Effect of cellulose ethers (HPMC) on the tensile adhesive strength of ceramic tile adhesives

Cellulose ethers, especially Hydroxypropyl Methylcellulose (HPMC), are widely used in commercial mortar as water retainers, thickeners, and binders.

Surface-treated cellulose ethers are designed for aqueous products. Due to market demands and production considerations, most HPMC products available are of the surface-treated type. Whether surface-treated cellulose ethers are suitable for cement-based products has been a concern for many mortar manufacturers.

Now, let’s examine the difference in tensile adhesive strength under different curing conditions between surface-treated and non-surface-treated cellulose ethers applied in tile adhesive.

Raw Materials and Experimental Methods

Materials and Basic Properties

Cement: Ordinary Portland cement, with basic properties listed in Table 1.

Table1 Basic Properties of Cement

Name

Compressive Strength (MPa)

Flexural Strength (MPa)

Setting Time (h:min)

3 Days

28 Days

3 Days

28 Days

Initial Set

Final Set

Cement

4.36

6.29

24.6

45.7

2:35

3:45

 

Silica Sand: Local river sand is used

Particle size distribution of two types of river sand (30-50 mesh and 50-140 mesh) As shown in Figure 1:

Figure 1 Particle size distribution of two kinds of sand

Cellulose ethers

We chose hydroxypropyl methyl cellulose ethers with a similar degree of substitution and a viscosity of around 120,000 for both surface-treated and non-surface-treated samples. There were 16 samples for each type. The key technical specifications of the samples are listed in Table 2.

Table 2: Key Technical Specifications of Two Types of Cellulose Ethers

Name

Viscosity mpa.s

Ash %

Moisture %

Methoxy%

Hydroxypropoxy%

Surface-treated

123389

2.57

3.66

20.03

8.84

Non-surface-treated

129483

2.54

3.82

21.63

8.73

The viscosity testing environment is at 20℃.

Experimental Methods

Experimental Design

The study refers to the most basic tile adhesive formula commonly found in the current market. To minimize influencing factors, other components and proportions remain unchanged. Latex powder is not added. Only the type of cellulose ether is varied to compare its effect on the tensile strength of cement-based tile adhesives. This formula is for reference only. Specific proportions of each component in the formula are listed in Table 3.

Table 3 Experimental Proportions

Raw Material

Ordinary Portland Cement

Medium Sand

Fine Sand

Cellulose Ether

 

 

30-50 mesh

50-140 mesh

 

Percentage %

35

24.6

40

0.4

Testing Methods

The molding, curing, and testing of specimens follow relevant methods outlined in industry-standard JC/T 547-2005 “Ceramic Tile Adhesive for Walls and Floors” to meet the standard consistency.  Water (23%) is added and mixed to achieve workability. The specimens are molded under standard conditions (23±2°C, 50±5% RH). The tensile strength of tile adhesive is tested under different curing conditions, including initial tensile bond strength (28 days standard curing), tensile bond strength after immersion in water (7 days standard curing + 21 days water immersion), tensile strength after 20 minutes of open time, and tensile strength under standard curing conditions for 2 days. In this study, tensile bond strength is tested using a tensile tester, with a loading rate of 250N/s for bonding to concrete substrates. The Vicat apparatus is used to measure the setting time of each sample.

Experimental Results and Analysis

Influence on Early Tensile Adhesive Strength

The tensile adhesive strength on concrete substrate surfaces under 48-hour standard laboratory curing conditions is examined. Sixteen samples of each of the two major categories of cellulose ethers listed in Table 3 are used in the tile adhesive proportions. Test specimens are formed and tested according to the methods described in JC/T 547-2005. Tensile adhesive strength is measured after 48 hours of curing under standard laboratory conditions.

The tensile adhesive test data for the same type of cellulose ether are grouped for statistical comparison between surface-treated and non-surface-treated cellulose ethers in terms of their influence on the early tensile adhesive strength of tile adhesive.

Plot of tensile strength effect under 2-days room temperature curing on concrete substrate

From the data perspective, there is little difference in the influence of surface-treated and non-surface-treated cellulose ethers on the early tensile adhesive strength of tile adhesive. The difference in mean values is only about 2%, with the non-surface-treated type slightly higher than the surface-treated type. It can be concluded that there is no significant difference in the effect of the two types of hydroxypropyl methyl cellulose ethers on the early tensile adhesive strength of tile adhesive.

Tensile Adhesive Bond Strength

Curing conditions involve 28-day standard laboratory conditions on concrete substrate surfaces.

Similarly, sixteen samples of each of the two major categories of cellulose ethers listed in Table 3 are used in the tile adhesive proportions. Test specimens are formed according to standard requirements and cured for 28 days under standard laboratory conditions. The tensile adhesive bond strength is then tested.

