POLYMERS IN THE ROLE OF POLLUTION CREATOR
Biodegradation of natural macromolecules, e.g., celluloses and proteins was a concern during the 1930s. At the time, polymer research concentrated on prevention and retardation of attack on polymers by bacteria, fungi, insects and various atmospheric parameters like moisture, heat, etc. However from the late eighties, use of synthetic inert and bioresistant polymers reached such a high level that their disposal became a concern. It is realized that use of long-lasting polymers for short-lived applications can cause problems toward preservation of living systems. In the seas, plastic rubbish-ropes, nets, packs, etc.-chokes and entangles marine mammals. Plastic debris has a costly impact on waste management for municipalities. Recycling of conventional plastics is one way of reducing the problems associated with plastic waste. However, many packaging materials made of plastics do not lend themselves to recycling, owing to contamination with the contents of the pack along with the ink, and the necessary cleaning prior to recycling is expensive. Furthermore, reprocessing often leads to a downgrading of the polymer used and an increased hold-up in the system. A lack of markets for recycled polymers (mostly lower quality) has led to large stock piles and the dumping of waste products.
The history of developing biodegradable plastics to replace indestructible and landfill squatting plastics has been full of expectations and disappointments along with some success over the last 20 years. There are at least 15 companies worldwide who are engaged in commercial development of degradable plastics. The polyvinyl alcohol (PVOH) based plastic Depart from Environmental Polymers (EPG) USA, Polylactic acid (PLA) in the name of Chronopol from Monsanto, copolyesters Ecoflex from BASF, Biomax from DUPONT, Biopol from ICI, (a copolymer of polyhydroxybutyrate) and valeric acid (PHB/V), Eastar Bio from Eastman Mater-Bi (starch with PVA) and Novon (starch with additive) are some of the known commercial grades of biodegradable plastics.
The bioplastics industry suffers from two major obstacles. One is the product's much higher cost than that of petroleum derived plastics. The second one is lack of transparency in the nature of biodegradation. Although these PVOH, PLA, polyesters, PHB/V, cellophane etc. meet the requirements for biodegradability, they could not meet the required permeability to moisture, oxygen and other barrier properties besides the higher cost element of the film. Natural polymers or biopolymers are largely based on renewable resources such as starch, cellulose, proteins and pectins. Synthetic polymers are made from petroleum and other feedstocks. Conventional petrochemical-based materials are not easily degraded in the environment because of their high molecular weight and hydrophobic character-hence the need of recycling, incineration etc. With no real relief from recycling and incineration, biodegradation remains the ultimate goal. In general, naturally occurring polymers are more biodegradable than synthetic polymers. Polymers containing ester functionality, particularly aliphatic polyesters are potentially biodegradable. It is believed that biodegradation of these polymers proceeds by attack of the ester groups by nonspecific esterases produced by ground micro flora combined with hydrolytic attack. Products of the degradation can be quickly metabolized by microorganisms.
USE OF POLYETHYLENE IN FLEXIBLE PACKAGING
Among the various plastics-both rigid containers and flexible packs, we have seen over the past four decades, the rapid penetration of LDPE (low-density polyethylene), HDPE (high-density polyethylene) and LLDPE (linear low-density polyethylene) in the carry bag market and flexible pouches. We are aware of huge day-to-day use for polyethylene films in the form of:
- Colored and uncolored polybag for groceries, stationery shops and vegetable market
- Black garbage bags
- Bread bags
- Hosiery/textile bags
- Woven sacks
- Household wrap film
- Poly pouches, etc.
Besides above, there is industrial use of polyethylene film as lamination film, agriculture film, construction film, heavy-duty sacks, stretch/shrink films, etc.
All these applications of polyethylene have derived from excellent techno-commercial features of polyethylene family and improvements in film extrusion technology. Development in catalyst and polymerization process technology continues unabated with metallocene systems and the anticipation of "new generation" linear polyethylene's combining improved processability and physical properties. These developments created new product and market opportunities for the polyethylene converter.
