By Ira Greiff Product Manager General Resins & Ancillaries
DEFFENITION OF POROSITY
Porosity is defined as very
fine air pockets that get trapped between the gel coat or mould surface or within the gelcoat film. Most often these only become visible when an FRP part is polished up using buffing pads.
They micro bubbles are very fine and usually too numerous to count.
There is normally a very fine gelcoat skin that encapsulates a micro bubble within the gelcoat film. Once buffing starts the "skin" is removed on the part which opens up the "pinhole or micro bubble". The polishing cream then gets trapped in these very fine pinholes which are usually less than 0.5mm in diameter and most notable on dark coloured gel coat. The pinholes are often concentrated in corners of moulds and radi and are too numerous to count.. The reason it’s most visible on dark coloured gelcoat is because white polishing cream gets trapped there and the contrast in colour becomes very noticeable... On white gelcoat this later gets highlighted only when dust gets trapped in the micro bubble. 2
CAUSE OF POROSITY
The following or combination of the following can be the cause of porosity:
These will each be expanded on separately:
1. Gelcoat "painted on" rather than the "brisk brush" method
2. Gelcoat temperature too cold bellow 18 deg C
3. Gelcoat applied too thick in single coat(more than 1mm)
4. Mould surface too cold bellow 18 deg C
5. Incorrect Spray application method
6. Incorrect nozzle size or worn nozzle
7. Incorrect pressure on MVP machine
8. Dust on mould
9. Dirt or soluble type of dust granules in buckets
10. Incorrect catalyst
11. Poor catalyst
12. Static electricity build-up on mould
13. Water or oil contamination from airlines
14. Out of Specification gelcoat or top coat
SOLUTIONS TO REDUCING POROSITY - BRUSH APPLICATIONS
SOLULTIONS FOR BRUSH APPLICATIONS
1. Incorrect Brush technique: Correctly trained operators will never "paint" gel coat on. Gel coat must be applied using the "brisk brush technique". This means short fast rapid brush strokes (left right and up and down) repeated at least 3-4 times on a 30cm x 30cm area before moving on at the correct thickness. This is because the rapid repeated strokes aids in lowering the viscosity and the bristles aid in "popping" the bubbles thus resulting in release of micro bubbles.
2. Gelcoat temperature too cold: Resulting in increased viscosity and more difficult for air release additives to work. Once again gelcoat becomes too thick. As gelcoat manufacturers we cannot formulate this to be thinner as this will cause other problems such as increased wrinkling, draining off vertical surfaces etc. Brushing with a spray version gelcoat is difficult because these are fast gelling and the operator would require practise because it’s easy to apply it too thin due to uneven nature of the brush.
3. Applying gelcoat too thick: This normally happens when someone "paints" gelcoat on instead of using the "brisk brush" stroke technique. Do not apply more than 600 microns(0.6mm). Some yacht builders require over 1mm for hulls (however rather apply this in a 2 step process).
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4. Incorrect Catalyst ratio: Usually too high or combined with very high temperatures over 30 deg C. This results in very fast gel times increasing the risk of porosity as there is not sufficient time for air release additives to work.
5. Mould surface too cold: (bellow 18 deg C). Mould surfaces that are cold will cause curing problems but will also increase viscosity making air release unlikely and difficult. The warmer the mould the better.
6. Dust on mould: This may not always be visible but obviously when buffing and cleaning will show up as a pinhole defects.
7. Not stirring the gelcoat before use: It is advisable to stir the gel coat first before adding catalyst. Gel coat that is stirred beforehand will cause the thixotropic properties to break down temporarily resulting in lowering of viscosity which in turn aids in air release. This is how gelcoat is formulated. The gelcoat will then "thicken up" once applied, preventing drainage.
8. Incorrect hand mix method: Do not use an "egg beater" method of stirring in Peroxide initiator. Rather use a flat bar and not a round or dirty stick. Very fast egg beating of Peroxide initiator with ribbon blade mixers brought to the surface will suck plenty air in to the gel coat. This substantially increases the amount of air trapped in the gelcoat.
9. Pouring gelcoat: Do not "pour" the bucket of gel coat on to the mould but rather dip the brush in to the gelcoat and apply accordingly.
