Development of novel HEMC cellulose ethers to reduce agglomeration in gypsum-based machine-sprayed plasters

Development of novel HEMC cellulose ethers to reduce agglomeration in gypsum-based machine-sprayed plasters

Gypsum-based machine-sprayed plaster (GSP) has been widely used in Western Europe since the 1970s. The emergence of mechanical spraying has effectively improved the efficiency of plastering construction while reducing construction costs. With the deepening of GSP commercialization, water-soluble cellulose ether has become a key additive. Cellulose ether endows GSP with good water retention performance, which limits the substrate’s absorption of moisture in the plaster, thereby obtaining a stable setting time and good mechanical properties. In addition, the specific rheological curve of cellulose ether can improve the effect of machine spraying and significantly simplify the subsequent mortar leveling and finishing processes.

Despite the obvious advantages of cellulose ethers in GSP applications, it can also potentially contribute to the formation of dry lumps when sprayed. These unwetted clumps are also known as clumping or caking, and they can adversely affect the leveling and finishing of the mortar. Agglomeration can reduce site efficiency and increase the cost of high-performance gypsum product applications. In order to better understand the effect of cellulose ethers on the formation of lumps in GSP, we conducted a study to try to identify the relevant product parameters that influence their formation. Based on the results of this study, we developed a series of cellulose ether products with a reduced tendency to agglomerate and evaluated them in practical applications.

Key words: cellulose ether; gypsum machine spray plaster; dissolution rate; particle morphology

1. Introduction

Water-soluble cellulose ethers have been successfully used in gypsum-based machine-sprayed plasters (GSP) to regulate water demand, improve water retention and improve the rheological properties of mortars. Therefore, it helps to improve the performance of the wet mortar, thereby ensuring the required strength of the mortar. Due to its commercially viable and environmentally friendly properties, dry mix GSP has become a widely used interior building material throughout Europe over the past 20 years.

Machinery for mixing and spraying dry-blend GSP has been successfully commercialized for decades. Although some technical features of equipment from different manufacturers vary, all commercially available spraying machines allow a very limited agitation time for water to mix with cellulose ether-containing gypsum dry-mix mortar. Generally, the entire mixing process takes only a few seconds. After mixing, the wet mortar is pumped through the delivery hose and sprayed onto the substrate wall. The whole process is completed within a minute. However, in such a short period of time, cellulose ethers need to be completely dissolved in order to fully develop their properties in the application. Adding finely ground cellulose ether products to gypsum mortar formulations ensures complete dissolution during this spraying process.

The finely ground cellulose ether builds up consistency quickly on contact with water during agitation in the sprayer. The rapid viscosity rise caused by the dissolution of the cellulose ether causes problems with the concurrent water wetting of the gypsum cementitious material particles. As the water begins to thicken, it becomes less fluid and cannot penetrate into the small pores between the gypsum particles. After the access to the pores is blocked, the wetting process of the cementitious material particles by water is delayed. The mixing time in the sprayer was shorter than the time required to fully wet the gypsum particles, which resulted in the formation of dry powder clumps in the fresh wet mortar. Once these clumps are formed, they hinder the efficiency of workers in subsequent processes: leveling mortar with clumps is very troublesome and takes more time. Even after the mortar has set, initially formed clumps may show up. For example, covering the clumps inside during construction will lead to the appearance of dark areas in the later stage, which we don’t want to see.

Although cellulose ethers have been used as additives in GSP for many years, their effect on the formation of unwetted lumps has not been studied much so far. This article presents a systematic approach that can be used to understand the root cause of agglomeration from a cellulose ether perspective.

2. Reasons for the formation of unwetted clumps in GSP

2.1 Wetting of plaster-based plasters

In the early stages of establishing the research program, a number of possible root causes for the formation of clumps in the CSP were assembled. Next, through computer-aided analysis, the problem is focused on whether there is a practical technical solution. Through these works, the optimal solution to the formation of agglomerates in GSP was preliminarily screened out. From both technical and commercial considerations, the technical route of changing the wetting of gypsum particles by surface treatment is ruled out. From a commercial point of view, the idea of replacing the existing equipment with a spraying equipment with a specially designed mixing chamber that can ensure sufficient mixing of water and mortar is ruled out.

Another option is to use wetting agents as additives in gypsum plaster formulations and we found a patent for this already. However, the addition of this additive inevitably negatively affects the workability of the plaster. More importantly, it changes the physical properties of the mortar, especially hardness and strength. So we didn’t delve too deeply into it. In addition, the addition of wetting agents is also considered to possibly have an adverse impact on the environment.

