FI58703C - YTBELAGT ELEKTRISKT ISOLERANDE PAPPER OCH FOERFARANDE FOER DESS FRAMSTAELLNING - Google Patents

Description

ΓβΊ rm ADVERTISEMENT conn 7 jgljft LBJ (11) UTLÄGGNINCSSKIUFT 3 8/0 3 ^ (45) Patent issued on 10 03 1931 ^ v ** (51) Hv.ik? / »Nt.a.3 H 01 B 3/52 ENGLISH —FINLAND (21) P «MnttH» k # mu * -Ptt ««> t «6knln | TU6 / T2 (22) HakemltpUvi — AiwIMcnlnisdag 20.03.72 (23) AltaipIM — GIM | h «t * d * f 20.03.72 (41) Tullut] ulklMk * l - Latches off« ntll | q £ ^ 0 J2 j · Registry Board (44) NShttviMp ^ oo]. Date of month

Petent · oeh registerstyrelsan Amekan uthgd oeh utUkrifun puUk «nd 28.11.80 (32) (33) (31)« uolk · ** - Bejlrd priorkM 05.0 ^ 4.71 USA (US) 13106o Proof-Styrkt (71) Kimberly-Clark Corporation, Delaware, US; Neenah, Wisconsin, USA (72) Kurt Suter, Carlisle, Pennsylvania, John H. Mathews, Lee, Massachusetts, USA (7 * 0 Oy Jalo Ant-Wuorinen Ab (5M Coated electrical insulating paper and method for its production - The present invention relates to paper, and more particularly to paper used as electrical insulation.

Papers used for electrical insulation that require high breakdown strength, such as in capacitors, are made on Fourdrinier paper machines, using a particularly pure, highly ground sulfate pulp. Such paper is substantially film-like in nature, i.e. it has no holes or cavities and is very poorly permeable to liquids. However, compared to films made from homogeneous systems, i.e., cellophane and a molded or extruded plastic film, the surface of the capacitor paper is rough or 'voicled' under microscopic examination. However, if two microscopic "valleys" on opposite sides of the paper are aligned, the result is a thin spot where insulation is poor and where there is a risk of breakthrough.

It is an object of the present invention to increase the puncture resistance of electrical insulating paper without increasing its thickness. It is important that the paper be thin when it is to be used as an insulating interlayer in the capacitor because, as is known, the smaller the insulating clearance between the capacitor electrodes, the higher its capacitance. The advantages of insulating papers with the highest possible penetration strength and at the same time as thin as possible are not limited to capacitors, but can generally be used in the manufacture of electrical equipment. For example, by wrapping such paper around the conductors forming the windings of the transformers, the size of the transformer can be reduced without reducing its capacity.

The present invention is based on the finding that if the surface of the paper is treated with dilute solutions of certain water-soluble film-forming polymers, the breakthrough strength of the paper is greatly increased. For example, an amount of coating that increases the paper weight by only less than 4% In other words, an improved paper having a certain total basis weight, including both a fibrous material and a surface coating according to the invention, is far superior in penetration strength than a conventional paper of the same weight consisting solely of fibers.

This rather unexpected result of using such small amounts of coatings probably suggests that the polymer solution tends to accumulate in the "valleys" on the surface of the paper, but if so, the "smoothing effect" is not such that the "peaks" on the surface are reduced so much as to impair the insulation paper. impregnation with liquid insulators.

Thus, the present invention must be distinguished from so-called lacquer insulating or capacitor papers, which use a continuous layer of paper surface insulation, whereby the effective thickness of the original paper increases.

There are, of course, a large number of substances that are both water-soluble and film-forming. However, in order to be useful in the present invention, the agent must be insoluble in the liquid insulators commonly used to impregnate insulating papers, such as cloned biphenyls, mineral oils, etc .; it must not in itself increase the loss factor of the paper insulation too much and must be substantially free of alkali metals, as the presence of these metals increases the electrical conductivity of the paper. As all otherwise suitable substances have been found to contain too much sodium, a commercially viable process for removing alkali metals from certain useful, water-soluble, film-forming agents has been developed, and the requirements of the removal process define and limit those useful under the conditions described above.

