DEGRADABLE FOAM MATERIALS

Abstract: Poly-hydroxy acid resin products foamed with hydrofluorocarbons (HFCs), which products are degradable and cause minimal environmental impact when discarded.

Description: TJTLE DEGRADABLE FOAM MATERIALS BACKGROUND OF THE INVENTION This invention relates to foam materials, particularly for food service and food packaging applications. The materials of this invention are degradable, both biodegradable and hydrolyzable. Furthermore., they contain no ozone-depleting chemicals nor chemicals that have significant photochemical reactivity. Also, they have little global warming potential compared to chlorofluorocarbons (CFCs) . Thus, this invention provides readily degradable foam packaging materials with minimal environmental impact. BACKGROUND OF THE INVENTION At the present time, the most commonly used food service and food packaging materials are foamed polystyrene, usually foamed with chlorofluorocarbon (CFC) or hydrocarbon (HC) blowing agents. These products pose serious environmental problems. First, polystyrene is not degradable, either in landfills or as roadside litter. Thus, unless collected and recycled, polystyrene foam products have an unlimited landfill and litter life. Second, CFC blowing agents trapped in the foam products eventually escape to the stratosphere where the chlorine enters an ozone-depletion cycle. Third, HC blowing agents, upon release from the foam are photochemically reactive and thus promote smog formation. Thus, there is a need for a foam of a degradable resin foamed by a blowing agent that does not enter into chemical reactions that cause environmental damage. SUMMARY OF THE INVENTION The foam packaging products of the present invention are foamed, degradable thermoplastic polymeric resin containing at least one hydroxy acid unit selected from the group consisting of: (1) (0(CR 2 R )nCO)p (2) (OCR1R2COOCR1R2CO) q (3) (OCRιR2CRι 2θCRιR 2 CO) r (4) (OCRιR 2 CFιR2θCRιR2CRιR 2 CO)H s wherein n is a whole number 2, 4 & 5, the total of p, q, r, s and t being 350 to about 5,000, and Ri or R 2 , which can be the same or different, is hydrogen, hydrocarbyl containing 1 to 12 carbon atoms, or substituted hydrocarbyl containing 1 to 12 carbon atoms. Preferably the PHA comprises at least 50% of units of formula (1) . The ratio p/q/r/s/t must be adjusted to impart crystallinity level and crystallization rates appropriate to oriented film production, as known in the art; containing a blowing agent of the formula wherein all A's are independently hydrogen or fluorine, and n = 1-4. Thus, the products of the present invention are a degradable resin foamed by an environmentally benign hydrofluorocarbon (HFC) blowing agent. In its simplest concept, the present invention is the use of HFCs of the formula (6) as a foam blowing agent for resins -of units of formulae (l)-(5) to produce highly useful, environmentally superior foam products. Preferred polymers are those of formula (l) for which R is either hydrogen or the methyl group, in which case the polymers are known as polyglycolide or polylactide, respectively. The polymers may be synthesized by polymerization directly from the acid precursors, or by the polymerization of the cyclic monomeric or dimeric (depending on the acid type) lactone. Polymerization from the cyclic lactone is preferred for producing high molecular weight polymers because equilibrium reactions with the intermediate, cyclic dimer and low molecular weight species place practical limits on the molecular weight of polymers produced directly from the acid. The polymers of this invention have molecular weights from about 50,000 to about 600,000; and preferably from about 100,000 to about 450,000. High molecular weight is required to provide sufficient viscosity and strength to enable the polymer to form a sustainable foam. If the molecular weight is too high, excessive degradation occurs at the temperatures required to melt process the composi¬ tions. The polymer compositions will normally contain some unreacted monomers and low molecular weight oligo ers. To avoid extrusion and foaming problems, it is desirable to keep low molecular weight, under 450, units in the polymer composition to less than about 7-1/2. Also, preferred polymer compositions are those having a major component of 50-97%, and most preferably from about 85 to 96 mol % of the L enantiomer of lactic acid corresponding to unit (2) above, where Ri is the methyl group, R 2 is the hydrogen atom. The minor component of these preferred