Prediction of Stress Relaxation in Laminated Leather Layers

Transcription

1 ISSN MATERIALS SCIENCE (MEDŽIAGOTYRA). Vol. 9, No.. 23 Predicion of Sress Relaxaion in Laminaed Leaher Layers D. Milašienė, V. Jankauskaiė, R. Arcišauskaiė 2 Deparmen of Clohing and Polymer Producs Technology, Kaunas Universiy of Technology, Sudenų 56, LT-33 Kaunas, Lihuania 2 Join Sock Company Sabalin, Žemaies 45, LT- 54 Šiauliai, Lihuania Received 2 November 22; acceped 3 January 23 Laminaed leaher, obained by he bonding of he microporous polyurehane film o spli leaher surface, was used for invesigaion. Such hybrid leaher is resisan o waer peneraion, bu shows high breahable properies. The effec of differen naure layers on mechanical properies and viscoelasic behaviour of laminaed leaher was examined. I was shown ha leaher backing mainly deermines he srengh of laminaed leaher, while polyurehane film deformabiliy. The combinaive srenghening is characerisic for hybrid sysem due o he repairing of leaher surface defecs by adhesive layers and dissipaion of kineic energy, which releases during he elemenary ac of he failure. Generalized Maxwell model, possessing a regular specrum of relaxaion imes, successfully describes sress relaxaion behaviour of he leaher in non-linear regions. The polyurehane film increases he sress relaxaion rae in he laminaed leaher, because rearrangemen a he inerface of wo layers arises. I was obained ha he viscoelasic naure of polyurehane film dominaes on he long-erm properies of laminaed leaher. Keywords: laminaed leaher, mechanical properies, sress relaxaion behaviour, generalized Maxwell model, discree specrum of relaxaion imes.. INTRODUCTION Leaher is used for many applicaions: foowear, gloves, clohing, purses, furniure upholsery, saddles, and a variey of oher uses. For each applicaion he leaher of differen physical and mechanical properies is obained by means of variaion in he processing echnology []. Tanned leaher is usually coaed wih hin pigmened or lacquer coaings. One of he purposes of such coaing is decoraive. The coaing may also change some physical properies of leaher: i may decrease waer and air permeabiliy, increase rigidiy, ec. [2]. Such changes depend on he coaing ype and coaing formaion echnology - he coaing may penerae deeply ino leaher or i may remain on he leaher surface [3]. There is also anoher quie differen mehod o form a surface coaing using separae moulded polymeric films. In one case microporous polymeric membranes are laminaed o he surfaces of exile, leaher, plasic or oher; in anoher - hese membranes are sandwiched beween an ouer maerial and lining [4, 5]. The range of such membranes is very wide - from waerproof and breahable films o surgical srapping for pos-operaive cicarices wih an aniviral barrier. Generally, he leaher is covered by a film, which is adhered o he surface by ho pressing [2]. Leaher laminaion is a mean of creaing a produc, which has unique properies such as resisance o waer, fire, ec. Phenomenological predicion of he mechanical behaviour of various maerials and heir sysems is a fundamenally imporan problem for engineering applicaion. To provide informaion abou he viscoelasic properies of maerials, various experimenal echniques Corresponding auhor. Tel.: ; fax: address: Daiva.Milasiene@ku.l (D. Milašienė) have been used, among which sress relaxaion is one of he simples. Equipmen designed for such research has been a coninuously improved and versaile exending machine wih compuer-aided processing of analysis resuls of he relaxaion processes of maerials open new experimenal opporuniies [6 8]. The ineres of he deformabiliy and relaxaion behaviour of leaher has been coninuously growing. Previous sudies concerning sress relaxaion in leaher have been focused on he effecs of hea and humidiy [9], pre-sain value [], parameers of shaping echnology []. Nowihsanding o abundance of research works performed in pracice, here is ofen a lack of boh heoreical and experimenal knowledge on relaxaion behaviour of leaher [6]. Especially i is ineresing o invesigae viscoelasic behaviour of relaively new sysem - laminaed leaher ha is composed from maerials wih quie differen naure and properies. The goal of his invesigaion is o sudy and o predic sress relaxaion in laminaed leaher in order o provide he possibiliy o invesigae he ime dependence shown by differen sysem layers and, hus, o gain an undersanding of heir viscoelasic behaviour. 2. THEORETICAL For maerials behaviour descripion, i is ofen necessary o fi experimenal daa o an analyic funcion. The objecive of his process is he deerminaion of he values of parameer ha in some sense represen he bes fi of he approximaing funcion o he experimenally obained resuls. Real polymeric maerials are characerised by coninuous specra of relaxaion imes. However, experimenally i is possible o deermine he specra only roughly. Due o his reason discree specra can be used, when relaxaion imes have cerain discree values [2]. 73

