The recycling of plastic, specifically PVC, is still in its early stages. Nevertheless, it is already being applied in different fields of industry.
Its main, most notable development has been in the production of plumbing pipes, profiles (such as windows), and signalling cones, etc. As to the use of recycled PVC in the production of footwear, it has been most notably used in the manufacture of soles, especially safety soles.
With this research, however, the Footwear Technological Centre of La Rioja (Spain), has managed to create a technical boot made with this new formulation. Specifically, it incorporates 60% of this type of recycled polymer made of residual plastics from automobiles.
The greatest merit from this project is that there is currently no similar product on the national or European market, and even less, one that rigorously complies with the demanding regulations and standards for this type of Personal Protective Equipment (PPE), such as ISO 20347 – ISO 20345.
Necessity of the Research
Plastics are not biodegradable, they do not dissolve in water, nor do they decay. This is both an advantage and a disadvantage; as they do not decompose, they do not release any harmful products, but at the end of their life they accumulate in landfills without disappearing.
Moreover, due to their lightness, they are very sensitive to wind and can therefore pollute any soil and water in the vicinity. Nowadays, plastic materials represent 7% of the total mass of waste and their disposal in landfill is not a sustainable solution, therefore, it is more appropriate to recycle them or to use their energetic value.
Among the numerous plastic waste to be recycled, the one that concerns this research in particular, PVC, has a very high importance. The relevance of its recycling is justified by the following three arguments:
1. In landfills, PVC can emit harmful substances such as polychlorinated di-phenyls that go into water and soil.
2. When incinerated, it can release highly carcinogenic furans, hydrochloric acid, and dioxins, meaning that the utilisation of the energy value of PVC is not a satisfactory solution either.
3. During its production, the additives and co-products added to PVC can spread into groundwater, soil, and air. Although these substances partially decompose in the environment, they can give rise to other eco-toxic products. The recovery and recycling of PVC products at the end of their life ensures that no more toxic substances are produced and released into the environment.
The dangerous impacts of PVC on the environment and the health of living beings force us to consider its recycling carefully and thoughtfully.
Nowadays, plastic waste from the automotive sector (mixtures of PVC and PU, among others) have a significant environmental impact, as well as giving this mixed polymer a negative value, since recycling companies must pay for sending it to landfill.
Thus, the CTCR proposed this innovative research that reuses the plastic waste from landfills, specifically from the automotive sector, to obtain the chippings needed to manufacture innovative footwear with good mechanical properties.
In addition to the novelty of using recycled PVC, another merit lies in the PVC’s origin in the automotive industry, which entails its own disadvantages in the separation of material. Given these disadvantages, until now, waste plastics from automobiles were reused with great difficulty.
Challenges and Development
The recovery and recycling rates of plastics are especially low, currently less than 10%, which is very little in relation to other materials of great consumption such as metal, glass, and paper. In this respect, the CTCR had to generate, as a challenge, a new formulation that would increase this percentage of “recyclability”, so that the margin resulting from the use of these products would be optimal. In this way, a much smaller amount of product would be used for recycling and the subsequent production of the desired final footwear.
On the other hand, it should be noted that plastic is not a single material and the numerous chemical formulas used for different products considerably complicates the separation of waste. There are a variety of heterogeneous materials that are difficult to remove, as they have different sizes, density, and even moisture content.
Mechanical recycling is a fairly simple technique when the waste is homogeneous or when its separation is possible and fast. Thus, the plastic waste is separated (by hand and/or with machines), shredded, cleaned, melted, and molded into pellets that can be used to manufacture new products. However, mechanical recycling of composite plastic waste is more difficult because the separation is very complicated. It consists of shredding the waste; its complete dissolution in a mixture of organic solvents; the recovery of the waste by precipitation, and then steam distillation to recover all the solvent. After drying, a recycled product is obtained, in powder form, of high quality, and with a composition similar to that of virgin PVC. Thus, the CTCR experts overcame the challenge of developing a recycling system that made it possible to generate virgin PVC and then formulate it according to their needs, without increasing production costs for the company, as this would make the product unviable in the future.
During this process, the researchers fought against an evident deficiency when using recycled PVC in other sectors of the industry, which involves its structural weakness and the formation of cracks. Throughout the phase of test specimens, a search for synergies was carried out by analysing different combinations of materials and looking for the complementarities between them. Thus, a mixture was found with the appropriate percentages of ‘virgin’ PVC, recycled PVC, and additives that showed good results in tensile strength and deformation at the breaking point by tension, bending, abrasion, elongation and tearing. It should be remembered that compliance with standards for this PPE was essential, for end users will have to face chemical elements, slippery surfaces, etc.
Characteristics of the New Product
Regarding the technical requirements that allowed the use of PVC residue as a component in footwear formulations, it is worth mentioning as a relevant aspect the compliance with the Reach (Registration, Evaluation, Authorisation and Restriction of Chemical Substances Regulations) and other legislation applicable to footwear such as CPSIA (Consumer Product Safety Improvement Act), Proposition 65 and CADS. The PVC residue complied with restrictions on substances such as azo-amines, phthalates, alkylphenol and alkylphenol ethoxylate, and polycyclic aromatic hydrocarbons.
The physical characteristics that also identify this residue as a raw material for injection into PVC soles for professional use are:
• Abrasion resistance of the sole according to ISO 4649, being less than 150 mm3 for materials with density higher than 0.9 gr/cm3 and less than 250 mm3 for materials with density lower than 0.9 gr/cm3. In the case of boots for protection against aqueous environments (all polymeric), the loss of volume should not exceed 250 mm3.
