高密度聚乙烯单聚合物复合材料的注塑工艺优化

Mao Qianchao，Wyatt Tom P.，Chen Jinnan，Wang Jian

1 INTRODUCTION

The traditional polymer composites，in which the reinforcement and the matrix are made from different materials，axe subjected to certain deficiencies such as the interfacial compatibility and recyclability. One of the alternative approaches to solve the problems is single-polymer composites (SPCs)，originally proposed in 1975. SPCs refer to a class of composites with the reinforce-ment and the matrix made from the same polymer type；therefore，they can be simply recycled by thermal processing which offers one of the routes to prepare environment-friendly materials. However，SPCs haven’t been widely used in industry due to processing difficulties. The difference between melting temperatures of the matrix and the reinforcement is generally small because of the same chemical structures of the constituents，which leads to narrow processing windows. Several articles reported on the preparation of SPCs using different processing methods including hot compaction of fibers or tapes，filmstacking followed by compression molding，and consolidation of co-extruded polypropylene (PP) tapes. It was reported that good interfacial bonding was promoted with higher interfacial shear strength than that of glass fiber-reinforced composites. The limited previous works to prepare SPCs primarily involve compaction processes，but the compaction process is not suitable for large scale production due to the relatively long process-ing cycle time，narrow processing temperature window，and post-forming requirement.

Injection molding is one of the most important industrialized techniques in the field of polymer composites. It provides advantages such as high production rates，repeatable high tolerances，and low labor cost. With the aim of adapting to the fast growing and high efficiency of industry，processes involving injection molding have been recently developed by Kmetty et al. to prepare SPCs. Pre-impregnated pellets which are hotcompacted from PP copolymer and highly oriented PP homopolymer fibers were injection molded into SPCs sheets with a processing range of~90℃. The yield stress achieved a maximum value of 38MPa which corresponded to a 52% improvement compared to the matrix materials. However，the fibers would suffer from heat loading and mechanical sheafing during the plastication and injection stages of the molding process，leading to the loss of fiber form and compromised ultimate strength.

In our preliminary study，an insert injection molding method was proposed to prepare PP SPCs. The PP fabric or fibers were pre-placed in the mold cavity and then the melt was injected into the mold and bonded with the fibers. It was reported that the PP SPCs achieved relatively good bonding with enhanced tensile and flexural properties. To better understand the process，different types of materials need to be investigated to further demonstrate the feasibility. In addition，optimized interfacial compatibility and bonding properties can be expected when the two composing materials are from the same polymers.

In this study，high-density polyethylene (HDPE) was chosen as a model system to further demonstrate the feasibility to prepare SPCs by insert injection molding. Melt-spun HDPE fibers made from the same resin as the matrix were heat treated to experimentally simulate the change of fiber properties during the injection molding process. Fibers were woven into fabric and further acted as the reinforcement in the HDPE SPCs. Mechanical and microstructural properties were investigated.

2 EXPERIMENTAL

2.1 Material

HDPE pellets，coded Marlex 9035，were provided by Chervon Phillips Chemical Company with a density of 0.952g/cm³and a melt flow index (MFI) of 40g/10min at 190℃.

2.2 Fiber Spinning and Heat Treatment of Fibers

Fiber spinning was performed by an Alex James and Associates piston extruder with 150mL capacity.The HDPE pellets were fed into the bore which was preheated to 200℃ and equilibrated for 1h.The molten HDPE was extruded through a 2.5mm die orifice at 200℃.The extrusion speed was about 7m/min.The molten thread line from the spinneret orifice was quenched through a glycerol bath maintained at room temperature and collected onto bobbins at a speed of 5m/min.The asspun HDPE fibers were heat drawn through heated glycerol at 83 ± 2℃ to a ratio of 18×.The total path length through the hot bath was 0.6m.

In the injection process，melt temperature，pressure in the cavity，and flow shear field can influence the fiber properties inside the mold，among which the melt temperature is considered to have the greatest effect. In order to investigate the changes in fiber properties upon exposure to the injected melt，heat treatment experiments on fibers were performed to approximate the conditions inside the mold. The fibers were submerged into the oil bath and then quickly removed from the bath. The contact time with hot oil was comparable to a short injection stage in injection molding. The designed pretreatment temperatures were 90~130℃ for fibers with tension and 90~125℃ for fibers without tension. A lower temperature was chosen in the latter case due to the immediate melting of fibers without tension at 130℃.

