What is the principle (working principle) of hot melt adhesive bonding? Explain the physical process from solid particles to cooling and solidification in one go

Many people are puzzled when using hot melt adhesive for the first time: how can a bag of solid small particles firmly stick the pages and paper box after heating? This article discusses the hot melt adhesiveAdhesive principle (working principle)Make it clear – it is essentially aA physical process driven by temperature changesThey don’t grow together through chemical reactions. With this understanding, many problems such as “poor adhesion, wire drawing, and carbonization” can be fundamentally understood.

1、 Let’s answer directly: Where does the adhesive force of hot melt adhesive come from

Hot melt adhesive (represented by EVA hot melt adhesive commonly used in Sunlife’s book and magazine line) isthermoplasticAdhesive: At room temperature, it is a solid particle, round particle, or block,When heated and melted, it becomes a flowable liquid, which is coated onto the material, soaked, pressurized, and then cooled back to a solid state, locking both sides together。

Key points:The entire process did not undergo any chemical reactions or produce any new substances——The statement in the “Instructions for Use” of Sunlife’s accompanying products that “the physical state changes with temperature, but the chemical state remains unchanged” means this. The adhesive force comes from the mechanical interlocking formed by the cooling and solidification of the molten adhesive and the infiltration and adhesion to the material surface, rather than relying on the reaction between the adhesive and the material.

2、 Take a look at the four steps separately

1. Solid particles (at room temperature)The factory produces cylindrical or round particles, which are convenient for transportation, measurement, and installation.
2. Heating and melting (160-180 ℃)Heat the glue pot to the operating temperature, and the particles melt into a low viscosity fluid. The higher the temperature, the lower the viscosity and the easier it is to flow and infiltrate; But beyond the optimal range, it will undergo thermal depolymerization, carbonization and brittleness (similar to “yellowing and discoloration”).
3. Infiltration and penetrationMolten glue is applied to the back and paper surface of the book, and with the help of pressure and opening time, it seeps into the gaps between the paper fibers and the milled back after being roughened——Penetration “is the key to whether books and periodicals are securely glued togetherPowder falling off the paper surface and milling too shallow can block penetration.
4. Cooling and solidification (about 3-5 minutes)After removing the heat source, the adhesive quickly cools down and returns to a solid state, mechanically locking both sides. Cutting and flipping before curing can cause sticking, deformation, and loose pages.

Simple note:Solid state → melting → infiltration → cooling and solidification, four steps to form adhesion。

3、 Why sometimes it doesn’t stick firmly? Once we return to the principle, we will understand

Many customers think that ‘not sticking firmly is due to poor adhesive’, but from the principle, it mostly lies in the middle two steps:

• Insufficient temperature → high viscosity → poor infiltrationThe actual temperature of the glue pot is lower than the set value (based on Sunlife’s after-sales experience, most cases of “not sticking firmly” are ultimately found to be due to equipment temperature control failure – the temperature controller displays 160 ℃, but the actual glue pot may only be 100 ℃). The glue is not fully melted, cannot flow, and naturally seeps into the paper seam.
• Not pressed during opening hours → surface conjunctivaAs soon as the opening time arrived, the surface of the adhesive began to crust, missing the pressure window and not sticking firmly on both sides.
• Paper and craftsmanship block penetrationCopperplate paper loses powder, milling back and pulling hair are not enough, the adhesive “hangs” on the surface instead of seeping in, and the initial adhesion looks good but falls off as soon as it is flipped over.

So the order of investigation should be:First check if the temperature is real, then check if the opening time is sufficient, and finally check the paper surface and milling backInstead of changing the glue as soon as it comes up. Specific investigation can refer toChecklist for checking if the binding glue of books and periodicals is not dry or firmly adhered。

4、 What does this mean for selection and operation

• Confirm the operating temperature before useThe optimal range for book and magazine adhesive 160–180℃Please refer to the accompanying “Instructions for Use/Process Card” for specific details; There are slight differences in the windows for different models.
• Select opening time based on machine speedLow speed (about 2000 RMB/hour) takes about 15-20 seconds, medium speed (2000-4000 RMB) takes about 5-10 seconds, and high speed (>4000 RMB) takes about 2-7 seconds – the faster the speed, the shorter the opening time of the adhesive, otherwise there will be no time to press and seal the film.
• The thickness of the adhesive layer depends on the secretaryThe adhesive layer for thin book cores of 10-12mm is about 0.4mm, and for thick book cores of 20mm or more, it is about 0.8-1.0mm. Thin book cores are actually the strongest, not the thicker the better.
• Don’t rush the curing timeUsually takes 3-5 minutes, cutting before it dries will cause sticking and deformation.

5、 Relationship with Sunlife’s real business

Sunlife hot melt adhesive covers copperplate paper, adhesive packaging paper, double adhesive paper, thin paper and other scenes. The back adhesive (292/296/295/298, 396/397/398), flat adhesive (391), test paper adhesive (39D), graphic store adhesive (393/395), pearl cotton adhesive (39B/399) and other models are all based on the same “thermoplastic physical bonding” principle, with differences in parameters such as flexibility, opening time, and temperature resistance. When customers encounter issues such as “adhesion failure, wire drawing, and carbonization”, Sunlife recommends prioritizing the four end judgment of “temperature opening time paper penetration adhesive layer”. Most of these issues can be resolved through parameter and process adjustments; Provide the ‘Instructions/Process Card’ along with the goods, and use ‘Temperature Control Calibration+Process Training’ as a support item to reduce such rework.

Explanation: The temperature described in this article is a general process key point, and the specific machine parameters are subject to Sunlife’s accompanying “Instructions/Process Card”; The flexibility, opening time, and temperature resistance of different models vary. Please refer to the actual model technical data for selection.