AU2642995A - Degradable polymers and polymer products - Google Patents

Description

DEGRADABLE POLYMERS AND POLYMER PRODUCTS

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of Application

Serial No. 08/258,280 filed June 10, 1994 which is a continuation of Application Serial No. 07/867,718 filed

July 9, 1992 now U.S. Patent No. 5,321,065 issued June 14,

1994.

BACKGROUND OF THE INVENTION

Polymer blending has become one of the most commercially important and inexpensive ways of developing new materials from readily available base polymers. The main aim of polyblending is the production of good performance materials at a reduced cost or the modification of some specific properties of polymers. This is achieved through the infinite blending possibilities, the ability to use existing flexible processing equipment, and the capacity to combine expensive polymers with ordinary and abundant ones.

Polyethylene is produced by polymerizing ethylene gas and the result is a joining together of the ethylene molecules into long polymer chains. The most common additives are heat and light stabilizers, slip, antiblock and antistatic agents, flame retardants and pigments. To protect against thermal oxidation which can be a problem during processing, antioxidants are usually added. Photo or light oxidation that occurs when natural PE is exposed to UV radiation is most often inhibited by the addition of carbon black and/or UV stabilizers.

PE is classified as either low and medium density (LDPE and LLDPE) or high density (linear) PE (HDPE) based on ASTM designation. LDPE and LLDPE have been used in several applications: film and sheeting, housewares, closures and containers, packaging materials, wire and cable coating, rotational molding, powder coating, pipe extrusion, refuse or garbage bags and extrusion coating. The primary processing techniques used to convert HDPE into end products are blow and injection molding for containers and lid closures, films and large containers.

Ethylene(vinyl) acetate (EVA) copolymer is generally obtained by adding vinyl acetate to PE. EVA is tougher, more flexible, softer and less heat resistant then LDPE. Being softer and more flexible than PE, the copolymers are often competitive with rubbers and plasticized PVC. At higher levels of comonomer incorporation, the EVA's are used as wax additives and as components in other formulations for hot melt coatings and adhesives. In these applications, the copolymers provide strength, improved barrier properties and better processing characteristics. Polypropylene (PP) in its natural form is particularly vulnerable to degradative attack by oxygen and sunlight. Stabilizers have been developed which allow PP to retain its balance of good mechanical properties at low cost, and to do so in severe environments. Phenolic antioxidants have the primary function of reacting with the polymer peroxy radicals to form more stable radicals, and thus stop the chain oxidative attack. To protect the polymer against long periods of outdoor exposure to UV radiation, UV absorbers are added before processing. These absorbers are colorless and transform UV radiation into harmless longer wavelength light. Common classes of UV additives include the benzophenones, benzotriazoles, salicylates, and phenyltriazines. Certain nickel salts also provide some degree of UV absorption and act as a free radical scavengers, preventing the propagation of the photo-chain degradation process. PP finds applications in molded products for automotive and appliance uses, packaging, fibers and fibrillated films, microporous filters and desalinization equipments, spun fibers, film and sheet and nonwovens.

Styrene is used primarily for the manufacture of thermoplastic resins, of which polystyrene (PS) and polystyrene copolymers are the most important. Polystyrene is the third most widely used thermoplastic resin, surpassed only by PVC and PE. Most polystyrene is

processed by rotational and injection molding, extrusion and thermoforming. Two types of polystyrene are currently used: crystal and impact polystyrene. Both types find uses in houseware applications, packaging, appliances, wall coverings and many specialty applications.

PVC is the most highly modifiable plastic known. Products can be formed with a broad range of mechanical properties. Being self-extinguishing, PVC also has inherent flame resistance. Plasticizers such as phtahlates and adipates contribute to its flexibility. The feel of PVC is controlled by the amount of plasticizers and/or filler material, as well as the type of resin. Impact modifiers can be included to increase breakage resistance.

