Degradation of Cross-Linked Unsaturated Polyesters by Using

Chapter 8

Degradation of Cross-Linked Unsaturated Polyesters by Using Subcritical Water 1

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Kanji Suyama , Masafumi Κubota , Masamitsu Shirai , and Hiroyuki Yoshida Downloaded by COLUMBIA UNIV on July 30, 2012 | http://pubs.acs.org Publication Date: January 1, 2009 | doi: 10.1021/bk-2009-1004.ch008

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Departments of Applied Chemistry and Chemical Engineering, Osaka Prefecture University, 1-1 Gakuen-Cho, Naka-Ku, Sakai, Osaka 599-8531, Japan

The degradation of unsaturated polyesters crosslinked with styrene was performed in sub-critical water (SCW) at 300°C in the absence and presence of organic additives. The unsaturated polyesters were partially de-crosslinked by hydrolysis of ester chains to form polystyrene derivatives. The de-crosslinking rate was enhanced in the presence of hydroxy compounds with a long alkyl chain and alkylamines, while carboxylic acids and benzenesulfonate salts were ineffective despite having a long alkyl chain. The degree of de-crosslinking was reduced in the presence of diamines and amino acids because re-crosslinking occurred. In the case of aminoalcohol, the amino group reacted prior to the hydroxy group, and a polystyrene derivative with side-chains having terminal hydroxy groups was obtained without re-crosslinking. The hydroxy groups could be modified with maleic anhydride, and polymerized with styrene to yield a network polymer again.

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© 2009 American Chemical Society

In Polymer Degradation and Performance; Celina, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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Downloaded by COLUMBIA UNIV on July 30, 2012 | http://pubs.acs.org Publication Date: January 1, 2009 | doi: 10.1021/bk-2009-1004.ch008

Introduction Chemical recycling of polymers is an important target of modern society that consumes a great deal of petroleum-based polymeric materials. Although the chemical recycling of thermosetting polymers is also required, it is difficult because of their inherent insoluble and infusible properties. Unsaturated polyesters (UPs) crosslinked with styrene are often used as a matrix of fiber reinforced plastics. Several reports treated the degradation of the crosslinked UPs with high temperature treatment in water (1,2), acetic acid (3), alcohols including glycols (4,5), and amines (6), often in the presence of catalysts. In these literatures, recovery of polymeric materials from the crosslinked UPs was not a main objective. However, in case we can hydrolyze polyester chains selectively, linear polystyrene derivatives can be obtained as recycled materials. Recently, we have attempted the degradation of crosslinked polymers in sub-critical region of water. At high temperature and pressure, water shows drastic decrease in dielectric constant, which improves the solubility of organic compounds into water. Furthermore, the magnitude of ion product of water increases compared to room temperature. These properties are advantageous for dissolution and hydrolysis of organic materials (7). However, the ion product of water decreased at supercritical region. Furthermore, polystyrene units in crosslinked UPs were also decomposed in monomers or other small molecules. These properties suggest the advantages of sub-critical water (SCW) than supercritical water from the viewpoint of recovery of polystyrene derivatives. The drawback of the degradation of polymers in SCW is its slower reaction rate than supercritical water. Recently, we have reported that the degradation of crosslinked UPs in SCW was enhanced by adding alcohols with a long alkyl chain (8) and amines (9). In this paper, we report the degradation behavior of a crosslinked UP using S C W treatment in the absence and presence of the additives and propose a new recycling system of polymers.

Experimental Instrumentation IR, U V , and Ή N M R spectra were obtained using Jasco FTIR-410, Shimadzu UV-2400PC, and Jeol JNM-GX270 (270 M H z ) spectrometers, respectively. Size exclusion chromatography (SEC) was carried out on a Jasco G P C equipment consisting of a PU-980 pump, an RI-930, and a Shodex K F 806M column with polystyrene as a standard and tetrahydrofiiran (THF) as an


90 eluent at 40°C. Thermogravimetric analysis (TGA) was carried out using a Shimadzu TGA50 thermogravimetric analyzer at a heating rate of 10 Κ / min under N . The onset temperature from T G A curves was adopted as thermal decomposition temperature (T ). Gas chromatography (GC) was performed on a Shimadzu GC2010 with a thermal conductivity detector. 2

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Materials Additives such as alcohols, phenols, and amines were used as received from Wako Chemical or Tokyo Chemical Industry, both in Japan. Maleic anhydride (MA) and phthalic anhydride (PA) were recrystallized from chloroform-hexane mixture and chloroform, respectively. 1,2-Propanediol (PG), styrene (St), and tetrahydrofuran (THF) were distilled before use.

Preparation of Crosslinked Unsaturated Polyesters (UP) A mixture of M A : P A : PG = 2 : 3 : 5.5 (mol/mol) was heated at 160°C for 4 h followed by heating at 210°C for 1 h under argon. After reprecipitation from chloroform / methanol system, UP1 was obtained in 36.1 % yield: T : 314°C, Mn: 2,600, Mw: 5,300. M A : P A : PG = 16.1 : 32.7 : 51.1 (mol/mol) from Ή NMR. In a glass tube, 2.95 g of UP 1 and 148 mg of A I B N were dissolved in 4.43 g of St monomer, argon-purged for 10 min, and heated at 60°C for 3 h. After cooling, the polymer was crushed, and the fraction of 0.25 ~ 2 mm diameter was separated using Iida testing sieves. Resulting grains were extracted by THF with a Soxhlet for 3 h to remove the non-crosslinked fraction. After drying, 6.92 g (92.6 %) of crosslinked UP2 was obtained. M A : P A : PG : St = 4 : 9 : 14 : 73 (mol/mol) from IR. T : 320°C. d

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SCW Treatment De-ionized water and additives (total 4.75 g), and 250 mg of UP2 were placed in a SUS316L stainless tube with 150 mm length and 8 mm inner diameter. After purging argon gas for 1 min, the tube was closed with Swagelok caps and plugs, and then heated in a salt bath of sodium nitrate and potassium nitrate mixture. After the SCW treatment, the tube was dipped in water at room temperature. S C W treating time is defined as the time between the moment of sintering the tube in the salt bath and that in water.


