What are the main reasons for the appearance of graying and blackening of PVC pipes?

July 5, 2019

 

What are the main reasons for the appearance of graying and blackening of PVC pipes?

 

The PVC pipe formulation is mainly composed of PVC resin and additives. Among them, the auxiliary agents are further divided into: heat stabilizers, lubricants, processing modifiers, impact modifiers, fillers, ageing agents, colorants, and the like.

    

What are the main reasons for the blackening of PVC pipes?
   

First, the quality of PVC products
 

Due to the presence of tertiary chlorine and allyl chloride in the molecular structure of PVC, PVC pipes are prone to decomposition during processing and later use. The de-HCL is easy to form conjugated double bonds, and multiple double bonds are formed, which causes the discoloration of the tubes to occur. The order of color change is: white-pink-light yellow-red-black (carbonized), so PVC quality, stabilizer quality (control decomposition) and dosage are essential.

 

Second, the quality is unqualified titanium dioxide induced discoloration
 

Whether or not the Ti surface is coated has an important influence on the photocatalytic reaction. Whether it is rutile or anatase TiO2, if the surface is not coated, it can undergo photocatalytic reaction and cause discoloration of PVC-u pipe; if it is coated with low activity (such as Al2O3 or ZRO2), it will block Tio2. The active point of the surface can largely inhibit or reduce the occurrence of surface photocatalytic reactions.
      

Generally, TiO2 has two structural forms: anatase and rutile. Due to its structure, use energy and aging resistance. The use of TiO2 as a PVC pipe must be rutile. However, due to the quality problems of TiO2 produced by some manufacturers or due to the uneven coating of TiO2 surface (SiO2, AL2O3 organic matter), TiO2 is prone to discoloration (graying) under photocatalysis. (The price of anatase is 1000 yuan per ton, and the import is 2000-4000 yuan per ton).
     

The active atomic groups generated during the photocatalytic reaction on the TiO surface undergo oxidation aging reaction with lead in the PVC blend.
(Reaction conditions: The photocatalytic reaction of TiO surface requires the interaction of multiple environmental conditions, such as sunlight exposure, humidity, temperature, etc., where temperature has a great influence on the chemical reaction rate, usually every 10 °C increase in temperature, chemical reaction The rate is increased by one time. In the hot summer of climatic, the surface of TiO: coloring material can reach 50-70 °C, which has a great influence on the photocatalytic reaction)
     

It can be seen from the above reaction process that TiO is under suitable conditions. The photocatalytic reaction occurring on the surface can be regarded as a periodic redox reaction. Due to the black lead produced by the redox reaction, grayish black spots appear on the surface of the light-colored PVC-U pipe. When the lighting is stopped for a certain period of time, the gray-black spots on the surface of the PVC-U pipe will slowly disappear.

in conclusion:
    

The reason for the gray-black spot on the surface of the light-colored PVC-U pipe is that it contains a lead compound, and the redox reaction of the lead compound upon photocatalytic reaction on the Tio surface produces black elemental lead, thereby producing grayish black spots. Rutile TiO can only slow down the speed of discoloration, but it cannot completely avoid the problem.
 

 

Third, sulfur pollution and lead salt stabilizer, cadmium stabilizer
 

With the development of industry, acid rain and sulfur pollution are becoming more and more serious, and the profiles may be in contact with sulfur and sulfide during storage and transportation, so the possibility of sulfide contamination is greater. Sulfur pollution is mainly
Pb2++S2-→PbS (black)
2S2-+PbCl2→Pb(ClS)2 (reddish brown)
The substance formed by the reaction is black or red ochre, which can cause discoloration of the white material, so the use of the lead salt stabilizer profile is prone to sulfur contamination. Cadmium stabilizers are toxic products, such as those used in pipes, which are prone to the following reactions in a sulfide environment:
S2-+Cd2+→CdS↓ (yellow)
Will lead to yellowing of the pipe, so cadmium stabilizers are rarely used.

 

Four. Discoloration caused by ultramarine (in the formulation of lead salt stabilizer)
 

Ultramarine is an aluminum silicate with a special structure containing sodium polysulfide. Ultramarine without special treatment has poor alkali resistance and acid resistance. When the pipe is exposed to acid rain, under the strong sunlight, the acid rain and the ultramarine remaining on the surface of the pipe form "HS ̄" ions, which are dissolved in water and further hydrolyzed to form S ̄ and the Pb 2+ present in the profile. The formation of a precipitate causes the surface of the tube to be colored to cause "vulcanization" contamination.
Pb2++S2 ̄→PbS↓ (crystal form: blue)
(amorphous: black)

 

 

Five. Influence of new toughening materials on weather resistance and discoloration of white profiles
 

CPE made PVC toughening materials are still used by most manufacturers, but CPE chlorinated polyethylene, due to its structural characteristics, is easy to decompose and discolor under the action of light, heat and oxygen, making the tube yellow. In addition, more importantly, the decomposition of CPE produces HCL and reacts with lead salt stabilizer to form PbCl2. Pb(ClS)2 and PbS are easily formed in a sulfur-contaminated environment, which causes the tube to turn reddish brown and black.
 
Six. Therefore, it is recommended that:
1 Qunqing must be processed, with alkali resistance and acid resistance
2 Do not use stabilizers containing sulfides
3 using rutile titanium dioxide
4 Using high quality CPE

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