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1.
Matthias Berger Manfred P. Schnelder 《Journal of the American Oil Chemists' Society》1992,69(10):961-965
Regioisomerically pure 1(3)-rac-monoacylglycerols are conveniently prepared in high yields (>75%) and in multigram quantities by enzymatic esterification
of glycerol in the presence of various lipases(Chromobacterium viscosum, Rhizopus delemar, Rhizomucor miehei) with a variety of different acyl donors, such as free fatty acids, fatty acid alkyl esters, vinyl esters and triacylglycerols,
as well as natural fats and oils. All reactions are carried out in aprotic organic solvents with low water content, namelyn-hexane, diethyl ether, tBuOMe or mixtures of these solvents. Essential for the success of these transformations were the
following two factors. First, the creation of an artificial interphase between the solvent-immiscible hydrophilic glycerol
and the hydrophobic reaction medium by its adsorption onto a solid support. Second, a facile system for the separation of
the desired monoacylglycerol from the reaction mixture, coupled with the continuous recycling of acyl donor and undesirable
by-products. 相似文献
2.
Michael J. Haas Pamela S. Fox Thomas A. Foglia 《European Journal of Lipid Science and Technology》2011,113(2):168-179
Immobilized Rhizomucor miehei lipase (Lipozyme RMIM) was employed in the synthesis of partial acylglycerols of acetoacetate (C4H6O3, 3‐oxobutanoic acid). Both 1(3)‐sn‐monoacetoacetyl glycerol (MAcG) and 1,3‐sn‐diacetoacetyl glycerol (DAcG) resulting from esterification of glycerol with ethylacetoacetate were isolated and identified by 1H and 13C NMR. An HPLC method for the separation and quantitation of these species was developed. The effects on product yield of coordinate variations in the amounts of enzyme, water, ethyl acetoacetate, and the ratio of immobilizing silica to glycerol were explored. This allowed creation of predictive equations relating these variables to product yield. Reaction conditions were thereby identified and validated under which maximum yields of MAcG and/or DAcG or total ester were predicted. The production of both MAcG and DAcG was markedly sensitive to water, with optimal yields obtained within a narrow range of added water contents. Substantial excesses of ethylacetoacetate ameliorated the inhibitory effect of water. MAcG was effectively produced with the lowest amount of RMIM investigated. It was necessary to use larger amounts of lipase to achieve high yields of DAcG, and even then actual yields were only 30% of theoretical maximum. In the absence of silica only MAcG was produced. Practical applications: Aceotacetyl esters of glycerol are potential prochiral building blocks for further chemical synthesis and exhibit the chemical reactivity of both the acetoacetyl moiety and of glycerol. They are thus of potential use in, for example, the production of biopolymers exhibiting any of a variety of features and functionalities. The use of enzymatic catalysis rather than chemical synthesis for their production offers advantages of reduced degradation and contamination due to the gentler reaction conditions characteristic of enzymatic catalysis, as well as a potential reduction in the costs to procure and dispose of catalyst. 相似文献
3.
Lipase-catalyzed synthesis of monoacylglycerols (MAG) was performed by transesterification reactions between fatty acid vinyl
esters and either glycerol (1) or 1,2-O-isopropylidene-rac-glycerol (2), without solvents or in the presence ofn-pentane. Vinyl decanoate, vinyl laurate, vinyl stearate and vinyl palmitate have been converted to the corresponding monoacylglycerols.
As expected for the reaction with1, a mixture of mono-, di- and triacylglycerols was synthesized. The highest concentrations of MAG were achieved with vinyl
stearate (30% 2-MAG and 15% 1-MAG). The reactions of fatty acid vinyl esters with the protected glycerol (2) led to the corresponding protected 3-monoacylglycerols with 100% conversion after short reaction times. The subsequent cleavage
of these acetonides was performed by four different methods. The fastest cleavage was found with trifluoroacetic acid as catalyst,
whereas the highest concentration of MAG (100%) was obtained for the boric acid-catalyzed hydrolysis of the acetonides. 相似文献
4.
