polyamid gf effect

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April 2008 The Arabian Journal for Science and Engineering, Volume 33, Number 1B 5

DYNAMIC MECHANICAL PROPERTIES OF TOUGHENED

POLYAMIDE COMPOSITESFares D. Alsewailem *

Petroleum and Petrochemicals Research Institute

King Abdulaziz City for Science and Technology (KACST)

P. O. Box 6086, Riyadh 11442, Saudi Arabia

:

(Thermoplastic rubber)(Composites)66(Virgin)(Recycled)

.

---(SEBS-g-MA)-(EP-g-MA)

(Maleic anhydride).(Rotational rheometer)

(Shear storage and loss moduli, G and G)

(Frequency).(G)

(Virgin composites)(Torsion test).

G and G(Recycled composites)

(Shear modulus)(Virgin composites)

(shear moduli).

(G)

.

(Transition)(G)

.

* To whom correspondence should be addressed

Tel. 9661-4814318, Fax 9661-4813670

E-mail [email protected]

Paper Received 28 June 2006; Revised 3 January 2007; Accepted 13 March 2007


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Fares D. Alsewailem

The Arabian Journal for Science and Engineering, Volume 33, Number 1B April 20086

ABSTRACT

The effect of incorporating thermoplastic rubber on the dynamic mechanical

properties, storage and loss moduli, of virgin and recycled glass-fiber-reinforcedpolyamide 66 has been investigated in this study. Styrene-EthyleneButylene-Styrene

and EthylenePropylene grafted with maleic anhydride were used as elastomers for

toughening. Dynamic mechanical properties of the composites were examined by therotational rheometry. Shear storage and loss moduli of recycled and virgin materials

were measured against frequency. Also the variation of storage modulus of the virgin

composites was measured against temperature by conducting a series of torsion tests.

Both dynamic storage and loss moduli of the composites were found to increase withincreasing glass fiber and rubber contents. Recycled composites had lower values of

dynamic modulus compared to that of virgin composites; however by proper

combining of fiber and rubber into the recycled material, its modulus fairly matches

that of the virgin material. Addition of rubber to virgin composites causes a reduction

in G as temperature increases. Rubber, which acts as a stress concentrator, had a

major effect on minimizing the overall modulus of the composites. The drop in Gversus temperature has been observed for all composites; however the temperature at

which the transition in G occurs decreases with increasing rubber content.

Key words:polyamide (PA), glass fiber, rubber, dynamic modulus.


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DYNAMIC MECHANICAL PROPERTIES OF TOUGHENED POLYAMIDE

COMPOSITES

1. INTRODUCTION

Glass-fiber-reinforced polyamide 66 (GFRPA66) has wide useful applications especially in the automotive industry.

In the interior parts of a car, it is used in steering column lock housings, door and window hardware, and pedals used forthe accelerator, clutch, and brake [1]. In the under-the-hood applications, GFRPA66 is used to manufacture many parts

around the engine. Radiator end tank, air manifold, and cooling fan are all made of GFRPA66.

Although neat PA66 is considered a tough material, there has been a great demand for further increasing its

toughness and that is due to its notch sensitivity and brittleness at low temperatures, which makes its resistance to crackpropagation very poor. The incorporation of rubber phase into polyamide (PA) via melt blending is an effective way to

obtain very tough PA. Typically, acid-functional elastomers at percentages ranging from 5 to 20 wt% are extrusion

blended with PA to enhance its toughness [1]. Maleic anhydride ethylene/propylene elastomers (EP-g-MA and EPDM-g-

MA), styrene/ethylene/butylene/styrene block copolymers (SEBS-g-MA), and core-shell rubbers are consideredimportant examples of rubbers that serve as impact modifiers for PA [1]. Anhydride and other functional groups in the

elastomers can react with PA during melt extrusion through the amine groups or through routes that involve the amide

linkage to produce PA grafted with elastomers. This process of grafting is expected to reduce the interfacial tensionbetween PA and the rubber phase and hence enhance the dispersion of rubber particles in the PA phase. The introduction

of ethylenepropylene copolymer to PA toughening was proposed by Roura [2].

