Introduction

Silicones (also known as polymerized siloxanes or polysiloxanes) are polymeric compounds with silicon atoms as well as carbon, hydrogen and oxygen atoms. Silicones can also include other elements. Because the silicon atom is larger than the carbon atom, it can form bonds that are longer and flexible with wider bond angles. Therefore, silicone-type polymers have flexible molecular structures and can form longer chains while maintaining their flexibility in comparison with organic polymers. Due to their flexible structure, siloxanes are able to align more effectively at interfaces and coat the surfaces effectively.

Because of their antimicrobial properties, hydrophobicity and biocompatibility, siloxanes are used in many industrial applications including electronics, automotive industry, textiles, construction, medical equipment, food industry, medicine, personal care products and cosmetics (Colas and Curtis 2006). The trends in patent application for silicone-based materials have shown an exponential increase as well as use of siloxanes use in consumer products (Fig. 1) (Google patents 2014).

Fig. 1
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Trend of silicone-based patents granted in the USA during 1957–2013 (Google Patents 2014)

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Silicon was discovered during investigation of reduction processes of potassium fluorosilicate with potassium by Berzelius in 1824 (Colas and Curtis 2006). The first silicon-containing organic compound was synthesized in 1863. Organosilicon chemistry flourished during the period from 1901 to 1930s when different types of silanes were synthesized. In the following decade, commercial materials were made available, when Dow Corning verified thermal stability and electrical resistance of resins made from silicone (Colas and Curtis 2006). Between the period of 1947 through 1950, silicone fluids and resins were developed, expanding the product range into antifoaming agents, surface coatings and textile impregnates for stain resistance (Wacker 2014; Tansel and Surita 2014).

Silicone use in consumer products reduces the carbon footprint by improving energy efficiency and reducing raw materials consumption due to extension of the lifetime of the products during use. Specifically, silicone use in the automotive, construction and solar energy industries has significantly reduced green house gas (GHG) emissions due to postproduction energy-saving technologies and sustainability via reduced water use, product life and energy consumption (Brandt et al. 2012). Products containing silicon have decreased GHG by approximately 54 million tons of CO2 (Brandt et al. 2012). Despite the reduction in GHG emission, the presence of siloxanes is a concern at landfill gas-to-energy facilities (Sevimoglu and Tansel 2013a, b). Accumulation of silicon dioxide deposits on engine components has been reported during combustion at landfill gas-to-energy facilities (Surita and Tansel 2014a; Naja et al. 2011; Arnold and Kajolinna 2010).

The objectives of this study are to evaluate the increase in the use of siloxanes in consumer products and estimate the projected siloxane content in municipal solid waste (MSW) based on the trends in patent applications for silicone-based materials and siloxane use in consumer products in the USA. The study was conducted based on the data compiled on silicone-based patents granted and use of silicone on consumer products in industrial applications in the USA. MSW trends were developed using the historical data for different MSW fractions in the USA. The silicone-containing waste products were categorized into five sectors. The analyses showed that the two most prevalent waste fractions with highest silicone use are paper/paperboard and construction wood. The fractions of silicone per silicone-containing products (i.e., paper/paperboard and construction wood) were used as the basis for comparing the projected silicone use and siloxane releases from landfills. The study was conducted during 2012–2014 in the Miami, Florida, USA.

Trend analysis of MSW in the USA was conducted for the period between 1960 and 2012, as reported by US Environmental Protection Agency (USEPA) (US EPA 2014). The categories representing the highest composition of waste in landfills included paper/paperboard, food waste, wood, total metals, plastics, textiles and rubber/leather as noted in Fig. 2. Although paper and paperboard use has decreased in recent years due to prevalent use of computers and Internet, use of siloxanes in paper products has increased. It should be noted that siloxanes have been incorporated into paper, wood and plastic components as coatings for repelling moisture or extending their lifetime. The significant increase in plastics deposited in landfill in recent years is due to rapid diffusion of electronic products and disposal of electronic waste (i.e., computers, televisions) as well as increasing use of plastic for packaging purposes (i.e., bottles and bags).

