Choosing polyurethane foam. Types, properties and features

Published: 02/26/2012 Category: Materials and technologies Views: 44514

Polyurethane foam. Types, properties and features

Often during construction or repair work there is a need for a material that can fill a gap, hole, joint or seam. This need arises when installing windows and doors, as well as other elements used in construction. The best option for solving these problems would be polyurethane foam . This article will provide detailed information on how to choose the necessary polyurethane foam, what you should pay attention to, and which of the many brands on the market to give preference to.

What is one-component polyurethane foam?

Based on their composition, there are two types of polyurethane foam: one-component and two-component. The first one is much more common, and its use is much simpler and more convenient. The one-component foam is packaged in an aerosol package and is essentially a polyurethane sealant. In fact, the substance in the cylinder is a mixture of several substances. The foam base is synthesized from an isocyanate and a polyol, the resulting substance being a prepolymer. The chemical reaction occurs partly inside the cylinder, but mostly on contact with air, resulting in the formation of polyurethane. The foaming agent for any of the polymers will be a mixture of liquefied gases (butane, isobutane, propane), which is called a propellant. Thanks to the same mixture, pressure is created under which the prepolymer leaves the container.

When leaving the container, the composition turns into foam due to contact with air and increases in volume by 20-40 times. Thanks to its rapid expansion, this foam can fill even the most difficult to reach cavities. Over a short period of time, the mass hardens due to the moisture contained in the air. To increase the speed of hardening, you can also moisten the surface on which the composition will be applied. It takes about a day for the composition to complete the chemical reaction - during this time the foam hardens, turning into a chemically stable polyurethane. The great advantage of this substance is its strength, resistance to wet environments, and harmlessness. Due to its porous structure, polyurethane is an excellent insulator.

High-quality polyurethane foam does not flow down the surface, but is well fixed to it. The finally hardened foam mass does not become brittle and brittle even when exposed to low temperatures.

You can use polyurethane foam when working with almost any building material: concrete, stone, iron, wood, plastic. This allows it to be used in the widest range of construction and installation works: installation of windows and doors, heat and sound insulation, sealing of joints, cracks, and drainage systems, as well as, if necessary, sealing of individual elements.

There are now many manufacturers of polyurethane foam on the market, but it is better to give preference to well-known brands:

• Henkel (Makroflex brand, Finland)
• Bison International (Netherlands)
• Den Braven (Netherlands)
• Soudal (Belgium) international concern Tremco illbruck
• Selena Group (trademarks Tytan, Hauser, Poland)
• Bau Master (Estonia)
• Domos (Estonia)
• Penosil (Estonia)
• Okyanus Kimya (trademark Soma Fix, Türkiye)
• Hermetic-Trade (trademarks Master Gvozd, CHIP, Putech, Russia)
• ULTIMA (Russia)

Please note that high-quality polyurethane foam, when applied, is immediately fixed to the surface and does not flow down it. Another indicator of quality is strength: even when exposed to low temperatures, high-quality polyurethane foam does not break or crumble.

An excellent domestic brand in the production of polyurethane foams can be called ULTIMA. Thus, professional summer foam with increased yield meets all the requirements for similar products, while being an affordable product.

Recommended foam for:

• any installation and construction work of large volumes;
• structures requiring high precision and lack of susceptibility to deformation during operation;
• ensuring the tightness of connections.

Due to its high adhesion to various materials, the range of uses of ULTIMA foam is very wide. But, like any other polyurethane product, it has low adhesion to some types of plastics, such as polyethylene and polypropylene, fluoroplastic and the like.

Popular polyurethane foam

This is an excellent product used in installing windows and doors in openings, foaming cracks and joints between building elements. After complete hardening, the foam can withstand repeated cycles of sudden temperature changes, high humidity and great physical activity.

The only disadvantage of polyurethane foam is the instability of the material to ultraviolet radiation, but this is a feature of any polyurethane foam. Therefore, in places exposed to sunlight, additional surface treatment is required.

The advantages of ULTIMA polyurethane foam are more significant:

• increased output volume up to 65 liters, which makes it possible to use for large-scale work;
• secondary expansion of up to 25% indicates minimal mobility of elements in the structure;
• forms a dense, finely porous structure when completely hardened;
• does not flow and does not settle, that is, suitable for vertical foaming;
• requires the use of a gun, which affects the ease of application.

