As you have learned, bonding with UHMWPE is EXTREMELY difficult. You’ve nailed the problem statement: non-melting, no-surface energy (lay term).
Following is reinforcement, perhaps much more than you wanted, using my understanding for the science of why “gluing” to PE is so hard, how this has been traversed with PEs before UHMWPE, and why UHMWPE is an even more difficult challenge.
All PE’s are essentially CH2 repeating units in very long chains, just “macromolecules” in the family of saturated (negligible C=C double bonds), linear hydrocarbon oils and waxes. Such saturated, linear hydrocarbons don’t surface bond usefully to anything. There is no molecular functionality (polarity and/or reactivity) available for bonding, either hydrogen or covalent.
How does one bond something to oil? By using surfactant molecules having a hydrocarbon end of the molecule and a functional end of the molecule comprising Oxygen, Phosphorous, or other non-hydrocarbon moiety. Think about a washing machine and detergent. One uses the “long enough” detergent molecules to provide (entropy driven) mixing on the oil side and hydrogen bonding on the water side.
Multi-layer plastic bottles use this approach to bond less expensive PE layers used for structural heft to various barrier layers used for limiting permeation of O2, CO2, water, etc. (typically nylons, EVOH, etc.). Mil-thickness tie layers, typically comprising expensive PE copolymers with Oxygen moieties, are placed between the PE layer and a very dissimilar barrier layer. The tie layers partly mix with the PE on one side and mix and/or bond to the barrier layer on the other side. It’s sort of like laundry detergent but in reverse. Instead of washing away the oil using surfactant to bond oils with water, the polymers are “frozen” (cooled to ambient) while in the mixed and/or bonded state. Food and beverage bottles and films only a couple of millimeters thick can have 11 layers of different structural, tie, and barrier polymer layers. (A useful bottle, but a recycling nightmare.)
Alas, the dissimilar layers are coupled while in the melt phase under high pressure, and this is not easily applicable to non-melting UHMWPE. See below.
Optionally, the linear paraffins, including UHMWPE, will MIX with select other molecules when not too dissimilar (without significant molecular polarity) – BUT!! this is useful only when the molecules are in liquid form. Think grease with oil, melted paraffin wax with gasoline, etc. But don’t think about mixing “frozen” wax or gelled diesel, unless willing to heat to a more liquid state. (And don’t put polar water or too much polar ethanol in your gas tank or they will phase separate.)
On the mixing thought, thick-walled HMWPE pipes for pressurized water and natural gas service can be “welded” to other pipe sections by thermal melting (liquefaction) of the pipe ends. (HMWPE is “extra” high MW for extra strength but it still meltable, unlike Ultra High MW for extreme strength.) The melted HMWPE PE molecules in their liquid states are forced one pipe end to the other, and then the joint is frozen in place by cooling. Although it is difficult to obtain uniform melting with enough contact time for molecular mixing without distortion and weak spots, heating rigs and supporting collars have been developed for such PE pipe welding. However, for residential and commercial construction, simple mechanical clamping methods are typically preferred as cheaper and more reliable for PE and PEX piping joints (unlike gluing or solvent bonding for PVC and CPVC piping).
The additional, intractable problem for such a melt mixing with UHMWPE is its strength. Literally. Although the Ultra High MW weight is essential for the incredible strength to weight ratio and abrasion resistance of this material, the very extreme MW means that the molecules are so large that heating UHMWPE tends to thermal decomposition before full melting (liquification); and even when melted the molecules are so large as to be incredibly viscous and not easily mixed for molecular entanglement.
This brings us to your idea of dissolving conventional HDPE to create a glue, which follows the lines of thought using 1) similar molecule type to be the binding agent and 2) solvation vs melting.
• Yes, conventional HDPE, LDPE and LLDPE can be dissolved, but even these “smaller” PE molecules are so large that they must be heated to almost their melting points in order to obtain solvation. Solvation temperatures ≥ about 100°C are needed.
• Acetone and other polar solvents are specifically adverse for solvation, because the PEs are so definitively non-polar.
• Instead, hydrocarbon solvents are useful for dissolving PEs. Xylene is often used in laboratory tests requiring partial or total solvation of PEs. Other hydrocarbons comprising hexane, cyclohexane, and decalin are used for industrial manufacture and fabrication processes for PEs.
