Recycling of Nylon From Carpet Waste.

MICHAEL D. AMIRIDIS [*]

Large amounts of post-consumer carpet are discarded every year. Most of this waste is currently landfilled, while a small percentage is incinerated. The face carpet fibers, consisting primarily of nylon 6 and nylon 6,6, represent the majority component in the carpet waste. Recent financial incentives and environmental constraints have motivated the industrial sector to develop recycling strategies for these fibers. Depolymerization into their constituent monomers is the most complex recycling route, but at the same time it produces the most valuable product. A second alternative involves the use of solvents for the extraction of carpet fiber components in their polymeric form. Finally, a third recycling option yields thermoplastic mixtures by melt blending the carpet waste. The recent literature on the recycling of nylon from carpet waste is reviewed in this paper. The paper also includes a section focusing on the current state of carpet recycling at the industrial level.

1. INTRODUCTION

In the United States alone, approximately 3.3 billion lbs of carpet fibers, including nylon, polyester, polypropylene, acrylic, wool and cotton, are produced annually [1]. Of this amount, approximately 65%--or 2 billion lbs--is composed of nylon 6 and nylon 6,6. The European annual production of carpet, including tufted, woven and needled products is approximately 1.5 million tons (3.3 billion lbs) [2]. The average life cycle of a carpet is between 8 and 12 years. Carpet production also generates a substantial amount of waste in the form of trimmings and cuts, which typically amounts to 12% of the total production [3, 4]. As a result, an enormous amount of synthetic waste is generated, most of which is disposed in landfills. Landfilling is not an environmentally friendly solution since carpet fibers, like many other synthetic polymers, are not biodegradable. Furthermore, the cost of disposal is increasing continuously owing to limited landfill capacity. Environmental concerns and governmental regulations have spurred efforts In the direction of recycling all non-biodegradable synthetic polymers, of which carpets and carpet fibers constitute a significant percentage.

The literature In the area of carpet recycling is relatively sparse. A review paper on the subject of nylon recycling was published in 1991 by Datye [4], and it provides a detailed insight into the recycling of post-production nylon 6 waste. This review, however, does not focus upon the issue of carpet recycling at both the post-manufacture and post-consumer levels, as this concept has emerged only over the last few years because of the large amounts of non-biodegradable carpet accumulated In landfills. Recycling of the entire carpet poses a significant challenge because of the inhomogeneous nature of the material. Moreover, it is even more difficult to recycle post-consumer carpet waste because of the dirt, cleaning chemicals, and other materials accumulated in the carpets over the years. Studies carried out in Europe reveal that post-consumer carpet is approximately 30% heavier than new carpet owing to dirt accumulated in the piles [5]. It also contains a significant amount of contaminants, mainly the chem icals used for cleaning purposes. It is this inhomogeneous nature of carpets that makes effective recycling a very difficult and costly process.

The recycling of polymers has been broadly classified into four categories [3]:

(a) Primary recycling or depolymerization, which converts the waste into products having a quality equivalent to that of the "virgin" polymer. This category encompasses methods to break down the long polymer chains into their original monomers that can be repolymerized.

(b) Secondary recycling, which involves recovering of the individual components of a polymeric mixture without necessarily breaking them down to the monomeric form. This category includes various extraction and separation methods.

(c) Tertiary recycling, which consists of preparing a thermoplastic mixture by melt-blending the entire carpet waste. This recycling route requires no fiber separation or latex removal, as the carpet components are mixed by means of reactive extrusion and compatibilization. Products of lower quality can be manufactured from this blend, by methods such as injection molding.

(d) Quaternary recycling, which involves only energy recovery during the incineration of the polymer waste.

The intention of this review is to provide an insight into the problem of carpet recycling, along with an analysis of the different methods being proposed or commercially utilized. The reviewed literature includes a limited amount of journal publications, which focus primarily on fundamental aspects, and a large number of patents, which describe the available technologies.

2. CARPETS--A BRIEF BACKGROUND

Artificial fibers have been created in an effort to improve the quality and availability of textiles, and to reduce the cost of several products for the consumer. Initial attempts were targeted towards the synthesis of fibers with properties similar to those of the natural materials. During the industrial revolution of the 19th century, the first patent for "artificial silk" was granted to a Swiss chemist in 1855 in England. It was not until the end of the 19th century that the "artificial silk" (rayon) started being produced on a commercial scale by French chemist Count Hilaire de Chardonnet.

