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PKG2006 Introduction to Packaging Polymers

2nd Semester 2018

SEO JONGCHUL, PhD

CHAPTER 4. Chemical Structure and Polymer Properties

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Chemical Structure Morphology

Chemical composition Structure Stereochemistry Arrangement

Physical form

AmorphousCrystalline(Semicrystalline)

Intermolecular and Intramolecular forces

Mechanical and Thermal properties

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Polymer heated to the melt state, shaped under high pressure and cooled down to room temperature (below Tg or Tm) to preserve its shape.

Shaping involves shear, bulk and elongational deformations of the polymer melt, which have different viscoelastic characteristics.

Polymerization process

Polymer

Product (Article)

PolymerProcessing

플라스틱 가공(Processing, Fabrication)

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플라스틱 가공(Processing, Fabrication)

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4.2. Fabrication Methods

Polymers have poor thermal conductivity and melt slowly

Rotating screw devices with a combination of external heaters and frictional heat

generated during transport

Three basic techniques: Molding, extrusion, or casting

A) Compression molding: heat and pressure (Fig 4.1)

1) Thermosetting polymers

2) In-situ crosslinking in the mold cavity

3) Partially cured (B-stage) Fully cured (C-stage)

Flow under pressure

4) Preform = Prepreg Polymeric binder + Reinforcing material (glass fiber)

+

Liquid polymer

FRP (Fiber-Reinforced Plastic)

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Fig 4.1 Compression molding

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B) Injection molding (Fig 4.2)

1) Thermoplastics

2) Screw is used to feed the polymer to the mold

3) Injection molding is faster than compression molding

Fig 4.2 Injection molding

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Fig 3.2 Polymer flow through a die orifice

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C) Reaction injection molding (RIM) (Fig 4.3)

1) Themosetting polymers

2) Monomer or low-MW polymeric precursor are mixed and injected into the mold

3) Reaction rates must be synchronized with the molding process

4) Advantages : -Polymerization prior to molding eliminated.

-Energy requirements for handling of the monomers are much lower than

those for viscous polymers.

D) Reinforced reaction injection molding (RRIM)

Reinforced fillers

Fig 4.3 Reaction injection molding (RIM)

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E) Blow molding (Fig 4.4)

1) Manufacturing bottles

2) Polymer tubing (parison) is blown by compressed air or drawn by vacuum into the

shape of the mold.

F) Casting

Pouring molten polymer into a mold and allowing the product to cool

G) Extrusion

1) Forcing compacted, molted polymer through a die shaped to give the desired object

2) Extruder resembles the injection molding apparatus (Fig 4.2) except that the mold cavity

is replaced with a die (Fig 3.2)

3) Useful for making elongated objects such as rods or pipes.

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Fig 4.4 Blow molding

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H) Polymer films

1) By casting from solution

2) By passing polymer under high pressure between hot rollers (calendering)

3) By extrusion through a slit die or through a ring-shaped die

I) Fibers by spinning

1) Polymer is forced under pressure through a perforated plate (spinneret) (Fig 4.5)

2) Melt spinning: polymer melt

The melt is cooled by cold air

3) Dry spinning: polymer solution

The solvent is removed by evaporation with heat

4) Wet spinning : polymer solution

The solvent is removed by leaching with another solvent

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Four-roll calendar

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Blown film molding process

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Fig 4.5 Fibers by spinning

Polymer is forced under pressure through a perforated plate (spinneret)

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J) Polymeric foams : by incorporation a blowing agent into the polymer

1) Physical blowing agents :

Gases (air, nitrogen, carbon dioxide) dissolved in the molted polymer under pressure

Low-boiling liquids (pentane) that vaporize on heating or under reduced pressure.

Chlorofluorocarbon, once the blowing agents of choice, were discontinued after

being linked to stratospheric ozone depletion

2) Chemical blowing agents :

Compounds that decompose on heating and give off nitrogen

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4.3. Mechanical Properties

Mechanical properties are dependent upon molecular weight.

a. For vinyl polymer(PE, PP, etc), molecular weight : 105

For polyamide, molecular weight : 20,000 ～ 50,000

b. For small molecular weight, properties of end group : significant In case of high molecular weight : negligible.

c. Properties and molecular weight.

