What transmits ultraviolet light is opaque. Nigeria is under control. Natural and synthetic coatings

In the late 1950s, immediately after its invention, it began to gain popularity. It is first used as a polymer container and UV protection in industry. Over time, polyethylene quickly finds use among flower and vegetable growers.

Advantages and disadvantages

At the moment, polyethylene film is the most popular and cheapest among all offers on the domestic market. The great demand for it is due to cost savings. But it has very few advantages over its analogues, although they exist:

• affordable price;
• 90% transmits sunlight;
• low coefficient of thermal expansion;
• over time, the strength of the material increases;
• does not lose its functionality at low temperatures.

The main drawback is that the film was not originally intended for these purposes. The coating usually lasts no more than a season, after which the film breaks and cracks. But this minus is compensated by the low cost of the film, so the greenhouse can be covered with new polyethylene every season.

There are other important disadvantages:

• ordinary polyethylene film is prone to rapid destruction under the influence of UV rays and high temperatures.
  If it is used as an additional covering under a polycarbonate or glass greenhouse, the service life of such a film will be approximately several years. If it is simply stretched over greenhouse arcs, it will hardly last four months;
• high temperatures and exposure to sunlight reduce the strength of the film, its frost resistance and light transmittance;
• high humidity in the greenhouse space collects condensation on the surface of the film, which blocks sunlight;
• the same condensation collects dust particles, which further aggravate the penetration of light;
• the temperature difference between the environment and the greenhouse space is large due to the fact that polyethylene does not transmit infrared rays that tend upward from the heated soil;
• a film stretched over a metal base is destroyed more severely due to the strong heating of the metal.

Modifications of polyethylene film

Due to its current nature, polyethylene for greenhouses has a fairly large number of varieties. It varies both in material strength and light transmittance.

Light-stabilized polyethylene

One of the components of this type of film is a special substance that stops the destruction of the coating due to an unfavorable environment. The service life of such film increases significantly compared to ordinary film - stabilized polyethylene withstands several seasons or can be used throughout the year.

It is impossible to distinguish a regular film from a modified one in appearance. When choosing the one you need, you should carefully study the label.

Hydrophilic polyethylene

This modification has a very important quality - it prevents condensation from accumulating on the surface of the polymer. The droplets are distributed evenly over the coating, so that this layer does not reduce light transmission and does not create droplets.

The merit of such advantages of the film is that it contains light and heat stabilizers, which not only increase the service life of the polymer several times, but also delay thermal radiation.

Another advantage is the increase in productivity in greenhouses with such a coating. According to research, in greenhouses with hydrophilic polyethylene, the yield and speed of ripening increase by about fifteen percent.

Foamed polyethylene

For those who have decided to make their own seasonal film for crops that are afraid of sudden temperature changes, it is recommended to pay attention to this type of film. It consists of two layers - monolith and foam material. The difference from conventional film is that this polyethylene transmits and scatters the sun's rays worse, thereby lowering the daytime ambient temperature. At night, the heat accumulated during the day slowly leaves the greenhouse, and it maintains a high temperature inside.

Reinforced polyethylene film

This film differs from other varieties in that it contains a triple layer of polymer. The thickness of polyethylene for greenhouses is small (from 15 to 300 microns), and the middle layer is a monofilament reinforcing mesh. Such a mesh may contain both fiberglass and other reinforcing elements, for example, lavsan.

It is worth noting that the film with a fine mesh and small cell size will have the greatest strength. However, a dense mesh reduces the light transmittance. The service life of such a film can be up to ten years.

What to choose

A large selection of modifications of polyethylene film should not be confusing, because each of them has its own specific properties. In the same time the choice of film coating will determine the entire seasonal harvest, therefore, this issue must be approached competently and fully armed. When choosing polyethylene for greenhouses, it is necessary, based on the budget, to determine the most suitable modification for specific tasks.

What is ultraviolet?

The main source of ultraviolet radiation is the Sun, although some sources of artificial lighting also have an ultraviolet component in their spectrum; in addition, it also occurs during gas welding work. The near range of UV rays, in turn, is divided into three components - UVA, UVB and UVC, which differ in their effect on the human body.

When exposed to living organisms, ultraviolet radiation is absorbed by the upper layers of plant tissue or the skin of humans and animals. Its biological action is based on chemical changes in biopolymer molecules caused both by their direct absorption of radiation quanta and, to a lesser extent, by interaction with the radicals of water and other low-molecular compounds formed during irradiation.

UVC is the shortest wavelength and highest energy ultraviolet radiation with a wavelength range from 200 to 280 nm. Regular exposure of living tissue to this radiation can be quite destructive, but fortunately it is absorbed by the ozone layer of the atmosphere. It should be taken into account that it is this radiation that is generated by bactericidal ultraviolet radiation sources and occurs during welding.