Use of Glycerol Carbonate in an Efficient, One-Pot and Solvent Free Synthesis of 1,3-sn-Diglycerides
Mojgan Kargar Rahim Hekmatshoar Mehdi Ghandi AbdolJalil Mostashari 《Journal of the American Oil Chemists' Society》2013,90(2):259-264
An efficient solvent-free synthesis of a variety of highly pure 1,3-sn-diglycerides (1,3-sn-diacylglycerols) in a two-step one pot process is described. Heating glycerol carbonate (4-hydroxymethyl-1,3-dioxolan-2-one) with fatty acid anhydrides 2a–d affords 1:1 mixtures of glycerol carbonate fatty esters 3a–3d and the corresponding fatty acids. Further heating the reaction mixtures in the presence of catalytic amounts of 1,4-diazabicyclo[2.2.2]octane (DABCO) at 195–200 °C yields highly pure 1,3-sn-diglycerides 4a–4d. 相似文献
5.
Transesterification of cocoa butter by fungal lipases: Effect of solvent on 1,3-specificity 总被引:1,自引:0,他引:1
Transesterification and alcoholysis reactions catalyzed by immobilized lipases fromMucor miehei andHumicola lanuginosa in hexane gave fatty acid esters that did not reflect the expected 1,3-specificity of the enzymes, due to competing acyl
migrations in the partial glyceride products. However, both lipases were 1,3-specific in reactions when diethyl ether was
used as a solvent, and this provided a convenient analytical methodology in combination with gas chromatography and nuclear
magnetic resonance spectroscopy for the determination of fatty acid distribution within triglycerides. 相似文献
6.
George J. Piazza Raymond G. Bistline Jr. Alexander Bilyk Stephen H. Feairheller Michael J. Haas 《Journal of the American Oil Chemists' Society》1993,70(7):727-729
The butyl amide of ricinoleic acid (N-n-butyl-12-hydroxy-(9Z)-octadecenamide) was prepared from a neat mixture of castor oil andn-butylamine (fatty ester/amine molar ratio, 1:1.3). No catalyst was required. The identity and purity of the amide was assessed
by thin-layer chromatography and confirmed by elemental analysis and by infrared and C13 nuclear magnetic resonance spectroscopy. High product yields were achieved at 45 and 65°C in 48 and 20 h, respectively. The
reaction was inhibited by the addition of trimethylpentane and dioxane, but not by water. An attempt was made to prepare the
amide from methyl ricinoleate, rather than castor oil; even after 10 d only partial conversion was achieved. Attempts to prepare
the amide from methyl-n-butylamine, rather thann-butylamine, were also unsuccessful. The ease with which secondary fatty amides can be produced from an oil that consists
primarily of the glycerol esters of hydroxylated fatty acids indicates that the described procedure has industrial utility. 相似文献
7.
Mark Rüsch gen. Klaas Siegfried Warwel 《European Journal of Lipid Science and Technology》2001,103(12):810-814
Using dialkyl carbonates as reagents for lipase‐catalyzed transesterification, the reaction is driven by the evolvement of carbon dioxide as the co‐product and thus no longer an equilibrium reaction. Therefore this transesterification method is faster and quantitative conversions can be obtained. Short‐chain dialkyl carbonates, like other short‐chain esters, are also suitable solvents for seed oil extraction. Thus, extraction and transesterification can be combined in a single reaction. This reaction, called reactive extraction, was carried out in a standard Soxhlet apparatus with rapeseed, linseed and calendula seed as the raw materials and with dimethyl and diethyl carbonate as extraction solvent and transesterification reagent at the same time. Fatty acid methyl esters and ethyl esters respectively were obtained with higher yields than those achieved by conventional two step extraction / transesterification. In the case of linseed fatty acid esters and especially calendula seed fatty acid esters, the iodine values of the products obtained by one‐pot‐two‐step reactive extraction were significantly higher. 相似文献
8.
Michael J. Haas Karen M. Scott William N. Marmer Thomas A. Foglia 《Journal of the American Oil Chemists' Society》2004,81(1):83-89
The production of simple alkyl FA esters by direct alkali-catalyzed in situ transesterification of the acylglycerols (AG) in soybeans was examined. Initial experiments demonstrated that the lipid in
commercially produced soy flakes was readily transesterified during agitation at 60°C in sealed containers of alcoholic NaOH.
Methyl, ethyl, and isopropyl alcohols readily participated in the reaction, suggesting that the phenomenon is a general one.
