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MECHANICAL STRENGTH OF INJECTION MOULDED POLYBUTYLENE

TEREPHTHALATE SAMPLES IN THE TEMPERATURE RANGE BETWEEN Tg AND Tm

Nicole Aegerter1, Nora Zimmerli1, Nicole Schai1 1ETH Zurich, Department of Materials, Laboratory of Polymer Technology, December 2013

ABSTRACT The following paper presents the results of the investigation of the mechanical strength of a semi-crystalline

polymer Polybutylene terephthalate (PBT). By varying the mould temperature and holding pressure of the injection moulding machine, different degrees of crystallinity should be obtained. As PBT shows high tendency for crystallization, DSC analysis of the different samples did not reveal any differences concerning the degree of crystallinity. Tensile testing at different testing temperatures was run in order to determine the mechanical strength. The results show a drop of the Young’s Modulus from 1715 MPa (room temperature) to 330 MPa (80 °C). The high toughness at low temperatures and high strength at high temperatures is remarkable. In connection with well reproducible degree of crystallization, these characteristics make PBT interesting for various applications.

Keywords: PBT, glass transition temperature, tensile strength, crystallinity, injection moulding

1. INTRODUCTION

HIS paper aims at introducing the mechanical stability of a semicrystalline polymer above the glass transition temperature (Tg).

In order to show the mechanical properties of a semicrystalline polymer above its glass transition temperature, Polybutylene terephthalate (PBT) was used. Its glass transition temperature is at 47°C, which allows measurements of mechanical strength below as well as above Tg by using simple laboratory equipment. PBT shows several technical advantages such as high degree of crystallinity, considerable toughness even below glass transition and little creep. Additionally, PBT is not sensible to slight changes in the manufacturing process, i.e. changes in mould temperature and injection velocity have only little effect on the final crystallinity and thus the mechanical properties. Due to high thermal conductivity, production rate can be increased in comparison to other technical polymers. Furthermore it is not prone to uptake of water. Again, this simplifies manufacturing processes and improper drying will not lead to failure during production. PBT can be used within a large temperature range from

40°C up to 125°C. Application above 70°C leads to a significant drop of strength [3]. In the industry, PBT is often processed by injection moulding. For PBT, the melt temperature should be set between 250-270°C [2]. Speed of injection influences the alignment of polymer chains along the mould. Injection velocity should therefore be high enough to prevent a change of properties at the surface of the PBT part. The temperature of the mould wall influences the rate of cooling and hence the degree of crystallization. Quenching reduces crystallinity. After injection, holding pressure has to be applied in order to prevent backflow of the melt. Additionally it compensates the shrinkage of the polymer during cooling. In order to achieve minimal production time, holding pressure should only be applied as long as necessary, i.e. until sealing point of injection. Form instabilities and shrinkage can be prevented by adjustment of the cooling time until the piece has reached the required temperature for demoulding.

Its field of application represents its stable and favorable

properties, ranging from cogwheels (dimensional stability), polymeric parts of cars and the electrical industry (high temperature ranges) to applications where substantial mechanical strength is needed, e.g. pumps. Due to its excellent flow characteristics PBT is especially suitable to produce thin-walled parts. In 2005 over 580.000 t of PBT were produced. Due to further improvements such as introduction of glass fibers or blends with other polymers, PBT can be used at even more extreme conditions [3].

T

FIGURE 1: Mechanical behaviour of Ultradur® B 4520 (BASF). at different temperatures. Similar experiments were run for this paper [4].

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Stress-strain diagrams for unreinforced Ultradur® at differenttemperatures (in accordance with ISO 527, take-off speed 50 mm/min)

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TABLE 1 MECHANICAL, THERMAL AND CHEMICAL PROPERTIES

Property Value Unit

density 1300 kg/m3 glass transition temperature 47 ° C melting temperature 223 ° C tensile E-Modulus 2500 MPa Yield stress 50 mm/min 55 MPa nominal fracture strain, 50 mm/min >50 % Intrinsic thermal conductivity 0.27 W/(mK) specific heat capacity 1250 J/(kg*K) moulding shrinkage (plate of 150*150*3 mm3)

1.5 %

Technical Data of Ultradur® B 4520 (BSAF). Please note that these are not the values that were measured as part of this paper. They were used as a guideline to efficiently plan and run the experiments [2].

