Spent tyre valorisation: new polymer modified asphalts for steel protection in an aggressive marine environment

Abstract

Waste valorisation is a topic that has been in the limelight since environmental concerns have promoted the better use of raw materials. Annually, millions of tons of waste tyres are generated worldwide and represent a great environmental threat; these waste tyres could be used in the development of novel materials for effective protection against corrosion. In this work, the performance of 8 new polymer modified asphalt (PMA)-based coatings made up of spent tyres was evaluated on steel samples. Accelerated tests (salt spray, humidity and accelerated aging), as well as adherence and impact studies at the laboratory scale, were carried out prior to a two-year exposure investigation in a very highly aggressive marine zone. Analysis of the results revealed the suitability of most of the PMA compositions in terms of anticorrosion protection, adherence and impact resistance. The suitability of these PMA compositions was validated by applying them in automobiles subjected to an aggressive marine environment.

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Introduction

Waste valorisation is a topic of great scientific interest to many researchers in different disciplines.^1–3 Every year, around 4.4 million tons of waste tyres in the world are discarded, which represent a great environmental threat. The presence of natural and synthetic rubbers in tyres accounts for approximately 60% of their composition, which makes this waste a potential raw material for a diversity of applications.^4,5 Therefore, the search for technically, economically and environmentally advantageous solutions is of paramount importance. Retreading, recycling and energy recovery are the three main treatments employed for tyre valorisation. With regard to recycling, several investigations have been focused on the development of rubber-modified bitumen where rubber particles act like modifying agents, improving the bitumen properties.^6,7 These types of materials are also known as composites and they are composed of a matrix and a filler material which reinforces the former.^8,9 This is an environmentally sustainable alternative in line with the current European Union policies (Directive 2000/53/EC), which state that at least 85% of the weight of end-of-life vehicles must be re-used or recycled by the present year 2015.¹⁰

Bitumen and bituminous binders are widely employed for roofing and road pavements because of their interesting adhesive and load resistance properties.¹¹

However, there are some pavement defects, such as rutting at high temperatures, crack initiation and propagation in the low temperature region and low elastic and viscous properties at high in-service temperature, that favour permanent deterioration, thus reducing the material's durability.^12,13 These distresses can be overcome by the addition of ground tyre rubber, which improves the material's mechanical properties.^7,13 Also, combinations of bitumen modifiers have been proposed to improve mechanical and rheological properties, namely rubber and recycled polyethylene,¹⁴ crumb rubber and a set of special additives,¹⁵ organic layered silicates¹⁶ and expanded vermiculite.¹⁷

The applicability in roofing, waterproofing, and the sealing industry, as well as the development of novel asphaltic modified emulsions, has been patented.^18,19 More specifically, rubber-modified asphalt applied by spraying has allowed the formation of a stable and tough waterproof layer over a surface, as already demonstrated for buildings and structure protection against moisture and rust.¹⁹ Recently, a new asphalt material constituted by a polymer and a polyol was developed and it turned out to confer remarkable waterproofing properties and protection from physical damage.²⁰

In general, the aforementioned studies have concentrated their efforts on evaluating the rheological and mechanical properties of different modified bitumen compositions for pavement or roofing applications. However, only a few investigations have referred to their anticorrosion properties and their application in waterproofing and rust proofing. Hence, these limitations have motivated the development of new compositions of rubber modified asphalt (PMA) at the laboratory scale. In this work, the protective properties of 8 new compositions were assessed through accelerated and outdoor exposure tests (aggressive marine environment), as well as investigating the adherence and impact resistance. Also, their waterproofing and sealant properties were evaluated, together with their anticorrosive capacity, as a prior step to their application in transport equipment such as bodywork.