UVB covers the wavelength range from 280 to 315 nm and is medium-energy radiation that is hazardous to human vision. It is UVB rays that contribute to tanning, photokeratitis, and in extreme cases, cause a number of skin diseases. UVB radiation is almost completely absorbed by the cornea, but some of it, in the range of 300–315 nm, can penetrate the internal structures of the eye.

UVA is the longest wavelength and least energetic component of UV radiation with l = 315–380 nm. The cornea absorbs some UVA radiation, but most of it is absorbed by the lens. This is the component that ophthalmologists and optometrists should primarily take into account, because it is the one that penetrates deeper than others into the eye and has a potential danger.

The eyes are exposed to a fairly wide range of UV radiation. Its short-wavelength part is absorbed by the cornea, which can be damaged by prolonged exposure to radiation waves with l = 290–310 nm. As ultraviolet wavelengths increase, the depth of its penetration into the eye increases, and most of this radiation is absorbed by the lens.

Light transmission of spectacle lens materials in the UV range

Although the spectral properties of spectacle lens materials are regularly discussed on the pages of professional publications, including Veko magazine, there are still persistent misconceptions about their transparency in the UV range. These incorrect judgments and ideas are expressed in the opinions of some ophthalmologists and even spill out onto the pages of mass publications. Thus, in the article “Sunglasses can provoke aggressiveness” by consultant ophthalmologist Galina Orlova, published in the St. Petersburg Vedomosti newspaper on May 23, 2002, we read: “Quartz glass does not transmit ultraviolet rays, even if it is not darkened. Therefore, any glasses with glass spectacle lenses will protect your eyes from ultraviolet radiation.” It should be noted that this is absolutely false, since quartz is one of the most transparent materials in the UV range, and quartz cuvettes are widely used to study the spectral properties of substances in the ultraviolet region of the spectrum. In the same place: “Not all plastic eyeglass lenses will protect against ultraviolet radiation.” We can agree with this statement.

In order to finally clarify this issue, let us consider the light transmission of basic optical materials in the ultraviolet region. It is known that the optical properties of substances in the UV region of the spectrum differ significantly from those in the visible region. A characteristic feature is a decrease in transparency with decreasing wavelength, that is, an increase in the absorption coefficient of most materials that are transparent in the visible region. For example, ordinary (non-spectacle) mineral glass is transparent at wavelengths above 320 nm, and materials such as uviol glass, sapphire, magnesium fluoride, quartz, fluorite, lithium fluoride are transparent in the shorter wavelength region [BSE].

Light transmission of spectacle lenses made of various materials:
1 - crown glass
2, 4 - polycarbonate
3 - CR-39 with light stabilizer
5 - CR-39 with a UV absorber in the polymer mass
In order to understand the effectiveness of protection from UV radiation of various optical materials, let us turn to the spectral light transmission curves of some of them. In Fig. the light transmission in the wavelength range from 200 to 400 nm is presented for five spectacle lenses made of various materials: mineral (crown) glass, CR-39 and polycarbonate. As can be seen from the graph (curve 1), most mineral spectacle lenses made of crown glass, depending on the thickness at the center, begin to transmit ultraviolet radiation from wavelengths of 280–295 nm, reaching 80–90% light transmission at a wavelength of 340 nm. At the border of the UV range (380 nm), the light absorption of mineral spectacle lenses is only 9% (see table).

This means that mineral spectacle lenses made from ordinary crown glass are unsuitable for reliable protection against UV radiation unless special additives are added to the batch for glass production. Crown glass spectacle lenses can only be used as sun filters after applying high-quality vacuum coatings.

The light transmission of CR-39 (curve 3) corresponds to the characteristics of traditional plastics that have been used for many years in the production of spectacle lenses. Such spectacle lenses contain a small amount of light stabilizer that prevents photodestruction of the polymer under the influence of ultraviolet radiation and atmospheric oxygen. Traditional spectacle lenses made of CR-39 are transparent to UV radiation from 350 nm (curve 3), and their light absorption at the boundary of the UV range is 55% (see table).

We would like to draw the attention of our readers to how much better traditional plastics are in terms of UV protection compared to mineral glass.

