The phenomenon of viscosity drop during paint storage is a common problem, especially after long-term storage, the viscosity of the paint decreases significantly, affecting the construction performance and product quality. The decrease in viscosity is related to many factors, such as temperature, humidity, solvent volatilization, polymer degradation, etc., but the interaction with the thickener cellulose ether is particularly critical.
1. Basic role of cellulose ether
Cellulose ether is a common thickener widely used in water-based paints. Their main functions include:
Thickening effect: Cellulose ether can form a swollen three-dimensional network structure by absorbing water, thereby increasing the viscosity of the system and improving the thixotropy and construction performance of the paint.
Suspension stabilization effect: Cellulose ether can effectively prevent the sedimentation of solid particles such as pigments and fillers in the paint and maintain the uniformity of the paint.
Film-forming property: Cellulose ether can also affect the film-forming property of the paint, making the coating have a certain toughness and durability.
There are many types of cellulose ethers, including methyl cellulose (MC), hydroxyethyl cellulose (HEC), hydroxypropyl methyl cellulose (HPMC), etc. These materials have different solubility, thickening ability and storage resistance in coatings.
2. Main reasons for viscosity reduction
During the storage of coatings, the viscosity reduction is mainly caused by the following reasons:
(1) Degradation of cellulose ethers
The thickening effect of cellulose ethers in coatings depends on the size of their molecular weight and the integrity of their molecular structure. During storage, factors such as temperature, acidity and alkalinity, and microorganisms may cause the degradation of cellulose ethers. For example, during long-term storage, the acidic or alkaline components in the coating may hydrolyze the molecular chain of cellulose ether, reduce its molecular weight, and thus weaken its thickening effect, resulting in a decrease in viscosity.
(2) Solvent volatilization and moisture migration
Solvent volatilization or moisture migration in the coating may affect the solubility state of cellulose ether. During storage, part of the water may evaporate or migrate to the surface of the coating, making the distribution of water in the coating uneven, thereby affecting the swelling degree of cellulose ether and causing a decrease in viscosity in local areas.
(3) Microbial attack
Microbial growth may occur in the coating when it is improperly stored or the preservatives become ineffective. Microorganisms can decompose cellulose ethers and other organic thickeners, weakening their thickening effect and causing the viscosity of the coating to decrease. Water-based coatings, in particular, are a good environment for microbial growth because they contain a large amount of water.
(4) High temperature aging
Under high temperature storage conditions, the physical or chemical structure of the cellulose ether molecular chain may change. For example, cellulose ethers are prone to oxidation or pyrolysis at higher temperatures, resulting in a weakening of the thickening effect. High temperatures also accelerate solvent volatilization and water evaporation, further affecting viscosity stability.
3. Methods to improve the storage stability of coatings
In order to reduce the decrease in viscosity during storage and extend the storage life of the coating, the following measures can be taken:
(1) Selecting the right cellulose ether
Different types of cellulose ethers have different performances in terms of storage stability. Cellulose ethers with high molecular weight generally have better thickening effects, but their storage stability is relatively poor, while cellulose ethers with lower molecular weight may have better storage performance. Therefore, when designing the formula, cellulose ethers with good storage stability should be selected, or cellulose ethers should be compounded with other thickeners to improve their storage resistance.
(2) Control the pH of the coating
The acidity and alkalinity of the coating system have an important influence on the stability of cellulose ethers. In the formulation design, the pH value of the coating should be controlled to avoid an overly acidic or alkaline environment to reduce the degradation of cellulose ethers. At the same time, adding an appropriate amount of pH adjuster or buffer can help stabilize the pH of the system.
(3) Increase the use of preservatives
In order to prevent microbial erosion, an appropriate amount of preservatives should be added to the coating. Preservatives can inhibit the growth of microorganisms, thereby preventing organic substances such as cellulose ether from being decomposed and maintaining the stability of the coating. Appropriate preservatives should be selected according to the coating formulation and storage environment, and their effectiveness should be checked regularly.
(4) Control the storage environment
The storage temperature and humidity of the coating have a direct impact on the viscosity stability. The coating should be stored in a dry and cool environment, avoiding high temperature and high humidity conditions to reduce solvent volatilization and cellulose ether degradation. In addition, well-sealed packaging can effectively reduce the migration and evaporation of water and delay the decrease in viscosity.
4. Other factors affecting viscosity
In addition to cellulose ethers, other components in the coating system may also affect the change in viscosity. For example, the type and concentration of pigments, the volatilization rate of solvents, and the compatibility of other thickeners or dispersants may affect the viscosity stability of the coating. Therefore, the overall design of the coating formula and the interaction between components are also key points that need to be paid attention to.
The decrease in viscosity during the storage of the coating is closely related to factors such as the degradation of cellulose ethers, solvent volatilization, and water migration. In order to improve the storage stability of the coating, appropriate cellulose ether varieties should be selected, the pH of the coating should be controlled, anti-corrosion measures should be strengthened, and the storage environment should be optimized. Through reasonable formula design and good storage management, the problem of viscosity decrease during the storage of the coating can be effectively reduced, and the product’s performance and market competitiveness can be improved.
Post time: Sep-27-2024