Although polymer materials are always destroyed by physical (such as heat, radiation) and chemical (oxygen, ozone) factors, there is a kind of substance called stabilizer that can effectively prevent, reduce or even basically stop the degradation of materials.
For the destruction process of PVC, people put forward various mechanisms: thermal oxidation decomposition; Cross-linking of large free radicals grown under anaerobic conditions; Effect of stereoregularity on degradation; Photodegradation; Oxidative dehydrochlorination; Radiation degradation; Molecular chain fracture caused by the critical stress introduced by the process; And the influence of branching points in PVC molecules on degradation.
Chemically, these mechanisms are very similar and can be directly related to the physical state of PVC.
The most important reason for the degradation of PVC is hydrogen peroxide.
As the dehydrochlorination process continues, the double bond of the yoke appears, and the wavelength of light absorbed by the polymer changes. When there are 6 or 7 polyene structures in a yoke system, PVC molecules absorb ultraviolet light and display.
At most, 0. 1% hydrogen chloride can be produced here.
As the degradation process continued, double bonds increased, the wavelength of absorbed light changed, and the color of PVC gradually became dark, dark, swaying and reddish brown until it turned black completely.
When the polymer is further damaged, it will be oxidized, broken and finally crosslinked.
In order to make up for the serious defects of PVC homopolymer and polymer, it is necessary to use stabilizers to eliminate the unstable parts that cause dehydrochlorination; Or as a scavenger of hydrogen chloride; Or react with free radicals when they are generated; Or as an antioxidant; Or changing the polyene structure to prevent discoloration, molecular chain breakage and crosslinking.
The stabilizer must be compatible with PVC system, which will not destroy the overall aesthetic feeling of the material system, and should also have the performance of adjusting lubrication.
Excellent PVC blends can be obtained by selecting stabilizers for specific resins, composite components and end uses.
The sensitivity of PVC resin and the stability or harmful effects of various chemicals may be different, so we need to pay attention to them one by one.
Therefore, we must pay attention to the zinc sensitivity of resin, the stability of metal soap lubricant, the working characteristics of epoxy and phosphorus-containing plasticizer, and the influence of various pigments and other components.
The process and product use determine the requirements for final stability, so it also determines the specific stabilizer type and dosage.
Attention must be paid to the equipment type, shear rate and possible thermal process of PVC admixture. While understanding the requirements of the regulatory authorities, we must also consider the physical appearance and durability of the finished product.
Types of stabilizers PVC stabilizers are usually inorganic or organometallic compounds, and the term itself indicates that they contain cations or organic compounds, which are usually classified according to chemical categories.
Generally, inorganic substances and organometallic compounds are alkaline (or primary) stabilizers, while organic substances are secondary or auxiliary stabilizers.
Stabilizers are mainly classified according to the mixture of tin, lead and group A metals such as barium, copper and zinc.
Tin stabilizer: Tetravalent tin compound containing 1 or two carbon-tin bonds, and the rest saturated with oxygen or sulfur-tin anion bonds, is the most effective stabilizer for PVC.
These compounds are the products of the reaction of organotin oxides or organotin chlorides with appropriate acids or esters.
The synergistic mixing of stabilizers is very common, which usually includes various liquid-based organotin compounds and wave-based salts (compounds) and additives, such as zinc soap, phosphite, epoxide, glyceride, ultraviolet absorber, antioxidant and so on.
Obviously, most synergistic compositions are specific, so they have not been found to be comprehensive.
Organotin stabilizers can be divided into sulfur-containing and sulfur-free categories.
Sulfur-containing stabilizers are outstanding in all aspects of stability, but there are problems of similar taste and cross-contamination with sulfur-containing compounds.
Typical sulfur-containing anions are: sulfhydryl compound -Sr thioester -S (CH) NCO or thioester -S (CH) NOCO or elemental sulfur.
Non-sulfur anions are usually based on maleic acid or maleic acid half ester. Non-sulfur organotin is an ineffective heat stabilizer, but it has better light stability.
Lead stabilizer: Typical lead stabilizers include the following compounds: dibasic lead stearate, hydrated tribasic lead sulfate, dibasic lead phthalate and dibasic lead phosphite. As a heat stabilizer, lead compounds will not damage the excellent electrical properties, low water absorption and outdoor weather resistance of PVC materials.
However, lead stabilizers have disadvantages, such as toxicity; Cross pollution, especially sulfur pollution; Lead chloride is generated, and stripes are formed on the finished product; This ratio is large, resulting in an unsatisfactory weight/volume ratio.
