Basic Chemical and Environmental Properties and Consideration in the Selection of Plastic Materials for Various Applications
The purpose of this article is to provide you with a basic understanding of plastics and rubber and to introduce a fundamental concepts related to this exciting topic. This article is conversational in the way the information is presented.
By John Gers
Gers Plastics and Rubber Machinery CC
Contents:
-Basic understanding of polymetric materials.
-What are the benefits of selecting the correct material for an application?
-What is the difference between plastic and rubber?
-How is plastic processed?
-How is rubber processed?
-How does rubber and plastic age?
-What additional characteristics can rubber present for engineers?
-How does one differentiate between potential environmental applications of different polymers?
-Consideration of the mechanical and thermal environment.
-How is a heat resistant rubber attained?
-What about plastics?
-Use of impact modifiers in plastics.
-Polymerization techniques to change properties.
-Effects of solvents on polymers.
-Factors affecting the selection and stress cracking in polyethylenes.
-What effect does the average molecular size have on a material?
-What is the effect of molecular size distribution?
-Electrical properties of polymers.
-Density.
-Indicative pricing of plastics.
-Environmental issues relating to polymers.
-Selection of a material for engineering applications.
-Resources required.
-Design safety discipline.
Basic understanding of polymeric materials
Polymeric materials consist of very large molecules, affording unique properties and allowing for the use in a variety of specialist applications. Plastics and rubber are within the field polymetric materials.
What are the benefits of selecting the correct material for an application?
In my experience, I have frequently reduced the cost of an item by a factor of as much as 10, and improved product quality at the same time. This was done by selecting the correct plastic/rubber material to replace a metal article.
What is the difference between plastic and rubber?
Plastic materials, when deformed, take on a new shape rather than return to its original form. Ideally, rubber is a material that tends to return to its original shape after it has been deformed. This is possible due to the large molecules having a specific shape, like that of a coil spring. You can stretch them but they will snap back to their original position.
How is plastic processed?
Plastics are heated to get a melt state and then they are formed into a desired shape. It is then cooled and retains the new shape. The process can be repeated over and over again. Of course one can expect that chemically active elements like oxygen may react with the hot melt, and so with each cycle some oxygen uptake may occur. Generally oxygen uptake results in scission of the giant molecules and this would then reduce the mechanical properties of the resultant product.
How is rubber processed?
Firstly rubber, in its virgin form, has to be heated to reduce it to a plastic state. The rubber molecules can now take on a new shape and retain that new shape. The new shape is then subjected to cross-linking of the molecules, commonly known as 'curing'. Once cross-linked, it cannot return to a plastic state again. This means that the recycling of rubber products is relatively limited.
How does rubber and plastic age?
Some rubbers have double bonds at regular intervals in the molecules. These double bonds are subject to attack by chemically active elements like ozone. There is always a small amount of ozone present in the atmosphere, resulting in the scission of bonds and a dramatic degradation in mechanical properties. For example rubber elastic bands that are old tend to crack and snap; they also feel tacky to the touch and may appear to have 'melted' together. In the case of rubber elastic bands, the problem can be solved by simply storing them in an airtight container. Similar storage restrictions are recommended should the rubber product be required to have a relatively long shelf life.
Modern chemists are well aware of this problem and have two solutions:
-Add a chemical called an anti-ozonant and a second additive to make an outer protective layer. The anti-ozonant will react preferentially with the ozone, and once depleted, the ozone will start attacking the rubber. In addition a wax can be added to the rubber. This forms a wax skin on the outside of the rubber and makes it difficult for the ozone to get at the rubber molecules. This does not work so well with rubber that is in dynamic operation.
-Use a rubber that does not have double bonds. These were first developed synthetically in the 1950's and do not have same elastic snap-back qualities as does, for example, natural rubber . So they are not so useful in dynamic applications such as car tyres but can be used with great success for car window seals for example.
What additional characteristics can rubber present for engineers?
Developments over the last 40 years have resulted in a series of thermoplastic rubbers. Thermoplastic rubber can be heated and put into a melt state, like plastics, and can then be shaped. Upon cooling it behaves like rubber typically does, having heat fugitive cross-links. A large proportion of thermoplastic rubbers are available with greatly varying degrees of elastic behaviour. These are called TPE's (Thermoplastic Elastomers) or TPR's (Thermoplastic Rubbers).
How does one differentiate between potential environmental applications of different polymers?
Consider first chemical environments. Polymers are traditionally built up of carbon and hydrogen; like paraffin or oil. So oily substances (non-polar substances) will cause them to swell and even possibly dissolve. However watery substances (polar substances) will have no effect on them unless they attack the chemical chain, like powerful oxidizing substances.