Materials and methods

Materials

Samples of low carbon steel (SAE-AISI 1010), with dimensions 150 mm × 100 mm × 1 mm and the chemical composition indicated in Table 1, were employed to classify the corrosive aggressiveness of the specific area under study. The surface cleaning consisted of degreasing with commercial grade naphtha and immersion for 3–5 min in 35 wt% hydrochloric acid solution (1 : 1 in distilled water), followed by thorough rinsing with distilled water. Then, samples were dried and kept in a desiccator with anhydrous CaCl₂ for at least 24 h. Finally, they were weighed and kept again in a desiccator until the beginning of the experiments, one part for corrosion study and the other part as a substrate of the PMA coatings.

Table 1 Chemical composition of the low carbon steel used

Elements (wt%)
Fe	Mn	C	S	P
99.30	0.40	0.07	0.05	0.016

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Rubber modified asphalt compositions

After preliminary investigations where several experimental designs were carried out and optimized, 8 new compositions were prepared to be used in the present work, following a patented procedure.²¹ Briefly, they are composed of a matrix (oxidized asphalt) and a reinforcement (powder particles of recycled tyre rubber of 0.21 mm in diameter (US mesh 70) joined through a binding agent). Rubber is a generic term that can describe elastomeric materials. The binding agents are insoluble soaps obtained from the saponification of soluble soaps (waste material) coming from aliphatic acid derivatives, from glycerides and waxes.²¹

The obtained PMAs are semisolids at room temperature and with a high softening point. The percentage of polymer (rubber) and the softening point for each composition are presented in Table 2. More details about the PMA composition are provided in a patent that is under submission. These data were obtained from the average of at least three measurements for each composition.

Table 2 Composition (%) of rubber in powder in each PMA variant and blending points (°C)

Variants	Powder rubber (wt%)	Blending point (°C)
1	33.33	126
2	38.00	123
3	36.00	130
4	33.33	135
5	32.00	127
6	34.00	123
7	34.67	128
8	36.63	127

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A uniform film over the substrate was prepared by applying the 8 melted PMAs with a spatula. Degradation of the polymers was observed at temperatures higher than 130 °C. Once the product was hardened, samples were packed and kept in a desiccator until the beginning of the experiments.

Four steel specimens coated with the PMA compositions were prepared (for each variant), three for the field study and the fourth (kept in a desiccator) as a blank for visual comparison. More samples were also prepared for laboratory tests. Replicates were always included in order to ease the achievement of statistically significant conclusions.

Laboratory tests

Laboratory tests were based on different normalized assessments, such as adherence and impact tests, as well as accelerated ones, which together with the outdoor exposure results allow the main objective of the present investigation to be achieved. Three replicates for each composition and each test were prepared and the data achieved were obtained from the average of three values.

Accelerated tests

Salt spray tests were performed in a climatic chamber, model Q-FOG C.C.T. where steel samples covered with PMAs were exposed. The required conditions for the test were the use of sodium chloride solution (50 ± 5 g L⁻¹) with a density at 25 °C between 1.0255 and 1.0400 g dm⁻³, pH: 6.5–7.2 and temperature 35 ± 2 °C, in agreement with the UNE-EN ISO standard 9227.²² The demands for sample assessment were 1000 hours (10 cycles of 100 hours each) in the chamber without the appearance of typical failures such as blistering, cracking, rusting and sensitive delamination, in accordance with the UNE-EN ISO standard 4628.²³ Before evaluation, coated specimens were rinsed with water (after each cycle) to remove the residues of salts on the surface.

Humidity tests were carried out in a humidity cabinet, model CCM/0/300, where the conditions of the assay were 40 ± 2 °C and 100% relative humidity with constant condensation over the steel specimens (covered with PMAs), according to the DIN EN ISO standard 62700.²⁴ The requirements for sample evaluation were the same as those mentioned for the salt spray test.

