Polyethylene glycol (PEG)

Properties keywords:  thermoplastic, Newtonian liquid, water-soluble

Analogue keywords: lava flows, lava domes, lava crust formation

Common names: Polyethylene glycol (PEG), Polyethylene oxide (PEO), Polyoxyethylene (POE), Carbowax

General Information:  Polyethylene glycol is a completely water-soluble, thermoplastic crystalline polymer.  The amount of monomers in the polymeric chain determines the molecular weight, which in turn determines viscosity.  PEG is a colourless viscous liquid when molecular weight is below 1000 and an opaque wax-like solid when over 1000 (Smythe et al., 1950; Harris, 1992).

PEG is non-toxic and readily available as it is commonly used in the medical industry and as adhesives, food packaging, lubricants and ceramic glazes. 

Properties

PEG 600, the most frequently used material in analogue studies of volcanological processes (Fink and Griffiths, 1992; Blake and Bruno, 2000; Griffiths et al, 2003; Cashman et al., 2006), solidifies at just below room temperature (freezing point 15-25°C, Griffiths et al., 2000; Cashman et al., 2006; Dow Chemical Company, 2011).

In its liquid form it is Newtonian (Fink and Griffiths, 1998), with a viscosity of 150 mPas at 20°C (Dow Chemical Company, 2011).  It is completely soluble in water and many organic solvents at room temperature e.g. ethanol and acetone but not ethyl ether, hexane or ethylene glycol (Harris, 1992).  Solution viscosity decreases as the ratio of PEG to solvent decreases.  As it cools PEG forms a solidified surface crust (Manga and Ventura, 2005).

In solid form, the viscosity, tensile strength, shear strength and Young’s modulus of PEG are all dependent on temperature (e.g. Figure 1, Bailey and Koleske, 1976), where all properties show a systematic decrease with increasing temperature (Soule and Cashman, 2000).  The viscosity and tensile strength are also dependent on strain-rate (Soule and Cashman, 2000).  The shear strength of PEG 600 at solidification temperature Ts is approximately 0.3 MPa.  The tensile strength is zero at Ts but, like viscosity, can be described by a power-law model.  For example, the tensile strength at 15°C and a strain-rate of 0.01 s-1 is 0.38 MPa (Soule and Cashman, 2000).

Figure1. a) Viscosity of PEG 600 against dimensionless Tc, where 0 is the solidification temperature and b) tensile strength against Tc, both from Soule and Cashman (2000).

Solid PEG can deform in a brittle or a ductile manner depending on temperature and strain-rate (Fink and Griffiths, 1992; Gregg and Fink, 2000; Soule and Cashman, 2000).  The transition between the two types of behaviour occurs at Tc (1-[T/Ts]) of approximately 0.5 and a strain-rate of ~ 0.05 s-1.  At this transition, PEG has a relaxation time of approximately 500 s (Soule and Cashman, 2000).

The density of PEG 600 at 20°C is 1130 kg m-3 and it has a flash point of 238-274°C (Dow Chemical Company, 2011).  Being thermoplastic, PEG can be shaped and re-used many times, making it ideal for experimentation.

Applications

PEG is a useful analogue for modelling cooling lava flows, due to its temperature dependent viscosity and formation of a solid crust, which will deform in a brittle or ductile manner depending on temperature and strain-rate.

Typically PEG is used to study the effect of a solidifying surface crust on flow advance, surface morphology, spreading behaviour, channel and tube formation and cooling of lava flows (e.g. Hallworth et al., 1987; Fink and Griffiths, 1990; 1992; 1998; Cashman et al., 2006; Fink and Gregg, 2012).  It has also been used as an analogue for compound lava flows, where crust formation governs the timing and location of lava breakouts (Blake and Bruno, 2000).

Experiments by Hallworth et al (1987) showed that it is possible to better replicate heat flow and surface tension characteristics of basaltic lava flows, resulting in more realistic lava morphologies, if PEG is injected into water rather than air (also Fink and Griffiths, 1990; 1992; Blake and Bruno, 2000; Griffiths et al., 2003).  Furthermore, Fink and Griffiths (1990, and later experiments) injected PEG into a solution of water and sucrose in order to minimise the density contrast and increase cooling rate.  This has also been used to simulate submarine eruptions (Fink and Gregg, 2012).

PEG has also been used as an analogue for silicic lava flows and domes (Griffiths and Fink, 1993; 1997; Fink and Griffiths, 1998).  An experiment by Weinberg and Leitch (1998) used PEG as a mafic magma analogue, into which they injected a colder, less dense, more viscous material to simulate magma mingling and mixing.