Fig. 2
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Categories and changes in composition of waste products in US landfills (US Environmental Protection Agency (USEPA) 2014)

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Siloxanes are primarily used as coatings and sealant in wood and paper products, and in some textiles for repelling moisture. Application of siloxanes in textiles has only recently been implemented and is not expected to contribute to significant quantities due to its slow rate of change in waste composition. Textiles are oftentimes recycled or reused. Similarly, food waste is not anticipated to contribute to elevated siloxane concentrations in landfill gas due to its rapid decomposition which leads to volatilization of siloxane gases into the atmosphere. Although plastics disposal in the waste stream has increased, the quantity of silicone-based material utilized in plastics manufacture is minimal, whereas the percentage of siloxanes by weight in paper/paperboard and construction (wood) industries is high (Brandt et al. 2012). Therefore, this study focused on the paper/paperboard and construction (wood) industries where the use of siloxanes has increase significantly; hence, the correlations between the silicone-based patents granted and siloxane quantities used in industrial applications can be more significant. The siloxane use in these two industries, siloxane quantities entering landfills and amounts released with landfill gas were evaluated and projected based on the use and decomposition rates (i.e., time to decompose) of paper/paperboard and wood waste in landfills.

In the USA, siloxane sales have reached over 236,000 tons in 2009 (Oxford Economics 2010). The number of cumulative number of silicone-related patents granted until 2009 was 169. Hence, the total tons of siloxane sold per patent was estimated at approximately 1397 tons per patent per year. Approximately 30.9% of silicone-containing products are used in construction industry primarily as exterior coatings (with about 300 square feet coverage per gal siloxane). Currently, the amount of silicone used in paper industry is relatively small (1.5%) (Brandt et al. 2012). However, because relative quantity of the paper products in waste streams is large and the product use lifetime is relatively small, siloxanes utilized in the paper industry have significant impact on the quantity of siloxanes entering landfills and the anticipated levels of siloxanes in landfill gas.

The management practices for the municipal waste fractions in Florida, USA, primarily involve landfill disposal and recycling (FDEP 2012). Based on the data provided by FDEP (2012), approximately 61% of construction waste is disposed in landfills and 39% is recycled (Table 1). Similarly, for the paper industry, an average of 72% waste is disposed in landfills and 28% is recycled. This includes corrugated cardboard, newspapers and office paper.

Table 1 Waste management practices used for paper products and construction and demolition wastes (FDEP 2012)
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Materials and methods

Silicone trend analysis

Siloxanes are man-made chemicals which are used in increasing quantities in industrial applications. The approach used in this study was to correlate the patents granted for silicone-based products to siloxanes used in consumer products. Although the waste quantities per capita are not increasing, the siloxane quantities used in consumer products and concentrations detected in landfill gas have been increasing over time. The potential effects of increasing siloxane-based patents and siloxane use in industrial applications were evaluated using three different growth models. The patent applications for silicone-containing products were evaluated in relation to the year of publication of the patent in the patent directory (Google Patents 2014). The number of silicone-related patents over time was estimated by regression analysis (Fig. 1). The trends of cumulative number of patents filed were estimated using three different growth models described below (Meadows et al. 1972).

Model 1: Exponential growth

This model assumes that the growth will continue according to the following exponential equation:

$$P = 2 \times 10^{ - 60} \cdot e^{0.0711x}$$
(1)

where P is total number of patents and x year (i.e., 2012).

Based on the current trends for patents granted, an exponential trend is observed (Fig. 1). However, the saturation growth curve is expected to be a more realistic scenario for patent filing rates and is suggestive of a smooth transition from exponential growth to steady state. While patent generation continues to increase, the resistance to further patent development is expected to concurrently increase. As such, a saturation growth-type model can be more appropriate to estimate the patent approval rates.

Model 2: Saturated growth

$$P = P_{\text{u}} (1 - e^{ - kt} )$$
(2)

where \(P_{\text{u}}\): ultimate patent generation count (assumed at 550 patents), \(k\): patent rate constant and \(t\): time since patent generation began (1957; t = 0).

The ultimate patent generation constant (\(P_{\text{u}}\)) was assumed as 550 based on numerical and visual analysis of patent trends, historical siloxane-based patents granted, and stabilization characteristics in patents obtained and market stabilization in demand. The numerical analyses indicate that the ultimate patent generation count would be about 550 for projections until the year 2050.

The patent rate constant \(\left( k \right)\) was calculated under the assumption that patent generation is finite. The maximum patent yield corresponds to the inflection point of the logistic curve; therefore, Eq. 3 can be used to estimate the patent rate constant where \(t_{d}\) is equivalent to the length of time to reach the half the maximum patent yield (assumed at 250 patents, approximately equivalent to 55 years).

$$k = \ln 2/t_{d}.$$
(3)

Observing the stability of patent development from 1957 until 1980, the model was revised to include the lag time before notable growth occurs. The corrected saturated growth curve expressed in Eq. 4 incorporates the lag time (\(t^{ *}\)) as 23 years based on the number of patent filings since the first silicone patents filed.