Under UV irradiation, and in the presence of oxygen and moisture, PVC undergoes a very fast dehydrochlorination and peroxidation process with the formation of polyenes and subsequent scission and/or crosslinking of the chains. Additives which have UV stabilization effect can be included to prevent degradation in sunlight. One such stabilizer is titanium dioxide ( iθ2) which provides adequate protection for most purposes and is most often introduced at levels up to 10 to 12 phr. At these levels, Ti02 can enhance the weathering properties of PVC products because of its ability to absorb to a certain degree UV radiation falling on the polymer. Tiθ2 is widely used in white and light color formulations. Tiθ2 is approximately 50% more costly than unplasticized PVC used for outdoor service and PVC producers are looking for ways to reduce their Ti0₂ consumption.

Current thermoplastic polymeric materials are generally disposed of by incineration. They can also be disposed of by recycling which can be achieved by increasing their oxidation temperature. Increasing the oxidation temperature can be achieved through the use of additives. Such additives have generally been known to have certain other drawbacks as when such materials must be incinerated, such additives generate toxic fumes necessitating an additional treatment step which increases the overall cost of disposal. In any event, with PVC, additional treatment for effluent gases is necessary.

Certain thermoplastic polymeric materials can either photodegrade or biodegrade. Generally, photodegradable thermoplastic polymeric materials are obtained by introducing photoactive additives into a base material such as for example polyolefin. These additives consist of molecules containing oxygen and/or heavy metals which play a role in the initiation and formation of free radicals under the action of ultraviolet (UV) radiation. The free radicals cause a rupture of ^{■ •} the chains of the polymer and therefore make the polymer fragile and mechanically degradable. Although frequently in use, such photoactive additives are generally strongly oxidizing which can cause the degradation of the plastic material to begin immediately after the manufacture of the material thus reducing the shelf life of the thermoplastic polymeric materials.

Generally, biodegradable plastic materials can be obtained by the introduction of a biopolymer such as starch. As starch can be attacked by microorganisms, the material becomes susceptible to degradation.

However, the incorporation of starch in such material can . have drawbacks as it can be partially decomposed during the processing and it is highly sensitive to water. Furthermore, starch is not compatible with most polymers, and its incorporation during polymer manufacture can render the final product brittle. Furthermore, in polymeric films with a particularly small thickness, the particle size of starch can be a limiting factor on the overall manufacturing process and the cost becomes prohibitive.

A number of biopolymers in addition to starch can also be used such as for example other carbohydrates with one major drawback , that upon blending with the polymeric material, the biopolymer can undergo various alterations such as oxidation and polycondensation. Such alteration to the biopolymer can have a negative effect on the mechanical properties of the polymeric materials. As an alternative to the foregoing, a biopolymer such as organosolv lignin can be incorporated with thermoplastic polymeric materials.

SUMMARY OF THE INVENTION

This invention provides for degradable polymers and polymer products having incorporated therein an organosolv lignin. The incorporation of the lignin enhances the mechanical properties of the polymers while causing them to degrade under certain conditions. The polymers of this invention can be disposed of without incineration or recycled, resulting in a savings in energy and minimal pollution.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The biopolymer employed in this invention is a lignin which is separated from plant biomass by a novel chemical delignification technology based on organic solvents, for example ethanol. Generally referred to as organosolv lignin, it is a free-flowing, non-toxic powder. It is soluble in aqueous alkali and in selected organic solvents. It is generally characterized by its hydrophobicity, purity, melt flow -properties and a low level of carbohydrates and inorganic contaminants. The lignins of this invention can be incorporated into various polymeric materials and can have various effects on the polymeric blend such as for example they can function as an antioxidant, as a stabilizer against ultraviolet radiation and can enhance the mechanical properties of these materials.

The lignins of this invention can stimulate degradation of the polymeric materials when photoadditives are added to the blend. The products can degrade by photodegradation of the polymeric materials and the lignin or alternatively by biodegradation of the lignin under composting conditions.

The lignins of this invention can be blended and compounded with polymers such as for example polyethylene, polypropylene, poly(vinyl) chloride and polystyrene copolymers in a weight ratio of from about 0.5% to about 40% with the polymer of choice. The blends can be then processed by extrusion, calendering, injection or known processes in the art to yield articles of manufacture having different utilities such as for example, film and molded products. Alternatively, the lignin can be blended with a copolymer such as for example ethylene (vinyl) acetate or styrene-butadiene copolymers. The resulting blend is a master batch which can be diluted by further blending with polymers such as for example polyethylene, polypropylene, poly(vinyl) choride and polystyrene copolymers. The blends can be processed using known methods in the arts to yield the desired final products. In a preferred embodiment, a master batch can be prepared by mixing with organosolv lignin of from 35% to about 85% on a weight basis with the copolymer of choice such as EVA, SBS or any other polymer which is known to have a glass transition temperature in the same range as that of the lignin. The master batch can be prepared by mixing all ingredients directly or in successive stages.

