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Effects of amorphous nylon on the properties of poly(butylene terephthalate) and 70/30 poly(butylene terephthalate)/nylon 6 blends.

INTRODUCTION

Polyamides and polyesters have been widely used as engineering plastics because of their good mechanical properties (1-3). Blends of them have also been extensively investigated to improve properties for more versatile applications in the plastics and the fiber industries (4-6). Blends of nylon 6 with poly(ethylene terephthalate)(PET) or poly(butylene terephthalate) (PBT) are typical examples (7-9). There are economic and ecological reasons for producing such blends, since they represent a potential means of recycling mixed waste of polyamides and polyesters (10,11). In particular, nylon 6/PBT blends have attracted much interest from the standpoints of both theory and industry (12,13). Since nylon 6 and PBT are incompatible, the use of a compatibilizer is essential to design a combination of two crystalline thermoplastics. A random copolymer of styrene, maleic anhydride, and glycidyl methacrylate(GMA), for instance, has been used to obtain a crystalline blend that improved the weak points of the two resin partners - dimensional stability in nylon 6, and hot-water hydrolysis in PBT (14,15).

In this work, amorphous nylon (a-nylon) was used as an impact modifier to blend with PBT, since the impact strength of PBT at room temperature is not high. The effects of adding small amounts of the amorphous nylon on the thermal and the mechanical properties of highly crystalline PBT have been investigated. The results were compared with the PBT/nylon 6 blends at the same composition bases. The compatibilizing effect of the a-nylon in the blend of PBT/nylon 6 of 70/30 composition by weight was also investigated.

EXPERIMENTAL

Nylon 6 and PBT were all commercially available grades. The a-nylon was kindly supplied by the Kolon Co., Korea. All the resins were dried before use at 105 [degrees] C, 105 [degrees] C, and 60 [degrees] C, respectively. The characteristics of the materials are given in Table 1. Our previous analyses by 1H-NMR spectroscopy showed that the a-nylon is a terpolymer prepared from 2:1 (by mol%) mixtures of (nylon 6 and nylon 6,6):nylon 12 prepolymers, even though the exact composition of the terpolymer has not been released to the public. A-nylon exhibits a very small heat of fusion around 100 [degrees] C on a differential scanning calorimeter (DSC) thermogram. The a-nylon showed broad amorphous halos on the X-ray diffractogram. It was noted by the supplier that the a-nylon has been developed to improve the impact strength of nylon 6 by reducing its crystallinity. Blends were prepared in a counter-rotating twin-screw extruder (Brabender Plasticorder, PLE-331). Blended samples were molded on an injection molding machine (Kum Sung Co.) for further testing and measurements. Table 1 summarizes the sample properties and the sample notations of the blends prepared in this work. The compositions of a-nylon or nylon 6 were varied from 0 to 20 phr based on 100 g of PBT. To prevent thermal aging of blended polymers during melt mixing, tri(2,4-di-t-butylphenyl)phosphite (Miwon Commercial Co., Ltd., Mianto P-650) was used as an antioxidant. In order to investigate the compatibilizing effect of a-nylon to the PBT/nylon 6 blend, ternary blends containing PBT, nylon 6, and a-nylon were also prepared. In this case, the composition of PBT and nylon 6 was fixed at 70/30 composition by weight.
Table 1. Sample Properties and Notations of the Blends.

Materials [M.sub.n] [M.sub.w] Source

PBT 31,400 71,280 Kolon Co. KP210
Nylon6 29,047 50,543 Kolon Co. KN120
amorphous nylon(AN) 43,364 65,372 Kolon Co. HMT

Notation PAN0 PAN7 PAN11 PAN15 PAN20 PN7 PN15 PN20

PBT (%) 100 100 100 100 100 100 100 100
AN(phr) 0 7 11 15 20
Nylon6 7 15 20

Notation PNA0 PNA7 PNA11 PNA15 PNA20

PBT (%) 70 70 70 70 70
Nylon 6 (%) 30 30 30 30 30
AN(phr) 0 7 11 15 20


Measurements

Molecular weight was determined by gel permeation chromatography (GPC; Waters 150C) at 140 [degrees] C using 1,2,4-trichlorobenzene (TCB) as an eluent. Thermal analyses were performed under nitrogen, with a heating rate of 10 [degrees] C/min, using a Perkin-Elmer differential scanning calorimeter(DSC7). Blend samples were initially heated from 50 [degrees] C to 280 [degrees] C at a heating rate of 10 [degrees] C/min, and then the heated samples were cooled to room temperature at a cooling rate of 10 [degrees] C/min; the second heating was done as for the first scan step. Tensile property measurements were done on a universal testing machine (UTM) (Instron model 4202) at room temperature following the procedure described in ASTM D638. A crosshead speed of 50 mm/min was used in measurements. Flexural property measurements were also done on the same universal testing machine using the ASTM D790 method. The cross-head speed was 300 mm/min. Notched Izod impact strength was measured using a TMI impact testing machine at room temperature. The specimens for the Izod impact strength measurements had dimensions of 63 x 12.5 x 3.1 mm with a notch 3 mm in radius.

