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The main causes of grey and blackening of PVC pipes

The main causes of grey and blackening of PVC pipes


PVC pipe formula is mainly composed of PVC resin and additives. According to their functions, additives are divided into heat stabilizers, lubricants (pe wax), processing modifiers, impact modifiers, fillers, anti-aging agents, colorants, etc.

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

1、 Quality of PVC products

Due to the existence of tertiary chlorine and allyl chlorine in the molecular structure of PVC, PVC pipes are easy to decompose in the process of processing and later use, and it is easy to form conjugated double bonds in the process of removing HCl. Multiple double bonds are formed, resulting in the discoloration of pipes. The order of color change is: white pink light yellow red black (carbonization), so the quality of PVC is high, The quality (decomposition control) and dosage of stabilizer are very important.

2、 Discoloration induced by unqualified titanium dioxide

Whether Ti surface is coated or not has an important influence on the photocatalytic reaction. If the surface of rutile or anatase  titanium dioxide is not coated, the photocatalysis reaction will take place and the PVC-U pipes will change color; If the active sites on the surface of  titanium dioxide are blocked by coating with low active materials(silica, aluminum oxide organic matter) , the surface photocatalytic reaction can be inhibited or reduced to a great extent.

Generally, titanium dioxide has two structural forms: anatase and rutile. Because of its different structure, use energy and aging resistance. As PVC pipe, titanium dioxide must be rutile. However, due to the quality problems of titanium dioxide produced by some manufacturers or the uneven coating of titanium dioxide surface (silica, aluminum oxide organic matter), titanium dioxide is prone to discoloration (ash change) under photocatalysis.

The active atoms produced in the photocatalytic reaction on the surface of titanium oxide and lead in PVC blends were oxidized and aged.

(reaction conditions: the photocatalytic reaction on the surface of TiO requires the joint action of multiple environmental conditions, such as sunlight exposure, humidity, temperature, etc., in which the temperature has a great influence on the chemical reaction rate. Generally, the chemical reaction rate increases by one time with every 10 ℃ increase in temperature. In hot summer, the surface of TiO: coloring material can reach 50 ~ 70 ℃, which has great influence on photocatalytic reaction


The reason of gray black spots on the surface of light color PVC-U pipe is that there are lead compounds in it. During the photocatalytic reaction on the surface of titanium oxide, the oxidation-reduction reaction of lead compounds produces black elemental lead, which produces gray black spots. Rutile titanium oxide can only slow down the discoloration rate, but can not completely avoid the problem.

3、 Sulfur pollution and lead salt stabilizer, cadmium stabilizer

With the development of industry, acid rain and sulfur pollution become more and more serious, and profiles may contact with sulfur and sulfide during storage and transportation, so sulfur pollution is more likely to occur. 

4、 Discoloration caused by ultramarine (in lead salt stabilizer formula)

Ultramarine is aluminum silicate with special structure containing sodium polysulfide. The alkali resistance, heat resistance and acid resistance of Ultramarine without special treatment are poor. When the pipe is drenched by acid rain, under the strong sunlight, the residual acid rain and the green action on the surface of the pipe form an ion, which is dissolved in water, further hydrolyzed with the binary lead action in the profile, forming sediment and coloring the surface of the pipe, which leads to the sulfur pollution phenomenon.

5. The influence of new toughening material on the weatherability and discoloration of white profile

CPE as PVC toughening material is 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 pipe yellow. In addition, the main reason is that HCl produced by CPE decomposition is easy to react with lead salt stabilizer to form lead chloride. It is easy to form lead chloride sulfide and lead sulfide in sulfur polluted environment, which will turn the pipe into reddish brown and black.

6、 Therefore, it is suggested that:

(1) Ultramarine must be treated, with alkali resistance, heat resistance and acid resistance

(2) No stabilizer containing sulfide is used

(3) Using rutile titanium dioxide

(4) CPE with high quality

Qingdao Sainuo Chemical Co., Ltd. We are manufacturer of PE wax, PP wax, OPE wax, EVA wax, PEMA, EBS, Zinc/Calcium Stearate…. Our products have passed the REACH, ROHS, PAHS, FDA testing. 

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