stirring paddle. Pulp-, ing temperature was maintained during pulping by circulat- ing 50 C water through the jacketed bowl, The ambient ph of the paper stock, approximately 8.5, was used unless indicated otherwise for the enzyme treatments. Control batches were, pulped both at ambient ph (with water and surfactant only) or adjusted to ph 10 with sodium hydroxide, the ph used in the protocol for the USPS adhesive evaluation. In our laboratory experiments, pulper stock was diluted to 1% consistency with 50 C water and stirred gently with a rotary mixer equipped with a slotted paddle for an additional 45 min. to simulate the holding time required for transfer of stock from the pulper to dump chest prior to pressure screening in a mill setting. Micro-stickie testing. Micro-stickies contained in processed water were measured after screening 100-g oven dried pulp furnished through a vibrating 6-cut (0.15-mm) Valley flat screen according to the Doshi Method (16). The dilute slurry of screened pulp was caught over 200-mesh screen and dewatered over a 25-L container. The screened pulp was diluted with 6 L of the water collected during screening to give approximately the 1.8% consistency indicated for the Doshi method. The pulp slurry was heated to 65 C on a hot plate and stirred at the lowest speed setting for 10 min. with a rotary stirrer equipped with a stirring pad- dle. Three squares of ovendried, weighed polyfoam were sus- pended into the pulp slurry on a weighted copper wire and stirring continued for 30 min. The polyfoam was removed, rinsed with cold water, and dried to` determine the weight of the micro-stickies adsorbed onto the polyfoam. DISCUSSION OF RESULTS More residual stickies were -detected in the cleaned pulps as the pulping ph increased. Figure 1 illustrates the effect of ph during recycling on contaminant removal. A batch. of Table I Residual stickies from acrylate PSA on unprinted office furnish after flotation and screening on 0.15-mm screen. Dyed handsheets were counted on an Optomax Testing. Handsheets were dyed with Morplas blue dye solubilized in Scanner for stickies greater than 0.02 mm 2 heptane, which stains stickies dark blue on a light blue fiber background. Dye was brushed on to the felt (rough) sides of 10 handsheets per sample run. The wire (smooth) sides were counted for specks greater than 0.02 mm 2 (in the 0.02 mm 2 to 4.0 mm 2 TAPPI dirt count size range) on an Optomax Speck Check scanner at 100 detection level, 600 dpi resolution, no shade compensation. Stickies 0.02 to < mm 2 mm 2 Alkaline control ph Lipase treated Cellulase treated Treated with combined cellulase/lipase 1997 Pulping Conference /689

4 Fig. 1. Effect of pulping ph on stickie removal. Residual stickies were counted after flotation and screening. Fig. 2. Comparative stickie removal after enzyme treatments at ph 8.5. Control was pulped at ph 10. office paper spiked with a rubber-based PSA. contained an initial stickie count of 1,714 ppm. At conventional ph 10 pulping, the stickie count was reduced to 364 ppm after flotation and screening. In contrast, the samples pulped with an enzyme preparation, a lipase in this case, at ambient ph contained approximately 170 ppm, 50% less. When sulfuric acid was added to the paper furnish to adjust to a more neutral ph Table II. Micro-stickies adsorbed from 6-L process water after (0.15 mm) screening of 100-g pulper samples using Doshi polyfoam adsorption method Weight of adsorbed cantaminants (mg) Control ph , the residual stickies decreased to 100 ppm using the Lipase 7.9 lipase preparation. However, variability in the results ob- Cellulase treated 5.1 tained from replicates showed that using the ambient ph of Combined 3.4 the paper furnish was almost as effective as were the results lipase/cellulase obtained when the ph was decreased to 7.8. Enzyme preparations were-more effective in removing stickie contaminants from recycled pulp than the conventional alkathat these small-scale experiments will translate well into, pilot plant trials using more typical mill processing. Clean recycled pulp is the obvious desired outcome of deink- ing. However, the relatively unexplored area of the fate of stickies solubilized during processing needs additional atten- tion. If stickies can be collected on screens or in flotation froth, that would he ideal. However, if the stickies are solubilized or converted into colloidal micro-stickies in the Figure 2 summarizes the trials optimizing enzyme dosage levels. These batches were spiked with half as much rubber- based PSA as was added in the work shown in Figure 1. The pulper batch contained approximately 900 ppm stickies. After pulping the control at ph 10, the residual stickies were reduced to 115 ppm. As expected, all the enzyme-treated