Manufacture and Application of High-Performance Geogrids with PP/PET Composite Covered Yarn

Introduction

The development of a grid is critical in civil, geotechnical, environmental and structural engineering applications. Composite yarns spun from multifilament and staple fibers have undergone much research and development, to produce favorable yarn characteristics and to increase the yield of the yarn [1]. The complex covered yarn is a kind of composite yarn, and it is manufactured using an innovative technology for weaving a grid. The selection of core yarns and covering materials is key to manufacturing qualified geogrids. The characteristics of the core yarn, including strength, elongation, Young's modulus, creep, thermal shrinkage and others, should be considered.

High-tenacity polyester filaments are generally used in geogrids. In some particular applications, high-tenacity vinylon and glass fibers are used. The use of a geogrid depends on the characteristics of the covering materials, including ultraviolet resistance, base resistance, acid resistance, bio-degradation resistance, toxicity, treatment temperature, adherence, strength and others. Polyvinyl chloride and other such as latex, acrylic and bitumen have been applied in civil engineering [2, 3]. Polyethylene terephalate (PET) is a thermoplastic material commonly used in the form of fibers and films, whose stability in environmental applications is of substantial interest [4]. PET must be protected to prevent its degradation by the environment. This work used polypropylene (PP) to wrap the PET and thus protect core yarn.

The friction as they apply to elastically or visco-elastically deformed materials, such as fibrous or polymeric materials, depend on the normal stress, the number of junctions in the region of contact or surface roughness, the shape of these junctions and their deformational behavior [6]. The polypropylene/polyester composite-covered yarn was spun using a rotor twister and the geogrids were weaved with the spun yarn to improve the performance of the geogrids. Numerous woven geogrids were tested and discussed to investigate the effects of process parameters on their mechanical properties and evaluate the optimal conditions.

Experimental

MATERIALS

In this work, 1000 D/120f polypropylene filaments and 2000 D/384f high-tenacity polyester filaments were used as covering and core yarn, respectively. The breaking tenacity and elongation of the polyester filaments were 8.0 g/d and 14.0 %; the maximum breaking tenacity and elongation of the polypropylene filaments were 3.0 g/d and 63.6 %, respectively.

The geogrids were woven with spun yarns using a rapier loom, and their mechanical characteristics were investigated.

METHODS

Manufacturing of the composite covered yarn

As presented in Fig. 1, the high-tenacity polyester filaments were put under tension with a tensioner and were wrapped in polypropylene filaments, installed on a rotor to spin the composite covered yarn. A tension meter (SHIMPO Limit 1000 gf) was used to measure the tension of the polyester filaments during twisting. In this experiment, the tension of the polyester filaments and the take-up speed were held constant; composite covered yarns were spun by changing the rotational rate of rotor.

The rate of rotation of the rotor was set to 6000, 8000, 10000, 12000, 14000, 16000 and 18000 rpm. The speed of take-up was fixed at 30 m/min and the polyester filaments were under a tension of 8 N.

Method of the geogrids

As indicated in Fig. 2, the wrapped yarns passed through the yarn collector, the tension device, the backrest, the heald, the reed and the breast beam. Then, they were fixed on the take-up roller. The width of the geogrid was adjusted using the yarn collector.

The texture of the geogrids was 2 by 2 basket weave with 25 25 mm^sup 2^ unit mesh size. The tension of the covering yarn and the twist multiple of the polypropylene/polyester composite covered yarn were set to 3 N and 8.3.

The geogrids were treated at various temperatures (180 C, 185 C, 190 C, 195 C and 200 C) for various periods (3.0 minutes, 3.5 minutes, 4.0 minutes, 4.5 minutes and 5.0 minutes).

TEST

The maximum breaking tenacities and elongations of the filaments and yarns were examined using a tensile tester (Textechno STATIMAT). The gauge between the clips was set to 250 mm and the tension was set to 300 mm/min. The test data were averaged over 20 samples.

The rib and junction strength were tested using the GRI-GG1 and GRI-GG2 testing methods. The tension rate was fixed at 50 mm/min. Data concerning each sample were collected ten times [2, 5].

Results and Discussion

EFFECTS OF TWIST MULTIPLE ON THE STRENGTH AND ELONGATION OF THE COMPOSITE COVERED YARN

In this test, when the take-up speed and the tension were fixed, increasing the rate of rotation of the rotor increased the count of composite covered yarns. When the rotational speed of the rotor exceeded 10,000 rpm (twist multiple = 8.3), the polypropylene filaments completely covered the polyester filaments. Figures 3(a) and 3(b) present a cross-sectional photograph of the composite covered yarn, respectively.

