Reasons for PVC profile yellowing and countermeasures

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In the production and application of PVC profiles, thermo-oxidative reactions occur; under the influence of heat and pressure, along with the presence of trace amounts of moisture, acids, bases, and other impurities, as well as oxidation from the air, the polymer macromolecules experience a decrease in molecular weight and a change in their structural configuration. This chemical change is called degradation; its most obvious manifestation is yellowing of the surface, a gradual loss of luster, and in severe cases, a grayish color. The discoloration of profiles leads customers to doubt the performance of PVC materials, and the consequences of this are severe. Discoloration of PVC profiles: I. Discoloration due to heat, oxygen, and light aging. Many people believe that to address the issue of inconsistent colors in profile processing, it is sufficient to reduce the processing temperature on the production lines where yellowing occurs, thereby reducing color variations. However, some believe that yellowing has the greatest impact on PVC, CPE, and stabilizers; different batches of these primary and auxiliary materials such as PVC, CPE, and stabilizers can result in varying color differences during processing. In this case, even adding a certain amount of whitening agent or anti-aging agent alone will not have much effect. The main manifestations of discoloration due to heat, oxygen, and light aging are yellowing of the profiles. Some stabilizer manufacturers claim in their promotional materials that using their products allows for a reduction in the amount of material needed and thus lowers costs. While this can result in profiles with a visually satisfactory appearance, during waste recycling or under stringent processing temperature requirements, the insufficient thermal stability of these products leads to damage to the PVC molecular structure, an effect that cannot be seen with the naked eye. As a result, the following issues occur during production: (1) The profiles show uneven coloring already during processing (a slight variation in color is acceptable) ； (2) The profile shows significant yellowing after heating deformation ； (3) It remains in good condition after heating, but the profiles turn gray shortly after being used to make doors and windows. The profiles do not turn yellow during normal production, but they do turn yellow when used to make window products later on. During the production of profiles, when basic conditions such as raw materials and molds remain unchanged, if the temperatures in the plasticizing and homogenizing stages are increased by about 5°C, the resulting profiles tend to yellow, which indicates insufficient thermal stability. It is therefore necessary to increase the amount of heat stabilizer used; especially when brightening agents such as fluorescent brighteners are included in the formula, insufficient heat stabilizers result in a weak brightening effect. II. Relationship between the molecular structure of PVC and color (1) Since PVC resin is a thermosensitive plastic with poor light stability, under the influence of heat and light, branch chains undergo dehydrochlorination reactions, resulting in polyene structures. When there are not too many conjugated double bonds in the main chain, slight color variations occur. Hydrogen chloride first reacts with surrounding substances that have acid-reactivity, and these conjugated double bonds become new active sites within the PVC molecular chain. Once these sites are activated by light to form macromolecular free radicals, PVC becomes susceptible to oxidation, leading to color changes. (2) The presence of a certain amount of low-molecular-weight components in PVC resin reduces the thermal stability of the polymer. The mechanisms underlying the decomposition of PVC include the free radical mechanism, the ionic mechanism, and the single-molecule mechanism. In addition to stabilizers, the decomposition of PVC can also be influenced by the quality of the PVC resin itself, such as whether there is an excessive amount of residual initiator present in it. (3) If certain impurities are present in the polymer, such as initiators, catalysts, acids, bases, etc., that were not completely removed during the polymerization process, or if moisture is absorbed during storage and transportation, this will reduce the stability of the polymer. Since these substances can trigger molecular–ionic degradation reactions, CPE contains a high amount of low-molecular-weight compounds such as Cl2 and HCl, which can accelerate the thermal decomposition of the resin. Therefore, for PVC CPE systems with poor stability, increasing the amount of stabilizer can also help eliminate some yellowing issues in the profiles. (4) Rigid PVC structural profiles are composed of a multiphase polymer mixture in which PVC serves as the continuous phase, along with heat stabilizers, light stabilizers, lubricants, fillers, and pigments. When such PVC profiles are exposed to the natural environment, once the PVC component loses hydrogen chloride, the original calcium carbonate component is converted into calcium chloride, which disperses within the layer of oxidized degradation products and creates potential sites for water absorption – this is one of the reasons why outdoor profiles turn yellow. (5) In addition to temperature, stress also plays a role; before molding, plastics are subjected to shear stress and tensile stress during processes such as high-speed mixing, cold mixing, and extrusion, which leads to thermal degradation. When addressing color differences, a single approach alone cannot resolve the issue effectively; considering all four aspects below can lead to a faster solution: (1) Reducing the processing temperature results in a whiter color compared to not reducing it ； (2) Adding a single lead salt alone to the composite stabilizer does not yield better results than increasing the amount of the composite stabilizer ； (3) Simply adding stabilizers is not as effective as using a combination of certain stabilizers along with brightening masterbatches or high-whiteness fillers, as this yields a faster improvement in brightness ； (4) Simply adding titanium dioxide along with an appropriate amount of internal lubricant yields better results. It is generally believed that low-molecular-weight substances have a plasticizing effect