C H A P T E R - 3

MAGNESIUM OXIDE/ZINC OXIDE COMBINATION

EFFICIENT STABILIZER FOR POLY(VINYL CHLORIDE)

50

MAGNESIUMOXIDE/ZINC OXIPE COMBINATIQN--EFFICIENT

sTApBIL1 ZER FOR POLY (VINYL g _C_HLQRID_E)

Poly(vinyl chloride) (PVC) surpasses every otherpolymer in diversity of applications. This versatilitystems from its ability to be compounded into various flexi­ble and rigid forms with good physical, chemical and weather­ing properties. In addition, PVC has broad processabilityincluding calendering, extrusion, moulding (injection,compression, rotational, blow), fluid (solution, latex,organosol, plastisol) coating and impregnation. This broadmenu of PVC technology has generated an almost endlessformulary of PVC compounds. PVC formulations can contain

varying amounts of the following ingredients in order togive the polymer the desired processing and end-use chara­cteristicsl.

PlasticizersMonomericsOligomerics

Polymeric_modifiersSolid plasticizersImpact modifiersHeat distortion improversProcessing aids

FillersHeat stabilizersLubricants

51

52

ColourantsLight stabilizersBlowing agentsFlame retardantsDeodourantsAntistats

Heat stabilizer is singly the most important‘ingredient in PVC compounds and it is incorporated in allPVC compositions to protect the polymer against thermaldegradation at the high temperatures of processing(generally ranging from 170°C to 220°C depending upon

the amount of plasticizer and other ingredients) and alsosubsequently in service.

PVC degradation and stabilization

If unstabilized PVC is exposed to heat (tempera­ture above 100°C), ultra-violet light or gamma radiation,then depending on the intensity of exposure and type of PVC,a cleavage of HCl accompanied by polyene sequence formation

and crosslinking along the chain or intermolecularly, canoccur2-4. As a result, the polymer discolours from yellowto dark brown, the solubility in various solvents decreases,and the original mechanical and electrical properties of thePVC deteriorate rapidly5’6. All these effects are intensi­fied in the presence of oxygen. The thermal degradation of

53

PVC is effected by an auto-catalytic dehydrochlorinationreaction due to the presence of labile sites in the polymerchains such as allyl chlorines and tertiary hydrogen andchlorine atoms, or by terminal end groups such as doublebonds or peroxide residues7. If PVC in reality did corres­pond to the idealized concept of regular repeating unitsshown in Fig.3.1(A) and nothing else, it would be expectedto be a polymer of remarkable inherent stability, asevidenced by thermogravimetric analysis and other studiesmade on similar low molecular weight model compounds suchas 1,3,5-trichlorohexane8’9. The fact that commercial PVC

does not perform in a manner predicted by the model system

led investigators to examine other model systems repre­sentative of the possible structures that might logicallybe expected to be present in PVC as structural irregularities.These studies have shown that internal allylic chlorides arethe least stable (most susceptible to replacement) of allthe groups examined, followed in order by tertiary chloridesterminal allylic chlorides and secondary chlorideslo. Fromthis and from other evidence, it is postulated that thedegradation of PVC is due initially to a loss of hydrogenchloride, which commences at a site on the molecule eithercontaining or adjacent to a tertiary or allylic chlorideatom, either of which can function as an activating groupas shown in Fig.3.l(B). The elimination of the first molecule

(3-—I

Cl’

i

-:1:

Cl

_

(7

6‘)

n1

I_._I__

I___

I__.Le ,

3

A PVC repeatlng unit

-—CH2-CHCl—CH2—CHCl— CH2— CHCl—-‘X-—

‘ 1-Hca-—CH2—CHCl— CH2—CHCl—CH=CH— x —

B. Unzlpplng of HCl under the actlvatlon of group X

-—CH=CH—CH=CH— CH=CH—X —

C A chaln segment of poly unsaturatlon

Flg 3 l Steps 1n PVC degradatlon

SS

of hydrogen chloride and the subsequent formation of anunsaturated double bond on the PVC chain then activates

