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DETERMINATION OF SILICONE DEFOAMERS
IN MILL PITCH DEPOSITS

Authors

Bruce Sithole and Denise Filion

Company

Pulp and Paper Research Institute of Canada, Pointe-Claire, QC, Canada

Keywords

foam inhibitors, deposits, solvents, solid phases, extraction, silica, model compounds, analysis, spectroscopy, infrared spectroscopy

 

 

 

 

ABSTRACT

Defoamers are often used to control or reduce foam problems in a variety of pulp and papermaking processes. It has been recognized that non-judicious use of oil-based defoamers containing amide can lead to undesirable deposition problems. Earlier work at Paprican led to the development of analytical methods for quantitative determination of these defoamers in pitch deposits. Amide-based defoamers have been largely supplanted by water-based or water-extended silicone defoamers that are claimed to be non-depositing. However, mill experience and research has shown otherwise. In order to determine the contribution of these defoamers to pitch deposition problems, we have developed methods for determination of low-molecular-weight silicone defoamer components in defoamer formulations and in mill deposits. The methods entail separation by solid-phase extraction and identification of the separated components by FTIR. Analysis of mill deposits showed that the contribution of hydrocarbon oils to the deposits was much higher than that of silicone oils, in spite of their relatively smaller proportions in defoamer formulations.

INTRODUCTION

Defoamers are often used to control or reduce foam problems in a variety of pulp and papermaking processes1. It has been recognized that non-judicious use of oil-based defoamers containing amide particles can lead to undesirable deposition problems. Earlier work at Paprican led to the development of analytical methods for quantitative determination of amide-based defoamers in pitch deposits2. Water-based or water-extended defoamers have largely supplanted amide-based defoamers. These defoamers often contain hydrocarbon oils, silicone oils, silica, plus traces of surfactants, and are claimed to be non-depositing3. However, mill experience and research has shown otherwise. For example, their propensity to deposit in laboratory systems was demonstrated by Clas and Allen4. In addition, a number of mills using these defoamers have experienced deposition problems.

In order to determine the contribution of these defoamers to pitch deposition problems, analytical procedures to determine silicone defoamer components in deposits are needed. Silicone oil, also named polydimethylsiloxane, comes in a variety of molecular weights. Thus a variety of defoamers with silicone oils of varying molecular weights can be envisaged5. In general, the defoamers are classified as low molecular weight (up to 10,000 Daltons), intermediate molecular weight (10–30,000 Daltons) and high molecular weight (>30,000 Daltons). Earlier defoamer formulations were based on low molecular weight silicone oils but, lately, more and more formulations based on high molecular weight silicone oils are being introduced into the pulp and paper industry.

We set out to develop analytical procedures for quantitative determination of components in deposits from mills using silicone-based defoamers. From past experience we know that mill deposits contain a variety of organic and inorganic matter. The organic matter includes oils, wood resin compounds, and metal soaps6. Thus the developed analytical procedures should be able to separate defoamer components from other deposit components. In this report we outline the development of procedures for quantitative separation of defoamer components from compounds commonly found in pitch deposits. The procedures were applied to defoamer formulations and pitch deposits from kraft mills that used silicone defoamers.

GENERAL APPROACH

We initially tried to measure the silicone oil by ash content using standard procedures, e.g., PAPTAC method G.11, but abandoned the technique because the ashing procedure led to volatilization and accumulation of silica particles all over the inner and outer surfaces of the crucibles and inside the furnace.

We then investigated the solubility of silicone oils in various solvents. Once a suitable solvent was identified, several methods were evaluated for measuring the amounts of silicone oil and silica present in defoamer formulations. These included liquid–liquid extraction, membrane filtration, and gravimetry after freeze-drying. Membrane filtration did not work as it resulted in blockage of the membrane by the fine particulate matter in the defoamer formulations. Liquid-liquid extraction was not suitable either because the extraction led to the formation of stable emulsions that could not be broken up.

We therefore explored the feasibility of using solid phase extraction (SPE) on silicone defoamers and on solvent extracts of pitch deposits. Although SPE has been used in the analysis of pulp and paper matrices, e.g., lipophilic extractives in wood chips7–9, additives and pitch deposits10-11, this is, as far as we know, the first attempt to use SPE in the analysis of silicone defoamers and silicone defoamer components in pitch deposits.

