Techniques for Oxidative Degradation of Polyethylene Richard 1. Spore' and Robert M. Betheaz Chemical Engineering Department, Texas Tech University, Lubbock, Tex. rQ40.9
The oxidative degradation of polyethylene in the temperature range 75-2OO0C and a t oxygen concentration levels of 0-1 00% was studied with gas chromatographic techniques. Twenty-four degradation products were identified during the study. O f these, 15 were aliphatic hydrocarbons, and nine were oxygenated organic compounds. Quantitative analysis of the degradation products indicated that their concentrations were directly proportional to oxygen concentration in the degradation atmosphere. The effect of temperature was not so clear-cut. Concentrations of nine of the degradation products were directly proportional to temperature increase, four reached a maximum within the temperature range, and 1 1 showed a combination of these two effects.
T h e number of thermooxidative degradation analyses of polymers has increased since the advent of the NASA manned space flight program. However, the bulk of these analyses has been limited to systems at or near conditions of pyrolysis. Little has been done to relate the effects of oxygen concentration on thermooxidative degradation and the types of degradation products evolved at lower temperatures. The purpose of the study reported here was twofold: first, to provide information on polymer degradation to fill the gap between the initial work performed by NASA and the more recent high temperature and pyrolysis studies; second, to study the effect of oxygen concentration on the amounts and types of degradation products. A commercial grade of polyethylene was chosen as the test polymer. Samples of the polymer were degraded a t four levels of oxygen concentration and six levels of temperature. Degradation products were identified by hydrogen flame ionization chromatography. Concentrations of the degradation products were obtained by the response factor chromatographic technique. Standards for degradation analysis adopted by NASA were used in all experimental work. literature
Diffusion in Polymers. A comprehensive review of diffusion in solid polymers and plastics has been presented (Smith, 1968). The dependence of diffusion rate in polymers on concentration was primarily dependent on the temperature, molecular size, and solvent power of the penetrant gas. Chain length, branching, and double bonds in the polymer also affect the rate of diffusion. It has been proposed (Gonella, 1964) that polymeric degradation products result from a combination of diffusional and chemical reaction processes. Four such processes were noted as most probable: Diffusion of entrapped gases from the interior of the polymer into the atmosphere Reaction of gases diffused from the interior of the polymer with oxygen in the atmosphere Surface reaction of polymeric materials and oxygen in the atmosphere Present address, Polyethylene Plant, n o w Chemical Co., Freeport, Tex. To whom correspondence should be addressed. 36
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 1, 1972
Interior reaction of entrapped gases with polymeric material, followed by diffusion of the reaction products into the atmosphere Figure 1illustrates these four processes. Infrared absorption bands in phenolic resins undergoing oxidative degradation were used to determine the nature of the process (Conley and Bieron, 1963). Their observations showed that the size of the oxidative infrared bands was directly proportional to sample thickness. They therefore concluded that degradation was diffusion controlled and occurred as a surface phenomenon. The bulk of degradation reactions studied has been attributed to some type of surface phenomenon. It has been reported (Isenberg, 1964) that the mass loss of a plastic material undergoing degradation occurred primarily a t the surface. H e proposed that a t a given temperature the rate of diffusion of gaseous material through the plastic controlled the rate of weight loss. Weight loss per unit time increased significantly with decreasing sample thickness. Mechanisms of Degradation. Most of the mechanisms of thermooxidative degradation of polymers have been attrihuted to either chain scission of the polymer or an attack by molecular oxygen on the polymer linkages. Studies (Bruck, 1964) of the effect of polymer cross-linking on degradation reactions showed that polymers having no cross-linking generally underwent scission of the chain structure and were cornpletely volatilized. Cross-linked polymers tended to carbonize under the same conditions and left residues of degradation amounting to as high as 50% of the original sample weight. Other evidence is available in the literature concerning the mechanisms of thermal degradations of various polymers under oxidative conditions (Cameron and Kerr, 1968; D u m e t al., 1968; Kelen et al., 1968; Schneider et al., 1968; Still e t al., 1969). The majority of these mechanisms is attributed to chain scission and the formation of some type of intermediate free-radical species. Thermal oxidation of polyolefins depends primarily on temperature and molecular structure (Winslow and Llatreyek, 1964). Reactions occur preferentially in amorphous regions where chain scission causes crystallization and step changes in physical properties. The mechanism of the reaction was determined (Winslow et al., 1964) by solution viscosity and gel fraction measurement. Changes in intrinsic viscosity
and density were closely correlated with changes in oxygen content. Antoxidation is diffusion controlled and occurs at random in the linear polymer (Winslow et al., 1966). The oxidation of linear polyethylene at 100°C consisted (Winslow e t SI., 1961) of a ranid . initial stage followed by a gradual decrease in react,ion rate as the clegree of crystallinity was increased by chain scission. In t kkat study significant variables were sample thickness and teniperature. A linear polyethylene, Marlex 6000, showed (Winslow e t al., 1963) no change in densit y when heated in B nitrogen atmosphere for 300 hr a t lo()"C. Replacement of the nitrogen by oxygen caused rapid de!nsity changes accompanied by the formation of an addition: tl crystalline phase. Analysis of Degrada tion Products. So far, only small mention has been made of the products of thermal degradation of polymers and the methods used to identify these products. Currently, the prim:iple means of such qualitative aualyses have been mass spectrometry, infrared spectroscopy, and gas chromatography. G: is chromatographic analysis has become increasingly popul;Pr in the last several years (Davison e t al., 1954; Barlow eb :al., 1963; Scholz et al., 1966; Tsuge et al., 1969). The in vacuo degradaicions of several polyoleiins have been reported (Tsuchiya and Sumi, 1968b, 1969a,h). Polymethylene was pyrolyzed hetwaen 375' and 465°C to yield n-alkanes, 1-alkenes, and alkadiene3 