Pan Dai，Wei Zhang，Yutong Pan，Jinnan Chen，Youjiang Wang，Donggang Yao
1 Introduction
The traditional composites，in which tile reinforcement and the matrix are made from different materials，are subjected to interfacial compatibility and recycling issues. One promising approach for improving interfacial compatibility and enhancing recyclability is the single polymer composites (SPCs) approach，originally proposed by Capiati and Porter. SPCs refer to the class of composite materials in which the matrix and the reinforcement come from the same polymer. The first SPC was prepared by embedding a highly orientated polyethylene fiber into molten polyethylene. Since then，a number of researchers utilized this concept to prepare SPCs. They all utilized the difference in melting temperature between oriented and non-oriented crystalline forms. However，this difference is typically small，resulting in a small processing temperature window.
An alternative manufacturing method based on hot compaction of fibers or fabrics has been developed by Hine and coworkers. Under hot compaction，the skin of the fibers melts but the central part does not. The melted polymer at the skin recrystallizes upon cooling and acts as a bonding agent among the fibers. Higher fiber volume fractions were achieved. However，this processing method is also subjected to a narrow processing temperature window，typically about 10℃ or smaller.
The concept of “overheating” above the fiber melting temperature by constraining fibers was introduced in preparing SPCs. Physically fixing the fiber ends can prevent shrinkage and molecular reorientation. To a certain extent，this method can enlarge the processing tem-perature window. The melting temperature of the con-strained PP fiber increased by about 20℃ compared to the unconstrained PP fiber. The overheating behavior of constrained fibers also has been reported for PA6 (polyamide 6) and PET (polyethylene terephthalate)，but only melting temperature shifts of 10℃ and 7℃，respectively，were observed.
To further enlarge the processing temperature window，researchers have also utilized polymers with same chemical composition but different chemical structures. Teishev et al. reinforced HDPE (high-density polyethylene) matrix with UHMWPE (ultra-high molecular weight polyethylene) fibers，and the process window was enlarged to 20℃. Devaux and Cazé reinforced LDPE (low-density polyethylene) with UHMWPE fibers，and the process window was further enlarged to about 40℃. Pegoretti et al. prepared SPCs based on liquid-crystalline fibers，Vectran®N and Vectran®HS. These two kinds of commercial fibers have the same chemical composition but different melting point.
The resulting temperature window for SPCs processing ranged from 260℃ to 285℃.Although manufacturability was greatly enhanced in these composite systems，the interfacial adhesion was found to be lower than the original SPC.Mead and Porter studied and found that the interfacial shear strength for HDPE films embedded in an LDPE matrix is 7.5MPa and for HDPE self-composites is 17MPa.In more rigorous definition，composites involving polymers with same chemical composition but different chemical structures are not true SPCs.
Recently，Yao et al. proposed to widen the processing temperature window utilizing the slow crystallization kinetics of some slowly crystallizing polymers such as PET and PLA(polylactide). Aslowly crystallizing polymer can be supercooled into a nearly amorphous phase. This amorphous material can then be liquefied by rapidly heating to a temperature well above the glass transition temperature(Tg) but considerably below the melting temperature(Tm) and combined with highstrength fibers to form an SPC. With this approach，the processing temperature window for PET SPCs was extended to about 70℃. However，there are two competing processes occurring when an amorphous polymer is heated. In order to avoid premature crystallization before fusion，the amorphous polymer needs to be heated rapidly throughout the entire thickness. This method is limit-ed to polymers with a relatively short crystallization halftime；it is difficult to apply it to fast crystallizing polymers，including PP，PE and PA6/66.
In this study，we investigated the feasibility of applying undercooled polymer melt in SPCs processing. It is known that semicrystalline polymer upon melting can typically be undercooled to a temperature well below the polymer melting temperature while crystallization is largely absent. The applicability of undercooled melt in SPCs processing is expected to be largely dependent on the degree of undercooling that the polymer can undergo without solidification；the larger the degree of undercooling，the less potential of heat damage to the strength of the polymer fiber. If successful，this approach may be applied to a variety of semi-crystalline polymers not limited to slowly crystallizing polymers.
2 Undercooling of polymer melt
Undercooling or supercooling refers to a process to cool a substance below a phase-transition temperature without the transition occurring. It is well know that some low molecular weight liquids such as water can be supercooled well below the freezing point without freezing. In general，polymer can be even easier to supercool because of their extremely high molecular weight and long molecular chain.