the neighbouring chlorine atom, which is now structurallylocated as an allylic chlorine, and thus facilitates thesubsequent elimination of another hydrogen chloride mole­cule, with the process continuing to repeat itself. Thisprogressive dehydrochlorination proceeds quite rapidly,soon leading to a chain segment of polyunsaturationll asshown in Fig.3.1(C). In the presence of air, the dehydro­chlorination is a nonlinear process with a tendency toincrease in rate in the course of the reaction, and the

polyene sequence can react with O2 to produce otherstructures that favour degradationl2. For this reason

degradation in the presence of heat and air is termedthermo-oxidative degradation. The mechanism of thermo­

oxidative degradation of PVC is easily explained in termsof the general oxidation of saturated hydrocarbons. Thedehydrochlorination reaction is also typical for the highenergy radiation degradation process of PVC13.

HCl acceptance is unquestionably the most importantfeature of any chemical substance used as a thermal stabilizerfor PVC because of the catalytic effect of HCl on degra­dation13'14. The second preventive function of a stabilizeris the exclusion of labile starting sites for dehydrochlori­nation in the polymer. Other mechanisms by which heat

56

stabilizers could function are, modification of chainreactions so as to prevent the evolution of HCl, limitingthe formation of conjugated double bond sequences bypromoting addition reactions or functioning as an oxi­dation catalyst to stabilize the colour of PVC andinactivation of impurities that may occur in the polymeri­zation and subsequent workls.

Heat stabilizers for_Py§

In addition to providing heat stability an idealstabilizer should also

1) be readily dispersible in the PVC compound andfully compatible with all its constituents even afterprolonged service,

2) have no adverse effect on processing properties,

3) be equally effective in PVC resins of all typesand from all sources,

4) be inexpensive and effective in small proportions,

5) pose no health hazards both in the general handlingand processing in the factory and in the subsequent use ofthe stabilized PVC material.

57

The most important classes of heat stabilizerscan be briefly summarized as followslzz

1. Organo tin stabilizers

These are the most effective stabilizers availableunequalled in the degree of stabilization and the claritythey confer on PVC compounds. Their greatest drawback is

their high cost. Another disadvantage is their toxicitythough some of them are allowed in food-contact applicationsThe commonly used systems are:

(a) Sulphur - containing tin stabilizers (organo tinmercaptides and sulphides)

Because of their excellent efficiency they areused in situations where temperatures of 200°C and aboveoccur. They can be used in all formulations, but becauseof the absence of a lubricative function, they have to beused with lubricants. Because of the minimum migrationascertained, they can be used in mixtures for food pack­aging and for pipes for drinking water. General structuralformulae of sulphur containing tin stabilizers are shownin Fig.3.2(A).

_R S-F? S­RI\ /S 2 I 2 I ISn or F21-Sn-S—R2 0" R1—Sn—S—Sn—RR2“ é-R é R Q2 -' 2 '_R1/ \S_

A Sulphur contalning tin stablllzers

F%1\\\ I//C)C:C)Fz2Sn

R1/ \OCOR2

B Sulphur free t1n stabllizers

(RCOO)2 Ba and (RCOO)2Cd

C. Barium-Cadmium stabilizers

(RCOO)2 Ba and (RCOO)2Zn

D Barium-Zlnc StablllZ€IS

(/"I

IU|\>

R2

F19 3 2 General formulae of lmportant classes of PVC stablllze IS

59

(b) Sulphur.free tin stabilizers (organo tin carboxylatesor salts of alkyl tin oxide with maleic acid or partlyesterified maleic acid)

In contrast to the sulphur containing tin compounds,the tin carboxylates have to be used with antioxidants.Their advantage over the sulphur containing ones is thegood photostability and the lack of odour. These stabili­zers, too, need additional lubricants. General structuralformulae of sulphur free tin stabilizers are shown inFig.3.2(B).

2. Barium-cadmium stabilizers

The two compounds are used together because of

the synergism (interplay by combination of the individualactions) in the various mixture ratios. Generally costabili­zers (phosphites), antioxidants (biphenols), and polyols ofthese mixtures are used to counteract the destabilizingeffect of cadmium chloride that results from the subsequentstabilization reaction. The greatest disadvantage of thisstabilizer system is the toxicity, predominantly of thecadmium part. General structural formulae of Ba-Cd stabili­zers are shown in Fig.3.2(C).