Samples

a) Silicone oils, wood resin components and metal soaps were used as model compounds for method development.

b) Defoamer #1 was a water-extended low molecular weight silicone oil defoamer from Supplier A.

c) Defoamer #2 was a water-based low molecular weight silicone oil defoamer from Supplier B.

d) Defoamer #3 was a water-based low molecular weight silicone oil defoamer from Supplier B.

e) Deposit A was collected from the #2 Centrisorter of a bleached kraft pulp mill. It was a brown, powdery and chunky mixture.

f) Deposit B was collected from the #1 primary Centrisorter of the same mill as deposit A. It was a dark brown, waxy and chunky solid.

g) Deposit C was collected from pulp M/C white water chest of a second bleached kraft pulp mill. It was a black and sticky solid deposit.

All deposit samples were kept frozen until use. The deposits and defoamers were freeze -dried prior to testing. Each sample was analyzed by FTIR to ascertain its composition.

Apparatus

  • Isolute brand SPE tubes were obtained from Chromatographic Specialties:
    - Silica SPE cartridge: 3 mL tubes loaded with 500 mg of silica particles
    - C18-silica SPE cartridge: 3 mL tubes loaded with silica particles bonded to C18 hydrocarbon chains.
  • A homemade gravity-elution rack made from Plexiglas and capable of accommodating 6 samples, was used to support the SPE tubes.
  • All solvents used were of high purity grade.
  • Solvent extraction: Soxtec Tecator.
  • Reacti-Therm evaporator.

Sample Preparation

  • a) Defoamers
    The solid content of each defoamer was determined by freeze-drying a known weight of the emulsion. An amount of defoamer emulsion equivalent to about 20 mg of dry defoamer was transferred to a pre-weighed vial and freeze-dried to determine its actual weight. The material was then used as a sample for SPE studies.
  • b) Deposits
    Solvent extraction: The deposits were dried before extraction and then homogenized by milling in a pestle and mortar. A weighed amount of the deposit was transferred to a thimble for solvent extraction. The sample was extracted sequentially with acetone and chloroform.
    After extraction, the extractives were re-dissolved in the corresponding extraction solvent and an aliquot containing about 20 mg was transferred to a pre-weighed vial. The samples were dried under nitrogen and then freeze-dried. The vial was weighed again and the weight recorded.

METHOD DEVELOPMENT

  • a) Selection of solvents for silicone oils
    A variety of solvents were tested to ascertain which ones would be good solvents for silicone oils. About 0.5 g of a low molecular weight silicone oil were shaken with 20 mL of solvent in a test tube at room temperature. Solubility of the silicone oil was assessed by visual inspection.

    The data in Table 1 seem to indicate that the best solvents for silicone oil are diethyl ether, hexane, and methyl ethyl ketone as they result in clear solutions that do not cause foam problems. Although acetone resulted in a cloudy solution, the dissolution of silicone oils was nevertheless quantitative. Due to the nature of the materials commonly found in pitch deposits, the solvent extraction scheme selected was acetone first followed by chloroform.
     
  • b) Solid phase extraction procedures
    A flow chart of the separation procedure is illustrated schematically in Figure 1. Published work on column chromatography on extractives and deposits helped in selecting the appropriate separation phases2,12. Silica is a polar medium that can be used to separate hydrocarbon oils from wood resin matter, hence our choice of silica SPE for normal phase separation (SPE is a modern form of column chromatography). C18-silica is a non-polar medium that is used to extract non-polar compounds from polar matrices (reverse phase separation). Important parameters in SPE are the amount of sample loaded onto the column and the solvent volumes used to rinse the columns.


