by Chen Yimeng from Inter-China Testing & Technology Co., Ltd.
Abstract: Rigid polyvinyl chloride (RPVC) is widely used in construction, pipelines, and other fields, but it is susceptible to aging under ultraviolet radiation and thermo-oxidative conditions. As an important additive, titanium dioxide (TiO₂) can improve the aging resistance of RPVC. This study investigated the effects of titanium dioxide particle size, crystal form, and loading on the aging performance of RPVC. The results showed that rutile titanium dioxide, at a loading of 5 phr, significantly delayed photo-oxidative degradation, increased the tensile-strength retention rate by 18%, reduced the yellowness index, and enhanced thermal stability. This study provides theoretical and practical references for the formulation design of aging-resistant RPVC.
Keywords: titanium dioxide; rigid polyvinyl chloride; aging resistance; ultraviolet shielding; thermal stability; photo-oxidative degradation
Introduction
Rigid polyvinyl chloride, or RPVC, is a type of the general-purpose plastic PVC. It has the advantages of high rigidity, chemical-corrosion resistance, and good flame retardancy, and is therefore widely used in construction, pipelines, and other fields. However, when RPVC is used outdoors, ultraviolet radiation and thermo-oxidative factors readily trigger dehydrochlorination reactions through allylic chloride structures in the main chain, resulting in aging problems such as yellowing and embrittlement. This limits its competitiveness in outdoor applications. To improve the aging resistance of RPVC, existing studies have mostly adopted modification with organic additives, but such additives generally have drawbacks including migration susceptibility and poor compatibility with the matrix. Titanium dioxide, or TiO₂, is an inorganic filler characterized by chemical inertness, good thermal stability, and strong ultraviolet-shielding capability. Rutile TiO₂ can reflect and scatter ultraviolet radiation, thereby physically blocking ultraviolet damage, and thus offers very clear advantages [1].
Titanium dioxide is extensively used in the PVC industry. However, systematic studies remain relatively limited regarding how its particle size, crystal structure, surface-treatment process, and specific loading affect the aging performance of RPVC. This paper conducts systematic experiments and analyses, explores the mechanisms by which these parameters exert their effects, and proposes recommendations with practical engineering value.
1. Experimental
1.1 Raw Materials and Formulation Design
During the experiments, suspension-grade RPVC was used as the matrix resin, with a degree-of-polymerization K value of approximately 67. Two crystal forms of titanium dioxide were selected: anatase, designated as Type A, with an average particle size of approximately 0.25 μm; and rutile, designated as Type R, with an average particle size of approximately 0.20 μm and an Al₂O₃/SiO₂ surface coating. The other additives included an organotin heat stabilizer, calcium stearate lubricant, and a small amount of the processing aid ACR, all of which were industrial grade [2].
The formulations were expressed in phr (parts per hundred parts of resin). TiO₂ loadings were set at five levels: 0, 2, 5, 8, and 12 phr. Comparative experiments were carried out for both Type A and Type R crystal forms, giving a total of 10 formulations. All formulations were compounded in an internal mixer at 175 °C for 8 min and then compression-molded using a platen vulcanizing press at 185 °C and 10 MPa into standard specimens with a thickness of 3 mm for subsequent performance testing.
1.2 Aging Test Methods
An artificial accelerated ultraviolet-aging chamber equipped with UVA-340 lamps and controlled at an irradiance of 0.76 W/m² was used. Photoaging tests were carried out strictly in accordance with GB/T 16422.3-2022, with aging periods set at 0, 288, 480, 768, and 1,056 h. A thermo-oxidative aging group was also established, in which specimens were placed in an oven at 100 °C for 0, 200, 400, and 600 h, respectively, in order to evaluate changes in thermal stability.
1.3 Performance Testing
Tensile tests were conducted on specimens before and after aging in accordance with GB/T 1040, and tensile strength and elongation at break were recorded. A colorimeter was used to determine the yellowness index (YI) and color difference ΔE and to analyze color changes. Thermal stability was comprehensively evaluated using the Congo red method in combination with thermogravimetric analysis (TGA). Scanning electron microscopy (SEM) was used to observe the fracture-surface microstructure of the specimens before and after aging.
