Relaxation behavior of polyethylene welded joints
© The Author(s). 2017
Received: 29 December 2016
Accepted: 7 April 2017
Published: 18 April 2017
The paper presents results of the investigation of structure relaxation and thermal properties of PE-80 and PE-100 polyethylene hot-tool butt welds. It was found that a weld with the re-crystallized crystalline structure is formed during the welding of dissimilar types of polyethylene. It is shown that within a long period (1 year) the relaxation occurs not only in amorphous but also in the crystalline phase (crystalline α-form transforms into mixed αβ-form), with respective changes in polyethylene properties.
Progress in the modern material science has caused wide expansion of thermoplastics application in various industries: chemical, construction, medical, radio-technical, electronic, foodstuff etc. [1–6].
Welding process for thermoplastic polymers happens with activation of welded surfaces either before bringing these surfaces in contact (hot tool, hot gas, or IR-radiation welding) or with activation of surfaces simultaneous with bringing surfaces in contact (friction or ultrasonic welding) .
Along the cooling of joint, the super-molecular structure is formed in the weld; such welds have the respective stress fields which are relaxing with time . These competing processes determine the final properties of the welded joints. The technological goal is to receive the joint with the properties as close as possible to the properties of the parent raw material.
Various physical and chemical transformations occur in the joint—melt fluidity is changing, orientation crystallization, re-crystallization, and even partial destruction occur, and as a result we receive heterogeneous structure of the welded joint .
Thereby, the goal of this work is a complex investigation of structure and property relaxation behavior in welded joints of dissimilar types of polyethylene, using the methods of wide- and small-angle X-ray scattering, differential scanning calorimetry, and thermal analysis.
Materials and processing
The following two types of high-density polyethylene (HDPE) specimens with different long-term minimum required strength (MRS, within 50 years at 20 °C) have been used for welding experiments and further structural, thermal, and field performance data investigations: PE-80 (MWbimodal 300,000 g/mol, density 0.953 g/cm3, MRS 8 MPa) and PE-100 (MWbimodal 300,000 g/mol, density 0.960 g/cm3, MRS 10 MPa).
Equipment and measurements
The heterogeneous structuring of these polymeric systems (at the nanometer level) was studied via small-angle X-ray scattering (SAXS) with a CRM-1 camera (Orel scientific equipment factory, Russia), having a slit collimator of the primary irradiation beam made via the Kratky method. The geometric parameters of the camera satisfied the condition of infinite height of the primary beam .
All X-ray structural investigations have been carried out using CuК α-emission monochromated by using Ni-filter, at temperature T = 20 ± 2 °C.
Thermal properties of the welds have been explored by means of differential scanning calorimetry (DSC) with TA Instruments DSC Q2000 (USA) in the inert nitrogen atmosphere under temperatures from 40 to 200 °C with linear heating rate 20 °C/min.
Thermal stability and thermal-oxidative degradation of welds have been investigated using TA Instruments TGA Q50 (USA) in dry air environment at temperatures from 30 to 700 °C with linear heating rate 20 °C/min. Thermomechanical behavior and deformation characteristics of welds have been investigated using TA Instruments TMA Q400 EM (USA) in dry air environment at 5 °C temperature modulation regime with heating rate 10 °C/min at temperatures from 30 to 250 °C. Measurements have been carried out in thermal expansion mode. Cylinder-shaped indenter with 2.8 ± 0.01 mm diameter has been used with loading on the specimen (10–1 MPa).
All the devices from TA Instruments have been certified according to the international standard ISO 9001:2000.
All investigations were repeated two times with different specimens for each time to enhance an accuracy of the measurements.
Results and discussion
Previously, it was reported  that WAXS diffraction analysis of PE-80, PE-100, and PE-80/PE-100 welds showed that all of them have amorphous-crystalline structure (Fig. 3), and the welding of dissimilar types of polyethylene gives a joint with texturized crystalline phase. It was explained by melting of crystallites with their further re-crystallization and simultaneous orientation. In Fig. 3 (for further comparison), this is indicated with the growth of diffraction maximum (200) whose angular position (2θ m ) on diffraction pattern is equal to 23.6°, and reduction of maximum (110) with angular position is 21.2°. X-ray diffraction patterns of these specimens are presented on Fig. 3 . Diffraction maxima at 2θ max = 21.2 i 23.6° have been used for the calculation.