Figure 1. SCHEMATIC FLOW DIAGRAM FOR SEPARATION OF PITCH DEPOSIT COMPONENTS BY SOLVENT EXTRACTION AND SOLID PHASE EXTRACTION TECHNIQUES

fIGURE 1

 

Table 1. SOLUBILITY OF SILICONE OIL IN VARIOUS BOILING SOLVENTS

Solvent

Comments

Acetone

Soluble, but cloudy solution

Acetonitrile

Not soluble

Chloroform

Clear and foaming

Dichloromethane

Soluble and foaming

Diethyl ether

Soluble

Ethyl acetate

Clear and foaming

Hexane

Soluble

Methanol

Not soluble

Methyl ethyl ketone

Soluble

Toluene

Clear and foaming

Water

Not soluble

 

a) Silica SPE:

  • The column is rinsed with 10 mL hexane to solvate the solid extraction phase and must not be allowed to dry in between solvents.
  • A pre-weighed vial is placed under the column to collect the eluate.
  • The sample in a vial is loaded onto the column with a minimum volume of hexane (~1 mL).
  • The vial is then rinsed with 5 ื 2 mL portions of hexane that are in turn used to rinse the SPE column by transfer via a Pasteur pipette. The solvent flow through the column is achieved by gravity.
  • The vial with hexane eluate is replaced with a fresh one and the column is rinsed with 10 mL of chloroform to elute the second fraction.
  • The third fraction is finally eluted with 10 mL of methanol into a third pre-weighed vial.
  • The vials containing the different fractions are dried under nitrogen and the extracts further dried by freeze-drying overnight.

 In general, hexane elutes hydrocarbon oil, chloroform elutes lipophilic wood extractives, and methanol elutes metal soaps (Na) and oxidized material (polar compounds). When silicone oil is present, the chloroform eluate contains both wood resin and silicone oil. To verify the separation, the fractions are dried and identified by FTIR spectroscopy. Wood resin is then separated from silicone oil using C18-silica SPE.

b) C18-silica SPE:

  • 10–20 mL of chloroform are used to wash the column to remove loose silica particles that may be present.
  • The column is then solvated by washing with methanol.
  • A pre-weighed vial is placed under the column to collect the first fraction.
  • The sample (chloroform fraction from the silica SPE containing silicone oil and wood resin) is then loaded onto the column by transferring with 1 mL of methanol.
  • The vial is then rinsed with 5 ื 2 mL portions of methanol that are in turn used to rinse the SPE column by transfer via a Pasteur pipette. The solvent flow through the column is achieved by gravity.
  • The pre-weighed vial (methanol fraction) is removed and replaced with a fresh one.
  • A second solvent, chloroform (10 mL), is used to rinse the vial and elute the second fraction (silicone oil).
  • The vials containing each solvent are dried under nitrogen, and the extracts further dried by freeze-drying overnight. The vials are then weighed and the identities of the extracts confirmed by FTIR.

During method development preliminary experiments were done using cartridges with luer endfittings, which were attached to empty plastic syringe barrels to serve as solvent reservoirs. However, FTIR analyses of the eluates showed the presence of unexpected silicone oil when blank samples were processed. The contamination originated from the silicone lubricant in the plastic syringe barrels. This problem was avoided by using vacuum type cartridges that do not require syringes and can be eluted by gravity flow or application of vacuum.

In addition, during initial use of the C18-silica SPE cartridges, we found that blank eluates were contaminated with silica particles. The silica particles, probably remnants of the manufacture of the C18-silica from silica particles, could be removed by washing of the SPE cartridges with 20 mL of chloroform prior to the solvating step with methanol.

c) Tests for selectivity of SPE using model compounds

Results for separation of model compounds found in pitch deposits are shown in Table 2. It is evident that silica SPE is ideal for separating hydrocarbon oils from silicone oils and wood resin compounds. A silicone oil/wood resin mixture can be fractioned by C18-silica SPE but some glycerides, waxes and calcium soaps may co-elute with silicone oil and interfere in its quantitative determination. This is, however, not a problem since in real samples wood resin components are separated from metals soaps by sequential extraction with acetone and chloroform6. FTIR analysis of the chloroform fractions of C18-silica SPE should be performed to ascertain the purity of the eluted silicone oil fraction from real mill samples. In reality though, glycerides are labile compounds that are rarely found in mill deposits.

