1、4776 (RP-1211) Mechanism for Reaction Between Polyolester Lubricant and Ferrous Metals, Part I: Literature Search Robert E. Kauffman ABSTRACT This paper summarizes the results of a literature search performed to gain an understanding of the reaction mecha- nism(s) responsible for the catalytic effec
2、ts offerrous metals on the thermal degradation of polyolesters. The referenced papers indicate that the reaction is highly temperature depen- dent and may be initiated by the presence of dissolved iron species. Reaction mechanism candidates based on iron cata- lyzed thermal degradation and hydrolysi
3、s/dehydration ofpoly- olesters are proposed to explain the reported experimental results of the cited researchers. The effects of temperature on the degradation reactions appear to be dependent on the decomposition temperatures of the produced iron carboxylate and polyolester reaction products. The
4、proposed reaction mechanisms will be tested andfinalized in the secondpart of the planned research program. INTRODUCTION Due to refrigeranthbricant compatibility issues such as solubility, the introduction of HFC refrigerants into the air- conditioning and refrigeration industry has required the rep
5、lacement of mineral oil-based lubricants with ester-based lubricants. Since esters have many properties making them better suited for lubrication than mineral oils, dibasic and polyol type ester basestocks have been used for several decades for the lubricating oils used in aircraft gas turbine engin
6、es (Randles 1993) and have been studied using a wide range of thermal, hydrolytic, and oxidation stability tests (Randles 1993; Beimesch 1999). However, it is also well known (Blake 1961) that the oxygen bonds present in an ester molecule greatly reduce its thermal and hydrolytic stabilities in comp
7、arison to mineral oils. For both dibasic and polyol type esters, the reaction mechanism with water (hydrolysis) is the same and repro- duces the carboxylic acid and alcohol used to synthesize the ester: The thermal breakdown mechanisms of dibasic esters involve the intramolecular abstraction of hydr
8、ogen from the carbon in the position of the alcohol portion (R) of the ester, producing an alkene and a carboxylic acid group by the following mechanism (Randles 1993): S B ; M II II II M n R-C-C -0-C-I II,C=Clli t 11-0 -C-R -C -0- R Il Alkene Acid (miiip I I li Iibaiic I !er P (2) Due to the hydrog
9、en abstraction mechanism, the ther- mal stability of dibasic esters are 5O0C-100“C lower than the thermal stabilities of polyolesters. Since polyolesters do not have a hydrogen, the thermal degradation is thought to occur by a free radical mechanism (Randles 1993), also producing an alkene and carbo
10、xylic acid: Robert Kauffman is a senior research chemist in the Department of Nonmetallic Materials, University of Dayton Research Institute, Dayton, 02005 ASHRAE. 378 Although the reaction mechanism is not well understood, it is well known (Blake 1961; Cuellar 1977) that the thermal degradation of
11、polyolester lubricants above 250C is acceler- ated in the presence of ferrous materials. Also, during recent research in which HFC refrigerants, polyolesters, and coupons of different refrigeration construction metals were heated inside sealed glass tubes (Field and Henderson 1998), the production o
12、fparticles and coupon weight loss indicating ther- mal degradation of the ester were only observed with steel coupons. The presence of ferrous coupons did not appear to accelerate the rate of lubricant degradation until the test temperatures exceeded 200C (392F). Since the mechanisms by which ferrou
13、s metals catalyze the thermal degradation of polyolesters is not understood and polyolesters are being used as lubricants in many current refrigerating systems and being proposed for future systems, a research project has been conducted to study the effects of HFC refrigerants, metal coupons, water,
14、 and carboxylic acids on the thermal degradation reactions of polyolesters. Prior to initiating the research phase of the project, a thorough litera- ture search, supplemented by computer searchers of chemistry and engineering data banks, was performed, focusing on the thermal degradation of esters
15、and carboxylic acids in the pres- ence of metal catalysts. This paper summarizes the literature search with the iden- tified references categorized into four areas of interest- refrigerant/polyolester thermal tests, polyolester thermal degradation reactions, polyolester wear reactions, and metal car
16、boxylatelcarbonyl oxygen interactions. Based on the results of the literature search, a reaction mechanism (to be tested in future sealed-tube tests) is proposed for the thermal degradation of polyolesters in the presence of iron. REFRIGERANTIPOLYOLESTER THERMAL TESTS Over the past 15 years, several
17、 researchers have investi- gated the thermal stability of polyolesters in the presence of R-134a refrigerant and metal catalysts. The majority of the research, summarized by Lilje (2000), was performed at 175C (347F) in sealed glass tubes, and it was found that the polyolesters were stable in the pr
