1.1 Preface
Polyvinyl chloride (PVC) plastics have excellent performance and low price. They are widely used, accounting for 40% of the total plastic consumption. They are the second-class plastic products in the world after polyethylene. However, since the molecular structure is unstable to heat, dehydrochlorination degradation reaction is likely to occur at temperatures above 100 ° C. The higher the temperature, the longer the heating time, and the more serious the degradation phenomenon. The characteristic of PVC degradation is that the color changes from white to pale pink, yellow, brown, and finally to black, and the physical properties of the products are degraded. One of the ways to improve the thermal stability of PVC is to add appropriate stabilizing additives to its processing formula to prevent or slow the thermal degradation of PVC during processing. In addition, it can prevent the light products from being light, heat, and during use. Oxygen-induced damage, maintaining its physical properties.
There are many types of heat stabilizers, and the classification methods are different. According to their chemical composition, there are mainly lead salts, metal soaps, organotins, rare earths, composites, organic and inorganic stabilizers.
Prior to the 1930s, PVC resins did not have much use due to the lack of effective heat stabilizers. Lead salt stabilizers are the first effective stabilizers developed by people. Lead salt stabilizers mainly refer to salt-based inorganic and organic acid lead salts, such as monobasic and tribasic lead sulfates, dibasic lead carbonates, dibasic lead phosphites, and dibasic bases. Lead stearate, etc., the main characteristics of such stabilizers are good thermal stability, excellent electrical insulation, low water absorption and a certain degree of lubrication, and good outdoor weatherability. The main disadvantages are poor transparency, poor initial hue, easy cross-contamination, toxic, and “frosting†in outdoor use. However, due to its low price, it has occupied most of the market for PVC heat stabilizers.
Cadmium salt stabilizer is a metal soap stabilizer, mainly cadmium stearate. The cadmium soap stabilizer is called primary stabilizer, which promotes the saponification reaction of PVC stearic acid, and the formed by-product cadmium chloride will deposit. In PVC, PVC is unstable, and even blackening causes bridging. Therefore, it is generally not used alone, and barium soap is often added to reduce the formation of precipitates. A compound system such as Cd/Ba, Cd/Ba/Zn, Cd/Ba/Pb or the like is formed. Cadmium salt stabilizers are widely used in the use of transparent products because of their small coloration, excellent transparency, good light and heat resistance, and certain self-slip properties. However, its shortcoming is that its long-term stability is poor, it is often completely degraded with little or no signs, and cadmium is highly toxic.
At present, under the pressure of the global green peace movement, countries around the world have paid more and more attention to the environmental pollution caused by PVC production and processing. The United States and Western European countries also abandoned their various production links, additives used in processing and PVC. The handling of the material has established strict regulations. Due to the toxicity of lead and cadmium stabilizers and their great harm to the environment, countries around the world have introduced new measures to restrict or prohibit the use of lead and cadmium stabilizers. For example, only PVC products with lead content less than 2×10_4 are allowed in the United States. The market, while the PVC industry in the EU is under pressure from the government and the public, promised to stop the use of cadmium stabilizers from March 1, 2001, and the use of lead will also be reduced. So far, the market share of lead and cadmium stabilizers has been decreasing.
1.2 Toxicity of lead and cadmium compounds
Lead compounds are one of the typical toxic heavy metal compounds. As early as 1976, the US Environmental Protection Agency (EPa) published 129 priority-controlled pollutants, and lead was on the list. In China, 52 kinds of toxic chemicals identified as priority control after screening and reviewing about 10,000 kinds of chemicals, lead ranked 27th with a comprehensive hazard score of 2.634. The State specifies the maximum allowable concentration of hazardous substances in the workshop in the "Design Regulations for Occupational Safety and Hygiene of Plastic Products Processing Enterprises" (GB-1532-92), lead smoke 0.03mg/m3, lead dust 0.05mg/m3.
Lead compounds mainly enter the body through the respiratory tract, digestive tract and skin, and then spread to other tissues through the blood, mainly concentrated in bone tissue, accounting for about 80-90% of the total. In addition, the content in tissues such as liver, kidney, and brain is also high, and these tissues are caused to have lesions. Although the amount of lead taken up by the mouth is only 10% or less absorbed by the digestive tract, once it is absorbed, it has a cumulative effect and cannot be quickly excreted from the body. When the cumulative content of lead in human blood reaches (0.6-0.8)×10-6, it will damage the liver, cause hematopoietic function to decline, and cause various lead poisoning diseases such as diarrhea, cramps, cerebral hemorrhage and chronic nephritis. Compared with organotins, rare earths, and metal soaps, the acute toxicity of lead and its compounds is not very high (the toxicity of some compounds is shown on the next page), but lead compounds accumulate in human tissues and have a half-life of up to 20 Years, therefore, the chronic harm to the human body is very serious.
Note: LD rough lethal dose; MLD minimum lethal dose; LD50 half lethal dose
The clinical manifestations of lead compounds on human health are:
1) Central nervous system: manifested as headache, insomnia, forgetfulness, easy to excite, etc., especially for children, it can be more severe, can cause ADHD, and seriously affect mental development.
2) Hematopoietic system: interferes with the synthesis of heme, which is anemia when severe.
3) Digestive system: loss of appetite, constipation, abdominal pain.
4) Liver and kidney system: liver enlargement, tenderness and renal hypertension can occur.
5) Reproductive system: especially affecting the fetus, it is easy to cause miscarriage, fetal abnormalities, etc. For men, there are sexual dysfunction, sperm reduction and other symptoms.