 

TABLE 2. SEPARATION OF MODEL COMPOUNDS BY SPE

Compound

Amount Eluted

Silica SPE

C18-silica SPE

Hexane

CHCl3

MeOH

MeOH

CHCl3

Silicone oil

 

100%

 

 

100%

Hydrocarbon oil

100%

 

 

 

100%

Calcium oleate

 

13%

51%

13%

74%

Calcium resinate

 

 

87%

69%

10%

Sodium oleate

 

 

95%

99%

 

Myristic acid

 

100%

 

99%

 

Linoleic acid

 

90%

 

97%

 

Docosanoic acid

 

93%

 

5%

91%

Dehydroabietic acid

 

100%

 

98%

 

Levopimaric acid

 

86%

15%

99%

 

Monopalmitin

 

3%

84%

95%

 

Dipalmitin

 

91%

 

 

100%

Tristearin

 

87%

 

 

100%

Cholesteryl stearate

 

94%

 

 

100%

 

d) Analysis of defoamer formulations

To evaluate the SPE methodology, several different types of defoamers were tested. Aliquots of dried defoamer samples were loaded onto silica SPE columns and washing with solvents eluted their component fractions. The fractionated components were dried, weighed and their identities determined by FTIR.

e) Spiked recoveries of silicone oil from deposits

 Previously analyzed deposits that did not contain silicone defoamers were spiked with known amounts of silicone oil and then processed through the extraction procedures to validate the recovery of the silicone oil. The results indicated that acetone extraction quantitatively recovered the spiked silicone oil (101 and 104% recoveries) despite the observation that the silicone/acetone mixture was cloudy. This confirms that acetone is a suitable solvent for extraction of low molecular weight silicone oils from pitch deposits.

f) Analysis of mill deposits

The deposits were extracted by sequential extraction with acetone and chloroform using a Soxtec extraction apparatus as described in a previous report13. The extracts were analysed by FTIR to ascertain their general composition and then fractionated by silica SPE (acetone extracts) and C18-silica SPE (chloroform extracts) to further separate the components present in them. The fractionated components were dried, weighed and their identities determined by FTIR.

RESULTS

a) Analysis of defoamer formulations

The results, shown in Table 3, indicate that defoamers contain 70–80% volatiles, 15–25% silicone oils, 0.3–0.4% hydrocarbon oils, and 2–3% silica particles. The FTIR spectra of the hexane fractions confirmed the presence of hydrocarbon oils as evidenced by the absorption bands at 1460 and 1375 cm−1 (Figure 2, top spectrum). The silicone oils in the chloroform fractions were confirmed by silicone absorption bands at 1261, 1098, 1022, and 802 cm−1 (Figure 2, middle spectrum). The FTIR spectra of the methanol fractions (Figure 2, bottom spectrum) seemed to indicate the presence of an ester peak (1737 cm−1): the ester peak was present in the chloroform fraction and was accentuated in the methanol fraction.

 

TABLE 3. ANALYSIS OF DEFOAMER SAMPLES BY SILICA SPE

Defoamer

Water and volatiles, %*

Silica SPE

Unknown fraction (methanol eluate), %**

Silica, %***

Defoamer 1-
Supplier A
Water extended

79.7

0.40

14.8

3.0

2.1

Defoamer 2 -
Supplier B
Water based

69.0

0.34

24.5

2.8

3.4

Defoamer 3 -
Supplier B
Water based

68.9

0.42

21.4

6.9

2.4

* Determined by freeze-drying.

** The unknown material is comprised of high molecular weight polymeric matter.

*** Determined by difference.

Figure 2. TYPICAL FTIR SPECTRA OF SILICONE OIL DEFOAMER COMPONENTS AFTER SEPARATION BY SOLID PHASE EXTRACTION

Figure 2

 

b) Analysis of mill deposits

The FTIR spectra of the acetone extractives of all the deposits suggested the presence of wood resin and silicone oil (see typical spectra in Figure 3 (a)). The chloroform extractives of deposits A and B contained calcium soaps. Subsequent analysis showed that calcium carbonate was a major component of the deposits.


Figure 3. FTIR SPECTRA OF COMPONENTS IN PITCH DEPOSIT A AFTER SOLVENT EXTRACTION (a) AND SEPARATION OF COMPONENTS IN THE EXTRACTS BY SILICA (b) AND C18-SILICA (c) SOLID PHASE EXTRACTION

fIGURE 3A

fIGURE 3B

fIGURE 3C

 

