What causes the surface of hot melt adhesive to turn white and frost (wax deposition)? how to handle

After using hot melt adhesive for books, magazines, or packaging, occasionally a layer of white “frost” or partial whitening can be seen on the surface of the adhesive layer, and there is a small powdery feeling when touched by hand. This is not because the glue is “broken”, in most cases it isWax precipitation (known as bloom/frosting in the industry)This physical phenomenon. It is two different things from the yellowing and blackening caused by high-temperature carbonization. Below are the reasons, differences from carbonization, and how to handle them clearly.

1、 First, let’s answer directly: What is white frost

Commonly used for binding books and periodicals EVA hot melt adhesiveFor example, in addition to EVA base material, the formula will also includeWax (paraffin/microcrystalline wax)andAntioxidantThe function of wax is to reduce viscosity, adjust opening time, and improve fluidity.

When the adhesive layer isLow temperature, prolonged storage, or experiencing repeated temperature fluctuationsAt that time, the low molecular weight wax components in the formula will gradually migrate and crystallize towards the surface, forming a white or grayish white frost like substance on the surface of the adhesive layer – this is called “wax precipitation/frosting”. It is usuallyPhysical changes, not chemical failuresThe adhesive strength of the adhesive body is generally not lost.

2、 Why did it appear? 4 common triggers

1. Storage and use at low or fluctuating temperatures

Wax is more prone to crystallization at low temperatures. If the temperature in the finished product warehouse or workshop is low, or if the adhesive layer undergoes repeated cycles of “hot during the day and cold at night”, the probability of wax precipitation increases. The normal operating temperature of book adhesive is 160–180℃(Please refer to the accompanying process card for details), long-term low glue temperature, repeated melting and cooling of the glue can also promote the occurrence.

2. The wax content in the formula is too high

The amount of adhesive wax used for different purposes varies. Wax with a tendency towards “fast drying, low viscosity, and good leveling” will be added more, and the risk of surface precipitation is relatively high. Sunlife will adjust the wax amount according to the purpose (book back glue, edge glue, graphic store, pearl cotton). If the selection is sensitive to the surface appearance, it should be explained in advance.

3. Thick adhesive layer and slow cooling

The thicker the adhesive layer, the longer the channel for internal wax to migrate to the surface. However, the slower the cooling of the thick adhesive layer and the longer the surface remains in a migratory state, the more obvious the precipitation will be. The thickness of the normal book core adhesive layer has a recommended range (based on the process card), and excessive thickness is not only adhesive intensive but also prone to frosting.

4. Repeated recycling of old materials

If the glue pot is not cleaned for a long time, the old materials will repeatedly melt, and low molecular wax will continuously precipitate and accumulate. White frost on the surface and burnt particles at the bottom of the pot will appear simultaneously. The instructions for using pearl cotton thread suggestClean the adhesive groove about once every two weeksAnti carbonization, the same goes for book and magazine lines.

3、 Key distinction: wax precipitation (white frost) ≠ carbonization (yellowing and blackening)

Many customers confuse two phenomena and handle them in completely different directions:

Simple note:White is wax, yellow and black is cokeYellowing and discoloration, as well as burnt particles in the glue pot, please refer to the category of “hot melt glue yellowing and discoloration” for troubleshooting; This article only deals with ‘whitening and frosting’.

4、 How to handle and prevent

1. Temperature control does not exceed the standardThe temperature on the machine should be set according to the process card, and the glue temperature should not be raised for a long time to “dry quickly” and avoid carbonization; Do not let the colloid repeatedly solidify and melt over a long period of time.
2. Improve storageThe finished product warehouse and workshop should be kept at a constant temperature as much as possible to avoid large temperature differences between the book core and adhesive layer; After opening, seal and prevent moisture according to the key points of “storage after opening”.
3. Periodic cleaning of glue potRegularly clean the burnt particles and old materials in the glue pot to avoid the accumulation of old wax repeatedly returning to the pot.
4. Select categories that are sensitive to appearance in advanceFor high-end products such as hardcover, picture books, and light colored covers that require high appearance of the adhesive layer, please indicate when placing an order that low wax separation or appearance optimization formula should be selected.
5. Finished products that have already started to frostMild white frost generally does not affect the bonding strength, and can be reduced by re pressing and appropriately increasing the ambient temperature to partially melt the wax; If there are high requirements for appearance, it is necessary to evaluate the replacement of glue or adjust the process.

5、 Relationship with Sunlife’s real business

Sunlife’s hot melt adhesive is mainly based on the EVA system, and there are corresponding models for book binding (copperplate paper/adhesive paper/double adhesive paper/thin paper), graphic printing, pearl cotton (EPE) and other scenarios. The wax amount and opening time in the formula are adjusted according to the purpose. When customers provide feedback on the “whitening of the adhesive surface” on site, priority should be given to judging based on the four aspects of “paper adhesive equipment environment”. Most of them belong to controllable wax precipitation phenomena and do not need to be directly judged as waste.

Explanation: The temperature and cleaning cycle described in this article are general process points, and the specific machine parameters are subject to Sunlife’s accompanying “Instructions/Process Card”; The wax quantity and opening time vary for different models. Please refer to the actual model technical data for selection.