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Abstract

Nano-CaCO₃-modified polypropylene (PP) nonwoven fabric composite is widely used as the filter material which could overcome the poor lightfastness and low weight of the pure one, while its aging performances would be influenced. In this article, the aging behavior of the nano-CaCO₃-modified PP nonwoven fabric composite was investigated. At first, the difference between untreated and CaCO₃-modified PP nonwoven fabric was analyzed by scanning electron microscopy, thermogravimetric/differential thermal analysis, and X-ray diffraction. Then the aging behaviors of these two PP nonwoven fabrics were studied in detail. The results showed that the PP nonwoven fabric exhibited good acid corrosion resistance. On the contrary, acid corrosion resistance of the modified PP nonwoven fabric was poor; although its weight and strength didn’t change under weak acid condition, its weight and strength loss both increased obviously under medium and strong acid condition with the prolongation of corrosion time. Due to the ultraviolet (UV) energy dissipation of nano-CaCO₃ particles, the UV aging resistance of the modified PP nonwoven fabric was improved, and the compound UV aging under weak acid condition was also superior to that of the untreated PP fabric. This article will provide experimental and theoretical base for the structural regulation of CaCO₃-modified PP fibers.

Keywords

Aging acidic nonwoven composite

Introduction

Filter fabrics have been widely applied in air purification systems nowadays. Among these filter fabrics, polypropylene (PP) nonwoven fabric accounts for a large proportion due to its good mechanical properties, low cost, and easy processing.^1,2 The pore size and distribution of the PP nonwoven can be easily controlled to meet different permeability in use. It can be used in depth filtration, and its air purification rate can be up to 99.9% in laboratory conditions.³ Meanwhile, filling micro- or nano-inorganic particles into the PP slice and then preparing the modified PP fiber through melt spinning process can change the original molecular chain arrangement of the PP and induce the generation of the β-PP crystal form.^4
-6 The strength, tenacity, thermal stability, antiaging, and other properties of the modified PP fibers have been further improved compared with the untreated PP fibers.⁷ At present, the micro- or nano-CaCO₃ particles are often used as a filler to modify PP in actual production for its lower cost.⁸

The CaCO₃-modified PP nonwoven fabric composites are processed into filter bags and used for exhaust gas purification and filter dedusting, especially in cement, chemical, automobile, and other industries; the effect of acid and ultraviolet (UV) radiation on its usage must be considered. Under the influence of acidic substances in the exhaust gas, such as SO _x and NO _x ,^3,9 the acid corrosion resistance of the CaCO₃ component itself and its distribution in the fiber may change the aging behavior of the modified PP products. The modified PP may be corroded to accelerate the aging process, resulting in fiber damages and shortening of the service life. At the same time, for the reason that exhaust gas treatment is usually exposed to sunlight, UV irradiation under acid conditions is a complex factor that must be considered. As far as we know, there is no relevant article published yet. In this article, the aging behaviors of CaCO₃-modified PP nonwoven fabric were investigated in terms of acid corrosion and UV aging under acid conditions. During this investigation, the weight loss rate, strength loss rate, and other indexes were used to evaluate its aging process. Therefore, this article will provide theoretical guidance for the rational selection of PP fiber filters under acid conditions and experimental and theoretical support for the structural regulation of CaCO₃-modified PP fibers.

Experiment

Materials and chemicals

Both PP and CaCO₃-modified PP nonwoven fabrics with the same areal density of 18 g m⁻² used in this experiment were provided by Cobes Industries (Hefei) Co., Ltd, China.

The 36% hydrochloric acid with Analytical Reagent (AR) grade was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd, China. The ultrapure water was used to prepare the solution in this experiment.

Experimental methods

Acid corrosion aging

Three different acid solutions with the pH of 1, 3.5, and 5.5 were prepared at first. Then both the PP nonwoven fabric and the CaCO₃-modified PP nonwoven fabric were immersed into each acid solution under the same condition. After being treated for different times, the fabrics were rinsed with clean water and dried. The rate of weight loss and breaking strength loss of each fabric were determined, and the morphology of fabrics after acid corrosion aging was measured.