2.3 Composite Sample Preparation

Self-spun HDPE fibers were made into fabric by plain weaving.The thickness of the fabric was about 0.2mm.Each yam had 16 filaments and each fabric had 5 warp yarns.The thread count of both the warp and weft direction was the same of 4 yam/cm.A commercial re-ciprocation screw injection molding machine (SE-18D，Sumitomo co.) was used to prepare HDPE SPCs.The mold used in the experiments contained a rectangular cavity of dimensions 63.5×9.5×1mm and was maintained at room temperature.Double-sided tape was used to affix the fabric onto the cavity wall with the warp yams along the injection direction.Two layers of fabric were used for each sample with one layer sticking to the wall of the moving mold and the other sticking to the static mold.The fiber volume fraction was about 46.4%.Fig.1 shows the schematic diagram of the gate and the coverage of fabric on the cavity wall.Injection temperature of 200，220，240，and 260℃ were chosen to study the influence of injection temperature on the properties of SPCs samples.The lowest temperature of 200℃ was chosen because the cavity was not able to be fully filled by the injected melt with relatively high viscosity.The highest temperature of 260℃ was chosen due to the excessive melt of the fabric inside the cavity.The injection and holding time was 1 s and 20 s，respectively.The injection pressure and holding pressure were 207 and 167MPa，respectively.The cooling time was 15s.In order to analyze the temperature distribution of the melt in the mold cavity，Moldflow analysis was performed using the same experimental conditions that were used for molding the SPCs.The number of elements was 44，601，and 3D tetrahedral mesh was used.

Fig.1 Schematic diagram of the gate and thecoverage of fabric on the cavity wall

2.4 Characterization

Differential scanning calorimetry (DSC) was performed on a TA Instruments Q200 DSC unit. HDPE pellets were subjected to a heat-cool-heat cycle in the range from 40 to 180℃ at 10℃/min. The second heating stage was chosen to obtain the thermal behavior of HDPE pellets. HDPE fibers were heated from 40 to 180℃ at 10℃/min. The DSC unit was purged with nitrogen at a flow rate of 50mL/min.

Wide angle X-ray diffraction(WAXD) data were collected on a Rigaku Micro Max 002(Cu Ky radiation，λ＝0.154nm) operating at 45kV and 0.65mA using an R axis Ⅳ++detector.Exposure time was 30min for each sample.The crystalline orientation factor was computed using the method developed by Wilchinsky.The 200 and 110 diffractions were used to determine the orientation factor based on the orthorhombic PE unit cell with dimensionsa＝0.742nm，b＝0.495nm，c＝0.255nm.

Tensile properties of the melt-spun fibers were measured using an Instron 5566 universal testing machine. Crosshead speed was 50mm/min with a 10-cm gauge length. At least six specimens were tested and averaged. HDPE SPCs samples were tested on an Instron 5166 universal testing machine with a crosshead speed of 20mm/min. The composites were cut into dumbbellshaped specimens using a dog-bone cutter along the injection direction. At least five specimens were tested for each SPCs samples. All the tensile tests were performed at room temperature.

HDPE SPCs samples for T-peel test were made by means of reducing the injection volume so that the cavity was about 10mm unfilled at the other end of the cavity away from the gate. The unfilled part of the SPCs sample acted as a subsequent starter crack. During testing，one layer of the fabric was fixed by the moving clamp of the tensile testing machine while the other layer of fabric and the matrix together were clamped together by the stationary clamper. The samples were pulled apart using an Instron 5166 universal testing machine with a crosshead speed of 20mm/min at room temperature.

The microstructure of the HDPE SPCs was observed by optical microscopy (Axio Observer Alm，Carl Zeiss，German).The transverse cross-sections of the SPCs samples were polished using different sizes of metallographic sandpaper from 200 # to 1500 # and different grain size of diamond abrasion paste from W2.5 to W0.5.

The fracture surfaces after peel testing and tensile failure of HDPE SPCs made at injection temperature of 240℃ were examined by scanning electronic microscope (SEM) (JSM6301F，JEOL co. Japan). Samples were gold sputtered before SEM observations.

3 RESULTS AND DISCUSSION

3.1 Melt-Spun Fiber Properties

Fig.2 shows the DSC thermograms for melt-spun HDPE fibers and the original HDPE pellets.The melting temperatures of fibers and the resin were 132.2℃ and 126.2℃，respectively.The crystallinity(calculated from the DSC data) of HDPE fibers was 67.5%，25.0% higher than that of the original HDPE pellets.Fig.3 shows the WAXD pattern of melt-spun HDPE fibers.The WAXD pattern indicates that a highly oriented crystalline structure was developed during hot draw-ing.The intense diffractions at 2θ of 21.4° and 23.7° correspond to the 110 and 200 planes，respectively，and axe indicative of the typical PE orthorhombic unit cell.The crystalline orientation factor of 0.89 suggested good crystalline c-axis alignment.In addition，the tensile strength and tensile modulus of the HDPE fibers were 460.0± 18.8MPa and 11.1± 1.07GPa，respectively.

