4. recycling of thermosetting polymers: their blends and ... fainleib.pdf · recycling of...

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Recent Developments in Polymer Recycling, 2011: 121-153 ISBN: 978-81-7895-524-7 Editors: A. Fainleib and O.Grigoryeva

4. Recycling of thermosetting polymers: Their blends and composites

Raju Thomas, Poornima Vijayan and Sabu Thomas

School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills P.O. Kottayam-686 560, Kerala, India

Abstract. In many applications thermosets are the materials of choice for long-term use because they are insoluble and infusible high-density networks. Recycling of thermosetting polymers is regarded as one of the urgent problems to be settled because of its technological difficulty. The increased production of thermoset blends and composites in recent years has greatly increased the amount of waste materials. The present chapter reviews the fundamental literature in the recycling of thermosetting polymers, their blends and composites.

1. Introduction

The use of polymer materials has simplified the modern life. At the same time, the extensive use of polymer materials in every walks of life have caused serious waste problems. The increased amount of polymer waste has become a serious issue globally and also caused depletion of petroleum resources without which the modern life become impossible for mankind. Among all the possible ways to manage polymer waste, a hierarchy could be established. The most preferred option is the minimization of waste, followed by reuse of materials in the same application, recycling in another application (including recovery of monomers or low-weight molecules), incineration with Correspondence/Reprint request: Dr. Sabu Thomas, School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills P.O., Kottayam-686 560, Kerala, India, E-mail: [email protected]

Raju Thomas et al. 122

energy recovery and finally incineration without energy recovery or land filling. A number of studies have been reported in the recycling of polymer wastes. The chapter deals with structure and properties of some important thermosetting polymers and their recycling studies.

2. Structure and properties of thermosetting polymers

2.1. Epoxy resins

The terminology ‘epoxy resin’ is generally applicable to both prepolymers as well as to cured resins. The former contain reactive epoxy groups whereas the cured resin may or may not contain reactive epoxy groups. While the term can be justified in the former case, the cured resins are also called epoxy resins. Epoxy resins typically contain a three membered ring with -O- atom. Different terminologies are also used to specify the group such as epoxide, oxirane and ethoxyline group, R CH CH2

O. Commercial

epoxy resins usually contain aliphatic, cycloaliphatic, or aromatic backbones. Epoxy resins are highly reactive presumably due to the strained three membered ring structures and react with many nucleophilic and electrophilic reagents. Therefore, a wide variety of organic compounds having active hydrogen atoms can be used as curatives. These include amines (both aliphatic/aromatic and primary/secondary), phenols, carboxylic acids, thiols, anhydrides etc. The general reactions of epoxy resin with these compounds are represented in Scheme 1.

CNH2 C+ R NHRHO

C C

O

OH+ R C CHO OR

alcohol

COOH+ R C CHO COOR

+ R SHC CHO SR

thiol

R

OO O C CHO

O

OC

RC

O

O

anhydride

acid

amine

Scheme 1. Reactions between epoxy and different curing agents.

Recycling of thermosetting polymers: Their blends and composites 123

Epoxy resins possess high resistance to chemicals and corrosion. Also, possess moderate toughness, flexibility and excellent mechanical and electrical behavior. It is also used as outstanding adhesives for different substrates. Epoxies are used in tooling, for laminates in flooring and to a small extent in moulding powders and in road surfacing. Epoxy resins are used for encapsulation of miniature components, particularly in space crafts. Epoxy resin laminates are useful in aircraft industry. Carbon fiber/epoxy resin composites are used for structural modification purpose in aeroplanes. Epoxy/aramid fibers find uses in the design of small boats. 2.2. Unsaturated polyester resins Linear unsaturated polyesters, which are often, called prepolymers find industrial applications. Unsaturation is introduced into the resin molecule using an unsaturated dicarboxylic acid such as maleic acid. For example, polyester of the following type is generated between ethylene glycol and maleic acid (cf. Scheme 2).

Scheme 2. Polyester from maleic acid and ethylene glycol. Commercial unsaturated polyesters are based on phthalic acid, maleic acid, ethylene glycol, and butanediol. The crosslink density, which represents the average number of crosslinks between polyester chains and the average length of the crosslinks, determine the mechanical properties of the product. The crosslink density, in turn, depends on the relative amounts of the unsaturated acids used to prepare the prepolymer. The average length of the crosslinks depends on the relative amounts of the prepolymer and monomer and on the copolymerization behavior of the two double bonds. For example, fumarate-styrene system yields a harder and tougher material than fumarate-methyl methacrylate system. The unsaturated polyester-styrene matrix is employed in fiber-reinforced plastics (FRP) structures. The resins are also useful for decorative coatings. The resin finds use in the manufacturing of large structures such as boats and car bodies since it is curable at room temperature. The powder form of the resin is used in solution or emulsion form as binders for glass-fiber performs and for the manufacture of pre-impregnated cloths.


2.3. Phenolic resins Phenolic polymers are obtained by the polymerization of phenol with formaldehyde [1]. The polycondensation reaction can be accelerated either by acids or by bases. The reaction yields resole prepolymers (resole phenolics) which are mixtures of mononuclear methylolphenols and various dinuclear and polynuclear compounds. Other products include substitution at o- and p- positions and the type of bridge between the rings (methylene versus ether). The typical ratio of formaldehyde to phenol is 1.2:1. Substituted phenols such as cresols (o-, m-, and p-), p-butylphenol, resorcinol, and bisphenol A are used for specific applications. Other aldehydes such as acetaldehyde, glyoxal, 2-furaldehyde are also used. The composition and molecular weights of the resole depend on the ratio of monomers, pH, temperature and other reaction conditions. For crosslinking a temperature as high as 180oC is necessary. During the curing process, methylene and ether bridges are formed between benzene rings to yield a network structure of the following type (Scheme 3).

OH OHCH2OCH2 CH2

OHCH2

CH2

CH2

O

OCH2

OH

CH2 CH2

CH2

CH2CH2

OH

CH2

OH

CH2

CH2

CH2

Scheme 3. Network structure formation in phenolic resins. Phenolic mouldings are hard, insoluble and heat resistant materials since they are highly crosslinked and interlocked [2]. The type of resin and filler influence the chemical resistance of the cured material. Cresol and xylenol- based resins are inert towards NaOH attack, whereas simple phenol-formaldehyde will be affected. Phenolic mouldings are resistant to acids except 50 % sulphuric acid, formic acid, and oxidizing acids, if the filler used is also resistant. The reins are stable up to 200oC.


