The effect of hydrazine hydrate on the thermal properties of polyvinyl alcohol.код для вставкиСкачать
Die Angewandte Makromolekulare Chemie 23 (1972) 1-8 ( N r . 291) From the Chemical Laboratory of Textile Fibres, Kyoto University of Industrial Arts and Textile Fibres, Matsugasaki, Sakyo-ku, Kyoto, Japan The Effect of Hydrazine Hydrate on the Thermal Properties of Polyvinyl Alcohol" By SEISHIMACHIDA, TSUTOMU SHINTANI, and HIROSHI NARITA (Eingegangen am 3. Mai 1971) SUMMARY: The effect of hydrazine hydrate on the thermal properties of polyvinyl alcohol was investigated. It was proved that the presence of carbonyl groups in the polymer chain is responsible for the thermal instability of polyvinyl alcohol, and that hydrazine combines with the carbonyl group to protect the polymer against autoxidation and dehydration on the heat-treatment. Hydrazine hydrate was suitable for prevention of colouring of polyvinyl alcohol on hegting at about 200 "C unless the carbonyl content is too high. Commercial polyvinyl alcohol was thus improved in the properties by the simple treating with hydrazine hydrate. ZUSAMMENFASSUNG : Es wurde der Effekt von Hydrazinhydrat auf die thermischen Eigenschaften von Polyvinylalkohol untersucht. Es zeigte sich, daB die Instabilitiit von Polyvinylalkohol gegen Warme vom Vorhandensein von Carbonylgruppen in der Polymerkette herruhrt, und da13 Hydrazin durch Verbindung mit den Carbonylgruppen die Polymeren bei der Warmebehandlung gegen Autoxidierung und Dehydrierung schutzt. Hydrazinhydrat ist geeignet, Verfiirbungen von Polyvinylalkohol bei der Warmebehandlung um 200 "C zu vermeiden, wenn der Carbonylgruppengehalt nicht zu hoch ist. Die Eigenschaften des kauflichen Polyvinylalkohols konnen also durch einfache Behandlung mit Hydrazinhydrat verbessert werden. Introduction In a previous paperl, it was reported t h a t the hydrolysis of polyvinyl acetate by hydrazine hydrate produces a polyvinyl alcohol of favourable properties. One of the properties is the heat-resistance. It was considered t h a t the protection against oxidation of the polymer with hydrazine hydrate results in the improvement on the thermal stability. The present investigation was undertaken t o find the function of hydrazine in the thermal properties of polyvinyl alcohol and t o improve the practical heatresistance of commercial polyvinyl alcohols by hydrazine hydrate. * Studies on water-soluble polymers, part 25. 1 S. MACHIDA,T. SHINTANI, and H. NARITA Protection of colouring of polyvinyl alcohol o n heating by hydrazine hydrate Polyvinyl alcohol was heated in an 80 yoaqueous solution of hydrazine hydrate2 a t 80 "C for half an hour, and the hydrazine-treated polyvinyl alcohol was isolated by pouring the reaction mixture into methanol. The film of the polymer was heated a t 190"Cand the colouring was followed by measuring the change in absorbance a t 420 mp. It was found that the sufficient stability of the polymer for colouration on heating is given by the treatment by hydrazine, as is shown in Fig. 1 . It is well-known that the commercial polyvinyl alcohol produced from alkaline hydrolysis of polyvinyl acetate suffers a disadvantage apt to be coloured by heat-treatment. It is said that a very small amount of carbonyl groups afforded unavoidably in the polymer chain (Fig. 4)is responsible for the thermal instability. YAMAGUCHI~ for example, proposed that the colouration of commercial polyvinyl alcohol on heating is induced by the trace amount of carbony1 groups. These reducing groups accelerat2 both autoxidation and dehydration of the polymer molecule to give more carbonyl and polyene groups along the polymer chain and causes the colouration. It is considered that the pro- 4 1 0 4 8 Time [ h ] 6 10 12 Fig. 1. Colouring of polyvinyl alcohol films on heating at 190°C. a : Original polyvinyl alcohol, b: Hydrazine-treated polyvinyl alcohol, c : Reduced polyvinyl alcohol, d: Oxidized polyvinyl alcohol (containing0.24 mole-yoof carbonyl groups), e : Oxidized polyvinyl alcohol (containing 1.74 mole-% of carbonyl groups), f : Oxidized and then hydrazine-treated polyvinyl alcohol (containing 0.24 mole-% of hydrazone groups), g : Oxidized and then hydrazine-treated polyvinyl alcohol (containing 1.74 mole-% of hydrazone groups). 