Thermoplastic polyurethane toughened polyacetal blends

Polymer Testing 19 (2000) 75–83

Material Properties

Thermoplastic polyurethane toughened polyacetal blends K. Palanivelu*, S. Balakrishnan, P. Rengasamy Central Institute of Plastics Engineering and Technology, Guindy, Chennai-600 032, India Received 4 July 1998; accepted 10 September 1998

Abstract Polyacetal/thermoplastic polyurethane blends at four different polyacetal wt% of 90, 80, 70 and 60 were made using a twin screw extruder. Mechanical, morphological and rheological properties of these blends were determined. The addition of thermoplastic polyurethane (TPU) to polyacetal produces a decrease of tensile and flexural strength of the blend material as the TPU wt% increases. The notched impact strength increases with the increase of TPU wt% in the blends. Scanning electron micrographs of impact fractured surfaces of these blends show droplet dispersion morphology. The melt flow curves for these blends show lower melt viscosity than those of feedstocks in the major range of experimental shear rates. The instrumented impact strength of these blends at 30 wt% TPU level are nearly nine times higher than that of polyacetal, and blends at 30 and 40 wt% TPU levels failed in a ductile manner, whereas the polyacetal failed in a brittle manner.  1999 Elsevier Science Ltd. All rights reserved.

1. Introduction Polymer blends is an important branch in polymer technology. Modification of a polymeric material by adding one or more other polymers is highly economical to obtain desired performance instead of synthesizing a new material. Polymer blends have received increasing attention from both the scientific and industrial community. The rate of growth of polymer blends/alloys per year is 12–14% whereas that for polymers is 6–8%. Many improvements have been considered to develop a polymer blend. Among them, improvement of toughness or notch resistance and processability are the important objectives for development of many commercial blends/alloys. * Corresponding author. Tel.: ⫹ 91-44-2342371; fax: ⫹ 91-44-2347120. 0142-9418/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 8 ) 0 0 0 7 2 - 5

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With the invention of melt processable thermoplastic elastomers, polymer blends have been widened from plastic/plastic systems to plastic/rubber ones, with toughness improvement of engineering thermoplastics receiving major emphasis. The development of rubber toughened thermoplastics is an important contribution to the commercial polymer industry. Polyacetal is one of the important engineering thermoplastics. It is a notch sensitive material. Hence, it is necessary to toughen polyacetal to extend its engineering applications. Reported literature [1] reveals that thermoplastic polyurethane is a better toughening component for polyacetal because of the significant mutual physical interactions in the blend. Patented literature [2–5] reports regarding the improvement of notch resistance of polyacetal by blending with TPU. Flexman [6], John et al. [7] and Kumar et al. [8] have studied fracture mechanics of the blend of polyacetal with TPU. Chiang and Lo [9] reported the physical and mechanical properties of polyacetal/TPU blends and established the property relationship by studying their morphology and compatibility. Chang and Yang [1] investigated polyacetal toughening with TPU in terms of rheological, mechanical and morphological properties. Chiang and Huang [10] investigated the mechanical, physical, thermal, dielectric and dynamic mechanical properties and morphology of polyacetal/TPU blends. Instrumented impact tests provide designers with a guide to see what service conditions a material can endure, how the part will fail (ductile vs brittle), and the margin of safety between incipient damage and total failure. Instrumented impact testing can more closely simulate actual service conditions than any other impact test methods and also reveal a lot more information about a materials behaviour. It was shown as early as 1964 [11] that PA 66 out-performs ABS, acetal homopolymer, polycarbonate and die cast zinc or aluminium in repeated impact. This work indicated that crystalline polymers have better fatigue performance than non-crystalline polymers and that different energy processes occur in single and multiple impact testing. It also showed clear difference between brittle and ductile failure. Commercial TPU’s are basically two different materials and made using two different low molecular weight polymers viz. polyethers and polyesters. All the published reports do not give details of the nature of TPU which were used for blending. The nature of the TPU definitely influences the partial compatibility in these blends. This partial compatibility is important for the better mechanical performance of these blends. In the present work polyester based TPU has been used and the mechanical, rheological, and fractured morphology of these blends are determined. Abrasion resistance of these blend materials is also reported. 2. Materials and experimental Polyacetal, Celcon M90 was used in this study and was procured from Hoechst Celanese. Thermoplastic polyurethane was obtained from Bayer Sanmar Ltd, India. The polyacetal/TPU blends were prepared using a Berstorff twin screw extruder and the processing temperature was in the range 170–190°C. Compositions were 90/10, 80/20, 70/30 and 60/40 the first number denoting polyacetal and coded as P90, P80, P70 and P60 respectively. The test specimens for mechanical properties determination were prepared by injection moulding. Tensile (ASTM D 638) and flexural (ASTM D 790) properties were carried out using a Lloyd UTM (LR 100 K). Notched Izod impact strength (ASTM D256) was performed using a Ceast impact tester. Heat deflection

