Znd. Eng. Chem. Res. 1993,32, 1754-1759
1754
Inhibition of Radiation Degradation by Hydrogen-Donating Hydroaromatics Junichi Kubo Central Technical Research Laboratory, Nippon Oil Company, Chidori-cho 8, Naka-ku, Yokohama, 231 J a p a n
The inhibiting effect of a multicomponent hydroaromatic type additive (HHAP) produced from petroleum which showed prominent radical-scavenging ability with DPPH (2,2-diphenyl-1picrylhydrazyl) was tested against the radiation degradation (y-ray in air a t room temperature) of mineral oil in comparison with the effect of a hindered phenolic antioxidant, 2,6-di-tert-butyl-pcresol (DBPC). The obvious effects of HHAP on the restriction of the increases of acid value and carbonyl absorbance were preserved up to 2500 kGy. However, the structural changes that occurred in DBPC were shown by analyses of the carbonyl absorbance and of the OH group absorbance by IR. DBPC itself was analyzed by gas chromatography as the irradiation dose accumulated. The differences in the inhibiting effects of a hindered phenolic antioxidant and HHAP between the thermal oxidation and radiation degradation of polyolefins are discussed from these results. HHAP, which does not have a functional group containing heteroatoms, can be considered to be resistant to radiation as well as t o heat.
Introduction Pure hydrocarbons without functional groups containing heteroatoms have not been used as deterioration inhibitors to date excluding very exceptional examples, although hydrogen-donating hydroaromatics (abbreviated as hydroaromatics) have been used in such processes as coal liquefaction and heavy oil upgrading in a reductive atmosphere. Hydroaromatics such as tetralin are considered to be easily oxidized (Robertson and Waters, 1948;Morton and Bell, 1958; Russell, 1955; Howard and Ingold, 1968), and they have not been used in oxidative atmosphere. However,the radical-scavengingabilities of hydroaromatic were confirmed even in the presence of oxygen by experiments using DPPH (2,2-diphenyl-l-picrylhydrazyl) (Kubo, 1992b; Kubo et al. 1991),and it was shown that a multicomponent type additive (HHAP) produced by the hydrogenation of a highly aromatic heavy fraction from petroleum had higher radical-scavenging ability than tetralin. It as also confirmed that HHAP clearly inhibited the radiation degradation of high-density polyethylene (PE) and isotactic polypropylene (PP) (Kubo and Otsuhata, 1992a), including postirradation oxidation (Kubo and Otsuhata, 1992b), as well as thermal and oxidative degradation of petroleum products (Kubo, 1991a),rubbers (Kubo, 1991b; Kubo, 1992a), and plastics (Kubo, 1991b; Kubo, 1991~). It was also reconfirmed in this series of tests that hydroaromatics were resistant to heat, as already demonstrated in coal liquefaction and other processes. On the basis of these facts, the inhibiting ability and durability of HHAP against radiation degradation of mineral oil (SAE-10)were investigated in comparison with those of the typical hindered phenolic antioxidant DBPC (2,6-di-tert-butyl-p-cresol).The results are discussed in relation with those of the oxidative and radiation degradation of polyolefins.
Experimental Section The properties of the base stock (SAE-10)used in these experiments are shown in Table I. Samples (25 g) were placed in glass bottles (inner volume 30mL) and irradiated by y-ray in air at room temperature without mixing. The samples after the irradiation were analyzed by an acid value analyzer (defined as JIS K2501; KOH (mg) was used
Table I. ProDerties of Base Stock (SAE-10) specific gravity 1514 OC viscosity, cSt at40OC at 100 OC viscosity index color ASTM pour point, OC smoke point, OC aniline point, OC refractive index na20 nitrogen, wppm sulfur, wppm analyses by column chromatography, w t % saturates aromatics resins
0.8636 23.01 4.545 111
L0.6 -10 -8 107 1.4691 3 4
95.1 4.8 0.1
