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Polyvinyl Acetate Adhesives Forestry 485 Lecture 3-4

Polyvinyl Acetate Adhesives Forestry 485 Lecture 3-4. Background. Vinyl group a.k.a. Ethylene. Self Addition Polymerization. Polyvinyl / Polyethylene. Background. Substituted Ethylene – replacing one H with an Acetate group New Monomer Resulting Molecule.

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Polyvinyl Acetate Adhesives Forestry 485 Lecture 3-4

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  1. Polyvinyl Acetate Adhesives Forestry 485 Lecture 3-4

  2. Background Vinyl group a.k.a. Ethylene Self Addition Polymerization Polyvinyl / Polyethylene

  3. Background Substituted Ethylene – replacing one H with an Acetate group New Monomer Resulting Molecule

  4. Background =

  5. Polyvinyl Acetate (PVAc) Properties: Polarity – PVAc is a more polar molecule than polyethylene Polymerization – performed in water, product forms globules that are suspended Bonding – Suspensions are stabilized Little H2O loss results in coalescence and bonding Bond formation in as little as 10 minutes Full strength in 24 hrs

  6. Creep Polyvinyl Acetate (PVAc) • Performance: • Thermoplastic in nature • High temp. softens adhesive • Moisture compromises performance • Under shear PVAc bonds as strong as maple • Perpendicular grain joints are allowed to “flex”

  7. Polyvinyl Acetate (PVAc) • Performance: • PVAc doesn’t fill gaps in an assembly well • Gaps can actually form – Island formation Emulsion is destroyed by freezing temps. • - Tell tale chalky appearance • (as opposed to translucent • appearance of normal cure) • - Application to “cold” surfaces • (< 60 oF) can also adversely • affect emulsion and cure

  8. Polyvinyl Acetate (PVAc) Modification: Aliphatics a.k.a. Carpenter’s Glue Aliphatics – Holdover term from “old days” of chemistry • Properties: • Faster Setting • Can tolerate lower temperatures • More resistant to creep • Less sensitive to moisture and heat • More thixotropic – Less squeeze out

  9. Monitoring the Gluing Operation Brief Overview See: Mara Chapter 10 “Quality Control” (Note: you don’t have this, but a copy will be provided upon request) Glue Prep Application Assembly Time Conditions Pressure Monitoring

  10. Glue Preparation Mixing: Manufacturer’s instructions Pre-Weigh / Pre-Measure Order of addition Mixing Temperature

  11. Glue Application Two Criteria: Ratio of surface area to weight of wood element Low = weight of adhesive per area of glue line Lbs. (liquid) / 1000 ft2 (US) High = weight of adhesive per weight of wood Lbs. (solids) / lbs wood (OD) Application QC – simple measurement One square foot, applied on both sides mass in grams X 1.1 = Lbs. / 1000 ft2 Alternate method – film thickness measurement

  12. Assembly Time Time Sensitivity: Glue may need time to “set up” Too much time - glue dries out Many factors involved in optimum time First layers and last layers most critical (as in plywood lay-up)

  13. Ambient Temperature / Relative Humidity Impact on Time Sensitivity: High temp and/or low humidity - decrease in optimum assembly time Measurement Wet and dry bulb thermometer Sling psychrometer

  14. Pressure Six Basic Mechanisms of Application: • Live Roll • Dead Weight • Spring • Screw • Ram • Pneumatic (Fluid) Pressure

  15. Pressure Terms to Clarify: Load = mass or force imposed on a system (lbs.) Pressure = mass or force imposed on a system over the area of the system (lbs/in2) Example – 200 lb. rock on a 1 ft2 panel Load = ? Pressure = ? Rigid versus Fluid Pressure – Rock versus Sandbag

  16. Pressure Dead Weight Systems – Direct reading systems Pressure = load divided by area Examples: A. 200 lb rock on 1 ft2 panel B. 55 gallon drum of water on same panel C. Same drum hung from lever with 4:1 advantage • 1.39 lb/in2 • 3.17 lb/in2 Load = 55x8.3 = 456.5 lbs • 12.68 lb/in2 Load = 456.5x4 = 1826 lbs

  17. Pressure Spring – Loaded Systems Maintain constant pressure over time “Follow up” Load depends on spring constant and amount of deflection Toggle or spring clamps used in shop- scale work

  18. Pressure Screw Presses Load a function of … Pitch, diameter, torque applied, friction Calibration: torque wrench and load cell Hand Screw Devices: C-Clamp, wood clamps, bar clamps No “follow up” ability inherent

  19. Pressure Hydraulic Presses: Liquid pressure transferred through a piston or ram Ram area x PSI on Ram = Assembly area x PSI on Assembly

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