Wang Jian，Du Ziran，Lian Tong
1 INTRODUCTION
The recyclability of composites still reminds a key topic to the pressures of resource and environment in recent years. However，heterogeneous compositions and special structures in traditional polymer composites strictly limit the realization of easy recycling. Benefiting from the same chemical compositions of matrix and reinforcement，single-polymer composites (SPCs) offer an easy access to deal with the recyclable challenge eliminating the complicated sorting and separation steps. The homology of SPCs also gives a route to overcome the poor interfacial adhesion between the matrix and reinforcement. Moreover，polymer fibers instead of glass，carbon，aramid，and natural fibers give the SPCs with reduced density. Therefore，SPCs can compete with traditional polymer composites in various application fields based on easy recycling，enhanced interfacial adhesion，reduced weight，and cost balance. Applications fall within a broad range of industries including automotive industry，industrial cladding，building and construction，cold temperature applications，audio products，personal protective equipment，sports good，and so on.
SPCs were first prepared based on polyethylene (PE). PE is one of the most widely used plastics；it has a simple structure，good chemical resistance，and processing properties. PE can be classified into ultrahigh molecular weight polyethylene (UHMWPE)，high-den-sity polyethylene (HDPE)，low-density polyethylene (LDPE)，and so on. Karger-Kocsis defined two subgroups of SPCs:(i) SPCs from the same polymer (one constituent SPCs) and (ii) SPCs from the same polymer type (two constituent SPCs). So far，for the one-constituent PE SPCs，HDPE SPCs and UHMWPE SPCs have been developed；for the two-constituent PE SPCs，according to the matrix/reinforcement form，LDPE/HDPE SPCs，LDPE/UHMWPE SPCs，LLDPE/UHM-WPE SPCs，LDPE-HDPE/UHMWPE SPCs and HDPE/UHMWPE SPCs were also developed successively. The main developed methods for preparing PE SPCs include hot compaction，solution impregnation，and film stacking. However，the abovementioned methods have some disadvantages，such as long cycle time，complicated procedure，expensive，and so on，and the common problem is that they cannot seem to escape from compression molding，greatly limiting the economics and the capabilities of PE SPCs. Injection molding and extrusion have been widely used in the field of polymer processing，they can also be used in the volume-production of SPCs. We reported an insert injection molding method to prepare one-constituent HDPE SPCs，a short cycle time and volume production can be realized but the structure and size of the products are still limited. Huang studied the continuous extrusion for producing self-reinforced HDPE relying on flow-induced crystallization in the die with a converging wedge channel，and the selfreinforced HDPE/UHMWPE was also achieved by Chen et al.through a specially designed fish-tail shaped extrusion die. More similar investigations were performed. However，the self-reinforced polymer is a different concept with composites because there is no specific phase difference on matrix and reinforcement，and it is mainly fo cused on the strength improvement by taking the advantage of molecular orientation during processing. Therefore，there still emerges a need for SPCs to develop a better processing protocol that is preferably compatible with standard high-throughput processes for polymers.
In this article，an extrusion-calendering process was developed to prepare PE SPCs. The UHMWPE fabric was inserted in a specifically designed extrusion die. It was coated by LDPE melt and then moved out into the calendering rolls to finally obtain the LDPE/UHMWPE SPC belt. This method surmounts the drawbacks of the existing methods and possesses the economical and practical significance to achieve continuous production of SPCs industrially. The effects of processing conditions on mechanical properties，interfacial properties，failure modes，and microstructures of the LDPE/UHMWPE SPCs were all investigated.
2 EXPERIMENTAL
2.1 Materials
The matrix material used in this work was LDPE granules(868-000) purchased from SINOPEC Maoming Company.The polymer had a density of 0.9205g/cm3 and a melt flow index(MFI) of 50.0g/10min.The UH-MWPE woven fabric(supplied by Shandong ICD High Performance Co.，Ltd.) was applied as the reinforcement.It had a warp density of 55 threads/10cm and a weft density of 93 threads/10cm，and its areal density was 284g/m2.The density of the UHMWPE fiber was 1.0111g/cm3.The tensile strength and modulus of the fiber could be up to 12 cN/dtex(1.176GPa) and 270 cN/dtex(26.46GPa).
