Is the problem with yellowing and burning of PVC products on the machine caused by the material or the machine?

doPVCThe bosses of products may have encountered such a scene: shortly after the material is put on the machine, the product begins to turn yellow, and even black spots of burnt material appear.

The first reaction is to adjust the temperature - lower it a little and then lower it a little, but the result is either still yellow or too much, which leads to poor plasticization. After repeatedly tossing and turning for half a day, the waste piled up all over the ground.

Poor thermal stability is indeed one of the most headache inducing issues in PVC processing. But this problem can never be solved simply by adjusting the temperature——raw materialThe formula and equipment temperature control need to be checked at both ends.

　　01

　　Formula is the foundation of thermal stability

　　Let's talk about the raw material side first.The formula is based on thermal stability, and if the foundation is not solid, no matter how much repair is made on it, it will be in vain.

PVC itself is a thermosensitive material that easily decomposes and releases hydrogen chloride (HCl) at processing temperatures, which in turn accelerates decomposition, forming a vicious cycle.

　　Heat stabilizerThe role ofIt means interrupting this cycle - absorbing and neutralizing HCl, replacing unstable chlorine atoms on the molecular chain, and inhibiting the growth of polyene structures, depending on the type of stabilizer. But the selection of stabilizer types, dosage ratios, and compatibility with other additives directly determine how long this layer of "protection" can last.

In the actual process of serving customers, there are also many cases where poor thermal stability is caused by unreasonable formula design: injection molded pipe fittings turning yellow on the machine, cable materials failing to meet thermal stability standards, etc. Behind these problems, there are often shortcomings in the design of stable systems - either due to improper selection of stabilizers or poor compatibility between additives, resulting in a narrow processing window.

Simply put, if the formula of the material itself has poor thermal stability, its safe processing temperature range may only be a dozen or even a few degrees. In this situation, no matter how finely you adjust the temperature on the device, it's like walking on a wire - a slight mistake can cause it to fall off.

　　02

　　Device temperature control is the 'execution layer'

　　Speaking of the device endThe formula determines the upper limit of thermal stability, but whether it can stably perform within this upper limit depends on the temperature control accuracy and process rationality of the equipment.

There are several common device side issues:

　　The temperature control accuracy is insufficient.The temperature fluctuation of many devices is within ± 5 ℃ or even greater, and setting 170 ℃ may actually fluctuate between 165-175 ℃. For PVC materials with already narrow thermal stability windows, this fluctuation is sufficient to cause local thermal decomposition. Regularly calibrating temperature control sensors and checking the status of heating elements are fundamental tasks.

　　The screw design is unreasonable.Excessive compression ratio or improper design of the shear zone can generate excessive shear heat, resulting in the actual temperature of the material being much higher than the set temperature. Especially when dealing with heat sensitive formulas, the shear history of the screw directly affects the risk of material decomposition. For formulas with strong heat sensitivity, using a lower screw compression ratio can help reduce shear heat.

　　Poor exhaust.During the processing of PVC, trace moisture and small molecular volatiles will be released. If the exhaust system is not awesome, these gases will be trapped in the melt, which may cause bubbles and crazes in the light, or accelerate local degradation in the heavy, resulting in black spot coke. If conditions permit, it is possible to consider adding vacuum exhaust to promptly extract trace small molecules released during processing, and the effect will be significantly different.

Reasonable process parameter settings are also important. Taking the common hard PVC injection molding as an example, the injection molding feed section is generally 140-150 ℃, and the plasticizing section is 160-175 ℃; In the extrusion process, the feeding section is generally at 130-150 ℃, the compression section at 150-170 ℃, and the homogenization section at 160-180 ℃. But the specific parameters need to be optimized based on the formula and equipment model, and cannot be copied from others' parameters.

　　03

　　Collaboration between both ends is the correct solution

The correct approach to solving thermal stability issues is to simultaneously focus on both ends:

On the raw material side, select modified particles with mature thermal stability system and wide processing window to ensure sufficient safety margin of the material itself.

On the equipment side, ensure precise temperature control, reasonable screw design, and smooth exhaust, allowing the equipment to operate stably within the material processing window.

Only when both ends are done well can headaches such as yellowing and burning be truly solved at the root. Staring at one end and exerting force will inevitably have limited effect.

　　Author/Zhang Ziyi