Shelf-life expert

Water activity: a better way to monitor moisture in plastics

Injection molding plastic parts with resin that has too much moisture in it can lead to brittle, unattractive or otherwise imperfect parts. Water activity can help. 

Most plastic resin looks dry, but looks can be deceiving. If plastic parts are injection-molded with resin that has too much moisture in it, problems can occur–problems like ugly-looking parts, or parts that are too brittle. Water activity is a more accurate way to monitor moisture in plastic resin, enabling manufacturers to avoid these problems. Why?

Water activity: a clear winner over moisture content

Usually, injection molders use loss-on-drying balances to check their products for moisture, but when a dry product is dried, the weight doesn’t change much. When we dried LEXAN™ resin for 4 hours at 100 ºC, here’s what happened:

Figure 1. Lexan resin dried for 4 hours at 100 ºC: weight only changed 7 mg or 0.1%

The sample weight changed by only 7 mg or 0.1%. Water Activity (aw) is a different way of looking at sample moisture. It measures the relative humidity of a sample instead of how much water is in it. When we measured the water activity of the same lexan sample, we saw this:

Figure 2. The water activity changed by more than 60% in the lexan resin sample dried for 4 hours at 100 ºC

The difference between the two Lexan samples is clear–the water activity changed by more than 60%.

Using the power of water activity to save time and money

Apply the extra power of water activity to a plastic resin sample using a simple process. First, take a water activity reading on the resin before drying. Then dry per the manufacturer’s specifications, and take a reading on the resin after drying. This gives a “target” water activity. For the next batch, simply take water activity readings on the resin every half hour or so. Once the “target” is reached, stop drying and start injection molding. Save time and also energy cost by not over-drying the resin.

Water activity is easy

It’s easy to use a water activity meter to measure moisture. Simply put 2-4 grams of sample in a sample cup, and close the sample chamber to get a reading in about 5 minutes. Operating the instrument doesn’t require any special training, reagents, sample preparation, or weighing.


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