Thus, in order to achieve the purposes of this invention, the film-forming agent must be insoluble in water in one temperature range but soluble in it in another temperature range. To facilitate the process, both of these temperature ranges are preferably, but not necessarily, less than 100 ° C »" Insoluble "means here that the substance must not swell considerably (gelatinize) or become sticky, but it must be permeable to water so that the ions can diffuse from the solid phase into the aqueous phase. Of the substances with these important properties, alkali metals can be easily removed simply by washing them with water having a low alkali metal content, keeping the temperature in an area where dissolution does not occur. The efficiency of the washing process can be improved by adding small amounts of "harmless" cations such as hydrogen or alkaline earth metal ions. These cations do not affect the electrical properties of the film-forming agent such as alkali metal ions, and they replace the alkali metal ions in the agent through cation exchange, thus greatly reducing the amount of wash required to reduce the alkali metal content to the desired value. sulfuric acid in the wash water about 0.01 to 0.1% by weight of the water, The washing process is also facilitated by the fact that the detergent is finely divided (high surface area / volume ratio) so that the diffusion rate of ions from the solid phase to the liquid phase is high.

The washing can be performed by any known method of washing for fines, such as batch dispersion, clarification and decantation; continuous washing and centrifugation; and countercurrently using rotating filters. Whatever the technique used, the extraction must be continued until the alkali metal content of the film-forming agent, which is usually about 1000 ppra before washing, has been reduced to a maximum of about 200 ppm, and preferably to a maximum of about 20 ppm.

When the washing is completed at a temperature at which the substance is insoluble, the substance is dispersed in the desired amount of water substantially free of alkali metals, after which the temperature is raised or lowered to a value at which dissolution occurs, as the case may be. Once the substance is dissolved, the necessary concentration and temperature corrections are made, and the solution is ready for use in making the improved paper of this invention.

Four substances suitable for use in the present invention are methylcellulose, starch, polyvinyl alcohol and protein.

Cellulose methyl ethers are manufactured in U, S, A by the Dow Chemical Company under the tradename "Methocel". These products are made from either cotton linters or wood pulp and are available in a wide variety of grades in terms of viscosity and degree of methoxylation. The preferred grades are those with a viscosity of 4 58703 of 10 to 25 centipoise (22 aqueous solution, 20 ° C) and a substitution of 1.64 to 1.92. This type of methylcellulose is insoluble in organic solvents and has the exceptional property of being insoluble in hot water but readily soluble in cold water. Thus, the washing process with this substance is carried out at n, 80 ° C, after which the slurry of washed methylcellulose is cooled to 0 ° to 10 ° C to obtain a gel-free solution. Once dissolved, the methylcellulose remains in solution if the temperature is raised to about 35 ° C and the viscosity of the solution decreases. The best temperature for handling paper is n, 30 ° C,

The most suitable starches are oxidized and chemically substituted types, such as hydroxyethylated starch, acetylated starch, etc., which are commonly used in surface sizing and as coating binders in the manufacture of printing and writing papers. Enzyme-modified pearl starches can also be used, but they are more cumbersome and the flow properties of their solutions are not as good as those of chemically modified starches. These starches are usually derived from cereals, but are also made from potatoes, millet, mili, tapioca, etc. Low and medium viscosity grades with a viscosity of less than about 200 centipoise (102 aqueous solution, 20 ° C). These starches are insoluble in cold or warm water, but dissolve when cooked at about 90 ° C for 20-30 minutes. Once dissolved, the starch remains in the solution upon cooling, and the temperature of the solution can be lowered to the processing temperature of the paper. The optical temperature of the treatment can be anything between 30 ° C and 90 ° C, depending on the concentration of the solution and the viscosity of the starch.

Polyvinyl alcohol is available in grades that are water soluble throughout the temperature range of 0 ° to 100 ° C, but the grade used in this invention is so-called "fully hydrolyzed" from which 982 or more of the original acyl groups have been removed by hydrolysis. i.e., it is insoluble in water at moderate temperatures but soluble when boiled at 80 ° C, depending on the molecular weight and the number of acetyl groups remaining. Products with a viscosity of about 25 to 125 centipoise (42 aqueous solution, 20 ° C) are considered most suitable. If the viscosity or degree of hydrolysis is lower, some solubility in cold water may occur, making the washing process more difficult.