Lead stabilizer often makes PVC products opaque immediately and discolors quickly after continuous heating.
Despite their toxicity and ecological defects, these stabilizers have been widely used.
For electrical insulation, lead is the preferred PVC stabilizer.
Based on the comprehensive function of this stabilizer, many flexible and rigid homopolymers and polymers can be realized.
Mixed metal stabilizer: Mixed metal stabilizer is an aggregate of various compounds, which is usually designed according to specific PVC uses and users.
This kind of stabilizer has developed from single barium succinate and cadmium palmitate to physical mixing of barium soap, cadmium soap, zinc soap, organic phosphite, antioxidant, solvent, extender, peptizer, colorant, ultraviolet absorber, brightener, viscosity control agent, lubricant, tackifier and artificial perfume.
In this way, there are quite a few factors that can affect the effect of the final stabilizer.
IIA metal stabilizers, such as barium, calcium and magnesium, can not protect the early color, but can be good long-term heat stabilizers for PVC.
This stable PVC is yellow/orange at first, then it turns brown gradually and finally becomes black after continuous heating.
Compounds of cadmium and zinc are first used as stabilizers because they are transparent and can keep the original color of PVC products.
The long-term thermal stability of pot and zinc is far less than that of barium compounds.
They often degrade suddenly and completely with little or no warning.
The function of barium steel stabilizer is not only related to the metal ratio, but also related to its anion.
Anions of stabilizers are the main factors affecting the following properties: lubricity, fluidity, transparency, color change of pigments and thermal stability of PVC.
The following are the anions of several common mixed metal stabilizers: 2- ethylhexanoate, phenolate, benzoate and stearate. With the need of technical innovation and use, calcium and zinc stabilizers have been developed.
At first, all PVC food packaging relied on government-approved calcium soap and zinc soap.
In order to meet the needs of consumers and develop market potential, PVC and melt equipment using this ineffective stabilizer were designed.
Auxiliary stabilizers can be used with these soaps.
Dihydropyridine and diketone are the latest auxiliaries.
Application of soft products: mainly mixed metal stabilizers, because of its low cost and easy addition of plasticizers.
The temperature adopted is consistent with the maximum stability of mixed metals.
Due to toxicity and environmental problems, barium-zinc and calcium-zinc stabilizers are rapidly replacing more effective barium tanks in most general situations.
New stabilizers with similar stability to copper systems are constantly being developed to realize steel-free stabilizers.
This situation is caused by government regulations and the high cost of waste disposal.
The combination of calcium and zinc stabilizer, food-grade phosphite and auxiliary ingredients has been applied to food packaging films.
Plasticizers used in most soft products contain epoxidized esters, such as epoxy glycerides and epoxy fatty acid esters.
Epoxide reacts with hydrogen chloride as an auxiliary stabilizer.
Because of its unique electrical properties, lead compounds dominate the wire and cable coating market, and some mixed metals are used as auxiliary stabilizers in coating applications.
Hard products: Although some mixed metals and liquid-based antimony esters are used in the hard PVC products market in North America, most of them are stabilizers containing organotin.
In other parts of the world, especially when used as profiles, lead stabilizer is gradually replacing barium stabilizer, which is due to the pot problem mentioned above.
However, due to potential environmental factors, calcium, zinc and organotin are gradually replacing lead in these applications.
Pipe: Rigid PVC pipe is the largest market unique to PVC, and most pipes are produced on twin-screw extruder.
Because of the short heating time, a low concentration of flow-based organotin stabilizer is used.
These pipeline-grade stabilizers can contain as little as 4- 10% of tin, and the dosage is usually 0.4 part per 100 part of polymer (in the case of twin-screw extrusion) and 0.6- 1.0 part in the case of single-screw extrusion.
Stabilizers used in drinking water pipelines must meet the requirements of independent certification bodies.
Injection molding: With the demand for suitable resin for reciprocating screw injection mold, an efficient stabilizer has been successfully developed and a very large part (35 pounds) has been made.
Although the lower the molecular weight of the resin, the easier it is, but the high shear of the injection mold usually requires that the organotin liquid-based ester contains tin 14%-25%.
Blow molding: The proper selection of organotin is very important for blow molding, because of the additional requirements of color, fragrance and transparency at the beginning, while butyl tin sulfate and methyl tin sulfate play a decisive role in general products.
Although methyl tin and ester tin have also obtained FDA approval, octyl tin is mainly used for food-grade applications.