to neutralize the total acidic compounds in the sample (g))by IR (JASCO FT/IR-7000,O.l-mm fixed cell) and by gas chromatography (SHIMADZUGC-SA: column, methylsilicone 0.25 mm Q X 25 m, carrier gas, He). The irradiation rate was 10 kGy/h from Co-60. The DBPC (produced by TOKYO KASEI KOGYO Co., Ltd.) was more than 99% in purity, and it was used as received. The HHAP (Kubo, 1991B)used as a hydrogen-donating hydroaromatic additive was produced by the hydrogenation of a highly aromatic heavy fraction which was obtained by thermal treatment (400-500 "C) and distillation of petroleum heavy oil. The hydrogenation was carried out at 350-400 "C and 10-15 MPa. This HHAP containsmore than 300 components, and it is difficult and meaningless to define the individual components (the individual components cannot be isolated even by gas chromatography). The properties of the mixtures are shown in Table 11. Most of the components contained in the HHAP have partly hydrogenated products (hydrogenation of aromatic rings: 55%) of condensed aromatic rings. The average molecular weight is 398, the boiling point is above 350 "C, and it has a yellow-brown color and solid/liquid state at room temperature. The added HHAP in these tests was the same one as was examined in the radical-scavenging tests with DPPH (Kubo, 1992b; Kubo et al., 19911, and its high radicalscavengingeffect has been confirmed. It was also the same one as was used in the tests for the oxidative deterioration (Kubo, 1991b;Kubo, 1991~) and the radiation degradation
0888-588519312632-1754$04.00/0 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 1756 Table 11. Promrties of Heavy Hydroaromatics from Petroleum (HHAP) distillation ("C) density (15 "C, glcmg) 1.028 IBP/5% 2.536 X l@ (25.36 cSt) viscosity (100 O C , m28-9 1.15 10130 carbon residue (wt %) Sol 70 27.5 pour point ("C) 90IEP 234 flash point ("C) composition elem analysis ( % ) saturates 89.7 C 10.1 aromatics H 0.08 1H-NMR of aromatics S N HJH H/C (atomic ratio) 1.35 W H avg mol weight 398 HdH 1.5978 HGIH refractive index fa (fraction of aromatic carbons) ~
(Kubo and Otsuhata, 1992a; Kubo and Otsuhata, 1992b) of polypropylene (PP) discussed below.
0.2
3481374 3851420 4471478 5251584 21.3 18.7 0.16 0.30 0.37 0.17 0.38
4
Results and Discussion The changes of acid values by dose are illustrated in Figure 1. The additive effects of both DBPC and HHAP are obvious, and the effect of HHAP is higher than that of DBPC. The addition of HHAP (0.5 wt % ) and DBPC (0.5 wt % ) together exhibited an intermediate effect between HHAP (1.0 wt %) and DBPC (1.0 wt %). The changes of carbonyl absorbances (1710 cm-1 by IR) by dose are shown in Figures 2 and 3. In the case of the addition of HHAP, the increases of carbonyl absorbances were clearly inhibited by the addition of HHAP until 2500 kGy (Figures 2 and 3), and the effects depended on the amounts of HHAP added (Figure 3). In the case of the addition of DBPC, on the contrary, the carbonyl absorbances were restricted up to 1000kGy; however, they were intensely increased at 2500 kGy, exceeding the blank (without addition) data (Figures 2 and 3). It can be considered that the increases of the carbonyl absorbances are due to quinones and other carbonyl compounds formed from the added DBPC (Cook, 1953). This is supported by the data in the case of the addition of HHAP (0.5 wt % ) and DBPC (0.5 wt % ) together (Figure 2). Next the DBPC contents (by gas chromatography) by dose were examined (Figure 4). The added DBPC (0.3, 1.0, 3.0 wt %) was certainly detected at dose 0, and the DBPC contents were abruptly decreased by the increase of dose. The hydroxyl groups contained in DBPC (3650 cm-1 by IR) were also examined by dose, and the results are shown in Figure 5. The hydroxyl groups were decreased by dose, the same as the DBPC contents. Information on the structural changes in HHAP could not be obtained because it has so many components, but it can be inferred from Figure 2 that such structural changes as with DBPC did not occur in HHAP. These results can be summarized as follows: (1)Both DBPC and HHAP were effective for the inhibition of the increase of acid values, and HHAP was more effective than DBPC. (2) Structural changes occurred in DBPC, and carbonyl compounds were formed by the accumulation of dose. (3) HHAP, which does not contain any functional groups, exhibited clear effects on the inhibition of the formation of carbonyl compounds up to 2500 kGy. Radiation degradation of hydrocarbons can be considered to be oxidation mediated with radicals formed by irradiation (Clough, 1988). As described in my previous paper (Kubo, 1992b), high radical-scavenging effects of hydroaromatics against DPPH and also hydrocarbons were confirmed even in the presence of oxygen, and these radical-scavenging effects are considered due to their dehydrogenation reactions. That is, the radicals are stabilized by the hydrogens liberated from hydroaromatics
0
Figure 1. Increases of acid values of mineral oil by irradiation with y-rays in air at room temperature (irradiation rate 10 kGylh from co-60): O, blank (without addition); m, addition of DBPC (1.0 wt %); A,addition of HHAP (1.0 wt %); X, addition of DBPC (0.5 w t %) and HHAP (0.5 wt %).
