Characteristics of PPTA Chipped Fiber/Fibrid and their Properties for Sheet Making

Transcription

1 Characteristics of PPTA Chipped Fiber/Fibrid and their Properties for Sheet Making Zhaoqing Lu, Ming Jiang, Meiyun Zhang, Xinxing Xia College of Light Industry & Energy, Shaanxi University of Science and Technology, Xi an Shaanxi CHINA Correspondence to: Zhaoqing Lu ABSTRACT Poly-p-phenylene diamine terephthalamide (PPTA) chipped fibers and fibrids can be made into insulation sheet materials by papermaking wet forming and hot calendaring operations. PPTA chipped fibers offer mechanical strength for the sheet, while PPTA fibrids act as binder. Novel PPTA fibrids and chipped fibers were investigated using SEM, a fiber quality analyzer and a fiber classifier. PPTA fibrids exhibited small sized pieces with wrinkled surfaces, uniform size and a high degree of fine fibrosis while short fiber was smooth and nearly cylindrical in shape. The effect of the fibrid to chipped fiber ratio on PPTA sheet mechanical and dielectric properties is investigated and an optimal ratio of 7:3 was determined. The pore structure and thermal stability of PPTA sheet were analyzed by mercury intrusion porosimetry and TG-DSC. The structure of PPTA sheet was similar to reinforced concrete structure; the pores were small and the bind between fibers was tight. The initial decomposition temperature and TG 10% of PPTA sheet are 535 C and 560 C, respectively, indicating excellent thermal properties. INTRODUCTION As one of the most important value-added materials, PPTA sheet has attracted increasing attention from major materials companies worldwide, including DuPont company in America [1], Teijin Company in Japan [2] and Taihe new material company in China [3]. With properties including high strength, high modulus, low density, excellent dielectric properties and thermal stability, PPTA sheet has become increasingly popular for use in such high performance products such as honeycomb structure material, high temperature insulating material and high performance electronic materials for applications in the aerospace and national defense industries [4-6]. Currently, commercial PPTA sheet is made from chipped fibers and pulp using papermaking, wet forming and hot calendaring operations. The chipped fibers act as reinforcement and the pulp serves as a matrix. The pulp, which is made from PPTA chipped fiber by mechanical treatment in water, has poor interfacial bonding with the chipped fibers because of an inert surface, high stiffness and high crystallinity, These properties result in loose sheet structure and low strength common to PPTA [7-8]. In order to overcome these drawbacks, a number of studies [9-10] have been done and a variety of techniques have been utilized, mainly focusing on chemical modification. These efforts have been limited by difficult reaction conditions and high production costs. PPTA fibrid is a new type of para-aramid fiber, which has been manufactured in recent years by precipitation methods. Previous research by the authors [11] has focused on the structure property relationships between PPTA pulp and fibrids and the resulting sheet. It was found that fibrids had better mechanical properties and were easier to incorporate into the sheet than pulp, which may offer some potential advantages in the PPTA sheets. However, there are few reports on PPTA fibrid and its application in PPTA sheet. The objective of this study was to investigate the characteristics of PPTA chipped fiber/fibrid and their properties for sheet making. The morphology characteristics of PPTA chipped fibers and fibrids were analyzed by SEM, fiber quality analyzer and fiber screening. The effect of the fibrid/fiber ratio on PPTA sheet properties is discussed and the pore structure and physical properties of novel PPTA sheet are characterized. Journal of Engineered Fibers and Fabrics 1

