Post-irradiation degradation of polypropylene: Comparison of hindered aromatic phenole and amine as gamma-stabilizer—I

Radiat. Phys. Chem. Vol.38, No. 3, pp. 295-301,1991 Int. J. Radiat. AppL lnstrum., Part C

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POST-IRRADIATION DEGRADATION OF POLYPROPYLENE: COMPARISON OF HINDERED AROMATIC PHENOLE AND AMINE AS GAMMA-STABILIZER--I A. A. KATBAB and A. Y. MOUSHIRABADI Polymer Engineering Department, Amirkabir University, Tehran, Iran and y-Ray Centre, Atomic Energy Organization, Iran (Received 9 January 1991) Abstract--Post-irradiation stability of 6°Co-~,-ray irradiated isotactic PP has been studied at 25 and 50 kGy. The effectiveness of 2,6-di-tert-butyl-p-cresol (BHT) and polymerized 2,2,4-trimethyl-l,2-dihydroquinoline (TMQ) at various concentrations is compared. The variation of crystallinity, carbonyl and hydroxyl index, mechanical properties, of stabilized and unstabilized PP was followed for 6 months after irradiation. Although the percentage of crystallinity of the unstabilized samples increased upon irradiation, but for all stabilized ones not only the initial value was retained but remained constant and lower to that of the unstabilized samples during storage. Tensile strength and bend strength was also retained. The extent of chain scission has been followed by measuring the change in apparent viscosity (pa) and flowability parameters, (K) and (n), using power law equation. Although both types of stabilizers were highly effective, the amine type imparts more stability at a longer time after irradiation as the corresponding nitroxyl and hydroxyl amine groups are formed.

INTRODUCTION

Irradiation sterilization both by v-ray and electron beam has been widely used for medical supplies. Polypropylene is one of the mostly used polymers for manufacturing disposable medical articles. This polymer suffers from severe deterioration of properties when irradiated under the required dose for sterilization (Williams et al., 1982; Dunn and Williams, 1983). The change in properties and chemical structure of the polymer continues after being irradiated which is mainly due to the residual radicals trapped in the crystalline phase of the polymer (Dunn et al., 1975; Zahrah et al., 1989). Oxidation degradation of PP can be lowered by the use of radical scavengers (Dunn and Williams, 1983) and by the addition of special additives (Wenzin et al., 1985). Hindered phenols have been reported to prevent radiation degradation of PP (Stevens and Gruse, 1941). In the present work, the post-irradiation stabilization of medical grade isotactic polypropylene with or without stabilizer irradiated by 6°Co-gamma ray at 25 and 50 kGy has been investigated. Different concentration to retard the change in chemical and mechanical properties of the irradiated PP on storage till 6 months has been studied. EXPERIMENTAL WORK

Materials Lightly stabilized isotactic PP powder was supplied by Hoechst Company (Germany) with the grade,

Hostalen PPT. The melting point and percentage crystallinity was 161.4°C and 45% measured by mettler TA3000 DSC. The type of stabilizers used were 2,6-di-tert-butyl p-crsol (BHT) obtained from Ciba-Geigy Co., and polymerized 2,2,4-trimethyl1,2 = dihydroquinoline (TMQ) obtained from Monsanto Co. CH3

OH tBu ~ ~ j , 7

t Bu

~

H

cHjCH3 n

CH3 Sample preparation and irradiation Laboratory size extruder model BXI2 made by "Axon ab" Company (Sweden) was used to mix stabilizers with different concentrations (0.05, 0.7, 0.15, 0.2wt%) with PP. The mix was extruded at the minimum screw speed (65 rpm) through a circular die (OD = 8 mm). The extruded PP was passed through a water bath and then ground by Retch Muhle grinder. To keep the heat history, the unstabilized PP was also processed at the same condition. Table 1 shows the codes used throughout the project. To prepare test samples, extruded PP was chipped and compression moulded into sheets of 1.5 and 0.04 mm thickness suitable for mechnical testing and I R spectroscopy respectively. Table 2 shows the moulding condition.

