Discussion on the hottest polyester bottle and its

2022-08-18
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Discussion on the quality of polyester bottles and their raw materials

polyester bottles are more and more widely used in the field of food and beverage packaging. The annual production capacity of domestic bottle polyester is also growing rapidly. Many fiber polyester production lines are equipped with solid-phase viscosity increasing devices to convert to bottle polyester. However, the quality of domestic bottle polyester is uneven, and the national standard of polyester bottle has just been launched. The trend of higher and higher terminal requirements for bottle polyester application is irreversible. Therefore, to improve the quality of bottle polyester, endow it with new properties and meet the new application needs of bottle polyester has become an important work for bottle polyester manufacturers to establish core technology and compete for market share

the scope of this paper involves conventional bottle polyester and polyester bottle quality indicators and internal quality characteristics

1 acetaldehyde (AA) content

acetaldehyde content in polyester bottles used for food and beverage packaging is strictly controlled. Water bottles have higher requirements for acetaldehyde content than carbonated beverages. For example, Coca Cola company requires its mass fraction to be 4 × Below 10-6. The acetaldehyde content measured in polyester is not the reason why the acetaldehyde content in the final bottle exceeds the standard. The real source of acetaldehyde in polyester bottles is the thermooxidative degradation of polyester caused by high temperature and screw cutting during the injection molding process, which is recommended by many industry experts. For example, the acetaldehyde mass fraction in polyester chips is 1 × Below 10-6, after injection molding into bottle blank, it rises significantly to (15 ~ 25) × 10-6, see Table 1

Table 1 change process of acetaldehyde mass fraction

Table 1 AA ma ss fraction changing course 10 - 6

samples enter the heater before solid-phase polycondensation, enter the reactor, and exit the reactor after billet injection

165 2 37. 9 5. 25 0. 92 16. 2

2 60. 8 27. 4 4. 46 0. 62 17. 8

3 53. 2 25. 3 2. 64 0. 53 22. 3

by reducing the screw temperature and back pressure, the acetaldehyde content in the injection part (bottle blank) can be effectively reduced. As shown in Table 2, the AA mass fraction in the bottle blank is increased from 9 nineteen × 10-6 to 7 seventy-two × 10- 6。

Table 2 acetaldehyde content under different process conditions

table 2 AA ma ss fraction under D ifferen t technology cond ition

process screw temperature/℃ screw speed/(R · min - 1) back pressure/mpa injection pressure/mpa cycle/s cooling temperature/℃ cooling time/s w (AA)/10 - 6

1 2956430 ~ 3580 ~ 8523 84 8~10 6 9. 19

2 290 62 28~32 80~83 23. 64 8~10 6 7. 72

note: the injection molding machine used: husky xl300 48 mold, and the bottle capacity is 1200 ml

polyester users should pay attention to the dust in chips. The accurate measurement method of dust is the so-called wet method, but the wet operation is very cumbersome and requires special instruments. Polyester manufacturers usually use dry method to measure dust, so the actual dust content is much higher than the index value. The filter in the dry air pipeline should be cleaned regularly, and some manufacturers have also added a dust collector, which can finally reduce the dust mass fraction in the slice to 10 × Below 10-6

dust is caused by polyester granulation and transportation process. Experimental data show that the existence of oxygen during viscosity increase forms peroxide on the resin surface, which is further oxidized during post-processing melting to produce acetaldehyde, especially in chip dust

the presence of acetaldehyde will also aggravate the reduction of antimony trioxide (Sb2O3) in polyester, make the resin gray, and cause the transparency of polyester bottle to decrease

acetaldehyde is the product of thermal oxidative degradation [1]. Stabilizer and antioxidant are added to polyester in the synthesis process, which helps to reduce the final acetaldehyde content

Wu Rongrui et al. [2] pointed out that the thermal oxidative degradation of pet can be inhibited by adding phenolic antioxidants, and the stabilizer phosphite and its esters can be used as hydrogen peroxide decomposition agents in pet macromolecules. Under certain conditions, some transition metals such as cobalt, manganese, copper, etc. also have the effect of inhibiting oxidation

the special polyester for low acetaldehyde water bottles developed by Zhuhai Yuhua polyester Co., Ltd. adds a special but low price acetaldehyde inhibition additive, which can meet the requirements of Nestle bottled water

2 heat resistance of polyester bottle

hot filling bottle is used for filling at high temperature (such as 88 ℃ and 93 ℃). The difference between hot filling bottle and ordinary polyester bottle is that hot filling bottle must undergo heat setting treatment to improve the crystallinity of all parts of the bottle. The crystallinity should generally be above 32%, and the glass transition temperature increases with the increase of crystallinity. If the crystallinity is below 28%, the hot filling temperature should be lower than 80 ℃

the crystallization speed of polyester chips from different manufacturers is different, which is reflected in their different thermal crystallization peak temperature and melting peak temperature, and their degree of supercooling and overheating are also different. The hot mold temperature and sticking time during bottle blowing and the heating temperature and time of crystallizer should be adjusted according to the above characteristics

the method to test the heat resistance of hot filling bottle is to inject hot water into the bottle at high temperature and measure its shrinkage. Different manufacturers have different detection methods for the shrinkage of bottles. The volumetric shrinkage is generally set at less than 3%, and the appearance cannot be deformed, meeting the requirements of top pressure and side pressure. After the bottle is stored for one month, it needs to be tested again. At this time, the volume shrinkage rate will increase, but it still cannot exceed 3%

bottle blanks should be placed for at least 48 hours after injection molding before bottle mouth crystallization and bottle blowing, otherwise it is very easy to cause insufficient crystallinity of bottle mouth and bottle body, affecting the dimensional stability and heat resistance of bottles. In northern China, because the weather is quite dry, the bottle storage environment also requires humidification to facilitate crystallization

the storage temperature and humidity of bottles have an impact on the heat resistance of bottles. The higher the temperature and humidity, the faster the heat resistance of bottles will decline, and the shorter the storage time

