Bondite or flexible primer for polyethylene

Q. We manufacture fiberglass duck hunting boats and are working on a different foam flotation than we've previously used. We are cutting and fitting polyethylene sheets and would like to coat them for abrasion resistance. I saw a posting some time ago on a duck hunting site about a product called "bondite". I would like to know more about this product or a similar product to use as a primer for the polyethylene or as a paint additive to give me a good bite and flexibility.

A. Louis,

Pardon my ignorance but I've never heard of Bondite and would think there sure just AIN'T no good adhesive or primer to allow you to SUCCESSFULLY paint onto PE or PP.

Then you mention you want to paint to improve the abrasion resistance. Holy B------- Cow! Polyethylene IS an abrasion resistant material, far, far better than any paint.

For years the mining industry used to use steel launders (ie. 20 ft to 40 ft long sloped hoppers) down which the ground-up and abrasive ores would slide. Then in the early sixties it was discovered that Polyethylene was far better. Mostly using the flexible low density PE. A Canadian invention. Heck, I recall selling LDPE launders to Ireland for the Mogul mine in those days.

The stiffer Polyethylenes might well be better such as the HDPE. The high density PE is now probably more easily available, too. But you sure can't get paint or epoxy or anything to 'glue' to it. Your supplier should also be able to get you the 'black' PE if U.V. attack is worrisome to you.

The only really good abrasion resistant paint is the 2-component Urethane. Pricey, too! But a painted surface is terribly thin. Further, rotationally moulded canoes from PE have been, as you know far better than I, available for many years ... and if you take that approach, you could build in a colour during the manufacturing process.

Cheers!

Freeman Newton - White Rock, British Columbia, Canada

Ed. note: Please keep Freeman in your thoughts & prayers. Message from Freeman, Dec. 2010

... looking over the Pacific

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A. There are many forms of polyethylene. One of the most abrasion resistant materials known to man is Ultra High Molecular Weight Polyethylene - UHMWPE or UHMW for short. It has such a high molecular weight and high viscosity that it is not readily processable like a normal plastic. This material is only available in sheet form (compression molded) and really cannot be melt processed into complex shapes like the other versions of PE. Common thicknesses are 1/8, 1/4, 3/8 and 1/2 inch thick, four foot by eight foot sheets. These can be bonded, bolted, riveted, screwed, counter-sunk-screwed etc over steel to allow rock and coal and oars to slide even in freezing rainy weather. The ones to be bonded have to have a rubber or cloth backing layer or be surface modified chemically for adhesion. Surprisingly, UHMW will OUT WEAR STEEL several time over in hopper liners or chutes.

Some of the other grades of Polyethylene are "High Density" (HDPE), "Medium Density" (MDPE) and "Low Density" (LDPE). The density is a function of level of crystallinity of the resin. The higher the crystallinity, the lower the fuel permeation and the higher the stiffness and strength but the lower the impact resistance. All of these are available in grades that can be blow molded into containers (fuel tanks) or injection molded into complex shaped objects (fenders). These all have good practical abrasion resistance but not nearly as good as UHMW

Fuel tanks for cars are all high density (crystals are impervious to fuel) and now incorporate an extra layer of high barrier polymer (EVOH; Eval or Sornal trade names) to meet EPA permeation (emission) targets. A sealed up portable gas can will really fume up your trunk unless it incorporates a barrier layer - which is today required by law in California. The "California Can" isn't just a couple of bucks but run cost into the teens. Plastic fuel tanks have been used in cars for years and for the past decade or so have had to incorporate the extra barrier layer per ever tightening EPA regs.

Fenders are likely a Medium Density PE to have sufficient impact resistance and flexibility but not be so flexible to appear floppy. LDPE is used in squeeze tubes and is likely too flexible for fenders.

For the true nerds out there, the density of PE is controlled by co-polymerizing other polymerizable monomers into the polymer chain. Polyethylene of the highest density is just polymerized ethylene ("homopolymer") and has no polymer chain branching. To get impact properties to the point where containers bounce when dropped, a small amount of alternative monomer has to be added. This causes what are called chain branches, which may be either short or long, and all serve to reduce crystallinity and thus density. The branches do not pack will into crystals and thus crystallinity and density both decrease as branching of any kind is added. FYI: most polymers can crystallize to a certain percent, never 100%, by a process called "chain folding".

All the manufacturers have patented their favorite ways to add in the branching to reduce density and add practical toughness. That's why blow molded polyethylene containers and fuel tanks bounce when dropped off the shelf or crashed.

Jack Wells
- Austin, Texas

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June 25, 2008 A. I'm not sure there is a Bondite, Louis :-) We've looked for years for a primary reference to it, but all we find is messages in forums where people think at some point somewhere they might have heard of the product from somebody for some reason, maybe, or something like that :-) It may just be a misspelling so people are referring to different products and we'll never find it. For example there is a "BondIt" and a "Bond It" and a "BondTite". However, you might consider Bulldog Tie Coat [linked by editor to product info at Amazon] or Krylon Fusion [linked by editor to product info at MisterArt], which is a paint designed specifically to be used on plastic. Of course, all plastics are not created equal and Freeman may be right that neither these nor the mythical Bondite will adhere to polyethylene. Good luck. Regards, Ted Mooney, P.E. finishing.com Brick, New Jersey

Q. OK - I'll revive a dead thread...This thread discussed this issue several years ago. Have there been any developments in the painting of HDPE since then? We are attempting to apply a clear coat on a HDPE product and are looking for some type of primer to increase (or a better term would be promote) adhesion. Anything out there......anybody?