Common problems and solutions for PVC sheet materials: deformation, aging and maintenance

In modern industrial production and daily life, PVC sheets have become an indispensable and important material in various fields such as architectural decoration, advertising production, medical devices, and food packaging due to their excellent physical and chemical properties, good processing characteristics, and affordable prices. According to the latest industry data, the global market size of PVC sheets is continuously expanding, and their application scope is constantly broadening. However, with the increasingly complex usage environment and the continuous improvement of usage requirements, the problems such as deformation, aging, and surface damage that PVC sheets exhibit during use have become increasingly prominent. These problems not only affect the appearance and performance of the products, but also may shorten their service life and increase the usage cost.

To deeply understand the various problems that occur during the use of PVC sheets, one must start from the characteristics of their base materials. PVC is a typical thermoplastic, and its molecular structure determines it to have a series of unique performance features. In terms of thermal properties, the glass transition temperature of PVC is approximately 80℃, which enables it to maintain good rigidity at room temperature, but it will start to soften at temperatures above 60℃, which is the main reason for its susceptibility to thermal deformation. Experimental data shows that a 1mm thick PVC sheet, under a 70℃ environment, will undergo approximately 2% permanent deformation within 24 hours with only 0.1MPa of continuous pressure. In terms of mechanical properties, the tensile strength of hard PVC can reach 40-60MPa, and the bending strength is 70-100MPa, indicating good mechanical strength. However, it is particularly important to note that the mechanical properties of PVC will significantly decrease with an increase in temperature. At 40℃, its strength may only be about 60% of that at room temperature.

Weather resistance is another characteristic that needs to be given special attention in the application of PVC sheets. Unmodified PVC will undergo photodegradation due to ultraviolet radiation when used outdoors, resulting in the breakage of molecular chains. PVC sheets exposed to ultraviolet rays for a long time will show obvious yellowing. Research has shown that after being exposed to the outdoors for one year, the impact strength of PVC may decrease by 30-50%. In addition, PVC has good tolerance to most inorganic acids, alkalis, and salt solutions, but it will swell or even dissolve in ketone and ester organic solvents. This property requires special attention during cleaning and maintenance.

In practical applications, deformation is one of the most common problems for PVC sheet materials. From a material science perspective, PVC, as a non-crystalline polymer material, undergoes re-arrangement and orientation of its molecular chains when heated or subjected to force, resulting in macroscopic dimensional changes. This deformation mainly takes three forms: thermal deformation, stress relaxation, and anisotropic deformation. Thermal deformation occurs when the environmental temperature exceeds the glass transition temperature of PVC, where the mobility of molecular chain segments increases, causing the material to transition from a glassy state to a highly elastic state. Under the action of an external force, irreversible deformation is easily induced. Stress relaxation is manifested as the molecular chain sliding and re-arrangement of PVC under continuous stress, leading to the gradual release of internal stress and the formation of a slow creep deformation. Anisotropic deformation results from the orientation effect of molecular chains during the processing, causing PVC sheet materials to exhibit different thermal expansion coefficients and contraction rates in different directions.

Through a large number of on-site case analyses, it was found that the main factors affecting the deformation of PVC sheets include temperature, stress distribution and material formulation. When the environmental temperature exceeds 60℃, the risk of deformation significantly increases. For example, PVC billboards installed in direct sunlight may have surface temperatures reaching 70-80℃ in the afternoon of summer, which is extremely prone to sagging deformation. Uneven stress distribution is also a key factor causing deformation. Approximately 65% of the deformation problems of PVC sheets result from unreasonable installation and fixation methods. In addition, factors such as the content of plasticizers and the type of fillers in the material formulation can significantly affect the deformation resistance of PVC. Data shows that the soft PVC with 20% plasticizers has a heat deformation temperature about 30℃ lower than that of hard PVC.

Regarding the deformation issue, we have proposed a systematic solution. In terms of material selection, for outdoor use, it is recommended to choose a deformation-resistant formula, such as PVC with methyl methacrylate copolymer modification, whose heat deformation temperature can be increased by 15-20℃; for applications requiring higher rigidity, a composite PVC sheet filled with calcium carbonate can be selected, with a filling rate of 20-30%, which can significantly improve the anti-rheological performance. In terms of structural design, for large-sized sheets (over 1.5m), it is recommended to design a reinforcement structure, which can increase the anti-deformation ability by more than 30%; using a honeycomb hollow structure design can reduce weight while maintaining good rigidity. In terms of installation process, it is recommended to use a metal frame with matching thermal expansion coefficient, using a multi-point fixation method (with the fixed point spacing not exceeding 50cm), and leaving a 1-2mm/m thermal expansion gap when installing in a high-temperature environment. For deformed sheets, the heat pressing shaping process can be adopted: heating the sheet to 90-100℃ and placing it in the shaping mold, maintaining a pressure of 0.3-0.5MPa, cooling to below 40℃ before releasing the pressure; for local deformation, an infrared heater combined with a shaping tool can be used for precise repair.

