RAS MAINS EXAM PAPER-2 STUDY NOTES
SCIENCE & TECHNOLOGY
- States of Matter
- Allotropes of carbon.
- pH Scale and importance of pH in daily life.
- Corrosion and its prevention
- Soap and Detergents – Cleansing action of soap.
- Polymers and their uses
States of Matter
Solids are formed when the attractive forces between individual molecules are greater than the energy causing them to move apart. Individual molecules are locked in position near each other, and cannot move past one another. The atoms or molecules of solids remain in motion. However, that motion is limited to vibration energy; individual molecules stay fixed in place and vibrate next to each other. As the temperature of a solid is increased, the amount of vibration increases, but the solid retains its shape and volume because the molecules are locked in place relative to each other. To view an example of this, click on the animation below which shows the molecular structure of ice crystals.
Liquids are formed when the energy (usually in the form of heat) of a system is increased and the rigid structure of the solid state is broken down. In liquids, molecules can move past one another and bump into other molecules; however, they remain relatively close to each other like solids. Often in liquids, intermolecular forces (such as the hydrogen bonds shown in the animation below) pull molecules together and are quickly broken. As the temperature of a liquid is increased, the amount of movement of individual molecules increases. As a result, liquids can “flow” to take the shape of their container but they cannot be easily compressed because the molecules are already close together. Thus, liquids have an undefined shape, but a defined volume. In the example animation below, we see that liquid water is made up of molecules that can freely move past one another, yet remain relatively close in distance to each other.
RAS MAINS EXAM PAPER-2 COMPLETE STUDY MATERIAL DOWNLOAD
Gases are formed when the energy in the system exceeds all of the attractive forces between molecules. Thus gas molecules have little interaction with each other beyond occasionally bumping into one another. In the gas state, molecules move quickly and are free to move in any direction, spreading out long distances. As the temperature of a gas increases, the amount of movement of individual molecules increases. Gases expand to fill their containers and have low density. Because individual molecules are widely separated and can move around easily in the gas state, gases can be compressed easily and they have an undefined shape.
Solids, liquids, and gases are the most common states of matter that exist on our planet. If you would like to compare the three states to one another, click on the comparison animation below. Note the differences in molecular motion of water molecules in these three states.
Plasmas are hot, ionized gases. Plasmas are formed under conditions of extremely high energy, so high, in fact, that molecules are ripped apart and only free atoms exist. More astounding, plasmas have so much energy that the outer electrons are actually ripped off of individual atoms, thus forming a gas of highly energetic, charged ions. Because the atoms in plasma exist as charged ions, plasmas behave differently than gases, thus representing a fourth state of matter. Plasmas can be commonly seen simply by looking upward; the high energy conditions that exist in stars such as our sun force individual atoms into the plasma state.
Allotropes of carbon
Allotropy is the property of some chemical elements to exist in two or more different forms, or allotropes, when found in nature. There are several allotropes of carbon.
Diamond is probably the most well known carbon allotrope. The carbon atoms are arranged in a lattice, which is a variation of the face-centered cubic crystal structure. It has superlative physical qualities, most of which originate from the strong covalent bonding between its atoms. Each carbon atom in a diamond is covalently bonded to four other carbons in a tetrahedron. These tetrahedrons together form a three-dimensional network of six-membered carbon rings in the chair conformation, allowing for zero bond-angle strain. This stable network of covalent bonds and hexagonal rings is the reason that diamond is as incredibly strong as a substance.
As a result, diamond exhibits the highest hardness and thermal conductivity of any bulk material. In addition, its rigid lattice prevents contamination by many elements. The surface of diamond is lipophillic and hydrophobic, which means it cannot get wet by water but can be in oil. Diamonds do not generally react with any chemical reagents, including strong acids and bases. Uses of diamond include cutting, drilling, and grinding; jewelry; and in the semi-conductor industry.
Graphite is another allotrope of carbon; unlike diamond, it is an electrical conductor and a semi-metal. Graphite is the most stable form of carbon under standard conditions and is used in thermo chemistry as the standard state for defining the heat of formation of carbon compounds.
Graphite has a layered, planar structure. In each layer, the carbon atoms are arranged in a hexagonal lattice with separation of 0.142 nm, and the distance between planes (layers) is 0.335 nm. The two known forms of graphite, alpha (hexagonal) and beta (rhombohedra), have very similar physical properties (except that the layers stack slightly differently). The hexagonal graphite may be either flat or buckled. The alpha form can be converted to the beta form through mechanical treatment, and the beta form reverts to the alpha form when it is heated above 1300 °C. Graphite can conduct electricity due to the vast electron delocalization within the carbon layers; as the electrons are free to move, electricity moves through the plane of the layers. Graphite also has self-lubricating and dry lubricating properties. Graphite has applications in prosthetic blood-containing materials and heat-resistant materials as it can resist temperatures up to 3000 °C.
Fullerenes and Nanotube
Carbon Nanomaterials make up another class of carbon allotropes. Fullerenes (also called buck balls) are molecules of varying sizes composed entirely of carbon that take on the form of hollow spheres, ellipsoids, or tubes. Bucky balls and buck tubes have been the subject of intense research, both because of their unique chemistry and for their technological applications, especially in materials science, electronics, and nanotechnology. Carbon Nanotube are cylindrical carbon molecules that exhibit extraordinary strength and unique electrical properties and are efficient conductors of heat. Carbon nanobuds are newly discovered allotropes in which fullerene-like “buds” are covalently attached to the outer side walls of a carbon Nanotube. Nan buds therefore exhibit properties of both Nanotube and fullerenes.
