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Recycling SMC scrap as a reinforcement.

Recycling SMC Scrap as a Reinforcement

Bulk molding composite (BMC) and sheet molding compound (SMC) consist of thermosetting unsaturated polyester resins reinforced with glass fibers and typically filled with calcium carbonates. These outstanding engineered materials are used to produce extremely durable and environmentally stable products noted for design flexibility, economical manufacturing, and resource conservation by virtue of weight reduction and low energy requirements for production, while also providing cost effectiveness and pleasing esthetics for the consumer.

The ultimate disposal of plastics in durable goods, such as glass-reinforced automotive thermoset composites, has not received major attention in the U.S. In contrast, significant technical and political efforts are under way within the European Economic Community to demonstrate the recyclability of thermoset composites. This level of effort will be duplicated in the U.S. in the next few years because it is in the strategic interest of both raw material suppliers and molders to take an active role in defining methods for the disposal of their products. If viable technical approaches to recycling thermoset composites can be demonstrated, their growth should continue.

Approaches to Recycling

The chemical, thermal, and mechanical stability of thermoset composites--which makes them the material of choice for many applications--also represents the challenge to recycling. Unlike thermoplastics and metals, thermosets cannot be simply remelted, but must be processed differently. Several technical approaches to disposal or recycling have been proposed; many deal only with the polyester resin, the organic portion, which makes up only 20% to 25% of a BMC or SMC formulation.

* Incineration--burning alone or as part of a larger waste stream with the recovery of the energy content of the cured polyester resin as heat.

* Chemical degradation--hydrolysis, glycolysis, or saponification of the cured resin with recovery of the organic raw materials for their chemical value.

* Pyrolysis--destructive distillation, directly or with molten salts, of the organic polyester with recovery of oil and gas for their energy content.

* Size reduction--mechanical particle size reduction of the cured composite with direct reuse of the mixture of glass fiber, filler, and ground cured resin. Reuse in new plastic composites is the most straightforward approach. However, a variety of other applications have been proposed, including use in paving and roofing asphalt, and in portland cement and polymer concrete.

This article describes a recycling process based on size reduction to produce a new raw material with maximum value. It has three primary objectives: first, to demonstrate techniques for mechanically processing BMC and SMC scrap--using the entire part without resorting--into a form suitable for reuse in composites; second, to develop formulations that incorporate the recycled thermoset into new composites, taking advantage of its glass reinforcement content; and third, to begin a database of the properties of parts molded from these formulations.

Steps in Recycling SMC

1. Collection. Three important sources of automotive BMC and SMC that must be dealt with are: 1) molding shop scrap consisting of start-up compound and off-specification part; 2) assembly shop scrap consisting of off-specification and damaged-in-process parts; and 3) composite parts from decommissioned vehicles. Efficient separation and collection--clearly the key to a commercially successful recycling program--must be addressed for all plastic components, especially for the third source, decommissioned vehicles.

2. Size Reduction. SMC parts as large as 15 to 20 [ft.sup.2] are not uncommon. Existing equipment can handle these sizes, shredding the SMC into pieces generally smaller than 2 x 8 in. Granulation to an appropriate size follows.

3. Formulation and Compounding. The entire sample of granulated BMC and SMC may be used as part or all of the reinforcement and filler in a BMC, SMC, or thermoplastic (TP) compound. Key criteria that must be considered are compound production, handling, moldability, and finished part mechanical properties.

4. Molding. Standard molding equipment with minor modifications has been used and found to be adequate.

Business Economics

The costs of physically manufacturing a part with recycled material are one factor in the long-term economics of recycling BMC and SMC. These direct manufacturing costs--steps 2 through 4 above--are readily developed. The economics of reduction to a size usable in BMC is outlined in Fig. 1. Premix, Inc., has itemized costs of using recycled materials as fillers in their molding compounds. During their tenure as a captive molder, the Harrison Radiator Division of General Motors consistently used regrind materials in production of BMC automotive parts.

Beyond these examples, however, there have been few attempts at commercialization of recycled thermoset composites. The first barrier is that of existing costs--today it is cheaper and easier to landfill scrap material than to recover it through recycling. As the price to landfill continues to rise, this becomes an opportunity cost for recycling economics. The second barrier is the establishment of a collection and sorting infrastructure. This is, of course, true for all industrial plastics and is being addressed by automotive OEMs, material suppliers, molders, and auto recyclers in both the U.S. and Europe.

Experimental

After removal of any metal fittings, a MAC "Saturn" shredder, equipped with a hydraulically driven, counter-rotating shredder head, was used to reduce a variety of large parts to approximately 2- x 8-in chunks. Further size reduction could be done by a variety of methods, such as ball or hammer milling, cryogenic grinding, and knife granulating. A rotating knife granulator (Nelmor Co.), with a range of screen sizes from 1-1/4 to 1/8 inch, was used. After a visual examination of the product, the 3/8-in (coarse) and 3/16-in (fine) screens were used to produce several thousand pounds of granulated SMC.

Both coarse and fine granulated materials were used as received in BMC and in TP composites. The fine particles (<8.2 x [10.sup.-3] in) were separated by additional screening and evaluated as a filler in SMC. BMC formulations are shown in Table 1. Polyethylene (Dow 08054N) and polypropylene (Himont SB-786) were used with granulated SMC loadings of 15%, 30%, and 50% by weight.

Thermoset BMCs (TS-BMC) were produced in a conventional Baker Perkins sigma blade mixer and molded in a conventional compression molding press into flat plaques and other parts. A dry blend of TP pellets and granulated SMC was plasticized into a thermoplastic BMC (TP-BMC) in a 100-mm diameter, reciprocating-screw extruder (Rose Machinery Co.) A "guillotine" cutoff allowed "logs" of variable length to be produced. The logs were "stamped" in cold molds in a fast closure hydraulic press.