Figures 4 and 5 depict the effect of the twist multiple on the strength and elongation of the composite covered yarn. The figures reveal that the strength of the composite covered yarn declined and elongation increased as the twist multiple was increased. In this work, the take-up speed and tension were fixed, so the twist multiple was increased by increasing the rate of rotation of the rotor. Increasing the rotational speed of the rotor increased the centrifugal force of the covered yarn, causing the core yarn to diverge form the center of the rotor and thus become twisted. The fibers were not parallel to the axle of the twisting yarn, so the strength of the core-yarn fell as cos^sup 2^?, and the elongation increased. The strength of the covered yarn depended mainly on the tenacity of core yarn. The data imply that the strengths of the covered yarns all exceeded those of the core yarn since the fibers in the core yarn were bound more tightly when covered with the covering yarn and so were more effective against tensional stress.

PROPORTION OF COVERING MATERIAL IN COMPOSITE YARN AND THE DIAMETER OF THE COVERED YARN

Finer covered yarn and more covering yarn corresponded to a greater increase in the covering count with the rate of rotation of the rotor, at a constant take-up speed. As stated in Table 1, the ratio of covering yarn to core yarn affected the performance of the reinforced geogrid. Lower covering yarn content led to a lower manufacturing cost. The covered yarn with an 8.3 twist multiple and a 30/70 covering/core ratio was lighter and cheaper than that typically used, which consists of PP staple and PET filament with a 30/70 covering/core ratio. The geogrids were woven using a PP/PET covered yarn with an 8.3 twist multiple, and the physical and chemical properties were investigated.

EFFECTS OF HEATING TEMPERATURE ON LONGITUDINAL STRENGTH, TRANSVERSE STRENGTH AND LONGITUDINAL ELONGATION

Figures 6 and 7 present the effect of heating time on longitudinal and transverse tension resistance and on the longitudinal elongation of geogrids. The results imply that the heating temperature did not influence the tension resistance. However, the resistance increased with the heating temperate because heat shrinking occurred at high temperatures.

EFFECT OF HEATING TEMPERATURE ON THE JUNCTION STRENGTH OF GEOGRIDS

Figure 8 plots the effect of heating temperature on the junction strength of geogrids. The maximum junction strength was at 190C because the PP fibers were melted and most desirable adhesion occurred at this temperature. The geogrids, which were heated at under 190C, exhibited lower strength because they had poorer adhesion. Figures 9 and 10 present the appearance of the geogrid heated to 190C and 180C. Experiments indicated that the optimal heating temperatures of the geogrids were 20C higher than the melting point of the covering yarn. In this experiment, the melting point of the covering yarn was 170C and the optimal heating temperature was 190C, consistent with empirical results. When the heating temperature exceeded 190C, the strength of the geogrids fell, perhaps because the melted PP lost the characteristics of the filaments and resulted in poorer strength.

EFFECTS OF HEATING TIME ON LONGITUDINAL STRENGTH, TRANSVERSE STRENGTH AND LONGITUDINAL ELONGATION

Figures 11 and 12 depict the effects of heating time on, longitudinal and transverse strengths, and longitudinal elongation. The results demonstrate that the heating time did not affect the longitudinal or transverse strength. However, the heating time affected the elongation since shrinkage occurred during heat-treatment.

EFFECT OF HEATING TIME ON THE JUNCTION STRENGTH OF GEOGRIDS

Figure 13 plots the effect of heating time on the interlace strength of geogrids at 190C, which strength was maximum when the heating time was 4 minutes, suggesting that the PP was melted. The junction strength of the geogrids declined as the heating time exceeded 4 minutes, due to the loss of the fiber characteristics.

Conclusion

In this investigation, PP/PET composite covered yarn was spun. Geogrids must exhibit high strength and low elongation, so PET and PP were selected as the materials herein, because of the economic and mechanical considerations. The PP/PET composite covered yarn produced by the rotor twister had several advantages, as shown in the following. Firstly, the covered yarn was of desirable strength. Secondly, the 30/70 (PP/PET) covered yarns were relatively cheap to manufacture and produced lighter geogrids, which could be more conveniently transported and constructed. Finally, unlike polyvinyl chloride, PP is not toxic and so does not pollute the environment. The findings reveal that a twist multiple of 8.3 and a heating time of 4 minutes were optimal for spinning composite covered yarn. The longitudinal and transverse tension resistances of this composite covered yarn were 46.4 kN/m and 23.2 kN/m; the junction strength reached 6.2 kN/m, and the longitudinal elongation was 11.4%. The experimental results indicate that the geogrids with the covered yarns under the specified conditions were more effective than the usual woven geogrids because they had a lighter unity weight.

ACKNOWLEDGEMENT

The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract NSC93-2212-E-035-024.