Resealing bulkhead

Discussion in 'General Paddling Discussions' started by pawsplus, Nov 6, 2015.

  1. Astoriadave

    Astoriadave Paddler

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    I'll check it out and respond later.
     
  2. Astoriadave

    Astoriadave Paddler

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    Mick,

    I read through those links. They give a broad brush treatment of the " theory of adhesives." What is missing is a tailored set of examples of commercially available adhesives in use for bonding materials (aka "substrates") to each other. You can think of the theoretical stuff as a toolkit having a bunch of tools in it, ready for use. You can think of an adhesion example as a car body missing a fender that needs reattaching, using the toolkit. What is needed to attach the fender to the body is an understanding of how you use the tools to do that.

    The links have no illustrations of how the theoretical tools help construct (or, "synthesize") a specific adhesive to stick two materials (for example, two kinds of plastic) together.

    I think approaching adhesion from an intermolecular forces standpoint is much simpler and more direct. Wikipedia has a pretty good treatise which takes that approach, for anybody wanting a deeper understanding of adhesion: https://en.m.wikipedia.org/wiki/Adhesion (Ignore the brief section on surface energy and dive into the discussion of "Mechanisms" of adhesion.)

    Back to the issue at hand: how does flaming a polyethylene or polypropylene surface prepare it for adhesion using the recommended adhesives.

    I think flaming does two things:

    1. Evaporates adsorbed smaller molecules such as water and solvents such as white gas, greases, and alcohols from the surface. These adsorbed molecules prevent the adhesive from directly contacting the surface.

    2. Converts some of the nonpolar polyethylene polymeric surface structure into other structures which are polar and have oxygen in them (the aldehydes and ketones). JKA's chosen adhesive, Sikaflex, is probably polar, and its polar molecules will have greater intermolecular attraction for other polar molecules ... produced by the oxidizing effect of flaming.

    The benetits of flaming degrade with time, especially when the plastic cools, allowing moisture and other materials to re-adsorb, preventing the Sikaflex from directly bonding to the polyethylene, modified or unmodified.
     
    Last edited: Oct 12, 2018
  3. mick_allen

    mick_allen Paddler & Moderator

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    That's very instructive . . . I think with your description of the polar properties being important that the form of bonding most likely here is hydrogen bonding - the weakest chemical bonding mentioned [the others 2 being ionic and covalent]. If that is so [or what you mean] then it makes some sense that the ability of the substrate to be wetted is of so much importance [ie the adhesive can get 'closer' to the substrate [plastic] as hydrogen bonding happens at very small intermolecular distances] .

    It is also mentioned in that wikipedia link that the bonds are quite brittle , which might mean that movement due to impacts might not be helpful for bulkhead adhesion. For example this might be a conflict between bulkhead adhesion [soft foam deflects a lot therebye large areas of adhesion compromised] VS hull impact stress conconcentration problems by using rigid bulkheads [which presumably would have less areas of compromised adhesion].

    Are those reasonable characterizations of anything?
     
  4. Astoriadave

    Astoriadave Paddler

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    The chemical jargon in those articles has lead you astray. I'll try to clean up the stuff that was misleading.

    Molecular polarity probably is how Sikaflex attaches itself to the oxidized polyethylene. Nonoxidized polyethylene is totally nonpolar. Flaming probably produces some polar molecular features on the surface of the polymeric polyethylene. But "molecular polarity" is due to the presence of dipoles in a molecule, often termed "polar bonds" or "polar covalent bonds." [Look up dipoles in Wikipedia, if needed.]

    In contrast, "hydrogen bonding" is miserably misleading chemical jargon for a specific intermolecular attraction in which a hydrogen atom forms a "bridge" between two atoms in separate molecules. It only occurs when those two atoms are H, N, or O, AND each of those two atoms is a partner in a "polar covalent bond." Example: one alcohol molecule can do this with a water molecule. The "to" is the "hydrogen bond."

    H3CH2C-O-H to O-H
    □□□□□□□□□□□□□|
    □□□□□□□□□□□□ H

    I hope my crude use of spaces and dashes comes out OK. (Ignore the boxes.) The upshot with oxidized polyethylene is it cannot participate in "hydrogen bonding." Thus it is primarily restricted to intermolecular attractions between its polar parts and the polar parts of the Sikaflex.

    The second confusion centers on "wetting" of a liquid to a solid surface. Up front: water is not needed for this. In the context of adhesion it refers to the spreading out a liquid does when it us attracted to a solid. Basically, polar liquids "wet" polar solids. And nonpolar liquids "wet" nonpolar solids.

    The Sikaflex being polar, it will "wet" a polar solid, but NOT a nonpolar solid. "Wetting" must occur for adhesion to begin. Consequently, oxidation of polyethylene to generate polar features on its surface allows Sikaflex to "wet" it, as a prelude to adhesion. Enough for now.
     
    Last edited: Oct 13, 2018
  5. mick_allen

    mick_allen Paddler & Moderator

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    Thanks Dave, your most helpful - I obviously need to do some more reading in order to get a better understanding of what you and the info is saying. Interesting stuff.
     
  6. Astoriadave

    Astoriadave Paddler

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    Mick, an intro level chemistry text with a decent chapter on covalent bonding and good coverage of intermolecular forces, as a reference, absorbed as you need it, is sufficient. A lot of chemical terminology, aka jargon, is argot which grew over time, and only parses well if you recognize each chemical idiom and interpret it correctly. User-friendly, it ain't!

    One error, previous post: "It only occurs when those two atoms are H, N, or O, ..."

    should read: " It only occurs when those two atoms are F, N, or O, "