295

296

A . A . KATBAB and A. Y. MOUSHIRABADI Table 1. Used codes for specimens

A FA = BP050 1.0

Stab. conc. (%) Stab. type Phenolic (BHT) Aminic (TMQ)

0.0

0.05

0.10

0.15

0.20

PPO PPO

PF1 PAl

PF2 PA2

PF3 PA3

PF4 PA4

0.9 0.8 0.7

IRRADIATION AND POST-IRRADIATION

Samples were irradiated in air with e°Co-7-ray at doses of 25 and 50 kGy and 15 kGy/h rate. All irradiated samples were kept in the presence of air at room temperature and tested at different intervals for 6 months.

0.6

(A)

I 1700

0.5 1800

Wovenumbers

I 1600 CM-1

Fig. l CARBONYL AND HYDROXYL INDEX

A FA : C F 4 5 0 1.0

The change in carbonyl and hydroxyl groups were followed by F T - I R model IFSL5 made by Bruker Company. The peak at 840 cm-~ was chosen as the reference peak for all samples as it remained constant during and after irradiation.

0.9 O.B 0.7

MEASUREMENT OF MECHANICAL PROPERTIES

Stress at yield (amax), stress at break (ab) and elongation to break (Eb) of all samples were measured by the curves obtained from the Zwick tensile tester model 1425, according to the ASTM D638 test method. Bend strength of samples was measured by the stress-angle curve obtained from the cast bending machine. The test carried out according to the ASTM D747 and results were recorded as the bending work to break (N. Rad/cm3).

0.6

(B) 0.5 1800

I 1700 Wovenumbers

I 1600 CM-1

Fig. 2 AFA:BP050

0.9 0.8

-

0.7

t

% CRYSTALLINITY

To measure percentage crystallinity of samples, differential scanning calorimetry (DSC) model TA3000 made by Mettler was used at a heating rate of 20°C/min. Percentage crystallinity was determined by comparing the enthalpy change (AHp) of the samples to that of a fully crystalline sample. The enthalpy change was obtained as the surface under the thermograph curve received from DSC.

0.6 0.5

(C)

3500

1 3400

I 3300

Wovenumber s CM-1

Fig. 3 AFA :CF450 1.0

RESULTS AND DISCUSSION 0.9

Figures 1-4 show the effect of a 0.2% phonolic type stabilizer (BHT) on the post-stabilization of PP irradiated at 50 kGy. The addition of 0.2% phenolic type stabilizer has substantially stabilized the polymer towards post-irradiation. The change in carbonyl index of irradiated PP and the effect of the stabilizer

0.8 0.7 0.6

(D)

0.5

Table 2. Moulding condition

Test Tensile Bending DMTA IR

Cavity depth (rnm)

Cavity width (ram)

1.5 1.5 1.5 0.04

150 150 150 --

Temp. Pressure (~C) (bar) 180 180 180 180

170 170 170 170

3500

Preheat Moulding time time (rain) (rain) 2 2 2 1

2 2 2 1

I 34-00

1 3300

W(:]Ve n UlTI be r s C M - 1

Fig. 4

concentration on the post-stabilization of this polymer are shown in Figs 5-8. It is obvious that both types of stabilizer are strongly effective especially at

Post-irradiation degradation of polypropylene FTIR 2 5 KGY

0.7 _

FTIR

0.7 0.6

0.6 0 ~P m

297

0.5

.j/

0.4

+

0

0.5 0.4

+~÷

I,....

0.3

h.