pulps contained less residual contaminants after processing. The cellulase treatments averaged about 70 ppm, the lipase 55 ppm, and combinations of both enzymes in different ra- tios averaged 52 ppm. Low levels of the cellulase were used: 0.02% to 0.05% based on ovendried pulp. Considerably higher levels of lipase were used: % to 0.5%; however, the lowest level of lipase performed best. Mixtures of both enzyme preparations gave the best removal. We assume pulper, appropriate attention has to be given to cleaning up the process water. Preliminary analysis of process water from 100 g of screened pulp from several experiments suggest, as expected, that when enzyme preparations are used at neutral ph range for repulping, fewer micro-stickies are created than when conventional alkaline repulping is used (Table II). Additional testing will be conducted on water samples collected in pilot plant scale-ups to verify our results. CONCLUDING REMARKS line repulping for the pressure sensitive adhesives (PSAs) that we evaluated in these laboratory trials. Not only was the pulp cleaner after flotation and screening when enzyme preparations were used in the pulper; but also fewer micro-stickies were contained in the process water following screening. We assume that repulping under mild conditions minimizes the tackiness and solubility of stickie particles, thus permitting them to be removed more readily by screening. This finding suggests that enzyme preparations are effective chemical substitutes for reprocessing office papers. 690/ TAPPI Proceedings

5 The scale-up trial of enzyme repulping will be done at ambient ph of the copy paper used in the USPS study with the comparable PSA level applied, was water will be col- 22, (1977). lected after pulping and after pressure screening for microstickie measurements, biological oxygen demand, and chemical oxygen demand for comparison with water gathered from the corresponding alkaline repulping trial. Sampling and testing will follow the FPL/USPS protocol for repulping PSAs. 8. Abraham, S., Stickies Pira International Research Programs, In: Paper Recycling Challenge Stickies, Vol. 1: 9. Hsu, N.N.C., Progress in Paper Recycling, Stickies The Importance of Their Chemical and Physical Properties, 6(1): 63 (1996). 10. Carre, B., Brun, J., and Galland, G., In: Secondary Stickies: Definition, Formation Mechanism and Characteri- ACKNOWLEDGMENT zation, In: Paper Recycling Challenge Stickies, Vol. 1: 57 (1997). We thank Neal Franks of Novo Nordisk Biochem of North America for his continuing counsel and supply of enzyme preparations for our research. REFERENCES 1. Ferguson, L.D., Why is a Sticky Like A Cockroach?,. In: 1996 Proceedings of Wastepaper VI, Chicago, IL. (1996). 2. Woodward, T. W., Controlling Contaminants in the Production and Use of Deinked Pulp, In: 1996 Proceedings of Wastepaper VI, Chicago, IL (1996). 3. McKinney, R.W.J., (ed.), Technology of Paper Recycling, Blackie Academic and Professional Press, London, p. 53, (1995). 4. Friberg, T., Progress in Paper Recycling, Cost Impact of Stickies, 6(1): 70 (1996): 5. Ling, T.F., Sutman, F.J., Richmann, S.K., and Letscher, M.K., Tappi Journal, Effect of Pulping Conditions on Stickies Behavior in Office Waste Deinking Systems, 77(7): 143 (1994): 6. Pikulin, M.A., Stickies and their impact on recycled fiber content, fine papers, In. Paper Recycling Challenge Stickies, M. Doshi and J. Dyer (eds.), Appleton, WI, Vol. 1: 89 (1997). 7. Cathie, K., Haydock, R., and Dias, I., Understanding the fundamental factors influencing stickies formation and deposition. 1st Research Forum on Recycling; CPPA, Toronto, Canada, p (1991). 11. Sykes, M., Klungness, J.H., Abubakar, S., and Tan, F., Progress in Paper Recycling, Upgrading Recovered Paper With Enzyme Pretreatment and Pressurized Peroxide Bleach ing, 5(4): 39,(1996). 12. Abubakr, S.,. Bormett, D., Sykes, M., and Klungness, J., Stickies: Problems and Solutions In: Paper Recycling Challenge Stickies, Vol. 1: 40 (1997). 13. Jeffries, T., Klungness, J.H., Sykes, M., and Cropsey, K., Tappi Journal, Comparison of Enzyme-Enhanced With Conventional Deinking of Xerographic and Laser-Printed Paper, 77(4): 173 (1994). 14. Jackson, L.S., Heitmann, J.A., and Joyce, T.W. Tappi Journal, Enzymatic Modifications of Secondary Fiber 76(3): 147(1993). 15. Pan, R., Paulsan, F., Johnson, D., Bousfield, D., and Thompson, E. A global model for predicting flotation efficiencies: In: Proceedings of TAPPI 1993 Pulping Conference, TAPPI PRESS, Atlanta, p (1993). 16. Doshi, M.R., Progress in Paper Recycling; Quantification of Stickies; 4(2): 103 (1995). 17. Ouellette, A.J., Troubleshooting for stickies using the Doshi method, In: Paper Recycling Challenge Stickies, Vol. : 252 (1996) Pulping Conference /691