Introduction Thermosets refer to the set of materials formed by polymers joined by chemical bonds acquiring a highly cross-linked final structure. The highly cross-linked structure of thermoset materials is directly responsible for the high mechanical and physical resistance (stresses or loads, temperature…) of these materials compared to thermoplastics and elastomers. On the …
Plastics are on the top of our economy & industry.
Combining excellent functional properties with low cost, we can find them in different applications, its global production volume is expected to continue growing well beyond the 2016 figure of 335 million tons.
In 2016, global production reached 335 metric tons, behind concrete or steel (Plastics Europe, 2018). Since its industrialization in 1950s, more than 83000 million tons have been produced, 70% (5800tn) have been turned into garbage and 84%, 4900 million tons have finished in landfills or in the environment (Geyer, Jambeck & Law, 2017).
As cheap and easy-to-produce materials, its indiscriminate use has been extended in all aspects of our economy. We produce plastics, we use them, and we dispose them.
The annual loss of material value is estimated at 70-105 billion euros worldwide, with a generation of 75,000 to 300,000 tons of annual microplastics ONLY in EU territory.
(1)Waste estimates for 2010 for the top 20 countries ranked by mass of mismanaged plastic waste(millions of tons per year, lower and upper bound estimates)
Not only do we live surrounded by plastic, but we are already made of them, or at least part of us, and not in the form of prostheses, but in the most literal sense.
All the alarms go off. If we add this to a total lack of information, we already have the perfect cocktail that demonizes this industry and puts it in the focus list.
So, the problem begins to be important and the debate is open: Is it a problem of use? Of environmental awareness? Economic? Of design? Can plastics really be recycled and limit their footprint? We will try to answer these questions, focusing on 3 of the most used plastic materials and in turn, complicated to recycle: PU, Epoxis and Silicones.
What is a plastic material? Are they all the same?
Plastics are polymers, derived directly from petroleum. They are organic materials (based on carbon chemistry) just like wood or cotton. In most cases they are synthetic alternatives to natural materials (such as glass, plant materials, skins, etc.) that are very simple and especially cheap to produce, with awesome properties.
Its name comes from the Greek “plastikos” which means moldable. This means that we can give them the way we want and once we have the final product, that form is maintained.
We can classify them as:
- Thermoplastics: are those that we can melt and give it the shape we need many times, without its properties being altered. For example PET (polyethylene), PVC, PS (polyester), PP (polypropylene). The simplest example would be a plastic bottle. They are the “easiest” to recycle.
- Thermosets: Once formed we can not “reuse” simply by melting and reshaping. Its physical chemical properties are much higher and therefore its recycling is much more complex, since it requires destroying its structure. We speak for example of epoxies (the resins of the boats, surfboards …), Teflon (kitchen utensils) and of course the PU (polyurethanes) and silicones that we will deal with later. Another added difficulty is that in many of its final uses, they are used in combination with thermoplastics (such as composite materials in the aviation / automobile industry), making their subsequent treatment very difficult. In its favor we can say that its average useful life (at least that of the material) is much longer than thermoplastic polymers.
Fuente : (1)The world plastics use
As a starting point we can ensure that all plastics are recyclable. This means that “theoretically” any polymeric material (plastic) can be treated for reuse. Obviously, this will have certain nuances.
Some of them require very complex treatments and especially with a high cost that until today neither the industry nor of course the end user, have wanted to assume.
Even more so when we talk about combined materials (for example, tetrabricks, electronic elements, vehicles, even food packaging) where we mix different natures without criteria
On the other hand, some materials will lose physical chemical properties in each recycling process, so their number of processes or useful life will be limited.
* Labeling according to the standard IRAM 13700
Therefore, we have materials that are potentially recyclable (or more recyclable than they are now), we know that we have to rationalize their use and optimize designs / applications and work to optimize their reuse either by optimizing methods (physical or chemical) as materials ( new materials that are “easier” to treat afterwards, such as biodegradable ones).
What is known as the four R strategy: Reduce, Reuse, Recycle and Energetically Recover.
Nowadays there are different recycling methodologies and economic and environmental costs associated
- Mechanical: Especially associated with thermosets. It is based on mechanical methods (crushing / milling/melting) and in most cases allows the circular economy that we are looking for.
The Swedish Environmental Protection Agency in a recent study showed that this type of recycling saves between 1-1.5 kg CO2 / kg of resin compared to virgin material systems, depending on (Stenmarck Å. Et al., 2018). Another study affects the energy costs per kg of 130 000 kJ (Rahimi & García, 2017).
- Chemicals: These are methods such as pyrolysis, solvent-based purification or depolymerization. They are methods that require a lot of energy and generate pollutants that need to be treated later (such as PAHS or Dioxins). Thermosets need this path in most cases. On the other hand, they allow the use of “contaminated” materials (mixtures of different natures), resulting in basic chemical raw materials, which can be reused in the petrochemical industry either as fuel (Onwudili, Insura & Williams, 2009) or in new synthesis (Aguado, Olazar, Gaisán, Prieto & Bilbao, 2003).
The resins themselves do not have any useful technical property until they are effectively hardened by double bond chemical reactions, then forming Thermosetting polymers. Epoxy resin is obtained by the reaction of Bisphenol and epichloridrin.
This resin reacts irreversibly with a catalyst / hardener, usually of Amina nature, resulting in a very hard material, with excellent chemical and mechanical properties (inert) and low weight.
Therefore, they are the most used for construction or structural materials such as aerospace, automotive or naval, reinforced by fibers (composite materials or composites), which require a long service life and maintaining their properties.
This gives us materials with excellent durability but at the cost of sustainability, since they are products that are hardly recyclable / reusable to date.
