What specific impact does temperature change have on the stability of HPMC in building materials, and how should it be addressed? ​

Hydroxypropyl methylcellulose (HPMC)As a key additive in building materials such as cement mortar, putty, coatings, and insulation mortar, its stability (such as viscosity retention, water retention, and film-forming properties) is significantly affected by temperature changes. The core mechanism is related to the molecular chain movement state, hydrogen bonding, and the interaction with environmental media (such as moisture and salt). The following are specific impacts and countermeasures:

1、 The Effect and Mechanism of Temperature Rise on the Stability of HPMC

The temperature rise in the construction or use environment of building materials (such as high temperature in summer, heat release from cement hydration, oven drying, etc.) can cause the following changes in the performance of HPMC:

1. Significant decrease in viscosity and weakened water retention

Mechanism: HPMC molecular chains form hydrogen bonds with water molecules through hydroxyl groups, forming a three-dimensional network structure, which is the core of their thickening and water retention. When the temperature rises (usually exceeding 50 ℃), molecular thermal motion intensifies, hydrogen bonds are broken, the network structure becomes loose, and molecular chains curl up, resulting in a significant decrease in solution viscosity (such as a 2% HPMC aqueous solution with a viscosity of 10000 mPa · s at 25 ℃, which may drop to below 3000 mPa · s at 80 ℃).

Consequences: The water retention of building materials systems (such as cement mortar) decreases, and water evaporation accelerates, which can easily lead to insufficient hydration of cement, surface sanding, and cracking; Coatings or putty may experience sagging and delamination due to low viscosity.

2. Thermal gel phenomenon (exceeding critical temperature)

mechanism:HPMCIt has "thermal gelling property" - when the temperature exceeds its critical gel temperature (usually 50~70 ℃, which increases with the degree of substitution), the hydrophobic effect of the molecular chain increases, and the system changes from a water-soluble state to an insoluble gel, and the system changes from a viscous liquid to a semi-solid state.

Consequences: HPMC gel will cause the system to lose fluidity and cannot be spread in the mortar under high temperature construction; If the gel is uneven, it will also form local hard blocks, which will damage the structural uniformity of building materials (such as uneven distribution of bubbles in thermal insulation mortar).

3. Accelerated degradation, long-term stability decline

Mechanism: Prolonged exposure to high temperatures (>60 ℃) and humid environments (such as underground engineering and steam curing) may lead to thermal oxidative degradation of HPMC molecular chains, ether bond breakage, and molecular weight reduction, resulting in the loss of core functions such as thickening and water retention.

Consequence: The performance of building materials deteriorates in the later stage, such as powdering of putty layer, decrease in mortar strength, and cracking of coatings.

2、 The Effect and Mechanism of Temperature Reduction on the Stability of HPMC

The impact of low-temperature environments (such as winter construction and cold regions) on HPMC is mainly reflected in its dissolution and construction performance:

1. The dissolution rate slows down and the dispersibility deteriorates

Mechanism: At low temperatures (<10 ℃), water molecules move slowly, and the hydrogen bonding efficiency between the hydrophilic groups (hydroxyl, ether groups) of HPMC molecular chains and water decreases, resulting in a prolonged dissolution time (such as 10 minutes for dissolution at 25 ℃ and over 30 minutes at 5 ℃), which can easily form "fish eyes" (partially dissolved particles).

Consequence: The dispersion of HPMC in the building materials system is uneven, resulting in abnormal local viscosity (high viscosity in high concentration areas, no thickening in low concentration areas), leading to mortar bleeding, putty clumping, and poor coating leveling.

2. Abnormal increase in viscosity and decrease in workability

Mechanism: At low temperatures, the HPMC molecular chains stretch more fully, the network structure becomes denser, and the system viscosity significantly increases (for example, the viscosity may increase by 50% to 100% at 0 ℃ compared to 25 ℃).

Consequence: The mixing resistance of cement mortar and putty increases, making it difficult to spread or apply; Paint is prone to brush marks, making it difficult to level and affecting surface smoothness.