10. Wrong catalyst: In Vinyl esters the peroxide initiators brakes down rapidly causing lots of gassing when the incorrect catalyst is used. If this is absolutely crucial switch over to Triganox 239. (THIS DOES NOT APPLY TO NCS VE TOOLING GELCOAT NORPOL GM RANGE)
11. Poor catalyst: Some catalysts manufacturers produce Peroxide Initiators that contain high MEK+ water content. This in itself reduces the shelf life and stability of the peroxide component and can result in early decomposition and oxygen being emitted which can cause porosity. Check the data sheet or batch number to ensure that moisture "MEK and Water" content is less than 5%. Standard Peroxide initiators should display this as:
33% Peroxide content; 63% DMP; 4% MEK + Water. There is always a small amount of water present during the manufacturing of catalyst. AKZO Nobel catalyst contain less than 5% MEK + water. Typically other manufacturers contain between 5% up to as much as 17% MEK + Water! This can cause other problems with gel coat .
12. Old catalyst that has become unstable can cause bubbling
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13. Static electricity: This is similar to the way a balloon charged with static hangs on to a wall, the same way micro bubbles in gelcoat get attracted to the mould surface instead of away. We know that the dielectric strength of composites, parts and moulds do not rapidly dissipate static charges. Build up of static takes place during buffing and preparation of release agents in moulds. How many times have you walked past a large mould and felt the hair on your arms stand up or heard a snap or spark jump from a large part lifted from a mould? If you have noticed static electricity in your plant then there is no way that any micro bubbles in the gelcoat will be able to escape! It will be trapped in the gelcoat and drawn to the mould surface, right there where it becomes visible upon buffing! Ensure that static build up on moulds are dissipated.
14. Gelcoat out of Specification: If gelcoat containers or kegs are not sealed properly the gelcoat will create a "skin" due to monomer evaporation. This will cause a thickening of the gelcoat and in turn will nullify the air release capabilities of the gelcoat due to thickening which makes it difficult for micro bubbles to pop, resulting in increased porosity.
It is believed amongst a few GRP manufacturers that gel coat manufacturers themselves can often be blamed when they experience porosity problems. Please take note of the following facts:
It is thought that gelcoat must be manufactured under vacuum to ensure that a GRP product is free from porosity. This is NOT TRUE, when vacuum is applied during the manufacturing of gelcoat it’s not for the end user to experience a porosity free product but for other manufacturing reasons.
Air release additives in gelcoat are NOT a requirement for porosity free products though it does help reduce the incidence of porosity. It can be proven that correct application methods using non vacuumed gel coats containing no air release additives can produce porosity free products. Additives are added only to facilitate the quicker release of micro bubbles in workshops that apply the correct applications procedure! Air release additives do not prevent porosity when application methods are poor.
ALL of NCS RESINS gelcoat and topcoats contain air release additives.
SOLULTIONS FOR REDUCING POROSITY- SPRAY APPLICATIONS
1. Apply to thick: Max thickness in one pass is 500 microns (0.5mm). Rather apply this as a "mist coat" of 150 microns per pass and build up 3-4 passes to get to 450-600 microns(0.4mm – 0.6 mm)
2. Nozzle size too large or worn: Use smaller spray tips as this force’s the operator to make more passes reducing the risk of porosity. Alternatively if you do not have access to smaller spray tip the spray tip distance from the mould needs to be increase whilst pressure on the fluid pump should also be increased. Once again four passes not one thick pass.
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3. Gelcoat too cold. Some of the MVP machines have inline heaters. Contrary to popular belief this is not to improve your production rate but aids in improving spray pattern, lower machine pressure, less fumes and off course reduced porosity as warm gelcoat expels micro bubbles much easier. The gelcoat falling on the mould surface usually attains the temperature of the mould and workshop in seconds, thus inline heaters do not improve cure time as some would believe. Mould temperature and workshop temperature do.
4. Peroxide level too high combined with high workshop temperatures. Spray gelcoat are typically formulated to gel much faster than brush at about 8 minutes-12 minutes. When Peroxide Initiator levels are increased to over 2.5%, combined with high workshop temperatures of over 30 degrees C the result will be a gel time of potentially under 2 minutes. This is not sufficient time for release of micro bubbles as gelcoat thickens and gels before this can take place.