Glassy or vitreous carbon is a class of carbon widely used as an electrode material in electrochemistry as well as in prosthetic devices and high-temperature crucibles. Its most important properties are high temperature resistance, hardness, low density, low electrical resistance, low friction, low thermal resistance, extreme resistance to chemical attack, and impermeability to gases and liquids.
- Catalyst, in chemistry, any substance that increases the rate of a reaction without itself being consumed. Enzymes are naturally occurring catalysts responsible for many essential biochemical reactions.
Most solid catalysts are metals or the oxides, sulfides, and halides of metallic elements and of the semi metallic elements boron, aluminum, and silicon. Gaseous and liquid catalysts are commonly used in their pure form or in combination with suitable carriers or solvents; solid catalysts are commonly dispersed in other substances known as catalyst supports.
In general, catalytic action is a chemical reaction between the catalyst and a reactant, forming chemical intermediates that are able to react more readily with each other or with another reactant, to form the desired end product. During the reaction between the chemical intermediates and the reactants, the catalyst is regenerated. The modes of reactions between the catalysts and the reactants vary widely and in solid catalysts are often complex. Typical of these reactions are acid–base reactions, oxidation–reduction reactions, formation of coordination complexes, and formation of free radicals. With solid catalysts the reaction mechanism is strongly influenced by surface properties and electronic or crystal structures. Certain solid catalysts, called polyfunctional catalysts, are capable of more than one mode of interaction with the reactants; bifunctional catalysts are used extensively for reforming reactions in the petroleum industry.
pH Scale and importance of pH in daily life
An acid is a substance which forms H+ ions as the only positive ion in aqueous solution.
Hydrochloric acid dissolved in water forms H+ and Cl– ions
HCl —> H+ + Cl–
Sulphuric acid dissolved in water forms H+ and SO42- ions
H2SO4 —>2H+ + SO42-
Nitric acid forms H+ and NO3– ions when dissolved in water
HNO3 —> H+ + NO3–
Ethanoic acid, also known as acetic acid, forms H+ and CH3COO– ions in water
CH3COOH —> H+ + CH3COO–
Acetone, also known as propanone, doesn’t form any ions in water, so it isn’t an acid.
CH3COCH3 just dissolves in water.
Methane, CH4, doesn’t form any ions in water, so this isn’t an acid either.
So just containing hydrogen doesn’t make something an Arrhenius acid.
Properties of Acids
- tastes sour
- acids change blue litmus to red
- their aqueous (water) solutions conduct electricity (i.e. they are electrolytes)
- react with bases to form salts and water as the only products
- evolve hydrogen gas (H2) upon reaction with an active metal, such as alkali metals, alkaline earth metals, zinc, iron, aluminum, forming a salt as the only other product
- Evolve carbon dioxide on reacting with metal carbonates.
An alkali is a substance which forms OH- ions as the only negative ion in aqueous solution. A base is an insoluble hydroxide.
Sodium hydroxide, when dissolved in water, forms Na+ and OH– ions
NaOH —> Na+ + OH–
Calcium hydroxide dissolves in water to give Ca+ and OH– ions
Ca (OH) 2 —> Ca2+ + 2 OH–
Ethanol CH3CH2OH does not form OH– ions when dissolved in water, so it isn’t a base.
Ethanol just dissolves. Strictly speaking, ethanol actually forms a tiny amount of H+ ions in water and is a very weak acid.
Properties of Bases
- taste bitter
- Feel slippery or soapy (But don’t touch them! They react with your skin to form soap.)
- bases turn red (acidified) litmus back to blue
- their aqueous (water) solutions conduct electricity (i.e. they are electrolytes)
- react with acids to form salts and water as the only products
A measure of the degree of the acidity or the alkalinity of a solution as measured on a scale (pH scale) of 0 to 14. The midpoint of 7.0 on the pH scale represents neutrality, i.e., a “neutral” solution is neither acid nor alkaline. Numbers below 7.0 indicate acidity; numbers greater than 7.0 indicate alkalinity. It is important to understand that pH is a measure of intensity, and not capacity; i.e., pH indicates the intensity of alkalinity in the same way temperature tells how hot something is, but not how much heat the substance carries.
The pH scale is logarithmic which means that moving on (unit either way on the pH scale results in a 10 fold increase in the degree of alkalinity or acidity.
Classification, pH, Product
- Hydrochloric, Sulfuric, Nitric Acids
- 1-2 Phosphoric, Sulfamic Acids
- 0 Citrus Fruit
- 0 Milk
- 0-7.5 Water, Sugar, Table Salt
- 0 Eggs
- 0 Ammonia
- 13-14 Caustic Soda, Degreasers
Products at the two extremes (less than pH 1 or greater than pH 13) are extremely oppressive and corrosive. Examples include sulfuric and hydrochloric acid on the acid end, and caustic soda on the alkaline end. Use solutions of phosphoric or sulfamic acid cleaners, typically in the pH range of slightly less than 2, may be described as “safe” acids comparison to the stronger acids. Of course, necessary safety precautions (eye and hand protection) as noted on the Material Safety Data Sheet should always be followed.