The protein used in the improved insulating paper may be from any source, provided that it is in such a form that its solubility behaves as required for washing. Purified soy protein, commonly known as "alpha protein", is preferred. The low viscosity form of this substance is most suitable. Unlike the other three substances listed above, ammonium hydroxide must be added to a pH of about 8 to dissolve the protein. -9, and heated to n, 50 ° C or higher All four water-soluble film-forming agents are approximately equal in their ability to increase the breakthrough strength of insulating paper, however, polyvinyl alcohol and protein slightly increase the loss factor of treated paper, resulting in two less suitable than methylcellulose and starch, for example, for the manufacture of capacitors where this property of electrical insulation is important.

Methylcellulose, in addition to increasing the puncture resistance of paper, also has a unique ability to prevent corona discharge and extend the life of paper insulation under conditions of high temperatures and high electrical loads. The improved performance of capacitors made using methylcellulose-treated paper insulators is discussed below. The water-soluble film-forming agent can be incorporated into the paper using any of the commonly used paper coating or surface treatment techniques, such as glue press, blade, roll, air knife coating, and the like.

It can be done either on a paper machine or afterwards, although the former is preferred for economic reasons.

The coating material can be applied either to one side of the paper only, or to both sides thereof. It is generally preferred to treat both sides to obtain the best treatment result and to avoid wrinkling. However, in some special cases, such as when making capacitor paper for vacuum metallization, it is preferable to vigorously treat only one side of the paper to make the surface to be metallized as smooth as possible, leaving the other side "rough" so that impregnating fluids can easily penetrate the capacitor winding.

The amount of surface treatment agent required to achieve a significant improvement in puncture resistance is at least about 2%, based on dry matter, of the total weight of the treated paper. The maximum amount that can be used is limited by treatment and drying techniques, but it is not considered desirable or practical to use larger amounts. than n, 15% of the total weight of the treated paper, with amounts of about 3% to 10% being preferred,

The following examples illustrate the uses of the invention and the objects achieved.

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Example I:

Methylcellulose having a substitution of 1.64 to 1.92 was washed as described above until its sodium content was reduced to 12 ppm.

It was then dissolved in water to a 4% by weight solution by cooling to 5 ° C.

The viscosity of this solution was about 120 cps at 20 ° C. The solution was treated with a capacitor paper web using a conventional gluing press placed on a paper machine. The basis weight of the obtained treated paper product was adjusted so that the weight was the same as before the treatment with methylcellulose solution. Both treated and untreated papers were supercalendered to a thickness of 0.5 mils (0.0127 mm) and a density of 1.0 g / cm 2.

It was found that a conventional capacitor paper having a basis weight of about 1.24 kg / 100 m 2 (5.3 lbs / 2000 ft 2) and a thickness of 0.51 mils had a breakdown strength of 1540 V / mil when tested According to ASTM-D-202-70 standard. Breakthrough strength of treated paper with a dry basis weight of n, 1.24 kg / 100 m (5.3 lhs / 2000 ft), of which 3.8% was methylcellulose and a thickness of 0.50 mil was 1850 V / mil.

Example II: The treated paper was prepared according to Example I except that a 3.8 wt.% Methylcellulose solution was used to coat the paper, which after supercalendering was 0.4 mils. Prior to coating, the paper had a basis weight of about 0.98 kg / 100 m 2 (4.2 lbs / 2000 ft 2), a thickness of 0.41 mils and a breakthrough strength of 1770 V / mil. The coated paper, which also had a basis weight of 2 2 about 0.98 kg / 100 m (4.2 lbs / 2000 ft), of which 3.7% was methylcellulose and a thickness of 0.40 mils, had a penetration strength of 2080 V / mil.

Coated and uncoated papers were also used to make conventional askarel-impregnated 5.0 pF capacitors that were loaded with DC voltage until damaged. Each capacitor was fabricated by winding sheet metal electrodes with paper tabs and placing the wound body in a housing that was evacuated and filled with ascarel. The average breakdown voltage of capacitors made of conventional untreated paper was 1690 volts DC, while the average breakdown voltage of units made of treated paper was 2140 volts DC,

Example III: Treated paper was prepared according to Example I except that hydroxyethylated starch was prepared by washing to 10 ppm sodium using the method described above and then dissolved in water by heating at 90 ° C for 30 minutes to give an 8% by weight solution.