The accelerated aging test was conducted in a QUV chamber (model QUV/SE) in accordance with ASTM D standard 4799.²⁵ Coated specimens were subjected to alternate cycles of radiation and humidity for 1000 hours. Fluorescent lamps were employed for UV-A radiation at a wavelength of 340 nm and a temperature between 60–80 °C (radiation cycle), followed by condensation at 50 °C (humidity cycle). Sample evaluation was performed at the end of every cycle (100 hours each). The absence of significant physical damage compared to the reference samples after 1000 hours was targeted.

Adherence tests

Adherence tests were performed following the pull-off method, as stated in the UNE-EN ISO standard 4624.²⁶ Measurements were performed by using an adhesion tester, model ERSAD.

Impact tests

The procedure described in the ASTM D2794 standard was followed²⁷ for impact resistance tests on coated panels. The tests were performed for a sphere of 0.9 kg in weight and 15.9 mm in diameter at three different heights (25, 50 and 100 cm). An impactometer (model 03040 10) was used for impact measurements.

Adherence and impact tests were performed over steel samples covered with PMA compositions at the initial time.

Outdoor exposure testing

Both steel and PMA samples were subjected to outdoor exposure testing at the testing station located at Playa Caleta hotel, as can be seen in Fig. 1.

Fig. 1 View of the outdoor exposure test station.

It is located in Varadero city (Matanzas, Cuba) 250 m from the North Coast. The latitude and longitude are 23° 9′ 13′′ N and 81° 15′ 5′′ W, respectively. This place is a marine area and fulfils the general requirements for this type of experiment. Specimens (steel and PMA-coated steel) were exposed in a rack, facing south, and at 45° to the horizontal, following the recommendations of the ISO 8565 standard.²⁸ Three steel samples (for corrosion rate determination at each exposure time) and three PMA specimens applied on steel substrates (for each PMA variant) were tested.

Determination of corrosion aggressiveness

Steel samples subjected to outdoor conditions for 24 months were picked up (at scheduled times) and cleaned in a specific solution (500 ml HCl, 500 ml distilled water and 3.5 g hexamethylenetetramine) until complete elimination of the corrosion products.²⁹ Subsequently, the specimens were rinsed, dried, packed and kept in a desiccator for 24 hours. Then, they were weighed with a weighing accuracy of ±1 mg and the weight loss obtained (average of three samples) was used to determine the corrosion rate, according to the demands of ISO standard 9226.³⁰ Classification of the corrosivity was carried out by means of the average of the annual corrosion rates, conforming to ISO standard 9223.³¹

Evaluation of PMA degradation

The 8 PMAs applied over the steel panels were subjected to outdoor exposure testing for 24 months. Failure evaluation of the PMA compositions was carried out following a specific scheduled time in agreement with the UNE-EN ISO standard 4628 (ref. 23) and ended when important defects were observed. Typical defects such as blistering, rusting, cracking and flaking were assessed with time by visual comparison between the samples studied and standard photographs specifically designed for each failure.²³ This test gives us quantitative information about coating deterioration with time, such as the quantity and size, as well as the intensity of blisters, cracks and flakes. In the case of rusting failure, the percentage of rusted area can be estimated. Results presented are the average of three tested coated panels for each PMA composition at each exposure time.

PMA applications in transport equipment

The most promising PMA compositions were applied in transport equipment, such as in car bodywork, under “high” (category C4) and “very high” (category C5) aggressive atmospheres for two years. Coating assessment was performed by visual inspection (once a year) with the aim to detect possible failures, following the UNE-EN ISO standard 4628.²³ Digital photography was also employed.

Results and discussion

Evaluation of PMA performance through laboratory tests

First of all, adequate compatibility of the PMAs with the substrate was confirmed after the salt spray test. After approximately 450 hours of exposure, some surface softening was observed, which led to a slight release of rubber particles from the coating that was not directly adhered to the metallic base. This phenomenon is more evident in this test than in the humidity one, probably due to the rinsing of the samples with water after each cycle to remove the residues of salts on the surface. As can be seen in Fig. 2, the 8 PMA specimens under study showed no rusting (metallic substrate) after 1000 hours in a salt fog chamber. Only some degree of oxidation was detected at the borders since they had not been protected in order to ease sample mounting inside the chamber.