Phenol- formaldehyde mouldings are widely used for domestic plugs and switches. Used in electrical industry where high electrical insulation properties are not needed. It is used for making cases, knobs, handles and telephones. In automobile industry, the resins are used for making fuse-box covers, distributor heads, and other applications where electrical insulation together with adequate heat resistance are needed. Heat resistant grade of the resins are used for saucepan handles, saucepan lid knobs, lamp housings, cooker handles, welding tongs, and electrical iron parts. Since the resin is hard and can be electroplated, it is used in the manufacture of ‘golf ball’ heads for typewriters. Bottle caps and closures are made from the resin in large quantities. Automatic compression presses and machines suitable for the injection mouldings of thermoplastics are manufactured out of phenol- formaldehyde resins. 2.4. Urea-formaldehyde resin It is an aminoplastic, a term generally used to represent resinous polymers formed by the interaction of amines or amides with aldehydes. The cured products form crosslinked insoluble and infusible thermoset. Compared to phenolic resins, the resins are cheaper, light in color, and have better resistance to electrical tracking. However, it exhibits higher water absorption and poor heat resistance. The reaction proceeds in the following route (cf. Scheme 4). The mono and dimethylol derivatives, formed during the reaction, further condense with urea to give the final resin structure.

NH2

NH2

HCHONHCH2OH

NH2

HCHONHCH2OH

NHCH2NH

C

C

O

O

+ CONHCH2OH

NH2

CO

NHCH2OH

NHCH2OHCO + NH2

NH2CO

NHCH2OHCO +

and so on

HCHO NH2CONH2 H NHCO NH CH2n

OHn + n H2O

H2O

Scheme 4. Urea-formaldehyde resin formation. There are many desirable properties for U-F moulding powders that enable to keep it in the highest application level. The wide range of colours is a reason for the widespread use of the material. U-F resins do not impart taste


and odour to foodstuffs and beverages with which they come in contact. Another added advantage is its good electrical insulation properties with particularly good resistance to tracking. The resin can resist continuous heat up to a temperature of 70oC. Some physical properties of urea-formaldehyde resins are tabulated in the following Table 1. The major application of urea-formaldehyde resin is in the field of electric and electronic applications. It is mainly used for making plugs, sockets and switches. In addition, it is used for domestic applications such as pot and panhandles and tableware. In the sanitary sector, the resins are used as toilet seats and miscellaneous bathroom equipment. The wide colour range and freedom from taste and odour make the material a good choice for the manufacture of bottle caps and closures. However, nowadays, its consumption in this area has been reduced by the development of new thermoplastics. Buttons are made from U-F moulding powders due to its resistance to detergents and dry-cleaning solvents. Miscellaneous uses include meat trays, toys, knobs, lampshades etc. The bulk of U-F resins are used as adhesives for particleboard, plywood and furniture industries. Another application of the resin is in the manufacture of chipboard. U-F resins are also used to make foams. U-F foams are used to place on airport runways to act as an arrester bed to stop aircraft that overshoot during emergency landings or abortive take-offs. Another large scale application of the resin lie in its manufacture of firelighters.

Table 1. Properties of urea-formaldehyde resins [2].

Property Units α-cellulose filled Wood flour filled Plasticized Translucent

Specific gravity — 1.5-1.6 1.5-1.6 1.5-1.6 1.5-1.55

Tensile strength 103lbf/in2

MPa 7.5-11.5 52-80

7-9.5 52-80

7-10 48-66

8-12 48-69

Impact strength ft/bf 0.20-0.35 0.16-0.35 0.16-0.24 0.14-0.2

Cross-breaking strength

103lbf/in2 11-17 11-16.5 13.5-15.5 13-17

Dielectric strength (90

oC)

V/0.001 in 120-200 60-180 100-200 70-130

Volume resistivity Ωm 1013-1015 1013-1015 1014-1015 —

Water absorption 24 h at 24

oC

30 min at 100oC

mg mg

50-130 180- 460

40-170 250-600

50-90 300-450

50-100 300-600


2.5. Melamine-formaldehyde resin Melamine and formaldehyde can also react to give methylol derivatives of melamine such as presented on Scheme 5:

C

N

NH2 HNCH2OH

NHCH2OHHOH2CHN

CC

N

N NH2H2N

+ 3 HCHOC

N

CC

N

N

Scheme 5. Formation of methylol derivatives of melamine. The methylol derivative with excess melamine undergoes polycondensation to give linear polymer, which forms three-dimensional network structure with further quantities of melamine monomer (cf. Scheme 6).

C

NC CN N

NHCH2NH

NH

NHCH2NH

NH

CH2

NHCH2NHNHCH2NH

CN

CC

N

N

CN

CC

N

N

C

NC CN N

Scheme 6. Melamine-formaldehyde resin formation. The M-F resins are characterized by superior properties. The mineral-filled resins are having low water absorption. The melamine resin is having better resistance to attaining by aqueous solutions such as fruit juice and beverages. Good electrical properties are maintained at elevated temperatures. Better heat resistance and greater hardness are the added advantages. They have a wide color range, track resistance and scratch resistance. Mineral-filled melamine based compositions have superior electrical insulation and heat resistance to the cellulose-filled grades. The resins are


used for the manufacture of decorative foils in compression moulding. The principal application of the resin is for the manufacture of tableware. A wide color range distribution, surface hardness and stain resistance are the reasons. Cellulose-filled compositions are used at small levels for the manufacture of trays, clock cases and radio cabinets. The mineral-filled compounding are used in electrical applications and knobs and handles for kitchen utensils. M-F resins are widely employed for laminating applications owing to their high hardness, good scratch resistance, freedom from color and heat resistance. They are also used as adhesives. Melamine-formaldehyde condensates are useful in textile industry. They are useful agents for permanent glazing, rot proofing, wool shrinkage control and, with phosphorus compounds, flame proofing. The resin can be used to prepare paper with enhanced wet-strength.