• Resistance to tearing of the sole according to ISO 20344, App. 8.2, should not be less than 8KN/m for materials with density higher than 0.9 gr/cm3 and 5kN for materials with density lower than 0.9 gr/cm3.
• Sole flexural strength, according to ISO 20344, Sec. 8.4, the incision increase should not be more than 4 mm, after 30,000 cycles.
• Determination of the tensile strength of the upper material, according to ISO 20344, App. 6.4.1., the modulus at 100% elongation should be 1.3 to 4.6 N/mm2 and the elongation ≥250%.
• The flexural strength of the upper, according to ISO 20344, App. 6.5, after 150,000 cycles should not crack.
Two separation techniques were used to obtain clean, recycled PVC, free of impurities. They are described below:
1. Separation by density after washing with water, obtaining the black PVC wet residue free of foamed polyurethane impurities, which had to be dried beforehand.
2. Separation by density using a controlled air cyclone, which obtained the dry residue free from impurities of grey foamed polyurethane.
In addition, the PVC residue was characterised by FTIR spectrometry and TGA, checking its percentage of plasticiser. They were tested to check compliance with Reach and the result was positive, complying with the required limits regarding the presence of Phthalates (DDINP, DEHP, DNOP, DIDP, BBO, DBP, DiBP), sum (DB+DEHP+DNO), sum (DIBP+DNOP+DINP), Alkylphenol and ethoxylated alkylphenol (NP, OP, NPEOn, OPEOn), Azo amines (22 amines), and Polycyclic aromatic hydrocarbons (eight substances). The residue was physically characterised through shoe hardness (A-73), density (1.20 gr/cm3), and tensile strength (159kg/cm2, ash 2.1%).
The laboratory validation of the developments was therefore successfully developed by performing the 60% mixture, according to the requirements necessary according to the standards for professional footwear ISO 20347 and ISO 20345.
PVC recovery and recycling
The main objective of the project was to develop a PVC pellet to be injected in the manufacture of manhole boots, from PVC waste from the automotive sector; always in compliance with the quality and resistance standards for the performance of a professional activity in aqueous environments, where, in addition, the protection in the sole and toe must be fundamental.
• Characterisation of automotive PVC, both in terms of visual and physical-chemical characteristics.
• Obtaining various formulations, with properties adapted for use in the world of footwear.
• Development and optimisation of the process conditions with the new developed pellets and adaptation of the production means.
• Life cycle analysis, before and after product development. Development of Type II Ecolabel and Green Marketing.
• Laboratory and industrial scale validation of the new products developed.
Achievement of the first manhole boots with 60% recycled PVC from the automotive sector.
The development of an innovative project has been confirmed, since the manufacture of a specific type of professional footwear has been achieved, for a very particular work use, that maintains sustainability and circular economy guidelines.
• Sustainability was achieved by increasing the level of recycling of plastic materials from non-renewable sources and closing the cycle with the consequent saving of raw materials and energy.
• Economically it has achieved the reduction of the final price of the pellets that can be used as raw material in the manufacture of injection shoe soles.
• The amount of ‘virgin’ PVC used in the manufacture of footwear was reduced.
• The possibility of offering companies that generate PVC waste from the automotive industry and/or those responsible for its management, a more sustainable treatment solution that incorporates environmental improvements, economic benefits and responds to the social demand to limit the generation of waste has been developed.
Almost two billion tonnes of waste are produced every year in the European Union and this figure is ever-increasing. The storage of this waste is not a sustainable solution, and its destruction is not satisfactory, due to the increasingly concentrated and polluting waste produced as by-products.
By way of conclusion, after the research was carried out, the best solution continues to consist of preventing the excessive production of waste and/or reintroducing it into the manufacturing cycle, by recycling its components, as long as the processes are sustainable from an ecological and economic point of view. In this respect, CTCR has been able to confirm this option as the most viable in the footwear sector. Specifically, it has achieved the fulfilment of a series of objectives set within the framework of a pioneering research, which has given rise to a typology of recycled technical products for professional use and intended for those who spend their daily lives in aqueous environments in the agricultural, construction, food, and other similar sectors.
In their research methodology, the expert biologists of CTCR carried out the arduous process of characterising the PVC coming from the automotive industry, both in terms of visual and physiochemical characteristics. They obtained several formulations, with properties adapted for use in footwear. They then developed and optimised the process conditions with the new chippings obtained, and adapted the production means until achieving the validation, both at laboratory and industrial scale, of the new safety boots.
The CTCR has achieved a series of business objectives focused, above all, on reducing the final price of the raw material used in the manufacturing process of the boots and, therefore, the amount of ‘virgin’ PVC used. This has led to an added bonus for the company in question, as it has the possibility of offering companies that generate PVC waste from the automotive industry and/or those responsible for its management, a more sustainable treatment solution that incorporates environmental improvements, with an economic benefit that responds, in turn, to the social demand of limiting the generation of waste. The environmental objectives of this innovative proposal include increasing the level of recycling of plastic materials from non-renewable sources and achieving the true closure of the life cycle, with the consequent saving of raw materials and energy. They also support management actions in the form of reuse, recycling, and recovery in general, set out in Law 22/2011 of 28 July, on waste and contaminated soils and other legislation in force in the State or enacted from Europe. As well as adhering to the demands of the Spanish National Integral Waste Plan and/or of the State Waste Prevention Programme 2014-2020, in minimising waste and reusing materials whose current destination is the landfill.