What Make PVC Tube Become Brittle, Soft, Discoloration

June 8, 2018


There are many auxiliaries in PVC pipe processing, the most important thing is that the selection is wrong and it is not used correctly. We should look at the material used for the performance of the pipe and the prescription for the material performance, otherwise, the performance of the pipe will be reduced.


Classification and selection of PVC modifiers


PVC modifier is used as glass amorphous PVC modifier according to its function and modification characteristics. It can be divided into 6 groups:


High-efficiency impact modifier: used in opaque impact mixture.


Transparent impact modifier: use this modifier when optical and impact resistance are required.


Hot deformation modifier: used to increase the processing temperature range of PVC mixture.


General modifier: used to improve impact resistance, high temperature strength and low temperature flexibility.


Weather resistant modifier: used in outdoor application to prevent UV photodegradation.


Processing aids: by reducing the melting time to improve the melt properties of PVC.


1. High efficiency impact modifier


High efficiency impact modifiers such as ABS and MBS have synergistic effect on the improvement of PVC impact resistance. Therefore, adding a small amount of modifier to PVC can obtain high impact resistance and increase the flexibility of PVC, but not obviously change the mechanical properties of the mixture. The molecular weight of PVC determines the amount of impact modifier. The higher the molecular weight, the smaller the amount of modifier needed. The final use of the product determines the molecular weight required for the PVC mixture. For example, low-molecular weight PVC is best processed by living mold, while high-molecular weight PVC is processed by tube extrusion. Typical Application of High efficiency impact resistant modifier It is used for PVC pipe, injection mixture and embossing opaque film and sheet.


2 transparent impact modifier


Transparent impact modifiers can provide additional properties such as light propagation, light fog, and yellowness index in PVC mixtures as opaque modifiers. Low folding white and discoloration and other optical properties. In the process of preparing ABS and MBS modifier, the optical properties needed to maintain transparency were obtained by equalizing the refractive index of PVC and modifier. The impact resistance was obtained by controlling the particle size of the rubber matrix in the narrow distribution range of 1000-3000 A, and the compatibility / incompatibility balance was obtained by the solubility parameters of the grafted S- / an or MMA / S. Impact characteristics. Typical applications of this modifier include transparent calendering film, packaging sheet and blowing PVC bottle.


3 hot deformation modifier


The thermal deformation modifier can increase the effective heat use temperature of PVC. Each addition of a modifier may improve ITs. The addition of thermal deformation modifier to PVC can also increase the stiffness and minimize the influence on the tensile strength, but it often weakens the impact strength. These modifiers are usually composed of poly- α-methyl styrene / acrylonitrile (AMSAN) or glutarimide. For AMS polymers, the thermal deformation of PVC was increased due to the steric resistance of methyl groups attached to styrene. Because of its heterocyclic structure, glutarimide polymer can increase the stiffness of the polymer chain, thereby increasing the thermal deformation of PVC matrix. Thermal deformation The application of modifier include vinyl panel, heat-resistant profile and automobile instrument liner which need pressing fastness.


4 general modifier


General modifier is a kind of semi-hard modifier used in semi-rigid PVC mixture. It is a typical ABS modifier with less butadiene and more ungrafted fully rigid poly (S / AN). These modifiers have two phases of rigidity and rubber, which can make semi-hard mixture have many properties. Butadiene rubber phase can increase the cracking resistance at low temperature, and the high molecular weight rigid S / an has the processing properties such as thermal formability and good performance retention. The typical use of general modifier is automobile dashboard sheet, luggage ABS cover material and automobile profile.


5 weathering modifier


Weatherproof impact modifiers prevent UV photodegradation. Butadiene modifiers, like MBS and ABS, are not suitable for outdoor use unless the outer layer is protected against ultraviolet rays. In the double chain of butadiene, UV can break the unsaturated carbon chain skeleton and make the modification brittle by oxidation and other degradation reactions. Modifiers with strong anti-UV degradation have similar functions as MBS and ABS, but they have grafted phases of acrylate or 2-ethylhexyl acrylate, the polymer chains of which do not contain double chains, There is no site where the degradation reaction begins. This type of modifier is commonly called Acrylic acid system modifier, and mainly used for PVC wall panels, window profiles and other applications requiring weather resistance. In outdoor use, they have some impact resistance, but not as effective as ABS or MBS.