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temperature (HDT) was carried out as per ASTM D 648. An Instron capillary rheometer was used to determine rheological behaviour of these blends. Scanning electron microscopy (SEM) observation of notched impact fractured surfaces was carried out using Jeol JSM 5300 SEM. The samples were sputter coated using Au–Pd alloy prior to SEM observation. Instrumented impact test was carried out using a Rosand IFW-4 impact test system. Samples in the form of plaques 5 cm dia. and 3 mm thickness were used. This instrumented impact test was carried out as per ASTM D 5420. Abrasion resistance values were determined as per ASTM D 1242. 3. Results and discussion 3.1. Mechanical properties Tensile stress vs strain curves for polyacetal, TPU and these blends were given in Fig. 1. The curve for polyacetal shows the highest tensile strength and characteristics of a semi-brittle material. The behaviour of TPU is of soft elastomeric characteristics and did not break even after 500% strain. The addition of TPU to polyacetal show progressive minor reduction of tensile strength with increase of TPU wt% levels. However, the breaking strain of these blends increases as the wt% level of TPU increased. The increase in breaking strain for these blends is an evidence for improvement in toughness. The tensile, flexural, notched impact strength values and heat deflection temperature of these

Fig. 1. Tensile stress–strain plots for blend materials.

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blends are reported in Table 1. Tensile, flexural strength values and heat deflection temperature of these blends are decreased with the increase of TPU wt%, as expected. This suggests progressive decrease of rigidity of polyacetal as the wt% increase of TPU. The notched Izod impact strength of standard specimens with 0.254 mm notch radius show increase with the increase of TPU wt%. The increase of impact strength and decrease of rigidity are competing properties in toughening of thermoplastics. Hence, an optimum level of elastomer in this blend system should be identified. The notched impact strength of this blend at 30 wt% TPU level is nearly 150% higher than that of virgin polyacetal. The rise of impact strength from 20 to 30 wt% TPU is much higher than any other similar level of TPU increment. SEM photomicrographs of fractured surfaces of polyacetal/TPU blends and polyacetal were observed and are shown in Fig. 2(a)–(e). Under Izod impact rate of about 3 m/s, polyacetal failed in a brittle manner. The blends also failed in a semi-brittle manner with minor characteristics of ductile fracture (plastic deformation or lateral constriction). The fractured surface of blends showed more stress whitened than polyacetal. The intensity or magnitude of stress whitening increased as the wt% of TPU increased. The polyacetal/TPU blends morphology is particle dispersion in the polyacetal matrix [12]. During fracture, some of the droplets have pulled away which leaves cavities as well as the proportion of the droplets still remaining on the fractured surface. During fracture, the dispersed TPU phase may act as an energy absorber in the continuous phase of polyacetal. The average dispersed particle size is as low as 0.1 ␮ in P90 blend to 2 ␮ in P60 blends. As far as the toughening of thermoplastics is concerned, there are two competing mechanisms viz. crazing and plastic deformation of dispersed domains, responsible for higher impact strength. As the morphology is droplets dispersion, crazing would be the predominant mechanism in this blend system. So, considerable improvement in notched impact strength is observed as the wt% of TPU increased. A controlled, repeatable impact is achieved by dropping a striker attached to a weight onto a sample in a defined manner and impact velocity. During the impact, the force exerted by the sample on the striker is measured as a function of time, and stored for subsequent display and analysis. This fracture event lasts, typically, for a few thousandths of a second. The instrumented impact test results for polyacetal/TPU blend materials are presented in Fig. 3. These curves show that up to 20 wt% addition of TPU to polyacetal does not show considerable Table 1 Mechanical properties of polyacetal/TPU blends Properties

