TPU is called thermoplastic polyurethane elastomer. There are mainly polyester type and polyether type, wide hardness range (60HA-85HD), wear resistance, oil resistance, transparent, good elasticity.
It is widely used in daily necessities, sporting goods, toys, decorative materials and other fields. However, people who contact TPU all know that the problem of yellowing and crystal point makes everyone "love and hate". Today, we will analyze the causes of yellowing and crystal point and the countermeasures.
TPU is a kind of yellowing resin. In ISO, MDI will turn yellow under ultraviolet irradiation. It can be seen that TPU yellowing is a property.
1. Add anti-yellow
In order to strengthen the TPU products Huang Nengli, tend to be added in the raw material of A kind of special fight yellow yellow agent, but is expensive, of the agent that fight yellow when we use yellow resistance agent should also measure its economic benefits, such as our body is not sensitive to yellowing is black type, so we can use cheaper did not add the resistance of yellow yellow material, due to the resistance to yellow agent is A kind of additive raw materials, add it in A component, we asked stir ingredients to uniform distribution, resistance to yellow effect, can appear otherwise local yellowing phenomenon.
2. Spray yellow resistant paint
The spray of paint usually has two forms, one is spray in mold, the other is spray out of mold. The spray of yellow-resistant paint will form a protective layer on the surface of TPU finished products to avoid yellowing pollution caused by contact between TPU epidermis and atmosphere. This form has been widely used at present.
3. Avoid pollution during manual operation
Many TPU products are polluted in the process of finishing or saving, resulting in yellowing, such as human sweat and organic solvents, etc. Therefore, TPU products should pay special attention to the cleanliness of the contact body and minimize finishing procedures as much as possible.
4. Avoid sunlight
TPUTPU storage area should be cool and ventilated, and the TPU can be placed in a plastic bag in a place with no sunlight. Relative to the problem of yellowing, the causes of the crystal point of TPU materials are more complex and varied.
The reason of more crystal points in polyester TPU may be related to the narrow and wide molecular weight distribution of polyester polyols. If the molecular weight distribution is too wide, it is difficult for the macromolecular part to be plasticized completely during processing, so the crystal points are generated. Inadequate compatibility with the additive itself will also lead to the formation of crystal points (incomplete plasticization) during the processing of polyester TPU.
The polyester polyol manufacturers are required to control the molecular weight distribution within a certain range. If the supplier fails to do so, the supplier should be replaced. To the compatibility of the additives themselves, do tests to check the compatibility of the additives and polyester polyols.
When the formula was designed, the R value was relatively high. Excessive isocyanate produced more cross-linking. Due to the difference of melting temperature, the TPU with high relative molecular weight was not easy to melt and form crystal points.
Do test check formula, make sure the R value is reasonable when designing.
Reasons for processing control
A. The amount of catalyst is relatively large, and excessive catalyst will continue to react in the twin screw to produce unfused crystal point. Due to the large amount of catalyst, the reaction temperature is set relatively low, the rotation speed is fast, and the material stays in the screw for a short time in the reaction process, which leads to insufficient crystal point formation in the reaction.
B. The unreasonable setting of process parameters leads to insufficient reaction to produce crystal points in TPU production;
C. Production flow fluctuation, flow fluctuation will lead to melting finger fluctuation, melting temperature is different to produce crystal point;
D. Screw block damage and screw block damage lead to uneven mixing and crystal points in the reaction process.
A. For enterprises with manual ingredients, the amount of raw materials, especially the amount of catalyst added is the key, and it is very important to review the catalytic dose.
B. Adjust process parameters and monitor parameters to ensure that the reaction temperature is set appropriately, the speed is reasonable, and the material does not stay in the screw for too short a time in the reaction process (resulting in insufficient reaction to produce crystal points).
C. Strengthen the regular maintenance of the flowmeter and the monitoring of flow fluctuation. If the flow fluctuation exceeds the range, the flow shall be stopped for inspection.
D. Check the screw regularly, and replace it in time when the screw block is damaged, so as to avoid uneven mixing and crystal points in subsequent processing.
Polyester type TPU produces crystal point for other reasons
1. Mixing, if different hardness (hardness difference is large)TPU is mixed, it is easy to produce crystal points in the use process, if the same type of mixing, may be degraded treatment, if different types of TPU mixing, such as ether mixed with ester, the whole batch of material can only be scrapped;
2. Inclean cleaning during formula switching will also lead to crystal points in the produced TPU;
3. The dehumidification and drying device is not cleaned cleanly during the product switch, which will also lead to the crystal point of the TPU sent to the customer;
4. Improper storage of raw materials will also lead to TPU crystal points. For example, improper storage of polyester polyols or pure MDI results in reduced reactivity and "failure" of catalysts.