UV aging and compound UV aging under acidic condition

ZP UV aging test chamber-spray type (Nanjing Huanke Equipment Co., Ltd, China) was used to determine the aging performance of the PP nonwoven fabrics. Eight UV-B tubes (HK TL40 W/L1200*d38, Nanjing Huanke Equipment Co., Ltd, China) were used as light source, and their luminescence spectra were described in our previous research paper.¹⁰ Before exposure to UV irradiation, both the PP nonwoven fabric and the CaCO₃-modified PP nonwoven fabric were cut into 300 × 75 mm² samples. During the UV aging, irradiation was carried out without any pause. During the compound UV aging under acid condition, irradiation was carried out for 102 min according to ISO 4892-2: 2006 and sprayed for the next 18 min using the previously prepared acid solution with the pH value of 5.5. After different irradiation cycles, the fabrics were rinsed and dried with the same method as described above. Then the same method was used to determine the breaking strength loss rate of each fabric.

Characterization

Morphology characterization

The morphology of PP nonwoven fabric and CaCO₃-modified PP nonwoven fabric before and after aging was measured by S-4800 scanning electron microscope (SEM; Hitachi Co., Ltd, Japan). Each fabric was treated by gold-sputtering before SEM test, and the difference of morphology between PP and CaCO₃-modified PP nonwoven fabric was analyzed.

X-ray diffraction characterization

X-ray diffraction (XRD) of PP nonwoven fabric and CaCO₃-modified PP nonwoven fabric before aging was measured, and the XRD patterns were recorded in a D8 series X-ray (powder) diffractometer (Bruker Instruments, Germany) using Cu Kα radiation (λ = 1.54 Å). It was operated in reflective mode at a voltage of 40 kV and a filament current of 30 mA. Radial scans of intensity versus diffraction angle (2θ) were recorded in the range of 5–60°C with a scanning rate of 2°C min⁻¹ and a step size of 0.02.

Differential thermal analysis and thermogravimetric analysis

Differential thermal analysis/thermogravimetric analysis (DTA/TG) was carried out with DTG-60 H microcomputer differential thermal balance (Shimadzu Corporation, Japan) under the nitrogen atmosphere with the heating rate of 20°C min⁻¹ in the range of 30–700°C.

UV-visible transmittance spectrum analysis

Transmittance spectra of PP nonwoven fabric and CaCO₃-modified PP nonwoven fabric were recorded in the range of 205–500 nm on a Lambda 1050 spectrophotometer (PerkinElmer, Waltham, Massachusetts, USA).

Weight loss rate analysis

In order to accurately measure the weight of fabric before and after aging, each fabric was dried absolutely and then balanced at 20 ± 2.0°C with a relative humidity of 65 ± 2.0% for 24 h, and the mean value of five measurements was measured. Weight loss rate (wr%) was calculated according to equation (1)

wr%= (1 - \frac{m_{i}}{m_{0}}) \times 100 %

where m ₀ is the original weight of fabric before aging and m_i is the weight of fabric after aging.

The test on breaking strength and analysis on strength loss rate

The breaking strength of PP nonwoven fabric and CaCO₃-modified PP nonwoven fabric before and after aging was tested according to the ASTM D 5035-1995(2003) standard by YG026D multifunctional electronic fabric strength machine (Wenzhou Darong Textile Instrument Co., Ltd, China). The mean value of five measurements was calculated. Strength loss rate (sr%) was calculated in accordance with equation (2)

sr%= (1 - \frac{F_{i}}{F_{0}}) \times 100 %

where F ₀ is the original breaking strength of fabric before aging and F_i is the breaking strength of fabric after aging.

