The study of the inhibiting properties of stabilizers by the thermomechanical method—II. Organotin compounds as stabilizers for polyvinylchloride under high temperature and γ-radiation conditions

THE STUDY OF THE INHIBITING PROPERTIES OF STABILIZERS BY THE THERMOMECHANICAL METHOD--II. ORGANOTIN COMPOUNDS AS STABILIZERS FOR POLYVINYLCHLORIDE UNDER HIGH TEMPERATURE AND 7-RADIATION CONDITIONS* M. N. SHTEDING and V. L. KARPOV L. Ya. Karpov Physicoehemical Scientific-l~esearch Institute (Received 3 July 1961)

THE stabilizing efficincy of organotin compounds, which improve the thermal stability and heat and light resistance of polymeric materials based on polyvinylchloride (PVC) has ben described more than once in the literature [1, 2]. In the organometallic-compound laboratory of the Karpov Institute we have used the thermomechanical method for the study and assessment of the inhibiting properties of organotin stabilizers, synthesized by a new method [3]. We were also interested in the suitability of this type of stabilizer for protection of PVC against y-radiation. In this case the added stabilizer, in accordance with the specific nature of the "ageing" of the polymer, should either retard and control the formation of crosslinkages between the polymer molecules, and the ratio of crosslinking to degradation, or remain passive, fulfilling only the role of acceptor of oxygen and hydrogen chloride. Kargin and Shteding [4, 5] showed by the termomechanical method t h a t the ageing effect in PVC and polymeric materials based on PVC is dependent to a considerable extent on the rapid development of the competing processes of degradation and crosslinking, and the efficiency of the stabilizer on its ability to retard and alter the ratio of the rates of these processes. In this work the thermomechanical method was used to study the properties of organotin compounds as ordinary heat stabilizers for PVC and as stabilizers against y-radiation from a e°Co source, of activity 20,000 g-equiv, of radium. The following organotin compounds were studied: di-n-butyltin maleate (I), di-n-butyltin dichloride (II), di-n-butyltin dilaurate (III), di-n-butyltin maleate prepared by a method different from that used for I (IV), di-n-butyltin disalicylate (V) and di-n-butyltin dimyristate (VI). The experiments were carried out on PVC grade PF-4. Two types of specimen were tested: type I--rigid, pressed tablets, 2.0 m m thick (PVC--100, dibutyl phthalate--5, and 2 parts by weight of stabilizer; the control specimens contained * Vysokomol. soyed 4: No. 12, 1806-1811, 1962. 561

562

M. N. SHTEDINGand V. L. KARPOV

no organotin stabilizer); type II--elastic films, 0.25 mm thick, obtained by milling (VPC--100, dibutyl phthalate--38, stabilizer--0.2 and 2.0 parts by weight; the control specimens contained no organotin stabilizer). The rigid specimens (tablets) were (a) heat treated for 2, 4, 6, and 8 hr at 160 ° and (b) subjected to 7-irradiation from a e°Co source with dosages of 10, 15 and 25 Mrads. The strain-temperature characteristics of the specimens were recorded by the method described previously [4] and from the shape of the curves an assessment was made of the nature of the structural changes in the polymer, of the speed of formation of bonds during "ageing" and accordingly of the protective efficiency of the organotin compounds being tested. For more complete characterization of the organotin compounds, other than as stabilizers and inhibitors, other film characteristics were determined with specimens of type II, such as (1) mechanical properties (tear strength, elongation at break) at 20 ° and 60°; (2) the same mechanical properties after ~-irradiation of the films with dosages of 50 and 100 Mrads; (3) the thermal stability of the films at 165 ° in minutes, determined by the State Standard Specification method and (4) the low-temperature resistance of the films (brittle point). Heating at 160° and irradiation were carried out in the presence of air.

DISCUSSION

The first series of deformation curves, Fig. la-g, were obtained with specimens heat-treated at 160 °, The curves in Fig. 1 (a) show the successivc changes in the shape of the curves for PVC without stabilizer. The polydispersity of the polymer heat treated for 2 hr increases sharply. Degradation is accompanied by Vhe formation of a small number of crosslinkages (the appearance of horizontal plateaux in the curves), which however do not prevent flow. The curve changes mainly in the region of high-elastic deformation. As the period of heat treatment is increased and the number of crosslinkages increases the polymer rapidly loses the ability to flow. The deformation curves of PVC after heat treatment for 6 and 8 hr present the typical picture of a fully crosslinked polymer. From the interpretation of the curves of Fig. la and of the subsequent series of deformation curves in Fig. lb-g, obtained with PVC specimens containing the various organotin compounds, the general conclusion can be drawn that the organotin compounds are good stabilizer-inhibitors. In PVC subjected to high temperatures alle of those to some degree retard the processes of structure formation, which are most harmful to the actual working conditions of a polymer, both directly and by alteration of the compatibility of the polymer with the usual plasticizers present. In addition to the general conclusion the strain-temperature characteristics give, on more detailed examination of the deformation curves, individual assessments of the protective efficiency of each of the stabilizers. In fact the inhibiting