FTIR spectra of the silica-SPE fractions (Figure 3 (b)) confirmed the separation of acetone extracts into their basic constituents. Further separation of the chloroform fractions of the silica SPE by C18-silica SPE and analysis by FTIR confirmed separation of the silicone oil from the wood resin components as can be seen in the FTIR spectra displayed in Figure 3 (c). The FTIR spectrum of the chloroform fraction showed a large peak at 1736 cm−1 that indicates the presence of esters. This peak was also observed in the defoamer formulations indicating that it is a component of the defoamers. A detailed breakdown of extractives and SPE fractions of all the three deposits is shown in Table 4. The results show that a significant portion of the deposits is comprised of hydrocarbon oils and smaller amounts of silicone oils. This seems to imply that hydrocarbon oils in the defoamer formulations are the nucleus for deposit formation, in accordance with previous findings that oils in oil-based defoamers contribute to pitch deposition problems14. The data also show that the total amounts of the SPE extracts (last column in Table 4) closely match the amounts of the original acetone extracts of the deposits obtained by solvent extraction. This is further demonstration of the accuracy of the SPE methodology. It should be borne in mind that the quantitative data obtained from separation followed by FTIR analyses is approximate (ฑ5%). More accurate quantitative data can be obtained for example by GC analyses of the extracts . In this case, FTIR analyses are sufficient for troubleshooting purposes.

 

TABLE 4. ANALYSIS OF MILL DEPOSITS BY SOLVENT EXTRACTION AND SILICA SPE

Sample

Solvent extraction

Silica SPE

C18-silica SPE

Total of
SPE
extracts

Acetone
extracts,

%

Chloroform
extracts

%

Hydro-
carbon
oil,
%

Oxidized
material,

%

Wood
resin,

%

Silicone
oil,

%

Deposit A

21.9

2.1

15.2

3.7

1.2

2.0

22.1

Deposit B

26.8

8.1

14.7

7.4

1.9

0.9

24.9

Deposit C

64.3

11.4

15.4

42.2

1.4

5.5

64.5

 

PROBLEMATIC DEPOSITS

Our analytical protocol broke down when applied to deposits from mills that were conducting trials on two new defoamer formulations; one based on high molecular weight silicone oils and the other on high molecular weight silicone oils plus amide wax particles. The results are summarized in Table 5. In deposit #1, amide in the defoamer interfered in the extraction and the separation of the silicone oils. Amide wax is a common constituent of oil-based defoamers. If present in deposits a different analytical scheme should be used: the sample should be extracted with hot chloroform, which is then cooled to precipitate out the amide wax that can subsequently be separated from wood resin and defoamer oils by filtration. Dorris et al.,2 previously evaluated the solubility of different amides and found that amides are partially soluble in acetone when extracted in a Soxhlet apparatus. We have since found that the solubility of amides is increased when a Soxtec extractor apparatus is used instead. This is because the extraction is done in boiling solvent instead of hot solvent used in Soxhlet extraction. However, the solubility of amide in boiling acetone is not quantitative. Thus if amide is present along with silicone oil, a separation scheme different from the one developed in this report (Figure 1) has to be used. It is not clear if silicone defoamers with amide wax will be commercially viable. Aside from the mill in question, we are not aware of any other mills that use this type of defoamer. In any case, the mill abandoned use of this defoamer because of the deposition problems that occurred. Consequently, we did not conduct further method development on this particular deposit.

In deposit #2, there was no amide present but silicone oil was detected in all the solvent extracts and, also, in the non-extractable matter. The silica SPE separated hydrocarbon oil from silicone oil (with hexane and chloroform, respectively). However, we also found some silicone in the methanol fraction. It is evident that a new analytical protocol is required to process deposits containing silicone defoamers made from high molecular weight silicone oils. There seems to be a shift towards these kinds of defoamers as they can be used at lower dosages than those containing low molecular weight silicone oils. We are currently developing methodologies to handle such deposits and the results will be reported in the near future.

 

TABLE 5. RESULTS FOE ANALYSIS OF TWO TROUBLESOME DEPOSITS

Sample

Solvent Extraction

Silica SPE

Acetone
Extracts,

%

Chloroform
Extracts

%

Non-
extractable
Matter,
%

Hexane
Fraction,

%

Chloroform
Fraction,

%

Methanol
Fraction,

%

Kraft mill
deposit #1

25.9

30.2

43.96

6.6

9.3

10.0

FTIR
identification

Amide,
Silicone
oil and
traces of
wood resin

Amide,
HMW
silicone
oil?

HMW
silicone
oil?