in the mole ratio of 1:2: 1. Polypropylene was studied und er vacuum a t temperatures of 360°, 380", and 400°C. The volatile products included paraffins and olefins in the C, to C12 ranee. The in vacuo deeradation " . . . . at polyisooutylene between 325 and 465°C yielded C, to CM hydrocarbons. Dimers, trimers,, and tetramers of isobutylene were among these products. All analyses were performed with a gas chromatograph. The combustion products os p V L y ~ ~ l I y ~ nIlalr lr Vrrll amined (Dotreppe-Grisard, 1968) with gas chromatography, thin-layer chromatography, and mass spectrometry. The mechanisms and degradation products of linear polyethyleue were investigated (Bailey and Liotta, 1964). When the polymer was pyrolyzed a t 415"C, eleven products were formed. The major ones were propylene, 30%; propane, 20%; ethane, 10%; and ethylene, 6%. At 600°C essentially only ethylene, 60%; propylene, 30%; and ethane, 10% were produced. Another pyrolysis-gas chromatography study of polyethylene has been reported (Trestianu and Sandulescu, 1967). Temperature of the degradatioii chamber was 600°C. They reported that eight degradation products were formed. The principal products and amounts were ethane, 30%; propane, 20%; n-hexane, 17.6%; n-butane, 8%; and isobutane, 12%. Normal pentane and pentene were reported in amounts less t,han 1%. The in vacuo thermal degradation of polyethylene at 40OoC yielded (Luff and White, 1968) a homologous series of hydrocarbons including paraffins, olefins, and higher-order acetyl. enes. Another study (Tsuchiya and Sumi, 1968a) employed gas chromatography in the analysis of the thermal degradation products of polyethylene under vacuum a t temperatures between 375" and 425°C. The sample was degraded for a 20-min. period. The 26 degradation products identified included nine of the C,C, alkanes, fourteen of the CrCg alkenes, and three of the Cspentadienes. ~~
I
.
~~
~~
. .
~
.
U
0 OW." E"frnW.d
(io*
8 acdom Rod"'! Figure 1 .
Probable polymeric degradation processes
~
Degradation and Aiialytical Equipment
Degradation Eqiuipment. Degradation Chamber. A 1-1. degradation chambe!r to he fashioned from borosilicate ghss
Figure 2.
Degradation chamber component parts
or 300 series stainless steel was proposed (Bethea, 1966). Lead gaskets were t o seal the chamber. In later work (Turner and Bethea, 1967) stainless steel chambers were used. Degrada. tion products were removed from the chamber through a sample septum with a 10OO-pl gas syringe and chromatographed. The degradation chamhers were subsequently modified (Lane, 1969) and fashioned from 2-1. stainless steel beakers. The degradation chamber design employed in the present work was a modified version of Lane's design. Four chamhers were built, three for actual degradation analyses of the polymer and one for experimental control. The component parts of the degradatioll chamber are shown in Figure 2. The main body of each chamber was a 2-1. stainless steel beaker. A round stainless steel plate 1/kin. thick was used to cover the open top of the chamber body. Gaskets were fashioned from afIG-in.sheet lead and placed between the cover plate and the chamber body t o provide a leakproof seal. T o provide gas sampling ports for the chambers, 8 hole was drilled through t h e wall of each beaker located 1-in. above the bottom. A 'Ia-in. Swagelok mole connector was then butt welded into the hole with Eutalloy microflow alloy. A Whitey angle pattern valve was attached t o the male connector as a chamber shutoff valve. A stainless steel female pipe-to-tuhing connector was attached t o the outlet of the valve to provide a connection for the sampling apparatus. After all connections had been made on the chambers, red Glyptal sealer was applied t o the cornleetion points. The chambers were then baked at 100°C for 24 hr to fix the sealer. Any possible contaminants evolved from the sealer during a Ind. Eng. Chem. Prod. Res. Develop., '401. 11, No. 1, 1 9 7 2
37
Table 1. Chromatographic C Column Solid wppc
no.
Liquid phose
1
Tricresvl nhosnhate Di-n-d&& phihalate and UCON 50HB 2000 Bentone 34 modified diisodecyl phthalate None
2
3 4
Chromosor Chromosor Chromosor Chromosorl
Acid washed. L)imethyldichlorosilanetreated
tne aeiivery 01 oxygen ana nelium Irom sorage cylinaers t o the degradation chambers. The delivery end of each piece of tubing was equipped with a Swagelok QC body assembly, which provided rapid coupling t o the stem assemblies on the sampling ay,paratus. Each gas line was also provided with a trap contain ing Ascarite and activated charcoal for gascleanup prior to use:. The traps were fabricated from 1-in. pipe and mor0 5 .._. in long. Equal volumes of Ascarite and activated .._._I charcoal were placed in each trap. Similar tubing and a QC assembly were used on the vacuum line for evacuation of the degradation chambers and sampling apparatus. A Cenco Megavac laboratory vacuum pump was used to draw the necessary vacuum. Oven. A Blue M type mechanical convection oven was used as the heating medium for the degradation chambers. Temperature could be varied from ambient to 23OOC. Wide double Odoors 011 the front of the oven provided easy access t o the degradation chambers. Analytical Equipment. Gas Chromatograph. The gas chromatograph used in this work was a Varian Aerngraph Moduline Model 1520B equipped with dual column connections and two hydrogen flame ionization detectors. Helium was employed as the carrier gas. Equal amounts of silica gel, activated charcoal, and molecular sieves were used in a trap fabricated from 1-in. pipe to provide final cleanup of the helium before its passage to the chromatographic columns. The trap was 10 in. long. Two Honeywell Electronik 16 strip-chart 0-1 mV recorders were used to record the chromatograms. With an individual recorder for each flame detector, dual analyses of a given degradation sample could be accomplished rapidly and with ease. Chromatographic Columns. Four different chromatographic columns were constructed for use in the analysis of the degradation samples, All columns were constructed of '/kin. copper refrigeration tubing. For ease in identification, the columns were tagged 1 , 2 , 3 , and 4 and will be referred to by such numbers in later sections of this work. Specifications for each of the coliimiis are given in Table I. The Chromosorb 102 column was chosen for its ability to separate light paraffinic and olefinic hydrocarbons. The column having di-n-decyl phthalate and UCON 50HB 2000 as a mixed liquid phase was employed to separate oxygenated compounds. The Bentone 34 modified diisodecyl phthalate column and the tricresyl phosphate columns were chosen hecause of their ability to separate a large variety of different compounds. The guidelines in the ASTM Standard E260-69 (1969) were followed in the preparation of all chromatographic columns. All columns were operated isothermally at a temperaI
Figure 3.
