ARTICLES

Single-component and two-component plasters

One bucket is good, but a bag is better
Times change, and with them, problems change. The problem of total shortages that plagued Soviet people has been replaced in today’s market economy by the no less painful problem of choice. Indeed, standing in today’s supermarket in front of endless shelves of similar products, it is difficult not to get confused and make a mistake.
Making the right choice is very important. When it comes to construction and renovation, it is especially important not to make a mistake when choosing materials for exterior finishing, which are exposed to harsh weather conditions that have been clearly intensifying recently.
In this article, we will try to help our readers decide on exterior plaster. In the most general sense, we will not compare specific brands or manufacturers, but only types of plaster. More specifically, we will compare single-component “acrylic” plaster in a bucket and two-component plaster, which is sold as a “bucket + bag” set. Let’s start from the beginning.

So what exactly is the difference?

 Indeed, how fundamentally different can these two types of plaster be? It turns out they can be. But before discussing the differences, we need to assess the similarities in order to understand whether these two materials can be compared at all.

So, let’s talk about the similarities. Both single-component and two-component polymer plasters contain a polymer dispersion (most often acrylic or acrylic-styrene), a filler and a filler (most often, beautiful snow-white marble of various sizes, simply sand and flour) and a pigment (dye). The content of these substances (common to both single-component and two-component materials) exceeds ¾ of the weight of the plaster, which allows us to consider them, if not close, then certainly distant relatives. Now we can see how they differ.

There is only one difference between our relatives, but it is very significant. Its name is CEMENT. It is present in two-component plasters, and in quite large quantities. In single-component plasters, it is not present at all. And here’s why. Single-component plaster is supplied ready for use, i.e., everything necessary is already mixed in, and nothing needs to be added. On the one hand, this is convenient: open, mix, and get to work. But on the other hand, it does not allow you to simultaneously add materials that interact with each other to the composition of single-component plaster. For example, water and cement: everyone knows what happens when they are mixed.

Water must be present in single-component plaster, as it is the basis of the polymer dispersion that determines all the pleasant and useful properties of such plaster. This means that cement has to be abandoned.

In two-component plaster, cement sits comfortably in a dry bag and only comes into contact with water in the form of a dispersion during mixing, i.e., immediately before the plaster is used. It interacts with water and hardens on the wall, which is only beneficial, as we will discuss below. For now, let’s just say the most important thing: the presence or absence of cement in plaster greatly changes its character. And this affects, first of all, its durability.

What is durability, and what does it mean?

As we know, builders come in all shapes and sizes. Not in the sense of “good and very good,” but in terms of specialization. Some builders specialize in construction itself, building houses and other structures that people need from scratch. The other part repairs what the first part has built. This is their profession, this is how they earn their living: every day, year after year, they do repairs.

But if you are not lucky enough to belong to this category of people, you are unlikely to enjoy doing repairs year after year. Especially in your own home. Especially with your own hard-earned money. You will have a legitimate and fair desire: the repairs in your home should last as long as possible. Until you get tired of them. Durability is precisely what is responsible for the realization of this desire.

Durability is the ability of a material to retain its original performance characteristics for a long time, despite various harmful influences. With regard to plaster, this means that it must retain its color, texture, integrity, and other useful properties, including the ability to adhere to the wall, while effectively resisting all kinds of weather influences. To simplify the further explanation, these influences can be reduced to two antagonistic pairs: “frost and sun” and “water and land.” So, let’s start with frost.

Oh, Frost-Frost…

One of the main parameters determining the durability of facade acrylic plaster is, of course, frost resistance. Freezing and thawing can quickly destroy even the strongest granite, let alone a polymer coating. Therefore, we will pay special attention to comparing the frost resistance of different acrylic plaster compositions.

First, it is necessary to determine the mechanisms of destruction under the influence of frost of one- and two-component plasters. And they differ greatly. These differences are due to the different hardening mechanisms of these two types of plaster.

As established by special studies, single-component acrylic plaster simply dries after application. No chemical processes such as “cross-linking” or polymerization occur, the only change is the removal of water layers that prevented the polymer molecules from sticking together. The resulting material (provided it is thoroughly dried) can best be described by the word “glass” in terms of its appearance and properties. Beautiful, transparent, but fragile. Especially in the cold. And at the same time, it is less durable than the window glass we are used to.

What does this mean? It means that the dried polymer will break in the same way as glass scratched with a glass cutter: any scratch or small defect is enough for a crack to form instantly in the cold. During a thaw, water will rush into this crack and fill it. And when it freezes, the ice, which has increased in volume, will complete the destruction.

In two-component plasters, hardening occurs in a completely different way. Of course, the drying process also takes place in them, but it is not the determining factor. The main role in the structure formation of such plasters is played by the hardening of cement. This is a complex, multi-phase process that leads to the formation of a branched three-dimensional structure consisting of very thin fibers of cement hardening products, which bind the filler grains together and are covered with a “coat” of polymer molecules. The fibers form a strong three-dimensional framework, reinforcing the entire volume of the plaster. And instead of simple brittle glass, we get what is called a “composite” in the language of building materials science.