2 EXPERIMENTAL Materials PPTA chipped fibers and fibrids were kindly provided by a chemical fiber company in Germany. Both of them are made by a condensation reaction and share chemical structure shown in Figure 1. PPTA chipped fibers were cut from spun PPTA yarn, which was formed in PPTA-H 2 SO 4 solution by the dry-jet-spinning method. PPTA fibrids were produced by precipitation from copolymer solutions of PPTA at low temperature. The average length of PPTA chipped fibers and fibrids were 5-6 mm and mm, respectively. The CSF freeness of fibrids was 175 ml. H 2 N O C NH 2 Para-phenylene diamine + Cl O C O C H N O C Terephthaloyl chloride Poly para-phenyleneterephthalamide FIGURE 1. Chemical structure of PPTA. Methods Used to Characterize the PPTA Fibers The surface morphology of PPTA fibers was studied using scanning electron microscopy (Hitachi S-4800, Japan) at 3.0 kv after the specimens were sputter coated with a thin layer of gold to render them conductive. Fiber length, curl index, kink index and fines content of PPTA fibers were analyzed using a Morfi Compact instrument from TECHPAP in Germany. The definition of Curl Index is the ratio of the true contour length L of the fiber to the projected length l of the fiber minus unity. Eq. (1) was used to calculate Curl Index [12]. L Curl index = ( 1 ) 100% (1) l The Kibblewhite equation, Eq. (2) [13], was used to calculate kink index. H N Cl n PPTA fibrids screening was conducted on Bauer McNett Instrument from PTI in Austria, according to Tappi standard method T233 (T233 cm-95). Ten grams of fibrids were used, and the average values of duplicate tests are reported. Preparation of PPTA Sheet PPTA sheets with basis weight of 45 g/m 2 were made using an ERNSTHAAGEBBS-3 hand sheet former. The proportion of PPTA chipped fibers and fibrids was adjusted according to Tappi standard method T205 (T 205 sp-95). The hand sheets were further calendared by a three-roller soft nip calendar at 240 C, 112 kn/m and 1.5 m/min. Methods Used to Characterize the PPTA Sheet The thickness of PPTA sheets was measured in accordance with Tappi standard method T 411 (T411 om-89) on a DC-HJY03 thickness tester. Tensile strengths and elongation of sheets were measured according to Tappi standard method T494 (T494 om-88) on an L&W SE-062 tensile tester with a load of 0.26 MPa. Tear strengths of sheet were measured according to Tappi standard method T414 (T414 om-88) on an MIT Elmendorf tester. Dielectric strengths of sheets were tested according to American Society for Testing and Materials Standard Method ASTM D149 (D149-09) on a CS2672D tester. Three specimens of each sheet were tested. The average of three replicates is reported. The surface morphology of PPTA fibers was characterized by SEM (Hitachi S-4800, Japan) at 3.0 kv. Cross sections of PPTA sheet were prepared by cryogenic cutting in liquid nitrogen and observed by SEM at an accelerating voltage of 15.0 kv. All of the samples were Au coated prior to examination. The freeze-dried samples of PPTA sheet were tested by an AutoPore IV 9500 Hg-porosimeter from Micromeritics in America using a pressure range of psi. The equilibrium time for each applied pressure level was 10 s. The maximum and minimum applied pressures correspond to cylindrical pore sizes of 5.0 nm and 1000 μm respectively. Eq. (3) was used for calculating fractal dimension [14]. Kink 2 N ( 21 ~ 45 ) + 3N ( 46 ~ 90 ) + 4N ( 91 ~ 180 ) index = (2) LTotal Journal of Engineered Fibers and Fabrics 2

3 dv ln = ( D 4 )ln p + ln k (3) dp Where V is specific intrusion volume, p is pressure, k is constant, and D is the fractal dimension. Thermogravimetric analysis (TGA) and differential scanning calorimetric analysis (DSC) were performed on a STA-409PCA simultaneous TG/DSC apparatus from Netzsch in Germany at a heating rate of 10 C/min from room temperature to 800 C under nitrogen atmosphere. RESULTS AND DISCUSSION Surface Morphology of PPTA Fibers The surface morphology of PPTA chipped fibers and fibrids, which mainly depends on the method of their preparation and processing, can greatly influence the properties of PPTA sheet [15]. with a small amount of granular adhesion, which is consistent with the fiber surface characteristics by dry-wet spinning method. The morphology of PPTA fibrids was shown in Figure 2-b. Unlike chipped fibers, the fibrids were rough and supple, being curled and branched. This permits physical entwining during wet forming in the sheet making process. The fibrids non-rigid filmy, ribbon-like particles with irregular shape, and can be visualized as a soft, flexible handkerchief. The average fiber length of the fibrids is 0.3 to 0.8 mm and the surface areas are as large as 7.35 m 2 /g. The large surface area and high softness of fibrids can provide opportunities for the development of good bonding between PPTA chipped fiber and fibrids, thus helping to enhance the strength of PPTA sheets. The Dimensions of PPTA Fibers by Morfi Compact and Classification of Fibrids Fiber length, curl index, kink index and fines content of PPTA chipped fibers and fibrids have a significant impact on sheet properties such as tensile index, tear index at bonding levels, formation, porosity and bulk. Morfi Compact was used to analyze their morphological parameters, and the results are shown in Table I. The weighted length of PPTA chipped fibers is mm, which may cause flocculation during sheet forming. Thus, satisfactory dispersion of chipped fibers will be difficult to achieve during wet forming. The average fiber length of PPTA fibrids, mm, was much shorter than that of chipped fibers, which made for easier dispersion in water during sheet making. The content of fines in the fibrids was as high as 68.9 %. The small size of fibrids implies a higher specific surface area and a higher absorptive capacity of water. This favors improved dispersion in water, better sheet formation and superior bonding during sheet calendering [16]. FIGURE 2. SEM images of fiber surface of (a) PPTA chipped fiber; (b) PPTA fibrid. Figure 2-a shows the line-like morphology of PPTA chipped fibers; the chipped fiber had a solid, nearly circular shape. The average diameter of the fibers was 20 µm and the surface was bright and clean, TABLE I. Morphological parameters of PPTA fiber. Sample Chipped Fibers Fibrids Arithmetic length, mm Length weighted length, mm Coarseness( dg ) Curl index, % Kink index, mm Fines content (<0.2 mm), % Journal of Engineered Fibers and Fabrics 3

