Wang Jian，Chen Jinnan，Dai Pan
1 Introduction
Single-polymer-composites (SPCs) refer to the class of composites in which tile matrix and tile reinforcement come from tile same polymer. Since the reinforcement and the matrix can be reprocessed together，the great advantage of SPCs is easy recyclability. Moreover，excellent fibre/matrix adhesion can be obtained without the help of any coupling agent. Different thermoplastic polymers，including polyethylene (PE)，polypropylene (PP)，polyethylene terephthalate (PET)，polyethylene naphthalate (PEN)，polylactic acid (PLA)，polyamide (PA)，polymethylmethacrylate (PMMA) and polytetrafiuoroethylene (PTFE)，were exploited to manufacture SPCs. Different processing methods have been developed for preparing SPCs，including hot compaction of fibres or tapes，film stacking，combination of hot compaction and film stacking，co-extrusion，and combination of compression molding and free sintering.
PEN is a polyester polymer which exhibits higher dimensional stability，shrinkage resistance，temperature stability and better barrier properties than PET. Applications for PEN are mainly in textile and industrial fibres，films，foamed articles，containers for beverages，laminate sails，tyres，ropes，cables，and food-contact applications. PEN fibres possess a higher initial stiffness than that of PET fibres and this makes them an attractive competitor for use in mooring ropes and other applications. PEN SPCs could be produced by using PEN fibre to reinforce PEN resin. PEN SPCs have the potential for an even greater improvement in performance compared to PET SPCs because oriented fibres can be produced with a significantly higher stiffness.
Small difference in melting temperature between the fibres and the matrix poses a big challenge during SPCs fabrication. The difference between the melting temperatures determines the processing temperature window. Hine et al. described the production of PEN SPCs by using hot compaction of PEN fibres. The optimum compacted PEN sheets were found to have an initial modulus close to 10 GPa and a strength of just over 200MPa. But the hot compaction method has a narrow processing temperature window，typically about 8℃ (268-276℃) for PEN SPCs. Furthermore，heat damage to the strength of fibre is unavoidable because the skin of fibres is melted. The major challenge is still the temperature sensitivity of the process. In our previous study，an approach of applying undercooled polymer melt to prepare SPCs was investigated. Undercooling refers to cooling a substance below a phase-transition temperature without the transition occurring. Polymers can be undercooled easily because of their extremely high molecular weight and long molecular chains. The key idea of applying an undercooled melt in SPCs processing is that the fibre can be introduced into a liquid matrix at a temperature well below the melting temperature of the matrix. The method was demonstrated using PP.
Aim of this paper is to investigate the possibility to prepare PEN SPCs based on the undercooling melt film stacking method. Differential scanning calorimetry (DSC) was used to determine the processing window. Tensile tests were used to determine the mechanical properties of the PEN SPCs. Various compaction temperature and holding time were used to determine the optimum processing conditions. The microstructure of the PEN SPCs was analyzed by scanning electron microscopy (SEM).
2 Experimental
2.1 Materials
PEN granules used in this study were supplied by Goodfellow Corporation (Oakdale，USA)，with a density of 1.36g/cm3at room temperature.PEN fibres were supplied by Performance Fibres Company (Huntersville，USA) in the form of a multifilament bundle of weight 1500g/denier.The fibre diameter is about 28μm.
2.2 Differential scanning calorimetry ( DSC)
A differential scanning calorimeter (Q200，TA Instruments) was applied to ascertain the thermal properties of the PEN matrix and fibre. The PEN granules were heated to 350℃ at 10℃/min and held for 2min to erase thermal and mechanical history，then cooled to 40℃ at the cooling rate of 10℃/min，immediately they were reheated up to 350℃ at 10℃/min and cooled to 40℃ at 10℃/min finally. The PEN fibres were heated to 300℃at a rate of 10℃/min and held for 5min，and then cooled to 40℃ at the cooling rate of 10℃/min. The flow rate of Nitrogen here was 60mL/min.
2.3 Composites preparation
PEN granules were dried in a vacuum oven at 120℃ for 24h to remove any adsorbed water，and then pressed into films with 0.5mm in thickness by compression molding at 300℃ and 1MPa.Continuous filaments were available for PEN fibres，a small loom (Vintage Weave Easy Hand Loom，Bandwagon Company) was used to produce bidirectional woven cloth.Each yarn consists of 210 bulked filaments with a diameter of about 28μm.The yarn was woven into plain weave fabric，the warp density and the weft density are 2.8 threads/cm and 15 threads/cm，respectively.
Fig.1 Schematic of the undercooling meltfilm stacking process for PEN SPCs
Undercooling melt film stacking is similar to film stacking，but one initial step，film undercooling，is needed before the pressing. Schematic of the process for PEN SPCs is shown in Fig.1，a customized two-station compression molding process was used. Two pieces of PEN films were first heated to 300℃ for 10min to obtain two layers of molten films on the first station. The molten films were then quickly transferred to the second station set at an undercooling temperature，where the molten films were undercooled. After the undercooled films were stabilized，the PEN woven cloth was sandwiched between the undercooled films and then pressed immediately under the undercooling temperature. Then the lamination was removed and cooled to room temperature. Both the heating temperature and undercooling temperature were determined by DSC analysis. The heating temperature is above the melting temperature of the matrix. The undercooling temperature is between the crystallization onset temperature of the matrix and the melting onset temperature of the fibres，so that the matrix can keep its liquid state and will not crystallize，and the fibres will not melt.