2 Macroscopic phenomenological models comprising Hookean spring and Newonian dashpos have been widely used o represen linear viscoelasic behaviour of various maerials [3]. However, behaviour of leaher is linear only under small srain. Oherwise, insananeous srains are pracically impossible o be applied. In hese cases models represening non-linear viscoelasiciy due o he involving non-hookean and non-newonian unis are more useful [4, 5]. On he oher hand, hey are complicaed and, herefore, ime consuming. So, for he simulaion of sress relaxaion behaviour of polymeric maerials an aemp o adap generalized Maxwell model, represening linear viscoelasiciy [6 8], was made. Generalized Maxwell model consiss of n Maxwell unis, possessing a differen relaxaion imes, and a single elasic Hookean spring wih elasiciy consan D ; all of hem are coupled in parallel (Fig. ). Every Maxwell uni consiss of Hookean spring wih elasiciy consan D i in series and Newonian viscous dashpo wih viscosiy consan η i of fluids inside he cylinders. Raio η I / D i is considered as relaxaion ime τ i of a Maxwell i-h elemen. D D η D i η i P D n η n Fig.. Generalized Maxwell model for sress relaxaion represenaion When he generalized Maxwell model is deformed under consan speed v = dε / d up o he consan level of is srain ε = ε, he relaxaion process can be expressed by he expression [6, 7]: σ = D ε + = D ε + v n i= n i= σ = i, () ε * Diτ i exp exp vτ i τ i where * is he ime couned from he insan a which he srain reaches he limi srain ε, i.e. * = ε v. The difficulies arise in analysing he relaxaion ime, because he change of any exernal facor influences on he change even of wo values - relaxaion ime and is inensiy of each model elemen. Besides i, he number of relaxaion imes in he specrum can vary, also. Due o hese reasons evaluaion of relaxaion behaviour becomes complicaed. The above menioned problems can be eliminaed by using regular discree specrum proposed in [6]. In his case relaxaion imes always have consan values, which can be obained according o he relaion: = i τ τi a, (2) where τ is minimal ime of relaxaion, a is consan (a > ). I is assumed ha he relaxaion imes of he Maxwell model differ significanly, i.e. τ << τ 2 << τ n. The number of relaxaion imes n depends on he ime range *, which is inended o be covered. In order o ensure ha wihin cerain ime ranges * relaxaion is refleced by one Maxwell elemen solely consan a admied o be equal o. Elasiciy consans of he model can be deermined according o mehod presened in [6, 7]. In he case of * >> τ n- sress relaxaion process can be described by one Maxwell elemen: * D n v Dn ngn exp σ ε = σ = τ, (3) τ n ε where = G n exp. vτ n The value of D, which defines he equilibrium sress σ on he model, can be calculaed from Eq. (3) using wo experimenal values of sress: σ n ha corresponds o he ime * τ n and σ n2 o he ime * 2 * u (* u is real ime of observaion) [6]. Thus, D * 2 * σ n2 σ n exp τ n =. (4) * ε 2 * exp τ n Correspondingly σ n Dε Dn =. (5) * vτ ngn exp τ n The values of sress σ n calculaed by Eq. (3), are subraced from experimenal values of σ σ n for he whole range of ime *. For * >> τ n 2, using he experimenal value of σ (n ) τ n 2, in a similar way he value D n and in urn he values of he oher consan D i are obained. The value of consan D, corresponding o he shores relaxaion ime τ, can be obained from he equaion σ e σ =, (6) ν τ G where σ e is he res of he experimenal values of sress for * = afer subracion of he calculaed values of σ 2 from he experimenal values of σ σ σ n σ 3. In such way for * = he calculaed and experimenally deermined values of sress fi ideally. 3. EXPERIMENTAL Maerials. Hybrid leaher - Permair leaher - used in hese sudies was a commercial grade produc. For invesigaion he semi-finished producs a he differen sages of Permair leaher manufacuring were used, also. 74

