In¯uence of benzoic acid on thermal, crystallization andmechanical properties of isotactic polypropylene under irradiation

Shamshad Ahmed, A.A. Basfar *

Institute of Atomic Energy Research, King Abdulaziz City for Science and Technology (KACST), P.O. Box 6086, Riyadh 11442,

Saudi Arabia

Abstract

Degree of super-cooling is denoted by the temperature di�erence between the melting temperature of the polymer

Tm, and peak crystallization temperature Tp. Upon addition of progressively increasing amounts of benzoic acid (BA)

to isotactic polypropylene {(is)-PP}, the degree of super-cooling was found to decrease, which leads to considerable

reduction in moulding cycle time and savings in production cost. Haze % was found to progressively decrease with the

corresponding increase in the amount of benzoic acid in (is)-PP, resulting in much improved transparency of the (is)-PP-

benzoic acid blends. Irradiation to an absorbed dose of 25 kGy a�ected the transparency of blends slightly.

Thermogravimetric analysis of (is)-PP-BA blends showed that there is no adverse e�ect on thermal stability of the

polypropylene. Also, the irradiation of (is)-PP-BA blends did not bring about any signi®cant changes in their thermal

stability. (is)-PP-BA blends demonstrated, in general, improved tensile strength when compared to pure (is)-PP.

Moreover, no signi®cant detrimental in¯uence of irradiation was observed on the tensile strength of (is)-PP-BA

blends. Ó 1999 Elsevier Science B.V. All rights reserved.

Keywords: c-Irradiation; (is)-Polypropylene; Benzoic acid; Thermo-mechanical; Crystallization

1. Introduction

The addition of nucleating agents changespolymerÕs crystallization temperature, spherulitesize, density, clarity and tensile properties [1,2].The in¯uence of addition of the nucleating agent(NA), t-(butyl) benzoic aluminium on crystalliza-tion parameters of a polypropylene (random) co-polymer containing 6% ethylene (PP-ran-Ethylene)was investigated by Shamshad and Ruimin [3].

Degree of super-cooling was found to decreaseprogressively with corresponding increase in theamount of the NA. Dongyuan et al. [4] reportedthat PP-benzoic acid composites have lower ulti-mate deformation, when compared with unirradi-ated samples. Kadir and coworkers [5] investigatedthe in¯uence of t-(butyl) benzoic aluminium on theradiation stability of polypropylene having di�er-ent molecular weights, and reported that thepolypropylenes with the nucleating agent are lessstable during radiation than polypropylene with-out nucleating agent, especially low molecularweight ones. In this work, the in¯uence of adding

Nuclear Instruments and Methods in Physics Research B 151 (1999) 169±173

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varying amounts of benzoic acid on the degree ofsupercooling, Tm and Tp (crystallization tempera-ture), thermal stability and tensile strength is re-ported. In¯uence of gamma irradiation to anabsorbed dose of 25 kGy on the clarity, thermaland tensile properties for various formulations ofpolypropylene and benzoic acid was also investi-gated.

2. Experimental

Isotactic polypropylene was obtained fromAldrich Chem. (UK) with melt ¯ow index of 4.5.The NA used was benzoic acid supplied by FlukaChemicals, UK. Melt ¯ow index (MFI) was de-termined using MFI unit Model 10 (Lloyds In-struments, UK) ¯ow rate in grams for 10 min at230°C under a load of 2.13 kg. Mixing of materialwas carried out using Brabender PL-2000 con-nected to an extruder Model 25/20 D at 40 rpmand 180°C. The mixture was later compressed intosheets or ®lms by moulding under a pressure of150 kg/mm2 for 3 min at 170°C using 0.5 mm thickmould after preheating for 5±7 min at the sametemperature. Formulations of various samplesprepared from blending of benzoic acid (BA) with(is)-PP are presented in Table 1. Haze measure-ments were carried out using an integrating typehazemeter made by Toyo Seiki, Seisakusho, Japan.The thickness of ®lms was �90 lm. Tensilestrength at yield point was determined from stress±strain curves measured using an Instron universaltesting machine Model 450 in accordance withASTM D638 at ambient temperature (20°C � 2).For thermogravimetric analysis, a Perkin Elmer

TGA-7 was used adopting a standard heating rateof 5°C/min. Di�erential Scanning Calorimetry(DSC) scans were measured with a Perking ElmerDSC-7 by heating each sample from ambient to185°C at a heating rate of 5°C/min in a ¯ow ofnitrogen. The pressed sheets were irradiated in airwith c-rays at a dose rate of 12 kGy/h to an ab-sorbed dose of 25 kGy and annealed for 2 h at80°C.

