Understanding Thermoplastics And Thermosets - A Look At Polyethylene

A group of designer materials that have altered our lives is polymers, particularly in the form of plastics. The properties of polymers can be modified. They can even make them to order by chemical structure manipulation. In selecting a polymer for a particular purpose, consideration must be given to the polymer's properties. 

Your company, Futuristic Materials, is involved in polymer research and production. Other companies that produce or use polymer based products routinely seek your expert advice when they are considering making some changes to the materials that they are currently using. 

1.PETE (polyethylene terephthalate) for its bottles. Which material would be better for this use?

 
2. You have been hired by a company that wants to start producing "soft" contact lenses. For many years this company has been manufacturing "hard" contact lenses, which arc made from poly(methyl methacrylate, PMMA). Would it be wise to produce "soft" contact lenses out of PMMA? Is thcrc another alternative? 

3. You have been hired by a company interested in finding an alternative to the material currently used to make 'dissolving sutures'. Dissolving sutures can be made of poly(glycolic acid). What properties make it a good choice? What alternatives might be useful? What direction should the company move in?

4. You have been hired by a clothing manufacturer. For many years this firm has been producing thermal underwear from cotton and/or wool. The market for this product is declining and the firm is considering producing a line of polypropylene thermals. Why is polypropylene more common now? Are there any synthetic alternatives to polypropylene?

5. You have been hired by a company that makes bicycle inner tubes of butyl rubber (which is a copolymer of isobutylene with a little isoprene). Should the company switch to an alternative material for these tubes? Why? 

6. You have been hired by a company that would like to branch into making climbing ropes. What options do they have for materials? What direction should the company move in?

7. You have been hired by a company that has been making plastic grocery bags. They would like to move towards a more environmentally friendly plastic bag. They are considering changing from polyethylene bags to a biodegradable polyethylene or a biopolymer. What direction should the company move in? 

The Benefits of Plastic Manufacturing with Polyethylene

Plastic is one of the most plentiful substances in the sphere.  It is a carbon-based amorphous compact and is favoured for its low-priced invention costs, thermal and mechanical capabilities, durability and stability. Plastics can be dealt with into two various classes in view of the thermal features: thermoset and thermoplastics plastics. Thermoplastics undergo reversible processes when heated and their linear sequence bits are linked end to end, building a big chain of particles.  Thermosets plastics are created by phase-growth polymerization under suitable situations, permitting the condensation of bi-functional fragments. Diverse chemical methods offer the material to have dissimilar chemical, mechanical or thermal elements. Polyethylene terephthalate is used to created peanut butter containers, water bottles, and food trays. High-density polyethylene (HDPE) is normally used for an inflexible jugs for products such as milk, juice, soaps and shampoo.  HDPE can be reprocessed into the drainage piping and cleanser bottles. The low-density polyethylene (LDPE) can be used to make plastic shopping bags (Bala, Kamaruddin, Napiah and Danlami 2018).

Polyethylene is the primary components of the contemporary plastic container.  Various variations of this solid are present, rising from the range of defects and branching. Polyethylene chains will arrange in a differing manner, influencing the end produce. Functional sets linked at the chains end will lessen the crystallinity and deter the chains from stuffing more closely. HDPE has rare flaws, permitting the chains to arrange closely. There is slight to no branch off, therefore the outcome is tougher plastic. LDPE has more blemishes than HDPE, which inhibits it from filling well.  Normally, the manacles of polyethylene have divisions that avoid it from crystallizing, these cause in a lesser strength and density.  But, it is more elastic and easier to process than HDPE. LDPE is branched but branches are frequent and smaller which increase the plastic density (Androsch, Di Lorenzo, Schick and Wunderlich 2010).

LDPE is well-defined by a compactness sort of 0.910-0.940g/cm3 and has a high point of short and long chain splitting, which shows that the chains do not pack into the crystal assembly as well. Thus, it has fewer robust intermolecular forces as the immediate dipole induced dipole pull is low which cause it to have increased ductility and lower tensile strength.  The high notch of subdivisions with the extended chain offers a liquefied LDPE desirable and unique flow aspects (Malpass 2010).

