Biodegradation of PLA/TiO₂ Nanocomposites in Controlled Composting: Functionalized Titania Accelerates Degradation

Introduction

Poly(lactic acid) (PLA) is a leading biodegradable polymer, prized for its renewable origin and potential in biomedical and consumer products. However, its relatively low heat distortion temperature, modest toughness, and slow degradation have limited its broader adoption. A common strategy to improve PLA performance is the incorporation of inorganic nanoparticles—such as nanoclays, carbon nanotubes, zinc oxide, and anatase TiO₂—which can enhance mechanical properties and modulate degradation behavior.

In our prior work, we prepared PLA/TiO₂ nanocomposites by melt blending PLA with lactic‑acid‑grafted TiO₂ (g‑TiO₂). Those composites exhibited improved strain at break and elasticity, and the grafted TiO₂ also influenced hydrolytic and photodegradation of PLA. Yet, the impact of TiO₂ on compost‑mediated biodegradation remains ambiguous, with reports of both accelerated and retarded degradation depending on filler type and loading.

Compost is a warm, moist, aerobic environment that accelerates the breakdown of organic materials. PLA degradation in compost proceeds via a two‑step mechanism: initial hydrolysis of ester bonds producing low‑molecular‑weight fragments, followed by microbial mineralization of these fragments to CO₂ and methane. The rate of water uptake, ester cleavage, and subsequent microbial activity governs the overall biodegradation kinetics.

While many studies have examined the effect of nanofillers on PLA biodegradation, the specific role of TiO₂ nanoparticles is still contested. This study systematically evaluates PLA/TiO₂ nanocomposites under controlled composting, measuring CO₂ evolution and physicochemical changes to determine how functionalized TiO₂ influences PLA degradation.

Methods

Materials

PLA (Natureworks® 4032D) had a weight‑average molecular weight (M_w) of 19,600 kDa and a polydispersity of 1.89 (GPC). The material was dried at 65 °C for 24 h under reduced pressure and stored in vacuum with a humidity absorber. Lactic acid (88 %) was distilled at 80 °C to remove water. Anatase TiO₂ nanoparticles (~20 nm) were supplied by Pangang Co., Ltd. The compost inoculum was derived from the organic fraction of municipal solid waste, provided by the China Plastics Processing Industry Association.

Sample Preparation

Functionalization of TiO₂ and preparation of PLA/TiO₂ nanocomposites followed our earlier protocol (16). G‑TiO₂ was produced by grafting lactic‑acid oligomers onto TiO₂ surfaces. Nanocomposites containing 0, 0.5, 1.0, 2.0, 5.0, 8.0, and 15.0 wt % g‑TiO₂ were melt blended in a co‑rotating twin‑screw extruder. Pure PLA underwent the same thermal history for consistency. Sheets (~0.5 mm thick) were produced by compression molding at 190 °C, 10 MPa, 4 min, then cooled at room temperature under 5 MPa. Specimens (5 mm × 5 mm) were weighed before testing.

Degradation Tests

A laboratory‑scale biodegradation test was conducted according to GB/T 19277–2003/ISO 14855‑1:2005, which quantifies ultimate aerobic biodegradability by measuring evolved CO₂. Each reactor (15 g reference material + 85 g inoculum + 320 g dry sea sand) was inoculated with microcrystalline cellulose (MCE) as a reference and run at 58 ± 2 °C in the dark for 90 days. CO₂ was captured in NaOH and quantified by titration against standard HCl. Data were averaged over three replicates.

Characterization

Microscope Examination

Scanning electron microscopy (SEM, Philips FEI INSPECT F, 5 kV) assessed surface morphology after sputter‑coating with gold.

Thermal Analysis

Differential scanning calorimetry (DSC, TA Q20) measured glass transition, cold crystallization, and melting temperatures under nitrogen flow (50 mL min⁻¹) from 20 °C to 200 °C (heating) and 200 °C to –50 °C (cooling) at 10 °C min⁻¹.

XRD Studies

X‑ray diffraction (XRD, DX‑1000, Cu Kα, λ = 0.154 nm) scanned 2–70° at 6° min⁻¹ to monitor crystallinity changes.

