The brittleness of PVC profiles is basically fully reflected in the physical and mechanical properties of the products. Its main features are: collapse at the time of cutting, cold burst and rupture. There are many reasons for the poor physical and mechanical properties of profile products, mainly as follows:
1. Formulation and mixing process are unreasonable
1, too much filler
In view of the current low profile prices on the market and the rising prices of raw materials, profile manufacturers are making a fuss about reducing costs. The formal profile manufacturers have optimized the combination of formulas to reduce costs without reducing the quality; Manufacturers are reducing costs while reducing product quality. Due to the formulation component, the most direct and effective method is to increase the filler. The filler commonly used in PVC-U plastic profiles is calcium carbonate. In the previous formulation system, most of the calcium is added, the purpose is to increase the rigidity and reduce the cost, but the heavy calcium is very different due to the irregular shape of the particles and the relatively small particle size and the poor compatibility of the PVC resin body. Low, and the number of parts increases the color and appearance of the profile. Nowadays, with the development of technology, most of the ultra-fine and light activated calcium carbonate, even nano-scale calcium carbonate, not only plays the role of increasing rigidity and filling, but also has the function of modification, but the filling amount is not Without limits, the proportion should be controlled. Some manufacturers now 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, resulting in the brittleness of the profile described in this chapter.
2. Types and quantities of impact modifiers added
The impact modifier is a high molecular polymer capable of increasing the total energy of the cracking of the polyvinyl chloride under the action of stress. At present, the main varieties of impact modifiers for rigid polyvinyl chloride are CPE, aCR, MBS, aBS, EVA, etc. Among them, the molecular structure of CPE, EVA and aCR modifiers do not contain double bonds, and the weather resistance is good. As outdoor building materials, they are blended with PVC, which can effectively improve the impact resistance, processability, weather resistance of hard PVC and improve the welding angle strength within a certain range.
In the PVC/CPE blend system, the impact strength increases with the increase in 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; the added amount increases most when it is 8-15 parts by mass; then the growth rate tends to be gentle. 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 blend system to form a network structure in which the phase separation is not separated, so that the blending is performed. 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 some CPE forms a gel, so that there is no suitable dispersed CPE particles at the interface of the two phases. To absorb the impact energy, the impact strength tends to increase slowly.
In PVC/aCR blends, aCR can significantly improve the impact resistance of the blend. At the same time, the “core-shell†particles can be uniformly dispersed in the PVC matrix. PVC is the continuous phase, aCR is the dispersed phase, and it is dispersed in the PVC continuous phase to interact with PVC, which acts as a processing aid to promote the plasticization of PVC. Gelation, short plasticizing time, and good processing properties. The forming temperature and plasticizing time have little effect on the notched impact strength, and the bending elastic modulus decreases little. Generally, the amount of the hard PVC product modified by aCR is 5-7 parts by mass, and has excellent room temperature impact strength or low temperature impact strength.
The experimental evidence shows that aCR has a high impact strength of about 30% compared with CPE. Therefore, the PVC/aCR blend system should be used as much as possible in the formulation, and the modification with CPE and the amount of less than 8 parts by mass tend to cause the profile to be brittle.
3. Too much or too little stabilizer
The role of the stabilizer is to inhibit degradation, or to react with the released hydrogen chloride and to prevent discoloration during processing of the polyvinyl chloride. Stabilizers vary depending on the type, but in general, too much use delays the plasticizing time of the material, which results in less plasticization of the material at the time of exiting the mold, and there is no complete fusion between the molecules in the formulation system. Its intermolecular structure is not strong. When the amount is too small, the relatively low molecular substances in the formulation system may be degraded or decomposed (also referred to as over-plasticization), and the stability of the intermolecular structure of each component may be destroyed. Therefore, the amount of stabilizer will also affect the impact strength of the profile. Too much or too little will cause the profile strength to decrease and cause the profile to become brittle.
4, excessive use of external lubricant
The external lubricant has low compatibility with the resin, can promote the sliding between the resin particles, thereby reducing the frictional heat and delaying the melting process. This action of the lubricant is early in the processing process (that is, external heating and internal friction heat generation). It is the largest before the resin is completely melted and the resin in the melt loses its signature. The external lubricant is divided into pre-lubrication and post-lubrication, and the over-lubricated material exhibits a poor shape under various conditions. If the amount of lubricant is not appropriate, it may cause flow marks, low yield, turbidity, poor impact, and rough surface. , adhesion, poor plasticization, etc. In particular, when the amount is too large, the compactness of the profile is poor, the plasticization is poor, and the impact property is poor, causing the profile to be brittle.
5, hot mix feeding sequence, temperature setting and curing time are also decisive factors for the performance of the profile
There are many components in the PVC-U formula. The order of addition should be beneficial to the role of each additive, and it is beneficial to increase the dispersion speed, and avoid the adverse synergistic effect. The order of additives should help improve the auxiliary. The synergistic effect of the agent overcomes the effect of phase gram elimination, so that the auxiliaries which should be dispersed in the PVC resin fully enter the interior of the PVC resin.
The typical lead salt stabilization system formulation order is as follows:
a. When running at low speed, add PVC resin to the hot mixing pot;
b. Stabilizer and soap are added at 60 ° C under high speed operation;
c. At 80 ° C, high-speed operation, adding internal lubricants, pigments, impact modifiers, processing aids;
d. Adding an external lubricant such as wax at a high speed of about 100 ° C;
e. Adding a filler at a high speed operation at 110 ° C;
f. Discharge the material to a cold mixing tank at a low speed of 110 ° C - 120 ° C for cooling;
g. When the temperature of the mixture is cooled to about 40 ° C, the material is discharged and sieved.