By incorporating fibers in a thermoplastic, an increase in stiffness and strength is normally achieved. However, the

drawback of this procedure is producing a material that is very poor in terms of handling impact loading. The area under

the stressstrain curve up to the failure point is a measure of the work of fracture. The conditions that lead to high

strength and stiffness usually result in low elongation to break, so that the work of fracture may be very low compared tothat of the matrix. The work of fracture depends on the existence of a mechanism for energy dissipation. Energy required

for fiber pull out is considered for composite impact fracture.

Thermoplastics are routinely blended with rubbery materials to enhance their toughness. Reinforcement materials, onthe other hand such as fibrous materials are added to polymeric materials in order to make them strong, stiff, and

dimensionally stable. In particular, properties such as impact and tensile strength can be altered by adding reinforcements

and/or tougheners. It is logical to expect a trade off between stiffness and toughness for a polymeric material whenincorporated with fibers, a high modulus type of materials, and a rubbery material which has a low value of modulus.

Combining both fibers and rubber with a thermoplastic matrix seems to be a good method in order to optimize theproperties of the final product. This has been an attractive subject to some researchers in the last decade, however these

studies seem to be fewer in number [312], and hence the synergic effect of combining both reinforcements and

tougheners into the base polymeric material is far from being completely understood. Composites with a superior

balance of strength, stiffness, toughness, and ductility may be achieved by the proper combination of glass fibers andrubber toughening [3].

It has been suggested that glass fibers inhibit crazing at rubber particles and rubber particles tend to promote crazingat the fibermatrix interface and also void initiation at fiber ends [10]. It was also believed that glass fibers contribute to

propagation toughness by fiber bridging of the matrix crack and by fiber pull out, this along with craze formation at the

fibermatrix interface which is promoted by rubber particles could lead to an increase in toughness of the composite at

high glass content [10].Although rubber toughening of neat PA66 has been extensively investigated in the past [1319], few studies are

available on rubber toughening of glass-fiber-reinforced PA66. In these studies, different kinds of elastomers have been

utilized. A system consisting styrene acrylonitrile of SAN and either butadiene or EPDM rubbers, which showed

incompatibility with the PA66 phase, has been used as tougheners [47]. Nair et al. [4] have found that the tensilestrength of fiber-reinforced PA66 toughened with ABS tends to increases with increasing rubber, i.e. acrylonitrile

butadiene styrene ABS, content in the composite up to (20/80) wt% (PA66/ABS). This is a positive deviation from the

rule of mixtures which predicts a linear decrease in the strength of the composites upon increasing rubber content.

Contrary to the behavior observed with tensile strength data, elongation at break of the glass-fiber-reinforced PA66toughened with ABS has shown a negative deviation from the rule of mixtures. The elongation at break of the fiber-


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reinforced PA66 was found to decrease with increasing rubber, i.e. ABS, content in the range from (20/80) wt%

(ABS/PA66) to 100 wt% ABS [4]. Other studies [9,12] have used EPDM rubber as a toughener for GFRPA66.

On the other hand, other rubbers such as SEBS-g-MA have proven to be good impact modifiers for PA66 [20-23]. Ithas been shown by others that blending PA66 with 20 wt% of SEBS-g-MA results in a super tough PA66 that has an

Izod impact strength of about 20 times that of neat PA66 [20]. This however, has resulted in about 50% reduction in the

tensile modulus of the un-toughened PA66 [20].While there is adequate data about toughening of un-reinforced PA66 by SEBS-g-MA and EP-g-MA, toughening of

GFRPA66 with SEBS-g-MA and EP-g-MA has received minor attention. Despite this fact, all characterizations

performed in the above studies concentrate greatly on static mechanical properties, whereas dynamic mechanical

properties for such important composites have not been given attention.

The current research aimed to investigate the effect of adding styreneethylenebutylenestyrene grafted with maleicanhydride (SEBS-g-MA) and EthylenePropylene grafted with maleic anhydride (EP-g-MA) on the dynamic mechanical

properties, i.e. storage modulus (G) and loss modulus (G), of the recycled and virgin GFRPA66.