compositions can be any of the other units (1) to (5) above, with the proviso that if additional lactic acid units are utilized, they be of the D enantiomeric configuration. The term "degradable" as used here with respect to the polylactate polymers means that the polylactate portion of the degradable material is biodegradable and, more importantly, degradable by hydrolysis. The degradation rate is consistent with its intended usage, i.e., the product does not significantly degrade in normal storage and usage, but will degrade significantly in a reasonable time after discarding. As is known in the art, hydrolysis degradation is dependent on the degree of crystallinity of the polymer; the higher crystallinity polymers degrade more slowly. For hydrolysis degradation, slightly acidic or basic conditions may be used advantageously. By hydrolysis degradation, monomer units can be recovered if desired for reconversion to useful polylactate polymers or can be discarded as an environmentally benign waste material. Acceptable blowing agents must have the following properties: environmental acceptability - low toxicity - appropriate volatility adequate solubility low reactivity acceptable diffusion rate relatively low molecular weight. Environmental acceptability means that the blowing agent, when released from the foam product, will have no potential for stratospheric ozone depletion. In this regard, blowing agents containing chlorine atoms are unacceptable. Also, when released, the blowing agent must have a minimal infrared energy absorbability and appropriate atmospheric lifetime so that it will not have significant global warming potential, and also have negligible photochemical reactivity so as not to promote smog formation. Low toxicity of the blowing agent is required to protect employees during foam manufacture. Also, it must not be toxic to uses of the foam products, which is of particular importance in food service and food packaging applications. The blowing agent must also have appropriate volatility for use with the thermoplastic resins of the present invention. Such blowing agents must provide the solution pressure required to expand and foam the viscous polymeric resin. The blowing agent must have adequate solubility, which means that the blowing agent is readily contained in the molten polymer when present in the concentration required for the needed degree of foaming. If the blowing agent separates from the polymeric resin before expansion of the blowing agent, gas pockets or non-uniform foam density can be caused. The blowing agent must have low reactivity, which means that it will not react with the resin or decompose under the typical temperatures and pressures conventionally used in thermoplastic resin foam production. Similarly, the blowing agent must be nonreactive under normal use so as to prevent product degradation. An acceptable diffusion rate is necessary for the blowing agents in the foams of the present invention. A very slow diffusion rate is preferred so that the thermoforming of foamed sheet into shaped objects such as hamburger containers, plates, etc. is easily achieved. A relatively low molecular weight blowing agent is desirable. Molecular weight determines the pounds of blowing agent needed to produce a given volume of gas. Thus, use bf a low molecular weight blowing agent minimizes the blowing agent cost per unit of production. It has been found that HFC blowing agents selected from those represented by formula (6) are ideally suited, with respect to the aforementioned properties, when used to make foam products with the polymeric resins of the units of formulae (l)-(5). Suitable HFC blowing agents of formula (6) used with a specific polymeric resin of the units of formulae (l)-(5) can be readily determined by conventional techniques. The following HFCs are suitable blowing agents for the foams of the present invention. HFC-32 CH 2 F 2 HFC-125 CF 3 CF2H HFC-134 ' HCF2CF2F HFC-134a CF 3 CFH2 HFC-143a CF3CH3 HFC-152 CH2F-CH2F HFC-152a CH 3 ~CHF 2 HFC-227 EA CF3-CHF-CF3 HFC-356 MFF CF 3 -CH 2 -CH -CF 3 HCC-365 MFC CF 3 _.CH -CF 2 -CH 3 The preferred HFCs are those containing one and two carbon atoms. The foam products of the present invention have a unique combination of properties making them environmentally acceptable both with respect to the atmosphere and also for disposal. Concerning disposal, they can be landfilled whereupon they will biodegrade and/or hydrolyze to harmless degradation products, or they can be recycled by