Continuing guidance on electrical materials. In this part again plastics: polyethylene, polypropylene and polystyrene.
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Depending on the synthesis conditions, the molecules of polyethylene can have a different structure, therefore they are distinguished: Low-density polyethylene (high pressure - according to the synthesis conditions). (LDPE - Low Density Polyethylene) Molecular chains have many branches. High density polyethylene (low pressure - according to synthesis conditions). (HDPE - High density Polyethylene) molecular chains are long and have little branch.
There are other options, ultrahigh molecular weight polyethylene UHMWPE, ultra low density polyethylene ULDPE and so on.
Polyethylene can still be “crosslinked” by PE-X, when chemically or physically, for example, with radiation, provoke the creation of transverse chemical bonds between long molecules of polyethylene. Molecules connected by transverse “bridges” give the products additional strength and heat resistance.
Physical properties depend on the type of polyethylene. LDPE is more flexible, stretches more before breaking. HDPE hard. Also important are various additives and fillers that are introduced into polyethylene, they can radically change the properties of the polymer.
Chemical properties are almost the same for all types of polyethylene.
, acids, alkalis, solvents do not affect him (for
rare exception). A plastic canister with evil chemistry is polyethylene. Canister
under the fuel - polyethylene. (Persistence is directly dependent on temperature, it swells up and dissolves in heated non-polar solvents.)Flexible
- allows you to make flexible bellows, dispensers, containers. The polyethylene canister can easily withstand a fall to the floor from a height of a couple of meters without destruction. Cases of car batteries are sometimes made of polyethylene (resistant to acid and does not crack when swelling cans). A polyethylene pipeline is not afraid of frost, if the water in such a pipe freezes, the pipe walls will simply stretch and not burst, as it happens with metals.Viscous.
Polyethylene, especially low density, is soft and stretches, while not prone
easy to tear (no zipper effect), which allows its use in armor. In some places it can replace rubber, for example, various dampers - shock absorbers. Construction helmets are made of polyethylene.Light bar (with additives only).
Unlike other types of polymers, it combines flexibility with UV resistance. Therefore, the wires for outdoor use PVC is replaced by polyethylene. The difference is especially noticeable in the cold, PVC dubeet stronger than polyethylene at low temperatures. Without additives, alas, is destroyed, the plastic film left outside for the third season turns into dust.Low adhesion
is a consequence of chemical resistance. This is both a plus and a minus. Polyethylene is extremely difficult to paint and glue, requires special tricks, surface treatment, the creation of intermediate layers. In this case, the adhesion is still extremely low, such bonding can not hold a high load. For the same reason, the color of polyethylene is usually produced in bulk in the manufacture of the addition of dye in the raw material, rather than painting the surface. This makes polyethylene an ideal material for the manufacture of tubes of glue, for example, a frozen "super glue" in such a tube is easy to clean off the spout. The impossibility of strong gluing determines the main method of joining polyethylene parts - welding.
How to distinguish polyethylene from other plastics? When burning smells of paraffin (candle).
It melts well. Polyethylene products are marked with signs: (Recycling signs are drawn by the user Tomina and taken from Wikipedia.)
It would seem that polyethylene is an ideal material for pipes, such pipes never rot, they do not burst in the cold. Actually made of polyethylene and make pipes for water supply for sewage. But only for cold water. Polyethylene softens at 80 ° , melts already at 135 ° (Approximate values, depending on raw materials, additives and modifications, PE-X cross-linked polyethylene is more heat-resistant). For this reason, the pipe from the unmodified
polyethylene for hot water under pressure can swell and tear.
However, cross-linked polyethylene pipes are used for plumbing, incl. and hot water (water coming from the boiler room can noticeably cool and does not develop at least 65 ° C in all houses). It can be limited to heating
Application examples (in electronic engineering)
Wire and cable insulation. Polyethylene with the addition of stabilizers and dyes - the material of wire insulation CIP - able to work outdoors under the sun for decades.Polyethylene drain pipe.Insulators RF connectors
, the insulation material of the inner core of the coaxial cable. When operating insulation with high frequency alternating current (more than 1 MHz), a number of specific material characteristics, such as, for example, dielectric absorption, come to the fore. As a result, what works well on direct current in high-frequency technology begins to warm up, to make losses.The terminal block insulation is made of polyethylene. Try not to use shitty (Everything is bad in them, and the design and material. Do not believe the nominal currents indicated on the package. A good terminal strip presses the wire with a plane and not a screw tip, and has a heat-resistant insulation.) insufficient and when heated, the insulation flows, besides burning. The insulation of the core of the coaxial cable is made of polyethylene, the outer black sheath is made of PVC.Cases of devices and products
, separators, holders. Various tanks for liquids, tubules.Packing material.