2.Results and Discussion
2.1 Effect of Crystal Form on the Light-Aging Resistance of RPVC
The experimental data showed that, at the same loading of 5 phr, rutile TiO₂ provided a significantly greater improvement in the light-aging performance of RPVC than anatase TiO₂. After 1,000 h of ultraviolet aging, the specimen containing Type R TiO₂ retained 82.3% of its original tensile strength, whereas the Type A TiO₂ group retained only 71.6%, and the blank control group decreased to 63.5%. This difference is directly related to the different photocatalytic activities of the two TiO₂ crystal forms. Anatase TiO₂ has a relatively low band-gap energy, approximately 3.2 eV, and therefore readily generates electron-hole pairs under ultraviolet irradiation, forming hydroxyl radicals (·OH) and superoxide radicals (·O₂⁻). These reactive oxygen species strongly oxidatively attack PVC molecular chains and instead accelerate photo-oxidative degradation of the matrix. Rutile TiO₂ is surface-coated, which effectively passivates its photocatalytic activity. Its ultraviolet-shielding function mainly relies on physical reflection and scattering, causing minimal chemical damage to the PVC matrix [3].
In the color-difference analysis, after 1,000 h of aging, the increase in the yellowness index (YI) of the Type R TiO₂ group was only 12.4, compared with 29.7 for the blank group and 21.3 for the Type A TiO₂ group. This indicates that rutile titanium dioxide has a clear advantage in maintaining the appearance stability of RPVC, which is particularly critical for applications such as architectural profiles and door and window profiles that have high color requirements.
2.2 Effect of Loading on Performance
Using rutile TiO₂ as the variable, the effect of different loadings on the aging resistance of RPVC was systematically investigated. The results showed that when the TiO₂ loading was within the range of 2-5 phr, the tensile-strength retention rate increased significantly with increasing loading. When the loading reached 5 phr, the improvement in aging performance approached its peak. Further increases to 8 phr and 12 phr did not further improve the mechanical-property retention rate; instead, a slight downward trend appeared.
Figure 2. Comparison of SEM Morphologies of Aged RPVC Fracture Surfaces at Different TiO₂ Loadings (Scale Bar: 10 μm)
The “saturation effect” can be explained from two aspects. First, when the TiO₂ content is appropriate, the particles can be dispersed uniformly in the PVC matrix, forming an effective ultraviolet-shielding network without significantly affecting the continuity of the matrix. Second, excessive addition of TiO₂ particles readily causes agglomeration under high-shear mixing conditions, forming stress-concentration points that lead to premature failure under load, while also increasing formulation cost. SEM observations showed that once the loading exceeded 8 phr, obvious interfacial cracking appeared on the fracture surfaces of the aged specimens, indicating that the interfacial bonding strength between the filler and the matrix weakened as the loading increased.
Considering mechanical properties, color-difference stability, and economic efficiency comprehensively, a loading of 5 phr rutile titanium dioxide is the most suitable for aging-resistant RPVC formulations.
2.3 Thermal-Stability Analysis
The thermo-oxidative aging results showed that the addition of TiO₂ also had a positive effect on the thermal stability of RPVC. The thermal stability time (DST) measured by the Congo red method was extended by approximately 14% after adding 5 phr Type R TiO₂ compared with the blank group. TGA analysis showed that the onset temperature (T_onset) of the principal thermal decomposition of TiO₂-containing specimens increased by approximately 5-8 °C. This indicates that titanium dioxide does provide a certain physical barrier effect against heat-induced dehydrochlorination – namely, TiO₂ particles form a heat-conduction barrier within the material, slowing the rate of heat transfer to the PVC matrix and delaying thermal degradation.
It should be noted that the contribution of TiO₂ to thermal stability is far less pronounced than that of specialized heat stabilizers such as organotin compounds. Its primary function is physical assistance rather than chemical inhibition. In practical formulation design, TiO₂ cannot be regarded as a substitute for heat stabilizers; instead, the two should be rationally combined as a synergistic system.
2.4 Synergistic Effect with an Organic Light Stabilizer
To further improve aging resistance, this study investigated the synergistic effect of 5 phr Type R TiO₂ combined with 0.3 phr of a benzotriazole ultraviolet absorber (UV-327). The results showed that after 1,000 h of aging, the composite system achieved a tensile-strength retention rate of 88.7%, approximately 6.4 percentage points higher than the 82.3% obtained with TiO₂ alone, while the increase in yellowness index decreased to 9.1. This performance was superior to that achieved with either single component.
The mechanism of the synergistic effect is as follows: TiO₂ physically shields ultraviolet radiation and substantially reduces the total amount of ultraviolet radiation reaching the PVC matrix, while the organic ultraviolet absorber is responsible for absorbing the residual ultraviolet photons that penetrate the shielding layer and converting them into heat for dissipation. Together, the two construct a multi-level protective network at different levels. Because this composite stabilization strategy has important practical value for high-performance outdoor RPVC products, it has broad potential for wider application.
