Recycled Materials Pa

FPS 761 MECHANICAL PROPERTIES OF FIBERS PhD FPS - NCSU

Recycled Polyamides. 1/15

RECYCLED POLYAMIDES, A LITERATURE REVIEW AND RESEARCH OPPORTUNITIES.

Edmir Silva.

With the strength of the green movement increasing daily, fiber manufacturers had to adapt and become more creative, developing ways to save and improve the environment. This article will review the current state of the art of recycling polyamide, its implications and opportunities that exist covering more ground of scientific exploration in this field. Brief discussion on testing is made and focus is given for the mechanical properties where multiples researchers results are compared. Finally recommendations are made for future works in the field.

Keywords: Recycled materials; Polyamide; re-used materials; Nylon recycling;

Contact information: College of Textiles, NCSU, email: [email protected].

1. INTRODUCTION

Worldwide Textile Mill consumption of Nylon is averaging 3.5 million tons yearly since the 90s and new investments in China announced recently, will drive the number even higher. With such consumption levels, the industry is forced to develop ways to re-use, re-cycle or just using the market language, become green at some extent. This data comes from yearly strategic research done by Unifi Inc.. The research also shows that polyester has 10 times more annual consumption worldwide than Nylon, on average.

The Figure one is a break down by staple and filament fiber. The filament is separated further into carpet, industrial and textile. It is clear that over the years Nylon staple demand has been low in quantity, where the filament side has a good balance between carpet, industrial and textile. Some growth is being seen on the industrial side for high value end-uses. The industry believes that raw material prices and availability



are the biggest factors influencing the Nylon business; the competitive market against polyester and vice-versa is a constant battle. In this context the re-use of the polyamide is clearly a way to get an extra source of raw material, and enable claims for carbon footprint reduction what aligns both industry and market needs.

Figure 1. Graph on demand by type.

Was observed by (3) that the majority of Nylon (Polyamide 6 and 66) is used on carpet, the recycling of carpet was thought and patented first by DuPont in 1944, even though the recycling of a dirty carpet represents a challenge still today. The collection and sorting of materials are the biggest challenges for the supply chain on recycling for that various methods on sorting are used, being the most common the checking the melting point of the polymer and infrared or near infra-red technologies (IR). The IR being a fast, accurate and non-destructive test are by far the most used. There many ways to re-use the polymer and few studies are available in the literature. The work from (1) at USC (University of South Carolina) did list four classes, (2) and (3) agrees with it:

1. Chemical recycling or De-polymerization method to break down the long chain of polymer into monomers than can be re-polymerized, which possibly converts

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the waste into products having a quality equivalent to that of the virgin polymer. Polyamide 6 can be depolymerized to its monomer caprolactam, by acidolysis, hydrolysis, aminolysis or catalyzed-de-polymerization in vaccum. Whilst aminolysis is now the preferred route being used by DuPont, catalyzed depolymerization in vacuum has recently been developing into promising process. Companies recycling polyamide 6 and 6.6 by depolymerization includes: DuPont, AlliedSignal, BASF and Novalis Fibers. This classic system for the closed-loop recycling of carpets, which in theory can proceed forever.

a. Acidolysis Nylon 6 depolymerized using an acid catalyst, the cut nylon 6 waste is melted in a continuous reactor and treated with steam, the monomer is formed by hydrolysis. After distillation and filtration the caprolactam is recovered read for further usage.

b. Hydrolysis Depolymerization of PA6 in high-pressure steam reactor (AlliedSignal), PA6 hidrolytically depolymerized in an aqueous system under pressure give yields around 70% of caprolactam, see figure 2, the expensive part on this process is the distillation to remove water.

c. Aminolysis DuPont identified ammonolysis as the best depolymerization option for scrap carpet, yields on this process can reach 80% in theory. All the preparation work is also required (backing separation, removal of dirt and contaminants), followed by shredding, chipping, going through a hammer mill, screening and then grounded to particles of 1.5mm. Water is added to the material to form a slurry and then further separation by density is performed, reaching ratios of 98.5% purity, this material is transferred to the de-polymerization reactor. In the ammonolysis reactor, the nylon is mixed with ammonia gas and phosphate catalyst, what is separated later by distillation. This process is versatile and can even process copolymers.