$page break $3 transparency of polyester bottle

the thicker the bottle blank, the harder it is to make a bottle with good transparency. This is because the farther the preform is from the surface layer, the slower and weaker the cooling speed is. Therefore, the more crystals are formed, the larger the crystals are, and the transparency decreases. The crystallization speed of polyester from different manufacturers is different, which will cause the transparency difference of bottles

the types and methods of catalysts and stabilizers added during polyester synthesis have a significant impact on transparency. The transparency of polyester using Sb2O3 as catalyst is obviously worse than that using germanium catalyst

polyester users should use chilled water instead of cooling water in the injection molding process, and the water temperature should be kept below 10 ℃. Some users directly use tap water to cool the mold, and the bottle blank cannot achieve the due transparency. However, if the cooling water is used, there is no need to require an air-conditioning environment. If the chilled water is used, the air-conditioning environment is required. Otherwise, there will be condensate on the mold surface, which will affect the bottle blank and bottle transparency, and even seriously affect the appearance. As mentioned above, the presence of acetaldehyde will also aggravate the reduction of antimony trioxide (Sb2O3) in polyester, make the resin gray, and reduce the transparency of polyester bottles

prolonging the cooling time of the bottle blank without affecting the production efficiency is helpful for its transparent spring tension and compression testing machine, spring change testing machine and spring fatigue testing machine. The industry standard is higher than the national standard

the author's research on a 20 l gap experimental device shows that using a composite catalyst with low antimony can improve the transparency of products (bottles)

Table 3 test results of light transmittance and color value of polyester bottle

sample 1 sample 2 sample 3

catalyst antimony trioxide antimony acetate composite catalyst

L value 8382 5 87

a value 3 2 - 0. 05 1. 5

b value 0 8 0. 620

light transmittance/% 84 5~88. 7 85. 0~88. 6 86. 3 ~ 90

4 diethylene glycol content

bottle polyester manufacturers generally do not provide users with diethylene glycol (DEG) content test results, but the DEG content is an index that has an impact on downstream processing

deg content has different effects on different users. For spinning users, DEG affects the dyeing performance of fabrics. If the DEG content is too high, the fabric will be dyed too deep, and if the DEG content is too low, the fabric will be dyed too light. For bottle users, DEG affects the fluidity of polyester melt in the injection molding process, and the melt with higher DEG has better fluidity. The flow property of melt will affect the internal stress of bottle blank, so the DEG content of polyester is indirectly related to the internal stress of bottle blank after molding, but too high DEG content will reduce the strength of products. DEG is formed by etherification reaction during polyester production. Generally speaking, in the esterification stage, lower esterification temperature and lower esterification pressure can reduce the formation of DEG, but the end carboxyl group will also increase. DEG content is an index that is difficult to control in the process of polyester production, and sometimes even out of control. Therefore, for downstream users, the emphasis is still on stability. Most of the 5th issue Zhuo junqian: Discussion on the quality of polyester bottles and their raw materials. The DEG mass fraction of 11 manufacturers is around 1%

deg content has a significant effect on the melting point of polyester, 0 The influence of 1% mass fraction is up to 0 42 ℃, which will also affect the screw temperature during processing

the author's research on a 20 l gap experimental device shows that sodium acetate (CH3COONa) or lithium acetate (CH3 cool I) has a significant inhibitory effect on the generation of DEG in polyester, as shown in Table 4. In Experiments 2 and 3, sodium acetate and lithium acetate were added respectively, resulting in a significant decrease in the content of DEG in the samples

Table 4 inhibitory effect of sodium acetate or lithium acetate on DEG

formula experiment 1 Experiment 2 experiment 3

pta/g 4000 4000

eg/g 19401 9401 940

w (CH3COONa)/10 - 6 - 280

w (ch3cooli)/10 - 6 - 250 -

sb2o3/g 1 2 1. 2 1. 2

Co (CH3COO) 2/g 0. 12 0. 12 0. 12

H3 PO4/g 0. 486 0. 486 0. 486

W (DEG)/% 4 in the sample 3 1. 46 1. 40

$page break $5 control of the amount of isophthalic acid (IPA)

the important role of adding IPA to bottle polyester is to slow down its crystallization speed and widen its operation window in post-processing, as shown in the area surrounded by the middle of the two shadows in Figure 1. With the increase of the amount of IPA, the operable temperature range of the bottle blank (that is, blow a transparent and qualified bottle) gradually expands

however, the increase in the amount of IPA means that the amorphous areas in the products increase, which are easier to relax and shrink than the molecular chains in the crystalline areas, and their moisture absorption capacity is relatively strong. The role of water can be understood as a plasticizer, which intensifies the relaxation and shrinkage behavior of the amorphous region with the extension of storage time, which is why the heat resistance and stress cracking resistance of polyester bottles will decline significantly with the extension of storage time. Therefore, the correct amount of IPA is to achieve a balance between the two, which often becomes the core technology of major bottle polyester manufacturers

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