Adhering Polyethylene to Various Substrates
“I need to attach this 1.58 mm polyethylene to another type of material such as wood-plastic-metal of various types. Any ideas?”
Polyethylene is a polyolefin and very difficult to achieve strong adhesion to. A common method to overcome this issue is to pre-treat the surface via corona discharge, gas plasma, flame treatment, or priming. These methods typically increase the surface energy of the substrate and the potential to adhere to it. Utilizing any of these pre-treatment methods will open up the choice of possible adhesive products. To bond surface-treated polyethylene to wood, plastic, or metal you can use a cyanoacrylate (RX-50 from Pacer, available through DYMAX), epoxy (Master Bond EP21), or polyurethane (Master Bond EP30D12).
The right adhesive choice for you is not only dependent on the dimension, design, and substrates you are trying to assemble, but also the environment the device/item is being subjected to. Is it being used indoors with no contact to moisture or outdoors with consistent contact to water? For a dry environment, a cyanoacrylate might be the right choice, whereas an epoxy might be better for a moist environment.
Outgassing During Cure of UV Medical Device Adhesives
“I need a recommendation of a medical grade non-cytotoxic UV adhesive for bonding together clear polystyrene moldings. What adhesive would you recommend? What is a simple but effective method to monitor if the curing reaction has gone to completion? Is there any dosimeter available to accurately measure the UV exposure? Would a post bake after UV curing help? Does anything “outgas” from the UV medical-grade acrylates during curing?”
Two options come to mind when looking for a medical-grade light-curable adhesive for polystyrene – 1201-M-SC and 1120-M-UR from DYMAX Corporation. These materials exhibit excellent adhesion to polystyrene, and are both ISO10993/USP Class VI tested. Product data sheets are available at www.dymax.com. One simple but effective method to monitor if the curing has gone to completion is incorporated into the 1201-M-SC product. This material uses a technology called “See-Cure”, where the material starts off with a brilliant blue color, and as it cures changes to clear. This is an excellent visual indicator that complete cure has been achieved in all parts of the bond line. Other methods to determine state of cure include destructive testing of the components to measure tensile force, or a drop of adhesive at the bond-line surface and using this droplet to measure for tack/semi-cure (a go/no-go measurement observed by the presence or absence of adhesive transfer onto a gloved finger). More complex methods include microscope FTIR analysis of the adhesive to identify the presence of the double-bonds peak (on the spectrum) before cure, and the removal of the double-bond peak after cure.
Dosimeters are necessary to accurately measure light exposure, and there are different versions, with different sensors, that measure different parts of the UV and visible light spectrum. While most light-curable adhesives cure with a combination of UV-A, UV-B, UV-C, and visible light, it is often convenient to reference the UV-A light spectrum coming from the light source. UV-A is commonly referred to as 365 nm, but actually covers a range of approximately 320-395 nm. This can be measured with an ACCU-CAL™ 50. If the polystyrene is UV blocking, then you would have to rely on the visible light spectrum of the lamp. The ACCU-CAL™ 50V measures 395-465 nm. Both units can give you average intensity (mW/cm2), peak intensity, and total energy (Joules/cm2). Other options are available like the ACCU-CAL™ 50 LED, which was developed for special lamps (such as LED lights which only emit a single wavelength at either 385 nm or 405 nm) to integrate around the center of the lamp spectrum.
A post bake is not necessary on most adhesives, but there are a few adhesives with a peroxide thermal initiator, which can use heat to cure areas not able to see light.
In regard to the question on outgassing of UV light-curable adhesives during cure, it is sometimes observed that a small amount of smoke comes up from the adhesive surface during the cure step. This is typical, as the adhesive may emit trace amounts of some of the ingredients (or fractions of the ingredients) contained in the formulation while light is shining on the adhesive and cure is taking place. Sometimes this can be overcome by varying the intensity and duration of cure, as well as adhesive choice and light source. This does not happen when the adhesive is used between two surfaces. Proper ventilation can help remove this smoke. If the smoke deposits onto a spot or flood lamp, then periodic cleaning of the end of the lightguide or lamp housing should be done to remove the film that may form there, as this thin film can reduce the intensity at the bond line.
Mechanical Properties of UV Adhesives
“In my application I have a process where I apply UV adhesive between two pieces of plastic and I am seeing a short contraction period followed by a longer expansion period. Is it possible for UV adhesive to behave this way? How much does UV adhesive shrink during cure? Could this cause a pulling force between two plastic materials? If under an opposite force could the UV adhesive relax and expand somewhat?”
When light-curable adhesives cure, whether curing with UV light or visible light, crosslinks are forming between polymer chains. This pulls the chemical chains closer to each other very rapidly. We typically see a 1-2% linear shrinkage, which could translate into a 2-5% volumetric shrinkage. This may stress some plastics or optical components. There is a relaxation effect, usually over the next few hours or overnight, where the chains relax slightly as they rotate into an optimum alignment. In the spirit of valentine’s day – polymer chains like to spoon together and snuggle. If they are at odd angles to each other, they are still touching, but want to find that alignment where they are in the same direction and bending the same way. Chemical bonds can stretch and spin around their axes and allow for this relaxation. Also good to note, a product with a low modulus will stretch easier under stress, and a product with a very high modulus will not stretch much at all. A silicone (on one extreme) can have a modulus as low as 300 psi, whereas an epoxy can have a modulus as high as 2,000,000 psi. Many UV-curable adhesives are urethane acrylates and can vary in their modulus’ over a very wide range. The product data sheet should list this value.