Polymer Science and Engineering course: Lecture 2

MMS330S, Spring 2001, Dr. S. Doroudiani 1

Polymers Additives

• Introduction

• Fillers

• Plasticisers and Softeners

• Stabilizers

• Colourants

• Flame Retardants

• Fibre Reinforcements

• Other Additives

MMS330S, Spring 2001, Dr. S.

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Introduction

Additives are added to polymers to change their bulk properties.

Additives play more important role in rubbers and some

plastics like PVC.

The additives may be solid, rubber, liquid and gas.

Additives should be:

– efficient in their function;

– stable under processing conditions;

– stable under service conditions;

– not bleed (into adjacent material) or bloom (out to the surface);

– non-toxic and not impart tast or odour;

– not expensive;

– not adversely affect the properties of the polymer.


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Fillers • Filler are solid additives, added to polymers toincrease bulk or modify

their physical properties (usually mechanical), such as: tensile and

compressive strengths, abrasion resistance, toughness, dimensional and

thermal stability.

• Classification:

– particulate fillers

• inert (extender filler); reduce cost, like: calcium carbonate, china clay, talk.

• active (reinforcing filler); improve mechanical properties, like: carbon black.

– rubbery fillers

• reactive

• non-reactive

– resins

• reactive

• non-reactive

– fibrous fillers

• laminar

• random

– cork


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Particulate filles are graded based on:

• Average particle size and size distribution: 10 nm-100s m

• Particle shape and porosity.

• Chemical nature of the surface.

• Impurities such as grit and metal ions.

Factors affecting the reinforcement power of fillers:

• An extensity factor: the total amount of surface area of filler.

• An intensity factor: the specific activity of the filler-polymer

interface, causing chemical and/or physical bonding.

• Geometric factors such as structure (aggregation) and porosity

of the particles.

Carbon blacks are the most powerful reinforcing fillers (for

equivalent particle size), also silicas.


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Coupling Agents

are used to enhance bonding (adhesion) between filler particle and

polymer. There are several methods:

• Improving the level of wetting the filler by polymer by coating the

filler particles, like calcium carbonate coated with stearic acid.

• Linking filler/polymer by covalent bonds using coupling agents.

• Most common coupling agents:

– silane

– titanium compounds


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Effects of Extender Fillers

• Increase in density;

• Increase in modulus of elasticity;

• Lower shrinkage;

• Increase in hardness;

• Increase in heat deflection temperature;

• Less temperature dependence of physical and mechanical

properties;

• Cost reduction.


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Effects of Reinforcing Fillers

• Increase in tensile strength, as well as compressive, shear and

flexural strength;

• Increase in modulus of elasticity and stiffness;

• Increase in heat deflection temperature and lowering of the T

dependence of mechanical properties;

• Lower shrinkage;

• Improvement in creep;

• Anisotropic effect.


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Application Criteria For Fillers

Different aspects should be considered when using fillers:

• Optimum particle size distribution;

• Possible catalytic activities at the fillers’ surface;

• Linkage with the plastics matrix;

• Abrasive action of the filler in processing machines;

• Safety industrial problems due to dust;

• Cost.


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Plasticisers • improve flexibility, ductility and toughness of polymers, and

generally reduce hardness and stiffness.

• External plasticisers are generally liquids with low vapour pressure

and low MM.

• Internal plasticising (copolymerisation) involves adding monomers

(whose homopolymers have a low Tg) to homopolymers with high Tg.

• Plasticisers lower Tg, so at ambient temperature polymer is in

rubbery situation.

• Plasticisers are usually used in polymers which are brittle at service

conditions such as PVC. Their molecules diffuse between chains

and by increasing the free volume lower Tg.

• The efficiency the plasticiser may be evaluated by a number of

different semiempirical tests, for example the depression of Tg.


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Plasticising Mechanism

There are several theories behind the palsticising mechanisms of

plasticisers. Each of these theories play a part in plsticising effects.

• Lubrication theory : palsticiser molecules diffuse between the

chains in amorphous regions and allow them to slip over each

other more easily.

• Solvation theory: palsticiser dissolves the polymer.

• Gel (thermodynamic) theory: palsticiser molecules diffuse between

the chains in amorphous regions, reducing the number of points of

attachment between chains, form weak bonds with them.