As an illustration of their non-aggressive behavior in comparison to stronger acids, products containing sulfamic or phosphoric acid were found to result in no chemical attacks on nylon carpets. On the other hand, a hydrochloric or powder acid-based product would basically dissolve the fibers.
Other than the two pH extremes, the pH scale becomes secondary to the inherent properties of the specific chemical in terms of corrosiveness. As an example, the pH of carbonated cola soda (which contains phosphoric acid) is in the 2.5 range. A concentrated (35%) hydrogen peroxide solution has a pH of approximately 3.5. Carbonated cola soda may be slightly irritating to the eyes, causing no permanent damage. However, a 35% hydrogen peroxide solution will cause chemical burns to the skin or mucous membranes. Thus, it is not the pH factor alone that causes corrosion of products to surfaces.
Corrosion and its prevention
Corrosion is defined as an attack on a material as a result of chemical, frequently electrochemical reaction, with the surrounding medium. According to this definition, the term corrosion can be applied to all materials, including non-metals. But in practice, the word corrosion is mainly used in conjunction with metallic materials.
Why do metals corrode? Apart from gold, platinum and a few others, metals do not occur in the nature in their pure form. They are normally chemically bound to other substances in ores, such as sulphides, oxides, etc. Energy must be expended (e.g. in a blast furnace) to extract the metals from the sulphides, oxides, etc to obtain pure metals.
Pure metals contain more bound energy, representing a higher energy state than that found in the nature as sulphides or oxides.
As all material in the universe strives to return to its lowest energy state, pure metals also strive to revert to their lowest energy state which they had as sulphides or oxides. One of the ways in which metals can revert to a low energy level is by corrosion. The products of corrosion of metals are often sulphides or oxides.
Chemical and electrochemical corrosion
Chemical corrosion can be seen as oxidation and occurs by the action of dry gases, often at high temperatures. Electrochemical corrosion on the other hand takes place by electrode reactions, often in humid environments, i.e. wet corrosion.
All metals in dry air are covered by a very thin layer of oxide, about 100Å (10-2µm) thick. This layer is built up by chemical corrosion with the oxygen in the air. At very high temperatures, the reaction with the oxygen in the air can continue without restraint and the metal will rapidly be transformed into an oxide.
At room temperature the reaction stops when the layer is thin. These thin layers of oxide can protect the metal against continued attack, e.g. in a water solution. In actual fact, it is these layers of oxide and/or products of corrosion formed on the surface of the metal that protect the metal from continued attack to a far greater extent that the corrosion resistance of the metal itself.
These layers of oxide may be more or less durable in water, for instance. We know that plain carbon steel corrodes faster in water than stainless steel. The difference depends on the composition and the penetrability of their respectively oxide layers. The following description of the corrosion phenomenon will only deal with electrochemical corrosion, i.e. wet corrosion.
How do metals corrode in liquids? Let us illustrate this, using a corrosion phenomenon called bimetal corrosion or galvanic corrosion. The bimetal corrosion cell can e.g. consist of a steel plate and a copper plate in electrical contact with one another and immersed in an aqueous solution (electrolyte).
The electrolyte contains dissolved oxygen from the air and dissolved salt. If a lamp is connected between the steel plate and the copper plate, it will light up. This indicates that current is flowing between the metal plates. The copper will be the positive electrode and the steel will be the negative electrode.
The driving force of the current is the difference in electrical potential between the copper and the steel. The circuit must be closed and current will consequently flow in the liquid (electrolyte) from the steel plate to the copper plate. The flow of current takes place by the positively charged iron atoms (iron ions) leaving the steel plate and the steel plate corrodes.
The corroding metal surface is called the anode. Oxygen and water are consumed at the surface of the copper plate and hydroxyl ions (OH-), which are negatively charged, are formed. The negative hydroxyl ions “neutralize” the positively charged iron atoms. The iron and hydroxyl ions form ferrous hydroxide (rust).
In the corrosion cell described above, the copper metal is called the cathode. Both metal plates are referred to as electrodes and the definition of the anode and the cathode are given below.
Anode: Electrode from which positive current flows into an electrolyte.
Cathode: Electrode through which positive electric current leaves an electrolyte.
When positive iron atoms go into solution from the steel plate, electrons remain in the metal and are transported in the opposite direction, towards the positive current.
The prerequisites for the formation of a bimetal cell are:
- Oxidation medium, such as dissolved oxygen (O2) or hydrogen ions (H+).
Electrode potential – Galvanic series
The electrode potential of a metal is an indication of the tendency of the metal to dissolve and corrode in a certain electrolyte.
Reference is also made to the “nobility” of the metal. The more noble the metal, the higher the potential is, the less the tendency it has to dissolve in an electrolyte.
The electrode potentials of different metals can be specified in relation to one another in galvanic series for different electrolytes.
Corrosion in micro-cells
The steel-copper example has shown how corrosion takes place when two different materials are connected in an aqueous solution. How does corrosion take place on the surface of a single metal? When the surface of a metal is studied under a microscope, it will be seen that it is not a single homogeneous metal. Differences in structure and grain size occur on the surface. The chemical composition may vary and various impurities may be present.