In composite materials, each component plays its role in a well-coordinated ensemble. The microfibers of cement hardening products take on the most dangerous tensile stresses, dampen emerging microcracks, redistribute loads, preventing them from growing to destructive values. And the dried polymer ensures the density of the structure and the transfer of external loads applied to the fibers.

According to building materials science, the strength and durability of the composite exceeds these characteristics for the individual components that make up its composition, and this effect is called synergistic. In our case, the frost resistance of plaster containing both cement and polymer will always be higher than that of pure polymer plaster.

Why plasterers don’t like to sunbathe.

Before delving further into the secrets of acrylic and cement, we need to take a brief detour into the world of everyday household appliances. Remember how beautiful the snow-white color of your newly purchased refrigerator was? Or a microwave oven, or another interior item with white plastic parts. But now, several years have passed, and upon closer inspection, it turns out that the whiteness is no longer the same, the once flawless plastic has taken on a dullness, even an unhealthy tint. And the reason here is not dirt and soot (or rather, not only that). The plastic has faded. Under the influence of sunlight, the surface of the plastic deteriorates, and there is nothing that can be done about it. Solar ultraviolet rays can break down any organic matter; it is only a matter of time.

Imagine: if this happened to plastic protected by double glazing in the greenhouse conditions of an air-conditioned kitchen, what happens to plaster, which takes the brunt of the sun’s rays? It doesn’t have it easy.

Of course, plaster manufacturers are aware of the harmful effects of ultraviolet rays on acrylic and try to protect it in every way possible, but all their tricks only lead to a slight extension of its service life. It is impossible to make a completely resistant polymer.

Now let’s remember what happens to cement under the influence of ultraviolet rays.

That’s right, NOTHING. Cement is cement, a purely inorganic material, rock solid. Not only is it not afraid of light, but it also does not allow it to penetrate deep into the plaster. Therefore, the destruction of two-component plaster under the influence of light is limited to a thin surface layer, and then the acrylic quietly serves in the shade of the cement frame.

About wet water and dry wind

At this stage of our analysis of plastering charms, we will have to recall philosophy and turn to common sense. And ask it, this very common sense, the following question: if you dry something wet and then put it back in water, what will happen to it? And common sense will answer: it will get wet again. It may even get soggy, that is, it will become unrecognizable.

Let’s try to apply this conclusion to acrylic plaster. What does acrylic dispersion do after being applied to the wall? That’s right, it dries. So, if you pour water on it, it will get wet. The degree of wetting and spoiling is directly proportional to the degree of previous drying. And since in real conditions the dispersion cannot dry completely, some part of it, albeit small but not zero, will spoil back to the state of dispersion and will be safely washed away by the rain. This unattractive phenomenon can sometimes be seen on facades that have been poorly plastered during the rainy season: under heavy rain, a whitish “milk” flows down from them, which is diluted dispersion. And what if it rains for a month? Given the current weather, this is quite likely…

Now let’s imagine what will happen to a piece of hardened cement if it is thrown into water. That’s right, NOTHING. No, of course, it will get wet and soak up water, but it will not dissolve. This is because during hardening, cement does not simply dry out, but interacts with water to form new substances that are insoluble in water. So even in the heaviest rain, the acrylic will still wash off the thin top layer. Further, the water will encounter a barrier of insoluble cement hardening products and will be forced to stop its destructive activity.

It would seem that if water is bad for plaster, then a dry warm breeze is a real blessing, right? Wrong! To understand this, we need to return to everyday phenomena. Only now we will go not to the kitchen, but to the bathroom. Where freshly washed, newly purchased jeans are drying. They fit perfectly when you tried them on. But what about after washing? That’s right. The jeans have shrunk. In scientific terms, shrinkage occurred during drying.

The terrible secret of plaster is that it also shrinks when it dries. And if during the first drying of freshly applied plaster this shrinkage is compensated by the elasticity of polymer molecules, then after a prolonged rain, which partially washed away and partially disrupted the integrity of the acrylic film, the elasticity of the plaster decreases. And after a certain number of cycles, shrinkage will prevail…

What about cement? Does it not shrink when it dries? Let’s be honest: it does. But to a much lesser extent than organic materials. At the same time, the three-dimensional cement framework is much stiffer than the soft acrylic film. And when the film tries to shrink, the framework does not allow it to do so.

What does this mean?

So, we have examined our “cousin” plasters from various angles. And we can now draw some conclusions. Yes, single-component plaster is more convenient to store and work with. But TWO-COMPONENT PLASTER IS SUPERIOR IN SERVICE, as it is much more resistant to various external influences and, as a result, has a longer service life.

The last condition remains to be specified: our conclusions are valid provided that the same dispersion is used in the same proportions. But is it possible to use a different, more effective dispersion, or increase its amount, or introduce some clever “chemistry” that will improve the characteristics of the plaster? Of course, it is possible. And conscientious manufacturers of single-component plasters are forced to use all three methods to increase the durability of their products. Unfortunately, all these methods lead to an increase in the cost of the finished plaster.

So, if you calculate everything correctly, a square meter of plaster made with a two-component composition will be either more durable or cheaper than the same square meter of single-component plaster, or even combine both of these pleasant qualities.