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Fig 4.6. Dependence of properties on MW

Where they level off depends on structure

Property

Molecular weight

Mechanical

Working range

Nonmechanical

Viscosity

(Density, refractive index, spectroscopic absorption)

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Tensile strength : Resistance to stretching

Compressive strength : Resistance to compression

Flexural strength : Resistance to bending (flexing)

Impact strength : Resistance to sudden stress, like a hammer blow

Fatigue : Resistance to repeated applications of tensile, flexural, or compressive stress

Main Mechanical Properties

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Tensile stress (σ)

where F = the force appliedA = the cross-sectional area

Tensile strain (ε)

where l0 = original lengthl = change in sample length

Tensile modulus (E) : resistance to tensile stress

A

F

0l

l

E

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Hooke's Lawσ = Eε

E = Young’s modulus

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Elastic properties of materials

Toughness: the resistance to fracture of a material when stressed. It is defined as the amount of energy per volume that a material can absorb before rupturing.

Stiffness: the resistance of an elastic body to deformation by an applied force

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Fig 4.7. Tensile stress-strain behavior

Beyond the yield point depends on the polymer’s initial morphology.

(1) High crystalline polymer: little change in morphology on drawing and

break soon after the yield point

(2)Amorphous or low crystalline polymer: apply stress crystallinity modulus

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Elastic and Plastic Deformation

REAL MATERIALS

Some brittle materials, like ceramics, at first

glance appear to be almost ideal in their behavior.

But, these materials are usually nowhere near as

strong as they should be.

Even more drastic deviations from ideal behavior are observed in ductile materials (e.g.

many metals), where a yield point occurs well before fracture.

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Stress/Strain of Polymers

For glassy and semi-crystalline polymers, we see this type of brittle and ductile behavior.

For elastomers, however, elastic but non-linear behavior is often observed up to strains of 500%!

Furthermore, even in the initial apparently Hookean regions of the stress/strain behavior of

polymers an elastic (time dependent) responses are usually observed!

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Stiffness

Material E (lbs/sq. inch) E (MPa)

Rubber 0.001 x 106 7

Polyethylene 0.2 x 106 150

Wood 2.0 x 106 14,000

Concrete 2.5 x 106 17,000

Glass 10.0 x 106 70,000

Steel 30.0 x 106 210,000

Diamond 170.0 x 106 1,200,000

Polymers aren’t very stiff!

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Stiffness and Structure

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Stiffness of Polymers

Factors:

-Crystallinity

-Cross-Linking

-Tg

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Fibers

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Tensile Strength

Material TS (psi) TS (Mpa)

Steel piano wire

High - tensile steel

Aluminium alloys

Titanium alloys

Wood (spruce),along grain

Wood (spruce),across grain

Ordinary glass

Ordinary brick

Ordinary cement

Nylon fiber

Kevlar 29 fiber

450,000

225,000

20,000 - 80,000

100,000 - 200,000

15,000

500

5,000 - 25,000

800

600

140,000

400,000

3,000

1,500

140 - 550

700 - 1,400

100

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30 - 170

5

4

950

2,800

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Brittle vs Ductile Materials

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Molecular analogies of viscoelasticity in plastic materials

A) Elasticity B) Viscous flow C) Chain uncoiling

T < Tg

Low ε High E Deformation of bond angle and bond length

Reversible

T > Tg

Sliding motion Energy of deformationis dissipated as heat Contributes to creepand stress relaxation

Irreversible

T Tg

High ε Low E Bond rotation and segmental motion

Reversible (delayed)

Spring Dashpot Spring and dashpot

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Fig 4.8. General tensile stress-strain curve for a typical thermoplastic ](for constant temperature)

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Fig 4.9. Effect of temperature on E of an amorphous thermoplastic

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Fig 4.10. Effects of temperature on E of various polymers

The higher the MW, the higher the temperature necessary to overcome the increased

molecular entanglements.