2.6. Polyimides Polyimides have the characteristic functional grouping of the following type [3, 4].

NCO

CO

The branched nature of the functional group facilitates the production of polymers. The backbone consists mainly of ring structures and hence high softening points. The polymers exhibit high thermal stability and hence valuable for high temperature applications. Aromatic polyimides are formed by the polycondensation of dianhydrides with diamines. For example, polycondensation of pyromellitic anhydride with p,p′-diamino diphenyl ether results in the synthesis of polyimides. The reaction is carried out in two steps. In the first step, the reaction is conducted in suitable solvents such as DMF, around 50oC, where polymerization takesplace with the formation of polyamic acid (cf. Scheme 7).

CO

COCO

COOO OH2N NH2n+

CO NH NHOOC

n

COOHHOOC

n

Scheme 7. Polyamic acid formation.


The polyamic acid is then casted as a film, by evaporating the solvent. It is then baked at 300oC in the atmosphere of nitrogen. Polycondensation takes place to form the following product in the second step. This product is converted in to the required shape. Polyimides, which can be either thermoplastic or thermoset, are widely used in aerospace applications. Thermosetting polyimides provide easier processing and higher thermal resistance, while thermoplastic polyimides offer greater toughness. A comparison of the properties of epoxy and polyimide thermoset matrices is furnished in Table 2. The polymer is having excellent resistance to oxidative degradation. Also inactive towards most chemicals other than strong bases and high-energy radiations. The principal application of polyimides is as compressor seals in jet engines. Also, used in data processing equipment such as pressure discs, sleeves, bearings, and as friction elements. They are also used as valve shafts in shut-off values. Due to the heat resistance capacity and deformation resistance of the polymers, they are used in soldering and welding equipment. However, the disadvantage of the polymer is that they may undergo hydrolysis and crack in water or steam at temperatures above 100oC. For such purposes, polyetheretherketones (PEEK) are employed.

Table 2. Properties of Composite matrices [2].

Property Epoxy Polyimide

Modulus, GPa 2.8-4.2 3.2

Tensile strength, MPa 55-130 56

Compressive strength, MPa 140 187

Density, g cm-3 1.15-1.2 1.43

Thermal expansion coefficient, 10-6 oC 45-65 50

3. Recycling of thermosetting polymers The use of plastic and other thermosetting polymers become enormous during the last decades. Hence, it becomes inevitable to clear them so that it may not turn into a major issue in the global conservation resources. The recycling of thermosetting polymers which are usually considered as


infusible and insoluble, aims to develop reworkable crosslinked polymer, which means crosslinkable-decrosslinkable systems. There are different ways of recycling treatments [5]. Incineration process with considerable non-combustible residues is one among them. The other includes a thermolysis process [6] with a poor value for decomposition products and a mechanical weakening [7]. Another method is mechanical recycling techniques based on granulation and comminution, leading to specific size fractions, which can be incorporated into new sheet moulding compounds (SMC) parts [8], in a thermoplastic matrix [9-11], or in concrete [12]. Solvolysis is another promising method for recovery of composite wastes. Solvolytic process of PET bottles are well-known and industrialized [13]. Chemical recycling process [14-18] was found to the most effective and promising method for thermosetting resins. 3.1. Epoxy resins It was reported that the possibility of recycling amine cured thermosetting resins [19] from epoxy-dissolved nitric acid was successful [20-21]. The decomposition behavior of amine cured bisphenol F type epoxy resin and its chemical recycling has been reported elsewhere [22-23]. Epoxy resin cured with amine hardener and its decomposition in nitric acid was investigated [24]. The purified residue was then recycled to prepare cured resin using an anhydride hardener. The yield of the recycled product as a function of time was studied by immersing in 4M and 6M nitric acid (cf. Figure 1).

Figure 1. Yield of decomposed products in 4M nitric acid solution, at 80oC [24].


It was examined that the resin decomposed rapidly and disappeared completely at about 100 hrs. The yield of the residue and extract was maximum at 50 hrs and 150 hrs, respectively. Thereafter it decreased. When the concentration of the acid solution was increased to 6M, the yield of the extract became higher as read from the Figure 2. The crystal was formed at 120 hrs due to the breakage of the main chain. In spite of the small decomposition rate and yield, 4M solution was found to be superior to 6M nitric acid. Size exclusion chromatographic analysis was employed to determine the molecular weight of extract collected from 4M and 6M solutions and the same is demonstrated in Figures 3 and 4.

Figure 2. Yield of decomposed products in 6M nitric acid solution, at 80oC [24].

Figure 3. Change of molecular weight distribution of extract with immersion time in 4M nitric acid solution, at 80

oC [24].


Figure 4. Change of molecular weight distribution of extract with immersion time in 6M nitric acid solution, at 80

oC [24].

Main peaks existed near log M=2.5, 2.7 and 2.73 for the extract collected from 4M nitric acid. For the extract from 6M nitric acid solution, prominent peaks observed near log M=2.5, 2.55, 2.7 and 2.77. The main product ‘extract’ was assumed to be a mixture of several lower molecular weight compounds and hence similar structures to monomer or dimmer of uncured BPA resin. In addition, the weight distribution from 4M nitric acid suggests that the residue is a mixture of intermediate products with higher molecular weight and can be decomposed again. Flexural strength and modulus of neat as well as recycled resin with different weight percentage of neutralized extract (NE) are furnished in Table 3. Flexural strength of recycled resin becomes maximum at a level of 10-20 % NE extracts. A repolymerization mechanism has been formulated in which the main chain of bisphenol A was easily broken since it possess quaternary carbon atom. Tertiary carbon atom is then generated by the attack of acid. The extract acts as a resin during repolymerization and reacts with curing agent to form crosslinked network. The fine network structure formed during the recycling resulted in rather improved mechanical properties.

Table 3. Comparison of mechanical properties [24].