If the electrode potential is measured across an apparently homogeneous surface, it will be found to vary considerably within areas of only fractions of a square millimeter. So cathodes and anodes, possibly small but sufficiently large to cause corrosion, can be formed on the same metal surface.
Parameters affecting the corrosion rate
- Some of the most important parameters affecting the corrosion rate of metals are outlined below.
Oxidizing agents: The corrosion process is conditional on an anodic reaction and a cathodic reaction taking place simultaneously. The anodic reaction causes the metal to dissolve. An oxidizing agent must be present for the cathodic reaction, and the most common agents are dissolved oxygen or hydrogen ions. If the availability of oxidizing agents is restricted, the corrosion process will be inhibited or will cease entirely. The hydrogen concentration can easily be measured as pH-value. Oxygen is normally present in water, but not in sewage due to the oxygen consuming bacteria.
The electric conductivity of the electrolyte: Corrosion involves electrochemical reactions, and an increase in the electrical conductivity of the electrolyte will therefore increase the corrosion rate. In sea water the chloride content causes rapidly increased conductivity.
Temperature: An increase in temperature will generally cause an increase in the corrosion rate. A rule of thumb is that temperature increases of 10°C will double the corrosion rate.
Concentration: An increased concentration will normally increase the corrosion rate up to a maximum level. Higher concentration above this will not give higher corrosion rate. E.g. a chloride concentration above approximately 1500 ppm will not increase the corrosion rate.
Different types of corrosion
Various forms of corrosion on metals and their characteristics are lined below.
General corrosion is characterized by an overall attack on the surface. The corrosion takes place without distinguished anodic and cathodic areas. The corrosion resistance of metallic materials can be illustrated in iso-corrosion diagrams. The curves indicate a corrosion rate of 0.1 mm/year in a specific liquid at different concentrations and temperatures. These diagrams are only valid for liquids in stagnant conditions. The corrosion rate will increase considerably in high velocity areas.
The opposite of general corrosion is local corrosion which is divided into different types e.g. pitting, crevice and Intergranular corrosion. In local corrosion, most of the metal surface is unaffected and only small areas are highly affected. It is much easier to compensate for uniform corrosion and to adopt preventive measures in the design than to make allowance for local corrosion attacks.
- When two different metals are electrically connected and in contact with an electrolyte (=liquid), they will form a galvanic cell where the more noble material is cathodic and the less noble anodic.
- The anodic material will corrode.
- The electro potentials of metals can be measured in different water solutions and listed in galvanic series, as for seawater in the diagram.
- The corrosion rate depends on:
- The surface area ratio between cathode and anode (a bigger anode area compared to the cathode area reduces the galvanic effects, e.g. stainless steel fasteners on a cast iron pump).
- The magnitude of potential difference (compare aluminium bronze in contact with stainless steel and cast iron in contact with stainless steel).
- The conductivity of the electrolyte (liquid).
Typical examples of pitting corrosion can be seen on aluminium and stainless steels in liquids containing chlorides, e.g. seawater. These materials are dependent on a thin surface oxide film for their corrosion protection. Mechanical damage or an inhomogeneous spot in the oxide film could be the starting point for corrosion attacks. The conditions in the pit are characterized by oxygen deficiency and low pH, which intensifies the attack and may also render it self-sustaining.
The rate of pitting corrosion can be very high with the attack being localized to a considerable depth. Pitting corrosion is most likely to occur in stagnant water. Stainless steels as AISI 316L (M 0344.2343.02) and AISI 329 (M 0344.2324.02) are not resistant to pitting corrosion in seawater. Other higher alloyed stainless steels such as UNS S31254 are considered to be resistant in seawater.
The mechanism for crevice corrosion is similar to that for pitting corrosion. Crevice corrosion takes place in confined liquid filled slots and crevices where the liquid circulation is prevented. Once corrosion has appeared, conditions in the crevice are changed; e.g. the pH-value is reduced and the chloride concentration increase. Accordingly the corrosiveness of the confined liquid will increase. Crevice corrosion mainly appears on stainless steel and aluminum in liquids containing chlorides.
Intergranular corrosion occurs between the grain boundaries inside a metal. This type of corrosion is well known for stainless steels which have been soaked for an excessive period of time at temperatures between 500 and 800 °C. At this temperature chromium will react with carbon at the grain boundaries and form carbides. This causes chromium depletion in the immediate vicinity of the grain boundaries. If the chromium content falls below 12 %, corrosion can easily start.
Corrosion is a combined effect of tensile stresses, either internal or applied, and a local corrosion attack. Tensile stresses arise for example during cold work of steel sheet or as a result of directly applied load. Stress corrosion is generally connected with austenitic stainless steels in contact with liquids containing chlorides. Cracks are however unlikely to occur below +60° C. Carbon and low alloy steels may be subject to stress cracking in caustic soda solutions at high concentrations and temperatures. To avoid stress corrosion, tensile stresses should be removed e.g by heat treatment after cold working or welding. Stress corrosion can also be avoided by the choice of a resistant material.
Erosion corrosion is a combination of electrochemical corrosion (i.e. general corrosion) and the action of a high speed fluid, eroding the corrosion product. The pits formed by erosion corrosion usually have bright surfaces free from corroded material. The attacks are generally localized to areas with turbulent flow and are promoted by gas bubbles and solid particles.