Structural characteristics of PE-80 and PE-100 polyethylene and of PE-80/PE-100 welded joint
Crystallinity level (DSC), %
Crystallinity level (WAXS), %
Crystallites size L (2θ max = 21.2°), nm
Crystallites size L (2θ max = 23.6°), nm
PE-100 welded joint
PE-100 welded joint (randomized)
PE-100 welded joint
PE-100 welded joint (randomized)
where I 1 and I 2—the intensity of WAXS scattering for PE-80 and PE-100 specimens; w 1 and w 2—weight parts of the components in the system (w 1 + w 2 = 1). From the comparison of experimental and calculated X-ray diffraction patterns of welds, a nonadditive change in the experimental diffraction curve is observed comparing with the calculated one (Fig. 3). This result is important since it confirms the interaction between macromolecules of both types of polyethylene in PE-80/PE-100 welded joint.
where ΔH°m—enthalpy of melting for completely crystallized polymer (for polyethylene ΔH°m = 286.7 J/g); ΔHm—enthalpy of melting of polymer, experimentally obtained from DSC curve.
This evaluation has shown that for both types of polyethylene as well as for the randomized specimen of welded joint the crystallinity level is practically the same (Table 1).
Detected peculiarities of PE-80 and PE-100 types of polyethylene and of PE-80/PE-100 welded joint are of interest for investigation of their structure heterogeneity.
for the welded joint is equal to approximately 27 nm.
where φ 1, φ 2—volume parts of micro-areas (φ 1 + φ 2 = 1). It was detected that l p value in the PE-80/PE-100 welded joint (l p = 20 nm) is bigger comparing with pure PE-80 (l p = 16 nm) and PE-100 (l p = 18 nm) specimens. In addition, it was found that heterogeneous structure of PE-80/PE-100 weld has not changed after 1 year (Fig. 5).
Thermal characteristics of the PE-80/PE-100 welded joint
T d , °C
T d max , °C
T m , °C
T m max. , °C
PE-80/PE-100 welded joint
1 year PE-80/
PE-100 welded joint
Thermomechanical behavior of the initial and aged welded joints is similar. The deformation of specimens is a result of polymer melting and the features of melting (described by deformation curves) reflect the changes of crystalline phase occured during welding and further aging. As it is seen from Fig. 7, the onset temperature (T m ) intensive deformation, as well as the maximum (T m max. ) of intensive deformation of PE-80/PE-100 weld decrease with time (Table 2). T m and T m max are parameters of the melting process of polymer at heating during TMA experiment, the specimen deformation occurs with melting of polymer. The differences in values of T m and T m max exists due to relaxation in time of polymer in the textured zone of the welded joint, and, probably, to transfer of its structure to the equilibrium state.
The paper presents results of the investigation of structure relaxation and thermal properties of PE-80 and PE-100 polyethylene hot-tool butt welds. It was found that during the welding of dissimilar types of polyethylene a process of re-crystallization of crystalline phase of the weld structure is observed. It is shown that within a long period (one-year) not only amorphous but also the crystalline phase relaxes (crystalline α-form transforms into mixed αβ-form). These features become apparent in relaxation of thermal and thermomechanical properties as well. Decreasing of the oxidative degradation onset and temperature maximum with time, as well as intensive deformation onset conditioned by polymer melting are revealed.
Submitted results were obtained during the fulfillment of the budget supported project in the Plastics Welding Department of the E.O.Paton Electric Welding Institute of the NAS of Ukraine.
VD and MI wrote the manuscript. VD fulfilled the WAXS and the SAXS investigations and analysis of the WAXS and the SAXS data. MI and AS provided valuable discussions and contributed in results analysis comparing to the structural peculiarities of the samples. AG performed butt fusion welding of PE pipes. All authors read and approved the final manuscript.
VD is PhD in Polymer Physics, staff researcher at the Plastics Welding Department of the E.O.Paton Electric Welding Institute of the NAS of Ukraine and at the Institute of Macromolecular Chemistry of the NAS of Ukraine. MI is Doctor in Physics, Head of Department, Plastics Welding Department of the E.O.Paton Electric Welding Institute of the NAS of Ukraine, and senior staff scientist at the Institute of Macromolecular Chemistry of the NAS of Ukraine. AS is PhD in Technical Sciences, staff researcher at the Plastics Welding Department of the E.O.Paton Electric Welding Institute of the NAS of Ukraine. AG is the leading engineer at the Plastics Welding Department of the E.O.Paton Electric Welding Institute of the NAS of Ukraine.
The authors declare that they have no competing interests.
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