5. Mould surface too cold (bellow 18 deg C). Mould surfaces that are cold will cause curing problems but will also increase viscosity making air release unlikely and difficult. The warmer the mould the better.
6. Dust on mould. This may not always be visible but obviously when buffing and cleaning will show up as a pinhole defects.
7. Not stirring the gelcoat before use: It is advisable to stir the gelcoat first before spraying. Note that gelcoat that is stirred beforehand will cause the thixotropic properties to break down slightly temporarily resulting in lowering of viscosity which in turn aids in air release. This is how gelcoat is formulated. The gelcoat will then "thicken up" once applied preventing drainage.
8. Incorrect spray fan. A thick spray fan will result in gelcoat applied too thickly which in turn could trap micro bubbles resulting in increased porosity. Increase your pump pressure or increase the gelcoat temperature.
9. Wrong catalyst: In Vinyl esters the peroxide brakes down rapidly causing lots of gassing when the incorrect catalyst is used. If this is absolutely crucial switch over to Triganox 239.(THIS DOES NOT APPLY TO NORPOL VE TOOLING GELCOAT)
10. Old catalyst that has become unstable can cause bubbling.
11. Poor catalyst: Some catalysts manufacturers produce Peroxide Initiators that contain high MEK+ water content. This in itself reduces the shelf life and stability of the peroxide component and can result in early decomposition and oxygen being emitted which can cause porosity. Check the data sheet or batch number to ensure that moisture "MEK and Water" content is less than 5%. Standard Peroxide initiators should display this as:
33% Peroxide content; 63% DMP; 4% MEK + Water. There is always a small amount of water present during the manufacturing of catalyst. AKZO Nobel catalyst contain less than 5% MEK + water. Usualy the water component is under 2%. Typically other manufacturers contain between 5% up to as much as 17% MEK + Water! This can cause other problems with gel coat .
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12. Static electricity: This is a very big cause of porosity and probably the single biggest cause on large moulds. Similar to the way a balloon charged with static hangs on to a wall, the same way micro bubbles in gelcoat get attracted to the mould surface instead of away from the mould surface. We know that the dielectric strength of composites, parts and moulds do not rapidly dissipate static charges. How many times have you walked past a mould and felt the hair on your arms stand up or heard a snap or spark jump from a large part lifted from a mould? If you have noticed static electricity in your plant then there is no way that any micro bubbles in the gelcoat will be able to escape! It will be trapped in the gelcoat and drawn to the mould surface!
13. Gelcoat out of Specification: If gelcoat containers or kegs are not sealed properly the gelcoat will create a "skin" due to monomer evaporation. This will cause a thickening of the gelcoat and in turn will nullify the air release capabilities of the gelcoat due to thickening which makes it difficult for micro bubbles to pop, resulting in increased porosity.
It is believed amongst a few GRP manufacturers that gel coat manufacturers themselves can often be blamed when they experience porosity problems. Please take note of the following facts:
It is thought that gelcoat must be manufactured under vacuum to ensure that a GRP product is free from porosity. This is NOT TRUE, when vacuum is applied during the manufacturing of gelcoat it’s not for the end user to experience a porosity free product but for other manufacturing reasons.
Air release additives in gelcoat are NOT a requirement for porosity free products. It can be proven that correct application methods using non vacuumed gel coats containing no additives can produce porosity free products. Additives are added only to facilitate the quicker release of micro bubbles in workshops that apply the correct applications procedure! Air release additives do not prevent porosity when application methods are poor.
ALL of NCS RESINS gelcoat and topcoats contain air release additives 7
HOW TO FIX A PART THAT ALREADY HAS POROSITY?
This is almost impossible to repair. No matter how hard you try by sanding and fairing you invariable break open the surface of the surrounding gelcoat creating an even larger area of pinholes. If the part sales value and cost is large enough then one can consider spray painting it.
References:
1) Reichhold Presentation and experiments on Porosity
2) Akzo Nobel. Training presentation on Peroxide Initiators
3) Tim Young - ACMA- Composites manufacturing "Good Basics are key to staying ahead" http://www.acmanet.org/cm/0906/feature_e0906.cfm
4) Ira Greiff personal experience in manufacturing and customer visits and trouble shooting
5) NCS Resins Internal technical liaisons with Rohan Mohabir and Mark Vandeyar
6) MVP discussions and experiences on spray equipment