2.2 Thermal Analysis
Differential scanning calorimetry (DSC) experiments were performed in Shimadzu DSC-60 to acquire the thermal behaviors of the LDPE granules and UHM-WPE fabric. The materials were heated with a rate of 10℃/min from 30℃ to 170℃ and maintained a constant temperature for 2min to eliminate the disturbance of thermal history，and then cooled down to 30℃ at 10℃/min.
2.3 Composites Preparation
The main experimental apparatus is a 20mm singlescrew extruder (RM-400B，Harbin Harp Electrical Technology Co.，Ltd.) with a length-to-diameter ratio of 25/1，and a calendering system (FYJ-30，Jinfangyuan Machinery Manufacturing Co.，Ltd) was used together.An extrusion die was specially designed to realize the coating process.The schematic of the extrusion-calendering system is shown in Fig.1.The circumferentiallinear distributing channel was made on the part of the external surface of the fabric guide.Inside the guide，there was a slit for the inserted fabric.The UHMWPE fabric with a width of 40mm was first introduced through the slit channel inside the guide and then withdrawn into the calendering system.The LDPE pellets were melted in the extruder and pushed into the die under the extrusion pressure.The UHMWPE fabric was coated with the LDPE melt in the die and then drawn out into the running calendering machine.The fabric was impregnated mainly with the two rollers.The die orifice had a width of 46mm and a thickness of 4mm.The gap between the rollers was kept at 2.7mm，and the roller temperature was at 20℃.The finally produced LDPE/UHMWPE SPC belt with a sandwiched structure was gradually cooled at room temperature.
The experimental processing conditions are summarized in Table 1. To determine the influences of different processing parameters(die temperature，screw rotational speed，and rolls speed)，an orthogonal experimental method was used then the single factor experiments were conducted. Sixteen groups of LDPE/UHMWPE SPC samples were produced. Nonreinforced LDPE samples were also prepared for comparison. It is noted that experimental no. 1~9 were the orthogonal experiments；T1，T2，T3，andT4were the single factor experiments at different die temperature；R1，R2，R3，andR4were the single factor experiments at different rolls speed；S1，S2，andS3were the single factor experiments at different screw speed.
Fig.1 Scheme of the experimental setup for manufacturing LDPE/UHMWPE SPCs：(a) extrusion die and (b) singleextruder and calendering system. [Color figure can be viewed at wileyonlinelibrary.com]
Table 1 Processing parameters of theexperiments to produce samples
2.4 Mechanical Testing
The tensile properties of the specimens were measured by a universal testing machine (XWW-20Kn，Beijing Jinshengxin Testing Instrument Co.，Ltd).Before the tests，the dumbbellshaped sample was cut by using a dog-bone cutter in accordance with GB 1040.2-2006 1BA.T-peel tests were also carded out to determine the interfacial bonding strength which is defined as the average load per 10mm width of the sample.The sample was cut into a rectangular geometry with 10 (width)×80mm (length) before testing.The unbonded ends of the sample with a length of 20mm were used as a starter crack for the test.The tests were performed at a crosshead speed of 10mm/min at room temperature.
2.5 Calculation
The UHMWPE fabric was cut into the tensile sample geometry by a dog-bone cutter，and then an electronic balance was used to measure the fabric weight，mf，which was 0.2796g.The weight of each tensile sample was also measured，so the fiber weight fraction，Wf，can be easily obtained.To determine the fiber volume fraction，the sample thickness was measured by a caliper，thus the density of the composite，ρc，can be obtained by
wheremc is the sample weight，A is the section area of the metallic cutter which is a constant value of 984.598mm2，and t is the sample thickness.Then the fiber volume fraction，Vf，could be calculated by
whereρfis the fiber density(1.0111g/cm3).It is noted that the fiber melting was not considered in the calculation.During the forming process，uncompleted infiltra-tion leads to voids existing in the composites.Based on the mass conversation，the volume fraction of voids can be predicted.The density of a composite can be obtained using the rule of mixtures given by
whereVvis the volume fraction of voids andρmis the matrix density(0.9205g/cm3).Equation 3can be rewritten as
The tensile properties of the LDPE/UHMWPE SPCs were strongly dependent on the processing parameters. The reinforcement efficiency factor，K，was used to evaluate the effectiveness of impregnation and the adhesion between the fabric and the matrix. Both the strength efficiency factor，Ks，and the modulus efficiency factor，KE，were calculated by using the following equations:
whereKsandKEdepend on the orientation of the fibers in the composite and the degree of adhesion between the fibers and the matrix.