• At home one would likely select xylene as the simplest, safest solvent for dissolving conventional PEs including HDPE, but even that can be hazardous since the needed 100°C is far above the (fire) flash point of xylene.
Alas, such a solution of HDPE would still not be a useful glue because it lacks any BONDING energy (enthalpy driven) to bind to the surface of the ambient UHMWPE fibers. Yes, the solution could in theory MIX (entropy driven) with the UHMWPE surface, but only if the UHMWPE surface were also liquid, which it distinctly is not — unless heated far hotter than 100°C, and near the point of UHMWPE thermal decomposition.
How else are conventional PEs bonded to other things such as printing inks and paper laminations, which we know exist as common products?
Often, this is done by converting a wee tiny bit of the surface of the PE to not-just-PE. Examples follow.
For adhering printing inks to PE bags, the surface of the PE film is converted into not-just-PE by air oxidation using a thermal or electrical corona treatment of the formed film. The surface oxidation adds a wee bit of acid, aldehyde and other Oxygen functionally into parts of some of the surface PE molecules; and these molecularly embedded polar moieties then bind with printing inks. A careful balancing is needed for the amount of antioxidant compounds included in the PE, which is needed to protect the extruder and die systems from oxidative fouling and to provide good shelf and service life of the PE film, versus the intensity of the corona treatment needed for film surface oxidation for ink bonding.
A similar surface (light) oxidation happens with air-stretch-blow-molding of PE for milk jugs, etc. A small amount of surface oxidation provides a weak linkage for applying pressure sensitive product labels. However, peal one of these labels from a PE milk jug and try to stick it to (unoxidized) UHMWPE materials. There will be a slight tack, but not the structural bond that we seek for making backpacking gear.
Lamination of PE films onto paper, aluminum foil, and other plastics also relies on a slight oxidation of the extruded web of molten PE before being pinched together with the other layer. (More expensive PE copolymers comprising Oxygen functionality (acids, esters, carbonyls, etc.) are used for more difficult lamination applications.)
This conversion of a very thin surface layer PE to not-just-PE sounds like your musing on cold plasma electrical device. You have hit on a very key concept here! However, obtaining sufficient and sufficiently uniform partial oxidation of the surface of the UHMWPE fabric to obtain a strongly glued bonding sounds extremely problematic at home. Has it even been done industrially?
As a practical demonstration of all this theory, reflect that “Cuben” fabrics are a lamination of dispersed (separated) UHMWPE fibers between 2 very thin layers of PET and that the PET layers are not really well bonded to the UHMWPE fiber, i.e., fiber “racking” occurs when laminate structure is stressed on bias.
Reflect further that the Cuben laminates reached commercial utility about 3 decades ago. If it were commercially “easy” to bond (glue) the UHMWPE fibers on 1-side only, as contrasted to sandwiching between 2 bonded layers of PET (or other) film, some type of “Ultra” would likely have become a commercial reality years ago.
I wonder how exactly Challenge Sail Cloth has bonded the urethane waterproofing layer to the UHMWPE fabric, but I’ve not dug for patents, and they are likely keeping much as trade secrets.
Reflect also that there are delamination stories already circulating, even with their “proprietary adhesive”.
Then reflect that the strength of the bonding of the waterproofing layer to the UHMWPE fabric need not be nearly as strong as would be needed to bond one piece of UHMWPE fabric to another to make a tent or backpack.
Why not make a UHMWPE that includes comonomers like organic acids, esters, etc. so that gluing is easier? Because no one has found a polymerization catalyst and process technology that can make at reasonable cost the extraordinarily (Ultra) high MWs needed for the exceptional mechanical properties when there is a comonomer other than ethylene.
In summary, gluing for UHMWPE is extremely difficult both conceptually and practically; but is it impossible? Perhaps not. Conceptually, a very uniform oxidation, sulfonation etc. of the fabric surface might be possible with industrial technology. Conceptually, melt phase coextrusion of fibers having UHMWPE gel core and a PE copolymer sheath might be possible to form and devolatilize. How else? How else? At what cost (and value vs. other material options)?