HDPE is denoted by a mass equal or more to 0.941g/cm3. It has a small mark of dividing and therefore, sturdier intermolecular powers and tensile forte. HDPE can be created by the silica/chromium, metallocene, or Ziegler-Natta catalysts. The deficiency of bifurcating is safeguarded by a suitable selection of catalyst (Malpass 2010).

Medium density polyethylene (MDPE) is categorised by a density array of between 0.915 to 0.950 g/cm3 which is a markedly linear polymer, with considerable numbers of short divisions, typically prepared from copolymerization of ethylene with tiny- chain alpha-olefins (Malpass 2010).

LDPE properties are very tough, semi-rigid, astonishing chemical resistance, weatherproof, little water absorption, translucent, simply managed by most techniques and low rate.  HDPE features include the weatherproof, flexible, translucent, good low-temperature toughness, low cost, good chemical resistance and easy to process by most methods (Malpass 2010).

Types of Polyethylene and Their Properties

Fuel and natural gas are the raw substances used in the manufacture of plastic. From the raw elements, two kinds of plastic are created.  Plastics have features that create them superlative for forming a range of merchandises particularly gears and other packing stuffs.  For instance, such features comprise the lightweight, thermal and electric lining, resilience to chemicals, and adaptability in what way they can be handled. This creates production easier and then suitable for users. There are diversity ways in which plastics can be produced into end products. Some illustrations are injection, extrusion and blow casting (Kasirajan and Ngouajio 2012). 

The possibility of substitute plastic gears is on the marketplace due to the conviction that current plastic hand baggage is harmful to the surrounding. Numerous replacements concentrate on reducing the period it takes for the bags to reduce by either putting in an additive or altering the materials to increase the degradation speed.  Biodegradable polymers: these are numbers of alternative by now are in the marketplace that users can use as an alternative to polyethylene plastics gears. Biodegradation is a regular procedure of degrading multifaceted organic mixtures by a microorganism such as microbes, into smaller and simpler carbon-based elements. Recyclable plastic gears can reduce in less than two years and therefore, there are good substitutes to polyethylene elastic bags in all grocery outlets, which can last in landfills for centuries.

Polymers with hydrolyzed pillars are vulnerable to biodegradation. The solitary high-weight molecular polyesters that are recyclable are the aliphatic polyesters. It has been established that polyesters with average sized monomers from carbon 6 to carbon 12 in dimension, can be quickly degraded by microorganism (Sarifuddin, Ismail and Ahmad 2013). Artificial polymers of this extent with elastic polymer manacles can be degraded as the polymers can apt into the enzyme initiation spots of these fungi. Polyglycolic acid and Polyglycolic acid-co-acetic acid are two biodegradable polyesters cases. The polymers are recognized to decompose through simple hydrolysis process of an ester support.  Due to the above, the substances are presently used as biodegradable sutures since the hydrolysis can happen over exposure to bodily liquids (Shameli et al. 2012).

Figure 1: starch polymer broken down (Shameli et al. 2012).

The concern with the starch-based yields is that at high temperatures about 150⁰C, the glucose connection starts to breaks apart.  One more concern with starch polymers is at a low temperature retrogradation processes happen. The retrogradation is where the hydrogen links reorganize and an aligning of the molecular sequence occurs due to freezing. This occurrence creates the subsequent starch-based flicks very hard. Due to this weakness, entirely starch-based films are not the best biodegradable polymers to totally substitute polyethylene (Mrosovsky, Ryan and James 2009).

The mixing of polyethylene and starch has received a lot of consideration for probable application in the unwanted disposal of polyethylene-based plastics. The concept behind the mixing is that if there is sufficient of the recyclable polymer, once it is eradicated by a microorganism in a waste dumping surrounding such as landfill, the residual polymer should lose its polymer veracity and break.  Granule starch has been in use to generate these kinds of mixed polymers.   In the surroundings having microorganism, the uncovered starch pellets on the superficial of the combined polymer substance can be enzymatically split apart.  When the starch is completely consumed by the microorganism, the samples will start to crumble.  This impact only happens for trials having 30% by volume or more of starch. However, the great quantity of starch will cause the film or plastic formed to have less ductile strength (Li, Wang, ExxonMobil Chemical Patents Inc 2018).

