What are polymers (Что такое полимеры). Учебное пособие
.pdf5.! ' !. "! ! %-* ! ! !.
) ) ) ), ) ),
),
6 . ' )
) ) ) )
: , 6 .
. 6
) ),
) . . )
) ) 6
. + )
) . ' ) )
) ) .
' 4 ) ) )
. .
. + ) 6
) .
6.* "!! -* ! !.
The glass transition temperature corresponds to the temperature where when you heat to and above this temperature, the polymer goes from a hard glassy material to a rubbery material. On a plot of strength vs. temperature, as you go through the glass transition temperature you lose mechanical strength.
A material’s glass transition temperature is the temperature below which molecules have little relative mobility and is usually applicable to wholly or partially amorphous phases.
Above the glass transition temperature, a motion started at one point on a polymer chain will produce results say 50 atoms along the chain away from where the initial disturbance occurred.
Thermoplastic (non-crosslinked) polymers are proved to be complex because, in addition to a melting temperature, Tm, above which all their crystalline structure disappears, such plastics have a second, lower Tg below which they become rigid and brittle, and can crack and shatter under stress.
31
7. * ! "# ' ' (.
Polymer bulk properties are strongly dependent upon their structure and mesoscopic behavior. A number of qualitative relationships between structure and properties are known.
Increasing chain length tends to decrease chain mobility, increase strength and toughness, and increase the glass transition temperature (Tg). This is a result of the increase in chain interactions such as Van der Waals attractions and entanglements that come with increased chain length. These interactions tend to fix the individual chains more strongly in position and resist deformations and matrix breakup, both at higher stresses and higher temperatures.
Crosslinking tends to increase Tg and increase strength and toughness because crosslinking includes formation of chemical bonds between chains.
Inclusion of plasticizers tends to lower Tg and increase polymer flexibility. Plasticizers are generally small molecules that are chemically similar to the polymer and create gaps between polymer chains for greater mobility and reduced interchain interactions.
8. ' "!! -* ' !+%.
1.The glass transition temperature is the temperature below which the physical properties of amorphous materials vary in a manner similar to those of a solid phase.
2.Proteins also possess a glass transition temperature below which both anharmonic motions and long-range correlated motion within a single molecule occur.
3.When a stress is applied to a viscoelastic material such as a polymer, parts of the long polymer chain change position.
4.Polymers remain a solid material even when these parts of their chains are rearranging in order to accompany the stress, and as this occurs, it creates a back stress in the material.
5.Vitrification (glass formation below the melting point) can occur when starting with a liquid such as water, usually through very rapid cooling or the introduction of agents that suppress the formation of ice crystals.
6.If a plastic with some desirable properties has a Tg which is too high, it can sometimes be combined with another in a copolymer or composite material with a Tg below the temperature of intended use.
32
7.The Space Shuttle Challenger disaster was caused by rubber O-rings that were below their glass transition temperature on an unusually cold Florida morning, and thus could not flex adequately to form proper seals between sections of the two solid-fuel rocket boosters.
8.The term «melting point» when applied to polymers suggests not a solid-liquid phase transition but a transition from a crystalline or semicrystalline phase to a solid amorphous phase.
9.The boiling point of a polymer substance is never defined due to the fact that polymers will decompose before reaching theoretical boiling temperatures.
10.Thermosetting polymers will decompose at high temperatures rather than melt.
9. ' ! -* ' !+%.
1.3 4 )
.
2. . 6
) ).
3. .) 6 ) )
) ) ) ), 4 ) . 4. . ) )
, ) 4
4 ).
5.% ) )
6 ) ) .
.
7.- ), ) ) )
) .
8. .) , (
), ) )
4 ).
9.' ) ) )
) ).
10. . )
) 6 ) ).
33
10. ! ! ! ! "# *! % %'.
binding energy — ) ) — ) — glass transition temperature — organized solid crystal —
) — crystal node — ) 4 —
— thermal motion — — neighboring molecules — ) — — energetically preferred points — ) — favored conformation — elastic deformation — — thermoplastic polymers — — viscous flow state — melting point — glassy solid — — rubber state
— — stress — — ordered system —
) — ) )
11. / + * ' !*:
1.Is there any difference between glass transition and crystallizing?
2.Describe three types of amorphous polymer phase states.
3.What structure of a polymer body corresponds to glassy state?
4.Does glass-transition temperature correspond to a single temperature point or temperature range?
5.Can we take a crystal (even a low molecular weight one) and transform it into a glassy state body?
12.! ! ! " "!! %
! % -* #.
34
Unit 5
PROCESSING OF POLYMERS
1. ! ! "#.
Processing is the source of much of the variability in properties of synthetic polymers. Different processing conditions determine the difference between a milk jug and, to a great extent, high strength PE fibers (ultra oriented). Processing is one component of a series of steps that lead to a plastic product or component from raw feed stocks. Although each commodity polymer has a somewhat different sequence of industrial stages involved in bringing it to market, a simple and fairly exemplary case is a polyethylene bottle of shampoo.
The various grade polymers are partly composed of blends of different branch content, molecular weight and density from different synthetic reaction conditions. For example, a film blowing grade of polyethylene might contain a blend of linear low density polyethylene, branch content metallocene polyethylene and low density polyethylene.
