[Science Stickers] Cause Analysis of the Brittleness of PVC-U Plastic Pipe (Part 1)

The brittleness of plastics has always been a factor that plagues the normal operation of some companies. The brittleness of pipes has more or less affected the market share and user reputation of these pipe companies in terms of cross-sectional appearance and installation approval. The brittleness of pipes is basically It is fully reflected in the physical and mechanical properties of the product.

This article discusses and analyzes the reasons for the brittleness of PVC-U plastic pipes from the formula, mixing process, extrusion process, mold and other external factors.

The main characteristics of PVC pipes becoming brittle are: cracking and rupture during cold punching during blanking.

There are many reasons for the poor physical and mechanical properties of pipe products, mainly as follows:

Unreasonable formula and mixing process

(1) Too much filler. In view of the current low profile prices on the market and the rising prices of raw materials, pipe manufacturers are all about reducing costs. Regular pipe manufacturers reduce costs without reducing quality by optimizing the combination of formulas; some Manufacturers have lowered the quality of their products while reducing costs. Due to the composition of the formula, the most direct and effective way is to add fillers. The filler commonly used in PVC-U plastic pipes is calcium carbonate.

In the previous formulation systems, most of them were filled with heavy calcium, the purpose of which was to increase rigidity and reduce costs. However, due to its irregular particle shape and relatively coarse particle size, heavy calcium has poor compatibility with the PVC resin body, so its addition is very high. Low, and when the number of copies increases, the color and appearance of the pipe will be affected.

Now with the development of technology, most of the use of ultra-fine lightweight activated calcium carbonate, or even nano-scale calcium carbonate, which not only plays the role of increasing rigidity and filling, but also has the role of modification , but its filling amount is not Infinite, its proportion should be controlled. Now some manufacturers add calcium carbonate to 20-50 parts by mass in order to reduce the cost, which greatly reduces the physical and mechanical properties of the profile and causes the tube to become brittle.

(2) Type and quantity of impact modifier added. Impact modifier is a high molecular polymer that can increase the total energy of PVC rupture under stress.

At present, the main varieties of impact modifiers for rigid PVC are CPE, ACR, MBS, ABS, EVA, etc. The molecular structure of CPE, EVA, ACR modifiers does not contain double bonds, and has good weather resistance and suitable As outdoor building materials, they are blended with PVC to effectively improve the impact resistance, processability and weather resistance of rigid PVC.

In the PVC/CPE blend system, its impact strength increases with the increase of the amount of CPE, showing an S-shaped curve. When the addition amount is less than 8 parts by mass, the impact strength of the system increases very little; when the addition amount is 8-15 parts by mass, the increase rate is the largest; after that, the increase rate tends to be flat.

When the amount of CPE is less than 8 parts by mass, it is not enough to form a network structure; when the amount of CPE is 8-15 parts by mass, it is continuously and uniformly dispersed in the blending system to form a phase-separated network structure that makes the blending The impact strength of the system increases the most; when the amount of CPE exceeds 15 parts by mass, continuous and uniform dispersion cannot be formed, but part of the CPE forms a gel, so that there will be no suitable CPE particles for dispersion at the two-phase interface To absorb the impact energy, so the impact strength growth tends to be slow.

In the PVC/ACR blend system, ACR can significantly improve the impact resistance of the blend system. At the same time, the "core-shell" particles can be uniformly dispersed in the PVC matrix. PVC is the continuous phase, and ACR is the dispersed phase. Dispersed in the continuous phase of PVC, it interacts with PVC and acts as a processing aid to promote the plasticization and plasticization of PVC. Gelation, short plasticization time, and good processing performance. The forming temperature and plasticizing time have little effect on the notched impact strength, and the decrease in the bending elastic modulus is also small.

The general dosage is 5-7 parts by mass. The hard PVC products modified by ACR have excellent room temperature impact strength or low temperature impact strength. However, it is proved by experiments that the impact strength of ACR is about 30% higher than that of CPE. Therefore, the PVC/ACR blending system should be used as much as possible in the formulation, and when modified with CPE and the amount is less than 8 parts by mass, the pipe will often become brittle.

