Rheological weldability

Rheological weldability ( PC ) is proposed as a concept for determining the reliability of the weldability of products from thermoplastics . From a practical point of view, PC of thermoplastic objects is estimated on the basis of their rheological properties: viscosity ( η ) and activation energy ( Е _А ). According to this criterion , the lower the viscosity ( η ) during the welding process and the smaller the absolute value of the activation energy (| Е _А |), the better the weldability . ^[1] ^[2]

According to the rheological concept, the formation of a welded joint includes two stages at the macro and micro levels. As the activated surfaces of the joined parts approach under pressure due to shear deformations, the polymer melt flows and the ingredients that prevent the approach and interaction of the macromolecules are removed from the contact zone. Due to the difference in melt flow rates, the macrovolumes of the melt are mixed in the contact zone.

After removal of the defective layers in the contact zone under the action of van der Waals forces of intermolecular and interatomic interaction, an interaction occurs between the macromolecules of the layers of the parts to be joined. This process occurs at the micro level. Interdiffusion of macromolecules is due to the energy potential and the uneven temperature gradient in the area of the welded surfaces.

Content

Viscosity

The smaller the η , the better the PC

Wetting is characterized by the degree of interfacial contact and is determined quantitatively through the contact angle ( θ _s ) for a liquid on the surface of a solid that is in equilibrium , as shown in FIG. 1. The relationship between the contact angle and surface friction in the equilibrium condition is determined by the equation: ^[3]

\gamma _{SG}\ =\gamma _{SL}+\gamma _{LG}\cos {\theta _{c}},

{\ displaystyle \ gamma _ {SG} \ = \ gamma _ {SL} + \ gamma _ {LG} \ cos {\ theta _ {c}},}

$\gamma _{SG}$ ${\ displaystyle \ gamma _ {SG}}$ - surface tension is solid - gaseous,
$\gamma _{SL}$ ${\ displaystyle \ gamma _ {SL}}$ - the surface tension of a solid-liquid
$\gamma _{LG}$ ${\ displaystyle \ gamma _ {LG}}$ - liquid-gas surface tension,
$\theta _{c}$ ${\ displaystyle \ theta _ {c}}$ - contact angle.

Ideally, for good wetting , the contact angle ( θ _s ) in equilibrium should have a minimum. However, this is permissible only in equilibrium conditions, and the equilibrium depends on the ratio between the wetting force and the viscosity of the liquid. For polymer melts, the viscosity can be very high. It will take a long time to reach an equilibrium wetting angle (dynamic contact angle).

To assess weldability, it is necessary to consider the melt viscosity of thermoplastics ( polymer melts), since welding is a fast process. The lower the viscosity during the welding process, the better the weldability .

Since viscosity ( η ) decreases with increasing temperature ( T ) and shear rate for most polymer melts, weldability is better where temperature and shear rate (movement) are higher in the cross section of the welding area. ^[1] ^[2]

| E _{, A} | the values of polymers such as PVC decreases with increasing shear rate, meaning better weldability , where the shear rate (movement) is higher, throughout the entire cross section of the welding area. ^[1] ^[2]

Viscosity at different polymer temperatures ( Е _A ) can be calculated using the Arrhenius equation : ^[4] ^[5] ^[6]

\eta =C\exp \left({\frac {-E_{a}}{RT}}\right),

{\ displaystyle \ eta = C \ exp \ left ({\ frac {-E_ {a}} {RT}} \ right),}

Where

η is the viscosity of the molten polymer,
C - frequency factor
R is the universal gas constant ,
T - temperature ( Kelvin ).

Note

² ¹ ² ³ O.Balkan, H.Demirer, A.Ezdesir, H.Yildirim. Butt welded PE, PP, and PVC sheets (English) // Polymer Engineering and Science: journal. - 2008. - Vol. 48 . - P. 732 . - ISSN 1548-2634 . - DOI : 10.1002 / pen.21014 .
↑ ¹ ² ³ O.Balkan, A.Ezdesir. Rheological Weldability of Polymers. - 12. International Materials Symposium (12.IMSP) Denizli, October 15–17, 2008. - P. 1046.
↑ Young, T. An Essay on the Cohesion of Fluids (Eng.) // Phil. Trans. R. Soc. Lond. : journal. - 1805. - Vol. 95 - P. 65-87 . - DOI : 10.1098 / rstl.1805.0005 .
↑ Arrhenius SA Chem. : magazin. - 1889. - Bd. 4 - S. 96-116 .
↑ Arrhenius, SA Über die Reaktionsgeschwindigkeit bei der Inversion von Rohrzucker durch Säuren (him) // ibid. : magazin. - 1889. - Bd. 4 - S. 226-248 .
↑ Laidler, KJ (1987) Chemical Kinetics , Third Edition, Harper & Row, p.42

Welding

The mechanism of formation of welded joints

[PES2008-1] ² ¹ ² ³ O.Balkan, H.Demirer, A.Ezdesir, H.Yildirim. Butt welded PE, PP, and PVC sheets (English) // Polymer Engineering and Science: journal. - 2008. - Vol. 48 . - P. 732 . - ISSN 1548-2634 . - DOI : 10.1002 / pen.21014 .

[IMSP2008-2] ¹ ² ³ O.Balkan, A.Ezdesir. Rheological Weldability of Polymers. - 12. International Materials Symposium (12.IMSP) Denizli, October 15–17, 2008. - P. 1046.

[3] Young, T. An Essay on the Cohesion of Fluids (Eng.) // Phil. Trans. R. Soc. Lond. : journal. - 1805. - Vol. 95 - P. 65-87 . - DOI : 10.1098 / rstl.1805.0005 .

[4] Arrhenius SA Chem. : magazin. - 1889. - Bd. 4 - S. 96-116 .

[5] Arrhenius, SA Über die Reaktionsgeschwindigkeit bei der Inversion von Rohrzucker durch Säuren (him) // ibid. : magazin. - 1889. - Bd. 4 - S. 226-248 .

[6] Laidler, KJ (1987) Chemical Kinetics , Third Edition, Harper & Row, p.42