Polymer crystallization can occur over a large tem-perature range fromTgtoTm. Crystallization typically experiences two distinct stages: nucleation and crystal growth. Fig.1 schematically shows the rate of nucleation，the rate of crystal growth and the rate of crystallization as a function of temperature. Lower temperature is favorable for the formation of nuclei while higher temperature is favorable for the growth of crystal. When the processing temperature is below and close to the melting tempera-ture，crystallization can be effectively suppressed. Because there are no nuclei near the melting temperature，the polymer cannot crystallize although the rate of crystal growth is high. Then the supercooling effect arises.
Fig.1 The rate of nucleation，the rate of crystallization and the rate of crystal growth as a function of temperature：A，nucleation；B，crystallization；C，crystal growth.
Thermal liquid crystal polymers (TLCPs) are known to supercool when cooled below their melting point. Done and Baird studied the rheology of liquid crystal polymers below their normal melting temperature by measuring dynamic mechanical properties. It was found that the TLCPs could be supercooled to 50℃ below their normal melting temperatures and can still be deformed. Extrusion studies on these materials were also carried out，and it was observed that in this supercooled state the polymer extrudate exhibited significant die swell. These results demonstrated that undercooled TLCP melts can be processed using normal melt processing techniques. Not only TLCPs but also typical thermoplastic polymers may be processed in a supercooled liquid state. This is supported by the typical crystallization thermograms observed in differentical scanning colorimetry (DSC). For instance，PP was found to exhibit a large degree of supercooling (~40℃) under normal cooling rates (~20℃/min) in DSC.
The key idea of applying an undercooled melt in SPCs processing is that the fiber can be introduced into a liquid matrix at a temperature well below the matrix melting temperature. Because the processing temperature is below the matrix melting temperature，and the fiber melting temperature is even higher than the matrix melting temperature (due to orientated crystals)，the fibers added to the matrix will not melt and therefore reinforce the matrix and form an SPC.
3 Experimental
3.1 Materials
PP granules were supplied by Phillips Sumika Polypropylene Company，with a density of 0.905g/cm3at room temperature and a melt flow rate of 3.8g/10min at 230℃.High-strength woven polypropylene cloth was supplied by Innegrity LLC (Simpsonville，SC).The weft and warp yarns were made from identical high-te nacity yarn with strength of 590MPa.Each yarn consisted of 225 bulked continuous filaments with a filament diameter of about 48pm.The yarn was woven into plain weave fabric of 423.8g/cm3，and the warp density and the weft density are 4.3 threads/cra and 6 threads/cm，respectively.
3.2 Sample preparation
Thin PP sheets，0.5mm in thickness，were pre-pared by compression molding the PP granules at 200℃and 1MPa for 5min followed by quenching at room temperature.The molded PP sheets were then melted and consolidated with the high-strength PP fabric to form an SPC using a customized two-station compression molding process，as schematically illustrated in Fig.2.The two-station process allows the PP sheets to be heated and melted at one temperature and then undercooled to a second temperature within a short period of time.Specifically，two pieces of PP sheets were first heated to200℃ for 10min on the first station to obtain two layers of molten PP sheets.The molten PP sheets were then quickly transferred to the second station set at a lower temperature，where the molten PP sheets were supercooled.After the undercooled PP melt sheets were stabilized on the second station，a PP fabric was inserted in between and the lamination was immediately compressed under a pressure of 9MPa for 10min.Then the lamination was removed and cooled to room temperature.The PP fabric was preheated to the same temperature as that of the second station before it was introduced to the undercooled PP melt layers.For all experiments，fiber preheating was performed in the uncontrained mode without applying fiber tension or surface pressure during the entire preheating stage.
3.3 Differential scanning calorimetry
A differential scanning calorimeter (Q200，TA Instruments) was employed for thermal analysis of PP fabric and PP matrix. The unconstrained PP fabric was heated from 40℃ to 200℃ at a rate of 10℃/min. The PP matrix was heated from 40℃ to 200℃ at arate of 10℃/min，held for 10min at 200℃，and then cooled back to 40℃ at varied coolingrates (1，10，20 and 30℃/min). The holding stage was considered necessary to erase possible effects of thermal history of the sample on the subsequent melt crystallization.
Isothermal crystallization behaviors of PP matrix at different temperature were also investigated. The matrix PP was heated to 200℃ and after held at this temperature for 10min rapidly cooled to a predetermined temperatureTp(125，130，135，140，145 and 150℃) for isothermal crystallization. At last the matrix PP was cooled to 40℃ at a rate of 10℃/min.