ULTIMA summer foam adequately copes with all its tasks.

Relevance of application

A composition with minimal expansion can be used for cracks up to 10 cm. However, for very small and larger cavities, completely different compositions are needed. Today, manufacturers produce dozens of different products that strictly meet individual requirements, from expansion coefficient to resistance to aggressive factors.

Expansion of polyurethane foam is an important characteristic that is responsible for how much the initial volume of the material changes. There are compositions with low coefficients, medium and regular ones, as well as highly expanding ones. The latter are used to fill large openings when it is necessary to provide sound insulation, protection from external factors and insulation. But they are not suitable for small holes.

The main area to use such polyurethane foam is gluing and filling small seams. Popular representatives are Titan, Soudal, Macroflex.

Polyurethane foam and its properties

The main property of polyurethane foam, which determines its value as a building material, is its ability to increase many times in volume , as discussed above. The degree of expansion varies from 10% to 60% for household polyurethane foam produced for home use, and from 180% to 300% for professional ones. This coefficient is influenced by various factors, such as air temperature and humidity, the rate of foam release from the container, the type of application (using an adapter or a gun) and even, oddly enough, the experience and abilities of the person who works with the foam.

When carrying out work, a container with household and professional mounting foam should be held “upside down” so that the light propellant gas can better mix with other mounting components and displace them from the container.

The packaging usually indicates the maximum possible foam yield. However, this volume is achievable only if all instructions are followed, as well as favorable environmental conditions.

When calculating the volume, it is also worth considering that foam expansion is of two types: primary and secondary. The primary occurs immediately upon exiting the container, while the secondary occurs before the completion of the polymerization process. Secondary expansion for high quality products is 20 - 30%. This fact must be taken into account when filling cracks and cavities: approximately 1/3 of the space should remain hollow so that the foam fills them, but does not go beyond.

There are also mounting foams with a lower coefficient of secondary expansion, which are excellent for installing windows and doors and guarantee no deformation in the seam. After all, the lower the secondary expansion, the less the foam increases in volume after application, which is especially important when installing structures where excessive foam pressure is not acceptable. Such products include summer professional polyurethane foam with increased yield from PENOSIL. PENOSIL Gold Gun 65 is also used in construction, when installing communications, installing windows, doors, and sealing structures.

The uniqueness of the composition of this product reveals certain advantages:

• fine porosity of polymerized foam - affects the mechanical strength of the connection or seal, as well as other performance properties;
• the foam yield, up to 65 liters, is improved not only quantitatively, but also qualitatively - the foam is supplied evenly and stably;
• secondary expansion up to 15%, which is much lower than the stated optimal parameters of professional polyurethane foams (20-30%);
• short period of time from application to polymerization;
• environmental friendliness of the composition - does not harm both health and the environment after installation work;
• good heat and sound insulation properties;
• high adhesion to most building materials.

Pay attention to the features of the foam that will help avoid problems during installation:

• Low secondary expansion will not allow filling voids in the structure; for this you need to use household foam.
• In addition, a gun is required to use professional foam.
• The hardened surface of any mounting foam in open areas requires secondary treatment to protect it from sunlight.

Advice. When working with foam, it is better to hold the container vertically down. This will allow the lighter gas propellant to mix with the rest, displacing the foam from the container in which it is located. For more efficient mixing of gases, the cylinder should be shaken from time to time. To avoid leakage of propellant gas, store cylinders in an upright position at an ambient temperature between 5ºC and 25ºC.

The density of foam that has undergone the process of secondary expansion and has finally hardened ranges from 15-25 kg/m³ for professional ones to 25-35 kg/m³ for household ones. It is impossible to say which indicator is better: it depends on your goals and objectives. The denser the resulting product, the lower the foam density is preferable. However, in some situations, high-density foam is preferable: for example, for seam insulation. In this case, the quality of the foam will be determined by the uniformity of its structure: all bubbles should be approximately the same size; otherwise, the use of the material will not give the desired effect.

Hardened high-quality polyurethane foam has a uniform fine-cell structure

Poor foam has a structure consisting of many large cells with large voids, inside which uncured foam may remain

In addition to expanding after leaving the container, polyurethane foam can also shrink . However, shrinkage should in no case be more than 5%, otherwise you will have to admit that you purchased a low-quality product. Large shrinkage can lead to deformation of the foam mass, and sometimes to ruptures, which, of course, is unacceptable.

