Ind. Eng. Chem. Res. 2004, 43, 6317-6324
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Chemical Recycling of Class A Surface Quality Sheet-Molding Composites Marie Vallee, Gilles Tersac,* Nade` ge Destais-Orvoen, and Ge´ rard Durand Laboratoire Chimie & Ge´ nie des Proce´ de´ s, Ecole Centrale Paris, Grande Voie des Vignes F92295 Chaˆ tenay-Malabry Cedex, France
This article describes the solvolysis of sheet-molding composites (SMC) consisting of an unsaturated polyester-styrene (UP) thermoset resin associated with glass fibers, filler (calcium carbonate), and a low-profile additive (LPA) (thermoplastic poly(vinyl acetate)). Preliminary studies have shown that glycols, diacids, and bisphenols are poor solvolytic reagents. On the contrary amino alcohols and polyamines allow much higher depolymerization yields, leading to a total digestion of the polymers. Diethylenetriamine (DETA), at boiling temperature (205 °C) was then chosen as solvolytic reagent. Pure poly(vinyl acetate) (PVAc) is readily converted to soluble products. A total dissolution of cured UP requires high yield of ester cleavage. Longer reaction times are needed with styrene content enhancing. The introduction of PVAc in the UP resin does not alter the kinetics of solvolysis of the thermoset but affects the rheological behavior in intermediate states and enhances the viscosity of the final liquid. A greater effect is observed with higher molecular weight poly(vinyl acetate). Treatment of SMC chips with DETA at 205 °C for 10-14 h leads to a mixture that can split into three fractions, viz. glass fibers, filler, and an organic liquid. Organic contamination of glass fibers and fillers is very low. The organic liquid may be used as a curing agent for epoxy resins. The course of the solvolytic process is affected by the nature of the LPA and the original sizing of the glass fibers. 1. Introduction Sheet-molding compounds (SMC) are semi-finished products associating 25-50 mm chopped glass fibers (GF) and a paste consisting typically of an unsaturated polyester diluted in styrene (UP), calcium carbonate (filler), a thermoplastic polymer acting as low profile additive (LPA), a curing catalytic system, a thickener, a releasing agent, and several other additives. SMCs can be molded in one step at a rate approaching that of the steel-stamping operations. This characteristic, coupled with favorable properties of the cured materialssuch as high strength-to-weight ratio, corrosion resistance, and metallic appearance when paintedshas made SMCs popular in the automotive and electrical industries. Nevertheless, one major obstacle to the use of SMCs is the difficulty of recycling such reinforced thermoset waste because of the inherent infusibility and insolubility of their structure. However, owing to European legislation regarding waste issued from end-of-life vehicles and electrical materials, recycling technologies are urgently needed. Typical ways of waste treatment1 are the following. (a) Incineration processes, but with considerable noncombustible residues. (b) Thermolysis processes2, but with a poor value for the decomposition products and a mechanical weakening.3 (c) Mechanical recycling techniques based on granulation and comminution, leading to specific size fractions that can be incorporated in new SMC parts,4 in a thermoplastic matrix5-7 or in concrete.8 However, these fine granulates behave as fillers and not as reinforcing fibers (which means that the inherent value of the fibers is lost). Alternatively, solvolysis is a promising route for recovery of composite waste. Solvolysis (chemical depo* To whom correspondence should be addressed. Tel.: + 331-41131191. Fax: + 33-1-41131597. E-mail:
[email protected].
