With the rapid development of the domestic wire and cable market, the current use of PVC insulation and PVC sheathing wire is more and more widely used. Different users will propose different color standards (RAL, MUNSELL, REA, etc.) for wire and cable, so that PVC wire Cable manufacturers put forward higher requirements on PVC coloring. At present, domestic PVC color masterbatch manufacturing is only the initial stage. The author has worked in the industry for many years and put forward some experiences and opinions on the production and manufacture of PVC masterbatch.
PVC resin is a heat-sensitive and shear-sensitive resin that can affect its application performance if it is "multiple processed". For various reasons, it is often encountered that the "one-time processing" product quality does not meet the requirements, and has to be processed multiple times. This not only affects production efficiency, increases raw material consumption, increases production costs, but may also affect the intrinsic quality of the product. Therefore, carefully analyze the causes of the "multiple processing" operation, understand the main factors affecting the "one-time processing" pass rate, and seek ways to improve the "one-time processing" pass rate, for the production of PVC cable color for high efficiency, low consumption and high quality. Masterbatch has important significance.
First, the status of processing of PVC cable masterbatch
PVC cable masterbatch has a wide variety, complex composition and different formulations. Some can be qualified by "one-time processing", and some have to be processed several times to meet the requirements. Among them, the color spectrum is different, and the "one-time processing" pass rate is quite different. The relationship between the pass rates is as follows:
White > Gray > Yellow > Orange > Brown > Black > Blue > Green > Red.
Second, the common "processing" problem in the production of PVC cable masterbatch
If the "one-time processing" of the PVC cable masterbatch is unqualified, the non-conformities are mainly concentrated in the following items:
Bad shape
Poor dispersion
Color deviation
Among them, the color deviation which leads to the "one-time processing" failure is mainly due to some problems in the formulation, which is beyond the scope of this article, because for a mature formula, the color should not be problematic first.
Causes of poor grain shape, classification and analysis of adverse conditions, the most common of which are the following three:
1. Extrusion strips are difficult to pull, when coarse and fine, the grain shape is not good.
2. There are protrusions on the surface of the extruded strip.
3. The surface of the extruded strip is extremely rough.
The first case is mainly due to the unstable feeding amount of the extruder. Two situations are often encountered in production:
(1) The premix is ​​agglomerated, which makes the feeding of the feeder difficult or the agglomerate is blocked at the feeding port of the extruder.
(2) The premixed packing density is too small, and the bottom is agglomerated due to the “air resistance†at the hopper feeding port, and the premix is ​​bridged in the hopper. In this way, the pressure of the melt of the extruder head is fluctuated greatly, and the material of the outlet die is often less and less uneven, and it is of course difficult to obtain a good grain shape.
In the second case, it usually appears on products with relatively low hardness. After some “secondary processingâ€, the smooth feeling of extruded strips is obviously improved, but some products have not improved significantly. If the dispersion is tested, it is satisfactory.
In the third case, it occurs in some products that use special plasticizers or certain pigments. At this point, the production condition may be good, but the surface of the extruded strip is quite rough, and only after repeated processing, the extruded strip feels smooth and flat.
As for the problem of poor dispersion, the simplest method is usually used in production, that is, "secondary processing".
Third, the main reasons affecting the "one-time processing" pass rate
There are quite a few factors affecting the "one-time processing" pass rate of PVC cable masterbatch products. In order to facilitate analysis and comparison, the effects of the three controllable factors of human, material and ring and the objectively immutable factor of the machine on the "one-time processing" pass rate are not considered. Each product is also integrated from the processing conditions of multiple batches of production, with a certain representativeness. If we analyze it from the perspective of formula, it will give us some inspiration.
1, the impact of organic pigment content
Sorting the masterbatch according to the content of organic pigment and the number of processing in descending order shows that when the content of organic pigment in the formula exceeds 2%, the pass rate of "one-time processing" is only 25%; and the content of organic pigment is less than 2 When it is %, its "one-time processing" pass rate is as high as 80%. This shows that the level of organic pigments has a significant impact on the “one-time processing†pass rate. And its critical concentration is about 2%. More than this value is mostly "secondary processing". One of the reasons is that the organic pigment has a small bulk density and a large volume, and the premix has a relatively small specific gravity during processing, which often causes difficulty in "feeding" and affects the extrusion operation; the second reason is that the dispersibility of the organic pigment is poor, and the content is more High, the more difficult the dispersion, resulting in poor product dispersion.
2. Influence of inorganic pigment content
Most PVC cables are required to have good hiding properties. In addition to organic pigments or inorganic pigments, an appropriate amount of titanium dioxide is usually added to improve the hiding property. Statistics show that the content of inorganic pigments is different, which also has an impact on the "one-time processing" pass rate.