A property such as adhesion characterizes the ability of foam to adhere to other surfaces . It is measured by moving the prototype and measuring the disruption of adhesive contact. The norm of foam shear resistance is about 0.4 - 0.48 MPa. Please note that the foam mass cannot stick to materials with an inert surface, such as Teflon, polyethylene, etc. However, most materials used in construction have good contact with polyurethane foam.

When foaming narrow crevices and small cavities, it is easier to dose the foam jet using a gun

To prevent loads from destroying the structure of the polyurethane foam, after hardening it must remain sufficiently elastic and, after eliminating the deforming factor, return to its previous position. This is important, especially considering the tendency of new houses to settle over time. High-quality material will easily withstand this process, will not crumble and will retain all its properties.

For humans, the foam is harmless: there are no fumes from it. The only case when it can release toxic substances is combustion, but the ignition temperature of polyurethane foam is at least 400 ºС.

An important indicator of quality is the color of the polyurethane foam. It should be light yellow. When exposed to direct sunlight, the stump darkens, acquiring an orange or brown tint. This is due to UV radiation destroying the foam structures. If this factor is not excluded, the foam will become brittle and lose most of its properties. However, the problem can be solved simply by plastering the area: this way the effect of light is neutralized, and the foam will serve you for a very, very long time.

Don't forget about safety precautions! When working with polyurethane foam, do not point the dispenser at people or animals; ventilate the room in which you are working. Protect your eyes with a special mask. It would also be a good idea to use a respirator or at least a cotton-gauze bandage to protect the respiratory tract.

Foam stability (stability):

The stability (stability) of foam is its ability to maintain the amount obtained, dispersion and the ability to prevent loss of liquid (syneresis). Most often, the object of measurement is a single bubble or a small volume of resulting foam.

The parameter depends on several conditions - the presence and type of pressure, the inclusion of surfactants in the composition. Thus, foams obtained from nonionic solutions are less stable, like those formed as a result of exposure to natural atmospheric pressure. Foam obtained under pressure, as well as those that contain anionic surfactants in the mixture, retain their state much longer.

Foam stability , expressed in time, is an important parameter that allows us to consider this coarse system as structured, i.e. possessing the properties of a solid. However, it directly depends on the type and presence of stabilizers.

Two-component polyurethane foam

Above we mainly discussed one-component foams. However, there are also two-component ones that must be mixed in a strictly defined proportion immediately before use. To do this, you must have a special mixing gun, or a special installation with a gun.

The difference between two-component foam is faster hardening , as well as good polymerization , regardless of air humidity. For example, excess foam produced by Makroflex Rapido (Henkel), lSoudafoam 2K (Soudal) or llbruck 2K (Tremco lllbruck) can be cut off within 10 minutes after application. The undoubted advantage of two-component foam is its greater yield . However, given the complexity of use and higher price, it is advisable for professionals to work with them.

Types and types of foams:

There are two main types of foams - chemical and air-mechanical.

Chemical foam is a solution where the dispersed phase is a mixture of alkali and acid, and the dispersed medium is water. To ensure higher strength and stability, stabilizers are added to the system. The resulting bubbles are filled with carbon dioxide.

Air-mechanical foam is a solution of water and a foaming agent, the bubbles of which are filled with air. Concentrated solutions of surfactants, which can be:

– synthetic;

– fluorosynthetic;

– protein;

– fluoroprotein.

Another well-known type of foam is solid . It received its name due to the inclusion of a solid dispersed medium in its composition, while the phase is still represented by a gaseous substance. This category includes pumice (a natural substance), foam concrete, foam glass, various insulating materials, sponge materials (foam, microporous rubber) and so on.

Which polyurethane foam to choose, household or better professional?

Undoubtedly, professional polyurethane foams are made from higher quality reagents. However, in general, household and professional foams are similar in composition, and both the first and the second are perfect for many types of work. In addition, for small jobs, containers with household foam will sometimes be more convenient: they are smaller in volume, and are also equipped with an adapter, thanks to which the foam can be applied locally to small areas (after completion of the work, this adapter must be rinsed with water).