Considering that cellulose ether is already part of the gypsum-based plaster formulation, optimizing cellulose ether itself becomes the best solution that can be selected. At the same time, it must not affect the water retention properties or adversely affect the rheological properties of the plaster in use. Based on the previously proposed hypothesis that the generation of non-wetted powders in GSP is due to the excessively fast increase in the viscosity of cellulose ethers after contact with water during stirring, controlling the dissolution characteristics of cellulose ethers became the main goal of our study .

2.2 Dissolving time of cellulose ether

An easy way to slow down the dissolution rate of cellulose ethers is to use granular grade products. The main disadvantage of using this approach in GSP is that particles that are too coarse do not dissolve completely within the short 10-second agitation window in the sprayer, which leads to a loss of water retention. In addition, the swelling of undissolved cellulose ether in the later stage will lead to thickening after plastering and affect the construction performance, which is what we don’t want to see.

Another option to reduce the dissolution rate of cellulose ethers is to reversibly crosslink the surface of cellulose ethers with glyoxal. However, since the crosslinking reaction is pH-controlled, the dissolution rate of cellulose ethers is highly dependent on the pH of the surrounding aqueous solution. The pH value of the GSP system mixed with slaked lime is very high, and the cross-linking bonds of glyoxal on the surface are quickly opened after contacting water, and the viscosity begins to rise instantly. Therefore, such chemical treatments cannot play a role in controlling the dissolution rate in GSP.

The dissolution time of cellulose ethers also depends on their particle morphology. However, this fact has not received much attention so far, although the effect is very significant. They have a constant linear dissolution rate [kg/(m2•s)], so their dissolution and viscosity build-up are proportional to the available surface. This rate can vary significantly with changes in the morphology of the cellulose particles. In our calculations it is assumed that full viscosity (100%) is reached after 5 seconds of stirring mixing.

Calculations of different particle morphologies showed that spherical particles had a viscosity of 35% of the final viscosity at half the mixing time. In the same time period, rod-shaped cellulose ether particles can only reach 10%. The disc-shaped particles just started to dissolve after 2.5 seconds.

Also included are ideal solubility characteristics for cellulose ethers in GSP. Delay initial viscosity build-up for more than 4.5 seconds. Thereafter, the viscosity increased rapidly to reach the final viscosity within 5 seconds of stirring mixing time. In GSP, such a long delayed dissolution time allows the system to have a low viscosity, and the added water can fully wet the gypsum particles and enter the pores between the particles without disturbance.

3. Particle morphology of cellulose ether

3.1 Measurement of particle morphology

Since the shape of cellulose ether particles has such a significant impact on solubility, it is first necessary to determine the parameters describing the shape of cellulose ether particles, and then to identify the differences between non-wetting The formation of agglomerates is a particularly relevant parameter.

We obtained the particle morphology of cellulose ether by dynamic image analysis technique. The particle morphology of cellulose ethers can be fully characterized using a SYMPATEC digital image analyzer (made in Germany) and specific software analysis tools. The most important particle shape parameters were found to be the average length of fibers expressed as LEFI(50,3) and the average diameter expressed as DIFI(50,3). Fiber average length data are considered to be the full length of a certain spread out cellulose ether particle.

Usually particle size distribution data such as the average fiber diameter DIFI may be calculated based on the number of particles (denoted by 0), length (denoted by 1), area (denoted by 2) or volume (denoted by 3). All particle data measurements in this paper are based on volume and are therefore indicated with a 3 suffix. For example, in DIFI(50,3), 3 means the volume distribution, and 50 means that 50% of the particle size distribution curve is smaller than the indicated value, and the other 50% is larger than the indicated value. Cellulose ether particle shape data are given in micrometers (µm).

3.2 Cellulose ether after particle morphology optimization

Taking into account the effect of the particle surface, the particle dissolution time of cellulose ether particles with a rod-like particle shape strongly depends on the average fiber diameter DIFI (50,3). Based on this assumption, development work on cellulose ethers was aimed at obtaining products with a larger average fiber diameter DIFI (50,3) to improve the solubility of the powder.

However, an increase in the average fiber length DIFI(50,3) is not expected to be accompanied by an increase in the average particle size. Increasing both parameters together will result in particles that are too large to dissolve completely within the typical 10-second agitation time of mechanical spraying.

Therefore, an ideal hydroxyethylmethylcellulose (HEMC) should have a larger average fiber diameter DIFI(50,3) while maintaining the average fiber length LEFI(50,3). We use a new cellulose ether production process to produce an improved HEMC. The particle shape of the water-soluble cellulose ether obtained through this production process is completely different from the particle shape of the cellulose used as the raw material for production. In other words, the production process allows the particle shape design of cellulose ether to be independent of its production raw materials.

Three scanning electron microscope images: one of cellulose ether produced by the standard process, and one of cellulose ether produced by the new process with a larger diameter of DIFI(50,3) than conventional process tool products. Also shown is the morphology of the finely ground cellulose used in the production of these two products.