3. Barium-zinc stabilizers

These are used in various mixtures because of thesame synergistic effect as that of the Ba—Cd stabilizers.

60

The conversion of Zn carboxylate into ZnCl2 during thestabilization action produces an even stronger destabili­

zation effect than that of CdCl2 (ZnCl2 is a much strongerLewis acid than CdCl2). Hence the comixture with phosphites9P0xidized fatty acid esters, and polyols in compoundsis indispensible. This comixture neutralizes the zincchloride in its action. Recently, because of the toxi­cological action of cadmium compounds, these stabilizershave found increasing use. General structural formulaeof Ba-Zn stabilizers are shown in Fig.3.2(D).

4- Calcium-Zin¢ Stabilizers

These have structural formulae similar to thatof the Ba-Cd or Ba-Zn stabilizers. They have recentlybeen finding many applications despite their very muchweaker effect, because of, two non-toxic compounds; calciumand zinc stearate. They are always used in combinationwith costabilizers, epoxy plasticizers and antioxidants.

5- Leas stséilizsss

The lead stabilizers are inorganic compounds whichcan be used with lead stearate in various combinations:

sulphates, phosphates, or carbonates. The most important

advantage of the stabilizing systems based on lead compounds

is the formation of lead chloride (PbCl2), which has no

61

destabilizing effect like that of ZnCl2 or CdCl2 and doesnot increase the conductivity properties of PVC. For thelatter reason, they are used in cable insulation.

6. Metal-freestabilizers

These are being increasingly discovered andinvestigated. The oldest of these are diphenyl thiourea

and at-phenyl indole. §-amino crotonic acid and its estersare newcomers in organic stabilizers.

7. Antioxidants

Antioxidants, mostly phenolic compounds such as

bisphenol A or 2,6-di-tert-butyl-p-cresol are sometimesadded to improve the action of the heat stabilizers.

8. UV absorbers

when PVC is used outside (window frames, foils,

sheeting, roofing etc.) UV absorbers are also generallyincorporated.

M9_Q/ 3119., §OM.BlNP~T10N FEAT. STE-‘:-13,11-*1,-ZEF FOR PVQ

MgO is an efficient acid acceptor and ZnO isassociated with good heat ageingls. Hence MgO/ZnO combination

in presence of a common lubricant, stearic acid, was triedas a heat stabilizer for PVC.

62

EXPerim@nP@l

1. Studies on_aBrabenderPlasticorde£

The study was conducted on a Brabender Plasticordermodel PL3S. The test parameters are shown in Table 3.1. PVCplasticized with SO phr (parts per hundred PVC resin) dioctyl

l

phthalate (DOP) was selected for investigation to keep themixing/test temperature around 170°C - 180°C.

Table 3.1

Test parameters

BraPender P1aS§i¢@¥deP

Roller Mixer : Type 30Mixer Temperature : 175°CRotor speed : 30 rev. min-1Measuring range : O-2500 metergrams

PVC 9°mPQHP§

Sample weight : 30 gmsPlasticizer content : 50 phrVariables : MgO, ZnO and stearic acidTest time : 15 mins.

63

PVC compounds with varying amounts of MgO, ZnO and stearic

acid were prepared using a high speed mixer and then run onthe Brabender plasticorder for 15 minutes and the torquecurves were taken. Initially, formulations containing MgOand ZnO at equal concentrations of 4 phr, 3 phr and 2 phr,keeping the concentration of stearic acid constant at 2 phrwere selected for study. For comparison, a formulationwith 4 phr of tribasic lead sulphate (TBLS), a conventionalPVC stabilizer, was also chosen. The torque curves of thesecompounds are shown in Fig.3.3. To find the most efficientcombination of MgO and ZnO, they were then added in differentconcentrations of 4 phr MgO/5phr ZnO, 3phr MgO/5phr ZnO,

2phr MgO/Sphr ZnO, Zphr MgO/4phr ZnO and 1 phr MgO/Sphr ZnO,

again keeping stearic acid at a constant level of 2 phr.The Brabender torque curves of these compounds are shown

in Fig.3.4.

