1. Field of the Invention
This invention relates to polyester polyols which are obtained by digesting scrap or recycled polyethylene terephthalate with polyols to form digested polyol mixtures and distilling ethylene glycol from the digested mixtures. The distilled polyols are useful in the preparation of cellular foam materials, particularly polyisocyanurate and polyurethane foams.
2. Description of the Prior Art
The preparation of foams characterized by isocyanurate and urethane linkages is well known in the art. Generally, these foams are prepared by reacting an organic polyisocyanate with a polyol in the presence of a blowing agent and a catalyst(s). Polyester polyols of many types can be used as the polyol components in the production of these foams.
U.S. Pat. No. 4,039,487, for example, discloses the use of aromatic polyester polyols to prepare polyisocyanurate foams. Although the foams of this patent have good fire resistance and low smoke evolution upon combustion, they have a relatively high friability. Furthermore, the polyols are comparatively expensive to manufacture.
U.S. Pat. No. 4,092,276 also discloses the use of rather costly aromatic polyester polyols in preparing polyisocyanurate foams. Another disadvantage of these foams is that they do not have especially high compressive strength. A further problem with using aromatic polyester polyols, particularly those of low molecular weight, is that the polyols tend to be solid at room temperature, or to be characterized by very high viscosity and poor solubility in resin mixtures, thus making them difficult to handle.
To remedy the above drawbacks, it has been proposed in U.S. Pat. No. 4,237,238 to use in the preparation of polyisocyanurate foams a minor amount of a cheap by-product type of liquid polyol mixture which is obtained by the transesterification, with a glycol of molecular weight from about 60 to 400, of a dimethyl terephthalate esterified oxidate reaction product residue. The polyisocyanurate foams produced are disclosed to be characterized by a high degree of fire resistance with low smoke evolution on combustion, a low foam friability and reasonably good compressive strength. U.S. Pat. No. 4,411,949 discloses cellular foams of exceptional quality producible by employing a polyol mixture derived from a by-product of dimethyl terephthalate production which has a substantially higher content of dimethyl terephthalate than the residue used in U.S. Pat. No. 4,237,238.
Another source of aromatic polyester polyols is available through the recovery of polyester wastes. High molecular weight polyesters of terephthalic acid and aliphatic dihydric alcohols are well known in the art. These high molecular weight polyesters, especially polyethylene terephthalate (PET), are used commercially for the manufacture of packaging film, fibers, electrical insulators, molded articles, such as PET beverage bottles, etc. The various manufacturing processes unfortunately generate considerable waste as the polyester is processed into commercial form. Also, the tremendous quantities of spent consumer goods containing the polyester constitute a huge supply of scrap polyester material.
There is a growing awareness of the need for energy conservation and the utilization of recyclable materials. It is realized that the judicious use of plastics can contribute significantly to energy savings. The industry has long recognized that recycling waste polyalkylene terephthalate would conserve raw materials, improve process economics, and eliminate the problem of waste disposal. Numerous processes have been proposed for recovering useful products from waste or scrap polyalkylene terephthalate by reducing or digesting the high molecular weight polymer to short-chain fragments. These short-chain fragments have been used principally in the production of additional polyester materials.
The use of a polyalkylene terephthalate digestion product in flexible polyurethane foam is described in U.S. Pat. No. 4,048,104. In this patent, the digestion product is employed to prepare polyisocyanate prepolymers, and not as a polyol ingredient in the manufacture of flexible polyurethane foam.
U.S. Pat. No. 4,223,068 discloses the manufacture of rigid polyurethane foam wherein 5 to 30 percent of the weight of the organic polyol used in the manufacture is a digestion product of polyalkylene terephthalate residues or scraps digested with organic polyols. The preparation of isocyanurate modified polyurethane foams utilizing a digestion product of polyalkylene terephthalate scrap dissolved in one or more organic polyols is described in U.S. Pat. No. 4,417,001. While these foams are characterized by desirable physical properties, deficiencies have been encountered in the fluidity and storage stability of polyol digestion products disclosed for use in preparing the foams. These deficiencies lead to inefficiency in the foam production.
One solution to the instability of the digestion products is to employ certain polycarboxylic acid components in the digestion of the scrap polyalkylene terephthalate, such as polyethylene terephthalate (PET), as disclosed in U.S. patent application Ser. No. 582,348, filed Feb. 22, 1984. However, cellular foams synthesized from the resulting improved digestion products have been found to be inferior in certain properties, e.g., foam density and thermal insulation capacity, to the generally acceptable foams of above-mentioned U.S. Pat. No. 4,411,949. It would be desirable if PET scrap could be utilized to produce an inexpensive polyol mixture having a good shelf stability for extended periods of time and the capacity to yield foamed products characterized by a broad range of desirable properties.