Discovered in 1931 by chemists at E. I. DuPont de Nemours and Company, nylon 6,6 quickly became one of the most used fiber materials. Nylon 6,6 is the polyamide formed by the reaction between hexamethylene diamine and adipic acid. Each of the monomers has 6 carbon atoms, thus the designation 6,6. Commercial production of nylon 6,6 by DuPont began in 1939. Its initial applications were sewing thread, parachute fabric, and women's hosiery. When the U.S. entered World War II, its nylon production was allocated for military use. In the post-war industry, nylon became one of the most widely used artificial materials, with a variety of applications. By the end of the 1940s, it was already being used for upholstery and in carpets. During the same period, Paul Schlack of the infamous I. G. Farben Company in Germany obtained a different form of polyamide by using caprolactam as the monomer, and called it "nylon 6." Most carpet fiber yarns in production today are either nylon 6 or 6,6, although small percentage is made of polyester, acrylics, polypropylene and other olefin fibers, wool or cotton [1, 6].

A typical carpet has four main layers (5) as shown in Fig. 1. The top layer, or face yarn, is composed of nylon fibers tufted through a primary backing, which is usually made of polypropylene. Other fibers such as jute, polyethylene, polyesters and rayon may also be used. Latex adhesive is applied under the primary backing in order to secure the face fiber. The adhesive is usually made of a styrene butadiene co-polymer (SBR), which is filled with inorganic materials such as [CaCO.sub.3] or [BaSO.sub.4] [3]. The fillers are added for soundproofing purposes. Finally, a secondary backing (same material as the primary backing) may optionally be added to the primary backing and bonded to it by the same SBR adhesive. The nylon face fibers, containing dyes, soil-repellents (to improve the resistance to stains), and possibly other additives to improve the quality of the carpet, usually account for about half of the total carpet weight.

3. CARPET RECYCLING

3.1 General Considerations

As mentioned in the Introduction, there are several recycling approaches, which differ by the type and quality of the product generated, and consequently by the type of process utilized. Depolymerization is the preferred route of carpet recycling, since it breaks down the carpet fibers (nylon 6 and/or nylon 6,6) into the corresponding monomers. This allows the recovery of the monomers that can be re-polymerized into new nylon products of high quality.

Alternatively, the nylon component can be separated from the carpet waste by extraction. In this process, the entire waste carpet material is dissolved in a solvent at elevated temperatures. During extraction, the nylon from the fibers is recovered in its polymeric form and can be reused in injection molding applications. The main problem associated with this approach is the selection of a suitable solvent that selectively dissolves the nylon fibers and does not react with or dissolve any of the other carpet components. Use of a partially selective solvent results in the recovery of nylon containing several impurities, and hence, having limited further use.

Melt blending of the entire carpet scrap generates a thermoplastic mixture that can be used for the manufacturing of a lower quality plastic material. Such a material can be utilized in less "demanding" products. The method consists of melting the entire carpet waste, without a previous separation into its components, to obtain a blend of different polymeric and inorganic materials. The low quality and lack of homogeneity of the resulting mixture are the main drawbacks of this method, restricting the number of applications in which its product can be used. Since carpets consist of several immiscible phases, compatibilizers (i.e. interfacial agents that decrease the surface tension and increase the interfacial adhesion between the different phases in a mixture) often must be added to the system in order to increase the miscibility of the various materials. The composition of the final product depends on the type and composition of the carpet recycled, varying significantly from one batch to another. Despite th ese problems, this recycling approach is still attractive because of its low cost and the utilization of the entire carpet waste, without a requirement for any prior separation.

Disassembling the face fibers off the used carpet material using different mechanical separation methods provides nylon 6 fibers that can be used in several applications. One example that has attracted some interest is their use in the reinforcement of concrete (7). Laboratory studies have shown that adding 1-2 wt% of short fibers separated from waste carpet to the concrete mix improves Its properties, such as its tensile strength and shrinkage.

Used together with landfilling, but to a much smaller extent, incineration is slowly being abandoned, mainly because of the air pollution problems it creates. The main idea behind incinerating this type of waste Is to at least recover some of the energy value that was input during the manufacturing of the carpet. Generally, this approach proves efficient when the energy needed to make the original material is less than twice the energy obtained by incinerating it (3). Otherwise, as is the case with polymeric materials, more advanced recycling techniques are preferred, which make use of the waste in a "smarter" way by transforming it into useful raw material for other applications. Hence, incineration of carpet waste is not regarded as a promising long-term alternative for carpet disposal.

In the following sections the technical advantages of each one of the most promising recycling techniques (depolymerization, extraction, melt blending, and mechanical separation) are reviewed in detail. An additional section is devoted to the current state of carpet recycling at the industrial level.

3.2 Depolymerization

During depolymerization, polymer chains are broken down into their monomeric constituents. A detailed description of the chemistry of the depolymerization of nylon 6 to its caprolactam monomer was presented by Agrawal in …

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