18、esence of R-134a refrig- erant, water, and metal catalysts (Kauffman 1993; Lilje However, Huttenlocher (1 992) and Sanvordenker (1 99 1) reported that polyolesters heated to 200C (392F) in the pres- ence of steel coupons and R-134a refrigerant produced carboxylic acids, carbon dioxide, and water. Hu
19、ttenlocher (1 992) reported that one particular polyolester underwent molecular changes at temperatures above 150C (300F) with the ester linkages undergoing significant decarboxylation. Sanvordenker (1 99 1) reported that a steel catalyst was neces- sary for the polyolesters to decompose at 200C (39
20、2F) and that at 250C (482F) the kinetics of the thermal decomposi- tion could be monitored. Huttenlocher (1 992) also stated that R-32 and R- 134a refrigerants were stable up to 200C (392F) 2000). in the presence of steel, i.e., no refrigerant decomposition products were detected under the test cond
21、itions. Spauchus (1 989) and Kaufian (1 993) also reported that R-134a was stable in the presence of metals and lubricants below 175C (347F). However, the lubricants tested by Spau- chus (1 989) underwent decomposition but were not identified. The additive-free lubricants tested by Kauffman (1993) w
22、ere stable in the presence of steel coupons for several days at 200C (392F). Gryglewicz et al. (1997) and Gryglewicz and Beran (2000) heated pentaerythritol esters based on C,-C carboxy- lic acids at 250C (482F) for 120 hours in the presence of R-134a refngerant, metal coupons, and water. Corrosion
23、of steel occurred at 50 pprn water, corrosion of copper occurred at 3,000 ppm water, and corrosion of aluminum was not observed. Large quantities of deposits and ester color changes were also noted. Analysis of gaseous products was not reported by Gryglewicz et al. (1997) and Gryglewicz and Beran (2
24、000). In a US Patent, Shimomura et al. (2001) reported heating pentaerythritol esters based on 2-ethylhex- anoic acid or 3,5,5 trimethylhexanoic acid for 2000 hours at 200C (392F) in the presence of R-134a refrigerant, 1000 ppm water, and metal catalysts (aluminum, copper, and iron). Although carbox
25、ylic acids were measured in the 2000- hour stressed oils, no corrosion of the metal coupons were observed by Shimomura et al. (2001). Finally, Field and Henderson (1 998) reported that branched acid polyolesters heated at 200C (392F) in the presence of R-l34a refrigerant and metal catalysts created
26、carboxylic acids, “copious” amounts of precipitates, and severe metal weight loss when cast iron or steel catalysts were used. The addition of water (1000 ppm) or hexanoic acid (500 and 5000 ppm) had little effect on the cast iron and steel cata- lyst tests. Minimal precipitates or metal corrosion w
27、ere reported for aluminum, brass (few pprn Zn in oil), or copper heated up to 175C (347F) in the absence or presence ofwater and hexanoic acid (carboxylic acid levels were elevated but stable). Although hydrogen and iron carboxylates were expected by-products of carboxylic acid corrosion of cast iro
28、n or steel, Field and Henderson (1998) were unable to detect hydrogen in the vapor phase of the heated polyolester, the precipitate appeared to contain ferrous oxide, and the precip- itate was not a metal carboxylate. Overall, the references on the ref-igerantpolyolester thermal tests indicated the
29、following: Thermal degradation of polyolesters does not occur in the absence of iron containing coupons below 200C (392F) and is not catalyzed by the addition of water or carboxylic acids. At or above 200C (392F) the presence of iron-containing coupons causes thermal degradation of the polyolester i
30、n the presence of R-134a refrigerant. The degree of ester degradation increased with temperature but was unaffected by the addition of carboxylic acid or water. ASHRAE Transactions: Research 379 3. The thermal degradation of the polyolester causes the corrosion (weight loss) of iron-containing coupo
31、ns, the production of carboxylic acids and iron-containing precip- itates in the liquid phase, and the production of carbon diox- ide, carbon monoxide, water, and volatile organic compounds in the vapor phase. The production ofhydrogen or iron carboxylates was not reported. Refrigerant R-32, or R-l3
32、4a, was stable up to 200C (392F) in the presence of polyolesters and iron-containing catalysts. 4. POLYOLESTER THERMAL DEGRADATION REACTIONS In contrast to the recent use of polyolesters for use in refrigeration applications, the use of polyolesters in aircraft engine environments has been studied f
33、or over 50 years (Cuel- lar 1977). A wide range of tests have been used to study the effects of metals on the thermal degradation of polyolesters. Cuellar (1977) used a stainless steel rotating cylinder to heat trimethylolpropane triheptanoate (TMP-THP) to 343C (650F) in dry and moist nitrogen atmos