The dynamic mechanical properties of blends were measured at 1 Hz using an RMS [Rheometric Mechanical Spectrometer (Rheometrics RMS 7700)] in a torsion rectangular dynamic mode. The samples were injection molded specimens with a thickness of 3.13 mm, width of 12.54 mm, and length of 45.37 mm. Strain was maintained at 10% for all the samples. The theological properties of blends were also measured using a capillary rheometer (Toyoseiki capirograph lB) at 230 [degrees] C and 250 [degrees] C. The length and diameter of the capillary were 10 mm and 1 mm, respectively.

The morphology of blend samples was observed with a scanning electron microscope (Cambridge JSM-35CF). Samples were cryogenically fractured in liquid nitrogen. The fractured surface of the specimens was directly observed by gold coating.

RESULTS AND DISCUSSION

Thermal Properties

DSC thermograms of PBT during first heating, cooling, and second heating scans are shown in Fig. 1. The PBT showed a single endotherm around 225.4 [degrees] C on the DSC thermogram at the first heating scan but exhibited a second endotherm at a temperature below that of the original endotherm as well as that of original higher-melting crystals at the second heating scan. It has been reported that the second lower-melting endotherm peak of PBT is often displayed in a subsequent scanning thermal analysis at a temperature below that of the original endotherm, which can be observed at first heating scan, when PBT is annealed. To explain the multiple peaks of PBT, Kim et al. noted that the crystallization of originally amorphous material is of two types, that coupled to preexisting crystals and that not coupled, and the coupled amorphous material cannot crystallize without molecular rearrangement within the crystalline material to which it is coupled (3,16-18). The amorphous material that can crystallize only at higher annealing temperature is likely to be coupled to pre-existing crystals. Thus, the lower-melting peak on DSC thermogram was said to have occurred at the expense of a higher-melting peak because of annealing on the second heating scan with no chemical change. Kim et al. proposed that the apparent transformation of high temperature to low temperature-melting material during annealing of PBT arises mainly from the coupled crystallization-recrystallization of the amorphous and the pre-existing crystalline material (3).

Table 2 summarizes the melting point and the changes of heat of fusion of PBT in PBT/a-nylon blends. When a-nylon was added, the original endotherm was remarkably affected, but the second endotherm was not appreciably changed; i.e., the heat of fusion at the original endotherm decreased linearly with increasing a-nylon content, meaning that the addition of a-nylon affects significantly the higher-melting crystals of PBT. The result suggests that the addition of a-nylon restricts the crystallization of higher-melting crystals in PBT, whereas it does not affect the coupling of crystallization-recrystallization of the amorphous and the preexisting crystalline material in PBT at all.

Table 3 shows the melting point and the changes of heat of fusion of PBT in PBT/nylon 6 blends. The temperature of the lower melting peak of PBT was observed to be superposed to the melting point of nylon 6, since the two temperatures are nearly identical to each other. Unlike PBT/a-nylon blends, in general, there was little change in the melting points and the heats of fusion of PBT and nylon 6 in all the PBT/nylon 6 blends, considering the experimental error ranges of measurements. The heat of crystallization of PBT on the cooling scan is, however, decreased slightly with increasing nylon 6 content. This result [TABULAR DATA FOR TABLE 2 OMITTED] [TABULAR DATA FOR TABLE 3 OMITTED] implies that the added nylon 6 acts as an impurity in the PBT matrix and reduces the crystallization of PBT, even though the nylon 6 itself is a highly crystallizable component. A comparison of Tables 2 and 3 suggests that a-nylon is partially compatibile with PBT but nylon 6 is not, as reported previously, from the fact that the addition of a-nylon affects the heat of reformed crystals and amorphous crystallization of PBT.

Mechanical Properties

Figure 2 shows the tensile modulus of PBT/a-nylon blends and the PBT/nylon 6 blend. In general, the tensile modulus decreased with increasing a-nylon content, whereas it increased slightly with increasing nylon 6 content. The typical flexural moduli of the PBT/a-nylon and PBT/nylon 6 blends are shown in Fig. 3.