For specific applications, the master batch can also be coextruded with a polymer of choice depending on the desired final product. Pellets can be produced with a core and a sheath with a variable composition. The core of the pellet can have the composition of the master batch while the sheath can have the polymer composition of the intended final product. The pellets can generally be obtained by granulating the filaments coming out of the extruder.

The core of the pellet can be manufactured by considering the nature of the biopolymer to be incorporated therein. When organosolv lignin is used, the pellet can be manufactured without causing any chemical or physical deterioration by mixing the lignin or a master batch polymer of particular interest such as for example EVA or SBS or any other master batch polymer which is known to have a glass transition temperature in the same range as that of the lignin. The core which comprises of from about 35% to about 85% lignin and 65% to 15% master batch polymer can be extruded at a temperature of from about 115°C to about 145°C to form a polymer sheath having a similar composition as the final product. It is believed that extrusion at the foregoing temperature will result in no damage to the lignin. Extrusion of the sheath generally requires a temperature of from about 170° to about 230°C. As a target, the overall composition of the coextruded compound is preferably equivalent to the composition of the finished product. The thickness of the sheath can be adjusted according to the diameter of the core which corresponds to the diameter of the central filament such that the level of lignin in the final coextruded product is of from about 0.5% to about 40%. In a preferred embodiment, for a core diameter of from about 1 mm to about 2 mm, the sheath thickness is of from about 4 mm to about 5 mm such that every individual pellet of compound comprises from about 4% to about 25% of lignin.

An advantage of using coextrusion at the compounding temperature of polyethylenes and polypropylenes is that the problem associated with the thermal decomposition and oxidation of the biopolymer is alleviated. In this particular instance, the coextruded compound upon extrusion, takes on the appearance of pellets which are heterogeneous under the microscope but still are more homogeneous overall by contrast to the appearance of pellets which result from the mechanical mixing of two different pellet compositions. The master batch of this invention can be processed by extrusion, blowing, injection or other processes known in the art. The machinery generally used requires adaptation to the processes of this invention to meet the shorter residence times which are required at critical temperatures such as for example the oxidation temperature. It is also believed that the processes of the invention can operate at a lower temperature mainly because of the additional heat protection from the sheath to the lignin-rich core which is easier to melt and the viscosity of which is not as sensitive to the temperature as the PE or PP.

In certain specific applications of this invention, organosolv lignin powder with a median particle size of from about 0.1 micron to about 100 microns and in a quantity of from about 0.5% to about 40% can be mixed with polyethylene or more generally an ethylene copolymer to manufacture homogeneous films having a thickness of from about 5 microns to about 100 microns. The polyethylene blends can be prepared by direct mixing or by using a master batch preparation. It has been observed that the resulting films can degrade when iron stearate or any other photoactive and/or oxidizing additives such as cerium salt is added in a range dependent on the target film shelf life and the conditions under which the film will be used. A preferred range is of from about 0.1% to about 0.5% of salt based on total weight of polymer blend. The plastic films thus obtained can be used for many agricultural applications, as well as for the manufacture of plastic bags for refuse, shopping baskets, etc. In agricultural applications, in which the stiffness of the film is essential, the lignin containing polyethylene film of the present invention appears particularly interesting to use since the degradation of the film over time is total, both for the surfaces which are outside the ground and for those which are buried inside the ground. Furthermore, in the agricultural applications field, the adsorption and absorption capacities of lignin, of essential oils, insecticides and the like, will permit then a use of lignin as an additive for the new fungicidal, rat-killing or other properties.

Likewise, the adsorption properties of lignin can be utilized so that the lignin can be incorporated into the photoactive products prior to its mixture with the copolymers, which has the advantage of increasing the homogeneity and the degradability of the film. On the other hand, this lignin containing plastic film can be coextruded, and can therefore be a part of a composite film.