The temperature of the solution was set at 50 ° C before it was treated with paper at a viscosity of 65 centipoise. The paper product, 7 58703 2 before treatment with starch solution, was found to have a surface pressure of n, 0.94 kg / 100 m (4.0 lbs./2000 ft ^) and a thickness of 0.4 mils and a breakthrough strength of 1520 volts / mil. The treated paper had a dry basis weight of n, 0.96 kg / 100 m (4.1 lbs / 2000 ft ^), of which 7.8% was starch, a thickness of 0.4 mil'iS and a penetration strength of 2030 V / mil.

Conventional 5.0 pF capacitors impregnated with ascarrel as above were fabricated using both treated and untreated paper and electrically loaded until damaged. Units of untreated paper had an average breakdown voltage of 1600 volts DC, while treated units had an average breakdown voltage of -stroke voltage was 1910 volts DC.

Example IV:

The treated paper was prepared as in Example I using Example III

hydroxyethylated starch in accordance with The surface weight of the paper prepared prior to incorporation of this solution was about 1.17 kg / 100 m (5.0 lbs / 2000 ft), thickness 0.5 mils) and the breakdown strength was 1365 V / mil. The dry basis weight of the treated paper was about 1.19 kg / 100 m ^ (5.1 lbs / 2000 ft ^), of which 7.7% was starch, its thickness was 0.5 mils and its breakthrough strength was 1855 V / mil.

Example V: The treated paper was prepared as in Example I except that a 7% by weight solution of alpha protein was used. This 7Z solution was prepared as described above by washing the sodium content to 14 ppm and then the pH of the solution was adjusted to 8-9 with ammonium hydroxide and heated to 60 ° C.

2

Prior to treatment, this solution had a paper basis weight of about 0.96 kg / 100 m (4.1 lbs / 2000 ft ^) and a thickness of 0.42 mils and a breakthrough strength of 1620 V / mil. The treated paper had a dry basis weight of about 0.96 kg / 100 m (4.1 lbs / 2000 ft) with 6.5% protein, a thickness of 0.42 mils and a breakthrough strength of 1960 V / mil.

Conventional 5.0 pF askarel-impregnated capacitors as described above were prepared using these papers. For units made with untreated papers, the average breakdown voltage was 2340 volts DC. The average breakdown voltage of the units made with the treated papers was 2650 volts DC.

Example VI:

Ordinary condenser paper was treated using a post-coating roll and a solution of "fully hydrolyzed" polyvinyl alcohol. The solution was prepared in advance by washing with a sodium content of 8 ppm and heating to 90 ° C.

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The resulting solution had a viscosity of 35 cps (20 ° C) and a concentration of 3.9% by weight solids. The untreated paper had a dry basis weight of about 1.01 kg / 100 m1 (A, 3 lbs / 2000 ft1), a thickness of 0.5 mils, and a breakthrough strength of 1780 V / mil. The treated paper contained 1.8% polyvinyl alcohol, had a thickness of 0.5 mils, and a breakdown strength of 1950 V / mil.

In all of the above examples, the surface weight and thickness of the coated paper were substantially the same as that of the uncoated paper. Nevertheless, the penetration strength of the coated samples was higher in each case than that of the uncoated sheets. It follows that if uncoated and coated sheets with the same penetration strength were produced, the coated sheets would be thinner than the uncoated ones.

It can also be inferred from the above examples that the percentage increase in the puncture strength of the coated paper due to the coating used is greater than the percentage increase in the weight of the paper due to the coating used. The following examples illustrate this immediately:

Example VII:

The treated paper was prepared using a 15% solution of acetylated starch prepared according to Example III. This solution was heated to 50 ° C and attached in a paper machine sizing press to a capacitor paper having a basis weight of about 0.97 kg / 100 m (A, 15 lbs / 2000 ft). The resulting treated product 2 had a basis weight of about 1.08 kg / 100 m (A, 61 lbs / 2000 ft). After a certain amount of treated paper was prepared, the addition of the starch solution was stopped and the amount of cellulose fiber in the sheet was increased to give a basis weight of n.

1.075 kg / 100 m 1 (A, 60 lbs / 2000 ft 1).

