Mold release agents play a vital role in the molded polyurethane foam sector, but their choice involves balancing a number of factors including environmental impact, cost and efficiency in use. Angel Rodriguez Guitart, Joaquim Serra Pica of Productos Concentrol consider some of the options.
There are many factors to consider when deciding on the right release system for a molded polyurethane part. The mold-release agent industry has been working hard to reduce the environmental impact, improve in-use economics and maintain the efficiency of mold release agents for the sector.
One of the most difficult and challenges of recent years has been to reduce the greasy finish which water-based release agents can give to parts.
The pressure to move away from solvent to water-based release agents has been strong: OEMs have put pressure on the industry to eliminate VOCs.
Additionally, worker exposure to solvents, measured through TLV/DNEL values related to solvents used in mold release, together with an improvement in the working area environment have helped drive the change from solvent-based to water-based release agents.
The mold-release industry has developed systems which offer improved finish of parts produced with water-based and co-solvent release agents. The finish of parts has become drier making it possible to glue on surface pieces, heating-pads and textiles auxiliaries without sacrificing release capacity.
It is not an either/or situation, between 100% solvent-based and 100% water-based release agents there are also two intermediate solutions: co-solvents and hybrids.
Co-solvent release agents contain at least 75-85% water and from 5 to 15% solvent and typically have shorter drying time and better emulsifications of waxes and active ingredients than 100% water-based systems.
Hybrids are based on a carrier system which is half water – half solvent. They are claimed to be at least like a solvent-based release agent in terms of functionality and finish of the demolded piece, but at least half of its formulations contains VOCs.
A third approach is to apply less, but more concentrated release agent. This allows users to reduce TLVs , VOCs and improve the working environment and use less.
A more sophisticated approach is to use electrostatic release agents.
With these the mold-release application gun is connected to an electricity pole which is negatively charged, while the working mold is connected to a positive pole. Upon application of a high voltage current, an electrostatic field is generated between the electrode on the tip of the gun and the mold.
When the applicator presses the trigger to start the flow of mold release agent, the aerosol particles are negatively charged.
Opposite charges attract: the mold attracts the mold release agent, efficiently covering its entire surface. This helps avoid unnecessary losses and so helps reduce consumption.
Any conventional solvent-based release agent can be converted into the electrostatic mold-coating process and this can be optimised to give an adequate conductivity solution that satisfies the needs of the customer or the application.
Nowadays electrostatic solutions do not present strong odours or additional toxicity, and they can be used with class I, II or III solvents.
Using electrostatic release agents can lead to a significant reduction of consumption of between 30 and 50% compared to conventional systems This is because the process avoids overspray onto the mold holders, floors, and surrounding environment. There is also a significant reduction of emitted VOCs which helps to improve the work environment.
Applying electrostatic mold release systems is more efficient through robotic systems — although manual application is possible. It is necessary to invest in need for specific equipment while the spray guns are larger and heavier than usual.
Using electrostatic release agents can also lead to a reduction of the flow rate of between 30-50%, this means that the generation of VOCs and TLVs are being reduced this percentage.
This helps to improve the workplace environment and there is a reduction in overall environmental impact from the molding process.
Tin Free release agent:
Release agent suppliers have been working to replace organotin compounds with other organometallic compounds. This has been moderately successful, but the newer materials have some limitations.
Important tin-based compounds are stannous octoate, dibutyltin dilaurate (DBTDL) and tin mercaptides. Certain salts of lead, mercury and antimony have also been used.
Substituting DBTDL as catalyst with other non-organotin substances is possible using a state-of-art knowledge, collaboration and customer trials to produce tailor-made products.
Flammability:
To reduce flammability risk, again, like with the case of VOCs/TLV , the best option is to change from solvent- to water-based release-agent, but there are alternatives: One is to upgrade the mold-relase system to use a less volatile solvent.
The EU polyurethane industry widely works to the solvent classification found in European Directive 67/548/CEE(4). This divides solvents in to three groups:
Class I: Easily flammable: Substances which its flash Points is below 21ºC.