Samples Flexural strength, MPa Flexural modulus, GPa Virgin resin 104.2 3.77

Recycled resin (5 %) 118.0 4.08




Figure 5. DSC scans of recycled resin in comparison with virgin resin [24]. Differential scanning calorimeter (DSC) was also employed to compare the Tg’s of recycled resin as well as that of virgin resin (Figure 5). Both show the existence of one Tg and also the Tg of recycled resin was higher than that of virgin resin which increased with increase in NE content. In a certain study [25], low-stress-type moulding epoxy resin powder containing silicone elastomer was subjected to recycling. The new recycled resin showed better thermal impact resistance than the original moulded resin. Besides, moulding resin powder was found to be suitable as filler for epoxy resin products such as insulating materials, paints and adhesives. The powder was also useful as a decorative agent for an acrylic-resin-type construction material. The study used two model-moulding resins. Model standard resin and model low–stress-type resin. The Table 4 shows the properties of original moulding resin and recycled moulding resin. The flow property of the recycled resin was reduced considerably (68 to 22 cm) where the standard resin powder was recycled into the original standard resin at a recycling ratio of 10 wt %. However, the flow properties improved using an epoxy resin with a melting temperature lower (55oC) than that of the original epoxy resin (70oC). But the moisture resistance and thermal impact resistance of the recycled resin were inferior to those of the original standard resin. The flow properties were also insufficient when the low-stress-type moulding resin powder was recycled in to the original standard resin. The property was improved where spherical silica powder was used, and up to the recycling ratio of 25 wt.%. In addition, heating the moulding resin powder (at 170oC for 8 hr) improved the flow property. From spectral analysis, it was


observed that the amount of epoxy groups in the low-stress-type moulding resin powder decreased by heating which lowed the surface reactivity. Nevertheless, the silane elastomer in the low-stress-type resin powder improved the thermal impact resistance of the recycled product. The mouldability, strength and insulating property of epoxy-resin type insulating material with the moulding resin powder were compared with reference material with fused silica filler and the results are tabulated in Table 5. The flexural strength and insulating property of the moulded material with the moulding resin powder were almost same as those the material with silica powder. However, the flow properties were not as good as with silica incorporated material. The surface treatment of moulding resin powder with silane coupling agents improved the strength and insulating property of the moulded material. This is because of the improvement in interfacial adhesion between the matrix and the resin powder. Epoxy silane coupling showed little effect compared to aminosilanes. Table 6 illustrates a comparative account of the strength and thermal expansion properties of epoxy resin moulded with moulding resin powder with that of compounds with conventional fillers such as CaCO3, talc powder and silica powder.

Table 4. Properties of original moulding resin and recycled moulding resin [25].

Original moulding resin

Recycled moulding resin Property

Standard resin

Low-stress-type resin

Standard resin with standard resin powder (10 wt.%)

Standard resin with low-stress-resin powder (25 wt.%)

Standard resin with low-stress-resin powder heated at 170oC for 8 hr

Mouldability Flow (cm) Barcol hardness Burr (mm)

69 74 0.6

73 58 1.0

68 58 1.3

47 55 1.3

61 62 2.3

Strength properties Flexural strength (kgf/mm2) Flexural modulus (kgf/mm2) Charpy impact strength (kgf/mm1)

14.7 1418 2.3

14.2 1240 2.3

17.3 1505 2.8

15.8 1391 3.6

14.2 1353 2.7

Thermal mechanical properties, Tg (oC) Thermal expansion coefficient, α1/α2 (10-5×oC-1)

155 2.0/6.5

171 1.8/7.4

139 2.2/6.3

154 2.0/6.6

154 2.1/6.3

Moisture resistance reliability 120oC, 2.3 atm, 200 hr 520 hr

0/10 1/10

0/10 0/10

0/10 4/10

0/10 0/10

0/10 0/10

Thermal impact resistance reliability, 150-60oC 100 cycles 150 cycles 200 cycles

3/10 5/10 8/10

0/10 0/10 0/10

7/10 9/10 -

0/10 0/10 1/10

0/10 0/10 0/10


Table 5. Properties of epoxy resin compounds with moulding resin powders for insulating materials [25].

Property

Silica powder 60 wt.%

Moulding resin powder 60 wt.%

Moulding resin treated with amino silane 0.5 wt.%

Moulding resin treated with amino silane 1.0 wt.%

Moulding resin treated with epoxy silane 1.0 wt.%

Mouldability Flow (cm) Gell time (s)

199 105

76 72

78 67

75 70

89 68

Flexural strength (kgf mm-2) Flexural modulus (kgf mm-2)

14.6 1057

14.8 696

16.1 722

16.1 742

14.0 678

Insulation resistance at 150oC (1013Ω cm)

4.0

2.0

24

22

4.0

Tg

141 142 138 143 141

Table 6. Properties of epoxy resin compounds with moulding resin powder for paints and adhesives [25].

Property None Moulding resin powder

Silica powder

CaCO3 powder

Talc powder

Average particle size of filler (μm)

— 16 10 9.2 11

Viscosity (Pa s) 2.5 48 14 12 206

Flexural strength (kgf cm-2)

817 800 787 610 577

Charpy impact strength (kgf cm-1)

2.3 2.4 1.6 2.5 2.5

Tensile strength (kgf cm-2)

440 474 428 336 280

Thermal expansion (10-5×oC-1)

13 11 11 14 9.5

Adhesive strength (kgf cm-2)

183 161 151 172 138

The strength of the moulded compound was comparable with that compounded with silica powder, but superior to those containing CaCO3 and talc powder. The thermal expansion was comparable with those having conventional fillers. Viscosity of the recycled resin was higher than those, which contain CaCO3 and silica powder, but lower, than the compound


containing talc powder. The adhesion property of the material containing moulding resin powder is almost equal to those containing conventional fillers. It was observed that the moulding resin powder is a useful decorating agent for resin-type construction materials. Table 7 furnishes the properties of moulded materials with the powders (maximum particle sizes, 1 and 5 mm). The strength of the materials with powders is sufficient for the construction materials even though they were not as good as that of the original material. The thermal deformation temperature and surface hardness of the materials compounded with powders were superior to that of the original material. Table 7. Properties of acrylic-resin-type construction materials with moulding resin powder [25].