91 Separation of Reaction Mixture The weight of recovered solid (mg) is obtained by filtering of the reaction mixture followed by drying in vacuum for ca. 30 min. The solid was conducted to Soxhlet extraction using T H F for ca. 3 h. Degree of de-crosslinking (%) is defined as (\-ITHF I 250) χ 100, where I HF is the weight (mg) of insoluble fraction in T H F . The T H F solution obtained from Soxhlet extraction was concentrated and poured in an excess hexane to afford non-crosslinked polymers whose weight (mg) was used to define the soluble fraction in THF (S ). T


THF

Re-crosslinking of a Polystyrene Derivative Recovered polystyrene derivative on SCW treatment of UP2 at 300°C in the presence of 5-amino-l-pentanol was dissolved in chloroform, added triethylamine and M A , and heated at 50°C for 24 h. Resulting solid was purified by reprecipitation and dissolved in St. After the addition of A I B N , the St solution was degassed and heated at 60°C for 3 h.

Results and Discussion SCW Treatment of 2 in the Absence of Additive In the absence of additives, 42.7 % of UP2 was de-crosslinked on S C W treatment at 300°C for 10 min (Table I entry 1). With an increase in S C W treating time, peaks at 1730 and 2500-3300 c m due to ester and carboxylic acid groups, respectively, decreased along with an increase of peaks at 1780 and 1860 cm" due to carboxylic anhydride group. These changes clearly indicate that dehydration between neighboring carboxylic acid groups proceeded after hydrolysis of ester groups as shown in Scheme 1. -1

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SCW Treatment of UP2 in the Presence of Additives The effect of alcohols and phenols on the degradation of UP2 is shown in Table I entries 2-11. The degree of de-crosslinking depended on the length of alkyl chain, and remarkable enhancement was observed for 1-undecanol, 1dodecanol, 1-tetradecanol, 1-hexadecanol, and w-pentadecylphenol. The balance of hydrophobicity and bulkiness of the alcohols and phenol might play a major role in penetrating into the crosslinked resin. The number average molecular



Scheme 1. Degradation and dehydration of UP2 in SCW.



93 weight of the polystyrene derivatives obtained by SCW treatment at 300°C in the presence of 1-tetradecanol was 12,800. When UP2 was SCW-treated in the presence of 0.24 g and 0.48 g of 1tetradecanol, the degree of de-crosslinking was 51.6 and 94.4 %, respectively, while that was 75.4 % in the presence of 0.71 g of w-pentadecylphenol. From the viewpoint of recovery of excessive additives from water, 1tetradecanol is advantageous because this alcohol is solid and insoluble in water at room temperature. More than 80 % of 1-tetradecanol was recovered from hexane phase when UP2 was treated at 300°C for 10 min. On the other hand, diols were ineffective for the enhancement of decrosslinking even though they have a long alkyl chain (entries 12-14). Sodium dodecylbenzenesulfonate, 1-tetradecanoic acid, and tetradecane did not enhance the de-crosslinking (Table I entries 15-17). These results indicate that the long alkyl chain was not effective to enhance the degradation, and functional groups that can react to ester groups would be crucial to promote the degradation. On the other hand, amines were very effective for the enhancement of the de-crosslinking (Table I entries 18,19). 1-Pentylamine was also effective even it has a short alkyl chain. The length of alkyl chain did not affect the degree of decrosslinking, and 1.3 equivalent of amino groups to ester units was enough to decrosslink completely. These results show that amino groups were highly reactive to ester groups. In fact, the de-crosslinking in the presence of 1-tetradecylamine needed shorter S C W treating time and lower S C W treating temperature compared to 1-tetradecanol. The effect of difunctional additives having an amino group at one end and other functional group at the other end was investigated (Table I entries 20-22). 5- Amino-l-pentanol was highly effective, while 1,8-octamethylenediamine and 6- aminohexanoic acid were not. These results in Table I can be explained by reactions in Scheme 2. As shown in Figures 2b and 2c, peaks at 1270 cm" due to C - O stretching band in conjugated ester disappeared, while peaks at 1730 cm" due to ester C=0 groups remained. These results indicate that the transesterification proceeded in the presence of alcohols and diols. In case of diol, the transesterification at both ends would afford re-crosslinking. A similar trend was observed for other difunctional additives such as diamine and amino acid. When amino compounds were added, recovered polymers had a peak at 1700 cm" as shown in Figures 2d and 2e due to cyclic imide moieties. When 5-amino-l-pentanol was used as an additive, amino groups reacted prior to hydroxy groups, leading to the formation of a polystyrene derivative with side-chains having terminal hydroxy groups. The hydroxy groups could be modified with M A , and polymerized with styrene to afford a network polymer again. 1

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Table I. Degradation of UP2 by SCW-Treatment at 300 °C for 10 min in the Presence of Additives.



Scheme 2. Plausible reactions ofUP2 in SCW in the presence of additives.


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Degradation of Cross-Linked Unsaturated Polyesters by Using

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