on polymers; one of the mechanisms is that the interactions between macromolecules and small molecules replace the interactions between macromolecules themselves, thereby making it much easier for the macromolecular chains to move. The aforementioned measures will prevent the decomposition of PVC to varying degrees and reduce color inconsistencies in profile processing; the most effective approach is to optimize the design of the stabilization system in order to fundamentally suppress the conjugated polyene chromophore structure in PVC. In production practice, using self-made ultraviolet lamps to subject the profiles to ultraviolet aging for 12 hours resulted in significant color changes. It is undoubtedly necessary to conduct natural climate exposure aging tests on products in a timely manner, in order to identify formulations with excellent weather resistance from a small number of candidates. III. Discoloration due to lead-sulfur contamination: When PVC profiles exposed to rain and strong sunlight outdoors, their surfaces may change color in certain areas. In some cases, on the same window, one of the profiles shows a more pronounced degree of discoloration in the areas where rainwater remains; the surface color becomes light gray or pale gray. Among other reasons, one of the causes is the uneven dispersion of ultramarine as a light blue coloring agent. Ultramarine, being a sulfur-containing compound with aluminum silicate, is resistant to alkalis but not to acids. When used in excessive amounts or accumulated in large quantities in PVC profiles, it can theoretically react with sulfur-containing compounds to form black lead sulfide, and this discoloration has occurred in many profile and window manufacturing plants. This is an issue of note for profile manufacturers that use lead salt series stabilizers along with lower-quality titanium dioxide and ultramarine. Some manufacturers have also reported that the profiles in contact with rubber seals experience significant discoloration, with this discoloration spreading to surrounding areas. In addition to low-quality seals containing oils and other materials that tend to leach out, another cause is the formation of lead sulfide as a result of the interaction between sulfur remaining in the rubber seals and lead. Whitening of profiled materials I. Titanium dioxide Titanium dioxide is the best white pigment and is widely used in plastic profiles; it is also an excellent light blocker. Rutile-type (R-type) titanium dioxide has a relative density of 4.26 and a refractive index of 2.72; it is capable of reflecting or refracting most visible light, as well as completely absorbing ultraviolet rays with wavelengths less than 410 nm. It boasts good heat and weather resistance, as well as strong UV-blocking properties, making it one of the key materials used to improve the weather resistance of profiles. It also helps to prevent PVC from releasing hydrogen chloride, thereby slowing down its decomposition. However, due to its high market price, some profile manufacturers reduce the amount of titanium dioxide used to 3.6 parts or even less in order to cut costs, which in turn reduces the weather resistance of the profiles. When the titanium dioxide content in the product is 8%, its coverage rate reaches a balance; adding more beyond this range is of no benefit. Since adding titanium dioxide to polymer materials increases viscosity, reduces fluidity, and raises torque, thereby making processing difficult, it is generally sufficient to add about 5 parts; if the whiteness is insufficient, a small amount of fluorescent brightener can be added. II. Fluorescent brighteners Fluorescent brighteners are special powdered organic substances that can absorb ultraviolet light with wavelengths below 400nm; they convert the absorbed energy into purple or blue fluorescence in the 400–500nm range, thereby compensating for the loss of purple or blue wavelengths reflected by the substrate. Therefore, it can be said that fluorescent brighteners not only whiten but also provide a certain degree of UV absorption. A small amount of blue pigment is added to white products in order to counteract yellowing. Fluorescent brighteners increase the amount of light reflected, thereby having a \"light-enhancing\" effect; this is different from the \"light-reducing\" effect of blue-based brighteners. As a result, the surface brightness of the products increases, making their colors clearer and more vivid, and achieving a brightening effect that gives the products a healthier appearance. The selection of fluorescent brightener types should take into account the melting point and decomposition temperature, lightfastness, solubility, and maximum absorption wavelength. Domestic general brighteners such as the PF type have a low decomposition temperature, with an initial decomposition temperature of 178°C and a maximum absorption wavelength of 363 nm; they also exhibit sublimation. Although their price is low, their performance is inadequate. III. Selection of processing aids: An inappropriate choice of processing aids can also lead to yellowing of the color. It is advisable to select processing aids that allow for rapid plasticization, good flow properties, low viscosity, and excellent oxidation resistance (such as WY-66/68 from Jiangsu Aiten Company). Due to the high viscosity, slow plasticization, poor flowability of the material, and long residence time, the color of the products turns yellow. IV. Selection of pigments: Many manufacturers choose inorganic pigments such as ultramarine for whitening and color adjustment, believing that these inorganic pigments have good lightfastness; however, their fastness rating is only 1-2 grade (as confirmed by the Guangzhou Aging Laboratory). Although ultramarine has good heat resistance, its acid resistance is poor. Since PVC decomposes to release hydrogen chloride during processing, uneven mixing can easily cause the polymer materials with high ultramarine content to decompose and change color, generating free sulfur. This sulfur then reacts with the lead salts used as stabilizers to form lead sulfide, resulting in a darkening of the color of the white profiles. Currently, there are few manufacturers in China that produce PVC masterbatches specifically for profiles. This is due to the high viscosity of PVC, its tendency to decompose, the resulting color variations in the finished products, and the high technical challenges involved.