Fig. 2 Visual appearance of the PMA samples after 1000 hours of exposure in a salt fog chamber.

An apparent superficial detachment of the material was detected after 400–500 hours of testing in the humidity test cabinet. This was more evident in the salt spray test. From 700 hours on (7 cycles), a hardening of the material was recorded, indicating a slightly superficial flexibility loss, which led to a slight cracking of the film surface at the end of the experiment. Then, after 1000 hours (10 cycles) and 24 hours more outside the humidity chamber, it can be stated that the proposed coatings fulfil the requirements of the standard.

Even though some hardening and loss of flexibility occurred after seven cycles in the QUV chamber, a suitable performance of the PMA compositions was demonstrated after 1000 hours under standard ultraviolet radiation and temperature conditions. Therefore, the coatings also passed the present test, in agreement with the demands established in the standard.

As observed during the accelerated tests, a slight release of rubber particles from the coating was demonstrated in the PMA compositions. This phenomenon was more evident in those compositions with a higher content of rubber particles (PMA 2 > PMA 8 > PMA 3, etc.).

The increase of rubber content in PMA contributes to promoting a lower adherence, so a higher content of this material should be evaluated. An adherence test was carried out for composition 2 with the highest relative rubber content. After the assessment, very low cracking and contraction because of flexibility loss was found, so a small release of material due to the rupture of the external layers was detected. The rupture occurred at 225 psi (1.5 MPa), and 100 psi (0.67 MPa) is the maximum stress, in agreement with the pull-off standard.²⁶ These results confirm that, even though a pressure higher than the maximum limit (more than double) was used, the observed damage is only superficial. Hence, adequate adherence of composition 2 to the steel substrate was demonstrated. Consequently, compositions with lower rubber content will pass the tests successfully and less release of material is expected to occur. The latter is supported by the greater relative content of oxidized asphalt and binding agent, thus leading to a higher adherence. This performance, in addition to the material's flexibility and ease of application with a spatula (when hot), widen the composition's application in joins: metal–metal, metal–mortar, metal–concrete and asbestos–cement.

The adherence test results suggest that lower rubber content will lead to higher adherence in the PMA compositions. These compositions are more suitable for application in liquid form by spray, because a greater rubber content could block the product output for spray applications.

On the other hand, the higher the rubber content in the PMA compositions, the greater the impact resistance. This is closely related to the flexibility of the PMA due to the polymer presence. Hence, a high rubber content composition was evaluated (PMA 8). The results of the impact test at three different heights (25, 50 and 100 cm) are summarized in Table 3.

Table 3 Results of impact test for three different heights

Weight (kg)	Height (cm)	Observations
0.9	25	The material remains without visible changes and supports the load
0.9	50	There is some detachment of the material
0.9	100	Total release of the material. The base is observed

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As can be seen, the sample tested presents a high endurance toward the applied load (0.9 kg) at 25 and 50 cm height, and no release of the coating is observed. However, a complete detachment of the coating at a 100 cm height is observed, probably due to the higher rubber quantity in the composition, which led to a decrease of the coating adherence to the steel base. Although the samples tested at 100 cm height failed, the results obtained for 25 and 50 cm heights are considered positive, bearing in mind that the material will not be exposed to such extreme conditions (0.9 kg weight and 100 cm height). Hence, the results confirmed the appropriateness of the coatings, as demanded by the standard.²⁶

Assessment of PMAs' deterioration when exposed to outdoor conditions

Before the evaluation of the PMAs' deterioration under outdoor conditions, the determination of the corrosive aggressiveness of a selected area was carried out. Following the classification system for the corrosivity of atmospheres established by ISO standard 9223,³¹ and once the annual average of the corrosion rate (CR) of the low carbon steel was calculated (671.95 g m⁻²), the area under study was concluded to be an environment with “very high” (category C5) corrosive aggressiveness. This experimental stage constitutes a fundamental step which will determine the surface preparation, design problems, protective systems, maintenance, etc. The weight loss after 1, 2, 3, 6 and 24 months of outdoor exposure was also determined, and is summarized in Table 4.

