The reason why plastic profiles are brittle is basically reflected in the physical and mechanical properties of plastic products. Among them, the main characteristics are as follows: breakdown during cutting, cold impact rupture and so on. There are many reasons for the poor physical and mechanical properties of profile products, such as unreasonable formulation and mixing process, unreasonable extrusion process, unreasonable die design, and the phenomenon of slit. The first part talks about unreasonable extrusion process, unreasonable formula and mixing process.
2. unreasonable formulation and mixing process
1. Overfilling
In view of the low profile prices and the rising raw material prices in the current market, profile manufacturers are writing articles on cost reduction. Formal profiles manufacturers reduce their costs by optimizing their formulations without lowering their quality; while some manufacturers reduce their costs, they also reduce the quality of their products.
Because of the composition of the formula, the most direct and effective way is to add fillers. The filler commonly used in PVC-U plastic profiles is calcium carbonate. In previous formulation systems, most of them were filled with heavier calcium to increase rigidity and reduce cost. However, due to irregular particle shape and poor compatibility with the bulk of PVC resin, the amount of heavier calcium added is very low, and the color and appearance of profiles will be affected by the increase of the amount of heavier calcium added.
With the development of technology, most of them use ultra-fine light activated calcium carbonate, even nano-sized calcium carbonate, which not only increases rigidity and filling, but also has the effect of modification, but its filling amount is not unlimited, and its proportion should be controlled. At present, some manufacturers add calcium carbonate to 20-50 parts in order to reduce costs, which greatly reduces the physical and mechanical properties of profiles, resulting in the brittle phenomenon of profiles mentioned in this chapter.
2. Types and quantities of impact modifiers
Impact modifier is a kind of macromolecule polymer which can increase the total energy of rupture of polyvinyl chloride under 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 does not contain double bonds, and their weatherability is good. They are suitable for outdoor building materials. By blending with PVC, the impact resistance, processability, weatherability and weld angle strength of rigid polyvinyl chloride can be effectively improved within a certain range.
In PVC/CPE blends, the impact strength increases with the increase of CPE dosage, showing a S-shaped curve. The impact strength of the system increases very little when the content of the additive is less than 8 parts of mass; the maximum increase is when the content of the additive is between 8 and 15 parts of mass; and then the increase tends to be gentle. When the amount of CPE is below 8 mass fraction, it is not enough to form a network structure. When the CPE content is 8-15 mass fraction, it will disperse uniformly in the blend system and form a separated phase network structure, so that the impact strength of the blend system will increase the most. When CPE dosage exceeds 15 mass fraction, it will not form continuous uniform dispersion, but some CPE will form gelatin. There will be no properly dispersed CPE particles at the interface to absorb impact energy, so the impact strength increases slowly.
In PVC/ACR blends, ACR can significantly improve the impact resistance of the blends. At the same time, the "core shell" particles can be dispersed uniformly in the PVC matrix, PVC is a continuous phase, ACR is dispersed phase, dispersed in PVC continuous phase and interact with PVC, play the role of processing aids, promote the plasticization and gelation of PVC, shorten the plasticizing time, and have good processability. The influence of forming temperature and plasticizing time on notch impact strength is small, and the decrease of bending modulus of elasticity is also small. Hard PVC products modified by ACR have excellent room temperature impact strength or low temperature impact strength.
The experimental results show that the impact strength of ACR is about 30% higher than that of CPE. Therefore, PVC/ACR blend system is used as far as possible in the formulation, and when the content of CPE is less than 8 parts, the profile will be brittle.
3. Too many or too few stabilizers: The role of stabilizers is to inhibit degradation, or react with released hydrogen chloride, and to prevent the discoloration of polyvinyl chloride during processing.
The amount of stabilizer varies according to the type, but generally speaking, excessive amount of stabilizer will delay the plasticizing time of the material and make the material less plasticized when it is exported to the die. The molecule in the formulation system does not fully dissolve with each other, resulting in the unstable structure of the molecule. When the dosage is too small, the relative low molecular weight substances in the formulation system will be degraded or decomposed (or superplasticized), which will destroy the stability of the intermolecular structure of each component. Therefore, the amount of stabilizer will also affect the impact strength of profiles, too much or too little will cause the reduction of the strength of profiles and the brittleness of profiles.
4. Excessive use of external lubricants
External lubricant has low compatibility with resin, which can promote the sliding between resin particles, thereby reducing friction heat and delaying melting process. This effect of lubricant is greatest at the early stage of processing (that is, before the external heating effect and internal friction heat make the resin completely melt and the resin in the melt lose its recognition characteristics).
External lubricants can be divided into pre-lubrication, post-lubrication and over-lubrication materials, which show poor shape under various conditions. If the amount of lubricant is not appropriate, it may cause flow marks, low output, turbidity, poor impact, rough surface, adhesion, poor plasticization and so on. Especially when the dosage is too much, it will result in poor compactness and plasticization of profiles, resulting in poor impact performance and brittleness of profiles.
5. The order of hot mixing, temperature setting and curing time also have decisive factors on the properties of profiles.
There are many components in the formulation of PVC-U. The order of feeding should be helpful to play the role of each additive and to improve the dispersing speed, while avoiding the harmful synergistic effect. The order of feeding should help to improve the complementary effect of additives, overcome the effect of mutual elimination and make the additives dispersed in PVC resin fully enter the interior of PVC resin.
The order of formula feeding for typical lead salt stabilization system is as follows:
When running at low speed, PVC resin is added to the hot mixing pot.
B. Adding stabilizers and soaps at 60 C under high-speed operation;
C. Internal lubricants, pigments, impact modifiers and processing aids are added under high-speed operation at about 80 C.
D. External lubricants such as wax are added at high speed at about 100 C.
E. Filling is added at 110 C under high-speed operation.
F. The material is discharged to the cold mixing barrel at low rotational speed from 110 ~120 (?)