Model 3: Corrected saturated growth

Since there was a lag in the number of silicon patent filings after the first silicon patent filing, the saturated growth model was modified to account for the lag time between the patent granting and the use in consumer products as follows:

$$P = P_{\text{u}} \left( {1 - e^{{ - k\left( {t - t^{ *} } \right)}} } \right)$$
(4)

where \(t^{ *}\) is lag time (years). Analysis of data showed that a \(t^{ *}\) value of 23 years was adequate to describe the trend of cumulative silicone patents.

Justification of sector alignment

A study covering silicon production, use and end-of-life analysis over Europe, North America and Japan was used as the framework for consolidating waste categories and for alignment purposes (Brandt et al. 2012) as presented in Table 2. The silicon use sectors included construction, electronics, industrial, personal and transportation as A through E, respectively. As illustrated in Fig. 3, silicone use in electronics (57%) and transportation sectors (7%) is anticipated to have long life spans; therefore, they were not considered in waste projections for the time period considered in this study. Personal care products are primarily discarded in wastewater streams as wash off products. Residual amounts of silicone-containing waste are found within containers that are landfilled (i.e., shampoo bottles, creams, makeup containers). Therefore, the personal care products category is also excluded from the analyses due to their relatively small contribution to siloxane amounts entering landfills.

Table 2 Consolidated waste categories for alignment
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Fig. 3
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Distribution of silicone use in industrial applications (Brandt et al. 2012)

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The primary landfilled waste components include paper/paperboard, wood, food waste and textiles. Therefore, these sectors were aligned with the components that are evaluated in this study. Table 3 provides silicone use in the two main sectors with silicone-containing products (Brandt et al. 2012). These sectors are Sector A, which represents construction (wood), and Sector C, which represents paper (paper/paperboard). Construction and demolition (C&D) waste is commonly buried in segregated landfills in the USA. With segregation, moisture, nutrients and seed, bacteria are reduced, thereby creating an environment with slower decomposition rates (Barlaz 2006).

Table 3 Silicone use in construction and other industrial applications
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Based on the current practices, paper and wood (construction) are the two industrial products where silicone-based materials are used extensively. In recent years, there has been a significant decrease in paper production due to the use of personal computers and other technologies. Quantities of construction wood have increased slightly but remained between 5 and 10 percentages of the waste composition during the past 25 years.

Municipal waste degradation

Wastes entering landfills degrade at different rates (Martin et al. 1997). In regard to wood degradation, Ximenes et al. (2007) demonstrated that laboratory research studies overestimate the decomposition of wood products in landfills. With optimum moisture content ranging from 40 to 70%, excavated samples from the study showed no visible deterioration of the majority of the wood recovered after 19–46 years. According to Jang and Townsend (2003), C&D waste is composed primarily of wood (33.2%), concrete (29.2%), roofing shingles (13.7%), drywall (12.4%) and cardboard (8.0%). Siloxanes and silanes are the most common penetrating sealers used in the construction industry.

Figure 4 illustrates the fate and estimated decomposition times for paper and C&D wastes as well as estimated release times of siloxanes during decomposition. Despite the extended degradation times of paper and wood products, siloxane volatilization occurs rather quickly because of its application to surfaces. The most common siloxanes detected in landfill gas are octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), hexamethyldisiloxane (L2) and trimethylsilanol (TMS) (Rasi et al. 2010; Sevimoglu and Tansel 2013a, b; Surita and Tansel 2014a). The composition of gas from MSW landfills is indicative of the siloxane compounds used in the paper/paperboard industry and coatings used in construction materials. Due to the high volatility of siloxanes, they are released from the wastes prior to decomposition of the wastes in the landfills. As presented in Fig. 4, the siloxane release is likely to occur with the 0.5–10 years for paper products and with the 5–35 years for C&D waste-based decomposition and release characteristics of siloxanes in landfill environment (Surita and Tansel 2014b).

Fig. 4
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Life expectancy of silicone-containing products from product manufacture to product decomposition

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Although C&D landfills are not considered for energy generation due to slow decomposition of construction wastes, the presence of siloxanes can affect the gas releases from the C&D landfills. Assuming first-order kinetics, D4 and D5 are estimated to degrade by 90% within 37 and 23 days, respectively (Surita and Tansel 2014b). Dow Corning (1999) had specified a general lifetime of volatile methylsiloxanes (VMS) to be within the range of 10–30 days. No biodegradation data were found pertaining to L2; however, it has been noted that they are resistant to biodegradation (Pubchem 2005). Half-lives of D4 in the atmosphere have been estimated to vary from 49 to 588 days at temperatures of 5°–25 °C (Wang et al. 2012). Pierce (2005) determined that siloxane concentrations decreased with waste age exponentially. The highest levels were estimated to be present 5 years after landfilling and decreasing by about 90% during the 10-year period after landfilling (i.e., waste age of 10 years).