Organotin compounds as stabilizers

563

properties of the organotin compounds, i.e. their ability to retard structure formation during heat treatment of PVC, differ. Dibutyltin dimaleate (I and IV) and diebutyltin dichloride are the most efficient stabilizers. In the deformation curves of PVC containing these additives (Fig. lb, c and d) there is practically no evidence of the presence of crosslinkages over the whole period of heat treatment. During heat treatment of P V C containing dibutyltin disalicylate (V) and dibutyltin dilaurate (III) the character of the deformation curves changes to some extent; the boundary of the region of flow shifts a littletoward the high temperature side, indicating the formation of a very small number of crosslinkages, which do not however prevent flow. Dibutyltin dimyristate (VI) is a less efficientstabilizerthan the others. It is clearly seen from Fig. Ig that after heat treatment for 8 kr P V C containing additive VI is crosslinked and has completely lost flow. Most probably the differences in inhibiting efficiency of these compounds, shown up by the thermomeehanical method, are associated with the successive decrease in the molar content of dibutyltin in the stabilizers. The second series of deformation curves (Fig. 2a-y) were obtained with P V C specimens of type If, containing the same organotin stabilizers and irradiated with ?-rays in successively increasing dosages (I0, 15 and 25 Mrads). It is seen from the diagrams that the deformation curves of P V C without stabilizer and with the various organotin compounds are almost the same. Addition of the stabiJizers does not affect the crosslinking reactions of the polymer. The organotin compounds are not inhibitors of the radiation crosslinking of the polymer under ?-irradiation and practically no individual differences are reflected in the shape of the deformation curves. The crosslinking b y ?-irradiation is slightly accelerated by the addition of additive I (Fig. 2b) and very slightly retarded by additive I I I (Fig. 2d). The characteristics of type I I PVC films with two of the organotin stabilizers are given in the Table. I t should be noted that all the organotin compounds studied markedly increase the thermal stability and satisfactorily suppress colour changes in PVC films (turning brown, darkening), both on heat treatment and under ?-irradiation, indicating t h a t t h e y function simultaneously as oxidation inhibitors and acceptors. The organotin stabilizers have a favourable effect on low-temperature resistance, and this is evidently both a result of the good compatibility of the stabilizers with PVC and a manifestation of their inhibiting effect during processing of the polymer at high temperatures. The authors express their gratitude to V. A. Kargin for collaboration in the work and to K. A. Koeheshkov, who provided for this work the organotin compounds, which were synthesized in his laboratory.

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M, •. SHTEDING and V. L. KARPOV

566

THE EFFECTOF ORGANOTINADDITIVESO1~THE PROPERTIESOF PVC FILMS

Type of test

Mechanical properties of films; at 20° at 60 ° After irradiation 50 Mrads at 20° 100 Mrads at 20° Thermal stability** in min at 165° Brittle point (low-temperature resistance °C)***

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CONCLUSIONS T h e stabilizing efficiency of a n u m b e r o f o r g a n o t i n c o m p o u n d s in p o l y v i n y l chloride has been studied b y the t h e r m o m e c h a n i c a l m e t h o d . I t is s h o w n t h a t the change in shape o f the e x p e r i m e n t a l d e f o r m a t i o n curves characterizes the efficiency o f the inhibiting a c t i o n o f each o f t h e o r g a n o t i n s~abilizers a n d their ability t o r e t a r d oxidative d e g r a d a t i o n processes a n d crosslinking w h e n the p o l y m e r is subjected t o high t e m p e r a t u r e s . W h e n the p o l y m e r is subjected t o ?-irradiation this t y p e o f stabilizer shows no inhibiting effect. H o w e v e r , as in t h e case o f h e a t t r e a t m e n t , t h e y m a i n t a i n their funcbion as stabilizer-acceptors of l i b e r a t a d h y d r o g e n chloride.

Translated by E. O. PHILLIPS REFERENCES 1. 2. 3. 4.

(L P. MACK, Kunststoffe 43: 94, 1953 G. P. MACK, Modern Plastics Encyclopedia, 1956. W. F. FISCHER and B. M. VANDERBILT, Modern Plastics 38: 165, 1956 A. V. ABRAMOVA, N. I. SHEVERDINA and K. A. KOCHESHKOV, Dokl. Akad. Nauk SSSR 123: 681, 1958 5. M. N. SHTEDING and V. A. KARGIN, Vysokomol. soyed. 4: 720, 1962