'~ 0.2

<

0.2

0.1

0.1 I 2 + PPO

I 4

Time ( m o n t h s ) C.PF1 APF2 xpF3

I 6

I 0 + PPO

vpg4

Fig. 5

0.6 +

0.5

+f+ f

OD

,,~ 0 . 4

{D

vPF4

FTIR 50KGY

0.7

0.6 0

I 6

Fig. 7

FTIR 2 5 KG Y

0.7

I

2 4 Time (months) OPF1 ~,PF2 xPF3

0.3

/°

0

0.5

<

0.4

,q. CO tO

•f~-

~

0.3

~

.~

-~

.x

<

"~ 0 . 2

0.2

0.1

v

v ---~-'-v~

0.1 I I 2 4 Time (month=) + PPO

oPAl

~PA2

xPA3

I 6 vPA4

Fig. 6

\

N. + PPo. + H20 / >

+ PPO

OPAl

aPA2

x PA3

vl:~4

Fig. 8

longer periods after irradiation. It has previously shown (Scott, 1980) that hindered phenols such as (BHT) are capable to stabilize oxidative degradation of polymer, and are converted to their corresponding bisphenols and bisquinone. This can explain the increase in the retarding effect of the stabilizer by time. Comparison of the effect of phenol and amine shows that although the amine type stabilizer prevents strongly the formation of carbonyl and hydroxy groups at a long time after irradiation, but is less effective during and early after irradiation. It has previously been shown (Carlsson et al., 1980; Thomas and Talman, 1962) that nitroxy radicals ( > N O . ) and hydroxylamine ( > N O H ) are the main product of the amine type stabilizers which are involved in the stabilization of polymer, and these products are strong scavengers for alkyl and peroxy radicals. Both types of stabilizers were significantly effective at 0.2% and the amine type did sensitise the polymer at concentration less that 0.1%. This could be explained to be due to the formation of polymeric alkoxy radicals (PPo.) according to the following reaction: > NH + PPooH--*

I I 2 4 Time(months)

0

o, NO.

% CRYSTALLINITY

The effect of molecular weight and its distribution (MW, MWD) on the rate of crystallinity of PP and PE has previously been investigated (Admad, 1982; Mark et al., 1984). According to these reports the decrease in molecular weight leads to the increase in the rate of crystallinity. In the present work, it was found that irradiation has a significant effect on the degree of crystallinity of both unstabilized and stabilized PP and therefore its post-stability (Figs 9-12). It is obvious that the degree of crystallinity of the unstabilized PP has increased upon irradiation (45% before radiation) and then declines by time. To measure the effect of radiation on the molecular weight, power law equation, a * = K ( ~ ) M, was used as the two parameters, K and n, has been shown to be related to the molecular weight and its distribution respectively (Admad, 1982; Mark et al., 1984; Deanin, 1972). Rheoscope (Ceast Model) was used for this purpose and the change in apparent viscosity and the values of: *a = shear stress, ~ = shear rate. K and n was followed from the a - ~ curves obtained, results are shown in Tables 3~5. The lower value for K in the case of unstabilized PP just after irradiation indicates the higher chain scission, therefore higher chain orderness in crystalline phase. The degree of crystallinity for all stabilized irradiated

298

A.A.

KATBAB a n d A . Y. MOUSHIRABADI

DSC 50 KGY

DSC 25 KGY

62 58

\

58

54

A

'2,

54

>, 5 0 .o =

50

~)

o 46

g

46 +

(3

42

42 36

I 2

0

+

I 4

36

1 2

0

Time

Ti me ( m o n t h s ) +PPO

OPF1

~PF2

I 4 (months)

XPF3

+ PPO

vPF4

+ PAl

,~ PAZ

Fig. 9

Fig.

OSC 5 0 KGY

~ PA3

v PA4

11

DSC 50KGY

58

\ A

54

54

~ 50 .E

46

~

__

+

A

&

46

x

_...~-x v

x

~)

?