All three capacitor papers were supercalendered and tested as in Example I except that each paper was loaded with two different thicknesses until they were damaged using increasing AC voltage. For the original 2 2 paper with a basis weight of n, 0.97 kg / 100 m (A, 15 lbs / 2000 ft) and a thickness of 0.1 A mil, the average breakdown voltage (two sheets) was 1285 volts AC.

The treated paper had a basis weight of about 1.08 kg / 100 m (A, 61 lbs / 2000 ft), of which 13% was starch, a thickness of 0.05 mils, and an average breakdown voltage of 1850 volts AC. This represents an increase of AA% in the breakthrough strength compared to the original paper with an increase in basis weight of 13%. The third paper 2 2 had a basis weight of about 1.09 kg / J.00 m (A, 65 lbs / 2000 ft) loose fiber, a thickness of 0, A5 mils and an average breakdown thickness of 1A50 volts AC. This represents an increase of 13% sn in the breakdown stress compared to the original paper, with an increase in basis weight of 12%, 9 58703

Example VIII;

The treated paper was prepared using a 4 Zsn methylcellulose solution as in Example I. The methylcellulose solution was incorporated into a capacitor paper 2 prepared to have a basis weight of n, 0.86 kg / 100 m (3.65 lbs / 2000 ft). The surface weight of the treated paper was about 0.89 kg / 100 m (3.8 lbs / 2000 ft2). After a certain amount of treated paper was made, the treatment was stopped and the cellulosic fiber was added until the basis weight was about 0.89 kg / 100 m 2 (3.8 lbs / 2000 ft 2).

All three papers produced were supercalarized, and the electrical breakdown strength of the two thicknesses of each paper was measured using an AC voltage. For the original paper, which had a basis weight of n, 0.86 kg / 100 m (3.65 lbs) and a thickness of 0.35 mil'iaj, the average breakdown voltage (two 2 sheets) was 1030 volts AC. The treated paper had a basis weight of about 0.89 kg / 100 m (3.8 lbs), a consistency of 0.36 mils, and an average breakdown voltage of 1270 volts. This represents, compared to the original paper, a 23% increase in breakthrough voltage compared to a 4Z increase in basis weight. The third paper, made entirely of cellulose fibers, had a basis weight of about 0.89 kg / 100 m (3.8 lbs), a thickness of 0.36 mils, and an average breakdown strength of 1080 volts. This represents an increase of 5Z compared to the original paper. to the breakdown voltage with a 4% increase in basis weight.

The accompanying drawings illustrate some advantages of the present invention. In the drawing, Figure 1 shows the breakthrough strength of untreated release paper and insulating paper treated in accordance with the invention; Figure 2 shows the relationship between the mean ages of capacitors made using both treated and untreated papers; Figure 3 is the loss factor of treated and untreated papers; and Figure 4 is the corona resistance of capacitors made from untreated papers and methylcellulose coated papers.

Figure 1 shows the electrical breakdown strength of two sheets of capacitor paper with a thickness ranging from 0.3 mils to 0.5 mils. Curve 1 shows the breakdown strength of conventional paper as a function of thickness, and curve II shows the corresponding ratio of paper treated with 4 weight-weight methylcellulose solution. Breakthrough tests were performed at 60 divisions AC voltage using an apparatus conforming to ASTM D-149-64.

Figure 2 graphically illustrates the ability of the above treated papers to resist the degrading effect of high electrical loads.

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Conventional 0.5 pF capacitors were constructed in each case using two different thicknesses of capacitor paper as insulation. Three different capacitor papers were used. Two of the selected papers were conventional commercial, particularly high quality capacitor papers, while the third paper was treated according to the process described in the examples using a 4% by weight methylcellulose solution. The capacitors were simultaneously impregnated with unmodified ascarel in a conventional vessel according to conventional methods. After impregnation, the pieces were wound for an electrical load test and placed in a protective housing to determine their duration. They were all loaded at 800 volts AC at the same time and allowed to stand there until all units were damaged. The time taken for the damage team was determined and is shown in Figure 2.