Degradation chamber and sampling apparatus
rleoradatmn a d wave w m n n t r r l .-..t"_-"
fnr hv ~ .". _II
~j * . Y~ nfI tho. o T t m n~. ~ y.X .Ilylll "~ "I
sphere in the blank chamber. A nressure nlat,r assemhlv - ~ square ' " rnniiiat,ins - ~ ~ ~ - of --~ a. --- m -~-.. -i 7~of steel plates and four 9-in. long bolts was used to s ea1 each chamber. One plate was placed on the top of the chaniber and one on the bottom. The bolts were then inserted throrigh holes ..,. in . m. > e corners or,m,. e piates. , aniieu ana a nut. ana. a washer placed over each bolt. With the nuts tightened down onto the top plate to a torque of 150 in.-lb, a vacuum of 27 in. of mercury could he drawn on each of the chambers. c"".,".,;"," h." TI." ".."."""+.." :^I. !"u""p'LY'q4 l l l r UL,LLl,'L"& appalauua 1b U l l U W l l 111 Figure 3. A 4-in. piece of '/+-in. 0.d. stainless steel tubing, equipped with Swagelok fittings on both ends, was connected t o one end of the'run of a stainless steel all-tube tee. A I/&. Whitey bar stock valve was attached to the branch port of the tee. The other rim connection of the tee was left open. The open side of the Whitey valve was then joined to one run connection of another stainless steel all-tube tee. The other part of this second tee was connected t o a '/&-in. Swagelok double-end shut-off quick-connect (QC) stem assembly. A Marsh compound pressure guage (30-in. mercury vacuum to 15 psi guage) was fixed to the branch part. of this second tee. The sampling apparatus t,hus had three open connections. The open end of the 4-in. section of 1/4-in. tubing could he easily connected to or removed from the female connector on the chamber shutoff valve of each degradation chamber. The open branch of the first tube tee was covered with a Swagelok nut and rubber septum when gas samples were to be taken from the degradation chambers. Finally, the QC stem assembly was used to attach both vacuum and cylinder gas lines to the chamber. The sampling apparatus and an assembled degradation chamber are shown in Figure 3. Peripheral Degradation Equipment. Gassing and Vacuum Equipment. Heavy-wall rubber tubing was used for I
~~~~
~
~~~
~~~~~~~~
.
.
" -----
nppW'ULUY.
38
Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 1, 1972
~
ture of 75°C. Optimum carrier gas flow rates at this temperature were obtained for each column and are given below: Column no.
Carrier gas flow rate, ml/min 80 50 100 150
It must be noted that the temperature chosen was not necessarily the optimum for each individual column. However, the loss of the optimum separation factor was more than compensated for by the ease of dual column analysis and column replacement. Operating Procedures and Analytical Techniques
Degradation Chamber Preparation. Preparation of the degradation chambers consisted of cleaning them prior to each experimental test. The cleaning was thorough since concentrations of the degradation products were expected to be in the ppm range. Any residues left from prior tests or handling would bias the next test. Chamber tops, gaskets, and beakers were all cleaned in the same manner. Visible residues were first removed by chipping with a small spatula and scouring with fine steel wool. Then, each chamber part was washed with hot soapy water. Distilled, ion-exchanged water was used to rinse each part after washing. Finally, each part was rinsed with two organic solvents. Pentane was applied first to remove nonpolar residues. After the pentane rinse, acetone was used to remove polar residues. All parts were then placed in the convection oven and dried a t 75°C for several hours. Polymer Preparation. For each test three samples of 20 i 0.5 grams were weighed out. The samples were placed in a desiccator filled with silica gel and allowed to stand for a period of 24 hr. The samples were then removed and weighed again. Since the test polymers were supplied by the manufacturer as being research grade, no other preparation (such as washing and drying) other than the desiccation was performed. Physical properties of the polymer are given in Table
11. Degradation of Polymer. After the polymer samples had been placed in the chambers and the chambers sealed, the degradation procedure was begun. The sampling apparatus was attached to the first chamber, and the vacuum line was connected to the QC assembly on the sampling apparatus. The chamber was then evacuated by applying a vacuum of 27 in. with the vacuum pump. The vacuum line was removed, and the desired oxygen atmosphere introduced by the gassing line. The chamber was evacuated again. This procedure was repeated a total of three times on each chamber to remove any trapped room air contaminants. After the third evacuation, the desired atmosphere was introduced and allowed to remain. The chamber shutoff valve was closed, and the sampling apparatus removed. Oxygen concentrations in the chambers for the four tests were set at 0, 33, 66, and 100 vol yo. Helium was chosen as the diluent gas for the 33 and 66% atmospheres. Pure helium was also used for the first test atmosphere. Helium was chosen as the diluent gas both because it is inert and because it was used as the carrier gas for the chromatograph. Amounts of both gases delivered to the chambers were metered with the compound gage on the sampling apparatus. Total pressure in each chamber was set at 1 psi gauge so that during sampling no room air would leak into the chambers.