• Free volume theory: palsticiser molecules diffuse between the

chains in amorphous regions and increase free volume.


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Choice of Plasticisers

depends mostly on the following

points:

• Cost;

• Compatibility with polymer;

• Processing characteristics;

• Thermal, electrical and

mechanical properties;

• Resistance to water, chemicals,

weathering, dirt and

microorganisms: plasticiser must

be stable against environmental

effects during polymer life;

• Effect on rheological properties;

• Health issue.


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Compatibility

• Plasticiser must be compatible with polymer in order to be effective for a long time. Factors controlling compatibility:

– Solubility

– Dipole moment

– Viscosity

Thermodynamics of polymr solubility: solubility is controlled primarily by H, the heat of mixing

H = n1 2 V1 (1 - 2)2 = E / V

n1: mole fraction of solvent, 2: volume fraction of solute, V1: the molar volume of the solution,

1 and 2: solubility parameters of the solvent and solute,

E: energy of vapourisation, V: molecular volume

A combination of a suitable solubility parameter and a similar dielectric constant often lead to selection of compatible plasticisers.


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Effects of Plasticisers on Processing

• Act as internal lubricants;

• Reduce adhesion to metal surfaces (extrnal lubricant);

• Lower processing temperature;

• Reduce melt viscosity;

• Lowers film forming temperature (in dispersion and latices);

• Improve wetting and dispersion of pigments and fillers.


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Effects on Properties of Final Products

• Plasticise polymers (improve flexibility);

• Decrease modulus of elasticity and tensile strength;

• Increase extensibility and elongation at break;

• Improve toughness and impact strength;

• Decrease Tg;

• Extend applicability of polymers to lower temperature;

• Improves adhesion to various substrates;

• Improve gloss and appearance of surfaces;

• Reduce electrostatic chargeability.


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Mechanical Effects

• Anti-plasticising effect;

• Plasticisers lower Tg:

W1 and W2 are weight

fractions of plasticiser

and polymer, Tg1 and Tg2

are Tg of pure plasticiser

and polymer.


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Stabilizers (Anti-aging additives)

The properties of polymers usually change during processing and

service life in an adverse manner (degradation). The corresponding

changes in the structure are:

• Chain scission, resulting in a loss in strength and toughness;

• Cross-linking, leading to hardening, brittleness, changes in

solubility;

• Development of chromophoric groups, leading to colour formation;

• Development of polar groups such as carbonyl in polyolefins.

• Classification of stabilizers:

– Antioxidants

– Antiozonants

– Light stabilisers


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Antioxidants

• The selection of antioxidant depends on the mechanism of oxidation of polymer. Oxidation of polymers (hydrocarbons, in particular) takes place in a free-radical chain reaction. As a result of shear stress, UV radiation, metal ions, polymer breaks down into two radicals:

R - R 2R• R• + O2 ROO• (fast reaction) ROO• + R-H ROOH + R• ……. chain reaction

possible mechanisms:

ROOH RO• + •OH 2ROOH RO• + ROO• + H2O ROOH + RH RO• + R• + H2O

chain reactions may terminate by:

2ROO• non-radical product

ROO• + R• ROOR (may lead to chain extension or cross-linking)

R• + R• R - R (may lead to chain extension or cross-linking)


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ROO• breakdown products

RO• breakdown products

direction of oxidation to decomposition or cross-linking depends

on the polymer and environment.

The role of antioxidant is to interrupt the oxidation reactions.

There are three types of antioxidants:

(1) Preventive; which prevent formation of radicals

(2) Inerrupting the chain reactions and terminating the reactions;

(3) Oxidation retarders, which lead the reactions to other slower

directions.


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Antiozonants

• Ozon cracking: ozon attacks dien rubbers under stressed

conditions and generates cracks. Antiozonants are used to

protect rubbers against ozon cracking.


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Light Stabilisers

Light and oxygen induce degradatin reactions in polymers that may

lead to discoloration, surface cracking, hardening and electrical

properties. In most cases these reactions are undesirable, so they

must be prevented or reduced. The greatest demage is caused by

short wavelengths (UV).