It should further be noted that the initial mechanical properties of the lignin containing degradable film of the present invention are comparable to those of a film which does not contain any lignin.

Generally and highly dependant on the process of preparation, the lignin thermally behayes by partially condensing with apparent fusion and without oxidizing in a temperature range of from about 125°C to about 200°C and on the other hand by oxidizing without condensation at about 160°C. The properties of the lignin are material to the processes of this invention. When the lignin condenses, it is believed that it is capable of generating water to approximately from about 1% to about 6% of its weight. Therefore, special attention must be given to eliminating water produced during the manufacture of the thermoplastic polymeric material.

The lignin and polymer can be mixed in an extruder which can be either a single or double screw. The mixing is preferably performed in a vented extruder such that any water vapor formed from the lignin is eliminated. The extrusion conditions are dependent on the scale of the process.

The rotation speed of the screw is an important parameter and is a function of the materials introduced upstream.

The temperature profile is also an important element of the success of a good mix since the lignin must be protected from oxidation and thermal degradation. This can be accomplished by adding the lignin to the already molten polymer or by using the master batch described herein. Upon mixing with the lignin, the lignin behaves as a thermal antioxidant which results in an increase in the oxidation temperature of the polymer. An increase in the oxidation temperature of the mixture enables the recycling of such material thus permitting it to be melted again for reuse without degradation. In the case where the material can no longer be recycled and it may prove necessary to effectively incinerate the material, addition of the lignin is beneficial as the heating value of the lignin is equivalent to that of the polymer used, thus allowing its destruction by incineration.

For example, when the polymer used is pure polyethylene, its oxidation temperature is about from 150°C to about 160°C. By contrast with about 10% lignin, the oxidation temperature is from about 185°C to about 195°C and with about 25% lignin, the oxidation temperature is from about 195°C to about 205°C. In another example, when the polymer used is polypropylene, the oxidation temperature is from about 210°C to about 220°C. By contrast with about 10% lignin, the oxidation temperature is from about 255°C to about 265°C.

The thermoplastic polymeric material of this invention can be used in applications known in the art for example in extrusion/blowing applications, calendering, injection molding to form films, plates, sheets, tubes, bottle caps, wrapping paper, car parts and the like. In order to improve the machining of the thermoplastic polymeric materials of this invention, plasticizers such as styrene butadiene rubber, zinc stearate, soybean oil to name a few can be added during fabrication.

Generally as thermoplastic polymeric materials cannot be perfumed, the invention provides for the addition of perfume material because of the presence of lignin or any other biopolymer which can absorb such scent additives. In one embodiment of this invention, lignin can be mixed hot or cold either alone or in conjunction with the thermoplastic polymeric material. In this embodiment, the lignin can be treated by maceration in solvents containing essential oils before it is blended with the polymeric materials. In another embodiment, a mixture of scents such as for example terpenes and citronella can be directly injected in one of the sections of the extruder during the manufacturing of the mixture.

It is to be noted that unexpectedly for polymers such as polyethylene, the addition of a biopolymer such as lignin to a polymeric material can lead to an improvement in the material's resilience to ultraviolet radiation. This is unexpected as one does not add an additive which would enable the resistance to photodegradation but rather the lignin plays a role in stabilizing the thermoplastic polymeric material to degradation by ultraviolet radiation as shown in Table 1. It is also to be noted that without the lignin, an increase in the length of exposure to ultraviolet radiation causes an important fragmentation of the polymer and a sudden and significant variation in the molecular weight of the polymer.

Table 1

Molecular Weight

Length Of Exposure To Polyethylene + 10%

UV Radiation (hours) Polvethylene Liαnin

0 320,000 300,000

30 240,000 242,000

200 35,000 120,000

In a preferred embodiment of this invention, PVC can be blended on a weight basis with from about 0.5 to about 40% organosolv lignin with a specific gravity of about 1.27 and a median particle size of from about 0.1 micron to about 100 microns. The final PVC/lignin blends have stronger mechanical properties and can degrade under the effect of light. The PVC blends can be used in medical, food, fashion and home applications. The following sets forth one particular embodiment for the blending of PVC with organosolv lignin. The PVC used is a commercial unplasticized resin (Geon 85862 from BF Goodrich Technical Center, Avon Lake, Ohio, USA) as a suspension polymer of high molecular weight (k = 67) and has the following formulation: PVC resin, 100 phr; stabilizer, 2 phr; processing aid, 1.5 phr; impact modifier, 6 phr; lubricants, 3.75 phr; Tiθ2, variable from 0 to 10 phr. With 10 phr Ti02, the PVC resin has a specific gravity of about 1.48.

