Keywords

1 Introduction

The optimization of battery safety, efficiency, and shatter resistance was the inspiration for starting this research. Polybutylene terephthalate (PBT) is a good candidate for the purposes mentioned. PBT is a thermoplastic engineering polymer with high strength and impact strength, low moisture absorption, water and chemical resistance, high thermal stability, hardness, abrasion, short molding time, and good surface [1].

PBT is widely used in electrical systems, including lamp holders, switches, circuit breakers, and motor housings. It has high heat resistance, mechanical strength and water resistance, and excellent insulating properties [2,3,4]. However, pure PBT is flammable. Glass fiber (GF) is added to PBT to prevent dripping and help maintain the integrity of combustion. Phosphorus-based compounds are added to PBT to improve fire resistance [5]. Melamine derivatives have been shown to synergize with phosphorus flame retardants to enhance the flame retardants of mixtures further [6, 7].

PBT is a suitable material for making battery cases. However, in this area, even if PBT has high rigidity to a certain extent, this property can be improved, and one way to do this is to add GF to reinforce PBT [8]. GF-modified PBT can manufacture electronic components that require operation under prolonged high-temperature conditions with high dimensional stability [9,10,11] and materials with high fire resistance [12, 13]. The effect of GF length and content has been reported in some polymers, such as polypropylene [14,15,16,17]. However, no publication has yet mentioned research on the effects of GF on its stiffness resistance to PBT.

The advantage of PBT is its resistance to deformation over time with a stable temperature. The hardness and bearing capacity are good, but poor fire resistance still has has disadvantages, so it cannot be widely used in some fields. GF is a suitable choice to improve the fire resistance of the material. If PBT is reinforced with GF, the impact toughness increases, contributing to the optimal properties.

2 Materials and Methods

Table 1. The composition of the sample (wt.%).
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This study used plastic materials PBT-30GF and PBT. PBT-30GF is produced by TA COMA CO., LTD, and PBT resin is supplied from Lanxess (Germany). PBT and PBT-30GF were mixed in the proportions in Table 1 and dried at 110 ℃ for about 4 h, reaching a humidity of less than 0.03%, using Toshiba's 100-ton injection molding method.

After injection molding, samples obtained at each ratio were used for impact strength measurement on the AG-X plus material tester, according to ASTM D256–10. Before proceeding with sizing, ensure the measuring medium is at a temperature of 23 ±  2 ℃ and a relative humidity of 50 ± 5% in the air. In addition, laboratory samples need to be stored for at least 40 h. Measurement: First, enter the measurement parameters into the machine at a speed of 5 mm/min. Next, fasten the V-notched sample to the device. After that, start measuring the impact length. Allowing the device to apply force to the upper part of the sample will cause the sample to break later. Finally, record the test data of the sample and remove the piece. Proceed to repeat the sequence of steps for the following models.

AG-X Plus is a PBT/GF blend impact strength tester. This device uses a method of converting energy from potential energy to kinetic energy to determine the impact resistance of a sample. The mass, length, and speed of movement of the striking hammer will affect the accuracy of the results when measuring. The AG-X Plus can compensate for errors caused by air friction resistance and hammer drop angle with the electronic control system. The ASTM D256-10 measurement method was used in this experiment's instructions and explanations in Fig. 1.

Fig. 1.
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V-notch impact strength testing

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3 Results and Discussion

Table 2. Average impact strength parameter of the test specimen
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The data in Table 2 show that the impact strength for each measurement is irregular. The GF5 prototype will gradually decrease from 4.69 kJ/m2 to GF15 of 3.73 kJ/m2 and progressively increase to 6.28 kJ/m2. This result may be due to the effect of the distribution density of GF on the test specimen. The standard deviation is in the range of 0.11-0.73, which is relatively small.

Fig. 2.
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Impact strength of PBT/GF blend

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Figure 2 describes the impact strength of the PBT/GF blend. Looking at the column chart, the impact strength of the sample containing 25% GF has a higher average impact strength than other samples. The sample containing 15% GF is the lowest. This result is due to the GF content present in this mixture. It is more flexible in GF5 and GF10 models with low GF content. The GF content is higher from GF20 and GF25 models and above, so it will be more rigid and less flexible. The GF15 model is shallow because the impact strength and flexibility are low, so breaking with a manageable force is easy. The impact strength will likely increase if the amount of GF increases. However, the material is very brittle when GF reaches its maximum. Our team will adjust below 30% GF in the mixture to limit defects and improve impact strength at the appropriate level.

Impact strength is influenced by GF content, which has been studied for the mechanical properties of PBT. The dispersion density of GF helps the mixture reduce plastic shrinkage and retain the shape of the sample. The impact strength of the mix has increased significantly with the addition of GF [15]. Although the impact strength is uneven, it is still 100% higher than PBT. The impact strength of PBT of 3.66 kJ/m2 is the lowest, lower than PBT/15GF of 3.71 kJ/m2. The reason for the increase in impact toughness is the presence of GF. The GF content further enhances the impact intensity. After measuring the impact strength, we will conduct a microscopic scan to observe compatibility between GF and PBT.

Fig. 3.
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SEM microstructure of the PBT/GF blend

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Figure 3 shows the SEM microstructure of the fracture surface of the PBT/GF mixture samples corresponding to the percentage of GF present in the PBT mixture. The increased density of GF on the PBT matrix shows the compatibility between PBT and GF in the mix. PBT stands out on GF surfaces as fault lines, meaning GF is highly compatible with PBT. Figure 3 shows the effect of GF content on the impact strength of PBT and PBT/GF mixtures. Compared with the neat PBT, the impact strength of the PBT/GF mixture moderately rises as the GF percentage increases. The nature of the hard GF relative to the matrix dominates the increase in impact strength. They can be caused by high dispersion and adhesion between GF and PBT matrices. Therefore, it can be seen that adding GF to PBT is an effective means of enhancing the properties of PBT.

At fiber content greater than 5 wt.%, the fiber length of the mixture is an essential factor for the mechanical strength of PBT/GF mixtures. From the results of Fig. 3, we have seen that the GF length of mixtures made with lower molecular weight PBT [16] is longer than that of mixtures produced with higher molecular weight PBT [17]. This behavior is probably due to the more severe decomposition of GF in the dense matrix than during mixing PBT and GF in plastic injection machines.

4 Conclusions

The mechanical properties are improved compared to 100% PBT sample. The slightest impact strength is 3.71 kJ/m2, and the highest is 6.28 kJ/m2. The impact strength decreases when GF reaches 15% but is still higher than 100% PBT. The additional filling increases friction between the elements of GF and uniform distribution throughout the mix, which helps to create strong bonds that increase the rigidity of the material; plus, the uniform distribution of GF throughout the mixture through SEM images has known the impact of GF. Flame retardant properties are also further improved due to the addition of GF scattered in the mix. PBT resin is additionally reinforced with GF, which can prevent the shrinkage of the mixture and create uniformity, which is a factor that clarifies the effect of the toughness of the improved material. This result makes the material less susceptible to environmental influences. This result is ideally suited for manufacturing battery cases without using expensive plastics while still having good properties and durability that save on fees. It even opens up new applications in sectors that require strength and resilience, such as the automotive or electronics industries. Overall, our research expands our understanding of how GF affects the mechanical properties of PBT and shows new prospects for developing materials with similar mechanical properties, which can be advanced development, as a premise for promoting rich applications in the manufacturing industry.