The only recycling methods that exist for this type of materials today are:
- Mechanical processing and subsequent storage in landfills
- Pyrolysis (2): The material in most cases is totally or partially destroyed (as in the case of carbon fiber composites, where there are already companies that can recover the fiber and reuse it. The resin is destroyed)
There are new research paths for the synthesis of epoxy resins that allow the subsequent total or partial recovery of both the thermosetting part and the thermoplastic part, from new catalysts (3) or catalysts that come from the recycling of other thermoplastics, such as the PU (4)
Polyurethane resin, like epoxies, is manufactured from the reaction of two components, in this case isocyanates and polyhydric alcohols (known as polyols). It is one of the most versatile polymers, since depending on its synthesis, we will obtain totally different products, with different physicochemical properties, from an elastic product such as foam from a mattress (foaming is its best known use) to one more hard for shoe soles or even paints.
It is also a thermoset polymer, so they irreversibly polymerize.
It has excellent physical / chemical properties as well as epoxies (and most thermosetting plastics) and comparatively, its chemical, temperature and UV resistance is greater than that of epoxy resins. On the other hand, its mechanical resistance (impact, scratching, etc.) and wear is lower, so they are not the most suitable in structural applications.
In fact, they are used in combination to give the mechanical resistance of the epoxy but the resistance to the external conditions of the PU.
In terms of recycling, PU gives us greater alternatives than epoxy resins.
Current recycling methods are based also on:
- Mechanical methods: PU waste is crushed and can be reused in final applications, especially thermal / acoustic insulation.
- Chemical Methods: Polyurethanes can be re-treated to transform them into Polyols for second life applications.
There are three technologies: hydrolysis, aminolysis and glycolysis (5). The latter being the most important for its efficiency and low environmental impact.
They process uncontaminated waste of known composition, mainly production waste. About 30% of the polyols used in rigid PU foam can proceed from glycolysis without affecting the quality of the product. (6)
Silicones are thermostable plastics that receive the special name of elastomers, due to their elastic properties that allow them to modify their shape and recover it again.
They are better known by the name of polysiloxanes:
They can be both monocomponent and bicomponent, requiring a catalyst to polymerize.
These catalysts can be platinum-catalyzed cure system, a condensation cure system, a peroxide cure system, or an oxime cure system.
Curing example of a silicone with a peroxide
Its properties make silicones an exceptional polymer:
- Temperature resistant from -55 to 300ºC maintaining properties
- Extremely easy to conform
- Elongation / compression superior to many other polymers
- Inert chemically, heat or ozone (which makes it ideal for extreme conditions where other polymers fail), in addition to sensitive applications such as sanitary material or in contact with food (of the few materials that the US Food and Drug Administration certified as Suitable for food contact.
- Durability: It is a polymer that does NOT break down and therefore does not generate waste that can pass to other organisms or contaminate the water cycle.
But on the other hand it provides us with a non-biodegradable material and very difficult to recycle, although with an infinitely longer useful life than other polymers.
There are currently two methods for total recycling:
- Mechanicals: The product is crushed and used as a load for other applications that require thermal / acoustic insulation or for vulcanized (to obtain rubber).
- Chemicals: There are methods for recycling, although there are few companies dedicated to it. They are based on depolymerizing silicon to obtain monomers and subsequently manufacturing other silicon derivatives, such as silicone oils, widely used as lubricants.
We know now that epoxy systems are one of the best alternatives in those applications that need high mechanical resistance, although it is one of the most complex polymers to recycle, which leads us to select their applications well beyond those necessary.
Polyurethanes, much easier to manage as waste (especially by mechanical methods), although it is true that their properties outside their resistance to UV light, are more limited than with epoxy systems.
With silicones, we have one of the materials with a longer lifespan that in addition to being inert it hinders its processing, new studies and new information are been generating now that more applications are being found for this polymer, such as putties to dissipate heat in electronic components, so we hope that the industry can lead to new recycling methods that are friendly to the environment.
- Item 25 Plast’ 21 No.102, May 2001, p.77-80 Spanish PYROLYSIS AS A METHOD FOR THE RECYCLING OF COMPOSITE MATERIALS de Marco I; Torres A; Laresgoiti M F; Caballero B M; Cabrero M A; Gonzalez A; Cambra J F; Legarreta J A; Chomon M J; Gondra K Pais Vasco,Universidad A pyrolysis technique was investigated as a method for the chemical recycling of glass fibre-reinforced unsaturated polyester SMC composites. The process yielded liquid products and gases and also a solid residue formed in the pyrolysis of glass fibres and fillers. The solid residue was used as a reinforcement/filler in unsaturated polyester BMC composites, and the influence on mechanical properties was studied in comparison with BMC prepared entirely
- 3 ACS Polymeric Materials Science and Engineering Fall Meeting.Volume 85. Chicago, IL, 26th-30th August 2001, p.506-7.012 CURING BEHAVIOR OF EPOXY RESIN WITH AMINOLYSIS PRODUCTS OF WASTE POLYURETHANES Lee D S; Hyun S W Chonbuk,National University (ACS,Div.of Polymeric Materials Science & Engng.) Rigid polyurethane foams were prepared at room temperature using commercial polyols and polymeric 4,4′- diphenyl methane diisocyanate, and used to study their recycling by aminolysis. The reaction products obtained by treatment with diethylene triamine at 180 C were evaluated as hardeners for epoxy resins. The exothermic heats of curing were determined over the temperature range 60-80 C by differential scanning calorimetry. A reaction order of 2.2-2.4 was obtained. 8 refs. KOREA Accession no.845621
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