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Thermal decomposition products of poly(vinyl alcohol)
Ser
TJ-I1
T2lr2
no.425
c .
2
BLDG
NATIONAL
RESEARCH
COUNCIL
OFCANADA
THERMAL DECOMPOSITION PRODUCTS OF
RICE 25 CENTS
POLY (VINYL ALCOHOU
BY
YOSHIO TSUCHIYA AND KlKUO SUM1
REPRINTED FROM
JOURNAL OF POLYMER SCIENCE
PART A
-
1. VOL. 7 , 1969 P. 3151-
9158RESEARCH PAPER NO. 425
O F THE
DIVISION OF BUILDING RESEARCH
OTTAWA
DECOMPOSITION THERMIQUE DES PRODUITS DIALCOOL POLYVINYLIQUE
La dkcomposition thermique dlalcool polyvinylique s e f a i t en deux Ctapes. Dans une etude d e l a ~ r e r n i ' e r e etape d e dB- composition, l a pyrolise du polym'ere a BtC f a i t e dans l e vide B 240°C durant q u a t r e h e u r e s e t l e s produits ont 6t6 d e t e r m i n e s B l'aide d e chromatographie en phase g a z e u s e . L e s principaux d e r i v e s sont d e lleau, d e s aldehydes ayant commeformulegCnCrale HGtCH = C H f C e t des cetones
o n
d e methyl a y a n t c o m m e f o r m u l e H,C -C <CH = CH-) CH, ,
I1
o n
a l o r s que n = 0, 1, 2, 3, etc. Les m e c a n i s m e s pour l a f o r - mation d e c e s compos6s carbonyles sont BtudiBs.
JOURSAI, 01; I'O1,YRIER SCIENCE: PART A - l VOL. '7, 3151-3138 (1969)
Thermal Decomposi~ion Products of
Poly (vinyl Alcohol)
yOSHIO TSUCHIYA and IiII(U0 SUBII, Fire Section, Division o$ Builcling Resea~cl~, Naliotzal R e s e a ~ c l ~ Cot~7zcil O$ Canacla, Ottawa, Canada
Synopsis
The thermal decomposition of poly(viny1 alcohol) is known to occur in two stages. I n a study of firststage decomposition, this polymer was pyrolyzed in vacuum a t 240°C for 4 hr and the products were determined by using gas chromatography. The main products were water, aldehydes having the general formula HC+CH=CII-fCI-13, and
I
I
n 0methyl ketones having the formula RJC-C+CH=CH-fCI-13, where n = 0, 1, 2, 3, etc.
I
I
n0
Mechanisms for the formation of these carbonyl compounds are discussed.
INTRODUCTION
T h e thermal deconlposition of poly(viny1 alcohol) occurs iri two stngcs.' The first stage, which begins a t about 200°C, is inairlly dehydratiori, ac- con~pnniccl by the formation of some volatile products; the residue is prc- dominantly macron~olccules havirig polycue s t r u c t ~ u c . T h e published d a t a on the volatile thermal decoinposition products of this polymer are riot only vcry limited, but arc also conflicting. Ynmnguchi and Aniagasa2 arialyzcd the products of the lirst-stagc dccomposition by chemical meth- ods and found acetaldehyde, crotonaldehyde, benzaldehyde, ancl aceto- phenone. I<aesche-I<rischer nr~d I-IeirlrichVound formalclehyde, acctal- dellyde, and acrolein. E t t r c and TTaradi4 used a pyrolj.sis-gas chromato- gmphic techniclue and repor1;ed that the main organic producls formed a t a pyrolysis temperature of 500°C m r e acetaldehyde and acetic acid. They also found smaller nnzo~uits of ethanol, methyl acetate, and some hgdro- carboiis. A t this temperature both stages of decon~positio~i must have occurred. I n second-stage deconlpositioi~ of poly(viny1 alcohol), the macro- molecules lzaving polyene stiucture are degraded to produce carbon and hydrocarbons. Gilbert and I<ipling%nnalyzed t,hc gases fornled in the second-stage decomposition of vinyl polymers.
T h e authors of this paper are primarily int,erested in first-stage decom- position. T h e only inlportant agreement in the dat,a presented by
was the finding t h a t acetaldehyde is one of the major products. T h e pur- 3151
3152 TSUCI-IIYA A N D SUM1
pose of the present investigation is to obtain rhore reliable inforination about the thermal decomposition products of poly(viny1 alcohol) by using the 1atest.techniques in gas chromatography. The experimental data will be used to discuss the nlechanisin of thermal decomposition of poly(viny1 alcohol).
EXPERIMENTAL Material
A commercial grade of poly(viny1 alcohol) (>99y0 lhydrolysis) was dried a t 80°C under vacuum for 48 hr, prior to pyrolysis. The molecular ~veight of the polymer was found to be about 130000 by a viscosity method.
