Is hot melt adhesive soft or hard after curing? How to choose softness and hardness (flexibility)

Customers often ask when choosing glue, ‘Is this glue soft or hard after it dries?’ This question is very accurate——The softness (flexibility) of the cured hot melt adhesive directly determines the flipping feel, book back stiffness, and tear resistance performanceIt is a finer selection dimension than ‘sticky’. Let’s clarify what determines it and how to choose it for different scenarios.

1、 First and foremost, the softness and hardness are determined by the formula

Whether hot melt adhesive is soft or hard after curing mainly depends on the formula composition:

• EVA grade (VA content)The higher the VA content, the softer and more flexible the colloid is; The lower the VA, the harder it is.
• Ratio of tackifying resin/wax/fillerAdding more fillers (such as 396 with fillers and 397 without fillers) will make the glue harder and lower the cost; Adding more flexible components makes it softer.
• Is the main focus on “flat/flexible”Like Sunlife S-391 This type of high-end flat adhesive has a formula that leans towards softness, allowing the book to be spread out 180 ° and has a good hand feel, making it suitable for business laptops.

in a word:Soft rubber is soft, hard rubber is firmThere is no ‘who is better’, only ‘which scene is more appropriate’.

2、 What is the difference in performance between soft glue and hard glue

3、 Select software and hardware according to the scene (corresponding to Sunlife)

1. Business notebook/requires 180 ° flat layout: Selectsoft rubberLike Sunlife S-391(High end flat, soft texture). Hard glue will bounce back and the book won’t close when spread flat.
2. Thick books, picture books, and book backs should be straight: SelectSlightly stiffThe back glue of the book provides good support and is not easy to collapse. The specific model is matched according to the weight of the paper (for copperplate paper/adhesive paper/double adhesive paper/thin paper, refer to the model table for comparison).
3. High speed binding of regular books and periodicals: Useoverall balanceConventional backing/edge glue that balances speed and basic flexibility, without pursuing extreme softness.
4. Pearl cotton (EPE) bonding: SelectModerateFlexible and foam compatible adhesive, too hard and prone to rebound and delamination (pearl cotton thread emphasizes not mixing different brands and models).

4、 Two points not to be ignored when selecting

• Soft and hard should be matched with the temperature on the machineFor the same adhesive, if the adhesive temperature is too high, it will be more “thin and soft”, and if it is too low, it will be more “hard and brittle”. So the soft and hard feel is also affected by the process, and the parameters are based on the accompanying “Process Card”.
• Soft ≠ Non stickIf the opening time and wettability of the soft adhesive do not match, it will still peel off; Hard glue can also crack if the temperature is not correct. The selection is based on the combination of software and hardness, opening time, and temperature, rather than just looking at one indicator.

5、 Relationship with Sunlife’s real business

Sunlife (Sunlife) hot melt adhesive covers books and magazines fromSoft Flat (S-391)arriveConventional back adhesive/edge adhesiveThe gradient of graphic and text stores (S-393 compared to the mid-range of Hede Miao, S-395 compared to the high-end low-temperature resistance of Henkel 3218) and pearl cotton (S-39B main, S-399 low price) also have their own soft and hard positioning. When a customer asks’ do you want something softer or harder ‘, first clarify whether it isFlat notebook, thick book brochure, or pearl cotton/packagingCorresponding to the model, do not blindly pursue softness or hardness.

Explanation: “Soft and hard” in this article is a relative concept. The specific flexibility performance, operating temperature, and opening time of a certain model are subject to Sunlife’s accompanying “Instructions/Process Card” and model technical data; Please consult Sunlife’s technical team for confirmation of the selection.