Material recycling of RIM-polyurethanes.

There is a choice of different methods to recycle RIM (reaction injection molding) polyurethanes. Rim-polyurethanes can be subjected to chemolytic processing methods (ref. 1) and become thus transferred into polyols and polyol/amine mixtures. Small amounts of these regenerates can be added to virgin material and reused for original applications. The mechanical properties of articles produced in this manner are somewhat reduced. Another alternative is the use of ground RIM-polyurethanes as a filler in RIM-parts (ref. 2) or thermoplastics such as polypropylene (ref. 2). Because of the crosslinked and duroplastic character of RIM-polyurethanes a true material recycling which has been realized in the public mind only for thermoplastics was thought to be impossible. It will be shown to you now that on the contrary, because of the crosslinked and duroplastic network of RIM-polyurethanes with physical and chemical bondings, material recycling is possible. Our new technology allows 100% reuse of recycled material (ref. 3).

What happens

Under heat and pressure crosslinked RIM polyurethanes can be reshaped. This thermoforming procedure is similar to the deep drawing of sheet steel for carpet panels. Figure 1 shows the result of this deep drawing of a RIM plaque. As can be seen from the deformation of the equidistant line pattern, there is material flow from areas, which are not directly exposed to heat and pressure into the cavity. No melting of the material takes place at all.

How does the process work?

Elastomeric RIM-polyurethanes comprise hard and soft segments which are arranged in cluster or domains. The stiffness of the hard segments results from physical bonding (hydrogen bonding) which are thermally reversible (ref. 4). Three functional polyether polyols or polyamines crosslink the soft segments. These crosslinkages are thermally not reversible and do not split under the processing conditions of this procedure.

Figures 2-4 show the schematic network of RIM-polyurethanes. Thermoforming of RIM-polyurethanes leads to new parts of stable geometry. The material does not pass a liquid stage during pressing. At the process temperature the physical (and some chemical) bondings in the hard segment domains open. The mechanism corresponds to a certain extent to the melting of thermoplastic polyurethanes. However in contrast to TPUs the chemical crosslinkages in the soft segments inhibit real melting. This performance excludes the options of conventional thermoplastic processing technologies. The soft segments form a matrix which allows orientation and reconfiguration of the "molten" hard segment domains. At cooling the hard segment domains crystallize again and thus stabilize the new geometry (see table 1).

Table 1 - conditions for deep drawing of
Bayflex 110/80 semifinished goods
Preheating time          0.2 - 15 min.
Preheating temperature   140 - 1800C
Mold temperature         180 - 1900C
Mold residence time        1 - 5 min.
Specific pressure          1 - 400 bar
Demolding temperature    180 - 190'C

Plaques of RIM and RRIM polyurethanes which have thinner cross sections can also be deep drawn by vacuum and heat.

Deep drawing of RIM and RRIM-polyurethanes leads to articles of new design with 100% recovery of properties. Sometimes the property profile is even improved provided specific processing conditions are strictly observed. This will be explained by figures 5 and 6.

A sensitive physical property: Dimensional stability

A very important property of the demolded article is their dimensional stability. The following figures demonstrate the dependence of shrinkage on various press temperatures, varying hard segment content (see figure 7) and the degree of crosslinking in the hard segment, respectively (see figure 8). The thickness of the samples is measured. The shrinkage is observed as increase in cross section.

Figure 7 demonstrates the shrinkage of the Bayflex 110 series. Practically no shrinkage is observed at mold temperatures above 150 [degrees] C, whereas between 130 [degrees] C and 150 [degrees] C it depends on the Detda content.

In the range of 140-160 [degrees] C shrinkage goes up at higher crosslinking density (figure 8) in the hard segment which was achieved by increasing the functionality of the isocyanates. Above 170 [degrees] C shrinkage does not occur anymore, below 170 [degrees] C shrinkage depends on the degree of chemical crosslinkage.

These investigations suggest that the physical bondings rearrange at temperatures between 160 [degrees] C and 170 [degrees] C. Thermoformed parts which are produced over 170 [degrees] C do not reveal frozen-in tensions. That means there is no stress on the polymer matrix.

In morphology and thermo-mechanical properties the articles correspond to post-cured RIM parts which are characterized by full relaxation of any tensions.

At temperatures above 170 [degrees] C no shrinkage is observed any more independently from the network's structure. This is because the physical network is complete and the chemical network is partially resolved.

The region between approx. 140-170 [degrees] C dominates the influence of the nature of the chemical network while the physical network is already resolved. Changes in the branching of the hard segment therefore have a significant influence on the shrinkage. Under 140 [degrees] C the physical network also remains intact and is influenced by the nature of both networks. Shrinkage is dominated mostly by its hard segment content. This shows the strong influence of the physical network at these temperatures.

So two mechanisms which control and stabilize the geometry of articles are under discussion. In addition to the physical factors, which are always active at temperatures above 160-170 [degrees] C splitting and reformation of chemical bondings can be observed (see figure 9).

Compression molding of granul9es

The interaction of physical and chemical factors permits the production of new articles form granules of RIM-polyurethanes. We call this procedure compression molding. The process is based on production waste or used parts which are chopped to a screen size of 2-3 mm on an appropriate cutting device. Granulating is done at ambient temperatures. No brittlening of the material by cooling to low temperature is necessary. For the molding process the granule is preheated in an oven and transferred into a steel tool with shear edges (see table 2). Its pronounced flowability allows undercuts and extended vertical areas in the tool.

Table 2 - table of typical compression molding
conditions
Grain size (screen size)     2 - 3 mm
Preheating time              1 - 12 min.
Preheating temperature     140 - 150 [degrees] C
Mold temperature           180 - 190 [degrees] C
Mold residence time          1 - 3 min.
Specific mold pressure     [is greater than or equal to] 350 bar
Demolding temperature      180 - 190 [degrees] C

The recombination and extent of unification of the granule grains is mainly responsible for property conservation of compression molded articles. Obtained property maintenance depends on the same conditions as deep drawing, especially pressure, preheating and mold temperatures. Mold residence time and mold temperatures also influence the surface roughness of new moldings.

The properties are also influenced by tool geometry, granule distribution and positioning in the tool (table 3).

[TABULAR DATA OMITTED]

All properties which depend on the orientation of the fibrous filler are averaged, which is the reason for the reduced flexural elasticity modulus.

Optimization of properties

By compression molding of a specific RIM material the properties can be recovered to a certain extent. With a given Bayflex 110 composition only minor property improvements can be achieved by further optimizing the conditions of compression molding. The level of physical properties of recycled materials is also dependent on the chemical composition of the Bayflex RIM systems. All components were varied separately. Also interactions between the components were considered. The objective was to maintain 100% properties after compression molding. The results are shown in table 4.

[TABULAR DATA OMITTED]

Summary

Compression molding technology provides a superb procedure to generate recycling articles from RIM-polyurethanes granules. It applies a well known technique and produces 100% recycled articles of three dimensional geometry. The property profile is well balanced. It can be further improved by using RIM systems which are optimized for recycling purposes. First commercial utilization is expected in the automotive industry this year.

References [1.] Bayer, G. 1991. Kunststoffe, 81:301 and therein cited literature. [2.] Meister, B. and H. Schaper. 1990. Kunststoffe, 80:1260. [3.] Ep's 310,896, 334,171, 348,760, 366,925, 371,330, Bayerag. [4.] Dieterich, D. 1983. Kunststoff-handbuch, vol. 7; G. Oertel, Carl Hanser Verlag, ed Polyurethane, Munchen, Wein, p. 3]f and therein cited literature.