Zeng Hua Zhao，Jin Nan Chen
1 Introduction
The original concept of single-polymer composites (SPCs) was presented by Capiati and Porter more than three decades ago. The two biggest advantages of SPCs are that they have good interface performance and they are easily recyclable materials. Interface problem has restricted the development of composite materials. A single-polymer composite is a kind of composite with matrix and reinforcement from the same polymer. Different kinds of polymer have been proposed for the processing of SPCs，including polyethylene (PE)，polypropylene (PP)，polyamides (PA)，and polyester. Different routes have been developed for the manufacture of SPCs. Much of the early research focused on how to enlarge the processing temperature window for the manufacture of SPCs. Particularly，Hine prepared a polyethylene/polyethylene composite by selective surface melting of the fibers via a suitable choice of compaction temperature.On cooling，the molten material recrystallizes to form a glue to bind the structure together. Lacroix used a low density polyethylene/xylene solution for crystallisation of matrix material on the fiber surfaces for processing PE/PE composites. Yao et al. prepared poly (ethylene terephthalate) (PET) SPCs by compression molding laminations of PET sheets. Among these methods，hot compression molding is the most widely used. In other words，SPCs are usually manufactured in a process of hot compaction and film stacking. In this process，only the surface fraction of each fiber is to melt under a comparatively low contact pressure，and then a substantially higher pressure is applied for a short time to achieve excellent consolidation of the structure. The difference of melting point between the fiber and the matrix is no more than 5℃. With this small processing temperature window，it is difficult to process SPCs. To resolve this prob-lem，a new route for the manufacture of SPCs by combining the processes of compression molding and free sintering was proposed.
PTFE，which has been applied in many areas，is a fluorocarbon compound and has a number of unique properties. The carbon backbone of PTFE molecule is totally covered by fluorine，which prevents any reaction between the backbone bonds and other aggressive media. In addition，PTFE is insoluble in all solvents，even at elevated temperatures. Besides the excellent chemical stability，the high crystallinity of the compound (90%~95%) results in high thermal stability and low surface energy. The melting temperature of PTFE is generally accepted to be 327℃. However，at this temperature its viscosity is still very high (of the order of magnitude 10m Pa s). Making use of this phenomenon，PTFE composites are prepared by combining the processes of compression molding and free sintering. There are many kinds of fillers for improving the properties of PT-FE. The most common are glass fibers，carbon fibers，asbestos，graphite，and bronze of different grades. Because of its excellent chemical stability，it is difficult to bond PTFE with fillers，resulting in composites of poor interfacial strength. PTFE SPCs can perhaps solve the bonding problem. In this study，PTFE SPCs were prepared for the first time by the new method of compression molding and free sintering. PTFE fibers wer eused as the reinforcing phase and PTFE powder was used as the matrix. Tensile and flexural tests were used to characterize the mechanical properties of the composites. The effects of preparation conditions on the tensile and flexural properties of PTFE SPCs were investigated. Finally，microscopic studies were used to describe the interface.
2 Experimental
2.1 Material
Two different physical forms of PTFE-perfect crystalline powder and highly crystalline fibers-were used in the experiments. The PTFE powder was provided by Tangshan Japan Feng Chemical Industry Ltd.，China. The average particle size of the powder was 50μm. The PTFE fibers with three different filament diameters of about 30，40，and 50μm were supplied by Fuxin Sheng Li Fluorine Polymer Material Co.，Ltd，China. The calorimetric measurements were performed using a DSC apparatus (Model ZCT-A Beijing Jing Hi-Tech Co.，Ltd.). The DSC curves are given in Fig.1. A heating rate of 10℃/rain was used in the DSC experiments. The structures of the PTFE powder and the PTFE fibers were examined by X-ray diffractometry (XRD). A 140kV/40mA laser plasma X-ray source based on a double stream xenon/helium gas puff irradiated target was used. The scanning mode is 2θ/θ，and the scanning type is continuous scanning. The scanning speed is 8°/min，and the scanning range is 10-110°. The XRD patterns are shown in Fig.2.
Fig.1 DSC curves for PTFE powder and fiber.A heating rate of 10℃/min was used
2.2 Compression molding and free sintering
PTFE has a very high melt viscosity that prevents PTFE from being processed by conventional melt processes，such as extrusion and injection molding. Therefore，processes inspired by powder metallurgy such as cold compaction followed by sintering have been developed. The PTFE SPCs were made by the cold compaction and sintering method. First，the PTFE fibers were added to the PTFE powder in various mass fractions，as indicated in Table 2，and then mixed by mechanical stirring. The mixture was directly shaped by cold compaction (27℃，15MPa) for 15min，followed by “free sintering” at 380℃. Crystal orientation of PTFE fiber has been almost no change after sintering. Due to fixed by matrix，the contraction of PTFE fiber are restrained，and the original strength of fiber are maintained in the sintering process. The sintering process was completed in a vacuum furnace (Hefei Risine Heaters Co. Ltd.，China). The temperature 380℃ is above the melting point of the virgin polymer and the fibers. These processes offered satisfactory solutions to bond the matrix and fibers together.
2.3 Orthogonal experimental design
Sintering time，mass fraction of PTFE fibers，and PTFE filament diameter are the main factors affecting the performance of single-polymer composites based on PTFE，among others. The sintering time is the holding time for 380℃. The mass fraction of PTFE fibers is the percentage of fibers in the SPCs. The filament diameter is the diameter of PTFE fibers. The orthogonal table L9 (34) was used to show the factors. The orthogonal factorial design is shownin Table 1.