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Polymers TREATED AS SOLIDS StrengthStiffness Toughness

POLYMERS TREATED AS FLUIDS Viscosity of polymer meltsElastic properties of polymer melts

VISCOELASTIC PROPERTIES Creep Stress Relaxation

Mechanical Properties of SolidsRheological Properties of Fluids

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Time dependent properties: Creep (cold flow), stress relaxation

Characteristic of viscoelastic materials

Arise from the slippage of polymer molecules

Creep: Increase in strain when a polymer sample is subjected to a constant stress

Stress relaxation: Decrease in stress when a sample is elongated rapidly to constant strain

In material science, creep is the tendency of a solid material to slowly move or deform

permanently under the influence of stresses. It occurs as a result of long term exposure to

high levels of stress that are below the yield strength of the material. Creep is more severe in

materials that are subjected to heat for long periods, and near melting point. Creep always

increases with temperature.

Stress relaxation describes how polymers relieve stress under constant strain. Because they

are viscoelastic, polymers behave in a nonlinear, non-Hookean fashion. This nonlinearity is

described by both stress relaxation and a phenomenon known as creep, which describes how

polymers strain under constant stress. (source from Wikipedia)

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Creep deformation under a constant load

as a function of time

Stress Relaxation constant deformation experiment

Experimental observations: Creep and stress relaxation

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Chain Stiffening

(Bulky side groups, cyclic units, crosslinking, crystallinity)

Tensile strength

Flexural strength

Compressive strength

Tensile elongation

Impact strength

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Table 4.1 Mechanical properties of common homopolymers

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Table 4.2. Fiber Properties

tex: weight in grams of 1000 m of fiberdenier: weight in grams of 9000 m of fiber

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Applications for high-strength PE fibers

- Cut-resistant industrial gloves

- Protective glove liners for health care workers

- Bulletproof vests

- Crash helmets

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A) Origins of decomposition

1) Bond breakage

Decreased materials properties

Depolymerization with many polymers

This is especially true with vinyl polymers

2) Aromatization upon heating

3) Thermal stability criterion

Td ≥ 400 °C

4.4 Thermal Stability

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B) Thermally Stable Polymers

Table 4.3 Representative thermally stable polymers

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1) Primarily a function of bond energy

a) T↑ ⇒ Bond vibration ↑ ⇒ Bond rupture

b) Cyclic repeating units: Breaking of one bond in a ring does not lead to a decrease

in MW and the probability of two bonds breaking within one ring is low

Ladder or semiladder polymers : Higher thermal stability than open-chain polymers

2) Thermooxidative stability

a) Initial onset may be lower than those in inert atmospheres

b) Initial decomposition is still via bond rupture,

then new mechanism involving oxidation may come into play

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Polybenzimidazole (Hoechst Celanese, trade name PBI)

Uses : Astronauts’ space suits and Firefighters’ protective clothing

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Rigid aromatic polymers :

High Tg, high melt viscosity, low solubility ∴ Intractable

<Solution 1>

Incorporation of “Flexibilizing groups” such as ether or sulfone into the backbone

Greater solubility and lower viscosity but thermal stability suffers

<Solution 2>

Introduction of cyclic aromatic groups that lie perpendicular to the planar aromatic backbone

Cardo polymers (from Lartin cardo : loop)

Improved solubility

No sacrifice of thermal properties

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<Solution 3>

Incorporation of reactive groups into the polymer backbone that undergo intramolecular

cycloaddition on heating (Scheme 4.1)

Improved processability

∵ Longer flow times are possible because little or no crosslinking takes place.

Tg ↑ because of chain stiffening

Scheme 4.1 Increasing Tg of a polyquinoxaline by intramolecular cycloaddition

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<Solution 4>

Aromatic oligomers or prepolymers capped with reactive end groups

End-capped oligomers melt at relatively low temp. and are soluble in a variety of solvents.