Cavitation corrosion appears in areas where vapour bubbles are formed due to low pressure. When the bubbles implode on a surface the protective oxide is destroyed and eroded away and after that built up again. The process is repeated and characteristic deep holes of cavitation corrosion are formed on the surface. It can usually be seen on the trailing edge of impellers and propellers.
- Selective corrosion occurs in metals in which the alloying elements are not uniformly distributed. Typical examples of this type of corrosion are:
- Dezincification of brass, whereby zinc is dissolved and leave behind a porous copper material.
- Graphitization of cast iron, whereby the iron is dissolved and leave behind a network of graphite of low mechanical strength
Soap and Detergents – Cleansing action of soap
Soap – Characteristics and Uses
Soaps are excellent cleansing agents and have good biodegradability. A serious drawback which reduces their general use is the tendency for the carboxylate ion to react with Ca+ and Mg+ ions in hard water. The result is a water insoluble salt which can be deposited on clothes and other surfaces. These hard water plaques whiten fabric colors and also create rings found in sinks and bath tubs. Another problem with using soaps is their ineffectiveness under acidic conditions. In these cases, soap salts do not dissociate into their component ions, and this renders them ineffective as cleansing agents.
Although primarily used for their cleansing ability, soaps are also effective as mild antiseptics and ingestible antidotes for mineral acid or heavy metal poisoning. Special metallic soaps, made from soap and heavier metals, are used as additives in polishes, inks, paints, and lubricating oils.
Detergent Physical Characteristics
The concentration at which micelles begin to form is the critical micelle concentration (CMC). The CMC is the maximum monomer concentration and constitutes a measure of the free energy of micelle formation. The lower the CMC, the more stable the micelle and the more slowly molecules are incorporated into or removed from the micelle. The structure of the hydrophobic region of the detergent can affect the micelle structure. An increase in the length of the hydrophobic hydrocarbon chain of ionic detergents results in an increased micelle size and a lower CMC, as fewer molecules are needed to construct a micelle.
The average number of monomers in a micelle is the aggregation number. The CMC and aggregation number values are highly dependent on factors such as temperature, pH, ionic strength, and detergent homogeneity and purity. Slight discrepancies in reported values for CMC and aggregation number may be the result of variations in the analytical methods used to determine the values. Aggregation number values are also shifted by concentration, since the number of detergent molecules per micelle may increase if the concentration is above the CMC.
Ease of removal or exchange is an important factor in the selection of a detergent. Some of the more common detergent removal methods include:
- Gel filtration chromatography
- Hydrophobic adsorption chromatography
- Protein precipitation
The CMC value associated with the detergent is a useful guide to hydrophobic binding strength. Detergents with higher CMC values have weaker binding and are subsequently easier to remove by dialysis or displacement methods. Detergents with low CMC values require less detergent in order to form micelles and solubilized proteins or lipids.
Another useful parameter when evaluating detergents for downstream removal is the micelle molecular weight, which indicates relative micelle size. Smaller micelles are more easily removed and are usually desirable when protein-detergent complexes are to be separated based on the molecular size of the protein. The micelle molecular weight may be calculated by multiplying the aggregation number by the monomer molecular weight.
The cloud point is the temperature at which the detergent solution near or above its CMC separates into two phases. The micelles aggregate, typically forming a cloudy phase with high detergent concentration, while the balance of the solution becomes detergent-depleted. The resulting two-phase solution can be separated, with the extracted protein being located in the detergent-rich phase. Detergents with low cloud point temperatures, such as TRITON® X-114 (cloud point ~23 °C) are recommended for use with proteins since high cloud point temperatures may denature solubilized proteins. The cloud point can be affected by changes in detergent concentration, temperature, and the addition of salt or polymers such as dextran and polyethylene glycol. Note that the detergent-rich phase is also contingent on the specific detergent(s) and salt concentration; under some conditions the phase may be clear rather than cloudy and be located as either the upper or lower phase of the solution. In non-ionic detergents, this behavior has been applied in the phase separation and purification of membrane proteins.2
Detergent Types and Selection
- When selecting a detergent, the first consideration is usually the form of the hydrophilic group:
Anionic and cationic detergents are considered biologically “harsh” detergents because they typically modify protein structure to a greater extent than neutrally charged detergents. The degree of denaturation varies with the individual protein and the particular detergent and concentration. Ionic detergents are more sensitive to pH, ionic strength, and the nature of the counter ion, and can interfere with downstream charge-based analytical methods.
Non-ionic detergents are considered to be “mild” detergents because they are less likely than ionic detergents to denature proteins. By not separating protein-protein bonds, non-ionic detergents allow the protein to retain its native structure and functionality, although detergents with shorter hydrophobic chain lengths are more likely to cause protein deactivation. Many nonionic detergents can be classified into three structure types:
- Poly (polyethylene) ethers and related polymers
- Bile salts
- Glycosidic detergents
Poly (polyethylene) ethers and related detergents have a neutral, polar head and hydrophobic tails that are ox ethylene polymers (e.g. Brij® and TWEEN®) or ethyleneglycoether polymers (e.g. TRITON®). The tert-octylphenol poly (ethyleneglycoether) series of detergents, which includes TRITON X-100 and IGEPAL® CA-630, have an aromatic head that interferes with downstream UV analysis techniques.