(3) Too much or too little stabilizer. The role of the stabilizer is to inhibit degradation, or react with the released hydrogen chloride, and prevent discoloration during processing of polyvinyl chloride.

The amount of stabilizer varies according to the type, but in general, too much dosage will delay the plasticization time of the material, so that the material is not plasticized when it is exported to the mold, and the molecules in the formula system are not completely fused. Causes its intermolecular structure to be weak.

When the dosage is too small, it will cause degradation or decomposition of the relatively low molecular weight in the formula system (also can be said to be over-plasticized), which will damage the stability of the intermolecular structure of each component. Therefore, the amount of stabilizer will also affect the impact strength of the pipe. Too much or too little will cause the strength of the pipe to decrease and cause the pipe to become brittle.

(4) Excessive amount of external lubricant. The external lubricant has low compatibility with the resin, which can promote the sliding between the resin particles, thereby reducing the frictional heat and delaying the melting process. This effect of the lubricant is in the early stage of the processing process (that is, the external heating effect and the frictional heat generated internally) Before the resin is completely melted and the resin in the melt loses its identifying characteristics) is the greatest.

External lubricants are divided into pre-lubrication and post-lubrication . Materials with excessive lubrication show poor appearance under various conditions. If the amount of lubricant is incorrect, it may cause flow marks, low output, turbidity, poor impact, and rough surface. , Adhesion, poor plasticization, etc. Especially when the amount is too large, it will cause poor compactness and poor plasticization of the profile, resulting in poor impact performance and brittleness of the pipe .

(5) The feeding sequence of hot mixing, temperature setting and curing time also have decisive factors on the performance of the profile. There are many components in the PVC-U formula. The selected order of addition should be conducive to the effect of each additive and increase the speed of dispersion, while avoiding its undesirable synergistic effect. The order of addition of the additives should help increase the auxiliary effect. The complementary effects of the agents overcome the effects of mutual elimination and elimination , so that the additives that should be dispersed in the PVC resin can fully enter the inside of the PVC resin.

The feeding sequence of a typical stable system formula is as follows:

a When  running at low speed, add PVC resin to the hot mixing pot;

b  Add stabilizer and soap under high-speed operation at 60°C;

c  Add internal lubricants, pigments, impact modifiers, and processing aids at around 80°C under high-speed operation;

d  Add external lubricants such as waxes at about 100°C and high speed;

e  Add filler under high-speed operation at 110°C;

f  Discharge the materials to the cold mixing tank for cooling at a low speed of 110°C-120°C;

g  Cold mix until the material temperature drops to about 40°C, then discharge. The above feeding sequence is more reasonable, but in actual production, it is also different according to their own equipment and various conditions. Most manufacturers add other additives together with the resin. There is also light activated calcium carbonate added together with the main ingredients and so on.

This requires the technical personnel of the enterprise to work out suitable processing technology and feeding sequence according to the characteristics of the enterprise.

Generally, the hot mixing temperature is around 120°C. When the temperature is too low, the materials will not be gelled and mixed uniformly. Above this temperature, some materials may decompose and volatilize, and the dry mixed powder will turn yellow. The mixing time is generally 7-10min before the material can achieve compaction, homogenization, and partial gelation. The cold mix is ​​generally below 40°C, and the cooling time is required to be short. If the temperature is greater than 40°C and the cooling speed is slow, the prepared dry mix will be less dense than the conventional one.

The maturation time of dry blends is generally 24 hours. If the material is longer than this time, it is easy to absorb water or agglomerate. If it is less than this time, the structure of the molecules between the materials is not stable, resulting in large fluctuations in the shape and wall thickness of the pipe during extrusion. . If the above links are not strengthened, the quality of the pipe products will be affected, and in some cases, the pipe will be brittle.

This article comes from the Internet, only for learning and communication, no commercial purpose.

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