2. Extracting recycling or Recovery of polymer components method to recover individual components of the polymeric mixture without reaching the monomer level. Includes multiples extraction and separation steps.



3. Mechanical recycling or Re-melting this method is the melt-blending of the entire structure. Used on carpets, for example, where a thermoplastic mixture is prepared from the melt-blending of the entire carpet waste, which produces a product with lower quality.

4. Thermal recycling or Energy generator which involves only energy recovery

during incineration of the polymer waste.

Figure 2. Diagram of Carpet Recycling (extracted from AlliedSignal Catalog).

An example of the chemical recycling is illustrated in Figure two. This recycle program, in use by AlliedSignal, defines that a new carpet is sold to a commercial building owner, when in need to be replaced a carpet installer removes the old worn carpeting and returns it to the carpet collector. The carpet collector uses a hand-held infrared scanner to sort and bale all the returned carpeting. A recycler picks up all the nylon 6 carpeting and takes it to the re-polymerization plant, where the worn carpeting is fed into the front end of the de-polymerization plant. Molded nylon 6 parts (even those with paint) can also be used. Super-heated steam is used to de-polymerize (chemically separate) the nylon 6 from the other components of the carpet, including the rubber backing. Finally the depolymerized nylon 6 is further processed to return it to its



caprolactam feedstock, which is then filtered for high purity and shipped to AlliedSignal polymerization plants to make their product branded INFINITYTM nylon 6 resin. Some considerations about the cited classes is that de-polymerization is the preferred route because it breaks the product into monomers which can then be re-polymerized into new high quality nylon. It can be argued that this process does cut some steps from the original monomer generation. When there is a blend and the nylon needs to be separated by a solvent, for example, it can be difficult to re-use the solvent since it will probably facilitate the dissolution of impurities that were on the outside of the polymer and hence limit the polymer usage or require filtering. In case of melting blends, where a different polymeric material is resultant, every time can restrict the future number of applications in which the product can be used. Blends result in batches and therefore quality levels can vary from time to time; still the possibility of avoiding separation makes this method attractive for some applications. In order to characterize what normally happens in the industrial processing of nylon, one has to understand how virgin and recycled material, blended with different ratios, behaves as raw material characterization, thermal and mechanical properties of the fiber. The work of (2) was the one that most approximate to a fiber production reality. The product variability is intrinsic to the amount of impurities present in the polymer since recycled products have more impurities than virgin materials which becomes an important point of control during the processing of such polymers. The work of (4) demonstrates this point and discusses that the durability and reliability of products using recyclate might be significantly reduced by the presence of impurities acting as stress concentrators. The absence of impurities is the key for reliable mechanical properties, therefore characterizing the size and concentration of impurities that the product can allow could be critical to the success of the processing of the recyclates. Degradation is another point that will occur in the process. The paper (5) recommended color readings to access the degree of degradation noting substantial differences after re-use of the polymer. In (6) the researcher observed the degradation phenomenon not only results in brittleness and deterioration of the mechanical properties of polymers, but also decreases stability and restricts the applications of the final



products. In the textile apparel for example, this would limit the shades that the fiber can be used in, normally light shades can be challenging.

2. TYPICAL PROPERTIES MEASURED

2.1. Moisture Content

Increase confidence of the drying process is normally checked before and after drying. The existence of moisture in melt spinning provokes hydrolytic scission of chains with consequent reduction on molecular weight and therefore, catastrophic reaction on fiber properties.