Different pressures of 3，6 and 9MPa were tried in the compaction preliminarily，little effects on the tensile properties were found due to very thin thickness (less than 1mm)，and so 6MPa was used as the compaction pressure in all the experiments. While in order to investigate the influence of compaction temperature and holding time，various compaction temperatures (220-260℃) and holding time (1-20min) were used.
2.4 Mechanical tests
Tensile tests were carried out by using a tensile test machine (Instron Universal Testing Machine 5166 Series，Instron Corp.，MA) at room temperature with a load of 30kN and a crosshead speed of 5mm/min.The prepared samples were cut into dog-bone shaped testing specimens using a cutting die according to DIN-53504.The PEN SPCs and non-reinforced PEN were all tested，and 5 specimens were tested for each sample.The warp density of the woven cloth is 2.8 threads/cra，very small because of the hand-woven，thus the SPCs were tested only in the weft direction.
2.5 Scanning electron microscopy ( SEM)
The specimens were freeze fractured in liquid Nitrogen. The fractured section in the middle part of the sample was gold coated and its morphology was observed using a SEM (Hitachi S-800) with an accelerating voltage of 8kV.
3 Results and discussion
3.1 Determination of processing window
In the undercooling melt film stacking method，the processing temperature window is not the melting temperature difference between matrix and fibres but the difference between crystallization onset temperature of the matrix and the melting onset temperature of the fibres. Therefore，compared with the traditional SPCs processing methods such as hot compaction and film stacking，the undercooling melt film stacking method could obtain a much larger processing temperature window.
Fig.2 shows the DSC thermograms of the PEN matrix and fibre. The PEN fibre begins to melt at 261℃，and the peak melting point Pis at 277℃. The peak melting point of PEN matrix is at 264℃. The fibre melting temperature is higher than the matrix melting temperature due to orientated crystals. The matrix begins to crystallize at 213℃，exhibiting a large capability of undercooling. Therefore，the processing temperature window could be up to 48℃(213~261℃)，which is much larger than the melting temperature difference of 13℃ (264~ 277℃). According to the processing window，the compaction exper-iments were carried out in the range of 220~260℃.(www.xing528.com)
Fig.2 DSC thermograms of the PEN matrix and fibre
3.2 Effects of compaction temperature on tensile properties of PEN SPCs
Fig.3 shows the tensile strength and modulus of the PEN SPCs made at the compaction temperature in the range of 220~260℃ and a holding time of 5min. As the compaction temperature increased，the tensile strength increased. It is correlated with the reduced viscosity and consequently improved wetting and infiltration then the coherency structure improved the strength. However，when the compaction temperature continued increasing，the tensile strength decreased. Since PEN belongs to polar polymer，the disoriented phase enhanced with higher compaction temperature leading to a fall in fibre strength. It can be recognized that high compaction temperature benefits wetting and infiltration，however much higher temperature causes embrittlement and weakens the fibre strength. The peak in the mechanical properties is at 245℃ which appears to be the best compaction temperature when the holding time is 5min.
The appearance of the broken tensile loaded specimens hints for the effects of compaction temperature. Photograph of the broken specimens is shown in Fig.4. The failure behavior is in close analogy with different matrix-fibre adhesion. The state of matrix-fibre adhesion can be quantitatively evaluated by checking the number of pull-out fibres. The more pull-out fibres，the worse is the adhesion. Specimens compacted at lower temperatures showed a greater strain to failure due to the lower interfacial strength，and more fibres were pull-out and debonding. The pull-out fibres were less and shorter when the temperature increased，because lower viscosity at higher temperature benefits infiltration. When the temperature increased to 250 and 260℃，there were barely any pull-out fibres but very clear fracture surfaces indi-cating the embrittlement occurred.
Fig.3 Tensile strength and modulus of the PEN SPCsmade at the compaction temperature in the rangeof 220~260℃ and a holding time of 5min
Fig.4 Photograph of the broken PEN SPCs made atdifferent temperatures of 220，230，240，250 and260℃ and the same holding time of 5min
3.3 Effects of holding time on tensile properties of PEN SPCs
The holding time also affects tensile properties of the PEN SPCs. Table 1 shows the dependence of the tensile strength and modulus with holding time at 235，240 and 245℃. The tensile strength increased first and then decreased as the holding time increased. It is noted that the adhesion could be improved by increasing holding time but further increase in holding time will lead to the increase of crystallinity of the matrix which is harmful to the matrix-fibre adhesion. Fig.5 shows the broken photograph of PEN SPCs made at the same temperature of 240℃ and different holding time of 1，5，10 and 15min. It suggests a very similar mechanism to Fig.4. The effects of the holding time on adhesion as the above discussed could be proved apparently.
