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Size Does Matter (but so do shape, rigidity and polarity)

December 31, 2010

Commercially important rejuvenation molecules arranged from fasted to slowest diffusionIn my December 29, 2010 post …Wonderer in the Wilderness inquired …

Q:

Isn’t it better to have a 100% reactive fluid? I understand that not all of the Novinium fluid components are water reactive. When all the fluid components are reacting with water, it seems like you should get better cable rejuvenation.

A:

In this post I will provide a more comprehensive answer. We learned from the abbreviated answer that water reactivity per se is a short-lived phenomenon and that the data provided by those who now croak of the importance of water scavenging confirms this is so. The water reactive functionality is gone in about two months for a 1/0 AWG conductor, it would last somewhat longer in bigger cable, but shorter still in a No.2 AWG cable. For decades-long life extension water reactivity over a couple of months has little direct impact. If water is not kept out by chemical reaction, how is it excluded? “Excluded” is such an absolute term and the second law of thermodynamics precludes such an absolute, but we can greatly reduce the quantity of water by preferential solubility. All of the components of all rejuvenation fluids are more soluble in the polymeric insulation than water. There is a finite amount of space between the amorphous polymer chains that constitute cable insulation. If those spaces are preferentially filled with rejuvenation fluid, there is that much less room for water. Furthermore the chemical properties of well chosen rejuvenation components can mitigate water damage even if there is some water present. Water is such a small molecule with such a large diffusion coefficient (See the nearby Figure.) that even if one could temporarily react with all of the water in the cable it would be back in a few short days. For a comprehensive understanding of the molecular thermodynamics of water in cables see …

IEEE-Molecular Thermodynamics of Water in Direct-Buried Power Cables

The real goal of including water reactivity in the formulation is to increase the size of the silane treatment components so that they diffuse slower once they get into the insulation. Slower diffusion means that the rejuvenation component is slower to exude out of the cable into the soil where it obviously provides no benefit. The suggestion of those who croak for 100% water reactivity is that if it doesn’t react, then it will exude.

Nothing could be further from reality. The only way to know how fast a component will migrate and exude from a cable is to make measurements of diffusion and solubility. At Novinium we have made thousands of such measurements.

Does size matter? Sorry boys, but yes it does. In general, the bigger the molecule the slower it diffuses, but size is only one of several parameters. In the Figure nearby I show to scale, two-dimensional representations of the non-catalytic components used in all URD treatment fluids. (I will deal with catalyst in a separate post and with feeder fluids in yet another post.) I have arranged the components from the fastest to diffuse to the slowest to diffuse. At first glance, the bigger the molecule the slower it diffuses, but that isn’t always the case. Included in the Figure alongside each molecular model is a short name for the component, its molecular weight (the absolute mass of a single molecule), and the approximate diffusion coefficient at 55°C (cm2/s). Some of the other factors that affect diffusion include the shape of the molecule, the flexibility/rigidity of the molecular structure, and its dipole moment (or internal charge imbalance). For a molecule to move through the insulation polymer it has to squeeze through very small spaces. Side chains and protrusions such as those designed into many of the molecules we use at Novinium improve longevity versus the more svelte molecules used in the first generation of technology.

Name in Figure Long name Comment
Water water ubiquitous
MeOH methanol hydrolysis by-product
TMMS trimethylmethoxysilane silane in Cablecure® XL fluid
PMDMS phenylmethyldimethoxysilane silane in Cablecure XL fluid and Cablecure iXL [Perfico® 011] fluid
2-ethyl-hexanol 2-ethyl-hexanol fast-to-diffuse component in Cablecure 732/733 [Ultrinium® 732] and Cablecure iXL fluids
GA< geranylacetone voltage stabilizer in Cablecure 732 fluid
TEMDMS tolylethylmethyldimethoxy silane silane in Cablecure 732 fluid
CBMDMS cyanobutylmethyldimethoxy silane silane in Cablecure 732 fluid
KV10 Irgastab® Cable KV10 anti-oxidant in Cablecure 732 fluid
PMH6 phenylmethylsiloxane hydrolyzate (linear DP6) typical reaction product of PMDMS
TEMH6 tolylethylmethylsiloxane hydrolyzate (linear DP6) typical reaction product of TEMDMS
Ferrocene ferrocene PD suppression and UVA in all Novinium® fluids
CBMH6 cyanobutylmethylsiloxane hydrolyzate (linear DP6) typical reaction product of CBMDMS
T1130m Tinuvin® 1130 monomer UVA in Cablecure 732 fluid
T1130d Tinuvin® 1130 dimer UVA in Cablecure 732 fluid
T123 Tinuvin® 123 HALS and methanolic corrosion suppression in Cablecure 732 and Cablecure iXL fluids

Ferrocene is an example of a small molecule with an anomalously small diffusion coefficient. Its rigidity and shape provides its anchor. KV10 stays in the cable much longer than suggested by its 10-8th diffusion coefficient. KV10 was designed with two long carbon-chain arms, which are highly soluble in the insulation polymer, to anchor it in place and reduce what cable manufactures that use KV10 call “sweat-out.” PMH6 has been in use for over two decades and enjoys long life in a cable. Any component below PMH6 lasts even longer. The smaller the diffusion coefficient, the longer it sticks around. The five bottom components are the non-water reactive components, but they enjoy the longest life.