3. Influence of plasticizer varieties
Plasticizer is one of the main components of PVC cable masterbatch. It affects the processability, heat resistance, cold resistance and softness of the product.
4, the impact of plasticizer content
Not only the variety of plasticizers affects the pass rate, but also the use of the same plasticizer, because of the different amount, will also affect the "one-time processing" pass rate, if the ratio of the weight of the resin and plasticizer is the parameter, When the ratio is less than 1, the "one-time processing" pass rate is significantly lower than the ratio greater than 1. Because when the amount of plasticizer increases, it will increase the difficulty of premixing operation, which will easily cause the premix to be “over-wet†and agglomerate. When the extrusion process is processed, it will block the feeding port and affect the operation or extrusion. "Soft" causes difficulty in pelletizing.
5, the impact of filler content
The addition of filler to the PVC cable masterbatch not only reduces the product cost, but also improves the flow properties of the melt, which is beneficial to control the production process conditions and facilitate operation. However, its content should also be appropriate, otherwise it will affect the qualification rate of "one-time processing". When the filler content is more than twice that of the resin, the "one-time processing" pass rate is less than 40%; if the content is 1 to 2 times that of the resin, the "one-time processing" pass rate is 75%. Because the heavy calcium carbonate filler has poor affinity with the resin after all, if the content is high, the "single processing" may not reach uniform distribution and dispersion in the melt, resulting in rough or bulging of the extruded strip. a feeling of. If after "secondary processing", the strip is obviously smooth.
6, the impact of hardness
The effect of hardness is more complicated. Because hardness is not an independent variable, it is governed by the method of preparation of the resin, the type and amount of plasticizer, the type and amount of filler and pigment. However, looking at the situation encountered in production, experience tells us that the high hardness can be considered, the "one-time processing" pass rate is also higher, which may be because of the high hardness, which is conducive to the premix operation and improve the dispersibility of the pigment. reason.
Fourth, methods and measures to improve the "one-time processing" pass rate
1. Improve the quality of premix
A good premix should be homogeneously mixed and looser without agglomerates. Therefore, during premix operation: Select the appropriate premixing equipment and properly select the weight of the premix.
Reasonably arrange the order of addition of each component in the formula:
a, first add resin, wax, lubricant, heat stabilizer, and premix high speed.
b. Add pre-mix, filler, and high-speed premix.
c, add rheology modifier, high speed premix.
d. Finally, add plasticizer at one time and mix at low speed for a short time.
2. Adjust the bulk density of the premix
The purpose is to reduce or eliminate the "air resistance" and bridging phenomenon at the feed port of the premix re-extruder. The methods that can be used are as follows:
Consider from the recipe:
When the user permits, reduce the dilution ratio at the time of use to reduce the content of the organic toner in the master batch.
Re-coloring, using organic pigments with higher color strength.
Under the premise of ensuring the hardness, the content of the plasticizer is reduced by increasing the content of the active filler.
When the hardness is allowed, add as much plasticizer as possible and add more filler.
In higher hardness formulations, little or no rheology modifier is added.
Process consideration
In the premixing process, first add about 20% of the same batch of finished particles of the same batch, which can increase the specific gravity of the premix and facilitate the feeding operation.
3, improve the feeding and measuring capacity
If the width of the gate of the feeding meter is designed to be adjustable, that is, when the PVC cable masterbatch is produced, the feeding speed can be controlled more conveniently and accurately by adjusting the width and height of the shutter. For example, when the width of the shutter is set to one-half of the original and the height of the shutter remains the same, the control range of the feed rate is doubled. This makes it possible to solve the problem of feeding of the "lighter" premix, thereby increasing the yield of "one-time processing".
4. Optimize process parameters
The important process parameters for the extrusion operation are temperature (screw, barrel, head, die), feed rate, screw speed, and the number of holes and holes in the die plate, as well as the temperature, torque and pressure of the melt. Different formulations should have their best operating parameters.
Five important recommendations
Improve the feeding metering device as soon as possible to solve the problem of uneven feeding speed
For the lighter proportion of the premix, the "mixed particles" method premixing process is adopted.
For the higher filler content, the "activation treatment" process is used to improve the surface roughness of the particles. There are two specific methods:
Filler online activation treatment
That is, in the premixing, the filler calcium carbonate is first added to the mixer, and then the coupling agent organic titanate or organosilane is added, and mixed at a high speed for a few minutes to obtain the active filler, and then premixed according to a conventional method.
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