Bile salts have a steroid core structure with a polar and a polar orientation, rather than the more obvious nonpolar tail structure of other detergents. Bile salts may be less denaturing than linear chain detergents with the same polar head group.
Glycosidic detergents have a carbohydrate, typically glucose or maltose, as the polar head and an alkyl chain length of 7-14 carbons as the polar tail.
Zwitterionic detergents have characteristics of both ionic and non-ionic detergent types. Zwitterionic detergents are less denaturing than ionic detergents and have a net neutral charge, similar to non-ionic detergents. They are more efficient than non-ionic detergents at disrupting protein-protein bonds and reducing aggregation. These properties have been used for chromatography, mass spectrometry, and electrophoresis methods, and solubilization of organelles and inclusion bodies.
Non-detergent sulfobetaines (NDSB), although not detergents, possess hydrophilic groups similar to those of zwitterionic detergents but with shorter hydrophobic chains. Sulfobetaines do not form micelles. They have been reported to improve the yield of membrane proteins when used with detergents and prevent aggregation of denatured proteins.
Uses for Detergent
Powder and liquid detergent can be used for other purposes besides cleaning clothes or dishes. This can save you money from having to buy multiple cleaning products.
Either form of detergent can be used to clean tiles, floors, counters, tubs and toilets. Mix 3/4 of a cup of bleach, 1 cup of detergent and 1 gallon of hot water together and pour it into spray bottles for a supply of all-purpose cleaner.
Sprinkle powdered detergent on moss that is growing in the cracks of your steps, sidewalk or driveway. Give it a few days to turn brown and then brush it from the cracks with a broom.
Powdered detergent can absorb oil that’s spilled on a garage floor or on the street.
Both types of cleaner can be added to carpet steam cleaners to make the carpet smell fresher and boost the appliance’s cleaning power
Instead of buying Drano to clean out a drain, put 1/4 cup of liquid detergent into the drain, then pour in a boiling pot of water after a minute to flush out to blockage.
If kids like to make bubbles with wants or play with bubble-making guns, one can make the bubble solution oneself by mixing liquid detergent with water.
Polymers and their uses
A molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetitions of units derived, actually or conceptually, from molecules of low relative molecular mass is called a polymer.
A polymer is a large molecule, or macromolecule, composed of many repeated subunits. Due to their broad range of properties, both synthetic and natural polymers play essential and ubiquitous roles in everyday life. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function. Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers. Their consequently large molecular mass relative to small molecule compounds produces unique physical properties, including toughness, viscoelasticity, and a tendency to form glasses and semi crystalline structures rather than crystals.
Polyethylene is a thermoplastic polymer with variable crystalline structure and an extremely large range of applications depending on the particular type. It is one of the most widely produced plastics in the world (tens of millions of tons are produced worldwide each year). The commercial process (the Ziegler-Natta catalysts) that made PE such a success was developed in the 1950s by German and Italian scientists Karl Ziegler and Giulio Natta.
There are vast arrays of applications for polyethylene in which certain types are more or less well suited. Generally speaking, High Density Polyethylene (HDPE) is much more crystalline, has a much higher density, and is often used in completely different circumstances than Low Density Polyethylene (LDPE). For example, LDPE is widely used in plastic packaging such as for grocery bags or plastic wrap. HDPE by contrast has common applications in construction (for example in its use as a drain pipe). Ultrahigh Molecular Weight Polyethylene (UHMW) has high performance applications in things such as medical devices and bulletproof vests.
What Are The Different Types of Polyethylene?
Polyethylene is commonly categorized into one of several major compounds of which the most common include LDPE, LLDPE, HDPE, and Ultrahigh Molecular Weight Polypropylene. Other variants include Medium Density Polyethylene (MDPE), Ultra-low-molecular-weight polyethylene (ULMWPE or PE-WAX), High-molecular-weight polyethylene (HMWPE), High-density cross-linked polyethylene (HDXLPE), Cross-linked polyethylene (PEX or XLPE), Very-low-density polyethylene (VLDPE), and Chlorinated polyethylene (CPE).
Low Density Polyethylene (LDPE) is a very flexible material with very unique flow properties that makes it particularly suitable to plastic film applications like shopping bags. LDPE has high ductility but low tensile strength which is evident in the real world by its propensity to stretch when strained.
Linear Low Density Polyethylene (LLDPE) is very similar to LDPE with the added advantage that the properties of LLDPE can be altered by adjusting the formula constituents and that the overall production process for LLDPE is typically less energy intensive than LDPE.
High Density Polyethylene (HDPE) is a strong, high density, moderately stiff plastic with a highly crystalline structure. It is frequently used as a plastic for milk cartons, laundry detergent, garbage bins, and cutting boards.
Ultrahigh Molecular Weight Polyethylene (UHMW) is an extremely dense version of polyethylene with molecular weights typically an order of magnitude greater than HDPE. It can be spun into threads with tensile strengths many times greater than steel and is frequently incorporated into high performance equipment like bulletproof vests.
What are the Characteristics of Polyethylene?