Table 4 Average, maximum, minimum and standard deviation values of the weight loss (g m⁻²) of the low carbon steel at different exposure times

	1 month	2 months	3 months	6 months	12 months	24 months
a Yearly average value starting the test in November.b Yearly average value starting the test in May.
Average	68.93	122.26	179.55	305.82	565.25	1024.70^b
	—	—	—	—	671.95^a	—
Maximum	167.37	255.35	270.32	420.31	580.35	1024.70
Minimum	28.26	57.24	88.83	224.49	550.15	1024.70
Standard deviation	33.35	27.43	35.10	30.40	21.35	0.00

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The monthly weight loss trend for carbon steel can be visualized in Fig. 3, where two different climate periods are clearly recorded. On the one hand, the winter or dry season (November–April) is characterized by a predominance of North-northeast winds, so the marine aerosol concentrations and the mass loss are increased. On the other hand, the summer or rainy season (May–October) leads to a reduction in the weight loss due to the weaker effect of the north winds, in agreement with previous studies.³²

Fig. 3 Monthly weight loss evolution for carbon steel samples exposed to an outdoor marine environment.

Afterwards, evaluation of the macroscopic defects present on the protective films after 24 months of exposure was performed, following the codes presented in Table 5.³³ These codes are related to specific values aiming at favouring the comparison among defects.

Table 5 Identification of the numerical values assigned according to UNE-EN ISO 4628-1 and equivalence with the scale detailed in ref. 32

Mark^32	Scale^a	Adherence^a	Blistering^a	Rusting^a	Cracking^a	Flaking^a
a UNE-EN ISO 4628-1: 2003.
100	0	No release	No blistering	Ri 0	No cracks	0%
80	1	<5%	Scarce	Ri 1	Scarce	0.1%
60	2	5–15%	2 S2–S5	Ri 2	2 S2–S5	0.3%
40	3	15–35%	3 S2–S5	Ri 3	3 S2–S5	1%
20	4	35–65%	4 S2–S5	Ri 4	4 S2–S5	3%
0	5	>65%	5 S2–S5	Ri 5	5 S2–S5	15%

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A summary of the PMA defects over the experimentation time is shown in Table 6, as obtained from the average of three tested coated panels for each scheduled time.

Table 6 PMA^a defects obtained after 24 months of outdoor exposure according to UNE-EN ISO 4628-1

PMA defects	Time (months)	PMA compositions
		1	2	3	4
a The PMA (5–8) results not presented are similar to the PMAs (2–3) shown in the table.
Blistering	3	0	0	0	0
	6	0	0	0	0
	12	0	0	0	0
	15	0	0	0	0
	21	0	0	0	0
	24	0	0	0	0
Rusting	3	Ri 0	Ri 0	Ri 0	Ri 0
	6	Ri 0	Ri 0	Ri 0	Ri 0
	12	Ri 0	Ri 0	Ri 0	Ri 0
	15	Ri 0	Ri 0	Ri 0	Ri 0
	21	Ri 0	Ri 0	Ri 0	Ri 0
	24	Ri 0	Ri 0	Ri 0	Ri 0
Cracking	3	2 S(3)	0	0	0
	6	2 S(3)	0	0	0
	12	2 S(3)	0	0	0
	15	2 S(3)	0	0	0
	21	2 S(3)	0	0	2 S(3)
	24	2 S(3)	0	0	2 S(3)
Flaking	3	0	0	0	0
	6	0	0	0	0
	12	0	0	0	0
	15	0	0	0	0
	21	0	0	0	0
	24	0	0	0	0

