Recovery Behavior of Thermoplastic Shape Memory polyurethane Based Laminates after Thermoforming- Varied Modulus of Polyurethanes

Abstract

In recent decades, a type of shape memory polymers (SMPs), namely thermoplastic shape memory polyurethane (shape memory TPU, using TPU for short) has drawn considerable attention for its excellent shape memory properties, versatile structure and good mechanical properties. Most recently, shape memory TPU films are envisioned as a replacement for automobile exterior and interior decorative applications in the forms of laminates through in-mold forming (IMF) process. However, for a better dimensional control of laminates during the IMF, the shape memory effect of laminates needs to be controlled such that its behaviour is only noted at the time of damage and is not an instigator of delamination. In order to investigate the shape memory behavior of TPU based laminates after they had experienced normal processing such as by thermoforming, the influence of different properties were examined, including TPU film modulus, substrate used (polypropylene (PP) versus acrylonitrile butadiene styrene (ABS)), ambient temperature and the extent of deep draw, on the recovery behaviour. The study included analyses through both experimental and modelling methods. A novel thermo-mechanical cycling method was proposed to examine the shape memory property of the TPU based laminates under stretching/bending conditions more similar to thermoforming. Recovery based on this method was defined using new terms of angle recovery ratio and recovery rate. The new test examined recovery at 15oC, 45oC and 65oC; these ambient conditions were selected above and below the glass transition temperature of the TPU. Results showed that the final angle recovery ratio and recovery rate of deformed laminates based on a new commercial class of TPU shape memory polymer increased with its modulus from low to high. Substrates of higher modulus (ABS) lowered the final angle recovery ratio and recovery rate achievable for a formed laminate. Furthermore, increasing the ambient temperature increased both the final angle recovery ratios and recovery rates of formed TPU based laminates. As the extent of draw changed from 6 mm to 10mm, the final angle recovery ratios and recovery rates of formed laminates increased for all TPU films but this trend was reversed when the draw further increased beyond 10mm. The laminate system was subsequently modelled using a linear viscoelastic (SLV) constitutive model to analyze the stress-strain relationship between the substrate and TPU film layers during recovery. A model parameter related to stress transfer across the interface of these two polymer layers was fitted to the experimental results with an excellent degree of fit. The model results fitted well with experimental data and showed that the final angle recovery ratios of formed TPU laminates were mainly dependant on the moduli of TPU and substrates layers as well as the stress transfer ratio through the adhesive layer (TR). The influence of the adhesive layer was not a trivial variable in the recovery nature of the laminate. The influence of ambient temperature on the recovery behaviour of laminates was mainly due to the temperature-dependent and time-dependent Young’s modulus and relaxation time of both TPU and substrate layers. Higher relaxation times for the TPU layer or lower relaxation time for the substrate layer yielded a higher recovery rate for the laminate during the first five minutes of recovery.