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Tuesday, March 30, 2010

Definition of Shelf Life

Shelf life is an arbitrary time for practical storage of a thermoset system.

Shelf life derives from the storage concept; i.e., how long can a thermoset be left on the shelf before it becomes difficult or even impossible to use in the intended application.

The term can refer to a one-can system (e.g., a phenolic molding compound must be molded within 1 year of compounding) or a two-can mix that must be set aside for a few hours before use.

Shelf life is also used to describe the storage stability of unmixed components of a thermosetting resin system if there is some threat to their reactivity as a consequence of the storage. For example, some curing agents are very hygroscopic and will lose reactivity if airborne moisture were to penetrate the storage container.

Thursday, March 25, 2010

Cleaning Solvents

Solvents such as acetone, methyl ethyl ketone and methanol are used in large quantities to clean equipment and tools. Of these, acetone is the most widely used. Many fabricators have begun to replace acetone with dibasic ester @BE). DBE is a mixture of the methyl esters of adipic, glutaric and succinic acids that is both less volatile and less flammable than acetone (Lucas 1988). Methylene chloride has been used widely for cleaning because it is an effective solvent for many cured resins, although its use has been declining due to health and safety concerns.

Solvents are used to remove uncured resins from spray equipment, rollers, brushes, tools, and finished surfaces. Typical solvents used include acetone, methanol, methyl ethyl ketone (MEK), toluene and xylene. Acetone and other similar solvents are used for general cleaning, as standard practice for most open-mold fabricators of fiberglass products. To clean the spray equipment, acetone is usually circulated through the lines after the spray operation is shut down for the day. A simple but effective method practiced by some fabricators to minimize wastes is placing the containers of solvent near the resin spray area to prevent spills and drippage for tool cleaning. Generally, the solvent is reused until the high concentration of resin contamination prevents effective cleaning. However, if the containers are left uncovered, solvent will evaporate, increasing air emissions as well as resin concentration. Methylene chloride is an effective solvent for cured resins, and has been used by plastics fabricators.

Although many other solvents have been tried, including multicomponent mixtures, these have had mixed results. The best way to minimize the need for this chemical is to clean equipment before the resin dries. Disposal of contaminated solvents represents a major hazardous waste management expense. In addition, fugitive air emissions during the curing and cleaning processes are also of concern.

Saturday, March 20, 2010

Release Agents Used in Composite Industry

A release agent is simply a coating that is applied to a surface to stop the material being moulded (usually a plastic) from sticking, i.e. it facilitates the easy release of the plastic from the mouldA lubricant, liquid, or powder (often silicone oils and waxes) used to prevent sticking of molded articles in the cavity.

Release Agent Requirements

• Guaranteed release
• Quick and easy to apply
• Low cost / part
• High quality finish
• Low defect rate
• No processing problems
• Ability to release a wide range of materials

Release Agents used in Composite Industry

i. Paste /Liquid Wax
Traditional products for open face, low temperature moulding processes, e.g. GRP. Usually carnauba wax based

ii. PVA (PolyVinylAlcohol dissolved in water)
Provides a release film; poor finish, guaranteed release

iii. Internal Mould Release Agent (IMR)
-Soaps, glycols or phosphates
-Used in pultrusion, abrasive processes or easy to release systems such as SMC

iv. Semi-Permanent Mould Release Agent (SPMRA)
-Polymer resins used extensively throughout advanced composite processes and where optimum productivity is paramount
-Lowest cost per released part

Monday, March 15, 2010

Storage and Handling of Prepregs

Prepregs should be stored as received in a cool dry place or in a refrigerator. After removal from refrigerator storage, prepreg should be allowed to reach room temperature before opening the polyethylene bag, thus preventing condensation (a full reel in its packaging can take up to 48 hours). Typically prepregs have a guaranteed shelf life at - 18 ºC of 12 months. Tack life at 23 ºC depends on the matrix, and is clearly defined on the relevant Product Data Sheet.
Prepregs are particularly low-risk in terms of handling hazards for the following reasons :

• Prepreg is covered on both sides by protective coverings, which are not removed until assembly lay-up. It should be cut to shape before removing the protective coverings and virtually no handling of the prepreg is necessary.

• Unlike wet lay-up methods of fibre reinforced composite manufacture, where dry fibre and liquid resin are used,uncured prepregs have no loose fibrous dust and are splash-free, leak-free and spillage free.

• Prepregs are volatile-free at normal room temperature.

• Prepregs have a moderate/low tack level at normal room temperature.

However, the usual precautions when handling synthetic resins should be observed, ie. always wear gloves and ensure arms are covered, thus avoiding skin contact with the product. Repeated unprotected touching of prepreg can cause an allergic reaction.
Dust from machining cured product will contain fibrous material, inhalation of which should be avoided. Provide positive dust extraction and collection from the cutting zone. Protect against fire and explosion by avoiding dust formation and ignition sources when machining cured product. Dust from products containing carbon fibre is electrically conductive.

Tuesday, March 9, 2010

Prepreg Material

Short for preimpregnated. A combination of mat, fabric, nonwoven material or roving with resin, usually cured to the B-stage, ready for molding. Can be redesignated as standard or net resin prepregs:

• Standard prepreg contains more resin than is desired in the finished part; excess resin is bled off during cure.

• Net resin prepreg contains the same resin content that is desired in the finished part; no resin bleed.

Prepreg containing a chemical thickening agent is called a mold-mat and those in sheet form are called sheet-molding compounds.

A prepreg consists of a combination of a matrix (or resin) and fibre reinforcement. It is ready to use in the component manufacturing process.
It is available in :

• UNIDIRECTIONAL (UD) form (one direction of reinforcement)

• FABRIC form (several directions of reinforcement).