Now that we know what it is used for, let’s examine some of the key properties of Polyethylene. PE is classified as a “thermoplastic” (as opposed to “thermoset”), and the name has to do with the way the plastic responds to heat. Thermoplastic materials become liquid at their melting point (110-130 degrees Celsius in the case of LDPE and HDPE respectively). A major useful attribute about thermoplastics is that they can be heated to their melting point, cooled, and reheated again without significant degradation. Instead of burning, thermoplastics like Polyethylene liquefy, which allows them to be easily [injection molded] and then subsequently recycled. By contrast, thermoset plastics can only be heated once (typically during the injection molding process). The first heating causes thermoset materials to set (similar to a 2-part epoxy) resulting in a chemical change that cannot be reversed. If you tried to heat a thermoset plastic to a high temperature a second time it would simply burn. This characteristic makes thermoset materials poor candidates for recycling.
Different types of Polyethylene exhibit wide variability in their crystalline structures. The less crystalline (the more amorphous) a plastic is, the more it demonstrates a tendency to gradually soften (i.e. they have a wider range between their glass transition temperature and their melting point). Crystalline plastics, by contrast, exhibit a rather sharp transition from solid to liquid.
Polyethylene is a homopolymer in that it is composed of a single monomer constituent (in this case ethylene: CH2=CH2).
Why is Polyethylene used so often?
Polyethylene is an incredibly useful commodity plastic. Because of the diversity of PE variants it is incorporated into a wide range of applications. Unless it is required for a specific application, we don’t typically use Polyethylene as part of the design process at Creative Mechanisms. For some projects, a part that will eventually be mass produced in PE can be prototyped with other more prototype-friendly materials like ABS.
PE is not available as a 3D printable material. It can be CNC machined or vacuum formed.
How is PE made?
Polyethylene, like other plastics, starts with the distillation of hydrocarbon fuels (ethane in this case) into lighter groups called “fractions” some of which are combined with other catalysts to produce plastics (typically via polymerization or polycondensation). You can read about the process in more depth here.
PE for Prototype Development on CNC Machines and 3D Printers:
- PE is available in sheet stock, rods, and even specialty shapes in a multitude of variants (LDPE, HDPE etc.)
- Making it a good candidate for subtractive machining processes on a mill or lathe.
- Colors are usually limited to white and black
- PE is not currently available for FDM or any other 3D printing process (at least not from the two major suppliers: Strategy’s and 3D Systems).
- PE is similar to PP in that it can be difficult to prototype with.
- You are pretty much stuck with CNC machining or vacuum forming if you need to use it in your prototype development process.
Is PE toxic?
In solid form, no. In fact, Polyethylene is often used in food handling. It could be toxic if inhaled and/or absorbed into the skin or eyes as a vapor or liquid (i.e. during manufacturing processes). Be careful and closely follow handling instructions for molten polymer in particular.
What are the Disadvantages of Polyethylene?
Polyethylene is generally more expensive than polypropylene (which can be used in similar part applications). PE is the second best choice for living hinges, behind PP at number one.
Common Uses of Polyethylene
Listed below are the top 5 most common uses for Polyethylene. These are the products containing Polyethylene which you are most likely to find already in your home and at the local supermarket.
At some point in our lives, everyone has used sandwich bags for one reason or another. What you may not know is that Polyethylene is used in the manufacturing process of these bags.
The polyethylene is used to make the plastic film that will eventually become the sandwich bags.
It is also used in the sandwich bags cousin, the freezer bag. More of a heavy duty polyethylene plastic is used in the freezer bags, as they need to be able to with stand extreme cold and still be able to protect the items within.
Almost every cling wrap you purchase at the supermarket is made from Polyethylene. The Polyethylene combined with other materials helps to make the cling wrap work and keep its hold.
While the cling wrap itself is actually very thin, low density Polyethylene is not used in its production.
Instead, during the manufacturing process the Polyethylene is stretched multiple times to give the cling wrap its normal, thin appearance.
The moisture barriers they use on construction sites and those that are used in crawl spaces under houses are made from Polyethylene.
Low density Polyethylene is not used in the manufacturing of these items, due to the fact that they need the highest quality of Polyethylene available to make sure that the areas are secured from any moisture getting inside.
You can also see non-moisture barriers made from Polyethylene on construction sites, and those are used to keep people out of certain areas or to help protect areas from damaging winds, dirt, or any other material that the workers want to keep off of the site.
No doubt you have been in the supermarket and found yourself searching for the best pack of ground beef or chicken.
You may notice that there is plastic wrapped around the meat to help keep it fresh before it can be sold and used.
That plastic wrapping is Polyethylene in a low density form. It is used to help make sure that the meat you are purchasing stays fresh until you can get it home and either freeze it or cook it.
The plastic wrapper is tightly fitted onto the packaging so that no other food particles or bacteria can contaminate the meats you are buying.
It can also be used in bakery wrapping to keep bread and other perishable sweet treats from spoiling before they can be sold and used.
You may notice that the bakery bags are a little different from the plastic wraps over your chicken, and that is due to the quality of Polyethylene that is used to make them. However, they both perform the same duty and keep your food fresh and contaminate free.
Polyethylene is also used to make the coatings that you see on fruit juice boxes. Though the container is mostly made of out cardboard type material with a small amount of plastic mixed in, the outer coating that keeps the box from falling apart once the liquid has been introduced is the Polyethylene.
It may look like a shiny coating for the box, but it is actually keeping that box tightly together so that kids can enjoy their juice boxes.
It is also used in the wrapping for the straw, and if you buy the juice boxes wrapped instead of in a large box, Polyethylene is also used to make the wrapping for the box.