3. Freezing causes functional failure

Mechanism: When the temperature drops below 0 ℃, the water in the building material system freezes, and the HPMC molecular chains are squeezed and destroyed by the ice crystals, causing the hydrogen bond network to break. After thawing, the original structure cannot be restored.

consequence:HPMCLosing thickening and water retention ability, mortar or putty is prone to cracking, sanding, and pinholes in the coating after drying.

3、 Targeted response measures

According to the influence of temperature changes, stability control can be achieved through HPMC selection, formula optimization, and process adjustment:

1. Measures to address the impact of high temperatures

Choose high temperature resistant HPMC model:

By increasing the degree of substitution of hydroxypropyl (MS) or methyl (DS), the hydrophobic interaction of molecular chains is enhanced, and the critical gel temperature is raised (e.g., from 50 ℃ to more than 70 ℃). For example, building grade high degree of substitution HPMC (with methoxy content of 28%~30% and hydroxypropyl content of 7%~12%) can still maintain a viscosity of over 80% at 60 ℃.

Control the dosage and formulation of HPMC:

Appropriately increase the dosage of HPMC in high temperature environments (such as from 0.2% to 0.3%~0.5%) to compensate for viscosity loss; Simultaneously compounding a small amount of other heat-resistant additives (such as guar gum ether and xanthan gum) to enhance the viscosity stability of the system through synergistic effects (the viscosity retention rate at high temperatures can be increased by 20% to 30% after compounding).

Delay the heating rate of the system:

For cement-based materials, adding retarders such as citric acid and sodium gluconate can reduce the hydration heat release rate and avoid sudden temperature rise in the system; Choose the low temperature period in the morning and evening during construction, or pre cool the substrate (such as spraying water for cooling).

2. Measures to address the impact of low temperatures

Choose low viscosity, instant HPMC:

Low molecular weight HPMC (viscosity 10000~50000 mPa · s) dissolves faster at low temperatures and the molecular chains are easier to stretch; The "instant HPMC" surface treated with etherification can reduce the formation of "fish eyes" and ensure uniform dispersion.

Optimize dissolution process:

When dissolving HPMC, use warm water (30-40 ℃, avoid exceeding 50 ℃) for pre dissolution to accelerate the stretching of molecular chains; Add a small amount of antifreeze (such as ethylene glycol, propylene glycol) to putty or paint to lower the freezing point and prevent water from freezing and damaging the HPMC structure.

Adjust the construction process:

Reduce the thickness of a single coating during low-temperature construction (such as reducing putty from 2mm to 1mm) and shorten the drying time; Insulate and maintain mortar or paint (such as covering with plastic film) to avoid sudden temperature drops.

3. Long term stability guarantee (for high temperature and high humidity environments)

Adding antioxidants: In building materials that have been exposed to high temperatures and humidity for a long time (such as underground engineering mortar), adding a small amount of antioxidants (such as BHT) can control the oxidative degradation of HPMC molecular chains and extend their functional life.

Control the salt content and pH of the system: avoid contact between HPMC and high concentrations of calcium ions (Ca ² ⁺) and magnesium ions (Mg ² ⁺) (such as reducing the amount of high alumina cement used), to prevent ion damage to hydrogen bonds; Keep the system neutral to weakly alkaline (pH 7-10) and avoid acidic environments that accelerate the hydrolysis of HPMC.

summary

Temperature change has a significant impact on the stability of HPMC in building materials: high temperature leads to viscosity reduction, gel and degradation, and low temperature leads to difficulty in dissolution and abnormal increase in viscosity. The core of response is "selection and adaptation+process adjustment": choose high substitution temperature resistant HPMC for high temperature environment and control the temperature rise; Choose instant HPMC for low-temperature environment and optimize the dissolution and construction temperature; In long-term use, stability is ensured through compounding and environmental regulation (such as antioxidant and salt control) to ensure that HPMC continues to play a role in thickening, water retention, and improving construction performance in building materials.