Fig.2 DSC thermograms of melt-spunHDPE fiber and the HDPE pellets

3.2 Heat Treatment of Fibers

To study how the fiber properties changed on exposure to high temperature，the HDPE fibers were heat treated in silicon oil to briefly simulate the temperature condition that occursinside the mold during injection process.(www.xing528.com)

Heat treatment trials showed that fibers partially melted while contacting with the heated oil at 130℃ for fibers with tension and fully melted at 125℃ for fibers without tension.The crystalline structures of heat-treated fibers were measured by WAXD.The 2D patterns of heat-treated fibers are shown in Fig.4.The lack of 2D pattern of fibers heat-treated without tension above 120℃ is because of the complete melting of the fibers.The results demonstrate that the crystalline structures of heat-treated fibers with and without tension below 120℃ were hardly changed compared to the tion original fiber.When pretreatment temperature further increased to 130℃，2D pattern of the fibers treated with tension showed a substantial change into concentric rings suggesting the loss of molecular orientation.This can be further verified by the orientation factor calculated from the total integration.For the fibers treated with tension below 130℃ and without tension below 120℃，the orientation factors remained the same to the original fiber at 0.89.The small extension applied during heat treatment is not large enough to rearrange or further orient the molecular chains.The orientation factor sharply dropped to 0.05 when the pretreatment temperature increased to 130℃for fibers with tension，corresponding well to the concentric rings observed in the 2D pattern.It is likely the result of molecular relaxations from the oriented state to entropically preferred random coil.For the fibers heattreated without tension at 125℃，fiber form was immediately lost upon exposure.

The thermal properties of heat-treated fibers were measured by DSC and are shown in Fig.5. For heat-treated fibers both with and without tension，melting temperatures and crystallinities demonstrate downward trends and the values are all under that of the original fi bers. This is because of the molecular relaxation occurred at relatively high pretreatment temperature. The tensile properties as a function of the pretreatment temperature for heat-treated fibers are shown in Fig.6. The tensile strength kept decreasing as the pretreatment temperature increases，corresponding well to the decreasing of the melting temperature and crystallinity. The tensile modulus showed no noticeable change for the fibers treated with tension below 130℃ and the fibers without tension below 120℃. Sharp drops were found at 130℃ for the fibers treated with tension and 120℃ for the fibers treated without tension. This is because of the loss of molecular orientation and the physical form of fibers.

Fig.3 WAXD pattern of melt-spun HDPE fiber：(a) 2D pattern；(b) total integration；miller indices correspondingto typical orthorhombic PE crystal planes are noted above the peak intensities. [Color figure can be viewed in theonline issue，which is available at wileyonlinelibrary.com.]

Fig.4 WAXD pattern of heat-treated fibers under various pretreatment temperature：(a) 100℃ with tension；(b) 120℃ with tension；(c) 130℃ with tension；(d) 100℃ without tension；(e) 120℃ without tension.The insert：calculated orientation factor (OF). [Color figure can be viewed in the online issue，which is available at wileyonlinelibrary.com.]

Fig.5 Thermal properties of heat-treated fibers at various pretreatmenttemperatures：(a) with tension；(b) without tension

The conclusions above have guiding significance to the insert injection molding process with fibers preplaced in the cavity.Fig.7 shows the temperature distribution of the melt in the mold cavity for injection temperature of 200，220，240，and 260℃.The temperature of the injected melt within 0.1mm away from the mold cavity is around or below 130℃ for the injection temperature of 200，220，240，and 260℃ at 1s after injection.According to the properties of heat-set fibers，the fibers within 0.15mm away from the wall axe able to maintain fiber form and properties during injection process.For fibers 0.1-0.2mm away from the wall，since the temperatures of the injected melt axe above 130℃for injection temperatures from 200 to 260℃，there are high possibilities of melting upon exposing to the melt.The pre-placed fibers act as an insulator between the mold and the injected melt，which influences the temperature distribution in the cross-section of the injected melt.The temperature of the melt touching the fabric might be much higher than 130℃ at 1s after injection because fabric has a lower thermal conductivity than mold.However，the temperature of the melt decreased sharply to below 120℃ within 2s after injection for all injection temperatures from 200 to 260℃.Therefore，the fibers are likely to avoid significant melting.In addition，it was reported that the melting temperature of constrained fibers would shift to a higher value because of reduced molecular relaxation.The fibers in the middle of the fabric can be considered to be constrained by the adjacent wept yarns and the doublesided tapes.This further provides the feasibility to keep the fibers retain the physical forms and properties during injection process and to prepare SPCs by insert injection molding.