On heating, they are converted to thermally stable network polymers

Phenylethynyl-terminated oligomers for aerospace applications

Chemistry of network polymer formation : Cycloaddition or addition polymerization

reactions of the end groups

Table 4.4 Reactive End Groups for converting oligomers to network polymers

Common name for 5-norbornene-2,3-dicarboximide

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Because synthetic polymers are used increasingly in construction and transportation,

considerable efforts has been expended to develop nonflammable polymers.

Inherently nonflammable : Poly(vinyl chloride)

Polymers having a high halogen contents

Self-extinguishing : Polycarbonate

Burn as long as a source of flame is present, but stop burning

when the flame is removed.

Flammable : Most polymers

4.5. Flammability and Flame Resistance

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A) Burning Steps

1. An external heat source increases the polymer temperature to a point where it begins to

decompose and release combustible gases.

2. Once the gases ignite, the temperature increases until the release of combustibles is rapid

enough for combustion to be self-sustaining so long as sufficient oxygen is available to

support the combustion process.

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B) Flame Chemistry

Flame contain atoms, free radicals, and ions

Dominant free radicals: H, HO and O, and hydrocarbon species

* Hydrogen combustion

Branching reactions

Chain or propagation reactions

Recombination reactions

* Methane combustion

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C) Release of combustible gases in polymers such as PS and PMMA

High in monomer ∵ Thermally induced depolymerization

Monomer breaks down further to lower MW combustible products, including hydrogen,

as it diffuses toward the flame.

Where depolymerization dose not occur, surface oxidation plays a role in generation of

combustible gases.

D) Approaches to flame resistance

1. Retarding the combustion process in the vapor phase

2. Causing “char” formation in the pyrolysis zone

3. Adding materials that decompose either to give nonflammable gases or

endothermically to cool the pyrolysis zone

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(1) Combustion occurs by a series of free radical propagation and radical transfer reactions

Reduce the concentration of radicals in the vapor by incorporating radical traps

into the polymer, e.g. Halogenated compounds

Hydrogen halide that is released reacts with free radicals to form less reactive halogen atoms.

Polyesters prepared with 5 decompose on heating by the retrograde Diels-Alder reaction, and

the resultant hexachlorocyclopentadiene, 6, suppresses radical formation in the vapor phase

and inhibits free radical depolymerization in the pyrolysis zone.

Antimony oxides are often used in combination with halogen compounds

∵ Synergistic effects arising from formation of antimony halides

Disadvantage of halogen compounds :

Toxicity of hydrogen halide formed during burning.

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(2) Char formation at the surface of polymer

Acts as a barrier to inhibit gaseous products from diffusing to the flame, and to shield the

polymer surface from the heat flux.

Aromatic polymers have a natural tendency toward char formation, which accounts for their

generally low flammability.

Crosslinking increases char formation: Introduction of chloromethyl groups onto polystyrene

∵ Crosslinking during pyrolysis

(3) Hydrated alumina, Al2O33H2O

Evolve water endothermically to cool the pyrolysis zone

Sodium bicarbonate decomposes to form carbon dioxide, which in turn dilutes the combustible gases.

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One of the problems that oil companies face with their huge petroleum storage tanks: rusting

away of the metal bottom from underneath.

<Remedy>

Spray-coating the inside floor of the tanks with glass fiber-reinforced unsaturated polyester

→ Lengthens the tank’s lifetime significantly

→ Avoids the expense of having to replace the tank bottom with steel

Two general approaches to increase chemical resistance

1. To increase the steric hindrance about the ester groups

2. To reduce the # of ester groups per unit chain length

Both increase the hydrophobic nature of the polyesters

4.6 Chemical Resistance

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Crystalline polymers: More resistant than amorphous counterparts

∵Close chain packing reduces permeability

Crosslinking increase solvent resistance

1) Particularly important in the microelctronics industry

2) Coating a substance with a polymer that crosslinks under the influence of light or

ionizing radiation.

3) A mask carrying the pattern to the transferred to the substrate is placed over the coated

surface is irradiated.