Property Acrylic resin + aluminium hydroxide (65 wt.%)

Acrylic resin+ moulding resin powder(less than 1mm; 10 wt.%)

Acrylic resin+ moulding resin powder(less than 1mm; 20 wt.%)

Acrylic resin+ moulding resin powder less than 5mm; 10 wt.%)

Flexural strength (kgf cm-2)

550

450

430

360

Charpy impact strength (kgf cm-1)

1.5

1.6

1.7

1.6

Tensile strength (kgf cm-2)

270

290

290

230

Thermal deformation temperature (oC)

94

93

93

93

Surface hardness 6 7 8 7

3.2. Unsaturated polyester resins The solvolysis of sheet-moulding composites (SMC) of unsaturated polyester–styrene thermoset resin incorporated with glass fibers, calcium carbonate filler and thermoplastic poly vinyl acetate as an additive has been reported [26]. SMC are semi-finished products consisting of 25-50 mm chopped glass fibers (GF) and a paste typically of an unsaturated polyester (UP) diluted in styrene. It also consists of fillers (calcium carbonate), a thermoplastic polymer (as a low profile additive), a curing catalytic system, a thickener and a releasing agent. It was observed that aminoalcohols and polyamines allow much higher depolymerization yields, resulting to total


digestion of polymers. Diethylenetriamine (DETA) was chosen as the solvolytic reagent. Treatment of SMC chips with solvents resulted in a mixture which contains three fractions i.e., glass fibers, fillers and an organic liquid. Glass fibers and fillers did not contain much organic contaminations and the organic liquid can be used as a curing agent for epoxy resin. Solvolysis reactions were done with different solvents on chips of different grades of SMC chips. Results showed that the solvent reactivity depends on solvent in the order polyamines and aminoalcohols > glycols > diacid derivatives. Analysis of the solid fraction is summarized below (Table 8). The study proved that the dismantlement of SMC composites by solvolysis is possible, leading to a valuable liquid fraction and allowing the recovery of two inorganic fractions i.e., the glass fibers and the fillers. Table 8. Analysis of the large sized solid fractions after solvolysis of SMC chips [26].

Reactant MEAa) DETAa) DEGa) 15% NaOH Initial composition

Time, temperature 20 hr, 170oC 14 hr, 200

oC 19 hr, 230

oC

Organic matter 0.7 % 0.2 % 9 % 24 %

CaCO3 2.4 % 2.9 % 34 % 47 %

GFa) 97 % 97 % 67 % 28 %

a) MEA – monoethanolamine, DETA– diethylenetriamine, DEG – diethyleneglycol, GF– glass fiber 3.3. Phenolic resins Phenolic resins are commonly used as industrial adhesives and in heavy-duty automotive parts such as the plastic trim on car bodies and the plastic containers that hold air filters. The resins are formed when phenol and formaldehyde are cured at high temperature and pressure in the presence of catalysts, so that the molecular chains form an interlocking 3D structure that is hard to break. As a result, they cannot be melted and remolded like other plastics. Instead, most of the 2.2 million tons that are manufactured worldwide every year end up in landfills.


Material recycling is to collect and to reuse the waste plastics as raw materials. Hisashi et al. [27] proposed two methods for the material recycling of waste containing phenolic resin. One is to mix the wastes with virgin compounds to make recycled plastics; another is to reuse them as a filler for thermoplastics. The results obtained are as follows:

i) The suppression of generating dust particles could be attained by adding alcohol at mixing crushed materials with virgin compounds. ii) Injection moulding was tried using the recycled plastics, which contained 30 wt.% crushed materials. This gave us satisfactory results in appearance, mechanical property, and moldability. iii) Reuse of the wastes as a filler for thermoplastics was also tried. The heat distortion temperature of the resultant resins was improved by 3 %, compared with that of polystyrene moulding materials, though their tensile strength and flexural stress decreased with increasing content of the wastes. iv) It was confirmed that this recycling technology could be put to practical use and would contribute toward reduction of environmental burdens.

There are two difficulties for undertaking material recycling method. The waste plastics should be classified when they are collected to retrieve genuine regenerated plastics. Another is found in the degradation of properties during recycling. Thermal recycling is to convert the waste plastic to fluid fuels. Osaki et al. [28] carried out the granulation of phenolic resin scraps like sprues and runners from injection moulding. The granules were blended with a binder of polypropylene bumper waste granules in the weight ratio 30/70-60/40, then melt extruded and hot-cut. The processability was fairly good up to the ratio 40/60.Satisfactory performance in practical use of the refuse–derived fuel compared with coal or other solid fuel made from town refuse. Because this method includes combustion of the product to obtain heat or power, this cannot be an ultimate recycling of plastics from the standpoint of effective uses of resources. Chemical recycling of phenol resin and its compounds has been carried out by some researchers. Supercritical water (SCW) has been growing in importance as a medium for chemical reactions for about ten years. It is well-known that water under supercritical conditions is much less polar and can homogenize substantial amounts of nonpolar organic compounds [29-31]. Sub- or supercritical fluids have been focused as reaction media for environmental applications from a view point of green chemistry. Supercritical water has been applied to the chemical degradation of phenol


resin and its model compounds [32-34]. Suzuki et al. [34] decomposed seven prepolymers and molding materials of phenol resin into their monomers by reaction in subcritical and supercritical water under an argon atmosphere. Seven prepolymers of phenol resin were decomposed into their monomers such as phenol, cresols, and p- isopropylphenol by reactions at 523-703 K under an argon atmosphere in subcritical and supercritical water. The total yield of identified products depended on the kind of prepolymers, and the maximum yield reached 78 % in the reaction at 703 K for 0.5 hr. The decomposition reactions were accelerated by the addition of Na2CO3, and the yields of identified monomers reached more than 90 %. Two kinds of molding materials of phenol resin whose content of phenol resin was less than 50 % were also decomposed mainly into phenol and cresols by the reaction in supercritical water. They have confirmed that not only prepolymers of phenol resin but also molding materials of phenol resin were decomposed into their monomers by the reaction in subcritical and supercritical water under an argon atmosphere. The decomposition reaction of thermoplastic resin in subcritical and supercritical water was known; therefore, a chemical recycling process in supercritical water can be applied to the mixture of various plastics including thermoplastic and thermosetting resins. Possible merits of utilizing supercritical methanol over supercritical water are found in the following respects. First, the operation condition will be milder because the critical temperature and pressure of methanol are lower than those of water. This would widen the selection of materials for the reactors. Second, the separation of products from the solvent is much easier than the case using supercritical water, because the boiling point of methanol is lower than that of water. Additionally, alteration or modification of the product distribution would also be expected by changing the solvent. Ozaki et al. [35] studied the chemical recycling of phenol resin by supercritical methanol. The objectives of their study are (1) to know how the conversion of phenol resin is influenced by the reaction conditions and (2) to see what kinds of species are included in the liquid product. In this study, they conducted a preliminary study on the liquefaction of phenol resin by supercritical methanol. The resin is one of the most difficult substances for recycling, because it has a highly cross-linked network structure. The obtained conclusions are as follows:

