Practice for Developing a Receiving and Storage Step - P154

 

Practice for Developing a Receiving and Storage Step
Practice document
Document Type	Practice
Document Identifier	154
MSTE workflow	Development
Prerequisites	Integrated Product Development Practice Practice for Developing a Process Step

Overview

This page discusses some of the key elements that are unique to shipping, receiving and handling composite materials. This includes procedures for handling rolls of material to avoid damage, how to properly store prepreg and dry fibre to avoid damage and extend the life of the material as much as possible. Lastly, discussing handling and storage of un-cured resin to promote longevity of the material.

Introduction

The raw materials required for composite production are sensitive to how they are handled and stored. Contaminations are easily introduced and will eventually lead to a reduction in the final part properties. Dry fibre is sensitive to external impact and contamination, while prepreg, resin and adhesive films are sensitive to temperature and moisture conditions. This page will bring forward some consideration for receiving and storing practices that are unique to composite manufacturing. Discussing the general practice of storing rolls of material before moving on to specifics for rolls of dry fibre and preimpregnated fibre rolls. Also, discussing the practices for storing un-cured resin.

Significance

Proper receiving and storing practices are crucial in order to maintain the material properties of the raw materials. It is the first step in the manufacturing of composites and has a trickle down effect on the final part properties. Proper procedures and documentation may also be part of quality assurance and/or certification programs.

Practice

Handling of Rolls of Material

Rolls of material should be labeled with important information such as the type of material, the weight of the roll, and the lot number for traceability purposes. This information should be recorded in a log or database for easy reference. Some manufacturers may choose to establish a RFID or QR code system for easier identification and tracking.

A large amount of materials used in composite manufacturing, from fibers to vacuum bags, come on cardboard rolls. These rolls need to be handled with care to prevent damage to the material and operators. When lifting rolls, it is best to use the cardboard center of the roll to minimize contact with the material. Keep rolls horizontal and do not put them vertically. For heavy rolls, a forklift with a mandrel is a safer option. Rolls of material are susceptible to crushing and tearing if dropped or stacked too high. Specifically, rolls of fibers are susceptible to fiber misalignment if handled improperly, leading to reduced properties of the final part further down the production line. When rolls of material are supplied from the manufacturer, it is not uncommon for them to arrive in cardboard boxes with foam supports inside to hold up the inner cardboard roll and the material itself.

Prepreg

When receiving prepreg materials, a log of temperatures that the material has experienced (thermal history) during shipping needs to be documented. Anytime the material has been exposed to temperatures above the storage temperature, this needs to be subtracted from the out-time of the material.

Prepreg and other rolls of uncured thermoset products, such as adhesive films, will advance their degree of cure at room temperature. Therefore, it is vital to store these products in a cold environment, usually at 0°F (-18°C), depending on the manufacturer's specifications. These materials also have a time limit on how long they can stay in the freezer and how long they can be exposed to room temperature. The shelf life at storage conditions in the freezer is normally between six months to one year. The time limit of the material out of the freezer is referred to as the "Out-Time" and can range from minutes to 30 days or more. It is essential to check the manufacturer's recommendations for these times.

Some materials have procedures in place to test whether they're still acceptable for use in production if their shelf life or out-time has expired. The characteristics of prepregs change over time depending on the remaining shelf life and out-time, including handling for layup and resin viscosity during cure.

These materials are also sensitive to moisture and condensation when thawing. Prepregs and adhesive films are packaged and sealed in moisture-proof bags, sometimes with desiccant packages. It is important to check these bags when receiving and storing materials because if the bags are punctured when thawing, it could introduce moisture to the surface of the material, leading to pre-bond moisture issues. When removing the material from the storage freezer, time must be allowed for the material to acclimate to room temperature before opening the moisture-proof bag. If the material is too cold, moisture may condense on it, which can make its way into the resin. A common rule of thumb is to wait 24 hours for a roll of material and to wipe condensation off the bag. If none reforms, it's safe to assume that the material is thawed. If the roll needs to be placed back in the freezer, ensure that the bag is properly re-sealed or use a new moisture-proof bag. This applies to plies that are cut and that need to be stored in the freezer until use.

Dry Fibre

In general, the fibre themselves do not have an expiry date, they can last indefinitely. However, some fibre types will have a shelf life as the fibre becomes discoloured and/or stiff and have a shelf life of somewhere between 6 months and one year. Some fibre types, like aramid, are very sensitive to UV degradation, where the surface of the fibres can deteriorate over time with exposure. It is recommended to store such fibres out of direct sunlight and preferably in a dark environment.