Figure 3 illustrates the effect of the treatments according to the examples described above on the loss coefficient of dry papers. Overlapping curves 1, 11, and III represent typical values obtained when the capacitor papers were treated with 4Z methylcellulose solution, 8%: Saliva starch solution, and using untreated capacitor papers. The overlapping curves IV and V represent the experimental results obtained on capacitor paper treated with 3.8Z polyvinyl alcohol and 7% alpha protein solutions. It should be noted that the methylcellulose and starch coating according to the invention has no effect on the loss factor.

Figure 4 illustrates the unique ability of methylcellulose-treated papers to dampen corona discharge in high voltage capacitors;

In Figure 4, curves I, II and III show the effect of electrical loading on the loss factor of normal absorbent-containing methylcellulose-treated capacitor papers, respectively.

The capacitors used were conventional 1.0 asf ascarel impregnated units with two sheets of 0.4 mil paper as insulation. Curve I (plain capacitor paper) shows the typical dependence of the capacitor loss factor on the load. Initially, the high loss factor values are due to the movements of ionic impurities present in small amounts in the saturation liquid phase of the capacitors during AC loading. As the load is increased, the velocity of the ions moves, resulting in a lower loss factor. Curve I in Figure 4 shows that capacitor papers made using ordinary papers have a minimum loss factor between 300 and 400 V / mil. When the AC load is increased above 400 V / mil for normal papers, the loss factor increases to 58703, indicating that the insulator does not withstand the applied load satisfactorily and that a small amount of corona discharge occurs, resulting in the formation of more ionic impurities from electrical discharges.

As this process continues, the loss factor increases at an accelerating rate.

In Figure 4, curve II represents newer, commercially used "additive" capacitor papers manufactured in accordance with U.S. Patents 3,090,705, 3,480,847, and 3,555,377. These papers are conventional capacitor papers with small percentages of finely divided mineral adsorbent. These substances have the desired property of purifying and retaining the above-mentioned ionic impurities from the liquid insulator by means of adsorptive forces, thus removing them from the liquid phase of the capacitor and immobilizing them under low AC loads. This results in a flat shape of the curve 11. However, this favorable cleaning effect does not prevent electric discharges from occurring when the load exceeds 400 V / mil. Thus, although resist corona discharges.

Curve III shows the effect of electrical load on the loss factor of capacitors when using the methylcellulose-treated paper according to the invention. It can be stated that from a slightly higher initial value of the loss factor, the curve quickly falls to its lowest value of the loss factor of 300 V / mil. Increasing the load up to 500 V / mil does not increase the loss factor and therefore only very little for extremely high loads of 800 V / mil and above.

The invention has been shown and described by way of example only in its preferred form and many modifications may be made which still fall within the scope of the invention. Thus, it is to be understood that the invention is not limited to any particular embodiment unless such limitations are included in the claims.

Claims (20)

1. Electrically insulating paper having a coating of such a substance which is insoluble in liquid dielectric substances, such as chlorinated biphenyls and mineral oils, and which is substantially free of alkali metals, which coating material is insoluble in water within a temperature range, characterized by it is soluble in water within another temperature range, that the coating material is 2-15% by weight of the total weight of the coated paper, the coated paper being thinner than an otherwise identical, non-coated paper having the same dielectric pour strength and that the percentage increase of the dielectric pour strength of the coated paper is greater than the percentage increase of the weight of the paper resulting from the coating. 2. Electrically insulating paper according to claim 1, characterized in that only one side of the paper is surface coated. 3 · Electrically insulating paper according to claim 1, characterized in that the gentle sides of the paper are coated. 4. Electrically insulating paper according to claim 1, characterized in that said coating does not contain more than 200 ppm of alkali metals. Electrically insulating paper according to claim 1, characterized in that said coating does not contain more than 20 ppn alkali metals. 6. An electrically insulating paper according to claim 1, characterized in that said coating material is present in an amount of 3-10% of the total weight of the coated paper. 7. An electrically insulating paper according to claim 1, characterized in that said coating material is methyl cellulose. β. Electrically insulating paper according to claim 7, characterized in that a 2% aqueous solution of said methyl cellulose at 20 ° C has a viscosity of about 10-25 centipoise, and a degree of substitution of 1.64-1. 92nd Electrically insulating paper according to claim 1, characterized in that said coating material is starch. 10. An electrically insulating paper according to claim 9, characterized in that a 10% aqueous solution of said starch at 20 ° C has a viscosity of less than about 200 centipoise.