Polymers offer certain unique problems in processing. They are viscoelastic fluids which display high viscosity and broad transition temperatures. Machinery is large and involves a significant energy input. Polymers degrade (chain breakup) resulting in a loss of properties with exposure to high temperatures and stress for an extended period of time.
Some basic processing operations and machinery are extrusion, screw extruder, fiber drawing, injection molding, blow molding, dip coating, calendering, mixing/dispersion, melting, modification, and introduction of additives.
$%% ! ! ! !. $"'' !") ! + ! *% ' %
' -
( ) ) 4,
) ), ) )
, ) ). "
4 ) ) (4 ) ). , ) ) 4,
, , ),
35
) , ) ). *
).
' , 4
4, , ) ), 6 )
) 4 .
. 4 ) )
(4), 4
) ,
) 4 ) ) ), ):
natural gas processing plant
? ←
↓
? ←
↓
? ←
↓
- ' ' ' -
/ ) ) 4
) ) )
). ' ) 4 )
4, ) )
4 , .
. 6
, ):
quaternary ammonium cation |
)) |
high pressure liquid |
# % |
chromatography |
|
, ) 4
) ): different processing conditions, high strength PE fibers, various grade polymers, different synthetic reaction conditions, film blowing grade, linear low density polyethylene, branch content metallocene polyethylene, broad transition temperatures, chain breakup 1.
36
2. ! . * - ! ! "() % ! ! % *" # -"! +.
6
) )
)
)
6
)
)
)
)
3. "!! . * ! "() % ! ! % - ! *" # -"! +.
variability in properties
high strength fiber
raw feed stock
37
commodity polymer blend
reaction conditions
branch content metallocene polyethylene
significant energy input loss of properties processing operations screw extruder
fiber drawing blow molding calendering
introduction of additives
4."!! % -* ! ! ! "' 1.
5.! ' !. "! ! %-* ! ! !.
& - 6 ,
4 ), 4
) , ) 4 )
(6 , ),
) ) ).
' ) ) ) ) 6
, , , , ,
. . , ) ) )
) ) ) 6
6. 5 ) ),
7 ) ) ) ) ), )
() )
38
, ) ) )
.
. 4 6
) :
-)
-.
6.* "!! -* ! !.
Polymer processing is of importance to a variety of parts of a fairly
intricate industrial complex. Understanding some of the features of polymer processing can be an asset in dealing with production problems encountered not only in the polymer and plastics industry but in almost every area of manufacturing ranging from biomedical devices to microelectronics.
Following the motives of other chemical engineering disciplines, polymer processing can be categorized according to type of equipment used (unit operations): extrusion, injection molding, blow molding, calendering, mixing/dispersion, rotational molding. The unit operations approach is a historic approach and was largely given up in the chemical engineering field with the rapid advent of new and diverse operations.
The modern chemical engineering approach is to study the elementary steps (or physical stages) of all unit operations in a single focal area and to then demonstrate how these elementary steps fit into each of the typical unit operations. For example, elementary steps of polymer processing may include: particulate solids processing, melting, pressurization and pumping, mixing, molding, devolatilization and etc.
7. * ! "# ' ' (.
Additives are used in polymers to prevent polymer degradation, resulting from exposure to heat, shear and light. They broaden the property and application range by providing higher functionality and enhanced performance.
Synergistic antioxidant blends, comprising phosphites or phosphonites and phenols, are used to ensure thermal and processing stability during pelletization, processing and end-use.
39
Ultraviolet light, the same type that causes skin to sunburn, can initiate chemical reactions in polymers, resulting in polymer chain destruction and loss of chemical and physical properties. The light stabilizers can improve the light stability of polypropylene. These are, for example, ultraviolet light absorbers or hindered amine/amide light stabilizers.
Besides additives for thermal, processing and light stability, other additives, such as nucleating agents, antistats, slip agents and colorants may also have to be added to achieve the desired property portfolio of a given polymer grade.
8. ' "!! -* ' !+%.
1.Because of its simplicity and ability to deliver high shear rates the screw extruder is a crucial component of almost every polymer processing operation.
2.Injection molding is a manufacturing technique for making parts from both thermoplastic and thermosetting plastic materials in production.
3.Typically, HDPE grades for blow molding are pelletized under high shear and high temperatures (often above 260 °C).
4.Extrusion is a manufacturing process used to create long objects of a fixed cross-sectional profile.
5.Vacuum forming is a process where a sheet of plastic is heated to a forming temperature, stretched onto or into a single-surface mold, and held against the mold by applying vacuum between the mold surface and the sheet.
6.Molten plastic is injected at high pressure into a mold, which is the inverse of the product’s shape.
7.Thermoforming is a manufacturing process for thermoplastic sheet or film. The sheet or film is heated between infrared, natural gas, or other heaters to its forming temperature.
8.Casting is a manufacturing process by which a liquid material such as a polymer melt is introduced into a mould, allowed to solidify within the mould, and then ejected or broken out to make a fabricated part.
9.Blow molding or blow forming is a manufacturing process by which hollow plastic parts are formed.
10.In extrusion blow molding, plastic is melted and extruded into a hollow tube that is then captured by closing it into a cooled metal mold.
40