Table II. Physical Properties of Polyethylene Test Sample
Trade name: “Marlex” polyethylene fluff Type: Resin no. 6006 Manufacturer: Phillips Petroleum Co. Physical property
W t av mol wt No. a v mol wt Density, g/cm3 Bulk density, Ib/fta Viscosity, poise hlelting point, O F Softening point, O F Brittleness temp, O F Melt index, g/10 min Flexural modulus, psia
Valuea
191,000 24,500 0.960 39 1.045 276 260 - 180 0.6 220,000
a All physical property values were determined and supplied by the Phillips Petroleum Co. Determinations were made on
compression molded specimens prepared in accordance with Procedure C of ASTM D1928-68 (1968).
The temperature chosen for each degradation was selected and set on the convective oven several hours before the first degradation chamber was placed in the oven. Chambers were introduced into the oven a t 2-hr intervals. It has been previously decided that chromatographic analysis of each degradation sample would take from 1-2 hr. To preserve the degradation time limitation, the chambers had to be introduced into the oven a t the previously mentioned intervals. The time limit for degradation of each sample was set a t 7 2 hr. Degradation Product Sampling. When the required time limit was reached for degradation of each sample, the sampling apparatus was attached t o the chamber shutoff valve on the bottom of the respective chamber. The vacuum line was then attached t o the QC stem assembly on the sampling apparatus, and the open branch of the tube tee capped with n Swagelok nut and rubber septum. The needle attached to a 1OOO-fil gas syringe was inserted through the septum into the sampling apparatus, and the vacuum and gassing shutoff valve on the apparatus was opened. A vacuum of 27 in. of mercury was then drawn on both the sampline; apparatus and the syringe t o remove room air contaminants from them so as not t o bias the sample. The vacuum and gassing shutoff valve on the sampling apparatus was next closed, and the chamber shutoff valve on the bottom of the degradation chamber opened, allowing a sample of the atmosphere in the chamber to flow into the sampling apparatus. The chamber shutoff valve was then closed, isolating the sample within the sampling apparatus. A portion of the sample was withdrawn by the inserted syringe and injected into the first column on the chromatograph. The remainder of the sample was flushed by again drawing vacuum on the apparatus. The entire procedure was repeated to obtain another sample from the same chamber for the second chromatographic column. All four chambers were sampled in the above manner. Each chamber was sampled in the same order as it was introduced into the oven. G a s Chromatographic Analyses. Library Retention Data. The degradation products identified during this work were analyzed with the use of gas chromatography. X library of retention times for various compounds was prepared for each chromatographic column. Retention times of the polymer degradation products were to be compared to the retenInd. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 1 , 1972
39
Table 111. Retention Time library Compound
Paraffins Methane Ethane Propane n-Butane Isobutane 2,2-Dimethylbutane 2,3-Dimethylbutane n-Pentane Isopentane Neopentane 24lethylpentane 3-Methylpentane 2,PDimethylpeiitane 2,2,4J?rimethylpentane 2,3,4Trimethylpentaiie n-Hexane 3-lllethylhexaiie 2,4-Dimethylhexane 2,5-Dimethylhexaiie 2,2,5-Trimethylhexane Cycloparaffins Cyclopentane Cyclohexane RIethylc yclohexane Unsaturates Ethylene Propylene 1-Butene cis-2-Butene trans-2-Butene Is0 butylene 2-llethylbutene-1 2-Nethylbutene-2 3-Rlethylbuteiie-1 2-E thylbutene-1 1-Penteiie cis-2-Pentene 2-llethylpentene-1 4-1Iethylpeiitene-1 4-hlethylpentene-2 1-Hesene 2-Hexene Cyclohexene Ketones Acetone Butaiione
Column retention time, min 1 2 3
4
2.65 2.83 3.13 3.83 3.43 3.52 4.22 5.38 4.78 3.74 7.33 8.09 9.90 8.12 13.68 8.83 7.93 19.10 18.13 13,61
0.63 0.68 0.76 0.94 0.85 1.44 1.71 1.36 1.24 0.96 1.98 2.21 2.84 3.64 6.02 2.39 3.39 5.71 5.48 6.62
0 . 7 5 0.58 0.78 1.48 0.80 4.58 0.91 16.55 0.86 12.65 1.07 1.51 1.16 1.08 0.94 1.45 1.56 1.90 2.73 4.15 1.63 2.55 3.40 3.30 4.62
9.41 16.80 24.83
2.25 4.20 6.40
1.51 2.48 3.58
2.83 3.24 4.06 4.65 4.43 4.06 3.56 4.15 4.96 6.27 3.40 3.89 5.74 4.59 4.79 5.74 6.67 24,35
0.68 0.74 0.98 1.10 1.05 0.99 1.26 1,52 1.25 2.36 1.26 1.42 2.21 1.76 1.85 2.21 2.54 5.45
0.78 0.81 0.93 0.98 0.95 0.90 1.22 1.31 1.04 1.79 1.22 1.27 1.71 1.48 1.53 1.71 1.90 2.93
3.06 5.73
2.83 4.15
1.19 4.10
tion library values to identify the degradation products. An attempt was made t o provide retention times for all library compounds on all four columns. In several cases this was not possible. The compounds selected for the library were chosen on the basis of their relative frequency of occurrence in the literature. Samples of the library compounds were obtained from three sources. The lighter compounds were taken from analytical grade bottled gases. A large amount of the heavier compounds (liquids a t room temperature) was obtained by sampling the vapors above the contained liquids. The third source was from pure liquid samples used oiily where the amount of the available compound was so small that no vapor could easily be obtained. 40
lnd. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 1 , 1972
Compound
3-Methyl-Zbutanone 3,3-Dimethyl-2-butanone 2-Pentanone 3-Pentanone 2-Hexanone Aldehydes Acetaldehyde Propionaldehyde n-Butyraldehyde Isobutyraldehyde Crotonaldehyde Ethers Ethyl ether %-Propyl ether Isopropyl ether Propyl isopropyl ether Alcohols Methanol Ethanol n-Propanol Isopropanol n-Bu t a no1 Isobutanol aec-Butanol tert-Butanol tert-Amyl alcohol Esters Methyl acetate Ethyl acetate Isopropyl acetate n-Propyl acetate n-Butyl acetate Isobutyl acetate sec-Butyl acetate tert-Butyl acetate Ethyl formate Propyl formate Allyl formate Methyl propionate n-Propyl propionate Isopropyl propionate Methyl butyrate E thy1 butryat e Methyl isobutyrate Other Methylal