Fig.2 XRD patterns for PTFE powder and fiber. Scanningspeed is 8°/min，and scanning range is 10°~110°
Table 1 Factors and levels for orthogonal design
2.4 Mechanical testing
Tensile tests were performed on an electronic multipurpose tensile tester (Model DXLL-3000，Shanghai D&G Measure Instrument Co.，Ltd.，China) according to GB 1040—1006 (Chinese National Standard). A stretching rate of 50mm/min was used to load these samples to failure. All samples were tested in air at room temperature. A minimum of five samples per group was tested. The tensile modulus was measured in these tests.
Three-point flexural tests were performed on the same instrument according to GB 8812—2007 (Chinese National Standard). The loading span was 60mm and the loading rate was 2mm/min. All samples were tested in air at room temperature. A minimum of five samples per group was tested. The flexural modulus was determined in these tests.(www.xing528.com)
For scanning electron microscopy observation，a Hitachi High-Technologies Corporation scanning electron microscope (SEM) was used in this study. Substrates of PTFE fibers were sputter coated with an ion sputter (Model E-1045，Hitachi High-Technologies Corporation，Japan) with a platinum target (5mA for 4min). These samples were then imaged using a cold field SEM with an accelerating voltage of 10kV and a working distance of approximately 5mm.
3 Results and discussion
3.1 DSC and XRD of material
The melting points(Tm) of the powder and fibers were approximately 335 and 325℃，respectively. Virgin PTFE powder has higher crystallinity than the fibers；the DSC curves are given in Fig.1. The XRD patterns are shown in Fig.2a and b. It can be seen that the patterns are very similar；however，the total intensity for PTFE fiber is lower than that for virgin PTFE powder. As shown in Fig.2c，the percentage of amorphous material is calculated by comparing the intensities of the two portions of the diffraction pattern. When the amorphous fraction is large，the crystallinity is low. Comparing Fig.2a and b，it is estimated that the virgin PTFE powder has much higher crystallinity than the fibers.
Table 2 Results of orthogonal experiments forsingle-polymer composites base onPTFE：mechanical testing
3.2 Tensile and flexural performance
The tensile strength and the flexural strength of the samples are listed in Table 2.K，k，R，andr were calculated and the results are listed in Tables 3 and 4，whereK is the sum of experimental values for each factor at every level，k is the average value，andR andr are the ranges ofK andk，respectively. As can be seen from Table 3，the effect on tensile strength is in the order:B>C>A，and the effect on tensile modulus is in the order C>B>A according to theR orr values. These results suggest that the mass fraction of fibers has a large effect on the tensile strength of the samples. The optimal mixing proportion was obtained to be A2B1C3；i. e.，sintering time，mass fraction of fibers，and filament diameter were 60min，5%，and 50μm，respectively. The effect on tensile modulus is similar to that on tensile strength in that factor C has the least effect on both the tensile strength and tensile modulus. The filament diameter has the largest effect on the tensile modulus of the samples，followed by the mass fraction of fibers. The optimal mixing proportion was obtained to be A2B3C2；i. e.，sintering time，mass fraction of fibers，and filament diameter were 60min，20%，and 40μm，respectively. But based onR andr of factor B on tensile modulus，k1areK3are more similar. Moreover，according toRandr of factor C on tensile strength，K2is almost equal toK3. In other words，going from level 2 to level 3 of C has little effect on the tensile strength. Therefore，the optimal mixing proportion can be adjusted to A2B1C2 if the tensile strength and the tensile modulus are considered at the same time.
Table 3 Range analysis on tensile tests ofsingle-polymer composites based on PTFE
It can be seen from Table 4 that the effect on flexural strength and the effect on fiexural modulus are similar，both are in the order C>B>A according to theR orr values. These results indicate that the filament diam-eter has a large effect on the flexural performance of the PTFE SPCs. The optimal mixing proportion for flexural strength was obtained to be A2B1C1；i. e.，sintering time，mass fraction of fibers，and filament diameter were 60min，5%，and 30μm，respectively. The effect on flexural modulus is a little different from that on flexural strength. The optimal level of factor A is 2. But forR andr of factor A，K2is almost equal toK1. In other words，there is little difference in the effect of the two levels on flexural modulus. So the optimal mixing proportion can be adjusted to A1B1C1 for cost saving.
The tensile and flexural properties of pure PTFE samples were also measured.The tensile strength，tensile modulus，flexural strength，and flexural modulus were 15.56，349.75，13.65，and 217.90MPa，respectively.These values are lower than the corresponding values for PTFE SPCs.
3.3 Scanning electron micrographs
Fig.3 displays the SEM micrographs of the surface of PTFE fiber in the single-PTFE Composites with different magnification. The SEM micrographs indicate that a strong bonding between the fiber and the matrix occurred.
Table 4 Range analysis on flexural tests ofsingle-PTFE composites
Fig.3 Scanning electron micrographs of PTFE fibers in SPCs
4 Conclusions
A new processing route has been developed for the preparation of a PTFE fiber/PTFE matrix composite. The novel idea is to prepare single-polymer composites based on PTFE by using cold compression molding and free sintering. The optimum processing conditions for preparing PTFE SPCs were derived from orthogonal experiments. The PTFE SPC prepared with a sintering time of 60min，mass fraction of fibers of 5%，and filament diameter of 40μm has the best tensile performance. The PTFE SPC prepared with a sintering time of 30min，mass fraction of fibers of 5%，and filament diameter of 30μm has the best flexural performance. The mechanical properties of PTFE SPCs are better than those of pure PTFE. Because of their good mechanical properties and other properties such as high thermal stability，low surface energy，biocompatibility，and high chemical stability，PTFE/PTFE composites are interesting for special applications.
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