Film and sheet: both extrusion and calendering are used to harden PVC films and sheets. Usually, films and sheets use the same stabilizers as those used in bottles.
Overlapping panels and external profiles: For PVC stabilizers used for overlapping panels and window frame materials, weather resistance and color durability are additional requirements.
Long-term research has determined the best organotin structure for these applications.
In North America, mono/bis organotin sulfate is the preferred stabilizer, and its attraction is increasing in Europe where metal mixtures are traditionally used.
In North America, a large amount of titanium dioxide is used to resist ultraviolet rays.
This fact, coupled with higher productivity, requires a better stability mechanism for organotin.
In order to prevent or delay the thermal aging of polymer materials such as PVC, we should either eliminate the initiators of thermal degradation in polymer materials, such as allyl chloride structure and unsaturated bonds in PVC; Only by eliminating all substances that can catalyze the non-chain-broken thermal degradation reaction, such as hydrogen chloride released by PVC, can the thermal degradation of this kind of polymer materials be prevented or delayed.
Therefore, the selected heat stabilizer should have the following functions:
(1) It can replace active atoms in polymer chains (such as chlorine atoms in allyl position in PVC) to obtain more stable chemical bonds and reduce the possibility of initiating dehydrochlorination reaction;
(2) It can quickly combine with falling hydrogen chloride and inhibit its autocatalytic action;
③ Addition reaction with unsaturated bonds in polymer materials to generate saturated polymer chains, thus improving the thermal stability of the materials; ④ It can inhibit the oxidation and crosslinking of polyene structure;
(5) It has affinity for polymer materials and is non-toxic or low-toxic;
⑥ Does not react with existing reagents in polymer materials, such as plasticizers, fillers, pigments, etc.
At present, the heat stabilizers used can not fully meet the above requirements, so it is necessary to select heat stabilizers with different properties in combination with the characteristics of different polymer materials.
Sometimes it must be combined with antioxidants, light stabilizers and other reagents to reduce the possibility of oxidative aging.
Qualitative, the alkaline lead salt inhibits its autocatalytic effect by capturing the falling hydrogen chloride.
On the one hand, fatty acid soap can capture hydrogen chloride, on the other hand, it can replace chlorine atoms in allyl chloride existing in PVC to generate relatively stable esters, thus eliminating the initiation source of dehydrochlorination in polymer materials.
Organotin compounds first coordinate with chlorine atoms in PVC molecular chains, and active chlorine atoms in polymer chains react with Y groups (acidic groups in organotin compounds) in the ligand electric field, thus inhibiting the thermal degradation reaction of dehydrochlorination of PVC.
Effect of heat stabilizer on refractive index of PVC 1 stabilizer
When the visible light refractive index of heat stabilizer and PVC resin is the same or close to 1.52~ 1.55, the transparency of PVC products is better, and vice versa.
2. Linear length of stabilizer molecule (molecular group)
The linear length of heat stabilizer molecules (or molecular groups) is less than the wavelength of visible light of 400~735nm, with less refracted light and higher transparency, and vice versa.
3. Solubility of heat stabilizer in PVC, that is, compatibility.
Compatibility refers to the mutual affinity of two or more substances when they are mixed.
Good compatibility means that it is possible to achieve molecular dispersion.
The heat stabilizer has good compatibility with PVC resin in molten state.
The shape is not two-phase, that is, there is no interface or the interface is not obvious, the refracted light is less, and the transparency of PVC products is higher.
The compatibility of liquid stabilizer in PVC is better than that of the corresponding solid metal soap, and the molecular line length is also smaller, so the transparency of PVC is higher.
The liquid organotin heat stabilizer has the best transparency, because the heat stabilizer itself, which did not participate in the thermal stabilization reaction, and R2SnCl2 _ 2, which participated in the stabilization reaction, have good compatibility in PVC resin.
Ba/Zn, Ba/Cd and Ca/Zn stearate soaps have certain compatibility in PVC and high light transmittance. However, due to their limited compatibility and long molecular linearity, the products after thermal stabilization are typical metal salts, such as CaCl2 _ 2 _ 2 and BaCl2 _ 2, which have poor compatibility with PVC. So the dosage is large, and they become turbid because of more refracted light, which affects their light transmittance.
The tribasic lead sulfate and dibasic lead phosphite with poor compatibility have large molecular groups, so PVC products are opaque.
However, lead stearate has certain compatibility, and it is translucent when used in a small amount. /bbs/viewthread.php? Tid = 6076extra = Page %3D3