What is the reason for the yellowing and discoloration of hot melt adhesive? How to avoid high temperature carbonization and improper storage

Sometimes when opening the glue bucket or opening the bound book, you may find that the hot melt adhesive turns yellow, brown, and even the back of the book becomes brittle. This is not just ‘glue color is like this’, it’s probablyExcessive heating or improper storageResulting in. Below are common reasons and prevention methods for yellowing and discoloration.

1、 Most common: The temperature of the glue pot is too high, and the glue is “burnt”

• Exceeding the safe zone of 160-180 ℃ → Thermal decomposition and carbonizationThe training points are clear – the viscosity and wettability of hot melt adhesive are optimal when used at a temperature of 160-180 ℃; No matter how high the temperature is, the EVA base material will thermally disintegrate, with light discoloration and yellowing, and heavy carbonization becoming brittle and breaking at a single fold.
• performanceThe color of the adhesive changes from translucent to yellow or brown, and the adhesive layer on the back of the book becomes brittle and prone to cracking; In severe cases, wire drawing and knife sticking may occur.
• The root cause often lies in temperature control failureThe actual temperature displayed at 160 ℃ may only be 100 ℃, or it may be the opposite – inaccurate temperature control leading to actual overheating. The key is to calibrate the true glue temperature with a thermometer.

2、 Repeated heating/long-term insulation, cumulative thermal aging

• The glue pot keeps on, and the glue repeatedly meltsThe glue that was not used up that day remained in a high-temperature molten state for a long time, continuously oxidized by heat, and gradually darkened in color.
• avoidAdd glue based on production estimate to reduce “boiling and dry burning”; Clean the pot before stopping, don’t let the glue stay in the pot all night.

3、 Insufficient oxidation of raw materials/antioxidants (batch issue)

• Insufficient antioxidant → prone to yellowing at high temperaturesAntioxidants in the hot melt adhesive formula are responsible for slowing down high-temperature oxidation; If the batch of antioxidants is insufficient or the raw materials are unstable, they will turn yellow when heated on the machine.
• judgeAt the same process temperature, the new batch of glue is significantly more yellow than the previous batch, and the batch itself is suspected first.

4、 Improper storage: damp, exposed to sunlight, high temperature environment

• Warehouse high temperature/exposure to sunlightThe rubber particles will slowly age and turn yellow when exposed to high temperatures for a long time; It should be stored in a cool place and avoid direct sunlight.
• Dampness ≠ yellowing, but it will clump and form white spotsMoisture mainly causes clumping, bubbles and white spots on the machine, and high-temperature yellowing, which are two different things. Don’t confuse them – but both point to “proper storage”.
• avoidTemperature control in the warehouse, keep away from the ground and walls, seal and store, and use up as soon as possible after opening.

5、 How to avoid yellowing and discoloration (practical checklist)

• Calibrate adhesive temperatureFirst, use a thermometer to confirm the true temperature of the glue pot, lock it at 160-180 ℃ (based on the process card), and do not blindly adjust it based on the meter display.
• Control timeReduce the prolonged high temperature insulation of the glue solution, add glue as needed, and stop the machine to clean the pot.
• Check the batchThe new batch has abnormal yellowing. First, compare the previous batches and investigate the issues with the raw material/antioxidant batches.
• Pipe storageCool, sun proof, moisture-proof, sealed, ready to use upon opening.
• Don’t mix brandsDifferent brands have different formula systems, and mixing them can cause reactions and performance drift.

Sunlife’s positioning: Clearly explain both temperature and batch

Sunlife hot melt adhesive is controlled for melting temperature and ratio according to the process card for each batch, matched with real model and batch testing, and tries to prevent “yellowing” before leaving the factory. If yellowing is found on the machine, don’t blame the glue yet – use a thermometer to measure the actual glue temperature, and most likely find the root cause of overheating; Confirmed to be a batch issue, Sunlife will replace the corresponding model according to the actual working conditions, provide the process card, and help you make the book back transparent and secure.

Frequently Asked Questions

Q: Can the glue still be used if it turns yellow?
A: Those with slight color difference and no significant change in performance can be considered for use up; If it has become brittle, brushed, and cannot stick firmly, it indicates severe carbonization. It is recommended to clean the pot and change the batch, and do not compromise.

Q: Is yellowing and moisture clumping the same thing?
A: No, it’s not. Yellowing is mostly caused by high-temperature carbonization/oxidation; Dampness causes clumping, bubbles, and white spots on the machine. Both rely on “temperature control+good storage” for prevention.

Q: How to quickly determine if it is caused by overheating?
A: Measure the true temperature of the glue pot with a thermometer. If it exceeds the 160-180 ℃ marked on the process card, it is basically due to overheating carbonization and yellowing; After calibrating the temperature, observe whether the color of subsequent batches has recovered.

The yellowing and discoloration of hot melt adhesive are most likely caused by “overheating carbonization”, prolonged boiling, batch oxidation, and improper storage. By calibrating the temperature, monitoring the batch, and ensuring stable warehouse management, the adhesive can remain transparent and firmly adhered.