1. Introduction
Materials that exhibit shape memory effect are getting more attention, that is due to their suitability for various applications and industries. This paper focuses on the development of shape memory foam based on polyester polymer composite.
Shape memory effect (SME) is the phenomenon whereby the original shape of a material is recovered in the presence of a suitable stimulus such as heat, light, electric, or magnetic fields [
1]. This has been observed in many different materials, including metallic alloys and polymers [
2]. SME consists of two different processes, as shown in
Figure 1: Programming process in which the material is deformed into a temporary shape, and the recovery process during which the material recovers its original shape.
SMPs and their foams are distinguished by their unique properties such as light weight, high elastic deformation [
3], ease of processing, low cost, and excellent shape recovery properties. Shape memory polymers are widely used for different applications, being employed as light actuators, structural parts with a reduced size during transport, and as expandable/deployable structures. Other potential applications are in the biomedical field, and these include drug delivery, biosensors, and biomedical devices. Moreover, since polymers can become biodegradable, they can be used as short term implants when removal by surgery is hazardous [
4].
Foams can be classified based on their cellular structures to open-cell foams like carbon, metallic and ceramic foams, and closed-cell foams like polyurethane and polystyrene foams. Common methods for producing foams are chemical [
5] or physical [
6,
7,
8] processes. These are complex methods, as they require the insertion of blowing agents. An emerging foaming process that does not require a chemical reaction or a physical foaming agent is known as solid-state foaming. This process was proposed and tested on epoxy polymers by Quadrini and his team [
9]. The main purpose of this foaming process is to simplify the process and reduce costs.
This research is motivated by the advantage of foam compared to the solid form of the polymer. Shape memory polymer foams have advantages over normal shape memory polymers due to their low density and their ability to be compressed. Yet, a major disadvantage of the foam is its low recovery force with a reduced stiffness and mechanical strength. This research introduces the use of composites to overcome this disadvantage. Shape memory composites have greater strength and stiffness [
10,
11,
12], as well as other special properties [
13] determined by the types of filler that are added. Lisuzzo et al. [
14] found that adding 10 wt% halloysite nanotubes to Mater-Bio plastic improved its elongation of the composite by 100%. Linul [
15] and his team found that Aluminum microfiber improved mechanical properties of polyurethane flexible foams. Another study by Meesorn [
16] proved that improved dispersion of cellulose nanocrystals (CNC) enhanced the mechanical properties of EO-EPI/CNC nanocomposites.
Previous work showed that The addition of Fe
3O
4 nanoparticles (NPs) caused an increase in the tensile strength of the poly(d,l-lactide) polymer [
11]. Similarly, another study showed improvement in the resultant compressive strength, when Fe
3O
4 NPs were added to polyimide [
17]. The results of previous studies motivate this research to study the effect of adding NPs to polyester-based foam. This research aims to find out the effect of adding Fe
3O
4 NPs to polyester-based polymer foam; specifically, the impact on the actuation load, and recovery speed of the composite foam. Polyester-based foam was introduced for the first time by Quadrini and Sque [
9], and its performance under microgravity was studied by Santo and Quadrini [
18]. Santo [
19] conducted further development to improve the foam performance. Santo and Tedde [
20] explored its application as an actuator.
The light weight and compressibility of polyester-based foam makes it suitable for space applications; this was investigated by Santo [
21]. The developments and potential of this shape memory foam also motivate this research to explore further improvements.
The next section, experimental method, demonstrates the use of a Taguchi Map as a method to guide the study to find the best parameter levels as well as the best combination of process parameters that lead to the optimal objectives. Results are shown in
Section 3 and discussed in
Section 4. It is clear that these NPs impact the mechanical properties of their host matrix. The paper concludes by
Section 5, where major findings are presented.
5. Conclusions
A solid-state foaming process, with no foaming agent, was tested on two different polymers: Namely, Corro-Coat PE Series 7® (CC) and Jotun Super Durable 2903® (JSD) and their composites with the inclusion of Fe3O4 NPs. Tablets were prepared at different levels as indicated by the Taguchi Map designed for this research. These tablets were foamed at different levels of foaming temperature and varied foaming times. Moreover, different tests were conducted on these foamed samples to measure their shape recovery speed (mm/min) and actuation loads (N).
The results showed that the insertion of NPs into the polymer matrix did not increase the shape recovery speed, and in fact, this caused a reduction in speed. This can be attributed to the fact that NPs do not possess shape memory behaviors as part of their nature, and they caused discontinuity within the polymer matrix.
Generally, the JSD polymer matrix showed higher actuation load values compared to CC, due to its higher yield strength and density. NPs insertion increased actuation load of the JSD composite, and reduced actuation load of the CC composite, this is because density increased for JSD samples, and decreased for CC. Increment in density means that the energy received upon compression was stored in smaller volume, and thus able to release more force when it recovered its shape.