In order to study the effect of stearic acid onstabilization, its concentration was varied, keeping theamounts of MgO and ZnO the same. Concentrations of stearic

acid tried were 2phr, 3phr, 4phr and Sphr, keeping theconcentrations of MgO and ZnO constant at 3phr and Sphr

respectively. To study the colour flexibility of thecompounds stabilized with MgO/ZnO/stearic acid system

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66

a formulation containing 2 phr titanium dioxide was alsoselected. The Brabender torque curves of these compoundsare shown in Fig.3.5.

Finally each of the three additives, MgO, ZnOand stearic acid was also completely withdrawn to assessthe importance of each additive on stabilization. Theformulations tried were 3 phr MgO and 5 phr ZnO with nostearic acid, 4 phr ZnO and 2 phr stearic acid with noMgO, and 4 phr MgO and 2 phr stearic acid with no ZnO.

The Brabender torque curves of these compounds are shownin Fig.3.6.

The sequence of colour development for each ofthe above compound during the 15 minutes of test run onthe Brabender Plasticorder was visually observed. Thenumbers in Fig.3.? represent the degree of colour develop­ment over a scale17 0-10 (O = colourless, 10 = black).

2. Oven stability_test18'19

The compounds, after the test run on the BrabenderPlasticorder, were subjected to the oven stability test at180°C (i5°C). The sequence of colour changes of the materialwas observed at intervals of 10 minutes for 110 minutes. The

results are shown in Fig.3.8. Formulations for Figs.3.7 and3.8 are given in Table 3.2.

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3- 90999 redtestle

Around 2 gms of the PVC compounds prepared for

the tests on the Brabender Plasticorder were subjected tothe Congo red test to assess whether there was any visualevidence of HCl evolution for 15 minutes.

Results and discussion

Torque curves of compounds with MgO and ZnO at

equal concentrations rose to a high torque level initially,dropped a little and then stabilized to a constant torquelevel (Fig.3.3). The initial unstable part may be the timetaken for fusion and homogenization. Hence, to a firstapproximation, the period over which the constant torquelevel continues may be taken to be a measure of the effi­ciency of the stabilizing system. For all the three compoundswith MgQ/ZnO system, this period extends over the test timeof 15 minutes. However, the stable torque value is slightlydifferent for the three compounds. This might indicate thatthe PVC phase is not fully homogenized at the temperature and/or shear rates encountered in the study. The higher torquevalues displayed by the compounds with higher amounts of MgO

and ZnO might imply that a larger amount of PVC melts when

the concentration of MgO and ZnO are increased due to theirlubricant action. The compound with 4 phr TBLS also shows astable behaviour as expected. However, its torque curve is

73

below those of the compounds containing MgO and ZnO, which

indicates that the lubricant action of 4 phr TBLS is notsufficient to homogenize the PVC phase fully at the tempera­ture and/or shear rate employed. For this compound the torqueis found to rise gradually during the test run which furthershows that the amount of fused and homogenized PVC increases

during the test run.

The behaviour of the PVC compounds with MgO and

ZnO at different concentrations is more or less similar(Fig.3.4). All the compounds, except the one with 1 phrMgO/5 phr ZnO display stable behaviour. The torque curveof the compound which contains 1 phr MgO and 5 phr ZnO

stabilizes to a constant torque level after the initialunstable part. But just before the end of the test-time,the torque is found to increase rapidly. This may be dueto the degradative crosslinking of the PVC matrix in turndue to the low level of MgO which is insufficient to provideefficient stabilization.

All the torque-curves in Fig.3.5 show a stablebehaviour. Varying the concentration of stearic acid,keeping the amounts of MgO and ZnO at the constant levelsof 3 phr and 5 phr respectively, does not seem to affectstability. 2 phr of stearic acid seems to be sufficient

74

for efficient stabilization. Increasing the amount ofstearic acid from 4 phr to 5 phr brings down the stabletorque value noticeably, indicating that at these levelsof stearic acid, the fusion of PVC particles is almostcomplete and hence the lubricant action alone has the sayin the torque value. Presence of 2 phr titanium dioxidedoes not seem to affect the stability or colour. Thismight imply that the colour flexibility of PVC compounds

will not be impaired when this stabilizer system is employed.

