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Modification method for PVC toughening and attack resistance

Elastomer toughened PVC:

In the field of elastomer toughening, two kinds of toughening agents have been developed, one is NBR, TPU, EVA, CPE, etc., which are characterized by being able to surround PVC primary particles to form a network structure, which is called a network polymer toughening agent; The other type is ABS, MBS, ACR, etc., which are present in the island (particle) phase in the PVC matrix and are referred to as discrete elastomer toughening modifiers.

1.1 Toughening mechanism of elastomer:

There are many theories for the modification of elastomers, and the following are mainly the following two types:

(1) Silver-shear band theory. It is shown that in plastic and rubber composites, shear yielding and silver streaking exist simultaneously. When the material is stressed, the modified particles act as a stress concentration body to induce a large amount of silver streaks and shear bands, thereby absorbing energy to achieve the purpose of toughening. At the same time, the elastomer particles and the shear band can terminate the silver streaks and prevent them from expanding into cracks, which improves the impact properties of the material [2]. This stress concentration can be attributed to the large difference in elastic modulus between the modifier particles and the polymer matrix.

(2) Elastomer impact modifiers absorb energy by extension or cavitation self-deformation [3].

In one aspect, the core-shell polymer can bridge crack growth;

On the other hand, the high elongation makes it difficult for the modifier to completely break with the matrix resin, and the formed crack remains as it is. At the same time, the cavitation causes the stress concentration to cause the shear band.

1.2 Factors affecting the toughening of PVC by elastomer

(1) Effect of the elastomer: The lower the glass transition temperature of the elastomer, the better the toughening effect.

(2) Effect of toughening agent content: The type of toughening agent is different, but the dosage is different, but it is about 5% to 20%.

(3) Effect of domain size of elastomer: According to the variety of PVC and elastomer, the optimal domain size is also different.

(4) Effect of PVC inclusions in elastomer particles: A suitable inclusion content will give the material an optimum impact strength.

The toughening impact modifiers that we have heard in the PVC industry are CPE, ACR, MBS, EVA, nitrile rubber, SBS rubber, etc., but we usually use CPE, ACR, MBS, nitrile rubber in actual production. At present, the most cost-effective is CPE, other low-temperature impact resistance is good, impact ACR weather resistance and impact resistance are the best, MBS, SBR weather resistance is poor.

The toughness of PVC products is poor. First of all, the reason for the overall formulation is to find that the composite stabilizer or excessive lubricant will cause poor toughness and impact resistance. The problem of toughening impact modifiers can be found by eliminating the formulation. Soft products have poor toughness. The general reason is that the filler is added too much, or the plasticizer is too small, and the processing temperature is too low. Generally, as long as the components in the formulation are suitable, the toughness is good. Hard PVC products have poor toughness, generally caused by excessive stabilizer or lubricant, resulting in poor plasticization, or the amount of toughening agent or improper selection.

MBS toughened modified PVC

MBS is a thermoplastic elastomer that combines the properties of plastics and rubber. It has good compatibility with PVC and is a good PVC toughening agent. It was first developed and researched by Rohm & Haas. Zhang Ying [5] of Nankai University and others mixed MBS with PVC (SG5, SG7, TH400). When the MBS content is 12%-20% of PVC, the notched impact strength of the blend is 8~ of pure PVC. 10 times. However, when the MBS content exceeds 20%, the impact strength decreases due to the phase reversal. When the MBS content is 12% of PVC (TH400), each performance index is optimal. Song Moudao of Nankai University and others used MBS as a compatibilizer in the PVC/SBS system. It was found that MBS not only has a good compatibilizing effect, but also has a synergistic toughening effect on PVC with SBS. Shen Jingwei of Sichuan University [7] and others used CPE/MBS composite system to toughen rigid PVC, suitable

The toughening agent composition ratio (RC is 60% to 80%) and suitable processing conditions, CPE / MBS synergistic toughening effect. It can be considered as a result of the synergy between the network toughening of CPE and the particle toughening mechanism of MBS. Peng Xiaotong [8] and others discussed the CPE/MBS system from the processing aspect and found that it is suitable to use this system for the production of injection products.

ACR toughened modified PVC

ACR is made by grafting methyl methacrylate onto an acrylate (ethyl ester, butyl ester or octyl ester) molecule. The most typical impact modifier ACR is a polybutyl acrylate crosslinked elastomer as a core, and the outer layer is grafted with a methyl methacrylate-ethyl acrylate polymer to form a copolymer having a core-shell structure. The shell layer has good compatibility with PVC, and its core plays a toughening role in the blend. The PVC/ACR composite is excellent in impact strength, flame resistance, dimensional stability, weather resistance, and weldability. Therefore, since the US Rohm & Haas Company first developed the successful ACR, its production growth is much faster than CPE and EVA. Liu Minjiang and others found that with the increase of ACR addition, the impact strength of rigid PVC materials gradually increased, especially in the range of 5-10 parts, the impact strength changed the fastest, while the maximum value of 10 parts reached the highest value, but ACR Addition reduces the tensile strength and hardness of HPVC. Chen Xiaomei et al. showed that when the ACR dosage is less than 8 parts, the cross section of the sample is “cloudy” structure, which is brittle fracture. Under the processing temperature of 176 ° C and the degree of gelation of 72.7%, PVC/ACR/CaCO3 (via surface) The maximum impact strength is obtained when the ratio is 100/8/5.

CPE toughened modified PVC

CPE (chlorine content of 30% to 40%) is a commonly used impact modifier for rigid PVC, and its application technology is relatively mature. It has many advantages in structure as an elastomer [18], thus giving PVC a good overall performance. The toughening effect of CPE is related to the molecular parameters of CPE (such as molecular mass, chlorine content and its distribution, residual crystallinity, etc.). He Peixin [19] and others reported that the solid phase CPE with a chlorine content of 36% to 42% has a better toughening effect on PVC, and CPE with a chlorine content of about 39% has the best toughening effect on PVC. Zhang Weihu [20] and others reported that the best effect was obtained when the amount of CPE was 8-16 parts. The increase of CPE dosage improved the performance of the blend, but the tensile strength and heat resistance decreased. In order to solve the problem that the delamination and low-temperature impact strength of the product are not ideal when the amount of CPE is large, Yang Jian [21] and others studied the CPE of different chlorination degree and the CPE-g-VC and PVC blending materials with different CPE dosage. At normal temperature impact performance, it is known that the increase in CPE-g-VC content can improve the impact properties of PVC, and the CPE-g-VC/PVC blend system has a microscopic network structure. Chen Jun [22] and others began to improve the compatibility of CPE-PVC. On the one hand, CPE-PVC (CV) graft copolymer was used instead of CPE, and on the other hand, CV/CPE composite toughened PVC. Shen Jingwei et al. used CPE/ACR and CPE/MBS composite toughened PVC. When the composition ratio of toughening agent is 60%, it can be considered that CPE and MBS or ACR play a synergistic toughening effect. Fang Shaopeng [23] et al. used CPE as a compatibilizer to modify PVDF (polyvinylidene fluoride) toughened PVC. It is considered that PVDF/CPE ratio (2.4-3.0)/1 has significant toughening synergistic effect. When the amount of PVC/PVDF/CPE is 100/12/5, the notched impact strength of the blend system is 29.85kJ/m2, which is 4.4 times that before unmodified. Tao Guoliang [24] and others used fine rubber powder to toughen PVC/CPE system, and found that rubber powder and CPE have synergistic toughening effect on PVC materials.

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