i) Supercritical methanol could liquefy phenol resin, and the reaction was obvious above 400°C to give a conversion higher than 80 %. ii) Both reaction conditions of a longer time at a temperature and a higher reaction temperature for a shorter time resulted in increasing the


conversion. However, the former condition turned out to be favourable for obtaining a higher yield of liquid products.

iii) The liquid product was found to include phenols. iv) The analysis of the solid products revealed a concentration of carbon atoms during the reactions.

Phenolic resins, phenol-formaldehyde polymers are previously thought to be nonbiodegradable. The first demonstration of biodegradation of phenol-formaldehyde polymers were done by Gusse et al. [36] White-rot fungus is known to decompose organic pollutants such as DDT, TNT, PCBs, and dioxins and can be used to clean up these toxins from the environment. The fungus produces ligninase enzymes, which can break down lignin, the compound that makes up the dry part of wood. Researchers in the department of biology at the University of Wisconsin studied whether the fungus could also degrade phenolic resins, which have a molecular structure similar to that of lignin. They used a generic industrial formula to make the polymers and placed resin chips in cultures with the fungi. The several hundred species of wood-rotting fungi fall into two broad categories, white-rot fungus and brown-rot fungus. The researchers tested 11 different fungi strains, including 5 species of white-rot and 1 species of brown rot fungus. All of the species used in the new research have been previously studied for their ability to biodegrade pollutants. The researchers first realized that the white-rot fungus was degrading the phenolic resins when their colour changed from yellow to a light pink, the colour of the phenol and formaldehyde subunits used to make the resins, says Adam Gusse, a biology graduate student and lead author of the study. They confirmed the presence of those subunits by gas chromatography/ mass spectrometry and by locating pockmarks on the surface of the resin chips with scanning electron microscopy (Figure. 6).

Figure 6. White-rot fungi caused the pockmarks on the surface of the phenolic resin plastic shown in this scanning electron microscope image [36].


3.4. Urea-formaldehyde Urea-formaldehyde (UF) accounts for about 15 % of the total thermoset resin production. Currently, one of its major applications is in molded products, including electrical equipment, dinnerware, buttons, cosmetic caps, and bottles. However, the same factors that make UF a good choice for many applications, namely its chemical, thermal, and mechanical stability, are also what make recycling such a big challenge. Urea formaldehyde grit (UFG) fillers can be used effectively in blends with HDPE, as one of the possible applications for this recycled thermoset [37]. When compared with more traditional fillers, the UFG is lighter and less expensive, and the modulus gains are similar, although the UFG-based systems are likely to be more brittle. The stress–strain behavior at various filler levels is represented in Figure 7.

Figure 7. Stress–strain behavior of UFG-filled HDPE at various filler levels (wt.%) [37].

Figure 8. Influence of ionomer modification on the relative tensile modulus of UFG-filled HDPE. The ratio of ionomer to UFG was held constant at 1 to 10 [37].


More over the addition of zinc-neutralized ionomer can produce significant increases in modulus, at only a modest cost in elongation to break (Figure 8). 3.5. Melamine-formaldehyde Melamine-formaldehyde (MF) resin is used to impregnate paper for the manufacture of both high pressure and particularly low pressure laminates [38]. After the preparation either of the impregnated paper or of the final laminate at the hot press a considerable amount of paper impregnated with non-fully-cured melamine resins is produced by square-trimming the paper. The amount of this dry waste is considerable, running up to 400 tons/year in just a medium sized paper impregnating factory. The fact that the MF resins impregnating the paper might present some residual activity renders possible to consider their utilisation as wood panel binders. This study deals then with the use of melamine-impregnated paper waste either (i) directly, in powder form, as a binder for particleboard or other wood panels, or (ii) indirectly by incorporating the finely ground, powdered resin impregnated paper in the resin itself during its preparation. Powdered melamine waste paper can be used successfully and directly as a binder of particleboard satisfying the relevant standards specification for water resistant and interior grade particleboard according to the relative proportion of waste paper, hence of resin added, and the residual activity of the MF resin impregnating the paper. Equally, powdered melamine waste paper can be used to substitute melamine in the formulation and during the preparation of MUF resin adhesives for wood. 3.6. Polyimides Polyimides are commonly prepared by the reaction of tetracarboxylic acid dianhydrides with diamines. Many of the polyimide-forming chemicals, particularly the dianhydrides, are very expensive. So the recovery of organic values from polyimides, and more particularly to the recycle of useful polyimide forming chemicals, has gained importance. One method for recycling polyimides which comprises heating polyimides with a solution of an alkali metal hydroxide in a dipolar aprotic solvent for a long time and at a temperature and pressure effective to convert polyimides to at least one tetracarboxylic acid or functional derivative thereof and at least one diamine or functional derivative thereof [39]. The product obtained by reaction of the alkali metal hydroxide with the polyimide comprises the polyimide reinforcing agent(s), if any, a functional


derivative which is usually the alkali metal salt of the tetracarboxylic acid from which the polyimide is derived, and the amine from which the polyimide is derived. Reinforcing agent, especially fibrous filler, may be isolated by washing with organic solvent and water by filtering and drying. When so isolated, it is frequently suitable for recycle to prepare further reinforced polyimide. The amine may be recovered by dissolution in a suitable organic solvent such as ether and/or ethyl acetate to form a solution which may be washed with water, acidified and stripped for recovery of the amine. For recovery of the tetracarboxylic acid, the alkali metal salt may be acidified and extracted with an organic solvent. The organic extracts contain the acid, which may be isolated by conventional means and converted, if desired to a functional derivative such as the dianhydride. Mormann et al. [40] carried out the ammonolysis of bismaleimide thermosets. Polyimides prepared by free radical polymerisation after ammonolysis at 160°C give the corresponding amines and linear polymers with unsubstituted imide and diamide units as demonstrated by Scheme 8.