Tensile strength (MPa) Flexural strength (MPa) Notched impact strength (J/m) HDT (°C)

Polyacetal composition (wt%) 100

90

80

70

60

56.9

46.6

39.6

33.6

27.7

73.8

68.2

55.4

45.1

35.3

95.9

126.6

164.5

232.6

255.71

92.3

86.6

81.1

78.2

74.9

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Fig. 2. (a)–(e) SEM photomicrographs for blend materials. (a) P90, (b) P80, (c) P70, (d) P60, (e) polyacetal.

difference in the pattern of curves compared with that of polyacetal. So, as compared with that of polyacetal, up to 20 wt% addition of TPU show low crack initiation energy. The nature of the fracture curve for 30 wt% TPU blend shows much higher crack initiation energy leading to crack propagation and then failure. The curve for 40 wt% TPU blend is similar. These suggest enormous impact strength improvement for 30 wt% TPU blend in this instrumented impact test mode. These impact test results are presented in Table 2. The instrumented impact strength for 30 wt% blend

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Fig. 2.

Continued.

is nearly nine times higher than that of virgin polyacetal. These values for blends up to 20 wt% TPU level are much lower compared with that at 30 wt% TPU level. Hence, this instrumented impact test method predicts relatively different impact behaviour compared with that of conventional pendulum impact tests. As stated earlier, instrumented tests can simulate more closely to a real event and these impact results can be useful for design purposes.

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Fig. 2.

Fig. 3.

Continued.

Instrumented impact test curves for blend materials.

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Table 2 Instrumented impact strength and abrasion resistance values for blend materials Blend materials

Impact strength (instrumented) (J)

POM P90 P80 P70 P60 TPU

Abrasion resistance (mg/1000 cycle)

3.52 5.39 9.09 30.8 43.02 —

26 24 22 21 19 15

The abrasion resistance values for these blend materials are given in Table 2. The abrasion resistance of TPU is considerably higher than that of polyacetal. Hence, as the wt% of TPU increases polyacetal/TPU blends show noticeable increase in abrasion resistance. The abrasion resistance of the blend at 40 wt% TPU level is the highest compared to any other composition level. 3.2. Rheological properties The viscosity vs shear rate plots for polyacetal, TPU and their blends at 190°C are illustrated in Fig. 4. TPU has higher melt viscosity than polyacetal throughout the range of shear rates studied. Viscosity of blends at very low shear rate is as high as that of TPU. In the higher shear rate region the viscosity of blends is lower than those of polyacetal and TPU. The viscosity behaviour suggest the blends are more shear rate sensitive than the feedstocks. Wu [13] studied droplet size in the visco–elastic polymer blend system and found the critical Weber number (the dispersed droplet size) is a function of viscosity ratio. The Weber number is minimum when the viscosity ratio is near one. The viscosity of the feedstocks are near to each other at high shear

Fig. 4. Melt flow curves for blend materials at 190°C.

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rates. The droplet size can be minimum at higher shear rates. Hence, preparation of these blends at high shear rates, i.e. blending in twin screw extruders can yield fine morphology. 4. Conclusions Tensile, flexural and HDT of the blends decreases as the increase in wt% of TPU. Notched Izod impact strength of polyacetal/TPU blends increases remarkably as the TPU wt% increases. The notched impact strength of this blend system at 30 wt% TPU level is 150% higher than that of virgin polyacetal. Instrumented impact test results show quite interesting trend. The instrumented impact strength is nearly nine times higher at 30 wt% TPU level compared to that of polyacetal. The rise of notched Izod as well as instrumented impact strengths from 20 to 30 wt% TPU levels was much higher than for any other similar levels of increment. Rheological study suggest that the higher shear rates can be ideal for the smaller droplet formation of the dispersed phase. Noticeable increase of abrasion resistance was observed with increase of TPU wt% level in polyacetal/TPU blends. The morphology of the fractured surfaces show fine droplet dispersion of TPU in the polyacetal matrix. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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