4 Fiber curl index and kink index can influence sheet properties. The degree of curl depends on the fiber manufacture. A certain amount of curl can promote physical entwinement of fibers, and improve the sheet strength without damaging the formation. Kink index reflects the tenacity of the fiber bends. The smaller the kink index is, the stiffer the fiber, and the better the paper properties [17]. In order to further analyze the fiber length distribution of PPTA fibrids, the Bauer McNett instrument was used to carry out screening. As shown in Figure 3, the fibrids had a narrow fiber length distribution, indicating a high uniformity of fiber length. The fibrids had a high ratio of screening components R50, indicating a good adhesion strength potential of fibrids in sheet [18]. The size of fibrids on the R100 and R200 screens and the lost fraction (<R200) reflects the range of fines in Morfi Compact system. The total content of 68.7 % was agreed well with the Mrofi Compact data. fibrid ratio R16 R R50 R100 R200 <R200 screening components FIGURE 3. Screening analysis of PPTA fibrids. The Mechanical and Dielectric Properties of Novel PPTA Sheet PPTA chipped fiber alone cannot be made into sheet. A certain amount of PPTA fibrids is needed to form a sheet fabric. Sheet properties mainly depend on the ratio of the two PPTA fiber types [19, 20]. As shown in Figure 4-a, both tensile index and tear index increased with the increase of the ratio of fibrid to chipped fiber, and reached maximum when the ratio was 7:3. As the fibrids content increased to this ratio, dispersion improved and sheet integrity increased, enhancing both the tensile and tear indices. As the ratio increased further, the fibrid contribution to uniformity remains constant, but the average length of fibers was reduced. This is likely responsible for the decrease in PPTA sheet strength at higher ratios of fibrid to fiber. 1:9 3:7 5:5 7:3 9:1 Fibrid/Chipped fiber ratio Journal of Engineered Fibers and Fabrics 4 Tensile index (N m/g) Elongation (%) :9 Tensile index Tear index Linear fit of dielectric strength Dielectric strength Elongation (a) (b) 0.8 y = x R 2 = x stands for fibrid content y stands for dielectric strength 0.4 3:7 5:5 7:3 9:1 Fibrid/Chipped fiber ratio Tear index (mn m 2 /g) Dielectric strength (kv/mm) FIGURE 4. Effect of fibrid/chipped fiber ratio on novel PPTA sheet properties: (a) tensile and tear index; (b) dielectric strength and elongation. As shown in Figure 4-b, the dielectric strength of PPTA sheet increases linearly with fibrid level. The dielectric strength was kv/m when the ratio of fibrid to chipped fiber was 7:3, and the PPTA sheet containing fibrids only exhibited the best dielectric strength. It is known that decreasing the amount of the pores in the fiber matrix helps to enhance the dielectric strength of PPTA sheet. Fibrids help to fill the pores in PPTA sheet because of the shorter fiber length. This may be responsible for the increased dielectric strength with increasing the ratio of fibrids. Therefore, the optimal ratio of fibrids to chipped fiber was 7:3 based on the combination of the mechanical and dielectric properties of novel PPTA sheet. The corresponding tensile index and tear index at this ratio are N m/g, mn m 2 /g, respectively, while the dielectric strength is kv/mm. The properties of the novel PPTA sheet and common PPTA sheet