2 Thermal Properties of Polyvinyl Alcohol tection of the reducing groups with hydrazine to form hydrazone results in the improvement on the resistance to colouring. For proving the consideration, the carbonyl groups were purposely introduced into the polyvinyl alcohol by oxidation4, and then the polymer was treated by hydrazine hydrate. The thermal properties of the polymers were shown in Fig. 1. The colouring of the oxidized polymer on heating a t 190°C increases with increasing of carbonyl content in the polymer chain, and the colouration is decreased by the treatment of the polymer with hydrazine hydrate. The absorbance a t 275 mp assigned to the carbonyl group in the ultra-violet absorption spectrum of the oxidized polyvinyl alcohol5 was vanished by the treatment with hydrazine hydrate, as Fig. 2 shows. However, it is shown in Fig. 1 that the 2.c Y 8 u 0, c 0 m 1.0 .- . .I I - 5 C I I- Y 250 a 0 300 350 4 Wave length [mp] Fig. 2. UV absorption spectra of polyvinyl alcohols. a: Original polyvinyl alcohol, d: Oxidized polyvinyl alcohol (containing 0.24 mole-% of carbonyl groups), f : Oxidized and then hydrazine-treated polyvinyl alcohol (containing0.24 mole- yo of hydrazone groups). treatment of polyvinyl alcohol with hydrazine hydrate is practically effective for its thermal stability only when the carbonyl content is small. The fact is probably due to that the oxidation brings about the depolymerization6 of the polymer as shown in Fig. 3 and 6. If the carbonyl group is responsible for the colouration, the colouration should be diminished also by reduction of the group. The polyvinyl alcohol was therefore reduced by sodium boron hydride7 in aqueous solution for 45 hrs. a t room temperature. The resulted polymer, after purification, showed sufficient stabilit y against colouration on heating under the present conditions, as shown in Fig. 1. The IR absorption spectrum of the reduced polyvinyl alcohol gives no band S. MACHIDA,T. SHINTANI, and H. NARITA a t 1720 and 1650 cm-1 for carbonyl and ethylenic groups respectively which are observed in that of the oxidized polyvinyl alcohol, as Fig. 4 shows. The absence of carbonyl groups or conjugated double bonds is considered to contribute to the thermal stability. 1.o 2 .o Carbonyl content (mole o/o) - 3.0 Fig. 3. Depolymerization of polyvinyl alcohol by oxidation. Thermal properties of polyvinyl alcohols The thermogravimetric analysis showed the difference in effect of hydrazine hydrate on the thermal stability between the original and oxidized polyvinyl alcohols, as shown in Fig. 5. Although the thermalstability ofthe original polymer isimproved byhydrazine hydrate up to that of the reduced polyvinyl alcohol, the oxidized and degraded polymer is scarcely affected in stability by the hydrazine hydrate. The oxidized and hydrazine-treated polyvinyl alcohol containing 1.74 mole-yo of hydrazone groups was heated a t 235 and 290°C for half an hour. The analysis of the decomposed polymer is shown in Table 1. Thenitrogenis foundtoremainin the polymerafter the heating, though its mode oflinkage is not yet revealed in detail. On the thermal reaction above 200°C, the carbonyl groups are perhaps generated along the polymer chain by autoxidation, and combined partly with hydrazone groups through inter- and intra-molecular reactions. The IR spectra shown in Fig. 4 would account for these reactions. The results of the differential thermal analysis of the polyvinyl alcohols are shown in Fig. 6. It is shown that the treatment of polyvinyl alcohol with hydrazine hydrate or sodium boron hydride brings about a little rise of melting 4 Thermal Propertiea of Polyvinyl Alcohol -$ u 0, c u Q .