Common problems and analysis in profile production

September 29, 2019

Common problems and analysis in profile production
 

In the production and use of PVC-U profiles, sometimes the problem of discoloration of the surface of the profile is encountered. If this problem is not analyzed and determined in time, it will often affect the production and product quality.

 

In the case of normal surface quality, the PVC-U profile is generally white with a smooth surface. According to the requirements of GB8814-88, the profile "should not have any defects such as scratches, bumps, cracks, impurities, etc.", and "the color of the profile should be uniform." If measured by a whiteness meter, the whiteness of a normally produced PVC-U profile is about 80.

 

The whiteness of the profile is affected by various factors such as raw and auxiliary materials, formulations, equipment, molds, and processes. In the case of other factors, the whiteness of the profiles produced is different due to the different PVC manufacturers; because the stability system is different, the surface whiteness of the profiles is different when different materials, molds, and processes are used to process different profiles. It is more difficult to produce profiles with the same whiteness. Only various measures can be taken to minimize the difference in whiteness of the profile surface, so that the "no obvious color difference" can be judged by the naked eye.

 

High-quality profile production is the result of a combination of factors
 

Analysis of the causes of discoloration of profiles

For various reasons, the color difference on the surface of the profile is a normal phenomenon. However, if the surface of the profile is discolored, it is not normal.

 

What is the cause of discoloration of the profile surface? In short, the reason for the discoloration of the surface of the profile is that the main component of the PVC resin undergoes thermal degradation and photodegradation under the action of heat and light, causing discoloration of the surface of the profile. There are two kinds of discoloration on the surface of the profile: one is the discoloration of the profile surface during the new formulation test or the occasional discoloration of the profile during normal production; the second is that the profile does not change color during normal production, and the surface discoloration occurs during use. The surface of the profile is discolored differently for different reasons.

 

Reason one: the formula design is not reasonable

 

The optical and thermal stability of the PVC resin is poor. The pure resin starts to decompose when heated to 100 ° C, and decomposes more rapidly when it exceeds 130 ° C. PVC is photooxidatively decomposed by the ultraviolet rays in the sun and the oxygen in the air. When the PVC resin is decomposed by heat and light, the resin is discolored. The order of color change is: white-pink-light yellow-red-brown-black.

 

In order to prevent the decomposition of PVC resin during processing, more than ten processing aids such as heat stabilizers, light stabilizers, antioxidants and lubricants must be added to the formulation. In the new formula test, if the surface of the profile is discolored, the main reason is that the formula design is unreasonable. The main reason may be: (1) insufficient amount of heat stabilizer, light stabilizer and antioxidant; (2) composite heat stabilizer Inappropriate selection and proportioning of components;

 

(3) improper selection and ratio of components of internal and external lubricants.

 

Reason 2: Ingredients and raw materials

 

For “old formulas” that have been scientifically proven and tested for a long time, if the profiles are occasionally discolored during production, the “unreasonable formula design” factor can be ruled out. If the special situation of the process change caused by the failure of the instrument is excluded, the reason is that the thermal stability of the system is poor, and the main reason should be to find the reason from the two aspects of the ingredients and raw materials.

In terms of ingredients, because PVC resin and other dozens of different processing aids are combined in different proportions, the batching work is not only complicated, but also has a large workload, and sometimes mismatches and mismatches are inevitable. If the worker forgets or adds less heat stabilizers, light stabilizers, antioxidants, and internal and external lubricants during the ingredients, the surface discoloration of the profile may occasionally occur.

 

In the case of raw materials, due to frequent replacement of raw material manufacturers and models, batch numbers, the surface discoloration of the profiles may also occur. Among the raw materials, the most influential components on the surface discoloration of the profile are: PVC resin, CPE and heat stabilizer.

