DETERMINATION OF SILICONE DEFOAMERS
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.
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 (1030,000 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.
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 liquidliquid 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 chips79, 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.
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.
a) Silica SPE:
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:
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.
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.
a) Analysis of defoamer formulations
The results, shown in Table 3, indicate that defoamers contain 7080% volatiles, 1525% silicone oils, 0.30.4% hydrocarbon oils, and 23% 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.
Figure 2. TYPICAL FTIR SPECTRA OF SILICONE OIL DEFOAMER COMPONENTS AFTER SEPARATION BY SOLID PHASE EXTRACTION
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.
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.
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.
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.
We thank Luc Lapierre for reviewing the manuscript.
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