Results and discussion

Difference between PP and CaCO₃-modified PP nonwoven fabric

Figure 1 shows the SEM images of PP nonwoven fabric and CaCO₃-modified PP nonwoven fabric. The PP nonwoven fibers possess smooth surface and uniform fineness, whereas the CaCO₃-modified PP nonwoven has extremely rough fiber surface, each fiber is covered with particles and protrusions, the uniformity of modified PP fibers is not as good as that of PP fibers, and the fineness of the modified PP fibers varies greatly. These particles and protrusions covered on the surface of modified PP fibers are caused by CaCO₃ particles due to its high surface energy and the tendency to agglomerate during the modification process,^11,12 resulting in the rough surface of modified PP fibers.

Figure 1.

Morphology of PP and modified PP nonwoven fabric: (a) PP nonwoven, (b) CaCO₃-modified PP nonwoven, and (c) magnification SEM.

TG and DTA are the common methods to characterize the thermal stability of polymers.¹³ Herein, the TG and DTA graphs of PP and modified PP nonwoven fabric are presented in Figure 2. TG graph shows that the ranges of weight loss of PP and modified PP nonwoven fabric are approximately 230–440°C and 250–460°C, respectively. Compared with PP nonwoven fabric, the starting thermal decomposition temperature of modified PP nonwoven shifts toward high temperature, indicating that its thermal stability is enhanced.^14,15 In addition, the weight retention rate of PP nonwoven remains at 0% when the temperature exceeds 440°C, showing that PP nonwoven has been completely pyrolyzed. In comparison, the weight retention rate of modified PP nonwoven fabric remains at 40% when the temperature exceeds 460°C, demonstrating that there was still residue after pyrolysis; the residue was CaCO₃ component with the content of 40% (m/m). DTA graph shows that PP nonwoven fabric and modified PP nonwoven fabric have similar endothermic signals during their melt transformation. However, there is a significant difference between the two curves in the range of 225–437°C; the curve of modified PP is basically stable without any exothermic or endothermic signals, whereas an obvious exothermic signal is shown in the curve of PP nonwoven fabric. In other words, PP nonwoven fabric began to decompose while the modified PP was still stable. The reason for improving the thermal stability of the modified PP is that the movement of the PP macromolecular chains is limited by the filling CaCO₃ particles during the thermal degradation, thereby its thermal decomposition is blocked.

Figure 2.

(a) TG and (b) DTA graphs of PP and modified PP nonwoven fabric.

As revealed in the XRD spectra in Figure 3, the main crystal of the PP nonwoven fabric is α-form with a strong diffraction intensity. In comparison, the diffraction signals of PP component and nano-CaCO₃ component were shown in the modified PP nonwoven fabric.¹⁶ The main crystal of the modified PP is also α-form with a small amount of β-form (2θ = 16° is the characteristic diffraction peak of β-form PP).^17,18 Besides, the diffraction intensity of modified PP nonwoven fabric is weaker than that without modification, which shows another difference between these two nonwoven fabrics. The weak diffraction intensity of the modified PP nonwoven fabric indicates that the spherulite size becomes tiny compared with the PP nonwoven,^19,20 which was attributed to the increased nucleation sites caused by the nano-CaCO₃ particles.

Figure 3.

XRD graph of PP and modified PP nonwoven fabric.

Acid corrosion aging

PP nonwoven fabric and modified PP nonwoven fabric were immersed into acid solutions with different pH value, and the weight and breaking strength of fabrics after aging for different time were measured carefully. Then, the rate of weight loss and breaking strength loss of each fabric were calculated according to equations (1) and (2), and the results were shown in Figures 4 and 5, respectively.

Figure 4.

Weight loss of PP and modified PP under acid corrosion aging.

Figure 5.

Break strength loss of PP and modified PP under acid corrosion aging.