3. Results and discussion

3.1. E�ect on the degree of super-cooling and otherparameters of (is)-PP

Degree of super-cooling of polypropylene isdenoted by TpÿTm. Tp is the peak crystallizationtemperature and occurs at the intercept of thetangents to the sides of the exothermic curve and isa measure of the temperature when the bulk of thepolymer has crystallized. Tp�c� is the constructedtemperature at the intercept of the tangent at thebaseline and the high temperature side of the ex-otherm. Fig. 1 shows a typical ``closed loop''thermogram of the melting and crystallization ofpure polypropylene wherein change in degree ofsupercooling caused by addition of a nucleatingagent is shown. As shown in Fig. 2, the Tp

increased with the addition of incremental con-centrations of BA. Consequently, an increase ofabout 7°C in Tp upon addition of 1% of the nu-cleating agent was observed leading to a decreaseof �10°C in supercooling. In addition withincrease in Tp, the corresponding increase in Tp�c�is also expected, which was indeed the case as

Table 1

Formulations of isotactic polypropylene

Formulation code

(unirradiated)

Benzoic acid content

(pphr)

Formulation code

(irradiated to 25 kGy)

Benzoic acid content

(pphr)

PP 0 PP/25 0

PP-1 0.1 PP-1/25 0.1

PP-2 0.2 PP-2/25 0.2

PP-3 0.4 PP-3/25 0.4

PP-4 1 PP-4/25 1

170 S. Ahmed, A.A. Basfar / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 169±173

demonstrated by the steady increase in values ofTp�c�. A progressive decrease in the value ofTp�c� ÿTp on addition of benzoic acid as presented

in Table 2, shows accelerated rate of nucleation.As presented in Table 2, Tm and Tm�c� (the con-structed melting temperature) were found to de-crease rather steadily, but, in general, nocorrelation was found between Tm, Tm�c� and thecontent of the nucleating agent in the (is)-PP for-mulations.

3.2. Transparency of unirradiated and irradiated(is)-PP-BA formulations

As seen in Fig. 3, haze percentage of all theformulations of (is)-polypropylene containing thenucleating agent was found to be lower than thatof (is)-PP alone, indicating that the transparencyof (is)-PP increased with the addition of the nu-cleating agent. Furthermore, the irradiation of the

Table 2

Tp, Tp�c�, Tm and Tm�c� of various polypropylene formulations

Sample Tp (°C) Tp�c� (°C) Tp�c� ÿTp (°C) Tm (°C) Tm�c� (°C)

PP 100.2 105.8 5.6 154.0 145.7

PP-1 102.9 107.9 5.0 152.4 142.3

PP-2 104.3 109.1 4.8 153.4 140.4

PP-3 104.8 109.0 4.2 151.2 143.6

PP-4 107.0 112.4 5.4 151.9 143.6

Fig. 1. Typical DSC curves of (is)-PP: 1(a) without NA; 5(a)

with NA (1%).

Fig. 2. Peak temperature Tp and constructed temperature Tp�c�as a function of concentration of nucleating agent.

Fig. 3. Haze % as a function of the concentration of nucleating

agent, benzoic acid.

S. Ahmed, A.A. Basfar / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 169±173 171

formulations containing NA to a dose of 25 kGyhad a little in¯uence on their clarity. Fig. 4 showsthe relative average clarity versus the peak tem-perature observed. Thus, increased clarity is asso-ciated with higher peak temperature andconsequently with less supercooling.

3.3. Thermal stabilities and tensile strength ofunirradiated and irradiated formulations

In Table 3, a combined evaluation based ontemperature of onset of degradation and Di�er-ential Thermogravimetric (DTG) peak maxima [6]shows that the negative in¯uence of the addition ofnucleating agent on thermal stability of (is)-PP isinsigni®cant. The table also shows that the sam-ples, when irradiated to a dose of 25 kGy, do notdemonstrate any changes in thermal stability.

Tensile strength at yield point presented inTable 4 shows that the addition of benzoic acidtends to increase slightly the tensile strength atyield. No correlation between the increase in ten-sile strength and the amount of NA seems to existin the concentration range studied. The observedvariation in tensile strength measurements mayoriginate from factors such as concentration ¯uc-tuations, impurities and small di�erences in ther-mal history. Upon irradiation of (is)-PP and(is)-PP-BA formulations to a dose of 25 kGy, thetensile strength was adversely in¯uenced as pre-

Fig. 4. Clarity % as a function of peak temperature Tp for

isotactic polypropylene.

Table 3

Thermogravimetric analysis of various polypropylene formulations

Formulation

code

Temp. for onset

of deg. (°C)

DTG peak

maximum (°C)

Formulationa

code

Temp. for onset

of deg. (°C)

DTG peak

maximum (°C)

PP 289.3 385.2 PP/25 293.7 376.9

PP-1 299.9 375.5 PP-1/25 301.3 375.2

PP-2 283.3 367.1 PP-2/25 290.2 369.3

PP-3 280.2 375.8 PP-3/25 290.4 360.7

PP-4 282.5 377.3 PP-4/25 282.0 355.7

a Samples were analysed after seven days of irradiation.

Table 4

Tensile strength for various polypropylene formulations

Formulation codea Tensile strength (MPa) Formulation code Tensile strength (MPa)

PP 38.3 � 2.1 PP/25 35.7 � 1.7

PP-1 37.6 � 1.6 PP-1/25 34.8 � 1.5

PP-2 40.0 � 1.5 PP-2/25 34.9 � 2.1

PP-3 39.1 � 2.2 PP-3/25 36.5 � 1.3

PP-4 39.4 � 3.1 PP-4/25 36.2 � 1.9

a Five samples per formulation were tested.

172 S. Ahmed, A.A. Basfar / Nucl. Instr. and Meth. in Phys. Res. B 151 (1999) 169±173

sented in Table 4. This may be attributed to scis-sion resulting from irradiation of the PP formu-lations in air.

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