Stabilizers Have A Variety Of Functions
Stabilizer is one of the important categories in plastic processing auxiliaries. Stabilizer is synchronized with the birth and development of PVC resin. Stabilizer It is mainly used in the processing of PVC resin. Therefore, the proportion of stabilizer and PVC resin and PVC in soft and hard products is closely related. PVC stabilizer used a small number of copies, but its role is huge. In the PVC processing using stabilizers can ensure that PVC is not easy to degradation, more stable. PVC processing commonly used stabilizers are basic lead salt stabilizer, metal soap stabilizer, organotin stabilizer, rare earth stabilizer, epoxy compounds and so on. PVC degradation mechanism is complex, the mechanism of different stabilizers is not the same, the stability effect is also different.
Hot polyvinyl chloride is considered to be one of the most versatile polymers due to compatibility with many other materials such as plasticizers, fillers and other polymers. Stabilizer The main drawback is the poor thermal stability. The use of additives can change the physical appearance and operating characteristics of polyvinyl chloride (PVC), but does not prevent the decomposition of the polymer. Although the physical material (such as heat, radiation) and chemical (oxygen, ozone) factors will always make the polymer material gradually destroyed, but called a stabilizer of a class of substances can effectively prevent, reduce or even stop the material Of the degradation. PVC in the 100 ~ 150 ℃ significant decomposition, ultraviolet light, Stabilizer mechanical power, oxygen, ozone, hydrogen chloride and some active metal salts and metal oxides, etc. will greatly accelerate the decomposition of PVC. PVC thermal oxygen aging is more complex, some literature reported PVC thermal degradation process is divided into two steps.
(A) dehydrochlorination: PVC polymer molecular chain on the removal of lively chlorine atoms to produce hydrogen chloride, while the formation of conjugated polyene;
(B) the formation of longer chains of polyolefins and aromatic rings: As the degradation progresses further, the chlorine atoms on the allyl groups are extremely unstable and easily removed to produce longer chain conjugated polyenes, the so-called " Zipper type "dehydrogenation, while a small amount of CC bond breakage, Stabilizer cyclization, resulting in a small amount of aromatic compounds. The decomposition of dehydrochlorination is the main cause of PVC aging. On the PVC degradation mechanism is more complex, there is no uniform conclusion, the researchers proposed the main free radical mechanism, ion mechanism and single molecular mechanism.
In the process of processing, the thermal decomposition of PVC for other properties change little, mainly affect the finished product color, adding stabilizer can inhibit the initial coloring of the product. When the removed HCl mass fraction reaches 0.1%, the color of the PVC begins to change. Depending on the number of conjugated double bonds formed, PVC will exhibit different colors (yellow, orange, red, brown, and black). If there is oxygen in the thermal decomposition of PVC, then there will be colloidal carbon, peroxide, carbonyl and ester compounds generated. However, during the long period of use of the product, the thermal degradation of PVC has a great influence on the properties of the material. Adding stabilizers can delay the degradation of PVC or reduce the degree of PVC degradation.
In the process of PVC processing by adding stabilizers can inhibit the degradation of PVC, then the main role of stabilizers play: by replacing the unstable chlorine atoms, Stabilizer hydrogen chloride absorption, and unsaturated parts of the addition reaction to inhibit the PVC molecules Of the degradation.
The ideal stabilizer should have a variety of functions:
(1) replacing a lively, unstable substituent, such as a chlorine atom or allyl chloride attached to a tertiary carbon atom, to produce a stable structure;
(2) to absorb and neutralize the process of PVC released in the process of HCl, Stabilizer to eliminate the automatic catalytic degradation of HCl;
(3) neutralizing or passivating metal ions and other harmful impurities that catalyze the degradation;
(4) through a variety of forms of chemical reactions can block the continued growth of unsaturated bonds, inhibit the degradation of coloring;
(5) the best protection against ultraviolet light shielding effect.
