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

Background =

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 H 2 O loss results in coalescence and bonding Bond formation in as little as 10 minutes Full strength in 24 hrs

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” Creep

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 o F) can also adversely affect emulsion and cure

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

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

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

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 ft 2 (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 ft 2 Alternate method – film thickness measurement

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)

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

Pressure Six Basic Mechanisms of Application:  Live Roll  Dead Weight  Spring  Screw  Ram  Pneumatic (Fluid) 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/in 2 ) Example – 200 lb. rock on a 1 ft 2 panel Load = ? Pressure = ? Rigid versus Fluid Pressure – Rock versus Sandbag Pressure

Dead Weight Systems – Direct reading systems Pressure = load divided by area Examples: A. 200 lb rock on 1 ft 2 panel B. 55 gallon drum of water on same panel C. Same drum hung from lever with 4:1 advantage Pressure A.1.39 lb/in 2 B.3.17 lb/in 2 Load = 55x8.3 = lbs C lb/in 2 Load = 456.5x4 = 1826 lbs

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 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 Pressure

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