The test data for the tensile adhesive bond strength of the same type of cellulose ether are grouped for statistical comparison between surface-treated and non-surface-treated cellulose ethers in terms of their influence on the 28-day tensile adhesive bond strength of tile adhesive.

Plot of tensile strength effect under 28-days room temperature curing on concrete substrate

From the data perspective, there is little difference in the influence of surface-treated and non-surface-treated cellulose ethers on the original tensile adhesive bond strength of tile adhesive. The difference in mean values is only about 1%, with the non-surface-treated type slightly lower than the surface-treated type. It can be concluded that both types of cellulose ethers have consistent effects on the 28-day original tensile adhesive bond strength of tile adhesive.

Tensile Adhesive Bond Strength after Immersion in Water

Curing conditions involve 7-day standard laboratory conditions followed by 21 days of immersion in water at (20±2)°C, on concrete substrate surfaces. The test data for the tensile adhesive bond strength of the same type of cellulose ether are grouped for statistical comparison between surface-treated and non-surface-treated cellulose ethers in terms of their influence on the tensile adhesive bond strength of tile adhesive after water immersion.

 Tensile strength effect plot after water immersion on cohesive substrate

Based on the data analysis, the impact of surface-treated and non-surface-treated HPMC on the tensile adhesive bond strength of tile adhesive after immersion in water is substantial. There’s an approximate 10% difference in mean values, with the surface-treated type showing significantly higher values compared to the non-surface-treated type. Therefore, it can be concluded that surface-treated HPMC is more effective in ensuring the water-resistant tensile adhesive bond strength of tile adhesive.

Tensile Adhesive Bond Strength after 20 Minutes of Open Time

Test specimens for the open time are formed according to the method described in JC/T 547-2005, with a required open time of 20 minutes. The specimens are stored under standard curing conditions for 28 days, and their tensile adhesive bond strength is tested. The test data for the tensile adhesive bond strength of the same type of cellulose ether are grouped for statistical comparison between surface-treated and non-surface-treated cellulose ethers in terms of their influence on the strength under conditions of 20 minutes of open time for tile adhesive.

Tensile Adhesive Strength Effect Plot on Coagulated Substrate, Airing Time 20 Minutes

From the data perspective, the influence of surface-treated and non-surface-treated cellulose ethers on the open time of tile adhesive is significant. The difference in mean values is approximately 20%, with the surface-treated type notably higher than the non-surface-treated type. It can be concluded that surface-treated cellulose ethers are more favorable for maintaining the surface condition of tile adhesive and extending the time for surface skin formation of the tile adhesive.

Setting Time

Similarly, sixteen samples of each of the two major categories of cellulose ethers listed in Table 3 are used in the tile adhesive proportions. With a fixed amount of water added, the setting time of the samples is measured using a Vicat apparatus. “IS” denotes the initial setting time, and “FS” denotes the final setting time.

Condensation time of different types of cellulose ethers

From the data perspective, both surface-treated and non-surface-treated cellulose ethers have a significant impact of nearly one hour on the initial and final setting times of tile adhesive. Surface-treated cellulose ethers exhibit noticeably weaker retarding properties compared to non-surface-treated ones.

 

Analysis and Conclusion

Surface-treated cellulose ethers undergo surface modification with formaldehyde, enabling them to disperse rapidly and uniformly in neutral cold water without clumping. This enhances their ability to form a homogeneous colloid in water. However, this protective effect of formaldehyde diminishes rapidly in alkaline environments, such as when the pH exceeds 9. In such conditions, cellulose ethers quickly dissolve to form colloids. Therefore, in cement-based materials, there is no difference in the dissolution rate between surface-treated and non-surface-treated cellulose ethers. However, these two types of cellulose ethers have different effects on the mechanical properties and setting time of cement-based materials.

Under standard conditions

  • The influence of both types of cellulose ethers on the tensile adhesive bond strength of tile adhesive is minimal, with no significant difference in strength data.

Water resistance

  • Tile adhesive formulated with surface-treated cellulose ethers exhibits better tensile adhesive bond strength compared to those formulated with non-surface-treated cellulose ethers.

Open time

  • Tile adhesive formulated with surface-treated cellulose ethers demonstrates significantly higher tensile adhesive bond strength and longer open time compared to those formulated with non-surface-treated cellulose ethers.

Setting time

  • Tile adhesive formulated with surface-treated cellulose ethers has a faster curing rate.

In summary, surface-treated cellulose ethers are not only suitable for aqueous products but also perform well in cement-based dry powder products. Some data even outperform those of non-surface-treated cellulose ethers.

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