Measuring surface tension of polymer melts

The surface (interfacial) tension of polymer melts is an important thermodynamic parameter that plays a key role in many technological processes such as wetting, coating, polymer blending, and the reinforcement of polymers with fibers. In our lab, we use an in-house development, based on a modified Wilhelmy technique to determine the surface tension of polymer melts at elevated temperatures.

Device for modified Wilhelmy - measurements

The modified Wilhelmy balance technique

The modified Wilhelmy balance technique is a versatile technique to measure indirectly either the surface (wetting) tension of polymer melts or the contact angle at the solid-polymer melt interface. We use thin fibers (diameter < 100 µm) as solid probes. The advantage of this arrangement is that the density of the polymer melt is not needed to calculate its surface tension.

Our instruments consist of supersensitive microbalances (accuracy: ± 2µg) and a motor driven movable table, on which a high-temperature cell containing the polymer sample is placed. The experiments can be performed under inert gas atmosphere and at elevated temperatures.

Scheme of the experimental set-up

Technical Data:
Diameter of fibers: 8 to100 µm
Net weight of solid polymer per gaging: ca. 50 mg
Measurement is carried out in an argon inert gas flow
Max. temperature: ca. 300°C
Control software: Custom-made TestPoint™ application

In the Wilhelmy experiment, a thin fiber is immersed into the polymer melt. The measured quantity is the force F per unit length of the fiber perimeter p. It is calculated from the increase in mass m due to the wetting of the fiber by the polymer melt which is recorded by an electrobalance. The force per unit length equals the wetting tension γlvcosθ.

Wetting of the fiber by a polymer melt

However, there are several difficulties that should be noted as the measured force on the electrobalance is the sum of gravitational, interfacial and buoyancy forces. Whereas the buoyancy force can be neglected using thin fibers and therefore also the knowledge of the melt density at elevated temperatures, the main difficulty in the experiment is the high viscosity of the polymer melt. Due to the forced fiber/ fluid motion, non-negligible force contributions result from the shear stress exerted by the viscous flow of the liquid on the fiber. To exclude these hydrodynamic effects in the Wilhelmy experiment, the fiber is held stationary at a constant immersion depth until viscous relaxation occurs.

Typical curve for a Wilhelmy experiment (acrylic resin melt that contains a leveling additve)

Compared to drop shape methods, a drawback of the Wilhelmy technique is that the surface tension is not measured directly. Since the measured quantity equals the wetting tension γlvcosθ complete wetting of the fiber by the polymer melt (contact angle =0°) is required to obtain the surface tension.

However, there are several applications, where the interaction between fibers or, generally, solid surfaces and polymer melts are of great technological interest. Examples are the interactions of powder coating melts with metal surfaces or the interactions between reinforcing fibers and polymer melts during the processing of fiber-reinforced polymer composites. In both cases, we used the modified Wilhelmy balance technique as a model system to study these interactions at elevated temperatures. Depending on the type of interaction at the solid/ liquid interface -physical or chemical interactions- we have found different wetting kinetics for unmodified and chemically modified polypropylene melts on differently sized glass fibers. Thus, the modified Wilhelmy balance technique seems to be a good way to simulate the complex wetting process under processing conditions. We assume that these measurements permit a more fundamental and quantitative understanding of the mechanism of interface formation in fiber-reinforced polymer composites.

Relevant publications

  • Grundke, K. ; Azizi, M. ; Ziemer, A. ; Michel, S. ; Pleul, D.Pleul ; Simon, F. ; Voit, B. ; Kreitschmann, M. ; Kierkus, P. Hyperbranched polyesters as potential additives to control the surface tension of polymers more Surface Coatings International / Part B: Coatings Transactions 88 (2005) 101-106
  • Ziemer, A. ; Azizi, M. ; Pleul, D.Pleul ; Simon, F. ; Michel, S. ; Kreitschmann, M. ; Kierkus, P. ; Voit, B. ; Grundke, K. Influence of Hyperbranched Polyesters on the Surface Tension of Polyols more Langmuir 20 (2004) 8096-8102
  • Synytska, A. ; Michel, S. ; Pleul, D.Pleul ; Bellmann, C. ; Schinner, R. ; Eichhorn, K.-J. ; Grundke, K. ; Neumann, A.W. ; Stamm, M. Monitoring the Surface Tension of Reactive Epoxy-Amine Systems Under Different Environmental Conditions more Journal of Adhesion 80 (2004) 667-683
  • Grundke, K. ; Michel, S. ; Synytska, A. A New Method to Determine Simultaneously Density and Surface Tension of Polymeric Liquids Including Reactive Systems more Adhesion and Interface 4 (2003) 30-36
  • Nygard, P. ; Grundke, K. ; Mäder, E. ; Bellmann, C. Wetting kinetics and adhesion strength between polypropylene melt and glass fibre: influence of chemical reactivity and fibre roughness more Journal of Adhesion Science and Technology 16 (2002) 1781-1808
  • Grundke, K. ; Michel, S. ; Eichhorn, K.-J. ; Beyerlein, D. ; Bayer, Th. Influence of Chemical Interactions on the Macroscopic Spreading of a Maleic Anhydride Copolymer Melt more Macromolecular Chemistry and Physics 203 (2002) 937-946
  • Amirfazli, A. ; Grundke, K. ; Michel, S. ; Neumann, A. W. A new method for simultaneous determination of surface tension and density of polymer melts from shape of a pendent drop more Polymeric Materials: Science and Engineering 82 (2000) 33-34
  • Wulf, M. ; Uhlmann, P. ; Michel, S. ; Grundke, K. How do Levelling Additives Affect Film Formation of Powder Coatings? more Powder R & D Newsletter 2 (2000) 299-307
  • Uhlmann, P. ; Wulf, M. ; Juckel, H. ; Michel, S. ; Grundke, K. 4th International Conference Advances in Coatings Technology ACT 2000, Katowice, 7 - 10 November 2000<br />Film Formation of Powder Coatings - Differences in the Surface Activity of Levelling Additives Correlate with Application Properties more Institute of Plastics and Paint Industry (2000) getr. Zählung, 9 Seiten
  • Wulf, M. ; Uhlmann, P. ; Michel, S. ; Grundke, K. How levelling additives affect the film formation of powder coatings more Modern Paint & Coatings 90 (2000) 16-20
  • Wulf, M. ; Uhlmann, P. ; Michel, S. ; Grundke, K. Surface tension studies of leveling additives in powder coatings more Progress in Organic Coatings 38 (2000) 59-66

M. Wulf, S. Michel, W. Jenschke, P. Uhlmann and K. Grundke
A new method for the simultaneous determination of surface tension and density of polymer melts
Phys. Chem. Chem. Phys. 1 (1999)   3899 - 3903

Wulf M, Michel S, Grundke K, del Rio OI, Kwok DY, Neumann AW
Simultaneous Determination of Surface Tension and Density of Polymer Melts Using Axisymmetric Drop Shape Analysis
Journal of Colloid Interface Science 210 (1999) 172-181

For further information, please contact:

Dr. Günter Auernhammer
Stefan Michel