The flat plaques were used in mechanical and physical property evaluations (Tables 2 and 3). Other parts that were molded and visually inspected included automotive radiator housings, 10- x 10-in trays, 6-in-diameter disks, 2-in x 4-in x 4-ft "lumber," and 2- and 6-in flower pots.

Shredding/Granulation

The size-reduction steps were rather straightforward. The shredder had more than sufficient power to shred SMC parts. Depending on initial part size, it may be beneficial to have a following ram in the hopper feeding the shredder. Also, because shredder cost rises substantially with increasing size, cutting parts in half prior to shredding may be economical for some molding facilities. The throughput for the granulation step depends upon the final material size required and the screen size used.

The coarse regrind appeared visually to retain a relatively high degree of glassfiber bundle integrity, whereas the fine regrind did not. These glass fibers can provide some reinforcement in the new composite.

The finer regrind had some "fuzz ball" clumping, typically seen with more intensive physical working of fibers such as occurs in hammer milling. Both materials were separated through standard sieves to determine their particle-size distributions. The results (Fig. 2) show relatively broad distributions. As produced, 25% and 45% of "fine-filler" type material (<8.2 x [10.sup.-3] in) was present in the coarse and fine regrind, respectively.

The interactive economic model for a size-reduction scheme using a shredder and a granulator having a 1000-lb/hr capacity (Fig. 1) yielded a calculated cost of $0.028/lb, not including collection, transportation, or packaging. It therefore models costs of an in-house scenario.

Formulations

The granulated SMC has a high resin demand compared with conventional fillers such as calcium carbonate. Its use thus requires reformulation. A typical TS-BMC (Table 1) contains about 20-wt% resin. In contrast, approximately 30-wt% resin was required when granulated SMC was used as 100% of the filler and reinforcement.

Similar results were observed in SMC formulation. When the "fine filler" obtained from sieving was substituted for calcium carbonate, resin-to-filler ratio was about 1/1.25 vs. 1/2.5 obtainable with calcium carbonate for the same relative paste viscosity. A more suitable way to use the regrind and the fine-filler portion would appear to be as a 5% to 10% substitution for the conventional fillers.

When a TP matrix was used, the melt index of the TP was an important factor in the wet-out of the reground SMC. Based on our experiments, a melt index of 7 or 8 or higher appears to be necessary. This may indicate that a mixed stream of recycled thermoplastics may not be a suitable matrix for incorporating granulated SMC materials.

Compounding/Molding

Both the coarse and fine granulated forms appeared suitable for use in TS-BMC compounding equipment. When the filler and reinforcement was 100% regrind, the resultant mix had a relatively "dry" appearance, lacked cohesiveness compared with standard BMC, and did not hold its shape well. In addition, the compound had a lower bulk density, which necessitated that the BMC be made in smaller batches.

If the granulated SMC were recycled back into SMC, a large number of carrier film tears could be expected unless the larger particles were removed by some initial screening.

Dry blending of the regrind with thermoplastic pellets caused fiber bundle degradation and "fuzz balls." The fuzz balls were partially dispersed by increasing the back pressure in the screw during compounding in the extruder. There was also some tendency for bridging in the 3-inch-square feed throat to the screw.

Both the thermoset and thermoplastic compounds were molded satisfactorily with conventional molding presses and molds.

Molded Part Evaluation

Table 2 shows that there is an across-the-board reduction in the mechanical properties of TS-BMC made with granulated SMC when compared with the control containing 20% 1/4-in chopped glass strand. Strengths and Izod impacts were the most affected.

In contrast, most properties of the polypropylene composite showed improvement over those of the neat resin control (Table 3). The granulated SMC acts much as a filler. The modulus was improved by a factor of 2 to 3; elongation and impact exhibited some loss.

Visually, surface quality deteriorated with increased loading of regrind in both thermoset and thermoplastic systems. The TS-BMC molded parts had a porcelain, granite-ware appearance.

Conclusions

SMC can be recycled. The use of shredded and granulated SMC as a reinforcement and filler in new thermoset and thermoplastic composites has been demonstrated. The mechanical properties of parts molded from such composites are not equivalent to those of standard formulations when 100% of the virgin filler and reinforcement is replaced by recycled SMC, but may be acceptable for certain applications. Optimization of formulations and/or processes should improve properties, for example, the granulated SMC could be "resized" with a coupling agent to improve properties.

Size reduction of SMC and reuse in new composites as an approach to recycling has several benefits:

* A full 100% of the SMC is reused--removing the entire waste stream from landfilling or incineration.

* Off-the-shelf size reduction equipment, formulation techniques, and molding technologies are employed.

* The capital required for size reduction equipment appears manageable, even for a modest-sized molding operation.

* No special recycling permits should be required.

Several issues must be addressed to allow this recycling technology to become commercially viable:

* The effect of metal, paint, and adhesive contamination and, if necessary, development of processes to remove them.

* Control of moisture in the granulated material.

* Segregation of various SMC formulations to provide material consistency.

* Development of viable SMC collection and recycled product marketing scenarios.
COPYRIGHT 1991 Society of Plastics Engineers, Inc.
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Title Annotation:sheet molding compound, bulk molding composite
Author:Jutte, Ralph B.; Graham, W. David.
Publication:Plastics Engineering
Date:May 1, 1991
Words:1983
Previous Article:Analyzing variations in SMC formulations.
Next Article:Tight-tolerance design.
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