lymerization) breaks cleavable bonds contained in the backbone of step-growth polymers by solvolytic processes such as alcoholysis, glycolysis, hydrolysis, and aminolysis. Solvolytic processes of PET bottles are wellknown9 and industrialized, leading back to monomers or specialty products (polyols, unsaturated polyesters, alkyds, plasticizers, etc.). Efficient depolymerization of the organic matrix could result in dismantlement of the composite, leading to two streams of valuable matter: (a) the solids, consisting of reinforcing fibers and the other inorganic parts, and (b) an organic liquid consisting of a mixture of polymer degradation products and excess reactive solvent. Chemically, the UPs result from the polycondensation of glycolsstypically propyleneglycol (PG)swith maleic anhydride (MA) and possibly other “saturated” diacid derivatives such as phthalic anhydride, isophthalic acid, or adipic acid. After cross-linking with a vinyl monomer like styrene, a copolymer network results (an idealized structure is shown in Scheme 1). As a consequence, the cleavage of ester linkages would result in a mixture of a phenylethylene-succinate oligomers, glycols, and derivatives of saturated acids. Some processes of solvolysis of cross-linked UPs have been reported. Although UPs are polyesters, it appears that their depolymerization requires drastic conditions. However, the characterizations always show10-12 that the formed phenylethylene-succinate oligomers are of low molecular weight (10 h). The kinetic curves (see Figure 2) show initially a fast release of free glycols until ca. 60% yield, with a rate nearly independent of the styrene content. The remainder of free glycols is gradually released during stages 2 and 3, more slowly for the styrene additived resins. The visual observations (Table 4) confirm that an increase in styrene percentage results in a slackening of the resin digestion. In all cases, the free glycol content reaches an asymptotic value, in good accordance with the theoretical amount for quantitative ester solvolysis. At equal styrene content, very similar kinetic curves were obtained for the three resins. A slightly lower reactivity of the MA-AdA/PG resin could be noticed from visual observations. The lower reactivity of styrene-rich resins could arise either from different swelling properties of the networks or from a lower solubility of oligomeric degradation products, or also from a lower concentration of cleavable ester linkages: the molar ratio of styrene/diester moieties is ca. 0.7 for unmodified UP, viz. ca. 2.0 for the styrene additived one. 3.4 DETA Solvolysis of Unfilled UP Resins Cured in the Presence of LPA. Knowing that PVAc is readily solvolyzed to a soluble mixture of PVOH and DETA acetamide, what is the effect of PVAc contained in SMC on solvolysis?
Figure 3. Free glycol release kinetic curve for DETA solvolysis, at 205 °C, of unfilled UP cured in the presence of LPA (DETA/ chips ) 2:1 w/w).
To answer this question, comparative solvolysis experiments were carried out with unfilled materials obtained by curing of UP-LPA mixtures. The results are summarized in Figure 3 and Table 5. At first, solvolysis of a resin 1/LPA 1 system shows that PVAc does not alter the PG release kinetic. However, there is a noticeable effect on the reaction mixture aspect. The fluidification time increases from less than 3 h to 6 h. During stage 2, much higher viscosity is observed and the resin grains, which were initially opaque, turn gradually to translucent, which could be attributed to a slow diffusion of PVAc or PVOH into the liquid phase. The final mixture after 24 h reaction is homogeneous but the dissolved PVOH leads to a more viscous fluid. Then, solvolysis of a resin 2/LPA 2 system was carried out. LPA 2 contains divinylbenzene. A homogeneous liquid cannot be obtained. The size of the solid grains decreases very slowly, to obtain, after 60 h reaction, a granular suspension of very fine particles that turns to a solid paste by cooling to room temperature. However the formation of free PG proves the effectiveness of ester groups cleavage (see Figure 3; note that the initial PG moieties content of this polymer is slightly lower than that for the others). Thus, the supplementary crosslinkage engendered by DVB prevents thermoset swelling and gel formation. Even after the ester linkage cleavage, the resin keeps a three-dimensional network structure. SEC characterization trials gave poor results. Samples of the crude solvolysis product were dissolved in tetrahydrofuran and eluted with the same solvent. Considerable peak tailing was observed. The only observed peaks correspond to molecular weights lower than about 500 g/mol. However, PVOH is not soluble in tetrahydrofuran and was not detected in similar analysis of the solvolysis product of pure PVAc. 3.5 DETA Solvolysis of SMC. Similar DETA solvolysis experiments with the four SMCs (see Table 1) were carried out. For a proper immersion of the SMC chips in the liquid phase, the weight ratio of solvent to solids may be enhanced from 2:1 to 4:1. Moreover, the polyester amount in SMC is only about 9%. Thus, the concentrations of free glycols released in the liquid phase are much lower and the accuracy of their GC determination may be estimated as only about 10%. Figure 4 shows that the free glycols release is more progressive than was observed with the unfilled model polymers (Figure 3). This apparent lower reactivity, especially at initial time, could result from a greater
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Table 5. DETA Solvolysis, at 200 °C, of Unfilled UP Resin Cured in Presence of LPA (DETA/Chips ) 2:1 w/w) Resin 1 no LPA
reactive systema
Resin 1 LPA 1
Resin 2 no LPA
PVAc wt % polyester wt % styrene to diester mole ratio
0 43 1.95
15 36 1.92
0 46 1.83
fluidification time aspect after 8 h reaction
2.75 h fluid many gels 76%
6h very viscous many gels 97%
3-3.5 h fluid many large gels 79%
approx. yield after 8 h reaction a
Resin 2 LPA 2 19 34 1.91 (+ 0.21 mol DVB) no pasty intermediate state white grains 88%
See Table 1.