Statistical experimental design methods and response surface regression analysis were used to optimize reaction conditions,
using methanol as alcohol. At 60°C, the highest yields of methyl ester with minimal contamination by FFA and AG were predicted
at a molar ratio of methanol/AG/NaOH of 226∶1∶1.6 with an approximately 8-h incubation. An increase in the amount of methanol,
coupled with a reduced alkali concentration, also gave high ester yields with low FFA and AG contamination. The reaction also
proceeded well at 23°C (room temperature), giving higher predicted ester yields than at 60°C. At room temperature, maximal
esterification was predicted at a molar ratio of 543∶1∶2.0 for methanol/AG/NaOH, again in 8 h. Of the lipid in soy flakes,
95% was removed under such conditions. The amount of FAME recovered after in situ transesterification corresponded to 84% of this solubilized lipid. Given the 95% removal of lipid from the soy flakes and
an 84% efficiency of conversion of this solubilized lipid to FAME, one calculates an overall transesterification efficiency
of 80%. The FAME fraction contained only 0.72% (mass basis) FFA and no AG. Of the glycerol released by transesterification,
93% was located in the alcoholic ester phase and 75 was on the post-transesterification flakes. 相似文献
9.
Roxana Irimescu Kiyomi Furihata Kazuhiko Hata Yugo Iwasaki Tsuneo Yamane 《Journal of the American Oil Chemists' Society》2001,78(3):285-290
A highly efficient enzymatic method for the synthesis of regioisomerically pure 1,3-dicapryloyl-2-docosahexaenoyl glycerol
(CDC) in two steps was established. 2-Monoglyceride (2-MG) formation by ethanolysis of tridocosahexaenoylglycerol (DDD) with
immobilized Candida antarctica lipase (Novozym 435) as catalyst was the key step of the synthesis. CDC was finally obtained by reesterification of 2-MG
with ethylcaprylate (EtC) catalyzed by Rhizomucor miehei lipase (Lipozyme IM). The regiospecificity of Novozym 435 depended on the type of reaction and the initial composition of
the reaction medium. It displayed strict 1,3-regiospecificity for ethanolysis at a high excess of ethanol in the reaction
mixture although it displayed no regiospecificity in transesterification and esterification reactions. The highest yield of
CDC (85.4%) was obtained by ethanolysis at a 4∶1 weight ratio of ethanol/DDD for 6 h followed by reesterification at a 20∶1
molar ratio of EtC/initial DDD for 1.5 h. The regioisomeric purity of CDC was 100%. Good results were obtained also for the
synthesis of 1,3-dicapryloyl-eicosapentaenoylglycerol (CEC) by the same method: 84.2% yield and 99.8% regioisomeric purity
at the same reactant ratios as above. The yield of the reesterification step and the regioisomeric purity of the product were
influenced by the molar ratio of the reactants for both CDC and CEC syntheses: higher excess of EtC favored higher yields
and regioisomeric purity of the products. 相似文献
10.
S. Basheer J. B. Snape K. Mogi M. Nakajima 《Journal of the American Oil Chemists' Society》1995,72(2):231-237
A simple kinetic model for the transesterification of triglycerides catalyzed by a modified lipase inn-hexane has been proposed. The model assumes that the enzyme has 1,3-positional specificity and does not distinguish among
the different fatty acid residues considered in this study. The model is based on material balances of consecutive second-order
reversible reactions and requires only one parameter that can easily be determined experimentally. The differential rate equations
have been solved analytically to give explicit equations that link the concentrations of all possible triglycerides to the
initial conditions and the reaction time. The model was in good agreement with experimental data for different biocatalyst
concentrations with the same value of the specific rate constant. The same value of specific rate constant also gave a good
fit with experimental data for an acidolysis reaction between a triglyceride and a fatty acid, implying that the modified
lipase did not distinguish between free fatty acids and fatty acid residues attached to the 1 and 3 positions of glycerol
backbone. 相似文献
11.