Scheme 8. Scheme of ammonolysis of bismaleimide thermosets. In terms of recycling, only the amines will be of immediate use while the linear polymers do not have applications so far. Beyond recycling, the analytical aspect is also important since characterization of the resulting linear polymer will give information on the degree of polymerisation of the maleimide moieties in the thermoset.


4. Recycling of thermosetting blends 4.1. Epoxy-thermoplastic blends A hybrid of thermosetting and thermoplastic resins improves the properties of the product during use while at the same time making it easy to decompose the resin at the end of its life cycle. The hybridization process involves blending the thermosetting resin with a small amount of thermoplastic resin, which can be broken down by organic solvents or heat, [41-42] schematically represented in Figure 9. Controlling the phase structure of the cured resin by changing the formation temperature, it is possible to obtain a thermosetting/thermoplastic hybrid in which the thermoplastic resin forms a continuous phase structure. A recent study on epoxy resin-PES (Polyether sulfone) blend having lower critical solution temperature (LCST) behaviors at higher formation temperatures (180oC) result in a continuous phase formation intermediate between thermosetting and thermoplastic resins. Comparative study of the decomposability by organic-solvent treatment with DMF of the cured unblended epoxy resin (Figure 10) with the cured epoxy/ PES hybrid (Figure 11), showed easy decomposition of later [42].

Figure 9. The concept of readily decomposable thermosetting resin [42].


5. Recycling of thermosetting composites Work on the recycling of thermoset composites can be divided into three categories. In most cases, thermoset composites are milled into very fine powders and are used as filler materials for polymers. For chemical recycling, the polymer is recovered as an organic compound, which may be used as a raw chemical material. Energy recovery means that the caloric content of the polymer matrix is usefully harvested by combustion. Most existing work on the recycling of thermoset composites emphasizes prepregs, but such technology could not be applied to cured systems.

Figure 10. Organic solvent treatment of cured thermosetting resin after treatment for 10 days at 23

oC [42].

Figure 11. Organic solvent treatment of cured thermosetting/thermoplastic hybrid after treatment for 50 hr at 23

oC [42].


5.1. Epoxy-glass fiber composites Disposal of plastic and glass fiber wastes is a serious solid waste problem [43-44]. By the enormous production of computer and communication hardware, epoxy/glass fiber waste become escalated. Thermoset glass fibre composites are milled into very fine powders and are used as filler materials for polymers. In a certain study [45], it was found that glass-fiber-reinforced epoxy resin boards could be used as fillers for epoxy resin products. They could yield products with better strength and thermal expansion properties than those made with conventional fillers. Concrete is one of the most promising substitutes for recycling epoxy/glass waste so as to minimize the environmental impact caused by direct land-fill disposal. Lee et al. [46] have evaluated the feasibility of adding epoxy/glass fiber waste in concrete and discussed its effectiveness and potential in construction application. The ground epoxy/glass particles (EG) were considered as fillers, in which coarser particles (φ>150 μm) partially replace fine aggregate and fine particles (φ<150 μm) acts as ultra fine fillers or supplementary binding material. A series of tests were conducted to assess the properties.

Table 9. Mix proportions (kg/m3) [46].

Mix no Water Cement ggbs Fly ash A0 225.0 262.5 90.0 22.5 A1 225.0 262.5 90.0 22.5 A2 222.3 262.5 90.0 22.5 A3 213.2 262.5 90.0 22.5 B0 225.0 225.0 77.1 19.3 B1 225.0 225.0 77.1 19.3 B2 221.0 225.0 77.1 19.3 B3 216.0 225.0 77.1 19.3 C0 225.0 196.9 67.5 16.9 C1 225.0 196.9 67.5 16.9 C2 222.0 196.9 67.5 16.9 C3 216.5 196.9 67.5 16.9

EG particles Mix no. (φ>150 μm ) (φ>75 μm )

Fine aggregate

Coarse aggregate

A0 0.0 0.0 890.7 763.8 A1 33.6 54.8 795.4 763.8 A2 66.7 108.7 701.6 763.8 A3 99.2 161.9 609.2 763.8 B0 0.0 0.0 941.5 763.8 B1 35.5 57.9 840.8 763.8 B2 70.5 114.9 741.6 763.8 B3 104.9 171.2 644.0 763.8 C0 0.0 0.0 979.6 763.8 C1 36.9 60.3 874.8 763.8 C2 73.3 119.6 771.6 763.8 C3 109.1 178.1 670.0 763.8


In their analysis epoxy/glass fiber waste were collected from copper-clad laminated printed circuit board after separating the metal magnetically. The chopped EG fiber wastes were ground to fine particles to mix with natural fiber aggregate. As observed by SEM the E-glass particles consists of filament shaped fibers with a diameter less than 10 μm and a length greater than 150 μm, which is surrounded by fine epoxy particles and epoxy matrix. In the recycled particles, the weight ratio of epoxy to glass is about 1.5. Epoxy/glass particles, in amounts of 10, 20 and 30 wt.% of fine aggregate, are separated into two parts based on the dividing size of 150 μm. Table 9 illustrates the detailed mix design. According to ASTM C39-99, C469-94 and C469-96 the specimens were tested at ages of 7, 28, 56 and 91 days for compression strength, splitting tensile strength and modulus of elasticity. The details are furnished in Table 10. EG particles, in general, improve concrete strength for a particular water/binder ratio. It is prominent in higher water/binder ratio mixes.

Table 10. Mechanical properties of concretes at various ages [46].

Compression strength (MPa) at different ages Mix no.