2.6 Scanning Electron Microscopy (SEM)
The tensile and peel fracture surfaces of the specimens prepared from different processing conditions were all observed by a microscopy (Quanta FEG250) with an accelerating voltage of 10kV. The specimens were pasted on the electron microscope holder by the double-sided carbon tape and their fracture surfaces were coated with gold before observation.
3 RESULTS AND DISCUSSION
3.1 Determination of Processing TemperatureWindow
According to the DSC curves presented in Fig.2，the LDPE had a melting temperature at 111℃. Two different melting temperatures of 147℃ and 153℃ appeared in the heat curve of the UHMWPE fabric，it is attributed to the imperfections of crystal in the crystalline polymer. In theory，the selected operating temperature should be higher than the melting temperature of the ma trix and lower than that of the reinforcement，thus the processing temperature window could be about 40℃(111~ 153℃). However，the temperature will decrease when the LDPE resin and UHMWPE fabric move out of the die. Meanwhile，the UHMWPE fabric moves through the die continuously，so it has a short heating time before calendering. The poor thermal conductivity of polymer will cause a temperature difference between the matrix and reinforcement. The large difference in melting temperature between the UHMWPE fiber and the LDPE matrix makes it possible to manufacture the composites with a wide processing temperature window. Therefore，the die temperature from 140℃ to 160℃ was selected in the experiments，and 135℃ was also used in the single factor experiments to determine the influence of the die temperature.
Fig.2 DSC curves of the LDPE matrix and UHMWPE fabric.[Color figure can be viewed at wileyonlinelibrary.com]
3.2 Tensile Properties(www.xing528.com)
Fig.3 shows the tensile strength and modulus of the LDPE/UHMWPE SPCs and nonreinforced LDPE prepared in the orthogonal processing conditions.The fiber weight fraction was from 13% to 15%.The tensile strength and modulus of the SPCs were 78.8 ± 7MPa and676.6 ± 116MPa，respectively.In comparison with nonreinforced LDPE，the maximum increment of tensile strength and modulus for LDPE/UHMWPE SPCs are 8 times and 4.8 times，respectively.
A summary of the tensile properties of LDPE/UHM-WPE SPCs produced by different methods are presented in Table 2. The comparison is not specific，because different original mate rials，reinforcement style，and testing conditions were used. But the rough comparison is feasible to know the features of these different processing methods. In comparison with the other methods including mixing-compression and solution-casting，the LDPE/UHMWPE SPCs produced by extrusion-calendering possessed comparable or superior mechanical properties.
The tensile properties of the LDPE/UHMWPE SPCs varied significantly under different experimental conditions；however，the bulk LDPE had almost no change.The best tensile strength was obtained by using die temperature of 150℃，screw speed of 45 rpm and rolls speed of 2.8m/min.For the analysis of orthogonal experiments，k which is the average value of the sum of experimental values for each factor at every level was calculated.Fig.4 shows the value ofk for tensile strength and modulus with different processing parameters.From the variation range ofk，we can see the effect degree of different factors in the following order: die temperature>rolls speed>screw speed.It suggests that the die temperature is the main influence factor while the screw speed is the least one.
