In the production of polyurethane products, it is necessary to use release agents to help the molded products detach from the molds. Among the release agents that can be used directly on the production line, the substance responsible for the release performance is called the effective release component. This component is usually wax, organosilicon, organic fluorine, etc., and generally accounts for less than 10% of the total. The majority, over 90%, consists of solvent components, which help the effective release components form a uniform film on the mold surface. In this article, let's explore the applications of solvents in polyurethane release agents.
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During the molding process of filled materials in molds, the materials come into contact with the mold surface, which may have minor defects due to the mold's working surface being uneven, leading to some friction resistance when the molded products detach from the mold. During the injection or extrusion process of filled materials, negative pressure may form between the filling material and the mold, or they may bond due to physical adsorption or chemical bonding, making it difficult to remove the molded material from the mold after forming. To weaken the adhesion or bonding between the product and the mold, additives (release agents) that can form an effective isolation film are often used. A release agent is an interface film layer used on the surfaces of two objects that are prone to bonding, forming isolation between the bonded substances and the releasing material, making it easier and more convenient for the product to detach from the mold.
Broadly speaking, release agents include many types, applied in various fields such as chemical engineering, metallurgy, building materials, etc. This discussion will focus only on the use of release agents in the chemical engineering field, specifically discussing the application changes of solvents in the release agents used for materials such as polyurethane.
Phase One:
In the initial stages of industrial production, environmental and safety considerations for raw materials were not a concern, and the focus was solely on high solubility and easy volatility. The requirements for solvent use were limited to considering them as diluents, requiring them to quickly evaporate after spraying on the mold to avoid foam surface defoaming. Therefore, some organic solvents became widely used due to their high solubility and easy volatility, such as:
1. Dichloromethane: Strong solvency, easy volatility at room temperature, chemically stable, and due to its lack of a flash point, it does not lead to combustion, and it is also inexpensive. It was extensively used in the early days. However, the high concentration of dichloromethane vapor can cause poisoning, and large emissions can also lead to the greenhouse effect. Before the widespread use of environmentally friendly release agents in the production of shoe materials in Jinjiang, which used dichloromethane, thousands of dichloromethane molecules were present in the air.
2. Petroleum Ether: It is a low-boiling fraction of petroleum, a mixture of low-level alkanes, usually used as a solvent with a boiling range of 30-60°C, highly volatile. It was commonly used in release agents in the early stages. Due to its low boiling point, it is easy to mix benzene and ketone compounds into the product, resulting in poor overall VOC (Volatile Organic Compounds) and odor performance.
3. Naphtha: Also known as crude gasoline, mainly composed of C5-C11 components of alkanes, flash point -2°C, extremely volatile and flammable. The vapor is irritating to the eyes and upper respiratory tract and can also cause environmental pollution.
4. Xylene: C8H10, isomers, boiling range around 140°C, high flash point, flammable, with a pungent odor, classified as a Group 3 carcinogen.
5. Quick-drying water: Cyclohexanone, a colorless or light yellow transparent liquid with a strong irritating odor, a Group 3 carcinogen. As we can see, the above products only focused on their usage characteristics but ignored their long-term effects on human health and the environment. Therefore, in today's environmentally themed society, their market share is getting smaller and they are sinking into low-end product usage. Some of them have been strictly prohibited from use as they are listed as carcinogens.
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Phase Two:
People with health awareness began looking for new solvent alternatives with lower toxicity. Heptane, a colorless and easily volatile liquid, gradually became widely used as a solvent for release agents due to its high fat solubility, high volatility, and strong degreasing ability. Commonly known as white gasoline, its chemical name is n-heptane, with a flash point of -4°C, making it a highly flammable organic solvent. Pure n-heptane is relatively expensive, so industrial-grade heptane emerged, which contains more impurities such as aromatic hydrocarbons, sulfur, nitrogen, etc., resulting in poor odor and VOC performance of the product. Also, in terms of toxicity, although heptane has lower toxicity than the initial solvents, with long-term use, there is still a risk of chronic poisoning, mainly manifested as damage to the nerves around the body. At this point, heptane gradually sank into the usage of low-end products.
