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About Wetness Indicator

1. Background

Diapers have widely applied by babies and incontinence adults for its stronger absorbent ability. However, the diaper needs to be exchanged after several absorbents. It is hard to tell when diapers should be exchanged just by their own observing, especially the new mothers must have some unhappy experience just tell by smell or touch. At the same time, if the diapers haven’t be exchanged in time, it will cause irritation even eczema.

Recently, for lots of customers, the easiest way to judge when should they exchange diaper for baby is observing wetness indicator. A wetness indicator is a common feature in many disposable diapers. It is a feature that reacts to exposure of liquid as an indicator for parents that a diaper needs changing.

2. Type

At present, there are two main types of wetness indicators, one is hot-melt type, the other one is ink type. Generally speaking, color change indicator( from yellow to green or blue) always adopts hot melt technology, while color fading and disappearing indicator adopt ink technology.

3. Principle

Hot melt wetness indicator adhesive works by organic combination of hot melt adhesive and substance with color-sensitive to PH value. The wetness indicator is covered on the inner side of the back sheet, which faces the absorbent core directly when the liquid soaks the core and transmits to hot melt adhesive, it leads to the PH value changes, then the color changes too. Through such a process, observing wetness indicator is achieved. It’s worth knowing that the transition interval ranges from 4 to 13, which means strong acid and strong alkali won’t cause the color change. It always is 1 to 3 stripes in the middle of the back sheet.

Ink indicator works by adding small-molecule substances with color into ink, when urine comes to them, the ink and small molecule substance are washed away, so the color disappears and the indicator effect is met. The ink indicator always is a picture

4. Composition

Adhesive: The base resin usually used in hot melt wetness indicator adhesive is polymer elastomer, such as SIS, SBS, SEBS, SEPS, etc. Phenylethylene and its segmented copolymer( SBS, SIS) are widely used in this field, neuroplasticity polymer elastomer usually as base resin in manufacturing the indicator, and equips with tacky resin, softener, antioxygen, PH value indicator.

Ink: the ink wetness indicator is composed of water, modified acrylic resin, organic&anic pigments, additives, wax, and solvents, ammonia /ammonia derivatives. Among these, water, modified acrylic resin, and organic & any pigments totally take up about 95%.

5. Characteristics


1)Safety, non-toxic, reliable.


3)Environmental stability.

4)Color change sharply

5)Resistant to wash

6)Fast reaction

7)Gradient function, the color changes from yellow to green then to blue per washing by urine, darker and darker. Customers can judge the absorbent level by color changing and exchange the diaper in time.


1)Non VOC, non-toxic, non smell, not stimulate the skin, clean, nonflammable, non-plosive.

2)Stable performance and long storage time.

3)High Solid content and saturation, excellent fluidity, less ink is needed.

4)Excellent adhesion


6)Intense color, high glossiness on surface.

7)Small grain size, orderly and closely distribution, fast dry off; wetness indicator function, color fading once encountering water.

6. Q&A

Question 1: If wetness indicator is “chemistry”?

First of all, we should be clear that chemical composition is a very board concept, most of daily using( even those are claimed as all-natural skincare products), more or less, will involve “chemical component”. But what we should pay attention to is those hurt “chemical component” instead of all of them. According to principles of wetness indicator, the hot-melt type indicator is a simple chemical reaction, while an ink indicator is just a physical reaction. Therefore, we come to a conclusion, indicators aren’t exactly chemical reaction, only color change type is a chemical reaction.

Question 2: If color change indicator will hurts baby?

Customers feel unsettled, once baby products connect with “chemical component”. They think the chemical components will cause irritation and hurt baby healthy because babies have tender skin, lower resistance, and immunity. Now that the color change indicator belongs to a chemical reaction, will it hurt the baby?

The answer is no. First of all, according to the characteristics of the hot melt adhesive indicator, it is non-toxic and stable, the PH value indicator, called bromocresol green, is non-toxic and non-irritation. What’s more, the baby diaper have three different parts, including top sheet, absorbent core, and back sheet. To help mother checks in time, the wetness indicator is covered on the back sheet, the farthest away layer from the baby. So the chemical components even have not to chance to touch baby skin, how can they cause irritation? To say the least, once the indicator colors off, it will be absorbed by SAP which will inflate to many times of original volume. It is extremely hard to pass through these strong “resin wall” to hurt the baby. Finally, international organizations should have introduced relevant policies to ban on using indicators, if it really hurts the baby.

7. Tips:

Tip 1:

Pay attention to the following points will help to improve application.

First of all, using the hot melt indicator adhesive required an independent glue machine, so the related glue machine system needs to be revised, but the diaper system with no need for revising.

Secondary, covering one square meter back sheet need 30 grams glue if one piece of baby diaper covered 2 stripes with 1.5mm width and 250mm length. And 10,000,000 pieces baby diaper need about 225kgs. Major glue manufacturers like Hankel, H.B fuller all have this type of product. Since we build a strategic cooperation relationship with Hankel and H.B fuller and get the promotion price, any inquiry will be welcomed.

Tip 2

What’s more, we suggest controlling the glue quantity adding into the glue machine by adding 3 to 4 pieces( 1 kg per piece) one time, then adding some more until the glue almost runs out. In order to avoid the temperature changes sharply, adding glue quantity should don’t be too many to affect the temperature of the glue box.

It happens, the machine will stop running, but still some glue in the glue box, in this situation, the glue manufacturer suggests to drain off the glue directly, but in order to save the cost, some manufacturers will turn down the temperature of glue machine to avoid aging.

Tip 3:

Ink wetness indicator needs independent printing, printed on inner of the back sheet( the side touch absorbent core directly), the print line should be thin and simple, or the wet indicator won’t color shade or disappearing.

Request for Quotation! (86)15659692670 Kelly

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