Column retention time, min 2 3
1
4
8 . 6 0 4.79 15.14 9.21 10.78 7 . 0 5 11.03 6.78 25.18 15.48 2.91 7.71 28.57 20.00
1.43 2.70 5.19 3.83 11.45
1.93 2.30 3.25 2.30 8.63
8.03 22.66 11.90 16.53
1.71 5.25 2.81 3.85
1.26 2.78 1.70 2.20
13.93 20.13 33.03 22.23
2.58 3.68 7.61 4.25 16.80 12.05 8.68 4.54 10,28
9.12 14.97 17.88
21.43
2 . 7 1 2.52 4 . 4 4 3.26 5.61 3.56 8 . 7 6 5.64 19.05 10.67 13.27 7 . 5 3 11.33 6.36 6 . 7 3 4.00 2.54 1.87 5.10 2 . 9 1 5.44 3.07 5.00 4.18 16.79 9 . 1 2 6.38 4.60 9 . 5 4 7.22 14.88 8 . 9 3 6.72 4.84
11.35
2.31 6 2 . 5
8.44 16.96 18,09 17.00
8.98
2.26 6.00 2 . 5 4 11.15 3.83 2.20 7.58 4.78 3.56 2.08 3.83
Gas syringes of varying sizes were used to obtain the library compounds from their sources and to introduce them into the chromatograph. For bottled gas samples, 100-pl syringes were used, and 100O-pl syringes for obtaining samples from vapors over liquids. Liquid samples were obtained with 10-pl syringes. Retention times in minutes for the library compounds 011 each column are given in Table 111. Response Factors. The response of a hydrogen flame ionization detector in a gas chromatographic device is extremely linear (McWilliams and Dewar, 1958; Novak and Janak, 1960; Onkiekong, 1960) over seven orders of magnitude. The response factor method for quantitative chromatographic analysis is based on the ljnearity of a chromatographic detector. The amount of a pure compound injected into the
chromatograph is related to the chromatogram peak area by the simple linear relationship where
C C A
=
fA
=
amount of injected component
= peak area of injected component f = response factor, amount/area
The response factor can be expressed on a weight, volume, or mole basis. Several authors (MeWilliams and Dewar, 1958; Ettre, 1962; Dietz, 1967) have obtained response factor data readily applicable to many chromatographic analyses. Response factors have been developed (Kuley, 1965) from plots of peak heights vs. component ppm in the sample. Linear relationships with a maximum deviation of 0.870 were obtained. Response factor data were collected for a wide variety of degradation products. Those compounds existing as gases at ambient conditions were chromatographed individually. Known amounts of each gas were analyzed, and the peak height data collected. Compounds in liquid form a.t ambient conditions were treated differently. Three component mixtures were made up by volume. Then, each mixture was injected into the chromatograph, and the peak height data were obtained. After the peak height tiat'a had been collected, plots were made of peak height vs. amount of injected component. The slopes of these plots were the response factors wit'h units of volume of compound per chromatographic peak height unit. The data obtained to calculate each response factor were then subjected to a linear regression analysis. Values of the linear regression correlation coefficient, r2, were at least 0.996 in all cases. The response factors for each compound are given in Table IV.
I 0
I
4
6
8
I
1
IO
12
1
14
1
16
1
,
I8
20
1
22
1
24
1
26
1
28
1
30
Retention Time. minuter
Figure 4a.
Separation of degradation products on Column 1 for 100°C and 100 vol 7 0 oxygen test 1.
2. 3.
Methane ( X 4 ) Isobutane (X4) n-Butane ( X 4 )
Table IV. Degradation Product Response Factors Compound
I n t e r p r e t a t i o n of E x p e r i m e n t a l Results
Qualitative Analysis. Twenty-four polyethylene degradation products were identified during the experimental study. Fourteen of these compounds were found on three of the four chromatographic columns. The remaining 10 compounds were identified on two chromatographic columns. The digital computer program CHROMO (Bentsen and Bethea, 1969) was used to facilitate the qualitative identification of each compound. The program compares unknown retention time data to library data for each column in the analysis. If an unknown compound is found on a t least two columns in the library, then the program furnished its name as output. Special features of the program enable it to correct library retention times biased by aging and to increase the number of columns needed for identification of unknowns. S o t all of the degradation products which showed up during the analyses were identified. There were eight unidentified peaks observed on Column 1, six on Column 2, nine on Column 3, and eight on Column 4. Table V lists all of the different degradation products identified. Quantitative Analysis. Degradation product concentrations for each compound were obt,aiiied by multiplying the chromatogram peak height times the respective response factor. -411 such concentrations were reported in part's per million. These coilcentrations are listed in Table VI for each compound at each particular test level. Sample chromatograms are given in Figures 4a-c to compare the effect of the increase in temperature on several of the identified degradation products. The effect of oxygen concent'ration on the same degradation products can be seen by the comparisoii of sample chromatograms in Figures 4c and 5a,b.
,
2
a
Response factor, PI/ peak ht counta
Methane 0 281 0 298 Ethane 0 122 Propane 0 153 %-Butane Isobutane 0 129 n-Pentane 0 199 ?e-He\;ane 0 242 0 238 2,4-Dirnethylpentane 0 229 3-Methylpentane 0 251 Ethylene 0 164 1-Butene 0 192 cis-2-But en e trans-2-Butene 0 179 0 168 33Iethylbutene-1 Methylal 0 702 Acetaldehyde 0 760 n-Butyraldehyde 0 675 Propyl ether 0 396 0 600 Isopropyl ether 0 839 2-Pentanone Ethanol 0 966 0 184 Isopropanol 1 026 n-Butanol Butyl acetate 1 188 Peak height counts are measured in 40 counts/in.