4) The pattern allows radiation through, and those portions of the polymer thus exposed

undergo crosslinking.

5) When the mask is removed, the unexposed parts of the polymer are dissolved with

solvents, leaving behind the desired pattern.

6) Negative resist: the exposed portions of the surface becomes resistant to solvent

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Sunlight brings about polymer degradation

Monomer containing ultraviolet-absorbing chromophores such as 2,4-dihydroxy-4’-

vinylbenzophenone have been incorporated into vinyl polymers to improve light stability.

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Most polymers: very durable Polymer waste products

Degradable by sunlight and soil microorganisms

Photodegradable Biodegradable

New markets in agriculture, medicine, and microelectronics for polymers that degrade at a predictable rate.

A) Photochemically degradable = Photodegradable

Incorporation of carbonyl groups

Absorb UV radiation to form excited states

Bond cleavage

4.7 Degradability

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Norrish type II reaction

Commercially available photodegradable packing materials employ this technology

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Norrish type I reaction

Norrish type II reaction

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B) Biodegradable polymers

1) Microorganisms degrade polymers by catalyzing hydrolysis and oxidation.

2) The lower the MW, the more rapidly the polymer degrades.

C) Photoresist (PR) technology and controlled release applications.

1) Degradable polymers are used for positive resists

2) Controlled release refers to the use of polymers containing agents of agricultural,

medicinal, or pharmaceutical activity, which are released into the environment of

interest at relatively constant rates over prolonged periods.

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Application for polymer degradability: Photoresist for IC. 1) Positive resists: radiation promotes degradation of the resist exposed by the mask. 2) Negative resists: radiation makes insoluble network.

Fig. 4.12 Lithographic procedure in manufacture of integrated circuits

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Controlled release based on polymer permeability to encapsulate the active reagent within a

polymeric membrane or a strip.

Fig 4.13 Membrane-controlled release devices

(a) Microencapsulation (b) Strip

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Transdermal patches: release drugs through the skin

Degradable polyesters: used as disappearing surgical sutures.

Smart polymers: polymers that respond in a predictable way to change in temperature or

polarity

e.g. Poly(N-isopropylacrylamide),

Shrinks reversibly in response to increase in temperature

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4.8 Electrical conductivity

Classification of electrical conductivity.

a. Insulator : σ < 10-8 S/ cm

b. Semiconductor: 10-7 < σ < 10-1 S/ cm

c. Conductor: σ > 102 S/ cm(σ=conductivity, S (simen)= 1/Ω)

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Table 4.5 Conductivities of Metals and Doped Polymers

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Purpose

(1) To alter the properties of

the polymer

(2) To enhance processability

4. 10 AdditivesTable 4.6 Polymer Addition

Table 4.7 Commonly used plasticizers

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고분자(플라스틱) 재료에 재질이 다른 것(첨가제, additive)을 혼합함으로써1) 플라스틱 그 자체가 지니고 있는 특성 개선하거나, 2) 가공성, 물성 등을 개량시켜 사용 목적에 적합하게 하거나, 3) 새로운 제품가치나 편리성을 부가

첨가제의 종류· 중화제 · 산화방지제 · UV 안정제· 열 안정제 · 대전방지제 · 활제(슬립제) · 블로킹 방지제 · 가소제 · 핵제· 난연제 · 착색제

플라스틱 첨가제(additives)

· 사용 플라스틱과 상용성이 좋고, 표면에 침출되어 외관 또는 기능을 손상시키지 않아야 함. · 가공 온도에 견딜 수 있어야 하며, 분해나 휘발이 발생하지 않아야 함. · 일반 사용 상태에서 효과가 지속적이어야 함.· 병용하여 사용되는 첨가제와 서로의 효과를 감소시키지 않아야 함.· 가공 중이나 사용 중에 착색이나, 변색되지 않아야 함.· 내수성, 내유성, 내약품성이 있어야 함. · 무취, 무미, 무색이어야 함. · 무독성이어야 함.

첨가제 선정시 고려 사항