2. MATERIALS USED

Recycled and virgin GFRPA66 were obtained from SDR Plastics and DuPont, USA respectively. Recycled materials

have been received as lot A and B. These materials, i.e. lot A and B, were waste materials generated during extrusioncompounding to manufacture DuPonts GFRPA66. The difference between lot A and B is that they have different

specific gravities due to the change in glass fiber content when switching to various batches during the course of

extrusion. The virgin materials have glass fiber content of 13 and 33 wt%. Working with virgin material would enable acomparison of its properties with those of the recycled one.

Rubbers used in this study were EP-g-MA (Exxelor VA 1801) and SEBS-g-MA (KRATON FG1901X). They were

supplied by ExxonMobil and KRATON Polymers of USA respectively. These two rubbers are semicrystalline and have

been produced by the maleic anhydride grafting process.

3. EXPERIMENTAL

3.1 Characterization of Recycled GFRPA66

The as received recycled GFRPA66 was initially characterized to determine glass fiber content and intrinsic

viscosity. Glass fiber contents of the recycled GFRPA66 designated as lot A and B as determined by ash test (ASTMD2584) were found to be 23.62 wt% and 14.79 wt% respectively. Ash test was done by burning a pre-weighed sample at

650 C and measuring the ash weight. Calculating intrinsic viscosity of recycled GFRPA66 can be used to estimate itsmolecular weight which would provide helpful information regarding the degradation that might occur during extrusion.The method of determining intrinsic viscosity of the recycled GFRPA66 was given elsewhere [24]. The intrinsic

viscosity value was 1.004 dl/g which indicates that it had a reasonable molecular weight, ~15 000 g/mol, as estimated by

the MarkHouwink formula.

3.2 Sample Preparation

Virgin GFRPA66 and recycled GFRPA66 were dry mixed and melt blended with various amounts of SEBS-g-MA

and EP-g-MA ranging form 5 wt% up to 20 wt% in a twin screw extruder. A C.W. Brabender continuous intermeshingcounter rotating twin screw extruder with 42 mm diameter screws was used. In order to minimize fiber attrition in the

extruder, a moderate screw speed, 40 rpm, was used. The extrusion temperature used was 275 C. Before each extrusion

run, samples were dried overnight at 82 C, and, when performing extrusion, the hopper was purged by argon gas toprevent degradation. The extrudates were then drawn into long strands in a water bath and then pelletized using a

Brabender strand pelletizer. Since virgin materials have glass fiber contents different from those of recycled materials,combining of the two virgin materials, i.e. 13 wt% and 33 wt%, was done in order to match the glass fiber content of the

recycled materials.

3.3 Measurements of Dynamic Moduli

A Rheometric Scientific Mechanical Spectrometer (RMS 800) was used to measure G and G of the toughened

PA66 composites. A parallel plate fixture with a diameter of 25 mm and 1 mm gap was used. Frequency sweep tests

were conducted for all samples at strain amplitude of 10 % and 275 C. This strain amplitude, i.e. 10 %, was within the

linear viscoelastic region as seen from the strain sweep tests conducted for all blends. G and G were measured versus

frequency. The variation ofG against temperature was measured by applying the torsion test utilizing the torsion fixture


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in the RMS 800 rheometer. The sample bars were mounted between the clamps of the fixture and a sinusoidal torsion at

1 rad/sec frequency and 0.1 % strain rate was applied fortemperature sweeps.

4. RESULTS AND DISCUSSION

G for virgin and recycled composites has been measured against dynamic shear rate (frequency) as shown in Figures

1 and 2. The storage modulus, G, which represents energy stored by the material due to elasticity increases with

increasing glass fiber and rubber contents; however, it is seen that in general the variation of G with frequency is fairlysteady, especially at high glass fiber and rubber contents. Independency of modulus with frequency is usually observed

with solid or pseudo-solid like materials and it is related to relaxation of polymer chains. Perhaps, the addition of rubber

at high content (i.e. 20 wt %) to the matrix material with high glass fiber loading (i.e. 23.62 wt %) is responsible for the

steadiness in storage modulus of the composites when shear rate is increased. Glass fiber which has a high modulus of

elasticity, is expected to contribute much to the overall storage modulus of the composite. Note here though that G

measurements have been conducted at melt state for the base material, i.e. PA66, which would have a different behavior