42

38

42

I 0

I 2

I 4

J 6

36

Time (months) +PPO

~ PF1

~ PF2

x PF3

I 2

0

v PF4

PPO

°PAl

F i g . 10

Unirr. 0 0.5 1 2 3 4 5 6

Unirr. 0 0.5 1 2 3 4 5 6

6 vPA4

12

Table 5. Rheoscope, (n) values, 25 kGy

PPO

PFI

PF2

PF3

PF4

9.5E - 3 1.5E - 3 8.0E-4 3.0E- 4 1.5E-4 I .SE - 4 1.5E-4 1.7E - 4 1.7E - 4

0.0112 2.4E - 3 1.4E-3 1.1E-3 1.3E-3 1.8E - 3 2.1E-3 1.2E - 3 1.4E - 3

0.0105 3.0E - 3 2.5E-3 1.7E- 3 2.1E- 3 2.0E - 3 2.1E-3 2.0E - 3 2.0E - 3

0.011 3.0E - 3 2.8E-3 2.3E- 3 2.2E- 3 3.2E - 3 3.5E- 3 4.2E - 3 3.0E - 3

0.0156 2.3E - 3 2,4E-3 2.0E- 3 2.9E- 3 2.4E - 3 2.3E- 3 2.8E - 3 2.4E - 3

Time (months)

PPO

PF 1

PF2

PF3

PF4

Unirr. 0 0.5 1 2 3 4 5 6

0.404 0.580 0.624 0.690 0.763 0,736 0.772 0.708 0.674

0.395 0.511 0.569 0.591 0.575 0.537 0.503 0.588 0.561

0.401 0.496 0.518 0.546 0.536 0.532 0.540 0.538 0.551

0.402 0.502 0,515 0.528 0.527 0.494 0.471 0.456 0.494

0.381 0.539 0.531 0,551 0.506 0.530 0.537 0.513 0.523

Table 4. Rheoscope, (K) values, 25 kGy Time (months)

i

4 Time [months) ~PA2 Y PA3 Fig.

Table 3. Rheoscope, (K) values, 25 kGy Time (months)

+

Table 6. Rheoscope, (n) values, 25 k G y

PPO

PAI

PA2

PA3

PA4

9.5E - 3 1.5E-3 8.0E-4 3.0E-4 1.5E-4 1.5E - 4 1.5E - 4 1.7E-4 1.7E-4

0.0136 2.2E-3 I.IE- 3 1.3E- 3 1.8E- 3 1.8E - 3 2.4E - 3 1.1E-3 1.2E- 3

8.5E - 3 2.0E-3 1.7E- 3 2.0E- 3 1.3E- 3 1.8E - 3 1.7E - 3 1.4E- 3 I.IE-3

0.0102 1.6E-3 2.0E- 3 2.1E- 3 2.1E- 3 1.7E - 3 1.9E - 3 1.6E- 3 1.4E- 3

0.0156 2.5E-3 2.tE-3 2.0E- 3 2.1E- 3 2.6E - 3 2.3E - 3 1.9E- 3 1.7E- 3

Time (months)

PPO

PA 1

PA2

PA3

PA4

Unirr. 0 0.5 1 2 3 4 5 6

0.404 0.580 0.624 0.690 0.763 0.736 0.722 0.708 0.674

0.345 0.532 0.618 0.585 0.551 0.531 0.494 0.614 0.596

0.411 0.533 0.548 0.523 0.588 0.544 0.552 0.581 0.617

0.388 0.573 0.528 0.522 0.523 0.555 0.552 0.569 0.579

0.322 0.514 0.537 0.534 0.539 0.505 0.526 0.544 0.557

Post-irradiation degradation o f polypropylene TensiLe 25KGY

6.0

11.