3 The characerisics of invesigaed maerials are presened in Table. Table. Characerisaion of maerials Sample Maerial Maerials characerisaion S S2 S3 S4 S5 Spli leaher Ground coaed spli leaher Ground coaed leaher wih adhesive layer Permair film Permair leaher Chrome-anned leaher.2 ±. mm of hickness Spli leaher grounded wih acrylic ground STUCO in amoun of 2 g/dm 2 Grounded leaher coaed wih polyurehane adhesive layer in amoun of 2 5 g/dm 2 ; conen of acrylic hardener in he adhesive composiion 3.2 % Microporous polyurehane (PU) membrane.4 ±.5 mm of hickness Hybrid leaher: micro-porous polyurehane membrane laminaed o spli leaher surface by adhesive Afer he leaher has been anned, hen i is spli from boh sides o he uniform hickness (S). Such leaher is grounded wih unpigmened acrylic ground (S2). The grounding layers penerae he surface of he corium and bond leaher inner layers. The polyurehane waer-born dispersion wih acrylic hardener was used for he bonding of microporous polyurehane film (S3). The polyurehane film consiss of inerconneced pores wih diameer of 5 µm (S4). Such film is resisan o waer peneraion, because usually diameer of waer drops is 5 5 µm (diameer of small drops 2 µm). Forces of srong ineracion keep waer molecules in a drop prevening heir spreading. Therefore, such drops are oo large o penerae hrough he membrane pores. Whereas molecules of waer seam, resuling from sweaing, are usually smaller han.3 µm. So, hey penerae hrough he membrane pores easily and hen drif away. The hybrid leaher was obained by he laminaion of he polyurehane film o he spli leaher surface by ho pressing a he emperaure of 7 75 C and pressure of.5. MPa for 2 s (S5). MICROPOROUS FILM ADHESIVE CORIUM OF LEATHER Waer droples W aer vapour molecules RH+ RH- Fig. 2. General scheme of Permair leaher srucure and acion The general scheme of Permair leaher srucure and is acion is presened in Fig. 2. Permair leaher is resisan for a long-erm waer impac as well as for many chemical maerials; i has absoluely no reacion wih he road sal Inerconnecing pores (NaCl); i may be characerised by high wear resisance and may be used a low emperaures (down o 25 C). Samples. The es pieces were cu in he shape of dumb-bell and he dimensions in accordance o he requiremens described in sandard ISO 3376:976 (lengh of l =5 mm, widh of b = mm). All specimens were condiioned for a leas 48 h a a sandard amosphere (emperaure T = 2 ± 2 C, humidiy ϕ = 65 ± 5 %) in accordance wih EN 2222:997. Tensile es. Mechanical properies ensile srengh and elongaion of invesigaed leaher were deermined by means of universal ensile esing machine FP-/ (Germany) wih crosshead speed of ± mm/min in accordance wih ISO 3376:976. Foe each experimenal poin 5 25 specimens of leaher and no less han specimens of polymeric film were esed. Sress-relaxaion es. Sress relaxaion ess were conduced using a universal ensile esing machine FP-/ a he sandard amosphere 2/65. Specimens were sreched a a speed ± mm/min up o srain level of 2 % and held in his posiion. The sress was recorded as a funcion of ime (from s up o 5 s). Few relaxaion imes were chosen (τ =.2, τ 2 = 2, τ 3 = 2, τ 4 = 2, τ 5 = 2) for experimenal daa approximaion. In all cases specimens were esed and average value repored. 4. RESULTS AND DISCUSSION 4.. Mechanical properies Mechanical properies of laminaed Permair leaher and is semi-finishing producs are presened in Table 2. Such properies as ensile srengh σ, elongaion a break ε b and elasiciy modulus a break E b were deermined. Besides, in he foowear manufacuring he case a σ s = 9.8 MPa is imporan. This value of sress appears in he leaher in is shaping by deformaion. Therefore, elongaion ε s and elasiciy modulus E s a 9.8 MPa of sress was deermined as well. Table 2. Mechanical properies of he laminaed leaher and is layers Sample σ, MPa Srain, %: Elasiciy modulus, MPa: ε s ε b E s E b S 2.6 ±.9 2 ± 38 ± S2 9. ±.3 2 ± 38 ± S3 5.9 ±.2 37 ± 2 53 ± S4* 7.5 ± ± S5 2.9 ±.7 6 ± 37 ± * Microporous polyurehane film ensile srengh is lower han 9.8 MPa; only Young modulus can be deermined (E Y =8 MPa a ε Y 8 %). From Table 2 i is eviden ha mechanical properies of he microporous PU film laminaed leaher depend on he naure of layers from which he sysem is composed. The ensile srengh σ of semi-finished leahers S-S3 is in he range of 6 22 MPa. The differences in he leaher 75