PVC blends were prepared with the composition set forth in Table 2.

Tabli 5 2

Blend # Ti02 (%) Orαanosolv Licrnin

1 9.09 0

2 6.81 2.27

3 4.54 4.54

4 2.27 6.82

5 0 0

6 0 4.54

7 0 6.82

8 0 9.09

9 0 13.63

10 0 18.18 The blends were prepared by melt compounding in a Haake Rheomix 600 equipped with roller blades at a temperature of about 195°C. The time of mixing was about 8 minutes at a speed of roller blades of about 65 rpm. PVC was added first and the lignin second after about 30 seconds. Several batches were prepared for each formulation and after melt mixing the obtained blends were ground to a particle size of from about 3 to about 5 mm. Sheets with a thickness of about 2 mm were molded by compression at about 195°C. After cooling with air and under pressure, the sheets were cut with a cutting die in shoulder shaped specimens for mechanical testing.

The mechanical properties, tensile strength and elongation at break were measured before and after 5 days and 20 days of artificial weathering and were correlated with the properties of PVC controls. They were measured in accordance with 7ASTM D 638 using an Instron universal testing machine.

The weathering of the samples was carried out using equipment known in the art such as a Q-Panel QUV. In this tester rain and dew are simulated by a condensation system and it contains a series of UV-A lamps with a peak emission at 343 nm and a spectral power distribution of from 295 to about 400 nm. All • the specimens were subjected to several cycles of 4 hours each of UV exposure at an equilibrium temperature of about 50°C alternating with condensation exposure at an equilibrium temperature of about 40°C. The number of days of accelerated weathering was 5 and 20. Table 3 shows the influence of lignin on the fusion characteristics the blends. The processability or the fusion characteristics of PVC blends is generally influenced by the type of resin and additives present. A change in formulation especially in the case of rigid PVC composition can affect the fusion characteristics of PVC blends and consequently their processability. Improper processability can have a negative effect on the mechanical properties of PVC and its weatherability. It has been found that the fusion characteristics of blends of PVC with lignin formulated with or without Tiθ2 present almost the same characteristics as PVC controls. One may conclude that the fusion characteristics of PVC-lignin blends in comparison with PVC controls are very close and the presence of lignin does not have a negative effect on the processability of PVC-lignin blends. The specimens of PVC lignin blends with Tiθ2 were colored from beige to tan and PVC lignin blends without Ti0₂ were dark brown.

Table 3

Blend Average Average Torque Value Average Temperature Type Fusion of the melt (°C) Time (S) Max. i !_t the end At max At the end (fusion) of 3 min. torgue of 8 min.

1 75 2500 1575 184 205

2 80 2530 1600 180 204

3 85 2570 1600 183 204

4 88 2425 1475 182 204

5 165 1960 1430 182 204

6 100 2280 1430 186 204

7 105 2350 1420 189 203

8 78 2370 1400 181 202 Table 4 shows the strain-stress data for PVC controls and PVC-lignin blends before and after 5 and 20 days. The lack of correlation between the values of the tensile stress-strain data predicted by the theoretical model elaborated by Nielsen (J. Appl. Polym. Sci., Vol. 10, 97-103 (1966)) particularly in the case of perfect adhesion between filler (in this case lignin) and polymer (in this case PVC) and the experimental values shown in Table 4 suggest a certain degree of interaction between the two polymers in the blend. Moreover, up to a certain level of about 6.81% lignin acts as a reinforcing agent without having a negative impact on the elongation. As can be seen afer weathering, all the blends show a higher tensile strength value than PVC controls, and the increased values can be correlated with lignin loading and weathering period. The elongation at break decreasing can be also correlated with weathering period and lignin loading. It can also be seen that after 20 days weathering period, all the blends regardless of their Tiθ2 level show a high degree of embrittlement and an increase in tensile strength with an almost lack in elongation. It is believed that the effect observed can be due to crosslinking.