Thermal Decomposition
The apparatus for the thermal decon~position of poly(viny1 alcohol) mas similar to that used for a previous study.G A sample ~veighing 1 g was placed in a Pyrex tube connected to a liquid nitrogen trap and a vacuum pump. After the system was evacuated to 1 X mm Hg, the vacuum line to the pump was closed and the sample was heated a t 240°C for 4 hr. Most of the volatile decomposition products were collected in a liquid nitro- gen trap. When the liquid nitrogen was removed, the conclerisl~tes in the trap separated into t n o layers: a water layer and an oil layer. The frac- tion volatile a t room tenlperature was transferred from the trap illto a gas sampling bottle of 1in0~1l volume. The three fractioils were arlalyzed separately, and the conlbinecl amounts of each compoilent were obtained.
The pyrolysis tube containing the solid residue was evacuated again, and the residue pyrolyzed a t 450°C for 4 hr. The products from the second- stage decomposition were arlalyzed in the same way as those from the first- stage decomposition.
Analysis
The chromatographic collditioils that were used to analyze the decom- position products are presented in Table I. The identification of the peaks was carried out as follo~vs: (a) comparison of retention times with those of known compourids by use of three different columns; (b) collection of products that yield certain pealis by preparative gas chromatography, fol- lowed by chemical nletllods of identification; (c) determination of carbon structure of products by catalytic l~ydrogcnation.~ For quantitative analysis, the areas under each peak were measured, and approximate sub- stance correction factors were a p ~ l i e d . ~
RESULTS AND DISCUSSION
The results of the products from the first-stage deconlposition of poly- (vinyl alcohol) are presented in Figure 1. The quantitative data are rep- resented by line charts, plotted against the retention indices. The reten-
3156 TSUCIIIYA AKD SUM1
tion index of each component was determined by using n-alkanes as stan- dards a t gas cl~romatographic condition 1.
The major pcalrs, Ao, A1, 8 2 , and Ag of Figure 1, are spaced unifornlly on
the retention indcx scale, suggesting that they bclong to the same homol- ogous series. The first threc were identified as acelaldchyde, crotonalde- hyde, and 2,4-hexadiene-I-a1 by comparing their retention indices with
700 900 1100 1300 1500 1700 1900 RETENTION INDEX
Fig. 1. First-silage decomposition producls of poly(vilry1 alcohol). Aren of peaks vs. retie~ltion i ~ ~ c l e s .
those of known con~pounds and by the determination of rlielting points of 2,4-dinitrophenyl11ydrazolles of the three products. Thc general fornlula of these compourlcls is I :
Peak A3 is believcd to bc due to 2,4,G-octntriene-1-4 the nest compour~d in thc samc series. This suggestion is supported by the fact that n-heptane was obtained on catalytic hydrogenation of this product.
Pealts KO, ICI, K?, and R3 are also uniformly spaced and thus appear to
be due to another l~omologous series. The first pealt, ICo, was due to ace- tone. The second pealt,
R1,
which yielded a positive iodoform test and produced pentane by catalytic hydrogenatio~l, was believed to be duc to 3- pentene-2-one. Although 2-pentanol and 2-pentanone should yield the snnle results on these txvo tests, their retention indices are different. The third peak, K T , nras identified as 3,5-heptadiene-2-one by comparing the re-POLY (VINYL ALCOI-101,) 3155 tention time with that from a synthesized s a n ~ p l e . V h e general formula for this homologous series is 11:
CII3-C+CII=CH j C H ,
I1 n
0 I1
TABLE I1
Thernlal Decomposition Products of Poly(viny1 Alcohol) (240°C, 4 hr) Wt.-70 of original Product polymer Water 33.4 Carbon monoxide 0.12 Carbon dioxide 0.18 Hydrocarbons C1-C3 0.01 Acetaldehyde 1.17 Acetone 0.38 Peak a 0.025 Ethanol 0.29 Benzene 0.06 Crotonaldehyde 0.76 K , (3-pentene-2-one) 0.19 2,4-hexadiene-1-a1 0.55 Peak b 0.014 3,5-Heptadiene-2-one 0.099 Benzaldehyde 0.022 Acetophenone 0.021 Peak c 0.017 113 (2,4,6-oclatriene-1-al) 0.11 Peak d 0.026 (3,5,7-nonatriene-2-one) 0.020 TABLE I11
Deconlposition Products of Poly(viny1 Alcohol) (Material Balance, in Weight Percentage of Original Polymer)
1st stage (240°C, 4 hr) 2nd stage (450°C, 4 hr) Volatiles 47.9 (LOSS 5.81 I (Loss 2.34 Water layer 33.4 ( ~ ~ & o m p d r . 1.56
I
Org. Conlpds. Analyzed 1.19Oil layer Volatiles
27.7 ( ~ o t analyzed 4.99 Water layer 0.60 I Oil layer 22.30 Gas 2.46 Residue Residue 52.1 24.4 -
3156 TSUCI-IIYA AND SUM1
The decomposition products reported by o t l ~ e r s ~ - ~ but not fouild in the present investigation were formaldehyde, acrolein, methyl acetate, and acetic acid. The difference in polymer samples could account for the dis- crepancy in Lhe production of acetic acid, because poly(viny1 alcohol) samples are li110\~11 to contain different amounts of acetyl groups.
