VIII. Resin. Fabric. Proportions and safety
Epoxy resin and fiberglass are used to restore stripped and recessed areas.
What is epoxy resin?
It is a thermosetting polymer containing a chain of identical links containing epoxy groups - three-membered heterocycles with one oxygen atom, capable of maintaining a three-dimensional structure under the action of hardeners a solid substance network - to cross-link ... let's try another way. This is a type of plastic that becomes hard when mixed with a hardener. When fiberglass is impregnated with epoxy resin, fiberglass is obtained (or carbon fiber, if fiberglass is replaced with carbon fiber).
The phrase “when mixed with a hardener” means that there are 2 components for preparing the finished resin (compound): resin (Component A) and hardener (Component B). Mixing is always done in parts by weight (i.e. by weight) according to the TDS (Technical Datasheet) of the compound manufacturer, specifically for the batch you bought! Scales are used for this operation. There is nothing to do without scales: it is impossible to obtain a finished resin with the required properties. Only torment and a bad result at the end. The main mistake in preparing the compound is that the mixing proportions were violated!
90% of the epoxy resins produced are called epoxy-diane, in honor of the Russian chemist A.P. Dianin, who first synthesized bisphenol A in 1891, who is also involved in the production of these 90% of resins.
The variety of resins is large, each is used in different conditions: on land, under water, thick or liquid. Before ordering resin, you should talk to the supplier's technologist about selecting resin for your processing conditions. What we plan to do, what molding technologies we use, what is the temperature and humidity of the room where the work is carried out, etc. God is in the details.
Fabric?
The fiberglass we use differs in density, type and direction of weaving. We are interested in structural fiberglass. Let's consider the classics in order.
Density: from 100 to 1500 g / m2. The higher the density, the easier it is to gain the required thickness of the laminate, the more resin will be absorbed by the fabric, the worse it is installed in places with a smaller radius and corners.
The lower the density, the less resin it absorbs, the longer it takes to set layers to the required thickness, the finer the texture of the finished surface (smoother). It fits better into recesses (corners) and is molded onto sharp angles of the structure.
Densities from 200 to 600 (usually) are used in boat building.
Fabric weaving types: plain (canvas), twill (serge), satin, roving (roofing, tow).
Each type of weave has its own characteristics.
Plain (canvas) - resembles a chessboard, the warp and weft threads alternate one after another. Such glass fabrics are the strongest, densest and most rigid, so they are used to reinforce heavily loaded areas. On the other hand, such fabrics cannot be used to reinforce areas of complex shapes, because they do not bend well and do not stretch.
Twill (twill) - this is a diagonal at 45°. These glass fabrics are more flexible than plain weave glass fabrics. They are characterized by lower density and greater stretchability. Twill fiberglass fabrics are also suitable for reinforcement and are convenient to use to create complex-shaped structures with angles, protrusions and bends.
Satin - a scattered diagonal. Satin fiberglass fabrics are obtained by loosely interlacing fibers. In this case, warp threads predominate on one side of the canvas, and weft threads on the other. Visually, diagonal stripes differ from twill weaving with an angle of 45°. These fabrics have high flexibility at low density. These fiberglass fabrics should be chosen when planning to manufacture a complex-shaped product.
Roving is worth considering separately. A fabric with a high density, where whole bundles are interlaced instead of threads. Suitable for structures that experience high loads, but it cannot be given a complex shape.
There is also glass mat. This is junk fiber, which we will not consider due to the lack of strength of this type of material. Also, it is not used with epoxy resins.
When ordering from a supplier, you must tell him that the fabric must be compatible with epoxy resin. This is due to fabric lubricants... I will not explain here - the Internet is big.
Classic types of weaving have a feature - anisotropic properties of finished fiberglass. This means that the properties along and across the fibers will be different: across the fibers, the structure is stronger in bending than along.
There is also multiaxial weaving: biaxial, quadraaxial. The properties of such weaves in the finished sheet of fiberglass, both lengthwise and crosswise, will be the same, but lower than crosswise in standard fiberglass weave. They are used for alternating loads. They are also good for laying. The advice is the same - before ordering fabric, we communicate with the supplier's technologists about the selection of weave types and fabric densities.
Volumes.
The required volume of resin always comes from the ability of the fiberglass to absorb resin. A denser fabric absorbs more resin than a less dense one.
The volume of the fabric is calculated based on the area (we measured the area with a tape measure), looked at the width of the roll of fabric from the supplier and calculated with a small reserve. The fabric in the layer is always laid with a small overlap, to ensure the strength of the finished structure (laminate). Also, the fabric is laid with a small reserve in terms of dimensions - the excess is cut off after polymerization.