The Advantages of Biodegradable Polymers

Viable thermoplastic polymers (non-biodegradable) have developed importance to the packing and foods sectors. The point that they are hydrophobic and organically inactive makes them faultless for such routines. Another method to create plastic decomposable is to add an ultraviolet light absorber which cause plastics biodegradable when open to sunlight. Figure below shows the polyethylene plastics gears that are not capable to be broken down into trivial portions (Sarifuddin, Ismail and Ahmad 2013).

Figure 2: polyethylene plastics bags that are not capable to be broken down (Sarifuddin, Ismail and Ahmad 2013).

Plastics gears and paper bags are both harmful to the surrounding. The formation procedure for a paper is less ecologically pleasant than that of a plastic carrier.  Actually, it necessitates four times the quantity of energy to produce a paper carrier than it does to create a elastic bag and twice the power to recondition a pound of paper than to reprocess a pound of elastic. The manufacture of paper bags consume up more power and build more air contamination than the plastic bag manufacture.  The primary concern of food wrapping is to preserve and guard raw foods from the microorganism and oxidative substance in order to prolong their shelf-life. Petrochemical centred elastic such as polyethylene have been used as wrapping substance due to their ability to block oxidative agents and to offer a good tensile strength. The huge application of synthetic packaging films leads to severe environmental concerns due to their non-degradability nature (Sarifuddin, Ismail and Ahmad 2013). 

One instance of a decomposable film is the mixture of starch, which is hydrophilic with the hydrophobic elastic medium. The adding of natural polymers such as starch to polyethylene forms starch-LDPE films enclosing up to 30% starch (Sarifuddin, Ismail and Ahmad 2013). These sheets have been revealed to be decomposable upon composting. The starch-LDPE film fit well into the ecology due to their total biodegradability form.  A number of aerobic and anaerobic bacteria have been recognised for biodegradation and the carbon sequence comprising biopolymer dilapidation as shown below.  

Figure 3: Biopolymers carbon cycle (Sarifuddin, Ismail and Ahmad 2013).

Tensile elements are obviously the finest for the densest testified polymers such as PGA.  It has been remarked that molecular mass can take part in the obtained mechanical properties.  In the figure below, one can discover the definite tensile strength range for various biopolymers. These figures were got by separating the original mechanical property by the density.  If either mechanical or density properties were not accessible within the similar locus, no particular facts were tabulated. This limit is important since it is probable that the polymers density is connected to its mechanical elements.  If one would wish to apply the biopolymer as structural elements without using reinforcement, particular aspects become crucial as they define the scopes important for a specific mechanical stiffness or strengths.

The litheness of amorphous polymers is minimised considerably when they are chilled below a typical transition temperature referred to as glass temperature. Usually, the handling temperature is 20-100C higher than the melting points. This sort majorly depends on application of additives that can avert thermal dilapidation of the polymer at elevated temperatures. A low course temperature may be beneficial when considering the energy rate of the manufacturing procedure.  Normally PGA requires a higher than 250C that would result to thermal deprivation of the flax.

Conclusion

It is projected that 500 billion to one trillion gears are used globally annually.  One of these bags is made up of LDPE owing to their low manufacturing and outstanding mechanical properties. They have ductility and tensile strength to be the substance of choice for many packaging uses. Polyethylene's chemical inactiveness makes sure that it will not restrict with the wrapping course.  Additives can be used to improve a plastic's capability to reduce, and they can be applied in connection with the deprivation of inducers like composting, ultraviolet radiation and heat dilapidation.  Plastics substances used for package, noticeable in all superstores, conserve favour and freshness due to the capability to seal out pollutants. Polyethylene material is beneficial over an extensive temperature, from icy to the microwavable package. Thus, due to these paybacks, the plastics have developed to be extensively used.  Nations and large company are starting to recognize the peril that these plastics gears bring to the planets. Several are suggesting or already passed guidelines and laws to drop the polyethylene plastics usage. Numerous grocery outlets are even recompensing users for carrying recyclable grocery bags as a substitute for the polyethylene plastic bags. 