 

The thermal stability of PVC resin is the most important cause of discoloration of the surface of the profile. The molecular weight of the PVC resin and its molecular weight distribution determine its thermal stability. Under the same conditions, the larger the molecular weight of the PVC resin, the smaller the molecular weight distribution, and the better the thermal stability of the resin. That is to say, the thermal stability of different types of PVC resin can be different, relatively speaking, it should be SG4>SG5>SG6. If the SG6 type PVC resin is processed according to the processing conditions of the SG5 type PVC, the produced profile may have surface discoloration. In addition, due to the different raw materials, process routes and production scales used by PVC resin manufacturers, the molecular weight distribution of PVC resins produced by different manufacturers is different; the PVC resin of the same manufacturer and different batches has different molecular weight distribution due to different process conditions. There will be some differences. The presence of a certain amount of a low molecular weight component in a PVC resin having a broad molecular weight distribution significantly reduces the thermal stability of the polymer, resulting in discoloration of the surface of the processed profile. Therefore, when the old formula is used for production, the profile is occasionally discolored. In the raw material, it is necessary to check whether the manufacturer and the batch number of the PVC resin have changed, and compare and compare the thermal stability of different manufacturers and batches of PVC resin.

 

Reason three: CPE quality

 

The quality of CPE is also an important factor affecting the discoloration of the surface of the profile. CPE is prepared by suspension chlorination of high density polyethylene (HDPE) in aqueous hydrochloric acid. CPE with a chlorine content of 36-38% has low crystallinity and glass transition temperature, and has good elasticity and PVC. Compatibility, widely used as an impact modifier for PVC-U profiles. In recent years, there have been many new SMEs producing CPE in various places. Some enterprises have high or low chlorine content in CPE due to various reasons. The product quality is unstable and contains many low molecules such as Cl2 and HCl.

 

The material is easy to accelerate the thermal decomposition of the PVC resin during the processing of the PVC-U profile, causing discoloration of the surface of the profile.

 

Reason 4: Quality and dosage of heat stabilizer

 

The quality and amount of heat stabilizers are also important causes of discoloration of the surface of the profile. At present, there are three types of heat stabilizers used in the processing of PVC-U profiled materials: lead-based, calcium-zinc composite stabilizers and rare earth organics stabilization systems. To compare the quality of the heat stabilizer, in addition to the actual comparison of production applications, it is mainly to test the content of its active ingredients.

 

The lead-based stabilizing system has two kinds of composite lead and one-component lead salt. It is very important to choose different types and ratios of lead salts. It not only affects the thermal stability effect, but also affects the surface color of the profiles. This is why some of the surface color of the profiles is always reddish.
Rare earth multi-functional heat stabilizers, although improving the overall effect of thermal stability and light stability, but resource shortages and prices.

 

Shangnuo CZ-2033P multi-functional environmental calcium-zinc composite heat stabilizer of Dongguan Shangnuo Co., Ltd., taking into account the respective roles and interactions of various components such as antioxidants and UV absorbers, and achieving thermal stability The combined effect of sex and light stability.

For the PVC-U system with poor thermal stability, appropriately increasing the amount of the heat stabilizer can eliminate the discoloration of the surface of the profile.

 

Reason 5: Quality of titanium dioxide

 

Titanium dioxide is known as titanium dioxide, and has two crystal forms of rutile and anatase. The rutile titanium dioxide has a soft texture, good weather resistance and heat resistance, strong ultraviolet shielding effect, is not easy to yellow, and has good water resistance, and is particularly suitable for outdoor plastic products.

 

Anatase titanium dioxide has poor heat resistance and light resistance, and promotes photoaging of polyvinyl chloride. It is mainly suitable for indoor use. Rutile titanium dioxide (TiO2) is the most ideal for white PVC-U profiles. The coloring agent combines the functions of shielding and scattering ultraviolet rays to improve the weather resistance and heat resistance of the profile. It should be noted that another anatase type TiO2 does not have the above characteristics of rutile type TiO2, and has poor light resistance and light fastness, and is used in plastic door and window profiles to easily cause discoloration and aging of the window frame. Therefore, in the case of the problem of discoloration of the profile during use, it is first necessary to check whether the model of the titanium dioxide is used incorrectly, and whether the counterfeit TiO2 is purchased.