TensiLe 5 0 KGY

5.6

A~A

5.5

299

5.2

5,0 0

=~4.5 e'g ~ 4 . 0

~

4.4

.~

4.0

E~

E~_

3.6

"g

3.2

~-

3.~

o =E

3.0 + 2.5

0

0

0

E.8 +

2.0 0 + PPO

I

I

I

2 Time (month=)

4

6

o PFI

~ PF2

x PF3

2.4 0 +PPO

v PF4

OPF1

I

1

2

4 Time ( m o n t h s ) A PF2 x PF3

t v PF4

Fig. 13

Fig. 15

sample was not only lower but remained almost constant after irradiation at 25 kGy. This indicates that less chain seission occurs during irradiation for samples having stabilizer.

mum stress (amx), bend strength, and elongation at break (Eb) of both unstabilized and stabilized polypropylene samples during and after irradiation has been investigated. Results are shown in Figs 13-20. For ductile samples, maximum stress is equivalent to the stress at break. The tensile strength of both unstabilized and stabilized polypropylene samples before irradiation was about 4.5 x 10-3psi. For unstabilized samples this value increased just after irradiation and decreased sharply during storage, while for all stabilized samples tensile strength remained constant by the time after being irradiated at 25 kGy. At 50kGy the amine type stabilizer was more effective, especially at 0.2 wt% concentration. At 0.05wt%, the tensile strength of the samples having amine type stabilizer decreased sharply for the first two months after irradiation. This is in accordance with the sharp increase in carbonyl index of these samples as shown in Fig. 8. Comparing the variation of ¢rm=x in Figs 13-16 to that of percentage crystallinity in Figs 9-12 shows that for the unstabilized samples the increase in the

MECHANICAL PROPERTIES

The mechanical properties of PP mainly depends on the percentage of crystallinity and its molecular weight (Dunn and Williams, 1983; Babic et al., 1983). Therefore the extent of the change occurring in the mechanical properties should be correlated to the variation of these two parameters. On the other hand, the amount of free radicals formed upon irradiation and trapped in the crystalline phase of PP depends on the percentage of crystallinity which is changed due to the chain scission. Therefore, attempts have been made in this work to correlate the mechanical changes (tensile strength, elongation, maximum bend angle) to the change of crystallinity and molecular weight for all samples. In this study, the effect of 6°Co-y-irradiation at 25 and 50 kGy on the maxi-

TensiLe

TensiLe 2 5 KGY

6.0

5.5

5.5 O.

5.0

+~

_

~

x

~ J a ~ .

x

4.0

~-

5 0 KGY

6.0

(~

5.0

~ ,a 4 . 0

3.5

E~

3.0

g

+

3E

2.5 2.0 0 + PPO

oPAl

+

2.5 2.0

I

I

J

2

4 Time (months)

6

aPA2

Fig. 14

× PA3

vPA4

a.o + +'~"~...+ I I 2 4 Time ( m o n t h s )

0 +PPO

OPAl

a PA2

Fig. 16

x PA3

I 6 v PA4

300

A. A. KATBAB and A. Y. MOUSHIRABADI

-

Bend

Ing

50

KG Y

Bending

9

<>

~-

2 5 KGY

v "~-,. 0

V

8 0

~7

~

E

7

o

,;6

g

z 5

~4 3 2 0 + PPO

o

I

I

t

2

4 Time ( m o n t h s )

6

PF1

~ PF2

x PF3

2

I

__

2

[

I

4

6

Time (months)

~ PF4

* PPO

~ PAl

~, PA2

× PA3

v

PA4

Fig. 17

Fig. 19

percentage of crystallinity just after irradiation leads to the increase in the maximum stress, but as the percentage of crystallinity tends to decrease during storage, the maximum stress declines sharply. The decrease in crystallinity and O'max confirms the occurrence of chain scission after irradiation for the unstabilized samples (Dunn et al., 1975). This can be concluded that the increase in crystallinity which resulted upon irradiation is the main cause of the sharp deterioration of O'max and E b as the number of free radicals trapped in the crystalline phase of the unstabilized samples increased. For all stabilized samples, O'max remained almost unchanged after being irradiated which is in accordance with the constancy of crystallinity in these samples (Figs 9-12). The variation of the bending work to break (equivalent to the change in stiffness) with time of storage is shown in Figs 17-20. The bend work to break is one of the most important parameters which estimates the extent of irradiation deterioration. The bend work to break of the both unstabilized and stabilized samples was about 6.5N.rad/cm 3

