Internal mold release

Siloxane containing polyester, poly(alkylene oxide) and polystyrene type copolymers have been used to improve the heat resistance, lubricity and flow properties of epoxy resin powder coatings 43). Thermally stable polyester-polysiloxane segmented copolymers have been shown to improve the flow, antifriction properties and scratch resistance of acrylic based auto repair lacquers 408). Organohydroxy-terminated siloxanes are also effective internal mold release agents in polyurethane reaction injection molding processes 409). 

Thermoset polyurethanes are cross-linked polymers, which are produced by casting or reaction injection molding (RIM). For cast elastomers, TDI in combination with 3,3,-dichloro-4,4,-diphen5lmethanediamine (MOCA) are often used. In the RIM technology, aromatic diamine chain extenders, such as diethyltoluenediamine (DETDA), are used to produce poly(urethane ureas) (47), and replacement of the polyether polyols with amine-terminated polyols produces polyureas (48). The aromatic diamines are soluble in the polyol and provide fast reaction rates. In 1985, internal mold release agents based on zinc stearate compatibilized with primary amines were introduced to the RIM process to minimize mold preparation and scrap from parts tom at demold. Some physical properties of RIM systems are listed in Table 7. 

An improvement in the production of foamed composite-door panels was reported by Schumacher et al (7). The process consists of the use of the SRIM process and a low-viscosity urethane foam system. In addition, vinyl- and glass-mat preform technology and IMR (internal mold release) were employed. The demold time at 75 C was 45 seconds, and molded densities were 400 to 800 g/cm. ... 

They are often blended into compounds that are developed for injection molding applications. Where such an additive is absent, molded components tend to stick to mold surfaces. To prevent this, mold surfaces must be treated between every cycle, or they can be coated with a range of semipermanent mold release treatments. Inclusion of an internal release agent prevents mold surface build-up, surface charring, and simplifies production. Internal mold release agents may include hydrocarbons, alcohols, carboxylic acids, halogenated compounds, ketones, carboxylic acid esters, amides, metal salts, and silicone compounds. 

This paper will explore the effect of these attributes toward making the RIM process competitive with other plastic molding techniques. It will also Illustrate the Impact that pending developments 1n RIM technology, such as Internal mold releasing systems, will have 1n making PU-RIM even more economically viable. 

An Ideal RIM Internal mold release (IMR) would essentially eliminate both the need to treat the tool between shots and mold... 

Internal mold release technology is well developed for nylon RIM systems while it is being developed for urethanes. Most urethanes require application of a mold release agent, a wax, a soap or a silicone, after every shot. Many mold releases cannot be added to the urethane reactant streams or the tanks because they react with the urethane chemicals. These reactions have caused problems of build-up in the mold, even with spray-on mold releases, necessitating periodic shutdowns to scrub the mold surfaces. Some newer systems use a third component stream to Introduce mold release at the mix head. Newer Internal mold release systems permit from 0 to 50 urethane shots to be made between applications of external mold release spray to the tool. Monsanto s nylon RIM system contains an internal mold release that is paintable. In certain applications where a high quality smooth show surface is required on a part, a... 

The problems surrounding part quality, while not always identified in a quantitive way, are certainly inherent in the use of external mold release sprays. A self-releasing system, made possible by the incorporation of an internal mold release agent in polyurethane chemicals offers a possible way of eliminating, or at least, minimizing the problems akin to external release methods. 

The gains which can be realized are distinct and real. The benefits derived from of a self-releasing system based on effective internal mold release agents are ... 

In an abstract of an unpublished paper, Dominquez (2) approached the question of Internal mold release by making use of an all - polyurea system. Specifics are not available on either the system or IMR agent employed. Further Information may be gleaned, however, from the work of Plevyak and Sobleskl (3). They show the utilization of the carboxy-functlonal dlmethylslloxane fluid In a two stream process operation In conjunction with a system which Is all polyurea. 

Table I. Formulation for Internal Mold Release System (4)...

Thus, to eliminate or minimize chemical adhesion the metal mold surfaces must be protected from polar materials such as polyurethanes by an interface which not only avoids the physical problems of adhesion, but those which are of a reactive chemical nature as well, i.e., a non-wetable, non-polar, non-reactive boundry layer or film. In terms of releasibility, therefore, such a boundry between the metal mold surface and the polyurethane must be present. It makes no difference whether this is achieved by the application of an external mold release (EMR) agent, or though the incorporation of an internal mold release (IMR) agent. 

Improved RIM Processing with Silicone Internal Mold Release Technology... 

This paper will discuss silicone internal mold release as a major contribution towards increased RIM productivity. The value of these internal release agents together with the necessary developmental parameters of such agents will be detailed. The paper also presents details of internal mold release agents developed by Dow Corning for polyurethane RIM. Field trial results including paint aging data will be presented. 

Finally, an update on more-recent commercial and experimental Dow Corning internal release agents, including two stream additives will be given. This paper should enable RIM processors to see how useful internal mold release agents can be in their polyurethane RIM process and the benefits they can realize by incorporating internal release agents into their systems. 

For internal mold release to be a viable processing alternative, it must fulfill a number of demanding requirements ... 

Figure 2 illustrates how the tailor-made silicones function as IMR agents in RIM molding. Due to the known surface tension reduction phenomenon of silicone fluids in urethanes and the differences In surface tension at the urethane/mold interface, internal mold release agents based upon silicone molecules can be developed to selectively migrate at different rates to the urethane/mold interface. There, it will orientate itself to build a protective layer which decreases the ability of the urethane to wet-out the mold surface. The inability of the urethane to wet-out the mold surface will facilitate release. 

Although many fluids can be developed to act as IMR for RIM, many would fail as commercial products due to their inability to impart paintability to the resulting molded part. In order to rectify this paintability concern with the use of silicone internal mold release agents, a functional moiety is designed into the silicone. This functionality, when correctly specified, reacts into the urethane matrix once the IMR has migrated to the urethane/mold interface. This reactivity causes the IMR to be immobilized in the urethane matrix, thus allowing paintability. 

Dow Corning has introduced DOW CORNING Q2-7119 Fluid (hereafter known as Q2-7119) as an IMR agent commercially available to the RIM industry. Field evaluations of Q2-7119 in amine and glycol extended systems demonstrated the ability of this functional silicone fluid to act as an internal release agent. Table I shows results of this material as an internal mold release agent in an amine extended system. 

Figure 2. How silicone-organic hybrid fluids function as internal mold release agents.

Cekoric M.E., Taylor R.P. and Barrickman C.E., "Internal Mold Release The Next Step Forward in RIM Productivity," Paper 830488, SAE Automotive Engineering Congress and Exposition, Detroit, MI, February 28 - March 4, 1983. 

The fillers are normally added after thorough dispersion of catalyst in the resin and other additives such as internal mold release, pigments, wetting agents. But the addition may precede the dispersion of catalyst if it is a shrink control filler. Table 3 gives some examples of the fillers used. 

Internal mold release agents are added to facilitate part ejection from the mold. The most common types of agents include metal stearates, fatty acids, fatty acid amides, and hydrocarbon waxes. Zinc stearate and calcium stearate are the widely used mold release agents in SMC and BMC. 

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