Answer:  

Mr E.I., who is working in the R&D department of YAMATO Co., Ltd., kindly answered the question above.
 
 
First of all, I will explain the reason why adhesives, such as glue, are able to attach things together.
  
When an adhesive is pasted on the surface of a material (adherend), the adhesive penetrates into the small gaps on the adherend surface, and then hardens and does not move any more inside the gap. It is thought that this inability to move is one of the reasons that things attach together. This effect is called the “anchor effect“, because like a ship with a dropped anchor the adhesive does not move. There are chemical bonds between adhesive molecules and adherend molecules, which also make them stick together. These bonds are, for example, hydrogen bond, Van der Waals bond, ionic bond, and covalent bond.
   

Thus, adhesion occurs not as a single effect but as the sum of several effects above. Polyvinyl pyrrolidone (PVP), the major component of a glue stick, also adheres by several effects such as anchor effect and intermolecular forces. There are also hydrogen bonds (O=C∙∙∙∙∙OH－) between carbonyl groups in PVP and hydroxyl groups in paper.

  
On the other hand, since the attraction force is generated when one molecule comes close to another molecule, it is necessary that the adhesive is brought near to the solid surface by liquid. Because the penetration of the adhesive into the adherend surface is important for adhesion, and the strength of adhesion depends on several effects above, we cannot always say, “to feel sticky = to have adhesive force”.
 
The reason why glue stick does not feel very sticky when you touch it would be that PVP is solidified. Fatty acid (sodium salt) is added for solidifying the glue stick, and glycols are added for smoothing. So the solid form of a glue stick does not feel very sticky to touch, but when you use the glue stick the solid form breaks up and stickiness increases.
 
 
Acknowledgement
We would like to thank Mr E.I. in YAMATO Co., Ltd. for his kind answer. Here, we will introduce Mr E.I briefly:
 
Mr E.I is in charge of new product development in his department. He works daily to realize management concepts of YAMATO Co., Ltd.
  
 

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