Furthermore, most fibre will come from the manufacturer with sizing on the fibre to assist in bonding to the polymer matrix. This sizing can be susceptible to breaking down over time and will be the main reason that dry fibre come with an expiry date. Sizing on glass fibre has been shown to be susceptible to elevated temperatures and humid environments. Causing a reaction to occur in the sizing making it less effective in maintaining the interfacial properties between the matrix and the fibre. For proper storage of dry fiber, it needs to be stored in a temperature stable environment with a relatively dry humidity (50%-70%). Ideally the fibre will be stored in their own plastic moisture proof bags, similar to prepreg.

Un-Cured Resin

Firstly, un-cured resins are usually considered hazardous materials and should be handled according to regulations. When receiving un-cured resin, it is best practice to write or label the container with information such as receipt date, manufacture date and expiry date. This is for rapid identification of resins and which state they are in, as well as allowing the containers to be arranged in a FIFO (First In, First Out) manner. Resins have a shelf life of somewhere between 6 months and 15 years if stored correctly.

For two part resins, store in a "cool-dry" place. This means that the resin and it's hardener or catalyst needs to be stored at room temperature, 18°C to 30, depending on manufacturer's specifications. Resins are susceptible to crystallization, which is when "lumps" of resin start forming in the neat resin without any hardener or catalyst being introduced. This happens when the resin has been stored for a long period of time and/or stored at temperatures lower than room temperature. Resins can also become more sensitive to crystallization if the temperature is not stable and it sees many changes, so it is important to keep the resin in an area that does not experience temperature swings and is not affected by direct sunlight. Keeping the resin stored off the floor will also aid in preventing temperature swings in the resin.

In epoxies it is possible to remove the formed crystals in the resin by gently heating up the material to around 50°C and stirring, however, it is important to check with the manufacturer before using the resin after this process. On the other hand, polyester resins will experience crystallization, where the resin becomes lumpy or jelly-like, however this is not a reversible process as it is the resin curing over time.

Another precaution of un-cured resins is to keep the resin in its original container and ensure the lid is securely sealed when not in use. This will prevent contaminants and moisture from entering the un-cured resin. Moisture and some contaminants can cause the resin to break down over time and become less effective. Oftentimes it is the hardener or catalyst that will fail first in the material system. This is due to the fact that catalysts contain important volatiles that have a significant impact on the cure kinetics, so when they are contaminated or degraded, the resin will not perform as intended.

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References

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Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

Pre-impregnated (prepreg) material refers to fibre that is already combined with resin. It is the most common material form used in aerospace.

During prepreg production, (e.g. fibres are run through a resin bath), prepreg is heated and partially cured to B Stage (< 5 % degree of cure). Thermoset prepregs (e.g. epoxy prepreg) have to be kept in a freezer at around -20 °C. At room temperature, the epoxy starts to cure.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Any manufacturing and/or decision making activity that occurs during any stage of the development design cycle (e.g. conceptual design to production).

In the context of Knowledge in Practice, practice refers to the systematic use of science based knowledge to reduce composites manufacturing risk, cost, and development time.

Thermosets are a class of polymer that undergo polymerization and crosslinking during curing with the aid of a hardening agent and heating or promoter. Initially they behave like a viscous fluid. During curing, they change from viscous fluid to rubbery gel (viscoelastic material) and finally glassy solid.

If heated after curing, initially they become soft and rubbery at high temperatures. If further heated, they do not melt but decompose (burn)

Comes in two parts: part A (resin) and B (hardener). When mixed, curing reaction starts and is not reversible.

Examples include epoxy or polyester.

Degree of cure (DOC) is an indication of how far the chemical curing reaction (crosslinking process) has advanced in a thermoset resin.

DOC is defined with a number between 0 and 1 (or 0% and 100%) where 100% is a fully cured resin. It does not have to fully reach 100% for the resin to become solid or the part to be used. In some aerospace applications, resins are only cured to about 90%. Higher the degree of cure, higher the mechanical properties.

In composites processing, viscosity is an indicator of how easily the resin matrix will mix with the reinforcement and how well it will stay in place during processing. The lower the viscosity, the more easily resin flows. Resin viscosity ranges considerably across chemistries and formulations.

By scientific definition, viscosity is a measure of a material’s resistance to deformation. For liquids, it is in response to imposed shear stresses.

Sizing or fibre sizing refers to a coating that is applied to fibre during manufacturing. Highly proprietary (formulation and process).

Sizing serves two functions:

• Protects and aids fibre during processing
• Aids in the bonding of fibre and matrix

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability