MATERIALS

POLYMERS

The term polymer means many parts. It is applied to large molecules or ions that consist of many distinct units linked together by covalent bonds. Polymer molecules are usually long chains, they can be linear or branched, cross–linked by covalent bonds or bonded with other chains by secondary bonds such as dipole–dipole or hydrogen bonds. The vast majority are carbon based, some important synthetic polymers are silicon based.

The term polymer is also applied to the materials and classes of materials that are composed of these large molecules, eg, polythene, polystyrene, nylon, rubber, and acrylics. Polymers can be synthesised with properties such as flexibility, hardness, melting point, colour, strength and density to match a particular use. Changing polymer properties can be done by

1) controlling certain features of the polymer’s molecular structure such as chain length, the extent of chain branching and cross linking and polarity of side groups. 2) adding substances to the pure polymer, eg UV absorbers.

Polymer advances make them more desirable than traditional materials

Eg polyethelene terephthalate (PET) has replaced glass as material of choice for soft drink bottles. It is cheaper and less dense.

POLYMERISATION REACTION

Polymers are formed from small molecules called monomers in reactions called polymerisation reactions. They are addition and condensation polymerisations to make addition polymers and condensation polymers.

ADDITION POLYMERS

Addition polymers are formed when monomers with C = C groups are reacted, under special conditions of temperature and pressure with the aid of initiators. The monomers add together to form single C =C bond links between monomer units.

Two important features of the Structure of an Additional Polymer

The molecules of an addition polymer consist of a carbon chain joined by single covalent bonds. Each carbon atom has non – metal atoms or groups of atoms attached to it by a single covalent bond

From the polymer the monomer unit can be deduced with the C – C replaced by a

C = C double bond

eg

CONDENSATION POLYMERS

Condensation polymers are formed from monomers with two functional groups per molecule that can undergo a condensation reaction. POLYESTERS Polyester molecules have ester links holding the polymer chain together

Ester link

Polyesters from diol and dicarboxylic acid monomers Eg

Polyesters from hydroxycarboxylic acid monomers

POLYAMIDES

Polyamide molecules have amide links holding the polymer chain together

Polyamides from diamine and dicarboxylic acid monomers

monomers

Polyamides from aminocarboxylic acid monomers

BONDING BETWEEN POLYMER CHAINS

The type and strength of bonding between polymer chains is determined by the polarity of side groups and main chain link groups.

Dispersion forces between chains

Chains that have non - polar side groups and main chain links bond to each other through dispersion forces. Because the chains are long, the dispersion forces are stronger than those of small molecules. While this is the weakest form of bonding between molecules, as chain length increases, the strength increases. Polymers formed from chains bonded only by dispersion forces are soft, flexible and non- elastic, they have low melting/softening points and can be softened and reshaped repeatedly, this makes them suitable for recycling.

Hydrogen Bonding between Chains

Hydrogen bonds can form between chains that have polar O – H, N – H and C = O groups present in side groups or link groups of the molecular structure, eg cellulose and polyamides. Hydrogen bonding imparts greatest strength, rigidity and elasticity to the polymer.

Covalent Bonding between Chains

Covalent bonds that exist between polymer chains are called cross – links. If there is extensive cross linking, a three dimensional network structure is established and the polymer material is called a resin. The extent of cross linking in a polymer can be controlled during production to meet property specifications for the end use of the polymer. Generally, the greater the cross linking, the greater the hardness, rigidity, brittleness and durability of the polymer material. Extensively cross linked polymers do not soften on heating. The introduction of covalent cross links between polymer chains can be achieved for polymers that have C = C groups present in the chains. Examples include vulcanised rubber,

Rubbers of different hardness and rigidity can be made by varying the proportions of natural rubber and sulphur in the vulcanisation process. Cross – linking is important in determining the properties of many different polymer materials. Silicone rubbers, polyurethanes and alkyd resins are important classes of polymers that are cross – linked.

THERMOPLASTIC AND THERMOSET POLYMERS

Thermoplastics – polymers soften when heated and return to their original condition when cooled. There are only dispersion forces or hydrogen bonds between the polymer chains of these materials. They are suited to recycling.

Thermosets – polymers that do not soften and change shape when heated. They are not suited

to recycling. They are highly cross - linked and must be shaped during polymerisation, they set irreversibly. One common effect of heating a thermoset polymer to a high temperature is charring and carbonisation.

Environmental Impact of Polymer Use

Raw material for organic polymers is crude oil, which is non renewable, therefore not sustainable. Most polymers and their additives are not biodegradable. In landfills the polymers will usually not break down. In environments, plastics have lead to deaths in wildlife. Some of the additives may leach from the polymer and poison water or land and may remain for a long time. Disposal of polymer wastes by incineration also presents environmental problems. During incineration, additives can be driven off, in toxic forms. Biodegradable polymers and less toxic additives are slowly being developed.

SILICATES

Silicon is the second most abundant element in the earth’s crust. It never occurs as the free element in nature, but is almost always combined with oxygen in the form of SiO4 tetrahedral units that can exist individually or linked into chains, rings, sheets or three dimensional covalent networks. These silicon oxygen structures, in the form of silicate minerals or silica, makes up about 95% of crustal rocks and their products, soils, clays and sands. Apart from silica, the silicate minerals often have complex polymeric structures in which silicate anions are bonded to metal cations. If the cations are transition metal ions, then the mineral may be coloured. The total positive charges of the metal cations in the mineral must balance exactly with the negative charges on the silicate anions. In a large number of silicate minerals, Al atoms can be found in place of some Si atoms, covalently bonded to oxygen atoms within the anion structure. These are called aluminosilicates. In other silicate minerals, aluminium is present as the Al+3 ion, where it contributes to the charge balance for the mineral.

The Formula of Silicate and Aluminosilicate anions

The complete formula of a silicate or aluminosilicate anion shows the ratio of Al: Si: O atoms in the ion and the overall charge. The formula can be found in 2 ways. Using the charge balance of the cations and anions in the mineral

Using the oxidation numbers of Aluminium, silicon and oxygen in the anion

Silicate ion Structures

The basic structural unit of silicate anion is the SiO4 tetrahedron shown below

The basic structural unit of all silicate anions is the SiO4 tetrahedron. SiO4 units can exist either as discrete structural entities or they can combine by corner sharing of oxygen atoms into larger units such as chains, sheets or three – dimensional networks. Linking of SiO4 tetrahedra in this way produces polymer silicate ions in which repeating units may be identified. As for polymer molecules, the formula of the repeating unit is used as the formula for the polymer ion. The joining of SiO4 is always through O – Si – O links, there are no direct

Si – Si bonds in the structures of silicate ions. The importance of the Si: O ration in a silicate anion The type of structure of the anion, its overall charge and the physical properties of the silicate can also be deduced from the silicon: oxygen ratio. Silicates with single and double chain structures are fibrous or in the form of prismatic crystals. Those with 2-D sheet structures are flaky or slippery to feel. Eg. Asbestos, Ca2 (Mg,Fe)5 (Si4O11)2 (OH)2

is an example of a double chain silicate. The Ca + Mg + Fe metal ions balance and bind the negatively charged double chains together into a neutral structure.

The bonds between double chains are much weaker than are the covalent bonds within the double chains, hence these minerals are fibrous.

Talc, Mg3Si4O10(OH)2 – sheet structure The bonds between sheets is weak, therefore they easily slide over each other hence they feel slippery

In silicates the oxidation states of oxygen and silicon are –2 and +4 respectively. Therefore the overall

charge on the silicate ion can be determined by adding up the total charge of silicon and oxygen atoms.

The charge on the silicate ion must be balanced out by metal ions to get an overall neutral charge. In this way we can determine how many of these metal ions are attached to a silicate given the formula of the silicate. Take for example Talc, it has a formula MgxSi4O10(OH)2 . To find the number of Mg ions that must be added we must first consider the charge on the rest of the ion Charge on silicate ion = -4 (4 x 4 + 10 x -2) Charge on (OH)2 = -2 (2 x –1) Overall charge = -6 Charge on Mg ion = +2 Therefore we need 3 magnesium ions to balance it. Therefore the formula for talc is Mg3Si4O10(OH)2

ALUMINOSILICATES

The aluminosilicates are common materials in which the anions can be thought of as silicate structures with Al atoms substituting for some of the silicon atoms. For each substitution, a negative charge is made. This is offset by a positive charge from a metal cation, commonly Ca+2, Na+, Al+3 and K+ The substitution of aluminium for silicon is most common in 3-D networks and sheet silicates, much less in single and double chains. Two of the most important aluminosilicate groups are the feldspar and zeolite minerals. Feldspar minerals constitute 60% of all minerals in the earth’s crust and zeolites have a wide range of commercial and industrial applications. Both Feldspar and zeolite anions have 3-D structures based on the SiO2 structure A third important group of aluminosilicates, the mica minerals have structures based on Si4O10 silicate sheets. The ratio (Si + Al):O in the aluminosilicate is the same as the Si:O ratio in the parent silicate. For each Al substituted into the formula of the silicate ion, the negative charge on the ion increases by one.

SOIL SILICATES

Clays are an important part of soils. They are made of particles about 2 microns in size, made up of hydrated aluminosilicate minerals with sheet structures forming layers. There are two types of sheets that form the layered structures, - tetrahedral and octahedral. Tetrahedral sheets have SiO4 tetrahedra showing corner oxygen Octahedral sheets consist of Aluminium atoms surrounded by oxygen and hydroxyls located at the corners of an octahedron. The sheets join to each other through common oxygen atoms. Some clays silicates have three layers – (tetra – octa – tetra) In some clay silicates, trivalent aluminium can substitute for some of the tetravalent silicon in the tetrahedral sheets and divalent magnesium can substitute for the trivalent aluminium in the octahedral layers. For each substitution, a surplus unit negative charge is generated and the layers of such silicate minerals acquire a surface negative charge. The substitution of one element for another in a clay silicate structure without alteration to the basic structure is called isomorphic substitution. The type of sheet structure and extent of isomorphic substitution in the minerals both influence the properties of clay.

Two important properties result from the presence of absorbed cations on layer surfaces. When water is added to the clay, cations become hydrated and some become detached from the surface. As they move into the spaces between the layers, they make the layer expand and hence the clay swells. If the clay particles are completely separated, then the clay is of lower quality and causes turbidity in water runoff. The dispersion can be reversed by treating the colloidal suspension with solutions of highly charged cations such as Fe+3 or Al+3. These ions attach strongly to negatively charged layers rather than occupy the spaces between, as a result, the gaps between layers close up and the colloidal particles group back together as larger particles that settle out of solution. This process is called Flocculation. When an aqueous solution with cations like NH4

+ or acid rain with a high H+ is added to the clay, they can replace some if the adsorbed ions in a process called cation exchange. Depending on the ions exchanged it can have a positive or negative effect. If a clay is treated with NH4

+, some will become adsorbed onto the silicate surface in place of K+ and Na+. The remaining NH4

+ ions remain in solution and in equilibrium with adsorbed NH4+. In

this way the silicate surface acts as a slow release source of nutrient NH4+ for plant growth.

With acid rain, H+ ions can replace Al+3 and so Al+3 ions go into solution and may be toxic to plants. Clays have a very fine particle size meaning that they will generally pass through filters. However these particles tend to have a negative surface charge. To remove these clay particles from water, salts of aluminium can be used. Aluminium has a large positive charge and will therefore attract and clump clay particles around it. This process in which large particles are formed from smaller ones is called flocculation.

Soil silicates When rocks weather to make soils Al replaces Si in silicates. This upsets the electrical neutrality of the soil (Al = 3+, Si = 4+). To balance the extra electron (1- charge) H+, NH4

+, K+, Ca2+, Mg2+ and Al3+ are attracted and held by the silicate. The ability to attract and hold cations is dependant on the amount of clay in the soil. Clay has a very small particle size and therefore has a large surface area. This allows it to hold a lot of cations and therefore have more nutrients available for plants to use. Equilibrium is set up between the cations held by the silicates and ions in the soil moisture allowing for cations such as Ca2+ to be available to plants.

(clay silicate)2-Ca2+(s) + 2H+

(aq) (clay silicate)2-(H+)2(s) + Ca2+(aq)

Clays have high cationic exchange and sands low cationic exchange due to differences in particle size and hence surface area. pH in the form of acid rain can have a dramatic effect on the health of the soil. When H+ enters the soil cations (K+, Ca2+, Mg2+) are released, this results in:

More acidic soil which inhibits plant growth.

Cations are leached from the soil and can no longer be used by plants.

It releases Al3+ from the silicate which is damaging to plants.

(clay silicate)3-Al3+(s) + 3H+

(aq) 3(clay silicate)- (H+) (s) + Al3+(aq)

H+ Mg

2+

Al3+

H+ Ca

2+

K+

NH4+

Ca2+

Mg2+

Water Softeners In hard water soap reacts with Ca2+ and Mg2+ to form scum. This stops the cleaning action of the soap before it begins.

2NaSt(aq) + Ca2+ CaSt2(s) + 2Na+ Sodium stearate in hard scum A soap water There are two ways to solve the problem:

Use soaps that don’t react with those cations (soapless detergents)

Remove the Ca2+ and Mg2+ cations from the water. Water softeners remove calcium and magnesium ions from water by cationic exchange. Silicates, often naturally occurring zeolites, are used for this process. They are open structures that cations can fit into

2Na+Resin(s) + Ca2+(aq) Ca2+Resin(s) + 2Na+

(aq)

The water is now soft because the Ca2+, Mg2+ cations have been removed. This is a reversible process and can be reversed by passing a sodium chloride solution through the water softener.

ZEOLITES

Zeolites are synthetic or naturally occurring hydrated aluminosilicates with symmetrically stacked Al2O3 and SiO2 tetrahedra resulting in an open and stable honeycomb structure. 45% of the structure can be open space, (therefore it is low density). Water molecules and exchangeable cations are bound within the pores of a zeolite. One use is to remove Ca+2 and Mg+2 ions from hard water. By passing the water through a bed of zeolite in a water softener. The ions replace Na+ on the surface of the zeolite. This softens the water. It can be re-generated by flushing the zeolite with a concentrate solution of sodium ions to replace the Ca+2 and Mg+2 ions.

CLEANING AGENTS

Stain Removal The method used to remove stains depends on the chemical nature of the stain. In broad terms, stains can be dissolved in a suitable solvent or removed by a surfactant.

Dissolving Stains When a stain is dissolved in a solvent, secondary bonds play an important role in the formation of the solution. When a solid or a liquid solute dissolves in a liquid solvent, molecules of each substance mix with each other. The ease with which this happens depends on the bonding between the particles The stronger the bonds between the solvent and solute, the greater the solubility.

Strong bonds between solute particles inhibit solubility unless the solvent can exert

comparable attraction. Similarly strong forces between solvent particles can prevent solute particles replacing

them unless there is strong solute-solvent particle attraction. So polar substances dissolve polar substances and non-polar dissolves non-polar. So like dissolves like.

A Polar Solvent and a Polar Solute The most common polar solvent is water. In water, there is extensive hydrogen bonding between molecules. For particles to be soluble in water, they must be charged or be polar molecules to form ion – dipole, dipole – dipole or hydrogen bonds with the water, therefore a stain that is polar can be dissolved in water.

A non – polar Solute and a non – polar Solvent There are a large number of non – polar organic solvents. Many home and commercial cleaning products have mixes of non – polar compounds. These solvents remove fats, oils, greases and waxes. They dissolve in non – polar solvents by dispersion forces, which is comparable to the dispersion forces attracting the molecules together.

A non – polar Solute and a polar Solvent

Fats, oils, greases and waxes do not dissolve in water. The non – polar fats and oils can’t form bonds with water to the same extent that water molecules can between themselves. Similarly, non – polar solvents can’t dissolve polar solutes Removal of stains can be more effectively done when the polar nature of the substance is known. Major difficulties arise when stains are a complex mixture of substances, eg, blood, wine, inks. Stain removal is even harder if the staining substance bonds with the stained material.

SOAPS AND DETERGENTS

Soaps and detergents are surface – acting cleaning agents, or surfactants, they are widely used to remove grease. They are long hydrocarbon chain sodium carboxylates (soap) or sulfurates ( detergents) with similar structures

The structural features that distinguish a soap anion from a detergent anion are The ionic head of a soap anion is a carboxylate ion while the detergent has a sulfonate

ion The hydrocarbon chain of the soap anion is a straight chain, the detergent is a straight

chain that includes a benzene ring and one branch

How do soaps and detergents remove grease?

The non – polar hydrocarbon chains of soap and detergent ions are soluble in non – polar grease, whereas the ionic heads are not soluble. These chains dissolve into the grease with the ionic heads sitting on the surface of the grease still in contact with the polar water. With agitation, the grease globules, with soap/detergent ions embedded are dislodged from the surface to which they have been attached. They are released into the water as spherical structures called micelles. The micelles repel each other because they have negatively charged surfaces. They remain suspended and can be washed away. Most dirt sticks to clothes and skin by means of a thin film of non – polar oil.

Formation of Soap from Fats and Oils

Soaps are produced by the alkaline hydrolysis of fats and oils. This is called saponification. On an industrial scale, animal fats and/or vegetable oils are boiled with concentrated sodium hydroxide solution, hydrolysing the triglycerides to glycerol and the carboxylate ions of the fatty acids. The products of the hydrolysis are all soluble in water. Solid soap is precipitated from the aqueous mixture by addition of a concentrated sodium chloride solution.

A Major disadvantage of Soaps

The major disadvantage is that soap reacts with Ca and Mg ions to form insoluble carboxylate salts that are collectively called scum. Ca and Mg ions are present in hard water.

Their formation removes soap, leaves unsightly deposits. Synthetic detergents do not form scums. This is an advantage of detergents.

Tripolyphosphate in detergent formulations

Modern detergents contain as little as 15% of anionic surfactants. The remainder is compounds to assist the surfactant clean. Water – soluble sodium Tripolyphosphate In structural terms, Tripolyphosphate ions can be Linear – a short chain linear polymer with three tetrahedral

Cyclic – a ring of three phosphate units linked by two common oxygen corners

Both the linear and cyclic tripolyphosphates are made when sodium hydrogenphosphate compounds undergo condensation reactions at carefully controlled temperatures:

linear

cyclic

Tripolyphosphate ions have three main purposes: They provide a slightly alkaline solution when added to water which provides a

favourable environment for detergent action

Alkaline conditions are favourable because:

- in alkaline conditions detergent anions remain in the ionic form. If the solution is acidic, the molecular form of the detergent is produced. This is non – polar and will not dissolve in water.

- alkaline conditions can hydrolyse grease making water soluble products they remove free Ca+2 and Mg+2 ions from hard water by forming

water – soluble complex ions. This stops scum formation

they act as deflocculants that keep clay particles in suspension rather than settle back on washed clothes

In recent years, the amount of tripolyphosphates used had been reduced. When in waterways it is a nutrient for algae.. This can cause Algae blooms to form, including Blue – Green algae. Blooms block out sunlight to the water, therefore no oxygen production, this leads to the death of the waterway. This is called Eutrophication Bacteria also decay algae and plants, further depleting oxygen levels in the water, producing anaerobic conditions. This produces poisonous gases such as H2S.

Enzymes in detergent formulations Approximately 50% of liquid detergents and 25% of powder detergents and almost all powdered bleach additives contain enzymes that assist the breakdown of stains that are hard to remove with surfactants only. The use of enzymes allows for low temperature and low mechanical energy approaches to be used for cleaning. Enzymes are also much more efficient than non – biological cleaning agents at breaking down stains such as blood, grass, milk and sweat. The criteria used for the inclusion of an enzyme in detergents are;

- the enzyme must be cheap - the enzyme must be stable under wash conditions - the enzyme must be safe to handle and use

The enzymes must be able to cope with the environment in a washing machine. They must be able to tolerate the presence of anionic surfactants, oxidants like bleaches, optical brighteners, pH ranges between 8 – 10.5 and other additives. The enzymes are proteins, yet they must remain active up to 600C Unlike inorganic chemical catalysts, enzymes are quite specific in their action. Each different enzyme is capable of breaking one type of chemical bond in a specific position within a biological molecule. The following are common enzymes included in detergent formations. Amylase – catalyses the hydrolysis of starches in stains to smaller water soluble saccharide

units. It effectively cuts the starch from the surface of the fabric piece by piece. The enzyme is active up to 850C and pH values up to 10.5

Proteases – catalyse the hydrolysis of protein molecules by breaking the peptide bonds that

hold amino acids together. Lipases – hydrolyse lipids (fats and oils), for fatty stains in clothes at low wash temperatures Celluloses – digest cellulose ‘ fine fluff released from cotton after repeated washing. Does not

hydrolyse the main fibres of the cloth

BLEACHES

Bleaches form another class of cleaning agent. They remove coloured stains by oxidising the coloured substances to colourless substances. Most coloured compounds owe their colour to a particular structure type, eg a series of alternating double bonds in a carbon chain, called conjugated double bonds. Bleaches oxidise the C = C functional group, thus breaking down the structure responsible for the colour. The chemical action of bleaches is quite different from the physical action of soaps and detergents. There are two main categories of bleaches:

- chlorine bleaches, hypochlorous acid, HCl0 - oxygen bleaches, hydrogen peroxide, H2O2.

Both HCl0 and H2O2

are oxidising agents that oxidise coloured stains. Hypochlorous acid is a stronger oxidising agent than molecular chlorine Cl2, and the hypochlorite ion, OCI-. The bleaching is enhanced when there is more hypochlorous acid in solution.

This can be done by adjusting the pH of the solution, thereby shifting the equilibrium position. Commercial bleaches use either sodium hypochlorite in solution or powdered calcium hypochlorite. Domestic bleaching solutions are made slightly acidic to remove the OH-, to increase the HOCl concentration. The bleaching activity of hydrogen peroxide and oxygen is based on the chemical oxidation of stains by these substances that are themselves reduced

In the reduction processes, there is a decrease in the oxidation number of oxygen Hydrogen peroxide

Oxygen Formation from H2O2

Acting as an oxidising agent under acid conditions

Acting as an oxidising agent under alkaline conditions

The most common source of hydrogen peroxide in laundry powders is sodium perborate tetrahydrate (NaBO3.4H2O). it forms by reacting sodium borate with hydrogen peroxide.

When dissolved in water the reverse reaction occurs. It occurs faster in hot water. It is often 15 – 30% by mass of the detergent formulation. It works best when clothes are allowed to soak with a high temperature 950C = 90% release 500C = 60% oxygen release, which is still considered to be effective.