polyvinyl acetate adhesives forestry 485 lecture 3-4
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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. - PowerPoint PPT PresentationTRANSCRIPT
Polyvinyl Acetate Adhesives
Forestry 485 Lecture 3-4
BackgroundVinyl group a.k.a. Ethylene
Self Addition Polymerization
Polyvinyl / Polyethylene
BackgroundSubstituted 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 H2O 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 natureHigh temp. softens adhesiveMoisture compromises performance
Under shear PVAc bonds as strong as maplePerpendicular 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 formationEmulsion is destroyed by freezing temps.
- Tell tale chalky appearance(as opposed to translucentappearance of normal cure)
- Application to “cold” surfaces(< 60 oF) can also adverselyaffect 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 temperaturesMore resistant to creepLess sensitive to moisture and heatMore thixotropic – Less squeeze out
Monitoring the Gluing OperationBrief Overview
See: Mara Chapter 10 “Quality Control”(Note: you don’t have this, but a copy will be provided upon request)
Glue PrepApplication
Assembly TimeConditionsPressure
Monitoring
Glue Preparation
Mixing:
Manufacturer’s instructionsPre-Weigh / Pre-MeasureOrder of additionMixing Temperature
Glue ApplicationTwo Criteria:
Ratio of surface area to weight of wood elementLow = weight of adhesive per area of glue lineLbs. (liquid) / 1000 ft2 (US)High = weight of adhesive per weight of woodLbs. (solids) / lbs wood (OD)Application QC – simple measurementOne square foot, applied on both sides mass in grams X 1.1 = Lbs. / 1000 ft2
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
timeMeasurement
Wet and dry bulb thermometerSling 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/in2)
Example – 200 lb. rock on a 1 ft2 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 ft2 panelB. 55 gallon drum of water on same panelC. Same drum hung from lever with 4:1 advantage
Pressure
A. 1.39 lb/in2
B. 3.17 lb/in2 Load = 55x8.3 = 456.5 lbsC. 12.68 lb/in2 Load = 456.5x4 = 1826 lbs
Spring – Loaded SystemsMaintain constant pressure over time “Follow up”Load depends on spring constant and
amount of deflectionToggle or spring clamps used in shop-
scale work
Pressure
Screw PressesLoad 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