Folahan O. Ayorinde Chukwuma P. Nwaonicha Voneil N. Parchment Kent A. Bryant Mahmoud Hassan Marcella T. Clayton 《Journal of the American Oil Chemists' Society》1993,70(2):129-132
cis-12,13-Epoxy-cis-octadecenoic (vernolic) acid occurs in triglycerides of the seed oil ofVernonia galamensis. The seeds also contain a lipase capable of hydrolyzing the triglycerides. Previous investigators incubated the seed ofVernonia anthelmintica and isolated 5.6% yield of 1,3-divernoloyl glycerol. We used crude lipase extract fromV. galamensis seed to synthesize 1,3-divernoloyl glycerol from vernonia oil in pentane at 40% yield. A 94% conversion of the 1,3-divernoloyl
glycerol to pure vernolic acid (5.34% oxirane = 98.9% purity) was achieved by a low-energy saponification process. The carbon-13
nuclear magnetic resonance (NMR) spectrum of the 1,3-divernoloyl glyceride indicates a potential for using carbon-13 NMR spectroscopy
in the identification of isomeric diglycerides. Thus the paper describes the synthesis, spectroscopic and chemical characterization
of 1,3-divernoloyl glycerol, in addition to providing quantitative carbon-13 NMR studies ofV. galamensis oil. 相似文献
12.
Enzymatic modification of trilinolein: Incorporation of n-3 polyunsaturated fatty acids 总被引:2,自引:0,他引:2
Casimir C. Akoh Brenda H. Jennings Dorris A. Lillard 《Journal of the American Oil Chemists' Society》1995,72(11):1317-1321
Two immobilized lipases, IM60 fromMucor miehei and SP435 fromCandida antarctica, were used as biocatalysts for the modification of trilinolein with n-3 polyunsaturated fatty acids (PUFA), such as eicosapentaenoic
acid (EPA) and docosahexaenoic acid (DHA), by using their ethyl esters as acyl donors (EEPA and EDHA, respectively). Transesterification
(ester-ester interchange) reactions were carried out in organic solvent. The products were analyzed according to their equivalent
carbon number and polarity by reverse-phase high-performance liquid chromatography, and the fatty acid profiles were determined
by gas-liquid chromatography. Modified triacylglycerol products contained 1 or 2 molecules of n-3 PUFA. With EEPA as the acyl
donor, the total EPA product yields with IM60 and SP435 as biocatalysts were 79.6 and 81.4%, respectively. However, with EDHA
as the acyl donor and IM60 and SP435 as biocatalysts, the total DHA product yields were 70.5 and 79.7%, respectively. Effects
of reaction parameters, such as type of solvent, enzyme load, time course, and molar ratio of substrates on the n-3 PUFA incorporation,
were followed with SP435 as the biocatalyst. High yields were obtained, even in the absence of organic solvent. These lipids
do hold promise for specialty nutrition and other therapeutic uses. 相似文献
13.
Regioisomerically Pure Mono- and Diacylglycerols as Synthetic Building Blocks Regioisomerically pure mono- and diacylglycerols are valuable, non-ionic surfactants for a multitude of applications e.g. in pharmaceutical or cosmetical industry. Furthermore they are attractive intermediates for the synthesis of more complex lipids like selectively functionalised triacylglycerols, glyco- or phospholipids. Unfortunately, this high synthetic potential of these molecules could not be exploited to any great extent so far due to the apparent lack of accessibility. Lipase-catalysed reactions often proceed with high regioselectivity. Therefore it seemed logic to investigate the potential of biocatalytic synthesis leading to the desired glycerides. The lipase-catalysed esterification of glycerol in presence of various acyl-donors (free fatty acids, fatty acid methyl esters, -vinyl esters, synthetic triglycerides, natural fats and oils) indeed provided regioisomerically pure 1,3-sn-di- and 1(3)-rac-monoacylglycerols in high yields and synthetically useful quantities. Modification of the free hydroxyl functions of the target molecules led to various, multifunctionalised C3-building blocks containing a prochiral center, selectively functionalized triacylglycerols and pharmacologically active compounds. 相似文献
14.