Examples of such rubbers are:
-NR (Natural rubber)
-PI (Poly-isoprene)
-SBR (Styrene-butadiene rubber)
-BR (Butadiene rubber)
To give rubbers resistance to organic solvents and oils chemists have introduced "polar groups" into the chain.
Examples are:
-Polychloproprene rubber or neoprene rubber (CR). A chlorine atom –Cl is inserted into each repeat unit of the chain.
-Nitrile butadiene rubber (NBR). A highly polar nitrile –CN group is introduced into each repeat unit. This is a preferred rubber for oil seals.
The two listed above are the most common oil resistant rubbers, but there is a large variety of others with polar groups which make them resistant to oils.
Consideration of the mechanical and thermal environment.
All polymeric materials are subject to 'creep'. Creep is the term used to describe the slow and permanent change in shape when a material is stressed. Be it by compression, extension or change in temperature. The higher the temperature the more rapid the material will creep. Chemically cross-linked rubbers are less subject to creep at raised temperatures whereas TRP's will creep considerably when heated.
How is a heat resistant rubber attained?
Should the chemical bonds within the molecule be weakened, either the heat will cause the bonds in the rubber chain to break or oxygen/other oxidizing agents will attack the molecule, breaking the long molecules structure.
This can be averted by using chains that do not react with oxygen at raised temperatures, such as found in:
-Silicone rubbers.
-Fluoro rubbers.
What about plastics?
Exactly the same considerations that apply to rubbers, apply to plastics, as they are also subject to creep.
There are varying solutions to preventing/eliminating creep:
-Adding a chemical called a 'coupling agent' considerably reduces creep. Particulate re-enforcement is done by adding carbon black, talc or lime. The coupling agent forms a chemical bond between the polymer molecules and the re-enforcing substance. Other properties are also enhanced, such as tensile strength and impact strength.
-Fibrous re-enforcement by use of fibres makes a large difference to creep and tensile strength. Further improvements are made by adding coupling agents to the fibre-plastic matrix. Typically glass fibres or carbon fibres are added.
-Molecular tailoring is done by increasing the average molecular mass (longer molecules) thus reducing creep and increasing tensile strength. Large molecular mass distributions are generally easier to process than narrow molecular distributions with the same average molecular mass. The small molecules in a large distribution act as a lubricant, allowing the large molecules to flow over each other. A narrow distribution would have beneficial mechanical properties but is difficult to process. This is where molecular tailoring comes in. The chemists introduce side groups to the main chains in a narrow distribution and this makes the material easier to process.
Use of impact modifiers in plastics.
All plastics tend to fail on impact. Some are very tough such as polycarbonate, and others are very brittle such a polystyrene. To improve the impact strength of plastics, impact modifiers are used.
These can be:
-Oils, such as Plasticizer (oil) in PVC to make Flexible PVC.
-Water.
-Nylon 6 and 66 has to be dry when processed. After processing it is quite brittle. This is overcome by boiling it in water for a pre-determined period. The water acts as a lubricant between the chains helping them to change conformation during impact and thus show elastic behaviour.
-Polymeric plasticizers. A polymeric plasticizer has to be non-compatible with the plastic body but yet it must also, on the edges connect with the polymer matrix.
In this way the following materials are made:
-ABS. (Acrylonitrile Butadiene Styrene)
-High Impact PVC.
-High Impact Acrylic.
-Super-tough Nylon.
Polymerization techniques to change properties. Polymers are made by joining together monomer molecules repeatedly. There are various ways in which monomer molecules can join.
In the case of a single monomer we get a homopolymer, but even then there are options; e.g. If a monomer molecule has a head and a tail section then you can get:
-Head-tail..Head-tail..Head-tail addition. (called isotactic)
-Head-tail..tail-Head..tail..Head..Head-tail in random sequence. (called atactic)
-Head-tail..tail-Head..Head-tail..tail-Head etc in alternating sequence. (called syndiotactic)
Each of these structures has different properties and therefore different applications. An example is isotactic polypropylene, which is a useful thermoplastic material. This is used for pipes in the chemical on water reticulation industry. Also jug kettles and transparent packaging of foods and clothing. It is used extensively in the automotive industry for air ducts, door panels, and bumpers.
Isotactic polypropylene is partially crystalline and this gives it its excellent mechanical properties. It goes from a glass to a tough material at about zero degrees Celsius but only melts when the crystals soften at around 170 C. On the other hand atactic polypropylene which is more like a tough wax, making it rather limited in application.