One can easily see that the flexural modulus also decreases as the a-nylon content increases. The reduction in tensile and flexural moduli should be expected as a result of the flexible and amorphous nature of the a-nylon. Figure 4 shows the tensile strength of the PBT/a-nylon and PBT/nylon 6 blends as a function of the content of a-nylon or nylon 6. As expected, the tensile strength increased with increasing nylon 6 content because the tensile strength of nylon 6 is higher than that of PBT. Of interest is that the tensile strength was slightly increased when a-nylon was added to PBT up to 15 phr. Usually, an increase of elastic modulus leads to a decrease of elongation at break and a small decrease in tensile strength. The slight increase in tensile strength can be explained by better adhesion between the dispersed a-nylon and the PBT matrix and thus by an increase of effective load-bearing cross section. These large effects indicate again that the a-nylon must be mixed with PBT at the interface. An increase of tensile strength is often observed when a graft copolymer is used as a compatibilizer for the component polymers, such as in low-density polyethylene(LDPE)/polystyrene(PS)/PS-g-LDPE blends (19). The small Increase of tensile strength with the addition of a-nylon may be In part also due to the Inherent high strength of the a-nylon, which is the ternary copolymer of nylon 6, nylon 66, and nylon 12.

The Izod impact strengths of the PBT/a-nylon as well as PBT/nylon 6 blends are shown In Fig. 5. Figure 5 shows that the Impact strength increases with Increasing amounts of a-nylon, whereas it decreases with IncreasIng nylon 6 content. Impact modification of thermosets and thermoplastics has been practiced widely in industry, usually by blendIng a plastic with an elastomer or a plasticizer.

A typical example is when polystyrene is modified with polybutadiene. Generally, however, such impact modification often sacrifices inherent thermal or mechanical strength properties of the matrix resin. The Increase of impact strength of PBT by a-nylon without sacrificing the tensile strength is therefore unusual and should be noted (5,14).

Rheological Behavior

The viscosity change at 250 [degrees] C against shear rate of PBT/a-nylon blends is shown In Fig. 6. It is seen that the viscosity decreases drastically with Increasing a-nylon content. The result implies that the added a-nylon acts as a plasticizer or a processing aid for PBT (20). The melt viscosity of PBT at 250 [degrees] C, however, did not decrease with Increasing nylon 6 content as remarkably as in the PBT/a-nylon blends, as shown in Fig. 7.

More interesting results are shown in the plot of viscosity change with constant shear stress difference for different blends. The viscosity drop with shear stress between PBT/nylon 6 and PBT/a-nylon is compared In Fig. 8 at 230 [degrees] C and 250 [degrees] C. In this Figure, dss(A)[prime] denotes the difference between the shear stress of A at 230 [degrees] C and shear stress of A at 250 [degrees] C, where A[prime] is the amount of a-nylon blended with PBT in phr. Similarly, dss(N)[double prime] denotes the difference between the shear stress of N at 230 [degrees] C and shear stress of N at 250 [degrees] C and N[double prime] relates to the amount a-nylon blended with PBT in phr. dv(A[prime]) and dv(N)[double prime] denote the viscosity differences of A and N at the two different temperatures, respectively. The viscosity drop is much higher for the a-nylon containing blends than for the nylon 6 containing blends, meaning that a-nylon is more the effective processability-improver for PBT.

Morphological Observation

Figures 9 and 10 show representative SEM micrographs of the fractured surfaces of PBT/a-nylon and PBT/nylon 6 blends having 20 phr of a-nylon and nylon 6, respectively. For the PBT/nylon 6 blends, one can clearly see that the blend shows gross phase separation because of their mutual incompatibility, whereas the SEM micrographs of the fractured surfaces of the PBT/a-nylon blends show the domain sizes of PBT are greatly reduced by adding small amounts of a-nylon, meaning that compatibilization was achieved for the PBT/a-nylon. The trend is more clearly observed as the a-nylon content increases. Thus, one can conclude that the morphology is closely related to the increase in tensile strength when a-nylon was added to PBT, as already discussed.

Figure 11 shows the tan [Delta] plot as a function of temperature for PBT and PBT/a-nylon blends. The [T.sub.g] of PBT, denoted by the maximum peak in the plot, is at 56 [degrees] C. The tan [Delta] peak shown in Fig. 11 also shows that the glass transition temperature of PBT is shifted at lower temperature as the content of added a-nylon is increased. That means that there is a partial compatibility between PBT and a-nylon (21). There is, however, no shift in the [T.sub.g] for the PBT/nylon 6 blends, implying incompatibility between the components.