The container with professional foam has a special applicator ring onto which the mounting gun is screwed. This device allows you to dose the foam output and regulate the pressure and power of the jet. The gun also has a long nose that penetrates even hard-to-reach places. Unfortunately, this device sometimes costs more than the cylinder itself. Its cost reaches 4000 rubles.

Cylinders with professional foam are larger in volume and have a significantly higher yield. They are used by finishers, builders and other specialists who need to use foam often and in large quantities.

Having foamed the joint between the window and the concrete enclosing structure of the balcony, the foam is given time to harden. Then, using a sharp knife, carefully cut off the excess foam mass (b) and plaster the surface (c), preparing it for final painting.

One example of a popular professional foam is summer REMONTIX PRO 65, the new formula of which allows you to achieve maximum filling capacity. The volume of foam in the container is 850 ml, which leads to a final fine-pored product yield of up to 65 liters. The foam has rapid polymerization, which significantly reduces installation time.

Clear advantages also include:

• secondary expansion is within acceptable limits for professional products, it is no more than 25%;
• high filling capacity;
• The universal operating temperature range for summer foams is from 5 to 30 ⁰С above zero.

The combination of good filling power and gun application technology results in excellent results from the use of REMONTIX PRO 65.

This mounting foam is recommended for:

• installation work in the construction and communications sector;
• fastening panels, polymer plates and other elements;
• insulation of structures, including building facades;
• formation of sound insulation;
• formation of a hydrobarrier;
• filling voids.

REMONTIX PRO 65 has features standard for polyurethane foams:

• deteriorates over time under the influence of ultraviolet rays, therefore they require additional processing;
• darkens and changes color when exposed to light;
• requires cutting off the protruding part after complete polymerization;
• complete polymerization time is 24 hours.

REMONTIX PRO 65, being a seasonal product, requires strict adherence to the rules of its use and storage, including maintaining temperature conditions. Hypothermia and overheating of containers, even short-term, are not desirable. This can lead to a significant change in performance properties.

Features of choice

A material with minimal secondary expansion is selected, taking into account some of the nuances of the upcoming work. Standard formulations are produced in convenient cylinders with a simple mechanism. The mixture exits the tube when a key is pressed. A pistol may also be used. Then a special cylinder is inserted into it. This makes it easier to apply the mixture into holes of any depth and thickness.

For a small amount of work, a regular cylinder is suitable. When you need to use polyurethane foam in large quantities, it is better to use a construction gun. On each cylinder you can read the unique characteristics of the composition and the rules for working with it. The main information in this key is also indicated there - the degree of expansion.

Features of using polyurethane foam in winter

Construction and installation work is often carried out in conditions that are not the most suitable for this, including at low temperatures. For such cases, manufacturers have developed polyurethane foam intended for use in winter. It hardens at lower air humidity, and can also be used in a wider temperature range. Its disadvantage is weaker expansion.

The main advantage of winter polyurethane foams is the extended range of operating temperatures, and therefore the period of construction work

If work needs to be carried out urgently, and special winter foams are not available, workers have to resort to tricks. For example, they can moisten the work surface with a spray bottle. However, such methods should be resorted to as a last resort: it would be more rational to purchase a special compound or wait for a more favorable time to carry out the work.

It is also worth considering that even when working with winter foam, the temperature of the cylinder should be about 20ºC. If the cylinders were stored in a cooler place, they must be brought to the required temperature in warm water, avoiding excessive heating (this can lead to the cylinder exploding).

Where is it important to use foam that does not expand much: for interior doors and not only

Non-expanding polyurethane foam is used to seal cracks from 10 to 100 mm. Gaps of these sizes can form when solving various problems : installing door frames and window frames, wiring communications and pipelines, sealing joints in various rooms and when carrying out external work, etc.

Polyurethane foam TYTAN Professional Lexy 60 all-season 750 ml. Photo Petrovich

It should be understood that to fill spaces of minimal dimensions it is necessary to use sealants rather than polyurethane foams. This choice is best for spaces with a depth of no more than 10 mm. Polyurethane foam is also suitable for fixing (gluing) delicate materials.

The problem of choice and secrets of use

Coming to a building materials store, any non-professional will be confused: the market offers the buyer polyurethane foams with a significant variation in both price and quality. How to make a choice?