Not only in the form of a film, but also in the form of sheets of polyethylene foam.
A polymer similar to polyethylene (additional lateral tail of the monomer molecule) but with slightly different properties. More heat resistant, more rigid, less chemically resistant.
Still bad (but already slightly better than polyethylene) glued and painted.
Pipes for cold and hot water are made from polypropylene (since the melting point of polypropylene is about 170 ° C, such pipes are sure to hold hot water, especially if they have a reinforcing layer, no additional crosslinking measures are needed like polyethylene). The pipes are welded together.
Unfortunately, polypropylene is very similar to high-density polyethylene in both physical and chemical properties, so I could not find a reliable way to distinguish between these two types of polymers. They differ slightly in smell when burning and in the softening temperature.
A huge amount of polypropylene is spent on all sorts of packaging - cups, blisters, etc.
Extruded polypropylene fiber is a material of filters resistant to moisture and aggressive chemicals.Polypropylene filter cartridge for water filters. Polypropylene fibers are wound and compressed so that particles larger than 5 microns are retained.
Polypropylene nonwoven fabric is a cheap substitute for fabric.
Non-woven fabric - a fabric obtained by a method similar to paper making - fiber covers a flat surface and fibers stick together in a chaotic manner. Additionally, the canvas can be “stitched” by sintering at points along the grid. Such a fabric is less durable than woven fabric, but is MUCH easier to manufacture and cheaper. Disposable clothing, filters, disposable wet wipes are all products from non-woven cloth.
In electronic engineering, polypropylene is used in the form of a film - insulator in film
capacitors.Various film capacitors. The white capacitor in the background has polypropylene insulation.
The strength and low cost of polypropylene pipes and the simplicity of their connection allows
create solid bulk constructions from them - from photoboxing to bunk nursery
Polystyrene, ABS plastic
In its pure form, fragile transparent plastic.
Optical polystyrene is one of the few polymers that has excellent optical properties and is suitable for the manufacture of lenses, prisms and other optical devices. Many other polymers, such as polyethylene, polypropylene transmit light, but the block made from them will be turbid on the light. In combination with low weight and less brittleness, compared to glass, completely replaced glass from glasses. The optical components of consumer electronics — lenses of photodetectors, flashlights, and flashlight diffusers — are made of optical polystyrene and then coated if required. Such optics are cheaper than glass.
The fragility of polystyrene is fought by introducing into it viscous elastic additives — elasticizers, for example, polybutadiene. Polystyrene modified in this way is listed as "high impact polystyrene" or high-impact polystyrene - HIPS (In Soviet literature, ABS refers to a variety of high impact polystyrenes, but in practice it is isolated separately).
If a copolymer acrylonitrile and butadiene are added to the styrene during polymerization during the polymerization, the resulting strong plastic is called ABS plastic (Acrylonitrile
Butadiene Styrene). Polybutadiene is rubber, in ABS plastic it is present in the form of the smallest inclusions, adding strength and elasticity.
Polystyrene foam, polystyrene foam, we all remember as “foam plastic”, packaging material, heat insulator in engineering and in construction. Highly flammable, which limits it
application in construction.
Links to parts of the manual:1
: Conductors: Silver, Copper, Aluminum.2
: Conductors: Iron, Gold, Nickel, Tungsten, Mercury.3
: Conductors: Carbon, nichrome, thermostable alloys, solders, transparent conductors.4
: Inorganic dielectrics: Porcelain, glass, mica, ceramics, asbestos, gas and water.5
: Organic semi-synthetic dielectrics: Paper, click, paraffin, oil and wood.6
: Synthetic dielectrics based on phenol-formaldehyde resins: carbolite (bakelite), getinax, textolite.7
: Dielectrics: Glass fiber (FR-4), varnished cloth, rubber and ebonite.8
: Plastics: polyethylene, polypropylene and polystyrene.9
: Plastics: polytetrafluoroethylene, polyvinyl chloride, polyethylene terephthalate and silicones.10
: Plastics: polyamides, polyimides, polymethyl methacrylate and polycarbonate. History of the use of plastics.11
: Insulating tapes and tubes.12