Table 4

Blend Tensile Strength Elonαation at Break

Type (MPA) (^; %)

Initial 5 Days 20 Davs Initial 5 Days : 20 Davs

5 42.30 48.36 50.52 280 289 47

1 40.15 45.76 49.73 332 292 269

2 43.85 48.30 49.86 281 193 61

3 44.14 49.40 50.58 326 110 45

4 47.05 50.41 51.60 312 91 30

8 48.66 50.26 51.67 182 51 29

In addition, after the weathering period the PVC blends without Ti02 and the PVC blends comprising lignin are characterized by a change in color observed only on the exposed side to UV light. In the case of PVC blends without Ti02, the color changed from white-grey to reddish yellow and in the case of the PVC blends comprising lignin the color changed to lighter tones. In the case of the PVC blends comprising 9.09% Ti02, the change in color after weathering is barely perceptible which is believed to be due to the effect of Tiθ2 on the weathering of PVC.

It is believed that the embrittlement and color change due to artificial weathering show the susceptibility of both interacting polymers to UV radiation. It is believed that the lignin photodegrade as a result of the formation of free radicals, mainly phenoxy radicals.

The PVC blends of this invention can be formulated to achieve good weatherability by blending synergistic levels of Ti02 and lignin.

The invention and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various modifications and changes can be made without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the specific materials, procedures and example hereinbefore described being merely preferred embodiments.

■ For example, by blending different kinds of PVC compounds with lignin, other types of blends can be formulated which would have- the advantage of a lower processing temperature and milder weatherability conditions. Suitable UV absorbers and/or light-thermal stabilizer systems can also be included in the formulations in order to achieve suitable mechanical properties before weathering and suitable shades of colors in the final blend. In another example, the lignin and Tiθ2 can be formulated together to further optimize the photochemical reaction of the lignin and Tiθ2 thus affecting the final photodegradability of the formulation.

Claims (15)

We claim:

1. A method of making a degradable thermoplastic polymeric blend comprising the step of mixing a polymeric material with an organosolv lignin in a weight ratio of from about 0.5 to about 40% on a weight basis with said polymeric material.

2. The method of claim 1 wherein said polymeric material is selected from the group consisting of polyethylene, polypropylene, poly(vinyl) chloride and polystyrene copolymers.

3. A degradable thermoplastic film comprising polyethylene and an organosolv lignin in a weight ratio of from about 0.5 to about 40% on a weight basis with said polyethylene.

4. The film of claim 3 further comprising an oxidizing additive in a weight ratio of from about 0.1% to about 0.5% with said polymeric blend.

5. The film of claim 4 wherein said lignin has a median particle size of from about 0.1 micron to about 100 microns.

6. The film of claim 5 having a thickness of from about 5 to about 100 microns.

7. A method of making a master batch comprising the steps of mixing an organosolv lignin with a polymeric material in a weight ratio of from about 35% to about 85% with said polymeric material.

8. The method of claim 7 wherein said polymeric material is selected from the group consisting of ethylene (vinyl) acetate and styrene butadiene copolymers.

9. A method of making a pellet, said method comprising the steps of:

mixing an organosolv lignin with a first polymeric material in a weight ratio of from about 35% to about 85% with said first polymeric material to form a master batch mixture; and

coextruding said master batch mixture with a second polymeric material to form said pellet, said pellet having a core and a sheath, said master batch forming said core and said second polymeric material forming said sheath.

10. The method of claim 9 wherein said pellet comprises of from about 0.5% to about 40% of said organosolv lignin on a weight basis with said first and second polymeric material.

11. The method of claim 10 wherein said first polymeric material is selected from the group consisting of ethylene (vinyl) acetate and styrene-butadiene copolymers.

12. The method of claim 11 wherein said second polymeric material is selected from the group consisting of polyethylene, polypropylene, poly(vinyl) chloride and polystyrene copolymers.

13. An article of manufacture according to the method of claim 1.

14. An article of manufacture according to the method of claim 7.

15. An article of manufacture according to the method of claim 9.