2.2. Color Readings

Degradation can have big impact on color and a way to perceive such changes is measuring the color of the chips or the as-spun fibers. The color readings of waste streams or raw material, as well as cutting edge inline color measurements (normally at the extruder, melted fiber), can enhance process control and allow containment in case of off-quality events.

2.3. Intrinsic Viscosity

Property checked in chips and as-spun samples using a capillary diameter viscometer. This measurement will enable the calculation of molecular weight. In order to determine material consistency, its recommended to combine rheology measurements with filtration measurements over time.

2.4. Density

Property measured on a density gradient column of carbon tethacloride with toluene, suggested three readings after 6h equilibrium. With the density the crystallinity fraction can be calculated. This measurement can indicate thermal



history in case there is significant variation in the readings between material to material.

2.5. DSC (Differential Scanning Calorimeter)

The curve provided in this test will allow crystallinity to be measured indirectly, glass transition region and melt point region on the samples analyzed, normally as-spun or drawn yarns. This test requires a considerable amount of time and precision to be able to use the results, one should not depend only on it, but use as a reference since it will detect significant molecular structural changes.

2.6. Birefringence

Provide clear information of the amorphous and crystalline regions hence orientation of the polymer. Normally defined by reflectance of light over the lamellas which will have 0 if they are perfectly oriented; normally fibers will vary around the 20 to 40.

2.7. Melt Flow Index

Measurements that can indicate melt viscosity changes and molecular weight changes working as a check against other measurements.

2.8. Linear Density

Property measured in yarns, normally Denier (g/9000m) or Dtex (g/10000m).

2.9. Mechanical Properties

For all the samples the tensile strength, modulus, and breaking elongation can be measured using ASTM 3822. This measurement will be used for comparison of



the many researchers reported in the literature. The well-known Instron machines or Statimat from Textechno are widely used in the industry and research laboratories today.

2.10. Shrinkage and Crimp properties

In case of yarns the shrinkage is an important factor and in case of textured yarns crimp is also good information. This paper will not discuss much of the thermal properties such as shrinkage and crimp, but since thermal history is rich in recycled materials, it is wise for the researcher to verify its effects mainly on properties that will require further thermal treatment. This paper discusses shrinkage and crimp of the yarn, but this should be carried out as far as the fabric dyeing and its thermal setting finishing processes before garment preparation.

3. DISCUSSION ON MECHANICAL PROPERTIES

For this paper the results of many authors experimental work will be presented in an indexed form. The initial property (the virgin material) value is set as an index one and the following values are the percent variation in relation to its virgin value. This

eliminates the need to state the units and allows an overall qualitative comparison. References are provided for further understanding of individual experiments performed. The properties that will be discussed are first, tensile strength, which is a measure of the steady force necessary to break a fiber and is given experimentally by the maximum load developed in a tensile test performed using ASTM 3822. The second is the elongation necessary to break a fiber, normally expressed as a percentage increase in length, also termed as elongation at break, using the same ASTM standard. The intention is to combine the available literature values into graphics that explain or indicate the to be expected behavior of the re-processing of polymer. By the end of the discussion an inclusion of waste will be presented, but the following graphs are re-processing of same polymer, which should demonstrate the thermal history behavior



existent on each polymer after re-processing. As in the well-known Boltzmanns

superposition principle, where his first conclusion was that the effect of load applied anytime continues forever, the thermal history suffered somewhat cannot be erased from the polymer.

Figure 3. Results on Tensile Stress versus number of cycles

As observed on Figure 2 obtained from (4), (5), and (6): From (4), while testing strength, the diameter was verified for aged and un-aged

samples showing limited influence on it. Since the exploratory work was considering influence of impurities diameter as well as cycles were verified that samples containing impurities above 100 micrometer, their tensile strength slightly decreases as the impurity diameter increases, which is expected as results of localized stress on the chains. Its also concluded that impurity sizes have a critical point for tensile properties and one should consider studying the impurity impact by its type. This material characterization can be visualized by a plot of Tensile strength versus (Diameter)(-1/2), suggested the author.

