Simple Analysis of Wetness Indicator

Simple Analysis of Wetness Indicator

1. Background

Diaper have widely applied by baby and incontinence adult for its stronger absorbent ability. However, the diaper need to be exchanged after several absorbents. It is hard to tell when diaper 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, 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 type 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 adopts 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. Wetness indicator is covered on inner side of back sheet, which faces the absorbent core directly, when the liquid soaks the core and transmits to hot melt adhesive, leads to the PH value changes, then the color changes too. Through such a process, observing wetness indicator is achieved. It’s worthy knowing that the transition interval is range from 4 to 13, which means strong acid and strong alkali won’t cause color change. It always is 1 to3 stripes in the middle of back sheet. (see picture 1)


Ink indicator works by adding small molecule substances with color into ink, when urine come to them, the ink and small molecule substance are washed away, so the color disappear and indicator effect is met. The ink indicator always is a picture( see picture 2)


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, pyroplasticity 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 & anic 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. Customer can judge the absorbent level by color changing and exchange 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 connected with “chemistry”?

First of all, we should be clear that chemical component is a very board concept, most of daily using( even those are claimed as all natural skin care 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, hot melt type indicator is a simple chemical reaction, while ink indicator is just a physical reaction. Therefore, we come to a conclusion, indicators aren’t exactly chemical reaction, only color change type is chemical reaction.

Question 2: If color change indicator will hurts baby?

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

The answer is no. First at all, according to the characteristics of 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, baby diaper is constructed by three different parts, including top sheet, absorbent core and back sheet. For convenience mother checks in time, the wetness indicator are covered on back sheet, the farthest away layer from baby. So the chemical components even have not chance to touch baby skin, how can they cause irritation? To say the least, once the indicator color 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 baby. Finally, international organization should have introduced relevant policy to ban on using indicator, if it really hurts baby.

7. Tips:

Tip 1: Pay attention to the following points will help to improve application.

First at all, using hot melt indicator adhesive required a 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 baby diaper is 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 products. Since we build strategic cooperation relationship with Hankel and H.B fuller and get the promotion price, any inquiry will be welcomed.

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

It happens, machine will stop running, but still some glue in 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 2:

Ink wetness indicator need independent printing, printed on inner of 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.