Biodegradation % (D_t)

The percentage of biodegradation (D_t %) was calculated as:
$$D_t=rac{(CO_2)_T-(CO_2)_B}{Th_{CO2}} imes100$$
where (CO₂)_T and (CO₂)_B are the CO₂ amounts from test and reference flasks, and Th_CO₂ is the theoretical CO₂ yield, computed by:
$$Th_{CO2}=M_{TOT} imes C_{TOT} imesrac{44}{12}$$

Molecular Weight Measurement

Gel permeation chromatography (GPC) determined number‑average (M_n) and weight‑average (M_w) molecular weights before and after composting, using chloroform as eluent at 0.8 mL min⁻¹, 30 °C, calibrated with polystyrene standards.

Results and Discussion

Visual inspection revealed distinct degradation patterns. Neat PLA transitioned from transparent to opaque within 10 days, with yellow‑brown plaques appearing after 30 days. In contrast, PLA/TiO₂ composites developed dark brown plaques and pronounced surface cracks as early as 6 days, indicating accelerated microbial colonization and hydrolytic erosion.

SEM images of (a) pure PLA and (b) PLA/TiO₂–2, –5, –8 nanocomposites at 0, 5, and 20 days. (a) 0 day; (b) 5 days; (c) 20 days.

DSC thermograms (Fig. 2) showed a slight decline in glass‑transition temperature (T_g) for all samples over time, reflecting increased chain mobility from hydrolysis and oligomer plasticization. The cold‑crystallization peak disappeared after 2 days, indicating rapid crystallization of degraded segments. The melting peak evolved from bimodal to monomodal, signaling the loss of small, imperfect crystals.

DSC first‑heating scans of degraded samples.

DSC cooling scans of degraded samples.

XRD patterns (Fig. 4) confirmed that the matrix was amorphous initially. After 2 days, distinct peaks at 2θ = 16.4°, 18.5°, 20.9°, and 23.6° emerged, characteristic of poly(L‑lactide) or poly(D‑lactide) crystallites. The increasing peak intensity with time reflected the preferential loss of amorphous material and the growth of crystalline domains.

XRD patterns of PLA and PLA/TiO₂ nanocomposites over time.

The biodegradation curves (Fig. 5) followed the expected lag‑phase, rapid‑phase, and plateau‑phase pattern. The lag phase for nanocomposites was shorter than for neat PLA, indicating that TiO₂ facilitates early hydrolysis. By day 80, neat PLA achieved ~79 % biodegradation, while PLA/TiO₂–5 reached ~98 %. Beyond 8 wt % TiO₂, aggregation reduced the benefits, leading to lower D_t values.

Percentage of biodegradation versus time for neat PLA and PLA/TiO₂ composites. Insert: biodegradation of MCE.

Molecular‑weight loss (Fig. 6) showed a rapid decline in M_n during the first 8 days for all samples, followed by a plateau. The rate constant (k) derived from ln M_n = ln M_n0 – kt (Eq. 3) revealed three distinct phases: an initial high‑rate phase (0–4 days), a slower intermediate phase (5–24 days), and a final plateau (post‑24 days). Nanocomposites exhibited higher k values during the first phase, confirming that TiO₂ accelerates hydrolytic scission.

Change of M_n over time.

Biodegradation rate constant k versus time.

Overall, the data indicate that uniformly dispersed g‑TiO₂ enhances water uptake, initiates hydrolysis, and accelerates microbial mineralization of PLA. The most effective composition was PLA/TiO₂–5, balancing high TiO₂ content with good dispersion to achieve the fastest degradation.

Conclusions

PLA/TiO₂ nanocomposites prepared with functionalized g‑TiO₂ retain PLA’s inherent biodegradability while exhibiting significantly faster degradation under controlled composting. TiO₂ nanoparticles promote water penetration and hydrolysis, especially at 5 wt % loading, resulting in > 97 % biodegradation within 80 days. These findings demonstrate that TiO₂ can be strategically used to tailor PLA biodegradation rates for compostable applications.

Abbreviations

DSC:

Differential scanning calorimetry

D_t:

Percent of biodegradation

GPC:

Gel permeation chromatography

g‑TiO₂:

Grafted TiO₂

MCE:

Microcrystalline cellulose

Mn:

Number‑average molecular weight

Mw:

Weight‑average molecular weight

PLA:

Poly (lactic acid)

SEM:

Scanning electron microscopy

T_cc:

Cold crystallization peak

T_g:

Glass transition temperature

XRD:

X‑ray diffraction