investigation of cross-linking of polyacrylontrile fibres by an isometric method

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  • INVEST IGAT ION OF CROSS-L INK ING OF POLYACRYLONTRILE

    F IBRES BY AN ISOMETRIC METHOD*

    M.A . Zharkova , G I . Kudryavtsev , I . F . Khudoshev , and T . A. Romanova

    UDC 677.494.745.32

    It is possible to change the major mechanical and thermomechanical propert ies of f ibres by means of s t ructura l -chemica l modification [1, 2]. Modifications leading to the production of chemical ly cross- l inked f ibres acquire a special significance.

    Previously the chemical cross- l inking of PAN f ibres with hydrazine hydrate has been investigated [3]. The present work studies the variat ion of certain deformation propert ies of chemical ly cross- l inked PAN f ibres by the isometr ic heating method. As has been shown, it is possible to determine the number of bonds in unit vol- ume of achainus ing the Mequilibrium contraction n curves of f ibres in ahighlyelast ic state. Hydrazine hydrate, semicarbaz ide and thiosemicarbazide were used as cross- l ink ing agents. The oriented f ibres based on PAN of 14.3 tex (N 70)t and 28.6 rex (N 35) were subjected to cross- l inking; the c ross- l ink ingwas control led by an iso- metr ic method since chemical methods give ambiguous results, owing to adequacy of formation of in t ra - and supermolecular structure.

    The curves on the isometr ic heating diagram (IHD) for the original and chemical ly cross- l inked f ibres are fundamentally different (Fig.l). The original PAN f ibre has a character ist ic bimodal curve, f i rst a maximum which corresponds approximately to the transit ion temperature in the viscoelast ic state, then a downward side character is ing the gradual transition f rom the highly elastic state to plastic flow. The ap- pearance of a second maximum at 280-300C is probably associated with the formation of intramolecular naphthyridine or s imi lar cyclic st ructures character ist ic of heat- t reated PAN.

    For chemical ly cross- l inked f ibres regard less of the nature of the cross- l ink ing agent the f i rst max- imum is displaced to the side of higher temperature (270-320C) then a second maximum appears at 380- 400C. It follows that structural processes are somehow involved here, which up to 270-320C impede fur - ther d isarrangement of the molecule. This impediment may be cross- l inkages, formed as the result of the processing of the f ibres with cross- l inking agents, while the position and magnitude of the maximum may vary depending on the nature and stabil ity of the cross- l inkages. The most stable cross- l inkages are formed, apparently, by the interaction of PAN with thiocarbazide (see Fig. 1, curve 4).

    The magnitudes of the maximum on the IHD curves can serve as a measure of the number of c ross - l inkages in the system. For example, chemical analysis data shows that PAN f ibres, cross- l inked with hy- drazine hydrate at various temperatures ,conta ind i f ferent quantities of combined hydrazine hydrate. F rom

    30 30 /50 2"/0 270 330

    Temp[ature,C

    Fig. 1. IH d iagram of c ross - linked PAN samples: 1) original; 2) cross- l inked byhydrazine hy- drate; 3) cross- l inked by semi - carbazide; 4) cross- l inked by thiosemicarbazide.

    Fig. 2, it is seen that the maximum tension ama x of all the c ross - linked f ibres at 280-290C is sharply decreased in compar ison with the tension of the original f ibre. The higher the reaction tempera- ture and, consequently, the more the cross- l inkage in the f ibre, the smal ler the maximum of the tension.

    Isometr ic heating d iagrams show not only the qualitative picture of the chemical cross- l inkages but also can be used for the determina- tion of the number of cross- l inkages (analogously to the determination of l inear oriented polymers [4]). If the kinetic nature of the force, originating at the stretching of the po lymers , is taken into considera- tion then in network po lymers it is possible to est imate the number of bonds by way of the formula of the kinetic theory of elast ic ity [5].

    * The second communication on the subject "Investigations of the structur isat ion propert ies of polyacrylonitr i le f ibres n. t Tex = g/1000 m, N = 1000/rex.

    (VNIIV) All-Union Scientific Research Institute of Synthetic F ibres. Translated f rom Khimicheskie Volokna, No. 2, pp. 21-23, March-Apr i l , 1969. Original art ic le submitted January 31, 1968.

    147

  • 30 90 tSg 2f0 270 330 390

    Temperature,C

    F ig. 2. IH d iagram of samples of PAN f ibre t reated with hydraz ine hydrate at var ious temperatures : 1) 1300; 2) 140C; 3) 150"0; 4) 160C; 5) or ig ina l .

    By invest igat ing po lar po lymers with chemica l c ross - l inkages it is poss ib le to es tab l i sh condit ions at which the react ions to a s t re tch ing fo rce show only nets of chemica l c ross - l inks .