First of all, you need to know for what kind of work the foam will be used. It is preferable to fill narrow spaces with foam without too significant expansion; for wide cavities, strength will be important, without which it is impossible to hold the structure. In the second case, however, you can use a trick and fill the space with wood, brick or crushed stone, and only then use foam with a slight expansion.

The shelf life of the product is also important. It rarely exceeds 18 months. An expired product will most likely not have the declared properties. The “older” the cylinder, the more viscous the composition is inside it. This can be used if dense foam with small cells is more suitable for your needs. In any case, an expired cylinder should not only be used, but also stored at home; It is better to dispose of them at special collection points for such containers. It is strictly forbidden to throw cylinders, even fully used ones, into fire.

Over time, the composition may “disperse”: heavier components will sink to the bottom of the cylinder, while lighter components will rise. To ensure that the foam is homogeneous, shake the can for about 30 seconds before use: then the components will mix again, and the resulting composition will be exactly as the manufacturers intended. It is also worth shaking the container periodically during operation, but in this case you should not reach the point of fanaticism.

The surfaces on which you plan to apply foam need pre-treatment. It was written above that foam simply will not fall on inert materials. Therefore, make sure that the area being treated is not covered with one of these substances. The surface can be damp, in some cases it is even recommended, but excess water is also harmful. In addition, when working at subzero temperatures, you need to make sure there is no ice or frost.

A container of polyurethane foam contains flammable substances, and the foam itself can ignite when brought to a certain temperature. And as we remember, the combustion products of polyurethane foam are harmful to human health. Therefore, you should be careful when working with foam, not to carry out parallel welding or other work that could cause a spark. And of course, smoking is prohibited during work.

Polyurethane foam cleaners are designed to effectively remove uncured foam stains, treat the valve and cylinder, as well as the applicator ring and gun. This is necessary to avoid problems with foam coming out of the gun.

After finishing working with foam, pay attention to the gun (in case of professional materials) or adapter (in case of using household foam). They must be washed, otherwise the composition will harden inside and reuse of the equipment will become impossible. Sometimes this requires special formulations, in which case additional information is indicated on the cylinder itself.

What is a non-expanding option?

Non-expanding foam is a kind of sealant, the expansion of which after hardening occurs no more than one and a half times. It is great for working with delicate materials.

There are several types of low expansion foam:

• Silicone - characterized by excellent interaction with most surfaces. Adhere well to smooth materials. The sealed seam, obtained with silicone mass, protects from atmospheric influences and is slightly deformed.

• Polyurethane - standard mounting foams.

• Acrylic - do not contain solvents; this mixture is good for filling small joints. However, acrylic-based compositions are less resistant to aggressive external factors.

For most work related to filling small openings, silicone mass is used. Such sealants are universal, have good elasticity, strength and durability. They do not expand and harden when exposed to a humid environment.

Attention - spaghetti effect

Installers and builders have a term called the spaghetti effect. It means that the polyurethane foam is not coming out of the container correctly . As a rule, this happens when using cheap, low-quality guns, the tips of which do not open completely, as a result of which the foam comes out in a thin stream. Thus, instead of the expected thick layer of foam, something comes out that in appearance most closely resembles thin noodles, which is where the name of the effect comes from. In this case, the displacing gas is wasted, and the mass is not sufficiently saturated with oxygen and is not even foam in the network. As a result, the volume of the finished product is much less than expected, and its quality leaves much to be desired.

Alternative materials

You can insulate and fill the seams in a wooden house using caulking made from natural materials:

• moss;
• jute;
• tow;
• flax wool

Eco-friendly heat insulators are more expensive, and implementation requires a lot of time, effort and professional skills. But unlike polyurethane foam, they have important advantages:

• safe and antibacterial composition;
• do not disturb the air exchange of the tree;
• have low thermal conductivity, due to which they retain heat in the room.

Among other analogues there are:

• Styrofoam;
• mineral wool;
• silicone sealant;
• epoxy resin;
• plaster mixture.

It is important to consider that the use of each of the substitutes is advisable under certain conditions . They can act as a complete analogue or additional material in sealing.

How not to make a mistake when choosing polyurethane foam: main points

Before going to the store, read specialized forums, or better yet, register on them and ask for advice from “experienced” people. Professionals in their field will certainly tell you which foam is best to choose for your case. Of course, this information can also be obtained from a sales consultant, but it is not a fact that he will not praise the products from the sale of which the largest percentage will go into his pocket.