The behaviour of the compounds when each of the

additives, MgO, ZnO and stearic acid, is completely with­drawn is interesting (Fig.3.6). The stability is found to 'be least when there is no MgO. Even before stabilizingproperly to a constant level, the torque curve rises againto a maximum value. This is obviously due to the increasein torque resulting from degradative crosslinking. Aftercrosslinking, the material breaks down to powder and hencethe torque reduces to zero. when there is no ZnO, the stableperiod seems to extend over the test time. However, thecompound becomes yellow in the Brabender plasticorder indi­cating inefficient stabilization. However the onset ofcrosslinking, indicating large scale degradation is notevident from the torque curve in this case. when there isno stearic acid, the stable period does not extend over the

75

test time. The torque increases before the end of testtime indicating severe degradation. Thus all the threeingredients MgO, ZnO and stearic acid are found to beessential for efficient stabilization.

An examination of the sequence of colour changesobserved while the compounds were under the test run onthe Brabender plasticorder (Fig.3.7 and Table 3.2) shows

that the compounds that became highly discoloured werethose in which MgO or stearic acid was completely absent(formulations 14 and 15). This further shows that thesetwo are the most essential ingredients of the stabilizersystem. The compound without ZnO (formulation 16) alsobecomes discoloured indicating that ZnO is also an indis­pensible ingredient. Other formulations which got badlydiscoloured indicating insufficient stabilization wereformulation 5 in which concentration of MgO was only 1 phrand formulation 1 in which the concentration of ZnO was

only 2 phr. These concentrations seem to be less than theminimum levels of MgO and ZnO required for efficient stabi­

lization. It may be inferred that a minimum of 2 phr MgO and3 phr of ZnO are required for efficient stabilization.

Results from the oven stability test (Fig.3.8 andTable 3.2) confirm the observations made above. It further

76

shows that compounds with 4 phr MgO/4 phr ZnO/2 phr stearic

acid, 3 phr MgO/5 phr ZnO/2 phr stearic acid, 4 phr MgQ/5 phr ZnO/2 phr stearic acid are very stable. Using MgOabove 4 phr and ZnO above 5 phr does not seem to improve

the efficiency any further. It seems that the optimum con­centration of MgO is about 3 to 4 phr and that of ZnO about4 to 5 phr. The concentration of stearic acid needs to beonly 2 to 3 phr.

The above conclusion was further substantiated bythe results of the Congo red test. Compounds in which MgO,ZnO or stearic acid was completely absent or MgO and ZnO

present in less than the minimum required level (formulations1, 5, 14, 15 and 16 of Table 3.2) were seen to be the leaststable.

The products from the tests on the Brabender Plasti­corder were subjected to natural weather conditions for sixmonths. Compounds containing more than the minimum levels

of MgO or ZnO with stearic acid showed barely any detectablechange. This might imply that this stabilizer system hasreasonable resistance against ultraviolet light and weathering.

Mechanism of stabilization

An attempt was made at explaining the possible mechanism

of stabilization by the MgQ/ZnO/stearic acid combination by

77

Infrared (IR) spectroscopy. IR spectra of the combinationsgiven in Table 3.3 were taken in KBr pellets on a PerkinElmer model 983 spectrophotometer. The concentration of theingredients in the combinations was based on one of the bestformulations obtained by the studies on the Brabender Plasti­corder (4 phr MgO/4 phr ZnO/2 phr stearic acid).

Table 3.3

Combinations employed for IR scan

1. Stearic acid (Fig.3.9)

2. MgO + Stearic acid, heated at 175°C for 5 minutes(Fig.3.1O)

3. ZnO + Stearic acid, heated at 175°C for 5 minutes(Fig.3.11)

4. MgO + ZnO + Stearic acid (Fig.3.12)

5. Mg0 + ZnO + Stearic acid, heated at 175°C for 5 minutes(Fig.3.13)

6. MgO + ZnO + Stearic acid heated at 175°C for 5 minutes +Hc1* (Fig.3.14)

*Mixture and PVC samples were sealed in extreme ends of thesame glass tube, and the side containing PVC inserted intoan oven so as to form HCl which can be reacted by the thermalstabilizer.

