What to do when TPE-coated ABS raw materials do not bond well
Source：德创化工Time：2019-08-14Hits：15587
Recently, a friend asked about the TPE-coated ABS material, summed up some personal opinions on TPE-coated ABS, and first analyzed the formula of TPE-coated ABS.
Basic formula: SEBS +SEBS-g-MAH+TPU+White Oil+Additive
1. People who make polymer materials all know that SEBS is a nonpolar polymer material, while ABS/PC is a low and medium polarity polymer material, while PA is a strong polarity engineering plastic. Under normal circumstances, pure TPE cannot encapsulate ABS/PC/PA and other engineering plastics (it cannot be glued). Why should TPE be used for encapsulation? Because in some occasions, especially in the part that is in contact with people's split seams, the coating material needs to have certain compliance and low hardness, which is what we usually call good handfeel, such as hardware tools, electric drills, saws, etc. If these tools do not include TPE, they are hard, good for short-term use, too much for long-term use, and also affect their work efficiency.
2. TPU has high hardness and poor compliance, but TPU is a thermoplastic elastomer with strong polarity and has good adhesion with polar engineering plastics such as PA/PC/ABS. By adding modifier (TPU) into the formula, the polarity of TPE can be improved, thus improving the adhesion between TPE and ABS. As the hardness of TPU is very high (Shore A: 82), adding TPU to the blend will increase its hardness, and with the increase of TPU content, the hardness of the material will continuously increase.
3. SEBS-g-MAH as compatilizer can well promote the mutual dissolution of SEBS and TPU or SEBS and ABS. Theoretically, TPU does not need to be added after adding compatilizer in the encapsulation process, which is not the case in actual production, mainly because TPU not only has good compatibility with ABS material, but also modifies the encapsulation material to enhance hardness and strength.
Therefore, the compatilizer also plays a role in promoting the compatibility of SEBS and TPU in the formula. Experimental data show that when 15% compatilizer is added, the 100% and 300% moduli of SEBS and TPU mixed materials are the largest (as shown in the following figure). Sometimes when we manufacture TPE coating materials, the amount of SEBS grafting agent added is insufficient (or there is too much difference with the ratio of TPU and SEBS), which leads to poor compatibility of the coating materials and delamination of openings during coating.
4. The function of white oil: to increase the processing fluidity of SEBS, adjust the hardness and strength of encapsulating material, and reduce the cost. Generally, in our formula of rubber coating, the oil filling ratio is 1, i.e. the same amount of oil as SEBS (it is also mentioned in the literature that when 100% oil filling is achieved, re-oil filling will not significantly improve the low temperature performance of SEBS/TPU blend). Excessive oil filling will cause uncomfortable hand feeling, oil leakage or stickiness. In addition, the stickiness between the coating material and ABS will be affected during coating.
5. Additives: Antioxidants are the most common additives. The main reason is that the stability of SEBS is relatively not very good, mainly in terms of oxidation resistance and thermal decomposition temperature. Generally, the temperature of encapsulation is about 200 degrees, and what's more, some of the temperatures of encapsulation PA reach about 260 degrees. Fillers, some rubber-coated materials will be slightly added with fillers such as calcium carbonate, usually the filling amount is about 5%, if the effect is to increase dimensional stability (reduce shrinkage), followed by some to enhance heat resistance.
6. Engineering plastics: some engineering plastics will be added to some formulations for modification, and the most common is to add that material to whatever material is wrapped, such as ABS. A small amount of ABS plastic will be added to the formulation of the encapsulating material, and the corresponding PA ingredients will be added to the encapsulating PA. In this way, the adhesion performance between TPE and ABS/PA in the encapsulating process can be promoted.
Therefore, we usually use the formula of encapsulating material (for reference only):
SEBS(30 parts) +TPU(15-20 parts) +SEBS-g-MAH(10-15 parts)+white oil (30 parts)+calcium carbonate (0-5 parts)+antioxidant (several)
Most users are concerned about the adhesion of TPE package ABS. I think there are mainly the following two points.
1. Formula of 1.TPE. The formula of TPE must contain components that are compatible or sticky with ABS, that is, not every brand of TPE coated with ABS can be used.
2.TPE secondary injection molding processing temperature. The injection molding temperature must ensure that the material is fully plasticized to achieve the fluidity required for injection molding, and at the same time, the activity of ABS compatilizer in TPE formula is increased, so that the TPE and ABS can penetrate each other at the encapsulation contact interface within a few tens of seconds, and the TPE and ABS can be firmly bonded.
ABS is a weak to medium polarity plastic with a certain polarity difference with TPE. By adding modifier (TPU) into the formula, the polarity of TPE can be improved, thus improving the adhesion between TPE, ABS and PC. In addition, the secondary injection temperature is recommended to be 180~230℃. The specific adjustment can be made according to the actual processing conditions.