Table 6. Effect of the LPA Composition on the DETA Solvolysis of SMC at 200 °C (1 × 1 × 0.3 cm3 chips; DETA/SMC ) 4:1 w/w)
a
SMC Da
SMC A
SMC Bb
aspect after 4 h reaction aspect after 6.5 h reaction aspect after 8 h reaction
very slightly swelled chips slightly swelled chips highly swelled chips abundant powder
some slightly swelled chips
GF organic contamination after 24 h reaction
0.4%
swelled chips highly swelled chips highly swelled chips many visible GF abundant powder 0.2%
swelled chips few visible GF abundant powder 12%
Contains high molecular weight PVAc. b contains DVB.
difficulty for the reactive solvent to enter the bulk material due to the high content of fillers in the composite. Other experiments with and without DBTO have confirmed that this compound has only a low catalytic effect. Comparison of the behavior of SMC A and SMC D shows the effect of molecular weight of PVAc, which is twice higher in composite D. Although similar reactivity of the (unfilled) UP parts of both SMCs was previously observed, the free glycols release rate is lower with the higher molecular weight PVAc, including the initial rate (see Figure 4). Lower rate of polymer swelling is also observed (see Table 6). This behavior could result from a slackening of either the diffusion of the solvent into the polymer matrix, or the diffusion of the reaction productssfree glycols and PVOHsfrom the matrix into the liquid phase. Finally, in both cases, after 24 h reaction, CaCO3 powder and glass fibers almost free of organic contamination were obtained. As expected, dismantlement of composite SMC B, which contains DVB, cannot be fully achieved (only organic contaminated filler powder may be recovered). Kinetics curves for SMC A and SMC B show nearly identical PG release rates in the initial step, but lower reactivity of SMC B in the later steps. Then, we have studied another composite, SMC C, which has the same composition as SMC A, except the sizing nature of the glass fibers. The PG release kinetic curves and the behavior during solvolysis processes are almost identical (apart from stirring problems, see below), leading finally in both cases to glass fibers almost free of organic contamination. However, the two systems differ greatly by the final aspect of the fibers (see Figure 5). In SMC A, the white GF monofilaments (diameter ca. 20 µm) are agglomerated in egg-shaped, rather dense, bundles. Inside the bundle most of the filaments are parallel to each other, whereas they are randomly oriented on the bundle surfaces. There are also several broken fibers (0.5 to 1 mm). In SMC C, the white GF monofilaments (diameter ca. 20 µm) are arranged in bundles having a low apparent density and a shape similar to the original chips. The high volume of the bundles can induce some stirring problems during
Figure 4. Free glycol release kinetic curve for DETA solvolysis of SMC at 205 °C (1 × 1 × 0.3 cm3 chips, DETA/chips ) 4:1 w/w).
the solvolysis. There are very few short fibers. Most of the filaments are parallel to each other and the bundle aspect is similar to those obtained by pyrolysis/CaCO3 acid digestion treatment of SMC. The fact that monofilaments are obtained proves that in both cases the original sizing (filament bonding agent) has disappeared, but this disappearance seems to occur later in the case of SMC C. Thus, in the initial times the released glass fibers keep arranged in strands, which protects them from mechanical damage due to stirring (and shearing). On the contrary in SMC A glass fibers are soon released as loose filaments which can separate from the chip, being sometimes broken, and reagglomerate around the partially solvolyzed chips to form egg-shaped bundles. 3.6 Solvolysis of Ground SMC. Solvolysis of large chips of SMC allows recovery of long fibers with a length similar to that of the initial chopped fibers. Mechanical comminution of SMC is an industrial recycling process. Solvolysis of comminuted fractions of SMC could also be of interest, for instance to valorize a polymer rich fraction,13 or to purify a short fiber rich fraction. Moreover, an enhanced reactivity could be hoped for. Thus, some DETA solvolysis trials were carried out. Three fractions of ground SMC A were investigated and
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Figure 5. Glass fibers bundles from SMC solvolysis at 205 °C (1 × 1 × 0.3 cm3 chips, DETA/chips ) 4:1 w/w (diameter of the money piece ) 2.1 cm): (a) SMC A, (b) SMC C. Table 7. Effect of Granulometry on DETA Solvolysis of SMC A (DETA/SMC ) 4:1 w/w) 1 × 1 × 0.3 cm3
0.4 < d