Comparing the electron micrographs of cellulose and cellulose ether produced by the standard process, it is easy to find that the two have similar morphological characteristics. The large number of particles in both images exhibits typically long, thin structures, suggesting that the basic morphological features have not changed even after the chemical reaction has taken place. It is clear that the particle morphology characteristics of the reaction products are highly correlated with the raw materials.

It was found that the morphological characteristics of the cellulose ether produced by the new process are significantly different, it has a larger average diameter DIFI (50,3), and mainly presents round short and thick particle shapes, while the typical thin and long particles in cellulose raw materials Almost extinct.

This figure again shows that the particle morphology of the cellulose ethers produced by the new process is no longer related to the morphology of the cellulose raw material – the link between the morphology of the raw material and the final product no longer exists.

4. Effect of HEMC particle morphology on the formation of unwetted clumps in GSP

GSP was tested under field application conditions to verify that our hypothesis about the working mechanism (that using a cellulose ether product with a larger mean diameter DIFI (50,3) would reduce unwanted agglomeration) was correct. HEMCs with mean diameters DIFI(50,3) ranging from 37 µm to 52 µm were used in these experiments. In order to minimize the influence of factors other than particle morphology, the gypsum plaster base and all other additives were kept unchanged. The viscosity of the cellulose ether was kept constant during the test (60,000mPa.s, 2% aqueous solution, measured with a HAAKE rheometer).

A commercially available gypsum sprayer (PFT G4) was used for spraying in the application trials. Focus on evaluating the formation of unwetted clumps of gypsum mortar immediately after it has been applied to the wall. Assessment of clumping at this stage throughout the plastering application process will best reveal differences in product performance. In the test, experienced workers rated the clumping situation, with 1 being the best and 6 being the worst.

The test results clearly show the correlation between the average fiber diameter DIFI (50,3) and the clumping performance score. Consistent with our hypothesis that cellulose ether products with larger DIFI(50,3) outperformed smaller DIFI(50,3) products, the average score for DIFI(50,3) of 52 µm was 2 (good) , while those with DIFI(50,3) of 37µm and 40µm scored 5 (failure).

As we expected, the clumping behavior in GSP applications depends significantly on the average diameter DIFI(50,3) of the cellulose ether used. Moreover, it was mentioned in the previous discussion that among all the morphological parameters DIFI(50,3) strongly affected the dissolution time of cellulose ether powders. This confirms that cellulose ether dissolution time, which is highly correlated with particle morphology, ultimately affects the formation of clumps in GSP. A larger DIFI (50,3) causes a longer dissolution time of the powder, which significantly reduces the chance of agglomeration. However, too long powder dissolution time will make it difficult for the cellulose ether to dissolve completely within the stirring time of the spraying equipment.

The new HEMC product with an optimized dissolution profile due to a larger average fiber diameter DIFI(50,3) not only has better wetting of the gypsum powder (as seen in the clumping evaluation), but also does not affect The water retention performance of the product. The water retention measured according to EN 459-2 was indistinguishable from HEMC products of the same viscosity with DIFI(50,3) from 37µm to 52µm. All measurements after 5 minutes and 60 minutes fall within the required range shown in the graph.

However, it was also confirmed that if DIFI(50,3) becomes too large, the cellulose ether particles will no longer dissolve completely. This was found when testing a DIFI(50,3) of 59 µM product. Its water retention test results after 5 minutes and especially after 60 minutes failed to meet the required minimum.

5. Summary

Cellulose ethers are important additives in GSP formulations. The research and product development work here looks at the correlation between the particle morphology of cellulose ethers and the formation of unwetted clumps (so-called clumping) when mechanically sprayed. It is based on the assumption of the working mechanism that the dissolution time of cellulose ether powder affects the wetting of gypsum powder by water and thus affects the formation of clumps.

The dissolution time depends on the particle morphology of the cellulose ether and can be obtained using digital image analysis tools. In GSP, cellulose ethers with a large average diameter of DIFI (50,3) have optimized powder dissolution characteristics, allowing more time for water to thoroughly wet the gypsum particles, thus enabling optimum anti-agglomeration. This type of cellulose ether is produced using a new production process, and its particle form does not depend on the original form of the raw material for production.

The average fiber diameter DIFI (50,3) has a very important effect on clumping, which has been verified by adding this product to a commercially available machine-sprayed gypsum base for on-site spraying. Furthermore, these field spray tests confirmed our laboratory results: the best performing cellulose ether products with large DIFI (50,3) were completely soluble within the time window of GSP agitation. Therefore, the cellulose ether product with the best anti-caking properties after improving the particle shape still maintains the original water retention performance.