Fig.3.9 shows the IR spectrum of stearic acid aloneand the absorption at around 1700 cm_1 of the carbonyl groupzo

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84

was made use of for this study. The reaction of MgO andstearic acid produces magnesium stearate as shown by theshifting of the carbonyl group to the new position20(Fig.3.1O). The reaction between ZnO and stearic acid alsoforms zinc stearate (Fig.3.11). Fig.3.12 and Fig.3.13look similar which indicates that the stearate formationcould take place even at room temperature. This showsthat the stabilization action of the MgQ/ZnO/stearic acidsystem might be similar to that of a mixture of magnesiumstearate and zinc stearate which in turn might be similarto the action of the barium-zinc stabilizers or the calcium­zinc stabilizers. Hence this study shows that instead of

using magnesium and zinc stearates proper, it is enough touse the corresponding metal oxides and stearic acid. whenHCl, evolved by the degradation of PVC, is passed over themixture, stearic acid is produced (Fig.3.14) necessarilywith the formation of the corresponding chlorides. Thepresence of chloride in the mixture has also been confirmedby chemical analysis: The overall chemical reaction couldthus be represented as

MgO + 2C17H35COOH .___; Mg(OCOC17H35)2 + H20

ZnO + 2C1.7H35COOI-I —-> Zn (ococ171-135)9_ + H20

Mg(OCOC17H35)2 + 2HCl -__9.MgCl2 + 2C17H35COOH

Zn (OCOC17I—I35)2 + 2I—ICl --> ZnCl2 + 2c171-135coo1-1

85

The overall mechanism might then be a synergismbetween the magnesium and zinc stearates as in the case ofbarium-zinc stabilizers or calcium-zinc stabilizers. Hencethe observations made from the Brabender studies could be

explained as follows.

1. The system was found to be least efficient when therewas no MgO. In this case, the zinc stearate formedcould rapidly fix the HCl evolved due to PVC degradation.However, the zinc chloride formed produces a strongdestabilization effectlz.

2. Stearic acid was found to be the second most importantingredient in the system. In the absence of stearicacid, the fixation of HCl by the metal oxides would beslow reducing the efficiency of stabilization.

3. when all the three ingredients were present in optimumconcentrations, the system was found to be an efficientstabilizer for plasticized PVC. In this case also zincstearate rapidly fixes the HCl evolved. However, zincchloride formed could immediately react with magnesium

stearate forming magnesium chloride and thereby regenerat­ing zinc stearate. Thus the system neutralises thedestabilizing effect of zinc chloride. Such a mechanismbased on the regeneration of cadmium or zinc alkyl­

86

carboxylate from their chlorides by the action ofbarium or calcium alkylcarboxylate is considered tobe the synergistic effect of barium-cadmium andcalcium-zinc soap mixtures21_23.

Yet another mechanism by which this system provides

stability could be by the replacement of labile tertiarychlorine atoms by ZnO and thereby crosslinking two PVC

molecules as in the case of the crosslinking of polychloro­prene rubber by ZnO24. lt has been verified by swellingstudies that a small amount of crosslinking, which does notinterfere with processing, does occur in the PVC matrixwhen the MgO/ZnO/stearic acid system is used for stabiliza­tion. Further, it has been established that zinc oxidecould act as a crosslinking agent for PVC in presence oftetra methylthiuram disulphide (TMTD)25. Such a small degreeof crosslinking might be present in the PVC phase stabilizedby conventional stabilizers like tribasic lead sulphate(TBLS) also. This is in conformity with the observation thatTBLS could act as a potential curative for polychloroprenerubber26 (Chapter 4).

gechanical properties of plasticized PVC stabilizedwith MgQ/ZnQ[Stearic acid system

4 phr MgO/4 phr ZnO/2 phr stearic acid, one of thebest combinations was chosen for determining the mechanical

87

properties of plasticized PVC with this stabilizer system.One compound with the conventional stabilizer, tribasiclead sulphate was also chosen for comparison. The formula­tions used for preparing test samples are shown in Table 3.4.

Table 3.4

Formulations used for determining mechanical properties

Formulation Q §PVC 100 100DOP 50 50MgO 4 -­ZnO 4 -­TBLS -— 4Stearic acid 2 2

The compounds were prepared on the Brabender Plasti­

corder at 175°C and pressed on a laboratory press at the sametemperature for2minutes using a mould which could be water

cooled immediately after pressing, keeping the sample stillunder compression. The dumb-bell specimens for testing tensileproperties were punched from these sheets. The tensile pro­perties were determined on a Zwick Universal Testing Machine

as per ASTM D412 (1980). The resistance to thermal ageing

88

of the samples was measured by keeping the samples at 100°C

for 48 hours in an air oven and then measuring the retentionin the tensile properties. The properties are shown inTable 3.5.