- .u 4- 5c Q e 4000 2400 1600 1200 800 Wave number [cm-'1 Fig. 4. IR absorption spectra of polyvinyl alcohols. a: Original polyvinyl alcohol, c : Reduced polyvinyl alcohol, e: Oxidized polyvinyl alcohol (containing 1.74 mole-yoof carbonyl groups), g : Oxidized and then hydrazine-treated polyvinyl alcohol (containing 1.74 mole-% of hydrazone groups), h : Oxidized and hydrazine-treated polyvinyl alcohol after heating at 235 "C for half an hour, i : Oxidized and hydrazine-treated polyvinyl alcohol after heating a t 290 "C for half an hour. point. The reaction of the treatment is considered to occur in the amorphous region of polymer. Conclusion It was proved that the thermal colouration of polyvinyl alcohol is protected by treating it with hydrazine hydrate, unless the content of carbonly groups in $he polymer is too high. The commercial polyvinyl alcohol containing a trace ibmount of carbonyl groups can readily be improved in the thermal properties by treatingwith hydrazine hydrate. The hydrolysis of polyvinyl acetate with hydrazine hydrate which has been reported in a previous paper' is, therefore, useful for 5 S. MACHIDA,T. SHINTANI, and H. NARITA 100 300 200 400 Temperature I"C] Fig. 5 . Thermogravimetric analysis of polyvinyl alcohol. a : Original polyvinyl alcohol, b : Hydrazine-treated polyvinyl alcohol, c : Reduced polyvinyl alcohol, e : Oxidized polyvinyl alcohol (containing 1.74 mole-% of carbonyl groups), g : Oxidized and then hydrazine-treated polyvinyl alcohol (containing 1.74 mole-% of hydrazone groups). Table 1. Elemental analysis of oxidized and hydrazine-treated polyvinyl alcohol before and after heating. C Before heating After heating a t 235°C After heating at 290°C IHIN [%I [%I 49.48 45.45 8.40 6.42 1.09 0.56 45.02 2.59 0.65 [%I the production of heat-stable polyvinyl alcohol. Even if some carbonyl groups are unavoidably formed in the polymer chain during the hydrolysis, the hydrazine combines with it immediately and protects the polymer against autoxidation and dehydration on heating. 6 Thermal Properties of Polyvinyl Alcohol b I 200 Fig. 6. I 250 Temperature [ " C ] 30 Differential thermal analysis of polyvinyl alcohols. a : Original polyvinyl alcohol, b : Hydrazine-treated polyvinyl alcohol, c : Reduced polyvinyl alcohol, d : Oxidized polyvinyl alcohol (containing 0.24 mole- Yo of carbonyl groups), f : Oxidized and then hydrazine-treated polyvinyl alcohol (containing 0.24 mole-?" of hydrazone groups). Experimental Sample of polyvinyl alcohol The commercially available polyvinyl alcohol was purified by extraction with extractor for 20 hrs. The average degree of poly.aqueous methanol in a SOXHLET merization obtained by viscosity determinations was 500. Reaction of polyvinyl alcohol with hydrazine hydrate The procedure used for the reactions was virtually identical with%hat described in a previous paper2 for polyacrylic hydrazide. The extent of substitution of carbonyl by hydrazone groups was determined by nitrogen analysis. Almost quantitative substitution was found within about half an hour under the reaction conditions. Oxidation of polyvinyl alcohol The polyvinyl alcohol was reacted with sodium hypochlorite in the manner deet al4. The amount of carbonyl groups in the oxidized polymer scribed by SHIRAISHI was determined by the hydroxylamine nicthodg. 7 S. MACHIDA,T. SHINTANI, and H. NARITA Reduction of polyvinyl alcohol To 500 g of a 1% aqueous solution of polyvinyl alcohol 1 g of sodium boron hydride7 was added, and the solution was allowed to stand for 45 hrs. at room temp. with occasional shaking. The solution acidified with hydrochloric acid solution was poured into methanol, and the precipitate was filtered and washed. The crude product was purified by extraction with methanol in a SOXHLET extractor6. The degree of polymerization was confirmed to be unchanged by the reduction. Colouring of polyvinyl alco?wl on heating The measurement was carried out by using a HITACHI 124 type double beam spectrophotometer. The data were converted to those of afilmof 0.1 mm thickness'. Absorption spectrophotomdry UV and visible spectra were taken with a HITACHI124 type double beam spectrophotometer. I R spectrum was taken with a SHIMAZU IR-27C type recording infrared spectrophotometer, using a KBr disk method. Thermogravimetric and di flerential thermal analyses CAHN Model No. 3600 type thermal balance and a SHIMAZU DT-2B type DTA apparatus were used. The measurements were carried out in air at a heating rate of lO"C/min. 2 3 4 5 6 7 * 9 8 S. MACHIDA, H. NARITA,and R. YAMAMURA, J. SOC.Fibre Sci. Techn. (Sen-i Gakkaishi) 26 (1970) 208. S. MACHIDA,Tappi 52 (1969) 1734. T. YAMAGUCHI, M. AMAGASA, S. KINUMAKI, and T. 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