Common polystyrene is atactic. It goes from a glassy plastic to a tough melt at just below 100 C. Above this temperature it is not useful. On the other hand it is possible to make isotactic polystyrene which will crystallize and have a melting temperature of 230 C. Of interest is syndiotactic polystyrene, which is also crystalline and has a melting point of 270 C. It is also fully transparent, which is an extremely desirable trait. The reason for its transparency is that the crystalline regions have the same density, 1.05mg/mm³, as the amorphous regions.
In the case of two different monomer molecules joining to make a polymer, there are again several possibilities.
-The monomer molecules A and B can join in an alternating way. Eg ABABAB
-The monomer molecules can join in a random way. Eg ABBABAAABABAAB.
An example of this is SBR (styrene butadiene rubber). This is used as a major ingredient in the manufacture of tyres. Another example is EVA (---ethylene vinyl acetate). This is a rubbery material with a hardness being defined by the relative percentages vinyl acetate to ethylene used.
A material of considerable interest to engineers is the ethylene propylene random copolymers that are rubbers which can be cross-linked. These rubbers have no residual double bonds and are therefore not subject to ozone attack. The monomers can join in blocks to give block copolymers. E.g. AAAAAAABBBBBBBAAAAAAAAAA.
There are many examples of these types of polymers and some useful criteria for engineers are the selection of impact grades of PP made in this way. When A = P (propylene) and B = E (ethylene) then a high impact grade of PP will result.
Another example is styrene butadiene (or isoprene) block co-polymer. The styrene blocks have a higher softening point that the rubbery butadiene (or isoprene) of -40 C and so the styrene blocks acts like heat fugitive cross-links to a rubbery material. The possibility also exists for the formation for graft copolymers. Here a chain of –AAAAAA- may graft onto a chain of –BBBBBBB-. An example of this is ABS where Butadiene rubber is dissolved in styrene and the styrene is co-polymerized with acrylonitrile. The resulting co-polymer of styrene and acrylonitrile grafts onto the chains of butadiene rubber.
A novel application of polymerization is to make ball polymers. In this process each polymer molecule grows spherically from a point. These ball polymers like ball bearings may possibly become the lubricants of the future.
Effects of solvents on polymers.
Generally polymers do not dissolve easily in solvents. They tend to swell in suitable solvents. Solvents and chemicals can have a very negative influence on plastics and can cause stress cracking.
For example PE (polyethylene) will stress crack in a watery detergent solution. The resistance to stress cracking in a standard test can vary from a few hours to over 1500 hours in rigorous tests, and the determining factor is the polymer construction and the length of the chains.
When a polymer is crystalline there is no solvent for it under the crystal melting point (above the crystal melting point there are no crystals) unless there is a specific interaction between the solvent and the polymer that is larger than the attraction of the polymer molecules for each other in their crystal arrangement.
Factors affecting the selection and stress cracking in polyethylenes.
Not all the polymer molecules in a sample have the same length. Rather, there is a huge variation in chain length of the different molecules making up a given sample. So there may be very long molecules and there may be very short molecules, and many in-between. The molecular size distribution for a given material will be the result of the process used to make it. Some processes make wide molecular size distributions and others make very narrow molecular size distributions. In addition the average size of the molecules will also be determined by the process conditions.
What effect does the average molecular size have on a material?
The larger the average molecular size, the higher the tensile strength, and the better the stress crack resistance. A larger molecular mass distribution makes for easier processing. The explanation is that the small molecules act as lubricants between the large molecules.
What is the effect of molecular size distribution?
The narrower the molecular mass distribution the stiffer the material and the higher the tensile strength. Conversely the material becomes more difficult to process. Narrow molecular mass distributions also make for excellent stress crack resistance.
The above factors are important from and engineering point of view. For example the materials for the coating of steel pipe cools around the pipe and shrinks. This causes a lot of stress in the material and if partial solvents or aggressive chemicals like soaps come into contact with the plastic is can stress crack. Developments in polymerization processes has resulted in PE grades that have high molecular mass and narrower distribution giving excellent stress crack resistance.
These materials also have lower creep and higher tensile yield stress. As a result of this it has become possible to decrease the wall thickness of PE pressure pipes.
An interesting example is found in polycarbonate which has excellent tensile strength and amazing impact strength and yet cannot be used in applications where the strain exceeds 0.7%, or severe stress cracking will occur.
Electrical properties of polymers.
Most polymers are excellent insulators. The extent to which they act as insulators arises out of their molecular constitution. For example polymers of a low polarity (those with C-H bonds) like PS, PE and PP are excellent insulators, while those with asymmetrical polar bonds (like amide –CONH- groups) tend to absorb moisture and have lesser insulating characteristics. Nylon 66 can absorb as much as 6% moisture, and high moisture content is essential for the nylon 66 to have optimal impact characteristics.