4 srengh properies can be relaed no only o he finishing procedure, bu also o iniial properies of he sample, opographical zone of leaher, defecs, ec. Meanwhile, he value of σ of he hybrid leaher is higher (σ 22 MPa), alhough i is laminaed wih low srengh PU film (σ 2.3 MPa). I can be aribued o he combinaive srenghening of laminaed sysem, when he ensile srengh of hybrid sysem is higher han ha of he separae layers even a low cohesion srengh of he medium layer [9]. This effec can be evaluaed by he coefficien of combinaive srenghening K s : σ σ =, (6) σ K s where σ is experimenally obained value of he ensile srengh of a laminaed sysem, σ is heoreically calculaed ensile srengh. Thus, in he insan of a highes srengh layer failure, he srains of layers wih lower srengh do no reach he limiary values ye. So, influence of hese layers on he sysem resisance o failure is deermined no by he ensile srengh of separae layers of hybrid sysem, bu by ensile srengh ha is reached a he insan of corresponding srains, which develop in he insan of he failure of a highes srengh layer. Therefore, ensile srengh of laminaed maerial should be compared no wih oal value σ of sysem consiuens, bu wih heir conribuion o he maerials resisance o failure. So, S S 2 Si σ k + σ 2 k2 + + σ i ki = σ, (7) S S S where σ i and S i are he ensile srengh and he crosssecion of i-h layer, respecively; k i is he raio of elasiciy modulus of i-h layer wih highes modulus. The evaluaion of Permair leaher mechanical properies shows ha he laminaion of microporous PU film considerably increases he srengh of laminaed sysem (K s = 4 23 %). I can be aribued o he leaher surface defecs "repairing" by he adhesive layer. Anoher possible explanaion of hese resuls may be relaed o he effec of "defecs locking", i.e., o he dissipaion of kineic energy, which releases during he ac of elemenary failure [9]. Sress-srain curves (σ ε) of he laminaed leaher and is layers a he differen sages of manufacuring are shown in Fig. 3. From he characer of σ ε curve follows ha mechanical properies of differen layers such as leaher and elasomeric PU film differ radically. The polymeric film has significanly higher elongaion a break (higher han 3 %) comparing o ha of he leaher (36 4 %). However, he adhesive increases he elongaion a break of he spli leaher abou.5 imes (sample S4 in Table 2). I may be considered ha low viscosiy elasomeric adhesive peneraes in o he leaher pores and oher gaps, affecs as plasicizing agen and repairs he surface defecs under he loading. The deformaion properies of he laminaed leaher S5 is very similar o hose of he spli or grounded leahers. However, i should be poined ou ha a he beginning he deformabiliy of he hybrid leaher is slighly lower (for S5 a σ s = 9.8 MPa elongaion ε s = 5 7 % comparing o 9 22 % of S and S2). I may be aribued o he ineracion beween wo layers, which resuls on he increase of surfaces siffness of conaced layers. σ, MPa ε, % Fig. 3. Sress-srain curves for laminaed leaher and is layers: PU film S4; 2 spli leaher wih adhesive layer S3; 3 grounded leaher S2; 4 laminaed leaher S5 Elasiciy moduli a break E b of all leaher samples are considerably higher han ha of PU film (Table 2). Moreover, in he case of spli leaher wihou or wih ground coaing or adhesive layer modulus E b is slighly higher han ha deermined a he shaping sress σ s = 9.8 MPa; whereas for he hybrid leaher he opposie phenomenon is observed. In his case E b is significanly higher han E s (68.8 MPa and 58.9 MPa, respecively). As can be seen from he daa, presened in Table 2, for PU film he markedly higher elasiciy modulus a low srain is characerisic, also. In his case Young modulus is E Y = 8 MPa, while modulus a break E b reaches only 2.3 MPa. Thus, above menioned variaion of hybrid leaher modulus can be explained by he influence of microporous PU film on is mechanical behaviour Sress relaxaion behaviour I is ineresing o deermine he sress relaxaion behaviour in he waerproof and breahable film laminaed leaher - relaively new maerial used in he manufacuring of foowear for professional use, spor and acive leisure. The load-elongaion and recovery behaviour of leaher and polyurehane film, which are used in Permair leaher manufacuring, shows considerable differences, which arise mainly from he