Phase Three:
The overwhelming momentum of environmental and safety inspections forced manufacturers to pay attention to environmental and safety issues. Relatively safe and environmentally friendly solvent products became the direction sought by release agent manufacturers. This is where the de-aromatized solvent oil for release agents came into play. De-aromatized solvent oil is mainly produced by using aviation kerosene through high-pressure hydrogenation at both ends, deep desulfurization, and then distillation. Desulfurization aims to eliminate odors and corrosiveness, while de-aromatization reduces harmful benzene-like substances and improves VOC performance. Currently, on the market, there are several types of de-aromatized solvent oils based on solvent flash points (such as D40, 60, etc., referred to as the D series solvent oils). Since pentanal has a low boiling point, the higher the distillation range, the lower the harmful substances in the solvent product itself.
De-aromatized solvent oil has low toxicity, which has almost no impact on personal safety and the production environment. Additionally, its higher flash point brings higher safety to production. However, high-boiling products evaporate relatively slowly, which can lead to foam defoaming on the surface of polyurethane products. Therefore, the initial scope of use is mostly limited to solvents that are relatively easy to volatilize.
Phase Four:
Environmental regulations are demanding higher safety for products, storage, and logistics in enterprises. Hazardous chemicals are finding less space to exist, and operating costs are rising. Transforming release agents from hazardous chemicals into non-hazardous chemicals has become a new challenge. Water-based release agents with water as the main solvent have long been considered by manufacturers. They are currently commonly used in fields such as shoe materials, metal die-casting, carpets, front-end sound insulation pads, steering wheels, etc. However, water evaporates relatively slowly, and even though water-based release agents can solve all environmental and safety issues brought by organic solvents once and for all, they face extremely stringent and inconvenient application processes due to the complex structures of seat foam production line molds, high-speed production, and the high rebound foam's sensitivity to water. Therefore, manufacturers tend to choose high-flash-point release agents with organic solvents as carriers for their convenience. According to the national classification standards for flammable liquids, liquids with flash points above 60°C are not hazardous chemicals and can be transported and stored as ordinary chemicals. Although high-flash-point release agents also evaporate slowly, through adjustment in the spraying process, they can still meet production needs. Therefore, in this phase, high-flash-point solvent oils entered the scope of use.
Phase Five:
Odor and VOC reduction become the main focus. High-end cars have extremely strict requirements for the interior environment, but changes are driven downward through a chain of suppliers. Therefore, higher requirements are placed on release agents, with the ideal goal being no odor and no VOC. While de-aromatized solvent oil already belongs to the trace VOC category, it still has a distinct solvent odor. Therefore, until the application of water-based release agents is fully mature, release agent manufacturers can only continue to search for alternatives in organic solvents. Eventually, isoalkanes suitable for personal care and cosmetics entered the application range of release agents. They are colorless, odorless, high-purity, single-component, free of aromatic hydrocarbons and sulfur, truly low-toxic, and have good solvency. These characteristics make isoalkanes the highest-end product in environmentally friendly solvent oils.
Currently, there are three main production processes for isoalkanes: straight-chain alkanes aromaticization, isobutene synthesis, and coal-to-liquid (CTL) Fischer-Tropsch synthesis of isoalkanes. Most imported products from abroad are produced by isobutene synthesis, where isobutene is cracked from naphtha, then undergoes isomerization, distillation, and hydrogenation to obtain high-purity synthetic isoalkanes that are completely odorless at room temperature and do not produce any odor when heated to a certain temperature. However, due to the high comprehensive cost of this process, the solvent prices are high, more than twice that of de-aromatized solvent oil. Currently, it can only be limited to the use of high-end products. Since China has abundant coal resources, coal-to-liquid (CTL) Fischer-Tropsch synthesis of isoalkanes has become the mainstream research route. With the gradual maturation of the later-stage process, it is believed that the price of domestically produced isoalkanes will significantly decrease, and the usage will see explosive growth.
In conclusion, in the release agent industry, you get what you pay for. Prices to some extent determine the quality of the products. Low prices cannot meet high-end demands, and high prices are also used to offset the upfront research and development costs.
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