of G versus shear rate. In ordinary cases, a polymer either semi-crystalline or amorphous which is a viscoelastic

material would exhibit an increase in both values of G and G when exposed to sinusoidal shearing. This also may

indicate a strong interface of matrix/fiber from one side and on the other side good compatibility between functionalized

rubber and polyamide composites. It can also be seen from Figures 1 and 2 that at a fixed shear rate, the value of G

increases with increasing rubber content at the two glass fiber loadings used in this study; however, higher values ofG

were observed at high contents of glass fiber and rubber ( i.e. 23.62 wt% and 20 wt% respectively). Recycled GFRPA66

toughened with both SEBS-g-MA and EP-g-MA exhibited lower values ofG than for those of virgin composites, except

at high glass fiber and rubber content (especially for composites with SEBS-g-MA). The lower values of modulus

observed for recycled materials when compared to those for virgin materials may be attributed to glass fiber attrition

during previous compounding processes.

Figure 1(a). Gforvirgin GFRPA66 at the two glass fiber loadings, solid lines: 14.79 wt%; no lines: 23.62 wt%, and various

wt% ofEP-g-MA rubber: 0 ; 5; 10; 15; + 20. (T = 275 C)

1 10 100

Frequency (rad/s)

G'(Pa)

106

105

104

103


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1 10 100

Frequency (rad/s)

G'(Pa)

10

6

105

104

103

Figure 1(b). Gfor recycled GFRPA66 at the two glass fiber loadings, solid lines: 14.79 wt%; no lines: 23.62 wt%, and

various wt% ofEP-g-MA rubber: 0 ; 5; 10; 15; + 20. (T = 275 C)


wt% ofSEBS-g-MA rubber: 0; 5; 10; 15; + 20. (T = 275 C)


various wt% ofSEBS-g-MA rubber: 0; 5; 10; 15; + 20. (T = 275 C)

1 10 100

Frequency (rad/sec)

G'(P

a)

103

106

105

104

1 10 100

Frequency (rad/s)

G'(Pa)

106

105

104

103


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G data which is plotted in Figures 3 and 4 shows different shear rate dependence behavior in contrast to the

behavior given by G. Energy dissipation when a polymer deforms, which is represented by G, increases rapidly with

increasing shear rate. Here, unlike the case with G, glass fibers will not be expected to play a major role since G

measures the response of viscosity rather than elasticity. This explains the great dependence ofG, which is dominated

by matrix properties, on shear rate.

It can be seen from Figures 3 and 4 that G increases when the shear rate (frequency) was increased, and this is atypical behavior ofG against shear rate for linear polymers. Also, Figures 3 and 4 show that at a fixed value of shear

rate, values ofG increase with increasing rubber content at both glass fiber loadings. Similar to what has been observed

with values ofG, values ofG for recycled composites were less than those of virgin composites.


wt% ofEP-g-MA rubber: 0 ; 5; 10; 15; + 20. (T = 275 C)


various wt% ofEP-g-MA rubber: 0 ; 5; 10; 15; + 20. (T = 275 C)

For a polymeric material, the variation ofG against temperature gives different behaviors depending on the nature of

the material. For example, contrary to the semi-crystalline case, an amorphous polymer exhibits a sharp drop in G at the

Tg. For a rubber, the shear modulus in the plateau region above Tg is related to the molecular weight between cross- links

[25]. For non-cross-linked polymers, G may be related to the molecular weight between entanglements [25].

1 10 100

Freqency (rad/s)

G"(Pa)

105

104

103

102

1 10 100

Frequency (rad/s)

G"(Pa)

105

104

103

102


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wt% ofSEBS-g-MA rubber: 0; 5; 10; 15; + 20. (T = 275 C)


various wt% ofSEBS-g-MA rubber: 0; 5; 10; 15; + 20. (T = 275 C)

When G is measured against temperature, the modulus goes through a transition at an important property of the

material that is the glass transition temperature, Tg. For the current study, the variations of G with temperature for thevirgin GFRPA66 toughened with SEBS-g-MA and EP-g-MA at two glass fiber loadings are shown in Figures 5 and 6.

