Chemical Foaming Agents Improve Performance and Productivity
Michael E. Reedy
Reedy International Corporation
Keyport, New Jersey, US
Chemical foaming agents can play a key role which enables both commodity and engineering polymers to process more easily and with improved properties for injection molding and extrusion processes. Both additive producers and resin companies have made improvements to their products. Compatibility and chemical reactions with blowing agent by-products are becoming more important considering desired improvements in part weight and impact strength.
Blowing Agent Process
What really happens when you add chemical foaming agents or blowing agents to your polymer process? The agents are blended with the resin or can be fed directly into the hopper and from there down through the barrel to the mold. Heat from the barrel causes a thermal decomposition of the material and may be either endothermic (heat absorbing) or exothermic (heat generating). Endothermic foaming agents primarily produce CO2 while exothermic mostly generate N2.
The liquid CO2 or N2 from the blowing agent is mingled among the liquid plastic resin molecules and is not typically considered to be miscible or a homogenous solution by itself. This solubility or miscibility of the liquid is influenced by the type of resin. Thermoplastic resin is classified as to its macromolecular structure. An amorphous or unstructured plastic is irregular with highly branched molecular chains and is transparent. Semi-crystalline thermoplastics have molecules in orderly or linear chains and are cloudy or semi transparent. Additives such as flame retardants, anti-oxidants, pigments, fillers, UV stabilizers, etc. also influence the solubility properties. The polymer system is influenced by the additives which effect surface energies or surface tension which can either promote compatibility and intimate mixing or destabilize the melt promoting separation and coalescing. However, if the resin and additive package, including blowing agents, are compatible you will be able to produce a polymeric emulsion, which is ideal for foam processing. Emulsion principles work as well for mayonnaise as for new design foamable semi-crystalline resins from Amoco or Montell.
The basic particle size of the blowing agent determines the size of the liquid/gas mixture in the melt. Large particles will tend to generate large liquid/gas mixtures and fine particles will tend to generate small liquid/gas mixtures. Under the right magnification levels, you can almost see beach balls vs. golf balls in the matrix.
The size of these liquid droplets and the amount mingled in the polymer mixture has a lubricating effect. This not only reduces shear-heating forces but also improves melt flow and consequently the polymer melt index. Fine particles that produce fine liquid droplets tend to produce fine emulsions.
Blowing Agent Compatability
Both additive and resin producers continue to make improvements in their product performance and compatibility. The basic chemical reactions in the polymer melt are becoming more critical when trying to lower part weight and maintain or improve impact strength. All chemical blowing agents decompose thermally to produce gas and chemical by-products. We need to recognize that these by-products can be either basic, neutral, or acidic and they can affect the pH balance of the complete polymer system. This has caused a number of problems with other additives especially flame retardants, stabilizers, and AZO based colorants.
Many of the low cost / low performance blowing agents in the market have caused problems with tool corrosion or part embrittlement. For endothermic agents, too large a concentration of either sodium bicarbonate or citric acid will cause a reaction with other additives such as bromine compounds or phosphate esters. This reaction can be with the thermoplastics resin or the mold tooling. For exothermic agents, the reactions are primarily basic which help explain why they are the preferred agents for naturally acidic PVC materials.
The key concept of using blowing agents is to match the agent with the resin system. When the proper match is achieved, there is a fantastic processing advantage. For example, a molder with a 15-pound shot of HIPS was experiencing a 3 _ minute cycle. Using a compatible endothermic blowing agent, the molder was able to blow out sinks and reduce the cycle to 2 _ minutes—a savings that amounted to almost $1.00 / part. This helps explain why there are over 150 endothermic formulations and over 200 exothermic formulations in the market today. Each blowing agent is attempting to offer the processor a unique economic or performance advantage.
In the future, we anticipate an increasing in endo/exothermics that match process and performance improvements. Similarly, the resin producers are interested in expanding their product lines to include more foam friendly resins and foam friendly additives.
Blowing Agent Use In Injection Molding
There are over 10 separate injection molding processes using blowing agents. Chemical blowing agents can play a key role and enable both commodity and engineering polymers to process more easily and with improved properties. These include:
- Straight Injection Molding
- Low Pressure Structural Foam Molding
- High Pressure Structural Foam Molding
- Gas Counter Pressure Structural Foam Molding
- Nitrogen Injection Structural Foam Molding
- Gas Co-Injection Structural Foam Molding
- Gas Assist Molding
- Chemical Gas Assist
- Over Molding Structural Foam Molding
Blowing agents in High Pressure Structural Foam Molding create a microcellular structure with a smooth solid skin around a fine cellular core. Particle size, distribution purity, and a controlled gas release are tailored to provide many, very small nucleation sites that create this fine and uniform microcellular structure.
Gas Counter Pressure Molding uses endo/exothermic agents to eliminate sink marks and improve processing economics including density reduction and cycle time with a class “A” finish. With CO2 based blowing agents, only a 35 p.s.i. counter pressure is needed to prevent splay.
CoralFoam is a new selective foaming process built around sophisticated tool design and endothermic agents. CoralFoam uses CO2 based blowing agents because of its low-pressure solubility and predictable post mold foaming characteristics.