as follows (tetralin as an example):
Accordingly, the additive effects of HHAP on acid value and carbonyl absorbance shown in this paper are considered to be due to their radical-scavenging effects. It is well-known that aromatic rings have the effect of absorbing radiation energy, and it was also confirmed by our experiments using model compounds that hydroaromatics (tetralin and octahydrophenanthrene) can lessen the damage caused by radiation to some extent (Soebianto et al., 1992). However, those effects can be considered to be relatively small, and the major additive effects of HHAP reported in this paper are attributed to its radicalscavenging ability. Hydroaromatics such as tetralin are considered to be easily oxidized, and they have not been used as antioxidants to date. However, as experimentally shown in my previous paper, the reaction pathways are changed by oxygen partial p r e s d e at the reaction sites, and reaction I) predominantly occurs when the oxygen partial pressure is relatively low. (1)
oxygerrcontainingcompounds
(10
In the experiments described in this paper, the samples were not mixed during the irradiation of y r a y in air, and
1756 Ind. Eng. Chem. Rea., Vol. 32, No. 8,1993
1
k
/
3.0
Dose (kGy)
Figure 2. Increaeee of carbonyl abeorbance (1710-' cm by IR)of mineral oil by irradiation with y-rays in air at room temperature (irradiationrata lOkGy/hfromCo-60,cellthickneseO.l mm):symbols are identical to those in Figure 1. 10.0
,-.
Dose (kGy)
I
.,
Figure 4. Changes of DBPC contents by irradiation with y-rays in air at room temperature (irradiation rate 10 kGy/h from Co-60): a,
v
'I
-
addition of DBPC (0.3 wt %); U, addition of DBPC (1.0wt %); addition of DBPC (3.0 wt %).
-. El f 5.0
//
f
e 2 i
0
2.W
1.000
0
3.000
Dose (kGy)
\
\
\\
\
10.0
\
\
h
HHAF
I
v
\
\
'I -. El f 5.0 8
I
Dose (kGy)
Figure 3. Cornpariron of theeffectaof DBPC and HHAP on inof carbonyl abnorbance (1710-'cm by IR)of mineral oil by irradiation with y-raye in air at room temperature (irradiation rate 10 kGy/h from Co-60, cell thicknw 0.1 mm): 0,blank (without addition); a, addition of DBPC (0.3 wt %); 0, addition of DBPC (1.0wt %); A, addition of HHAP (0.3 w t %); A,addition of HHAP (1.0wt %); A, addition of HHAP (3.0 wt %).
the oxygen partial pressure at the reaction sites is not defined; however, it can be estimated to be low because of the low diffusion rates of oxygen molecules into the liquid (lower than 0.02 MPa). Furthermore, it should be noted that the oxygen partial pressure at the sites where oxidation reactions occur in such hydrocarbon products as petroleum, rubbers, and
Figure 5. Changes of OH absorbance in DBPC (3660-1 cm by IR) by irradiation with y-rays in air at room temperature (irradiation rate 10 kGy/h from Co-60, cell thickness 0.1 mm): symbols are identical to those in Figure 4.
plastics is much lower than the air pressure in most of the practical uses. The additive effects of HHAP are quite obvious in the experiments described in this paper. This can be understood by the results in my previous paper (Kubo, 1992b). In the case of the addition of DBPC, the structural changes occurred in DBPC as the irradiation doses were accumulated (Figure 3). The main oxidation products from DBPC can be judged from other papers (Cook, 1953; Korcek et al., 1983) to be quinones, and the inhibiting
Ind. Eng. Chem. Res., Vol. 32, No.8,1993 1757 4.0
r
t I
0
100
200
300 400 Deterioration Time at 120'c (h)
500
700
h
500
x v
10
20
30
50
40
60
Deterioration Time at 270T (min.)
Figure 7. Changes in melt flow rate of polypropylene by the addition of inhibitors: symbols am identical to those in Figure 6.
0
800 r
I
0
200
100
300
400
500
Deterioration Time a t 120T (h)
Figure 6. Changes of elongation at break and tensile strength at yielding point of polypropylene by the addition of inhibitors: 0 , blank (withoutaddition);A, additionof HHAP (0.1wt % );A,addition of HHAP (0.5 wt %); X, addition of Irganox 1010 (0.1 wt %).