Fig.2 Experimental setup for PP SPCs manufacturing
3.4 X-ray measurement
The changes in fiber orientation were studied using wide angle X-ray diffraction (WAXD).WAXD patterns were obtained on multifilament bundles by Rigaku Micromax-007 (operated at 45kV，66mA，wavelength＝1.5418 Å) using Rigaku R-axis IV++detection system.The diffraction patterns were analyzed using AreaMax V.2.00 and MDI Jade 7.1.
3.5 Rheological measurement
Dynamic rheological properties were measured on a parallel-plate rotational rheometer (AR2000ex，TA Instruments). The plate diameter and the gap between the plates were 25mm and 1mm，respectively. The strain applied was 1%. The PP sheets were melted and equilibrated at 200℃. The initial gap was set to a value e-quivalent to the final gap plus 50μm. After the excessive sample squeezed out was carefully trimmed off，the upper plate was moved to the final gap size. To remove the existing crystallization and residual stress，the melted PP sheet was held for about 10min at the heating temperature and then cooled at a rate of 10℃/min for temperature ramp rheological measurements. The PP sheets were also cooled to the predetermined temperature at a rate of 10℃/min for time sweep rheological measurements.
3.6 Tensile test
Tensile tests were carried out on a tensile test machine (Instron Universal Testing Machine 5166 Series，Instron Corp.，MA) at room temperature with a crosshead speed of 5mm/min. The PP sheet and its SPCs were cut into dog-bone shaped testing specimens using a cutting die according to DIN-53504. The SPCs were tested in the weft and warp directions，and 5 specimens were tested for each sample.(www.xing528.com)
3.7 Dynamic mechanical analysis
A dynamic mechanical analyzer (Q800，TA Instruments) was employed for dynamic mechanical thermal analysis (DMTA) of PP fiber，PP sheet and PP SPC.The measurements were carried out at a strain of0.1%，a frequency of 1Hz，and a temperature ramping rate of 1℃/min.
Fig.3 DSC thermograms of PP fiber and PP matrix. PPmatrix was heated at 10℃/min，and then cooled at 10℃/min；PP fiber was heated at 10℃/min
4 Results and discussion
4.1 Determination of processing window
In order to prepare PP SPCs，the processing temperature window was determined by using differential scanning calorimetry and rheological measurements.Fig.3 shows the DSC thermograms of the starting materials(the PP fiber and the PP matrix).There are three important observations.First，the PP fiber begins to melt at 152℃，and there are two melting peaks.One is at 161℃ and the other is at 177℃.The integrated X-ray diffraction intensity of PP fiber is shown in Fig.4；only intensity peaks for the typical α-form PP crystals were observed，and the two strong intensity peaks at 2θ of16.2°and 21.2° generated by theβ-crystal form are not observed.The 2D wide angle X-ray image of PP fiber is shown in Fig.5a，and it indicates a highly orientated crystalline structure.Schwenker et al.also reported that the drawn PP fiber has two thermal peaks:one at 158℃and the other at 173℃.They thought that the first peak is crystallization orientation release rising，and the second peak is crystallite melt generating.Second，the PP matrix only has one distinct melting peak at 167℃，and crystallites in PP matrix can be completely melted before 200℃.Third，the PP matrix exhibits a large capability of supercooling.It is observed that the PP matrix begins to crystallize at 124℃，significantly below the melting point.The PP matrix will remain in a molten state or a supercooled molten state until it is cooled to 124℃.
Fig.4 Integrated wide angle X-ray diffractionintensity of PP fiber
Fig.5 X-ray fiber photographs of heat-treated PP fibers
With employment of an undercooled melt in SPCs processing，less damage to the fiber strength is anticipated. As shown in the DSC results，the PP fiber begins to melt at 152℃. Since mechanical properties of fibers are related not only to crystallinity but also to crystallization orientation，one needs to further check the level of orientation before determining a suitable process window. The crystallization orientations of the original PP fiber and the heat-treated PP fibers were measured by Xray. The PP fibers were taken out of the PP fabric heattreated at different temperature (140，150，and160℃) for 10min. Fig.5 shows that the crystallite orientation of PP fibers heated at 140℃ and 150℃ were hardly changed comparing with the original PP fibers. However，a substantial change was observed when the heating temperature further increased to 160℃. Therefore，the processing temperature should not exceed 150℃. Otherwise，a large reduction in tensile strength of fibers is expected.