Process Improvement Using CO2
The process benefits are all based in chemistry and the chemical reactions discussed earlier, as well as the physical nature of the gases. CO2 is a low vapor pressure gas with low-pressure solubility. It can become a super critical fluid at relatively low-pressure. Typically, this is in the range of 1,000 to 1,700 p.s.i. In the super critical state, CO2 is a super solvent and can lower the TG of most resins. In the case of PVC, studies have shown reductions of 50∞C. The solubility rate of liquid CO2.is still being studied. This rate is important in injection molding processes because the dwell time is very short. What we have typically been able to observe is that a lowering of the process temperature by 10∞ to 20∞F, overall for most resin processes, is achievable.
In many cases, the endothermic agent is not used to make foams, but will be used to improve the melt behavior of the polymer as a processing aid. CO2 acts as a lubricant improving melt flow, which, for injection molding, improves mold filling. Blowing agents are often used to eliminate sink marks. The CO2 will seek out area of lowest resistance, which are the hottest areas. Cellular expansion fills the voids left from the cooling polymer. One key concept to remember is that you must change the process parameters to take advantage of foam formation. In most cases, excess pack pressure will prevent foaming.
Probably the most important advantage of lowering viscosity is the possibility to achieve equivalent flow characteristics at lower melt temperatures. By lowering the viscosity of the polymer melt, one has the choice of reducing the processing temperature or utilizing the improved melt flow at the same temperature. Improved melt flow can mean fewer gates, thinner wall sections, less molded-in-stress and reduced burning through shear heat. In certain cases, the uses of endothermic agents make production of multi-cavity tools easier to balance.
The saturated CO2 of the emulsion exerts an internal pressure that enables the processor to feed polymeric material very consistently. Since the product is still saturated with CO2 and is significantly plasticized, lower molding stresses are evident in the product. The plastic will orientate in the direction of the flow (laminar flow) and the blowing agent will act as an expanding spear causing a molecular disorientation. This results in far less stress cracking in the finished part and improved impact strength.
Troubleshooting Using Chemical Foaming
One easy way to discuss process improvements using chemical blowing agents is to highlight the problems and the important variables in resolving them. Key foam processing troubleshooting actions include:
Foam Post Blow—Because foaming agents are so efficient, parts removed from the mold will continue to swell. The solution is to lower the process temperatures, which, as I mentioned earlier, will allow you to reduce cooling time which, is the largest time component in the processing cycle. You may also simply be using too much blowing agent for the part.
Elephant Skin—This is a surface roughness often appearing near the end of the fill. This is usually an indication that the melt or mold temperatures are too low.
Sink Marks—As thermoplastic materials cool, they shrink and thick portions of ribs or bosses shrink the most. Add a little more blowing agent and make sure that the pack pressure is released so the foam has room to expand.
Warpage—-This is when the parts side walls bow in or out. This is often because the temperature and injection pressures are too high. Also the part is not cooling or setting-up completely, so increased cooling will help.
Voids—These are areas of missing material, usually hollow areas formed from gas pockets. The solution is usually to increase melt temperatures and lower the injection speed. You may want to avoid pressure drops that will result in inadequate gas counter pressure on the melt face and cause prefoaming and other anomalies inside the mold. It is very important to match the foaming agent to the resin system.
Plate Out—Plate out or deposits on the screw and screen pack is a problem with exothermic blowing agents. This can have a number of causes including the resin and additive packages, but the quickest solution is to reduce the amount of blowing agent used. Again, it is important to ensure that the blowing agent is matched to the resin emulsion system.
Impact and Brittleness—This is a problem when a part does not meet the physical properties requirement. Typical fixes include lower temperatures, decrease the amount of blowing agent or change the blowing agent to a different type. You may want to contact the blowing agent producer for specific recommendations.
Flash—This is the excess material usually at the mold knit lines seen after demolding. The solution is to lower the process temperatures, reduce shot size or lower the blowing agent dosage. Avoid sharp transitions of section thickness; especially close to the point of injection because it will create more difficulty with regard to surface gassing. If possible, smooth transitions should be used.
Burning—This is when the part is discolored and the surfaces show unevenness. This can be fixed by lowering temperatures and opening the gates and vents. In general you want to avoid flow constrictions that may cause excessive shear heat and an undesired reaction of the blowing agent. Remember gates should be as generous as possible but should still remain not more than two thirds of the product thickness that is being fed.
We have briefly discussed how chemical blowing agents can improve performance and productivity in injection molding processes. Compatibility of the blowing agent and the complete polymer system is the key consideration. The effect of CO2 on the flow behavior of a polymer has been well documented and can be used by the injection molder to produce products with improved physical properties and process economics.
We are also studying how chemical foaming agent technology utilizes unique surfactants and high surface area particles that can launch supercritical CO2 into new areas of material science. These involve using supercritical CO2 to improve polymer to polymer compatibility through interpenetrating networks. Also, supercritical CO2 solutions are being evaluated as emulsifiers for organic and inorganic blowing agents and their contribution to foamed materials.
Now with increased environmental pressure and a new emphasis on the use of CO2, I am sure endothermic chemical agents will continue to have tremendous growth in the plastic industry. New opportunities are available –it only takes foam.
The author would like to acknowledge the following people and companies
for their assistance.
---Mike Caropresso, Consultant
---Larry Currie, Mack Molding Company
---Walt Harfmann, Harfmann Technologies
---Larry Novack, BASF
---Milko Guergov, M & C Advanced Processing
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