OPTIONS FOR BIODEGRADABLE POLYETHYLENE
Though the general belief is in favor of using paper as an environmentally friendly biodegradable packaging material over plastic, the paper suffers from two-and-a-half time energy loss in its manufacturing, loss of trees from environment, hence spoiling soil bank and gaseous imbalance in atmosphere. On the above account, polyethylene is the choice, provided it can be made biodegradable and here comes starch blended LDPE film.
Out of the three major manufacturing routes of producing starch blended polyethylene film,(1) modified starch incorporation into polyethylene, (2) starch incorporation into modified polyethylene, and (3) blending of polyethylene and starch in presence of a coupling agent, the third method is reported to be cost effective. Some of the worldwide known manufacturers of such starch-LDPE biodegradable films are AMCO Plastics, USA; Novamont, Italy; Exxon Chemicals, Belgium, etc. Using low-density polyethylene (60 %), cornstarch (30 %) and additives (10%), the starch-blended polyethylene film has been produced. The novel proprietary coupling system developed by Maiti and coworkers7 resulted promising cost effective LDPE-starch blend film. Starch will be eaten by soil microorganisms in a landfill and therefore, the plastic matrix will be broken down into smaller particles.8 Such starch blended polyethylene film though produced in commercial scale, the film had inadequacy for printing and packaging applications. The ink anchorage on the starch blended polyethylene film is poor and inadequate. The regular practice of corona treating the LDPE-starch film surface did not result in ink anchorage onto the film.
GRAFTING AND CHARACTERIZATION OF GRAFTED LDPE - STARCH FILM
Grafting on LDPE - Starch Film LDPE-starch film under a stream of nitrogen gas was soaked with Ceric Amonium Nitrate (CAN) in acidic solution. After the excess CAN solution was drained out from the reaction flask and vinyl acetate was added dropwise on the film surface under constant sparging of nitrogen gas into the flask.9 The graft copolymerization was continued for three hours at 30 °C. After 3 hours, the film was washed thoroughly with water and acetone. The homopolymer, if formed, was removed completely with hot acetone (45 - 50 °C) for two to three hours. Finally, the grafted films were dried and weighed.
CHARACTERIZATION OF GRAFTED FILM
Characterization of the vinyl acetate grafted LDPE-starch film was done by measurement of percent grafting, haziness and color development on the film, UV-VIS Spectroscopy, X-ray diffraction, ATR-IR and Scanning Electron Microscope (SEM). Up to a maximum of 12% vinyl acetate grafting was carried out, when CAN concentration was at 0.912 moles/ litre. Since the objective is to develop better anchorage/printability without affecting the biodegradability of the LDPE-starch film, higher or extra-amount grafting was unwanted. Greater grafting is supposed to pile on the film surface, thereby decreasing the biodegradability. At 12% vinyl acetate grafting, the film has provided satisfactory printability and ink anchorage (which is reported in a subsequent part).
The UV-VIS spectra of the vinyl acetate grafted LDPE-starch film with absorption maxima at 209.5 nm is indicative of the extended coverage of vinyl acetate grafting onto starch-LDPE film. The ATR-IR spectra with the additional broad peak at 1747 cm-1 shows the presence of ester group of vinyl acetate grafted onto LDPE-starch film. From the X-ray diffractogram, it is seen that grafting of vinyl acetate onto LDPE-starch film does not change the overall shape of the diffraction pattern. The crystallinity of polyvinyl acetate grafted LDPE-starch film is higher than that of the ungrafted LDPE-starch film.
From the Scanning Electron Microscope (Figure 1), it is established that the surface of the LDPE-starch film becomes masked with grafted polymer. Thus grafting 10 of the vinyl acetate monomer onto starch blended LDPE film opened up the possibility of better surface characteristics of film to make it printable.
STUDIES OF POLYETHYLENE PRINT & LAMINATE PROPERTIES AND THEIR DEGRADATION IN SOIL
Polyethylene finds two major applications in making flexible packaging-one for surface printing where properties, e.g., printability, ink adhesion, gloss, solvent retention in the print, etc. have been studied. Besides surface printing properties, polyethylene films act as a sealable layer in polyethylene laminates for which laminate bond, heat sealability, solvent retention etc. have been studied.