If a special UV absorber is added to the reaction mixture, then the spectacle lens transmits radiation with a wavelength of 400 nm and is an excellent means of protection against ultraviolet radiation (curve 5). Spectacle lenses made of polycarbonate are distinguished by high physical and mechanical properties, but in the absence of UV absorbers they begin to transmit ultraviolet radiation at 290 nm (that is, similar to crown glass), reaching 86% light transmission at the boundary of the UV region (curve 2), which makes them unsuitable for use as a UV protection agent. With the introduction of a UV absorber, spectacle lenses cut off ultraviolet radiation down to 380 nm (curve 4). In table 1 also shows the light transmission values ​​of modern organic spectacle lenses made of various materials - highly refractive and with average refractive index values. All these spectacle lenses transmit light radiation starting only from the edge of the UV range - 380 nm, and reach 90% light transmission at 400 nm.

It must be taken into account that a number of characteristics of spectacle lenses and design features of frames affect the effectiveness of their use as means of UV protection. The degree of protection increases with increasing area of ​​the spectacle lenses - for example, a spectacle lens with an area of ​​13 cm2 provides 60–65% degree of protection, and with an area of ​​20 cm2 – 96% or even more. This occurs by reducing side illumination and the possibility of UV radiation entering the eyes due to diffraction at the edges of spectacle lenses. The presence of side shields and wide temples, as well as the choice of a more curved frame shape that matches the curvature of the face, also contribute to increasing the protective properties of glasses. You should be aware that the degree of protection decreases with increasing vertex distance, since the possibility of rays penetrating under the frame and, accordingly, getting into the eyes increases.

Cutting limit

I would like to believe that we have finally convinced everyone that ordinary mineral spectacle lenses, and even more so quartz glass, are significantly inferior to organic lenses in terms of ultraviolet cutting efficiency.

On the pages of this information resource, the need to protect polyethylene products, in particular semi-finished products (polyethylene rods, sheets, plates, etc.) made of various grades of polyethylene, as well as other materials of the polyolefin family, from the harmful effects of UV radiation during operation has already been noted outdoor products.

The harmful effects of UV radiation are expressed in a change in the color of the material (fading), as well as in a change in its mechanical properties - the material becomes brittle and cracks, even without mechanical load.

It should be noted that these processes (fading and changes in mechanical properties) are not related to each other - fading characterizes, first of all, the durability of the dyes used in the production of materials, and therefore the loss of the original color of the product does not always mean a change in the mechanical properties of the material.

As noted above, to make polyolefins resistant to UV radiation, special UV stabilizers (HALS inhibitors) are introduced into their composition during the production process.

In general, we can say that the resistance of a material to UV radiation, and, therefore, the service life of products, depends on the amount and effectiveness of UV stabilizers used, as well as on the intensity of UV radiation - at higher latitudes the intensity of UV radiation is lower, than in lower ones. Additionally, the intensity of UV radiation can enhance, for example, its reflection from the water surface.

The combination of stabilizers and dyes introduced into the composition of the material can also have a significant impact on the service life of products, for example, a soot-based dye introduced into the composition of polyethylene products is itself a good UV stabilizer, so the service life of black polyethylene products is the longest .

Leading manufacturers of engineering thermoplastics regularly test their materials to determine the effect of UV radiation on their properties. In general, we can say that the target period during which there should be no significant change in the properties of materials is 10 years.

However, taking into account the fact that, as noted above, the intensity of UV radiation is different for different climatic zones, for places with high radiation intensity the actually achievable value of this indicator may be significantly lower.

On the other hand, for products containing a carbon black-based dye, the service life can be significantly longer - on average up to 20 years, without significant changes in the properties of the material.

Separately, it is worth dwelling on the issue of fading of the material. This effect can be observed to a greater or lesser extent, depending on the intensity of UV radiation and the durability of the dyes used. At the same time, the resistance of organic dyes used recently is, as a rule, significantly lower than the resistance of dyes based on heavy metals (for example, cadmium). Therefore, more modern materials are not always more resistant to fading.

In everyday life, we often use ready-made blocks of knowledge acquired in childhood, often at school. We practically do not analyze them, a priori considering them indisputable, requiring no additional evidence or analysis. And if you ask us, for example, whether glass transmits ultraviolet light, the majority will confidently answer: “No, it doesn’t, we memorized that at school!”

But one day our friend will appear and say: “You know, I spent the whole day driving yesterday, the sun was merciless, my entire forearm on the side of the window was tanned!” And in response to a skeptical smile, he rolls up his shirt sleeve, showing his reddened skin... This is how stereotypes are destroyed, and a person remembers that by nature he is a researcher.

And yet - what to do with our question? After all, we know that it is ultraviolet radiation that causes skin tanning in people. The answer is not as clear-cut as it might seem at first. And it will sound like this: “It depends on what glass and what ultraviolet!”