Another weatherproof modifier for outdoor use is CPE. The modifier is not very effective and the modification effect is not very good. Increase the toughness of PVC matrix through mechanisms similar to plasticizing (or a penetrating network).


6 processing aids


The melting and melt properties can be increased by adding processing aids to PVC blends. Typical processing aids are polymers with very high molecular weight, such as MMA / EA, styrene, MMA / Sr / an or S / A. Mainly used in PVC blends, the amount of PVC dry blends is generally 1 phr or less. Its function is to promote the mixture melting by increasing the friction between PVC and the metal inner surface of the mixing equipment. In PVC foaming, it is very effective to control melt viscosity because of high molecular weight poly (San) and poly (MMA / P / an). In the plastics industry, these different modifier divisions In order to choose the modifier correctly, each specific polymer should be correctly selected in various aspects such as impact resistance, fluidity, cost, stability and particle size control.


Properties and usage of PVC modifiers


At present, CPE is mainly used as impact resistant agent for PVC in China, while MBS and ACR (acrylates copolymer) are mainly used abroad. There are ABS (acrylonitrile / butadiene styrene copolymer) and EVA (ethylene vinyl acetate copolymer). MBS resin is a kind of impact modifier with excellent comprehensive properties, especially transparency and coordination.


The comparison is as follows:


Transparency: MBS > ACR


Processability: MBSU ACR > CPE > EVA


Compatibility: MBS > ACR > CPE > EVA


The impact resistance of MBS resin is better than that of EVA, but inferior to that of ACR-CPE and ABS. Weatherability MBS and ABS near, but inferior to the other three. The optical and thermal stability of MBS was similar to that of ABS, but lower than that of ACR. And CPE. It should be emphasized that MBS resin plays an important role in the processing of transparent PVC resin products, which can not be replaced by .MBS resin, which can increase the impact strength of PVC by 5-6 times, and has little effect on tensile strength and elongation.


PVC impact modifiers sometimes also play a role in plasticizing, so can also be seen as plasticizers. The impact modifiers used in PVC resin are as follows:


(1) CPE


At present, CPE is the dominant impact modifier in China, especially in the production of PVC pipes and profiles, most factories use CPE. The dosage of CPE is 5-15 phr. CPE can be used with other toughening agents, such as rubber EVA, the effect is better, but rubber additives can not resist aging.


Selection of CPE impact modifier


1) domestic CPE models are generally identified by such as 135AZ140BX 239C, in which the first digit numbers 1 and 2 represent the residual crystallinity (TAC).) the size of 1 represents a TAC value of 0 to 10 and a value of 2 indicates a TAC value greater than 10, and the second digit number 3 indicates the chlorine content. For example, 35 indicates that the chlorine content is 35 and the last one is the letter ABC, which is used to indicate that the molecular weight of PE is maximum and C is the smallest.


2) the influence of molecular weight on the molecular weight of type A compound of CPE is the largest, and the melt viscosity is also large. Its viscosity is the most suitable for PVC, and it has the best dispersion effect in PVC, which can form an ideal network form of dispersion. Therefore, the type A compound of CPE is generally chosen as the modifier of PVC.


3) TAC value: the TAC value indicates the PE crystallization and the amorphous content in the CPE molecule, which reflects the uniform distribution of the chlorine atoms on the CPE molecule, the TAC value is large, which indicates that the PE chain segment is more crystalline, and the PE chain segment is poor in compatibility with PVC, and the TAC value is less CPE than that of PVC, and the selectivity is less than 5 as an anti shock modifier.


The grade of CPE is not uniform, there are two kinds of poor quality CPE, one is because some factories do not have the technical conditions and the chlorination process is backward. The other is directly mixed with calcium carbonate or talc powder in pure CPE for unfair competition. In the processing of PVC, the use of these inferior CPE as modifiers for PVC processing, in order to enable PVC products to achieve the purpose of impact and toughening, the amount of CPE is bound to increase, which will inevitably lead to the increase of plasticizing time and the decrease of equilibrium torque. Thus, the time to achieve balance torque is increased. All of which will result in a deterioration in the overall performance of the product, The rate of waste has increased. Therefore, the use of chea