Fig.6 Tensile properties of heat-treated fibers under various pretreatment temperatures：(a) with tension；(b) without tension

Fig.7 Temperature distribution of the melt in the mold cavity for injection temperature of 200，220，240，and 260℃：(a) 1 s after injection；(b) 2 s after injection

Fig.8 Tensile property of HDPE SPC with lab-made fabric：(a) Tensile strength as a function ofinjection temperature；(b) a comparison of HDPE SPCs with neat HDPE

3.3 Mechanical Property of HDPE SPCs

Fig.8a shows the tensile strength of HDPE SPCs with labmade fabric as a function of injection temperature in the range from 200 to 260℃.The samples were tensile tested parallel to the warp direction (injection direction).The tensile strength increased with increasing injection temperature and achieved a maximum value of 50 ± 3.8MPa for the SPCs samples made at injection temperature of 240℃.The increase in tensile strength may be ascribed to the lower melt viscosity at higher injection temperature and therefore improved permeability into the fabric.Furthermore，higher melt temperature is related to a better pressure transmission along the flow path which benefits the penetration of the melt.The tensile strength of SPCs sample made at injection temperature of 260℃ decreased sharply to a value around the strength of the neat HDPE due to the excessive melting of the fabric.The representative tensile stress-strain curves of HDPE SPCs with lab-made fabric and the neat HDPE both made at injection temperature of 240℃ are shown in Fig.8b.The maximum tensile strength of the HDPE SPCs around 50MPa was 2.8 times that of the neat HDPE (18MPa).From this point of view，HDPE SPCs can be processed by insert injection molding within the injection temperature range from 200 to 240℃.

3.4 Interfacial Property of HDPE SPCs

Fig.9 shows the peel load traces of HDPE SPCs prepared at different injection temperature in the range of 200~240℃.The traces show a sawtooth appearance with the load rising to a peak and then dropping to a lower value，most likely due to the fabric structure with the uneven surface.The peel strength is 6.67，10.23，and 16.14N/cm for the HDPE SPCs made at injection temperature of 200，220，and 240℃，respectively.The increased peel strength indicates the increased bonding property as the injection temperature goes higher.Generally，at lower injection temperature (e.g.，200℃)，the viscosity of the injected melt in the cavity is relatively higher which leads to poorer penetration ability of the melt and hence the poorer bonding between the matrix and the fabric.This corresponds to the adhesive failure mode proposed by Alcock et al.As the injection temperature increases，the viscosity of the injected melt decreases with enhanced wetting property.In addition，the surface of the fibers began to become“tacky” due to the partial melting of the fiber caused by the heat transferred from the melt.As a result，the fibers and the matrix started to fuse together and the molecular interdiffusion began to occur，which caused a combination of adhesive failure and cohesive failure corresponding to the increased peel strength.As the injection temperature further increased to 240℃，the wetting and bonding properties between the matrix and the fibers achieved the maximum.This is partially due to the relatively low viscosity of the melt.On the other hand，the fibers can be partially melted by the heat from the injected melt at high injection temperature The melted and recrystallized HDPE melt may act as a “bridge” to combine the fiber and the matrix which can improve the interfacial adhesion between the two phases.Therefore，the interfacial bonding property of SPCs samples made at 240℃ achieved a maximum value，corresponding to the cohesive failure mode.

Fig.9 Interfacial shear strengths of HDPE SPC with labmade fabric made at different injection temperatures：(a) 220℃；(b) 220℃；(c) 240℃

It is noted that the trend of the peel load traces went up with increasing extension for the SPCs samples made at injection temperatures of 240℃. This is correlated to the character of the injection molding process. Since the mold was maintained at room temperature around 25℃，the injected melt in the cavity was cooling down during the filling process，which led to the increase of melt viscosity. Higher melt viscosity would cause the reduced wetting and permeability between the two phases. In addition，the SPCs samples for peel tes ting were made by reducing the injection volume so that the cavity was not fully filled at the end of the cavity away from the gate. As a result，the pressure in the cavity would decrease at the position away from the gate. The lower pressure would lead to the poorer penetration and hence the poorer bonding properties between the matrix and the fabric.