Fig.3 Tensile properties of LDPE/UHMWPE SPCs and nonreinforced LDPE prepared in the orthogonal processing conditions：(a) tensile strength and (b) tensile modulus
Table 2 Tensile properties of the “optimum” LDPE/UHMWPE SPCs producedby extrusion-calendering compared with other methods
Fig.4 The value ofk for(a) tensile strength，(b) tensile modulus，(c) fiber volume fraction，and(d) void content with different processing parameters
Fig.5 Tensile strength and modulus of LDPE/UHMWPE SPCs as a function of(a) die temperature(T1，T2，T3，T4) and(b) rolls speed(R1，R2，R3，R4)
In Fig.4a，the tensile strength increased，then decreased with the increase of die temperature. At higher temperature，the LDPE melt with a lower viscosity is easy to permeate through the void spaces between fibers，and the accelerated interdiffusion also occurs in the interface between the matrix and fabric. The improved interfacial bonding can effectively lead to the stress transfer from the matrix to fiber；the UHMWPE fiber with superior mechanical strength could enhance the tensile strength of the SPCs. As the temperature has exceeded the melting point of the fiber，a substantial fraction of fibers may be melted and the relaxation of fibers will inevitably lead to the decrease of tensile strength. Furthermore，the fabric due to high temperature is easy to be deformed even disrupted under the pulling of the rotational rolls. Fig.4b shows that tensile modulus decreased as the temperature increased from 140℃ to 160℃. To determine the variation of the tensile modulus，single factor experiments as a function of die temperature from 135℃ to 160℃ were conducted. As shown in Fig.5a，the variation of modulus shows that the temperature for the greatest tensile modulus was lower than for the greatest strength. This can be explained by the molecular relaxation and the reduction of molecular weight.
Fig.5b presents the influence of the rolls speed on tensile properties of the LDPE/UHMWPE SPCs.As the rolls speed increased from 0.8 to 3.8m/min，both tensile strength and modulus followed the variation of increasing first and then decreasing.Similar to the effect of die temperature，the rolls speed for the greatest tensile strength was a little faster than for the greatest modulus.The squeezing force generated by the two rolls is similar to the direct pressure from the hot pressing meth-od.Increasing rolls speed is equivalent to increasing the pressure exerting on the interlayer，which promotes the LDPE melts to infiltrate into the void spaces between fibers.It will result in a stronger mechanical locking and improved interfacial bonding.Meanwhile，when the fabric coated with LDPE matrix is withdrawn into the gap between the two rolls，the lamination suffers from the effect of longitudinal shear and radial squeezing.Molecular orientation due to rolls shear will result in a significant variation of physical properties，especially in the direction of calendering.Thus，the increase of rolls speed enhanced the tensile properties.Nevertheless，the extrusion speed should be parallel to the rolls speed，otherwise much higher rolls speed will make the fabric drawn out without enough matrix.Moreover，the temper ature difference between the matrix and fabric is larger at higher rolls speed because of the shorter residence time in the die.On the contrary，much lower rolls speed will cause nonuniform distribution of thickness because the melt will accumulate in the rolls gap.The appearance quality of the products and a smooth production process should also be considered，thereby the rolls speed should be changed in accordance with the screw speed.
The orthogonal analysis shows that the screw rotational speed had a little effect on the tensile properties. The screw speed is mainly used to regulate the output of extrusion melts to make sure the integrity of products as a lower screw speed cannot provide adequate LDPE melts for the UHMWPE fabric timely. Much faster screw speed，however，will lead to the increase of flow output and shear heat. The excess melt will dissipate axially due to the squeezing of the rolls，but the excess matrix may reduce the fiber volume fraction and thus decrease the tensile strength. The shear heat generated by the screw rotation is limited because the matrix with high temperature will undergo a certain degree of cooling when it is extruded out of the die.
Therefore，the mutual cooperation of those three parameters on the sample quality is important. A temperature around the melting point of the fiber and the equivalent screw-rolls speed should be selected to improve the impregnation，interdiffusion，and interfacial properties.
3.3 Fiber Volume Fraction， Void Content， Strength， and Modulus Efficiency
The impregnating/flow behavior of the polymer is controlled by the processing parameters and may result in poor or good impregnation/interdiffusion.Besides temperature，the balance between screw and roller speeds may strongly influence the resulted fiber volume fraction and void content and affect the mechanical properties.The fiber volume fraction and void contents of the SPC samples are shown in Table 3.The calculated fiber volume fraction was around 12%，and the highest void content was 7.5%.Fig.4c and d show the value of k for fiber volume fraction and void content with different processing parameters.The effect of the three processing parameters on the void content was significant.Higher die temperature and rolls speed caused low viscosity and good flow behavior which allowed better wetting of the fibers，and then the void content reduced.