5 are shown in Table II. It is clear that the novel PPTA sheet shows higher mechanical and dielectric properties than common PPTA sheet [21]. The possible reason was that the fibrids have good interfacial bonding with chipped fibers in sheet because of the soft shape and large surface area. The Structure of the Novel PPTA Sheet The surface morphology and cross sectional images of PPTA sheet made of fibrids/chipped fiber in a ratio of 7:3 are shown in Figure 5-a and 5-b. The chipped fibers, which act as a skeleton, were evenly dispersed in the PPTA sheet. Some chipped fibers, which were deformed to some extent by calendering, filled more voids and arranged well with the fibrids to yield a compact paper structure. Fibrids, acting as binder, were distributed around the chipped fibers. The fibrids were softened and appear to adhere to the chipped fibers during the high temperature and high pressure encountered during calendaring. The resulting structure is similar to that of reinforced concrete, which likely contributes to the observed physical properties of the PPTA sheet. It is also notable that many irregular pores exist on the surface and through the cross-section of the PPTA sheet. Mercury intrusion porosimetry was employed to investigate the pore structure of PPTA sheet. The parameters of pore structure for the novel PPTA sheet and the common PPTA sheet are found in Table II. The average pore diameter and porosity of novel PPTA sheet are µm and %, respectively, slightly lower than those of common PPTA sheet. These differences can be explained by the softer shape and higher surface activity of the fibrids, and enhanced interfacial bonding of PPTA sheet. Hence, the performance of the resulting paper will be improved. FIGURE 5. SEM images of novel PPTA sheet: (a) surface morphology; (b) cross sectional images Fractal geometry has been widely used in many areas of modern science. The fractal dimension is the key quantity in fractal geometry [22, 23]. It can be used as a parameter to evaluate the pore structure of PPTA sheet. The fractal dimension of novel PPTA sheet, calculated by dividing the slope of linear range curve by Eq. (3), was found to be 2.519, lower than that of the common PPTA sheet (Table II). The smaller the fractal dimension indicates tighter a tighter sheet structure, and thus higher interfacial bonding strength of PPTA sheet [24]. Journal of Engineered Fibers and Fabrics 5

6 TABLE II. Measurement of novel and common PPTA sheet. Sample Novel Sheet Common Sheet [21] basis weight, g/m Tensile index, N m/g Tear index, mn m 2 /g Dielectric strength, kv/mm Average pore diameter, µm Porosity, % Fractal dimension Crystallinity, % TG 10%, C Initial decomposition temperature, C Reference This work Prior work The Thermal Stability of Novel PPTA Sheet The thermal stability of the PPTA sheet is a direct indicator of the suitability of the material for such applications as high temperature filtration or as a honeycomb structure material [25]. As shown in Figure 6, a small amount of weight loss at around 100 C can be found from TG curve because of the loss of water molecules from the ambient environment. It is observed that the initial decomposition and 10% decomposition temperature (TG 10% ) of the novel PPTA sheet are 535 C and 560 C, respectively. The total mass loss was % at 800 C. The DSC curve shows a broad exothermic peak, which was a pyrolysis peak, indicating that the novel PPTA sheet has excellent thermal properties. As shown in Table II, the crystallinity of the common PPTA sheet was similar to that of the novel sheet, whereas its thermal properties were less desirable. It is possible that higher porosity of the common PPTA sheet could be promoting thermal conductivity within the sheet structure and accelerating decomposition. Weight (100 %) TG quality change 60.20% DSC Temperature/ FIGURE 6. TG-DSC curves of novel PPTA sheet. DSC Exothermic CONCLUSION In this study, PPTA fibrid, a new type of fiber, and chipped fiber were investigated and used to produce a novel PPTA sheet. The pore structure and physical properties of the novel PPTA sheet were evaluated. PPTA chipped fiber exhibited a smooth and clean surface and was circular in shape. PPTA fibrid consists of flat particles with wrinkled surface, has a large specific surface area, high content of fines and narrow fiber length distribution. These characteristics are attributed to the method of preparation and resulted in high interfacial bonding strength of the PPTA sheet. The optimal ratio of A fibrid to chipped fiber ratio of 7:3 in the PPTA sheet resulted in the best combination of physical, electrical and thermal properties. Compared to the common PPTA sheet, the novel PPTA sheet exhibited tighter structure, higher mechanical strength and improved thermal and electrical properties. These property improvements make the novel PPTA sheet an excellent candidate for high performance applications such as honeycomb structure material, high temperature insulating material and high performance electronic materials for applications in the aerospace and national defense industries. Journal of Engineered Fibers and Fabrics 6