As shown in Figure 4, PP nonwoven fabric possesses good acid corrosion resistance; its weight loss is very low and is hardly affected by acid strength and aging time. Nevertheless, the modified PP nonwoven fabric is very sensitive to medium and strong acid, especially under the strong acidic condition with the pH of 1, the weight loss rate of fabric exceeds 10% after aging for 4 h, and the weight loss further increases with the prolonging of the aging time, the weight loss rate of fabric is close to 15% after aging for 48 h. Compared with the strong acid corrosion aging, the weight loss of modified PP nonwoven fabric reduces significantly under moderate acid condition with a pH of 3.5, but the fabric weight loss is still close to 4% after aging for 48 h. Although the weight loss of modified PP nonwoven fabric increases with the prolonging of the aging time under weak acid condition with a pH of 5.5, the value remains at a relatively low level, less than 0.5% even after aging for 48 h. Therefore, we can conclude that weak acid aging has little effect on modified PP nonwoven fabric, while moderate and strong acid aging have a severe impact on modified fabric in terms of weight loss.

According to Figure 5, similar to the weight loss change shown in Figure 4, the breaking strength of the PP nonwoven fabric is also hardly affected by the acid corrosion aging. Whereas the breaking strength loss of the modified PP nonwoven fabric is very obvious, for instance, the breaking strength of the modified PP nonwoven fabric (aging for 48 h under the strong acid condition with the pH of 1) is more than 35%, indicating that this fabric has been damaged seriously. The strength loss change of modified PP nonwoven fabric is consistent with its weight loss change during the aging process, but the strength loss change of modified PP nonwoven fabric under weak acid condition is very different from that under medium and strong acid condition. The strength loss rate is 0% which won’t increase with the aging time under weak acid condition, while the strength loss rate increases with the aging time under medium and strong acid condition, indicating that modified PP nonwoven fabric is resistant to weak acid corrosion. Combined with analysis on the weight loss rate, we can conclude that only the surface of the modified PP fiber will be corroded under weak acid condition while the fiber body won’t be damaged, so that its mechanical properties will not be reduced.

The morphology of modified PP nonwoven fabric after aging for different time under different acid conditions was investigated, and the result is shown in Figure 6. Weak acid aging only worked on the surface of the modified PP fiber; with the aging time prolonging, the particles or protrusions covered on the surface of modified PP fibers were dissolved gradually. The main body of the fiber remained and there was no fiber breakage in this study. Nevertheless, the CaCO₃ particles or protrusions covered on the surface of modified PP fibers were first dissolved by the strong acid, and then with the aging time prolonging, strong acid penetrated into the interior of the fiber through the micropores or the interfacial gap and continued to dissolve the CaCO₃ compound existed there, leading to the fiber breakage. The breakage of the modified PP fibers was more likely to occur at a position where the CaCO₃ component was agglomerated or concentrated.

Figure 6.

SEM images of modified PP nonwoven fabric under acid corrosion aging: (a) weak acid aging (pH: 5.5) and (b) strong acid aging (pH: 1).

Compound UV aging under acidic condition

Compound UV aging under weak acid condition with a pH of 5.5 was conducted on PP nonwoven fabric and CaCO₃-modified PP nonwoven fabric; the strength of each aged fabric was measured and the strength loss percent was calculated in line with equation (2). In the meantime, the difference on the effect of compound UV aging under weak acid condition and UV aging was analyzed, and the result is shown in Figure 7.

Figure 7.

Strength loss of PP and modified PP nonwoven after UV aging.

PP nonwoven fabric possesses poor UV aging resistance.^21,22 As shown in Figure 7, the breaking strength loss rate increased significantly with the extension of UV aging time. There was no significant difference in terms of the strength loss of PP nonwoven fabric between the UV aging and UV aging under weak acid condition within 10 h, but with the aging continuing, the effect on the strength loss of aged fabric of UV aging under weak acid condition was stronger than of UV aging. Acid solution sprayed on the fabric during the UV aging could penetrate into the fiber through cracks on the fiber surface, and react with the aging product of PP such as esters, carbonyl compounds, and hydroxyl groups, thus promoting the breakage of PP fibers.^23,24 Due to less cracks on the surface of PP fibers at the beginning of UV aging, the acid solution could hardly enter the interior of the fibers. Therefore, the effect of UV aging under weak acid condition is gradually higher than UV aging as the aging time prolonging.