The efficiency factors—calculated from Eqs 5 and 6—are also reported in Table 3. Both Ksand KEare dependent on several factors such as the fiber aspect ratio，fiber orientation relative to the loading direction，and the degree of adhesion between the fibers and the polymeric matrix. The fabric has a specific orientation which can increase the strength and resistance to deformation of the polymer. The composites are strongest when the fibers are parallel to the force being exerted but are weak when the fibers are perpendicular，so the highest Kswas a round 50% due to the plain structure of the fab-ric. The lower Ksvalue is due to the influence of the processing parameters. The effect of processing parameters on Ks and KEwas the same with the effect on tensile strength and modulus，respectively.
Table 3 Fiber weight fraction，fiber volumefraction，void content，strength，andmodulus efficiency factors of the LDPE/UHMWPE SPC samples
3.4 Interfacial Properties
The peel load is closely bound up with the interfacial adhesion.Fig.6a shows the peel load traces for the SPC samples produced in different die temperature.The peel strength of 22.4N/10mm was achieved at 160℃；it was much improved compared to 4.8N/10mm obtained at 140℃.The peel load curve for 140℃ was constant with small peaks and troughs.For 150℃，the average load was higher and the load variation was greater.It suggests that lower temperature results in interfacial debonding and inefficient load transfer.It also provides the evidence of inferior tensile strength and modulus.With the increase of die temperature，both improved permeating and interdiffusion enhanced the interfacial bonding.For the die temperature of 160℃，the average load was the highest，but the fluctuation of the trace became much greater.More fibers at the much high temperature melted and then consolidated with the matrix，so the sample made at 160℃ was broken off and the T-peel test stopped earlier.
Fig.6b shows the peel load traces for the LDPE/UHMWPE SPC samples produced in different rolls speed.Relying on the stronger shear and squeezing force produced by higher rolls speed，the LDPE matrix can infiltrate into the intervals of adjacent fiber bundles.It enhances the interfacial bonding.For the much higher rolls speed of 2.8m/min，the peel load curve twisted and turned to be unstable，although the final peel load was higher.This demonstrates that a suitable rolls speed should be set to achieve not only an excellent interfacial bonding strength but also a steady process.Compared to the die temperature and rolls speed，the screw speed had little influence on the peel strength.In Fig.6c，three peel load curves maintained at the same level with different degrees of overlap，they all reached an average value of about 11N/10mm.
Fig.6 Peel load traces for the LDPE/UHMWPE SPCsmanufactured in different(a) die temperature(t2，t3，t4)，(b) rolls speed(R1，R2，R3)，and(c) screw speed(S1，S2，S3).[Color figure can be viewed at wileyonlinelibrary.com]
3.5 Morphology Observations
Fig.7 shows the tensile fracture cross-section taken from the samples produced at different die temperature. Fiber bundles with different morphology can beclearly seen in the sandwiched structures (square in Fig.7ac). Only a small number of fibers bonded together at 140℃，the clear interface (arrows in Fig.7d) reveals that lower temperature only affected partial fibers at the fabric surface. The low interfacial bonding led to a fracture mode that the LDPE layers disintegrated firstly then the fibers were easily pulled out (Fig.7a and d). As expected，increased temperature progressively led to the melting of fibers. Fig.7b illustrates that more fibers melted at 150℃. Fiber drawing and fracture coexisted. Higher temperature resulted in continuity and integration of fiber bundles. Fiber interface was also improved. In Fig.7c and e，the brittle fracture mode indi-cates that an excellent bonding existed in the interface (arrows in Fig.7e)，and the fibers consolidated with matrix tightly. The fibers almost fused together in Fig.7f and tiny particles (arrows in Fig.7g) can be seen on the fiber bundles. Much higher temperature (160℃) deprived of the original shape of the fiber bundles and deteriorated the high strength properties of the reinforcement，and thus there was a decrease in tensile strength.