The experilnental data on the thermal decomposition products of poly- (vinyl alcohol) are presented in Table 11, and the dataon material balance in Table 111. The main decon~position products of the first stage were x a t e r and carbonyl compounds. The distribution of oxygen compounds in the products, based on 100 monoiner units in the original polymer, as: water, 86.4 mole; aldellydes, 1.9 mole; ketones, 0.5 mole; other, 0.7 mole. Water is formed by a mechanism similar to that forming hydrogen chloride from poly(viny1 chloride) and acetic acid from poly(viny1 acetate), leaving a residue having conjugated polyene structure [eq ( I )
1.
Scission of some of the C-C bonds results in the formation of the carbonyl ends. Aldehyde and methyl lietone end formation are expected to proceed as shown in eqs. (2) and (3), respectively.
The formation of acetaldehyde, crotonaldehyde, 2,4-hexadiene-1-al, and 2,4,6-octatriene-1-d is plausible from the aldehyde ends, and the formation of acetone, 3-pentene-%one, and 3,5-heptadiene-%one is expected from the methyl ltetone ends.
Considerably more aldehydes than lietones were found in the products. This difference mas predictable from these reactions, because the aldehyde ends can be formed by both reactions (2) and (3), ~ ~ ~ h e r e a s the ketone ends are formed by the latter reaction only.
Yamaguchi and Amagasalo predicted the formation of the series of alde- hydes and ltetones (found in the present study) from the mechanisms of
POI,Y (VINYL ALCOI-IOL) 3157 decomposition that they proposed. They suggested that the methyl lietolle ends are formed from the carbonyl groups that arc present i11 the original polymer [eq (4)
1.
w-CII-CIIa-C-CITZ-CII-CIIZ- ~4
I
011 0 I I 011 I
4
The present authors suggest that the transfer of a hydrogen atom from a tertiary carbon follon-ed by a decomposition reaction (3) co~ild also accouiit for the formation of methyl lietone ends.
A large portion of the oil layer was not analyzed (as iiotcd in Table III), becwse n ~ o s t of these products n-ere not sufficiently volatile for the gas chroinatographic conditio~ls uscd. The conversioil of the cnrbonyl com- pounds in the oil layer to 2,4-di1litrophcnyll1~-dlzzones resulted in n large yield, illclicating that the carbonyl compounds represented a large portion of this layer.
The volatile deconlposition products of the second stage were mainly hydrocarboiis; series of n-alkanes, n-allienes, and aromatic hydrocarbons
II ere found. The second-stage pyrolysis products of poly(viny1 chloride)
and poly(viny1 acetate) n-ere also determined for comparison n-ith those from poly(viny1 alcohol). The similarity in both the qualitative ancl quan- titative data of the products from the three polymers indicates that the salne mechanism applies to the second-stage decomposition of each of the three virlyl polymers.
CONCLUSION
Analysis by gas chromatogmphy sho5ved that the first-stage thermal dc- con~position products of poly(viny1 alcohol) arc mainly composed of water, aldehydes having the general formula CHtCH=CH+CHZ and methyl
I I
0
lietones having the formula CH3-CtCH=CH+CH3, where 7z =
I I
0
0 , 1, 2, 3, etc. According to the inechai~isms proposed in this study, de- hydration is accompanied by some scission of the polymer chai11, resulting in the formation of aldehyde ends by one type of reaction and of both alde- hyde and methyl ketone ends by another type. These n~echanisms explain the formation of these carbonyl compounds.
This papcr is a contribution from the Division of Buildill:: Research, National Researcl~ Cour~cil of Canada, and is published with the approval of the Director of the Divisior~.
TSUCI-IIYA AND SUM1 References
1. J. B. Gilbert, J. J. Kipling, 13. McEtlaliey aiid J. N. Sherwood, 3, 1 (1962).
2. T. Yarnaguchi and M. Amagasa, Ziobz~nsili Iiclgalcu, 18, 645 (1961).
3. B. Kaesche-Krischer and 11. J. Ileinrich, Z . Pivysilc. Cileln. (Frunlcjurl), 23, 202 (1960).
4. I<. Ettre and P. I?. Varadi, A r ~ n l . Cilein., 35, 69 (1963). 5. J. B. Gilbert and J. J. Kipling, Fuel, 41, 249 (1'362). 6. Y. Tsuchiya and K. Sumi, J. P o l y n ~ . Sci. A-1, 7, 813 (1969). 7. M. Berozn, .-lnnl. Cile~r~., 34, 1801 (1962).
8. R. Kaiser, Chror~~alogrnpl~ie irl der Gasphase. 111, Bibliographisches Institut,
RIal~rlheim, 1962, 11. 136.
9. L. I<. Evniis and A. G. Gillam, J. Chern. Sac., 1945, 432.
10. T. Yarnaguchi and >I. Amagasn, ZCobunsiti Iiagaku, 18, 653 (1961).
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