Table V. Polyethylene Degradation Products
Methane Ethane Propane n-Butane Isobutane n-Pentane %Hexane 2,4-Dimethylpentane 33lethylpen tane
Ethylene 1-Butene cis-2-Butene trans-2-Butene 3-llethylbutene-1
Methylal -4cetaldehyde n-Butyraldehyde Propyl ether Isopropyl ether 2-Pentanone Ethanol Isopropanol n-Butanol
Three different behavior patterns concerning the effect of temperature and oxygen concentration on degradation product concentrations were observed from the data. The first pattern involved compounds whose concentrations were nondecreasing over the entire range of test conditions studied. Among this group were methane, ethane, n-butane, n-pentane, 3-methylbutene-1, methylal, acetaldehyde, n-butyl acetate, and n-propyl ether. Figures 6 and 7 illustrate an example Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 1 , 1972
41
Table VI. Polyethylene Degradation Product Concentrations
Concentrations, ppm Oxygen concn, VOI %,
75
Temp, O C 125 150
100
175
200
Oxygen concn, VOI %
75
Methane 0 33 66 100
NE5 NE NE
0 33 66 100
NE YE NE
0 33 66 100
NE NE NE
0 33 66 100
NE NE NE
0 33 66 100
NE NE NE
0 33 66 100
NE NE NE
15
37 Ethane
NE NE NE
b b b
1
b
b
NE NE KE
0.8 5 11 22
4 3 4 0.5
3 Propane
NE NE NE 3 n-Butane
NE NE NE
b b b
4
b
1
100
1
b
NE NE NE 25 n-Pentane
NE NE NE 0.9
b
b b b
0.9 108 268 2270 b
18 45 280
NE NE KE 2570
NE NE NE 280
1 270 337 3120
0 33 66 100
NE NE NE
b
62 48 522
0 33 66 100
NE NE NE
5 8 8 16
0 33 66 100
NE NE NE
0.4 11 14 56
0 33 66 100
NE NE NE
0.5 172 81 357
0 33 66 100
NE NE NE
0 33 66 100
NE NE NE
0 33 66
NE NE NE
b
b
NE NE NE b
b
b b b
NE NE NE b
NE NE SE
0.3 3 5 24
NE NE NE
0.4 41 67 512
NE NE NE
24
38
b b
42
b
8 21 6 47 3-Methylpentane
NE NE NE
b
b b
b
5 10 331
10
b
NE NE NE
b
5 5 16
44
NE NE NE 101
b
18 16 84
b
b
NE NE NE
b
7 38 85
b
7110 1-Butene
552 1070 8460
NE NE NE
b
b b
b
6060
NE NE NE
b b
b
1
b
NE NE NE
2
b
323 499 5860 b
0.5 2 5
cis-2-Butene
b
2 4 12
412
NE NE NE 19
b
5 8 26
b b
b
68
NE NE NE
b b
78
yo
Methane (X640) Ethylene (X640) Acetaldehyde (X640) 4. Propane ( X 80) 5. Irobutane ( X 4 ) 6. n-Butane (X4) 7. 1-Butene ( X 4 ) 8. frons-2-Butene ( X 4 )
200
Ethylene 4 6 17 31
1 5
Figure 4b. Separation of degradation products on Column 1 for 15OO.C and 100 vol oxygen test 1. 2. 3.
NE NE NE
b
b
b
b
9. cis-2-Butene (X4) 10. 3-Methylbutene-1 (XED) 1 1 . n-Pentane (X4) 15. Methylal ( X 4 ) 17. Isopropanol (X4) 18. Propyl ether ( X 4 ) 19. n-Butyraldehyde (X4)
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 1 , 1 9 7 2
b
100
NE NE NE
b b
b b
NE NE NE
b
b b
b
0.7 2 b 8 trans-2-Butene
b
%-Hexane 0 33 66 100
175
2 ,CDimet h ylpentane
Isobutane 0.2 0.5 2 5
Temp, OC 125 150
~
b b
b
2 3-Methylbutene-1
b b
b
NE NE
b
8 17 82
b
b
NE1
NE NE NE
b
2 4 5
6
NE NE NE 3
NE NE NE 99
b
0.6 3 6 b
30 28 133 continued
Separation of degradation products on Column 1 for 200°C and 100 VOI % oxygen test
Figure 4c. 1. 2. 3. 4.
5. 6. 7.
8.
9.
Methane (X640) Ethylene ( X640) Acetaldehyde (X640) Propane (X80) lrobutane (X80) n-Butane ( X 4 ) 1 -Butene (X4) frans-2-Butene ( X 4 ) cis-2-Butene ( X 4 )
10. 11. 12. 13.
14. 15.
16.
17. 18. 19.
3-Methylbutene-1 (X80) n-Pentane ( X 4 ) 3-Methylpeqtane ( X 4 ) n-Hexane ( X 4 ) 2,4-Dimethylpentane ( X 4 ) Methylal ( X 4 ) Isopropyl ether (X4) Isopropanol ( X 4 ) Propyl ether ( X 4 ) n-Butyraldehyde [ X 4 )
Table VI.