Fig.7 Tensile fracture section of LDPE/UHMWPE SPCs manufactured mainly at different die temperature：(a-e)amples No. 2，4，9，3，7；(f) the high magnification image of section A in (e)；(g) the high magnification of section B in (f). Note：some unbroken fiber bundles were cut by scissors in (a) and (b). [Color figure can be viewed atwileyonlinelibrary.com]
Fig.8 Peeling fracture surfaces of LDPE/UHMWPE SPCs made at different die temperature：(a)~(c)t2，t3，andt4；(e)t4andt2in the same image；(d) the high magnification image of section A in(e)；(f) the high magnificationimage of section B in(e). [Color figure can be viewed at wileyonlinelibrary.com]
Fig.8 compares the variation of peeling fracture surfaces with different die temperature. As die temperature increased，the adjacent matrix and fibers fused together and molecular interdiffusion caused a combination failure within the fiber/matrix interface and the fiber/fiber interface. Many fibrils show the good interfacial adhesion. In Fig.8c and d，large parts of fused fibers were peeled off；the cohesive failure testifies the better inter facial strength. Once again，the molecular relaxation and interdiffusion increase with increasing temperature，and thus the peel strength increased. However，it also causes much shrinkage of the fiber bundles and destruction of fiber orientation，so the tensile strength decreased at the higher temperature.
Fig.9 shows the relationship between rolls speed and the morphology of peeling fracture surface.Compared with the sample made at 0.8 m/min，better interfacial adhe sion can be found in the samples (Fig.9e and f) made at higher rolls speed，but the difference between the sample made at 1.8 and 2.8 m/min was not significant.In Fig.10，there is no significant difference in the morphological deformation caused by the vari-ation of screw speed.
Fig.9 Peeling fracture surfaces of LDPE/UHMWPE SPCs made at different rolls speed：(a-c)R1，R2，andR3；(d)~(f) the magnification images of(a)~(c)
Fig.10 Peeling fracture surfaces of LDPE/UHMWPE SPCs made at different screw speed：(a)S1，(b)S3；(c) the magnification image of(a)；(d) the magnification image of(b)
4 CONCLUSIONS
The process of extrusion-calendering was introduced to realize the continuous production of SPCs.The two-constituent LDPE/UHMWPE SPCs were successfully prepared.The difference in melting temperature of LDPE and UHMWPE was also applied to enlarge the processing temperature window.An operating temperature range from 140℃ to 160℃ was achieved.In comparison with other methods such as hot compaction，solution impregnation，and film stacking，this approach makes it possible to produce SPCs continuously and avoids the drawbacks involving a long cycle time，inconvenient operation and expensive cost.It also gives huge opportunities to be compatible with the volumeproduction of SPCs.Based on the results of orthogonal and single factor experiments，it was found that the effect of die temperature on tensile and interfacial properties was significant，followed by the rolls speed.For the given die temperatures of 150℃ and 160℃，the tensile strength and modulus of the LDPE/UHMWPE SPCs could be up to 78.8 ± 7MPa and 676.6 ± 116MPa，8 and 4.8 times higher than that of the nonreinforced LDPE，respective ly.Much lower or higher temperature will all reduce the tensile properties，but higher temperature always increases the peel strength.The peel strength was significantly improved，22.4N/10mm was achieved at 160℃.Increasing rolls speed can promote the interfacial adhesion then lead to the improvement of tensile and interfacial properties.The screw speed had a negligible effect.The mutual cooperation of those three parameters on the sample quality is important.A temperature around the melting point of the fiber and the equivalent screw-rolls speed should be selected.The optimum processing conditions for achieving low void content and great tensile strength were die temperature of 150℃，rolls speed of 2.8m/min and screw speed of 45rpm.Finally，the obser vations of microscopic morphology also provided the evidence of theoretical analysis.In the future，the multiplayer LDPE/UHMWPE SPC parts could be also realized via coextrusion-calendering process.
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