Compared with PP nonwoven fabric, CaCO₃-modified PP nonwoven fabric has better anti-UV aging properties, especially in the first 10 h of UV aging, and the strength loss rate is less than 7.5%. Meanwhile, UV aging under weak acid condition has no more serious impact on the strength loss of modified PP nonwoven fabrics. The reason why the modified PP nonwoven fabric has better UV aging resistance than PP fabric is that the dissipation of UV energy is increasing by the addition of nano-CaCO₃ particles.^25,26

The transmittance spectra could be used to explain the interaction between the photon energy and the fiber. Herein, the UV transmission spectra of PP and modified PP nonwoven fabric have been measured and shown in Figure 8. The transmittance of the modified PP is significantly lower than that of the PP fabric, and UV energy applied to the PP macromolecules is less, resulting in the improvement of the UV aging resistance of the modified PP nonwoven fabric. For the same reason as PP nonwoven fabric, the effect of UV aging under weak acid condition on the strength loss of the modified PP fabric will be greater than the UV aging with the extension of the aging time.^27,28 Above all, compared with PP nonwoven fabric, the CaCO₃-modified PP nonwoven fabric has better resistance to UV aging under weak acid condition or UV aging.

Figure 8.

Transmittance spectra of PP and modified PP nonwoven fabrics.

Conclusions

The chemical structure and aging behaviors of the pure and CaCO₃-modified PP nonwoven fabrics have been investigated in this research. The surface of CaCO₃-modified PP fibers was covered with CaCO₃ particles or protrusions, resulting in the increase of fiber surface roughness; the aggregation structure of modified PP changed, and its thermal stability was enhanced. The weight loss and strength loss of the modified PP nonwoven fabric increased rapidly with the prolongation of the ageing time under medium or strong acid condition, indicating that modified PP nonwoven fabrics do not have the ability to resist medium or strong acid corrosion aging. However, the modified PP nonwoven fabric possessed weak acid corrosion resistance because its weight and strength did not change under the weak acid corrosion aging. Due to the UV energy dissipation of nano-CaCO₃ particles, the UV aging resistance of the modified PP nonwoven fabric was improved, and the compound UV aging under weak acid condition was also superior to the unmodified PP nonwoven fabric. The damage caused by the compound UV aging under weak acid condition on the modified PP nonwoven fabric was greater than UV light aging alone, especially with the prolongation of the ageing time.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was financially supported by the Key Research and Development Plan Project of Anhui Province (1804a09020077); Open Project Program of Anhui Province College of Anhui Province College Key Laboratory of Textile Fabrics, Anhui Engineering and Technology Research Center of Textile (2018AKLTF14); the Natural Science Foundation of Anhui Province (1908085QE225); and the Scientific Research Fund of Talent Introduction of Anhui Polytechnic University (2018YQQ010).

ORCID iD

Yinchun Fang

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Aging behavior of nano-CaCO 3 -modified polypropylene nonwoven fabric composites

Abstract

Keywords

Introduction

Experiment

Materials and chemicals

Experimental methods

Acid corrosion aging

UV aging and compound UV aging under acidic condition

Characterization

Morphology characterization

X-ray diffraction characterization

Differential thermal analysis and thermogravimetric analysis

UV-visible transmittance spectrum analysis

Weight loss rate analysis

The test on breaking strength and analysis on strength loss rate

Results and discussion

Difference between PP and CaCO3-modified PP nonwoven fabric

Acid corrosion aging

Compound UV aging under acidic condition

Conclusions

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References

Difference between PP and CaCO₃-modified PP nonwoven fabric