The aim of this paper is to quest for the most appropriate thermoplastic biopolymer. Density and temperature seem to be restraining criteria for the selection of an appropriate polymer. As mentioned, the mechanical strengths are not crucial for this specific application.  PGA is definitely eradicated as a contender as the combined matrix o melting point and density are too high in the order of saving energy.  Moreover, the higher handling temperature would cause the flax deprivation.  PGA polymer seems to easily degraded, which shows the sign of low corrosion resistance, and besides, they are amorphous which would resultant to poor adhesion with the strengthening fibre.  PLA is amorphous and glass transition temperature is too stumpy to warrant resistance to marginally raised temperatures. PLA seems to notch well on all the deliberated properties:  polymers densities, tensile elements and glass transition temperature are satisfactory and their melting point is almost perfect to create flax fibre reinforce mixtures.  Therefore, preferences should be given to PLA rather than PGA since the latter is more costly and PLA is also already commercially accessible.

References

Androsch, R., Di Lorenzo, M.L., Schick, C. and Wunderlich, B., 2010. Mesophases in polyethylene, polypropylene, and poly (1-butene). Polymer, 51(21), pp.4639-4662, Available from: https://www.sciencedirect.com/science/article/pii/S0032386110006464, [Accessed on 23 October 2018].

Bala, N., Kamaruddin, I., Napiah, M. and Danlami, N., 2018. Polyethylene Polymer Modified Bitumen: Process Optimization and Modeling of Linear Viscoelastic Rheological Properties Using Response Surface Methodology. Journal of Engineering and Applied Sciences, 13(9), pp.2818-2827, Available from: https://docsdrive.com/pdfs/medwelljournals/jeasci/2018/2818-2827.pdf, [Accessed on 23 October 2018].

Kasirajan, S. and Ngouajio, M., 2012. Polyethylene and biodegradable mulches for agricultural applications: a review. Agronomy for Sustainable Development, 32(2), pp.501-529 , Available from: https://link.springer.com/article/10.1007/s13593-011-0068-3, [Accessed on 23 October 2018].

Li, W. and Wang, X.C., ExxonMobil Chemical Patents Inc, 2018. Polyethylene Films and Processes for Making Them. U.S. Patent Application 15/775,752. , Available from: https://patents.google.com/patent/US20180265657A1/en, [Accessed on 23 October 2018].

Malpass, D. B. (2010). Introduction to industrial polyethylene: properties, catalysts, and processes (Vol. 45). John Wiley & Sons, Available from: https://books.google.com/books?hl=en&lr=&id=nhObWJ3rKrcC&oi=fnd&pg=PR9&dq=polyethylene&ots=kq7-eH8uFe&sig=hUyRfni_sr0fzoCIojvY2cjFL7g, [Accessed on 23 October 2018].

Mrosovsky, N., Ryan, G.D. and James, M.C., 2009. Leatherback turtles: the menace of plastic. Marine pollution bulletin, 58(2), pp.287-289, Available from: https://www.sciencedirect.com/science/article/pii/S0025326X08005031, [Accessed on 23 October 2018].

Sarifuddin, N., Ismail, H. and Ahmad, Z., 2013. The Effect of Kenaf Core Fibre Loading on Properties of Low Density Polyethylene/Thermoplastic Sago Starch/Kenaf Core Fiber Composites. Journal of Physical Science, 24(2), Available from:  https://web.usm.my/jps/24-2-13/24-2-7.pdf, [Accessed on 23 October 2018].

Shameli, K., Bin Ahmad, M., Jazayeri, S.D., Sedaghat, S., Shabanzadeh, P., Jahangirian, H., Mahdavi, M. and Abdollahi, Y., 2012. Synthesis and characterization of polyethylene glycol mediated silver nanoparticles by the green method. International journal of molecular sciences, 13(6), pp.6639-6650, Available from: https://www.mdpi.com/1422-0067/13/6/6639/htm, [Accessed on 23 October 2018].