Properties of ultraviolet rays

Ultraviolet radiation has wavelengths ranging from approximately 10 to 400 nm. This is a fairly large spread, and, accordingly, rays in different parts of this range will have different properties. Physicists divide the entire ultraviolet spectrum into three different types:

1. Type C or hard UV radiation . Characterized by a wavelength from 100 to 280 nm. This radiation got its name for a reason; it is extremely dangerous for humans, leading to skin cancer or rapid eye burns. Fortunately, the range's rays are almost completely blocked by the Earth's atmosphere. A person can only encounter them very high in the mountains, but even here they are extremely weakened.
2. Type B or medium UV radiation . Its wavelength is from 280 to 315 nm. These rays cannot be called gentle towards humans either; they are similar in their properties to the previous type, but still act less destructively. Like type C, they are also lost in the atmosphere, but are less retained by it. Therefore, 20% of them still reach the surface of the planet. It is this type of rays that causes tanning on our skin. But this radiation is not able to penetrate ordinary glass.
3. Type A or soft UV radiation . From 315 to 400 nm. It doesn’t care about the atmosphere, and it passes unhindered to ocean level, sometimes penetrating even through light clothing. This radiation perfectly overcomes the layer of ordinary window glass, appearing in our apartments and offices, leading to fading of wallpaper, carpets and furniture surfaces. But “A rays” cannot in any way lead to tanning of a person’s skin!

True, extreme ultraviolet radiation with a wavelength below 100 nanometers is also released, but it manifests itself only in conditions close to a vacuum, and in conditions of the earth’s surface it can be neglected.

What should you answer to your motorist friend? Why did his forearm get tanned?

Different types of glass

And here we come to the second part of our answer: “Look at the glass!” After all, glass is different: both in composition and in thickness. For example, quartz allows all three types of UV radiation to pass through it. The same picture is observed when using plexiglass.
And silicate, used in window frames, and in cars, transmits only “soft radiation”.

However, there is one important “BUT” here! If the glass is very thin or very transparent, highly polished (as is the case with a car), it will let in a small fraction of the “B radiation” responsible for our tanning. This is not enough to tan after standing near the window for an hour. But if the driver has spent many hours behind the wheel, exposing his skin to the sun, then it will tan even through closed windows. Especially if the skin is delicate and the case occurs high in relation to sea level.

And now, having heard the question whether ultraviolet radiation passes through glass, we can answer in a very ambiguous way - it does, but only in a limited part of the spectrum, and only if we talk about ordinary window glass.

To answer this question, let’s understand the nature of such a phenomenon as ultraviolet radiation, and the nature of a material such as plexiglass.

Until we get to the detailed characteristics, we will answer the question - Does plexiglass transmit ultraviolet radiation? Yes, he lets him through!

Ultraviolet radiation is rays that are located just beyond the visible spectrum in wavelength. The wavelength range for ultraviolet is 10-400 nm. The range of 10-200 nm is called vacuum or “far”, since rays with this wavelength are present exclusively in outer space and are absorbed by the planet’s atmosphere. The remaining part of the range is called “near” ultraviolet, which is divided into 3 categories of radiation:

• wavelength 200-290 nm - short wavelength;
• wavelength 290-350 nm - medium wave;
• wavelength 350-400 nm - long wavelength.

Each type of ultraviolet radiation produces different effects on living organisms. Short-wave radiation is the most high-energy radiation; it damages biomolecules and causes DNA destruction. Medium wave radiation causes burns to the skin of humans; plants tolerate short-term irradiation without consequences, but over a long period of time, vital functions are suppressed and die.

Long-wavelength is practically harmless to the vital functions of the human body, safe and beneficial for plants. The short-wave ultraviolet range and part of the mid-wave range are absorbed by our “protective armor” - the ozone layer. Part of the medium-wave radiation range and the entire long-wave range, i.e., reaches the surface of the planet, the habitat of living beings and plants. spectrum of beneficial rays and those not harmful during short-term irradiation.

Plexiglas is a chemical synthetic polymer structure of methyl methacrylate and is a transparent plastic. Light transmission is slightly lower than that of ordinary silicate glass, easy to machine, and light weight. Plexiglas is not resistant to certain solvents - acetone, benzene and alcohols. Produced based on standard chemical composition. The differences between brands and manufacturers lie in the imparting of specific properties: impact resistance, heat resistance, UV protection, etc.

Standard plexiglass allows ultraviolet light to pass through. Its radiation is characterized by transmittance:

• no more than 1%, for a wavelength of 350 nm;
• not less than 70%, for a wavelength of 400 nm.

Those. plexiglass transmits only long-wave radiation, at the very edge of the wavelength range, which is the safest and most useful for living organisms.

It is worth noting that plexiglass has low resistance to mechanical stress. Over time, when abrasive particles come into contact with it during the cleaning process, the surface is damaged, the glass becomes dull and reduces its ability to transmit both visible light and ultraviolet radiation.