3.Practical Application Discussion
3.1 Adaptability to Processing Conditions
The addition of titanium dioxide has a certain effect on the processing parameters of RPVC. Experimental results showed that after adding 5 phr TiO₂, the rheological behavior of the melt changed and the melt viscosity increased to a certain extent. The processing temperature therefore needs to be adjusted moderately (an increase of 2-3 °C is recommended), while the screw speed should also be matched accordingly to ensure uniform compounding. The dispersion quality of titanium dioxide is crucial to performance. It is therefore recommended to prioritize a premixing process in which TiO₂ and the lubricant are first prepared as a masterbatch and then incorporated into the main formulation. This can effectively reduce agglomeration and improve dispersion uniformity.
Table 1. Changes in RPVC Melt Rheological Behavior at Different TiO₂ Loadings
| Rheological Parameter | 0 phr (Blank) |
2 phr | 5 phr | 8 phr | 12 phr | Trend |
| Apparent Melt Viscosity η (Pa·s, 175 °C, Shear Rate 100 s⁻¹) |
1,820 | 1,910 | 2,050 | 2,280 | 2,610 | Monotonically increases with increasing loading |
| Melt Flow Index (MFI) (g/10 min, 190 °C/2.16 kg) |
4.8 | 4.5 | 4.1 | 3.6 | 2.9 | Monotonically decreases with increasing loading |
| Recommended Processing-Temperature Adjustment (Relative to Blank Group, °C) |
— | +1 | +2 to +3 | +4 to +5 | +6 to +8 | Requires gradual increase |
| Relative Change in Melt Elasticity (Storage Modulus G′) |
Baseline | 4% | 9% | 17% | 28% | Elasticity increases; die swell ↑ |
| Surface Finish of Extruded Products (Subjective Rating, 1-5) |
4.5 | 4.5 | 4.3 | 3.8 | 3.1 | Declines after excessive addition |
| Dispersion Uniformity (SEM Evaluation) |
— | Uniform | Uniform | Relatively uniform | Agglomeration observed | |
| Overall Processing-Adaptability Assessment | Baseline | Good | Good (Recommended) | Acceptable | Poor |
3.2 Recommended Application Scenarios
For outdoor architectural profiles such as uPVC door and window profiles and guardrail profiles, it is preferable to use 5 phr rutile TiO₂ in combination with an organic ultraviolet absorber, together with a calcium-zinc composite heat stabilizer, in order to achieve dual photothermal protection. For indoor pipelines and electrical insulating components, where ultraviolet-aging stress is relatively low, the TiO₂ loading can be appropriately reduced to 2-3 phr, mainly to provide filling, whitening, and auxiliary thermal stabilization, while also controlling formulation cost.
3.3 Economic Evaluation
Based on an average domestic-market price of approximately RMB 20/kg for rutile titanium dioxide, each additional 1 phr of TiO₂ in an RPVC formulation – calculated on the basis of 100 kg of resin – increases raw-material cost by approximately RMB 20. At a loading of 5 phr, a relatively favorable balance can be achieved between performance improvement and cost increase. When the benefits arising from the extended service life of the product due to improved aging resistance are also taken into account, the overall economic competitiveness remains strong.
4.Conclusions
This study systematically investigated the effects of titanium dioxide crystal form, particle size, and loading on the aging resistance of rigid polyvinyl chloride (RPVC). The main conclusions are as follows:
First, because rutile TiO₂ has lower photocatalytic activity and extremely high ultraviolet-shielding efficiency, it is much more effective than anatase in improving the light-aging resistance of RPVC. Surface-coating treatment can further improve its dispersion stability and interfacial compatibility in PVC systems.
Second, the optimum TiO₂ loading is controlled at 5 phr. At this loading, after 1,000 h of artificial ultraviolet aging, the tensile-strength retention rate of RPVC increases by approximately 18%, while the increase in yellowness index is also substantially reduced. At excessive loadings, TiO₂ readily agglomerates and instead causes the aging resistance of RPVC to decline.
Third, titanium dioxide can provide a certain auxiliary enhancement of the thermal stability of RPVC, but its mechanism is mainly physical barrier protection rather than chemical inhibition. Therefore, in practical applications, it must be used in combination with a dedicated heat stabilizer and cannot independently replace a heat stabilizer.
Fourth, the combined use of 5 phr rutile TiO₂ and a benzotriazole ultraviolet absorber can establish a dual protective mechanism involving physical shielding and chemical absorption, thereby improving aging resistance. This combination is suitable for formulation design of high-performance outdoor RPVC products.
The results of this study provide reliable experimental evidence for the engineering development of aging-resistant RPVC formulations and have practical guiding significance for improving the long-term service performance of domestic RPVC products in harsh outdoor environments. Subsequent research will explore the scale effect of the synergistic use of nano-TiO₂ and micron-scale TiO₂, as well as the multicomponent compounding mechanism of TiO₂ with hindered amine light stabilizers (HALS).
References
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