The amount of resin required for work comes from the amount of fiberglass. Resin consumption by fiberglass can be found in the TDS (Technical Datasheet) of the fabric manufacturer, it depends on the density of the fabric and the method of fabric molding by us. In this case, we will have contact manual molding. You can find a lot of useful information about proportions and molding methods in fiberglass reference books. In our case (manual molding of fabric, on a laminated base), this proportion will be 50:50, i.e. approximately 1 kg of resin will be used per 1 kg of fiberglass. You can do several tests before work and calculate the proportion empirically to see how well the fabric is impregnated with resin. Too much resin is also bad - these are the worst strength indicators and a source of microcracks in the future.
For pits after osmosis, we build up a layer from the inside out. The first layer is a small patch, the second is larger, and so on, until we reach the “height” of a healthy surface. Sometimes you can use fillers, but you need to be careful with them: the filler does not provide strength. We do not use fillers, we demand quality. =)
The second point is the use of fabrics of different density from dense to less dense. This is due to the texture of the fabric. Less dense fabric is needed on the outer, visible surface for greater smoothness and evenness of the relief - to reduce the texture.
External factors.
Temperature. An important indicator for working with composite and multicomponent polymers. 20 degrees. The temperature can be any, provided that it is equal to 20 degrees Celsius (68 degrees Fahrenheit). The temperature may differ from this by +-10 degrees. It must be constant. When ordering materials, it is necessary for technologists to know the temperature of your workshop during work. Then they will select components that work in these conditions. All materials and the object itself must be at the same temperature. If we brought materials to the workshop, then they must equalize in temperature with the workshop. During work, the temperature should not change more than +-2 (3) degrees. It is possible to work at a temperature of +5, but with major reservations and mixing the finished resin (compound) will be problematic and of poor quality.
Relative humidity is also important. From 30 to 70% humidity. Without condensation on work surfaces. Ideally 40-60%.
Ventilation of the room is necessary.
Tool.
- Durable plastic (disposable) tableware. Cups for small volumes, buckets for large ones;
- Stirring tools. Wooden sticks for small volumes. Mixers with stirrers for small and medium ones. Screwdrivers, drills with whisks for large ones;
- Electronic scales;
- Means for dosing components (if necessary). These can be plastic mugs, etc.;
- Disposable protective suit;
- Nitrile gloves;
- A large number of paper towels;
- A mask with a respirator or glasses and a half mask;
- Flat brushes with natural bristles for painting from a hardware store, 10, 25, 50 mm wide;
- Paint tray;
- You will also need a trash can with a bag inside and a lid;
- Large scissors.
From the equipment, we will need a large table covered with film or masking paper.
Production process:
Get dressed in overalls. Put on nitrile gloves and a mask.
The body is dry, dust is removed. The prepared surface is clean.
On a clean surface, unroll the fiberglass and try the fabric on the body with a small margin. Cut off the required amount. Do not cut along the grain line. The cut is made at an angle, then the threads will not fall out of the canvas.
Cut off all the necessary layers, lay them out in the required order on the table. Then put them on the scales to find out the mass.
By the fabric-resin ratio, we find out the mass of the resin. In our case, it is 50:50 (50 parts by weight of fabric and 50 parts by weight of resin) (your values may differ).
Next, take a clean bucket (it is disposable), install it on the scale. Reset the readings (or press the "tare" button). Calculate the proportion of components A and B based on the mass of the resin according to the documentation of the resin manufacturer. You should not take a 5-liter bucket if you need to mix 100 grams of resin. Everything should be comparable, otherwise there will be no good mixing. (I use 5-liter buckets. The minimum volume of resin or glue for me is 1 kg). By the way, you should not mix 10 kg at once. The resin will start to boil! And in 30 minutes you will not be able to use it all (the life of the finished resin is limited).
Dosing the resin (component A) into the bucket to the required weight according to the scale readings.
Then dosing the hardener (component B) to the required final weight of the finished resin. Component B is poisonous. If it gets on the skin, it causes a chemical burn. If it gets inside or into the eyes... Be careful and don't forget about PPE! And we start mixing everything with a mixer whisk. The mixing speed is low - you cannot allow air to be captured from the surface of the resin deep inside. Be sure to mix with a mixer near the walls and the bottom of the container. If the process is done well, then you can work with this container. If not, then it is better to pour it into a clean one. If the mixture is of poor quality, unreacted components will remain on the walls, giving an unclear result in work (strength, polymerization time, etc.)
Pour the finished resin into a paint tray and apply a layer of the finished resin to our body with a brush (or roller). Next, apply the fiberglass. Give it some time to soak. Smooth and pat the fiberglass in the corners and radii with a brush. You can also use spatulas (rubber, plastic) for this operation. Our task is to lay out the fabric evenly, without bubbles, and let it soak well. This happens with the first layer of fabric - we lay the fabric evenly, with a slight overlap on each other, otherwise the structure will not be strong.
Then coat the first layer with a brush with resin. Apply the next one, etc. until the fabric and resin run out. =)