From (5), the tensile strength as a function of a number of processes, a small increment in the tensile strength property of PA6 is observed when the number of processes increases. The increase is considered significant by the author.

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From (6), the load at the yield point and draw point increases with increased cycles of injection molding. The overall increment in the yield stress from the 1st to the 16th cycle is approximately 25%, and a higher load at the cold drawing region is observed for the 8th and the 16th processed PA6 samples.

Figure 4. Results on Elongation at Break versus number of cycles

Figure 3 illustrates the elongation behavior from the work of (4), (5), (6), (7) and (8).

From (4), the elongation is similar or lower compared with re-extruded reference samples. Again, the existence of impurity explains the reduction on elongation and was also noted in a relationship of the elongation with the diameter of impurities, suggesting that there is a critical diameter after what cause catastrophic failure.

From (5), the re-processing has an obvious reduction on elongation, making it the most abrupt decrement.

From (6), the elongation at break is somewhat stable up to the 12th cycle and then has an abrupt reduction in contrast with all the other works. The author also noted the increase of the standard deviation after the 13th cycle on top of its value decrement, which can be the characterization of a non-stable polymer chain.

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From (7), relates its tensile properties decrease to be directly related to molecular weight reduction.

From (8), just note that this work was performed on nano-composites; the elongation decrease follows the other tests trends.

The mechanical properties are affected to a different extent as a result of reprocessing operations. The results show that there is an increment in tensile yield stress, flexural strength and consequently modulus, therefore it is possible to observe that the mechanical properties of PA6 were gradually modified from soft and tough to hard and brittle after each processing cycle. The modification in mechanical properties should be thus attributed to the macromolecular chain scission and broad chain length distribution, propose (6). During the recycling of the injection molding (5), a darkness patterning was observed from the 4th re-processing time increasing to the 10th time, therefore it concludes that the cause of decrement on physical-mechanical properties of PA6 was polymer degradation, and added further PA6 can be processed up to seven times without effecting its physical-mechanical properties and morphology; the only change registered was the color, but can be used if the end-use is directed only to highly pigmented items. Furthermore, the author does not recommend exceeding 10 cycles of re-processing, which is not in agreement with the practical world since this cannot necessarily can be measured. The observation made by (7) was that the degradation is due to the thermo-mechanical stress acting on the molten polymer. Re-processing of wet material provokes

a drastic reduction of molecular weight by hydrolytic chain scission, thus the usage of additives is recommended to remove water, making it possible to recycle PA (and other poly-condensation polymer) in wet conditions. The discussion proposed by (8) is interesting for its detailed measures taken of ductility of the material at break, as well as viscosity, molecular weight, reprocessing at different temperatures, and dispersion versus shear stress. The only pure textile driven work was presented by (2) where the approach differs from all the others by use of a % of waste combined with the virgin material, or namely a



blend, was observed that by the increasing the fraction of recycle PA6 in the samples, glass transition temperature decreases which suggested that amorphous regions were extended with increase in waste material. The analyses was made in two phases, as-spun and drawn fibers, the tensile properties of as-spun yarns such as tensile strength, breaking elongation, and modulus were made and statistical studies on the samples indicate that there is no significant difference between tensile properties of different samples at 95% confidence level. Therefore, they were consider to have similar behavior as can be seen in Figure 5.

Figure 5. Tensile Properties of As-spun Fiber versus blend

The second part of the study was performed on drawn yarns where statistical analyses of tensile properties of drawn samples show that strength and modulus of 0% (virgin material) samples is different from those of other samples at a 95% confidence level, but elongation at break of drawn samples are the same. As seen in Figure 6, tensile strength and elongation of virgin material are somewhat higher than yarns containing re-used PA6-drawn yarns, of course drawing as-spun yarns results in the increase of tensile strength and modulus with consequent decrease of breaking elongation, which can be explained by the orientation and crystallinity increase of as-spun yarn subjected to the drawing process.