7 ACKNOWLEDGEMENT The authors would like to acknowledge the financial support to this research from the National high technology research and development program 863 major projects of China (2012AA03A208), the Shaanxi province as a whole the innovation project of science and technology plan projects (2011ktcg01-19), and the Science & Research Startup project of Shaanxi University of Science and Technology (BJ12-17). REFERENCES [1] Tokarsky, E.W.; High density para-aramid papers, US Patent , [2] Akikage, K.; Completely aromatic-polyamide-fiber synthesized paper sheet, CN Patent , [3] Qiu, Z.M.; and Ma, Q.L.; Preparation method of pure p-aramid paper, CN Patent , [4] Peter, J.D.; and Warren, M.F.; Aramid fiber filtration sheet, US Patent , [5] Cornbower, T.; High temperature paper, US Patent , [6] Han, L.; Study of the rheological properties of Nomex fibrids, College of Engineering and Mineral Resources at West Virginia University, [7] You, X.L.; and Liu, Z.F.; Development on technology of PPTA-pulp preparation; Journal of Polym Mater Sci and Eng 2003, 19, [8] Sufeng, Z., Liang, J., and Meiyun, Z.; Characteristics of aramid fiber/fibrids and their properties of sheet making; Nordic pulp and paper research Journal 2010, 25, [9] Huapeng, Z., Jianchun, Z., Jianyong, C., and Xinmin, H. Effect of solar UV irradiation on the tensile properties and structure of PPTA fiber; Polymer degradation and stability 2006, 91, [10] Wang, J.; Chen, P.; and Li, H.; Surface characteristic of poly (p-phenylene terephthalamide) fibers with oxygen plasma treatment; Surface and Interface Analysis 2008, 40, [11] Jiang, M.; Zhang, M. Y.; and Lu, Z. Q.; Study on the performances of the Para-Aramid Fibrid; Journal of functional materials 2006, 37, [12] Zeng, X. L.; Elias, R.; and Sabine, H.; fiber deformations induced by different mechanical treatments and their effect on zero-span strength; Nordic pulp and paper research Journal 2012, 27, [13] Sutton, P.; Joss, C.; and Crossely, B.; Factors affecting fiber characteristics in pulp, Proceedings of 4 th international conference on new available techniques and common Stockholm 1992, [14] Provatas, N.; Haataja, M.; and Asikainen, J.; Fiber deposition models in two and three spatial dimensions; Colloids and surface Analysis 2000, 165, [15] Miguel, A.; Montes, M.; and Juan, I.P.; Surface Characterization of PPTA Fibers Using Inverse Gas Chromatography; Macromolecules 2002, 35, [16] Joutsimo, O.; Wathen, R.; and Tamminen, T.; Effects of fiber deformations on pulp sheet properties and fiber strength; Paperi ja puu 2005, 87, [17] Zhang, S. F.; Liu, W.; and Zhang, M. Y.; Effect of aramid fiber and pulp morphologies on properties of sheet; Journal of Southwest Jiao tong University 2010, 45, [18] Lai, Y.M.; and Xie, Y.M.; Configuration of PMIA-pulp and its effect on aramid paper; China pulp & paper 2010, 29, 1-5. [19] Zhang, S.F.; Zhang, M.Y.; Li, K.C.; Adhesion characteristics of aramid fibre-fibrids in a sheet hot pressing process; Appita Journal 2010, 63, [20] Nazhad, N.M.; Harris, E.J.; and Dodson, C.J.; The influence of formation on tensile strength of paper made from mechanical pulp; Tappi Journal 2000, 83, [21] Yang, B.; Characterization and research of PPTA-pulp properties and structure and sheet forming performance; Shaanxi university of science and technology Press: Xi an, [22] Matthew, J.W.; and Karen, J.E.; An experimental study of gas adsorption on fractal surface; Langmuir 2005, 21, [23] Moura, M.J.; Ferreira, P.J.; and Figueiredo, M.M.; Mercury intrusion porosimetry in pulp and paper technology; powder technology 2005, 160, Journal of Engineered Fibers and Fabrics 7

8 [24] Jaroniec, M. Evaluation of the fractal dimension from a single adsorption isotherm; Langmuir 1995, 11, [25] Tadao, K.; Yoshikazu, T.; and Toshiaki, H. Heat resistance properties of poly (p-phenylene-2, 6-benzobisoxazole) Fiber; Appl Polym Sci 1997, 65, AUTHORS ADDRESSES Zhaoqing Lu Ming Jiang Meiyun Zhang Xinxing Xia College of Light Industry & Energy, Shaanxi University of Science and Technology Weiyang District Xi an, Shaanxi CHINA Journal of Engineered Fibers and Fabrics 8