Silicone-containing waste projections

The projected silicone-containing waste per sector in a given year is estimated as the product of annual silicone production, the percentage of silicone use per sector (i.e., paper, construction) and number of patents, estimated from corrected saturated growth curve (Model 3), as follows:

$${\text{PSW}}_{s} = {\text{ SPF}} \cdot {\text{ PSS }} \cdot P$$
(5)

where PSW s is projected silicone waste per sector in a given year (tons), SPF total annual silicone production factor (1397 tons/patent), PSS percentage of silicone present per sector (i.e., % silicon/product in paper or C&D), and P number of patents filed (estimated by Eq. 4).

Results and discussion

Based on the saturated growth rate model, the rate of patents granted will decrease, while cumulative number of patents will increase (Model 3). Although the exponential growth model fits the current patent generation trend, saturation is anticipated at approximately 550 patents (with doubling time of 55 years and growth rate constant of approximately 0.0126) based on the historical trends. The saturation growth curve fits the predicted trend (until 2050), indicating a resistance to future patent development potential and market saturation. The model was adjusted for the lag time between granting of the patent and its use in consumer products. Figure 5 provides an illustration of the models leading to the use of Model 3 for estimating the cumulative number of patents incorporating a lag time of 23 years. The figure identifies the actual cumulative patents approved beginning in 1957 and culminating in 2013.

Fig. 5
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Cumulative patents observed from 1957 to 2013 and prediction models for exponential and saturated growth curves

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Table 4 presents the estimated disposal of silicone-containing wastes from the paper/paperboard and C&D industries. Although the amount of paper deposited in landfills is decreasing, the quantity of siloxanes used in paper products is increasing. It was assumed that the siloxane use in paper and wood products will follow a pattern consistent with the actual patents granted during this time period according to the saturated growth model. The amount of wastes containing siloxane has been estimated assuming an average 1-year lag time for paper/paperboard and a 30-year lag time for construction (wood) materials prior to landfill disposal.

Table 4 Estimated and predicted silicone use quantities in paper/paperboard and C&D industries
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Siloxane releases based on the estimated 5 and 15% release scenarios are presented in Fig. 6. These projections indicate that the siloxane content of landfill gas is likely to increase by 29% during the next 10 years and by 78% during the next 25 years based on the assumption that 5% of the siloxanes entering the landfills will partition the landfill gas. Figure 6 compares for the scenarios corresponding to 5 and 15% release of siloxanes into the gas phase from wastes originating from paper/paperboard products and construction (wood). The analyses indicate that siloxane content in landfill gas is expected to increase significantly after 2020. This increase is likely to result in development of control measures at waste-to-energy facilities to remove siloxanes from landfill gas prior to combustion.

Fig. 6
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Projected siloxane release quantities based on the trends of silicone patents and siloxane use in consumer products assuming 5 and 15 percentages of siloxane is released as VMS during MSW decomposition at landfills: a MSW originating from paper/paperboard products, b MSW originating from construction (wood)-related activities

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Conclusion

The projected siloxane quantities in MSW were estimated based on the use of siloxane-based materials in consumer products, trends in historical MSW composition, as well as trends in patent applications for silicone-based inventions (i.e., chemicals, products). The projections were performed using the exponential growth curve since inception of silicone patents in 1957 with a lag period of 23 years (simplified logistic growth curve).

The primary sectors with the highest silicone content are the paper and construction industries. The fractions of silicone per silicone-containing product were used as the basis for comparing the use of siloxane-containing wastes to project the amounts of siloxane releases from landfills during decomposition of the waste products.

Based on the historical trends, it is estimated that the quantities of siloxanes entering MSW landfills will increase by approximately 29% in conjunction with a 5% increase in silicone-containing products within the next 10 years and 78% within the next 25 years. These percentages were estimated by taking into account the lag time between the use and decomposition of consumer products. The lag times used were one year for the paper/paperboard industry and 30 years for the construction materials.

Siloxane-based products are still in development stage, and they are entering the markets as new products with new applications. Therefore, data on siloxane use in consumer products and quantities entering to landfills are not well documented. The biogas from landfills shows that siloxane levels are increasing in biogas, affecting the biogas-to-energy recovery processes. The analyses conducted provide insight into the anticipated increase in siloxanes that will enter landfills and their effect on biogas quality over time. It should be noted that the method developed was based on a number of assumptions; hence, the results provided have uncertainties due to limitations of the assumptions used.