introduction
Polyvinyl chloride (CLEAR PVC) is the one with the largest output, the most versatile and the lowest price among the general large-variety plastics. According to different application requirements, it can be divided into hard products and soft products. It is widely used in the manufacture of plates (sheets), pipes, profiled materials, leather, film, and various decorative materials. Some products, such as hoses, sheaths, films and shoe materials, still need to be made of soft transparent CLEAR PVC. The formulation of soft CLEAR PVC transparent material has been reported in the literature. Commonly used compounding agents in the listed formulations are generally heat stabilizers such as organotin, antimony mercaptan and phosphate esters, auxiliary stabilizers such as epoxy soybean oil and epoxy resin, stearic acid lubricant and MBS modifier Wait. The effects of these compounding agents on the short-term and long-term transparency of soft transparent CLEAR PVC products and the changes in transparency after thermal oxygen and light aging are still lacking in sufficient research.

Processing technology and performance test conditions
CLEAR PVC and plasticizers and other various compounding agents are kneaded uniformly in a high-speed mixer, and the plasticizing well is kneaded uniformly on an open mill at about 140°C. The flat vulcanizing agent is used to compress test pieces with a thickness of 1 to 2 mm under the conditions of 170°C×10min×15MPa, and pressure cooling. Or the kneaded material is shaken and extruded on the extruder to extrude a 0.2mm thick sheet. The extruder's standby temperature is 140, 170, 175℃, and the head is 180℃. The speed is 30r/min. The light transmittance is measured on the 751G spectrophotometer, and the visible light wavelength range is 400~600nm (taking air as 100% light transmittance as reference). The thermal aging experiment conditions were 100℃ ×24, ×4 B, and ×96h, respectively. The weather aging condition is that the sample is exposed to the sun for a cumulative number of days of 80 days.

Results and discussion
1. Stearic acid
Stearic acid is a common lubricant in CLEAR PVC processing. According to literature reports, the addition amount in soft transparent CLEAR PVC products is generally 0.3 to 0.5 parts. In some documents, stearic acid is added in more than 1 part. This article examines the effect of stearic acid on transparency shown in Figure 1. It can be seen that in the formula with 0.4 parts of stearic acid, the transparency of the material tends to decrease. The basic formula is CLEAR PVC100, DOP30, DBP5, epoxy soybean oil 3 and 2.5 parts mercaptan methyl saw. Quqian 2 is 0.4 parts stearic acid added on the basis of curve 1 formula, curve 4 is 0.4 stearic acid added on the basis of curve 3 formula. Compared with the two sets of curves, the degree of decrease is roughly the same, and there is no doubt that the decrease in transparency is caused by stearic acid. In addition, we found that the surface of the transparent sheet added with stearic acid had blooming phenomenon after the transparent sheet was placed for a week.

2. Dibutyltin dilaurate
In soft transparent CLEAR PVC products, dibutyltin dilaurate is generally used as the main stabilizer, which can be used alone or in combination with other auxiliary stabilizers. As shown in Figure 2, when used alone, its dosage has a greater impact on transparency. When the dosage of dibutyltin dilaurate is small, its transparency will gradually increase with the increase of dosage, but when the dosage of dibutyltin dilaurate is 4.0 parts, its transparency will suddenly and greatly decrease. Therefore, the amount of dibutyltin dilaurate in the formulation of soft transparent CLEAR PVC products should be appropriately controlled. In general, dibutyl tin dimonthly silicate is often used in conjunction with auxiliary stabilizers, and the effect is better. We selected epoxy soybean oil and epoxy resin and used them together, and the results are shown in Figure 3. It can be seen that the stable system used in combination with epoxy soybean oil is better than the stable system used in combination with epoxy resin.

3. Epoxidized soybean oil
The effect of epoxy soybean oil alone as a heat stabilizer on transparency is shown in Figure 4. Epoxidized soybean oil itself is slightly yellow. It can be seen from Figure 4 that when 1 part of epoxy soybean oil is added, its light transmittance is significantly reduced. When adding 3 parts, its light transmittance is close to the blank, and when adding 4 parts, its transparency is the best. Continue to increase the amount of epoxidized soybean oil, and its light transmittance tends to decrease. The reason may be related to the thermal stability of epoxidized soybean oil and its own pollution. When a small amount of epoxidized soybean oil is used, the thermal stabilization effect is not obvious. Over time, its own pollution tends to prevail. Therefore, the dosage of epoxidized soybean oil should not exceed 4 parts. Comparing the results of Figure 4 and Figure 2, it can be seen that the effect of epoxidized soybean oil when used alone is worse than when used in combination with dibutyltin dilaurate.

4. Comparison of methyl tin mercaptan and dibutyl tin dilaurate
The main stabilizers were methyl tin mercaptan and butyl tin dilaurel, and epoxy soybean oil and epoxy resin were used as auxiliary stabilizers to study their effects on transparency. It can be seen from the results in Figure 5 that the combined effect of using dibutyltin dilaurate as the main stabilizer and epoxy soybean oil and epoxy resin as auxiliary stabilizers is significantly better than that of methyl tin mercaptan and epoxy resin respectively. Combination system of soybean oil and epoxy resin. In addition, for dibutyltin dilaurate, the compounding effect with epoxy soybean oil is obviously better than that with epoxy resin. The opposite is true for methyl tin mercaptide.

5.MBS
MBS is a ternary graft copolymer of methyl methacrylate (M), butadiene (B) and styrene (S). It is often used as a modifier for rigid transparent CLEAR PVC products. The research situation of MBS in soft transparent CLEAR PVC products is rarely reported. Comparing the curves 1 and 2 and 3 and 4 in Fig. 6, it can be seen that when 5 parts of MBS are added, the transparency is reduced. However, the 80-day sun aging experiment shows (compare 1 and 1', 2 and 2', 3 and 3', and 4 and 4', respectively. After light aging, the transparency of the samples with MBS increased, but no The light transmittance of MBS samples all showed a downward trend. Why the transparency of MBS samples increased after light exposure needs to be further explored.

6. The effect of thermal aging on transparency
The thermal aging test results of epoxy soybean oil and dibutyltin dilaurate as the stabilizer of soft CLEAR PVC transparent sheet are listed in Table 1. Without adding any stabilizer blank formula, after 100 ℃, 24, 48 and 96h thermal oxygen aging, its transparency gradually declines. When adding 4 parts of epoxy soybean oil, or adding 3 parts of dibutyltin dilaurate, the effect of heat aging tends to be constant, and the transparency reaches the maximum respectively. It can be seen that in soft CLEAR PVC transparent products, it is also feasible to use epoxy soybean oil or dibutyltin dilaurate separately as stabilizers.

in conclusion
In soft CLEAR PVC transparent products, dibutyltin dilaurate and epoxy soybean oil are better transparency stabilizers, which can be used alone or in combination, and the combined effect is better. The addition of MBS will reduce the transparency of soft CLEAR PVC, but the long-term light weather resistance is better. Stearic acid will reduce the transparency of soft CLEAR PVC, and it is easy to bloom, so it should be used with caution in soft products.