 

In the general climate of China, the amount of rutile-type TiO2 added to the PVC-U window frame profile is 4-6 parts (when 100 parts of PVC). The application of rutile TiO2 in the profile of calcium-zinc composite thermal stability system can improve the light stability and weather resistance of the profile. The rutile type TiO2 is applied to the profile of the organotin stabilizing system. Since the organotin is poor in light stability and weather resistance, the amount of TiO2 should be 12-15 parts (100 parts of PVC).

 

PVC-U thermal stability, mixing and other analysis

 

PVC-U thermal stability

 

PVC resin not only has poor thermal stability, but also poor light stability. Under the action of ultraviolet light and oxygen, PVC resin will produce photooxidation, decompose and dehydrogenate to cause aging, resulting in discoloration of the surface of the profile.

 

Therefore, in the processing of PVC resin, enough photothermal stabilizer should be added to eliminate the discoloration of the surface of the profile. In the normal production, the profile does not change color, and the surface discoloration of the profile occurs during use, reflecting that the thermal stability of the system is acceptable, and the light stability is poor, so it is necessary to find the reason from the light stability.

 

To analyze the light stability of different PVC resins, it is necessary to compare the light stability of different batches of PVC resin from different manufacturers and the same manufacturer.

 

Mixing process

 

Mixing is the first step in the production of PVC profiles, and it is also a key process. PVC resin, stabilizer, modifier, lubricant, titanium dioxide, etc. are generally mixed by high-speed mixer to achieve uniform dispersion of pigments and components. .

 

Inventory and packaging transportation

 

It is necessary to pack, store and transport PVC-U profiles in strict accordance with the requirements of GB8814. GB8814 stipulates that: “Profiles should be wrapped with kraft paper or plastic film and tied tightly, and the profiles should not be exposed”. “Profiles should be stored in warehouses with flat ground and temperatures below 40 ° C.” Profiles are “packaged, stored and transported”. During the process, the product is not allowed to be exposed to strong light or sunlight.

 

The discoloration of PVC profiles is the result of a combination of factors. In addition to the internal and external influencing factors and additives discussed above, factors that cause discoloration of the profile in the production process and equipment molds should also be considered. Mainly to make PVC under the conditions of thermal processing, mechanical shearing, etc., to minimize thermal oxygen degradation, to ensure better stability of the profile under light, oxygen and weathering conditions. This includes control of thermal oxygen degradation by various processes such as mixing, extrusion, plasticization, shaping, and cooling.

 

Improvement measures 

(1) Adjust the formula, adopt scientific and advanced formula system and non-toxic and high-efficiency heat stability system to improve the comprehensive stability of heat, light and oxygen of formula;
(2) Establishing a raw material laboratory to improve the analysis ability and quick judgment ability for the cause of discoloration of the profile surface;
(3) Establish reliable and reliable raw material supply channels to ensure the quality of raw materials;
(4) Appropriately increase the amount of heat stabilizer, light stabilizer and antioxidant, and timely deal with the problem of surface discoloration caused by changes in raw materials;
(5) When packaging, storing and transporting PVC-U profiled materials, it shall be carried out in strict accordance with the requirements of GB8814.

 

The discoloration of PVC-U profiles is the result of a combination of internal and external factors. The external conditions include ultraviolet radiation, certain temperature, humidity and oxygen conditions, air pollution (dust sulfur, acid rain, etc.). Dry and hot weather conditions have an accelerated effect on the degradation of PVC-U. The internal factor is that the PVC resin degrades and decolorizes the HCl under light and oxygen conditions. Therefore, controlling the quality of PVC resin, especially controlling the degradation of PVC resin and the formation of polyene structure, is the primary problem to solve the discoloration of profiles. In addition, various additives in the PVC profile production formula, including stabilizers, antioxidant systems, titanium dioxide, pigments and fillers, also have discoloration factors due to their respective qualities. By optimizing the stable system in the formulation and cooperating with the necessary weathering specification rutile titanium dioxide, it is necessary to reduce the discoloration of the profile by controlling the quality of the resin and various additives in the formulation and its own possible discoloration factors.

 

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