differences in he layers srucure (Fig. 4). Under he loading he srucural elemens of leaher shif wih respec o each oher. Consequenly, sress reduces and residual srains are closely conneced o he leaher opographical zone, naure and size of defecs, sor and age of cale, ec. Meanwhile, elasomers are characerized by srong reversible deformaion ha impedes he fixaion of given shape. I is supposed ha sress in polymeric maerials becomes seady afer s of relaxaion [2]. From Fig. 4, a i seems ha sresses in all invesigaed samples become nearly consan afer 5 s of relaxaion. However, he ransfer of experimenal resuls ino he 'σ log ' scale demonsraes ha he process is sill in progress boh in he leaher and polyurehane film (Fig. 4, b). 76

5 The aemp o obain saisfacory predicion of longerm viscoelasic behaviour of he hybrid leaher using generalized Maxwell model (Fig. ) was made. Using he experimenal daa, presened in Fig. 4, he sress relaxaion behaviour was described by Eq. (). Calculaion of elasiciy consan D of a single spring of Maxwell model by he mehod proposed in [6], leads o conclude ha sress σ 5 comprises % of he sress equilibrium value σ. Alhough D of he microporous polyurehane film has he lowes value (6 imes lower comparing o ha of esed leahers), i has no pracical impac on D value of hybrid leaher i is in he same level as in he cases of semi-finished leahers (Table 3). 2 nex periods he elasiciy consans of all maerials o be esed become similar. Fig. 4 compares he prediced and measured sress relaxaion daa. The heoreical sress relaxaion curves were obained by inlaying in Eq. () he calculaed values of elasiciy consans D and D i. I is eviden ha heoreical predicion is excellen. On he oher hand, consans D and D i may be used parially for he inerpreaion of he relaxaion process as well. D, MPa σ, MPa σ, MPa , s a,, s Fig. 5. Dependence of consans D i of Maxwell model of laminaed leaher layers on relaxaion period τ i. (Curves descripion as in Fig. 4) The inadequacy dispersion and he error of measuremen [2] were used for he evaluaion of adequacy of he heoreical sress changes o he experimenal daa. The accuracy daa of he sress relaxaion behaviour evaluaion in he maerials analysed are presened in Table 4. The comparison of he prediced and measured sress relaxaion daa shows ha heoreical predicion by he generalized Maxwell model, which represens linear viscoelasiciy, successfully describes leaher behaviour in he non-linear regions. Table 4. The inadequacy dispersions s 2 ad and measuremen error e of sress relaxaion evaluaion by Maxwell model Sample s 2 ad e, s b Fig. 4. Sress relaxaion in he laminaed leaher and is layers (,,,, experimenal poins; heoreical curves by Eq. ()): S, S2, S3, S4, S5; a σ ; b σ log Table 3. Elasiciy consan D (MPa) of Maxwell model Samples S S2 S3 S4 S Changes of elasiciy consan D i during he relaxaion ime, deermined by he approximaion of sress relaxaion curves by regular specrum of discree relaxaion periods, are presened in Fig. 5. As can be seen, he inensiy of he shores ime of relaxaion for he PU film also markedly differs from ha of leahers (values of D ). However, in he S S S S S In order o demonsrae he inensiy of reducion of sress in maerials he rae of sress relaxaion can be used [2, 5]. Fig. 6 shows values of σ / σ as a funcion of ime (where σ is a maximum sress a he insan =, i.e., he insan a which he desired srain is reached, and σ is a sress a subsequen imes ) for all invesigaed samples. The rae of sress relaxaion decreases in ime for boh he laminaed leaher and is layers. I was observed ha sress relaxaion rae of semi-finished leahers is lower han in he case of laminaed leaher or film, and pracically did no depend on heir finishing. Alhough sress occurring in PU film is he lowes, he inensiy of sress decreasing is highes; already afer 5 s sress in he film reduces in 22 %, while in spli leahers only in 4 %. I may be supposed ha polymeric film increases he sress relaxaion rae of he laminaed leaher. 77