Evaluation of film, print and laminate properties of vinyl acetate grafted LDPE-starch film (GBP) was done in comparison to that of prints on industrial standard corona-treated LDPE film (STP) and starch-blended LDPE film (SBP).
FILM SURFACE TENSION
Surface tension of the polyethylene films were measured by Visking solution using formamide and cellosolve mixture and contact angle following sessile drop method 11 with water and formamide as the probe liquids. The surface energy of GBP film, 40 dynes/cm was found to be as high as that of STP film, hence encouraging for printing purpose. Whereas the surface tension of SBP film is at 32 dynes/cm.
PRINT AND LAMINATE PROPERTIES
Two ink formulations were studied for surface printing on STP, SBP and GBP films. The print on STP was taken as an acceptable reference standard. Ink (1), which is predominantly based on co-solvent polyamide resin, phthalocyanine b blue pigment, and iso propyl alcohol, butyl alcohol solvents offers adhesion and satisfactory printability on GBP film. Whereas the SBP film does not provide the adequate anchorage. Proper adhesion ensures the color retention on the printed film. The improvement in ink adhesion is noticeable on print of GBP in comparison to print of SBP film, which shows the utility of vinyl acetate grafted surface. The ink (2), which is based on predominantly nitrocullulose resin, phthalocyanine b blue pigment, and ethyl alcohol, ethyl acetate solvents has shown satisfactory film surface wetting and printability. It is evident that the problem of poor adhesion on SBP film with ink (2) has been overcome by the vinyl acetate grafted surface of GBP.
Lower gloss level of both the inks (1) and (2) on SBP was caused by more porosity of the starch-blended surface of polyethylene, for penetration of resinous ink into the film. The higher gloss values 56.3 and 50.9 by Glossometer at 750 on prints of GBP film with inks (1) & (2), indicates the uniformity of film surface causing less penetration of ink resins into pores of the film and resulting in the desired higher gloss values. Comparison prints of SBP film had gloss values 29 & 32 of ink (1) & (2) respectively.
Higher solvent retention in the print is not a desirable packaging property. The lower the retained or entrapped value, the better it is. The very high solvent retention value 56.68 mgm/sm found in SBP is due to the presence of starch element in the film. However grafting with vinyl acetate in GBP caused lowering of solvent retention to a great extent at 29.97 mgm/sm, where solvent penetration was prevented by the grafted polyvinyl acetate layer on the film.
The polyester film was reverse printed with ink (3), based on vinyl chloride/vinyl acetate copolymer and ethlylene vinyl acetate copolymer resins, phthalocyanine b blue pigment and MEK & toluene solvents. The laminate made with polyester print and GBP film, had bond value of 500 g/15 mm, which is encouraging. The laminates of both SBP and GBP have shown higher solvent retention, which are attributed to starch and vinyl acetate components. The polyethylene films in both the laminates of SBP and GBP have shown sealability, which is a requirement for pouch making.
ISOLATION OF MICROORGANISMS FROM SOIL
The soil sample from the testing field was characterized for its types of microbes present. Microbial densities were determined by the dilution plate count technique.12 Soil (10 g) was suspended in sterile distilled water (90 ml) and shaken vigorously for 15 minutes. Appropriate series of 10 fold dilutions were performed and aliquots of 1ml were placed in petridishes containing different media. The plates were incubated at 30-32ºC for 10 days. Single isolated colonies were counted after 10 days of incubation and soil microbial population was calculated in terms of colony forming units per gram of soil. For bacteria, actinomycetes and fungi, nutrient agar, glycerol-asparagine agar and potato dextrose agar were used respectively. All media were sterilized by autoclaving at 15 psi for 15 minutes. The bacteria, microbes etc. present in the soil were thus evaluated (Table 2 ) by the culture of the soil.
SOIL BURIAL TESTS
What happens to plastics packaging that are dumped into landfill is very much debated. Landfill sites vary, and within any given site there can be considerable variation. Stories of 20-year-old newspapers still being legible and undecomposed carrots are still talked about and are attributable to low water availability. The various breakdown or degradation mechanism steps involving hydrolysis, solubilization, thermal degradation, oxidative degradation, mechanical degradation, photodegradation, biodegradation etc. are believed to operate simultaneously.
