PLASTICS WITH MEMORY EFFECT

Thermoplastics can be bent, pulled, or squeezed into various useful shapes. But eventually — especially if you add heat — they return to their original form. This is known as plastic memory. Plastic memory offers some interesting design possibilities.

Thermoplastics never forget. You deform them; and after a while, depending on temperature, they move back toward their original shape.

When most materials are bent, stretched, or compressed, they somehow alter their molecular structure or grain orientation to accommodate the deformation — permanently. Not so with polymers. Polymers temporarily assume the deformed shape but always maintain internal stresses that want to force the material back to its original shape. Usually, this desire to change shape is called plastic memory.

This so-called memory is often undesirable. Sometimes people prefer that thermoplastic parts forget their original shape and stay put — especially when the parts must be formed, machined, or rapidly cooled. However, this memory, or instability, can be used advantageously.

The time/temperature-dependent change in mechanical properties results from stress relaxation and other viscoelastic phenomena typical of polymers. When the change is an unwanted limitation, it is called creep. When the change is skilfully adapted to the overall design, it is called plastic memory.

Most plastic parts can be produced with a built-in memory. That is, the tendency to move into a new shape is included as an integral part of the design. So then, after the parts are assembled in place, a small amount of heat can make them change shape. Seals, gaskets and seamless covers for tubing and wiring are typical examples.

In other applications, plastic parts can be deformed during assembly, and then allowed to return to their original shape. In this case, parts can be stretched around obstacles without permanent damage.

Potential memory exists in all thermoplastics. Polyolefin, neoprene, silicone, and some other polymers can be given a memory either by radiation or by a chemical change.

Memory can be exploited in four ways:

The component is deformed at room temperature. Upon heating, the component recovers its original dimensions.

The component is deformed at an elevated temperature and — while held in the strained condition — it is cooled to room temperature so that the deformation is “frozen in”. Upon reheating, the component returns to its original dimensions.

The component is used in a confined situation under constant stress. The deformed sections try to return to their original dimensions or form. Since the part is restrained from doing this, a stress — in addition to the normal elasticity— is produced which is most often used for sealing.

The component is deformed for a short interval, and then the stress is removed. After a time, at room temperature, most of the deformation is recovered. This condition is often used for installation of parts over obstructions.

2.Find in the text English equivalents to the following words and word combinations:

можливості, термопластики, деформувати, первинна форма, змінювати молекулярний склад, полімери, внутрішня напруга, труднощі, прокладка, безшовна оболонка, первинні розміри.

3.Make word combinations from the following words. Translate them into Ukrainian.

Form, molecular, original, structure, shape, plastic, original, memory, dimensions, original.

4.Put the verbs in brackets either in Present Perfect or in Present Perfect Continuous Tense.

1. Thermoplastics just (to move back) toward their original shape.

2. These materials (to alter) their molecular structure since 2002.

3. Polymers (to assume) the deformed shape yet.

4. People (to use) plastics since 1980s.

5. Potential memory (to exist) in all thermoplastics since that moment.

6. Memory already (to be exploited) in several days.

7. The component just (to be deformed) at an elevated temperature.

5.Answer the following questions:

1. What is a plastic memory?

2. In what cases is this memory undesirable?

3. What phenomena does the change in mechanical properties of polymers result from?

4. List four ways of plastic memory exploitation.

Section 6

1.Read and translate the following text:

FIBRES

Fibres are probably the oldest engineering materials used by man. Jute, flax, and hemp have been used for "engineered" products such as rope, cordage, nets, water hose, and containers since antiquity. Other plant and animal fibres have been used for felts, paper, brushes, and heavy structural cloth.

The fibre industry is clearly divided between natural fibres (from plant, animal, or mineral sources) and synthetic fibres. Many synthetic fibres have been developed specifically to replace natural fibres, because synthetics often behave more predictably and are usually more uniform in size.

For engineering purposes, glass, metallic, and organically derived synthetic fibres are most significant. Nylon is used for belting, nets, hose, rope, parachutes, webbing, ballistic cloths, and as reinforcement in tyres.

Metal fibres are used in high-strength, high-temperature, light weight composite materials for aerospace applications. Fibre composites improve the strength-to-weight ratio of base materials such as titanium and aluminium. Metal-fibre composites are used in turbine compressor blades, heavy-duty bearings, pressure vessels and spacecraft re-entry shields. Boron, carbon, graphite, and refractory oxide fibres are common materials used in high-strength fibre composites.

Glass fibers are probably the most common of all synthetic engineering fibers. These fibers are the finest of all fibers, typically 1 to 4 microns in diameter. Glass fibers are used for heat, sound, and electrical insulation; filters; reinforcements for thermoplastics and thermoset resins and for rubber (such as in tyres), fabrics; and fiber optics.

2.Find in the text English equivalents to the following words and word combinations:

волокно, джут, конопля, такелаж, шланг, фетр, синтетичні волокна, скло, ремені, тасьма, зміцнення, композитні матеріали, турбокомпресор, скловолокно, тепло, звуко та електроізоляція, нейлон, термопластик.

3.Make word combinations from the following words. Translate them into Ukrainian.

Animal, applications, bearings, natural, fibre, heavy-duty, thermoset, pressure, fibres, engineering, purposes, turbine, ratio, composite, insulation; materials, aerospace, strength-to-weight, industry, compressor, vessels, heat, fibres, resins.

4.Make up sentences from the following words and translate them.

1. Flax, have been use, jute, and hemp, for, such, as, “engineered”, products, rope, water hose, cordage, nets, since, antiquity.

2. Brushes, have been used, other, plant, fibers, and, animal, heavy, structural, and, close, paper, for, felts.

3. Synthetic, fibers, glass, common, fibers, of all, engineering, are, probably, the most.

5.Turn the following sentences from Direct Speech into Indirect Speech:

1. The scientist said: “Fibers have been already used as engineering materials for centuries”.

2. They declared: “The fiber industry is clearly divided between natural fibers and synthetic fibers”.

3. He declares: “For engineering purposes glass, metallic and organically derived synthetic fibers are the most significant”.

4. The engineer said: “Glass fibers are probably the most common of all synthetic engineering fibers”.

5. He said: “We used glass fibers for heat insulation”.

5. Answer the following questions:

1. Why are fibres the oldest engineering materials used by man?

2. Where were plant and animal fibres used?

3. How is the fibre industry divided?

4. Why do synthetic fibres replace natural fibres?

5. Where is nylon used?

6. What are the properties of the metal fibres? Where are metal fibres used?

7. What fibres are the most common of all synthetic engineering fibers?

Section 7

1.Read and translate the following text:

Date: 2015-01-02; view: 2724