Continued
Concentrations, ppm Oxygen concn, VOJ
Temp,
%
100
75
Oxygen concn,
‘C
125
150
175
200
VOI
%
75
Temp, 125
100
b
NE NE XE
b
7
NE
b
NE NE
b
b
NE NE NE
b
4 4 14
b
23
7 17 26
NE ?;E NE 950
162 244 1160
0 33 66 100
NE XE YE
0 33 66 100
NE NE NE
0 33 66 100
NE XE TE
0 33 66 100
SE SE NE
0 33 66 100
XE SE NE
b
NE
b
NE
NE NE NE
0 33 66 100
NE NE NE
NE b ?;E 44 b KE 95 12 23 503 n-Butyraldehyde b NE b b NE b b NE 6 b b
b
b
b
b
b
70 Ethanol
SE XE SE
b
b b
454
b
b
NE NE NE
0 33 66 100
NE NE NE
XE KE NE
b
NE SE
b
38 b 85 7 58 168 Isopropyl ether b
b
NE
b
6
b
b
h
b
b
b
b
80 157 256
NE 202
b
?r’E SE TE
b
0 3 2 18
b
478 n-Butanol
,
,
I
2
4
6
P I
?;E XE NE
6 b b
NE 163
n‘E SE NE 1180
263 240 688
b
SE
b
SE
74 60 82
NE 59 1
NE SE
b
28 84 259
165
b
b
b
106 314 122
b
108 77 92
b
i8
NE 90
69 396
13uty1 acetate
NE NE NE
b
13 25 39
b
15 14 17
27
,
,
,
,
,
14
16
18
20
22
KE NE NE
b
b 6
10 33 66
20
6
b
XE NE NE
h
b
34 29 140
104
Indicates no measurable amount of compound was observed a t the particular
Indicates the particular test level was not examined. test level.
IO,
b
247 b 487 48 38 1110 Isopropanol b
Propyl ether 0 33 66 100
82 137 244
YE NE XE
b
YE SE
b
NE
b b
b
Acetaldehyde 0 33 66 100
150
2-Pentanone
Methylal 0 33 66 100
“C
,
,
,
24
26
28
L
,
I
0
B
10
12
30
Refenlion Time. minuleg
Figure 5 a . Separation of degradation products on Column 1 for 200°C and 66 vol %oxygen test 1. 2. 3. 4.
5. 6. 7. 9.
Methane ( X 64) Ethylene (X4) Acetaldehyde (X4) Propane (X4) lsobutane (X64) n-Butane (X4) 1 -Butene (X4) cis-2-Butene (X4)
10. 11. 12. 15. 17. 18. 19.
3-Methylbutene-1 (X4) n-Pentane ( X 4 ) 3-Methylpentane ( X 4 ) Methylal ( X 4 ) Isopropanol (X4) Propyl ether ( X 4 ) n-Butyraldehyde ( X 4 )
of this behavior pattern. Methane is the degradation product example. The serond pattern exhibited was one in which the degradation product concentrations reached a maximum value within the temperature range investigated and then decreased a t the higher levels of the range. This pattern included 2,4dimethylpentane, ethylene, isopropanol, and 2-pentanone. Figures 8 and 9 show this behavior pattern with ethylene as
0
2
4
6
8
12
10
14
16
18
20
22
24
26
28
30
Refenlion Time. minulea
Figure 5b. Separation of degradation products on Column 1 for 2 0 0 ° C and 0 vol oxygen test
yo
1. 5. 6.
Methane ( X 4 ) lsobutone (X4) n-Butane ( X 4 )
the sample compound. Kote that in Figure 9 the 2OOOC isotherm lies below the 15OOC isotherm for all levels of oxygen concentration. The final behavior pattern observed was a combination of the first two patterns. h t the first three levels of oxygen concentration examined, the compounds propane, Isobutane, 3-methylpentane, and cis-2-butene showed noiidecreasing ppm concentrations over the entire temperature range investigated. At the 100 vol % oxygen concentration these products reached maximum concentration values within the temperature range and then decreased a t the higher levels of temperature qtudied. Isopropyl ether and ethanol exhibited Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 1, 1972
43
3w0ml 1
8000
2500
7000
6000
.g
5000
L
g E 4000 3000
2000
Figure 6. Effect of temperature a t constant oxygen concentration on concentration of methane
A
w
1000
0
33%0xygen 44% Oxygen 1OO%Oxygen
Olygan C o n c m l r a t i o n . Vobnn X
Figure 9. Effect of oxygen concentration a t constant temperature on concentration of ethylene 0
U A
2500
l0O0C 15OoC 200°C
L i 2000
E
p
1500
0
1000
500 0 Oxygen Concentration, Volume Y
Figure 7. Effect of oxygen concentration at constant temperature on concentration of methane 0
A
w
T m w o t u r e , 'C
l0O0C 150°C 20O0C
Figure 10. Effect of temperature at constant oxygen concentration on concentration of isobutane
A
w
8000
33%0xygen 66%0xygen ~OO%Oxygen
7000
6000
5000
,E
0
4000
3000
2000
1000
0
k 100
125
I50
175
200
Tmpuolure, 'C
Figure 8. Effect of temperature at constant oxygen concentration on concentration of ethylene A
w
44
33%0xygen 4670 Oxygen 100% Oxygen
Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 1 , 1 9 7 2
the same general behavior. However, the maximum concentration values were first observed a t the 66 vol yo oxygen concentration level rather than the 100% level. The behavior pattern of this last group is given by isobutane in Figures 10 and 11. Note that the 15OOC isotherm crosses over the 200°C isotherm above 66 vol % oxygen in Figure 11. Not enough data were available to establish a behavior pattern for the compounds hexane, 1-butene, trans-2-butene, and n-butyraldehyde. The data for n-butanol were so erratic that it was not placed among any particular pattern group. It must be noted here that Figures 6-11 are in no way intended to establish a statistical correlation of the data. They have been given solely to portray the trends observed in the experimental data. The behavior patteriis exhibited by the data seem to suggest degradation processes somewhat like those proposed (Gonella, 1964). The compounds which followed the first pattern (degradation concentrations were nondecreasillg over the entire range of test conditions) are possibly the results of a continuing surface reaction of polymer with oxygen in the
E 400
n
i
Bentsen, P. C., Bethea, R. hl., J . Chromatogr. Sei., 7, 399--407 (1969).