Table 3.5

Tensile properties of plasticized PVC

Property A BBefgre ageing

Tensile strength (MPa) 14.7 14.1Elongation at break (%> 260.10 276.26100% modulus (MPa) 8.8 8.3

after ageing

Tensile strength (MPa) 16.0 15.1Elongation of break (%) 232.12 245.54100% modulus (MPa) 9.1 8.7

The tensile properties of both samples A and B arecomparable. The PVC stabilized with Mg0/ZnO system shows

slightly higher tensile strength but slightly lower elongationat break. This might be due to a slightly higher degree of

89

crosslinking in the former sample. The properties afterthermal ageing suggest that the polymer has not been adverselyaffected by the ageing conducted at 100°C. Tensile strengthhas slightly improved in both samples A and B and elongationat break has come down a little. This suggests an increasein the crosslink density in the PVC matrix for both A and Bduring ageing.

Efficiency of MgO/ZnO stabilizers in presence of otherplasticizersand lubricants

Some plasticizers like epoxidized soyabean oil hasa stabilizing action on PVC in addition to the plasticizingaction27'28. The migration resistance of this plasticizeris also high, comparable to that of the polymeric plasti­cizers. MgQ/ZnO/stearic acid stabilizerswere tried in presenceof epoxidized soyabean oil to determine whether there was anyimprovement in the efficiency of stabilization. Calciumstearate has a dual role in PVC formulations both as a lubri—cant and heat stabilizer17. Hence it was also examinedwhether calcium stearate could improve the efficiency of theMgO/ZnO/stearic acid system if substituted in place of stearicacid. It has been stated that MgO and ZnO get converted totheir corresponding stearates and then fix the HCl. Hence,it would be interesting to note whether the same efficiencywould be retained if stearic acid is substituted with other

90

fatty acids like lauric acid. To compare the efficiency ofstabilization of these systems a dynamic stability test29was performed on the Brabender Plasticorder. The stabilitytimes of the various systems are given in Table 3.6. The mixertemperature was 175°C and rotor speed 30 rev/minute, and thesample weight 30 gms.

The stability time observed for MgO/ZnO/stearic acid system(A) is 37 minutes (Fig.2.1) . when DOP is substituted by epoxidized

soyabean oil (B) the stability time goes upto 47 minutes. Henceas with other systems, epoxidized soyabean oil behaves synergisti­cally with the MgO/ZnO/stearic acid system also. When stearicacid is substituted with calcium stearate (C) there is a markedimprovement in the efficiency of stabilization, as expected.Almost the same stability time is obtained, when stearic acidis substituted by lauric acid (D). This shows that the MgQ/ZnOsystem could function as an efficient stabilizer in presence offatty acids other than stearic acid.

‘gdyantages of MgO/ZnO/stearic acid system compared toother conventional stabilizers

The MgO/ZnO/stearic acid system has been identifiedas an efficient stabilizer system for plasticized PVC. Ithas the following advantages.

Table 3.6

91

Stability times of MgO/ZnO based stabilizer systems

Formulations

PVC - 100DOP - 50MgO - 4ZnO - 4Stearic acid - 3

PVC — 100Epoxidizedsoyabean oil - 50MgO - 4ZnO - 4Stearic acid - 3

PVC - 100DOP - 50MgO - 4ZnO - 4Calciumstearate - 3PVC — 100DOP - 50MgO - 4ZnO - 4Lauric acid - 3

Stability time,mins.

37

47

55

37

Non-toxic: hence could be used in food contactapplications

Could be used in blends of rubbers with PVC,

since all the three ingredients MgO, ZnO andstearic acid are conventional ingredients inrubber compounding

Inexpensive

Does not impair colour

Odourless

Stable in the range of processing temperaturesfor PVC

Non-sulphide staining, hence could be used inblends of PVC with sulphur vulcanizable rubbers

93

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