    That is obtained when the po lymer is found in a highly e las t i c s tate . It is poss ib le to cons ider the temperature of the max imum on the i sometr i c heat ing d iagram as the t rans i t ion temperature in the bonds of h ighly e las t i c s ta te . The number of mo lecu lar network bonds N in unit vo lume are found by means of the fo rmula :

    ,\,, :=: ~m:Lx fo r /, . /'1

    where Xt is the length fo rmed af ter the appl icat ion of a s t re tch ing fo rce be fore IH, X2 is the same a f te r equ i l ib r ium cont ract ion at a temperature above the temperature of the max imum; K is the Bo l tz - mann constant ; T is the absolute temperature ; emax is the tension at the max imum on the curve of the d iagram.

    METHOD OF INVEST IGAT ION

    The spec imen is subjected to orientating stretching at a pressure of 400 kg /em 2, fixed with the c lamps of the isometr ic heating apparatus and gradual ly heated to a temperature cor respond ing to the max i - mum on the IH d iagram. After reaching this, max imum tens ionswere produced by the "equi l ibr ium contrac- tion" of the spec imen on gradual ly raising the temperature of the spec imen over a 20-30 range.

    An est imation of the number of fibre networks (cross-l inked with hydraz ine hydrate at 130, 150, and 160C for 2 h), cor respond ing to the arbitrary designations weak ly cross-l inked, optimally cross- l inked and strongly cross-l inked, is presented. In order to distinguish the intermolecular network, fo rmed by chemica l bonding (cross-links), f rom the networks existing in the fibres as the result of the interweaving ('over-lash") of macromolecu les , the propert ies of control fibres, heated at the same temperature but without cross- l inking agents, were determined. The number of cross- l inks was determined by the difference in the number of bonds in the molecu lar networks of the c ross - l inked f ib res and the contro l samples ca lcu - la ted by the fo rmula quoted above (see Table 1).

    F rom Table 1 it is seen that with increase of the heat ing temperature of the contro l spec imens the amount of in terweav ing fa l l s , s ince there is an increase in the mobi l i ty of the macromoleeu les and hence bet ter condit ions for re laxat ion are es tab l i shed . On heat ing the c ross - l inked f ib re the op- pos i te effect is observed-w i th increas ing temperature number of networks increases at the expense of c ross - l inkages . This is in good agreement with the phys icomechan ica l p roper t ies of c ross - l inked f ib res (Table 2).

    F rom Table 2 i t is seen that with increase of heat ing temperature the PAN f ibre obtained increases i ts thermostab i l i ty , par t i cu la r ly at 200C. The in i t ia l PAN at this temperature has pract i ca l ly no s tab i l i ty , that

    TABLE 1. Number of Cross -L inks in a Unit Vo lume of Di f ferent Samples of F ib re (Po ly - mer )

    Sample of PAN fibre

    Control at 130"C . . . . . . . . . . . . Cross-linked at 130"C . . . . . . . . Control at 150C . . . . . . . . . . . . Cross-linked at 150"C . . . . . . . . Control at 160"C . . . . . . . . . . . . Cross-linked at 160"C . . . . . . . . * For o0= 400 kg/em 2.

    a* at 280C, kg/em 2

    148 194 128.5 225 108 263

    1.15 1.11 1.14 1.12 1.16 1.12

    No. (.102) in 1 cm ~

    of bonds in the molecular network

    41.5 75.5 37.6 79.5 28.6 92.5

    of cro~s-links

    34.0

    41.6

    63.9

    No. of cross-links in 100 monomer units

    26

    32

    47

    148

  • TABLE 2. Dependence of Physicomechanical Properties of PAN Fibre on the Temperature of Heating

    PAN fibre

    Original . . . . . . . . . . . . Processed with hydrazine

    hydrate . . . . . . . . . . . The same . . . . . . . . . .

    . . . . . . . . . .

    t t

    N n , o . o . . , o . .

    Temp.. C

    130 140 150 160

    Physicomechanieal properties at temperatures, C

    20

    strength, km 1 breaking / leng,h___

    45.5 I

    extension,%

    14.4

    46.4 18,4 40.0 21,9 35.9 23.0 35.5 24,2 14.3 13.1

    150 200

    degree of re- tention of strength, %

    56.6

    54.4 60.6 64.5 85.5 93.5

    degree ofre- tention of ex- tensivity, %

    143

    136 115 106 98 95.5

    degree of re- degree of re- tention of tention ofex- strength, % tensivity,%

    6.3 186

    19.3 133 49.0 108 48.0 91 79.5 1o0 72.0 88

    * At 100*C in concentrated H2SO 4 over a period of 30 rain.

    Shrinkage *, %

    Dissolves

    Swells 70 46 36 14

    heated at 130C retains about 50% stabil ity, andat 150C ~79%. The breaking temperature of the f ibre also inc reases with the increase of heating temperature . Thus the more molecu lar bonds are fo rmed in the