7 days 28 days 56 days 91 days

A0 13.6 28.3 31.9 33.2 A1 14.6 29.2 33.2 34.2 A2 14.0 30.3 35.7 36.9 A3 — 15.2 — — B0 9.1 20.9 25.9 27.0 B1 9.6 22.0 27.8 28.5 B2 11.3 23.6 28.9 30.7 B3 — 11.7 — — C0 7.1 15.1 18.8 20.1 C1 7.3 16.9 21.3 23.2 C2 8.9 19.1 23.6 25.5 C3 — 9.3 — —

Elastic modulus (GPa) at different ages Splitting tensile strength (MPa) at different ages

Mix no. 28 days 56 days 91 days 28 days 56 days 91 days A0 15.7 18.2 20.4 2.7 3.2 3.2 A1 17.2 19.5 22.2 3.1 3.4 3.7 A2 19.8 21.4 24.0 3.8 3.7 4.2 A3 5.1 — — 2.1 — — B0 14.5 16.3 16.9 1.9 2.7 2.8 B1 16.2 22.0 27.8 2.5 3.4 3.4 B2 11.3 18.8 20.2 2.6 3.6 3.7 B3 4.7 — — 1.6 — — C0 13.4 15.8 16.1 1.7 2.1 2.1 C1 15.9 17.1 19.5 15.9 17.1 19.5 C2 17.8 20.4 20.7 2.4 2.9 3.0 C3 4.4 — — 1.5 — —


The compressive strength index ( )''

fcfc control

, which represents the

ratio of compressive strength of EG specimen over the compressive strength of control specimen (A0, B0, and C0 mixes), shows that C1 and C2 mix increase about 10 and 20 % respectively, in comparison to the control mix. The filling or complementary cementitious effect of ultra fine EG particles increase in EG1 and EG2, whereas for EG3 a decline is observed due to the inhomogeneous mixture. In addition, the ratio of splitting tensile strength to compressive strength ( )'ft

fcincreases with the percentage of EG particles.

It was also noted that concretes with 10 and 20 % of EG particles have higher pulse velocity than control specimens for specified concrete ages. The absorption that reflects the condition of the concrete surface porous decreases with the increase in EG particle content. However, it was observed that all mixes using 20 % EG particles have almost the absorption of 4.1 % irrespective of the waster/binder ratio. Surface conditions were significantly improved ultra fine particles, in higher water/binder ratio mixes. The percentage of resistivity increases with EG particles in EG1 and EG2 mixes. Both results suggests that the proper use of fine EG particles can effectively modify the internal micro or meso porous system and reduce concrete permeability. The addition of EG particles shows improvement in sulfate resistance of concrete. Specimens with 20 % EG particles show a reduction of 50 % in weight loss. EG particles reduce concrete porosity and inhibit ions penetration. 5.2. Epoxy–carbon fiber composite Carbon fiber reinforced composites (CFRCs) have increasing number of applications in the aerospace industry and high-grade application products. It also provides the drivers for a recycling solution. Emerging technologies will focus on recovering long, high modulus fibres since this is the most valuable form of CF’s. Around 40 % of long fiber, pre-impregnated (“pre-preg”) material is wasted as off cuts during fabrication, so the recycling process presents, potentially, a high economic value [47]. Milled fiber products are readily available on the market and have little intrinsic value. Landfill taxes and the European Union (EU) end-of-life vehicle directive will penalize those industries failing to take up appropriate recycling technologies [48-49]. Adherent Technologies Inc. has developed a chemical recycling technology that allows the recovery of carbon fibers and also produces a phenolic oil mixture that can be used for the formulation of phenol formaldehyde resins [50].


Hernanz et al. [51] investigated the chemical recycling of carbon fiber reinforced composites (CFRCs) using sub critical and supercritical alcohols as reactive-extraction media. The epoxy resin that joins the fibers is degraded during the process, producing fibres that retain 85–99 % of the strength of the virgin fibres. The reactive dissolution of the resin is a non-steady process in which five main mass transfer steps are found (Figure 12). (1) and (2) are diffusion (or dissolution) of the reagent to the surface of the fiber (double-film theory), (3) is the reaction at the surface of the reinforced fiber, (4) is the diffusion (or dissolution) of the products to the bulk fluid and (5) is the external mass transfer by convection in the bulk fluid. The capacity of methanol, ethanol, 1-propanol and acetone as solvent-reagents for the chemical recycling of carbon fibre reinforced composites has been investigated. The process has worked in batch and in semi-continuous mode at temperatures from 200 to 450oC. The batch system did not have any agitation to avoid fiber damaging. Under these conditions, mass transfer was not favoured. Using a flow system in the semi continuous mode and an alkali as catalyst, the degradation process was improved by increasing the total rate of reaction. Flow rates was in the range from 1.1 to 2.5 kg-alcohol/kg-fiber/min and the addition of alkali catalysts such as NaOH, KOH and CsOH from 0.016 to 0.50M were enough to degrade more than 95 % of the resin in less than 15 min. The use of a flow reactor is recommended to enhance the

Figure 12. Mass transfer steps in the reaction–extraction process of the epoxy resin from CFRCs. Concentration profile in a single carbon filament [51].


Figure 13. SEM micrograph (4000×) of the recycled carbon fibres treated with 1-propanol at 350 oC. The fibres appear to be very clean and almost resin-free (98.0 wt.% eliminated resin) [51]. mass transfer without damaging the fibres. SEM analyses (Figure 13) and tensile strength tests on the fibres post-treatment showed clean fibres retaining 85–99 % of the strength of raw fibres. Preliminary GPC analyses assisted by NMR and FTIR spectroscopy were used to estimate the composition of the liquid degradation product stream, however a further investigation in this way is strongly recommended. Conclusions Recycling of thermoset polymers is a main concern of modern world and a number of methodologies have been developed in this area. The chapter is dedicated to recycling characteristics of thermoset polymers, their blends and composites. Structure and properties of some of the common thermoset polymers are discussed in the first half of this chapter. These include epoxy resin, unsaturated polyester resins, phenolic resins, urea/melamine–formaldehyde reins, silicone polymers and polyimides. These polymers have a wide spectrum of applications from domestic to industrial levels. The structure of these polymers, their desirable properties and specific applications are briefed. This is followed by the details of their recycling characteristics. Different types of recycling methods such as incineration, thermolysis and chemical process are available. The properties of original


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