The order of feeding above is more reasonable, but in the actual production process, according to its own equipment and various conditions, most manufacturers add other additives in addition to the resin. There is also a light activated calcium carbonate added together with the main ingredient and the like. This requires the enterprise technicians to develop their own processing technology and feeding sequence according to the characteristics of the enterprise.
Generally, the hot mixing temperature is about 120 ° C. When the temperature is too low, the material does not reach gelation and the mixture is uniform. Above this temperature, some materials may decompose and volatilize, and the dry mixed powder turns yellow.
The mixing time is generally 7-10 min to 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 higher than 40 ° C and the cooling rate is slow, the prepared dry mix will be inferior to the conventional compactness. The curing time of the dry mix is ​​generally 24 hours. Above this time, the material is easy to absorb water or agglomerate. Below this time, the structure between the molecules is not stable, resulting in large fluctuations in profile size and wall thickness during extrusion. . If the above links are not strengthened, the quality of the profile products will be affected. In some cases, the profiles will be brittle.
Second, the extrusion process is unreasonable
1. Material plasticization is excessive or insufficient
This is related to the process temperature setting and the feed ratio. If the temperature is set too high, the material will be over-plasticized. Some of the components with lower molecular weight will decompose and volatilize. If the temperature is too low, there will be no molecules between the components. Completely fused, the molecular structure is not strong. However, the feeding ratio is too large, which causes the heating area and shearing of the material to increase, and the pressure increases, which easily causes over-plasticization; if the feeding ratio is too small, the heated area and shearing of the material are reduced, which may cause plasticization. Whether it is over plasticization or under plasticization, it will cause the cutting of the profile to collapse.
2, the head pressure is insufficient
On the one hand, it is related to the mold design (this is described separately below). On the other hand, it is related to the feed ratio and temperature setting. When the pressure is insufficient, the compactness of the material is poor, and the looseness of the material appears to be loose. The metering feed speed and the extrusion screw speed should be adjusted to control the head pressure between 25Mpa and 35Mpa.
3. The low molecular components in the product are not discharged.
There are generally two ways to produce low molecular weight components in a product, one being produced during hot mixing, which can be discharged through a dehumidification and exhaust system during hot mixing. The second is partially residual and extruded water and hydrogen chloride gas generated when heated. This is generally forced out through the forced exhaust system of the exhaust section of the main engine. The vacuum is generally between -0.05Mpa and 0.08Mpa. If it is not open or too low, low molecular components will remain in the product, resulting in a decrease in the mechanical properties of the profile.
4, the screw torque is too low
The torque of the screw is the value of the reaction machine under the force state. The value of the process temperature is set. The feed ratio is directly reflected in the screw torque value. The screw torque is too low to reflect to some extent. The temperature is low or the feed ratio is small, so that the material is not fully plasticized in the degree of extrusion, and the mechanical properties of the profile are also lowered. According to different extrusion equipment and molds, the screw torque is generally between 60% and 85% to meet the requirements.
5. The traction speed does not match the extrusion speed.
If the traction speed is too fast, the mechanical properties of the profile wall will decrease, and the traction speed will be too slow. The resistance of the profile is large, and the product is in a high tensile state, which will also affect the mechanical properties of the profile.
Third, the mold design is not reasonable
1. The design of the die section is unreasonable, especially the distribution of the internal ribs and the treatment of the interface angle. This will cause stress concentration, and it is necessary to improve the design and eliminate the right angle and acute angle at the interface.
2. The die pressure is insufficient. The pressure at the die is directly determined by the compression ratio of the mold, especially the length of the straight section of the mold. If the compression ratio of the die is too small or the straight section is too short, the product will not be dense and affect the physical properties. On the one hand, the change of the die pressure can adjust the flow resistance by changing the length of the flat section of the die; on the other hand, different compression ratios can be selected to change the extrusion pressure during the mold design stage, but it must be noted that the compression ratio of the head should be The compression ratio of the extruder screw is adapted; the melt pressure can also be changed by changing the formulation, adjusting the extrusion process parameters, and increasing the perforated plate.
3. For the performance degradation caused by the poor convergence of the split ribs, the length of the ribs and the outer surface, the ribs and the ribs at the confluence should be appropriately increased, or the compression ratio should be increased to solve.
4. The die discharge is uneven, resulting in inconsistent wall thickness of the profile or inconsistent compactness. This also caused a difference in the mechanical properties between the two faces of the profile. We sometimes failed to pass the test when it was cold-punched, which just proved this. As for non-standard profiles such as thin walls, I won’t say more here.
5. The cooling rate of the sizing die. The cooling water temperature often does not cause enough attention. The function of the cooling water is to cool and shape the macromolecular chain of the profile in time to achieve the purpose of use. Slow cooling allows the molecular chain to stretch for a sufficient amount of time to facilitate shaping. With rapid cooling, the temperature difference between the water temperature and the extruded parison is too large, and the product is subject to quenching, which is not conducive to the improvement of the low temperature performance of the product. From the explanation of polymer physics, the PVC macromolecular chain undergoes curling and stretching under the action of temperature and external force. When the temperature and external force are withdrawn, the macromolecular chain does not recover in a free state in time and is outside the glass state. The disordered arrangement of the chains causes low-temperature impact performance of the macroscopic products. From the plastic processing technology to explain the pvc profile after extrusion, the product has a stress relaxation process after removing the temperature and external force. A suitable cooling water temperature is beneficial to this process. If the cooling water temperature is too low, the stress in the product will not be eliminated, resulting in product performance.
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