Testing of polyvinyl alcohol and starch mixtures as biodegradable polymeric materials

doi:10.1016/S0142-9418(99)00049-5

Polymer Testing

Volume 19, Issue 7, August 2000, Pages 785-799

https://doi.org/10.1016/S0142-9418(99)00049-5 Get rights and content

Abstract

The mechanical properties, thermal behaviour and biodegradation of some polymeric materials based on polyvinyl alcohol and starch, in the presence of urea (used as a micro-organism nitrogen source/plasticizer) and glycerine (used as a plasticizer) have been studied. The chemical composition optimization of the analysed blends was carried out using a second order factorial experimental design. The progress of biodegradation was evidenced by means of scanning electronic microscopy, differential scanning calorimetry and thermogravimetric measurements. The evaluation of the weight loss of the samples at the end of the biodegradation process was also carried out. The weight loss after 18 days of inoculation with microorganisms is between 38 and 73% by weight and is influenced by the composition of the blend as well as by the nature of the microorganism used. During biodegradation, plasticisers (glycerine and urea) and starch are consumed together with some amorphous structures from polyvinyl alcohol. The crystalline structures of polyvinyl alcohol which remained after biodegradation show high values of entalpy, and contribute to an increase in the apparent thermal stability of the biodegraded samples.

Introduction

Biodegradable polymers have been a subject of interest for many years because of their potential to protect the environment by reducing non-biodegradable synthetic plastic waste [1], [2], [3], [4], [5]. Biodegradation involves enzymatic and chemical degradation by living microorganisms [6], [7], [8].

In essence, the enzymatic degradation of polymers takes place by hydrolysis and oxidation. Most synthetic polymers cannot be degraded by microorganisms. To increase their biodegradability, non-biodegradable polymers are associated with fully biodegradable polymers, such as aliphatic polyesters [9], [10], or natural polymers [11], [12].

Products from agricultural sources, with reference to starch and protein especially, constitute a good alternative in developing degradable materials [13]. Starch, a mixture of amylose and amylopectin, is a renewable and fully biodegradable polymer, readily available in high purity and at low cost. Unfortunately, the starch has no physico-mechanical characteristics, as well as processing properties, good enough to allow the whole replacement of the composite materials based on petroleum hydrocarbons. The products from starch are mostly water soluble and brittle. Some properties of starch can be improved by blending with synthetic polymers.

There are few data published regarding the biodegradability of polyvinyl alcohol (PVA)/starch mixtures [14], [15], [16]. Particular attention was given to the biodegradation of the starch component as a function of the amylose/amilopectin ratio [17]. The mixtures of PVA/starch were studied regarding their processability [14] or mechanical properties [18] especially.

In this paper we report on the results of preliminary findings regarding the mechanical properties as well as the thermal behaviour of blends based on starch in mixture with PVA. The synthesis of the blends was carried out with the main objective of obtaining some experimental parameters, which can then be used to study the biodegradation process of the polymeric materials. The biodegradation of PVA/starch mixtures was carried out in the presence of some bacteria and fungi localized in sediments obtained from activated sludges of municipal sewage plant and from landfill.

Section snippets

Materials

The raw materials used in our experiments are available as commercial products. Industrial corn starch, a white fine powder with 2% by weight moisture content, acidity 2.0 (cm³ 0.01 N NaOH solution) and around 1% by weight proteins and lipids was provided by Amidex-Tg, Secuiesc, Romania. The PVA polymer, with hydrolysis degree 88%, polymerization degree 1200, saponification index 140±30 mg KOH/g, K value 65±5 and 2% by weight ash, was obtained from Romacryl-Rasnov, Romania. Other materials used

Mechanical properties

The tensile strength as well as the elongation at break were evaluated using a mechanical dynamometer of TIRA TEST-2200 type (Germany). The samples (100×10×0.2 mm) were conditioned in a vacuum oven containing CaCl₂, at 50°C, residual pressure 0.01 kgf/cm² for 2 h. Three determinations were carried out for each sample, and the results were taken as their average values.

Scanning electronic microscopy (SEM)

The morphology of the surface of the films, before and after biodegradation, was investigated using a scanning electronic

Results and discussion

Data processing on an IBM compatible PC computer led to the following regression equations:

(a) tensile strength (Y₁): $Y_{1} =9.0675+1.765X_{1} +0.6854X^{2}_{1} −0.0075X_{1} X_{2} −0.42X_{1} X_{3} +0.415X_{1} X_{4} −1.1342X_{2} +0.1079X^{2}_{2} +0.3587X_{2} X_{3} +0.5688X_{2} X_{4} −1.1992X_{3} +0.1104X^{2}_{3} +0.1238X_{3} X_{4} +1.3808X_{4} +0.1129X^{2}_{4}$

(b) elongation at break (Y₂): $Y_{2} =101.105−22.6313X_{1} +3.5591X^{2}_{1} +6.0106X_{1} X_{2} +2.6769X_{1} X_{3} −1.3431X_{1} X_{4} −6.7296X_{2} −2.5247X^{2}_{2} −2.0519X_{2} X_{3} −2.0319X_{2} X_{4} +4.1596X_{3} +6.4753X^{2}_{3} −2.6156X_{3} X_{4} −2.0896X_{4} +0.0791X^{2}_{4}$

On the basis of the regression equations the level

Conclusions

Some preliminary results were obtained before biodegradation of blends based on starch, PVA, urea and glycerine. Generally, the tensile strength increases with increasing PVA amount, as well as starch amount and decreases with increasing glycerine and urea amount in the blends. An explanation of this behaviour can be due to the glycerine, which acts as a plasticizer, leading to a greater mobility of both the starch and PVA macromolecular chains and the presence of some strong intermolecular

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Material BehaviorTesting of polyvinyl alcohol and starch mixtures as biodegradable polymeric materials

Abstract

Introduction

Section snippets

Materials

Mechanical properties

Scanning electronic microscopy (SEM)

Results and discussion

Conclusions

Int. Biodeterior. Biodegrad.

Carbohydr. Polym.

Emerging technologyes for materials and chemicals from biomass

ACS Symposium Series

Polym.-Plast. Technol. Eng.

In: Biodegradable polymers from agricultural products

ACS Symposium Series

J. Environ. Polym. Degrad.

Material Behavior
Testing of polyvinyl alcohol and starch mixtures as biodegradable polymeric materials