hydrolysis, preferably under slightly acidic or basic conditions, to form monomeric units that can be used in the production of polymer. The foam products of the present invention can be made by conventional techniques. Thus, the polymeric resin with the blowing agent therein can be pressure extruded at an appropriate temperature below the degradation or reaction temperature of the resin and the blowing agent. The foamed product can be in the form of a sheet or a shaped article such as hamburger containers, trays, plates, boxes and the like. These products are particularly useful in food packaging and food service containers because of the combined properties of low toxicity and environmental acceptability. In addition to polymer and blowing agent, conventional adjuvants can be included. Typical nucleants such as calcium silicate talc; processing aids such as mineral oils; extrusion aids such as dioctylphthalate (DOP) ; and color concentrates. The concentrations of the additives are generally independent of the amount of blowing agent. HFC manufacture is known in the art. See "Aliphatic Fluorine Compounds", A. M. Lovelace et al. (1958) , p. 55. EXAMPLE 1 Continuous production at the rate of 500 lbs./hr. of a foam sheet product is prepared by feeding to a 4-1/2"-6" tandem extrusion system with annular die and mandrel a mixture of 2-1/2 lbs. per hour of calcium silicate talc nucleating agent and 472.5 lbs./hr. of molten polylactic acid (85% L, the remaining 15% being D/L and a small amount of low molecular weight lactide and oligomer, the polymer having a molecular weight of about 300,000). Into the molten polymer in the extruder is introduced 25 lbs. per hr. of CF 3 -CFH 2 (HFC-134a) . The temperature of the material in the 4-1/2" extruder is maintained at 400-'F, and the pressure at 2500 lbs./sq. in. gauge. Prior to exiting the 6" extruder the temperature is dropped low enough to prevent cell collapse upon discharge from the extruder. The HFC comes out of solution in the polymer, forming an oriented degrad¬ able thermoplastic foam product. This product is rigid foam sheet 44 in. wide and 100 mils thick, having a density of 6 lbs. per cubic ft. The sheet foam is then fed through a thermoformer where it is stamped into 12 in. circular plates. Following the same procedures, sheets ranging from 50-100 mils thick are prepared and thermoformed into hinged food service containers. EXAMPLES 2-6 Following the procedure of Example 1, there are formed plates and service containers of PHA/HFC foams from the ingredients listed below: Calcium PLA Silicate Blowing Composi- Talc Ex. Agent M.W. #/hr. tions* #/hr. Nucleator 2 CH 2 F2 52 2.55 85% L 494.95 2.5 (HFC-32) 3 CH 3 CHF 2 66 3.24 70% L 494.26 2.5 (HFC-152a) 4 CF 3 -CH 2 F 102 5 60% L 492.5 2.5 (HFC-134a) 5 CF3CHFCF3 170 8.33 75% L 489.17 2.5 (HFC-227 EA) 6 CF3CH2CF2CH3* 166 8.14 50% L 489.36 2.5 (HFC-365 MFC) * The remaining PLA composition is D polylactic acid isomer plus less than 7% low molecular weight lactide and oligomer.

Claims: E CLAIM: 1. A product comprising foamed, degradable resin and a hydrofluorocarbon blowing agent. 2. The product of Claim 1 wherein the acid resin contains at least one acid unit selected from the formula (1) (0(CR 2 R 2 )nCO) p (2) (0CRιR2C00CRιR 2 C0)q (3 ) (0CRιR2CRι 2 0CRιR2C0) r (4 ) (OCRιR 2 CFι.R 2 OCRιR CRιR 2 CO) H s wherein n is a whole number 2, 4 & 5, the total of p, q, r, s and t being 350 to about 5,000, and Ri or R , which can be the same or different, is hydrogen, hydrocarbyl containing 1 to 12 carbon atoms, or substituted hydrocarbyl containing 1 to 12 carbon atoms. 3. The product of Claim 1 wherein the poly-hydroxy acid units are selected from the group consisting of polylactide and polyglycolide. 4. The product of Claim 1 wherein the poly-hydroxy acid units comprise at least 50% of units of formula (1) . 5. The product of Claim 1 where over 50% of the polylactate resin is L-isotropic form polymer units. 6. The product of Claim 4 wherein the polymer resin contains less than 7% lactide and lactyllactate oligo er units of less than 450 molecular weight. 7. The product of Claim 1 wherein the blowing agent is selected from the formula wherein all A's are independently hydrogen or fluorine and n = 1-4. 8. The product of Claim 7 wherein n = 1. 9. The product of Claim 7 wherein n = 2

Inventors: HAMMEL HOWARD SIMS, YORK ROBERT OLIVER, HAMMEL, HOWARD, SIMS, YORK, ROBERT, OLIVER
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1991
Main classification: C08J9
IPC codes: C08J9