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2. SUBJECT & METHODS The main focus of the following experiments was placed

upon the mechanical strength of Ultradur® B 4520 (BASF). The injection moulding was run by an injection moulding machine of Arburg.

First, the sealing point had to be determined. Melt

temperature, moulding wall temperature, holding pressure and cooling time were adjusted. This procedure is essential in order to find the maximal production rate without losing the tight range of mass distribution of the parts due to backflow. Sealing time and demoulding temperature were used to calculate the effective thermal conductivity. The neat thermal conductivity of PBT can be taken from literature and thus the sealing time could be calculated (1, B. Equations), the thermal conductivity behaves slightly different within the mould injection machine. Little changes occur due the different thermal resistance from the melt to the mould surface.

Initial measurements showed, that mainly mould wall

temperature and holding pressure affected the behavior of the sample. Consequently, nine batches of samples were prepared. Table 2 summarizes the conditions, at which the samples were created.

The samples were named as follows: Batch - mould wall

temperature - holding pressure (e.g. A-30-1200 is a sample prepared at 30 °C wall temperature and 1200 bar holding pressure). Samples were labeled and stored at room temperature.

2.1 Influence on weight distribution and shrinkage

After demoulding, the samples were cooled down to room temperature. Of each batch, the masses of five samples were measured using a Mettler Toledo precision laboratory scale. About 1/2 hour after demoulding, the length of the samples was recorded. As the size of the mould is known precisely, shrinkage could be calculated. In order to compare weight distribution, in mould pressure profiles of the injection process were recorded.

2.2 DSC for crystallinity

DSC analysis was run for each batch to determine the degree of crystallinity. The samples were collected using a drill. By drilling through the core of the moulded pieces, the center of the samples was analyzed. The degree of crystallinity was calculated by comparing the heat of fusion of the samples to the heat of fusion of 100% crystalline PBT.

2.3 Injection simulation

The injection moulding of PBT was further analyzed by using Autodesk Moldflow Synergy® simulation. Parameters were set as listed in Table 2. Technical data used for the simulation had already been imported by the program. The simulations were compared to the actual experiments. Furthermore, the simulations allowed the visualization of stress, pressure and shrinkage distribution.

2.4 Tensile testing

The main purpose of the investigation was to show the significant strength of PBT even above glass transition temperature. Tensile tests were run at room temperature and elevated temperatures (40 °C, 60 °C, 80 °C) for five samples of each batch. Initial tests had shown no difference on the E-Modulus at strain rates of 5 mm/min or 50 mm/min. All measurements were therefore run at 50 mm/min. The tests at elevated temperatures were realized in a special oven. The temperature was further controlled with an external thermo element and kept within a range of ±1 °C. The samples were preheated on a heating plate to the temperature at which they were tested. After each change of sample, the system was allowed at least two minutes to restore the temperature within the oven and of the sample itself. The apparatus used was Zwick Z020. Force curves were recorded by the corresponding testXpert ® software. In order to measure at elevated temperature, a special set up was used. Special clamps allowed the fixation of the samples. A heating chamber was prepared and put around the tensile testing machine. The tensile rods entered and left the chamber through specially isolated openings in the walls of the heating chamber.

TABLE 2 SETTINGS FOR PRODUCTION OF SAMPLES

Property Value Unit

melt temperature 260 ° C speed of injection 50 mm/min injection pressure 1800 bar temperature of hot channel 270 ° C holding time 10 sec cooling time 2 sec mould temperature 30

50 70

° C ° C ° C

holding pressure 800 1200 1600

bar bar bar

Production parameters for the injection moulded samples made from PBT. Parameters were set as listed, nine batches were created, one at each mould temperature and every holding pressure.

FIGURE 2: Injection moulding machine, Arburg Allrounder 270 U.

FIGURE 2: Deformation by necking of the sample after the tensile test.