Hydro-
carbon
oil

Silicone
oil

Amide,
traces of
silicone

 

Kraft mill
deposit #2

2.8

2.8

75.2

5.4

7.5

3.7

FTIR
identification

Silicone
oil, wood
resin

HMW
silicone
oil?

HMW
silicone
oil?

Hydro-
carbon
oil

Silicone
oil

Silicone
oil, metal
(Na) soaps

HMW = high molecular weight.

 

CONCLUSIONS

We have developed methods for quantitative determination of silicone defoamer components in mill deposits and in defoamer formulations. The methods entail separation by solid phase extraction and identification of the separated components by FTIR. They are applicable to low molecular weight defoamers (up to 10,000 Daltons) and enable complete characterization of deposits from mills that use such defoamers. Analysis of mill deposits show that the contribution of oils to the deposits was much higher than that of silicone oils, in spite of their relatively smaller proportions in defoamer formulations. The methods, however, are not applicable to deposits that contain high molecular weight silicone oils. Analytical procedures for these are under development and will be reported in the future.

ACKNOWLEDGEMENTS

We thank Luc Lapierre for reviewing the manuscript.

REFERENCES

1. Allen, S.L., Allen, L.H. and Flaherty, T., Defoaming in the Pulp and Paper Industry, in Defoaming: Theory and Industrial Applications, PR Garrett (Ed.), Ch. 3, Marcel Dekker Inc., New York, pp. 151–175, 1993.

2. Dorris, G.M., Douek, M. and Allen, L.H., Analysis of Amide Defoamers in Kraft Mill Pitch Deposits, J. Pulp Pap. Sci., 11(5): J149–154, 1985.

3. Brandt, C.S., Teasley, J.G. and Anderson-Norris, A., Water-Based Silicone Defoamers; New Generation of Defoamers, Paper Age, 112 (10): 24, 1996.

4. Clas, S.-D. and Allen, L.H., Comparison of the Performances of Water and Oil-based Defoamers, Pulp Paper Can., 95(1): 33–36, 1994.

5. Mudaly, G., Bubreak Siloxane Technology: the Key to Profitable Pulping, TAPPSA J., Jan 2002: 17–19.

6. Dorris, G.M., Douek, M. and Allen, L.H., Analysis of Metal Soaps in Kraft Mill Brown Stock Pitch Deposits, J. Pulp Pap. Sci., 9(1): TR1–7, 1983.

7. Chen, T., Wang, Z., Zhou, Y., Breuil, C., Aschim, O.K., Yee, E., and Nadeau, L., Using Solid-Phase Extraction to Assess Why Aspen Causes More Pitch Problems Than Softwoods in Kraft Pulping, Tappi J., 78(10): 143–149, 1995.

8. Chen, T., Breuil, C., Carriere, S., and Hatton, J.V., Solid-Phase Extraction can Rapidly Separate Lipid Classes From Acetone Extracts of Wood and Pulp, Tappi J., 77(3): 235–240, 1994.

9. Robinson, M.J., Anderson, S.M., Judd, M.C., and Stuthridge, T.R., Advanced Extraction Techniques for the Analysis of Pulp and Paper Matrices, Proceedings, 53rd Appita Annual Conf., Vol. 2, pp. 765–771, 1999.

10. Sweeney, K.M., Solid-Phase Extraction Techniques in the Pulp and Paper Industry, Tappi J., 71(1): 137–140, 1988.

11. Gutierrez, A., del Rio, J.C., Gonzalez-Vila, F.J., and Martin, F., Analysis of Lipophilic Extractives From Wood and Pitch Deposits by Solid-Phase Extraction and Gas Chromatography, J. Chromatogr., A, 823(1&2): 449–455, 1998.

12. Zinkel, D.F., Quantitative Separation of Ether-Soluble Acidic and Neutral Materials, J. Wood Chem. Technol., 3(2): 131–143, 1983.

13. Sithol้, B.B., Vollstaedt, P. and Allen, L.H., Comparison of Soxtec and Soxhlet Systems for Determining Extractives Content, Tappi J., 74(11): 187–191, 1991.

14. Allen, L.H., Pitch Control in Pulp Mills, in Pitch Control, Wood Resin and Deresination, E. Back & L.H. Allen (Eds), TAPPI Press, Atlanta, ch. 11, 2000.

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