Plastic components are exposed to various environmental influences during their service life which can significantly change their material properties and reduce their life cycle . These influences are classified as “internal” factors (chemical degradation, physical structure, etc.) or “external” factors (temperature, atmospheric and biological burdens, etc.) .
Generally, multiple factors come into play at the same time. Plastics are chemically and thermomechanically stressed by, e.g., temperature and humidity, both during production/processing and later operating directly on the component. This may result in oxidative and/or hydrolytic chain degradation of the materials. The change in chemical structure may cause changes in mechanical, thermal and rheological property; rigidity, impact strength, thermal and chemical stability, and the elasticity of a material will be reduced . Particularly plastics containing ester groups, such as bio-based PLA, are prone to a hydrolytic degradation process (figure 1). Via cleavage of the molecular chains, reactive acid end-groups are developed which are then responsible for further chain reactions.
In order to prevent premature failure of a component, stabilizers are often employed in plastics processing that improve long-term stability in relation to external influences . Such additives, however, also mean higher costs for material manufacturers and ultimately also for the product itself.
Information on the effectiveness of stabilizers is therefore crucial for material development and the target-oriented application of additives . Already slight differences in concentration can change the properties and determine the price of the material depending on the application field.
In order to analyze the influence of various levels of PLA hydrolysis stabilizer concentration, several modified PLA compounds were manufactured, characterized and subjected to aging tests.
The material used was a semicrystalline PLA homopolymer (PLLA) to which additives were added in different concentrations (1% / 1.5 wt%) for protection against hydrolysis during extrusion. The additive thus protects the biobased plastic from molecular chain degradation both during processing and in the finished product. The hydrolysis stabilizer reacts with the acid end groups of the PLLA and so prevents the acidcatalyzed ester hydrolysis .
The recorded thermomechanical properties are presented and compared in figure 2. Hydrolysis protection (1 - 1.5 wt%) has no influence on the tensile strength, tensile modulus or heat deflection temperature. The impact strength rises with increasing concentration of the stabilizer, indicating a change in crystal structure. The flowability is reduced by 1.5 wt/% due to the cross-linking effect of the additive, what might have a negative impact on the cycle time. This also suggests a reduction in the degree of crystallization .
In order to be able to judge the influence of the additive on crystallization, the materials were analyzed by means of DSC (NETZSCH DSC 204 F1 Phoenix®). The samples were dynamically heated (heating/ cooling rate: 10 K/min) in a closed aluminum crucible (approx. 7 mg) under a nitrogen atmosphere (flow rate: 20 ml/min) to a temperature above the melting point (Tm = 170°C).
As can be seen from the 2nd heating curves (figure 3), the melting enthalpy (ΔHm) is reduced as the concentration of the hydrolysis stabilizer increases. Less energy is required for melting the crystalline portions of the material. On the basis of ΔHm and the literature value for a completely crystallized PLA (ΔHLit. = 93 J/g [7, 8, 9]), the degree of crystallization (K) can be calculated using the formula:
K defines the stiffness as a function of the crystal structure. It is generally the case that the higher the value for K, the stiffer and more brittle a material is. In comparison with PLLA, there is a tendency for decline in the degree of crystallization of the stabilized samples. This might be a reason for the increase of impact strength as well as of the cycle time (figure 2).
In a water immersion test, all samples were continuously aged at 65°C over a period of 750 hours.
By means of DSC, the melting ranges of the materials were recorded in order to draw conclusions about the degradation behavior. Molecular chain degradation generally yields shorter chain segments that melt at lower temperature and thus result in a lowered melting point, evaluated as maximum peak of the melting effects (Tm).
With the 2nd DSC heating curve (closed Al crucible, sample masses: 5.5 - 6 mg, N2 flow rate: 20 ml/min, heating/cooling rate: 10 K/min), hydrolytic degradation of the material becomes noticeable (figure 4).
The samples exposed to water exhibit a shift of Tm to lower temperatures (max. ΔT = 21°C) after 750 h. As follows, a higher concentration of the stabilizer offers longer protection against material degradation (1.5 wt%, ΔT = 15°C).
The influence of the water immersion test on Tm from 0 - 750 h (figure 5) illustrates that the PLLA already exhibits a drastic degradation of the molar mass after 50 h in H2O. This means a complete loss of the material's properties and functional failure of a component. The stabilized PLLAs exhibit the beginning of material degradation after 300 h (1 wt%) and 500 h (1.5 %wt%), whereby the longterm resistance to H2O is significantly improved.
In order to be able to make any declarations about the stabilizer's effect and the concentration with respect to O2, isothermal and dynamic OIT measurements were carried out comparatively using the DSC 204 F1 Phoenix®. These in turn should allow for conclusions to be drawn about the oxidation behavior due to exothermal reactions with oxygen [1, 10, 11].
For determination of the Oxidation Induction Temperature (OIT), the sample was initially heated to 200°C (10 K/min) in an open aluminum crucible (sample mass: 13 mg) under a nitrogen atmosphere (flow rate: 20 ml/min). Following an isothermal phase (5 min), the purge gas was switched to O2 and the sample was dynamically heated to 330°C (10 K/min) at a flow rate of 50 ml/min.
As the exothermal reaction (figure 6) shows, the non-stabilized PLLA is irreparably damaged at a significantly lower temperature (< 260°C). In contrast, the stabilized samples react with oxygen at a temperature >290°C. The subtle differences between concentrations of 1 wt% (> 290°C) and 1.5 wt% (> 310°C) can clearly be seen. This means that the PLLA has been successfully stabilized.
For determination of the Oxidation Induction Time, the samples (15 mg) were dynamically heated to a temperature over Tm (10 K/min) in an open aluminum crucible under a nitrogen atmosphere (flow rate: 20 ml/min) and, beginning at 225°C, isothermally purged with O2 (flow rate: 50 ml/min, 50 min). The time from the first contact with O2 until the start of oxidation was measured .
Fig. 7 shows that the performance of pure PLLA against O2 can be improved by using hydrolysis stabilizers. Compared to the stabilized samples, PLLA exhibits an oxidative reaction already after a short time (OIT = 1.6 min) and is irreparably damaged.
The two stabilized samples (1 - 1.5 wt%), in contrast, exhibit no oxidative reaction.
The resistance of PLLA to O2 and H2O can be prolonged with the help of hydrolysis stabilizers. The additive protects a material – when subjected to external factors – against molecular degradation and therefore also against the loss of material's properties. Even the smallest of differences in concentration (0.5 wt%) can influence the material properties (impact strength, flowability, degree of crystallization) and determine the long-term stability and the life span depending on the application. Target-oriented metering of expensive additives additionally offers possible savings in terms of material costs.
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