Cadmium is an environmental pollutant and is a toxic element of the human body. In 1974, the United Nations Environment Program (UNEP) and the International Committee for Work Heavily Heavy Metals discussed cadmium as a key environmental pollutant. The American Toxicology Administration (aTSDR) has listed it as the sixth toxic substance that threatens human health. According to EU Directive 67/548/EEC, most cadmium compounds are environmentally harmful (ecologically toxic), some cadmium compounds are highly harmful, and some compounds are carcinogens (category 2), cadmium is persistent, some Cadmium compounds accumulate in some organisms. 
Cadmium compounds can enter the body through the respiratory tract and digestive tract. Cadmium, a cadmium-containing protein that enters the body, reaches the body through blood circulation and selectively accumulates in the kidney and liver. The kidney is the target organ for cadmium poisoning, and the kidney can accumulate 1/3 of the cadmium absorption. In addition, there is a certain accumulation in the spleen, pancreas, thyroid, testis and hair. Cadmium can damage the renal tubules, causing symptoms such as diabetes, proteinuria and amino aciduria, and increasing the output of urinary calcium and uric acid. Renal insufficiency affects the activity of vitamin D3, which hinders the growth and metabolism of bones, resulting in loose bones, atrophy, deformation, and breakage. The autopsy of Japan's famous public nuisance disease - pain patients also shows that there are multiple fractures in the bones. From animal experiments and population epidemiological investigations, cadmium can also cause aberrations in warm-blooded animals and human chromosomes. The teratogenic and carcinogenic effects of cadmium have also been confirmed by experiments.
The clinical manifestations of cadmium compounds on human health are:
1) Exercise system: muscle pain in the lower extremities, pain after compression of the bones, osteoporosis, multiple pathological fractures under micro-collision, and then bone deformation.
2) Respiratory system: cough, difficulty breathing, causing pneumonia, pulmonary edema and pulmonary fibrosis.
3) Urinary system: renal tubular reabsorption dysfunction, renal tubular proteinuria, chronic interstitial nephritis, chronic renal failure.
4) Digestive system: nausea, vomiting, abdominal pain, diarrhea, liver damage.
5) Nervous system: headache, dizziness, burnout, aversion to cold, sweating, dysosmia, restlessness, collapse, and even shock.
1.3 Impact on the environment
The use of heavy metal stabilizers in PVC products has attracted increasing attention in the last 10 years, mainly for the following two reasons:
First, because of the chemical reaction of heavy metal stabilizers during the use of PVC products, the environmental pollution caused by migration. If the UPVC profile door and window using the composite lead heat stabilizer loses luster and turns yellow after a period of use, the plastic surface gradually starts to be powdered, deteriorated and decomposed under the action of photoaging, and the dust formed by decomposition can be found through inspection. Contains lead compounds. As the lead salt stabilizer migrates slowly and slowly, the lead-containing dust formed on the surface of the plastic will accumulate and cause harm to the environment. NASA's Space Flight Center and the military Aberdeen Test Center confirmed that UPVC doors and windows using lead salts as stabilizers in the United States and other places, with the addition of more than 10 parts of titanium dioxide, using electron microscopy It observes that after a period of time, the lead-containing resin powder formed by decomposition will still be found on the plastic surface. Similarly, the lead salt stabilizer used in the UPVC drain pipe is slowly impregnated under the action of oxygen in the air and oxides in the water, causing environmental pollution.
Second, because of the environmental hazards of heavy metals in PVC products during the processing of PVC waste plastics. As all plastic products will eventually lose their use value as the lifespan and recycling times increase, when these products are recycled to the final stage, they often cause local governments and environmental organizations to be uneasy because they are ultimately Incineration treatment is either buried underground or thrown into the sea to degrade naturally. These processes will cause heavy metal pollution to the marine and groundwater systems, and the number is huge, which is a huge threat to the human living environment. If available, 10% of the lead salt stabilizer can be released from a flexible PVC cable that includes a variety of mixed plasticizers in the filler field. Today, with great emphasis on the peaceful coexistence of man and nature, this issue has increasingly attracted the attention of governments.
In 1999, China's annual output of 300,000 tons of UPVC doors and windows, lead content is calculated according to UPVC doors and windows 1%, UPVC doors and windows life expectancy in 50 years, 50 years later, there will be 3,000 tons of lead compounds along with scrapped plastic doors and windows left in our children and grandchildren The home of survival. Not only that, but how many tons of powdered lead compounds are floating in the environment every year? These lead compounds will cause serious pollution to the atmosphere and the surrounding environment, and cause harm to human health. With the promotion of China's plastic building materials, the demand for UPVC doors and windows is increasing. If lead salt is used as a stabilizer, the lead compounds floating in the environment will increase geometrically, and the consequences are unimaginable.
At present, China produces nearly 300,000 tons of UPVC drainage pipes every year. If the lead content is calculated as 1% of the drainage pipe, China's annual lead of 3,000 tons of lead is mixed in the drainage pipe. These lead slowly and increasingly flow into the discharged sewage, especially the outdoor rain leaking tube is decomposed faster due to photoaging, and the lead compound is more easily leached from the completely decomposed UPVC pipe, which seriously pollutes the environment.
Dissipated lead compounds are injected into rivers and lakes with rainwater or discharged sewage, infiltrated into the ground, into fish ponds, pastures, farmland, orchards and other places, recycling into the human food chain, which will seriously endanger human health. The death of a musician like Beethoven has always been a mystery. Recently, scientists have confirmed that Beethoven is dying of lead poisoning. There is a background to this: there were many lead-making plants on the banks of the Danube and the Rhine at the time, and the river was polluted by lead. But Beethoven
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