effects on the formation of carboxylic acids are observed to decline as the doses increase (Figure 2). In summary, it can be said that hindered phenols can easily be structurally changed into quinones because their OH groups can easily be attacked by irradiation. However, pure hydrocarbon type HHAP, which does not have functional groups containingheteroatoms, is more resistant to irradiation. This will be made clearer by the comparison of the data for the oxidation and the radiation degradation of polypropylene. It was already reported in my previous papers (Kubo, 1991b;Kubo, 1991~) that a hindered phenolic antioxidant
/
x
Irganox 1010
exhibited higher inhibiting effects against the oxidative degradation of polyolefins than did HHAP. To elucidate this again,the experimental results on the inhibitingeffects of the hindered phenolic antioxidant and HHAP on the changes of elongation and tensile strength of PP by oxidative degradation are shown in Figure 6. It can be seen that the addition of 0.1 wt % of the hindered phenolic antioxidant was as effective as the addition of 0.5 wt % of HHAP, and the addition of 0.1 wt 96 of HHAP was not so effective. The melt flow rates of PP at 270 OC in air
,
c
20
W
40
&
80
1w
Dam (kCy)
Figure 8. Changes in elongation at break of polypropyleneby dose (electron beam 2400 kGy/h) at room temperature in air: 0, blank (without addition);A, addition of "AP (0.3 wt %); 0, addition of HHAP (1.0 wt %); 0 , addition of Irganox 1010 (0.1 wt %).
are also illustrated in Figure 7. These data are consistent with the results in Figure 6. The additive effects of the same conventional antioxidant and HHAP on the radiation degradation of PP were already reported (Kubo and Otauhata, 1992a; Kubo and Otsuhata, 1992b). The changes of elongation of PP during irradiation are shown in Figure 8. The addition of 0.3w t % and 1.0 wt % of HHAP is clearly effective;however, the addition of 0.1 wt % of the conventional antioxidant is not so effective when compared with the case of oxidative degradation (Figures 5 and 6). The effects of these additives on the inhibition of postirradiation oxidation (Kubo and Otsuhata, 1992b)are elucidated in Figure 9. From these data, it can be seen that the conventional antioxidant is not BO effective in postirradiationoxidation when compared with ita excellent inhibiting ability exhibited in oxidative degradation. It can be understood from these results that the hindered phenolic antioxidant cannot exhibit as prominent inhib-
1768 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 Blank ( 0 kGv)
EB 30kGy
0-0-
Looi 2oo
0 '
0
A A
D
U
-
t
L
0
1.5
3
4.5
6 Aging Time (month)
Aging Time (month)
EB 50kGy
EB BOkGy
Bw(f-O\ 600
0
1.5
3
4.5
6
0
Aging Time (month)
" 1.5
3
4.5
9 6
Aging Time (month)
Figure 9. Changes in elongation at break of polypropylene with aging time (electron beam 2400 kGy) at room temperature: symbols are identical to those in Figure 8.
iting ability in the radiation degradation of P P as in the oxidative degradation. It was already pointed out (Allen et al., 1987;Horng and Klemcbuk, 1984) that hindered phenolic antioxidants, including Irganox 1010, were destroyed at lower doses in the case of PP and that the extent of the destruction depended on the nature of both the antioxidants and the polymers. The previously mentioned difference in the effects of the hindered phenolic antioxidant between the oxidative deterioration and the radiation degradation seems to be due to the destruction of the antioxidant. This is supported by the experimental results elucidated in Figures 4 and 5, and the differences in the destruction rates between our data and the literature cited seem to depend on the antioxidants (Irganox 1010 and DBPC), the base material (PPand mineral oil), and the processing history of the samples. In contrast, hydroaromatics exhibit prominent inhibiting ability against radiation degradation of PP up to higher doses because they do not contain functional groups and they are resistant to radiation, as illustrated in Figures 1 and 2. It is well-known that oxidation occupies the greater part in radiation degradation, especially in postirradiation degradation, and the antioxidants which are resistant to radiation can be effectively used as antiradiation material. In addition, the properties of hydroaromatics which are resistant to radiation as well as their prominent radicalscavenging abilities can be effectively used in such radiation processes as cross-linking. Pure hydrocarbonswithout functionalgroups have never been used as radical scavengers to date except in such processes as coal liquefaction and heavy oil upgrading. However, the obvious radical-scavenging abilities of hydroaromatics were confirmed even in the presence of oxygen (Kubo, 199213; Kubo et al., 1991), and their prominent inhibiting abilities against the thermal, oxi-
dative, and radiation degradation of various hydrocarbon products were also shown. Now,it was shown by these tests that hydroaromatics are resistant to radiation compared with the conventional hindered phenolic antioxidants.