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3. RESULTS The injection moulding process as well as the prepared

samples were analyzed. In the following, the obtained results will be presented. 3.1. Filling pressure

The pressure near the sprue was measured by an internalized measuring device and read out for analysis. Figure 3 shows the curves obtained at constant pressure (1200 bar) and the three different mould temperatures.

3.2 DSC for crystallinity The crystallinity of the samples was measured by DSC.

The heat of fusion was compared to 100% crystalline PBT and the percentage of crystallinity was calculated for each sample. Explicit values are listed in table 3.

3.3 Injection simulation

The sealing time was calculated by Autodesk Moldflow Synergy® for each pressure and mould temperature. Please note the quite fast sealing times listed in table 4. 3.5 Tensile testing

Tensile tests were run at a speed of 50 mm/min. There were no different results obtained at lower speeds. Of each batch and set up, five samples were tested. Figure 4 and Figure 5 show the averaged testing curves as well as standard deviations. All samples exhibited necking at some point in the tensile test.

In addition to the stress-strain curves, the Young’s Modulus was calculated directly by the software testXpert ® by Zwick. The following results show the change in the Young’s modulus for different ambient temperatures at constant mould temperature (Figure 6) and constant pressure (figure 7). Figure 8 shows the change in Young’s modulus for varying sampling temperatures at constant pressure.

TABLE 3 CRYSTALLINITY VS. MOULD TEMPERATURE

Mould temperature [°C]

Average crystallinity [%]

20 27.4 ± 0.0 30 26.6 ± 1.2

50 26.2 ± 0.7 70 27.3 ± 1.3

Crystallinity of PBT tensile samples produced at different mould temperatures and measured by DSC analysis. The heat of fusion of 100% crystalline PBT was taken to be 144.5 J/g [1].

TABLE 4 SIMULATED SEALING TIME

Sample Sealing time [s] Mass [g] A-70-0800 3.50 11.30 A-70-1200 4.11 11.51 A-70-1600 4.47 11.67 A-50-0800 2.63 11.33 A-50-1200 3.02 11.52 A-50-1600 4.13 11.68 A-30-0800 3.35 11.37 A-30-1200 4.29 11.58 A-30-1600 3.80 11.73

Simulated sealing times of the samples.

FIGURE 3: Analysis of pressure near sprue during the filling process of the injection mould. Constant pressure p=1200 bar, mould temperature 30°C (blue), 50°C (green), 70°C (red). FIGURE 4: Engineering stress-strain curves of different samples

for one batch (A-30-0800). Samples neck at quite different strain values, hence the standard deviation is comparably large.

FIGURE 5: Averaged engineering stress-strain curves of A-30-0800 samples at different testing temperatures. Average and standad deviation were calculated from five samples for each condition.

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4. DISCUSSION / CONCLUSIONS Already during preparation, some parameters were

measured and analyzed. Especially the pressure near the sprue is of interest. The sealing point was determined at 30° C mould temperature by increasing holding time to 12 s where no change of weight of the samples could be seen. Additionally, computer simulations revealed even lower sealing times. However, the pressure curves reveal some loss of pressure inside the mould, once the holding pressure is released. This might be due to some backflow of PBT out of the mould before reaching the sealing point. Another interesting feature can be seen after 7 seconds. The cause of the instability of pressure is still unclear and needs to be investigated by further experiments.

4.1 DSC for crystallinity

The crystallinity of the samples calculated from the DSC analysis is constant around 26 % - 27 % for all mould temperatures (table 3). Hence, varying only this production parameter is not sufficient to be able to regulate the crystallinity of the material. This property of PBT could prove to be interesting for industrial manufacturing since the injection molding process can be performed over a wide range of mould temperature and still produce parts with unchanged mechanical properties. 4.2 Simulation of injection

Compared to the sealing times calculated from empirical data, i.e. the 13 seconds used for the sample preparation, the

simulated values are significantly smaller, which indicates that more simulation parameters might be needed to produce results closer to the experimental data (table 4). However, PBT shows very low sealing time in general. Concluding from these results, PBT conducts heat perfectly and a high degree of crystallinity cannot be prevented. 4.3 Tensile tests

The tensile tests recorded for different testing temperature show the expected behavior. At room temperature, PBT is below its glass transition temperature and shows highest strength. At lower temperatures, the different samples show lower correlation during necking among each other and hence the standard deviation is high. This behavior is explained by the molecular structure of PBT. Below glass transition temperature, the amorphous part is not flexible. Small defects and irregularities such as air bubbles thus have a larger influence on onset of sample necking. As the chains get more flexible, these defects are no longer primary source of failure and the sample behavior becomes more reproducible. Additionally, the high toughness of PBT below glass transition is remarkable. Figure 5 shows that the stress on the samples after necking varies less than a factor of 2 between room temperature and 80° C.