34、pheres. Temperatures above 315C (600F) were required to increase the acid content (n-heptanoic acid) of the bulk ester, and the amount of acids was increased by the presence of water. However, Cuel- lar (1 977) reported that the majority of the n-heptanoic acid was concentrated in the cold trap (nit
35、rogen flow) and the stain- less steel cylinder did not experience residue buildup or corro- sion. A series of studies by Cvitkovic et al. (1979), Klaus et al. (1970), and Naidu et al. (1988) used the Penn State microre- actor to evaluate the thermal stabilities of polyolesters as thin films on metal
36、 catalysts. The heated esters were tested as films to increase the metal to ester ratio and, presumably, to increase the catalytic effects of metal on ester thermal degradation. However, Cvitkovic et al. (1979) reported no significant ther- mal degradation of TMP-THP heated from 175C (347F) up to 25
37、0C (482F) on aluminum and low carbon steel surfaces in a 20 mL per minute nitrogen flow. Naidu et al. (1988) also heated films of TMP-THP in a 20 mL per minute nitrogen flow on the metal catalyst. They monitored thermal degradation using gel permeation chromatography to detect molecular weight chang
38、es in the stressed ester and reported that low carbon steel slightly increased thermal degradation in compar- ison to copper or aluminum catalysts in the temperature range of 200C (392F) to 250C (482F). In contrast to Cvitkovic et al. (1979) and Naidu et al. (1 988), Klaus et al. (1 970) heated a wi
39、de range of esters using a static nitrogen atmosphere to allow analysis of the volatile degradation by-products. The presence of combined metal catalysts (M-10 steel, 52-100 steel, and naval bronze) did not affect the decomposition rates of dibasic esters at 3 15C (600F). However, the combined catal
40、ysts greatly increased the degradation rate of TMP-THP and pentaerythritol esters at 3 15C (600F) and the catalysts experienced weight losses in the following decreasing order: 52-100 steelM-lO steelnavalbronze. Klausetal. (1970) also reported that the presence of the metal catalysts greatly increas
41、ed the system pressure, assumed to be hydrogen and carbon dioxide and later verified by Naidu et al. (1988). Naidu et al. (1 988) also reported that in previous research by Klaus et al. (1 970), the TMP-THP decomposed in the pres- ence of low carbon steel to produce n-heptanoic acid and a product si
42、milar in weight to trimethylol propane diheptanoate, i.e., TMP triester (THP) loses a carboxylic acid (HP) to produce diester (DHP). Klaus et al. (1970) also reported that constant volume systems (the liquid degradation products remain in contact with the heated ester and steel catalyst) increased t
43、he rate of degradation (measured rate of ester deple- tion and metal weight loss, which were comparable) versus constant pressure systems (products allowed to escape to maintain atmospheric pressure). Of the esters tested by Klaus et al. (1970), dimethyl sebacate and methyl laurate (no carbon on alc
44、ohol) had much higher thermal stability than TMP-THP and pentaerythritol esters (have carbon but no hydrogen) and underwent minor degradation at 37 1 “C (700F) with or without metal catalysts, i.e., thermal stability and degradation mechanism similar to hydrocarbon. Blake (1961) used a static isoten
45、iscope (closed system) to monitor the vapor pressure of compounds heated under nitro- gen. Blake defined the decomposition point as the temperature at which 10% decomposition occurred using a heating rate of 4C per minute. Blake reported that the addition of a 52-100 steel coupon to the isoteniscope
46、 decreased the decomposition point of the esters-bis (2-ethylhexyl) sebacate ( hydrogen) from 283C to 227“C, bis (1 -methylcyclohexylmethyl) seba- cate (no hydrogen) from 328C to 295“C, and TMP-THP (no hydrogen) from 317C to 256C. The presence of 52-100 steel had no effect on the decomposition point
47、 of the hydro- carbon octacosane (347C to 350C). Cottington and Ravner (1 969) performed thermal tests in sealed glass cells (closed system) of various unidentified esters at 260C (500F) with and without the metal catalysts: AMs-5504 steel, mild steel, and high purity iron. For every esterlmetal com
48、bination, the ester degradation rate increased (carboxylic acids increased) in the presence of the metal and the catalyst experienced weight loss. The presence of the metal catalysts did not affect the thermal degradation of white petro- leum oil. Jones et al. (1970) performed thermal tests in seale
49、d mild steel capsules (closed system) of pentaerythritol tetracaproate (PE-TC) ester at 260C (500F). The evolution of hydrogen (carboxylic acid reacts with steel surface to form carboxylate and hydrogen) through the steel wall into an evacuated cham- ber was used to monitor the reaction rate of the ester degrada- tion. After 24 hours at 260C (500“F), a dark, resinous solid was observed on the steel walls. After 96 hours of heating at 260C (500F), Jones et al. (1970) reported the remaining solid contained fin
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