Effects of a-Nylon on the Miscibility and Properties of PBT/Nylon 6 Blends

The enhancement of the miscibility in the 70/30 PBT/nylon 6 blend by a-nylon was also confirmed by morphological studies. Figure 12 shows the SEM micrographs of the PBT/nylon 6/a-nylon ternary blends of 70/30 composition by weight. It was already seen that the PBT/nylon 6 blend is incompatible and the phase is grossly separated. When small amounts of a-nylon, 7 and 20 phr, were added to the PBT/nylon 6 binary blend, the morphology changes ([ILLUSTRATION FOR FIGURES 12B AND C OMITTED], respectively).

The SEM micrographs of the ternary blend consisting of a-nylon shows a finer domain structure than the binary blend without a-nylon, and the trend is much clearer as the content of a-nylon becomes higher. One can see that compatibilization was achieved in the PBT/nylon 6 blend of 70/30 composition by weight in the presence of a-nylon (22-25).

The mechanical and rheological properties of the PBT/nylon 6 blend can be also affected by the addition of a-nylon. Table 4 shows DSC thermogram data of PBT/nylon 6/a-nylon ternary blends. Two melting peaks were observed for the ternary blends. The melting peak of nylon 6 and the lower melting peak of PBT may be superposed at almost the same temperature ranges. When a-nylon was added, the higher-melting endotherm was remarkably affected but the lower melting endotherm was not appreciably changed, suggesting [TABULAR DATA FOR TABLE 4 OMITTED] also that the addition of a-nylon restricts the crystallization of higher-melting crystals in PBT, as in the PBT/a-nylon binary blends, whereas it does not affect the coupling of crystallization-recrystallization of amorphous and pre-existing crystalline material in PBT at all as well as nylon 6 itself.

Figure 13 shows the mechanical properties of PBT/ nylon 6/a-nylon ternary blends. The tensile moduli of PBT/nylon 6 blends decreased with increasing a-nylon content in general. The highest tensile modulus occurred when 3 phr of a-nylon was added. The exact reason for this maximum tensile strength at that a-nylon content is not clear at the present moment, even though the result is of great practical interest. The complicated mechanical behavior may be caused by a number of factors acting independently and simultaneously, including the decreasing crystallinity of PBT by blending with a-nylon and the partial compatibility between a-nylon to PBT, and so on. The tensile strength of the PBT/nylon 6 blend decreased with a-nylon content up to 7 phr but increased with further increasing a-nylon content. This trend may be closely related to the results in Fig. 4, exhibiting an increase in tensile strength of PBT with added a-nylon because of the partial miscibility of a-nylon with PBT. In Fig. 14, the flexural modulus of the 70/30 PBT/nylon 6 blend decreased with increasing a-nylon content.

Figure 15 shows the effect of a-nylon content on the viscosity change at 250 [degrees] C against shear rate of PBT/nylon 6/a-nylon ternary blends. The viscosity decreased slightly with increasing a-nylon content. The result means that the added a-nylon still served as a plasticizer or a processing aid for the PBT/nylon 6 blend, just as in the PBT/a-nylon blends. This result shows that a-nylon can be effectively used as a processability-improver for a PBT/nylon 6 blend as well as for PBT homopolymer.

CONCLUSIONS

It was found that a small amount of a-nylon significantly affected the flow, thermal, and mechanical properties of PBT. The results were compared with PBT/nylon 6 blends. The tensile and flexural modulus decreased with an increase in the content of a-nylon, but the tensile strength was increased by the addition of a-nylon up to 15 phr. The complicated mechanical properties of a-nylon containing binary blends were discussed, along with their morphological properties.

The PBT thermogram showed a single endotherm around 225.4 [degrees] C in the first heating scan but exhibited a second endotherm at a temperature below that of the original endotherm during the second heating scan. When a-nylon was added, the original endotherm was remarkably affected, but the second endotherm was not appreciably changed, showing that the addition of a-nylon significantly affects only the higher-melting crystals of PBT.

The impact strength of PBT was increased with increasing content of a-nylon. The morphological studies show that PBT has much better compatibility with a-nylon than with nylon 6. The addition of a-nylon was also effective In enhancing miscibility between PBT and nylon 6 in the PBT/nylon 6 blend of 70/30 composition by weight. Finally, it was found that the melt viscosity of PBT and the 70/30 PBT/nylon 6 blend decreased with increasing a-nylon content, implying that the added a-nylon acted as a plasticizer or a processing aid for PBT and for the 70/30 PBT/nylon 6 blend.

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Author:Kang, Tae-Kyu; Kim, Yang; Cho, Won-Jei; Ha, Chang-Sik
Publication:Polymer Engineering and Science
Date:Oct 1, 1996
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