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l33tminion) wrote2006-12-07 01:55 am
l33tminion) wrote2006-12-07 01:55 am
Scratch-Resistance Chemistry
I've been planning to write a bit about my work a bit for a while. So here's an explanation of my MatSci final project. For the project, I studied scratch-resistant plastics:
There are two main applications for transparent, scratch-resistant coatings for plastics:
1. Keeping plastic casings scratch-free (pretty)
2. Keeping plastic lenses scratch-free (working)
Glass (mostly amorphous SiO2 (amorphous means it's irregular, unlike quartz)) has most of the desired properties: It's transparent and relatively scratch-resistant. However, glass is brittle (which means it's prone to cracking, which is as bad as scratching). As it turns out, this problem can be fixed. A simple solution is laid out in US Patent 4027073, filed by the Dow Corning Corporation in 1976.
The process relies the following reaction:
1. Combine glacial acetic acid, colloidal silica, and methyltrimethoxysilane.
2. Let stand four days.
3. Coat the resulting mixture on plastic.
4. Bake.
The ingredients require a more detailed explanation:
1. Acetic acid is the acid in vinegar. Glacial, in this context, means 100% pure (no water).
2. Colloidal silica means very small particles of silica (SiO2) suspended in water.
3. Methyltrimethoxysilane is Si(OCH3)3CH3. In other words, it's a silicon atom connected to three methoxy groups (OCH3) and one methyl group (CH3).
The methoxy groups get replaced by silica in the presence of acid, producing methanol. However, the methyl groups are stable, they don't get replaced by anything. Thus, after step 2 in the process you end up with a solution of large molecules made of silicon, oxygen, and methyl groups in water and methanol. After step 4, the water and methanol have evaporated away, and the reaction has completed, leaving a thin film deposited on the plastic. If some methyltrimethoxysilane is unreacted during step 2, it probably binds directly to the plastic molecules during step 4. Otherwise, the film is held to the plastic by hydrogen bonding (mostly between hydrogen in the plastic and oxygen in the film).
The silicon and oxygen in the film give it many of the properties of glass, making it transparent and scratch-resistant. The methyl groups give it some of the properties of plastic, making it more flexible and resistant to cracking.
There are several other good things about this reaction: The ingredients are cheap and fairly safe, and all the byproducts (water, methanol, and acetic acid) are fairly harmless.
The cool conclusion to this is that Ducker (my lab partner) and I were able to reproduce the process described in the patent, and it worked amazingly well. The coating stuck to the plastic, bent along with the plastic, and noticeably increased the resistance to abrasion.
There are two main applications for transparent, scratch-resistant coatings for plastics:
1. Keeping plastic casings scratch-free (pretty)
2. Keeping plastic lenses scratch-free (working)
Glass (mostly amorphous SiO2 (amorphous means it's irregular, unlike quartz)) has most of the desired properties: It's transparent and relatively scratch-resistant. However, glass is brittle (which means it's prone to cracking, which is as bad as scratching). As it turns out, this problem can be fixed. A simple solution is laid out in US Patent 4027073, filed by the Dow Corning Corporation in 1976.
The process relies the following reaction:
1. Combine glacial acetic acid, colloidal silica, and methyltrimethoxysilane.
2. Let stand four days.
3. Coat the resulting mixture on plastic.
4. Bake.
The ingredients require a more detailed explanation:
1. Acetic acid is the acid in vinegar. Glacial, in this context, means 100% pure (no water).
2. Colloidal silica means very small particles of silica (SiO2) suspended in water.
3. Methyltrimethoxysilane is Si(OCH3)3CH3. In other words, it's a silicon atom connected to three methoxy groups (OCH3) and one methyl group (CH3).
The methoxy groups get replaced by silica in the presence of acid, producing methanol. However, the methyl groups are stable, they don't get replaced by anything. Thus, after step 2 in the process you end up with a solution of large molecules made of silicon, oxygen, and methyl groups in water and methanol. After step 4, the water and methanol have evaporated away, and the reaction has completed, leaving a thin film deposited on the plastic. If some methyltrimethoxysilane is unreacted during step 2, it probably binds directly to the plastic molecules during step 4. Otherwise, the film is held to the plastic by hydrogen bonding (mostly between hydrogen in the plastic and oxygen in the film).
The silicon and oxygen in the film give it many of the properties of glass, making it transparent and scratch-resistant. The methyl groups give it some of the properties of plastic, making it more flexible and resistant to cracking.
There are several other good things about this reaction: The ingredients are cheap and fairly safe, and all the byproducts (water, methanol, and acetic acid) are fairly harmless.
The cool conclusion to this is that Ducker (my lab partner) and I were able to reproduce the process described in the patent, and it worked amazingly well. The coating stuck to the plastic, bent along with the plastic, and noticeably increased the resistance to abrasion.