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Among the eight compositions tested, only 1 and 4 showed cracking (type 2 S(3))³³ after 3 and 21 months of exposure, respectively. The morphology of the cracks was without a preferential direction.³³ This behaviour is considered acceptable, in agreement with the evaluation scale (Table 5), and it can be visualized in Fig. 4. The aforementioned is supported due to the beneficial effect of rubber on the PMAs. The addition of rubber, from recycled automobile tyres, to an asphalt binder, substantially reduces the rate of oxidative hardening of asphalt and increases its useful life.³⁴

Fig. 4 Evolution of cracking in PMAs over 24 months of outdoor exposure.

This failure is not considered important due to its superficial nature. Also, these PMA compositions showed neither blistering nor rusting and flaking after two years of exposure in a very highly aggressive location. However, both compositions were discarded due to the cracks detected, which can promote the access of water and pollutants through the layer, thus affecting its anticorrosive protection.

In relation to the rest of the compositions tested, a promising protective performance was concluded after 24 months under a very high aggressive environment, as can be seen in Fig. 5.

Fig. 5 Visual appearance of the PMAs after 24 months of outdoor exposure. Some cracks are identified with blue arrows. The width of each figure is equivalent to 10 cm in reality.

This is in compliance with the UNE-EN ISO standard 12944-5 (ref. 35) referring to the selection of coating systems, since a durability from 2 to 5 years without failure is required.

Evaluation of PMA applications in transport equipment

From the results obtained, both PMA 2 (with a high amount of rubber) and PMA 5 (with the lowest quantity of rubber) were selected to be applied as semisolid coatings. Hence, they were produced at a pilot plant existing in our research group. Composition 2, a semisolid PMA (DISTIN 403), was applied in the metal–metal joins of car fenders for coating the car floor (inside part), as well as for sealing some parts of the car body (Fig. 6). Once applied, an average thickness around 1 to 2 mm was reached. After yearly visual inspections, the material was demonstrated to act as an anticorrosive and waterproofing coating providing resistance to water and aggressive media. It was flexible, mouldable and helped to simultaneously reduce the noise produced outside the car.

Fig. 6 PMA application (A) in the sealing of a car floor (DISTIN 403), (B) and (C) as a stone chip coating on the outer part of the car floor (DISTIN 403 L).

Composition 5, a liquid PMA of solvent type (DISTIN 403 L) with a semisolid base, was sprayed on the outer part of the car floor. The new coating was able to withstand the impact of water and particles and it also showed excellent adherence and corrosion resistance, for an average thickness of 138 ± 10 μm. After more than one year of exposure, the PMA applied on the automobiles subjected to an aggressive environment confirmed its excellent performance as a stone chip coating (Fig. 6), and also provided a reduction of noise derived from the impact of stones with the exterior surface of the car floor. In summary, as a result of all these tests, two types of PMA with specific characteristics and applications have been identified as follows:

• PMA composition as an anticorrosive coating for transport equipment.

• PMA composition as a waterproofing and sealant coating for floors.

Conclusions

In this work, we have demonstrated the suitability of polymer modified asphalts (PMAs) containing spent tyre-powder to protect steel substrates against corrosion under very highly aggressive environments. From the results achieved, a suitable adherence and impact resistance was demonstrated. Accelerated and outdoor exposure tests under a marine atmosphere confirmed the promising protective properties, since no blistering, rusting, cracking and flaking were detected for most of the PMA compositions evaluated. Additionally, the application of PMA compositions (2 and 5) in automobiles proved their rust proofing, sealant, waterproofing and impact resistance properties, thus demonstrating the potential of these new coatings.

Acknowledgements

The authors are very grateful to the University of Vigo (Spain) for funding through a PhD grant. Also, we are thankful for the valuable support received from the CEAT Research group of the University of Matanzas (Cuba) and the ENCOMAT group from the University of Vigo.

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