Microorganism, bacteria, fungi and algae are living catalysts, which help a large number of chemical processes to occur in water and soil. Fungi and bacteria decompose chemical compounds to simpler species and thereby drive the energy requirements for their growth and metabolism.
Krupp and Jewell studied biodegradability13 of following modified plastic films in controlled biological environments. Polyethylene with 6% ADM Starch, (AMCO Plastic Inc., USA), Biothene (Weisstech Corporation) with 6% starch in polyethylene matrix was reported to degrade in landfills. BIOPOL (Zeneca/ICI) and polyethylene with 10-12% starch (Eco-matrix Ltd.) are other examples. Their study concluded biodegradations in aerobic and anaerobic bioreactors.
Although biodegradation is rather simple and naturally occurring phenomenon, the various microorganisms, e.g., bacteria, fungi, actinomycetes etc., from natural sources (water, soil, air, etc.) are quite complex in their behavior and activity. Degradation of starch blended polyethylene in soil burial test have been studied and confirmed under various compost field environment.14,15 Hence there lies a necessity for the study of biodegradation of printed polyethylene films.
Similar to what happens in real life garbage, the printed films and laminates were put in soil burial test where soil based microorganisms and enzymes are active. A pit was dug and the soil of the pit was made free of dirts, stones, polymeric materials etc. The surface prints on STP, SBP and GBP of both the inks (1) and (2) and laminates of STP, SBP and GBP were kept at the bottom of the pit. The pit was filled up with loose, moist and soft soil. The prints and laminates were studied on their degradation at three months intervals for one year after digging the pit and taking out the prints and laminates.
Characterization of polymer degradation was done by studying the degree of degradation or loss in polyethylene. The photographs of the prints of ink (1) on STP, SBP and GBP after keeping in soil for three days and 12 months are shown in Figure 2 (STP/SBP/GBP). The STP prints do not show any loss of print color and polyethylene film. The prints on SBP show a number of pores, developed after six months and the pores have increased with time. The development of pores signals the disintegration of SBP film. The color of ink in general has been retained in the print. The prints on GBP also have shown the pores development, signaling the much-desired disintegration of the GBP film. The ink color of GBP print however has been retained like SBP print.
The physical changes on the prints of ink (2) on STP, SBP and GBP, after soil burial for three days and 12 months are shown in Figure 2. The ink (2) printed SBP and GBP films showed much removal or loss of colored print area in comparison to Ink (1) printed films. The cellulosic resin formulation of ink (2) has resulted in the desired greater loss of ink materials.
The polyester laminates with the three grades polyethylene films STP, SBP and GBP, after keeping in soil did not show any change and loss of laminate body. The ink color remained mostly unchanged in the laminates. However in the laminates of SBP and GBP, the polyethylene part have shown development of pores. The degradation of polyethylene in SBP and GBP prints and their laminates are significant which is a desired phenomenon. The soil burial test also clearly establishes the inertness of STP film toward degradation. In the laminates, the printed polyester film did not show any degradation.
CONCLUSION
The optimized nitrocellose based ink and grafted starch-LDPE combination lead to development of surface printed polyethylene package, where both ink resins and film degrade in soil burial conditions, whereas the conventional polyethylene and ink resins do not degrade. Since no degradation of polyester film (used in laminate of polyester and polyethylene) was noticed, work may be initiated to look into solution through alternative film, ink development and or newer packaging option for such laminates.
The novelty in the present investigation lies in the modification of the starch blended polyethylene film by vinyl acetate grafting and subsequent printing and biodegradability study of the printed film using optimized ink composition. There is tremendous scope for further investigation and development in this topic. The area of establishing the nature of degradation, identifying the microbe type and quantifying the degree of microbial attack demand special attention.
Dr. Rabindra Nath Ghosh can be reached via e-mai at rabin.ghosh@cal.coates.co.in
Correspondence: 49/1 P.B.SHAH ROAD, KOLKATA 700033, INDIA.
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