Pay attention to the appearance of the cylinders: they should not have defects, dents or chips. Otherwise, it can be assumed that the foam was stored in inappropriate conditions, which affected its quality. Pay attention to the expiration date, as well as information about the nuances of use that are indicated on the cylinder, as well as the manufacturer’s contact information - you can find a lot of useful tips on their website.

These articles may also be of interest to you:

1. How to choose building materials for building a house
2. Types and properties of sealants. Which sealant is better to choose?
3. Material for insulating steam rooms in baths or saunas
4. Installation technology for interior swing doors

Ultimate shear tension:

This parameter is usually expressed in terms of foam , i.e. its ability to perceive and withstand certain mechanical loads that affect their deformation or change in the final volume. Even foams whose films are quite liquid exhibit pronounced rigidity, because with any, even insignificant, external influence, the minimum surface energy responsible for the state of equilibrium of the solution increases.

Methods and methods for producing foams:

It is impossible to obtain stable foam without the use of stabilizers (foaming agents), therefore almost all methods of mixing this dispersed liquid include their use. The process itself is quite simple: when the dispersed phase and the medium are combined, several gas bubbles first appear, on the surface of which the adsorption of stabilizer molecules (most often surfactants) begins. This allows one top layer of small bubbles to form. Each subsequent pop-up bubble puts pressure on this layer from below, but surfactants prevent the rupture of the films of already formed bubbles, and a hemispherical dome appears with a rapidly increasing surface. Adsorption continues, the films retain gas or air inside the bubbles, and they are surrounded by two layers of foaming agent, between which a film of liquid is located.

Thanks to the stability and strength of the films, which is provided by surfactants, the bubbles formed in the upper layer come together. Their shape undergoes changes, turning from spherical to multifaceted, the thickness of the films decreases, and liquid films appear. The result is the addition of subsequent bubbles to one layer and the transformation of the entire liquid into voluminous foam.

You can get foam, like other disperse systems, in two ways:

– dispersion methods from coarsely dispersed systems;

– condensation methods from true solutions.

What to consider when choosing

It is important to consider the expansion rate of the foam.
The most important thing for waterproofing is the product's ability to act as a barrier to water . If the material allows moisture to pass through, it is not suitable for these purposes. You should also pay attention to the following characteristics:

• the severity of the primary and secondary expansion of the composition (a significant parameter for accurate filling of cavities);
• volume of material output;
• fire resistance;
• temperature values ​​at which repair and operation of this foam are possible.

Speed ​​of drying is also important. It is recommended to study the manufacturer's installation instructions, in particular how to remove excess material.

Destruction of the formed foam:

For this purpose, chemical and non-chemical methods are used.

Chemical means the use of antifoaming agents - special substances capable of extinguishing foam that meet the following criteria:

– having the ability to destroy foam even with a small amount and prevent its reappearance for a long period of time;

– do not react with the substances that formed the initial solution, and, accordingly, do not change its main production indicators, do not influence the technological process, do not change the operating parameters of the equipment;

– do not change your own parameters when changing storage conditions, regardless of its duration, as well as physical parameters (for example, when heating);

– for some industries it is important to comply with requirements such as sterility, non-toxicity, etc.

It should be noted that defoamers should be selected taking into account the specifics of production, because not all of them are effective for various coarse compositions.

Non-chemical methods are divided into physical and mechanical.

Physical methods include :

– an increase in the temperature of the mixture, at which the liquid from the solution evaporates and the film of bubbles is destroyed;

– exposure to ultrasound – an acoustic method is possible, which is not always reliable and is not suitable for quickly formed mixtures, but is considered the most common in small industries; it is also possible to use sound sirens;

– exposure to electric current is currently an experimental method, because allows foam to settle almost instantly, but at the same time requires high safety measures.

Mechanical methods are divided into:

– centrifugal – when moving, the foam collides with a stationary surface;

– aerodynamic – destruction of foam by a gas jet ejected under pressure;

– hydrodynamic – similar to the previous one, but using a jet of liquid;

– barometric – changes in pressure in special devices.

They are not often used in production, because they usually require complex, bulky equipment and significant energy costs. Also, such methods are almost ineffective for highly stable disperse systems with low expansion, and all others do not completely destroy the foam, but only reduce its amount.