Table 1 Tensile strength and modulus of the PEN SPCs made at different holding timeof 1，5，10，15 and 20min and different compaction temperatures of 235，240 and 245℃.
Fig.5 Photograph of the broken PEN SPCsmade at the same temperature of 240℃ and differentholding time of 1，5，10 and 15min
Interestingly in Table 1，the optimum holding time is different at different compaction temperature，and it decreased as the compaction temperature increased.The underline indicates the optimum holding time.It demonstrates that a longer holding time is needed when the compaction temperature is lower in order to achieve the best tensile strength.Similar trend in tensile modulus can be noted.The optimum processing is at the compaction temperature of 240℃ and the holding time of 10min，which could obtain a tensile strength of 224MPa and a tensile modulus of 8.4GPa.
3.4 Comparison
Hine et al. produced PEN SPCs using hot compaction of PEN fibres. The tensile strength was between 140 and 230MPa in the temperature range of 5℃ (268~273℃). In this paper，the tensile strength was between 149 and 224MPa in the temperature range of 40℃ (220~260℃). The processing window for good samples is 20℃ (240~260℃). It can be seen that the processing temperature window was enlarged significantly by using the undercooling melt film stacking method. In addi tion，for the hot compaction the holding time is shorter (2min) due to the matrix being produced around fibres and so there is no diffusion time required. However，the undercooling temperature is a desired isothermal temperature but usually unstable in related to the time and so a longer holding time is needed in the undercooling melt film stacking.
Fig.6 Comparison of the tensile stress-strainbehavior for the PEN SPC and the non-reinforced PEN
Fig.6 shows the comparison of the tensile stressstrain behavior for the PEN SPC and the non-reinforced PEN.The specimens were all compacted under the optimum processing.The non-reinforced PEN yielded in a ductile manner when failing，plastic deformation happened.The stress-strain curve contains a yielding and flow region，the maximum stress is 63MPa.Whereas the PEN SPC exhibited a brittle behavior，elastic deformation occurred.The specimen was broken suddenly when the stress reached up to 224MPa.The tensile strength is 3.6 times higher than that of the non-reinforced PEN.The initial linear elastic region also shows a higher yielding strength for the PEN SPC.The tensile modulus is 2 times higher than that of the non-reinforced PEN.Fig.7 compares the different failure mechanisms.Elastic deformation and plastic deformation were respectively observed in the failure section of the pure PEN and the PEN SPC.It can be deduced that good interfacial bonding was achieved in the PEN SPC.
Fig.7 Comparison of the failure mode for thePEN SPC and the non-reinforced PEN
3.5 Morphological properties
The micrographs of the fracture surface of PEN SPCs are reported in Fig.8. Most fibres still maintain their original size suggesting the fibres were not molten at the undercooling temperature. Lots of matrix particles can be clearly seen on the fibre surfaces，and many fibres were split in their axial direction. All these observa-tions indicate that the matrix melt penetrated into fibre webs very well，and the PEN SPCs had good interfacial compatibility and bonding properties. As the temperature increased to 250℃ (see Fig.8b)，the parts of matrix on the fibre surfaces became larger，and some fibres were closely associated by the matrix. It illustrates that higher compaction temperature improved the interfacial adhesion. Fig.8c and d show different fracture morphologies，fibres are all surrounded by matrix. A matrix particle and a large part of matrix on the fibre surface can be observed in Fig.8c. Many micro links in the matrix-fibre interface can be seen in Fig.8d，some surface layers of the fibres were peeled off. It further proves good interfacial compatibility and bonding properties.
4 Conclusions
The excellent properties of PEN fibre and resin are available in the PEN SPCs.The possibility of the undercooling melt film stacking method was investigated for the production of PEN SPCs.The reinforcements was sandwiched between the undercooled films and then pressed at a temperature which is between the melting onset temperature of the fibre and the crystallization onset temperature of the matrix.The processing temperature window was determined to be 48℃ (213~ 261℃)，which was enlarged significantly.PEN SPCs were successfully produced within 220~260℃.Suitable com-paction temperature and holding time could improve matrix-fibres adhesion then enhance mechanical properties.At a lower compaction temperature，longer holding time is needed to ensure good mechanical properties.The optimum processing condition (compaction temperature of 240℃，holding time of 10min and pressure of 6MPa) was obtained.The optimum compacted PEN SPCs were found to have a tensile strength of 224MPa and a tensile modulus of 8.4GPa，which are much higher than that of the non-reinforced PEN.The morphological properties of the PEN SPCs illustrated good interfacial compatibility and bonding properties.
Fig.8 Micrographs of the fracture surface of PEN SPCs
Acknowledgements
The project was financed by the Basic Research Fund of geijing Institute of Technology (Project No. 3100012211304). Many experiments were carried out in Georgia Institute of Technology，the authors thank Prof. Donggang Yao and Prof. Youjiang Wang for their helpful suggestions and comments.
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