The aging of PVC sheets is a complex physical and chemical process, mainly consisting of three mechanisms: photodegradation, thermal oxidative aging, and plasticizer migration. Photodegradation refers to the cleavage of C-Cl bonds in PVC molecules under the action of ultraviolet rays, generating free radicals, which then trigger a chain reaction. This process leads to a decrease in molecular weight, color change, and deterioration of mechanical properties. Experimental data shows that the molecular weight of PVC exposed to the outdoors for 2 years may decrease by 40%, the yellowness index increases from the initial 5 to above 30, and the impact strength decreases by 50-70%. Thermal oxidative aging occurs when PVC undergoes chain breakage and cross-linking reactions under the combined action of high temperature and oxygen. Studies have shown that after accelerated aging at 80°C for 1000 hours, the fracture elongation rate of PVC may drop to 30% of the initial value. Plasticizer migration is a specific aging phenomenon of soft PVC, where the plasticizers gradually volatilize or migrate over time, causing the material to become harder and more brittle. Data shows that for PVC with DEHP plasticizers, the loss of plasticizers can reach 15-20% within 1 year at 60°C.

The environmental factors influencing the aging of PVC include ultraviolet radiation intensity, temperature and humidity. The aging rate in equatorial regions is 2-3 times faster than that in temperate regions. For every 10℃ increase in temperature, the aging rate increases by approximately 1 fold. High humidity environments accelerate the release of HCl and promote degradation. In terms of material factors, the stabilizer system, pigment type and surface treatment all affect the aging process. Lead salt stabilizers have better long-term thermal stability than calcium-zinc stabilizers. Titanium dioxide and other pigments can provide certain UV shielding effects. Fluorocarbon coatings can significantly improve weather resistance.

Regarding the issue of aging, modern material technology offers a variety of advanced protection solutions. In terms of material selection, for outdoor use, UV-stable PVC is recommended, such as formulations that contain light stabilizers like Tinuvin; consider using weather-resistant PVC sheets produced by ASA co-extrusion; choose PVC composite materials modified with nano-TiO2, which can increase the UV resistance rate to over 90%. In terms of surface protection technology, applying PVDF coating can ensure a weathering life of over 15 years; using nano-SiO2 modified polyurethane topcoat can simultaneously enhance wear resistance and weathering performance; consider adhering PET fluorocarbon film, which is both aesthetically pleasing and has excellent weathering properties. During daily maintenance, it is recommended to clean the surface once every 6 months to remove contaminants; inspect the surface coating condition once every 2 years; avoid using strong alkaline cleaners (with pH > 9). Aging assessment can be conducted using various methods: regularly measure the yellowing index (consider protective measures when it exceeds 15); use a portable infrared spectrometer to detect the content of C=O groups; conduct simple bending tests to check for brittleness.

Scientific storage management can significantly extend the service life of PVC sheets. In terms of environmental control, the temperature should be maintained at 15-25℃, the humidity at 40-60%RH, and direct sunlight should be avoided. Good air circulation should be maintained. Regarding storage methods, when placed horizontally, the sheets should be separated by foam paper, and the stacking height should not exceed 1.5m, with the spacing between the bottom support plates not exceeding 50cm; when placed vertically, the inclination angle should not exceed 15°, and a dedicated support should be used for support, with support points set every 1m. Inventory management should follow the "first in, first out" principle. Pay attention to the longest storage period (hard PVC not exceeding 2 years, soft PVC not exceeding 1 year), and regularly check the status of the inventory materials.

With the continuous advancement of material technology, the performance of PVC sheets will continue to improve, and their application fields will also expand continuously. Through scientific material selection, reasonable design and standardized maintenance, problems such as deformation, aging and surface damage can be effectively addressed, maximizing the advantages of PVC materials and creating greater economic and social value for various industries. In the future, with the development of nanotechnology, self-repairing technology and intelligent monitoring technology, PVC sheets will undoubtedly exhibit more excellent performance and usage experience, bringing more possibilities to industrial production and daily life.