|
|
Syntetiske overfladefleece | Why use a syntethic veil? |
A lightweight non-woven fabric that is used in the manufacture of Fiber Reinforced Plastics
The veil can provide the following characteristics or properties to an FRP part: 1. Improved surface appearance and profile 2. Improved corrosion resistance and service life 3. Improved abrasion resistance and impact resistance 4. Improved thermal shock properties 5. Improved dye wears and reduced pull forces in pultrusion 6. Prevent underlying Glass of weathered FRP parts from blooming to the surface 7. Serve as Print Blocker of underlying reinforcements 8. Provide graphics, deeper shades of color and various other properties such as resin effecting chemistries and conductive properties
| | What is a synthetic veil? |
A Synthetic Veil is a lightweight fabric that is produced from man-made fibers such as PET “Polyester”, Polyamide “Nylon”, Acrylic, Para Aramid or other man-made fibers utilizing various non-woven engineering technologies to form into a light-weight fabric.
| | The history of synthetic veil in corrosion |
In the late 1960’s early 1970’s C-Glass Veil was predominantly used to improve corrosion resistance of FRP parts that were subjected to harsh environments. The surfacing veil was conducive to producing a resin rich layer when present.
In the early 1970’s it was found that synthetic veils performed better than C-Glass Veils in certain harsh corrosive environments such as hydrofluoric acid, sodium hypochlorite, other hypochlorites, chlorine scrubber fluids and strong caustic environments to name a few. In 1974 Burlington Industries introduced a binder free synthetic polyester veil known as Nexus®. The chemical industry quickly adopted Nexus® as the synthetic veil of choice for us in chlorine scrubbers. Nexus® was very different from C-Glass Veil, as it is made from polyester stable fibers not glass fibers. The first step in producing synthetic veils is to form the synthetic fibers into a fabric by either hydroentangling or thermally bonding the fibers. Depending on the fiber forming technology used other steps are employed to produce the right characteristic of fiber density, area weight, thickness, stretch, and strength. As a result the synthetic veil is very soft, pliable, but still has very good tensile strength. The use of synthetic veils grew rapidly in special corrosive environments with as many 10–14 plies of Nexus being used to obtain longer service life in very aggressive environments. Because of good handling characteristics, synthetic veils began getting used in applications not involving corrosive chemical environments – such as food handling and pharmaceutical applications where glass contamination could be an issue.
| | Why is Nexus better than C-glass veil? |
Synthetic veils are dramatically different in construction from C-Veils. Nexus synthetic veils are fiber dense, can be produced binder free or with binders to increase stiffness, and are 3 Dimensional in nature vs. C Glass Veils that always contain Binder, are more 2 Dimensional in nature, and are not as fiber dense. The specific gravity of synthetic veils like Nexus is 1.38 vs. 2.56 for glass. The 3-Dimensional nature of Nexus ® synthetic surfacing veils helps improve interlaminar bond strength of surface layers and ultimately corrosion resistance. Some synthetic veil fabricating techniques work very well and others fail miserably. It is important to note that synthetic veils must be handled differently than C-Glass veils when molding or processing. When C-Veil is wetted with a styrenated polyester or vinyl ester resin, the resin passes straight through the veil very easily. The resin softens or dissolves the binder in the glass at the fiber intersections, leaving the fibers free to lie flat on the mold or wrap around a sticker roller and the air is expelled. When synthetic veil is wetted with the same resin, air is trapped in the spaces between the fiber. It helps to wet-out a synthetic veil from the mold side. The mechanical properties of a synthetic veil do not change significantly with the presence of resin; therefore the properties of springiness and integrity are retained. Synthetic veils such as Nexus have excellent tensile and elongation properties that allow FRP fabricators to fully utilize increased resilience and toughness of corrosion resistant resins. C-Glass Veils and mats are composed of glass filaments with a break elongation of about 2.5% - 4.5% which is less than the 5 – 7% range of tensile elongation of resilient corrosion resistant resins. The inability of glass surfacing mat filaments to elongate under thermal or mechanical stress to the same extent as the surrounding resin matrix can contribute to the premature failure of the resin-rich corrosion barrier of FRP corrosion structures often subjected to dynamic stresses and thermal shock. | | How do you manufacture using synthetic veil? |
Elimination of air bubbles and pockets
The best way to minimize air bubbling when working with synthetic veils is to bring the excess amount of resin from the mold side out through the synthetic veil, expelling the air ahead of the resin. On rotating mandrels of all sizes, this can be done by taking advantage of the high strength of the veil and winding the veil on the underside of the mandrel under even tension while keeping a bead of resin between the mandrel and the veil. The synthetic veil has excellent wet-out properties via this method since the resin is forced through the veil from the mold side by the veil tension. On larger mandrels over-the-top winding can be used with the resin being sprayed on the mandrel just ahead of the veil contact line. In either case, with proper tension, only very light rolling is required. On stationary molds the mold should be pre-wetted with a slight excess of resin by spray or brush before the veil is applied. Light rolling of the veil is used to wet the veil. The procedure is repeated for each layer of veil to be applied. Light to moderate pressure should always be used in order to achieve best results. Care should be taken to have line contact and avoid point to point contact and high pressure since resin will be forced out when the veil is heavily compressed and air will be taken back in as the veil springs up behind the roller. Point contact with high local pressure results when rollers are angled on small and medium diameter mandrels or when straight rollers are used on compound curvatures. Spring activated flexible rollers should be used on small-radius compound curved molds. If chopped strand mats are laid over the wet-out veil layers, ends of the dry chopped strand glass will suck resin from the veil layers as they start to wet out. This lets air back into the layers. Veil surfaces should always be covered by brush or sprayed with resin before the chopped-strand mat is applied. The wrong way is to put dry veil on the mold apply high viscosity resin and roll hard with lots of point contact and then put the dry chopped-strand glass fiber direct on the wet veil liner. Preforming: The mechanical binding of synthetic veil and the thermoplasticity of the fibers can be used as an advantage to preform the veil to contour to compound curves before use. Hat-shaped preforms can be made by pulling the veil down over a male form. Even better forming is obtained if heat is applied to the synthetic veil. Always drape, shape, fit and cut the veil before prewetting the mold. The wrong way is to wet the veil and then try to brush and roll it over compound curves. back References: “Synthetic Veil: Why and How to use it” by D.G. Chandler, George Overholt, and Tom Anderson Modern Plastics April 1984 McGraw-Hill, Inc. “Thermal Shock Properties of Corrosion Resistant FRP” by Terry S McQuarrie “The Role of Synthetic Veil in the Wear Factor of Corrosion Resistant Laminates” T.O. Baustista Reinforced Plastics/Composites Institute 1980 | | |
|
|
|
|
