by Julius Vogel and David Grewell, Agricultural and Biosystems Engineering, Iowa State University
With the growing demand for environmentally friendly biorenewable resources, there has been a parallel growth in the development of bioplastics. These include commercially available starch- derived plastics and plastics derived from renewable oil and proteins. As with any plastic, these new materials often must be joined to produce final products. This article reviews impulse and ultrasonic welding of PLA as well as friction welding of plant protein-based plastics. It was found that each of these plastics can be welded with weld strengths matching the parent material strengths.
Bioplastics are currently defined by the BioPreferred for semi-durable plastic films with a minimum content level of 45 percent and disposable cutlery of 48 percent of renewable feedstock. While the materials that meet this minimum requirement may help offset the negative impacts of petrochemical plastics, plastics that are 100 percent renewable represent more environmentally friendly plastics. The reason for the ambiguity on the environmental impact is the ongoing studies on the life cycle analysis that define the impact of any product on the environment. Because it is commonly reported that many of the materials such as PLA (polylactic acid) and plant protein plastics result in less green house gas production and energy usage, this article reviews the welding of these materials for assembly of commercial products.
Polylactic acid is a biodegradable polymer derived from the monomer lactic acid. It is made from 100-percent renewable resources such as sugar beets, wheat, corn or other starch-rich crops. During the production, the corn (or other feedstocks) is initially milled, then slurried in water and heated – a process called mashing. This process separates the starch from the hulls of the corn, sterilizes the corn and swells the starch granules for depolymerization into lower molecular-weight molecules sugars. The starch is then further depolymerised to simple fermentable sugars through an enzymatic reaction called saccharification. The sugar molecules (dextrose or glucose) are then converted to lactic acid through fermentation. The lactic acid is then polymerized, either through a condensation reaction or through a ring opening process. Condensation polymerization involves the removal of water through condensation and the use of a solvent under a high temperature in vacuum. Condensation polymerization in a high-boiling-point solvent results in a high molecular-weight PLA, while the condensation polymerization at lower temperatures generally produces lower molecular-weight polymers, which can be additionally treated with coupling reagents to produce high molecular PLA.
In addition to converting starch into polymers, other researchers have studied the use of plant proteins, a natural polymerized amino acid, as plastics. These polymers have the advantage of being renewable, biodegradable and not linked to depleting fossil fuels. The thermo-mechanical and water-absorption properties of soy protein-based plastics depend heavily on the plasticizers used (e.g., glycerol, ethylene glycerol, butanediol, sorghum wax and sorbitol), addition of cross-linking agents (e.g., zinc sulfate, acedic anhydride, formaldehyde) and processing parameters (e.g., extrusion pressure and temperature and initial moisture content).
These polymers can be engineered for structural applications by incorporating nanoscale reinforcements, such as biocompatible nanoclays, to create a composite. The addition of the nanoclay also lowers the overall cost. Another, cost effective matrix material is defatted soy flour (which contains approximately 53-percent protein), which historically has inferior mechanical properties to the already low cost soy protein isolate (ca. 92-percent protein). Because of its low cost, soy protein-based plastics in particular have the potential to replace traditional plastics in applications ranging from packaging to disposable road signs to drug delivery.
Because most applications cannot be molded as a single part, joining of sub-components is often required. While there are a number of methods for joining plastics, it is common to use ultrasonic, vibration, hot plate and impulse welding to join films and sealing applications. Researchers have shown that while hot plate welding is not effective in joining plant protein plastics, vibration welding can produce welds with near parent material strengths. Figure 1 shows photographs of various fracture surfaces of the vibration welded lap joints. It is seen in sample (A), where a relatively strong weld was noted (0.45 MPa (66 psi)), that there is evidence of significant material shearing pull out at the faying surface. This suggests that the weld is actually near the parent material strength. In contrast, samples with (B) intermediate (0.4 MPa (58 psi)) and (C) weak weld strengths (0.3 MPa) show some shearing and pull out and very little shear and pull out, respectively.