It is worth mentioning that both DSC and WAXD were conducted on unconstrained fabrics/fibers. In actual compression molding，pressure and/or tension can be applied to the fabric during heating. In the constrained mode，the fiber melting temperature is expected to be raised. Therefore，the upper temperature limits determined by DSC and WAXD could have been higher if lateral constraints should be applied.
Fig.6 Complex viscosity of PP matrix as a function oftemperature during cooling from molten state
Fig.6 shows the cooling behavior of the PP matrix in a plot of complex viscosity versus temperature. The PP matrix was cooled from 200℃ at a rate of 10℃/min. The PP matrix begins to solidify (crystallize) at 125℃，indicated by an abrupt increase in complex viscosity. In this case，the degree of supercooling for the PP matrix exceeded 40℃，compared with its DSC melting point of 167℃. This large degree of supercooling is consistent with the calorimetric results shown in Fig.3. These results indicate that PP is processable at temperatures well below its melting point，but not below 125℃. Because PP fibers begin to melt at 153℃，the temperature window for processing PP SPCs is between 150℃ and 125℃. The two-station process of preparing PP SPCs mentioned above took only about 10s，It means that the cooling rate during processing must be higher than 10℃/min. Fig.7 shows the effect of cooling rate on the supercooling of PP. As shown in Fig.7，the temperature of melt crystallization decreases from 138℃ to 118℃，as the cooling rate increases from 1℃/min to 30℃/min，suggesting that the degree of PP supercooling can be changed by the cooling rate. This result is consistent with the result of Beck and Ledbetter on the effect of cooling rate on the supercooling of polypropylene. Their result showed that there existed a 19℃ difference in peak temperature between the cooling rates of 1℃/min and 33℃/min. To sum up，the higher the cooling rate，the higher the degree of supercooling for polypropylene.
4.2 Properties of PP single polymer composites
Fig.7 Effect of cooling rate on supercooling of PP
Fig.8 A comparison of stress-strain curves for PPSPC and non-reinforced PP sheet
Fig.9 DMTA temperature scans for PP SPCand non-reinforced PP sheet
Fig.8 shows a comparison of stress-strain curves for PP SPC sheets (the weft direction) and non-reinforced PP sheets.The PP SPC was obtained by compression molding at 135℃.The thickness of the PP SPC is 0.8mm，and the weight percentage of PP fabric is approximately 42%.As shown in Fig.8，the non-reinforced PP sheet is ductile and its stress-strain curve contains a yielding and flow region，whereas the PP SPC exhibits a brittle behavior.The maximum stress for PP SPC is 134MPa，significantly higher than the value of 33MPa for the non-reinforced PP.The initial linear elastic region also shows a higher yielding strength for the PP SPC than for the non-reinforced PP.Fig.9 shows the DMTA results for PP fibers，PP SPC sheets (the weft direction) and non-reinforced PP sheets.The storage modulus of the PP SPC is around 7GPa at 30℃，falling monotonically to just under 2GPa at 100℃.The storage modulus of the PP sheet was found to be 1.8GPa at 30℃ and 0.2GPa at 100℃.The large decrease in storage modulus in the PP SPC can be correlated with the corresponding modulus reduction of the PP fiber；the storage modulus of the PP fiber dropped from 17GPa to 8GPa，as the temperature increased from 30℃ to 100℃.In summary，the PP SPC has much improved mechanical properties than the non-reinforced PP: three times improvement in tensile strength and three times improvement in storage modulus.
4.3 Effect of processing temperature on tensilestrength of PP SPCs
As is well known，when preparing fabric-reinforced thermoplastic composites by melt processing，it takes time for the matrix to penetrate the fabric and wet the fibers.When an undercooled polymer melt is used as a matrix material in composites processing，additional concerns would arise since the polymer is now processed below its melting temperature.The critical issue here is whether the molten state can be well kept over time.To address this issue，the isothermal crystallization kinetics of the PP matrix at different temperatures (within the processing temperature window) was studied.Fig.10shows the heat flow curves for the isothermal crystallization (isothermal stage) of the PP matrix at different temperatures followed by cooling to 40℃ (cooling stage).As shown in Fig.10，for both isothermal crystallization temperatures of 125℃ and 130℃，the PP matrix were completely crystallized during isothermal crystallization (for a period of 30min)，as indirectly indicated by the absence of crystallization peaks in the cooling stage.In fact，at 125℃，the crystallization process rapidly completes within the first 2.5min，indicating that the molten state of the PP matrix is difficult to be maintained at this temperature.At 130℃，the PP matrix begins to crystallize after 1.3min，and the whole crystallization process takes 14min to complete.This implies that the molten state can be maintained during the first 1.3min.When the isothermal crystallization temperature increases，the crystallization process takes a longer time to complete.At 135℃，PP only partially crystallizes during the first 30min，as indicated by the small crystallization peak in the cooling stage.For even higher isothermal crystallization temperatures，e.g.，145℃ and 150℃，there were no crystallization observable during the 30min isothermal stage，and there were large crystallization peaks observed during the cooling stage.This means that the molten state of PP can be well maintained for at least half an hour at these temperatures，a condition that is desirable for preparing PP SPCs.