The fracture surfaces of the SPCs samples made at injection temperature from 200 to 240℃ after peel testing are shown in Fig.10. At injection temperature of 200℃，the individual fibers can still retain their physical form without deforming. Some particles of the matrix can be found adhesive on the surface of the fibers indicating relatively good bonding between the fibers and the matrix. As the injection temperature is increased to 220℃，more quantities of the matrix were adhesive on the peeled fibers and some matrix was able to be found between the fibers. It suggests that the viscosity of the matrix was low enough to penetrate the webs of fibers which can enhance the wetting and bonding properties. As the injection temperature further increased to240℃，the surface of the peeled fibers became rough and fibrillation occurred. This evidence of stress transfer confirms the good bonding between the two phases，corresponding to the highest peel strength as shown in Fig.9(c).

3.5 Optical Microscope Observation of HDPE SPCs

Some optical micrographs of polished and unetched transverse cross-sections of HDPE SPCs with labmade fabric made at injection temperature of 200 and 240℃ are shown in Fig.11. Weft fibers in strip shape and warp fibers in circular shape in the fabric can be seen in the pictures [Fig.11 (a) and (e)]. At injection temperature of 200℃，voids between warp fibers and considerable gaps between the fibers and the matrix were easily observed，which indicates compromised penetration and bonding properties of the SPCs samples. It was seen at higher magnification that part of the fiber surface was slightly melted and most of the fibers were able to reserve their physical forms. As injection temperature increased to 240℃，the number of the voids between the fibers became less and the arrangement of the fibers tended to be closer. Some of the fibers were pressed into polygon shapes by the adjacent fibers due to the relatively high injection pressure [Fig.11 (f)]. It can be seen in Fig.11 (g) that the edges of the fibers were blurring which suggests that more fibers were partially melted as the injection temperature increased. Furthermore，the webs between the fibers were fully filled with the matrix. The matrix between the fibers is probably composed of two parts:the partially melted fibers and the origi nal injected melt. As can be found at higher magnification in Fig.11 (h)，the webs between the fibers were completely penetrated by the matrix. Since the fibers were spun from exact the same resin with the matrix，it is expected that the compatibility of the two phases is good enough for bonding without phase separation. Therefore，it can be concluded that good interfacial properties were achieved for the HDPE SPCs made at injection temperature of 240℃.

Fig.10 Fracture surfaces of SPCs samples made at various injection temperatures afterpeel testing：(a) 200℃；(b) 220℃；(c) 240℃

Fig.11 Optical micrographs of polished and un-etched transverse cross-sections of HDPESPCs made at injection temperature of：(a，b，c，d) 200℃；(e，f，g，h) 240℃.

Fig.12 SEM images of HDPE SPCs made at 240℃：(a，c) fracture surface aftertensile failure；(b，d) partially enlarged view of (a，c) respectively

3.6 SEM Observation of HDPE SPCs

The SEM images of the fracture surfaces of HDPE SPCs with lab-made fabric made at 240℃ after tensile failure are shown in Fig.12. It can be seen from Fig.12a and b that the fiber next to the matrix was wetted by the matrix and no gap was observed between the two phases，which indicates that good interfacial bonding was achieved during injection molding. As shown in Fig.12 (c) and d，fibers still remained in the matrix after tensile failure and the matrix was adhesive on the surface of the fibers. It suggests the good compatibility and bonding properties between the fibers and the matrix.

4 CONCLUSIONS

The feasibility of making HDPE SPCs by insert injection molding was demonstrated.HDPE fibers were melt-spun in the laboratory using exact the same resin with the matrix.Heat treatment of HDPE fibers showed that fibers were able to maintain their physical integrity and preserve useful tensile properties during injection molding process.The tensile strength of HDPE SPCs with 30 wt % fibers made at injection temperature of 240℃achieved 50MPa，2.8 times that of non-reinforced HDPE.The peel strength of HDPE SPCs increased with increasing injection temperature from 200 to 240℃ and achieved a maximum value of 16.14N/cm，which indicated relatively good interfacial property.Optical micrographs of polished transverse cross-sections showed that higher injection temperature is beneficial to the wetting and permeation properties of the matrix.Webs of the fibers could be fully filled with the matrix at injection temperature of 240℃.SEM photographs suggested good bonding and compatibility between the fibers and the matrix.

5 ACKNOWLEDGMENT

The authors acknowledge the contribution of Dongjie Chen at Beijing Institute of Technology for assistance with SEM observation and polishing samples for optical microscope.