In PVDF the dielectric properties are frequency dependant, and this limits its use as an electrical insulator. PVDF has however piezoelectric properties 3 to 5 times greater than crystalline quartz. Thus films are used to generate ultrasound waves up to microwave frequencies. PVDF also has pyroelectric properties and is used in pyroelectric detectors.
Thermoset polyesters have in the past 20 years replaced phenolics in low voltage switchgear. The polyesters in which styrene is a large ingredient, show superior tracking resistance and as a result the components can be made much smaller.
Conductive plastics are a reality and the author has made PE compounds with conductive carbon blacks with volume resistivity in the range of 40 Ohm cm. Such conductive plastics are important in areas where static removal is imperative. Another way to make plastics conductive is to add metal or carbon fibres. Highly conductive polymers have been made but their ability to be processed remains a problem.
The dielectric properties of plastics dependant on:
-Temperature.
-Frequency.
-Presence of plasticizers.
Density.
The density of most plastics range from 0.9mg/mm³ to 1,4mg/mm³.
The addition of inorganic fillers allow for density of plastics to range up to 4mg/mm³. For example Barytes filled FPVC have been made with a density of 2.2mg/mm³. Thus plastics are much lighter than metals and offer the possibility of fuel savings such as is seen in the automotive and aerospace industries.
Many articles have a certain spatial function. As plastics weigh about 1/8th of brass and steel, the weight of the final product will be reduced 8 times. As most plastics products are made in one operation, quite unlike their metal counterparts, additional production cost savings occur.
Indicative pricing of plastics:
-PE's and PP's cost from R8 per kg to R14 per kg
-PS, HIPS and ABS cost in the range
-PA6 and PA66 cost in the range
-PMMA
-PBT
-PPO's
-PC
-PA11 and PA12
-PEI
Environmental issues relating to polymers.
The use of synthetic polymers in a variety of industrial items results in many environmental issues and concerns. Also bringing a threat to the health of all living creatures on earth. Petroleum, a non-renewable resource, is the main source of raw material in making synthetic polymers. Most polymers are non-biodegradable. When plastics such as polyvinyl chloride (PVC) are burned, toxic fumes such as dioxins and furans are emitted. Dioxins and furans are also produced naturally when there are forest fires or volcanic eruptions. However, most of the fumes that are produced in the atmosphere are caused by human activity, including disposal of plastics/polymers.
Illnesses that are linked to toxic chemicals emitted by polymers are:
-Birth deformities.
-Reproductive disorders.
-Liver disorders.
-Skin disorders. (eg chloracne)
-Damage to immune system.
-Cancer.
-Respiratory diseases.
The two main areas of focus are:
1. Physical effects.
Spilled plastics from factories and post consumer waste find their way to the oceans of the world. Harmful to all sea life if consumed/swallowed because the polymer cannot be digested, thus causing afflicted creatures to literally starve to death even though their stomachs are full.
Solutions include pellet traps at factories, massive environmental education programs and strict hygiene practices enforced by the plastics and rubber industries. The global concern has supported a world-wide movement towards researching better recycling methods and minimizing waste. As a vital part of any business, every product manufactured absolutely must have a thorough ecological study done to predict and minimize its impact on the fauna and flora.
2. Chemical effects.
Certain plastics and additives are said to have a phyto-hormonal effect on humans, animals and plant-life. The first material steps have been taken by the banning of a specific plasticizer from children's toys as this causes early maturity in girls and developmental retardation in infants.
Alligators in Florida were found to have males with reduced genitalia and vice versa with the females. This was tracked back to a spill of DDT in that area. Scientific studies suggest that certain plastics may have similar effects on humans.
Much research and chemical testing must be done prior to the production of polymetric materials/products to avoid harming humans, fauna and flora.
For more information you can view the following website: http://www.sciencedirect.com/science/article/pii/S0003347204001782
Selection of a material for engineering applications.
The first step is to define the key properties that the product must have.
Factors to consider are:
-Lifespan.
-Chemical properties.
-Mechanical properties (tensile, flexural, impact, speed, fequency)
-Thermal properties.
-Electrical requirements.
-Cost.
-Appearance. (aesthetics)
-Safety function.
-Recyclability
Resources required:
-Material knowledge
-Material data sheets finite element analysis
Design safety discipline.
This consists of a re-iterative failure mode analysis. The failure mode analysis is carried out until there are no failures.
Adapted from a lecture at The Institute of Mechanical Engineers, presented by John Gers
Edited by: Beáta Gers
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