before irradiation. Although this value increased to 7.5 N.rad/cm 3 just after irradiation for the unstabilized samples, but decreased to 2.9 N.rad/cm 3 after 6 months in storage. For the stabilized samples as can be seen from the figures, this property remained almost unchanged which is in accordance with the constancy of the maximum stress and percent of crystallinity which was discussed before. For the samples having amine type stabilizer, the variation of the bend strength was lower than those containing phenol type, especially at 50 kGy, but the discolouring for the amine was higher than the phenolic stabilizer. CONCLUSION Both chemical and physical structure of PP together with its mechanical properties deteriorate upon 6°Co-y-ray irradiation under the dose required for sterilization. Chain scission and therefore the decrease in crystallinity was found to be the major cause of the post-deterioration of the mechanical

Bending 2 5 KGY

Bending 50KGY

9

8.

t~

v

7

7

Z

v

~

E

E o

~e

+

g

o

5

25

~4

~4

3 I2 0

t

I

I

2

4

6

+

21

I

o

Time ( m o n t h s ) + PPO

OPF1

~PF2

Fig. 18

xPF3

vPF4

÷

PPO

___

t

+

2 4 Time ( m o n t h s ) ,> PAl

a PA2

Fig. 20

x PA3

6 v PA4

Post-irradiation degradation of polypropylene properties of unstabilized PP. It was found that both 2,6-di-tertiary-butyl phenol and polymerized 2,2-4trimethyl-1, 2-dihydroquinoline are effective Gamma stabilizers for polypropylene, although the latter imparts discoloration. These type of stabilizers render their stabilizing effect by means of retarding chain scission and therefore the change in crystallinity of the polymer during storage after being irradiated. The addition of both types of stabilizer caused the decrease in the degree of crystallinity of PP and therefore the number of the radicals formed upon irradiation and trapped in the crystalline phases. A sensitizing effect was also observed for the amine type at low concentration. REFERENCES

Admad M. (1982) Polypropylene Fibres Science and Tech. Elsevier, Amsterdam. Allars D. L. and Hawkins W. L. (1978) Stabilization and Degradation of Polymers. Am. Chem. Sot., Washington, D.C.

L s ~ 38/3,---B

301

Babic D., Safrenj I. A., Markovich V. and Kotoskil D., Radiat. Phys. Chem. 27, 659. Carlsson D. J. et al. (1982) d. Polym. Sci. Polym. Chem. Ed. 20, 575-582. Deanin R. D. (1972) Polymer Structure, Properties and Applications. Cahners Publishing Co. Dunn T. S. and Williams J. L. (1983) Ind. Irrad. Tech. I, 33. Dunn T. S., Eperson B. J., Sugg H. W., Stannett V. T. and Williams J. L. (1975) Proc. 2nd International Meeting on Radiation Proceedings, Miami, November 1978. Radiat. Phys. Chem. 14, 625 (1975). Mark J. E. et al. (1984) Physical Properties of Polyms. Am. Chem. Soc., Washington, D.C. Scott G. (1980) Developments in Polymer Stabilization.

Applied Science Publishers, London. Stevens D. R. and Gruse W. A. (1941) U.S. Patent 2, 263, 582. Thomas C. A. and Tolman J. A. (1962) Chem. Soc. 84, 2930. Wenzin C., Goldman J. P. and Silverman J. J. (1985) Radiat. Phys. Chem. 25, 317. Williams J. L., Dunn T. S. and Stannett V. T. (1982) Radiat. Phys. Chem. 19, 291. Zahrah, Karir A., Yashii F., Makuuchi K. and Ishigaki I. (1989) Polymer 30 (August), 1425-1432.