Limitations and tips for use

The major limitation of PEG is its lack of yield strength above solidification temperature.  This affects its usefulness as a silicic lava analogue, as the molten material cannot sustain large shear stresses (Fink and Griffiths 1998).  This also leads to dome models not replicating natural fracture processes.  It has been suggested that the addition of suspended kaolin powder to PEG can overcome these issues (Fink and Griffiths 1998; Lyman et al., 2005).

Furthermore, PEG, being Newtonian in its liquid state, does not account for the non-Newtonian behaviour expected for many lava flows, especially those with high crystal and bubble contents (Bagdassarov and Pinkerton, 1994).

Some issues with scaling exist, namely that the strength of a PEG solid crust does not scale directly to that of a natural lava (Fink and Griffiths, 1990), meaning it can be difficult to reproduce some textural features of lava flows in the laboratory (Gregg and Fink, 2000).

It should be noted that PEG is insoluble in water at high temperatures and that strength of solid PEG can vary significantly between batches.

Polyethylene glycol is non-toxic (FDA approved for internal consumption), however, the use of eye protection, gloves and protective clothing is advised.

References

Bagdassarov  N and Pinkerton H (2004) Transient phenomena in vesicular lava flows based on laboratory experiments with analogue materials. Journal of Volcanology and Geothermal Research 132: 115-136

Bailey FE and Koleske JV (1976) Poly(ethylene) Oxide. New York: Academic Press

Blake S and Bruno BC (2000) Modelling the emplacement of compound lava flows. Earth and Planetary Science Letters 184: 181-197

Cashman KV, Kerr RC, and Griffiths RW (2006) A laboratory model of surface crust formation and disruption on lava flows through non-uniform channels. Bulletin of Volcanology 68: 7-8, 753-770

Dow Chemical Company technical data sheet CARBOWAXTM Polyethylene Glycol (PEG) 600. 2011. Available from: http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0887/0901b80380887904.pdf?filepath=polyglycols/pdfs/noreg/118-01800.pdf&fromPage=GetDoc

Fink JH and Gregg TKP (2012) Quantification of submarine lava-flow morphology through analog experiments. Geology 1995: 23, 73-76

Fink JH and Griffiths RW (1990) Radial spreading of viscous-gravity currents with solidifying crust. Journal of Fluid Mechanics 221(1): 485-509.

Fink JH and Griffiths RW (1992) A laboratory analog study of the surface morphology of lava flows extruded from point and line sources. Journal of Volcanology and Geothermal Research 54(1): 19-32

Fink JH and Griffiths RW (1998) Morphology, eruption rates, and rheology of lava domes: Insight from laboratory models. Journal of Geophysical Research 103(B1): 527-545

Gregg TKP and Fink JH (2000) A laboratory investigation into the effects of slope on lava flow morphology. Journal of Volcanology and Geothermal Research 96(1): 145-159

Griffiths RW and Fink JH (1993) Effects of surface cooling on the spreading of lava flows and domes. Journal of Fluid Mechanics 252: 667-702

Griffiths RW and Fink JH (1997) Solidifying bingham extrusions; a model for the growth of silicic lava domes. Journal of Fluid Mechanics 347: 13-36

Griffiths RW, Kerr RC, and Cashman KV (2003) Patterns of solidification in channel flows with surface cooling. Journal of Fluid Mechanics 496: 33-62

Hallworth MA, Huppert HE, and Sparks RSJ (1987) A laboratory simulation of basaltic lava flows. Modern Geology 11: 93–107

Harris JM (1992). Poly(ethylene Glycol) Chemistry: Biotechnical and Biomedical applications. New York. Plenum Press. 2-3

Lyman AW, Kerr RC, and Griffiths RW (2005) Effects of internal rheology and surface cooling on the emplacement of lava flows. Journal of Geophysical Research 110(1): 1-16

Smyth HF, Carpenter CP, and Weil CS (1950) The toxicology of the Polyethylene Glycols. Journal of American Pharmaceutical Association 39(6): 349-354

Soule SA and Cashman KV (2004) The mechanical properties of solidified polyethylene glycol 600, an analog for lava crust. Journal of Volcanology and Geothermal Research 129(1): 139-153

Weinberg RF and Leitch AM (1998) Mingling in mafic magma chambers replenished by light felsic inputs: fluid dynamical experiments. Earth and Planetary Science Letters 157: 41-56