6 In all cases experimenal poins in he curves σ / σ appear o lie on he wo sraigh lines. The firs line refers o a greaer slope and applies for he shor imes, while he second line is for longer imes. The iniial relaxaion may arise due o rearrangemen or reorienaion in he maerial srucure. The differences of he mechanism of sress relaxaion of various maerials may be esimaed by he locaion of he inersecion of lines. I should be poined ou ha he locaion of inersecion of wo lines depends on he maerials naure and represens he changes over ime, a which he mechanism operaing a shor ime exhaused. I is obvious ha he rearrangemens in he leaher do no depend on he finishing processes (Fig. 6). So, he inersecion of lines for all semi-finished leahers is observed approximaely a he same ime l 5 s. Meanwhile, for PU film his ime equals only f = s. So, combining elasic leaher wih viscoelasic PU film he changes in hybrid sysem long-erm behaviour may be expeced. I confirms resuls, presened in Fig. 6: PU film reduces he ime of iniial relaxaion in he laminaed leaher. Due o ha he posiion of lines inersecion of he laminaed leaher curve decreases from 5 s down o h = 2 s. I follows, by similar reasoning, ha iniial relaxaion region arises due o he rearrangemen or reorienaion a he leaher and polyurehane film inerface. σ/σ, % f h l, s Fig. 6. Relaive sress relaxaion curves for laminaed leaher and is layers. (Curves descripion as in Fig. 4) Thus, he esimaion of he conribuion of polyurehane film on he sress relaxaion indicaes ha viscoelasic naure of he film dominaes principally on he long-erm properies of he hybrid leaher. The shape sabiliy of laminaed leaher will considerably differ from common finished leahers. I may be noiced in he sage of design and manufacure of leaher producs. CONCLUSIONS Mechanical properies and viscoelasic behaviour of microporous polyurehane film laminaed leaher depend on he srucure of he layers from which i is composed. Polyurehane film provides leaher wih he enire complex of high performance parameers such as resisance o waer and high breahable feaures. A he same ime i changes sysem properies and behaviour under he loading. Leaher backing mainly influences on he mechanical behaviour of he hybrid sysem, while polymeric film deformabiliy. For laminaed leaher combinaive srenghening is characerisic due o he repairing of leaher surface defecs by adhesive layer and kineic energy dissipaion during failure. The linear viscoelasic model such as generalized Maxwell model can be used o predic non-linear sress relaxaion behaviour a consan srain levels up o abou 2 %. For his purpose regular discree relaxaion ime specrum can be used. I is expeced ha his mehod can be applied for he predicion of creep behaviour of laminaed leaher. Sress relaxaion rae is markedly higher in he polymeric film han in he leaher. The film increases laminaed leaher sress relaxaion rae due o he rearrangemen or reorienaion a he leaher and polyurehane film inerface. So, he viscoelasic naure of he film dominaes on he long-erm properies of he hybrid leaher. Acknowledgmens Auhors express heir acknowledgemen o qualiy conrol manager D. Danilaiiene of Join Sock Company Šiaulių Sumbras for he provided maerial samples necessary for he research. REFERENCES. Thorsensen, T. C. Pracical Leaher Technology, 4 h ed. New York, Shoe Trade Publishing, 993: 34 p. 2. Coaings Technology Handbook, 2 nd ed. Ed. by D. Saas, A. A.Tracon. New York, Marcel Dekker Inc., 2: 92 p. 3. Sarkar, K. T. Reanning, Dyeing and Finishing of Leaher. New York, Shoe Trade Publishing, Proline Ihr Haunaher Schuz, Lainiere de Picardie, Laminaed Texiles Deparmen, France, Huren, J., Spijkers, J. Laminierung von Sympaex Membranen Adhasion Kleben & Dichen 2 997: pp Wolf, H. Spanungs - und Dehnungsrelaxaion an Leder Leder- und Hauemark 2 998: pp Burmisrov, A. G., Kocherov, A. V. Evaluaion of Maerials Qualiy by Compuer-aided Complex Relax Leaher and Foowear Manufacuring 998: pp. 7 9 (in Russian). 8. Burmisrov, A. G., Chiursin, V. I., Manukian, A. M. Auomaic Conrol of Deformaion Properies of Leaher by Means of he Apparaus Relax Leaher and Foowear Manufacuring 4 2: pp (in Russian). 9. Bukin, A. N., Kalia, A. N. Influence of Heaing and Temperaure of Foowear Maerials on heir Relaxaion Behaviour Knowledge of Universiies. Technology of Ligh Indusry 2 98: pp (in Russian).. Gvoziakaja, V. A., Adigezalov, L. I., Shvarc, A. S. Influence of Pre-srain on he Relaxaion Properies of Chrome-anned Leaher Leaher and Foowear Manufacuring 8 98: pp. 58 (in Russian).. Kravchenko, A. D. Influence of Sress Relaxaion in Chrome-anned Leaher on he Foowear Shape Sabiliy and use Properies Leaher and Foowear Manufacuring 2 972: pp (in Russian). 78

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