Three methods are used against photodegradations:

– Light screens: absorb damaging radiation before reaches the polymer;

– UV absorbers: is a form of screen that absorb primarily in the UV range;

– Quenching agants: react with activated polymer molecules


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Colourants Methods for colouring polymers:

• Surface coating (painting);

• Surface dyeing;

• introduction of colour-forming groups into the polymer molecule;

• Mass colouration (the most common method).

Colourants may be insoluble (pigments) or soluble (dyestuffs) in the

polymer. Pigments have greater significance in polymer industry.

Pigments are solids with particles size of ~0.01-1 m, are organic or

inorganic.

They might be added to the polymer during polymerisation, in such a

case it shoud not interfere with the polymerisation reaction.

Colourants may adversely affect plymer properties and stability, such

as oxidation resistance


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Available forms of pigments for processing:

• Liquid pigment concentrate; example: plasticiser paste for colouration

of PVC.

• Solid pigment concentrates; pigment is dispersed in a solid carrier.

This form needs more time of mixing than liquid concentrate.

• Special pigment mixtures; are combinations of dispersible inorganic

pigments and/or fillers with organic pigment of lower dispersibility

General properties of colourant/polymer system:

• Thermal stability; colourants must be stable at high T of processing.

There are standard tests to evaluate the thermal stability of colourants.

• Lightfastness; colourants can be degraded by light and the colour

changes. There are standard tests (both accelerated and natural) to

evaluate the lightfastness of colourants.

• Weathering resistance; other factors are UV, oxygen, heat, moisture,

atmospheric impurities such as SO2, SH2, ozone. Standard outdoor

exposure tests and accelerated indoor tests are available.


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• Migration; as a result of partial solubility in polymer, are three types:

– Solvent bleeding; release of colourants (mainly dyes) into an organic liquid.

– Contact bleeding; in a solid/solid system.

– Blooming; migration of pigment particlesduring storage at room T to the

surface

There is no migration for pigmented glassy polymers (T<Tg).

• Plate out; deposion of pigment-containning polymers on mould or die

surface. It can be prevented by:

– Improving the dispersion.

– Adding surface active substances.

– Modifying the formulation (lubricant, stabiliser, etc.).

– Changing the pigment.

• Effect on rheological properties; soluble dyes in small quantities have

no effect on melt viscosity, but at higher concentration may have

significant effect. Pigments increase viscosity the same way as fillers.


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Flame Retardants Polymers are combustible. In some applications such as construction,

furniture, transportation and electrical equipment, polymers should

be resistance against fire.

Flame retardants can impair the properties of polymer, so a

compromise should be found between decrease in performance and

desired improvement in fire safety.

An ideal flame retardant should be:

– compatible with polymer (not bleed out);

– not alter mechanical properties;

– colourless and odourless;

– resistant to aging, hydrolysis and photodegradation;

– starts to be effective at temperatures below decomposition T of polymer;

– not corrosive;

– thermal resistant;

– not emit toxic gases and emit only low levels of smoke


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Mechanisms of Flame Retardancy

Flame retardants function by four mechanisms:

• Chemically interferring with the flame propagation mechanism;

• producing large volume of incombustible gases which dilute air;

• reacting, decomposing or changing state endothermically

(absorbing heat);

• forming impervious fire-resistant coating and preventing access

of oxygen to polymer.

Important classes of flame retardants:

• Halogen-containing (Cl-, Br-);

• Phosphorous-containing;

• Inorganic; like Al(OH)3

Standard methods are available to access flame retardants and

combustion of polymers.


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Fibre Reinforcements

• Fibre reinforcements are added to polymers to enhance

their stiffness and strength. This leads to formation of

composite materials. Composites are very important class

of materials and are studied in a seperate chapter.


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Other Additives • Lubricants and flow promoters:

– Internal lubricants; reduce melt viscosity and promote flow.

– external lubricants; prevent sticking melt to processing equipment.

• Blowing agents: release gas at processing conditions and form

cellular (foamed) structure.

• Cross-linking agents: are added to thermoset polymers and rubers

(vulcanising agents) to cross-link them.

• Antistatic agents: reduce the chargability of polymer, which is

critical in some applications such as packing and handling

electronic chips.

• Nucleating agents: promote nucleation of spherulites in crystalline

polymers, result in higher crystallinity and smaller spherulites.

Polymer Science and Engineering course: Lecture 2

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