PVC stabilizers are usually inorganic or organometallic compounds, and the term itself is meant to contain cations, or organic compounds, Stabilizer which are usually classified by chemical class. In general, inorganic and metal organic compounds are basic (or major) stabilizers, while organic compounds are secondary or auxiliary stabilizers.
Stabilizers are classified according to tin, lead and blood A family metals such as barium, copper and zinc.
Tin stabilizer: containing one or two carbon-tin bond, the remaining price of oxygen or sulfur - tin anion bond saturated tetravalent tin compounds, PVC is the most effective stabilizer. These compounds are products of organotin oxide or organotin chloride with the reaction of an appropriate acid or ester.
Stabilizer synergistic mixtures are common and generally include various flow-based organotin compounds and wave-based salts (compounds) and auxiliary additives such as zinc soap, phosphite, epoxide, glyceride, UV absorbers, Agent and so on. It is clear that most of the synergistic compositions are specific and have not yet been found to have a general nature.
Organotin stabilizers are classified into sulfur and sulfur-free. Sulfur stabilizers are excellent in all stable properties, but there are problems with tastes and crossings that are similar to sulfur-containing compounds. Typical sulfur-containing anions are:
Thiolate - SR
Mercapto ester - S (CH) nCOOR
Mercapto ester - S (CH) nOCO
Or elemental sulfur.
Non-sulfur anions are usually based on maleic acid or maleic acid half esters, non-sulfur organotin is less effective stabilizer, but it has good light stability.
Lead stabilizers: Typical lead stabilizers include the following compounds: lead salts of dihydroxy salts, hydrazine salts of trihydrochloride, lead salts of diacyl phthalate, lead
As a stabilizer, the lead compound does not impair the excellent electrical properties, low water absorption and outdoor weather resistance of the PVC material. However, lead stabilizers have drawbacks, such as toxic; cross-contamination, especially cross-contamination with sulfur; generation of lead chloride, Stabilizer formation of stripes on the finished product; greater weight, resulting in unsatisfactory weight / volume ratio. Lead stabilizers often make the PVC products become opaque and change color soon after heating.
Despite the toxic and ecological deficiencies, these stabilizers are still widely used. For electrical insulation, lead is the preferred PVC stabilizer. Based on the combined effect of this stabilizer, there are many flexible and rigid, homopolymer and copolymer formulations that can be achieved.
Mixed metal stabilizers: Mixed metal stabilizers are aggregates of various compounds, usually designed for specific PVC applications and users. These stabilizers have been developed by the addition of barium succinate and cadmium citrate to the addition of barium soap, cadmium soap, zinc soap, organic phosphites, antioxidants, solvents, extenders, Colorants, UV absorbers, brighteners, viscosity control agents, lubricants, tackifiers, and artificial flavors. In this way, there are quite a number of factors that can affect the effect of the final stabilizer.
Group II metal stabilizers such as barium, calcium and magnesium do not protect the early colors but provide a good long term stabilizer for PVC. Steady PVC in this way starts with yellow / orange, and then continues to heat, gradually becomes check / brown, and finally black.
Cadmium and zinc compounds are first used as stabilizers because they are transparent and can retain the original color of the PVC product. The long-term thermal stability provided by cadmium and zinc is much smaller than that of barium compounds. They are often in a very small precursor or without aura case, Stabilizer the sudden complete degradation.
In addition to the proportion of metal, the effect of barium-steel stabilizer is also related to its anion. Stabilizer Anions are the main factors affecting the following properties: lubricity, migration, transparency, pigment color change, and the thermal stability of PVC. The following are the common anions of several common mixed metal stabilizers: 2-ethylhexanoate, phenate, benzoate, stearate
With the development of processing technology and the need for the use of calcium-zinc stabilizer has developed. At first, all PVC food packaging relied on government approved calcium soap, zinc soap. In order to meet the needs of consumers and the development of market potential, the design of the use of this less effective stabilizer of the PVC formula and melt manufacturing equipment. Auxiliary stabilizers can be used with these soaps. Dihydropyridine and diketone are the latest auxiliary additives.