Other work has studied the welding of PLA with impulse and ultrasonic welding. The results showed that relatively high weld strengths could be achieved with impulse welding over a relatively wide range of parameters. In addition, ultrasonic welding produced samples of relatively high strength. However, while this process can be used with faster cycle times, it was less robust. In detail, ultrasonic-welded samples of a thickness of 254 µm that were welded with a cycle time of 0.25 s had an average strength of approximately 160 N (36 lbs), while the results showed a standard deviation up to 50 N (11 lbs), see Figure 2.
In impulse welding, samples of 100 µm thickness welded at 2 and 3 s had a strength of approximately 75 N (17 lbs), while the deviation was approximately 3 or 4 N (0.7 or 0.9 lbs), see Figure 3. It also was seen that sample thickness affected the optimized welding parameters as well as ultimate strength. A thickness of 305 µm had a weld strength of 80 N (18 lbs) while the strength was 25-30 N (5.6 to 6.7 lbs) at a thickness of 200 and 254 µm.
In this work, ultrasonic welding of PLA was further studied. Ultrasonic welding is an important process in the industry because it is fast, economical and has a high repeatability to produce high-quality joints. While it can be used to join crystalline materials, it is mostly used for joining amorphous materials, such as polystyrene. Examples of ultrasonic-welded applications are electrical switches, vacuum and pressure valves, floats and aerosol containers. During ultrasonic welding, mechanical vibrations at high frequencies of 20 40 kHz at a low amplitude (usually between 20 100 µmp-p) are applied to the parts to be joined. The heat generated by the cyclical deformation of the thermoplastic material is highest at the interface of the parts because of surface asperities resulting in intermolecular friction to melt and fusion-bond the plastics. Typical joint designs include energy directors and shear joints for rigid components. However for welding films, it is common not to incorporate a joint design and rely on asperity peaks and interfacial heating.
In impulse welding, one or more electrically heated bars/elements are pressed against the surfaces of the films to be welded until they melt and bond at the faying surfaces. Temperature, time and pressure are the main process parameters. Most impulse equipment include a nichrom heating element with a small thermal mass that is heated quickly with electrical current (~ While there are a wide range of welding processes, this work evaluated the use of vibration and hot plate welding to join soybean protein-based nano-composite polymers that were exfoliated with high-powered ultrasonics. Vibration welding is a well-established process that can create large, sealed, mechanically strong weld seams with cycle times of only a few seconds. It is mainly used when short cycle times are demanded and the part is too large for the use of ultrasonic welding. Often, vibration welding is preferable to heated-tool welding due to its relatively short cycle times. In vibration welding, the two parts to be joined are clamped together under a relatively high force. At a preset force, one part is vibrated relative to the second, usually with a displacement between 0.5 and 1.5 mm. The motion results in frictional heating. This heat results in the joining surfaces melting and fusing together.
Hot plate welding is one of the most popular methods for joining thermoplastics because it is a simple, reliable and economical way of producing strong welds. Hot plate welding works by placing the two components to be welded against or near a heated tool. The weld surface is then heated by conduction, convection, and radiation to promote melting. Once a certain amount of melt is built up at the faying surfaces, the heat source is removed and the two surfaces are brought together. Usually there is a certain amount of squeeze flow in order to assure proper fusion and help remove any contamination. The interface is then allowed to solidify resulting in a weld.
While bioplastics have only made a marginal impact in displacing petrochemical plastics, their use will certainly grow. This growth will only be possible by a better understanding of these materials, including processing and secondary operations such as welding, sealing and cutting. Researchers are currently developing this knowledge for a wide range of processes, including heated tool, impulse, vibration and ultrasonic welding. Some of this work is imperial while some of it is also fundamental in nature. For example, researchers are developing models to predict weld strength based on molecular diffusion and activation energy. The knowledge gained from this work will promote the use of bioplastics as it reduces the risks of adopting new materials. While bioplastics are not truly carbon natural, their adoption is important in reducing dependence on petrochemical feedstock, as well as reducing production of green house gases.