Class II: Flammable: Substances which its Flash Point is between 21ºC and 55ºC.
Class III: Combustible. Substances which its Flash Point is above 55ºC.
For example, if a molder were using a Class I release agent, which may contain heptane, with a flash point of less than or 0ºC; the logical step is to change it to a Class II release agent with a flash point of 28oC. This may contain a naphtha C9-C10.
Changing again from a Class II to Class III system, based on isoparaffin with a flash point above 55ºC, would significantly decrease volatility. Changing to higher flash-point solvents is a good way to quickly reduce VOC and TLV but, there is a price.
The drying time increases with the increase in flash point temperature and way of working in the production line becomes little more critical. This is because longer drying time is needed in the molds before pouring the PU into them.
VOC and FOG
VOCs are one of the most important pollutants present in the atmosphere, because they are key substances in the formation of tropospheric or ground-level ozone and they play an important part in the formation of highly globally warming nitrogen oxides.
The chemical reactions involved in tropospheric ozone formation are a series of complex cycles in which carbon monoxide and VOCs are oxidised to water vapour and carbon dioxide, both of which contribute to climate change.
The European Union and the World Health Organisation defines a VOC as “any organic compound having an initial boiling point less than or equal to 250 °C (482 °F) measured at a standard atmospheric pressure of 101.3 kPa.”
Boiling point EU/WHO designation
< 50ºC Very Volatile Organic Compounds (VVOC)
>50ºC-<250ºC Volatile Organic Compounds (VOC)
>250ºC-<400ºC Semivolatile Organic Compound (SVOC)
>400ºC Particle Organic Matter (POM)
Volatile Organic Compounds are organic chemicals containing carbon atoms that have a high vapour pressure at ordinary room temperature. This vapour pressure is related to boiling point. The lower a liquid’s boiling point the faster its molecules evaporate into the surrounding air.
In the automotive sector, Germany’s VDA regulations are highly influential. The VDA regulations address the organic emissions from automotive components, and when analysing them two classes of compounds are distinguished: VOCs & FOG’s.
According to the VDA tests: the amount of VOC emitted is given from the sum of VVOC and VOC which easily evaporate from sample at test-temperature 25 <<100ºC and with in-car concentration at least twice as high as the exterior concentration. The test defies FOG as the sum of VOCs and SVOCs which evaporate from sample at test-temperature >90ºC.
Box TLV, DNEL
Threshold Limit Values (TLVs)are the concentrations of substances suspended in the air. They are important to users of mold-release agents because most of these are sprayed onto the mold.
TLVs represent conditions under which it is believed that almost all workers can be repeatedly exposed day after day without showing adverse health effects.
There are several types of TLV. The time weighted average (TWA) is the mean concentration, weighted over time for a normal working day of 8 hours and a 40 hour week, that workers can be repeatedly exposed to without adverse health effects. These are useful in defining the size and power of the plant’s extraction capacity to ensure the TLV for each solvent is not breached
The derived no-effect level (DNEL) is the level of exposure to a substance above which humans should not be exposed.
According to EU Reach legislation, manufacturers and importer of chemical substances must calculate DNELs as part of their Chemical Safety Assessment (CSA)(2) for any chemicals used in quantities of 10 tonne/year or more.
The DNEL measures the potential of the substance to cause adverse health effects. This potential will vary depending on the exposure pattern to the substance, which is usually defined by a combination of the following elements:
- The population likely to be exposed to the chemical, i.e. workers, consumers or humans exposed through the environment. In some cases, specific vulnerable subpopulations can be considered such as pregnant women or children.
- The frequency and duration of the exposure;
- The route of exposure: dermal, inhalation or oral.
Taking a dearomatised C9-C10 hydrocarbon, which is a standard aliphatic solvent commonly used as the carrier in release agents it has the following: DNEL (skin contact) of 208 mg/kg bw/day and DNEL (inhalation) 871 mg/m3.
If a Class III C11-C12 hydrocarbon is used the DNEL disappears.
This is an edited version of a paper presented at UTECH Las Americas, April 4-6, Centro Banamex, Mexico City.