As indicated by Figures 5 and 6, addition of rubber to glass fiberreinforced nylon 66 causes a reduction in G as

temperature increases. Note here that the torsion test was conducted on solid bars where the temperature range is below

the melting temperature of the base material; therefore increasing rubber (which acts as a stress concentrator), contenthas a major effect on minimizing the overall modulus of the composite. Going from low to high glass fiber contents does

not seem to affect values ofG.

Composites toughened with EP-g-MA have a different behavior at high rubber content (e.g. 10 wt %) than that of

composites toughened with SEBS-g-MA. These composites of glass-fiber-reinforced nylon 66 with EP-g-MA at high

1 10 100

Frequency (rad/s)

G"(Pa)

105

104

103

102

1 10 100Frequency (rad/s)

G"(Pa)

105

104

10

3

102


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rubber content exhibit two plateau regions. The change in G at Tg has been observed for all composites, however the

temperature at which the transition in G occurs decreases with increasing rubber content.

Figure 5(a). Variation of Gversus temperature forvirgin GFRPA66 with 14.79 wt% glass fiber and various wt% ofEP-g-MA

rubber: 0 ; 5; 10; 15; + 20

Figure 5(b). Variation of Gversus temperature forvirgin GFRPA66 with 23.62 wt% glass fiber and various wt% ofEP-g-MA

rubber: 0 ; 5; 10; 15; + 20

0 25 50 75 100 125 150 175 200 225

Temperature (C)

G'(Pa)

109

108

107

0 25 50 75 100 125 150 175 200 225Temperature (C)

G'(Pa)

109

108

107


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Figure 6(a). Variation of Gversus temperature forvirgin GFRPA66 with 14.79 wt% glass fiber and various wt% ofSEBS-g-

MA rubber: 0 ; 5; 10; 15; + 20

Figure 6(b). Variation of Gversus temperature forvirgin GFRPA66 with 23.62 wt% glass fiber and various wt% ofSEBS-g-

MA rubber: 0 ; 5; 10; 15; + 20

5. CONCLUSIONSThe current study has shown that the storage and loss moduli, G and G, of the GFRPA66 toughened with SEBS-g-

MA and EP-g-MA increase with increasing rubber content at the two glass fiber contents; however, higher values of G

and G were observed at high contents of glass fiber and rubber (i.e. 23.62 wt% and 20 wt% respectively). The values of

G have been seen to be independent of dynamic shear rate (frequency) especially at high content of both glass fiber and

rubber, and this may be an implication of a strong matrix/fiber/rubber interface. On the other hand, the loss modulus,

G, was found to increase rapidly with increasing shear rate. Here, unlike the case with G, glass fibers are not expected

to play a major role since G measures the response of viscosity rather than elasticity. This explains the great

dependence ofG, which is dominated by matrix properties, on shear rate. Recycled GFRPA66 toughened with both

SEBS-g-MA and EP-g-MA have lower values ofG in comparison to those of virgin composites with the exception of at

0 25 50 75 100 125 150 175 200 225

Temperature (C)

G'(Pa)

109

108

107

0 25 50 75 100 125 150 175 200 225

Temperature (C)

G'(Pa)

107

109

108


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high glass fiber and rubber content (especially for composites with SEBS-g-MA rubber type). The retention of modulus

of the recycled material at combined higher contents of fiber and rubber gives a good strategy to enhance its properties inorder to match that of the virgin material, i.e. GFRPA66, which is mainly used in the under-the-hood applications in

automotive industry. Fiber attrition during processing of the recycled GFRPA66 may be the cause for the reductions in

G and G. Addition of rubber to virgin GFRPA66 causes a reduction in G as temperature increases. Increasing rubber.

(which acts as a stress concentrator), content has a major effect on minimizing the overall modulus of the composite.

Going from low to high glass fiber contents does not seem to affect values of G. The change in G at Tg has beenobserved for all composites; however the temperature at which the transition in G occurs decreases with increasingrubber content.

ACKNOWLEDGMENTS

The author would like to thank his employer, King Abdulaziz City for Science and Technology (KACST), for

financial support.

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[19] S. Wu Formation of Dispersed Phase in Incompatible Polymer Blends: Interfacial and Rheological Effects, Polym.Eng Sci., 27 (1987), pp. 335343.

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polyamid gf effect

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