Enzymatic synthesis of high-purity structured lipids with caprylic acid at 1,3-positions and polyunsaturated fatty acid at 2-position 总被引:2,自引:0,他引:2
Akiko Kawashima Yuji Shimada Miwa Yamamoto Akio Sugihara Toshihiro Nagao Sadao Komemushi Yoshio Tominaga 《Journal of the American Oil Chemists' Society》2001,78(6):611-616
We attempted to synthesize high-purity structured triacylglycerols (TAG) with caprylic acid (CA) at the 1,3-positions and
a polyunsaturated fatty acid (PUFA) at the 2-position by a two-step enzymatic method. The first step was synthesis of TAG
of PUFA (TriP), and the second step was acidolysis of TriP with CA. Candida antarctica lipase was effective for the first reaction. When a reaction medium of PUFA/glycerol (3∶1, mol/mol) and 5% immobilized Candida lipase was mixed for 24 h at 40°C and 15 mm Hg, syntheses of TAG of γ-linolenic, arachidonic, eicosapentaenoic, and docosahexaenoic
acids reached 89, 89, 88, and 83%, respectively. In these reactions, the lipase could be used for at least 10 cycles without
significant loss of activity. In the second step, the resulting trieicosapentaenoin was acidolyzed at 30°C for 48h with 15
mol parts CA using 7% of immobilized Rhizopus delemar lipase. The CA content in the acylglycerol fraction reached 40 mol%. To increase the content further, the acylglycerols were
extracted from the reaction mixture with n-hexane and were allowed to react again with CA under conditions similar to those of the first acidolysis. After three successive
acidolysis reactions, the CA content reached 66 mol%. The content of dicapryloyl-eicosapentaenoyl-glycerol reached 86 wt%
of acylglycerols, and the ratio of 1,3-dicapryloyl-2-eicosapentaenoyl-glycerol to 1(3),2-dicapryloyl-3(1)-eicosapentaenoyl-glycerol
was 98∶2 (w/w). In this reaction, the lipase could be used for at least 20 cycles without significant loss of activity. Repeated
acidolysis of the other TriP with CA under similar conditions synthesized 1,3-dicapryloyl-2-γ-linolenoyl-glycerol, 1,3-dicapryloyl-2-arachidonoyl-glycerol,
and 1,3-dicapryloyl-2-docosahexaenoyl-glycerol in yields of 58, 87, and 19 wt%, respectively. 相似文献
15.
T. J. Mangos K. C. Jones T. A. Foglia 《Journal of the American Oil Chemists' Society》1999,76(10):1127-1132
Because of their unique fatty acid specificities and regioselectivities, lipases have been found to be effective catalysts
for the synthesis of structured lipids that have a predetermined composition and distribution of fatty acyl groups on the
glycerol backbone. The prospective plant-derived lipase found in the exudate of Carica papaya is known for its shortchain acyl group specificity, 1,3-glycerol regioselectivity, and sn-3 stereoselectivity. Carica papaya latex (CPL) was therefore examined for its potential ability to synthesize structured lowcalorie short- and long-chain triacylglycerols
(SLCT). In this paper, we describe the utility of CPL in the lipase-catalyzed interesterification reaction of triacetin and
hydrogenated soybean oil. Normal-phase high-performance liquid chromatography, combined with mass spectrometry, was used to
distinguish the structured SLCT synthesized using the lipase from the corresponding SLCT produced by chemical synthesis. 相似文献
16.
Substrate preferences for lipase-mediated acyl-exchange reactions with butteroil are concentration-dependent 总被引:1,自引:0,他引:1
Substrate preferences for pancreatic lipase-mediated acyl-exchange reactions with butteroil were concentration-dependent for
the series of acyl donors and alcohol acceptors evaluated. For acidolysis reactions, the initial reaction rates and percent
reaction yields after 18 h at 50 μmol acyl donor per gram substrate mixture were similar forn-fatty acids and their methyl and glycerol esters. At 400–500 μmol g−1 (and greater), order of initial reaction rates and percent reaction yield was fatty acid glycerol esters > fatty acid methyl
esters > fatty acids. At concentrations above 300–500 μmol g−1, reaction inhibition was observed for fatty acid substrates, and inhibition took place at lower concentrations for the shorter-chainlength
fatty acids of those evaluated (5–17 carbons). Inhibition was primarily attributed to acidification of the microaqueous environment
of the lipase. Desorption of water by the fatty acid substrate may be a secondary mode of inhibition. The concentration dependence
of initial reaction rates and percent reaction yield was similar for then-alcohol substrates evaluated (2–15 carbons) for alcoholysis reactions with butteroil. Optimum alcohol concentration was 375–500
μmol g−1 (except for butanol, which was 1 mmol g−1, above which reaction inhibition was observed. Inhibition was attributed to desorption of water from the enzyme by the alcohol
substrate. Relative reactivity of classes of alcohols for this reaction system was primary alcohols > secondary alcohols >
tertiary alcohols. Generally, alcoholysis reactions were faster than acidolysis reactions, and triacylglycerols were the best
substrates for acidolysis reactions with butteroil at high levels (up to 2 mmol g−1) of acyl donor substrate. 相似文献
17.
