# Qualifying Propylene Glycol amounts for specific volume

1. Feb 8, 2014

### cjweber

Hello all, I am a newbie to this site and have found some interesting discussions herein, so I thought it worthwhile asking the collective wisdom of this group about Hygroscopic liquid calculations that I am struggling to correlate.

It has been several years now since I worked as an electronics engineer for the science industry, and am now a bit rusty in my skill set. As a hobby, I am evaluating many types of desiccants (and humectants) for assisting in the maintenance of wooden acoustic musical instruments. I have been reviewing a product that appears to use Propylene Glycol as a main ingredient, within a method of controlling relative humidity and I cannot seem to qualify the claims made by the manufacturer, leaving me to suspect it is a poor consideration for these types of instruments.

Dow has a 'guide to Glycols' that I am using:
http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0047/0901b803800479d9.pdf?filepath=propyleneglycol/pdfs/noreg/117-01682.pdf&fromPage=GetDoc [Broken]

The product under review is found at:
http://www.humicase.com/customercare/humicase_product_care_brochure.pdf [Broken]

This company suggests "Saturate each sponge with approximately 1 oz. of Activation Solution" and identify an ingredient of the solution as "CONTAINS PROPYLENE GLYCOL".

The Dow document Figure 3 provides percent by weight (PBW) of glycol in water solution required to maintain equilibrium in contact with air of various temperatures and humidity’s. I have extrapolated formula from calculations Dow provides in their 'Humectancy Example' as;

x = quantity of water required
y = quantity of solution needed
z = total amount of PG required

y = x / (1 - PBW/100)
z = y - x

The wikipedia definition of 'Humectancy' "is one of a group of hygroscopic substances used to keep things moist; it is the opposite of a desiccant." This product appears to be suggesting it is a solution towards maintaining an equilibrium, regardless of whether their customers keep acoustic musical instruments in dry/arid or humid environments, and makes no mention of vapour pressure, etc.

Referring to http://www.thomasnet.com/articles/chemicals/liquid-desiccant-drying, Propylene Glycal (PG) does not seem an ideal choice for that purpose as it requires higher concentrations to achieve equilibrium and may also evaporate (is typically used in lower temperature operations in which evaporation rates are lower). A significantly larger amount of solution is necessary to work as a desiccant; only so far as its volumetric nature allows it to gather water, the capacity of PG to absorb water is very minimal.

HumiCase - humidification guidelines: (HumiCase Product Care Brochure, n.d.)
• Y = 2 * 1oz 'solution' = 60ml activation solution
• 'compound' treated Sponge

The percentage by weight ratio of water to Propylene Glycol is very important for specifics environmental conditions. For 50% RH @ 27 ºC, the ratio of y = (z / 0.81) for a solution of y. "Assuming" that this product has taken this into account, I am still left with the question;

Q. At what volume of environment would this be likely to be effective and for how long?

A few other concerns evolve about storing any quantity of liquid with a wooden acoustic instrument and the ratio of solution from head to body of the guitar since a guitar body is ~90% of the volume; we want to equalise the RH between ends.

Last edited by a moderator: May 6, 2017