Most of the time you will see the juice boxes being wrapped when you purchase them from a larger grocery chain.
Almost anything that is either wrapped in plastic or coated in plastic is made from polyethylene.
Polyethylene is made into millions of products every year, most of which we as consumers use on an everyday basis.
And if you are like most people, you did not realize the number of items that can be made from Polyethylene, but it is actually the number one used plastic in the world.
You would be surprised if you looked around at the world today and actually noticed everything that is made from Polyethylene.
Companies are even using it to make cable insulators to keep moisture and animals off of the actual cable wires.
Right now, low density Polyethylene is gaining a lot of ground and is being used in more and more places around the world.
Though it is mainly used in the manufacturing of coatings and cable insulators, it is also being considered for another large industry.
Manufacturers of plastic toys have been looking into using low density Polyethylene to create their toys.
Polyethylene is also being considered in the manufacturing of more household goods, though some household goods are already using the plastic.
What is Polyvinyl Chloride (PVC) and what is it used for?
Polyvinyl Chloride (PVC) is one of the most commonly used thermoplastic polymers in the world (next to only a few more widely used plastics like PET and PP). It is a naturally white and very brittle (prior to the additions of plasticizers) plastic. PVC has been around longer than most plastics having been first synthesized in 1872 and commercially produced by B.F. Goodrich Company in the 1920s. By comparison, many other common plastics were first synthesized and became commercially viable only in the 1940s and 1950s. It is used most commonly in the construction industry but is also used for signs, healthcare applications, and as a fiber for clothing.
PVC is produced in two general forms, first as a rigid or unplasticized polymer (RPVC or uPVC), and second as a flexible plastic. Flexible, plasticized or regular PVC is softer and more amenable to bending than uPVC due to the addition of plasticizers like phthalates (e.g. diisononyl phthalate or DINP). Flexible PVC is commonly used in construction as insulation on electrical wires or in flooring for homes, hospitals, schools, and other areas where a sterile environment is a priority, and in some cases as a replacement for rubber. Rigid PVC is also used in construction as pipe for plumbing and for siding which is commonly referred to by the term “vinyl” in the United States. PVC pipe is often referred to by its “schedule” (e.g. Schedule 40 or Schedule 80). Major differences between the schedules include things like wall thickness, pressure rating, and color.
Some of PVC plastic’s most important characteristics include its relatively low price, its resistance to environmental degradation (as well as to chemicals and alkalies), high hardness, and outstanding tensile strength for a plastic in the case of rigid PVC. It is widely available, commonly used and easily recyclable (categorized by resin identification code “3”).
What are the Characteristics of Polyvinyl Chloride (PVC)?
Some of the most significant properties of Polyvinyl Chloride (PVC) are:
Density: PVC is very dense compared to most plastics (specific gravity around 1.4)
Economics: PVC is readily available and cheap.
Hardness: Rigid PVC is very hard.
Strength: Rigid PVC has extremely good tensile strength.
Polyvinyl Chloride is a “thermoplastic” (as opposed to “thermoset”) material which has to do with the way the plastic responds to heat. Thermoplastic materials become liquid at their melting point (a range for PVC between the very low 100 degrees Celsius and higher values like 260 degrees Celsius depending on the additives). A major useful attribute about thermoplastics is that they can be heated to their melting point, cooled, and reheated again without significant degradation. Instead of burning, thermoplastics like polypropylene liquefy, which allows them to be easily injection molded and then subsequently recycled. By contrast, thermoset plastics can only be heated once (typically during the injection molding process). The first heating causes thermoset materials to set (similar to a 2-part epoxy) resulting in a chemical change that cannot be reversed. If you tried to heat a thermoset plastic to a high temperature a second time it would simply burn. This characteristic makes thermoset materials poor candidates for recycling.
Why is Polyvinyl Chloride (PVC) used so often?
Rigid PVC in particular has very high density for a plastic making it extremely hard and generally very strong. It is also readily available and very economical which combined with the long-lasting characteristics of most plastics make it an easy choice for many industrial applications like construction.
What Are The Different Types of PVC?
Polyvinyl Chloride is widely available in two broad categories: rigid and flexible.
How is PVC made?
Polyvinyl Chloride is made from one of three emulsion processes:
- Suspension polymerization
- Emulsion polymerization
Polyvinyl Chloride for Prototype Development on CNC Machines, 3D Printers, & Injection Molding Machines: There are two main issues working with PVC that make it fairly problematic and not generally recommended for use by non-professionals. The first is the emission of toxic and corrosive gases when melting the material. This happens to some extent or another while 3D Printing, CNC machining, and injection molding. We recommend you take a look at the MSDS data sheets for different chlorinated hydrocarbon gases like chlorobenzene and discuss the production process with a professional manufacturer. Second is the corrosive nature of PVC. This is problematic when PVC is repeatedly coming into contact with metal nozzles, cutters, and/or mold tools that are made from a material other than stainless steel or some other similarly corrosion resistant metal.
Polyvinyl Chloride is available in filament form as a plastic welding rod (the material used for welding) but it is not presently retrofit for specific use in 3D printing. Although there are a growing number of plastics and plastic substitutes available for 3D printing, by far the two most common are still ABS and PLA. At Creative Mechanisms we typically 3D print with ABS. For a list of reasons why and a comparison of