Holding pressure and mould temperature do not seem to

have any effect on the samples Young’s modulus (Figure 8). This result was to be expected after the DSC analysis, which showed no significant changes in crystallinity for different

FIGURE 6 FIGURE 7

FIGURE 8

FIGURES 6-8: (6) Young’s modulus for constant mould temperature 50 °C. The different colours refer to the different sampling temperatures. 80 °C (blue), 60 °C (red), 40 °C (green), room temperature (purple). (7) Young’s modulus for a constant pressure of 1200 bar. The different colours refer to the different sampling temperatures. 80 °C (purple), 60 °C (green), 40 °C (red), room temperature (blue). (8) Young’s modulus for different sampling temperatures. The colours refer to the different mould temperatures. Note that the obtained results are nearly equal for different mould temperatures. The pressure is kept constant at 1200 bar.

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mould temperatures. The expected decrease of the Young’s modulus for increasing sampling temperatures as seen in Figure 6 underlines the glass transition at 47 °C. At room temperature, all samples show a high Young’s modulus of about 1700 MPa. At 40 °C (close to Tg), some parts of the amorphous part of the polymer seem to be flexible already. A big drop of the Young’s modulus is obtained when rising temperature above Tg. However, even at elevated temperatures, PBT shows impressive strength due to the high degree of crystallinity given that only one quarter of the volume is solid. 4.4 Other experimental data

In addition to the DSC analysis, simulation and tensile tests, some sample dimensions and weights were recorded but negligibly low shrinkage of around 2 % and no significant changes in mass could be detected. Furthermore, charpy impact tests of all samples were performed but did not produce meaningful results. At room temperature, PBT is too tough to be fractured. At temperatures below -70 °C, samples did fracture but not in a reproducible way. 4.5 Conclusion

In summary, the experiments prove the remarkable toughness and strength of semi-crystalline polymers above Tg. Hence, polymers such as PBT can be used at elevated temperatures. The fast rate of crystallization impairs difficulties upon the analysis of the influence of crystallinity on the behavior of the Young’s modulus at different temperatures. The experiments run for this investigation cannot prove, that the obtained behavior of the Young’s modulus results from crystallinity. However, the stable and reproducible characteristics of PBT even at varying preparation conditions are favorable for industry. In connection with low cost, wide temperature range for application and low humidity uptake, PBT is perfect for various applications but as a consequence of its inflexibility

in crystallinity and mechanical properties unsuitable for lab course. B. Equations

In order to calculate the cooling time (tk) at different mould temperatures (ϑW) and melt temperatures (ϑM), the following equation was used, where s is the mould wall thickness, ϑE is the maximum demoulding temperature and a is the effective thermal conductivity, which was calculated to be 0.131 W/(mK).

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ACKNOWLEDGMENTS We would like to thank Thomas Schweizer, who greatly

contributed to the presented results. Furthermore, we thank Werner Schmidheiny, who prepared the special clamps used for the tensile testing within the heating chamber.

REFERENCES [1] A. Deschamps, D. Grijpma, J. Feijen (2001).

Poly(ethylene oxide)/poly(butylene terephthalate) segmented block copolymers: the effect of copolymer composition on physical properties and degradation behavior. Polymer 42, 9335-9345

[2] BASF (2012). Ultradur® B 4520 Produkt Information Data. BASF SE

[3] W. Kaiser (2006). Kunststoffchemie für Ingineure. Carl Hanser Verlag München Wien.

[4] BASF (2013). Mechanische Eigenschaften von Ultradur® (PBT, PBT/ASA) Retrieved December 4th 2013, from http://www.plasticsportal.net

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