Conclusions The inhibiting ability and durability of HHAP (a multicomponent, hydroaromatic additive produced by hydrogenating a highly aromatic heavy fraction from petroleum) against the radiation degradation of mineral oil were investigated compared with DBPC, and the following results were obtained: 1. The inhibiting effects of HHAP on the increases of acid value and carbonyl absorbance were higher than those of DBPC. 2. Structural changes in DBPC and the formation of carbonyl compounds were observed as the dose accumulated. 3. Such structural changes could not be found in the case of HHAP, which contains no functional group, and its inhibiting ability was maintained up to a high dose (2500 kGy). 4. The insufficient inhibiting ability of the hindered phenolic antioxidant against the radiation degradation of polyolefins despite its excellent effects on oxidative degradation described in our previous papers can be elucidated from the structural changes in, DBPC by radiation. Acknowledgment
I thank Dr. Yoneho Tabata, professor of Tokai University, and Dr. Yousuke Katsumura, assistant professor at Tokyo University, who provided the opportunity to conduct the experiments. I also thank Dr. Kazushige
Ind. Eng.Chem.Res.,Vol. 32, No. 8,1993 1759 Otsuhata of Raytech Corp. and the staff of the Central Technical Research Laboratory of Nippon Oil Company who were involved in the tests and analyses.
Literature Cited Allen, D. W.; Leathard, D. A.; Smith, C.; McGuiness, J. D. Effects of Gamma-Irradiation on Hindered Phenol Antioxidants in Poly(Viylchloride) and Polyolefiis. Chem. Znd. 1987,198-199. Clough, R. L. Encyclopedia of Polymer Science and Engineering, 2nd ed.; Wiley: New York, 1988; Vol. 13, p 667. Cook, C. D. Oxidation of Hindered Phenols. 1. Oxidation of and Oxidation Inhibition by 2,BDi-tert Butyl-dMethylphenol. J . Org. Chem. 1963,18, 261-266. Horng, P.; Klemcbuk, P. Stabilizers in Gamma-Irradiated Polypropylene. Plastic Eng. 1984, April, 36-37. Howard, J. A,; Ingold,K. U. AbsoluteRate Constants for Hydrocarbon Oxidation. The Reactions of Tertiary Peroxy Radicals. Can. J . Chem. 1968,2666-2660. Korcek, S.; Mahoney, L. R.; Jensen, R. K.; Zinbo, M.; Willermet, P. A.; Kandah, S. K. Report 1983, AFOSR-TR-83-0987, p 214. Kubo, J. Inhibiting Abilities of Hydroaromatics against Deterioration of Hydrocarbon Products-1. Inhibition of Thermal Deterioration and Ita ApplicationtoOilProducta. FuelProcess. Technol. 1991a, 263-277. Kubo, J. Inhibiting Abilities of Hydroaromatics against Deterioration of Hydrocarbon Products-2. Application to Rubbers and Plastics. Fuel Process. Technol. 1991b, 19-34. Kubo, J. Inhibition of Deterioration of Plastics by Hydroaromatics. J. Appl. Polym. Sci. 19910, 61-60.
Kubo, J. Inhibition of Deterioration of Rubbers by Hydroaromatics. J. Appl. Polym. Sci. 1992a, 926-936. Kubo, J. Radical Scavenging by Hydroaromatics in the Presence of Oxygen. Znd. Eng. Chem. Res. 1992b, 2687-2693. Kubo, J.; Otsuhata, K. Inhibition of Radiation Degradation of Polyolefi by Hydroaromatics. Radiat. Phys. Chem. 1992a,261268. Kubo, J.; Otauhata, K. Effects of Hydroaromatics on Post Oxidation of Polyolefi Induced by Electron Irradiation. Radiat. Phys. Chem. 1992b, 477-483. Kubo, J.; Miyagawa, R.; Takashashi, S. Radical ScavengingAbilities of Hydroaromatics. J. Jpn. Petrol. Znst. 1991,473-476. Morton, F.; Bell, R. T. T. The Low Temperature Liquid Phase Oxidation of Hydrocarbons: A Literature Survey. J . Znst. Petrol. 1968,260-272. Robertson, A.; Waters, W. A. Studiesof the Autoxidation of Tetralin. Part 1. Investigation of Autoxidation Products. J . Chem. SOC. 1948,1674-1690. Ruseell, G. A. The Rate of Oxidation of Aralkyl Hydrocarbons. Polar Effects in Free Radical Reactions. J . Am. Chem.SOC.1966,10471064. Soebianto, Y. S.; Katsumura, Y.;Ishigure, K.; Kubo, J.; Koizumi, T. Protection in Radiolysis of n-Hexadecane-2. Radiolysis of nHexadecane in the Presence of Additives. Radiat. Phys. Chem. 1992,461-469. Received for review November 9, 1992 Revised manuscript received April 29, 1993 Accepted May 10,1993