Bethea, R. M.,Report No. LWP 178, NASA Langley Research Center, Hampton, Va., January 13, 1966. Bruck, S. D., Paper presented at the National Meeting of the American Chemical Society, Philadelphia, Pa., April 1964. Cameron, G. C., Kerr, G. P., Eur. Polym. J., 4,709-17 (1968). Conley, R. T., Bieron, J. F., J . Appl. Polym. Sci., 7, 103-17 (1963).
Davison, W. H. T., Slaney, S., Wragg, A . L., Chetn. Znd., 44, 1356 (1954).
Dietz, W. A., J . Gas Chromatogr., 5,68-71 (1967). Dotreppe-Grisard, hI., Rev. Belge Matieres Plast., 9, 79-81 (1968).
Dunn, A. S., Coley, R. L., Duncalf, B., SCZ Jlonogr., 30, 20821, (1968).
Oxygen Concentration, Volurno X
Figure 1 1. Effect of oxygen concentration a t constant temperature on concentration of isobutane 0
A
l0ODC
150°C 2OOOC
degradation atmosphere. These compounds could be considered primary degradation products. Those compounds which followed the second pattern (concentrations exhibited a maximum value and then decreased) seem to support the existence of several other degradation processes. The first of these could be the reaction of a primary degradation product with oxygen t o yield a secondary degradation product. The second process might then be the reaction of two secondary products; and the final process, the reactmionof a secondary product with oxygen. . h y one of the last three processes could yield tertiary degradation product's. Before any decision may be reached concerning the actual nature of the degradation processes, a much more discriminating investigation will be necessary. Conclusions
Oxidative degradation of polyethylene yields both aliphatic and oxygenated organic compounds as degradation products. The concentrations of polyethylene degradation products increase Kith increasing oxygen concentration over the temperature range of 75-200°C. The effect of temperature on the concentrations of polyethylene degradation products strongly indicates that several different degradation processes occur \Tithin the t'emperature range of 75-200OC. literature Cited ASTIll Stand. 711928-68, Philadelphia, Pa., 1968. ASTAW Stand. E260-69. PhiladelDhia. Pa.. 1969. Bailey, W.J., Liotta, C., iimer.'Chem. Soc., Diu. Polym. Chem., Prrp., 5 , 333-45 (1964). Barlow, A., Lehrle, 11. S., Ilobb, J. C., SCZ rlfonogr., 17, 267-83 (1963).
Ettre, L. S., J . Chromatogr., 8 , 52.5-30 (1962). Gonella, N. T., Paper presented at the 15th Annual Mid-American Symposium on Spectroscopy, Chicago, Ill., 1964. Isenberg, L., Chem. Eng. Progr. Symp. Ser., No. 52, 60, 68-78 (1964).
Kelen, T., Balint, P., Tudos, F., J l a g y . Iiem. Lapja, 23, 610-16 f1968i. Chpm. Abstr.. 70. 3825811 11969).
Kufey, C: J., J . P&m. hi.; Part Cy IO, 103-11 (1965). Lane, R. E., M S report, Texas Tech University, Lubbock, Tex.,
_""". 1464
Luff, P. O., White, M., Vacuum, 18,437-44 (1968). RlcWilliams, I. G., Dewar, It. A., in "Gas Chromat,ography," D. H. Ilesty, Ed., Butterworths, London, England, l9d8, pp 142-7. Novak, J., Janak, J., J . Chromatogr., 4, 249 (1960). Onkiekong, L., Doctorate thesis, Technical University, Eindhoven, The Netherlands, 1960. Schneider, I. A,, Vasile, C., Furnica, D., Onu, A , , Jlakromol. Chem., 117,41-9 (1968). Scholz, It. G., Bednarczyk, J., Yaniauchi, T., Anal. Chcm., 38, 331-4 (1966).
Smith, T. G., NASA Contract Report 97218, September 1968. &ill, It. H., Jones, P. B., Mansell, A. L., J . A p p l . Polym. Sei., 13, 401-1-5 (1969).
Trestianu, S., Sandulescu, I)., Rev. Chim., 18, 419-24 (1967); Chem. Abstr., 68, 78635j (1968).
Tsuchiya, Y., Sumi, K., J . Polym. Sei., Parl A - f , 6, 415-24 (1968a).
Tauchiya, Y.,Sumi, K., J . Polym. Sci., Part A - f , 7, 813-36 (1969a).
Tsuchiya, Y.,Sumi, K., J. Polym. Sei., Part A-f, 7, 1599-607 (196913).
Tsuchiya, Y., Sumi, K., Polym. Lrtl., 6,357-61 (1968)b. Tsuge, S., Oniumoto, T., Takeuchi, T., J . Chromatogr. Sei., 7 , 250-2 (1969).
Turner, It. L., Rethea, It. lf.,Report No. Lm'P-462, NASA Langley Research Center, Hanipton, Ya., August 1, 1967. Winslow, F. H., Rlatreyek, W., Amer. Chcm. Soc., Uio. Pol?/m. Chem., Prepr., 5 (2), 5.52-7 (1964); Chcna. Abstr., 64, 12825g (1966).
Winslow, F. H., Hawkins, W. L., Natreyek, W., Amcr. Chcm. Soc., Diu. Polurn. Chcm., Prepr., 2 ( I ) , 186-0 (1961); Chem. Ahstr., 57, 1.5332h (1962). Winslow, F. H., Hellman, 11. Y., Matreyek, W.,Salovey, It., Amer. Chem. Sac., Diu. Polyni. Chem., Prcpr., 5 ( l ) , 47-31 (1964).
Winslow, F. H., Hellman, JI. Y., Matreyek, W., Stills, S. ll., Polym. Eng. Sci., 6 (3), 273-8 (1966).
Winslow, F. H., Aloisio, C. J., Hawkins, W. L., lIatreyek, W., Natsuoka, S., Chem. Znd. (London), 1963, p 146.5. RICCKIVI;D for review June 1, 1971 ACCEPTI:DOctober 28, 1971
Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 1, 1 9 7 2
45
