Paying for Water: What Six Percent Moisture Costs Across Twenty Tonnes

A purchaser in Guangzhou compared two suppliers on price and chose the one at 1,980 dollars a tonne over the one at 2,090. Eighteen months later, an audit of the receiving records showed the winning supplier averaged 7.9 percent volatile content against 2.8 percent for the other. On 240 tonnes a year, that is 12.2 tonnes of water bought at polymer prices, or about 24,000 dollars, which is more than twice the apparent saving.

Moisture is the easiest specification to leave out of a purchase order and one of the most expensive to ignore. This article sets out the arithmetic, explains why moisture varies between suppliers, and gives a 105 C test your own lab can run in two hours.

The Arithmetic: 1.2 Tonnes of Nothing

Six percent moisture on a 20 tonne lot is 1.2 tonnes of water. At 2,100 dollars a tonne, that is 2,520 dollars, or 126 dollars per tonne of product you can actually use. Ten lots a year and the number reaches 25,000 dollars, which is usually more than the entire annual difference between the cheapest and the most expensive qualified supplier.

The correction is simple: dry-basis price equals the as-received price divided by one minus the moisture fraction. At 6 percent, divide by 0.94, which raises the effective price by 6.4 percent. At 8 percent, divide by 0.92, which raises it by 8.7 percent. Two quotes 5 percent apart in headline price can reverse once this is applied.

Why Moisture Varies: Drying Cost Versus Freight

Polyacrylate leaves the reactor as a gel at 35 to 45 percent solids, is dried to a sheet or crumb, ground and classified. Drying to 2 percent costs energy and limits throughput; drying to 8 percent is cheaper per tonne and gives a slightly higher yield because the material is less brittle in the mill. A supplier running a tight dryer will quote more, and the difference is real, not padding.

Freight pushes the other way. Moisture is free weight in a container, and on a 24 tonne container from Asia to Europe, 6 percent moisture is 1.4 tonnes of water shipped across an ocean. Some suppliers deliberately ship at 6 to 8 percent because the freight is charged on gross weight and the customer does not check. That is not fraud, but it is a cost you are carrying.

A 105 C Test Your Own Lab Can Run in Two Hours

Weigh 5.00 g of granules into a pre-dried aluminium dish, dry at 105 C for 2 hours, cool in a desiccator for 20 minutes and reweigh. The loss is volatile content, which is mostly water plus a small amount of residual monomer and oligomer. Repeat on a second sample from a different position in the bag, because the top of a bulk bag equilibrates with the air and can read 2 points lower than the bottom.

Record the result against the certificate for the first ten deliveries. A consistent gap of more than 1.5 points means either the supplier’s sample point is unrepresentative or the bag is picking up moisture in transit, which points at liner integrity. Both are worth knowing before you sign a 12-month agreement.

Moisture Clauses That Survive a Re-Test

Write the ceiling as 5 percent maximum with a target of 3 percent, state the method as 2 hours at 105 C to constant weight, and add a price adjustment of 1 percent of invoice value for each point above the ceiling. Add a re-test right: if your result differs by more than 1 point from the certificate, a joint sample goes to an agreed third-party lab, and the loser pays for it.

That last clause matters more than the number, because it changes the conversation from assertion to measurement. On a 20 tonne lot, 1 percent of invoice value is about 420 dollars per point, which is enough to make a supplier check their dryer rather than argue.

Moisture also affects how the granule behaves in a feeder, so a stable 3 percent product is worth more than its dry-basis price suggests. The polyacrylate granules supplied to a stated volatile content are listed with the test method alongside.