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Further work was performed on textured yarns by (2), but potential miss conceptions about the texturing process could have misled the researcher. This paper will not discuss the results, in order to be fair to the author a series of questions were forwarded to him with no answers so far.

4. OPPORTUNITIES FOR FUTURE RESEARCH

The academic research carried up to now focused on the re-use of the same material over and over; just one or two groups explored the blends. In the industrial reality waste stream is not readily available and definitely not constant or homogenous. It is necessary to study and characterize waste streams and discuss methods to validate its quality over time in an online and non-destructive way. An example is to receive a material from a certified source and overtime verify its filtration levels and consistency as well as intrinsic viscosity variation; in case of Nylon, watch batch to batch moisture levels. Further than that can be the discussion of formulation of recipes where one can take multiple batches and combine them (blend) in ratios that can assure a reasonable consistency for the fiber producer. If the blend is considered, the number of cycles may not be a measurable factor since the material does not seem to carry an aging

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characteristic in regards to number of cycles suffered besides the degradation of its properties.

Little research consideration was noted in regards to the uniformity of the measurements or lack of sound statistical analyses on uniformity over re-processing which could have shown signs of degradation of polymer, which was confirmed when the color was considered. The only exception was work done by (2). A more robust statistical approach is therefore recommended in order to establish definite root causes and factors that truly need to be controlled, examples include: impurity levels, coloration, machine and process parameters, waste stream usage and its fraction on the entire blend, the nature of the blend itself and its uniformity. An important factor for a recycled product is its enriched thermal history which can be studied in the form of the polymer, yarn as-spun, drawn yarn or drawn-textured, dyeing and fabric framing or finishing. Its believed that the recycled products will generate a lot of challenges for all those involved in these processes. Studies need to verify ways to better treat those materials that have somewhat suffered some thermal stress in their past.

5. CONCLUSION

The paper discussed briefly the current state on polyamide recycling and its relevance in volume and technology. Emphasis was made on the various tests and procedures employed by different researchers on this field, citing and comparing the mechanical properties analyzed by them, overall was observed that there is no obstacle in recycling fibers. This work did not discuss the thermal properties but do recommend to not be forgotten by users of recycled fibers. Furthermore thoughts for further research are shared on the last part of the paper.



6. REFERENCES

(1) Mihut C, Captain DK, Gadala-Maria F, Amiridis MD. Review: Recycling of nylon from carpet waste. Polym.Eng.Sci. 2001;41(9):1457-1470.

(2) Meyabadi TF, Mohaddes Mojtahedi MR, Mousavi Shoushtari SA. Melt spinning of reused nylon 6: structure and physical properties of as-spun, drawn, and textured filaments. Journal of the Textile Institute 2010;101(6):527-537.

(3) Scheirs J. Polymer recycling : science, technology, and applications. New York: Wiley; 1998.

(4) Eriksson PA, Albertsson AC, Boydell P, Mnson JAE. Influence of impurities on mechanical properties of recycled glass fiber reinforced polyamide 66. Polym.Eng.Sci. 1998;38(5):749-756.

(5) LozanoGonzlez M. Physicalmechanical properties and morphological study on nylon6 recycling by injection molding. J Appl Polym Sci 2000;76(6):851-858.

(6) Su KH, Lin JH, Lin CC. Influence of reprocessing on the mechanical properties and structure of polyamide 6. J.Mater.Process.Technol. 2007;192:532-538.

(7) La Mantia FP, Curto D, Scaffaro R. Recycling of dry and wet polyamide 6. J Appl Polym Sci 2002;86(8):1899-1903.

(8) Goitisolo I, Eguiazbal JI, Nazbal J. Effects of reprocessing on the structure and properties of polyamide 6 nanocomposites. Polym.Degrad.Stab. 2008;93(10):1747-1752.

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