Study On Structure And Properties Of Pipes Formed By HDPE Induced Systems With HMWPE Under Complex Stress Field

HDPE pipes are widely used sectional materials. Conventional pipes, however, have lowcircumferential strength, thus are usually uncomfortable to transmit high press fluid. So far, broadresearches on pipe reinforcing technology have been conducted by scholars at home or abroad.However, the research that relatively low molecular weight HDPE with small amount ofrelatively high molecular HDPE is reinforced by induced crystallization is just at the preliminarystage. Our research group has engaged in this field for a long term, and this paper is completedon basis of former research. Self-reinforced pipes of induced systems with small amount ofHMWPE are extruded from self-made shear-tensile bidirectional combined stress field equipment,and further researches are conducted on their structure and properties.First of all, researches are conducted on structure properties of pipes which areextruded by HDPE pure materials of three different molecular weights under thesame temperature and different circumferential shear rotational speed. The resultsshow that under the complex stress field, tensile strength both in the circumferentialand axial direction are significantly enhanced, with more significant enhancement inthe circumferential direction. Compared to regularly extruded pipes, circumferentialtensile strength and axial tensile strength of HDPE (2480) have the maximumincrease of 24.1% and 12.7 respectively. Internal shape and structure of samples are characterized by SEM, WAXD, DSCand etc.; it is found that under complex stress field, internal structure of pipes is stillspherocrystal, but with fined grains, increased crystallinity and crystal orientationwhich is at a certain angle from the circumferential direction.This experiment focuses on the research that pipes of induced systems(relatively low molecular weight HDPE with small amount of HMWPE) are extrudedthrough complex stress field. It is found that compared to pipes formed with foresaidpure materials under the same conditions, those prepared with inducted systemsthrough complex stress field have bigger increase of bidirectional self-reinforcement:Herein, compared to pure material system extruded through complex stress field,induced system (HDPE 5421B: HDPE 2480=2:98) has the increase ofcircumferential and axial tensile strength by 32% and 2.3% respectively;Compared to pure material system extruded through complex stress field,induced system (HDPE 5421B: HDPE 2480=6:94) has the increase ofcircumferential and axial tensile strength by 49% and 6.9% respectively;Compared to pure material system extruded through complex stress field,induced system (HDPE 5421B: HDPE 2480=10:90) has the increase ofcircumferential and axial tensile strength by 35% and 6.4% respectively;Internal shape and structure of pipes are characterized by SEM, WAXD andDSC. It is validated that the reinforcing mechanism belongs to shear inducedcrystallization reinforcement.In this experiment, induced system with HMWPE content of 6% has best effectof induced crystallization. With too few of HMWPE, line cores are insufficient,which can't lead to sound induced crystallization; while with too many HMWPE,they can be self crystallized or their molecular chains can twist together, rather thanplay a role of crystallographic axis promoting crystallization of HDPE.Every induced system has an optimal shear casing rotational speed (generally20r/min in this experiment). With low rotational speed, it can cause low shear forcefield for circumferential orientation and result in unnoticeable orientation; with theincrease of shear casing rotational speed, shearing thermogenesis increases, which exacerbates disorientation and leads to decomposition of oriented crystallizationstructure.Every induced system has an optimal shearing stress field temperature toobtain optimal mechanical properties. At too low temperature, epitaxial lamellamolecules can't obtain sufficient active capacity to form sound interlocking structure,which can also result in hard extrusion, poor product appearance and obvious weldlines. At too high temperature, deorientation is aggravated, which can result in thedecomposition of oriented crystalline structure.Different cooling methods can result in the difference of mechanical strength ofpipes, i.e. pipes obtained through natural cooling by air have better mechanicalstrength than those through emergency cooling by water.Therefore, research on this subject has active significance in improvingproperties of HDPE pipes and discovering the relationship of mechanical propertiesand microscopic structure and the relationship of processing parameters andperformance when HDPE pipes of induced systems extruded through complex stressfield.