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 cheap but poor quality CPE to reduce or increase production costs, the results are self-evident!

 

(2) Active Cavity Radiometer

 

Alias: PVC processing aid, PVC solid polymer plasticizer

 

Main ingredients: methacrylate, acrylate, etc.

 

Properties: this product is white powder, viscosity 0.2-13.5 ~ 40 mesh sieve rate > 99.0, apparent density 0.35-0.55, volatile matter < 1.5

 

Functions:

 

1. It has good compatibility with PVC, good dispersity, intertwined with PVC molecular chain, promotes PVC melt plasticization, effectively reduces the melting processing temperature of PVC, and improves the weathering resistance of the products on the basis of small energy saving;

 

(2) the rheological behavior of PVC melt can be changed, and the fluidity of PVC material can be improved, thus it is easier to form and extrude, and the stability of long processing molding is guaranteed.

 

(3) it can improve the melt strength of PVC material, avoid melt fracture, solve the surface problems such as shark skin, and improve the internal quality and surface glossiness of PVC products.

 

4, it can effectively prevent pressure fluctuations and flow marks caused by extrusion and injection molding in extrusion and injection molding, and effectively avoid surface problems such as ripple and zebra line.

 

(5) the surface gloss of the product can be improved because of the plasticization uniformity, and the mechanical properties such as tensile strength, impact strength, elongation at break and so on can be improved at the same time.

 

6, PVC products can significantly reduce all kinds of additives such as stabilizers, pigments, calcium powder and other in-process surface deposition.

 

(7) foaming regulator can adjust the density and size of foam effectively, increase the melt strength of PVC material, effectively wrap the foaming gas, form a uniform honeycomb structure, prevent the gas from escaping, and decrease the density of the product.

 

8, good metal peeling, because belongs to the high-molecular material, does not cause the precipitation and so on like the lubricant.

 

Classification:

 

According to the viscosity, it can also be divided into: low viscosity, medium viscosity, high viscosity; low viscosity 0.2-4.0 and transparent film, sheet, etc; medium viscosity from 4.0 to 11.0, generally used for profile, pipe fittings, etc. High viscosity in the general viscosity of 11.0-13.5, generally used in foaming boards, foamed wood and plastic.

 

According to their functions, they can be divided into common processing aids (for example, HF-401HF-401H175C20801, etc.) and foaming regulators (HF-9090192921, etc.); in China, foaming agents generally refer to acrylic processing aids with viscosity higher than 10.0 or even higher than 11.0, with a reference dosage of 4.0-12.0 or more. General processing aids include foaming regulators, lubricating processing aids, general processing aids, reference dosage of 0.5-2.5 phr.

 

The products can be used with stabilizers, internal, external lubricants, fillers and pigments, with better synergy and no adverse reactions. The performance of the products used in the dosage is obviously improved, the effect is not obviously improved, even the melt strength is too large, the processing fluidity is worse, and the product yellowing.

 

(3) Mutual Broadcasting System

 

MBS resin is a terpolymer of methyl methacrylate, butadiene and styrene, which is mainly used in the processing of PVC and improves its impact strength without damage to the inherent characteristics of PVC resin. The transmittance of products can reach 90%. Transparency and UV aging resistance are its outstanding advantages.

 

MBS is a toughening agent for core-shell structure. The outer shell is MMA, the inner core is rubber, the shell: compatible MMA: good compatibility with the applied matrix, the core: rubber (butadiene rubber is used for absorbing shock, the product is white amorphous powder, the particle is uniform, It has good chemical stability and thermoplasticity, insoluble in water, ethanol and swelling in butanone and chloroform.

 

As one of the most important impact modifiers for PVC, MBS has a solubility parameter of 94-9.5, which is close to the solubility parameter of PVC, so it has good compatibility with PVC. The biggest characteristic of MBS is that it can be made into transparent products after adding PVC. Generally, the impact strength of PVC can be increased 6-15 times by adding 10-17 phr of PVC, but when the amount of MBS is more than 30 phr, the impact strength of PVC decreases. MBS itself has good impact property, good transparency, and the transmittance of PVC can reach 90% or more. At the same time, other properties of the resin, such as tensile properties, were improved. Strength, elongation at break, etc. The price of MBS is relatively high. It is often used with other impact modifiers, such as EAV / CPE / SBS, etc., which has poor heat resistance and weathering resistance, and is not suitable for long-term outdoor use. Generally do not do plastic doors and windows profile production impact modifier use.

 

Mbs commonly used brand: Rohm Haas 2620, Korea LG EM500, Japan Zhong Yuan M 521 701 and so on.

 

(4) anti-skid brake system.

ABS is styrene (40%-50%), butadiene (25% - 30%) and acrylonitrile (25%-30%) three terpolymer. It is mainly used as engineering plastics. It is also used as PVC impact modification. The effect of low temperature impact modification is also good. When.ABS is added to 50 copies, the impact strength of PVC is generally 5 to 20 copies of the equivalent.ABS of pure ABS, and the weatherability of ABS is poor. Suitable for long term outdoor use products, generally do not use as impact modifier produced by plastic door and window profiles.

 

(5) EVA

 

Eva is a copolymer of ethylene and vinyl acetate. The introduction of vinyl acetate changes the crystallinity of polyethylene. The content of vinyl acetate is very poor, and the refractive index of EVA and PVC is different, so it is difficult to obtain transparent products. EVA and other impact resin are often used to add less than 10 phr EVA.

 

In addition, rubber impact modifiers are also excellent toughening agents. The main varieties are: EPP, EPDM, NBR and SBR, natural rubber, cis-butadiene rubber, Chloroprene rubber, polyisobutylene. Butadiene rubber, among which EPRP EPDMN NBR is the most commonly used, is to improve low temperature impact resistance, but not aging resistance, plastic door and window profiles generally do not use this impact modifier.

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