Improved synthesis of sucrose fatty acid monoesters 总被引:3,自引:0,他引:3
M. Angeles Cruces Francisco J. Plou Manuel Ferrer Manuel Bernabé Antonio Ballesteros 《Journal of the American Oil Chemists' Society》2001,78(5):541-546
The base-catalyzed synthesis of four sucrose fatty acid esters (caprylate, laurate, myristate, and palmitate) was performed
in dimethylsulfoxide by transesterification of sucrose with the corresponding vinyl esters using disodium hydrogen phosphate
as catalyst. In using a molar ratio sucrose/vinyl ester 4∶1 and mild reaction conditions (40°C and atmospheric pressure),
yields were higher than 85%. The isolated sucroesters had a higher percentage of monoesters (≥90%) and a lower content of
diesters in comparison with commercial derivatives. In all cases, 2-O-acylsucrose was the major product (≥60%) in the monoester fraction. 相似文献
18.
Hydroxy fatty acid (HFA) esters of long-chain alcohols, such as hydroxy stearates, have potential applications from lubricants
to cosmetics. These esters were synthesized enzymatically to overcome the problems associated with chemical processes. An
immobilized lipase, Rhizomucor miehei, was employed as catalyst in the esterification reaction between hydroxy-stearic acid as a source of HFA and monohydric fatty
alcohols (C8–C18). The yields of esters were in the range of 82–90% by conducting the reactions at 65±2°C, 2–5 mm Hg pressure, and 10% lipase
concentration. The products were analyzed by infrared spectroscopy, and some of their analytical characteristics were determined. 相似文献
19.
The transesterification of soybean oil with glycerol, 1,2-propanediol, and methanol by an immobilized lipase in flowing supercritical
carbon dioxide for the synthesis of monoglycerides is described. A lipase from Candida antarctica was used to catalyze the reaction of soybean oil with glycerol, 1,2-propanediol, ethylene glycol, and methanol. Reactions
were performed in supercritical carbon dioxide at a density of 0.72 g/L and at a flow rate of 6 μL/min (expanded gas). The
substrates were added at flows ranging from 2.5 to 100 μL/min. Monoglycerides were obtained at up to 87 wt%, and fatty acid
methyl esters at nearly 100 wt%. The reactivity of the alcohols paralleled the solubility of the substrate in liquid carbon
dioxide. Glycerol has the slowest reaction rate, only 2% of that of methanol. 相似文献
20.
Transesterification of trimethylolpropane and rapeseed oil methyl ester to environmentally acceptable lubricants 总被引:7,自引:0,他引:7
Esa Uosukainen Yu-Yen Linko Merja Lämsä Tommi Tervakangas Pekka Linko 《Journal of the American Oil Chemists' Society》1998,75(11):1557-1563
Biodegradable trimethylolpropane [2-ethyl-2-(hydroxymethyl)-1,3-propanediol] esters of rapeseed oil fatty acids were synthesized
by transesterification with rapeseed oil methyl ester both by enzymatic and chemical means, both in bench and pilot scales.
Nearly complete conversions were obtained with both techniques. A reduced pressure of about 2 to 5 kPa, to remove the methanol
formed during transesterification, was critical for a high product yield. The quantity of added water was also critical in
the biocatalysis. Candida rugosa lipase was used as biocatalyst and an alkaline catalyst in chemical transesterifications. In biocatalysis the maximum total
conversion to trimethylolpropane esters of up to 98% was obtained at 42°C, 5.3 kPa, and 15% added water. The maximum conversion
of about 70% to the tri-ester was obtained at the slightly higher temperature of 47°C. The reaction time was longer in the
biocatalysis, but considerably higher temperatures were required in chemical synthesis. In the chemical synthesis tri-ester
yields increased when the temperature was first held at 85 to 110°C for 2.5 h and subsequently increased to up to 120°C for
8 h. The trimethylolpropane esters obtained were tested as biodegradable hydraulic fluids and compared to commercially available
hydraulic oils. The hydraulic fluids based on trimethylolpropane esters of rapeseed oil had good cold stability, friction
and wear characteristics, and resistance against oxidation at elevated temperatures. 相似文献