Fig.10 The heat flow curves of PP matrix during isothermal crystallization at different temperatures followed by coolingto 40℃. The different temperatures are：A，125℃；B，130℃；C，135℃；D，140℃；E，145℃；and F，150℃
Fig.11 Complex viscosities of PP matrix as a function oftime during cooling to different set temperatures
In composites processing，high fluidity of the ma trix is desired for fiber wetting. In order to determine the fluidity of the PP matrix during processing，the complex viscosities of the PP matrix were measured as a function of time，as the PP matrix was cooled to different set temperatures. The results are shown in Fig.11. All the samples were cooled at a rate of 10℃/min，the maximum cooling rate achievable by the parallel-plate rotational rheometer. It took the sample 2~3min to arrive at the stable temperature when the sample was cooled to predetermined temperatures for time sweep rheological measurements. Time sweep rheological measurements cannot be made at 125℃ and 130℃，because the PP matrix solidifies within 2~3min at these temperatures. However，there could still be sufficient time to prepare PP SPCs，because it took less than 10s to move the PP sheet from the high temperature platen to the low temperature platen. As shown in Fig.11，the complex viscosities of PP matrix do not change for half an hour at 145℃ and 150℃，again indicating that the PP matrix can keep its molten state well，consistent with the DSC results shown in Fig.11. The complex viscosity is higher at 145℃ than at 150℃. The complex viscosity begins to increase after 10min at the predetermined temperature of 140℃，suggesting that the PP matrix can keep its molten state for about 10min. The complex viscosity increases rapidly at135℃，an indication of occurrence of crystallization，and it takes 7min for the crystallization to finish. To sum up，the lower the predetermined temperature，the more difficult it is for the PP matrix to keep its molten state.
Fig.12 shows the tensile strength of the samples made at temperatures in the range from 125 to 150℃. The samples were tested parallel to both the warp and weft directions. The tensile strength in the weft direction is higher than that in the warp direction. This is not surprising since the weft density is larger than the warp density. As the processing temperature increased，the tensile strength increased in both the weft and warp directions. The increase in tensile strength may be correlated with the reduced viscosity and consequently improved wetting of the PP fabric by the PP matrix. As seen in Fig.11，the lower the processing temperature，the higher the complex viscosity of the PP matrix. The high viscosity can lead to low permeability of the PP matrix and poor wetting of the PP fabric.
5 Conclusions
PP SPCs were successfully prepared by applying undercooled polymer melt. With the aid of DSC and parallel-plate rheometry，a processing temperature window of at least 25℃，from 125℃ to 150℃，was established for processing PP SPCs. Within this processing temperature window，high fluidity of the matrix PP can be ob-tained without significantly reducing the fiber properties. The SPC molded at 150℃ containing 50% by weight of PP fabric achieved tensile strengths of approximately 220 and 180MPa in the weft and wrap directions，much higher than the value of 30MPa for the nonreinforced PP. Likewise，a significant improvement in storage modulus was achieved in the PP SPCs over the non-reinforced PP. The processing temperature was found to not only affect the feasibility of processing，but also affect the quality of the SPC. In particular，the tensile strength of PP SPCs decreased in both the weft and warp directions as the processing temperature decreased. This decrease in tensile strength may be correlated with increased viscosity and consequently reduced wetting quality of the PP fabric by the PP matrix.
Fig.12 Tensile strength of SPC as a function of processing temperature：(a) tested parallel to the warp direction，and (b) tested parallel to the weft direction
Acknowledgements
The authors acknowledge the scholarship fund (for Dai) from the China Scholarship Council and partial financial support (for Yao) from the Georgia TIP (Traditional Industries Program). The authors are also grateful to Yaodong Liu for his assistance in X-ray diffrac-tion. The authors are indebted to Dr. Elizabeth Cates of Innegrity LLC (Simpsonville，SC) for generously supplying polypropylene fabrics for this research.
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